INTERNEURONS OF THE GANGLIONIC LAYER IN THE MORMYRID ELECTROSENSORY LATERAL-LINE LOBE - MORPHOLOGY, IMMUNOHISTOCHEMISTRY, AND SYNAPTOLOGY

This is the second paper in a series that describes the morphology, im munohistochemistry, and synaptology of the mormyrid electrosensory lat eral li:ne lobe (ELL). The ELL is a highly laminated cerebellum-like s tructure in the rhombencephalon that subserves an active electric sens e: Objects in the nearby environment of the fish are detected on the b asis of changes in the reafferent electrosensory signals that are gene rated by the animal's own electric organ discharge. The present paper describes interneurons in the superficial (molecular, ganglionic, and plexiform) layers of the ELL cortex that were analyzed in the light an d electron microscopes after Golgi impregnation, intracellular labelin g, neuroanatomical tracing, and gamma-aminobutyric acid (GABA) immunoh istochemistry. The most numerous interneurons in the ganglionic layer are GABAergic medium-sized ganglionic (MG) cells and small ganglionic (SG) cells. MG cells have 10-20 spiny apical dendrites in the molecula r layer, a cell body of 10-12 mu m diameter in the ganglionic layer, a single basal dendrite that gives rise to fine, beaded, axon-like bran ches in either the plexiform layer (MG(1) subtype) or the deeper granu lar layer (MG(2) subtype), and an axon that terminates in the plexifor m layer. Their apical dendritic tree has 12,000-22,000 spines that are contacted by GABA-negative terminals, and it receives 1,250-2,500 GAB A-positive contacts on the smooth dendritic surface between the spines . The average ratio of GABA-negative to GABA-positive contacts on the interneuron apical dendrites (14:1) is significantly higher than that for the efferent projection cells that have been described previously (Grant et al. [1996] J. Comp. Neurol., this issue). The somata and bas al dendrites of MG cells receive a low to moderate density of GABAergi c synaptic input, and their axons make GABAergic synaptic contacts wit h the somata and cell bodies of MG as well as with large ganglionic (L G) cells. SG cells probably represent immature, growing MG cells. Othe r interneurons in the superficial ELL layers include GABAergic stellat e cells in the molecular layer, two types of non-GABAergic cells with smooth dendrites in the deep molecular layer that are named thick-smoo th dendrite cells and deep molecular layer cells, and horizontal cells that are encountered particularly in the plexiform layer. Comparison with the ELL of waveform gymnotiform fish, which is another group of a ctive electrolocating teleosts that has been investigated thoroughly, shows striking differences. In these fish, no GABAergic interneurons a re found in the ganglionic (pyramidal) layer of the ELL, and GABA-nega tive interneurons with smooth dendrites in the molecular layer also se em to be lacking. At present, the phylogenetic origin of the described superficial interneurons in the mormyrid ELL is uncertain. (C) 1996 W iley-Liss, Inc.