Involvement of cannabinoid receptors in peripheral and spinal morphine analgesia

Cannabinoid CB 2 receptors in primary sensory neurons are implicated in CB 2 agonist-mediated suppression of paclitaxel-induced neuropathic nociception and sexually-dimorphic sparing of morphine tolerance

Cannabinoid CB 2 agonists show therapeutic efficacy without the unwanted side effects commonly associated with direct activation of CB 1 receptors. The G protein-biased CB 2 receptor agonist LY2828360 attenuates the maintenance of chemotherapy-induced neuropathic nociception in male mice and blocks the development of morphine tolerance in this model. However, the specific cell types involved in this phenomenon have never been investigated and whether this therapeutic profile is observed in female mice remains poorly understood. We used conditional deletion of CB 2 receptors from specific cell populations to determine the population(s) mediating the anti-allodynic and morphine-sparing effects of CB 2 agonists. Anti-allodynic effects of structurally distinct CB 2 agonists (LY2828360 and AM1710) were present in paclitaxel-treated CB 2 f/f mice of either sex. The anti-allodynic effect of the CB 2 agonists were absent in conditional knockout (KO) mice lacking CB 2 receptors in peripheral sensory neurons (Advillin CRE/+ ; CB 2 f/f ) but preserved in mice lacking CB 2 receptors in CX3CR1 expressing microglia/macrophages (CX3CR1 CRE/+ ; CB 2 f/f ). The morphine-sparing effect of LY28282360 occurred in a sexually-dimorphic manner, being present in male mice but absent in female mice of any genotype. In mice with established paclitaxel-induced neuropathy, prior LY2828360 treatment (3 mg/kg per day i.p. x 12 days) blocked the subsequent development of morphine tolerance in male CB 2 f/f mice but was absent in male (or female) Advillin CRE/+ ; CB 2 f/f mice. LY2828360-induced sparing of morphine tolerance was preserved in male CX3CR1 CRE/+ ; CB 2 f/f mice, but this effect was not observed in female CX3CR1 CRE/+ ; CB 2 f/f mice. Similarly, co-administration of morphine with a low dose of LY2828360 (0.1 mg/kg per day i.p. x 6 days) reversed tolerance to the anti-allodynic efficacy of morphine in paclitaxel-treated male CB 2 f/f mice, but this effect was absent in female CB 2 f/f mice and Advillin CRE/+ ; CB 2 f/f mice of either sex. Additionally, LY2828360 (3 mg/kg per day i.p. x 8 days) delayed, but did not prevent, the development of paclitaxel-induced mechanical and cold allodynia in either CB 2 f/f or CX3CR1 CRE/+ ; CB 2 f/f mice of either sex. Our studies reveal that CB 2 receptors in primary sensory neurons are required for the anti-allodynic effects of CB 2 agonists in a mouse model of paclitaxel-induced neuropathic nociception. We also find that CB 2 agonists acting on primary sensory neurons produce a sexually-dimorphic sparing of morphine tolerance in males, but not female, paclitaxel-treated mice.

Dexmedetomidine alleviates osteoarthritis inflammation and pain through the CB2 pathway in rats

As a common joint disease, osteoarthritis (OA) is often associated with chronic pain. Synovial inflammation is correlated with OA progression and pain. Synovial inflammation can produce a series of destructive substances, such as inflammatory factors and pain mediators, which aggravate cartilage injury and further accelerate the progression of OA. Although many studies investigated the effects of synovial inflammation on the onset and progression of OA, there are limited reports regarding slowing the progression of OA and relieving pain by modulating synovial inflammation. Therefore, there is an urgent need to search for safe and effective drugs to alleviate synovial inflammation. Dexmedetomidine, a selective α₂ agonist, has been shown to have anti-inflammatory and analgesic effects. However, its role and mechanism in OA remain unclear. Here, the effects and mechanisms of dexmedetomidine in OA synovial inflammation were investigated both in vivo and in vitro. We observed that dexmedetomidine stunted LPS-induced migration and invasion of FLSs and the expression of inflammatory factors by upregulating cannabinoid receptor type 2 (CB₂) expression. Surprisingly, the application of AM630 (CB₂ antagonist) reversed this therapeutic effect. The results of the animal experiments showed that dexmedetomidine reduced synovial inflammation and increased the pain threshold in an OA rat model. These preliminary results imply that dexmedetomidine may be an effective compound for OA treatment.

ASRA Pain Medicine consensus guidelines on the management of the perioperative patient on cannabis and cannabinoids

BACKGROUND

The past two decades have seen an increase in cannabis use due to both regulatory changes and an interest in potential therapeutic effects of the substance, yet many aspects of the substance and their health implications remain controversial or unclear.

METHODS

In November 2020, the American Society of Regional Anesthesia and Pain Medicine charged the Cannabis Working Group to develop guidelines for the perioperative use of cannabis. The Perioperative Use of Cannabis and Cannabinoids Guidelines Committee was charged with drafting responses to the nine key questions using a modified Delphi method with the overall goal of producing a document focused on the safe management of surgical patients using cannabinoids. A consensus recommendation required ≥75% agreement.

RESULTS

Nine questions were selected, with 100% consensus achieved on third-round voting. Topics addressed included perioperative screening, postponement of elective surgery, concomitant use of opioid and cannabis perioperatively, implications for parturients, adjustment in anesthetic and analgesics intraoperatively, postoperative monitoring, cannabis use disorder, and postoperative concerns. Surgical patients using cannabinoids are at potential increased risk for negative perioperative outcomes.

CONCLUSIONS

Specific clinical recommendations for perioperative management of cannabis and cannabinoids were successfully created.

