Glutamate increases the [Ca]i but stimulates Ca-independent release of [H]GABA in cultured chick retina cells

The effect of glutamate of [Ca²⁺]_i and on [³H]γ-aminobutyric acid (GABA) release was studied on cultured chick embryonic retina cells. It was observed that glutamate (100 μM) increases the [Ca²⁺]_i by Ca²⁺ influx through Ca²⁺ channels sensitive to nitrendipine, but not to ω-conotoxin GVIA (ω-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 [³H]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 [³H]GABA was significantly depressed by Ca_o²⁺, but it was dependent on Na_o⁺, just as was observed for the [³H]GABA release induced by veratridine (50 μM). The veratridine-induced release could be fully inhibited by TTX, but this toxin had no effect on the glutamate-induced [³H]GABA release. Both veratridine- and glutamate-induced [³H]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 [³H]GABA evoked by glutamate. Continuous depolarization with 50 mM K⁺ induced maximal release of [³H]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 μM) stimulated [³H]GABA release induced by 50 mM K⁺, and this effect was blocked by MK-801, suggesting that the effect of K⁺ on [³H]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 [³H]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.