Comparison of novel cannabinoid partial agonists and SR141716A in the guinea-pig small intestine

A Cannabinoid-1 Receptor Antagonist MJ08 with Different Effects in Stomach and Small Intestine

Background: To investigate the inverse agonistic effect of a novel type 1 cannabinoid (CB₁) receptor antagonist, MJ08, on the gastrointestinal tract (GIT). Methods: In vivo, carbon propulsion within the stomach of mice was undertaken to investigate the effects of MJ08. In vitro, the effects of MJ08 were investigated on the contraction of smooth muscle on the isolated gastric fundus, gastric body, duodenum, jejunum, and ileum. Results: Western blotting results showed that MJ08 (0.62 mg/kg body weight) reversed WIN55,212-2 (1.0 mg/kg)-induced reduction of carbon transit. MJ08 (1.25, 2.5 mg/kg) stimulated carbon transit dose dependently, demonstrating an inverse agonistic effect. In vitro experiments showed that the expression of MJ08 increased the contraction of small intestine, and that its inverse agonistic effect was significantly stronger than that of SR141716A, but no effect was noted on the gastric body. Western blotting showed that the MJ08 increased the expression of CB₁ receptor in different GIT segments. Conclusion: MJ08 is not only an antagonist but also an inverse agonist of the CB₁ receptor. MJ08 and SR141716A can enhance motility in the small intestine and increase the expression of CB₁ receptor in the small intestine.

Development of Oxygen-Bridged Pyrazole-Based Structures as Cannabinoid Receptor 1 Ligands

In this work, the synthesis of the cannabinoid receptor 1 neutral antagonists 8-chloro-1-(2,4-dichlorophenyl)-N-piperidin-1-yl-4,5-dihydrobenzo-1H-6-oxa-cyclohepta[1,2-c]pyrazole-3-carboxamide 1a and its deaza N-cyclohexyl analogue 1b has led to a deepening of the structure-activity studies of this class of compounds. A series of novel 4,5-dihydrobenzo-oxa-cycloheptapyrazoles analogues of 1a,b, derivatives 1c–j, was synthesized, and their affinity towards cannabinoid receptors was determined. Representative terms were evaluated using in vitro tests and isolated organ assays. Among the derivatives, 1d and 1e resulted in the most potent CB₁ receptor ligands (K_iCB₁ = 35 nM and 21.70 nM, respectively). Interestingly, both in vitro tests and isolated organ assays evidenced CB₁ antagonist activity for the majority of the new compounds, excluding compound 1e, which showed a CB₁ partial agonist behaviour. CB₁ antagonist activity of 1b was further confirmed by a mouse gastrointestinal transit assay. Significant activity of the new CB₁ antagonists towards food intake was showed by preliminary acute assays, evidencing the potentiality of these new derivatives in the treatment of obesity.

The Structure-Function Relationships of Classical Cannabinoids: CB1/CB2 Modulation

The cannabinoids are members of a deceptively simple class of terpenophenolic secondary metabolites isolated from Cannabis sativa highlighted by (-)-Δ(9)-tetrahydrocannabinol (THC), eliciting distinct pharmacological effects mediated largely by cannabinoid receptor (CB1 or CB2) signaling. Since the initial discovery of THC and related cannabinoids, synthetic and semisynthetic classical cannabinoid analogs have been evaluated to help define receptor binding modes and structure-CB1/CB2 functional activity relationships. This perspective will examine the classical cannabinoids, with particular emphasis on the structure-activity relationship of five regions: C3 side chain, phenolic hydroxyl, aromatic A-ring, pyran B-ring, and cyclohexenyl C-ring. Cumulative structure-activity relationship studies to date have helped define the critical structural elements required for potency and selectivity toward CB1 and CB2 and, more importantly, ushered the discovery and development of contemporary nonclassical cannabinoid modulators with enhanced physicochemical and pharmacological profiles.

Endocannabinoids in the Gut

Cannabis has been used medicinally for centuries to treat a variety of disorders, including those associated with the gastrointestinal tract. The discovery of our bodies' own "cannabis-like molecules" and associated receptors and metabolic machinery - collectively called the endocannabinoid system - enabled investigations into the physiological relevance for the system, and provided the field with evidence of a critical function for this endogenous signaling pathway in health and disease. Recent investigations yield insight into a significant participation for the endocannabinoid system in the normal physiology of gastrointestinal function, and its possible dysfunction in gastrointestinal pathology. Many gaps, however, remain in our understanding of the precise neural and molecular mechanisms across tissue departments that are under the regulatory control of the endocannabinoid system. This review highlights research that reveals an important - and at times surprising - role for the endocannabinoid system in the control of a variety of gastrointestinal functions, including motility, gut-brain mediated fat intake and hunger signaling, inflammation and gut permeability, and dynamic interactions with gut microbiota.

