Raphe glucose-sensing serotonergic neurons stimulate KNDy neurons to enhance LH pulses via 5HT2CR: rat and goat studies.

Dysfunction of central serotonergic neurons is known to cause depressive disorders in humans, who often show reproductive and/or glucose metabolism disorders. This study examined whether dorsal raphe (DR) serotonergic neurons sense high glucose availability to upregulate reproductive function via activating hypothalamic arcuate (ARC) kisspeptin neurons (= KNDy neurons), a dominant stimulator of gonadotropin-releasing hormone (GnRH)/gonadotropin pulses, using female rats and goats. RNA-seq and histological analysis revealed that stimulatory serotonin-2C receptor (5HT2CR) was mainly expressed in the KNDy neurons in female rats. The serotonergic reuptake inhibitor administration into the mediobasal hypothalamus (MBH), including the ARC, significantly blocked glucoprivic suppression of luteinizing hormone (LH) pulses and hyperglycemia induced by intravenous 2-deoxy-D-glucose (2DG) administration in female rats. A local infusion of glucose into the DR significantly increased in vivo serotonin release in the MBH and partly restored LH pulses and hyperglycemia in the 2DG-treated female rats. Furthermore, central administration of serotonin or a 5HT2CR agonist immediately evoked GnRH pulse generator activity, and central 5HT2CR antagonism blocked the serotonin-induced facilitation of GnRH pulse generator activity in ovariectomized goats. These results suggest that DR serotonergic neurons sense high glucose availability to reduce gluconeogenesis and upregulate reproductive function by activating GnRH/LH pulse generator activity in mammals.

Ethanol deprivation and central 5-HT deficiency differentially affect the mRNA editing of the 5-HT2C receptor in the mouse brain.

BACKGROUND: Serotonin (5-HT) 5-HT2C receptor mRNA editing (at five sites, A-E), implicated in neuropsychiatric disorders, including clinical depression, remains unexplored during alcohol abstinence-often accompanied by depressive symptoms. METHODS: We used deep sequencing to investigate 5-HT2C receptor editing in mice during early ethanol deprivation following prolonged alcohol exposure and mice lacking tryptophan hydroxylase (TPH)2, a key enzyme in central 5-HT production. We also examined Tph2 expression in ethanol-deprived animals using quantitative real-time PCR (qPCR). RESULTS: Cessation from chronic 10% ethanol exposure in a two-bottle choice paradigm enhanced immobility time and decreased latency in the forced swim test (FST), indicating a depression-like phenotype. In the hippocampus, ethanol-deprived "high ethanol-drinking" mice displayed reduced Tph2 expression, elevated 5-HT2C receptor editing efficiency, and decreased frequency of the D mRNA variant, encoding the less-edited INV protein isoform. Tph2-/- mice showed attenuated receptor editing in the hippocampus and elevated frequency of non-edited None and D variants. In the prefrontal cortex, Tph2 deficiency increased receptor mRNA editing at site D and reduced the frequency of AB transcript, predicting a reduction in the corresponding partially edited VNI isoform. CONCLUSIONS: Our findings reveal differential effects of 5-HT depletion and ethanol cessation on 5-HT2C receptor editing. Central 5-HT depletion attenuated editing in the prefrontal cortex and the hippocampus, whereas ethanol deprivation, coinciding with reduced Tph2 expression in the hippocampus, enhanced receptor editing efficiency specifically in this brain region. This study highlights the interplay between 5-HT synthesis, ethanol cessation, and 5-HT2C receptor editing, providing potential mechanism underlying increased ethanol consumption and deprivation.

Discovery of Novel Oleamide Analogues as Brain-Penetrant Positive Allosteric Serotonin 5-HT2C Receptor and Dual 5-HT2C/5-HT2A Receptor Modulators.

