Size-related colocalization of glycine and glutamate immunoreactivity in 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 postembedding immunocytochemical methods on serial semithin sections. In Scarpa's ganglion of the frog, all cell bodies were glutamate immunoreactive. About 17% of the cells per section were also glycine immunoreactive, but none were GABA or beta-alanine immunoreactive. The mean diameter of glycine-positive cell bodies (26.7 +/- 6.9 microns; N = 130) was significantly (P0.0001) larger than that of glycine-negative cell bodies (15.7 +/- 5.4 microns; N = 272). The intensity of glutamate immunostaining decreased with cell diameter, whereas the intensity of glycine immunostaining increased with cell diameter. As a result, the staining intensities for glutamate and glycine in a given cell were negatively correlated. Glycine immunoreactivity was also present in a size-related manner in distal and proximal afferent fibers. The majority of thin fibers (4 microns) was glycine negative, whereas most of the thick fibers (10 microns) were glycine positive. Glycine-positive fibers were observed in the sensory epithelial of all end organs in the inner ear. The saccular macula and its nerve, however, contained only few glycine immunoreactive structures. In Scarpa's ganglion of the rat, all cells were immunoreactive for glutamate, about 12% for colocalized glycine, and none for GABA or beta-alanine. Glycine-positive cell bodies were significantly (P0.0001) larger (32.2 +/- 5.2 microns; N = 82) than glycine-negative cell bodies (25.1 +/- 5.3 microns; N = 274). Cell bodies in the spiral ganglion were only glutamate immunoreactive, whereas staining for glutamate, glycine, and GABA was dense in the ventral cochlear nucleus. These results demonstrate that thicker vestibular afferent fibers represent a particular subpopulation that differs from the majority of thinner afferents due to their glycine immunoreactivity.