Synergistic anti-allodynic effects of nociceptin/orphanin FQ and cannabinoid systems in neuropathic mice

Synergistic action between a synthetic cannabinoid compound and tramadol in neuropathic pain rats

In the present study the interaction of cannabinoid, PhAR-DBH-Me [(R, Z)-18-((1S,4S)-5-methyl-2,5-diazabicyclo[2.2.1]heptan-2-yl)-18-oxooctadec-9-en-7-ylphenyl-acetate] and tramadol in two neuropathy models, as well as their possible toxic effects, was analyzed. The anti-allodynic effect of PhAR-DBH-Me, tramadol, or their combination, were evaluated in neuropathic rats. Furthermore, the effective dose 35 (as the 35 % of the anti allodynic effect) was calculated from the maximum effect of each drug. Moreover, the isobolographic analysis was performed to determine the type of interaction between the drugs. A plasma acute toxicity study was carried out to assess the hepatic, renal, and heart functions after an individual or combined administration of the drugs, as well as histology using the hematoxylin-eosin or Masson-trichome method. PhAR-DBH-Me, tramadol, and their combination produced an antiallodynic effect on spinal nerve ligation (SNL) and cisplatin-induced neuropathic pain in rats. Moreover, PhAR-DBH-Me and tramadol combination showed a synergistic interaction in neuropathic pain rats induced by SNL but not for cisplatin-induced neuropathy. On the other hand, changes in renal and hepatic functions were not observed. Likewise, analysis of liver, kidney and heart histology showed no alterations compared with controls. Results show that the combination of PhAR-DBH-Me and tramadol attenuates the allodynia in SNL rats; the acute toxicology analysis suggests that this combination could be considered safe in administered doses.

A Crosstalk between the Cannabinoid Receptors and Nociceptin Receptors in Colitis-Clinical Implications

Inflammatory bowel diseases (IBD) refer to a group of gastrointestinal (GI) disorders with complex pathogenesis characterized by chronic intestinal inflammation with a variety of symptoms. Cannabinoid and nociceptin opioid receptors (NOPs) and their ligands are widely distributed in the GI tract. The nociceptin opioid receptor is a newly discovered member of the opioid receptor family with unique characteristics. Both cannabinoid and NOP systems exhibit antinociceptive and anti-inflammatory activity and contribute to maintaining proper motility, secretion and absorption in the GI tract. Furthermore, they influence high and low voltage calcium channels, which play a crucial role in the processing of pain, and share at least two kinases mediating their action. Among them there is NF-κB, a key factor in the regulation of inflammatory processes. Therefore, based on functional similarities between cannabinoid and nociceptin receptors and the anti-inflammatory effects exerted by their ligands, there is a high likelihood that there is an interaction between cannabinoid receptors 1 and 2 and the nociceptin receptor in colitis. In this review, we discuss potential overlaps between these two systems on a molecular and functional level in intestinal inflammation to create the basis for novel treatments of IBD.

Non-opioid Analgesics and the Endocannabinoid System

Non-steroidal anti-inflammatory drugs produce antinociceptive effects mainly through peripheral cyclooxygenase inhibition. In opposition to the classical non-steroidal anti-inflammatory drugs, paracetamol and dipyrone exert weak anti-inflammatory activity, their antinociceptive effects appearing to be mostly due to mechanisms other than peripheral cyclooxygenase inhibition. In this review, we classify classical non-steroidal anti-inflammatory drugs, paracetamol and dipyrone as “non-opioid analgesics” and discuss the mechanisms mediating participation of the endocannabinoid system in their antinociceptive effects. Non-opioid analgesics and their metabolites may activate cannabinoid receptors, as well as elevate endocannabinoid levels through different mechanisms: reduction of endocannabinoid degradation via fatty acid amide hydrolase and/or cyclooxygenase-2 inhibition, mobilization of arachidonic acid for the biosynthesis of endocannabinoids due to cyclooxygenase inhibition, inhibition of endocannabinoid cellular uptake directly or through the inhibition of nitric oxide synthase production, and induction of endocannabinoid release.

