8

Progress in Opiate Pharmacology

 

 
Gavril W. Pasternak
The Cotzias Laboratory of Neuro-Oncology, Memorial Sloan-Kettering Cancer Center and Departments of Neurology and Neuroscience, and Pharmacology, Cornell University Medical College, New York, New York 10021

Pain remains a major problem in medicine and opiates are the mainstay for its treatment, but not without problems. Side effects, particularly in tolerant patients taking high doses of drugs, coupled with a misplaced concern for the addictive potential of these agents, often interfere with their appropriate use. Until recently, all clinically used opioid analgesics were considered pharmacologically equivalent to morphine with respect to both analgesic mechanisms and side effects, differing only in potency, duration of action, or oral availability. However, this is an oversimplification that can no longer be supported. Seven distinct subtypes of opioid receptors have been identified, of which at least six are capable of modulating pain perception (Table 1). Compounds acting through many of the recently discovered subtypes have been developed and display unique pharmacological profiles. Furthermore, recent evidence now indicates that many of the drugs used clinically act in part through these newly identified receptor subtypes, suggesting new approaches to their rational use. Thus, the presence of multiple opioid receptors opens new perspectives in the actions of analgesics and in the development of new, highly selective analgesics with fewer side effects. This review summarizes recent advances in the area of opioid receptor heterogeneity and their implications in the management of pain.

CLASSIFICATION OF OPIOID RECEPTORS

Opioid receptors have been classified by their selectivity in binding and pharmacological assays (Tables 1 and 2) and, more recently, through a series of selective antagonists (Table 3). Mu receptors were first defined by their high selectivity for morphine in pharmacological studies (53). Over the years a number of highly selective antagonists for mu receptors have been developed, including b-funaltrexamine (b-FNA) (83), which have been instrumental in the study of mu actions.

A number of years ago we proposed two subdivisions of mu receptors (mu1 and mu2) to explain a novel binding site first identified in 1975 (28,68,69,114). Since then, a number of groups using different approaches have confirmed the presence of these binding sites (22,51,52,107). Rothman (personal communication) has proposed a mu/delta complex (93,95), which corresponds mu1 receptor. Although both sites potently bind morphine and other mu ligands, their overall binding selectivity profiles differ significantly (12). The two subtypes are also differentiated by the mu1-selective antagonists naloxazone and naloxonazine (12,31,47,67,69). In contrast to b-FNA, which antagonizes both mu receptor subtypes, naloxazone and naloxonazine can selectively reverse mu actions (89). Although binding studies provide strong evidence for the presence of two discrete subpopulations of mu receptors, pharmacological studies are even more convincing, as described below.

Delta receptors were first proposed from comparisons of the enkephalins with morphine in bioassays (50). These early studies were hampered by the lability of the natural enkephalins. The development of stable analogs has enabled extensive studies into both the binding and the pharmacology of these agents (11,19,23,29,88,92). [3H][D-Pen2,D-Pen5]enkephalin ([3H]DPDPE) binding has been well documented (Table 2) and the pharmacology of DPDPE and other analogs extensively studied. The availability of highly selective antagonists, including naltrindole and ICI174,864 (20), has also proven quite useful.

Of all the classes of opioid receptors, the kappa class is the most complex (1,10,13,14,25,30,40,41,57,63,76,94,104,117). It was initially proposed by Martin et al. (53) following detailed pharmacological studies. The development of highly selective agonists U50,488H and U69,593 (14,41,111) and the antagonist nor-binaltorphimine (norBNI) (104) helped to define the kappa receptor. More recent studies utilizing complex binding paradigms have uncovered several U50,488H-insensitive kappa sites. The first one described, termed kappa (118), has been confirmed in binding studies from several laboratories. Unfortunately, its pharmacological relevance remains unknown since there are no highly selective ligands and the use of mu and delta competitors cannot be extrapolated into pharmacological studies. More recently we reported yet another U50,488H-insensitive kappa receptor, kappa, using a novel opiate probe, naloxone benzoylhydrazone (NalBzoH) (13,85). The very high density of this receptor, typically twice the levels of either mu or delta receptors, makes kappa3 site the predominant opioid receptor in the brain. Unlike the kappa2 receptor, the kappa3 receptor has been extensively characterized pharmacologically, as discussed below. The kappa site probably corresponds to the kappaa site observed in detailed competition studies (94).
Chapter 8 table 1: Classes of opioid receptors
 
