A peripheral cannabinoid mechanism suppresses spinal fos protein expression and pain behavior in a rat model of inflammation

Negative allosteric modulation of CB1 cannabinoid receptor signaling suppresses opioid-mediated tolerance and withdrawal without blocking opioid antinociception

The direct blockade of CB1 cannabinoid receptors produces therapeutic effects as well as adverse side-effects that limit their clinical potential. CB1 negative allosteric modulators (NAMs) represent an indirect approach to decrease the affinity and/or efficacy of orthosteric cannabinoid ligands or endocannabinoids at CB1. We recently reported that GAT358, a CB1-NAM, blocked opioid-induced mesocorticolimbic dopamine release and reward via a CB1-allosteric mechanism of action. Whether a CB1-NAM dampens opioid-mediated therapeutic effects such as analgesia or alters other unwanted opioid side-effects remain unknown. Here, we characterized the effects of GAT358 on nociceptive behaviors in the presence and absence of morphine in male rats. We examined the impact of GAT358 on formalin-evoked pain behavior and Fos protein expression, a marker of neuronal activation, in the lumbar spinal cord. We also assessed the impact of GAT358 on morphine-induced slowing of colonic transit, tolerance, and withdrawal behaviors in male mice. GAT358 attenuated morphine antinociceptive tolerance without blocking acute antinociception and reduced morphine-induced slowing of colonic motility without impacting fecal boli production. GAT358 also produced antinociception in the presence and absence of morphine in the formalin model of inflammatory nociception and reduced the number of formalin-evoked Fos protein-like immunoreactive cells in the lumbar spinal cord. Finally, GAT358 mitigated the somatic signs of naloxone-precipitated, but not spontaneous, opioid withdrawal following chronic morphine dosing. Our results support the therapeutic potential of CB1-NAMs as novel drug candidates aimed at preserving opioid-mediated analgesia while preventing their unwanted side-effects. Our studies also uncover previously unrecognized antinociceptive properties associated with an arrestin-biased CB1-NAM.

Palmitoylethanolamide-Incorporated Elastic Nano-Liposomes for Enhanced Transdermal Delivery and Anti-Inflammation

Palmitoylethanolamide (PEA) exhibits multiple skincare functions such as anti-nociceptive and anti-inflammatory effects. However, its topical application is limited due to its difficulty in bypassing the stratum corneum barrier, relatively low bioavailability, and low stability. Herein, elastic nano-liposomes (ENLs) with excellent deformability and elasticity were utilized as a novel drug delivery system to encapsulate PEA to overcome the abovementioned issues and enhance the biological effects on the skin. ENL was prepared with phosphatidylcholine, cholesterol, and cetyl-PG hydroxyethyl palmitamide with a molar ratio mimicking skin epidermal lipids, and PEA was loaded. The PEA-loaded ENL (PEA-ENL) demonstrated efficient transdermal delivery and enhanced skin retention, with negligible cytotoxicity toward HaCaT cells and no allergic reaction in the human skin patch test. Notably, PEA-ENL treatment increased cell migration and induced significant regulation in the expression of genes associated with anti-nociceptive, anti-inflammatory, and skin barrier repair. The mechanism of the anti-nociceptive and anti-inflammatory effects of PEA was further investigated and explained by molecular docking site analysis. This novel PEA-ENL, with efficient transdermal delivery efficiency and multiple skincare functionalities, is promising for topical application.

Negative allosteric modulation of cannabinoid CB 1 receptor signaling suppresses opioid-mediated tolerance and withdrawal without blocking opioid antinociception

The direct blockade of CB 1 cannabinoid receptors produces therapeutic effects as well as adverse side-effects that limit their clinical potential. CB 1 negative allosteric modulators (NAMs) represent an indirect approach to decrease the affinity and/or efficacy of orthosteric cannabinoid ligands or endocannabinoids at CB 1 . We recently reported that GAT358, a CB 1 -NAM, blocked opioid-induced mesocorticolimbic dopamine release and reward via a CB 1 -allosteric mechanism of action. Whether a CB 1 -NAM dampens opioid-mediated therapeutic effects such as analgesia or alters other unwanted side-effects of opioids remain unknown. Here, we characterized the effects of GAT358 on nociceptive behaviors in the presence and absence of morphine. We examined the impact of GAT358 on formalin-evoked pain behavior and Fos protein expression, a marker of neuronal activation, in the lumbar dorsal horn. We also assessed the impact of GAT358 on morphine-induced slowing of colonic transit, tolerance, and withdrawal behaviors. GAT358 attenuated morphine antinociceptive tolerance without blocking acute antinociception. GAT358 also reduced morphine-induced slowing of colonic motility without impacting fecal boli production. GAT358 produced antinociception in the presence and absence of morphine in the formalin model of inflammatory nociception and reduced the number of formalin-evoked Fos protein-like immunoreactive cells in the lumbar spinal dorsal horn. Finally, GAT358 mitigated the somatic signs of naloxone-precipitated, but not spontaneous, opioid withdrawal following chronic morphine dosing in mice. Our results support the therapeutic potential of CB 1 -NAMs as novel drug candidates aimed at preserving opioid-mediated analgesia while preventing their unwanted side-effects. Our studies also uncover previously unrecognized antinociceptive properties associated with an arrestin-biased CB 1 -NAMs.

Highlights

CB 1 negative allosteric modulator (NAM) GAT358 attenuated morphine tolerance GAT358 reduced morphine-induced slowing of colonic motility but not fecal productionGAT358 was antinociceptive for formalin pain alone and when combined with morphineGAT358 reduced formalin-evoked Fos protein expression in the lumbar spinal cordGAT358 mitigated naloxone precipitated withdrawal after chronic morphine dosing.

Conditional deletion of CB2 cannabinoid receptors from peripheral sensory neurons eliminates CB2-mediated antinociceptive efficacy in a mouse model of carrageenan-induced inflammatory pain

CB2 cannabinoid receptor agonists suppress pathological pain in animal models and lack unwanted side effects commonly associated with direct activation of CB1 receptors. However, the types of pain most responsive to CB2 agonists are incompletely understood and cell types which underlie CB2-mediated therapeutic efficacy remain largely unknown. We previously reported that the CB2 receptor agonist LY2828360 reduced neuropathic nociception induced by toxic challenge with chemotherapeutic and anti-retroviral agents in mice. Whether these findings generalize to models of inflammatory pain is not known. Here we show that LY2828360 (10 mg/kg i.p.) reversed the maintenance of carrageenan-induced mechanical allodynia in female mice. Anti-allodynic efficacy was fully preserved in global CB1 knock out (KO) mice but absent in CB2 KO mice. The anti-allodynic efficacy of LY2828360 was absent in conditional KO (cKO) mice lacking CB2 receptors in peripheral sensory neurons (AdvillinCRE/+; CB2f/f) and preserved in cKO mice lacking CB2 receptors in microglia/macrophages expressing C-X3-C Motif Chemokine Receptor 1 (CX3CR1CRE/+; CB2f/f). Intraplantar administration of LY2828360 (30 μg i.pl.) reversed carrageenan-induced mechanical allodynia in CB2f/f but not AdvillinCRE/+; CB2f/f mice of both sexes. Thus, CB2 receptors in peripheral sensory neurons likely underlie the therapeutic effects of LY2828360 injection in the paw. Lastly, qRT-PCR analyses revealed that LY2828360 reduced carrageenan-induced increases in IL-1β and IL-10 mRNA in paw skin. Our results suggest that LY2828360 suppresses inflammatory nociception in mice through a neuronal CB2-dependent mechanism that requires peripheral sensory neuron CB2 receptors and suggest that the clinical applications of LY2828360 as an anti-hyperalgesic agent should be re-evaluated.

Cannabinoids, the endocannabinoid system, and pain: a review of preclinical studies

ABSTRACT

This narrative review represents an output from the International Association for the Study of Pain's global task force on the use of cannabis, cannabinoids, and cannabis-based medicines for pain management, informed by our companion systematic review and meta-analysis of preclinical studies in this area. Our aims in this review are (1) to describe the value of studying cannabinoids and endogenous cannabinoid (endocannabinoid) system modulators in preclinical/animal models of pain; (2) to discuss both pain-related efficacy and additional pain-relevant effects (adverse and beneficial) of cannabinoids and endocannabinoid system modulators as they pertain to animal models of pathological or injury-related persistent pain; and (3) to identify important directions for future research. In service of these goals, this review (1) provides an overview of the endocannabinoid system and the pharmacology of cannabinoids and endocannabinoid system modulators, with specific relevance to animal models of pathological or injury-related persistent pain; (2) describes pharmacokinetics of cannabinoids in rodents and humans; and (3) highlights differences and discrepancies between preclinical and clinical studies in this area. Preclinical (rodent) models have advanced our understanding of the underlying sites and mechanisms of action of cannabinoids and the endocannabinoid system in suppressing nociceptive signaling and behaviors. We conclude that substantial evidence from animal models supports the contention that cannabinoids and endocannabinoid system modulators hold considerable promise for analgesic drug development, although the challenge of translating this knowledge into clinically useful medicines is not to be underestimated.

