Anandamide amidohydrolase reacting with 2-arachidonoylglycerol, another cannabinoid receptor ligand

Membrane microdomains and metabolic pathways that define anandamide and 2-arachidonyl glycerol biosynthesis and breakdown

Anandamide (AEA) and 2-arachidonyl glycerol (2-AG), endogenous ligands for the CB1 and CB2 cannabinoid receptors, are referred to as endocannabinoids because they mimic the actions of delta9-tetrahydrocannabinol (Delta9-THC), a plant-derived cannabinoid. The processes by which AEA and 2-AG are biosynthesized, released, taken up by cells and hydrolyzed have been of much interest as potential therapeutic targets. In this review we will discuss the progress that has been made to characterize the primary pathways for AEA and 2-AG formation and breakdown as well as the role that specialized membrane microdomains known as lipid rafts play in these processes. Furthermore we will review the recent advances made to track and detect AEA in biological matrices.

Endocannabinoids in the retina: from marijuana to neuroprotection

The active component of the marijuana plant Cannabis sativa, Delta9-tetrahydrocannabinol (THC), produces numerous beneficial effects, including analgesia, appetite stimulation and nausea reduction, in addition to its psychotropic effects. THC mimics the action of endogenous fatty acid derivatives, referred to as endocannabinoids. The effects of THC and the endocannabinoids are mediated largely by metabotropic receptors that are distributed throughout the nervous and peripheral organ systems. There is great interest in endocannabinoids for their role in neuroplasticity as well as for therapeutic use in numerous conditions, including pain, stroke, cancer, obesity, osteoporosis, fertility, neurodegenerative diseases, multiple sclerosis, glaucoma and inflammatory diseases, among others. However, there has been relatively far less research on this topic in the eye and retina compared with the brain and other organ systems. The purpose of this review is to introduce the "cannabinergic" field to the retinal community. All of the fundamental works on cannabinoids have been performed in non-retinal preparations, necessitating extensive dependence on this literature for background. Happily, the retinal cannabinoid system has much in common with other regions of the central nervous system. For example, there is general agreement that cannabinoids suppress dopamine release and presynaptically reduce transmitter release from cones and bipolar cells. How these effects relate to light and dark adaptations, receptive field formation, temporal properties of ganglion cells or visual perception are unknown. The presence of multiple endocannabinoids, degradative enzymes with their bioactive metabolites, and receptors provides a broad spectrum of opportunities for basic research and to identify targets for therapeutic application to retinal diseases.

Endocannabinoid signaling in microglial cells

The endocannabinoid signaling system (eCBSS) is composed of cannabinoid (CB) receptors, their endogenous ligands (the endocannabinoids, eCB) and the enzymes that produce and inactivate these ligands. Neurons use this signaling system to communicate with each other and Delta9-tetrahydrocannabinol (THC), the main psychotropic ingredient of Cannabis sativa, induces profound behavioral effects by impinging on this communication. Evidence now shows that microglia, the macrophages of the brain, also express a functional eCBSS and that activation of CB receptors expressed by activated microglia controls their immune-related functions. This review summarizes this evidence, discusses how microglia might use the eCBSS to communicate with each other and neighboring cells, and argues that compounds selectively targeting the eCBSS expressed by microglia constitute valuable therapeutics to manage acute and chronic neuroinflammation, without inducing the psychotropic effects and underlying addictive properties commonly associated with THC.

A COX-2 metabolite of the endogenous cannabinoid, 2-arachidonyl glycerol, mediates suppression of IL-2 secretion in activated Jurkat T cells

Previous studies from this laboratory have demonstrated that a COX-2 metabolite of the endogenous cannabinoid, 2-arachidonyl glycerol (2-AG), inhibits IL-2 secretion in activated T cells through PPARgamma activation independent of the cannabinoid receptors, CB1/CB2. Because numerous cyclooxygenase (COX) products have been shown to activate PPARgamma, the primary purpose of the present studies was to determine the role of COX metabolism in the inhibition of IL-2 secretion by 2-AG. Pretreatment with nonselective and COX-2-specific inhibitors completely abrogated 2-AG-mediated suppression of IL-2 secretion. In contrast, pretreatment with COX-1-specific inhibitors had no effect upon 2-AG-mediated inhibition of IL-2 secretion. Interestingly, the current studies also demonstrate that while the potency of 2-AG is comparable between human Jurkat T cells and murine splenocytes, anandamide (AEA) is markedly more potent in suppressing IL-2 production in Jurkat T cells compared to murine splenocytes. Additionally, the present studies also demonstrate that COX-2 protein is readily detectable in resting Jurkat T cells, which is in contrast to resting murine splenocytes in which COX-2 protein is virtually undetectable. Furthermore, COX-2 protein and mRNA levels are significantly increased over basal levels by 2h following activation of Jurkat cells, whereas increases in COX-2 protein in murine splenocytes are not observed until 4h after cellular activation. These studies suggest that the potency of AEA in the suppression of IL-2 secretion may correlate with COX-2 protein levels in different T cell models. The present studies are also significant in that they demonstrate 2-AG-mediated inhibition of IL-2 secretion is dependent upon COX-2 metabolism.

Fatty acid amide hydrolase inhibition enhances the anti-allodynic actions of endocannabinoids in a model of acute pain adapted for the mouse

Cannabinoid ligands have been shown to be anti-nociceptive in animal models of acute and chronic pain by acting at the two known cannabinoid receptors, cannabinoid-1 receptor (CB-1) and cannabinoid-2 receptor (CB-2). A major concern with the use of cannabinoids for pain relief is that they activate receptors at sites other than those involved in the transmission of nociceptive stimuli. An alternative approach is to target the naturally occurring endocannabinoids, such as anandamide (AEA), 2-arachidonylglycerol (2-AG) and N-arachidonylglycine (N-AG). However in vivo results obtained with these compounds appear to be weak, most probably due to their rapid degradation and subsequent short half-life. The predominant enzyme responsible for the hydrolysis of anandamide (and some other endocannabinoids) in the brain is fatty acid amide hydrolase (FAAH). Recently, the alpha-ketoheterocycle OL135 has been synthesized and shown to be a highly potent and selective inhibitor of FAAH with efficacy in pain models in vivo. In the present study, we have adapted the mild thermal injury (MTI) model of acute pain for the mouse and pharmacologically characterized this model by showing significant reversal of the tactile allodynia by morphine (3, 5 and 10 mg kg(-1) s.c.), gabapentin (100 and 300 mg kg(-1) i.p.), ibuprofen (100 mg kg(-1) i.p.) and OL135 (10, 30 and 100 mg kg(-1) i.p.). Furthermore we have demonstrated, using this model, that a subtherapeutic dose of OL135 can enable the endocannabinoids AEA and 2-AG, but not N-AG to be active at doses where they are otherwise nonanalgesic (20 mg kg(-1) i.p.). The implications of this model in the study of pain in mice, and the therapeutic potential of FAAH inhibition to provide analgesia without the undesirable side effects of direct agonism of cannabinoid receptors are discussed.

Interplay between endocannabinoids, steroids and cytokines in the control of human reproduction

The use of marijuana, which today is the most used recreational drug, has been demonstrated to affect adversely reproduction. Marijuana smokers, both men and women, show impaired fertility, owing to defective signalling pathways, aberrant hormonal regulation, or wrong timing during embryo implantation. Anandamide (N-arachidonoylethanolamine, AEA) and 2-arachidonoylglycerol (2-AG) mimic Delta(9)-tetrahydrocannabinol (THC), the psychoactive principle of Cannabis sativa, by binding to both the brain-type (CB(1)) and the spleen-type (CB(2)) cannabinoid receptors. These 'endocannabinoids' exert several actions either in the central nervous system or in peripheral tissues, and are metabolised by specific enzymes that synthesise or hydrolyse them. In this review, we shall describe the elements that constitute the endocannabinoid system (ECS), in order to put in a better perspective the role of this system in the control of human fertility, both in females and males. In addition, we shall discuss the interplay between ECS, sex hormones and cytokines, which generates an endocannabinoid-hormone-cytokine array critically involved in the control of human reproduction.

