Ligands that target cannabinoid receptors in the brain: from THC to anandamide and beyond

A major finding--that (-)-trans-Delta(9)-tetrahydrocannabinol (Delta(9)-THC) is largely responsible for the psychotropic effects of cannabis--prompted research in the 1970s and 1980s that led to the discovery that this plant cannabinoid acts through at least two types of cannabinoid receptor, CB(1) and CB(2), and that Delta(9)-THC and other compounds that target either or both of these receptors as agonists or antagonists have important therapeutic applications. It also led to the discovery that mammalian tissues can themselves synthesize and release agonists for cannabinoid receptors, the first of these to be discovered being arachidonoylethanolamide (anandamide) and 2-arachidonoylglycerol. These 'endocannabinoids' are released onto their receptors in a manner that appears to maintain homeostasis within the central nervous system and sometimes either to oppose or to mediate or exacerbate the unwanted effects of certain disorders. This review provides an overview of the pharmacology of cannabinoid receptors and their ligands. It also describes actual and potential clinical uses both for cannabinoid receptor agonists and antagonists and for compounds that affect the activation of cannabinoid receptors less directly, for example by inhibiting the enzymatic hydrolysis of endocannabinoids following their release.

Inhibition of human neutrophil chemotaxis by endogenous cannabinoids and phytocannabinoids: evidence for a site distinct from CB1 and CB2

Here, we show a novel pharmacology for inhibition of human neutrophil migration by endocannabinoids, phytocannabinoids, and related compounds. The endocannabinoids virodhamine and N-arachidonoyl dopamine are potent inhibitors of N-formyl-l-methionyl-l-leucyl-l-phenylalanine-induced migration of human neutrophils, with IC(50) values of 0.2 and 8.80 nM, respectively. The endocannabinoid anandamide inhibits human neutrophil migration at nanomolar concentrations in a biphasic manner. The phytocannabinoid (-)-cannabidiol is a partial agonist, being approximately 40 fold more potent than (+)-cannabidiol; abnormal-cannabidiol is a full agonist. Furthermore, the abnormal-cannabidiol (CBD) analog trans-4-[3-methyl-6-(1-methylethenyl)-2-cyclohexen-1-yl]-5-methyl-1,3-benzenediol (O-1602) inhibits migration, with an IC(50) value of 33 nM. This reported profile of agonist efficacy and potency parallels with the pharmacology of the novel "abnormal-cannabidiol" receptor or a related orphan G protein-coupled receptor, which are already known to modulate cell migration. Although having no effect alone, N-arachidonoyl l-serine attenuated inhibition of human neutrophil migration induced by anandamide, virodhamine, and abnormal-CBD. Our data also suggest that there is cross-talk/negative co-operativity between the cannabinoid CB(2) receptor and this novel target: CB(2) receptor antagonists significantly enhance the inhibition observed with anandamide and virodhamine. This study reveals that certain endogenous lipids, phytocannabinoids, and related ligands are potent inhibitors of human neutrophil migration, and it implicates a novel pharmacological target distinct from cannabinoid CB(1) and CB(2) receptors; this target is antagonized by the endogenous compound N-arachidonoyl l-serine. Furthermore, our findings have implications for the potential pharmacological manipulation of elements of the endocannabinoid system for the treatment of various inflammatory conditions.

GPR55: a new member of the cannabinoid receptor clan?

In this issue of the British Journal of Pharmacology, Ryberg et al. present convincing in vitro evidence that the orphan GPCR, GPR55, is a cannabinoid receptor. GPR55 was activated by a range of plant, synthetic and endogenous cannabinoids and blocked by the non-psychoactive phytocannabinoid, cannabidiol. Their experiments have revealed several differences between the pharmacology of GPR55 and the established cannabinoid CB1 and CB2 receptors. For example, the CB1 receptor antagonist, AM251, activated GPR55 and the main psychoactive constituent of cannabis, Delta9-tetrahydrocannabinol, displayed greater efficacy at GPR55 than at CB1 or CB2 receptors. They also compared the distribution of GPR55 and CB1 mRNA in mouse and report that GPR55 couples to Galpha13, that it is activated by virodhamine, palmitoylethanolamide and oleoylethanolamide, and that virodhamine displays relatively high efficacy as a GPR55 agonist. Still to be identified are the main roles played by GPR55 in health and disease and any potential therapeutic benefits of activating or blocking this receptor.

