Serotonergic involvement in methamphetamine-induced locomotor activity: A detailed pharmacological study

Pharmacological Treatments for Methamphetamine Use Disorder: Current Status and Future Targets

The illicit use of the psychostimulant methamphetamine (METH) is a major concern, with overdose deaths increasing substantially since the mid-2010s. One challenge to treating METH use disorder (MUD), as with other psychostimulant use disorders, is that there are no available pharmacotherapies that can reduce cravings and help individuals achieve abstinence. The purpose of the current review is to discuss the molecular targets that have been tested in assays measuring the physiological, the cognitive, and the reinforcing effects of METH in both animals and humans. Several drugs show promise as potential pharmacotherapies for MUD when tested in animals, but fail to produce long-term changes in METH use in dependent individuals (eg, modafinil, antipsychotic medications, baclofen). However, these drugs, plus medications like atomoxetine and varenicline, may be better served as treatments to ameliorate the psychotomimetic effects of METH or to reverse METH-induced cognitive deficits. Preclinical studies show that vesicular monoamine transporter 2 inhibitors, metabotropic glutamate receptor ligands, and trace amine-associated receptor agonists are efficacious in attenuating the reinforcing effects of METH; however, clinical studies are needed to determine if these drugs effectively treat MUD. In addition to screening these compounds in individuals with MUD, potential future directions include increased emphasis on sex differences in preclinical studies and utilization of pharmacogenetic approaches to determine if genetic variances are predictive of treatment outcomes. These future directions can help lead to better interventions for treating MUD.

Methamphetamine and REM sleep deprivation interact to affect behavioral performance in adult and adolescent rats

Drug addiction may result in sleep problems. Importantly, sleep deprivation (SD) is known as an important risk factor for relapse to drug abuse as SD mimics the effects of psychostimulants on dopamine system of the brain. Moreover, aging may affect sleep and drug addiction. This study, therefore, set out to assess the effects of methamphetamine (METH) and REM sleep deprivation (RSD) on locomotor activity, anxiety-like behavior and spatial memory in adult and adolescent rats. Adult and adolescent male Wistar rats received a neurotoxic METH regimen; four subcutaneous injections of 6 mg/kg, at 2 h intervals. Five days later, the animals underwent a 48-h RSD episode using the multiple platforms method. They were then examined using the open field (OF), elevated plus maze (EPM) and Y-maze tasks. We found that the METH and RSD paradigms showed synergistic effects to increase locomotion and risk-taking behavior in both adult and adolescent animals, while only adolescent rats revealed RSD-induced anxiety-like behavior. Moreover, adolescent animals revealed greater sensitization for vertical activity following METH plus RSD episode. In addition, METH and RSD paradigms revealed synergistic effects to impair spatial working memory, but neither METH nor RSD alone affected performance of animals in the Y-maze task. Our findings may indicate that there are important relationships between METH and RSD to induce hyperlocomotion, risk-taking behavior and spatial memory impairment, particularly in adolescent animals. Moreover, it seems that adolescent rats may be more susceptible to anxiety-like behavior and hyperlocomotion than adults.

Heterozygous and homozygous gene knockout of the 5-HT1B receptor have different effects on methamphetamine-induced behavioral sensitization

The psychostimulant drug methamphetamine (METH) causes euphoria in humans and locomotor hyperactivity in rodents by acting on the mesolimbic dopamine (DA) pathway and has severe abuse and addiction liability. Behavioral sensitization, an increased behavioral response to a drug with repeated administration, can persist for many months after the last administration. Research has shown that the serotonin 1B (5-HT1B) receptor plays a critical role in the development and maintenance of drug addiction, as well as other addictive behaviors. This study examined the role of 5-HT1B receptors in METH-induced locomotor sensitization using 5-HT1B knockout (KO) mice. To clarify the action of METH in 5-HT1B KO mice the effects of METH on extracellular levels of DA (DAec) and 5-HT (5-HTec) in the caudate putamen (CPu) and the nucleus accumbens (NAc) were examined. Locomotor sensitization and extracellular monoamine levels were determined in wild-type mice (5-HT1B +/+), heterozygous 5-HT1B receptor KO (5-HT1B +/-) mice and homozygous 5-HT1B receptor KO mice (5-HT1B -/-). Behavioral sensitization to METH was enhanced in 5-HT1B -/- mice compared to 5-HT1B +/+ mice but was attenuated in 5-HT1B +/- mice compared to 5-HT1B +/+ and 5-HT1B -/- mice. In vivo, microdialysis demonstrated that acute administration of METH increases DAec levels in the CPu and NAc of 5-HT1B KO mice compared to saline groups. In 5-HT1B +/- mice, METH increased 5-HTec levels in the CPu, and DAec levels in the NAc were higher than in others.5-HT1B receptors play an important role in regulating METH-induced behavioral sensitization.

