Chronic androgen treatment increases action potential duration in the electric organ of Sternopygus

Abstract

The quasi-sinusoidal electric organ discharge (EOD) of the weakly electric fish Sternopygus is involved in communication and orientation. Each monophasic pulse of the low-intensity EOD is a compound action potential (AP) from the simultaneously firing electrocytes of the electric organ. EOD frequency is lower and EOD pulse duration longer in sexually mature males than in sexually mature females; exogenous androgen lowers EOD frequency and increases EOD pulse duration. In order to determine the contribution of single electrocyte spikes to the entire EOD pulse, APs were induced by intracellular current injection in single electrocytes of isolated pieces of electric organ. Each AP looks very similar to the externally recorded EOD pulses, and AP duration (APD) is significantly correlated with EOD pulse duration (r = 0.48; p less than 0.0005). The APD is slightly longer when compared to the EOD pulse duration, but this difference is likely due to the stimulation paradigm. Fish treated with dihydrotestosterone showed a decrease in EOD frequency, increase in EOD pulse duration, and corresponding increase in APD; control fish showed random, insignificant changes in EOD wave form and APD. Evidence presented here shows that changes in the passive membrane properties are unlikely to be responsible for the APD increase. The possibility is discussed that androgens act directly upon the electric organ, ultimately altering the ionic currents that produce the AP.