Clinical Psychopharmacology · Paper A
Drug mechanisms (MOA)
This concept tests the specific molecular action of frequently-examined psychiatric drugs, exactly which receptor or enzyme each one hits. The mechanism then explains the drug's effects and quirks, e.g. a drug for anxiety that takes weeks to work and causes no sedation at all. That unusual profile belongs to buspirone specifically, not a typical anxiolyticA drug that reduces anxiety.Read more → drug.
Buspirone is a 5-HT1AA serotonin receptor subtype whose partial activation is linked to reducing anxiety.Read more → partial agonistA drug that activates a receptor but produces a smaller effect than a full agonist, even at a high dose.Read more →; its anxiolyticA drug that reduces anxiety.Read more → (anxiety-reducing) effect takes 2-4 weeks and it causes no sedation or dependence. Memantine blocks the NMDA receptorA glutamate receptor in the brain involved in learning and memory, and a target of drugs like ketamine and memantine.Read more →, dampening harmful glutamate over-signalling in Alzheimer's. Lithium inhibits the enzyme inositol monophosphatase (and another enzyme, GSK-3β), quietening certain cell-signalling cascades. Carbamazepine and lamotrigine block sodium channels in nerve membranes. A patient started on buspirone who expects immediate relief within days will be disappointed, since nothing shifts for at least a fortnight. Clinically, patients need counselling that its benefit builds gradually over weeks.
Many psychiatric drugs get lumped into broad categories, 'antidepressant', 'mood stabiliser', '', but exam questions and real prescribing decisions both hinge on the exact molecular target: which receptor, transporter or enzyme a given drug hits. This matters because two drugs in the same broad category can have completely different mechanisms, onset times and side-effect profiles, so knowing only the class name is not enough to predict how a drug will actually behave in a patient.
A RECEPTOR is a protein on or in a cell that a chemical messenger binds to, triggering a response. A TRANSPORTER (or 'reuptake pump') is a protein that moves a back into the nerve cell that released it. An ENZYME is a protein that speeds up a chemical reaction, here often the breakdown or production of a signalling molecule. An AGONIST activates a receptor; an blocks a receptor without activating it; a activates a receptor, but only weakly, so it can either boost a quiet receptor or dampen an over-active one depending on context, a stabilising effect.
Buspirone is a 5-HT1A (a serotonin receptor subtype) , and its anxiolytic (anxiety-reducing) effect takes two to four weeks to build, with no sedation and no dependence, an unusual profile among anxiety treatments. For example, a drug for anxiety with a slow onset and no sedative punch at all almost always points to buspirone in a question. Memantine is an (NMDA is a glutamate receptor subtype), used in Alzheimer's disease to dampen harmful excess glutamate signalling that damages nerve cells over time. Lithium inhibits the enzyme inositol monophosphatase, and also inhibits GSK-3 beta (glycogen synthase kinase 3 beta), quietening intracellular signalling cascades involved in mood stability; a question describing a drug that depletes inositol and inhibits GSK-3 beta is describing lithium specifically. Carbamazepine and lamotrigine, two anticonvulsant mood stabilisers, both work mainly by blocking voltage-gated sodium channels in the nerve cell membrane, stabilising overexcitable nerve firing.
Clozapine is a broad-spectrum receptor blocker, hitting many different receptor types, but notably with relatively LOW for the dopamine compared with typical antipsychotics, which is thought to explain both its lower rate of movement side effects and part of its unique efficacy in treatment-resistant psychosis. Bupropion blocks the noradrenaline and dopamine reuptake transporters, an unusual mechanism among antidepressants, which is also why it is used for smoking cessation. Acamprosate acts on both NMDA (glutamate) and systems to reduce alcohol craving during abstinence, whereas naltrexone is a straightforward opioid receptor , blocking the rewarding effect of both opioids and alcohol.
Consider a patient started on buspirone who expects immediate relief within days: they will be disappointed, since nothing shifts for at least a fortnight, unlike a benzodiazepine which works within the hour. Clinically, patients need counselling that buspirone's benefit builds gradually over weeks, otherwise they are likely to stop it prematurely, judging it 'not working' before it has had a fair trial.
SSRIs (selective serotonin reuptake inhibitors) and SNRIs (serotonin-noradrenaline reuptake inhibitors) are both reuptake inhibitors and both take two to four weeks for full antidepressant effect, but SNRIs additionally block the noradrenaline transporter, giving them a broader side-effect profile including a small blood-pressure-raising effect that SSRIs largely lack. MAOIs (monoamine oxidase inhibitors) are enzyme inhibitors, blocking the enzyme that breaks down monoamine , entirely different from a reuptake mechanism, and requiring dietary restriction to avoid a hypertensive crisis with tyramine-containing foods.
This is a knowledge and vignette-recognition concept rather than a diagnostic pathway: exam stems typically describe a distinctive onset time, side-effect pattern, or biochemical clue, such as 'no sedation, no dependence, slow onset' or 'depletes inositol', and expect the exact drug and its receptor or enzyme target to be identified, not just its broad class.
In practice, knowing the exact mechanism predicts both onset of action and expected side effects, guiding choices such as picking bupropion over an when sexual side effects are a major concern (SSRIs' serotonin reuptake blockade commonly causes them, while bupropion's dopamine/noradrenaline mechanism rarely does), or picking buspirone over a benzodiazepine in a patient with a substance-misuse history, per the Maudsley Prescribing Guidelines in Psychiatry and BNF guidance on and antidepressant choice.
When a stem gives an unusual side-effect profile or an unusual onset time rather than just a drug name, it is testing the specific receptor, transporter or enzyme mechanism, not the drug's broad class label.
Each psychiatric drug has a specific receptor, transporter or enzyme target, and that exact target, not its broad class, is what predicts its onset of action, its side effects and when it should be chosen over a same-class alternative.