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 visualize the distribution of melanocortin (MC) receptors 3 and 4 in sagittal sections of the rat brain. The method takes advantage of the MC3 and MC4 receptor selective compounds, gamma1-MSH and HS014. First, we characterized the binding of gamma1-MSH, HS014 and the radioligand [125I]NDP-MSH to the rat MC3 and MC4 receptors expressed in COS cells. [125I]NDP-MSH was found to be non-selective, whereas gamma1-MSH showed a 40-fold preference for the rat MC3 receptor, and HS014 an over 300-fold preference to the rat MC4 receptor. Second, to discriminate between the MC3 and MC4 receptors in rat brain sections, the sections were incubated with [125I]NDP-MSH in the presence of graded concentrations of the MC3 selective ligand, gamma1-MSH, or the MC4 selective ligand, HS014. From the autoradiograms thus made, competition curves of gamma1-MSH and HS014 could be constructed for different regions of the rat brain. Our results indicate that in the nucleus accumbens shell, the medial preoptic area, and the ventromedial nucleus of the hypothalamus, there is a clear dominance of the MC3 receptor, whereas in the lateral septum and the olfactory tubercle, there seem to be present both MC3 and MC4 receptors, although the MC3 receptor may still be the dominating subtype. In the optic layer of the superior colliculus, our data indicate a more abundant expression of the MC4 receptor. In the ventral tegmental area, there might be an additional MSH-peptide binding site of unknown origin.