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

Endogenous beta-endorphin induces thermal analgesia at the initial stages of a murine osteosarcoma

Transient thermal, but not mechanical, hypoalgesia appears at the early stages of the development of an hyperalgesic murine osteosarcoma. This hypoalgesia is suppressed by the administration of naloxone, its peripherally acting analog naloxone methiodide, the mu- and delta-opioid receptor antagonists cyprodime and naltrindole, or the CRF receptor antagonist, alpha-helical CRF (9-41). When immunohistochemical assays were performed with an anti-beta-endorphin antibody, whose in vivo administration suppressed the analgesia, labeled mononuclear immune cells appeared both inside and surrounding the tumoral tissue. In conclusion, the peripheral action of beta-endorphin, released in response to the osteosarcoma seems responsible for the observed thermal analgesia.

Non-psychoactive CB2 cannabinoid agonists stimulate neural progenitor proliferation

Cannabinoids, the active components of marijuana and their endogenous counterparts, act on the brain and many other organs through the widely expressed CB1 cannabinoid receptor. In contrast, the CB2 cannabinoid receptor is abundant in the immune system and shows a restricted expression pattern in brain cells. CB2-selective agonists are, therefore, very attractive therapeutic agents as they do not cause CB1-mediated psychoactive effects. CB2 receptor expression in brain has been partially examined in differentiated cells, while its presence and function in neural progenitor cells remain unknown. Here we show that the CB2 receptor is expressed, both in vitro and in vivo, in neural progenitors from late embryonic stages to adult brain. Selective pharmacological activation of the CB2 receptor in vitro promotes neural progenitor cell proliferation and neurosphere generation, an action that is impaired in CB2-deficient cells. Accordingly, in vivo experiments evidence that hippocampal progenitor proliferation is increased by administration of the CB2-selective agonist HU-308. Moreover, impaired progenitor proliferation was observed in CB2-deficient mice both in normal conditions and on kainate-induced excitotoxicity. These findings provide a novel physiological role for the CB2 cannabinoid receptor and open a novel therapeutic avenue for manipulating neural progenitor cell fate.

Neuronal Control of Skin Function: The Skin as a Neuroimmunoendocrine Organ

This review focuses on the role of the peripheral nervous system in cutaneous biology and disease. During the last few years, a modern concept of an interactive network between cutaneous nerves, the neuroendocrine axis, and the immune system has been established. We learned that neurocutaneous interactions influence a variety of physiological and pathophysiological functions, including cell growth, immunity, inflammation, pruritus, and wound healing. This interaction is mediated by primary afferent as well as autonomic nerves, which release neuromediators and activate specific receptors on many target cells in the skin. A dense network of sensory nerves releases neuropeptides, thereby modulating inflammation, cell growth, and the immune responses in the skin. Neurotrophic factors, in addition to regulating nerve growth, participate in many properties of skin function. The skin expresses a variety of neurohormone receptors coupled to heterotrimeric G proteins that are tightly involved in skin homeostasis and inflammation. This neurohormone-receptor interaction is modulated by endopeptidases, which are able to terminate neuropeptide-induced inflammatory or immune responses. Neuronal proteinase-activated receptors or transient receptor potential ion channels are recently described receptors that may have been important in regulating neurogenic inflammation, pain, and pruritus. Together, a close multidirectional interaction between neuromediators, high-affinity receptors, and regulatory proteases is critically involved to maintain tissue integrity and regulate inflammatory responses in the skin. A deeper understanding of cutaneous neuroimmunoendocrinology may help to develop new strategies for the treatment of several skin diseases.

Identification of in vitro metabolites of JWH-015, an aminoalkylindole agonist for the peripheral cannabinoid receptor (CB2) by HPLC-MS/MS

The in vitro microsomal metabolism of JWH-015, a ligand that exhibits a high binding affinity at the peripheral cannabinoid receptor CB(2), has been studied. A total of 22 metabolites were identified and structurally characterized. The metabolites are products of: 1) monohydroxylation on the naphthalene ring (m/z 344, M20 and M21), indole ring (m/z 344, M17 and M18), or the N-alkyl group (m/z 344, M14); 2) arene oxidation leading to dihydrodiols (m/z 362, M12 and M15); 3) dihydroxylation on the naphthalene ring (m/z 360, M7) or indole ring (m/z 360, M13), resulting from a combination of monohydroxylations on both the naphthalene and indole rings (m/z 360, M16), or a combination of monohydroxylations on the naphthalene ring and on the N-propyl group (m/z 360, M9); 4) trihydroxylation (m/z 378, M1, M3, M4, M6, and M10); 5) N-dealkylation (m/z 286, M19); 6) N-dealkylation and monohydroxylation on the naphthalene ring (m/z 302, M11); 7) N-dealkylation and dihydrodiol formation from arene oxidation (m/z 320, M2 and M5); 8) dehydrogenation after monohydroxylation on the N-alkyl group (m/z 326, M22); 9) dehydrogenation and monohydroxylation on the indole ring (m/z 342, M8).

