AUTORADIOGRAPHIC DISCRIMINATION OF MELANOCORTIN RECEPTORS INDICATES THAT THE MC3 SUBTYPE DOMINATES IN THE MEDIAL RAT-BRAIN

In the present study, we developed an autoradiographic method to visua lize the distribution of melanocortin (MC) receptors 3 and 4 in sagitt al sections of the rat brain. The method takes advantage of the MC3 an d MC4 receptor selective compounds, gamma 1-MSH and HS014. First, we c haracterized the binding of gamma 1-MSH, HS014 and the radioligand [I- 125]NDP-MSH to the rat MC3 and MC4 receptors expressed in COS cells. [ I-125]NDP-MSH was found to be non-selective, whereas yl-MSH showed a 4 0-fold preference for the rat MC3 receptor, and HS014 an over 300-fold preference to the rat MC4 receptor. Second, to discriminate between t he MC3 and MC4 receptors in rat brain sections, the sections were incu bated with [I-125]NDP-MSH in the presence of graded concentrations of the MC3 selective ligand, gamma 1-MSH, or the MC4 selective ligand, HS 014. From the autoradiograms thus made, competition curves of gamma 1- MSH and HS014 could be constructed for different regions of the rat br ain. Our results indicate that in the nucleus accumbens shell, the med ial preoptic area, and the ventromedial nucleus of the hypothalamus, t here is a clear dominance of the MC3 receptor, whereas in the lateral septum and the olfactory tubercle, there seem to be present both MC3 a nd MC4 receptors, although the MC3 receptor may still be the dominatin g subtype. In the optic layer of the superior colliculus, our data ind icate a more abundant expression of the MC4 receptor. in the ventral t egmental area, there might be an additional MSH-peptide binding site o f unknown origin. (C) 1998 Elsevier Science B.V. All rights reserved.