The serotonin 5-HT2A receptor (5-HT2AR) and 5-HT2CR localize to the brain and share overlapping signal transduction facets that contribute to their roles in cognition, mood, learning, and memory. Achieving selective targeting of these receptors is challenged by the similarity in their 5-HT orthosteric binding pockets. A fragment-based discovery approach was employed to design and synthesize novel oleamide analogues as selective 5-HT2CR or dual 5-HT2CR/5-HT2AR positive allosteric modulators (PAMs). Compound 13 (JPC0323) exhibited on-target properties, acceptable plasma exposure and brain penetration, as well as negligible displacement to orthosteric sites of ∼50 GPCRs and transporters. Furthermore, compound 13 suppressed novelty-induced locomotor activity in a 5-HT2CR-dependent manner, suggesting 5-HT2CR PAM, but not 5-HT2AR, activity at the level of the whole organism at the employed doses of 13. We discovered new selective 5-HT2CR PAMs and first-in-class 5-HT2CR/5-HT2AR dual PAMs that broaden the pharmacological toolbox to explore the biology of these vital receptors.

Antidepressant effect of 4-Butyl-alpha-agarofuran via HPA axis and serotonin system.

Depression is a leading cause of disability worldwide and the psychiatric diagnosis most commonly associated with suicide. 4-Butyl-alpha-agarofuran (AF-5), a derivative of agarwood furan, is currently in phase III clinical trials for generalized anxiety disorder. Herein, we explored the antidepressant effect and its possible neurobiological mechanisms in animal models. In present study, AF-5 administration markedly decreased the immobility time in mouse forced swim test and tail suspension test. In the sub-chronic reserpine-induced depressive rats, AF-5 treatment markedly increased the rectal temperature and decreased the immobility time of model rats. In addition, chronic AF-5 treatment markedly reversed the depressive-like behaviors in chronic unpredictable mild stress (CUMS) rats by reducing immobility time of forced swim test. Single treatment with AF-5 also potentiated the mouse head-twitch response induced by 5-hydroxytryptophan (5-HTP, a metabolic precursor to serotonin), and antagonized the ptosis and motor ability triggered by reserpine. However, AF-5 had no effect on yohimbine toxicity in mice. These results indicated that acute treatment with AF-5 produced serotonergic, but not noradrenergic activation. Furthermore, AF-5 reduced adrenocorticotropic hormone (ACTH) level in serum and normalized the neurotransmitter changes, including the decreased serotonin (5-HT) in hippocampus of CUMS rats. Moreover, AF-5 affected the expressions of CRFR1 and 5-HT2C receptor in CUMS rats. These findings confirm the antidepressant effect of AF-5 in animal models, which may be primarily related to CRFR1 and 5-HT2C receptor. AF-5 appears to be promising as a novel dual target drug for depression treatment.

Pharmacologic Activity of Substituted Tryptamines at 5-Hydroxytryptamine (5-HT)2A Receptor (5-HT2AR), 5-HT2CR, 5-HT1AR, and Serotonin Transporter.

