THE ORIGIN OF CALCIUM OVERLOAD OF NEURONS AFTER TOXIC GLUTAMATE TREATMENT

The study of the dynamics of intracellular free calcium concentration ([Ca2+]i) during and after toxic glutamate treatment (TGT) has been ca rried out on single cultured hippocampal neurons and cerebellar granul e cells using the fura 2/AM spectrofluorimetric technique. The experim ents showed that the persistent elevation of [Ca2+]i ([Ca2+]i plateau) after the 15-min glutamate (50 mM, 0 Mg2+) application was insensitiv to the inhibition of Na+/Ca2+ exchange caused by the substitution of external Na+ by Li+ in the postglutamate period. A decrease in externa l [Ca2+] (up lo approximately 20 nM by adding 50 nM EGTA to the nomina lly Ga2+ free solution) caused in most cells only a very slow and smal l decay of [Ga2+]i. These observations suggest that the Ca2+ overload induced by TGT results mainly from the inefficiency of Ca2+ extrusion systems rather than from the stable increase in Ca2+ permeability of c ell membrane as it has been supposed earlier. The reason for this inhi bition of Ca2+ extrusion systems (Ca2+ pump and Na+/Ca2+ exchanger) ma y be a decrease in cytoplasmic pH observed during and after the GLU tr eatment and/or the dramatic reduction in the ATP level under the influ ence of GLU.