Cannabinoid WIN 55,212-2 regulates TRPV1 phosphorylation in sensory neurons

Cannabinoids are known to have multiple sites of action in the nociceptive system, leading to reduced pain sensation. However, the peripheral mechanism(s) by which this phenomenon occurs remains an issue that has yet to be resolved. Because phosphorylation of TRPV1 (transient receptor potential subtype V1) plays a key role in the induction of thermal hyperalgesia in inflammatory pain models, we evaluated whether the cannabinoid agonist WIN 55,212-2 (WIN) regulates the phosphorylation state of TRPV1. Here, we show that treatment of primary rat trigeminal ganglion cultures with WIN led to dephosphorylation of TRPV1, specifically at threonine residues. Utilizing Chinese hamster ovary cell lines, we demonstrate that Thr(144) and Thr(370) were dephosphorylated, leading to desensitization of the TRPV1 receptor. This post-translational modification occurred through activation of the phosphatase calcineurin (protein phosphatase 2B) following WIN treatment. Furthermore, knockdown of TRPA1 (transient receptor potential subtype A1) expression in sensory neurons by specific small interfering RNA abolished the WIN effect on TRPV1 dephosphorylation, suggesting that WIN acts through TRPA1. We also confirm the importance of TRPA1 in WIN-induced dephosphorylation of TRPV1 in Chinese hamster ovary cells through targeted expression of one or both receptor channels. These results imply that the cannabinoid WIN modulates the sensitivity of sensory neurons to TRPV1 activation by altering receptor phosphorylation. In addition, our data could serve as a useful strategy in determining the potential use of certain cannabinoids as peripheral analgesics.

Neurophysiological, neuroimmunological, and neuroendocrine basis of pruritus

Pruritus (itch) can be defined as an unpleasant cutaneous sensation associated with the immediate desire to scratch. Recent findings have identified potential classes of endogenous "itch mediators" and establish a modern concept for the pathophysiology of pruritus. First, there in no universal peripheral itch mediator, but disease-specific sets of involved mediators. Second, numerous mediators of skin cells can activate and sensitize pruritic nerve endings, and even modulate their growth. Our knowledge of itch processing in the spinal cord and the involved centers in the central nervous system is rapidly growing. This review summarizes the current information about the significance of neuron-skin interactions, ion channels, neuropeptides, proteases, cannabinoids, opioids, kinins, cytokines, biogenic amines, neurotransmitters, and their receptors in the pathobiology of pruritus. A deeper understanding of these circuits is required for the development of novel antipruritic strategies.

The endocannabinoid system as an emerging target of pharmacotherapy

The recent identification of cannabinoid receptors and their endogenous lipid ligands has triggered an exponential growth of studies exploring the endocannabinoid system and its regulatory functions in health and disease. Such studies have been greatly facilitated by the introduction of selective cannabinoid receptor antagonists and inhibitors of endocannabinoid metabolism and transport, as well as mice deficient in cannabinoid receptors or the endocannabinoid-degrading enzyme fatty acid amidohydrolase. In the past decade, the endocannabinoid system has been implicated in a growing number of physiological functions, both in the central and peripheral nervous systems and in peripheral organs. More importantly, modulating the activity of the endocannabinoid system turned out to hold therapeutic promise in a wide range of disparate diseases and pathological conditions, ranging from mood and anxiety disorders, movement disorders such as Parkinson's and Huntington's disease, neuropathic pain, multiple sclerosis and spinal cord injury, to cancer, atherosclerosis, myocardial infarction, stroke, hypertension, glaucoma, obesity/metabolic syndrome, and osteoporosis, to name just a few. An impediment to the development of cannabinoid medications has been the socially unacceptable psychoactive properties of plant-derived or synthetic agonists, mediated by CB(1) receptors. However, this problem does not arise when the therapeutic aim is achieved by treatment with a CB(1) receptor antagonist, such as in obesity, and may also be absent when the action of endocannabinoids is enhanced indirectly through blocking their metabolism or transport. The use of selective CB(2) receptor agonists, which lack psychoactive properties, could represent another promising avenue for certain conditions. The abuse potential of plant-derived cannabinoids may also be limited through the use of preparations with controlled composition and the careful selection of dose and route of administration. The growing number of preclinical studies and clinical trials with compounds that modulate the endocannabinoid system will probably result in novel therapeutic approaches in a number of diseases for which current treatments do not fully address the patients' need. Here, we provide a comprehensive overview on the current state of knowledge of the endocannabinoid system as a target of pharmacotherapy.

Effects of a cannabinoid agonist on spinal nociceptive neurons in a rodent model of neuropathic pain

The effects of the synthetic cannabinoid WIN 55,212-2 on heat-evoked firing of spinal wide dynamic range (WDR) neurons were examined in a rodent model of neuropathic pain. Fifty-eight WDR neurons (1 cell/animal) were recorded from the ipsilateral spinal dorsal horns of rats with chronic constriction injury (CCI) and sham-operated controls. Relative to sham-operated controls, neurons recorded in CCI rats showed elevations in spontaneous firing, noxious heat-evoked responses, and afterdischarge firing as well as increases in receptive field size. WIN 55,212-2 (0.0625, 0.125, and 0.25 mg/kg, intravenous) dose-dependently suppressed heat-evoked activity and decreased the receptive field areas of dorsal horn WDR neurons in both nerve injured and control rats with a greater inhibition in CCI rats. At the dose of 0.125 mg/kg iv, WIN 55,212-2 reversed the hyperalgesia produced by nerve injury. The effect of intravenous administration of WIN 55,212-2 appears to be centrally mediated because administration of the drug directly to the ligated nerve did not suppress the heat-evoked neuronal activity in CCI rats. Pretreatment with the cannabinoid CB(1) receptor antagonists SR141716A or AM251, but not the CB(2) antagonist SR144528, blocked the effects. These results provide a neural basis for reports of potent suppression by cannabinoids of the abnormal sensory responses that result from nerve injury.

