GLUTAMATE INCREASES THE [CA-2+]I BUT STIMULATES CA-2+-INDEPENDENT RELEASE OF [H-3] GABA IN CULTURED CHICK RETINA CELLS

The effect of glutamate on [Ca2+]i and on [H-3]gamma-aminobutyric acid (GABA) release was studied on cultured chick embryonic retina cells. It was observed that glutamate (100 muM) increases the [Ca2+]i by Ca2+ influx through Ca2+ channels sensitive to nitrendipine, but not to omega-conotoxin GVIA (omega-Cg Tx) (50%), and by other channels insensitive to either Ca2+ Channel blocker. Mobilization of Ca2+ by glutamate required the presence of external Na+, suggesting that Na+ mobilization through the ionotropic glutamate receptors is necessary for the Ca2+ channels to open. The increase in [Ca2+]i was not related to the release of [H-3]GABA induced by glutamate, suggesting that the pathway for the entry of Ca2+ triggered by glutamate does not lead to exocytosis. In fact, the glutamate-induced release of [H-3]GABA was significantly depressed by Ca(o)2+, but it was dependent on Na(o)+, just as was observed for the [H-3]GABA release induced by veratridine (50 muM). The veratridine-induced release could be fully inhibited by TTX, but this toxin had no effect on the glutamate-induced [H-3]GABA release. Both veratridine- and glutamate-induced [H-3]GABA release were inhibited by 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridine-carboxylic acid (NNC-711), a blocker of the GABA carrier. Blockade of the NMDA and non-NMDA glutamate receptors with MK-801 and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), respectively, almost completely blocked the release of [H-3]GABA evoked by glutamate. Continuous depolarization with 50 mM K+ induced maximal release of [H-3]GABA of about 1.5%, which is much smaller than the release evoked by glutamate under the same conditions (6.0-6.5%). Glycine (3 muM) stimulated [H-3]GABA release induced by 50 mM K+, and this effect was blocked by MK-801, suggesting that the effect of K+ on [H-3]GABA release was partially mediated through the NMDA receptor which probably was stimulated by glutamate released by K+ depolarization. We conclude that glutamate induces Ca2+-independent release of [H-3]GABA through reversal of the GABA carrier due to Na+ entry through the NMDA and non-NMDA, TTX-insensitive, channels. Furthermore the GABA carrier seems to be inhibited by Ca2+ entering by the pathways open by glutamate. This Ca2+ does not lead to exocytosis, probably because the Ca2+ channels used are located at sites far from the active zones.