Amphibian melanotrope subpopulations respond differentially to hypothalamic secreto-inhibitors

The melanotrope population of the frog intermediate lobe consists of two subtypes of cells, referred to as high-(HD) and low-density (LD) melanotrope cells, which differ markedly in their basal morphofunctional features as well as their in vitro response to hypothalamic factors, such as the stimulator thyrotropin-releasing hormone (TRH) and the inhibitor dopamine. In this study, we have investigated whether other major hypothalamic regulators of the release of alpha-melanocyte-stimulating hormone (alpha-MSH), such as gamma-aminobutyric acid (GABA) and neuropeptide Y (NPY), also differentially regulate frog melanotrope subpopulations. Our results show that in LD cells, both factors markedly inhibited proopiomelanocortin (POMC) mRNA accumulation and alpha-MSH secretion. In contrast, the secretory and biosynthetic activity of HD cells was not modified by GABA. NPY inhibited POMC transcript accumulation and tended to reduce alpha-MSH secretion in HD cells, yet these effects were less pronounced than those evoked in LD cells. In addition, GABA and NPY inhibited the KCl-induced rise in cytosolic free calcium levels in both subpopulations. Taken together, these results further indicate that frog melanotrope subpopulations are differentially regulated by the hypothalamus and strongly suggest that the intensity of such regulation is directly related to the activity of the cell subset. Thus, the LD subpopulation represents a highly responsive cell subset which is regulated by multiple neuroendocrine factors (TRH, dopamine, GABA and NPY), whereas the hormone storage HD subpopulation shows a moderate response to single stimulatory (TRH) and inhibitory (NPY) inputs.