Modulation of Ca2+-stimulated glutamate release from synaptosomes by Na+ entry through tetrodotoxin-sensitive channels

Abstract

Tityustoxin (TsTX), a toxin obtained from the venom of the Brazilian scorpion Tityus serrulatus, stimulates Na+ influx through tetrodotoxin (TTX)-sensitive Na+ channels which, in turn, promotes both Ca2+-dependent and Ca2+-independent release of glutamate from rat cerebrocortical synaptosomes. The level of Ca2+-dependent glutamate release after addition of 0.5 μM TsTX is greater than that produced by a maximally depolarizing concentration of KCl. This effect of TsTX, which is entirely dependent on Na+ entry, suggests that Na+ has a role in modulating Ca2+ entry and glutamate release that is not simply related to membrane depolarization. In order to investigate possible modulatory role(s) of Na+ on Ca2+-dependent glutamate release, we compared the effects of TsTX with those of KCl and the Na+ ionophore gramicidin D. When used alone, 100 nM gramicidin D produced a larger increase in intrasynaptosomal free Na+ than did 0.5 μM TsTX, and a similar rise in intrasynaptosomal free Ca2+, but was much less effective in promoting glutamate release. Even the combination of membrane depolarization (by 33 mM KCl) and elevation of intrasynaptosomal free Na+ (by 100 nM gramicidin) was still less effective than TsTX at causing Ca2+-dependent glutamate release. These data suggest that localized Na+ entry, through TTX-sensitive Na+ channels, exerts a modulatory role on Ca2+-dependent glutamate release from nerve endings in the cerebral cortex.