Melanocortin receptors mediate α-MSH-induced stimulation of neurite outgrowth in Neuro 2A cells

Melanocortins (MC), neuropeptides derived from pro-opiomelanocortin, have been implicated in enhancing neurite outgrowth via an as yet unknown mechanism. Recently, five MC receptors have been identified, three of which, the MC₃-R, the MC₄-R and the MC₅-R, are expressed in the nervous system. In this study, α-MSH and the melanocortin analog [D-Phe⁷]ACTH (4-10) were able to stimulate neurite outgrowth in the neuroblastoma cell line Neuro 2A. ACTH (4-10), γ₂-MSH and ORG2766 were inactive. In addition, the MC₄-R antagonist [D-Arg⁸]ACTH (4-10), inhibited the α-MSH effect, indicating that the MC₄-R mediated stimulation of neurite outgrowth by α-MSH. Indeed, the presence of MC₄-R mRNA in Neuro 2A cells was demonstrated by a RNase protection assay. Heterologous expression of the MC₅-R in Neuro 2A cells lead to the recruitment of a responsiveness to γ₂-MSH, but did not increase the effect of α-MSH on neurite outgrowth. This finding indicated that the function of MC₄-R can also be exerted by another MC receptor, suggesting that the coupling to G(s), which they have in common, plays an essential role in the neurite outgrowth promoting effect. This was further substantiated by the fact that forskolin treatment per se induced neurite outgrowth in a similar fashion. These data imply that the neurotrophic properties of α-MSH are likely to result from G(s)-coupled MC receptor activity in neuronal cells.