CB2 cannabinoid receptor activation produces antinociception by stimulating peripheral release of endogenous opioids

Cannabis-based medicines in multiple sclerosis--a review of clinical studies

For some years a mixture of anecdotal report and data from animal models have implied a potential role for cannabis-based medicines in ameliorating a variety of symptoms of multiple sclerosis. Only recently however have large randomised controlled trials (RCTs) examined these potential effects rigorously. At present the results of RCTs have lacked a coherent message to the prescribing clinician and reasons for such heterogeneity in cannabinoid trials are discussed.

Spinal and peripheral analgesic effects of the CB2 cannabinoid receptor agonist AM1241 in two models of bone cancer-induced pain

BACKGROUND AND PURPOSE

The activation of CB(2) receptors induces analgesia in experimental models of chronic pain. The present experiments were designed to study whether the activation of peripheral or spinal CB(2) receptors relieves thermal hyperalgesia and mechanical allodynia in two models of bone cancer pain.

EXPERIMENTAL APPROACH

NCTC 2472 osteosarcoma or B16-F10 melanoma cells were intratibially inoculated to C3H/He and C57BL/6 mice. Thermal hyperalgesia was assessed by the unilateral hot plate test and mechanical allodynia by the von Frey test. AM1241 (CB(2) receptor agonist), AM251 (CB(1) receptor antagonist), SR144528 (CB(2) receptor antagonist) and naloxone were used. CB(2) receptor expression was measured by Western blot.

KEY RESULTS

AM1241 (0.3-10 mg.kg(-1)) abolished thermal hyperalgesia and mechanical allodynia in both tumour models. The antihyperalgesic effect was antagonized by subcutaneous, intrathecal or peri-tumour administration of SR144528. In contrast, the antiallodynic effect was inhibited by systemic or intrathecal, but not peri-tumour, injection of SR144528. The effects of AM1241 were unchanged by AM251 but were prevented by naloxone. No change in CB(2) receptor expression was found in spinal cord or dorsal root ganglia.

CONCLUSIONS AND IMPLICATIONS

Spinal CB(2) receptors are involved in the antiallodynic effect induced by AM1241 in two neoplastic models while peripheral and spinal receptors participate in the antihyperalgesic effects. Both effects were mediated by endogenous opiates. The use of drugs that activate CB(2) receptors could be a useful strategy to counteract bone cancer-induced pain symptoms.

A cannabinoid 2 receptor agonist attenuates bone cancer-induced pain and bone loss

AIMS

Cannabinoid CB(2) agonists have been shown to alleviate behavioral signs of inflammatory and neuropathic pain in animal models. AM1241, a CB(2) agonist, does not demonstrate central nervous system side effects seen with CB(1) agonists such as hypothermia and catalepsy. Metastatic bone cancer causes severe pain in patients and is treated with analgesics such as opiates. Recent reports suggest that sustained opiates can produce paradoxical hyperalgesic actions and enhance bone destruction in a murine model of bone cancer. In contrast, CB(2) selective agonists have been shown to reduce bone loss associated with a model of osteoporosis. Here we tested whether a CB(2) agonist administered over a 7day period inhibits bone cancer-induced pain as well as attenuates cancer-induced bone degradation.

MAIN METHODS

A murine bone cancer model was used in which osteolytic sarcoma cells were injected into the intramedullary space of the distal end of the femur. Behavioral and radiographic image analysis was performed at days 7, 10 and 14 after injection of tumor cells into the femur.

KEY FINDINGS

Osteolytic sarcoma within the femur produced spontaneous and touch evoked behavioral signs of pain within the tumor-bearing limb. The systemic administration of AM1241 acutely or for 7days significantly attenuated spontaneous and evoked pain in the inoculated limb. Sustained AM1241 significantly reduced bone loss and decreased the incidence of cancer-induced bone fractures.

SIGNIFICANCE

These findings suggest a novel therapy for cancer-induced bone pain, bone loss and bone fracture while lacking many unwanted side effects seen with current treatments for bone cancer pain.

Antinociceptive effects of racemic AM1241 and its chirally synthesized enantiomers: lack of dependence upon opioid receptor activation

Cannabinoid CB(2) receptors represent a therapeutic target that circumvents unwanted central side effects (e.g., psychoactivity and/or addiction) associated with activation of CB(1) receptors. One of the primary investigative tools used to study functions of the CB(2) receptor is the aminoalkylindole (R,S)-AM1241. However, (R,S)-AM1241 has been described as an atypical CB(2) agonist which produces antinociception mediated indirectly by opioid receptors. (R,S)-AM1241 and its enantiomers, (R)-AM1241 and (S)-AM1241, were evaluated for antinociception in response to thermal (Hargreaves) and mechanical (von Frey) stimulation. Pharmacological specificity was established using antagonists for CB(1) (rimonabant [SR141716]) and CB(2) (SR144528). The opioid antagonist naloxone was administered locally in the paw or systemically to evaluate the contribution of opioid receptors to CB(2)-mediated antinociception produced by (R,S)-AM1241, (R)-AM1241, and (S)-AM1241. Comparisons were made with the opioid analgesic morphine. (R,S)-AM1241, (R)-AM1241, and (S)-AM1241 (0.033-10 mg/kg i.p.) produced antinociception to thermal, but not mechanical, stimulation of the hindpaw in naive rats. Antinociception produced by (R,S)-AM1241 and (S)-AM1241 exhibited an inverted U-shaped dose response curve. (R)-AM1241 produced greater antinociception than either (S)-AM1241 or (R,S)-AM1241 at the lowest (0.033 and 0.1 mg/kg i.p.) and highest (10 mg/kg i.p.) doses. Similar levels of antinociception were observed at intermediate doses. (R,S)-AM1241, (R)-AM1241, and (S)-AM1241 each produced CB(2)-mediated antinociception that was blocked by SR144528 but not by rimonabant. Local and systemic naloxone blocked morphine-induced antinociception but did not block antinociceptive effects of (R,S)-AM1241, (R)-AM1241, or (S)-AM1241. The antinociceptive effects of the CB(2)-selective cannabinoid (R,S)-AM1241 and its enantiomers, (R)-AM1241 and (S)-AM1241, are not dependent upon opioid receptors.