The Endogenous Cannabinoid System: A Budding Source of Targets for Treating Inflammatory and Neuropathic Pain

A great need exists for the development of new medications to treat pain resulting from various disease states and types of injury. Given that the endogenous cannabinoid (that is, endocannabinoid) system modulates neuronal and immune cell function, both of which play key roles in pain, therapeutics targeting this system hold promise as novel analgesics. Potential therapeutic targets include the cannabinoid receptors, type 1 and 2, as well as biosynthetic and catabolic enzymes of the endocannabinoids N-arachidonoylethanolamine and 2-arachidonoylglycerol. Notably, cannabinoid receptor agonists as well as inhibitors of endocannabinoid-regulating enzymes fatty acid amide hydrolase and monoacylglycerol lipase produce reliable antinociceptive effects, and offer opioid-sparing antinociceptive effects in myriad preclinical inflammatory and neuropathic pain models. Emerging clinical studies show that 'medicinal' cannabis or cannabinoid-based medications relieve pain in human diseases such as cancer, multiple sclerosis, and fibromyalgia. However, clinical data have yet to demonstrate the analgesic efficacy of inhibitors of endocannabinoid-regulating enzymes. Likewise, the question of whether pharmacotherapies aimed at the endocannabinoid system promote opioid-sparing effects in the treatment of pain reflects an important area of research. Here we examine the preclinical and clinical evidence of various endocannabinoid system targets as potential therapeutic strategies for inflammatory and neuropathic pain conditions.

The monoacylglycerol lipase inhibitor KML29 with gabapentin synergistically produces analgesia in mice

BACKGROUND AND PURPOSE

Gabapentin is commonly prescribed for nerve pain but may also cause dizziness, sedation and gait disturbances. Similarly, inhibition of the endogenous cannabinoid enzyme monoacylglycerol lipase (MAGL) has antinociceptive and anti-inflammatory properties but also induces sedation in mice at high doses. To limit these side effects, the present study investigated the analgesic effects of coadministering a MAGL inhibitor with gabapentin.

EXPERIMENTAL APPROACH

Mice subjected to the chronic constriction injury model of neuropathic pain were administered the MAGL inhibitor KML29 (1-40 mg·kg^-1 , i.p.), gabapentin (1-50 mg·kg^-1 , i.p.) or both compounds. Mice were tested for mechanical and cold allodynia. The function and expression of cannabinoid CB₁ receptors in whole brain homogenates and lipid profile of spinal cords were assessed after repeated drug administration.

KEY RESULTS

The combination of low-dose KML29:gabapentin additively attenuated mechanical allodynia and synergistically reduced cold allodynia. The CB₁ antagonist, rimonabant, partially reversed the anti-allodynic effects of KML29:gabapentin in mechanical allodynia but not cold allodynia. The anti-allodynic effects of KML29:gabapentin did not undergo tolerance in mechanical allodynia after repeated administration but produced mild tolerance in cold allodynia. High dose KML29 alone reduced CB₁ receptor expression and function, but KML29:gabapentin reduced the density of CB₁ receptors but did not alter their function. KML29:gabapentin influenced additional signalling pathways (including fatty acids) other than the pathways activated by a higher dose of either drug alone.

CONCLUSION AND IMPLICATIONS

These data support the strategy of combining MAGL inhibition with a commonly prescribed analgesic as a therapeutic approach for attenuating neuropathic pain.

Bifunctional opioid/nociceptin hybrid KGNOP1 effectively attenuates pain-related behaviour in a rat model of neuropathy

A bifunctional peptide containing an opioid and nociceptin receptor-binding pharmacophore, H-Dmt-D-Arg-Aba-β-Ala-Arg-Tyr-Tyr-Arg-Ile-Lys-NH2 (KGNOP1), was tested for its analgesic properties when administered intrathecally in naïve and chronic constriction injury (CCI)-exposed rats with neuropathy-like symptoms. KGNOP1 significantly increased the acute pain threshold, as measured by the tail-flick test, and also increased the threshold of a painful reaction to mechanical and thermal stimuli in CCI-exposed rats. Both of the effects could be blocked by pre-administration of [Nphe1]-Nociceptin (1-13)-NH2 (NPhe) or naloxone, antagonists for nociceptin and opioid receptors, respectively. This led us to conclude that KGNOP1 acts as a dual opioid and nociceptin receptor agonist in vivo. The analgesic effect of KGNOP1 proved to be more powerful than clinical drugs such as morphine and buprenorphine. Repeated daily intrathecal injections of KGNOP1 led to the development of analgesic tolerance, with the antiallodynic action being completely abolished on day 6. Nevertheless, the development of tolerance to the antihyperalgesic effect was delayed in comparison to morphine, which lost its efficacy as measured by the cold plate test after 3days of daily intrathecal administration, whereas KGNOP1 was efficient up to day 6. A single intrathecal injection of morphine to KGNOP1-tolerant rats did not raise the pain threshold in any of the behavioural tests; in contrast, a single intrathecal dose of KGNOP1 significantly suppressed allodynia and hyperalgesia in morphine-tolerant rats.

