Amphibian melanotrope subpopulations respond differentially to hypothalamic secueto-inhibitors

The melanotrope population of the frog intermediate lobe consists of two su btypes of cells, referred to as high(HD) and low-density (LD) melanotrope c ells, which differ markedly in their basal morphofunctional features as wel l as their in vitro response to hypothalamic factors, such as the stimulato r thyrotropin-releasing hormone (TRH) and the inhibitor dopamine. In this s tudy, we have investigated whether other major hypothalamic regulators of t he release of a-melanocyte-stimulating hormone (alpha -MSH), such as gamma -aminobutyric acid (GABA) and neuropeptide Y (NPY), also differentially reg ulate frog melanotrope subpopulations. Our results show that in LD cells, b oth factors markedly inhibited proopiomelanocortin (POMC) mRNA accumulation and alpha -MSH secretion. In contrast, the secretory and biosynthetic acti vity of HD cells was not modified by GABA. NPY inhibited POMC transcript ac cumulation and tended to reduce alpha -MSH secretion in HD cells, yet these effects were less pronounced than those evoked in LD cells. In addition, G ABA and NPV inhibited the KCl-induced rise in cytosolic free calcium levels in both subpopulations. Taken together, these results further indicate tha t frog melanotrope subpopulations are differentially regulated by the hypot halamus and strongly suggest that the intensity of such regulation is direc tly related to the activity of the cell subset. Thus, the LD subpopulation represents a highly responsive cell subset which is regulated by multiple n euroendocrine factors (TRH, dopamine, GABA and NPY), whereas the hormone st orage HD subpopulation shows a moderate response to single stimulatory (TRH ) and inhibitory (NPY) in puts. Copyright (C) 2001 S. Karger AG, Basel.