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Vol. 60, Issue 4, 816-827, October 2001
-Opioid Agonist and Inverse-Agonist Binding Capacity
Département de Biochimie (G.P., M.A., M.B.), Pharmacologie
(A.D.L.), and Institut des Recherches Cliniques de Montréal
(P.S.), Université de Montréal, Montréal, Canada
This study assessed the effects of short-term treatment (30-min)
with inverse agonists on receptor protein levels and on the ability of
agonists, inverse agonists, and neutral antagonists to bind to the
human
-opioid receptor (h
OR). Incubation of human embryonic
kidney 293s cells stably expressing h
OR with the inverse agonist
ICI174864 (1 µM) induced reciprocal changes in agonist and
inverse-agonist binding. The total number of binding sites recognized
by the agonists [3H]bremazocine and
[3H][D-Pen2,D-Pen5]-enkephalin
was reduced by 33 and 57%, respectively, whereas binding capacity for
the radiolabeled inverse-agonist
[3H]Tyr-TicY[CH2NH]Cha-Phe-OH
increased by 44%. In contrast, total receptor protein and sites
labeled by neutral antagonists [3H]naltrindole and
[3H]Tyr-D-Tic-Phe-Phe-OH remained unchanged.
Pertussis toxin (PTX) and 5-guanylylimidodiphosphate (GppNHp) mimicked
the outcome of ICI174864 pretreatment in promoting the loss of agonist
binding sites. The lack of an additive effect on
[3H]bremazocine binding when these three agents were
combined indicates that inverse agonists may, in part, share the
mechanism by which GppNHp and PTX reduce agonist binding capacity.
Spontaneous recovery of maximal agonist binding capacity after
inverse-agonist treatment was slow, suggesting a decrease in the
isomerization rate between agonist- and inverse agonist-preferring
conformations. Overall, the data presented are consistent with the idea
that h
ORs exist in multiple states capable of discriminating among
ligands of different levels of efficacy and show that, after short-term
treatment with an inverse agonist, the receptor ability to adopt
conformations preferentially induced by agonist ligands is reduced.
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