A prototypic intracellular calcium antagonist, TMB-8, protects cultured cerebellar granule cells against the delayed, calcium-dependent component of glutamate neurotoxicity

The effect(s) of a prototypic intracellular Ca2+ antagonist, 8-(N,N-diethylamino) octyl-3,4,5-trimethoxybenzoate (TMB-8), on glutamate-induced neurotoxicity was investigated in primary cultures of mouse cerebellar granule cells. Glutamate evoked an increase in cytosolic free-Ca2+ levels ([Ca2+](i)) that was dependent on the extracellular concentration of Ca2+ ([Ca2+](o)). In addition, this increase in [Ca2+](i) correlated with a decrease in cell viability that was also dependent on [Ca2+](o). Glutamate-induced toxicity, quantified by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) staining, was shown to comprise two distinct components, an ''early'' Na+/Cl--dependent component observed within minutes of glutamate exposure, and a ''delayed'' Ca2+-dependent component (ED(50) similar to 50 mu M) that coincided with progressive degeneration of granule cells 4-24 h after a brief (5-15 min) exposure to 100 mu M glutamate. Quantitative analysis of cell viability and morphological observations identify a ''window'' in which TMB-8 (at >100 mu M) protects granule cells from the Ca2+-dependent, but not the Na+/Cl--dependent, component of glutamate-induced neurotoxic damage, and furthermore, where TMB-8 inhibits glutamate-evoked increases in [Ca2+](i). These findings suggest that Ca2+ release from a TMB-8-sensitive intracellular store may be a necessary step in the onset of glutamate-induced excitotoxicity in granule cells. However, these conclusions are compromised by additional observations that show that TMB-8 (1) exhibits intrinsic toxicity and (2) is able to reverse its initial inhibitory action on glutamate-evoked increases in [Ca2+](i) and subsequently effect a pronounced time-dependent potentiation of glutamate responses. Dantrolene, another putative intracellular Ca2+ antagonist, was completely without effect in this system with regard to both glutamate-evoked increases in [Ca2+](i) and glutamate-induced neurotoxicity.