Review of Neurotransmitters: The Foundation of CNS Pharmacology

DCM 2101 · CNS Pharmacology

Review of Neurotransmitters: The Foundation of CNS Pharmacology

Almost every medicine that changes pain, consciousness, sleep, mood, movement, behaviour, memory or seizure activity works by changing neurotransmission. Before a learner can understand opioids, local anaesthetics, anti-convulsants, antidepressants, antipsychotics, sedatives or medicines used in Parkinson disease, they must understand the chemical language used by the nervous system. A neurotransmitter is an endogenous chemical messenger released from a neuron that acts on a target cell—another neuron, a muscle cell or a gland cell—by binding to a specific receptor. The important clinical question is never simply “what does this chemical do?” It is: where is it made, where is it released, which receptor does it act on, how is its action stopped, and at which point can a medicine modify the process?

Learning objectives

After studying this lesson, the learner should be able to:

Define neurotransmitter, synapse, receptor, agonist, antagonist, reuptake and enzyme inhibition.
Describe the sequence of chemical synaptic transmission from synthesis to termination.
Differentiate ionotropic from metabotropic receptors and explain why the distinction matters clinically.
Classify the important CNS neurotransmitters and relate each to major functions, pathways, receptors, drugs and disease.
Explain how pharmacological agents alter neurotransmitter synthesis, storage, release, receptor action, reuptake and metabolism.
Apply neurotransmitter physiology to common clinical problems including epilepsy, Parkinson disease, depression, psychosis, dementia, pain, addiction and poisoning.

1. The nervous system communicates at synapses

A neuron carries an electrical signal (the action potential) along its axon. At the end of the axon, it must pass information to another cell. The specialised junction where this occurs is a synapse. Most clinically important synapses are chemical synapses.

Part of a chemical synapse	What it contains or does	Why it matters in pharmacology
Presynaptic terminal	The axon ending. It contains transmitter-synthesising enzymes, storage vesicles, voltage-gated calcium channels and transporters.	Drugs may alter synthesis, vesicular storage, calcium-dependent release or presynaptic feedback receptors.
Synaptic vesicles	Small membrane sacs that concentrate neurotransmitter and protect it from metabolism until release.	Botulinum toxin, amphetamines, vesicular transport inhibitors and many toxins act before or during release.
Synaptic cleft	The tiny gap between cells. Released transmitter diffuses across it in milliseconds.	Enzymes and reuptake transporters remove transmitter from this space; their inhibition can increase signalling.
Postsynaptic membrane	Contains receptors, ion channels and second-messenger systems that convert a chemical message into a cellular response.	Most medicines are receptor agonists, partial agonists, antagonists or allosteric modulators.
Glial cells	Astrocytes and other glia buffer potassium, take up transmitters such as glutamate and GABA, and support neuronal metabolism.	Failure of glial clearance can contribute to excitotoxicity; glia are active partners, not passive support cells.

Chemical versus electrical synapses

Chemical synapse Uses a neurotransmitter. It is slightly slower than an electrical synapse but is highly flexible: the signal can be amplified, inhibited, prolonged, reversed, learned from or modified by drugs. Nearly all CNS pharmacology focuses here.

Electrical synapse Uses gap junctions: direct protein channels linking neighbouring cells. Ions flow rapidly from one cell to another. They are useful where synchronisation is required, but they are less common as drug targets.

2. The life cycle of a neurotransmitter

Neurotransmission is a repeating cycle. A medicine can increase or decrease signalling at almost every stage. Learn this sequence well—it is the organising principle for the entire CNS pharmacology chapter.

Synthesis: A precursor is taken into the neuron and converted into a transmitter by enzymes. For example, tyrosine is converted to dopamine; glutamate is converted to GABA.
Storage: The transmitter is transported into vesicles. Storage allows a rapid, controlled pulse of release rather than continuous leakage.
Arrival of the action potential: Depolarisation reaches the presynaptic terminal.
Calcium entry: Voltage-gated Ca²⁺ channels open. Calcium influx triggers vesicles to dock and fuse with the presynaptic membrane.
Exocytosis: Vesicles release transmitter into the synaptic cleft.
Receptor binding: The transmitter binds a matching receptor. Depending on the receptor and ion involved, the result may be excitatory, inhibitory or modulatory.
Termination: The message ends by reuptake into the nerve terminal or glial cell, enzymatic breakdown, diffusion away, or uptake into surrounding tissue.
Recycling: Components are reused to make more transmitter or vesicles.

