Properties of a selective kappa agonist, U-50,488H

Life Sci. 1982 Nov 15-22;31(20-21):2257-60. doi: 10.1016/0024-3205(82)90132-1.


U-50,488H has been shown to be a naloxone antagonizable analgesic in rodents. However, the dose of naloxone needed for antagonism is higher than it is for morphine. U-50,488H does not produce physical dependence; however it does produce tolerance upon chronic administration. U-50,488H is cross tolerant with bremazocine but not with morphine. Monkeys trained to discriminate ethylketocyclazocine (EKC) from saline show a complete generalization to U-50,488H but not to morphine. The evaluation of U-50,488H in 3H-EKC site-selective binding indicated that U-50,488H has a high affinity for the kappa receptor (Ki = 114 nM) and a low affinity for the mu receptor (Ki = 6100 nM). The ratio of Ku/Kk was 0.08 for morphine, 0.4 for dynorphin, and 53.5 for U-50,488H. The data suggest that U-50,488H is a selective agonist at the opioid kappa receptor.

Publication types

  • Comparative Study

MeSH terms

  • Analgesics, Opioid / metabolism
  • Animals
  • Biological Assay
  • Cyclazocine / analogs & derivatives
  • Cyclazocine / metabolism
  • Enkephalins / chemical synthesis*
  • Enkephalins / pharmacology
  • Ethylketocyclazocine
  • Guinea Pigs
  • Ileum / drug effects
  • Indicators and Reagents
  • Kinetics
  • Male
  • Muscle Contraction / drug effects
  • Rats
  • Receptors, Opioid / drug effects
  • Receptors, Opioid / metabolism
  • Structure-Activity Relationship
  • Vas Deferens / drug effects


  • Analgesics, Opioid
  • Enkephalins
  • Indicators and Reagents
  • Receptors, Opioid
  • Ethylketocyclazocine
  • Cyclazocine