Mechanism of action of opioids

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Opioid agonists produce analgesia by binding to G protein-coupled receptors (GPCRs) in the brain and spinal cord regions involved in the transmission and modulation of pain
Opioid drugs mimic endogenous compounds: endorphins, enkephalins, and dynorphins that bind to opioid receptors.

Four opioid receptor types: mu (μ), kappa (κ), delta (δ), and sigma (σ) → GPCR → membrane hyperpolarization → blocks release/response of excitatory neurotransmitters (acetylcholine, substance P) → inhibits nociceptive neurons
Inhibition can occur at several points along pain pathway: periphery → afferent neurons → dorsal horn of spinal cord → secondary neuron

Agonist-antagonists (nalbuphine, butorphanol, and pentazocine) have less efficacy than full agonists (fentanyl)