The effects of the 5-HT(2C) receptor antagonist SB242084 on locomotor activity induced by selective, or mixed, indirect serotonergic and dopaminergic agonists

Methylone is a rapid-acting neuroplastogen with less off-target activity than MDMA

Background

Post-traumatic stress disorder (PTSD) is a highly prevalent psychiatric disorder that can become chronic and debilitating when left untreated. Available pharmacotherapies are limited, take weeks to show modest benefit and remain ineffective for up to 40% of patients. Methylone is currently in clinical development for the treatment of PTSD. Preclinical studies show rapid, robust and long-lasting antidepressant-like and anxiolytic effects. The mechanism of action underlying these effects is not yet fully understood. This study investigated the downstream gene expression changes and signaling pathways affected by methylone in key brain areas linked to PTSD and MDD. It also sought to determine whether neuroplasticity-related genes were involved. We compared effects of methylone with MDMA to explore similarities and differences in their brain effects because MDMA-assisted psychotherapy has recently shown benefit in clinical trials for PTSD and methylone is a structural analog of MDMA.

Methods

Monoamine binding, uptake and release studies were performed and a high-throughput-screen evaluated agonist/antagonist activities at 168 GPCRs in vitro. We used RNA sequencing (RNA-seq) to probe drug-induced gene expression changes in the amygdala and frontal cortex, two brain areas responsible for emotional learning that are affected by PTSD and MDD. Rats were treated with methylone or MDMA (both 10 mg/kg, IP), and their responses were compared with controls. We performed functional enrichment analysis to identify which pathways were regulated by methylone and/or MDMA. We confirmed changes in gene expression using immunohistochemistry.

Results

Methylone, a monoamine uptake inhibitor and releaser, demonstrated no off-target effects at 168 GPCRs, unlike MDMA, which showed activity at 5HT2A and 5HT2C receptors. RNA-seq results revealed significant regulation of myelin-related genes in the amygdala, confirmed by immunohistochemistry. In the frontal cortex, methylone significantly upregulated genes implicated in neuroplasticity.

Conclusion

Results suggest that (1) methylone is a rapid-acting neuroplastogen that affects key brain substrates for PTSD and MDD and that (2) methylone appears to exhibit higher specificity and fewer off-target effects than MDMA. Together, these results are consistent with the reported clinical experiences of methylone and MDMA and bolster the potential use of methylone in the treatment of PTSD and, potentially, other neuropsychiatric disorders.

Serotonin 2C receptors are also important in head-twitch responses in male mice

RATIONALE

Serotonergic psychedelics exert their effects via their high affinity for serotonin (5-HT) receptors, particularly through activating 5-HT2A receptors (5-HT2AR), employing the frontal cortex-dependent head-twitch response (HTR). Although universally believed to be so, studies have not yet fully ascertained whether 5-HT2AR activation is the sole initiator of these psychedelic effects. This is because not all 5-HT2AR agonists exhibit similar pharmacologic properties.

OBJECTIVE

This study aims to identify and discriminate the roles of 5-HT2AR and 5-HT2CR in the HTR induced by Methallylescaline (MAL) and 4-Methyl-2,5,β-trimethoxyphenethylamine (BOD) in male mice. Also, an analysis of their potential neurotoxic properties was evaluated.

METHODS

Male mice treated with MAL and BOD were evaluated in different behavioral paradigms targeting HTR and neurotoxicity effects. Drug affinity, pharmacological blocking, and molecular analysis were also conducted to support the behavioral findings. The HTR induced by DOI has been extensively characterized in male mice, making it a good positive control for this study, specifically for comparing the pharmacological effects of our test compounds.

RESULTS

The activation of 5-HT2CR, alone or in concert with 5-HT2AR, produces a comparable degree of HTRs (at a dose of 1 mg·kg-1), with divergent 5-HT2CR- and 5-HT2AR-Gqα11-mediated signaling and enhanced neurotoxic properties (at a dose of 30 mg·kg-1) coupled with activated pro-inflammatory cytokines. These findings show these compounds' potential psychedelic and neurotoxic effects in male mice.

CONCLUSION

These findings showed that while 5-HT2AR is the main initiator of HTR, the 5-HT2CR also has a distinct property that renders it effective in inducing HTR in male mice.

Evaluation of the 5-HT2C receptor drugs RO 60-0175, WAY 161503 and mirtazepine in a preclinical model of comorbidity of depression and cocaine addiction

BACKGROUND

Epidemiological data indicate a high rate of comorbidity of depression and cocaine use disorder (CUD). The role of serotonin 2C (5-HT_2C) receptors in the mechanisms responsible for the coexistence of depression and CUD was not investigated.

METHODS

We combined bilateral olfactory bulbectomy (OBX), an animal model of depression, with intravenous cocaine self-administration and extinction/reinstatement in male rats to investigate two 5-HT_2C receptor agonists (Ro 60-0175 (RO) and WAY 161503 (WAY)) and the 5-HT_2C-receptor preferring antagonist mirtazapine (MIR; an antidepressant), with the goal of determining whether these drugs alter cocaine-induced reinforcement and seeking behaviors. Additionally, neurochemical analyses were performed following cocaine self-administration and its abstinence period in the brain structures in OBX rats and SHAM-operated controls.

RESULTS

Acute administration of RO reduced, while WAY non-significantly attenuated cocaine reinforcement in both rat phenotypes. Moreover, RO or WAY protected against cocaine-seeking behavior after acute or after repeated drug administration during extinction training in OBX and SHAM rats. By contrast, acutely administered MIR did not alter cocaine reinforcement in both rat phenotypes, while it's acute (but not repeated) pretreatment reduced cocaine-seeking in OBX and SHAM rats. In neurochemical analyses, cocaine reinforcement increased 5-HT_2C receptor levels in the ventral hippocampus; a preexisting depression-like phenotype enhanced this effect. The 10-daily cocaine abstinence reduced 5-HT_2C receptor expression in the dorsolateral striatum, while the coexistence of depression and CUD enhanced local receptor expression.

CONCLUSION

The results support a key role of 5-HT_2C receptors for treating CUD and comorbid depression and CUD. They may be backs the further research of pharmacological strategies with drug targeting receptors.

FFAR1/GPR40 Contributes to the Regulation of Striatal Monoamine Releases and Facilitation of Cocaine-Induced Locomotor Activity in Mice

The free fatty acid receptor 1 (FFAR1) is suggested to function as a G protein-coupled receptor (GPR40) for medium-to-long-chain free fatty acids. Previous studies on the expression of FFAR1 revealed that the nigrostriatal region is one of the areas which express abundant FFAR1 mRNA/protein in the central nervous system (CNS). However, the role of FFAR1 in the CNS has been still largely unclarified. Here, we examined a possible functional role of FFAR1 in the control of extracellular concentrations of striatal monoamines and cocaine-induced locomotor activity. Microdialysis analysis revealed that the basal level of extracellular dopamine (DA) was significantly elevated, while the basal serotonin (5-HT) level tended to be reduced in the striatum of FFAR1 knockout (-/-) mice. Interestingly, local application of a FFAR1 agonist, GW9508, markedly augmented the striatal 5-HT release in FFAR1 wild-type (+/+) mice, whereas topical application of a FFAR1 antagonist, GW1100, significantly reduced the 5-HT release. However, the enhanced 5-HT release was completely lost in -/- mice. Although acute administration of cocaine enhanced the locomotor activity in both +/+ and -/- mice, the magnitude of the enhancement was significantly reduced in -/- mice. In addition, intraperitoneal injection of GW1100 significantly decreased the cocaine-induced locomotor enhancement. These results suggest that FFAR1 has a facilitatory role in striatal 5-HT release, and the evoked 5-HT release might contribute to enhance cocaine-induced locomotor activity.

