Nucleus accumbens inflammation mediates anxiodepressive behavior and compulsive sucrose seeking elicited by saturated dietary fat

Abstract

Objective: The incidence of depression is significantly compounded by obesity. Obesity arising from excessive intake of high-fat food provokes anxiodepressive behavior and elicits molecular adaptations in the nucleus accumbens (NAc), a region well-implicated in the hedonic deficits associated with depression and in the control of food-motivated behavior. To determine the etiology of diet-induced depression, we studied the impact of different dietary lipids on anxiodepressive behavior and metabolic and immune outcomes and the contribution of NAc immune activity.

Methods: Adult C57Bl/6 mice were subjected to isocaloric high-fat/high-sucrose diets (HFD), enriched in either saturated or monounsaturated fat, or a control low-fat diet (LFD). Metabolic responses, anxiodepressive behavior, and plasma and NAc inflammatory markers were assessed after 12 weeks. In separate experiments, an adenoviral construct inhibiting IKKβ, an upstream component of the nuclear factor kappa-b (NFkB) pathway, was a priori injected into the NAc.

Results: Both HFDs resulted in obesity and hyperleptinemia; however, the saturated HFD uniquely triggered anxiety-like behavior, behavioral despair, hyperinsulinemia, glucose intolerance, peripheral inflammation, and multiple pro-inflammatory signs in the NAc, including reactive gliosis, increased expression of cytokines, antigen-presenting markers and NFкB transcriptional activity. Selective NAc IKKβ inhibition reversed the upregulated expression of inflammatory markers, prevented anxiodepressive behavior and blunted compulsive sucrose-seeking in mice fed the saturated HFD.

Conclusions: Metabolic inflammation and NFкB-mediated neuroinflammatory responses in the NAc contribute to the expression of anxiodepressive behavior and heightened food cravings caused by a diet high in saturated fat and sugar.

Keywords: Anxiety; Depression; Diet-induced obesity; Dietary fatty acids; Food reward; Neuroinflammation; Nuclear factor kappa-b.

Graphical abstract

Figure 1

Saturated high-fat feeding potentiates metabolic impairments and inflammation. (A–C) Final body weights (n = 23–24/diet), body mass composition (n = 7–8/diet), and adipose tissue depositions (n = 8/diet) following 12 weeks of low-fat diet (Control), saturated (Palm) or monounsaturated (Olive) high-fat feeding. (D) Average weekly caloric intake on each diet (n = 8/diet). (E–H) Plasma levels of free fatty acids, leptin, insulin and glucose at time of sacrifice (n = 6–8/diet). (I) Oral glucose tolerance test in overnight fasted mice receiving a dose of dextrose (2 g/kg) (n = 7–8/diet). (J) Insulin tolerance test in 4 h fasted mice injected with insulin (n = 4–8/diet). (K–N) Plasma levels of C-reactive protein, tumor necrosis factor (TNF), interleukin-1bêta (IL-1β) and corticosterone at time of sacrifice (n = 7–13/diet). Group mean ± SEM, One-way analysis of variance, Bonferroni post hoc; *p < 0.05, **p < 0.01, ***p < 0.005.

Figure 2

Stimulation of anxiodepressive behavior by a saturated, high-fat diet. (A, B) Time spent and number of entries into the open arms of the elevated-plus maze (n = 15–16/diet). (C) Immobility time during the 4 last minutes of the forced swim test (n = 6–8/diet). (D) Swimming velocity during the first 2 min of the forced swim test (n = 6–8/diet). Group mean ± SEM; one-way analysis of variance, Bonferroni post hoc; *p < 0.05, ***p < 0.005.

Figure 3

Saturated, high-fat feeding triggers inflammation in the nucleus accumbens. (A) Relative nucleus accumbens gene expression of glial fibrillary acidic protein (GFAP), ionized calcium binding adaptor molecule-1 (Iba-1), tumor necrosis factor (TNF), interleukin-1bêta (IL-1β), interferon-gamma (IFN-γ), CD45, CD11b, and heat-shock protein-72 (HSP-72). (B) βgal (red) immunofluorescence on nucleus accumbens coronal slices of NFkB-LacZ (βgal) reporter mice fed one of 3 diets. 20× magnification; 50 μm scale bars. (C) Quantification of βgal signal density in nucleus accumbens (n = 4/diet). (D, E) Staining and quantification of GFAP + cells in the nucleus accumbens (n = 7–10/diet). (F, G) Staining and quantification of Iba-1+ cells in the nucleus accumbens (n = 8–14/diet). (H, I) Length and number of processes of Iba-1+ cells (n = 9–11/diet). (J–L) Ratio (minimum ferret/maximum ferret), density and perimeter of Iba-1+ cells (n = 10–11/diet). Cell quantification per surface area of 0.078 mm^2; magnification of 20× and 63× (inserts); 50 μm scale bars. Group mean ± SEM; one-way analysis of variance, Bonferroni post hoc; *p < 0.05, **p < 0.01.

Figure 4

Inhibition of IKKβ/NFkB in the nucleus accumbens prevents diet-induced anxiodepressive behavior and neuroinflammation. (A) Experimental layout depicting start of diets, viral injection and testing. (B) Final body weights after testing (n = 21–25/group). (C, D) Time spent and entries in open arms of the elevated-plus maze (n = 13–20/group). (E) Distance traveled in the elevated-plus maze. (F) Immobility time during the last 4 min of the forced swim test (n = 13–19/group). (G) Swimming velocity during the first 2 min of the forced swim test (n = 13–19/group). (H) Nucleus accumbens relative gene expression of inhibitor of kappa-B kinase-bêta (IKKβ), glial fibrillary acidic protein (GFAP), ionized calcium binding adaptor molecule-1 (Iba-1), interleukin-1bêta (IL-1β), interferon-gamma (IFN-γ), CD45, CD11b, heat shock protein-72 (HSP-72) vimentin, tumor necrosis factor (TNF), major histocompatibility complex (MHC)-1 and MHC-II (n = 7–9/group). Group mean ± SEM; two-way analysis of variance, Bonferroni post hoc; *p < 0.05, **p < 0.01, ***p < 0.005.

Figure 5

Nucleus accumbens IKKβ/NFkB inhibition decreases sucrose reward and compulsive sucrose seeking in mice fed the saturated high-fat diet. (A) Breakpoint values in operant responding for sucrose pellets before (Pre) and after (Post) viral injection (n = 3–6/group); two-way analysis of variance, Bonferonni post hoc. (B) Variation in breakpoint ratio before and after viral injection; unpaired two-tailed t-test (Palm^GFP vs. Palm^IKKdn). (C) Preference for correct over incorrect lever in operant responding for sucrose pellets before and after viral injection. (D) Protocol for cue-induced suppression of sucrose seeking. (E) Variation in lever pressing for sucrose before and after aversive conditioning (n = 5–6/group); two-way analysis of variance, Bonferonni post hoc. Group mean ± SEM; *p < 0.05, **p < 0.01.