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 colorectal irritation in rats. In this study we characterize the modulating effect of chemical and mechanical colonic irritation on this reflex response to peritoneal irritation induced by diluted acetic acid (HAc) in conscious C57BL/6N mice. Pain responses were scored by counting the number of abdominal contractions during the 30-min period after intraperitoneal (i.p.) injection of either vehicle or HAc. Abdominal contractions were induced by 0.6% but not by 0.3% HAc. Chemical irritation of the colon by intraluminal 25% turpentine did not produce abdominal contractions by itself, but significantly increased the effect of both 0.3 and 0.6% i.p. HAc, administered 60 min after the luminal stimulus. Mechanical stimulation of the anorectum and colon by insertion of a balloon did not modify the effect of 0.6% HAc, while the insertion plus the inflation to 0.1 and 0.2 ml (30 s on/30 s off for 10 min) reduced the response to i.p. HAc by 35 and 88%, respectively. This inhibitory 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 demonstrate that chemical irritation of visceral afferents in the colonic mucosa and peritoneum of mice interact to enhance viscerosomatic pain responses, while the activation of colonic mechanoreceptors inhibits peritoneal irritation-induced pain responses and induces a freezing behavior by a naloxone-sensitive mechanism.