Introduction to Behavioral Neuroscience · Basic Neurochemistry

Neurotransmitters Made from Amino Acids

7 min read

Neurotransmitter/receptor mechanisms and drug actions are described as commonly taught reference concepts; verify doses, indications, and clinical guidance against current texts and prescribers.

On this page 9 sections

In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

Amino acids are the raw material for most of the brain's "classical" small-molecule neurotransmitters, and they work in two distinct ways. First, a few amino acids are themselves fast neurotransmitters: , , and carry most fast excitatory and inhibitory signaling in the CNS. Second, other amino acids serve as precursors converted into transmitters — dopamine, norepinephrine, and epinephrine (catecholamines, from tyrosine), serotonin (from tryptophan), and histamine (from histidine).

Because they are small, stored in vesicles, and recycled at synapses, they follow the life cycle from Topic 1; what distinguishes them is their chemistry, their locations, and what happens when their levels go wrong.

Why this matters

Glutamate and GABA mediate most fast synaptic transmission, and their balance determines whether the brain is excitable or calm: too much excitation relative to inhibition is the physiological basis of seizures. The monoamines are the targets of the most widely prescribed psychiatric drugs — antidepressants (serotonin, norepinephrine), antipsychotics (dopamine), stimulants (dopamine, norepinephrine), and sleep/wake medications (histamine, serotonin). Dopamine loss in Parkinson's disease and dopamine overactivity in schizophrenia are classic textbook models of a disturbed messenger system.

The college version

Core Concepts

Glutamate: the brain's main excitatory transmitter

Glutamate is the most abundant excitatory neurotransmitter in the central nervous system. It is made from the amino acid glutamine by the enzyme glutaminase, with glial cells participating in a glutamine–glutamate recycling cycle. It acts through:

Ionotropic receptors: AMPA and kainate receptors produce the fast excitatory postsynaptic potential; NMDA receptors are slower and voltage-dependent — they open only when glutamate binds and the membrane is depolarized (expelling a magnesium ion blocking the pore). NMDA lets calcium in and is central to plasticity and learning.
Metabotropic receptors (mGluR1–8): modulate the cell more slowly through G proteins.

Glutamate is essential, but too much is toxic: — excessive glutamate over-activating NMDA receptors and flooding the cell with calcium — damages neurons after stroke, traumatic brain injury, and in neurodegeneration.

GABA: the brain's main inhibitory transmitter

GABA (gamma-aminobutyric acid) is the principal fast inhibitory transmitter in the adult brain. It is synthesized from glutamate itself by the enzyme , which needs vitamin B6 as a cofactor. GABA acts through:

GABA-A receptors: ionotropic chloride channels. When GABA opens them, chloride enters and the cell becomes harder to excite. This is the receptor targeted by benzodiazepines, barbiturates, alcohol, anesthetics, and antiseizure drugs — all of which enhance GABA-A function rather than mimicking GABA.
GABA-B receptors: metabotropic, slower inhibitory signaling.

The everyday picture is a balance: glutamate is the gas pedal, GABA is the brake. Seizures occur when excitation outruns inhibition; many antiseizure and anti-anxiety drugs strengthen the GABA brake.

Glycine: spinal cord and brainstem inhibition

Glycine is a second inhibitory amino-acid transmitter, most important in the spinal cord and brainstem, where it acts at ionotropic chloride channels; it is also a required co-agonist at NMDA receptors. The plant toxin strychnine blocks glycine receptors, removing inhibition from motor circuits and causing violent muscle spasms — a classic demonstration of inhibition failing.

Monoamines: one amino acid, one messenger family

Catecholamines (dopamine, norepinephrine, epinephrine) from tyrosine: converts tyrosine to L-DOPA (the rate-limiting step), which becomes dopamine; dopamine can then become norepinephrine and epinephrine. Dopamine pathways are famous: the nigrostriatal pathway controls movement (it degenerates in Parkinson's disease) and the mesolimbic pathway mediates reward and motivation. Norepinephrine from the locus coeruleus drives arousal, attention, and fight-or-flight.
from tryptophan: tryptophan hydroxylase converts tryptophan to 5-HTP, which becomes serotonin. Raphe-nucleus neurons project widely and influence mood, appetite, sleep, and pain. Selective serotonin reuptake inhibitors (SSRIs) block the serotonin transporter (SERT), leaving more serotonin in the cleft.
Histamine from histidine: histidine decarboxylase makes histamine in the tuberomammillary nucleus, promoting wakefulness. Older "first-generation" antihistamines cross into the brain, block histamine receptors, and cause drowsiness.

Packaging, recycling, and breakdown

Monoamines are loaded into vesicles by the and broken down by and COMT. The historical antipsychotic reserpine blocks VMAT and depletes monoamine stores — a drug acting at the packaging stage. Every step of a monoamine's career is a place where drugs act.

