Selective blockade of peripheral delta opioid agonist induced antinociception by intrathecal administration of delta receptor antisense oligodeoxynucleotide

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.

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.

Role of nociceptor αCaMKII in transition from acute to chronic pain (hyperalgesic priming) in male and female rats

We have previously shown that activation of protein kinase Cε (PKCε) in male rats induces a chronic, long-lasting change in nociceptors such that a subsequent exposure to proinflammatory mediators produces markedly prolonged mechanical hyperalgesia. This neuroplastic change, hyperalgesic priming, is dependent on activation of cytoplasmic polyadenylation element-binding protein (CPEB), downstream of PKCε, and consequent translation of mRNAs in the peripheral terminal of the nociceptor. Since α calmodulin-dependent protein kinase II (αCaMKII), a molecule implicated in neuroplasticity, is a target of CPEB and can also affect CPEB function, we investigated its role in the transition from acute to chronic pain. Priming induced by direct activation of PKCε can be prevented by inhibition of αCaMKII. In addition, direct activation of αCaMKII induces priming, which was not prevented by pretreatment with PKCε antisense, suggesting that αCaMKII is downstream of PKCε in the induction of priming. Activation of ryanodine receptors (RyRs), which can lead to activation of αCaMKII, also induced priming, in a calcium- and αCaMKII-dependent manner. Similarly, inhibition of the RyR and a calcium buffer prevented induction of priming by PKCε. Unlike activation of PKCε, ryanodine and αCaMKII induced priming in female as well as male rats. Our results demonstrate a contribution of αCaMKII to induction of hyperalgesic priming, a phenomenon implicated in the transition from acute to chronic pain.

Delta opioid receptor analgesia: recent contributions from pharmacology and molecular approaches

Delta opioid receptors represent a promising target for the development of novel analgesics. A number of tools have been developed recently that have significantly improved our knowledge of δ receptor function in pain control. These include several novel δ agonists with potent analgesic properties, and genetic mouse models with targeted mutations in the δ opioid receptor gene. Also, recent findings have further documented the regulation of δ receptor function at cellular level, which impacts on the pain-reducing activity of the receptor. These regulatory mechanisms occur at transcriptional and post-translational levels, along agonist-induced receptor activation, signaling and trafficking, or in interaction with other receptors and neuromodulatory systems. All these tools for in-vivo research, and proposed mechanisms at molecular level, have tremendously increased our understanding of δ receptor physiology, and contribute to designing innovative strategies for the treatment of chronic pain and other diseases such as mood disorders.

Hydrogen sulphide induces micro opioid receptor-dependent analgesia in a rodent model of visceral pain

BACKGROUND

Hydrogen sulphide (H2S) is a gaseous neuro-mediator that exerts analgesic effects in rodent models of visceral pain by activating KATP channels. A body of evidence support the notion that KATP channels interact with endogenous opioids. Whether H2S-induced analgesia involves opioid receptors is unknown.

METHODS

The perception of painful sensation induced by colorectal distension (CRD) in conscious rats was measured by assessing the abdominal withdrawal reflex. The contribution of opioid receptors to H2S-induced analgesia was investigated by administering rats with selective mu, kappa and delta opioid receptor antagonists and antisenses. To investigate whether H2S causes mu opioid receptor (MOR) transactivation, the neuronal like cells SKNMCs were challenged with H2S in the presence of MOR agonist (DAMGO) or antagonist (CTAP). MOR activation and phosphorylation, its association to beta arrestin and internalization were measured.

RESULTS

H2S exerted a potent analgesic effects on CRD-induced pain. H2S-induced analgesia required the activation of the opioid system. By pharmacological and molecular analyses, a robust inhibition of H2S-induced analgesia was observed in response to central administration of CTAP and MOR antisense, while kappa and delta receptors were less involved. H2S caused MOR transactivation and internalization in SKNMCs by a mechanism that required AKT phosphorylation. MOR transactivation was inhibited by LY294002, a PI3K inhibitor, and glibenclamide, a KATP channels blocker.

