Serotonin2C receptor localization in GABA neurons of the rat medial prefrontal cortex: implications for understanding the neurobiology of addiction

Psilocybin-enhanced fear extinction linked to bidirectional modulation of cortical ensembles

The serotonin 2 receptor (5HT2R) agonist psilocybin displays rapid and persistent therapeutic efficacy across neuropsychiatric disorders characterized by cognitive inflexibility. However, the impact of psilocybin on patterns of neural activity underlying sustained changes in behavioral flexibility has not been characterized. To test the hypothesis that psilocybin enhances behavioral flexibility by altering activity in cortical neural ensembles, we performed longitudinal single-cell calcium imaging in the retrosplenial cortex across a five-day trace fear learning and extinction assay. A single dose of psilocybin induced ensemble turnover between fear learning and extinction days while oppositely modulating activity in fear- and extinction- active neurons. The acute suppression of fear-active neurons and delayed recruitment of extinction-active neurons were predictive of psilocybin-enhanced fear extinction. A computational model revealed that acute inhibition of fear-active neurons by psilocybin is sufficient to explain its neural and behavioral effects days later. These results align with our hypothesis and introduce a new mechanism involving the suppression of fear-active populations in the retrosplenial cortex.

Open Label Pilot of Lorcaserin (a serotonin 2C-receptor agonist) for Cannabis Use Disorder

Background And Objective

Cannabis Use Disorder (CUD) has no FDA approved treatment. Serotonin-2c (5HT2c) agonists have preclinical and human laboratory evidence for potential efficacy for CUD. We assessed the tolerability and effects of lorcaserin (5HT2c agonist) on CUD.

Methods

In a 10-week, open label, uncontrolled trial, the tolerability of lorcaserin was tested in outpatients with CUD. Adverse events (AE) were assessed weekly. Cannabis use was assessed twice weekly by the Timeline follow-back and quantitative urine metabolites.

Results

17 participants enrolled, and 14 received medication. Participants' average age was 35 years; majority were male (N=12). The medication was well tolerated in males. There were no serious adverse events (SAE). The most common AE's were headache/migraine (N=4, all females), anorexia (N=3), and irritability (N=2). Participants decreased their frequency of cannabis use significantly (p < 0.001), adjusted for baseline use. By the end of the trial, participants decreased by 1.76 (SE=0.47) cannabis using days/week. Average daily amount of cannabis and urine THC metabolite levels did not change significantly.

Conclusions

Lorcaserin was well tolerated in males but not females suggesting possible sex differences. Future trials of other 5HT2c agonists (lorcaserin was withdrawn at the request of the FDA) should consider longer dose titration phases.

Trial Registration

NCT02932215.

Effects of 5-HT2C receptor stimulation in male mice on behaviour and Fos expression: feeding, reward and impulsivity

Serotonin modulates many motivated behaviours via multiple receptor subtypes. Agonists at 5-HT_2C receptors have potential for treating behavioural problems associated with obesity and drug use. In this work we examined the impact of the 5-HT_2C receptor agonist lorcaserin on several motivated behaviours related to feeding, reward and waiting impulsivity, and on neuronal activation in key brain areas mediating those behaviours. In male C57BL/6J mice effects of lorcaserin (0.2, 1 and 5mg/kg) were examined on feeding, and on operant responding for a palatable reward. Feeding was reduced only at 5mg/kg, whereas operant responding was reduced at 1mg/kg. At a much lower dose range lorcaserin 0.05-0.2mg/kg also reduced impulsive behaviour measured as premature responding in the 5-choice serial reaction time (5-CSRT) test, without affecting attention or ability to perform the task. Lorcaserin induced Fos expression in brain regions related to feeding (paraventricular nucleus and arcuate nucleus), reward (ventral tegmental area), and impulsivity (medial prefrontal cortex, VTA) although these effects did not show the same differential sensitivity to lorcaserin as the behavioural measures. These results indicate a broad profile of action of 5-HT_2C receptor stimulation on brain circuitry and on motivated behaviours, but with clear evidence of differential sensitivity across behavioural domains. This is exemplified by that fact that impulsive behaviour was reduced at a much lower dose range than was feeding behaviour. Along with previous work, and some clinical observations, this work supports the idea that 5-HT_2C agonists may be useful for behavioural problems associated with impulsivity.

The costs and benefits of psychedelics on cognition and mood

Anecdotal evidence has indicated that psychedelic substances may acutely enhance creative task performance, although empirical support for this claim is mixed at best. Clinical research has shown that psychedelics might have enduring effects on mood and well-being. However, there is no neurocognitive framework that ties acute changes in cognition to long-term effects in mood. In this review, we operationalize creativity within an emerging cognitive control framework and assess the current empirical evidence of the effects of psychedelics on creativity. Next, we leverage insights about the mechanisms and computations by which other psychoactive drugs act to enhance versus impair cognition, in particular to those that act on catecholamines, the neurophysiological consequences of which are relatively well understood. Finally, we use the same framework to link the suggested psychedelic-induced improvements in creativity with enduring psychedelic-induced improvements in mood.

Role of neuropeptide neuromedin U in the nucleus accumbens shell in cocaine self-administration in male rats

The nucleus accumbens shell (NAcSh) and its afferent and efferent neuronal projections control key aspects of motivation for cocaine. A recently described regulator of γ-aminobutyric acid (GABA) projections from the dorsal raphe nucleus (DRN) to the NAcSh (DRN → NAcSh) is the neuropeptide neuromedin U (NMU). Here, we find that systemic administration of NMU decreases breakpoint for cocaine on a progressive ratio schedule of reinforcement in male rats. Employing a retrograde adeno-associated virus (AAV), we found that RNAi-mediated knockdown of the NMU receptor 2 (NMUR2) in afferent DRN projections to the NAcSh increases the breakpoint for cocaine. Our previous studies demonstrated that NMU regulates GABA release in the NAcSh, and our current investigation found that systemic NMU administration suppresses cocaine-evoked GABA release in the NAcSh and increases phosphorylated c-Fos expression in neurons projecting from the NAcSh to the ventral pallidum (VP). To further probe the impact of NMU/NMUR2 on neuroanatomical pathways regulating motivation for cocaine, we employed multi-viral transsynaptic studies. Using a combination of rabies virus and retrograde AAV helper virus, we mapped the impact of NMU across three distinct brain regions simultaneously and found a direct connection of GABAergic DRN neurons to the NAcSh → VP pathway. Together, these data reveal that NMU/NMUR2 modulates a direct connection within the GABAergic DRN → NAcSh → VP circuit that diminishes breakpoints for cocaine. These findings importantly advance our understanding of the neurochemical underpinnings of pathway-specific regulation of neurocircuitry that may regulate cocaine self-administration, providing a unique therapeutic perspective.

Epilepsy: Expert opinion on emerging drugs in phase 2/3 clinical trials

INTRODUCTION

Despite the existence of over 30 anti-seizure medications (ASM), including 20 over the last 30 years, a third of patients with epilepsy remain refractory to treatment, with no disease-modifying or preventive therapies until very recently. The development of new ASMs with new mechanisms of action is therefore critical. Recent clinical trials of new treatments have shifted focus from the traditional common epilepsies to rare, genetic epilepsies with known mechanistic targets for treatment and disease-specific animal models.

AREAS COVERED

ASMs in phase 2a/b and 3 clinical trials target cholesterol, serotonin, sigma-1 receptors, potassium channels and metabotrobic glutamate receptors. Neuroinflammation, protein misfolding, abnormal thalamocortical firing, and molecular deficiencies are among the targeted pathways. Clinically, the current phase 2a/b-3 agents hold promise for variety of epilepsy conditions, from developmental epileptic encephalopathies (Dravet Syndrome, Lennox-Gastaut syndrome, CDKL5 and PCDH19, Rett's Syndrome), Infantile Spasms, Tuberous Sclerosis as well as focal and idiopathic generalized epilepsies and acute rescue therapy for cluster seizures.

