ω-grammotoxin SIA blocks multiple, voltage-gated, Ca²⁺ channel subtypes in cultured rat hippocampal neurons

ω-Grammotoxin SIA is a peptide isolated from tarantula venom on the basis of its ability to block the voltage-gated Ca²⁺ channels that mediate glutamate release. To determine the Ca²⁺ channel subtype selectivity of ω-grammotoxin SIA, whole-cell Ba²⁺ current (I_Ba) was measured in cultured rat hippocampal neurons. Selective Ca²⁺ channel blockers were used to identify components of I_Ba mediated by Ca²⁺ channel subtypes. ω-Agatoxin IVA at 30 nm, 1 μm ω-conotoxin GVIA, and 3 μm-conotoxin MVIIC, applied consecutively, each elicited a fractional increase in the cumulative block of I_Ba, identifying components of I_Ba mediated by P-, N-, and Q-type calcium channels. ω-Grammotoxin at 1 μm, a maximally effective concentration, blocked 52% of I_Ba. ω-Conotoxin MVIIC and the combination of ω-conotoxin GVIA and micromolar ω-agatoxin IVA blocked 52% and 54% of I_Ba, respectively, and block of I_Ba by ω-grammotoxin SIA was mutually occlusive of block of I_Ba by either treatment, both of which block N-, P-, and Q-type Ca²⁺ channels. The L channel blocker nimodipine produced identical block of I_Ba in the presence and absence of ω-grammotoxin SIA. These results indicate that ω-grammotoxin SIA blocks N-, P-, and Q-type but not L-type voltage-gated calcium channels. Block of I_Ba by ω-grammotoxin SIA was faster in onset and less sensitive to external divalent cation concentrations than was block by ω-conotoxin MVIIC, and it was rapidly and substantially reversible. Rapid onset, relative insensitivity to divalent cation concentrations, and reversibility render ω-grammotoxin SIA a useful tool for inhibition of neuronal voltage-gated Ca²⁺ channels.