Our lab focuses on the mechanism of action of opioid drugs. Our
work has involved studies that target opioid drug action; beginning
with the interaction of opioids with specific opioid receptors and
culminating with functional drug effects. As with many other
neurotransmitter systems, opioid receptors have dynamic properties
and can be actively regulated by exposure to agonists and
antagonists. Although, the potency of opioids can be affected by a
number of elements in the cascade leading from receptor to effect,
the properties and density of receptors can have important effects
on drug potency and action. The overall focus of our lab has been
the regulation of opioid receptors by opioid drugs. We have been
particularly interested in how changes in receptor characteristics
and the intracellular messengers coupled to receptors, as well as
the proteins involved in receptor trafficking, can impact on opioid
drug potency. Throughout our studies, a systems molecular
pharmacological approach is employed.
Studies in our lab have examined the regulation of opioid
receptors by both opioid agonists and antagonists. The results of
these studies have shown that opioid receptor regulation by opioid
agonists and antagonists appears to rely on different mechanisms.
Chronic treatment with the opioid antagonists (naloxone,
naltrexone) increases the density (upregulation) opioid receptors
as determined in receptor binding assays. However, there is no
corresponding increase in receptor abundance determined in
immunoassays (Yoburn et al, 2004). The increase in receptor density
is closely associated with increased potency of opioid agonists
such as morphine. Recent data indicate that the opioid
antagonist-induced increase in receptor density in vivo does not
depend upon changes in receptor gene expression.
Opioid agonists also regulate opioid potency and receptors.
Chronic treatment with an opioid agonist such as morphine typically
produces a reduction in opioid agonist potency (i.e., tolerance).
While tolerance is regularly observed following both acute and
chronic opioid agonist treatment protocols, decreases in receptor
number (downregulation) are not a necessary condition for the
expression of tolerance. However, decreases in receptor number can
be produced by chronic exposure to opioid agonists which have high
intrinsic efficacy; a property which can be roughly described as
the ability of a drug to produce an effect when bound to a
receptor. Opioid agonist-induced changes in opioid receptor density
have been shown to be associated with a change in opioid receptor
gene expression, as well as a decreased in receptor abundance in
immunoassays. Recently, we have begun to characterize "receptor
density-independent" and "receptor density-dependent" pathways for
tolerance in vivo. Our data (Shen et al., 2000; Stafford et al,
2001) suggest that Protein Kinase A and other signaling proteins
(e.g., G i α2) play a major role in receptor desensitization, but
are minimally important in mediating opioid agonist-induced
u-opioid receptor downregulation. Furthermore, it seems clear that
changes in receptor density (e.g., downregulation) have
consequences on opioid potency (see Stafford et al., 2001).
Taken together, chronic opioid antagonist treatment increases
the density of opioid receptors and the potency of opioid agonists,
but does not require a change in receptor gene expression.
Tolerance to opioid agonists develops in the absence of changes in
receptor density, but when agonist-induced receptor downregulation
occurs, changes in receptor gene expression and increases in
tolerance have been found. These results suggest that opioid
agonist potency can depend upon several cellular systems and that
opioid receptor regulation can occur via different pathways.
Recent studies are concerned with examining the intracellular
signaling substrates for receptor regulation and opioid tolerance.
Using several approaches, the role of G-proteins in opioid
tolerance and receptor regulation has been probed. Our results
support suggestions that PTX sensitive G-proteins and G i α2 in
particular, are necessary for tolerance and acute opioid effects
(see Gomes et al., 2002; Yoburn et al, 2003). However,
agonist-induced downregulation and antagonist-induced upregulation
are independent of these G-proteins. In recent experiments using
the mouse spinal cord as a model for receptor regulation and
tolerance, we have found that opioid antagonist-induced u-opioid
receptor upregulation is mediated by downregulation of GRK2 and
Dynamin2 (Patel et al, 2002a; Zhang et al., in press), whereas
agonist-induced downregulation depends upon increases in dynamin 2
abundance (Patel et al, 2002b; Yoburn et al., 2004). Overall, our
studies suggest markedly different cellular pathways and mechanisms
that mediate opioid tolerance and receptor regulation by opioid
agonists and antagonists. Finally, our data indicate that
regulation of opioid receptors in the intact mouse engages systems
that modify the abundance and expression of numerous signaling and
trafficking proteins.