Receptors coupled to ionic channels: the glutamate receptor family Gregory P. Gasic and Stephen Heinemann The Salk Institute,
La Jolla, California,
USA
Glutamate-gated ion channels belong to a complex family of receptors containing several pharmacological subtypes. They are thought to be essential for the acquisition of associative memory and for activitydependent synaptogenesis, and have been implicated in several central nervous system diseases. Within the past year, molecular cloning of the first glutamate receptor channel and several related subunits has opened new approaches for understanding the basis of these important phenomena.
Current Opinion in Neurobiology
Introduction Receptors composed of l&and-gated ion channels mediate rapid (in the order of milliseconds) transduction events at chemical synapses by causing the selective influx of about lo* cations or anions per millisecond and thereby altering the resting membrane potential of excitable cells (for reviews see [1*,261X In general, the l&and-gated channels activated by either acetylcholine or L-glutamate (Glu) cause cell excitation whereas glycine or y-aminobutyric acid (GABA) cause inhibition of cell firing. Unlike the GTP-binding protein (G protein)-coupled receptors, the l&and-gated channels do not require second messenger systems for signal transduction. Acetylcholine mediates synaptic excitatory transmission between nerve and muscle, at autonomic ganglia, chromaflin cells and at a subset of central nervous system (CNS) synapses, whereas Glu is thought to be the major excitatory neurotransmitter in the brain. In the spinal cord and brainstem glycine is the dominant inhibitory neurotmnsmitter, whereas in higher brain regions, GABA is dominant. Activation of the muscle nicotinic acetylcholine (r&h) receptor (R) channel and the GluR channel leads to the influx of small monovalent and divalent cations that depolarize the cell. In contrast, activation of the glycine (Gly)R channel or GABA*R channel opens chloride than nels, which often leads to inhibition. The chloride channels gated by these two receptors have similar biophysical properties. The pentameric skeletal muscle (or Torpedo electric organ) nAchR [subunits: 012, p, Y(E),&], is the best characterized l&and-gated channel in electrophysiological, biochemical and molecular terms [1*,2,3] and serves as the prototype for the l&and-gated channel receptors [ 241.
1991, 1:20-26
Although the deduced protein sequences of the nAchR, GlyR and GABA, R reveal a similar structural organization (e.g. hydrophobic segments) and sign&ant amino acid sequence identity [3-6], the designation of a common evolutionary origin for these receptors on the basis of conserved structure is still debated. Receptor subtype diversity in the CNS has emerged as a common theme for the l&and-gated ion channel receptors as well as for the voltage-gated and G-protein-coupled receptors [ 7*]. Two mechanisms used to generate diversity are: the expression of distinct genes encoding receptor subtypes, and alternative splicing of exons of the same gene. Structural heterogeneity of receptor subtypes is created by combinatorial assemblies of different subunits which then translates into functional diversity. It has been suggested that the multiplicity and diversity of these CNS receptors increases the information handling capacity of neurons and contributes to neural plasticity 17’1.
In this review, we focus on the glutamate receptors as they are essential to the signal transduction mechanisms which underlie several important biological and pathophysiological processes in the vertebrate brain. Within the past year, molecular cloning of the first glutamategated ion channel receptor (designated GluRl) from rat brain has been achieved by functional expression in oocytes. Additional members of the ionotropic GluR family (GluR2-GluRS) were discovered and cloned by lowstringency screening of rat brain cDNA libraries and polymerase chain reaction technology. What follows is a brief description of the pharmacological basis of GluR subtypes, properties of the GluR channels, the biological and pathophysiological Importance of the ionotropic GluR, and recent studies of this family of receptors.
Abbreviations ACPD-tram-1-amino-cyclopentane-1,3 dicarboxylate; AMPA--cc-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid; L-AP4-2-amino-4-phosphonobutyric acid; BDNMrain-derived neurotrophic factor; CNS-central nervous system; GABA-y-aminobutyric acid; G protein--CTP-binding protein; Clu-t-glutamate; Cly-glycine; RA-kainic acid; nAch--nicotinic acetylcholine; NC&nerve growth factor; NMDA-N-methyl-o-aspartic acid; R-receptor. 20
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The glutamate receptor family Casic and Heinemann
Pharmacological basis of GluR subtypes
Biological significance
GluRs have been differentiated on pharmacological grounds into five distinct subtypes named after their most selective agonists: N-methyl-D-aspartic acid (NMDA), kainic acid (KA), a-amino-3-hydroxy5-methyl-4-isoxazole propionic acid (AMPA), 2-amino-4-phosphonobutyric acid (L-AP4) and fransl-amino-cyclopentane-1,3 dicarboxylate (ACPD) [81. Three of these subtypes, NMDA, KA and AMPA, represent l&and-gated channels. The LAP4 subtype is inferred from the specific depressant effect L-AP4 has on some excitatoty amino acid pathways in the retina, brain and spinal cord without any antagonist effect on Glu-induced responses. Evidence from studies of retinal depolarizing bipolar cells suggests than an LAP4 receptor acts via a G protein to increase the hydrolysis of cGMP, resulting in the closure of ion channels conducting an inward current [9]. The Wth subtype, ACPD, is a pertussis toxin-sensitive G-protein-coupled receptor which is linked to inositol phosphate/diacylglycerol formation and release of Ca2+ from internal stores.
More than forty years ago, Hebb proposed that synaptic modification required conjoint presynaptic and postsynaptic activity [19]. Such a Hebbian synapse appears to be essential for some forms of long-term potentiation (see review by S Siegelbaum and E Kandel, this issue, pp113-120) and the dynamic phase of synaptogenesis [20]. GluRs are ubiquitously distributed in the mammalian brain and are apparently fundamental to synaptic networks and some kinds of learning [15,1921,22**-24**,25*].GluRs equip synapses with the potential for use-dependent modilications of synaptic efficacy [ 15,211.
Properties of GiuR channels Studiesusing cultured neurons indicate that NMDAactivated channels are permeable to Na+ , K+ and Ca2+ but at resting potentials are largely blocked by physiological concentrations of extracellular Mg2+ [lo,1 11. KA- and AMPA-activated conductances are permeable to Na+ and K+ [lO,ll] and in some studies to Ca2+ [12,13,14*]. In most membrane patches and in the absence of Mg2+, Glu activates ion channels with multiple conductance states (in the range of < l-50 pS). KA activates low-conductance states while NMDA activates at least live main conductance states in the range of lO-5OpS [10,11,15]. Glycine, in micromolar concentrations, seems to be required for the activation of the NMDA-receptor chanThe function of the glycine binding nel [10,11,15,16]. site is unclear, although some experiments suggest that it may modulate desensitization. This site has also been proposed to bind a ‘glycine-like’ ligand which displaces glycine and leads to receptor inactivation.
NMDA receptors are required to induce changes in synaptic efficacy at some synapses which are correlated with associative learning and activity-dependent modifications of synaptic connections du&g Glu > KAfor inhibition of binding) [42*]. Despite this nanomolar AMPA binding, these receptor channels respond to Glu, quisqualateand AMPAat much higher concentrations [42*,43*]. The results blur previous pharmacological distinctions between KA and AMPAsubclasses [ 81. Additional support for a continuum of KA-AMPAreceptors comes from electrophysiological studies in rat dorsal root ganglion neurons j47.1. Coexpression of GluR subunits in X~~@LS oocytes or 293 cells results in a synergistic effect (two-fourfold potentiation). Different subunit combinations exhibit disparate current/voltagerelationships and rectification characteristics [42*,43=,45]. GluR2 in combination with GluRl or GluR3 produced an almost linear current/voltagerelation similar to hippocampal poly A+ RNA These data suggest that in s&u.,heteromeric combinations of GluR subunits may generate functional receptor diversity. Alternativesplicingof adjacent exons of the GluR1-GluR4 genes to produce subunitswhich differin a smalldomain (the alternativeforms are designated Yip’and ‘flop’) preceding the putativetransmembrane4 domain, introduces further functional receptor diversityand pharmacoIogical complexity [48**]. Saturatingconcentrations of Glu and KA evoked similarcurrents for the flip versions, whereas KA evoked currents were greater than Glu-mediatedcurrents for the flop versions. The disparity between the flip and flop channels was observed for the desensitizing and steady-statecomponents of the current response to Glu. Glu activatedchannels four to five times more e&ctivelywhen encountering the Rip version. Native GluRs may be composed of heteromeric assemblies of different subunits which contain either the flip or flop exons. In such combinations, one subunit can be responsible for the fast desensitizingcomponent while the other can mediate the steady-statecomponent These observations support the evidence that KA and AMPAact via a common receptor. Evidence for splice variantshas also been found for the GluR5 subunit [46*].
