Spinal Cannabinoid Research Articles

Spinal cannabinoid receptor 2 activation alleviates neuropathic pain by regulating microglia and suppressing P2X7 receptor.

Neuropathic pain (NP) is the chronic pain in patients resulting from injuries or diseases in the somatosensory nervous system. However, effective treatment remains limited to opioids. Currently, there is an urgent need to develop new specific pharmaceuticals with low abuse potentiality. Cannabinoid receptor 2 (CB2R) is one of the significant receptors in the endocannabinoid system. It is widely expressed in the central nervous system, especially enriched in glial cells, and plays an important role in the occurrence and development of inflammation in the nervous system. CB2R activation has a neuroprotective effect on nerve injury. In this study, we report increased and more reactive microglia (with larger cell body, shorter processes, and fewer endpoints) observed in the spinal dorsal horn of spared nerve injury (SNI) rats. Continuous intrathecal administration of CB2R agonist PM226 attenuated mechanical and cold hyperalgesia in rats and prevented the transition of microglia to the proinflammatory stage. Thus, microglia transitioned into the neuroprotective stage. Meanwhile, the proinflammatory factors TNF-α and iNOS decreased, and the levels of anti-inflammatory factors Arg-1 and IL-10 increased. The content of P2X7 receptors in the spinal dorsal horn of rats increases with time after SNI. After continuous intrathecal administration of PM226, the content of P2X7 protein decreases significantly. The administration of P2X7 inhibitor A-438079 alleviated the mechanical hyperalgesia of rats, reduced the number of microglia, and decreased the content of P2X7. These results indicate that P2X7 is involved in the neuroprotective effect caused by CB2R activation. In conclusion, this study provides new insights into the neuroprotective mechanism of CB2R activation.

Spinal cannabinoid receptor 2 activation reduces hypersensitivity associated with bone cancer pain and improves the integrity of the blood–spinal cord barrier

BackgroundDisruption of the blood–spinal cord barrier (BSCB) can facilitate inflammation that results in pain hypersensitivity. Proinflammatory cytokines produced by activated microglia and astrocytes damage the BSCB. This study aims to...

Involvement of Spinal Cannabinoid CB2 Receptors in Exercise-Induced Antinociception

Muscle pain affects approximately 11–24% of the global population. Several studies have shown that exercise is a non-pharmacological therapy to pain control. It has been suggested that the endocannabinoid system is involved in this antinociceptive effect. However, the participation of this pathway is unclear. The present study aimed to investigate whether spinal cannabinoid CB2 receptors participate in the exercise-induced antinociception. The inflammatory muscle pain model was induced by the intramuscular injection of carrageenan. Tactile allodynia and thermal hyperalgesia were determined with the von Frey filaments and hot-plate tests. C57BL/6J female mice underwent a swimming training protocol that lasted 3 weeks. This protocol of exercise reduced carrageenan-induced tactile allodynia and thermal hyperalgesia and this effect was prevented by the cannabinoid CB2 receptors inverse agonist AM630 and potentiated by MAFP (inhibitor of the enzyme that metabolizes endocannabinoids) and minocycline (microglia inhibitor). In addition, exercise increased the endocannabinoid anandamide levels and cannabinoid CB2 receptors expression whereas it reduced Iba1 (microglial marker) protein expression as well as pro-inflammatory cytokines (TNF-α and IL-1β) in the spinal cord of mice with inflammatory muscle pain. Swimming training also reduced muscle temperature of carrageen-treated animals. The present study suggests that activation of spinal cannabinoid CB2 receptors and reduction of activated microglia are involved in exercise-induced antinociception.