A comprehensive profile of brain enzymes that hydrolyze the endocannabinoid 2-arachidonoylglycerol

Endogenous ligands for cannabinoid receptors ("endocannabinoids") include the lipid transmitters anandamide and 2-arachidonoylglycerol (2-AG). Endocannabinoids modulate a diverse set of physiological processes and are tightly regulated by enzymatic biosynthesis and degradation. Termination of anandamide signaling by fatty acid amide hydrolase (FAAH) is well characterized, but less is known about the inactivation of 2-AG, which can be hydrolyzed by multiple enzymes in vitro, including FAAH and monoacylglycerol lipase (MAGL). Here, we have taken a functional proteomic approach to comprehensively map 2-AG hydrolases in the mouse brain. Our data reveal that approximately 85% of brain 2-AG hydrolase activity can be ascribed to MAGL, and that the remaining 15% is mostly catalyzed by two uncharacterized enzymes, ABHD6 and ABHD12. Interestingly, MAGL, ABHD6, and ABHD12 display distinct subcellular distributions, suggesting that they may control different pools of 2-AG in the nervous system.

Endocannabinoid metabolism and uptake: novel targets for neuropathic and inflammatory pain

Cannabinoid CB1 and CB2 receptors are located at key sites involved in the relaying and processing of noxious inputs. Both CB1 and CB2 receptor agonists have analgesic effects in a range of models of inflammatory and neuropathic pain. Importantly, clinical trials of cannabis-based medicines indicate that the pre-clinical effects of cannabinoid agonists may translate into therapeutic potential in humans. One of the areas of concern with this pharmacological approach is that CB1 receptors have a widespread distribution in the brain and that global activation of CB1 receptors is associated with adverse side effects. Studies of the endogenous cannabinoids (endocannabinoids) have demonstrated that they are present in most tissues and that in some pain states, such as neuropathic pain, levels of endocannabinoids are elevated at key sites involved in pain processing. An alternative approach that can be used to harness the potential therapeutic effects of cannabinoids is to maximise the effects of the endocannabinoids, the actions of which are terminated by re-uptake and metabolism by various enzymes, including fatty acid amide hydrolase (FAAH), monoacylglycerol lipase (MAGL) and cyclooxygenase type 2 (COX2). Preventing the metabolism, or uptake, of endocannabinoids elevates levels of these lipid compounds in tissue and produces behavioural analgesia in models of acute pain. Herein we review recent studies of the effects of inhibition of metabolism of endocannabinoids versus uptake of endocannabinoids on nociceptive processing in models of inflammatory and neuropathic pain.

Therapeutic Potential of Endocannabinoid-Hydrolysing Enzyme Inhibitors

The specific protein target of delta9-tetrahydrocannabinol (delta9-THC), the main active ingredient of Cannabis sativa L., was characterized from rat brain nearly 20 years ago, and several endogenous compounds and proteins comprising the endocannabinoid (eCB) system have since been discovered. It has become evident that the eCB system consists of at least two cannabinoid receptors (i.e. the CB1 and CB2 receptors), in addition to their endogenous ligands (the eCBs) and several enzymes involved in the biosynthesis and catabolism of the eCBs. The two well-established eCBs, N-arachidonoylethanolamide (AEA) and 2-arachidonoylglycerol (2-AG), are produced by neurons on demand, act near their sites of synthesis and are effectively metabolized by fatty acid amide hydrolase (FAAH) and monoglyceride lipase (MGL), respectively. Inhibitors specifically targeting these enzymes could offer novel therapeutic approaches (e.g. for the treatment of pain and movement disorders). This MiniReview summarizes the literature concerning the potential therapeutic potential of FAAH and MGL inhibitors.

The contribution of cyclooxygenase-2 to endocannabinoid metabolism and action

The development of sensitive analytical methods for measurement of endocannabinoids, their metabolites, and related lipids, has underlined the complexity of the endocannabinoid system. A case can be made for an 'endocannabinoid soup' (akin to the inflammatory soup) whereby the net effect of a pathological state and/or a pharmacological intervention on this system is the result not only of changes in endocannabinoid levels but also of their metabolites and related compounds that affect their function. With respect to the metabolism of anandamide and 2-arachidonoylglycerol, the main hydrolytic enzymes involved are fatty acid amide hydrolase and monoacylglycerol lipase. However, other pathways can come into play when these are blocked. Cyclooxygenase-2 derived metabolites of anandamide and 2-arachidonoylglycerol have a number of properties, including effects upon cell viability, contraction of the cat iris sphincter (an effect mediated by a novel receptor), mobilization of calcium and modulation of synaptic transmission. Nonsteroidal anti-inflammatory agents, whose primary mode of action is the inhibition of cyclooxygenase, can also interact with the endocannabinoid system both in vitro and in vivo. Other enzymes, such as the lipoxygenase and cytochrome P450 oxidative enzymes, can also metabolize endocannabinoids and produce biologically active compounds. It is concluded that sensitive analytical methods, which allow for measurement of endocannabinoids and related lipids, should provide vital information as to the importance of these alternative metabolic pathways when the primary hydrolytic endocannabinoid metabolizing enzymes are inhibited.

The N-acylethanolamine-hydrolyzing acid amidase (NAAA)

Bioactive N-acylethanolamines, including the endocannabinoid anandamide and anti-inflammatory N-palmitoylethanolamine, are hydrolyzed to fatty acids and ethanolamine in animal tissues by the catalysis of fatty acid amide hydrolase (FAAH). We recently cloned cDNA of N-acylethanolamine-hydrolyzing acid amidase (NAAA), another enzyme catalyzing the same reaction, from human, rat, and mouse. NAAA reveals no sequence homology with FAAH and belongs to the choloylglycine hydrolase family. The most striking catalytic property of NAAA is pH optimum at 4.5-5, which is consistent with its immunocytochemical localization in lysosomes. In rat, NAAA is highly expressed in lung, spleen, thymus, and intestine. Notably, the expression level of NAAA is exceptionally high in rat alveolar macrophages. The primary structure of NAAA exhibits 33-35% amino acid identity to that of acid ceramidase, a lysosomal enzyme hydrolyzing ceramide to fatty acid and sphingosine. NAAA actually showed a low, but detectable ceramide-hydrolyzing activity, while acid ceramidase hydrolyzed N-lauroylethanolamine. Thus, NAAA is a novel lysosomal hydrolase, which is structurally and functionally similar to acid ceramidase. These results suggest a unique role of NAAA in the degradation of N-acylethanolamines.

Expression of endocannabinoid synthetic enzyme mRnas is correlated with cannabinoid 1 receptor mRNA in the mouse brain

The two principal endocannabinoids, N-arachidonoyl-ethanolamine (anandamide) and 2-arachidonoyl-glycerol (2-AG), are synthesized from arachidonic acid (AA) and AA is released as they are degraded. Therefore, the function of endocannabinoids is closely linked to AA, but the exact relationships have not been clarified, especially with respect to endocannabinoid metabolism. In the present study, oil rich in AA was administered (0, 100, 200 and 300 microl) orally to male mice for 7 days. Phospholipase D (PLD), fatty acid amide hydrolase (FAAH), diacyl-glycerol lipase (DAGL), monoacyl-glycerol lipase (MAGL) and cannabinoid 1 (CB1) receptor mRNA expressions were determined in the whole brain. No changes in the expression of any gene investigated were detected following AA treatment. However, it was demonstrated that the expression of the CB1 receptor was positively correlated with PLD, FAAH and DAGL expression. This suggests that expression of the CB1 receptor is closely coordinated with that of the enzymes which synthesize its ligands.