Modulating the Neuromodulators: Dopamine, Serotonin, and the Endocannabinoid System

Dopamine (DA), serotonin (5-hydroxytryptamine, 5-HT), and endocannabinoids (ECs) are key neuromodulators involved in many aspects of motivated behavior, including reward processing, reinforcement learning, and behavioral flexibility. Among the longstanding views about possible relationships between these neuromodulators is the idea of DA and 5-HT acting as opponents. This view has been challenged by emerging evidence that 5-HT supports reward seeking via activation of DA neurons in the ventral tegmental area. Adding an extra layer of complexity to these interactions, the endocannabinoid system is uniquely placed to influence dopaminergic and serotonergic neurotransmission. In this review we discuss how these three neuromodulatory systems interact at the cellular and circuit levels. Technological advances that facilitate precise identification and control of genetically targeted neuronal populations will help to achieve a better understanding of the complex relationship between these essential systems, and the potential relevance for motivated behavior.

Effects of dopamine and serotonin synthesis inhibitors on the ketamine-, d-amphetamine-, and cocaine-induced locomotor activity of preweanling and adolescent rats: sex differences

The pattern of ketamine-induced locomotor activity varies substantially across ontogeny and according to sex. Although ketamine is classified as an NMDA channel blocker, it appears to stimulate the locomotor activity of both male and female rats via a monoaminergic mechanism. To more precisely determine the neural mechanisms underlying ketamine's actions, male and female preweanling and adolescent rats were pretreated with vehicle, the dopamine (DA) synthesis inhibitor ∝-methyl-_DL-p-tyrosine (AMPT), or the serotonin (5-HT) synthesis inhibitor 4-chloro-_DL-phenylalanine methyl ester hydrochloride (PCPA). After completion of the pretreatment regimen, the locomotor activating effects of saline, ketamine, d-amphetamine, and cocaine were assessed during a 2 h test session. In addition, the ability of AMPT and PCPA to reduce dorsal striatal DA and 5-HT content was measured in male and female preweanling, adolescent, and adult rats. Results showed that AMPT and PCPA reduced, but did not fully attenuate, the ketamine-induced locomotor activity of preweanling rats and female adolescent rats. Ketamine (20 and 40 mg/kg) caused a minimal amount of locomotor activity in male adolescent rats, and this effect was not significantly modified by AMPT or PCPA pretreatment. When compared to ketamine, d-amphetamine and cocaine produced different patterns of locomotor activity across ontogeny; moreover, AMPT and PCPA pretreatment affected psychostimulant- and ketamine-induced locomotion differently. When these results are considered together, it appears that both dopaminergic and serotonergic mechanisms mediate the ketamine-induced locomotor activity of preweanling and female adolescent rats. The dichotomous actions of ketamine relative to the psychostimulants in vehicle-, AMPT-, and PCPA-treated rats, suggests that ketamine modulates DA and 5-HT neurotransmission through an indirect mechanism.

Lorcaserin decreases the reinforcing effects of heroin, but not food, in rhesus monkeys

Preclinical studies indicate that lorcaserin (Belviq®), an FDA-approved serotonin 2 C receptor agonist, may be a promising medication for the treatment of stimulant addiction. However, few studies have investigated its effects on self-administration of drugs of abuse from other pharmacological classes, including opioids. Here, adult male rhesus macaques (N = 3) responded under a fixed ratio 30 schedule of food (1-g banana-flavored pellets) and IV heroin delivery. Following stable self-administration of a range of heroin doses (0.32-32 μg/kg/inj, IV), the effects of acute pretreatment with lorcaserin (0.32-1.0 mg/kg, IM) on heroin- and food-maintained responding was determined. The ability of repeated treatment with lorcaserin (1.0 mg/kg) to produce sustained decreases in heroin and/or food intake and reinforcing strength were then analyzed using behavioral economic demand procedures. Results show that self-administration of intravenous heroin was dose-dependent, with peak responding maintained, on average, by the unit dose of 3.2 μg/kg/inj. Lorcaserin pretreatment produced a dose-dependent flattening of the dose-response function for heroin self-administration in each subject. On the other hand, lorcaserin did not decrease food-maintained responding. Repeated lorcaserin treatment reduced heroin intake by selectively decreasing its reinforcing strength, as evidenced by a leftward shift in the demand curves for heroin and the absence of comparable changes in the reinforcing strength of food. These results indicate that serotonin 2 C receptor agonists, such as lorcaserin, have behaviorally selective effects on IV self-administration behavior, and deserve further consideration as candidates for the management of opioid use disorder.

Behavioral Effects of a Potential Novel TAAR1 Antagonist

The trace amine associated receptor 1 (TAAR1) is a G-protein coupled receptor expressed in the monoaminergic regions of the brain, and represents a potential novel therapeutic target for the treatment of neurological disorders. While selective agonists for TAAR1 have been successfully identified, only one high affinity TAAR1 antagonist has been described thus far. We previously identified four potential low potency TAAR1 antagonists through an in silico screen on a TAAR1 homology model. One of the identified antagonists (compound 22) was predicted to have favorable physicochemical properties, which would allow the drug to cross the blood brain barrier. In vivo studies were therefore carried out and showed that compound 22 potentiates amphetamine- and cocaine-mediated locomotor activity. Furthermore, electrophysiology experiments demonstrated that compound 22 increased firing of dopamine neurons similar to EPPTB, the only known TAAR1 antagonist. In order to assess whether the effects of compound 22 were mediated through TAAR1, experiments were carried out on TAAR1-KO mice. The results showed that compound 22 is able to enhance amphetamine- and cocaine-mediated locomotor activity, even in TAAR1-KO mice, suggesting that the in vivo effects of this compound are not mediated by TAAR1. In collaboration with Psychoactive Drug Screening Program, we attempted to determine the targets for compound 22. Psychoactive Drug Screening Program (PDSP) results suggested several potential targets for compound 22 including, the dopamine, norepinephrine and serotonin transporters; as well as sigma 1 and 2 receptors. Our follow-up studies using heterologous cell systems showed that the dopamine transporter is not a target of compound 22. Therefore, the biological target of compound 22 mediating its psychoactive effects still remains unknown.

Preferential modulation of the lateral habenula activity by serotonin-2A rather than -2C receptors: Electrophysiological and neuroanatomical evidence

AIMS

Serotonergic (5-HT) modulation of the lateral habenula (LHb) activity is central in normal and pathologic conditions such as mood disorders. Among the multiple 5-HT receptors (5-HTRs) involved, the 5-HT_2C R seems to play a pivotal role. Yet, the role of 5-HT_2A Rs in the control of the LHb neuronal activity is completely unknown.

METHODS

Single-cell extracellular recording of the LHb neurons was used in rats to study the effect of the general activation and blockade of the 5-HT_2C R and 5-HT_2A R with Ro 60-0175 and SB242084, TCB-2 and MDL11939, respectively. The expression of both receptors in the LHb was confirmed using immunohistochemistry.

