Fish melanocortin system.

Melanocortin signalling is mediated by binding to a family of G protein-coupled receptors that positively couple to adenylyl cyclase. Tetrapod species have five melanocortin (MC(1)-MC(5)) receptors. The number of receptors varies in fish, zebrafish, for example, having six melanocortin receptors, with two copies of the melanocortin MC(5) receptor, while pufferfish have 4 receptors with no melanocortin MC(3) receptor and one copy of melanocortin MC(5) receptor.