Coordination of EOD frequency and pulse duration in a weakly electric wave fish: the influence of androgens

Abstract

1. The electric organ discharge (EOD) of wave-type weakly electric fish is generated as an extremely regular series of electric organ pulses. Measurements of pulse duration and EOD frequency were made in the speciesSternopygus andEigenmannia. Pulse duration is highly inversely correlated with EOD frequency in a population of fish, so that the EOD waveform remains quasisinusoidal over the species range of EOD frequencies. This places most of the energy of the EOD within the fundamental harmonic to which the electroreceptors are most sensitive. 2. Treatment with the anesthetic MS-222 transiently lowers EOD frequency, but does not change pulse duration. This demonstrates that pulse duration is independent of immediate EOD frequency and, therefore, of the medullary pacemaker nucleus (PMN). 3. InSternopygus, small monophasic potentials could be recorded outside the tails of curarized fish. These were of constant duration and shape across all fish regardless of the fish's EOD frequency. The location along the body where this potential reversed polarity was close to, although not identical with, the isopotential line for the EOD pulse. These potentials were partially blocked by the curare. They probably represent the summed psps of the synchronously-firing electrocytes. This result supports the hypothesis that differences in EOD waveform, specifically pulse duration, arise from electrocyte membrane events following the psp (postsynaptic potential). 4. Implantation ofSternopygus with 5-α-dihydrotestosterone (DHT) in silastic capsules results in decreased EOD frequencies, as previously reported for this species (Meyer and Zakon 1982; Meyer 1983). In addition, corresponding significant increases in EOD pulse duration occurred (a mean increase of about 1.3 ms or 24%). No changes were measured in controls implanted with empty capsules. The maximum decrease in EOD frequency and increase in pulse duration plateau at different times, suggesting that they are the result of different processes. After removal of the hormone capsules, EOD frequency and pulse duration reverted to baseline levels within a few weeks. 5. These results demonstrate a degree of coordination between the PMN and the electric organ not previously recognized. It is likely that androgens play a role in this process. If, as proposed, the electrocyte membrane properties determine the characteristics of the waveform, then the androgen must act on both the PMN and the electrocytes to keep EOD frequency and pulse duration in register.