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.