Occurrence and biosynthesis of endogenous cannabinoid precursor, N-arachidonoyl phosphatidylethanolamine, in rat brain

Cannabinoid CB1 receptors and ligands in vertebrate retina: localization and function of an endogenous signaling system

CB1, a cannabinoid receptor enriched in neuronal tissue, was found in high concentration in retinas of rhesus monkey, mouse, rat, chick, goldfish, and tiger salamander by using a subtype-specific polyclonal antibody. Immunolabeling was detected in the two synaptic layers of the retina, the inner and outer plexiform layers, of all six species examined. In the outer plexiform layer, CB1 was located in and/or on cone pedicles and rod spherules. Labeling was detected in some amacrine cells of all species and in the ganglion cells and ganglion cell axons of all species except fish. In addition, sparse labeling was found in the inner and/or outer segments of the photoreceptors of monkey, mouse, rat, and chick. Using GC/MS to detect possible endogenous cannabinoids, we found 3 nmol of 2-arachidonylglycerol per g of tissue, but no anandamide was detectable. Cannabinoid receptor agonists induced a dramatic reduction in the amplitude of voltage-gated L-type calcium channel currents in identified retinal bipolar cells. The presence and distribution of the CB1 receptor, the large amounts of 2-arachidonylglycerol found, and the effects of cannabinoids on calcium channel activity in bipolar cells suggest a substantive role for an endogenous cannabinoid signaling system in retinal physiology, and perhaps vision in general.

N-Acylethanolamines in signal transduction of elicitor perception. Attenuation Of alkalinization response and activation of defense gene expression

In a recent study of N-acylphosphatidylethanolamine (NAPE) metabolism in elicitor-treated tobacco (Nicotiana tabacum L.) cells, we identified a rapid release and accumulation of medium-chain N-acylethanolamines (NAEs) (e.g. N-myristoylethanolamine or NAE 14:0) and a compensatory decrease in cellular NAPE (K.D. Chapman, S. Tripathy, B. Venables, A.D. Desouza [1998] Plant Physiol 116: 1163-1168). In the present study, we extend this observation and report a 10- to 50-fold increase in NAE 14:0 content in leaves of tobacco (cv Xanthi) plants treated with xylanase or cryptogein elicitors. Exogenously supplied synthetic NAE species affected characteristic elicitor-induced and short- and long-term defense responses in cell suspensions of tobacco and long-term defense responses in leaves of intact tobacco plants. In general, synthetic NAEs inhibited elicitor-induced medium alkalinization by tobacco cells in a time- and concentration-dependent manner. Exogenous NAE 14:0 induced expression of phenylalanine ammonia lyase in a manner similar to fungal elicitors in both cell suspensions and leaves of tobacco. NAE 14:0, but not myristic acid, activated phenylalanine ammonia lyase expression at submicromolar concentrations, well within the range of NAE 14:0 levels measured in elicitor-treated plants. Collectively, these results suggest that NAPE metabolism, specifically, the accumulation of NAE 14:0, are part of a signal transduction pathway that modulates cellular defense responses following the perception of fungal elicitors.

Biosynthesis and inactivation of N-arachidonoylethanolamine (anandamide) and N-docosahexaenoylethanolamine in bovine retina

N-Arachidonoylethanolamine (anandamide; AEA) and 2-arachidonoylglycerol (2-AG), the two proposed endogenous agonists of cannabinoid receptors, and the putative AEA biosynthetic precursor, N-arachidonoylphosphatidylethanolamine (NArPE), were identified in bovine retina by means of gas chromatography-electron impact mass spectrometry (GC-EIMS). This technique also allowed us to identify N-docosahexanoylethanolamine (DHEA) and 2-docosahexanoylglycerol (2-DHG), two derivatives of docosahexaenoic acid (DHA), one of the most abundant fatty acids esterified in retina phospholipids and necessary for optimal retinal function. N-Docosahexaenoylphosphatidylethanolamine (NDHPE), the potential biosynthetic precursor for DHEA, was also found. The fatty acid composition of the sn-1 and sn-2 positions of bovine retina's most abundant phospholipid classes, also determined here, were in agreement with a phospholipid-dependent mechanism for 2-AG, 2-DHG, AEA, and DHEA biosynthesis, as very high levels of polyunsaturated fatty acids, including DHA, were found on the sn-2 position of phosphatidylcholine (PC) and -ethanolamine (PE), and measurable amounts of di-docosahexanoyl-PC and -PE, two potential biosynthetic precursors of NDHPE, were detected. Accordingly, we found that isolated particulate fractions from bovine retina could release AEA and DHEA in a time-dependent fashion. Finally, a fatty acid amide hydrolase (FAAH)-like activity with subcellular distribution and pH dependency similar to those reported for the brain enzyme was also detected in bovine retina. This activity was inhibited by FAAH inhibitors, phenylmethylsulfonyl fluoride and arachidonoyltrifluoromethylketone, and appeared to recognize DHEA with a lower efficiency than AEA. These data indicate that AEA and its congeners may play a physiological role in the mammalian eye.