EXPERT OPINION

New delivery mechanisms increase potency and site-specificity of existing drugs. Novel mechanisms of action involve cholesterol degradation, mitochondrial pathways, anti-inflammation and neuro-regeneration. Earlier identification of genetic conditions through genetic testing will allow for earlier use of disease specific and disease-modifying therapies.

Lorcaserin for Dravet Syndrome: A Potential Advance Over Fenfluramine?

Lorcaserin, a selective serotonin 5-HT_2C receptor agonist, was developed as an appetite suppressant with the rationale of minimizing the risk of cardiovascular toxicity associated with non-selective serotoninergic agents such as fenfluramine. Eight years after FDA approval, however, it was withdrawn from the market, when a large safety study suggested a potential cancer risk. Following in the fenfluramine footsteps and utilizing the repurposing approach coupled with the regulatory orphan drug designation, lorcaserin is currently in clinical development for the treatment of epilepsy. This potential novel indication builds on the evidence that 5-HT_2C receptor stimulation can protect against seizures, and accounts at least in part for fenfluramine's antiseizure effects in Dravet syndrome models. In animal models, lorcaserin shows a narrower range of antiseizure activity than fenfluramine. In particular, lorcaserin is inactive in classical acute seizure tests such as maximal electroshock and subcutaneous pentylenetetrazole in mice and rats, and the 6-Hz stimulation model in mice. However, it is active in the GAERS absence seizure model, and in mutant zebrafish models of Dravet syndrome. Preliminary uncontrolled studies in patients with Dravet syndrome have yielded promising results, and a phase III, double-blind, placebo-controlled, parallel group trial is currently ongoing to assess its efficacy and safety in children and adults with Dravet syndrome.

The interplay between 5-HT2C and 5-HT3A receptors in the dorsal periaqueductal gray mediates anxiety-like behavior in mice

The monoamine neurotransmitter serotonin (5-HT) modulates anxiety by its activity on 5-HT_2C receptors (5-HT_2CR) expressed in the dorsal periaqueductal gray (dPAG). Here, we investigated the presence of 5-HT_3A receptors (5-HT_3AR) in the dPAG, and the interplay between 5-HT_2CR and 5-HT_3AR in the dPAG in mediating anxiety-like behavior in mice. We found that 5-HT_3AR is expressed in the dPAG and the blockade of these receptors using intra-dPAG infusion of ondansetron (5-HT_3AR antagonist; 3.0 nmol) induced an anxiogenic-like effect. The activation of 5-HT_3ABR by the infusion of mCPBG [1-(m-Chlorophenyl)-biguanide; 5-HT₃R agonist] did not alter anxiety-like behaviors. In addition, blockade of 5-HT_3AR (1.0 nmol) prevented the anxiolytic-like effect induced by the infusion of the 5-HT_2CR agonist mCPP (1-(3-chlorophenyl) piperazine; 0.03 nmol). None of the treatment effects on anxiety-like behaviors altered the locomotor activity levels. The present results suggest that the anxiolytic-like effect exerted by serotonin activity on 5-HT_2CR in the dPAG is modulated by 5-HT_3AR expressed in same region.

A selective serotonin reuptake inhibitor ameliorates obsessive-compulsive disorder-like perseverative behavior by attenuating 5-HT2C receptor signaling in the orbitofrontal cortex

Perseveration is a characteristic of patients with obsessive-compulsive disorder (OCD). Clinically, neuronal activity in the lateral orbitofrontal cortex (OFC) is increased in OCD patients. Successful treatment with selective serotonin reuptake inhibitors (SSRIs) reduces activity in the lateral OFC of OCD patients, but the precise mechanisms underlying this effect are unclear. Previously, we reported that repeated injection of the dopamine D₂ receptor agonist quinpirole (QNP) resulted in OCD-like deficits, including perseveration in a reversal learning task. QNP-treated mice showed hyperactivity in lateral OFC pyramidal neurons. The present study demonstrated that 4-week administration of an SSRI increased the rate of correct choice in a reversal learning task. Using the electrophysiological approach, we revealed that an SSRI decreased the activity of lateral OFC pyramidal neurons in QNP-treated mice by potentiating inhibitory inputs. The 4-week administration of an SSRI inhibited the potentiation of neuronal activity induced by a 5-HT_2C receptor agonist. Additionally, both 4-week administration of SSRI and acute application of 5-HT_2C receptor antagonist prevented the QNP-induced potentiation of inhibitory inputs to fast-spiking interneurons in the lateral OFC. Administration of a 5-HT_2C receptor antagonist to mice for 4 days increased the rate of correct choice in a reversal learning task. Collectively, these results indicate that chronic SSRI ameliorated perseverative behavior in QNP-treated mice by modulating inhibitory inputs in the lateral OFC. Short-term 5-HT_2C receptor blockade also ameliorated QNP-induced behavioral and neurological abnormalities by, at least in part, a common mechanism with chronic SSRI.

Serotonin 2C Antagonism in the Lateral Orbitofrontal Cortex Ameliorates Cue-Enhanced Risk Preference and Restores Sensitivity to Reinforcer Devaluation in Male Rats

Previous research has indicated that reward-paired cues can enhance disadvantageous risky choice in both humans and rodents. Systemic administration of a serotonin 2C receptor antagonist can attenuate this cue-induced risk preference in rats. However, the neurocognitive mechanisms mediating this effect are currently unknown. We therefore assessed whether the serotonin 2C receptor antagonist RS 102221 is able to attenuate cue-enhanced risk preference via its actions in the lateral orbitofrontal cortex (lOFC) or prelimbic (PrL) area of the medial prefrontal cortex (mPFC). A total of 32 male Long–Evans rats were trained on the cued version of the rat gambling task (rGT), a rodent analog of the human Iowa gambling task, and bilateral guide cannulae were implanted into the lOFC or PrL. Intra-lOFC infusions of the 5-HT_2C antagonist RS 102221 reduced risky choice in animals that showed a preference for the risky options of the rGT at baseline. This effect was not observed in optimal decision-makers, nor those that received infusions targeting the PrL. Given prior data showing that 5-HT_2C antagonists also improve reversal learning through the same neural locus, we hypothesized that reward-concurrent cues may amplify risky decision-making through cognitive inflexibility. We therefore devalued the sugar pellet rewards used in the cued rGT (crGT) through satiation and observed that decision-making patterns did not shift unless animals also received intra-lOFC RS 102221. Collectively, these data suggest that the lOFC is one critical site through which reward-concurrent cues promote risky choice patterns that are insensitive to reinforcer devaluation, and that 5-HT_2C antagonism may optimize choice by facilitating exploration.

5-HT Receptors and the Development of New Antidepressants

Serotonin modulates several physiological and cognitive pathways throughout the human body that affect emotions, memory, sleep, and thermal regulation. The complex nature of the serotonergic system and interactions with other neurochemical systems indicate that the development of depression may be mediated by various pathomechanisms, the common denominator of which is undoubtedly the disturbed transmission in central 5-HT synapses. Therefore, the deliberate pharmacological modulation of serotonergic transmission in the brain seems to be one of the most appropriate strategies for the search for new antidepressants. As discussed in this review, the serotonergic system offers great potential for the development of new antidepressant therapies based on the combination of SERT inhibition with different pharmacological activity towards the 5-HT system. The aim of this article is to summarize the search for new antidepressants in recent years, focusing primarily on the possibility of benefiting from interactions with various 5-HT receptors in the pharmacotherapy of depression.

