All subjects · Biology · WBI15 (Unit 5)

Drugs, synapses and brain chemistry

Many drugs work by changing what happens at synapses. Imbalances in brain chemicals such as dopamine and serotonin can cause illness, and understanding them guides new treatments.

Drugs that affect nerve transmission

Drugs can mimic a neurotransmitter, block its receptors, block ion channels, or change how much neurotransmitter is in the synapse.

Drug	How it acts	Effect
Nicotine	Mimics acetylcholine, binding to nicotinic ACh receptors and opening their channels	Stimulates neurones; leads to more dopamine release in the brain's reward pathway, so it is addictive
Lidocaine	Blocks voltage-gated Na+ channels in neurone membranes	No action potentials in sensory neurones: local anaesthetic
Cobra venom (alpha-cobratoxin)	Binds to ACh receptors at the neuromuscular junction and blocks ACh from binding	Muscles cannot be stimulated: paralysis, including of breathing muscles
L-DOPA (levodopa)	A precursor that is converted to dopamine in the brain	Raises dopamine levels: treats Parkinson's disease
MDMA (ecstasy)	Blocks serotonin reuptake transporters and makes them run in reverse	More serotonin in the synapse: mood changes; may damage serotonin neurones

Dopamine and Parkinson's disease

In Parkinson's disease, neurones that make dopamine in part of the brain that controls movement die. Low dopamine causes tremor, muscle stiffness and slow movement.

Dopamine itself cannot cross from the blood into the brain, but L-DOPA can. In the brain it is converted to dopamine. It is often given with a drug (such as carbidopa or benserazide) that stops it being broken down before it reaches the brain. Other treatments include drugs that mimic dopamine (dopamine agonists).

Serotonin and depression

Low levels of serotonin at synapses in parts of the brain are linked to depression. Drugs called SSRIs (selective serotonin reuptake inhibitors) block the transporters that take serotonin back into the presynaptic neurone, so serotonin stays in the synapse longer.

Knowing which brain chemicals are out of balance, and how synapses work, lets scientists design new drugs that target specific receptors, transporters or enzymes.

Worked example

Explain why lidocaine stops a patient feeling pain during dental treatment.

Lidocaine blocks voltage-gated Na+ channels in sensory neurones.
Na+ cannot enter, so the membrane cannot depolarise to threshold.
No action potentials travel to the brain.

Answer: No impulses reach the brain from the treated area, so no pain is felt.

Key idea: Nicotine mimics ACh; lidocaine blocks Na+ channels (local anaesthetic); cobra toxin blocks ACh receptors (paralysis); L-DOPA becomes dopamine (Parkinson's); MDMA raises synaptic serotonin. Low dopamine causes Parkinson's; low serotonin is linked to depression (SSRIs).

Check yourself

How does nicotine affect synapses?
- It blocks Na+ channels
- It destroys acetylcholine
- It binds to acetylcholine receptors and stimulates the postsynaptic neurone
- It blocks Ca2+ channels in the presynaptic knob
Show answer
It binds to acetylcholine receptors and stimulates the postsynaptic neurone. Nicotine mimics ACh at nicotinic receptors.
How does lidocaine act as a local anaesthetic?
- It blocks voltage-gated Na+ channels so action potentials cannot form
- It increases serotonin
- It binds to dopamine receptors
- It speeds up nerve impulses
Show answer
It blocks voltage-gated Na+ channels so action potentials cannot form. No Na+ entry means no depolarisation.
Why can a cobra bite cause paralysis?
- The toxin increases acetylcholine release
- The toxin blocks dopamine production
- The toxin blocks acetylcholine receptors at neuromuscular junctions
- The toxin dissolves myelin
Show answer
The toxin blocks acetylcholine receptors at neuromuscular junctions. ACh cannot bind, so muscles are not stimulated.
Why is L-DOPA given to people with Parkinson's disease rather than dopamine?
- Dopamine is too cheap
- L-DOPA can enter the brain from the blood and is converted to dopamine there
- L-DOPA blocks dopamine receptors
- Dopamine causes paralysis
Show answer
L-DOPA can enter the brain from the blood and is converted to dopamine there. Dopamine cannot cross into the brain; its precursor can.
How does MDMA increase serotonin at synapses?
- It destroys serotonin receptors
- It blocks Na+ channels
- It is converted into dopamine
- It blocks serotonin reuptake transporters and makes them run in reverse, releasing serotonin into the synapse
Show answer
It blocks serotonin reuptake transporters and makes them run in reverse, releasing serotonin into the synapse. More serotonin stays in the synaptic cleft.
What causes the symptoms of Parkinson's disease?
- Too much serotonin
- Loss of dopamine-producing neurones
- Damage to the myelin sheath of all neurones
- Too much acetylcholinesterase
Show answer
Loss of dopamine-producing neurones. Low dopamine affects control of movement.
How do SSRIs help treat depression?
- They destroy serotonin
- They block dopamine release
- They act as local anaesthetics
- They block serotonin reuptake, so serotonin stays in the synapse longer
Show answer
They block serotonin reuptake, so serotonin stays in the synapse longer. More serotonin at synapses.

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Last updated 7 November 2026. Spotted a mistake? Tell us.