GABA Agonists and Antagonists David I. B. Kerr and Jennifer Ong Department of Anaesthesia and lntensive Care, The University of Adelaide, Adelaide, South Australia 5000, Australia I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. GABAA- and GABAB-Receptors . . . . . . . . . . . . . . . . . . . . .................................. B. GABAA-Receptor Complexes . . C. Heterogeneity of GABAA-Recep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. GABAA-Receptor Agonists . . . . . . . . . . . . . . . . . . . . . . . . A. Simple Aliphatic GABA Analogs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Altered Acidic and Basic Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Analogs of Restricted Conformation . . . . . . . . . . . . . . . . . . . . :. . . . D. Muscimol and Muscimol Analogs ................... 111. GABAA-Receptor Antagonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Competitive GABAA-Receptor Antagonists . . . . . B. Noncompetitive GABAA-ReceptorAntagonists . . . . . . . . . . . . . . . . . . . . . . . C. Ligands for Picrotoxin Sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. GABAA-ReceptorModulators . . . . . . . . . . . . . . A. Benzodiazepines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. P-Carbolines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Barbiturates A. GABAB-ReceptorAgonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. GABAB-ReceptorAntagonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

593 594 595 596 596 597 598 598 600 602 602 606 609 611 611 613 615 616 620 622 624 628 629 629

I. INTRODUCTION GABA (4-aminobutanoic acid) is present within a large proportion of neurons in the central nervous system (CNS), where it is a major inhibitory transmitter controlling synaptic transmission and neuronal excitability. Evidence for this has steadily accumulated over the 40 years since it was first identified in brain extracts,'J and GABA can now be considered to have fulfilled all the criteria for identification as a neurotransmitter within the central and peripheral nervous ~ y s t e m s . ~In- ~practice, such criteria, as laid down by Werman et aL7 (1966), have seldom been realized in their entirety for any of the acknowledged central transmitters. Rather, most have first been identified through the use of antagonists, and only subsequently have the additional criteria been established and refined. While GABA was known to exist in the CNS, and much was known of its biochemistry, GABA was not generally accepted as a transmitter until it was found that many of its actions could be antagonized by bicuculline, which Medicinal Research Reviews, Vol. 12, No. 6, 593-636 (1992) 0 1992 John Wiley & Sons, Inc.

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was introduced as the prototypical competitive GABA antagonist by Curtis et aI.8 (1970). The subsequent development of a variety of agonists and antagonists has led to the notion of heterogeneity among GABA r e c e p t ~ r s . ~As - l ~a result, at least two broad classes of receptor for GABA are now known: GABAA- and GABA,-recept~rs.~ Each of these receptor types has distinctive GABA-binding properties on neuronal membranes, and each mediates characteristically different functional responses to GABA, although both are ultimately concerned with inhibition in the CNS.

A. GABAA- and GABA,-Receptors By definition, GABAA-receptors are linked to chloride channels, and are activated by isoguvacine, modulated by barbiturates and benzodiazepines, and antagonized by bicuculline. l4 On the other hand, GABA,-receptors are linked to calcium or potassium channels, and are activated by (-)ba~lofen,9,~5 and antagonized by phaclofen and 2 - h y d r o x y ~ a c l o f e n , ~but ~-~~ are insensitive to bic~culline.~ Both GABAA-and GABA,-receptors are found as presynaptic receptors, including autoreceptors, and postsynaptic receptors. Of these, presynaptic receptors modulate transmitter release from synaptic terminals, where autoreceptors inhibit the release of GABA itself, whereas the postsynaptic receptors are responsible for inhibiting excitability of the postsynaptic cells (Fig. 1). Using a variety of biochemical, neurochemical, anatomical, and electrophysiological techniques, combined with the latest advances in molecular biology, a ubiquitous distribution of heterogeneous GABAA- and GABA,receptors has been identified, at least in the brain. Some actions of GABA, and of GABA analogs with folded conformations, cannot be adequately described within this framework of GABAA- and GABA,-receptors, and such effects have been ascribed by Johnston to a different class of receptor, the GABA,-receptor. l6 Characteristically, these are insensitive to bicuculline, recalling the invertebrate receptor. l7 Whether or not these GABA,-receptors are in any way related to the newly described18 picrotoxin-sensitive, bicuculline-insensitive GABA-receptors of retinal origin is not clear. Such diverse GABA-receptor subtypes may be responsible for the complexities of GABA in mediating neural inhibitory mechanisms. David Kerr completed his Ph.D. in 1953 on structure-action relations in benzazole central relaxants at the Department of Physiology and Pharmacology, University of Adelaide. One-time Chairman of the Department and Dean of Science, he is now an Honorary Visiting Research Fellow in the Department of Anaesthesia and lntensive Care, University of Adelaide, and an Honora y Associate of the Department of Pharmacology, University of Sydney, which he joined in 1986. He has been a Visiting Scientist at UCLA, University of Oregon Medical School, and University of Edinburgh, as well as Visiting Fellow at John Curtin School of Medicine, Canberra, and the Department of Pharmacology, University College, London. Jennifer Ong is a Australian Research Council Research Fellow in the Department of Anaesthesia and Intensive Care, University of Adelaide. She graduated in 1986 with a Ph.D. in G A B A pharmacology at the University of Adelaide. She joined the Department of Pharmacology, University of Sydney in 1986, and was awarded a Queen Elizabeth 11 Research Fellowship by the Australian Research Council. Retaining an affiliation with the Department of Pharmacology, University of Sydney, she moved to the Department of Anaesthesia and lntensive Care at Adelaide in 1989, where she has continued her research on GABA,-receptors.

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Presynaptic Receptor

G&A-

Synaptic Cleft .

I

GA

Postsynaptic Neuron

, *Postsynaptic

Receptor

4K.B

u Kf

Figure 1. GABAA-and GABAB-receptors are subdivided into presynaptic and postsynaptic, and auto-receptors. Presynaptic receptors not only modulate the release of GABA, but can also inhibit the release of a variety of other transmitters.

Because GABA research to date is so wide, this review will focus more on recent advances made in the area of the development of newer compounds, emphasizing where possible the requirements for selective agonist, antagonist, and modulatory actions at these receptors. Several general reviews on ligands for GABAA-receptorsand their modulatory sites are available, covering the earlier l i t e r a t ~ r e , * which ~~~~ is ~so~extensive ~ - ~ ~ that no exhaustive treatment is possible here, even for the most recent literature. Instead, the present discussion is intended to highlight some of the controversies, and to introduce selected aspects being most actively pursued.

B. GABAA-Receptor Complexes The GABAA-receptor is known to be a site of action of several classes of pharmacologically active compounds, including important therapeutic agents such as benzodiazepines and barbiturates, which potentiate the actions of GABA.14 It is also well established that the GABAA-receptorcomplex plays a significant role in the pharmacology of several anxiolytic, anticonvulsant, sedative-hypnotic, and anaesthetic drugs. The receptor is classically a heterooligomeric complex comprised of at least four types of multiple allosterically interacting binding sites, together with an integral chloride ion channel. On the receptor, the allosteric binding sites identified are as follows: (1) the GABA agonist/antagonist site; (2) the benzodiazepine site, ligands for which can be further subdivided into anxiolytic agonists and anxiogenic inverse agonists; (3) the barbiturate site, which mediates some of the anaesthetic and anticonvulsant actions of barbiturates; and (4) the picrotoxin site, where many convulsant agents bind. Each of these sites is thought to be physically

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distinct, and can be occupied simultaneously to induce their pharmacological effects through allosteric interactions. Additional sites on the GABA,-receptor complex have been proposed for avermectin Bla, an ant helm inti^,^^,^^ and steroid^.^^,^^ In particular, some of the anaesthetic and sedative, as well as proconvulsant actions of endogenous and synthetic steroids can be attributed to their modulatory effects on the GABAA-receptorcomplex, either potentiating or inhibiting GABAA-receptormediated neurotransmission at relatively low concentration^.^^,^^,^^

C. Heterogeneity of GABAA-Receptor In revealing the number and function of native GABA,-receptor subunits, molecular biological approaches have painted a more complex picture of GABAA-receptors than first predicted by conventional pharmacology. Indeed, recent advances in molecular cloning technology have contributed to major progress in the understanding of GABA,-receptor structure, particularly in the field of isolation, purification, and reconstitution of the different subunits. Various subunit types of the brain GABA,-receptor have been cloned, and their primary structures defined. This receptor is a member of the ligand-gated superfamily of receptors, sharing strong homology and common architectural features with other neurotransmitter receptor types such as glycine and nicotinic receptor^.^^,^^ The cloning of multiple types and subtypes of polypeptides reinforces the considerable complexity of the GABA,-re~eptor,~~,~~ and to date it is clear that at least four different major classes of subunits exist (a,p, y, and 6), with multiple variants existing within each of these classes.32Most of the subunits can be activated by GABA when expressed individually in Xenopus ~ o c y t e s , ~ ~ the resultant GABA-activated choride channels showing multiple conductance levels with potentiation by pentobarbital and inhibition by p i ~ r o t o x i n . ~ ~ The roles of the y and 6 subunits are less well characterized than those of the a and p, but the y2 subunit seems to be essential for normal modulation of GABAA-receptor complexes by b e n z ~ d i a z e p i n e s . ~Recombinant ~,~~ coexpression of these subunits has produced varied receptor subtypes that reflect the heterogeneity of GABA,-receptors. 33,37 There appears to be sigruficant regional variation in the distribution of mRNAs for the different GABA, receptor polypeptides in the brain, consistent with pharmacological and biochemical evidence of receptor heterogeneity. However, the functional significance of these diverse receptor complexes is still not understood, but it can be anticipated that distinctive subtypes will be found, characteristic of their different locations and inhibitory synaptic actions at presynaptic and postsynaptic sites. 11. GABAA-RECEPTOR AGONISTS GABA itself can assume a wide range of configurations due to its inherent rotational flexibility. Considerable effort has been made to define the optimal GABA conformation for activity at postsynaptic GABA,-receptors, but little is known of any specific requirements at presynaptic receptors or autoreceptors. The question of the particular configuration, folded as against extended (Fig. 2), for GABA interaction with different types of GABA,-receptor is not trivial,

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since GABA dysfunction is obviously linked to a wide range of neurological and psychiatric conditions, and agents with a specificity for these subtypes may well have considerable therapeutic interest.38 In particular, it is likely that definite distinctions will eventually be made between postsynaptic and presynaptic GABA,-receptors, with autoreceptors forming a separate group. A major caveat in all such studies is the possibility that the observed activity of particular ligands may in fact be greatly influenced by uptake mechan i s m ~This . ~ ~is especially so if the agents themselves are inhibitors of this process, when their apparent activity is likely to be confounded by accumulation of endogenous GABA in the region of the receptors at GABAergic synapses. Indeed, even GABA itself is considerably more potent in the presence of glial and neuronal uptake inhibitor^.^^

A. Simple Aliphatic GABA Analogs In defining the requirements for activity at postsynaptic GABA,-receptors, considerable advances have resulted from the study of GABA analogs with restricted c o n f ~ r m a t i o nand , ~ ~from systematic variation of the acidic or basic functions. Of the aliphatic analogs (Fig. 2), GABA itself is the most active, and either lengthening, shortening, or substituting the butyryl chain results in substantial loss of agonist activity, although 5-aminopentanoic acid is a weak ag~nist.'~ From , ~ ~the point of view of defining the receptor, GABA analogs with alkyl or halogen substituents on C2-442 are of minor interest as GABAAreceptor ligands, since they retain the full conformational flexibility of GABA. Of more immediate importance are the hydroxy analogs of GABA, both 2OH-GABA and 3-OH-GABA being active at GABAA-receptors.21,43,44 Indeed, 3-OH-GABA (P-hydroxy-GABA, GABOB) was once seriously proposed as the endogenous GABA-like inhibitory t r a n ~ m i t t e rrather , ~ ~ than GABA itself. As a GABAA-agonist, its activity in some preparations is partly confounded by additional partial agonist/antagonist properties at GABA,-receptors. Earlier work with 3-OH-GABA was complicated through the use of the racemate, but stereospecific syntheses have been d e ~ i s e d ,and ~ ~ the , ~ S-( ~ +)-enantiomer ( S -

H2N-

COOH

GABA

SGABOB

+HzN

EDAC

+

H2N

4 F! H2Nu--S03H r c W H

+H$J

r C W H

A r S

H2N

3-APS

H2N

GNAA

H2N

JCOOH

S

TUAA

Figure 2. GABA analogs active as agonists at GABAA-receptors.

TWA

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GABOB) is the most potent at GABA,-receptors. The absolute configuration of these stereoisomers has also been ascertained, so that, although retaining flexibility, they are of some use in structure-action studies.44

B. Altered Acidic and Basic Groups Replacements of the carboxyl group that still give GABAA-receptoractivity are found in the sulfonic and sulfinic analogs, whereas the phosphonic and phosphinic acids are selective ligands for GABA,-receptors, and are inactive at GABA,-re~eptors.~~,~~ Partly due to lack of uptake, 3-aminopropylsulfonic acid (3-APS) is more potent than GABA.50 Curiously enough, the totally unrelated compound ethylenediamine has been proposed as an "agonist" for GABA,-rece~tors.~~ However, in the presence of carbon dioxide, ethylenediamine is rapidly converted to the carbamate (EDAC), which resembles GABA, and is either itself a GABA,-agonist or possibly rapidly releases GABA by a homoexchange m e c h a n i ~ m ~certainly, ~ , ~ ~ ; in some peripheral preparations, depletion of endogenous GABA reduces the activity of ethylenediamine in the presence of carbon d i o ~ i d e . ~ ~ , ~ ~ In general, the amino group of GABA cannot be substituted without severe loss of activity at GABA,-receptors, although N-benzoyl and pivaloyl GABA derivatives are of interest since they can enter the brain where they are hydrolyzed and exhibit anticonvulsive activity.56The only alterations of the amino group that still show activity at GABAA-receptorsare those where it is replaced by a charge-delocalized guanidino group, or by the corresponding Sthioureyl residue with similar charge delo~alization.~~ The guanidino analogs become more active as the chain length is shortened, so that the overall charge separation more nearly resembles that of GABA itself, as in guanidinoacetic acid (GNAA, Fig. 2). Such analogs were among the first examined for GABA-mimetic proper tie^.^^ By contrast, however, S-thioureylacetic acid (TUAA) is a weaker GABA displacer than the next higher Sthioureylpropionic acid (TUPA). Unfortunately, few of these thioureyl compounds appear to have been examined in functional tests, but become of considerable interest when conformational restriction is introduced by unsaturation in the corresponding thioureylpropenoic acids.