X-linked serotonin 2C receptor is associated with a non-canonical pathway for sudden unexpected death in epilepsy

Sudden Unexpected Death in Epilepsy is a leading cause of epilepsy-related mortality, and the analysis of mouse Sudden Unexpected Death in Epilepsy models is steadily revealing a spectrum of inherited risk phenotypes based on distinct genetic mechanisms. Serotonin (5-HT) signalling enhances post-ictal cardiorespiratory drive and, when elevated in the brain, reduces death following evoked audiogenic brainstem seizures in inbred mouse models. However, no gene in this pathway has yet been linked to a spontaneous epilepsy phenotype, the defining criterion of Sudden Unexpected Death in Epilepsy. Most monogenic models of Sudden Unexpected Death in Epilepsy invoke a failure of inhibitory synaptic drive as a critical pathogenic step. Accordingly, the G protein-coupled, membrane serotonin receptor 5-HT2C inhibits forebrain and brainstem networks by exciting GABAergic interneurons, and deletion of this gene lowers the threshold for lethal evoked audiogenic seizures. Here, we characterize epileptogenesis throughout the lifespan of mice lacking X-linked, 5-HT2C receptors (loxTB Htr2c). We find that loss of Htr2c generates a complex, adult-onset spontaneous epileptic phenotype with a novel progressive hyperexcitability pattern of absences, non-convulsive, and convulsive behavioural seizures culminating in late onset sudden mortality predominantly in male mice. RNAscope localized Htr2c mRNA in subsets of Gad2+ GABAergic neurons in forebrain and brainstem regions. To evaluate the contribution of 5-HT2C receptor-mediated inhibitory drive, we selectively spared their deletion in GAD2+ GABAergic neurons of pan-deleted loxTB Htr2c mice, yet unexpectedly found no amelioration of survival or epileptic phenotype, indicating that expression of 5-HT2C receptors in GAD2+ inhibitory neurons was not sufficient to prevent hyperexcitability and lethal seizures. Analysis of human Sudden Unexpected Death in Epilepsy and epilepsy genetic databases identified an enrichment of HTR2C non-synonymous variants in Sudden Unexpected Death in Epilepsy cases. Interestingly, while early lethality is not reflected in the mouse model, we also identified variants mainly among male Sudden Infant Death Syndrome patients. Our findings validate HTR2C as a novel, sex-linked candidate gene modifying Sudden Unexpected Death in Epilepsy risk, and demonstrate that the complex epilepsy phenotype does not arise solely from 5-HT2C-mediated synaptic disinhibition. These results strengthen the evidence for the serotonin hypothesis of Sudden Unexpected Death in Epilepsy risk in humans, and advance current efforts to develop gene-guided interventions to mitigate premature mortality in epilepsy.

Effects of Withdrawal from Cocaine Self-Administration on Rat Orbitofrontal Cortex Parvalbumin Neurons Expressing Cre recombinase: Sex-Dependent Changes in Neuronal Function and Unaltered Serotonin Signaling

The orbitofrontal cortex (OFC) is a brain region involved in higher-order decision-making. Rodent studies show that cocaine self-administration (CSA) reduces OFC contribution to goal-directed behavior and behavioral strategies to avoid drug intake. This change in OFC function persists for many weeks after cocaine withdrawal, suggesting involvement in the process of addiction. The mechanisms underlying impaired OFC function by cocaine are not well-understood. However, studies implicate altered OFC serotonin (5-HT) function in disrupted cognitive processes during addiction and other psychiatric disorders. Thus, it is hypothesized that cocaine impairment of OFC function involves changes in 5-HT signaling, and previous work shows that 5-HT_1A and 5-HT_2A receptor-mediated effects on OFC pyramidal neurons (PyNs) are impaired weeks after cocaine withdrawal. However, 5-HT effects on other contributors to OFC circuit function have not been fully investigated, including the parvalbumin-containing, fast-spiking interneurons (OFC^PV), whose function is essential to normal OFC-mediated behavior. Here, 5-HT function in naive rats and those withdrawn from CSA were evaluated using a novel rat transgenic line in which the rat parvalbumin promoter drives Cre-recombinase expression to permit identification of OFC^PV cells by fluorescent reporter protein expression. We find that whereas CSA altered basal synaptic and membrane properties of the OFC^PV neurons in a sex-dependent manner, the effects of 5-HT on these cells were unchanged by CSA. These data suggest that the behavioral effects of dysregulated OFC 5-HT function caused by cocaine experience are primarily mediated by changes in 5-HT signaling at PyNs, and not at OFC^PV neurons.

Basolateral Amygdala Serotonin 2C Receptor Regulates Emotional Disorder-Related Symptoms Induced by Chronic Methamphetamine Administration

Globally, methamphetamine (MA) is the second most abused drug, with psychotic symptoms being one of the most common adverse effects. Emotional disorders induced by MA abuse have been widely reported both in human and animal models; however, the mechanisms underlying such disorders have not yet been fully elucidated. In this study, a chronic MA administration mouse model was utilized to elucidate the serotonergic pathway involved in MA-induced emotional disorders. After 4 weeks of MA administration, the animals exhibited significantly increased depressive and anxious symptoms. Molecular and morphological evidence showed that chronic MA administration reduced the expression of the 5-hydroxytryptamine (5-HT) rate-limiting enzyme, tryptophan hydroxylase 2, in the dorsal raphe and the concentrations of 5-HT and its metabolite 5-hydroxyindoleacetic acid in the basolateral amygdala (BLA) nuclei. Alterations in both 5-HT and 5-HT receptor levels occurred simultaneously in BLA; quantitative polymerase chain reaction, western blotting, and fluorescence analysis revealed that the expression of the 5-HT2C receptor (5-HT_2CR) increased. Neuropharmacology and virus-mediated silencing strategies confirmed that targeting 5-HT_2CR reversed the depressive and anxious behaviors induced by chronic MA administration. In the BLA, 5-HT_2CR-positive cells co-localized with GABAergic interneurons. The inactivation of 5-HT_2CR ameliorated impaired GABAergic inhibition and decreased BLA activation. Thus, herein, for the first time, we report that the abnormal regulation of 5-HT_2CR is involved in the manifestation of emotional disorder-like symptoms induced by chronic MA use. Our study suggests that 5-HT_2CR in the BLA is a promising clinical target for the treatment of MA-induced emotional disorders.

CB1/5-HT/GABA interactions and food intake regulation

Despite historically the serotonergic, GABAergic, and cannabinoid systems have been shown to play a crucial role in the central regulation of eating behavior, interest in the study of the interactions of these neurotransmission systems has only now been investigated. Current evidence suggests that serotonin may influence normal and pathological eating behavior in significantly more complex ways than was initially thought. This knowledge has opened the possibility of exploring the potential clinical utility of new therapeutic strategies more effective and safer than the current approaches to treat pathological eating behavior. Furthermore, the nature and complexity of the interactions between these neurotransmitter systems have provided a better understanding of the pathophysiological mechanisms not only of eating behavior and eating disorders but also of some of the comorbidities associated with modulation of cortical circuits, which are involved in high order cognitive processes. Accordingly, in the present chapter, the clinical and experimental findings of the interactions between serotonin, GABA, and cannabinoids are synthesized, emphasizing the pharmacological, neurophysiological, and neuroanatomical aspects that could potentially improve the current therapeutic approaches against pathological eating behavior.

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.

Glutamatergic and Serotonergic Modulation of Rat Medial and Lateral Orbitofrontal Cortex in Visual Serial Reversal Learning

Adapting behavior to a dynamic environment requires both steadiness when the environment is stable and behavioral flexibility in response to changes. Much evidence suggests that cognitive flexibility, which can be operationalized in reversal learning tasks, is mediated by cortico-striatal circuitries, with the orbitofrontal cortex (OFC) playing a prominent role. The OFC is a functionally heterogeneous region, and we have previously reported differential roles of lateral (lOFC) and medial (mOFC) regions in a touchscreen serial visual reversal learning task for rats using pharmacological inactivation. Here, we investigated the effects of pharmacological overactivation of these regions using a glutamate transporter 1 (GLT-1) inhibitor, dihydrokainate (DHK), which increases extracellular glutamate by blocking its reuptake. We also tested the impact of antagonism of the serotonin 2A receptor (5-HT_2AR), which modulates glutamate action, in the mOFC and lOFC on the same task. Overactivation induced by DHK produced dissociable effects in the mOFC and lOFC, with more prominent effects in the mOFC, specifically improving performance in the early, perseveration phase. Intra-lOFC DHK increased the number of omitted responses without affecting errors. In contrast, blocking the 5-HT_2AR in the lOFC impaired reversal learning overall, while mOFC 5-HT_2AR blockade had no effect. These results further support dissociable roles of the rodent mOFC and lOFC in deterministic visual reversal learning and indicate that modulating glutamate transmission through blocking the GLT-1 and the 5-HT_2AR have different roles in these two structures.

