Pain remains a key therapeutic area with intensive efforts directed toward finding effective and safer analgesics in light of the ongoing opioid crisis. Amongst the neurotransmitter systems involved in pain perception and modulation, the mu-opioid receptor (MOR), a G protein-coupled receptor, represents one of the most important targets for achieving effective pain relief. Most clinically used opioid analgesics are agonists to the MOR, but they can also cause severe side effects. Medicinal plants represent important sources of new drug candidates, with morphine and its semisynthetic analogues as well-known examples as analgesic drugs. In this study, combining in silico (pharmacophorebased virtual screening and docking) and pharmacological (in vitro binding and functional assays, and behavioral tests) approaches, we report on the discovery of two naturally occurring plant alkaloids, corydine and corydaline, as new MOR agonists that produce antinociceptive effects in mice after subcutaneous administration via a MOR-dependent mechanism. Furthermore, corydine and corydaline were identified as G protein-biased agonists to the MOR without inducing β-arrestin2 recruitment upon receptor activation. Thus, these new scaffolds represent valuable starting points for future chemical optimization towards the development of novel opioid analgesics, which may exhibit improved therapeutic profiles. Naturally occurring opioid alkaloids, such as morphine (Fig. 1), have been used for centuries for severe and chronic pain relief 1. Over several decades, new opioids with diverse scaffolds were synthesized, pharmacologically evaluated and clinically used as the most effective class of analgesic drugs 2-5. However, all currently available opioid analgesics share a similar spectrum of undesirable side effects, including respiratory depression, constipation, sedation, nausea and analgesic tolerance 5,6. Additionally, the potential for addiction and abuse of opioids has seriously hindered their clinical application, with a huge rise in opioid misuse and overdose deaths resulting in an ongoing and rapidly emerging opioid epidemic worldwide 7,8. Currently, intensive research focuses on finding new, innovative medications and technologies to treat opioid addiction, together with the discovery of safe, effective, non-addictive drugs to manage chronic pain 9-12. Opioids produce their pharmacological effects through the activation of opioid receptors, which include three main types, mu (MOR), delta (DOR) and kappa (KOR) 13,14 , of which the MOR type is the primary target of most clinically used opioid analgesics 3,5. Opioid receptors share high homology and belong to the superfamily of seven transmembrane-spanning G protein-coupled receptors (GPCRs). Because of its therapeutic relevance, the MOR is among the few GPCRs determined in different activation states, with the first X-ray crystal structure of the receptor protein bound to β-funaltrexamine (Fig. 1), an irreversible antagonist (PDB entry 4DKL) 15 , and the 3D-structure in the active confo...