ACETYLCHOLINE AUTOEXCITES THE RELEASE OF PROOPIOMELANOCORTIN-DERIVED PEPTIDES FROM MELANOTROPE CELLS OF XENOPUS-LAEVIS VIA AN M(1) MUSCARINIC RECEPTOR

The release of POMC-derived peptides from the melanotrope cells in the pituitary pars intermedia of Xenopus laevis is regulated by multiple inhibitory and stimulatory neurochemical messengers. In this study we examined the role of acetylcholine (ACh) in regulating melanotrope cel l activity. The origin of ACh was studied in in vitro biosynthesis exp eriments, where it was demonstrated that dissociated melanotropes can synthesize ACh. In addition, immunocytochemical studies showed the pre sence of the ACh-synthesizing enzyme choline acetyltransferase in the cytoplasm of melanotrope cells. Oscillations in the concentration of i ntracellular free calcium ions are thought to be the driving force for the secretion of POMC-derived peptides from Xenopus melanotrope cells . A technique was developed to study simultaneously calcium oscillatio ns and peptide secretion from the same population of single melanotrop e cells in vitro. ACh concentration-dependently raises the intracellul ar free calcium concentration by increasing both the frequency and amp litude of the calcium oscillations as well as the release of POMC-deri ved peptides. The actions of ACh on the melanotrope cell were mimicked by muscarine, but not by nicotine, indicating the exclusive presence of a muscarinic cholinergic receptor. Furthermore, selective muscarini c receptor antagonists showed that the actions of ACh are mediated thr ough an M(1)-subtype muscarinic receptor. Immunofluorescence cytochemi stry in combination with confocal laser scanning microscopy visualized muscarinic receptors on the surface of melanotrope cells. It is concl uded that ACh stimulates the release of POMC-derived peptides from the Xenopus melanotrope cell in an autocrine way, acting via a muscarinic M(1) receptor on the intracellular free calcium concentration.