2 Neuroscience2.5 Neurochemistry and neurophysiology

2.5 Neurotransmitters and receptors

Creator: Resident Doctor Committee
Published: 2026-10-14T21:05:16.259Z

1 min read1 figure

Written by Resident Doctor Committee · Published 14 October 2026

Chemical classes of transmitter

Chemical messengers are classed by structure into small-molecule neurotransmitters and peptide messengers, which include peptide or protein hormones as well as neuropeptides. A messenger's role in the brain does not decide its class.

Class	Members	Structural group
Amino acids	Glutamate, aspartate, GABA, glycine	Small molecule
Monoamines (biogenic amines)	Dopamine, noradrenaline, adrenaline, serotonin, histamine	Small molecule
Acetylcholine	A choline ester broken down by acetylcholinesterase	Small molecule
Neuropeptides	Oxytocin, vasopressin, cholecystokinin, orexins	Peptide
Gases	Nitric oxide, carbon monoxide	Made on demand

Amino acid transmitters are released as the amino acid itself.
GABA (gamma-aminobutyric acid) is made by decarboxylation but keeps one carboxyl group beside its amino group, so it is still an amino acid.
Histamine is a monoamine, not a peptide. Decarboxylation of histidine leaves an imidazole ring joined to an ethylamine chain.

Neuropeptides

Neuropeptides are made in the cell body and stored in dense-core vesicles, whereas single amino acid transmitters sit in small clear vesicles.
Oxytocin is a chain of nine amino acids joined by peptide bonds.
Orexin A is 33 amino acids cut from prepro-orexin. Orexin neurones sustain wakefulness through the locus coeruleus, raphe nuclei and tuberomammillary nucleus.

Gaseous transmitters

Gases diffuse across membranes and act at intracellular targets, and nitric oxide can signal retrogradely.
Carbon monoxide is released when haem oxygenase breaks down haem. It shares nitric oxide's guanylyl cyclase mechanism but has a different source.

Receptors

Feature	Ionotropic	Metabotropic
Structure	Ligand-gated ion channel	Linked to intracellular signalling, usually G protein-coupled
Onset	Usually rapid	Often slower

Two-panel schematic. On the left, glutamate binds an AMPA receptor channel in the membrane and sodium passes through its pore. On the right, dopamine binds a seven-helix D1 receptor coupled to a Gs protein, which activates adenylyl cyclase, leading to cAMP, protein kinase A and protein phosphorylation. — Compare the two panels: at the ligand-gated ion channel (left), glutamate opens the pore and sodium enters within about a millisecond, while at the G protein-coupled receptor (right), dopamine acts through Gs, adenylyl cyclase and cAMP, with an onset of hundreds of milliseconds to seconds. Drawn for MRCPsychiatrist

Gs proteins raise cyclic adenosine monophosphate, Gi/o proteins lower it, and Gq proteins activate phospholipase C.
Autoreceptors respond to the neurone's own transmitter, and heteroreceptors respond to another transmitter.

Revision material for the MRCPsych examinations. Not clinical guidance, and not for use in patient care.