Neuropeptide Y inhibits thyrotropin-releasing hormone-induced stimulation of melanotropin release from the intermediate lobe of the frog pituitary.

Previous studies have shown that the release of melanotropin from frog neurointermediate lobes is under the control of two neuropeptides: thyrotropin-releasing hormone (TRH) stimulates, while neuropeptide Y (NPY) inhibits alpha-melanocyte-stimulating hormone (alpha-MSH) secretion from intact neurointermediate lobes in vitro. The aim of the present study was to investigate possible interactions between the two regulatory peptides at the pituitary level. Whole neurointermediate lobes or acutely dispersed pars intermedia cells from Rana ridibunda were perifused in vitro for 2 to 7.5 hr and the concentrations of alpha-MSH released into the effluent perifusate were monitored by radioimmunoassay. Administration of TRH (10(-7) M) or NPY (10(-7) M) to dispersed cells induced, respectively, marked stimulation or inhibition of alpha-MSH release. The effects of the two neuropeptides were similar to those observed using intact neurointermediate lobes, suggesting that TRH and NPY act directly on melanotropic cells. Perifused whole neurointermediate lobes were exposed to NPY (10(-8) to 3 x 10(-7) M) for 120 min and a single dose of TRH (10(-8) M) was administered during the prolonged infusion of NPY. Using this procedure, we observed a dose-dependent inhibition of TRH-evoked alpha-MSH release. These data support the concept that TRH and NPY act through a common intracellular pathway to regulate alpha-MSH release.