Effects of serotonin 2C receptor agonists on the behavioral and neurochemical effects of cocaine in squirrel monkeys

Innate cocaine-seeking vulnerability arising from loss of serotonin-mediated aversive effects of cocaine in rats

Cocaine blocks dopamine reuptake, thereby producing rewarding effects that are widely studied. However, cocaine also blocks serotonin uptake, which we show drives, in rats, individually variable aversive effects that depend on serotonin 2C receptors (5-HT2CRs) in the rostromedial tegmental nucleus (RMTg), a major GABAergic afferent to midbrain dopamine neurons. 5-HT2CRs produce depolarizing effects in RMTg neurons that are particularly strong in some rats, leading to aversive effects that reduce acquisition of and relapse to cocaine seeking. In contrast, 5-HT2CR signaling is largely lost after cocaine exposure in other rats, leading to reduced aversive effects and increased cocaine seeking. These results suggest a serotonergic biological marker of cocaine-seeking vulnerability that can be targeted to modulate drug seeking.

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.

Clinical neuropharmacology of cocaine reinforcement: A narrative review of human laboratory self-administration studies

Cocaine use is an unrelenting public health concern. To inform intervention and prevention efforts, it is crucial to develop an understanding of the clinical neuropharmacology of the reinforcing effects of cocaine. The purpose of this review is to evaluate and synthesize human laboratory studies that assess pharmacological manipulations of cocaine self-administration. Forty-one peer-reviewed, human cocaine self-administration studies in which participants received a pretreatment drug were assessed. The pharmacological action and treatment regimen for all drugs reviewed were considered. Drugs that increase extracellular dopamine tend to have the most consistent effects on cocaine self-administration. The ability of nondopaminergic drugs to impact cocaine reinforcement might be related to their downstream effects on dopamine, though it is difficult to draw conclusions because pharmacologically selective compounds are not widely available for human testing. The ability of acute versus chronic drug treatment to differentially affect human cocaine self-administration was not determined because buprenorphine was the only pretreatment drug that was assessed under both acute and chronic dosing regimens. Future research directly comparing acute and chronic drug treatment and/or comparing drugs with different mechanisms of action, is needed to make more conclusive determinations about the clinical neuropharmacology of cocaine reinforcement.

Environmental influence on the preclinical evaluation of substance use disorder therapeutics

Substance use disorders (SUD) develop as a result of complex interactions between the environment, the subject, and the drug of abuse. Preclinical basic research investigating each of these tripartite components of SUD individually has resulted in advancements in our fundamental knowledge regarding the progression from drug abuse to SUD and severe drug addiction and the underlying behavioral and neurobiological mechanisms. How these complex interactions between the environment, the subject, and the drug of abuse impact the effectiveness of candidate or clinically used medications for SUD has not been as extensively investigated. The focus of this chapter will address the current state of our knowledge how these environmental, subject, and pharmacological variables have been shown to impact candidate or clinical SUD medication evaluation in preclinical research using drug self-administration procedures as the primary dependent measure. The results discussed in this chapter highlight the importance of considering environmental variables such as the schedule of reinforcement, concurrent availability of alternative nondrug reinforcers, and experimental housing conditions in the context of SUD therapeutic evaluation. The thesis of this chapter is that improved understanding of environmental variables in the context of SUD research will facilitate the utility of preclinical drug self-administration studies in the evaluation and development of candidate SUD therapeutics.

Pilot study of extended-release lorcaserin for cocaine use disorder among men who have sex with men: A double-blind, placebo-controlled randomized trial

OBJECTIVE

To determine if men who have sex with men (MSM) with cocaine use disorder (CUD) and actively-using cocaine could be enrolled and retained in a pharmacologic intervention trial of lorcaserin-a novel 5-HT2cR agonist-and determine the degree to which participants would adhere to study procedures.

METHODS

This was a phase II randomized, double-blind, placebo-controlled pilot study with 2:1 random parallel group assignment to daily extended-release oral lorcaserin 20 mg versus placebo (clinicaltrials.gov identifier-NCT03192995). Twenty-two of a planned 45 cisgender MSM with CUD were enrolled and had weekly follow-up visits during a 12-week treatment period, with substance use counseling, urine specimen collection, and completion of audio-computer assisted self-interview (ACASI) behavioral risk assessments. Adherence was measured by medication event monitoring systems (MEMS) caps and self-report. This study was terminated early because of an FDA safety alert for lorcaserin's long-term use.

RESULTS

Eighty-six percent completed the trial, with 82% of weekly study follow-up visits completed. Adherence was 55.3% (lorcaserin 51.6% vs. placebo 66.2%) by MEMS cap and 56.9% (56.5% vs. placebo 57.9%) by self-report and did not differ significantly by treatment assignment. Intention-to-treat analyses (ITT) did not show differences in cocaine positivity by urine screen between the lorcaserin and placebo groups by 12 week follow-up (incidence risk ratio [IRR]: 0.96; 95%CI = 0.24-3.82, P = 0.95). However, self-reported cocaine use in timeline follow-back declined more significantly in the lorcaserin group compared to placebo (IRR: 0.66; 95%CI = 0.49-0.88; P = 0.004).

CONCLUSION

We found that it is feasible, acceptable, and tolerable to conduct a placebo-controlled pharmacologic trial for MSM with CUD who are actively using cocaine. Lorcaserin was not associated with significant reductions in cocaine use by urine testing, but was associated with significant reductions in self-reported cocaine use. Future research may be needed to continue to explore the potential utility of 5-HT2cR agonists.

Serotonin/dopamine interaction: Electrophysiological and neurochemical evidence

The interaction between serotonin (5-HT) and dopamine (DA) in the central nervous system (CNS) plays an important role in the adaptive properties of living animals to their environment. These are two modulatory, divergent systems shaping and regulating in a widespread manner the activity of neurobiological networks and their interaction. The concept of one interaction linking these two systems is rather elusive when looking at the mechanisms triggered by these two systems across the CNS. The great variety of their interacting mechanisms is in part due to the diversity of their neuronal origin, the density of their fibers in a given CNS region, the distinct expression of their numerous receptors in the CNS, the heterogeneity of their intracellular signaling pathway that depend on the cellular type expressing their receptors, and the state of activity of neurobiological networks, conditioning the outcome of their mutual influences. Thus, originally conceptualized as inhibition of 5-HT on DA neuron activity and DA neurotransmission, this interaction is nowadays considered as a multifaceted, mutual influence of these two systems in the regulation of CNS functions. These new ways of understanding this interaction are of utmost importance to envision the consequences of their dysfunctions underlined in several CNS diseases. It is also essential to conceive the mechanism of action of psychotropic drugs directly acting on their function including antipsychotic, antidepressant, antiparkinsonian, and drug of abuse together with the development of therapeutic strategies of Alzheimer's diseases, epilepsy, obsessional compulsive disorders. The 5-HT/DA interaction has a long history from the serendipitous discovery of antidepressants and antipsychotics to the future, rationalized treatments of CNS disorders.