SYSTEMIC CANNABIDIOL DOES NOT REDUCE COMPOUND 48/80-INDUCED ITCHING BEHAVIOR IN MICE

Aims: Cannabinoids are chemical compounds including natural cannabinoids found in the Cannabis plant, their synthetic counterparts, and endocannabinoids. Cannabidiol, a phytocannabinoid derived from the Cannabis plant, exerts anticonvulsant, anxiolytic, anti-inflammatory, neuroprotective, analgesic effects. Although there are many similarities between the pathophysiological mechanisms of pain and itch, researches that investigate the effect of cannabinoids on itching are insufficient. Here, we aimed to examine the antipruritic effect of cannabidiol and the contribution of spinal cannabinoid receptors. Methods: Male Balb/c mice, weighing 20-30 g, were used. Itching behavior was produced by intradermal injection of compound 48/80 (100 μg/50 μl); cannabidiol (1, 3, 10 mg/kg, ip) was administered 30 minutes before compound 48/80 injections. Then, scratching of the injected site by the hind paws was videotaped for 30 minutes. Locomotor performances were assessed using a rotarod apparatus. Results: Cannabidiol had no effect on compound 48/80-induced itching behavior at any dose given; moreover, cannabidiol did not produce any impairment on motor function. AM-251, a cannabinoid receptor type 1 antagonist, and AM-630, a cannabinoid receptor type 2 antagonist were administered intrathecally to observe the contribution of spinal cannabinoid receptors to the antipruritic action of cannabidiol. We observed that cannabidol did not possess any effect on itching behaviour. Conclusion: Our results indicate that systemic administration of cannabidiol does not attenuate compound 48/80 induced itching behavior in mice.

Involvement of spinal cannabinoid receptors in the antipruritic effects of WIN 55,212-2, a cannabinoid receptor agonist.

Cannabinoids have been used for their analgesic and euphoric effects for millennia, but recently the antipruritic effects of cannabis have been discovered. Considering the similarities between pain and itch sensations, we hypothesized that cannabinoid receptors may play a role in the antipruritic effects of cannabinoids. To analyse the role of the spinal cannabinoid receptors, CB1 and CB2, in the antipruritic effects of the cannabinoid agonist WIN 55,212-2. Male Balb/c mice weighing 20-30 g were used. Scratching behaviour in the mice was produced by injection of serotonin 5 μg/50 μL intradermally into the nape of the neck. Scratching of the site of injection by the hind paws was video-recorded for 30 min. After testing different doses of WIN 55,212-2 [1, 3 and 10 mg/kg intraperitoneally (IP)], the effects of the CB1 receptor antagonist, AM-251 [1 μg/mouse administered intrathecally (IT)] and the CB2 receptor antagonist AM-630 (4 μg/mouse IT) on the antipruritic effects of WIN 55,212-2 were studied using a rotarod apparatus. WIN 55,212-2 (1, 3 or 10 mg/kg IP) dose-dependently decreased serotonin-induced scratches. The receptor antagonist CB1 partially reversed the effects of WIN 55,212-2 (P < 0.05); whereas CB2 had no statistically significant effect. WIN 55,212-2 impaired motor function only at the highest dose given (10 mg/kg, P < 0.05). Our findings support prior researches indicating that cannabinoids exert antipruritic effects. Moreover, our results show that the antipruritic effects of cannabinoids are partially mediated by spinal CB1 receptors.

CB1 Receptors Mediated Inhibition of ATP-Induced [Ca2+]i Increase in Cultured Rat Spinal Dorsal Horn Neurons.