Cannabinoid CB1 receptor facilitation of substance P release in the rat spinal cord, measured as neurokinin 1 receptor internalization

The contribution of CB1 receptors in the spinal cord to cannabinoid analgesia is still unclear. The objective of this study was to investigate the effect of CB1 receptors on substance P release from primary afferent terminals in the spinal cord. Substance P release was measured as neurokinin 1 (NK1) receptor internalization in lamina I neurons. It was induced in spinal cord slices by dorsal root stimulation and in live rats by a noxious stimulus. In spinal cord slices, the CB1 receptor antagonists AM251, AM281 and rimonabant partially but potently inhibited NK1 receptor internalization induced by electrical stimulation of the dorsal root. This was due to an inhibition of substance P release and not of NK1 receptor internalization itself, because AM251 and AM281 did not inhibit NK1 receptor internalization induced by exogenous substance P. The CB1 receptor agonist ACEA increased NK1 receptor internalization evoked by dorsal root stimulation. The effects of AM251 and ACEA cancelled each other. In vivo, AM251 injected intrathecally decreased NK1 receptor internalization in spinal segments L5 and L6 induced by noxious hind paw clamp. Intrathecal AM251 also produced analgesia to radiant heat stimulation of the paw. The inhibition by AM251 of NK1 receptor internalization was reversed by antagonists of mu-opioid and GABA(B) receptors. This indicates that CB1 receptors facilitate substance P release by inhibiting the release of GABA and opioids next to primary afferent terminals, producing disinhibition. This results in a pronociceptive effect of CB1 receptors in the spinal cord.

2,3-Dihydro-1-benzofuran derivatives as a series of potent selective cannabinoid receptor 2 agonists: design, synthesis, and binding mode prediction through ligand-steered modeling

We recently discovered and reported a series of N-alkyl-isatin acylhydrazone derivatives that are potent cannabinoid receptor 2 (CB(2)) agonists. In an effort to improve the druglike properties of these compounds and to better understand and improve the treatment of neuropathic pain, we designed and synthesized a new series of 2,3-dihydro-1-benzofuran derivatives bearing an asymmetric carbon atom that behave as potent selective CB(2) agonists. We used a multidisciplinary medicinal chemistry approach with binding mode prediction through ligand-steered modeling. Enantiomer separation and configuration assignment were carried out for the racemic mixture for the most selective compound, MDA7 (compound 18). It appeared that the S enantiomer, compound MDA104 (compound 33), was the active enantiomer. Compounds MDA42 (compound 19) and MDA39 (compound 30) were the most potent at CB(2). MDA42 was tested in a model of neuropathic pain and exhibited activity in the same range as that of MDA7. Preliminary ADMET studies for MDA7 were performed and did not reveal any problems.

The analgesic potential of cannabinoids

Historically and anecdotally cannabinoids have been used as analgesic agents. In recent years, there has been an escalating interest in developing cannabis-derived medications to treat severe pain. This review provides an overview of the history of cannabis use in medicine, cannabinoid signaling pathways, and current data from preclinical as well as clinical studies on using cannabinoids as potential analgesic agents. Clinical and experimental studies show that cannabis-derived compounds act as antiemetic, appetite modulating, and analgesic agents. However, the efficacy of individual products is variable and dependent upon the route of administration. As opioids are the only therapy for severe pain, analgesic ability of cannabinoids may provide a much-needed alternative to opioids. Moreover, cannabinoids act synergistically with opioids and act as opioid sparing agents, allowing lower doses and fewer side effects from chronic opioid therapy. Thus, rational use of cannabis-based medications deserves serious consideration to alleviate the suffering of patients due to severe pain.

Cannabinoid-opioid interactions during neuropathic pain and analgesia

Opiates and exogenous cannabinoids, both potent analgesics used for the treatment of patients with neuropathic pain, bind to and activate class A G-protein-coupled receptors (GPCRs). Several lines of evidence have recently suggested that opioid and cannabinoid receptors can functionally interact in the central nervous system (CNS). These interactions may be direct, such as through receptor heteromerization, or indirect, such as through signaling cross-talk that includes agonist-mediated release and/or synthesis of endogenous ligands that can activate downstream receptors. Interactions between opioid and cannabinoid receptors may mediate many of the behavioral phenomena associated with the use of these drugs, including the production of acute antinociception and the development of tolerance and cross-tolerance to the antinociceptive effects of opioid and cannabinoid-specific ligands. This review summarizes behavioral, anatomical, and molecular data characterizing these interactions during the development of neuropathic pain and during antinociceptive treatment with these drugs alone or in combination. These studies are critical for understanding how the receptor systems involved in pain relief are altered during acute or chronic pain, and for designing better antinociceptive drug therapies, such as the combined use of opioid and cannabinoid receptor agonists or selective activation of receptor heteromers, that directly target the altered neurophysiology of patients experiencing pain.