The diverse CB1 and CB2 receptor pharmacology of three plant cannabinoids: delta9-tetrahydrocannabinol, cannabidiol and delta9-tetrahydrocannabivarin

Cannabis sativa is the source of a unique set of compounds known collectively as plant cannabinoids or phytocannabinoids. This review focuses on the manner with which three of these compounds, (-)-trans-delta9-tetrahydrocannabinol (delta9-THC), (-)-cannabidiol (CBD) and (-)-trans-delta9-tetrahydrocannabivarin (delta9-THCV), interact with cannabinoid CB1 and CB2 receptors. Delta9-THC, the main psychotropic constituent of cannabis, is a CB1 and CB2 receptor partial agonist and in line with classical pharmacology, the responses it elicits appear to be strongly influenced both by the expression level and signalling efficiency of cannabinoid receptors and by ongoing endogenous cannabinoid release. CBD displays unexpectedly high potency as an antagonist of CB1/CB2 receptor agonists in CB1- and CB2-expressing cells or tissues, the manner with which it interacts with CB2 receptors providing a possible explanation for its ability to inhibit evoked immune cell migration. Delta9-THCV behaves as a potent CB2 receptor partial agonist in vitro. In contrast, it antagonizes cannabinoid receptor agonists in CB1-expressing tissues. This it does with relatively high potency and in a manner that is both tissue and ligand dependent. Delta9-THCV also interacts with CB1 receptors when administered in vivo, behaving either as a CB1 antagonist or, at higher doses, as a CB1 receptor agonist. Brief mention is also made in this review, first of the production by delta9-THC of pharmacodynamic tolerance, second of current knowledge about the extent to which delta9-THC, CBD and delta9-THCV interact with pharmacological targets other than CB1 or CB2 receptors, and third of actual and potential therapeutic applications for each of these cannabinoids.

Meta-analysis of cannabinoid ligand binding affinity and receptor distribution: interspecies differences

A meta-analysis, unlike a literature review, synthesizes previous studies into new results. Pooled data from 211 studies measured ligand binding affinities at human (Hs) or rat (Rn) cannabinoid receptors CB1 and CB2. Cochrane methods were modified for this non-clinical analysis. Meta-regression detected data heterogeneity arising from methodological factors: use of sectioned tissues, lack of PMSF and choice of radioligand. Native brain tissues exhibited greater affinity (lower nM) than transfected cells, but the trend fell short of significance, as did the trend between centrifugation and filtration methods. Correcting for heterogeneity, mean Ki values for delta 9-tetrahydrocannabinol differed significantly between HsCB1 and RnCB1 (25.1 and 42.6 nM, respectively) but not between HsCB1 and HsCB2 (25.1 and 35.2). Mean Kd values for HsCB1, RnCB1 and HsCB2 of CP55,940 (2.5, 0.98, 0.92) and WIN55,212-2 (16.7, 2.4, 3.7) differed between HsCB1 and RnCB1 and between HsCB1 and HsCB2. SR141716A differed between HsCB1 and RnCB1 (2.9 and 1.0 nM). Anandamide at HsCB1, RnCB1 and HsCB2 (239.2, 87.7, 439.5) fell short of statistical differences due to heterogeneity. We consider these Kd and Ki values to be the most valid estimates in the literature. Sensitivity analyses did not support the numerical validity of cannabidiol, cannabinol, 2-arachidonoyl glycerol and all ligands at RnCB2. Aggregate rank order analysis of CB(1) distribution in the brain (pooled from 119 autoradiographic, immunohistochemical and in situ hybridization studies) showed denser HsCB1 expression in cognitive regions (cerebral cortex) compared to RnCB1, which was relatively richer in movement-associated areas (cerebellum, caudate-putamen). Implications of interspecies differences are discussed.

The synthetic cannabinoid HU210 induces spatial memory deficits and suppresses hippocampal firing rate in rats

BACKGROUND AND PURPOSE

Previous work implied that the hippocampal cannabinoid system was particularly important in some forms of learning, but direct evidence for this hypothesis is scarce. We therefore assessed the effects of the synthetic cannabinoid HU210 on memory and hippocampal activity.

EXPERIMENTAL APPROACH

HU210 (100 microg kg(-1)) was administered intraperitoneally to rats under three experimental conditions. One group of animals were pre-trained in spatial working memory using a delayed-matching-to-position task and effects of HU210 were assessed in a within-subject design. In another, rats were injected before acquisition learning of a spatial reference memory task with constant platform location. Finally, a separate group of animals was implanted with electrode bundles in CA1 and CA3 and single unit responses were isolated, before and after HU210 treatment.

