PARTICIPATION OF CAPSAICIN-SENSITIVE AFFERENT NEURONS IN GASTRIC MOTOR INHIBITION CAUSED BY LAPAROTOMY AND INTRAPERITONEAL ACID

Stimulation of somatic or visceral nociceptors causes changes in gastrointestinal motor activity and blood pressure. The present study examined the possible participation of capsaicin-sensitive afferent and noradrenergic efferent neurons in the blood pressure and gastric motor responses to laparotomy and intraperitoneal injection of capsaicin or hydrochloric acid in the rat. Gastric motor activity was measured by recording the intragastric pressure of phenobarbital-anaesthetized rats via an oesophageal catheter. Laparotomy as well as intraperitoneal injection of capsaicin (33 and 330-mu-M) or hydrochloric acid (30 mM) caused a transient reduction of gastric motor activity stimulated by intravenous infusion of bombesin (200 pmol/min) and a brief fall of blood pressure (depressor effect). The depressor effect of laparotomy was followed by prolonged hypertension. Defunctionalization of capsaicin-sensitive afferent neurons by systemic pretreatment of rats with capsaicin (0.4 mmol/kg) prevented the depressor effect and gastric motor inhibition elicited by laparotomy, intraperitoneal capsaicin (33-mu-M) or intraperitoneal hydrochloric acid (30 mM). However, the effects of 330-mu-M capsaicin on blood pressure and gastric motility were only partially reduced by capsaicin pretreatment. Blockade of noradrenergic sympathetic neurons by pretreating rats with guanethidine (0.225 mmol/kg) prevented the gastric motor inhibition and depressor effects of laparotomy and intraperitoneal injection of hydrochloric acid (30 mM). The inhibition of gastric motility caused by capsaicin (33 and 330-mu-M) was only partially reduced by guanethidine pretreatment. The secondary hypertension following the depressor effect of intraperitoneal capsaicin or hydrochloric acid was enhanced in guanethidine-pretreated rats whereas the prolonged hypertension induced by laparotomy was left unchanged. These observations indicate that laparotomy and intraperitoneal injection of algesic chemicals inhibit gastric motor activity and elicit a depressor effect by a neural reflex. The afferent limb of the reflex is constituted by capsaicin-sensitive afferent neurons. which comprise unmyelinated primary afferents connected to chemoceptors and polymodal nociceptors, whilst the efferent reflex limb involves noradrenergic sympathetic neurons. This neural wiring may have a bearing on pathological inhibition of gastrointestinal motility.