The endocannabinoid system and pain

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 CB(1) and CB(2) receptor subtypes. Endocannabinoids, the brain's own cannabis-like substances, share the same molecular target as Delta(9)-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 and 2-arachidonoylglycerol, the two best characterized endocannabinoids identified to date, in controlling nociceptive responding. The roles of anandamide and 2-arachidonoylglycerol, 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.

Dynamic regulation of the endocannabinoid system: implications for analgesia

The analgesic effects of cannabinoids are well documented, but these are often limited by psychoactive side-effects. Recent studies indicate that the endocannabinoid system is dynamic and altered under different pathological conditions, including pain states. Changes in this receptor system include altered expression of receptors, differential synthetic pathways for endocannabinoids are expressed by various cell types, multiple pathways of catabolism and the generation of biologically active metabolites, which may be engaged under different conditions. This review discusses the evidence that pain states alter the endocannabinoid receptor system at key sites involved in pain processing and how these changes may inform the development of cannabinoid-based analgesics.

Adjuvant topical therapy with a cannabinoid receptor agonist in facial postherpetic neuralgia

BACKGROUND

Postherpetic neuralgia is a frequent adverse event in herpes zoster patients and difficult to treat. Conventional analgetic therapy often fails to reduce the burning pain transmitted by unmyelinated nerve fibers. These nerves express cannabinoid receptors which exert a role in modulation of nociceptive symptoms. Therefore, topical therapy with cannabinoid receptor agonist seems likely to suppress local burning pain.

PATIENTS AND METHODS

In an open-labeled trial, 8 patients with facial postherpetic neuralgia received a cream containing the cannabinoid receptor agonist N-palmitoylethanolamine. The course of symptoms was scored with the visual analog scale.

RESULTS

5 of 8 patients (62.5 %) experienced a mean pain reduction of 87.8 %. Therapy was tolerated by all patients. No unpleasant sensations or adverse events occurred.

CONCLUSIONS

Topical cannabinoid receptor agonists are an effective and well-tolerated adjuvant therapy option in postherpetic neuralgia.

[Cannabinoid and opioid system in the mechanisms and control of pain]

Opium and Hashish have been classically employed for the control of pain. The pharmacologic rationale for the use of these substances lies in the fact that they are able to modulate the endogenous opioid and cannabinoid systems respectively. Both systems, which depress the central nervous system (CNS), are capable of producing analgesia both in experimental animals and in humans by interfering with the transmission of pain signals (nociceptive) from the periphery to the superior centers of the CNS. We will review the main theories that explain the peripheral effects on both systems and its possible interest from the treatment of musculoskeletal pain standpoint.

The endocannabinoid system of the skin in health and disease: novel perspectives and therapeutic opportunities

The newly discovered endocannabinoid system (ECS; comprising the endogenous lipid mediators endocannabinoids present in virtually all tissues, their G-protein-coupled cannabinoid receptors, biosynthetic pathways and metabolizing enzymes) has been implicated in multiple regulatory functions both in health and disease. Recent studies have intriguingly suggested the existence of a functional ECS in the skin and implicated it in various biological processes (e.g. proliferation, growth, differentiation, apoptosis and cytokine, mediator or hormone production of various cell types of the skin and appendages, such as the hair follicle and sebaceous gland). It seems that the main physiological function of the cutaneous ECS is to constitutively control the proper and well-balanced proliferation, differentiation and survival, as well as immune competence and/or tolerance, of skin cells. The disruption of this delicate balance might facilitate the development of multiple pathological conditions and diseases of the skin (e.g. acne, seborrhea, allergic dermatitis, itch and pain, psoriasis, hair growth disorders, systemic sclerosis and cancer).

Central antinociception induced by mu-opioid receptor agonist morphine, but not delta- or kappa-, is mediated by cannabinoid CB1 receptor

BACKGROUND AND PURPOSE

It has been demonstrated that cannabinoids evoke the release of endogenous opioids to produce antinociception; however, no information exists regarding the participation of cannabinoids in the antinociceptive mechanisms of opioids. The aim of the present study was to determine whether endocannabinoids are involved in central antinociception induced by activation of mu-, delta- and kappa-opioid receptors.

EXPERIMENTAL APPROACH

Nociceptive threshold to thermal stimulation was measured according to the tail-flick test in Swiss mice. Morphine (5 microg), SNC80 (4 microg), bremazocine (4 microg), AM251 (2 and 4 microg), AM630 (2 and 4 microg) and MAFP (0.1 and 0.4 microg) were administered by the intracerebroventricular route.