Biosynthesis and inactivation of endocannabinoids: relevance to their proposed role as neuromodulators

The two putative endogenous ligands of cannabinoid receptors, anandamide and 2-arachidonoylglycerol, are synthesized by and released from neurons in a Ca2+-dependent fashion, and re-uptaken and catabolized by both neurons and astrocytes. These biochemical features of the endocannabinoids, as well as some of their pharmacological effects in both central and peripheral nervous systems, suggest a role as neuromodulators for these metabolites. This neuromodulatory role is supported by the brain regional distribution of anandamide, its biosynthetic precursor and its major inactivating enzyme, and by the existence of possible regulatory mechanisms for the biosynthesis and inactivation of endocannabinoids, which are reviewed in this article.

N-Acylethanolamines in seeds. Quantification Of molecular species and their degradation upon imbibition

N-Acylethanolamines (NAEs) were quantified in seeds of several plant species and several cultivated varieties of a single species (cotton [Gossypium hirstutum]) by gas chromatography-mass spectroscopy. The total NAE content of dry seeds ranged from 490 +/- 89 ng g(-1) fresh weight in pea (Pisum sativum cv early Alaska) to 1,608 +/- 309 ng g(-1) fresh weight in cotton (cv Stoneville 7A glandless). Molecular species of NAEs in all seeds contained predominantly 16C and 18C fatty acids, with N-linoleoylethanolamine (NAE18:2) being the most abundant (approaching 1,000 ng g(-1) fresh weight in cottonseeds). Total NAE levels dropped drastically following 4 h of imbibition in seeds of pea, cotton, and peanut (Arachis hypogea cv Virginia), and this decline was most pronounced for NAE18:2. A novel enzyme activity was identified in cytosolic fractions of imbibed cottonseeds that hydrolyzed NAE18:2 in vitro. NAE degradation was optimal at 35 degrees C in 50 mM MES buffer, pH 6.5, and was inhibited by phenylmethylsulfonyl fluoride and 5, 5'-dithio-bis(2-nitrobenzoic acid), which is typical of other amide hydrolases. Amidohydrolase activity in cytosolic fractions exhibited saturation kinetics toward the NAE18:2 substrate, with an apparent K(m) of 65 &mgr;M and a V(max) of 83 nmol min(-1) mg(-1) protein. Total NAE amidohydrolase activity increased during seed imbibition, with the highest levels (about four times that in dry seeds) measured 2 h after commencing hydration. NAEs belong to the family of "endocannabinoids," which have been identified as potent lipid mediators in other types of eukaryotic cells. This raises the possibility that their imbibition-induced metabolism in plants is involved in the regulation of seed germination.

Finding of the endocannabinoid signalling system in Hydra, a very primitive organism: possible role in the feeding response

Hydra (Cnidaria) is the first animal organism to have developed a neural network, which has been proposed to control, inter alia, the "feeding response", i.e. a mechanism through which the coelenterate opens and then closes its mouth in the presence of prey and/or glutathione. Here, we report that Hydra contains: (i) selective cannabinoid binding sites; (ii) the endogenous cannabinoid receptor ligand, anandamide (arachidonoylethanolamide); (iii) a fatty acid amide hydrolase-like activity catalysing anandamide hydrolysis; and (iv) the putative biosynthetic precursor of anandamide, N-arachidonoylphosphatidylethanolamine. We suggest that this "endogenous cannabinoid system" is involved in the modulation of the "feeding response". Anandamide (1 nM-1 microM) potently inhibited (up to 45%) the glutathione-induced "feeding response" by accelerating Hydra vulgaris mouth closure. The effect was maximal at 100 nM anandamide and was reversed by the selective antagonist of the CB1 subtype of mammalian cannabinoid receptors, SR 141716A (50-100 nM). Specific cannabinoid binding sites were detected in membranes from Hydra polyps by using [3H]SR 141716A (Kd= 1.87 nM, Bmax = 26.7 fmol/mg protein), and increasing anandamide concentrations were found to displace the binding of [3H]SR 141716A to these membranes (Ki = .505 nM). Hydra polyps were also found to contain amounts of anandamide (15.6 pmol/g) and N-arachidonoylphosphatidylethanolamine (32.4 pmol/g), as well as the other "endocannabinoid" 2-arachidonoylglycerol (11.2 nmol/g), comparable to those described previously for mammalian brain. Finally, a fatty acid amide hydrolase activity (Vmax = 3.4 nmol/min/mg protein), with subcellular distribution, pH dependency and sensitivity to inhibitors similar to those reported for the mammalian enzyme, but with a lower affinity for anandamide (Km = 400 microM), was also detected in Hydra polyps. These data suggest that the endocannabinoid signalling system plays a physiological role in Hydra that is to control the feeding response. Hydra is the simplest living organism described so far to use this recently discovered regulatory system.

