The electric organ discharge (EOD) of gymnotiform electric fish is produced by the simultaneous electric discharges of individual electrocytes (μEODs). In the gymnotiform Brachyhypopomus pinnicaudatus, the EOD and its constituent μEODs are biphasic sinusoids. The biphasic μEOD is produced by the sequential action potentials of the electrocyte’s two excitable membranes. The first action potential (AP1) occurs on the electrocyte’s posterior membrane, and contributes to the positive first phase (P1) of the μEOD. The second action potential (AP2) occurs on the electrocyte’s anterior membrane, producing the negative second phase (P2) of the μEOD. Masculine EODs show greater overall amplitude and greater repolarization time of P2 (tau-P2). In males, the masculinity of the waveform is enhanced on a circadian rhythm, and within minutes in response to stressors and changes in the social environment thus altering the signal’s communication function. We reported previously (SFN 2004) that in vivo intramuscular inject...