Rimonabant, gastrointestinal motility and obesity

Background:

Obesity and overweight affect more than half of the US population and are associated with a number of diseases. Rimonabant, a cannabinoid receptor 1 blocker in the endocannabinoid (EC) system, was indicated in Europe for the treatment of obesity and overweight patients with associated risk factors but withdrawn on Jan, 2009 because of side effects. Many studies have reported the effects of rimonabant on gastrointestinal (GI) motility and food intake.

The aims of this review are:

to review the relationship of EC system with GI motility and food intake;

to review the studies of rimonabant on GI motility, food intake and obesity;

and to report the tolerance and side effects of rimonabant.

Methods:

the literature (Pubmed database) was searched using keywords: rimonabant, obesity and GI motility.

Results:

GI motility is related with appetite, food intake and nutrients absorption. The EC system inhibits GI motility, reduces emesis and increases food intake; Rimonabant accelerates gastric emptying and intestinal transition but decreases energy metabolism and food intake. There is rapid onset of tolerance to the prokinetic effect of rimonabant. The main side effects of rimonabant are depression and GI symptoms.

Conclusions:

Rimonabant has significant effects on energy metabolism and food intake, probably mediated via its effects on GI motility.

Cannabinoid receptors: nomenclature and pharmacological principles

The CB1 and CB2 cannabinoid receptors are members of the G protein-coupled receptor (GPCR) family that are pharmacologically well defined. However, the discovery of additional sites of action for endocannabinoids as well as synthetic cannabinoid compounds suggests the existence of additional cannabinoid receptors. Here we review this evidence, as well as the current nomenclature for classifying a target as a cannabinoid receptor. Basic pharmacological definitions, principles and experimental conditions are discussed in order to place in context the mechanisms underlying cannabinoid receptor activation. Constitutive (agonist-independent) activity is observed with the overexpression of many GPCRs, including cannabinoid receptors. Allosteric modulators can alter the pharmacological responses of cannabinoid receptors. The complex molecular architecture of each of the cannabinoid receptors allows for a single receptor to recognize multiple classes of compounds and produce an array of distinct downstream effects. Natural polymorphisms and alternative splice variants may also contribute to their pharmacological diversity. As our knowledge of the distinct differences grows, we may be able to target select receptor conformations and their corresponding pharmacological responses. Importantly, the basic biology of the endocannabinoid system will continue to be revealed by ongoing investigations.

Cyclooxygenase-2 prostaglandins mediate anandamide-inhibitory action on nitric oxide synthase activity in the receptive rat uterus

Anandamide, an endocannabinoid, prostaglandins derived from cyclooxygenase-2 and nitric oxide synthesized by nitric oxide synthase (NOS), are relevant mediators of embryo implantation. We adopted a pharmacological approach to investigate if anandamide modulated NOS activity in the receptive rat uterus and if prostaglandins mediated this effect. As we were interested in studying the changes that occur at the maternal side of the fetal-maternal interface, we worked with uteri obtained from pseudopregnant rats. Females were sacrificed on day 5 of pseudopregnancy, the day in which implantation would occur, and the uterus was obtained. Anandamide (2 ng/kg, i.p.) inhibited NOS activity (P<0.001) and increased the levels of prostaglandin E(2) (P<0.001) and prostaglandin F(2α) (P<0.01). These effects were mediated via cannabinoid receptor type 2, as the pre-treatment with SR144528 (10 mg/kg, i.p.), a selective cannabinoid receptor type 2 antagonist, completely reverted anandamide effect on NOS activity and prostaglandin levels. The pre-treatment with a non-selective cyclooxygenase inhibitor (indomethacin 2.5mg/kg, i.p.) or with selective cyclooxygenase-2 inhibitors (meloxicam 4 mg/kg, celecoxib 3mg/kg, i.p.) reverted anandamide inhibition on NOS, suggesting that prostaglandins are derived from cyclooxygenase-2 mediated anandamide effect. Thus, anandamide levels seemed to modulate NOS activity, fundamental for implantation, via cannabinoid receptor type 2 receptors, in the receptive uterus. This modulation depends on the production of cyclooxygenase-2 derivatives. These data establish cannabinoid receptors and cyclooxygenase enzymes as an interesting target for the treatment of implantation deficiencies.

