Glutamate Synthesis Pathways

• Glutamate (Glu) is the primary excitatory neurotransmitter in the central nervous system (CNS).
• It is present in the cytosol, mitochondria, cells, and vesicles.

Glutamate Synthesis Pathways

• Three main pathways exist for synthesizing glutamate.

Glutamate Dehydrogenase Pathway (GDH)

• α-ketoglutarate and NH₄⁺ are the substrates.
• Glutamate dehydrogenase (GDH) is the enzyme.
• NADPH or NADH acts as a cofactor.
• α-ketoglutarate is aminated by NH₄⁺ with GDH.
• The reaction is reversible and contributes to nitrogen balance and serves as a major Glu source when ammonia is high.

Aminotransferase Pathway (Transamination)

• α-ketoglutarate and another amino acid (aspartate or alanine) act as substrates.
• Aminotransferase (Aspartate Aminotransferase or Alanine Aminotransferase) is the enzyme.
• The amino group is transferred from an amino acid (such as aspartate) to α-ketoglutarate.
• This generates glutamate and a keto acid (e.g., oxaloacetate).
• It balances amino acid and neurotransmitter levels in neurons.

Glutamine-Glutamate Cycle (Astrocytes → Neurons)

• Glutamine acts as the substrate.
• Glutamine Synthetase (in astrocytes) converts glutamate to glutamine in an ATP-dependent reaction.
• Glutaminase (in neurons) converts glutamine back to glutamate.
• Astrocytes uptake excess glutamate and convert it into glutamine and neurons then uptake glutamine and convert it back to glutamate for neurotransmission.

Vesicular Packaging of Glutamate

• V-ATPase, located on the vesicle membrane, pumps protons into the vesicle.
• VGluT1, 2, and 3 move glutamine into the vesicle and pump H+ out to balance acidity.
• VGluT3 is present in cholinergic, GABAergic, and monoaminergic vesicles.
• Glu packaging into vesicles relies on VGluT3, which both antiports with protons and is ATP-dependent.
• VGluT3 contains 10 transmembrane domains.

Glutamate Receptors

AMPA Receptors

• Typically composed of 4 subunits, commonly 2x GluA1 and 2x GluA2, but GluA3 and GluA4 also exist.
• Conduct Na+ and K+ ions, and Ca2+ if GluR2 is absent.
• Trigger fast excitatory postsynaptic potentials (EPSPs) lasting 1-10 ms.
• Exhibit rapid desensitization.

Kainate Receptors

• Composed of 5 subunits.
• Frequently located presynaptically where they modulate glutamate release.
• Conduct Na+ and K+ ions and trigger fast EPSPs.

NMDA Receptors

• Consist of 4 subunits, usually 2x GluN1 and 2x GluN2.
• Require a co-agonist, typically glycine, which binds to the NR1 subunit and glutamate binds to the NR2 subunit.
• Primarily conduct Ca2+ and also conduct Na+ (entering) and K+ (exiting).
• Receptor is voltage-dependent and blocked by Mg2+ at resting membrane potential which is removed upon depolarization.
• Function as coincidence detectors, requiring simultaneous presynaptic glutamate release and postsynaptic depolarization.
• Transmit slow synaptic potentials (100 ms).
• Modulated or blocked by both endogenous and exogenous ligands.

Role of NMDA Receptors

Synaptogenesis

• Involved in memory formation and synaptic connections.
• Blocking NMDA receptors disrupts normal synaptic development.

Excitotoxicity

• Excessive glutamate release overactivates NMDA receptors, leading to excessive Ca2+ and Na+ influx.
• Na+ influx causes edema (neuron bursting and necrosis).
• Ca2+ influx activates proteases, leading to proteolytic damage and cell death.

Therapeutic Implications

• NMDA receptor blockers may help treat stroke and Parkinson’s disease.
• NMDA receptor potentiators may be beneficial for treating Alzheimer’s disease.

Metabotropic Glutamate Receptors (mGluRs)

• There are 8 mGluR subtypes, divided into three classes.

Group I (excitatory)

• Includes mGluR1 and mGluR5, primarily found postsynaptically.
• Activate phospholipase C (PLC), producing IP3 and increasing intracellular Ca2+ release.

Group II (inhibitory)

• Includes mGluR2 and mGluR3, primarily found presynaptically.
• Inhibit adenylyl cyclase (AC), reducing cAMP production.

Group III (inhibitory)

• Includes mGluR4, mGluR6, mGluR7, and mGluR8, also primarily presynaptic.
• These receptors work through similar inhibitory mechanisms as Group II.
• Play a role in strengthening glutamatergic networks by modulating neuronal excitability.
• Receptor activation triggers transcriptional and translational changes that enhance network function.

Removal of Glutamate from the Synapse

• Removal is necessary to prevent excitotoxicity.
• Glu is taken up back into the presynaptic terminal into astrocytes via high affinity EAATs (co-transporter that involves uptake of 2 Na+ ions with every 1 Glu).
• EAAT activity is an important feature of astrocytes regulation of Glu and Na+ in the synapse.
• Once in astrocytes, Glu can be converted to alpha-ketoglutarate (alpha KG) by Glu dehydrogenase (GluD) or into glutamine (Gln) by glutamine synthetase (ATP dependant).
• Alpha KG then enters the TCA cycle or Gln is transported to GABAergic neurons, where it is converted to glutamate, then GABA + CO2 by Glu decarboxylase.