Anandamide receptors

Effect of an unstirred layer on the membrane permeability of anandamide

To study the effect of an unstirred layer (UL), we have investigated the exchange efflux kinetics of anandamide at 0 degrees C, pH 7.3, from albumin-free as well as from albumin-filled human red blood cell ghosts to media of various BSA concentrations ([BSA](o)). The rate constant (k(m)) of unidirectional flux from the outer membrane leaflet to BSA in the medium increased with the square root of [BSA](o) in accordance with the existence of a UL, which is a water layer adjacent to the membrane that is not subject to the same gross mixing that takes place in the rest of the medium. From k(m), it is possible to calculate the rate constant of anandamide dissociation from BSA (k(1)) if we know the membrane binding of anandamide, the equilibrium dissociation constant of BSA-anandamide complexes, and the diffusion constant of anandamide. We estimated k(1) to be 3.33 +/- 0.27 s(-1). The net flux of [(3)H]anandamide is balanced by an equal and opposite movement of nonradioactive anandamide in exchange efflux experiments. This means that our results are also valid for uptake. We show that for anandamide with rapid membrane translocation, UL causes a significant resistance to cellular uptake. Depicting the rate of anandamide uptake as a function of equilibrium water phase concentrations results in a parabolic uptake dependence. Such apparent "saturation kinetics" is often interpreted as indicating the involvement of transport proteins. The validity of such an interpretation is discussed.

Review article: endocannabinoids and their receptors in the enteric nervous system

The therapeutic actions of cannabinoids have been known for centuries. In the last 25 years this area of research has grown exponentially with the discovery of specific cannabinoid receptors and endogenous ligands. In the enteric nervous system of gastrointestinal tract, cannabinoid receptors are located on enteric nerve terminals where they exert inhibitory actions on neurotransmission to reduce motility and secretion. Endogenous cannabinoids are present in the enteric nervous system, as are the degradative enzymes necessary to inhibit their action. The cellular mechanism of action of endocannabinoids has not been established in the enteric nervous system. Endocannabinoids not only act at cannabinoid receptors, but potentially also at vanilloid and 5-HT3 receptors, both of which are expressed in the gastrointestinal tract. The interactions between endocannabinoids and these other important receptor systems have not been extensively investigated. A greater understanding of the endocannabinoid system in the enteric nervous system could lead to advances with important therapeutic potential in the treatment of gastrointestinal disorders such as irritable bowel syndrome, inflammatory bowel disease, secretory diarrhoea and gastro-oesophageal reflux disease.

Mechanisms of HIV-1 Inhibition by the Lipid MediatorN-Arachidonoyldopamine

Several linear fatty acid dopamides (N-acyldopamines) have been identified recently in the brain. Among them, N-arachidonoyldopamine (NADA) is an endogenous lipid mediator sharing endocannabinoid and endovanilloid biological activities. We have reported previously that NADA exerts some of its biological activities through inhibition of the NF-kappaB pathway and, because this transcription factor plays a key role in HIV-1-long terminal repeat (LTR) trans activation, we have evaluated the anti-HIV-1 activity of NADA. In this study, we show that NADA inhibits vesicular stomatitis virus-pseudotyped HIV-1 infection in the human leukemia T cell line Jurkat, in primary T cells, and in the human astrocytic cell line U373-MG. Other endocannabinoids such as anandamide, 2-arachidonoylglycerol, and noladin ether did not show inhibitory activity in the HIV-1 replication assays. The anti-HIV-1 activity of NADA was independent of known cannabinoid and vanilloid receptor activation. In addition, NADA did not affect reverse transcription and integration steps of the viral cycle, and its inhibitory effect was additive with that of the reverse transcriptase inhibitor azidothymidine. NADA inhibited both TNF-alpha and HIV-1 trans activator protein-induced HIV-1-LTR activation. We also show that NADA counteracts the TNF-alpha-mediated trans activation capacity of the p65 NF-kappaB subunit without affecting its physical association to the HIV-1-LTR promoter. Moreover, NADA inhibited the p65 transcriptional activity by specifically targeting the phosphorylation of this NF-kappaB subunit at Ser(536). These findings provide new mechanistic insights into the biological activities of NADA, and highlight the potential of lipid mediators for the management of AIDS.

Characterization of the endocannabinoid system in boar spermatozoa and implications for sperm capacitation and acrosome reaction

Anandamide (AEA) is the endogenous ligand of cannabinoid (CB) receptors, and as such it plays several central and peripheral activities. Regulation of female fertility by AEA has attracted growing interest, yet a role for this endocannabinoid in controlling sperm function and male fertility in mammals has been scarcely investigated. In this study we report unprecedented evidence that boar sperm cells have the biochemical machinery to bind and degrade AEA, i.e. type-1 cannabinoid receptors (CB1R), vanilloid receptors (TRPV1), AEA-synthesizing phospholipase D (NAPE-PLD), AEA transporter (AMT) and AEA hydrolase (FAAH). We also show that the non-hydrolyzable AEA analogue methanandamide reduces sperm capacitation and, as a consequence, inhibits the process of acrosome reaction (AR) triggered by the zona pellucida, according to a cyclic AMP-dependent pathway triggered by CB1R activation. Furthermore, activation of TRPV1 receptors seems to play a role of stabilization of the plasma membranes in capacitated sperm, as demonstrated by the high incidence of spontaneous AR occurring during the cultural period when TRPV1 activity was antagonized by capsazepine. We show that sperm cells have a complete and efficient endocannabinoid system, and that activation of cannabinoid or vanilloid receptors controls, at different time-points, sperm functions required for fertilization. These observations open new perspectives on the understanding and treatment of male fertility problems.

Targeted lipidomics: fatty acid amides and pain modulation

Mass spectrometric approaches to the identification and quantification of lipid signalling molecules are reviewed. Fatty acid amides are an important new class of lipid signalling molecules which include oleamide, the endocannabinoid anandamide, the endovanilloid/endocannabinoid N-arachidonoyldopamine (NADA) and the endovanilloid N-oleoyldopamine (OLDA) among many others. This diverse group of endogenous compounds comprises combinations of acyl backbones coupled by an amide bond to any of a variety of different small polar molecules such as ethanolamine, various amino acids, and catecholamines. Many fatty acid amides appear to play a role in pain and inflammation. Targeted lipidomics of fatty acid amides aims to identify new members of this diverse class of compounds, of which only a few representative molecules have been characterized to date. This effort has been made feasible by advances in chromatography and mass spectrometry, which permits: (1) identification of compounds present in complex mixtures, (2) astronomical increases in sensitivity due to miniaturization of HPLC components, and (3) novel scanning modes that permit the identification of compounds exhibiting similar structural components. Insofar as lipid signalling molecules such as prostanoids, leukotrienes and endocannabinoids operate via G-protein coupled receptors (GPCR), it appears likely that many of the numerous lipids awaiting identification may serve as ligands for any of the greater than 150 orphan GPCRs.

The endocannabinoid system in the physiology and pathophysiology of the gastrointestinal tract

Numerous investigations have recently demonstrated the important roles of the endocannabinoid system in the gastrointestinal (GI) tract under physiological and pathophysiological conditions. In the GI tract, cannabinoid type 1 (CB1) receptors are present in neurons of the enteric nervous system and in sensory terminals of vagal and spinal neurons, while cannabinoid type 2 receptors are located in immune cells. Activation of CB1 receptors was shown to modulate several functions in the GI tract, including gastric secretion, gastric emptying and intestinal motility. Under pathophysiological conditions induced experimentally in rodents, the endocannabinoid system conveys protection to the GI tract (e.g. from inflammation and abnormally high gastric and enteric secretions). Such protective activities are largely in agreement with anecdotal reports from folk medicine on the use of Cannabis sativa extracts by subjects suffering from various GI disorders. Thus, the endocannabinoid system may serve as a potentially promising therapeutic target against different GI disorders, including frankly inflammatory bowel diseases (e.g. Crohn's disease), functional bowel diseases (e.g. irritable bowel syndrome) and secretion- and motility-related disorders. As stimulation of this modulatory system by CB1 receptor agonists can lead to unwanted psychotropic side effects, an alternative and promising avenue for therapeutic applications resides in the treatment with CB1 receptor agonists that are unable to cross the blood-brain barrier, or with compounds that inhibit the degradation of endogenous ligands (endocannabinoids) of CB1 receptors, hence prolonging the activity of the endocannabinoid system.