The glutamate receptor family Gasic and Heinemann
Gly R
GABAAR
nAchR-a,
Eight amino acids between TM2 and TM3
Fig. 1. Amiio
acid residue number versus relative hydropathy for glycine receptor (CIyR), y-aminobutyric acid A receptor (CABAAR), neuronal nicotinic acetylcholine receptor CnAchR) (u-3), and glutamate receptor (CIuRH. Amino acid sequences of the CIyR, GABAAR, nAchR, and CluRl were analyzed using the relative hydropathy algorithm of Kyte and Doolittle. The hydropathy profiles were inspected and putative transmembrane domains (Ml-M4) were chosen on the basis of hydropathy and on the assumption that these receptors can be classed into a family of ligand-gated receptors with four transmembrane domains (based on nAchR studies). The shaded regions represent ‘best guess’assignments of putative Ml-M4. For the CIuR, two models A and B are presented and discussed in greater detail in [37*1. The GluR has almost twice as many amino acid residues as the other three receptors and what appears to be a large extracellular domain spanning approximately the amino-terminal half. A pattern common to all these receptors is the clustering of putative Ml-M3, a hydrophilic domain and then putative M4.
23
24
Signalling mechanisms
Ca2+ permeability subunit
of KA-AMPA
channels and
composition
NMDA-receptor channels are permeable to Ca2+ and blocked by Mg2 + in a voltage-dependent fashion, while their KA-AMPA counterparts are thought to have negligible divalent cation permeability and are not blocked by Mg2+ [10,15]. Two recent studies, employing Ca2+ imaging in rat astrocytes [12] and electrophysiological techniques in hippocampal neurons [13] observed sigticant Ca2+ conductances mediated by a subset of KA receptors [13] which was abolished by a quinoxaline derivative, non-NMDA receptor antagonist, CNQX [12]. Cloned GluR subunits were used to investigate this property further. Oocytes expressing GluRl , GluR3, and GluRl plus GluR3 but not GluRl plus GluR2 or GluR3 plus GluR2 responded to KA and AMPAwith inward Ca2+ currents [14*]. The experiments suggest that, if some neurons express these non-NMDA GluR subunit assemblies, Glu can trigger Ca2+ conductances mediated by nonNMDA receptors.
CluRs
in neurokdocrine
morphine
tolerance
regulation,
and
References
reading
Papers of special interest, published within the annual period of review, have been highlighted as: . of interest of outstanding interest .. KARUNA Explorations of the Nicotinic Acetyicholine Receptor. Hurvqr Lectures 1991, 85: in press. &is well written chapter describes what has been learned over the past 25 years about the structure of the electric organ (end plate) nicotinic acetylcholine receptor, the prototype and best characterized of ligandgated ion channels. Much of the work described comes from the author’s own laboratory. 1.
2.
GW JL, REVAHF, BEG.% A, CHANGEUXJP: Functional Architecture of the Nicotinic Acetylcholine Receptor. From the Electric Organ to Brain. Annu Rev Phwnuwl Toxicd 1991, 31: in press.
3.
STROUDRM, MCCARTHYMP, SHWTER M: Nicotinic AcetyIcholine Receptor SuperfamiIy of Ligand-Gated Ion Ghannels. Eiocbemisfy 1990, 29:11009-11023.
4.
UNWINN: The Structure of Ion ChanneIs in Membranes of Excitable Cells. Neuron 1989, 3665676.
5.
6.
and addiction
Experimental evidence suggests that non-NMDA receptors are involved in neuroendocrine regulation while NMDA receptors may mediate the development of opiate tolerance and dependence. Converging experiments employing immunocytochemistly, Caz+imaging physiology, and intracellular recordings implicate Glu as the dominant neurotransmitter in neuroendocrine regulation in the hypothalamus [49**]. Also, in situ hybridization experiments demonstrate differential expression of GluR subunits in the pituitary, hypothalamus and adrenal glands and suggest a dominant role for these receptors in neuroendocrine regulation (J Hermans-Borgmeyer et al, unpublished data). The non-competitive NMDA receptor antagonist, MK-801, attenuated the development of morphine-induced tolerance and dependence without affecting analgesia [ 50**]. As there is a link between glutamate@ and dopaminergic systems [ 311, could GluRs be involved in cocaine tolerance and dependence?
and recommended
DINGIEDINER, MYERS SJ, NICHOLLSRA: Molecular
ogy of Mammalian Amino Acid Receptors. 4:2636-2645.
BiolFASEB J 1990,
BEE H: Ligand-Gated Ion Channels in the Brain The Amino Acid Receptor Superfamily. Neuron 1990, 5:3&X3392.
SCHOFIELD PR, SHIVERS BD, SEEBURGPH: The Role of Receptor Subtype Diversity in the CNS. Trends Neurasci 1990, 13:%11. A well written and intriguing perspective on the role of receptor subtype diversity(ligand-gated ion channels, voltage-gated ion channels and G-protein-coupled receptors). The authors postulate that this diversity serves as a means for increasing the information handling capacity of neurons and ultimately, may contribute to neural plasticity in the CNS.
7. .
8.
WATKINSJC, KR~GSSGAARD-LARSEN P, HONORE T: StructureActivity Relationships in the Development of Excitatory Amino Acid Receptor Agonists and Competitive Antagonists. Trends Pkarmuwl Sci 1990, 11:2>33.
9.
NAWY S, JAHR CE: Suppression by Glutamate of cGMPActivated Conductance in Retinal Bipolar Cells. Nature 1990, 346:26%271.
10.
MAYERML, WES?BROOKGL The Physiology of Excitatory Amino Acids in the Vertebrate Central Nervous System. Prog Neurcbiol 1987, 28:197-276.
11.
SANWMMSP, USHERWOODPNR: Single Channel Studies of Glutamate Receptors. Int Rev Neurobiol 1990, 32:51-106.
12.
GLUM SR, HOLZWRA~JA, MILLER RJ: Glutamate Receptors Activate Ca*+ Mobilization and Ca2+ Itiux Into Astrocytes. Pm Nat1Acad Sci USA1990, 8734543458.
Conclusion
13.
efforts are underway to elucidate the molecular structure of the NMDA receptor and to use molecular tools to gain knowledge of the function of the nonNMDA receptors. The next challenge will be to determine the stoichiometries, structures, and cellular localization of these glutamate receptors in vivo. In addition, experiments to map the chromosomal locations of the glutamate receptor genes may find linkages to several CNS diseases. ultimately, these studies should provide clues about the role of glutamate receptors in the regulation of important processes at the synapse.
IINO MS, OZAWAS, Tsuxna K: Permeation of Calcium Through Excitatory Amino Acid Receptor Channels in Cultured Rat Hippocampal Neurons. J P&mid 1990, 424:151-165.
