Enhanced delta-opioid receptor-mediated antinociception in mu-opioid receptor-deficient mice

Antinociceptive Effects of Aaptamine, a Sponge Component, on Peripheral Neuropathy in Rats

Aaptamine, a natural marine compound isolated from the sea sponge, has various biological activities, including delta-opioid agonist properties. However, the effects of aaptamine in neuropathic pain remain unclear. In the present study, we used a chronic constriction injury (CCI)-induced peripheral neuropathic rat model to explore the analgesic effects of intrathecal aaptamine administration. We also investigated cellular angiogenesis and lactate dehydrogenase A (LDHA) expression in the ipsilateral lumbar spinal cord after aaptamine administration in CCI rats by immunohistofluorescence. The results showed that aaptamine alleviates CCI-induced nociceptive sensitization, allodynia, and hyperalgesia. Moreover, aaptamine significantly downregulated CCI-induced vascular endothelial growth factor (VEGF), cluster of differentiation 31 (CD31), and LDHA expression in the spinal cord. Double immunofluorescent staining showed that the spinal VEGF and LDHA majorly expressed on astrocytes and neurons, respectively, in CCI rats and inhibited by aaptamine. Collectively, our results indicate aaptamine's potential as an analgesic agent for neuropathic pain. Furthermore, inhibition of astrocyte-derived angiogenesis and neuronal LDHA expression might be beneficial in neuropathy.

Alleviating pain with delta opioid receptor agonists: evidence from experimental models

The use of opioids for the relief of pain and headache disorders has been studied for years. Nowadays, particularly because of its ability to produce analgesia in various pain models, delta opioid receptor (DOPr) emerges as a promising target for the development of new pain therapies. Indeed, their potential to avoid the unwanted effects commonly observed with clinically used opioids acting at the mu opioid receptor (MOPr) suggests that DOPr agonists could be a therapeutic option. In this review, we discuss the use of opioids in the management of pain in addition to describing the evidence of the analgesic potency of DOPr agonists in animal models.

A Novel Mu-Delta Opioid Agonist Demonstrates Enhanced Efficacy With Reduced Tolerance and Dependence in Mouse Neuropathic Pain Models

Numerous studies have demonstrated a physiological interaction between the mu opioid receptor (MOR) and delta opioid receptor (DOR) systems. A few studies have shown that dual MOR-DOR agonists could be beneficial, with reduced tolerance and addiction liability, but are nearly untested in chronic pain models, particularly neuropathic pain. In this study, we tested the MOR-DOR agonist SRI-22141 in mice in the clinically relevant models of HIV Neuropathy and Chemotherapy-Induced Peripheral Neuropathy (CIPN). SRI-22141 was more potent than morphine in the tail flick pain test and had equal or enhanced efficacy versus morphine in both neuropathic pain models, with significantly reduced tolerance. SRI-22141 also produced no jumping behavior during naloxone-precipitated withdrawal in CIPN or naïve mice, suggesting that SRI-22141 produces little to no dependence. SRI-22141 also reduced tumor necrosis factor-α and cyclooxygenase-2 in CIPN in the spinal cord, suggesting an anti-inflammatory mechanism of action. The DOR-selective antagonist naltrindole strongly reduced CIPN efficacy and anti-inflammatory activity in the spinal cord, without affecting tail flick antinociception, suggesting the importance of DOR activity in these models. Overall, these results provide compelling evidence that MOR-DOR agonists could have strong efficacy with reduced side effects and an anti-inflammatory mechanism in the treatment of neuropathic pain. PERSPECTIVE: This study demonstrates that a MOR-DOR dual agonist given chronically in chronic neuropathic pain models has enhanced efficacy with strongly reduced tolerance and dependence, with a further anti-inflammatory effect in the spinal cord. This suggests that MOR-DOR dual agonists could be effective treatments for neuropathic pain with reduced side effects.

Synthesis and Evaluation of a Novel Bivalent Selective Antagonist for the Mu-Delta Opioid Receptor Heterodimer that Reduces Morphine Withdrawal in Mice

A major limitation in the study of the mu-delta opioid receptor heterodimer (MDOR) is that few selective pharmacological tools exist and no heteromer-selective antagonists. We thus designed a series of variable-length (15-41 atoms) bivalent linked peptides with selective but moderate/low-affinity pharmacophores for the mu and delta opioid receptors. We observed a U-shaped MDOR potency/affinity profile in vitro, with the 24-atom spacer length (D24M) producing the highest MDOR potency/affinity (<1 nM) and selectivity (≥89-fold). We further evaluated D24M in mice and observed that D24M dose-dependently antagonized tail flick antinociception produced by the MDOR agonists CYM51010 and Deltorphin-II, without antagonizing the monomer agonists DAMGO and DSLET. We also observed that D24M sharply reduced withdrawal behavior in models of acute and chronic morphine dependence. These findings suggest that D24M is a first-in-class high-potency MDOR-selective antagonist both in vitro and in vivo.

Usefulness of knockout mice to clarify the role of the opioid system in chronic pain

Several lines of knockout mice deficient in the genes encoding each component of the endogenous opioid system have been used for decades to clarify the specific role of the different opioid receptors and peptide precursors in many physiopathological conditions. The use of these genetically modified mice has improved our knowledge of the specific involvement of each endogenous opioid component in nociceptive transmission during acute and chronic pain conditions. The present review summarizes the recent advances obtained using these genetic tools in understanding the role of the opioid system in the pathophysiological mechanisms underlying chronic pain. Behavioural data obtained in these chronic pain models are discussed considering the peculiarities of the behavioural phenotype of each line of knockout mice. These studies have identified the crucial role of specific components of the opioid system in different manifestations of chronic pain and have also opened new possible therapeutic approaches, such as the development of opioid compounds simultaneously targeting several opioid receptors. However, several questions still remain open and require further experimental effort to be clarified. The novel genetic tools now available to manipulate specific neuronal populations and precise genome editing in mice will facilitate in a near future the elucidation of the role of each component of the endogenous opioid system in chronic pain.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

The Delta Opioid Receptor in Pain Control

Nowadays, the delta opioid receptor (DOPr) represents a promising target for the treatment of chronic pain and emotional disorders. Despite the fact that they produce limited antinociceptive effects in healthy animals and in most acute pain models, DOPr agonists have shown efficacy in various chronic pain models. In this chapter, we review the progresses that have been made over the last decades in understanding the role played by DOPr in the control of pain. More specifically, the distribution of DOPr within the central nervous system and along pain pathways is presented. We also summarize the literature supporting a role for DOPr in acute, tonic, and chronic pain models, as well as the mechanisms regulating its activity under specific conditions. Finally, novel compounds that have make their way to clinical trials are discussed.

