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

Abstract

Levels of y-amino-butyric acid, glycine, glutamate and aspartate were measured in samplesdissected from freeze-dried sections of cat cochlear nuclei and the immediate surroundings.Refined methods of sampling and record-keeping were systematically applied at the histologiclevel. A detailed three-dimensional map of the distribution of the amino acids was obtainedfor one cochlear nucleus, and the results were compared with selected samples from five othernuclei. On a dry weight basis, y-amino-butyric acid showed a 40-fold range, with highest levelsin the molecular and fusiform cell layers of the dorsal cochlear nucleus. Glycine had a similardistribution as y-amino-butyric acid although less striking. The aspartate maps differed fromthose of the other amino acids in that levels were lower in granular regions than in the ventralcochlear 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 didthe olivocochlear bundle, but some unusually high glycine levels were found in parts of thevestibular nerve. The highest y-amino-butyric acid levels were in the lateral vestibularnucleus, where glutamate levels were remarkably low.