Role of cAMP-dependent protein kinase (PKA) in opioid agonist-induced ?-opioid receptor downregulation and tolerance in mice

Abstract

Studies suggest that acute and chronic opioids can regulate the cAMP‐dependent protein kinase (PKA) signaling pathway and that changes in this pathway may be involved in opioid tolerance. In the present study, we examined the role of cAMP‐PKA on μ‐opioid receptor downregulation and tolerance in mice. Mice were injected intracerebroventricular (i.c.v.) and intrathecal (i.t.) once a day with an antisense oligodeoxynucleotide directed at the mRNA for the α catalytic subunit of mouse PKA. Controls were treated with saline or a mismatch oligodeoxynucleotide. On day 2 of treatment, mice were implanted s.c. with a 25‐mg morphine pellet and an osmotic minipump infusing morphine (40 mg/kg/day) for 3 days. Other mice were implanted with an osmotic minipump infusing etorphine (125, 250 μg/kg/day) for 2 days. Control mice were implanted s.c. with inert placebo pellets. At the end of treatment, pumps and pellets were removed and mice tested for morphine or etorphine analgesia. Other mice were sacrificed and μ‐opioid receptor binding assays conducted in whole brain. Both infusion doses of etorphine produced significant tolerance (ED50 shift = 3.6 and 6.3‐fold). The higher etorphine infusion produced downregulation of μ‐receptor density (≈30%) while the lower infusion dose of etorphine did not. Morphine treatment also produced significant tolerance in mice (ED50 shift = 4.5‐fold), but no receptor downregulation. Antisense to PKA partially blocked tolerance induced by the higher dose of etorphine, but had no effect on receptor downregulation. On the other hand, antisense to PKA completely blocked tolerance induced by morphine and the lower infusion dose of etorphine. The mismatch oligodeoxynucleotide had no effect on any measure. These results suggest that PKA has a limited role in opioid agonist‐induced receptor downregulation. However, the partial block of tolerance for the high infusion dose of etorphine and the complete block of tolerance for morphine and the low infusion dose of etorphine suggests that PKA may play a critical role in tolerance that is “receptor‐regulation‐independent.” Synapse 38:322–327, 2000. © 2000 Wiley‐Liss, Inc.

Agonist-directed Interactions with Specific β-Arrestins Determine μ-Opioid Receptor Trafficking, Ubiquitination, and Dephosphorylation

“…Our studies showing that DAMGO, but not morphine, induces ubiquitination of the MOR could indicate that the DAMGO-bound receptor is then degraded, whereas the morphine-bound receptor is not. This would be in agreement with several studies that have observed that chronic treatment with morphine does not cause MOR down-regulation in whole mouse brain or brainstem (19,(51)(52)(53), which may be due to morphine's inability to promote MOR ubiquitination.…”

Agonist-directed Interactions with Specific β-Arrestins Determine μ-Opioid Receptor Trafficking, Ubiquitination, and Dephosphorylation

“…Our studies showing that DAMGO, but not morphine, induces ubiquitination of the MOR could indicate that the DAMGO-bound receptor is then degraded, whereas the morphine-bound receptor is not. This would be in agreement with several studies that have observed that chronic treatment with morphine does not cause MOR down-regulation in whole mouse brain or brainstem (19,(51)(52)(53), which may be due to morphine's inability to promote MOR ubiquitination.…”

Role of Giα2‐protein in opioid tolerance and μ‐opioid receptor downregulation in vivo

“…Thus, etorphine produces tolerance via receptor downregulation and via receptor desensitization. These data support the role of both receptor density-dependent and receptor density-independent mechanisms of tolerance and agree with previous results using antisense to PKA (Shen et al, 2000).…”

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

“…Although opioid agonists produce tolerance, it has been suggested that only high intrinsic efficacy agonists produce opioid receptor internalization and down-regulation (Yoburn et al, 1993;Duttaroy and Yoburn, 1995;Yabaluri and Medzihradsky, 1997;Burford et al, 1998;Shen et al, 2000;Zaki et al, 2000;Stafford et al, 2001; however, see Alvarez et al, 2002). The present results support the role for intrinsic efficacy in OR regulation, although morphine and etorphine differ in several other respects (e.g., receptor selectivity and kinetics) in addition to intrinsic efficacy.…”

Agonist-directed Interactions with Specific β-Arrestins Determine μ-Opioid Receptor Trafficking, Ubiquitination, and Dephosphorylation

“…Our studies showing that DAMGO, but not morphine, induces ubiquitination of the MOR could indicate that the DAMGO-bound receptor is then degraded, whereas the morphine-bound receptor is not. This would be in agreement with several studies that have observed that chronic treatment with morphine does not cause MOR down-regulation in whole mouse brain or brainstem (19,(51)(52)(53), which may be due to morphine's inability to promote MOR ubiquitination.…”

Role of Giα2‐protein in opioid tolerance and μ‐opioid receptor downregulation in vivo

“…Thus, etorphine produces tolerance via receptor downregulation and via receptor desensitization. These data support the role of both receptor density-dependent and receptor density-independent mechanisms of tolerance and agree with previous results using antisense to PKA (Shen et al, 2000).…”

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

“…Although opioid agonists produce tolerance, it has been suggested that only high intrinsic efficacy agonists produce opioid receptor internalization and down-regulation (Yoburn et al, 1993;Duttaroy and Yoburn, 1995;Yabaluri and Medzihradsky, 1997;Burford et al, 1998;Shen et al, 2000;Zaki et al, 2000;Stafford et al, 2001; however, see Alvarez et al, 2002). The present results support the role for intrinsic efficacy in OR regulation, although morphine and etorphine differ in several other respects (e.g., receptor selectivity and kinetics) in addition to intrinsic efficacy.…”

Agonist-directed Interactions with Specific β-Arrestins Determine μ-Opioid Receptor Trafficking, Ubiquitination, and Dephosphorylation

“…Our studies showing that DAMGO, but not morphine, induces ubiquitination of the MOR could indicate that the DAMGO-bound receptor is then degraded, whereas the morphine-bound receptor is not. This would be in agreement with several studies that have observed that chronic treatment with morphine does not cause MOR down-regulation in whole mouse brain or brainstem (19,(51)(52)(53), which may be due to morphine's inability to promote MOR ubiquitination.…”

Role of Giα2‐protein in opioid tolerance and μ‐opioid receptor downregulation in vivo

“…Thus, etorphine produces tolerance via receptor downregulation and via receptor desensitization. These data support the role of both receptor density-dependent and receptor density-independent mechanisms of tolerance and agree with previous results using antisense to PKA (Shen et al, 2000).…”

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

“…Although opioid agonists produce tolerance, it has been suggested that only high intrinsic efficacy agonists produce opioid receptor internalization and down-regulation (Yoburn et al, 1993;Duttaroy and Yoburn, 1995;Yabaluri and Medzihradsky, 1997;Burford et al, 1998;Shen et al, 2000;Zaki et al, 2000;Stafford et al, 2001; however, see Alvarez et al, 2002). The present results support the role for intrinsic efficacy in OR regulation, although morphine and etorphine differ in several other respects (e.g., receptor selectivity and kinetics) in addition to intrinsic efficacy.…”