Synthesis and Opioid Activity of Tyr1 -ψ[(Z)CF=CH]-Gly2 and Tyr1 -ψ[(S)/(R)-CF3 CH-NH]-Gly2 Leu-enkephalin Fluorinated Peptidomimetics

We describe the design, synthesis, and opioid activity of fluoroalkene (Tyr1 -ψ[(Z)CF=CH]-Gly2 ) and trifluoroethylamine (Tyr1 -ψ[(S)/(R)-CF3 CH-NH]-Gly2 ) analogues of the endogenous opioid neuropeptide, Leu-enkephalin. The fluoroalkene peptidomimetic exhibited low nanomolar functional activity (5.0±2 nm and 60±15 nm for δ- and μ-opioid receptors, respectively) with a μ/δ-selectivity ratio that mimics that of the natural peptide. However, the trifluoroethylamine peptidomimetics, irrespective of stereochemistry, did not activate the opioid receptors, which suggest that bulky CF3 substituents are not tolerated at this position.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

Tetrapeptide Endomorphin Analogs Require Both Full Length and Truncated Splice Variants of the Mu Opioid Receptor Gene Oprm1 for Analgesia

The mu opioid receptor gene undergoes extensive alternative splicing. Mu opioids can be divided into three classes based on the role of different groups of splice variants. Morphine and methadone require only full length seven transmembrane (7TM) variants for analgesia, whereas IBNtxA (3'-iodobenzyol-6β-naltrexamide) needs only truncated 6TM variants. A set of endomorphin analogs fall into a third group that requires both 6TM and 7TM splice variants. Unlike morphine, endomorphin 1 and 2, DAPP (Dmt,d-Ala-Phe-Phe-NH₂), and IDAPP (3'-iodo-Dmt-d-Ala-Phe-Phe-NH₂) analgesia was lost in an exon 11 knockout mouse lacking 6TM variants. Restoring 6TM variant expression in a knockout mouse lacking both 6TM and 7TM variants failed to rescue DAPP or IDAPP analgesia. However, re-establishing 6TM expression in an exon 11 knockout mouse that still expressed 7TM variants did rescue the response, consistent with the need for both 6TM and 7TM variants. In receptor binding assays, ¹²⁵I-IDAPP labeled more sites (B_max) than ³H-DAMGO ([d-Ala²,N-MePhe⁴,Gly(ol)⁵]-enkephalin) in wild-type mice. In exon 11 knockout mice, ¹²⁵I-IDAPP binding was lowered to levels similar to ³H-DAMGO, which remained relatively unchanged compared to wild-type mice. ¹²⁵I-IDAPP binding was totally lost in an exon 1/exon 11 knockout model lacking all Oprm1 variant expression, confirming that the drug was not cross labeling non-mu opioid receptors. These findings suggested that ¹²⁵I-IDAPP labeled two populations of mu binding sites in wild-type mice, one corresponding to 7TM variants and the second dependent upon 6TM variants. Together, these data indicate that endomorphin analogs represent a unique, genetically defined, and distinct class of mu opioid analgesic.

Recent advances on the δ opioid receptor: from trafficking to function

Within the opioid family of receptors, δ (DOPrs) and μ opioid receptors (MOPrs) are typical GPCRs that activate canonical second-messenger signalling cascades to influence diverse cellular functions in neuronal and non-neuronal cell types. These receptors activate well-known pathways to influence ion channel function and pathways such as the map kinase cascade, AC and PI3K. In addition new information regarding opioid receptor-interacting proteins, downstream signalling pathways and resultant functional effects has recently come to light. In this review, we will examine these novel findings focusing on the DOPr and, in doing so, will contrast and compare DOPrs with MOPrs in terms of differences and similarities in function, signalling pathways, distribution and interactions. We will also discuss and clarify issues that have recently surfaced regarding the expression and function of DOPrs in different cell types and analgesia.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Spinal μ and δ opioids inhibit both thermal and mechanical pain in rats

The expression and contribution of μ (MOPR) and δ opioid receptors (DOPR) in polymodal nociceptors have been recently challenged. Indeed, MOPR and DOPR were shown to be expressed in distinct subpopulation of nociceptors where they inhibit pain induced by noxious heat and mechanical stimuli, respectively. In the present study, we used electrophysiological measurements to assess the effect of spinal MOPR and DOPR activation on heat-induced and mechanically induced diffuse noxious inhibitory controls (DNICs). We recorded from wide dynamic range neurons in the spinal trigeminal nucleus of anesthetized rats. Trains of 105 electrical shocks were delivered to the excitatory cutaneous receptive field. DNICs were triggered either by immersion of the hindpaw in 49°C water or application of 300 g of mechanical pressure. To study the involvement of peptidergic primary afferents in the activation of DNIC by noxious heat and mechanical stimulations, substance P release was measured in the spinal cord by visualizing neurokinin type 1 receptor internalization. We found that the activation of spinal MOPR and DOPR similarly attenuates the DNIC and neurokinin type 1 receptor internalization induced either by heat or mechanical stimuli. Our results therefore reveal that the activation of spinal MOPR and DOPR relieves both heat-induced and mechanically induced pain with similar potency and suggest that these receptors are expressed on polymodal, substance P-expressing neurons.

Bradykinin controls pool size of sensory neurons expressing functional δ-opioid receptors

Analgesics targeting the δ-opioid receptor (DOR) may lead to fewer side effects than conventional opioid drugs, which mainly act on μ-opioid receptors (MOR), because of the less abundant expression of DOR in the CNS compared with MOR. Analgesic potential of DOR agonists increases after inflammation, an effect that may be mediated by DOR expressed in the peripheral sensory fibers. However, the expression of functional DOR at the plasma membrane of sensory neurons is controversial. Here we have used patch-clamp recordings and total internal reflection fluorescence microscopy to study the functional expression of DOR in sensory neurons from rat trigeminal (TG) and dorsal root ganglia (DRG). Real-time total internal reflection fluorescence microscopy revealed that treatment of TG and DRG cultures with the inflammatory mediator bradykinin (BK) caused robust trafficking of heterologously expressed GFP-tagged DOR to the plasma membrane. By contrast, treatment of neurons with the DOR agonist [d-Ala(2), d-Leu(5)]-enkephalin (DADLE) caused a decrease in the membrane abundance of DOR, suggesting internalization of the receptor after agonist binding. Patch-clamp experiments revealed that DADLE inhibited voltage-gated Ca(2+) channels (VGCCs) in 23% of small-diameter TG neurons. Pretreatment with BK resulted in more than twice as many DADLE responsive neurons (54%) but did not affect the efficacy of VGCC inhibition by DADLE. Our data suggest that inflammatory mediator-induced membrane insertion of DOR into the plasma membrane of peripheral sensory neurons may underlie increased DOR analgesia in inflamed tissue. Furthermore, the majority of BK-responsive TG neurons may have a potential to become responsive to DOR ligands in inflammatory conditions.

Behavioural and anatomical characterization of mutant mice with targeted deletion of D1 dopamine receptor-expressing cells: response to acute morphine

Considerable topographic overlap exists between brain opioidergic and dopaminergic neurons. Pharmacological blockade of the dopamine D(1) receptor (Drd1a) reverses several behavioural phenomena elicited by opioids. The present study examines the effects of morphine in adult mutant (MUT) mice expressing the attenuated diphtheria toxin-176 gene in Drd1a-expressing cells, a mutant line shown previously to undergo post-natal striatal atrophy and loss of Drd1a-expression. MUT and wild-type mice were assessed behaviourally following acute administration of 10 mg/kg morphine. Treatment with morphine reduced locomotion and rearing similarly in both genotypes but reduced total grooming only in MUT mice. Morphine-induced Straub tail and stillness were heightened in MUT mice. Chewing and sifting were decreased in MUT mice and these effects were not modified by morphine. Loss of striatal Drd1-positive cells and up-regulated D(2)-expression, as reflected in down-regulated D(1)-like and up-regulated D(2)-like binding, respectively, is not uniform along the cranio-caudal extent in this model but appears to be greater in the caudal striatum. Preferential caudal loss of µ-opioid-expression, a marker for the striosomal compartment, was seen. These data indicate that Drd1a-positive cell loss modifies the exploratory behavioural response elicited by morphine, unmasking novel morphine-induced MUT-specific behaviours and generating a hypersensitivity to morphine for others.