Receptor Agonists Analgesia Other actions
Mu 
  Mu1		 Morphine,
  DAMGO,
  Morphiceptin
  PL_017

Supraspinal

 Prolactin release, acetylcholine release 
(brain), free feeding, deprivation-induced feeding, catalepsy
   Mu2   Spinal Respiratory depression, inhibition of GI 
transit, most signs of physical dependence, guinea pig ileum bioassay, most cardiovascular effects, dopamine turnover (striatum)
Kappa
   Kappa1		 Dynorphin A
U50,488H, U69,593		 Spinal Diueresis
   Kappa2 Bremazocine unknown unknown
   Kappa3 NalBzoH, nalrophine Supraspinal Feeding
Delta 
   Delta1		 DSLET, DADL
DPDPE

Spinal

 Mouse vas deferenes bioassay
 
   Delta2 [D-Ala2]deltaphin II Supraspinal  
 

MECHANISMS OF OPIOID ANALGESIA

The perception of pain associated with an injury is highly dependent on the situation in which it occurs. In landmark studies from World War II, Beecher documented lower analgesic requirements for wounded soldiers than for civilians undergoing surgery back in the United States, despite the greater severity of the battle wounds (3). These differences illustrate the ability of the brain to ``filter'' nociceptive input. Experimental studies have confirmed the presence of pain-modulating systems within the CNS. Electrical stimulation of the periaqueductal gray and periventricular gray in rodents (58) and humans (91) produces analgesia that is readily reversed by the selective opiate antagonist naloxone, implying the release of morphine-like, or opioid, compounds. There is now extensive evidence for a large family of endogenous opioid peptides, composed of the enkephalins, dynorphins and b-endorphin (20), which, together with their receptors, constitute a complex system capable of influencing the subjective sensation of pain. The criteria for activating these systems physiologically remain unclear. Pharmacologically, these systems are activated by the opioid analgesics.

The discovery of multiple subclasses of opiate receptors in binding studies was soon followed by the demonstration of a number of discrete analgesic systems that were capable of independently relieving pain: mu, mu, kappa, kappa, and delta receptors. Defining these distinct systems has rested on a number of approaches, including selective antagonists and cross tolerance.

Morphine is a potent analgesic when given systemically. In addition to its sensitivity toward traditional antagonists such as naloxone, morphine analgesia is also effectively antagonized by b-FNA, a mu-specific antagonist (112). Morphine analgesia is unaffected by antagonists selective for kappa (norBNI) (104) or delta (naltrindole) receptors (84). Systemic morphine analgesia has been subclassified as mu receptors in both rats and mice, based on its sensitivity toward naloxazone and naloxonazine (47,66). However, the mu analgesic system is complex and subsequent studies also have verified the importance of spinal mu mechanisms, as described below. The ability of naloxonazine to antagonize systemic morphine analgesia implies that the mu system is more sensitive.

One additional aspect of morphine analgesia deserves mentioning. Morphine is readily converted to a number of metabolites, of which the glucuronides appear to be most important (34). Early studies suggested that the 6b-glucuronide metabolite had analgesic activity (98), whereas the 3-glucuronide was inactive. We examined this question and observed that morphine-6b-glucuronide is quite active (65,73). Indeed, when administered directly into the CNS, it is more than 100-fold more active than morphine itself.

U50,488H is highly selective for kappa receptors and is a powerful analgesic in mice when given systemically (111). It is closely related to spiradoline, another kappa analgesic that is active in humans. Unlike morphine, U50,488H analgesia is not antagonized by b-FNA, but is sensitive to norBNI (105). Neither naloxonazine nor naltrindole reverses U50,488H analgesia. Thus, U50,488H analgesia has a unique antagonist profile.

NalBzoH is a mixed agonist/antagonist that has proven useful in studying kappa actions. Although it is an antagonist against other receptors, NalBzoH is a potent kappa agonist (27,71). Given systemically, NalBzoH produces analgesia in mice and rats, although some strains are more sensitive than others (95). NalBzoH analgesia is readily reversed by traditional opiate antagonists such as WIN44,441, but not by the selective antagonists b-FNA (mu), norBNI (kappa), or naltrindole (delta). Delta mechanisms mediating analgesia also have been demonstrated (21,38,75).
 

