DƯỢC LÍ Goodman & Gilman's The Pharmacological Basis of Therapeutics 12th, 2010

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SECTION II
NEUROPHARMACOLOGY
Nocistatin
Orphanin
98-127 Nocistatin MPRVRSLFQEQEEPEPGMEEAGEMEQKQLQ
130-146 Orphanin
149-165 Orphanin-2
FGGFTGARKSARKLANQ
FSEFMRQYLVLSMQSSQ
Figure 18–2. Human pro-orphanin-derived peptides.
Orphanin-2
also possess two conserved cysteine residues in the first and second
extracellular loops, which form a disulfide bridge.
Structural Correlates of Binding/Coupling
Requirements for Opiate Ligands
Opiate Receptor Structures. Studies of chimeric receptors and sitedirected
mutagenesis of cloned receptors have provided definitive
insights into structural determinants of opioid ligand–receptor interaction.
Though there is significant complexity (Kane et al., 2006),
several general principles define binding and selectivity. First, all
opioid receptors display a binding pocket formed by TM 3
-TM 7
.
Second, the pocket in the respective receptor is partially covered by
the extracellular loops, which together with the extracellular termini
of the TM segments, provide a gate conferring selectivity, allowing
ligands, particularly peptides, to be differentially accessible to the
different receptor types. Thus, alkaloids (e.g., morphine) bind in
the core of the transmembrane portion of the receptor, whereas large
peptidyl ligands bind at the extracellular loops. As noted, it is the
extracellular loops that show the greatest structural diversity across
receptors. Third, selectivity has been attributed to extracellular loops:
first and third for the MOR, second for the KOR, and third for the
DOR (Waldhoer et al., 2004). Alkaloid antagonists are thought to
bind deeper into the pocket sterically hindering conformational
changes leading to a functional antagonism.
Structure-Activity Relationships. Receptor selectivity by the various
opiate agonists is commonly explained in terms of the “message-address”
concept (Takemori and Portoghese, 1992). Thus,
elements shared by all structures (reflecting agents that bind at all
sites, such as naltrexone) represent the “message,” while elements
associated with a ligand binding at a specific receptor represent the
structural “address.” The common structural features constituting
the message are:
• a protonated nitrogen
• a phenolic ring (that, with the protonated nitrogen, forms
tyramine)
• a hydrophobic domain
To this “message” are added a variable “linker” region and the
“address” that specifies opiate receptor selectivity (Figure 18–3).
Refinements of this message-linker-address model have led to the
synthesis of new compounds with the predicted specificity. For the
KOR and DOR, elements constituting the address have been defined.
Thus, for the KOR, a second basic hydrophobic group is implicated
in forming a specific salt bridge; for the DOR, a hydrophobic group
such as an indole forms the address. MOR ligands such as morphine
lack a common chemical moiety and thus other elements are thought
to contribute to ligand specificity at that receptor (Kane et al., 2006).
Opiate Receptor Coupling to Membrane Function
Agonist binding results in conformational changes in the GPCR, initiating
the G protein activation/inactivation cycle (Chapter 3). The μ,
δ, and κ receptors largely couple through pertussis toxin-sensitive,
G i
/G o
proteins (but occasionally to G s
or G z
). Upon receptor activation,
the G i
/G o
coupling results in a large number of intracellular
events, including:
• Inhibition of adenylyl cyclase activity
• Reduced opening of voltage-gated Ca 2+ channels
• Stimulation of K + current though several channels including G
protein-activated inwardly rectifying K + channels (GIRKs)
• Activation of PKC and PLC β
As with other GPCRs, the second intracellular loop is involved in
the efficacy of G-protein activation while the third loop defines the
α subunit that is activated (Gether, 2000).
Regulation of Opiate Receptor Disposition
Like other GPCRs, MOR and DORs can undergo rapid agonistmediated
internalization via a classic endocytic, β-arrestin-mediated
pathway, whereas KORs do not internalize after prolonged agonist
exposure (Chu et al., 1997). Internalization of the MOR and DORs
apparently occurs via partially distinct endocytic pathways, suggesting
receptor-specific interactions with different mediators of intracellular
trafficking. These processes may be induced differentially as a
function of the structure of the ligand. For example, certain agonists,
such as etorphine and enkephalins, cause rapid internalization of the
receptor, whereas morphine does not cause MOR internalization,
even though it decreases adenylyl cyclase activity equally well. In
addition, a truncated receptor with normal G-protein coupling recycles
constitutively from the membrane to the cytosol (Segredo et al.,
1997), suggesting that activation of signal transduction and internalization
are controlled by distinct molecular mechanisms. These studies
also support the hypothesis that different ligands induce different
conformational changes in the receptor that result in divergent intracellular
events, and they may provide an explanation for differences
in the spectrum of effects of various opioids.
Functional Consequences of Acute and Chronic
Opiate Receptor Activation
The loss of effect with exposure to opiates occurs over short- and
long-term intervals.
Desensitization. Acute agonist occupancy of the opiate receptors
results in activation of the intracellular signaling outlined previously.
In the face of a transient activation (minutes to hours), a phenomenon
called acute tolerance or desensitization can be observed that is specific
for that receptor and disappears with a time course parallel to the
clearance of the agonist. Short-term desensitization probably involves
phosphorylation of the receptors resulting in an uncoupling of the
receptor from its G-protein and/or internalization of the receptor.
Tolerance. Sustained administration of an opiate agonist (days to
weeks) leads to progressive loss of drug effect. Here tolerance refers
to a decrease in the apparent effectiveness of a drug with continuous