S: Calcium PermeabiIHOW M, HARTIEYM, HEINEMANN ity of KA-AMPA-Gated Ion Channels Depends on Receptor Subunit Composition. Science 1991, 251: in press. At physiological resting potentials, KA and AMPAelicited inward Ca2+ currents in oocytes which expressed GluRl, GIuR3, and GluRl plus GluR3 subunits but not in those which expressed GluRl plus GluR2 or GluR2 plus GluR3 subunits. These ksuk rake the possibility that some non-NMDA receptors may also cause flux of signilicant amounts of calcium into the cell. Because oocytes possess Caz+-activated chloride channels, single-channel studies are required to determine the Ca2+ conductance contribution to the observed current
Intense
14. .
The glutamate receptor family Gasic and Heinemann
15.
COLUNGRIDGE CL, LESTERRAJ: Excitatory Amino Acid Recep tots in tbe Vertebrate Nervous System. Pbarmacol Rev 1989, 40:143-210.
16.
THOMSONAM: GIycine is a CoagonIst at the NMDA Recep-
tor/channel
Complex. Prog Neurobid
1990, 35:5%74.
Duhnr!s A, PINJP, OOMAGARI K, SEBBENM, BOC~C~ERT J: Arachidonic Acid Released Front Striatal Neurons by Joint Stimulation of Ionotropic and Metabotropic QuisquaIate Receptors. Nature 1990, 347:182-184. Releaw of arachidonic acid (whose metabolite(s) are postulated to act as local messengers> from stdatal neurons occurs only upon simultaneous stimulation of the ionotropic and G-protein-coupled glutamate receptors. This work demonstrates that CNS neurons have the capability to integrate fast (ionotropic) and intermediate (G-protein-coupled) GluR-mediated signals to generate a locally active messenger. Such a messenger may ultimately be involved in the trans.synaptic s&nailing events leading to changes in yptic plasticity. 17. .
18. ..
BE~B JM, STEVENSCF: Computational Implications of NMDA Receptor Channels. Cold Spring Harbor S’ Quant Bid 1990, 55: in press. Simply and elegantiy presented, this article crystallizes the unique properties of the NMDA receptor(s) (i.e. requirement for glycine, voltagedependent Mg2+ gating, lengthy open time, and significant Ca2+ conductance) which impart computational capabilities on the synapses that possess them. The chapter references seminal contributions that span nearly a decade of work on this intricate and biologically important glutamate receptor. 19.
HEBB DO: 7& Organization
of Behavior
21.
26.
DING~DINE R, McBm CJ, MCNAMARA JO: Excitatory Amino Acid Receptors in Epilepsy. Trenuk Pbarmacol Sci 1990, 11:334-338.
27.
&IN
CONST~-PATON M, CLINE HT, DEBSKIE: Patterned Activity, Synaptic Convergence, and the NMDA Receptor in Developing Visual Pathways. Annu Rev Neurasci 1990, 13129-154. COUJNGRIDGE GL, SINGERW: Excitatory Amino Acid Recep tars and Synaptic PIasticity. Trends Pbarmacol Sci 1990, 11:29&2%.
CLINEHT, CONSTANTINE-PATON M: NMDA Receptor Agonists and Antagonists Alter Retinal Ganglion Cell Arbor Structure in the Developing Frog RetinotectaI Projection. J Neurosci 1990, 10:1197-1216. Retinalganglion cell arbors in Runupipiensundergo structural changes during normal development. Chronic NMDA application through increased NMDAreceptor activation has been linked with decreased dendritic branch initiation and increased synapse and branch survival. This study proposes that the NMDA receptor senses tierent coactivity and translates it into structural modifications of the arbors which ultimately have physiological consequences.
23. ..
24. ..
UDINSB, SCHERER WJ: Restoration of the Plasticity of BinocuIar yaps by NMDA After the Critical Period in Xenopus Science 1990, 249~66-72. In Xen@us kaeut the visual maps in the optic tectum develop properly with binocular visual input during a critical period. Shit&g the eye position to produce a mismatch in the tecral maps produces a compensatory adjustment in the ipsilateral tectal map only during this critical developmental window. Continuous NMDAtreatment, beyond the critical period, appears to restore the ability to realign the visual map. 25. .
ZAFRAF, HJINGERER B, LEIBROCK J, THO~NENH, ~~HOLM D: Activitydependent Regulation of BDNF and NGF mRNAs in
Huntington’s Disease.
28.
MEIDRUMB, GARTHWA~TE J: Excitatory Amino Acid Neumtoxicity and Neurodegenerative Dis&e. Trends Pbarmacd Sci 1990, 11:37!+387.
29.
CHOI DW, ROTHMAN SM: The Role of Glutamate Neurotoxic-
ity in Hypoxic-ischemic 1990, 13171-182.
Neuronal Death. Annu Rev Neuraui
._.
:
30.
MOGHADDAM B, GRUENRJ, ROTH RH, Bu& BS, ADRN: Effect of L-glutamate on the Release of StriataI Dopamine: in uivo Dialysis and Electrochemical Studies. Brain Res 1990, 518:55-60.
31.
WACHTEL H, TURSKI L: Glutamate: a New Target Schizophrenia? Trends Pbarmacd Sci 1990, 11:219-220.
32.
CHENJ-W, CUNNINGHAM MD, GALTONSN, ~QCHAEL~S EK: Immune LabeIling and Purification of a 71 kD Glutamate-Binding Protein from Rat Brain. J BioI @em 1988, 263417426.
33.
HAMF5ONDR, WENTHOLDRJ: A Kainic Acid Receptor from Frog Brain PuriIied by Domoic Acid AlTinity Cbromatogra phy. J Biol &em 1988, 2632500-2505.
34.
GREGOR P, ESHAR N, ORTEGA A, TEICHEIERG VI: Isolation,
in
Immunochemical Characterization and Localization of the Kainate Subclass of Glutamate Receptor from Chick Cerebellum. EMBO J 1988, 726732679.
22. ..
Mom RGM, DAVISS, BUTCHERSP: HippocampaI Synaptic Plasticity and NMDA Receptors: a Role in Information Storage? Pbih Trans R Sot Land 1990, 329:187-204. This article reviews current evidence for the idea that changes in synpatic efficacy form the neural basis for information storage. Some of the fundamental work described is from the author’s own laboratory. Although, retrieval of established memories is not a&ted by D-APV,an NMDA receptor competitive antagonist, activation of NMDA receptors is essential for some kinds of learning, especially those dependent on spatial orientation of the animal in its environment.
RL, YOUNG AB, PENNYJB, HANDEIJNB, BALFOLJRR, ANDERSON KD, MARKEL DS, TO~RTEILOTE WW, REINER A: Abnormalities of StriataI Projection Neurons and N-methyl-D-aspartate Receptors in Presymptomatic New En@ J Med 1989, 322:12931298.
[book]. New York:
John Wdey 1949. 20.
the Rat Hippocampus is Mediated by Non-NMDA Glutamate Receptors. EMBO J 1990, 935453550. Both BDNF and NGF are necessary for the survival of certain types of neurons in culture. This paper demonstrates that non-NMDh glutamate-gated receptor activitymediates the levels of mRNAsfor these two growth factors.
35.
IKINAF, KLOOGY, SOKOLovsKYM: NMDA/PhencycI.idine Receptor Complex of Rat Forebrain: Purification and Biochemical Characterization. Biochemistry 1990, 29:229&2295.
HOLLMANN M, O’SHEA-GREENFIELJJ A, ROGERSS, HEINEMANN S: Cloning by Functional Expression of a Member of the Glutamate Receptor Family. Nature 1989, 342:64-. Cloning by functional expression of the first member of the ionotropic GluR famiIy represented a technically demanding challenge. This approach took advantage of: directional cloning of cDNAs to derive sense transcripts from pools of phage clones, voltage recording of small agonist-induced depolarizations in injected Xenc@us oocytes (electrophysiological techniques are about four orders of magnitude more sensitive than l&and-binding assays [5] >, successive fractionation of pools of clones yielding GluR responses in oocytes, and the apparent formation of homo-oligomeric glutamate-gated channels. This study served as a spring board for additional subunit cloning efforts by several laboratories [42*-44* ,,45 46*48**], , 36. ..
