Activation of peripheral cannabinoid CB1 and CB2 receptors suppresses the maintenance of inflammatory nociception: a comparative analysis

BACKGROUND AND PURPOSE

Effects of locally administered agonists and antagonists for cannabinoid CB(1) and CB(2) receptors on mechanical and thermal hypersensitivity were compared after the establishment of chronic inflammation.

EXPERIMENTAL APPROACH

Carrageenan was administered unilaterally to the rat hindpaw on day 1. Prophylactic efficacy of locally administered CB(1)- and CB(2)-selective agonists -arachidonyl-2-chloroethylamide (ACEA) and (R,S)-(2-iodo-5-nitro-phenyl)-[l-(l-methyl-piperidin-2-ylmethyl)-lH-ubdik-3-yl]-methanone ((R,S)-AM1241), respectively- on mechanical and thermal hypersensitivity were compared on day 2. Pharmacological specificity was evaluated using locally administered CB(1) and CB(2)-selective antagonists -N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamidehydrochloride (SR141716A) and N-[(1S)-endo-1,3,3-trimethyl bicycle [2.2.1] heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide (SR144528), respectively.

KEY RESULTS

Administration of either ACEA or AM1241 to the inflamed but not noninflamed paw suppressed the maintenance of carrageenan-evoked mechanical hyperalgesia and tactile allodynia and attenuated thermal hyperalgesia. The ACEA-induced suppression of mechanical and thermal hypersensitivity was blocked by local injection of SR141716A but not SR144528. AM1241 suppressed mechanical hypersensitivity with the reverse pharmacological specificity. The AM1241-induced suppression of thermal hyperalgesia was blocked by SR144528 and to a lesser extent by SR14176A. Co-administration of ACEA with AM1241 in the inflamed paw increased the magnitude but not the duration of thermal antihyperalgesia compared to intraplantar administration of either agonist alone.

CONCLUSIONS AND IMPLICATIONS

Cannabinoids act locally through distinct CB(1) and CB(2) mechanisms to suppress mechanical hypersensitivity after the establishment of chronic inflammation, at doses that produced modest changes in thermal hyperalgesia. Additive antihyperalgesic effects were observed following prophylactic co-administration of the CB(1)- and CB(2)-selective agonists. Our results suggest that peripheral cannabinoid antihyperalgesic actions may be exploited for treatment of inflammatory pain states.

Effects of the CB1R agonist WIN-55,212-2 and the CB1R antagonists SR-141716 and AM-1387: Open-field examination in rats

This study examined the open-field (O-F) effects in rats of the cannabinoid 1 receptor (CB1R) agonist WIN-55,212-2 (WIN; 1 to 5.6 mg/kg) and its interaction with the CB1R antagonist/inverse agonist SR-141716 (1 to 5.6 mg/kg). Additionally, separate studies examined the O-F effects of SR-141716 (1 to 10 mg/kg) and a newly synthesized CB1R selective antagonist/inverse agonist AM-1387 (3 and 10 mg/kg) when these ligands were administered alone. Both antagonists are characterized in vitro by decreased of GTPgammaS binding and increased cAMP accumulation (inverse agonism). WIN dose dependently reduced ambulation (horizontal activity) and rearing (vertical activity); SR-141716 completely (WIN 3 mg/kg) or partially (WIN 5.6 mg/kg) normalized these behaviors. WIN alone resulted in circling and in an increased latency to leave the start area of the O-F, effects blocked by all doses of SR-141716. Both the increased scratching and grooming, associated with SR-141716 administration, were attenuated but not abolished by WIN. SR-141716 alone tended to reduce ambulation (significant at 10 mg/kg) and rearing (non-significant), had no effect on latency, and increased scratching and grooming (both frequency and duration), at doses of 3 mg/kg and up. At the doses examined, AM-1387 had no effect on ambulation, rearing, latency but significantly increased scratching (10 mg/kg); there was also a trend for increased grooming (both frequency and duration). The O-F profile of WIN suggests more similarity with the effects of THC rather than methanandamide (and presumably also anandamide). Intrinsic activity (scratching and grooming) by SR-141716 was re-affirmed and seemed to be associated with administration of AM-1387 as well. AM-1387 was less potent than SR-141716.