KEY RESULTS

HU210 treatment had no effect on working or short-term memory. Relative to its control Tween 80, deficits in acquisition of a reference memory version of the water maze were obtained, along with drug-related effects on anxiety, motor activity and spatial learning. Deficits were not reversed by the CB(1) receptor antagonists SR141716A (3 mg kg(-1)) or AM281 (1.5 mg kg(-1)). Single unit recordings from principal neurons in hippocampal CA3 and CA1 confirmed HU210-induced attenuation of the overall firing activity lowering both the number of complex spikes fired and the occurrence of bursts.

CONCLUSIONS AND IMPLICATIONS

These data provide the first direct evidence that the underlying mechanism for the spatial memory deficits induced by HU210 in rats is the accompanying abnormality in hippocampal cell firing.

Anti-inflammatory property of the cannabinoid receptor-2-selective agonist JWH-133 in a rodent model of autoimmune uveoretinitis

Previous studies have shown that cannabinoids have anti-inflammatory and immune-modulating effects, but the precise mechanisms of action remain to be elucidated. In this study, we investigated the effect of JWH 133, a selective agonist for cannabinoid receptor 2, the main receptor expressed on immune cells, in a model of autoimmune disease, experimental autoimmune uveoretinitis (EAU). JWH 133 suppressed EAU in a dose-dependent manner (0.015-15 mg/kg), and the suppressive effect could be achieved in the disease-induction stage and the effector stage. Leukocytes from mice, which had been treated with JWH 133, had diminished responses to retinal peptide and mitogen Con A stimulation in vitro. In vivo JWH 133 treatment also abrogated leukocyte cytokine/chemokine production. Further in vitro studies indicated that JWH 133 down-regulated the TLR4 via Myd88 signal transduction, which may be responsible for its moderate, suppressive effect on antigen presentation. In vivo JWH 133 treatment (1 mg/kg) also suppressed leukocyte trafficking (rolling and infiltration) in inflamed retina as a result of an effect on reducing adhesion molecules CD162 (P-selectin glycoprotein ligand 1) and CD11a (LFA-1) expression on T cells. In conclusion, the cannabinoid agonist JWH 133 has a high in vivo, anti-inflammatory property and may exert its effect via inhibiting the activation and function of autoreactive T cells and preventing leukocyte trafficking into the inflamed tissue.

Interactions of cannabidiol with endocannabinoid signalling in hippocampal tissue

The phytocannabinoid cannabidiol (CBD) possesses no psychotropic activity amid potentially beneficial therapeutic applications. We here characterized interactions between CBD (1 microM) and the endocannabinoid system in cultured rat hippocampal cells. The CBD-induced Ca2+ rise observed in neurons and glia was markedly reduced in the presence of the endogenous cannabinoid anandamide in neurons, with no alteration seen in glia. Neuronal CBD responses were even more reduced in the presence of the more abundant endocannabinoid 2-arachidonyl glycerol, this action was maintained in the presence of the CB1 receptor antagonist AM281 (100 nM). Neuronal CBD responses were also reduced by pre-exposure to glutamate, expected to increase endocannabinoid levels by increasing in [Ca2+]i. Application of AM281 at 1 microM elevated CBD-induced Ca2+ responses in both cell types, further confirming our finding that endocannabinoid-mediated signalling is negatively coupled to the action of CBD. However, upregulation of endogenous levels of endocannabinoids via inhibition of endocannabinoid hydrolysis (with URB597 and MAFP) could not be achieved under resting conditions. Because delta9-tetrahydrocannabinol did not mimic the endocannabinoid actions, and pertussis toxin treatment had no effect on CBD responses, we propose that the effects of AM281 were mediated via a constitutively active signalling pathway independent of CB1 signalling. Instead, signalling via G(q/11) and phospholipase C appears to be negatively coupled to CBD-induced Ca2+ responses, as the inhibitor U73122 enhanced CBD responses. Our data highlight the interaction between exogenous and endogenous cannabinoid signalling, and provide evidence for the presence of an additional pharmacological target, sensitive to endocannabinoids and to AM281.

