Up-regulation and trafficking of delta opioid receptor in a model of chronic inflammation: implications for pain control

The delta agonists DPDPE and deltorphin II recruit predominantly mu receptors to produce thermal analgesia: a parallel study of mu, delta and combinatorial opioid receptor knockout mice

Delta-selective agonists have been developed to produce potent analgesic compounds with limited side-effects. DPDPE and deltorphin II are considered prototypes, but their delta-selectivity in vivo and the true ability of delta receptors to produce analgesia remain to be demonstrated. Here we have performed a parallel analysis of mu, delta and combinatorial opioid receptor knockout mice, in which we found no obvious alteration of G-protein coupling for remaining opioid receptors. We compared behavioural responses in two models of acute thermal pain following DPDPE and deltorphin II administration by intracerebroventricular route. In the tail-immersion test, both compounds were fully analgesic in delta knockout mice and totally inactive in mu knockout mice. In the hotplate test, the two compounds again produced full analgesia in delta knockout mice. In mu knockout mice, there was significant, although much lower, analgesia. Furthermore, DPDPE analgesia in the delta knockout mice was fully reversed by the mu selective antagonist CTOP in both tests. Together, this suggests that mu rather than delta receptors are recruited by the two agonists for the tail withdrawal and the hotplate responses. Finally, deltorphin II slightly prolonged jump latencies in double mu/kappa knockout mice (delta receptors only) and this response was abolished in the triple knockout mice, demonstrating that the activation of delta receptors alone can produce weak but significant mu-independent thermal antinociception.

Morphine-induced changes in delta opioid receptor trafficking are linked to somatosensory processing in the rat spinal cord

An in vivo fluorescent deltorphin (Fluo-DLT) internalization assay was used to assess the distribution and regulation of pharmacologically available delta opioid receptors (deltaORs) in the rat lumbar (L4-5) spinal cord. Under basal conditions, intrathecal injection of Fluo-DLT resulted in the labeling of numerous deltaOR-internalizing neurons throughout dorsal and ventral horns. The distribution and number of Fluo-DLT-labeled perikaryal profiles were consistent with that of deltaOR-expressing neurons, as revealed by in situ hybridization and immunohistochemistry, suggesting that a large proportion of these cells was responsive to intrathecally administered deltaOR agonists. Pretreatment of rats with morphine for 48 hr resulted in a selective increase in Fluo-DLT-labeled perikaryal profiles within the dorsal horn. These changes were not accompanied by corresponding augmentations in either deltaOR mRNA or (125)I-deltorphin-II binding levels, suggesting that they were attributable to higher densities of cell surface deltaOR available for internalization rather than to enhanced production of the receptor. Unilateral dorsal rhizotomy also resulted in increased Fluo-DLT internalization in the ipsilateral dorsal horn when compared with the side contralateral to the deafferentation or to non-deafferented controls, suggesting that deltaOR trafficking in dorsal horn neurons may be regulated by afferent inputs. Furthermore, morphine treatment no longer increased Fluo-DLT internalization on either side of the spinal cord after unilateral dorsal rhizotomy, indicating that microOR-induced changes in the cell surface availability of deltaOR depend on the integrity of primary afferent inputs. Together, these results suggest that regulation of deltaOR responsiveness through microOR activation in this region is linked to somatosensory information processing.

Mu-opioid receptor knockout prevents changes in delta-opioid receptor trafficking induced by chronic inflammatory pain

Previous studies from our laboratory have demonstrated that both chronic inflammatory pain, induced by intraplantar injection of complete Freund's adjuvant (CFA), and prolonged (48 h) stimulation of mu-opioid receptors (muOR) by systemic administration of a variety of selective agonists, resulted in enhanced plasma membrane targeting of delta-opioid receptors (deltaOR) in neurons of the dorsal spinal cord. To determine whether deltaOR trafficking induced by chronic inflammation was dependent on the activation of muOR, we investigated by immunogold cytochemistry the effects of intraplantar CFA injection on the plasma membrane density of deltaOR in muOR knockout (KO) mice. In untreated wild-type (WT) mice, only a small proportion of deltaOR was associated with neuronal plasma membranes in the dorsal horn of the spinal cord. The CFA-induced inflammation produced a significantly higher ratio of plasma membrane to intracellular receptors, as well as a 75% increase in the membrane density of immunoreactive deltaOR, in dendrites of the ipsilateral dorsal horn as compared to untreated mice. This increase in the membrane density of deltaOR was likely due to a recruitment of receptors from intracellular stores since no difference in the overall deltaOR immunolabeling density was evident between CFA-treated and untreated mice. Most importantly, the CFA-induced changes in deltaOR plasma membrane insertion seen in WT animals were not present in the spinal cord of muOR KO mice. These results demonstrate that the integrity of muOR is necessary for CFA-induced changes in deltaOR trafficking to occur and suggest that these changes could be elicited by stimulation of muOR by endogenous opioids released in response to chronic inflammatory pain.

