Anomalous Levels of Cl⁻ Transporters in the Hippocampal Subiculum from Temporal Lobe Epilepsy Patients Make GABA Excitatory

The mRNA levels of NKCC1, an inwardly directed Na⁺, K⁺-2Cl⁻ cotransporter that facilitates the accumulation of intracellular Cl⁻, and of KCC2, an outwardly directed K⁺-Cl⁻ cotransporter that extrudes Cl⁻, were studied in surgically resected brain specimens from drug-resistant temporal lobe (TL) epilepsy (TLE) patients. Quantitative RT-PCR analyses of the mRNAs extracted from the human TLE-associated brain regions revealed an up-regulation of NKCC1 mRNA and a down-regulation of KCC2 mRNA in the hippocampal subiculum, compared with the hippocampus proper or the TL neocortex, suggesting an abnormal transcription of Cl⁻ transporters in the TLE subiculum. In parallel experiments, cell membranes isolated from the same TLE-associated brain regions were injected into Xenopus oocytes that rapidly incorporated human$GABA_{A}$receptors into their surface membrane. The GABA currents elicited in oocytes injected with membranes from the subiculum had a more depolarized reversal potential ($E_{GABA}$) compared with the hippocampus proper or the neocortex. The NKCC1 blocker bumetanide or a temperature decrease of 10°C shifted the GABA-current$E_{GABA}$more negative in oocytes injected with membranes from TLE hippocampal subiculum, matching the$E_{GABA}$of TL neocortex-injected oocytes. We conclude that the anomalous expression of both Cl⁻ transporters, KCC1 and NKCC2, in TLE hippocampal subiculum probably causes altered Cl⁻ transport in the "epileptic" neurons, as revealed in the microtransplanted Xenopus oocytes, and renders GABA aberrantly "exciting," a feature that may contribute to the precipitation of epileptic seizures.