NKCC-1 mediated Cl(−) uptake in immature CA3 pyramidal neurons is sufficient to compensate phasic GABAergic inputs

Activation of GABA(A) receptors causes in immature neurons a functionally relevant decrease in the intracellular Cl(−) concentration ([Cl(−)](i)), a process termed ionic plasticity. Amount and duration of ionic plasticity depends on kinetic properties of [Cl(−)](i) homeostasis. In order to characterize the capacity of Cl(−) accumulation and to quantify the effect of persistent GABAergic activity on [Cl(−)](i), we performed gramicidin-perforated patch-clamp recordings from CA3 pyramidal neurons of immature (postnatal day 4–7) rat hippocampal slices. These experiments revealed that inhibition of NKCC1 decreased [Cl(−)](i) toward passive distribution with a time constant of 381 s. In contrast, active Cl(−) accumulation occurred with a time constant of 155 s, corresponding to a rate of 15.4 µM/s. Inhibition of phasic GABAergic activity had no significant effect on steady state [Cl(−)](i). Inhibition of tonic GABAergic currents induced a significant [Cl(−)](i) increase by 1.6 mM, while activation of tonic extrasynaptic GABA(A) receptors with THIP significantly reduced [Cl(−)](i.). Simulations of neuronal [Cl(−)](i) homeostasis supported the observation, that basal levels of synaptic GABAergic activation do not affect [Cl(−)](i). In summary, these results indicate that active Cl(−)-uptake in immature hippocampal neurons is sufficient to maintain stable [Cl(−)](i) at basal levels of phasic and to some extent also to compensate tonic GABAergic activity.