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8.2.7Topic 8 · Grey Matter

Action of Drugs on Synapses — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 18 of 22 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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8.2.7 Action of Drugs on Synapses — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe how stimulant, depressant and other drugs alter synaptic transmission, using agonists, antagonists and reuptake inhibitors.A synapse can be altered at several stages: synthesis of the neurot…

  • dependence
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Drugs that bind to receptors and mimic the neurotransmitter are agonists
  • Others affect enzymes that break down the neurotransmitter, or the transporters that take it back

Section 1 of 9Ways a drug can affect a synapse

A synapse can be altered at several stages: synthesis of the neurotransmitter, its storage and release, its binding to postsynaptic receptors and its removal from the cleft.

Agonists
Drugs that bind to receptors and mimic the neurotransmitter are agonists.
Antagonists
Drugs that bind and block the receptor without activating it are antagonists (they act like competitive inhibitors of the transmitter).
Others affect enzymes that break down the neurotransmitter, or the transporters that take it back.

Section 2 of 9Examples

DrugActionEffect
NicotineShaped like acetylcholine; binds to nicotinic receptors (agonist) and stimulates release of dopamineStimulant, addictive reward
CocaineBlocks reuptake of dopamine, leaving it in the cleftProlonged stimulation of reward pathways
Amphetamines / MDMAIncrease release of dopamine or serotonin and inhibit reuptakeStimulant effects, mood change
Opioids (heroin, morphine)Agonists on opioid receptorsPain relief, euphoria
Alcohol, benzodiazepinesIncrease effect of the inhibitory transmitter GABADepressant: relaxation, slowed reactions
Beta-blockersAntagonists at adrenaline receptorsSlower heart rate

Section 3 of 9Reward pathways and addiction

Many drugs of abuse increase dopamine in the brain's reward pathway.

1
The pleasant feeling reinforces drug use.
2
With repeated use, the postsynaptic neurones respond by reducing the number of receptors, or the presynaptic neurones produce less transmitter,
  • so the same dose has a weaker effect.
3Tolerance
This is tolerance, which leads to higher doses.
4Dependence
When the drug is stopped, the brain is short of normal stimulation, producing withdrawal symptoms and craving, which is dependence.

Section 4 of 9Explaining a drug effect

A good answer follows the stageswhere the drug acts, what it does to neurotransmitter level or binding, how the postsynaptic response changes and what the result is for the person.

Explaining a drug effect
For example, an antagonist with a similar shape to the neurotransmitter binds to the receptor; no depolarisation occurs;
  • and fewer action potentials are generated in the postsynaptic neurone.
Reasoning from shape and binding applies the same ideas as enzyme inhibition.

Section 5 of 9Links

Drugs are also used to treat disease, as with SSRIs and L-dopa. Understanding the dose, side effects and the way drugs vary between individuals leads to personalised medicine. The same mechanisms explain why some poisons act quickly, by blocking receptors at neuromuscular junctions.

Section 6 of 9Synaptic transmission revision and a worked mechanism

To explain drug action you must be secure on normal transmission at a cholinergic synapse.

1
An action potential arrives at the presynaptic knob and opens voltage-gated calcium ion channels.
2
Calcium ions enter by diffusion and cause synaptic vesicles to fuse with the presynaptic membrane and release acetylcholine by exocytosis.
3
Acetylcholine diffuses across the cleft and binds to receptors on sodium ion channels in the postsynaptic membrane,
  • which open,
  • so sodium ions enter and depolarise the membrane.
4
If the threshold is reached, an action potential is generated.
5
Acetylcholinesterase then breaks down acetylcholine and the products are taken back into the presynaptic neurone to be resynthesised.

Each step is a possible drug target.

1
A drug that stops calcium channels opening reduces release (this is how some toxins work).
2
A drug that inhibits acetylcholinesterase,
  • such as some insecticides and nerve gases, leaves acetylcholine in the cleft,
  • causing continued stimulation and muscle spasm.
3
A drug that blocks the receptor, such as curare, prevents depolarisation and causes paralysis.
4
Learning to place a named drug at the correct step and to predict the result is an effective way of scoring marks.

In the central nervous system, summation matters too. A few receptors blocked may only reduce the response, because several neurones can add their effects (spatial summation) or one neurone can fire repeatedly (temporal summation). Drugs that change the amount of transmitter alter how easily the threshold is reached, and so change behaviour.

Caffeine is another example

  • it is an antagonist of adenosine receptors, so it blocks the signal that promotes sleepiness.
  • Remember that the effect of any drug depends on the dose, the way it is taken and how quickly the body breaks it down and removes it.

Section 7 of 9Exam-style questions

  1. Explain how cocaine produces its effect at a synapse. [3]3 marks

  2. Distinguish between an agonist and an antagonist. [2]2 marks

  3. Explain why an addict may need larger doses over time. [2]2 marks

Section 8 of 9Key terms

TermMeaning
AgonistSubstance that binds to a receptor and triggers a response
AntagonistSubstance that binds to a receptor and blocks the response
Reuptake inhibitorDrug blocking return of transmitter into the presynaptic neurone
ToleranceReduced response to a drug requiring larger doses
DependenceNeed for a drug to function normally, with withdrawal if stopped
GABAInhibitory neurotransmitter

Section 9 of 9Exam technique

1
Describing every drug as blocking a receptor; some mimic the transmitter (agonists).
2
Confusing tolerance with dependence.
3
Saying drugs "stop" the synapse working; state exactly which step is changed.
4
Forgetting that reuptake inhibition keeps the transmitter in the cleft.

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