Hippocampal GSK3β as a Molecular Link Between Obesity and Depression. 
Obesity is considered as a risk factor for mood disorders including depression.  Nevertheless, the mechanisms underlying this association are not clearly understood.  To address this issue, we investigated the impact of high-fat (HF)-diet-induced obesity on depressive-like behavior and on serotonin (5-HT)-dependent Akt/glycogen synthase kinase 3β (GSK3β) signaling in the dentate gyrus (DG) of the hippocampus, which has been associated with mood regulation.  We first showed that a HF diet induced significant overweight and hyperglycemia as well as a depressive-like behavior in adult Wistar rats.  By using an ex vivo approach on brain slices, we demonstrated that 5-HT activates the Akt/GSK3β cascade in the DG of control chow (C) diet-fed animals and that a 16-week HF diet feeding abolishes this activation, concurrently with a desensitization of leptin and insulin signaling in the same region.  Furthermore, depressive-like behavior inversely correlated with 5-HT-induced phosphorylation of GSK3β in the subgranular neurons of the DG.  Interestingly, a substitution of HF with C diet for 6 weeks induced a total loss of depressive symptoms, whereas body weight and glycemia remained significantly higher compared to control rats.  In addition, food restoration led to a recovery of the Akt/GSK3β signaling pathway activation in the DG.  In parallel, we observed a negative correlation between body weight and cell proliferation in the subgranular zone of the DG.  To conclude, we provide evidence for a desensitization of 5-HT-induced Akt/GSK3β signaling and an impaired cell proliferation in the DG by HF diet, suggesting novel molecular mechanisms linking obesity to depression.