Safety and Preliminary Efficacy of Lorcaserin for Cocaine Use Disorder: A Phase I Randomized Clinical Trial

Background and Objectives: Preclinical studies show serotonin (5-HT) 5-HT_2C receptor (5-HT_2CR) agonists reduce cocaine-seeking and cocaine intake. This study examined safety of the 5-HT_2CR agonist lorcaserin administered with cocaine in participants with cocaine use disorder (CocUD). Secondarily, subjective response to cocaine and choice of cocaine vs. money were examined. Methods: A double-blind, randomized, placebo-controlled trial of 25 inpatient non-treatment seeking participants with CocUD. Participants were randomized to either lorcaserin (n = 17) or placebo (n = 8). Primary outcome measures included cardiovascular measures and plasma cocaine levels. Secondary measures of subjective response to cocaine were assessed using a visual analog scale (VAS) and cocaine vs. money progressive ratio choice sessions. Results: Thirteen randomized participants were included in the final analysis. No serious or unexpected adverse events were related to lorcaserin. There were no significant interactions between cocaine and lorcaserin on cardiovascular measures, plasma cocaine, or subjective ratings. After multiple comparisons correction, cocaine significantly increased blood pressure, heart rate, and QTc. Lorcaserin significantly decreased VAS ratings of "feel irritable," "feel hungry," and "I am craving." For the cocaine vs. money choice procedure, there was a significant interaction between choice (cocaine vs. money) and lorcaserin. Participants treated with lorcaserin were more likely to choose cocaine. Discussion and Conclusions: This study showed safety of lorcaserin administered with cocaine but lack of efficacy to reduce the reinforcing effects of cocaine. Scientific Significance: This study is the first to show a disconnect between effects of 5-HT_2CR agonists on craving and cocaine choice in human cocaine users.

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.

Targeting the 5-HT2C Receptor in Biological Context and the Current State of 5-HT2C Receptor Ligand Development

Serotonin (5-HT) 5-HT2C receptor (5-HT2CR) is recognized as a critical mediator of diseaserelated pathways and behaviors based upon actions in the central nervous system (CNS). Since 5-HT2CR is a class A G protein-coupled receptor (GPCR), drug discovery efforts have traditionally pursued the activation of the receptor through synthetic ligands with agonists proposed for the treatment of obesity, substance use disorders and impulse control disorders while antagonists may add value for the treatment of anxiety, depression and schizophrenia. The most significant agonist discovery to date is the FDAapproved anti-obesity medication lorcaserin. In recent years, efforts towards developing other mechanisms to enhance receptor function have resulted in the discovery of Positive Allosteric Modulators (PAMs) for the 5-HT2CR, with several molecule series now reported. The biological significance and context for signaling and function of the 5-HT2CR, and the current status of 5-HT2CR agonists and PAMs are discussed in this review.

Effects of morphine/CP55940 mixtures on an impulsive choice task in rhesus monkeys

µ-Opioid receptor agonists are commonly used to treat pain despite their adverse effects. In preclinical studies, cannabinoid receptor agonists increase the potency of opioids for producing antinociceptive but not reinforcing effects. It is unknown whether other adverse effects of these drugs, such as impairment of complex behavior, are enhanced by their co-administration. This study characterized the effects of morphine (µ-opioid receptor agonist; 0.32-5.6 mg/kg, subcutaneously) and CP55940 (CB1/CB2 cannabinoid receptor agonist; 0.0032-0.32 mg/kg, subcutaneously), alone and in mixtures, in monkeys (n=3) choosing between one pellet delivered immediately and two pellets delivered after a delay. Two consecutive choices of the immediate or delayed reward decreased or increased, respectively, the delay. The median adjusted delay, indicating indifference between the immediate and delayed reinforcers, was increased by morphine (3.2 mg/kg) and CP55940 (0.01-0.032 mg/kg). Performance after administration of morphine (0.32 and 1 mg/kg)/CP55940 (0.0032-0.032 mg/kg) mixtures was not different from performance after CP55940 alone. Neither morphine, CP55940, nor mixtures decreased the median adjusted delay (i.e. increased impulsivity). These findings failed to confirm previous studies showing that morphine increases impulsivity, perhaps because of procedural differences among studies. Treatment of pain often requires repeated drug administration; thus, it remains to be determined whether the present findings predict the effects of chronically administered morphine/CP5540 mixtures on impulsive choice.

Effects of lorcaserin on reinstatement of responding previously maintained by cocaine or remifentanil in rhesus monkeys

Drug abuse remains a serious public health issue, underscoring the need for additional treatment options. Agonists at serotonin (5-HT)2C receptors, particularly lorcaserin, are being considered as pharmacotherapies for abuse of a variety of drugs, including cocaine and opioids. The current study compared the capacity of lorcaserin to attenuate reinstatement of extinguished responding previously maintained by either cocaine or an opioid; this type of procedure is thought to model relapse, an important aspect of drug abuse. Six rhesus monkeys responded under a fixed-ratio schedule for cocaine (0.032 mg/kg/infusion) or remifentanil (0.00032 mg/kg/infusion). Reinstatement of extinguished responding was examined following administration of noncontingent infusions of cocaine (0.32 mg/kg) or heroin (0.0032-0.1 mg/kg) combined with response-contingent presentations of the drug-associated stimuli, or heroin alone without presentation of drug-associated stimuli. When combined with drug-associated stimuli, cocaine and heroin increased extinguished responding. On average, monkeys emitted fewer reinstated responses following 0.32 mg/kg cocaine, compared with the number of responses emitted when cocaine was available for self-administration or when extinguished responding was reinstated by 0.032 mg/kg heroin. When drug-associated stimuli were not presented, heroin did not increase responding. Lorcaserin dose dependently attenuated reinstated responding, and its potency was similar regardless of whether cocaine or heroin was given before reinstatement sessions. The generality of this effect of lorcaserin across pharmacological classes of abused drugs might make it particularly useful for reducing relapse-related behaviors in polydrug abusers. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Effects of long-term high-fat food or methamphetamine intake and serotonin 2C receptors on reversal learning in female rhesus macaques

Perseverative behavior has been highly implicated in addiction. Activation of serotonin 2C receptors (5-HT_2CRs) attenuates cocaine and high caloric food intake, but whether a 5-HT_2CR agonist can reduce high caloric diet (HCD) or methamphetamine (METH) intake and response perseveration remains unknown. Clarifying the role of 5-HT_2CRs in these behaviors will improve knowledge of neurochemical processes that regulate flexible decision-making and whether improvements in decision-making are accompanied by decreases in HCD or METH intake. This study evaluated the effects of long-term HCD and METH intake on reversal learning in female rhesus monkeys. The effects of the 5-HT_2CR agonist WAY163909 on reversal learning before and after extended HCD or METH intake, and on food intake, was also tested. Moreover, we examined whether the 5-HT_2CR is necessary for the effects of WAY163909. WAY163909 was given prior to reversal learning at baseline and after extended HCD or METH intake, and prior to measures of food intake. Extended intake of METH or the HCD increased perseverative errors during reversal. WAY163909 increased correct responses and decreased perseverative errors, both before and after extended HCD or METH intake. Similarly, WAY163909 decreased consumption of a HCD, but not a low caloric diet. The effects of WAY163909 on all these measures were blocked by co-administration with a 5-HT_2CR antagonist. These data indicate that long-term HCD or METH intake disrupts flexible decision-making. Further, the results suggest that reductions in food intake produced by WAY163909 are associated with parallel improvements in decision-making strategies, underscoring the role of the 5-HT_2CR for these behavioral effects.