Irt>t schedule controlled behavior in ‘learned-helpless’ rats: Effects from a cannabinoid agonist

Human depression is partly a congenital disorder. Aspects of the behavior accompanying depression can be magnified by genetic manipulation of bred animal species. Learned Helplessness (LH) is a trait-mark behavior that successfully breeds in rodents. Here, 'congenital' LH (cLH) rats were trained to recognize and respond to 12s long interval cues (irt>12s schedule). Rats compliant to an irt>t schedule will space responses evenly and respond rhythmically. Irt>t schedule derived data are plotted in histograms showing irt (interresponse time) frequencies. A pause response peak emerges, for outbred rats, at irt values approximating the minimum interval for reinforcement. cLH rats [n=9] complied poorly to schedule contingencies when diluent (vehicle) was injected before testing. Moderate and high dose injections of a CB 1 receptor selective agonist drug (AM 411), however, increased operant schedule compliance and normalized the cLH rats' irt>t histogram distributions. Performance indicators for cLH rats are presented alongside coordinate measures from a comparison group [n=5] of normally bred Sprague-Dawley (SD) rats. In both cLH and SD rats, treatment session histograms revealed shifts of the pause response peak not accompanied by a change in motor responsiveness. The irt>12s histogram shifts were absent when AM 411 dosages were arranged to follow pre-medication injections of a CB 1 receptor selective antagonist drug (AM 251). In short, AM 411 increased timing acuity in rats prone to behavioral despair but had opposite timing effects in normally bred SD rats.

Purification and mass spectroscopic analysis of human CB1 cannabinoid receptor functionally expressed using the baculovirus system

The cannabinoid receptor 1 (CB1) cannabinoid receptor is an essential component of the cannabinergic system. It has been recognized as a therapeutic target for treating numerous diseases and is currently receiving considerable attention by the pharmaceutical community. Target-based drug design, utilizing three-dimensional information of receptor structure and ligand-binding motifs, requires significant amounts of purified protein. To facilitate the purification of CB1, we have expressed the receptor fused to various epitope tags using the baculovirus expression system. In addition, expression levels and ligand-binding profiles corresponding to the expressed fusion proteins have been compared. C-terminal histidine (His)-tagged CB1 gave a Bmax higher than most other systems previously reported in the literature, and was selected for subsequent metal affinity chromatography purification and mass spectroscopic (MS) analysis. Moreover, cells expressing C-terminal His-tagged CB1 were shown to inhibit forskolin-stimulated cyclic adenosine 3',5'-monophosphate (cAMP) production in a concentration-dependent manner in the presence of CP-55,940, confirming the expressed receptor's functional characteristics. A Western blot analysis of the purified receptor showed several forms of CB1, the most abundant being a 57 kDa monomeric protein. The purified CB1 preparations were subjected to protein digestion followed by MS. Fragments corresponding to >70% of the receptor were identified by this method, confirming the identity and purity of the expressed protein. The work presented here demonstrates that epitope-tagged CB1 can be expressed in sufficient amounts and purified to homogeneity for MS analysis. Moreover, these results will serve as a basis for future experiments aimed at characterizing the ligand-binding domains using covalently reacting receptor probes.

The novel cannabinoid agonist AM 411 produces a biphasic effect on accuracy in a visual target detection task in rats

Cannabinoid agonists have been shown to produce dose-related impairments in several measures of cognitive performance. However, it is unclear if low doses of cannabinoid CB1 agonists, or CB1 antagonists, can facilitate aspects of stimulus detection. The present study employed an operant procedure involving visual stimulus detection in rats. The task was found to be sensitive to the muscarinic acetylcholine antagonist scopolamine. The CB1 antagonist AM 251 did not affect stimulus detection processes across a broad range of doses. However, the novel CB1 agonist AM 411 produced a biphasic effect, with the two lowest doses (0.25 and 0.5 mg/kg) enhancing accuracy. AM 411 changed patterns of responding toward runs of consecutive errors on only one of the two levers. It produced a biphasic effect on consecutive errors on the lever associated with a higher level of errors, with decreases in errors following the lower doses (0.25 and 0.5 mg/kg) and increases following the highest dose (2.0 mg/kg). These effects were not accompanied by changes in measures of bias commonly used to uncover such patterns in rodent operant models of cognitive performance. In contrast to the cognitive impairment seen after administration of moderate to high doses of CB1 agonists, it appears that low doses of some CB1 agonists may be capable of enhancing stimulus detection processes.

