The therapeutic potential of cannabinoids has been the topic of extensive investigation following the discovery of cannabinoid receptors and their endogenous ligands. Cannabinoid receptors and their endogenous ligands are present at supraspinal, spinal and peripheral levels. Cannabinoids suppress behavioral responses to noxious stimulation and suppress nociceptive processing through activation of cannabinoid CB1 and CB2 receptor subtypes. Endocannabinoids, the brain’s own cannabis-like substances, share the same molecular target as Δ9-tetrahydrocannabinol, the main psychoactive component in cannabis. Endocannabinoids serve as synaptic circuit breakers and regulate multiple physiological and pathological conditions, e.g. regulation of food intake, immunomodulation, inflammation, analgesia, cancer, addictive behavior, epilepsy and others. This review will focus on uncovering the roles of anandamide (AEA) and 2-arachidonoylglycerol (2-AG), the two best characterized endocannabinoids identified to date, in controlling nociceptive responding. The roles of AEA and 2-AG, released under physiological conditions, in modulating nociceptive responding at different levels of the neuraxis will be emphasized in this review. Effects of modulation of endocannabinoid levels through inhibition of endocannabinoid hydrolysis and uptake is also compared with effects of exogenous administration of synthetic endocannabinoids in acute, inflammatory and neuropathic pain models. Finally, the therapeutic potential of the endocannabinoid signaling system is discussed in the context of identifying novel pharmacotherapies for the treatment of pain.

Cannabinoids suppress behavioural responses to noxious stimulation and suppress nociceptive transmission through activation of CB 1 and CB 2 receptor subtypes. CB 1 receptors are expressed at high levels in the central nervous system (CNS), whereas CB 2 receptors are found predominantly, but not exclusively, outside the CNS. CB 2 receptors are also upregulated in the CNS and dorsal root ganglia by pathological pain states. Here, we review behavioural, neurochemical and electrophysiological data, which identify cannabinoid CB 2 receptors as a therapeutic target for treating pathological pain states with limited centrally, mediated side effects. The development of CB 2 -selective agonists (with minimal affinity for CB 1 ) as well as mutant mice lacking CB 2 receptors has provided pharmacological and genetic tools required to evaluate the effectiveness of CB 2 agonists in suppressing persistent pain states. This review will examine the efficacy of cannabinoid CB 2 -selective agonists in suppressing acute, inflammatory and neuropathic nociception following systemic and local routes of administration. Data derived from behavioural, neurochemical and neurophysiological approaches are discussed to better understand the relationship between antinociceptive effects induced by CB 2 -selective agonists in behavioural studies and neural mechanisms of pain suppression. Finally, the therapeutic potential and possible limitations of CB 2 -based pharmacotherapies for pathological pain states induced by tissue and nerve injury are discussed.

Background Mixed cannabinoid CB1/CB2 agonists such as Δ9-tetrahydrocannabinol (Δ9-THC) can produce tolerance, physical withdrawal, and unwanted CB1-mediated central nervous system side effects. Whether repeated systemic administration of a CB2-preferring agonist engages CB1 receptors or produces CB1-mediated side effects is unknown. Methods We evaluated anti-allodynic efficacy, possible tolerance, and cannabimimetic side effects of repeated dosing with a CB2-preferring agonist AM1710 in a model of chemotherapy-induced neuropathy produced by paclitaxel using CB1KO, CB2KO, and WT mice. Comparisons were made with the prototypic classical cannabinoid Δ9-THC. We also explored the site and possible mechanism of action of AM1710. Results Paclitaxel-induced mechanical and cold allodynia developed equivalently in CB1KO, CB2KO, and WT mice. Both AM1710 and Δ9-THC suppressed established paclitaxel-induced allodynia in WT mice. Unlike Δ9-THC, chronic AM1710 did not engage CB1 activity or produce antinociceptive tolerance, CB1-mediated cannabinoid withdrawal, hypothermia, or motor dysfunction. Anti-allodynic efficacy of systemic AM1710 was absent in CB2KO mice or WT mice receiving the CB2 antagonist AM630, administered either systemically or intrathecally. Intrathecal AM1710 also attenuated paclitaxel-induced allodynia in WT but not CB2KO mice, implicating a possible role for spinal CB2 receptors in AM1710 anti-allodynic efficacy. Finally, both acute and chronic treatment with AM1710 decreased mRNA levels of tumor necrosis factor alpha and monocyte chemoattractant protein-1 in lumbar spinal cord of paclitaxel-treated WT mice. Conclusions Our results highlight the potential of prolonged use of CB2 agonists for managing chemotherapy-induced allodynia with a favorable therapeutic ratio marked by sustained efficacy and absence of tolerance, physical withdrawal, or CB1-mediated side effects.