Formation of B and T cell subsets require the cannabinoid receptor CB2

A recent and surprising body of research has linked changes in immune function to biologic and therapeutic targeting of cannabinoid receptors, which prototypically respond to delta-9 tetrahydrocannabinol. The peripheral cannabinoid receptor CB2 is highly expressed in immune cell types (macrophages, dendritic cells, and B cells), and pharmacologically alters their cytokine production and responsiveness. Accordingly, cannabinoid agonists can powerfully alter susceptibility to certain microbial infections, atherosclerosis, and cancer immunotherapy. What is unknown is the physiologic role of natural levels of endocannabinoids and their receptors in normal immune homeostasis. Galphai2-/- mice are deficient in the formation of certain B and T cell subsets and are susceptible to immune dysregulation, notably developing inflammatory bowel disease. A key issue is the identity of the Gi-coupled receptors relevant to this Galphai2-signaling pathway. We find that mice deficient in CB2, the Gi-coupled peripheral endocannabinoid receptor, have profound deficiencies in splenic marginal zone, peritoneal B1a cells, splenic memory CD4+ T cells, and intestinal natural killer cells and natural killer T cells. These findings partially phenocopy and extend the lymphocyte developmental disorder associated with the Galphai2-/- genotype, and suggest that the endocannabinoid system is required for the formation of T and B cell subsets involved in immune homeostasis. This noncompensatable requirement for physiologic function of the endocannabinoid system is novel. Because levels of endocannabinoids are highly restricted microanatomically, local regulation of their production and receptor expression offers a new principle for regional immune homeostasis and disease susceptibility, and extends and refines the rationale for CB2-targeted immunotherapy in immune and inflammatory diseases.

In vitro pharmacological characterization of AM1241: a protean agonist at the cannabinoid CB2 receptor?

BACKGROUND AND PURPOSE

The CB2 receptor has been proposed as a novel target for the treatment of pain, and CB2 receptor agonists defined in in vitro assays have demonstrated analgesic activity in animal models. Based on its in vivo analgesic efficacy, AM1241 has been classified as a CB2-selective agonist. However, in vitro characterization of AM1241 in functional assays has not been reported.

EXPERIMENTAL APPROACH

In this study, AM1241 was characterized across multiple in vitro assays employing heterologous recombinant receptor expression systems to assess its binding potencies at the human CB2 and CB1 receptors and its functional efficacies at the human CB2 receptor.

KEY RESULTS

AM1241 exhibited distinct functional properties depending on the assay conditions employed, a unique profile in contrast to those of the agonist CP 55,940 and the inverse agonist SR144528. AM1241 displayed neutral antagonist activities in FLIPR and cyclase assays. However, when cyclase assays were performed using lower forskolin concentrations for stimulation, AM1241 exhibited partial agonist efficacy. In addition, it behaved as a partial agonist in ERK (or MAP) kinase assays.

CONCLUSIONS AND IMPLICATIONS

The unusual phenomenon of inconsistent functional efficacies suggests that AM1241 is a protean agonist at the CB2 receptor. We postulate that functional efficacies displayed by protean agonists in various assay systems may depend on the levels of receptor constitutive activities exhibited in the assay systems, and therefore, efficacies observed in in vitro assays may not predict in vivo activities.

Characterization of chronic constriction of the saphenous nerve, a model of neuropathic pain in mice showing rapid molecular and electrophysiological changes

Neuropathic pain is one of the most inextricable problems encountered in clinics, because few facts are known about its etiology. Nerve injury often leads to allodynia and hyperalgesia, which are symptoms of neuropathic pain. The aim of this study was to understand some molecular and electrophysiological mechanisms of neuropathic pain after chronic constriction of the saphenous nerve (CCS) in mice. After surgery, CCS mice displayed significant allodynia and hyperalgesia, which were sensitive to acute systemic injection of morphine (4 mg/kg), gabapentin (50 mg/kg), amitriptyline (10 mg/kg), and the cannabinoid agonist WIN 55,212-2 (5 mg/kg). These behavioral changes were accompanied after surgery by an increase of c-Fos expression and by an overexpression of mu-opioid and cannabinoid CB1 and CB2 receptors in the spinal cord and the dorsal hind paw skin. In combination with the skin-nerve preparation, this model showed a decrease in functional receptive fields downstream to the injury and the apparition of A-fiber ectopic discharges. In conclusion, CCS injury induced behavioral, molecular, and electrophysiological rearrangements that might help us in better understanding the peripheral mechanisms of neuropathic pain. This model takes advantage of the possible use in the future of genetically modified mice and of an exclusively sensory nerve for a comprehensive study of peripheral mechanisms of neuropathic pain.

