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 sy naptosomes incubated in Ca2+-free medium caused no detectable acetylch oline (ACh) release. However, subsequent addition of Ca2+ caused a lar ge ACh release that depended on time of exposure, dose of CTX and on [ Ca2+]. Tetrodotoxin completely prevented CTX-induced Ca2+-dependent AC h release. Simultaneous blockade of Ca2+ channel subtypes by FTX, a to xin extracted from the venom of the spider Agelenopsis aperta, omega-c onotoxin and Gd3+ did not prevent ACh release caused by CTX, upon addi tion of Ca2+. These results suggest that CTX activates the reversed op eration of the Na+/Ca2+ exchange system allowing the entry of Ca2+ in exchange for Na+. It is concluded that Torpedo synaptosomes are endowe d with Na+ channels sensitive to pico- to nanomolar concentrations of CTX.