NH₄⁺ triggers the release of astrocytic lactate via mitochondrial pyruvate shunting

Neural activity is accompanied by a transient mismatch between local glucose and oxygen metabolism, a phenomenon of physiological and pathophysiological importance termed aerobic glycolysis. Previous studies have proposed glutamate and K⁺ as the neuronal signals that trigger aerobic glycolysis in astrocytes. Here we used a panel of genetically encoded FRET sensors in vitro and in vivo to investigate the participation of NH₄⁺, a by-product of catabolism that is also released by active neurons. Astrocytes in mixed cortical cultures responded to physiological levels of NH₄⁺ with an acute rise in cytosolic lactate followed by lactate release into the extracellular space, as detected by a lactate-sniffer. An acute increase in astrocytic lactate was also observed in acute hippocampal slices exposed to NH₄⁺ and the somatosensory cortex of anesthetized mice in response to i.v. NH₄⁺. Unexpectedly, NH₄⁺had no effect on astrocytic glucose consumption. Parallel measurements showed simultaneous cytosolic pyruvate accumulation and NADH depletion, suggesting the involvement of mitochondria. An inhibitor-stop technique confirmed a strong inhibition of mitochondrial pyruvate uptake that can be explained by mitochondrial matrix acidification. These results show that physiological NH₄⁺diverts the flux of pyruvate from mitochondria to lactate production and release. Considering that NH₄⁺ is produced stoichiometrically with glutamate during excitatory neurotransmission, we propose that NH₄⁺ behaves as an intercellular signal and that pyruvate shunting contributes to aerobic lactate production by astrocytes.