MODE OF ACTION OF PALYTOXIN ON THE RELEASE OF ACETYLCHOLINE FROM RAT CEREBROCORTICAL SYNAPTOSOMES

Palytoxin (PTX; 10(-14)-10(-6) M) caused a dose-dependent increase in the release of [H-3]acetylcholine ([H-3]ACh), cytosolic free Ca2+ concentration ([Ca2+]i), and uptake of Na-22+ and decrease in membrane potential in rat cerebrocortical synaptosomes. The dose-response curves for the PTX-induced increases in [H-3]ACh release and in [Ca2+]i were depressed by removing extracellular Ca2+ or by decreasing extracellular Na+ concentrations. The release of [H-3]ACh induced by concentrations of PTX < 10(-10) M was more dependent on the simultaneous presence of both Ca2+ and Na+ than the release induced by higher concentrations of PTX. The PTX-induced increase both in [H-3]ACh release and in [Ca2+]i was almost completely abolished by the combination of Ca2+ deprivation and Na+ concentration reduction. All responses to PTX were highly resistant to 10(-6) M tetrodotoxin. These results suggest that low concentrations of PTX cause depolarization as a result of an increase in Na+ permeability through tetrodotoxin-insensitive channels. This, in turn, increases Ca2+ influx and leads to an increase in the release of ACh. It appears that at high concentrations PTX increases the release of [H-3]ACh by directly increasing the influx of Ca2+ into synaptosomes and by releasing Ca2+ from intracellular storage sites via an Na+-Ca2+ exchange mechanism.