BINDING-SITES FOR MELANIN-CONCENTRATING HORMONE (MCH) IN BRAIN SYNAPTOSOMES AND MEMBRANES FROM PERIPHERAL-TISSUES IDENTIFIED WITH HIGHLY TRITIATED MCH

Melanin-concentrating hormone (MCH) is a neuropeptide occurring in the brain of all vertebrate species. In chromatophores of teleost fishes it induces pigment granule aggregation. In mammals, however, its physi ological function is not yet clear. Attempts to identify the site(s) o f its action by binding analysis failed because radioiodinated MCH wit h the natural sequence was devoid of biological activity. We have now synthesized an analogue of rat/human MCH, [Pra(4,8,12,19)]-MCH, contai ning four L-propargylglycine (Pra) residues in positions 4, 8, 12, and 19 for catalytic tritiation to norvaline ([H-3(4)]Nva) residues, each of which containing four tritium atoms. The resulting [H-3]-MCH ([(H- 3(4))Nva(4,8,12,19)]-MCH) had a specific radioactivity of approx. 12,2 00 GBq/mmol (330 Ci/mmol) and retained a biological activity of 10% as compared to rat/human MCH when tested in the carp scale assay. A seri es of qualitative binding studies performed with rat crude membranes f rom brain and peripheal tissues as well as with rat brain synaptosomes using the [H-3]-MCH radioligand provided the first evidence for the p resence of MCH receptors in mammalian tissues. The data showed that sp ecific binding is present in the hypothalamus, hippocampus and in the adrenal gland while none was detected in the brain cortex or spleen. O wing to the tendency of [H-3]-MCH to non-specific binding to tissue, g lass and plastic surfaces, a saturation binding analysis with this rad ioligand was not possible.