Novel psychoactive substances, including synthetic substituted tryptamines, represent a potential public health threat. Additionally, some substituted tryptamines are being studied under medical guidance as potential treatments of psychiatric disorders. Characterizing the basic pharmacology of substituted tryptamines will aid in understanding differences in potential for harm or therapeutic use. Using human embryonic kidney cells stably expressing 5-hydroxytryptamine (5-HT)1A, 5-HT2A, and 5-HT2C receptors (5-HT1AR, 5-HT2AR, and 5HT2CR, respectively) or the serotonin transporter (SERT), we measured affinities, potencies and efficacies of 21 substituted tryptamines. With the exception of two 4-acetoxy compounds, substituted tryptamines exhibited affinities and potencies less than one micromolar at the 5-HT2AR, the primary target for psychedelic effects. In comparison, half or more exhibited low affinities/potencies at 5-HT2CR, 5-HT1AR, and SERT. Sorting by the ratio of 5-HT2A to 5-HT2C, 5-HT1A, or SERT affinity revealed chemical determinants of selectivity. We found that although 4-substituted compounds exhibited affinities that ranged across a factor of 100, they largely exhibited high selectivity for 5-HT2ARs versus 5-HT1ARs and 5-HT2CRs. 5-substituted compounds exhibited high affinities for 5-HT1ARs, low affinities for 5-HT2CRs, and a range of affinities for 5-HT2ARs, resulting in selectivity for 5-HT2ARs versus 5-HT2CRs but not versus 5-HT1ARs. Additionally, a number of psychedelics bound to SERT, with non-ring-substituted tryptamines most consistently exhibiting binding. Interestingly, substituted tryptamines and known psychedelic standards exhibited a broad range of efficacies, which were lower as a class at 5-HT2ARs compared with 5-HT2CRs and 5-HT1ARs. Conversely, coupling efficiency/amplification ratio was highest at 5-HT2ARs in comparison with 5-HT2CRs and 5-HT1ARs. SIGNIFICANCE STATEMENT: Synthetic substituted tryptamines represent both potential public health threats and potential treatments of psychiatric disorders. The substituted tryptamines tested differed in affinities, potencies, and efficacies at 5-hydroxytryptamine (5-HT)2A, 5-HT2C, and 5HT1A receptors and the serotonin transporter (SERT). Several compounds were highly selective for and coupled very efficiently downstream of 5-HT2A versus 5-HT1A and 5-HT2C receptors, and some bound SERT. This basic pharmacology of substituted tryptamines helps us understand the pharmacologic basis of their potential for harm and as therapeutic agents.

Amyloid ß oligomers disrupt piriform cortical output via a serotonergic pathway.

Although olfactory deficits have been found in patients with early-stage Alzheimer's disease (AD), the underlying mechanisms remain unclear. Here we investigated whether and how human amyloid ß (Aß) oligomers affect neural activity in the piriform cortex (PC) slices of adult mice. We found that oligomeric Aß1-42 decreased the excitability of pyramidal neurons in the anterior PC. The effect was not blocked by glutamate or GABAA receptor antagonists, suggesting that Aß1-42-induced hypoactivity is independent of glutamatergic and GABAergic transmission. Interestingly, the hypoexcitability was occluded by serotonin (5-HT) and blocked by antagonists of 5-HT2C receptors, phospholipase C (PLC), and calcium-activated potassium (BK) channels. Furthermore, Aß1-42 oligomers failed to increase K+-channel currents in the presence of a BK channel blocker. Finally, 5-HT2C receptor antagonist improved olfactory memory and odor discrimination in APP/PS1 mice. The above data indicate that Aß disrupts olfactory information output from the PC via the 5-HT-5-HT2C receptor-PLC-BK channel pathway. This study reveals that serotonergic modulation is a potential novel therapeutic target for olfactory damage in AD.

Constitutive 5-HT2C receptor knock-out facilitates fear extinction through altered activity of a dorsal raphe-bed nucleus of the stria terminalis pathway.

Serotonin 2C receptors (5-HT2CRs) are widely distributed throughout the brain and are strongly implicated in the pathophysiology of anxiety disorders such as post-traumatic stress disorder (PTSD). Although in recent years, a considerable amount of evidence supports 5-HT2CRs facilitating effect on anxiety behavior, the involvement in learned fear responses and fear extinction is rather unexplored. Here, we used a 5-HT2CR knock-out mouse line (2CKO) to gain new insights into the involvement of 5-HT2CRs in the neuronal fear circuitry. Using a cued fear conditioning paradigm, our results revealed that global loss of 5-HT2CRs exclusively accelerates fear extinction, without affecting fear acquisition and fear expression. To investigate the neuronal substrates underlying the extinction enhancing effect, we mapped the immediate-early gene product cFos, a marker for neuronal activity, in the dorsal raphe nucleus (DRN), amygdala and bed nucleus of the stria terminalis (BNST). Surprisingly, besides extinction-associated changes, our results revealed alterations in neuronal activity even under basal home cage conditions in specific subregions of the DRN and the BNST in 2CKO mice. Neuronal activity in the dorsal BNST was shifted in an extinction-supporting direction due to 5-HT2CR knock-out. Finally, the assessment of DRN-BNST connectivity using antero- and retrograde tracing techniques uncovered a discrete serotonergic pathway projecting from the most caudal subregion of the DRN (DRC) to the anterodorsal portion of the BNST (BNSTad). This serotonergic DRC-BNSTad pathway showed increased neuronal activity in 2CKO mice. Thus, our results provide new insights for the fear extinction network by revealing a specific serotonergic DRC-BNSTad pathway underlying a 5-HT2CR-sensitive mechanism with high significance in the treatment of PTSD.