Direct suppression of CNS autoimmune inflammation via the cannabinoid receptor CB1 on neurons and CB2 on autoreactive T cells

The cannabinoid system is immunomodulatory and has been targeted as a treatment for the central nervous system (CNS) autoimmune disease multiple sclerosis. Using an animal model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE), we investigated the role of the CB(1) and CB(2) cannabinoid receptors in regulating CNS autoimmunity. We found that CB(1) receptor expression by neurons, but not T cells, was required for cannabinoid-mediated EAE suppression. In contrast, CB(2) receptor expression by encephalitogenic T cells was critical for controlling inflammation associated with EAE. CB(2)-deficient T cells in the CNS during EAE exhibited reduced levels of apoptosis, a higher rate of proliferation and increased production of inflammatory cytokines, resulting in severe clinical disease. Together, our results demonstrate that the cannabinoid system within the CNS plays a critical role in regulating autoimmune inflammation, with the CNS directly suppressing T-cell effector function via the CB(2) receptor.

Cannabidiol displays unexpectedly high potency as an antagonist of CB1 and CB2 receptor agonists in vitro

BACKGROUND AND PURPOSE

A nonpsychoactive constituent of the cannabis plant, cannabidiol has been demonstrated to have low affinity for both cannabinoid CB1 and CB2 receptors. We have shown previously that cannabidiol can enhance electrically evoked contractions of the mouse vas deferens, suggestive of inverse agonism. We have also shown that cannabidiol can antagonize cannabinoid receptor agonists in this tissue with a greater potency than we would expect from its poor affinity for cannabinoid receptors. This study aimed to investigate whether these properties of cannabidiol extend to CB1 receptors expressed in mouse brain and to human CB2 receptors that have been transfected into CHO cells.

EXPERIMENTAL APPROACH

The [35S]GTPS binding assay was used to determine both the efficacy of cannabidiol and the ability of cannabidiol to antagonize cannabinoid receptor agonists (CP55940 and R-(+)-WIN55212) at the mouse CB1 and the human CB2 receptor.

KEY RESULTS

This paper reports firstly that cannabidiol displays inverse agonism at the human CB2 receptor. Secondly, we demonstrate that cannabidiol is a high potency antagonist of cannabinoid receptor agonists in mouse brain and in membranes from CHO cells transfected with human CB2 receptors.

CONCLUSIONS AND IMPLICATIONS

This study has provided the first evidence that cannabidiol can display CB2 receptor inverse agonism, an action that appears to be responsible for its antagonism of CP55940 at the human CB2 receptor. The ability of cannabidiol to behave as a CB2 receptor inverse agonist may contribute to its documented anti-inflammatory properties.

The psychoactive plant cannabinoid, Delta9-tetrahydrocannabinol, is antagonized by Delta8- and Delta9-tetrahydrocannabivarin in mice in vivo

BACKGROUND AND PURPOSE

To follow up in vitro evidence that Delta(9)-tetrahydrocannabivarin extracted from cannabis (eDelta(9)-THCV) is a CB(1) receptor antagonist by establishing whether synthetic Delta(9)-tetrahydrocannabivarin (O-4394) and Delta(8)-tetrahydrocannabivarin (O-4395) behave as CB(1) antagonists in vivo.

EXPERIMENTAL APPROACH

O-4394 and O-4395 were compared with eDelta(9)-THCV as displacers of [(3)H]-CP55940 from specific CB(1) binding sites on mouse brain membranes and as antagonists of CP55940 in [(35)S]GTPgammaS binding assays performed with mouse brain membranes and of R-(+)-WIN55212 in mouse isolated vasa deferentia. Their ability to antagonize in vivo effects of 3 or 10 mg kg(-1) (i.v.) Delta(9)-tetrahydrocannabinol in mice was then investigated.

KEY RESULTS

O-4394 and O-4395 exhibited similar potencies to eDelta(9)-THCV as displacers of [(3)H]-CP55940 (K (i)=46.6 and 64.4 nM, respectively) and as antagonists of CP55940 in the [(35)S]GTPgammaS binding assay (apparent K (B)=82.1 and 125.9 nM, respectively) and R-(+)-WIN55212 in the vas deferens (apparent K (B)=4.8 and 3.9 nM respectively). At i.v. doses of 0.1, 0.3, 1.0 and/or 3 mg kg(-1) O-4394 and O-4395 attenuated Delta(9)-tetrahydrocannabinol-induced anti-nociception (tail-flick test) and hypothermia (rectal temperature). O-4395 but not O-4394 also antagonized Delta(9)-tetrahydrocannabinol-induced ring immobility. By themselves, O-4395 and O-4394 induced ring immobility at 3 or 10 mg kg(-1) (i.v.) and antinociception at doses above 10 mg kg(-1) (i.v.). O-4395 also induced hypothermia at 3 mg kg(-1) (i.v.) and above.

