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Vol. 302, Issue 2, 502-509, August 2002
Department of Pharmacological and Pharmaceutical Sciences,
University of Houston, Houston, Texas
Morphine tolerance in vivo is reduced following blockade of the
orphanin FQ/nociceptin (OFQ/N)/opioid receptor-like 1 (ORL1) receptor
system, suggesting that OFQ/N contributes to the development of
morphine tolerance. We previously reported that a 60-min activation of
ORL1 receptors natively expressed in BE(2)-C cells desensitized both µ and ORL1 receptor-mediated inhibition of cAMP. Investigating the mechanism(s) of OFQ/N-mediated µ and ORL1 receptor cross-talk, we
found that pretreatment with the protein kinase C inhibitor, chelerythrine chloride (1 µM), blocked OFQ/N-mediated homologous desensitization of ORL1 and heterologous desensitization of µ opioid receptors. Furthermore, depletion of PKC by
12-O-tetradecanoylphorbol-13-acetate exposure (48 h, 1 µM) also prevented OFQ/N-mediated µ and
ORL1 desensitization. OFQ/N pretreatment resulted in translocation of
PKC-
, G protein-coupled receptor kinase 2 (GRK2) and GRK3 from the
cytosol to the membrane, and this translocation was also blocked by
chelerythrine. Reduction of GRK2 and GRK3 levels by antisense, but not
sense DNA treatment blocks ORL1 and µ receptor desensitization. This
suggests that PKC-
is required for GRK2 and GRK3 translocation to
the membrane, where GRK can inactivate ORL1 and µ opioid receptors
upon rechallenge with the appropriate agonist. Our results demonstrate
for the first time the involvement of conventional PKC isozymes in
OFQ/N-induced µ-ORL1 cross-talk, and represent a possible mechanism
for OFQ/N-induced anti-opioid actions.
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