Glutamate uptake shapes low-[Mg2+] induced epileptiform activity in juvenile rat hippocampal slices

A wide range of data support a role for ambient glutamate (Glu) in epilepsy, although temporal patterns associated with the cellular uptake of Glu have not been addressed in detail. We report on the effects of Glu uptake inhibitors on recurrent seizure-like events (SLEs) evoked by low-[Mg2+] condition in juvenile rat hippocampal slices. Effects were compared for inhibitors such as l-trans-pyrrolidine-2,4-dicarboxylate (tPDC), dl-threo-β-benzyloxyaspartate (dl-TBOA) and dihydrokainic acid (DHK), representing different transporter specificity and transportability profiles. Latency to the first SLE after drug application was shortened by the inhibitors (in % of control: 500 μM tPDC: 54 ± 7, 15 μM dl-TBOA: 74 ± 5, 50 μM dl-TBOA: 70 ± 6, 100 μM DHK: 69 ± 4, 300 μM DHK: 71 ± 7). Further SLEs were frequently aborted by higher inhibitor concentrations applied (500 μM tPDC: 2/6, 50 μM TBOA: 5/5, 100 μM DHK: 6/8, 300 μM DHK: 3/3). Simultaneous field potential and whole-cell voltage recordings showed depolarization-induced inactivation of CA3 pyramidal neurons during inhibitor application. In the presence of inhibitors, the amplitude of forthcoming SLE was also decreased (in % of control: 500 μM tPDC: 66 ± 9, 15 μM dl-TBOA: 88 ± 5, 50 μM dl-TBOA: 59 ± 6, 100 μM DHK: 67 ± 4, 300 μM DHK: 68 ± 1). Dependent on type and concentration of the inhibitor, the duration of the first SLE of drug application either increased (100 μM DHK: 375 ± 90 %; 100 μM tPDC: 137 ± 13 %) or decreased (50 μM TBOA: 62 ± 13 %; 300 μM DHK: 60 ± 15 %) reflecting differences in subtype-specificity or mechanism of action of the inhibitors. Our findings suggest a role for ambient Glu in the genesis and maintenance of recurrent epileptiform discharges.