Abstracts

An interictal-like activity is spontaneously generated in the subiculum of slices obtained from patients after temporal lobe epilepsy and hippocampal sclerosis surgery. A minority of subicular pyramidal cells exhibit depolarizing responses to GABA indicating Cl- homeostasis is perturbed. We studied the role of Cl- homeostasis in GABAergic signaling in human epileptic tissue, focusing on the cation-chloride cotransporters NKCC1 and KCC2, which are major Cl- intracellular concentration regulators., Slices of mesial human temporal lobe tissue containing the subiculum were prepared after surgery performed for pharmacoresistant epilepsy. Field potentials and unit activity were recorded with extracellular electrodes. Intracellular pipettes contained biocytin so that recorded cells could be anatomically identified. KCC2 and NKCC1 expression was studied with in situ hybridization and immunochemistry., In pyramidal cells, GABAA IPSPs reversal potentials were asymmetrically distributed with a mode near -70 mV and most depolarized values up to -50 mV. The driving force was positive with respect to resting potential in 20% of pyramidal cells. Neither morphological nor cellular characteristics differed for pyramidal cells with depolarizing or hyperpolarizing GABAergic signaling. In situ hybridization did not detect KCC2 mRNA in 30% of subicular pyramidal cells, contrasting with a constant presence in dentate gyrus or CA2 pyramidal cells. Combined biocytin labeling and immunohistochemistry showed that 6/6 cells with hyperpolarizing responses to GABA expressed KCC2, while only 4/7 cells excited during epileptiform bursts were immunonegative for KCC2. Almost all pyramidal cells expressed NKCC1, suggesting that uptake of Cl- was effective. Consistent with this, bumetanide, at doses which specifically block NKCC1, produced a hyperpolarizing shift in GABA reversal potentials and suppressed interictal activity., We conclude that changes in Cl- transporter expression contribute to human interictal activity and that molecules inhibiting Cl- uptake may be useful anti-epileptic drugs, rehabilitating inhibitory responses to GABA., (Supported by INSERM; EUROPEAN COMMUNITY.)