Circulating total and intact GDF-15 levels are not altered in response to weight loss induced by liraglutide or lorcaserin treatment in humans with obesity.

BACKGROUND: Growth differentiation factor 15 (GDF-15) is a stress-response cytokine proposed to be associated with body weight regulation. AIMS: The primary aim was to investigate changes of circulating intact GDF-15 (wildtype, non-carrier of the rs1058587 polymorphism coding for the H2O2D mutation) and total GDF-15 (measured irrespective of the mutation) in response to liraglutide (GLP-1 receptor agonist) and lorcaserin (5-HT2C receptor agonist), two pharmacologic agents that induce food intake and weight reduction. In addition, we perform exploratory correlations of total and intact GDF-15 with clinical, hormonal and metabolo-lipidomic parameters in humans with obesity. MATERIALS AND METHODS: We utilized two studies: 1) Study 1, a randomized, double-blinded, cross-over trial of liraglutide and placebo administration for 5 weeks in subjects with obesity (n = 20; BMI = 35.6 ± 5.9 kg/m2), in escalating doses starting at 0.6 mg/day on week 1 and increased every week, up to the highest dose of 3.0 mg/day during week 5. b) Study 2, a randomized, double-blinded trial of lorcaserin 10 mg twice daily, or placebo for 12-weeks in humans with obesity (n = 34 BMI = 37.4 ± 6.1 kg/m2). Total and intact GDF-15 levels were measured with novel enzyme-linked immunosorbent assays and the metabolomics and lipidomics analysis was performed with nuclear magnetic resonance spectroscopy. RESULTS: Total and intact GDF-15 were positively correlated with diabetes risk index and trimethylamine N-oxide and negatively with eGFR. Despite significant changes in body weight, total and intact GDF-15 were not altered in response to liraglutide or lorcaserin treatment in subjects with obesity. CONCLUSIONS: Total and intact GDF-15 levels are not altered in response to liraglutide or lorcaserin therapy and are thus not directly involved in the metabolic feedback loop pathways downstream of GLP1 or 5-HT2C receptor agonists. Since neither total nor intact GDF-15 levels were altered in response to weight loss, future studies are needed to elucidate the pathways activated by GDF-15 in humans and its role, if any, in body weight regulation and energy homeostasis.

Agomelatine rescues lipopolysaccharide-induced neural injury and depression-like behaviors via suppression of the Gαi-2-PKA-ASK1 signaling pathway.