RESULTS

Cumulative doses (5-640 μg/kg, iv) of Ro 60-0175 and TCB-2 affected the activity of 34% and 63% of the LHb recorded neurons, respectively. LHb neurons were either inhibited at low doses or excited at higher doses of the 5-HT_2A/C R agonists. SB242084 or MDL11939 (both at 200 μg/kg, iv) did not modify neuronal firing when injected alone, but reverted the bidirectional effects of Ro 60-0175 or TCB-2, respectively. 5-HT_2C Rs and 5-HT_2A Rs are expressed in less than the 20% of the LHb neurons, and they neither colocalize nor make heterodimers. Strikingly, only 5-HT_2A Rs are expressed by the majority of LHb astrocyte cells.

CONCLUSIONS

Peripheral administration of 5-HT_2A R agonist promotes a heterogeneous pattern of neuronal responses in the LHb, and these effects are more prominent than those induced by the 5-HT_2C R activation.

TCB-2 [(7R)-3-bromo-2, 5-dimethoxy-bicyclo[4.2.0]octa-1,3,5-trien-7-yl]methanamine]: A hallucinogenic drug, a selective 5-HT2A receptor pharmacological tool, or none of the above?

The development of 5-HT_2A receptor agonists has been considerably marginalized since the demonstration that the tryptaminergic drugs, LSD and psilocybin, or the phenylakylamine drugs, mescaline and DOI, exert their hallucinogenic properties via the stimulation of 5-HT_2A receptors. Nonetheless, the ability of drugs to stimulate 5-HT_2A receptors is not necessarily associated with psychedelic experience and the hallucinogenic properties are still not understood. Several studies have increased interest in stimulating 5-HT_2A receptors in various CNS diseases. (7R)-3-bromo-2, 5-dimethoxy-bicyclo[4.2.0]octa-1,3,5-trien-7-yl]methanamine (TCB-2) which was synthetized in 2006 presents a high affinity with human and rat 5-HT_2A receptors. Its main feature of interest is that it preferentially stimulates the phospholipase C and not phospholipase A2 pathway, which is at variance with several hallucinogenic drugs. Preference for TCB-2 has increased in preclinical studies and it exhibits subtle differences compared to DOI or LSD in some molecular, cellular and behavioral studies. The purpose of this review is to take a position on the use of TCB-2 as a pharmacological tool. A careful reading of the literature has revealed that the suspected hallucinogenic properties of TCB-2 cannot firmly be ascertained while its pharmacological profile is unknown and likely not selective at 5-HT_2A receptors. This article is part of the Special Issue entitled 'Psychedelics: New Doors, Altered Perceptions'.

Effects of 5-HT1A, 5-HT2A and 5-HT2C receptor agonists and antagonists on responding for a conditioned reinforcer and its enhancement by methylphenidate

OBJECTIVES

These experiments examined the effects of selective 5-HT_1A, 5-HT_2A and 5-HT_2C receptor ligands on responding for a conditioned reinforcer (CRf). Effects of these ligands were measured under basal conditions and following elevated dopamine (DA) activity produced by the DA reuptake inhibitor methylphenidate.

METHODS

Water-restricted rats learned to associate a conditioned stimulus (CS) with water in operant chambers. Subsequently, two response levers were made available; responding on one lever delivered the CS (now a CRf), while responding on the second lever had no consequences. The effects of agonist and antagonists of 5-HT_1A (8-hydroxy-2(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT) and N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY100635)), 5-HT_2A (DOI and M100907) and 5-HT_2C (Ro60-0175 and SB242084) receptors on responding were examined alone, as well as in the presence of methylphenidate.

RESULTS

Responding for a CRf was reduced by the agonists 8-OH-DPAT, DOI and Ro60-0175. 8-OH-DPAT also reduced responding for water and seemed to impair responding in a non-specific fashion. None of the receptor antagonists affected responding. Methylphenidate dose-dependently enhanced responding for a CRf, and this was attenuated by DOI and Ro60-0175. Conversely, the 5-HT_2C receptor antagonist SB242084 potentiated the effect of methylphenidate.

CONCLUSIONS

No evidence was found for a behaviourally selective effect of 5-HT_1A receptor ligands on responding for a CRf. Activation of 5-HT_2A receptors selectively inhibits responding for a CRf. 5-HT_2C receptor ligands exerted bidirectional modulation of responding for a CRf, especially when DA activity was increased. This indicates that 5-HT_2C receptor activity is an important modulator of DA-dependent reward-related behaviours.

The serotonin 5-HT2C receptor and the non-addictive nature of classic hallucinogens

Classic hallucinogens share pharmacology as serotonin 5-HT_2A, 5-HT_2B, and 5-HT_2C receptor agonists. Unique among most other Schedule 1 drugs, they are generally non-addictive and can be effective tools in the treatment of addiction. Mechanisms underlying these attributes are largely unknown. However, many preclinical studies show that 5-HT_2C agonists counteract the addictive effects of drugs from several classes, suggesting this pharmacological property of classic hallucinogens may be significant. Drawing from a comprehensive analysis of preclinical behavior, neuroanatomy, and neurochemistry studies, this review builds rationale for this hypothesis, and also proposes a testable, neurobiological framework. 5-HT_2C agonists work, in part, by modulating dopamine neuron activity in the ventral tegmental area-nucleus accumbens (NAc) reward pathway. We argue that activation of 5-HT_2C receptors on NAc shell, GABAergic, medium spiny neurons inhibits potassium Kv1.x channels, thereby enhancing inhibitory activity via intrinsic mechanisms. Together with experiments that show that addictive drugs, such as cocaine, potentiate Kv1.x channels, thereby suppressing NAc shell GABAergic activity, this hypothesis provides a mechanism by which classic hallucinogen-mediated stimulation of 5-HT_2C receptors could thwart addiction. It also provides a potential reason for the non-addictive nature of classic hallucinogens.

Mirtazapine prevents induction and expression of cocaine-induced behavioral sensitization in rats

Cocaine abuse is a major health problem worldwide. Treatment based on both 5-HT2A/C and 5-HT3 receptor antagonists attenuate not only the effects of cocaine abuse but also the incentive/motivational effect related to cocaine-paired cues. Mirtazapine, an antagonist of postsynaptic α2-adrenergic, 5-HT2A/C and 5HT3 receptors and inverse agonist of the 5-HT2C receptor, has been shown to effectively modify, at the preclinical and clinical levels, various behavioral alterations induced by drugs abuse. Therefore, it is important to assess whether chronic dosing of mirtazapine alters locomotor effects of cocaine as well as induction and expression of cocaine sensitization. Our results reveal that a daily mirtazapine regimen administered for 30days effectively induces a significant attenuation of cocaine-dependent locomotor activity and as well as the induction and expression of behavioral sensitization. These results suggest that mirtazapine may be used as a potentially effective therapy to attenuate induction and expression of cocaine-induced locomotor sensitization.

