GLUTAMATE IMMUNOREACTIVITY IN THE CAT RETINA - A QUANTITATIVE STUDY

Immunocytochemical methods were used to visualize glutamate immunoreac tivity in the cat retina and to compare its localization with that of aspartate, GABA, and glycine. The cellular and subcellular distributio n of glutamate was analyzed at the light-microscopic level by optical densitometry and at the electron-microscopic level by immunogold quant ification. The findings were consistent with the proposed role for glu tamate as the neurotransmitter of photoreceptors and bipolar cells as particularly high concentrations of staining were found in synaptic te rminals of these cells. Ganglion cells were also consistently stained. Aspartate was totally colocalized with glutamate in neuronal cell bod ies but the synaptic levels of aspartate were much lower than for glut amate. In addition to the staining of photoreceptor, bipolar, and gang lion cells, glutamate immunoreactivity was also observed in approximat ely 60% of the amacrine cells. These cells exhibited colocalization wi th either GABA or glycine. The elevated levels of Glu in amacrine cell s may reflect its role as a transmitter precursor in GABAergic cells a nd as an energy source for mitochondria in glycinergic cells.