Compensatory Enhancement of Intrinsic Spiking upon NKCC1 Disruption in Neonatal Hippocampus

Abstract

Depolarizing and excitatory GABA actions are thought to be important in cortical development. We show here that GABA has no excitatory action on CA3 pyramidal neurons in hippocampal slices from neonatal NKCC1 Ϫ/Ϫ mice that lack the Na-K-2Cl cotransporter isoform 1. Strikingly, NKCC1 Ϫ/Ϫ slices generated endogenous network events similar to giant depolarizing potentials (GDPs), but, unlike in wild-type slices, the GDPs were not facilitated by the GABA A agonist isoguvacine or blocked by the NKCC1 inhibitor bumetanide. The developmental upregulation of the K-Cl cotransporter 2 (KCC2) was unperturbed, whereas the pharmacologically isolated glutamatergic network activity and the intrinsic excitability of CA3 pyramidal neurons were enhanced in the NKCC1 Ϫ/Ϫ hippocampus. Hence, developmental expression of KCC2, unsilencing of AMPA-type synapses, and early network events can take place in the absence of excitatory GABAergic signaling in the neonatal hippocampus. Furthermore, we show that genetic as well as pharmacologically induced loss of NKCC1-dependent excitatory actions of GABA results in a dramatic compensatory increase in the intrinsic excitability of glutamatergic neurons, pointing to powerful homeostatic regulation of neuronal activity in the developing hippocampal circuitry.

The Necessity of NKCC1: Loss of the Chloride Cotransporter in a Knock-Out Model and Potential Compensatory Mechanisms

“…The findings however, deviate from those of Sipilä et al (2009) on several key points. NKCC1 Ϫ/Ϫ pyramidal neurons in Pfeffer et al's (2009) recordings still exhibited a depolarizing, be it much attenuated, GABAergic current.…”

“…The differences may be due to environmental factors, or they may stem from variations in the underlying genetic backgrounds. Sipilä et al (2009) point out that their strain of mouse is different to the one used by Pfeffer et al (2009). This distinction opens the possibility of indirect effects occurring through "modifier" genes, allelic variations in genes other than the target knock-out gene that can alter the final phenotype (Sigmund, 2000).…”

“…Rather than imposing a premature, hyperpolarizing Cl Ϫ gradient on neurons, Sipilä et al (2009) used a transgenic mouse model in which the NKCC1 gene Slc12a2 had been knocked out, to produce a homozygous NKCC1-null (NKCC1 Ϫ/Ϫ ) mutant. Through electrophysiological recordings, they then investigated the role of the cotransporter in maintaining the depolarizing effect of GABA and the impact of this on signaling and developmental mechanisms in the neonatal brain.…”

“…Next, Sipilä et al (2009) compared the relative network activity of NKCC1 Ϫ/Ϫ and WT slices by patching onto neurons from both genotypes using a low-chloride (4 mM) pipette solution. When clamped at 0 mV, WT neurons produced spontaneous bursts of outward current typically associated with GABAergic GDPs.…”

GABAergic synchronization in the limbic system and its role in the generation of epileptiform activity

“…First, accumulation of intracellular Cl − ([Cl − ] i ) -eventually favored by decreased KCC2 activity -can shift the E GABA to values positive to resting membrane potential; indeed, since dendritic receptor activation often produces depolarization, it was proposed that [Cl − ] i in cortical cell dendrites could be higher than in the soma, where hyperpolarizations are commonly recorded (Misgeld et al, 1986). Second, [Cl − ] i accumulation is caused (at least in immature neurons) by the activation of NKCC1 cotransporter that imports Na + , K + , and Cl − (Achilles et al, 2007;Sipilä et al, 2009). As discussed in Section 9, this mechanism may play an important role in neonatal seizures (Dzhala et al, 2005(Dzhala et al, , 2010Nardou et al, 2009).…”