The cannabinoid WIN 55,212-2 inhibits transient receptor potential vanilloid 1 (TRPV1) and evokes peripheral antihyperalgesia via calcineurin

Cannabinoids can evoke antihyperalgesia and antinociception at a peripheral site of action. However, the signaling pathways mediating these effects are not clearly understood. We tested the hypothesis that certain cannabinoids directly inhibit peripheral capsaicin-sensitive nociceptive neurons by dephosphorylating and desensitizing transient receptor potential vanilloid 1 (TRPV1) via a calcium calcineurin-dependent mechanism. Application of the cannabinoid WIN 55,212-2 (WIN) to cultured trigeminal (TG) neurons or isolated skin biopsies rapidly and significantly inhibited capsaicin-activated inward currents and neuropeptide exocytosis by a mechanism requiring the presence of extracellular calcium. The inhibitory effect did not involve activation of G protein-coupled cannabinoid receptors, because neither pertussis toxin nor GDPbetaS treatments altered the WIN effect. However, application of WIN-activated calcineurin, as measured by nuclear translocation of the nuclear factor of activated T cells (NFAT)c4 transcription factor, dephosphorylated TRPV1. The WIN-induced desensitization of TRPV1 was mediated by calcineurin, because the application of structurally distinct calcineurin antagonists (calcineurin autoinhibitory peptide and cyclosporine/cyclophilin complex) abolished WIN-induced inhibition of capsaicin-evoked inward currents and neuropeptide exocytosis. This mechanism also contributed to peripheral antinociceptive/antihyperalgesic effects of WIN because pretreatment with the calcineurin antagonist calcineurin autoinhibitory peptide (CAIP) significantly reduced peripherally mediated WIN effects in two behavioral models. Collectively, these data demonstrate that cannabinoids such as WIN directly inhibit TRPV1 functional activities via a calcineurin pathway that represents a mechanism of cannabinoid actions at peripheral sites.

The endocannabinoid system: current pharmacological research and therapeutic possibilities

In the relatively short period of time since the discovery of cannabinoid receptors and their endogenous ligands, the endocannabinoids, an intensive research effort has resulted in the identification of agents that affect all aspects of the endocannabinoid system. The cannabinoid(1) receptor antagonist rimonabant is in phase III clinical trials for the treatment of obesity and as an aid to smoking cessation, and cannabinoid(2) receptor agonists are promising in animal models of inflammatory and neuropathic pain. In the present MiniReview, the endocannabinoid system is described from a pharmacological perspective. The main topics covered are: the mechanism of action of cannabinoid(2) receptor agonists; identification of the endocannabinoid(s) involved in retrograde signalling; the elusive mechanism(s) of endocannabinoid uptake; therapeutic possibilities for fatty acid amide hydrolase inhibitors; and the cyclooxygenase-2 and lipoxygenase-derived biologically active metabolites of the endocannabinoids.

The neurobiology of itch

The neurobiology of itch, which is formally known as pruritus, and its interaction with pain have been illustrated by the complexity of specific mediators, itch-related neuronal pathways and the central processing of itch. Scratch-induced pain can abolish itch, and analgesic opioids can generate itch, which indicates an antagonistic interaction. However, recent data suggest that there is a broad overlap between pain- and itch-related peripheral mediators and/or receptors, and there are astonishingly similar mechanisms of neuronal sensitization in the PNS and the CNS. The antagonistic interaction between pain and itch is already exploited in pruritus therapy, and current research concentrates on the identification of common targets for future analgesic and antipruritic therapy.

CB2 receptor-mediated antihyperalgesia: possible direct involvement of neural mechanisms

In mouse the cannabinoid receptor 2 (CB2) agonists L768242 and (+)-AM1241, at doses of 30 mg/kg i.p. and 1 and 3 mg/kg i.v., respectively, reduced the second phase of nocifensive behaviors elicited by formalin intraplantar injection. This effect was counteracted by the selective CB2 antagonist SR144528 (1 mg/kg i.p.). In rat (+)-AM1241 (3 and 6 mg/kg i.v.) and L768242 (30 mg/kg i.p.) reduced allodynia elicited by L5-L6 spinal nerve ligation. SR144528 reverted these effects, supporting a CB2-mediated action. To clarify the mechanisms underlying these effects we investigated CB2 gene expression and function in the nervous system. CB2 mRNA was expressed in spinal cord and dorsal root ganglia (DRG) of both sham and neuropathic rats and was up-regulated in the ipsilateral spinal cord of neuropathic rats. Expression studies demonstrated the presence of CB2 mRNA in culture of spinal cord microglia. A biomarker, CGRP, was used to investigate modulation of DRG primary afferents by CB2 agonists. Both L768242 and (+)-AM1241 dose dependently (EC50 of 3.6 and 4.5 nM, respectively) reduced capsaicin-induced calcitonin gene-related peptide (CGRP) release. Coadministration of SR144528 resulted in a rightforward shift (pKB 8.1 and 8.2 for (+)-AM1241 and L768242, respectively) of the dose-response curve. Experiments on capsaicin-induced CGRP release in tissue from CB1-/- mice ruled out a CB1-mediated effect. These results confirm that CB2 is present in the central nervous system and suggest that CB2 agonists may elicit their analgesic effect by acting not only at non-neuronal peripheral sites but also at neural level, making CB2 an attractive target for chronic pain treatment.

