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-HT2CR in the BLA, but not CeA, has been implicated anxiogenic behaviors and anxiety disorders. Here, we tested the hypothesis that 5-HT2CR in the BLA plays a critical role in CeA plasticity and neuropathic pain behaviors in the rat spinal nerve ligation (SNL) model. Local 5-HT2CR knockdown in the BLA with stereotaxic injection of 5-HT2CR 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-HT2CR knockdown blocked the increase in neuronal activity (increased responsiveness, irregular spike firing, and increased burst activity) in SNL rats. At the synaptic level, 5-HT2CR 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-HT2CR knockdown in control and SNL conditions to a similar degree. The findings can be explained by immunohistochemical data showing increased expression of 5-HT2CR in non-GABAergic BLA cells in SNL rats. The results suggest that increased 5-HT2CR in the BLA contributes to neuropathic-pain-related amygdala plasticity by driving synaptic excitation of CeA neurons. As a rescue strategy, 5-HT2CR 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-HT2C contributes critically to the imbalance between excitatory and inhibitory drive of amygdala output neurons. Local viral vector-mediated 5-HT2CR 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.

Functional status of the serotonin 5-HT2C receptor (5-HT2CR) drives interlocked phenotypes that precipitate relapse-like behaviors in cocaine dependence.

Relapse vulnerability in cocaine dependence is rooted in genetic and environmental determinants, and propelled by both impulsivity and the responsivity to cocaine-linked cues ('cue reactivity'). The serotonin (5-hydroxytryptamine, 5-HT) 5-HT2C receptor (5-HT2CR) within the medial prefrontal cortex (mPFC) is uniquely poised to serve as a strategic nexus to mechanistically control these behaviors. The 5-HT2CR functional capacity is regulated by a number of factors including availability of active membrane receptor pools, the composition of the 5-HT2CR macromolecular protein complex, and editing of the 5-HT2CR pre-mRNA. The one-choice serial reaction time (1-CSRT) task was used to identify impulsive action phenotypes in an outbred rat population before cocaine self-administration and assessment of cue reactivity in the form of lever presses reinforced by the cocaine-associated discrete cue complex during forced abstinence. The 1-CSRT task reliably and reproducibly identified high impulsive (HI) and low impulsive (LI) action phenotypes; HI action predicted high cue reactivity. Lower cortical 5-HT2CR membrane protein levels concomitant with higher levels of 5-HT2CR:postsynaptic density 95 complex distinguished HI rats from LI rats. The frequency of edited 5-HT2CR mRNA variants was elevated with the prediction that the protein population in HI rats favors those isoforms linked to reduced signaling capacity. Genetic loss of the mPFC 5-HT2CR induced aggregate impulsive action/cue reactivity, suggesting that depressed cortical 5-HT2CR tone confers vulnerability to these interlocked behaviors. Thus, impulsive action and cue reactivity appear to neuromechanistically overlap in rodents, with the 5-HT2CR functional status acting as a neural rheostat to regulate, in part, the intersection between these vulnerability behaviors.

Serotonin 2C receptors in pro-opiomelanocortin neurons regulate energy and glucose homeostasis.

Energy and glucose homeostasis are regulated by central serotonin 2C receptors. These receptors are attractive pharmacological targets for the treatment of obesity; however, the identity of the serotonin 2C receptor-expressing neurons that mediate the effects of serotonin and serotonin 2C receptor agonists on energy and glucose homeostasis are unknown. Here, we show that mice lacking serotonin 2C receptors (Htr2c) specifically in pro-opiomelanocortin (POMC) neurons had normal body weight but developed glucoregulatory defects including hyperinsulinemia, hyperglucagonemia, hyperglycemia, and insulin resistance. Moreover, these mice did not show anorectic responses to serotonergic agents that suppress appetite and developed hyperphagia and obesity when they were fed a high-fat/high-sugar diet. A requirement of serotonin 2C receptors in POMC neurons for the maintenance of normal energy and glucose homeostasis was further demonstrated when Htr2c loss was induced in POMC neurons in adult mice using a tamoxifen-inducible POMC-cre system. These data demonstrate that serotonin 2C receptor-expressing POMC neurons are required to control energy and glucose homeostasis and implicate POMC neurons as the target for the effect of serotonin 2C receptor agonists on weight-loss induction and improved glycemic control.

