Stimulation of serotonin (5-HT)1A receptors attenuates the locomotor, but not the discriminative, effects of amphetamine and cocaine in rats

Mirtazapine decreased cocaine-induced c-fos expression and dopamine release in rats

Introduction

Chronic cocaine exposure induces an increase in dopamine release and an increase in the expression of the Fos protein in the rat striatum. It has been suggested that both are necessary for the expression of cocaine-induced alterations in behavior and neural circuitry. Mirtazapine dosing attenuated the cocaine-induced psychomotor and reinforcer effects.

Methods

The study evaluates the effect of chronic dosing of mirtazapine on cocaine-induced extracellular dopamine levels and Fos protein expression in rats. Male Wistar rats received cocaine (10 mg/Kg; i.p.) during the induction and expression of locomotor sensitization. The mirtazapine (30 mg/Kg; MIR), was administered 30 minutes before cocaine during the cocaine withdrawal. After each treatment, the locomotor activity was recorded for 30 minutes. Animals were sacrificed after treatment administration. Dopamine levels were determined by high-performance liquid chromatographic (HPLC) in the ventral striatum, the prefrontal cortex (PFC), and the ventral tegmental area (VTA) in animals treated with mirtazapine and cocaine. The quantification of c-fos immunoreactive cells was carried out by stereology analysis.

Results

Mirtazapine generated a decrease in cocaine-induced locomotor activity. In addition, mirtazapine decreased the amount of cocaine-induced dopamine and the number of cells immunoreactive to the Fos protein in the striatum, PFC, and VTA.

Discussion

These data suggest that mirtazapine could prevent the consolidation of changes in behavior and the cocaine-induced reorganization of neuronal circuits. It would explain the mirtazapine-induced effects on cocaine behavioral sensitization. Thus, these data together could support its possible use for the treatment of patients with cocaine use disorder.

Evaluation of multitarget drugs on the expression of cocaine-induced locomotor sensitization in male rats: A comparative study

Purpose

- Cocaine use disorder (CUD) is a complex disease. Several studies have shown the efficacy of multitarget drugs used to treat CUD. Here we compare the efficacy of mirtazapine (MIR), pindolol (PIN), fluoxetine (FLX), risperidone (RIS), trazodone (TRZ), ziprasidone (ZPR), ondansetron (OND), yohimbine (YOH), or prazosin (PRZ), to reduce long-term cocaine-induced locomotor activity and the expression of cocaine-induced locomotor sensitization in rats.

Methods

- The study consists of four experiments, which were divided into four experimental phases. Induction (10 days), cocaine withdrawal (30 days), expression (10 days), and post-expression phase (10 days). Male Wistar rats were daily dosed with cocaine (10 mg/kg; i.p.) during the induction and post-expression phases. During drug withdrawal, the MIR, PIN, FLX, RIS, TRZ, ZPR, OND, YOH, or PRZ were administered 30 min before saline. In the expression, the multitarget drugs were administered 30 min before cocaine. After each administration, locomotor activity for each animal was recorded for 30 min.During the agonism phase, in experiment four, 8-OH-DPAT, DOI, CP-809-101, SR-57227A, or clonidine (CLO) was administered 30 min before MIR and 60 min before cocaine. After each administration, locomotor activity for each animal was recorded for 30 min.

Results

-MIR, FLX, RIS, ZPR, OND, or PRZ attenuated the cocaine-induced locomotor activity and cocaine locomotor sensitization. PIN, TRZ, and YOH failed to decrease cocaine locomotor sensitization. At the optimal doses used, PIN, FLX, RIS, TRZ, ZPR, OND, YOH, or PRZ failed to attenuate long-term cocaine locomotor activation. MIR generated a decrease in cocaine-induced locomotor activity of greater magnitude and duration than the other multitarget drugs evaluated.

Conclusion

- At the optimal doses of multitarget drugs evaluated, MIR was the multitarget drug that showed the greatest long-term cocaine-induced behavior effects compared to other multitarget drugs.

NLX-112, a highly selective 5-HT1A receptor biased agonist, does not exhibit misuse potential in male rats or macaques

NLX-112 (a.k.a. F13640 or befiradol) exhibits nanomolar affinity, exceptional selectivity and biased agonism at serotonin 5-HT_1A receptors. NLX-112 displays robust analgesic activity in a number of rodent models of pain, and is currently developed as a treatment for l-DOPA-induced dyskinesia (LID) in Parkinson's disease (PD) patients. Noteworthy, PD patients can suffer from comorbid chronic pain, thus necessitating the use of analgesic drugs, such as opioids, which have potential for misuse. Additionally, dopamine agonists used to treat PD can produce cocaine-like effects in preclinical assays of misuse potential. The present study investigated whether NLX-112 possesses misuse potential of its own using two behavioural assays routinely used for this purpose: intracranial self-stimulation (ICSS) in rats, and cocaine discrimination in macaque monkeys. In rats, low doses of NLX-112 (0.03 and 0.1 mg/kg p.o.) did not alter ICSS frequency-rate curves, while higher doses (0.3 and 1.0 mg/kg) shifted the curve to the right and flattened it, i.e., reduced ICSS. As expected, cocaine (10 mg/kg i.p.) shifted the curve to the left, i.e., facilitated ICSS, but NLX-112 (0.03 and 0.1 mg/kg p.o.) did not further enhance cocaine-induced facilitation of ICSS. In monkeys trained to discriminate cocaine (0.4 mg/kg i.m.) from saline, NLX-112 (0.01-0.1 mg/kg p.o.) did not substitute for cocaine. Taken together, these results suggest that NLX-112, at doses displaying anti-dyskinetic activity in rat, marmoset and macaque models of LID, is free from misuse potential. From a translational perspective, this is a desirable property for a compound destined to be used in PD patients, who can suffer from comorbid chronic pain necessitating the use of potentially misused analgesic drugs.

Toluene Inhalation Causes Early Anxiety and Delayed Depression with Regulation of Dopamine Turnover, 5-HT1A Receptor, and Adult Neurogenesis in Mice

Inhaled solvents such as toluene are of particular concern due to their abuse potential that is easily exposed to the environment. The inhalation of toluene causes various behavioral problems, but, the effect of short-term exposure of toluene on changes in emotional behaviors over time after exposure and the accompanying pathological characteristics have not been fully identified. Here, we evaluated the behavioral and neurochemical changes observed over time in mice that inhaled toluene. The mice were exposed to toluene for 30 min at a concentration of either 500 or 2,000 ppm. Toluene did not cause social or motor dysfunction in mice. However, increased anxiety-like behavior was detected in the short-term after exposure, and depression-like behavior appeared as delayed effects. The amount of striatal dopamine metabolites was significantly decreased by toluene, which continued to be seen for up to almost two weeks after inhalation. Additionally, an upregulation of serotonin 1A (5-HT_1A) receptor in the hippocampus and the substantia nigra, as well as reduced immunoreactivity of neurogenesis markers in the dentate gyrus, was observed in the mice after two weeks. These results suggest that toluene inhalation, even single exposure, mimics early anxiety- and delayed depression-like emotional disturbances, underpinned by pathological changes in the brain.

