9.6 - Effects of drugs on the nervous system

What are agonist & anatagonist drugs?

• Drugs that stimulate a synapse are called agonists

• Drugs that inhibit a synapse are called antagonists

How does nicotine affect synapses?

Agonist stimulant:

• absorbed in the lungs & travels to the brain via the bloodstream

• similar shape to acetylcholine, so binds to cholinergic receptors on the postsynaptic membrane

• causes Na⁺ ion channels to open, which leads to depolarisation & an action potential → causes the release of more neurotransmitters (e.g. adrenaline, dopamine & endorphins)

How does lidocaine affect synpases?

Antagonist:

• blocks voltage-gated Na⁺ ion channels in the postsynaptic membrane → Na⁺ ions cannot enter the cell when neurotransmitter binds

• action potentials cannot form in the postsynaptic cell → prevents postsynaptic neuron from depolarising

• prevents transmission of pain signals, so acts as an anaesthetic

How does cobra venom affect synpases?

Antagonist:

• binds to acetylcholine receptors, which prevents binding of acetylcholine

• postsynaptic membrane is not stimulated to depolarise → can cause paralysis

How does alcohol affect synapses?

Agonist of GABA (inhibitory neurotransmitter) receptors → binds to them:

• more Cl^- channels open in the postsynaptic membrane

• membrane potential becomes even more negative → results in reduced brain activity, sedation & lowers anxiety (i.e. acts as a depressant)

How does tetrodotoxin affect synapses?

Antagonist (produced by blue-ringed octopus & pufferfish):

• blocks Na⁺ ion channels → no or less influx of Na⁺ ions into the postsynaptic membrane

• no depolarisation, so no action potential → results in muscle paralysis

How does sarin (nerve agent) affect synapses?

Agonist:

• binds irreversibly to the enzyme, acetylcholinesterase

• acetylcholine (ACh) is not hydrolysed → more of it is left in the synapse for longer

• muscles cannot relax/lack of muscle control → leads to quick death (e.g. cardiac arrest, breathing muscle paralysis)