Serotonin receptor 2B induces visceral hyperalgesia in rat model and patients with diarrhea-predominant irritable bowel syndrome

BACKGROUND Serotonin receptor 2B(5-HT2B receptor)plays a critical role in many chronic pain conditions.The possible involvement of the 5-HT2B receptor in the altered gut sensation of irritable bowel syndrome with diarrhea(IBS-D)was investigated in the present study.AIM To investigate the possible involvement of 5-HT2B receptor in the altered gut sensation in rat model and patients with IBS-D.METHODS Rectosigmoid biopsies were collected from 18 patients with IBS-D and 10 patients with irritable bowel syndrome with constipation who fulfilled the Rome IV criteria and 15 healthy controls.The expression level of the 5-HT2B receptor in colon tissue was measured using an enzyme-linked immunosorbent assay and correlated with abdominal pain scores.The IBS-D rat model was induced by intracolonic instillation of acetic acid and wrap restraint.Alterations in visceral sensitivity and 5-HT2B receptor and transient receptor potential vanilloid type 1(TRPV1)expression were examined following 5-HT2B receptor antagonist adminis-tration.Changes in visceral sensitivity after administration of the TRPV1 antago-INTRODUCTION Irritable bowel syndrome(IBS)is a chronic functional bowel disorder characterized by recurrent abdominal pain with altered bowel habits that affects approximately 15%of the population worldwide[1].IBS significantly impacts the quality of life of patients.Although the pathogenesis of IBS is not completely understood,the role of abnormal visceral sensitivity in IBS has recently emerged[2,3].5-Hydroxytryptamine(5-HT)is known to play a key role in the physiological states of the gastrointestinal tract.Plasma 5-HT levels in IBS with diarrhea(IBS-D)patients were greater than those in healthy controls[4],suggesting a possible role of 5-HT in the pathogenesis of IBS-D.The serotonin receptor 2(5-HT2 receptor)family comprises three subtypes:5-HT2A,5-HT2B,and 5-HT2c.All 5-HT2 receptors exhibit 46%-50%overall sequence identity,and all of these receptors preferentially bind to Gq/11 to increase inositol phosphates and intracellular calcium mobilization[5].5-HT2B receptors are widely expressed throughout the gut,and experimental evidence suggests that the primary function of 5-HT2B receptors is to mediate contractile responses to 5-HT through its action on smooth muscle[6].The 5-HT2B receptor is localized to both neurons of the myenteric nerve plexus and smooth muscle in the human colon.The 5-HT2B receptor mediates 5-HT-evoked contraction of longitudinal smooth muscle[6].These findings suggest that the 5-HT2B receptor could play an important role in modulating colonic motility,which could affect sensory signaling in the gut.Other laboratories have shown that the 5-HT2B receptor participates in the development of mechanical and formalin-induced hyperalgesia[7,8].A 5-HT2B receptor antagonist reduced 2,4,6-trinitrobenzene sulfonic acid(TNBS)and stress-induced visceral hyperalgesia in rats[9,10].However,the role of the 5-HT2B receptor in IBS-D patients and in acetic acid-and wrap restraint-induced IBS-D rat models was not investigated.

Association of Serotonin Receptors with Attention Deficit Hyperactivity Disorder： A Systematic Review and Meta-analysis

Attention deficit hyperactivity disorder （ADHD） is one of the most common mental disorders in childhood, with a high heritability about 60% to 90%. Serotonin is a monoamine neurotransmitter. Numerous studies have reported the association between the serotonin receptor family （5-HTR） gene polymorphisms and ADHD, but the results are still controversial. In this study, we conducted a meta-analysis of the association between 5-HTRIB, 5-HTR2A, and 5-HTR2C genetic variants and ADHD. The results showed that the 861G allele of 5-HTRIB SNP rs6296 could significantly increase the risk of ADHD （OR= 1.09, 95% CI： 1.01-1.18）; the 5-HTR2C gene rs518147 （OR=1.69, 95% CI： 1.38-2.07） and rs3813929 （OR = 1.57, 95% CI： 1.25-1.97） were all associated with the risk of ADHD. In addition, we also carried on a case- control study to explore the relevance between potential candidate genes 5-HTR1A, 5-HTRIE, 5-HTR3A and ADHD. The results indicated that 5-HTRIA rs6295 genotype （CC＋CG vs. GG OR=Z00, 95% CI： 1.23-3.27） and allele （OR=1.77, 95% CI： 1.16-2.72） models were statistically significantly different between case group and control group. This study is the first comprehensive exploration and summary of the association between serotonin receptor family genetic variations and ADHD, and it also provides more evidence for the etiology of ADHD.