Impact of serotonin (5-HT)2C receptors on executive control processes.

Although the serotonin (5-hydroxytryptamine, 5-HT) neurotransmitter system has been implicated in modulating executive control processes such as attention, response inhibition, and behavioral flexibility, the contributions of particular serotonin receptors remain unclear. Here, using operant-based behavioral paradigms, we demonstrate that mice with genetically ablated 5-HT2C receptors (2CKO mice) display deficits in executive functions. 2CKO mice were impaired in the acquisition of a visuospatial attention task as assessed in the 5-choice serial reaction time task (5-CSRTT). In this task, 2CKO mice exhibited marked impairment of attentional processes, with normal response inhibition. We assessed dynamic changes in neurotransmitter levels within the nucleus accumbens (NAc) by in vivo microdialysis in task-performing animals. Extracellular dopamine concentrations were elevated in the NAc of 2CKO mice during task performance, indicating that 5-HT2C receptors impact dopamine homeostasis during a visuospatial attention task. These findings raise the possibility that disinhibition of mesolimbic dopamine pathways contributes to impaired attention and perturbed task performance in 2CKO mice. Additionally, in a spatial reversal learning task, 2CKO mice failed to improve their performance over a series of reversals, indicating that intact 5-HT2C receptor signaling is required to accurately respond to repeated changes in reward contingencies. In contrast to the 2CKO phenotype in the 5-CSRTT, wild-type mice treated with the 5-HT2C receptor antagonist SB242084 exhibited diminished response inhibition, suggesting differing effects of acute pharmacological blockade and constitutive loss of 5-HT2C receptor activity. Altogether, these findings provide insights into the serotonergic regulation of executive control processes and suggest that impaired 5-HT2C receptor signaling during development may predispose to executive function disorders.

A serotonin and melanocortin circuit mediates D-fenfluramine anorexia.

D-Fenfluramine (D-Fen) increases serotonin (5-HT) content in the synaptic cleft and exerts anorexigenic effects in animals and humans. However, the neural circuits that mediate these effects are not fully identified. To address this issue, we assessed the efficacy of D-Fen-induced hypophagia in mouse models with manipulations of several genes in selective populations of neurons. Expectedly, we found that global deletion of 5-HT 2C receptors (5-HT(2C)Rs) significantly attenuated D-Fen-induced anorexia. These anorexigenic effects were restored in mice with 5-HT(2C)Rs expressed only in pro-opiomelanocortin (POMC) neurons. Further, we found that deletion of melanocortin 4 receptors (MC4Rs), a downstream target of POMC neurons, abolished anorexigenic effects of D-Fen. Reexpression of MC4Rs only in SIM1 neurons in the hypothalamic paraventricular nucleus and neurons in the amygdala was sufficient to restore the hypophagic property of D-Fen. Thus, our results identify a neurochemically defined neural circuit through which D-Fen influences appetite and thereby indicate that this 5-HT(2C)R/POMC-MC4R/SIM1 circuit may yield a more refined target to exploit for weight loss.

No link of serotonin 2C receptor editing to serotonin transporter genotype.

RNA editing is a post-transcriptional process, which has the potential to alter the function of encoded proteins. In particular, serotonin 2C receptor (5-HT2cR) mRNA editing can produce 24 protein isoforms of varying functionality. Rodent studies have shown that 5-HT2cR editing is dynamically modulated in response to environmental challenges. Basal extracellular serotonin, which is strongly influenced by serotonin transporter (SERT), was proposed as a potential trigger for this modulation; however, the data remain inconclusive. Here, 5-HT2cR editing is evaluated in SERT mutant versus wild-type rats, and in humans with different SERT genotypes. Our findings argue against the hypothesis that 5-HT2cR editing efficiency is regulated by extracellular serotonin levels.

