MTOR and major depressive disorder: Considering the interest in novel and fast-acting antidepressant agents, the purposes of this study were: (1) to investigate if systemic GUO induces fast-onset antidepressant effects in the OBX model of depression in mice, and (2) verify the involvement of the mechanistic target of rapamycin (RAP), the mammalian target of rapamycin (mTOR) pathway, a key target in the fast onset effect of ketamine (Ket)12.