Repeated restraint stress potentiates methylphenidate and modafinil-induced behavioral sensitization in rats

Stress increases the susceptibility of drug abuse and drugs of abuse impair behavioral tolerance. It has been shown that stress exposure enhances the sensitivity to the reinforcing properties of drugs, augments locomotor sensitization effects of drugs of abuse and impairs behavioral tolerance. Previously, it has been shown that long-term administration of psychostimulants (Methylphenidate and Modafinil) induced locomotor sensitization effect that was more pronounced after 13 days of drug administration and was greater at high dose. The present study is designed to investigate the relationship between restraint stress and psychostimulants (Methylphenidate and Modafinil) that induced sensitization. Methylphenidate (10 mg/kg/day twice a day), modafinil (75 mg/kg/day once daily), and saline (0.9% NaCl; 1 ml/kg/day) were administered orally to treated and control animals. Rats were exposed to immobilization stress for 30 days (until locomotor sensitization produced) to monitor any change in drug-induced behavioral sensitization. The motor activity was compared daily by using familiar environment of home cage and weekly by novel environment of open field. The results show that the methylphenidate and modafinil-induced locomotor sensitization is enhanced and impaired behavioral tolerance in repeated restrained rats. It shows that the psychostimulants like methylphenidate and modafinil produce greater locomotor sensitization in stressful environment, suggesting addictive effects of stress and psychostimulants (methylphenidate/modafinil) on dopaminergic neurotransmission. These finding may be helpful to develop potential pharmacotherapies for the patients with co-occurring depression and substance abuse/dependence disorder.

Intracerebroventricular microinjection of the 5-HT1F receptor agonist LY 344864 inhibits methamphetamine conditioned place preference reinstatement in rats

Relapse following a prolonged period of drug cessation is a key barrier in the treatment of methamphetamine (METH) addiction, for which pharmacological treatment exhibits little efficacy. Previous studies have suggested that this process involves alterations in levels of serotonin (5-HT) in the brain. Although the 5-HT1F receptor has been implicated in the reward pathway, its physiological functions remain unknown. In the present study, we examined the effect of the 5-HT1F agonist LY 344864 on the reinstatement of METH-seeking behavior in rats using a conditioned place preference (CPP) paradigm. The CPP paradigm was first used to determine the effective doses of LY and METH. Four groups were then conditioned with METH (5 mg/kg; i.p.), while the sham group received saline. METH-induced CPP was subsequently extinguished. On the 13th day of extinction, the rats received either METH (0, 1, or 2.5 mg/kg; i.p.) plus vehicle or priming METH plus LY (2 μg/5 μL; i.c.v.). On reinstatement day, preference scores were calculated as the difference in time spent in the drug-paired and vehicle-paired compartments. Rats conditioned with the lowest effective dose of METH (5 mg/kg) exhibited significant differences in pre- and post-testing preference scores. Preference scores were significantly higher in the saline + METH group than in the control group. Furthermore, preference scores were significantly higher in rats that had received priming METH treatment, and pre-treatment with LY significantly attenuated the reinstatement of METH-seeking behavior. These findings suggest that future studies should evaluate the therapeutic potential of 5-HT1F agonists for preventing relapse in individuals with METH addiction.

Investigating the Role of Serotonin in Methamphetamine Psychosis: Unaltered Behavioral Effects of Chronic Methamphetamine in 5-HT1A Knockout Mice

Methamphetamine (Meth) is a widely abused stimulant drug, but this abuse is associated with an increased risk of developing psychosis. In addition to its well-known action on brain dopamine, Meth also affects serotonergic (5-HT) neurons. The aim of this study was to investigate this role in mice, which lack one of the main serotonin receptors, the 5-HT_1A receptor, which has been implicated in both schizophrenia and Meth-induced psychosis. Male and female wild-type or 5-HT_1A knockout (KO) mice received daily treatment with increasing doses of methamphetamine from 6 to 9 weeks of age (1-4 mg/kg/day twice a day). At least 2 weeks after the last injection, the mice underwent a battery of behavioral tests focusing on psychosis-related behaviors, including Meth-induced hyperactivity, prepulse inhibition (PPI), social interaction, elevated plus maze (EPM), and Y-maze. Meth pretreatment resulted in significantly increased hyperlocomotion in response to an acute Meth challenge, but this effect was independent of genotype. Chronic Meth treatment resulted in decreased levels of anxiety in the EPM in both sexes, as well as increased startle responses in female mice only, again independent of genotype. 5-HT_1A KO mice showed an increased locomotor response to acute Meth in both sexes, as well as increased PPI and decreased startle responses in female mice only, independent of Meth pretreatment. In conclusion, the effects of chronic Meth appear unaffected by the absence of the 5-HT_1A receptor. These results do not support a role of the 5-HT_1A receptor in Meth-induced psychosis.

Serotoninergic and dopaminergic modulation of cortico-striatal circuit in executive and attention deficits induced by NMDA receptor hypofunction in the 5-choice serial reaction time task

Executive functions are an emerging propriety of neuronal processing in circuits encompassing frontal cortex and other cortical and subcortical brain regions such as basal ganglia and thalamus. Glutamate serves as the major neurotrasmitter in these circuits where glutamate receptors of NMDA type play key role. Serotonin and dopamine afferents are in position to modulate intrinsic glutamate neurotransmission along these circuits and in turn to optimize circuit performance for specific aspects of executive control over behavior. In this review, we focus on the 5-choice serial reaction time task which is able to provide various measures of attention and executive control over performance in rodents and the ability of prefrontocortical and striatal serotonin 5-HT_1A, 5-HT_2A, and 5-HT_2C as well as dopamine D₁- and D₂-like receptors to modulate different aspects of executive and attention disturbances induced by NMDA receptor hypofunction in the prefrontal cortex. These behavioral studies are integrated with findings from microdialysis studies. These studies illustrate the control of attention selectivity by serotonin 5-HT_1A, 5-HT_2A, 5-HT_2C, and dopamine D₁- but not D₂-like receptors and a distinct contribution of these cortical and striatal serotonin and dopamine receptors to the control of different aspects of executive control over performance such as impulsivity and compulsivity. An association between NMDA antagonist-induced increase in glutamate release in the prefrontal cortex and attention is suggested. Collectively, this review highlights the functional interaction of serotonin and dopamine with NMDA dependent glutamate neurotransmission in the cortico-striatal circuitry for specific cognitive demands and may shed some light on how dysregulation of neuronal processing in these circuits may be implicated in specific neuropsychiatric disorders.