Effects ofN-acylethanolamines on the respiratory chain and production of reactive oxygen species in heart mitochondria

Long-chain N-acylethanolamines (NAEs) have been found to uncouple oxidative phosphorylation and to inhibit uncoupled respiration of rat heart mitochondria [Wasilewski, M., Wieckowski, M.R., Dymkowska, D. and Wojtczak, L. (2004) Biochim. Biophys. Acta 1657, 151-163]. The aim of the present work was to investigate in more detail the mechanism of the inhibitory effects of NAEs on the respiratory chain. In connection with this, we also investigated a possible action of NAEs on the generation of reactive oxygen species (ROS) by respiring rat heart mitochondria. It was found that unsaturated NAEs, N-oleoylethanolamine (N-Ole) and, to a greater extent, N-arachidonoylethanolamine (N-Ara), inhibited predominantly complex I of the respiratory chain, with a much weaker effect on complexes II and III, and no effect on complex IV. Saturated N-palmitoylethanolamine had a much smaller effect compared to unsaturated NAEs. N-Ara and N-Ole were found to decrease ROS formation, apparently due to their uncoupling action. However, under specific conditions, N-Ara slightly but significantly stimulated ROS generation in uncoupled conditions, probably due to its inhibitory effect on complex I. These results may contribute to our better understanding of physiological roles of NAEs in protection against ischemia and in induction of programmed cell death.

The sperm, a neuron with a tail: 'neuronal' receptors in mammalian sperm

A number of plasma membrane receptor types originally thought to be specific to neurons have been found in other somatic cells. More surprisingly, the mammalian sperm and neuron appear to share many of these 'neuronal' receptors. The morphology, chromosome number, genomic activity, and functions of those two cell types are as unlike as any two cells in the body, but they both achieve their highly disparate goals with the aid of a number of the same receptors. Exocytosis in neurons and sperm is essential to the functions of these cells and is strongly influenced by similar receptors. 'Neuronal' receptor types in sperm may also play a role in the control of sperm motility (a function of course not shared by neurons). This review will consider the evidence for the presence of sperm plasma membrane 'neuronal' receptors and for their significance to mammalian sperm function. The persuasiveness of the evidence varies depending on the receptor being considered, but there is strong experimental support for the presence and importance of a number of 'neuronal' receptors in sperm.

Finding of endocannabinoids in human eye tissues: Implications for glaucoma

Cannabinoid CB(1) receptors are involved in ocular physiology and may regulate intraocular pressure (IOP). However, endocannabinoid levels in human ocular tissues of cornea, iris, ciliary body, retina, and choroid from normal and glaucomatous donors have not been investigated. Anandamide (N-arachidonoylethanolamine; AEA), 2-arachidonoylglycerol (2-AG), and the anandamide congener, palmitoylethanolamide (PEA), were detected in all the human tissues examined. In eyes from patients with glaucoma, significantly decreased 2-AG and PEA levels were detected in the ciliary body, an important tissue in the regulation of IOP. The findings suggest that these endogenous compounds may have a role in this disease, particularly with respect to regulation of IOP.

Endocannabinoids and exercise

Exercise induces changes in mental status, particularly analgesia, sedation, anxiolysis, and a sense of wellbeing. The mechanisms underlying these changes remain unknown. Recent findings show that exercise increases serum concentrations of endocannabinoids, suggesting a possible explanation for a number of these changes. This article provides an overview of this emerging field.

Cannabinoid receptor systems: therapeutic targets for tumour intervention

The past decade has witnessed a rapid expansion of our understanding of the biological roles of cannabinoids and their cognate receptors. It is now certain that Delta9-tetrahydrocannabinol, the principle psychoactive component of the Cannabis sativa plant, binds and activates membrane receptors of the 7-transmembrane domain, G-protein-coupled superfamily. Several putative endocannabinoids have since been identified, including anandamide, 2-arachidonyl glycerol and noladin ether. Synthesis of numerous cannabinomimetics has also greatly expanded the repertoire of cannabinoid receptor ligands with the pharmacodynamic properties of agonists, antagonists and inverse agonists. Collectively, these ligands have proven to be powerful tools both for the molecular characterisation of cannabinoid receptors and the delineation of their intrinsic signalling pathways. Much of our understanding of the signalling mechanisms activated by cannabinoids is derived from studies of receptors expressed by tumour cells; hence, this review provides a succinct summary of the molecular pharmacology of cannabinoid receptors and their roles in tumour cell biology. Moreover, there is now a genuine expectation that the manipulation of cannabinoid receptor systems may have therapeutic potential for a diverse range of human diseases. Thus, this review also summarises the demonstrated antitumour actions of cannabinoids and indicates possible avenues for the future development of cannabinoids as antitumour agents.

Molecular Characterization of N-Acylethanolamine-hydrolyzing Acid Amidase, a Novel Member of the Choloylglycine Hydrolase Family with Structural and Functional Similarity to Acid Ceramidase

Bioactive N-acylethanolamines, including anandamide (an endocannabinoid) and N-palmitoylethanolamine (an anti-inflammatory and neuroprotective substance), are hydrolyzed to fatty acids and ethanolamine by fatty acid amide hydrolase. Moreover, we found another amidohydrolase catalyzing the same reaction only at acidic pH, and we purified it from rat lung (Ueda, N., Yamanaka, K., and Yamamoto, S. (2001) J. Biol. Chem. 276, 35552-35557). Here we report complementary DNA cloning and functional expression of the enzyme termed "N-acylethanolamine-hydrolyzing acid amidase (NAAA)" from human, rat, and mouse. The deduced primary structures revealed that NAAA had no homology to fatty acid amide hydrolase but belonged to the choloylglycine hydrolase family. Human NAAA was essentially identical to a gene product that had been noted to resemble acid ceramidase but lacked ceramide hydrolyzing activity. The recombinant human NAAA overexpressed in HEK293 cells hydrolyzed various N-acylethanolamines with N-palmitoylethanolamine as the most reactive substrate. Most interestingly, a very low ceramide hydrolyzing activity was also detected with NAAA, and N-lauroylethanolamine hydrolyzing activity was observed with acid ceramidase. By the use of tunicamycin and endoglycosidase, NAAA was found to be a glycoprotein. Furthermore, the enzyme was proteolytically processed to a shorter form at pH 4.5 but not at pH 7.4. Expression analysis of a green fluorescent protein-NAAA fusion protein showed a lysosome-like distribution in HEK293 cells. The organ distribution of the messenger RNA in rats revealed its wide distribution with the highest expression in lung. These results demonstrated that NAAA is a novel N-acylethanolamine-hydrolyzing enzyme that shows structural and functional similarity to acid ceramidase.

Effects of anandamide on the binding and signaling properties of M1 muscarinic acetylcholine receptors

This study investigated the effects of the endocannabinoid, anandamide, on M(1) muscarinic acetylcholine receptors (mAChRs) expressed in Chinese hamster ovary (CHO) cells. In the presence of anandamide, [(3)H]N-methylscopolamine ([(3)H]NMS) inhibition binding was characterized by Hill coefficients greater than 1 while saturation binding isotherms were characterized by a reduction in radioligand B(max). Anandamide did not affect the potency of classic agonists, antagonists or allosteric modulators to inhibit [(3)H]NMS binding, indicating that the site of anandamide action did not involve receptor regions recognized by these compounds. Although the mode of binding of anandamide was reversible, the order of ligand addition was important; the inhibitory effect was greatest when anandamide was equilibrated with the receptor prior to radioligand addition, and weakest in the converse situation. Interestingly, the inhibitory potency of anandamide was reduced on pre-equilibration with non-transfected CHO cell membranes, prior to addition of M(1) mAChR-transfected membranes. In phosphoinositide (PI) hydrolysis assays, anandamide significantly reduced the maximal response to acetylcholine, but at higher concentrations than those needed to fully inhibit radioligand binding. Studies utilizing a range of agonists with varying intrinsic activities showed that the inhibitory effects of anandamide on agonist function were most pronounced with the lowest efficacy agonists. These findings suggest that the mechanism of action of anandamide at the M(1) mAChR involves perturbation of the receptor via the membrane in a manner that is sensitive to the conformation of the receptor (occupied versus vacant).

