Abstract
In the studies on Xenopus laevis presented here, the role of acetylcholine (ACh) in the regulation of pituitary activity was examined. The results are discussed in connection with other results on cholinergic regulation of pituitary functioning in vertebrates. It is demonstrated that dissociated melanotropes can synthesize ACh in vitro. In addition, immunocytochemical studies show the presence of the ACh-synthesizing enzyme choline acetyltransferase (ChAT) in the cytoplasm of melanotrope cells. ACh concentration-dependently raises the intracellular free calcium concentration as well as the release of proopiomelanocortin (POMC)-derived peptides. Selective muscarinic receptor antagonists showed that the actions of ACh are mediated through an M1-subtype muscarinic receptor. Immunofluorescence cytochemistry 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, excitatory way. Finally, the possible mechanism through which ACh is released from melanotrope cells and the physiological significance of ACh in the pituitary gland are discussed.
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