INHIBITORY EFFECT OF ENDOMORPHIN-1 AND ENDOMORPHIN-2 ON ACETYLCHOLINE-RELEASE FROM MYENTERIC PLEXUS OF GUINEA-PIG ILEUM

Endomorphin-1 and -2, putative endogenous ligands for the mu-opioid re ceptor, inhibited acetylcholine (ACh) release evoked by electrical fie ld stimulation (EFS) at 1 Hz, which partially activates muscarinic aut oreceptors, but not at 10 Hz, which fully activates muscarinic autorec eptors, in longitudinal muscle with the myenteric plexus (LMMP) prepar ations of guinea pig ileum. After blockade of autoinhibition by atropi ne, the peptides also inhibited EFS-evoked ACh release at 10 Hz. The i nhibitory effects on ACh release were abolished by the mu-opioid antag onist cyprodime. These results suggest that endomorphin-1 and -2 inhib it ACh release from LMMP preparations of guinea pig ileum and that the mechanism of the inhibition must have a component in common with musc arinic autoinhibition.