Lipid rafts control signaling of type-1 cannabinoid receptors in neuronal cells. Implications for anandamide-induced apoptosis

Several G protein-coupled receptors function within lipid rafts plasma membrane microdomains, which may be important in limiting signal transduction. Here we show that treatment of rat C6 glioma cells with the raft disruptor methyl-beta-cyclodextrin (MCD) doubles the binding efficiency (i.e. the ratio between maximum binding and dissociation constant) of type-1 cannabinoid receptors (CB1R), which belong to the rhodopsin family of G protein-coupled receptors. In parallel, activation of CB1R by the endogenous agonist anandamide (AEA) leads to approximately 3-fold higher [35S]GTPgammaS binding in MCD-treated cells than in controls, and CB1R-dependent signaling via adenylate cyclase, and p42/p44 MAPK is almost doubled by MCD. Unlike CB1R, the other AEA-binding receptor TRPV1, the AEA synthetase NAPE-PLD, and the AEA hydrolase FAAH are not modulated by MCD, whereas the activity of the AEA membrane transporter (AMT) is reduced to approximately 50% of the controls. We also show that MCD reduces dose-dependently AEA-induced apoptosis in C6 cells but not in human CHP100 neuroblastoma cells, which mirror the endocannabinoid system of C6 cells but are devoid of CB1R. MCD reduces also cytochrome c release from mitochondria of C6 cells, and this effect is CB1R-dependent and partly mediated by activation of p42/p44 MAPK. Altogether, the present data suggest that lipid rafts control CB1R binding and signaling, and that CB1R activation underlies the protective effect of MCD against apoptosis.

Cholesterol-dependent modulation of type 1 cannabinoid receptors in nerve cells

Type 1 cannabinoid receptors (CB1R) are G-protein-coupled receptors that mediate several actions of the endocannabinoid anandamide (N-arachidonoylethanolamine; AEA) in the central nervous system. Here we show that cholesterol enrichment of rat C6 glioma cell membranes reduces by approximately twofold the binding efficiency (i.e., the ratio between maximum binding and dissociation constant) of CB1R and that activation of CB1R by AEA leads to approximately twofold lower [(35)S]GTPgammaS binding in cholesterol-treated cells than in controls. In addition, we show that CB1R-dependent signaling via adenylate cyclase and p42/p44 mitogen-activated protein kinase is almost halved by cholesterol enrichment. Unlike CB1R, the other AEA-binding receptor TRPV1, the AEA synthetase NAPE-PLD, and the AEA hydrolase FAAH are not modulated by cholesterol, whereas the catalytic efficiency (i.e., the ratio between maximal velocity and Michaelis-Menten constant) of the AEA membrane transporter AMT is almost doubled compared with control cells. These data demonstrate that, among the proteins of the "endocannabinoid system," only CB1R and AMT critically depend on membrane cholesterol content. This observation may have important implications for the role of CB1R in protecting nerve cells against (endo)cannabinoid-induced apoptosis.

The biosynthesis, fate and pharmacological properties of endocannabinoids

The finding of endogenous ligands for cannabinoid receptors, the endocannabinoids, opened a new era in cannabinoid research. It meant that the biological role of cannabinoid signalling could be finally studied by investigating not only the pharmacological actions subsequent to stimulation of cannabinoid receptors by their agonists, but also how the activity of these receptors was regulated under physiological and pathological conditions by varying levels of the endocannabinoids. This in turn meant that the enzymes catalysing endocannabinoid biosynthesis and inactivation had to be identified and characterized, and that selective inhibitors of these enzymes had to be developed to be used as (1) probes to confirm endocannabinoid involvement in health and disease, and (2) templates for the design of new therapeutic drugs. This chapter summarizes the progress achieved in this direction during the 12 years following the discovery of the first endocannabinoid.

Up-regulation of the endocannabinoid system in the uterus of leptin knockout (ob/ob) mice and implications for fertility

The levels of the endocannabinoids anandamide and 2-arachidonoylglycerol (2-AG) are under the negative control of leptin in the rodent hypothalamus. As leptin and endocannabinoids play opposite roles in the control of reproduction, we have investigated whether the impaired fertility typical of leptin-defective ob/ob mice is due, in part, to enhanced uterine endocannabinoid levels. We found that levels of both anandamide and 2-AG in the uterus of ob/ob mice are significantly elevated with respect to wild-type littermates, due to reduced hydrolase activity in the case of anandamide, and to reduced monoacylglycerol lipase and enhanced diacylglycerol lipase activity in the case of 2-AG. Furthermore, the process mediating endocannabinoid cellular uptake was also impaired in ob/ob mice, whereas the levels of cannabinoid and anandamide receptors were not modified. Although ineffective in wild-type mice, treatment of ob/ob mice with leptin re-established endocannabinoid levels and enzyme activities back to the values observed in wild-type littermates. Finally, treatment of ob/ob females with the CB1 receptor antagonist SR141716A did not improve their fertility, and inhibition of endocannabinoid inactivation with the endocannabinoid uptake inhibitor OMDM-1 in wild-type females did not result in impaired fertility.

Biosynthesis of anandamide and N-palmitoylethanolamine by sequential actions of phospholipase A2 and lysophospholipase D

Anandamide (an endocannabinoid) and other bioactive long-chain NAEs (N-acylethanolamines) are formed by direct release from N-acyl-PE (N-acyl-phosphatidylethanolamine) by a PLD (phospholipase D). However, the possible presence of a two-step pathway from N-acyl-PE has also been suggested previously, which comprises (1) the hydrolysis of N-acyl-PE to N-acyl-lysoPE by PLA1/PLA2 enzyme(s) and (2) the release of NAEs from N-acyllysoPE by lysoPLD (lysophospholipase D) enzyme(s). In the present study we report for the first time the characterization of enzymes responsible for this pathway. The PLA1/PLA2 activity for N-palmitoyl-PE was found in various rat tissues, with the highest activity in the stomach. This stomach enzyme was identified as group IB sPLA2 (secretory PLA2), and its product was determined as N-acyl-1-acyl-lysoPE. Recombinant group IB, IIA and V of sPLA2s were also active with N-palmitoyl-PE, whereas group X sPLA2 and cytosolic PLA2a were inactive. In addition, we found wide distribution of lysoPLD activity generating N-palmitoylethanolamine from N-palmitoyl-lysoPE in rat tissues, with higher activities in the brain and testis. Based on several lines of enzymological evidence, the lysoPLD enzyme could be distinct from the known N-acyl-PE-hydrolysing PLD. sPLA2-IB dose dependently enhanced the production of N-palmitoylethanolamine from N-palmitoyl-PE in the brain homogenate showing the lysoPLD activity. N-Arachidonoyl-PE and N-arachidonoyl-lysoPE as anandamide precursors were also good substrates of sPLA2-IB and the lysoPLD respectively. These results suggest that the sequential actions of PLA2 and lysoPLD may constitute another biosynthetic pathway for NAEs, including anandamide.

[Endocannabinoids in the central nervous system]

The major psychoactive component of cannabis derivatives, delta9-THC, activates two G-protein coupled receptors: CB1 and CB2. Soon after the discovery of these receptors, their endogenous ligands were identified: lipid metabolites of arachidonic acid, named endocannabinoids. The two major main and most studied endocannabinoids are anandamide and 2-arachidonyl-glycerol. The CB1 receptor is massively expressed through-out the central nervous system whereas CB2 expression seems restricted to immune cells. Following endocannabinoid binding, CB1 receptors modulate second messenger cascades (inhibition of adenylate cyclase, activation of mitogen-activated protein kinases and of focal-adhesion kinases) as well as ionic conductances (inhibition of voltage-dependent calcium channels, activation of several potassium channels). Endocannabinoids transiently silence synapses by decreasing neurotransmitter release, play major parts in various forms of synaptic plasticity because of their ability to behave as retrograde messengers and activate non-cannabinoid receptors (such as vanilloid receptor type-1), illustrating the complexity of the endocannabinoid system. The diverse cellular targets of endocannabinoids are at the origin of the promising therapeutic potentials of the endocannabinoid system.

