Effect of endogenous opioid peptides on acetylcholine release from the cat superior cervical ganglion: selective effect of a heptapeptide.

The present experiments show the presence of both metenkephalin-like and met-enkephalin-Arg6-Phe7-like immunoreactivity in the superior cervical ganglion of the cat; this was determined by radioimmunoassay after high-pressure liquid chromatography separation of tissue extracts. There was measurable efflux of both peptides, as determined by radioimmunoassay of ganglionic perfusates; this measure was increased by thiorphan, an enkephalinase inhibitor. The effect of the 2 peptides on ACh release was determined: The stable analog of methionine-enkephalin, D-Ala2-methionine-enkephalinamide, did not affect ACh release from the ganglion; in contrast, methionine-enkephalin-Arg6-Phe7 significantly depressed evoked ACh release. The effect of met-enkephalin-Arg6-Phe7 to decrease ACh release was antagonized, although only partially, by the opioid antagonist naloxone. Thus, it appears that methionine-enkephalin-Arg6-Phe7 alters ACh release from the superior cervical ganglion by acting, at least in part, on a presynaptic opioid receptor. The results suggest that in the cat superior cervical ganglion, the heptapeptide enkephalin might have a significant role in the regulation of synaptic transmission, which is unrelated to its potential function as a precursor for methionine-enkephalin.