Energy metabolism in the rat cortex under thiopental anaesthesia measured in vivo by 13C MRS at 14.1T
Sarah Sonnay1, Nathalie Just2, Rolf Gruetter3, and João M.N. Duarte4

1Laboratory of Functional and Metabolic Imaging (LIFMET), Ecole Polythechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland, 2Universitätsklinikum Münster, Münster, Germany, 3Center for Biomedical Imaging (CIBM) and Laboratory of Functional and Metabolic Imaging (LIFMET), Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland, 4Laboratory of Functional and Metabolic Imaging (LIFMET), Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

Synopsis

Cerebral function relies on cooperative interaction between neuronal and glial cells. Anaesthetics modulate basal neuronal activity, and therefore the so-called neurometabolic coupling, by targeting different receptors, such as γ-aminobutyric acid type A (GABAA) receptors, in the case of thiopental or α-chloralose anaesthesia. We investigated cortical metabolism in vivo using 13C magnetic resonance spectroscopy (MRS) during infusion of [1,6-13C]glucose under thiopental anesthesia. Data indicate glycolysis inhibition, decreased mitochondrial oxidative metabolism and possible oxidation of three-carbon molecules, namely lactate, which plasma concentration increased two-fold (compared to α-chloralose anaesthesia).

Introduction

Brain metabolism relies on synchronized interaction between neurons and astrocytes [1]. Anaesthetics modulate basal neuronal activity, and therefore the so-called neurometabolic coupling, by acting on different molecular targets, such as γ-aminobutyric acid type A (GABAA) receptors, in the case of α-chloralose [2] or thiopental [3] anaesthesia. The aim of this study was to characterize neuronal and astrocytic pathways of energy metabolism in the rat cortex under thiopental anesthesia in vivo by direct 13C MRS.

Materials and Methods

Sprague Dawley rats (n=7, 267±23 g) under thiopental (50 mg/kg bolus followed by a continuous intravenous infusion rate of 90 mg/kg/h) anesthesia were stereotaxically fixed and positioned in a homebuilt holder. MRS experiments were performed on a 14.1 T/26 cm horizontal bore magnet with a homebuilt coil consisting of a 1H quadrature surface coil combined to a 13C linearly polarized surface coil. After FAST(EST)MAP shimming, localized 1H and 13C spectra (during infusion of [1,6-13C]glucose [1]) were acquired from a 94 μL volume in the cortex with STEAM and semi-adiabatic distortionless enhancement by polarization transfer (DEPT) combined with 3D-ISIS for 1H localization [4], respectively. LCModel was used for analysis of both 1H and 13C spectra [5]. The scaling of 13C fractional enrichment (FE) curves was based on MRS of brain extracts [1]. Data was fitted to a two-compartment model and variance of parameters was determined by Monte-Carlo analyses [6].

Results

The administration of [1,6-13C]glucose under thiopental anaesthesia increased plasma lactate concentration by 104% compared to experiments under α-chloralose anaesthesia [1,7]. After 4 hours of [1,6-13C]glucose infusion, plasma and brain lactate FE was increased by 36% and 33%, respectively, in the thiopental compared to α-chloralose anaesthesia. However, at the end of the experiment, FE of glutamate and glutamine C4, C3 and C2, and aspartate C3 and C2 was similar under both types of anaesthesia, as measured ex vivo from the brain extracts. Glucose transport analysis resulted in an apparent Michaelis constant (Kt) of 4.5±8.1 mM, in an apparent maximum transport (Tmax) of 2.6±0.6 µmol/g/min and in a cerebral metabolic rate (CMRglc) of 0.26±0.09 µmol/g/min. Under thiopental anaesthesia, the rate of glutamatergic neurotransmission i.e. the glutamate-glutamine cycle (VNT) was 0.06±0.01 µmol/g/min and glutamine synthetase (VGS) was 0.12±0.02 µmol/g/min. The flux through the tricarboxylic acid cycle in astrocytes (VTCAg) and neurons (VTCAn) were 0.27±0.03 µmol/g/min and 0.52±0.02 µmol/g/min, respectively, resulting in a total cerebral metabolic rate of glucose (CMRglc(ox)) of 0.42±0.02 µmol/g/min.

Discussion

Thiopental data from 13C MRS in vivo indicate glycolysis reduction as well as decreased CMRglc(ox), although still substantial as indicated by similar amino acids FE at the end of the experiment. The mismatch between glycolytic flux and global mitochondrial oxidative metabolism, depicted by CMRglc(ox), suggests that other substrates are substantially oxidized, probably lactate that was increased two-fold in the plasma of rats anaesthetised with thiopental, relative to experiments under α-chloralose anaesthesia [1,7].

Conclusion

Preliminary metabolic flux analysis of brain metabolism under thiopental anaesthesia assessed in vivo using 13C MRS suggests that neuron-glia interactions are similar to those under α-chloralose anaesthesia. However, the reduction of glycolysis under thiopental anaesthesia implies a substantial utilisation of substrates other than glucose. Since lactate concentration was high, it may be the carbon source that complements glucose in sustaining cortical oxidative metabolism under thiopental anaesthesia.

Acknowledgements

This work was supported by National Competence Center in Biomedical Imaging, Swiss National Science Foundation, and Centre d’Imagerie BioMédicale of the UNIL, UNIGE, HUG, CHUV, EPFL, the Leenaards and Jeantet Foundations.

References

[1] Duarte et al. (2011) Front Neuroenergetics 3:3 ; [2] Garrett & Gan (1998) J. Pharmacol. Exp. Ther. ; [3] Rho et al, (1996) J. Physiol. (Lond.) ; [4] Henry et al. (2003) MRM 50:684 ; [5] Henry et al. (2003) NMR Biomed 16:400 ; [6] Duarte et al, (2014) J Neurosci Res ; [7] Sonnay et al, ISMRM 2015 abstract #4627.

Figures

Figure 1 : (A) Anatomical images showing the localization of the voxel of interest (94 µl) in the cortex. (B) Typical in vivo 13C MRS spectra acquired in the cortex during [1,6-13C]glucose infusion under thiopental anesthesia. (C) Turnover curves of aliphatic carbons of glutamate, glutamine and aspartate and their respective best fits.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
3968