Reduced serotonin transporter binding in binge eating women

RATIONALE: There is evidence that abnormalities in brain dopamine, norepinephrine and serotonin metabolism may play an important role in binge eating. Serotonin-active antidepressant drugs have also been found to decrease binge eating. OBJECTIVE: We investigated serotonin transporter binding in obese binge-eating women. Eleven obese binge-eating and seven obese control women participated in the study. The subjects were not taking any medication known to affect serotonin (5-HT) transporters. METHODS: We used single-photon emission tomography (SPECT) with the radioligand 123I-labelled nor-beta-CIT, which specifically labels 5-HT transporters. RESULTS: Obese binge-eating women showed significantly decreased 5-HT transporter binding in the mid-brain compared with obese controls (2.1 +/- 0.5 versus 2.9 +/- 0.5, respectively). CONCLUSIONS: SPECT imaging with a ligand specific for 5-HT transporters can be used to assess altered serotonin transporter binding in the living human brain. The results tentatively suggest that 5-HT transporter binding is decreased in binge-eating women.     

Opposing changes in serotonin and norepinephrine transporter mRNA levels after serotonin depletion

We and others have earlier shown that severe serotonin depletion leads to a compensatory down-regulation in the expression of the serotonin transporter (5HTT) gene. We have now investigated the expression of both the 5HTT and the norepinephrine transporter (NET) gene to assess the possible interaction between the noradrenergic and the serotonergic neurotransmitter systems. Acute severe serotonin depletion induced by p-chlorophenylalanine (PCPA) treatment leads to enhanced NET_Long mRNA levels and reduced 5HTT mRNA level. This change in transporter mRNA expression was paralleled by a non-significant change in protein expression. Chronic severe serotonin depletion combined with treatment with the antidepressant imipramine leads to enhanced NET_Long mRNA levels. Acute treatment with the monoamine oxidase A inhibitor clorgyline, acute moderate NE reduction (-methyl-p-tyrosine treatment) or less severe depletion for 3 weeks have no effect on the gene expression of the transporters. Taken together, the present data demonstrate that the NET gene expression is enhanced in case of severe serotonin depletion.     

Hippocampal serotonin depletion facilitates the enhancement of prepulse inhibition by risperidone: possible role of 5-HT(2C) receptors in the dorsal hippocampus

Abnormalities in both the hippocampal region and in serotonergic transmission are evident in patients with schizophrenia. We previously found that rats with serotonergic lesions targeting the dorsal hippocampus show altered psychotropic drug-induced hyperlocomotion and prepulse inhibition (PPI), behavioural paradigms relevant to aspects of schizophrenia. The present study explored the effect of serotonin depletion (>70%) along the dorsoventral axis of the hippocampus, or of partial serotonin depletion (∼50%) in the ventral hippocampus, on PPI modulation by acute antipsychotic drug treatment. We also used receptor binding autoradiography to investigate the neurochemical basis of behavioural effects. Following micro-injection of 5,7-dihydroxytryptamine, neither hippocampal serotonin depletion or partial serotonin depletion in the ventral hippocampus altered baseline PPI, startle magnitude or startle habituation. Acute treatment with clozapine or haloperidol had minimal effects on PPI in these lesioned rats or sham-operated controls. In contrast, risperidone treatment increased PPI to a significantly greater extent in rats with hippocampal serotonin depletion, an effect which was most prominent at low prepulse intensities. Partial serotonin depletion in the ventral hippocampus did not alter PPI modulation by risperidone. Neither type of serotonergic lesion altered the densities of 5-HT_1A or 5-HT_2A receptors in the hippocampus; serotonin transporters or 5-HT_1A autoreceptors on raphe cell bodies; or dopamine transporters, D₁ or D₂ receptors in forebrain regions efferent to the hippocampus and implicated in schizophrenia, such as the nucleus accumbens. However, levels of [³H]mesulergine binding to 5-HT_2C receptors were increased by approximately 70% in the dorsal hippocampus of rats with serotonin depletion in this region, while those in the ventral hippocampus were unaffected. Therefore, despite intact baseline PPI, abnormal PPI regulation in rats with >70% serotonin depletion in the hippocampus was unmasked by acute risperidone treatment. Selective upregulation of 5-HT_2C receptors in the dorsal, but not ventral, hippocampus of these lesioned rats suggests that hippocampal 5-HT_2C receptors vary in their adaptability to changes in serotonergic tone along the dorsoventral axis. These findings suggest that 5-HT_2C receptors in the dorsal hippocampus may contribute to risperidone-induced enhancement of PPI.This article is part of a Special Issue entitled ‘Serotonin’.     

