STRUCTURAL AND FUNCTIONAL-ASPECTS OF THE FAST ELECTROSENSORY PATHWAY IN THE ELECTROSENSORY LATERAL-LINE LOBE OF THE PULSE FISH GYMNOTUS-CARAPO

The fast electrosensory pathway (FEP) of gymnotiform fish is mediated by tuberous electroreceptor organs innervated by ganglion cells that s ynapse with spherical cells of the electrosensory lateral line lobe (E LL). Spherical cells project to the magnocellular mesencephalic nucleu s. The electrosensory environment was represented somatotopically with in ELL. The mandibular (MN) and the supraorbital (SON) nerves projecte d to rostral ELL (occupying 19-28% and 4-10%, respectively), and the p osterior branch of the anterior lateral line nerve (pALLN) projected t o caudal ELL (occupying 56-64%). Labeling with horseradish peroxidase or biotinylated dextran-amine demonstrated three kinds of synaptic end ings coupling primary afferents to spherical cells: multiple synaptic knobs, medium-sized calyxes, and very large calyxes. Multiple synaptic knobs arose from MN and SON primary afferents and were found in a nar row rostral area covering the centrolateral (CLS) and lateral (LS) seg ments of ELL. Medium and large calyxes, proceeding from the same nerve s, predominated in the remaining parts of the three segments of ELL co ntaining spherical cells. Calyx-type endings were also found in the LS -occupying regions in which the pALLNs projected. Calyx-type endings f ormed gap junctions but also contained vesicles and showed submembrane specializations typical of chemical synapses. The postsynaptic spheri cal cells were linked by dendrosomatic gap junctions and were also con tacted by unlabeled en passant synaptic boutons, whose fine structure suggested chemical transmission. Electrophysiological studies indicate d that spherical cell responsiveness diminished after electrosensory s timulation. This apparently inhibitory phenomenon may be subserved by the unlabeled synaptic boutons, which possibly originate from interneu rons that have yet to be identified. (C) 1998 Wiley-Liss, Inc.