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Short answer
An inhibitory neurotransmitter is a chemical messenger that makes the receiving neuron less likely to fire. In the mammalian central nervous system the two main ones are GABA (γ-aminobutyric acid) and glycine. GABA is now regarded as the most important inhibitory transmitter in the mammalian brain, and glycine is a vital inhibitory transmitter in the spinal cord and higher (supraspinal) regions.
Definition and scope
“Inhibitory” describes what a transmitter does at a particular receptor, not a fixed property of the molecule. A transmitter inhibits when its receptor opens ion channels that move the postsynaptic membrane away from the firing threshold. The two classic mechanisms are an increase in chloride (Cl⁻) flux and an increase in potassium (K⁺) conductance, both of which produce hyperpolarization.
The weight of GABA in the brain is large. Bowery and Smart (2006) write that “at least 40% of inhibitory synaptic processing in the mammalian brain uses GABA”, so it is a lower bound rather than an exact count. This guide focuses on these two transmitters. For the cells and structures involved, see our guides on the discipline of neuroscience, the axon and axon hillock and neurobiology.
Fast and slow inhibition: GABA-A, GABA-B and glycine receptors
The same transmitter can act through receptors that work on different timescales. The table summarises the receptor types described by Bowery and Smart (2006).
| Receptor | Type | Effect on the neuron |
|---|---|---|
| GABA-A | Ionotropic; gates neuronal Cl⁻ channels | Fast hyperpolarization from increased Cl⁻ flux |
| GABA-B | Acts through increased K⁺ conductance | Slow hyperpolarization; also reduces calcium (Ca²⁺) conductance, which lowers transmitter release at axon terminals |
| Glycine receptor | Pentamer of α and β subunits; ionotropic Cys-loop ligand-gated Cl⁻ channel | Inhibition in the spinal cord and supraspinal regions |
Two points follow. First, “inhibition” can occur on the postsynaptic cell (hyperpolarization) or at the axon terminal (reduced release), so the site matters when interpreting an experiment. Second, GABA-A and glycine receptors both open chloride channels, so they cannot be told apart by the ion they carry alone; the experiments below separate them by pharmacology.
How labs establish that a transmitter is inhibitory
Calling a molecule “the inhibitory transmitter” at a synapse is an experimental conclusion. Matthews and Wickelgren (1979) illustrate the logic in lamprey reticulospinal neurons, where they compared two candidates, glycine and GABA, using the following checks.
- Apply the candidate and look for the same effect as the synaptic potential. Both glycine and GABA hyperpolarized the cell bodies and reduced input resistance.
- Test the ion dependence. The inhibitory postsynaptic potential (IPSP) and the drug responses appeared to involve an increase in chloride conductance, and reversal potentials shifted with internal and external chloride.
- Test whether the two share a receptor. The receptors for GABA and glycine appeared to be different because there was no cross-desensitization.
- Use antagonists to separate them. Low doses of picrotoxin and bicuculline blocked GABA responses, while strychnine blocked glycine responses.
- Ask which antagonist abolishes the real synaptic potential. At 20 μM, strychnine abolished the IPSP, whereas picrotoxin and bicuculline did not.
From this the authors concluded that glycine was a better candidate than GABA for the inhibitory transmitter onto the Müller cells they studied. Bowery and Smart (2006) likewise describe strychnine as the most potent and selective competitive glycine-receptor antagonist, used to diagnose glycine-receptor involvement.
Checklist: antagonist logic from the two sources
| Observation | Points toward | Source |
|---|---|---|
| Response blocked by low-dose picrotoxin or bicuculline | GABA receptor involvement | Matthews and Wickelgren 1979 |
| Response blocked by strychnine | Glycine receptor involvement | Matthews and Wickelgren 1979; Bowery and Smart 2006 |
| IPSP reversal shifts with chloride concentration | A chloride-conductance mechanism | Matthews and Wickelgren 1979 |
| No cross-desensitization between the two agonists | Distinct receptors | Matthews and Wickelgren 1979 |
The lamprey result is specific to that preparation. It shows why identity has to be tested tissue by tissue rather than assumed from a textbook list.
Where it matters for labs
The lamprey work shows what a defensible identification rests on: varying the chloride concentration to test the ionic mechanism, and comparing antagonists against the real synaptic potential rather than against applied drugs alone. Because transmitter identity was tested in one specific preparation, the preparation and the ionic conditions belong in the methods of any such experiment. For the wider field, see the overview of pharmacology.
Common misunderstandings
- “GABA is the only inhibitory neurotransmitter.” Glycine is also an established inhibitory transmitter, described by Bowery and Smart as vital in the spinal cord and supraspinal regions.
- “Inhibitory” means the molecule is inherently inhibitory. The effect depends on the receptor and the ions it moves. GABA acts through fast Cl⁻-based GABA-A receptors and slow K⁺-based GABA-B receptors, and Bowery and Smart note that GABA can also depolarize primary afferent terminals, which decreases transmitter release.
- “All GABA responses are the same.” GABA-A and GABA-B differ in speed, mechanism and site of action.
- “The 40% figure is exact.” It is a lower bound (“at least”) from a 2006 review of the mammalian brain, not a measurement for every region or species.
Frequently asked questions
What is the main inhibitory neurotransmitter in the brain?
GABA. Bowery and Smart (2006) describe it as established as the most important inhibitory neurotransmitter in the mammalian central nervous system.
Is glycine an inhibitory neurotransmitter?
Yes. Glycine acts on a ligand-gated chloride channel and is described as a vital inhibitory transmitter in the spinal cord and supraspinal regions.
What is the difference between GABA-A and GABA-B receptors?
GABA-A receptors gate chloride channels and give fast hyperpolarization. GABA-B receptors increase potassium conductance and give slow hyperpolarization.
How do researchers tell GABA inhibition from glycine inhibition?
By antagonist pharmacology. In the lamprey study, low doses of picrotoxin and bicuculline blocked GABA responses and strychnine blocked glycine responses; strychnine is also used as a diagnostic indicator of glycine-receptor involvement.
Is GABA always inhibitory?
Its effect depends on the receptor and the site. Bowery and Smart note that GABA can depolarize primary afferent terminals, which reduces transmitter release, so the end result at the terminal is still inhibition of release.
Sources
- Bowery NG, Smart TG. GABA and glycine as neurotransmitters: a brief history. British Journal of Pharmacology 2006;147(Suppl 1):S109-S119. https://pmc.ncbi.nlm.nih.gov/articles/PMC1760744 (checked 2026-10-29).
- Matthews G, Wickelgren WO. Glycine, GABA and synaptic inhibition of reticulospinal neurones of lamprey. Journal of Physiology 1979;293:393-415. https://pmc.ncbi.nlm.nih.gov/articles/PMC1280720 (checked 2026-10-29).
