Effects of site-specific infusions of methionine sulfoximine on the temporal progression of seizures in a rat model of mesial temporal lobe epilepsy

Abstract

SUMMARY Glutamine synthetase (GS) in astrocytes is critical for metabolism of glutamate and ammonia in the brain, and perturbations in the anatomical distribution and activity of the enzyme are likely to adversely affect synaptic transmission. GS is deficient in discrete regions of the hippocampal formation in patients with mesial temporal lobe epilepsy (MTLE), a disorder characterized by brain glutamate excess and recurrent seizures. To investigate the role of site-specific inhibition of GS in MTLE, we chronically infused the GS inhibitor methionine sulfoximine (MSO) into one of the following areas of adult laboratory rats: (1) the angular bundle, n = 6; (2) the deep entorhinal cortex (EC), n = 7; (3) the stratum lacunosum-moleculare of CA1, n = 7; (4) the molecular layer of the subiculum, n = 10; (5) the hilus of the dentate gyrus, n = 6; and (6) the lateral ventricle, n = 6. Twelve animals were infused with phosphate buffered saline (PBS) into the same areas to serve as controls. All infusions were unilateral, and animals were monitored by continuous video-intracranial EEG recordings for 3 weeks to capture seizure activity. All animals infused with MSO into the entorhinal–hippocampal area exhibited recurrent seizures that were particularly frequent during the first 3 days of infusion and that continued to recur for the entire 3 week recording period. Only a fraction of animals infused with MSO into the lateral ventricle had recurrent seizures, which occurred at a lower frequency compared with the other MSO infused group. Infusion of MSO into the hilus of the dentate gyrus resulted in the highest total number of seizures over the 3-week recording period. Infusion of MSO into all brain regions studied, with the exception of the lateral ventricle, led to a change in the composition of seizure severity over time. Low-grade (stages 1–3) seizures were more prevalent early during infusion, while severe (stages 4–5) seizures were more prevalent later. Thus, the site of GS inhibition within the brain determines the pattern and temporal evolution of recurrent seizures in the MSO model of MTLE.

Astrocytes and Glutamine Synthetase in Epileptogenesis

“…We next assessed whether the anatomical site of GS inhibition was a determining factor for induction of epileptogenesis and for the epileptic phenotype. MSO or phosphate buffered saline (PBS) were infused unilaterally into different mesial temporal lobe regions of adult rats, including (1) the angular bundle, (2) the deep entorhinal cortex (EC), (3) the stratum lacunosum‐moleculare of CA1, (4) the molecular layer of the subiculum, (5) the hilus of the dentate gyrus, (6) the lateral ventricle, and (7) the central nucleus of the amygdala (Dhaher et al, ; Gruenbaum et al, ). All animals infused with MSO into the brain tissue exhibited recurrent seizures that were particularly frequent during the first 3 days of infusion and continued to recur for the entire 3‐week EEG‐recording period.…”

“…Thus, inhibition of GS focally in the hippocampal formation triggers a process of epileptogenesis characterized by gradual worsening of seizure severity over a period of several weeks (Dhaher et al, ). This gradual progress in epileptogenesis is accompanied by structural changes that evolve over time and include numerous brain areas both within and outside the GS‐inhibited brain region (Albright et al, ; Wang et al, ).…”

Network evolution in mesial temporal lobe epilepsy revealed by diffusion tensor imaging

“…Male Sprague-Dawley rats (370-430 g) were implanted with a cannula into the right entorhinal-hippocampal region for continuous infusion of either MSO (n = 18) or PBS (n = 13) as detailed in Dhaher et al 12 Epidural screw electrodes were implanted in the skull to record cortical EEG activity.…”

Normal and epilepsy-associated pathologic function of the dentate gyrus

“…While it is currently known that astrogliosis is present in the hippocampus, and specifically, in the DG of patients with temporal lobe epilepsy and in animal models of the disease, the exact contributions of this pathology in altering circuit function within the DG are currently unknown. However, in one recent study, Dhaher et al (2015) pharmacologically mimicked the glutamine synthetase-reducing effects of astrogliosis, by infusing the glutamine synthetase inhibitor, methionine sulfoximine into different hippocampal substructures, including the angular bundle of the perforant path, deep layers of the EC, the DG, CA1, subiculum, and the lateral ventricle and monitored animals for seizures via video and EEG recordings. Among all tested structures, methionine sulfoximine infusion into the DG produced the highest number of seizures over the recording period, highlighting both the importance of the DG in seizure generation, and its potential susceptibility for gliosis-mediated disinhibition in epilepsy.…”