Structure-activity relationship of a series of inhibitors of monoacylglycerol hydrolysis--comparison with effects upon fatty acid amide hydrolase

A series of 32 heterocyclic analogues based on the structure of 2-arachidonoylglycerol (2-AG) were synthesized and tested for their ability to inhibit monoacylglycerol lipase and fatty acid amide hydrolase activities. The designed compounds feature a hydrophobic moiety and different heterocyclic subunits that mimic the glycerol fragment. This series has allowed us to carry out the first systematic structure-activity relationship study on inhibition of 2-AG hydrolysis. The most promising compounds were oxiran-2-ylmethyl (5Z,8Z,11Z,14Z)-icosa-5,8,11,14-tetraenoate (1) and tetrahydro-2H-pyran-2-ylmethyl (5Z,8Z,11Z,14Z)-icosa-5,8,11,14-tetraenoate (5). They inhibited cytosolic 2-oleoylglycerol (2-OG) hydrolysis completely (IC50 values of 4.5 and 5.6 muM, respectively). They also blocked, albeit less potently, 2-OG hydrolysis in membrane fractions (IC50 values of 19 and 26 muM, respectively) and anandamide hydrolysis (IC50 values of 12 and 51 muM, respectively). These compounds will be useful in delineating the importance of the cytosolic hydrolytic activity in the regulation of 2-AG levels and, hence, its potential as a target for drug development.

Inhibition of [3H]batrachotoxinin A-20alpha-benzoate binding to sodium channels and sodium channel function by endocannabinoids

A number of putative endocannabinoids were found to modify the binding of [(3)H]batrachotoxinin A-20alpha-benzoate ([(3)H]BTX-B) to site 2 on voltage-gated sodium channels of mouse brain and achieve functional inhibition of sodium channels in vitro. 2-Arachidonoyl-glycerol (2-AG), arachidonoyl glycerol ether (AGE), N-arachidonoyl-dopamine (NADA) gave almost complete inhibition of [(3)H]BTX-B binding with IC(50) values of 90.4, 51.2 and 20.7 microM, respectively. The CB1 receptor antagonist AM251 (2 microM) had no effect on the displacement of radioligand by these endocanabinoids. Arachidonoyl-glycine (A-Gly) and arachidonoyl-GABA (A-GABA) were apparently less effective inhibitors of [(3)H]BTX-B binding giving 14.8+/-2.2 and 23.9+/-4.8% inhibition at 100 microM. Phenylmethanesulphonylfluoride (PMSF) did not alter the inhibitory effects of 2-AG, AGE, NADA and A-Gly on binding, but the efficacy of 100 microM A-GABA was increased by 60.3+/-6.3% (P<0.05). Scatchard analyses showed that 2-AG, AGE and NADA reduce the binding of [(3)H]BTX-B by lowering B(max) although increases in K(D) were also evident for AGE and NADA. Our kinetic experiments found that 2-AG, AGE and NADA increase the dissociation velocity of radioligand from site 2 on sodium channels demonstrating that these endocannabinoids operate as allosteric inhibitors of [(3)H]BTX-B binding. 2-AG, AGE and NADA inhibited veratridine-dependent (TTX-suppressible) depolarization of the plasma membrane of synaptoneurosomes at low micromolar concentrations and again the capacities of A-Gly and A-GABA to inhibit this response were less pronounced. The three most effective endocannabinoids (2-AG, AGE and NADA) were then examined in a synaptosomal transmitter release assay where they were observed to inhibit sodium channel- (veratridine-dependent) release of l-glutamate and GABA in the low micromolar range. These effects also occurred through a mechanism that was not influenced by 2 microM AM251. It is concluded that direct inhibition of sodium channel function leading to reduced neuronal excitation and depression of presynaptic release of amino acid transmitters is a property shared by several endocannabinoids.

Vanilloid receptor 1 (TRPV1/VR1) co-localizes with fatty acid amide hydrolase (FAAH) in retinal amacrine cells

Fatty acid amide hydrolase (FAAH) is the degradative enzyme for anandamide (AEA), an endogenous ligand for the vanilloid receptor (TRPV1) and cannabinoid receptor 1. As FAAH and TRPV1 are integral membrane proteins, FAAH activity could modulate the availability of AEA for TRPV1 activation. Previous studies in this laboratory reported an extensive endocannabinoid system in goldfish retina. Immunocytochemistry was used to determine the relative distributions of FAAH-immunoreactivity (IR) and TRPV1-IR in goldfish retina. Here, we show the first example in an intact neural system in which TRPV1-IR co-localizes in subpopulations of FAAH-immunoreactive neurons, in this case amacrine cells. These cells are rare and consist of three subtypes: 1. ovoid cell with granular-type dendrites restricted to sublamina a, 2. pyriform cell with smooth processes in sublamina b, and 3. fusiform cell with smooth processes that project to sublaminae a and b. The varied appearances of reaction product in the dendrites suggest different subcellular localization of TRPV1, and hence function of FAAH activity regarding TRPV1 stimulation among the cell types. Ovoid and pyriform amacrine cells, but not fusiform cells, labeled with GAD-IR and constituted subsets of GABAergic amacrine cells. TRPV1 amacrine cells, though rare, are represented in the ON, OFF and ON/OFF pathways of the retina. As TRPV1 stimulation increases intracellular calcium with numerous downstream effects, co-localization of TRPV1 and FAAH suggests an autoregulatory function for anandamide. Due to the rarity of these cells, the three vanilloid amacrine cell types may be involved in global effects rather than feature extraction, for example: sampling of ambient light or maintaining homeostasis.

Hydrolysis of Prostaglandin Glycerol Esters by the Endocannabinoid-Hydrolyzing Enzymes, Monoacylglycerol Lipase and Fatty Acid Amide Hydrolase

Cyclooxygenase-2 (COX-2) can oxygenate the endocannabinoids, arachidonyl ethanolamide (AEA) and 2-arachidonylglycerol (2-AG), to prostaglandin-H2-ethanolamide (PGH2-EA) and -glycerol ester (PGH2-G), respectively. Further metabolism of PGH2-EA and PGH2-G by prostaglandin synthases produces a variety of prostaglandin-EA's and prostaglandin-G's nearly as diverse as those derived from arachidonic acid. Thus, COX-2 may regulate endocannabinoid levels in neurons during retrograde signaling or produce novel endocannabinoid metabolites for receptor activation. Endocannabinoid-metabolizing enzymes are important regulators of their action, so we tested whether PG-G levels may be regulated by monoacylglycerol lipase (MGL) and fatty acid amide hydrolase (FAAH). We found that PG-Gs are poor substrates for purified MGL and FAAH compared to 2-AG and/or AEA. Determination of substrate specificity demonstrates a 30-100- and 150-200-fold preference of MGL and FAAH for 2-AG over PG-Gs, respectively. The substrate specificity of AEA compared to those of PG-Gs was approximately 200-300 fold higher for FAAH. Thus, PG-Gs are poor substrates for the major endocannabinoid-degrading enzymes, MGL and FAAH.

The pharmacology of the cannabinoid system--a question of efficacy and selectivity

Our knowledge of the function of the cannabinoid system in the body has been aided by the availability of pharmacological agents that affect its function. This has been achieved by the design of agents that either directly interact with the receptor (agonists and antagonist/inverse agonists) and agents that indirectly modulate the receptor output by changing the levels of the endogenous cannabinoids (endocannabinoids). In this review, examples of the most commonly used receptor agonists, antagonists/inverse agonists, and indirectly acting agents (anandamide uptake inhibitors, fatty acid amide hydrolase inhibitors, monoacylglycerol lipase inhibitors) are given, with particular focus upon their selectivity and, in the case of the directly acting compounds, efficacy. Finally, the links between the endocannabinoid and cyclooxygenase pathways are explored, in particular, with respect to agents whose primary function is to inhibit cyclooxygenase activity, but which also interact with the endocannabinoid system.

