Serotonin Uptake and Serotonin Uptake Inhibition RAY W. FULLER A N D DAVID T. WONG Lilly Research Laboratories Eli Lilly and Company Lilly Corporate Center Indianapolis, Indiana 46285
INTRODUCTION After serotonin is released from nerve terminals and has acted on post-synaptic receptors, it is transported out of the synaptic cleft back into the nerve terminal via the action of a specific membrane uptake carrier. During the past fifteen years, several compounds have been described that inhibit this serotonin uptake carrier with high selectivity, and their availability has helped in understanding the physiologic role of the uptake carrier. Administration of these uptake inhibitors to animals does not result in depletion of brain serotonin content, and in fact leads to a reduction in brain serotonin formation and release, suggesting that the physiological role of the neuronal uptake carrier is not to conserve serotonin but instead to inactivate serotonin by removing it from the synaptic cleft. In the brain, serotonin uptake carriers also occur on glial cells, but the physiologic roles of these uptake carriers and their possible influence on synaptic function are still not well understood.' Blood platelets also have a serotonin uptake carrier, which is the means by which platelets acquire serotonin, since they do not synthesize it.2 Inhibition of the serotonin uptake carrier on blood platelets over a prolonged time results in depletion of platelet (and whole blood) concentrations of ~ e r o t o n i n . ~ This chapter will focus on brain serotonin neurons and the ways in which serotonin uptake inhibition and the functional results of that inhibition have been studied.
CHEMICAL CLASSES OF SEROTONIN UPTAKE INHIBITORS The chemical structures of several selective inhibitors of serotonin uptake are shown in FIGURE 1. The compounds cover a wide range of chemical types, with little similarity among them overall. All have primary or substituted amino groups separated from an aromatic nucleus by cyclic, branched or straight-chain connecting groups. Several, but not all, have halogen substituents on the aromatic nucleus.
SELECTIVITY OF SEROTONIN UPTAKE INHIBITORS I N VZTRO Many of the earlier uptake inhibitors, especially the tricyclic antidepressant drugs, were relatively nonselective in blocking the uptake of neurotransmitter rnonoamines. They inhibited norepinephrine and serotonin uptake, and some antidepressant drugs 1 are such as nomifensin also inhibited dopamine ~ p t a k eThe . ~ compounds in FIGURE 68
FULLER & WONG: SEROTONIN UPTAKE AND ITS INHIBITION
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FLUOXETINE
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69
I C' ~Hl~HICHIN(CHl~l
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FIGURE 1. Chemical structures of some selective inhibitors of serotonin uptake.
all highly selective in blocking the uptake of serotonin, having effects on norepinephrine and dopamine uptake only at much higher concentrations in vitro. In several cases, the substituent on the nitrogen is an important determinant of selectivity. The removal of a methyl group from clomipramine results in a selective inhibitor of norepinephrine
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uptake, not serotonin uptake! On the other hand, the removal of a methyl group from zimelidine increases potency of serotonin uptake inhibition.’ The substituent on the aromatic nucleus also is an important determinant of selectivity. The absence of the chlorine atom in clomipramine reduces the potency and selectivity of serotonin uptake i n h i b i t i ~ nReplacement .~ of the p-trifluoromethyl substituent on the phenoxy nucleus of fluoxetine, a selective inhibitor of serotonin uptake,6 with an o-methyl or o-methoxy group results in selective inhibitors of norepinephrine uptake.’.’
DEMONSTRATION OF SEROTONIN UPTAKE INHIBITION I N VWO Serotonin uptake inhibition can be demonstrated ex vivo by measuring serotonin uptake i n vitro in brain slices or synaptosomes prepared from animals treated with the uptake inhibitor in viva The percentage inhibition measured is probably not a true reflection of the percentage inhibition that occurred in the intact brain, since this type of experiment is to some extent a bioassay of the amount of drug present in the brain preparation used to measure uptake. Such experiments are especially useful to demonstrate the ability of serotonin uptake inhibitors to reach the brain, their potency and selectivity relative to one another, and their duration of action.’ A less direct method for evaluating uptake inhibition in brain, but one that measures an effect in the intact brain as opposed to in vitro, is the antagonism of brain serotonin depletion by a drug whose depleting action requires the membrane uptake carrier. Four such serotonin-depleting drugs whose antagonism by serotonin uptake inhibitors has been used as a measure of uptake inhibition i n vivo a r e p-chloromethamphetamine,“ p-chloroamphetamine,” fenfluramine,I2and H75/ 12.13 The accumulation of these drugs into serotonin neurons via the membrane uptake carrier apparently is required for their depletion of brain serotonin, since that depletion is blocked in a dose-dependent manner by serotonin uptake inhibitors. Inhibition of the serotonin uptake carrier on blood platelets can also be demonstrated by measuring uptake in vitro after administration of the uptake inhibitor in vivo. This technique can be applied in human studies, since blood platelets are readily accessible, as well as in animal studies. Serotonin becomes depleted from blood platelets after continued administration of a serotonin uptake inhibitor, because uptake is the means by which platelets acquire serotonin. Inhibition of platelet serotonin uptake ex vivo and reduction of platelet serotonin content have been demonstrated in humans given selective inhibitors of serotonin uptake.14
UPTAKE INHIBITORS AS RADIOLABELS FOR SEROTONIN UPTAKE CARRIERS Radioactive forms, especially tritiated forms, of several selective inhibitors of serotonin uptake have been used as radioligands to label the serotonin uptake carrier. Radiolabels used for i n vitro binding studies have included i m i ~ r a m i n e , ’ ~ nitroimipramine,I6fluoxetine,” citalopram,” norzimelidine,” and paroxetine.20Radiolabels used for quantitative autoradiography in vivo or in brain slices include imipramine,*’ nitroimipramine,22 p a r ~ x e t i n e ,citalopram,” ~~ i n d a l ~ i n e , *and ~ cyanoimipramine.25
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EVIDENCE FOR INCREASED SEROTONIN CONCENTRATIONS IN THE SYNAPTIC CLEFT AFTER SEROTONIN UPTAKE INHIBITION As a consequence of administration of a serotonin uptake inhibitor, serotonin concentration in the synaptic cleft is expected to increase since it cannot be removed from the synaptic cleft by the membrane uptake carrier. Although the technology for measuring serotonin concentration directly in the synaptic cleft does not exist currently, several experimental approaches have documented increased extracellular concentrations of serotonin after uptake inhibition. Geyer et presented histofluorescence evidence for increased extraneuronal concentrations of serotonin in raphe regions of rat brain after fluoxetine administration. Marsden et aL2’ showed that i n vivo voltammetric signals believed to be serotonin increased in rat brain after fluoxetine administration. Guan and McBride” reported several-fold increases in serotonin concentration in extracellular fluid obtained from push-pull cannulae in the nucleus accumbens of rats after administration of fluoxetine. Perhaps the most extensive evidence for increased serotonin concentrations i n the synaptic cleft and increased activation of synaptic receptors for serotonin comes from the numerous and varied functional effects that have been produced by serotonin uptake inhibitors.
