Neural control of the sphincter of Oddi. Physiologic role of enkephalins on the regulation of basal sphincter of Oddi motor activity in the cat.

The effect and physiologic role of enkephalins on the sphincter of Oddi was studied in the cat. Leucine- and methionine-enkephalin caused an initial sphincter of Oddi contraction followed by a more prolonged relaxation. The excitatory effect of leucine-enkephalin was antagonized partially by atropine and by methysergide infusion. It was completely blocked by a combination of atropine and methysergide infusion, by tetrodotoxin, by 5-hydroxytryptamine depletion induced by reserpine, and by 5-hydroxytryptamine tachyphylaxis. Naloxone infusion decreased sphincter of Oddi motor activity, which was reversed by tetrodotoxin. Maximal sphincter of Oddi relaxation was induced by doses of naloxone (40 micrograms/kg) that antagonized a maximal dose of morphine. Higher doses of naloxone (640 micrograms/kg) were needed to block maximal doses of enkephalin (8 micrograms/kg). The enkephalin inhibitory action was blocked only by tetrodotoxin. These findings indicate that (a) leucine-enkephalin has two receptor sites on the sphincter of Oddi--one at the serotonergic neurons and the other at noncholinergic, nonadrenergic inhibitory neurons, and (b) opioid peptides participate in the intramural excitatory pathway to the sphincter of Oddi.