The Significance of Multiple Inhibitory Mechanisms Converging on the Melanotrope Cell of Xenopus laevis

The melanotrope cell of the pituitary pars intermedia secretes peptides derived from the precursor protein proopiomelanocortin (POMC), including alpha -melanophore-stimulating hormone ( alpha -MSH). In amphibians alpha -MSH stimulates dispersion of the black pigment melanin in dermal melanophores and is responsible for inducing darkening of the skin in animals on a black background. We are studying the regulation of secretion of POMC-derived peptides from the melanotrope cell of the aquatic toad Xenopus laevis. The secretory activity of this cell is regulated by multiple factors, both classical neurotransmitters and neuropeptides. Of particular importance are inhibitory neurons in the hypothalamic suprachiasmatic nucleus, the terminals of which make synaptic contact with the melanotrope cells. These terminals contain coexisting inhibitory transmitter substances dopamine, gamma -aminobutyric acid (GABA), and neuropeptide Y (NPY); GABA is found in electron-lucent vesicles, whereas dopamine and NPY are sequestered together within dense-core vesicles. Postsynaptically on the melanotrope cell GABA inhibits secretion through action on both GABA sub(A) and GABA sub(B) receptors; dopamine inhibits by a D sub(2) receptor and NPY utilizes a Y sub(1) receptor to inhibit the secretory process. Efforts to attach functional significance to these multiple inhibitory receptor mechanisms, by looking for potentiating interactions among them, showed in fact that the inhibitions induced by these receptors are strictly additive. The adenylyl cyclase system plays a central role in the regulation of alpha -MSH secretion from the Xenopus melanotrope cell. The D sub(2), Y sub(1), and GABA sub(B) receptor mechanisms have all been shown to inhibit cyclic adenosine monophosphate (cAMP) production in this cell while the release-stimulating neuropeptide corticotropin-releasing hormone (CRH) stimulates production of the cyclic nucleotide. Moreover, an active cAMP-dependent protein kinase is required for the Xenopus melanotrope to display intracellular Ca super(2+) oscillations; these oscillations are believed to be the driving force for peptide secretion. To further pursue possible significance of the multiple inhibitory mechanisms we have recently considered the possibility that they could have differential actions on melanotrope cell sensitivity to stimulation by cAMP. Therefore, we have studied the effects of the inhibitory mechanisms on the melanotrope.