Spinal cannabinoid receptor 1 (CB1R) and purinergic P2X receptors (P2XR) play a critical role in the process of pathological pain. Both CB1R and P2XR are expressed in spinal dorsal horn (DH) neurons. It is not clear whether CB1 receptor activation modulates the function of P2X receptor channels within dorsal horn. For this reason, we observed the effect of CP55940 (cannabinoid receptor agonist) on ATP-induced Ca2+ mobilization in cultured rat DH neurons. The changes of intracellular calcium concentration ([Ca2+]i) were detected with confocal laser scanning microscopy using fluo-4/AM as a calcium fluorescent indicator. 100μM ATP caused [Ca2+]i increase in cultured DH neurons. ATP-evoked [Ca2+]i increase in DH neurons was blocked by chelating extracellular Ca2+ and P2 purinoceptor antagonist PPADS. At the same time, ATP-γ-S (a non-hydrolyzable ATP analogue) mimicked the ATP action, while P2Y receptor agonist ADP failed to evoke [Ca2+]i increase in cultured DH neurons. These data suggest that ATP-induced [Ca2+]i elevation in cultured DH neurons is mediated by P2X receptor. Subsequently, we noticed that, in cultured rat DH neurons, ATP-induced Ca2+ mobilization was inhibited after pretreated with CP55940 with a concentration-dependent manner, which implies that the opening of P2X receptor channels are down-regulated by activation of cannabinoid receptor. The inhibitory effect of CP55940 on ATP-induced Ca2+ response was mimicked by ACEA (CB1R agonist), but was not influenced by AM1241 (CB2R agonist). Moreover, the inhibitory effect of CP55940 on ATP-induced Ca2+ mobilization was blocked by AM251 (CB1 receptor antagonist), but was not influenced by AM630 (CB2 receptor antagonist). In addition, we also observed that forskolin (an activator of adenylate cyclase) and 8-Br-cAMP (a cell-permeable cAMP analog) reversed the inhibitory effect of CP55940, respectively. In a summary, our observations raise a possibility that CB1R rather than CB2R can downregulate the opening of P2X receptor channels in DH neurons. The reduction of cAMP/PKA signaling is a key element in the inhibitory effect of CB1R on P2X-channel-induced Ca2+ mobilization.

Osteoarthritis pain mechanisms: basic studies in animal models

Osteoarthritis (OA) is a complex and painful disease of the whole joint. At present there are no satisfying agents for treating OA. To promote OA research and improved treatment, this review summarizes current preclinical evidence on the development of OA. Preclinical OA research was searched and key findings are summarized and commented. Mechanisms of OA-associated pain have been studied in rodent knee OA models produced by intra-knee injection of the chondrocyte glycolytic inhibitor mono-iodoacetate (MIA), surgery, or spontaneous development in some species. These models are clinically relevant in terms of histological damage and functional changes, and are used to study mechanisms underlying mechanical, thermal, ambulatory, body weight supporting-evoked, and ongoing OA pain. Recent peripheral, spinal, and supraspinal biochemical and electrophysiological studies in these models suggest that peripheral pro-inflammatory mediators and neuropeptides sensitize knee nociceptors. Spinal cytokines and neuropeptides promote OA pain, and peripheral and spinal cannabinoids inhibit OA pain respectively through cannabinoid-1 (CB1) and CB1/CB2 receptors. TRPV1 and metalloproteinases contribute and supraspinal descending facilitation of 5-hydroxytryptamine (5-HT)/5-HT 3 receptors may also contribute to OA pain. Conditioned place preference tests demonstrate that OA pain induces aversive behaviors, suggesting the involvement of brain. During OA, brain functional connectivity is enhanced, but at present it is unclear how this change is related to OA pain. Animal studies demonstrate that peripheral and central sensitization contributes to OA pain, involving inflammatory cytokines, neuropeptides, and a variety of chemical mediators. Interestingly, brainstem descending facilitation of 5-HT/5-HT3 receptors plays a role OA pain.

Activation of spinal cannabinoid CB2 receptors inhibits neuropathic pain in streptozotocin-induced diabetic mice

