Fig. 2.

In vivo ¹³C MRS experiments reporting rates of neurotransmitter cycling [V_cyc(tot)] and glucose oxidation in neurons [CMR_glc(ox),N] and astrocytes [CMR_glc(ox),A]. (A) Values of V_cyc(tot) and CMR_glc(ox),N for rat (red circles) and human (blue triangles) cerebral cortex. The rat data represented many activity levels in the somatosensory cortex, whereas the human data were from the awake resting state in the visual cortex with varying degrees of gray vs. white matter partial volume (Table S3). Linear trends between changes in V_cyc(tot) and CMR_glc(ox),N suggest strong coupling between neurotransmitter activity and energy metabolism [R² = 0.92, where CMR_glc(ox),N = 0.87 V_cyc(tot) + 0.10], where the finite intercept ranging between 0.05 and 0.15 μmol/g per minute indicates energy consumption for nonsignaling conditions. (B) Ratios of V_cyc(tot)/CMR_glc(ox),N in the nonanesthetized awake resting state (filled bars) (Table S3) and CMR_glc(ox),A/CMR_glc(ox) for all conditions above isoelectricity (open bars) (Table S4) in rat (red), human (blue), and overall (purple). Similarity between the V_cyc(tot)/CMR_glc(ox),N ratios (filled bars) in rat and human suggests that the relationship between V_cyc(tot) and CMR_glc(ox),N in both species may be conserved. Likewise, correspondence between the CMR_glc(ox),A/CMR_glc(ox) ratios (open bars) suggests that the relationship between CMR_glc(ox),A and CMR_glc(ox) across species could be similar.