The cannabinoid CB1 antagonist AM 251 produces food avoidance and behaviors associated with nausea but does not impair feeding efficiency in rats

RATIONALE

A growing body of evidence suggests that cannabinoid CB1 receptor antagonists have potential therapeutic utility as appetite suppressants. However, the specific mechanisms underlying the reduction in food intake produced by these drugs are not well understood.

OBJECTIVE

Considering the known antiemetic and motor-suppressive effects of CB1 agonists, the present studies were conducted to determine if the reductions in food intake induced by the CB1 antagonist AM 251 could result from nausea or impairments in intake-related motor control, rather than solely from appetite suppression.

METHODS

Three experiments were conducted to examine the effects of AM 251 (2.0, 4.0, or 8.0 mg/kg or vehicle) on detailed parameters of food intake, on the development of conditioned taste avoidance, and on taste reactivity.

RESULTS

In the first experiment, acute administration of AM 251 dose-dependently decreased food intake; nevertheless, feeding rate (grams consumed per time spent eating) and food handling were unaffected, which suggests that food intake was not reduced because of severe motor impairments. In the second experiment, AM 251 dose-dependently reduced intake of a flavor with which it had previously been associated, indicating that conditioned taste avoidance had developed. Lastly, AM 251 was found to induce conditioned rejection reactions in a dose-dependent manner.

CONCLUSIONS

The CB1 antagonist AM 251 may reduce food intake in part by inducing nausea or malaise, but not because of incoordination or motor slowing related to feeding.

Purification and mass spectroscopic analysis of human CB2 cannabinoid receptor expressed in the baculovirus system

The cannabinergic system is present in a variety of organs and tissues that perform a wide range of essential physiologic functions making it an inherently important therapeutic target for drug discovery. In order to augment our knowledge regarding the interactions between cannabinoid receptors (CBs) and their ligands, efficient and effective tools are essential for robust expression and purification of these membrane-bound proteins. In this report, we describe a suitable method for purification of the human cannabinoid receptor 2 (CB2) to a qualitative and quantitative level sufficient for mass spectral analysis. We utilized a baculovirus expression system, incorporating several epitope tags to facilitate purification and to ameliorate the effect the tags have on CB2 expression and function. Expressed protein encoded by a carboxy (C)-terminal His-tagged CB2 construct displayed a B(max) value of 9.3 pmol/mg with a K(D) of 7.30 nM using [3(H)]CP-55(940), a standard cannabinoid radioligand, and was selected for subsequent purification experiments. Western blot analysis of purified membrane protein yielded several forms of CB2, the most abundant being a 41 kDa peptide. A second protein species was observed with an apparent molecular weight of 46 kDa representing a glycosylated form of CB2. In addition, a CB2 homodimer was also identified. The purified receptor was subjected to mass spectroscopic analysis to confirm its identity and purity. Mass spectra corresponding to the intracellular, extracellular and transmembrane domains were obtained. These experiments exemplify the importance of high-level expression systems when developing membrane-bound protein purification strategies. This work will aid in the identification of receptor-ligand binding sites, the characterization of molecular features involved in receptor activation, and the elucidation of the CB2 receptor tertiary structure.

Structural requirements for cannabinoid receptor probes

The discovery and cloning of CB1 and CB2, the two known G(i/o) protein-coupled cannabinoid receptors, as well as the isolation and characterization of two families of endogenous cannabinergic ligands represented by arachidonoylethanolamide (anandamide) and 2-arachidonoylglycerol (2-AG), have opened new horizons in this newly discovered field of biology. Furthermore, a considerable number of cannabinoid analogs belonging to structurally diverse classes of compounds have been synthesized and tested, thus providing substantial information on the structural requirements for cannabinoid receptor recognition and activation. Experiments with site-directed mutated receptors and computer modeling studies have suggested that these diverse classes of ligands may interact with the receptors through different binding motifs. The information about the exact binding site may be obtained with the help of suitably designed molecular probes. These ligands either interact with the receptors in a reversible fashion (reversible probes) or alternatively attach at or near the receptor active site with the formation of covalent bonds (irreversible probes). This review focuses on structural requirements of cannabinoid receptor ligands and highlights their pharmacological and therapeutic potential.