Electrospray ionization mass spectrometric method for the determination of cannabinoid precursors: N-acylethanolamine phospholipids (NAPEs)

N-Acylethanolamine phospholipids (NAPEs) serve as endogenous precursors of N-acylethanolamines (NAEs), e.g. N-arachidonoylethanolamine (anandamide) and N-palmitoylethanolamine that are endogenous ligands of cannabinoid receptors. Under physiological conditions, NAPE is found in very low concentrations in mammalian tissue (3-12 nmol g(-1)). However, pathophysiological conditions may increase the endogenous NAPE levels, which again may cause an increase in endocannabinoid concentrations. This paper presents a simple and selective method for the determination of NAPE standards using negative electrospray ionization mass spectrometry (ESI-MS). The procedure provides complete positioning of all acyl and alkenyl groups contained in each NAPE species. The calibration curve for standard NAPE was linear over the range 100 fmol-50 pmol (0.1-50 ng) per injection. The lower limit of detection (signal-to-noise ratio of 3) was 100 fmol, implying that this method is superior to previous methods for the determination of NAPE. These results suggest that this ESI-MS method can be used to identify and quantify NAPE species in mammalian tissues and provide information on the corresponding NAEs to be released from the endogenous NAPE pool.

GC/MS analysis of anandamide and quantification of N-arachidonoylphosphatidylethanolamides in various brain regions, spinal cord, testis, and spleen of the rat

Anandamide [N-arachidonoylethanolamide (NAE)] was initially isolated from porcine brain and proposed as an endogenous ligand for cannabinoid receptors in 1992. Accumulating evidence has now suggested that, in the tissue, NAE is generated from N-arachidonoylphosphatidylethanolamides (N-ArPEs) by phosphodiesterase. In this study a sensitive and specific procedure was developed to quantify NAE and N-ArPE, including organic solvent extraction, reverse-phase C-18 cartridge separation, derivatization, and gas chromatography/mass spectrometry (GC/MS) analysis. NAE is converted by a two-step derivatization procedure to a pentafluorobenzoyl ester followed by pentafluoropropionyl acylation. Quantification was performed by isotope dilution GC/MS using deuterium-labeled NAE (NAE-2H8) as an internal standard. The same chemical derivatization was applicable to N-ArPE quantification. The separated N-ArPE fractions were converted by a two-step cleavage/derivatization procedure into the pentafluorobenzoyl ester of NAE and then to its pentafluoropropionyl amide. The derivative was quantified by GC/MS using deuterium-labeled 1,2-[2H8]dioleoyl-sn-glycero-3-phospho(arachidonoyl)ethanolamid e as an internal standard. Using these methods, we have found that endogenous NAE levels in rat brain, spleen, testis, liver, lung, and heart were below the level of quantification achievable (0.1 pmol/mg of protein) but that N-ArPE is readily quantifiable and is widely distributed in the rat CNS with the highest level in the spinal cord. The striatum, hippocampus, and accumbens contain intermediate concentrations of N-ArPE, whereas the value is lowest in the cerebellum.

Effects of the endogeneous cannabinoid, anandamide, on neuronal activity in rat hippocampal slices

1. The arachidonic acid derivative arachidonylethanolamide (anandamide) is an endogeneous ligand of cannabinoid receptors that induces pharmacological actions similar to those of cannabinoids such as delta9-tetrahydrocannabinol (THC). We examined whether anandamide can influence excessive neuronal activity by investigating stimulation-induced population spikes and epileptiform activity in rat hippocampal slices. For this purpose, the effects of anandamide were compared with those of the synthetic cannabinoid agonist WIN 55,212-2 and its inactive S(-)-enantiomer WIN 55,212-3. 2. Both anandamide (1 and 10 microM) and WIN 55,212-2 (0.1 and 1 microM) decreased the amplitude of the postsynaptic population spike and the slope of the field excitatory postsynaptic potential (field e.p.s.p.) without affecting the presynaptic fibre spike of the afferents. At a concentration of 1 microM, WIN 55,212-2 completely suppressed the postsynaptic spike, whereas the S(-)-enantiomer WIN 55,212-3 produced only a slight depression. The CB1 receptor antagonist SR 141716 blocked the inhibition evoked by the cannabinoids. SR 141716 had a slight facilitatory effect on neuronal excitability by itself. 3. Anandamide shifted the input-output curve of the postsynaptic spike and the field e.p.s.p. to the right and increased the magnitude of paired-pulse facilitation indicating a presynaptic mechanism of action. 4. Anandamide and WIN 55,212-2, but not WIN 55,212-3, attenuated both stimulus-triggered epileptiform activity in CA1 elicited by omission of Mg2+ and spontaneously occurring epileptiform activity in CA3 elicited by omission of Mg2+ and elevation of K+ to 8 mM. The antiepileptiform effect of these cannabinoids was blocked by SR 141716. 5. In conclusion, cannabinoid receptors of the CB1 type as well as their endogeneous ligand, anandamide, are involved in the control of neuronal excitability, thus reducing excitatory neurotransmission at a presynaptic site, a mechanism which might be involved in the prevention of excessive excitability leading to epileptiform activity.

Dopamine activation of endogenous cannabinoid signaling in dorsal striatum

We measured endogenous cannabinoid release in dorsal striatum of freely moving rats by microdialysis and gas chromatography/mass spectrometry. Neural activity stimulated the release of anandamide, but not of other endogenous cannabinoids such as 2-arachidonylglycerol. Moreover, anandamide release was increased eightfold over baseline after local administration of the D2-like (D2, D3, D4) dopamine receptor agonist quinpirole, a response that was prevented by the D2-like receptor antagonist raclopride. Administration of the D1-like (D1, D5) receptor agonist SKF38393 had no such effect. These results suggest that functional interactions between endocannabinoid and dopaminergic systems may contribute to striatal signaling. In agreement with this hypothesis, pretreatment with the cannabinoid antagonist SR141716A enhanced the stimulation of motor behavior elicited by systemic administration of quinpirole. The endocannabinoid system therefore may act as an inhibitory feedback mechanism countering dopamine-induced facilitation of motor activity.

