The potassium ion, K, a neuronal signal that is released during excitatory synaptic activity, produces acute activation of glucose consumption in cultured astrocytes, a phenomenon mediated by the sodium bicarbonate cotransporter NBCe1 ( SLC4A4). We have explored here the relevance of this mechanism in brain tissue by imaging the effect of neuronal activity on pH, glucose, pyruvate and lactate dynamics in hippocampal astrocytes using BCECF and FRET nanosensors. Electrical stimulation of Schaffer collaterals produced fast activation of glucose consumption in astrocytes with a parallel increase in intracellular pyruvate and biphasic changes in lactate . These responses were blocked by TTX and were absent in tissue slices prepared from NBCe1-KO mice. Direct depolarization of astrocytes with elevated extracellular K or Ba mimicked the metabolic effects of electrical stimulation. We conclude that the glycolytic pathway of astrocytes in situ is acutely sensitive to neuronal activity, and that extracellular K and the NBCe1 cotransporter are involved in metabolic crosstalk between neurons and astrocytes. Glycolytic activation of astrocytes in response to neuronal K helps to provide an adequate supply of lactate, a metabolite that is released by astrocytes and which acts as neuronal fuel and an intercellular signal.

Glycolysis is the primary step for major energy production in the cell. There is strong evidence suggesting that glucose consumption and rate of glycolysis are highly modulated by cytosolic pH The high energy requirement of the mammalian brain is primarily fueled by the degradation of blood-derived glucose. Astrocytes, the major glial cells in the brain, take up a large portion of blood glucose, and by aerobic glycolysis, they preferably produce more lactate (1, 2). Accumulating evidence suggests that astrocytes metabolically support neurons by releasing lactate, which then could be taken up by neighboring neurons (3, 4). Therefore, glucose metabolism in astrocytes was proposed to be modulated by various means of signals presumably originating from neurons (5, 6). It is well documented that astrocytes respond to neuronal activity with an intracellular alkalinization (7-9). Recently, it was shown that the glycolytic rate in astrocytes can be significantly enhanced by an alkaline pH i shift derived from extracellular K ϩ -induced membrane depolarization enhancing bicarbonate uptake via the electrogenic sodium bicarbonate cotransporter NBCe1 (10), which is highly expressed in mouse cortical astrocytes (11). In fact, cytoplasmic pH is well known to be a potential regulator of glucose metabolism, particularly the glycolytic rate, in various cell types (12)(13)(14)(15) ResultsWe have approached the question whether glucose consumption and glycolysis are stimulated by a fall in [H Ϫ transport into and out of astrocytes, we employed a genetic mouse model with a deletion in the gene for the electrogenic sodium bicarbonate cotransporter NBCe1 (SLC4A4) (21), which is highly expressed in mouse cortical astrocytes (11,22