Neurotransmitters System I: Glutamate

What key requirements must a molecule meet to be considered a neurotransmitter?

It must be synthesised in the presynaptic neuron.

It must be stored in presynaptic vesicles.

It must be released from the axon terminal upon stimulation (usually Ca²⁺-dependent).

It must bind and activate postsynaptic receptors, producing a response.

A neurotransmitter also requires a mechanism for removal (reuptake or enzymatic degradation) and must produce reproducible physiological effects when experimentally applied.

What are the steps of neurotransmission?

1. Neurotransmitter synthesis (soma or terminal).

2. Loading into vesicles.

3. Vesicle fusion and release after AP-evoked Ca²⁺ entry.

4. Binding to receptors on the postsynaptic cell.

Receptors may be ionotropic or metabotropic, enabling fast or slow signalling respectively.

Why was glutamate slow to be recognised as a neurotransmitter?

It sits at metabolic crossroads, participating in multiple pathways (TCA cycle, amino acid metabolism), making it hard to prove its signalling-specific role.

Glutamate exists at high concentrations in all cells, so demonstrating vesicular release and synaptic specificity required advanced methods.

What is glutamate’s major role in the CNS?

It is the major excitatory neurotransmitter, responsible for the majority of fast excitatory synaptic transmission.

How is glutamate synthesised in neurons?

1. From glutamine (major pathway):

Glutamine → Glutamate

Enzyme: Glutaminase

Occurs in nerve terminals.

2. From α-ketoglutarate:

Via transamination reactions in the TCA cycle.

Glutamine is supplied by astrocytes → core of the glutamate–glutamine cycle, preventing excitotoxicity.

How is glutamate stored before release?

Loaded into synaptic vesicles by Vesicular Glutamate Transporters (VGLUTs).

VGLUTs maintain high intravesicular concentrations so that vesicles release quantal packets of glutamate.

How is extracellular glutamate removed and recycled?

Excitatory Amino Acid Transporters (EAATs) on glia and neurons remove glutamate.

In astrocytes:

Glutamate → Glutamine via glutamine synthetase.

Glutamine returned to neurons for reuse.

EAAT2 (GLT-1) is the main glial transporter; dysfunction contributes to ALS and epilepsy.

What are the two major families of glutamate receptors?

1. Ionotropic (ligand-gated ion channels):

AMPA, NMDA, Kainate

2. Metabotropic (G-protein coupled):

mGluR Groups I, II, III

Ionotropic receptors mediate fast EPSCs; metabotropic receptors modulate excitability and plasticity.

What are AMPA receptors made of?

Heterotetramers composed of GluA1–GluA4 subunits.

Typically: 2 GluA2 + 2 of GluA1/3/4 (“dimer of dimers”).

How do AMPA receptors signal?

Activated by glutamate binding.

Require two occupied binding sites for channel opening.

Conduct mainly Na⁺ influx, causing fast depolarisation (fast EPSCs).

AMPA receptors lacking GluA2 allow Ca²⁺ influx, increasing plasticity but also vulnerability.

Why is the GluA2 subunit important?

Presence of GluA2 prevents Ca²⁺ entry, protecting neurons from excitotoxicity.

Ca²⁺-permeable AMPARs appear during development, injury, and some forms of LTP.

What conditions are needed for NMDA receptor activation?

1. Ligand requirement: glutamate + co-agonist (glycine or D-serine).

2. Voltage requirement: depolarisation removes Mg²⁺ block.

This dual requirement makes NMDA receptors coincidence detectors, essential for LTP.

What are the NMDA receptor subunits?

GluN1, GluN2 (A–D), GluN3 (A–B).

Usually: 2 GluN1 + 2 GluN2.

GluN3 subunits reduce receptor activity and alter Ca²⁺ permeability.

How do NMDA receptor EPSCs compare to AMPA?

Slower onset, longer lasting.

Conduct significant Ca²⁺, triggering intracellular signalling.

What characterises kainate receptors?

Ionotropic glutamate receptors with five subunits (GluK1–5).

GluK1–3 form homomers or heteromers; GluK4–5 require co-assembly.

Less widely distributed than AMPA/NMDA.

They have mixed pre- and postsynaptic roles, modulating neurotransmitter release.

What are the three groups of mGluRs and their main features?

Group I (mGlu1, mGlu5): Postsynaptic, Gq-coupled → ↑IP₃, ↑Ca²⁺.

Group II (mGlu2, mGlu3): Presynaptic, Gi-coupled → ↓cAMP, inhibit glutamate release.

Group III (mGlu4, 6, 7, 8): Presynaptic, Gi-coupled → similar inhibition of transmitter release.

Group I = excitatory modulators; Groups II/III = inhibitory autoreceptors controlling synaptic strength.

What is unique about mGluR dimerisation?

Form obligate dimers:

Homomers (e.g., mGlu1-mGlu1)

Heteromers (e.g., mGlu2-5-HT2A)

Dimer composition changes receptor pharmacology and signalling.

What is the difference between an EPSC and an EPSP?

EPSC: actual postsynaptic ionic current.

EPSP: change in membrane voltage caused by the EPSC.

EPSCs via NMDA and Kainate receptors are slower and longer-lived than AMPA-mediated EPSCs.

What is excitotoxicity?

Pathological process where excessive excitatory stimulation (especially Ca²⁺ influx via NMDA) leads to neuronal injury and death.

Seen in stroke, traumatic brain injury, and Alzheimer’s disease where glutamate clearance is impaired.

Define long-term potentiation (LTP).

Persistent strengthening of synaptic transmission following repeated patterns of activity.

LTP is a cellular correlate of learning and memory.

What are the molecular steps underlying early-phase LTP?

1. Glutamate activates AMPA receptors → Na⁺ influx → depolarisation.

2. Depolarisation removes Mg²⁺ block from NMDA receptors.

3. NMDA receptors open → Ca²⁺ influx.

4. Ca²⁺ activates CaMKII and PKC.

5. These kinases promote insertion of new AMPA receptors into postsynaptic membrane.

6. Result: increased AMPA receptor number and conductance → stronger synaptic response.

CaMKII autophosphorylation provides a molecular “memory,” stabilising the potentiated state.

Why is NMDA Ca²⁺ entry crucial for LTP induction?

Ca²⁺ acts as a second messenger activating kinases (CaMKII, PKC).

These kinases drive structural and functional synaptic changes.

Without Ca²⁺, synapses cannot enter a potentiated state even if glutamate is present.

Why are EAATs essential for preventing glutamate toxicity?

They rapidly clear glutamate, preventing prolonged receptor activation and Ca²⁺ overload.

Astrocytes convert glutamate → glutamine, safely removing it from synaptic space.

Impaired EAAT function is implicated in ALS.

What factors control whether glutamate produces physiological signalling vs excitotoxicity?

Duration and concentration of glutamate exposure.

Presence of Ca²⁺-permeable receptors (e.g., AMPARs lacking GluA2).

Efficiency of EAAT-mediated clearance.

Mitochondrial ability to buffer Ca²⁺ influx.