The Cl- buffering system at the GABAergic tripartite synapses and a role of astrocytes expressing NKCC1

Atsuo Fukuda

Department of Neurophysiology, Hamamatsu University School of Medicine – Japan

Abstract

GABA is a main inhibitory neurotransmitter, however, excitatory GABA actions have been observed in brain slices of patients with temporal lobe epilepsy and in many in-vitro seizure models. This is caused by neuronal GABAA receptor-mediated Cl- accumulation and consequent collapse of Cl- gradient at the inhibitory postsynapses. Astrocytic processes tightly wrap around inhibitory synapses, enabling astrocytes to modulate synaptic cleft ion concentration. In contrast to neuron, astrocytic Cl- concentration is high due to abundance in Na+-K+-2Cl- cotransporter type 1 (NKCC1), so that the astrocytic GABAA receptor activation leads to Cl- efflux. This Cl- efflux from the astrocyte could buffer the synaptic cleft Cl- concentration ([Cl-]o) to stabilize the postsynaptic Cl- gradient during intense activation of GABAergic synapses (Egawa et al., J. Physiol. 2013). We therefore investigated the function of astrocytes in modulating the postsynaptic GABA action in epilepsy. We used the astrocyte-specific conditional NKCC1 knock-out (astroNKCC1 KO) mice, in which astrocytic GABAA receptor-mediated Cl- efflux would be reduced. The decrease in astrocytic Cl- efflux is expected to lower the [Cl-]o, hence hasten the collapse of postsynaptic Cl- gradient. The seizure-like events (SLEs) in CA1 pyramidal neurons were triggered by tetanic stimulation of stratum radiatum in hippocampus. We found that the threshold of stimulation intensity for SLEs is significantly lower in astroNKCC1 KO, compared to wild-type littermates (WT). In addition, in astroNKCC1 KO the duration of SLEs was significantly longer and the bicuculline-sensitive excitatory GABA current was significantly larger. Consistent with these in vitro results, in vivo pilocarpine-induced seizure model indicated astroNKCC1 KO mice are significantly seizure-prone with the lower induction dose and the higher score of Racine scale. Thus, our findings suggest a protective role of astrocytic NKCC1 in excitatory GABA-mediated seizures. This might provide an answer to the question why some of the clinical trials of bumetanide, a cell-type nonselective NKCC1 inhibitor, failed to show clear appreciable results.

Invited par Igor Medyna

Wednesday 5 April 2023 at 11 am, in the conference room