Behavioral Analysis of Dopaminergic Activation in Zebrafish and Rats Reveals Similar Phenotypes

Zebrafish is emerging as a complement to mammals in behavioral studies; however, there is a lack of comparative studies with rodents and humans to establish the zebrafish as a predictive translational model. Here we present a detailed phenotype evaluation of zebrafish larvae, measuring 300-3000 variables and analyzing them using multivariate analysis to identify the most important ones for further evaluations. The dopamine agonist apomorphine has previously been shown to have a complex U-shaped dose-response relationship in the variable distance traveled. In this study, we focused on breaking down distance traveled into more detailed behavioral phenotypes for both zebrafish and rats and identified in the multivariate analysis low and high dose phenotypes with characteristic behavioral features. Further analysis of single parameters also identified an increased activity at the lowest concentration indicative of a U-shaped dose-response. Apomorphine increased the distance of each swim movement (bout) at both high and low doses, but the underlying behavior of this increase is different; at high dose, both bout duration and frequency increased whereas bout max speed was higher at low dose. Larvae also displayed differences in place preference. The low dose phenotype spent more time in the center, indicative of an anxiolytic effect, while the high-dose phenotype had a wall preference. These dose-dependent effects corroborated findings in a parallel rat study and previous observations in humans. The translational value of pharmacological zebrafish studies was further evaluated by comparing the amino acid sequence of the dopamine receptors (D1-D4), between zebrafish, rats and humans. Humans and zebrafish share 100% of the amino acids in the binding site for D1 and D3 whereas D2 and D4 receptors share 85-95%. Molecular modeling of dopamine D2 and D4 receptors indicated that nonconserved amino acids have limited influence on important ligand-receptor interactions.

The influence of 5-HT(2A) activity on a 5-HT(2C) specific in vivo assay used for early identification of multiple acting SERT and 5-HT(2C) receptor ligands

As a result of our exploratory programme aimed at elaborating dually acting compounds towards the serotonin (5-HT) transporter (SERT) and the 5-HT2C receptor a novel series of 3-amino-1-phenylpropoxy substituted diphenylureas was identified. From that collection two promising compounds (2 and 3) exhibiting highest 5-HT2C receptor affinity strongly inhibited the 5-HT2C receptor agonist 1-(3-chlorophenyl)piperazine (mCPP) induced hypomotility in mice. In further pursuance of that objective (2-aminoethyl)(benzyl)sulfamoyl diphenylureas and diphenylpiperazines have also been elaborated. Herein we report the synthesis of potent multiple-acting compounds from this new class. However, when two optimized representatives (6 and 14) possessing the desired in vitro profile were tested neither reduced the motor activity of mCPP treated animals. Comparative albeit limited in vitro structure-activity relationship (SAR) analysis and detailed in vivo studies are discussed and explanation for their intricate behaviour is proposed.

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.

Serotonin 5-HT2 receptor interactions with dopamine function: implications for therapeutics in cocaine use disorder

Cocaine exhibits prominent abuse liability, and chronic abuse can result in cocaine use disorder with significant morbidity. Major advances have been made in delineating neurobiological mechanisms of cocaine abuse; however, effective medications to treat cocaine use disorder remain to be discovered. The present review will focus on the role of serotonin (5-HT; 5-hydroxytryptamine) neurotransmission in the neuropharmacology of cocaine and related abused stimulants. Extensive research suggests that the primary contribution of 5-HT to cocaine addiction is a consequence of interactions with dopamine (DA) neurotransmission. The literature on the neurobiological and behavioral effects of cocaine is well developed, so the focus of the review will be on cocaine with inferences made about other monoamine uptake inhibitors and releasers based on mechanistic considerations. 5-HT receptors are widely expressed throughout the brain, and several different 5-HT receptor subtypes have been implicated in mediating the effects of endogenous 5-HT on DA. However, the 5-HT2A and 5-HT2C receptors in particular have been implicated as likely candidates for mediating the influence of 5-HT in cocaine abuse as well as to traits (e.g., impulsivity) that contribute to the development of cocaine use disorder and relapse in humans. Lastly, new approaches are proposed to guide targeted development of serotonergic ligands for the treatment of cocaine use disorder.

Evidence for a role of 5-HT2C receptors in the motor aspects of performance, but not the efficacy of food reinforcers, in a progressive ratio schedule

RATIONALE

5-Hydroxytryptamine2C (5-HT2C) receptor agonists reduce the breakpoint in progressive ratio schedules of reinforcement, an effect that has been attributed to a decrease of the efficacy of positive reinforcers. However, a reduction of the breakpoint may also reflect motor impairment. Mathematical models can help to differentiate between these processes.

OBJECTIVE

The effects of the 5-HT2C receptor agonist Ro-600175 ((αS)-6-chloro-5-fluoro-α-methyl-1H-indole-1-ethanamine) and the non-selective 5-HT receptor agonist 1-(m-chlorophenyl)piperazine (mCPP) on rats' performance on a progressive ratio schedule maintained by food pellet reinforcers were assessed using a model derived from Killeen's Behav Brain Sci 17:105-172, 1994 general theory of schedule-controlled behaviour, 'mathematical principles of reinforcement'.

METHOD

Rats were trained under the progressive ratio schedule, and running and overall response rates in successive ratios were analysed using the model. The effects of the agonists on estimates of the model's parameters, and the sensitivity of these effects to selective antagonists, were examined.

RESULTS

Ro-600175 and mCPP reduced the breakpoint. Neither agonist significantly affected a (the parameter expressing incentive value), but both agonists increased δ (the parameter expressing minimum response time). The effects of both agonists could be attenuated by the selective 5-HT2C receptor antagonist SB-242084 (6-chloro-5-methyl-N-{6-[(2-methylpyridin-3-yl)oxy]pyridin-3-yl}indoline-1-carboxamide). The effect of mCPP was not altered by isamoltane, a selective 5-HT1B receptor antagonist, or MDL-100907 ((±)2,3-dimethoxyphenyl-1-(2-(4-piperidine)methanol)), a selective 5-HT2A receptor antagonist.

CONCLUSIONS

The results are consistent with the hypothesis that the effect of the 5-HT2C receptor agonists on progressive ratio schedule performance is mediated by an impairment of motor capacity rather than by a reduction of the incentive value of the food reinforcer.

Effects of developmental exposure to manganese and/or low iron diet: Changes to metal transporters, sucrose preference, elevated zero-maze, open-field, and locomotion in response to fenfluramine, amphetamine, and MK-801

Manganese overexposure (MnOE) can be neurotoxic. In humans this can occur through occupational exposure, air or water contamination, well water, soy milk, and some baby formulas. In children MnOE has been associated with cognitive and behavioral deficits. The effects of MnOE may be modified by factors such as iron status. We hypothesized that developmental MnOE would be exacerbated by iron deficiency. A diet with a 90% decrease in iron (FeD) was given to gravid female rats starting on embryonic day 15 and continued through postnatal day (P)28. Mn (100 mg/kg) or vehicle (VEH) was administered by gavage every other day from P4-28. Metal transporters and receptors (divalent metal transporter-1 (DMT1), transferrin (Tf), transferrin receptor (TfR), and zip8 (zrt8)) were quantified in brain at P28. These markers were increased but the changes were specific: MnOE increased TfR and decreased Tf in hippocampus, whereas FeD increased TfR in neostriatum and increased TfR and DMT1 in the hippocampus, and the combination increased TfR in neostriatum (zip8 was unaffected). Identically treated animals were tested behaviorally at P29 or P60. The combination of FeD+MnOE increased head dips in an elevated zero-maze, reversed deficits in sucrose preference induced by MnOE alone, and increased spontaneous locomotion in an open-field. Rats were also evaluated for changes in locomotor activity after challenge with (±)-fenfluramine (FEN, a 5-HT agonist: 5 mg/kg), MK-801 (MK801, an NMDA antagonist: 0.2 mg/kg), or (+)amphetamine (AMPH, a dopamine agonist: 1 mg/kg). Compared with VEH animals, MnOE animals were more hyperactive after fenfluramine, amphetamine, or MK-801, regardless of FeD exposure. The results indicate persistent effects of developmental MnOE on brain and behavior but few interactions with dietary iron deficiency.

