Melanin-concentrating hormone (MCH) and
alpha-melanocyte-stimulating hormone (
alpha-MSH) are known to exhibit mostly functionally antagonistic, but in some cases agonistic activities, e.g., in pigment cells and in the brain.
Neuropeptide E-I (NEI) displays functional MCH-antagonist and
MSH-agonist activity in different behavioral paradigms; the role of
neuropeptide G-E (NGE) is not known. This study addressed the question of possible molecular interactions between
alpha-MSH, MCH and the MCH-precursor-derived
peptides NEI and NGE at the level of the pigment cell
MCH receptor subtype (MCH-Rpc) and the different
melanocortin (MC) receptors. Radioreceptor assays using [125I]MCH, [125l]
alpha-MSH and [125I]NEI as radioligands and bioassays were performed with MCI-R-positive and MC1-R-negative mouse
B16 melanoma cells and with COS cells expressing the different MC receptors. The IC50s of
alpha-MSH and NEI or NGE for [125I]MCH displacement from mouse MCH-Rpc were 80-fold and, respectively, >300-fold higher than that of MCH, and the IC50s for MCH and NEI or NGE for [125I]
alpha-MSH displacement from mouse MC1-R were 50,000-fold and >200,000-fold higher than that of
alpha-MSH. No high-affinity binding sites for NEI were detected on
B16 melanoma cells and there was no significant displacement of [1251]
alpha-MSH by MCH, NEI or NGE with MC3-R, MC4-R and MC5-R expressed in COS cells. At concentrations of 100 nM to 10 microM, however, MCH, NEI and NGE induced cAMP formation and
melanin synthesis which could be blocked by
agouti protein or inhibitors of
adenylate cyclase or
protein kinase A. This shows that mammalian MCH-precursor-derived
peptides may mimic
MSH signalling via MC1-R activation at relatively high, but physiologically still relevant concentrations, as e.g. found in autocrine/paracrine signalling mechanisms.