Effect of light stimulation on the synthesis and release of GABA in rat and frog retinae

The effect of light stimulation on the incorporation of label from [14C]glucose and [3H]acetate into aspartate, glutamate, glutamine and GABA, both in vivo and in vitro, was studied in dark-adapted frog and rat retinae. With [14C]glucose as the precursor, but not with [3H]acetate, flashes of light appeared to increase the in vivo syntheses of both GABA and glutamine at the expense of glutamate in frog retina, while attenuating the labelling of aspartate and glutamate in isolated frog tissues. Amino acid synthesis from these radioactive substrates in rat retina was not altered under these conditions. Also, the turnover rates of amino acids derived from [14C]-glucose in rat retina in vivo were closely similar in light and darkness. Isolated retinae from both species accumulated [14C]GABA from the incubation medium. In each case the subsequent release of this compound was augmented by electric field stimulation, high K+ and by a high concentration of cold GABA in the medium, but not by a flashing light. Similarly, application of a light stimulus was unable to evoke the liberation of [14C]GABA derived from [14C]glucose in either species.