PHARMACOLOGIC EFFECTS OF SEROTONIN UPTAKE INHIBITORS IN ANIMALS Serotonin uptake inhibitors produce various functional effects on laboratory animals alone or in combination with 5-hydroxytryptophan (SHTP). These are thought to be due to the enhanced activation of synaptic receptors for serotonin, either presynaptic autoreceptors or postsynaptic receptors. Neurochemical Efects Serotonin uptake inhibitors cause a decrease in serotonin turnover, which has been measured in various ways, including a decrease in incorporation of radioactive tryptophan into 5-hydroxyindoles. a decrease in the accumulation of SHTP after decarboxylase inhibition, a decrease in the steady state concentration of 5-hydroxyindoleacetic acid (SHIAA) and in the accumulation of 5HIAA after administration of probenecid to block its efflux from brain.” The decrease in serotonin turnover occurs in parallel with a decrease in firing of serotonin neurons measured electrophysiol~gically.~~ The decrease in serotonin turnover may occur partly or entirely via activation of autoreceptors on serotonin neurons that regulate serotonin synthesis and release. Other neurochemical effects of serotonin uptake inhibitors probably result from increased activation of postsynaptic receptors by serotonin that has accumulated in the synaptic cleft. For example, serotonin uptake inhibitors potentiate the increase in striatal dopamine metabolites caused by neuroleptic drugs.” They also increase hypothalamic norepinephrine metabolite^,'^ likely due to facilitatory input to noradrenergic n e u r o n ~ , ~especially ~.’~ when combined with 5HTP.35 Neuroendocrine Efects Serotonin uptake inhibitors increase serum corticosterone concentration in rats” by increasing corticotropin-releasing hormone release and increasing plasma adreno-
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corticotrophin (ACTH).” Pretreatment of rats with serotonin uptake inhibitors antagonizes the elevation of serum corticosterone concentration by p-chloroamphetamine3* or f e n f l ~ r a m i n by e ~ blocking ~ the carrier-mediated release of serotonin by these agents, but potentiates the elevation of serum corticosterone concentration by L-5-hydroxytryptophan (L-SHTP)’~by inhibiting the inactivation of serotonin by the uptake carrier. Serotonin uptake inhibitors alone do not increase serum prolactin concentration in ~’ rats,40 but they antagonize the elevation of serum prolactin by f e n f l ~ r a m i n e and potentiate the elevation of serum prolactin by L-SHTP.~’ Behavioral Efects Animals treated with serotonin uptake inhibitors look normal in gross appearance, but subtle behavioral effects can be detected in suitable experimental paradigms. For example, serotonin uptake inhibitors inhibit muricidal behavior in rats4’ and improve passive avoidance behavior in some but not all behavioral tests in bulbectomized rats4’ Serotonin uptake inhibitors potentiate behavioral effects of SHTP, such as head twitch and the discriminative cue stimulus property of SHTP in rats.4s in Eflect on Food Intake Serotonin uptake inhibitors decrease food intake after single doses in animal^.^^-^' The decrease in food intake persists after daily treatment, and body weight or weight gain is decreased after repeated administration to normal or obese rats and m i ~ e . ~ ’ ~ ~ ” Other Eflects Selective inhibitors of serotonin uptake have been shown to reduce rapid eye movement sleep in rats and to enhance memory processing in mice,” and to produce analgesic effects or potentiate opioid-induced a n a l g e ~ i a . ~The ~ ~ multiplicity ’~ of effects that can be elicited by enhancing serotonin function through uptake inhibition results from the widespread distribution of serotonergic projections to many different brain regions involved in various functions controlled by the brain.56
THERAPEUTIC EFFECTS OF SEROTONIN UPTAKE INHIBITORS IN HUMANS Serotonin uptake inhibitors are used or have shown promise for use in treating numerous psychiatric illnesses and other conditions. Some of these therapeutic uses are outlined below. Depression There is now extensive literature on the efficacy of selective inhibitors of serotonin uptake in treating mental depres~ion.~’~’~ A major clinical advantage over most tricyclic antidepressant drugs is the lack of side effects related to anticholinergic, antihistaminergic, or anti-adrenergic actions, since most serotonin uptake inhibitors
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have little affinity for muscarinic, histamine H1, alpha-1 adrenergic or other neurotransmitter receptors.59Selective inhibitors of serotonin uptake generally lack direct effects on the heart, in contrast to many tricyclic antidepressant drugs6' Weight gain, a relatively common side effect of earlier antidepressant drugs, is also not a problem with selective inhibitors of serotonin uptake used to treat depression.6' Zimelidine was the first selective inhibitor of serotonin uptake to be marketed for the treatment of depression worldwide, but it had to be removed from clinical use due to some infrequent but severe side effects including the Guillain-Barre syndrome. The adverse effects were not thought to be related to serotonin uptake inhibition but were somehow due to the particular chemical structure of zimelidine. Fluoxetine was the first selective inhibitor of serotonin uptake to be introduced as an antidepressant drug in the U.S. market. Other compounds in this group are in clinical use outside the U S . and their development is continuing. Postanoxic Intention Myoclonus This frequently severe movement disorder that can follow brain injury by anoxia is thought to be related to impaired serotonergic neurotransmission and has been treated with L - ~ H T P . ~Fluoxetine ' alone had minimal benefit in these patients, but the combination of fluoxetine and a low dose of L-SHTP (given with carbidopa to block its decarboxylation in peripheral tissues) resulted in enhanced antimyoclonic activity with fewer side effects.63 Obsessive-Compulsive Disorder White et al.64have described a patient with severe obsessive-compulsive disorder refractory to treatment with a tricyclic antidepressant or a monoamine oxidase inhibitor who responded dramatically and repeatedly to treatment with two serotonin uptake inhibitors, clomipramine and citalopram. Turner et al.65 had reported that fluoxetine was effective in patients with obsessive-compulsive disorder. Panic Disorder Zimelidine, a selective inhibitor of serotonin uptake, was superior to placebo in most rating scales in the treatment of patients with agoraphobia with panic attacks.'"' In this study, imipramine was not shown to be superior to placebo. Fluvoxamine has also decreased panic attacks in a study in which a norepinephrine uptake inhibitor did not.67Fluoxetine has caused complete cessation of panic attacks in some patients." Obesity Serotonin uptake inhibitors decrease food intake in rats, acting synergistically with L - ~ H T PUnlike . ~ ~ anorectic drugs acting via non-serotonergic mechanisms, serotonin uptake inhibitors have been shown in animal studies to suppress stress-induced eating,69to selectively suppress carbohydrate intake,7oand to suppress insulin-induced hyperphagia as well as 2-deoxyglucose-induced h~perphagia.~' Therapeutic utility of serotonin uptake inhibitors in the treatment of obesity in humans has been reported for zimelidine,'* fern~xetine,'~ f l u ~ o x a m i n and e ~ ~f l ~ o x e t i n e . ~ ~