CONCLUSIONS

This study provides pharmacological and molecular evidence that antinociception exerted by H2S in a rodent model of visceral pain is modulated by the transactivation of MOR. This observation provides support for development of new pharmacological approaches to visceral pain.

Morphine peripheral analgesia depends on activation of the PI3Kgamma/AKT/nNOS/NO/KATP signaling pathway

Morphine is one of the most prescribed and effective drugs used for the treatment of acute and chronic pain conditions. In addition to its central effects, morphine can also produce peripheral analgesia. However, the mechanisms underlying this peripheral action of morphine have not yet been fully elucidated. Here, we show that the peripheral antinociceptive effect of morphine is lost in neuronal nitric-oxide synthase null mice and that morphine induces the production of nitric oxide in primary nociceptive neurons. The activation of the nitric-oxide pathway by morphine was dependent on an initial stimulation of PI3Kgamma/AKT protein kinase B (AKT) and culminated in increased activation of K(ATP) channels. In the latter, this intracellular signaling pathway might cause a hyperpolarization of nociceptive neurons, and it is fundamental for the direct blockade of inflammatory pain by morphine. This understanding offers new targets for analgesic drug development.

Endogenous kappa-opioid receptor systems inhibit hyperalgesia associated with localized peripheral inflammation

Peripheral inflammation evokes functional and biochemical changes in the periphery and spinal cord which result in central sensitization and hypersensitivity. Inhibitory control systems from the rostral ventromedial medulla (RVM) are also activated. The present study investigates whether endogenous kappa-opioid receptor (KOPr) systems contribute to these neuroadaptations. Inflammation was induced by intraplantar injection of complete Freund's adjuvant (CFA) into one hindpaw. Mechanical and thermal thresholds were determined using the Von Frey and radiant heat tests, respectively. KOPr gene deletion in mice or systemic administration of the long-acting KOPr antagonist, norbinaltorphimine (norBNI) significantly exacerbated mechanical and thermal hypersensitivity of the ipsilateral, inflamed paw. Thermal and mechanical thresholds of the non-inflamed, contralateral hindpaw were unaffected by CFA treatment. However, gene deletion as well as norBNI treatment resulted in mechanical, but not thermal hypersensitivity of the non-inflamed paw. Similar results were obtained when norBNI was administered intrathecally or into the RVM in rats. These data demonstrate a previously unrecognized role of endogenous KOPr systems in inhibiting hyperalgesia during inflammation. Furthermore, they demonstrate that decreased KOPr activity in either the spinal cord or RVM not only enhances mechanical and thermal hyperalgesia of the inflamed limb but also leads to an unmasking of mechanical hyperalgesia at a site remote from inflammation. The differential effects of KOPr antagonism on mechanical versus thermal thresholds for the non-inflamed paw support the notion that distinct neuroanatomical or neurochemical mechanisms modulate the processing of thermal versus mechanical stimuli.

Antinociceptive effect of antisense oligonucleotides against the vanilloid receptor VR1/TRPV1

To examine the role of the vanilloid receptor TRPV1 in neuropathic pain, we assessed the effects of the receptor antagonist thioxo-BCTC and antisense oligonucleotides against the TRPV1 mRNA in a rat model of spinal nerve ligation. In order to identify accessible sites on the mRNA of TRPV1, the RNase H assay was used, leading to the successful identification of binding sites for antisense oligonucleotides. Cotransfection studies using Cos-7 cells were employed to identify the most effective antisense oligonucleotide efficiently inhibiting the expression of a fusion protein consisting of TRPV1 and the green fluorescent protein in a specific and concentration-dependent manner. In an in vivo rat model of spinal nerve ligation, intravenous application of the TRPV1 antagonist thioxo-BCTC reduced mechanical hypersensitivity yielding an ED(50) value of 10.6mg/kg. Intrathecal administration of the antisense oligonucleotide against TRPV1, but not the mismatch oligonucleotide or a vehicle control, reduced mechanical hypersensitivity in rats with spinal nerve ligation in a similar manner. Immunohistochemical analysis revealed neuropathy- and antisense-associated regulation of TRPV1 protein expression in spinal cord and dorsal root ganglia. Our data demonstrate comparative analgesic effects of a TRPV1 anatagonist and a rationally designed TRPV1 antisense oligonucleotide in a spinal nerve ligation model of neuropathic pain and thus, lend support to the validation of TRPV1 as a promising target for the treatment of neuropathic pain.