The role of serotonin in drug use and addiction

The use of psychoactive drugs is a wide spread behaviour in human societies. The systematic use of a drug requires the establishment of different drug use-associated behaviours which need to be learned and controlled. However, controlled drug use may develop into compulsive drug use and addiction, a major psychiatric disorder with severe consequences for the individual and society. Here we review the role of the serotonergic (5-HT) system in the establishment of drug use-associated behaviours on the one hand and the transition and maintenance of addiction on the other hand for the drugs: cocaine, amphetamine, methamphetamine, MDMA (ecstasy), morphine/heroin, cannabis, alcohol, and nicotine. Results show a crucial, but distinct involvement of the 5-HT system in both processes with considerable overlap between psychostimulant and opioidergic drugs and alcohol. A new functional model suggests specific adaptations in the 5-HT system, which coincide with the establishment of controlled drug use-associated behaviours. These serotonergic adaptations render the nervous system susceptible to the transition to compulsive drug use behaviours and often overlap with genetic risk factors for addiction. Altogether we suggest a new trajectory by which serotonergic neuroadaptations induced by first drug exposure pave the way for the establishment of addiction.

Extending therapeutic use of psychostimulants: focus on serotonin-1A receptor

INTRODUCTION

Despite a number of medicinally important pharmacological effects, the therapeutic use of psychostimulants is limited because of abuse potential and psychosis following long term use. Development of pharmacological agents for improving and extending therapeutic use of psychostimulants in narcolepsy, attention deficit hyperactivity disorder, Parkinson's disease, obesity and as cognitive enhancer is an important research imperative. In this regard, one potential target system is the 5-hydroxytryptamine (5-HT; serotonin) neurotransmitter system. The focus of the present article is to evaluate a potential role of 5-HT-1A receptor in the alleviation of abuse potential and psychosis-induced by prescription psychostimulants amphetamines and apomorphine.

METHOD

Synaptic contacts between dopamine systems and 5-HT-1A receptors are traced. Studies on serotonin-1A influences on the modulation of dopamine neurotransmission and psychostimulant-induced behavioral sensitization are accumulated.

RESULTS

Inhibition of amphetamine and apomorphine-induced behavioral sensitization by co administration of 5-HT-1A agonists cannot be explained in terms of direct activation of 5-HT-1A receptors, because activation of pre- as well as postsynaptic 5-HT-1A receptors tends to increase dopamine neurotransmission.

CONCLUSION

Long term use of amphetamine and apomorphine produces adaptive changes in 5-HT-1A receptor mediated functions, which are prevented by the co-use of 5-HT-1A agonists. In view of extending medicinal use of psychostimulants, it is important to evaluate the effects of co-use of 5-HT-1A agonists on potential therapeutic profile of amphetamine and apomorphine in preclinical research. It is also important to evaluate the functional significance of 5-HT-1A receptors on psychostimulant-induced behaviors in other addiction models such as drug self-administration and reinstatement of drug seeking behavior.

Efficacy of buspirone for attenuating cocaine and methamphetamine reinstatement in rats

BACKGROUND

There are no approved pharmacotherapies for preventing psychomotor stimulant relapse. The operant reinstatement model has been suggested as a screen for identifying candidate medications. The present study examined if the anxiolytic buspirone could attenuate reinstatement of extinguished responding in Long-Evans rats that previously self-administered intravenous cocaine or methamphetamine.

METHODS

Rats were trained in 2-h daily sessions to self-administer 0.5mg/kg cocaine or 0.1mg/kg methamphetamine infusions followed by 12 days of instrumental extinction. Reinstatement was evoked by 17mg/kg i.p. cocaine primes or response-contingent cocaine-paired cues in cocaine-reinforced rats, and by 1mg/kg i.p. methamphetamine primes or response-contingent methamphetamine-paired cues in methamphetamine-reinforced rats.

RESULTS

Buspirone (1 and 3mg/kg) significantly (p<0.05) attenuated cocaine cue but not cocaine prime reinstatement. Buspirone (1 and 3mg/kg) also significantly attenuated methamphetamine cue reinstatement. Buspirone (3mg/kg) significantly attenuated methamphetamine prime reinstatement. During all reinstatement tests, 3mg/kg buspirone reduced levels of inactive lever pressing relative to those of vehicle, significantly so during the cocaine cue-induced reinstatement tests.

CONCLUSIONS

Given the complexity of buspirone's neuropharmacology consisting of serotonin 5-HT1A receptor partial agonist activity, and dopamine D2, D3 and D4 receptor antagonist effects, it is uncertain which of these activities or their combination is responsible for the present results. Overall, these results suggest that buspirone may reduce the likelihood of relapse to cocaine and methamphetamine use under some conditions, although this speculation must be interpreted with caution given buspirone's similar potency to attenuate inactive-lever responding.

Potential serotonin 5-HT(1A) and dopamine D(4) receptor modulation of the discriminative stimulus effects of amphetamine in rats

Activation of the dopaminergic system underlies the behavioral effects of (+)-amphetamine, and plays a major role in its discriminative stimulus properties. Although serotonin receptors modulate dopamine levels in the brain, and 5-HT(1A) and 5-HT(2) receptor agonists do not mimic (+)-amphetamine, pretreatment with 5-HT(2A/2C) agonists significantly potentiates the (+)-amphetamine cue. Further, 5-HT(2) antagonists do not modify the discriminative stimulus effect of (+)-amphetamine, but 5-HT(1A) antagonists have never been tested in (+)-amphetamine-trained rats. This study sought to characterize the effects of the 5-HT(1A) antagonist WAY 100635 on (+)-amphetamine-induced discriminative stimulus effects. Male Sprague-Dawley rats were trained in a two-lever, fixed ratio 50, food-reinforced task with (+)-amphetamine sulfate (1.0 mg/kg, i.p., 30 min pretreatment time) as the discriminative stimulus. Substitution and combination tests with WAY 100635 were then performed. WAY 100635 did not produce substitution in amphetamine-trained rats, but significantly increased behavioral disruption. In combination tests 0.4 and 5.4 mg/kg doses of WAY 100635 potentiated the amphetamine cue. We suggest that low doses of WAY 100635 potentiated the (+)-amphetamine cue by blockade of 5-HT(1A) receptors, but stimulation of the dopamine D(4) receptor by higher doses of WAY 100635 may be responsible for potentiation of amphetamine-induced behavioral sensitization. The high percentage of behavioral disruption in substitution tests might suggest that rats trained to discriminate (+)-amphetamine from saline show behavioral sensitization that is not detectable by the drug discrimination assay but may be expressed as hyperactivity and stereotypic behavior that disrupts operant behavior.

