Impaired Cl−Extrusion in Layer V Pyramidal Neurons of Chronically Injured Epileptogenic Neocortex

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Abstract
In the mature brain, the K+/Cl−cotransporter KCC2 is important in maintaining low [Cl−]i, resulting in hyperpolarizing GABA responses. Decreases in KCC2 after neuronal injuries result in increases in [Cl−]iand enhanced neuronal excitability due to depolarizing GABA responses. We used the gramicidin perforated-patch technique to measure ECl(∼ EGABA) in layer V pyramidal neurons in slices of partially isolated sensorimotor cortex of adult rats to explore the potential functional consequence of KCC2 downregulation in chronically injured cortex. EGABAwas measured by recording currents evoked with brief GABA puffs at various membrane potentials. There was no significant difference in EClbetween neurons in control and undercut animals (–71.2 ± 2.6 and –71.8 ± 2.8 mV, respectively). However, when loaded with Cl−by applying muscimol puffs at 0.2 Hz for 60 s, neurons in the undercut cortex had a significantly shorter time constant for the positive shift in EClduring the Cl−loading phase (4.3 ± 0.5 s for control and 2.2 ± 0.4 s for undercut, P < 0.01). The positive shift in ECl3 s after the beginning of Cl−loading was also significantly larger in the undercut group than in the control, indicating that neurons in undercut cortex were less effective in maintaining low [Cl−]iduring repetitive activation of GABAAreceptors. Application of furosemide eliminated the difference between the control and undercut groups for both of these measures of [Cl−]iregulation. The results suggest an impairment in Cl−extrusion resulting from decreased KCC2 expression that may reduce the strength of GABAergic inhibition and contribute to epileptogenesis.