Cannabinoid receptor agonists from Conus venoms alleviate pain-related behavior in rats

Cannabinoid (CB) receptor agonists show robust antinociceptive effects in various pain models. However, most of the clinically potent CB1 receptor-active drugs derived from cannabis are considered concerning due to psychotomimetic side effects. Selective CB receptor ligands that do not induce CNS side effects are of clinical interest. The venoms of marine snail Conus are a natural source of various potent analgesic peptides, some of which are already FDA approved. In this study we evaluated the ability of several Conus venom extracts to interact with CB1 receptor. HEK293 cells expressing CB1 receptors were treated with venom extracts and CB1 receptor internalization was analyzed by immunofluorescence. Results showed C. textile (C. Tex) and C. miles (C. Mil) samples as the most potent. These were serially subfractionated by HPLC for subsequent analysis by internalization assays and for analgesic potency evaluated in the formalin test and after peripheral nerve injury. Intrathecal injection of C. Tex and C. Mil subfractions reduced flinching/licking behavior during the second phase of formalin test and attenuated thermal and mechanical allodynia in nerve injury model. Treatment with proteolytic enzymes reduced CB1 internalization of subfractions, indicating the peptidergic nature of CB1 active component. Further HPLC purification revealed two potent antinociceptive subfractions within C. Tex with CB1 and possible CB2 activity, with mild to no side effects in the CB tetrad assessment. CB conopeptides can be isolated from these active Conus venom-derived samples and further developed as novel analgesic agents for the treatment of chronic pain using cell based or gene therapy approaches.

The naked mole-rat has a functional purinergic pain pathway despite having a non-functional peptidergic pain pathway

Naked mole-rats (Heterocephalus glaber) have adaptations within their pain pathway that are beneficial to survival in large colonies within poorly ventilated burrow systems, with lower O2 and higher CO2 ambient levels than ground-level environments. These adaptations ultimately lead to a partial disruption of the C-fiber pain pathway, which enables naked mole-rats to not feel pain from the acidosis associated with CO2 accumulation. One hallmark of this disruption is that naked mole-rats do not express neuropeptides, such as Substance P and calcitonin gene-related peptide in their cutaneous C-fibers, effectively making the peptidergic pain pathway hypofunctional. One C-fiber pathway that remains unstudied in the naked mole-rat is the non-peptidergic, purinergic pathway, despite this being a key pathway for inflammatory pain. The current study aimed to establish the functionality of the purinergic pathway in naked mole-rats and the effectiveness of cannabinoids in attenuating pain through this pathway. Cannabinoids can manage chronic inflammatory pain in both humans and mouse models, and studies suggest a major downstream role for the purinergic receptor, P2X3, in this treatment. Here we used Ca2+-imaging of cultured dorsal root ganglion neurons and in vivo behavioral testing to demonstrate that the P2X3 pathway is functional in naked mole-rats. Additionally, formalin-induced inflammatory pain was reduced by the cannabinoid receptor agonist, WIN55 (inflammatory, but not acute phase) and the P2X3 receptor antagonist A-317491 (acute and inflammatory phases). This study establishes that the purinergic C-fiber pathway is present and functional in naked mole-rats and that cannabinoid-mediated analgesia occurs in this species.

Cannabinoid Receptor Type 1 and Its Role as an Analgesic: An Opioid Alternative?

Understanding how the body regulates pain is fundamental to develop rational strategies to combat the growing prevalence of chronic pain states, opioid dependency, and the increased financial burden to the medical care system. Pain is the most prominent reason why Americans seek medical attention and extensive literature has identified the importance of the endocannabinoid pathway in controlling pain. Modulation of the endocannabinoid system offers new therapeutic opportunities for the selective control of excessive neuronal activity in several pain conditions (acute, inflammatory, chronic, and neuropathic). Cannabinoids have a long history of medicinal use and their analgesic properties are well documented; however, there are major impediments to understanding cannabinoid pain modulation. One major issue is the presence of psychotropic side effects associated with D9-tetrahydrocannabinol (THC) or synthetic derivatives, which puts an emphatic brake on their use. This dose-limiting effect prevents the appropriate degree of analgesia . Animal studies have shown that the psychotropic effects are mediated via brain cannabinoid type 1 (CB1) receptors, while analgesic activity in chronic pain states may be mediated via CB1R action in the spinal cord, brainstem, peripheral sensory neurons, or immune cells. The development of appropriate therapies is incumbent on our understanding of the role of peripheral versus central endocannabinoid-driven analgesia. Recent physiological, pharmacological, and anatomical studies provide evidence that one of the main roles of the endocannabinoid system is the regulation of gamma-aminobutyric acid (GABA) and/or glutamate release. This article will review this evidence in the context of its implications for pain. We first provide a brief overview of CB1R's role in the regulation of nociception, followed by a review of the evidence that the peripheral endocannabinoid system modulates nociception. We then look in detail at regulation of central-mediated analgesia, followed up with evidence that cannabinoidmediated modulation of pain involves modulation of GABAergic and glutamatergic neurotransmission in key brain regions. Finally, we discuss cannabinoid action on non-neuronal cells in the context of inflammation and direct modulation of neurons. This work stands to reveal long-standing controversies in the cannabinoid analgesia area that have had an impact on failed clinical trials and implementation of therapeutics targeting this system.

Brain-Permeant and -Impermeant Inhibitors of Fatty Acid Amide Hydrolase Synergize with the Opioid Analgesic Morphine to Suppress Chemotherapy-Induced Neuropathic Nociception Without Enhancing Effects of Morphine on Gastrointestinal Transit

Opioid-based therapies remain a mainstay for chronic pain management, but unwanted side effects limit therapeutic use. We compared efficacies of brain-permeant and -impermeant inhibitors of fatty acid amide hydrolase (FAAH) in suppressing neuropathic pain induced by the chemotherapeutic agent paclitaxel. Paclitaxel produced mechanical and cold allodynia without altering nestlet shredding or marble burying behaviors. We compared FAAH inhibitors that differ in their ability to penetrate the central nervous system for antiallodynic efficacy, pharmacological specificity, and synergism with the opioid analgesic morphine. (3'-(aminocarbonyl)[1,1'-biphenyl]- 3-yl)-cyclohexylcarbamate (URB597), a brain-permeant FAAH inhibitor, attenuated paclitaxel-induced allodynia via cannabinoid receptor 1 (CB₁) and cannabinoid receptor 2 (CB₂) mechanisms. URB937, a brain-impermeant FAAH inhibitor, suppressed paclitaxel-induced allodynia through a CB₁ mechanism only. 5-[4-(4-cyano-1-butyn-1-yl)phenyl]-1-(2,4-dichlorophenyl)-N-(1,1-dioxido-4-thiomorpholinyl)-4-methyl-1H-pyrazole-3-carboxamide (AM6545), a peripherally restricted CB₁ antagonist, fully reversed the antiallodynic efficacy of N-cyclohexyl-carbamic acid, 3'-(aminocarbonyl)-6-hydroxy[1,1'- biphenyl]-3-yl ester (URB937) but only partially reversed that of URB597. Thus, URB937 suppressed paclitaxel-induced allodynia through a mechanism that was dependent upon peripheral CB₁ receptor activation only. Antiallodynic effects of both FAAH inhibitors were reversed by N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251). Antiallodynic effects of URB597, but not URB937, were reversed by 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl](4-methoxyphenyl)methanone (AM630). Isobolographic analysis revealed synergistic interactions between morphine and either URB597 or URB937 in reducing paclitaxel-induced allodynia. A leftward shift in the dose-response curve of morphine antinociception was observed when morphine was coadministered with either URB597 or URB937, consistent with morphine sparing. However, neither URB937 nor URB597 enhanced morphine-induced deficits in colonic transit. Thus, our findings suggest that FAAH inhibition may represent a therapeutic avenue to reduce the overall amount of opioid needed for treating neuropathic pain with potential to reduce unwanted side effects that accompany opioid administration.

Endocannabinoid System and Migraine Pain: An Update

The trigeminovascular system (TS) activation and the vasoactive release from trigeminal endings, in proximity of the meningeal vessels, are considered two of the main effector mechanisms of migraine attacks. Several other structures and mediators are involved, however, both upstream and alongside the TS. Among these, the endocannabinoid system (ES) has recently attracted considerable attention. Experimental and clinical data suggest indeed a link between dysregulation of this signaling complex and migraine headache. Clinical observations, in particular, show that the levels of anandamide (AEA)—one of the two primary endocannabinoid lipids—are reduced in cerebrospinal fluid and plasma of patients with chronic migraine (CM), and that this reduction is associated with pain facilitation in the spinal cord. AEA is produced on demand during inflammatory conditions and exerts most of its effects by acting on cannabinoid (CB) receptors. AEA is rapidly degraded by fatty acid amide hydrolase (FAAH) enzyme and its levels can be modulated in the peripheral and central nervous system (CNS) by FAAH inhibitors. Inhibition of AEA degradation via FAAH is a promising therapeutic target for migraine pain, since it is presumably associated to an increased availability of the endocannabinoid, specifically at the site where its formation is stimulated (e.g., trigeminal ganglion and/or meninges), thus prolonging its action.

ZLc002, a putative small-molecule inhibitor of nNOS interaction with NOS1AP, suppresses inflammatory nociception and chemotherapy-induced neuropathic pain and synergizes with paclitaxel to reduce tumor cell viability

Elevated N-methyl-D-aspartate receptor activity contributes to central sensitization. Our laboratories and others recently reported that disrupting protein-protein interactions downstream of N-methyl-D-aspartate receptors suppresses pain. Specifically, disrupting binding between the enzyme neuronal nitric oxide synthase and either its upstream (postsynaptic density 95 kDa, PSD95) or downstream (e.g. nitric oxide synthase 1 adaptor protein, NOS1AP) protein partners suppressed inflammatory and/or neuropathic pain. However, the lack of a small-molecule neuronal nitric oxide synthase-NOS1AP inhibitor has hindered efforts to validate the therapeutic utility of disrupting the neuronal nitric oxide synthase-NOS1AP interface as an analgesic strategy. We, therefore, evaluated the ability of a putative small-molecule neuronal nitric oxide synthase-NOS1AP inhibitor ZLc002 to disrupt binding between neuronal nitric oxide synthase and NOS1AP using ex vivo, in vitro, and purified recombinant systems and asked whether ZLc002 would suppress inflammatory and neuropathic pain in vivo. In vitro, ZLc002 reduced co-immunoprecipitation of full-length NOS1AP and neuronal nitric oxide synthase in cultured neurons and in HEK293T cells co-expressing full-length neuronal nitric oxide synthase and NOS1AP. However, using a cell-free biochemical binding assay, ZLc002 failed to disrupt the in vitro binding between His-neuronal nitric oxide synthase1-299 and glutathione S-transferase-NOS1AP400-506, protein sequences containing the required binding domains for this protein-protein interaction, suggesting an indirect mode of action in intact cells. ZLc002 (4-10 mg/kg i.p.) suppressed formalin-evoked inflammatory pain in rats and reduced Fos protein-like immunoreactivity in the lumbar spinal dorsal horn. ZLc002 also suppressed mechanical and cold allodynia in a mouse model of paclitaxel-induced neuropathic pain. Anti-allodynic efficacy was sustained for at least four days of once daily repeated dosing. ZLc002 also synergized with paclitaxel when administered in combination to reduce breast (4T1) or ovarian (HeyA8) tumor cell line viability but did not alter tumor cell viability without paclitaxel. Our results verify that ZLc002 disrupts neuronal nitric oxide synthase-NOS1AP interaction in intact cells and demonstrate, for the first time, that systemic administration of a putative small-molecule inhibitor of neuronal nitric oxide synthase-NOS1AP suppresses inflammatory and neuropathic pain.