KEY RESULTS

The CB(1)-selective cannabinoid receptor antagonist AM251 completely reversed the central antinociception induced by morphine in a dose-dependent manner. In contrast, the CB(2)-selective cannabinoid receptor antagonist AM630 did not antagonize this effect. Additionally, the administration of the anandamide amidase inhibitor, MAFP, significantly enhanced the antinociception induced by morphine. In contrast, the antinociceptive effects of delta- and kappa-opioid receptor agonists were not affected by the cannabinoid antagonists. The antagonists alone caused no hyperalgesic or antinociceptive effects.

CONCLUSIONS AND IMPLICATIONS

The results provide evidence for the involvement of cannabinoid CB(1) receptors in the central antinociception induced by activation of mu-opioid receptors by the agonist morphine. The release of endocannabinoids appears not to be involved in central antinociception induced by activation of kappa- and delta-opioid receptors.

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.

Cannabinoids against pain. Efficacy and strategies to reduce psychoactivity: a clinical perspective

The clinical use of cannabinoids is currently a topic of interest not exclusively, but most importantly, concerning different areas of pain therapy. One of the major obstacles in developing clinically acceptable compounds is the cannabimimetic side-effect profile of delta-9-tetrahydrocannabinol (THC) and other cannabinoids. This article gives a brief overview of the endocannabinoid system, its components and functions and explains the current approaches to avoiding cannabimimetic side effects by separating them from the therapeutic effects. One of these approaches is the addition of cannabidiol (CBD) as well as the use of preparations suitable for oromucosal application. Also cannabinoids, which primarily stimulate peripheral cannabinoid-1 (CB1) receptors or selectively cannabinoid-2 (CB2) receptors, can further separate analgesic activity from cannabimimetic activity. Local or topical modes of application are another attempt aiming in the same direction. Modulating the endogenous cannabinoid tone (via the inhibition of endocannabinoid-metabolising enzymes) is another strategy. The combination of THC in low, non-psychoactive doses with opioids has a synergistic effect and reduces opioid tolerance effects. Available data from these approaches are summarised and their more and less promising aspects are discussed.

Interaction of the cannabinoid and opioid systems in the modulation of nociception

Cannabinoids and opioids produce antinociceptive synergy. Cannabinoids such as Delta-9-tetrahydrocannabinol (THC) release endogenous opioids and endocannabinoids such as anandamide (AEA) also alter endogenous opioid tone. Opioids and cannabinoids bind distinct receptors that co-localize in areas of the brain involved with the processing of pain signals. Therefore, it is logical to look at interactions of these two systems in the modulation of both acute and chronic pain. These drugs are often co-abused. In addition, the lack of continued effectiveness of opioids due to tolerance development limits the use of such drugs. The cost to society and patients in terms of dollars, loss of productivity, as well as quality of life, is staggering. This review summarizes the data indicating that with cannabinoid/opioid therapy one may be able to produce long-term antinociceptive effects at doses devoid of substantial side effects, while preventing the neuronal biochemical changes that accompany tolerance. The clinical utility of modulators of the endocannabinoid system as a potential mimic for THC-like drugs in analgesia and tolerance-sparing effects of opioids is a critical future direction also addressed in the review.

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.

Mode of action of cannabinoids on nociceptive nerve endings

In recent years, cannabinoids have emerged as attractive alternatives or supplements to therapy for chronic pain states. However, in humans the activation of cannabinoid receptors in neurons of the central nervous system is associated with psychotropic side effects, temporary memory impairment and dependence, which arise via the effects of cannabinoids on forebrain circuits. For clinical exploitation of the analgesic properties of cannabinoids, a major challenge is to devise strategies that reduce or abolish their adverse effects on cognitive, affective and motor functions without attenuating their analgesic effects. The cannabinoid receptor family currently includes two cloned metabotropic receptors: CB1, CB2 and possibly GPR55 which are distributed widely across many key loci in pain-modulating pathways, including the peripheral terminals of primary afferents. Modulation of transducer ion channels expressed at nociceptive terminals occurs upon activation of metabotropic cannabinoid receptors, but direct cannabinoid action on ion channels involved in sensory transduction or regulation of neuron excitability likely contributes to the peripheral cannabinoid effects.

Antinociceptive effect of the cannabinoid agonist, WIN 55,212-2, in the orofacial and temporomandibular formalin tests

Orofacial pain disorders are frequent in the general population and their pharmacological treatment is not always adequately resolved. Cannabinoids have demonstrated their analgesic effect in several pain conditions, both in animal models and in clinical situations. The aim of the present study was to evaluate the cannabinoid-mediated antinociception in two inflammatory models of orofacial pain (orofacial and temporomandibular joint (TMJ) formalin test) and to compare it with a spinal inflammatory model (paw formalin test). WIN 55,212-2 (0.5, 1mg/kg), a synthetic cannabinoid agonist, was intraperitoneally (i.p.) administered prior to formalin and significantly reduced the nociceptive behavioural responses in these inflammatory tests. To elucidate which subtype of receptor could be involved in such effect, two selective cannabinoid antagonists were administered prior to WIN. SR141716A (1mg/kg i.p.), the CB1 receptor-selective antagonist, was able to prevent the cannabinoid-induced analgesia in all three models, whereas SR144528 (1mg/kg i.p.), the CB2 receptor-selective antagonist, only prevented it in the paw formalin test. A comparison with the antinociceptive effects of morphine (2.5, 5, 10mg/kg, i.p.), indomethacin (2.5, 5mg/kg, i.p.) and ketamine (25, 50mg/kg, i.p.) was also performed. Morphine displayed a dose-dependent reduction of acute and inflammatory pain in all three models, whereas indomethacin and ketamine only attenuated inflammatory pain at the highest tested doses. These results indicate that the cannabinoid-induced antinociception in the orofacial region is mediated by activation of CB1 cannabinoid receptor. Moreover WIN was as effective as morphine and more effective than indomethacin and ketamine, in oral inflammatory pain.