Clinical framework

Think “more signal” or “less signal”

A medicine may produce more transmitter effect by increasing synthesis or release, blocking reuptake, blocking metabolism, directly stimulating a receptor, or positively modulating receptor activity. It may produce less transmitter effect by reducing synthesis or release, blocking a receptor, accelerating breakdown, or negatively modulating receptor activity. The desired direction depends on the disease: increasing dopamine helps Parkinson disease, whereas blocking excessive dopamine signalling helps psychosis.

Key pharmacology words

Term	Meaning	Simple example
Agonist	Binds a receptor and activates it, imitating the endogenous transmitter.	Morphine activates μ-opioid receptors; salbutamol activates β₂ receptors.
Partial agonist	Activates a receptor, but less strongly than a full agonist. In a high-transmitter state it may reduce the overall effect.	Buprenorphine is a partial μ-opioid receptor agonist.
Antagonist	Binds without activating the receptor and blocks an agonist or transmitter.	Naloxone blocks opioid receptors; many antipsychotics block D₂ receptors.
Reuptake inhibitor	Blocks the transporter that normally returns transmitter to the nerve terminal or glia, increasing transmitter in the synaptic cleft.	SSRIs inhibit serotonin reuptake; cocaine blocks monoamine reuptake.
Enzyme inhibitor	Prevents enzymatic destruction of a transmitter, prolonging its action.	Acetylcholinesterase inhibitors increase acetylcholine; MAO inhibitors increase monoamines.
Allosteric modulator	Binds at a site different from the transmitter-binding site and changes receptor response.	Benzodiazepines positively modulate GABA_A receptors.

3. Receptors: how the message becomes a response

A neurotransmitter has no effect unless the target cell has an appropriate receptor. The receptor determines the speed, direction and duration of the response. A single transmitter can be excitatory in one place and inhibitory in another because receptor subtype—not the transmitter name alone—determines the final effect.

Ionotropic receptors

Fast, direct ion channels

The receptor itself is an ion channel. When transmitter binds, the channel opens within milliseconds and ions move across the membrane.

Excitatory example: AMPA or nicotinic acetylcholine receptors permit positive ion entry, moving the cell toward depolarisation.
Inhibitory example: GABA_A and glycine receptors permit Cl⁻ movement, usually making firing less likely.
Clinical relevance: benzodiazepines, barbiturates, anaesthetics and many anti-seizure medicines change fast ion-channel-mediated inhibition or excitation.

Metabotropic receptors

Slower, longer-lasting G-protein signalling

These receptors activate intracellular G proteins and second messengers such as cyclic AMP, IP₃ and DAG. They can open channels indirectly, alter enzyme activity, change gene transcription and remodel synapses.

Examples: muscarinic, dopamine, adrenergic, serotonin (except 5-HT₃), metabotropic glutamate, GABA_B, opioid and cannabinoid receptors.
Clinical relevance: many antidepressants, antipsychotics, antiparkinsonian agents and opioids have their main action through these receptors.

Excitation and inhibition: an important caution

Excitatory means the net response makes action-potential firing more likely; inhibitory means it makes firing less likely. This is not an absolute property of a chemical. The outcome depends on receptor subtype, ion gradient, location and the neuronal circuit. For example, acetylcholine can excite skeletal muscle through nicotinic receptors but slow the heart through M₂ muscarinic receptors. Therefore, avoid memorising “one transmitter = one effect.”