Effects of acute treatments with the serotonin 2A antagonist M100907 alone or in combination with the serotonin 2C agonist WAY163909 on methamphetamine self-administration in rhesus monkeys

BACKGROUND

Serotonin 5-HT2A receptor antagonists and 5-HT2C receptor agonists have been proposed as important candidates for the development of pharmacotherapies for psychostimulant abuse, with evidence suggesting that those receptors may act together to control behavior. However, the role of 5-HT2A receptors on the reinforcing effects of psychostimulant drugs has not been fully elucidated.

METHODS

In the present study, we investigated the effects of the selective 5HT2A receptor antagonist M100907 alone or in combination with the selective 5HT2C agonist WAY 163909 on intravenous methamphetamine self-administration in rhesus macaques (N = 3). Methamphetamine self-administration (0.01-0.03 mg/kg/inf) was evaluated under a fixed-ratio 20-schedule of reinforcement, and acute pretreatments were conducted 1 h (M100907) or 45 min (WAY 163,909) prior to the beginning of self-administration sessions at the EDMax dose of methamphetamine once stability criteria were met.

RESULTS

Pretreatment with M100907 (0.03-0.3 mg/kg, i.m.) dose-dependently attenuated methamphetamine self-administration, with the highest dose significantly decreasing response rates compared to vehicle. Combined administration of ineffective doses of M100907 and WAY 163,909 had no effects on methamphetamine self-administration.

CONCLUSIONS

Our study indicates that acute selective 5-HT2A receptor blockade decreases peak methamphetamine intake in nonhuman primates. Combination approaches with sub-threshold doses of 5-HT2A receptor antagonists and 5-HT2C receptor agonists, on the other hand, do not seem to be effective in decreasing methamphetamine reinforcement. Further studies are needed in order to investigate the effects of chronic treatments with M100 on complete METH SA dose-response curves.

Modelling Differential Vulnerability to Substance Use Disorder in Rodents: Neurobiological Mechanisms

Despite the prevalence of drug use within society, only a subset of individuals actively taking addictive drugs lose control over their intake and develop compulsive drug-seeking and intake that typifies substance use disorder (SUD). Although research in this field continues to be an important and dynamic discipline, the specific neuroadaptations that drive compulsive behaviour in humans addicted to drugs and the neurobiological mechanisms that underlie an individual's innate susceptibility to SUD remain surprisingly poorly understood. Nonetheless, it is clear from research within the clinical domain that some behavioural traits are recurrently co-expressed in individuals with SUD, thereby inviting the hypothesis that certain behavioural endophenotypes may be predictive, or at least act in some way, to modify an individual's probability for developing this disorder. The analysis of such endophenotypes and their catalytic relationship to the expression of addiction-related behaviours has been greatly augmented by experimental approaches in rodents that attempt to capture diagnostically relevant aspects of this progressive brain disorder. This work has evolved from an early focus on aberrant drug reinforcement mechanisms to a now much richer account of the putatively impaired cognitive control processes that ultimately determine individual trajectories to compulsive drug-related behaviours. In this chapter we discuss the utility of experimental approaches in rodents designed to elucidate the neurobiological and genetic underpinnings of so-called risk traits and how these innate vulnerabilities collectively contribute to the pathogenesis of SUD.

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.

Ayahuasca and Its DMT- and β-carbolines - Containing Ingredients Block the Expression of Ethanol-Induced Conditioned Place Preference in Mice: Role of the Treatment Environment

Ayahuasca is a hallucinogenic beverage produced from the decoction of Banisteriopsis caapi (Bc) and Psychotria viridis (Pv), β-carboline- and N,N-dimethyltryptamine(DMT)-containing plants, respectively. Accumulating evidence suggests that ayahuasca may have therapeutic effects on ethanol abuse. It is not known, however, whether its effects are dependent on the presence of DMT or if non-DMT-containing components would have therapeutic effects. The aim of the present study was to investigate the rewarding properties of ayahuasca (30, 100, and 300 mg/kg, orally), Bc (132, 440, and 1320 mg/kg, orally) and Pv (3.75, 12.5 and 37.5 mg/kg, i.p.) extracts and their effects on ethanol (1.8 g/kg, i.p.) reward using the conditioned place preference (CPP) paradigm in male mice. Animals were conditioned with ayahuasca, Bc or Pv extracts during 8 sessions. An intermediate, but not a high, dose of ayahuasca induced CPP in mice. Bc and Pv did not induce CPP. Subsequently, the effects of those extracts were tested on the development of ethanol-induced CPP. Ayahuasca, Bc or Pv were administered before ethanol injections during conditioning sessions. While Bc and Pv exerted no effects on ethanol-induced CPP, pretreatment with ayahuasca blocked the development of CPP to ethanol. Finally, the effects of a post-ethanol-conditioning treatment with ayahuasca, Bc or Pv on the expression of ethanol-induced CPP were tested. Animals were conditioned with ethanol, and subsequently treated with either ayahuasca, Bc or Pv in the CPP environment previously associated with saline or ethanol for 6 days. Animals were then reexposed to ethanol and ethanol-induced CPP was quantified on the following day. Treatment with all compounds in the ethanol-paired environment blocked the expression of ethanol-induced CPP. Administration of an intermediate, but not a high, dose of ayahuasca and Bc, as well as Pv administration, in the saline-paired compartment blocked the expression of ethanol-induced CPP. The present study sheds light into the components underlying the therapeutic effects of ayahuasca on ethanol abuse, indicating that ayahuasca and its plant components can decrease ethanol reward at doses that do not exert abuse liability. Importantly, the treatment environment seems to influence the therapeutic effects of ayahuasca and Bc, providing important insights into clinical practice.

