Increased branched-chain amino acids at baseline and hours before a spontaneous seizure in the human epileptic brain

Abstract

The objective of this study was to monitor the extracellular brain chemistry dynamics at baseline and in relation to spontaneous seizures in human patients with refractory epilepsy. Thirty patients with drug-resistant focal epilepsy underwent intracranial electroencephalography and concurrent brain microdialysis for up to 8 continuous days. Extracellular brain glutamate, glutamine, and the branched-chain amino acids (BCAAs) valine, leucine, and isoleucine were quantified in the dialysis samples by liquid chromatography-tandem mass spectrometry. Extracellular BCAAs and glutamate were chronically elevated at baseline by approximately 1.5-3-fold in brain regions of seizure onset and propagation versus regions not involved by seizures. Moreover, isoleucine increased significantly above baseline as early as 3 h before a spontaneous seizure. BCAAs play important roles in glutamatergic neurotransmission, mitochondrial function, neurodegeneration, and mammalian target of rapamycin signaling. Because all of these processes have been implicated in epilepsy, the results suggest a novel role of BCAAs in the pathogenesis of spontaneous seizures.

Keywords: ictogenesis; metabolism; microdialysis; neurochemistry; seizure prediction.

Figure 1.

A: Example of an implanted, combined EEG-microdialysis probe projected onto a magnetic resonance image of the brain. The 10 mm long dialysis membrane is situated between electrode contacts 1 and 2 in the left hippocampus. B: Patient characteristics. All values are reported as mean ± SD, except the number of probes, which is reported as median (range). C: Mean basal, extracellular chemistry concentrations in non-epileptic (light color/cross-hatched bars) and epileptic (dark color/solid bars) brain regions. Error bars = standard error of the mean. *p < 0.05, **p < 0.001, n.s. not significant. D: Hourly extracellular chemistry concentrations in non-epileptic (n = 4 seizures) and epileptic (n = 9 seizures) brain regions, starting 6 hours before a seizure (red arrow) and ending 6 hours after. There is a significant increase in isoleucine (from baseline) at 3 hours before the seizure (*) and at the time of the seizure (*). There is also a significant increase in glutamate, glutamine, valine and leucine at 1 hour before the seizure (*). Graph colors correspond to the color codes in C.

Figure 2.

Branched-chain amino acid (BCAA) - glutamate - glutamine shuttle, and the proposed mechanisms underlying the effects of BCAAs on seizures. Anticonvulsant effects: (1) decrease in neuronal glutamate concentration; (2) increased glutamate clearance via glutamine – BCAA obligate transport; (3) increased gamma-aminobutyric acid (GABA) production in GABAergic neurons; (4) increased uptake of extracellular glutamate via excitatory amino acid transporter 3 (EAAT3). Proconvulsant effects: (5) increased ammonia production. Abbreviations: Glu: glutamate, Gln: glutamine, NH3: ammonia, Pyr: pyruvate, OAA: oxaloacetate, aKG: alpha-ketoglutaric acid, BCKA: branched-chain alpha-ketoacid, PAG: phosphate-activated glutaminase, GAD: glutamic acid decarboxylase, PC: pyruvate carboxylase, TCA: tricarboxylic acid cycle, BCATm: mitochondrial branched-chain amino acid transaminase, GDH: glutamate dehydrogenase, GOT: glutamic oxaloacetic transaminase, GS: glutamine synthetase, BCATc: cytoplasmic branched-chain amino acid transaminase. From Gruenbaum et al. Reprinted with permission from Springer Nature Switzerland AG.