Involvement of the opioid and cannabinoid systems in pain control: new insights from knockout studies

The endogenous opioid and cannabinoid systems are involved in the physiological inhibitory control of pain and are of particular interest for the development of therapeutic approaches for pain management. The involvement of these endogenous systems in pain control has been studied from decades by the use of compounds with different affinities for each cannabinoid and opioid receptor or for the different enzymes involved in endocannabinoid and endogenous opioid metabolism. However, the selectivity of these pharmacological tools in vivo has represented an important limitation for these studies. The generation of genetically modified mice with selective mutations in specific components of the endocannabinoid and endogenous opioid system has provided important advances in the identification of the specific contribution of each component of these endogenous systems in the perception of noxious stimuli and the development of pathological pain states. Different lines of constitutive and conditional knockout mice deficient in specific cannabinoid and opioid receptors, specific precursors of the endogenous opioid peptides and the main enzymes involved in endocannabinoid and endogenous opioid degradation are now available. These knockout mice have also been used to evaluate the contribution of each component of the endocannabinoid and opioid system in the antinociceptive effects of cannabinoid and opioid agonists, including those currently used to treat pain in humans. This review summarizes the main advances provided in the last 15 years by the use of these genetic tools in the knowledge of the physiological control of pain and the pharmacology of cannabinoid and opioid compounds for pain management.

Monogenic mouse models of social dysfunction: implications for autism

Autism is a pervasive disorder characterized by a complex symptomatology, based principally on social dysfunction. The disorder has a highly complex, largely genetic etiology, involving an impressive variety of genes, the precise contributions of which still remain to be determined. For this reason, a reductionist approach to the study of autism has been proposed, employing monogenic animal models of social dysfunction, either by targeting a candidate gene, or by mimicking a single-gene disorder characterized by autistic symptoms. In the present review, we discuss this monogenic approach by comparing examples of each strategy: the mu opioid receptor knock-out (KO) mouse line, which targets the opioid system (known to be involved in the control of social behaviors); and the Fmr1-KO mouse, a model for Fragile X syndrome (a neurodevelopmental syndrome that includes autistic symptoms). The autistic-relevant behavioral phenotypes of the mu-opioid and Fmr1-KO mouse lines are described here, summarizing previous work by our research group and others, but also providing novel experimental evidence. Relevant factors influencing the validity of the two models, such as sex differences and age at testing, are also addressed, permitting an extensive evaluation of the advantages and limits of monogenic mouse models for autism.

Disruption of δ-opioid receptor phosphorylation at threonine 161 attenuates morphine tolerance in rats with CFA-induced inflammatory hypersensitivity

OBJECTIVE

Our previous study identified Threonine 161 (Thr-161), located in the second intracellular loop of the δ-opioid receptor (DOR), as the only consensus phosphorylation site for cyclin-dependent kinase 5 (Cdk5). The aim of this study was to assess the function of DOR phosphorylation by Cdk5 in complete Freund's adjuvant (CFA)-induced inflammatory pain and morphine tolerance.

METHODS

Dorsal root ganglion (DRG) neurons of rats with CFA-induced inflammatory pain were acutely dissociated and the biotinylation method was used to explore the membrane localization of phosphorylated DOR at Thr-161 (pThr-161-DOR), and paw withdrawal latency was measured after intrathecal delivery of drugs or Tat-peptide, using a radiant heat stimulator in rats with CFA-induced inflammatory pain.

RESULTS

Both the total amount and the surface localization of pThr-161-DOR were significantly enhanced in the ipsilateral DRG following CFA injection. Intrathecal delivery of the engineered Tat fusion-interefering peptide corresponding to the second intracellular loop of DOR (Tat-DOR-2L) increased inflammatory hypersensitivity, and inhibited DOR- but not µ-opioid receptor-mediated spinal analgesia in CFA-treated rats. However, intrathecal delivery of Tat-DOR-2L postponed morphine antinociceptive tolerance in rats with CFA-induced inflammatory pain.

CONCLUSION

Phosphorylation of DOR at Thr-161 by Cdk5 attenuates hypersensitivity and potentiates morphine tolerance in rats with CFA-induced inflammatory pain, while disruption of the phosphorylation of DOR at Thr-161 attenuates morphine tolerance.

Decreased pain responses of C-C chemokine receptor 5 knockout mice to chemical or inflammatory stimuli

Chemokines are small chemotactic cytokines that elicit many physiological and pathological effects through binding to their corresponding receptors. Recent studies have suggested that C-C chemokine receptor (CCR) 5 interacts with μ-opioid receptor and modifies a nociceptive reaction. We examined effects of CCR5 deficiency on pain responses by employing CCR5 knockout (KO) mice. We found that pain responses of CCR5 KO mice to chemical or inflammation stimuli were milder than those of CCR5 wild type (WT) mice. However, there was no remarkable change in thermal nociception. To prove the involvement of CCR5 deletion in lowered nociception, we examined pain reactions with CCR5 WT mice following treatment of a CCR5 antagonist (D-Ala(1)-peptide T-NH(2,) DAPTA). Chemical or inflammatory pain behavior was significantly relieved by intracerebroventricular infusion of the inhibitor. When we assessed expression level of μ-opioid receptor (MOR) in the periaqueductal gray where the receptors are critical for analgesic effects, immunoreactivity of MOR was significantly higher in CCR5 KO mice than WT mice without change in phosphorylation level of the receptor. Reduced nociceptive responses in CCR5 KO mice were moderated by administration of naloxone and d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP), MOR antagonists. Our data indicate that CCR5 deficiency is related to up-regulation of MOR without an increase in the receptor desensitization which might result in increased analgesic effects against chemical or inflammatory stimuli. Alternatively, higher amount of opioid ligands in CCR5 mice might be linked to these results. Therefore, CCR5 appears to be a therapeutic target for treatment of pain related diseases such as inflammatory hyperalgesia.

Nociceptive behaviour upon modulation of mu-opioid receptors in the ventrobasal complex of the thalamus of rats

The role of mu-opioid receptors (MORs) in the inflammatory pain processing mechanisms within the ventrobasal complex of the thalamus (VB) is not well understood. This study investigated the effect of modulating MOR activity upon nociception, by stereotaxically injecting specific ligands in the VB. Nociceptive behaviour was evaluated in two established animal models of inflammatory pain, by using the formalin (acute and tonic pain) and the ankle-bend (chronic monoarthritic pain) tests. Control (saline intra-VB injection) formalin-injected rats showed acute and tonic pain-related behaviours. In contrast, intrathalamic administration of [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin acetate (DAMGO), a MOR-specific agonist, induced a statistically significant decrease of all tonic phase pain-related behaviours assessed until 30-35min after formalin hind paw injection. In the acute phase only the number of paw-jerks was affected. In monoarthritic rats, there was a noticeable antinociceptive effect with approximately 40min of duration, as denoted by the reduced ankle-bend scores observed after DAMGO injection. Intra-VB injection of D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP), a specific MOR antagonist, or of CTOP followed, 10min after, by DAMGO had no effects in either formalin or ankle-bend tests. Data show that DAMGO-induced MOR activation in the VB has an antinociceptive effect in the formalin test as well as in chronic pain observed in MA rats, suggesting an important and specific role for MORs in the VB processing of inflammatory pain.