Thoracic epidural anaesthesia vs intrathecal morphine in dogs undergoing major thoracic and abdominal surgery: clinical study

BACKGROUND

There is scant clinical research on neuraxial analgesia in dogs undergoing major surgery. With this study we compared the perioperative analgesic effects of thoracic epidural anaesthesia (TEA) and intrathecal morphine (ITM) in dogs scheduled for thoracic or cranial abdominal surgery. The dogs received methadone and dexmedetomidine, were anaesthetized with propofol maintained with sevoflurane, and randomly assigned to receive either TEA (ropivacaine 0.5% at 0.2 mg/kg and morphine 0.1 mg/kg administered at T₁₂-T₁₃) or ITM (morphine 30 μg/kg administered at L₆-L₇). Intraoperative rescue analgesia (iRA) was fentanyl 1 μg/kg administered if heart rate or mean arterial pressure increased by 30% above the pre-stimulation level. Glasgow Pain Composite Scale score (GPCS) dictated the use of postoperative rescue analgesia (pRA) with methadone 0.2 mg/kg.

RESULTS

There was a statistically significant difference in iRA, median time to first fentanyl bolus, median fentanyl dose after surgical opening, and median GPCS score at 30 minutes (min), 1 ,2, 4, 6, and 8 hours (h) between the two groups (p<0.001; p<0.001; p<0.001; p<0.01; p<0.01; p<0.001; p<0.01; p=0.01; p=0.01, respectively). Fewer TEA than ITM group dogs required iRA during surgical opening and pRA: 5% (1/18) and 2/18 (11%), respectively, in the TEA and 83% (16/18) and 10/18 (55%), respectively, in the ITM group. Side effects were urinary retention in 3/18 (16%) TEA group dogs and 2/18 (11%) ITM group dogs and prolonged sedation in 2/18 (11%) in ITM group dogs. TEA and ITM were effective in managing perioperative pain in dogs undergoing thoracic or cranial abdominal surgery.

Bradykinin induces peripheral antinociception in PGE2-induced hyperalgesia in mice

BACKGROUND

Bradykinin (BK) is an endogenous peptide involved in vascular permeability and inflammation. It has opposite effects (inducing hyperalgesia or antinociception) when administered directly in the central nervous system. The aim of this study was to evaluate whether BK may also present this dual effect when injected peripherally in a PGE₂-induced nociceptive pain model, as well as to investigate the possible mechanisms of action involved in this event in mice.

METHODS

Male Swiss and C57BL/6 knockout mice for B₁ or B₂ bradykinin receptors were submitted to a mechanical paw pressure test and hyperalgesia was induced by intraplantar prostaglandin E₂ (2 µg/paw) injection.

RESULTS

Bradykinin (20, 40 and 80 ng/paw) produced dose-dependent peripheral antinociception against PGE₂-induced hyperalgesia. This effect was antagonized by bradyzide (8, 16 and 32 μg/paw), naloxone (12.5, 25 and 50 μg/paw), nor-binaltorphimine (50, 100 and 200 μg/paw) and AM251 (20, 40 and 80 μg/paw). Bestatin (400 µg/paw), MAFP (0.5 µg/paw) and VDM11 (2.5 µg/paw) potentiated the antinociception of a lower 20 ng BK dose. The knockout of B₁ or B₂ bradykinin receptors partially abolished the antinociceptive action of BK (80 ng/paw), bremazocine (1 μg/paw) and anandamide (40 ng/paw) when compared with wild-type animals, which show complete antinociception with the same dose of each drug.

CONCLUSION

The present study is the first to demonstrate BK-induced antinociception in peripheral tissues against PGE₂-induced nociception in mice and the involvement of κ-opioid and CB₁ cannabinoid receptors in this effect.

Role of primary sensory neurone cannabinoid type-1 receptors in pain and the analgesic effects of the peripherally acting agonist CB-13 in mice

BACKGROUND

Cannabinoid type-1 receptors (CB1Rs) are expressed in primary sensory neurones, but their role in pain modulation remains unclear.

METHODS

We produced Pirt-CB1R conditional knockout (cKO) mice to delete CB1Rs in primary sensory neurones selectively, and used behavioural, pharmacological, and electrophysiological approaches to examine the influence of peripheral CB1R signalling on nociceptive and inflammatory pain.

RESULTS

Conditional knockout of Pirt-CB1R did not alter mechanical or heat nociceptive thresholds, complete Freund adjuvant-induced inflammation, or heat hyperalgesia in vivo. The intrinsic membrane properties of small-diameter dorsal root ganglion neurones were also comparable between cKO and wild-type mice. Systemic administration of CB-13, a peripherally restricted CB1/CB2R dual agonist (5 mg kg-1), inhibited nociceptive pain and complete Freund adjuvant-induced inflammatory pain. These effects of CB-13 were diminished in Pirt-CB1R cKO mice. In small-diameter neurones from wild-type mice, CB-13 concentration-dependently inhibited high-voltage activated calcium current (HVA-ICa) and induced a rightward shift of the channel open probability curve. The effects of CB-13 were significantly attenuated by AM6545 (a CB1R antagonist) and Pirt-CB1R cKO.

CONCLUSION

CB1R signalling in primary sensory neurones did not inhibit nociceptive or inflammatory pain, or the intrinsic excitability of nociceptive neurones. However, peripheral CB1Rs are important for the analgesic effects of systemically administered CB-13. In addition, HVA-ICa inhibition appears to be a key ionic mechanism for CB-13-induced pain inhibition. Thus, peripherally restricted CB1R agonists could have utility for pain treatment.

Determination of the temperature causing a nociceptive response in the tail of albino BALB/c mice

INTRODUCTION

Designs for determining nociceptive response in rodents are of great use in neurology and experimental neuroscience. Immersing mice's tails in warm water is one of the most widely used procedures to evaluate this response; however, a wide range of temperatures are used in different studies. Knowing the temperature that produces a powerful nociceptive response in the tail of BALB/c mice is extremely useful.

