– Tianeptine metabolite MC5, an active derivative of the atypical antidepressant tianeptine produced through β-oxidation, exhibits selective G protein activation in bioluminescence resonance energy transfer (BRET) assays. Utilizing HEK293T cells expressing the human μ-opioid receptor (MOR) over the human δ-opioid receptor (DOR; EC50s = 0.454 and >100 µM, respectively), it demonstrates specificity. Moreover, this metabolite reduces immobility in wild-type mice during the forced swim test at a dose of 30 mg/kg, an effect not observed in MOR knockout mice, suggesting its antidepressant-like activity is MOR-dependent.