Retrograde endocannabinoid inhibition of goldfish retinal cones is mediated by 2-arachidonoyl glycerol

A functional role for retinal endocannabinoids has not been determined. We characterized retrograde suppression of membrane currents of goldfish cones in a retinal slice. Whole-cell recordings were obtained from cone inner segments under voltage clamp. I(K(V)) was elicited by a depolarizing pulse to +54 mV from a holding potential of -70 mV. A fifty-millisecond puff of saline with 70 mM KCl or Group I mGluR agonist DHPG was applied through a pipette directly at a mixed rod/cone (Mb) bipolar cell body. The amplitude of I(K(V)) decreased 25% compared to the pre-puff control. Retrograde suppression of I(K(V)) was blocked by CB1 receptor antagonist, SR141716A. The FAAH inhibitor URB597 had no effect on the suppression of I(K(V)), whereas nimesulide, a COX-2 inhibitor, prolonged the effects of the K+ puff 10-fold. Orlistat, a blocker of 2-AG synthesis, blocked the effect of the K+ puff. Group I mGluR activation of Gq/11 was demonstrated in that a puff with DHPG decreased I(K(V)) of cones by 32%, an effect blocked by SR141716A. The effect of DHPG was not blocked by the mGluR5 antagonist MPEP, indicating involvement of mGluR1. The suppressive effect of the K+ puff vanished in a Ca2+-free, 2 mM Co2+ saline. TMB-8 or ryanodine, blocked the effect of DHPG, but not that of the K+ puff, showing that calcium influx or release from intracellular stores could mediate retrograde release. We suggest that retrograde suppression of cone I(K(V)) is mediated by Ca2+-dependent release of 2-AG from Mb bipolar cell dendrites by separate mechanisms: (1) voltage-dependent, mimicked by the K+ puff, that may be activated by the depolarizing ON response to light; (2) voltage-independent, occurring under ambient illumination, mediated by tonic mGluR1 activation. The negative feedback of this latter mechanism could regulate tonic glutamate release from cones within narrow limits, regardless of ambient illumination.

Interaction of ligands for the peroxisome proliferator-activated receptor gamma with the endocannabinoid system

BACKGROUND AND PURPOSE

There is good evidence that agents interacting with the endocannabinoid system in the body can also interact with the peroxisome proliferator-activated receptor gamma. The present study was designed to test whether the reverse is true, namely whether peroxisome proliferator-activated receptor gamma ligands have direct effects upon the activity of the endocannabinoid metabolizing enzyme fatty acid amide hydrolase.

EXPERIMENTAL APPROACH

Fatty acid amide hydrolase activity was measured in rat brain homogenates, C6 glioma and RBL2H3 basophilic leukaemia cells. Cellular uptake of anandamide was also assessed in these cells.

KEY RESULTS

Peroxisome proliferator-activated receptor gamma activators inhibited the metabolism of the endocannabinoid anandamide in rat brain homogenates with an order of potency MCC-555 > indomethacin approximately ciglitazone approximately 15-deoxy-Delta(12,14)-prostaglandin J(2) approximately pioglitazone > rosiglitazone > troglitazone. The antagonists BADGE, GW9662 and T0070907 were poor inhibitors of anandamide hydrolysis. The inhibition by ciglitazone was competitive and increased as the pH of the assay buffer was decreased; the K(i) value at pH 6.0 was 17 microM. In intact C6 glioma cells assayed at pH 6.2, significant inhibition of anandamide hydrolysis was seen at 3 microM ciglitazone, whereas 100 microM was required to produce significant inhibition at pH 7.4. Ciglitazone also interacted with monoacylglycerol lipase as well as with cannabinoid CB(1) and CB(2) receptors.

CONCLUSIONS AND IMPLICATIONS

Ciglitazone may be useful as a template for the design of novel dual action anti-inflammatory agents which are both inhibitors of fatty acid amide hydrolase and agonists at the peroxisome proliferator-activated receptor gamma.

Interactions between endocannabinoids and stress-induced decreased sensitivity to natural reward

Since endocannabinoids modulate reward processing and the stress response, we tested the hypothesis that endocannabinoids regulate stress-induced decreased sensitivity to natural reward. Restraint was used to produce stress-induced reductions in sucrose consumption and preference in male mice. Central cannabinoid receptor (CB(1)) signaling was modulated pharmacologically prior to the application of stress. The preference for sucrose over water was significantly decreased in mice exposed to restraint. Treatment of mice with a cannabinoid receptor agonist (CP55940) or fatty acid amide hydrolase inhibitor (URB597) attenuated, while the CB(1) receptor antagonist/inverse agonist, rimonabant (SR141716), enhanced, stress-induced decreases in sucrose preference. These data are consistent with a tonically active, stress-inhibitory role for the CB(1) receptor. Mice treated with 10 daily episodes of restraint showed reduced sucrose preference that was unaffected by CP55940 and URB597. However, rimonabant produced a greater reduction in sucrose preference on day 10 compared to day 1. These data suggest that on day 10, endocannabinoid signaling is maximally activated and essential for reward sensitivity. The findings of the present study indicate that the CB(1)/endocannabinoid signaling system is an important allostatic mediator that both modulates the responses of mice to stress and is itself modulated by stress.

Identification of a novel endocannabinoid-hydrolyzing enzyme expressed by microglial cells

The endocannabinoids (eCBs) anandamide and 2-arachidonoyl glycerol (2-AG) are inactivated by a two-step mechanism. First, they are carried into cells, and then anandamide is hydrolyzed by fatty acid amide hydrolase (FAAH) and 2-AG by monoacylglycerol lipase (MGL). Here we provide evidence for a previously undescribed MGL activity expressed by microglial cells. We found that the mouse microglial cell line BV-2 does not express MGL mRNA and yet efficiently hydrolyzes 2-AG. URB597 (3'-carbamoyl-biphenyl-3-yl-cyclohexylcarbamate) reduces this hydrolysis by 50%, suggesting the involvement of FAAH. The remaining activity is blocked by classic MGL inhibitors [[1,1-biphenyl]-3-yl-carbamic acid, cyclohexyl ester (URB602) and MAFP (methylarachidonyl fluorophosphate)] and is unaffected by inhibitors of COXs (cyclooxygenases), LOXs (lipooxygenases), and DGLs (diacylglycerol lipases), indicating the involvement of a novel MGL activity. Accordingly, URB602 leads to selective accumulation of 2-AG in intact BV-2 cells. Although MGL expressed in neurons is equally distributed between the cytosolic, mitochondrial, and nuclear fractions, the novel MGL activity expressed by BV-2 cells is enriched in mitochondrial and nuclear fractions. A screen for novel inhibitors of eCB hydrolysis identified several compounds that differentially block MGL, FAAH, and the novel MGL activity. Finally, we provide evidence for expression of the novel MGL by mouse primary microglia in culture. Our results suggest the presence of a novel, pharmacologically distinct, MGL activity that controls 2-AG levels in microglia.

Presynaptic monoacylglycerol lipase activity determines basal endocannabinoid tone and terminates retrograde endocannabinoid signaling in the hippocampus

Endocannabinoids function as retrograde messengers and modulate synaptic transmission through presynaptic cannabinoid CB1 receptors. The magnitude and time course of endocannabinoid signaling are thought to depend on the balance between the production and degradation of endocannabinoids. The major endocannabinoid 2-arachidonoylglycerol (2-AG) is hydrolyzed by monoacylglycerol lipase (MGL), which is shown to be localized at axon terminals. In the present study, we investigated how MGL regulates endocannabinoid signaling and influences synaptic transmission in the hippocampus. We found that MGL inhibitors, methyl arachidonoyl fluorophosphonate and arachidonoyl trifluoromethylketone, caused a gradual suppression of cannabinoid-sensitive IPSCs in cultured hippocampal neurons. This suppression was reversed by blocking CB1 receptors and was attenuated by inhibiting 2-AG synthesis, indicating that MGL scavenges constitutively released 2-AG. We also found that the MGL inhibitors significantly prolonged the suppression of both IPSCs and EPSCs induced by exogenous 2-AG and depolarization-induced suppression of inhibition/excitation, a phenomenon known to be mediated by retrograde endocannabinoid signaling. In contrast, inhibitors of other endocannabinoid hydrolyzing enzymes, fatty acid amide hydrolase and cyclooxygenase-2, had no effect on the 2-AG-induced IPSC suppression. These results strongly suggest that presynaptic MGL not only hydrolyzes 2-AG released from activated postsynaptic neurons but also contributes to degradation of constitutively produced 2-AG and prevention of its accumulation around presynaptic terminals. Thus, the MGL activity determines basal endocannabinoid tone and terminates retrograde endocannabinoid signaling in the hippocampus.