Brain regional distribution of endocannabinoids: implications for their biosynthesis and biological function

The amounts, in nine different rat brain regions, of the two endocannabinoids, anandamide (arachidonoylethanolamide, AEA) and 2-arachidonoylglycerol (2-AG), and of the putative AEA precursor N-arachidonoyl-phosphatidylethanolamine (NArPE), were determined by isotope-dilution gas chromatography-mass spectrometry and compared to the number of cannabinoid binding sites in each region. The distribution of NArPE, reported here for the first time, exhibited a good correlation with that of AEA, the former metabolite being 3-13 times more abundant than the endocannabinoid in all regions. The highest amounts of both metabolites (up to 358.5 and 87 pmol/g wet weight tissue, respectively) were found in the brainstem and striatum, and the lowest in the diencephalon, cortex, and cerebellum. These data support the hypothesis that, in the brain, AEA is a metabolic product of NArPE and may reach levels compatible with its proposed neuromodulatory function. The brain distribution of 2-AG, also described in this study for the first time, was found to correlate with that of AEA with levels ranging from 2.0 to 14.0 nmol/g (in the diencephalon and brainstem, respectively). The distribution of the endocannabinoids did not match exactly with that of cannabinoid binding sites, suggesting either that these compounds are not necessarily produced near their molecular targets, or that they play functional roles additional to the activation of cannabinoid receptors. Regional differences in the ligand/receptor ratios may also lead to predict corresponding differences in the efficiency of receptor activation, as shown by previous studies.

Chronic ethanol increases the cannabinoid receptor agonist anandamide and its precursor N-arachidonoylphosphatidylethanolamine in SK-N-SH cells

In an earlier study, we demonstrated that chronic ethanol (EtOH) exposure down-regulated the cannabinoid receptors (CB1) in mouse brain synaptic plasma membrane. In the present study, we investigated the effect of chronic EtOH on the formation of anandamide (AnNH), an endogenous cannabimimetic compound, and its precursor N-arachidonoylphosphatidylethanolamine (N-ArPE) in SK-N-SH cells that were prelabeled with [3H]arachidonic acid. The results indicate that exposure of SK-N-SH cells to EtOH (100 mM) for 72 h significantly increased levels of [3H]AnNH and [3H]N-ArPE (p < 0.05) (1.43-fold for [3H]AnNH and 1.65-fold for [3H]N-ArPE). Exposure of SK-N-SH cells to EtOH (100 mM, 24 h) inhibited initially the formation of [3H]AnNH at 24 h, followed by a progressive increase, reaching a statistical significance level at 72 h (p < 0.05). [3H]N-ArPE increased gradually to a statistically significant level after 48 and 72 h (p < 0.05). Incubation with exogenous ethanolamine (7 mM) and EtOH (100 mM, 72 h) did not result in an additive increase in the formation of [3H]AnNH. The formation of [3H]AnNH and [3H]N-ArPE by EtOH was enhanced by the Ca2+ ionophore A23187 or by the depolarizing agent veratridine and the K+ channel blocker 4-aminopyridine. Further, the EtOH-induced formation of [3H]AnNH and [3H]N-ArPE was inhibited by exogenous AnNH, whereas only [3H]AnNH formation was inhibited by the CB1 receptor antagonist SR141716A and pertussis toxin, suggesting that the CB1 receptor and G(i/o) protein mediated the regulation of AnNH levels. The observed increase in the levels of these lipids in SK-N-SH cells may be a mechanism for neuronal adaptation and may serve as a compensatory mechanism to counteract the continuous presence of EtOH. The present observation taken together with our previous results indicate the involvement of the endocannabinoid system in mediating some of the pharmacological actions of EtOH and may constitute part of a common brain pathway mediating reinforcement of drugs of abuse including EtOH.

Evidence that the cannabinoid CB1 receptor is a 2-arachidonoylglycerol receptor. Structure-activity relationship of 2-arachidonoylglycerol, ether-linked analogues, and related compounds

An endogenous cannabimimetic molecule, 2-arachidonoylglycerol, induces a rapid, transient increase in intracellular free Ca2+ concentrations in NG108-15 cells through a cannabinoid CB1 receptor-dependent mechanism. We examined the activities of 24 relevant compounds (2-arachidonoylglycerol, its structural analogues, and several synthetic cannabinoids). We found that 2-arachidonoylglycerol is the most potent compound examined so far: its activity was detectable from as low as 0.3 nM, and the maximal response induced by 2-arachidonoylglycerol exceeded the responses induced by others. Activities of HU-210 and CP55940, potent cannabinoid receptor agonists, were also detectable from as low as 0.3 nM, whereas the maximal responses induced by these compounds were low compared with 2-arachidonoylglycerol. Anandamide was also found to act as a partial agonist in this assay system. We confirmed that free arachidonic acid failed to elicit a response. Furthermore, we found that a metabolically stable ether-linked analogue of 2-arachidonoylglycerol possesses appreciable agonistic activity, although its activity was apparently lower than that of 2-arachidonoylglycerol. We also confirmed that pretreating cells with various cannabinoid receptor agonists nullified the response induced by 2-arachidonoylglycerol, whereas pretreating cells with other neurotransmitters or neuromodulators did not affect the response. These results strongly suggested that the cannabinoid CB1 receptor is originally a 2-arachidonoylglycerol receptor, and 2-arachidonoylglycerol is the intrinsic physiological ligand for the cannabinoid CB1 receptor.