BACKGROUND: Agomelatine has been shown to be effective in the treatment of depression, but the molecular mechanisms underlying its antidepressant effects have yet to be elucidated. Identification of these molecular mechanisms would not only offer new insights into the basis for depression but also provide the foundation for the development of novel treatments for this disorder. METHODS: Intraperitoneal injection of LPS was used to induce depression-like behaviors in rats. The interactions of the 5-HT2C reporter and Gαi-2 were verified by immunoprecipitation or immunofluorescence assay. Inflammatory related proteins, autophagy related proteins and apoptosis markers were verified by immunoblotting or immunofluorescence assay. Finally, electron microscopy analysis was used to observe the synapse and ultrastructural pathology. RESULTS: Here, we found that the capacity for agomelatine to ameliorate depression and anxiety in a lipopolysaccharide (LPS)-induced rat model of depression was associated with an alleviation of neuroinflammation, abnormal autophagy and neuronal apoptosis as well as the promotion of neurogenesis in the hippocampal dentate gyrus (DG) region of these rats. We also found that the 5-HT2C receptor is coupled with G alphai (2) (Gαi-2) protein within hippocampal neurons and, agomelatine, acting as a 5-HT2C receptor antagonist, can up-regulate activity of the Gαi-2-cAMP-PKA pathway. Such events then suppress activation of the apoptosis signal-regulating kinase 1 (ASK1) pathway, a member of the mitogen-activated protein kinase (MAPK) family involved in pathological processes of many diseases. CONCLUSION: Taken together, these results suggest that agomelatine plays a neuroprotective role in regulating neuroinflammation, autophagy disorder and apoptosis in this LPS-induced rat model of depression, effects which are associated with the display of antidepressant behaviors. These findings provide evidence for some of the potential mechanisms for the antidepressant effects of agomelatine.

Serotonin 5-HT2C receptor knockout in mice attenuates fear responses in contextual or cued but not compound context-cue fear conditioning.

Previous findings have proposed that drugs targeting 5-HT2C receptors could be promising candidates in the treatment of trauma- and stress-related disorders. However, the reduction of conditioned freezing observed in 5-HT2C receptor knock-out (KO) mice in previous studies could alternatively be accounted for by increased locomotor activity. To neutralize the confound of individual differences in locomotor activity, we measured a ratio of fear responses during versus before the presentation of a conditioned stimulus previously paired with a footshock (as a fear measure) by utilizing a conditioned licking suppression paradigm. We first confirmed that 5-HT2C receptor gene KO attenuated fear responses to distinct types of single conditioned stimuli (context or tone) independently of locomotor activity. We then assessed the effects of 5-HT2C receptor gene KO on compound fear responses by examining mice that were jointly conditioned to a context and a tone and later re-exposed separately to each. We found that separate re-exposure to individual components of a complex fear memory (i.e., context and tone) failed to elicit contextual fear extinction in both 5-HT2C receptor gene KO and wild-type mice, and also abolished differences between genotypes in tone-cued fear extinction. This study delineates a previously overlooked role of 5-HT2C receptors in conditioned fear responses, and invites caution in the future assessment of molecular targets and candidate therapies for the treatment of PTSD.

The Regulatory Role of the Central and Peripheral Serotonin Network on Feeding Signals in Metabolic Diseases.

Central and peripheral serotonin (5-hydroxytryptamine, 5-HT) regulate feeding signals for energy metabolism. Disruption of central 5-HT signaling via 5-HT2C receptors (5-HT2CRs) induces leptin-independent hyperphagia in mice, leading to late-onset obesity, insulin resistance, and impaired glucose tolerance. 5-HT2CR mutant mice are more responsive than wild-type mice to a high-fat diet, exhibiting earlier-onset obesity and type 2 diabetes. High-fat and high-carbohydrate diets increase plasma 5-HT and fibroblast growth factor-21 (FGF21) levels. Plasma 5-HT and FGF21 levels are increased in rodents and humans with obesity, type 2 diabetes, and non-alcohol fatty liver diseases (NAFLD). The increases in plasma FGF21 and hepatic FGF21 expression precede hyperinsulinemia, insulin resistance, hyperglycemia, and weight gain in mice fed a high-fat diet. Nutritional, pharmacologic, or genetic inhibition of peripheral 5-HT synthesis via tryptophan hydroxylase 1 (Tph1) decreases hepatic FGF21 expression and plasma FGF21 levels in mice. Thus, perturbing central 5-HT signaling via 5-HT2CRs alters feeding behavior. Increased energy intake via a high-fat diet and/or high-carbohydrate diet can upregulate gut-derived 5-HT synthesis via Tph1. Peripheral 5-HT upregulates hepatic FGF21 expression and plasma FGF21 levels, leading to metabolic diseases such as obesity, insulin resistance, type 2 diabetes, and NAFLD. The 5-HT network in the brain-gut-liver axis regulates feeding signals and may be involved in the development and/or prevention of metabolic diseases.

