COMPARATIVE EFFECTS OF MU-OPIATE AND KAPPA-OPIATE AGONISTS ON THE CECOCOLIC MOTILITY IN THE PONY

The electrical and mechanical activity of the large intestine and its response to administration of opiate mu and kappa agonists were assess ed from electrodes and inductograph coils chronically implanted on the cecocolic segment in six ponies given a diet of hay and concentrates. Before the drugs were given, migrating complexes propagating from the cecum into the colon occurred at the rate of 1.5 to 16/hour. During t his propulsive activity, the cecocolic sphincter opened and closed all owing the outflow of cecal contents and preventing the backflow of col ic contents. Each pony was used as its own control and was given fenta nyl (0.01 and 0.05 mg/kg of body weight, IV) and U50488H (0.1 and 0.5 mg/kg, IV) at weekly intervals. The mu agonist fentanyl elicited a mar ked phase of inhibition of the propulsive activity and a closure of th e cecocolic sphincter that lasted one to two hours depending on the do se. The kappa agonist U50488H induced an inhibition of the short spiki ng activity, i.e. of the resting muscle tone. It did not disturb the o ccurrence of migrating complexes nor that of the openings of the cecoc olic sphincter. These kappa compounds may be drugs of choice to allevi ate visceral pain in colic stases without inducing delay of transit un like mu compounds.