The localization and metabolism of glutamate, aspartate and GABA in the rat retina.

Radiolabelled glucose and glutamine have been used to study the metabolism of the neuroactive amino acids glutamate, aspartate and GABA in the rat retina. Endogenous amino acids have been measured by double label dansylation. Significant increases in the concentrations of all three amino acids have been found on light-adaptation. In vitro , glutamine is metabolized alongside glucose and, at 600 ?M (the concentration in the rat vitreous), is a major precursor of the neuroactive amino acids. GABA is formed principally in the amacrine and inner plexiform layers of the tissue, whereas, in photoreceptor cells, glutamate and aspartate are heavily labelled. Evidence has been obtained for decreased turnover of all three amino acids on light stimulation. The data are consistent with a role for glutamate and/or aspartate as photoreceptor neurotransmitters. Glucose utilization is reduced in the light-stimulated retina.