This study further supports dissociable roles of specific orbitofrontal subregions, as well as glutamatergic and serotonergic transmission in these subregions, in cognitive flexibility. This knowledge will add to the understanding of specific neural mechanisms underlying inflexible behaviour across psychiatric disorders.

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.

Convergent neural connectivity in motor impulsivity and high-fat food binge-like eating in male Sprague-Dawley rats

Food intake is essential for survival, but maladaptive patterns of intake, possibly encoded by a preexisting vulnerability coupled with the influence of environmental variables, can modify the reward value of food. Impulsivity, a predisposition toward rapid unplanned reactions to stimuli, is one of the multifaceted determinants underlying the etiology of dysregulated eating and its evolving pathogenesis. The medial prefrontal cortex (mPFC) is a major neural director of reward-driven behavior and impulsivity. Compromised signaling between the mPFC and nucleus accumbens shell (NAcSh) is thought to underlie the cognitive inability to withhold prepotent responses (motor impulsivity) and binge intake of high-fat food (HFF) seen in binge eating disorder. To explore the relationship between motor impulsivity and binge-like eating in rodents, we identified high (HI) and low impulsive (LI) rats in the 1-choice serial reaction time task and employed a rat model of binge-like eating behavior. HFF binge rats consumed significantly greater calories relative to control rats maintained on continual access to standard food or HFF. HI rats repeatedly exhibited significantly higher bingeing on HFF vs. LI rats. Next, we employed dual viral vector chemogenetic technology which allows for the targeted and isolated modulation of ventral mPFC (vmPFC) neurons that project to the NAcSh. Chemogenetic activation of the vmPFC to NAcSh pathway significantly suppressed motor impulsivity and binge-like intake for high-fat food. Thus, inherent motor impulsivity and binge-like eating are linked and the vmPFC to NAcSh pathway serves as a 'brake' over both behaviors.

Acute and chronic interactive treatments of serotonin 5HT2C and dopamine D1 receptor systems for decreasing nicotine self-administration in female rats

A variety of neural systems are involved in the brain bases of tobacco addiction. Animal models of nicotine addiction have helped identify a variety of interacting neural systems involved in the pathophysiology of tobacco addiction. We and others have found that drug treatments affecting many of those neurotransmitter systems significantly decrease nicotine self-administration. These treatments include dopamine D₁ receptor antagonist, histamine H1 antagonist, serotonin 5HT_2C agonist, glutamate NMDA antagonist, nicotinic cholinergic α4β2 partial agonist and nicotinic cholinergic α3β4 antagonist acting drugs. It may be the case that combining treatments that affect different neural systems underlying addiction may be more efficacious than single drug treatment. In the current study, we tested the interactions of the D₁ antagonist SCH-23390 and the serotonin 5HT_2c agonist lorcaserin, both of which we have previously shown to significantly reduce nicotine self-administration. In the acute interactions study, both SCH-23390 and lorcaserin significantly reduced nicotine self-administration when given alone and had additive effects when given in combination. In the chronic study, each drug alone caused a significant decrease in nicotine self-administration. No additive effect was seen in combination because SCH-23390 given alone chronically was already highly effective. Chronic administration of the combination was not seen to significantly prolong reduced nicotine self-administration into the post-treatment period. This research shows that unlike lorcaserin and SCH-23390 interactions when given acutely, when given chronically in combination they do not potentiate or prolong each other's effects in reducing nicotine self-administration.

Zebrafish studies identify serotonin receptors mediating antiepileptic activity in Dravet syndrome

Dravet syndrome is a life-threatening early-onset epilepsy not well controlled by antiepileptic drugs. Drugs that modulate serotonin (5-HT) signalling, including clemizole, locaserin, trazodone and fenfluramine, have recently emerged as potential treatment options for Dravet syndrome. To investigate the serotonin receptors that could moderate this antiepileptic activity, we designed and synthesized 28 novel analogues of clemizole, obtained receptor binding affinity profiles, and performed in vivo screening in a scn1lab mutant zebrafish (Danio rerio) model which recapitulates critical clinical features of Dravet syndrome. We discovered three clemizole analogues with 5-HT receptor binding that exert powerful antiepileptic activity. Based on structure–activity relationships and medicinal chemistry-based analysis, we then screened an additional set of known 5-HT receptor specific drug candidates. Integrating our in vitro and in vivo data implicates 5-HT_2B receptors as a critical mediator in the mechanism of seizure suppression observed in Dravet syndrome patients treated with 5-HT modulating drugs.

Serotonin Regulation of the Prefrontal Cortex: Cognitive Relevance and the Impact of Developmental Perturbation

The prefrontal cortex is essential for both executive function and emotional regulation. The interrelationships among these behavioral domains are increasingly recognized, as well as their sensitivity to serotonin (5-hydroxytryptamine, 5-HT). Prefrontal cortex receives serotonergic inputs from the dorsal and median raphe nuclei and is modulated by multiple subtypes of 5-HT receptor across its layers and cell types. Extremes of serotonergic modulation alter mood regulation in vulnerable individuals, yet the impact of serotonin under more typical physiological parameters remains unclear. In this regard, new tools are permitting a closer examination of the behavioral impact of the serotonin system. Optogenetic and chemogenetic manipulations of dorsal raphe 5-HT neurons reveal that serotonin has a greater impact on executive function than previously appreciated. Domains that appear sensitive to fluctuations in 5-HT neuronal excitability include patience and cognitive flexibility. This work is broadly consistent with ex vivo research investigating how 5-HT regulates prefrontal cortex and its output projections. A growing literature suggests 5-HT modulation of these prefrontal circuits is unexpectedly flexible to alteration during development by genetic, behavioral, environmental or pharmacological manipulations, with lasting repercussions for cognition and emotional regulation. Here, we review the cellular and circuit mechanisms of prefrontal serotonergic modulation, investigate recent research into the cognitive consequences of the serotonergic system, and probe the lasting consequences of developmental perturbations. Understanding both the complexity of the prefrontal serotonin system and its sensitivity during development are essential to learn more about the vulnerabilities of this system in mood and anxiety disorders and the underappreciated cognitive consequences of these disorders and their treatment.

Endogenous Serotonin 5-HT2A and 5-HT2C Receptors Associate in the Medial Prefrontal Cortex

The 5-HT_2A receptor (5-HT_2AR) and 5-HT_2CR are localized to the same neurons within the medial prefrontal cortex (mPFC), which regulates executive function, decision-making, and reward-guided learning and memory processes. The 5-HT_2AR and 5-HT_2CR coimmunoprecipitate in the mPFC of male Sprague-Dawley rats, while in vitro studies demonstrate the presence of a physical interaction between the 5-HT_2AR and 5-HT_2CR. The purpose of this study was to identify mPFC subregions in which the 5-HT_2AR and 5-HT_2CR physically interact ex vivo in the male Sprague-Dawley rat. We established the expression patterns of 5-HT_2AR and 5-HT_2CR in layers I-VI of the anterior cingulate cortex (ACC), prelimbic (PL), and infralimbic (IL) subregions using double-label fluorescence immunohistochemistry in male rats. We then employed the proximity ligation assay (PLA) to test the hypothesis that the 5-HT_2AR and 5-HT_2CR form a close, physical association within these mPFC subregions. Our results demonstrate subregion- and layer-specific expression of the 5-HT_2AR and 5-HT_2CR proteins using immunofluorescence and single recognition PLA, and a spatially close (within 40 nm) interaction between the 5-HT_2AR and 5-HT_2CR that occurs along a dorsal-ventral gradient in the rat mPFC.