Acute and constitutive increases in central serotonin levels reduce social play behaviour in peri-adolescent rats

Rationale Serotonin is an important modulator of social behaviour. Individual differences in serotonergic signalling are considered to be a marker of personality that is stable throughout lifetime. While a large body of evidence indicates that central serotonin levels are inversely related to aggression and sexual behaviour in adult rats, the relationship between serotonin and social behaviour during peri-adolescence has hardly been explored. Objective To study the effect of acute and constitutive increases in serotonin neurotransmission on social behaviour in peri-adolescent rats. Materials and methods Social behaviour in peri-adolesent rats (28–35 days old) was studied after genetic ablation of the serotonin transporter, causing constitutively increased extra-neuronal serotonin levels, and after acute treatment with the serotonin reuptake inhibitor fluoxetine or the serotonin releasing agent 3,4-methylenedioxymethamphetamine (MDMA). A distinction was made between social play behaviour that mainly occurs during peri-adolescence, and non-playful social interactions that are abundant during the entire lifespan of rats. Results In serotonin transporter knockout rats, social play behaviour was markedly reduced, while non-playful aspects of social interaction were unaffected. Acute treatment with fluoxetine or MDMA dose-dependently inhibited social play behaviour. MDMA also suppressed non-playful social interaction but at higher doses than those required to reduce social play. Fluoxetine did not affect non-playful social interaction. Conclusions These data show that both acute and constitutive increases in serotonergic neurotransmission reduce social play behaviour in peri-adolescent rats. Together with our previous findings of reduced aggressive and sexual behaviour in adult serotonin transporter knockout rats, these data support the notion that serotonin modulates social behaviour in a trait-like manner.     

L-Tryptophan's effects on mouse killing, feeding, drinking, locomotion, and brain serotonin

Injections of the serotonin precursor l-tryptophan (25, 50, and 100 mg/kg IP), inhibited mouse killing behavior in rats, as indicated by a dose dependent increase in latencies to attack and kill mice. Tests in 24 hr food deprived rats revealed that feeding behavior was also significantly decreased by about 30% by tryptophan injections (50--100 mg/kg IP). Concomitant with the behavioral changes were increased levels of brain serotonin and its metabolite 5-hydroxyindoleacetic acid. Drinking, latencies to sniff mice, and ability to locomote on a rotating rod were not affected by l-tryptophan injections, although spontaneous activity in an open field was reliably reduced by 33% with a dose of 100 mg/kg. Thus, while the degree of selectivity for tryptophan's effects on behavior remains open to question, these findings are consistent with hypotheses of an inhibitory role for central serotonergic systems, particularly in mouse killing and feeding behaviors.     

Implications of genetic research on the role of the serotonin in depression: emphasis on the serotonin type 1<Subscript>A</Subscript> receptor and the serotonin transporter

Serotonin systems appear to play a key role in the pathophysiology of major depressive disorder. Consequently, ongoing research determines whether serotonin related genes account for the very robust differential behavioral and neural mechanisms that discriminate patients with depression from healthy controls. Serotonin type 1_A receptors and the serotonin transporters are reduced in depression, and recent genetic research in animals and humans has implicated both in depression. Preclinical studies have utilized a variety of animal models that have been used to explain pathophysiological mechanisms in humans, although it is not clear at all whether these models constitute relevant models for depression in humans. However, data from preclinical studies can generate hypotheses that are tested in humans by combining genetic data with behavioral and physiological challenge paradigms and neuroimaging. These studies will enhance our understanding about combined influences from multiple interacting genes, as well as from environmental factors on brain circuits and their function, and about how these mechanisms may contribute to the pathophysiology of neuropsychiatric disorders.     

