Cooccupancy of the outer vestibule of voltage-gated sodium channels by micro-conotoxin KIIIA and saxitoxin or tetrodotoxin.

The guanidinium alkaloids tetrodotoxin (TTX) and saxitoxin (STX) are classic ligands of voltage-gated sodium channels (VGSCs). Like TTX and STX, micro-conotoxin peptides are pore blockers but with greater VGSC subtype selectivity. micro-Conotoxin KIIIA blocks the neuronal subtype Na(V)1.2 with nanomolar affinity and we recently discovered that KIIIA and its mutant with one fewer positive charge, KIIIA[K7A], could act synergistically with TTX in a ternary peptide x TTX x Na(V) complex. In the complex, the peptide appeared to trap TTX in its normal binding site such that TTX could not readily dissociate from the channel until the peptide had done so; in turn, the presence of TTX accelerated the rate at which peptide dissociated from the channel. In the present study we examined the inhibition of Na(V)1.2, exogenously expressed in Xenopus oocytes, by STX (a divalent cation) and its sulfated congener GTX2/3 (with a net +1 charge). Each could form a ternary complex with KIIIA and Na(V)1.2, as previously found with TTX (a monovalent cation), but only when STX or GTX2/3 was added before KIIIA. The KIIIA x alkaloid x Na(V) complex was considerably less stable with STX than with either GTX2/3 or TTX. In contrast, ternary KIIIA[K7A] x alkaloid x Na(V) complexes could be formed with either order of ligand addition and were about equally stable with STX, GTX2/3, or TTX. The most parsimonious interpretation of the overall results is that the alkaloid and peptide are closely apposed in the ternary complex. The demonstration that two interacting ligands ("syntoxins") occupy adjacent sites raises the possibility of evolving a much more sophisticated neuropharmacology of VGSCs.