Regional specificity of obesity-induced neurochemical modifications measured in vivo by proton MRS at 14.1 T
João M.N. Duarte1, Blanca Lizarbe1, Rolf Gruetter1,2,3, and Ana Francisca Soares1

1LIFMET, EPFL, Lausanne, Switzerland, 2UNIL, Lausanne, Switzerland, 3UNIGE, Geneva, Switzerland

Synopsis

Insulin resistance has deleterious effects on memory performance, brain morphology and the neurochemical profile of the cortex and hippocampus. We now investigated the neurochemical modifications in the hippocampus, cortex and hypothalamus of mice exposed to high-fat diet, a model of obesity-associated insulin resistance. In long-term high-fat diet-exposed mice, obesity-associated insulin resistance affects the neurochemical profiles of the hippocampus, cortex and hypothalamus in a region-specific manner.

PURPOSE

The increasing prevalence of metabolic disorders is associated to the consumption of highly palatable food products rich in calories. Metabolic disorders, particularly insulin-resistant diabetes mellitus, affect brain function and lead to cognitive impairment [1]. Recent work in an animal models of insulin resistance demonstrated its deleterious effect on memory performance, brain morphology and the neurochemical profile of the cortex and hippocampus [2-4]. Furthermore, a direct link between consumption of high-fat and/or high-sucrose containing diets and cognitive dysfunction has been reported [5-7]. We now investigated the neurochemical modifications in the hippocampus, cortex and hypothalamus of mice exposed to long-term hypercaloric high-fat diet (HFD), relative to mice exposed to a regular diet (RD).

METHODS

C57BL/6J mice (males; 3 months old) were fed either a HFD (60% kcal from fat, n=7) or a RD (10% kcal from fat, n=5) for 6 months. Metabolic dysfunction and insulin resistance were inferred from measurements of 6-hour fasting glycemia and insulinemia, and from a glucose tolerance test (1.5 g/kg glucose gavage). Brain function was assessed by measuring hippocampal-dependent spontaneous alternation in a Y-maze and exploratory behaviour in an open field arena [4]. Then, localized 1H MRS was performed on a 14.1 T, 26 cm VNMRS spectrometer using a home-built 8 mm diameter quadrature surface coil (used both for RF excitation and signal reception) as previously described [3]. C57BL/6 mice were anaesthetized under 1-2% isoflurane in 50% O2 in air. Briefly, field homogeneity was adjusted by FASTMAP, and 1H spectra were acquired from VOIs placed in the hippocampus, cortex and hypothalamus, using SPECIAL with TE of 2.8 ms and TR of 4 s [8], and metabolite concentrations were estimated with LCModel [9]. The entire neurochemical profile was analysed with ANOVA, followed by Student t-tests for paired comparisons.

RESULTS

Mice exposed to HFD displayed increased weight gain over the period of diet exposure (P<0.01), higher fasting glycemia (P<0.05) and insulinemia (P<0.01), and impaired glucose clearance in the glucose tolerance test (P<0.01), relative to controls. HFD affected mnemonic function, as revealed by a reduction of 10% in the hippocampal-dependent spontaneous alternation in the Y-maze (P<0.05). A neurochemical profile composed of 19 metabolites was determined in vivo in the mouse hippocampus, cortex, and hypothalamus (fig.1). In the hippocampus, HDF induced an increase in the concentrations of creatine (+17%, P<0.05) but not phosphocreatine, of GABA (+15%, P<0.01), of glutamine (+20%, P<0.05), of taurine (+14%, P<0.01), and of glucose (+159%, P<0.001). In the cortex, HDF induced an increase in the concentrations of creatine (+13%, P<0.05) accompanied by a reduction of phosphocreatine levels (-20%, P<0.05), and also a reduction in glycine content (-29%, P<0.01). In the hypothalamus, long-term HFD caused an increase in levels of creatine (+20%, P<0.05) but not phosphocreatine, of GABA (+16%, P<0.05), of myo-inositol (+21%, P<0.01), and of ascorbate (+106%, P<0.01), compared to controls.

DISCUSSION

The present results demonstrate that a mouse model of obesity-induced insulin-resistance that displays brain dysfunction, exhibits region-specific metabolic modifications in the brain. Due to higher glycaemia, compared to controls, glucose tended to be elevated in the brain HFD-exposed mice, as observed in other diabetes models [2,3], with exception of the hypothalamus, probably related to its glucose sensing function. Nevertheless, HFD-exposed mice displayed a global reduction of the ratio of phosphocreatine to creatine, suggesting reduced energy availability, which is in line with impaired mitochondrial function in the diabetic brain [1]. The hippocampus is importantly affected by insulin resistance [1]. In the particular case of this region, there was an elevation of taurine in HFD vs. RD. Diabetes-induced taurine elevations were reported in the hippocampus of other diabetes models [2,3], and could play a role in the osmotic adaptation to sustained high glucose levels upon uncontrolled diabetes. Notably, HFD was associated to high hypothalamic myo-inositol levels, which could be associated to the known inflammatory response of this region to excessive lipid exposure [10].

CONCLUSION

In HFD-exposed mice, obesity-associated insulin resistance affects the neurochemical profiles of the hippocampus, cortex and hypothalamus in a region-specific manner.

Acknowledgements

Supported by Swiss National Science Foundation (grant 148250) and Centre d’Imagerie BioMédicale (CIBM) of the UNIL, UNIGE, HUG, CHUV, EPFL and the Leenaards and Jeantet Foundations.

References

[1] Duarte (2015) Aging Dis 6(5):304; [2] Duarte & Gruetter (2014) Diabetologia 57:s276; [3] Duarte et al. (2009) J Neurochem 111(2):368; [4] Duarte et al. (2012) Plos One 7(4):e21899; [5] Calvo-Ochoa et al., JCBFM 34:1001, 2014; [6] Moy & McNay, Physiol Behav 109:69, 2013; [7] Soares et al., Neurosci 250:565, 2013; [8] Mlynárik et al. (2006) Mag Reson Med 56, 965.; [9] Provencher (1993) Mag Reson Med 30:672; [10] Thaler et al. (2012) J. Clin. Invest 122(1):153.

Figures

Fig.1: neurochemical profiles in the hippocampus, cortex, and hypothalamus of mice exposed of 6 months of high-fat (HFD) or regular (RD) diets. Data is shown as mean and SEM. * P<0.05, **P<0.01 for HFD vs. RD.



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