Absolute levels of some free amino acids in normal and biologically fractionated retinas

Using isolated (1) normal retinas, (2) receptorless retinas, and (3) retinas with markedly reduced inner layers, respectively, derived from 90 day old pigmented mice were normal, or possessed of a genetic receptor dystrophy, or treated postnatally with monosodium glutamate, extracts were assayed with an animo acid analyzer and/or by fluorometric ultramicro techniques. In millimoles per kilogram protein, the glycine, ananine, and GABA levels were, respectively, 17.0, 4.7, and 18.7 in (1), 38.0, 8.7, and 26.0 in (2), and 9.0, 2.9, and 6.0 in (3). These data suggest a relative concentration of these amino acids in the inner retina with glycine and GABA levels in (2) matching or exceeding published values for any central nervous system region. Taurine, however, had concentrations of 410, 150, and 500 in (1), (2), and (3), respectively, thus suggesting high levels everywhere and a relative concentration in the outer retina. In some groups, glutamate, GABA glutamine, or aspartate levels were lower in receptor containing retinas from dark adapted as compared to light adapted animals. Glutamate concetrations were similar in (1), (2) and (3) and aspartate and glutamine plus serine somewhat elevated in (2). Relative neurophysiologic inactivity and/or the different lesion mechanisms in the two abnormal retinas must affect these data. However, when large concentration differences between (1) and (2) and between (1) and (3) are in opposing directions, true distribution differences in (1) are suggested. The data may bear on amino acids as possible neural transmitters and/or on local metabolic specializations.