MELANOCORTIN RECEPTORS MEDIATE ALPHA-MSH-INDUCED STIMULATION OF NEURITE OUTGROWTH IN NEURO 2A CELLS

Melanocortins (MC), neuropeptides derived from pro-opiomelanocortin, h ave been implicated in enhancing neurite outgrowth via an as yet unkno wn mechanism. Recently, five MC receptors have been identified, three of which, the MC(3)-R, the MC(4)-R and the MC(5)-R, are expressed in t he nervous system. In this study,alpha-MSH and the melanocortin analog [D-Phe(7)]ACTH (4-10) were able to stimulate neurite outgrowth in the neuroblastoma cell line Neuro 2A. ACTH (4-10), gamma,-MSH and ORG2766 were inactive. In addition, the MC(4)-R antagonist [D-Arg(8)]ACTH (4- 10), inhibited the alpha-MSH effect, indicating that the MC(4)-R media ted stimulation of neurite outgrowth by alpha-MSH. Indeed, the presenc e of MC(4)-R mRNA in Neuro 2A cells was demonstrated by a RNase protec tion assay. Heterologous expression of the MC(5)-R in Neuro 2A cells l ead to the recruitment of a responsiveness to gamma(2)-MSH, but did no t increase the effect of alpha-MSH on neurite outgrowth. This finding indicated that the function of MC(4)-R can also be exerted by another MC receptor, suggesting that the coupling to G(5), which they have in common, plays an essential role in the neurite outgrowth promoting eff ect. This was further substantiated by the fact that forskolin treatme nt per se induced neurite outgrowth in a similar fashion. These data i mply that the neurotrophic properties of alpha-MSH are likely to resul t from G(s)-coupled MC receptor activity in neuronal cells.