4. Classification of clinically important neurotransmitters

Group	Main examples	General features
Amino acids	Glutamate, GABA, glycine	Very common, rapid synaptic signalling. Glutamate is the main excitatory transmitter in the CNS; GABA is the main inhibitory transmitter in the brain; glycine is a major inhibitory transmitter in spinal cord and brainstem.
Acetylcholine	Acetylcholine (ACh)	Acts in the autonomic nervous system, neuromuscular junction and important CNS pathways for attention and memory.
Monoamines	Dopamine, noradrenaline, adrenaline, serotonin, histamine	Usually work through G-protein-coupled receptors and have widespread modulatory effects on mood, arousal, movement, appetite and autonomic function.
Neuropeptides	Endorphins, enkephalins, substance P, neuropeptide Y	Often co-released with a small-molecule transmitter. They act more slowly and can produce prolonged effects such as analgesia, stress responses and appetite changes.
Purines	ATP, adenosine	ATP can signal through purinergic receptors; adenosine usually suppresses neuronal activity and promotes sleep pressure.
Unconventional transmitters	Nitric oxide, carbon monoxide, endocannabinoids	Not stored in ordinary vesicles in the same way. Some are made on demand and can signal backwards from postsynaptic to presynaptic neuron.

5. Glutamate: the principal excitatory transmitter

Glutamate is the major excitatory neurotransmitter in the brain and spinal cord. It is essential for normal sensation, learning, memory, motor control and synaptic plasticity. It is also dangerous in excess: excessive glutamate stimulation allows too much calcium into neurons and may cause excitotoxicity, contributing to neuronal injury after stroke, traumatic brain injury, prolonged seizures and some neurodegenerative disorders.

Synthesis and recycling: neurons release glutamate; astrocytes take it up and convert much of it to glutamine. Glutamine returns to neurons, where it can be converted back to glutamate. This glutamate–glutamine cycle prevents toxic extracellular accumulation.
AMPA receptors: fast ionotropic receptors responsible for much rapid excitatory transmission.
NMDA receptors: ionotropic receptors permeable to Ca²⁺ and important in learning, memory and plasticity. At resting membrane potential they are partly blocked by magnesium; depolarisation removes this block. They therefore act as coincidence detectors: both glutamate release and postsynaptic depolarisation are needed.
Kainate and metabotropic glutamate receptors: contribute to excitation and circuit modulation.

Clinical connection

Glutamate excess and seizures

Seizures represent abnormal, excessive synchronised neuronal firing. Many anti-seizure medicines reduce excitation, enhance GABA inhibition, stabilise voltage-gated sodium channels or reduce calcium-dependent transmitter release. In severe or prolonged seizures, continued glutamate-driven activity can injure neurons; this is why status epilepticus is a medical emergency.

6. GABA and glycine: the brakes of the nervous system

Gamma-aminobutyric acid (GABA) is the main inhibitory transmitter in the brain. It is synthesised from glutamate by glutamic acid decarboxylase (GAD), packaged into vesicles and released at inhibitory synapses. Its major role is to prevent excessive firing and to shape normal network rhythms.

GABA_A An ionotropic chloride-channel receptor. Activation usually hyperpolarises or stabilises the neuron and quickly reduces firing. Benzodiazepines, barbiturates, many general anaesthetics and alcohol-related sedative effects involve GABA_A signalling. Important: benzodiazepines do not replace GABA; they increase the receptor response to GABA, which gives a ceiling to their direct effect compared with barbiturates.

GABA_B A metabotropic receptor that opens potassium channels and reduces calcium entry, producing slower inhibition. Baclofen is a GABA_B agonist used mainly for spasticity.

Glycine A major inhibitory transmitter in the spinal cord and brainstem. Its receptor is an ionotropic chloride channel. Glycine also acts with glutamate at NMDA receptors. Toxin example: strychnine blocks glycine receptors, removing spinal inhibition and causing severe painful muscle spasms.

7. Acetylcholine: attention, memory, autonomic function and movement

Acetylcholine (ACh) is made from choline and acetyl-coenzyme A by choline acetyltransferase. It is broken down in the synaptic cleft by acetylcholinesterase into choline and acetate; choline is then recycled into the nerve terminal.

