Ad. Blake et al., DIFFERENTIAL OPIOID AGONIST REGULATION OF THE MOUSE MU-OPIOID RECEPTOR, The Journal of biological chemistry, 272(2), 1997, pp. 782-790
mu opioid receptors mediate the analgesia induced by morphine. Prolong
ed use of morphine causes tolerance development and dependence. To inv
estigate the molecular basis of tolerance and dependence, the cloned m
ouse mu opioid receptor with an amino-terminal epitope tag was stably
expressed in human embryonic kidney (HEK) 293 cells, and the effects o
f prolonged opioid agonist treatment on receptor regulation were exami
ned, In HEK 293 cells the expressed mu receptor showed high affinity,
specific, saturable binding of radioligands and a pertussis toxin-sens
itive inhibition of adenylyl cyclase. Pretreatment (1 h, 3 h, or overn
ight) of cells with 1 mu M morphine or [D-Ala(2)MePhe(4),Gly(ol)(5)] e
nkephalin (DAMGO) resulted in no apparent receptor desensitization, as
assessed by opioid inhibition of forskolin-stimulated cAMP levels. In
contrast, the morphine and DAMGO pretreatments (3 h) resulted in a 3-
4-fold compensatory increase in forskolin-stimulated cAMP accumulation
. The opioid agonists methadone and buprenorphine are used in the trea
tment of addiction because of a markedly lower abuse potential. Pretre
atment of mu receptor-expressing HEK 293 cells with methadone or bupre
norphine abolished the ability of opioids to inhibit adenylyl cyclase.
No compensatory increase in forskolin-stimulated cAMP accumulation wa
s found with methadone or buprenorphine; these opioids blocked the com
pensatory effects observed with morphine and DAMGO. Taken together, th
ese results indicate that methadone and buprenorphine interact differe
ntly with the mouse mu receptor than either morphine or DAMGO. The abi
lity of methadone and buprenorphine to desensitize the mu receptor and
block the compensatory rise in forskolin stimulated cAMP accumulation
may be an underlying mechanism by which these agents are effective in
the treatment of morphine addiction.