Blocking serotonin 2A (5-HT2A) receptors attenuates the acquisition of methamphetamine-induced conditioned place preference in adult female rats

Methamphetamine withdrawal can induce intense cravings leading to relapse. Contexts/cues paired with chronic methamphetamine use develop incentive motivational properties, promoting future drug-seeking and taking behavior. Research has shown that, in adult male rats, the selective 5-HT_2A receptor antagonist M100907 attenuates the acquisition of methamphetamine-induced conditioned place preference (CPP), a measure that examines conditioned associations between the rewarding properties of drugs and contexts. However, these findings have not been extended to adult female rats. The present study investigated the effects of M100907 on the acquisition of methamphetamine-CPP in adult female rats. During conditioning, rats were administered M100907 (0, 0.025, 0.25 mg/kg, i.p.) 15 min before methamphetamine (1 mg/kg, i.p.) and then placed into their initially non-preferred chamber for 30 min, or administered saline and placed into their initially preferred chamber for 30 min. Conditioning sessions were separated by four hours. Following four days of conditioning, the effects of M100907 on the acquisition of methamphetamine-CPP were assessed during a 15 min drug-free test trial. Pretreatment with M100907 dose-dependently attenuated the acquisition of methamphetamine-induced CPP. Blocking 5-HT_2A receptors with a low dose of the selective antagonist M100907 attenuated the rewarding effects of methamphetamine in adult female rats. These data provide further evidence that the 5-HT_2A receptor subtype is involved in the behavioral effects of methamphetamine.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Antagonizing serotonin 2A (5-HT2A) receptors attenuates methamphetamine-induced reward and blocks methamphetamine-induced anxiety-like behaviors in adult male rats

BACKGROUND

Methamphetamine is a highly addictive and abused psychostimulant. Symptoms of methamphetamine withdrawal including drug craving and anxiety that can drive relapse. Currently, there is no FDA approved treatment for methamphetamine use disorder, highlighting the need for research examining the neural mechanisms underlying psychostimulant-induced behaviors. Research indicates that the 5-HT_2A receptor antagonist M100907 attenuates several psychostimulant-induced behaviors, including conditioned place preference (CPP). However, these findings have not been extended to methamphetamine. The present study investigated the effects of M100907 on acquisition of methamphetamine-CPP and methamphetamine-induced anxiety-like behavior.

METHODS

Adult male rats were tested across eight consecutive days. Prior to methamphetamine administration (0 or 1 mg/kg, i.p.), rats were pretreated with their assigned dose of M100907 (0, 0.0025 .025 or 0.25 mg/kg, i.p.) and were placed into their initially non-preferred chamber. After four methamphetamine conditioning sessions, the effects of M100907 on methamphetamine-induced CPP were assessed. Following CPP testing, rats were screened for anxiety-like behaviors in the elevated plus-maze.

RESULTS

Pretreatment with M100907 attenuated methamphetamine-induced CPP without producing any observable rewarding or aversive effects in methamphetamine naïve rats. Additionally, M100907 blocked methamphetamine-induced increases in anxiety-like behavior and attenuated some indices of anxiety in methamphetamine naïve rats.

