Identification of avian α-melanocyte-stimulating hormone in the eye: Temporal and spatial regulation of expression in the developing chicken

The presence and possible physiological roles of α-melanocyte-stimulating hormone (α-MSH) in the peripheral tissues of birds have not been established. By a combination of RT-PCR, immunocytochemistry and in situ hybridization, we have examined α-MSH expression in the eye of the chicken during development. In the 1-day-old chick, α-MSH was expressed in the retinal pigment epithelial (RPE) cells, and also at a lower level in the cone cells. The melanocortin receptor subtypes, CMC1, CMC4 and CMC5, were expressed in the layers of the choroid and the neural retina, but not in the RPE cells. It is probable that the RPE cells secrete α-MSH to exert paracrine effects on the choroid and neural retina. During embryonic development, α-MSH immunoreactivity in the RPE cells was initially detected at embryonic day 10, and increased in intensity as development proceeded. No cone cells were stained with anti-α-MSH antiserum in any of the embryonic stages tested. The immunoreactivities for two prohormone convertases, PC1 and PC2, were co-localized to the RPE cells with a pattern of staining similar to that of α-MSH. Despite containing α-MSH immunoreactivity, the RPE cells in 1-day-old chicks expressed no immunoreactivity for the endoproteases. Furthermore, in a 3-day-old chick, proopiomelanocortin mRNA was detectable by in situ hybridization only in the photoreceptor layer and not in the RPE cells. These results suggest that the RPE cells and the cone cells are intraocular sources of α-MSH in the embryonic and postnatal life of the chicken respectively. Embryonic expression of α-MSH in the RPE cells implies a possible role for the peptide in ocular development.