SUMMARY Cisplatin, a platinum-derived chemotherapeutic agent, produces mechanical and cold allodynia reminiscent of chemotherapy-induced neuropathy in humans. The endocannabinoid system represents a novel target for analgesic drug development. The endocannabinoid consists of endocannabinoids (e.g. anandamide (AEA) and 2-arachidonoylglycerol (2-AG)), cannabinoid receptors (e.g. CB1 and CB2) and the enzymes controlling endocannabinoid synthesis and degradation. AEA is hydrolyzed by fatty-acid amide hydrolase (FAAH) whereas 2-AG is hydrolyzed primarily by monoacylglycerol lipase (MGL). We compared effects of brain permeant (URB597) and impermeant (URB937) inhibitors of FAAH with an irreversible inhibitor of MGL (JZL184) on cisplatin-evoked behavioral hypersensitivities. Endocannabinoid modulators were compared with agents used clinically to treat neuropathy (i.e. the opioid analgesic morphine, the anticonvulsant gabapentin and the tricyclic antidepressant amitriptyline). Cisplatin produced robust mechanical and cold allodynia but did not alter responsiveness to heat. After neuropathy was fully established, groups received acute intraperitoneal (i.p.) injections of vehicle, amitriptyline (30 mg/kg), gabapentin (100 mg/kg), morphine (6 mg/kg), URB597 (0.1 or 1 mg/kg), URB937 (0.1 or 1 mg/kg) or JZL184 (1, 3 or 8 mg/kg). Pharmacological specificity was assessed by coadministering each endocannabinoid modulator with either a CB1 (AM251 3 mg/kg), CB2 (AM630 3 mg/kg), TRPV1 (AMG9810 3 mg/kg) or TRPA1 (HC030031 8 mg/kg) antagonist. Effects of cisplatin on endocannabinoid levels and transcription of receptors (CB1, CB2, TRPV1, TRPA1) and enzymes (FAAH, MGL) linked to the endocannabinoid system were also assessed. URB597, URB937, JZL184 and morphine reversed cisplatin-evoked mechanical and cold allodynia to pre-cisplatin levels. By contrast, gabapentin only partially reversed the neuropathy while amitriptyline, administered acutely, was ineffective. CB1 or CB2 antagonist completely blocked the anti-allodynic effects of both FAAH (URB597, URB937) and MGL (JZL184) inhibitors to mechanical and cold stimulation, while TRPV1 antagonist AMG9810 blocked only the anti-allodynic efficacy of both FAAH inhibitors, but not the MGL inhibitor,. By contrast, the TRPA1 antagonist HC30031 did not attenuate anti-allodynic efficacy of any endocannabinoid modulator. When the levels of endocannabinoids were examined, cisplatin increased both anandamide (AEA) and 2-arachidonoylglycerol (2-AG) levels in the lumbar spinal cord and decreased 2-AG levels (but not AEA) in dorsal hind paw skin. RT-PCR showed that mRNA for FAAH, but not other markers, was upregulated by cisplatin treatment in dorsal root ganglia. The present studies demonstrate that cisplatin alters endocannabinoid tone and that inhibition of endocannabinoid hydrolysis alleviates chemotherapy-induced mechanical and cold allodynia. The anti-allodynic effects of FAAH and MGL inhibitors are mediated by CB1 and CB2 cannabinoid receptors, whereas TRPV1, but not TRPA1, -dependent mechanisms ...