CONCLUSIONS AND IMPLICATIONS

O-4394 and O-4395 exhibit similar in vitro potencies to eDelta(9)-THCV as CB(1) receptor ligands and as antagonists of cannabinoid receptor agonists and can antagonize Delta(9)-tetrahydrocannabinol in vivo.

[Significance of cardiac cannabinoid receptors in regulation of cardiac rhythm, myocardial contractility, and electrophysiologic processes in heart]

Intravenous administration of cannabinoid (CB) receptor agonists (HU-210, 0.1 mg/kg; ACPA, 0.125 mg/kg; methanandamide, 2.5 mg/kg; and anandamide, 2.5 mg/kg) induced bradycardia in chloralose-anesthetized rats irrespective of the solubilization method. Methanandamide, HU-210, and ACPA had no effect on the electrophysiological activity in the heart, while anandamide increased the duration of the QRS complex. The negative chronotropic effect of HU-210 was due to CB1 receptor activation since is was not observed after CB1 receptor blockade by SR141716A (1 mg/kg intravenously) but was present after pretreatment with CB2 receptor antagonist SR144528 (1 mg/kg intravenously). CB receptor antagonists SR141716A and SR144528 had no effect on cardiac rhythm or ECG indices. Hence, in the intact heart, endogenous CB receptor agonists are not involved in the regulation of cardiac rhythm and electrophysiological processes. The chronotropic effect of CBs was independent of the autonomic nervous system because it remained significant after autonomic ganglion blockade by hexamethonium (1 mg/kg intravenously). CB receptor activation by HU-210 (0.1 and 1 microM) in vitro decreased the rate and force of isolated heart contractions, the rates of contraction and relaxation, and end diastolic pressure. The negative chronotropic effect of HU-210 was less pronounced in vitro than in vivo. The maximum inotropic effect of HU-210 was reached at the concentration of 0.1 microM.

Effect of sublingual application of cannabinoids on intraocular pressure: a pilot study

PURPOSE

The purpose of this study was to assess the effect on intraocular pressure (IOP) and the safety and tolerability of oromucosal administration of a low dose of delta-9-tetrahydrocannabinol (Delta-9-THC) and cannabidiol (CBD).

PATIENTS AND METHODS

A randomized, double-masked, placebo-controlled, 4 way crossover study was conducted at a single center, using cannabis-based medicinal extract of Delta-9-THC and CBD. Six patients with ocular hypertension or early primary open angle glaucoma received a single sublingual dose at 8 AM of 5 mg Delta-9-THC, 20 mg CBD, 40 mg CBD, or placebo. Main outcome measure was IOP. Secondary outcomes included visual acuity, vital signs, and psychotropic effects.

RESULTS

Two hours after sublingual administration of 5 mg Delta-9-THC, the IOP was significantly lower than after placebo (23.5 mm Hg vs. 27.3 mm Hg, P=0.026). The IOP returned to baseline level after the 4-hour IOP measurement. CBD administration did not reduce the IOP at any time. However, the higher dose of CBD (40 mg) produced a transient elevation of IOP at 4 hours after administration, from 23.2 to 25.9 mm Hg (P=0.028). Vital signs and visual acuity were not significantly changed. One patient experienced a transient and mild paniclike reaction after Delta-9-THC administration.

CONCLUSIONS

A single 5 mg sublingual dose of Delta-9-THC reduced the IOP temporarily and was well tolerated by most patients. Sublingual administration of 20 mg CBD did not reduce IOP, whereas 40 mg CBD produced a transient increase IOP rise.

Differential effects of cannabis extracts and pure plant cannabinoids on hippocampal neurones and glia

