Significance: A near one-to-one relationship had previously been observed between increments in the fluxes of the glutamate−glutamine neurotransmitter cycle and neuronal glucose oxidation in the tricarboxylic acid (TCA) cycle. This flux relationship was consistent with a hypothesized mechanism involving glycolytic ATP in astrocytes and astrocyte-to-neuron lactate shuttling. Here, 2-fluoro-2-deoxy-d-glucose was used to evaluate the glucose flux through glycolysis and the TCA cycle in nerve terminals isolated from the brains of rats under baseline and high-activity conditions. In a direct contradiction of this hypothesis, the results show that nerve terminals metabolize significant amounts of glucose.

Major depressive disorder (MDD) is a leading cause of distress, disability, and suicides. As per the latest WHO report, MDD affects more than 260 million people worldwide. Despite decades of research, the underlying etiology of depression is not fully understood. Glutamate and γ-aminobutyric acid (GABA) are the major excitatory and inhibitory neurotransmitters, respectively, in the matured central nervous system. Imbalance in the levels of these neurotransmitters has been implicated in different neurological and psychiatric disorders including MDD.¹H nuclear magnetic resonance (NMR) spectroscopy is a powerful non-invasive method to study neurometabolites homeostasisin vivo. Additionally,¹³C-NMR spectroscopy together with an intravenous administration of non-radioactive¹³C-labeled glucose or acetate provides a measure of neural functions. In this review, we provide an overview of NMR-based measurements of glutamate and GABA homeostasis, neurometabolic activity, and neurotransmitter cycling in MDD. Finally, we highlight the impact of recent advancements in treatment strategies against a depressive disorder that target glutamate and GABA pathways in the brain.

Acetate is a well-known astrocyte-specific substrate that has been used extensively to probe astrocytic function in vitro and in vivo. Analysis of amino acid turnover curves from¹³C-acetate has been limited mainly for estimation of first-order rate constants from exponential fitting or calculation of relative rates from steady-state¹³C enrichments. In this study, we used¹H-[¹³C]-Nuclear Magnetic Resonance spectroscopy with intravenous infusion of [2-¹³C]acetate-Na⁺in vivo to measure the cerebral kinetics of acetate transport and utilization in anesthetized rats. Kinetics were assessed using a two-compartment (neuron/astrocyte) analysis of the¹³C turnover curves of glutamate-C4 and glutamine-C4 from [2-¹³C]acetate-Na⁺, brain acetate levels, and the dependence of steady-state glutamine-C4 enrichment on blood acetate levels. The steady-state enrichment of glutamine-C4 increased with blood acetate concentration until 90% of plateau for plasma acetate of 4 to 5 mmol/L. Analysis assuming reversible, symmetric Michaelis–Menten kinetics for transport yielded 27±2mmol/L and 1.3±0.3 μmol/g/min for K_tand T_max, respectively, and for utilization, 0.17±0.24 mmol/L and 0.14±0.02 μmol/g/min for K_{M_util}and V_{max_util}, respectively. The distribution space for acetate was only 0.32±0.12 mL/g, indicative of a large excluded volume. The astrocytic and neuronal tricarboxylic acid cycle fluxes were 0.37±0.03 μmol/g/min and 1.41±0.11 μmol/g/min, respectively; astrocytes thus comprised ∼21%±3% of total oxidative metabolism.

The ¹³C nuclear magnetic resonance (NMR) studies together with the infusion of ¹³C-labeled substrates in rats and humans have provided important insight into brain energy metabolism. In the present study, we have extended a three-compartment metabolic model in mouse to investigate glutamatergic and GABAergic tricarboxylic acid (TCA) cycle and neurotransmitter cycle fluxes across different regions of the brain. The ¹³C turnover of amino acids from [1,6-¹³C₂]glucose was monitored ex vivo using ^qH-[¹³C]-NMR spectroscopy. The astroglial glutamate pool size, one of the important parameters of the model, was estimated by a short infusion of [2-¹³C]acetate. The ratio V_cyc/V_TCA was calculated from the steady-state acetate experiment. The ¹³C turnover curves of [4-¹³C]/[3-¹³C]glutamate, [4-¹³C]glutamine, [2-¹³C]/[3-¹³C]GABA, and [3-¹³C]aspartate from [1,6-¹³C₂]glucose were analyzed using a three-compartment metabolic model to estimate the rates of the TCA cycle and neurotransmitter cycle associated with glutamatergic and GABAergic neurons. The glutamatergic TCA cycle rate was found to be highest in the cerebral cortex (0.91±0.05 μmol/g per minute) and least in the hippocampal region (0.64±0.07 μmol/g per minute) of the mouse brain. In contrast, the GABAergic TCA cycle flux was found to be highest in the thalamus-hypothalamus (0.28±0.01 μmol/g per minute) and least in the cerebral cortex (0.24±0.02 μmol/g per minute). These findings indicate that the energetics of excitatory and inhibitory function is distinct across the mouse brain.