Discriminative stimulus, subject-rated and cardiovascular effects of cocaine alone and in combination with aripiprazole in humans

Aripiprazole is a dopamine D(2) receptor partial agonist undergoing evaluation as a pharmacotherapy for stimulant-use disorders. Acutely administered aripiprazole attenuates the discriminative stimulus and other behavioral effects of d-amphetamine in humans; however, whether aripiprazole attenuates the effects of more commonly abused stimulants is unknown. The aim of this experiment was to assess the discriminative stimulus, subject-rated and cardiovascular effects of oral cocaine alone and following acute administration of aripiprazole in humans. Eight cocaine-dependent subjects learned to discriminate 150 mg cocaine from placebo. After acquiring the discrimination, the effects of cocaine (0, 25, 50, 100 and 200 mg) administered alone and in combination with aripiprazole (15 mg) were determined. Significant effects of cocaine were observed for the drug discrimination task, stimulant-like subject-rated effects and heart rate. Limited effects of aripiprazole were revealed. However, for most measures, fewer doses of cocaine were significantly greater than placebo when combined with aripiprazole, suggesting a reduction in the discriminative stimulus, self-reported and cardiovascular effects of cocaine. These data are consistent with previous studies that have tested acutely administered aripiprazole in combination with d-amphetamine and suggest that the ability of aripiprazole to modify stimulant effects is a function of the duration of treatment (acute vs. chronic).

5-HT(1A)-like receptor activation inhibits abstinence-induced methamphetamine withdrawal in planarians

No pharmacological therapy is approved to treat methamphetamine physical dependence, but it has been hypothesized that serotonin (5-HT)-enhancing drugs might limit the severity of withdrawal symptoms. To test this hypothesis, we used a planarian model of physical dependence that quantifies withdrawal as a reduction in planarian movement. Planarians exposed to methamphetamine (10 μM) for 60 min, and then placed (tested) into drug-free water for 5 min, displayed less movement (i.e., withdrawal) than either methamphetamine-naïve planarians tested in water or methamphetamine-exposed planarians tested in methamphetamine. A concentration-related inhibition of withdrawal was observed when methamphetamine-exposed planarians were placed into a solution containing either methamphetamine and 5-HT (0.1-100 μM) or methamphetamine and the 5-HT(1A) receptor agonist 8-hydroxy-N,N-dipropyl-2-aminotetralin (8-OH-DPAT) (10, 20 μM). Planarians with prior methamphetamine exposure displayed enhanced withdrawal when tested in a solution of the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridyl)cyclohexanecarboxamide (WAY 100635) (1 μM). Methamphetamine-induced withdrawal was not affected by the 5-HT(2B/2C) receptor agonist meta-chlorophenylpiperazine (m-CPZ) (0.1-20 μM). These results provide pharmacological evidence that serotonin-enhancing drugs inhibit expression of methamphetamine physical dependence in an invertebrate model of withdrawal, possibly through a 5-HT(1A)-like receptor-dependent mechanism.

Behavioral evidence for the significance of serotoninergic (5-HT) receptors in cocaine addiction

Cocaine addiction has somatic, psychological, psychiatric, socio-economic and legal implications in the developed world. Presently, there is no medication approved for the treatment of cocaine addiction. In recent years, data from the literature (pre-clinical studies and clinical trials) have provided several lines of evidence that serotonin (5-HT) and 5-HT receptors play a modulatory role in the mechanisms of action of cocaine. Here we review the contribution of 5-HT receptor subtypes to cocaine sensitization, discrimination, conditioned place preference, self-administration, reinstatement of seeking behavior and withdrawal symptoms in laboratory animals. Additionally, the consequences of chronic cocaine exposure on particular 5-HT receptor-assigned functions in pre-clinical studies are presented.

Serotonergic mechanisms in addiction-related memories

Drug-associated memories are a hallmark of addiction and a contributing factor in the continued use and relapse to drugs of abuse. Repeated association of drugs of abuse with conditioned stimuli leads to long-lasting behavioral responses that reflect reward-controlled learning and participate in the establishment of addiction. A greater understanding of the mechanisms underlying the formation and retrieval of drug-associated memories may shed light on potential therapeutic approaches to effectively intervene with drug use-associated memory. There is evidence to support the involvement of serotonin (5-HT) neurotransmission in learning and memory formation through the families of the 5-HT(1) receptor (5-HT(1)R) and 5-HT(2)R which have also been shown to play a modulatory role in the behavioral effects induced by many psychostimulants. While there is a paucity of studies examining the effects of selective 5-HT(1A)R ligands, the available dataset suggests that 5-HT(1B)R agonists may inhibit retrieval of cocaine-associated memories. The 5-HT(2A)R and 5-HT(2C)R appear to be integral in the strong conditioned associations made between cocaine and environmental cues with 5-HT(2A)R antagonists and 5-HT(2C)R agonists possessing potency in blocking retrieval of cocaine-associated memories following cocaine self-administration procedures. The complex anatomical connectivity between 5-HT neurons and other neuronal phenotypes in limbic-corticostriatal brain structures, the heterogeneity of 5-HT receptors (5-HT(X)R) and the conflicting results of behavioral experiments which employ non-specific 5-HT(X)R ligands contribute to the complexity of interpreting the involvement of 5-HT systems in addictive-related memory processes. This review briefly traces the history of 5-HT involvement in retrieval of drug-cue associations and future targets of serotonergic manipulation that may reduce the impact that drug cues have on addictive behavior and relapse.

Disparate cocaine-induced locomotion as a predictor of choice behavior in rats trained in a delay-discounting task

Heightened impulsivity and differential sensitivity to a drug's behavioral effects are traits that, individually, have been associated with chronic drug use and dependence. Here, we used an animal model to test whether individual differences in cocaine-induced activity are predictive of impulsive choice behavior. Adult, male Sprague-Dawley rats were given cocaine (10mg/kg, i.p.) and classified into low or high cocaine responders (LCRs or HCRs, respectively) based on their locomotor response in an open-field arena. Rats were then trained in a delay-discounting task that offers a choice between immediately delivered, but smaller reinforcements, or larger reinforcements that are delivered after a delay. We also examined the effects of amphetamine (AMPH; 0.3-1.0mg/kg) and the 5-HT1A agonist 8-OH-DPAT (0.3-1.0mg/kg) on delay-discounting. Lastly, all rats were retested in the open-field to determine if phenotypes were stable. We observed baseline differences in choice behavior between the groups, with HCRs behaving more impulsively (i.e., choosing the small reinforcement) compared to LCRs. AMPH decreased choice of the large reinforcement in LCRs, but did not alter choice in HCRs. Impulsive choice was increased in both phenotypes following 8-OH-DPAT, with LCRs exhibiting changes across a wider range of delays. When cocaine-induced open-field behavior was retested, responses in LCRs were similar whereas HCRs showed evidence of tolerance. Our results suggest that differential sensitivity to cocaine-induced locomotion is predictive of impulsivity and the potential neurobiological differences in LCRs and HCRs may provide insight into mechanisms contributing to vulnerability for chronic drug use and/or dependence.