The effect of anandamide on uterine nitric oxide synthase activity depends on the presence of the blastocyst

Nitric oxide production, catalyzed by nitric oxide synthase (NOS), should be strictly regulated to allow embryo implantation. Thus, our first aim was to study NOS activity during peri-implantation in the rat uterus. Day 6 inter-implantation sites showed lower NOS activity (0.19±0.01 pmoles L-citrulline mg prot⁻¹ h⁻¹) compared to days 4 (0.34±0.03) and 5 (0.35±0.02) of pregnancy and to day 6 implantation sites (0.33±0.01). This regulation was not observed in pseudopregnancy. Both dormant and active blastocysts maintained NOS activity at similar levels. Anandamide (AEA), an endocannabinoid, binds to cannabinoid receptors type 1 (CB1) and type 2 (CB2), and high concentrations are toxic for implantation and embryo development. Previously, we observed that AEA synthesis presents an inverted pattern compared to NOS activity described here. We adopted a pharmacological approach using AEA, URB-597 (a selective inhibitor of fatty acid amide hydrolase, the enzyme that degrades AEA) and receptor selective antagonists to investigate the effect of AEA on uterine NOS activity in vitro in rat models of implantation. While AEA (0.70±0.02 vs 0.40±0.04) and URB-597 (1.08±0.09 vs 0.83±0.06) inhibited NOS activity in the absence of a blastocyst (pseudopregnancy) through CB2 receptors, AEA did not modulate NOS on day 5 pregnant uterus. Once implantation begins, URB-597 decreased NOS activity on day 6 implantation sites via CB1 receptors (0.25±0.04 vs 0.40±0.05). While a CB1 antagonist augmented NOS activity on day 6 inter-implantation sites (0.17±0.02 vs 0.27±0.02), a CB2 antagonist decreased it (0.17±0.02 vs 0.12±0.01). Finally, we described the expression and localization of cannabinoid receptors during implantation. In conclusion, AEA levels close to and at implantation sites seems to modulate NOS activity and thus nitric oxide production, fundamental for implantation, via cannabinoid receptors. This modulation depends on the presence of the blastocyst. These data establish cannabinoid receptors as an interesting target for the treatment of implantation deficiencies.

Structural and pharmacological analysis of O-2050, a putative neutral cannabinoid CB(1) receptor antagonist

Rimonabant, the prototypic antagonist of cannabinoid CB(1) receptors, has been reported to have inverse agonist properties at higher concentrations, which may complicate its use as a tool for mechanistic evaluation of cannabinoid pharmacology. Consequently, recent synthesis efforts have concentrated on discovery of a neutral antagonist using a variety of structural templates. The purpose of this study was to evaluate the pharmacological properties of the putative neutral cannabinoid CB(1) receptor antagonist O-2050, a sulfonamide side chain analog of Δ(8)-tetrahydrocannabinol. O-2050 and related sulfonamide cannabinoids exhibited good affinity for both cannabinoid CB(1) and CB(2) receptors. While the other sulfonamide analogs produced cannabinoid agonist effects in vivo (e.g., activity suppression, antinociception, and hypothermia), O-2050 stimulated activity and was inactive in the other two tests. O-2050 also decreased food intake in mice, an effect that was reminiscent of that produced by rimonabant. Unlike rimonabant, however, O-2050 did not block the effects of cannabinoid agonists in vivo, even when administered i.c.v. In contrast, O-2050 antagonized the in vitro effects of cannabinoid agonists in [(35)S]GTPγS and mouse vas deferens assays without having activity on its own in either assay. Further evaluation revealed that O-2050 fully and dose-dependently substituted for Δ(9)-tetrahydrocannabinol in a mouse drug discrimination procedure (a cannabinoid agonist effect) and that it inhibited forskolin-stimulated cyclic AMP signaling with a maximum efficacy of approximately half that of the full agonist CP55,940 [(-)-cis-3-[2-hydroxy-4(1,1-dimethyl-heptyl)phenyl]-trans-4-(3-hydroxy-propyl)cyclohexanol]. Together, these results suggest that O-2050 is not a viable candidate for classification as a neutral cannabinoid CB(1) receptor antagonist.

