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

Studies suggest that acute and chronic opioids can regulate the cAMP-depend ent 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 mu -opioid receptor downregulation and tolerance in mi ce. Mice were injected intracerebroventricular (i.c.v.) and intrathecal (i. t.) once a day with an antisense oligodeoxynucleotide directed at the mRNA for the alpha catalytic subunit of mouse PKA. Controls were treated with sa line or a mismatch oligodeoxynucleotide. On day 2 of treatment, mice were i mplanted 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 osmo tic minipump infusing etorphine (125, 250 mug/kg/day) for 2 days. Control m ice 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 mu -opioid receptor binding assay s conducted in whole brain. Both infusion doses of etorphine produced signi ficant tolerance (ED50 shift = 3.6 and 6.3-fold). The higher etorphine infu sion produced downregulation of mu -receptor density ( approximate to 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 rec eptor downregulation. Antisense to PKA partially blocked tolerance induced by the higher dose of etorphine, but had no effect on receptor downregulati on. On the other hand, antisense to PKA completely blocked tolerance induce d by morphine and the lower infusion dose of etorphine. The mismatch oligod eoxynucleotide had no effect on any measure. These results suggest that PKA has a limited role in opioid agonist-induced receptor downregulation. Howe ver, 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." (C) 2000 Wiley-Liss, Inc.