Neuroprotectant effects of iso-osmolar d-mannitol to prevent Pacific ciguatoxin-1 induced alterations in neuronal excitability: A comparison with other osmotic agents and free radical scavengers

The basis for the neuroprotectant effect of d-mannitol in reducing the sensory neurological disturbances seen in ciguatera poisoning, is unclear. Pacific ciguatoxin-1 (P-CTX-1), at a concentration 10 nM, caused a statistically significant swelling of rat sensory dorsal root ganglia (DRG) neurons that was reversed by hyperosmolar 50 mM d-mannitol. However, using electron paramagnetic resonance (EPR) spectroscopy, it was found that P-CTX-1 failed to generate hydroxyl free radicals at concentrations of toxin that caused profound effects on neuronal excitability. Whole-cell patch-clamp recordings from DRG neurons revealed that both hyper- and iso-osmolar 50 mM d-mannitol prevented the membrane depolarisation and repetitive firing of action potentials induced by P-CTX-1. In addition, both hyper- and iso-osmolar 50 mM d-mannitol prevented the hyperpolarising shift in steady-state inactivation and the rise in leakage current through tetrodotoxin (TTX)-sensitive Nav channels, as well as the increased rate of recovery from inactivation of TTX-resistant Nav channels induced by P-CTX-1. d-Mannitol also reduced, but did not prevent, the inhibition of peak TTX-sensitive and TTX-resistant INa amplitude by P-CTX-1. Additional experiments using hyper- and iso-osmolar d-sorbitol, hyperosmolar sucrose and the free radical scavenging agents Trolox® and l-ascorbic acid showed that these agents, unlike d-mannitol, failed to prevent the effects of P-CTX-1 on spike electrogenesis and Nav channel gating. These selective actions of d-mannitol indicate that it does not act purely as an osmotic agent to reduce swelling of nerves, but involves a more complex action dependent on the Nav channel subtype, possibly to alter or reduce toxin association.