Adrenergic Stimulation Mediates Chronic Visceral Pan Induced By Heterotypic Intermittent Stress In Rats

Background&Aims: Chronic stress exacerbates or causes relapse of symptoms suchas abdominal pain and cramping in patients with irritable bowel syndrome （IBS）.However, the precise molecular mechanism remains unknown. We thereforeinvestigated whether adrenoceptor was involved in visceral hypersensitivity in a ratmodel of heterotypical intermittent stress （HIS）. We also determined whether HISenhanced expression of P₂X₃receptors and reduced function of voltage-gated potassiumchannels in colon-specific thoracolumbar dorsal root ganglion （DRG） neurons (T₁₃-L₂)in rats following chronic stress.Methods: Visceral hypersensitivity was induced in6⁸week old male SD rats by a9day stress protocol comprised of3randomly arranged stressors: cold restraint stressor,forced swimming stressor and water avoidance stressor. Behavioral response tocolorectal distension （CRD） was measured by visual observation of abdominalwithdrawal reflex （AWR） scores. Whole cell patch-clamp recording technique wasemployed to record excitability and current density of voltage-gated potassium channelsof acutely dissociated colon DRGs. Western blotting was conducted to analyze P₂X₃receptor expression in colon related DRGs.Results: HIS significantly increased the visceromoter response to CRD, which startedat6h, peaked at24h and return to normal level48h after HIS. Nonselectiveβ-adernoreceptor antagonist, but not α-adernoreceptor antagonist, attenuatedstress-induced visceral hypersensitivity in a dose dependent manner. Further, blockadeof β₂-adernoreceptor attenuated AWR scores while administration of β₁-orβ₃-adernoreceptor antagonists had little effect on stress-induced visceral hypersensitivity. Patch-clamp recording showed that resting membrane potential wasdepolarized, rheobase was decreased, and number of action potentials was significantlyincreased in colon specific DRG neurons from HIS rats. Under voltage-clampconditions, neurons from both groups exhibited transient A-type （I_A） and sustainedoutward rectifier K+currents （I_K）. Compared with controls, the average IKbut not theaverage IAdensity was markedly reduced in the HIS group （P <0.05）. These datasuggest that HIS increases the excitability of colon-specific DRG neurons bysuppressing IKdensity. Incubation of norepinephrine （NE） increased the excitability ofcolon-specific DRGs from healthy rats. Expression of P₂X₃receptors in colon DRGs(T₁₃-L₂) was significantly increased at6h,24h after termination of HIS. This expressionwas returned to normal level one week after HIS. Most importantly, β-adernoreceptorantagonist reversed the enhanced excitability and reduced the upregulation of P₂X₃receptors in colon specific DRGs induced by HIS.Conclusions: The present data suggest that suppression of IKdensity may accountfor the enhanced excitability of colon specific DRG neurons, thus contributing to thevisceral hypersensitivity induced by heterotypic intermittent stress. Our data alsosuggest that the visceral hypersensitivity induced by HIS is mediated, at least in a largepart, by β₂-adernoreceptors which leading to sensitization of P₂X₃receptors. Thepresent and future studies might provide insight into the molecular mechanismsunderlying chronic visceral pain in patients with functional gastrointestinal diseasessuch as IBS.