A neurotoxin is a toxin that acts specifically on nerve cells [1] (neurons), usually by interacting with membrane proteins such as ion channels. Some sources are more general, and define the effect of neurotoxins as occurring at nerve tissue. [2] Bungarotoxin, which is considered a neurotoxin, [3] has its effect at the motor end plate.

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Classification and examples

Venoms

Many of the venoms and other toxins that organisms use in defense against vertebrates are neurotoxins. A common effect is paralysis, which sets in very rapidly. The venom of bees, scorpions, pufferfish, spiders and snakes can contain many different toxins.

Channel inhibitors

Many neurotoxins act by affecting voltage-dependent ion channels:

Agent

Channel |- | tetrodotoxin and batrachotoxin

sodium channels |- | maurotoxin, agitoxin, charybdotoxin, margatoxin, slotoxin, scyllatoxin and hefutoxin

potassium channels |- | calciseptine, taicatoxin, calcicludine, and PhTx3

calcium channels

A potent neurotoxin such as batrachotoxin affects the nervous system by causing depolarization of nerve and muscle fibers due to increased sodium ion permeability of the excitable cell membrane.

A very potent neurotoxin is tetrodotoxin. This chemical acts to block sodium channels in neurons, preventing action potentials. This leads to paralysis and eventually death.

Another very potent neurotoxin is taipoxin. The toxin causes a gradual reduction to complete stop of evoked and spontaneous release of acetylcholine from motor nerve terminals. The victim dies from asphyxia caused by paralysis of the respiratory muscles.

Botulinum neurotoxins are the most potent natural toxins known. They are produced by various toxigenic strains of Clostridium botulinum and act as metalloproteinases that enter peripheral cholinergic nerve terminals and cleave proteins that are crucial components of the neuroexocytosis apparatus, causing a persistent but reversible inhibition of neurotransmitter release resulting in flaccid muscle paralysis. ^[4]

Nerve agents

A number of artificial neurotoxins, known as nerve agents, have been developed for use as chemical weapons.

Neurotoxin sources

Exogenous

Toxins ingested from the environment are described as exogenous and include gases (such as carbon monoxide), metals (such as mercury ^[5]), liquids (ethanol) and numerous solids. Many neurotoxins are found in plant and animal matter found in nature; for example the neurotoxin aesculin is found in the horse chestnut (Aesculus hippocastanum ), ^[6] and the California buckeye tree. ^[7] When exogenous toxins are ingested, the effect on neurons is largely dependent on dosage.

Endogenous

Neurotoxicity also occurs from substances produced within the body, known as endogenous neurotoxins. ^[8] An example of an endogenous neurotoxin is the primary neurotransmitter glutamate, which, when levels reach too high, can result in excitotoxicity and cause neuronal death by apoptosis.

See also

• Toxin
• Microbial toxins
• Sea anemone neurotoxin
• Snake toxins

References

1. neurotoxin - Definitions from Dictionary.com
2. Dorlands Medical Dictionary:neurotoxin
3. Identification of alpha-bungarotoxin (A31) as the major postsynaptic neurotoxin, and complete nucleotide identity of a genomic DNA of Bungarus candidus from Java with exons of the Bungarus multicinctus alpha-bungarotoxin (A31) gene
4. Microbial Toxins: Current Research and Future Trends
5. United States Environmental Protection Agency: Mercury
6. ''Plant poisons: Aesculin''
7. C.Michael Hogan (2008) ''Aesculus californica'', Globaltwitcher.com, ed. N. Stromberg
8. Pharmacology of endogenous neurotoxins: a handbook