DISTRIBUTION OF GABA, GLYCINE, AND GLUTAMATE IMMUNOREACTIVITIES IN THE VESTIBULAR NUCLEAR-COMPLEX OF THE FROG

This study describes the localization of gamma-aminobutyric acid (GABA ), glycine, and glutamate immunoreactive neurons, fibers, and terminal -like structures in the vestibular nuclear complex (VNC) of the frog b y using a postembedding procedure with consecutive semithin sections a t the light microscopic level. For purposes of this study, the VNC was divided into a medial and a lateral region. Immunoreactive cells were observed in all parts of the VNC. GABA-positive neurons, generally sm all, in size, were predominantly located in the medial part of the VNC . Glycine-positive cells, more heterogeneous in size than GABA-positiv e cells, were scattered throughout the VNC. A quantitative analysis of the spatial distribution of GABA or glycine immunoreactive cells reve aled a complementatory relation between the density of GABA and glycin e immunoreactive neurons along the rostrocaudal extent of the VNC. In about 10% of the immunolabeled neurons, GABA and glycine were colocali zed. Almost all vestibular neurons were, to a variable degree, glutama te immunoreactive, and colocalization of glutamate with GABA and/or gl ycine was typical. GABA, glycine, or glutamate immunoreactive puncta w ere found in close contact to somata and main dendrites of vestibular neurons. A. quantitative analysis revealed a predominance of glutamate -positive terminal-like structures compared to glycine or GABA contain ing profiles. A small proportion of terminal-like structures expressed colocalization of GABA and glycine or glycine and glutamate. The resu lts are compared with data from mammals and discussed in relation to v estibuloocular and vestibule-spinal projection neurons, and vestibular interneurons. GABA and glycine are the major inhibitory transmitters of these neurons in frogs as well as in mammals. The differential dist ribution of GABA and glycine might reflect a compartmentalization of n eurons that is preserved to some extent from the early embryogenetic s egmentation of the hindbrain. (C) 1997 Wiley-Liss, Inc.