Steroid influences upon the discharge frequencies of a weakly electric fish

Summary

Sternopygus dariensis, a weakly electric gymnotiform fish, possesses sexually different electric organ discharges (EODs), with females emitting EODs at higher frequencies than males. This difference can be accounted for by the effects of gonadal steroids: laboratory experiments indicate that daily injections (0.25 to 20 μg/g body weight) of androgens lower discharge frequencies while injections of estrogens raise discharge frequencies. Clear changes in discharge frequencies can be noted over the course of days, and persist until several days after injections are terminated. These effects are noted irrespective of age or sex, and can be seen in both intact and gonadectomized animals. The discharge frequencies of these animals are established by a neuronal oscillator, the medullary pacemaker nucleus. Thus, the steroid-elicited changes in discharge frequencies are indicative of changes in the activity of the pacemaker nucleus. Steroids may be exerting their effects either through indirect effects upon those neurons which provide input to the pacemaker, or through direct effects upon the spontaneous depolarizations of the pacemaker itself.