Behavioral mechanisms underlying the maternal disruptive effect of serotonin 5-HT2A receptor activation in Sprague-Dawley rats

Recent evidence indicates that acute activation of 5-HT_2A receptors causes a disruption of maternal behavior in rats. However, the behavioral mechanisms underlying such a disruption are not known. We addressed this issue using two behavioral approaches targeting the maternal motivational and emotional processing systems. First, we used the pup-separation technique to increase maternal motivation to see whether pup separation is capable of reducing the maternal disruptive effect of TCB-2 (a high-affinity 5-HT_2A agonist) treatment. On postpartum days 4 and 6, different groups of Sprague-Dawley dams were treated with the TCB-2 (5.0 mg/kg, sc) or vehicle and their maternal behaviors were tested after either a 4-h pup-separation or no-pup-separation condition. Although acute TCB-2 injection disrupted maternal behavior, this disruption was not attenuated by pup separation, even after we optimized the timing of separation to maximize its increase on maternal motivation. Acute TCB-2 also impaired the retrieval of food pellets, suggesting a general effect on motivated behaviors. Next, we used a pup preference test and found that dams treated with TCB-2 exhibited an even stronger preference to pups over a male conspecific than vehicle-treated dams, indicating an enhanced motivational and emotional processing of the rewarding property of pups. These findings suggest that TCB-2 has a disruptive effect on rat maternal behavior, and this disruption is not likely due to the drug's effect on mothers' motivational and emotional processing of the incentive salience of pups, although this motivational suppression account cannot be completely ruled out. Future work could explore other possible behavioral mechanisms, such as the drug's effect on executive function.

Synthesis and Evaluation of Pyridyloxypyridyl Indole Carboxamides as Potential PET Imaging Agents for 5-HT2C Receptors

Nine pyridyloxypyridyl indole carboxamides were synthesized and displayed high affinities for 5-HT_2C receptors and high selectivity over 5-HT_2A and 5-HT_2B. Among them, 6-methyl-N-[6-[(2-methyl-3-pyridinyl)oxy]-3-pyridinyl]1H-indole-3-carboxamide (8) exhibits the highest 5-HT_2C binding affinity (K_i = 1.3 nM) and high selectivity over 5-HT_2A (∼1000 times) and 5-HT_2B (∼140 times). [¹¹C]8 was synthesized by palladium-catalyzed coupling reaction between pinacolboranate 16 and [¹¹C]CH₃I with an average radiochemical yield of 27 ± 4% (n = 8, decay-corrected from end of [¹¹C]CH₃I synthesis). MicroPET imaging studies in rhesus monkeys showed regional uptake of [¹¹C]8 in the choroid plexus, whereas the bindings in all other brain regions were low. The specific binding in the choroid plexus was confirmed by administration of a blocking dose of 0.1 mg/kg of the 5-HT_2C antagonist SB-242084.

The behavioral pharmacology and therapeutic potential of lorcaserin for substance use disorders

Substance abuse is serious public health problem for which there are few effective pharmacotherapies. Traditional strategies for drug development have focused on antagonists to block the abuse-related effects of a drug at its site of action, and agonists to replace/mimic the effects of the abused substance. However, recent efforts have targeted receptors, such as serotonin (5-HT)₂ receptors, that can indirectly modulate dopamine neurotransmission with the goal of developing a pharmacotherapy that might be effective at reducing the abuse-related effects of drugs more generally. Lorcaserin is a 5-HT_2C receptor-preferring agonist that is approved by the US Food and Drug Administration for the treatment of obesity. Mounting evidence from preclinical and clinical studies suggests that lorcaserin might also be effective at reducing the abuse-related effects of drugs with different pharmacological mechanisms (e.g., cocaine, heroin, ethanol, and nicotine). Lorcaserin represents a promising and important first step towards the development a new class of pharmacotherapies that have the potential to dramatically improve the treatment of substance abuse. This article will review the behavioral pharmacology of 5-HT_2C receptor-preferring agonists, with a focus on lorcaserin, and evaluate the preclinical evidence supporting the development of lorcaserin for treating substance abuse. This article is part of the Special Issue entitled 'Psychedelics: New Doors, Altered Perceptions'.

Inhibition of Cocaine and 3,4-Methylenedioxypyrovalerone (MDPV) Self-Administration by Lorcaserin Is Mediated by 5-HT2C Receptors in Rats

Lorcaserin is a serotonin (5-HT)_2C receptor-preferring agonist approved by the US Food and Drug Administration to treat obesity. Lorcaserin decreases cocaine self-administration in rats and monkeys. Although this effect is partially inhibited by a 5-HT_2C receptor antagonist (SB242084), lorcaserin also has effects at 5-HT_2A and 5-HT_1A receptors, and the relative contribution of these receptors to its anti-cocaine effects has not been investigated. The goals of this study were to determine 1) the potency and effectiveness of lorcaserin to decrease self-administration of cocaine and 3,4-methylenedioxypyrovalerone (MDPV), a common “bath salts” constituent; and 2) the receptor(s) mediating the effects of lorcaserin on cocaine and MDPV self-administration. Male Sprague-Dawley rats (n = 6) were trained to self-administer MDPV under a progressive ratio schedule of reinforcement and maintained under this schedule with daily access to 0.32 mg/kg per infusion of cocaine or 0.032 mg/kg per infusion of MDPV. Dose-response curves for the effects of lorcaserin on cocaine and MDPV self-administration were generated by administering lorcaserin (0.1–5.6 mg/kg) 25 minutes before the start of the session. To assess the effects of 5-HT_2C (SB242084, 0.1 mg/kg), 5-HT_2A (MDL100907, 0.1 mg/kg), and 5-HT_1A (WAY100635, 0.178 mg/kg) receptor antagonists, they were administered 15 minutes before lorcaserin. Lorcaserin decreased cocaine and MDPV self-administration with equal potency. Antagonism of 5-HT_2C (but not 5-HT_1A or 5-HT_2A) receptors blocked the effects of lorcaserin on cocaine and MDPV self-administration. Taken together, these data provide additional support for further development of 5-HT_2C receptor agonists, such as lorcaserin, for the treatment of stimulant abuse.

Effects of lorcaserin (Belviq®) on nicotine- and food-maintained responding in non-human primates

BACKGROUND

Accumulating evidence suggests that the FDA-approved serotonin 5-HT2C receptor agonist, lorcaserin (Belviq®), may be a promising candidate for the management of substance use disorders, including nicotine addiction. The present study was conducted to determine the efficacy and selectivity of acute or continuous lorcaserin treatment for decreasing the reinforcing effects of nicotine in a primate species.

METHODS

Adult rhesus monkeys (n=4) with a history of nicotine self-administration (>2years) responded for injections of nicotine (0.32-100μg/kg IV) or food pellets under a fixed-ratio schedule of reinforcement during daily 100-min sessions. When responding was stable, lorcaserin was administered either as an acute pretreatment (0.1-1.0mg/kg, IM) or by continuous infusion (0.1mg/kg/hr, SC for 3-5days). Daily activity patterns were also monitored immediately following experimental sessions.