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-HT2C receptors (5-HT2CR) expressed in the dorsal periaqueductal gray (dPAG). Here, we investigated the presence of 5-HT3A receptors (5-HT3AR) in the dPAG, and the interplay between 5-HT2CR and 5-HT3AR in the dPAG in mediating anxiety-like behavior in mice. We found that 5-HT3AR is expressed in the dPAG and the blockade of these receptors using intra-dPAG infusion of ondansetron (5-HT3AR antagonist; 3.0 nmol) induced an anxiogenic-like effect. The activation of 5-HT3ABR by the infusion of mCPBG [1-(m-Chlorophenyl)-biguanide; 5-HT3R agonist] did not alter anxiety-like behaviors. In addition, blockade of 5-HT3AR (1.0 nmol) prevented the anxiolytic-like effect induced by the infusion of the 5-HT2CR 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-HT2CR in the dPAG is modulated by 5-HT3AR expressed in same region.

Gαi/o-coupled Htr2c in the paraventricular nucleus of the hypothalamus antagonizes the anorectic effect of serotonin agents.

The anorexigenic effect of serotonergic compounds has largely been attributed to activation of serotonin 2C receptors (Htr2cs). Using mouse genetic models in which Htr2c can be selectively deleted or restored (in Htr2c-null mice), we investigate the role of Htr2c in forebrain Sim1 neurons. Unexpectedly, we find that Htr2c acts in these neurons to promote food intake and counteract the anorectic effect of serotonergic appetite suppressants. Furthermore, Htr2c marks a subset of Sim1 neurons in the paraventricular nucleus of the hypothalamus (PVH). Chemogenetic activation of these neurons in adult mice suppresses hunger, whereas their silencing promotes feeding. In support of an orexigenic role of PVH Htr2c, whole-cell patch-clamp experiments demonstrate that activation of Htr2c inhibits PVH neurons. Intriguingly, this inhibition is due to Gαi/o-dependent activation of ATP-sensitive K+ conductance, a mechanism of action not identified previously in the mammalian nervous system.

Amberlyst-15 catalysed synthesis of novel indole derivatives under ultrasound irradiation: Their evaluation as serotonin 5-HT2C receptor agonists.

A series of indole based novel Schiff bases was designed as potential agonists of 5-HT2C receptor that was supported by docking studies in silico. These compounds were synthesized via Amberlyst-15 catalysed condensation of an appropriate pyrazole based primary amine with the corresponding indole-3-aldehyde under ultrasound irradiation at ambient temperature. A number of target Schiff bases were obtained in good yields (77-87%) under mild conditions within 1 h. Notably, the methodology afforded the corresponding pyrazolo[4,3-d]pyrimidin-7(4H)-one derivatives when the primary amine was replaced by a secondary amine. Several Schiff bases showed agonist activity when tested against human 5-HT2C using luciferase assay in HEK293T cells in vitro. The SAR (Structure-Activity-Relationship) studies suggested that the imine moiety was more favorable over its cyclic form i.e. the corresponding pyrazolopyrimidinone ring. The Schiff bases 3b (EC50 1.8 nM) and 3i (EC50 5.7 nM) were identified as the most active compounds and were comparable with Lorcaserin (EC50 8.5 nM). Also like Lorcaserin, none of these compounds were found to be PAM of 5-HT2C. With ∼24 and ∼150 fold selectivity towards 5-HT2C over 5-HT2A and 5-HT2B respectively the compound 3i that reduced locomotor activity in zebrafish (Danio rerio) larvae model emerged as a promising hit molecule for further study.

Melanocortin 4 receptor stimulation prevents antidepressant-associated weight gain in mice caused by long-term fluoxetine exposure.