TOLERANCE AND CROSS TOLERANCE

Tolerance is observed with all opioids following chronic administration, regardless of the receptor class involved (100). However, tolerance develops independently for each receptor system (71,100). For example, mice tolerant to morphine are cross tolerant to other mu drugs, but not to agents that act through other receptor classes, such as U50,488H (kappa), NalBzoH (kappa), or DPDPE (delta). Similarly, mice tolerant at kappa receptors following chronic administration of U50,488H retain their analgesic sensitivity toward morphine and NalBzoH. Daily injections of NalBzoH also produce tolerance at kappa receptors without affecting the actions of morphine or U50,488H. This lack of cross tolerance among the various receptor classes provides further evidence for distinct analgesic systems and may help to explain some clinically relevant issues.

Most clinically used analgesics relieve pain through more than one receptor mechanism. Even morphine, a relatively selective mu ligand, activates both mu and mu systems. Other agents, such as levorphanol, pentazocine, and nalbuphine (79,106), utilize kappa systems either in addition to or instead of mu receptors. This lack of selectivity for some agents might help to explain the concept of incomplete cross tolerance. For years clinicians have noted that switching a tolerant patient to an alternative drug often restores analgesic effectiveness. A number of issues probably play a role in this effect, but the concept of receptor multiplicity offers an interesting possibility. As noted above, tolerance develops to each receptor subtype independently from the others. Thus, tolerance to morphine may be primarily restricted to mu systems. Giving a patient a less selective opiate capable of activating non-mu receptors might restore analgesia by activating these additional, nontolerant receptor systems. This concept has been demonstrated in an animal model (61).

Clinically, tolerance typically is overcome by increasing drug dosages. Escalation of dose is usually limited by the appearance of side effects rather than the loss of analgesic actions. In the clinical setting the degree of tolerance can be extraordinary. Many of our highly tolerant cancer patients require intravenous morphine infusions at rates greater than 100 mg/hr and, in rare instances, more than 1,000 mg/hr have been needed. These higher doses, which can be 10- to 100-fold greater than those used in naive patients, may relieve pain by more effectively activating the tolerant receptors. However, at the high doses used in tolerant patients, the drugs will lose their selectivity for specific opioid receptor subtypes and activate additional classes with which they normally would not interact. Thus, their ability to continue to relieve pain might be due to their recruitment of additional analgesic systems utilizing alternative receptor classes.

The mechanism of opioid tolerance remains unclear. Although some evidence from tissue culture studies reveals decreased effects on second messenger systems (5,36,43,44,86,87,97,100), other studies have implicated the activation of antagonistic neuronal systems (8,18,21,64,78,101,113). Although a number of agents, including cholecystokinin antagonists and antidepressants, can ``reverse'' morphine tolerance, they probably do not influence the basic mechanisms involved, since they potentiate morphine analgesia in naive, as well as tolerant, subjects. Recently, it was observed that the N -methyl-[smd[nm-aspartate (NMDA) antagonist MK801 prevents the development of tolerance to morphine (108). A close association has been noted between many NMDA actions and nitric oxide (NO), a newly described neurotransmitter produced by activation of NO synthase (9,17,24,42). We examined the effects of an NO synthase inhibitor, N ;xG-nitro-[sml[nm-arginine (NOArg), on opioid tolerance (38,39). After daily administration of 5 mg/kg of morphine, the analgesic response in mice declines from its initial value of 60% to 0%. Coadministration of morphine with NOArg (2 mg/kg) prevents tolerance for longer than 4 weeks. NOArg also can reverse preexisting tolerance. The actions of NOArg are restricted to mu systems, since it had no effect on either kappa or kappa analgesia. NOArg also minimizes the development of dependence (37,39).

SITES OF OPIOID ACTION

Opiates typically act in several regions of the CNS. For simplicity, it is easiest to localize central systems either supraspinally or spinally. At the spinal level, the opioids act within the dorsal horn, an area containing both opioid peptides and their receptors (20). Supraspinally, the situation is not as clear. Although regions sensitive to morphine have been extensively mapped (45,75), we have little information regarding the supraspinal regions involved with other receptor systems. Finally, evidence also is accumulating for peripheral actions. When considering the sites of opioid action, it also is important to remember that many of these systems synergistically interact with one another. The descending mu systems from the brain stem to the spinal cord are an excellent example. Furthermore, virtually all clinical analgesics act through at least two receptor subtypes. Together, these observations underscore the complexity of opioid analgesia.

Supraspinal Analgesia

Microinjection studies of the brain have identified a number of regions sensitive to morphine, including the periaqueductal gray, locus coeruleus, nucleus raphe magnus, and nucleus gigantocellularis (33,56,75). Additional studies have subclassified these actions as mu based on the activity of selected agonists and their sensitivity toward naloxonazine (7). Supraspinal mu analgesic mechanisms also are important in mice (32,46,70,80).