RESULTS

Results indicate that acute lorcaserin pretreatment produced significant and dose-dependent decreases in nicotine-maintained responding across a >100-fold range of self-administered nicotine doses. Continuous lorcaserin treatment decreased intake of 10μg/kg/inj nicotine to about 50% of baseline values. Food-maintained responding was only moderately decreased in 3 of 4 subjects after acute administration and unaffected in all subjects during continuous treatment. Daily activity also was significantly decreased-to ≤50% of control values-following experimental sessions in which acute lorcaserin was administered.

CONCLUSIONS

These data indicate that lorcaserin reduces IV self-administration of nicotine at a dose that decreases motoric activity but less consistently disrupts food-maintained responding. Further research into lorcaserin's potential utility for the management of nicotine dependence is warranted.

Role of impulsivity and reward in the anti-obesity actions of 5-HT2C receptor agonists

The selective 5-HT_2C receptor agonist lorcaserin entered clinical obesity trials with the prevalent view that satiety was a primary mechanism of action. Subsequent Phase II and III trials demonstrated efficacy in terms of weight loss, although the overall effect size (~3% placebo-corrected change) is considered modest. Lorcaserin has been approved by the FDA for the treatment of obesity with lifestyle modification, but since its introduction in 2013 its sales are in decline, probably due to its overall modest effect. However, in some individuals, lorcaserin has a much more clinically significant effect (i.e. >10% placebo-corrected change), although what common features, if any, define these high responders is presently unknown. In the present article we highlight the evidence that alternative mechanisms to satiety may contribute to the anti-obesity effect of lorcaserin, namely effects on constructs of primary and conditioned reward and impulsivity. This may better inform the clinical evaluation of lorcaserin (and any future 5-HT_2C receptor agonists) to subgroups of obese subjects characterized by overeating due to maladaptive impulsivity and reward mechanisms. One such population might be individuals diagnosed with binge eating disorder.

Pharmacological Modulation of 5-HT2C Receptor Activity Produces Bidirectional Changes in Locomotor Activity, Responding for a Conditioned Reinforcer, and Mesolimbic DA Release in C57BL/6 Mice

Converging lines of behavioral, electrophysiological, and biochemical evidence suggest that 5-HT_2C receptor signaling may bidirectionally influence reward-related behavior through an interaction with the mesolimbic dopamine (DA) system. Here we directly test this hypothesis by examining how modulating 5-HT_2C receptor activity affects DA-dependent behaviors and relate these effects to changes in nucleus accumbens (NAc) DA release. In C57BL/6 mice, locomotor activity and responding for a conditioned reinforcer (CRf), a measure of incentive motivation, were examined following treatment with three 5-HT_2C receptor ligands: the agonist CP809101 (0.25-3 mg/kg), the antagonist SB242084 (0.25-1 mg/kg), or the antagonist/inverse agonist SB206553 (1-5 mg/kg). We further tested whether doses of these compounds that changed locomotor activity and responding for a CRf (1 mg/kg CP809101, 0.5 mg/kg SB242084, or 2.5 mg/kg SB206553) also altered NAc DA release using in vivo microdialysis in anesthetized mice. CP809101 reduced locomotor activity, responding for a CRf, and NAc DA release. In contrast, both SB242084 and SB206553 enhanced locomotor activity, responding for a CRf, and NAc DA release, although higher doses of SB206553 produced opposite behavioral effects. Pretreatment with the non-selective DA receptor antagonist α-flupenthixol prevented SB242084 from enhancing responding for a CRf. Thus blocking tonic 5-HT_2C receptor signaling can release serotonergic inhibition of mesolimbic DA activity and enhance reward-related behavior. The observed bidirectional effects of 5-HT_2C receptor ligands may have important implications when considering the 5-HT_2C receptor as a therapeutic target for psychiatric disorders, particularly those presenting with motivational dysfunctions.

Effects of the serotonin 2C receptor agonist WAY163909 on the abuse-related effects and mesolimbic dopamine neurochemistry induced by abused stimulants in rhesus monkeys

RATIONALE

Accumulating evidence shows that the serotonergic system plays a major role in psychostimulant abuse through its interactions with the dopaminergic system. Studies indicate that serotonin 5-HT2C receptors are one of the main classes of receptors involved in mediating the influence of serotonin in drug abuse.

OBJECTIVE

The aim of the present study was to evaluate the effects of the selective serotonin 5-HT2C receptor agonist WAY163909 on the behavioral neuropharmacology of cocaine and methamphetamine in adult rhesus macaques.

METHODS

Cocaine or methamphetamine self-administration and reinstatement were evaluated under second-order and fixed-ratio schedules of reinforcement, respectively. Cocaine- and methamphetamine-induced increases in dopamine were assessed through in vivo microdialysis targeting the nucleus accumbens.

RESULTS

Pretreatment with WAY163909 dose-dependently attenuated cocaine and methamphetamine self-administration and drug-induced reinstatement of extinguished behavior previously maintained by cocaine or methamphetamine delivery. In an additional experiment, WAY163909 induced a dose-dependent attenuation of cocaine- or methamphetamine-induced dopamine overflow in the nucleus accumbens.

CONCLUSIONS

Our data indicate that selective 5-HT2C receptor activation decreases drug intake and drug-seeking behavior in nonhuman primate models of psychostimulant abuse through neurochemical mechanisms involved in the modulation of mesolimbic dopamine.

Effects of a Serotonin 2C Agonist and a 2A Antagonist on Actigraphy-Based Sleep Parameters Disrupted by Methamphetamine Self-Administration in Rhesus Monkeys

Sleep disorders and substance abuse are highly comorbid and we have previously shown that methamphetamine self-administration significantly disrupts activity-based sleep parameters in rhesus monkeys. To the best of our knowledge, no study has evaluated the effectiveness of any pharmacological intervention to attenuate the effects of methamphetamine on nighttime activity under well-controlled conditions in laboratory animals. Thus, we examined the effects of a 5-HT_2C receptor agonist, WAY163909, and a 5-HT_2A receptor antagonist, M100907, given alone and in combination, on actigraphy-based sleep parameters disrupted by methamphetamine self-administration in non-human primates. Adult male/female rhesus monkeys self-administered methamphetamine (0.03 mg/kg/injection, i.v.) under a fixed-ratio 20 schedule of reinforcement (60-min sessions once a day, 5 days per week). Nighttime activity was evaluated using Actiwatch monitors. WAY163909 (0.1, 0.3, and 1.0 mg/kg), M100907 (0.03, 0.1, and 0.3 mg/kg), and a combination (0.1 mg/kg M100+0.3 mg/kg WAY) were administered i.m. before lights-out. Each dose was given for five consecutive days during which self-administration took place in the morning. Both drugs improved activity-based sleep measures disrupted by methamphetamine by decreasing sleep latency and increasing sleep efficiency compared with vehicle. By combining these drugs, their individual effects were significantly enhanced. Agonists at the 5-HT_2C receptor and antagonists at the 5-HT_2A receptor show promise as potential treatments for the sleep-disrupting effects of stimulants when used alone and in combination. Combining subthreshold doses of WAY and M100 produced significant improvements in nighttime activity measures while avoiding the general motor-decreasing effects of the high dose of WAY.