METHODS

Eight 2-month-old male BALB/c mice were used. A 14-cm high beaker was filled with water up to 13cm. The animals' tails were immersed in the container with a starting temperature of 36°C. The water temperature was raised in 1°C increments until we identified the temperatures that produced nociceptive responses. That response was determined by counting the time taken before the mouse shook its tail to remove it from the water.

RESULTS

Six of the 8 mice began shaking their tails at the temperature of 51°C. All animals removed their tails from the water at the temperatures of 54°C, 55°C, and 56°C, taking a mean time of 8.54, 7.99, and 5.33seconds, respectively. ANOVA applied to the response times for each of the 3 temperatures indicated revealed a value of F=2.8 (P=.123).

CONCLUSIONS

The response time was statistically similar for the temperatures of 54°C, 55°C, and 56°C; however, the data were less dispersed for the latter temperature.

The Endocannabinoid System Contributes to Electroacupuncture Analgesia

The extensive involvement of the endocannabinoid system (ECS) in vital physiological and cognitive processes of the human body has inspired many investigations into the role of the ECS and drugs, and therapies that target this system and its receptors. Activation of cannabinoid receptors 1 and 2 (CB₁ and CB₂) by cannabinoid treatments, including synthetic cannabinoids, alleviates behavioral responses to inflammatory and neuropathic pain. An increasing body of scientific evidence details how electroacupuncture (EA) treatments achieve effective analgesia and reduce inflammation by modulating cannabinoid signaling, without the adverse effects resulting from synthetic cannabinoid administration. CB₁ receptors in the ventrolateral area of the periaqueductal gray are critically important for the mechanisms of the EA antinociceptive effect, while peripheral CB₂ receptors are related to the anti-inflammatory effects of EA. This review explores the evidence detailing the endocannabinoid mechanisms involved in EA antinociception.

Endogenous opioid and cannabinoid systems contribute to antinociception produced by administration of NSAIDs into the insular cortex of rats

Pain sensation is characterized as a complex experience, dependent on sensory processes as well as the activation of limbic brain areas involved in emotion, among them anterior insula. This cortical area is involved in the perception and response to painful stimuli. We investigated if this area contributes to antinociception produced by NSAIDs, and underlying mechanisms. We found that administration of NSAIDs into the anterior insular cortex in rats reduced mechanical and heat hyperalgesia produced by intraplantar injection of formalin, and this was attenuated by pre- or post-treatment with the opioid receptor antagonists, naloxone and CTOP, and the cannabinoid receptor (CB1) antagonist AM-251. These data support the concept that NSAID-evoked antinociception is mediated via descending endogenous opioid and cannabinoid systems inhibiting spinal paw withdrawal reflexes in rodents.

The cannabinoid CB2 receptor agonist LY2828360 synergizes with morphine to suppress neuropathic nociception and attenuates morphine reward and physical dependence

The opioid crisis has underscored the urgent need to identify safe and effective therapeutic strategies to overcome opioid-induced liabilities. We recently reported that LY2828360, a slowly signaling G protein-biased cannabinoid CB₂ receptor agonist, suppresses neuropathic nociception and attenuates the development of tolerance to the opioid analgesic morphine in paclitaxel-treated mice. Whether beneficial effects of LY2828360 are dependent upon the presence of a pathological pain state are unknown and its impact on unwanted opioid-induced side-effects have never been investigated. Here, we asked whether LY2828360 would produce synergistic anti-allodynic effects with morphine in a paclitaxel model of chemotherapy-induced neuropathic pain and characterized its impact on opioid-induced reward and other unwanted side-effects associated with chronic opioid administration. Isobolographic analysis revealed that combinations of LY2828360 and morphine produced synergistic anti-allodynic effects in suppressing paclitaxel-induced mechanical allodynia. In wildtype (WT) mice, LY2828360 blocked morphine-induced reward in a conditioned place preference assay without producing reward or aversion when administered alone. The LY2828360-induced attenuation of morphine-induced reward was absent in CB₂ knockout (CB₂KO) mice. In the absence of a neuropathic pain state, LY2828360 partially attenuated naloxone-precipitated opioid withdrawal in morphine-dependent WT mice, and this withdrawal was itself markedly exacerbated in CB₂KO mice. Moreover, LY2828360 did not reliably alter morphine-induced slowing of colonic transit or attenuate tolerance to morphine antinociceptive efficacy in the hot plate test of acute nociception. Our results suggest that cannabinoid CB₂ receptor activation enhances the therapeutic properties of opioids while attenuating unwanted side-effects such as reward and dependence that occur with sustained opioid treatment.

Dermorphin [D-Arg2, Lys4] (1-4) amide inhibits below-level heat hypersensitivity in mice after contusive thoracic spinal cord injury

