6- effects of drugs on the nervous system

Where do most medicinal and recreational drugs affect the nervous system?

The synapses

What are drugs that stimulate a synapse called?

Agonists

What are drugs that inhibit a synapse called?

Antagonists

What are the ways a drug can act, give an example of each.

1. Mimic a neurotransmitter e.g. nicotine, levodopa

2. Stimulate the release of a neurotransmitter e.g. cocaine, caffeine

3. Open a neuroreceptor channel e.g. alcohol, marijuana, salbutamol

4. Block a neuroreceptor channel e.g. a-neurotoxins, curare, opioids

5. Inhibit the breakdown enzyme e.g. DDT

Which of the ways a drug can act have an agonist effect?

• Mimic a neurotransmitter

• Stimulate the release of a neurotransmitter

• Open a neuroreceptor channel

• Inhibit the breakdown enzyme

Which of the ways a drug can act has an antagonist effect?

• Block a neuroreceptor channel

How does nicotine affect the nervous system?

It has a similar structure to acetylcholine (neurotransmitters) so binds to cholinergic neuroreceptors in the CNS and switches them on, like acetylcholine.

So, it is acting as an agonist but in an uncontrolled way.

The affected neuroreceptors have complex functions involving the release of other neurotransmitters in the CNS. So by strongly switching on these channels, it causes the release of more neurotransmitters, including:

• adrenaline, which triggers the fight or flight response

• dopamine, which stimulates the reward system in the brain, leading to feelings of pleasure

• endorphins, also causes feelings of well-being but leads to withdrawal symptoms, so causes addiction

How does cobra venom (a-neurotoxin) affect the nervous system?

It binds tightly and irreversibly to the acetylcholine receptors in neuromuscular junctions, stopping the receptors from opening, so stopping the skeletal muscles from contracting.

So, the victim is paralysed and is killed if the toxin reaches the breathing muscles.

How does lidocaine affect the nervous system?

It blocks voltage-gated sodium channels (acetylcholine receptors), stopping all action potentials.

In low doses it only has a local effect at the site of administration, as it is quickly broken down. So it is often used as a local anaesthetic e.g. in dental surgery by inhibiting sensory neurones and as a muscle relaxant by inhibiting motor neurones.

But in high doses it has a more widespread effect and can be fatal.