Endogenous opioids are neuropeptides produced within the body that bind opioid receptors (μ, δ, κ) to inhibit pain transmission and modulate stress, reward, and autonomic functions.
Major Families
- Endorphins (β‑endorphin) – from POMC in pituitary/hypothalamus; “runner’s high,” stress analgesia.
- Enkephalins (Met/Leu) – widespread in CNS/GI; short‑acting, modulate spinal pain gates.
- Dynorphins – κ‑receptor preferring; dysphoria, stress response, appetite regulation.
- Endomorphins – high μ selectivity; potent analgesia.
- Nociceptin/Orphanin FQ – NOP receptor; pronociceptive/anxiolytic, distinct from classic opioids.
Physiologic Roles
- Pain inhibition (descending pathways).
- Stress response (HPA axis modulation).
- Reward, motivation, addiction circuitry.
- Gut motility, immune modulation, cardiovascular regulation.
Clinical Relevance
- Exercise‑induced analgesia, placebo effect.
- Chronic pain states may involve endogenous opioid dysregulation.
- Opioid tolerance/dependence involves receptor downregulation.
- Naloxone reverses both exogenous and endogenous opioid effects.