The endocannabinoid system and its therapeutic exploitation

The term 'endocannabinoid' - originally coined in the mid-1990s after the discovery of membrane receptors for the psychoactive principle in Cannabis, Delta9-tetrahydrocannabinol and their endogenous ligands - now indicates a whole signalling system that comprises cannabinoid receptors, endogenous ligands and enzymes for ligand biosynthesis and inactivation. This system seems to be involved in an ever-increasing number of pathological conditions. With novel products already being aimed at the pharmaceutical market little more than a decade since the discovery of cannabinoid receptors, the endocannabinoid system seems to hold even more promise for the future development of therapeutic drugs. We explore the conditions under which the potential of targeting the endocannabinoid system might be realized in the years to come.

Differential effects of endogenous and synthetic cannabinoids on alpha7-nicotinic acetylcholine receptor-mediated responses in Xenopus Oocytes

The effects of endogenous and synthetic cannabinoid receptor agonists, including 2-arachidonoylglycerol (2-AG), R-methanandamide, WIN55,212-2 [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenylcarbonyl)-6H-pyrrolo[3,2,1ij]quinolin-6-one], and CP 55,940 [1alpha,2beta-(R)-5alpha]-(-)-5-(1,1-dimethyl)-2-[5-hydroxy-2-(3-hydroxypropyl) cyclohexyl-phenol], and the psychoactive constituent of marijuana, Delta9-tetrahydrocannabinol (Delta9-THC), on the function of homomeric alpha7-nicotinic acetylcholine (nACh) receptors expressed in Xenopus oocytes was investigated using the two-electrode voltage-clamp technique. The endogenous cannabinoid receptor ligands 2-AG and the metabolically stable analog of anandamide (arachidonylethanolamide), R-methanandamide, reversibly inhibited currents evoked with ACh (100 microM) in a concentration-dependent manner (IC50 values of 168 and 183 nM, respectively). In contrast, the synthetic cannabinoid receptor agonists CP 55,940, WIN55,212-2, and the phytochemical Delta9-THC did not alter alpha7-nACh receptor function. The inhibition of alpha7-mediated currents by 2-AG was found to be non-competitive and voltage-independent. Additional experiments using endocannabinoid metabolites suggested that arachidonic acid, but not ethanolamine or glycerol, could also inhibit the alpha7-nACh receptor function. Whereas the effects of arachidonic acid were also noncompetitive and voltage-independent, its potency was much lower than 2-AG and anandamide. Results of studies with chimeric alpha7-nACh-5-hydroxytryptamine (5-HT)3 receptors comprised of the amino-terminal domain of the alpha7-nACh receptor and the transmembrane and carboxyl-terminal domains of 5-HT3 receptors indicated that the site of interaction of the endocannabinoids with the alpha7-nAChR was not located on the N-terminal region of the receptor. These data indicate that cannabinoid receptor ligands that are produced in situ potently inhibit alpha7-nACh receptor function, whereas the synthetic cannabinoid ligands, and Delta9-THC, are without effect, or are relatively ineffective at inhibiting these receptors.

Stearoylethanolamide exerts anorexic effects in mice via downregulation of liver stearoyl‐coenzyme A desaturase‐1 mRNA expression

Given the recent demonstration that oleoylethanolamide (OEA), a cannabinoid receptor-inactive N-acylethanolamine, decreases food intake by activating the nuclear receptor PPARalpha (peroxisome proliferator-activated receptor alpha) in the periphery, we here evaluated the effects of both saturated and unsaturated C18 N-acylethanolamides (C18:0; C18:1; C18:2) in mice feeding behavior after overnight starvation. Our results show stearoylethanolamide (SEA, C18:0) exerts, unlike other unsaturated C18 homologs, a marked dose-dependent anorexic effect evident already at 2 h after its intraperitoneal administration. In addition, oral administration of SEA (25 mg/kg) was also effective in reducing food consumption, an effect ascribed to the molecule itself and not to its catabolites. Moreover, although the anorexic response to oral administered SEA was not associated with changes in the levels of various hematochemical parameters (e.g., glucose, cholesterol, triglycerides, leptin) nor in liver mRNA expression of peroxisome proliferator-activated receptors (PPARs) including PPARalpha, the anorexic effect of SEA was interestingly accompanied by a reduction in liver stearoyl-CoA desaturase-1 (SCD-1) mRNA expression. As SCD-1 has been recently proposed as a molecular target for the treatment of obesity, the novel observation provided here that SEA reduces food intake in mice in a structurally selective manner, in turn, correlated with downregulation of liver SCD-1 mRNA expression, has the potential of providing new insights on a class of lipid mediators with suitable properties for the pharmacological treatment of over-eating dysfunctions.

A new strategy to block tumor growth by inhibiting endocannabinoid inactivation

Endocannabinoid signaling has been shown to be enhanced in several cancer tissues and malignant cells, and studies in cell lines have shown that this up-regulation might serve the purpose of providing transformed cells with a further means to inhibit their proliferation. Here we investigated the effect of inhibitors of endocannabinoid degradation on the growth of rat thyroid tumor xenografts induced in athymic mice. VDM-11, a selective inhibitor of endocannabinoid cellular re-uptake, and arachidonoyl-serotonin (AA-5-HT), a selective blocker of endocannabinoid enzymatic hydrolysis, both inhibited the growth in vivo of tumor xenografts induced by the subcutaneous injection of rat thyroid transformed (KiMol) cells. This effect was accompanied by significantly enhanced endocannabinoid concentrations in the tumors excised at the end of the in vivo experiments. Endocannabinoids, as well as VDM-11 and AA-5-HT, inhibited the growth in vitro of the transformed rat thyroid cells used to induce the tumors in vivo, and their effect was reversed at least in part by the cannabinoid CB1 receptor antagonist SR141716A. This compound, however, when administered alone, did not enhance, but instead slightly inhibited, the growth of rat thyroid transformed cells both in vitro and in tumor xenografts induced in vivo. These findings indicate that endocannabinoids tonically control tumor growth in vivo by both CB1-mediated and non-CB1-mediated mechanisms and that, irrespective of the molecular mechanism of their anti-proliferative action, inhibitors of their inactivation might be used for the development of novel anti-cancer drugs.

A comparison of the discriminative stimulus effects of delta(9)-tetrahydrocannabinol and O-1812, a potent and metabolically stable anandamide analog, in rats

Efforts to determine whether Delta(9)-tetrahydrocannabinol (Delta(9)-THC) and anandamide elicit similar discriminative stimulus effects have yielded conflicting results. The difficulty in establishing a discriminative cue to anandamide may be due to its metabolic instability. Rats were trained to discriminate either Delta(9)-THC or O-1812, a metabolically stable anandamide analog, from vehicle to avoid this issue. O-1812 and Delta(9)-THC substituted for each other; however, both drugs were more potent in the O-1812-trained rats. Further, O-1812 only substituted for Delta(9)-THC at response rate decreasing doses. The CB(1) antagonist, SR141716A, blocked the discriminative stimulus effects of both drugs but augmented their rate effects. O-1839, a VR(1) agonist, failed to substitute for either cannabinoid. These results suggest that the discriminative stimulus effects of Delta(9)-THC and O-1812 are similar, but subtle differences also exist.

N-cyclohexanecarbonylpentadecylamine: a selective inhibitor of the acid amidase hydrolysing N-acylethanolamines, as a tool to distinguish acid amidase from fatty acid amide hydrolase

Anandamide ( N-arachidonoylethanolamine) and other bioactive N-acylethanolamines are degraded to their corresponding fatty acids and ethanolamine. This hydrolysis is mostly attributed to catalysis by FAAH (fatty acid amide hydrolase), which exhibits an alkaline pH optimum. In addition, we have identified another amidase which catalyses the same reaction exclusively at acidic pH values [Ueda, Yamanaka and Yamamoto (2001) J. Biol. Chem. 276, 35552-35557]. In attempts to find selective inhibitors of this acid amidase, we screened various derivatives of palmitic acid, 1-hexadecanol, and 1-pentadecylamine with N-palmitoylethanolamine as substrate. Here we show that N-cyclohexanecarbonylpentadecylamine inhibits the acid amidase from rat lung with an IC50 of 4.5 microM, without inhibiting FAAH at concentrations up to 100 microM. The inhibition was reversible and non-competitive. This compound also inhibited the acid amidase in intact alveolar macrophages. With the aid of this inhibitor, it was revealed that rat basophilic leukaemia cells possess the acid amidase as well as FAAH. Thus the inhibitor may be a useful tool to distinguish the acid amidase from FAAH in various tissues and cells and to elucidate the physiological role of the enzyme.

