Differential effects of chemical and mechanical colonic irritation on behavioral pain response to intraperitoneal acetic acid in mice

Abdominal contractions are a viscerosomatic reflex response to noxious colo rectal irritation in rats. In this study we characterize the modulating eff ect of chemical and mechanical colonic irritation on this reflex response t o peritoneal irritation induced by diluted acetic acid (HAc) in conscious C 57BL/6N mice. Pain responses were scored by counting the number of abdomina l contractions during the 30-min period after intraperitoneal (i.p.) inject ion of either vehicle or HAc. Abdominal contractions were induced by 0.6% b ut not by 0.3% HAc. Chemical irritation of the colon by intraluminal 25% tu rpentine did not produce abdominal contractions by itself, but significantl y increased the effect of both 0.3 and 0.6% i.p. HAc, administered 60 min a fter the luminal stimulus. Mechanical stimulation of the anorectum and colo n by insertion of a balloon did not modify the effect of 0.6% HAc, while th e insertion plus the inflation to 0.1 and 0.2 ml (30 s on/30 s off for 10 m in) reduced the response to i.p. HAc by 35 and 88%, respectively. This inhi bitory effect was reversed by naloxone (5 mg/kg, s.c.) pretreatment, while naloxone alone did not modify the effect of 0.6% HAc. These results demonst rate that chemical irritation of visceral afferents in the colonic mucosa a nd peritoneum of mice interact to enhance viscerosomatic pain responses, wh ile the activation of colonic mechanoreceptors inhibits peritoneal irritati on-induced pain responses and induces a freezing behavior by a naloxone-sen sitive mechanism. (C) 1999 International Association for the Study of Pain. Published by Elsevier Science B.V.