Quantitative histochemical mapping of candidate transmitter amino acids in cat cochlear nucleus.

Abstract

Levels of GABA, glycine, glutamate and aspartate were measured in samples dissected from freeze-dried sections of cat cochlear nuclei and the immediate surroundings. Refined methods of sampling and record-keeping were systematically applied at the histologic level. A detailed 3-dimensional map of the distribution of the amino acids was obtained for 1 cochlear nucleus, and the results were compared with selected samples from 5 other nuclei. On a dry weight basis, GABA showed a 40-fold range, with highest levels in the molecular and fusiform cell layers of the dorsal cochlear nucleus. Glycine had a similar distribution as GABA although less striking. The aspartate maps differed from those of the other amino acids in that levels were lower in granular regions than in the ventral cochlear nucleus. The distribution of glutamate was least impressive and, on a volume basis, was almost uniform. The auditory nerve lacked high levels of any of the amino acids, as did the olivocochlear bundle, but some unusually high glycine levels were found in parts of the vestibular nerve. The highest GABA levels were in the lateral vestibular nucleus, where glutamate levels were remarkably low.