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

Activation of GABA receptors causes in immature neurons a functionally relevant decrease in the intracellular Cl concentration ([Cl]), a process termed ionic plasticity. Amount and duration of ionic plasticity depends on kinetic properties of [Cl] homeostasis. In order to characterize the capacity of Cl accumulation and to quantify the effect of persistent GABAergic activity on [Cl], 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] 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]. Inhibition of tonic GABAergic currents induced a significant [Cl] increase by 1.6 mM, while activation of tonic extrasynaptic GABA receptors with THIP significantly reduced [Cl]. Simulations of neuronal [Cl] homeostasis supported the observation, that basal levels of synaptic GABAergic activation do not affect [Cl]. In summary, these results indicate that active Cl-uptake in immature hippocampal neurons is sufficient to maintain stable [Cl] at basal levels of phasic and to some extent also to compensate tonic GABAergic activity.