In silico target fishing for rationalized ligand discovery exemplified on constituents of Ruta graveolens

The identification of targets whose interaction is likely to result in the successful treatment of a disease is of growing interest for natural product scientists. In the current study we performed an exemplary application of a virtual parallel screening approach to identify potential targets for 16 secondary metabolites isolated and identified from the aerial parts of the medicinal plant RUTA GRAVEOLENS L. Low energy conformers of the isolated constituents were simultaneously screened against a set of 2208 pharmacophore models generated in-house for the IN SILICO prediction of putative biological targets, i. e., target fishing. Based on the predicted ligand-target interactions, we focused on three biological targets, namely acetylcholinesterase (AChE), the human rhinovirus (HRV) coat protein and the cannabinoid receptor type-2 (CB (2)). For a critical evaluation of the applied parallel screening approach, virtual hits and non-hits were assayed on the respective targets. For AChE the highest scoring virtual hit, arborinine, showed the best inhibitory IN VITRO activity on AChE (IC (50) 34.7 muM). Determination of the anti-HRV-2 effect revealed 6,7,8-trimethoxycoumarin and arborinine to be the most active antiviral constituents with IC (50) values of 11.98 muM and 3.19 muM, respectively. Of these, arborinine was predicted virtually. Of all the molecules subjected to parallel screening, one virtual CB (2) ligand was obtained, i. e., rutamarin. Interestingly, in experimental studies only this compound showed a selective activity to the CB (2) receptor ( Ki of 7.4 muM) by using a radioligand displacement assay. The applied parallel screening paradigm with constituents of R. GRAVEOLENS on three different proteins has shown promise as an IN SILICO tool for rational target fishing and pharmacological profiling of extracts and single chemical entities in natural product research.

Pathogenesis and treatment of pruritus in cholestasis

Pruritus is an enigmatic, seriously disabling symptom accompanying cholestatic liver diseases and a broad range of other disorders. Most recently, novel itch-specific neuronal pathways, itch mediators and their relevant receptors have been identified. In addition, new antipruritic therapeutic strategies have been developed and/or are under evaluation. This review highlights recent experimental and clinical findings focusing on the pathogenesis and actual treatment of pruritus in cholestatic liver disease. Evidence-based therapeutic recommendations, including the use of anion exchange resins cholestyramine, colestipol and colesevelam, the microsomal enzyme inducer rifampicin, the opioid receptor antagonists naltrexone and naloxone, and the serotonin reuptake inhibitor sertraline, are provided.

Crucial role of CB(2) cannabinoid receptor in the regulation of central immune responses during neuropathic pain

Neuropathic pain is a clinical manifestation of nerve injury difficult to treat even with potent analgesic compounds. Here, we used different lines of genetically modified mice to clarify the role played by CB(2) cannabinoid receptors in the regulation of the central immune responses leading to the development of neuropathic pain. CB(2) knock-out mice and wild-type littermates were exposed to sciatic nerve injury, and both genotypes developed a similar hyperalgesia and allodynia in the ipsilateral paw. Most strikingly, knock-outs also developed a contralateral mirror image pain, associated with an enhanced microglial and astrocytic expression in the contralateral spinal horn. In agreement, hyperalgesia, allodynia, and microglial and astrocytic activation induced by sciatic nerve injury were attenuated in transgenic mice overexpressing CB(2) receptors. These results demonstrate the crucial role of CB(2) cannabinoid receptor in modulating glial activation in response to nerve injury. The enhanced manifestations of neuropathic pain were replicated in irradiated wild-type mice reconstituted with bone marrow cells from CB(2) knock-outs, thus demonstrating the implication of the CB(2) receptor expressed in hematopoietic cells in the development of neuropathic pain at the spinal cord.

Opioids and sensory nerves

This chapter reviews the expression and regulation of opioid receptors in sensory neurons and the interactions of these receptors with endogenous and exogenous opioid ligands. Inflammation of peripheral tissues leads to increased synthesis and axonal transport of opioid receptors in dorsal root ganglion neurons. This results in opioid receptor upregulation and enhanced G protein coupling at peripheral sensory nerve terminals. These events are dependent on neuronal electrical activity, and on production of proinflammatory cytokines and nerve growth factor within the inflamed tissue. Together with the disruption of the perineurial barrier, these factors lead to an enhanced analgesic efficacy of peripherally active opioids. The major local source of endogenous opioid ligands (e.g. beta-endorphin) is leukocytes. These cells contain and upregulate signal-sequence-encoding messenger RNA of the beta-endorphin precursor proopiomelanocortin and the entire enzymatic machinery necessary for its processing into the functionally active peptide. Opioid-containing immune cells extravasate using adhesion molecules and chemokines to accumulate in inflamed tissues. Upon stressful stimuli or in response to releasing agents such as corticotropin-releasing factor, cytokines, chemokines, and catecholamines, leukocytes secrete opioids. Depending on the cell type, this release is contingent on extracellular Ca(2+) or on inositol triphosphate receptor triggered release of Ca(2+) from endoplasmic reticulum. Once secreted, opioid peptides activate peripheral opioid receptors and produce analgesia by inhibiting the excitability of sensory nerves and/or the release of proinflammatory neuropeptides. These effects occur without central untoward side effects such as depression of breathing, clouding of consciousness, or addiction. Future aims include the development of peripherally restricted opioid agonists, selective targeting of opioid-containing leukocytes to sites of painful injury, and the augmentation of peripheral opioid peptide and receptor synthesis.