Molecular biology of cannabinoid receptors

To date, two cannabinoid receptors have been isolated by molecular cloning. The CB1 and CB2 cannabinoid receptors are members of the G protein-coupled receptor family. There is also evidence for additional cannabinoid receptor subtypes. The CB1 and CB2 receptors recognize endogenous and exogenous cannabinoid compounds, which fall into five structurally diverse classes. Mutagenesis and molecular modeling studies have identified several key amino acid residues involved in the selective recognition of these ligands. Numerous residues involved in receptor activation have been elucidated. Regions of the CB1 receptor mediating desensitization and internalization have also been discovered. The known genetic structures of the CB1 and CB2 receptors indicate polymorphisms and multiple exons that maybe involved in tissue and species-specific regulation of these genes. The cannabinoid receptors are regulated during chronic agonist exposure, and gene expression is altered in disease states. There is a complex molecular architecture of the cannabinoid receptors that allows a single receptor to recognize multiple classes of compounds and produce an array of distinct downstream effects.

Cannabinoids and the digestive tract

In the digestive tract there is evidence for the presence of high levels of endocannabinoids (anandamide and 2-arachidonoylglycerol) and enzymes involved in the synthesis and metabolism of endocannabinoids. Immunohistochemical studies have shown the presence of CB1 receptors on myenteric and submucosal nerve plexuses along the alimentary tract. Pharmacological studies have shown that activation of CB1 receptors produces relaxation of the lower oesophageal sphincter, inhibition of gastric motility and acid secretion, as well as intestinal motility and secretion. In general, CB1-induced inhibition of intestinal motility and secretion is due to reduced acetylcholine release from enteric nerves. Conversely, endocannabinoids stimulate intestinal primary sensory neurons via the vanilloid VR1 receptor, resulting in enteritis and enhanced motility. The endogenous cannabinoid system has been found to be involved in the physiological control of colonic motility and in some pathophysiological states, including paralytic ileus, intestinal inflammation and cholera toxin-induced diarrhoea. Cannabinoids also possess antiemetic effects mediated by activation of central and peripheral CB1 receptors. Pharmacological modulation of the endogenous cannabinoid system could provide a new therapeutic target for the treatment of a number of gastrointestinal diseases, including nausea and vomiting, gastric ulcers, secretory diarrhoea, paralytic ileus, inflammatory bowel disease, colon cancer and gastro-oesophageal reflux conditions.

Inverse agonism and neutral antagonism at cannabinoid CB1 receptors

There are at least two types of cannabinoid receptor, CB1 and CB2, both G protein coupled. CB1 receptors are expressed predominantly at nerve terminals and mediate inhibition of transmitter release whereas CB2 receptors are found mainly on immune cells, one of their roles being to modulate cytokine release. Endogenous cannabinoid receptor agonists also exist and these "endocannabinoids" together with their receptors constitute the "endocannabinoid system". These discoveries were followed by the development of a number of CB1- and CB2-selective antagonists that in some CB1 or CB2 receptor-containing systems also produce "inverse cannabimimetic effects", effects opposite in direction from those produced by cannabinoid receptor agonists. This review focuses on the CB1-selective antagonists, SR141716A, AM251, AM281 and LY320135, and discusses possible mechanisms by which these ligands produce their inverse effects: (1) competitive surmountable antagonism at CB1 receptors of endogenously released endocannabinoids, (2) inverse agonism resulting from negative, possibly allosteric, modulation of the constitutive activity of CB1 receptors in which CB1 receptors are shifted from a constitutively active "on" state to one or more constitutively inactive "off" states and (3) CB1 receptor-independent mechanisms, for example antagonism of endogenously released adenosine at A1 receptors. Recently developed neutral competitive CB1 receptor antagonists, which are expected to produce inverse effects through antagonism of endogenously released endocannabinoids but not by modulating CB1 receptor constitutive activity, are also discussed. So too are possible clinical consequences of the production of inverse cannabimimetic effects, there being convincing evidence that released endocannabinoids can have "autoprotective" roles.

Rapid tolerance to the intestinal prokinetic effect of cannabinoid CB1 receptor antagonist, SR 141716 (Rimonabant)

The cannabinoid CB(1) receptor antagonist, SR 141716 (Rimonabant), has been reported to stimulate, when acutely administered, intestinal motility in mice. The present study was aimed at determining whether tolerance develops to its repeated administration. Mice were treated twice a day for up to 8 consecutive days with 0, 3 and 5.6 mg/kg SR 141716 (i.p.). On days 1, 3, 5 and 8, separate groups of mice were treated intragastrically with a non-absorbable colored marker (carmine). The distance traveled by the head of the marker in the small intestine was recorded. On day 1, SR 141716 markedly activated intestinal peristalsis, but complete tolerance to this effect developed within the third day of treatment. The results may have some relevance to the proposed future clinical use of SR 141716.