The role of spinal cannabinoid systems in neuropathic pain of streptozotocin (STZ)-induced diabetic mice was studied. In normal mice, injection of the cannabinoid receptor agonist WIN-55,212-2 (1 and 3μg, i.t.) dose-dependently prolonged the tail-flick latency, whereas there were no changes with the injection of either cannabinoid CB1 (AM 251, 1μg, i.t.) or CB2 (AM 630, 4μg, i.t.) receptor antagonists. AM 251 (1μg, i.t.), but not AM 630 (4μg, i.t.), significantly inhibited the prolongation of the tail-flick latency induced by WIN-55,212-2 (3μg, i.t.). In STZ-induced diabetic mice, the tail-flick latency was significantly shorter than that in normal mice. A low dose of WIN-55,212-2 (1μg, i.t.) significantly recovered the tail-flick latency in STZ-induced diabetic mice. The effect of WIN-55,212-2 (1μg, i.t.) in STZ-induced diabetic mice was significantly inhibited by AM 630 (4μg, i.t.), but not AM 251 (1μg). The selective cannabinoid CB2 receptor agonist L-759,656 (19 and 38μg, i.t.) also dose-dependently recovered the tail-flick latency in STZ-induced diabetic mice, and this recovery was inhibited by AM 630 (4μg, i.t.). The protein levels of cannabinoid CB1 receptors, CB2 receptors and diacylglycerol lipase α (DGL-α), the enzyme that synthesizes endocannabinoid 2-arachidonoylglycerol, in the spinal cord were examined using Western blotting. The protein levels of both cannabinoid CB1 and CB2 receptors were increased in STZ-induced diabetic mice, whereas the protein level of DGL-α was significantly decreased. These results indicate that spinal cannabinoid systems are changed in diabetic mice and suggest that cannabinoid CB2 receptor agonists might have an ability to recover diabetic neuropathic pain.

Spinal Cannabinoid Receptor Type 2 Agonist Reduces Mechanical Allodynia and Induces Mitogen-Activated Protein Kinase Phosphatases in a Rat Model of Neuropathic Pain

Peripheral nerve injury generally results in spinal neuronal and glial plastic changes associated with chronic behavioral hypersensitivity. Spinal mitogen-activated protein kinases (MAPKs), eg, p38 or extracellular signal-regulated kinases (ERKs), are instrumental in the development of chronic allodynia in rodents, and new p38 inhibitors have shown potential in acute and neuropathic pain patients. We have previously shown that the cannabinoid type 2 receptor agonist JWH015 inhibits ERK activity by inducing MAPK phosphatase (MKP)-1 and MKP-3 (the major regulators of MAPKs) in vitro in microglial cells. Therefore, we decided to investigate the role of these phosphatases in the mechanisms of action of JWH015 in vivo using the rat L5 nerve transection model of neuropathic pain. We observed that peripheral nerve injury reduced spinal MKP-1/3 expression and activity and that intrathecal JWH015 reduced established L5 nerve injury-induced allodynia, enhanced spinal MKP-1/3 expression and activity, and reduced the phosphorylated form of p38 and ERK-1/2. Triptolide, a pharmacological blocker of MKP-1 and MKP-3 expression, inhibited JWH015’s effects, suggesting that JWH015 exerts its antinociceptive effects by modulating MKP-1 and MKP-3. JWH015-induced antinociception and MKP-1 and MKP-3 expression were inhibited by the cannabinoid type 2 receptor antagonist AM630. Our data suggest that MKP-1 and MKP-3 are potential targets for novel analgesic drugs. PerspectiveMAPKs are pivotal in the development of chronic allodynia in rodent models of neuropathic pain. A cannabinoid type 2 receptor agonist, JWH015, reduced neuropathic allodynia in rats by reducing MAPK phosphorylation and inducing spinal MAPK phosphatases 1 and 3, the major regulators of MAPKs.

Antinociceptive effect of intrathecal cannabinoid receptor agonist WIN 55,212-2 in a rat bone tumor pain model