Intracellular studies on the effects of systemic administration of serotonin agonists on rat facial motoneurons

Intracellular recordings were made from facial motoneurons of anesthetized rats during systemitic administration of 5-methoxy-N,N-dimethyltryptamine N-dimethyltryptamine (5-MeODMT) and p-chloroamphetamine (PCA), drugs which elicit the behavioral serotonin syndrome. Both drugs caused a slow depolarization, increased input resistance, and increased excitability of facial motoneurons. These changes are identical to those caused by direct microiontophoretic application of serotonin to these neurons, and probably underlie some of the components of the behavioral serotonin syndrome.     

Peripherally administered serotonin decreases food intake in rats

We report that intraperitoneal injection of serotonin produces a dose-related decrease in the food intake of hungry rats. The efficacy of serotonin was increased by prior treatment with clorgyline, a type A monoamine oxidase inhibitor. Doses of serotonin which were anorectic did not significantly impair locomotor activity of sensorimotor performance. Further, 2 mg/kg serotonin (ED50 on food intake) did not produce a conditioned taste aversion when paired repeatedly with sucrose ingestion. We conclude that the anorectic effects of serotonin are not secondary to nonspecific effects of the agent, and suggest that peripheral serotonin may play a role in normal satiation.     

Hyperphagia following intraventricular p-chlorophenylalanine-, leucine- or tryptophan-methyl esters: Lack of correlation with whole brain serotonin levels

The methyl ester hydrochlorides of DL-p-chlorophenylalanine (PCPA), L-leucine and L-tryptophan were intraventricularly administered to rats. All compounds produced increased food intake compared to saline administration. PCPA and leucine administration significantly decreased serotonin levels by 15–18%, while no serotonin depletion occurred following tryptophan injections. The data suggest that intraventricular injections of large quantities of neutral amino acid methyl esters may cause hyperphagia in rats through non-serotonergic effects on brain function.     

p-chloroamphetamine: behavioral effects of reduced cerebral serotonin in rats.

p-Chloroamphetamine (PCA), a serotonin depletor, was given to rats at least 24 h prior to testing in an open field or a shock avoidance Y-maze task. In the open field PCA groups showed hypoactivity and increased defecation up to 30 days after drug administration. These same animals plus independent groups of PCA animals, showed facilitated avoidance acquisition in the Y-maze up to 15 days after PCA administration. At the beginning of behavioral testing serotonin levels in PCA animals were reduced 70 percent and were still reduced 41 percent after 38 days in whole brain. These results suggest a separation between shock and non-shock effects of brain serotonin depletion. The facilitated avoidance also provides support for the role of serotonin as an inhibitory neurotransmitter mediating a behavioral suppression system.     

Tianeptine,a selective enhancer of serotonin uptake in rat brain

Summary Tianeptine is a tricyclic agent provided with antidepressant activity in experimental models and in clinical trials. In vitro tianeptine and its two principal metabolites have no effects on monoamine uptake, release or neurotransmitter receptor binding. The biochemical effect of tianeptine in vivo after acute or repeated treatment indicates an enhanced serotonin uptake in cortex and hippocampus but not in mesencephalon, with no effect on noradrenaline or dopamine uptake. This enhanced serotonin uptake is not due to decrease in serotonin release, but is related to increase in the V _max of the uptake carrier for serotonin. The fact that enhancers as well as inhibitors of serotonin uptake are provided with antidepressant activity challenge simple conclusion as to their mechanism of action. The possibility that increased serotonin uptake after repeated treatment may be related to the antidepressant activity exerted by these drugs, and drugs enhancing serotonin uptake might have antidepressant activity, with an earlier onset, is proposed. Send offprint requests to T. Mennini     

