Apamin increases excitability of CA1 hippocampal pyramidal neurons

The effects of bath application of apamin, a neurotoxin from bee venom, on CA1 hippocampal pyramidal neurons of young rabbits were examined in vitro using the intracellular sharp-electrode recording technique. Apamin dose-dependently reduced the AHP amplitude [F(5, 79) = 3.33, P < 0.009]. The mean percent of AHP reduction at 250 - 1000 nM was ˜ 20%. A small reduction of the spike-frequency adaptation was also observed. These significant, but small and dose-dependent, reductions of both the AHP amplitude and spike-frequency adaptation, reflecting increases in hippocampal pyramidal neuron excitability, may have been overlooked in previous reports. The results suggest that the apamin-sensitive AHP contributes little directly to the learning-induced reduction of the AHP and accommodation observed in hippocampal pyramidal neurons, but may contribute indirectly by enhancing cholinergic input to the hippocampus.