We have shown previously that the plant cannabinoid cannabidiol (CBD) elevates intracellular calcium levels in both cultured hippocampal neurones and glia. Here, we investigated whether the main psychotropic constituent of cannabis, Delta(9)-tetrahydrocannabinol (THC) alone or in combination with other cannabis constituents can cause similar responses, and whether THC affects the responses induced by CBD. Our experiments were performed with 1 microM pure THC (pTHC), with 1 microM pure CBD (pCBD), with a high-THC, low CBD cannabis extract (eTHC), with a high-CBD, low THC cannabis extract (eCBD), with a mixture of eTHC and eCBD (THC:CBD=1:1) or with corresponding 'mock extracts' that contained only pTHC and pCBD mixed in the same proportion as in eTHC, eCBD or the 1:1 mixture of eTHC and eCBD. We detected significant differences in neurones both between the effects of pTHC and eTHC and between the effects of pCBD and eCBD. There were also differences between the Ca(2+) responses evoked in both neurones and glia by eTHC and mock eTHC, but not between eCBD and mock eCBD. A particularly striking observation was the much increased response size and maximal responder rates induced by the mixture of eTHC and eCBD than by the corresponding 1:1 mixture of pTHC and pCBD. Our data suggest that THC shares the ability of CBD to elevate Ca(2+) levels in neurones and glia, that THC and CBD interact synergistically and that the cannabis extracts have other constituents yet to be identified that can significantly modulate the ability of THC and CBD to raise Ca(2+) levels.

The bioassay of cannabinoids using the mouse isolated vas deferens

The mouse isolated vas deferens is a nerve-smooth muscle preparation that serves as a highly sensitive and quantitative functional in vitro bioassay for cannabinoid CB1 receptor agonists. Additionally, it is commonly used as a bioassay for competitive surmountable CB1 receptor antagonists, and also provides a means for distinguishing neutral CB1 antagonists from CB1 inverse agonists. The bioassay of CB1 receptor agonists relies on the ability of these ligands to produce concentration-related decreases in the amplitude of electrically evoked contractions of the vas deferens. This they do by acting on naturally expressed prejunctional neuronal CB1 receptors to inhibit release of the contractile neurotransmitters, noradrenaline and ATP, that is provoked by the electrical stimulation. The bioassay of competitive surmountable CB1 receptor antagonists involves determining the ability of these compounds to produce parallel dextral shifts in CB1 receptor agonist log concentration-response curves in electrically stimulated tissues. The mouse vas deferens has also been used to measure the ability of anandamide to activate vanilloid (TRPV1) receptors, to monitor modulation by cannabinoids such as 6"-azido-2"-yne-cannabidiol and abnormal cannabidiol of contractions elicited in electrically unstimulated tissues by agonists for P2X purinoceptors or alpha1-adrenoceptors, and as a bioassay for the nonpsychoactive plant cannabinoid cannabidiol.

Scopolamine and MK801-induced working memory deficits in rats are not reversed by CBD-rich cannabis extracts

Smoking marijuana causes working and short-term memory deficits, an effect that is mediated by cannabinoid receptor (CB1) activation in the brain. While this may be due to the main psychoactive constituent Delta9-tetrahydrocannabinol (Delta9-THC), plant extracts also contain other cannabinoid and terpenoid compounds with unknown properties. Towards this end, we have recently shown that high concentrations of plant extracts rich in cannabidiol (CBD) can reverse working memory deficits induced by Delta9-THC which is a remaining contaminant of this extract [Fadda P, Robinson L, Fratta W, Pertwee RG, Riedel G. Differential effects of THC- and CBD-rich cannabis-extracts on working memory in rats. Neuropahrmacology 2004;47:1170-9]. Since this effect was dose-dependent and indicative of memory enhancing qualities of the CBD-rich extract, this prompted a wider investigation into the effects of CBD on other forms of amnesia in order to determine the mechanism of action and to reveal its potency against anticholinergic and antiglutamatergic agents. We employed a spatial delayed matching to position task in the open-field water maze. Both scopolamine (0.2 mg/kg i.p.) and dizocilpine (MK801: 0.1mg/kg i.p.) impaired working memory at delays of 30 s and 4 h. Two doses of CBD-rich extracts (5 and 10 mg/kg), which did not affect working memory when given alone, were unable to reverse these deficits when co-administered with scopolamine or MK801. These data suggest that reversal of working memory deficits by CBD-rich extracts are specific to the cannabinoid system and do not compensate for acutely induced cholinergic or glutamatergic receptor hypoactivity.