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Bulimia Bulimia is an eating disorder with episodes of binge eating followed by self-induced vomiting. Recent evidence indicates that serotonin uptake inhibitors are effective in treating bulimia.76
Alcoholism In rats, selective inhibitors of serotonin uptake including fluoxetine, zimelidine, fluvoxamine and sertraline, have reduced ethanol drinking in rats selected or bred to prefer alcohol when given a choice of drinking f l ~ i d s . ~In~ humans, -~~ zimelidine decreased the number of drinks consumed and increased the number of abstinent days in nondepressed heavy Similar effects have been reported with another serotonin uptake inhibitor, citalopram.82
Other Disorders Serotonin uptake inhibitors including fluoxetine and clomipramine have been reported to be effective in treating cataplexy, sudden brief paralysis of voluntary movement and loss of muscle tone following a momentary decrease in alertne~s;'~ fluoxetine had earlier been effective in treating cataplexy in dogs.84There is extensive evidence that central serotonergic pathways are involved in the reception and processing of pain messages,85and serotonin uptake unhibitors may be useful in treatment of some types of pain, such as chronic pain.86 It is possible that depletion of platelet serotonin by serotonin uptake inhibitors will have therapeutic benefit in reducing migraine attacks.87
SUMMARY Serotonin uptake carriers occur on serotonin neurons, on glial cells and on blood platelets. The uptake carrier on serotonin neurons inactivates serotonin that has been released into the synaptic cleft by transporting it back into the nerve terminal. The serotonin uptake carrier is the means by which blood platelets acquire serotonin, since they do not synthesize it. The function of the serotonin uptake carrier on glial cells is poorly understood. Selective inhibitors of serotonin uptake enhance neurotransmission via serotonergic neurons and have been useful pharmacologic tools for studying physiologic roles of serotonin neurons. Some serotonin uptake inhibitors are finding therapeutic uses in mental depression and other psychiatric disorders and in treating obesity and bulimia; other therapeutic applications continue to be evaluated. REFERENCES 1986. Occurrence and functional significance of serotonin and catecholamine uptake by astrocytes. Biochem. Pharmacol. 3 5 2273-228 1. 2. PLETSCHER, A. 1987. The 5hydroxytryptamine system of blood platelets: physiology and pathophysiology. Int. J. Cardiol. 14: 177-178. 3. PETERSEN, E. N., E. BECHGAARD, R. J. SORTWELL & L. WETTERBERG. 1978. Potent 1.
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J., H. E. SAVAKI, C. MALGOURIS, C . LAPLACE, c . MARGELIDON, M. DANIEL, J. 24. BENAVIDES, COURTEIX, A. UZAN,C. GUEREMY & G. LE FUR.1985. Quantitative autoradiographyof [3H]indalpine binding sites in the rat: I. Pharmacological characterization. J. Neurochem. 4 5 514-520. D. J. BRUNSWICK & A. FRAZER.1988. Quantitative 25. KOVACHICH.G. B., C. E. ARONSON, autoradiography of serotonin uptake sites in rat brain using [3H]cyanomipramine. Brain Res. 4 5 4 78-88. 1978. Fading: A new cytofluorimetric 26. GEYER,M. A,, W. J. DAWSEY& A. J. MANDELL. measure quantifying serotonin in the presence of catecholamines at the cellular level in brain. J . Pharmacol. Exp. Ther. 207: 650-667. C. A,, J. CONTI,E. STROPE,G. CURZON& R. N. ADAMS.1979. Monitoring 27. MARSDEN, 5-hydroxytryptaminerelease in the brain of the freely moving unanaesthetized rat using in vivo voltammetry. Brain Res. 171: 85-99. 28. GUAN,X.-M. & W. J. MCBRIDE.1988. Fluoxetine increases the extracellular levels of serotonin in the nucleus accumbens. Brain Res. Bull. 21: 43-46. 29. FULLER,R. W. & D. T. WONG. 1977. Inhibition of serotonin reuptake. Fed. Proc. 3 6 2154-2158. 30. CLEMENS, & B. CERIMELE. 1977. Further evidence that serotonin is a J. A., B. D. SAWYER neurotransmitter involved in the control of prolactin secretion. Endocrinology 1 0 0 692698. P. A. & A. A. DELIXI-STULA. 1979. Serotonin-dopamine interactions in the 31. WALDMEIER, nigrostriatal system. Eur. J. Pharmacol. 5 5 363-373. T. DENNIS,A. SERRANO, S. ARBILLA, 32. SCATTON,B., Y. CLAUSTRE,D. GRAHAM, C. PIMOULE, H. SCHOEMAKER, D. BIGG& S. Z. LANGER.1988. SL 81.0385: A novel selective and potent serotonin uptake inhibitor. Drug Develop. Res. 12: 29-40. 33. MCRAE-DEGUEURCE, 1985. Regulation of A., T. DENNIS,L. LEGER& B. SCATTON. noradrenergic neuronal activity in the rat locus coeruleus by serotoninergic afferents. Physiol. Psychology 1 3 188-196. T. J. & G. HERTTING. 1986. Serotonin (5-HT) enhances hippocampal 34. FEUERSTEIN, noradrenaline (NA) release: Evidence for facilitatory 5-HT receptors within the CNS. Naunyn-Schmiedeberg’s Arch. Pharmacol. 3 3 3 19 1-197. 35. SMYTHE, G. A,, R. M. GLEESON& G. M. STEAD.1988. Mechanisms of 5-hydroxy-Ltryptophan-induced adrenocorticotropin release: A major role for central noradrenergic drive. Neuroendocrinology 47: 389-397. R. W. 1981. Serotonergic stimulation of pituitary-adrenocortical function in rats. 36. FULLER, Neuroendocrinology 32: 118-127. 37. Glees, D. M. & W. VALE. 1983. Effect of the serotonin reuptake inhibitor fluoxetine on corticotropin-releasing factor and vasopressin secretion into hypophysial portal blood. Brain Res. 2 8 0 176-179. 38. FULLER, R. W. & H. D. SNODDY.1980. Effect of serotonin-releasing drugs on serum corticosterone concentration in rats. Neuroendocrinology 31: 96-100. J. F., J. M. MILLER& J. S. MEYER.1984. Fenfluramine,p-chloroamphetamine 39. MCELROY, and p-fluoroamphetamine stimulation of pituitary-adrenocortical activity in rats: Evidence for differences in site and mechanism of action. J. Pharmacol. Exp. Ther. 228: 593-599. 40. KRULICH, L. 1975. The effect of a serotonin uptake inhibitor (Lilly 110140) on the secretion of prolactin in the rat. Life Sci. 17: 1141-1 144. 41. VAN DE KAR,L. D., J. H. URBAN, K. D. RICHARDSON & C. L. BETHEA.1985. Pharmacological studies on the serotoninergic and nonserotonin-mediated stimulation of prolactin and corticosterone secretion by fenfluramine. Neuroendocrinology 41: 283-288. 42. MOLINA, V. A., S. GOBAILLE & P. MANDEL.1986. Effects of serotonin-mimetic drugs on mouse-killing behavior. Aggress. Behav. 12: 201-21 1. 43. JOLY,D. & D. J. SANGER. 1986. The effects of fluoxetine and zimeldine on the behavior of olfactory bulbectomized rats. Pharmacol. Biochem. Behav. 2 4 199-204. R., P.C. WALDMEIER, 44. ORTMANN, E. RADEKE, A. FELNER& A. DELINI-STULA. 1980. The
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47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66.
