Exacerbation of Epilepsy by Astrocyte Alkalization and Gap Junction Uncoupling

Abstract

Seizures invite seizures. At the initial stage of epilepsy, seizures intensify with each episode; however, the mechanisms underlying this exacerbation remain to be solved. Astrocytes have a strong control over neuronal excitability and the mode of information processing. This control is accomplished by adjusting the levels of various ions in the extracellular space. The network of astrocytes connected via gap junctions allows a wider or more confined distribution of these ions depending on the open probability of the gap junctions. K⁺ clearance relies on the K⁺ uptake by astrocytes and the subsequent diffusion of K⁺ through the astrocyte network. When astrocytes become uncoupled, K⁺ clearance becomes hindered. Accumulation of extracellular K⁺ leads to hyperexcitability of neurons. Here, using acute hippocampal slices from mice, we uncovered that brief periods of epileptiform activity result in gap junction uncoupling. In slices that experienced short-term epileptiform activity, extracellular K⁺ transients in response to glutamate became prolonged. Na⁺ imaging with a fluorescent indicator indicated that intercellular diffusion of small cations in the astrocytic syncytium via gap junctions became rapidly restricted after epileptiform activity. Using a transgenic mouse with astrocyte-specific expression of a pH sensor (Lck-E²GFP), we confirmed that astrocytes react to epileptiform activity with intracellular alkalization. Application of Na⁺/HCO₃^- cotransporter blocker led to the suppression of intracellular alkalization of astrocytes and to the prevention of astrocyte uncoupling and hyperactivity intensification both in vitro and in vivo Therefore, the inhibition of astrocyte alkalization could become a promising therapeutic strategy for countering epilepsy development.SIGNIFICANCE STATEMENT We aimed to understand the mechanisms underlying the plastic change of forebrain circuits associated with the intensification of epilepsy. Here, we demonstrate that first-time exposure to only brief periods of epileptiform activity results in acute disturbance of the intercellular astrocyte network formed by gap junctions in hippocampal tissue slices from mice. Moreover, rapid clearance of K⁺ from the extracellular space was impaired. Epileptiform activity activated inward Na⁺/HCO₃^- cotransport in astrocytes by cell depolarization, resulting in their alkalization. Our data suggest that alkaline pH shifts in astrocytes lead to gap junction uncoupling, hampering K⁺ clearance, and thereby to exacerbation of epilepsy. Pharmacological intervention could become a promising new strategy to dampen neuronal hyperexcitability and epileptogenesis.

Keywords: Na+/HCO3– cotransporter; astrocyte; epilepsy; gap junction; pH; potassium.