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

Abstract

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.