Opioid use for chronic pain is limited by severe central adverse effects. We examined whether activating mu-opioid receptors (MORs) in the peripheral nervous system attenuates spinal cord injury (SCI) pain-like behavior in mice. We produced a contusive SCI at the T10 vertebral level and examined motor and sensory dysfunction for 6 weeks. At 6 weeks, we tested the effect of subcutaneous (s.c.) injection of dermorphin [D-Arg2, Lys4] (1-4) amide (DALDA), a peripherally acting MOR-preferring agonist, on mechanical and heat hypersensitivity. Basso mouse scale score was significantly decreased after SCI, and mice showed hypersensitivity to mechanical and heat stimulation at the hind paw beginning at 2 weeks, as indicated by increased paw withdrawal frequency to mechanical stimulation and decreased paw withdrawal latency to heat stimulation. In wild-type SCI mice, DALDA (1 mg/kg, s.c.) attenuated heat but not mechanical hypersensitivity. The effect was blocked by pretreatment with an intraperitoneal injection of methylnaltrexone (5 mg/kg), a peripherally restricted opioid receptor antagonist, and was also diminished in Pirt-MOR conditional knockout mice. DALDA did not adversely affect exploratory activity or induced preference to drug treatment in SCI mice. In vivo calcium imaging showed that DALDA (1, 10 mg/kg, s.c.) inhibited responses of small dorsal root ganglion neurons to noxious heat stimulation in Pirt-GCaMP6s mice after SCI. Western blot analysis showed upregulation of MOR in the lumbar spinal cord and sciatic nerves at 6 weeks after SCI. Our findings suggest that peripherally acting MOR agonist may inhibit heat hypersensitivity below the injury level with minimal adverse effects.

In vivo mapping of a GPCR interactome using knockin mice

With over 30% of current medications targeting this family of proteins, G-protein-coupled receptors (GPCRs) remain invaluable therapeutic targets. However, due to their unique physicochemical properties, their low abundance, and the lack of highly specific antibodies, GPCRs are still challenging to study in vivo. To overcome these limitations, we combined here transgenic mouse models and proteomic analyses in order to resolve the interactome of the δ-opioid receptor (DOPr) in its native in vivo environment. Given its analgesic properties and milder undesired effects than most clinically prescribed opioids, DOPr is a promising alternative therapeutic target for chronic pain management. However, the molecular and cellular mechanisms regulating its signaling and trafficking remain poorly characterized. We thus performed liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses on brain homogenates of our newly generated knockin mouse expressing a FLAG-tagged version of DOPr and revealed several endogenous DOPr interactors involved in protein folding, trafficking, and signal transduction. The interactions with a few identified partners such as VPS41, ARF6, Rabaptin-5, and Rab10 were validated. We report an approach to characterize in vivo interacting proteins of GPCRs, the largest family of membrane receptors with crucial implications in virtually all physiological systems.

Investigation of the Involvement of the Endocannabinoid System in TENS-Induced Antinociception

Transcutaneous electrical nerve stimulation (TENS) promotes antinociception by activating the descending pain modulation pathway and consequently releasing endogenous analgesic substances. In addition, recent studies have shown that the endocannabinoid system controls pain. Thus, the present study investigated the involvement of the endocannabinoid system in TENS-induced antinociception of cancer pain using a cancer pain model induced by intraplantar (i.pl.) injections of Ehrlich tumor cells in male Swiss mice. Low- and high-frequency TENS was applied for 20 minutes to the mice's paws, and to investigate the involvement of the endocannabinoid system were used the N-(peperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pitazole-3-carboixamide (AM251), a cannabinoid CB₁ receptor antagonist and (5Z,8Z,11Z,14Z)-5,8,11,14-eicosatetraenyl-methylester phosphonofluoridic acid (MAFP), an inhibitor of the endocannabinoid metabolizing enzyme fatty acid amide hydrolase, injected by via i.pl., intrathecal (i.t.), and intradorsolateral periaqueductal gray matter (i.dl.PAG). Furthermore, liquid chromatography-tandem mass spectrometry, western blot, and immunofluorescence assays were used to evaluate the endocannabinoid anandamide levels, cannabinoid CB₁ receptor protein levels, and cannabinoid CB₁ receptor immunoreactivity, respectively. Low- and high-frequency TENS reduced the mechanical allodynia induced by Ehrlich tumor cells and this effect was reversed by AM251 and potentiated by MAFP at the peripheral and central levels. In addition, TENS increased the endocannabinoid anandamide levels and the cannabinoid CB₁ receptor protein levels and immunoreactivity in the paw, spinal cord, and dorsolateral periaqueductal gray matter. These results suggest that low- and high-frequency TENS is effective in controlling cancer pain, and the endocannabinoid system is involved in this effect at both the peripheral and central levels. PERSPECTIVE: TENS is a nonpharmacological strategy that may be used to control cancer pain. Identification of a new mechanism involved in its analgesic effect could lead to the development of clinical studies as well as an increase in its application, lessening the need for pharmacological treatments.

Differential Expression of Neuroinflammatory mRNAs in the Rat Sciatic Nerve Following Chronic Constriction Injury and Pain-Relieving Nanoemulsion NSAID Delivery to Infiltrating Macrophages

The neuroinflammatory response to peripheral nerve injury is associated with chronic pain and significant changes in the molecular expression profiles of mRNAs in neurons, glia and infiltrating immune cells. Chronic constriction injury (CCI) of the rat sciatic nerve provides an opportunity to mimic neuropathic injury and quantitatively assess behavior and differential gene expression in individual animals. Previously, we have shown that a single intravenous injection of nanoemulsion containing celecoxib (0.24 mg/kg) reduces inflammation of the sciatic nerve and relieves pain-like behavior for up to 6 days. Here, we use this targeted therapy to explore the impact on mRNA expression changes in both pain and pain-relieved states. Sciatic nerve tissue recovered from CCI animals is used to evaluate the mRNA expression profiles utilizing quantitative PCR. We observe mRNA changes consistent with the reduced recruitment of macrophages evident by a reduction in chemokine and cytokine expression. Furthermore, genes associated with adhesion of macrophages, as well as changes in the neuronal and glial mRNAs are observed. Moreover, genes associated with neuropathic pain including Maob, Grin2b/NMDAR2b, TrpV3, IL-6, Cacna1b/Ca_v2.2, Itgam/Cd11b, Scn9a/Na_v1.7, and Tac1 were all found to respond to the celecoxib loaded nanoemulsion during pain relief as compared to those animals that received drug-free vehicle. These results demonstrate that by targeting macrophage production of PGE₂ at the site of injury, pain relief includes partial reversal of the gene expression profiles associated with chronic pain.