Mu-opioid receptors are not necessary for nortriptyline treatment of neuropathic allodynia

Tricyclic antidepressants (TCAs) are among the first line treatments clinically recommended against neuropathic pain. However, the mechanism by which they alleviate pain is still unclear. Pharmacological and genetic approaches evidenced a critical role of delta-opioid receptors (DORs) in the therapeutic action of chronic TCA treatment. It is however unclear whether mu-opioid receptors (MORs) are also necessary to the pain-relieving action of TCAs. The lack of highly selective MOR antagonists makes difficult to conclude based on pharmacological studies. In the present work, we thus used a genetic approach and compared mutant mice lacking MORs and their wild-type littermates. The neuropathy was induced by unilateral sciatic nerve cuffing. The threshold for mechanical response was evaluated using von Frey filaments. MOR-deficient mice displayed the same baseline for mechanical sensitivity as their wild-type littermates. After sciatic nerve cuffing, both wild-type and MOR-deficient mice displayed an ipsilateral mechanical allodynia. After about 10 days of treatment, nortriptyline suppressed this allodynia in both wild-type and MOR-deficient mice. MORs are thus not critical for nortriptyline action against neuropathic pain. An acute injection of the DOR antagonist naltrindole induced a relapse of neuropathic allodynia in both wild-type and MOR-deficient mice, thus confirming the critical role of DORs in nortriptyline action. Moreover, morphine induced an acute analgesia in control and in neuropathic wild-type mice, but was without effect in MOR-deficient mice. While MORs are crucial for morphine action, they are not critical for nortriptyline action. Our results highlight the functional difference between DORs and MORs in mechanisms of pain relief.

Differential noxious and motor tolerance of chronic delta opioid receptor agonists in rodents

In the present study, we asked whether multiple intrathecal injections of deltorphin II, a selective delta opioid receptor (DOPR) agonist, induced DOPR tolerance in three behavioral assays. Unilateral inflammation caused by complete Freund's adjuvant (CFA) injection into the rat or mouse hind paw (CFA model) induced thermal hyperalgesic response that was transiently and dose-dependently reduced by intrathecal administration of deltorphin II or morphine. In both rodent species, the effect of deltorphin II was not modified by a single prior administration of deltorphin II, suggesting an absence of acute tolerance in this paradigm. Repeated administration of intrathecal deltorphin II or s.c. SB-235863 (five consecutive injections over 60 h) also failed to impair the antihyperalgesic response to delta opioid receptor agonist, whereas repeated intrathecal or s.c. injections of morphine induced a significant decrease in the subsequent thermal antihyperalgesic response to morphine. In mice, deltorphin II also induced a rapid, transient motor incoordination/ataxia-like behavior as tested with the accelerating rotarod. In contrast to the antihyperalgesic responses, tolerance to the motoric effect of deltorphin II was evident in mice previously exposed to multiple intrathecal agonist injections, but not multiple saline administrations. Using the tail flick antinociceptive test, we found that DOPR-mediated analgesia was significantly reduced by repeated exposure to deltorphin II. Altogether, these observations suggest that repeated injections of DOPR agonists induce differential tolerance effects on antihyperalgesic, antinociceptive, and motor incoordination/ataxia-like behaviors related to DOPR activation by deltorphin II.

Cell surface targeting of mu-delta opioid receptor heterodimers by RTP4

Mu opioid receptors are G protein-coupled receptors that mediate the pain-relieving effects of clinically used analgesics, such as morphine. Accumulating evidence shows that mu-delta opioid heterodimers have a pharmacologic profile distinct from those of the mu or delta homodimers. Because the heterodimers exhibit distinct signaling properties, the protein and mechanism regulating their levels have significant effects on morphine-mediated physiology. We report the characterization of RTP4, a Golgi chaperone, as a regulator of the levels of heterodimers at the cell surface. We show that the association with RTP4 protects mu-delta receptors from ubiquitination and degradation. This leads to increases in surface heterodimer levels, thereby affecting signaling. Thus, the oligomeric organization of opioid receptors is controlled by RTP4, and this governs their membrane targeting and functional activity. This work is the first report of the identification of a chaperone involved in the regulation of the biogenesis of a family A GPCR heterodimer. The identification of such factors as RTP4 controlling dimerization will provide insight into the regulation of heterodimers in vivo. This has implications in the modulation of pharmacology of their endogenous ligands, and in the development of drugs with specific therapeutic effects.

Inflammatory pain is enhanced in delta opioid receptor-knockout mice

To examine the involvement of opioid receptors in inflammatory pain, we compared Complete Freund's Adjuvant-induced hyperalgesia in mice lacking mu, delta or kappa receptors under the same experimental conditions. Mechanical allodynia and thermal hyperalgesia were measured using von Frey filaments and the plantar test, respectively. All three receptor-knockout mice, as well as wild-type animals, developed inflammatory hyperalgesia following Complete Freund's Adjuvant administration. Mu-receptor mutants showed similar hyperalgesia to wild-types in the two tests. Kappa-receptor mutants exhibited enhanced mechanical allodynia compared with wild-type mice but similar thermal hyperalgesia. In contrast, mechanical allodynia and thermal hyperalgesia were both markedly augmented in delta-receptor mutants, indicating a role for an endogenous delta-receptor tone in the control of inflammatory pain. Treatment with the delta-selective agonist SNC80 produced antihyperalgesia, and this effect was abolished in the delta-receptor knockout mice. Altogether, these data demonstrate that delta receptors inhibit inflammatory pain when activated either endogenously or exogenously. We have previously shown enhanced neuropathic pain in delta-receptor knockout mice. The delta receptor definitely represents a promising target for treating chronic pain conditions.

Delta opioid receptor mRNA expression is changed in the thalamus and brainstem of monoarthritic rats

Changes in the mRNA expression of neurotransmitters receptors under chronic pain conditions have been described in various areas of the central nervous system (CNS). Delta opioid receptors (DORs) have been implicated in pain mechanisms but, although its mRNA expression has been studied in the rat CNS, there are no reports describing its distribution in specific thalamic and brainstem nuclei during chronic inflammatory pain. Here, in situ hybridization for DOR mRNA was performed in brain sections from control and monoarthritic (MA) rats with 2, 4, 7 and 14 days of inflammation. Grain densities were determined bilaterally in the ventrobasal complex (VB), posterior (Po), centromedial/centrolateral (CM/CL) and reticular (Rt) nuclei of the thalamus, and in the dorsal reticular (DRt), lateral reticular (LRt) and parvocellular reticular (PCRt) nuclei of the brainstem. Control animals exhibited weak mRNA expression in the VB, Po and CM/CL, as well as in PCRt, while moderate grain densities were observed in the Rt, DRt and LRt. During MA, DOR mRNA expression was significantly decreased (22%) in the Rt contralateral to the affected joint at both 7 and 14 days of inflammation, as compared to controls. A bilateral reduction (35%) was also observed in the DRt at 14 days of MA, while a contralateral increase was found in the PCRt at 7 days (+39%). No significant changes were observed in the other regions analyzed. Thus, data show changes in the DOR mRNA expression during the development of chronic inflammatory pain, in thalamic and brainstem nuclei implicated in pain processing mechanisms.