Biochemistry of the endogenous ligands of cannabinoid receptors

In 1992 the discovery of the first endogenous ligand of cannabinoid receptors, anandamide, provided conclusive support to the hypothesis that an "endogenous cannabinoid regulatory system" exists in mammalian nervous tissue. Anandamide (N-arachidonoyl-ethanolamine) was the first of a series of long-chain fatty acid derivatives, including two other polyunsaturated N-acylethanolamines and 2-arachidonoyl-glycerol, found to exert cannabimimetic properties in either central or peripheral tissues. Here we review the current knowledge on the biochemical bases of the formation and inactivation of endogenous cannabinoid ligands as well as of their interaction with cannabinoid receptor subtypes.

Endocannabinoids: endogenous cannabinoid receptor ligands with neuromodulatory action

The existence of an endogenous cannabinoid system was demonstrated conclusively with the discovery of endogenous brain constituents capable of activating the cannabinoid receptors functionally. These compounds are synthesized by neuronal cells and inactivated through re-uptake and enzymatic hydrolysis by both neurons and astrocytes. In analogy with the endorphins they can be referred to as endocannabinoids. Apart from the identification of their metabolic pathways, research carried out in the past six years has focused on the possible cellular and molecular targets for the actions of endocannabinoids. These studies have confirmed a similarity between the endocannabinoids and the psychoactive substance in marijuana, delta9(-)-tetrahydrocannabinol, and have suggested a role for endocannabinoids in the modulation of neurotransmitter action and release.

A new perspective on cannabinoid signalling: complementary localization of fatty acid amide hydrolase and the CB1 receptor in rat brain

CB1-type cannabinoid receptors in the brain mediate effects of the drug cannabis. Anandamide and sn-2 arachidonylglycerol (2-AG) are putative endogenous ligands for CB1 receptors, but it is not known which cells in the brain produce these molecules. Recently, an enzyme which catalyses hydrolysis of anandamide and 2-AG, known as fatty acid amide hydrolase (FAAH), was identified in mammals. Here we have analysed the distribution of FAAH in rat brain and compared its cellular localization with CB1-type cannabinoid receptors using immunocytochemistry. High concentrations of FAAH activity were detected in the cerebellum, hippocampus and neocortex, regions of the rat brain which are enriched with cannabinoid receptors. Immunocytochemical analysis of these brain regions revealed a complementary pattern of FAAH and CB1 expression with CB1 immunoreactivity occurring in fibres surrounding FAAH-immunoreactive cell bodies and/or dendrites. In the cerebellum, FAAH was expressed in the cell bodies of Purkinje cells and CB1 was expressed in the axons of granule cells and basket cells, neurons which are presynaptic to Purkinje cells. The close correspondence in the distribution of FAAH and CB1 in rat brain and the complementary pattern of FAAH and CB1 expression at the cellular level provides important new evidence that FAAH may participate in cannabinoid signalling mechanisms of the brain.

Endocannabinoids

The background knowledge leading to the isolation and identification of anandamide and 2-arachidonoyl glycerol, the principal endocannabinoids is described. The structure-activity relationships of these lipid derivatives are summarized. Selected biochemical and pharmacological topics in this field are discussed, the main ones being levels of endocannabinoids in unstimulated tissue and cells, biosynthesis, release and inactivation of endocannabinoids, the effects of 'entourage' compounds on the activities of anandamide and 2-arachidonoyl glycerol, their signaling mechanisms and effects in animals.

An analgesia circuit activated by cannabinoids

Although many anecdotal reports indicate that marijuana and its active constituent, delta-9-tetrahydrocannabinol (delta-9-THC), may reduce pain sensation, studies of humans have produced inconsistent results. In animal studies, the apparent pain-suppressing effects of delta-9-THC and other cannabinoid drugs are confounded by motor deficits. Here we show that a brainstem circuit that contributes to the pain-suppressing effects of morphine is also required for the analgesic effects of cannabinoids. Inactivation of the rostral ventromedial medulla (RVM) prevents the analgesia but not the motor deficits produced by systemically administered cannabinoids. Furthermore, cannabinoids produce analgesia by modulating RVM neuronal activity in a manner similar to, but pharmacologically dissociable from, that of morphine. We also show that endogenous cannabinoids tonically regulate pain thresholds in part through the modulation of RVM neuronal activity. These results show that analgesia produced by cannabinoids and opioids involves similar brainstem circuitry and that cannabinoids are indeed centrally acting analgesics with a new mechanism of action.