Preparation of bivalent agonists for targeting the mu opioid and cannabinoid receptors

In order to obtain novel pharmacological tools and to investigate a multitargeting analgesic strategy, the CB₁ and CB₂ cannabinoid receptor agonist JWH-018 was conjugated with the opiate analgesic oxycodone or with an enkephalin related tetrapeptide. The opioid and cannabinoid pharmacophores were coupled via spacers of different length and chemical structure. In vitro radioligand binding experiments confirmed that the resulting bivalent compounds bound both to the opioid and to the cannabinoid receptors with moderate to high affinity. The highest affinity bivalent derivatives 11 and 19 exhibited agonist properties in [³⁵S]GTPγS binding assays. These compounds activated MOR and CB (11 mainly CB₂, whereas 19 mainly CB₁) receptor-mediated signaling, as it was revealed by experiments using receptor specific antagonists. In rats both 11 and 19 exhibited antiallodynic effect similar to the parent drugs in 20 μg dose at spinal level. These results support the strategy of multitargeting G-protein coupled receptors to develop lead compounds with antinociceptive properties.

Evidence for the involvement of opioid and cannabinoid systems in the peripheral antinociception mediated by resveratrol

Despite all the development of modern medicine, around 100 compounds derived from natural products were undergoing clinical trials only at the end of 2013. Among these natural substances in clinical trials, we found the resveratrol (RES), a pharmacological multi-target drug. RES analgesic properties have been demonstrated, although the bases of these mechanisms have not been fully elucidated. The aim of this study was to evaluate the involvement of opioid and cannabinoid systems in RES-induced peripheral antinociception. Paw withdrawal method was used and hyperalgesia was induced by carrageenan (200 μg/paw). All drugs were given by intraplantar injection in male Swiss mice (n = 5). RES (100 μg/paw) administered in the right hind paw induced local antinociception that was antagonized by naloxone, non-selective opioid receptor antagonist, and clocinnamox, μOR selective antagonist. Naltrindole and nor-binaltorfimine, selective antagonists for δOR and kOR, respectively, did not reverse RES-induced peripheral antinociception. CB₁R antagonist AM251, but not CB₂R antagonist AM630, antagonized RES-induced peripheral antinociception. Peripheral antinociception of RES intermediate-dose (50 μg/paw) was increased by: (i) bestatin, inhibitor of endogenous opioid degradation involved-enzymes; (ii) MAFP, inhibitor of anandamide amidase; (iii) JZL184, inhibitor of 2-arachidonoylglycerol degradation involved-enzyme; (iv) VDM11, endocannabinoid reuptake inhibitor. Acute and peripheral administration of RES failed to affect the amount of μOR, CB₁R and CB₂R. Experimental data suggest that RES induces peripheral antinociception through μOR and CB₁R activation by endogenous opioid and endocannabinoid releasing.

Targeting pain at its source in sickle cell disease

Sickle cell disease (SCD) is a genetic disorder associated with hemolytic anemia, end-organ damage, reduced survival, and pain. One of the unique features of SCD is recurrent and unpredictable episodes of acute pain due to vasoocclusive crisis requiring hospitalization. Additionally, patients with SCD often develop chronic persistent pain. Currently, sickle cell pain is treated with opioids, an approach limited by adverse effects. Because pain can start at infancy and continue throughout life, preventing the genesis of pain may be relatively better than treating the pain once it has been evoked. Therefore, we provide insights into the cellular and molecular mechanisms of sickle cell pain that contribute to the activation of the somatosensory system in the peripheral and central nervous systems. These mechanisms include mast cell activation and neurogenic inflammation, peripheral nociceptor sensitization, maladaptation of spinal signals, central sensitization, and modulation of neural circuits in the brain. In this review, we describe potential preventive/therapeutic targets and their targeting with novel pharmacologic and/or integrative approaches to ameliorate sickle cell pain.

Determination of the temperature causing a nociceptive response in the tail of albino BALB/c mice

INTRODUCTION

Designs for determining nociceptive response in rodents are of great use in neurology and experimental neuroscience. Immersing mice's tails in warm water is one of the most widely used procedures to evaluate this response; however, a wide range of temperatures are used in different studies. Knowing the temperature that produces a powerful nociceptive response in the tail of BALB/c mice is extremely useful.

METHODS

Eight 2-month-old male BALB/c mice were used. A 14-cm high beaker was filled with water up to 13 cm. The animals' tails were immersed in the container with a starting temperature of 36°C. The water temperature was raised in 1°C increments until we identified the temperatures that produced nociceptive responses. That response was determined by counting the time taken before the mouse shook its tail to remove it from the water.

RESULTS

Six of the 8 mice began shaking their tails at the temperature of 51°C. All animals removed their tails from the water at the temperatures of 54°C, 55°C, and 56°C, taking a mean time of 8.54, 7.99, and 5.33seconds, respectively. ANOVA applied to the response times for each of the 3 temperatures indicated revealed a value of F=2.8 (P=.123).

CONCLUSIONS

The response time was statistically similar for the temperatures of 54°C, 55°C, and 56°C; however, the data were less dispersed for the latter temperature.