Essential role of mu opioid receptor in the regulation of delta opioid receptor-mediated antihyperalgesia

Analgesic effects of delta opioid receptor (DOR) -selective agonists are enhanced during persistent inflammation and arthritis. Although the underlying mechanisms are still unknown, membrane density of DOR was shown to be increased 72 h after induction of inflammation, an effect abolished in mu opioid receptor (MOR) -knockout (KO) mice [Morinville A, Cahill CM, Kieffer B, Collier B, Beaudet A (2004b) Mu-opioid receptor knockout prevents changes in delta-opioid receptor trafficking induced by chronic inflammatory pain. Pain 109:266-273]. In this study, we demonstrated a crucial role of MOR in DOR-mediated antihyperalgesia. Intrathecal administration of the DOR selective agonist deltorphin II failed to induce antihyperalgesic effects in MOR-KO mice, whereas it dose-dependently reversed thermal hyperalgesia in wild-type mice. The antihyperalgesic effects of deltorphin II were blocked by naltrindole but not d-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP) suggesting that this agonist was mainly acting through DOR. SNC80-induced antihyperalgesic effects in MOR-KO mice were also attenuated as compared with littermate controls. In contrast, kappa opioid receptor knockout did not affect deltorphin II-induced antihyperalgesia. As evaluated using mice lacking endogenous opioid peptides, the regulation of DOR's effects was also independent of beta-endorphin, enkephalins, or dynorphin opioids known to be released during persistent inflammation. We therefore conclude that DOR-mediated antihyperalgesia is dependent on MOR expression but that activation of MOR by endogenous opioids is probably not required.

Morphine priming in rats with chronic inflammation reveals a dichotomy between antihyperalgesic and antinociceptive properties of deltorphin

We previously showed that prolonged morphine treatment and chronic inflammation both enhanced delta opioid receptor (deltaOR) cell surface density in lumbar spinal cord neurons. Here, we sought to determine whether administration of morphine to rats with chronic inflammation would further increase the bio-availability of deltaOR, and thereby the analgesic properties of the deltaOR agonist deltorphin, over that produced by inflammation alone. We found that chronic inflammation produced by injection of complete Freund's adjuvant (CFA) into the hind paw resulted in a bilateral increase in the binding and internalization of fluorescent deltorphin in neurons of the lumbar spinal cord as did prolonged morphine treatment [Morinville A, Cahill CM, Aibak H, Rymar VV, Pradhan A, Hoffert C, Mennicken F, Stroh T, Sadikot AF, O'Donnell D, Clarke PB, Collier B, Henry JL, Vincent JP, Beaudet A (2004a) Morphine-induced changes in delta opioid receptor trafficking are linked to somatosensory processing in the rat spinal cord. J Neurosci 24:5549-5559]. This effect was accompanied by an increase in the antinociceptive efficacy of intrathecal deltorphin as measured using the tail-flick test. Treatment of CFA-injected rats with morphine decreased the cell surface availability of deltaOR in neurons of the dorsal horn of the lumbar spinal cord as compared with treatment with CFA alone. Behaviorally, it significantly enhanced the antihyperalgesic effects of deltorphin (plantar test; % maximum possible antihyperalgesic effect (MPAHE)=113.5%+/-32.4% versus 26.1%+/-11.6% in rats injected with CFA alone) but strongly reduced the antinociceptive efficacy of the drug (tail-flick test; % maximum possible antinociceptive effect (MPE)=29.6%+/-3.6% versus 66.6%+/-6.3% in rats injected with CFA alone) suggesting that the latter, but not the former, is linked to the deltaOR trafficking events observed neuroanatomically. These results demonstrate that in chronic inflammation, the antihyperalgesic effects of deltaOR agonists may be enhanced by morphine pre-treatment. They also reveal a dichotomy between mechanisms underlying antihyperalgesic and antinociceptive effects of deltaOR agonists.

Trafficking of delta-opioid receptors and other G-protein-coupled receptors: implications for pain and analgesia

A cell can regulate how it interacts with its external environment by controlling the number of plasma membrane receptors that are accessible for ligand stimulation. G-protein-coupled receptors (GPCRs) are the largest superfamily of cell surface receptors and have a significant role in physiological and pathological processes. Much research effort is now focused on understanding how GPCRs are delivered to the cell surface to enhance the number of 'bioavailable' receptors accessible for activation. Knowing how such processes are triggered or modified following induction of various pathological states will inevitably identify new therapeutic strategies for treating various diseases, including chronic pain. Here, we highlight recent advances in this field, and provide examples of the importance of such trafficking events in pain.

Induction of δ Opioid Receptor Function by Up-Regulation of Membrane Receptors in Mouse Primary Afferent Neurons

It is not clear whether primary afferent neurons express functional cell-surface opioid receptors. We examined delta receptor coupling to Ca2+ channels in mouse dorsal root ganglion neurons under basal conditions and after receptor up-regulation. [D-Ala2,Phe4,Gly5-ol]-enkephalin (DAMGO), [D-Ala2,D-Leu5]-enkephalin (DADLE), trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl) benzene-acetamide methanesulfonate (U-50,488H; 1 microM), and baclofen (50 microM) inhibited Ca2+ currents, whereas the -selective ligands [D-Pen2,Pen5]-enkephalin (DPDPE) and deltorphin II (1 microM) did not. The effect of DADLE (1 microM) was blocked by the mu-antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 300 nM) but not by the -antagonist Tyr-1,2,3,4-tetrahydroisoquinoline-Phe-Phe-OH (300 nM), implicating mu receptors. Despite a lack of functional delta receptors, flow cytometry revealed cell-surface receptors. We used this approach to identify conditions that up-regulate receptors, including mu receptor gene deletion in dorsal root ganglion neurons of mu-/- mice and 18-h incubation of mu+/+ neurons with CTAP followed by brief (10-min) DPDPE exposure. Under these conditions, the expression of cell-surface delta receptors was up-regulated to 149 +/- 9 and 139 +/- 5%, respectively; furthermore, DPDPE and deltorphin II (1 microM) inhibited Ca2+ currents in both cases. Viral replacement of mu receptors in mu-/- neurons reduced delta receptor expression to mu+/+ levels, restored the inhibition of Ca2+ currents by DAMGO, and abolished receptor coupling. Our observations suggest that receptor-Ca2+ channel coupling in primary afferent fibers may have little functional significance under basal conditions in which mu receptors predominate. However, up-regulation of cell-surface delta receptors induces their coupling to Ca2+ channels. Pharmacological approaches that increase functional delta receptor expression may reveal a novel target for analgesic therapy.

Opioid receptors

Opioid receptors belong to the large superfamily of seven transmembrane-spanning (7TM) G protein-coupled receptors (GPCRs). As a class, GPCRs are of fundamental physiological importance mediating the actions of the majority of known neurotransmitters and hormones. Opioid receptors are particularly intriguing members of this receptor family. They are activated both by endogenously produced opioid peptides and by exogenously administered opiate compounds, some of which are not only among the most effective analgesics known but also highly addictive drugs of abuse. A fundamental question in addiction biology is why exogenous opioid drugs, such as morphine and heroin, have a high liability for inducing tolerance, dependence, and addiction. This review focuses on many aspects of opioid receptors with the aim of gaining a greater insight into mechanisms of opioid tolerance and dependence.

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.

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.