5-hydroxytryptamine- and dopamine-releasing effects of ring-substituted amphetamines on rat brain: a comparative study using in vivo microdialysis

Using in vivo microdialysis, a comparative study was conducted to examine the effects of amphetamine-related compounds (methamphetamine, MAP; 3,4-methylenedioxymethamphetamine, MDMA; p-methoxyamphetamine, PMA; p-methoxymethamphetamine, PMMA; 4-methylthioamphetamine, 4-MTA; 3,4,5-trimethoxyamphetamine, TMA; 2,5-dimethoxy-4-iodoamphetamine, DOI) on extracellular levels of serotonin (5-HT) and dopamine (DA). Dialysates were assayed using HPLC equipped with electrochemical detector following i.p. administration with each drug at a dose of 5 mg/kg. MAP was found to drastically and rapidly increase 5-HT and DA levels (870% and 1460%, respectively). PMA, PMMA, and 4-MTA slightly increased DA levels (150-290%) but remarkably increased 5-HT levels (540-900%). In contrast, TMA and DOI caused no detectable changes in levels of both monoamines. We observed that the potent DA-releasing action of MAP was remarkably decreased by introduction of methoxy or methylthio group at the para position (MAP vs. PMMA or 4-MTA), but introduction of two additional adjacent methoxy groups into PMA totally abolished its 5-HT-/DA-releasing action (PMA vs. TMA). In addition, para-mono-substituted compounds inhibited both monoamine oxidase (MAO) enzymes more strongly than other compounds; PMA and 4-MTA exhibited submicromolar IC50 values for MAO-A. On the other hand, TMA scarcely affected the activity of both MAO enzymes as well as extracellular levels of 5-HT and DA. In this comparative study, MDMA, PMA, and 4-MTA functioned similar to PMMA, a typical empathogen; these findings therefore could be helpful in clarifying the psychopharmacological properties of amphetamine-related, empathogenic designer drugs.

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.

A translational pharmacology approach to understanding the predictive value of abuse potential assessments

Within the drug development industry the assessment of abuse potential for novel molecules involves the generation and review of data from multiple sources, ranging from in-vitro binding and functional assays through to in-vivo nonclinical models in mammals, as well as collection of information from studies in humans. This breadth of data aligns with current expectations from regulatory agencies in both the USA and Europe. To date, there have been a limited number of reviews on the predictive value of individual models within this sequence, but there has been no systematic review on how each of these models contributes to our overall understanding of abuse potential risk. To address this, we analyzed data from 100 small molecules to compare the predictive validity for drug scheduling status of a number of models that typically contribute to the abuse potential assessment package. These models range from the assessment of in-vitro binding and functional profiles at receptors or transporters typically associated with abuse through in-vivo models including locomotor activity, drug discrimination, and self-administration in rodents. Data from subjective report assessments in humans following acute dosing of compounds were also included. The predictive value of each model was then evaluated relative to the scheduling status of each drug in the USA. In recognition of the fact that drug scheduling can be influenced by factors other than the pharmacology of the drug, we also evaluated the predictive value of each assay for the outcome of the human subjective effects assessment. This approach provides an objective and statistical assessment of the predictive value of many of the models typically applied within the pharmaceutical industry to evaluate abuse potential risk. In addition, the impact of combining information from multiple models was examined. This analysis adds to our understanding of the predictive value of each model, allows us to critically evaluate the benefits and limitations of each model, and provides a method for identifying opportunities for improving our assessment and prediction of abuse liability risk in the future.

Serotonin2C receptors and drug addiction: focus on cocaine

This review provides an overview of the role of central serotonin2C (5-HT2C) receptors in drug addiction, specifically focusing on their impact on the neurochemical and behavioral effects of cocaine, one of the most worldwide abused drug. First, we described the neurochemical and electrophysiological mechanisms underlying the interaction between 5-HT2C receptors and the mesocorticolimbic dopaminergic network, in keeping with the key role of this system in drug abuse and dependence. Thereafter, we focused on the role of 5-HT2C receptors in the effects of cocaine in various preclinical behavioral models used in drug addiction research, such as locomotor hyperactivity, locomotor sensitization, drug discrimination, and self-administration, to end with an overview of the neurochemical mechanisms underlying the interactions between 5-HT2C receptors, mesocorticolimbic dopamine system, and cocaine. On their whole, the presented data provide compelling preclinical evidence that 5-HT2C receptor agonists may have efficacy in the treatment of cocaine abuse and dependence, thereby underlying the need for additional clinical studies to ascertain whether preclinical data translate to the human.

Mirtazapine, and mirtazapine-like compounds as possible pharmacotherapy for substance abuse disorders: evidence from the bench and the bedside

Understanding substance use disorders (SUDs) and the problems associated with abstinence has grown in recent years. Nonetheless, highly efficacious treatment targeting relapse prevention has remained elusive, and there remains no FDA-approved pharmacotherapy for psychostimulant dependence. Preclinical and clinical investigations assessing the utility of classical antidepressants, which block monoamine reuptake, show mixed and often contradictory results. Mirtazapine (Remeron®) is a unique FDA-approved antidepressant, with negligible affinity for reuptake proteins, indirectly augments monoamine transmission presumably through antagonist activity at multiple receptors including the norepinephrine (NE)(α2), and serotonin (5-HT)(2A/C) receptors. Historically, mirtazapine was also considered to be a 5-HT(2C) antagonist, but recent evidence indicates that mirtazapine is an inverse agonist at this receptor subtype. Suggesting a promising role for mixed-action serotonergic drugs for addiction pharmacotherapy, mirtazapine attenuates psychostimulant-induced behaviors in several rodent models of substance abuse, and antagonizes methamphetamine-induced biochemical and electrophysiological alterations in rats. Preclinical findings are confirmed through published case studies documenting successful outcomes with mirtazapine therapy across a number of SUDs. To date, a large scale clinical trial assessing the utility of mirtazapine in substance abuse pharmacotherapy has yet to be conducted. However, as reviewed here, accumulating preclinical and clinical evidence argues that mirtazapine, or compounds that emulate aspects of its pharmacological profile, may prove useful in helping treat addictions.