Activation of Cannabinoid CB2Receptors Suppresses C-Fiber Responses and Windup in Spinal Wide Dynamic Range Neurons in the Absence and Presence of Inflammation

Effects of the CB2-selective cannabinoid agonist AM1241 on activity evoked in spinal wide dynamic range (WDR) neurons by transcutaneous electrical stimulation were evaluated in urethane-anesthetized rats. Recordings were obtained in both the absence and the presence of carrageenan inflammation. AM1241, administered intravenously or locally in the paw, suppressed activity evoked by transcutaneous electrical stimulation during the development of inflammation. Decreases in WDR responses resulted from a suppression of C-fiber–mediated activity and windup. Aβ- and Aδ-fiber–mediated responses were not reliably altered. The AM1241-induced suppression of electrically evoked responses was blocked by the CB2antagonist SR144528 but not by the CB1antagonist SR141716A. AM1241 (33 μg/kg intraplantar [ipl]), administered to the carrageenan-injected paw, suppressed activity evoked in WDR neurons relative to groups receiving vehicle in the same paw or AM1241 in the opposite (noninflamed) paw. The electrophysiological effects of AM1241 (330 μg/kg intravenous [iv]) were greater in rats receiving ipl carrageenan compared with noninflamed rats receiving an ipl injection of vehicle. AM1241 failed to alter the activity of purely nonnociceptive neurons recorded in the lumbar dorsal horn. Additionally, AM1241 (330 μg/kg iv and ipl; 33 μg/kg ipl) reduced the diameter of the carrageenan-injected paw. The AM1241-induced decrease in peripheral edema was blocked by the CB2but not by the CB1antagonist. These data demonstrate that activation of cannabinoid CB2receptors is sufficient to suppress neuronal activity at central levels of processing in the spinal dorsal horn. Our findings are consistent with the ability of AM1241 to normalize nociceptive thresholds and produce antinociception in inflammatory pain states.

(???)-Adamantyl-??8-tetrahydrocannabinol (AM-411), a selective cannabinoid CB1 receptor agonist: effects on open-field behaviors and antagonism by SR-141716 in rats

(-)-Adamantyl-Delta8-tetrahydrocannabinol (AM-411) is a 'classical' tricyclic cannabinoid CB1 receptor agonist in which the C-3 alkyl side-chain has been replaced with an adamantyl group. The compound is cannabinoid CB1 receptor subtype selective (CB1 Ki=6.86 nmol/l, CB2 Ki=52.0 nmol/l). We examined the effects of AM-411 alone and in combination with the cannabinoid CB1 receptor antagonist/inverse agonist, SR-141716, on open-field behaviors of rats. The lowest effective dose of AM-411, 3 mg/kg, suppressed ambulation (horizontal activity) and rearing (vertical activity) and increased circling frequency compared to vehicle control levels. Co-administration of SR-141716 normalized these changes. SR-141716 (3 and 5.6 mg/kg) also produced significant increases in scratching and grooming (both frequency and duration), effects that were not eliminated in the presence of AM-411. Coupled with previous drug discrimination data, the open-field profile of AM-411 suggests that this high-affinity CB1 cannabinoid receptor agonist induces behavioral effects similar to the natural cannabinoid Delta9-tetrahydrocannabinol and different from (R)-methanandamide, a chiral analog of the endogenous ligand anandamide.