Inhibition by oxytocin of methamphetamine-induced hyperactivity related to dopamine turnover in the mesolimbic region in mice

Accumulated data have shown the neuroactive properties of oxytocin (OT), a neurohypophyseal neuropeptide, and its capability of reducing the abuse potential of drugs. The present study investigated the effect of OT on methamphetamine (MAP)-induced hyperactivity in mice and its possible mechanism of action. Locomotor activity was measured after administered with MAP using an infrared sensor. High-performance liquid chromatography with electrochemical detection (HPLC-ECD) was used to detect the content of monoamines and their metabolites in the striatum and accumbens and prefrontal cortex in mice after the behavioral test. OT (0.1, 0.5, and 2.5 microg/mouse, i.c.v.) had no effect on locomotor activity in naïve mice, but inhibited, in a dose-dependent manner, the hyperactivity induced by acute administration of MAP. Atosiban (Ato) (2.0 microg/mouse, i.c.v.), the selective inhibitor of OT receptor, attenuated the inhibitory effect of OT on MAP. A marked reduction of the ratios of 3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) to dopamine (DA) was observed in the striatum and accumbens of mice after acute administration of MAP. OT (2.5 microg, i.c.v.) significantly inhibited the reduction of DOPAC/DA and HVA/DA ratios. However, Ato decreased the ratio of DOPAC/DA significantly in mice compared with OT (2.5 microg) in combination with MAP. There was no significant change in serotonin (5-HT) metabolism in mice after a single administration of MAP. These results suggested that OT inhibited the MAP-induced hyperactivity by altering the DA turnover in the mesolimbic region of mice.

The 5-HT3 receptor partial agonist MD-354 (meta-chlorophenylguanidine) enhances the discriminative stimulus actions of (+)amphetamine in rats

The effect of the 5-HT3 receptor partial agonist MD-354 (meta-chlorophenylguanidine) was examined on the discriminative stimulus produced by (+)amphetamine. Using male Sprague-Dawley rats trained to discriminate 1.0 mg/kg (i.p.) of (+)amphetamine from saline vehicle (VI 15-s schedule of reinforcement) in a two-lever operant procedure for appetitive reward, tests of stimulus generalization (substitution) and antagonism showed that MD-354 neither substituted for, nor antagonized, the amphetamine stimulus at the doses evaluated. Administration of (+)amphetamine doses in combination with a fixed (i.e., 1.0 mg/kg) dose of MD-354 shifted the (+)amphetamine dose-response curve to the left such that, following 0.3 mg/kg of (+)amphetamine, stimulus generalization occurred. Furthermore, MD-354 doses of 0.1, 0.3 and 1.0 mg/kg, but not doses of 0.01, 0.5, 1.5 or 3.0 mg/kg (i.p.), administered in combination with the ED(50) dose (0.33 mg/kg) of (+)amphetamine resulted in stimulus generalization (i.e., >80% drug-appropriate responding). It is concluded that even though MD-354 lacks amphetamine-like central stimulant actions of its own it can modulate the discriminative stimulus effects of (+)amphetamine in rats.

Serotonin and psychostimulant addiction: Focus on 5-HT1A-receptors

Serotonin(1A)-receptors (5-HT(1A)-Rs) are important components of the 5-HT system in the brain. As somatodendritic autoreceptors they control the activity of 5-HT neurons, and, as postsynaptic receptors, the activity in terminal areas. Cocaine (COC), amphetamine (AMPH), methamphetamine (METH) and 3,4-methylenedioxymethamphetamine ("Ecstasy", MDMA) are psychostimulant drugs that can lead to addiction-related behavior in humans and in animals. At the neurochemical level, these psychostimulant drugs interact with monoamine transporters and increase extracellular 5-HT, dopamine and noradrenalin activity in the brain. The increase in 5-HT, which, in addition to dopamine, is a core mechanism of action for drug addiction, hyperactivates 5-HT(1A)-Rs. Here, we first review the role of the various 5-HT(1A)-R populations in spontaneous behavior to provide a background to elucidate the contribution of the 5-HT(1A)-Rs to the organization of psychostimulant-induced addiction behavior. The progress achieved in this field shows the fundamental contribution of brain 5-HT(1A)-Rs to virtually all behaviors associated with psychostimulant addiction. Importantly, the contribution of pre- and postsynaptic 5-HT(1A)-Rs can be dissociated and frequently act in opposite directions. We conclude that 5-HT(1A)-autoreceptors mainly facilitate psychostimulant addiction-related behaviors by a limitation of the 5-HT response in terminal areas. Postsynaptic 5-HT(1A)-Rs, in contrast, predominantly inhibit the expression of various addiction-related behaviors directly. In addition, they may also influence the local 5-HT response by feedback mechanisms. The reviewed findings do not only show a crucial role of 5-HT(1A)-Rs in the control of brain 5-HT activity and spontaneous behavior, but also their complex role in the regulation of the psychostimulant-induced 5-HT response and subsequent addiction-related behaviors.

Psychostimulant-induced attenuation of hyperactivity and prepulse inhibition deficits in Adcyap1-deficient mice

Psychostimulants, including amphetamine, act as antihyperkinetic agents in humans with hyperkinetic disorder such as attention-deficit hyperactivity disorder and are known to be effective in enhancing attention-related processes; however, the underlying mechanisms have not been adequately addressed. Mice lacking the Adcyap1 gene encoding the neuropeptide pituitary adenylate cyclase-activating polypeptide (Adcyap1(-/-)) display psychomotor abnormalities, including increased novelty-seeking behavior and hyperactivity. In this study, Adcyap1(-/-) mice showed sensory-motor gating deficits, measured as deficits in prepulse inhibition (PPI), and showed normal PPI in response to amphetamine. Amphetamine also significantly decreased hyperlocomotion in Adcyap1(-/-) mice, and this paradoxical antihyperkinetic effect depended on serotonin 1A (5-HT(1A)) receptor signaling. c-Fos-positive neurons were increased in the prefrontal cortex in amphetamine-treated Adcyap1(-/-) mice, suggesting increased inhibitory control by prefrontal neurons. Additionally, amphetamine produced an antihyperkinetic effect in wild-type mice that received the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin. These results indicate that Adcyap1(-/-) mice act as a model of hyperlocomotion and PPI deficits and suggest that 5-HT(1A)-mediated pathways are important determinants of the psychostimulant-elicited, rate-dependent effects that are in a negative function of the baseline rate of activity.