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effects of 5-HT uptake- and MAO-inhibitors on L-5-HTP- induced excitation in rats. Naunyn-Schmiedeberg’s Arch. Pharmacol. 311: 185-192. BARRETT, R. J., M. A. BLACKSHEAR & E. SANDERS-BUSH. 1982. Discriminative stimulus properties of L-5-hydroxytryptophan: Behavioral evidence for multiple serotonin receptors. Psychopharmacology 76 29-35. GOUDIE, A. J., E. W. THORNTON & T. J. WHEELER. 1976. Effects of Lilly 110140, a specific inhibitor of 5-hydroxytryptamine uptake, on food intake and on 5-hydroxytryptophaninduced anorexia. Evidence for serotonergic inhibition of feeding. J. Pharm. Pharmacol. 2 8 318-320. DUMONT, C., J. LAURENT, A. GRANDADAM & J. R. BOISSIER. 1981. Anorectic properties of a new long acting serotonin uptake inhibitor. Life Sci. 28: 1939-1945. LUCKI,I., M. S. KREIDER & K. J. SIMANSKY. 1988. Reduction of feeding behavior by the serotonin uptake inhibitor sertraline. Psychopharmacology 9 6 289-295. YEN,T. T., D. T. WONG& K. G. BEMIS.1987. Reduction of food consumption and body weight of normal and obese mice by chronic treatment with fluoxetine: A serotonin reuptake inhibitor. Drug Develop. Res. 1 0 37-45. WONG,D. T. & R. W. FULLER. 1987. Serotonergic mechanisms in feeding. Int. J. Obesity 11(SUppl. 3): 125-133. SLATER, I. H., G. T. JONES& R. A. MOORE.1978. Inhibition of REM sleep by fluoxetine, a specific inhibitor of serotonin uptake. Neuropharmacology 17: 383-389. PASTEL,R. H. & J. D. FERNSTROM. 1987. Short-term effects of fluoxetine and trifluoromethylphenylpiperazine on electroencephalographic sleep in the rat. Brain Res. 436 92102. FLOOD,J. F. & A. CHERKIN. 1987. Fluoxetine enhances memory processing in mice. Psychopharmacology 9 3 36-43. MESSING,R. B., L. PHEBUS,L. A. FISHER& L. D. LYTLE.1975. Analgesic effect of fluoxetine hydrochloride (Lilly 1 10140), a specific inhibitor of serotonin uptake. Psychopharmacol. Commun. 1: 51 1-521. SUGRUE, M. F. 1979. On the role of 5-hydroxytryptamine in drug-induced antinociception. Br. J. Pharmacol. 6 5 677-681. AZMITIA,E. C. & P. J. GANNON.1986. The primate serotonergic system: A review of human and animal studies and a report on Macaca fascicularis. Adv. Neurol. 4 3 407468. LEMBERGER, L., R. W. FULLER& R. L. ZERBE.1985. Use of specific serotonin uptake inhibitors as antidepressants. Clin. Neuropharmacol. 8 299-3 17. ASBERG,M., B. ERIKSSON, B. MARTENSSON, L. TRASKMAN-BENDZ & A. WAGNER. 1986. Therapeutic effects of serotonin uptake inhibitors in depression. J. Clin. Psychiatr. SUPPI. PI. 4): 23-35. E. & A. NELSON.1984. Antagonism by antidepressants of neurotransmitter RICHELSON, receptors of normal human brain in v i m . J. Pharmacol. Exp. Ther. 230 94-102. PARY,R., C. R. TOBIAS& S. LIPPMANN. 1989. Antidepressants and the cardiac patient. Postgrad. Med. 85: 267-276. 1988. Antidepressants and weight gain. Appetite RUSS, M. J. & S. H. ACKERMAN. 1 0 103-1 17. VAN WOERT,M. H. & D. ROSENBAUM. 1979. L-5-Hydroxytryptophantherapy in myoclonus. In Cerebral Hypoxia and Its Consequences. S. Fahn, J. N. Davis and L. P. Rowland, Eds. New York: Raven Press, pp. 107-122. VAN WOERT,M. H., I. MAGNWSSEN, D. ROSENBAUM & E. CHUNG.1983. Fluoxetine in the treatment of intention myoclonus. Clin. Neuropharmacol. 6 49-54. WHITE,K.,P. E. KECK,JR. & J. LIPINSKI.1986. Serotonin-uptake inhibitors in obsessivecompulsive disorder: A case report. Comprehensive Psychiatry 27: 21 1-214. TURNER, S. M., R. G. JACOB, D. C. BEIDEL& J. HIMMELHOCH. 1985. Fluoxetine treatment of obsessive-compulsivedisorder. J. Clin. Psychopharmacol. 5: 207-21 2. EVANS,L., J. KENARDY, P. SCHNEIDER & H. HOEY.1986. Effect of a selective serotonin uptake inhibitor in agoraphobia with panic attacks. A double-blind comparison of zimeldine, imipramine and placebo. Acta Psychiatrica Scand. 73: 49-53.