Chronic morphine administration results in tolerance to delta opioid receptor-mediated antinociception

Delta opioid receptor agonists produce only a moderate degree of antinociception, possibly reflecting the predominantly intracellular location of delta opioid receptor. However, recent studies suggest that short term morphine pretreatment can increase delta opioid receptor-mediated antinociception by promoting the translocation of delta opioid receptor to the cell surface. Even more striking sensitization has been reported after long term morphine pretreatment and withdrawal in locomotor tests. In the present study we therefore examined the effects of longer term morphine pretreatment and withdrawal on delta opioid receptor-mediated antinociception in the formalin test. Male adult rats were pretreated daily with morphine (10 mg/kg s.c.) or saline for 10 days, and were tested acutely with the delta opioid receptor agonist [D-Ala2,Glu4]-deltorphin (intrathecal) at 0, 7 and 14 days of withdrawal. Unexpectedly, chronic morphine pre-exposure resulted in tolerance to [D-Ala2,Glu4]-deltorphin-induced antinociception, and this occurred at 0 and 7 but not 14 days of morphine withdrawal. Morphine challenge at withdrawal day 7 confirmed the presence of tolerance to the antinociceptive effects of this drug. Chronic morphine pretreatment also resulted in tolerance to the locomotor stimulant effect of [D-Ala2,Glu4]-deltorphin (given i.c.v.), contrary to a previous report of sensitization. However, consistent with previous reports, short term (2 day) pretreatment with morphine did result in sensitization to [D-Ala2,Glu4]-deltorphin. Subsequent in vitro analysis, using [125I][D-Ala2,Glu4]-deltorphin or guanosine 5'(gamma-35S-thio) triphosphate autoradiography, did not reveal any changes in delta opioid receptor binding or function resulting from chronic morphine pretreatment. In conclusion, chronic morphine pretreatment caused tolerance to delta opioid receptor-mediated behavioral effects with no clear change at the receptor level.

An efficient intrathecal delivery of small interfering RNA to the spinal cord and peripheral neurons

We have developed a highly effective method for in vivo gene silencing in the spinal cord and dorsal root ganglia (DRG) by a cationic lipid facilitated delivery of synthetic, small interfering RNA (siRNA). A siRNA to the delta opioid receptor (DOR), or a mismatch RNA, was mixed with the transfection reagent, i-Fect (vehicle), and delivered as repeated daily bolus doses (0.5 microg to 4 microg) via implanted intrathecal catheter to the lumbar spinal cord of rats. Twenty-four hours after the last injection, rats were tested for antinociception by the DOR selective agonist, [D-Ala(2), Glu(4)]deltorphin II (DELT), or the mu opioid receptor (MOR) selective agonist, [D-Ala(2), N-Me-Phe(4), Gly-ol(5)]enkephalin (DAMGO). Pretreatment with the siRNA, but not the mismatch RNA or vehicle alone, blocked DELT antinociception dose-dependently. The latter was concomitant with a reduction in the spinal immunoreactivity and receptor density of DOR, and in DOR transcripts in the lumbar DRG and spinal dorsal horn. Neither siRNA nor mismatch RNA pretreatment altered spinal immunoreactivity of MOR or antinociception by spinal DAMGO, and had no effect on the baseline thermal nociceptive threshold. The inhibition of function and expression of DOR by siRNA was reversed by 72 hr after the last RNA injection. The uptake of fluorescence-tagged siRNA was detected in both DRG and spinal cord. The low effective dose of siRNA/i-Fect complex reflects an efficient delivery of the siRNA to peripheral and spinal neurons, produced no behavioral signs of toxicity. This delivery method may be optimized for other gene targets.