The pharmacology of cannabinoid receptors and their ligands: an overview

Mammalian tissues express at least two cannabinoid receptor types, CB1 and CB2, both G protein coupled. CB1 receptors are found predominantly at nerve terminals where they mediate inhibition of transmitter release. CB2 receptors occur mainly on immune cells, one of their roles being to modulate cytokine release. Endogenous agonists for cannabinoid receptors also exist, and are all eicosanoids. The first-discovered of these 'endocannabinoids' was arachidonoylethanolamide and there is convincing evidence that this ligand and some of its metabolites can activate vanilloid VRI (TRPV1) receptors. Certain cannabinoids also appear to have TRPV1-like and/or non-CB1, non-CB2, non-TRPV1 targets. Several CB1- and CB2-selective agonists and antagonists have been developed. Antagonists include the CB1-selective SR141716A, AM251, AM281 and LY320135, and the CB2-selective SR144528 and AM630. These all behave as inverse agonists, one indication that CB1 and CB2 receptors can exist in a constitutively active state. 'Neutral' cannabinoid receptor antagonists have also been developed. CB1 and/or CB2 receptor activation appears to ameliorate inflammatory and neuropathic pain and certain multiple sclerosis symptoms. This might be exploited clinically by using CB1, CB2 or CB1/CB2 agonists, or inhibitors of the membrane transport or catabolism of endocannabinoids that are released in increased amounts, at least in animal models of pain and multiple sclerosis. We have recently discovered the presence of an allosteric site on the CB1 receptor. Consequently, it may also prove possible to enhance 'autoprotective' effects of released endocannabinoids with CB1 allosteric enhancers or, indeed, to reduce proposed 'autoimpairing' effects of released endocannabinoids such as excessive food intake with CB1 allosteric antagonists.

Evidence that the plant cannabinoid Delta9-tetrahydrocannabivarin is a cannabinoid CB1 and CB2 receptor antagonist

Delta9-tetrahydrocannabivarin (THCV) displaced [(3)H]CP55940 from specific binding sites on mouse brain and CHO-hCB(2) cell membranes (K(i)=75.4 and 62.8 nM, respectively).THCV (1 microM) also antagonized CP55940-induced stimulation of [(35)S]GTPgammaS binding to these membranes (apparent K(B)=93.1 and 10.1 nM, respectively). In the mouse vas deferens, the ability of Delta9-tetrahydrocannabinol (THC) to inhibit electrically evoked contractions was antagonized by THCV, its apparent K(B)-value (96.7 nM) approximating the apparent K(B)-values for its antagonism of CP55940- and R-(+)-WIN55212-induced stimulation of [(35)S]GTPgammaS binding to mouse brain membranes. THCV also antagonized R-(+)-WIN55212, anandamide, methanandamide and CP55940 in the vas deferens, but with lower apparent K(B)-values (1.5, 1.2, 4.6 and 10.3 nM, respectively).THCV (100 nM) did not oppose clonidine, capsaicin or (-)-7-hydroxy-cannabidiol-dimethylheptyl-induced inhibition of electrically evoked contractions of the vas deferens. Contractile responses of the vas deferens to phenylephrine hydrochloride or beta,gamma-methylene-ATP were not reduced by 1microM THCV or R-(+)-WIN55212, suggesting that THCV interacts with R-(+)-WIN55212 at prejunctional sites. At 32 microM, THCV did reduce contractile responses to phenylephrine hydrochloride and beta,gamma-methylene-ATP, and above 3 microM it inhibited electrically evoked contractions of the vas deferens in an SR141716A-independent manner. In conclusion, THCV behaves as a competitive CB(1) and CB(2) receptor antagonist. In the vas deferens, it antagonized several cannabinoids more potently than THC and was also more potent against CP55940 and R-(+)-WIN55212 in this tissue than in brain membranes. The bases of these agonist- and tissue-dependent effects remain to be established.

Cannabinoid pharmacology: the first 66 years

Research into the pharmacology of individual cannabinoids that began in the 1940s, several decades after the presence of a cannabinoid was first detected in cannabis, is concisely reviewed. Also described is how this pharmacological research led to the discovery of cannabinoid CB(1) and CB(2) receptors and of endogenous ligands for these receptors, to the development of CB(1)- and CB(2)-selective agonists and antagonists and to the realization that the endogenous cannabinoid system has significant roles in both health and disease, and that drugs which mimic, augment or block the actions of endogenously released cannabinoids must have important therapeutic applications. Some goals for future research are identified.