Hallucinogen-Like Action of the Novel Designer Drug 25I-NBOMe and Its Effect on Cortical Neurotransmitters in Rats

NBOMes are N-benzylmethoxy derivatives of the 2C family hallucinogens. 4-Iodo-2,5-dimethoxy-N-(2-methoxybenzyl)phenethylamine (25I-NBOMe) is one of the commonly used illicit drugs. It exhibits high binding affinity for 5-HT_2A/C and 5-HT_1A serotonin receptors. Activation of 5-HT_2A receptor induces head-twitch response (HTR) in rodents, a behavioral marker of hallucinogen effect in humans. There is not much data on neurochemical properties of NBOMes. Therefore, we aimed to investigate the effect of 25I-NBOMe on extracellular level of dopamine (DA), serotonin (5-HT), and glutamate (GLU) in the rat frontal cortex, tissue contents of monoamines, and hallucinogenic activity in rats. The extracellular levels of DA, 5-HT, and GLU were studied using microdialysis in freely moving animals. The tissue contents of DA, 5-HT and their metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) were determined in the rat frontal cortex. We also tested a drug-elicited HTR. 25I-NBOMe at doses 1, 3, and 10 mg/kg (sc) increased extracellular DA, 5-HT, and GLU levels, enhanced tissue content of 5-HT and 5-HIAA, but did not affect tissue level of DA and its metabolites. The compound exhibited an inverted U-shaped dose-response curve with respect to the effect on extracellular DA and 5-HT levels, but a U-shaped dose-response curve was observed for its effect on GLU release and HTR. The data from our study suggest that hallucinogenic activity of 25I-NBOMe seems to be related with the increase in extracellular GLU level-mediated via cortical 5-HT_2A receptors. The influence of 25I-NBOMe on 5-HT_2C and 5-HT_1A receptors may modulate its effect on neurotransmitters and HTR.

MK212, a 5-hydroxytryptamine 2C receptor agonist, inhibits conditioned avoidance responses independent of blocking endogenous dopamine release in rats

Although it is widely accepted that 5-hydroxytryptamine (5-HT) 2C receptor agonists produce antipsychotic effects by reducing endogenous dopamine release from presynaptic neurons, no direct evidence supports this. The aim of the present study was to investigate whether the antipsychotic effects induced by 5-HT_2C receptor agonists are dependent on the inhibition of endogenous dopamine release. We developed a novel conditioned avoidance response paradigm to test this hypothesis. In this assay, rats in which dopamine was depleted by reserpine failed to show conditioned avoidance responses, and the acute administration of quinpirole reversed the disruption of avoidance responses induced by reserpine. This suggests that animals successfully showed conditioned avoidance responses independent of endogenous dopamine release under these experimental conditions. Our results revealed that MK212 (0.5 mg/kg) reduced avoidance responses triggered by quinpirole in dopamine-depleted rats. Therefore, 5-HT_2C receptor agonists can inhibit conditioned avoidance responses independent of blocking endogenous dopamine release. Furthermore, the 5-HT_2C receptor agonist, MK212, decreased the extracellular concentration of glutamate in the nucleus accumbens, indicating that this mechanism may be critical for the antipsychotic effects of 5-HT_2C receptor agonists.

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.

Persistent attenuation of nicotine self-administration in rats by co-administration of chronic nicotine infusion with the dopamine D1 receptor antagonist SCH-23390 or the serotonin 5-HT2C agonist lorcaserin

Tobacco addiction each year causes millions of deaths worldwide. Brain nicotinic acetylcholine receptors have been shown to be central to tobacco addiction. Nicotine replacement therapy aids tobacco cessation, but the success rate is still far too low. This may in part be due to the fact that neurons with nicotinic receptors are not the only neural systems involved in tobacco addiction. Interacting neural systems also play important roles in tobacco addiction. Nicotine increases the release of a variety of neurotransmitters, including dopamine and serotonin. Dopamine, in particular dopamine D₁ receptors, has been shown to be involved in the reinforcing action of nicotine. Serotonin through its actions on 5-HT_2C receptors has been shown to play a key role in modulating the reinforcement of addictive drugs, including nicotine and alcohol. Combination of treatments could provide greater treatment efficacy. These studies were conducted to evaluate combination therapies utilizing nicotine replacement therapy in conjunction with either a dopamine D₁ receptor antagonist SCH-23390 or a serotonin 5-HT_2C receptor agonist, lorcaserin. Female Sprague-Dawley rats were given access to self-administer nicotine via IV infusions. Osmotic pumps were implanted to reproduce the kinetic of chronic nicotine patch therapy. SCH-23390 (0.02 mg/kg) or lorcaserin (0.6 mg/kg) were administered prior to nicotine self-administration sessions. Reproducing earlier findings SCH-23390, lorcaserin and nicotine replacement therapy were effective at reducing IV nicotine self-administration. 5HT_2C agonist treatment had additive effects with chronic nicotine infusion for significantly lowering nicotine self-administration. This study demonstrates the feasibility of combination of chronic nicotine with therapies targeting non-nicotinic receptors as treatment options for tobacco addiction.

Effect of Group Density on the Physiology and Aggressive Behavior of Male Brandt's Voles (Lasiopodomys brandtii)

Xin Dai, Ling-Yu Zhou, Jie-Xia Cao, Yan-Qi Zhang, Feng-Ping Yang, Ai-Qin Wang, Wan-Hong Wei, and Sheng-Mei Yang (2018) Population density is well known to influence animal physiology and behavior. How population density affects the aggressive behavior of the Brandt's vole (Lasiopodomys brandtii) is, however, little known. The aim of this study was to investigate the effect of group density on physiologic responses and aggressive behavior of male Brandt's voles and their potential underlying neuro-mechanism. The results show that increasing group density led to elevated serum corticosterone levels and increased spleen weight; it also induced more male-male aggressive behavior. By contrast, it had a negative effect on body growth and the weight of testis and epididymis. Aging also increased male-male aggressive behavior. Higher density reduced mRNA levels of tryptophan hydroxylase 2 (TPH2), 5-hydroxytryptamine receptor 1A (5HT1A), and 5-hydroxytryptamine receptor 1B (5HT1B) in the amygdala and the dorsal raphe nucleus (DRN). Our results demonstrate that higher population density can intensify stress reactions and male-male aggressive behavior in Brandt's voles at the price of inhibiting body growth and reproduction. Serotonergic systems in the amygdala and the DRN may take part in the control of aggressive behavior among male voles. Our results provide novel insights into the neuro-mechanism underlying the influence of population density on aggressive behavior in Brandt's vole, and imply that aggressive behavior may play an important role in the population fluctuation of the animal.

Early antipsychotic treatment in juvenile rats elicits long-term alterations to the adult serotonin receptors

BACKGROUND

Antipsychotic drug (APD) prescription/use in children has increased significantly worldwide, despite limited insight into potential long-term effects of treatment on adult brain functioning. While initial long-term studies have uncovered alterations to behaviors following early APD treatment, further investigations into potential changes to receptor density levels of related neurotransmitter (NT) systems are required.

METHODS

The current investigation utilized an animal model for early APD treatment with aripiprazole, olanzapine, and risperidone in male and female juvenile rats to investigate potential long-term changes to the adult serotonin (5-HT) NT system. Levels of 5-HT1A, 5-HT2A, and 5-HT2C receptors were measured in the prefrontal cortex (PFC), caudate putamen (CPu), nucleus accumbens (NAc), and hippocampus via Western Blot and receptor autoradiography.

RESULTS

In the male cohort, long-term changes to 5-HT2A and 5-HT2C receptors were found mostly across hippocampal and cortical brain regions following early aripiprazole and olanzapine treatment, while early risperidone treatment changed 5-HT1A receptor levels in the NAc and PFC. Lesser effects were uncovered in the female cohort with aripiprazole, olanzapine and risperidone to alter 5-HT1A and 5-HT2A receptors in NAc and hippocampal brain regions, respectively.

CONCLUSION

The results of this study suggest that early treatment of various APDs in juvenile rats may cause gender and brain regional specific changes in 5-HT2A and 5-HT2C receptors in the adult brain.