The pain of antisense: in vivo application of antisense oligonucleotides for functional genomics in pain and analgesia

As the genomic revolution continues to evolve, there is an increasing demand for efficient and reliable tools for functional characterization of individual gene products. Antisense oligonucleotide-mediated knockdown has been used successfully as a functional genomics tool in animal models of pain and analgesia yet skepticism regarding the validity and utility of antisense technology remains. Contributing to this uncertainty are the lack of systematic studies exploring antisense oligonucleotide use in vivo and the many technical and methodological challenges intrinsic to the method. This article reviews the contributions of antisense oligonucleotide-based studies to the field of pain and analgesia and the general principles of antisense technology. A special emphasis is placed on technical issues surrounding the successful application of antisense oligonucleotides in vivo, including sequence selection, antisense oligonucleotide chemistry, DNA controls, route of administration, uptake, dose-dependence, time-course and adequate evaluation of knockdown.

Spinal delivery of analgesics in experimental models of pain and analgesia

Systemic administration of analgesics can lead to serious adverse side effects compromising therapeutic benefit in some patients. Information coding pain transmits along an afferent neuronal network, the first synapses of which reside principally in the spinal cord. Delivery of compounds to spinal cord, the intended site of action for some analgesics, is potentially a more efficient and precise method for inhibiting the pain signal. Activation of specific proteins that reside in spinal neuronal membranes can result in hyperpolarization of secondary neurons, which can prevent transmission of the pain signal. This is one of the mechanisms by which opioids induce analgesia. The spinal cord is enriched in such molecular targets, the activation of which inhibit the transmission of the pain signal early in the afferent neuronal network. This review describes the pre-clinical models that enable new target discovery and development of novel analgesics for site-directed pain management.

Ten years of antisense inhibition of brain G-protein-coupled receptor function

Antisense oligonucleotides (AOs) are widely used as tools for inhibiting gene expression in the mammalian central nervous system. Successful gene suppression has been reported for different targets such as neurotransmitter receptors, neuropeptides, ion channels, trophic factors, cytokines, transporters, and others. This illustrates their potential for studying the expression and function of a wide range of proteins. AOs may even find therapeutic applications and provide an attractive strategy for intervention in diseases of the central nervous system (CNS). However, a lack of effectiveness and/or specificity could be a major drawback for research or clinical applications. Here we provide a critical overview of the literature from the past decade on AOs for the study of G-protein-coupled receptors (GPCRs). The following aspects will be considered: mechanisms by which AOs exert their effects, types of animal model system used, detection of antisense action, effects of AO design and delivery characteristics, non-antisense effects and toxicological properties, controls used in antisense studies to assess specificity, and our results (failures and successes). Although the start codon of the mRNA is the most popular region (46%) to target by AOs, targeting the coding region of GPCRs is almost as common (41%). Moreover, AOs directed to the coding region of the GPCR mRNA induce the highest reductions in receptor levels. To resist degradation by nucleases, the modified phosphorothioate AO (S-AO) is the most widely used and effective oligonucleotide. However, the end-capped phosphorothioate AOs (ECS-AOs) are increasingly used due to possible toxic and non-specific effects of the S-AO. Other parameters affecting the activity of a GPCR-targeting AO are the length (mostly an 18-, 20- or 21-mer) and the GC-content (mostly varying from 30 to 80%). Interestingly, one-third of the AOs successfully targeting GPCRs possess a GC/AT ratio of 61-70%. AO-induced reductions in GPCR expression levels and function range typically from 21 to 40% and 41 to 50%, respectively. In contrast to many antisense reviews, we therefore conclude that the functional activity of a GPCR after AO treatment correlates mostly with the density of the target receptors (maximum factor 2). However, AOs are no simple tools for experimental use in vivo. Despite successful results in GPCR research, no general guidelines exist for designing a GPCR-targeting AO or, in general, for setting up a GPCR antisense experiment. It seems that the correct choice of a GPCR targeting AO can only be ascertained empirically. This disadvantage of antisense approaches results mostly from incomplete knowledge about the internalisation and mechanism of action of AOs. Together with non-specific effects of AOs and the difficulties of assessing target specificity, this makes the use of AOs a complex approach from which conclusions must be drawn with caution. Further antisense research has to be carried out to ensure the adequate use of AOs for studying GPCR function and to develop antisense as a valuable therapeutic modality.