Lorcaserin Suppresses Oxycodone Self-Administration and Relapse Vulnerability in Rats

Opioid use disorder (OUD) is a major public health problem. High relapse rates and poor treatment retention continue to pose major challenges in OUD treatment. Of the abused opioids, oxycodone is well described to maintain self-administration and evoke the durable conditioned responses ("cue reactivity") that result from pairing of opioid-related stimuli (e.g., paraphernalia) with repeated abuse. Serotonin (5-HT) neurotransmission, particularly through the 5-HT_2C receptor (5-HT_2CR), regulates psychostimulant reward and cue reactivity, and in the present experiments, we investigated the hypothesis that the selective 5-HT_2CR agonist lorcaserin, which is approved by the United States Food and Drug Administration (FDA) for the treatment of obesity, will suppress oxycodone self-administration and oxycodone-associated cue reactivity in rats. We found that lorcaserin inhibited oxycodone intake, an effect blocked by the selective 5-HT_2CR antagonist SB242084. Lorcaserin also decreased responding for the discrete cue complex ("cue reactivity") previously associated with delivery of oxycodone (i.e., stimulus lights, infusion pump sounds) in both abstinence and extinction-reinstatement models. The selected dose range of lorcaserin (0.25-1 mg/kg) does not overtly alter spontaneous behaviors nor operant responding on inactive levers in the present study. Taken together, the ability of lorcaserin to reduce the oxycodone self-administration and decrease cue reactivity associated with relapse highlights the therapeutic potential for lorcaserin in the treatment of OUD.

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.

Repeated 7-Day Treatment with the 5-HT2C Agonist Lorcaserin or the 5-HT2A Antagonist Pimavanserin Alone or in Combination Fails to Reduce Cocaine vs Food Choice in Male Rhesus Monkeys

Cocaine use disorder is a global public health problem for which there are no Food and Drug Administration-approved pharmacotherapies. Emerging preclinical evidence has implicated both serotonin (5-HT) 2C and 2A receptors as potential mechanisms for mediating serotonergic attenuation of cocaine abuse-related neurochemical and behavioral effects. Therefore, the present study aim was to determine whether repeated 7-day treatment with the 5-HT_2C agonist lorcaserin (0.1-1.0 mg/kg per day, intramuscular; 0.032-0.1 mg/kg/h, intravenous) or the 5-HT_2A inverse agonist/antagonist pimavanserin (0.32-10 mg/kg per day, intramuscular) attenuated cocaine reinforcement under a concurrent 'choice' schedule of cocaine and food availability in rhesus monkeys. During saline treatment, cocaine maintained a dose-dependent increase in cocaine vs food choice. Repeated pimavanserin (3.2 mg/kg per day) treatments significantly increased small unit cocaine dose choice. Larger lorcaserin (1.0 mg/kg per day and 0.1 mg/kg/h) and pimavanserin (10 mg/kg per day) doses primarily decreased rates of operant behavior. Coadministration of ineffective lorcaserin (0.1 mg/kg per day) and pimavanserin (0.32 mg/kg per day) doses also failed to significantly alter cocaine choice. These results suggest that neither 5-HT_2C receptor activation nor 5-HT_2A receptor blockade are sufficient to produce a therapeutic-like decrease in cocaine choice and a complementary increase in food choice. Overall, these results do not support the clinical utility of 5-HT_2C agonists and 5-HT_2A inverse agonists/antagonists alone or in combination as candidate anti-cocaine use disorder pharmacotherapies.

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.

Effects of Lorcaserin on Cocaine and Methamphetamine Self-Administration and Reinstatement of Responding Previously Maintained by Cocaine in Rhesus Monkeys

Stimulant abuse is a serious public health issue for which there is no effective pharmacotherapy. The serotonin_2C [5-hydroxytryptamine_2C (5-HT_2C)] receptor agonist lorcaserin decreases some abuse-related effects of cocaine in monkeys and might be useful for treating stimulant abuse. The current study investigated the effectiveness of lorcaserin to reduce self-administration of either cocaine or methamphetamine and cocaine-induced reinstatement of extinguished responding. Four rhesus monkeys responded under a progressive-ratio (PR) schedule in which the response requirement increased after each cocaine infusion (32-320 μg/kg/infusion). A separate group of four monkeys responded under a fixed-ratio (FR) schedule for cocaine (32 μg/kg/infusion) and reinstatement of extinguished responding was examined following administration of noncontingent infusions of cocaine (0.1-1 mg/kg) that were combined with response-contingent presentations of the drug-associated stimuli. Finally, three monkeys responded under a FR schedule for methamphetamine (0.32-100 μg/kg/infusion). Lorcaserin (3.2 mg/kg) significantly decreased the final ratio completed (i.e., decreased break point) in monkeys responding under the PR schedule and reduced the reinstatement of responding for drug-associated stimuli following a noncontingent infusion of cocaine; these effects did not appear to change when lorcaserin was administered daily. The same dose of lorcaserin decreased responding for methamphetamine in two of the three monkeys, and the effect was maintained during daily lorcaserin administration; larger doses given acutely (10-17.8 mg/kg) significantly decreased responding for methamphetamine, although that effect was not sustained during daily lorcaserin administration. Together, these results indicate that lorcaserin might be effective in reducing cocaine and methamphetamine abuse and cocaine relapse at least in some individuals.

Decreased Incentive Motivation Following Knockout or Acute Blockade of the Serotonin Transporter: Role of the 5-HT2C Receptor

Acute pharmacological elevation of serotonin (5-hydroxytryptamine; 5-HT) activity decreases operant responding for primary reinforcers, suggesting that 5-HT reduces incentive motivation. The mechanism by which 5-HT alters incentive motivation is unknown, but parallel evidence that 5-HT2C receptor agonists also reduce responding for primary reinforcers implicates this receptor as a potential candidate. These experiments examined whether chronic and acute disruptions of serotonin transporter (SERT) activity altered incentive motivation, and whether the 5-HT2C receptor mediated the effects of elevated 5-HT on behavior. To assess incentive motivation, we measured responding for three different reinforcers: a primary reinforcer (saccharin), a conditioned reinforcer (CRf), and an unconditioned sensory reinforcer (USRf). In the chronic condition, responding was compared between SERT knockout (SERT-KO) mice and their wild-type littermates. In the acute condition, responding was examined in wild-type mice following treatment with 10 or 20 mg/kg citalopram, or its vehicle. The ability of the selective 5-HT2C antagonist SB 242084 to prevent the effects of SERT-KO and citalopram on responding was subsequently examined. Both SERT-KO and citalopram reduced responding for saccharin, a CRf, and a USRf. Treatment with SB 242084 enhanced responding for a CRf and a USRf in SERT-KO mice and blocked the effects of citalopram on CRf and USRf responding. However, SB 242084 was unable to prevent the effects of SERT-KO or citalopram on responding for saccharin. These results support a powerful inhibitory function for 5-HT in the control of incentive motivation, and indicate that the 5-HT2C receptor mediates these effects of 5-HT in a reinforcer-dependent manner.