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1988. Effect of a serotonin and noradrenaline 67. DENBOER,J. A. & H. G. M. WESTENBERG. uptake inhibitor in panic disorder; A double-blind study with fluvoxamine and maprotiline. Int. Clin. Psychopharmacol. 3 59-74. J. M., M. R. LIEBOWITZ, A. J. FYER,D. GOETZ,R. B. CAMPEAS, M. R. FYER,S. 68. GORMAN, 0. DAVIES& D. F. KLEIN.1987. An open trial of fluoxetine in the treatment of panic attacks. J. Clin. Psychopharmacol. 7: 329-332. S. M., N. ROWLAND & C. KOCAN.1981. Anorectics: Lack of cross tolerance 69. ANTELMAN, among serotonergic drugs and sensitization of amphetamine’s effect. In Anorectic Agents: Mechanisms of Action and Tolerance. S. Garattini & R. Samanin, Eds. New York: Raven Press, pp. 45-62. J. J. & R. J. WURTMAN. 1977. Fenfluramine and fluoxetine spare protein 70. WURTMAN. consumption while suppressing caloric intake by rats. Science 198: 1178-1 180. M. O., S. RICCIARDI, P. SPANO & P. MANTEGAZZA. 1985. Dopaminergic and 71. CARRUBA, serotoninergic anorectics differentially antagonize insulin- and 2DG-induced hyperphagia. Life Sci. 3 6 1739-1 749. R.J., D. J. LAWTON, M. H. WATT& B. TIPLADY. 1981. Effect of zimelidine, a new 72. SIMPSON, antidepressant, on appetite and body weight. Br. J. Clin. Pharmacol. 11: 96-98. J., P. CHRISTIANSEN & B. SKRUMSAGER. 1981. Treatment of obesity by 73. SMEDEGAARD, femoxetine, a selective 5HT reuptake inhibitor. Int. J. Obesity 5 377-378. 74. ABELL,C.A., D.L. FARQUHAR, S. M. GALLOWAY, F. STEVEN, A. E. PHILIP& J. F. MUNRO. 1986. Placebo controlled double-blind trial of fluvoxamine maleate in the obese. J. Psychosom. Res. 3 0 143-146. & J. BOSOMWORTH. 1987. Use of a serotonin re-uptake 75. LEVINE,L. R., S. ROSENBLATT inhibitor, fluoxetine, in the treatment of obesity. Int. J. Obesity ll(Supp1. 3): 185-190. 76. FREEMAN, C. P. L. & M. HAMPSON. 1987. Fluoxetine as a treatment for bulimia nervosa. Int. J. Obesity ll(Supp1. 3): 171-177. 77. ROCKMAN, G. E., Z. AMIT, Z. W. BROWN,C. BOURQUE& S.-0. OGREN.1982. An investigation of the mechanisms of action of 5-hydroxytryptamine in the suppression of ethanol intake. Neuropharmacology 21: 341-347. 78. GILL,K., Z. AMIT& B. K. KOE. 1988. Treatment with sertraline, a new serotonin uptake inhibitor, reduces voluntary ethanol consumption in rats. Alcohol 5 349-354. G. J. GATTO,W. J. MCBRIDE,L. LUMENG& T.-K. LI. 79. MURPHY,J. M., M. B. WALLER, 1985. Monoamine uptake inhibitors attenuate ethanol intake in alcohol-preferring (P) rats. Alcohol 2 349-352. 80. NARANJO, C. A., E. M. SELLERS, C. A. ROACH,D. V. WOODLEY, M. SANCHEZ-CRAIG & K. SYKORA. 1984.Zimelidine-inducedvariations in alcohol intake by nondepressed heavy drinkers. Clin. Pharmacol. Ther. 3 5 374-381. G. ROCKMAN, K. GILL & N. SELVAGGI. 1985. 81. AMIT, Z., Z. BROWN,A. SUTHERLAND, Reduction in alcohol intake in humans as a function of treatment with zimelidine: Implications for treatment. In Research Advances in New Psychopharmacological Treatments for Alcoholism. C. A. Naranjo and E. M. Sellers, Eds. Amsterdam: Elsevier Science Publishers BV, pp. 189-198. 82. NARANJO, C. A.. E. M. SELLERS,J. T. SULLIVAN, D. V. WOODLEY,K. KADLEC& K. SYKORA. 1987. The serotonin uptake inhibitor citalopram attenuates ethanol intake. Clin. Pharmacol. Ther. 41: 266-274. N., J. SHINDLER, J. D. PARKES& S. BANDAK. 1986. Fluoxetine in the treatment 83. LANGDON, of cataplexy. Sleep 9 370-371. D. A., E. L. NARVER,W. C. DEMENT& M. M. MITLER.1976. Effects of 84. BABCOCK, imipramine, chlorimipramine, and fluoxetine on cataplexy in dogs. Pharmacol. Biochem. Behav. 5 599-602. .~ 85. ROBERTS, M. H. T. 1984. 5-Hydroxytryptamine and antinociception. Neuropharmacology 23: 1529-1 536. 86. GOURLAY, G. K., D. A. CHERRY, M. J. COUSINS,B. L. LOVE,J. R. GRAHAM & M. 0. MCLACHLAN. 1986. A controlled study of a serotonin reuptake blocker, zimelidine, in the treatment of chronic pain. Pain 2 5 35-52. 87. SYVALAHTI, E., P. KANGASNIEMI & S. B. ROSS. 1979. Migraine headache and blood serotonin levels after administration of zimelidine, a selective inhibitor of serotonin uptake. Curr. Ther. Res. 25: 299-310.
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DISCUSSION OF THE PAPER
T. MENNINI(Milan): I would like just to add a brief comment because in my opinion the pharmacology of serotonin uptake inhibitors is not always so simple and it probably cannot be generalized. It is important to characterize the pharmacological and biochemical effects of serotonin uptake inhibitors by seeing if they are antagonized by serotonin antagonists in order to establish a mechanism of action. For example, indalpine is a good inhibitor of serotonin uptake but does not decrease food intake in rats. So this is just a comment in order to remind us that the pharmacology of serotonin uptake can not be oversimplified. FULLER: Thank you. QUESTION:After treatment with serotonin uptake inhibitors to reduce body weight or to decrease alcohol consumption, do the effects persist or does the weight go back to normal and does the alcohol intake go back to normal? FULLER:I don’t think there is much data available for alcoholism because of limited clinical studies. Naranjo and Sellers have shown that serotonin uptake inhibitors decrease alcohol intake significantly, but I don’t think they have studied whether the effect persists. There’s also limited clinical data available on weight reduction at this point. Longer term studies are being done with fluoxetine and other selective inhibitors of serotonin uptake. Even if weight loss is maintained for as long as the drug is administered, I doubt that weight would stay down after the drug is discontinued unless other factors such as dietary intake and exercise patterns are changed. E F R A ~ AZMITIA N (New York University, New York City): You suggested that uptake inhibitors such as fluoxetine block the pharmacological effects of a serotonin releaser, p-chloroamphetamine, by preventing the p-chloroamphetamine from getting into the terminal. Is there strong evidence supporting that, or is that just an hypothesis? FULLER:It is a hypothesis that, in my judgment is the best one to fit all of the available data. There are no direct data to show that compounds which release serotonin are selectively accumulated in serotonin nerve terminals. But many effects which they produce are blocked by uptake inhibitors, including not only short-term effects whether the blocks might be explained in some other way but also long-term effects which I find difficult to explain any other way. It is a hypothesis that I favor but which is not yet proven. GEORGEANDERSON(Yale Medical School, New Haven, C T ) :Could you comment on whether the uptake inhibitors are acting directly at the 5-HT uptake site or at a second regulatory site? FULLER:Dr. David Wong wants to respond to that question. WONG:Most of the selective inhibitors of serotonin uptake show complete competitive inhibitors of serotonin uptake and they competitively inhibit the binding of radioligands to the uptake carrier, suggesting they act directly on the serotonin uptake site. There have been suggestions of a regulatory site, but I think the existence of such a site remains to be established. ANDERSON: There may be differences among uptake inhibitors. WONG:Some of the evidence for regulatory sites has been based on biphasic inhibition of tritiated imipramine binding. The low affinity binding site for tritiated imipramine may not be associated with the serotonin uptake carrier.
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R. MURPHY(New York University, New York City): I’m just curious about the anorectic activity of tluoxetine in the rat. How robust is the effect in deprived vs. non-deprived rats or in rats fed different diets? FULLER:Anorexic effects with serotonin uptake inhibitors have been shown by many investigators in various types of experiments. Goudie et af. [J. Pharm. Pharmacol. 28: 3 18, 19761 were the first to report anorectic effects of fluoxetine in rats fed a single daily meal. Anorectic effects have also been demonstrated in rats fed different diets or induced to eat in some way. For example, Wurtman and Wurtman [Curr. Med. Res. Opin. 6(Suppl. 1): 28, 19791 have shown that serotonin uptake inhibitors and other serotonergic drugs selectively suppressed carbohydrate consumption in rats given a choice between high protein and low protein diets. Rowland and others [Life Sci 36: 2295, 19851 have shown anorectic effects of serotonin uptake inhibitors in various models and after chronic administration. But Dr. Mennini pointed out that not all serotonin uptake inhibitors have shown anorectic effects in all types of experiments, and the exceptions are not well understood. J. CARUSO (SUNY, Stony Brook. N Y ) : Have the glial uptake sites been characterized and are they the same as the 5-HT sites that are located on the 5-HT neuron? FULLER:I haven’t been involved directly with this study of glial uptake of serotonin, as some other people in the room, such as Dr. Whitaker-Azmitia, have been, but my reading of the literature is that to the extent uptake sites on glia have been characterized they appear to be similar in their specificity to the uptake sites on neurons.