Supraspinal kappa systems are limited to kappa, and not kappa, receptors (27,71,104). The analgesia from systemically administered NalBzoH is reversed by more than 1,000-fold more potently by antagonists given intracerebroventricularly than intrathecally (71). However, the specific supraspinal regions mediating kappa analgesia remain unknown.

The delta story is more complex (81,82). The delta ligand DPDPE is far less active supraspinally than spinally, leading some investigators to speculate that supraspinal delta systems are not very important in pain modulation. However, evidence now suggests the presence of two subdivisions of delta receptors based upon the delta-selective agonists DPDPE and [D-Ala2]deltorphin II and their differential antagonism by naltrindole-5;pr-isothiocyanate and [D-Ala2,Leu5]enkephalin-Cys6 (DALCE) (35). The site selective for DPDPE (delta) seems to be most effective at the spinal level, whereas the delta site may be more important supraspinally. However, these associations remain tentative and will require more investigation.

Spinal Analgesia

The spinal mechanisms of opioid analgesia are extremely interesting and have been extensively studied (96,109). Morphine is a potent analgesic when administered intrathecally and/or epidurally in animals (114) and in humans (15). The sensitivity of this response in rodents to b-FNA is consistent with a mu classification, whereas its insensitivity toward naloxonazine implicates a role for mu, and not mu, receptors (32,46,70,79). Thus, different mu receptors mediate spinal and superspinal analgesia. The sensitivity of systemic morphine analgesia to naloxonazine indicates that the supraspinal mu system is more sensitive than spinal ones.

Early studies with the kappa-selective agonist U50,488H and its antagonist norBNI demonstrated a spinal site of action for kappa analgesia (3,71). Administered systemically, U50,488H analgesia is reversed far more effectively by antagonists given intrathecally than intracerebroventricularly. Delta ligands such as DPDPE also are extremely potent analgesics at the spinal level in animals and the delta peptide [D-Ala2,D-Leu5]enkephalin (DADL) is a potent analgesic when given intrathecally in humans (62). Unfortunately, DADL is not as selective for delta receptors as the newer peptides such as DPDPE, leaving its association with delta receptors less firm.

Regional Interactions

The presence of multiple receptor subtypes capable of producing pain relief independently of each other only starts to reveal the full complexity of pain modulatory systems. Although each receptor class is able to modulate pain independently, evidence also supports profound interactions among these various systems. Studies as early as 1980 revealed synergistic, or multiplicative, interactions between supraspinal and spinal morphine (115). Given intracerebroventricularly, the morphine dose required to produce analgesia in 50% of animals (ED) was approximately 10 ng, whereas the ED at the level of the spinal cord was approximately 4 ng. However, if the morphine dose were equally split between the brain and spinal cord, the total dose of morphine required to produce the same analgesic response was only 0.7 ng (0.35 ng administered both i.c.v. and i.t.). These multiplicative interactions have great clinical importance. For example, epidural or intrathecal morphine may prove far more efficacious when given with a small dose of systemic drug.

More recent studies looking in microinjection of opiates into the brain suggest a similar synergistic interaction between the periaqueductal gray and locus coeruleus, two supraspinal regions (6; G. Rossi, R. Bodnar, and G. W. Pasternak, unpublished observations ). Other work also suggests multiplicative interactions between supraspinal kappa receptors and spinal mu receptors as well as between supraspinal kappa and spinal kappa receptors in mice (C. G. Pick and G. W. Pasternak, unpublished observations ). The presence of these synergistic interactions among receptor classes emphasize the complexity of analgesic mechanisms and raise the possibility that activation of more than one receptor class may greatly enhance analgesia.

Peripheral Analgesia

Although opiates certainly have potent central actions, they also exert actions peripherally (4,16,103). The presence of opioid receptors on peripheral nerves has long been established. Indeed, opioids inhibit the electrically stimulated muscle contractions in the guinea pig ileum and mouse vas deferens bioassays through activation of receptors localized to the plexus surrounding the contracting muscles. Opioids also possess peripheral analgesic actions in an inflammatory pain model in rodents and, more recently, in humans (102). In this study morphine given intra-articularly following arthroscopic surgery of the knee proved more effective in reducing pain than the same morphine dose given intravenously. Although many questions remain, peripheral mechanisms of systemically administered opioids should be seriously considered.