The endocannabinoid system is implicated in a variety of physiological and pathological conditions (inflammation, immunomodulation, analgesia, cancer and others). The main active ingredient of cannabis, D 9-tetrahydrocannabinol (D 9 -THC), produces its effects through activation of CB1 and CB2 receptors. CB1 receptors are expressed at high levels in the central nervous system (CNS), whereas CB2 receptors are concentrated predominantly, although not exclusively, in cells of the immune system. Endocannabinoids are endogenous lipid-signalling molecules that are generated in the cell membrane from phospholipid precursors. The two best characterized endocannabinoids identified to date are anandamide (AEA) and 2-arachidonoylglycerol (2-AG). Here we review the relationship between the endocannabinoid system and anti-tumour actions (inhibition of cell proliferation and migration, induction of apoptosis, reduction of tumour growth) of the cannabinoids in different types of cancer. This review will focus on examining how activation of the endocannabinoid system impacts breast, prostate and bone cancers in both in vitro and in vivo systems. The therapeutic potential of cannabinoids for cancer, as identified in clinical trials, is also discussed. Identification of safe and effective treatments to manage and improve cancer therapy is critical to improve quality of life and reduce unnecessary suffering in cancer patients. In this regard, cannabis-like compounds offer therapeutic potential for the treatment of breast, prostate and bone cancer in patients. Further basic research on anti-cancer properties of cannabinoids as well as clinical trials of cannabinoid therapeutic efficacy in breast, prostate and bone cancer is therefore warranted. LINKED ARTICLESThis article is part of a themed issue on Cannabinoids in Biology and Medicine. To view the other articles in this issue visit http://dx.doi.org/10.1111/bph.2011.163.issue-7 Abbreviations 2-AG, 2-arachidonoylglycerol; AC, adenylyl cyclase; ACEA, arachidonyl-2′-chloroethylamide; AEA, anandamide; AKT, protein kinase B; AR, androgen receptor; ATP, adenosine triphosphate; Bax, pro-apoptotic protein; Bcl2, anti-apoptotic protein; brca1, breast cancer susceptibility gene product; cAMP, cyclic adenosine monophosphate; CBD, cannabidiol; CB, cannabinoid; CB1, cannabinoid receptor 1; CB2, cannabinoid receptor 2; CBN, cannabinol; Cdc2, p34 cyclindependent kinase 1; CDK, cyclin-dependent kinases; CNS, central nervous system; DALN, desacetyllevonantradol; D 9 -THC, delta 9-tetrahydrocannabinol; DRG, dorsal root ganglion; EGF, epidermal growth factor receptor; EPEA, eicosapentaenoyl ethanolamide; ERK, extracellular regulated kinase; FAAH, fatty-acid amide hydrolase; GPR55, G-protein-coupled receptor 55; H2O2, hydrogen peroxide; p27/KIP1, cyclin kinase inhibitor; p38MAPK, p38 mitogenactivated protein kinase; PRL, prolactin receptor; MET, methanandamide; MGL or MAGL, monoacylglycerol lipase; OTFP, 3-octylthio-1,1,1-trifluoropropan-2-one; p53, p53 protein; p21ras, p21 ras protein; PEA, palmito...

