SIZE-RELATED COLOCALIZATION OF GLYCINE AND GLUTAMATE IMMUNOREACTIVITYIN FROG AND RAT VESTIBULAR AFFERENTS

Presence and distribution of glutamate, glycine, GABA and beta-alanine in VIIIth nerves of frogs and rats were investigated with postembeddi ng immunocytochemical methods on serial. In Scarpa's ganglion of the f rog, all cell bodies were glutamate immunoreactive. About 17% semithin sections. of the cells per section were also glycine immunoreactive, but none were GABA or p-alanine immunoreactive The mean diameter of gl ycine-positive cell bodies (26.7 +/- 6.9 mu m; N = 130) was significan tly (P < 0.0001) larger than that of glycine-negative cell bodies (15. 7 +/- 5.4 mu m; N = 272). The intensity of glutamate immunostaining de creased with cell diameter, whereas the intensity of glycine immunosta ining increased with cell diameter. As a result, the staining intensit ies for glutamate and glycine in a given cell were negatively correlat ed. Glycine immunoreactivity was also present in a size-related manner in distal and proximal afferent fibers. The majority of thin fibers ( <4 mu m) was glycine negative, whereas most of the thick fibers (>10 m u m) were glycine positive. Glycine-positive fibers were observed in t he sensory epithelia of ail end organs in the inner ear. The saccular macula and its nerve, however, contained only few glycine-immunoreacti ve structures. In Scarpa's ganglion of the rat, all cells were immunor eactive for glutamate, about 12% for colocalized glycine, and none for GABA or beta-alanine Glycine-positive cell bodies were significantly (P < 0.0001) larger (32.2 +/- 5.2 mu m; N = 82) than glycine-negative cell bodies (25.1 +/- 5.3 mu m; N = 274). Cell bodies in the spiral ga nglion were only glutamate immunoreactive, whereas staining for glutam ate, glycine, and GABA was dense in the ventral cochlear. These result s demonstrate that thicker vestibular afferent fibers represent a part icular subpopulation that differs from the majority of thinner afferen ts due to their glycine immunoreactivity. (C) 1994 Wiley-Liss, Inc.