Common Confusions

Do Not Confuse	With	Difference
"Glutamate is only a food additive (MSG)"	Glutamate as the brain's main excitatory transmitter	Same molecule, different pools: dietary glutamate is largely kept out of the brain (commonly taught)
"GABA and glutamate are unrelated molecules"	GABA is made from glutamate	GAD converts glutamate into GABA
"Dopamine is made from tryptophan"	Dopamine comes from tyrosine	Tyrosine → dopamine → norepinephrine → epinephrine; tryptophan → serotonin only
"NMDA receptors open whenever glutamate binds"	NMDA also needs depolarization	The Mg²⁺ block must be expelled, making NMDA a coincidence detector
"Glycine is only an inhibitory transmitter"	Glycine also co-activates NMDA receptors	Glycine inhibits at its own channels but is required at NMDA receptors
"All monoamines are made in the same brain region"	Each has its own nuclei	Dopamine: substantia nigra/VTA; serotonin: raphe; norepinephrine: locus coeruleus; histamine: tuberomammillary nucleus

Eli explains

The same idea, in plain words

Explain it like I’m 10

Amino acids are like LEGO bricks: the brain snaps a few of them together to build special messengers. Some amino acids — glutamate, GABA, glycine — are messengers all by themselves: glutamate shouts "go!" and GABA whispers "stop!" Others, like tyrosine and tryptophan, are starter bricks for messengers such as dopamine and serotonin, which help you move, feel, sleep, and pay attention.

Worked example

Imagine a person with acute anxiety is prescribed a benzodiazepine (educational example — real prescribing is a clinician's decision). The drug does not add GABA; it binds a site on the GABA-A receptor so that receptor responds more strongly when the brain's own GABA arrives. More inhibition flows through the same chloride channels, the gas-pedal/brake balance shifts toward calm, and anxiety-related arousal is dampened. Contrast with an SSRI: it does not touch GABA — it blocks the serotonin transporter so serotonin lingers in the cleft longer, gradually boosting serotonergic signaling. One life cycle, two intervention points (receptor enhancement vs. reuptake blockade), two messenger systems. If glutamate signaling became pathologically strong instead, the result could be a seizure — why many antiseizure drugs act on GABA or glutamate systems.

Key takeaways

Glutamate = main excitatory; GABA = main inhibitory transmitter in the CNS; glycine inhibits mainly in the spinal cord/brainstem.
GABA is synthesized from glutamate by GAD — the two are chemical relatives.
NMDA receptors need glutamate plus depolarization (to expel the Mg²⁺ block) plus glycine — they are coincidence detectors central to plasticity.
Excessive glutamate → excitotoxicity → neuronal damage (stroke, neurodegeneration).
Benzodiazepines, barbiturates, alcohol, and many anesthetics enhance GABA-A receptor function (the chloride channel).
Dopamine and norepinephrine come from tyrosine, serotonin from tryptophan, histamine from histidine; the hydroxylase enzymes are rate-limiting.
Parkinson's disease = dopamine neuron loss (substantia nigra); many antipsychotics block D2 receptors; SSRIs block serotonin reuptake.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

Which two amino-acid transmitters mediate most fast excitation and inhibition in the CNS, and how are they chemically related?
Show answer
Glutamate (excitation) and GABA (inhibition). They are related because GABA is synthesized from glutamate by glutamic acid decarboxylase (GAD).
Why does the require depolarization before it will pass calcium?
Show answer
A magnesium ion normally blocks the channel pore; a depolarized membrane repels the Mg²⁺, so the receptor opens only when glutamate is present and the cell is already active — a coincidence detector for plasticity.
What is excitotoxicity, and when is it most relevant?
Show answer
Excessive glutamate over-activates receptors (especially NMDA), flooding the cell with calcium and damaging it; relevant after stroke, traumatic brain injury, and in neurodegeneration.
Trace the synthesis pathway from tyrosine to epinephrine, naming the rate-limiting enzyme.
Show answer
Tyrosine → (tyrosine hydroxylase, rate-limiting) → L-DOPA → dopamine → norepinephrine → epinephrine.
Which precursor gives rise to serotonin, and what class of antidepressants acts on its reuptake?
Show answer
Tryptophan (→ 5-HTP → serotonin); selective serotonin reuptake inhibitors (SSRIs) block the serotonin transporter (SERT).
Name two different life-cycle stages at which drugs act on the GABA or monoamine systems, with an example of each.
Show answer
Examples: receptor enhancement (benzodiazepines boosting GABA-A), reuptake blockade (SSRIs on SERT), degradation blockade (MAO inhibitors), or packaging blockade (reserpine on VMAT).

Keep learning

Ready to build on this? Continue to the next lesson.

Study toolsKey vocabulary

Key vocabulary

glutamate: The amino acid that is the brain's main fast excitatory transmitter
GABA: The amino acid derived from glutamate that is the main fast inhibitory transmitter
glycine: An amino-acid transmitter that inhibits mainly in the spinal cord/brainstem
monoamine: A transmitter with one amino group, made from a single amino-acid precursor
catecholamine: Monoamine family made from tyrosine (dopamine, norepinephrine, epinephrine)
serotonin (5-HT): Monoamine made from tryptophan; affects mood, appetite, sleep, pain
NMDA receptor: Glutamate receptor needing glutamate + depolarization + glycine; lets calcium in
excitotoxicity: Neuron damage from excessive glutamate stimulation
glutamic acid decarboxylase (GAD): The enzyme that converts glutamate into GABA
tyrosine hydroxylase: Rate-limiting enzyme converting tyrosine toward dopamine
vesicular monoamine transporter (VMAT): Pumps monoamines into vesicles for storage
monoamine oxidase (MAO): Enzyme that degrades monoamines

Sources & references

openstax.org — Introduction Behavioral Neuroscience

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.