Induction of CB1 cannabinoid receptor by inflammation in primary afferent neurons facilitates antihyperalgesic effect of peripheral CB1 agonist

Cannabinoids act on various regions in the nervous system to modulate neuronal activity including nociception. Here, we investigated CB1 receptor expression in primary afferent neurons in the dorsal root ganglion (DRG) and the efficacy of a local (intraplantar) application of the selective CB1 agonist, 2-arachidonyl-2-chloroethylamide (ACEA), on inflammatory thermal hyperalgesia. In situ hybridization showed normal CB1 mRNA expression in 28% of DRG neurons. Peripheral inflammation by CFA (complete Freund's adjuvant) significantly increased the ratio of CB1 mRNA-positive neurons to 43%, primarily with increase in NF200-negative C-fiber nociceptors. Furthermore, CB1 and TRPV1 (transient potential receptor vanilloid subtype-1) co-localization was increased from 41% before inflammation to 67% two days after inflammation. Inflammation also increased CB1 immunoreactivity in DRG neurons and in nerve fibers of the hindpaw dermis, indicating increased CB1 transport from the cell body to the peripheral nerve. The intraplantar application of ACEA attenuated CFA-induced thermal hyperalgesia. The antinociceptive properties of ACEA became more prominent at 2 days after inflammation, compared with those in non-inflamed and inflamed animals at 8 h. These results suggest that CB1 expression in primary afferent neurons is increased by inflammation and that the subsequent increase in CB1 transport to peripheral axons contributes to the increased antihyperalgesic efficacy of locally administered CB1 agonist.

New anti-inflammatory agents to reduce atherosclerosis

The concept that inflammation plays a major role in atherogenesis has become accepted in recent years (Hansson 2005). As a result, anti-inflammatory agents may become increasingly important in the treatment of atherogenesis, atherosclerosis, and possibly even acute coronary or ischemic syndromes. This presentation reviews two types of molecules associated with the diagnosis, development, or treatment of atherosclerosis: C-reactive protein (CRP), and cannabis.

Modulation of Neuropathic and Inflammatory Pain by the Endocannabinoid Transport Inhibitor AM404 [N-(4-Hydroxyphenyl)-eicosa-5,8,11,14-tetraenamide]

The endocannabinoid system may serve important functions in the central and peripheral regulation of pain. In the present study, we investigated the effects of the endocannabinoid transport inhibitor AM404 [N-(4-hydroxyphenyl)-eicosa-5,8,11,14-tetraenamide] on rodent models of acute and persistent nociception (intraplantar formalin injection in the mouse), neuropathic pain (sciatic nerve ligation in the rat), and inflammatory pain (complete Freund's adjuvant injection in the rat). In the formalin model, administration of AM404 (1-10 mg/kg i.p.) elicited dose-dependent antinociceptive effects, which were prevented by the CB(1) cannabinoid receptor antagonist rimonabant (SR141716A; 1 mg/kg i.p.) but not by the CB2 antagonist SR144528 (1 mg/kg i.p.) or the vanilloid antagonist capsazepine (30 mg/kg i.p.). Comparable effects were observed with UCM707 [N-(3-furylmethyl)-eicosa-5,8,11,14-tetraenamide], another anandamide transport inhibitor. In both the chronic constriction injury and complete Freund's adjuvant model, daily treatment with AM404 (1-10 mg/kg s.c.) for 14 days produced a dose-dependent reduction in nocifensive responses to thermal and mechanical stimuli, which was prevented by a single administration of rimonabant (1 mg/kg i.p.) and was accompanied by decreased expression of cyclooxygenase-2 and inducible nitric-oxide synthase in the sciatic nerve. The results provide new evidence for a role of the endocannabinoid system in pain modulation and point to anandamide transport as a potential target for analgesic drug development.

The emerging role of the endocannabinoid system in endocrine regulation and energy balance

During the last few years, the endocannabinoid system has emerged as a highly relevant topic in the scientific community. Many different regulatory actions have been attributed to endocannabinoids, and their involvement in several pathophysiological conditions is under intense scrutiny. Cannabinoid receptors, named CB1 receptor and CB2 receptor, first discovered as the molecular targets of the psychotropic component of the plant Cannabis sativa, participate in the physiological modulation of many central and peripheral functions. CB2 receptor is mainly expressed in immune cells, whereas CB1 receptor is the most abundant G protein-coupled receptor expressed in the brain. CB1 receptor is expressed in the hypothalamus and the pituitary gland, and its activation is known to modulate all the endocrine hypothalamic-peripheral endocrine axes. An increasing amount of data highlights the role of the system in the stress response by influencing the hypothalamic-pituitary-adrenal axis and in the control of reproduction by modifying gonadotropin release, fertility, and sexual behavior. The ability of the endocannabinoid system to control appetite, food intake, and energy balance has recently received great attention, particularly in the light of the different modes of action underlying these functions. The endocannabinoid system modulates rewarding properties of food by acting at specific mesolimbic areas in the brain. In the hypothalamus, CB1 receptor and endocannabinoids are integrated components of the networks controlling appetite and food intake. Interestingly, the endocannabinoid system was recently shown to control metabolic functions by acting on peripheral tissues, such as adipocytes, hepatocytes, the gastrointestinal tract, and, possibly, skeletal muscle. The relevance of the system is further strenghtened by the notion that drugs interfering with the activity of the endocannabinoid system are considered as promising candidates for the treatment of various diseases, including obesity.

