INS and diabetes mellitus: Consistently, insulin increased the levels of serotonin and its metabolite in the brain [59], while the oral antidiabetic metformin, glyburide, pioglitazone, and sitagliptin increased serotonin levels in the hippocampus and cortex, serotonergic neuronal activity in the dorsal raphe, and mitigated depression-like behavior in rodent models of diabetes [57, 60].