Changes in nociceptin/orphanin FQ levels in rat brain regions after acute and chronic cannabinoid treatment in conjunction with the development of antinociceptive tolerance

It has been indicated that acute and chronic morphine administrations enhance nociceptin/orphanin FQ (N/OFQ) levels in the brain, which might play role in the development of tolerance to the antinociceptive effect of morphine. Accordingly, N/OFQ receptor (NOP) antagonists have been shown to prevent the development of antinociceptive tolerance to morphine. Our aim is to observe whether cannabinoids, similarly to opioids, enhance N/OFQ levels in pain-related brain regions and whether antagonism of NOP receptors attenuates the development of tolerance to the antinociceptive effect of cannabinoids. Hot plate and Tail flick tests are used to assess the antinociceptive response in Sprague-Dawley rats. N/OFQ levels are measured in cortex, amygdala, hypothalamus, periaqueductal gray, nucleus raphe magnus and locus coeruleus of rat brains using Western blotting and immunohistochemistry. Within 9 days, animals became completely tolerant to the antinociceptive effect of the cannabinoid agonist WIN 55,212-2 (2, 4, 6 mg/kg, i.p.). Chronic administration of JTC-801, a NOP receptor antagonist, at a dose that exerted no effect on its own (1 mg/kg, i.p.), attenuated development of tolerance to the antinociceptive effect of WIN 55,212-2 (4 mg/kg, i.p.). Western blotting and immunohistochemistry results showed that N/OFQ levels significantly increased in amygdala, periaqueductal gray, nucleus raphe magnus and locus coeruleus of rat brains when WIN 55,212-2 was combined with JTC-801. We hypothesize that, similar to opioids, chronic cannabinoid + NOP antagonist administration may enhance N/OFQ levels and NOP receptor antagonism prevents development of tolerance to cannabinoid antinociception.

Peripheral interactions between cannabinoid and opioid receptor agonists in a model of inflammatory mechanical hyperalgesia

Activation of opioid and cannabinoid receptors expressed in nociceptors induces effective antihyperalgesia. In this study, we examined whether combinations of opioid and cannabinoid receptor agonists directed at the injured site would enhance therapeutic effectiveness. Behavioral pharmacology experiments were performed to compare the effects of DAMGO, a selective agonist for μ-opioid receptor (MOR), ACPA, a specific agonist for CB1, and combinations of DAMGO and ACPA in attenuating complete Freund's adjuvant (CFA)-induced mechanical hyperalgesia in the rat hindpaw. DAMGO (1μg-1mg) or ACPA (1μg-2mg) was administered into the inflamed paw when mechanical hyperalgesia was fully developed. When administered individually, DAMGO and ACPA dose-dependently reversed the mechanical hyperalgesia. DAMGO displayed a lower ED50 value (57.4±2.49μg) than ACPA (111.6±2.18μg), but ACPA produced longer lasting antihyperalgesic effects. Combinations of DAMGO and ACPA also dose-dependently attenuated mechanical hyperalgesia, but the antihyperalgesic effects were partial and transient even at high doses. Using isobolographic analysis, we determined that combined treatment with DAMGO and ACPA produced antagonistic effects with the observed ED50 of 128.4±2.28μg. Our findings showed that MOR and CB1 agonists directed at the inflamed site effectively attenuate mechanical hyperalgesia when administered individually, but exert opposing effects when administered together. The antagonistic interactions between the two classes of drugs at the inflamed site suggest distinct mechanisms unique to peripheral nociceptors or inflamed tissue, and therefore require further studies to investigate whether the therapeutic utility of the combined drug treatments in chronic pain conditions can be optimized.

Contribution of nociceptin/orphanin FQ receptors to the anti-nociceptive and hypothermic effects of dipyrone

BACKGROUND

Dipyrone is one of the most commonly used non-opioid analgesic and antipyretic drug. Its anti-nociceptive and hypothermic effects have long been suspected to be centrally mediated. The involvement of the most recently discovered opioid peptide, nociceptin/orphanin FQ (N/OFQ), and its receptor (NOP) in pain transmission is controversial. It appears to be pro-nociceptive when administered supra-spinally, but exerts anti-nociceptive effects when injected spinally or systemically.

OBJECTIVE

Investigation of the role of the N/OFQ system in paracetamol-induced anti-nociception and hypothermia led us to determine its role in the anti-nociceptive and hypothermic effects of dipyrone. Material and Methods Hot-plate and tail-flick tests were used to assess nociception, and a rectal thermometer was used to measure rectal temperature in mice.

