This project is to provide new knowledge of how the cochlear nucleus processes information coming from the organ of hearing through the auditory nerve. We described in previous Reports our evidence that the excitatory amino acids glutamate and aspartate are neurotransmitter candidates for the auditory nerve synapses in the cochlear nucleus. We also reported on our hypothesis that the enzymes glutaminase (GLNase) and aspartate aminotransferase (AATase) may serve as markers for glutamergic and aspartergic neurons. We have continued our immunocytochemical studies on the distribution of these two enzymes in the central nervous system of guinea pigs and rats using mainly immunoperoxidase techniques, with light and electron microscopy for visualization of the immunoreactivities. We have now added studies of the distribution of AATase-like and GLNase-like immunoreactivities in cerebellum, neocortex and hippocampus and of AATase-like immunoreactivity in retina. Findings from these studies provide evidence for the hypothesis that GLNase may serve as an immunocytochemical marker for excitatory amino acid neurons and that AATase-like immunoreactivity may define sub-populations of excitatory amino acid neurons or, perhaps, GABAergic neurons. Further studies are needed to confirm, or refute, the hypothesis. Our in vitro studies of auditory nerve synapses in the brain stem have been continued. Chamber mounted slices of the brainstem of chickens were prepared, and drugs were applied to synapses. Antidromic and orthodromic responses were evoked; field potentials were recorded using a glass electrode filled with artificial cerebro-spinal fluid. The results indicate that, as in mammals, an excitatory amino acid is released from the chicken auditory nerve and has its action terminated by an uptake process, but that this amino acid activates a kainate-type receptor on nucleus magnocellularis neurons. In the mammal, the corresponding receptor is of the NMDA-type.