In general, a neurotoxin is a toxin that poisons a neuron's ion channels in some way. Given the importance of Na+ and K+ channels for neuronal excitation, it's not surprising that many organisms have evolved channel-specific toxins as mechanisms for self-defense or for capturing their prey. The best-known channel toxin is tetrodotoxin, produced by certain puffer fish and other animals. Tetrodotoxin produces a very strong and specific obstruction of the Na+ channels responsible for action potential generation, thereby paralyzing the animals unfortunate enough to ingest it. Saxitoxin, a homologue of tetrodotoxin produced by dinoflagellates, has a similar action. The potentially lethal effects of eating "red tide" shellfish are due to saxitoxin.

Scorpions paralyze their prey by injecting peptide toxins, including the α-toxins, which slow inactivation of Na+ channels; the toxin prolongs the action potential, scrambling information flow within the nervous system of the soon-to-be-devoured victim. Other peptides, called β-toxins, shift the critical voltage levels of Na+ channel activation, causing the channels to open at potentials much more negative than normal. Some alkaloid toxins combine both these actions, such as batrachotoxin; some tribes of South American Indians use this poison on their arrow tips. Some plants produce similar alkaloids, such as aconitine, from buttercups, veratridine, from lilies, and many insecticidals from chrysanthemums and rhodonendrons.

Potassium channels are also targeted by some toxins. Peptide toxins affecting K+ channels include dendrotoxin, from wasps, apamin, from bees, and charybdotoxin, also from scorpions. These toxins block K+.

Neuroscience, Sinaur Associates (QP355.2.N487 1997)

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