Dissociable contribution of 5-HT1A and 5-HT2A receptors in the medial prefrontal cortex to different aspects of executive control such as impulsivity and compulsive perseveration in rats

Serotonin (5-HT) receptors are increasingly recognized as major targets for cognitive enhancement in schizophrenia. Several lines of evidence suggest a pathophysiological role for glutamate NMDA receptors in the prefrontal cortex in schizophrenia and associated disorders in attention and executive functioning. We investigated how the interactions between 5-HT1A and 5-HT2A and glutamate NMDA receptor mechanisms in the medial prefrontal cortex (mPFC) contribute to the control of different aspects of attentional performance. Rats were trained on a five-choice serial reaction time (5-CSRT) task, which provides indices of attentional functioning (percentage of correct responses), executive control (measured by anticipatory and perseverative responses), and speed. The competitive NMDA receptor antagonist CPP (50 ng/side) was infused directly into the mPFC 5 min after infusion of either 8-OH-DPAT (30 and 100 ng/side) or M100907 (100 and 300 ng/side) into the same brain area. Impairments in attentional functioning induced by CPP were completely abolished by both doses of 8-OH-DPAT or M100907. In addition, M100907 abolished the CPP-induced anticipatory responding but had no effects on perseverative over-responding, while 8-OH-DPAT reduced the perseverative over-responding but had no effects on anticipatory responding induced by CPP. The selective 5-HT(1A) receptor antagonist WAY100635 (30 ng/side) antagonized the effects of 8-OH-DPAT (100 ng/side). 8-OH-DPAT at 30 ng/side reduced the latency of correct responses in controls and CPP-injected rats and lowered the percentage of omissions in CPP-injected rats. The data show that 5-HT1A and 5-HT2A receptors in the mPFC exert opposing actions on attentional functioning and demonstrate a dissociable contribution of 5-HT1A and 5-HT2A receptors in the mPFC to different aspects of executive control such as impulsivity and compulsive perseveration.

Modification of cocaine-induced behavioral and neurochemical effects by serotonin1A receptor agonist/antagonist in mice

Administration of cocaine causes a locomotor stimulant effect and increases extracellular levels of serotonin (5-HT) and dopamine (DA) in the brains of rodents. Previous studies show that 5-HT1A receptor agonist and antagonist modify the cocaine-induced behavioral and neurochemical effects in the rats. However, the role of the 5-HT system on the effects of cocaine has not been studied in the prefrontal cortex. The present study examined in ddY-strain male mice the effects of the 5-HT1A receptor agonist osemozotan and the receptor antagonist WAY100635 on cocaine-induced locomotor stimulant effect and increases in extracellular levels of 5-HT and DA in the prefrontal cortex. The cocaine-induced locomotor stimulant effect was attenuated by osemozotan and enhanced by WAY100635. The cocaine-induced increase in extracellular levels of 5-HT was attenuated by osemozotan, and enhanced by WAY100635. The cocaine-induced increase in extracellular levels of DA was enhanced by osemozotan, but not affected by WAY100635. These results suggest that the prefrontal 5-HT system plays a pivotal role in the locomotor stimulant effect of cocaine in mice.

Interactions between serotonin and dopamine in the control of impulsive choice in rats: therapeutic implications for impulse control disorders

Forebrain serotonergic lesions attenuate the ability of d-amphetamine to decrease impulsivity in a delay-discounting paradigm, potentially through interactions between the serotonin (5-HT) and dopamine (DA) systems. Nucleus accumbens (NAC) lesions increase impulsivity, but the extent to which accumbal DA is involved in regulating impulsive choice is unknown. In the current study, the effects of intra-accumbal infusions of 6-hydroxydopamine (6-OHDA) on impulsive choice were evaluated, in combination with d-amphetamine and serotonergic drugs, in order to investigate the importance of 5-HT : DA interactions in the control of impulsive behavior. Following training on a delay-discounting task, animals received intra-NAC 6-OHDA or sham surgery. Postoperatively, subjects received systemic injections of d-amphetamine (0, 0.3, 1.0, 1.5 mg/kg) and the 5-HT(1A) receptor agonist 8-OH-DPAT (0, 0.1, 0.3, 1.0 mg/kg). Intra-NAC 6-OHDA, which reduced local DA and NA levels by 70-75%, had no effect on delay-discounting, but transiently potentiated the d-amphetamine-induced decrease in impulsive choice. 8-OH-DPAT (1.0 mg/kg) increased impulsivity in sham-operated controls, an effect which was blocked by the 5-HT(1A) receptor antagonist WAY 100635. However, 8-OH-DPAT had no effect on impulsivity in 6-OHDA NAC lesioned rats. 8-OH-DPAT (0.3 mg/kg), which did not itself alter task performance, blocked the effect of d-amphetamine in sham-operated controls, while WAY 100635 augmented the effect of amphetamine in all subjects. In an additional experiment, intracerebroventricular administration of the selective serotonergic toxin 5,7-dihydroxytryptamine, which decreased forebrain 5-HT levels by 85-90%, did not block 8-OH-DPAT's ability to increase impulsive choice. These data suggest a significant role for 5-HT : DA interactions within the NAC in the control of impulsivity, and in the mechanism by which amphetamine decreases impulsive choice.

The 5-HT1A receptor modulates the effects of cocaine on extracellular serotonin and dopamine levels in the nucleus accumbens

The regulation of extracellular levels of serotonin (5-HT) and dopamine in response to cocaine by 5-HT1A receptors was examined using in vivo microdialysis and the 5-HT1A receptor antagonist 4-(2'-methoxy-)-phenyl-1-[2'-(N-2''-pyridinyl)-p-fluorobenzamido-]ethyl-piperazine (p-MPPF). Pretreatment with p-MPPF significantly augmented the increase in extracellular levels of both 5-HT and dopamine in the nucleus accumbens produced by systemic administration of cocaine. Levels of 5-HT or dopamine were unaffected by p-MPPF given alone. Extracellular levels of 5-HT and dopamine were increased dramatically by cocaine infused locally into the nucleus accumbens. Systemic injection of cocaine given during the cocaine infusion reduced 5-HT and dopamine levels, presumably by activating inhibitory 5-HT and dopamine autoreceptors outside of the locus of infusion. The reduction of 5-HT and dopamine levels by systemic cocaine during accumbal infusion was blocked by pretreatment with the 5-HT1A receptor antagonist p-MPPF. Taken together, these findings suggest that the 5-HT1A autoreceptor acts to modulate the effects of cocaine on both 5-HT and dopamine levels in the nucleus accumbens.