CONCLUSIONS

Results suggest that blocking 5-HT_2A receptors with the selective antagonist M100907 attenuates the rewarding effects of methamphetamine and does not produce any rewarding or aversive effects alone. Further, M100907 pretreatment blocked the anxiety-inducing effects of methamphetamine. Collectively, these data suggest that the 5-HT_2A receptor subtype represents a novel target for treating methamphetamine use disorder.

Lack of dopamine D4 receptor participation in mouse hyperdopaminergic locomotor response

Chronic methamphetamine (METH) treatment induces behavioral sensitization in rodents. During this process, hyperactivation of the mesolimbic dopamine system plays a central role, and dopamine D2-like receptor-based antipsychotics are known to alleviate the behavioral hyperactivity. The atypical antipsychotic, clozapine (Clz), acts partially as a dopamine D4 receptor (D4R) antagonist and mitigates hyperdopaminergic drug addiction and/or comorbid psychotic symptoms; however, it remains unclear whether D4R blockade contributes to the therapeutic effects of Clz. Here, we evaluated the potential role of D4R in regulating hyperdopaminergia-induced behavioral hyperactivity in METH behavioral sensitization and dopamine transporter (DAT) knockdown (KD) mice. Clz or a D4R-selective antagonist, L-745,870, were co-administered to mice with daily METH in a METH sensitization model, and Clz or L-745,870 were administered alone in a DAT KD hyperactivity model. Locomotor activity and accumbal D4R expression were analyzed. Clz suppressed both the initiation and expression of METH behavioral sensitization, as well as DAT KD hyperactivity. However, repetitive Clz treatment induced tolerance to the suppression effect on METH sensitization initiation. In contrast, D4R inhibition by L-745,870 had no effect on METH sensitization or DAT KD hyperactivity. Accumbal D4R expression was similar between METH-sensitized mice with and without Clz co-treatment. In sum, our results suggest the mesolimbic D4R does not participate in behavioral sensitization encoded by hyperdopaminergia, a finding which likely extends to the therapeutic effects of Clz. Therefore, molecular targets other than D4R should be prioritized in the development of future therapeutics for treatment of hyperdopaminergia-dependent neuropsychiatric disorders.

REM sleep deprivation impairs retrieval, but not reconsolidation, of methamphetamine reward memory in male rats

Susceptibility to interference can be a result of memory retrieval and reconsolidation. Given the fact that addiction develops through the neural mechanisms of learning and memory, it would not be surprising that a consolidated drug reward memory may also be susceptible to interference following retrieval/reconsolidation. Due to the critical role of sleep in memory consolidation, sleep deprivation (SD) has been shown to impair memory. Therefore, the major objective of this study was to investigate the effect of rapid eye movement (REM) sleep deprivation (RSD) on the retrieval and reconsolidation of methamphetamine (METH) reward memory in male rats. The animals were trained to acquire METH-induced CPP (2 mg/kg, i.p.). METH reward memory was then reactivated/retrieved in the drug-paired chamber during a drug-free (memory reactivation) session. A period of 48-h RSD paradigm using the multiple platform technique resulted in persistent deficits in the retrieval of METH reward memory. Nevertheless, the same protocol of RSD, which was conducted immediately after the memory reactivation, did not affect the reconsolidation of METH reward memory. Additionally, the RSD episode induced a temporary potentiation of METH-induced hyperlocomotion. Our findings would seem to suggest that sleep is involved in the retrieval, but not reconsolidation, of METH reward memory. The results may also demonstrate that RSD mimics the effects of METH on locomotor activity. The results of this study, therefore, support the idea that sleep is involved in the processing of METH reward memory which can be considered for further investigations to manage the relapse associated with drug-related memory.

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.