Anandamide transport is independent of fatty-acid amide hydrolase activity and is blocked by the hydrolysis-resistant inhibitor AM1172

The endogenous cannabinoid anandamide is removed from the synaptic space by a high-affinity transport system present in neurons and astrocytes, which is inhibited by N-(4-hydroxyphenyl)-arachidonamide (AM404). After internalization, anandamide is hydrolyzed by fatty-acid amide hydrolase (FAAH), an intracellular membrane-bound enzyme that also cleaves AM404. Based on kinetic evidence, it has recently been suggested that anandamide internalization may be mediated by passive diffusion driven by FAAH activity. To test this possibility, in the present study, we have investigated anandamide internalization in wild-type and FAAH-deficient (FAAH(-/-)) mice. Cortical neurons from either mouse strain internalized [(3)H]anandamide through a similar mechanism, i.e., via a rapid temperature-sensitive and saturable process, which was blocked by AM404. Moreover, systemic administration of AM404 to either wild-type or FAAH(-/-) mice enhanced the hypothermic effects of exogenous anandamide, a response that was prevented by the CB(1) cannabinoid antagonist rimonabant (SR141716A). The results indicate that anandamide internalization in mouse brain neurons is independent of FAAH activity. In further support of this conclusion, the compound N-(5Z, 8Z, 11Z, 14Z eicosatetraenyl)-4-hydroxybenzamide (AM1172) blocked [(3)H]anandamide internalization in rodent cortical neurons and human astrocytoma cells without acting as a FAAH substrate or inhibitor. AM1172 may serve as a prototype for novel anandamide transport inhibitors with increased metabolic stability.

Behavioral effects of inhibition of cannabinoid metabolism: The amidase inhibitor AM374 enhances the suppression of lever pressing produced by exogenously administered anandamide

Biochemical investigations have identified putative enzymatic pathways for the synthesis and metabolism of endogenous cannabinoids. Anandamide amidase is an enzyme that metabolizes anandamide into arachadonic acid and ethanolamine. Using in vitro methods, various inhibitors of amidase have been identified. The present studies were undertaken to determine if the amidase inhibitor AM 374 could enhance the effects of intraperitoneal (IP) injections of anandamide. Three studies were conducted to investigate the effects of various drug treatments on fixed ratio 5 operant lever pressing for food reinforcement. In the first study, the effects of different doses of anandamide were assessed, and it was demonstrated that 5.0 and 10.0 mg/kg anandamide IP significantly suppressed lever pressing, while 2.5 mg/kg produced very little effect. The second study tested the effects of intraventricular (ICV) injections of AM 374, and it was observed that doses up to 10.0, 20.0 and 40 microg AM 374 had no significant effect upon lever pressing. The third study investigated the combined effect of AM374 with a low dose of anandamide. Rats received two drug injections: one ICV and one IP. Four different drug treatments were assessed: 1) ICV vehicle + IP vehicle, 2) ICV vehicle + 2.5 mg/kg anandamide IP, 3) ICV 20.0 microg AM 374 + IP vehicle, and 4) ICV 20 microg AM 374 + 2.5 mg/kg anandamide IP. Combined administration of AM 374 plus anandamide led to a significant decrease in lever pressing compared to either AM374 or anandamide administered alone. Observations of the animals treated with the combination of AM374 plus anandamide indicated that the drug combination resulted in motor slowing, which is consistent with the notion that stimulation of cannabinoid receptors produced a motor deficit that interfered with lever pressing. Although AM374 produced no effect on its own, this amidase inhibitor did enhance the behavioral effect of a low dose of anandamide. These results are consistent with the notion that AM 374 inhibited the enzymatic breakdown of exogenously injected anandamide. This type of procedure can be used to assess a variety of different compounds for their ability to inhibit cannabinoid metabolism.

The cannabinoid CB1 antagonists SR 141716A and AM 251 suppress food intake and food-reinforced behavior in a variety of tasks in rats

