Neurotransmitters – Agonists & Antagonists
What are agonists?
- An agonist is a molecule that binds to a synaptic receptor and activates it to promote a reaction e.g. with neurotransmission this reaction takes place within the synapse
- Drugs can act as agonists i.e. they affect the degree of a neurotransmitter’s effect (as they are made outside of the body they are known as exogenous agonists)
- If a drug (e.g. an anti-depressant) increases the effect of a neurotransmitter (e.g. serotonin) it is known as an agonist
- Selective Serotonin Reuptake Inhibitors (SSRIs) are serotonin agonists as they prevent the re-absorption of serotonin back into the presynaptic neuron and thus increase the amount of serotonin available to travel around the brain
- SSRIs are commonly used to treat depression but they can also be prescribed to treat anxiety disorders such as OCD and PTSD
The action of an SSRI is an example of an agonist in action.
What are antagonists?
- An antagonist is a molecule that binds to a synaptic receptor but this time the effect is the opposite to that of an agonist: it decreases the effect of the neurotransmitter
- Drugs can act as antagonists i.e. they affect the degree of a neurotransmitter’s effect (as they are made outside of the body they are known as exogenous antagonists)
- If a drug (e.g ecicopam) decreases the effect of a neurotransmitter e.g. dopamine it is known as an antagonist
- An antagonist reduces the action of what would normally happen when a substance binds with and effectively blocks the receptor
- Dopamine antagonists stop dopamine binding with receptors in the post-synaptic neuron which then prevents the usual feelings of reward, pleasure and motivation from occurring
- Dopamine antagonists are used to treat drug addiction but they can also be used to treat schizophrenia, psychosis and bi-polar disorder
Examiner Tip
Remember that agonists and antagonists do not change the effect of a neurotransmitter. An antagonist will not change an excitatory neurotransmitter into an inhibitory one; it will just lower the degree of the excitatory response.
Which studies focus on agonists & antagonists?
- Crockett et al. (2010) in a lab experiment which investigated the role of SSRIs in prosocial behaviour
- Romach et al. (1999) used a natural experiment to investigate the effectiveness of a dopamine antagonist on participants with cocaine addiction
Crockett and Romach’s studies are available as separate Key Studies – just navigate the Brain and Behaviour section of this topic to find them.
Examiner Tip
It is hugely important that you don’t confuse agonists with antagonists in the exam as the two are NOT the same! Try to find your own way of remembering which is which e.g. ‘My aunt is an antagonist as she decreases my fun’ = antagonists decrease the effect of a neurotransmitter.
Drug treatment may be necessary to break addiction to other drugs such as cocaine.
Worked example
Explain the role of one agonist in human behaviour. [9]
Take a look at this paragraph for an example of how to present the role of the SSRI and in an exam answer:
Selective Serotonin Reuptake Inhibitors (SSRIs) are agonists (antidepressant drugs) for serotonin as they block the brain’s re-absorption of serotonin. What this means is that during the process of synaptic transmission some molecules of serotonin do not cross the synaptic gap but instead are absorbed back into the presynaptic neuron (the neuron that fired the electric action potential to start the process of neurotransmission). Some SSRIs are agonists as they block the receptor responsible for the re-uptake of serotonin back into the presynaptic neuron (leaving it in the synapse to bind with the receptor again) and therefore they are used to treat affective disorders such as depression and anxiety as these illnesses have been linked to irregular levels of serotonin in the brain.