The influence of aripiprazole and venlafaxine on the antidepressant-like effect observed in prenatally stressed rats (animal model of depression)

Depression is a nosological entity which may appear alone or concomitantly (e.g. in schizophrenia). Analysis of data from both clinical and experimental studies allows a conclusion that atypical antipsychotics, such as aripiprazole (ARI), may also be effective in treating depression in addition to antidepressants. The aim of the studies was to determine antidepressant efficacy of ARI, venlafaxine (VEN) and combined therapy using both drugs, in prenatally stressed rats (animal depression model) and control group. In addition, this article was aimed at determining the effect of these drugs on locomotor activity of these animals. The effect of chronic stress used in pregnant rats and the use of drugs such as ARI (1.5 mg/kg) and VEN (20 mg/kg) were studied in forced swimming test (FST; antidepressant effect) and locomotor activity test. Performed tests confirmed the antidepressant effect of ARI, VEN and efficacy of combined drugs in FST in both prenatally stressed rats (effect present upon single administration and after 7, 14 and 21 days of testing) and control group rats (effect present upon single administration and 7 days of testing). Moreover, upon single administration of the used drugs to prenatally stressed rats, it was found sedative effect - reduced animals' locomotor activity. Study results have proven antidepressant and sedative efficacy of ARI, VEN and combined administration of these drugs. Due to the small amount of data on the above preparations, in particular in the context of animal depression models, further studies in this respect are recommended.

Sex differences and serotonergic mechanisms in the behavioural effects of psilocin

Psilocybin has recently attracted a great deal of attention as a clinical research and therapeutic tool. The aim of this paper is to bridge two major knowledge gaps regarding its behavioural pharmacology - sex differences and the underlying receptor mechanisms. We used psilocin (0.25, 1 and 4 mg/kg), an active metabolite of psilocybin, in two behavioural paradigms - the open-field test and prepulse inhibition (PPI) of the acoustic startle reaction. Sex differences were evaluated with respect to the phase of the female cycle. The contribution of serotonin receptors in the behavioural action was tested in male rats with selective serotonin receptor antagonists: 5-HT1A receptor antagonist (WAY100635 1 mg/kg), 5-HT2A receptor antagonist (MDL100907 0.5 mg/kg), 5-HT2B receptor antagonist (SB215505 1 mg/kg) and 5-HT2C receptor antagonist (SB242084 1 mg/kg). Psilocin induced dose-dependent inhibition of locomotion and suppression of normal behaviour in rats (behavioural serotonin syndrome, impaired PPI). The effects were more pronounced in male rats than in females. The inhibition of locomotion was normalized by 5-HT1A and 5-HT2B/C antagonists; however, PPI was not affected significantly by these antagonists. Our findings highlight an important issue of sex-specific reactions to psilocin and that apart from 5-HT2A-mediated effects 5-HT1A and 5-HT2C/B receptors also play an important role. These findings have implications for recent clinical trials.

The antipsychotic-like effects in rodents of YQA31 involve dopamine D3 and 5-HT1A receptor

BACKGROUND

We previously reported that YQA31 is a dopamine D3 receptor antagonist with modest 5-HT1A receptor affinity and that it exhibits antipsychotic properties in animal models of schizophrenia. However, the contributions of D3 and 5-HT1A receptors in the anti-psychotic effects of YQA31 are not clear. The current study evaluated the role of these two receptors in the effect of YQA31 on the hyperactivity and novel object recognition deficit in mice.

METHODS

We used dopamine D3 receptor knockout mice and 5-HT1A receptor antagonist WAY100635 pretreatment, respectively, to investigate the involvement of these receptors in the effects of YQA31. The anti-psychotic effects were tested by inducing hyperlocomotion with methamphetamine or MK-801 and by inducing novel object recognition deficit with MK-801, which are the animal models to represent a positive symptom and a cognitive disorder.

RESULTS

YQA31 significantly inhibited MK-801-induced hyperlocomotion and novel object recognition deficit in WT mice, which was significantly inhibited by dopamine D3 receptor knockout. The 5-HT1A receptor antagonist, WAY100635, also blocked the effect of YQA31 in MK-801-induced novel object recognition deficit but not hyperlocomotion. The effect of YQA31 on methamphetamine-induced hyperlocomotion was not reversed by either dopamine D3 receptor knockout or WAY100635 pretreatment.

