Effect of Tyr-Pro-Leu-Gly-NH 2 on melanotropin-stimulating hormone release in the frog and rat

Abstract

The tetrapeptide Tyr-Pro-Leu-Gly-NH 2 (Tyr-MIF I) has been recently characterized in the hypothalamus and pineal of the rat. Since the concentration of Tyr-MIF I in the brain is increased by pinealectomy and is higher when the animals are in the dark, the possibility that Tyr-MIF I could be a physiological regulator of melanotropin secretion has been investigated. For this study a well-defined perifusion model has been applied, using whole neurointermediate lobes from male frogs ( Rana ridibunda Pallas) or male Wistar rats. The amount of α-MSH released in the effluent perifusate was measured by means of a sensitive and specific radioimmunoassay method. For concentrations ranging from 10 −10 to 10 −6 M, Tyr-MIF I did not significantly alter the spontaneous release of α-MSH in the frog nor did it alter the release of α-MSH in the rat. Since it has been recently demonstrated that the tripeptide pGlu-His-Pro-NH 2 (mammalian TRH) is a specific MSH-releasing factor in the frog, the possibility that Tyr-MIF I could modulate the response of the intermediate lobe of the frog to TRH has also been investigated. No alteration of TRH-induced α-MSH release was observed in the presence of a 100-fold excess of Tyr-MIF I. In addition, Tyr-MIF I was found to be unable to lighten the skin of the frog ( Rana pipiens) when applied directly to the pituitary of the darkened animals. Thus these results definitively rule out the possibility that Tyr-MIF I is the melanotropin-release inhibiting factor in the frog or rat.