Vol. 62, Issue 6, 1464-1470, December 2002

Opioid Agonists Differentially Regulate µ-Opioid Receptors and Trafficking Proteins in Vivo

Minesh B. Patel, Chintan N. Patel, Vikram Rajashekara, and Byron C. Yoburn

Department of Pharmaceutical Sciences, College of Pharmacy and Allied Health Professions, St. John's University, Queens, New York

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 µ-opioid receptor (µOR) 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 µg/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-Ala²,N-Me-Phe⁴,Gly⁵-ol]-enkephalin (DAMGO) analgesia. In other mice, spinal cord was removed for [³H]DAMGO binding studies or GRK-2 and DYN-2 protein and mRNA abundance were determined. Both etorphine and morphine produced significant tolerance (ED₅₀ shift = 7.6- and 7.3-fold for morphine and etorphine, respectively). Etorphine decreased spinal µOR density by 30%, whereas morphine did not change µOR density. Etorphine increased (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.