C. Analogs of Restricted Conformation From the point of view of attempting to find some optional configuration for activity of GABA at GABA,-receptors, Johnston and his colleagues have surveyed the possible ways in which conformational restriction can be achieved.40 Few of the compounds they reviewed show any activity, but among the more interesting are those with a double bond on the butyryl chain (Fig. 3). In particular, the 4-aminocrotonic acids point to an extended GABA conformation as the optimal for GABAA-receptoractivation, as the (trans) Eisomer (TACA) is comparable to GABA as a bicuculline-sensitive depressant, whereas the (cis) Z-isomer (CACA) is a bicuculline-insensitive d e p r e ~ s a n t . ~ ~ In 4-aminotetrolic acid (4-ATTA), the chain is linearly constrained, and its activity at GABA,-receptors confirms the notion that an extended rather than folded conformation of GABA is preferred at GABAA-receptors. Unlike the saturated analogs, a number of derivatives of E-4-aminocrotonic acid with

GABA AGONISTS AND ANTAGONISTS H2Nf l C O O H

H

2

N

jCOOH H2N,

599

-

CACA

TACA

A'ITA

ZAPA

ATPA

ACPA

COOH H2N$CooH CH3

S-MTAC

ACPEA

Figure 3. GABAA-receptoragonists with restricted conformations.

substituents on the "GABA backbone" show significant activity at these receptors. In particular, the 2-F, C1, Br, and Me, the 3-C1, and the 4-Me derivatives are active. The latter is of particular interest since the S-(-)-isomer (S-MTAC) is some 40 times more potent than R-(+)-isomer in GABA, binding40 However, all these analogs retain some degree of conformational flexibility that limits their use in defining the GABA,-receptor pharmacophore, beyond stressing that an extended conformation of GABA is favored. Nevertheless, the (S)-isomer of the 4-Me derivative has been used for this purpose.38 Alteration of the basic function in such unsaturated analogs, as in 2-3-(amidinothio)-propenoic acid (ZAPA) provides extremely potent agonist activity at low-affinity GABAA sites60;unfortunately, such S-thioureyl analogs are inherently unstable due to ready oxidation, which severely limits their use. From the structure-action point of view, however, ZAPA is again of considerable interest since the double bond restricts the available configurations that it can assume, emphasizing the preference for an extended form, while the corresponding E-analog is inactive. More definitive restricted conformations of GABA are provided by incorporating part of the GABA backbone into carbocyclic ring structures where there is a marked reduction of conformational mobility. Consideration of these again points to an essentially "extended" GABA conformation as being most favorable for activation of GABA,-receptors. The smallest of these, with a cyclopropane ring, retains activity in the amino-methyl trans isomer of the 1carboxylate (ATPA), whereas the corresponding trans-cyclobutane analog is less active.61,62However, activity becomes more potent in the 3-amino-cyclopentane derivatives,63,@ where the trans acids again show the higher a~tivity.4~These analogs have been synthesized as stereoisomers, (+)-1S,3S-3-aminocyclopentane-l-carboxylic acid (ACPA) being a GABA agonist comparable with muscimol in potency. The corresponding analog with a conjugated double bond, (+)-(4S)-4-aminocyclopent-l-ene-l-carboxylic acid (ACPEA), is somewhat less active, although still slightly more potent than GABA i t ~ e l f . 6Moving ~ the double bond to the 2 position in the (+)-1R,4Ranalog reduces the GABA, agonist potency to some fourfold less than GABA. Enlarging the ring to cyclohexane causes a fundamental loss of activity. Of all these cyclic analogs, the conformationally restricted (+)-4S-4-aminocyclopent-1-ene-1-carboxylic acid is of particular interest as a potential conformationally restricted template structure for the fitting of GABA analogs to possible conformations suited to GABAA-receptors.65

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P-4s

IGUV

DHP4S

4-PIOL

Figure 4. GABA analogs with a cyclized basic functionality.

Incorporation of the basic function of GABA into a five-membered pyrrolidine ring system (Fig. 4) markedly reduces GABAA agonist activity, as in P-proline.66 However, homo-P-proline (pyrrolidin-3-ylacetic acid; P-3A) retains some activity, while the sulfonic analog of homo-p-proline (P-3SA) is a relatively potent GABA, a g ~ n i s tIf. ~the ~ cyclized basic function is replaced by an imidazole ring, as in imidazole-4-acetic acid (IAA), there is a marked increase in potency at GABAA-receptors.@This is evidently related to an altered charge distribution over the planar ring, with considerable delocalization of the positive charge. Imidazole-4-acetic acid occurs naturally as a metabolite of histamine, and was early found to be a GABA agonist.68 Upon completely incorporating the GABA backbone into a six-membered ring system, there is considerable activity at GABA,-receptors. In particular, an acidic functionality at the 4 position of the six-membered piperidine ring yields a number of active GABA, agonists. Of these, piperidine-4-carboxylicacid (isonipecotic acid; P-4C) is of moderate activity,69and piperidine-4-sulfonic acid (P-4s) is more potent than GABA as a bicuculline-sensitive a g ~ n i s tThe .~~ latter sulfonic analog is unusual in its interactions with benzodiazepine binding, in that it deactivates such binding at 0 "C in the presence of chloride ions, but exhibits normal potentiation at 37 "C. The corresponding compound with a 3-isoxazolol (4-PIOL) replacing the sulfonic group is also anomalous in benzodiazepine binding. 7O Introducing unsaturation into the ring of isonipecotic acid and piperidine-4sulfonic acid further increases their potency. Of these, the carboxylic analog isoguvacine (IGUV) is a GABA, agonist of choice in many binding assays, including those for GABA,-receptors. The sulfonic analog DH-P4S has been less used in functional studies on GABA,-receptors, but in view of the peculiar binding properties of the saturated piperidine-4-sulfonic acid, this may also be a useful agent for probing modulatory interactions at these receptors.21 D. Muscimol and Muscimol Analogs

Probably the single greatest advance in structure-action studies at GABA,receptors has been provided by the discovery that the naturally occurring isoxazolol muscimol (MUSC) is a potent agonist at these receptor^,^^ with a strong structural resemblance to GABA.71Of all the related analogs (Fig. 5),

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MUSC

DHMUSC

TMUSC

ISTHAZ

Figure 5. Muscimol and muscimol analogs

muscimol is the most potent, but thiomuscimol (TMUSC) and S-(+)-dihydromuscimol (DHMUSC) are also among the more potent known GABAAreceptor agonists. Muscimol represents a GABA agonist where there is considerable loss of flexibility at the isoxazolol head, the carboxyl group being replaced by a negative charge-delocalized ring structure. The molecule does, however, retain rotational flexibility about the aminomethyl side chain, sufficient to allow some activity at GABA,- as well as GABA,-receptors. An optimal "muscimol conformation" has been suggested that differs somewhat from the "bicuculline conformation" proposed for agonist interaction at GABA,-receptom6' Nevertheless, a reasonable overlap of muscimol and bicuculline structures can be achieved, perhaps defining the range of active conformations at bicuculline-sensitive GABA,-receptors. This remaining flexibility of the muscimol molecule has been further reduced by incorporating the basic function into a ring structure (THIP), where the conformation is essentially locked within the constraints of the bicyclic system.50THIP is a moderately potent GABA, agonist that has undergone clinical trials and perhaps represents the true muscimol conformation for activating low-affinity GABA,-receptors. This is of importance, since some GABA,-receptor subtypes show markedly different affinity for THIP, and the more active of these are likely to be lowaffinity functional GABA,-receptors. Many other muscimol analogs have been prepared, but most are uniformly poorly active or totally inactive as GABA, agonists; for instance, enlarging the "piperidyl ring" converts THIP to a mixed GABAA-and glycine receptor antagonist (THAZ), as is the related iso-THAZ (ISTHAZ).72However, if the piperidyl moiety of THIP is detached, the resultant 5-(4-piperidyl)-3-isoxazolol (4-PIOL; Fig 4) is a GABA, agonist, analogous to isonipecotic and piperidine-4-sulfonic acid~.~O Like the latter, 4-PIOL exhibits effects on benzodiazepine binding to synaptosomal membranes which differ from those of the conventional GABA, agonists. Such compounds are of considerable interest for examining the nature of the GABA-receptor/benzodiazepinereceptor interaction, but they have been little used in neuropharma~ology.~~ In this regard, 4-PIOL is of particular interest since there are regiospecific differences

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in its behavior at GABA,-receptors and associated benzodiazepine sites. Spinal GABA,-receptors are activated by 4-PIOL, whereas supraspinal receptors behave as though they are antagonized by this analog. Moreover, the effects of 4-PIOL on benzodiazepine binding are a mirror image of those with muscimol, suggesting that it may in some way act as an "inverse agonist" at these sites (see Sec. 1V.A on benzodiazepines), which would account for its apparent antagonist properties in some regions. The GABA, agonist properties of 4-PIOL in the spinal cord may then reflect activity at GABAA-receptorsthat are poorly coupled to benz~diazepines,~~ following an earlier suggestion that such receptors exist.20

111. GABAA-RECEPTOR ANTAGONISTS At first, the only GABA, antagonist available was picrotoxin, which was originally examined on invertebrate preparations. 73 However, its poor aqueous solubility and lack of any charge on the molecule precludes its iontophoretic application. Consequently, it never came into general use as an antagonist, although there are early reports of antagonist actions in the vertebrate nervous system.74Nevertheless, the site at which it acts on the receptor complex has become of major interest, more particularly in binding studies with membrane preparations. Functionally, it is a noncompetitive antagonist showing interesting interactions with other modulators of GABA-induced responses. 75-79

A. Competitive GABAA-ReceptorAntagonists Bicuculline, a phthalide isoquinoline alkaloid, was the first competitive GABAA-receptorantagonist shown to be specific against GABA.8,80,81It was the introduction of this antagonist that eventually led to general acceptance of the notion that GABA is an inhibitory transmitter in the mammalian CNS, although curiously enough, bicuculline-insensitive GABA,-receptors were only hinted at and never identified in any of these earlier studies. Antagonism with bicuculline is stereospecific, (lS, 9R)-( +)-bicuculline [lS, 9R-( +)BIC; Fig. 61 being the active enantiomer, as against the much weaker (lR, 9s)

lS,gR-(+)-BIC

(+)-HYDR

BCC

Figure 6. Phthalide isoquinoline GABAA-receptor antagonists.

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isomer. The two methylenedioxy bridges are not in themselves essential for activity of these phthalide isoquinolines. Corlumine is a weaker antagonist with the same (IS, 9R) absolute configuration, where the methylenedioxy is replaced by two methoxy groups on the tetrahydroisoquinoline moiety. However, (+)-hydrastine [(+)-HYDR], with two methoxy groups replacing the methylenedioxy bridge of the phthalide benzoisofuranone moiety, is even more active than (+)-bicuculline as a GABA,-receptor antagonist, and the most active known phthalide isoquinoline antagonist.82 Opening the lactone ring of bicuculline to give bicucine (BCC) totally inactivates its antagonist activity. Since the lactone readily opens under unfavourable conditions of pH,83 this property originally contributed to considerable confusion over early attempts to identify synapses where bicuculline-sensitive GABAergic inhibition might operate. Eventually the more stable and soluble quaternary derivatives (BJCM, Fig. 6) bicuculline methochloride and bicuculline methiodides4fs5were introduced, although the lactone ring can still open even in these derivatives. Furthermore, it should be pointed out that bicuculline and its quaternary derivatives do show effects that suggest some mode(s) of action unrelated to its GABA, antagonist or anticholinesterase properties. 86-88 It is a characteristic feature of bicuculline interaction with GABAA-receptors that this antagonist preferentially combines with low-affinity sites,s9 and that chaotropic agents (e.g., thiocyanate) enhance the ability of bicuculline to displace [3H]-GABA from its low-affinity binding sites. 90,91 Maksay and Ticku9* have further suggested that this effect arises through the chaotropic agents acting to promote interconversion of GABA-receptor sites from a hydrophilic (agonist) state to a hydrophobic (antagonist) state. As a result, the antagonists then bind more readily through such hydrophobic accessory sites, consistent with the original generalized suggestion by Ariens et and the proposal by Mohler and Okada93that GABA,-receptor affinity sites exist in two states, one binding agonists and the other binding antagonists. Since bicuculline appears to occupy only such low-affinity binding sites on the GABA,-receptor complex, affinity labels or irreversible inactivators related to bicuculline would be of considerable interest as investigational tools for these sites. The most encouraging compounds so far are bicuculline derivatives with substituents in the 5 position (AMSB),94derived from the original attempts to prepare such derivative^,^^ but these require a more thorough characterization. Following the pioneer effort by Andrews and Johnston61(1979) to establish the manner in which bicuculline or muscimol might interact with the GABA,receptor, there have been further attempts at defining the nature of this interacti0n.96-~~ In regard to the suggestion that some feature of the bicuculline structure serves to block the chloride channel after combination with the recept0r,9~it is worth recalling that bicuculline does not behave as a chloride channel blocker, nor does it alter the kinetics of the channel. Its action is quite different from that of picrotoxin which does in some way alter the coupling between receptor and channel opening, holding the channel for longer periods in the closed state. Furthermore, bicuculline behaves as a true competitive antagonist in pharmacological tests, competing for the GABA binding site, as it does with binding of GABA agonists in isolated membrane

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preparations. In contrast, the true ion channel modifiers (Sec. 1II.B) are all noncompetitive antagonists. For a time, bicuculline was the only known competitive GABAA-receptor antagonist, acting at the GABA recognition site rather than the differently located picrotoxin site. However, a number of other antagonists are now known (Fig. 7), including a novel steroid derivative, RU 5135 (3cx-hydroxy16-imino-5~-17-aza-androstan-ll-one), which is a potent antagonist of GABA-stimulated [3H]-diazepambinding, some 500 times more potent than bicuculline,99 and is a potent antagonist of GABA-receptor binding. '00 Structurally, RU 5135 bears an amidine moiety on a discernable GABA skeleton within the steroid ring structure. RU 5135 is not only extremely potent in binding studies with benzodiazepines, muscimol and b i c u ~ u l l i n e , ~but ~~'~~ also exhibits strong competitive antagonism of GABAA-receptor-mediatedresponses in rat cuneate slices or guinea pig ileum, where it is some 100 times more potent than bicuculline (PA, = 8.0-8.3)101,102;this makes it the most potent competitive GABAA antagonist known. Despite such potent competitive antagonism, which suggests considerable specificity in its action, unfortunately RU 5135 is also an antagonist at strychnine-sensitive glycine receptor^,'^^,^^^ and this lack of selectivity limits its use as a GABA antagonist. Better specificity of GABAA-receptor antagonism is exhibited by pitrazepin (PITRAZ), which is some 3-10 times more potent than bic ~ c u l l i n edepending ,~~~ on the test preparation,105but pitrazepin atypically inhibits benzodiazepine binding. More recently, a new series of freely water-soluble and stable GABA,-

H

RU5135

PITRAZ

SECN CH2NH2 I

SR 42461:Cl ;SR 95531:OMe

DIMEF

2-ITDZ

3-AMP

DPA-3S

TRP-P-2

Figure 7. GABAA-receptor antagonists unrelated to bicuculline.

GABA AGONISTS AND ANTAGONISTS

605

receptor antagonists has been found. Io6 These pyridazinyl GABA derivatives, designated SR 42641 and SR 95531, represent an apparently optimal configuration for GABA,-receptor antagonism in this series, where SR 95531 is the most active, some two times more potent than b i c u ~ u l l i n eNotably, . ~ ~ ~ ~they ~~ all contain an intact GABA sequence, and lengthening, shortening, or substituting on this GABA backbone deletes activity to a greater or lesser extent. Isosteric manipulation of the pyridazine ring to give thiadiazole analogs of SR 95531 results in GABAA antagonists (2-ITDZ)less active than the parent compound.'O* SR 95531 is of interest in that, unlike bicuculline, it appears to have affinity for both high- and low-affinity GABAA-receptorsites. We have found it to be an effective antagonist against GABAA-receptor-mediatedresponses in the guinea pig isolated ileum, more potent than bicuculline itself.'Og In relation to the possible binding states for antagonists at GABAA-receptors, members of the polar SR-antagonist series with a free carboxyl group do not show enhancement of binding in the presence of chaotropic agents,lo6but do so when the acidic function is replaced by a cyano group.11oThis supports the notion that the large, relatively nonpolar structures that replace the carboxy group in most GABAA antagonists may indeed bind at some hydrophobic region newly exposed by the chaotropic agents, as discussed above. A further class of GABAA-receptor antagonist has been introduced by Beutler et aZ.l" (1985). These are indolizidine alkaloids of the securinine group, of which the most active member is securinine itself (SECN).However, securinine is some five times less potent than bicuculline and, moreover, has been little studied for the nature and distribution of its antagonist properties in various brain regions. Nevertheless, it is of interest since it contains a "ybutyro-lactone" structure, recalling the isofuranone ring of the phthalide moiety in bicuculline. Incidentally, aminomethyl benzoisofuranones (3-AMP), directly related to bicuculline, were originally studied by Johnston and his colleagues,ll* but were inactive at GABAA-receptorsin their tests.'12 Apart from the GABAA-receptor antagonists discussed above, there are several diverse compounds that appear to act in a similar manner. Few of these have been investigated in any depth, but of possible physiological significance are the sulfated metabolites of dopamine (dopamine-3-O-sulfate; DPA-3S, and 4-O-sulfate).113 Both of these induce convulsions upon intraventricular administration, additive with bicuculline, which are prevented by diazepam as well as muscimol. Also, both metabolites displace 13H]-GABA binding from synaptic membranes. It has therefore been suggested that they are bicucullinelike GABA antagonists, although no detailed functional studies appear to have been made. Likewise, the polyacetylenic alcohol ~ u n a n i o l l is '~ a convulsant of comparable potency with picrotoxin, but, apart from a demonstration of GABA antagonism in the cat spinal cord, cunaniol does not seem to have been studied further and it is not clear if it is a competitive antagonist. The analeptic dimefline (DIMEF)also exhibits GABA antagonism, but this is confounded by atropiniclike action^."^ Benzylpenicillin is a weak GABAA-receptorantagonist, as is tubocurarine; but these are also glycine antagonists, particularly the latter.116,117 This raises the question of which part of the receptor complex might be the site where these "nonspecific" antagonists act, remembering that both GABAA- and glycine-activated chloride channels belong to a superfamily with strong ho-

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mologies. A similar lack of specificity between GABA and glycine is exhibited by iso-THIP and iso-THAZ (Fig. 5), although these two analogs of restricted conformation are more interesting in that relationships to GABA are clearly evident in their structure. The same resemblance to GABA is true for 5-guanidinopentanoic acidll8 and the taurine analog guanidinoethanesulfonate,ll9 both of which are convulsants. Most recently, a series of amino-ycarbolines (TRP-T-2;Fig. 7) has been described that are probably true GABA,receptor antagonists, not benzodiazepine inverse agonists or picrotoxinlike agents,lZ0and these too deserve better characterization. Although a number of antagonists acting at the GABA/bicuculline site are known, of which bicuculline itself is best characterized, nevertheless, very little is known of the differential antagonist potencies at presynaptic, postsynaptic, or autoreceptors among the various compounds, and this deserves more attention. In particular, specific agents for autoreceptors might have considerable therapeutic application since these regulate GABA release, and antagonists might well increase GABAergic inhibition.

