Interactions among Toxins That Inhibit N-type and P-type Calcium Channels

A number of peptide toxins from venoms of spiders and cone snails are high affinity ligands for voltage-gated calcium channels and are useful tools for studying calcium channel function and structure. Using whole-cell recordings from rat sympathetic ganglion and cerebellar Purkinje neurons, we studied toxins that target neuronal N-type (Ca_V2.2) and P-type (Ca_V2.1) calcium channels. We asked whether different toxins targeting the same channels bind to the same or different sites on the channel. Five toxins (ω-conotoxin-GVIA, ω-conotoxin MVIIC, ω-agatoxin-IIIA, ω-grammotoxin-SIA, and ω-agatoxin-IVA) were applied in pairwise combinations to either N- or P-type channels. Differences in the characteristics of inhibition, including voltage dependence, reversal kinetics, and fractional inhibition of current, were used to detect additive or mutually occlusive effects of toxins. Results suggest at least two distinct toxin binding sites on the N-type channel and three on the P-type channel. On N-type channels, results are consistent with blockade of the channel pore by ω-CgTx-GVIA, ω-Aga-IIIA, and ω-CTx-MVIIC, whereas grammotoxin likely binds to a separate region coupled to channel gating. ω-Aga-IIIA produces partial channel block by decreasing single-channel conductance. On P-type channels, ω-CTx-MVIIC and ω-Aga-IIIA both likely bind near the mouth of the pore. ω-Aga-IVA and grammotoxin each bind to distinct regions associated with channel gating that do not overlap with the binding region of pore blockers. For both N- and P-type channels, ω-CTx-MVIIC binding produces complete channel block, but is prevented by previous partial channel block by ω-Aga-IIIA, suggesting that ω-CTx-MVIIC binds closer to the external mouth of the pore than does ω-Aga-IIIA.