Bone tumor pain is a poorly controlled pain comprising background and severe pain on moving or weight-bearing postures that decreases the quality of life for cancer patients; thus, more effective analgesics are clearly needed. This study evaluated the efficacy of a cannabinoid (CB) receptor agonist (WIN 55,212-2) on bone tumor pain in the spinal cords of rats, and clarified the roles of the CB1 and CB2 receptors in WIN 55,212-2-induced antinociception at the spinal level. Bone tumor pain was induced by injecting MRMT-1 tumor cells (1 × 10 5) into the right tibias of female Sprague–Dawley rats under sevoflurane anesthesia. Bone tumor development was monitored radiologically. Under sevoflurane anesthesia, a polyethylene catheter was inserted into the intrathecal space for drug administration. To assess pain, the withdrawal threshold was measured by applying a von Frey filament to the tumor cell inoculation site. The effect of intrathecal WIN 55,212-2 was investigated. Next, the WIN 55,212-2-mediated antinociception was reversed using CB1 (AM 251) and CB2 (AM 630) receptor antagonists. The intratibial injection of MRMT-1 tumor cells produced radiologically confirmed bone tumors. The paw withdrawal threshold decreased significantly (mechanical allodynia) with tumor development; however, intrathecal WIN 55,212-2 dose-dependently increased the withdrawal threshold. The antinociceptive effect of WIN 55,212-2 was reversed by both CB1 and CB2 receptor antagonists. Intrathecal WIN 55,212-2 reduced bone tumor-related pain behavior mediated via spinal CB1 and CB2 receptors. Therefore, spinal CB receptor agonists may be novel analgesics in the treatment of bone tumor pain.

Recent advances in pain medicine: from bench to bed; August 17, 2009, Kobe, Japan

In late 2000, the US Congress passed into law a provision, which the President signed, that declared a 10-year period as the Decade of Pain Control and Research which began from January 1, 2001. The American Pain Society has actively supported the Decade of Pain Control and Research, and it has been a focal point for the development of numerous programs to heighten awareness and treatment of pain and research funding. Cutaneous sensation produced by noxious and innocuous stimuli is transmitted to the dorsal horn of the spinal cord via primary afferent fibers and recognized as pain and touch in the somatosensory cortex. While acute pain has a fundamental role as a protective system against the occurrence or threat of injuries and diseases, chronic pain associated with diseases such as postherpetic neuralgia, and rheumatoid arthritis often outlasts its biological usefulness and in itself causes functional disturbance. Neuropathic pain is a significant and largely unresolved medical problem characterized by spontaneous pain, hyperalgesia, and tactile pain (allodynia). Over several decades, non-steroidal antiinflammatory drugs (NSAIDs) and opioids have been used for clinical management of pain. Neuropathic pain is persistent and can be particularly problematic because they are often poorly managed by NSAIDs and conventional opioid analgesics. Kohno demonstrated that although excitatory transmission remained intact, presynaptic GABA release was reduced in lamina II neurons of an isolated adult rat spinal cord slice preparation in neuropathic pain models. He also reviewed the role of neuron–microglia interactions in the spinal cord in neuropathic pain [1]. Pain associated with cancer, particularly when tumors metastasize to bone, is often severe and debilitating. Better understanding of the neurobiological mechanisms underlying cancer pain are likely to lead to the development of more effective treatments. Kawamata et al. demonstrated that transient receptor potential vanilloid subfamily 1 (TRPV1) was up-regulated, and the l opioid receptor was down-regulated in peripheral neurons in bone cancer. Activation of spinal cannabinoid receptor type 1 (CB1) has been shown to inhibit glutamatergic excitatory postsynaptic currents in the spinal cord slices of naive rats and C-fiber-evoked neuronal response of the dorsal horn. He found that CB1 was located in the axon terminals of excitatory spinal interneuron, and its expression is preserved in bone cancer. Furthermore, he reported that spinal CB1 activation by an exogenously administered CB1 agonist reduced bone cancer-related pain. Therefore, spinal T. Minami (&) Department of Anesthesiology, Osaka Medical College, 2-7 Daigaku-machi, Takatsuki, Osaka 569-8686, Japan e-mail: ane022@poh.osaka-med.ac.jp

Spinal cannabinoids – a double‐edged sword? (Commentary on Zhang et al.)