Role of 5-HT2A Receptors in Immunomodulation in Animal Models of Aggressive Behavior

Serotonin 5-HT2A receptors are playing an important role in the pathophysiology of aggressive behaviors and in the control of immune function. In the present study, we analyzed the effects of activation and blockade of 5-HT2A receptors with selective ligands on the immune response formation in animals with aggressive behaviors induced by genetic factors (rats selected for the increased aggressiveness toward human) or by chronic social stress (mice of the CBA/Lac strain engaged in 10 days of social confrontations). Activation of 5-HT2A receptors with DOI at 1.0 mg/kg reduced the immune response level both in aggressive rats and mice compared to the corresponding vehicle-treated groups, while DOI administration did not alter the immune reaction in nonaggressive animals. The blockade of 5-HT2A receptors with ketanserin at 1.0 mg/kg resulted in immunostimulation both in mice of the CBA strain not subjected to social stress (the controls) and in nonaggressive rats selected for elimination of aggressiveness. On the other hand, its administration to CBA mice demonstrating offensive aggression enhanced the immune reaction, while the same dose of ketanserin did not modify the immune response level in rats with genetic predisposition to the increased defensive aggression. Thus, our data suggest that the role of 5-HT2A receptors in immunomodulation depends on the specific type of aggression that may be taking into account in the treatment of some neuropsychiatric disorders with the antipsychotic drugs and antidepressants targeting 5-HT2A receptors.

Molecular mechanism of noradrenaline during the stress-induced major depressive disorder

Chronic stress-induced depression is a common hallmark of many psychiatric disorders with high morbidity rate.Stress-induced dysregulation of noradrenergic system has been implicated in the pathogenesis of depression.Lack of monoamine in the brain has been believed to be the main causative factor behind pathophysiology of major depressive disorder（MDD） and several antidepressants functions by increasing the monoamine level at the synapses in the brain.However,it is undetermined whether the noradrenergic receptor stimulation is critical for the therapeutic effect of antidepressant.Contrary to noradrenergic receptor stimulation,it has been suggested that the desensitization of β-adrenoceptor is involved in the therapeutic effect of antidepressant.In addition,enhanced noradrenaline（NA） release is central response to stress and thought to be a risk factor for the development of MDD.Moreover,fast acting antidepressant suppresses the hyperactivation of noradrenergic neurons in locus coeruleus（LC）.However,it is unclear how they alter the firing activity of LC neurons.These inconsistent reports about antidepressant effect of NA-reuptake inhibitors（NRIs） and enhanced release of NA as a stress response complicate our understanding about the pathophysiology of MDD.In this review,we will discuss the role of NA in pathophysiology of stress and the mechanism of therapeutic effect of NA in MDD.We will also discuss the possible contributions of each subtype of noradrenergic receptors on LC neurons,hypothalamic-pituitary-adrenal axis（HPA-axis） and brain derived neurotrophic factor-induced hippocampal neurogenesis during stress and therapeutic effect of NRIs in MDD.

Discovery of a Novel 5-HT_(2A) Inhibitor by Pharmacophore-based Virtual Screening

The serotonin 2A（5-HT2A） receptor has been implicated in several neurological conditions and potent 5-HT2A antagonists have therapeutic effects in the treatment of schizo phrenia and depression.In this study,a potent novel 5-HT2A inhibitor 05245768 with a Ki value of （593.89±34.10） nmol/L was discovered by integrating a set of computational approaches and experiments（protein structure prediction,pharmacophore-based virtual screening,automated molecular docking and pharmacological bioassay）.The 5-HT2A receptor showed a negatively charged bin-ding pocket.The binding mode of compound 05245768 with 5-HT2A was obtained by GOLD docking procedure,which revealed the conserved interaction between protonated nitrogen in compound 05245768 and carboxylate group of D3.32 at the active site of 5-HT2A.

Effects of Estradiol on 5-HTsA and 5-HT2c Receptor Immunolabeling in Rat Hippocampus

Steroid hormones participate in the modulation of serotonergic transmission, including the regulation of synthetic and metabolic enzyme production, as well as receptor and transporter activity. The changes of 5-HT5A and 5-HT2c immunolabeling induced by steroids in the hippocampus ofovariectomized rats were studied in this work. Densitometric analysis in rat hippocampi were carried out for adjacent brain coronal immunolabeled sections after treatment with subcutaneous injections of vehicle, estradiol, progesterone or the combination of both steroids in ovariectomized rats. Exposure to estradiol and the combination of estradiol and progesterone significantly reduced the 5-HT5A-like immunosignal in the CA 1 region while progesterone did not induce changes. On the other hand, exposure to the combination of estradiol and progesterone or estradiol alone increased the 5-HT2c immunosignal in the same region. These results indicate that estradiol is involved in the discrete regulation of serotonin receptors 5-HT5A and 5-HT2c in rat hippocampus.