New therapeutic opportunities for 5-HT2C receptor ligands in neuropsychiatric disorders

The 5-HT2C receptor (R) displays a widespread distribution in the CNS and is involved in the action of 5-HT in all brain areas. Knowledge of its functional role in the CNS pathophysiology has been impaired for many years due to the lack of drugs capable of discriminating among 5-HT2R subtypes, and to a lesser extent to the 5-HT1B, 5-HT5, 5-HT6 and 5-HT7Rs. The situation has changed since the mid-90s due to the increased availability of new and selective synthesized compounds, the creation of 5-HT2C knock out mice, and the progress made in molecular biology. Many pharmacological classes of drugs including antipsychotics, antidepressants and anxiolytics display affinities toward 5-HT2CRs and new 5-HT2C ligands have been developed for various neuropsychiatric disorders. The 5-HT2CR is presumed to mediate tonic/constitutive and phasic controls on the activity of different central neurobiological networks. Preclinical data illustrate this complexity to a point that pharmaceutical companies developed either agonists or antagonists for the same disease. In order to better comprehend this complexity, this review will briefly describe the molecular pharmacology of 5-HT2CRs, as well as their cellular impacts in general, before addressing its central distribution in the mammalian brain. Thereafter, we review the preclinical efficacy of 5-HT2C ligands in numerous behavioral tests modeling human diseases, highlighting the multiple and competing actions of the 5-HT2CRs in neurobiological networks and monoaminergic systems. Notably, we will focus this evidence in the context of the physiopathology of psychiatric and neurological disorders including Parkinson's disease, levodopa-induced dyskinesia, and epilepsy.

Lorcaserin Reduces the Discriminative Stimulus and Reinforcing Effects of Cocaine in Rhesus Monkeys

Cocaine abuse and obesity are serious public health problems, and studies suggest that both dopamine and serotonin systems are involved in regulating the consumption of drugs and food. Lorcaserin has serotonin (5-HT)2C receptor agonist actions, is approved by the U.S. Food and Drug Administration for treating obesity, and might be effective for treating cocaine abuse. These studies characterized the pharmacokinetic and behavioral profiles of lorcaserin (intragastric administration) and determined the effectiveness of lorcaserin to alter discriminative stimulus and reinforcing effects of cocaine (intravenous administration) in rhesus monkeys. Administered acutely, lorcaserin dose-dependently increased the occurrence of yawning while decreasing spontaneous activity and operant responding for food. These effects appeared within 30-60 minutes of administration and began to dissipate by 240 minutes, a time course closely matching plasma concentrations of lorcaserin. In monkeys discriminating cocaine from saline, lorcaserin alone did not occasion cocaine-appropriate responding but shifted the cocaine dose-response curve to the right and down in two of three monkeys. When administered acutely, lorcaserin dose-dependently decreased the rate at which monkeys responded for infusions of cocaine. When administered chronically, 3.2 mg/kg lorcaserin reduced the rate of cocaine-maintained responding by 50% for the duration of a 14-day treatment period. Together, these results show that lorcaserin attenuates the discriminative stimulus effects of cocaine after acute administration and the reinforcing effects of cocaine after acute and repeated administration, consistent with the view that it might have utility in treating cocaine abuse.

Role of 5-HT₂C receptors in effects of monoamine releasers on intracranial self-stimulation in rats

RATIONALE

Many monoamine releasers are abused by humans and produce abuse-related facilitation of intracranial self-stimulation (ICSS) in rats. Facilitation of ICSS in rats can be limited by monoamine releaser-induced serotonin (5-HT) release, but receptors that mediate 5-HT effects of monoamine releasers are unknown.

OBJECTIVES

The aim of this study is to investigate whether 5-HT2C receptor activation is necessary for rate-decreasing effects produced in an ICSS procedure in rats by the 5-HT-selective monoamine releaser fenfluramine and the non-selective releasers napthylisopropylamine (PAL-287) and (+)-3,4-methylenedioxymethamphetamine ((+)-MDMA).

METHODS

Adult male Sprague-Dawley rats with electrodes implanted in the medial forebrain bundle were trained to lever press for brain stimulation under a "frequency-rate" ICSS procedure. Effectiveness of the 5-HT2C antagonist SB 242,084 was evaluated to block rate-decreasing effects produced by (1) the 5-HT2C agonist Ro 60-0175, (2) the 5-HT-selective releaser fenfluramine, and (3) the mixed-action dopamine (DA)/norepinephrine (NE)/5-HT releasers PAL-287 (1.0-5.6 mg/kg) and (+)-MDMA (1.0-3.2 mg/kg). For comparison, effectiveness of SB 242,084 to alter rate-decreasing effects of the kappa-opioid receptor agonist U69,593 and rate-increasing effects of the DA>5-HT releaser amphetamine was also examined.

RESULTS

SB 242,084 pretreatment blocked rate-decreasing effects of Ro 60-0175 and fenfluramine, but not the rate-decreasing effects of U69,593 or the rate-increasing effects of amphetamine. SB 242,084 blunted the rate-decreasing effects and enhanced expression of rate-increasing effects of PAL-287 and (+)-MDMA.

CONCLUSIONS

These data suggest that 5-HT2C receptor activation contributes to rate-decreasing effects that are produced by selective and mixed-action 5-HT releasers in rats and that may oppose and limit the expression of abuse-related ICSS facilitation by these compounds.

Genotype-Dependent Difference in 5-HT2C Receptor-Induced Hypolocomotion: Comparison with 5-HT2A Receptor Functional Activity

In the present study behavioral effects of the 5-HT2C serotonin receptor were investigated in different mouse strains. The 5-HT2C receptor agonist MK-212 applied intraperitoneally induced significant dose-dependent reduction of distance traveled in the open field test in CBA/Lac mice. This effect was receptor-specific because it was inhibited by the 5-HT2C receptor antagonist RS102221. To study the role of genotype in 5-HT2C receptor-induced hypolocomotion, locomotor activity of seven inbred mouse strains was measured after MK-212 acute treatment. We found that the 5-HT2C receptor stimulation by MK-212 decreased distance traveled in the open field test in CBA/Lac, C57Bl/6, C3H/He, and ICR mice, whereas it failed to affect locomotor activity in DBA/2J, Asn, and Balb/c mice. We also compared the interstrain differences in functional response to 5-HT2C and 5-HT2A receptors activation measured by the quantification of receptor-mediated head-twitches. These experiments revealed significant positive correlation between 5-HT2C and 5-HT2A receptors functional responses for all investigated mouse strains. Moreover, we found that 5-HT2A receptor activation with DOI did not change locomotor activity in CBA/Lac mice. Taken together, our data indicate the implication of 5-HT2C receptors in regulation of locomotor activity and suggest the shared mechanism for functional responses mediated by 5-HT2C and 5-HT2A receptors.