B. Noncompetitive GABAA-Receptor Antagonists Specific picrotoxin sites are likely found on all subunits of the GABA, receptor complex, certainly on the a and p subunits where they are closely associated with the GABA-receptor region.34Picrotoxin is an equimolar mixture of picrotoxinin (PIC; Fig. 8), PIC the active convulsant constituent, and the less active picrotin (PCRT), that are ligands for these PTX sites on the receptor complex. Picrotoxinin is one of a number of closely related compounds of plant origin including coriamyrtin (COR).lZ1 Consideration of

PIC

COR

PCRT

DHP

CHGBL

BFGBL

Figure 8. Picrotoxin-related GABAA-receptor antagonists.

607

GABA AGONISTS AND ANTAGONISTS

these structures indicates that the essential elements for antagonists of this type are the cyclohexane ring and its lactone bridge that forms a y-butyrolactone bearing a P-isopropenyl group (CHGBL).122 Hydrogenation to dihydropicrotoxinin (DHP), with an isopropyl group replacing the p-isopropenyl side chain, provides only a very weak antagonist that nevertheless has been used as a tritiated ligand for the antagonist site.123 It is thus the p-isopropenyl substituent that in large part confers antagonism on this series of picrotoxin-related compounds. Indeed, it has been sugg e ~ t e dthat l ~ ~the active moiety may in fact be the p-isopropenyl-y-butyrolactone ring (BPGBL), rather than the cyclohexane ring skeleton (CHGBL) to which it is attached. This is supported by x-ray ~ r y s t a l l o g r a p h y which ,~~~ clearly shows the p-isopropenyl-y-butyrolactone ring as an exposed face that would be readily accessible to some binding site within the receptor complex. However, differences in potency between the various naturally occurring picrotoxin analogs indicates that the remainder of the structure is also of importance in guiding this active lactone ”face” to the binding site and ensuring its binding capacity. Simple p-substituted y-butyrolactones form a class of convulsants sharing features with picrotoxinin in their action^.^^^,^^^ In general, such p-substitution confers convulsant properties regardless of any other substitutions on the lactone ring. Interestingly, y-butyrolactones substituted at the a- or ypositions, or both, are anticonvulsants rather than convulsants as seen with p-substituted Moreover, the convulsant effects of p- or a,P-substituted lactones can be reversed by potent a-substituted anticonvulsant y-butyrolactones,124,127,’29-131 the implication .being that all such compounds act at the picrotoxinin site of the GABAA-receptorcomplex. Ligands for the PTX site are now known that can either diminish or augment GABAA-receptor-mediatedresponses, consistent with either ”agonist” or “inverse agonist,” as well as ”mixed antagonidinverse agonist” proper tie^.^^^ Interestingly, a,a-di-isopropyl-y-butyrolactone (DPGBL; Fig. 9) appears to be a pure “antagonist” at the PTX site, which has no effect on GABA-induced responses, but blocks the actions of both agonists and inverse agonists (convulsants and anticonvulsants respectively) at this site.132Thus ligands for the PTX site exhibit the same continuum from agonists to inverse agonists to antagonists that is found at benzodiazepine sites on the GABAAreceptor complex. The functional implications of this continuum at the PTX site have yet to be defined, but PTX sites seem to be present on all subunits of the GABAA-receptorcomplex, pointing to their importance for the normal

DPGBL

ESM

ANIS

DECL

Figure 9. Noncompetitive GABAA-receptor antagonists.

BEM

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operation of the receptor complex. As with benzodiazepines, some admixtures of a and P GABAA-receptor subunits when expressed in Xenopus oocytes show anomalous behavior, where picrotoxin induces a chloride current, rather than blocking it as is ~ s u a 1 .Such l ~ ~ behavior is compatible with the ability of the PTX site to be modulated in a bidirectional manner. Thiolactones and dithiolactones, analogous to the y-butyrolactones, also show anticonvulsant or convulsant properties depending on the presence of a or P alkyl s u b s t i t ~ e n t s , ~ The ~ ~ -heteroatom ~ ~ ~ , ~ ~ ~can also be an ionizable N in the corresponding lactam, or more often, the imide, as in ethosuximide (aethyl-a-methylsuccinimide; ESM). In these imides, the nature of the a-alkyl substituent again governs the properties at the PTX site, the related a,a,a',attetramethylsuccinimide being a convulsant that is reversible by the "inverse agonists" such as a-ethyl-a-methyl-y-butyrolactone.126~127~130~135 It can be seen that the at,a' substituents in one sense correspond to the P substituent of the y-butyrolactones. Relatively little work has been done on the activity at the PTX site of analogs with larger rings. The most familiar is the convulsant analeptic bemegride (P-ethyl-P-methylglutarimide (BEM), while anisatin (ANIS) is a relatively potent displacer of TBPS binding and an antagonist at GABAA-receptors.136 Anisatin contains a six-membered lactone ring substituted at what corresponds to the a and P carbons of the y-butyrolactones. 137 At one time, it was assumed that longer-chain o-amino acid analogs of GABA are antagonists at GABAA-receptors.58In fact, there is no strong evidence for this, and the mechanism of this "anti-GABA property of the longchain w-amino acids remains obscure. However, when such long-chain oamino acids are cyclized, the corresponding lactams are antagonists acting at the PTX site. The parent caprolactam is relatively inactive, but becomes more active upon alkyl substitution. Our own studies have included "P"-substituted caprolactams which are powerful convulsants, in particular, 6-ethylcaprolactam and 6-ethyl-4-ethyl-caprolactam (DECL; Fig. 9).138-140These convulsant lactams are noncompetitive GABAA antagonists, and their actions are reversed by barbiturates just as with picrotoxinin i t ~ e 1 f . lInterestingly, ~~ the 3- and 7-alkyl substituted ("a")analogs of caprolactam are anticonvulsants. 138~142In retrospect, these compounds with opposing actions may represent earlier examples of what have now become described as agonists and inverse agonists at the PTX site. Although barbiturates have been suggested to act at a special barbiturate site on the receptor, nevertheless, the specific reversal of PTX by certain barbiturates strongly recalls the inverse-agonist activity of the a-substituted y-butyrolactones which act directly at the PTX site. Certainly, the barbiturate and PTX sites, even if different, are very closely coupled since ligands for each mutually prevent the actions of one another. This is illustrated at the functional level where the GABA-potentiating barbiturates such as pentobarbitone and thiopentone are effective picrotoxin "antagonists," in that they restore the slope and maximum of GABA dose-response curves in its presence.141 There are several caveats to be borne in mind when using barbiturates in such studies, not the least of which is the possibility that they can suppress transmitter release, as with excitatory amino acids,143which may explain their anticonvulsant but not their anaesthetic properties. Moreover,

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because of such additional actions, there is in general no good correlation between sedative-hypnotic or anaesthetic actions and the ability to reverse picrotoxin in a range of barbiturate^.'^^

C. Ligands for Picrotoxin Sites Apart from the various lactones related to picrotoxinin, a diverse series of quite unrelated convulsant compounds exists, which are also ligands for the PTX site on the GABA,-receptor complex (Fig. 10). Of these, the most studied are the cage convulsants, including some of the most potent convulsants available (TBOB, TBPO, TBPS). Uniformly, the cage convulsants are based on symmetrical phosphonates with a short alkyl s u b s t i t ~ e n t .Because ~ ~ , ~ ~they ~ can be prepared in labeled form and are considerably more potent than picrotoxinin, the cage convulsants have replaced [3H]-dihydropicrotoxininas probes for the PTX site, which is now more often referred to as the TBPS binding site.137Until recently, the exact location of this site on the oligomeric complex of the GABA, receptor was unknown. However, this is now amenable to affinity labeling by isothiocyanate derivatives of tert-butylbicycloorthobenzoate (TBOB), particularly the p-isothiocyanate (TBOBT).146This does not appear to have been done, but if suitably labeled, this should allow an exact identification of the peptide sequence at the PTX binding site on the particular subunits of the complex. In addition to these cage convulsants, a range of tetrazoles, glutarimides, succinimides, and benzodiazepines are also active at the PTX sitel47 (Figs. 10 and 11). None of these have been at all well characterized, in particular by using some functional response to assess their mode of action or sensitivity to barbiturate-induced reversal. Tetrazole based analeptics are among the more familiar antagonists acting at the PTX site.148Of particular interest are the 1,5dialkyl tetrazoles where either analeptic or depressant activity is found depending on the nature of the s u b ~ t i t u e n t s , 'a~ ~property that recalls the bidirection actions of substituted y-butyrolactones. The best known tetrazol analeptic is pentylenetetrazole (1,5-pentamethylenetetrazole, metrazole, MTZ; Fig. lo), which is in fact only a weak displacer of TBPS binding. However, 6,6'dichoropentamethylene tetrazole (DCMTC), 8-t-butylpentamethylenetetrazole (DBMTZ)150(Fig. 10) and camphortetrazole (6-methyl-6,9-isopropyliden-pentamethylene-tetrazole)147 are all potent displacers of TBPS binding. An interesting variant on the cage convulsants is the highly symmetrical molecule TMTD (Fig. lo), which we originally proposed as a GABA antag~ n i ~ t although , ~ * ~the~action ~ , of ~ TMTD ~ ~ appears nonspecific since it is also a blocker of glycine-activated chloride channels. In CPK space-filling models, TMDT is a ball-like structure that may be a candidate for a true chloride channel blocker, although it has not yet been used in patch-clamp studies. The benzodiazepine Ro 5-3663 (Fig. 11) evidently acts at the PTX site, and the chlorodiazepam Ro 5-4864 has been suggested to do 1 i k e ~ i s e .Neither l~~ of these is sensitive to the benzodiazepine antagonist flumazenil. Cyclodiene insecticides (Fig. 11) also act at the PTX site. Among the more potent are endrin (ENDN) and endosulfan (ESNS) which block GABA-induced chloride flux and inhibit TBPS binding,15l as does deltamethrin.152It is to be antici-

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x=m:TBm

TBOB

TBOBT

X=PS: TBPS

m

TMTD

Dcm

Figure 10. Ligands for the picrotoxin site.

pated that further groups of compounds will continue to be found that act at the PTX site. For example, some excitant, convulsant quinolone carboxylic antibacterial agents have recently been shown to be effective GABA antago n i s t ~Norfloxacin .~~~ (NFXC), the most potent of these, is a noncompetitive picrotoxinlike antagonist at GABAA-receptors, suggesting that such structures could be refined for this property. Since PTX sites are ubiquitous on the different subunits of the GABAAreceptor complex, they are likely subserving some physiological/pharmacological functions, either potentiating or antagonizing GABA-induced responses. But the nature of the naturally occurring ligands for these sites is still controversial. At first sight, a naturally occurring PTX-like GABAA-receptor antagonist is difficult to envisage. However, some steroids are extremely potent GABA antagonists, among the most potent being cortisone (CTSN; Fig. 17)154and Reichsteins "Substance S (RSS; Fig. 16),155and possibly represent the endogenous ligands for the PTX site. H

O

CI

RO-5-3663

-

RO-54864

ENSN

ANwCHzCH3

&o

CI

HN

F

CI

' /

COOH

CI

0

NFXC

CI

ENDN

Figure 11. Convulsants acting at the picrotoxin site

611

GABA AGONISTS AND ANTAGONISTS IV. GABAA-RECEPTOR MODULATORS A. Benzodiazepines

Chlordiazepoxide (CDZO) and diazepam (DZPM), the prototypical benzodiazepines (Fig. 12) were introduced as anxiolytics long before GABA was thought to be an inhibitory transmitter.156Only some 15 years later was their site of action shown to be associated with GABA receptor^,^^^-'^^ and the benzodiazepine receptor (BZR) defined.160 Importantly, the imidazodiazepine flumazenil (Ro-15-1788)was found to be a benzodiazepine-receptor antagonist161 with no intrinsic activity but able to displace other benzodiazepines, thus introducing the modern era of benzodiazepine pharmacology. Almost countless variations of the fundamental benzodiazepine structure have been synthesized and tested, in an effort to produce better therapeutic agents.20 Benzodiazepines modulate the actions of GABA at GABA,-receptors, acting at a site different from the GABA recognition site or the barbiturate site.14 Functionally, they enhance GABA,-receptor-mediated inhibition by altering the rate of chloride channel opening that follows activation of the receptor, and consequently elicit anxiolytic, anticonvulsant, sedative-hypnotic, and muscle relaxant effects. Confirmatory evidence for a specific benzodiazepine site comes from molecular biology, where the a, p, and y subunits of the receptor have been assembled in various cell lines.32 Expression of receptors constructed from ct, p, or ct + p subunits leads to functional chloride channels that are operated by GABA, with enhancement by barbiturates and antagonism by picrotoxinin. However, these are atypically modulated by agonist benzodiazepines in an inverse agonist manner, but typical benzodiazepine modulation is found if y2 subunits are also present, indicating that this sub-

CDZO

RO 5-4200

DZPM

CL 218.872

CGS 8216

RO 15-1788

PK 11195

Figure 12. Benzodiazepines and ligands for BZR sites

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unit type is necessary for expression of fully functional GABAA-receptors.In addition to which, differential sensitivity to various classes of benzodiazepinelike modulators seems to be imparted by different OL subunits in the complex.162 While the presence of benzodiazepine receptor sites in GABAA-receptor complexes is generally accepted with little controversy, the explanation of the full range of activities exhibited by various ligands for these sites remains contentious. Suffice it to say that a spectrum of ligands for benzodiazepine receptor sites is now recognized, ranging from agonists through partial agonists to partial inverse agonists, and full inverse agonists. These ”inverse agonists” precipitate anxiety and are proconvulsants or have frankly convulsant actions, and in this sense are ”antagonists” at GABAA-receptors.However, true antagonists at benzodiazepine sites are known, that block each of these previous actions, without themselves eliciting any influence on GABAA-receptor-mediatedresponses.22 The present nomenclature for describing bidirectional allosteric modulation at benzodiazepine receptor sites is rather unsatisfactory, possibly misleading. As K e n a k i ~ has ~ l ~pointed ~ out, these terms agonist, inverse agonist, or antugonist, when applied to presumably allosteric interactions between GABA and benzodiazepine receptor sites, may be more fruitfully approached within the simplistic framework of affinity and intrinsic efficacy. In this regard, homomeric GABA,-receptors, from gene expression in oocytes, occasionally show anomalous activity toward benzodiazepine ligands, as though intermediate states exist where altering the equilibrium can reverse the normal ”res p o n ~ e . Indeed, ” ~ ~ one can reflect that if the p-carboline convulsant ”inverse agonists” had been found first, then they would be carboline ”agonists” that antagonize GABA, and the benzodiazepines would then have become “inverse agonists” that potentiate GABA. Not surprisingly, in view of the heterogeneity of subunits that constitute GABAA-receptor complexes, at least two types of benzodiazepine receptors are recognized: BZ,R, with high affinity for the triazolopyridazine CL 218,872, the pyrazoloquinolone CGS 8216 (Fig. 12), and p-carboline-3-carboxylate ethyl ester (PCCE; Fig. 14); and a second type, BZ2R, with less selectivity but high affinity for flunitrazepam and low affinity for CL 218,872 and P-carboline-3carboxylate ester^.'^,'^^ There is a further subtype, BZ,R, preferentially recognizing 4’-chlorodiazepam (Ro 5-4864; Fig. 11)and the isoquinoline carboxamide PK 11195 (Fig. 12).166These “peripheral” BZ,R receptors are located on glial cells, rather than neurons in the CNS, and the convulsant properties of Ro 5-4864 seem unrelated to any actions at these glial ~ i t e 9 . l ~ ~ A considerable effort has been made to define the pharmacophore for agonists at benzodiazepine sites,168but less is known of that for the inverse a g ~ n i s t salthough , ~ ~ ~ ligands for both categories likely bind within common domains on the relevant GABAA-receptorsubunits. Knowledge of this pharmacophore structure is highly desirable, since is may assist in the identification of naturally occurring ligands for benzodiazepine receptors, a number of which have been proposed.170 Originally, the only ligands of plant origin to be identified with a reasonable potency were the biflavanoid amentoflavon (AMTF; Fig. 13),17’and the flavone chrysin (CRSN).More recently, a series of 0-quinonoid diterpenes, of which miltirone (MLT) from Suluia rniltiorrhiza is

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GABA AGONISTS AND ANTAGONISTS

\ OH

I

OH

0

AMTF

MLT

I 0

CRSN

MMLT

APMLT

Figure 13. Benzodiazepine receptor agonists of plant origin.

the most p0tent,'~~,'~3 have been identified as ligands for the benzodiazepine sites. These miltirone related compounds deserve wide examination, particularly since the 4 methylene analog (MMLT) may be an antagonist. Miltirone and the more potent acephenanthrylene analog (APMLT)are probably partial agonists lacking myorelaxant properties at benzodiazepine sites.

