Limited Access to a High Fat Diet Alters Endocannabinoid Tone in Female Rats

Abstract

Emerging evidence suggest an impaired endocannabinoid activity in the pathophysiology of binge eating disorder (BED). Herein, we investigated whether endocannabinoid tone could be modified as a consequence of dietary-induced binge eating in female rats. For this purpose, brain levels of the endocannabinoids anandamide (AEA) and 2-arachidonoyl glycerol (2-AG), as well as two endocannabinoid-like lipids, oleoylethanolamide (OEA) and palmitoylethanolamide (PEA), were assessed in different brain areas involved in the hedonic feeding (i.e., prefrontal cortex, nucleus accumbens, amygdala, hippocampus, and hypothalamus). The brain density of cannabinoid type-1 receptors (CB₁) was also evaluated. Furthermore, we determined plasma levels of leptin, ghrelin, and corticosterone hormones, which are well-known to control the levels of endocannabioids and/or CB1 receptors in the brain. To induce binge eating behavior, rats were subject to an intermittent and limited access to a high fat diet (HFD) (margarine). Three experimental groups were used, all with ad libitum access to chow: control (CTRL), with no access to margarine; low restriction (LR), with 2 h margarine access 7 days/week; high restriction (HR), with 2 h margarine access 3 days/week. Bingeing was established when margarine intake in the HR group exceeded that of the LR group. Our results show that, compared to CTRL, AEA significantly decreased in the caudate putamen, amygdala, and hippocampus of HR group. In contrast, 2-AG significantly increased in the hippocampus while OEA decreased in the hypothalamus. Similar to the HR group, AEA and OEA decreased respectively in the amygdala and hypothalamus and 2-AG increased in the hippocampus of LR group. Moreover, LR group also had AEA decreased in the prefrontal cortex and increased in the nucleus accumbens. In both groups we found the same reduction of CB₁ receptor density in the prefrontal cortex compared to CTRL. Also, LR and HR groups showed alterations in both ghrelin and corticosterone levels, while leptin remained unaltered. In conclusion, our findings show a modified endocannabinoid tone due to margarine exposure, in several brain areas that are known to influence the hedonic aspect of food. Even if not uniquely specific to binge eating, margarine-induced changes in endocannabinoid tone could contributes to the development and maintenance of this behavior.

Keywords: 2-arachidonoyl glycerol; anandamide; binge eating disorder; cannabinoid type-1 receptors; high fat diet.

Figure 1

Schematic representation of the experimental timeline: (A) Dietary groups; (B) Induction of binge eating behavior.

Figure 2

Schematic representation (adapted from Paxinos and Watson, ; the Rat Brain in Stereotaxic Coordinates) of the prefrontal cortex (Cingulate cortex Cg1 and Cg3), caudate-putamen (CPu), Nucleus accumbens (Nacc Core and Nacc Shell), Hippocampus (CA1 field of Ammon's horn; CA2, field of Ammon's horn; CA3, field of Ammon's horn; DG, dentate gyrus of hippocampus), Amygdala (AMY) and Hypotalamus (lateral LH and ventro-medial VHM).

Figure 3

Induction of binge-type eating. Data are presented as mean kcal ± SEM (n = 12 rats per group). (A) Margarine intake during the limited (2 h) access: HR group consumed more margarine compared to LR group (Two-way ANOVA: ^#P < 0.01 and ^§P < 0.001, Bonferroni test vs. LR group); (B) Chow intake during the limited (2 h) access: LR group and HR group consumed less chow compared to CTRL group (one way ANOVA: ^*P < 0.05, ^#P < 0.01 and ^§P < 0.001, Newman–Keuls test vs. CTRL group). (C) Chow intake after the limited (2 h) access: no difference was found between groups.

Figure 4

Endocannabinoid levels: data are presented as mean ± SEM (n = 6 rats per group). (A) AEA: HR group showed a decrease in Caudate Putamen, Amygdala and Hippocampus compared to CTRL group (one-way ANOVA: ^*P < 0.05 and ^**P < 0.01, Newman–Keuls test vs. CTRL); LR group showed a decrease in Prefrontal Cortex and Amygdala as well as an increase in the Nucleus accumbens compared to CTRL group (one-way ANOVA: ^*P < 0.05 and ^**P < 0.01, Newman–Keuls multiple comparison test vs. CTRL group). (B) 2-AG: both LR and HR groups showed an increase compared to CTRL group in the hippocampus (one-way ANOVA: ^**P < 0.01, Newman–Keuls test vs. CTRL group).

Figure 5

Endocannabinoid-like compounds levels: data are presented as mean ± SEM (n = 6 rats per group). (A) OEA: both LR and HR groups showed a decrease in the hypothalamus compared to CTRL group (one-way ANOVA: ^**P < 0.01, Newman–Keuls test vs. CTRL group). LR group showed an increase in the Prefrontal Cortex compared to CTRL group (one-way ANOVA: ^**P < 0.01, Newman–Keuls test vs. CTRL group). (B) No changes in the levels of PEA were found between all three experimental groups.

Figure 6

CB₁ receptor density in the cingulate cortex Cg1 and Cg3. Data are expressed as mean fmol/mg protein of [3H]CP55940 ± SEM (n = 6 per group). Both LR and HR groups showed a decrease in the Cg1 and Cg3 compared to CTRL group (one-way ANOVA: ^*P < 0.05 and ^§P < 0.001, Newman–Keuls test vs. CTRL group).

Figure 7

Plasma hormone levels. (A) Leptin: data are expressed as mean ng/ml ± SEM (n = 6 per group). No changes were found between all three experimental groups. (B) Ghrelin: data are expressed as mean ng/ml ± SEM (n = 6 per group). Both LR and HR groups showed a decrease of plasma ghrelin levels compared to CTRL group (one-way ANOVA: ^*P < 0.05, Newman–Keuls multiple comparison test vs. CTRL group). (C) Corticosterone: data are expressed as mean pg/ml ± SEM (n = 6 per group). Both LR and HR groups showed an increase compared to CTRL group (one-way ANOVA: ^*P < 0.05, Newman–Keuls multiple comparison test vs. CTRL group).