Contrasting with the predicted anorexigenic effect of increasing brain serotonin signaling, long-term use of selective serotonin reuptake inhibitor (SSRI) antidepressants correlates with body weight (BW) gain. This adverse outcome increases the risk of transitioning to obesity and interferes with treatment compliance. Here, we show that orally administered fluoxetine (Flx), a widely prescribed SSRI, increased BW by enhancing food intake in healthy mice at 2 different time points and through 2 distinct mechanisms. Within hours, Flx decreased the activity of a subset of brainstem serotonergic neurons by triggering autoinhibitory signaling through 5-hydroxytryptamine receptor 1a (Htr1a). Following a longer treatment period, Flx blunted 5-hydroxytryptamine receptor 2c (Htr2c) expression and signaling, decreased the phosphorylation of cAMP response element-binding protein (CREB) and STAT3, and dampened the production of pro-opiomelanocortin (POMC, the precursor of α-melanocyte stimulating hormone [α-MSH]) in hypothalamic neurons, thereby increasing food intake. Accordingly, exogenous stimulation of the melanocortin 4 receptor (Mc4r) by cotreating mice with Flx and lipocalin 2, an anorexigenic hormone signaling through this receptor, normalized feeding and BW. Flx and other SSRIs also inhibited CREB and STAT3 phosphorylation in a human neuronal cell line, suggesting that these noncanonical effects could also occur in individuals treated long term with SSRIs. By defining the molecular basis of long-term SSRI-associated weight gain, we propose a therapeutic strategy to counter this effect.

Overfeeding during development induces temporally-dependent changes in areas controlling food intake in the brains of male Wistar rats.

AIMS: We sought to evaluate the effects of overfeeding during lactation on the feeding behavior and expression of specific regulatory genes in brain areas associated with food intake in 22- and 60-day old male rats. METHODS: We evaluated body weight, food intake of standard and palatable diet, and mRNA expression of dopamine receptor D1 (DDR1), dopamine receptor (DDR2), melanocortin 4 receptor (MC4R), the µ-opioid receptor (MOR), neuropeptide Y (NPY), agouti-related protein (AGRP), proopiomelanocortin (POMC), cocaine-and amphetamine-regulated transcript (CART), serotonin (5-hydroxytryptamine; 5-HT) transporter (SERT), 5-hydroxytryptamine receptor 1B (5-HT1B), 5-hydroxytryptamine receptor 2C receptor (5-HT2C), Clock (CLOK), cryptochrome protein 1 (Cry1) and period circadian protein homolog 2 (Per2) in the striatum, hypothalamus and brainstem of male rats at post-natal days (PND) 22 and 60. KEY FINDINGS: Overfeeding resulted in significantly increased body weight through PND60, and a 2-fold increase in palatable food intake at PND22, but not at PND60. We observed significant increases in DDR1, DDR2, and MC4R gene expression in the striatum and brainstem and POMC/CART in the hypothalamus of the OF group at PND22 that were reversed by PND60. Hypothalamic levels of 5-HT1B, 5-HT2C and NPY/AGRP on the other hand were decreased at PND22 and increased at PND60 in OF animals. Clock genes were unaffected by OF at PND22, but were significantly elevated at PND60. SIGNIFICANCE: Overfeeding during early development of the rat brain results in obesity and altered feeding behavior in early adulthood. The altered behavior might be the consequence of the changes in food intake and reward gene expression.

5-HT2C agonists and antagonists block different components of behavioral responses to potential, distal, and proximal threat in zebrafish.