Endovanilloids. Putative endogenous ligands of transient receptor potential vanilloid 1 channels

Endovanilloids are defined as endogenous ligands of the transient receptor potential vanilloid type 1 (TRPV1) protein, a nonselective cation channel that belongs to the large family of TRP ion channels, and is activated by the pungent ingredient of hot chilli peppers, capsaicin. TRPV1 is expressed in some nociceptor efferent neurons, where it acts as a molecular sensor of noxious heat and low pH. However, the presence of these channels in various regions of the central nervous system, where they are not likely to be targeted by these noxious stimuli, suggests the existence of endovanilloids. Three different classes of endogenous lipids have been found recently that can activate TRPV1, i.e. unsaturated N-acyldopamines, lipoxygenase products of arachidonic acid and the endocannabinoid anandamide with some of its congeners. To classify a molecule as an endovanilloid, the compound should be formed or released in an activity-dependent manner in sufficient amounts to evoke a TRPV1-mediated response by direct activation of the channel. To control TRPV1 signaling, endovanilloids should be inactivated within a short time-span. In this review, we will discuss, for each of the proposed endogenous ligands of TRPV1, their ability to act as endovanilloids in light of the criteria mentioned above.

Occurrence, metabolism, and prospective functions of N-acylethanolamines in plants

N-Acylethanolamines (NAEs) are fatty acid amides that are derived from an N-acylated phoshatidylethanolamine presursor, a minor membrane lipid constituent of plant and animal cells. Historically, the formation of N-acylethanolamines was associated with cellular stress and tissue damage in mammals, but more recently has been shown to be part of the endocannabinoid signaling system that regulates a variety of normal physiological functions, including neurotransmission, immune responses, vasodilation, embryo development and implantation, feeding behavior, cell proliferation, etc. The widespread regulation of vertebrate physiology by this class of lipid mediators and the conservation of the mechanisms for NAE formation, perception and degradation in higher plants raises the possibility that the metabolism of NAEs represents an evolutionarily conserved lipid signaling pathway that regulates an array of physiological processes in multicellular eukaryotes. Here the recent information on NAEs in plants is reviewed in the context of the occurrence, metabolism and functions of this bioactive class of lipid mediators.

Effects of N-acylethanolamines on mitochondrial energetics and permeability transition

Effects of N-acylethanolamines (NAEs): N-arachidonoylethanolamine (anandamide), N-oleoylethanolamine and N-palmitoylethanolamine, on energy coupling and permeability of rat heart mitochondria were investigated. In nominally Ca2+-free media, these compounds exerted a weak protonophoric effect manifested by dissipation of the transmembrane potential and stimulation of resting state respiration. The strongest action was exhibited by N-arachidonoylethanolamine, followed by N-oleoylethanolamine, whereas N-palmitoylethanolamine was almost inactive. These protonophoric effects were resistant to cyclosporin A (CsA) and were much weaker than those of corresponding nonesterified fatty acids. In uncoupled mitochondria N-arachidonoylethanolamine and N-oleoylethanolamine partly inhibited mitochondrial respiration with glutamate and succinate but not with tetramethyl-p-phenylenediamine (TMPD) plus ascorbate as respiratory substrates. In mitochondria preloaded with small amounts of Ca2+, NAEs produced a much stronger dissipation of the membrane potential and a release of accumulated calcium, both effects being inhibited by CsA, indicative for opening of the mitochondrial permeability transition pore (PTP). Again, the potency of this action was N-arachidonoylethanolamine>N-oleoylethanolamine>N-palmitoylethanolamine. However, in spite of making the matrix space accessible to external [14C]sucrose, N-arachidonoylethanolamine and N-oleoylethanolamine resulted in only a limited swelling of mitochondria and diminished the rate of swelling produced by high Ca2+ load.

Noladin ether, a putative endocannabinoid, attenuates sensory neurotransmission in the rat isolated mesenteric arterial bed via a non-CB1/CB2 G(i/o) linked receptor

1 Noladin ether has recently been reported to be an endocannabinoid, with selectivity for the cannabinoid (CB) CB1 receptor. In the present study, we investigated the effects of noladin ether in the rat isolated mesenteric arterial bed, cultured dorsal root ganglia (DRG) cells and human vanilloid (TRPV1)-receptor-expressing HEK293 cells (TRPV1-HEK293 cells). 2 Electrical field stimulation of the mesenteric bed evoked frequency-dependent vasorelaxation due to the action of calcitonin gene-related peptide (CGRP) released from sensory nerves. Noladin ether (0.1-3 microm) attenuated sensory neurogenic relaxation in a concentration-dependent manner. Noladin ether (1 microm) reduced vasorelaxation at a submaximal frequency (8 Hz), from 57.3+/-6.8 to 23.3+/-3.8% (P<0.05, n=4). 3 The inhibitory effects of noladin ether were unaffected by the CB1 antagonists SR141716A and LY320135, and the CB2 antagonist SR144528 (1 microm). 4 Noladin ether had no effect on vasorelaxation elicited by exogenous CGRP or capsaicin. These data suggest that noladin ether is acting at a prejunctional site and no interaction with TRPV1 is involved. 5 In mesenteric beds from pertussis toxin (PTX)-pretreated rats, the inhibitory actions of noladin ether on sensory neurotransmission were abolished, indicating the involvement of G(i/o) protein-coupled receptors. 6 Noladin ether evoked a concentration-dependent increase in intracellular Ca2+ concentration in TRPV1-HEK293 cells at 10 microm (36.5+/-3.2% of maximal capsaicin-induced response), but it was a less potent agonist than both capsaicin and anandamide and at 1 microm it was essentially inactive. Noladin ether (1 microm) had no effect on capsaicin-evoked Ca2+ responses in DRG cells, and produced no response alone, indicating it neither modulates nor acts directly on TRPV1 receptors. 7 These data demonstrate that noladin ether attenuates sensory neurotransmission in rat mesenteric arteries via a non-CB1 non-CB2 PTX-sensitive prejunctional site, independently of TRPV1 receptors.

The endogenous cannabinoid system protects against colonic inflammation

Excessive inflammatory responses can emerge as a potential danger for organisms' health. Physiological balance between pro- and anti-inflammatory processes constitutes an important feature of responses against harmful events. Here, we show that cannabinoid receptors type 1 (CB1) mediate intrinsic protective signals that counteract proinflammatory responses. Both intrarectal infusion of 2,4-dinitrobenzene sulfonic acid (DNBS) and oral administration of dextrane sulfate sodium induced stronger inflammation in CB1-deficient mice (CB1(-/-)) than in wild-type littermates (CB1(+/+)). Treatment of wild-type mice with the specific CB1 antagonist N-(piperidino-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-pyrazole-3-carboxamide (SR141716A) mimicked the phenotype of CB1(-/-) mice, showing an acute requirement of CB1 receptors for protection from inflammation. Consistently, treatment with the cannabinoid receptor agonist R(-)-7-hydroxy-Delta(6)-tetra-hydrocannabinol-dimethylheptyl (HU210) or genetic ablation of the endocannabinoid-degrading enzyme fatty acid amide hydrolase (FAAH) resulted in protection against DNBS-induced colitis. Electrophysiological recordings from circular smooth muscle cells, performed 8 hours after DNBS treatment, revealed spontaneous oscillatory action potentials in CB1(-/-) but not in CB1(+/+) colons, indicating an early CB1-mediated control of inflammation-induced irritation of smooth muscle cells. DNBS treatment increased the percentage of myenteric neurons expressing CB1 receptors, suggesting an enhancement of cannabinoid signaling during colitis. Our results indicate that the endogenous cannabinoid system represents a promising therapeutic target for the treatment of intestinal disease conditions characterized by excessive inflammatory responses.

