DOMOIC ACID-INDUCED RELEASE OF [H-3] GABA IN CULTURED CHICK RETINA CELLS

The effect of the neurotoxin domoic acid (DOM), a structural analogue of kainic acid, on the release of [H-3]gamma-aminobutyric acid (GABA) and on the [Ca2+]i was studied in cultured chick retina cells. DOM stimulated dose-dependently the release of [H-3]GABA with an EC50 of 2.5 muM. In Ca2+-containing medium (1 mM). DOM (5 muM) increased the [Ca2+]i by about 190 nM and evoked the release of 11.8 +/- 1.3% of the intracellular [H-3]GABA, while in the absence of extracellular Ca2+ DOM induced the release of only 7.9 +/- 1.4% of the accumulated [H-3]GABA. The Ca2+-independent release of [H-3]GABA was blocked by the non-competitive inhibitor of the GABA carrier 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridine-carboxylic acid hydrochloride (NNC-711), but a component of Ca2+-dependent release remains. DOM evoked Ca2+-independent release of [H-3]GABA was significantly depressed in the absence of external Na+ and completely blocked by the non-selective antagonist of the non-NMDA glutamate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Similarly, CNQX decreased the [Ca2+]i response to DOM, whereas L(+)-2-amino-3-phosphonopropionic acid (L-AP3), an antagonist of the metabotropic glutamate receptors, was without effect. MK-801 did not affect the release of [H-3]GABA stimulated by DOM. Taken together our results indicate that DOM evokes both Ca2+-dependent and Ca2+-independent release of [H-3]GABA, most likely by activating kainate receptors.