title: Enhancement of morphine analgesic effect with induction of mu-opioid receptor endocytosis in rats.
abstract: BACKGROUND
  Morphine can desensitize mu-opioid receptor (MOR), but it does not cause internalization of the receptor after binding. Acute desensitization of MOR impairs the efficiency of signaling, whereas the receptor internalization restores the cell responsiveness to the agonists. Thereby, the property of morphine may limit the analgesic effects of this opiate drug. It has been shown that [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAMGO), a potent MOR agonist inducing the internalization, facilitates morphine to internalize MOR, suggesting that MOR agonists with low relative activity versus endocytosis (RAVE) values such as DAMGO can potentiate analgesic effects of morphine through stimulating MOR internalization. The authors examined whether the acute analgesic effect of morphine can be potentiated by low relative activity versus endocytosis agonists DAMGO and fentanyl.
  
  
  METHODS
  Rats injected intrathecally with opioids were subjected to a hot plate test for antinociceptive effect. Immunostained spinal dorsal horn was analyzed by confocal microscopy.
  
  
  RESULTS
  Fentanyl induced MOR internalization to a lesser extent than DAMGO at equianalgesic doses. Coadministration of fentanyl promoted morphine-induced MOR internalization. The analgesic effect of morphine was greatly potentiated together with decrease in the relative activity versus endocytosis value when MOR internalization was induced by coadministration of a subanalgesic dose of DAMGO or fentanyl. In contrast, the combination of DAMGO and fentanyl increased neither the analgesic effect nor the internalization of MOR.
  
  
  CONCLUSIONS
  The results suggest that the coadministration of morphine with MOR-internalizing agonist is clinically applicable to develop successful pain-management regimens to achieve satisfactory analgesia using less morphine.
authors.affiliation: Department of Anesthesiology, Shimane University Faculty of Medicine, Japan. hashit@med.shimane-u.ac.jp
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Hashimoto
authors.forename: Yoji
authors.initials: Y
authors.lastname: Saito
authors.forename: Kazuo
authors.initials: K
authors.lastname: Yamada
authors.forename: Nobumasa
authors.initials: N
authors.lastname: Hara
authors.forename: Yumiko
authors.initials: Y
authors.lastname: Kirihara
authors.forename: Mikako
authors.initials: M
authors.lastname: Tsuchiya
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D004705:Endocytosis / Q000187:drug effects*; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D005283:Fentanyl / Q000494:pharmacology; D008297:Male; D009020:Morphine / Q000493:pharmacokinetics / Q000494:pharmacology*; D020671:Posterior Horn Cells / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000378:metabolism*
chemical_list: D000701:Analgesics, Opioid; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D009020:Morphine; D005283:Fentanyl