Electric fish generate an electric organ discharge (EOD) by which they convey information about gender and individual identity, reproductive condition, and social dominance. In some species, the amplitude of the EOD increases during the dark phase of the photoperiod when fish are socially active, and in response to the presence of another male. Such increases in EOD amplitude can be mimicked by systemic injection with the peptide adrenocorticotropic hormone (ACTH) (Markham, Haskell-Luevano & Stoddard, SFN 2004). ACTH also increases the amplitude of the extracellularly recorded potential when directly applied to an isolated electric organ, demonstrating that ACTH exerts its effects locally on the electric organ. ACTH binds to melanocortin receptors (MCRs) and activates the adenylyl cyclase/PKA signaling pathway. This suggests that ACTH increases EOD amplitude via activation of PKA. Indeed, treatment of isolated electric organ cells with 8bromo cAMP (8Br cAMP) increases the magnitude of the Na+ current under...