Age- and sex-dependent effects of methamphetamine on cognitive flexibility and 5-HT2C receptor localization in the orbitofrontal cortex of Sprague-Dawley rats

Adolescents and females experience worse outcomes of drug use compared to adults and males. This could result from age- and sex-specific consequences of drug exposure on brain function and cognitive behavior. In the current study, we examined whether a history of intravenous methamphetamine (METH) self-administration impacted cognitive flexibility and 5-HT_2CR localization in the orbitofrontal cortex (OFC) in an age- and sex-dependent manner. Strategy shifting was assessed in male and female Sprague-Dawley rats that had self-administered METH (0.08 mg/kg/inf) or received non-contingent infusions of saline during periadolescence or young adulthood. After all rats reached adulthood, they were tested in an operant strategy shifting task and their brains were subsequently analyzed using immunofluorescence to quantify co-localization of 5-HT_2C receptors with parvalbumin interneurons in the OFC. We found that adolescent-onset females were the only group impaired during discrimination and reversal learning, but they did not exhibit changes in localization of 5-HT_2C receptors. In contrast, adult-onset males exhibited a significant increase in co-localization of 5-HT_2C receptors within parvalbumin interneurons in the left hemisphere of the OFC. These studies reveal that age and sex differences in drug-induced deficits in reversal learning and 5-HT_2CR co-localization with parvalbumin interneurons are dissociable and can manifest independently. In addition, these data highlight the potential for certain treatment approaches to be more suitable in some populations compared to others, such as alleviating drug-induced cognitive deficits as a focus for treatment in adolescent females.

Fear extinction in an obsessive-compulsive disorder animal model: Influence of sex and estrous cycle

Obsessive-compulsive disorder (OCD) is a neuropsychiatric condition that affects men and women equally, but with a sexually dimorphic pattern of development. Reproductive cycle events can influence symptom severity of OCD in females, indicating that ovarian hormones or their interaction with distinct neurotransmitter systems may play a role in OCD pathophysiology. Clinical studies and animal models have confirmed the importance of the serotonergic (5-HT) system in the neurobiology and treatment of OCD. Accordingly, the non-selective 5-HT2c agonist, meta-chlorophenylpiperazine (mCPP), exacerbates symptoms in untreated OCD patients. In rodents, it evokes repetitive behaviors that engage brain areas that are homologous with those found to be dysfunctional in OCD patients. These regions, including the medial prefrontal and anterior cingulate cortices, are also involved in fear inhibition, which is impaired in OCD. Here, we treated rats with mCPP (0.5 and 3.0 mg/kg) to evaluate its influence on self-grooming behavior and assess potential fear extinction retention deficits, taking into account sex differences and females' estrous cycle. We found that mCPP exacerbated grooming in male and female rats, irrespective of the estrous cycle phase. Fear extinction retention, however, was impaired only in females. Moreover, females undergoing fear extinction training during the metestrus/diestrus phases of the estrous cycle were more sensitive to the impairments induced by mCPP. Our results indicate that mCPP can induce OCD-like symptoms, exacerbating self-grooming and impairing fear extinction. It suggests that changes in 5-HT signaling through 5-HT2c receptors may have an important role in the OCD pathophysiology and that the influence of gonadal hormones in OCD should be further investigated.

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.

Clemizole and modulators of serotonin signalling suppress seizures in Dravet syndrome

Dravet syndrome is a catastrophic childhood epilepsy with early-onset seizures, delayed language and motor development, sleep disturbances, anxiety-like behaviour, severe cognitive deficit and an increased risk of fatality. It is primarily caused by de novo mutations of the SCN1A gene encoding a neuronal voltage-activated sodium channel. Zebrafish with a mutation in the SCN1A homologue recapitulate spontaneous seizure activity and mimic the convulsive behavioural movements observed in Dravet syndrome. Here, we show that phenotypic screening of drug libraries in zebrafish scn1 mutants rapidly and successfully identifies new therapeutics. We demonstrate that clemizole binds to serotonin receptors and its antiepileptic activity can be mimicked by drugs acting on serotonin signalling pathways e.g. trazodone and lorcaserin. Coincident with these zebrafish findings, we treated five medically intractable Dravet syndrome patients with a clinically-approved serotonin receptor agonist (lorcaserin, Belviq®) and observed some promising results in terms of reductions in seizure frequency and/or severity. Our findings demonstrate a rapid path from preclinical discovery in zebrafish, through target identification, to potential clinical treatments for Dravet syndrome.

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.

Managing epilepsy-associated depression: Serotonin enhancers or serotonin producers?

Depression is one of the major psychiatric comorbidities having a major impact on the quality of life in people with epilepsy (PWE). Selective serotonin reuptake inhibitors (SSRIs) are considered as safest therapy for the treatment of depression in PWE. Although administration of SSRIs increases the synaptic serotonin levels, it decreases the overall serotonin synthesis in the brain. Long-term therapy with SSRIs has been reported to decrease serotonin synthesis, which may be the possible reason for lessening of their antidepressant effect over time as well as elevated seizure outcomes observed in PWE. Thus the present scenario warrants streamlined studies to explore the safety and efficacy of SSRIs as well as approaches beyond SSRIs for treatment of depression in epilepsy. In this review, we outline the approaches which may restore serotonin levels rather than a pseudo enhancement of serotonin with SSRIs. The potential of various anti-inflammatory approaches such as selective cyclooxygenase-2 inhibitors, inflammatory cytokine inhibitors, and indoleamine 2,3-dioxygenase inhibitors pertaining to their serotonin restoring effects is discussed as possible therapy for treatment of depression in epilepsy.

5-HT2C Receptor Knockdown in the Amygdala Inhibits Neuropathic-Pain-Related Plasticity and Behaviors

Neuroplasticity in the amygdala drives pain-related behaviors. The central nucleus (CeA) serves major amygdala output functions and can generate emotional-affective behaviors and modulate nocifensive responses. The CeA receives excitatory and inhibitory inputs from the basolateral nucleus (BLA) and serotonin receptor subtype 5-HT_2CR in the BLA, but not CeA, has been implicated anxiogenic behaviors and anxiety disorders. Here, we tested the hypothesis that 5-HT_2CR in the BLA plays a critical role in CeA plasticity and neuropathic pain behaviors in the rat spinal nerve ligation (SNL) model. Local 5-HT_2CR knockdown in the BLA with stereotaxic injection of 5-HT_2CR shRNA AAV vector decreased vocalizations and anxiety- and depression-like behaviors and increased sensory thresholds of SNL rats, but had no effect in sham controls. Extracellular single-unit recordings of CeA neurons in anesthetized rats showed that 5-HT_2CR knockdown blocked the increase in neuronal activity (increased responsiveness, irregular spike firing, and increased burst activity) in SNL rats. At the synaptic level, 5-HT_2CR knockdown blocked the increase in excitatory transmission from BLA to CeA recorded in brain slices from SNL rats using whole-cell patch-clamp conditions. Inhibitory transmission was decreased by 5-HT_2CR knockdown in control and SNL conditions to a similar degree. The findings can be explained by immunohistochemical data showing increased expression of 5-HT_2CR in non-GABAergic BLA cells in SNL rats. The results suggest that increased 5-HT_2CR in the BLA contributes to neuropathic-pain-related amygdala plasticity by driving synaptic excitation of CeA neurons. As a rescue strategy, 5-HT_2CR knockdown in the BLA inhibits neuropathic-pain-related behaviors.SIGNIFICANCE STATEMENT Neuroplasticity in the amygdala has emerged as an important pain mechanism. This study identifies a novel target and rescue strategy to control abnormally enhanced amygdala activity in an animal model of neuropathic pain. Specifically, an integrative approach of gene transfer, systems and brain slice electrophysiology, behavior, and immunohistochemistry was used to advance the novel concept that serotonin receptor subtype 5-HT_2C contributes critically to the imbalance between excitatory and inhibitory drive of amygdala output neurons. Local viral vector-mediated 5-HT_2CR knockdown in the amygdala normalizes the imbalance, decreases neuronal activity, and inhibits neuropathic-pain-related behaviors. The study provides valuable insight into serotonin receptor (dys)function in a limbic brain area.