Cannabinoid CB1 receptor agonists, including delta-9-tetrahydrocannabinol (Delta 9-THC) (the main psychoactive ingredient in marijuana) have been shown to increase feeding in rats and humans. Conversely, it has been reported that acute administration of the CB1 receptor antagonist SR 141716A reduces food intake in rats. Based upon this observation, it has been suggested that CB1 antagonists could be useful as appetite suppressant drugs. The present studies were designed to provide a detailed examination of the effects of CB1 antagonists on food intake across a range of paradigms. Two CB1 antagonists (SR 141716A and AM 251) were administered to rats trained on fixed-ratio schedules with two different ratio requirements (fixed-ratio 1 and fixed-ratio 5). Both drugs produced a dose-dependent decrease in lever pressing, and had a relatively long duration of action (T1/2: SR 141716A, 15.1 h; AM 251, 22.0 h). Furthermore, intake of three diets with differing macronutrient composition (lab chow, high fat, high carbohydrate) was studied. Both drugs significantly suppressed intake of all three foods, and there were no significant interactions between drug dose and diet type. These findings support the hypothesis that CB1 receptor antagonists could be useful pharmacological tools for the suppression of appetite.

Selective activation of cannabinoid CB(2) receptors suppresses spinal fos protein expression and pain behavior in a rat model of inflammation

Activation of cannabinoid CB(2) receptors attenuates thermal nociception in untreated animals while failing to produce centrally mediated effects such as hypothermia and catalepsy [Pain 93 (2001) 239]. The present study was conducted to test the hypothesis that activation of CB(2) in the periphery suppresses the development of inflammatory pain as well as inflammation-evoked neuronal activity at the level of the CNS. The CB(2)-selective cannabinoid agonist AM1241 (100, 330 micrograms/kg i.p.) suppressed the development of carrageenan-evoked thermal and mechanical hyperalgesia and allodynia. The AM1241-induced suppression of carrageenan-evoked behavioral sensitization was blocked by the CB(2) antagonist SR144528 but not by the CB(1) antagonist SR141716A. Intraplantar (ipl) administration of AM1241 (33 micrograms/kg ipl) suppressed hyperalgesia and allodynia following administration to the carrageenan-injected paw but was inactive following administration in the contralateral (noninflamed) paw, consistent with a local site of action. In immunocytochemical studies, AM1241 suppressed spinal Fos protein expression, a marker of neuronal activity, in the carrageenan model of inflammation. AM1241 suppressed carrageenan-evoked Fos protein expression in the superficial and neck region of the dorsal horn but not in the nucleus proprius or the ventral horn. The suppression of carrageenan-evoked Fos protein expression induced by AM1241 was blocked by coadministration of SR144528 in all spinal laminae. These data provide evidence that actions at cannabinoid CB(2) receptors are sufficient to suppress inflammation-evoked neuronal activity at rostral levels of processing in the spinal dorsal horn, consistent with the ability of AM1241 to normalize nociceptive thresholds and produce antinociception in inflammatory pain states.

Ligand based structural studies of the CB1 cannabinoid receptor

The structural characterization of G-protein coupled receptors (GPCRs) is quite important as these proteins represent a vast number of therapeutic targets involved in drug discovery. However, solving the three-dimensional structure of GPCR has been a significant obstacle in structural biology. A variety of reasons, including their large molecular weight, intricate interhelical packing, as well as their membrane-associated topology, has hindered efforts aimed at their purification. In the absence of pure protein, available in the native conformation, classical methods of structural analysis such as X-ray crystallography and nuclear magnetic resonance spectroscopy cannot be utilized successfully. Alternative methods must therefore be explored to facilitate the structural features involved in drug-receptor interactions. The methods described herein detail the use of covalent probes, or affinity labels, capable of binding covalently to a target GPCR at its binding site(s). Our approach involves the incorporation of a number of reactive moieties in different regions of the ligand molecule each of which is expected to react with different amino acid residues. Information obtained from such work coupled with computer modeling and validated by the use of site-directed mutagenesis of GPCRs allows for three-dimensional mapping of the receptor binding site. It also sheds light on the different possible binding motifs for the various classes of agonists and antagonists and identifies amino acid residues involved with GPCR activation or inactivation.