CANNABINOID RECEPTORS AS THERAPEUTIC TARGETS

CB1 and CB2 cannabinoid receptors are the primary targets of endogenous cannabinoids (endocannabinoids). These G protein-coupled receptors play an important role in many processes, including metabolic regulation, craving, pain, anxiety, bone growth, and immune function. Cannabinoid receptors can be engaged directly by agonists or antagonists, or indirectly by manipulating endocannabinoid metabolism. In the past several years, it has become apparent from preclinical studies that therapies either directly or indirectly influencing cannabinoid receptors might be clinically useful. This review considers the components of the endocannabinoid system and discusses some of the most promising endocannabinoid-based therapies.

Drugs and alcohol: treating and preventing abuse, addiction and their medical consequences

Recent advances in the fields of genetics, molecular biology, behavioral neuropharmacology, and brain imaging have dramatically changed our understanding of the addictive process and why relapse occurs even in the face of catastrophic consequences. Addiction is now recognized as a chronic brain disease that involves complex interactions between repeated exposure to drugs, biological (i.e., genetic and developmental), and environmental (i.e., drug availability, social, and economic variables) factors. Its treatment, therefore, requires, in general, not only a long-term intervention but also a multipronged approach that addresses the psychiatric, medical, legal, and social consequences of addiction. Also, because addiction usually starts in adolescence or early adulthood and is frequently comorbid with mental illness, we need to expand our treatment interventions in this age group both for substance abuse and psychiatric disorders.

Towards a therapeutic use of selective CB2 cannabinoid receptor ligands for atherosclerosis

Atherosclerosis remains the primary cause of heart disease and stroke, causing approximately 50% of all deaths in Western countries. The identification of promising novel anti-atherosclerotic therapies is therefore of great interest and represents a continued challenge to the medical community. Cannabinoids, such as Δ9-tetrahydrocannabinol (THC), which is the major psychoactive compound of marijuana, modulate immune functions and might therefore be of therapeutic use for the treatment of inflammatory diseases. The authors have demonstrated recently that oral treatment with low dose THC inhibits atherosclerosis progression in mice through pleiotropic immunomodulatory effects on inflammatory cells. All these effects were mediated via the cannabinoid receptor CB2, the main cannabinoid receptor expressed on immune cells. However, these promising results are in conflict with the known health risks of smoking marijuana, as THC binds to and activates both cannabinoid receptors, CB1 and CB2. The identification and characterization of cannabinoid derivative that selectively activate CB2 receptors and are devoid of adverse effects might offer a novel therapeutic strategy for the treatment of atherosclerosis.

Kappa- and delta-opioid receptor functional activities are increased in the caudate putamen of cannabinoid CB1 receptor knockout mice

The purpose of this study was to examine the functional interaction between endogenous opioid and cannabinoid receptor systems in the caudate putamen and nucleus accumbens. We therefore examined by autoradiography the functional activity and density of micro-, kappa- and delta-opioid receptors in both brain regions of cannabinoid CB1 receptor knockout mice. Functional activity was estimated by measuring agonist-stimulated [35S]GTPgammaS binding. Results showed that deletion of the CB1 cannabinoid receptor markedly increased kappa-opioid (50%) and delta-opioid (42%) receptor activities whereas no differences were found in micro-opioid receptor in the caudate putamen. In contrast, binding autoradiography showed a similar density of micro-, kappa- and delta-opioid receptors between mutant and wild-type mice. No differences were found in densities or activities of micro-, kappa- and delta-opioid receptors between mutant and wild-type mice in the nucleus accumbens. Taken together, our results revealed that deletion of CB1 cannabinoid receptors produced a pronounced increase in the activity of kappa- and delta-opioid receptors in the caudate putamen. This endogenous interaction between opioid and cannabinoid receptors may be relevant to further understand a variety of neuroadaptative processes involving the participation of opioid receptors, such as motor behaviour, emotional responses and drug dependence.

Endogenous opioid analgesia in peripheral tissues and the clinical implications for pain control

Opioid receptors are widely expressed in the central and peripheral nervous system as well as in numerous nonneuronal tissues. Both animal models and human clinical data support the involvement of peripheral opioid receptors in analgesia, particularly in inflammation where both opioid receptor expression and efficacy are increased. Immune cells have been shown to contain numerous opioid peptides such as beta-endorphin (END), met-enkephalin (ENK), and dynorphin-A (DYN), although the predominant opioid peptide involved in immune-cell mediated antinociception is thought to be END. These opioid-containing immune cells migrate to inflamed tissues during a complex process of recruitment by chemokines, adhesion, and extravasation. In these tissues, opioid peptide is released from the immune cells upon stimulation with corticotrophin-releasing factor (CRF), noradrenaline, and interleukin 1beta (IL-1beta), and the immune cells return to the local lymph node depleted of peptide. Consistent with this model, systemic immunosuppression may lead to impaired endogenous analgesia as competent immune cells are essential to achieve release of endogenous opioid peptides within inflamed tissue. A further level of complexity is added by the observation that exogenous opioids may impair immune cell function, although there is some evidence to suggest that endogenous opioid peptides do not share this immunosuppressive effect. Improving our understanding of endogenous opioid mechanisms will provide valuable insight towards the development of novel treatments for pain with improved side effect profiles.

