A4olecular and Cellular Endocrinology, 50 (1987) 203-209 Elsevier Scientific Publishers Ireland, Ltd. 203 MCE 01624 Biphasic effect of thyrotropin-releasing factor (TRH) on a-melanotropin secretion from frog intermediate lobe in vitro Marek Lamacz, Marie-Christine Tonon, Jean-Michel Danger, Bruce Jenks ‘, Gotfryd Kupryszewski 2 and Hubert Vaudry Groupe de Recherche ett Endocrinologie Molkuiaire, UA CNRS 6S0, Unrth Allihe ir I’INSERM, Faculty des Sciemw. Uniclersrt6 de Rouen, 76130 Mont-Saint-Aignan, Frmce, ’ Departmertt of Zoologv, Focul~v of Sciences, Catholic Urtrc~ersrt,~, hS.?S ED Nijnwge~l, The Netherlands, md .’ Laborator?, of Bioorganrc Chemistr)?. Imtrtute of Chemistyv, lJniwrsi!v of Gdamh. Poland (Received 24 July 19X6; accepted 27 November 19X6) Kqv words; Intermediate lobe; (Pituitary): sin; Perifusion; (Amphibians) Summary Desensitization; Melanotropin; Thyrotropin-releasing hormone: Cycloheximide: Monen- The kinetics of cw-MSH secretion induced by prolonged TRH infusion were studied using perfused frog neurointermediate lobe (NIL). During a 2 h administration of TRH (lo-’ M), the secretion rate of a-MSH displayed two phases. During the first phase, secretion of (u-MSH increased rapidly reaching a maximum within 20 min and then, despite continued TRH infusion, this secretion slowly declined. The second phase was characterized as plateau of elevated release (relative to basal secretion); within this second phase there was often a small peak of released a-MSH occurring at about 100 min. Exposure of NIL to another TRH (lop8 M) pulse 90 min later induced a normal stimulation of a-MSH secretion, thus demonstrating the viability of tissue in perifusion. Continuous infusion of cycloheximide (10 5 M) during a 5 h period totally inhibited the biosynthetic activity of NIL but did not influence TRH-induced a-MSH secretion. In particular, cycloheximide had no effect on the second phase of the response to prolonged infusion of TRH. Similarly, during continuous infusion of the monovalent carboxylic ionophore monensin (lop6 M), the biphasic response to prolonged infusion of TRH (10-s M) was still observed. Administra- tion of a short pulse of TRH (lo-’ M) during the declining part of the first phase or during the second phase of prolonged TRH (lo-* M) infusion induced a significant enhancement of (u-MSH stimulation. From these results we conclude that (1) prolonged TRH infusion causes a-MSH release in a biphasic manner; (2) attenuation of the secretory response to continuous TRH administration does not result from exhaustion of the releasable pool of wMSH; (3) the biphasic response depends neither on de novo biosynthesis nor on intracellular transit of the prohormone from the endoplasmic reticulum to the Golgi complex. Thus, we propose that prolonged exposure of frog NIL to TRH may cause transconformational changes of membrane TRH receptors from high to low affinity states. Address for correspondence: M. Laman, Groupe de Re- cherche en Endocrinologie Moleculaire, UA CNRS 650, Unite Alliee a I’INSERM, Faculte des Sciences, Universite de Rouen, 76130 Mont-Saint-Aignan, France. Introduction There is clear evidence that, during constant exposure to hypothalamic releasing factors, pitu- 0303-7207/87/$03.50 0 1987 Elsevier Scientific Publishers Ireland. Ltd