Recent advances in drug action and therapeutics: relevance of novel concepts in G-protein-coupled receptor and signal transduction pharmacology

PROBLEM STATEMENT

During especially the past two decades many discoveries in biological sciences, and in particular at the molecular and genetic level, have greatly impacted on our knowledge and understanding of drug action and have helped to develop new drugs and therapeutic strategies. Furthermore, many exciting new drugs acting via novel pharmacological mechanisms are expected to be in clinical use in the not too distant future.

SCOPE AND CONTENTS OF REVIEW

In this educational review, these concepts are explained and their relevance illustrated by examples of drugs used commonly in the clinical setting, with special reference to the pharmacology of G-protein-coupled receptors. The review also addresses the basic theoretical concepts of full and partial agonism, neutral antagonism, inverse agonism and protean and ligand-selective agonism, and the relevance of these concepts in current rational drug therapy. Moreover, the mechanisms whereby receptor signalling (and eventually response to drugs) is fine-tuned, such as receptor promiscuity, agonist-directed trafficking of receptor signalling, receptor trafficking, receptor 'cross-talk' and regulators of G-protein signalling (RGSs) are discussed, from theory to proposed therapeutic implications.

CONCLUSIONS

It is concluded that the understanding of molecular receptor and signal transduction pharmacology enables clinicians to improve their effective implementation of current and future pharmacotherapy, ultimately enhancing the quality of life of their patients.

Attenuation of mechanical but not thermal hyperalgesia by electroacupuncture with the involvement of opioids in rat model of chronic inflammatory pain

Opioid peptides have been proven effective in reducing the sign of hyperalgesia associated with inflammation. Electroacupuncture (EA) produces antinociception via release of endogenous opioid peptides in normal rats. Moreover, intrathecal injection of dynorphin has antinociceptive effect in rats. The present study was designed to examine whether EA has effect on the thermal and mechanical hyperalgesia in rat model of complete Freund's adjuvant (CFA)-induced inflammatory pain. The results are the following: (1) single session of 100Hz EA (0.5-1.0-1.5 mA, 10 min for each intensity) at both Zusanli (ST 36) and Sanyinjiao acupoints (SP 6) significantly increased mechanical withdrawal threshold determined by von Frey filaments but not with thermal withdrawal latency that is determined by hot plate (52 +/- 0.2 degrees C); (2) 100 Hz EA applied twice a week for 4 weeks and showed a significant decrease in the mechanical hyperalgesia at the third and fourth week, with no effect on thermal hyperalgesia; (3) naloxone (20 mg kg(-1)) had the ability to reverse the inhibition of the mechanical hyperalgesia produced by a single session of EA. In conclusion, the present results indicate that a single or repetitive EA could reduce mechanical hyperalgesia, but not thermal hyperalgesia, in CFA-inflammatory pain rats, and the opioid system might be involved in these effects.