Effects of Cannabinoid Type 2 Receptor Agonist AM1241 on Morphine-Induced Antinociception, Acute and Chronic Tolerance, and Dependence in Mice

Morphine is a potent opioid analgesic used to alleviate moderate or severe pain, but the development of drug tolerance and dependence limits its use in pain management. Previous studies showed that cannabinoid type 2 (CB₂) receptor ligands may modulate opioid effects. However, there is no report of the effect of CB₂ receptor agonist on acute morphine tolerance and physical dependence. We therefore investigated the effect of a CB₂ receptor agonist (AM1241) on morphine-induced morphine tolerance and physical dependence in mice. Repeated coadministration of AM1241 (1 or 3mg/kg intraperitoneally) and morphine (10mg/kg subcutaneously) for 7days increased the mechanical paw withdrawal threshold in mice as measured by the von Frey filament test, and 3mg/kg AM1241 in combination with morphine increased the thermal paw withdrawal latency as measured by the hot-plate test. Combination with 3mg/kg AM1241 and morphine increased acute morphine antinociception. Coadministration of 1 or 3mg/kg AM1241 and morphine reduced acute morphine tolerance, and 3mg/kg AM1241 reduced chronic morphine tolerance. Coadministration of 1 or 3mg/kg AM1241 and morphine reduced naloxone-precipitated withdrawal jumping, but not diarrhea. Coadministration of AM1241 and morphine did not inhibit spontaneous locomotor activity. Pretreatment with 3mg/kg AM1241 decreased the chronic morphine-induced Iba1 expression in spinal cord. Coadministration of AM1241 (3 mg/kg) reduced the production of interleukin-1β, tumor necrosis factor-α, and interleukin-6 induced by long-term and acute morphine treatment. Our findings suggest that the coadministration of the CB₂ receptor agonist and morphine could increase morphine antinociception and reduce morphine tolerance and physical dependence in mice.

PERSPECTIVE

The combination of a CB₂ agonist and morphine may provide a new strategy for better treatment of acute and chronic pain and prevention of opioid tolerance and dependence. This finding may also provide a clue for the treatment of opioid tolerance and dependence in clinics.

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.

Slowly Signaling G Protein-Biased CB2 Cannabinoid Receptor Agonist LY2828360 Suppresses Neuropathic Pain with Sustained Efficacy and Attenuates Morphine Tolerance and Dependence

The CB₂ cannabinoid agonist LY2828360 lacked both toxicity and efficacy in a clinical trial for osteoarthritis. Whether LY2828360 suppresses neuropathic pain has not been reported, and its signaling profile is unknown. In vitro, LY2828360 was a slowly acting but efficacious G protein-biased CB₂ agonist, inhibiting cAMP accumulation and activating extracellular signal-regulated kinase 1/2 signaling while failing to recruit arrestin, activate inositol phosphate signaling, or internalize CB2 receptors. In wild-type (WT) mice, LY2828360 (3 mg/kg per day i.p. × 12 days) suppressed chemotherapy-induced neuropathic pain produced by paclitaxel without producing tolerance. Antiallodynic efficacy of LY2828360 was absent in CB₂ knockout (KO) mice. Morphine (10 mg/kg per day i.p. × 12 days) tolerance developed in CB₂KO mice but not in WT mice with a history of LY2828360 treatment (3 mg/kg per day i.p. × 12 days). LY2828360-induced antiallodynic efficacy was preserved in WT mice previously rendered tolerant to morphine (10 mg/kg per day i.p. × 12 days), but it was absent in morphine-tolerant CB₂KO mice. Coadministration of LY2828360 (0.1 mg/kg per day i.p. × 12 days) with morphine (10 mg/kg per day × 12 days) blocked morphine tolerance in WT but not in CB₂KO mice. WT mice that received LY2828360 coadministered with morphine exhibited a trend (P = 0.055) toward fewer naloxone-precipitated jumps compared with CB₂KO mice. In conclusion, LY2828360 is a slowly signaling, G protein-biased CB₂ agonist that attenuates chemotherapy-induced neuropathic pain without producing tolerance and may prolong effective opioid analgesia while reducing opioid dependence. LY2828360 may be useful as a first-line treatment in chemotherapy-induced neuropathic pain and may be highly efficacious in neuropathic pain states that are refractive to opioid analgesics.

Low-Dose Cannabinoid Type 2 Receptor Agonist Attenuates Tolerance to Repeated Morphine Administration via Regulating μ-Opioid Receptor Expression in Walker 256 Tumor-Bearing Rats

BACKGROUND

Morphine is widely used in patients with moderate and severe cancer pain, whereas the development of drug tolerance remains a major problem associated with opioid use. Previous studies have shown that cannabinoid type 2 (CB2) receptor agonists induce morphine analgesia, attenuate morphine tolerance in normal and neuropathic pain animals, induce transcription of the μ-opioid receptor (MOR) gene in Jurkat T cells, and increase morphine analgesia in cancer pain animals. However, no studies of the effects of CB2 receptor agonists on morphine tolerance in cancer pain have been performed. Therefore, we investigated the effect of repeated intrathecal (IT) injection of the low-dose CB2 receptor agonist AM1241 on the development of morphine tolerance in walker 256 tumor-bearing rats. We also tested the influence of the CB2 receptor agonist AM1241 on MOR protein and messenger ribonucleic acid (mRNA) expression in the rat spinal cord and dorsal root ganglia (DRG).

METHODS

Walker 256 cells were implanted into the plantar region of each rat's right hindpaw. Tumor-bearing rats received IT injection of the CB2 receptor agonist AM1241 or antagonist AM630 with or without morphine subcutaneously twice daily for 8 days. Rats receiving drug vehicle only served as the control group. Mechanical paw withdrawal threshold and thermal paw withdrawal latency were assessed by a von Frey test and hot plate test 30 minutes after drug administration every day. MOR protein and mRNA expression in the spinal cord and DRG were detected after the last day (day 8) of drug administration via Western blot and real-time reverse transcription polymerase chain reaction. The data were analyzed via analysis of variance followed by Student t test with Bonferroni correction for multiple comparisons.