Peripheral nerve injury induces cannabinoid receptor 2 protein expression in rat sensory neurons

We have localized cannabinoid receptor 2 protein in rat and mouse somatic sensory nervous system, using an antibody that recognizes mouse cannabinoid receptor 2. Little or no cannabinoid receptor 2 immunoreactivity was found in sections of naive rat or mouse dorsal root ganglia or spinal cord. This was in accord with the lack of detectable cannabinoid receptor 2 mRNA in (dorsal root ganglion) neurons by in situ hybridization experiments described in the literature. However, we could detect cannabinoid receptor 2 immunoreactivity following unilateral nerve damage-either by sciatic nerve section, or by spinal nerve ligation. It was localized to the superficial laminae of the dorsal horn of the spinal cord, ipsilateral to the nerve damage, coincident with the area of termination of damaged afferents which was marked by loss of isolectin B4 binding. This upregulation was not seen in cannabinoid receptor 2 null mice. The cannabinoid receptor 2 protein in spinal cord appeared to be expressed on sensory neuron afferent terminals as it colocalized with two markers of damaged afferents, namely growth associated protein-43 and the neuropeptide galanin. Moreover, it did not colocalize with markers of activated microglial cells (OX-42) or astroglial cells (glial fibrillary acidic protein) in rat spinal cord. In the peripheral nerve, accumulation of cannabinoid receptor 2 immunoreactivity was seen in nerve sections proximal, but not distal, to the ligation site, suggesting transport down the nerve from the cell bodies. Although convincing cannabinoid receptor 2 immunoreactivity was seen in neither uninjured nor injured dorsal root ganglion neuron cell bodies in tissue sections, expression was detectable in isolated, cultured neurons that had received a prior axotomy in vivo. This clear demonstration of CB(2) receptors on sensory neurons suggests an additional cellular target for CB(2) agonist induced analgesia, at least in neuropathic models.

A role for cannabinoid receptors, but not endogenous opioids, in the antinociceptive activity of the CB2-selective agonist, GW405833

Several recent reports have demonstrated a role for selective cannabinoid CB2 receptor agonists in pain modulation, showing both analgesic and antihyperalgesic activities. While the mechanism of action is poorly understood, it has been postulated that these effects may be indirect, involving release of endogenous opioids. We have previously reported that administration of the selective cannabinoid CB2 receptor agonist GW405833 (2,3-dichloro-phenyl)-[5-methoxy-2-methyl-3-(2-morpholin-4-yl-ethyl)-indol-1-yl]-methanone) to rats elicits potent and efficacious antihyperalgesic effects against neuropathic and inflammatory pain and, at high dose (100 mg/kg), is analgesic and ataxic [Valenzano, K.J., Tafesse, L., Lee, G., Harrison, J.E., Boulet, J., Gottshall, S.L., Mark, L., Pearson, M.S., Miller, W., Shan, S., Rabadi, L., Rotstheyn, Y., Chaffer, S.M., Turchin, P.I., Elsemore, D.A., Toth, M., Koetzner, L., Whiteside, G.T., 2005. Pharmacological and pharmacokinetic characterization of the cannabinoid receptor 2 agonist, GW405833, utilizing rodent models of acute and chronic pain, anxiety, ataxia and catalepsy. Neuropharmacology 48, 658-672]. In the current study, we confirm these properties using mouse models and investigate the role of cannabinoid CB2 receptors using knockout animals. Furthermore, we provide evidence that the antinociceptive properties of GW405833 are opioid independent. GW405833 elicited robust antihyperalgesic effects in mouse models of inflammatory (Freund's complete adjuvant) and neuropathic (Seltzer) pain. In contrast, GW405833 showed no antihyperalgesic activity against Freund's complete adjuvant-mediated inflammatory pain in cannabinoid CB2 receptor knockout mice. As in rats, high-dose GW405833 (100 mg/kg) showed both analgesic and sedative activities in wild-type mice, activities that were also apparent in cannabinoid CB2 receptor knockout mice. In rats, neither the antihyperalgesic effect in the Freund's complete adjuvant model nor the analgesic effects in tail flick and hot plate assays were inhibited by pre-treatment with the non-selective opioid receptor antagonist, naltrexone. These data demonstrate that the antihyperalgesic effects of GW405833 are mediated via the cannabinoid CB2 receptor, whereas the analgesic and sedative effects are not. Furthermore, these data suggest that the mechanism of action for GW405833 does not depend on the release of endogenous opioids.

The therapeutic potential of drugs that target cannabinoid receptors or modulate the tissue levels or actions of endocannabinoids

There are at least 2 types of cannabinoid receptor, CB(1) and CB(2), both G protein coupled. CB(1) receptors are expressed predominantly at nerve terminals and mediate inhibition of transmitter release, whereas CB(2) receptors are found mainly on immune cells, their roles including the modulation of cytokine release and of immune cell migration. Endogenous agonists for cannabinoid receptors also exist. These "endocannabinoids" are synthesized on demand and removed from their sites of action by cellular uptake and intracellular enzymic hydrolysis. Endocannabinoids and their receptors together constitute the endocannabinoid system. This review summarizes evidence that there are certain central and peripheral disorders in which increases take place in the release of endocannabinoids onto their receptors and/or in the density or coupling efficiency of these receptors and that this upregulation is protective in some disorders but can have undesirable consequences in others. It also considers therapeutic strategies by which this upregulation might be modulated to clinical advantage. These strategies include the administration of (1) a CB(1) and/or CB(2) receptor agonist or antagonist that does or does not readily cross the blood brain barrier; (2) a CB(1) and/or CB(2) receptor agonist intrathecally or directly to some other site outside the brain; (3) a partial CB(1) and/or CB(2) receptor agonist rather than a full agonist; (4) a CB(1) and/or CB(2) receptor agonist together with a noncannabinoid, for example, morphine or codeine; (5) an inhibitor or activator of endocannabinoid biosynthesis, cellular uptake, or metabolism; (6) an allosteric modulator of the CB(1) receptor; and (7) a CB(2) receptor inverse agonist.

