Thyrotropin releasing hormone (TRH) selectively binds and activates the melanocortin 1 receptor Helgi B. Schio ¨th a, *, Peteris Prusis a , Ruta Muceniece a,b , Felikss Mutulis a,c , Ilze Mutule a,b , Jarl E.S. Wikberg a a Department of Pharmaceutical Pharmacology, Uppsala University, Uppsala, Sweden b Laboratory of Pharmacology, Institute of Organic Synthesis, Riga, Latvia c Department of Medicinal Chemistry, Institute of Organic Synthesis, Riga, Latvia Received 12 June 1998; accepted 30 September 1998 Abstract We tested the endogenous tripeptide TRH (thyrotropin releasing hormone) ability to bind to MC (melanocortin) receptor subtypes. We discovered that TRH binds to the human MC1 receptor expressed in COS cells and to murine B16 melanoma cells with 5790  1010 nM and 6370  1260 nM Ki’s, respectively. TRH did not bind to the human MC3, MC4 or MC5 receptor subtypes. Moreover, TRH also stimulated cAMP production in murine B16 melanoma cells reaching the same maximum level of cAMP as found for -MSH. However, several analogues of TRH, including TRH-OH, TRH-Gly-NH 2 and other analogues, where each of the three amino acid residues in TRH had been exchanged by a related residue, did not bind to any of the MC receptors tested in this study. C atoms of molecular models of TRH and the core of a MSH peptide were aligned with r.m.s. of 0.01 Å. Moreover, TRH could be docked into a binding pocket of a molecular model of the MC1 receptor at only a little higher energy than a short cyclic MSH peptide. The data indicate a similarity in the mode of TRH and MSH activation of the MC1 receptor. © 1999 Elsevier Science Inc. All rights reserved. Keywords: TRH; Melanocortin (MC) receptor subtypes; MSH (melanocyte stimulating hormone); Ligand binding; cAMP 1. Introduction The melanocortic (MC) receptors are known to have the melanocortic peptides, ACTH (adrenocorticotropin) and MSH (melanocyte stimulating hormone) as natural ligands. These peptides are primarily known for their role in the regulation of adrenal steroid production [31] and skin pig- mentation [7]. The MSH and ACTH peptides also induce a variety of other peripheral and central effects. Five mela- nocortin receptor subtypes (MC1–5) have been cloned [5,6, 8,9,17] and these receptors are all G-protein coupled. The MC1 receptor is found in melanocytes where it has a role for pigmentation and is also believed have a role in immu- nomodulation [16]. The MC2 receptor is the adrenal gland ACTH receptor and is distinguishable from the other MC receptor subtypes by not binding to the MSH peptides [17,23]. The MC3 receptor is mainly expressed in the brain [8,22] and the MC4 receptor is exclusively expressed in the central nervous system [9,18]. Recent findings, using knock-out techniques [11] have related the MC4 receptor to feeding behavior and weight homeostasis. The MC5 recep- tor is also found in the brain but, more importantly, it has a wide peripheral distribution and is believed to have role for exocrine gland function [4]. Thyrotropin releasing hormone (TRH, thyroliberin, pro- tirelin) is a hypothalamic hormone that releases thyrotropin from the anterior pituitary. TRH is also known to have paracrine regulatory function and to be a neurotransmitter/ neuromodulator. TRH is a tripeptide that binds to and acti- vates the TRH receptor, that also is a G-protein coupled receptor [10]. TRH is also known to induce proopiomela- nocortin biosynthesis and release of -MSH in amphibians and fishes [14,15]. Currently, no non-peptides are known that bind to any of * Corresponding author. Tel.: +46-18-174123; fax: +46-18-559718. E-mail address: helgis@bmc.uu.se (H.B. Schio ¨th) Peptides 20 (1999) 395– 400 0196-9781/99/$ – see front matter © 1999 Elsevier Science Inc. All rights reserved. PII: S0196-9781(99)00168-5