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Table 2 Selectivity of Opioid receptors to the Endogenous Opioid Peptides and Exogenous Opioid Drugs

From: Molecular and cellular mechanisms of the age-dependency of opioid analgesia and tolerance

Opioid receptor Endogenous opioid peptides Opioid drugs
Μ-receptor β-endorphin morphine
  enkephalin fentanyl
δ-receptor Met-enkephalin deltorphin
  Leu-enkephalin [D-Pen2,D-Pen5]enkephalin
κ-receptor dynorphin A ethylketocyclazocine (EKC)
  dynorphin B buprenorphine
   U 50,488
ORL1-receptor nociceptin None