37. .
HOI~MANN M, ROGERS SW, O’SHEA-GREENFIELD4 DENER~~ES, HUGHESTE, G~slc GP, HEINEMANN S: The Glutamate Recep
tor GluR-Kl: Structure, Function and Expression in tbe Brain. G&i @ring Harb Symp Quant Bid 1990, 55: in press. This chapter provides a compendium of the properties of the firscloned glutamate receptor subunit (GluRl; formerly GluR-Kl) from DNA to protein. Oocytes expressing this subunit respond to both KA and quisqual 38.
WADA K, DECHESNECJ,. Sm BUONANNO4
S, KING RG, KUSANOK,
HAMPSONDR, BANNERC, WENIHOLD RJ: Se-
quence and Expression of a Frog Brain Complementary DNA Encoding a Kainate-Binding Protein. Narure 1989, 342684-689.
25
26
Signallingmechanisms 39.
GREGORP, MANO I, M&o2 I, MCKEOWNM, TEICHBERGVI: Mokcuiar Structure of the Chick Cerebellar Kainate-Bindlng Subunit of a Putative Glutamate Receptor. N&we 1989, 342689-692.
40. .
ROGERSSW, HUGHESTE, How M, GGIC GP, DENERI~ES, HJZINEMANN S: Antibodies to a Cloned Glutamate Receptor Reveal Bxcitatory Circuits in the Rat Brain. / Neumsci 1991, in press. Fusion protein antibodies to the domains of cloned GluRl have been used to determine the synaptic localization and distribution of this subunit in rat brain. Predominantly neuronal and posrsynpatic, GluRl appears to be abundantly expressed in the cerebellum and parts of the llmbic system. While inhenzntlymore difficult,antibody studies of these receptor subunits are necessary because discordances between mRNA and protein expression do exist and receptor cytoarchitecture may provide imlxxtant clues to function. 41.
ANDERsEN P: Synaptic hte8ration in Hippocanlpal CA1 Plm+ mkial Neurons. Prog Bruin Res 1990, 83215-222.
TA, KEINANEN K, WI~DENW, SOMMER B, WERNERP, VERLXIORN SAKMANN B, SEEBURGPH: A Family of AMPA-Selective Glutamate Receptors. Science 1990, 24955560. The authors employed a new nomenclature for GluR-Kl, calling it GluRA, and presented sequence and expression data for three additional GluR subunits - GluRB (GluR2), GIURC (GluR3), GluRD (GluR4) - with about 70% sequence identity to GluR-Kl (GluRl). On the basii of agonist biding studies with GluR subunits expressed in mammalian cells, this paper argues in favor of calling these subunits AMPA receptors. Transiently expressed in embryonic kidney cells, disparate combinations of these subunits exhibit tierent agonist e&a ties and currenr/voltage relationships. 42.
.
BOIJLER J, HOUMANNM, O’SHW-GREENFIEU) A, HARTIEYM, DENE~USES, MARONC, HEINEMANN S: Molecular Cloning and Functipnal Bxpression of Glutamate Receptor Subunit _ en&. Science 1990, 249~10351037. Iike [42-l, this article presents StruCNre function studies of additional cloned subunits expressed in oocytes. In concert with the rat brain GluR subunit in situ hybridization data from this study, and in context with GluR electrophysioloey data from cdNted neurons, the conclusion that the in vim GluR is most likely a hetero-oligomer emerges. 43. .
44. .
NAKANLsHl N, SCHNEIDER NA, ABEL R A Family of Glutamate Receptor Genes: Evidence for the Formation of Heteromultimeric Receptors with Distinct Channel Properties. Neuron 1990, 5569-581. GluRGluR3 subunits appear to have a conserved region which has some sequence slm&ulity with the glutamine-binding domain of the Es&e&&a cdi glutamine permease. 45.
m K, BUJO H, ARAKIK, YAMAUK~M, MEGUROH, W~HINA A, NUMAS, M.+~HINA M: Functional Expression &om Cloned cDNAs of Glutamate Receptor Species Responsive to Kalnate and Qulsqualate. FEBS Len 1990, 272:73-80.
46. .
BET~~ER B, BOUL.TERJ, HERMANS-BORGMEYER I, O’SHEAGREENFEIDA, DENERIS ES, Mou C, BORG~R U, HOUMANN
M, HEINEMANN S: Cloning of a Novel Glutamate Receptor Subunit, GluRs: Expression ln the Netvous System During Development. Neunm 1990, 5583-595. GluR5 shares only 40% amino acid sequence identity with the other subunits and when expressed in oocytes, responds only weakly to L-glutamate. GluR5 ls expressed ln subsets of neurons in developing and adult CNS and PNS. During embryogenesis, GluR5 transcripts are apparent in zones undergoing neuronal differentiation and synapse formation. HIJET~NERJE: Glutamate Receptor Channels in Rat DRG Neurons: Activation by Kalnate and Qulsqualate and Block of Desensitization by ConA. Neuron 1990, 5255-266. Rapid application of Glu, qulsqualate and AMPA to dorsal root 8an& neurons produces a transient current and densen.u‘tiz.esthesecellsto subsequent applications of KA or domoate. In contrast to CNS neurons, the action of KA in dorsal root ganglla neurons was more potent than Glu. Concanawlin A treatment of these cells abolishes desensitition to glutamate class agonists and all f& agonists evoked similar currents. 47. .
48. ..
TA, WI~DENW, BURNASHEV SOh4MER B, KEINANEN K, VERDOOaN N, HERB A, KOHIER M, TAKAGIT, SAKMANN B, SEEBURG PH: Flip and Flop: a Cell-Specilic Functional switch in Glutamate-Operated Channels of the CNS. Science 1990, 24915-1585. This important arricle demonstrates that funcrional diversity can arise from alternative splicing of exons of the same gene. A 38.amino-acid stretch preceding putative transmembrane domain 4 has been demonstrated to exist in two alternative forms, designated ‘flip’and ‘flop’, and encoded by adjacent exons of the respective GluR gene. Despite the WhhSiCd tide, this ekgant SttUcNfdutmiOII study is fat fmm a ‘80~‘. A more flexible nomenclature for alternativelyspliced vadants of a given receptor subunit gene is GluRX. Y (X = l-n; Y = l-n) as described by Bettler er al. [46.1.Among% the ligand-gated channels, the GluRs are unique because alternative splicing produces functional diversity. Purthermore, evidence is presented for a cell-specific pattern for altemaw tively spliced versions. VANDEN POLAN, WUARIN JP, DUDEKFE: Glutamate, the Dominant Excitatory Transmitter in Neuroendocrine Regulation. Science 1990, 250:1276-1278. This ele@nt SNdy employs ukta9t~~Nd immunocytochemistry, physiolo@cal experiments with Ca2+ dye imaging, and intracellular recordlng to demonstrate a dominant role for glutamate and non-NMDA receptors in control of neurons of the neumendocrine hypothalamus. 49.
..
KA, AKU H: Inhibition of Morphine Tolerance and Dependence by the NMDAReceptor Antagonist MK-801. Science 1991, 251:85-87. Compellingevidence is presented for the invotient of NMDA re-
50.
..
Trwm
ceptors in the development of morphine tolerance and dependence ln rats without affecting morphine-induced ana&sii Further studies are needed to determine whether non-NMDA receptors are also involved and whether this e&t is mediated pdmarily by NMDAreceptors in the locus coeruleus or other CNS loci.
GP Gasic and S Heinemann, Molecular Neurobiology Laboratory, Howard Hughes Medical Institute, The Salk Institute, 10010 North Torrey Pines Road, Ia Jolla, California 92307, USA.