[Anandamide and R-(+)-methanandamide prevent development of ischemic and reperfusion arrhythmia in rats by stimulation of CB2-receptors]

It has been found that prior intravenous administration of the endocannabinoid anandamide (10 mg/kg) or its synthetic analogue R-(+)-methanadamide (5 mg/kg) prevents a development of ischemic and reperfusion arrhythmias in rats. The prior injection of the CB1 receptor antagonist, SR 141716A (3 mg/kg), did no affect the antiarrhythmic action of both cannabinoids. Pretreatment with the CB2 receptor antagonist, SR 144528 (1 mg/kg), completely abolished antiarrhythmic effect of anandamide and R-(+)-methanandamide. Both CB antagonist had no effect on the arrhythmias itself. Pretreatment with the NO-synthase inhibitor, L-NAME (50 mg/kg), had no effect on the antiarrhythmic action of cannabinoids. We therefore conclude that CB2 receptor stimulation increases the heart tolerance to ischemic and reperfusion arrhythmias.

Endogenous cannabinoids improve myocardial resistance to arrhythmogenic effects of coronary occlusion and reperfusion: a possible mechanism

Stimulation of cannabinoid receptors with endogenous cannabinoid anandamide and its enzyme-resistant analogue R-(+)-methanandamide improved cardiac resistance to arrhythmias induced by coronary occlusion and reperfusion. This antiarrhythmic effect was not associated with activation of NO synthase, since pretreatment with NG-nitro-L-arginine methyl ester had no effect on the incidence of ischemia/reperfusion-induced arrhythmias. Blockade of ATP-dependent K+ channels with glybenclamide did not abolish the antiarrhythmic effect of R-(+)-methanandamide. Antiarrhythmic activity of endogenous cannabinoids is probably associated with their direct effects on the myocardium.

Preparation of optically pure 4-(hydroxymethyl)-2-pentadecyl-1,3-dioxolanes and their corresponding phosphodiester ether lipid derivatives

Semi-preparative HPLC on a chiral stationary phase (Chiracel OD) was utilized in the course of this synthesis to separate the four possible diastereomers [cis-(2R,4S)-2a, trans-(2S,4S)-2b, cis-(2S,4R)-2a', and trans-(2R,4R)-2b'] of a 2,4-disubstituted-1,3-dioxolane into optically pure forms (100% de, 100% ee). The syntheses of phosphodiester head group derivatives from each of these four conformationally constrained diastereomeric dioxolanes gave phospholipids which are monocyclic ether lipid analogs. First, the series of four [[(2-pentadecyl-1,3-dioxolan-4-yl)methyl]oxy]phosphocholines 5 were synthesized to give optically pure conformationally constrained analogues of ET-16-OCH(3). A head group variation was also demonstrated by the syntheses of the four diastereomeric [[(2-pentadecyl-1,3-dioxolan-4-yl)-methyl]oxy]phospho-beta-(N-methylmorpholino)ethanols 6.

(R)-methanandamide and Delta 9-THC as discriminative stimuli in rats: tests with the cannabinoid antagonist SR-141716 and the endogenous ligand anandamide

RATIONALE AND OBJECTIVES

(R)-methanandamide (AM-356), a metabolically more stable chiral analog of the endocannabinoid ligand anandamide, was used as a representative of fatty acid ethanolamide CB1 receptor ligands to characterize the discriminative stimulus functions of anandamides.

METHODS

Rats discriminated between 10 mg/kg (R)-methanandamide and vehicle administered IP 15 min prior to session onset. Another group of rats was initially trained to discriminate between 3 mg/kg Delta9-THC and vehicle given IP 30 min prior to session onset; for anandamide testing, the animals were retrained with 1.8 and 5.6 mg/kg Delta9-THC. A two lever operant methodology (FR10) was used.

