Acetylcholine autoexcites the release of proopiomelanocortin-derived peptides from melanotrope cells of Xenopus laevis via an M1 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 cell activity. The origin of ACh was studied in in vitro biosynthesis experiments, where it was demonstrated that dissociated melanotropes can synthesize ACh. In addition, immunocytochemical studies showed the presence of the ACh-synthesizing enzyme choline acetyltransferase in the cytoplasm of melanotrope cells. Oscillations in the concentration of intracellular 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 oscillations and peptide secretion from the same population of single melanotrope cells in vitro. ACh concentration-dependently raises the intracellular free calcium concentration by increasing both the frequency and amplitude of the calcium oscillations as well as the release of POMC-derived 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 muscarinic receptor antagonists showed that the actions of ACh are mediated through an M1-subtype muscarinic receptor. Immunofluorescence cytochemistry in combination with confocal laser scanning microscopy visualized muscarinic receptors on the surface of melanotrope cells. It is concluded that ACh stimulates the release of POMC-derived peptides from the Xenopus melanotrope cell in an autocrine way, acting via a muscarinic M1 receptor on the intracellular free calcium concentration.