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.

IGF-I is expressed in somatotrophs, and IGF-I receptors are expressed in most somatotrophs and some corticotrophs in the mouse pituitary gland. Our recent study demonstrated that IGF

Alpha-melanocyte-stimulating hormone (α-MSH) stimulates prolactin (PRL) release and mammotrope proliferation in mouse anterior pituitary glands. The present study investigated the regulation of melanocortin-3 receptor (MC3-R) mRNA levels in mice using semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. Relative MC3-R mRNA levels in the anterior lobes of female mice increased between 20 and 45 days of age, and a significant difference in MC3-R mRNA expression between sexes was seen at 45 days. Ovariectomy decreased MC3-R mRNA expression in the female anterior lobes, and estradiol-17β (E₂) treatment increased MC3-R mRNA levels in ovariectomized mouse anterior lobes and cultured anterior pituitary cells. E₂ treatment increased proopiomelanocortin (POMC) mRNA levels in ovariectomized mouse neurointermediate lobes and cultured neurointermediate pituitary cells. On the other hand, E₂ treatment did not affect POMC mRNA expression in mouse anterior lobes or cultured anterior pituitary cells. These results suggest that α-MSH directly stimulates PRL release and mammotrope proliferation through MC3-Rs expressed in mammotropes, while estrogen stimulates MC3-R gene transcription in the anterior pituitary cells and POMC gene transcription in the intermediate lobes. In lactating mice, POMC mRNA levels in the neurointermediate lobes were elevated compared with in non-lactating mice. The present study suggests that α-MSH is involved in augmented PRL secretion by estrogen and during lactation.