Nucleus accumbens serotonin1A receptors control cocaine-induced hyperactivity but not local serotonin increase: an in vivo microdialysis study

The nucleus accumbens (Nac) is an important structure for cocaine-induced hyperactivity and receives a dense serotonergic (5-HT) innervation. Previous studies showed that a systemic activation of 5-HT(1A) receptors potentiates cocaine-induced hyperlocomotion, but attenuates the cocaine-induced 5-HT increase in the Nac. In order to address the role of Nac 5-HT(1A) receptors in the control of cocaine-induced and spontaneous behavioural activity and local 5-HT release, we used in vivo microdialysis in freely moving rats. The 5-HT(1A)-receptor agonist, 8-OH-DPAT (0, 1 and 10 microM), was applied locally into the Nac by reverse dialysis followed by a cocaine (10 mg/kg) or saline i.p. injection. The Nac 5-HT(1A)-receptor activation potentiated cocaine-induced hyperlocomotion, but attenuated rearing behaviour dose-dependently. Parallel to that, the cocaine-induced increase in Nac 5-HT dialysate level was unaffected, as were the decreases in 5-HIAA and DOPAC dialysate levels after cocaine. In saline treated rats, the local application of 8-OH-DPAT into the Nac affected neither spontaneous behavioural activity nor 5-HT, 5-HIAA or DOPAC dialysate levels in the Nac. These data suggest that Nac 5-HT(1A) receptors exert a bi-directional control of cocaine-induced hyperactivity, while not affecting spontaneous behaviour. Furthermore, accumbal 5-HT(1A) receptors do not appear to be directly involved in the acute effects of cocaine on 5-HT, 5-HIAA or DOPAC levels in the Nac.

Differential roles of 5-HT receptor subtypes in cue and cocaine reinstatement of cocaine-seeking behavior in rats

The 5-HT indirect agonist, d-fenfluramine, attenuates cue reinstatement of extinguished cocaine-seeking behavior. To investigate the role of 5-HT receptor subtypes in this effect, we examined whether the attenuation is reversed by either a 5-HT(1A), 5-HT(2A/C), or 5-HT(2C) receptor antagonist. We also examined the effects of the antagonists alone on both cue and cocaine-primed reinstatement. Rats that had been trained to press a lever for cocaine (0.75 mg/kg/0.1 ml, i.v.) paired with light and tone cues underwent daily extinction sessions during which responding had no consequences. We then examined the effects of WAY 100635 (0-1.0 mg/kg, s.c.), ketanserin (0-10.0 mg/kg, i.p.), or SB 242,084 (0-1.0 mg/kg, i.p.) with and without d-fenfluramine (1.0 mg/kg, i.p.) pretreatment on cue reinstatement. Subsequently, we examined the effects of the antagonists on cocaine-primed (7.5 or 15.0 mg/kg, i.p.) reinstatement. The 5-HT(1A) antagonist, WAY 100635, failed to alter cue reinstatement, but attenuated cocaine reinstatement. Conversely, the 5-HT(2A/C) antagonist, ketanserin, attenuated cue reinstatement, but failed to alter cocaine reinstatement. The 5-HT(2C)-selective antagonist, SB 242,084, did not alter cue or cocaine reinstatement, but was the only drug that reversed the d-fenfluramine-induced attenuation of cue reinstatement. The findings suggest that stimulation of 5-HT(1A) receptors plays a critical role in cocaine-primed, but not cue, reinstatement. Furthermore, 5-HT(2A) and 5-HT(2C) receptors may play oppositional roles in cue reinstatement. The SB 242,084 reversal of the d-fenfluramine attenuation suggests that stimulation of 5-HT(2C) receptors inhibits cue reinstatement, whereas the ketanserin-induced attenuation of cue reinstatement suggests that decreased stimulation of 5-HT(2A) receptors inhibits this behavior.

Serotonin1A-receptor agonism attenuates the cocaine-induced increase in serotonin levels in the hippocampus and nucleus accumbens but potentiates hyperlocomotion: an in vivo microdialysis study

The hippocampus and the nucleus accumbens (Nac) are important structures for the modulation of spontaneous locomotor activity. Both structures receive a serotonergic (5-HT) innervation. We have previously reported that the 5-HT(1A)-receptor antagonist WAY 100635 blocked cocaine-induced hyperactivity, while potentiating cocaine-induced 5-HT increases in the hippocampus and the Nac. In order to further investigate the relationship between extracellular 5-HT concentration and cocaine-induced behaviour, we used in vivo microdialysis to measure the effects of the 5-HT(1A)-receptor agonist 8-OH-DPAT on cocaine-induced changes in the extracellular 5-HT concentration in the hippocampus and the Nac and on behavioural activity. Following a pilot pretest in which we determined the lowest effective dose of 8-OH-DPAT for potentiating cocaine-induced hyperlocomotion, four groups of rats were given one of the following drug treatments: 8-OH-DPAT (0.2 mg/kg) and cocaine (10 mg/kg), saline and cocaine (10 mg/kg), 8-OH-DPAT (0.2 mg/kg) and saline, or saline and saline. The injections were administered i.p. and spaced 30 min apart. We found that the 5-HT(1A)-receptor agonist 8-OH-DPAT attenuated the cocaine-induced increases in 5-HT in the hippocampus and the Nac, but potentiated cocaine-induced hyperlocomotion. 5-HT metabolite measurements revealed a complex role for the 5-HT(1A)-receptor in the broad spectrum of cocaine's neurochemical effects. Altogether, these observations support an important role of the 5-HT(1A)-receptor in the hippocampus and Nac in the modulation of cocaine stimulant effects.

Cocaine-conditioned behavioral effects: a role for habituation processes

Cocaine has potent locomotor stimulant effects in rodents, which seemingly can become conditioned to test environment cues. In two experimental protocols, we measured the effects of cocaine on locomotor activity and grooming behavior, and subsequently tested whether these cocaine effects became conditioned to contextual cues. In the first experiment, three groups of rats received 14 injections of either saline or cocaine (10 mg/kg) paired or unpaired to the test environment. Cocaine increased locomotion and decreased grooming during treatment and on the conditioning test. Over the course of the treatment phase, however, the saline- and cocaine-unpaired groups but not the cocaine paired group developed progressively lower locomotion and higher grooming scores indicative of substantial habituation effects. To examine whether the cocaine may have impaired the acquisition of habituation effects rather than induce a Pavlovian cocaine conditioned response, an additional experiment was conducted in which two additional non-habituation saline and cocaine control groups were added to the experimental design. On a conditioning test, the two non-habituation control groups were equivalent in activity and grooming behavior to the cocaine-paired group. The findings were consistent with a failure by cocaine-paired animals to acquire habituation effects, which could transfer to the non-cocaine state. The connection between cocaine and novelty/habituation may have substantial importance for understanding cocaine effects.