Comparative antinociceptive effect of arachidonylcyclopropylamide, a cannabinoid 1 receptor agonist & lignocaine, a local anaesthetic agent, following direct intrawound administration in rats

Background & objectives:

Treatment of inflammatory pain with opioids is accompanied by unpleasant and, at times, life-threatening side effects. Cannabis produces antinociception as well as psychotropic effects. It was hypothesized that peripheral cannabinoid receptors outside the central nervous system could be selectively activated for relief of pain. This study was undertaken to measure the antinociceptive effect of type 1 cannabinoid receptor (CB1r) agonist arachidonylcyclopropylamide (ACPA) in a rat model of inflammatory pain after intrawound administration and the effects were compared with lignocaine.

Methods:

Wounds were produced under controlled conditions by an incision in the right hind paw in rats. ACPA (10, 30 or 100 μg/10 μl) was administered directly into the wound. Antinociception was evaluated by guarding, allodynia and thermal hyperalgesia. This was compared to lignocaine (30 μg/10 μl). Reversal of ACPA (30 μg)-mediated antinociceptive effect was attempted by intrawound AM251 (100 μg), a CB1r antagonist. Antinociception was also evaluated after contralateral administration of ACPA (30 μg). Primary afferent nociceptive input to the spinal cord was investigated by c-Fos expression after ACPA treatment (100 μg).

Results:

ACPA, but not lignocaine, inhibited guarding behaviour, which was locally mediated. Conversely, lignocaine, but not ACPA, inhibited thermal hyperalgesia and mechanical allodynia. ACPA-mediated inhibitory effect was reversible and dose dependent. It was associated with a decreased c-Fos expression. Locomotor activity was unaffected following ACPA (100 μg) treatment.

Interpretation & conclusions:

Lignocaine attenuated evoked pain behaviour whereas ACPA decreased guarding score. This difference was likely due to blockade of sodium ion channels and the activation of peripheral CB1r, respectively. Central side effects were absent after ACPA treatment. Further studies need to be done to assess the effect of ACPA treatment in clinical conditions.

Small molecule inhibitors of PSD95-nNOS protein-protein interactions suppress formalin-evoked Fos protein expression and nociceptive behavior in rats

Excessive activation of NMDA receptor (NMDAR) signaling within the spinal dorsal horn contributes to central sensitization and the induction and maintenance of pathological pain states. However, direct antagonism of NMDARs produces undesirable side effects which limit their clinical use. NMDAR activation produces central sensitization, in part, by initiating a signaling cascade that activates the enzyme neuronal nitric oxide synthase (nNOS) and generates the signaling molecule nitric oxide. NMDAR-mediated activation of nNOS requires a scaffolding protein, postsynaptic density protein 95kDa (PSD95), which tethers nNOS to NMDARs. Thus, disrupting the protein-protein interaction between PSD95 and nNOS may inhibit pro-nociceptive signaling mechanisms downstream of NMDARs and suppress central sensitization while sparing unwanted side effects associated with NMDAR antagonists. We examined the impact of small molecule PSD95-nNOS protein-protein interaction inhibitors (ZL006, IC87201) on both nociceptive behavior and formalin-evoked Fos protein expression within the lumbar spinal cord of rats. Comparisons were made with ZL007, an inactive analog of ZL006, and the NMDAR antagonist MK-801. IC87201 and ZL006, but not ZL007, suppressed phase 2 of formalin-evoked pain behavior and decreased the number of formalin-induced Fos-like immunoreactive cells in spinal dorsal horn regions associated with nociceptive processing. MK-801 suppressed Fos protein expression in both dorsal and ventral horns. MK-801 produced motor ataxia in the rotarod test whereas IC87201 and ZL006 failed to do so. ZL006 but not ZL007 suppressed paclitaxel-induced mechanical and cold allodynia in a model of chemotherapy-induced neuropathic pain. Co-immunoprecipitation experiments revealed the presence of the PSD95-nNOS complex in lumbar spinal cord of paclitaxel-treated rats, although ZL006 did not reliably disrupt the complex in all subjects. The present findings validate use of putative small molecule PSD95-nNOS protein-protein interaction inhibitors as novel analgesics and demonstrate, for the first time, that these inhibitors suppress inflammation-evoked neuronal activation at the level of the spinal dorsal horn.

Antihyperalgesic effect of CB1 receptor activation involves the modulation of P2X3 receptor in the primary afferent neuron

Cannabinoid system is a potential target for pain control. Cannabinoid receptor 1 (CB₁) activation play a role in the analgesic effect of cannabinoids once it is expressed in primary afferent neurons. This study investigates whether the anti-hyperalgesic effect of CB₁ receptor activation involves P2X₃ receptor in primary afferent neurons. Mechanical hyperalgesia was evaluated by electronic von Frey test. Cannabinoid effect was evaluated using anandamide or ACEA, a non-selective or a selective CB₁ receptor agonists, respectively; AM251, a CB₁ receptor antagonist, and antisense ODN for CB₁ receptor. Calcium imaging assay was performed to evaluated α,β-meATP-responsive cultured DRG neurons pretreated with ACEA. Anandamide or ACEA administered in peripheral tissue reduced the carrageenan-induced mechanical hyperalgesia. The reduction in the carrageenan-induced hyperalgesia induced by ACEA was completely reversed by administration of AM251 as well as by the intrathecal treatment with antisense ODN for CB₁ receptor. Also, ACEA reduced the mechanical hyperalgesia induced by bradykinin and by α,β-meATP, a P2X₃ receptor non-selective agonist, but not by tumor necrosis factor alpha (TNF-α), interleukin-1 beta (IL-1β) and chemokine-induced chemoattractant-1 (CINC-1). Finally, CB₁ receptors are co-localized with P2X₃ receptors in DRG small-diameter neurons and the treatment with ACEA reduced the number of α,β-meATP-responsive cultured DRG neurons. Our data suggest that the analgesic effect of CB₁ receptor activation is mediated by a negative modulation of the P2X₃ receptor in the primary afferent neurons.

Peripherally Selective Cannabinoid 1 Receptor (CB1R) Agonists for the Treatment of Neuropathic Pain

Alleviation of neuropathic pain by cannabinoids is limited by their central nervous system (CNS) side effects. Indole and indene compounds were engineered for high hCB1R affinity, peripheral selectivity, metabolic stability, and in vivo efficacy. An epithelial cell line assay identified candidates with <1% blood-brain barrier penetration for testing in a rat neuropathy induced by unilateral sciatic nerve entrapment (SNE). The SNE-induced mechanical allodynia was reversibly suppressed, partially or completely, after intraperitoneal or oral administration of several indenes. At doses that relieve neuropathy symptoms, the indenes completely lacked, while the brain-permeant CB1R agonist HU-210 (1) exhibited strong CNS side effects, in catalepsy, hypothermia, and motor incoordination assays. Pharmacokinetic findings of ∼0.001 cerebrospinal fluid:plasma ratio further supported limited CNS penetration. Pretreatment with selective CB1R or CB2R blockers suggested mainly CB1R contribution to an indene's antiallodynic effects. Therefore, this class of CB1R agonists holds promise as a viable treatment for neuropathic pain.

Design, synthesis and biological evaluation of novel analgesic agents targeting both cyclooxygenase and TRPV1

Multitarget-directed ligands might offer certain advantages over traditional single-target drugs and/or drug combinations. In the present study, a series of novel analgesic agents targeting both cyclooxygenase and TRPV1 were prepared and evaluated in an effort to optimize properties of previously described lead compounds from piperazine, ethanediamine cores. These compounds were evaluated for antagonism of hTRPV1 activation by capsaicin and the ability to inhibit Ovine COX-1 and human recombinant COX-2 in vitro. The favorable potentials of these test compounds were further characterized in preliminary analgesic and side-effects tests in vivo. On the basis of comprehensive evaluations, compound 8d which showed strong TRPV1 antagonistic activity, middle COX-2 inhibition, weak ulcerogenic action and had no hyperthermia side-effect was considered as a safe candidate for the further development of analgesic drugs.

