Compartmentalised energy metabolism supporting glutamatergic neurotransmission in response to increased activity in the rat cerebral cortex: A 13C MRS study in vivo at 14.1 T

Many tissues exhibit metabolic compartmentation. In the brain, while there is no doubt on the importance of functional compartmentation between neurons and glial cells, there is still debate on the specific regulation of pathways of energy metabolism at different activity levels. Using C-13 magnetic resonance spectroscopy (MRS) in vivo, we determined fluxes of energy metabolism in the rat cortex under alpha-chloralose anaesthesia at rest and during electrical stimulation of the paws. Compared to resting metabolism, the stimulated rat cortex exhibited increased glutamate-glutamine cycle (+67 nmol/g/min,+95%, P< 0.001) and tricarboxylic (TCA) cycle rate in both neurons (+62 nmol/g/min,+12%, P< 0.001) and astrocytes (+68 nmol/g/min,+22%, P = 0.072). A minor, non-significant modification of the flux through pyruvate carboxylase was observed during stimulation (+5 nmol/g/min,+8%). Altogether, this increase in metabolism amounted to a 15% (67 nmol/g/min, P< 0.001) increase in CMRglc(ox), i. e. the oxidative fraction of the cerebral metabolic rate of glucose. In conclusion, stimulation of the glutamate-glutamine cycle under alpha-chloralose anaesthesia is associated to similar enhancement of neuronal and glial oxidative metabolism.