Involvement of vanilloid-like receptors in the effects of anandamide on motor behavior and nigrostriatal dopaminergic activity: in vivo and in vitro evidence

The administration of the endocannabinoid anandamide to rats produces hypokinesia in parallel to a decrease in the activity of nigrostriatal dopaminergic neurons. It was earlier hypothesized that this effect was mediated through the activation of CB(1) receptors, although these receptors have not been found in dopaminergic neurons, but in striatal projection neurons connected with them. However, two recent discoveries: (i) that anandamide is also able to activate vanilloid VR(1) receptors, and (ii) that VR(1) receptors are located on nigrostriatal dopaminergic neurons, allow to re-evaluate this hypothesis and suggest that the activation of vanilloid-like receptors rather than CB(1) receptors might be responsible of anandamide-induced hypokinesia and decreased nigrostriatal dopaminergic activity. To validate this new hypothesis, we carried out two different experiments. First, we explored whether the inhibitory effects of anandamide on motor activity and dopaminergic transmission were reversed by capsazepine, an antagonist for vanilloid-like receptors. Our data demonstrated that anandamide reduced ambulation, stereotypies and exploration, measured in the open-field test, whereas it increased the time spent in inactivity. All these effects were completely reversed by capsazepine, which had no effect by itself. Anandamide also caused a significant decrease in nigrostriatal dopaminergic activity, reflected by a reduction in DOPAC contents in the caudate-putamen, which was also reversed by capsazepine. As a second objective, we explored whether anandamide is able to directly influence nigrostriatal dopaminergic function by examining its effects on in vitro dopamine (DA) release using perifused striatal fragments. Our data confirmed that anandamide significantly decreased K(+)-stimulated dopamine release from nigrostriatal terminals and that this effect was vanilloid-like receptor-mediated since it was prevented by capsazepine. This in vitro inhibitory effect was not seen with a classic cannabinoid agonist that does not bind vanilloid-like receptors. In summary, anandamide behaves as a hypokinetic substance, thus producing motor depression in the open-field test, presumably related to a decrease in nigrostriatal dopaminergic activity. These effects were completely reversed by the vanilloid-like receptor antagonist capsazepine, thus indicating a role of these receptors, which are located on dopaminergic neurons, in mediating hypokinetic effects of anandamide. In vitro studies, using perifused striatal fragments, support this vanilloid-like receptor-mediated direct action, which would not be available for classic cannabinoid agonists.

Prostaglandin ethanolamides (prostamides): in vitro pharmacology and metabolism

We investigated whether prostaglandin ethanolamides (prostamides) E(2), F(2alpha), and D(2) exert some of their effects by 1) activating prostanoid receptors either per se or after conversion into the corresponding prostaglandins; 2) interacting with proteins for the inactivation of the endocannabinoid N-arachidonoylethanolamide (AEA), for example fatty acid amide hydrolase (FAAH), thereby enhancing AEA endogenous levels; or 3) activating the vanilloid receptor type-1 (TRPV1). Prostamides potently stimulated cat iris contraction with potency approaching that of the corresponding prostaglandins. However, prostamides D(2), E(2), and F(2alpha) exhibited no meaningful interaction with the cat recombinant FP receptor, nor with human recombinant DP, EP(1-4), FP, IP, and TP prostanoid receptors. Prostamide F(2alpha) was also very weak or inactive in a panel of bioassays specific for the various prostanoid receptors. None of the prostamides inhibited AEA enzymatic hydrolysis by FAAH in cell homogenates, or AEA cellular uptake in intact cells. Furthermore, less than 3% of the compounds were hydrolyzed to the corresponding prostaglandins when incubated for 4 h with homogenates of rat brain, lung, or liver, and cat iris or ciliary body. Very little temperature-dependent uptake of prostamides was observed after incubation with rat brain synaptosomes or RBL-2H3 cells. We suggest that prostamides' most prominent pharmacological actions are not due to transformation into prostaglandins, activation of prostanoid receptors, enhancement of AEA levels, or gating of TRPV1 receptors, but possibly to interaction with novel receptors that seem to be functional in the cat iris.

Enhancement of potency and efficacy of NADA by PKC-mediated phosphorylation of vanilloid receptor

The search for an endogenous ligand for the vanilloid receptor (VR or TRPV1) has led to the identification of N-arachidonyl dopamine (NADA). This study investigates the role of protein kinase C (PKC)-mediated phosphorylation on NADA-induced membrane currents in Xenopus oocytes heterologously expressing TRPV1 and in dorsal root ganglion (DRG) neurons. In basal state, current induced by 10 microM NADA is 5-10% of the current induced by 1 microM capsaicin or protons at pH 5. However, PKC activator, phorbol 12,13-dibutyrate (PDBu) strongly potentiated ( approximately 15-fold) the NADA-induced current. Repeated application of NADA at short intervals potentiated its own response approximately fivefold in a PKC-dependent manner. PKC inhibitor, bisindolylmaleimide (BIM, 500 nM), a mutant TRPV1 (S800A/S502A), and maximal activation of PKC abolished the potentiation induced by repeated application of NADA. As a further confirmation that NADA could stimulate PKC, pretreatment with NADA potentiated the response of protons at pH 5 (approximately 20 fold), which was dramatically reduced in the mutant TRPV1. In DRG neurons, capsaicin (100 nM) induced a approximately 15 mV depolarization and initiated a train of action potentials compared with 1 microM NADA that produced a approximately 5 mV response. Pretreatment with PDBu induced significantly larger depolarization and potentiated NADA-induced current. Furthermore, exposure of NADA to the intracellular surface of the membrane-induced larger currents suggesting inaccessibility to the intracellular binding site might contribute to its weaker action. These results indicate that NADA is a potent agonist of VR when the receptor is in the PKC-mediated phosphorylation state.

Multiple actions of anandamide on neonatal rat cultured sensory neurones

1. We have investigated the effects of the endocannabinoid anandamide (AEA) on neuronal excitability and vanilloid TRPV1 receptors in neonatal rat cultured dorsal root ganglion neurones. 2. Using whole-cell patch-clamp electrophysiology, we found that AEA inhibits high-voltage-activated Ca(2+) currents by 33+/-9% (five out of eight neurones) in the absence of the CB(1) receptor antagonist SR141716A (100 nM) and by 32+/-6% (seven out of 10 neurones) in the presence of SR141716A. 3. Fura-2 fluorescence Ca(2+) imaging revealed that AEA produced distinct effects on Ca(2+) transients produced by depolarisation evoked by 30 mM KCl. In a population of neurones of larger somal area (372+/-20 microM(2)), it significantly enhanced Ca(2+) transients (80.26+/-13.12% at 1 microM), an effect that persists after pertussis toxin pretreatment. In a population of neurones of smaller somal area (279+/-18 microM(2)), AEA significantly inhibits Ca(2+) transients (30.75+/-3.54% at 1 microM), an effect that is abolished by PTX pretreatment. 4. Extracellular application of 100 nM AEA failed to evoke TRPV1 receptor inward currents in seven out of eight neurones that responded to capsaicin (1 microM), with a mean inward current of -0.94+/-0.21 nA. In contrast, intracellular application of 100 nM AEA elicited robust inward currents in approximately 62% of neurones, the mean population response was -0.85+/-0.21 nA. When AEA was applied to the intracellular environment with capsazepine (1 microM), the mean population inward current was -0.01+/-0.01 nA. Under control conditions, mean population current fluctuations of -0.09+/-0.05 nA were observed.

Molecular Characterization of a Phospholipase D Generating Anandamide and Its Congeners

Anandamide (N-arachidonoylethanolamine) is known to be an endogenous ligand of cannabinoid and vanilloid receptors. Its congeners (collectively referred to as N-acylethanolamines) also show a variety of biological activities. These compounds are principally formed from their corresponding N-acyl-phosphatidylethanolamines by a phosphodiesterase of the phospholipase D-type in animal tissues. We purified the enzyme from rat heart, and by the use of the sequences of its internal peptides cloned its complementary DNAs from mouse, rat, and human. The deduced amino acid sequences were composed of 393-396 residues, and showed that the enzyme has no homology with the known phospholipase D enzymes but is classified as a member of the zinc metallohydrolase family of the beta-lactamase fold. As was overexpressed in COS-7 cells, the recombinant enzyme generated anandamide and other N-acylethanolamines from their corresponding N-acyl-phosphatidylethanolamines at comparable rates. In contrast, the enzyme was inactive with phosphatidylcholine and phosphatidylethanolamine. Assays of the enzyme activity and the messenger RNA and protein levels revealed its wide distribution in murine organs with higher contents in the brain, kidney, and testis. These results confirm that a specific phospholipase D is responsible for the generation of N-acylethanolamines including anandamide, strongly suggesting the physiological importance of lipid molecules of this class.