CONCLUSIONS

These results demonstrate the different roles of dopamine D3 and 5-HT1A receptors in the anti-psychotic effects of YQA31. Both dopamine D3 and 5-HT1A receptors contributed to the effects of YQA31 on the inhibition of MK-801-induced novel object recognition deficit, and the dopamine D3 receptor mediated the inhibiting effect of YQA31 on hyperlocomotion induced by MK-801.

Synthesis and Structure-Activity Relationships of Tool Compounds Based on WAY163909, a 5-HT2C Receptor Agonist

The development of probe molecules that can be used to investigate G protein-coupled receptor (GPCR) pharmacology, trafficking, and relationship with other GPCRs is an important and growing area of research. Here, we report the synthesis of analogues of the known selective serotonin (5-HT) 5-HT_2C receptor (5-HT_2CR) agonist WAY163909 which were designed to allow for the attachment of a second ligand, signaling or reporter molecules, as well as immobilization agents to the parent molecule with the maintenance of agonist activity. This goal was accomplished by the synthesis of novel molecules in which sites a-d were modified and resulting compounds were analyzed pharmacologically in vitro.

A short history of the 5-HT2C receptor: from the choroid plexus to depression, obesity and addiction treatment

This paper is a personal account on the discovery and characterization of the 5-HT_2C receptor (first known as the 5-HT_1C receptor) over 30 years ago and how it translated into a number of unsuspected features for a G protein-coupled receptor (GPCR) and a diversity of clinical applications. The 5-HT_2C receptor is one of the most intriguing members of the GPCR superfamily. Initially referred to as 5-HT_1CR, the 5-HT_2CR was discovered while studying the pharmacological features and the distribution of [³H]mesulergine-labelled sites, primarily in the brain using radioligand binding and slice autoradiography. Mesulergine (SDZ CU-085), was, at the time, best defined as a ligand with serotonergic and dopaminergic properties. Autoradiographic studies showed remarkably strong [³H]mesulergine-labelling to the rat choroid plexus. [³H]mesulergine-labelled sites had pharmacological properties different from, at the time, known or purported 5-HT receptors. In spite of similarities with 5-HT₂ binding, the new binding site was called 5-HT_1C because of its very high affinity for 5-HT itself. Within the following 10 years, the 5-HT_1CR (later named 5-HT_2C) was extensively characterised pharmacologically, anatomically and functionally: it was one of the first 5-HT receptors to be sequenced and cloned. The 5-HT_2CR is a GPCR, with a very complex gene structure. It constitutes a rarity in the GPCR family: many 5-HT_2CR variants exist, especially in humans, due to RNA editing, in addition to a few 5-HT_2CR splice variants. Intense research led to therapeutically active 5-HT_2C receptor ligands, both antagonists (or inverse agonists) and agonists: keeping in mind that a number of antidepressants and antipsychotics are 5-HT_2CR antagonists/inverse agonists. Agomelatine, a 5-HT_2CR antagonist is registered for the treatment of major depression. The agonist Lorcaserin is registered for the treatment of aspects of obesity and has further potential in addiction, especially nicotine/ smoking. There is good evidence that the 5-HT_2CR is involved in spinal cord injury-induced spasms of the lower limbs, which can be treated with 5-HT_2CR antagonists/inverse agonists such as cyproheptadine or SB206553. The 5-HT_2CR may play a role in schizophrenia and epilepsy. Vabicaserin, a 5-HT_2CR agonist has been in development for the treatment of schizophrenia and obesity, but was stopped. As is common, there is potential for further indications for 5-HT_2CR ligands, as suggested by a number of preclinical and/or genome-wide association studies (GWAS) on depression, suicide, sexual dysfunction, addictions and obesity. The 5-HT_2CR is clearly affected by a number of established antidepressants/antipsychotics and may be one of the culprits in antipsychotic-induced weight gain.

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.

