Gangliosides normalize distorted single-cell intracellular free Ca2+ dynamics after toxic doses of glutamate in cerebellar granule cells.

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Abstract
Glutamate-induced delayed neurotoxicity after abusive and paroxismal activation of its receptors has been proposed to depend upon a sustained increase in intracellular free Ca2+ [( Ca2+]i). To elucidate the temporal and causal relationship between glutamate-induced changes in [Ca2+]i and neuronal death, we simultaneously studied the dynamics of [Ca2+]i changes in single neurons with the acetoxymethyl ester of fura-2 and the cell viability by imaging the nuclear penetration of propidium iodide. The main difference between toxic (50 microM) and nontoxic (5 microM) doses of glutamate is the lack of regulation in [Ca2+]i 20 min after glutamate is removed. This protracted rise in [Ca2+]i in a single cell is correlated with (r = 0.87, P less than 0.01, Spearman's test), and consequently predictive of, the time of appearance of neuronal death, as measured by propidium iodide fluorescence. In addition, the glutamate receptor antagonists dibenzocyclohepteneimine (MK-801) and 3,3-(2-carboxypiperazine-4-yl)propyl 1-phosphate reduce the acute increase of [Ca2+]i induced by glutamate but fail to revert the protracted increase of [Ca2+]i, elicited by toxic doses of glutamate. In contrast, the ganglioside GM1 and the semisynthetic lysoGM1 with N-acetylsphingosine (LIGA-4) and lysoGM1 with N-dichloroacetylsphingosine (LIGA-20) failed to change the immediate rise of [Ca2+]i elicited by glutamate but prevented the protracted increase in [Ca2+]i after toxic doses of glutamate. Voltage-dependent Ca2+ channel blockers (nifedipine, etc.) did not change the initial or protracted responses to glutamate.