A PROTOTYPIC INTRACELLULAR CALCIUM-ANTAGONIST, TMB-8, PROTECTS CULTURED CEREBELLAR GRANULE CELLS AGAINST THE DELAYED, CALCIUM-DEPENDENT COMPONENT OF GLUTAMATE NEUROTOXICITY
Cs. Malcolm et al., A PROTOTYPIC INTRACELLULAR CALCIUM-ANTAGONIST, TMB-8, PROTECTS CULTURED CEREBELLAR GRANULE CELLS AGAINST THE DELAYED, CALCIUM-DEPENDENT COMPONENT OF GLUTAMATE NEUROTOXICITY, Journal of neurochemistry, 66(6), 1996, pp. 2350-2360
The effect(s) of a prototypic intracellular Ca2+ antagonist, 8-(N,N-di
ethylamino) octyl-3,4,5-trimethoxybenzoate (TMB-8), on glutamate-induc
ed neurotoxicity was investigated in primary cultures of mouse cerebel
lar granule cells. Glutamate evoked an increase in cytosolic free-Ca2 levels ([Ca2+](i)) that was dependent on the extracellular concentrat
ion of Ca2+ ([Ca2+](o)). In addition, this increase in [Ca2+](i) corre
lated with a decrease in cell viability that was also dependent on [Ca
2+](o). Glutamate-induced toxicity, quantified by (4,5-dimethylthiazol
-2-yl)-2,5-diphenyltetrazolium bromide (MTT) staining, was shown to co
mprise two distinct components, an ''early'' Na+/Cl--dependent compone
nt observed within minutes of glutamate exposure, and a ''delayed'' Ca
2+-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 via
bility and morphological observations identify a ''window'' in which T
MB-8 (at >100 mu M) protects granule cells from the Ca2+-dependent, bu
t not the Na+/Cl--dependent, component of glutamate-induced neurotoxic
damage, and furthermore, where TMB-8 inhibits glutamate-evoked increa
ses in [Ca2+](i). These findings suggest that Ca2+ release from a TMB-
8-sensitive intracellular store may be a necessary step in the onset o
f glutamate-induced excitotoxicity in granule cells. However, these co
nclusions are compromised by additional observations that show that TM
B-8 (1) exhibits intrinsic toxicity and (2) is able to reverse its ini
tial inhibitory action on glutamate-evoked increases in [Ca2+](i) and
subsequently effect a pronounced time-dependent potentiation of glutam
ate responses. Dantrolene, another putative intracellular Ca2+ antagon
ist, was completely without effect in this system with regard to both
glutamate-evoked increases in [Ca2+](i) and glutamate-induced neurotox
icity.