Regulation of gene transcription and keratinocyte differentiation by anandamide

Anandamide (AEA) is a member of an endogenous class of lipid mediators, known as endocannabinoids, which are involved in various biological processes. In particular, AEA regulates cell growth, differentiation, and death. Accumulating evidence demonstrates that AEA controls also epidermal differentiation, one of the best characterized mechanisms of cell specialization. Indeed, the epidermis is a keratinized multistratified epithelium that functions as a barrier to protect the organism from dehydration, mechanical trauma, and microbial insults. Its function is established during embryogenesis and is maintained during the whole life span of the organism, through a complex and tightly controlled program, termed epidermal terminal differentiation (or cornification). Whereas the morphological changes that occur during cornification have been extensively studied, the molecular mechanisms that underlie this process remain poorly understood. In this chapter, we summarize current knowledge about the molecular regulation of proliferation and terminal differentiation in mammalian epidermis. In this context, we show that endocannabinoids are finely regulated by, and can interfere with, the differentiation program. In addition, we review the role of AEA in the control of cornification, and show that it occurs by maintaining a transcriptional repression of gene expression through increased DNA methylation.

Targeting the cannabinoid system to produce analgesia

Cannabinoid receptors are present at key sites involved in the relay and modulation of nociceptive responses. The analgesic effects of the cannabinoid CB₁ receptor are well described. The widespread distribution of these receptors in the brain does, however, also explain the side-effects associated with CB₁ receptor agonists. The cannabinoid CB₂ receptor also produces analgesic effects in models of acute, inflammatory and neuropathic pain. The sites and mechanisms of CB₂ receptor-mediated analgesia are described herein. In addition to targeting cannabinoid receptors directly, protection of endocannabinoids (eCBs) from metabolism also produces analgesic effects. Indeed, reports that noxious stimulation elevates levels of eCBs in the spinal cord and brain provide further rationale for this approach. The effects of inhibition of fatty acid amide hydrolase (FAAH) on nociceptive responses in models of inflammatory and neuropathic pain are discussed.

Endocannabinoid receptor pharmacology

This chapter will review the basic pharmacology of endocannabinoid receptors. As the best-described cannabinoid receptors are G-protein-coupled receptors (GPCRs), those will be the focus of this chapter. We will start with a basic review of GPCR signaling, as these concepts are critical to understanding the function of cannabinoid receptors. Next, several features of cannabinoid receptor signaling will be presented, with an emphasis on the effectors modulated by cannabinoid receptors. Finally, we will finish with a discussion of cannabinoid receptor agonists and antagonists and future directions. The aim of this chapter is to introduce the cannabinoid receptor pharmacology that will be necessary to appreciate the intricacies of endocannabinoid signaling presented in later chapters.

A pilot trial of low-dose naltrexone in primary progressive multiple sclerosis

A sixth month phase II multicenter-pilot trial with a low dose of the opiate antagonist Naltrexone (LDN) has been carried out in 40 patients with primary progressive multiple sclerosis (PPMS). The primary end points were safety and tolerability. Secondary outcomes were efficacy on spasticity, pain, fatigue, depression, and quality of life. Clinical and biochemical evaluations were serially performed. Protein concentration of beta-endorphins (BE) and mRNA levels and allelic variants of the mu-opiod receptor gene (OPRM1) were analyzed. Five dropouts and two major adverse events occurred. The remaining adverse events did not interfere with daily living. Neurological disability progressed in only one patient. A significant reduction of spasticity was measured at the end of the trial. BE concentration increased during the trial, but no association was found between OPRM1 variants and improvement of spasticity. Our data clearly indicate that LDN is safe and well tolerated in patients with PPMS.

Peripheral mechanisms of pain and analgesia

This review summarizes recent findings on peripheral mechanisms underlying the generation and inhibition of pain. The focus is on events occurring in peripheral injured tissues that lead to the sensitization and excitation of primary afferent neurons, and on the modulation of such mechanisms. Primary afferent neurons are of particular interest from a therapeutic perspective because they are the initial generator of noxious impulses traveling towards relay stations in the spinal cord and the brain. Thus, if one finds ways to inhibit the sensitization and/or excitation of peripheral sensory neurons, subsequent central events such as wind-up, sensitization and plasticity may be prevented. Most importantly, if agents are found that selectively modulate primary afferent function and do not cross the blood-brain-barrier, centrally mediated untoward side effects of conventional analgesics (e.g. opioids, anticonvulsants) may be avoided. This article begins with the peripheral actions of opioids, turns to a discussion of the effects of adrenergic co-adjuvants, and then moves on to a discussion of pro-inflammatory mechanisms focusing on TRP channels and nerve growth factor, their signaling pathways and arising therapeutic perspectives.

