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Ciguatoxin enhances quantal transmitter release from frog motor nerve terminals
Author(s) -
Molgó Jordi,
Cornelia Joan X.,
Legrand Anne Marie
Publication year - 1990
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1990.tb12991.x
Subject(s) - tetrodotoxin , depolarization , neuromuscular junction , biophysics , marine toxin , ciguatoxin , electrophysiology , chemistry , motor nerve , anatomy , membrane potential , neurotoxin , grenouille , ciguatera , biology , neuroscience , toxin , biochemistry , salientia , xenopus , fishery , fish <actinopterygii> , gene
1 Ciguatoxin (CTX), a marine toxin produced by the benthic dinoflagellate Gambierdiscus toxicus , is responsible for a complex endemic disease in man known as ciguatera fish poisoning. In the present study we have investigated the effects of purified CTX extracted for Gymnothorax javanicus moray‐eel liver on frog isolated neuromuscular preparations with conventional electrophysiological techniques. 2 CTX (1–2.5 n m ) applied to cutaneous pectoris nerve‐muscle preparations induced, after a short delay, spontaneous fibrillations of the muscle fibres that could be suppressed with 1 μ m tetrodotoxin (TTX) or by formamide to uncouple excitation‐contraction. 3 In preparations treated with formamide, CTX (1–2.5 n m ) caused either spontaneous or repetitive muscle action potentials (up to frequencies of 60–100 Hz) in response to a single nerve stimulus. Recordings performed at extrajunctional regions of the muscle membrane revealed that during the repetitive firing a prolongation of the repolarizing phase of the action potential occurred. At junctional sites the repetitive action potentials were triggered by repetitive endplate potentials (e.p.ps). 4 CTX (2.5 n m ) caused a TTX‐sensitive depolarization of the muscle membrane. 5 In junctions equilibrated in solutions containing high Mg 2+ + low Ca 2+ , addition of CTX (1.5 n m ) first induced an average increase of 239 ± 36% in the mean quantal content of e.p.ps. Subsequently CTX reduced and finally blocked nerve‐evoked transmitter release irreversibly. 6 CTX (1.5–2.5 n m ) increased the frequency of miniature endplate potentials (m.e.p.ps) in junctions bathed either in normal Ringer, low Ca 2+ ‐high Mg 2+ medium or in a nominally Ca 2+ ‐free solution containing EGTA. Extensive washing with toxin‐free solutions did not reverse the effect. Furthermore, Cd 2+ (0.1 m m ), a potent calcium channel blocker, neither antagonized nor abolished the increase in transmitter release caused by CTX. 7 TTX (1 μ m ) completely prevented the effect of CTX (2.5 n m ) on m.e.p.p. frequency. This effect was independent of the presence of extracellular Ca 2+ . TTX, when added after CTX (2.5 n m ) exposure, antagonized the increase in m.e.p.p. frequency. The antagonism was complete in Ca 2+ ‐free medium. These results strongly suggest that increased permeability of the nerve terminal to Na + is responsible for the increase in m.e.p.p. frequency caused by CTX. It is likely that CTX may trigger calcium release from internal stores due to an increase of intraterminal Na + concentration. 8 It is concluded that CTX exerts, in the nanomolar concentration range, a selective action on sodium channels of the neuromuscular junction causing both pre‐ and postsynaptic effects.