INTRODUCTION After serotonin is released from nerve terminals and has acted on post-synaptic receptors, it is transported out of the synaptic cleft back into the nerve terminal via the action of a specific membrane uptake carrier. During the past fifteen years, several compounds have been described that inhibit this serotonin uptake carrier with high selectivity, and their availability has helped in understanding the physiologic role of the uptake carrier. Administration of these uptake inhibitors to animals does not result in depletion of brain serotonin content, and in fact leads to a reduction in brain serotonin formation and release, suggesting that the physiological role of the neuronal uptake carrier is not to conserve serotonin but instead to inactivate serotonin by removing it from the synaptic cleft. In the brain, serotonin uptake carriers also occur on glial cells, but the physiologic roles of these uptake carriers and their possible influence on synaptic function are still not well understood.' Blood platelets also have a serotonin uptake carrier, which is the means by which platelets acquire serotonin, since they do not synthesize it.2 Inhibition of the serotonin uptake carrier on blood platelets over a prolonged time results in depletion of platelet (and whole blood) concentrations of ~ e r o t o n i n . ~ This chapter will focus on brain serotonin neurons and the ways in which serotonin uptake inhibition and the functional results of that inhibition have been studied.
CHEMICAL CLASSES OF SEROTONIN UPTAKE INHIBITORS The chemical structures of several selective inhibitors of serotonin uptake are shown in FIGURE 1. The compounds cover a wide range of chemical types, with little similarity among them overall. All have primary or substituted amino groups separated from an aromatic nucleus by cyclic, branched or straight-chain connecting groups. Several, but not all, have halogen substituents on the aromatic nucleus.
SELECTIVITY OF SEROTONIN UPTAKE INHIBITORS I N VZTRO Many of the earlier uptake inhibitors, especially the tricyclic antidepressant drugs, were relatively nonselective in blocking the uptake of neurotransmitter rnonoamines. They inhibited norepinephrine and serotonin uptake, and some antidepressant drugs 1 are such as nomifensin also inhibited dopamine ~ p t a k eThe . ~ compounds in FIGURE 68
FULLER & WONG: SEROTONIN UPTAKE AND ITS INHIBITION
C
F
l
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e OCHCHICHINHCH,
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FLUOXETINE
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69
I C' ~Hl~HICHIN(CHl~l
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, ~-CH,CH,CH,CH,OCH, ~ CH,CH,CH,N(CHI)I
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,
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FIGURE 1. Chemical structures of some selective inhibitors of serotonin uptake.
all highly selective in blocking the uptake of serotonin, having effects on norepinephrine and dopamine uptake only at much higher concentrations in vitro. In several cases, the substituent on the nitrogen is an important determinant of selectivity. The removal of a methyl group from clomipramine results in a selective inhibitor of norepinephrine
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uptake, not serotonin uptake! On the other hand, the removal of a methyl group from zimelidine increases potency of serotonin uptake inhibition.’ The substituent on the aromatic nucleus also is an important determinant of selectivity. The absence of the chlorine atom in clomipramine reduces the potency and selectivity of serotonin uptake i n h i b i t i ~ nReplacement .~ of the p-trifluoromethyl substituent on the phenoxy nucleus of fluoxetine, a selective inhibitor of serotonin uptake,6 with an o-methyl or o-methoxy group results in selective inhibitors of norepinephrine uptake.’.’
DEMONSTRATION OF SEROTONIN UPTAKE INHIBITION I N VWO Serotonin uptake inhibition can be demonstrated ex vivo by measuring serotonin uptake i n vitro in brain slices or synaptosomes prepared from animals treated with the uptake inhibitor in viva The percentage inhibition measured is probably not a true reflection of the percentage inhibition that occurred in the intact brain, since this type of experiment is to some extent a bioassay of the amount of drug present in the brain preparation used to measure uptake. Such experiments are especially useful to demonstrate the ability of serotonin uptake inhibitors to reach the brain, their potency and selectivity relative to one another, and their duration of action.’ A less direct method for evaluating uptake inhibition in brain, but one that measures an effect in the intact brain as opposed to in vitro, is the antagonism of brain serotonin depletion by a drug whose depleting action requires the membrane uptake carrier. Four such serotonin-depleting drugs whose antagonism by serotonin uptake inhibitors has been used as a measure of uptake inhibition i n vivo a r e p-chloromethamphetamine,“ p-chloroamphetamine,” fenfluramine,I2and H75/ 12.13 The accumulation of these drugs into serotonin neurons via the membrane uptake carrier apparently is required for their depletion of brain serotonin, since that depletion is blocked in a dose-dependent manner by serotonin uptake inhibitors. Inhibition of the serotonin uptake carrier on blood platelets can also be demonstrated by measuring uptake in vitro after administration of the uptake inhibitor in vivo. This technique can be applied in human studies, since blood platelets are readily accessible, as well as in animal studies. Serotonin becomes depleted from blood platelets after continued administration of a serotonin uptake inhibitor, because uptake is the means by which platelets acquire serotonin. Inhibition of platelet serotonin uptake ex vivo and reduction of platelet serotonin content have been demonstrated in humans given selective inhibitors of serotonin uptake.14
UPTAKE INHIBITORS AS RADIOLABELS FOR SEROTONIN UPTAKE CARRIERS Radioactive forms, especially tritiated forms, of several selective inhibitors of serotonin uptake have been used as radioligands to label the serotonin uptake carrier. Radiolabels used for i n vitro binding studies have included i m i ~ r a m i n e , ’ ~ nitroimipramine,I6fluoxetine,” citalopram,” norzimelidine,” and paroxetine.20Radiolabels used for quantitative autoradiography in vivo or in brain slices include imipramine,*’ nitroimipramine,22 p a r ~ x e t i n e ,citalopram,” ~~ i n d a l ~ i n e , *and ~ cyanoimipramine.25
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EVIDENCE FOR INCREASED SEROTONIN CONCENTRATIONS IN THE SYNAPTIC CLEFT AFTER SEROTONIN UPTAKE INHIBITION As a consequence of administration of a serotonin uptake inhibitor, serotonin concentration in the synaptic cleft is expected to increase since it cannot be removed from the synaptic cleft by the membrane uptake carrier. Although the technology for measuring serotonin concentration directly in the synaptic cleft does not exist currently, several experimental approaches have documented increased extracellular concentrations of serotonin after uptake inhibition. Geyer et presented histofluorescence evidence for increased extraneuronal concentrations of serotonin in raphe regions of rat brain after fluoxetine administration. Marsden et aL2’ showed that i n vivo voltammetric signals believed to be serotonin increased in rat brain after fluoxetine administration. Guan and McBride” reported several-fold increases in serotonin concentration in extracellular fluid obtained from push-pull cannulae in the nucleus accumbens of rats after administration of fluoxetine. Perhaps the most extensive evidence for increased serotonin concentrations i n the synaptic cleft and increased activation of synaptic receptors for serotonin comes from the numerous and varied functional effects that have been produced by serotonin uptake inhibitors.