RESULTS

Repeated morphine treatments reduced the mechanical withdrawal threshold and thermal latency. Coadministration of a nonanalgetic dose of the CB2 receptor agonist AM1241 with morphine significantly inhibited the development of morphine tolerance and increased the MOR protein expression in the spinal cord and DRG and mRNA expression in the spinal cord in tumor-bearing rats.

CONCLUSIONS

Our findings indicate that IT injection of a nonanalgetic dose of a CB2 receptor agonist increased the analgesia effect and alleviated tolerance to morphine in tumor-bearing rats, potentially by regulating MOR expression in the spinal cord and DRG. This receptor may be a new target for prevention of the development of opioid tolerance in cancer pain.

The Selective Monoacylglycerol Lipase Inhibitor MJN110 Produces Opioid-Sparing Effects in a Mouse Neuropathic Pain Model

Serious clinical liabilities associated with the prescription of opiates for pain control include constipation, respiratory depression, pruritus, tolerance, abuse, and addiction. A recognized strategy to circumvent these side effects is to combine opioids with other antinociceptive agents. The combination of opiates with the primary active constituent of cannabis (Δ(9)-tetrahydrocannabinol) produces enhanced antinociceptive actions, suggesting that cannabinoid receptor agonists can be opioid sparing. Here, we tested whether elevating the endogenous cannabinoid 2-arachidonoylglycerol through the inhibition of its primary hydrolytic enzyme monoacylglycerol lipase (MAGL), will produce opioid-sparing effects in the mouse chronic constriction injury (CCI) of the sciatic nerve model of neuropathic pain. The dose-response relationships of i.p. administration of morphine and the selective MAGL inhibitor 2,5-dioxopyrrolidin-1-yl 4-(bis(4-chlorophenyl)methyl)piperazine-1-carboxylate (MJN110) were tested alone and in combination at equieffective doses for reversal of CCI-induced mechanical allodynia and thermal hyperalgesia. The respective ED50 doses (95% confidence interval) of morphine and MJN110 were 2.4 (1.9-3.0) mg/kg and 0.43 (0.23-0.79) mg/kg. Isobolographic analysis of these drugs in combination revealed synergistic antiallodynic effects. Acute antinociceptive effects of the combination of morphine and MJN110 required μ-opioid, CB1, and CB2 receptors. This combination did not reduce gastric motility or produce subjective cannabimimetic effects in the drug discrimination assay. Importantly, combinations of MJN110 and morphine given repeatedly (i.e., twice a day for 6 days) continued to produce antiallodynic effects with no evidence of tolerance. Taken together, these findings suggest that MAGL inhibition produces opiate-sparing events with diminished tolerance, constipation, and cannabimimetic side effects.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 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 (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 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 (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).

Cannabinoid receptor-specific mechanisms to alleviate pain in sickle cell anemia via inhibition of mast cell activation and neurogenic inflammation

Sickle cell anemia is a manifestation of a single point mutation in hemoglobin, but inflammation and pain are the insignia of this disease which can start in infancy and continue throughout life. Earlier studies showed that mast cell activation contributes to neurogenic inflammation and pain in sickle mice. Morphine is the common analgesic treatment but also remains a major challenge due to its side effects and ability to activate mast cells. We, therefore, examined cannabinoid receptor-specific mechanisms to mitigate mast cell activation, neurogenic inflammation and hyperalgesia, using HbSS-BERK sickle and cannabinoid receptor-2-deleted sickle mice. We show that cannabinoids mitigate mast cell activation, inflammation and neurogenic inflammation in sickle mice via both cannabinoid receptors 1 and 2. Thus, cannabinoids influence systemic and neural mechanisms, ameliorating the disease pathobiology and hyperalgesia in sickle mice. This study provides 'proof of principle' for the potential of cannabinoid/cannabinoid receptor-based therapeutics to treat several manifestations of sickle cell anemia.

Receptome: Interactions between three pain-related receptors or the "Triumvirate" of cannabinoid, opioid and TRPV1 receptors

A growing amount of data demonstrates the interactions between cannabinoid, opioid and the transient receptor potential (TRP) vanilloid type 1 (TRPV1) receptors. These interactions can be bidirectional, inhibitory or excitatory, acute or chronic in their nature, and arise both at the molecular level (structurally and functionally) and in physiological processes, such as pain modulation or perception. The interactions of these three pain-related receptors may also reserve important and new therapeutic applications for the treatment of chronic pain or inflammation. In this review, we summarize the main findings on the interactions between the cannabinoid, opioid and the TRPV1 receptor regarding to pain modulation.

Inhibition of the regulator of G protein signalling RGS4 in the spinal cord decreases neuropathic hyperalgesia and restores cannabinoid CB1 receptor signalling

BACKGROUND AND PURPOSE

Regulators of G protein signalling (RGS) are major determinants of metabotropic receptor activity, reducing the lifespan of the GTP-bound state of G proteins. Because the reduced potency of analgesic agents in neuropathic pain may reflect alterations in RGS, we assessed the effects of CCG 63802, a specific RGS4 inhibitor, on pain hypersensitivity and signalling through cannabinoid receptors, in a model of neuropathic pain.

EXPERIMENTAL APPROACH

The partial sciatic nerve ligation (PSNL) model in male Sprague Dawley rats was used to measure paw withdrawal thresholds to mechanical (von Frey hairs) or thermal (Hargreaves method) stimuli, during and after intrathecal injection of CCG 63802. HEK293 cells expressing CB1 receptors and conditional expression of RGS4 were used to correlate cAMP production and ERK phosphorylation with receptor activation and RGS4 action.