The selective serotonin(1A)-receptor antagonist WAY 100635 blocks behavioral stimulating effects of cocaine but not ventral striatal dopamine increase

An increase in the extracellular dopamine (DA) concentration is generally accepted as an important neurochemical mediator of the behavioral effects of cocaine. Cocaine induced increases in serotonergic (5-HT) activity also appears to be involved in these effects. Here we describe the effects of the 5-HT(1A)-receptor antagonist WAY 100635 on the behavioral and neurochemical effects of cocaine. In-vivo microdialysis was used in behaving rats to measure extracellular concentration of DA in the nucleus accumbens (Nac). Four groups of animals received one of the following drug combinations: WAY 100635 (0.4 mg/kg) and cocaine (10 mg/kg), saline and cocaine (10 mg/kg), WAY 100635 (0.4 mg/kg) and saline, or saline and saline. The injections were administered i.p. and spaced 20 min apart. The pretreatment with WAY 100635 significantly attenuated the locomotor stimulant effects of cocaine without altering the DA overflow in the Nac. WAY 100635 itself did not modify locomotion or the extracellular DA concentration in the Nac. These results indicate that (1) the 5-HT(1A)-receptor is an important component in the mediation of cocaine locomotor stimulant effects, and (2) an increase in the extracellular DA concentration in the Nac might be a necessary but is not a sufficient condition for the locomotor stimulant effects of cocaine.

Attenuation of the amphetamine discriminative cue in rats with the atypical antipsychotic olanzapine

Sixteen male Sprague-Dawley rats were trained to discriminate between saline and amphetamine injections (1.0 mg/kg ip) using a standard two-lever (FR10) drug discrimination paradigm. A baseline dose-effect curve was generated for amphetamine administration alone, using doses both above and below the training dose (0.0-2.2 mg/kg ip). Once completed, a single dose of olanzapine (OLZ; 1.5 mg/kg sc) was tested for its ability to attenuate the amphetamine cue. OLZ pretreatment (60 min) successfully interfered with an animal's ability to discriminate amphetamine injections across various doses. The percentage of correct responding on the amphetamine lever and rate of responding were both significantly decreased across some but not all of the amphetamine doses. Therefore, we believe that this preliminary investigation has successfully shown that an OLZ dose of 1.5 mg/kg sc at 60 min can interfere with an animal's ability to detect some subjective cue(s) associated with amphetamine administration.

Involvement of serotonin in nicotine dependence: processes relevant to positive and negative regulation of drug intake

The neurobiological substrate of nicotine dependence has been the subject of extensive preclinical and clinical research. Many experimental reports have implicated the brain serotonin (5-HT) systems in processes relevant to nicotine dependence, but the specific role of this neurotransmitter system largely remains to be elucidated. This review will focus on the role of 5-HT in the acute and chronic effects of nicotine. In particular, the evidence for a role of 5-HT neurotransmission in brain processes thought to be involved in positive and negative control of nicotine use will be examined, and potential clinical implications discussed.

Effect of oleamide on sleep and its relationship to blood pressure, body temperature, and locomotor activity in rats

Oleamide (cis-9,10-octadecenoamide) is a brain lipid that has recently been isolated from the cerebral fluid of sleep-deprived cats. Intracerebroventricular and intraperitoneal administration of oleamide induces sleep in rats. However, it is unclear whether oleamide's hypnogenic effects are mediated, in part, by its actions on blood pressure and core body temperature. Here we show that systemic administration of oleamide (10 and 20 mg/kg) in rats increased slow-wave sleep 2, without affecting blood pressure and heart rate. In addition, oleamide decreased body temperature and locomotor activity in a dose-dependent manner. These latter effects were not correlated in time with the observed increases in slow-wave sleep. These data suggest that the hypnogenic effects of oleamide are not related to changes in blood pressure, heart rate, or body temperature.

Effects of the serotonin 5-HT(2) antagonist, ritanserin, and the serotonin 5-HT(1A) antagonist, WAY 100635, on cocaine-seeking in rats

Manipulations of serotonergic systems have been shown to modify many of the behavioral effects of cocaine. It was recently demonstrated that serotonin (5-HT) depletions produced by inhibition of tryptophan hydroxylase reduced cocaine-seeking in an animal model. The present study was designed to determine whether pretreatment with specific 5-HT antagonists might also decrease cocaine-seeking. The effect of pretreatment with the 5-HT(2) antagonist, ritanserin (0.0, 1.0, or 10.0 mg/kg), or the 5-HT(1A) antagonist, WAY 100635 (0. 0, 0.1, 0.3, or 1.0 mg/kg), on cocaine (5.0, 10.0, or 20.0 mg/kg)-produced reinstatement of extinguished drug-taking behavior was measured. Although ritanserin was ineffective, WAY 100635 attenuated cocaine-produced reinstatement in a dose-dependent manner. These effects of WAY 100635 appeared to be specific since responding maintained by saccharin self-administration remained high following pretreatment with 0.3 or 1.0 mg/kg WAY 100635. These data suggest a role of 5-HT(1A), but not 5-HT(2), receptors in cocaine-seeking.

Activation of serotonin (5-HT)1A receptors inhibits amphetamine sensitization in mice

The effects of serotonin (5-HT)1A drugs on the development and expression of sensitization to the locomotor effect of amphetamine (AMPH) were studied in mice. 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A agonist, dose-dependently reduced the expression of AMPH (2.5 mg/kg)-induced sensitization. The latter inhibitory effect of 8-OH-DPAT was reversed by (S)-N-tert-butyl-3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenyl propamine (WAY 100135), a 5-HT1A antagonist. WAY 100135 given alone did not affect expression of AMPH sensitization. Combined injections of 8-OH-DPAT, but not WAY 100135, with AMPH (2.5 mg/kg) during the development of sensitization, protected against the expression of sensitization to a challenge dose of AMPH (2.5 mg/kg) 3 days after withdrawal. The above inhibitory effect of 8-OH-DPAT on the development of AMPH sensitization was blocked by pretreatment with WAY 100135. The AMPH-induced conditioned locomotion was unaffected by pretreatment with 8-OH-DPAT. These results indicate that 5-HT1A receptors are not involved in AMPH-induced sensitization per-se, whereas their pharmacological activation leads to the inhibition of both the development and the expression of AMPH-induced sensitization.

Behavioral effects of flibanserin (BIMT 17)

Flibanserin is a 5-HT1A agonist that, in contrast to other 5-HT1A receptor agonists, is capable of activating 5-HT1A receptors in frontal cortex. Flibanserin also behaves as an antagonist at 5-HT2A receptors. This compound has been described to be a putative fast-acting antidepressant owing to these properties. In the present study, the effect of flibanserin was investigated in several behavioral paradigms different from animal models of depression. Intraperitoneal flibanserin, at doses of 4-8 mg/kg, antagonized d-amphetamine- and (+)SKF-10047- induced hypermotility in mice and rats. At doses of 816 mg/kg, flibanserin exerted anxiolytic-like effects in the light/dark exploratory test and stress-induced hyperthermia in mice, and antagonized d-amphetamine- and apomorphine-induced stereotypy in rats. At the dose of 16 mg/kg, flibanserin reduced spontaneous motor activity in rats. At the dose of 32 mg/kg, flibanserin did not exert any clear effect on spontaneous motor activity in mice, or on the elevated plus-maze and the water maze in rats.