Behavioral actions of androgens and androgen receptor expression in the electrocommunication system of an electric fish, Eigenmannia virescens.

Gymnotiform electric fish emit an electric organ discharge (EOD) that, in many species, is sexually dimorphic and used in gender recognition. The glass knife fish Eigenmannia has been a major neuroethological model system, and there have been reports that its EOD is sexually dimorphic. However, no study has examined the role of steroids in establishing this dimorphism. We tested the effect of three androgens, testosterone, dihydrotestosterone, and 11-ketotestosterone (11kT), on the EOD of Eigenmannia virescens. Implants of any of these three androgens induced a decrease in EOD frequency, consistent with several studies showing that males have a lower EOD frequency than females. In a separate experiment, we found that 11kT treatment also increases EOD pulse duration, but has no effect on the duty cycle, the relative duration of the positive and negative phases of the wave, or the harmonic content of the EOD. Using immunocytochemistry, we found that the cells of the electric organ (the electrocytes) that produce the EOD and control its pulse duration label positively with an antibody to the androgen receptor. These experiments indicate that androgens decrease the firing frequency of the medullary pacemaker and alter the ion current kinetics in the electrocytes. Moreover, the presence of nuclear androgen receptors in electrocytes suggests that androgens act directly on the electric organ to modify EOD pulse duration.