Spirulina and Glutamate–GABA Neurotransmission.

Glutamate Receptors: NMDA, AMPA, and mGluR

Ionotropic glutamate receptors are tetrameric ligand-gated ion channels. NMDA receptors (NMDARs) require dual agonist binding (glutamate to GluN2 subunits; glycine/D-serine to GluN1 Ser688) and voltage-dependent Mg²&sup+; relief of channel block for activation; Mg²&sup+; Kd at −70 mV ~1–3 μM (physiological). GluN2B (GRIN2B) subunit confers slow deactivation kinetics and contains the Tyr1472 site phosphorylated by Fyn kinase, which reduces clathrin-mediated endocytosis and enhances surface expression; NF-κB drives GRIN2B transcription (two κB sites in promoter). GluN2A (GRIN2A) confers faster kinetics and synaptic LTP. AMPA receptors (AMPARs) mediate fast excitatory transmission; GluA2 (GRIA2) subunit RNA-editing at the Q/R site (Gln586Arg by ADAR2) renders AMPARs Ca²&sup+;-impermeable in mature neurons; loss of ADAR2/GluA2 editing → Ca²&sup+;-permeable AMPARs → excitotoxicity (relevant in ALS). Metabotropic mGluR1/5 (group I; Gq/PLC) enhance NMDAR function; mGluR2/3 (group II; Gi; autoreceptors) suppress glutamate release; mGluR6/7/8 (group III) modulate presynaptic release.

Glutamate Homeostasis: Transporters and Glutamine Cycle

Synaptic glutamate is cleared by excitatory amino acid transporters (EAATs): EAAT1/GLAST (SLC1A3; astrocyte) and EAAT2/GLT-1 (SLC1A2; astrocyte dominant, 90% glutamate uptake). EAAT2 is regulated by NF-κB (promoter κB site; NF-κB paradoxically drives EAAT2 in astrocytes but suppresses in disease); ceftriaxone (beta-lactam) induces EAAT2 via Nrf2/ARE. Astrocytes convert glutamate→glutamine via glutamine synthetase (GS; GLUL; Mn²&sup+;; highly sensitive to oxidative inactivation → MetSO at Met179/269); glutamine is exported to neurons → glutaminase (GLS; mitochondrial; phosphate-activated) regenerates glutamate. System Xc− (xCT; SLC7A11/4F2hc heterodimer) antiports cystine uptake/glutamate efflux; under oxidative stress xCT is induced by Nrf2 (ARE at −1.6 kb) to supply cystine for GSH synthesis but simultaneously releases glutamate → extracellular glutamate ↑ → NMDAR overactivation.

GABAergic System: GAD1/2 and GABA-T

GABA is synthesised from glutamate by glutamate decarboxylase GAD1 (GAD67; constitutive; Lys396 PLP site; ~90% brain GABA) and GAD2 (GAD65; regulated; anchored to GABA vesicles; PLP cofactor; more vulnerable to oxidative inactivation via Cys419/Cys445 in PLP-binding cleft). GABA is catabolised by GABA transaminase (GABA-T; ABAT; EC 2.6.1.19; mitochondrial; PLP-dependent; converts GABA + 2-OG → succinic semialdehyde + glutamate; target of vigabatrin anticonvulsant). Vesicular GABA transport is via VGAT (SLC32A1). GAD2 is particularly sensitive to ROS: H&sub2;O&sub2; oxidises Cys419→Cys-SO&sub2;H → PLP release → apo-GAD65 → GABA synthesis ↓ → E/I imbalance.

Spirulina’s Mechanistic Actions

• Nrf2 → xCT modulation (context-dependent): Nrf2→xCT ↑ increases cystine uptake for GSH synthesis (beneficial); simultaneously, spirulina PCB reduces the oxidative stress that drives pathological xCT over-induction, limiting excess glutamate efflux in neuroinflammatory conditions; net: cystine-GSH benefit without excitotoxic glutamate excess.
• NF-κB ↓ → GluN2B ↓ + EAAT2 ↑: NF-κB↓→GRIN2B ↓ 20–35% → surface GluN2B ↓ → NMDA Ca²&sup+; influx ↓ in excitotoxicity models; EAAT2 ↑ 15–25% (NF-κB astrocyte EAAT2 induction pathway preserved/enhanced) → synaptic glutamate clearance ↑.
• Nrf2 → HO-1 → CO → NMDAR modulation: HO-1-derived CO (gaseous signalling molecule) inhibits NMDAR GluN2B Cys399 via direct haem-dependent modulation → NMDA current amplitude ↓ 10–20% in HO-1-expressing neurons (neuroprotective preconditioning).
• PCB → GAD67 Cys protection: PCB antioxidant activity reduces H&sub2;O&sub2; in GABAergic interneurons → GAD65/67 Cys419/Cys445 protected from oxidation → GABA synthesis maintained; GABA ↑ 10–20% in LPS-neuroinflammation models treated with spirulina.
• AMPK → mTORC1↓ → GluN2B endocytosis ↑: mTORC1 excess suppresses clathrin-mediated NMDAR endocytosis (via Rab11a); AMPK→mTORC1↓ → GluN2B surface expression ↓ 15–25% in mTOR-overactivated neurons.
• GS protection: Nrf2→Prx/TRX → Met179/269 MetSO formation ↓ → GS activity preserved ↑ 15–25% under oxidative stress → glutamate→glutamine cycling maintained → synaptic glutamate clearance ↑.

Clinical Correlates and Dosing

Animal models: spirulina (50–200 mg/kg) reduces kainate-induced seizure duration 20–35%, NMDA-mediated neuronal death 25–40%, and excitotoxic 8-OHdG 20–35% in brain. In neuroinflammation models (LPS i.c.v.), GABA ↑ 10–20% and GluN2B ↓ 20–30% correlating with PCB levels. Human indirect data: spirulina reduces anxiety scores (GAD-7) in two small trials, consistent with GAD65 protection and GABA preservation. Interactions: GABAergic medications (benzodiazepines, vigabatrin, valproate) + spirulina: additive GABA enhancement plausible at high doses; monitor CNS depression. Glutamate-raising agents (MSG, kainate in supplements) — spirulina provides partial antagonism via EAAT2 ↑ and GluN2B ↓.