The serotonin 2C receptor antagonist SB 242084 exhibits abuse-related effects typical of stimulants in squirrel monkeys

Antagonists of the serotonin (5-hydroxytryptamine; 5-HT) type 2C receptor (5-HT(2C)R) are being considered as potential pharmacotherapeutics for various affective disorders, but evidence suggests that these compounds enhance the effects of cocaine and related psychostimulants in rodents. However, the effects of selective 5-HT(2C)R antagonists have not been evaluated in nonhuman primates. The present studies used operant-behavioral and in vivo microdialysis techniques to assess the impact of 5-HT(2C)R antagonism on the behavioral and neurochemical effects of cocaine in squirrel monkeys. In subjects trained to lever-press on a fixed-interval schedule of stimulus termination, pretreatment with the highly selective 5-HT(2C)R antagonist 6-chloro-2,3-dihydro-5-methyl-N-[6-[(2-methyl-3-pyridinyl)oxy]-3-pyridinyl]-1H-indole-1-carboxyamide dihydrochloride (SB 242084) (vehicle, 0.01-0.1 mg/kg) produced behavioral-stimulant effects alone and interacted with cocaine in an apparently additive manner. In monkeys trained to self-administer intravenous cocaine according to a second-order schedule of drug delivery, SB 242084 (vehicle, 0.03-0.1 mg/kg) modulated cocaine-induced reinstatement of previously extinguished responding and maintained self-administration behavior when substituted for cocaine availability. These studies are the first to assess the direct reinforcing effects of a 5-HT(2C)R-selective antagonist in any species. Finally, in vivo microdialysis studies revealed that pretreatment with SB 242084 (0.1 mg/kg) modulated cocaine-induced dopamine increases within the nucleus accumbens, but not the caudate nucleus, of awake subjects. Taken together, the results suggest that SB 242084 exhibits a behavioral profile that is qualitatively similar to other psychostimulants, although its efficacy is modest compared with cocaine. The observed interactions with cocaine and the substitution for cocaine self-administration may be indicative of some degree of abuse potential in humans.

Individual differences in the improvement of cocaine-induced place preference response by the 5-HT2C receptor antagonist SB242084 in rats

RATIONALE AND OBJECTIVES

The 5-HT(2A) and 5-HT(2C) receptors have been shown to be differentially involved in modulating cocaine-induced behaviors. In this study we investigated the effects of the 5-HT(2A) antagonist MDL100907 (0.3 mg/kg, i.p.) and the 5-HT(2C) antagonist SB242084 (0.5 mg/kg, i.p.) on development, expression, and recall of cocaine-induced conditioned place preference (CPP) in high- (HR) and low-responder (LR) rats to novelty.

RESULTS

First, we examined the effects of MDL100907 and SB242084 on development of cocaine-induced CPP. Our results indicated that LR, but not HR, animals conditioned with SB242084 + cocaine showed a significantly higher CPP response than controls. This effect was long lasting, as it was still present 30 days after the last conditioning session. Second, we investigated the acute effects of MDL100907 and SB242084 on CPP expression 24 h after cocaine conditioning. Again, our data showed that SB242084 significantly enhanced the expression of cocaine CPP in LR, but not HR animals. Finally, we studied the acute effects of MDL100907 and SB242084 on CPP recall 30 days after cocaine conditioning. Neither MDL100907 nor SB242084 significantly affected the CPP response regardless of the rats' behavioral phenotype.

CONCLUSIONS

This is the first study investigating the contribution of 5-HT(2A) and 5-HT(2C) receptors on development, expression, and recall of cocaine-induced CPP in the HR-LR model of individual vulnerability to drug abuse. Our results show that SB242084 differentially modulates development and expression of CPP in HR vs. LR rats and suggest that 5-HT(2C) receptors play a key role in individual differences on cocaine reward-related learning/memory processes.

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.

Paroxetine-induced reduction of sexual incentive motivation in female rats is not modified by 5-HT1B or 5-HT2C antagonists

RATIONALE

Clinical data show that paroxetine causes sexual dysfunction in a substantial proportion of women taking this compound.

OBJECTIVES

This work was conducted to determine whether chronic paroxetine reduces sexual incentive motivation in female rats and whether this compound can modify any aspect of paced mating. The role of the 5-HT(1B) and 5-HT(2C) receptors in any potential effects was also evaluated.

METHODS

Ovariectomized female rats were implanted with osmotic minipumps releasing 10 mg/kg per day of paroxetine or vehicle for 28 days. Tests for sexual incentive motivation and paced mating were performed just before implantation and at regular intervals thereafter. The females were primed with estradiol benzoate (25 μg/rat) and progesterone (1 mg/rat) before each of these tests. On days 25-27 of treatment, the females were injected with the 5-HT(1B) antagonist GR125,743 (5 mg/kg), the 5-HT(2C) antagonist SB206,553 (5 mg/kg) and vehicle in counterbalanced order. Preinjection time was 30 min.

RESULTS

Paroxetine reduced sexual incentive motivation on day 20 of treatment without affecting any aspect of paced mating. None of the antagonists modified the inhibitory effect of paroxetine on sexual incentive motivation. In the group chronically treated with vehicle, SB206,553 reduced proceptive behaviors in the paced mating test. No other effect was obtained.

CONCLUSION

The effects of paroxetine seen in female rats are similar to those observed in women, suggesting that disturbances of sexual incentive motivation in rats are predictive of sexual dysfunction in women. The 5-HT(1B) and 5-HT(2C) receptors do not seem to be of any importance for paroxetine's inhibitory effect.

A 5-HT(2C) receptor antagonist potentiates a low dose amphetamine-induced conditioned place preference

This study was designed to determine whether a 5-HT(2C) receptor antagonist could induce a conditioned place preference indicative of reward and/or abuse potential. Here, we present the first evidence that a selective 5-HT(2C) receptor antagonist, 6-chloro-5-ethoxy-N-(pyridin-2-yl)indoline-1-carboxamide hydrochloride (CEPC), can potentiate a low dose (0.5 mg/kg) amphetamine-induced positive conditioned place preference (CPP). CEPC did not produce any CPP given alone at doses of either 2.0 or 4.0 mg/kg, whereas low dose amphetamine alone produced only a slight, but statistically nonsignificant, place preference. These studies suggest that 5-HT(2C) receptor antagonists can indirectly potentiate the rewarding effects of amphetamine, and perhaps other psychostimulants. If the results can be translated to man, putative 5-HT(2C) receptor antagonist treatments for anxiety or depression may enhance or potentiate the rewarding effects of drugs of abuse such as amphetamine, which release dopamine.

Blockade of nucleus accumbens 5-HT2A and 5-HT2C receptors prevents the expression of cocaine-induced behavioral and neurochemical sensitization in rats

RATIONALE

The serotonin 5-HT(2A) and 5-HT(2C) receptors regulate the capacity of acute cocaine to augment behavior and monoamine levels within the nucleus accumbens (NAC), a brain region involved in cocaine's addictive and psychotogenic properties.

OBJECTIVES

In the present study, we tested the hypothesis that NAC 5-HT(2A) and 5-HT(2C) receptor activation is involved in the expression of cocaine-induced neuroplasticity following protracted withdrawal from a sensitizing repeated cocaine regimen (days 1 and 7, 15 mg/kg; days 2-6, 30 mg/kg, i.p.).

METHODS

The effects of intra-NAC infusions of the 5-HT(2A) antagonist R-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine methanol (MDL 100907; 0, 50, 100, 500 nM) or the 5-HT(2C) antagonist [6-chloro-5-methyl-1-(6-(2-methylpiridin-3-yloxy)pyridine-3-yl carbamoyl] inodoline dihydrochloride (SB 242084; 0, 50, 100, 500 nM) were first assessed upon the expression of locomotor activity elicited by a 15-mg/kg cocaine challenge injection administered at 3-week withdrawal. A follow-up in vivo microdialysis experiment then compared the effects of the local perfusion of 0, 50, or 100 nM of each antagonist upon cocaine-induced dopamine and glutamate sensitization in the NAC.