Therapeutic Potential of 5-HT2C Receptor Agonists for Addictive Disorders

The neurotransmitter 5-hydroxytryptamine (5-HT; serotonin) has long been associated with the control of a variety of motivated behaviors, including feeding. Much of the evidence linking 5-HT and feeding behavior was obtained from studies of the effects of the 5-HT releaser (dex)fenfluramine in laboratory animals and humans. Recently, the selective 5-HT2C receptor agonist lorcaserin received FDA approval for the treatment of obesity. This review examines evidence to support the use of selective 5-HT2C receptor agonists as treatments for conditions beyond obesity, including substance abuse (particularly nicotine, psychostimulant, and alcohol dependence), obsessive compulsive, and excessive gambling disorder. Following a brief survey of the early literature supporting a role for 5-HT in modulating food and drug reinforcement, we propose that intrinsic differences between SSRI and serotonin releasers may have underestimated the value of serotonin-based pharmacotherapeutics to treat clinical forms of addictive behavior beyond obesity. We then highlight the critical involvement of the 5-HT2C receptor in mediating the effect of (dex)fenfluramine on feeding and body weight gain and the evidence that 5-HT2C receptor agonists reduce measures of drug reward and impulsivity. A recent report of lorcaserin efficacy in a smoking cessation trial further strengthens the idea that 5-HT2C receptor agonists may have potential as a treatment for addiction. This review was prepared as a contribution to the proceedings of the 11th International Society for Serotonin Research Meeting held in Hermanus, South Africa, July 9-12, 2014.

Protein kinase C inhibition rescues manic-like behaviors and hippocampal cell proliferation deficits in the sleep deprivation model of mania

BACKGROUND

Recent studies revealed that bipolar disorder may be associated with deficits of neuroplasticity. Additionally, accumulating evidence has implicated alterations of the intracellular signaling molecule protein kinase C (PKC) in mania.

METHODS

Using sleep deprivation (SD) as an animal model of mania, this study aimed to examine the possible relationship between PKC and neuroplasticity in mania. Rats were subjected to SD for 72 h and tested behaviorally. In parallel, SD-induced changes in hippocampal cell proliferation were evaluated with bromodeoxyuridine (BrdU) labeling. We then examined the effects of the mood stabilizer lithium, the antipsychotic agent aripiprazole, and the PKC inhibitors chelerythrine and tamoxifen on both behavioral and cell proliferation impairments induced by SD. The antidepressant fluoxetine was used as a negative control.

RESULTS

We found that SD triggered the manic-like behaviors such as hyperlocomotion and increased sleep latency, and reduced hippocampal cell proliferation. These alterations were counteracted by an acute administration of lithium and aripiprazole but not of fluoxetine, and only a single administration of aripiprazole increased cell proliferation on its own. Importantly, SD rats exhibited increased levels of phosphorylated synaptosomal-associated protein 25 (SNAP-25) in the hippocampus and prefrontal cortex, suggesting PKC overactivity. Moreover, PKC inhibitors attenuated manic-like behaviors and rescued cell proliferation deficits induced by SD.

CONCLUSIONS

Our findings confirm the relevance of SD as a model of mania, and provide evidence that antimanic agents are also able to prevent SD-induced decrease of hippocampal cell proliferation. Furthermore, they emphasize the therapeutic potential of PKC inhibitors, as revealed by their antimanic-like and pro-proliferative properties.

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.

Drugs that prevent mouse sleep also block light-induced locomotor suppression, circadian rhythm phase shifts and the drop in core temperature

Exposure of mice to a brief light stimulus during their nocturnal active phase induces several simultaneous behavioral or physiological responses, including circadian rhythm phase shifts, a drop in core body temperature (Tc), suppression of locomotor activity and sleep. Each response is triggered by light, endures for a relatively fixed interval and does not require additional light for expression. The present studies address the ability of the psychostimulant drugs, methamphetamine (MA), modafinil (MOD) or caffeine (CAF), to modify the light-induced responses. Drug or vehicle (VEH) was injected at CT11 into constant dark-housed mice then exposed to 5-min 100μW/cm(2) light or no light at CT13. Controls (VEH/Light) showed approximately 60-min phase delays. In contrast, response was substantially attenuated by each drug (only 12-15min delays). Under a 12-h light:12-h dark (LD12:12) photoperiod, VEH/light-treated mice experienced a Tc drop of about 1.3°C coincident with locomotor suppression and both effects were abolished by drug pre-treatment. Each drug elevated activity during the post-injection interval, but there was also evidence for CAF-induced hypoactivity in the dark prior to the photic test stimulus. CAF acutely elevated Tc; MA acutely lowered it, but both drugs reduced Tc during the early dark (ZT12.5-ZT13). The ability of the psychostimulant drugs to block the several effects of light exposure is not the result of drug-induced hyperactivity. The results raise questions concerning the manner in which drugs, activity, sleep and Tc influence behavioral and physiological responses to light.