B. P-Carbolines Proconvulsant P-carboline derivatives (BZR inverse agonists; Fig. 14) were serendipitously found in attempts to isolate naturally occurring ligands resembling benzodiazepines. The prototypical ligand P-carboline-3-carboxylate ethyl ester (P-CCE) was obtained from ethanol treatment of human urine extract at low P H . ' ~This ~ arose as an artefact during the isolation by esterification of ~-carboline-3-carboxylicacid, which itself is derived from tryptophan by ring closure. Nevertheless, the propyl ester P-CCP has been isolated under conditions more nearly precluding any such esterification, and is likely to be a true endogenous ligand of this type,*75but an inverse agonist anticonvulsant. Since then, a variety of P-carboline-3-carboxylatederivatives have been investigated for activity at benzodiazepine receptor sites. The most potent convulsant/inverse agonist amongst these is methyl,6,7,-dimethoxy-4ethyl-P-carboline-3-carboxylate(DMCM).176Subsequently, P-carboline derivatives have been found that exhibit the full range of agonist, inverse agonist, partial agonist, partial inverse agonist, and competitive antagonist activity at benzodiazepine receptor sites. However, not all inverse agonists are simple Pcarboline carboxylic esters. The related 3-hydroxymethyl-P-carboline (BCHM) and 3-methoxycarbonylamino-~-carboline (BCMCA) are also inverse agonists; the latter is capable of reversing the sedative actions of benzodiazepines without anxiogenic or proconvulsant side effects, probably since it is a partial agonist. 177

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BCC-E,P,B

DMCM CI

H

ZK 93,423

H

ZK 93,426

H

CGS 9896

CE C N

GH414

Figure 14. Benzodiazepine receptor inverse agonists and related agonists.

Strong agonist activity, resembling that of benzodiazepines, is found in ZK 93423 (6-benzyloxy-4-methoxymethyl-~-carboline-3-carboxylate ethyl ester; Fig. 14).178 The depressant actions of this analog are antagonized by Ro15-1788 (flumazenil), the classical antagonist benzodiazepine, indicative of benzodiazepine receptor involvement in its actions. Minor modification on the P-carboline-3-carboxylate structure also yields compounds having no significant intrinsic effects, yet are capable of antagonizing benzodiazepines, as well as P-carboline agonists and inverse agonists. Of these, the simple npropyl ester PCCP and the 3-t-butyl ester PCCtB are active inhibitors of benzodiazepine binding, as is 5-isopropoxy-4-methyl-~-carboline-3-carboxylate ethyl ester (ZK 93426; Fig. 14).179 Of greater relevance here is the finding of further classes of compounds that are convulsant/inverse agonists at benzodiazepine receptor sites. Among these are pyrazoloquinolines (CGS 8216; Fig. 12 and CGS 9896; Fig. 14), thienylpyrazoloquinolines, and diindoles, where relatively simple changes again convert these molecules to agonists or antagonists rather than inverse agonists. 168 It is increasingly evident that partial agonists and partial antagonists, from among such series, may yield therapeutic agents of an improved profile; in particular, some are anxiolytics without the muscle relaxant and ataxic properties of the original benzodiazepines. Little pharmacological or electrophysiologicalwork has been done with any of these inverse agonists or true antagonists, yet potentially they may yield considerable insight into the

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GABA AGONISTS AND ANTAGONISTS

mode of actions of ligands that modulate GABAA-receptor-mediatedinhibition through the benzodiazepine receptor sites. There is yet another group of compounds, based on the synthetic pyrethroid insecticide deltamethrin, which markedly enhance benzodiazepine binding to synaptosomal membranes. The most active is cycloprothrin (GH 414; Fig. 14).ISowhich considerably enhances TBPS binding in the manner of benzodiazepine agonists, and inhibits binding of the benzodiazepine antagonist Ro 15-1788. These particular pyrethroid analogs thus may represent a class of benzodiazepine agonists of unique structure which deserves further study.

C. Barbiturates Barbitone (5,5-diethylbarbituric acid; BARB, Fig. 15) and phenobarbitone (5-ethyl-5-phenylbarbituric acid; PBARB) were the first barbiturates introduced as anaesthetics around 1900. Since that time, some thousands of barbiturates have been prepared, and many have become used as hypnotics, anaesthetics or anticonvulsants. The first suggestions that the actions of barbiturates might be related to GABA-mediated inhibition come from early studies on spinal cord presynaptic inhibition, which barbiturates potentiate.181,182They also reverse the actions of picrotoxin and metrazole as GABA antagonists. Apart from their actions in reversing GABA antagonism at the PTX site (see Sec. III.B), barbiturates potentiate responses to GABA and other ligands at the GABA,-receptor complex. This is reflected in a barbiturate-induced enhancement of GABA binding in the presence of ~ h l o r i d e ,and ~ ~correlates ~,~~~ with potentiation of muscimol-induced responses in slices of the rat cuneate nucleus.ls5 In such assays, pentobarbitone is very much more potent than phenobarbitone, which is an anticonvulsant rather than an anaesthetic. Indeed, the anticonvulsant activity of barbiturates seems unrelated to any ac-

PBARB

BARB

CART

ETAZ

TRAC

ETOM

Figure 15. Barbiturates, and pyrazolopyridines acting at the barbiturate site.

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tion at GABAA-receptors,but rather reflects their ability to depress excitatory amino acid release. Potentiation is also seen with GABA-induced contractions of the guinea-pig isolated ileum, mediated through GABAA-receptors,141but it is not known if this effect is found in all subtypes of GABAA-receptor, although barbiturates are well known to potentiate presynaptic inhibition due to GABA.1s2,1s6,1s7This potentiation of GABA by barbiturates has a different structure-action profile from that found in the PTX reversal property (see Sec. III.B),143and the two effects are probably independent. Potentiation of GABA-induced responses is related to a prolongation of chloride channel open-time, for a given agonist concentration.lS8More importantly, at higher concentrations, some barbiturates directly activate GABAA-receptors,resulting in GABA-mimetic responses.1ss Little work has been done on any structure-activity correlation of these various effects of barbiturates at GABAA-receptors,and the subject deserves a more thorough investigation, along with other compounds such as g l u t a r i m i d e ~ that ~ ~have ~ some structural resemblance to barbiturates. Furthermore, the differing actions of the chiral enantiomers of barbiturates also deserves greater emphasis in any such study, although the convulsant properties found in some of these may have no relation to GABA,-receptors. There are marked interactions between barbiturates and benzodiazepines at the GABAA-receptorcomplex, which probably reflect mutually interacting allosteric effects. Subanaesthetic concentrations of pentobarbitone (PTBARB; Fig. 15) stimulate GABA-enhanced benzodiazepine binding, whereas anaesthestic concentrations enhance benzodiazepine binding in the absence of added GABA,164J90an effect no doubt related to the GABA-mimetic actions of barbiturates at higher concentrations. Curiously, the anticonvulsant phenobarbitone does not show this enhancing activity on benzodiazepine binding, but instead appears to prevent the action of those barbiturates that do enhance. As with the reversal of picrotoxin, there is no particular correlation between anaesthetic or anticonvulsant barbiturate actions and such effects on benzodiazepine binding, and the multiplicity of such interactions emphasises the inherent complexity of allosteric interactions in general at the GABA, receptor complex. Cartazolate (CART), etazolate (ETAZ), and tracazolate (TRAC) are pyrazolopyridines (Fig. 15) that enhance benzodiazepine binding in a chloride-sensitive manner. It is possible that this occurs through an interaction at barbiturate sites, since this action is sensitive to picrotoxin, and they themselves influence dihydropicrotoxinin binding.137J64,191Although having a more than superficial resemblance to 6-carbolines, these pyrazolopyridines are evidently not themselves ligands at benzodiazepine sites. Etomidate (ETOM) is a further compound that enhances benzodiazepine binding, but this is not sensitive to picrotoxin and etomidate is unlikely to act at the barbiturate site.137J92Again, despite their inherent interest, little functional work has been done with any of these compounds. D. Steroids and Other Modulatory Agents

It is now some 50 years since Seyle described the rapid and reversible depressant and anaesthetic actions of various steroids (Fig. 16), including

GABA AGONISTS AND ANTAGONISTS

PROG

HDHP

617

HYDN

PNDN

RSS

Figure 16. Steroidal modulators of GABAA-receptors.

progesterone (PROG).193These studies eventually led to the development of steroidal anaesthetics such as hydroxydione (HYDN)194and alphaxalone (ALF) before it was known that they might act on GABA-induced inhibition in the CNS.195 It is now recognized that some of the anaesthetic, sedativehypnotic, anticonvulsant, and anxiolytic effects of such steroids may involve enhancement or mimicry of the action of GABA at the GABAA-receptorcomplex. Conversely, the pro-convulsant/convulsantand anxiogenic steroids may be antagonists at GABAA-receptors.196,197 Several endogenous steroids are now known to be potent modulators of GAB.A-mediatedsynaptic inhibition, and thus are potentially able to influence brain function under physiological and pathophysiological states. In particular, certain reduced metabolites of progesterone and deoxycorticosterone have potent anticonvulsant and anaesthetic properties. 198 Among these are 3a-hydroxy-5a-dihydroprogesterone (HDHP) and 3a,5a-tetrahydrodeoxycorticosi erone (PNDN), which inhibit TBPS binding and increase benzodieizepine binding, as well as directly stimulating chloride uptake into isolated brain vesicles. They also potentiate the inhibitory actions of GABA in cultured rat hippocampal and spinal cord neurons. 199 The neurosteroids pregnenolone sulfate (PGNS; Fig. 17) and dehydroepiandrosterone sulphate (DEASS) appear to be endogenous antagonists of GAB.4-induced actions,200-202 while dehydroepiandrostenone itself (DEAS)203and Reichstein's "Substance S" (RSS; Fig. 16) both induce generalized convulsions in experimental animals.19*r204 The so-called neurosteroids are products of glial metabolism in the brain, where their synthesis is under the influence of the diazepam-binding inhibitor peptide (DBI), which is a ligand for the "peripheral" benzodiazepine binding sites.167 It is generally agreed that convulsant and excitant steroids act at the picrotoxin site, whereas

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some depressant steroids so closely resemble barbiturates in their actions as to suggest that they too act at some site closely related to the latter, if not a unique site on the complex. Structure-activity studies indicate that of a number of steroids related to the neurosteroids, 5a-pregnan-3a-ol-20-one(5A-HDHP)is the most potent in modulating TBPS binding sites, which appear to be functionally coupled to a steroid "modulatory" site.205These sites are found in particular on the a and y2 subunit types.206 The specificity of 5a,3a- as against 5P,3a-pregnane steroids may seem remarkable, but the 5a- and 5P-isomers (5B-HDHP) differ markedly in the conformational orientation of the ring bearing the 3a substituents (Fig. 17). Similarly, the activity of the 5a steroids is lower in the 3P form. Nevertheless, GABA potentiation by such steroids is enhanced as their oxidation levels increase, and the structure-action relationships are more complex than at first t h o ~ g h t . ~ ~ , ~ ~ Some 17-hydroxycorticosteroids such as cortisol (CTSL; Fig. 17) and cortisone (CTSN) have proconvulsive properties. 196 We have recently reported potent biphasic effects of cortisol on GABAA-receptor-mediatedcontractile responses in the guinea pig isolated ileum. Cortisol enhances the contractile responses at picomolar concentrations and inhibits at nanomolar concentrat i o n ~as, has ~ ~ been ~ confirmed by others.208Interestingly, although cortisone does not differ sterically from cortisol, it behaves only as an antagonist of GABA-induced contractions at nanomolar concentrations, which puts it among the more potent GABA,-receptor a n t a g 0 n i ~ t s .On l ~ ~the other hand, corticosterone is less potent than either cortisol or cortisone in modulating GABA-induced response^.^^,^* It is possible that circulating steroid hormones released from the adrenal gland may well induce profound alterations in

@ /H @ o

F@ 03s0

HO

5A-HDHF'

PGNS

5B-HDHP

%so-

CH20H

CH3

HO

-

CTSL

DEAS DEASS

CHpOH I

o@EH

CTSN

Figure 17. Neurosteroids acting at GABAA-receptors.

GAB,$ AGONISTS AND ANTAGONISTS

619

brain excitability by actions at the GABA,-receptor ensuring arousal and alertness in response to sudden acute stress.28 In the same way, the metabolites of the sex steroids may contribute to mood alterations during the menstrual cycle. ALFX, Fig. 16) proAlphaxalone (3a-hydroxy-5a-pregnane-11,20-dione; duces surgical anaesthesia in humans at low micromolar plasma concentrations. 209 Due to adverse side effects, neither hydroxydione nor alphaxalone have survived as anaesthetic agents, although the latter is of interest since it provides considerable insight into the mode of action of these agents. Its involvement in GABA actions was recognized after it was shown to prolong the G{ABA-mediateddorsal root potential associated with presynaptic inhibition in the cat,210and enhance GABA-mediated inhibitory potentials in the olfactory cortex.211 It also selectively augments GABA-evoked chloridedependent responses in rat cuneate slice preparations,212as well as potentiating GABA-activated chloride conductances in bovine chromaffin cells213and in rat spinal and hippocampal n e ~ r o n e s . The ~ ~ nonanaesthetic ~,~~~ 3P-hydroxy analog is inactive. In the guinea pig isolated ileum where GABAA-receptor-mediatedcontractile responses can readily be established, alphaxalone at low concentrations potentiates GABA-induced contractions.216However, at relatively higher concentrations, alphaxalone itself directly activates GABAA-receptors and Overall, the actions of alphaxmimics GABA-induced alone closely resemble those of barbiturates such as pentobarbital, but at substantially lower concentration^.^^^,^^^ Recent neurochemical studies, however, suggest that alphaxalone and other naturally occurring depressant steroids modulate the GABA,-receptor complex at a site distinct from that for barbiturate^.^^^,^^^

Apart from alphaxalone and other steroids described above, additional compounds (Fig. 18) with CNS depressant and anaesthetic properties dependent on GABAA-receptorsare ethanol, a-chloralose (CHLS), chlormethiazole (CMTZ), propanidid (PPND), and possibly the closely related propinal (PPNL), as well as propofol (PRPF). All these compounds markedly enhance GABAergic neurotransmission and potentiate the synaptic effects of GABA.219At low concentrations, ethanol enhances GABA-dependent chloride influx,220and at higher concentrations mimics muscimol-stimulated chloride uptake into synaptosomes.221While it might be thought, from its chemical nature, that the actions of ethanol on GABA,-receptors would be rather

CO\H

kc -cc'3 I HO-C-H I

H-C-OH I

H-C-OH I

HpC- OH

CHLS

(CHzhu

CMTZ

CH2COOPr PPND

CHpCHpCH3 PPNL

PRPF

Figure 18. Activators of GABAA-receptors, unrelated to GABA.

