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

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

Purpose

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

Methods

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

Results

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

Conclusion

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

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.

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.

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.

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.

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.

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.

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.