Synthetic omega-conotoxin blocks synaptic transmission in the hippocampus in vitro

The effects of some organic calcium channel blockers and a toxin on synaptic transmission were examined in hippocampal slices from the guinea pig. omega-Conotoxin fraction GVIA, a novel peptide that blocks N and L type voltage-sensitive calcium channels, blocked synaptic transmission from mossy fibers to CA3 neurons at very low concentration (100 nM). However, organic calcium channel blockers such as verapamil (100 microM) and nifedipine (10 microM), which block L type calcium channels, had little effect on the synaptic transmission. Phenytoin (100 microM, T type calcium channel blocker) was also ineffective to block the synaptic transmission. These results suggest that presynaptic calcium channels which cause transmitter release may be N type calcium channel.