Spinal cannabinoids – a double‐edged sword? (Commentary on Zhang <i>et al.</i>)

Reduction of Bone Cancer Pain by Activation of Spinal Cannabinoid Receptor 1 and Its Expression in the Superficial Dorsal Horn of the Spinal Cord in a Murine Model of Bone Cancer Pain

Bone cancer pain has a strong impact on the quality of life of patients, but it is difficult to treat. Therefore, development of a novel strategy for the treatment of bone cancer pain is needed for improvement of patient quality of life. This study examined whether selective spinal cannabinoid receptor 1 (CB1) activation alleviates bone cancer pain and also examined the spinal expression of CB1. A bone cancer pain model was made by implantation of sarcoma cells into the intramedullary space of the mouse femur. In behavioral experiments, the authors examined the effects of activation of spinal CB1 and inhibition of metabolism of endocannabinoid on bone cancer-related pain behaviors. Immunohistochemical experiments examined the distribution and localization of CB1 in the superficial dorsal horn of the spinal cord using specific antibodies. Spinal CB1 activation by exogenous administration of a CB1 agonist arachidonyl-2-chloroethylamide reduced bone cancer-related pain behaviors, including behaviors related to spontaneous pain and movement-evoked pain. In immunohistochemical experiments, although mu-opioid receptor 1 expression was reduced in the superficial dorsal horn ipsilateral to the site of implantation of sarcoma cells, CB1 expression was preserved. In addition, CB1 was mainly expressed in the axon terminals, but not in the dendritic process in the superficial dorsal horn. Spinal CB1 activation reduced bone cancer-related pain behavior. Presynaptic inhibition may contribute to the analgesic effects of spinal CB1 activation. These findings may lead to novel strategies for the treatment of bone cancer pain.

Spinal Cannabinoid Receptor Type 2 Activation Reduces Hypersensitivity and Spinal Cord Glial Activation after Paw Incision

Cannabinoids bind to cannabinoid receptors type 1 and 2 and produce analgesia in several pain models, but central side effects from cannabinoid 1 receptors limit their clinical use. Cannabinoid 2 receptors reduce inflammatory responses in the periphery by acting on immune cells, and they are present on glia in the central nervous system. This study tested whether spinal cannabinoid activation would induce analgesia, glial inhibition, and central side effects in a postoperative model or incisional pain. Rats underwent paw incision surgery, with intrathecal injections of cannabinoid agonists and antagonists and assessment of withdrawal thresholds and behavioral side effects. Spinal glial activation was determined by immunohistochemistry. Intrathecal administration CP55940 reduced postoperative hypersensitivity (91 +/- 9% maximum possible effect; P < 0.05), and this was prevented by intrathecal administration of both cannabinoid 1 receptor (AM281) and cannabinoid 2 receptor (AM630) antagonists. CP55940 also caused several behavioral side effects, and these were prevented by the cannabinoid 1 receptor but not by the cannabinoid 2 receptor antagonist. Intrathecal injection of the cannabinoid 2 receptor agonist JWH015 reversed postoperative hypersensitivity (89 +/- 5% maximum possible effect; P < 0.05), and this was reversed by the cannabinoid 2 but not by the cannabinoid 1 receptor antagonist. JWH015, which did not induce behavioral side effects, reduced paw incision induced microglial and astrocytic activation in spinal cord (P < 0.05). These data indicate that intrathecal administration of cannabinoid receptor agonists may provide postoperative analgesia while reducing spinal glial activation, and that selective cannabinoid 2 receptor agonists may do so without central side effects.

Involvement of cannabinoid (CB 1)-receptors in the development and maintenance of opioid tolerance

Sustained exposure to opioid agonists such as morphine increases levels of calcitonin gene-related peptide (CGRP) in the spinal dorsal horn, a response implicated in the development of opioid tolerance and physical dependence. Recent evidence suggests that both the opioid-induced increase in CGRP and the development of opioid physical dependence are suppressed by blockade of spinal cannabinoid (CB 1)-receptors. The present study examined whether CB 1-receptor activity also has a role in the development of opioid tolerance. In rats implanted with spinal catheters, repeated acute injections of morphine (15 μg) delivered over 4 h resulted in a rapid decline of thermal and mechanical antinociception and a significant loss of analgesic potency, reflecting development of acute opioid tolerance. In another set of experiments, chronic administration of spinal morphine (15 μg) once daily for 5 days produced a similar loss of analgesic effect and a marked increase in CGRP-immunoreactivity in the superficial laminae of the dorsal horn. Consistent with the in vivo findings, primary cultures of adult dorsal root ganglion (DRG) neurons exposed to morphine for 5 days showed a significant increase in the number of CGRP-immunoreactive neurons. Co-administration of acute or chronic morphine with a CB 1-receptor antagonist/inverse agonist, 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide (AM-251), inhibited the development of both acute and chronic analgesic tolerance. In animals already exhibiting tolerance to morphine, intervention with AM-251 restored morphine analgesic potency. Co-administration with AM-251 attenuated the morphine-induced increase in CGRP-immunoreactivity in the spinal cord and in DRG cultured neurons. Collectively, the results of this study suggest that activity of endocannabinoids, mediated via CB 1-receptors, contributes to both the development and maintenance of opioid tolerance by influencing the opioid-induced increase in spinal CGRP.