Cannabidiol-induced intracellular Ca2+ elevations in hippocampal cells

The phytocannabinoid cannabidiol (CBD) is at the forefront of therapeutic cannabinoid research due to its non-psychotropic properties. Research supports its use in a variety of disorders, yet the cellular mechanisms of its action remain unclear. In this study, the effect of CBD upon Ca2+ homeostasis in hippocampal cells was characterised. CBD (1 microM) elevated intracellular Ca2+ ([Ca2+]i) by approximately +45% of basal Ca2+ levels in both glia (77% responders) and neurones (51% responders). Responses to CBD were reduced in high excitability HEPES buffered solution (HBS), but not affected in low excitability/low Ca2+ HBS. CBD responses were also significantly reduced (by 50%) by the universal Ca2+ channel blocker cadmium (50 microM) and the L-type specific Ca2+ channel blocker nifedipine (20 microM). Interestingly, intracellular store depletion with thapsigargin (2 microM) had the most dramatic effect on CBD responses, leading on average to a full block of the response. Elevated CBD-induced [Ca2+]i responses (>+100%) were observed in the presence of the CB1 receptor antagonist, AM281 (1 microM), and the vanilloid receptor antagonist, capsazepine (CPZ, 1 microM). Overall, our data suggest that CBD modulates hippocampal [Ca2+]i homeostasis via intracellular Ca2+ stores and L-type VGCC-mediated Ca2+ entry, with tonic cannabinoid and vanilloid receptor signalling being negatively coupled to this pathway.

Pharmacological actions of cannabinoids

Mammalian tissues express at least two types of cannabinoid receptor, CB1 and CB2, both G protein coupled. CB1 receptors are expressed predominantly at nerve terminals where they mediate inhibition of transmitter release. CB2 receptors are found mainly on immune cells, one of their roles being to modulate cytokine release. Endogenous ligands for these receptors (endocannabinoids) also exist. These are all eicosanoids; prominent examples include arachidonoylethanolamide (anandamide) and 2-arachidonoyl glycerol. These discoveries have led to the development of CB1- and CB2-selective agonists and antagonists and of bioassays for characterizing such ligands. Cannabinoid receptor antagonists include the CB1-selective SR141716A, AM251, AM281 and LY320135, and the CB2-selective SR144528 and AM630. These all behave as inverse agonists, one indication that CB1 and CB2 receptors can exist in a constitutively active state. Neutral cannabinoid receptor antagonists that seem to lack inverse agonist properties have recently also been developed. As well as acting on CB1 and CB2 receptors, there is convincing evidence that anandamide can activate transient receptor potential vanilloid type 1 (TRPV1) receptors. Certain cannabinoids also appear to have non-CB1, non-CB2, non-TRPV1 targets, for example CB2-like receptors that can mediate antinociception and "abnormal-cannabidiol" receptors that mediate vasorelaxation and promote microglial cell migration. There is evidence too for TRPV1-like receptors on glutamatergic neurons, for alpha2-adrenoceptor-like (imidazoline) receptors at sympathetic nerve terminals, for novel G protein-coupled receptors for R-(+)-WIN55212 and anandamide in the brain and spinal cord, for novel receptors for delta9-tetrahydrocannabinol and cannabinol on perivascular sensory nerves and for novel anandamide receptors in the gastro-intestinal tract. The presence of allosteric sites for cannabinoids on various ion channels and non-cannabinoid receptors has also been proposed. In addition, more information is beginning to emerge about the pharmacological actions of the non-psychoactive plant cannabinoid, cannabidiol. These recent advances in cannabinoid pharmacology are all discussed in this review.

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

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

Synthesis of long-chain amide analogs of the cannabinoid CB1 receptor antagonist N-(piperidinyl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716) with unique binding selectivities and pharmacological activities

An extended series of alkyl carboxamide analogs of N-(piperidinyl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl- 1H-pyrazole-3-carboxamide (SR141716; 5) was synthesized. Each compound was tested for its ability to displace the prototypical cannabinoid ligands ([3H]CP-55,940, [3H]2; [3H]SR141716, [3H]5; and [3H]WIN55212-2, [3H]3), and selected compounds were further characterized by determining their ability to affect guanosine 5'-triphosphate (GTP)-gamma-[35S] binding and their effects in the mouse vas deferens assay. This systematic evaluation has resulted in the discovery of novel compounds with unique binding properties at the central cannabinoid receptor (CB1) and distinctive pharmacological activities in CB1 receptor tissue preparations. Specifically, compounds with nanomolar affinity which are able to fully displace [3H]5 and [3H]2, but unable to displace [3H]3 at similar concentrations, have been synthesized. This selectivity in ligand displacement is unprecedented, in that previously, compounds in every structural class of cannabinoid ligands had always been shown to displace each of these radioligands in a competitive fashion. Furthermore, the selectivity of these compounds appears to impart unique pharmacological properties when tested in a mouse vas deferens assay for CB1 receptor antagonism.

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

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