Premium
Effect of excitatory amino acids on gamma‐aminobutyric acid release from frog horizontal cells.
Author(s) -
Cunningham J R,
Neal M J
Publication year - 1985
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1985.sp015662
Subject(s) - kainate receptor , glutamate receptor , excitatory postsynaptic potential , nmda receptor , biophysics , gamma aminobutyric acid , chemistry , biology , biochemistry , medicine , ampa receptor , receptor
The effects of excitatory amino acids, analogues and K on [3H]gamma‐aminobutyric acid [3H]GABA) release from horizontal cells of the isolated superfused frog retina were studied. Exposure of the retina to medium containing high concentrations (25‐100 mM) of KCl increased the release of [3H]GABA to a maximum which was 40 times the spontaneous resting release. The K‐evoked release of [3H]GABA was almost abolished in high‐Mg/low‐Ca medium. Glutamate, aspartate, kainate and quisqualate also stimulated the release of [3H]GABA from horizontal cells, the maximum evoked release being similar to that produced by KCl. The release of [3H]GABA evoked by glutamate, aspartate, kainate and quisqualate was abolished in high‐Mg/low‐Ca medium and by Na‐free medium. The evoked releases of [3H]GABA were not reduced by tetrodotoxin. N‐Methyl‐D‐aspartate (NMDA) at concentrations up to 10 mM had virtually no effect on [3H]GABA release from horizontal cells. In Mg‐free medium, NMDA stimulated [3H]GABA release, but the maximum release was only 10% of that produced by other agonists. Mg‐free medium did not significantly affect the evoked release of [3H]GABA by other agonists. NMDA apparently possessed affinity for the kainate receptor, because in normal medium it antagonized the effects of kainate but not glutamate, aspartate or quisqualate. The non‐selective antagonist of excitatory amino acids, (+/‐)‐cis‐2,3‐piperidine dicarboxylic acid (PDA) antagonized the action of glutamate, aspartate, kainate and quisqualate on horizontal cell [3H]GABA release. D(‐)‐2‐Amino‐4‐phosphonobutyrate (APB) and D‐gamma‐glutamylglycine (D‐gamma‐GG) antagonized the actions of kainate on horizontal cell [3H]GABA release at concentrations which had little affect on quisqualate‐evoked responses. Approximate estimates of pA2 values (Schild, 1947) showed that the specificity and potency of the antagonists was low. Nevertheless, the retinal 'non‐NMDA' receptors can probably be subdivided into kainate and quisqualate types. Glutamate diethylester (GDEE) did not affect the action of any agonist. We conclude that glutamate (and aspartate) probably stimulate the release of [3H]GABA from frog horizontal cells by activating receptors of the non‐NMDA type. This activation may trigger the opening of tetrodotoxin‐insensitive Na channels, resulting in the depolarization of the cell membrane and an increase in the conductance of voltage‐sensitive Ca‐channels. An influx of Ca ions would then trigger the release of [3H]GABA. Our results are not consistent with previous suggestions that GABA release from horizontal cells involves an outwardly directed transport process.