In vivo regulation of ?-opioid receptor density and gene expression in CXBK and outbred Swiss Webster mice

Abstract

Chronic in vivo treatment with the opioid agonist etorphine downregulates -opioid receptor density, produces tolerance, and regulates gene expression in the mouse. After cessation of treatment, there is an increase in -opioid receptor mRNA level associated with the recovery of -opioid receptors. However, the effect of etorphine on the regulation of mRNA during treatment is currently not known. In this study, etorphine-induced changes in -opioid receptor density, mRNA, and opioid analgesic potency were determined in two mouse strains that differ in basal -opioid receptor density in brain. CXBK mice (-opioid receptor deficient) and outbred Swiss Webster mice were implanted s.c. with placebo pellets (controls) or etorphine minipumps (250 g/kg/day) for 1-7 days and -opioid receptor density or mRNA levels in whole brain were assessed or mice were tested for etorphine analgesia following 7 days of treatment. In control CXBK mice, -receptor density was Ϸ40% less than that for the Swiss Webster, although mRNA abundance was similar in both strains. Etorphine's potency was 4-fold greater in control Swiss Webster compared to CXBK mice. Etorphine treatment decreased (Ϸ25-40%) -receptor density similarly in both strains throughout treatment. The magnitude of analgesic tolerance to etorphine was 8-fold in both mouse strains. Etorphine produced a biphasic effect on receptor mRNA in both strains with levels decreased (25%) by 3 days and increased (30 -40%) at 7 days. mRNA levels remained elevated (55%) 16 h following the end of the 7 day etorphine treatment. Taken together, these data suggest that in vivo etorphine treatment that produces -opioid receptor downregulation and tolerance, can regulate -opioid receptor mRNA abundance. Receptor downregulation may initially induce decreases in mRNA levels since downregulation preceded a decrease in gene expression. Prolonged (Ͼ3 days) receptor downregulation may be responsible for increasing message levels and may be important in recovery of receptors following treatment. In addition, the magnitude of changes in receptor density, mRNA, and tolerance were similar in both CXBK and Swiss Webster mice, indicating that the mechanisms required for receptor regulation and its functional consequences are independent of basal -opioid receptor density. Synapse 37:118 -124, 2000.

Deletion of the M 5 muscarinic acetylcholine receptor attenuates morphine reinforcement and withdrawal but not morphine analgesia

“…⌻he density of central -opioid receptors in membrane homogenates from selected brain regions was measured using [ 3 H]DAMGO (5 nM) as a radioligand, essentially as described (26). D1 and D2 dopamine receptor levels were determined in the presence of 4 nM [ 3 H]SCH23390 and 2 nM [ 3 H]spiperone, respectively, following a published protocol (27).…”

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

“…However, the mechanisms involved in opioid receptor regulation after opioid agonist and antagonist treatment are different. For example, opioid agonist treatment is associated with changes in gene expression of opioid receptors (Kim et al, 1995;Sehba et al, 1997;Duttaroy and Yoburn, 2000) but antagonist treatment is not (Jenab et al, 1995;Unterwald et al, 1995;however, see Brodsky et al, 1995;Castelli et al, 1997;Duttaroy et al, 1999). Thus, it was not certain that GRK-2 and DYN-2 would be involved in opioid agonist-induced OR regulation in vivo.…”

Mu opioid receptor and orexin/hypocretin mRNA levels in the lateral hypothalamus and striatum are enhanced by morphine withdrawal

“…Different studies have examined the effect of chronic opioid agonists or their withdrawal on MOP-r mRNA levels in different brain regions (Brodsky et al 1995, Buzas et al 1996, Castelli et al 1997, Sehba et al 1997, Duttaroy & Yoburn 2000. The results obtained are conflicting; a decrease , Duttaroy & Yoburn 2000, an increase (Sehba et al 1997), or no change (Brodsky et al 1995, Buzas et al 1996, Castelli et al 1997 have been reported, and these apparently contradictory results may depend on differences in the brain regions examined, exposure time, dose, and route of the opioid agonist administered.…”

“…The results obtained are conflicting; a decrease , Duttaroy & Yoburn 2000, an increase (Sehba et al 1997), or no change (Brodsky et al 1995, Buzas et al 1996, Castelli et al 1997 have been reported, and these apparently contradictory results may depend on differences in the brain regions examined, exposure time, dose, and route of the opioid agonist administered. In many of the previous studies, morphine or opioid agonists were chronically administered by minipumps or pellets (Brodsky et al 1995, Sehba et al 1997, Duttaroy & Yoburn 2000. This differs from our morphine experiments using an intermittent pattern of administration, in which the animal could experience both rewarding effects and chronic stress induced by repeated morphine injection and withdrawal.…”