GENETICS OF OPIOID SENSITIVITY

Patients vary enormously in their analgesic needs. This variability is oftentimes ascribed to cultural and psychological differences, but evidence for a genetic component has been mounting. Genetic differences in opioid sensitivity in animals can be very dramatic (2,54,74,77,90,95,99,110) (Table 4). For example, the analgesic sensitivity of the CXBK strain of mouse to morphine is more than 25-fold lower than most typical mouse strains, such as BALB/c and CD-1. Furthermore, the sensitivity of the various opioid subtypes appear to be under independent genetic control. The CD-1 strain is quite sensitive to mu, kappa, and kappa drugs. The BALB/c strain is even more sensitive to morphine but is markedly less sensitive to the kappa drugs, while the Swiss-Webster strain shows a similar decreased sensitivity toward all agents. Genetic differences have even been observed between mu and mu analgesic systems. As noted above, the CXBK strain is markedly insensitive toward systemic morphine. This is carried over to intracerebroventricular injections, where doses more than 20-fold greater than the ED in CD-1 mice still produce analgesia in less than 15% of CXBK mice. Despite this dramatic supraspinal mu insensitivity toward morphine, the spinal mu system in CXBK mice is just as sensitive to morphine as CD-1 mice. These differences in sensitivity correspond to the decreased levels of mu, but not mu, binding sites in the CXBK mice (59,60). Thus, the genetic variability of opioid sensitivity in mice is quite dramatic and, based on these studies, it is reasonable to expect genetic differences among patient groups, although this has not been verified experimentally. The importance of cultural, emotional, and situational factors may make studies of the genetic sensitivity of patient groups difficult. However, the potential of genetic variability should reinforce the need to individualize the treatment of patients and their pain.
  Chapter 8 table 4 Sensitivity of mouse strains to mu, kappa1, and kappa3 analgesics
Strain Mu Kappa3 Kappa1 (%)
CD-1  76  54  70
Swiss-Webster  40  29  30
BALB/c  90  14  10
C57/bgJ  62  0  0
C57/+  40  0  0
CXBK  0  0  0
HS  62  0  0
Mice were given a single dose of the mu drug morphine (5 mg/kg, s.c.), the kappa3 agent NalBzoH (50 mg/kg, s.c.), or the kappa1 compound U50,488H (5 mg/kg, s.c.) and tested for analgesia in the tail flick test. Analgesia was defined as a doubling or greater of baseline latencies

Adapted from ref. 37
 
 

OTHER OPIATE ACTIONS

Opiates produce a number of actions other than analgesia, many of which limit their use. Sedation, nausea, vomiting, and constipation can be problematic, as can significant respiratory depression seen in patients with compromised pulmonary function. Like analgesia, many of these actions can be attributed to specific receptor subtypes. In rodents, both constipation and respiratory depression are mediated through mu receptors (26,32,48,49,72). The subtypes responsible for sedation, nausea, and vomiting have not been identified. However, with multiple classes of opiate receptors it is reasonable to expect that drugs highly selective for various subtypes might retain their analgesic activity with markedly different side-effect profiles.

CONCLUSION

Opioid receptor heterogeneity opens many new doors in the design and use of analgesics. In addition to the obvious use of selective agents, the localization of many of these subtypes to different parts of the neuraxis offer additional opportunities in pain control. The widespread use of epidural and intrathecal morphine now can be justified scientifically. The presence of other analgesic systems involving kappa and delta receptors at the level of the spinal cord opens many additional possibilities. Perhaps kappa drugs such as spiradoline should be administered at the spinal level to optimize its analgesic actions while minimizing dysphoria and psychomimetic actions that are elicited supraspinally. Similarly, preliminary studies of DADL in patients suggest that delta drugs may have a significant role in spinal analgesia. The availability of highly selective agents presumably also would lower the incidence of side effects, such as constipation and respiratory depression, that are commonly seen with mu drugs. The presence of synergistic regional interactions may also prove helpful. For example, the effectiveness of epidural opiates may be enhanced enormously by the concurrent administration of low doses of systemic drugs. Potential interactions between peripheral and complex central opioid systems also have not been fully explored. While our understanding of opioid analgesia has expanded remarkably over the past decade, many important questions remain.

ACKNOWLEDGMENTS

I thank Dr. J. B. Posner for his assistance and support. GWP is supported by a Research Career Development Award from the National Institute on Drug Abuse (DA00139).

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