Activity‐dependent astrocyte swelling is mediated by pH‐regulating mechanisms

Abstract

During neuronal activity in the mammalian brain, the K released into the synaptic space is initially buffered by the astrocytic compartment. In parallel, the extracellular space (ECS) shrinks, presumably due to astrocytic cell swelling. With the Na /K /2Cl cotransporter and the Kir4.1/AQP4 complex not required for the astrocytic cell swelling in the hippocampus, the molecular mechanisms underlying the activity-dependent ECS shrinkage have remained unresolved. To identify these molecular mechanisms, we employed ion-sensitive microelectrodes to measure changes in ECS, [K ] and [H ] /pH during electrical stimulation of rat hippocampal slices. Transporters and receptors responding directly to the K and glutamate released into the extracellular space (the K /Cl cotransporter, KCC, glutamate transporters and G protein-coupled receptors) did not modulate the extracellular space dynamics. The HCO3--transporting mechanism, which in astrocytes mainly constitutes the electrogenic Na / HCO3- cotransporter 1 (NBCe1), is activated by the K -mediated depolarization of the astrocytic membrane. Inhibition of this transporter reduced the ECS shrinkage by ∼25% without affecting the K transients, pointing to NBCe1 as a key contributor to the stimulus-induced astrocytic cell swelling. Inhibition of the monocarboxylate cotransporters (MCT), like-wise, reduced the ECS shrinkage by ∼25% without compromising the K transients. Isosmotic reduction of extracellular Cl revealed a requirement for this ion in parts of the ECS shrinkage. Taken together, the stimulus-evoked astrocytic cell swelling does not appear to occur as a direct effect of the K clearance, as earlier proposed, but partly via the pH-regulating transport mechanisms activated by the K -induced astrocytic depolarization and the activity-dependent metabolism.

Cell Volume Control in Healthy Brain and Neuropathologies

“…Fluctuations of TMA + /TEA + concentrations under steady-state conditions reflects changes in the volume of the interstitial liquid. Using this type of assay, several groups found that electrical stimulation of neurons causes significant reductions in the extracellular space (cell swelling) in brain preparations from lower vertebrates and mammals [see for example (Svoboda & Sykova, 1991; Jing, Aitken, & Somjen, 1994; Larsen et al, 2014; Larsen & MacAulay, 2017) and comprehensive review by (Sykova & Nicholson, 2008)].…”

“…12.3C). Moderate elevations in [K + ] o during physiological excitation are thought to be the key reason for the dynamic changes in astrocyte volume in healthy brain tissue [section 12.4, but see alternative view in (Larsen & MacAulay, 2017)]. Work in astrocyte cultures suggests that supraphysiologic increases in [K + ] o (up to 10–15 mM) trigger cell swelling via stimulation of the bumetanide-sensitive NKCC1 (Larsen et al, 2014).…”

“…Recent studies from the N. MacAulay laboratory (Larsen et al, 2014; Larsen & MacAulay, 2017), proposed alternative mechanisms of astrocytic swelling in hippocampal slices. They found that electrical stimulation of neuronal networks causes increases in intracellular [K + ] and shrinkage of the interstitial space, reflecting cellular (astrocyte?)…”

“…In brain slices, cell swelling was largely insensitive to the inhibitors of glutamate transporters (TBOA), K + channels (Ba 2+ ), and NKKC1 (bumetanide). Instead, it was lessened by the non-selective inhibitor of the Na + -bicarbonate cotransporter NBCe1, DIDS, and poorly selective monocarboxylate transporter blocker, α-cyano-4-hydroxycinnamate (Larsen & MacAulay, 2017), suggesting the contribution of these two latter transport mechanisms. Although this work remains to be validated using more selective molecular biology and genetics tools, the underlying ideas are supported by the literature.…”

Brain active transmembrane water cycling measured by MR is associated with neuronal activity

“…However, simultaneous changes (e.g., in cell morphology and volume) likely occur for any perturbation of membrane water transport. Experiments with long‐term neuronal activity suppression (AP 5,o plus DNQX o , TTX o , and low [ K o + ]) showed significant cell volume reduction, which might be caused by the reduction in neuronal and glial cell swelling associated with neuronal activity . From Equation , k io (p) is inversely proportional to cell diameter (assuming the cell shape is little changed and P W (p) is constant).…”