[125I]-S36057: a new and highly potent radioligand for the melanin-concentrating hormone receptor.

Shortened, more stable and weakly hydrophobic analogues of melanin-concentrating hormone (MCH) were searched as candidates for radioiodination. Starting from the dodecapeptide MCH(6 - 17), we found that: (1) substitution of Tyr(13) by a Phe residue; (2) addition of a 3-iodo-Tyr residue at the N-terminus; and (3) addition of a hydrophilic spacer 8-amino-3,6-dioxyoctanoyl between the 3-iodo-Tyr and MCH(6 - 17) (compound S36057), led to an agonist more potent than MCH itself in stimulating [35S]-GTPgammaS binding at membranes from HEK293 cells stably expressing the human MCH receptor. Specific binding of [125I]-S36057 was found in HEK293 and CHO cell lines stably expressing the human MCH receptor. This radioligand recognized a similar number of binding sites (ca. 800 fmol mg(-1)) than [125I]-[3-iodo Tyr(13)]-MCH. However, the K(D) for [125I]-S36057 obtained from saturation studies (0.037 nM) or from binding kinetics (0.046 nM) was at least 10 fold higher to that of [125I]-[3-iodo Tyr(13)]-MCH (0.46 nM). Affinities determined for a series of MCH analogues were similar with both radioligands, S36057 being the most potent compound tested (K(i)=0.053 nM). Finally, [125I]-S36057 also potently labelled the MCH receptor in membranes from whole rat brain (K(D) 0.044 nM, B(max)=11 fmol mg(-1)). In conclusion, [125I]-S36057 is a more potent and more stable radioligand than [125I]-[3-iodo Tyr(13)]-MCH that will represent a reliable tool for binding assays in the search of novel MCH ligands. It should also provide great help for autoradiographic studies of the MCH receptor distribution in the central nervous system.