Kappa opioid receptors in rat spinal cord: sex-linked distribution differences

Activation of kappa opioid receptors (KORs) in the spinal cord can diminish nociception. Humans and rodents show sex differences in the analgesia produced by KOR agonists, and female rats show fluctuations in KOR density and sensitivity across the estrous cycle. However, it is unclear whether there are sex differences in the amount and/or distribution of spinal KORs. In the present study, immunocytochemically labeled KORs were examined in laminae I and II of the lumbosacral spinal dorsal horn of male and normally cycling female Sprague-Dawley rats. The basic pattern of KOR labeling was determined in both sexes using qualitative electron microscopy (EM), and sex-linked differences in the density and subcellular distribution of KOR immunoreactivity were determined with quantitative EM and light microscopy. KOR labeling was visualized with immunoperoxidase for optimally sensitive detection, or with immunogold for precise subcellular localization. By EM, the general pattern of KOR immunoreactivity was similar in males and females. KOR immunoreactivity was common in dendrites, axons, and axon terminals, and was in a few glia and neuronal somata. Most KOR-immunoreactive (-ir) axons were fine-diameter and unmyelinated. Most KOR-ir terminals were small or medium-sized, and a minority formed asymmetric or symmetric synapses with unlabeled dendrites. KOR immunoreactivity was associated both with the plasma membrane and with cytoplasmic organelles, notably including dense core vesicles in terminals. Light microscopic densitometry revealed that KOR immunoreactivity was significantly denser in estrus and proestrus females than in males. By EM, the distribution of KOR-immunogold labeling within axon terminals differed, with a greater proportion of cytoplasmic KOR labeling in estrus females compared with males. In contrast, the abundance and types of KOR-immunoperoxidase-labeled profiles did not show sex-linked differences. We conclude that in both sexes, KORs are positioned to influence both pre- and postsynaptic neurotransmission and are present in morphologically heterogeneous neuron populations. These findings are consistent with complex consequences of KOR activation in the spinal cord. In addition, the presence of increased KOR density and proportionally elevated intracellular KORs in proestrus/estrus females suggests a basis for sex-linked differences in KOR-mediated antinociception.

Phylogenetic changes in the expression of delta opioid receptors in spinal cord and dorsal root ganglia

To assess the validity of rodent models for investigating the role of delta opioid receptors (DOR) in analgesia, the distribution of DOR binding and mRNA were compared between rodent and primate spinal cord and dorsal root ganglia (DRG), using receptor autoradiography and in situ hybridization, respectively. In mouse and rat spinal cord, [(125)I]-deltorphin-labeled DOR binding sites were detected throughout the gray matter. In contrast, in primate and particularly in human spinal cord, DOR binding was mainly present in laminae I-II, with little to no binding in deeper layers. Accordingly, in rodent spinal cord, DOR mRNA was expressed by a large number of neurons distributed throughout the ventral and dorsal horns, whereas in the primate, DOR expression was significantly lower, as evidenced by a moderate number of labeled cells throughout the gray matter in monkey and by only few labeled cells in human, mainly in Clarke's column and lamina IX. Major species differences in DOR expression were also observed in primary afferent cells bodies. In rat DRG, intense DOR mRNA hybridization was primarily observed over large ganglion cells immunopositive for neurofilament 200. In contrast, in monkey and human DRG, DOR mRNA was primarily detected over small and medium-sized ganglion cells. These results demonstrate major differences in the expression and distribution of DOR in the spinal cord and DRG between mammalian species. Specifically, they point to a progressive specialization of DOR toward the regulation of primary somatosensory, namely nociceptive, inputs during phylogeny and suggest that the effects of DOR agonists in rodents may not be entirely predictive of their action in humans.

Evidence for a selective role of the delta-opioid agonist [8R-(4bS*,8aalpha,8abeta, 12bbeta)]7,10-Dimethyl-1-methoxy-11-(2-methylpropyl)oxycarbonyl 5,6,7,8,12,12b-hexahydro-(9H)-4,8-methanobenzofuro[3,2-e]pyrrolo[2,3-g]isoquinoline hydrochloride (SB-235863) in blocking hyperalgesia associated with inflammatory and neuropathic pain responses

