Wednesday, February 01, 2017

Involvement of host defense mechanisms against Toxoplasma gondii infection in anhedonic and despair-like behaviors in mice

 2017 Jan 30. pii: IAI.00007-17. doi: 10.1128/IAI.00007-17. [Epub ahead of print]


Toxoplasma gondii is a pathogen relevant to psychiatric disorders. We recently showed that reactivation of chronic T. gondii induced depressive-like behaviors in mice. Further, it has been hypothesized that depression-like behaviors are mediated via a host defense mechanism against invading pathogens; proximate mechanisms of this behavioral hypothesis remain unclear. In the present study, we investigate the contribution of indoleamine 2, 3-dioxygenase (IDO), inflammation and interferon-gamma (IFN-γ) on anhedonic- and despair-related behaviors in T. gondii-infected mice using sucrose preference and forced swim tests, respectively. First, we confirmed BALB/c mice exhibited both sickness and depression-like behaviors during acute infection. Treatment of infected wild-type mice with minocycline (anti-inflammatory drug) abated sickness and anhedonic- and despair-like behaviors; whereas in T. gondii-infected mice, treatment normalized kynurenine/tryptophan (Kyn/Trp) ratios in both plasma and brain tissue. Additionally, T. gondii infection failed to induce anhedonic and despair-like behaviors or increase Kyn/Trp ratio in immunocompromised (IFN-γ-/-) mice; whereas, sickness behavior was observed in both immunocompetent and IFN-γ-/- mice following infection. Furthermore, treatment with 1-methyl tryptophan (an IDO inhibitor) did not affect locomotor activity, attenuated clinical score and anhedonic- and despair-like behaviors, and resulted in normal Kyn/Trp ratios in T. gondii-infected wild-type mice. Although low levels of serotonin and dopamine were observed in the brain during acute and chronic infection, anhedonic- and despair-like behaviors were not detected in the chronic stage of infection. Collectively, our results demonstrated that immune enhancement in response to infection with T. gondii resulted in IFN-γ production, IDO activation, and inflammation associated with anhedonic- and despair-like behaviors.

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