Genetic and pharmacological evidence that 5-HT2C receptor activation, but not inhibition, affects motivation to feed under a progressive ratio schedule of reinforcement.

Previous work showed that 5-HT(2C) receptor agonists reduce cocaine self-administration on a progressive ratio (PR) schedule of reinforcement, whereas a 5-HT(2C) receptor antagonist enhances responding for cocaine. The present experiments examined the effects of Ro60-0175 (5-HT(2C) agonist) and SB242084 (5-HT(2C) receptor antagonist) in rats on responding for food on a PR schedule; responding was also determined in mice lacking functional 5-HT(2C) receptors. In food-restricted rats, lever pressing reinforced by regular food pellets or sucrose pellets was reduced by Ro60-0175. This effect was blocked by SB242084, and was absent in mice lacking functional 5-HT(2C) receptors. A number of studies examined the effects of SB242084 on responding for food under a variety of conditions. These included manipulation of food type (regular pellets versus sucrose pellets), nutritional status of the animals (food restriction versus no restriction), and rate of progression of the increase in ratio requirements on the PR schedule. In all cases there was no evidence of enhanced responding for food by SB242084. Mice lacking functional 5-HT(2C) receptors did not differ from wildtype mice in responding for food in either food-restricted or non-restricted states. The effects of Ro60-0175 are consistent with its effects on food consumption and motivation to self-administer cocaine. Unlike their effects on cocaine self-administration, pharmacological blockade of 5-HT(2C) receptors, and genetic disruption of 5-HT(2C) receptor function do not alter the motivation to respond for food. Because the 5-HT(2C) receptor exerts a modulatory effect on dopamine function, the differential effects of reduced 5-HT(2C) receptor mediated transmission on responding for food versus cocaine may relate to a differential role of this neurotransmitter in mediating these two behaviours.

The serotonin 2C receptor potently modulates the head-twitch response in mice induced by a phenethylamine hallucinogen.

RATIONALE: Hallucinogenic serotonin 2A (5-HT(2A)) receptor partial agonists, such as (+ or -)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI), induce a frontal cortex-dependent head-twitch response (HTR) in rodents, a behavioral proxy of a hallucinogenic response that is blocked by 5-HT(2A) receptor antagonists. In addition to 5-HT(2A) receptors, DOI and most other serotonin-like hallucinogens have high affinity and potency as partial agonists at 5-HT(2C) receptors. OBJECTIVES: We tested for involvement of 5-HT(2C) receptors in the HTR induced by DOI. RESULTS: Comparison of 5-HT(2C) receptor knockout and wild-type littermates revealed an approximately 50% reduction in DOI-induced HTR in knockout mice. Also, pretreatment with either the 5-HT(2C) receptor antagonist SB206553 or SB242084 eradicated a twofold difference in DOI-induced HTR between the standard inbred mouse strains C57BL/6J and DBA/2J, and decreased the DOI-induced HTR by at least 50% in both strains. None of several measures of 5-HT(2A) receptors in frontal cortex explained the strain difference, including 5-HT(2A) receptor density, Galpha(q) or Galpha(i/o) protein levels, phospholipase C activity, or DOI-induced expression of Egr1 and Egr2. 5-HT(2C) receptor density in the brains of C57BL/6J and DBA/2J was also equivalent, suggesting that 5-HT(2C) receptor-mediated intracellular signaling or other physiological modulators of the HTR may explain the strain difference in response to DOI. CONCLUSIONS: We conclude that the HTR to DOI in mice is strongly modulated by 5-HT(2C) receptor activity. This novel finding invites reassessment of hallucinogenic mechanisms involving 5-HT(2) receptors.

Impact of serotonin 2C receptor null mutation on physiology and behavior associated with nigrostriatal dopamine pathway function.

