C. Zöllner • C. Stein (✉)
Klinik für Anaesthesiologie und operative Intensivmedizin, Charit´e–Universitätsmedizin Berlin, Campus Benjamin Franklin, Hindenburgdamm 30, 12200 Berlin, Germany firstname.lastname@example.org
Abstract Opioids are the most effective and widely used drugs in the treatment of severe pain. They act through G protein-coupled receptors. Four families of endogenous ligands (opioid peptides) are known. The standard exogenous opioid analgesic is morphine. Opi- oid agonists can activate central and peripheral opioid receptors. Three classes of opioid
receptors (μ, δ, κ) have been identiﬁed. Multiple pathways of opioid receptor signaling (e.g.,
Gi/o coupling, cAMP inhibition, Ca++ channel inhibition) have been described. The dif-
ferential regulation of effectors, preclinical pharmacology, clinical applications, and side
effects will be reviewed in this chapter.
Keywords Central and peripheral opioid receptors · Signal transduction ·
Clinical applications · Pharmacokinetics and side effects · Routes of administration
Opium is an extract of the exudate derived from seedpods of the opium poppy, Papaver somniferum. The poppy plant was cultivated in the ancient civilizations of Persia, Egypt, and Mesopotamia, and the ﬁrst known written reference to the poppy appears in a Sumerian text dated around 4000 bc. Opium was inhaled or given through punctures in the skin, which subsequently led to analgesia, but also to respiratory depression and death due to variable rates of absorption. Opium is a complex chemical cocktail containing sugars, proteins, fats, water, plant wax, latex, gums, and numerous alkaloids, most notably morphine (10%–15%), codeine (1%–3%), noscapine (4%–8%), papaverine (1%–3%), and thebaine (1%–2%). Many alkaloids are used medicinally to treat pain (morphine, codeine), cough (noscapine, codeine), and visceral spasms (papaverine). The chemist Sertürner was the ﬁrst to publish the isolation of morphine, which he named after the god of dreams (Schmitz 1985). Morphine is a potent analgesic and it was more predictable than opium, which subsequently led to its widespread use in the treatment of acute and chronic pain.
Early binding studies and bioassays deﬁned three types of opioid receptors
(Lord et al. 1977; Martin et al. 1976; Pert and Snyder 1973), the μ-, δ- and
κ-receptors. A number of other receptor types have been proposed (e.g., sigma, epsilon, orphanin) but are currently not considered “classical” opioid recep- tors (Kieffer and Gavériaux-Ruff 2002). The identiﬁcation of opioid receptor complementary DNA (cDNA) allowed for the independent study of individual opioid receptor types with regard to pharmacological proﬁle, cellular effec- tor coupling, anatomical distribution, and regulation of expression. The ﬁrst cDNA encoding an opioid receptor was isolated simultaneously by two lab- oratories in 1992 (Evans et al. 1992; Kieffer et al. 1992). When expressed in transfected cells, the protein showed the expected pharmacological proﬁle of
the δ-receptor. Expression in mammalian cells indicated that the δ-receptor has structural characteristics similar to the family of seven transmembrane
G protein-coupled receptors (GPCRs) (Fig. 1). Subsequently, the μ- and the κ-receptor were cloned (Meng et al. 1993; Wang et al. 1993). The μ-receptor gene shows approximately 50%–70% homology to the genes encoding for the δ-receptor and κ-receptor.
The concept of receptor subtypes has emerged from classical pharmacolog-
ical data to explain biphasic binding characteristics of opioid receptor ligands
Fig. 1 The seven α-helical transmembrane (TM) domains characteristic of the μ-receptor. N-terminal tail and extracellular loops (ECL) are above the TM domains.A disulﬁde bond connects ECL1 and ECL2. The intracellular loops (ICL) 1–3 and the C-terminal tail contain multiple serines and threonines that are potential phosphorylation sites for protein kinases
(Pasternak 2004). Using radiolabeled agonists and antagonists different high- afﬁnity binding sites were detected and termed as μ1- and μ2-receptor (Paster- nak 2004). In vivo pharmacological studies proposed δ1- and δ2-receptor sub-
types (Mattia et al. 1991) and κ1-, κ2, and κ3-receptor subtypes (Attali et al.