La Jolla, California,
USA
Glutamate-gated ion channels belong to a complex family of receptors containing several pharmacological subtypes. They are thought to be essential for the acquisition of associative memory and for activitydependent synaptogenesis, and have been implicated in several central nervous system diseases. Within the past year, molecular cloning of the first glutamate receptor channel and several related subunits has opened new approaches for understanding the basis of these important phenomena.
Current Opinion in Neurobiology
Introduction Receptors composed of l&and-gated ion channels mediate rapid (in the order of milliseconds) transduction events at chemical synapses by causing the selective influx of about lo* cations or anions per millisecond and thereby altering the resting membrane potential of excitable cells (for reviews see [1*,261X In general, the l&and-gated channels activated by either acetylcholine or L-glutamate (Glu) cause cell excitation whereas glycine or y-aminobutyric acid (GABA) cause inhibition of cell firing. Unlike the GTP-binding protein (G protein)-coupled receptors, the l&and-gated channels do not require second messenger systems for signal transduction. Acetylcholine mediates synaptic excitatory transmission between nerve and muscle, at autonomic ganglia, chromaflin cells and at a subset of central nervous system (CNS) synapses, whereas Glu is thought to be the major excitatory neurotransmitter in the brain. In the spinal cord and brainstem glycine is the dominant inhibitory neurotmnsmitter, whereas in higher brain regions, GABA is dominant. Activation of the muscle nicotinic acetylcholine (r&h) receptor (R) channel and the GluR channel leads to the influx of small monovalent and divalent cations that depolarize the cell. In contrast, activation of the glycine (Gly)R channel or GABA*R channel opens chloride than nels, which often leads to inhibition. The chloride channels gated by these two receptors have similar biophysical properties. The pentameric skeletal muscle (or Torpedo electric organ) nAchR [subunits: 012, p, Y(E),&], is the best characterized l&and-gated channel in electrophysiological, biochemical and molecular terms [1*,2,3] and serves as the prototype for the l&and-gated channel receptors [ 241.
1991, 1:20-26
Although the deduced protein sequences of the nAchR, GlyR and GABA, R reveal a similar structural organization (e.g. hydrophobic segments) and sign&ant amino acid sequence identity [3-6], the designation of a common evolutionary origin for these receptors on the basis of conserved structure is still debated. Receptor subtype diversity in the CNS has emerged as a common theme for the l&and-gated ion channel receptors as well as for the voltage-gated and G-protein-coupled receptors [ 7*]. Two mechanisms used to generate diversity are: the expression of distinct genes encoding receptor subtypes, and alternative splicing of exons of the same gene. Structural heterogeneity of receptor subtypes is created by combinatorial assemblies of different subunits which then translates into functional diversity. It has been suggested that the multiplicity and diversity of these CNS receptors increases the information handling capacity of neurons and contributes to neural plasticity 17’1.
In this review, we focus on the glutamate receptors as they are essential to the signal transduction mechanisms which underlie several important biological and pathophysiological processes in the vertebrate brain. Within the past year, molecular cloning of the first glutamategated ion channel receptor (designated GluRl) from rat brain has been achieved by functional expression in oocytes. Additional members of the ionotropic GluR family (GluR2-GluRS) were discovered and cloned by lowstringency screening of rat brain cDNA libraries and polymerase chain reaction technology. What follows is a brief description of the pharmacological basis of GluR subtypes, properties of the GluR channels, the biological and pathophysiological Importance of the ionotropic GluR, and recent studies of this family of receptors.
Abbreviations ACPD-tram-1-amino-cyclopentane-1,3 dicarboxylate; AMPA--cc-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid; L-AP4-2-amino-4-phosphonobutyric acid; BDNMrain-derived neurotrophic factor; CNS-central nervous system; GABA-y-aminobutyric acid; G protein--CTP-binding protein; Clu-t-glutamate; Cly-glycine; RA-kainic acid; nAch--nicotinic acetylcholine; NC&nerve growth factor; NMDA-N-methyl-o-aspartic acid; R-receptor. 20
@ Current Biology ltd ISSN 09594388
The glutamate receptor family Casic and Heinemann
Pharmacological basis of GluR subtypes
Biological significance
GluRs have been differentiated on pharmacological grounds into five distinct subtypes named after their most selective agonists: N-methyl-D-aspartic acid (NMDA), kainic acid (KA), a-amino-3-hydroxy5-methyl-4-isoxazole propionic acid (AMPA), 2-amino-4-phosphonobutyric acid (L-AP4) and fransl-amino-cyclopentane-1,3 dicarboxylate (ACPD) [81. Three of these subtypes, NMDA, KA and AMPA, represent l&and-gated channels. The LAP4 subtype is inferred from the specific depressant effect L-AP4 has on some excitatoty amino acid pathways in the retina, brain and spinal cord without any antagonist effect on Glu-induced responses. Evidence from studies of retinal depolarizing bipolar cells suggests than an LAP4 receptor acts via a G protein to increase the hydrolysis of cGMP, resulting in the closure of ion channels conducting an inward current [9]. The Wth subtype, ACPD, is a pertussis toxin-sensitive G-protein-coupled receptor which is linked to inositol phosphate/diacylglycerol formation and release of Ca2+ from internal stores.
More than forty years ago, Hebb proposed that synaptic modification required conjoint presynaptic and postsynaptic activity [19]. Such a Hebbian synapse appears to be essential for some forms of long-term potentiation (see review by S Siegelbaum and E Kandel, this issue, pp113-120) and the dynamic phase of synaptogenesis [20]. GluRs are ubiquitously distributed in the mammalian brain and are apparently fundamental to synaptic networks and some kinds of learning [15,1921,22**-24**,25*].GluRs equip synapses with the potential for use-dependent modilications of synaptic efficacy [ 15,211.
Properties of GiuR channels Studiesusing cultured neurons indicate that NMDAactivated channels are permeable to Na+ , K+ and Ca2+ but at resting potentials are largely blocked by physiological concentrations of extracellular Mg2+ [lo,1 11. KA- and AMPA-activated conductances are permeable to Na+ and K+ [lO,ll] and in some studies to Ca2+ [12,13,14*]. In most membrane patches and in the absence of Mg2+, Glu activates ion channels with multiple conductance states (in the range of < l-50 pS). KA activates low-conductance states while NMDA activates at least live main conductance states in the range of lO-5OpS [10,11,15]. Glycine, in micromolar concentrations, seems to be required for the activation of the NMDA-receptor chanThe function of the glycine binding nel [10,11,15,16]. site is unclear, although some experiments suggest that it may modulate desensitization. This site has also been proposed to bind a ‘glycine-like’ ligand which displaces glycine and leads to receptor inactivation.
NMDA receptors are required to induce changes in synaptic efficacy at some synapses which are correlated with associative learning and activity-dependent modifications of synaptic connections du&g Glu > KAfor inhibition of binding) [42*]. Despite this nanomolar AMPA binding, these receptor channels respond to Glu, quisqualateand AMPAat much higher concentrations [42*,43*]. The results blur previous pharmacological distinctions between KA and AMPAsubclasses [ 81. Additional support for a continuum of KA-AMPAreceptors comes from electrophysiological studies in rat dorsal root ganglion neurons j47.1. Coexpression of GluR subunits in X~~@LS oocytes or 293 cells results in a synergistic effect (two-fourfold potentiation). Different subunit combinations exhibit disparate current/voltagerelationships and rectification characteristics [42*,43=,45]. GluR2 in combination with GluRl or GluR3 produced an almost linear current/voltagerelation similar to hippocampal poly A+ RNA These data suggest that in s&u.,heteromeric combinations of GluR subunits may generate functional receptor diversity. Alternativesplicingof adjacent exons of the GluR1-GluR4 genes to produce subunitswhich differin a smalldomain (the alternativeforms are designated Yip’and ‘flop’) preceding the putativetransmembrane4 domain, introduces further functional receptor diversityand pharmacoIogical complexity [48**]. Saturatingconcentrations of Glu and KA evoked similarcurrents for the flip versions, whereas KA evoked currents were greater than Glu-mediatedcurrents for the flop versions. The disparity between the flip and flop channels was observed for the desensitizing and steady-statecomponents of the current response to Glu. Glu activatedchannels four to five times more e&ctivelywhen encountering the Rip version. Native GluRs may be composed of heteromeric assemblies of different subunits which contain either the flip or flop exons. In such combinations, one subunit can be responsible for the fast desensitizingcomponent while the other can mediate the steady-statecomponent These observations support the evidence that KA and AMPAact via a common receptor. Evidence for splice variantshas also been found for the GluR5 subunit [46*].