Antinociceptive effect of spinally administered cannabinergic and 2-adrenoceptor drugs on the formalin test in rat: possible interactions

The current experiments were designed to study the antinociceptive effects of intrathecal (i.t.) administration of cannabinoid CB1 receptor and 2-adrenoceptor drugs in the nociceptive processing and also their receptor interactions. Different doses of a cannabinoid receptor agonist, CP 55,940, and an 2-adrenoceptor agonist, clonidine induced a dose-dependent antinociception in both phases of the formalin test.CP 55,940-induced antinociception was reduced by pretreatment of a selective cannabinoid CB1 receptor antagonist, SR 141716A, but not by pretreatment with an 2-adrenoceptor antagonist, yohimbine in both phases of the test. However, yohimbine and SR 141716A attenuated the antinociception induced by clonidine in the early phase but not in the late phase of the test. While SR 141716A by itself did not influence pain behaviour, the reversal effect of clonidine by SR 141716A indicate that clonidine stimulate the release of endocannabinoid(s). In conclusion, our findings may suggest that: (1) spinal cannabinoid and 2-adrenoceptor systems are able to induce antinociception in both phases of formalin test, and (2) the cannabinoid system may be involved in the antinociception induced by adrenoceptors in the early phase.

Central glucocorticoid receptors modulate the expression of spinal cannabinoid receptors induced by chronic morphine exposure

Central cannabinoid receptors (CBRs) have been implicated in the opioid analgesic effects. However, it remains unclear as to whether the expression of central CBRs would be altered after repeated morphine exposure. Here, we show that chronic intrathecal treatment with morphine (10 μg, twice daily for 6 days) induced a time-dependent upregulation of both CB-1 and CB-2 receptors within the spinal cord dorsal horn. This morphine-induced CB-1 and CB-2 upregulation was dose-dependently attenuated by the intrathecal co-administration of morphine with the glucocorticoid receptor (GR) antagonist RU38486 (0.25, 0.5, or 2 μg). The intrathecal RU38486 treatment regimen also attenuated the development of morphine tolerance. These results indicate that the expression of spinal CBRs was altered following repeated morphine exposure and regulated by the activation of central GRs.

Pharmacologic interaction between cannabinoid and either clonidine or neostigmine in the rat formalin test.

Although spinal cannabinoid receptor agonist (WIN 55,212-2) has been shown to encounter various models of pain, the role of two subtypes of cannabinoid receptor for the antinociceptive effect of cannabinoids has not been investigated at the spinal level. Spinal alpha 2 receptor agonist (clonidine) and cholinesterase inhibitor (neostigmine) are also active in the modulation of nociception. The authors examined the properties of drug interaction after coadministration of WIN 55,212-2-clonidine, and intrathecal WIN 55,212-2-neostigmine, and further clarified the role of cannabinoid 1 and 2 receptors in cannabinoid-induced antinociception at the spinal level. Catheters were inserted into the intrathecal space of male Sprague-Dawley rats, and 50 microl of 5% formalin solution was injected into the hind paw to evoke the pain. Isobolographic analysis was used for evaluation of pharmacologic interaction. Intrathecal 55,212-2, clonidine, and neostigmine dose-dependently suppressed the flinching observed during phase 1 and 2 in the formalin test. Isobolographic analysis revealed a synergistic interaction after intrathecal delivery of WIN 55,212-2-clonidine or WIN 55,212-2-neostigmine mixture in both phases. The antinociceptive effect of WIN 55,212-2 was antagonized by cannabinoid 1 receptor antagonist (AM 251) but not by cannabinoid 2 receptor antagonist (AM 630). No antinociceptive effect was seen after intrathecal administration of cannabinoid 2 receptor agonist (JWH 133). Intrathecal 55,212-2, clonidine, and neostigmine attenuate the facilitated state and acute pain. WIN 55,212-2 interacts synergistically with either clonidine or neostigmine. The antinociception of WIN 55,212-2 is mediated through the cannabinoid 1 receptor, but not the cannabinoid 2 receptor, at the spinal level.