Receptor mechanisms in increased sensitivity to serotonin agonists after dihydroxytryptamine shown by electronic monitoring of muscle twitches in the rat

Muscle twitches and autonomic changes were induced by systemic injections of L-5-hydroxytryptophan (5-HTP) or the serotonin agonist 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) in rats previously lesioned with intracranial 5,7-dihydroxytryptamine (5,7-DHT) after desmethylimipramine. Movements were recorded sensitively and continuously by an electronic activity monitor. Spontaneous locomotor activity was strongly reduced after 5-HTP in both intact and lesioned rats, so that electronically recorded activity correlated very closely with disordered jerking movements scored by a behavioral rating scale. This myoclonus was dependent on the doses of 5-HTP and of 5,7-DHT and was strongly inhibited by serotonin antagonists. In lesioned rats, myoclonus occurred with unaltered activity of monoamine oxidase (MAO) and after only small increases in serotonin levels after 5-HTP, but even large increases in availability of serotonin in intact rats, or strong inhibition of serotonin uptake failed to induce myoclonus unless MAO was first inhibited. The response to 5-HTP in lesioned rats was attenuated by repeated injections of 5-HTP or 5-MeO-DMT. This decreased response was in turn blocked by repeated doses of a serotonin antagonist, but appeared not to be due to altered metabolism of 5-HTP or of serotonin; repeated pretreatment with cyproheptadine potentiated the myoclonic response to 5-HTP after DHT. Changes in postsynaptic receptors may be important in the behavioral supersentivity following 5,7-DHT, and restitution of serotonin or stimulation of its receptors after presynaptic denervation may suppress an evolving supersensitivity at receptive postsynaptic membranes.     

5-Methoxy-N,N-diisopropyltryptamine (Foxy), a selective and high affinity inhibitor of serotonin transporter

5-Methoxy-N,N-diisopropyltryptamine (5-MeO-DIPT) is a synthetic orally active hallucinogenic tryptamine derivative, known also as Foxy or Foxy methoxy. However, few studies have examined its effects in vitro. In the present study, we investigated the actions of 5-MeO-DIPT against monoamine neurotransmitter transporters, including the transporters for dopamine (DAT), norepinephrine (NET), and serotonin (SERT), using COS-7 cells heterologously expressing these transporters and rat brain synaptosomes. 5-MeO-DIPT specifically inhibited the uptake of [³H]serotonin (5-HT) by the SERT-expressing COS-7 cells and rat striatal synaptosomes in a high affinity manner at concentrations similar to those for cocaine. The effect was reversible and competitive. 5-MeO-DIPT failed to stimulate reverse transport of [³H]5-HT through SERT, while it prevented the releasing action of methamphetamine. 5-MeO-DIPT induced cell toxicity at high concentrations in COS-7 cells, and it was not influenced by the expression of SERT. These results demonstrated that 5-MeO-DIPT acts as a competitive SERT inhibitor and has an inability to cause reverse transport, underlying its serotonergic actions.     

Possible involvement of serotonin in extinction.

In Experiment 1, rats were trained to leverpress for continuous reinforcement with food; half were then intubated with the serotonin synthesis inhibitor parachlorophenylalanine (PCPA: 400 mg/kg) and half with water. In extinction the PCPA-treated rats responded at a higher rate. In Experiment 2, rats were trained on a random interval schedule and then assigned to two groups, treated as in Experiment 1, and tested in extinction. There was no significant difference in the resistance to extinction of the two groups. In Experiment 3, the responding of rats trained in a punished stepdown response paradigm and then given an intragastric injection of PCPA took longer to recover than the responding of water-injected controls. These observations suggest that serotonergic neurons might play a role in extinction processes.     

Clozapine—A serotonin antagonist?