The glutamate receptor family Gasic and Heinemann
Gly R
GABAAR
nAchR-a,
Eight amino acids between TM2 and TM3
Fig. 1. Amiio
acid residue number versus relative hydropathy for glycine receptor (CIyR), y-aminobutyric acid A receptor (CABAAR), neuronal nicotinic acetylcholine receptor CnAchR) (u-3), and glutamate receptor (CIuRH. Amino acid sequences of the CIyR, GABAAR, nAchR, and CluRl were analyzed using the relative hydropathy algorithm of Kyte and Doolittle. The hydropathy profiles were inspected and putative transmembrane domains (Ml-M4) were chosen on the basis of hydropathy and on the assumption that these receptors can be classed into a family of ligand-gated receptors with four transmembrane domains (based on nAchR studies). The shaded regions represent ‘best guess’assignments of putative Ml-M4. For the CIuR, two models A and B are presented and discussed in greater detail in [37*1. The GluR has almost twice as many amino acid residues as the other three receptors and what appears to be a large extracellular domain spanning approximately the amino-terminal half. A pattern common to all these receptors is the clustering of putative Ml-M3, a hydrophilic domain and then putative M4.
23
24
Signalling mechanisms
Ca2+ permeability subunit
of KA-AMPA
channels and
composition
NMDA-receptor channels are permeable to Ca2+ and blocked by Mg2 + in a voltage-dependent fashion, while their KA-AMPA counterparts are thought to have negligible divalent cation permeability and are not blocked by Mg2+ [10,15]. Two recent studies, employing Ca2+ imaging in rat astrocytes [12] and electrophysiological techniques in hippocampal neurons [13] observed sigticant Ca2+ conductances mediated by a subset of KA receptors [13] which was abolished by a quinoxaline derivative, non-NMDA receptor antagonist, CNQX [12]. Cloned GluR subunits were used to investigate this property further. Oocytes expressing GluRl , GluR3, and GluRl plus GluR3 but not GluRl plus GluR2 or GluR3 plus GluR2 responded to KA and AMPAwith inward Ca2+ currents [14*]. The experiments suggest that, if some neurons express these non-NMDA GluR subunit assemblies, Glu can trigger Ca2+ conductances mediated by nonNMDA receptors.
CluRs
in neurokdocrine
morphine
tolerance
regulation,
and
References
reading
Papers of special interest, published within the annual period of review, have been highlighted as: . of interest of outstanding interest .. KARUNA Explorations of the Nicotinic Acetyicholine Receptor. Hurvqr Lectures 1991, 85: in press. &is well written chapter describes what has been learned over the past 25 years about the structure of the electric organ (end plate) nicotinic acetylcholine receptor, the prototype and best characterized of ligandgated ion channels. Much of the work described comes from the author’s own laboratory. 1.
2.
GW JL, REVAHF, BEG.% A, CHANGEUXJP: Functional Architecture of the Nicotinic Acetylcholine Receptor. From the Electric Organ to Brain. Annu Rev Phwnuwl Toxicd 1991, 31: in press.
3.
STROUDRM, MCCARTHYMP, SHWTER M: Nicotinic AcetyIcholine Receptor SuperfamiIy of Ligand-Gated Ion Ghannels. Eiocbemisfy 1990, 29:11009-11023.
4.
UNWINN: The Structure of Ion ChanneIs in Membranes of Excitable Cells. Neuron 1989, 3665676.
5.
6.
and addiction
Experimental evidence suggests that non-NMDA receptors are involved in neuroendocrine regulation while NMDA receptors may mediate the development of opiate tolerance and dependence. Converging experiments employing immunocytochemistly, Caz+imaging physiology, and intracellular recordings implicate Glu as the dominant neurotransmitter in neuroendocrine regulation in the hypothalamus [49**]. Also, in situ hybridization experiments demonstrate differential expression of GluR subunits in the pituitary, hypothalamus and adrenal glands and suggest a dominant role for these receptors in neuroendocrine regulation (J Hermans-Borgmeyer et al, unpublished data). The non-competitive NMDA receptor antagonist, MK-801, attenuated the development of morphine-induced tolerance and dependence without affecting analgesia [ 50**]. As there is a link between glutamate@ and dopaminergic systems [ 311, could GluRs be involved in cocaine tolerance and dependence?
and recommended
DINGIEDINER, MYERS SJ, NICHOLLSRA: Molecular
ogy of Mammalian Amino Acid Receptors. 4:2636-2645.
BiolFASEB J 1990,
BEE H: Ligand-Gated Ion Channels in the Brain The Amino Acid Receptor Superfamily. Neuron 1990, 5:3&X3392.
SCHOFIELD PR, SHIVERS BD, SEEBURGPH: The Role of Receptor Subtype Diversity in the CNS. Trends Neurasci 1990, 13:%11. A well written and intriguing perspective on the role of receptor subtype diversity(ligand-gated ion channels, voltage-gated ion channels and G-protein-coupled receptors). The authors postulate that this diversity serves as a means for increasing the information handling capacity of neurons and ultimately, may contribute to neural plasticity in the CNS.
7. .
8.
WATKINSJC, KR~GSSGAARD-LARSEN P, HONORE T: StructureActivity Relationships in the Development of Excitatory Amino Acid Receptor Agonists and Competitive Antagonists. Trends Pkarmuwl Sci 1990, 11:2>33.
9.
NAWY S, JAHR CE: Suppression by Glutamate of cGMPActivated Conductance in Retinal Bipolar Cells. Nature 1990, 346:26%271.
10.
MAYERML, WES?BROOKGL The Physiology of Excitatory Amino Acids in the Vertebrate Central Nervous System. Prog Neurcbiol 1987, 28:197-276.
11.
SANWMMSP, USHERWOODPNR: Single Channel Studies of Glutamate Receptors. Int Rev Neurobiol 1990, 32:51-106.
12.
GLUM SR, HOLZWRA~JA, MILLER RJ: Glutamate Receptors Activate Ca*+ Mobilization and Ca2+ Itiux Into Astrocytes. Pm Nat1Acad Sci USA1990, 8734543458.
Conclusion
13.
efforts are underway to elucidate the molecular structure of the NMDA receptor and to use molecular tools to gain knowledge of the function of the nonNMDA receptors. The next challenge will be to determine the stoichiometries, structures, and cellular localization of these glutamate receptors in vivo. In addition, experiments to map the chromosomal locations of the glutamate receptor genes may find linkages to several CNS diseases. ultimately, these studies should provide clues about the role of glutamate receptors in the regulation of important processes at the synapse.
IINO MS, OZAWAS, Tsuxna K: Permeation of Calcium Through Excitatory Amino Acid Receptor Channels in Cultured Rat Hippocampal Neurons. J P&mid 1990, 424:151-165.