CB1 Knockout Mice Unveil Sustained CB2-Mediated Antiallodynic Effects of the Mixed CB1/CB2 Agonist CP55,940 in a Mouse Model of Paclitaxel-Induced Neuropathic Pain

Cannabinoids suppress neuropathic pain through activation of cannabinoid CB1 and/or CB2 receptors; however, unwanted CB1-mediated cannabimimetic effects limit clinical use. We asked whether CP55,940 [(-)-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-4-(3-hydroxypropyl)cyclohexanol], a potent cannabinoid that binds with similar affinity to CB1 and CB2 in vitro, produces functionally separable CB1- and CB2-mediated pharmacological effects in vivo. We evaluated antiallodynic effects, possible tolerance, and cannabimimetic effects (e.g., hypothermia, catalepsy, CB1-dependent withdrawal signs) after systemic CP55,940 treatment in a mouse model of toxic neuropathy produced by a chemotherapeutic agent, paclitaxel. The contribution of CB1 and CB2 receptors to in vivo actions of CP55,940 was evaluated using CB1 knockout (KO), CB2KO, and wild-type (WT) mice. Low-dose CP55,940 (0.3 mg/kg daily, i.p. ) suppressed paclitaxel-induced allodynia in WT and CB2KO mice, but not CB1KO mice. Low-dose CP55,940 also produced hypothermia and rimonabant-precipitated withdrawal in WT, but not CB1KO, mice. In WT mice, tolerance developed to CB1-mediated hypothermic effects of CP55,940 earlier than to antiallodynic effects. High-dose CP55,940 (10 mg/kg daily, i.p.) produced catalepsy in WT mice, which precluded determination of antiallodynic efficacy but produced sustained CB2-mediated suppression of paclitaxel-induced allodynia in CB1KO mice; these antiallodynic effects were blocked by the CB2 antagonist 6-iodopravadoline (AM630). High-dose CP55,940 did not produce hypothermia or rimonabant-precipitated withdrawal in CB1KO mice. Our results using the mixed CB1/CB2 agonist CP55,940 document that CB1 and CB2 receptor activations produce mechanistically distinct suppression of neuropathic pain. Our study highlights the therapeutic potential of targeting cannabinoid CB2 receptors to bypass unwanted central effects associated with CB1 receptor activation.

A lipid gate for the peripheral control of pain

Cells in injured and inflamed tissues produce a number of proalgesic lipid-derived mediators, which excite nociceptive neurons by activating selective G-protein-coupled receptors or ligand-gated ion channels. Recent work has shown that these proalgesic factors are counteracted by a distinct group of lipid molecules that lower nociceptor excitability and attenuate nociception in peripheral tissues. Analgesic lipid mediators include endogenous agonists of cannabinoid receptors (endocannabinoids), lipid-amide agonists of peroxisome proliferator-activated receptor-α, and products of oxidative metabolism of polyunsaturated fatty acids via cytochrome P450 and other enzyme pathways. Evidence indicates that these lipid messengers are produced and act at different stages of inflammation and the response to tissue injury, and may be part of a peripheral gating mechanism that regulates the access of nociceptive information to the spinal cord and the brain. Growing knowledge about this peripheral control system may be used to discover safer medicines for pain.

Cannabinoid receptor 2 agonist attenuates pain related behavior in rats with chronic alcohol/high fat diet induced pancreatitis

Background

Chronic Pancreatitis (CP) is a complex and multifactorial syndrome. Many contributing factors result in development of dysfunctional pain in a significant number of patients. Drugs developed to treat a variety of pain states fall short of providing effective analgesia for patients with chronic pancreatitis, often providing minimal to partial pain relief over time with significant side effects. Recently, availability of selective pharmacological tools has enabled great advances in our knowledge of the role of the cannabinoid receptors in pathophysiology. In particular, cannabinoid receptor 2 (CB2) has emerged as an attractive target for management of chronic pain, as demonstrated in several studies with inflammatory and neuropathic preclinical pain models. In this study, the analgesic efficacy of a novel, highly selective CB2 receptor agonist, LY3038404 HCl, is investigated in a chronic pancreatitis pain model, induced with an alcohol/high fat (AHF) diet.

Results

Rats fed the AHF diet developed visceral pain-like behaviors detectable by week 3 and reached a maximum at week 5 that persists as long as the diet is maintained. Rats with AHF induced chronic pancreatitis were treated with LY3038404 HCl (10 mg/kg, orally, twice a day for 9 days). The treated animals demonstrated significantly alleviated pain related behaviors after 3 days of dosing, including increased paw withdrawal thresholds (PWT), prolonged abdominal withdrawal latencies (ABWL), and decreased nocifensive responses to noxious 44°C hotplate stimuli. Terminal histological analysis of pancreatic tissue sections from the AHF chronic pancreatitis animals demonstrated extensive injury, including a global pancreatic gland degeneration (cellular atrophy), vacuolization (fat deposition), and fibrosis. After the LY3038404 HCl treatment, pancreatic tissue was significantly protected from severe damage and fibrosis. LY3038404 HCl affected neither open field exploratory behaviors nor dark/light box preferences as measures of higher brain and motor functions.

Conclusion

LY3038404 HCl, a potent CB2 receptor agonist, possesses tissue protective and analgesic properties without effects on higher brain function. Thus, activation of CB2 receptors is suggested as a potential therapeutic target for visceral inflammation and pain management.

No more pain upon Gq-protein-coupled receptor activation: role of endocannabinoids

Marijuana has been used to relieve pain for centuries. The analgesic mechanism of its constituents, the cannabinoids, was only revealed after the discovery of cannabinoid receptors (CB1 and CB2) two decades ago. The subsequent identification of the endocannabinoids, anandamide and 2-arachidonoylglycerol (2-AG), and their biosynthetic and degradation enzymes discloses the therapeutic potential of compounds targeting the endocannabinoid system for pain control. Inhibitors of the anandamide and 2-AG degradation enzymes, fatty acid amide hydrolase and monoacylglycerol lipase, respectively, may be superior to direct cannabinoid receptor ligands as endocannabinoids are synthesized on demand and rapidly degraded, focusing action at generating sites. Recently, a promising strategy for pain relief was revealed in the periaqueductal gray (PAG). It is initiated by Gq-protein-coupled receptor (Gq PCR) activation of the phospholipase C-diacylglycerol lipase enzymatic cascade, generating 2-AG that produces inhibition of GABAergic transmission (disinhibition) in the PAG, thereby leading to analgesia. Here, we introduce the antinociceptive properties of exogenous cannabinoids and endocannabinoids, involving their biosynthesis and degradation processes, particularly in the PAG. We also review recent studies disclosing the Gq PCR-phospholipase C-diacylglycerol lipase-2-AG retrograde disinhibition mechanism in the PAG, induced by activating several Gq PCRs, including metabotropic glutamatergic (type 5 metabotropic glutamate receptor), muscarinic acetylcholine (M1/M3), and orexin 1 receptors. Disinhibition mediated by type 5 metabotropic glutamate receptor can be initiated by glutamate transporter inhibitors or indirectly by substance P, neurotensin, cholecystokinin and capsaicin. Finally, the putative role of 2-AG generated after activating the above neurotransmitter receptors in stress-induced analgesia is discussed.

Anti-inflammatory Effect of Isaria sinclairii Glycosaminoglycan in an Adjuvant-treated Arthritis Rat Model

The anti-inflammatory effects of glycosaminoglycan (GAG) derived from Isaria sinclairii (IS) and of IS extracts were investigated in a complete Freund’s adjuvant (CFA)-treated chronic arthritis rat model. Groups of rats were treated orally with 30 mg/kg one of the following: [1] saline control, extracts of [2] water-IS, [3] methanol-IS, [4] butanol-IS, [5] ethyl acetate-IS, or [6] Indomethacin® as the positive control for a period of two weeks. The anti-paw edema effects of the individual extracts were in the following order: water-IS ex. > methanol ex. > butanol ex. > ethyl acetate ex. The water/methanol extract from I. sinclairii remarkably inhibited UV-mediated upregulation of NF-κB activity in transfected HaCaT cells. GAG as a water-soluble alcohol precipitated fraction also produced a noticeable anti-edema effect. This GAG also inhibited the pro-inflammatory cytokine levels of prostaglandin E₂-stimulated lipopolysaccharide in LAW 264.7 cells, cytokine TNF-α production in splenocytes, and atherogenesis cytokine levels of vascular endothelial growth factor (VEGF) production in HUVEC cells in a dose-dependent manner. In the histological analysis, the LV dorsal root ganglion, including the articular cartilage, and linked to the paw-treated IS GAG, was repaired against CFA-induced cartilage destruction. Combined treatment with Indomethacin® (5 mg/kg) and IS GAG (10 mg/kg) also more effectively inhibited CFA-induced paw edema at 3 hr, 24 hr, and 48 hr to levels comparable to the anti-inflammatory drug, indomethacin. Thus, the IS GAG described here holds great promise as an anti-inflammatory drug in the future.

Anti-inflammatory effect of glycosaminoglycan derived from Gryllus bimaculatus (a type of cricket, insect) on adjuvant-treated chronic arthritis rat model

Anti-inflammatory effects of glycosaminoglycan (GAG) derived from cricket (Gryllus bimaculatus, Gb) were investigated in a complete Freund's adjuvant (CFA)-treated chronic arthritic rat model. This GAG produced a significant anti-edema effect as evidenced by inhibition of C-reactive protein (CRP) and rheumatoid factor, and interfered with atherogenesis by reducing proinflammatory cytokine levels of (1) vascular endothelial growth factor (VEGF) production in human umbilical vein endothelial cells (HUVEC), (2) interleukin-6, (3) prostaglandin E2-stimulated lipopolysaccharide in RAW 264.7 cells, and (4) tumor necrosis factor (TNF)-α production in normal splenocytes, in a dose-dependent manner. This GAG was also found to induce nitric oxide (NO) production in HUVEC cells and elevated endothelial nitric oxide synthase (eNOS) activity levels. Histological findings demonstrated the fifth lumbar vertebrae (LV) dorsal root ganglion, which was linked to the paw treated with Gb GAG, was repaired against CFA-induced cartilage destruction. Further, combined indomethacin (5 mg/kg)-Gb GAG (10 mg/kg) inhibited more effectively CFA-induced paw edema at 3 h and 2 or 3 d after treatment to levels comparable to only the anti-inflammatory drug indomethacin. Ultraviolet (UV)-irritated skin inflammation also downregulated nuclear factor κB (NFκB) activity in transfected HaCaT cells. Data suggest that the anti-inflammatory effects of GAG obtained from cricket (Gb) may be useful for treatment of inflammatory diseases including chronic arthritis.

