Melanotropin receptors in the brain are differentially distributed and recognize both corticotropin and alpha-melanocyte stimulating hormone.

Melanotropinergic neurons in the brain may mediate the known modulatory effects of alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropic hormone (ACTH) on brain functions including thermoregulation, pituitary regulation, learning and behavior. To determine the distribution of brain melanotropin receptors, we used biologically active [125I]Nle4, D-Phe7-alpha-MSH ([125I]NDP-MSH) for in situ binding and autoradiography in frozen rat brain sections. Specific (alpha-MSH-inhibitable) [125I]NDP-MSH binding sites were distributed in a region-specific pattern, and were present in numerous structures within the septal area, hypothalamus, thalamus, epithalamus, olfactostriatal complex, and midbrain. Each brain structure studied showed a characteristic, reproducible distribution and relative intensity of binding. Receptor peptide selectivity was assessed by comparing the dose-response relationships for inhibition of binding by alpha-MSH, NDP-MSH and ACTH. In all brain structures studied, the 3 peptides gave comparable maximal inhibition of tracer binding, indicating that all detectable binding sites recognized all 3 melanotropins. The respective relative potencies were: NDP-MSH (EC50 = 1.7 +/- 0.6 nM) greater than alpha-MSH (EC50 = 46.9 +/- 11.7 nM) = ACTH. These results provide a preliminary neuroanatomic map of potential target sites for melanotropin actions, and indicate that these sites are capable of recognizing multiple products of the intrinsic melanotropinergic system of the brain.