Venoms and the nervous system

Dani Bancroft gets to grips with the neurotoxins released by organisms to defend themselves against threats or to kill prey

A blue-ringed octopus

A blue-ringed octopus.

Credit:

Cropped from a photo by Angell Williams on Flickr

Venom is a fluid mixture of chemicals; venoms have evolved as a defence mechanism in many different organisms. Some components of venoms, called neurotoxins, affect the nervous system; others, called haemotoxins, affect the blood. For more on haemotoxins, see ‘Venoms and the blood’.

What do neurotoxins do?

Neurotransmitters, chemicals such as acetylcholine, transmit information across the gap (synapse) between neurons (nerve cells). Chemicals that are similar in structure to neurotransmitters can also stimulate the postsynaptic receptors of an adjacent neuron. This binding action can be agonistic (stimulatory), in which case the impulse is still sent: venoms that act in this way can overstimulate neurons, causing muscle spasms (tetany) and an increased heart rate.

Some toxins have an antagonistic (inhibitory) effect. In this case, the nerve impulse is blocked and symptoms include muscle paralysis and a low heart rate (bradycardia).

Some neurotoxins inhibit the enzyme that breaks down the neurotransmitter–receptor complex, which also causes overstimulation of that neural pathway. This is seen in some types of rattlesnake or mamba venom.

Which venoms affect the nervous system?

Tetrodotoxin is a notorious poison found in some animals that are famous for being venomous, such as puffer fish, blue-ringed octopuses and poison dart frogs.

If tetrodotoxin enters the body, it blocks motor neuron pathways to skeletal (voluntary) muscles, causing paralysis. (It also blocks the vagus nerve, which causes the resting heart rate to climb to more than 100 beats per minute.) Paralysis of the intercostal muscles of the lungs and diaphragm means that the victim cannot breathe and goes into a coma or dies owing to a lack of oxygen to the brain. This poison is extremely toxic, and there is no known cure.

Chlorotoxin has the opposite, agonistic (stimulatory) effect. This neurotoxin is found in scorpion venom and causes convulsions (extreme tetany). However, it has been used in cancer research to develop a drug delivery system to treat a type of brain cancer called glioblastoma. The toxin is combined with a type of IgG antibody (immunoglobulin) to make a specific structure that binds only to the tumour cells to target them for treatment.

References

Questions for discussion

  • What is the difference between poisonous and venomous animals?
  • Are there any plant toxins that cause similar neurological symptoms to tetrodotoxin or chlorotoxin? Biochemically, why might these be similar?
  • What are some of the ways in which humans have exploited poisons such as tetrodotoxin and snake venoms in the past?

Further reading

About this resource

This resource was first published in ‘Proteins’ in January 2014.

Topics:
Cell biology, Health, infection and disease, Biotechnology and engineering
Issue:
Proteins
Education levels:
16–19, Continuing professional development