The cannabinoid system and its pharmacological manipulation--a review, with emphasis upon the uptake and hydrolysis of anandamide

Although cannabis has been used both recreationally and for medicinal purposes since ancient times, it was not until the 1990s that the receptors responsible for many of the actions of Delta(9)-tetrahydrocannabinol, the main psychoactive ingredient of cannabis, were cloned. Since then, our knowledge of the endogenous cannabinoid system, its physiology, pharmacology and therapeutic potential have expanded enormously. In the present review, the cannabinoid system is described, with particular emphasis on the mechanisms of removal and metabolism of the endocannabinoid signalling molecule anandamide. The current literature shows that cells can accumulate anandamide, and that this process can be disrupted pharmacologically, but that the nature of the mechanism(s) involved remains a matter of some debate. The main enzyme for the hydrolysis of anandamide, fatty acid amide hydrolase, is well characterized, and molecules selectively inhibiting this enzyme have potential therapeutic utility in a number of areas, in particular for the treatment of pain conditions.

Temporal variation in CB2R levels following T lymphocyte activation: Evidence that cannabinoids modulate CXCL12-induced chemotaxis

Cannabinoids have long been proposed to affect the immune system, especially as one of the cannabinoid receptors, the cannabinoid receptor-2 (CB(2)R) has been found almost exclusively on immune cells. Here, using human in vitro activated peripheral blood-derived T lymphocytes we investigated the long-term changes in cannabinoid receptor protein expression following cellular activation and the effects of cannabinoids on migration. We report that resting T lymphocytes do not detectably express either the cannabinoid receptor-1 (CB(1)R) or CB(2)R at the protein level. However, CB(2)R protein expression is upregulated in a biphasic manner in T lymphocytes following activation by superantigen. The cannabinoids 2-AG and JWH-133 were found to elicit activation of downstream biochemical effectors (as assessed by the phosphorylation of the ERK1/2 MAP kinases). Neither 2-AG nor JWH-133 induced chemotaxis in day 5 activated T lymphocytes, when receptor expression was at its highest. Interestingly, both 2-AG and JWH-133 inhibited CXCL12-induced chemotaxis, suggesting a modulatory role for cannabinoids in activated T lymphocytes.

Visualization of 2-arachidonoylglycerol accumulation and cannabinoid CB1 receptor activity in rat brain cryosections by functional autoradiography

In neuronal signalling mediated by the endocannabinoid 2-arachidonoylglycerol, both synthetic and inactivating enzymes operate within close proximity to the G(i/o)-coupled pre-synaptic CB(1) receptors, thus allowing for rapid onset and transient duration of this lipid modulator. In rat brain, 2-arachidonoylglycerol is inactivated mainly via hydrolysis by serine hydrolase inhibitor-sensitive monoacylglycerol lipase activity. We show in this study that comprehensive pharmacological elimination of this activity in brain cryosections by methyl arachidonylfluorophosphonate or hexadecylsulphonyl fluoride results in endocannabinoid-mediated CB(1) receptor activity, which can be visualized by functional autoradiography. URB597, a specific inhibitor of anandamide hydrolysis proved ineffective. TLC indicated that the bioactivity resided in 2-arachidonoylglycerol-containing fraction and gas chromatography-mass spectroscopy detected elevated levels of monoacylglycerols, including 2-arachidonoylglycerol in this fraction. Although two diacylglycerol lipase inhibitors, tetrahydrolipstatin (THL) and RHC80267, blocked the bulk of 2-arachidonoylglycerol accumulation in methyl arachidonylfluorophosphonate-treated sections, only THL reversed the endocannabinoid-dependent CB(1) receptor activity. Further studies indicated that at the used concentrations, THL rather specifically antagonized the CB(1) receptor. These findings confirm that in brain sections there is preservation of enzymatic pathways regulating the production of endogenous receptor ligands. Furthermore, the presently described methodology may serve as an elegant and intuitive approach to identify novel membrane-derived lipid modulators operating in the CNS.

Endothelium-dependent metabolism by endocannabinoid hydrolases and cyclooxygenases limits vasorelaxation to anandamide and 2-arachidonoylglycerol

BACKGROUND AND PURPOSE

The endocannabinoids, N-arachidonoylethanolamide (anandamide) and 2-arachidonoylglycerol (2-AG) are rapidly degraded by fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MGL). Whilst these lipid mediators are known to modulate vascular tone, the extent to which they are inactivated via local metabolism in the vasculature remains unclear.

EXPERIMENTAL APPROACH

In rat isolated small mesenteric arteries, the regulatory role of FAAH, MGL and cyclooxygenase (COX) in relaxant responses to anandamide and 2-AG was evaluated by using inhibitors of these enzymes. Relaxations to non-hydrolysable analogues of endocannabinoids and arachidonic acid were also examined.

KEY RESULTS

Relaxation to anandamide but not 2-AG was potentiated by the selective FAAH inhibitor, URB597 (1 microM). In contrast, MAFP (10 microM; an inhibitor of FAAH and MGL) enhanced responses to both anandamide and 2-AG. Inhibition of COX-1 by indomethacin (10 microM) potentiated relaxations to 2-AG, whereas inhibition of COX-2 by nimesulide (10 microM) potentiated anandamide-induced relaxation. With the exception of MAFP, effects of FAAH and COX inhibitors were dependent on the endothelium. Relaxation to methanandamide and noladin ether, the non-hydrolysable analogues of anandamide and 2-AG respectively, were insensitive to the enzyme inhibitors.

CONCLUSION AND IMPLICATIONS

This study shows that local activity of FAAH, MGL and COX, which is present largely in the endothelium, limits the vasodilator action of endocannabinoids in rat small mesenteric arteries. Despite the differential roles played by these enzymes on relaxation to anandamide versus 2-AG, our results suggest that inhibitors of these enzymes enhance the vascular impact of endocannabinoids.

Analgesic effects of fatty acid amide hydrolase inhibition in a rat model of neuropathic pain

Cannabinoid-based medicines have therapeutic potential for the treatment of pain. Augmentation of levels of endocannabinoids with inhibitors of fatty acid amide hydrolase (FAAH) is analgesic in models of acute and inflammatory pain states. The aim of this study was to determine whether local inhibition of FAAH alters nociceptive responses of spinal neurons in the spinal nerve ligation model of neuropathic pain. Electrophysiological studies were performed 14-18 d after spinal nerve ligation or sham surgery, and the effects of the FAAH inhibitor cyclohexylcarbamic acid 3-carbamoyl biphenyl-3-yl ester (URB597) on mechanically evoked responses of spinal neurons and levels of endocannabinoids were determined. Intraplantar URB597 (25 microg in 50 microl) significantly (p < 0.01) attenuated mechanically evoked responses of spinal neurons in sham-operated rats. Effects of URB597 were blocked by the cannabinoid 1 receptor (CB1) antagonist AM251 [N-1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide] (30 microg in 50 microl) and the opioid receptor antagonist naloxone. URB597 treatment increased levels of anandamide, 2-arachidonyl glycerol, and oleoyl ethanolamide in the ipsilateral hindpaw of sham-operated rats. Intraplantar URB597 (25 microg in 50 microl) did not, however, alter mechanically evoked responses of spinal neurons in spinal nerve ligated (SNL) rats or hindpaw levels of endocannabinoids. Intraplantar injection of a higher dose of URB597 (100 microg in 50 microl) significantly (p < 0.05) attenuated evoked responses of spinal neurons in SNL rats but did not alter hindpaw levels of endocannabinoids. Spinal administration of URB597 attenuated evoked responses of spinal neurons and elevated levels of endocannabinoids in sham-operated and SNL rats. These data suggest that peripheral FAAH activity may be altered or that alternative pathways of metabolism have greater importance in SNL rats.