Nucleotide signaling and cutaneous mechanisms of pain transduction

Sensory neurons that innervate the skin provide critical information about physical contact between the organism and the environment, including information about potentially-damaging stimuli that give rise to the sensation of pain. These afferents also contribute to the maintenance of tissue homeostasis, inflammation and wound healing, while sensitization of sensory afferents after injury results in painful hypersensitivity and protective behavior. In contrast to the traditional view of primary afferent terminals as the sole site of sensory transduction, recent reports have lead to the intriguing idea that cells of the skin play an active role in the transduction of sensory stimuli. The search for molecules that transduce different types of sensory stimuli (mechanical, heat, chemical) at the axon terminal has yielded a wide range of potential effectors, many of which are expressed by keratinocytes as well as neurons. Emerging evidence underscores the importance of nucleotide signaling through P2X ionotropic and P2Y metabotropic receptors in pain processing, and implicates nucleotide signaling as a critical form of communication between cells of the skin, immune cells and sensory neurons. It is of great interest to determine whether pathological changes in these mechanisms contribute to chronic pain in human disease states such as complex regional pain syndrome (CRPS). This review discusses recent advances in our understanding of communication mechanisms between cells of the skin and sensory axons in the transduction of sensory input leading to pain.

Role of ionotropic cannabinoid receptors in peripheral antinociception and antihyperalgesia

Despite the wealth of information on cannabinoid-induced peripheral antihyperalgesic and antinociceptive effects in many pain models, the molecular mechanism(s) for these actions remains unknown. Although metabotropic cannabinoid receptors have important roles in many pharmacological actions of cannabinoids, recent studies have led to the recognition of a family of at least five ionotropic cannabinoid receptors (ICRs). The known ICRs are members of the family of transient receptor potential (TRP) channels and include TRPV1, TRPV2, TRPV4, TRPM8 and TRPA1. Cannabinoid activation of ICRs can result in desensitization of the TRPA1 and TRPV1 channel activities, inhibition of nociceptors and antihyperalgesia and antinociception in certain pain models. Thus, cannabinoids activate both metabotropic and ionotropic mechanisms to produce peripheral analgesic effects. Here, we provide an overview of the pharmacology of TRP channels as ICRs.

Peripheral delta opioid receptors require priming for functional competence in vivo

Although centrally acting opioid analgesics produce profound antinociception under basal conditions, the antinociceptive properties of peripherally restricted opioid analgesics are generally only detectable after inflammation or injection of inflammatory mediators. Despite considerable research, the cellular mechanisms regulating the functional competence of peripheral opioid receptor systems for inhibition of nociception remain unclear. Recent work has demonstrated that brief pre-treatment (priming) with bradykinin, arachidonic acid, protease-activated receptor-2 agonists, or direct activators of protein kinase C (PKC) are capable of inducing the functional competence of the opioid receptor system in cultures of primary sensory neurons in vitro. Here we report that the peripheral delta opioid receptor system also requires PKC-dependent priming to inhibit prostaglandin E(2) (PGE(2))-induced thermal allodynia in the rat. Peripheral hindpaw injection of [D-Pen(2,5)]-enkephalin (DPDPE), a selective delta opioid receptor agonist, did not alter PGE(2)-induced thermal allodynia. However, following priming (15 min) with bradykinin or arachidonic acid, DPDPE produced a significant reduction in allodynia that was antagonist reversible, peripherally restricted, and exhibited a typical dose-response relationship. Furthermore, the bradykinin priming effect was blocked by the PKC inhibitors, bisindolylmaleimide I and chelerythrine. Collectively, these data support prior in vitro findings that, although present on primary sensory neurons, peripheral opioid receptor systems are functionally inactive under basal conditions and require activation of a PKC- and arachidonic acid-dependent signaling pathway to develop functional competence in vivo.

Cannabinoid modulation of cutaneous Adelta nociceptors during inflammation

Previous studies have demonstrated that locally administered cannabinoids attenuate allodynia and hyperalgesia through activation of peripheral cannabinoid receptors (CB(1) and CB(2)). However, it is currently unknown if cannabinoids alter the response properties of nociceptors. In the present study, correlative behavioral and in vivo electrophysiological studies were conducted to determine if peripheral administration of the cannabinoid receptor agonists arachidonyl-2'-chloroethylamide (ACEA) or (R)-(+)-methanandamide (methAEA) could attenuate mechanical allodynia and hyperalgesia, and decrease mechanically evoked responses of Adelta nociceptors. Twenty-four hours after intraplantar injection of complete Freund's adjuvant (CFA), rats exhibited allodynia (decrease in paw withdrawal threshold) and hyperalgesia (increase in paw withdrawal frequency), which were attenuated by both ACEA and methAEA. The antinociceptive effects of these cannabinoids were blocked by co-administration with the CB(1) receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophen yl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) but not with the CB(2) receptor antagonist 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-y l](4-methoxyphenyl)methanone (AM630). ACEA and methAEA did not produce antinociception under control, non-inflamed conditions 24 h after intraplantar injection of saline. In parallel studies, recordings were made from cutaneous Adelta nociceptors from inflamed or control, non-inflamed skin. Both ACEA and methAEA decreased responses evoked by mechanical stimulation of Adelta nociceptors from inflamed skin but not from non-inflamed skin, and this decrease was blocked by administration of the CB(1) receptor antagonist AM251. These results suggest that attenuation of mechanically evoked responses of Adelta nociceptors contributes to the behavioral antinociception produced by activation of peripheral CB(1) receptors during inflammation.

