CALCIUM INFLUX VIA L-TYPE VOLTAGE-GATED CHANNELS MEDIATES THE DELAYED, ELEVATED INCREASES IN STEADY-STATE C-FOS MESSENGER-RNA LEVELS IN CEREBELLAR GRANULE CELLS EXPOSED TO EXCITOTOXIC LEVELS OF GLUTAMATE

The altered kinetics of steady-state c-fos mRNA production in cultured cerebellar granule cells under excitotoxic conditions was investigate d in neurons subjected to depolarising stimuli, namely, high KCI and L -glutamate (Glu), in which Ca2+ influx occurs by differing routes. Inc reases in intracellular-free calcium levels ([Ca2+](i)) stimulated by nontoxic or toxic levels of Glu were blocked by selective N-methyl-D-a spartate (NMDA) receptor antagonism; were blocked only partially by th e L-type channel blocker, nifedipine; and were unaffected by alpha-ami no-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptor a ntagonists. Glu-induced cell death was prevented only by NMDA receptor blockade, Exposure of cells to nontoxic levels of Glu resulted in a t ransient increase in c-fos mRNA levels, whereas an excitotoxic dose pr oduced a delay in the appearance of c-fos mRNA but a subsequent, progr essive, and sustained (>4 hr) increase. An excitotoxic dose of Glu in combination with either nifedipine or selective NMDA receptor antagoni sts resulted in the normal, transient increase of c-fos mRNA levels. C hronic exposure to 55 mM KCI caused no cytotoxicity, although it resul ted in a delayed, elevated increase in c-fos mRNA levels that was unaf fected by NMDA receptor blockade but reverted to the normal, transient profile of c-fos mRNA formation when it was coadministered with nifed ipine, The KCl-induced increase in [Ca2+](i) levels was inhibited dram atically by nifedipine but was unaffected by any of the ionotropic Glu receptor antagonists. The results support the notion that the appeara nce of a delayed but elevated increase in steady-state c-fos mRNA leve ls following exposure to excitotoxic doses of Glu is mediated specific ally by calcium influx via L-type voltage-gated channels, (C) 1998 Wil ey-Liss, Inc.