Neuropharmacology of the endocannabinoid signaling system-molecular mechanisms, biological actions and synaptic plasticity

The endocannabinoid signaling system is composed of the cannabinoid receptors; their endogenous ligands, the endocannabinoids; the enzymes that produce and inactivate the endocannabinoids; and the endocannabinoid transporters. The endocannabinoids are a new family of lipidic signal mediators, which includes amides, esters, and ethers of long-chain polyunsaturated fatty acids. Endocannabinoids signal through the same cell surface receptors that are targeted by Delta(9)-tetrahydrocannabinol (Delta(9)THC), the active principles of cannabis sativa preparations like hashish and marijuana. The biosynthetic pathways for the synthesis and release of endocannabinoids are still rather uncertain. Unlike neurotransmitter molecules that are typically held in vesicles before synaptic release, endocannabinoids are synthesized on demand within the plasma membrane. Once released, they travel in a retrograde direction and transiently suppress presynaptic neurotransmitter release through activation of cannabinoid receptors. The endocannabinoid signaling system is being found to be involved in an increasing number of pathological conditions. In the brain, endocannabinoid signaling is mostly inhibitory and suggests a role for cannabinoids as therapeutic agents in central nervous system (CNS) disease. Their ability to modulate synaptic efficacy has a wide range of functional consequences and provides unique therapeutic possibilities. The present review is focused on new information regarding the endocannabinoid signaling system in the brain. First, the structure, anatomical distribution, and signal transduction mechanisms of cannabinoid receptors are described. Second, the synthetic pathways of endocannabinoids are discussed, along with the putative mechanisms of their release, uptake, and degradation. Finally, the role of the endocannabinoid signaling system in the CNS and its potential as a therapeutic target in various CNS disease conditions, including alcoholism, are discussed.

Actions of the FAAH inhibitor URB597 in neuropathic and inflammatory chronic pain models

While cannabinoid receptor agonists have analgesic activity in chronic pain states, they produce a spectrum of central CB(1) receptor-mediated motor and psychotropic side effects. The actions of endocannabinoids, such as anandamide are terminated by removal from the extracellular space, then subsequent enzymatic degradation by fatty-acid amide hydrolase (FAAH). In the present study, we compared the effect of a selective FAAH inhibitor, URB597, to that of a pan-cannabinoid receptor agonist HU210 in rat models of chronic inflammatory and neuropathic pain. Systemic administration of URB597 (0.3 mg kg(-1)) and HU210 (0.03 mg kg(-1)) both reduced the mechanical allodynia and thermal hyperalgesia in the CFA model of inflammatory pain. In contrast, HU210, but not URB597, reduced mechanical allodynia in the partial sciatic nerve-ligation model of neuropathic pain. HU210, but not URB597, produced a reduction in motor performance in unoperated rats. The effects of URB597 in the CFA model were dose dependent and were reduced by coadministration with the cannabinoid CB1 antagonist AM251 (1 mg kg(-1)), or the CB2 and SR144528 (1 mg kg(-1)). Coadministration with AM251 plus SR144528 completely reversed the effects of URB597. These findings suggest that the FAAH inhibitor URB597 produces cannabinoid CB1 and CB2 receptor-mediated analgesia in inflammatory pain states, without causing the undesirable side effects associated with cannabinoid receptor activation.

The antinociceptive effects of intraplantar injections of 2-arachidonoyl glycerol are mediated by cannabinoid CB2 receptors

BACKGROUND AND PURPOSE

2-arachidonoyl glycerol (2-AG) is an endogenous cannabinoid with central antinociceptive properties. Its degradation is catalysed by monoacylglycerol lipase (MGL) whose activity is inhibited by URB602, a new synthetic compound. The peripheral antinociceptive effects of 2-AG and URB602 in an inflammatory model of pain are not yet determined. We have evaluated these effects with and without the cannabinoid CB(1) (AM251) and CB(2) (AM630) receptor antagonists.

EXPERIMENTAL APPROACH

Inflammation was induced in rat hind paws by intraplantar injection of formalin. Nociception was assessed behaviourally over the next 60 min, in 19 experimental groups: (1) control; (2-6) 2-AG (0.01-100 microg); (7) AM251 (80 microg); (8) AM251+2-AG (10 microg); (9) AM630 (25 microg); (10) AM630+2-AG (10 microg); (11-16) URB602 (0.1-500 microg); (17) 2-AG+URB602 (ED(50)); (18) AM251+URB602 (ED(50)); (19) AM630+URB602 (ED(50)). Drugs were injected s.c. in the dorsal surface of the hind paw (50 microl), 15 min before formalin injection into the same paw.

KEY RESULTS

2-AG and URB602 produced dose-dependent antinociceptive effects for the late phases of the formalin test with ED(50) of 0.65+/-0.455 mug and 68+/-14.3 microg, respectively. Their combination at ED(50) doses produced an additive antinociceptive effect. These effects were inhibited by AM630 but not by AM251 for 2-AG and by the two cannabinoid antagonists for URB602.

CONCLUSIONS AND IMPLICATIONS

Locally injected 2-AG and URB602 decreased pain behaviour in a dose-dependent manner in an inflammatory model of pain. The antinociceptive effect of 2-AG was mediated by the CB(2) receptor.

Lack of selectivity of URB602 for 2-oleoylglycerol compared to anandamide hydrolysis in vitro

BACKGROUND AND PURPOSE

Two compounds, URB602 and URB754, have been reported in the literature to be selective inhibitors of monoacylglycerol lipase, although a recent study has questioned their ability to prevent 2-arachidonoyl hydrolysis by brain homogenates and cerebellar membranes. In the present study, the ability of these compounds to inhibit monoacylglycerol lipase and fatty acid amide hydrolase has been reinvestigated.

EXPERIMENTAL APPROACH

Homogenates and cell lines were incubated with test compounds and, thereafter, with either [(3)H]-2-oleoylglycerol or [(3)H]-anandamide. Labelled reaction products were separated from substrate using chloroform: methanol extraction.

KEY RESULTS

In cytosolic fractions from rat brain, URB602 and URB754 inhibited the hydrolysis of 2-oleoylglycerol with IC(50) values of 25 and 48 microM, respectively. Anandamide hydrolysis by brain membranes was not sensitive to URB754, but was inhibited by URB602 (IC(50) value 17 microM). Hydrolysis of 2-oleoylglycerol by human recombinant monoacylglycerol lipase was sensitive to URB602, but not URB754. The lack of selectivity of URB602 for 2-oleoylglycerol compared to anandamide hydrolysis was also observed for intact RBL2H3 basophilic leukaemia cells. C6 glioma expressed mRNA for monoacylglycerol lipase, and hydrolyzed 2-oleoylglycerol in a manner sensitive to inhibition by methyl arachidonoyl fluorophosphonate but not URB754 or URB597. MC3T3-E1 mouse osteoblastic cells, which did not express mRNA for monoacylglycerol lipase, hydrolyzed 2-oleoylglycerol in the presence of URB597, but the hydrolysis was less sensitive to methyl arachidonoyl fluorophosphonate than for C6 cells.

CONCLUSIONS AND IMPLICATIONS

The data demonstrate that the compounds URB602 and URB754 do not behave as selective and/or potent inhibitors of monoacylglycerol lipase.

Endocannabinoid mechanisms of pain modulation

Cannabinoids are antinociceptive in animal models of acute, tissue injury-, and nerve injury-induced nociception. This review examines the biology of endogenous cannabinoids (endocannabinoids) and behavioral, neurophysiological, and neuroanatomical evidence supporting the notion that cannabinoids play a role in pain modulation. Behavioral pharmacological approaches, in conjunction with the identification and quantification of endocannabinoids through the use of liquid and gas chromatography mass spectrometry, have provided insight into the functional roles of endocannabinoids in pain modulation. Here we examine the distribution of cannabinoid receptors and endocannabinoid-hydrolyzing enzymes within pain modulatory circuits together with behavioral, neurochemical, and neurophysiological studies that suggest a role for endocannabinoid signaling in pain modulation. This review will provide a comprehensive evaluation of the roles of the endocannabinoids 2-arachidonoylglycerol and anandamide in stress-induced analgesia. These findings provide a functional framework with which to understand the roles of endocannabinoids in nociceptive processing at the supraspinal level.

