Endogenous Ligands Pentapeptides that mimic opioid activity and activate opioid receptors were initially isolated from brain extracts. Two isoforms were identiﬁed: Met- enkephalin (Tyr-Gly-Gly-Phe-Met) and Leu-enkephalin (Tyr-Gly-Gly-Phe-Leu) (Hughes et al. 1975). The same and other opioid peptides (endorphins, END; dynorphins, DYN) were later isolated from brain, spinal cord, pituitary gland, adrenals, immune cells, and other tissues. The amino-terminal of these opi- oid peptides contains the Tyr-Gly-Gly-Phe-[Met/Leu] sequence. Three dis- tinct opioid precursors have been identiﬁed so far: prodynorphin (PDYN), proopiomelanocortin (POMC), and proenkephalin (PENK) (Comb et al. 1982; Kakidani et al. 1982; Nakanishi et al. 1979). Each of these precursors un- dergoes processing by proteolytic enzymes into DYN, END, and ENK, re- spectively. The POMC gene is transcribed and translated into a prohormone of 267 amino acids. Posttranslational processing of this large precursor pep-tide produces several smaller peptides, including β-endorphin, α-melanocyte- stimulating hormone (α-MSH), and ACTH. Posttranslational processing of PENK results in Met-enkephalin, Leu-enkephalin, and a number of other enkephalins, including bovine adrenal medulla peptide 22. The PDYN gene produces a prohormone of 254 amino acids, which encodes the opioid pep- tides dynorphin A and B (Höllt 1993). END, ENK, and DYN can bind to any of the known opioid receptors with varying afﬁnities. In addition, a novel family of endogenous opioid peptides have been discovered and termed en- domorphins (Zadina et al. 1997). Endomorphin-1 (Tyr-Pro-Trp-Phe) and endomorphin-2 (Tyr-Pro-Phe-Phe) bind to the μ-receptor with high afﬁnity (Horvath 2000).
Endogenous opioid peptides can be released from neurons and axon termi- nals by depolarization and can exert pre- and postsynaptic effects. In addition, endogenous opioids are produced in many nonneuronal tissues, notably also in lymphocytes, monocytes, and granulocytes in inﬂamed tissue (Rittner et al.
2001). In models of inﬂammatory pain, opioid peptide-containing immune cells migrate from the circulation to the injured tissue. Selectins and integrins
have been identiﬁed as important adhesion molecules which regulate the mi- gration of opioid-containing immune cells (Machelska et al. 1998, 2002). Upon
exogenous stressful stimuli or releasing agents (e.g., cold water swim stress, postoperative pain, corticotropin releasing factor, catecholamines), these pep- tides can be locally released and subsequently bind to opioid receptors on sen- sory neurons. By inhibiting the excitability of these neurons, they can increase
nociceptive thresholds (i.e., produce analgesia; Stein et al. 1993). Preclinical research using protease (“enkephalinase”) inhibitors to increase concentra-
tions of endogenous opioid peptides in various central and peripheral tissues is ongoing but has not resulted in clinical applications for pain treatment so far (Roques 2000).
Exogenous opioid ligands can be classiﬁed into three groups: full agonists, partial agonists/antagonists, and full antagonists. The standard to which all other opioid analgesics are compared is morphine. Although the alkaloid mor- phine was isolated in the early 1800s the structure of morphine was identiﬁed only in 1925 by Gullard and Robinson (Fig. 3a). It was hypothesized that the piperidine ring is essential for its pharmacological activity. The nitro- gen atom of this ring is normally positively charged and can interact with
Fig. 3a–d Structures of classical opioid receptor agonists: a morphine, b the 4- phenylpiperidine fentanyl, and c the diphenylpropylamine methadone. d Structure of the classical opioid receptor antagonist naloxone
a negatively charged counterpart. Many pharmacologists and chemists tried to produce more potent synthetic opioids with less physical and psychological dependence. The 4-phenylpiperidines (e.g., fentanyl; Fig. 3b), the diphenyl- propylamines (e.g., methadone; Fig. 3c), and many derivatives (e.g., alfentanil, sufentanil, remifentanil) have different structures from morphine. Ligands at μ-receptors produce potent analgesia; however, they also produce side ef- fects like respiratory depression, constipation, physical dependence, and tol- erance. In 1942 the substitution of an allyl group for the methyl group on the nitrogen atom of morphine produced the ﬁrst opioid receptor antago- nist, nalorphine. Nalorphine not only countered the effects of morphine but
also produced limited analgesia mediated through κ-opioid receptors. An ad- ditional member of this mixed agonist/antagonist group is buprenorphine, a semi-synthetic opiate derivative of thebaine. Buprenorphine is a partial ag-
onist at the μ-receptor. At low doses it produces typical morphine-like ef- fects, and at higher doses it has a reduced intrinsic activity compared to pure agonists (Cowan et al. 1977). Buprenorphine also has the properties
of a κ-receptor antagonist (Negus et al. 1989). Naloxone was developed as a pure opioid receptor antagonist, which has a structure similar to that of morphine (Fig. 3d). Several newer compounds with restricted access to the CNS have been synthesized. These were designed with the aim to activate pe-
ripheral opioid receptors exclusively, without the occurrence of central side effects (Binder et al. 2001; Furst et al. 2005; Kumar et al. 2005; Whiteside et al. 2004).
Sites of Action
Supraspinal and Spinal Sites
Various major brain regions containing opioid peptides and opioid receptors have been identiﬁed including the periaqueductal gray, the locus coeruleus, and the rostral ventral medulla (Heinricher and Morgan 1999; Fig. 4a). All three opioid receptors are also present in the dorsal horn of the spinal cord, which is in another area important for opioid-induced analgesia (Fig. 4b).
