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Neuropeptide Y Inhibits Spontaneous α-Melanocyte-Stimulating Hormone (α-MSH) Release via a Y5 Receptor and Suppresses Thyrotropin-Releasing Hormone-Induced α-MSH Secretion via a Y1 Receptor in Frog Melanotrope Cells

2002; Oxford University Press; Volume: 143; Issue: 5 Linguagem: Inglês

10.1210/endo.143.5.8761

1945-7170

Ludovic Galas, Marie‐Christine Tonon, Delphine Beaujean, Robert Fredriksson, Dan Larhammar, Isabelle Lihrmann, Sylvie Jégou, Alain Fournier, Nicolas Chartrel, Hubert Vaudry,

Hypothalamic control of reproductive hormones

In amphibians, the secretion of α-MSH by melanotrope cells is stimulated by TRH and inhibited by NPY. We have previously shown that NPY abrogates the stimulatory effect of TRH on α-MSH secretion. The aim of the present study was to characterize the receptor subtypes mediating the action of NPY and to investigate the intracellular mechanisms involved in the inhibitory effect of NPY on basal and TRH-induced α-MSH secretion. Y1 and Y5 receptor mRNAs were detected by RT-PCR and visualized by in situ hybridization histochemistry in the intermediate lobe of the pituitary. Various NPY analogs inhibited in a dose-dependent manner the spontaneous secretion of α-MSH from perifused frog neurointermediate lobes with the following order of potency porcine peptide YY (pPYY) > frog NPY (fNPY) > porcine NPY (pNPY)-2–36) > pNPY-(13–36) > [d-Trp32]pNPY > [Leu31,Pro34]pNPY. The stimulatory effect of TRH (10−86 m) on α-MSH release was inhibited by fNPY, pPYY, and [Leu31,Pro34]pNPY, but not by pNPY-(13–36) and [d-Trp32]pNPY. These data indicate that the inhibitory effect of fNPY on spontaneous α-MSH release is preferentially mediated through Y5 receptors, whereas the suppression of TRH-induced α-MSH secretion by fNPY probably involves Y1 receptors. Pretreatment of neurointermediate lobes with pertussis toxin (PTX; 1 μg/ml; 12 h) did not abolish the inhibitory effect of fNPY on cAMP formation and spontaneous α-MSH release, but restored the stimulatory effect of TRH on α-MSH secretion, indicating that the adenylyl cyclase pathway is not involved in the action of fNPY on TRH-evoked α-MSH secretion. In the majority of melanotrope cells, TRH induces a sustained and biphasic increase in cytosolic Ca2+ concentration. Preincubation of cultured cells with fNPY (10−7m) or ω-conotoxin GVIA (10−7m) suppressed the plateau phase of the Ca2+ response induced by TRH. However, although fNPY abrogated TRH-evoked α-MSH secretion, ω-conotoxin did not, showing dissociation between the cytosolic Ca2+ concentration increase and the secretory response. Collectively, these data indicate that in frog melanotrope cells NPY inhibits spontaneous α-MSH release and cAMP formation through activation of a Y5 receptor coupled to PTX- insensitive G protein, whereas NPY suppresses the stimulatory effect of TRH on α-MSH secretion through a Y1 receptor coupled to a PTX-sensitive G protein-coupled receptor.