Convergence of electrotonic club endings, GABA- and serotoninergic terminals on second order neurons of the electrosensory pathway in mormyrid fish, Gnathonemus petersii and Brienomyrus niger (Teleostei).

Previous electrophysiological data indicate that the afferent electrosensory impulses conveyed towards the mesencephalon are blocked in the rhombencephalic electrosensory lateral line nucleus (nELL) by the concomitantly occurring EOD (electric organ discharge) command-associated (corollary) discharge. Electron-microscopic observations and anterograde labeling with horseradish peroxidase show that the primary electrosensory fibers terminate with club endings on the adendritic soma of the nELL cells and form gap junctions with the postsynaptic membrane. The remaining part of the soma and the initial segment membrane of nELL cells are covered with a large number of boutons showing chemical synaptic profiles. The GABA-ergic (gamma-aminobutyric-acid) nature of the majority of the boutons is revealed immunocytochemically by anti-GABA and anti-glutamic acid decarboxylase (anti-GAD) antisera, as seen in the light microscope. Electron-microscopic examination confirms the GABAergic nature of most of the bouton-like terminals, whereas club endings show negative immunoreactivity. In addition, serotonin-immunoreactive fibers and boutons are found in the same nucleus, between and next to the nELL cells. It is suggested that the GABAergic endings are the morphological basis for the inhibition that occurs in the nELL and that is mediated by the corollary discharge.