Association between the serotonin 1A receptor C（-1019）G polymorphism and major depressive disorder in the northern Han ethnic group in China

Background Recent studies have suggested that susceptibility to major depressive disorder （MDD） might be related to the serotonin 1A receptor （5-HTR1A） C（-1019）G polymorphism. In this study, we aimed to assess the association between 5-HTR1A C（-1019）G polymorphism and MDD in the Northern Han ethnic group of China. Methods The C（-1019）G of 5-HTR1A was detected with polymerase chain reaction （PCR） in 400 patients with MDD and 400 unrelated age- and sex-matched healthy control subjects. Association between the C（-1019）G and MDD was statistically analyzed. Results There was a statistically significant difference between MDD patients and controls in both the genotype distribution （X^2=10.913, df=2, ,P=0.004） and the allele frequency （X^2=10.379, df=1, P=0.001 ）, and a significant difference in the genotype distribution and the allele frequency was found both in the female subjects （Genotype distribution： X^2=15.406, df=2, P=0.000; allele frequency： X^2=15.552, df=1, P=0.000） and the late-onset subjects （Genotype distribution X^2=7.771, df=2, P=0.021 ; allele frequency： X^2=8.007, df=1, P=0.005） in the two groups.Conclusion These results suggest that 5-HTR1A C（-1019）G polymorphism is probably associated with MDD and it is likely to be the susceptible gene locus for the female and late-onset MDD.

Serotonin 1A receptor inhibits the status epilepticus induced by lithium-pilocarpine in rats

Status epilepticus （SE） is a life-threatening neurological emergency associated with a high mortality rate. The serotonin 1A （5-HTIA） receptor is a possible target for the treatment of SE, but its role in animal models and the precise area of brain involved remain controversial. The hippocampus is a candidate site due to its key role in the development of SE and the existence of a high density of 5-HT1A receptors. Therefore, we investigated the effects of subcutaneous and intrahippocampal activation of 5-HT1A receptors in lithium-pilocarpine-induced SE, and tested whether the hippocampus is a true effector site. We developed SE in male Sprague-Dawley rats by giving lithium chloride （LiCI; 3 meq/kg, i.p.） 22-24 h prior to pilocarpine （25 mg/kg, i.p.）, and found that 8-OH-DPAT, a 5-HT1A receptor agonist administered subcutaneously （s.c.） at 0.5 or 1.0 mg/kg 1 h before pilocarpine injection increased the latency to the first epileptiform spikes, the electrographic SE, and the behavioral generalized seizures （GS）, while reducing the total EEG seizure time （P 〈0.01）. The duration of GS was shortened only by 1.0 mg/kg 8-OH-DPAT s.c. （P 〈0.05）. All these effects were inhibited by combined administration of WAY-100635 （1.0 mg/kg, s.c.） （P 〈0.05）, an antagonist of the 5-HT1A receptor, but WAY-100635 alone and low doses of 8-OH- DPAT （0.01 and 0.1 mg/kg） did not alter seizure activity. Furthermore, intrahippocampal 8-OH-DPAT only shortened the GS duration （P 〈0.05）. These findings imply that the 5-HT1A receptor is a promising therapeutic target against the generation and propagation of SE, and hippocampal receptors are involved in reducing the seizure severity.

5-HT receptors mediating external carotid vasoconstriction in vagosympathectomized dogs

One specific example reflecting the completity ofcardiovascular responses induced by serotonin(5-hydroxytryptamine;5-HT)and the progress achieved inthe pharmacological characterization of the receptorsinvolved can be illustrated by the effects of 5-HT on thecanine external carotid artery bed.Within thisframework,it has been shown that the external carotidvasoconstrictor response to 5-HT in the dog is mediatedby'5-HT_(1)-like'receptors,which being blocked by the5-HT_(1B/1D)receptor antagonist GR127935,resemble 5-HT_(1B/1D)(previously called 5-HT_(1B/1D))receptors.It was proposed that these receptors coilld belong to the 5-HT_(lB),rather than the 5-HT_(lD),subtype on the basis of their resistance to blockade by a high dose of ritanserin(a potential 5-HT_(lD)receptor antagomst and the presence of mRNA for 5-HT_(lB)(5-HT_(lDB))receptors,but not for 5-HT_(lD)(5-HT_(lD))receptors,in vascalar smooth muscle.With the advent of subtype-selective antagonists it was subsequently shown that the external carotid vasocenstriction to 5-HT and surmantriptan is dose-dependently antagonized by the selective 5-HT_(lB)receptor antagonist SB224289[2,3,6.7-telrahydro-I'-methyl-5-[2'-methyl-4(5-methyl-1,2,4-oxadiazol-3-y1)biphenyl-4-carbonyl]]furo[2,3-f]indole-3-spiro-4'-piperidine hydrochloride],whereas the selective 5-HT_(lD)receptor antagonist BRL15572[l(3-chloropheny1)-4-[3,3-diphenyl(2-(S,R)hydroxypropany1)piperazine]hydrochloride was inefective.These findings represent the first in vivo evidence showing that vascular constriction induced by 5-HT and sumatriptan is mediated primarily via 5-HT_(lB),but not 5-HT_(lD)receptors.The pharmacological profile of these receptors could be similar(isolated human temporal artery and porcine carotid arteriovenous anastomoses)to other putative 5-HT_(lB)receptors mediating vasoconstrictor responses.In view of the putative pathophysiologic role of external carotid(and extracerebral)vasodilation in migraine,the constriction of these blood vessels by sumatriptan via 5-HT_(lB)receptors may be,at least partly,responsible for its therapeutic efficacy in migraine.