Vol. 297, Issue 2, 582-589, May 2001

Inverse Agonist Action of Leu-Enkephalin at ₂-Opioid Receptors Mediates Spinal Antianalgesia

Jodie J. Rady , Blythe B. Holmes, Leon F. Tseng and James M. Fujimoto

Research Service, Veterans Affairs Medical Center, Milwaukee, Wisconsin (J.J.R., J.M.F.); and Departments of Pharmacology and Toxicology (J.J.R., B.B.H., J.M.F.) and Anesthesiology (L.F.T.), Medical College of Wisconsin, Milwaukee, Wisconsin

Dynorphin A(1-17) given intrathecally releases spinal cholecystokinin to produce an antianalgesic action against spinal morphine in the tail-flick test in CD-1 mice. The present study showed that following the cholecystokinin step, a ₂-opioid inverse agonist action of Leu-enkephalin (LE), was involved. Pretreatment with intrathecal LE antiserum eliminated dynorphin and cholecystokinin-8s antianalgesia. A small dose of LE intrathecally produced antianalgesia that like that from dynorphin A(1-17) and cholecystokinin was eliminated by naltriben but not 7-benzylidenenaltrexone (₂- and ₁-opioid receptor antagonist, respectively). This LE step was followed by N-methyl-D-aspartate (NMDA) receptor activation. MK801, an NMDA receptor antagonist, eliminated the antianalgesia from dynorphin A(1-17), cholecystokinin-8s, and LE. Furthermore, none of the three were effective against morphine analgesia in 129S6/SvEv mice possibly because of their deficiency in NMDA receptor response. In 129S6/SvEv mice, [D-Ser²]-Leu-enkephalin-Thr analgesia was not attenuated by LE; thus, this ₂-analgesic agonist and LE inverse agonist action did not occur through competition at the same ₂-receptor in CD-1 mice. In CD-1 mice, a linear sequence of dynorphin A(1-17)  cholecystokinin  LE  NMDA receptors was indicated: cholecystokinin antiserum inhibited cholecystokinin but not LE; naltriben inhibited LE but not NMDA. The uniqueness of LE in linking dynorphin A(1-17), cholecystokinin, ₂-opioid, and NMDA receptor activation may unify the separate known mechanisms involved in the antiopioid actions of these components against morphine.