1982). However, only three opioid receptor genes have been characterized so
far. Subtypes may result from alternative processing, splice variants, or a com- bination of the two. Splicing of μ-receptor messenger RNA (mRNA) has been observed in a variety of species. The cytoplasmic tail of the rat μ-receptor undergoes alternative splicing leading to two isoforms, μ-receptor1 and μ- receptor1B. These receptor variants share 100% amino acid sequence identity up to amino acid 386, but differ from residue 387 to the carboxyl terminus. When expressed in human embryonic kidney (HEK293) cells, both isoforms exhibit similar pharmacological proﬁles; however, μ-receptor1B appears to be more resistant to agonist-induced desensitization than μ-receptor1 (Koch et al.
1998). In addition to alternative splicing, posttranslational modiﬁcations of the gene product (glycosylation, palmitoylation, phosphorylation) or receptor
dimerization to form homomeric and heteromeric complexes might explain pharmacologically deﬁned differences in opioid receptor binding (Gavériaux- Ruff and Kieffer 1999b).
In addition to classic pharmacological methodology, the contribution of
each receptor to opioid actions in vivo can be assessed by genetic approaches. For example, synthetic antisense oligodeoxynucleotides hybridize to comple- mentary sequences in the target gene or its mRNA, thereby leading to reduced transcription, translation, and protein levels. Antisense studies have conﬁrmed
the contribution of μ-, δ-, and κ-receptors in opioid-induced analgesia (Ki- effer 1999). The availability of techniques to knock out genes now permits
unprecedented selectivity in the removal of responses mediated by the respec- tive encoded protein. While this approach circumvents some shortcomings of conventional pharmacology (e.g., limited duration of action and variable selectivity of agonists, antagonists, or antibodies) it is itself limited by com-
pensatory developmental changes during embryogenesis and adolescence and by variable genetic backgrounds (Kieffer and Gavériaux-Ruff 2002). Knockout studies have shown that the removal of any single opioid receptor does not
result in major changes of basal pain thresholds or other behaviors, but they have conﬁrmed that all three classes of opioid receptors mediate analgesia induced by their respective agonists, and that there is only one gene encoding
for each receptor (Kieffer 1999). Those data have also demonstrated a critical role of the μ-receptor in cannabinoid and alcohol reinforcement, an impor- tant role for the δ-receptor in emotional behaviors, and an involvement of κ-receptors in dysphoric responses (Gavériaux-Ruff and Kieffer 2002). Mice lacking all three opioid receptors are viable and healthy (Simonin et al. 2001). Triple mutants allow for exploration of the molecular basis of unusual opioid
receptor subtypes or nonclassical opioid responses. For example, it was shown that κ-2 receptor binding is most likely related to binding to the three known
opioid receptors and that no additional receptor is required to explain κ-2 receptor pharmacology (Simonin et al. 2001). In addition, it was shown that the immunosuppressive action of naltrindole is not mediated by opioid recep- tors (Gavériaux-Ruff et al. 2001). Triple knockout mutants may also help us to explore the molecular basis for nonopioid activity of different opioid peptides (Narita and Tseng 1998).
Structural Features of Opioid Receptors
Among GPCRs, bovine rhodopsin is the only receptor whose structure has been solved at high resolution (Palczewski et al. 2000). Comprehensive biochemical and mutational analyses of the transmembrane segments of all three opioid receptors and the ability to align receptor sequences by highly conserved residues in each helix support the use of the rhodopsin structure as a template to model other GPCRs, including the opioid receptors. Opioid receptors have a high similarity in transmembrane (TM) domain 2, 3, 5, 6, and 7, the three intracellular loops, and a short region of the C-terminal tail (Fig. 1). Almost no homology is found in the extracellular loops (ECL) or in the N-terminal tails. One of the few features preserved throughout the GPCR family is a disulﬁde bond connecting the second extracellular loop and TM 3. This disulﬁde bridge is found in more than 90% of all GPCRs and may be important in receptor function (Karnik et al. 2003). We demonstrated that an intact disulﬁde bond between a highly conserved cysteine residue in the second ECL and an equally well-conserved cysteine residue in the third ECL is an important regulator of μ-receptor activity (Zhang et al. 1999). For the chemoattractant C5a receptor, another GPCR, it was suggested that the ECL act as a ﬁlter and regulate the ability of the ligand to interact with the binding pocket (Massotte and Kieffer
2005). Evidence supports the notion that, in addition to guiding ligands on their way to the binding pocket, the ECL may regulate the “on-off” transition
in the absence of ligands (Klco et al. 2005).