KEY RESULTS

Treatment of PSNL rats with CCG 63802, twice daily for 7 days after nerve injury, attenuated thermal hyperalgesia during treatment. Spinal levels of anandamide were higher in PSNL animals, irrespective of the treatment. Although expression of CB1 receptors was unaffected, HU210-induced CB1 receptor signalling was inhibited in PSNL rats and restored after intrathecal CCG 63802. In transfected HEK cells expressing CB1 receptors and RGS4, inhibition of cAMP production, a downstream effect of CB1 receptor signalling, was blunted after RGS4 overexpression. RGS4 expression also attenuated the CB1 receptor-controlled activation of ERK1/2.

CONCLUSIONS AND IMPLICATIONS

Inhibition of spinal RGS4 restored endogenous analgesic signalling pathways and mitigated neuropathic pain. Signalling through CB1 receptors may be involved in this beneficial effect.

The spinal anti-inflammatory mechanism of motor cortex stimulation: cause of success and refractoriness in neuropathic pain?

Background

Motor cortex stimulation (MCS) is an effective treatment in neuropathic pain refractory to pharmacological management. However, analgesia is not satisfactorily obtained in one third of patients. Given the importance of understanding the mechanisms to overcome therapeutic limitations, we addressed the question: what mechanisms can explain both MCS effectiveness and refractoriness? Considering the crucial role of spinal neuroimmune activation in neuropathic pain pathophysiology, we hypothesized that modulation of spinal astrocyte and microglia activity is one of the mechanisms of action of MCS.

Methods

Rats with peripheral neuropathy (chronic nerve injury model) underwent MCS and were evaluated with a nociceptive test. Following the test, these animals were divided into two groups: MCS-responsive and MCS-refractory. We also evaluated a group of neuropathic rats not stimulated and a group of sham-operated rats. Some assays included rats with peripheral neuropathy that were treated with AM251 (a cannabinoid antagonist/inverse agonist) or saline before MCS. Finally, we performed immunohistochemical analyses of glial cells (microglia and astrocytes), cytokines (TNF-α and IL-1β), cannabinoid type 2 (CB2), μ-opioid (MOR), and purinergic P2X4 receptors in the dorsal horn of the spinal cord (DHSC).

Findings

MCS reversed mechanical hyperalgesia, inhibited astrocyte and microglial activity, decreased proinflammatory cytokine staining, enhanced CB2 staining, and downregulated P2X4 receptors in the DHSC ipsilateral to sciatic injury. Spinal MOR staining was also inhibited upon MCS. Pre-treatment with AM251 blocked the effects of MCS, including the inhibitory mechanism on cells. Finally, MCS-refractory animals showed similar CB2, but higher P2X4 and MOR staining intensity in the DHSC in comparison to MCS-responsive rats.

Conclusions

These results indicate that MCS induces analgesia through a spinal anti-neuroinflammatory effect and the activation of the cannabinoid and opioid systems via descending inhibitory pathways. As a possible explanation for MCS refractoriness, we propose that CB2 activation is compromised, leading to cannabinoid resistance and consequently to the perpetuation of neuroinflammation and opioid inefficacy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-014-0216-1) contains supplementary material, which is available to authorized users.

Genetic Manipulation of the Endocannabinoid System

The physiological and pathophysiological functions of the endocannabinoid system have been studied extensively using transgenic and targeted knockout mouse models. The first gene deletions of the cannabinoid CB(1) receptor were described in the late 1990s, soon followed by CB(2) and FAAH mutations in early 2000. These mouse models helped to elucidate the fundamental role of endocannabinoids as retrograde transmitters in the CNS and in the discovery of many unexpected endocannabinoid functions, for example, in the skin, bone and liver. We now have knockout mouse models for almost every receptor and enzyme of the endocannabinoid system. Conditional mutant mice were mostly developed for the CB(1) receptor, which is widely expressed on many different neurons, astrocytes and microglia, as well as on many cells outside the CNS. These mouse strains include "floxed" CB(1) alleles and mice with a conditional re-expression of CB(1). The availability of these mice made it possible to decipher the function of CB(1) in specific neuronal circuits and cell populations or to discriminate between central and peripheral effects. Many of the genetic mouse models were also used in combination with viral expression systems. The purpose of this review is to provide a comprehensive overview of the existing genetic models and to summarize some of the most important discoveries that were made with these animals.

Neuropathic cancer pain: What we are dealing with? How to manage it?

Cancer pain is a serious health problem, and imposes a great burden on the lives of patients and their families. Pain can be associated with delay in treatment, denial of treatment, or failure of treatment. If the pain is not treated properly it may impair the quality of life. Neuropathic cancer pain (NCP) is one of the most complex phenomena among cancer pain syndromes. NCP may result from direct damage to nerves due to acute diagnostic/therapeutic interventions. Chronic NCP is the result of treatment complications or malignancy itself. Although the reason for pain is different in NCP and noncancer neuropathic pain, the pathophysiologic mechanisms are similar. Data regarding neuropathic pain are primarily obtained from neuropathic pain studies. Evidence pertaining to NCP is limited. NCP due to chemotherapeutic toxicity is a major problem for physicians. In the past two decades, there have been efforts to standardize NCP treatment in order to provide better medical service. Opioids are the mainstay of cancer pain treatment; however, a new group of therapeutics called coanalgesic drugs has been introduced to pain treatment. These coanalgesics include gabapentinoids (gabapentin, pregabalin), antidepressants (tricyclic antidepressants, duloxetine, and venlafaxine), corticosteroids, bisphosphonates, N-methyl-D-aspartate antagonists, and cannabinoids. Pain can be encountered throughout every step of cancer treatment, and thus all practicing oncologists must be capable of assessing pain, know the possible underlying pathophysiology, and manage it appropriately. The purpose of this review is to discuss neuropathic pain and NCP in detail, the relevance of this topic, clinical features, possible pathology, and treatments of NCP.