The specific involvement of the delta-opioid receptor in the control of nociception was explored by investigating the pharmacological activity in vivo of a selective, orally active, and centrally penetrant delta-opioid agonist. [8R-(4bS*,8aalpha,8abeta,12bbeta)]7,10-dimethyl-1-methoxy-11-(2-methylpropyl)oxycarbonyl 5,6,7,8,12,12b-hexahydro-(9H)-4,8-methanobenzofuro[3,2-e]pyrrolo[2,3-g]isoquinoline hydrochloride (SB-235863) is a new pyrrolomorphinan with high affinity (Ki = 4.81 +/- 0.39 nM) for the delta-opioid receptor, full agonist activity, and binding selectivity versus the mu- and kappa-opioid receptors of 189-fold and 52-fold, respectively. Perorally administered SB-236863 was inactive in the rat tail-flick and hot-plate tests of acute pain response, but potently reversed thermal hyperalgesia in rats resulting from a carrageenan-induced inflammatory response. This activity could be blocked by the delta-opioid antagonist naltrindole (3 mg/kg s.c.), but selective mu- and kappa-opioid antagonists were ineffective. Naltrindole (1 microg i.c.v.) also blocked the activity of 10 mg/kg (p.o.) SB-235863, showing that the compound activates delta-opioid receptor sites in the central nervous system. SB-235863 was additionally effective at reversing chronic hyperalgesia in the Seltzer rat model of partial sciatic nerve ligation after peroral administration. These data show that the delta-opioid receptor plays a selective role in regulating evoked and lasting changes in nociceptive pain signaling. Classical side effects of mu- and kappa-opioid receptor activation (slowing of gastrointestinal transit and motor incoordination, respectively) were not observed after administration of 70 mg/kg (p.o.) SB-235863, nor was evoked seizure activity affected. These results suggest a selective and limited role of delta-opioid receptors in the modulation of nociception.

The Role of Nuclear Factor κB in Tumor Necrosis Factor-Regulated Transcription of the Human μ-Opioid Receptor Gene

Opioids and their receptors are key players in a cross-talk between the nervous and immune systems. For example, the endogenous opioid system is activated during inflammation as a physiological feedback mechanism to attenuate inflammatory pain. Herein, we report that in primary human T lymphocytes, Raji B cells, U937 monocytes, primary human polymorphonuclear leukocytes, and mature dendritic cells, the proinflammatory cytokine tumor necrosis factor induced mu-opioid receptor gene transcription. Transcriptional induction of the gene in immune cells was mediated via tumor necrosis factor receptor type 2. Using selective in vivo disruption of possibly involved transcription factors with decoy oligonucleotides, nuclear factor-kappaB was identified as the factor responsible for induction of the gene in immune cells, whereas activator protein-1 was found to be uninvolved. Nuclear factor-kappaB also mediates up-regulation of mu-opioid receptors in neuronal cells stimulated with tumor necrosis factor. Among six putative nuclear factor-kappaB binding sites on the mu-opioid receptor gene promoter, three cis-active elements at nt -2174, -557, and -207 were identified using transfection experiments of reporter gene constructs, electrophoretic mobility shift assays, and in vivo binding studies with decoy oligonucleotides. An allelic variation within the -557 element significantly reduced its trans-activating potency, which may affect regulation of the mu-opioid receptor gene in persons carrying this mutation. This study suggests a regulatory function of tumor necrosis factor in opioid-mediated processes in neuronal and immune cells, with possible impact on the complex of inflammation-induced analgesia.

The expression of delta- and kappa-opioid receptor is enhanced during intestinal inflammation in mice

In the gut, mu-, delta-, and kappa-opioid receptors are present in the submucous and myenteric plexi and in enterocytes. Using pharmacological methods, our group has shown that intestinal inflammation enhances the antitransit and antisecretory effects of systemic opioids. The aim of the present study was to evaluate whether the enhanced antisecretory effects of delta and kappa-agonists were associated with an increased transcription and/or expression of these receptors at central (brain and spinal cord) and/or peripheral sites (gut); we also evaluated the expression of delta- and kappa-opioid receptors in dissected sections of the gut containing the myenteric (MP/LM) or submucous (SP/M) plexi. The mRNA and protein levels of both opioid receptors were determined using a reverse-transcriptase polymerase chain reaction and immunoprecipitation/Western blot, respectively. Intestinal inflammation significantly augmented the transcription of delta-opioid receptors in the spinal cord (34%) and in the whole gut (102%). Also increased mRNA and protein levels of delta-opioid receptors in the MP/LM and SP/M preparations. The kappa-opioid receptors gene transcription was not altered by inflammation, whereas kappa-opioid receptors protein levels were significantly enhanced in the SP/M preparation. No changes in gene transcription or protein levels for delta- and kappa-opioid receptors could be demonstrated in the brain. These results suggest that local transcriptional and post-transcriptional changes of the delta- and kappa-opioid receptors genes could be responsible for the enhanced antisecretory effects of delta- and kappa-opioid agonists during intestinal inflammation.