Basic Neurosciences · Paper A
Neurotransmitter pharmacology
Several drugs work by targeting one specific neurotransmitterA chemical messenger that nerve cells release to signal to the next cell.Read more → system, and matching each drug to its exact target and effect is more useful than a vague sense of 'it affects brain chemicals'. Riluzole, e.g. the drug used specifically in motor neurone disease, works by reducing the release of one particular excitatory transmitter. In practice, matching drug to mechanism precisely is exactly what these questions test.
Riluzole, used in motor neurone disease, inhibits the release of glutamate, reducing excitotoxicity (nerve cell damage from excessive excitatory signalling). Memantine and ketamine both block NMDA glutamate receptors, though for different clinical purposes: memantine for dementia and ketamine as an anaesthetic and rapid-acting antidepressant. Glycine is a major inhibitory transmitter in the spinal cord and brainstem, blocked by the poison strychnine, and also acts as a co-agonistA drug or chemical that activates a receptor to produce an effect.Read more → at NMDA receptorsA glutamate receptor in the brain involved in learning and memory, and a target of drugs like ketamine and memantine.Read more →. GABAThe brain's main calming (inhibitory) chemical messenger, boosted by benzodiazepines and alcohol.Read more → is the main inhibitory transmitter of the central nervous system (the brain and spinal cord) more broadly. Nitric oxide, made from the amino acid L-arginine, acts as a retrograde gaseous messenger, signalling backwards across a synapseThe tiny gap between two nerve cells where a chemical signal passes from one to the other.Read more → rather than forwards. A patient who ingests strychnine, e.g. one who develops severe, painful muscle spasms, shows glycine blockade in action. Recognising strychnine poisoning by its specific glycine-blocking mechanism, distinct from other toxins, therefore guides urgent treatment.
A vague statement like "this drug affects brain chemicals" is almost useless clinically and rarely enough to pass an exam question. What actually matters, and what a fresh graduate needs to be able to do at the bedside, is matching each drug precisely to the system it targets, the specific receptor or process involved, and the clinical use that follows logically from that mechanism.
Six systems recur constantly in psychiatric pharmacology: glutamate (the principal excitatory neurotransmitter in the central nervous system, the brain and spinal cord), and glycine (the two main inhibitory transmitters), acetylcholine and dopamine (transmitters acting through their own distinct receptor families), and nitric oxide (an unusual gaseous signalling molecule). Each has its own receptors, its own named drugs, and its own clinical indications, and treating them as interchangeable is exactly the vague thinking this topic is designed to catch.
A single receptor can be the target of two completely different clinical uses. Memantine and ketamine both block the NMDA glutamate receptor but are used for entirely different purposes: memantine is used in moderate-to-severe Alzheimer's disease, following NICE guidance, to reduce glutamate-driven excitotoxicity (nerve cell damage caused by excessive excitatory signalling) associated with disease progression, while ketamine is used as an anaesthetic agent and, at low doses, has an emerging role as a rapid-acting treatment for severe, treatment-resistant depression. Knowing the receptor is not enough; you must also know the indication being tested.
Glutamate acts mainly through NMDA and AMPA receptors, two different types of glutamate receptor. Riluzole, used specifically in motor neurone disease (a progressive condition causing degeneration of motor neurons), works by inhibiting the release of glutamate, which reduces excitotoxicity thought to contribute to motor neuron death. (gamma-aminobutyric acid) is the main inhibitory of the central nervous system more broadly, acting at two receptor types: GABA-A, a fast, ionotropic (directly ligand-gated ion channel) receptor targeted by benzodiazepines and barbiturates, and GABA-B, a slower, metabotropic (G-protein-coupled) receptor. Glycine is a separate, major inhibitory transmitter concentrated in the spinal cord and brainstem rather than acting broadly across the whole brain like GABA; it also plays a second role as a co- required alongside glutamate at the .
Consider a patient who has ingested strychnine, a plant-derived poison that blocks glycine receptors, whether accidentally or deliberately, and presents with severe, painful muscle spasms and a rigid, arched posture, sometimes with a fixed grimacing expression from facial muscle spasm. Because strychnine blocks glycine's normal inhibitory action in the spinal cord, motor neurons fire unopposed, producing this dramatic clinical picture of generalised muscle over-activity without any accompanying loss of consciousness. Working through the mechanism step by step, from receptor blockade, to loss of inhibition, to unopposed motor firing, explains the whole clinical picture from one single fact: which receptor strychnine blocks.
Strychnine poisoning is easily mistaken for a generalised tonic-clonic seizure or for tetanus, since both also produce severe muscle rigidity and spasm. The key differential is consciousness and trigger pattern: strychnine poisoning classically spares consciousness and spasms are often triggered or worsened by stimuli such as noise or touch, whereas a generalised seizure causes loss of consciousness. Tetanus, caused by a bacterial toxin blocking a different inhibitory pathway, typically has a slower onset with a preceding history of a wound, rather than the rapid onset following ingestion seen with strychnine. Separately, acetylcholine and dopamine are easy to muddle because both have multiple receptor subtypes; remember acetylcholine acts at nicotinic (fast, ionotropic) and muscarinic (slower, metabotropic) receptors, while dopamine acts exclusively through G-protein-coupled D1 to D5 receptors, with no fast ionotropic dopamine receptor at all.
Diagnosis in this topic means correctly identifying which transmitter system a clinical picture or drug mechanism points to, rather than a single test. For strychnine poisoning specifically, diagnosis rests on history (a recent ingestion) and the characteristic examination findings of spasm with preserved consciousness, supported where available by toxicology screening. For the pharmacology questions more broadly, "diagnosis" means correctly reasoning from a described mechanism, for example a signal travelling backwards from the postsynaptic to the presynaptic neuron, to the correct transmitter, in that example nitric oxide, which is unlike every other transmitter discussed here: a small molecule made from the amino acid L-arginine that diffuses freely and acts as a retrograde messenger rather than being stored in vesicles and released in the conventional forward direction.
Management in this topic means using the mechanism to predict or explain the actual treatment used. Riluzole reduces glutamate release in motor neurone disease; memantine blocks in moderate-to-severe Alzheimer's disease per NICE guidance, while ketamine's NMDA blockade is used instead for anaesthesia or, increasingly, treatment-resistant depression; cholinesterase inhibitors raise acetylcholine levels for milder Alzheimer's disease; and antipsychotic medication works mainly through D2 dopamine receptor blockade, explaining both its therapeutic effect and its extrapyramidal side effects. For strychnine poisoning, management is supportive: investigation focuses on history and toxicology, and treatment includes benzodiazepines to control muscle spasm via the separate -A system, alongside general poisoning management such as airway support and cooling for associated hyperthermia.
A question describing a signal travelling backwards across a should immediately point you to nitric oxide rather than any conventional transmitter such as glutamate or . A question describing painful spasm with preserved consciousness after a poisoning should point you to strychnine and glycine, not a seizure. And whenever two drugs share a receptor, memantine and ketamine at NMDA being the classic pair, expect the question to hinge on indication, not mechanism.
Match each drug to its specific transmitter target rather than a vague "affects brain chemicals" answer: glutamate (riluzole, memantine, ketamine), and glycine (benzodiazepines, strychnine), acetylcholine and dopamine (cholinesterase inhibitors, antipsychotics), and nitric oxide, the odd one out as a gaseous, retrograde messenger.