Increased extracellular brain
glutamate has been implicated in the pathophysiology of human refractory
temporal lobe epilepsy (TLE), but the cause of the excessive
glutamate is unknown. Prior studies by us and others have shown that the
glutamate degrading
enzyme glutamine synthetase (GS) is deficient in astrocytes in the epileptogenic hippocampal formation in a subset of patients with TLE. We have postulated that the loss of GS in TLE leads to increased
glutamate in astrocytes with elevated concentrations of extracellular
glutamate and recurrent
seizures as the ultimate end-points. Here we test the hypothesis that the deficiency in GS leads to increased
glutamate in astrocytes. Rats were chronically infused with
methionine sulfoximine (MSO, n=4) into the hippocampal formation to induce GS deficiency and recurrent
seizures. A separate group of rats was infused with
0.9% NaCl (saline) as a control (n=6). At least 10days after the start of infusion, once recurrent
seizures were established in the MSO-treated rats, the concentration of
glutamate was assessed in CA1 of the hippocampal formation by immunogold electron microscopy. The concentration of
glutamate was 47% higher in astrocytes in the MSO-treated vs. saline-treated rats (p=0.02), and the ratio of
glutamate in astrocytes relative to axon terminals was increased by 74% in the MSO-treated rats (p=0.003). These data support our hypothesis that a deficiency in GS leads to increased
glutamate in astrocytes. We additionally propose that the GS-deficient astrocytes in the hippocampal formation in TLE lead to elevated extracellular brain
glutamate either through decreased clearance of extracellular
glutamate or excessive release of
glutamate into the extracellular space from these cells, or a combination of the two.