Nociceptin activation of ORL1 (opioid receptor-like 1 receptor) has been shown to antagonize l receptor-mediated analgesia at the supraspinal level. ORL1 and l-opioid receptor (lR) are co-expressed in several subpopulations of CNS neurons involved in regulating pain transmission. The amino acid sequence of ORL1 also shares a high degree of homology with that of l receptor. Thus, it is hypothesized that ORL1 and lR interact to form the heterodimer and that ORL1/lR heterodimerization may be one molecular basis for ORL1-mediated antiopioid effects in the brain. To test this hypothesis, myc-tagged ORL1 and HA-tagged lR are co-expressed in human embryonic kidney (HEK) Grisel et al. 1996;Mogil et al. 1996;Calo et al. 1998;Wang et al. 1999). It has also been reported that intracerebroventricular administration of [Nphe 1 ]nociceptin(1-13)NH2, a selective ORL1 antagonist, potentiates l receptor agonist-induced analgesia and produces a naloxone-resistant antinociceptive effect (Calo et al. 2000a). These results suggest that the functional antagonism of l receptor-mediated analgesia by ORL1 is tonically active at the supraspinal level.A growing number of biochemical, biophysical and functional investigations demonstrated that heterodimerization of distinct G protein-coupled receptors occurs in the plasma membrane and that pharmacological and signaling Received April 20, 2004; revised manuscript received August 13, 2004; accepted October 6, 2004. Address correspondence and reprint requests to Hung-Li Wang, Department of Physiology, Chang Gung University School of Medicine, Kwei-San, Tao-Yuan, Taiwan. E-mail: hlwns@mail.cgu.edu.twAbbreviations used: AT1-R, type 1 angiotensin II receptor; B2-R, type 2 bradykinin receptor; CHO, Chinese hamster ovary; DA, dopamine; DAMGO, [D-Ala 2 ,N-methyl-Phe 4 ,Gly-ol 5 ]enkephalin; GRK2, G proteincoupled receptor kinase 2; HEK, human embryonic kidney; MAPK, mitogen-activated protein kinase; lR, l-opioid receptor; ORL1, opioid receptor-like 1 receptor; PAG, periaqueductal gray; PBS, phosphatebuffered saline; SDS-PAGE, sodium dodecyl sulphate polyacrylamide gel electrophoresis.
