PRESYNAPTIC INHIBITORY EFFECT OF MET-ENKEPHALIN ON [C-14] ACETYLCHOLINE-RELEASE FROM THE MYENTERIC PLEXUS AND ITS INTERACTION WITH MUSCARINIC NEGATIVE FEEDBACK INHIBITION

Abstract

The effect of Met-enkephalin on the release of radioactivity (14C) from a myenteric plexus-longitudinal muscle preparation loaded with [14C]choline was investigated under different conditions, when the muscarinic receptor mediated negative feedback inhibition was operative or when it was completely excluded by atropine. Separation of the [14C]acetylcholine (ACh) and [14C]choline components of the released radioactivity revealed that during 45-min incubation periods about 3.2% of the ACh store became labeled and that during stimulation only the release of [14C]ACh increased above resting level. The fractional release at rest measured in the 5-min collection period was 1.07 .+-. 0.09 .times. 10-2 in the absence and 1.56 .+-. 0.07 .times. 10-2 in the presence of physostigmine. Met-enkephalin had no effect on the release of ACh evoked by a 2-Hz stimulation when cholinesterase was inhibited by physostigmine. However, in the presence of atropine or in the absence of cholinesterase inhibition, the release by stimulation was significantly higher and subject to inhibition by Met-enkephalin. Met-enkephalin is apparently able to reduce ACh release only under those conditions in which the negative feedback modulation is negligible and the release is not yet reduced completely. In the myenteric plexus there is no independent population of cholinergic neurons exclusively sensitive to either ACh or to Met-enkephalin; cholinergic varicosities are equipped with at least both types of receptors studied. When the effect of Met-enkephalin on twitch tension and on the release of radioactivity was studied simultaneously, a positive correlation was found. The responses of the smooth muscle to stimulation were more readily inhibited than the release of ACh, indicating that the safety margin of this neuro-effector transmission site is less than 1.