Definition of Botulinum toxin

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Botulinum toxin: A toxin produced by the bacterium Clostridium botulinum that is the most poisonous biological substance known. Botulinum toxin acts as a neurotoxin. It binds to the nerve ending at the point where the nerve joins a muscle, blocking the release by the nerve of the chemical acetylcholine (the principal neurotransmitter at the neuromuscular junction), preventing the muscle from contracting. The result is weakness and paralysis of the muscle. The muscle atrophies. The blockage of acetylcholine release is irreversible. Function can be recovered by the sprouting of nerve terminals and the formation of new synaptic contacts, which usually takes 2 to 3 months.

Very small amounts of botulinum toxin can cause botulism in one of two ways. One way is by ingesting the toxin itself (food borne botulism), as in canned foods. The other way is by infection with the bacterial spores that produce and release the toxin in the body (infectious botulism). The infection may occur in the intestine (intestinal botulism), as in a newborn, or deep within a wound (wound botulism).

There is more than one type of botulinum toxin. Different strains of the bacteria produce seven distinct neurotoxins designated A through G. All seven types have a similar molecular weight and structure, consisting of a heavy chain and a light chain joined by a disulphide bond. All seven types act in a similar manner. Only types A, B, E and F are known to cause botulism in humans. The toxin is heat labile and can be destroyed if heated at 80°C for 10 minutes or longer.

Purified botulinum toxin A was the first bacterial toxin to be used as a medicine. The US Food & Drug Administration (FDA) approved purified botulinum toxin A in 1989 for treating two eye conditions: blepharospasm and strabismus. It is marketed under the trade name Botox. It has since found other medical and cosmetic uses.

See also: Botox; Botulism; Clostridium; Clostridium botulinum.