Animal models of serotonergic psychedelics

The serotonin 5-HT(2A) receptor is the major target of psychedelic drugs such as lysergic acid diethylamide (LSD), mescaline, and psilocybin. Serotonergic psychedelics induce profound effects on cognition, emotion, and sensory processing that often seem uniquely human. This raises questions about the validity of animal models of psychedelic drug action. Nonetheless, recent findings suggest behavioral abnormalities elicited by psychedelics in rodents that predict such effects in humans. Here we review the behavioral effects induced by psychedelic drugs in rodent models, discuss the translational potential of these findings, and define areas where further research is needed to better understand the molecular mechanisms and neuronal circuits underlying their neuropsychological effects.

Characterization of the hypothermic effects of imidazoline I₂ receptor agonists in rats

BACKGROUND AND PURPOSE Imidazoline I(2) receptors have been implicated in several CNS disorders. Although several I(2) receptor agonists have been described, no simple and sensitive in vivo bioassay is available for studying I(2) receptor ligands. This study examined I(2) receptor agonist-induced hypothermia as a functional in vivo assay of I(2) receptor agonism. EXPERIMENTAL APPROACH Different groups of rats were used to examine the effects of I(2) receptor agonists on the rectal temperature and locomotion. The pharmacological mechanisms were investigated by combining I(2) receptor ligands and different antagonists. KEY RESULTS All the selective I(2) receptor agonists examined (2-BFI, diphenyzoline, phenyzoline, CR4056, tracizoline, BU224 and S22687, 3.2-56 mg·kg(-1) , i.p.) dose-dependently and markedly decreased the rectal temperature (hypothermia) in rats, with varied duration of action. Pharmacological mechanism of the observed hypothermia was studied by combining the I(2) receptor agonists (2-BFI, BU224, tracizoline and diphenyzoline) with imidazoline I(2 ) receptor/ α(2) adrenoceptor antagonist idazoxan, selective I(1) receptor antagonist efaroxan, α(2) adrenoceptor antagonist/5-HT(1A) receptor agonist yohimbine. Idazoxan but not yohimbine or efaroxan attenuated the hypothermic effects of 2-BFI, BU224, tracizoline and diphenyzoline, supporting the I(2) receptor mechanism. In contrast, both idazoxan and yohimbine attenuated hypothermia induced by the α(2) adrenoceptor agonist clonidine. Among all the I(2) receptor agonists studied, only S22687 markedly increased the locomotor activity in rats. CONCLUSIONS AND IMPLICATIONS Imidazoline I(2) receptor agonists can produce hypothermic effects, which are primarily mediated by I(2) receptors. These data suggest that I(2) receptor agonist-induced hypothermia is a simple and sensitive in vivo assay for studying I(2) receptor ligands.

5-HT(2A) receptor blockade and 5-HT(2C) receptor activation interact to reduce cocaine hyperlocomotion and Fos protein expression in the caudate-putamen