Cannabinoid pharmacology in the cardiovascular system: potential protective mechanisms through lipid signalling

Cannabinoids include not only plant-derived compounds (of which delta9-tetrahydrocannabinol is the primary psychoactive ingredient of cannabis), but also synthetic agents and endogenous substances termed endocannabinoids which include anandamide (2-arachidonoylethanolamide) and 2-arachidonoylglycerol. Cannabinoids act on specific, G-protein-coupled, receptors which are currently divided into two types, CB1 and CB2. Relatively selective agonists and antagonists for these receptors have been developed, although one agent (SR141716A) widely used as an antagonist at CB1 receptors has non-cannabinoid receptor-mediated effects at concentrations which are often used to define the presence of the CB1 receptor. Both cannabinoid receptors are primarily coupled to Gi/o proteins and act to inhibit adenylyl cyclase. Stimulation of CB1 receptors also modulates the activity of K+ and Ca2+ channels and of protein kinase pathways including protein kinase B (Akt) which might mediate effects on apoptosis. CB, receptors may activate the extracellular signal-regulated kinase cascade through ceramide signalling. Cannabinoid actions on the cardiovascular system have been widely interpreted as being mediated by CB1 receptors although there are a growing number of observations, particularly in isolated heart and blood vessel preparations, that suggest that other cannabinoid receptors may exist. Interestingly, the currently identified cannabinoid receptors appear to be related to a wider family of lipid receptor, those for the lysophospholipids, which are also linked to Gi/o protein signalling. Anandamide also activates vanilloid VR1 receptors on sensory nerves and releases the vasoactive peptide, calcitonin gene-related peptide (CGRP), which brings about vasodilatation through its action on CGRP receptors. Current evidence suggests that endocannabinoids have important protective roles in pathophysiological conditions such as shock and myocardial infarction. Therefore, their cardiovascular effects and the receptors mediating them are the subject of increasing investigative interest.

Cannabinoids and the human uterus during pregnancy

OBJECTIVE

The aim of this study was to investigate the expression of cannabinoid receptors in human uterine smooth muscle during pregnancy and to evaluate the effects of endogenous and exogenous cannabinoids on myometrial contractility in vitro.

STUDY DESIGN

Human myometrial biopsy specimens were obtained at elective cesarean delivery and snap frozen or mounted for isometric recording under physiologic conditions. Cumulative doses of the endogenous cannabinoid anandamide or the exogenous cannabinoid Delta(9) (indicates a double bond between carbons 9 and 10) tetrahydrocannabinol were added in the range 1 nmol/L to 100 micromol/L. Selectivity of the cannabinoid receptor agonists was investigated with specific antagonists for the CB(1) and the CB(2) receptors. Reverse transcription-polymerase chain reaction with primers for the CB(1) and CB(2) receptors was performed on messenger RNA that was isolated from human pregnant myometrium.

RESULTS

Both anandamide and Delta(9)-tetrahydrocannabinol exerted a direct relaxant effect on human pregnant myometrium in vitro, which was of equal potency for both compounds. This relaxant effect was antagonized by the specific CB(1) receptor antagonist, SR 141716, but not by the specific CB(2) receptor antagonist, SR 144528 (n=6 specimens, P<.01). Both the CB(1) and CB(2) receptors are expressed in human myometrium.

CONCLUSIONS

Both endogenous and exogenous cannabinoids exert a potent and direct relaxant effect on human pregnant myometrium, which is mediated through the CB(1) receptor. This highlights a possible role for endogenous cannabinoids during human parturition and pregnancy. These results also support the view that the use of exogenous cannabinoids during pregnancy is not linked independently with preterm labor.

