SIZE-RELATED PROPERTIES OF VESTIBULAR AFFERENT-FIBERS IN THE FROG - UPTAKE OF AND IMMUNOREACTIVITY FOR GLYCINE AND ASPARTATE GLUTAMATE

Vestibular afferent fibers and their somata in the ganglion of Scarpa colocalize glutamate and glycine in a size-related manner.(17) In this study tritiated aspartate, glycine or GABA was injected in the vestib ular nuclear complex of frogs to investigate the uptake by afferent fi bers and the retrograde transport of these amino acids to the cell bod ies in the ganglion by autoradiographical methods. Ganglion cells were labeled by [H-3]aspartate or [H-3]glycine but not by [H-3]GABA. The i ntensity of labeling with [H-3]glycine increased and the intensity of labeling with [H-3]aspartate decreased with cell size. On consecutive semithin sections the immunoreactivity of the same neurons was investi gated with antibodies against glutamate or glycine. The results of thi s combined study showed that smaller, strongly glutamate immunopositiv e ganglion cells exhibited only weak or no labeling with [H-3]glycine whereas larger, less strongly glutamate immunopositive ganglion cells were more intensely labeled with [H-3]glycine. A similar size-related labeling pattern was observed in ganglion cells for [H-3]aspartate and glycine-immunoreactivity. Both glycine uptake and glutamate immunorea ctivity, as well as aspartate uptake and glycine-immunoreactivity, ten ded to be inversely correlated with the size of a given ganglion cell. These results provide evidence for a specific, size-related uptake of aspartate and glycine and are compatible with our hypothesis that the two amino acids are coreleased by thick but not by thin vestibular af ferents. In an accompanying paper [Straka H. et al. (1995) Neuroscienc e 70, 697-707], we provide evidence for a size-related, monosynaptic a ctivation of different glutamate receptors by vestibular afferent fibe rs.