Abstract
Although peripheral low-grade inflammation has been associated with a high incidence of mood symptoms in patients with metabolic syndrome (MetS), much less is known about the potential involvement of brain activation of cytokines in that context. Recently we showed in a mouse model of MetS, namely the db/db mice, an enhanced hippocampal inflammation associated with increased anxiety-like behavior (Dinel et al., 2011). However, depressive-like behavior was not affected in db/db mice. Based on the strong association between depressive-like behavior and cytokine-induced brain activation of indoleamine 2,3-dioxygenase (IDO), the enzyme that metabolizes tryptophan along the kynurenine pathway, these results may suggest an impairment of brain IDO activation in db/db mice. To test this hypothesis, we measured the ability of db/db mice and their healthy db/+ littermates to enhance brain IDO activity and depressive-like behavior after a systemic immune challenge with lipopolysaccharide (LPS). Here we show that LPS (5 μg/mouse) significantly increased depressive-like behavior (increased immobility time in a forced-swim test, FST) 24h after treatment in db/+ mice, but not in db/db mice. Interestingly, db/db mice also displayed after LPS treatment blunted increase of brain kynurenine/tryptophan ratio compared to their db/+ counterparts, despite enhanced induction of hippocampal cytokine expression (interleukin-1β, tumor necrosis factor-α). Moreover, this was associated with an impaired effect of LPS on hippocampal expression of the brain-derived neurotrophic factor (BDNF) that contributes to mood regulation, including under inflammatory conditions. Collectively, these data indicate that the rise in brain tryptophan catabolism and depressive-like behavior induced by innate immune system activation is impaired in db/db mice. These findings could have relevance in improving the management and treatment of inflammation-related complications in MetS.