S: Calcium PermeabiIHOW M, HARTIEYM, HEINEMANN ity of KA-AMPA-Gated Ion Channels Depends on Receptor Subunit Composition. Science 1991, 251: in press. At physiological resting potentials, KA and AMPAelicited inward Ca2+ currents in oocytes which expressed GluRl, GIuR3, and GluRl plus GluR3 subunits but not in those which expressed GluRl plus GluR2 or GluR2 plus GluR3 subunits. These ksuk rake the possibility that some non-NMDA receptors may also cause flux of signilicant amounts of calcium into the cell. Because oocytes possess Caz+-activated chloride channels, single-channel studies are required to determine the Ca2+ conductance contribution to the observed current
Intense
14. .
The glutamate receptor family Gasic and Heinemann
15.
COLUNGRIDGE CL, LESTERRAJ: Excitatory Amino Acid Recep tots in tbe Vertebrate Nervous System. Pbarmacol Rev 1989, 40:143-210.
16.
THOMSONAM: GIycine is a CoagonIst at the NMDA Recep-
tor/channel
Complex. Prog Neurobid
1990, 35:5%74.
Duhnr!s A, PINJP, OOMAGARI K, SEBBENM, BOC~C~ERT J: Arachidonic Acid Released Front Striatal Neurons by Joint Stimulation of Ionotropic and Metabotropic QuisquaIate Receptors. Nature 1990, 347:182-184. Releaw of arachidonic acid (whose metabolite(s) are postulated to act as local messengers> from stdatal neurons occurs only upon simultaneous stimulation of the ionotropic and G-protein-coupled glutamate receptors. This work demonstrates that CNS neurons have the capability to integrate fast (ionotropic) and intermediate (G-protein-coupled) GluR-mediated signals to generate a locally active messenger. Such a messenger may ultimately be involved in the trans.synaptic s&nailing events leading to changes in yptic plasticity. 17. .
18. ..
BE~B JM, STEVENSCF: Computational Implications of NMDA Receptor Channels. Cold Spring Harbor S’ Quant Bid 1990, 55: in press. Simply and elegantiy presented, this article crystallizes the unique properties of the NMDA receptor(s) (i.e. requirement for glycine, voltagedependent Mg2+ gating, lengthy open time, and significant Ca2+ conductance) which impart computational capabilities on the synapses that possess them. The chapter references seminal contributions that span nearly a decade of work on this intricate and biologically important glutamate receptor. 19.
HEBB DO: 7& Organization
of Behavior
21.
26.
DING~DINE R, McBm CJ, MCNAMARA JO: Excitatory Amino Acid Receptors in Epilepsy. Trenuk Pbarmacol Sci 1990, 11:334-338.
27.
&IN
CONST~-PATON M, CLINE HT, DEBSKIE: Patterned Activity, Synaptic Convergence, and the NMDA Receptor in Developing Visual Pathways. Annu Rev Neurasci 1990, 13129-154. COUJNGRIDGE GL, SINGERW: Excitatory Amino Acid Recep tars and Synaptic PIasticity. Trends Pbarmacol Sci 1990, 11:29&2%.
CLINEHT, CONSTANTINE-PATON M: NMDA Receptor Agonists and Antagonists Alter Retinal Ganglion Cell Arbor Structure in the Developing Frog RetinotectaI Projection. J Neurosci 1990, 10:1197-1216. Retinalganglion cell arbors in Runupipiensundergo structural changes during normal development. Chronic NMDA application through increased NMDAreceptor activation has been linked with decreased dendritic branch initiation and increased synapse and branch survival. This study proposes that the NMDA receptor senses tierent coactivity and translates it into structural modifications of the arbors which ultimately have physiological consequences.
23. ..
24. ..
UDINSB, SCHERER WJ: Restoration of the Plasticity of BinocuIar yaps by NMDA After the Critical Period in Xenopus Science 1990, 249~66-72. In Xen@us kaeut the visual maps in the optic tectum develop properly with binocular visual input during a critical period. Shit&g the eye position to produce a mismatch in the tecral maps produces a compensatory adjustment in the ipsilateral tectal map only during this critical developmental window. Continuous NMDAtreatment, beyond the critical period, appears to restore the ability to realign the visual map. 25. .
ZAFRAF, HJINGERER B, LEIBROCK J, THO~NENH, ~~HOLM D: Activitydependent Regulation of BDNF and NGF mRNAs in
Huntington’s Disease.
28.
MEIDRUMB, GARTHWA~TE J: Excitatory Amino Acid Neumtoxicity and Neurodegenerative Dis&e. Trends Pbarmacd Sci 1990, 11:37!+387.
29.
CHOI DW, ROTHMAN SM: The Role of Glutamate Neurotoxic-
ity in Hypoxic-ischemic 1990, 13171-182.
Neuronal Death. Annu Rev Neuraui
._.
:
30.
MOGHADDAM B, GRUENRJ, ROTH RH, Bu& BS, ADRN: Effect of L-glutamate on the Release of StriataI Dopamine: in uivo Dialysis and Electrochemical Studies. Brain Res 1990, 518:55-60.
31.
WACHTEL H, TURSKI L: Glutamate: a New Target Schizophrenia? Trends Pbarmacd Sci 1990, 11:219-220.
32.
CHENJ-W, CUNNINGHAM MD, GALTONSN, ~QCHAEL~S EK: Immune LabeIling and Purification of a 71 kD Glutamate-Binding Protein from Rat Brain. J BioI @em 1988, 263417426.
33.
HAMF5ONDR, WENTHOLDRJ: A Kainic Acid Receptor from Frog Brain PuriIied by Domoic Acid AlTinity Cbromatogra phy. J Biol &em 1988, 2632500-2505.
34.
GREGOR P, ESHAR N, ORTEGA A, TEICHEIERG VI: Isolation,
in
Immunochemical Characterization and Localization of the Kainate Subclass of Glutamate Receptor from Chick Cerebellum. EMBO J 1988, 726732679.
22. ..
Mom RGM, DAVISS, BUTCHERSP: HippocampaI Synaptic Plasticity and NMDA Receptors: a Role in Information Storage? Pbih Trans R Sot Land 1990, 329:187-204. This article reviews current evidence for the idea that changes in synpatic efficacy form the neural basis for information storage. Some of the fundamental work described is from the author’s own laboratory. Although, retrieval of established memories is not a&ted by D-APV,an NMDA receptor competitive antagonist, activation of NMDA receptors is essential for some kinds of learning, especially those dependent on spatial orientation of the animal in its environment.
RL, YOUNG AB, PENNYJB, HANDEIJNB, BALFOLJRR, ANDERSON KD, MARKEL DS, TO~RTEILOTE WW, REINER A: Abnormalities of StriataI Projection Neurons and N-methyl-D-aspartate Receptors in Presymptomatic New En@ J Med 1989, 322:12931298.
[book]. New York:
John Wdey 1949. 20.
the Rat Hippocampus is Mediated by Non-NMDA Glutamate Receptors. EMBO J 1990, 935453550. Both BDNF and NGF are necessary for the survival of certain types of neurons in culture. This paper demonstrates that non-NMDh glutamate-gated receptor activitymediates the levels of mRNAsfor these two growth factors.
35.
IKINAF, KLOOGY, SOKOLovsKYM: NMDA/PhencycI.idine Receptor Complex of Rat Forebrain: Purification and Biochemical Characterization. Biochemistry 1990, 29:229&2295.
HOLLMANN M, O’SHEA-GREENFIELJJ A, ROGERSS, HEINEMANN S: Cloning by Functional Expression of a Member of the Glutamate Receptor Family. Nature 1989, 342:64-. Cloning by functional expression of the first member of the ionotropic GluR famiIy represented a technically demanding challenge. This approach took advantage of: directional cloning of cDNAs to derive sense transcripts from pools of phage clones, voltage recording of small agonist-induced depolarizations in injected Xenc@us oocytes (electrophysiological techniques are about four orders of magnitude more sensitive than l&and-binding assays [5] >, successive fractionation of pools of clones yielding GluR responses in oocytes, and the apparent formation of homo-oligomeric glutamate-gated channels. This study served as a spring board for additional subunit cloning efforts by several laboratories [42*-44* ,,45 46*48**], , 36. ..
