Altered Neuronal Chloride Homeostasis and Excitatory GABAergic Signaling in Human Temporal Lobe Epilepsy

Cation-Chloride Cotransporters and GABAergic Innervation in the Human Epileptic Hippocampus. Munoz A, Mendez P, DeFelipe J, Alvarez-Leefmans FJ. Epilepsia 2007;48(4):663–673. Intracellular chloride concentration, [Cl]i, determines the polarity of GABAA-induced neuronal Cl currents. In neurons, [Cl]i is set by the activity of Na+, K+, 2Cl cotransporters (NKCC) such as NKCC1, which physiologically accumulate Cl in the cell, and Cl extruding K+, Cl cotransporters like KCC2. Alterations in the balance of NKCC1 and KCC2 activity may determine the switch from hyperpolarizing to depolarizing effects of GABA, reported in the subiculum of epileptic patients with hippocampal sclerosis. We studied the expression of NKCC (putative NKCC1) and KCC2 in human normal temporal neocortex by Western blot analysis and in normal and epileptic regions of the subiculum and the hippocampus proper using immunocytochemistry. Western blot analysis revealed NKCC and KCC2 proteins in adult human neocortical membranes similar to those in rat neocortex. NKCC and KCC2 immunolabeling of pyramidal and nonpyramidal cells was found in normal and epileptic hippocampal formation. In the transition between the subiculum with sclerotic regions of CA1, known to exhibit epileptogenic activity, double immunolabeling of NKCC and KCC2 revealed that approximately 20% of the NKCC-immunoreactive neurons do not express KCC2. In these same areas, some neurons were distinctly hyperinnervated by parvalbumin (PV) positive hypertrophic basket formations that innervated mostly neurons expressing NKCC (74%) and to a lesser extent NKCC-immunonegative neurons (26%). Hypertrophic basket formations also innervated KCC2-positive (76%) and -negative (24%) neurons. The data suggest that changes in the relative expression of NKCC1 and KCC2 in neurons having aberrant GABAergic hyperinnervation may contribute to epileptiform activity in the subicular regions adjacent to sclerotic areas of the hippocampus. Perturbed Chloride Homeostasis and GABAergic Signaling in Human Temporal Lobe Epilepsy. Huberfeld G, Wittner L, Clemenceau S, Baulac M, Kaila K, Miles R, Rivera C. J Neurosci 2007;27(37):9866–9873. Changes in chloride (Cl-) homeostasis may be involved in the generation of some epileptic activities. In this study, we asked whether Cl- homeostasis, and thus GABAergic signaling, is altered in tissue from patients with mesial temporal lobe epilepsy associated with hippocampal sclerosis. Slices prepared from this human tissue generated a spontaneous interictal-like activity that was initiated in the subiculum. Records from a minority of subicular pyramidal cells revealed depolarizing GABAA receptor-mediated postsynaptic events, indicating a perturbed Cl-homeostasis. We assessed possible contributions of changes in expression of the potassium–chloride cotransporter KCC2. Double in situ hybridization showed that mRNA for KCC2 was absent from 30% of CaMKII (calcium/calmodulin-dependent protein kinase II)-positive subicular pyramidal cells. Combining intracellular recordings with biocytin-filled electrodes and KCC2 immunochemistry, we observed that all cells that were hyperpolarized during interictal events were immunopositive for KCC2, whereas the majority of depolarized cells were immunonegative. Bumetanide, at doses that selectively block the chloride-importing potassium–sodium–chloride cotransporter NKCC1, produced a hyperpolarizing shift in GABAA reversal potentials and suppressed interictal activity. Changes in Cl- transporter expression thus contribute to human epileptiform activity, and molecules acting on these transporters may be useful antiepileptic drugs.