Anatomical identification of neurons responsive to nociceptive stimuli

We describe methods for labeling and identifying neurons within the central nervous system involved in the transmission of nociceptive stimuli. The most reliable methods are physiological identification followed by intracellular injection or immunocytochemical detection of stimulus-induced markers such as Fos. These latter strategies are used with appropriate controls to distinguish neurons activated secondarily (e.g., motor response or inhibitory neurons) by the nociceptive stimuli. Other methods include location and morphology as determined by standard cytological and tracing methods and/or the presence of specific neurochemical markers such as substance P determined by immunocytochemistry.

mu-Opioid receptor knockout mice display reduced cocaine conditioned place preference but enhanced sensitization of cocaine-induced locomotion

The mu-opioid receptor (OPRM1) is expressed in brain regions implicated in reward and locomotor processes. Reduced reward, not only from opiates, but also from several other abused substances has been observed in mice with lifelong deletions of the OPRM1 gene. To further define the roles of mu-opioid receptors in psychostimulant actions, cocaine psychomotor stimulant and rewarding effects were examined in wild-type (WT), heterozygous and homozygous mu-opioid receptor knockout mice. While mu-opioid receptor knockout did not affect basal locomotion, locomotor stimulant effects of cocaine were enhanced in a within-subjects dose-response experiment. However, further study revealed that in mice injected with 20 mg/kg for the first time, there was no difference in the locomotor-stimulating effects of cocaine between knockout and wild-type mice. In a sensitization study (modeled after the conditions in the dose-response experiment) although not observed in WT mice, OPRM1-/- mice did exhibit cocaine sensitization. By stark contrast, and similar to the effects of other rewarding drugs in OPRM1 KO mice, cocaine reward, as assessed by conditioned place preference, was reduced in both homozygous and heterozygous OPRM1 KO mice. The present results confirm a central role of the mu-opioid receptor in drug reward but opposing effects on locomotor sensitization. The reduced cocaine reward identified in heterozygous mu-opioid receptor knockout mice supports the possibility that humans with fewer available mu-opioid receptors might experience less cocaine reward.

Regulation of delta-opioid receptor trafficking via mu-opioid receptor stimulation: evidence from mu-opioid receptor knock-out mice

We recently demonstrated that prolonged treatment with morphine increases the antinociceptive potency of the delta-opioid receptor (deltaOR) agonist deltorphin and promotes cell surface targeting of deltaORs in neurons of the dorsal horn of the rat spinal cord (Cahill et al., 2001b). In the present study we examined whether these effects were mediated selectively via muOR. Using the same intermittent treatment regimen as for morphine, we found that methadone and etorphine, but not fentanyl, enhanced [D-Ala2]-deltorphin-mediated antinociception. However, continuous delivery of fentanyl for 48 hr resulted in augmented deltaOR-mediated antinociception when compared with saline-infused animals. Time course studies confirmed that a 48 hr treatment with morphine was necessary for the establishment of enhanced deltaOR-mediated antinociception. The observed increases in deltaOR agonist potency and deltaOR plasma membrane density were reversed fully 48 hr after discontinuation of morphine injections. Wild-type C57BL/6 mice pretreated with morphine for 48 hr similarly displayed enhanced deltaOR-mediated antinociception in a tonic pain paradigm. Accordingly, the percentage of plasma membrane-associated deltaOR in the dorsal horn of the spinal cord, as assessed by immunogold electron microscopy, increased from 6.6% in naive to 12.4% in morphine-treated mice. In contrast, morphine treatment of muOR gene knock-out (KO) mice did not produce any change in deltaOR plasma membrane density. These results demonstrate that selective activation of muOR is critical for morphine-induced targeting of deltaOR to neuronal membranes, but not for basal targeting of this receptor to the cell surface.

DPDPE-UK14,304 synergy is retained in mu opioid receptor knockout mice

When agonists to alpha(2)adrenergic receptor (AR) and delta opioid receptor (DOR) are co-administered, they act synergistically to inhibit nociceptive elicited behavior. Some previous studies of synergism have used the DOR-selective agonist [D-Pen(2),D-Pen(5)]-enkehphalin (DPDPE), however, DPDPE has been shown to be less potent in mu opioid receptor-knockout (MOR-KO) mice. It is possible, therefore, that MOR contributes to the synergism of DPDPE with the alpha(2)AR agonists. We compared the interactions of spinally administered DPDPE with an alpha(2)AR-adrenergic agonist in MOR-KO and MOR-wildtype (WT) mice. In these mice, morphine is ineffective and the potency of spinally administered DOR agonists, deltorphin II (DELT II) and DPDPE decreased 16- and 250-fold, respectively. Antagonism studies using the MOR-selective antagonist, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH(2) (CTOP) and the DOR-selective antagonist, naltrindole HCl (naltrindole) demonstrated that while DOR mediates DPDPE-induced antinociception in MOR-KO, both MOR and DOR participate in DPDPE antinociception in WT mice, suggesting that DPDPE is less selective for DOR than previously observed in binding studies when given in vivo. The potency of the alpha(2)AR agonist UK14,304 was equivalent in WT and MOR-KO, demonstrating that the loss of opioid-mediated antinociception in the MOR-KO was not due to generalized impairment of antinociceptive processing. Interestingly, isobolographic analysis showed that, despite substantial loss of DPDPE potency in MOR-KO, DPDPE-UK14,304 synergism is fully retained. Collectively, these experiments demonstrate that although MOR participates in DELT II- and DPDPE-mediated spinal antinociception, DOR independently participates in synergistic antinociception with alpha(2)AR. Resolution of the roles of the opioid receptor subtypes in opioid agonist-induced effects may require comparison of the effects of multiple selective agonists in knockout animals.

The alpha2A-adrenoceptor subtype is not involved in inflammatory hyperalgesia or morphine-induced antinociception

The purpose of the present study was to investigate the role of the alpha(2A)-adrenoceptor subtype in inflammatory hyperalgesia, and in adrenergic-mu-opioid interactions in acute pain and inflammatory hyperalgesia. Behavioral responses to mechanical and thermal stimuli were studied in alpha(2A)-adrenoceptor knockout mice and their wild-type controls. Thermal nociception was evaluated as paw withdrawal latencies to radiant heat applied to the hindpaws. Mechanical nociception was measured using von Frey monofilament applications to the hindpaws. Mechanical and thermal hyperalgesia, induced with intraplantar carrageenan (1 mg/40 microl) were compared in alpha(2A)-adrenoceptor knockout and wild-type mice. The effects of the systemically administered mu-opioid receptor agonist morphine (1-10 mg/kg) were evaluated on mechanical withdrawal responses under normal and inflammatory conditions in knockout and wild-type mice. Withdrawal responses to radiant heat and von Frey monofilaments were similar in alpha(2A)-adrenoceptor knockout and wild-type mice before and after the carrageenan-induced hindpaw inflammation. Also, the antinociceptive effects of morphine in mechanical nociceptive tests were similar before and after carrageenan-induced hindpaw inflammation. Our observations indicate that alpha(2A)-adrenoceptors are not tonically involved in the modulation of inflammation-induced mechanical and thermal hyperalgesia. In addition, alpha(2A)-adrenoceptors do not appear to play an important role in mu-opioid receptor-mediated antinociception or antihyperalgesia.

Acute antinociceptive responses in single and combinatorial opioid receptor knockout mice: distinct mu, delta and kappa tones

We have examined responses of mice lacking mu, delta and kappa opioid receptor (MOR, DOR and KOR, respectively) genes, as well as combinatorial mutants, in several pain models. This is the first truly comparative study of all three opioid receptor-deficient mice, with genotypes and gender analysis using mice on the hybrid 50% 129/SV : 50% C57BL/6 genetic background. In the tail-immersion test, only KOR-/- females showed decreased withdrawal latencies. This modification was also found in MOR/KOR and MOR/DOR/KOR, but not MOR/DOR mutants. The hotplate test revealed increased nociceptive sensitivity for MOR-/-, a phenotype which was also observed in double mutants involving the MOR deletion, and in the triple mutants. The tail-pressure test showed increased response for both MOR-/- and DOR-/- mutants, a modification which was enhanced in the triple-mutant mice. In the formalin test, MOR-/- and DOR-/- mice showed increased responses in the early and late phases, respectively, while the triple mutant tended to show enhanced nociception in both phases. Finally, the enhanced response of KOR-/- mice in the writhing test, which we have demonstrated previously, was confirmed in double MOR/KOR- and triple-mutant mice. Together, the data support the existence of an antinociceptive opioid tone. Each receptor presents a distinct pattern of activities, with mu receptors influencing responses to mechanical, chemical and thermal nociception at a supraspinal level, kappa receptors involved in spinally mediated thermal nociception and chemical visceral pain, and delta receptors modulating mechanical nociception and inflammatory pain. Phenotypes of mutant mice were subtle, suggesting a low endogenous opioid tone in the regulation of physiological pain.