The μ-, δ-, and κ-opioid receptors are mainly located in the upper laminae, particularly the substantia gelatinosa (laminae I and II). In addition, δ-opioid
receptors are found in the deeper laminae of the dorsal horn and in the ven- tral horn (Gouarderes et al. 1993). Opioid receptor activation mostly results in depression of neuronal ﬁring. Presynaptically, opioids inhibit Ca2+ inﬂux and the subsequent release of glutamate and neuropeptides (e.g., substance P,
calcitonin gene-related peptide) from primary afferent terminals. Postsynap- tically, opioids hyperpolarize ascending projection neurons by increasing K+ conductance.
Fig. 4a An autoradiographic receptor binding technique showing the distribution of μ- receptors within rat brain slices. An increase in μ-receptor density (indicated with arrows) shows their distribution in various regions, including the cortex, thalamus, hypothalamus, and brainstem. b Immunocytochemical studies showing μ-receptors within laminae I–II of spinal cord dorsal horns from rats (kindly provided by Dr. S. Mousa). c Immunocyto- chemical studies from rat hindpaw preparations indicating μ-receptors on primary afferent neurons (also kindly provided by Dr. S. Mousa)
It became clear in the late 1980s that opioid receptors and opioid peptides are also located in the peripheral nervous system, including primary afferent neurons and dorsal root ganglia (DRG) (Stein et al. 1989; Stein et al. 2003). Opioid receptors have been shown mainly on small- to medium-diameter neu- ronal cell bodies of sensory neurons (Mousa et al. 2000). After synthesis in the DRG, opioid receptors are transported to the peripheral nerve terminals of primary afferent neurons (Hassan et al. 1993; Fig. 4c). Opioid receptors are also expressed by neuroendocrine (pituitary, adrenals), immune, and ecto- dermal tissues (Slominski et al. 2000). Although opioids increase potassium currents in the CNS it is still controversial whether this occurs in DRG neu-
rons. Rather, it was shown that the modulation of Ca2+ currents is the principal mechanism for the inhibitory effect of opioids on sensory neurons (Akins and
McCleskey 1993). Recently, G protein-coupled inwardly rectifying potassium channels (GIRK2) and μ-opioid receptors were colocalized on sensory nerve endings in epidermis, and it was proposed that endothelin-B receptors trigger the release of endorphin from keratinocytes, suppressing pain via opioid re- ceptors coupled to GIRK channels (Khodorova et al. 2003). In addition, it was shown that opioids activate inhibitory Gi/o proteins, which leads to a decrease of cAMP in peripheral sensory neurons (Chen et al. 1997).
Peripheral Opioid Receptors and Inflammation
In animal experiments, local application of opioid receptor agonists elicits a more pronounced antinociceptive effect under painful inﬂammatory condi- tions than in noninﬂamed tissue. It was shown that subcutaneous inﬂamma- tion can induce an upregulation of μ-opioid receptor mRNA within the lumbar spinal cord (Maekawa et al. 1996) and DRG (Puehler et al. 2004). In addition, the expression of opioid receptors in sensory neurons increases time-dependently during inﬂammation (Zöllner et al. 2003). Subsequently, the axonal transport of opioid receptors to the peripheral nerve terminals is augmented (Hassan et al.
1993; Laduron and Castel 1990). This increase might be related to cytokines
(e.g., interleukin 4) through the binding of STAT-6 transcription factors to the μ-opioid receptor gene promoter (Kraus et al. 2001). Other potential mecha- nisms contributing to enhanced antinociceptive efﬁcacy include an increase in the number of opioid receptor bearing peripheral sensory nerve terminals (Stein et al. 2003), an increase in G protein coupling (Shaqura et al. 2004; Zöllner et al. 2003), a disruption of the perineural barrier (Antonijevic et al.
1995), and an enhanced opioid receptor trafﬁcking to the neuronal membrane
(Patwardhan et al. 2005).
Opioids are the most broadly effective analgesics and are used in both acute and chronic pain. Typical acute pain situations include intraoperative, post- operative, and posttraumatic pain. In those situations opioids are used pre- emptively (i.e., before occurrence of an anticipated noxious stimulus, e.g., during induction of anesthesia) or therapeutically (i.e., after occurrence of noxious stimulation). Whereas acute pain is generally amenable to drug ther- apy, chronic pain is a complex disease in its own right and needs to be differ- entiated into malignant (cancer-related) and nonmalignant (e.g., neuropathic,
inﬂammatory) pain. Acute and cancer-related pain are commonly responsive to opioids. Chronic nonmalignant pain requires a multidisciplinary approach encompassing various pharmacological and nonpharmacological (e.g., psy- chological, physiotherapeutic) treatment strategies. Various routes of opioid administration (e.g., oral, intravenous, subcutaneous, intrathecal, epidural, topical, intraarticular, transnasal) are used, depending on the clinical circum- stances. The opioid effect is a selective one on nociception. Touch, pressure, and other sensory modalities are generally unaffected. After systemic adminis- tration, forebrain mechanisms play a prominent role in the clinical effects, and a common clinical manifestation of opioid analgesia is a change in the affective response to pain. Spinal mechanisms become proportionally more important when opioids are given by neuraxial injection. Patients given systemic opioids will typically say that pain is still present, but the intensity is reduced and it no longer bothers them as much. Mental clouding and dissociation from pain is often accompanied by mood elevation.