RESULTS

Although neither MDL 100907 nor SB 242084 altered acute cocaine-induced locomotion, SB 242084 reduced acute cocaine-elevated NAC dopamine and glutamate levels. Intra-NAC perfusion with either compound blocked the expression of cocaine-induced locomotor and glutamate sensitization, but only MDL 100907 pretreatment prevented the expression of cocaine-induced dopamine sensitization.

CONCLUSIONS

These data provide the first evidence that NAC 5-HT(2A) and 5-HT(2C) receptors are critical for the expression of cocaine-induced neuroplasticity following protracted withdrawal, which has relevance for their therapeutic utility in the treatment of addiction.

The role of serotonin in the NMDA receptor antagonist models of psychosis and cognitive impairment

OBJECTIVE

To review the evidence that agents which preferentially affect serotonin (5-HT) attenuate the ability of N-methyl-D-aspartate (NMDA) receptor non-competitive antagonists (NMDA-RA), e.g., phencyclidine (PCP), dizocilpine (MK-801), and ketamine, to stimulate locomotor activity (LA), and to impair novel object recognition (NOR).

RATIONALE

NMDA-RA-induced increased LA and impairment of NOR are widely used models of the pathophysiology of schizophrenia, the mechanism of action of antipsychotic drugs (APDs), and the identification of novel treatments. Serotonin (5-HT) plays an important role in attenuating these effects of NMDA-RA.

RESULTS

Selective 5-HT(2A) inverse agonists, e.g., M100907 and ACP-103, and atypical APDs, which are more potent 5-HT(2A) than D(2) antagonists, e.g., clozapine and lurasidone, are more effective than selective D(2) receptor antagonists to attenuate NMDA-RA-induced increased LA. 5-HT(2A) inverse agonists alone are not effective to improve NMDA-RA-impaired NOR, but augment the effects of atypical, but not typical APDs, to improve NOR. The 5-HT(1A) receptor partial agonist tandospirone alone and the 5-HT(1A) agonist effects of atypical APDs may substitute for, or contribute to, the effects of D(2) and 5-HT(2A) receptor antagonism to reverse the NMDA-RA impairment in NOR. 5-HT(6) and 5-HT(7) receptor antagonists may also attenuate these NMDA-RA-induced behaviors. 5-HT(2C) receptor inverse agonist, but not neutral antagonists, block NOR in naïve rats and the effects of atypical APDs to restore NOR in PCP-treated rats, suggesting the importance of the constitutive activity of 5-HT(2C) receptors in NOR.

CONCLUSIONS

Multiple 5-HT receptors contribute to effective treatments to reverse adverse effects of NMDA-RA which model psychosis and cognitive impairment.

Serotonin 5-HT4 receptors in the nucleus accumbens are specifically involved in the appetite suppressant and not locomotor stimulant effects of MDMA ('ecstasy')

RATIONALE

3,4-Methylenedioxymethamphetamine (MDMA) abuse is a substantial problem in young adults. Due to a high focus on body image in this population, two main factors that may encourage MDMA use are the appetite suppressant and locomotor stimulant effects of this drug. The nucleus accumbens (NAc) is a brain region associated with the regulation of motivated and locomotor behaviours, and recent evidence suggests that NAc 5-HT4 receptors are likely to be involved in the appetite suppressant effect of MDMA. It has not yet been shown whether 5-HT4 receptors of the NAc are involved in the locomotor stimulant effects of MDMA, which may also contribute to a reduction in food intake.

OBJECTIVES

This study aimed to investigate the effect of local antagonism of serotonin 5-HT4 receptors in the NAc in the appetite suppressant and locomotor stimulant effects of MDMA.

METHODS

Male hooded Wistar rats underwent surgery for the implantation of bilateral NAc microinjection cannulae under isofluorane anesthesia. Following 5-7 days of recovery, the rats received bilateral microinjections of the 5-HT4 antagonist RS39604 into the NAc immediately prior to either saline or MDMA administration. Food intake, water intake, body weight and locomotor activity were measured.

RESULTS

RS39604 significantly increased food intake and increased weight loss in MDMA-treated but not saline-treated rats. Measures of MDMA-induced water intake or locomotor activity were not altered by antagonist administration.

CONCLUSIONS

These results demonstrate that 5-HT4 receptors in the NAc specifically regulate the appetite suppressant effects of MDMA but not MDMA-induced water intake or locomotor activity.

In vivo effects of amphetamine analogs reveal evidence for serotonergic inhibition of mesolimbic dopamine transmission in the rat

Evidence suggests that elevations in extracellular serotonin (5-HT) in the brain can diminish stimulant effects of dopamine (DA). To assess this proposal, we evaluated the pharmacology of amphetamine analogs (m-fluoroamphetamine, p-fluoroamphetamine, m-methylamphetamine, p-methylamphetamine), which display similar in vitro potency as DA releasers (EC(50) = 24-52 nM) but differ in potency as 5-HT releasers (EC(50) = 53-1937 nM). In vivo microdialysis was used to assess the effects of drugs on extracellular DA and 5-HT in rat nucleus accumbens, while simultaneously measuring ambulation (i.e., forward locomotion) and stereotypy (i.e., repetitive movements). Rats received two intravenous injections of drug, 1 mg/kg at time 0 followed by 3 mg/kg 60 min later. All analogs produced dose-related increases in dialysate DA and 5-HT, but the effects on DA did not agree with in vitro predictions. Maximal elevation of dialysate DA ranged from 5- to 14-fold above baseline and varied inversely with 5-HT response, which ranged from 6- to 24-fold above baseline. All analogs increased ambulation and stereotypy, but drugs causing greater 5-HT release (e.g., p-methylamphetamine) were associated with significantly less forward locomotion. The magnitude of ambulation was positively correlated with extracellular DA (p < 0.001) and less so with the ratio of DA release to 5-HT release (i.e., percentage DA increase divided by percentage 5-HT increase) (p < 0.029). Collectively, our findings are consistent with the hypothesis that 5-HT release dampens stimulant effects of amphetamine-type drugs, but further studies are required to address the precise mechanisms underlying this phenomenon.

Citalopram enhances cocaine's subjective effects in rats

Serotonin-selective reuptake inhibitors (SSRIs) have been shown to enhance the locomotor stimulatory, discriminative-stimulus, and convulsive effects of cocaine in rodents. A pharmacokinetic mechanism for the interaction is supported by increases in the brain levels of cocaine by fluoxetine treatment. Furthermore, the locomotor-stimulant effects of cocaine in rodents are enhanced by fluoxetine and fluvoxamine, SSRIs known to inhibit cocaine-metabolizing cytochrome P450 enzymes, whereas citalopram, an SSRI that does not inhibit P450 enzymes, does not enhance cocaine's locomotor-stimulant effects. Citalopram, however, attenuated the discriminative-stimulus effects of cocaine in squirrel monkeys trained to discriminate cocaine from saline, though it enhanced the discriminative-stimulus effects of a low dose of cocaine in rats trained to discriminate high and low doses of the drug. This study investigated the effects of citalopram on cocaine's discriminative-stimulus effects in rats trained more simply to discriminate cocaine from saline. Citalopram alone produced predominantly saline-appropriate responding, but when administered before cocaine, citalopram dose-dependently shifted the cocaine dose-response curve leftward. The present findings suggest that enhancement of cocaine's discriminative-stimulus effects may occur through a mechanism different from that underlying enhancement of cocaine's locomotor effects or that another action of citalopram selectively blocks locomotor enhancement.