Receptor type	Where / action	Clinical relevance
Nicotinic receptors	Ionotropic cation channels at the neuromuscular junction, autonomic ganglia and selected CNS pathways.	Nicotine activates them; neuromuscular blockers act at skeletal-muscle nicotinic receptors. Nicotine dependence is related to reward-circuit activation.
Muscarinic receptors	Metabotropic receptors in parasympathetic organs and CNS. M₁, M₃ and M₅ are generally Gq-linked; M₂ and M₄ are generally Gi-linked.	Atropine blocks muscarinic effects; muscarinic adverse effects include salivation, lacrimation, bronchospasm, bradycardia, diarrhoea and miosis.

In the CNS, cholinergic neurons from the basal forebrain project widely to cortex and hippocampus and support attention and memory. Degeneration of these pathways occurs in Alzheimer disease. Medicines such as donepezil, rivastigmine and galantamine inhibit acetylcholinesterase, modestly increasing available ACh and improving symptoms for some patients; they do not cure the underlying neurodegeneration.

Poisoning pattern

Too much acetylcholine

Organophosphate pesticides inhibit acetylcholinesterase and can cause a life-threatening cholinergic crisis: salivation, sweating, bronchorrhoea, bronchospasm, vomiting, diarrhoea, miosis, bradycardia, fasciculations, weakness, seizures and coma. Immediate decontamination, airway support and protocol-based antidotal treatment are essential. This is an emergency, not a condition for home management.

8. Dopamine: movement, motivation, reward and psychosis

Dopamine is synthesised from tyrosine: tyrosine → L-DOPA → dopamine. It is cleared mainly by reuptake through the dopamine transporter (DAT) and metabolised by monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT).

Dopamine pathway	Normal role	Clinical correlation
Nigrostriatal (substantia nigra → striatum)	Initiation and smooth control of movement.	Loss of dopamine neurons causes Parkinson disease: bradykinesia, rigidity, resting tremor and postural instability. D₂ blockade may cause extrapyramidal adverse effects.
Mesolimbic (midbrain → limbic structures)	Reward, salience, motivation and reinforcement learning.	Excess signalling is linked to positive psychotic symptoms; many addictive drugs increase dopamine in reward pathways.
Mesocortical (midbrain → frontal cortex)	Cognition, planning, attention and affect.	Reduced or dysregulated signalling may contribute to cognitive and negative symptoms in psychotic disorders.
Tuberoinfundibular (hypothalamus → pituitary)	Dopamine suppresses prolactin release.	D₂ antagonists may cause hyperprolactinaemia: galactorrhoea, menstrual disturbance, sexual dysfunction and reduced bone density over time.

Dopamine receptors are metabotropic. D₁-like receptors (D₁, D₅) generally increase cAMP, while D₂-like receptors (D₂, D₃, D₄) generally reduce cAMP. The direction of effect depends on the circuit. Levodopa replenishes dopamine precursor in Parkinson disease; antipsychotics commonly reduce D₂-mediated signalling. Both treatment benefit and adverse effects can be predicted from the pathways above.

9. Noradrenaline and adrenaline: arousal, stress and autonomic control

Noradrenaline (norepinephrine) is made from dopamine. In the CNS, neurons in the locus coeruleus project widely and help regulate alertness, attention, vigilance, sleep–wake state, anxiety and the stress response. Adrenaline is produced from noradrenaline in a smaller group of CNS neurons and in the adrenal medulla.

α₁ receptors: generally Gq-linked; often increase smooth-muscle contraction and neuronal excitation.
α₂ receptors: generally Gi-linked presynaptic inhibitory receptors; their activation can reduce noradrenaline release. Clonidine is an α₂ agonist.
β₁, β₂ and β₃ receptors: generally Gs-linked; increase cAMP. They are central to cardiovascular and respiratory pharmacology.

SNRIs and many tricyclic antidepressants increase synaptic noradrenaline by blocking reuptake. Excess sympathetic activation may produce tremor, anxiety, insomnia, tachycardia and hypertension; too little central noradrenergic drive may contribute to reduced alertness and concentration.

10. Serotonin (5-hydroxytryptamine): mood, sleep, appetite and pain modulation

Serotonin (5-HT) is synthesised from tryptophan. Most body serotonin is found in the gastrointestinal tract and platelets, but CNS serotonergic neurons are concentrated in the raphe nuclei of the brainstem and project widely to cortex, limbic structures and spinal cord.

