Melanocortin‐3 Receptor mRNA Expression in Pro‐Opiomelanocortin Neurones of the Rat Arcuate Nucleus

The melanocortins α ‐ and γ ‐melanocyte‐stimulating hormones ( α ‐ and γ ‐MSH) derive from the pro‐opiomelanocortin (POMC) precursor. Melanocortins exert a wide range of biological activities in the brain through activation of at least three distinct melanocortin receptor (MC‐R) subtypes. In order to determine whether POMC neurones can modulate their own activity, we looked for the possible expression of the MC3‐R gene in POMC‐positive cell bodies in the rat hypothalamus. In situ hybridization experiments revealed that the density of MC3‐R mRNA is particularly high in the arcuate nucleus which contains the main population of POMC neurones in the brain. The occurrence of MC3‐R mRNA in POMC‐positive cell bodies was demonstrated using a double‐labelling in situ hybridization technique. The proportion of POMC neurones expressing MC3‐R mRNA was significantly higher in the most rostral (43.5%) than in the most posterior part of the arcuate nucleus (8.2%). These results indicate that melanocortins likely exert a direct regulatory feedback on POMC neurones through activation of MC3‐R receptors. Our data also suggest that MC3‐R may be involved in the neuroendocrine responses induced by centrally administered melanocortins.