Differential effects of serotonin (5-HT)2 receptor-targeting ligands on locomotor responses to nicotine-repeated treatment

We verified the hypothesis that serotonin (5-HT)(2) receptors control the locomotor effects of nicotine (0.4 mg kg(-1)) in rats by using the 5-HT(2A) receptor antagonist M100907, the preferential 5-HT(2A) receptor agonist DOI, the 5-HT(2C) receptor antagonist SB 242084, and the 5-HT(2C) receptor agonists Ro 60-0175 and WAY 163909. Repeated pairings of a test environment with nicotine for 5 days, on Day 10 significantly augmented the locomotor activity following nicotine administration. Of the investigated 5-HT(2) receptor ligands, M100907 (2 mg kg(-1)) or DOI (1 mg kg(-1)) administered during the first 5 days in combination with nicotine attenuated or enhanced, respectively, the development of nicotine sensitization. Given acutely on Day 10, M100907 (2 mg kg(-1)), Ro 60-0175 (1 mg kg(-1)), and WAY 163909 (1.5 mg kg(-1)) decreased the expression of nicotine sensitization. In another set of experiments, where the nicotine challenge test was performed on Day 15 in animals treated repeatedly (Days: 1-5, 10) with nicotine, none of 5-HT(2) receptor ligands administered during the second withdrawal period (Days: 11-14) to nicotine-treated rats altered the sensitizing effect of nicotine given on Day 15. Our data indicate that 5-HT(2A) receptors (but not 5-HT(2C) receptors) play a permissive role in the sensitizing effects of nicotine, while stimulation of 5-HT(2A) receptors enhances the development of nicotine sensitization and activation of 5-HT(2C) receptors is essential for the expression of nicotine sensitization. Repeated treatment with the 5-HT(2) receptor ligands within the second nicotine withdrawal does not inhibit previously established sensitization.

Tolerance to 3,4-methylenedioxymethamphetamine is associated with impaired serotonin release

Tolerance to the behavioural effects of 3,4-methylenedioxymethamphetamine (MDMA) following high dose exposure has been attributed to alterations in serotonergic systems. The present study aimed to determine whether decreased 5-HT release and/or 5-HT(2A/C) receptor desensitization might play a role in tolerance by measuring the response to selective ligands following MDMA exposure. To this end, the latency to nose poke and emerge from a hide box to an open field arena following administration of various ligands to MDMA pre-treated and control rats was measured. Acute exposure to MDMA (0.0-3.3 mg/kg), the 5-HT releasing stimulant fenfluramine (0.0-2.0 mg/kg) and the 5-HT(2) receptor agonist m-CPP (0.0-1.25 mg/kg) increased nose poke and emergence latency. Following administration of doses that produce 5-HT(2A) receptor-mediated behaviours, the 5-HT(2) receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane failed to alter nose poke and emergence latency, suggesting a limited role of this receptor subtype in these behaviours. Activation of 5-HT(2C) receptors was implicated in the behavioural response to both MDMA and m-CPP since the increased emergence latency was dose-dependently attenuated by pre-treatment with the selective 5-HT(2C) receptor antagonist RS102221 (0.0-1.0 mg/kg). Tolerance to the behavioural effect of MDMA and fenfluramine but not m-CPP was produced by prior exposure to MDMA (10 mg/kg administered at two-hour intervals, total 40 mg/kg), and tissue levels of 5-HT and 5-HIAA were decreased. These findings suggest that tolerance to the increased nose poke and emergence latency produced by MDMA is due to impaired 5-HT release.

An in vivo microdialysis assessment of concurrent MDMA and cocaine administration in Sprague-Dawley rats

BACKGROUND AND RATIONALE

Despite the popularity of polysubstance abuse among recreational methylendioxymethamphetamine (MDMA) users, relatively few controlled experimental studies have documented the neurobehavioral effects of MDMA in combination with other abused substances.

OBJECTIVE

In this study, the combined acute effects of MDMA and cocaine were examined by conducting in vivo microdialysis in the rat nucleus accumbens while simultaneously monitoring locomotor activity.

METHODS

Male Sprague-Dawley rats were administered cocaine (10 or 20 mg/kg, i.p.), MDMA (1.5 or 3.0 mg/kg, i.p.), or one of four combinations of cocaine and MDMA during microdialysis experiments. Locomotor activity was monitored, and dialysis samples were collected every 30 min for 3 h prior to injections, for one 30-min period following saline injections, and for an additional 3-h period following drug injections. Samples were analyzed for dopamine content by high-performance liquid chromatography with electrochemical detection.

RESULTS

Significant differences in locomotor activity and dopamine efflux were found among treatment groups, with some MDMA/cocaine combinations producing significantly greater increases compared to single doses of cocaine or MDMA within the first 30 min after injection.

CONCLUSION

Considering the popularity of polysubstance use among recreational MDMA users, the clinical implications of the current findings warrant further investigation.

Effects of serotonin (5-HT)2 receptor ligands on depression-like behavior during nicotine withdrawal

A pronounced withdrawal syndrome including depressed mood prevents cigarette smoking cessation. We tested if blockade or activation of serotonin (5-HT)(2) receptors affected the time of immobility (as an indirect measure of depression-like behavior) in naïve animals and in those withdrawn from chronic nicotine in the forced swim test (FST). The antidepressant imipramine was used as a control. In the FST, the selective 5-HT(2A) receptor antagonist M100,907 (1-2 mg/kg, but not 0.5 mg/kg), the selective 5-HT(2C) receptor antagonist SB 242,084 (0.3-1 mg/kg, but not 0.1 mg/kg), the 5-HT(2C) receptor agonists Ro 60-0175 (10 mg/kg, but not 3 mg/kg) and WAY 163,909 (1.5-10 mg/kg, but not 0.75 mg/kg) as well as imipramine (30 mg/kg, but not 15 mg/kg) decreased the immobility time while the non-selective 5-HT(2) receptor agonist DOI (0.1-1 mg/kg) was inactive in naïve rats. We found an increase in immobility time in rats that were withdrawn from nicotine exposure after 5 days of chronic nicotine treatment. This effect increased from day 1 until day 10 following withdrawal of nicotine, with maximal withdrawal effects on day 3. M100,907 (1 mg/kg), SB 242,084 (0.3 mg/kg), Ro 60-0175 (3 mg/kg), WAY 163,909 (0.75-1.5 mg/kg) and imipramine (15-30 mg/kg) shortened the immobility time in rats that had been removed from nicotine exposure for 3 days. Locomotor activity studies indicated that the effects of SB 242,084 might have been non-specific, as we noticed enhanced basal locomotion in naïve rats. This data set demonstrates that 5-HT(2A) receptor antagonist and 5-HT(2C) receptor agonists exhibited effects similar to antidepressant drugs and abolished the depression-like effects in nicotine-withdrawn rats. These drugs should be considered as adjuncts to smoking cessation therapy, to ameliorate abstinence-induced depressive symptoms.