Science Topics – 168

During seizure-like events (SLEs), action of the inhibitory neurotransmitter GABA switches from inhibition to excitation. This is due to hyperactivity of GABA_A receptor-mediated chloride accumulation, resulting in a depolarization of the chloride reversal potential.
Moreover, accumulation of intracellular Cl^- is enhanced because the sodium-potassium-chloride cotransporter type 1 (NKCC1) maintains additional inward chloride flow into the cell. The specific NKCC1 inhibitor bumetanide can reduce the intracellular Cl^- concentration and thereby sustain the inhibitory effect of GABA in in vitro experiments. However, NKCC1 knock-out mice exhibited more status epilepticus than WT littermates. Since it has been shown that astrocytes also express NKCC1 it is possible that astrocytic NKCC1 contributes to these seizures. To investigate this, we used the astrocyte conditional NKCC1 knock-out(astroNKCC1KO) mice. SLEs were obtained from CA1 pyramidal neurons. In addition, the pilocarpine-induced seizure model was evaluated using the Racine scale in vivo.
The AstroNKCC1KO mice were prone to seizures with lower threshold and longer duration of SLEs and larger GABA_AR-mediated depolarization underlying the SLEs, accompanied by higher Racine-scored seizures. Bumetanide reduced these indicators of seizure in AstroNKCC1KO mice (which still express neuronal NKCC1), but not in the WT, both in vitro and in vivo. Our findings suggest a protective role of astrocytic NKCC1 in excitatory GABA-mediated seizures. The results of this study suggest that the development of therapeutic agents that selectively inhibit the function of NKCC1, which is expressed in neurons, holds promise as a new treatment for epilepsy and autism.

Astrocytic NKCC1 inhibits seizures by buffering Cl⁻ and antagonizing neuronal NKCC1 at GABAergic synapses.
Trong Dao Nguyen, Masaru Ishibashi, Adya Saran Sinha, Miho Watanabe, Daisuke Kato, Hiroshi Horiuchi, Hiroaki Wake, Atsuo Fukuda.
Epilepsia, 2023. (http://doi.org/10.1111/epi.17784)

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＜Figure Legends＞
Schematic diagram of astrocyte-mediated Cl^- buffering of the synaptic cleft. (A) Wild Type(left) and Astrocyte conditional NKCC1 knock-out (right) mice are prone to decrease in pore Cl^- concentration due to the absence of Cl^- buffering from astrocytes to the synaptic cleft when excessive synaptic transmission occurs (A right panel). (B) Schematic diagram of bumetanide action. In wild-type mice, the seizure-reducing effect of NKCC1 is less likely to be achieve because of the seizure-enhancing effect of NKCC1 on astrocytes, in addition to the seizure-reducing effect of NKCC1 on postsynaptic neurons (B left panel). In astrocyte conditional NKCC1 knock-out mice, bumetanide acts only on NKCC1 in postsynaptic neurons, resulting in seizure reduction (B right panel).

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Department of Neurophysiology, Hamamatsu University School of Medicine, Hamamatsu, Japan