Glutamate immunoreactivity in the cat retina: A quantitative study

Immunocytochemical methods were used to visualize glutamate immunoreactivity in the cat retina and to compare its localization with that of aspartate, GABA, and glycine. The cellular and subcellular distribution of glutamate was analyzed at the light-microscopic level by optical densitometry and at the electron-microscopic level by immunogold quantification. The findings were consistent with the proposed role for glutamate as the neurotransmitter of photoreceptors and bipolar cells as particularly high concentrations of staining were found in synaptic terminals of these cells. Ganglion cells were also consistently stained. Aspartate was totally colocalized with glutamate in neuronal cell bodies but the synaptic levels of aspartate were much lower than for glutamate. In addition to the staining of photoreceptor, bipolar, and ganglion cells, glutamate immunoreactivity was also observed in approximately 60% of the amacrine cells. These cells exhibited colocalization with either GABA or glycine. The elevated levels of Glu in amacrine cells may reflect its role as a transmitter precursor in GABAergic cells and as an energy source for mitochondria in glycinergic cells.