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

Pharmacological and physiological evidence supports a role for delta (delta) opioid receptors in the nociceptive mechanisms of inflammation. However, few data exist regarding delta opioid receptor expression and localization in such conditions. In this study, we have assessed the distribution and function of delta opioid receptors in the rat spinal cord following induction of chronic inflammation by intraplantar injection of complete Freund's adjuvant (CFA). Intrathecal administration of the selective delta opioid receptor agonist, D-[Ala(2), Glu(4)] deltorphin, dose-dependently reversed thermal hyperalgesia induced by CFA. In situ hybridization and Western blotting experiments revealed an increase in delta opioid receptor mRNA and protein levels, respectively, in the dorsal lumbar spinal cord ipsilateral to the CFA injection site compared to the contralateral side and sham-injected controls. By electron microscopy, immunopositive delta opioid receptors were evident in neuronal perikarya, dendrites, unmyelinated axons and axon terminals. Quantification of immunopositive signal in dendrites revealed a twofold increase in the number of immunogold particles in the ipsilateral dorsal spinal cord of CFA-injected rats compared to the contralateral side and to sham-injected rats. Moreover, the relative frequency of immunogold particles associated with or in close proximity to the plasma membrane was increased in the ipsilateral dorsal spinal cord, indicating a more efficient targeting of delta opioid receptors to neuronal plasma membranes. These data demonstrate that CFA induces an up-regulation and increased membrane targeting of delta opioid receptors in the dorsal spinal cord which may account for the enhanced antinociceptive effects of delta opioid receptor agonists in chronic inflammatory pain models.

The role of mu-opioid receptors in inflammatory hyperalgesia and alpha 2-adrenoceptor-mediated antihyperalgesia

The purpose of the present study was to investigate the role of mu-opioid receptor in inflammatory hyperalgesia in intact and in spinalized animals and the interaction between mu-opioid and alpha2-adrenergic receptor in acute pain and inflammatory hyperalgesia. Behavioral responses to mechanical and heat stimuli were studied in mu-opioid receptor knockout mice and wildtype control mice. Thermal nociception was evaluated by measuring paw withdrawal latencies to radiant heat applied to the hindpaws. Mechanical nociception was measured by von Frey monofilament applications to the hindpaws. Intraplantar carrageenan-induced (1 mg/40 microl) mechanical and heat hyperalgesia were compared in micro-opioid knockout and wildtype mice. The effect of systemically administered alpha2-adrenergic receptor agonist dexmedetomidine (1-10 microg/kg) was evaluated on mechanical and thermal withdrawal responses under normal and inflammatory state in knockout and wildtype mice. The role of micro-opioid receptor in descending modulation of nociception was studied by assessing mechanical and heat withdrawal responses before and after mid-thoracic spinalization. Withdrawal responses to radiant heat and von Frey monofilaments were similar in mu-opioid knockout and wildtype mice before and after the carrageenan induced hindpaw inflammation. Also, antinociceptive effects of dexmedetomidine in thermal and mechanical nociceptive tests were similar before carrageenan induced hindpaw inflammation. However, the potency of dexmedetomidine was significantly reduced in carrageenan-induced mechanical hyperalgesia in mu-opioid knockout mice compared to the wildtype control mice. Thermal and mechanical withdrawal responses were similar between mu-opioid knockout and wildtype mice before and after mid-thoracic spinalization. Our observations indicate that the mu-opioid receptors do not play an important role in alpha2-adrenergic receptor agonist-mediated acute antinociception. In addition, micro-opioid receptors are not tonically involved in the modulation of inflammation-induced mechanical and thermal hyperalgesia, and the supraspinal control of spinal reflexes. However, in the presence of inflammation, mu-opioid receptors play an important role in the antihyperalgesic actions of an alpha2-adrenergic receptor agonist.

Opioid control of inflammatory pain regulated by intercellular adhesion molecule-1

Pain can be effectively controlled by endogenous mechanisms based on neuroimmune interactions. In inflamed tissue immune cell-derived opioid peptides activate opioid receptors on peripheral sensory nerves leading to potent analgesia. This is brought about by a release of opioids from inflammatory cells after stimulation by stress or corticotropin-releasing hormone (CRH). Immunocytes migrate from the circulation to inflamed tissue in multiple steps, including their rolling, adhesion, and transmigration through the vessel wall. This is orchestrated by adhesion molecules on leukocytes and vascular endothelium. Intercellular adhesion molecule-1 [ICAM-1 (or CD54)] is expressed by endothelium and mediates adhesion and extravasation of leukocytes. The goal of this study was to show that ICAM-1 regulates the homing of opioid-producing cells and the subsequent generation of analgesia within sites of painful inflammation. This was accomplished using immunofluorescence, flow cytometry, and behavioral (paw pressure) testing. We found that ICAM-1 is upregulated on the vascular endothelium, simultaneously with an enhanced immigration of opioid-containing immune cells into inflamed paw tissue. The intravenous administration of a monoclonal antibody against ICAM-1 markedly decreased the migration of opioid-containing leukocytes and of granulocytes, monocytes-macrophages, and T cells to the inflamed tissue. At the same time, circulating immunocytes increased in numbers, and macroscopic inflammation (hyperalgesia, paw volume, and paw temperature) remained primarily unchanged. Most importantly, peripheral opioid analgesia elicited either by cold water swim stress or by intraplantar administration of CRH was dramatically reduced. Together, these findings indicate that ICAM-1 expressed on vascular endothelium recruits immunocytes containing opioids to promote the local control of inflammatory pain.

Quantitative autoradiography of adenosine receptors and NBTI-sensitive adenosine transporters in the brains and spinal cords of mice deficient in the mu-opioid receptor gene

There is a large body of evidence indicating important interactions between the adenosine and opioid systems in regulating pain at both the spinal and supraspinal level. Mice lacking the mu-opioid receptor (MOR) gene have been successfully developed and the animals show complete loss of analgesic responses to morphine as well as differences in pain sensitivity. To investigate if there are any compensatory alterations in adenosine systems in mutant animals, we have carried out quantitative autoradiographic mapping of A(1) and A(2A) adenosine receptors and nitrobenzylthioinosine (NBTI) sensitive adenosine transporters in the brains and spinal cords of wild type, heterozygous and homozygous mu-opioid receptor knockout mice. Adjacent coronal sections were cut from the brains and spinal cords of +/+, +/- and -/- mice for the determination of binding of [3H]DPCPX, [3H]CGS21680 or [3H]NBTI to A(1) and A(2A) adenosine receptors and NBTI-sensitive adenosine transporters, respectively. A small but significant reduction in [3H]DPCPX and [3H]NBTI binding was detected in mutant mice brains but not in spinal cords. No significant change in A(2A) binding was detected in mu-opioid receptor knockout brains. The results suggest there may be functional interactions between mu-receptors and A(1) adenosine receptors as well as NBTI-sensitive adenosine transporters in the brain but not in the spinal cord.