The Endocannabinoid System as a Target for the Treatment of Visceral Obesity and Metabolic Syndrome

The endogenous cannabinoid system is a novel, remarkably elaborate physiological signaling system, comprising the recently identified endogenous cannabinoid ligands, their corresponding selective receptors, and the machinery of proteins and enzymes that is involved in their biosynthesis, release, transport, and degradation. This system extends widely in both the central nervous system (CNS) and the periphery and exhibits a variety of actions implicated in vital functions (e.g., behavioral, antinociceptive, neuroprotective, immunosuppressive, cardiovascular, and metabolic). Particular interest has been focused on the apparent participation of endocannabinoids in metabolic homeostasis by modulating the activity of CNS circuits that control food intake and energy expenditure, the neuroendocrine response of the stress system, and the metabolic functions of crucial peripheral tissues, such as the adipose tissue, the gastrointestinal tract, the liver, and the skeletal muscles. These effects are predominantly CB(1) receptor mediated and, thus, selective antagonists of this receptor subtype are being vigorously investigated as potential therapeutic agents for the treatment of various metabolic derangements (e.g., obesity, insulin resistance, dyslipidemia, and metabolic syndrome). The first selective CB(1) receptor antagonist, rimonabant, has already successfully completed phase III clinical trials as adjunctive obesity treatment, with significant improvements in several associated metabolic and cardiovascular risk factors that led to the recent approval of its clinical use by the Food and Drug Administration.

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.

A novel assay for monoacylglycerol hydrolysis suitable for high-throughput screening

A simple assay for monoacylglycerol hydrolysis suitable for high-throughput screening is described. The assay uses [(3)H]2-oleoylglycerol as substrate, with the tritium label in the glycerol part of the molecule and the use of phenyl sepharose gel to separate the hydrolyzed product ([(3)H]glycerol) from substrate. Using cytosolic fractions derived from rat cerebella as a source of hydrolytic activity, the assay gives the appropriate pH profile and sensitivity to inhibition with compounds known to inhibit hydrolysis of this substrate. The assay could also be adapted to a 96-well plate format, using C6 cells as the source of hydrolytic activity. Thus the assay is simple and appropriate for high-throughput screening of inhibitors of monoacylglycerol hydrolysis.

Cannabinoid receptors and endocannabinoids: evidence for new players

It is now well established that the psychoactive effects of Cannabis sativa are primarily mediated through neuronal CB1 receptors, while its therapeutic immune properties are primarily mediated through CB2 receptors. Two endocannabinoids, arachidonoylethanolamide and 2-arachidonoylglycerol, have been identified, their action on CB1 and CB2 thoroughly characterized, and their production and inactivation elucidated. However, many significant exceptions to these rules exist. Here we review the evidence suggesting that cannabinoids can modulate synaptic transmission, the cardiovascular system, and the immune system through receptors distinct from CB1 and CB2, and that an additional "independent" endocannabinoid signaling system that involves palmitoylethanolamide may exist.

Inhibitors of fatty acid amide hydrolase reduce carrageenan-induced hind paw inflammation in pentobarbital-treated mice: comparison with indomethacin and possible involvement of cannabinoid receptors

The in vivo effect of inhibitors of fatty acid amide hydrolase (FAAH) upon oedema volume and FAAH activity was evaluated in the carrageenan induced hind paw inflammation model in the mouse. Oedema was measured at two time points, 2 and 4 h, after intraplantar injection of carrageenan to anaesthetised mice. Intraperitoneal (i.p.) injections of the FAAH inhibitor URB597 (0.1, 0.3, 1 and 3 mg kg(-1)) 30 min prior to carrageenan administration, dose-dependently reduced oedema formation. At the 4 h time point, the ED(50) for URB597 was approximately 0.3 mg kg(-1). Indomethacin (5 mg kg(-1) i.p.) completely prevented the oedema response to carrageenan. The antioedema effects of indomethacin and URB597 were blocked by 3 mg kg(-1) i.p. of the CB(2) receptor antagonist SR144528. The effect of URB597 was not affected by pretreatment with the peroxisome proliferator-activated receptor gamma antagonist bisphenol A diglycidyl ether (30 mg kg(-1) i.p.) or the TRPV1 antagonist capsazepine (10 mg kg(-1) i.p.), when oedema was assessed 4 h after carrageenan administration. The CB(1) receptor antagonists AM251 (3 mg kg(-1) i.p.) and rimonabant (0.5 mg kg(-1) i.p.) gave inconsistent effects upon the antioedema effect of URB597. FAAH measurements were conducted ex vivo in the paws, spinal cords and brains of the mice. The activities of FAAH in the paws and spinal cords of the inflamed vehicle-treated mice were significantly lower than the corresponding activities in the noninflamed mice. PMSF treatment almost completely inhibited the FAAH activity in all three tissues, as did the highest dose of URB597 (3 mg kg(-1)) in spinal cord samples, whereas no obvious changes were seen ex vivo for the other treatments. In conclusion, the results show that in mice, treatment with indomethacin and URB597 produce SR144528-sensitive anti-inflammatory effects in the carrageenan model of acute inflammation.

[Endogenous cannabinoid receptor ligands--anandamide and 2-arachidonoylglycerol]

Marijuana has been used as a traditional medicine and a pleasure-inducing drug for thousands of years around the world, especially in Asia. Delta(9)-tetrahydrocannabinol, major psychoactive component of marijuana, has been shown to interact with specific cannabinoid receptors, thereby eliciting a variety of pharmacological responses in experimental animals and human. In 1990, the gene encoding a cannabinoid receptor (CB1) was cloned. This prompted the search for endogenous ligands. In 1992, N-arachidonoylethanolamine (anandamide) was isolated from pig brain as an endogenous ligand, and in 1995, 2-arachidonoylglycerol was isolated from rat brain and canine gut as another endogenous ligand. Both anandamide and 2-arachidonoylglycerol exhibit various cannabimimetic activities. The results of structure-activity relationship experiments, however, revealed that 2-arachidonoylglycerol, but not anandamide, is the intrinsic natural ligand for the cannabinoid receptor. 2-arachidonoylglycerol is a degradation product of inositol phospholipids that links the function of the cannabinoid receptors with the enhanced inositol phospholipid turnover in stimulated tissues and cells. The possible physiological roles of cannabinoid receptors and 2-arachidonoylglycerol in various mammalian tissues such as those of the nervous and inflammatory cells are demonstrated. Furthermore, the future development of therapeutic drugs coming from this endocannabinoid system are discussed.

Anandamide reduces infarct size in rat isolated hearts subjected to ischaemia-reperfusion by a novel cannabinoid mechanism

Although the endocannabinoids 2-arachidonoylglycerol (2-AG) and anandamide share a similar pharmacology, 2-AG reportedly limits myocardial ischaemia-reperfusion injury whereas anandamide does not. We therefore investigated whether or not anandamide reduces infarct size and which, if any, of the known cannabinoid-signalling pathways are involved. Rat isolated perfused hearts were subjected to global, no-flow ischaemia (30 min) and reperfusion (1 h). Agonists were present from 5 min before ischaemia until the end of reperfusion. Antagonists, where used, were present throughout the protocol. Recovery of left ventricular developed pressure and coronary flow was incomplete in control hearts and not significantly affected by any drug treatment. In vehicle-treated hearts, 26+/-3% (n=13) of the left ventricle was infarcted at the end of reperfusion. Infarction of the left ventricle was significantly reduced after 1 microM anandamide (10+/-1%, n=7) or 1 microM methanandamide (12+/-4%, n=6) but not 1 microM HU210. Neither ACPA (1 microM; CB1 receptor agonist) nor JWH133 (1 microM; CB2 receptor agonist), individually or combined significantly affected infarct size. Anandamide (1 microM) did not reduce infarct size in the presence of the CB1 receptor antagonist rimonabant (SR141716A, 1 microM) or the CB2 receptor antagonist, SR144528 (1 microM). Despite sensitivity to CB1 and CB2 receptor antagonists, the infarct-limiting action of anandamide was not mimicked by agonists selective for CB1 or CB2 receptors suggesting the involvement of a novel cannabinoid site of action.

