Opioid agonists differentially regulate mu-opioid receptors and trafficking proteins in vivo.
Chronic opioid agonist treatment produces tolerance and in some cases opioid receptor internalization and down-regulation. Both morphine and etorphine induce tolerance; however, only etorphine produces mu-opioid receptor (muOR) down-regulation. In vitro studies implicate dynamin-2 (DYN-2) and G-protein receptor kinase-2 (GRK-2) in these processes. Therefore, we examined etorphine and morphine effects on regulation of GRK-2 and DYN-2 in mouse spinal cord. Mice were treated for 7 days with etorphine (200 microg/kg/day infusion) or morphine (40 mg/kg/day infusion + one 25-mg implant pellet). Controls were implanted with a placebo pellet. On the 7th day after implantation mice were tested for i.t. [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) analgesia. In other mice, spinal cord was removed for [(3)H]DAMGO binding studies or GRK-2 and DYN-2 protein and mRNA abundance were determined. Both etorphine and morphine produced significant tolerance (ED(50) shift = 7.6- and 7.3-fold for morphine and etorphine, respectively). Etorphine decreased spinal muOR density by approximately 30%, whereas morphine did not change muOR density. Etorphine increased ( approximately 70%) DYN-2 protein abundance and decreased its mRNA (31%), whereas it had no effect on GRK-2 protein and mRNA abundance. Morphine had no effect on either DYN-2 or GRK-2 protein or mRNA abundance. These data raise the possibility that unequal receptor regulation by etorphine and morphine might be due to differential regulation of trafficking proteins. Overall, receptor down-regulation associated with chronic etorphine treatment may accelerate dynamin-related activity. Finally, the decrease in DYN-2 mRNA may be related to stabilization of DYN-2 protein abundance, which might inhibit transcription.
Patel
Minesh B
MB
Department of Pharmaceutical Sciences, College of Pharmacy and Allied Health Professions, St. John's University, Queens, New York 11439, USA.
Patel
Chintan N
CN
Rajashekara
Vikram
V
Yoburn
Byron C
BC
D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D001665:Binding Sites; D017868:Cyclic AMP-Dependent Protein Kinases / Q000235:genetics / Q000378:metabolism*; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D005048:Etorphine / Q000494:pharmacology; D054770:G-Protein-Coupled Receptor Kinase 3; D008297:Male; D051379:Mice; D009020:Morphine / Q000494:pharmacology; D012333:RNA, Messenger / Q000187:drug effects / Q000378:metabolism; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000378:metabolism*; D051552:beta-Adrenergic Receptor Kinases
D000701:Analgesics, Opioid; D012333:RNA, Messenger; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D005048:Etorphine; D009020:Morphine; D017868:Cyclic AMP-Dependent Protein Kinases; D054770:G-Protein-Coupled Receptor Kinase 3; C517850:GRK3 protein, mouse; D051552:beta-Adrenergic Receptor Kinases