Glycine stimulates calcium-independent release of 3H-GABA from isolated retinas of Xenopus laevis.
A perfusion system was used to monitor the release of [3H]-GABA from isolated retinas of Xenopus laevis. Measurable release was stimulated by glycine at concentrations as low as 200 microM. Glycine-stimulated release was blocked by strychnine, and was not reduced in "calcium-free" Ringer's solution (0 Ca2+/20 mM Mg2+). Glutamate also stimulated calcium-independent release, using concentrations as low as 100 microM. In contrast, release stimulated by 25 mM potassium was reduced by 80% in calcium-free medium. In most experiments, agonists were applied in six consecutive 4-min pulses separated by 10-min washes with Ringer's solution. Under these conditions, the release stimulated by 0.5 mM glutamate or 25 mM potassium decreased by at least 50% from the first to the second pulse, and then gradually decreased with successive applications. In contrast, the response to 0.5 mM glycine at first increased and then only gradually decreased with successive pulses. These patterns of response to different agonists were similar in calcium-free medium. Somatostatin (-14 or -28) also stimulated release, and this effect was inhibited by AOAA, an inhibitor of GABA degradation. In the presence of AOAA, somatostatin had little effect, except at high concentrations of somatostatin (5 microM), which increased both basal and glycine-stimulated release. In contrast to somatostatin, glycine-stimulated release was much larger in the presence of AOAA. Autoradiography was used to investigate which cell types released [3H]-GABA under our conditions. Autoradiograms showed that horizontal cells and a population of apparent "off" bipolar cells were well-labeled by [3H]-GABA high-affinity uptake. In addition, light labeling was seen over numerous amacrine cells. After application of glycine, glutamate, or potassium, there was a decrease in label density over horizontal cells.
Smiley
J F
JF
Cullen Eye Institute, Baylor College of Medicine, Houston, TX 77030.
Basinger
S F
SF
D000625:Aminooxyacetic Acid / Q000494:pharmacology; D000818:Animals; D001345:Autoradiography; D002118:Calcium / Q000494:pharmacology; D002855:Chromatography, Thin Layer; D018756:GABA Antagonists; D005971:Glutamates / Q000494:pharmacology; D005998:Glycine / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D011188:Potassium / Q000494:pharmacology; D012160:Retina / Q000187:drug effects / Q000378:metabolism*; D013004:Somatostatin / Q000037:antagonists & inhibitors / Q000494:pharmacology; D013331:Strychnine / Q000494:pharmacology; D014982:Xenopus laevis; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D018756:GABA Antagonists; D005971:Glutamates; D000625:Aminooxyacetic Acid; D013004:Somatostatin; D005680:gamma-Aminobutyric Acid; D013331:Strychnine; D011188:Potassium; D002118:Calcium; D005998:Glycine