Selective labeling of spiral ganglion and granule cells with D-aspartate in the auditory system of cat and guinea pig.

The present study sought to locate putative glutamatergic or aspartatergic pathways in the auditory system of cats and guinea pigs. We injected 0.06 to 3 mM D-[3H] aspartate (D-Asp) in the cochlear nucleus before preparation for light microscopic autoradiography. At short survival times (15 and 40 min) there was heavy labeling of astrocytic somata. Labeling patterns typical of cochlear nerve endings decorated neurons in the cochlear nucleus, e.g., cell bodies and dendritic trunks of octopus cells. Labeling patterns consistent with retrograde axonal transport by the parallel fibers of granule cells appeared in the molecular layer of the dorsal cochlear nucleus and in the external granular layer. Retrograde labeling of the cochlear nerve root fibers also occurred. Consistent with these results are companion biochemical findings on the rapidly dissected cochlear nuclei of guinea pigs. The dorsal, anteroventral, and posteroventral cochlear nuclei, each, evinced uptake of D-Asp. Subsequently, electrical stimulation of each nucleus released a portion of the accumulated amino acid. Most of this release probably came from synaptic endings. Another group of experiments compared autoradiographic localization of 0.06 to 3 mM D-Asp to that of horseradish peroxidase (HRP) 6 hr to 2 d after injections in the cochlear nucleus. Astroglial cell bodies were no longer labeled by D-Asp, but spiral ganglion cell bodies in the cochlea and granule cell bodies in the cochlear nucleus were. Perikarya of the periolivary and ventral cochlear nuclei projecting to the dorsal cochlear nucleus were labeled by HRP and not by D-Asp. Thus, comparisons with the HRP findings indicate that D-Asp labeling resulted from a selective retrograde transport. There was no evidence for a selective anterograde axonal transport. The present observations support the hypothesis that cochlear nerve fibers and granule cells may use L-glutamate and/or L-aspartate as a transmitter in the cochlear nucleus.