Targeting the cannabinoid system for pain relief?

Marijuana has been used to relieve pain for centuries, but its analgesic mechanism has only been understood during the past two decades. It is mainly mediated by its constituents, cannabinoids, through activating central cannabinoid 1 (CB1) receptors, as well as peripheral CB1 and CB2 receptors. CB2-selective agonists have the benefit of lacking CB1 receptor-mediated CNS side effects. Anandamide and 2-arachidonoylglycerol (2-AG) are two intensively studied endogenous lipid ligands of cannabinoid receptors, termed endocannabinoids, which are synthesized on demand and rapidly degraded. Thus, inhibitors of their degradation enzymes, fatty acid amide hydrolase and monoacylglycerol lipase (MAGL), respectively, may be superior to direct cannabinoid receptor ligands as a promising strategy for pain relief. In addition to the antinociceptive properties of exogenous cannabinoids and endocannabinoids, involving their biosynthesis and degradation processes, we also review recent studies that revealed a novel analgesic mechanism, involving 2-AG in the periaqueductal gray (PAG), a midbrain region for initiating descending pain inhibition. It is initiated by Gq-protein-coupled receptor (GqPCR) activation of the phospholipase C (PLC)-diacylglycerol lipase (DAGL) enzymatic cascade, generating 2-AG that produces inhibition of GABAergic transmission (disinhibition) in the PAG, thereby leading to analgesia. This GqPCR-PLC-DAGL-2-AG retrograde disinhibition mechanism in the PAG can be initiated by activating type 5 metabotropic glutamate receptor (mGluR5), muscarinic acetylcholine (M1/M3), and orexin (OX1) receptors. mGluR5-mediated disinhibition can be initiated by glutamate transporter inhibitors, or indirectly by substance P, neurotensin, cholecystokinin, capsaicin, and AM404, the bioactive metabolite of acetaminophen in the brain. The putative role of 2-AG generated after activating the above neurotransmitter receptors in stress-induced analgesia is also discussed.

P2X7 receptor mediates activation of microglial cells in prostate of chemically irritated rats

PURPOSE

Evidence shows that adenosine triphosphate (ATP) is involved in the transmission of multiple chronic pain via P2X7 receptor. This study was to investigate the P2X7 and microglial cells in the chronic prostatitis pain.

MATERIALS AND METHODS

Rats were divided into control group and chronic prostatitis group (n = 24 per group). A chronic prostatitis animal model was established by injecting complete Freund's adjuvant (CFA) to the prostate of rats, and the thermal withdrawal latency (TWL) was detected on days 0, 4, 12 and 24 (n = 6 at each time point in each group). Animals were sacrificed and the pathological examination of the prostate, detection of mRNA expression of P2X7 and ionized calcium binding adaptor molecule 1 (IBA-1) and measurement of content of tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) in the dorsal horn of L5-S2 spinal cord were performed on days 0, 4, 12 and 24. In addition, the content of TNF-α and IL-1β in the dorsal horn of L5-S2 spinal cord was measured after intrathecal injection of inhibitors of microglial cells and/or P2X7 for 5 days.

RESULTS

The chronic prostatitis was confirmed by pathological examination. The expression of P2X7 and IBA-1 and the content of TNF-α and IL-1β in rats with chronic prostatitis were significantly higher than those in the control group. On day 4, the expressions of pro-inflammatory cytokines became to increase, reaching a maximal level on day 12 and started to reduce on day 24, but remained higher than that in the control group. Following suppression of microglial cells and P2X7 receptor, the secretion of TNF-α and IL-1β was markedly reduced.

CONCLUSION

In chronic prostatitis pain, the microglial cells and P2X7 receptor are activated resulting in the increased expression of TNF-α and IL-1β in the L5-S2 spinal cord, which might attribute to the maintenance and intensification of pain in chronic prostatitis.

Modulating the endocannabinoid system in human health and disease--successes and failures

The discovery of the endocannabinoid system, comprising the G-protein coupled cannabinoid 1 and 2 receptors (CB1/2), their endogenous lipid ligands or endocannabinoids, and synthetic and metabolizing enzymes, has triggered an avalanche of experimental studies implicating the endocannabinoid system in a growing number of physiological/pathological functions. These studies have also suggested that modulating the activity of the endocannabinoid system holds therapeutic promise for a broad range of diseases, including neurodegenerative, cardiovascular and inflammatory disorders; obesity/metabolic syndrome; cachexia; chemotherapy-induced nausea and vomiting; and tissue injury and pain, amongst others. However, clinical trials with globally acting CB1 antagonists in obesity/metabolic syndrome, and other studies with peripherally-restricted CB1/2 agonists and inhibitors of the endocannabinoid metabolizing enzyme in pain, have introduced unexpected complexities, suggesting that a better understanding of the pathophysiological role of the endocannabinoid system is required to devise clinically successful treatment strategies.

Characterization of cannabinoid-induced relief of neuropathic pain in a rat model of cisplatin-induced neuropathy

Clinical use of antineoplastic drugs is associated with the development of numerous adverse effects that many patients find intolerable, including peripheral neuropathy. Cannabinoids have relieved neuropathic pain in different animal models. But their therapeutic activities could be affected by their psychoactive properties. The aim of this work was to determine the effect of cannabinoids in cisplatin-evoked neuropathy. For this purpose, the non-selective agonist WIN 55,212-2 (WIN), the CB1-selective agonist ACEA or the CB2-selective agonist JWH133 (or their vehicle) was either systemically administered at a non-psychoactive dose or locally injected in cisplatin-treated rats. Selective CB1 and CB2 cannabinoid antagonists (AM251 and SR144528, respectively) were used to characterize cannabinoid effects. Cisplatin-treated rats showed mechanical allodynia but not thermal hyperalgesia. Cannabinoid agonists alleviated mechanical allodynia. This effect was mediated by both CB1 and CB2 cannabinoid receptors when the cannabinoid was systemically applied. At the dose used, cannabinoid agonists had no psychoactive effect. The local effect of the drug involved the activation of peripheral CB1 receptors whereas involvement of CB2 receptors was less clear. In a rat model of cisplatin-induced neuropathy, cannabinoids have an antinociceptive effect, but the cannabinoid receptors involved could be different depending on the route of administration. Non-psychoactive doses of cannabinoid agonists are capable of alleviating the signs of peripheral neuropathy when systemically applied. Interestingly, local administration of selective CB1 agonists or systemic administration of CB2 agonists, which are non-psychoactive, may serve as new therapeutic alternatives for symptom management in painful neuropathy associated with cisplatin treatment.

The role of endocannabinoids in pain modulation

The endocannabinoid system (ES) is comprised of cannabinoid (CB) receptors, their endogenous ligands (endocannabinoids), and proteins responsible for their metabolism. Endocannabinoids serve as retrograde signaling messengers in GABAergic and glutamatergic synapses, as well as modulators of postsynaptic transmission, that interact with other neurotransmitters. Physiological stimuli and pathological conditions lead to differential increases in brain endocannabinoids that regulate distinct biological functions. Furthermore, endocannabinoids modulate neuronal, glial, and endothelial cell function and exert neuromodulatory, anti-excitotoxic, anti-inflammatory, and vasodilatory effects. Analgesia is one of the principal therapeutic targets of cannabinoids. Cannabinoid analgesia is based on the suppression of spinal and thalamic nociceptive neurons, but peripheral sites of action have also been identified. The chronic pain that occasionally follows peripheral nerve injury differs fundamentally from inflammatory pain and is an area of considerable unmet therapeutic need. Over the last years, considerable progress has been made in understanding the role of the ES in the modulation of pain. Endocannabinoids have been shown to behave as analgesics in models of both acute nociception and clinical pain such as inflammation and painful neuropathy. The framework for such analgesic effects exists in the CB receptors, which are found in areas of the nervous system important for pain processing and in immune cells that regulate the neuro-immune interactions that mediate the inflammatory hyperalgesia. The purpose of this review is to present the available research and clinical data, up to date, regarding the ES and its role in pain modulation, as well as its possible therapeutic perspectives.

Effects of gonadal hormones on the peripheral cannabinoid receptor 1 (CB1R) system under a myositis condition in rats

In this study, we assessed the effects of peripherally administered cannabinoids in an orofacial myositis model, and the role of sex hormones in cannabinoid receptor (CBR) expression in trigeminal ganglia (TG). Peripherally administered arachidonylcyclopropylamide (ACPA), a specific CB1R agonist, significantly attenuated complete Freund's adjuvant (CFA)-induced mechanical hypersensitivity in the masseter muscle in male rats. The ACPA effect was blocked by a local administration of AM251, a specific CB1R antagonist, but not by AM630, a specific CB2R antagonist. In female rats, a 30-fold higher dose of ACPA was required to produce a moderate reduction in mechanical hypersensitivity. CFA injected in masseter muscle significantly upregulated CB1R mRNA expression in TG in male, but not in female, rats. There was a close correlation between the CB1R mRNA levels in TG and the antihyperalgesic effect of ACPA. Interleukin (IL)-1β and IL-6, which are elevated in the muscle tissue following CFA treatment, induced a significant upregulation of CB1R mRNA expression in TG from male rats. The upregulation of CB1R was prevented in TG cultures from orchidectomized male rats, which was restored by the application of testosterone. The cytokines did not alter the CB1R mRNA level in TG from intact as well as ovariectomized female rats. Neither estradiol supplement nor estrogen receptor blockade had any effects on CB1R expression. These data indicate that testosterone, but not estradiol, is required for the regulation of CB1Rs in TG under inflammatory conditions, which provide explanations for the sex differences in the antihyperalgesic effects of peripherally administered cannabinoids.