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unspecific, nevetheless, ethanol has been shown to interact quite specifically with an 8-amino-acid sequence of the yZLsubunits of GABAA-receptorsexpressed in Xenopus oocytes, resulting in enhanced responses to GABA. It is probably of great relevance that this same sequence is phosphorylated by protein kinase C.222 Chlormethiazole, a synthetic precursor of vitamin B,, shows sedative, hypnotic, anxiolytic, and anticonvulsant properties, and has been used in the treatment of alcohol withdrawal. Although chlormethiazole and propanidid both mimic GABA actions at GABAA-receptorsites, and enhance their function, it is not clear if these actions are mediated through some distinct site or the benzodiazepine/barbiturate sites on the complex. Of all these compounds acting at GABAA-receptors,currently the most interesting is propofol, which is a novel short-acting anaesthetic coming into more general use. Propofol (2,6-diisopropylphenol)is chemically unrelated to other general anaesthetics, or to any previously found agents acting at GABA,-receptors. It too enhances GABA-induced responses, which may be the basis of its anaesthetic and sedative actions.223,224 GABAA-receptor complexes seem unique in the number and diversity of their modulatory sites, and in the complexity of interactions between the various ligands for these sites, although many of the modulatory interactions seen in neurochemistry have so far received scant attention at the functional level. As more and more agents have been found to act at the complex, the ri are different sites have had to be postulated where they could act. The spectrum from agonist to inverse agonist at some sites should, however, alert us to the possibility that, in reality, the subunits of the complex may have but a small number of domains at which the various ligands can act to alter the behavior of the chloride channels. That these recognition sites are often paradoxically relatively nonselective can be seen with GABAA-receptoragonists. Despite rather stringent structural requirements for optimal agonist activity, nevertheless, there is a group of compounds with cul-iously disparate structures quite unrelated to GABA (Fig. 18) that are still able to activate the GABAA site. It may well be, then, that the endogenous ligands for the various other modulatory sites, as for example with the avermectins, in fact bear no relationship to the presently known ligands, and have yet to be identified. V. GABA,-RECEPTORS

Nearly a decade after the first review on the physiological actions of baclofen on GABA,-receptors, and its possible mechanisms of action as a muscle relaxant and an antispastic agent,225the picture which emerges has changed considerably.226Despite intensive efforts over a number of years to discover successive generations of potent and selective GABA agonists, it is somewhat ironic that only baclofen is in general use as a therapeutic agent,227 and that this has turned out to be a specific agonist for a new type of GABAreceptor. Although much is now known of the physiology and pharmacology of the GABA,-receptor,226~228 nevertheless, by comparison with GABAA-receptors, relatively little has been achieved in the field of structure-action studies of the GABA,-receptor. This section summarizes the salient features, so far as they are known.

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After a synaptic role for GABA in central inhibitory mechanisms was established, it became apparent that GABA mimetics might be of therapeutic interest in controlling spasticity and abnormal movements. GABA itself does not cross the blood-brain barrier, which prompted the design of an analog, P-p-chlorophenyl-GABA (baclofen), with a lipophilic substituent on the GABA backbone, that would penetrate the brain and act on GABA-receptors.229,230 It was soon discovered that baclofen could depress spinal reflexes after oral administration. Baclofen not only reduces skeletal muscle tone and inhibits spinal monosynaptic and polysynaptic reflex activity, due to a neuronal depressant activity in the spinal cord,231,232but also depresses activity in higher brain But curiously enough, these actions of baclofen were found to be insensitive to bicuculline and thus apparently not acting through GABA,-receptors, despite it being a GABA analog. This paradox was not resolved until 1980 when Bowery and co-workers showed that baclofen mediates a depression of transmitter release by activating a class of bicuculline-insensitive GABA-re~eptors~~~ which they called GABABreceptors. Evidence for the functional existence of GABA,-receptor sites was initially obtained in peripheral tissues, where their activation by GABA and baclofen diminishes the evoked release of neurotransmitter, consistent with a presynaptic location for GABA,-receptors on autonomic nerve terminal^.^^^,^^^ This action can be distinguished from those at GABA,-receptors, since it is not mimicked by GABA analogs such as isoguvacine (IGUV, Fig. 4), which is a specific ligand at GABA,-receptor sites, and most importantly the response is insensitive to bicuculline. Neurochemical studies have supported the existence of bicuculline-insensitive GABAB binding sites, using [3H]-GABA and [3H]-ba~lofen,~ such binding being dependent on the presence of calcium and In most areas of the CNS, GABA, binding sites are more abundant than the GABAB type, but significant numbers of the latter are found in many brain areas.239 Previously, it was unclear whether the therapeutic actions of baclofen as an antis pastic agent might lie solely in the depression of transmitter release from presynaptic terminals, as a result of the activation of presynaptic bicucullineinsensitive GABAB-receptors.This uncertainty was due to the lack of any selective GABA,-receptor antagonists. Since then, substantial advances have been made in the pharmacological characterization and classification GABABreceptors through the availability of antagonists and of more refined neurophysiological and neurochemical techniques. From electrophysiologicalstudies, GABA,-receptors are indirectly coupled to either calcium or potassium channels, where they mediate a presynaptic reduction in calcium i n f l ~ x or ~ a~ postsynaptic , ~ ~ ~ increase in potassium conductance,242 while GTP-binding proteins act as an essential biochemical link in these actions243(Fig. 19). Postsynaptic inhibitory responses to activation of GABAB-receptors(late IPSPs) are now known to result from activation of potassium c h a n n e l ~ . ~The ~ 2 formal demonstration of the associated late, bicuculline-insensitive, inhibitory postsynaptic potentials has relied on the use of our GABAB-receptorantagonists.244Originally this was done in hippocampal slices with phaclofen,2@,245but subsequently in a variety of preparations with 2-hydroxysaclofen, which is more potent. 12,13,246-250 Functional

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& GABA

Kt

u G -prptei n s 4

2nd Messengers

Figure 19. Organization of GABAB-receptors, showing their coupling to calcium and/or potassium channels through G proteins and second messenger systems.

GABA,-receptors coupled to potassium channels have recently been expressed in o ~ c y t e s but , ~ ~the ~ native receptor with its associated regulatory G proteins has not yet been isolated. Interestingly, there is also some evidence for a functional coupling of GABA,-receptors to chloride channel^,^^^,^^^ although the ultimate physiological implications of such channels is not understood and requires further investigation.

A. GABA,-Receptor Agonists GABA itself activates both GABAA-and GABA,-receptors, evidently due to its inherent flexibility, which permits a large number of conformations. Nevertheless, the structural requirements for GABA,-receptor activation are surprisingly stringent238r253(Fig. 20). This is illustrated in baclofen (P-p-chlorophenyl-GABA), which is the prototypical selective GABA,-receptor agonist. Here, the p-chlorophenyl analog is far more active than those with 0or rn-chlorophenyl substituents, and introduction of other halogens in the pposition leads to a loss of activity.238Moving the p-chlorophenyl moiety to the a or y positions on the GABA backbone, or elongation of the GABA chain, likewise results in a loss of activity (although some compounds of altered chain length are GABA,-receptor antagonists); only y-guanidino-baclofen retains any reasonable agonist activity.229 Baclofen has a chiral @-carbon, and its enantiomers show a differential potency at GABA,-re~eptors.~~~ Racemic (X,S)-baclofen is essentially equipotent with GABA at GABA,-receptors, while X-(-)-baclofen [R-(-)-BAC] is the active enantiomer that is some 2 times more potent than GABA, and the S-(+)-isomer is virtually inactive254;the latter may even be a GABA,-antagonist,255,256presumably because it is a partial agonist. The absolute configuration of X-(-)-badofen is known, with the suggestion that the essentially extended GABA backbone seen in x-ray crystallography of the hydrochloride may represent the conformation for activation of GABA,-receptors, although such crystalline structures may bear little relationship to active conformations

623

GAB,4 AGONISTS AND ANTAGONISTS

HZN,

COOH H2N BAC

HZN,

(R)-(-)-BAC

COOH

Cn;B

MBFG

HzN

BPG

COOH

GBFT

DHIB

COOH

H2N

DCBAC

Figure 20. Baclofen and GABAB-receptor ligands.

in solution.257Interestingly, R-(-)-GABOB (Fig. 20) has the opposite configuration to that of R-(-)-BAC but is more active than S-(+)-GABOB(Fig. 1) at GABA,-receptors. For some time, the des-chloro baclofen analog P-phenylGABA (BPG) has been known to have central activity, particularly in mood alteration.258Our own investigations indicate that it is a partial agonist/antagonist at central and peripheral GABA,-receptor~.~~~ As with baclofen itself, the major activity resides with R-(-)-4-amino-3-phenylbutanoic acid, with the same absolute configuration as R-( -)-baclofen.260,261 More recently, two variants of the P-p-chlorophenyl substituent have been explored for activity at GABAB-receptors, 4-amino-3-(5-methoxybenzo [b]fuuran-2-yl) butanoic acid (MBFG) and 3-(5-chlorothien-2-y1)-GABA (CTC;B).262 Of these, the benzofuran analogs are partial agonist/antag0nists.263,~M Interestingly, calculations indicate a preference for the banzofuran ring system to be lying in the plane of the GABA backbone so that a van der Waals bond can form between the basic functionality and the oxygen of the b e n ~ o f u r a nThis . ~ ~may ~ account for the partial agonist property of the benzofuran analogs, since it is known from x-ray crystallography and conformational energy calculations that the p-chlorophenyl moiety of baclofen is preferentially orientated perpendicularly across the plane of the GABA backbone. The importance of this feature has been tested by locking the baclclfen molecule in the latter configuration, as in the dihydroindenyl analog (DHIB), but this only leads to a loss of activity.266This dihydroindenyl structure recalls the spiro-GABA analog Gabapentin (GBPT) which, although an anticonvulsant, is also inactive at either GABA, or GABA,-receptors. Incorporating an ortho chlorine into baclofen (DCBAC) to restrict rotation about the C, bond, again results in considerable loss of activity.266

KERR AND ONG Few variants of the p-chlorophenyl substituent in baclofen have ever been used. So far, only the thienyl analog (CTGB) shows any reasonable activity, the P-thiophene substituent evidently having sufficient aromaticity to behave as a p-chlorophenyl analog since it is a GABA,-receptor a g ~ n i s tthe ~ ~corre~; sponding furanyl analog lacks this character and is inactive.265Interestingly, the 5-chloro substituent in CTGB corresponds in space to the p-chloro substituent in baclofen, and shifting the chlorine to the 4 position markedly reduces the activity in binding studies, as it does in the rn-chloro baclofen. There are no other strict baclofen analogs of note as GABA,-receptor agonists, since the sulphonic and phosphonic analogs are antagonists. However, some phosphinic analogs of GABA itself are highly effective in GABA, binding and as agonists in functional tests.268-273This may be because the phosphinic acidic head more closely resembles the carboxylic group of GABA and baclofen than is found in the tetrahedral sulfonic or the dibasic phosphonic analogs. Since the parent phosphinic analog (3-APPiA; Fig. 21) of GABA is inherently prone to oxidation into the corresponding phosphonate, the Palkyl substituent analogs are potentially of more use. Of these, 3-aminopropane-1-(P-methyl)-phosphinicacid (SKF 97541; Fig. 21) is a stable and effective a g ~ n i s tThese . ~ ~ phosphinic ~ analogs are readily converted to antagonists by longer chain alkyl-P substitution. B. GABA,-Receptor Antagonists

Over the years, the lack of a selective antagonist for GABA,-receptors has been a major source of difficulty in assessing their physiological and pharmacological significance. It is a feature of these receptors that no GABAA-antagonists show any significant GABA,-receptor antagonist activity, not even SR 95531, which might be expected to be a GABA, antagonist since it retains practically the full flexibility of the GABA backbone. This implies a great specificity of the binding site for the basic functionality of any GABA,-receptor agonist or antagonist, and in general, N-substitution abolishes all activity at these receptors. Earlier, 6-aminovaleric acid and homotaurine (3-APS; Fig. 2) were proposed as antagonist^,^^^-^^^ but these are relatively weak and suffer from the drawback of also being active at GABA,-receptor sites.274-277Eventually, our in-

9

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I

H

OH

CH3

3-APPiA

SKF 97541

9

3-APPA

OH

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CGP-35348

Figure 21. Phosphonic and phosphinic GABA analogs active at GABAB-receptors.

GABA AGONISTS AND ANTAGONISTS CI

CI

PHACLO

SACLO

625 CI

2-HO-SAC

Figure 22. Competitive GABAB-receptorantagonists related to baclofen.

troduction in 1987 of the antagonist phaclofen (PHACLO; Fig 22), the phosphonic analog of baclofen,'O followed by the more potent sulphonic analogs saclofen (SACLO) and 2-hydroxysaclofen (2-HO-SAC) provided the first physiological evidence for GABA,-receptor-mediated actions in the CNS.253 Phaclofen is only a weak antagonist (pA2=4.0),278,279 but nevertheless was the first agent able to selectively inhibit neural postsynaptic hyperpolarization induced by baclofen, and antagonize GABA,-receptor-mediated late I P S P S . ~ ~ In addition, phaclofen antagonizes the depression of synaptic transmission by both baclofen and GABA in a range of peripheral tissues.279The more potent 2-hydroxysaclofen (pA2=5.0) is also an effective competitive antagonist of central and peripheral GABA,-receptors.11-13,2&,2*7 Indeed, it remains amongst the most potent antagonists so far available, the more particularly so if it were resolved to provide the active enantiomer. In developing phaclofen, -sclofen and 2-hydroxysaclofen, a great number of GABA and baclofen analogs were explored,253but the only compounds of real interest until recently have been 3-aminopropanephosphonic acid (3APPA; Fig. 21), 4-aminobutanephosphonic acid (4-ABPA),279and the active baclofen analogs phaclofen, saclofen, and 2-hydroxysaclofen. More recently, it has been found that an intermediate in the synthesis of 3-aminopropanephosphinic acid, 3-aminopropane-l-(P-diethoxymethyl)-phosphinic acid (CGP 35348), is an effective GABA,-receptor antagonist,280as is the corresponding (n-hexy1)phosphinic acid (3-APHPiA),271,272 pointing to a hydrophobic region adjacent to the anion-recognition site as being of importance for imparting antagonist properties in these phosphinic analogs. CGP 35348 has been thoroughly characterized as a GABA,-antagonist in a variety of test preparations, both in vivo and in vitro,280 but when compared to 2-hydroxysaclofen, its potency varies between preparations, suggestive of GABA,-receptor heterogeneity. However, neither CGP 35348 nor 3-APHPiA shows any great improvement over 2-hydroxysaclofen in isolated preparat i o n ~ . ~ ~ ~ A totally unrelated series of phthalide 3-aminomethylbenzoisofuranones, related to those derived from the bicuculline structure by Johnston,l12 provides a further class of GABA,-receptor antagonists281(Fig. 23). In these (ACBF, ADCF), there is some restriction of conformation, and analogs of this series may provide selective antagonists at subclasses of the receptor. In addition, a series of shortened baclofen analogs (Fig. 23) derived from Pphenyl-P-alanine (BPBA) has recently been described as GABAg antagonists, among which are a number of active compounds that readily cross the blood-

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Q Ri=CI: ACBF R;,R~=cL: ADBF

SPHAC

BCF'T

BF'T

3-HO-BAC

ZACPPS

Figure 23. Unconventional GABAB-receptor antagonists.

brain barrier and are active parenterally.282,283 It was originally suggested that P-phenyl-P-alanine might not be a true GABAB antagonist, despite it being effective against the centrally mediated baclofen-induced hypersecretion of gastric hydrochloric Interestingly, the original binding data leading to the use of this compound, in fact, referred to very weak GABAA-receptor binding, and not to GABAB-receptorbinding in the presence of calcium or magnesium.284 From the structure-action point of view, the interest in these compounds (Fig. 23) lies in the conversion of inactive precursors, p-alanine, taurine, and ciliatine, into GABA,-receptor antagonists by the incorporation of a P-phenyl substituent (BPBA, BPC, BPT), or of the P-p-chlorophenyl substituent (BCPA, BCPC, BCPT). The most active analog is again the sulphonic derivative 2amino-2-(p-chlorophenylethane)-l-sulphonic acid (BCPT) which is comparable to phaclofen in potency.285Nevertheless, these analogs are potentially useful, despite their relatively low potency, mainly because (3-phenyl-P-alanine and P-p-chlorophenyl-P-alanine both readily penetrate into the brain. The converse strategy of extension of the GABA backbone of baclofen has also provided GABAB-receptor antagonists. In these, the acidic functionality is further removed from the carbon bearing the p-chlorophenyl substituent by the introduction of an ether linkage,286the phosphono analog SPHAC being most active. However, none of these is more active than phaclofen. A major drawback in all the work so far, based on baclofen analogs as GABA,-receptor antagonists, is that only racemates have been available for use in the test systems. There is thus a real need for the development of some generalized method for the resolution of such w-amino-baclofen analogs, so that the activity of the enantiomers can be defined and preferably compared with their absolute configurations obtained from x-ray crystallography. In the

