Enhanced synaptic activity and epileptiform events in the embryonic KCC2 deficient hippocampus

Abstract

The neuronal potassium-chloride co-transporter 2 [indicated thereafter as KCC2 (for pro- tein) and Kcc2 (for gene)] is thought to play an important role in the post natal excitatory to inhibitory switch of GABA actions in the rodent hippocampus. Here, by studying hippocampi of wild-type (Kcc2 +/+) and Kcc2 deficient (Kcc2 -/-) mouse embryos, we unexpectedly found increased spontaneous neuronal network activity at E18.5, a developmental stage when KCC2 is thought not to be functional in the hippocampus. Embryonic Kcc2 -/- hippocampi have also an augmented synapse density and a higher frequency of spontaneous glutamatergic and GABA-ergic postsynaptic currents than naïve age matched neurons. However, intracellular chloride concentration ([Cl -]i) and the reversal potential of GABA- mediated currents (E GABA) were similar in embryonic Kcc2 +/+ and Kcc2 -/- CA3 neurons. In addition, KCC2 immunolabeling was cytoplasmic in the majority of neurons suggesting that the molecule is not functional as a plasma membrane chloride co-transporter. Collectively, our results show that already at an embryonic stage, KCC2 controls the formation of synapses and, when deleted, the hippocampus has a higher density of GABA-ergic and glutamatergic synapses and generates spontaneous and evoked epileptiform activities These results may be explained either by a small population of orchestrating neurons in which KCC2 operates early as a chloride exporter or by transporter independent actions of KCC2 that are instrumental in synapse formation and networks construction. ©2011Khalilov, Chazal, Chudotvorova, Pellegrino, Corby, Ferrand, Gubkina, Nardou,Tyzio, Yamamoto, Jentsch, Hübner, Gaiarsa, Ben-Ariand Medina.