title: Inverse agonistic effect of ICI-174,864 on the cloned delta-opioid receptor: role of G protein and adenylyl cyclase activation.
abstract: Previous studies have established that the delta-selective antagonist ICI-174,864 exhibits negative intrinsic activity at the delta-opioid receptors in NG108-15 membranes. To determine whether ICI-174,864 can function as a true inverse agonist in intact cells, its ability to stimulate cAMP accumulation was examined in a human embryonic kidney 293 cell line (293/DOR) expressing the cloned murine delta-opioid receptor. Forskolin-stimulated cAMP accumulation in the 293/DOR cells was dose-dependently suppressed by the delta-selective agonist [D-Pen2, D-Pen5]enkephalin, and such inhibition was abolished by pertussis toxin or the opiate antagonist naloxone. In contrast, ICI-174,864 significantly potentiated the forskolin response. The ICI-174,864-induced enhancement of the forskolin response exhibited dose-dependency and was antagonized by [D-Pen2,D-Pen5]enkephalin and blocked by pertussis toxin. Neither ICI-174,864 nor pertussis toxin elevated the basal level of cAMP accumulation in the absence of forskolin. Other opiate antagonists, such as naloxone and naltrindole, were ineffective in enhancing the forskolin-stimulated cAMP accumulation. Elevation of cAMP levels in response to the activation of Gs (through either ligand-bound receptor or point mutation on alpha(s)) was also potentiated by ICI-174,864. Our results indicate that ICI-174,864 behaves as an inverse agonist in human embryonic kidney 293 cells stably expressing the delta-opioid receptor. The inverse agonistic effect of ICI-174,864 seemed to require Gi proteins and was clearly manifested when adenylyl cyclase was activated.
authors.affiliation: Department of Biology, Hong Kong University of Science and Technology, Kowloon.
authors.forename: T T
authors.initials: TT
authors.lastname: Chiu
authors.forename: L Y
authors.initials: LY
authors.lastname: Yung
authors.forename: Y H
authors.initials: YH
authors.lastname: Wong
mesh_terms: D002460:Cell Line; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D004347:Drug Interactions; D004743:Enkephalin, Leucine / Q000031:analogs & derivatives* / Q000494:pharmacology; D006801:Humans; D009292:Narcotic Antagonists / Q000494:pharmacology*; D017465:Receptors, Opioid, delta / Q000037:antagonists & inhibitors*
chemical_list: D009292:Narcotic Antagonists; D017465:Receptors, Opioid, delta; D005576:Colforsin; D004743:Enkephalin, Leucine; C040643:N,N-diallyl-tyrosyl-alpha-aminoisobutyric acid-phenylalanyl-leucine; D000242:Cyclic AMP