Reflex adrenergic inhibition of colonic motility in anesthetized rat caused by nociceptive stimuli of peritoneum. An alpha 2-adrenoceptor-mediated response.

Nociceptive stimulation of the peritoneum inhibits colonic motility via a sympathoadrenergic reflex. This sympathetic reflex most probably induces the inhibition indirectly via a presynaptic blockade of cholinergic transmission. To further clarify this reflex, colonic motility was continuously recorded by a volume method in rats exposed to nociceptive abdominal stimulation and intravenous drug administration. Intraabdominally applied HCl inhibited colonic motility, which was unaffected by nonselective beta-adrenoceptor blockade (propranolol). However, the nonselective alpha-adrenoceptor antagonist phentolamine and the selective alpha 2-adrenoceptor antagonist yohimbine restored motility. Spontaneous colonic motility was unaffected by the nonselective beta-adrenoceptor agonist (isoproterenol) and the selective alpha 1-adrenoceptor agonist (L-phenylephrine). On the other hand, a nonselective alpha-agonist (noradrenaline) and a selective alpha 2-adrenoceptor agonist (clonidine) inhibited spontaneous colonic motility. It is suggested that in the rat sympathetic reflex inhibition of colonic motility caused by abdominal nociception is mediated via presynaptic alpha 2-adrenoceptors inhibiting the excitatory cholinergic neurons.