APOPTOSIS IN CEREBELLAR GRANULE CELLS IS BLOCKED BY HIGH KCL, FORSKOLIN, AND IGF-1 THROUGH DISTINCT MECHANISMS OF ACTION - THE INVOLVEMENT OF INTRACELLULAR CALCIUM AND RNA-SYNTHESIS

Cerebellar granule cells deprived of depolarizing concentration of ext racellular potassium, [K+](o), undergo apoptosis. We here report that this apoptotic process is associated with an immediate and permanent d ecrease in the levels of free intracellular calcium, [Ca2+](i). Althou gh forskolin and IGF-1 are both able to prevent apoptosis, only forsko lin is able to counteract the instantaneous decrease of [Ca2+](i). How ever, the early effect of forskolin on [Ca2+](i) is lost after longer incubation in low [K+](o). The calcium antagonist nifedipine is able t o inhibit the survival effect of high [K+](o), while not affecting for skolin and IGF-1 promoted survival, as assessed by viability and genom ic DNA analysis. Accordingly, the L-type calcium channels agonist Bay K8644 significantly enhanced the survival of low KCI treated neurons. To temporally characterize the signal transduction events and the esse ntial transcriptional step in cerebellar granule cells apoptosis, we d etermined the time course of the rescue capacity of high [K+](o), fors kolin, IGF-1, and actinomycin D. Addition of high KCI, forskolin, or I GF-1 6 hr after the initial KCI deprivation saves 50% of cells. Remark ably, 50% of neurons loss the potential to be rescued by actinomycin D after only 1 hr in low [K+](o). Finally, we show that the survival pr omoting activities of high [K+](o), forskolin, and IGF-1 do not requir e RNA synthesis. We conclude that [Ca2+](i) is involved in the surviva l promoting activity exerted by high [K+](o) but not in those of forsk olin and IGF-I, and that all three agents, although rescuing neurons f rom apoptosis through distinct mechanisms of action, do not necessitat e RNA transcription.