Background and purpose: 2-arachidonoyl glycerol (2-AG) is an endogenous cannabinoid with central antinociceptive properties. Its degradation is catalysed by monoacylglycerol lipase (MGL) whose activity is inhibited by URB602, a new synthetic compound. The peripheral antinociceptive effects of 2-AG and URB602 in an inflammatory model of pain are not yet determined. We have evaluated these effects with and without the cannabinoid CB 1 (AM251) and CB 2 (AM630) receptor antagonists. Experimental approach: Inflammation was induced in rat hind paws by intraplantar injection of formalin. Nociception was assessed behaviourally over the next 60 min, in 19 experimental groups: (1) control; (2-6) 2-AG (0.01-100 mg); (7) AM251 (80 mg); (8) AM251 þ 2-AG (10 mg); (9) AM630 (25 mg); (10) AM630 þ 2-AG (10 mg); (11-16) URB602 (0.1-500 mg); (17) 2-AG þ URB602 (ED 50 ); (18) AM251 þ URB602 (ED 50 ); (19) AM630 þ URB602 (ED 50 ). Drugs were injected s.c. in the dorsal surface of the hind paw (50 ml), 15 min before formalin injection into the same paw. Key results: 2-AG and URB602 produced dose-dependent antinociceptive effects for the late phases of the formalin test with ED 50 of 0.6570.455 mg and 68714.3 mg, respectively. Their combination at ED 50 doses produced an additive antinociceptive effect. These effects were inhibited by AM630 but not by AM251 for 2-AG and by the two cannabinoid antagonists for URB602. Conclusions and implications: Locally injected 2-AG and URB602 decreased pain behaviour in a dose-dependent manner in an inflammatory model of pain. The antinociceptive effect of 2-AG was mediated by the CB 2 receptor.

Systemic injections of TTX diminished pain behaviour in a dose-dependent manner in models of inflammatory, visceral and neuropathic pain without causing adverse events, whereas morphine analgesia was associated with heavy sedation. TTX is a very promising substance for the treatment of various types of pain but needs further evaluation.

The endocannabinoid 2-arachidonoylglycerol (2-AG) is degraded primarily by monoacylglycerol lipase (MGL). We compared peripheral antinociceptive effects of JZL184, a novel irreversible MGL inhibitor, with the reversible MGL-preferring inhibitor URB602 and exogenous 2-AG in rats. EXPERIMENTAL APPROACHNociception in the formalin test was assessed in groups receiving dorsal paw injections of vehicle, JZL184 (0.001-300 mg), URB602 (0.001-600 mg), 2-AG (ED50), 2-AG + JZL184 (at their ED50), 2-AG + URB602 (at their ED50), AM251 (80 mg), AM251 + JZL184 (10 mg), AM630 (25 mg) or AM630 + JZL184 (10 mg). Effects of MGL inhibitors on endocannabinoid accumulation and on activities of endocannabinoid-metabolizing enzymes were assessed. KEY RESULTSIntra-paw administration of JZL184, URB602 and 2-AG suppressed early and late phases of formalin pain. JZL184 and URB602 acted through a common mechanism. JZL184 (ED50 Phase 1: 0.06 Ϯ 0.028; Phase 2: 0.03 Ϯ 0.011 mg) produced greater antinociception than URB602 (ED50 Phase 1: 120 Ϯ 51.3; Phase 2: 66 Ϯ 23.9 mg) or 2-AG. Both MGL inhibitors produced additive antinociceptive effects when combined with 2-AG. Antinociceptive effects of JZL184, like those of URB602, were blocked by cannabinoid receptor 1 (CB1) and cannabinoid receptor 2 (CB2) antagonists. JZL184 suppressed MGL but not fatty-acid amide hydrolase or N-arachidonoyl-phosphatidylethanolamine phospholipase D activities ex vivo. URB602 increased hind paw 2-AG without altering anandamide levels. CONCLUSIONS AND IMPLICATIONSMGL inhibitors suppressed formalin-induced pain through peripheral CB1 and CB2 receptor mechanisms. MGL inhibition increased paw skin 2-AG accumulation to mediate these effects. MGL represents a target for the treatment of inflammatory pain.