Behavioral, pharmacological and neuroanatomical analysis of serotonin 2C receptor agonism on maternal behavior in rats

As a highly motivated social behavior, maternal behavior in rats has been routinely used to study psychoactive drugs for clinical, neuroscience and pharmacological purposes. Recent evidence indicates that acute activation of serotonin 2C (5-HT_2C) receptors causes a disruption of rat maternal behavior. The present study was designed to elucidate the behavioral, pharmacological mechanisms and neuroanatomical basis of this 5-HT_2C effect. First, we replicated the finding that acute MK212 injection (2.0mg/kg, a highly selective 5-HT_2C agonist) disrupts maternal behavior, especially on pup retrieval. Interestingly, this disruption was significantly attenuated by 4-h pup separation (a procedure putatively increased maternal motivation). MK212 also suppressed food retrieval, indicating that it has a general effect on motivated behaviors. Second, we showed that MK212 disrupts maternal behavior by specifically activating 5-HT_2C receptor, as pretreatment with a 5-HT_2C receptor antagonist SB242084 (0.6 and 1.0mg/kg) alleviated MK212-induced disruption on pup retrieval. Third, we microinjected MK212 into various brain regions implicated in the regulation of maternal behavior: nucleus accumbens shell (25, 75, 250ng/0.5μl/side), medial prefrontal cortex (25 and 250ng, 1, 2 and 5μg/0.5μl/side), and medial preoptic area (MPOA, 75ng, 1 and 5μg/0.5μl/side). Pup retrieval and other maternal responses were not affected by any of these manipulations. Finally, we used c-Fos immunohistochemistry to identify the central mechanisms of the acute and repeated MK212 effects on maternal behavior. Acute MK212 (2.0mg/kg) disrupted pup retrieval and concurrently decreased c-Fos expression in the ventral part of lateral septal nucleus (LSv), MPOA, dentate gyrus (DG) and dorsal raphe (DR), but increased it in the central amygdala (CeA). Five days of repeated MK212 (2.0mg/kg) treatment produced a persistent disruption of pup retrieval and only decreased c-Fos expression in the DR. These findings not only confirm a role of 5-HT_2C receptor in rat maternal behavior, but also suggest that the coordinated 5-HT_2C activity in various limbic (e.g., LSv, DG, CeA), hypothalamic regions (e.g., MPOA) and brainstem areas (e.g. DR), is likely involved in the mediation of important psychological processes (e.g. motor function, motivation) necessary for the normal expression of maternal behavior.

Comparative Evaluation of Partial α2 -Adrenoceptor Agonist and Pure α2 -Adrenoceptor Antagonist on the Behavioural Symptoms of Withdrawal after Chronic Alcohol Administration in Mice

As an addictive drug, alcohol produces withdrawal symptoms if discontinued abruptly after chronic use. Clonidine (CLN), a partial α2 -adrenergic agonist, and mirtazapine (MRT), an antagonist of α2 -adrenoceptor, both clinically aid alcohol withdrawal. Considering different mechanisms of action of the two drugs, this study was designed to see how far these two mechanistically different drugs differ in their ability to decrease the severity of ethanol withdrawal syndrome. The effect of CLN and MRT on ethanol withdrawal-induced anxiety, depression and memory impairment was analysed using EPM, FST and PAR tests, respectively. Animals received distilled water, ethanol and/or either of the drugs (CLN and MRT) in different doses. Relapse to alcohol use was analysed by CPP test. Animals received ethanol as a conditioning drug and distilled water, CLN or MRT as test drug. CLN and MRT both alleviated anxiety in a dose-dependent manner. MRT (4 mg/kg) was more effective than CLN (0.1 mg/kg) in ameliorating the anxiogenic effect of alcohol withdrawal. However, CLN treatment increased depression. It significantly decreased swimming time and increased immobility time, whereas MRT treatment decreased immobility time and increased climbing and swimming time during abstinence. The effect was dose dependent for both drugs. The results of PAR test show that CLN treatment worsens working memory. Significant increase in SDE and TSZ and decrease in SDL were observed in CLN-treated animals. MRT treatment, on the other hand, improved working memory at both doses. Further, both CLN and MRT alleviated craving. A significant decrease in time spent in the ethanol-paired chamber was seen. MRT treatment at both doses showed better effect than CLN in preventing the development of preference in CPP test. These findings indicate a potential therapeutic use and better profile of mirtazapine over clonidine in improving memory, as well as in alleviating depression, anxiety and craving associated with alcohol withdrawal.

5-HT2C receptors in psychiatric disorders: A review

5-HT2Rs have a different genomic organization from other 5-HT2Rs. 5HT2CR undergoes post-transcriptional pre-mRNA editing generating diversity among RNA transcripts. Selective post-transcriptional editing could be involved in the pathophysiology of psychiatric disorders through impairment in G-protein interactions. Moreover, it may influence the therapeutic response to agents such as atypical antipsychotic drugs. Additionally, 5-HT2CR exhibits alternative splicing. Central serotonergic and dopaminergic systems interact to modulate normal and abnormal behaviors. Thus, 5HT2CR plays a crucial role in psychiatric disorders. 5HT2CR could be a relevant pharmacological target in the treatment of neuropsychiatric disorders. The development of drugs that specifically target 5-HT2C receptors will allow for better understanding of their involvement in the pathophysiology of psychiatric disorders including schizophrenia, anxiety, and depression. Among therapeutic means currently available, most drugs used to treat highly morbid psychiatric diseases interact at least partly with 5-HT2CRs. Pharmacologically, 5HT2CRs, have the ability to generate differentially distinct response signal transduction pathways depending on the type of 5HT2CR agonist. Although this receptor property has been clearly demonstrated, in vitro, the eventual beneficial impact of this property opens new perspectives in the development of agonists that could activate signal transduction pathways leading to better therapeutic efficiency with fewer adverse effects.

Effects of lack of microRNA-34 on the neural circuitry underlying the stress response and anxiety

Stress-related psychiatric disorders, including anxiety, are complex diseases that have genetic, and environmental causes. Stressful experiences increase the release of prefrontal amygdala neurotransmitters, a response that is relevant to cognitive, emotional, and behavioral coping. Moreover, exposure to stress elicits anxiety-like behavior and dendritic remodeling in the amygdala. Members of the miR-34 family have been suggested to regulate synaptic plasticity and neurotransmission processes, which mediate stress-related disorders. Using mice that harbored targeted deletions of all 3 members of the miR-34-family (miR-34-TKO), we evaluated acute stress-induced basolateral amygdala (BLA)-GABAergic and medial prefrontal cortex (mpFC) aminergic outflow by intracerebral in vivo microdialysis. Moreover, we also examined fear conditioning/extinction, stress-induced anxiety, and dendritic remodeling in the BLA of stress-exposed TKO mice.

We found that TKO mice showed resilience to stress-induced anxiety and facilitation in fear extinction. Accordingly, no significant increase was evident in aminergic prefrontal or amygdala GABA release, and no significant acute stress-induced amygdalar dendritic remodeling was observed in TKO mice. Differential GRM7, 5-HT2C, and CRFR1 mRNA expression was noted in the mpFC and BLA between TKO and WT mice. Our data demonstrate that the miR-34 has a critical function in regulating the behavioral and neurochemical response to acute stress and in inducing stress-related amygdala neuroplasticity.

