Abstracts

Rationale: Rationale:  In vivo brain microdialysis studies of epilepsy patients demonstrate elevated basal glutamate levels in epileptogenic foci (Ann Neurol 2016; 80: 35-45). We hypothesize this results from a deficiency in glutamine synthetase, the enzyme that metabolizes glutamate to glutamine, as this enzyme has been shown to be deficient in epileptogenic foci (Lancet 2004; 363: 28-37).  The objective of this study was to evaluate the effect of inhibition of glutamine synthetase on basal and seizure associated levels of extracellular glutamate, glutamine, and GABA and seizure occurrence in the GS-deficient vs. GS-intact hippocampus. Methods: Methods: Male Sprague Dawley rats were implanted with an osmotic pump continuously infusing either phosphate buffered saline (PBS) or glutamine synthetase inhibitor, methionine sulfoximine (MSO), into the entorhinal cortex for a period of up to 28 days.  Guide cannula were also placed in the hippocampus proper both ipsilateral and contralateral to the infusion location. Some animals were also implanted with unipolar stainless-steel depth electrodes in bilateral hippocampi.   Results:  Results: Basal levels of glutamate were significantly higher on the contralateral side of the MSO treated rats compared to the ipsilateral side (p=0.01), and compared to the PBS control (p=0.02).  Glutamine was significantly lower in the ipsilateral (p=0.004) and contralateral hippocampi (p=0.01) in the MSO treated animals compared to the PBS control. GABA levels were not significantly different between the groups. In the contralateral hippocampus, the ratio of glutamate and GABA in the MSO treated rats gradually increased from 5 hours before the seizure, peaking three hours after the seizure (p<0.0001), followed by a 29% drop (p=0.006) and stabilization.  In the ipsilateral hippocampus, the ratio remained stable from seven hours prior to the seizure, increased by 56 % at the third hour following the seizure (p<0.01), followed by a decrease to baseline levels at the fifth hour after the seizure (p=0.01).  The depth EEG clearly showed seizure initiation from the ipsilateral hippocampus and propagation to the contralateral hippocampus but with the time for intrahippocampal propagation decreasing through epileptogenesis.    Conclusions: Conclusions:  GS-deficient and GS-intact hippocampi of the MSO model of temporal lobe epilepsy share both similarities and differences with respect to both chemistry and seizure expression.  Seizures consistently initiated from the ipsilateral GS-deficient hippocampus with the delay to involvement of the GS-intact but glutamate elevated contralateral hippocampus decreasing over time. Furthermore, the ratio of glutamate and GABA progressed similarly around the seizure in both the ipsilateral and contralateral hippocampi.  These findings are significant because they help to elucidate the effect of unilateral glutamine synthetase inhibition on bilateral basal glutamate, glutamine, and GABA levels, and seizure initiation and propagation.  Funding: Source of Funding: NINDS K08 NS058674, R01 NS070824, and NCATS UL1 RR024139