kappa -opioid receptor agonists modulate visceral nociception at a novel, peripheral site of action

kappa-opioid receptor agonists (kappa-ORAs) have been shown to modulate visceral nociception through an interaction with a peripheral, possibly novel, kappa-opioid-like receptor. We used in the present experiments an antisense strategy to further explore the hypothesis that kappa-ORA effects in the colon are produced at a site different from the cloned kappa-opioid receptor (KOR). An antisense oligodeoxynucleotide (ODN) to the cloned rat KOR was administered intrathecally (12.5 microg, twice daily for 4 d) to specifically knock-down the cloned KOR. Efficacy of the KOR antisense ODN treatment was behaviorally evaluated by assessing the antinociceptive effects of peripherally administered kappa- (EMD 61, 753 and U 69,593), mu- (DAMGO) and delta- (deltorphin) ORAs in the formalin test. Intrathecal antisense, but not mismatch ODN blocked the actions of EMD 61,753 and U 69,593 without affecting the actions of DAMGO or deltorphin; a complete recovery of antinociceptive actions of the kappa-ORA EMD 61,753 was observed 10 d after the termination of antisense ODN treatment. In contrast, the ability of EMD 61,753 to dose-dependently attenuate responses of pelvic nerve afferent fibers to noxious colonic distension was unaffected in the same rats in which the antisense ODN effectively knocked-down the KOR as assessed in the formalin test. Additionally, Western blot analysis demonstrated a significant downregulation of KOR protein in the L4-S1 dorsal root ganglia of antisense, but not mismatch ODN-treated rats. The present results support the existence of a non-kappa-opioid receptor site of action localized in the colon.

delta opioid receptor modulation of several voltage-dependent Ca(2+) currents in rat sensory neurons

Endogenous enkephalins and delta opiates affect sensory function and pain sensation by inhibiting synaptic transmission in sensory circuits via delta opioid receptors (DORs). DORs have long been suspected of mediating these effects by modulating voltage-dependent Ca(2+) entry in primary sensory neurons. However, not only has this hypothesis never been validated in these cells, but in fact several previous studies have only turned up negative results. By using whole-cell current recordings, we show that the delta enkephalin analog [D-Ala(2), D-Leu(5)]-enkephalin (DADLE) inhibits, via DORs, L-, N-, P-, and Q-high voltage-activated Ca(2+) channel currents in cultured rat dorsal root ganglion (DRG) neurons. The percentage of responding cells was remarkably high (75%) within a novel subpopulation of substance P-containing neurons compared with the other cells (18-35%). DADLE (1 microM) inhibited 32% of the total barium current through calcium channels (I(Ba)). A delta (naltrindole, 1 microM), but not a mu (beta-funaltrexamine, 5 microM), antagonist prevented the DADLE response, whereas a DOR-2 subtype (deltorphin-II, 100 nM), but not a DOR-1 (DPDPE, 1 microM), agonist mimicked the response. L-, N-, P-, and Q-type currents contributed, on average, 18, 48, 14, and 16% to the total I(Ba) and 19, 50, 26, and 20% to the DADLE-sensitive current, respectively. The drug-insensitive R-type current component was not affected by the agonist. This work represents the first demonstration that DORs modulate Ca(2+) entry in sensory neurons and suggests that delta opioids could affect diverse Ca(2+)-dependent processes linked to Ca(2+) influx through different high-voltage-activated channel types.