System-specific O2 sensitivity of the tandem pore domain K+ channel TASK-1

Hypoxic inhibition of TASK-1, a tandem pore domain background K+ channel, provides a critical link between reduced O2 levels and physiological responses in various cell types. Here, we examined the expression and O2 sensitivity of TASK-1 in immortalized adrenomedullary chromaffin (MAH) cells. In physiological (asymmetrical) K+ solutions, 3 μM anandamide or 300 μM Zn2+ inhibited a strongly pH-sensitive current. Under symmetrical K+ conditions, the anandamide- and Zn2+-sensitive K+ currents were voltage independent. These data demonstrate the functional expression of TASK-1, and cellular expression of this channel was confirmed by RT-PCR and Western blotting. At concentrations that selectively inhibit TASK-1, anandamide and Zn2+ were without effect on the magnitude of the O2-sensitive current or the hypoxic depolarization. Thus TASK-1 does not contribute to O2 sensing in MAH cells, demonstrating the failure of a known O2-sensitive K+ channel to respond to hypoxia in an O2-sensing cell. These data demonstrate that, ultimately, the sensitivity of a particular K+ channel to hypoxia is determined by the cell, and we propose that this is achieved by coupling distinct hypoxia signaling systems to individual channels. Importantly, these data also reiterate the indirect O2 sensitivity of TASK-1, which appears to require the presence of an intracellular mediator.

The endocannabinoid system: a general view and latest additions

After the discovery, in the early 1990s, of specific G-protein-coupled receptors for marijuana's psychoactive principle Delta(9)-tetrahydrocannabinol, the cannabinoid receptors, and of their endogenous agonists, the endocannabinoids, a decade of investigations has greatly enlarged our understanding of this altogether new signalling system. Yet, while the finding of the endocannabinoids resulted in a new effort to reveal the mechanisms regulating their levels in the brain and peripheral organs under physiological and pathological conditions, more endogenous substances with a similar action, and more molecular targets for the previously discovered endogenous ligands, anandamide and 2-arachidonoylglycerol, or for some of their metabolites, were being proposed. As the scenario becomes subsequently more complicated, and the experimental tasks to be accomplished correspondingly more numerous, we briefly review in this article the latest 'additions' to the endocannabinoid system together with earlier breakthroughs that have contributed to our present knowledge of the biochemistry and pharmacology of the endocannabinoids.

The endogenous cannabinoid system and its role in nociceptive behavior

The analgesic properties of exogenous cannabinoids have been recognized for many years and suggest a regulatory role for the endogenous cannabinoid ("endocannabinoid") system in mammalian nociceptive pathways. The endocannabinoid system includes: (1) at least two families of lipid signaling molecules, the N-acyl ethanolamines (e.g., anandamide) and the monoacylglycerols (e.g., 2-arachidonoyl glycerol); (2) multiple enzymes involved in the biosynthesis and degradation of these lipids, including the integral membrane enzyme fatty acid amide hydrolase; and (3) two G-protein coupled receptors, CB1 and CB2, which are primarily localized to the nervous system and immune system, respectively. Here, we review recent genetic, behavioral, and pharmacological studies that have tested the function of the endocannabinoid system in pain sensation. Collectively, these investigations support a role for endocannabinoids in modulating behavioral responses to acute, inflammatory, and neuropathic pain stimuli.

The endogenous cannabinoid, anandamide, activates the hypothalamo-pituitary-adrenal axis in CB1 cannabinoid receptor knockout mice

The purpose of this study was to investigate the effects of the endogenous cannabinoid arachidonoyl-ethanolamide, anandamide (AEA), on the activity of the hypothalamo-pituitary-adrenal (HPA) axis in cannabinoid receptor (CB(1) receptor) inactivated (KO) mice. A low dose (0.01 mg/kg i.p.) of AEA significantly increased plasma corticotropin (ACTH) and corticosterone concentrations in both wild-type (+/+) and in mutant (-/-) animals. In each case, hormone levels reached their peaks at 90 min after AEA administration. In a parallel experiment, AEA administration was preceded by the injection of SR 141716A (1.0 mg/kg), a selective and potent CB(1) receptor antagonist, or of capsazepine (5.0 mg/kg), a potent vanilloid receptor of type 1 (VR1) antagonist. The latter drugs did not prevent the effects of AEA on the HPA axis. Using Fos protein immunohistochemistry, we observed that the parvocellular part of the hypothalamic paraventricular nucleus (PVN) was activated as early as 45 min after AEA injection and reached peak levels after 60 min in both +/+ and -/- mice. Furthermore, the CB(1) and VR1 receptor antagonists did not block the effects of AEA on Fos immunoreactivity. The results strongly support the view that activation of the HPA axis produced by AEA possibly occurs via a currently unknown (CB(x)) cannabinoid receptor present in PVN.

The endocannabinoid system in the basal ganglia and in the mesolimbic reward system: implications for neurological and psychiatric disorders

To date, N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol are the best studied endocannabinoids and are thought to act as retrograde messengers in the central nervous system (CNS). By activating presynaptic cannabinoid CB1 receptors, they can reduce glutamate release in dorsal and ventral striatum (nucleus accumbens) and alter synaptic plasticity, thereby modulating neurotransmission in the basal ganglia and in the mesolimbic reward system. In this review, we will focus on the role of the endocannabinoid system within these neuronal pathways and describe its effect on dopaminergic transmission and vice versa. The endocannabinoid system is unlikely to directly affect dopamine release, but can modify dopamine transmission trough trans-synaptic mechanisms, involving gamma-aminobutyric acid (GABA)-ergic and glutamatergic synapses, as well as by converging signal transduction cascades of the cannabinoid and dopamine receptors. The dopamine and endocannabinoid systems exert a mutual control on each other. Cannabinergic signalling may lead to release of dopamine, which can act via dopamine D1-like receptors as a negative feedback mechanism to counteract the effects of activation of the cannabinoid CB1 receptor. On the other hand, dopaminergic signalling via dopamine D2-like receptors may lead to up-regulation of cannabinergic signalling, which is likely to represent a negative feedback on dopaminergic signalling. The consequences of these interactions become evident in pathological conditions in which one of the two systems is likely to be malfunctioning. We will discuss neurological and psychiatric disorders such as Parkinson's and Huntington's disease, drug addiction and schizophrenia. Furthermore, the possible role of the endocannabinoid system in disorders not necessarily depending on the dopaminergic system, such as eating disorders and anxiety, will be described.

CB1 cannabinoid receptors and on-demand defense against excitotoxicity

Abnormally high spiking activity can damage neurons. Signaling systems to protect neurons from the consequences of abnormal discharge activity have been postulated. We generated conditional mutant mice that lack expression of the cannabinoid receptor type 1 in principal forebrain neurons but not in adjacent inhibitory interneurons. In mutant mice,the excitotoxin kainic acid (KA) induced excessive seizures in vivo. The threshold to KA-induced neuronal excitation in vitro was severely reduced in hippocampal pyramidal neurons of mutants. KA administration rapidly raised hippocampal levels of anandamide and induced protective mechanisms in wild-type principal hippocampal neurons. These protective mechanisms could not be triggered in mutant mice. The endogenous cannabinoid system thus provides on-demand protection against acute excitotoxicity in central nervous system neurons.

Anandamide and vanilloid TRPV1 receptors

A large body of evidence now exists to substantiate that the endocannabinoid, anandamide, activates TRPV1 receptors. It is a low intrinsic efficacy TRPV1 agonist that behaves as a partial agonist in tissues with a low receptor reserve, while in tissues with high receptor reserve and in circumstances associated with certain disease states, it behaves as a full agonist. The efficacy of anandamide as a TRPV1 agonist is influenced by a succession of factors including receptor reserve, phosphorylation, metabolism and uptake, CB1 receptor activation, voltage, temperature, pH and bovine serum albumin. There are indications that the endocannabinoid system may play a role in the modulation of TRPV1 receptor activation. The activation of TRPV1 receptors by anandamide has potential implications in the treatment of inflammatory, respiratory and cardiovascular disorders. The relative importance of anandamide as a physiological and/or pathophysiological TRPV1 receptor agonist in comparison to other potential candidates has yet to be revealed.