Platelet- and macrophage-derived endogenous cannabinoids are involved in endotoxin-induced hypotension

Macrophages are the primary cellular targets of bacterial lipopolysaccharide (LPS), but the role of macrophage-derived cytokines in LPS-induced septic shock is uncertain. Recent evidence indicates that activation of peripheral CB1 cannabinoid receptors contributes to hemorrhagic hypotension and that macrophage-derived anandamide as well as unidentified platelet-derived substances may be contributing factors. Here we demonstrate that rat platelets contain the endogenous cannabinoid 2-arachidonyl glyceride (2-AG), as identified by reverse phase high-performance liquid chromatography, gas chromatography, and mass spectrometry, and that in vitro exposure of platelets to LPS (200 microg/ml) markedly increases 2-AG levels. LPS-stimulated, but not control, macrophages contain anandamide, which is undetectable in either control or LPS-stimulated platelets. Prolonged hypotension and tachycardia are elicited in urethane-anesthetized rats treated 1) with LPS (15 mg/kg i.v.); 2) with macrophages plus platelets isolated from 3 ml of blood from an LPS-treated donor rat; or 3) with rat macrophages or 4) platelets preincubated in vitro with LPS (200 microg/ml). In all four cases, the hypotension but not the tachycardia is prevented by pretreatment of the recipient rat with the CB1 receptor antagonist SR141716A (3 mg/kg i.v.), which also inhibits the hypotensive response to anandamide or 2-AG. The hypotension elicited by LPS-treated macrophages or platelets remains unchanged in the absence of sympathetic tone or after blockade of nitric oxide synthase. These findings indicate that platelets and macrophages generate different endogenous cannabinoids, and that both 2-AG and anandamide may be paracrine mediators of endotoxin-induced hypotension via activation of vascular CB1 receptors.

Control of pain initiation by endogenous cannabinoids

The potent analgesic effects of cannabis-like drugs and the presence of CB1-type cannabinoid receptors in pain-processing areas of the brain and spinal cord indicate that endogenous cannabinoids such as anandamide may contribute to the control of pain transmission within the central nervous system (CNS). Here we show that anandamide attenuates the pain behaviour produced by chemical damage to cutaneous tissue by interacting with CB1-like cannabinoid receptors located outside the CNS. Palmitylethanolamide (PEA), which is released together with anandamide from a common phospholipid precursor, exerts a similar effect by activating peripheral CB2-like receptors. When administered together, the two compounds act synergistically, reducing pain responses 100-fold more potently than does each compound alone. Gas-chromatography/mass-spectrometry measurements indicate that the levels of anandamide and PEA in the skin are enough to cause a tonic activation of local cannabinoid receptors. In agreement with this possibility, the CB1 antagonist SR141716A and the CB2 antagonist SR144528 prolong and enhance the pain behaviour produced by tissue damage. These results indicate that peripheral CB1-like and CB2-like receptors participate in the intrinsic control of pain initiation and that locally generated anandamide and PEA may mediate this effect.

Accumulation of various N-acylethanolamines including N-arachidonoylethanolamine (anandamide) in cadmium chloride-administered rat testis

Changes in the levels of various molecular species of N-acylethanolamine in CdCl2-administered rat testis were examined. We found that the levels of various N-acylethanolamines including anandamide (N-arachidonoylethanolamine), an endogenous cannabinoid receptor ligand, were dramatically increased in CdCl2-admin-istered rat testis. Such changes were particularlyprominent for saturated and monoenoic species such as N-palmitoyl species (39-fold at 9 h) and N-stearoyl species (21-fold at 9 h), compared with unsaturated fatty acid-containing species such as anandamide (5-fold at 9 h). Noticeably, increased levels were observed of not only N-acylethanolamines but also several species of N-acylphosphatidylethanolamine, potential precursors for N-acylethanolamines. We confirmed that the rat testis microsomal fraction contains phosphodiesterase activity catalyzing the release of N-acylethanolamine from N-acylphosphatidylethanolamine and transacylase activity catalyzing the formation of N-acylphosphatidylethanolamine from phosphatidylethanolamine and phosphatidylcholine. These enzyme activities were not dramatically different in the microsomal fraction obtained from CdCl2-administered rat testis compared with that in the case of control rat testis, at least when estimated in cell-free assay systems, suggesting that the accessibility of the substrates to the enzymes may be increased in CdCl2-administered rat testis to generate a large amount of N-acylethanolamine. Possible pathophysiological implications of the augmented generation of N-acylethanolamine including anandamide in CdCl2-administered rat testis were discussed.

'Endocannabinoids' and other fatty acid derivatives with cannabimimetic properties: biochemistry and possible physiopathological relevance

The only endogenous substances isolated and characterised so far that are capable of mimicking the pharmacological actions of the active principle of marijuana, (-)-Delta9-tetrahydrocannabinol, are amides and esters of fatty acids. Some of these compounds, like anandamide (N-arachidonoylethanolamine) and 2-arachidonoylglycerol, act as true 'endogenous cannabinoids' by binding and functionally activating one or both cannabinoid receptor subtypes present on nervous and peripheral cell membranes. The metabolic pathways and molecular mode of actions of these metabolites, as well as their possible implication in physiopathological responses, are reviewed here.