Attenuation of cocaine-induced reinstatement of drug seeking in squirrel monkeys by direct and indirect activation of 5-HT2C receptors

RATIONALE

5-Hydroxytryptamine (5-HT) transport inhibitors can attenuate the abuse-related effects of cocaine, and the mechanisms underlying this attenuation may involve activation of 5-HT2C receptors.

OBJECTIVES

The objective of this study was to investigate the consequences of direct and indirect pharmacological activation of 5-HT2C receptors on reinstatement of cocaine-seeking behavior induced by cocaine priming and a cocaine-paired stimulus.

METHODS

Monkeys were trained to self-administer cocaine under a second-order schedule in which responding was maintained by i.v. cocaine injections and a cocaine-paired stimulus. Drug seeking was extinguished by replacing cocaine with vehicle and eliminating the cocaine-paired stimulus. During reinstatement tests, the animals received a priming injection of cocaine along with restoration of the cocaine-paired stimulus, but only vehicle was available for self administration.

RESULTS

Pretreatment with either the 5-HT transport inhibitor fluoxetine (5.6 mg/kg) or the 5-HT2C receptor agonist Ro 60-0175 (1 mg/kg) attenuated reinstatement of drug seeking by cocaine priming. The reinstatement-attenuating effects of both drugs were reversed by the 5-HT2C receptor antagonist SB 242084 (0.03-0.56 mg/kg). Ro 60-0175 (1 mg/kg) attenuated cocaine-induced reinstatement of drug seeking regardless of whether priming injections were or were not accompanied by restoration of the cocaine-paired stimulus. Ro 60-0175 (0.56 mg/kg) was equally effective whether it was administered acutely or chronically. Finally, Ro 60-0175 (0.3-1 mg/kg) had observable behavioral effects suggestive of anxiolytic-like properties.

CONCLUSIONS

5-HT2C receptor mechanisms play a key role in the modulation of cocaine-induced reinstatement by fluoxetine and Ro 60-0175. Direct activation of 5-HT2C receptors may offer a novel, tolerance-free therapeutic strategy for the prevention of cocaine relapse.

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.

Serotonin2C receptors modulate dopamine transmission in the nucleus accumbens independently of dopamine release: behavioral, neurochemical and molecular studies with cocaine

In keeping with its ability to control the mesoaccumbens dopamine (DA) pathway, the serotonin2C receptor (5-HT2C R) plays a key role in mediating the behavioral and neurochemical effects of drugs of abuse. Studies assessing the influence of 5-HT2C R agonists on cocaine-induced responses have suggested that 5-HT2C Rs can modulate mesoaccumbens DA pathway activity independently of accumbal DA release, thereby controlling DA transmission in the nucleus accumbens (NAc). In the present study, we assessed this hypothesis by studying the influence of the 5-HT2C R agonist Ro 60-0175 on cocaine-induced behavioral, neurochemical and molecular responses. The i.p. administration of 1 mg/kg Ro 60-0175 inhibited hyperlocomotion induced by cocaine (15 mg/kg, i.p.), had no effect on cocaine-induced DA outflow in the shell, and increased it in the core subregion of the NAc. Furthermore, Ro 60-0175 inhibited the late-onset locomotion induced by the subcutaneous administration of the DA-D2 R agonist quinpirole (0.5 mg/kg), as well as cocaine-induced increase in c-Fos immunoreactivity in NAc subregions. Finally, Ro 60-0175 inhibited cocaine-induced phosphorylation of the DA and c-AMP regulated phosphoprotein of Mr 32 kDa (DARPP-32) at threonine residues in the NAc core, this effect being reversed by the selective 5-HT2C R antagonist SB 242084 (0.5 mg/kg, i.p.). Altogether, these findings demonstrate that 5-HT2C Rs are capable of modulating mesoaccumbens DA pathway activity at post-synaptic level by specifically controlling DA signaling in the NAc core subregion. In keeping with the tight relationship between locomotor activity and NAc DA function, this interaction could participate in the inhibitory control of cocaine-induced locomotor activity.

Role of cues and contexts on drug-seeking behaviour

Environmental stimuli are powerful mediators of craving and relapse in substance-abuse disorders. This review examined how animal models have been used to investigate the cognitive mechanisms through which cues are able to affect drug-seeking behaviour. We address how animal models can describe the way drug-associated cues come to facilitate the development and persistence of drug taking, as well as how these cues are critical to the tendency to relapse that characterizes substance-abuse disorders. Drug-associated cues acquire properties of conditioned reinforcement, incentive motivation and discriminative control, which allow them to influence drug-seeking behaviour. Using these models, researchers have been able to investigate the pharmacology subserving the behavioural impact of environmental stimuli, some of which we highlight. Subsequently, we examine whether the impact of drug-associated stimuli can be attenuated via a process of extinction, and how this question is addressed in the laboratory. We discuss how preclinical research has been translated into behavioural therapies targeting substance abuse, as well as highlight potential developments to therapies that might produce more enduring changes in behaviour.

Effects of pharmacologic dopamine β-hydroxylase inhibition on cocaine-induced reinstatement and dopamine neurochemistry in squirrel monkeys

Disulfiram has shown promise as a pharmacotherapy for cocaine dependence in clinical settings, although it has many targets, and the behavioral and molecular mechanisms underlying its efficacy are unclear. One of many biochemical actions of disulfiram is inhibition of dopamine β-hydroxylase (DBH), the enzyme that converts dopamine (DA) to norepinephrine (NE) in noradrenergic neurons. Thus, disulfiram simultaneously reduces NE and elevates DA tissue levels in the brain. In rats, both disulfiram and the selective DBH inhibitor nepicastat block cocaine-primed reinstatement, a paradigm which is thought to model some aspects of drug relapse. This is consistent with some clinical results and supports the use of DBH inhibitors for the treatment of cocaine dependence. The present study was conducted to confirm and extend these results in nonhuman primates. Squirrel monkeys trained to self-administer cocaine were pretreated with disulfiram or nepicastat prior to cocaine-induced reinstatement sessions. Neither DBH inhibitor altered cocaine-induced reinstatement. Unexpectedly, nepicastat administered alone induced a modest reinstatement effect in squirrel monkeys, but not in rats. To investigate the neurochemical mechanisms underlying the behavioral results, the effects of DBH inhibition on extracellular DA were analyzed in the nucleus accumbens (NAc) using in vivo microdialysis in squirrel monkeys. Both DBH inhibitors attenuated cocaine-induced DA overflow in the NAc. Hence, the attenuation of cocaine-induced changes in accumbal DA neurochemistry was not associated with altered cocaine-seeking behavior. Overall, the reported behavioral effects of DBH inhibition in rodent models of relapse did not extend to nonhuman primates under the conditions used in the current studies.

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.