PROJECTION NEURONS OF THE MORMYRID ELECTROSENSORY LATERAL-LINE LOBE -MORPHOLOGY IMMUNOHISTOCHEMISTRY, AND SYNAPTOLOGY

This paper describes the morphological, immunohistochemical, and synap tic properties of projection neurons in the highly laminated medial an d dorsolateral zones of the mormyrid electrosensory lateral line lobe (ELL), These structures are involved in active electrolocation, i.e., the detection and localization of objects in the nearby environment of the fish on the basis of changes in the reafferent electrosensory sig nal generated by the animal's own electric organ discharge. Electrosen sory, corollary electromotor command-associated signals (corollary dis charges), and a variety of other inputs are integrated within the ELL microcircuit. The organization of ELL projection neurons is analyzed a t the light and electron microscopic levels based on Golgi impregnatio ns, intracellular labeling, neuroanatomical tracer techniques, and gam ma-aminobutyric acid (GABA), gamma-aminobutyric acid decarboxylase (GA D), and glutamate immunohistochemistry. Two main types of ELL projecti on neurons have been distinguished in mormyrids: large ganglionic (LG) and large fusiform (LF) cells. LG cells have a multipolar cell body ( average diameter 13 mu m) in the ganglionic layer, whereas LF cells ha ve a fusiform cell body (on average, about 10 x 20 mu m) in the granul ar layer. Apart from the location and shape of their soma, the morphol ogical properties of these cell types are largely similar. They are gl utamatergic and project to the midbrain torus semicircularis, where th eir axon terminals make axodendritic synaptic contacts in the lateral nucleus. They have 6-12 apical dendrites in the molecular layer, with about 10,000 spines contacted by GABA-negative terminals and about 3,0 00 GABA-positive contacts on the smooth dendritic surface between the spines. Their somata and short, smooth basal dendrites, which arborize in the plexiform layer (LG cells) or in the granular layer (LF cells) , are densely covered with GABA-positive, inhibitory terminals. Correl ation with physiological data suggests that LG cells are I units, whic h are inhibited by stimulation of the center of their receptive fields , and LF cells are E units, excited by electric stimulation of the rec eptive field center. Comparison with the projection neurons of the ELL of gymnotiform fish, which constitute another group of active electro locating teleosts, shows some striking differences, emphasizing the in dependent development of the ELL in both groups of teleosts. (C) 1996 Wiley-Liss, Inc.