PHARMACOLOGIC EFFECTS OF SEROTONIN UPTAKE INHIBITORS IN ANIMALS Serotonin uptake inhibitors produce various functional effects on laboratory animals alone or in combination with 5-hydroxytryptophan (SHTP). These are thought to be due to the enhanced activation of synaptic receptors for serotonin, either presynaptic autoreceptors or postsynaptic receptors. Neurochemical Efects Serotonin uptake inhibitors cause a decrease in serotonin turnover, which has been measured in various ways, including a decrease in incorporation of radioactive tryptophan into 5-hydroxyindoles. a decrease in the accumulation of SHTP after decarboxylase inhibition, a decrease in the steady state concentration of 5-hydroxyindoleacetic acid (SHIAA) and in the accumulation of 5HIAA after administration of probenecid to block its efflux from brain.” The decrease in serotonin turnover occurs in parallel with a decrease in firing of serotonin neurons measured electrophysiol~gically.~~ The decrease in serotonin turnover may occur partly or entirely via activation of autoreceptors on serotonin neurons that regulate serotonin synthesis and release. Other neurochemical effects of serotonin uptake inhibitors probably result from increased activation of postsynaptic receptors by serotonin that has accumulated in the synaptic cleft. For example, serotonin uptake inhibitors potentiate the increase in striatal dopamine metabolites caused by neuroleptic drugs.” They also increase hypothalamic norepinephrine metabolite^,'^ likely due to facilitatory input to noradrenergic n e u r o n ~ , ~especially ~.’~ when combined with 5HTP.35 Neuroendocrine Efects Serotonin uptake inhibitors increase serum corticosterone concentration in rats” by increasing corticotropin-releasing hormone release and increasing plasma adreno-
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corticotrophin (ACTH).” Pretreatment of rats with serotonin uptake inhibitors antagonizes the elevation of serum corticosterone concentration by p-chloroamphetamine3* or f e n f l ~ r a m i n by e ~ blocking ~ the carrier-mediated release of serotonin by these agents, but potentiates the elevation of serum corticosterone concentration by L-5-hydroxytryptophan (L-SHTP)’~by inhibiting the inactivation of serotonin by the uptake carrier. Serotonin uptake inhibitors alone do not increase serum prolactin concentration in ~’ rats,40 but they antagonize the elevation of serum prolactin by f e n f l ~ r a m i n e and potentiate the elevation of serum prolactin by L-SHTP.~’ Behavioral Efects Animals treated with serotonin uptake inhibitors look normal in gross appearance, but subtle behavioral effects can be detected in suitable experimental paradigms. For example, serotonin uptake inhibitors inhibit muricidal behavior in rats4’ and improve passive avoidance behavior in some but not all behavioral tests in bulbectomized rats4’ Serotonin uptake inhibitors potentiate behavioral effects of SHTP, such as head twitch and the discriminative cue stimulus property of SHTP in rats.4s in Eflect on Food Intake Serotonin uptake inhibitors decrease food intake after single doses in animal^.^^-^' The decrease in food intake persists after daily treatment, and body weight or weight gain is decreased after repeated administration to normal or obese rats and m i ~ e . ~ ’ ~ ~ ” Other Eflects Selective inhibitors of serotonin uptake have been shown to reduce rapid eye movement sleep in rats and to enhance memory processing in mice,” and to produce analgesic effects or potentiate opioid-induced a n a l g e ~ i a . ~The ~ ~ multiplicity ’~ of effects that can be elicited by enhancing serotonin function through uptake inhibition results from the widespread distribution of serotonergic projections to many different brain regions involved in various functions controlled by the brain.56
THERAPEUTIC EFFECTS OF SEROTONIN UPTAKE INHIBITORS IN HUMANS Serotonin uptake inhibitors are used or have shown promise for use in treating numerous psychiatric illnesses and other conditions. Some of these therapeutic uses are outlined below. Depression There is now extensive literature on the efficacy of selective inhibitors of serotonin uptake in treating mental depres~ion.~’~’~ A major clinical advantage over most tricyclic antidepressant drugs is the lack of side effects related to anticholinergic, antihistaminergic, or anti-adrenergic actions, since most serotonin uptake inhibitors
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have little affinity for muscarinic, histamine H1, alpha-1 adrenergic or other neurotransmitter receptors.59Selective inhibitors of serotonin uptake generally lack direct effects on the heart, in contrast to many tricyclic antidepressant drugs6' Weight gain, a relatively common side effect of earlier antidepressant drugs, is also not a problem with selective inhibitors of serotonin uptake used to treat depression.6' Zimelidine was the first selective inhibitor of serotonin uptake to be marketed for the treatment of depression worldwide, but it had to be removed from clinical use due to some infrequent but severe side effects including the Guillain-Barre syndrome. The adverse effects were not thought to be related to serotonin uptake inhibition but were somehow due to the particular chemical structure of zimelidine. Fluoxetine was the first selective inhibitor of serotonin uptake to be introduced as an antidepressant drug in the U.S. market. Other compounds in this group are in clinical use outside the U S . and their development is continuing. Postanoxic Intention Myoclonus This frequently severe movement disorder that can follow brain injury by anoxia is thought to be related to impaired serotonergic neurotransmission and has been treated with L - ~ H T P . ~Fluoxetine ' alone had minimal benefit in these patients, but the combination of fluoxetine and a low dose of L-SHTP (given with carbidopa to block its decarboxylation in peripheral tissues) resulted in enhanced antimyoclonic activity with fewer side effects.63 Obsessive-Compulsive Disorder White et al.64have described a patient with severe obsessive-compulsive disorder refractory to treatment with a tricyclic antidepressant or a monoamine oxidase inhibitor who responded dramatically and repeatedly to treatment with two serotonin uptake inhibitors, clomipramine and citalopram. Turner et al.65 had reported that fluoxetine was effective in patients with obsessive-compulsive disorder. Panic Disorder Zimelidine, a selective inhibitor of serotonin uptake, was superior to placebo in most rating scales in the treatment of patients with agoraphobia with panic attacks.'"' In this study, imipramine was not shown to be superior to placebo. Fluvoxamine has also decreased panic attacks in a study in which a norepinephrine uptake inhibitor did not.67Fluoxetine has caused complete cessation of panic attacks in some patients." Obesity Serotonin uptake inhibitors decrease food intake in rats, acting synergistically with L - ~ H T PUnlike . ~ ~ anorectic drugs acting via non-serotonergic mechanisms, serotonin uptake inhibitors have been shown in animal studies to suppress stress-induced eating,69to selectively suppress carbohydrate intake,7oand to suppress insulin-induced hyperphagia as well as 2-deoxyglucose-induced h~perphagia.~' Therapeutic utility of serotonin uptake inhibitors in the treatment of obesity in humans has been reported for zimelidine,'* fern~xetine,'~ f l u ~ o x a m i n and e ~ ~f l ~ o x e t i n e . ~ ~
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Bulimia Bulimia is an eating disorder with episodes of binge eating followed by self-induced vomiting. Recent evidence indicates that serotonin uptake inhibitors are effective in treating bulimia.76