The monoacylglycerol lipase inhibitor JZL184 suppresses inflammatory pain in the mouse carrageenan model

AIM

The present study tested whether the selective monoacylglycerol lipase (MAGL) inhibitor JZL184 would reduce allodynia and paw edema in the carrageenan test.

MAIN METHODS

The anti-edematous and anti-allodynic effects of JZL184 were compared to those of PF-3845, an inhibitor of fatty acid amide hydrolase (FAAH), and diclofenac, a non-selective cyclooxygenase inhibitor. Cannabinoid receptor involvement in the anti-edematous and anti-allodynic effects of JZL184 was evaluated by administration of the respective CB1 and CB2 receptor antagonists rimonabant and SR144528 as well as with CB1(-/-) and CB2(-/-) mice. JZL184 (1.6, 4, 16, or 40mg/kg) was administered for six days to assess tolerance.

KEY FINDINGS

JZL184 administered before or after carrageenan significantly attenuated carrageenan-induced paw edema and mechanical allodynia. Complementary genetic and pharmacological approaches revealed that the anti-allodynic effects of JZL184 required both CB1 and CB2 receptors, but only CB2 receptors mediated its anti-edematous actions. Importantly, both the anti-edematous and anti-allodynic effects underwent tolerance following repeated injections of high dose JZL184 (16 or 40mg/kg), but repeated administration of low dose JZL184 (4mg/kg) retained efficacy.

SIGNIFICANCE

These results suggest that the MAGL inhibitor JZL184 reduces inflammatory nociception through the activation of both CB1 and CB2 receptors, with no evidence of tolerance following repeated administration of low doses.

Characterization of cannabinoid-induced relief of neuropathic pain in rat models of type 1 and type 2 diabetes

Diabetic neuropathy is a frequent complication of diabetes mellitus with a tremendous impact on patients' quality of life, and it remains poorly treated. Cannabinoids relieve the signs of diabetic neuropathy in different experimental models, including streptozotocin- (STZ-) induced type 1 diabetic rodents, and they may also relieve neuropathic signs in type 2 diabetic animals. This study compares the effect of the non-selective cannabinoid agonist WIN 55,212-2 (WIN) in Zucker Diabetic Fatty (ZDF) rats (type 2 diabetes) and in STZ-injected Wistar rats (type 1 diabetes). WIN (or its vehicle) was either systemically administered at a non-psychoactive dose or locally injected. Selective CB1 and CB2 cannabinoid antagonists were used to characterize WIN antineuropathic effects. Both type 1 and type 2 diabetic rats showed mechanical allodynia but not thermal hyperalgesia. WIN alleviated mechanical allodynia in both models of diabetes. In STZ-treated rats, both cannabinoid receptors were involved, whereas in ZDF rats, WIN effects seemed to mainly involve the activation of CB1 receptors. Higher doses of WIN were needed to significantly relieve mechanical allodynia upon intraplantar administration in ZDF vs. STZ-injected rats. Cannabinoids, acting on systemic and/or peripheral receptors, may serve as a new therapeutic alternative for symptom management in painful neuropathy associated with both type 1 and type 2 diabetes. Additionally, our results highlight the need for appropriate selection of diabetic experimental models because the results from studies in STZ-induced diabetic rodents might not be applicable in all diabetic situations.

Effects of anandamide in migraine: data from an animal model

Systemic nitroglycerin (NTG) produces spontaneous-like migraine attacks in migraine sufferers and induces a condition of hyperalgesia in the rat 4 h after its administration. Endocannabinoid system seems to be involved in the modulation of NTG-induced hyperalgesia, and probably, in the pathophysiological mechanisms of migraine. In this study, the analgesic effect of anandamide (AEA) was evaluated by means of the formalin test, performed in baseline conditions and following NTG-induced hyperalgesia in male Sprague-Dawley rats. AEA was administered 30 min before the formalin injection. In addition, the effect of AEA (administered 30 min before NTG injection) was investigated on NTG-induced Fos expression and evaluated 4 h following NTG injection. AEA induced a significant decrease in the nociceptive behavior during both phases of the formalin test in the animals treated with vehicle, while it abolished NTG-induced hyperalgesia during the phase II. Pre-treatment with AEA significantly reduced the NTG-induced neuronal activation in nucleus trigeminalis caudalis, confirming the results obtained in our previous study, and in area postrema, while the same treatment induced an increase of Fos expression in paraventricular and supraoptic nuclei of the hypothalamus, parabrachial nucleus, and periaqueductal grey. The study confirms that a dysfunction of the endocannabinoid system may contribute to the development of migraine attacks and that a pharmacological modulation of CB receptors can be useful for the treatment of migraine pain.

Is lipid signaling through cannabinoid 2 receptors part of a protective system?

The mammalian body has a highly developed immune system which guards against continuous invading protein attacks and aims at preventing, attenuating or repairing the inflicted damage. It is conceivable that through evolution analogous biological protective systems have been evolved against non-protein attacks. There is emerging evidence that lipid endocannabinoid signaling through cannabinoid 2 (CB₂) receptors may represent an example/part of such a protective system/armamentarium. Inflammation/tissue injury triggers rapid elevations in local endocannabinoid levels, which in turn regulate signaling responses in immune and other cells modulating their critical functions. Changes in endocannabinoid levels and/or CB₂ receptor expressions have been reported in almost all diseases affecting humans, ranging from cardiovascular, gastrointestinal, liver, kidney, neurodegenerative, psychiatric, bone, skin, autoimmune, lung disorders to pain and cancer, and modulating CB₂ receptor activity holds tremendous therapeutic potential in these pathologies. While CB₂ receptor activation in general mediates immunosuppressive effects, which limit inflammation and associated tissue injury in large number of pathological conditions, in some disease states activation of the CB₂ receptor may enhance or even trigger tissue damage, which will also be discussed alongside the protective actions of the CB₂ receptor stimulation with endocannabinoids or synthetic agonists, and the possible biological mechanisms involved in these effects.

Paradoxical effects of the cannabinoid CB2 receptor agonist GW405833 on rat osteoarthritic knee joint pain

OBJECTIVE

The present study examined whether local administration of the cannabinoid-2 (CB(2)) receptor agonist GW405833 could modulate joint nociception in control rat knee joints and in an animal model of osteoarthritis (OA).

METHOD

OA was induced in male Wistar rats by intra-articular injection of sodium monoiodo-acetate with a recovery period of 14 days. Immunohistochemistry was used to evaluate the expression of CB(2) and transient receptor potential vanilloid channel-1 (TRPV1) receptors in the dorsal root ganglion (DRG) and synovial membrane of sham- and sodium mono-iodoacetate (MIA)-treated animals. Electrophysiological recordings were made from knee joint primary afferents in response to rotation of the joint both before and following close intra-arterial injection of different doses of GW405833. The effect of intra-articular GW405833 on joint pain perception was determined by hindlimb incapacitance. An in vitro neuronal release assay was used to see if GW405833 caused release of an inflammatory neuropeptide (calcitonin gene-related peptide - CGRP).

RESULTS

CB(2) and TRPV1 receptors were co-localized in DRG neurons and synoviocytes in both sham- and MIA-treated animals. Local application of the GW405833 significantly reduced joint afferent firing rate by up to 31% in control knees. In OA knee joints, however, GW405833 had a pronounced sensitising effect on joint mechanoreceptors. Co-administration of GW405833 with the CB(2) receptor antagonist AM630 or pre-administration of the TRPV1 ion channel antagonist SB366791 attenuated the sensitising effect of GW405833. In the pain studies, intra-articular injection of GW405833 into OA knees augmented hindlimb incapacitance, but had no effect on pain behaviour in saline-injected control joints. GW405833 evoked increased CGRP release via a TRPV1 channel-dependent mechanism.

CONCLUSION

These data indicate that GW405833 reduces the mechanosensitivity of afferent nerve fibres in control joints but causes nociceptive responses in OA joints. The observed pro-nociceptive effect of GW405833 appears to involve TRPV1 receptors.

Anandamide suppresses pain initiation through a peripheral endocannabinoid mechanism

Peripheral cannabinoid receptors exert a powerful inhibitory control over pain initiation, but the endocannabinoid signal that normally engages this intrinsic analgesic mechanism is unknown. To address this question, we developed a peripherally restricted inhibitor (URB937) of fatty acid amide hydrolase (FAAH), the enzyme responsible for the degradation of the endocannabinoid anandamide. URB937 suppressed FAAH activity and increased anandamide levels outside the rodent CNS. Despite its inability to access brain and spinal cord, URB937 attenuated behavioral responses indicative of persistent pain in rodent models of peripheral nerve injury and inflammation and prevented noxious stimulus-evoked neuronal activation in spinal cord regions implicated in nociceptive processing. CB₁ cannabinoid receptor blockade prevented these effects. These results suggest that anandamide-mediated signaling at peripheral CB₁ receptors controls the access of pain-related inputs to the CNS. Brain-impenetrant FAAH inhibitors, which strengthen this gating mechanism, might offer a new approach to pain therapy.

