Contributions of Astrocyte and Neuronal Volume to CA1 Neuron Excitability Changes in Elevated Extracellular Potassium

Abstract

Rapid increases in cell volume reduce the size of the extracellular space (ECS) and are associated with elevated brain tissue excitability. We recently demonstrated that astrocytes, but not neurons, rapidly swell in elevated extracellular potassium (∧[K+]o) up to 26 mM. However, effects of acute astrocyte volume fluctuations on neuronal excitability in ∧[K+]o have been difficult to evaluate due to direct effects on neuronal membrane potential and generation of action potentials. Here we set out to isolate volume-specific effects occurring in ∧[K+]o on CA1 pyramidal neurons in acute hippocampal slices by manipulating cell volume while recording neuronal glutamate currents in 10.5 mM [K+]o + tetrodotoxin (TTX) to prevent neuronal firing. Elevating [K+]o to 10.5 mM induced astrocyte swelling and produced significant increases in neuronal excitability in the form of mixed α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/N-methyl-D-aspartate (NMDA) receptor mEPSCs and NMDA receptor-dependent slow inward currents (SICs). Application of hyperosmolar artificial cerebrospinal fluid (ACSF) by addition of mannitol in the continued presence of 10.5 mM K+ forced shrinking of astrocytes and to a lesser extent neurons, which resisted swelling in ∧[K+]o. Cell shrinking and dilation of the ECS significantly dampened neuronal excitability in 10.5 mM K+. Subsequent removal of mannitol amplified effects on neuronal excitability and nearly doubled the volume increase in astrocytes, presumably due to continued glial uptake of K+ while mannitol was present. Slower, larger amplitude events mainly driven by NMDA receptors were abolished by mannitol-induced expansion of the ECS. Collectively, our findings suggest that cell volume regulation of the ECS in elevated [K+]o is driven predominantly by astrocytes, and that cell volume effects on neuronal excitability can be effectively isolated in elevated [K+]o conditions.

Perspectives on the basis of seizure-induced respiratory dysfunction

“…Although the relationship between sleep, sleep problems, and epilepsy have long been appreciated (Diaz-Negrillo, 2013;Wang et al, 2018), the basis for these associations is not clear. Based on evidence that regulation of the ECV is coupled to sleep-wake status (Ding et al, 2016) and decreased ECV positively correlates with neural activity (Walch et al, 2022), we speculate that disruption of sleep (as seen in Dravet syndrome; Kalume et al, 2015) will decrease ECV, lower seizure threshold and favor propagation of SD (Figure 2).…”

The Neurovascular Unit Dysfunction in the Molecular Mechanisms of Epileptogenesis and Targeted Therapy

“…The reduction of ESC increases extracellular ion concentration and strengthens tactile interactions between neurons, leading to more synchronous firing. Neurons are extremely sensitive to changes in ESC and osmotic pressure, and such abnormal conditions can lead to neuronal damage [ 55 ]. Consistently, an increased number of spontaneous seizures and prolonged duration of seizures are observed in AQP4 knockout mice [ 56 ].…”

Astrocyte regulation of extracellular space parameters across the sleep-wake cycle

“…Astrocyte volume change coincides with the maximum and minimum of sleep drive, where sleep pressure is also at its maximum and minimum, and state switches occur ( Figure 1A ). Regulation of CSF components, specifically K + , controls cerebral ECS volume and neuronal excitability ( Figure 1B ; Amzica et al, 2002 ; Rasmussen et al, 2019 ; Walch et al, 2022 ). ECS volume is inversely proportional to neuronal excitability such that expansion of the ECS, which ultimately decreases cortical tortuosity allowing for “washing” of ISF, is associated with the sleep state ( Xie et al, 2013 ; Tuura et al, 2021 ; Thomas, 2022 ).…”

“…This space then shrinks upon entering wakefulness ( Haj-Yasein et al, 2012 ; Ding et al, 2016 ; Sherpa et al, 2016 ). Astrocyte morphology and ex vivo experimentation favor astrocytes as a key contributor to ECS volume dynamics ( Lauderdale et al, 2015 ; Reed and Blazer-Yost, 2022 ; Walch et al, 2022 ). While cellular volume is not dichotomous, rather existing on a continuum, distinct stages of this spectrum may also occur within cortical states.…”

“…While local increases occur directly after neuronal activity ( Barron and Kim, 2019 ), arousal signals such as norepinephrine (NE) globally elevate extracellular K + in vivo through an independent pathway ( Ding et al, 2016 ; Rasmussen et al, 2019 ). Elevated extracellular K + produces intracellular K + accumulation generating osmotically driven astrocyte volume increase ( Walz, 2000 ; MacVicar et al, 2002 ; Walch et al, 2022 ). Evidence suggests this K + entrance into the cell is mainly via the NKA ( Wang et al, 2012 ; Larsen et al, 2014 ; Walch et al, 2020 ), which is enhanced by adrenergic stimulation ( Hertz et al, 2015 ), leading to intracellular K + accumulation and increased astrocytic volume during wakefulness.…”