Incubation of cocaine cue reactivity associates with neuroadaptations in the cortical serotonin (5-HT) 5-HT2C receptor (5-HT2CR) system

Intensification of craving elicited by drug-associated cues during abstinence occurs over time in human cocaine users while elevation of cue reactivity ("incubation") is observed in rats exposed to extended forced abstinence from cocaine self-administration. Incubation in rodents has been linked to time-dependent neuronal plasticity in the medial prefrontal cortex (mPFC). We tested the hypothesis that incubation of cue reactivity during abstinence from cocaine self-administration is accompanied by lower potency and/or efficacy of the selective serotonin (5-HT) 5-HT2C​ receptor (5-HT2CR) agonist WAY163909 to suppress cue reactivity and a shift in the subcellular localization profile of the mPFC 5-HT2CR protein. We observed incubation of cue reactivity (measured as lever presses reinforced by the discrete cue complex) between Day 1 and Day 30 of forced abstinence from cocaine relative to sucrose self-administration. Pharmacological and biochemical analyses revealed that the potency of the selective 5-HT2CR agonist WAY163909 to suppress cue reactivity, the expression of synaptosomal 5-HT2CR protein in the mPFC, and the membrane to cytoplasmic expression of the 5-HT2CR in mPFC were lower on Day 30 vs. Day 1 of forced abstinence from cocaine self-administration. Incubation of cue reactivity assessed during forced abstinence from sucrose self-administration did not associate with 5-HT2CR protein expression in the mPFC. Collectively, these outcomes are the first indication that neuroadaptations in the 5-HT2CR system may contribute to incubation of cocaine cue reactivity.

Interaction between CHL1 and serotonin receptor 2c regulates signal transduction and behavior in mice

The serotonergic system plays important roles in multiple functions of the nervous system and its malfunctioning leads to neurological and psychiatric disorders. Here, we show that the cell adhesion molecule close homolog of L1 (CHL1), which has been linked to mental disorders, binds to a peptide stretch in the third intracellular loop of the serotonin 2c (5-HT2c) receptor through its intracellular domain. Moreover, we provide evidence that CHL1 deficiency in mice leads to 5-HT2c-receptor-related reduction in locomotor activity and reactivity to novelty, and that CHL1 regulates signaling pathways triggered by constitutively active isoforms of the 5-HT2c receptor. Furthermore, we found that the 5-HT2c receptor and CHL1 colocalize in striatal and hippocampal GABAergic neurons, and that 5-HT2c receptor phosphorylation and its association with phosphatase and tensin homolog (PTEN) and β-arrestin 2 is regulated by CHL1. Our results demonstrate that CHL1 regulates signal transduction pathways through constitutively active 5-HT2c receptor isoforms, thereby altering 5-HT2c receptor functions and implicating CHL1 as a new modulator of the serotonergic system.

Mice Lacking Serotonin 2C Receptors Have increased Affective Responses to Aversive Stimuli

Although central serotonergic systems are known to influence responses to noxious stimuli, mechanisms underlying serotonergic modulation of pain responses are unclear. We proposed that serotonin 2C receptors (5-HT2CRs), which are expressed within brain regions implicated in sensory and affective responses to pain, contribute to the serotonergic modulation of pain responses. In mice constitutively lacking 5-HT2CRs (2CKO mice) we found normal baseline sensory responses to noxious thermal, mechanical and chemical stimuli. In contrast, 2CKO mice exhibited a selective enhancement of affect-related ultrasonic afterdischarge vocalizations in response to footshock. Enhanced affect-related responses to noxious stimuli were also exhibited by 2CKO mice in a fear-sensitized startle assay. The extent to which a brief series of unconditioned footshocks produced enhancement of acoustic startle responses was markedly increased in 2CKO mice. As mesolimbic dopamine pathways influence affective responses to noxious stimuli, and these pathways are disinhibited in 2CKO mice, we examined the sensitivity of footshock-induced enhancement of startle to dopamine receptor blockade. Systemic administration of the dopamine D2/D3 receptor antagonist raclopride selectively reduced footshock-induced enhancement of startle without influencing baseline acoustic startle responses. We propose that 5-HT2CRs regulate affective behavioral responses to unconditioned aversive stimuli through mechanisms involving the disinhibition of ascending dopaminergic pathways.

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.

The role of 5-HT2C receptors in touchscreen visual reversal learning in the rat: a cross-site study

RATIONALE

Reversal learning requires associative learning and executive functioning to suppress non-adaptive responding. Reversal-learning deficits are observed in e.g. schizophrenia and obsessive-compulsive disorder and implicate neural circuitry including the orbitofrontal cortex (OFC). Serotonergic function has been strongly linked to visual reversal learning in humans and experimental animals but less is known about which receptor subtypes are involved.

OBJECTIVES

The objectives of the study were to test the effects of systemic and intra-OFC 5-HT2C-receptor antagonism on visual reversal learning in rats and assess the psychological mechanisms underlying these effects within novel touchscreen paradigms.

METHODS

In experiments 1-2, we used a novel 3-stimulus task to investigate the effects of 5-HT2C-receptor antagonism through SB 242084 (0.1, 0.5 and 1.0 mg/kg i.p.) cross-site. Experiment 3 assessed the effects of SB 242084 in 2-choice reversal learning. In experiment 4, we validated a novel touchscreen serial visual reversal task suitable for neuropharmacological microinfusions by baclofen-/muscimol-induced OFC inactivation. In experiment 5, we tested the effect of intra-OFC SB 242084 (1.0 or 3.0 μg/side) on performance in this task.

RESULTS

In experiments 1-3, SB 242084 reduced early errors but increased late errors to criterion. In experiment 5, intra-OFC SB 242084 reduced early errors without increasing late errors in a reversal paradigm validated as OFC dependent (experiment 4).

CONCLUSION

Intra-OFC 5-HT2C-receptor antagonism decreases perseveration in novel touchscreen reversal-learning paradigms for the rat. Systemic 5-HT2C-receptor antagonism additionally impairs late learning-a robust effect observed cross-site and potentially linked to impulsivity. These conclusions are discussed in terms of neural mechanisms underlying reversal learning and their relevance to psychiatric disorders.

Sedative-Hypnotic and Receptor Binding Studies of Fermented Marine Organisms

This study was performed to investigate the sedative-hypnotic activity of γ-aminobutyric acid (GABA)-enriched fermented marine organisms (FMO), including sea tangle (FST) and oyster (FO) by Lactobacillus brevis BJ20 (L. brevis BJ20). FST and FO were tested for their binding activity of the GABAA-benzodiazepine and 5-HT2C receptors, which are well-known molecular targets for sleep aids. We also measured the sleep latency and sleep duration during pentobarbital-induced sleep in mice after oral administration of FST and FO. In GABAA and 5-HT2C receptor binding assays, FST displayed an effective concentration-dependent binding affinity to GABAA receptor, similar to the binding affinity to 5-HT2C receptor. FO exhibited higher affinity to 5-HT2C receptor, compared with the GABAA receptor. The oral administration of FST and FO produced a dose-dependent decrease in sleep latency and increase in sleep duration in pentobarbital-induced hypnosis. The data demonstrate that FST and FO possess sedative-hypnotic activity possibly by modulating GABAA and 5-HT2C receptors. We propose that FST and FO might be effective agents for treatment of insomnia.

Serotonergic Regulation of Prefrontal Cortical Circuitries Involved in Cognitive Processing: A Review of Individual 5-HT Receptor Mechanisms and Concerted Effects of 5-HT Receptors Exemplified by the Multimodal Antidepressant Vortioxetine

It has been known for several decades that serotonergic neurotransmission is a key regulator of cognitive function, mood, and sleep. Yet with the relatively recent discoveries of novel serotonin (5-HT) receptor subtypes, as well as an expanding knowledge of their expression level in certain brain regions and localization on certain cell types, their involvement in cognitive processes is still emerging. Of particular interest are cognitive processes impacted in neuropsychiatric and neurodegenerative disorders. The prefrontal cortex (PFC) is critical to normal cognitive processes, including attention, impulsivity, planning, decision-making, working memory, and learning or recall of learned memories. Furthermore, serotonergic dysregulation within the PFC is implicated in many neuropsychiatric disorders associated with prominent symptoms of cognitive dysfunction. Thus, it is important to better understand the overall makeup of serotonergic receptors in the PFC and on which cell types these receptors mediate their actions. In this Review, we focus on 5-HT receptor expression patterns within the PFC and how they influence cognitive behavior and neurotransmission. We further discuss the net effects of vortioxetine, an antidepressant acting through multiple serotonergic targets given the recent findings that vortioxetine improves cognition by modulating multiple neurotransmitter systems.