Arachidonyl dopamine as a ligand for the vanilloid receptor VR1 of the rat

The vanilloid receptor VR1 is a nonspecific Ca(2+) channel, expressed in sensory neurons in the peripheral nervous system and in various brain regions, which is believed to be an important molecular integrator of several chemical (acid, vanilloids) and physical stimuli (heat) that cause pain. Recently, several endogenous ligands for VR1 have been identified such as arachidonyl ethanolamide (anandamide) and the more potent arachidonyl dopamine (AA-DO). Here, we further characterize AA-DO as a ligand for rat VR1, heterologously expressed in CHO and HEK293 cells. AA-DO inhibited the binding of [3H]RTX to VR1 with a K(d) value of 5.49 +/- 0.68 microM and with positive cooperativity (p = 1.89 +/- 0.27), indicating that AA-DO was about 5-fold more potent than anandamide in this system. The K(d) (9.7 +/- 3.3 microM), and p values (1.54 +/- 0.04) for the binding of AA-DO to spinal cord membranes are in good correlation with the CHO-VR1 data. AA-DO stimulated 45Ca(2+) uptake on CHO-VR1 and HEK-VR1 cells with EC(50) values of 4.76 +/- 1.43 and 7.17 +/- 1.64 microM and Hill coefficients of 1.28 +/- 0.11 and 1.13 +/- 0.13, respectively, consistent with the binding measurements. In contrast to anandamide, AA-DO induced virtually the same level of 45Ca(2+) uptake as did capsaicin (90 +/- 6.6% in the CHO cells expressing VR1 and 89.3 +/- 9.4% in HEK293 cells expressing VR1). In a time dependent fashion following activation, AA-DO partially desensitized VR1 both in 45Ca(2+) uptake assays (IC(50) = 3.24 +/- 0.84 microM, inhibition is 68.5 +/- 6.85%) as well as in Ca(2+) imaging experiments (35.8 +/- 5.1% inhibition) using the CHO-VR1 system. The extent of desensitization was similar to that caused by capsaicin itself. We conclude that AA-DO is a full agonist for VR1 with a potency in the low micromolar range and is able to significantly desensitize the cells in a time and dose dependent manner.

Anandamide regulates neuropeptide release from capsaicin-sensitive primary sensory neurons by activating both the cannabinoid 1 receptor and the vanilloid receptor 1 in vitro

The effect of anandamide, which activates both the cannabinoid 1 (CB1) receptor and the vanilloid receptor 1 (VR1), was studied on calcitonin gene-related peptide (CGRP) release from cultured primary sensory neurons, the majority of which coexpress the CB1 receptor and VR1. Concentrations of anandamide < 1 micro m produced a small but significant CB1 receptor-mediated inhibition of basal CGRP release while higher concentrations induced VR1-mediated CGRP release. The excitatory effect of anandamide was potentiated by the CB1 receptor antagonist SR141716A. In the presence of SR141716A at concentrations < 100 nm, anandamide was equipotent with capsaicin in stimulating CGRP release. However, at higher concentrations anandamide produced more CGRP release than equimolar concentrations of capsaicin. Three and ten nanomolar anandamide inhibited the capsaicin-evoked CGRP release. In the presence of SR141716A, treatments which activated protein kinase A, protein kinase C and phospholipase C significantly potentiated the anandamide-evoked CGRP release at all anandamide concentrations. Although this potentiation was reduced when the CB1 receptor antagonist was omitted from the buffer, the CGRP release evoked by 300 nm and 1 micro m anandamide was still significantly larger than that seen with nonpotentiated cells. These data indicate that anandamide may regulate CGRP release from capsaicin-sensitive primary sensory neurons in vivo, and that the net effect of anandamide on transmitter release from capsaicin-sensitive primary sensory neurons depends on the concentration of anandamide and the state of the CB1 receptor and VR1. These findings also suggest that anandamide could be one of the molecules responsible for the development of inflammatory heat hyperalgesia.

Novel selective and metabolically stable inhibitors of anandamide cellular uptake

Novel aromatic analogues of N-oleoylethanolamine and N-arachidonoylethanolamine (anandamide, AEA) were synthesized and, based on the capability of similar compounds to interact with proteins of the endocannabinoid and endovanilloid signaling systems, were tested on: (i) cannabinoid CB(1) and CB(2) receptors; (ii) vanilloid VR1 receptors; (iii) anandamide cellular uptake (ACU); and (iv) the fatty acid amide hydrolase (FAAH). The (R)- and, particularly, the (S)-1'-(4-hydroxybenzyl) derivatives of N-oleoylethanolamine and AEA (OMDM-1, OMDM-2, OMDM-3, and OMDM-4) inhibited to a varied extent ACU in RBL-2H3 cells (K(i) ranging between 2.4 and 17.7 micro M), the oleoyl analogues (OMDM-1 and OMDM-2, K(i) 2.4 and 3.0 micro M, respectively) being 6- to 7-fold more potent than the arachidonoyl analogues (OMDM-3 and OMDM-4). These four compounds exhibited: (i) poor affinity for either CB(1) (K(i)> or = 5 micro M) or CB(2) (K(i)>10 micro M) receptors in rat brain and spleen membranes, respectively; (ii) almost no activity at vanilloid receptors in the intracellular calcium assay carried out with intact cells over-expressing the human VR1 (EC(50)> or = 10 micro M); and (iii) no activity as inhibitors of FAAH in N18TG2 cell membranes (K(i)>50 micro M). The oleoyl- and arachidonoyl-N'-(4-hydroxy-3-methoxybenzyl)hydrazines (OMDM-5 and OMDM-6), inhibited ACU (K(i) 4.8 and 7.0 micro M, respectively), and were more potent as VR1 agonists (EC(50) 75 and 50nM, respectively), weakly active as CB(1) receptor ligands (K(i) 4.9 and 3.2 micro M, respectively), and inactive as CB(2) ligands (K(i)>5 micro M) as well as on FAAH (K(i)> or = 40 micro M). In conclusion, we report two novel potent and selective inhibitors of ACU (OMDM-1 and OMDM-2) and one "hybrid" agonist of CB(1) and VR1 receptors (OMDM-6). Unlike other compounds of the same type, OMDM-1, OMDM-2, and OMDM-6 were very stable to enzymatic hydrolysis by rat brain homogenates.

ASK1-p38 MAPK/JNK signaling cascade mediates anandamide-induced PC12 cell death

Anandamide is a neuroimmunoregulatory molecule that triggers apoptosis in a number of cell types including PC12 cells. Here, we investigated the molecular mechanisms underlying anandamide-induced cell death in PC12 cells. Anandamide treatment resulted in the activation of p38 mitogen-activated protein kinase (MAPK), c-Jun N-terminal kinase (JNK), and p44/42 MAPK in apoptosing cells. A selective p38 MAPK inhibitor, SB203580, or dn-JNK, JNK1(A-F) or SAPKbeta(K-R), blocked anandamide-induced cell death, whereas a specific inhibitor of MEK-1/2, U0126, had no effect, indicating that activation of p38 MAPK and JNK is critical in anandamide-induced cell death. An important role for apoptosis signal-regulating kinase 1 (ASK1) in this event was also demonstrated by the inhibition of p38 MAPK/JNK activation and death in cells overexpressing dn-ASK1, ASK1 (K709M). Conversely, the constitutively active ASK1, ASK1DeltaN, caused prolonged p38 MAPK/JNK activation and increased cell death. These indicate that ASK1 mediates anandamide-induced cell death via p38 MAPK and JNK activation. Here, we also found that activation of p38 MAPK/JNK is accompanied by cytochrome c release from the mitochondria and caspase activation (which can be inhibited by SB203580), suggesting that anandamide triggers a mitochondrial dependent apoptotic pathway. The caspase inhibitor, zVAD, and the mitochondrial pore opening inhibitor, cyclosporine A, blocked anandamide-induced cell death but not p38 MAPK/JNK activation, suggesting that activation of these kinases may occur upstream of mitochondrial associated events.

The therapeutic potential of cannabis in multiple sclerosis

There has been renewed interest in the therapeutic applications of cannabis, and people, particularly those with multiple sclerosis, claim that it may offer benefit in symptom control. Cannabis exerts many of its effects because it taps into an endogenous cannabinoid system. Recent advances have begun to shine light on the biology of this system and may support some of the anecdotal medical claims. The problem with cannabis as a drug is that both the positive and negative aspects are largely the work of the same receptor. However, it may be possible to avoid these through modulation of the endogenous system. Cannabinoids provide a novel therapeutic target, not only for controlling symptoms, but also slowing disease progression through inhibition of neurodegeneration, which is the cause of accumulating irreversible disability.