Axonal and non-axonal immunolocalization of cytosolic aspartate aminotransferase (cAATase), GABA and glutamic acid decarboxylase (GAD) in the rat cochlear nucleus.

The localization of both cAATase activity, by histoenzymological method, and the immunoreactivity against cAATase, were investigated in the cochlear nucleus of rats. The immunohistochemical determination of cAATase was carried out using the PAP method, with an antiserum obtained from rabbits immunized with porcine cAATase. It was also studied both the immunolocalization of GABA-like and GAD-like substances. Our observations, with light microscope, revealed weak cAATase activity in the small neurons, and a more intense one in the fibers surrounding neuronal bodies. The large neurons presented a very weakly activity within their neuronal bodies and dendrites, but it was strongly found in granulations that surround the perikaryon and dendrites. cAATase immunoreactivity presents the same distribution as the enzymological activity. In the same way, we have investigated the pattern of distribution of both GABA- and GAD-like substances. Immunolocalization of these substances was similar to that found for cAATase. In the control sections incubated with Gostatin (0.05 mM), cAATase activity was absent. The immunoreactivity was also negative in every immunohistochemical control sections. These facts suggest that aspartate could intervene as a co-neurotransmitter or neuromodulator in the rat cochlear nucleus, and that axonic endings could contain cAATase, GABA and GAD. It was also found immunoreactivity against cAATase, GABA and GAD, in neuronal bodies, dendrites and glial processes, in close association with capillary wall. These observations have led us to suggest the possible co-localization and co-release of both GABA and aspartate from synaptic and non-synaptic sites in the cochlear nucleus.