Opioid receptors in GtoPdb v.2025.1

Opioid and opioid-like receptors are activated by a variety of endogenous peptides including[Met]enkephalin (met), [Leu]enkephalin (leu), β-endorphin (β-end), α-neodynorphin, dynorphin A(dynA), dynorphin B (dynB), big dynorphin (Big dyn), nociceptin/orphanin FQ (N/OFQ); endomorphin-1 and endomorphin-2 are also potential endogenous peptides. The Greek letter nomenclature for theopioid receptors, μ, δ and κ, is well established, and NC-IUPHAR considers this nomenclatureappropriate, along with the symbols spelled out (mu, delta, and kappa), and the acronyms, MOP,DOP, and KOP [126, 103, 94]. However the acronyms MOR, DOR and KOR are still widely used in theliterature. The human N/OFQ receptor, NOP, is considered 'opioid-related' rather than opioidbecause, while it exhibits a high degree of structural homology with the conventional opioidreceptors [310], it displays a distinct pharmacology. Currently there are numerous clinically useddrugs, such as morphine and many other opioid analgesics, as well as antagonists such as naloxone.The majority of clinically used opiates are relatively selective μ agonists or partial agonists, thoughthere are some μ/κ compounds, such as butorphanol, in clinical use. κ opioid agonists, such as thealkaloid nalfurafine and the peripherally acting peptide difelikefalin, are in clinical use for itch.