RESULTS

Mice injected with dipyrone (150, 300, 600 mg/kg, i.p.) displayed dose-related anti-nociception and hypothermia. The NOP receptor antagonist JTC-801 (3 mg/kg, i.p.), at a dose that exerted no effect when used alone, alleviated dipyrone-induced anti-nociception but did not reverse dipyrone-induced hypothermia.

CONCLUSION

We conclude that NOP receptors participate in the anti-nociceptive, but not in the hypothermic, effects of dipyrone.

Nociceptin/orphanin FQ peptide receptor antagonist JTC-801 reverses pain and anxiety symptoms in a rat model of post-traumatic stress disorder

BACKGROUND AND PURPOSE

Single-prolonged stress (SPS), a rat model of post-traumatic stress disorder (PTSD), also induces long-lasting hyperalgesia associated with hypocortisolism and elevated nociceptin/orphanin FQ (N/OFQ) levels in serum and CSF. Here, we determined the effect of JTC-801 (N-(4-amino-2-methylquinolin-6-yl)-2-(4-ethylphenoxymethyl) benzamide monohydrochloride), a nociceptin/orphanin FQ peptide (NOP) receptor antagonist, on symptoms of pain and anxiety in rats after SPS exposure, and examined N/OFQ-NOP receptor system changes.

EXPERIMENTAL APPROACH

Male Sprague Dawley rats received JTC-801 (6 mg kg(-1) i.p., once daily) during days 7-21 of SPS. The ability of JTC-801 to inhibit N/OFQ-stimulated [(35) S]-GTPγS binding was confirmed in rat brain membranes. Anxiety-like behaviour and pain sensitivity were monitored by changes in elevated plus maze performance and withdrawal responses to thermal and mechanical stimuli. Serum corticosterone and N/OFQ content in CSF, serum and brain tissues were determined by radioimmunoassay; NOP receptor protein and gene expression in amygdala, hippocampus and periaqueductal grey (PAG) were examined by immunoblotting and real-time PCR respectively.

KEY RESULTS

JTC-801 treatment reversed SPS-induced mechanical allodynia, thermal hyperalgesia, anxiety-like behaviour and hypocortisolism. Elevated N/OFQ levels in serum, CSF, PAG and hippocampus at day 21 of SPS were blocked by JTC-801; daily JTC-801 treatment also reversed NOP receptor protein and mRNA up-regulation in amygdala and PAG.

CONCLUSION AND IMPLICATIONS

JTC-801 reversed SPS-induced anxiety- and pain-like behaviours, and NOP receptor system up-regulation. These findings suggest that N/OFQ plays an important role in hyperalgesia and allodynia maintenance after SPS. NOP receptor antagonists may provide effective treatment for co-morbid PTSD and pain.

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.

The endocannabinoid system as a potential therapeutic target for pain modulation

Although cannabis has been used for pain management for millennia, very few approved cannabinoids are indicated for the treatment of pain and other medical symptoms. Cannabinoid therapy re-gained attention only after the discovery of endocannabinoids and fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL), the enzymes playing a role in endocannabinoid metabolism. Nowadays, research has focused on the inhibition of these degradative enzymes and the elevation of endocannabinoid tonus locally; special emphasis is given on multi-target analgesia compounds, where one of the targets is the endocannabinoid degrading enzyme. In this review, I provide an overview of the current understanding about the processes accounting for the biosynthesis, transport and metabolism of endocannabinoids, and pharmacological approaches and potential therapeutic applications in this area, regarding the use of drugs elevating endocannabinoid levels in pain conditions.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Involvement of descending serotonergic and noradrenergic pathways in CB1 receptor-mediated antinociception

Cannabinoids produce antinociceptive and antihyperalgesic effects mainly through activation of the inhibitory CB1 receptors. The demonstration that antinociceptive effects of systemic cannabinoids are significantly diminished following surgical dorsolateral funiculus lesion provides evidence that supraspinal sites and descending pain modulatory pathways play crucial roles in systemic cannabinoid analgesia. In this review, we will firstly provide a background, brief overview of descending modulatory pathways followed by descending pathways implicated in cannabinoid analgesia. We will then describe the recent evidence of the involvement of descending serotonergic and noradrenergic pathways in CB1 receptor-mediated antinociception. This review will provide evidences that systemically administered cannabinoids reinforce the descending serotonergic and noradrenergic pathways to produce acute antinociceptive effects via spinal 5-HT7, 5-HT2A and alpha-2 adrenoceptors activation.