The impact of serotonergic neurotransmission on brain dopaminergic pathways has substantial relevance to many neuropsychiatric disorders. A particularly prominent role has been ascribed to the inhibitory effects of serotonin 2C receptor (5-HT(2C)R) activation on physiology and behavior mediated by the mesolimbic dopaminergic pathway, particularly in the terminal region of the nucleus accumbens. The influence of this receptor subtype on functions mediated by the nigrostriatal dopaminergic pathway is less clear. Here we report that a null mutation eliminating expression of 5-HT(2C)Rs produces marked alterations in the activity and functional output of this pathway. 5-HT(2C)R mutant mice displayed increased activity of substantia nigra pars compacta (SNc) dopaminergic neurons, elevated baseline extracellular dopamine concentrations in the dorsal striatum (DSt), alterations in grooming behavior, and enhanced sensitivity to the stereotypic behavioral effects of d-amphetamine and GBR 12909. These psychostimulant responses occurred in the absence of phenotypic differences in drug-induced extracellular dopamine concentration, suggesting a phenotypic alteration in behavioral responses to released dopamine. This was further suggested by enhanced behavioral responses of mutant mice to the D(1) receptor agonist SKF 81297. Differences in DSt D(1) or D(2) receptor expression were not found, nor were differences in medium spiny neuron firing patterns or intrinsic membrane properties following dopamine stimulation. We conclude that 5-HT(2C)Rs regulate nigrostriatal dopaminergic activity and function both at SNc dopaminergic neurons and at a locus downstream of the DSt.

Loss of cholecystokinin and glucagon-like peptide-1-induced satiation in mice lacking serotonin 2C receptors.

To investigate the role of serotonin 2C receptors (2CR), which are expressed only in the central nervous system, in the satiating actions of the gut peptides CCK and glucagon-like peptide 1 (GLP-1), we examined 1) the effect of null mutations of serotonin 2CR (2CR KO) on the eating-inhibitory potencies of dark-onset intraperitoneal injections of 0.9, 1.7, or 3.5 nmol/kg (1, 2, or 4 microg/kg) CCK and 100, 200, and 400 nmol/kg (33, 66, or 132 microg/kg) GLP-1, and 2) the effects of intraperitoneal injections of 1.7 nmol//kg CCK and 100 nmol/kg GLP-1 on neuronal activation in the brain, as measured by c-Fos expression. All CCK and GLP-1 doses decreased 30-min food intake in wild-type (WT) mice, but none of them did in 2CR KO mice. CCK increased the number of cells expressing c-Fos in the nucleus tractus solitarii (NTS) of WT, but not 2CR KO mice. CCK induced similar degrees of c-Fos expression in the paraventricular (PVN) and arcuate (Arc) nuclei of the hypothalamus of both genotypes. GLP-1, on the other hand, increased c-Fos expression similarly in the NTS of both genotypes and increased c-Fos expression more in the PVN and Arc of 2CR KO mice, but not WT mice. These results indicate that serotonin signaling via serotonin 2CR is necessary for the full satiating effects of CCK and GLP-1. In addition, they suggest that the satiating effects of the two peptides are mediated by different neural mechanisms.

Pharmacological, but not genetic, disruptions in 5-HT(2C) receptor function attenuate LPS anorexia in mice.

Peripheral administration of bacterial lipopolysaccharide (LPS) elicits anorexia in several species, including rats and mice. There is strong evidence that antagonism of serotonergic activity at 2C receptors (5-HT(2C)R) attenuates LPS anorexia in rats. Here we used pharmacological and genetic approaches to examine the role of the 5-HT(2C)R in LPS anorexia in mice. In Experiment 1, SB 242084, a potent and selective 5-HT(2C) antagonist (0.3 mg/kg) was injected intraperitoneally 15 min before intraperitoneal LPS (2 microg/kg) injections just prior to dark onset in c57BL/6 mice. Food intake was recorded 1, 2 and 4 h after LPS administration. In Experiment 2, we recorded 2, 4 and 24 h food intake following dark onset intraperitoneal LPS (0.125, 0.25, 0.5, 1 and 2 microg/kg) injections in mice with a genetic deletion of 5-HT(2C)R and their WT controls. Our pharmacological results suggest that at least part of the anorexia following peripheral LPS administration is mediated by an increase in 5-HT-ergic activity at the 5-HT(2C)R. Our genetic data, in contrast, suggest that 5-HT(2C)R is not a necessary part of LPS anorexia.

mCPP-induced hyperactivity in 5-HT2C receptor mutant mice is mediated by activation of multiple 5-HT receptor subtypes.

