Excitability And TRPV1and P2X3Receptors Expression Of Colon-specific Dorsal Root Ganglion Neurons In A Rat Model Of Colonic Visceral Hypersensitivity

Objective Irritable bowel syndrome is a functional disorder of intestinal which has themain symptoms of abdominal pain or discomfort or the change of defecation.Epidemiological survey show the incidence of IBS is approximately5%to10%[1].Theincidence in Europe and other developed countriesis is high when Beijing, Guangzhou,Shanghai, Zhejiang in China is high too. IBS severely affects the quality of life. But itspathogenesis is not clear. Now scholar think the pathogenesis of IBS may be associatedwith visceral hypersensitivity, disorders of intestinal motility, inflammation, nerveendocrine system, genetic and so on in which visceral hypersensitivity is considered asa major pathophysiological. In the course of visceral perception, it has long beenspeculated that the pain from the gastrointestinal tract is composed of spinal afferentfibers[2]. Pain signals originate from the endings of peripheral nociceptive receptorswhich locate in dorsal root ganglion neurons. Nociceptors can be transformed into asignal of noxious stimulus via transient receptor potential vanilloid receptor1or P2Xpurinergic receptors and this signal can be transmitted from the outer nervous system tocentral nervous system which caused a corresponding reflex. But presently, it remainsunclear that the mechanism between visceral hypersensitivity and TRPV1and P2XRs.In this study, visceral hypersensitivity was induced by colonic injection of0.5%aceticacid.Colon-specific DRG neurons were retrogradely labeled by injection of DiI. Toinvestigate the excitability and TRPV1and P2X3receptors expression of thecolon-specific DRG neurons in a rat model of colonic visceral hypersensitivity. Methods Visceral hypersensitivity was induced by colonic injection of0.5%aceticacid (AA) in10-day-old rats while control(NS) induced with0.9％normal saline. Theabdominal withdrawal reflex (AWR), inuced by colorectal distention, was used toquantify the level of colonic sensitivity in adult rats. Colon-specific DRG neurons wereretrogradely labeled by injection of DiI into the colon wall and observed byfluorescence microscope after separated into a single cell with digestive enzyme.Rheobase and the frequency of action potentials(APs) which represent excitability inresponse to a current stimulation twice the rheobase from neurons were measured usingwhole patch clamp techniques. Expression of TRPV1and P2X3receptors wereobserved by immunofluorescence techniques and calculate the correlation betweenAWR scores and TRPV1and P2X3receptors positive rate and the frequency of actionpotentials and the rheobase of the colon-specific DRG neurons in AA group.Results1.Rats in AA group appeared abnormal stools after daily acetic acid stimulation, ratswere irritabile and fearing and screaming when acetic acid stimulation was ongoing.With the increase of days, rats were irritated and aggressive.2. Colonic sensitization with acetic acid increased the AWR to innocuous levels ofcolorectal distention (5.332、7.998、10.664kPa)(P<0.05).3. DiI-labeled positive neurons distributed between35to45μm in AA group whilebetween25to35μm in control.4. The average rheobase of AA neurons was significantly lower than control(P<0.05).The frequency of APs in AA neurons in response to a2rheobase stimulation wassignificantly higher than control(P<0.05).5. In AA group, the proportion expressing of TRPV1and P2X3receptors wassignificantly higher in the colon-specific DRG neurons than controls (P<0.01). 6.The AWR scores in response to different pressures in AA group were positivecorrelated with the proportion expressing of TRPV1and P2X3receptors（P<0.01）andthe frequency of Aps(P<0.01),but were negative correlated with rheobase（P<0.05）.Conclusion1. Visceral hypersensitivity was induced by colonic injection of0.5%acetic acid.2. Colon-specific DRG neurons in a rat model increased in size and excitability aftercolonic visceral hypersensitivity.3. TRPV1and P2X3receptors were increased in colon-specific dorsal root ganglionneurons in a rat model of colonic visceral and there is a close relationship betweenvisceral hypersensitivity and expression of TRPV1and P2X3receptors andexcitability of colon-specific DRG neurons.