Multiple channel interactions explain the protection of sympathetic neurons from apoptosis induced by nerve growth factor deprivation

We investigated the neuroprotective properties of two M-type K⁺ channel blockers, linopirdine and its analog XE991, in rat sympathetic neurons deprived of nerve growth factor (NGF). Linopirdine and XE991 promoted sympathetic neuronal survival 48-72 hr after NGF withdrawal in a concentration-dependent manner. Both drugs prevented neuronal apoptosis by blocking the pathway leading to the release of cytochrome c and development of "competence-to-die" after NGF deprivation. Fura-2 Ca²⁺ imaging showed no significant difference in intracellular free Ca²⁺ ([Ca²⁺]_i) in the presence or absence of NGF; linopirdine and XE991, on the other hand, caused membrane depolarization and increases in [Ca²⁺]_i. Whole-cell recordings showed that linopirdine and XE991 selectively blocked the M current at neuroprotective concentrations, although they additionally inhibited other K⁺ currents at high concentrations. Membrane depolarization and [ca²⁺]_i increases induced by linopirdine and XE991 were blocked by the Na⁺ channel blocker tetrodotoxin (TTX) or by the L-type Ca²⁺ channel antagonist nifedipine. TTX and nifedipine also prevented the neuroprotection elicited by linopirdine or XE991. We propose that Na ⁺ channel activation amplifies the membrane depolarization produced by M channel blockade and is essential for subsequent Ca²⁺ entry via the L-type Ca²⁺ channel. The interaction of these three classes of ion channels highlights an integrated anti-apoptosis mechanism in sympathetic neurons.