**4.3. Changes in the levels of TRP and its metabolites in the serum and frontal cortex of mice following chronic Ifn-γ gene expression**

In order to further elucidate the relationship between the IDO1-induced KYN pathway and the development of depression-like behavior in mice transfected with the pCpG-Muγ plasmid, we measured TRP metabolites in the serum and frontal cortex of these mice.

The serum and the frontal cortex were corrected from mice immediately following behavioral testing to determine the levels of TRP, KYN, KA, 3-HK, 3-HAA, and AA (**Figure 5b** and **c**). The concentration of serum TRP was significantly decreased in IFN-γ transfected (+) mice compared to IFN-γ-transfected (−) mice. In contrast, the levels of serum KYN and 3-HK were significantly increased in the IFN-γ-transfected (+) mice (**Figure 5b**). In the frontal cortex, IFN-γ transfected (+) mice had significantly higher KYN and 3-HK levels than the IFN-γ-transfected (−) mice. The TRP and KA levels in the frontal cortex tended to be lower in the IFN-γ-transfected (+) mice (**Figure 5c**). The activation of IDO1 by *Ifn-γ* gene transfer significantly modified the levels of TRP and its metabolites not only in the serum, but also in the frontal cortex of mice. These results suggest that an alternative explanation for the participation of IDO1 in IFNγ-induced depression-like behavior is the generation of neuroactive TRP metabolites. This interpretation is consistent with our clinical data and previous studies by O'Connor et al. and Wichers et al. [32, 35].
