Ciguatoxin extracted from poisonous moray eels Gymnothorax javanicus triggers acetylcholine release from Torpedo cholinergic synaptosomes via reversed Na(+)-Ca2+ exchange.

Ciguatoxin (CTX) (0.1 pM to 10 nM) added to a suspension of Torpedo synaptosomes incubated in Ca(2+)-free medium caused no detectable acetylcholine (ACh) release. However, subsequent addition of Ca2+ caused a large ACh release that depended on time of exposure, dose of CTX and on [Ca2+]. Tetrodotoxin completely prevented CTX-induced Ca(2+)-dependent ACh release. Simultaneous blockade of Ca2+ channel subtypes by FTX, a toxin extracted from the venom of the spider Agelenopsis aperta, omega-conotoxin and Gd3+ did not prevent ACh release caused by CTX, upon addition of Ca2+. These results suggest that CTX activates the reversed operation of the Na+/Ca2+ exchange system allowing the entry of Ca2+ in exchange for Na+. It is concluded that Torpedo synaptosomes are endowed with Na+ channels sensitive to pico- to nanomolar concentrations of CTX.