A peripherally restricted cannabinoid receptor agonist produces robust anti-nociceptive effects in rodent models of inflammatory and neuropathic pain

Cannabinoids are analgesic in man, but their use is limited by their psychoactive properties. One way to avoid cannabinoid receptor subtype 1 (CB1R)-mediated central side-effects is to develop CB1R agonists with limited CNS penetration. Activation of peripheral CB1Rs has been proposed to be analgesic, but the relative contribution of peripheral CB1Rs to the analgesic effects of systemic cannabinoids remains unclear. Here we addressed this by exploring the analgesic properties and site of action of AZ11713908, a peripherally restricted CB1R agonist, in rodent pain models. Systemic administration of AZ11713908 produced robust efficacy in rat pain models, comparable to that produced by WIN 55, 212-2, a CNS-penetrant, mixed CB1R and CB2R agonist, but AZ11713908 generated fewer CNS side-effects than WIN 55, 212-in a rat Irwin test. Since AZ11713908 is also a CB2R inverse agonist in rat and a partial CB2R agonist in mouse, we tested the specificity of the effects in CB1R and CB2R knock-out (KO) mice. Analgesic effects produced by AZ11713908 in wild-type mice with Freund's complete adjuvant-induced inflammation of the tail were completely absent in CB1R KO mice, but fully preserved in CB2R KO mice. An in vivo electrophysiological assay showed that the major site of action of AZ11713908 was peripheral. Similarly, intraplantar AZ11713908 was also sufficient to induce robust analgesia. These results demonstrate that systemic administration of AZ11713908, produced robust analgesia in rodent pain models via peripheral CB1R. Peripherally restricted CB1R agonists provide an interesting novel approach to analgesic therapy for chronic pain.

Inhibitors of monoacylglycerol lipase, fatty-acid amide hydrolase and endocannabinoid transport differentially suppress capsaicin-induced behavioral sensitization through peripheral endocannabinoid mechanisms

Monoacylglycerol lipase (MGL) and fatty-acid amide hydrolase (FAAH) degrade the endocannabinoids 2-arachidonoylglycerol (2-AG) and anandamide (AEA), respectively. Pharmacological inhibition of these enzymes in the periphery may elucidate the role of endocannabinoids in controlling nociceptive transmission. We compared effects of the MGL inhibitor JZL184, the FAAH inhibitor URB597, and the endocannabinoid uptake inhibitor VDM11, administered locally in the paw, on behavioral hypersensitivities produced by capsaicin, the pungent ingredient in hot chili peppers. Intradermal capsaicin (10 microg i.pl.) produced nocifensive behavior, thermal hyperalgesia, and mechanical allodynia in rats. JZL184 (100 microg i.pl.) suppressed capsaicin-induced nocifensive behavior and thermal hyperalgesia without altering capsaicin-evoked mechanical allodynia. Effects of JZL184 were blocked by either the CB(1) antagonist AM251 (80 microg i.pl.) or the CB(2) antagonist AM630 (25 microg i.pl.). URB597 (75 microg i.pl.) suppressed capsaicin-induced mechanical allodynia without altering capsaicin-evoked thermal hyperalgesia or nocifensive behavior. Effects of URB597 were blocked by AM251 (80 microg i.pl.), but not by AM630 (25 microg i.pl.). VDM11 (100 microg i.pl.) suppressed capsaicin-evoked hypersensitivity for all three dependent measures (nocifensive behavior, thermal hyperalgesia, and mechanical allodynia), suggesting an additive effect following putative elevation of both AEA and 2-AG. The VDM11-induced suppression of capsaicin-evoked nocifensive behavior and thermal hyperalgesia was blocked by either AM251 (80 microg i.pl.) or AM630 (25 microg i.pl.), as observed with JZL184. The VDM11-induced suppression of capsaicin-evoked mechanical allodynia was blocked by AM251 (25 microg i.pl.) only, as observed with URB597. Thus, peripheral inhibition of enzymes hydrolyzing 2-AG and AEA suppresses capsaicin-evoked behavioral sensitization with distinct patterns of pharmacological specificity and in a non-overlapping and modality-specific manner. Modulation of endocannabinoids in the periphery suppressed capsaicin-evoked nocifensive behavior and thermal hyperalgesia through either CB(1) or CB(2) receptor mechanisms but suppressed capsaicin-evoked mechanical allodynia through CB(1) mechanisms only. Inhibition of endocannabinoid transport was more effective in suppressing capsaicin-induced sensitization compared to inhibition of either FAAH or MGL alone. These studies are the first to unveil the effects of pharmacologically increasing peripheral endocannabinoid levels on capsaicin-induced behavioral hypersensitivities. Our data suggest that 2-AG, the putative product of MGL inhibition, and AEA, the putative product of FAAH inhibition, differentially suppress capsaicin-induced nociception through peripheral cannabinoid mechanisms.

Regulation of inflammatory pain by inhibition of fatty acid amide hydrolase

Although cannabinoids are efficacious in laboratory animal models of inflammatory pain, their established cannabimimetic actions diminish enthusiasm for their therapeutic development. Conversely, fatty acid amide hydrolase (FAAH), the chief catabolic enzyme regulating the endogenous cannabinoid N-arachidonoylethanolamine (anandamide), has emerged as an attractive target for treating pain and other conditions. Here, we tested WIN 55212-2 [(R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de)-1,4-benzoxazin-6-yl]-1-napthalenylmethanone], a cannabinoid receptor agonist, and genetic deletion or pharmacological inhibition of FAAH in the lipopolysaccharide (LPS) mouse model of inflammatory pain. WIN 55212-2 significantly reduced edema and hot-plate hyperalgesia caused by LPS infusion into the hind paws, although the mice also displayed analgesia and other central nervous system effects. FAAH(-/-) mice exhibited reduced paw edema and hyperalgesia in this model without apparent cannabimimetic effects. Transgenic mice expressing FAAH exclusively on neurons continued to display the antiedematous, but not the antihyperalgesic, phenotype. The CB(2) cannabinoid receptor (CB(2)) antagonist SR144528 [N-[(1S)-endo-1,3,3-trimethyl bicyclo [2.2.1] heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide] blocked this non-neuronal, anti-inflammatory phenotype, and the CB(1) cannabinoid receptor (CB(1)) antagonist rimonabant [SR141716, N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide] blocked the antihyperalgesic phenotype. The FAAH inhibitor URB597 [cyclohexylcarbamic acid 3'-carbamoylbiphenyl-3-yl ester] attenuated the development of LPS-induced paw edema and reversed LPS-induced hyperalgesia through the respective CB(2) and CB(1) mechanisms of action. However, the transient receptor potential vanilloid type 1 antagonist capsazepine did not affect either the antihyperalgesic or antiedematous effects of URB597. Finally, URB597 attenuated levels of the proinflammatory cytokines interleukin-1beta and tumor necrosis factor alpha in LPS-treated paws. These findings demonstrate that simultaneous elevations in non-neuronal and neuronal endocannabinoid signaling are possible through inhibition of a single enzymatic target, thereby offering a potentially powerful strategy for treating chronic inflammatory pain syndromes that operate at multiple levels of anatomical integration.

Effects of cannabinor, a novel selective cannabinoid 2 receptor agonist, on bladder function in normal rats

BACKGROUND

Cannabinoid (CB) receptors may be involved in the control of bladder function; the role of CB receptor subtypes in micturition has not been established.

OBJECTIVES

Our aim was to evaluate the effects of cannabinor, a novel CB2 receptor agonist, on rat bladder function.

DESIGN, SETTING, AND PARTICIPANTS

Sprague Dawley rats were used. Distribution of CB2 receptors in sensory and cholinergic nerves of the detrusor was studied. Selectivity of cannabinor for human and rat CB receptors was evaluated. Effects of cannabinor on rat detrusor and micturition were investigated.

MEASUREMENTS

Immunohistochemistry, radioligand binding, tritium outflow assays, organ bath studies of isolated bladder tissue, and cystometry in awake rats were used.

RESULTS AND LIMITATIONS

CB2 receptor immunoreactivity was expressed in the urothelium and in sensory and cholinergic bladder nerves. Cannabinor exhibited similar binding at human and rat CB2 receptors and a 321-fold functional selectivity for the CB2 receptor versus the CB1 receptor. Cannabinor had no effect on isolated detrusor muscle function. In vivo, cannabinor 3.0mg/kg increased micturition intervals and volumes by 52% (p<0.05) and 96% (p<0.01), respectively, and increased threshold and flow pressures by 73% (p<0.01) and 49% (p<0.001), respectively. Cannabinor 0.3 or 1.0mg/kg or vehicle did not affect urodynamic parameters.

CONCLUSIONS

Considering that CB2 receptors are localized on sensory nerves and on the urothelium and that cannabinor had effects on "afferent" urodynamic parameters, peripheral CB2 receptors may be involved in sensory functions of rat micturition. Effects of cannabinor on cholinergic nerve activity in normal bladder tissue appear to be limited.

Opioid receptor and NO/cGMP pathway as a mechanism of peripheral antinociceptive action of the cannabinoid receptor agonist anandamide

AIMS

In this study, we investigated whether the opioid system and the nitric oxide pathway were involved in the peripheral antinociception induced by a cannabinoid receptor agonist anandamide.

MAIN METHODS

Hyperalgesia was induced by a subcutaneous injection of carrageenan (250 microg) into the plantar surface of the rat's hindpaw and measured by the paw pressure test 3h after injection. The weight in grams (g) required to elicit a nociceptive response, paw flexion, was determined as the nociceptive threshold.

KEY FINDINGS

Anandamide elicited a dose-dependent (50, 75, and 100 ng per paw) antinociceptive effect. The highest dose of anandamide did not produce antihyperalgesia in the contralateral paw, indicating a peripheral site of action. The CB(1) receptor antagonist AM251 (20, 40, 80 and 160mug per paw) antagonized peripheral antihyperalgesia induced by anandamide (100 ng), in a dose-dependent manner, suggesting CB(1) receptor activation. Anandamide-induced peripheral antihyperalgesia was reverted by blockers of the l-arginine/NO/cGMP pathway N(G)-nitro-l-arginine (NOARG; 24, 36 and 48 microg per paw) and 1H-[1,2,4] Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 25, 50 and 100 microg per paw), in a dose-dependent manner. Furthermore, opioid receptor antagonist naloxone (12.5, 25 and 50 microg per paw) antagonized the peripheral antihyperalgesia induced by anandamide.

