G protein-first mechanism of activation for opioid receptors and their cognate Gi protein. Σ0: In the absence of ligand and Gi protein, the opioid receptors adopt the inactive conformation, featuring a tight hydrogen bond between the cytosolic ends of TM3 and TM6 that keeps the cytoplasmic region tightly closed. Σ1: Before agonist binding, the inactive Gi protein tightly bound to GDP couples to inactive opioid receptor, to form a pre-coupled opioid receptor-Gi (GDP) complex. Σ2: Interactions between inactive opioid receptor and inactive Gi (GDP) leads to breaking the TM3-TM6 hydrogen bond and opening the cytoplasmic region of the receptors to accommodate the Gi protein. As a result, the pre-activated state (Σ2) emerges, which remains at this resting state until an agonist binds the receptor. Σ3′: agonist bound to the pre-activated state induces the Gi (GDP) to be activated. Activation of the Gi protein is associated with a remarkable opening in the cleft between AH and Ras-like domains of Gα, providing an exit path for GDP release or exchange with a GTP. Σ4′: Upon GDP release of exchange, the agonist-opioid receptor-Gi protein evolves to its fully active state.