The effect of clozapine on the central serotonergic transmission system was studied by investigation of open-field motility of rats after microinjection of drugs into nucleus accumbens and median raphe nucleus. Previous work has shown that LSD in low doses potentiates apomorphine-induced hypermotility and that this LSD effect is induced by a serotonin agonist action in median raphe nucleus. Clozapine, injected into median raphe nucleus (0.05 micrograms), suppressed the LSD effect in the same manner as serotonin antagonists did. Since alpha-adrenergic drugs, injected into median raphe nucleus, caused locomotor stimulant effects, an alpha- adrenalytic action of clozapine was excluded. Clozapine, injected into nucleus accumbens (0.2 micrograms), increased apomorphine-induced hypermotility, whereas the dopamine antagonist haloperidol suppressed it. Our results suggest a serotonin antagonist action of clozapine.     

Release of serotonin in the locus coeruleus of normotensive and spontaneously hypertensive rats (SHR)

The aim of the present work was to clarify whether differences exist between the release of endogenous serotonin in the locus coeruleus of normotensive and hypertensive rats. The locus coeruleus was superfused with artificial cerebrospinal fluid (aCSF) through a push-pull cannula and serotonin and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) were determined in the superfusate by HPLC combined with electrochemical detection. Compared with normotensive Wistar-Kyoto (WKY) rats, the basal release rate of serotonin in the locus coeruleus of spontaneously hypertensive rats (SHR) was increased more than twofold. Intravenous infusion of noradrenaline (4 μg/kg min) increased mean arterial blood pressure to the same extent in hypertensive and normotensive rats. The pressor response was associated with an increased serotonin release. In WKY rats, the release of serotonin in the locus coeruleus evoked by noradrenaline infusion was more pronounced than in SHR. In WKY rats, intravenous infusion of sodium nitroprusside (150 μg/kg min) led to a fall in blood pressure which was less pronounced and lasted shorter than in SHR. The depressor response was associated with decreased serotonin release. In WKY rats, the decrease in serotonin release evoked by sodium nitroprusside was more pronounced and lasted longer than in SHR. Neither noradrenaline nor sodium nitroprusside influenced the outflow of 5-HIAA. The sensory stimuli noise and tail pinch led to a slight rise in arterial blood pressure which was similar in WKY rats and SHR. These stimuli enhanced the release rate of serotonin and the outflow of 5-HIAA to the same extent in the locus coeruleus of normotensive and hypertensive rats. The findings suggest that the enhanced release of serotonin in the locus coeruleus of genetically hypertensive rats reflects a mechanism counteracting the disturbed blood pressure homeostasis. Stressors influence blood pressure and release of serotonin in the locus coeruleus of SHR and WKY rats to the same extent. Received: 16 November 1998 / Accepted: 22 February 1999     

Effects of serotonin depletion on the volitional alcohol intake of rats during a condition of psychological stress

Summary Non-preferred solutions of ethyl alcohol, that were slightly above preference threshold, were offered together with water to rats which were trained to avoid foot shock by pressing a lever. Intake of alcohol increased significantly when the rats were stressed by intermittent, random unavoidable shock. When levels of brain serotonin were lowered by p-chlorophenylalanine (pCPA), a potent inhibitor of tryptophan hydroxylase, the selection of alcohol declined during the period of random punishment to the pre-stress baseline intakes. The depletion of serotonin during the stress period reduced alcohol preference particularly in those rats which drank the largest amounts of alcohol. These findings extend those of an earlier report that pCPA lowers alcohol intake in non-stressed rats which drank alcohol solutions within the normal range of preference.This research was supported in part by National Science Foundation Grant GB 7906, ONR Contract N-00014-67-A-0226-003 and by a grant from the Wallace Laboratories. We are indebted to P. Curzon and W. L. Veale for their assistance.A part of the data in this paper was included in a Ph. D. dissertation by T. J. Cicero completed at Purdue University, August, 1968. Present address: Department of Psychiatry, Washington University School of Medicine, 4940 Audubon Avenue, St. Louis, Missouri 63110.     