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rare examples where this has been done, R-( -)-baclofen is an agonist whereas S-(+)-baclofen may be a weak antagonist, and R-( -)-4-amino-3-phenylbutanoic acid ((3-phenyl-GABA)is also the agonist enantiomer. The enantiomers of 3-hydroxyGABA (GABOB) also show differing activities at GABA,-receptors.& It is worth recalling that the higher affinity R-( -)-GABOB has a stereoconfiguration about the f3-carbon opposite to that of R-( -)-baclofen (Fig. 20). No doubt this is of relevance for the antagonist properties of 2-hydroxysaclofen, since the corresponding racemic baclofen analog, 3hydroxybaclofen (3-HO-BAC; Fig. 23), has only half the agonist activity of baclofen itself.287 So far, there is little evidence for what might be the antagonist conformation for baclofen or GABA analogs at GABA,-receptors. However, there are suggestions that this may be represented by a more folded conformation than that predicted from x-ray crystallography of baclofen hydrochloride. For example, such a folded conformation would be more readily adopted by the prop-1-enylsulfonic acid (ZACPPS; antagonist Z-3-amino-2-(p-chlorophenyl) Fig. 23), which has a partially restricted conformation favoring a folded configuration. The relatively active GABA, antagonist Z-5-aminopent-2-enoic acid (ZAPEA), an analog of 5-aminopentanoic acid, can also readily assume a folded form,288as can the P-phenyl- and (3-p-chlorophenyl-(3-alanineanalogs, which also best match a folded rather than an extended configuration of the antagonists phaclofen and saclofen; when extended, they show a mismatch with the basic functionalities of the latter due to their shortened chain length. These pioneering efforts in the development of GABA,-receptor antagonists have enabled a more thorough characterization of GABA,-receptors into different subtypes. Elucidation of such heterogeneity of GABA,-receptors is of importance in studying the functional and clinical significance of GABA,-receptors, which can now be subdivided into presynaptic receptors, including autoreceptors, and postsynaptic receptors. For example, in the hippocampal slice, presynaptic and postsynaptic GABA,-receptors have different pharmacological properties, the presynaptic depression of the excitatory postsynaptic potential elicited by CA1 afferent stimulation is resistant to phaclofen whereas the slow inhibitory postsynaptic potentials are sensitive to p h a ~ l o f e nAn . ~ ~insensitivity ~ to phaclofen at presynaptic GABA,-receptors has also been seen in cultured hippocampal n e ~ r o n e sSimilarly, . ~ ~ ~ the presynaptic depressant action of baclofen is not affected by phaclofen in rat motoneurones.290 Earlier, a differential sensitivity of phaclofen on presynaptic and postsynaptic actions of baclofen was found in the cat spinal c0rd.l" Nevertheless, it must be emphasized that phaclofen is only a weak antagonist, and these peculiarities may merely reflect an inability to achieve proper antagonistic concentrations in some areas where perhaps high concentrations of GABA are reached in the synaptic cleft. A presynaptic, GABA,-receptor-mediated regulation of GABA release has been demonstrated in both rat and human synaptosomes, and from rat brain slices, sensitive to p h a c l ~ f e n . The ~~~ ability - ~ ~of~ GABA, ligands to depress release is more evident when the frequency of electrical stimulation or depolarizing concentration of potassium is Subsequent studies have shown that presynaptic GABA,-receptors are present on the terminals of GABAergic n e ~ r o n e s since , ~ ~ ~baclofen effectively reduces the amplitude of

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inhibitory postsynaptic potentials (IPSPs) recorded at inhibitory synapses between embryonic rat hippocampal neurones. These autoreceptors are insensitive to p h a c l ~ f e nexcept , ~ ~ ~ at very high concentrations, but they are sensitive to 2-hydro~ysaclofen.~~~ This phenomenon is of considerable interest as it focuses on the role of GABA and baclofen in activating autoreceptors, and hence the regulation of endogenous GABA release. The time is now ripe for examining the structural requirements for activation and blockade of both GABAA- and GABA,-autoreceptors in appropriate and specific functional models as an aid in characterizing their potential clinical importance, since they may be useful for targeting new drugs of potential therapeutic value. At the biochemical level, a number of studies suggest a modulatory role of GABA,-receptors in signal transduction mechanisms, through intracellular messenger systems such as adenylate cyclase activity, and cyclic AMP accumulation induced by other neurotransmitters. Only some of these actions are sensitive to phaclofen, again suggestive of the existence of a heterogeneity of G A B A , - r e ~ e p t o r s . ~All ~ ~this - ~ ~reinforces ~ the notion that GABA,-receptors on presynaptic and postsynaptic membranes are likely distinct from one another. Thus the possibility should not be overlooked that presynaptic and postsynaptic GABA,-receptors might well show differential sensitivities to the antagonists, particularly since these receptors are linked through different G proteins to calcium and potassium channels, respectively. These receptor subtypes are quite different, for only the postsynaptic receptor appears to be Thus it is highly sensitive to G protein ribosylation with pertussis likely that specific agonists and antagonists will be found for each subtype, as has been done with other transmitters such as acetylcholine, 5-hydroxytryptamine, and dopamine. Indeed, there is a considerable therapeutic potential for specific presynaptic receptor or autoreceptor agonists controlling GABA release in the CNS. VI. CONCLUSION Based on our current knowledge and understanding of the structure and function of GABA-receptors, it is now clear that GABAA-receptorsand their modulatory agents such as benzodiazepines, barbiturates, and steroids mediate a wide spectrum of clinically important actions. Their anxiolytic, anticonvulsant, sedative-hypnotic, and anaesthetic properties are all mediated complexes. Likewise, it is through the GABAA-receptor-chloride-ionophore becoming apparent that GABA,-receptors are also involved in physiological actions of considerable clinical and therapeutic p ~ t e n t i a lranging ,~ from anticonvulsant and analgesic actions to mood regulation and memory consolidation, the latter involving induction of long-term potentiation and synaptic plasticity. But before these can be fully realized, there is a need for a systematic and comparative approach using the various available ligands in animals and in humans. Furthermore, a great deal of attention has already been focussed on the role of GABAergic mechanisms in neural as well as nonneural tissues in the p e r i p h e ~ y ~these , ~ ~ ~include ; the enteric nervous system, the reproductive tract, and the endocrine tissues as well as the lungs, the latter in relation to antiasthmatic effects. In the last decade, we have seen considerable advances in the knowledge

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and understanding of the role of GABA,- and GABA,-receptors in nervous system function. With GABA,-receptors, the major breakthrough has largely been in the area of molecular biology where, by using the latest cloning techniques to identify the various subunits, we have come to appreciate the structural diversities of multiple GABAA-receptor subtypes, as well as the existence of different modulatory sites on these receptors. Although GABA,receptors, on the other hand, are relatively novel, nevertheless, much progress has been made in this area as a result of the discovery of selective antagonists, together with the developing concept of their heterogeneity. However, the molecular biology of GABA,-receptors is still in its infancy, despite attempts being made in the solubilization and isolation of these receptors. It is most likely that we will see a major growth in this area in the near future. The further challenge that awaits us is to find more potent and more selective agonists and antagonists at the pre-, post- and autoGABA,-receptor subtypes in order to probe the properties and functional significance of these receptors. We acknowledge the fruitful collaboration with Professor R. H. Prager over the years, in particular his contribution to the syntheses of the various GABAB agonists and antagonists. We also thank the Australian Research Council for the award of a Research Fellowship to Dr. Jennifer Ong.

REFERENCES E. Roberts and S. Frankel, 1. Bid. Chem. 187, 55 (1950). J. Awapara, A. J. R. Landua, J. Fuerst, and B. Seale, J. B i d . Chem. 187, 35 (1950). D. R. Curtis and G. A. R. Johnston, Ergebn. Physiol. 69, 97 (1974). C. Tanaka, Life Sci. 37, 2221 (1985). D. I. B. Kerr and J. Ong. Current Opinion Neurol. Neurosurg. 4, 566 (1991). J. Ong and D. I. B. Kerr, Life Sci. 46, 1489 (1990). R. Werman, Comp. Biochem. Physiol. 18, 745 (1966). D. R. Curtis, A. W. Duggan, D. Felix, and G. A. R. Johnston, Nature (London) 226, 1222 (1970). 9. D. R. Hill and N. G. Bowery, Nature (London) 290, 149 (1981). 10. D. I. 8.Kerr, J. Ong, R. H. Prager, 8.D. Gynther, andD. R. Curtis, BrainRes. 405,150(1987). 11. D. I. B. Kerr, J. Ong, G. A. R. Johnston, J. Abbenante, and R. H. Prager, Neurosci. Lett. 92,92 (1988). 12. D. R. Curtis, 8.D. Gynther, D. T. Beattie, D. I. 8.Kerr, and R. H. Prager, Neurosci. Lett. 92, 97 (1988). 13. D. I. B. Kerr, J. Ong, G. A. R. Johnston, J. Abbenante, and R. H. Prager, Neurosci. Lett. 107, 239 (1989). 14. R. W. Olsen, 1. Neurochem. 37, l(1981). 15. L. Sivilotti and A. Nistri, Prog. Neurobiol. 36, 35 (1991). 16. BenzodiazepinelGABA Receptors and Chloride Channels, R. W. Olsen and J. C. Venter, Eds., Liss, New York, 1986, p. 57. 17. A. Nistri and A. Constanti, Prog. Neurobiol. 13, 117 (1979). 18. L. Polenzani, R. M. Woodward, and R. Miledi, Proc Natl. Acad. Sic. U S A 88, 4318 (1991). 19. R. D. M a n and G. A. R. Johnston, Med. Res. Rev. 3, 91 (1983). 20. W. Haefely, E. Kyburz, M. Gerecke, and H. Mohler, Adv. Drug Res. 14, 166 (1985). 21. P. Krogsgaard-Larsen Med. Res. Rev. 8, 27 (1988). 22. C. R. Gardner, Prog. Neurobiol. 31, 425 (1988). 23. R. W. Olsen and A. M. Snowman, 1. Neurochem. 44, 1074 (1985). 24. D. I. B. Kerr and J. Ong, Neurosci. Lett. 65, 7 (1986). 25. J. J. Lambert, J. A. Peters, and G. A. Cottrell, Trends Phamacol. Sci. 8 , 224 (1987). 1. 2. 3. 4. 5. 6. 7. 8.

630

KERR AND ONG

26. D. M. Turner, R. W. Ransom, J. S.-J. Yang, and R. W. Olsen, J. Pharmacol. Exp. Ther. 248,960 (1989). 27. G. A. R. Johnston, D. I. B. Kerr and J. Ong, in Pharmacology, M. J. Rand and C. Raper, Eds., Elsevier, Division, Amsterdam, 1987, p. 121. 28. D. I. 8. Kerr, J. Ong, and G. A. R. Johnston, Stress Anxiety 13, 209 (1991). 29. P. R. Schofield, M. G. Darlison, N. Fujita, D. R. Burt, F. A. Stephenson, H. Rodriguez, L. M. Rhee, J. Ramachandran, V. Reale, T. A. Glencorse, P. H. Seeburg, andE. A. Barnard, Nature (London) 328 (221) (1987). 30. P. R. Schofield, Trends Pharmacol Sci. 10, 476 (1989). 31. D. B. Prichett, H. Sontheimer, C. M. Gorman, H. Kettenmann, P. H. Seeburg, and P. R. Schofield, Science 242, 1306 (1988). 32. R. W. Olsen and A. J. Tobin, Fed. Am. SOC.Exp. Bid. J. 4, 1469 (1990). 33. E. S. Levitan, P. R. Schofield, D. R. Burt, L. M. Rhee, W. Wisden, M. Kohler, N. Fujitan, H. F. Rodriguez, A. Stephenson, M. G. Darlison, E. A. Barnard, P. H. Seeburg, Nature (London) 335, 76 (1988). 34. L. A. C. Blair, E. S. Levitan, J. Marshall, V. E. Dione, and E. A. Barnard, Science 242, 577 (1988). 35. D. 8. Prichett, H. Sontheimer, B. D. Shivers, S. Ymer, H. Kettenham, P. R. Schofield, and P. H. Seeburg, Nature (London) 338, 582 (1989). 36. H. Luddens and W. Wisden, Trends Pharmacol. Sci. 12, 49 (1991). 37. P. R. Schofield, B. D. Shivers, and P. H. Seeburg, Trends Neurosci. 13, 8 (1990). 38. P. Krogsgaard-Larsen, H. Hjeds, E. Falch, F. S. Jorgensen, and L. Nielsen, Ada Drug Res. 17, 382(1988). 39. J. Ong, Br. J. Pharmacol. 91, 9 (1987). 40. G. A. R. Johnston, R. D. Allan, S. M. E. Kennedy, and B. Twitchin, in GABA-Neurotransrnitters. Pharmacological, Biochemical and Pharmacological Aspects, P. Krogsgaard-Larsen, J. ScheelKruger, and H. Kofod, Eds., Munksgaard, Copenhagen, 1979, p. 149. 41. S. Luzzi, C. A. Maggi, S. Spagnesi, P. Santicioli, and L. Zilletti, J. Auton. Pharrnacol. 5, 65 (1985). 42. A. Galli, L. Zilletti, M. Scotton, G. Adenbrif, and A. Giotti, J. Neurochem. 32, 1123 (1979). 43. E. Falch, A. Hedegaard, L. Nielsen, B. R. Jensen, H. Hjeds, and P. Krogsgaard-Larsen, J. Neurochem. 47, 398 (1986). 44. U. Kristiansen and B. Fjalland, Pharmacol. Toxicol. 68, 332 (1991). 45. T. Hayashi, J. Physiol (London) 145, 570 (1959). 46. K. Bock, I. Lundt, and C. Pederson, Acta Chem. Scand. B 37, 341 (1983). 47. B. E. Rossiter and K. B. Sharpless, J. Org. Chem. 49, 3707 (1984). 48. D. R. Curtis and J. C. Watkins, Pharmacol. Rev. 17, 347 (1965). 49. D. R. Curtis and J. C. Watkins, J. Neurochem. 6, 117 (1960). 50. P. Krogsgaard-Larsen, P. Jacobsen, and E. Falch, in The GABA Receptors, S. J. Enna, Ed., Humana, New Jersey, 1983, p. 149. 51. M. N. Perkins and T. W. Stone, Br. J. Pharmacol. 75, 93 (1982). 52. D. I. 8. Kerr and J. Ong, Br. J. Pharmacol. 83, 169 (1984). 53. D. I. 8. Kerr and J. Ong, in GABAergic Mechanisms in the Periphery, S. L. Erdo and N. G. Bowery, Eds., Raven, New York, 1986, p. 153. 54. D. I. B. Kerr and J. Ong, Br. J. Pharmacol. 90, 763 (1987). 55. J. Ong and D. I. B. Kerr, Eur. J. Pharrnacol. 134, 349 (1987). 56. L. Galzigna, G. Manani, L. Garbin, C. Aldino, and G. Quadro, Aggressologie 22, 219 (1981). 57. C. G. Wermuth and K. Biziere, Trends Pharmacol. Sci. 7, 421 (1986). 58. D. P. Purpura, M. Girado, T. G. Smith, D. A. Callan, and H. Grundfest, J. Neurochem. 3,238 (1959). 59. G. A. R. Johnston, D. R. Curtis, P. M. Beart, C. J. A. Game, R. M. McCulloch, and B. Twitchin, J. Neurochem. 24, 157 (1975). 60. R. D. Allan, H. W. Dickenson B. P. Hiem, G. A. R. Johnston, and R. Kazlauskas, Br. J. Pharmacol. 88, 379 (1986). 61. P. R. Andrews and G. A. R. Johnston, Biochem. Pharmacol. 28, 2697 (1979). 62. R. D. Allan, D. R. Curtis, P. M. Headley, G. A. R. Johnston, D. Lodge, and B. Twitchin, J. Neurochem. 34, 652 (1980). 63. J. P. ODonnell, D. A. Johnston, and A. T. Azzaro, J. Med. Chem. 23, 1142 (1980).