37. .
HOI~MANN M, ROGERS SW, O’SHEA-GREENFIELD4 DENER~~ES, HUGHESTE, G~slc GP, HEINEMANN S: The Glutamate Recep
tor GluR-Kl: Structure, Function and Expression in tbe Brain. G&i @ring Harb Symp Quant Bid 1990, 55: in press. This chapter provides a compendium of the properties of the firscloned glutamate receptor subunit (GluRl; formerly GluR-Kl) from DNA to protein. Oocytes expressing this subunit respond to both KA and quisqual 38.
WADA K, DECHESNECJ,. Sm BUONANNO4
S, KING RG, KUSANOK,
HAMPSONDR, BANNERC, WENIHOLD RJ: Se-
quence and Expression of a Frog Brain Complementary DNA Encoding a Kainate-Binding Protein. Narure 1989, 342684-689.
25
26
Signallingmechanisms 39.
GREGORP, MANO I, M&o2 I, MCKEOWNM, TEICHBERGVI: Mokcuiar Structure of the Chick Cerebellar Kainate-Bindlng Subunit of a Putative Glutamate Receptor. N&we 1989, 342689-692.
40. .
ROGERSSW, HUGHESTE, How M, GGIC GP, DENERI~ES, HJZINEMANN S: Antibodies to a Cloned Glutamate Receptor Reveal Bxcitatory Circuits in the Rat Brain. / Neumsci 1991, in press. Fusion protein antibodies to the domains of cloned GluRl have been used to determine the synaptic localization and distribution of this subunit in rat brain. Predominantly neuronal and posrsynpatic, GluRl appears to be abundantly expressed in the cerebellum and parts of the llmbic system. While inhenzntlymore difficult,antibody studies of these receptor subunits are necessary because discordances between mRNA and protein expression do exist and receptor cytoarchitecture may provide imlxxtant clues to function. 41.
ANDERsEN P: Synaptic hte8ration in Hippocanlpal CA1 Plm+ mkial Neurons. Prog Bruin Res 1990, 83215-222.
TA, KEINANEN K, WI~DENW, SOMMER B, WERNERP, VERLXIORN SAKMANN B, SEEBURGPH: A Family of AMPA-Selective Glutamate Receptors. Science 1990, 24955560. The authors employed a new nomenclature for GluR-Kl, calling it GluRA, and presented sequence and expression data for three additional GluR subunits - GluRB (GluR2), GIURC (GluR3), GluRD (GluR4) - with about 70% sequence identity to GluR-Kl (GluRl). On the basii of agonist biding studies with GluR subunits expressed in mammalian cells, this paper argues in favor of calling these subunits AMPA receptors. Transiently expressed in embryonic kidney cells, disparate combinations of these subunits exhibit tierent agonist e&a ties and currenr/voltage relationships. 42.
.
BOIJLER J, HOUMANNM, O’SHW-GREENFIEU) A, HARTIEYM, DENE~USES, MARONC, HEINEMANN S: Molecular Cloning and Functipnal Bxpression of Glutamate Receptor Subunit _ en&. Science 1990, 249~10351037. Iike [42-l, this article presents StruCNre function studies of additional cloned subunits expressed in oocytes. In concert with the rat brain GluR subunit in situ hybridization data from this study, and in context with GluR electrophysioloey data from cdNted neurons, the conclusion that the in vim GluR is most likely a hetero-oligomer emerges. 43. .
44. .
NAKANLsHl N, SCHNEIDER NA, ABEL R A Family of Glutamate Receptor Genes: Evidence for the Formation of Heteromultimeric Receptors with Distinct Channel Properties. Neuron 1990, 5569-581. GluRGluR3 subunits appear to have a conserved region which has some sequence slm&ulity with the glutamine-binding domain of the Es&e&&a cdi glutamine permease. 45.
m K, BUJO H, ARAKIK, YAMAUK~M, MEGUROH, W~HINA A, NUMAS, M.+~HINA M: Functional Expression &om Cloned cDNAs of Glutamate Receptor Species Responsive to Kalnate and Qulsqualate. FEBS Len 1990, 272:73-80.
46. .
BET~~ER B, BOUL.TERJ, HERMANS-BORGMEYER I, O’SHEAGREENFEIDA, DENERIS ES, Mou C, BORG~R U, HOUMANN
M, HEINEMANN S: Cloning of a Novel Glutamate Receptor Subunit, GluRs: Expression ln the Netvous System During Development. Neunm 1990, 5583-595. GluR5 shares only 40% amino acid sequence identity with the other subunits and when expressed in oocytes, responds only weakly to L-glutamate. GluR5 ls expressed ln subsets of neurons in developing and adult CNS and PNS. During embryogenesis, GluR5 transcripts are apparent in zones undergoing neuronal differentiation and synapse formation. HIJET~NERJE: Glutamate Receptor Channels in Rat DRG Neurons: Activation by Kalnate and Qulsqualate and Block of Desensitization by ConA. Neuron 1990, 5255-266. Rapid application of Glu, qulsqualate and AMPA to dorsal root 8an& neurons produces a transient current and densen.u‘tiz.esthesecellsto subsequent applications of KA or domoate. In contrast to CNS neurons, the action of KA in dorsal root ganglla neurons was more potent than Glu. Concanawlin A treatment of these cells abolishes desensitition to glutamate class agonists and all f& agonists evoked similar currents. 47. .
48. ..
TA, WI~DENW, BURNASHEV SOh4MER B, KEINANEN K, VERDOOaN N, HERB A, KOHIER M, TAKAGIT, SAKMANN B, SEEBURG PH: Flip and Flop: a Cell-Specilic Functional switch in Glutamate-Operated Channels of the CNS. Science 1990, 24915-1585. This important arricle demonstrates that funcrional diversity can arise from alternative splicing of exons of the same gene. A 38.amino-acid stretch preceding putative transmembrane domain 4 has been demonstrated to exist in two alternative forms, designated ‘flip’and ‘flop’, and encoded by adjacent exons of the respective GluR gene. Despite the WhhSiCd tide, this ekgant SttUcNfdutmiOII study is fat fmm a ‘80~‘. A more flexible nomenclature for alternativelyspliced vadants of a given receptor subunit gene is GluRX. Y (X = l-n; Y = l-n) as described by Bettler er al. [46.1.Among% the ligand-gated channels, the GluRs are unique because alternative splicing produces functional diversity. Purthermore, evidence is presented for a cell-specific pattern for altemaw tively spliced versions. VANDEN POLAN, WUARIN JP, DUDEKFE: Glutamate, the Dominant Excitatory Transmitter in Neuroendocrine Regulation. Science 1990, 250:1276-1278. This ele@nt SNdy employs ukta9t~~Nd immunocytochemistry, physiolo@cal experiments with Ca2+ dye imaging, and intracellular recordlng to demonstrate a dominant role for glutamate and non-NMDA receptors in control of neurons of the neumendocrine hypothalamus. 49.
..
KA, AKU H: Inhibition of Morphine Tolerance and Dependence by the NMDAReceptor Antagonist MK-801. Science 1991, 251:85-87. Compellingevidence is presented for the invotient of NMDA re-
50.
..
Trwm
ceptors in the development of morphine tolerance and dependence ln rats without affecting morphine-induced ana&sii Further studies are needed to determine whether non-NMDA receptors are also involved and whether this e&t is mediated pdmarily by NMDAreceptors in the locus coeruleus or other CNS loci.
GP Gasic and S Heinemann, Molecular Neurobiology Laboratory, Howard Hughes Medical Institute, The Salk Institute, 10010 North Torrey Pines Road, Ia Jolla, California 92307, USA.