2-Arachidonoylglycerol, an endogenous cannabinoid receptor agonist: identification as one of the major species of monoacylglycerols in various rat tissues, and evidence for its generation through CA2+-dependent and -independent mechanisms

The molecular species compositions of monoacylglycerols obtained from various rat tissues were examined by reverse-phase high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) analyses. We confirmed that 2-arachidonoylglycerol, an endogenous cannabinoid receptor agonist, is one of the most abundant molecular species of monoacylglycerols in the brain. Substantial amounts of 2-arachidonoylglycerol were also found in the liver, spleen, lung and kidney, but the levels were considerably lower than that in the brain. We found that a small amount of 2-arachidonoylglycerol was generated in a brain homogenate during incubation in the absence of Ca2+. Importantly, the generation of 2-arachidonoylglycerol was markedly augmented in the presence of Ca2+, suggesting that Ca2+ plays a key role in regulation of the generation of 2-arachidonoylglycerol in this tissue.

Cannabinoid receptors and their endogenous agonists

Marijuana has been in use for over 4000 years as a therapeutic and as a recreational drug. Within the past decade, two cannabinoid receptor types have been identified, their signal transduction characterized, and an endogenous lipid agonist isolated from mammalian tissues. The CB1 cannabinoid receptor is widely distributed in mammalian tissues, with the highest concentrations found in brain neurons. CB1 receptors are coupled to modulation of adenylate cyclase and ion channels. The CB2 receptor is found in cells of the immune system and is coupled to inhibition of adenylate cyclase. Both receptor types selectively bind delta 9-THC, the active principle in marijuana, and anandamide (arachidonylethanolamide), an endogenous cannabimimetic eicosanoid. Progress is being made in the development of novel agonists and antagonists with receptor subtype selectivity, mice with genetic deletion of the cannabinoid receptors, and receptor-specific antibodies, which should help in providing a better understanding of the physiological role of the cannabinoid receptors.

Alternative pathways of anandamide biosynthesis in rat testes

We have investigated the biosynthesis of long-chain N-acylethanolamines (NAEs) from endogenous substrates in rat testes membranes with special emphasis on anandamide (20:4n-6 NAE), a cannabinoid receptor agonist. Incubation of various membrane preparations with 5 mM Ca2+ produced both N-acyl phosphatidylethanolamine (N-acyl PE) and NAE with primarily (approximately 85%) N-palmitoyl groups (16:0 NAE) and less than 2% 20:4n-6 NAE. In contrast, incubation of these membranes with 5 mM EGTA and 10 mM ethanolamine had little effect on N-acyl PE composition but yielded NAEs whose major constituent (32-37%) was anandamide. Incubations with [1,1,2,2,-2H4]ethanolamine in media containing 40% H2(18)O showed that the Ca(2+)-independent NAE synthesis occurred by direct condensation of ethanolamine with free fatty acids present in the membrane preparation. This biosynthetic activity occurred at ethanolamine concentrations as low as 50 microM and exhibited substrate selectivity for arachidonate which increased with increasing ethanolamine concentrations. The results of inhibitor experiments suggest that the Ca(2+)-independent NAE synthesis was catalyzed by the NAE amidohydrolase acting in reverse. This condensation reaction could be important in agonist-induced anandamide synthesis for cell signalling through cannabinoid receptors.

Isotope dilution GC/MS determination of anandamide and other fatty acylethanolamides in rat blood plasma

Anandamide and allied fatty acylethanolamides (AEs) may act as signalling molecules in brain and peripheral tissues. In the present study, we describe an electron-impact gas chromatography/mass spectrometry (GC/MS) method based on isotope dilution, which may be used for the identification and quantification of anandamide and other AEs in biological matrices. The calibration curves for standard AEs were linear over the range 0-1000 pmol (r2 = 0.99) with a coefficient of variation of 4% at 2.5 pmol. Detection and quantification limits were in the high fmol to low pmol range for all AEs. Using this method we measured nanomolar concentrations of three endogenous AEs in deproteinated rat blood plasma (anandamide: 5.2 pmol/ml; palmitylethanolamide: 16.7 pmol/ml; oleylethanolamide: 8.1 pmol/ml). These results are consistent with a regulatory role of anandamide and other AEs in peripheral tissues.

Cannabinoid receptor-mediated inhibition of the rat tail-flick reflex after microinjection into the rostral ventromedial medulla

Systemic administration of cannabinoids produce profound antinociception in rodents. The purpose of this study was to examine the contribution of the rostral ventromedial medulla (RVM) to cannabinoid-mediated inhibition of the tail-flick reflex. Rats received direct injections of two selective cannabinoid agonists, WIN55,212-2 and HU210, into the RVM. Both compounds significantly elevated tail-flick latencies by over 50%. WIN55,212-3, the inactive enantiomer, was without effect. Furthermore, co-administration of the selective cannabinoid receptor antagonist, SR141716A greatly attenuated the antinociception produced by HU210. Finally, injections of WIN55,212-2 outside the region of the RVM did not affect tail-flick latencies. These results demonstrate that the cannabinoid receptor system participates in the descending control of nociception and raise the possibility that actions of endogenous cannabinoids in the RVM may modulate nociceptive responsiveness.

