Effects of Lipoic Acid on High-Fat Diet-Induced Alteration of Synaptic Plasticity and Brain Glucose Metabolism: A PET/CT and 13C-NMR Study

Abstract

High-fat diet (HFD)-induced obesity is accompanied by insulin resistance and compromised brain synaptic plasticity through the impairment of insulin-sensitive pathways regulating neuronal survival, learning, and memory. Lipoic acid is known to modulate the redox status of the cell and has insulin mimetic effects. This study was aimed at determining the effects of dietary administration of lipoic acid on a HFD-induced obesity model in terms of (a) insulin signaling, (b) brain glucose uptake and neuronal- and astrocytic metabolism, and (c) synaptic plasticity. 3-Month old C57BL/6J mice were divided into 4 groups exposed to their respective treatments for 9 weeks: (1) normal diet, (2) normal diet plus lipoic acid, (3) HFD, and (4) HFD plus lipoic acid. HFD resulted in higher body weight, development of insulin resistance, lower brain glucose uptake and glucose transporters, alterations in glycolytic and acetate metabolism in neurons and astrocytes, and ultimately synaptic plasticity loss evident by a decreased long-term potentiation (LTP). Lipoic acid treatment in mice on HFD prevented several HFD-induced metabolic changes and preserved synaptic plasticity. The metabolic and physiological changes in HFD-fed mice, including insulin resistance, brain glucose uptake and metabolism, and synaptic function, could be preserved by the insulin-like effect of lipoic acid.

Figure 1

Effects of lipoic acid and HFD on body weight gain and insulin resistance. Mice were fed with 9-week HFD or normal diet, with or without lipoic acid administration in drinking water (0.23% w/v) and different parameters were monitored weekly or at the end of the 9-week period. (A) Body weight changes; (B) Body weight gain; At the end of the study (week 9) the following parameters were examined: (C) Plasma triglyceride levels; (D) Fasting glucose level; (E) Glucose tolerance test; (F) Area-under the curve values of glucose tolerance test; (G) Fasting insulin levels; (H) Insulin resistance index, HOMA-IR. Data presented as mean ± SD, n ≥ 5, *p < 0.05, **p < 0.01.

Figure 2

Effect of lipoic acid on brain glucose uptake. (A) Representative images of brain glucose uptake ([¹⁸F]-FDG microPET/CT) in control, lipoic acid, HFD, HFD + lipoic acid groups at the last scanning point; (B) Glucose standard uptake values (SUV) and (C) glucose standard uptake rate (SUV/time). Data presented as mean ± SD, n ≥ 5, *p < 0.05, **p < 0.01.

Figure 3

Effect of lipoic acid on HFD-induced alterations in the expression of glucose transporters and the insulin signaling. Expression of (A) membrane associated GLUT3 and GLUT4; (B) Total levels of GLUT3 and GLUT4. (C) Representative western blots of brain tissue from control, +lipoic acid, HFD, and HFD + lipoic acid with (+) or without (−) ex vivo insulin stimulation. Na⁺/K⁺ ATPase and β-actin were used as loading control for membrane fraction and whole homogenate. Data presented as mean ± SD respectively, n ≥ 3 animals, *p < 0.05, **p < 0.01.

Figure 4

¹³C enrichment percentage of the major metabolites isotopomers after [1-¹³C]-glucose and [1,2-¹³C]-acetate infusion. These 18- bar graph show the different ¹³C labeled glutamate (column A), glutamine (column B), GABA (column C), and Aspartate (column D) isotopomers after infusion of [1-¹³C]-glucose+[1,2-¹³C]-acetate for 60 min. The enrichment of each metabolite isotopomers was calculated as described in Materials and methods section. Data presented as mean ± SD, n ≥ 4, *p < 0.05, **p < 0.01.

Figure 5

Neuronal–astrocytic metabolic interactions. Labeling pattern after [1-¹³C]-glucose+[1,2-¹³C]-acetate infusion. Effect of HFD on enrichments of different isotopomers indicated by open arrows; effect of lipoic acid on enrichments of different isotopomers on HFD-feeding mice indicated by close arrows.

Figure 6

Metabolic ratios quantified after [1-¹³C]-glucose+[1,2-¹³C]-acetate infusion. Metabolic ratios were calculated: (Ai) Percentage of glycolytic activity was calculated from the levels of [3-¹³C]-alanine; (Aii) TCA cycle activity was based on glutamate formation from [1-¹³C]-glucose; (Bi and Bii) ¹³C glucose cycling ratio for glutamate and glutamine respectively; (Ci and Cii) ¹³C acetate cycling ratio for glutamate and glutamine, respectively; (Di) transfer of glutamine from astrocytes to glutamatergic neurons, and (Dii) transfer of glutamine to GABAergic neurons. Data presented as mean ± SD, n ≥ 4, *p < 0.05, **p < 0.01.

Figure 7

Effects of lipoic acid on hippocampal synaptic plasticity. Input/Output (I/O) and changes after induction of LTP in all groups. (A) I/O relationship curves at increasing stimulus intensities; Bar graphs showing the minimum (B) and maximum (C) fEPSP slope values at 100 and 400 µA; (D) Baseline fEPSP slopes and those after induction of LTP; (E) The measured LTP using %EPSP for the last 5 min of the response to TBS stimulation. For each sub-figure: total n ≥ 8 slices/group and at least 3–4 animals/group. Data presented as mean ± SD, n ≥ 5, *p < 0.05, **p < 0.01.