Para-chlorophenylalanine,serotonin and killing behavior

Both p-chlorophenylalanine (PCPA) and PCPA methyl ester were found to reliably induce mouse-killing in non-killer rats only when unusually large doses were used (three successive daily injections of 300 mg/kg) and brain serotonin (5-HT) concentration was drastically reduced (about 90 percent). Neither three doses of 100 mg/kg of PCPA nor p-chloroamphetamine (3 × 3.5 mg/kg) caused similar effects in spite of the fact that these compounds depleted brain 5-HT by 85 percent and 60 percent, respectively. PCPA-induced mouse killing was reversed by 5-HTP (100 mg/kg) only when this serotonin precursor completely restored levels of 5-HT. The topography of PCPA-induced killing did not resemble normal interspecies aggression and was also directed toward rat pups. These findings suggest that 5-HT depletion might facilitate nonspecific killing reactions, but is not a sufficient condition to induce the species-specific predatory behavior in rats.     

The effects of para-chlorophenylalanine on brain serotonin,food intake,and U-Maze behavior

The effects of the serotonin depleting drug dl para-chlorophenylalanine on conditioned alternation in the U-Maze were examined. Rats given para-chlorophenylalanine and nondrugged rats received reward contingent upon alternating choices between the left and right maze arms.To evaluate the effectiveness of the reward contingency, the spontaneous alternation level was determined by observing the alternation behavior of a group of nondrugged yoked subjects (Ss) that received noncontingent rewards. Training was given for 30 days with a 30 min intertrial interval.Because there was no effect of the reward contingency on alternation scores after 30 days of training with a 30 min intertrial interval, the intertrial interval was reduced to 30 sec for an additional 20 days of training. The reward contingent Ss did perform at higher alternation levels than the yoked rats when the intertrial interval was reduced to 30 sec. There was no difference in alternation performance between the drugged and nondrugged contingent reward groups during the experiment. This study thus failed to support the notion that serotonin contributes to memory processes.The drug reduced food intake. However, because asymptotic running times were unaffected by para-chlorophenylalanine, it was concluded that the drug had no effect on food motivation.This experiment was part of a doctoral thesis submitted to the University of Houston. Thanks are due to Daniel Sheer, thesis adviser, and to Mrs. Dorothy L. Taylor, who generously performed the serotonin assays.     

Effects of dietary tryptophan variations on extracellular serotonin in the dorsal hippocampus of rats

Rationale There is evidence to suggest that the efficacy of selective serotonin reuptake inhibitors (SSRIs) in depression is dependent on the availability of serotonin (5-HT) in the brain. Moreover, there is circumstantial evidence suggesting that plasma tryptophan (TRP) levels can predict response to SSRIs. These findings suggest that dietary TRP variations may affect the efficacy of SSRI in major depression.Objectives To study the neurochemical and behavioral effects of dietary TRP variations in rats.Methods In vivo microdialysis in the hippocampus was performed in conscious rats randomly assigned to receive a diet containing low, normal or high levels of TRP. Basal levels of 5-HT and 5-HIAA were measured using HPLC. Fenfluramine and fluvoxamine were infused locally to determine the effect of the diet on 5-HT availability and release. In a parallel group of rats, the differential reinforcement of low rate 72 s (DRL-72 s) schedule was used to assess the behavioral effects of the dietary manipulations.Results 5-HT and 5-HIAA levels were significantly decreased after TRP low diet and 5-HT, but not 5-HIAA, levels were significantly increased after TRP high diet. 5-HT release after fluvoxamine and fenfluramine was significantly diminished in rats on a TRP low diet, and significantly enhanced after fenfluramine in rats on a TRP high diet. DRL-72 performance in rats was decreased by a TRP low diet, whereas a TRP high diet increased DRL performance similar to fluvoxamine administration.Conclusion The amount of 5-HT released after a 5-HT releasing agent and the effect of an SSRI on extracellular 5-HT are dependent on the nutritional availability of TRP. Moreover, increased availability of TRP affects behavior in a manner similar to SSRI administration. These findings suggest that nutritional factors have behavioral and neurochemical effects, relevant for the treatment of depression.