Enzymology of cottonseed microsomal N-acylphosphatidylethanolamine synthase: kinetic properties and mechanism-based inactivation

An ATP-, Ca2+-, and CoA-independent acyltransferase activity, designated "N-acylphosphatidylethanolamine (NAPE) synthase", was reported to catalyze the direct acylation of phosphatidylethanolamine (PE) with free fatty acids (FFAs) in cottonseed microsomes [K.D. Chapman, T.S. Moore, Jr., Plant Physiol. 102 (3) (1993) 761-769]. Here, NAPE synthase was purified 138, 176-fold from crude cottonseed homogenates to a specific activity of 5.98 mumol min-1 mg-1 protein by immobilized artificial membrane chromatography. Enzyme purity was confirmed by the presence of a 64 kDa polypeptide in fractions analyzed by tricine-SDS-PAGE. Initial velocity measurements with various free fatty acids ([14C]-linoleic, -palmitic, -oleic, -stearic and -myristic acids) and saturating concentrations of dioleoyl-PE revealed non-Michaelis-Menten, biphasic kinetics with high and low affinity sites demonstrating positive cooperativity specific for each [14C]-FFA. In contrast to FFA substrates, no kinetic differences were observed for two different molecular species of PE, (18:1,18:1)-PE and (16:0,18:2)-PE, and biphasic curves were not pronounced. Neither [14C]-dipalmitoylphosphatidylcholine nor [14C]-palmitoyl-CoA served as acyl donors for the synthesis of NAPE, indicating a preference for FFAs as the acyl donor. Also, neither ethanolamine nor sphingosine functioned as acyl acceptor molecule to form N-acylethanolamine or ceramide, respectively, indicating specificity for the phospholipid PE. NAPE synthase was inactivated in a time- and concentration-dependent manner by diisopropylfluorophosphate (DFP) through the apparent modification of one serine residue. Palmitic acid protected the enzyme from DFP-inactivation and [14C]-DFP incorporation, suggesting that a serine residue probably binds FFAs in the enzyme's active site forming an acyl-enzyme intermediate. Collectively, these results provide new information on the kinetic behavior of a purified, integral membrane enzyme which synthesizes a bilayer-stabilizing product from two lipid-soluble substrates. The biochemical properties of cottonseed NAPE synthase are consistent with a possible free fatty acid scavenging role in vivo. (c) 1998 Elsevier Science B.V.

Endocannabinoids: a new class of vasoactive substances

Endogenous cannabinoids (endocannabinoids) have recently been identified in the CNS and attention has now turned to their cardiovascular actions. The prototypic endocannabinoid, anandamide, derived from arachidonic acid, has been shown to be a vasorelaxant, particularly in the resistance vasculature. This vasorelaxation has been shown to be both endothelium-independent and -dependent, depending on the vascular bed. It has been proposed that an endocannabinoid may mediate the nitric oxide- and prostanoid-independent component of endothelium-dependent relaxations, as these responses are sensitive to a cannabinoid receptor antagonist and show similarities to anandamide-induced relaxations. This hypothesis has generated much controversy and the emerging conflicts in the literature are discussed in this article by Michael Randall and David Kendall. Despite this controversy, it has recently been shown that anandamide is produced by endothelial cells. Clearly, much work is required to adequately define the physiological significance of endocannabinoids in the cardiovascular system.

Functional role of high-affinity anandamide transport, as revealed by selective inhibition

Anandamide, an endogenous ligand for central cannabinoid receptors, is released from neurons on depolarization and rapidly inactivated. Anandamide inactivation is not completely understood, but it may occur by transport into cells or by enzymatic hydrolysis. The compound N-(4-hydroxyphenyl)arachidonylamide (AM404) was shown to inhibit high-affinity anandamide accumulation in rat neurons and astrocytes in vitro, an indication that this accumulation resulted from carrier-mediated transport. Although AM404 did not activate cannabinoid receptors or inhibit anandamide hydrolysis, it enhanced receptor-mediated anandamide responses in vitro and in vivo. The data indicate that carrier-mediated transport may be essential for termination of the biological effects of anandamide, and may represent a potential drug target.

A second endogenous cannabinoid that modulates long-term potentiation

Cannabinoid receptors are molecular targets for marijuana and hashish, the widespread drugs of abuse. These receptors are expressed in areas of the central nervous system that contribute in important ways to the control of memory, cognition, movement and pain perception. Indeed, such functions can be strongly influenced by cannabinoid drugs, with consequences that include euphoria, analgesia, sedation and memory impairment. Although the pharmacology of cannabinoid drugs is now beginning to be understood, we still lack essential information on the endogenous signalling system(s) by which cannabinoid receptors are normally engaged. An endogenous ligand for cannabinoid receptors, anandamide, has been described. Here we report that sn-2 arachidonylglycerol (2-AG), a cannabinoid ligand isolated from intestinal tissue, is present in brain in amounts 170 times greater than anandamide. 2-AG is produced in hippocampal slices by stimulation of the Schaffer collaterals, an excitatory fibre tract that projects from CA3 to CA1 neurons. Formation of 2-AG is calcium dependent and is mediated by the enzymes phospholipase C and diacylglycerol lipase. 2-AG activates neuronal cannabinoid receptors as a full agonist, and prevents the induction of long-term potentiation at CA3-CA1 synapses. Our results indicate that 2-AG is a second endogenous cannabinoid ligand in the central nervous system.