Non-opioid antinociception produced by brain stem injections of improgan: significance of local, but not cross-regional, cannabinoid mechanisms

Improgan, a cimetidine derivative which lacks activity at known histamine, opioid or cannabinoid receptors, acts by an unknown mechanism in the periaqueductal gray (PAG) and raphe magnus (RM) to stimulate descending, analgesic circuits. These circuits may utilize cannabinoid mechanisms. To characterize further the nature of these circuits, the effects of intracerebral (i.c.) microinjections of rimonabant (a CB(1) receptor inverse agonist) were studied on antinociceptive responses following i.c. microinjections of improgan and the cannabinoid agonist WIN 55,212 (WIN) in rats. Separate intra-RM injections of improgan (30 microg) and WIN (8 microg) produced near-maximal antinociception on both the hot plate (HP) and tail flick (TF) nociceptive tests. Pretreatment with intra-RM rimonabant (20 microg) antagonized the antinociception produced by both intra-RM improgan and intra-RM WIN, but had no effects when given alone. Similar studies with improgan demonstrated rimonabant-sensitive sites within the dorsal and ventrolateral PAG. However, intra-RM pretreatment with rimonabant had no effect on antinociceptive responses following intra-PAG improgan. These studies show that improgan activates pain-relieving mechanisms in the PAG and the RM, both of which may utilize local cannabinoid mechanisms.

MDA7: a novel selective agonist for CB2 receptors that prevents allodynia in rat neuropathic pain models

BACKGROUND AND PURPOSE

There is growing interest in using cannabinoid type 2 (CB(2)) receptor agonists for the treatment of neuropathic pain. In this report, we describe the pharmacological characteristics of MDA7 (1-[(3-benzyl-3-methyl-2,3-dihydro-1-benzofuran-6-yl)carbonyl]piperidine), a novel CB(2) receptor agonist.

EXPERIMENTAL APPROACH

We characterized the pharmacological profile of MDA7 by using radioligand-binding assays and in vitro functional assays at human cannabinoid type 1 (CB(1)) and CB(2) receptors. In vitro functional assays were performed at rat CB(1) and CB(2) receptors. The effects of MDA7 in reversing neuropathic pain were assessed in spinal nerve ligation and paclitaxel-induced neuropathy models in rats.

KEY RESULTS

MDA7 exhibited selectivity and agonist affinity at human and rat CB(2) receptors. MDA7 treatment attenuated tactile allodynia produced by spinal nerve ligation or by paclitaxel in a dose-related manner. These effects were selectively antagonized by a CB(2) receptor antagonist but not by CB(1) or opioid receptor antagonists. MDA7 did not affect rat locomotor activity.

CONCLUSION AND IMPLICATIONS

MDA7, a novel selective CB(2) agonist, was effective in suppressing neuropathic nociception in two rat models without affecting locomotor behaviour. These results confirm the potential for CB(2) agonists in the treatment of neuropathic pain.

Modulation of the anti-nociceptive effects of 2-arachidonoyl glycerol by peripherally administered FAAH and MGL inhibitors in a neuropathic pain model

BACKGROUND AND PURPOSE

There are limited options for the treatment of neuropathic pain. Endocannabinoids, such as anandamide and 2-arachidonoyl glycerol (2-AG), are promising pain modulators and there is recent evidence of interactions between anandamide and 2-AG biosynthesis and metabolism. It has been clearly demonstrated that 2-AG degradation is mainly catalysed not only by monoacylglycerol lipase (MGL) but also by a fatty acid amide hydrolase (FAAH). Inhibitors specifically targeting these two enzymes have also been described: URB602 and URB597, respectively. However, the anti-nociceptive effects of the combination of peripherally injected 2-AG, URB602 and URB597 in a neuropathic pain model have not yet been determined. This was performed in the presence or absence of cannabinoid CB(1) (AM251) and CB(2) (AM630) receptor antagonists.

EXPERIMENTAL APPROACH

Mechanical allodynia and thermal hyperalgesia were evaluated in 213 male Wistar rats allocated to 32 different groups. Drugs were injected subcutaneously in the dorsal surface of the hind paw (50 muL) 15 min before pain tests.

KEY RESULTS

2-AG, URB602 and URB597 significantly decreased mechanical allodynia and thermal hyperalgesia with ED50 of 1.6+/-1.5 and 127+/-83 mug for 2-AG and URB602, respectively. These effects were mediated locally and were mostly inhibited by the two cannabinoid antagonists.

CONCLUSIONS AND IMPLICATIONS

The combination of the three compounds did not produce any greater anti-allodynic or anti-hyperalgesic effects, suggesting that FAAH inhibition could reduce or limit the anti-nociceptive effects of 2-AG. Peripheral administration of endocannabinoids or MGL/FAAH inhibitors is a promising analgesic approach requiring further investigation.