Different effects of opioid and cannabinoid receptor agonists on C-fiber-induced extracellular signal-regulated kinase activation in dorsal horn neurons in normal and spinal nerve-ligated rats

Nerve injury results in neuropathic pain, a debilitating pain condition. Whereas cannabinoids are consistently shown to attenuate neuropathic pain, the efficacy of opioids is highly controversial. Molecular mechanisms underlying analgesic effects of opioids and cannabinoids are not fully understood. We have shown that the signaling molecule ERK (extracellular signal-regulated kinase) is activated by C-fiber stimulation in dorsal horn neurons and contributes to pain sensitization. In this study, we examined whether opioids and cannabinoids can affect C-fiber-induced ERK phosphorylation (pERK) in dorsal horn neurons in spinal cord slices from normal and spinal nerve-ligated rats. In normal control spinal slices, capsaicin induced a drastic pERK expression in superficial dorsal horn neurons, which was suppressed by morphine (10 microM), the selective mu-opioid receptor agonist DAMGO [[d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (1 microM)], and the selective CB1 receptor ACEA agonist [arachidonyl-2'-chloroethylamide (5 microM)]. One week after spinal nerve ligation when neuropathic pain is fully developed, capsaicin induced less pERK expression in the injured L(5)-spinal segment. This pERK induction was not suppressed by morphine (10 microM) and DAMGO (1 microM) but was enhanced by high concentration of DAMGO (5 microM). In contrast, ACEA (10 microM) was still very effective in inhibiting capsaicin-induced pERK expression. In the adjacent L(4) spinal segment, both DAMGO and ACEA significantly suppressed pERK induction by capsaicin. These results indicate that, after nerve injury, opioids lose their capability to suppress C-fiber-induced spinal neuron activation in the injured L(5) but not in the intact L(4) spinal segment, whereas cannabinoids still maintain their efficacy.

The cannabinoid CB2 receptor selective agonist JWH133 reduces mast cell oedema in response to compound 48/80 in vivo but not the release of beta-hexosaminidase from skin slices in vitro

In a recent study so far published in abstract form, it was reported that the CB(2) receptor selective agonist AM1241 diminishes oedema produced as a result of mast cell degranulation in vivo. It is, however, not known whether other structurally different CB(2) agonists share this effect, and whether this is due to a direct effect on mast cell function. In the present study, we have investigated the effects of JWH133, a CB(2) receptor selective agonist, together with the anti-inflammatory agent palmitoylethanolamide and its analogue palmitoylisopropylamide, on compound 48/80-induced oedema and degranulation in vivo and in vitro. JWH133 (20 and 200 microg/mouse i.p.) significantly reduced the ability of compound 48/80 to induce oedema in vivo in the anaesthetised mouse following its injection into the ear pinna. Palmitoylethanolamide (200 microg/mouse i.p) also reduced the response to compound 48/80, whereas no firm conclusions could be drawn for palmitoylisopropylamide (20 and 200 microg/mouse i.p.). The CB(2) selective antagonist/inverse agonist SR144528 (60 microg/mouse i.p.) appeared to produce anti-inflammatory effects per se in this model, making it hard to interpret the effects of JWH133 in terms of CB(2) receptor mediated activation. In contrast to the situation in vivo, neither JWH133 (0.3 and 3 microM) nor palmitoylethanolamide (10 microM) affected mast cell degranulation, measured by following the release of the granular protein beta-hexosaminidase, produced by compound 48/80 in vitro in mouse skin slices. The two compounds were also ineffective in inhibiting the binding of [(3)H]pyrilamine to histamine H(1) receptors in vitro. It is concluded that the ability of JWH133 to affect mast cell dependent inflammation in vivo may be mediated by an indirect action upon the mast cells.

Therapeutic potential of cannabinoid receptor agonists as analgesic agents

Increasing data emerging from controlled clinical trials support an analgesic activity of cannabinoids. However, the psychotropic side effects associated with tetrahydrocannabinol or synthetic derivatives essentially puts a brake on their use, possibly limiting the degree of analgesia that can be achieved as well as providing regulatory hurdles. Animal studies show that although these side effects are mediated via central cannabinoid type 1 (CB(1)) receptors, the analgesic activity in chronic pain states may be mediated via spinal CB(1) and potentially CB(2) receptors, as well as peripheral CB(1) and CB2 receptors on sensory nerves or immune cells. The design of novel compounds that either specifically target peripheral CB(1) receptors or display high selectivity for CB(2) receptors may offer avenues for harnessing the analgesic effect of CB receptor agonists while avoiding the central adverse events seen with cannabinoid structures. Clinical trials with such compounds are required to determine whether either approach can provide the level of analgesia required to fulfil the unmet medical need left by current therapies for chronic pain.