α-melanocyte-stimulating hormone stimulates prolactin secretion through melanocortin-3 receptors expressed in mammotropes in the mouse pituitary

The intermediate lobe of rodent pituitaries is involved in the regulation of prolactin (PRL) secretion from the anterior lobe. In a previous study, we demonstrated the stimulatory effect of α-melanocyte-stimulating hormone (α-MSH) on PRL release and the expression of melanocortin-3 receptors (MC3-Rs) in cultured mouse pituitary cells. The aim of the present study was to clarify whether α-MSH directly stimulates PRL release through the MC3-Rs by determining the cell type of MC3-R-expressing cells in the mouse pituitary anterior lobe. Northern blot analysis revealed a 2.7-kb transcript for MC3-R mRNA in the anterior and neurointermediate lobes of pituitary glands of adult male and female mice. Dual cellular localization of MC3-R mRNA and PRL or growth hormone (GH) in the mouse pituitary glands was performed by in situ hybridization analysis of MC3-R mRNA followed by immunocytochemical detection of PRL or GH. MC3-R mRNA was detected in most mammotropes and some somatotropes. α-MSH increased PRL release and stimulated DNA replication in mammotropes, and these effects were blocked by SHU9119, an antagonist of MC3-R and MC4-R. These results indicate that α-MSH stimulates PRL release and proliferation of mammotropes through MC3-Rs, and suggest that α-MSH from intermediate lobes can regulate mammotrope functions in the mouse pituitary.