Both the 5-HT(2A) receptor (R) antagonist M100907 and the 5-HT(2C) R agonist MK212 attenuate cocaine-induced dopamine release and hyperlocomotion. This study examined whether these drugs interact to reduce cocaine hyperlocomotion and Fos expression in the striatum and prefrontal cortex. We first determined from dose-effect functions a low dose of both M100907 and MK212 that failed to alter cocaine (15 mg/kg, i.p.) hyperlocomotion. Subsequently, we examined whether these subthreshold doses given together would attenuate cocaine hyperlocomotion, consistent with a 5-HT(2A)/5-HT(2C) R interaction. Separate groups of rats received two sequential drug injections 5 min apart immediately before a 1-h locomotion test as follows: (1) saline + saline, (2) saline + cocaine, (3) 0.025 mg/kg M100907 + cocaine, (4) 0.125 mg/kg MK212 + cocaine, or (5) cocktail combination of 0.025 mg/kg M100907 and 0.125 mg/kg MK212 + cocaine. Brains were extracted for Fos immunohistochemistry 90 min after the second injection. We next examined the effects of 0.025 mg/kg M100907 and 0.125 mg/kg MK212, alone and in combination, on spontaneous locomotor activity. While neither drug given alone produced any effects, the M100907/MK212 cocktail attenuated cocaine hyperlocomotion as well as cocaine-induced Fos expression in the dorsolateral caudate-putamen (CPu), but had no effect on spontaneous locomotion. The findings suggest that 5-HT(2A) Rs and 5-HT(2C) Rs interact to attenuate cocaine hyperlocomotion and Fos expression in the CPu, and that the CPu is a potential locus of the interactive effects between these 5-HT(2) R subtypes on behavior. Further research investigating combined 5-HT(2A) R antagonism and 5-HT(2C) R agonism as a treatment for cocaine dependence is warranted.

Pharmacological and genetic interventions in serotonin (5-HT)(2C) receptors to alter drug abuse and dependence processes

The present review provides an overview on serotonin (5-hydroxytryptamine; 5-HT)(2C) receptors and their relationship to drug dependence. We have focused our discussion on the impact of 5-HT(2C) receptors on the effects of different classes of addictive drugs, illustrated by reference to data using pharmacological and genetic tools. The neurochemical mechanism of the interaction between 5-HT(2C) receptors, with focus on the mesocorticolimbic dopaminergic system, and drugs of abuse (using cocaine as an example) is discussed. Finally, we integrate recent nonclinical and clinical research and information with marketed products possessing 5-HT(2C) receptor binding affinities. Accordingly, available nonclinical data and some clinical observations targeting 5-HT(2C) receptors may offer innovative translational strategies for combating drug dependence.This article is part of a Special Issue entitled: Brain Integration.

Effects of the 5-HT2C receptor agonist Ro60-0175 and the 5-HT2A receptor antagonist M100907 on nicotine self-administration and reinstatement

The reinforcing effects of nicotine are mediated in part by brain dopamine systems. Serotonin, acting via 5-HT(2A) and 5-HT(2C) receptors, modulates dopamine function. In these experiments we examined the effects of the 5-HT(2C) receptor agonist Ro60-0175 and the 5-HT(2A) receptor antagonist (M100907, volinanserin) on nicotine self-administration and reinstatement of nicotine-seeking. Male Long-Evans rats self-administered nicotine (0.03 mg/kg/infusion, IV) on either a FR5 or a progressive ratio schedule of reinforcement. Ro60-0175 reduced responding for nicotine on both schedules. While Ro60-0175 also reduced responding for food reinforcement, response rates under drug treatment were several-fold higher than in animals responding for nicotine. M100907 did not alter responding for nicotine, or food, on either schedule. In tests of reinstatement of nicotine-seeking, rats were first trained to lever press for IV infusions of nicotine; each infusion was also accompanied by a compound cue consisting of a light and tone. This response was then extinguished over multiple sessions. Injecting rats with a nicotine prime (0.15 mg/kg) reinstated responding; reinstatement was also observed when responses were accompanied by the nicotine associated cue. Ro60-0175 attenuated reinstatement of responding induced by nicotine and by the cue. The effects of Ro60-0175 on both forms of reinstatement were blocked by the 5-HT(2C) receptor antagonist SB242084. M100907 also reduced reinstatement induced by either the nicotine prime or by the nicotine associated cue. The results indicate that 5-HT(2C) and 5-HT(2A) receptors may be potential targets for therapies to treat some aspects of nicotine dependence.