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

The presence and possible physiological roles of alpha -melanocyte-stimulat ing hormone (alpha -MSH) in the peripheral tissues of birds have not been e stablished. By a combination of RT-PCR, immunocytochemistry and in situ hyb ridization, we have examined alpha -MSH expression in the eye of the chicke n during development. In the 1-day-old chick, alpha -MSH was expressed in t he 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 th e RPE cells. It is probable that the RPE cells secrete alpha -MSH to exert paracrine effects on the choroid and neural retina. During embryonic develo pment, alpha -MSH immunoreactivity in the RPE cells was initially detected at embryonic day 10, and increased in intensity as development proceeded. N o cone cells were stained with anti-alpha -MSH antiserum in any of the embr yonic 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 alpha -MSH. Despite containing alpha -MSH immunoreactiv ity, the RPE cells in 1-day-old chicks expressed no immunoreactivity for th e endoproteases. Furthermore, in a 3-day-old chick, proopiomelanocortin mRN A 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 alpha -MSH in the embryonic and postn atal life of the chicken respectively. Embryonic expression of alpha -MSH i n the RPE cells implies a possible role for the peptide in ocular developme nt.