NEUROTOXINS

Neurotoxin effectively means "nerve poison". Neurotoxins act directly on neurons or nerve cells by interfering with membrane proteins and ion channels.

A neuron carries a signal as a miniature electric current. Ions carry charges, and when they move across the cell membrane in a specific region of a neuron at a rapid rate they change the electrical potential in that region. The rapid movement of ions migrates along the neuron and propagates an electrical signal (action potential). When this signal reaches the end of the neuron, it must trigger a response in the next neuron. In a few cases, neurons are packed closely enough so that the charge associated with the moving action potential directly excites the next neuron. In most cases, the first neuron releases chemicals called neurotransmitters that diffuse across a small gap (synaptic cleft) and interact with the next neuron, triggering its response. Many neurotoxins, work by disrupting this communication process.

There are two common mechanisms by which nerve signaling is disrupted. The cell that receives the signal does so when receptors within its membrane interact with the neurotransmitters. Some neurotoxins act by blocking these receptors, making it impossible for them to receive signals. When signaling stops, nerve function is impaired or eliminated and, the neurotoxin has caused its damage.

There are special enzymes in the synaptic cleft that break down certain neuro transmitters. Some neurotoxins block the actions of the hydrolytic enzymes, thereby preventing the removal of acetylcholine, leading to continuous stimulation of the neurons and, ultimately, cell death.

The effect of various neurtoxins on the brain's neurons depends primarily on the dosage, but typically results in loss of muscle control, loss of mental abilities, loss of feeling and, sometimes loss of consciousness. Low levels of ethanol, for instance, are mildly neurotoxic and result only in drunkenness, not poisoning; but prolonged exposure to it weakens and kills neurons.

Exogenous neurotoxins

Toxins ingested from the environment are described as exogenous and include gases (such as carbon monoxide), metals (mercury),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), and the California buckeye tree. When exogenous toxins are ingested, the effect on neurons is largely dependent on dosage.

Endogenous neurotoxins

Neurotoxicity also occurs from substances produced within the body, known as endogenous neurotoxins. 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.

Some of the common neurotoxins found in day-to-day life are:

Neurotoxin Found in

Albuterol Breathing medicine

Aflatoxins Peanuts

Captopril Blood pressure medicine

Codeine Phosphate Cough syrup

Digoxin Heart drug

Fluoxetine Prozac

Ethanol Alcohol

Neurotoxins are found in venoms of animals and reptiles for their defense, where the animals inject their venoms and paralyze. The most common effect of the neurotoxins used by organisms is rapid-setting paralysis, useful to snakes and other venomous predators as it keeps the prey from running away.

Below are some neurotoxins and their action on the nervous system.

Name of the toxin Source Neuronal Action

Apamin Honey bee Blocks potassium channels

Anatoxin Algae Acetyl choline receptor agonist

Agitoxin Scorpian Blocks potassium channels

Capsaicin Cayenne Pepper Excites peripheral nerve endings

Cobrotoxin Cobra Blocks nicotinic receptors

Phoneutriatoxin Banana spider Slows sodium channel activation

Ciguatoxin Dinoflagellate Opens sodium channels

Botulinum toxin Bacteria Blocks acetyl choline release

Philathotoxin Predaceous Wasp Blocks glutamate receptors

Homobatrachotoxin Pitohui (bird) Activates sodium channels

rTamapin Indian Red Scorpion Blocks potassium channels

Tetrodotoxin (TTX) Pufferfish Blocks sodium channels

Neurotoxins from blue-green algae present in certain foods or water can accumulate in proteins and might cause brain diseases like Alzheimer's after many years, suggests a new study.

ByAnand.D

Source: Internet (I MSc)