Heterogeneity of brain melanocortin receptors suggested by differential ligand binding in situ

Abstract The existence of multiple brain melanocortin receptor types has been postulated, based on the complex pharmacology of intracerebrally administered melanocortin (melanocyte-stimulating hormone-related) peptides. In this study, this hypothesis was tested by determining whether different brain melanocortin receptor populations can be discriminated on a pharmacologic or neuroanatomic basis. The abilities of various pharmacologically active native melanocortins and structural analogs, as well as other test substances, to compete with biologically active [ 125 I]Nle 4 , d -Phe 7 -α-MSH([ 125 I]NDP-MSH) for binding to melanocortin receptors was determined, by in vitro binding and autoradiography in frozen rat brain tissue sections. We have previously shown that native melanocortins including α-MSH, γ-MSH and ACTH 1–39 compete with [ 125 I]NDP-MSH for binding to brain tissue sites. In the present studies, each of the melanocortin peptides α-MSH, des-acetyl-α-MSH, β-MSH and ACTH 1–24 when present at 1 μM virtually eliminated [ 125 I]NDP-MSH binding in each of a series of brain structures, including medial preoptic area, caudate putamen, olfactory tubercle, bed nucleus of the stria terminalis, ventral part of the lateral septal nucleus, hypothalamic periventricular and paraventricular nuclei, dorsal anterior amygdaloid area, substantia innominata and thalamic paraventricular nucleus; as well as in extraorbital lacrimal gland, a peripheral melanocortin target. In contrast, the behaviorally and neurotrophically active melanocortin analogs Met(O 2 ), d -Lys,Phe 9 -α-MSH 4–9 (Org2766), ACTH 4–9 and the antipyretic peptide α-MSH 11–13 did not affect [ 125 I]NDP-MSH binding at concentrations up to 100 μM, implying that the receptors or receptor binding sites which mediate the actions of these analogs must comprise additional types, distinct from those which bind [ 125 I]NDP-MSH. Binding of [ 125 I]NDP-MSH was also unaffected by the nonmelanotropic peptides ACTH 1–4 , ACTH 34–39 and vasoactive intestinal polypeptide (VIP) and by the antipyretic drugs acetaminophen and lysine-salicylate. Although some of the brain structures are known to express mRNA encoding a γ-MSH-preferring melanocortin receptor type known as MC3, the relative order of binding affinities of melanocortins, determined in concentration-response studies, wasNdp-MSH≥ACTH 1–24 ⩾α-MSH>γ-MSH>ACTH 4–10 probably account for most of the [ 125 I]NDP-MSH binding detectable in the brain. Furthermore, the potency relationships between these respective peptides and the binding activity of D -Trp 7 , D -Phe 10-α-MSH 6–11 amide, a synthetic α-MSH antagonist, varied considerably among the brain sites and peripheral (lacrimal and melanoma) tissues studied, suggesting some degree of heterogeneity of ligand binding properties among [ 125 I]NDP-MSH-binding melanocortin receptor populations in different regions of the brain. Considered together with the available data on the pharmacologic actions of melanocortins and the molecular biology of melanocortin receptors, the present results provide evidence for the existence of multiple, pharmacologically distinct classes of melanocortin receptors in the brain, potentially providing a basis for the pharmacological targeting of specific populations of central melanocortin receptors.