Functions: mood, anxiety, sleep, appetite, sexual function, nausea/vomiting, thermoregulation and descending pain modulation.
Receptors: most 5-HT receptors are metabotropic; 5-HT₃ is the important ionotropic cation channel receptor.
Drug examples: SSRIs block serotonin reuptake; SNRIs affect both serotonin and noradrenaline; ondansetron blocks 5-HT₃ receptors to reduce vomiting; triptans act mainly at selected 5-HT₁ receptors in migraine.

Safety alert

Serotonin toxicity

Combining multiple serotonergic medicines or substances can rarely produce serotonin syndrome: agitation, confusion, sweating, fever, diarrhoea, tremor, hyperreflexia and clonus. Severe cases can be life-threatening. The pattern differs from simple adverse effects and requires urgent clinical assessment, stopping the triggering agents and supportive treatment.

11. Histamine, adenosine, peptides, nitric oxide and endocannabinoids

Histamine Histaminergic neurons from the tuberomammillary nucleus promote wakefulness, attention and appetite regulation. First-generation H₁ antihistamines cross the blood–brain barrier and often cause sedation; this is why they may impair driving and learning.

Adenosine Adenosine accumulates during prolonged wakefulness and generally reduces neuronal activity, helping create sleep pressure. Caffeine blocks adenosine receptors, promoting alertness. Excess caffeine may produce tremor, palpitations, anxiety and insomnia.

Endogenous opioids Endorphins, enkephalins and dynorphins act at opioid receptors. They reduce pain transmission, alter stress and reward circuits, and can slow gastrointestinal motility and respiration when opioid receptors are strongly activated by medicines or illicit opioids.

Substance P A neuropeptide involved in nociception, neurogenic inflammation and vomiting pathways. NK₁ receptor antagonists may be used in selected antiemetic regimens.

Nitric oxide (NO) A gas made on demand rather than stored in vesicles. It diffuses easily across membranes and can act as a retrograde messenger. It is important in vascular tone, learning and some forms of neuronal signalling.

Endocannabinoids Anandamide and 2-AG are lipid-derived messengers made on demand by postsynaptic cells. They can travel backwards to presynaptic CB₁ receptors and reduce transmitter release. Cannabis-related products affect this system and may alter memory, coordination, mood and psychosis risk.

12. How CNS medicines alter neurotransmission

Target step	What a drug can do	Examples and clinical use
Synthesis	Provide a precursor or inhibit a synthesising enzyme.	Levodopa increases dopamine precursor availability in Parkinson disease; metyrosine reduces catecholamine synthesis in selected settings.
Vesicular storage	Alter transporter-mediated uptake into vesicles.	Reserpine depletes monoamines from vesicles; its use is now limited because of adverse effects including depression.
Release	Increase or reduce calcium-dependent exocytosis.	Botulinum toxin blocks acetylcholine release; gabapentinoids reduce release of several excitatory transmitters by acting on calcium-channel subunits.
Reuptake	Block transporters and prolong transmitter action.	SSRIs, SNRIs, tricyclic antidepressants, cocaine and methylphenidate affect monoamine transporters in different ways.
Metabolism	Inhibit degradation enzymes.	Donepezil inhibits acetylcholinesterase; MAO-B inhibitors and COMT inhibitors prolong dopamine-related signalling.
Receptors	Activate, partially activate, block or modulate a receptor.	Opioid agonists relieve pain; naloxone reverses opioid effect; antipsychotics reduce D₂ signalling; benzodiazepines enhance GABA_A response.
Ion channels	Change action-potential generation or transmitter release indirectly.	Local anaesthetics block voltage-gated sodium channels; many anti-seizure medicines limit repetitive firing or calcium entry.

13. Drug abuse and the reward pathway: an introduction

Psychoactive substances alter perception, mood, consciousness, cognition or behaviour. They do not all act at the same receptor, but many converge on the mesolimbic dopamine reward pathway, particularly projections from the ventral tegmental area to the nucleus accumbens. Repeated exposure can change synaptic strength, learning, stress systems and cue-response behaviour.