RESULTS

Delta9-THC was more potent than (R)-methanandamide at both 15 and 30 min post-injection, irrespective of the training drug used. Additional tests with 10 and 18 mg/kg (R)-methanandamide suggested that the effects were declining by 1 h. The cannabinoid antagonist SR 141716 (0.3 and 1 mg/kg) produced rightward shifts in the Delta9-THC dose-response curve for Delta9-THC-appropriate responding and for (R)-methanandamide-appropriate responding (surmountable antagonism). SR-141716 (0.3 and 1 mg/kg) antagonized the ability of (R)-methanandamide to occasion either Delta9-THC-appropriate responding or (R)-methanandamide-appropriate responding. This antagonism was surmountable only at a dose of 0.3 mg/kg SR-1421716 in the (R)-methanandamide-trained rats. SR-141716 did not antagonize the rate-decreasing effects of (R)-methanandamide in either the Delta9-THC or the (R)-methanandamide trained rats. Response suppression precluded testing doses higher than 30 mg/kg (R)-methanandamide. Tests with SR-141716 (1 and 10 mg/kg) alone resulted in <3% Delta9-THC-appropriate responding. With 10 mg/kg SR-141716, response rate was significantly lower as compared to the rate observed during a vehicle test. Tests with anandamide (10 and 18 mg/kg) resulted in 41% and 85% (R)-methanandamide-appropriate responding at a 3-min pre-treatment time, but in a maximum of 15% (R)-methanandamide-appropriate responding at a longer (15 min) pre-treatment time. In the Delta9-THC (1.8 and 5.6 mg/kg) trained rats, anandamide never produced more than about 20% Delta9-THC-appropriate responding.

CONCLUSION

The results add to a growing body of evidence indicating that there are both similarities and dissimilarities between classical cannabinoids such as THC and endogenous fatty acid ethanolamides.

Endocannabinoids acting at vascular CB1 receptors mediate the vasodilated state in advanced liver cirrhosis

Advanced cirrhosis is associated with generalized vasodilation of unknown origin, which contributes to mortality. Cirrhotic patients are endotoxemic, and activation of vascular cannabinoid CB1 receptors has been implicated in endotoxin-induced hypotension. Here we show that rats with biliary cirrhosis have low blood pressure, which is elevated by the CB1 receptor antagonist SR141716A. The low blood pressure of rats with CCl4-induced cirrhosis was similarly reversed by SR141716A, which also reduced the elevated mesenteric blood flow and portal pressure. Monocytes from cirrhotic but not control patients or rats elicited SR141716A-sensitive hypotension in normal recipient rats and showed significantly elevated levels of anandamide. Compared with non-cirrhotic controls, in cirrhotic human livers there was a three-fold increase in CB1 receptors on isolated vascular endothelial cells. These results implicate anandamide and vascular CB1 receptors in the vasodilated state in advanced cirrhosis and indicate a novel approach for its management.

Stereochemical selectivity of methanandamides for the CB1 and CB2 cannabinoid receptors and their metabolic stability

Several chiral, analogues of the endogenous cannabinoid receptor ligand, arachidonylethanolamide (anandamide), methylated at the 2,1' and 2' positions using asymmetric synthesis were evaluated in order to study (a) stereoselectivity of binding to CB1 and CB2 cannabinoid receptors; and (b) metabolic stability with regard to anandamide amidase. Enantiomerically pure 2-methyl arachidonic acids were synthesized through diastereoselective methylation of the respective chiral 2-oxazolidinone enolate derivatives and CB1 and CB2 receptor affinities of the resulting chiral anandamides were evaluated using a standard receptor binding assay. Introduction of a single 2-methyl group increased affinity for CB1, led to limited enantioselectivity and only modestly improved metabolic stability. However, a high degree of enantio- and diastereoselectivity was observed for the 2,1'-dimethyl analogues. (R)-N-(1-methyl-2-hydroxyethyl)-2-(R)-methyl-arachidonamide (4) exhibited the highest CB1 receptor affinity in this series with a K(i) of 7.42 nM, an at least 10-fold improvement on anandamide (K(i)=78.2 nM). The introduction of two methyl groups at the 2-position of anandamide led to no change in affinity for CB1 but somewhat enhanced metabolic stability. Conversely, chiral headgroup methylation in the 2-gem-dimethyl series led to chiral analogues possessing a wide range of CB1 affinities. Of these the (S)-2,2,2'-trimethyl analogue (12) had the highest affinity for CB1 almost equal to that of anandamide. In agreement with our previous anandamide structure-activity relationship work, the analogues in this study showed high selectivity for the CB1 receptor over CB2. The results are evaluated in terms of stereochemical factors affecting the ligand's affinity for CB1 using receptor-essential volume mapping as an aid. Based on the results, a partial CB1 receptor site model is proposed, that bears two hydrophobic pockets capable of accommodating 1'- and 2-methyl groups