Excitatory/inhibitory neuronal metabolic balance in mouse hippocampus upon infusion of [U-¹³C₆]glucose

Abstract

Hippocampus plays a critical role in linking brain energetics and behavior typically associated to stress exposure. In this study, we aimed to simultaneously assess excitatory and inhibitory neuronal metabolism in mouse hippocampus in vivo by applying 18FDG-PET and indirect 13C magnetic resonance spectroscopy (1H-[13C]-MRS) at 14.1 T upon infusion of uniformly 13C-labeled glucose ([U-13C6]Glc). Improving the spectral fitting by taking into account variable decoupling efficiencies of [U-13C6]Glc and refining the compartmentalized model by including two γ-aminobutyric acid (GABA) pools permit us to evaluate the relative contributions of glutamatergic and GABAergic metabolism to total hippocampal neuroenergetics. We report that GABAergic activity accounts for ∼13% of total neurotransmission (VNT) and ∼27% of total neuronal TCA cycle (VTCA) in mouse hippocampus suggesting a higher VTCA/VNT ratio for inhibitory neurons compared to excitatory neurons. Finally, our results provide new strategies and tools for bringing forward the developments and applications of 13C-MRS in specific brain regions of small animals.

In Vivo Metabolism of [1,6-13C2]Glucose Reveals Distinct Neuroenergetic Functionality between Mouse Hippocampus and Hypothalamus

“…As such, by using additional information from plasma measurements, with interpolation of minimal time-points, we have been able to assess CMR Glc , V dil in and V dil out . We have previously reported that the inclusion of a dilution flux (V ex i ), that corresponds to the labeling exchange between two distinct GABA pools, provides a more accurate means of fitting the GABA labeling curves [ 20 ]. As such, inclusion of this flux in the GABAergic model would allow us to assess hypothalamic GABAergic metabolism in a more detailed way than previously described [ 19 ].…”

“…In particular, carbon-13 ( 13 C) MRS allows the investigation of metabolism with cell-specific information through dynamic assessment of metabolic fluxes in vivo upon administration of a 13 C-labeled substrate [ 17 , 18 ]. We have previously shown that studying region-specific brain metabolism of mouse was feasible at ultra-high field using indirect carbon-13 MRS ( 1 H-[ 13 C]-MRS) with 13 C-labeled substrates such as [1,6- 13 C 2 ]glucose ([1,6- 13 C 2 ]Glc) [ 16 , 19 ] or [U- 13 C 6 ]glucose ([U- 13 C 6 ]Glc) [ 20 ]. Because variations in the acquisition protocol or mathematical modeling of brain metabolic fluxes can lead to slight variations in the results that render region-to-region statistical comparisons challenging, we sought of evaluating hippocampus and hypothalamus metabolism under similar experimental and analytical conditions.…”

Deletion ofCrtc1leads to hippocampal neuroenergetic impairments associated with depressive-like behavior

“…In vivo 18 FDG positron emission tomography ( 18 FDG-PET) Dynamic non-invasive fluorodeoxyglucose positron emission tomography ( 18 FDG-PET) was performed as described previously 59,92 . Briefly, mice under 1-2% (vol/vol) isoflurane anesthesia in O2 were positioned in the scanner after tail vein cannulation and remained monitored for temperature and breathing rate throughout the experiment.…”

“…The VOI included the bilateral dorsal hippocampus (2x5.5x1.5 mm 3 ) and led to a typical water linewidth of 20±1Hz after field homogeneity adjustment. 1 H-[ 13 C]-MRS spectra were acquired using the full intensity SPECIAL-BISEP sequence (TE=2.8ms, TR=4000ms, averages=8) as previously described 59,95,96 . The non-edited (proton, 1 H) and inverted spectra (editing OFF and ON) were obtained using an interleaved acquisition and were subtracted in the post processing steps to obtain the edited spectra (protons bound to carbon 13, 1 H- for wild-type and Crtc1 -/respectively, mean±s.e.m).…”

“…For both models, the cerebral metabolic rate of glucose (CMRGlc) was set to the value obtained in the same voxel from the 18 and from blood acetate (Vdil g ); the pyruvate dehydrogenase activity of excitatory (VPDH e ) and inhibitory (VPDH i ) neurons; a transmitochondrial flux for excitatory (Vx e ) and inhibitory (Vx i ) neurons; a neurotransmission flux for excitatory (VNT e ) and inhibitory (VNT i ) neurons; glutamate decarboxylase activity (VGAD); and two exchange fluxes between two Gln or two GABA pools (Vex g and Vex i ). Values of pyruvate carboxylase activity (VPC), glial tricarboxylic acid cycle (Vg) and glial transmitochondrial flux (Vx g ) were fixed to known values and glial Gln efflux (Veff) was set equal to VPC, as described in Cherix et al 59 . The other parameters were calculated from the estimated fluxes through mass-balance equations, assuming metabolic steady-state (i.e.…”