Cannabinoid CB 1 receptor inhibition of mechanically evoked responses of spinal neurones in control rats, but not in rats with hindpaw inflammation

Spinally administered cannabinoid receptor agonists are anti-nociceptive in a variety of models of acute and persistent pain. The present study investigated the effects of activation of spinal cannabinoid CB 1 receptors on mechanically evoked responses of spinal neurones in acute and inflammatory pain states. In vivo electrophysiology studies were carried out in anaesthetised rats. Effects of spinal administration of a selective cannabinoid CB 1 receptor agonist, arachidonyl-2-chloroethylamide (ACEA), on mechanically evoked responses of dorsal horn neurones in control rats and rats with peripheral hindpaw carrageenan-induced inflammation were compared. ACEA (0.27 nM–27 μM) significantly inhibited innocuous and noxious mechanically evoked responses of dorsal horn neurones in control rats. Pre-administration of the CB 1 receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1(2,4-dichlorophenyl)-4-methyl-1- H-pyrazole-3-carboxyamide, SR141716A, (0.43 μM) attenuated the inhibitory effects of ACEA (27 μM). ACEA did not alter mechanically evoked responses of dorsal horn neurones in rats with hindpaw carrageenan-induced inflammation. Following peripheral inflammation, there is a loss of spinal CB 1 receptor-mediated inhibition of mechanically evoked responses, which is suggestive of a functional down-regulation of CB 1 receptors under these conditions.

Topical cannabinoid enhances topical morphine antinociception

Opioids and cannabinoids produce antinociception through both spinal and supraspinal action. Both opioids and cannabinoids also have important peripheral action. Many previous studies indicate that systemically administered cannabinoids enhance antinociceptive properties of opioids. Experiments were conducted to test the hypothesis that topical cannabinoids would enhance the topical antinociceptive effects of morphine. Antinociception was measured in the radiant tail-flick test after immersion of the tail of mice into a solution of dimethyl sulfoxide (DMSO) containing WIN 55, 212-2, a cannabinoid agonist and morphine, an opioid agonist. Morphine and WIN 55, 212-2 produce time dependent topical analgesic effects limited to the portion of the tail exposed to drugs. WIN 55, 212-2 had a potency lower than that of morphine. The topical antinociceptive effects of WIN 55, 212-2 were blocked by systemic pretreatment of cannabinoid CB1 receptor selective antagonist, AM 251. This suggests that topical antinociceptive effects of WIN 55, 212-2 involve CB1 receptors. Combination of topical WIN 55, 212-2 with topical morphine yielded significantly greater analgesic effects than that of topical morphine alone. The ability of the CB1 receptor antagonist AM 251 to antagonize the enhancement of antinociception of morphine by WIN 55, 212-2 indicates that WIN 55, 212-2 acts through a CB1 receptor to enhance the potency of topical morphine. Additionally, spinally administered ineffective doses of WIN 55, 212-2 potentiated the antinociceptive effects of topical morphine. These results demonstrate an antinociceptive interaction between topical opioids with topical, and spinal cannabinoids. These observations are significant in using of topical combination of cannabinoid and morphine in the management of pain.