The serotonin receptor agonist mCPP induces hyperlocomotion in 5-HT2C receptor knockout (KO) mice or in the presence of a 5-HT2C receptor antagonist. In the present group of experiments, we evaluate the role of 5-HT1A, 5-HT1B and 5-HT2A receptors in mCPP-induced hyperactivity in 5-HT2C KO mice. We also assess the ability of agonists at these receptors to induce hyperactivity in wildtype (WT) mice pre-treated with a selective 5-HT2C receptor antagonist. As previously reported, mCPP (3 mg/kg) induced hyperactivity in 5-HT2C KO mice. A combination of the 5-HT1B receptor agonist CP-94,253 (20 mg/kg) and the 5-HT1A receptor agonist 8-OH-DPAT (0.5 mg/kg) induced marked hyperactivity in WT but not in 5-HT2C KO mice, nor in mice treated with the selective 5-HT2C receptor antagonist, SB 242084 (1.5 mg/kg). Neither CP-94,253 nor 8-OH-DPAT had any intrinsic effect on locomotion in WTs. mCPP-induced hyperactivity was attenuated in 5-HT2C KO mice by the 5-HT1B receptor antagonist SB 224289 (2.5 mg/kg), and the 5-HT2A receptor antagonists ketanserin (0.3 mg/kg) and M100907 (0.01 mg/kg) but not by the 5-HT1A receptor antagonist WAY 100635 (1 mg/kg). The 5-HT(2A/2B/2C) receptor agonist, Ro 60-0175 (3 mg/kg), induced a modest increase in locomotor activity in WT mice pre-treated with SB 242084. However, the combination of Ro 60-0175 with CP-94,253 induced a substantial increase in activity in 5-HT2C KO mice, an effect comparable to mCPP-induced hyperactivity. Thus, joint activation of 5-HT1A and 5-HT1B receptors stimulates locomotion in WT mice but this response is dependent on a functional 5-HT2C receptor population and hence is absent in 5-HT2C KO mice. By contrast, mCPP-induced hyperactivity depends on the inactivation of a separate 5-HT2C receptor population and is mediated by 5-HT2A and 5-HT1B receptor activation.

The 5-HT2C receptor as a target for mood disorders.

The serotonin (5-hydroxytryptamine2C [5-HT2C]) receptor is one of the 5-HT receptors with a G-protein-coupled intracellular signalling pathway. A large number of compounds showing antidepressive, antipsychotic and anxiolytic properties, and affecting sleep patterns, feeding behaviour and neuroendocrine functions, target this subtype of receptor. The potential use of 5-HT2C receptor ligands in psychiatry has been suggested as a result of a number of observations from animal and in vitro experiments. The results of studies performed suggest that some of the therapeutic effects of the selective serotonin re-uptake inhibitors (SSRIs) may be mediated, at least in part, by the 5-HT2C receptor. The long-term downregulation of 5-HT2C receptors is associated with their interaction with some SSRIs and may lead to disinhibition of the mesolimbic dopamine system, which may be partly responsible for their antidepressant action. Nevertheless, current evidence does not allow complete definition of 5-HT2C receptor functions and properties. Concerning genetics, there are no unequivocal results for the involvement of polymorphisms of the 5-HT2C receptor, and no studies on their association with an antidepressant response have ever been performed. This paper reviews some of the studies on this 5-HT receptor subtype, focusing on its possible importance as a target for mood disorder therapy.