GABA AGONISTS AND ANTAGONISTS

631

64. R. D. Allan, D. R. Curtis, P. M. Headley, G. A. R. Johnston, S. M. E. Kennedy, D. Lodge, and 8 . Twitchin, Neurochem. Res. 5, 393 (1980). 65. R. D. Allan, H. W. Dickenson, and J. Fong, Eur. 1. Pharrnacol. 122, 339 (1986). 66. P. Thorbek, H. Hjeds, and K. Schaumburg, Acta Chem. Scand. Ser. B 35, 473 (1981). 67. P. Krogsgaard-Larsen, E. Falch, A. Schousboe, D. R. Curtis, and D. Lodge, J. Neurochem. 34, 756 (1980). 68. E. G. McGeer, P. L. McGeer, and H. McLennan, J. Neurochem. 8, 36 (1961). . ' l Krogsgaard-Larsen, G. A. R. Johnston, D. Lodge, and D. R. Curtis, Nature (London) 268, 69. 53 (1977). 70. E. Falch, P. Jacobsen, P. Krogsgaard-Larsen, and D. R. Curtis, 1. Neurochem. 44, 68 (1985). 71. L. Brehm, H. Hjeds, and P. Krogsgaard-Larsen, Acta Chem. Scand. 26, 1298 (1972). 72. L. Brehm, P. Krogsgaard-Larsen, K. Schaumburg, J. S. Johansen, E. Falch, and D. R. Curtis, J. Med. Chem. 29, 224 (1986). 73. J. Robbins, and W. G. Van der Kloot, 1. Physiol. (London) 143, 541 (1958). 74. A. Galindo, Brain Res. 14, 763 (1969). 75. M. A. Simmonds, Br. J. Pharmacol. 73, 739 (1981). 76. A. Krantis and D. I. B. Ken; Naunyn-Schmiedeberg's Arch. Pharmacol. 137, 257 (1981). 77. J. Ong and D. I. B. Kerr, Eur. 1. Pharmacol. 86, 9 (1983). 78. J. Ong and D. I. B. Kerr, Eur. 1. Pharmacol. 94, 305 (1983). 79. J. Ong and D. I. B. Kerr, Eur. J. Pharmacol. 103, 327 (1984). 80. D. R. Curtis, A. W. Duggan, D. Felix, and G. A. R. Johnston, Brain Res. 32, 169 (1971). 81. D. R. Curtis, A. W. Duggan, D. Felix, G. A. R. Johnston, and H. McLennan, Brain Res. 33,57 (1971). 82. J-H. Huang and G. A. R. Johnston, Br. J. Pharmacol. 99, 727 (1990). 83. R. W. Olsen, M. Ban, J. Miller, and G. A. R. Johnston, Brain Res. 98, 383 (1975). 84. G. A. R. Johnston, P. M. Beart, D. R. Curtis, C. J. A. Game, R. M. McCulloch, and R. M. MacLachlan, Nature (London) 240, 219 (1972). 85. S. F. Pong and L. T Graham, Brain Res. 42, 486 (1972). 86. G. M. Frigo, A. Galli, S. Lecchini, and M. Marcoli, Br. J. Pharmacol. 90, 31 (1987). 87. M. Tonini, G. De Petris, L. Onori, L. Manzo, C. A. Rizzi, and A. Crema, Br. J. Pharmacol. 97, 556 (1989). 88. A. Bartolini, A. Giotti, S. Giuliani, P. Malmberg-Aiello, anu R. Patacchini, Gen. Pharmacol. 21, 277 (1990). 89. R. W. Olsen and A. M. Snowman, J. Neurochem. 41, 1653 (1983). 90. S. J. Enna and S. H. Snyder, Mol. Pharmucol. 13, 442 (1977). 91. G. Maksay and M. K. Ticku, 1. Neurochem. 43, 261 (1984). 92. E. J. Ariens, A. D. Beld, J. F. R. Miranda, and A. M. Simonis, in The Receptors, R. D. OBrien, Ed., Plenum, New York, 1987, p. 47. 93. H. Mohler and T. Okada, Mol. Pharmacol. 14, 256 (1978). 94. R. D. Allan and C. Apostopoulos, Aust. J. Chem. 43, 1259 (1990). 95. A. Bhattacharyya, K. M. Madyastha, P. K. Bhattacharyya, and M. S. Devanandan, lnd. J. Biochem. Biophys. 18, 171 (1981). 96. G. W. Pooler and E. G. Steward, J. Mol. Struct. 156, 247 (1987). 97. M. H. Aprison and K. B. Lipkowitz, J. Neurosci. Res. 23, 129 (1989). 98. K. B. Lipkowitz, R. D. Gilardi, and M. H. Aprison, J. Mol. Struct. 195, 65 (1989). 99. P. Hunt and S. Clements-Jewery, Neuropharmacology 20, 357 (1981). 100. R. W. Olsen, Eur. J. Pharmacol. 103, 333 (1984). 101. M. A. Simmonds and J. P. Turner, Br. J. Pharmacol. 84, 631 (1985). 102. J. Ong and D. I. B. Kerr, Neurosci. Lett. 101, 203 (1989). 103. D. R. Curtis and R. Malik, Eur. 1. Pharmacol. 110, 383 (1985). 104. B. H. Gahwiler, R. Maurer, and H. J. Wuthrich, Neurosci. Lett. 45, 311 (1984). 105. J. A. Kemp, G. R. Marshall, E. H. F. Wong, and G. N. Woodruff, Br. J. Pharmacol. 85, 237P (1985). 106. C-G. Wermuth, J-J. Bourguignon, G. Schlewer, J-P. Gies, A. Schoenfelder, A. Melikian, M-J. Bouchet, D. Chantreux, J-C. Molimard, M. Heaulme, J-P. Chambon, and K. Biziere, J. Med. Chem. 30, 239 (1987). 107. M. Heaulme, J-P. Chambon, R. Leyris, J-C. Molimard, C. G. Wermuth, and K. Biziere, Brain Res. 384, 224 (1986).

KERR AND ONG 108. R. D. Allan, C. Apostopoulos, and J. A. Richardson, Aust. 1. Chem. 43, 1767 (1990). 109. J. Ong and D. I. 8. Kerr, unpublished. 110. C. G. Wermuth, J. P. Chambon, M. Heaulme, A. Melikian, G. Schlewer, R. Leyris, and K. Biziere, Eur. J. Phamacol. 144, 375 (1987). 111. J. A. Beautler, E. W. Karbon, A. N. Brubaker, R. Malik, D. R. Curtis, and S. J. Enna, Brain Res. 330, 135 (1985). 112. G. A. R. Johnston, in Receptors in Pharmacology, J. R. Smythies and R. J. Bradley, Eds., Marcel Dekker, New York, 1978, p. 296. 113. N. T. Buu, J. Duhaime, and 0. Kuchel, Life Sci. 35, 1083 (1984). 114. J. P. Quilliam and R. Stables, Pharmacol. Res. Commun. 1, 7 (1969). 115. J. Ong and D. I. B. Kerr, unpublished. 116. R. A. Davidoff, Brain Res. 36, 218 (1972). 117. R. G. Hill, M. A. Simmonds, and D. W. Straughan, Br. 1. Pharmacol. 47, 642 (1973). 118. I. Yokoi, K. Tsuruta, H. Shigara, and A. Mori, Epilepsy Res. 1, 114 (1987). 119. A. 5. Herranz, J. M. Solis, 0. Herreras, N. Menendez, E. Ambrosio, L. M. Orensanz, and R. del Rio Martin, J. Neurosci. Res. 26, 98 (1990). 120. Y. Kanai, 0. Wada, and 5. Manabe, J. Pharmucol. Exp. Ther. 252, 1269 (1990). 121. D. R. Curtis and G. A. R. Johnston, in Neuropoisons, Vol. 2, L. L. Simpson and D. R. Curtis, Eds., Plenum, New York, 1974, p. 207. 122. C. H. Jarboe, L. A. Porter and R. T. Buckler, J. Med. Chem. 11, 729 (1968). 123. M. K. Ticku, M. Ban and R. W. Olsen, Mol. Pharmacol. 14, 391 (1978). 124. W. E. Klunk, D. F. Covey, and J. A. Ferrendelli, Mol. Phamacol. 22, 444 (1982). 125. B. M. Craven, Acta Crystallogr. Sect. B Struct. Crystullogr. Cryst. Chem. 15, 387 (1962). 126. W. E. Klunk, D. F. Covey and J. A. Ferrendelli, Mol. Pharmacol, 22, 438 (1982). 127. W. E. Klunk, B. L. Kalman, J. A. Ferrendelli, and D. F. Covey, Mol. Pharmacol. 23,511 (1983). 128. D. J. Canney, K. D. Holland, J. A. Levine, A. C. McKeon, J. A. Ferrendelli, and D. F. Covey. 1. Med. Chem. 34, 1460 (1991). 129. J. A. Levine, J. A. Ferrendelli, and D. F. Covey, Biochem. Pharmacol. 23, 4187 (1985). 130. K. D. Holland, A. C. McKeon, D. F. Covey, and J. A. Ferrendelli, 1. Pharmacol. Exp. Therap. 254, 578 (1990). 131. K. D. Holland, J. A. Ferrendelli, D. F. Covey, and 5. M. Rothman, J. Neurosci. 10,1719 (1990). 132. K. D. Holland, K-W. Yoon, J. A. Ferrendelli, D. F. Covey, and S. M. Rothman, Mol. Pharmacol. 39, 79 (1991). 133. R. W. Olsen, M. Bureau, M. Khrestchatisky, A. J. MacLennan, M-Y. Chiang, A. J. Tobin, W. Xu, M. Jackson, C. Sternini, and N. Brecha, in GABA and Benzodiazepine Receptor Subtypes, G. Biggio and E. Costa, Eds., Raven, New York, 1990, p. 35. 134. K. Baker, J. Yang, D. F. Covey, D. 8. Clifford, and C. F. Zorumski, Neurosci. Lett. 87, 133 (1988). 135. D. A. Coulter, J. F. Huguenard, and D. A. Prince, Br. 1. Pharmacol. 100, 807 (1990). 136. Y. Kudo, J-I. Oka, and K. Yamada, Neurosci. Lett. 25, 83 (1981). 137. R. F. Squires, J. E. Casida, M. Richardson, and E. Saedrup, Mol. Phamacol. 23, 326 (1983). 138. T. Duong, R. H. Prager, J. M. Tippet, A. D. Ward, and D. I. B. Kerr, Aust. 1. Chem. 29,2667 (1976). 139. D. I. B. Kerr, 8 . J. Dennis, E. L. M. Breuker, R. H. Prager, A. D. Ward, andT. Duong, Brain Res. 110, 413 (1976). 140. D. I. B. Kerr, J. Ong, R. H. Prager, and D. A. Ward, Eur. 1. Pharmacol. 124, 203 (1986). 141. J. Ong and D. I. B. Kerr, Eur. 1. Pharmacol. 103, 327 (1984). 142. J. H. Skemtt, G. A. R. Johnston, S. Chen Chow, R. L. Macdonald, R. H. Prager, and A. D. Ward, Brain Res. 331, 225 (1985). 143. M. A. Simmonds and A. L. Horne, in Excitatory Amino Acids in Health and Disease, D. Lodge, Ed., Wiley, Chichester, 1988, p. 219. 144. N. G. Bowery, J. F. Collins, R. G. Hill, and S. Pearson, Br. J. Phamucol. 57, 435P (1976). 145. J. E. Casida, M. Eto, A. D. Moscioni, J. L. Engel, D. 5. Milbrath, and J. G. Verkade, Toxicol. Appl. Phurmacol. 36, 261 (1976). 146. B. R. de Costa, A. H. Lewin, K. C. Rice, P. Skolnick, and J. A. Schoenheimer, 1. Med. Chem. 34, 1531 (1991). 147. M. G . Wong and P. R. Andrews, Eur. 1. Med. Chem. 24, 323 (1989). 148. F. Hildebrandt, Handbk. Exp. Pharmacol. 5, 128 (1937).

GABA AGONISTS AND ANTAGONISTS

633

E. G. Gross and R. M. Featherstone, 1. P h m c o l . 87, 299 (1946). R. Ramanjaneyulu and M. K. Ticku, Eur. 1. P h m o l . 98, 337 (1984). D. 8 . Gant, M. E. Eldefrawi, and A. T Eldefrawi, Toricol. Appl. Phrmucol. 88, 313 (1987). L. J. Lawrence and J. E. Casida, Science 211, 1399 (1983). C. Blandizzi, M. De Bona, G. Natale, C. Agen, and M. Del Tacca, Eur. 1. Phrmacol. 201, 35 (1991). 154. J. Ong, D. I. B. Kerr, H. R. Capper, and G. A. R. Johnston, 1. P h r m . Pharmacol. 42, 662 (1990). 155. J. Ong, D. I. B. Kerr, and G. A. R. Johnston, unpublished. 156. L. 0. Randall, W. Schallek, G. A. Heuse, E. F. Keith, and R. E. Bagdon, 1. Pharmucol. Exp. Therap. 129, 163 (1960). 157. R. F. Schmidt, M. E. Vogel, and M. Zimmerman, Naunyn-Schmiedeberg’s Arch. Exp. Path. Pharmuk. 258, 69 (1967). 158. P. Polc and W. Haefely, Naunyn-Schmiedeberg’s Arch. Phrmacol. 294, 121 (1976). 159. P. Polc and W. Haefely, Neurosci. Lett. 28, 193 (1982). 160. R. F. Squires and C. Braestrup, Nature (London) 266, 732 (1977). 161. P. Polc, J. P. Laurent, R. Scherschlicht, and W. Heafely, Naunyn-Schmiedeberg’s Arch. P h r mucol. 316, 317 (1981). 162. D. 8 . Prichett and P. H. Seeburg, Proc. Natl. Acad. Sci. U S A 88, 1421 (1991). 163. T. Kenakin, Trends Pharmucol. Sci. 8, 423 (1987). 164. L. M. F. Leeb-Lundberg and R. W. Olsen, Mol. Phrmacol. 23, 315 (1983). 165. P. H. Seeburg, in GABA and Benzodiazepine Receptor Subtypes, G. Biggio and E. Costa, Eds., Raven, New York, 1990, p. 15. 166. A. Guidotti, M. D. Antonacci, P. Giusti, M. Massotti, M. Memo, and J. L. Schlichting, in GABA and Benzodiuzepine Receptor Subtypes, G. Biggio and E. Costa, Eds., Raven, New York, 1990, p. 15. 167. E. Costa and A. Guidotti, Life Sci. 49, 325 (1991). 168. H. Diaz-Arauzo, K. F. Koehler, T. J. Hagen, and J. M. Cook, Life Sci. 49, 207 (1991). 169. M. S. Allen, T. J. Hagen, M. L. Trudell, P. W. Codding, P. Skolnick, and J. M. Cook. 1.Med. Chem. 31, 1854 (1988). 170. J. H. Medina, C. Pena, M. L. De Stein, C. Wolfman, and A. C. Paladini, Biochem. Biophys. Res. Commun. 165, 547 (1989). 171. M. Nielsen, S. Frokjaer, and C. Braestrup, Biochem. Phrmucol. 37, 3285 (1988). 172. C-M Lee, H. N. C. Wong, K-Y. Chui, T. F. Choang, P-M. Hon, and H-M. Chang, Neurosci. Lett. 127, 237 (1991). 173. H. M. Chang, K-Y. Chui, F. W. L. Tan, Y. Yang, Z. P. Zhong, C. M. Lee, H. L. Sham, and H. N. C. Wong, 1. Med. Chem. 34, 1675 (1991). 174. M. Nielsen, 0. Gredal, and C. Braestrup, Life Sci, 25, 679 (1979). 175. C. Braestrup, Neurochem. Znt. 13, 21 (1988). 176. C. Braestrup, R. Schmieden, G. Neef, M. Nielsen, and E. N. Petersen, Science 216, 1241 (1982). 177. D. J. Nutt and R. G. Lister, Eur. 1. Pharmacol. 165, 135 (1989). 178. S. P. Hollinshead, M. L. Trudell, P. Skolnick, and J. M. Cook. 1.Med. Chem, 33, 1062 (1990). 179. L. H. Jensen and E. N. Petersen, 1. Neural Transm. 58, 183 (1983). 180. G. A. R. Johnston, R. D. Allan, A. D. Benton, S. Chen Chow, C. A. Drew, B. P. Hiem, G. Holan, R. Kazlauskas, H. Rzeznickzak, and R. P. Weatherby, Proc. 9th Int. Congr. P h r m a d (London), 3, 179 (1984). 181. J. C. Eccles, R. F. Schmidt, and W. D. Willis, 1. Physiol (London) 168, 500 (1963). 182. R. F. Schmidt, Pfuergers Arch. 277, 325 (1963). 183. M. Willow and G. A. R. Johnston, 1. Neurosci. 1, 364 (1981). 184. M. Willow and G. A. R. Johnston, 1. Neurochem. 37, 1291 (1981). 185. N. L. Hamson and M. A. Simmonds, Br. 1. Pharmacol. 80, 387 (1983). 186. R. A. Nicoll, 1. Physiol (London) 223, 803 (1972). 187. R. A. Nicoll, Proc. Natl. Acad. Sci. 72, 1460 (1975). 188. R. L. Macdonald and J. L. Barker, Neurology 29, 432 (1979). 189. G. M. Nicholson, I. Spence, and G. A. R. Johnston, Neurophrmacology 24, 461 (1985). 190. F. Leeb-Lundberg and R. W. Olsen, Mol. Pharmacol. 21, 320 (1982). 191. M. K. Ticku and W. C. Davis, 1. Neurochem. 38, 1180 (1982). 149. 150. 151. 152. 153.

