Autoradiographic evidence of primary projections to the caudal cochlear nucleus in cats.

Six adult cats received unilateral cochlear injections of 30–70 μCi 3H-leucine (3-H-leu) in saline. After 20–48 hours, their brains were prepared for autoradiography. The octopus cell area (OCA) and the dorsal region (DCN) of the cochlear nucleus of both the injected and uninjected sides were studied in detail. Grain counts of autoradiographs always showed much greater label in the injected side. Autoradiography confirmed uniform distribution of primary afferents in the OCA, as seen in grain counts over the whole area (both somata and neuropil); however, grains were more densely packed over somata than neuropil of the OCA. In DCN, grain counts showed a gradient of label from the deep to superficial layers, a greater density of label over somata than neuropil of the deep DCN, and uniform distribution of label over the whole fusiform cell layer of the DCN. These results showed (1) best resolution and localization in cats that survived 24 hours and with exposures of two weeks, (2) no significant diffusion of label to other CNS regions than the auditory nuclei,and (3) transneuronaltransport of label after 48-hour survival times. Liquid scintillation counts (LSC) of cochlear nerve roots ipsilateral and contralateral to the injection showed at least a 10:1 ratio in all cats. This report not only gives new autoradiographic evidence of the distribution of primary afferents within the caudal cochlear nucleus, but also provides a useful approach to the study of distribution of specific amino acids implicated in central neurotransmission by cochlear terminals.