Background and Purpose
Glucose has been thought as the principal metabolic substrate of the brain. However, Magestritti et al. proposed an astrocyte-neuron lactate shuttle hypothesis, which claims the metabolic role of lactate in the brain, especially in neurons 1 . This model hypothesizes that neuronal activation stimulates the anaerobic production of lactate in neighboring astrocytes. The produced lactate in astrocytes is then transferred to the neuron, which then uses the lactate as an energy substrate aerobically. However, this hypothesis has not been clearly elucidated yet. Therefore, in this study, the role of lactate was evaluated by “Bioradiography” system with 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG.), which is a positron emitting radiotracer for glucose metabolism measurement. “Bioradiography” is the dynamic living brain slice imaging system for positron-emitter labeled compounds. We investigated the brain energy metabolism under steady state and neural activated conditions.
Methods
The “Bioradiography” was performed according to the method of Murata et al. 2 . Two groups of rat brain slices were incubated with [18F]FDG in oxygenated Krebs-Ringer solution (including 10 mM glucose) for 150 min. To the one group, 0.5 mM of lactate transporter inhibitor, alpha-cyano-4-hydroxycinnamate (4-CIN), was loaded (4-CIN group, n=8), and to the other group, this inhibitor was not added (control group, n=7). 4-CIN inhibits the lactate transport to neuron via monocarboxylate transporter-2 (MCT2). After 60 min incubation, 50 mM KCl was added to both groups to activate the neuronal metabolism. During the incubation, serial [18F]FDG images of the slices were obtained on imaging plates at every 10 min. [18F]FDG uptake rate (R) was obtained from the slope of time activity curve. The lactate concentration of the incubation medium was also measured in every 20 min.
Results
[18F]FDG uptake increased with time in both 4-CIN added and control conditions. No significant difference was observed in the [18F]FDG uptake rate between both conditions before KCl addition (R=0.013 min−1 for both 4-CIN and control group). These observations suggest that neurons do not use exogenous lactate as a dominant energy substrate in steady state. On the other hand, KCl elevated the uptake rate in both 4-CIN and control group, but increased extent was significantly larger for the 4-CIN added condition (R=0.075 min−1 for 4-CIN and 0.040 min−1 for control group). This indicates that activated neuron consume lactate for energy substrate. The lactate concentration in the incubation medium was increased with KCl treatment in both groups and the extent was slightly greater in 4-CIN group.
Conclusion
These results suggested that:
The brain mainly uses glucose, not lactate, as an energy substrate in steady state. When neuron is stimulated, excess amounts of lactate are produced in astrocytes and the lactate is mobilized as an energy substrate.