634

KERR AND ONG

D. Ashton, R. Geerts, C. Waterkeyn, and J. E. Leysen, Life Sci. 29, 2631 (1981). H. Selye, Proc. Soc. Exp. Biol. Med. 46, 116 (1941). F. J. Murphy, N. P. Guadagni, and F. de Bon, J. A m . Med. Assoc. 158, 1412 (1955). K. J. Child, J. P. Curie, 8 . Davis, M. G. Dodds, D. R. Pearce, and D. J. Twissell, Br. 1. Anaes. 43, 2 (1971). 196. M. D. Majewska, Biochem. Pharmucol. 36, 3781 (1987). 197. J. J. Lambert, J. A. Peters, and G. A. Cottrell, Trends Pharmacol. Sci. 8, 224 (1987). 198. L. Gymerk, J. Iriarte, and P. Crabbe, 1. Med. Chem. 11, 117 (1968). 199. M. D. Majewska, N. L. Harrison, R. D. Schwartz, J. L. Barker, and S. M. Paul, Science 232, 1004 (1986). 200. J. Ong, D. 1. 8. Kerr, and G. A. R. Johnston, Eur. J. Pharmacol. 142, 461 (1987). 201. M. D. Majewska and R. D. Schwartz, Brain Res. 404, 355 (1988). 202. M. D. Majewska, J-M. Mienville, and S. Vicini, Neurosci. Lett. 90, 279 (1988). 203. H. Witzel, Ztschr. Vitamin-, Hormon-u. Fementforsch. 10, 46 (1959). 204. G. Heuser and E. Eidelberg, Endocrinology 69, 915 (1961). 205. K. W. Gee, M. B. Bolger, R. E. Brinton, H. Coirini, and 8. S. McEwen, J. Pharmacol. Exp. Ther. 246, 803 (1988). 206. R. Shingai, M. L. Sutherland, and E. A. Barnard, Eur. 1. Pharmacol. (Mol Pharmacol.) 206, 77 (1991). 207. J. Ong, D. I. 8. Kerr, and G. A. R. Johnston, Neurosci Lett. 82, 101 (1987). 208. F. Andres-Trelles, V. Bibby, S. Lustman, and M. A. Simmonds, Neuropharmacology 28, 705 (1989). 209. J. W. Sear and C. Prys-Roberts, Br. J. Anaes. 51, 861 (1979). 210. D. Lodge and N. A. Anis, Br. J. Anaes. 56, 1143 (1984). 211. C. N. Scholfield, Pflugers Arch. 383, 249 (1980). 212. N. L. Harrison and M. A. Simmonds, Brain Res. 323,287 (1984). 213. G. A. Cottrell, J. J. Lambert, and J. A. Peters, Br. 1. Pharmacol. 90, 491 (1987). 214. J. L. Barker, N. L. Harrison, G. D. Lange, and D. G. Owen, J. Physiol (London) 386, 485 (1987). 215. N. L. Harrison, S. Vicini, and J. L. Barker, J. Neurosci. 7, 604 (1987). 216. J. Ong, D. I. B. Kerr, and G . A. R. Johnston, Br. J. Pharmacol. 95, 33 (1988). 217. E. F. Kirkness and A. J. Turner, Eur. J. Pharmacol. 150, 385 (1988). 218. D. M. Turner, R. W. Ransom, J. S-J. Yang, and R. W. Olsen, J. Pharmacol. Exp. Ther. 248,960 (1989). 219. P. E. Keane and K. Biziere, Life Sci. 41, 1437 (1987). 220. A. K. Mehta and M. K. Ticku, 1. Pharmacol. Exp. Ther. 246, 558 (1988). 221. P. D. Suzdak, R. D. Schwartz, P. Skolnick, and S. M. Paul, Proc. Natl. Acad. Sci. U S A 83,4071 (1986). 222. K. A. Wafford, D. M. Burnett, N. J. Leidenheimer, D. R. Burt, J. B. Wang, P. Kofuji, T. V. Dunwiddie, R. A. Harris, and J. M. Sikela, Neuron 7, 27 (1991). 223. A. Concas, G. Santoro, M. P. Mascia, M. Serra, E. Sanna, and G. Biggio, 1. Neurochm. 55, 2135 (1990). 224. A. Concas, G. Santoro, M. Serra, E. Sanna, and G. Biggio, Brain Res. 542, 225 (1991). 225. N. G. Bowery, Trends PharrnacoI. Sci. 3, 400 (1982). 226. GABAB Receptors in Mammalian Function, N. G. Bowery, H. Bittiger, and H-R. Olpe, Eds., Wiley, Chichester, 1990. 227. Spasticity-A Topical Survey, W. Birkmayer, Ed., Hans Huber, Vienna, 1972. 228. N. G. Bowery, Trends Pharmacol. Sci. 10, 401 (1989). 229. H. J. Bein, in SpasticityA Topical Survey, W. Birkmayer, Ed., Hans Huber, Vienna, 1972, p. 76. 230. H. Keberle and J. W. Faigle, in Spasticity-A Topical Survey, W. Birkmayer, Ed., Hans Huber, Vienna, 1972, p. 90. 231. F-K. Pierau and P. Zimmerman Brain Res. 54, 376 (1973). 232. R. A. Davidoff and E. S . Sears, Neurology 24, 957 (1974). 233. D. R. Curtis, C. J. A. Game, G. A. R. Johnston, and R. M. McCuIloch, Brain Res. 70, 493 (1974). 234. J. Davies and J. C. Watkins, Brain Res. 70, 501 (1974). 235. N. G. Bowery, D. R. Hill, A. L. Hudson, A. Doble, D. N. Middlemiss, J. Shaw, and M. J. Turnbull Nature (London) 283,92 (1980). 192. 193. 194. 195.

GABA AGONISTS AND ANTAGONISTS

635

236. N. G. Bowery, A. Doble, D. R. Hill, A. L. Hudson, J. S. Shaw, M. J. Turnbull, and R. Warrington, Eur. J. Phamacol. 71, 53 (1981). 237. D. I. B. Kerr and J. Ong, in GABA-Basic Research and Clinical Applications, N. G. Bowery and G. Nistico, Eds., Pythagora, Italy, 1989, p. 239. 238. N. G. Bowery, D. R. Hill and A. L. Hudson, Br. J. Pharmacol. 78, 191 (1983). 239. G. W. Price and N. G. Bowery, IS1 Atlas of Science: Pharmacology, 136 (1988). 240. K. Dunlap, Br. J. Phrmacol. 74, 579 (1981). 241. A. C. Dolphin and R. H. Scott, Br. J. Pharmacol. 88, 213 (1986). 242. N. R. Newberry and R. A. Nicoll, Nature (London) 308, 405 (1984). 243. A. C. Dolphin, S. M. McGuirk, and R. H. Scott, Br. J. Pharmacol. 97, 263 (1989). 244. P. Dutar and R. A. Nicoll, Nature (London) 332, 156 (1988). 245. P. Dutar and R. A. Nicoll, Neuron 1, 585 (1988). 246. N. A. Lambert, N. L. Harrison, D. I. B. Kerr, J. Ong, R. H. Prager, and T. J. Teyler, Neurosci. Lett. 107, 125 (1989). 247. N. L. Hamson, D. M. Lovinger, N. A. Lambert, T. J. Teyler, R. H. Prager, J. Ong, and D. I. 8. Ken; Neurosci. Lett. 119, 272 (1990). 248. C. A. Drew, G. A. R. Johnston, D. I. B. Ken; and J. Ong, Neurosci. Lett. 113, 107 (1990). 249. S. Aram and D. L. Hammond, J. Pharmacol. Exp. Ther. 257, 360 (1991). 250. T. Akema and F. Kimura, Brain Res. 546, 143 (1991). 251. M. Sekiguchi, H. Sakuta, K. Okamoto, and Y. Sakai, Mol. Brain Res. 8, 301 (1990). 252. L. Hahner, S. McQuilkin, and R. A. Harris, Fed. Am. Soc. Exp. Biol. 1. 2468, 2466 (1991). 253. D. I. 8. Ken; J. Ong, and R. H. Prager, in GABAB Receptors in Mammalian Function, N. G. Bowery, H. Bittiger, and H-R. Olpe, Eds. Wiley, Chichester, 1990, p. 29. 254. N. G. Bowery, D. R. Hill, and A. L. Hudson, Neuropharrnacology, 24, 207 (1985). 255. J. Sawynok and C. Dickson, Pharmacology 31, 248 (1985). 256. G. H. Fromm, T. Shibuya, M. Nakata, and C. F. Terrence, Neuropharrnacology 29,249 (1990). 257. C-H. Chang, D. S. C. Yang, C. S. Yoo, B-C. Wang, J. Pletcher, M. Sax, and C. Terrence, Acta Cryst. B 38, 2065 (1982). 258. I. A. Sytinsky, A. T. Soldatenkov, and L. A. Lajtha, Prog. Neurobiol. 10, 89 (1978). 259. J. Ong, D. I. B. Kerr, and G. A. R. Johnston, Neurosci. Lett. 77, 109 (1987). 260. R. D. Allan, M. C. Bates, C. A. Drew, R. K. Duke, T. W. Hambley, G. A. R. Johnston, K. N. Mewett, and I Spence, Tetrahedron 46, 2511 (1990). 261. J. Ong, D. I. 8. Ken; R. K. Duke, and G. A. R. Johnston, unpublished. 262. P. Berthelot, C. Vaccher, A. Musadad, N. Flouquet, M. Debaert, and M. Luyckx, J. Med. Chem. 30, 743 (1987). 263. D. I. 8. Kerr, J. Ong, G. A. R. Johnston, P. Berthelot, M. Debaert, and C. Vaccher, Eur. J. Pharmacol. 164, 361 (1989). 264. D. T. Beattie, D. R. Curtis, M. Debaert, C. Vaccher, and P. Berthelot, Neurosci. Lett. 100, 292 (1989). 265. P. Berthelot, C. Vaccher, N. Flouquet, M. Debaert, M. Luyckx, and C. Brunet, J. Med. Chem. 34, 2557 (1991). 266. A. Mann, T. Boulanger, B. Brandau, F. Durant, G. Evvard, M. Heaulme, E. Desaulles, and C-G. Wermuth, J. Med. Chem. 34, 1307 (1991). 267. J. Ong, D. I. B. Kerr, P. Berthelot, and M. Debaert, unpublished. 268. G . D. Pratt, C. Knott, R. Davey, and N. G. Bowery, Br. J. Pharmucol. 96, 141P (1989). 269. J. Ong, D. I. 8 . Ken, G. A. R. Johnston, and R. G. Hall, Neurosci. Lett. 109, 169 (1990). 270. J. Ong, N. L. Harrison, R. G. Hall, J. L. Barker, G. A. R. Johnston, and D. I. B. Kerr, Brain Res. 526, 138 (1990). 271. G. R. Seabrook, W. Howson, and M. G. Lacey, Br. 1. Pharmacol. 101, 949 (1990). 272. J. M. Hills, A. J. Sellers, J. Mistry, M. Broekman, and W. Howson, Br. J. Phrmacol. 102, 5 (1991). 273. J. M. Hills, M. M. Larkin, and W. Howson, Br. J. Pharmacol. 102, 631 (1991). 274. M. Muhyaddin, P. J. Roberts, and G. N. Woodruff, Br. J. Pharmacol. 77, 163 (1982). 275. J. Ong and D. I. B. Kerr, Eur. J. Pharmacol. 94, 305 (1983). 276. A. Giotti, S. Luzzi, S. Spagnesi, and L. Zilletti, Br. J. Phrmacol. 79, 855 (1983). 277. R. D. Allan and H. W. Dickenson, Eur. J. Pharmacol. 120, 119 (1986). 278. D. I. B. Kerr, J. Ong, G. A. R. Johnston, and R. H. Prager, Brain Res. 480, 312 (1989). 279. D. I. B. Ken, J. Ong, and R. H. Prager, Br. J. Phamacol. 99, 422 (1990).

KERR AND ONG 280. H. Bittiger, W. Froestl, R. Hall, G. Karlsson, K. Klebs, H-R. Olpe, M. F. Pozza, M. S. Steinmann, and H. Van Riezen, in GABAB Receptors in Mammalian Function N. G. Bowery, H. Bittiger, and H-R. Olpe, Eds., Wiley, Chichester, 1990, p. 47. 281. D. I. B. Kerr, J. Ong, and R. H. Prager, Mol. Neuropharmacol, (in press). 282. N. Hara, Y. Hara, and Y. Goto, lap. J. Pharmacol, 52, 345 (1990). 283. N. Hara, Y. Hara, Y. Natsume, and Y. Goto, Eur. J. Pharmacol. 179, 17 (1990). 284. G. K. Matheson, E. Freed, and G. Tunniclif, Gen. Pharmacol, 18, 269 (1987). 285. J. Ong, D. I. B. Kerr, Abbenante, and R. H. Prager, Eur. J. Pharmacol. (in press.) 286. R. D. Allan, C. A. Drew, J. Ong, and H. W. Tran, Neurosci. Lett. 112, 223 (1990). 287. J. Ong, D. I. 8. Kerr, and R. H. Prager, unpublished. 288. H. W. Dickenson, R. D. Allan, J. Ong, and G. A. R. Johnston, Neurosci. Lett. 86,351 (1988). 289. N. L. Harrison, J. Physiol, (London) 442, 433 (1990). 290. M. Y. Wang, and N. J. Dun, Br. J. Pharrnacol. 99, 413 (1990). 291. G. Bonanno, G. Fontana, and M. Raiteri, Eur. J. Pharmacol. 154, 223 (1988). 292. G. Bonanno, P. Cavazzani, G. C., Andrioli, D. Asaro, G. Pellegrini, and M. Raiteri, Br. J. Pharmacol. 96, 341 (1989). 293. G. Bonanno, G. Pellegrini, D. Asaro, G. Fontana, and M. Raiteri, Eur. J. Phrmacol. (Mol Pharmacol. Section) 172, 41 (1989). 294. M. Raiteri, M. R. Giralt, G. Bonanno, A. Pittaluga, E. Fedele, and G. Fontana, in GABAB Receptors in Mammalian Function, N. G. Bowery, H. Bittiger, and H-R. Olpe, Eds., Wiley, Chichester, 1990, p. 81. 295. N. L. Harrison, G. D. Lange, and J. L. Barker, Neurosci. Lett. 85, 105 (1988). 296. N. L. Harrison, D. M. Lovinger, N. A. Lambert, T. J. Teyler, and D. I. B. Ken, Soc. Neurosci. Abstr. 16, 1040P (1990). 297. W. J. Wojcik and N. H. Neff, Mol. Pharmacol. 25, 24 (1984). 298. E. W. Karbon and S . J. Enna, Mol. Phurmacol, 27, 53 (1985). 299. M. Nishikawa and K. Kuriyama, Neurochern. Int. 14, 85 (1989). 300. GABAergic Mechanisms in the Mammalian Periphery, S. L. Erdo and N. G. Bowery, Eds., Raven, New York, 1986.

GABA agonists and antagonists.

GABA Agonists and Antagonists David I. B. Kerr and Jennifer Ong Department of Anaesthesia and lntensive Care, The University of Adelaide, Adelaide, So...
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