Modulation of Na+-channels by neurotoxins produces different effects on [3H]ACh release with mobilization of distinct Ca2+-channels

1. Voltage-gated Na+ channels are responsible for initiation and conduction of action potentials. The arrival of an action potential at nerve terminal increases intracellular Na+ and Ca2+ concentrations. Calcium entry into neurons through voltage-dependent calcium channels is associated with a variety of intracellular processes. Scorpion neurotoxins have been used as tools to investigate mechanisms involved in neurotransmitter release. Tityustoxin (TsTX) is an alpha-type toxin that delays Na+-channel inactivation. Toxin-gamma (TiTX-gamma)is a beta-type toxin that induces Na+-channel activation at resting potentials. 2. In the present work, we describe the effects of both toxins on [H-3] acetylcholine ([H-3]ACh) release from rat cerebrocortical synaptosomes, in the presence or absence of the calcium channels blockers: omega-conotoxin-GVIA (omega-CgTx), 1 muM; omega-agatoxin-IVA (omega-Aga), 30 nM; omega-conotoxin-MVIIC (omega-MVIIC), 1 muM; or verapamil, 1 muM. 3. TsTX evokes [H-3] ACh release in a concentration-dependent manner with a gradual increase up to saturation at concentrations of 500 nM. However, release of ACh evoked by TiTX-gamma was not linear regarding the toxin concentration. The [H-3]-ACh release evoked by TsTX or TiTX-gamma was partially inhibited by omega-CgTx or omega-Aga, and blocked with omega-MVIIC. Verapamil (1 muM) had no effect. Tetrodotoxin blocked [H-3] ACh release evoked by both toxins. 4. These results show that different actions on Na+-channels produce different effects on [H-3] ACh release with involvement of distinct presynaptic Ca2+-channels, which supports the idea that sodium channels may modulate neurotransmitter release.