Alcoholism In rats, selective inhibitors of serotonin uptake including fluoxetine, zimelidine, fluvoxamine and sertraline, have reduced ethanol drinking in rats selected or bred to prefer alcohol when given a choice of drinking f l ~ i d s . ~In~ humans, -~~ zimelidine decreased the number of drinks consumed and increased the number of abstinent days in nondepressed heavy Similar effects have been reported with another serotonin uptake inhibitor, citalopram.82
Other Disorders Serotonin uptake inhibitors including fluoxetine and clomipramine have been reported to be effective in treating cataplexy, sudden brief paralysis of voluntary movement and loss of muscle tone following a momentary decrease in alertne~s;'~ fluoxetine had earlier been effective in treating cataplexy in dogs.84There is extensive evidence that central serotonergic pathways are involved in the reception and processing of pain messages,85and serotonin uptake unhibitors may be useful in treatment of some types of pain, such as chronic pain.86 It is possible that depletion of platelet serotonin by serotonin uptake inhibitors will have therapeutic benefit in reducing migraine attacks.87
SUMMARY Serotonin uptake carriers occur on serotonin neurons, on glial cells and on blood platelets. The uptake carrier on serotonin neurons inactivates serotonin that has been released into the synaptic cleft by transporting it back into the nerve terminal. The serotonin uptake carrier is the means by which blood platelets acquire serotonin, since they do not synthesize it. The function of the serotonin uptake carrier on glial cells is poorly understood. Selective inhibitors of serotonin uptake enhance neurotransmission via serotonergic neurons and have been useful pharmacologic tools for studying physiologic roles of serotonin neurons. Some serotonin uptake inhibitors are finding therapeutic uses in mental depression and other psychiatric disorders and in treating obesity and bulimia; other therapeutic applications continue to be evaluated. REFERENCES 1986. Occurrence and functional significance of serotonin and catecholamine uptake by astrocytes. Biochem. Pharmacol. 3 5 2273-228 1. 2. PLETSCHER, A. 1987. The 5hydroxytryptamine system of blood platelets: physiology and pathophysiology. Int. J. Cardiol. 14: 177-178. 3. PETERSEN, E. N., E. BECHGAARD, R. J. SORTWELL & L. WETTERBERG. 1978. Potent 1.
KIMELBERG, H. K.
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4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
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DISCUSSION OF THE PAPER
T. MENNINI(Milan): I would like just to add a brief comment because in my opinion the pharmacology of serotonin uptake inhibitors is not always so simple and it probably cannot be generalized. It is important to characterize the pharmacological and biochemical effects of serotonin uptake inhibitors by seeing if they are antagonized by serotonin antagonists in order to establish a mechanism of action. For example, indalpine is a good inhibitor of serotonin uptake but does not decrease food intake in rats. So this is just a comment in order to remind us that the pharmacology of serotonin uptake can not be oversimplified. FULLER: Thank you. QUESTION:After treatment with serotonin uptake inhibitors to reduce body weight or to decrease alcohol consumption, do the effects persist or does the weight go back to normal and does the alcohol intake go back to normal? FULLER:I don’t think there is much data available for alcoholism because of limited clinical studies. Naranjo and Sellers have shown that serotonin uptake inhibitors decrease alcohol intake significantly, but I don’t think they have studied whether the effect persists. There’s also limited clinical data available on weight reduction at this point. Longer term studies are being done with fluoxetine and other selective inhibitors of serotonin uptake. Even if weight loss is maintained for as long as the drug is administered, I doubt that weight would stay down after the drug is discontinued unless other factors such as dietary intake and exercise patterns are changed. E F R A ~ AZMITIA N (New York University, New York City): You suggested that uptake inhibitors such as fluoxetine block the pharmacological effects of a serotonin releaser, p-chloroamphetamine, by preventing the p-chloroamphetamine from getting into the terminal. Is there strong evidence supporting that, or is that just an hypothesis? FULLER:It is a hypothesis that, in my judgment is the best one to fit all of the available data. There are no direct data to show that compounds which release serotonin are selectively accumulated in serotonin nerve terminals. But many effects which they produce are blocked by uptake inhibitors, including not only short-term effects whether the blocks might be explained in some other way but also long-term effects which I find difficult to explain any other way. It is a hypothesis that I favor but which is not yet proven. GEORGEANDERSON(Yale Medical School, New Haven, C T ) :Could you comment on whether the uptake inhibitors are acting directly at the 5-HT uptake site or at a second regulatory site? FULLER:Dr. David Wong wants to respond to that question. WONG:Most of the selective inhibitors of serotonin uptake show complete competitive inhibitors of serotonin uptake and they competitively inhibit the binding of radioligands to the uptake carrier, suggesting they act directly on the serotonin uptake site. There have been suggestions of a regulatory site, but I think the existence of such a site remains to be established. ANDERSON: There may be differences among uptake inhibitors. WONG:Some of the evidence for regulatory sites has been based on biphasic inhibition of tritiated imipramine binding. The low affinity binding site for tritiated imipramine may not be associated with the serotonin uptake carrier.
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R. MURPHY(New York University, New York City): I’m just curious about the anorectic activity of tluoxetine in the rat. How robust is the effect in deprived vs. non-deprived rats or in rats fed different diets? FULLER:Anorexic effects with serotonin uptake inhibitors have been shown by many investigators in various types of experiments. Goudie et af. [J. Pharm. Pharmacol. 28: 3 18, 19761 were the first to report anorectic effects of fluoxetine in rats fed a single daily meal. Anorectic effects have also been demonstrated in rats fed different diets or induced to eat in some way. For example, Wurtman and Wurtman [Curr. Med. Res. Opin. 6(Suppl. 1): 28, 19791 have shown that serotonin uptake inhibitors and other serotonergic drugs selectively suppressed carbohydrate consumption in rats given a choice between high protein and low protein diets. Rowland and others [Life Sci 36: 2295, 19851 have shown anorectic effects of serotonin uptake inhibitors in various models and after chronic administration. But Dr. Mennini pointed out that not all serotonin uptake inhibitors have shown anorectic effects in all types of experiments, and the exceptions are not well understood. J. CARUSO (SUNY, Stony Brook. N Y ) : Have the glial uptake sites been characterized and are they the same as the 5-HT sites that are located on the 5-HT neuron? FULLER:I haven’t been involved directly with this study of glial uptake of serotonin, as some other people in the room, such as Dr. Whitaker-Azmitia, have been, but my reading of the literature is that to the extent uptake sites on glia have been characterized they appear to be similar in their specificity to the uptake sites on neurons.
