Ionic currents that contribute to a sexually dimorphic communication signal in weakly electric fish

Weakly electric fish produce a communication signal, the electric organ dis charge, that is species specific, and in many species, sexually dimorphic. Because the neural circuit that controls the electric organ discharge is re latively simple, it is an excellent model in which to study both the biophy sical mechanisms underlying a rhythmic behavior and the neuroendocrine cont rol of a sexually dimorphic behavior. By studying the effects of ion channe l blockers on neurons in the medullary pacemaker nucleus, I pharmacological ly characterized three ionic currents that influence the pacemaker rhythm, and thus electric organ discharge frequency, in the gymnotiform fish, Apter onotus leptorhynchus. These currents included a tetrodotoxin-sensitive sodi um current; a potassium current that was sensitive to 4-aminopyridine; and a calcium current that was sensitive to nickel and cadmium, but resistant t o specific blockers of L-, N-, P-, and Q-type calcium currents. The pharmac ological profiles of the ionic currents in the pacemaker nucleus are simila r to those of ionic currents involved in pacemaking in other neuronal oscil lators. Because these ionic currents dramatically influence pacemaker firin g frequency, which is directly related to electric organ discharge frequenc y, these ionic currents are likely targets of steroid hormone action in pro ducing Sexual dimorphisms in electric organ discharge frequency. Additional studies are needed to determine how these ionic currents interact to gener ate the electric organ discharge rhythm and to investigate the possibility that sexual dimorphism in the electric organ discharge results from the act ions of gonadal steroids on these ionic currents.