During periods of increased neuronal activity, the brain's energy demand rises substantially. In addition to D-glucose, L-lactate is an important energy substrate for the brain. Astrocytes, a type of glial cell, support neurons with energy substrates, including L-lactate. In astrocytes, L-lactate is produced from D-glucose through aerobic glycolysis, a process that occurs despite sufficient oxygen availability. The resulting L-lactate is then transported from astrocytes to neurons via specific transporters, where L-lactate is further metabolized through oxidative phosphorylation for energy production. Brain metabolism can be regulated through activation of G protein-coupled receptors (GPCRs) on brain cells. Noradrenaline, a neuromodulator that binds to adrenergic receptors, which are GPCRs, plays an important role in regulating brain metabolism. Activation of adrenergic receptors in astrocytes triggers intracellular signalling pathways and metabolic responses such as facilitation of aerobic glycolysis, production and release of L-lactate. L-lactate can then enter neurons for energy production or acts extracellularly as a signalling molecule, enhancing aerobic glycolysis through its interaction with lactate sensitive GPCRs in astrocytes and perhaps also neurons. It is not yet clear whether activation of other GPCR types can also lead to aerobic glycolysis in astrocytes. Moreover, it remains unknown whether aerobic glycolysis can also be triggered in neurons through GPCR signaling.
To study how GPCR activation affects neuronal and astrocytic aerobic glycolysis in the context of the whole brain, we used the fruit fly Drosophila melanogaster as a model organism. The fruit fly is a valuable model for investigating brain metabolism, as the fundamental mechanisms of nervous system function are evolutionarily conserved between fruit flies and mammals. We selectively expressed the genetically encoded FRET-based lactate sensor Laconic in fruit fly neurons and astrocytes. Intracellular L-lactate dynamics were then monitored in the optic lobes of fruit fly brain using real-time confocal microscopy upon application of different GPCR agonists, including octopamine (homologue of mammalian noradrenaline), dopamine, acetylcholine, and Smart 009. By exposing brains to elevated extracellular L-lactate levels we also assessed whether L-lactate can enter brain cells.
Stimulation of fruit fly brains with GPCR agonists octopamine, dopamine, and acetylcholine led to a statistically significant increase in cytosolic L-lactate levels in neurons, indicating facilitation of aerobic glycolysis. Smart 009 did not induce significant changes in L-lactate levels in neurons. In astrocytes only octopamine triggered a significant increase in cytosolic L-lactate. Both neurons and astrocytes effectively took up extracellular L-lactate, with astrocytes being more effective. We conclude that in the optic lobes of the fruit fly brain, neurons, rather than astrocytes, are the primary site of GPCR-mediated aerobic glycolysis.
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