Malfunction of astrocytic K+ regulation contributes to the breakdown of extracellular K+ homeostasis during ischemia and spreading depolarization events. Studying astroglial K+ changes is, however, hampered by a lack of suitable techniques. Here, we combined results from fluorescence imaging, ion-selective microelectrodes, and patch-clamp recordings in murine neocortical slices with the calculation of astrocytic [K+]. Brief chemical ischemia caused a reversible ATP reduction and a transient depolarization of astrocytes. Moreover, astrocytic [Na+] increased by 24 mM and extracellular [Na+] decreased. Extracellular [K+] increased, followed by an undershoot during recovery. Feeding these data into the Goldman–Hodgkin–Katz equation revealed a baseline astroglial [K+] of 146 mM, an initial K+ loss by 43 mM upon chemical ischemia, and a transient K+ overshoot of 16 mM during recovery. It also disclosed a biphasic mismatch in astrocytic Na+/K+ balance, which was initially ameliorated, but later aggravated by accompanying changes in pH and bicarbonate, respectively. Altogether, our study predicts a loss of K+ from astrocytes upon chemical ischemia followed by a net gain. The overshooting K+ uptake will promote low extracellular K+ during recovery, likely exerting a neuroprotective effect. The resulting late cation/anion imbalance requires additional efflux of cations and/or influx of anions, the latter eventually driving delayed astrocyte swelling.

Recent work has established that glutamatergic synaptic activity induces transient sodium elevations in grey matter astrocytes by stimulating glutamate transporter 1 (GLT-1) and glutamate-aspartate transporter (GLAST). Glial sodium transients have diverse functional consequences but are largely unexplored in white matter. Here, we employed ratiometric imaging to analyse sodium signalling in macroglial cells of mouse corpus callosum. Electrical stimulation resulted in robust sodium transients in astrocytes, oligodendrocytes and NG2 glia, which were blocked by tetrodotoxin, demonstrating their dependence on axonal action potentials (APs). Action potential-induced sodium increases were strongly reduced by combined inhibition of ionotropic glutamate receptors and glutamate transporters, indicating that they are related to release of glutamate. While AMPA receptors were involved in sodium influx into all cell types, oligodendrocytes and NG2 glia showed an additional contribution of NMDA receptors. The transporter subtypes GLT-1 and GLAST were detected at the protein level and contributed to glutamate-induced glial sodium signals, indicating that both are functionally relevant for glutamate clearance in corpus callosum. In summary, our results demonstrate that white matter macroglial cells experience sodium influx through ionotropic glutamate receptors and glutamate uptake upon AP generation. Activity-induced glial sodium signalling may thus contribute to the communication between active axons and macroglial cells.

support-information-section).

Emerging evidence indicates that neuronal activity-evoked changes in sodium concentration in astrocytes Naa represent a special form of excitability, which is tightly linked to all other major ions in the astrocyte and extracellular space, as well as to bioenergetics, neurotransmitter uptake, and neurovascular coupling. Recently, one of us reported that Naa transients in the neocortex have a significantly higher amplitude than those in the hippocampus. Based on the extensive data from that study, here we develop a detailed biophysical model to further understand the origin of this heterogeneity and how it affects bioenergetics in the astrocytes. In addition to closely fitting the observed experimental Naa changes under different conditions, our model shows that the heterogeneity in Naa signaling leads to substantial differences in the dynamics of astrocytic Ca2+ signals in the two brain regions, and leaves cortical astrocytes more susceptible to Na+ and Ca2+ overload under metabolic stress. The model also predicts that activity-evoked Naa transients result in significantly larger ATP consumption in cortical astrocytes than in the hippocampus. The difference in ATP consumption is mainly due to the different expression levels of NMDA receptors in the two regions. We confirm predictions from our model experimentally by fluorescence-based measurement of glutamate-induced changes in ATP levels in neocortical and hippocampal astrocytes in the absence and presence of the NMDA receptor's antagonist (2R)-amino-5-phosphonovaleric acid.

No abstract