Comparative effects of opiate agonists on proximal and distal colonic motility in dogs

The colonic motility index was measured in dogs, by using strain-gauge transducers, before and after administration of opiate agonists. Fentanyl, a mu-compound, ethylketazocine (EKC), a kappa-compound, and [D-Ala2,Leu5]enkephalin (DADLE), a delta-agonist, were administered by intravenous (i.v.) or intrathecal (i.t.) routes. Fentanyl (2-10 nmol X kg-1 i.v.) induced a dose-related period of hyperactivity characterized by an increase in both tone and frequency of contractions in the distal portion of the colon, whereas a period of hypomotility following a short-lived period of increased activity was elicited on the proximal colon. Fentanyl (0.1, 0.2 nmol X kg-1 i.t.) had inhibitory effects on both proximal and distal colon. Small doses of EKC (2-4 nmol X kg-1) administered i.t. or larger doses (20-40 nmol X kg-1 i.v.) inhibited colonic motility for a dose-related duration, the effects of EKC being more marked on the distal colon than on the proximal colon. The administration of DADLE, 1-2 nmol X kg-1 i.t., and 40-20 nmol X kg-1 i.v., inhibited and stimulated the colonic contractions, respectively, in both proximal and distal colon. The results demonstrated that the colonic opiate-mediated responses may vary according to the route of administration, the portion of the colon studied and the opioid agonist used. Opiate agonists when administered centrally show a tendency to modulate colonic activity in a similar way whatever the type of agonist.