A comparison of the potential role of the tetrodotoxin-insensitive sodium channels, PN3/SNS and NaN/SNS2, in rat models of chronic pain

Alterations in sodium channel expression and function have been suggested as a key molecular event underlying the abnormal processing of pain after peripheral nerve or tissue injury. Although the relative contribution of individual sodium channel subtypes to this process is unclear, the biophysical properties of the tetrodotoxin-resistant current, mediated, at least in part, by the sodium channel PN3 (SNS), suggests that it may play a specialized, pathophysiological role in the sustained, repetitive firing of the peripheral neuron after injury. Moreover, this hypothesis is supported by evidence demonstrating that selective "knock-down" of PN3 protein in the dorsal root ganglion with specific antisense oligodeoxynucleotides prevents hyperalgesia and allodynia caused by either chronic nerve or tissue injury. In contrast, knock-down of NaN/SNS2 protein, a sodium channel that may be a second possible candidate for the tetrodotoxin-resistant current, appears to have no effect on nerve injury-induced behavioral responses. These data suggest that relief from chronic inflammatory or neuropathic pain might be achieved by selective blockade or inhibition of PN3 expression. In light of the restricted distribution of PN3 to sensory neurons, such an approach might offer effective pain relief without a significant side-effect liability.

Tetrodotoxin-resistant Na+ currents and inflammatory hyperalgesia

Several mechanisms have been identified that may underlie inflammation-induced sensitization of high-threshold primary afferent neurons, including the modulation of voltage- and Ca2+-dependent ion channels and ion channels responsible for the production of generator potentials. One such mechanism that has recently received a lot of attention is the modulation of a tetrodotoxin (TTX)-resistant voltage-gated Na+ current. Evidence supporting a role for TTX-resistant Na+ currents in the sensitization of primary afferent neurons and inflammatory hyperalgesia is reviewed. Such evidence is derived from studies on the distribution of TTX-resistant Na+ currents among primary afferent neurons and other tissues of the body that suggest that these currents are expressed only in a subpopulation of primary afferent neurons that are likely to be involved in nociception. Data from studies on the biophysical properties of these currents suggest that they are ideally suited to mediate the repetitive discharge associated with prolonged membrane depolarizations. Data from studies on the effects of inflammatory mediators and antinociceptive agents on TTX-resistant Na+ currents suggest that modulation of these currents is an underlying mechanism of primary afferent neuron sensitization. In addition, the second-messenger pathways underlying inflammatory mediator-induced modulation of these currents appear to underlie inflammatory mediator-induced hyperalgesia. Finally, recent antisense studies have also yielded data supporting a role for TTX-resistant Na+ currents in inflammatory hyperalgesia. Although data from these studies are compelling, data presented at the Neurobiology of Pain colloquium raised a number of interesting questions regarding the role of TTX-resistant Na+ currents in inflammatory hyperalgesia; implications of three of these questions are discussed.

Effects of U-69,593, a kappa-opioid receptor agonist, on carrageenin-induced peripheral oedema and Fos expression in the rat spinal cord

In an attempt to study the anti-inflammatory and the antinociceptive effects of a kappa1-opioid receptor agonist (U-69,593: trans-3,4-dichloro-N-methyl-N-[7-(1-pyrrolidinyl)cycloexil]benzene acetamide methanesulfonate), we used a combination of the measurement of peripheral oedema (with a calliper) and Fos immunodetection in the carrageenin model of inflammation. The intraplantar injection of carrageenin-induced the development of a peripheral oedema, associated with an increase in Fos-like immunoreactivity at the level of the dorsal horn of the spinal cord. U-69,593 administered intravenously (i.v.) 10 min before carrageenin administration over the dose range 0.75, 1.5 and 3 mg/kg, reduced both paw and ankle oedema in a non dose-dependent manner. The maximal decrease was observed at the highest dose and did not exceed 21% and 20% for the paw and the ankle respectively. These effects were kappa-opioid receptor specific since the anti-inflammatory effect of 1.5 mg/kg i.v. of U-69,593 was antagonised by a specific kappa-opioid receptor antagonist nor-binaltorphimine. Pre-treatment with U-69,593 strongly decreased the number of Fos-like Immunoreactive neurones of the spinal cord in a dose-dependent, antagonist reversible manner; maximal effect was 65%. The disparate results between the anti-inflammatory effects and the depressive effects on Fos expression suggest that anti-inflammatory effects of kappa-opioid receptor agonist are of minor importance for the antinociceptive effects of this compound.