Tolerance: a larger dose is needed to obtain the previous effect. It may reflect receptor desensitisation, receptor down-regulation, altered intracellular signalling or increased drug metabolism.
Dependence: the body adapts to the presence of a substance; abrupt withdrawal produces symptoms that are often opposite to the acute effect.
Addiction / substance use disorder: compulsive use despite harm, impaired control, craving and continued use even when health, relationships, education or work are damaged.
Withdrawal: may be dangerous. Alcohol, benzodiazepine and severe opioid withdrawal require proper assessment and evidence-based treatment; do not advise unsupervised abrupt cessation for a dependent person.

Understanding receptor pharmacology helps clinicians respond without stigma. Substance-use disorders are treatable health conditions that require assessment of intoxication, overdose, withdrawal risk, psychiatric comorbidity, social circumstances and safeguarding needs. The next lesson expands the definition, risk factors, major drugs of abuse and management.

14. Clinical integration: predict the effect from the transmitter

Clinical situation	Core transmitter problem	Pharmacological principle
Parkinson disease	Reduced nigrostriatal dopamine.	Increase dopamine precursor/effect or reduce acetylcholine-dominant imbalance; monitor dyskinesia, hallucinations and orthostatic symptoms.
Psychosis	Dysregulated dopamine signalling, especially mesolimbic pathways.	Reduce D₂-mediated effects while watching for extrapyramidal symptoms, metabolic effects and prolactin changes.
Epilepsy	Excess excitation and/or inadequate inhibition.	Reduce glutamatergic activity, stabilise ion channels, reduce release or enhance GABA-mediated inhibition.
Anxiety / acute seizures	Need for increased inhibitory network control.	Benzodiazepines enhance GABA_A-mediated inhibition; they can cause sedation, falls, tolerance and dependence.
Depression / anxiety disorders	Complex circuit dysfunction involving serotonin, noradrenaline and sometimes dopamine.	SSRIs/SNRIs modify monoamine signalling; therapeutic benefit is delayed because adaptive circuit changes matter, not only immediate transmitter rise.
Alzheimer disease	Loss of cholinergic neurons among other pathological changes.	Acetylcholinesterase inhibition may provide symptomatic benefit; it does not reverse the disease process.
Opioid overdose	Excess opioid receptor activation suppresses brainstem respiratory drive.	Urgent airway and ventilation support; naloxone competitively blocks opioid receptors but monitoring is needed because its duration may be shorter than the opioid.

15. High-yield exam and OSCE points

Glutamate is the major excitatory transmitter in the CNS; GABA is the major inhibitory transmitter in the brain.
Glycine is an important inhibitory transmitter in the spinal cord and brainstem.
GABA_A is a ligand-gated chloride channel; GABA_B is a G-protein-coupled receptor.
The only major serotonin receptor that is ionotropic is 5-HT₃.
Acetylcholine is broken down by acetylcholinesterase; monoamines are cleared largely by reuptake and metabolised by MAO and, for catecholamines, COMT.
Dopamine deficiency in the nigrostriatal pathway produces Parkinsonian motor features; D₂ blockade may produce extrapyramidal adverse effects.
Do not say a transmitter is always excitatory or always inhibitory. The response depends mainly on receptor subtype and neural circuit.
All new confusion, seizures, severe rigidity, hyperthermia, respiratory depression or suspected poisoning requires urgent clinical assessment and supportive management alongside the specific antidote or medicine.

Knowledge Check Summary

Vital facts to remember

A neurotransmitter is released from a presynaptic neuron and acts on receptors on another neuron, muscle or gland cell.
The core cycle is synthesis → storage → calcium-dependent release → receptor action → termination → recycling.
Ionotropic receptors act rapidly by opening ion channels; metabotropic receptors act more slowly through G proteins and second messengers.
Glutamate drives most fast CNS excitation, whereas GABA provides most CNS inhibition.
Neurotransmitter pathways explain both the therapeutic effect and adverse effects of CNS medicines.
Drug treatment must be combined with assessment, supportive care, safety monitoring and current local treatment guidance.

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Review of Neurotransmitters: The Foundation of CNS Pharmacology