Glutamate

Primary excitatory neurotransmitter in the vertebrate Central Nervous System. If GABA is “brakes”, glutamate is “accelerator”, virtually every fast excitatory synapse runs on it. Synthesized primarily from Glutamine via Glutaminase in the presynaptic terminal, and secondarily from the TCA cycle intermediate $\alpha$-ketoglutarate via transamination.

Receptors

Two primary classes acted through; ionotropic(fast/ligand) and metabotropic(slow/GPCR)

Ionotropic

AMPA receptor: mediates bulk of fast EPSPs. Primarily $N{a}^{+}$/$K^{+}$ permeable; GluR2 subunit determines $C{a}^{2+}$ impermeability. Rapid kinetics, millisecond timescale. main vehicle for information throughput.

NMDA receptor: coincidence detector. Requires two conditions simultaneously: ligand binding (glutamate + glycine co-agonist) and membrane depolarization to expel $M{g}^{2+}$ . Once open, it’s highly $C{a}^{2+}$ permeable, making it the entry point for plasticity signals. This is the biophysical substrate of Hebbian learning; synapse strengthens when pre- and postsynaptic activity coincide. fire together wire together!

Kainate receptor; not as well studied. mostly presynaptic modulation / interneuron circuits. Contributes to high frequency bursting, seizure susceptibility.

Metabotropic

Three functional groups based on coupling and location:

Group	Receptors	Location	Effect
I	mGluR1, mGluR5	postsynaptic	excitatory (Gq/IP3/$C{a}^{2+}$)
II	mGluR2, mGluR3	presynaptic autoreceptor	inhibitory (Gi, reduces release)
III	mGluR4, mGluR6, mGluR7, mGluR8	presynaptic	inhibitory (Gi)

Group II autoreceptors are the endogenous feedback mechanism: excess synaptic glutamate activates mGluR2/3, which suppresses further vesicle release. Fasoracetam exploits this to reduce glutamate tone systemically.

Synthesis and Recycling

doesn’t freely diffuse in and out, majority is recycled through glutamate-glutamine cycle.

astrocytes surrounding synapse take up released Glu via EAATs (primarily EAAT2/GLT-1, which accounts for ~90% of CNS glutamate clearance), convert it to glutamine via glutamine synthetase, shuttle it back to the presynaptic neuron where glutaminase reconverts it.

keeps extracellular glutamate at nanomolar levels; necessary, because tonic receptor activation at higher concentrations is excitotoxic.

Role in Plasticity

LTP and LTD are cellular substrates of learning / memory, both gated by NMDA receptor activation.

sequence: high-frequency stimulation → AMPA Receptor mediated depolarization → $M{g}^{2+}$ unblock → $C{a}^{2+}$ influx through NMDA → CaMKII activation → AMPA receptor phosphorylation and trafficking → strengthened synapse.

LTD is reverse: low-frequency, mild Ca²⁺ elevation activates phosphatases instead.

TAK-653 and ACD-856 act downstream of this by potentiating AMPA receptor kinetics, amplifying the depolarization step and the downstream BDNF/mTOR cascade.

E/I Balance

Glutamate and GABA are intrinsically coupled: pyramidal cells (glutamatergic) drive GABAergic interneurons, which in turn inhibit pyramidal cells. ratio of excitatory to inhibitory tone determines whether circuit oscillates, fires tonically, or silent. Disrupted E/I balance partially behind schizophrenia (NMDA hypofunction → disinhibition), epilepsy (excess excitation), implicated in autism spectrum disorder.

Excitotoxicity

Pathological glutamate release; from ischemia, trauma, or EAAT failure, causes sustained NMDA activation, massive $C{a}^{2+}$ influx, and mitochondrial dysfunction.

Ca²⁺ overload → cytochrome c release → caspase activation → apoptosis or necrosis.

core mechanism in stroke, TBI, and neurodegenerative disease progression.