Exploring the opioid system by gene knockout

The endogenous opioid system consists of three opioid peptide precursor genes encoding enkephalins (preproenkephalin, Penk), dynorphins (preprodynorphin, Pdyn) and beta-endorphin (betaend), proopiomelanocortin (POMC) and three receptor genes encoding mu-opiod receptor (MOR), delta-opiod receptor (DOR) and kappa-opiod receptor (KOR). In the past years, all six genes have been inactivated in mice by homologous recombination. The analysis of spontaneous behavior in mutant mice has demonstrated significant and distinct roles of each gene in modulating locomotion, pain perception and emotional behaviors. The observation of opposing phenotypes of MOR- and DOR-deficient mice in several behaviors highlights unexpected roles for DOR to be further explored genetically and using more specific delta compounds. The analysis of responses of mutant mice to exogenous opiates has definitely clarified the essential role of MOR in both morphine analgesia and addiction, and demonstrated that DOR and KOR remain promising targets for pain treatment. These studies also show that prototypic DOR agonists partially require MOR for their biological activity and provide some support for the postulated mu-delta interactions in vivo. Finally, data confirm and define a role for several genes of the opioid system in responses to other drugs of abuse, and the triple opioid receptor knockout mutant allows exploring non-classical opioid pharmacology. In summary, the study of null mutant mice has extended our previous knowledge of the opioid system by identifying the molecular players in opioid pharmacology and physiology. Future studies should involve parallel behavioral analysis of mice lacking receptors and peptides and will benefit from more sophisticated gene targeting approaches, including site-directed and anatomically-restricted mutations.

Opioid receptor genes inactivated in mice: the highlights

The opioid system controls nociception, stress responses, and addictive behaviors. Exogenous alkaloid opiates and endogenous opioid peptides stimulate mu-, delta- and kappa-opioid receptors, whose activities have long been analyzed by pharmacological tools. Mice lacking opioid receptor and opioid peptide precursor genes have now been produced by gene targeting. Behavioral analysis of mutant animals in the absence of drug has highlighted a distinct role of opioid receptors or peptides in nociception and revealed an important role for delta receptors in emotional behaviors. The examination of responses to drugs has clarified involvement of each receptor as molecular targets for exogenous opiates in vivo. Those data have also demonstrated the critical role of mu-receptor in cannabinoid and alcohol reinforcement and confirmed the involvement of kappa receptor in several dysphoric responses. Ongoing studies therefore help in understanding the molecular basis of opioid-controlled behaviors and will contribute to the development of novel therapeutics for pain, anxiety, and drug abuse.

Naloxone fails to produce conditioned place aversion in mu-opioid receptor knock-out mice

There is growing evidence that tonic activity of the opioid system may be important in the modulation of affective state. Naloxone produces a conditioned place aversion in rodents, an effect that is centrally mediated. Previous pharmacological data using antagonists with preferential actions at mu-, delta-, and kappa-opioid receptors indicate the importance of the mu-opioid receptor in mediating this effect. We sought to test the mu-opioid receptor selectivity of naloxone aversion using mu-opioid receptor knock-out mice. mu-Opioid receptor knock-out and wild-type mice were tested for naloxone (10 mg/kg, s.c.) aversion using a place conditioning paradigm. As a positive control for associative learning, knock-out mice were tested for conditioned place aversion to a kappa agonist, U50,488H (2 mg/kg, s.c.). Naloxone produced a significant place aversion in wild-type mice, but failed to have any effect in mu-opioid receptor knock-out mice. On the other hand, both knock-out and wild-type mice treated with U50,488H spent significantly less time in the drug-paired chamber compared to their respective vehicle controls. We conclude that the mu-opioid receptor is crucial for the acquisition of naloxone-induced conditioned place aversion. Furthermore, in a separate experiment using C57BL/6 mice, the delta-selective antagonist naltrindole (10 or 30 mg/kg, s.c.) failed to produce conditioned place aversion.Taken together, these data further support the notion that naloxone produces aversion by antagonizing tonic opioid activity at the mu-opioid receptor.

Respiratory function in adult mice lacking the mu-opioid receptor: role of delta-receptors

Mice lacking the mu-opioid receptor (MOR) provide a unique model to determine whether opioid receptors are functionally interactive. Recent results have shown that respiratory depression produced by delta-opioid receptor agonists is suppressed in mice lacking the mu-opioid receptor. Here we investigated the involvement of mu- and delta-opioid receptors in the control of ventilation and mu/delta receptor interactions in brainstem rhythm-generating structures. Unrestrained MOR-/- and wild-type mice showed similar ventilatory patterns at rest and similar chemosensory responses to hyperoxia (100% O2), hypoxia (10% O2) or hypercapnia (5%CO2-95%O2). Blockade of delta-opioid receptors with naltrindole affected neither the ventilatory patterns nor the ventilatory responses to hypoxia in MOR-/- and wild-type mice. In-vitro, respiratory neurons were recorded in the pre-Bötzinger complex of thick brainstem slices of MOR-/- and wild-type young adult mice. Respiratory frequency was not significantly different between these two groups. The delta2 receptor agonist deltorphin II (0.1-1.0 microM) decreased respiratory frequency in both groups whereas doses of the delta1 receptor agonist enkephalin[D-Pen2,5] (0.1-1.0 microM) which were ineffective in wild-type mice significantly decreased respiratory frequency in MOR-/- mice. We conclude that deletion of the mu-opioid receptor gene has no significant effect on ensuing respiratory rhythm generation, ventilatory pattern, or chemosensory control. In MOR-/- mice, the loss of respiratory-depressant effects of delta2-opioid receptor agonists previously observed in vivo does not result from a blunted response of delta receptors in brainstem rhythm-generating structures. These structures show an unaltered response to delta2-receptor agonists and an augmented response to delta1-receptor agonists.

Pronociceptive actions of dynorphin maintain chronic neuropathic pain

Whereas tissue injury increases spinal dynorphin expression, the functional relevance of this upregulation to persistent pain is unknown. Here, mice lacking the prodynorphin gene were studied for sensitivity to non-noxious and noxious stimuli, before and after induction of experimental neuropathic pain. Prodynorphin knock-out (KO) mice had normal responses to acute non-noxious stimuli and a mild increased sensitivity to some noxious stimuli. After spinal nerve ligation (SNL), both wild-type (WT) and KO mice demonstrated decreased thresholds to innocuous mechanical and to noxious thermal stimuli, indicating that dynorphin is not required for initiation of neuropathic pain. However, whereas neuropathic pain was sustained in WT mice, KO mice showed a return to baselines by post-SNL day 10. In WT mice, SNL upregulated lumbar dynorphin content on day 10, but not day 2, after injury. Intrathecal dynorphin antiserum reversed neuropathic pain in WT mice at post-SNL day 10 (when dynorphin was upregulated) but not on post-SNL day 2; intrathecal MK-801 reversed SNL-pain at both times. Opioid (mu, delta, and kappa) receptor density and G-protein activation were not different between WT and KO mice and were unchanged by SNL injury. The observations suggest (1) an early, dynorphin-independent phase of neuropathic pain and a later dynorphin-dependent stage, (2) that upregulated spinal dynorphin is pronociceptive and required for the maintenance of persistent neuropathic pain, and (3) that processes required for the initiation and the maintenance of the neuropathic pain state are distinct. Identification of mechanisms that maintain neuropathic pain appears important for strategies to treat neuropathic pain.