Monoacylglycerol lipase inhibition by organophosphorus compounds leads to elevation of brain 2-arachidonoylglycerol and the associated hypomotility in mice

Three components of the cannabinoid system are sensitive to selected organophosphorus (OP) compounds: monoacylglycerol (MAG) lipase that hydrolyzes the major endogenous agonist 2-arachidonoylglycerol (2-AG); fatty acid amide hydrolase (FAAH) that cleaves the agonist anandamide present in smaller amounts; the CB1 receptor itself. This investigation considers which component of the cannabinoid system is the most likely contributor to OP-induced hypomotility in mice. Structure-activity studies by our laboratory and others rule against major involvement of a direct toxicant-CB1 receptor interaction for selected OPs. Attention was therefore focused on the OP sensitivities of MAG lipase and FAAH, assaying 19 structurally diverse OP chemicals (pesticides, their metabolites and designer compounds) for in vitro inhibition of both enzymes. Remarkably high potency and low selectivity is observed with three O-alkyl (C1, C2, C3) alkylphosphonofluoridates (C8, C12) (IC50 0.60-3.0 nM), five S-alkyl (C5, C7, C9) and alkyl (C10, C12) benzodioxaphosphorin oxides (IC50 0.15-5.7 nM) and one OP insecticide metabolite (chlorpyrifos oxon, IC50 34-40 nM). In ip-treated mice, the OPs at 1-30 mg/kg more potently inhibit brain FAAH than MAG lipase, but FAAH inhibition is not correlated with hypomotility. However, the alkylphosphonofluoridate-treated mice show dose-dependent increases in severity of hypomotility, inhibition of MAG lipase activity and elevation of 2-AG. Moderate to severe hypomotility is accompanied by 64 to 86% MAG lipase inhibition and about 6-fold elevation of brain 2-AG level. It therefore appears that OP-induced MAG lipase inhibition leads to elevated 2-AG and the associated hypomotility.

Inhibition of fatty acid amide hydrolase and monoacylglycerol lipase by the anandamide uptake inhibitor VDM11: evidence that VDM11 acts as an FAAH substrate

There is some dispute concerning the extent to which the uptake inhibitor VDM11 (N-(4-hydroxy-2-methylphenyl) arachidonoyl amide) is capable of inhibiting the metabolism of the endocannabinoid anandamide (AEA) by fatty acid amide hydrolase (FAAH). In view of a recent study demonstrating that the closely related compound AM404 (N-(4-hydroxyphenyl)arachidonylamide) is a substrate for FAAH, we re-examined the interaction of VDM11 with FAAH. In the presence of fatty acid-free bovine serum albumin (BSA, 0.125% w v(-1)), both AM404 and VDM11 inhibited the metabolism of AEA by rat brain FAAH with similar potencies (IC(50) values of 2.1 and 2.6 microM, respectively). The compounds were about 10-fold less potent as inhibitors of the metabolism of 2-oleoylglycerol (2-OG) by cytosolic monoacylglycerol lipase (MAGL). The potency of VDM11 towards FAAH was dependent upon the assay concentration of fatty acid-free bovine serum albumin (BSA). Thus, in the absence of fatty acid-free BSA, the IC(50) value for inhibition of FAAH was reduced by a factor of about two (from 2.9 to 1.6 microM). A similar reduction in the IC(50) value for the inhibition of membrane bound MAGL by both this compound (from 14 to 6 microM) and by arachidonoyl serinol (from 24 to 13 microM) was seen. An HPLC assay was set up to measure 4-amino-m-cresol, the hypothesised product of FAAH-catalysed VDM11 hydrolysis. 4-Amino-m-cresol was eluted with a retention time of approximately 2.4 min, but showed a time-dependent degradation to compounds eluting at peaks of approximately 5.6 and approximately 8 min. Peaks with the same retention times were also found following incubation of the membranes with VDM11, but were not seen when the membranes were preincubated with the FAAH inhibitors URB597 (3'-carbamoyl-biphenyl-3-yl-cyclohexylcarbamate) and CAY10401 (1-oxazolo[4,5-b]pyridin-2-yl-9-octadecyn-1-one) prior to addition of VDM11. The rate of metabolism of VDM11 was estimated to be roughly 15-20% of that for anandamide. It is concluded that VDM11 is an inhibitor of FAAH under the assay conditions used here, and that the inhibition may at least in part be a consequence of the compound acting as an alternative substrate.

Modulators of endocannabinoid enzymic hydrolysis and membrane transport

Tissue concentrations of the endocannabinoids N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG) are regulated by both synthesis and inactivation. The purpose of this review is to compile available data regarding three inactivation processes: fatty acid amide hydrolase, monoacylglycerol lipase, and cellular membrane transport. In particular, we have focused on mechanisms by which these processes are modulated. We describe the in vitro and in vivo effects of inhibitors of these processes as well as available evidence regarding their modulation by other factors.

Decrease in prostaglandin level is a prerequisite for the expression of cannabinoid withdrawal: A quasi abstinence approach

Cannabinoid withdrawal has been indicated in both human and animal subjects. One of pathways proposed to facilitate cannabinoid action is the arachidonic acid cascade. Previously, we have shown that prostaglandin attenuated the expression of withdrawal signs in tetrahydrocannabinol-dependent mice. It follows that the cascade might participate in the expression of cannabinoid withdrawal. We utilized a quasi abstinence approach (the induction of a state of cannabinoid withdrawal without giving any cannabinoid substances in a naïve animal) to describe the relationship between the change in prostaglandin level, an end product of the arachidonic acid cascade, and the expression of cannabinoid withdrawal. Administration of 10 mg/kg diclofenac, a prostaglandin synthesis inhibitor, i.p. 30 min before SR 141716A induced cannabinoid withdrawal signs in naïve mice, which were comparable to the true abstinence in cannabinoid-tolerant mice. In turn, 10 mg/kg Delta(8)-THC i.p., given 15 min prior to SR 141716A, blocked the expression of these signs. These results suggested that the decrease in prostaglandin level is a prerequisite for the expression of cannabinoid withdrawal.

Influence of the degree of unsaturation of the acyl side chain upon the interaction of analogues of 1-arachidonoylglycerol with monoacylglycerol lipase and fatty acid amide hydrolase

Little is known as to the structural requirements of the acyl side chain for interaction of acylglycerols with monoacylglycerol lipase (MAGL), the enzyme chiefly responsible for the metabolism of the endocannabinoid 2-arachidonoylglycerol (2-AG) in the brain. In the present study, a series of twelve analogues of 1-AG (the more stable regioisomer of 2-AG) were investigated with respect to their ability to inhibit the metabolism of 2-oleoylglycerol by cytosolic and membrane-bound MAGL. In addition, the ability of the compounds to inhibit the hydrolysis of anandamide by fatty acid amide hydrolase (FAAH) was investigated. For cytosolic MAGL, compounds with 20 carbon atoms in the acyl chain and 2-5 unsaturated bonds inhibited the hydrolysis of 2-oleoylglycerol with similar potencies (IC50 values in the range 5.1-8.2 microM), whereas the two compounds with a single unsaturated bond were less potent (IC50 values 19 and 21 microM). The fully saturated analogue 1-monoarachidin did not inhibit the enzyme, whereas the lower side chain analogues 1-monopalmitin and 1-monomyristin inhibited the enzyme with IC50 values of 12 and 32 microM, respectively. The 22-carbon chain analogue of 1-AG was also potent (IC50 value 4.5 microM). Introduction of an alpha-methyl group for the C20:4, C20:3, and C22:4 compounds did not affect potency in a consistent manner. For the FAAH and the membrane-bound MAGL, there was no obvious relationship between the degree of unsaturation of the acyl side chain and the ability to inhibit the enzymes. It is concluded that increasing the number of unsaturated bonds on the acyl side chain of 1-AG from 1 to 5 has little effect on the affinity of acylglycerols for cytosolic MAGL.