2-iodomelatonin prevents apoptosis of cerebellar granule neurons via inhibition of A-type transient outward K+ currents

Compelling evidence indicates that excessive K+ efflux and intracellular K+ depletion are key early steps in apoptosis. Previously, we reported that apoptosis of cerebellar granular neurons induced by incubation under low K+ (5 mm) conditions was associated with an increase in delayed rectifier outward K+ current (I-K) amplitude and caspase-3 activity. Moreover, the melatonin receptor antagonist 4P-PDOT abrogated the effects of 2-iodomelatonin on I-K augmentation, caspase-3 activity and apoptosis. Here, we show that incubation under low K+/serum-free conditions for 6 hr led to a dramatic increase in the A-type transient outward K+ current (I-A) (a 27% increase; n = 31); in addition, fluorescence staining showed that under these conditions, cell viability decreased by 30% compared with the control. Treatment with 2-iodomelatonin inhibited the I-A amplitude recorded from control and apoptotic cells in a concentration-dependent manner and modified the I-A channel activation kinetics of cells under control conditions. Moreover, 2-iodomelatonin increased the viability of cell undergoing apoptosis. Interestingly, 4P-PDOT did not abrogate the effect of 2-iodomelatonin on I-A augmentation under these conditions; in the presence of 4P-PDOT (100 mum), 2-iodomelatonin reduced the average I-A by 41 +/- 4%, which was similar to the effect of 2-iodomelatonin alone. These results suggest that the neuroprotective effects of 2-idomelatonin are not only because of its antioxidant or receptor-activating properties, but rather that 2-iodomelatonin may inhibit I-A channels by acting as a channel blocker.