The metabolism of glutamate, the most likely neurotransmitter of vestibular ganglion cells, includes synthesis from glutamine by the enzyme glutaminase. We used microdissection combined with a fluorometric assay to measure glutaminase activity in the vestibular nerve root and nuclei of rats with unilateral vestibular ganglionectomy. Glutaminase activity in the lesioned-side vestibular nerve root decreased by 62% at 4 days after ganglionectomy and remained at similar values through 30 days. No change occurred in the contralateral vestibular nerve root. Glutaminase activity changes in the vestibular nuclei were lesser in magnitude and more complex, including contralateral increases as well as ipsilateral decreases. At 4 days after ganglionectomy, glutaminase activity was 10-20% lower in individual lesioned-side nuclei compared with their contralateral counterparts. By 14 and 30 days after ganglionectomy, there were no statistically significant differences between the nuclei on the two sides. This transient asymmetry of glutaminase activities in the vestibular nuclei contrasts with the sustained asymmetry in the vestibular nerve root and suggests that intrinsic, commissural, or descending pathways are involved in the recovery of chemical symmetry. This recovery resembles our previous finding for glutamate concentrations in the vestibular nuclei and may partially underlie central vestibular compensation after peripheral lesions.

Exposure to intense sound often leads to tinnitus, the perception of a monotonous sound not actually present. Increased neural spontaneous activity in the central auditory system found in animal models of tinnitus should have a basis in their chemistry. Most chemical studies so far have focused on neurotransmitters, by which neurons communicate with each other, because alteration of this chemistry could easily lead to abnormal neural activity that might be perceived as tinnitus. Although increased spontaneous activity has been observed in the hamster dorsal cochlear nucleus (DCN) a month after intense tone exposure, we did not find increased glutamate concentrations in the 3 layers of the hamster dorsal DCN at that time. We did, however, find decreased glutamate concentrations 2 days after exposure that might correlate with slightly decreased spontaneous activity observed then. Others have provided evidence for decreased glutamate release in the chinchilla DCN 2 days after intense sound exposure. Other intense-sound-induced changes are increased choline acetyltransferase activity in some cochlear nucleus regions, increased acetylcholine receptor sensitivity in some DCN neurons, and some changes in the g-aminobutyric acid (GABA) neurotransmitter system in the inferior colliculus. There is a need for more study of these and other neurotransmitter systems to determine their possible roles in tinnitus.

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