SIGNIFICANCE

This study provides evidence that the peripheral antinociceptive effect of the cannabinoid receptor agonist anandamide may result from l-arginine/NO/cGMP pathway activation and that the opioid system is also involved.

Peripheral and central sites of action for the non-selective cannabinoid agonist WIN 55,212-2 in a rat model of post-operative pain

BACKGROUND AND PURPOSE

Activation of cannabinoid (CB) receptors decreases nociceptive transmission in inflammatory or neuropathic pain states. However, the effects of CB receptor agonists in post-operative pain remain to be investigated. Here, we characterized the anti-allodynic effects of WIN 55,212-2 (WIN) in a rat model of post-operative pain.

EXPERIMENTAL APPROACH

WIN 55,212-2 was characterized in radioligand binding and in vitro functional assays at rat and human CB(1) and CB(2) receptors. Analgesic activity and site(s) of action of WIN were assessed in the skin incision-induced post-operative pain model in rats; receptor specificity was investigated using selective CB(1) and CB(2) receptor antagonists.

KEY RESULTS

WIN 55,212-2 exhibited non-selective affinity and agonist efficacy at human and rat CB(1) versus CB(2) receptors. Systemic administration of WIN decreased injury-induced mechanical allodynia and these effects were reversed by pretreatment with a CB(1) receptor antagonist, but not with a CB(2) receptor antagonist, given by systemic, intrathecal and supraspinal routes. In addition, peripheral administration of both CB(1) and CB(2) antagonists blocked systemic WIN-induced analgesic activity.

CONCLUSIONS AND IMPLICATIONS

Both CB(1) and CB(2) receptors were involved in the peripheral anti-allodynic effect of systemic WIN in a pre-clinical model of post-operative pain. In contrast, the centrally mediated anti-allodynic activity of systemic WIN is mostly due to the activation of CB(1) but not CB(2) receptors at both the spinal cord and brain levels. However, the increased potency of WIN following i.c.v. administration suggests that its main site of action is at CB(1) receptors in the brain.

Targeting CB2 receptors and the endocannabinoid system for the treatment of pain

The endocannabinoid system consists of the cannabinoid (CB) receptors, CB(1) and CB(2), the endogenous ligands anandamide (AEA, arachidonoylethanolamide) and 2-arachidonoylglycerol (2-AG), and their synthetic and metabolic machinery. The use of cannabis has been described in classical and recent literature for the treatment of pain, but the potential for psychotropic effects as a result of the activation of central CB(1) receptors places a limitation upon its use. There are, however, a number of modern approaches being undertaken to circumvent this problem, and this review represents a concise summary of these approaches, with a particular emphasis upon CB(2) receptor agonists. Selective CB(2) agonists and peripherally restricted CB(1) or CB(1)/CB(2) dual agonists are being developed for the treatment of inflammatory and neuropathic pain, as they demonstrate efficacy in a range of pain models. CB(2) receptors were originally described as being restricted to cells of immune origin, but there is evidence for their expression in human primary sensory neurons, and increased levels of CB(2) receptors reported in human peripheral nerves have been seen after injury, particularly in painful neuromas. CB(2) receptor agonists produce antinociceptive effects in models of inflammatory and nociceptive pain, and in some cases these effects involve activation of the opioid system. In addition, CB receptor agonists enhance the effect of mu-opioid receptor agonists in a variety of models of analgesia, and combinations of cannabinoids and opioids may produce synergistic effects. Antinociceptive effects of compounds blocking the metabolism of anandamide have been reported, particularly in models of inflammatory pain. There is also evidence that such compounds increase the analgesic effect of non-steroidal anti-inflammatory drugs (NSAIDs), raising the possibility that a combination of suitable agents could, by reducing the NSAID dose needed, provide an efficacious treatment strategy, while minimizing the potential for NSAID-induced gastrointestinal and cardiovascular disturbances. Other potential "partners" for endocannabinoid modulatory agents include alpha(2)-adrenoceptor modulators, peroxisome proliferator-activated receptor alpha agonists and TRPV1 antagonists. An extension of the polypharmacological approach is to combine the desired pharmacological properties of the treatment within a single molecule. Hopefully, these approaches will yield novel analgesics that do not produce the psychotropic effects that limit the medicinal use of cannabis.

The mu-opioid receptor agonist morphine, but not agonists at delta- or kappa-opioid receptors, induces peripheral antinociception mediated by cannabinoid receptors

BACKGROUND AND PURPOSE

Although participation of opioids in antinociception induced by cannabinoids has been documented, there is little information regarding the participation of cannabinoids in the antinociceptive mechanisms of opioids. The aim of the present study was to determine whether endocannabinoids could be involved in peripheral antinociception induced by activation of mu-, delta- and kappa-opioid receptors.

EXPERIMENTAL APPROACH

Nociceptive thresholds to mechanical stimulation of rat paws treated with intraplantar prostaglandin E2 (PGE2, 2 microg) to induce hyperalgesia were measured 3 h after injection using an algesimetric apparatus. Opioid agonists morphine (200 microg), (+)-4-[(alphaR)-alpha-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80) (80 microg), bremazocine (50 microg); cannabinoid receptor antagonists N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) (20-80 microg), 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl(4-methoxyphenyl) methanone (AM630) (12.5-100 microg); and an inhibitor of methyl arachidonyl fluorophosphonate (MAFP) (1-4 microg) were also injected in the paw.

KEY RESULTS

The CB1-selective cannabinoid receptor antagonist AM251 completely reversed the peripheral antinociception induced by morphine in a dose-dependent manner. In contrast, the CB2-selective cannabinoid receptor antagonist AM630 elicited partial antagonism of this effect. In addition, the administration of the fatty acid amide hydrolase inhibitor, MAFP, enhanced the antinociception induced by morphine. The cannabinoid receptor antagonists AM251 and AM630 did not modify the antinociceptive effect of SNC80 or bremazocine. The antagonists alone did not cause any hyperalgesic or antinociceptive effect.

CONCLUSIONS AND IMPLICATIONS

Our results provide evidence for the involvement of endocannabinoids, in the peripheral antinociception induced by the mu-opioid receptor agonist morphine. The release of cannabinoids appears not to be involved in the peripheral antinociceptive effect induced by kappa- and delta-opioid receptor agonists.

Therapeutic potential of cannabis in pain medicine

Advances in cannabis research have paralleled developments in opioid pharmacology whereby a psychoactive plant extract has elucidated novel endogenous signalling systems with therapeutic significance. Cannabinoids (CBs) are chemical compounds derived from cannabis. The major psychotropic CB delta-9-tetrahydrocannabinol (Delta(9)-THC) was isolated in 1964 and the first CB receptor (CB(1)R) was cloned in 1990. CB signalling occurs via G-protein-coupled receptors distributed throughout the body. Endocannabinoids are derivatives of arachidonic acid that function in diverse physiological systems. Neuronal CB(1)Rs modulate synaptic transmission and mediate psychoactivity. Immune-cell CB(2) receptors (CB(2)R) may down-regulate neuroinflammation and influence cyclooxygenase-dependent pathways. Animal models demonstrate that CBRs play a fundamental role in peripheral, spinal, and supraspinal nociception and that CBs are effective analgesics. Clinical trials of CBs in multiple sclerosis have suggested a benefit in neuropathic pain. However, human studies of CB-mediated analgesia have been limited by study size, heterogeneous patient populations, and subjective outcome measures. Furthermore, CBs have variable pharmacokinetics and can manifest psychotropism. They are currently licensed as antiemetics in chemotherapy and can be prescribed on a named-patient basis for neuropathic pain. Future selective peripheral CB(1)R and CB(2)R agonists will minimize central psychoactivity and may synergize opioid anti-nociception. This review discusses the basic science and clinical aspects of CB pharmacology with a focus on pain medicine.

Peripheral cannabinoid CB1 receptors inhibit evoked responses of nociceptive neurones in vivo

We investigated the effect of peripheral administration of a selective cannabinoid CB1 receptor agonist arachidonyl-2-choroethylamide (ACEA), on evoked responses of primary afferents in vivo. Extracellular recordings were made from filaments of the saphenous nerve that responded to noxious mechanical stimulation of their receptive fields and effects of ACEA (30 and 50 microg/100 microl, i.a.) were studied. ACEA significantly inhibited evoked responses, effects that were blocked by co-administration of the cannabinoid CB1 receptor antagonist AM251 (30 microg/100 microl). These results demonstrate a cannabinoid CB1 receptor-mediated inhibition of primary afferent nociceptor excitability and provide further support for a peripheral site of action of cannabinoids.

Cannabilactones: a novel class of CB2 selective agonists with peripheral analgesic activity

The identification of the CB2 cannabinoid receptor has provided a novel target for the development of therapeutically useful cannabinergic molecules. We have synthesized benzo[ c]chromen-6-one analogs possessing high affinity and selectivity for this receptor. These novel compounds are structurally related to cannabinol (6,6,9-trimethyl-3-pentyl-6 H-benzo[ c]chromen-1-ol), a natural constituent of cannabis with modest CB2 selectivity. Key pharmacophoric features of the new selective agonists include a 3-(1',1'-dimethylheptyl) side chain and a 6-oxo group on the cannabinoid tricyclic structure that characterizes this class of compounds as "cannabilactones." Our results suggest that the six-membered lactone pharmacophore is critical for CB2 receptor selectivity. Optimal receptor subtype selectivity of 490-fold and subnanomolar affinity for the CB2 receptor is exhibited by a 9-hydroxyl analog 5 (AM1714), while the 9-methoxy analog 4b (AM1710) had a 54-fold CB2 selectivity. X-ray crystallography and molecular modeling show the cannabilactones to have a planar ring conformation. In vitro testing revealed that the novel compounds are CB2 agonists, while in vivo testing of cannabilactones 4b and 5 found them to possess potent peripheral analgesic activity.