Abstract
Knifefish of the genus Eigenmannia (Gymnotiformes, Teleostei) are seasonal breeders that spawn only during the tropical rainy season. Both sexes modulate their otherwise constant wave-like electric organ discharges (EODs) in the context of courtship and aggressive behavior by “chirps,” abrupt frequency modulations with brief interruptions. Play-backs of recordings of male courtship chirps can induce spawning in gravid females (Hagedorn and Heiligenberg, 1985). The EOD, produced by a specialized electric organ, is under the control of a pacemaker nucleus (Pn) in the medulla oblongata. Injections of HRP into the Pn label only a small cluster of cells bilaterally at the boundary of diencephalon and mesencephalon, constituting the prepacemaker nucleus, PPn (Heiligenberg et al., 1981). Microstimulation experiments have shown that chirp-like EOD modulations can be elicited from a subnucleus of the PPn, the PPn-C (Kawasaki and Heiligenberg, 1988; Kawasaki et al., 1988). By retrograde HRP labeling, we investigated the dependence of the PPn's morphology upon the sexual maturity of the fish. The most prominent effect was that, during the breeding season, females developed an abundance of “varicosities,” swellings 1–4 microns in diameter in distal regions of dendrites at a density of approximately 1/10 microns. In contrast to mature females, immature females had none or only a few varicosities. Such a clear correlation between the abundance of varicosities and relative gonadal weight was not found in males, most likely because testicular and dendritic development are not strictly synchronized. After the onset of the simulated dry season, however, relative gonadal weight, as well as the number of varicosities, was reduced drastically in both sexes. This reduction in the number of varicosities is accompanied by a decrease in their diameter. Varicosities may represent growing buds or regions of synaptic input from afferent areas or both. Reconstruction of individual PPn-C neurons showed maturity-dependent changes in the pattern of dendritic proliferation in females. Such dynamic changes in the structure of neurons might subserve seasonal modifications in an animal's propensity to execute specific behaviors.