Serotonin (5-HT) receptors have been implicated in responses to aversive stimuli in mammals and fish, but its precise role is still unknown. Moreover, since at least seven families of 5-HT receptors exist in vertebrates, the role of specific receptors is still debated. Aversive stimuli can be classified as indicators of proximal, distal, or potential threat, initiating responses that are appropriate for each of these threat levels. Responses to potential threat usually involve cautious exploration and increased alertness, while responses to distal and proximal threat involve a fight-flight-freeze reaction. We exposed adult zebrafish to a conspecific alarm substance (CAS) and observed behavior during (distal threat) and after (potential threat) exposure, and treated with the 5-HT2C receptor agonists MK-212 or WAY-161503 or with the antagonist RS-102221. The agonists blocked CAS-elicited defensive behavior (distal threat), but not post-exposure increases in defensive behavior (potential threat), suggesting inhibition of responses to distal threat. MK-212 blocked changes in freezing elicited by acute restraint stress, a model of proximal threat, while RS-102221 blocked changes in geotaxis elicited this stressor. We also found that RS-102221, a 5-HT2C receptor antagonist, produced small effect on behavior during and after exposure to CAS. Preprint: https://www.biorxiv.org/content/10.1101/2020.10.04.324202; Data and scripts: https://github.com/lanec-unifesspa/5-HT-CAS/tree/master/data/5HT2C.

Central 5-HTR2C in the Control of Metabolic Homeostasis.

The 5-hydroxytryptamine 2C receptor (5-HTR2C) is a class G protein-coupled receptor (GPCR) enriched in the hypothalamus and the brain stem, where it has been shown to regulate energy homeostasis, including feeding and glucose metabolism. Accordingly, 5-HTR2C has been the target of several anti-obesity drugs, though the associated side effects greatly curbed their clinical applications. Dissecting the specific neural circuits of 5-HTR2C-expressing neurons and the detailed molecular pathways of 5-HTR2C signaling in metabolic regulation will help to develop better therapeutic strategies towards metabolic disorders. In this review, we introduced the regulatory role of 5-HTR2C in feeding behavior and glucose metabolism, with particular focus on the molecular pathways, neural network, and its interaction with other metabolic hormones, such as leptin, ghrelin, insulin, and estrogens. Moreover, the latest progress in the clinical research on 5-HTR2C agonists was also discussed.

Impact of specific serotonin receptor modulation on behavioral flexibility.

Serotonin (5-HT) is known to play a critical role in regulation of essential neural processes, whereas more recent research highlights serotonin's modulatory effects on cognition and executive functioning. Current examinations have identified specific serotonin receptors for their direct impact on behavioral flexibility. Providing definitive evidence for the impact of specific receptor targets on behavioral flexibility is difficult, due to the range of behavioral tests used. Due to limited studies and the sheer amount of different serotonin receptor targets, beginning to bring these studies together is important for the field. Our current review of the literature aims to differentiate how modulation of specific 5-HT receptors affects behavioral flexibility. Although more studies have examined 5-HT2A, 5-HT2C, and 5-HT6 receptors, it is unclear why this is the case. Above all, there are some paradoxical results pertaining to these receptor targets. There is a clear distinction between 5-HT2A and 5-HT2C, which conveys that these two receptor subtypes have inverse effects when compared to each other. In addition, some findings support one another, such as upregulation of 5-HT6 receptors impairs flexibility, while blockade alleviates this impairment in both drug-induced and disease model rodent studies. Further understanding how modulatory effects of specific 5-HT receptors impact behavioral flexibility is imperative to advance the development of new therapeutics for neuropsychiatric disorders afflicted by behavioral inflexibility.

Nucleoprotein (P40) Binding to 5HT2C Receptors (5HT2CR) is the Key Point in the Pathogenesis of BoDV-1-Infected Hosts.

Nucleoprotein (P40) is one of the most important proteins of Borna disease virus 1 (BoDV-1), but which proteins it would bind to in the pathogenesis of BoDV-1-infected hosts is unknown. We used lentivirus LV5-P40 overexpressing P40 to infect primary hippocampal neurons and characterized the interactome of P40 with co-immunoprecipitation (Co-IP) followed by mass spectrometry (MS) analysis. These interacting protein partners revealed the pathogenesis of BoDV-1-infected hosts. We also show for the first time that P40 interacts with 5HT2CR in rat neurons, which may be the molecular basis leading to neuropsychiatric diseases such as anxiety disorders and behavioral abnormalities after BoDV-1 infection of hosts.