Background: The actual mechanisms underlying human hippocampal epileptogenicity, a process ultimately mediated by neurochemical events, remains to be fully elucidated. We submit early insight data regarding microdialysis (MD) recovery of the neuroactive amino acids glutamate, aspartate and gamma-aminobutyric acid (GABA) from the intraoperative and intact, spontaneously epileptiform human hippocampus.
Method: Generally anaesthetised temporal lobe epilepsy (TLE) patients (N=7) undergoing therapeutic and anatomically standardised resective surgery were also subjected to ipsilateral anterior hippocampal MD with concomitant hippocampal electrocorticography (ECoG). Recovered 10-min dialysate samples were quantified for glutamate, aspartate and GABA using high-performance liquid chromatography; corresponding ECoG data was assessed for epileptiform activity (EA); mesial resection tissue was postoperatively examined and graded for hippocampal sclerosis.
Findings: Mean 'Sample 3' dialysate absolute recovery of glutamate, aspartate and GABA from hippocampi with minimal EA (N=5) was ( micro M+/-SEM): 6.406+/-2.143, 0.600+/-0.215, and 0.357+/-0.093, respectively. In contrast, 'Sample 3' dialysate absolute glutamate, aspartate and GABA levels ( micro M) from the hippocampi of two patients with vigorous EA were: 101.099 and 211.861, 21.860 and 14.482, and 4.241 and 4.817, respectively. Mesial resection tissue in all cases demonstrated hippocampal sclerosis, though the histopathological degree of sclerosis varied between patients.
Interpretation: These preliminary intraoperative findings suggest that dialysate glutamate, aspartate, and GABA levels from the sclerotic anterior hippocampus likely reflects the functional status of the sampled tissue - i.e., lower levels of these neuroactive amino acids are to be expected during quiescent or minimal EA versus considerably higher levels corresponding to vigorous EA.