Modulation of electrically evoked acetylcholine release through cannabinoid CB1 receptors: evidence for an endocannabinoid tone in the human neocortex

Cannabinoids are known to inhibit neurotransmitter release in the CNS through CB1 receptors. The present study compares the effects of synthetic cannabinoids on acetylcholine (ACh) release in human and mice neocortex. We further investigated a possible endocannabinoid tone on CB1 receptors in human neocortex caused by endogenous agonists like anandamide or 2-arachidonylglycerol. Brain slices, incubated with [3H]-choline, were superfused and stimulated electrically under autoinhibition-free conditions to evoke tritium overflow assumed to represent ACh release. The first series of experiments was performed with 26 pulses, 60 mA, at 0.1 Hz. In mice neocortical slices, the cannabinoid receptor agonist WIN55212-2 decreased ACh release (pIC50=6.68, I(max)=67%). In the human neocortex the concentration-response curve of WIN55212-2 was bell-shaped and flat (I(max observed) approximately 30%). The estimated maximum possible inhibition, however, was much larger: I(max derived)=79%. Lec, the negative logarithm (lg) of the biophase concentration of endocannabinoids in 'WIN55212-2 units,' was -6.52, the pKd of WIN55212-2 was 7.47. The CB1 receptor antagonist/inverse agonist SR141716 enhanced ACh release in the human neocortex (by 38%) and prevented the inhibitory effect of WIN55212-2. The concentration-response curve of WIN55212-2 was changed in its shape including a shift to the right due to the presence of SR141716. A pA2 of this antagonist between 11.60 and 11.18 was obtained. SR141716 alone had no effect in mice neocortical slices. A partial agonist without inverse agonistic activity, O-1184, enhanced ACh release in the human neocortex. The endocannabinoid uptake-inhibitor AM404 decreased ACh release in human, but not in mice, neocortical slices. Change of the stimulation parameters (eight trains of pseudo-one-pulse bursts (4 pulses, 76 mA, 100 Hz), spaced by 45 s intervals) led to a stronger inhibitory effect of WIN55212-2, and abolished the disinhibitory effect of SR141716 and O-1184. The results show that activation of CB1 cannabinoid receptors leads to inhibition of ACh release in the human and mouse neocortex. The endocannabinoid tone is high in the human, but not in the mouse neocortex and is dependent on neuronal activity. SR141716 acts as a competitive CB1 receptor antagonist.

Hypersensitization of the Orexin 1 receptor by the CB1 receptor: evidence for cross-talk blocked by the specific CB1 antagonist, SR141716

In the present study, we observed evidence of cross-talk between the cannabinoid receptor CB1 and the orexin 1 receptor (OX1R) using a heterologous system. When the two receptors are co-expressed, we observed a major CB1-dependent enhancement of the orexin A potency to activate the mitogen-activated protein kinase pathway; dose-responses curves indicated a 100-fold increase in the potency of orexin-mediated mitogen-activated protein kinase activation. This effect required a functional CB1 receptor as evidenced by the blockade of the orexin response by the specific CB1 antagonist, N-(piperidino-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-pyrazole-3-carboxamide (SR141716), but also by pertussis toxin, suggesting that this potentiation is Gi-mediated. In contrast to OX1R, the potency of direct activation of CB1 was not affected by co-expression with OX1R. In addition, electron microscopy experiments revealed that CB1 and OX1R are closely apposed at the plasma membrane level; they are close enough to form hetero-oligomers. Altogether, for the first time our data provide evidence that CB1 is able to potentiate an orexigenic receptor. Considering the antiobesity effect of SR141716, these results open new avenues to understand the mechanism by which the molecule may prevent weight gain through functional interaction between CB1 and other receptors involved in the control of appetite.

Cannabinoid modulation of sensory neurotransmission via cannabinoid and vanilloid receptors: roles in regulation of cardiovascular function

Capsaicin-sensitive sensory nerves are widely distributed in the cardiovascular system. They are activated by a variety of physical and chemical stimuli, characteristically by capsaicin acting via the vanilloid receptor VR1, and have a role in the regulation of peripheral vascular resistance and maintenance of homeostasis via their afferent and efferent functions. Cannabinoids, a recently discovered family of extracellular signalling molecules, can act at cannabinoid (CB) receptors expressed on sensory nerves, to cause inhibition of sensory neurotransmitter release. There is recent evidence, however, that anandamide, an endogenous cannabinoid, can activate VR1, coexpressed with CB receptors on the same sensory nerve terminals, causing a release of sensory neurotransmitter, vasorelaxation and hypotension. Hence, anandamide can elicit opposite actions, inhibition via CB receptors and excitation via VR1, on sensory neurotransmission. The possible biological significance of this is discussed.

Cannabinoid receptor agonism inhibits transient lower esophageal sphincter relaxations and reflux in dogs

BACKGROUND & AIMS

Transient lower esophageal sphincter relaxations (TLESRs) are the major cause of gastroesophageal acid reflux, and are triggered by postprandial gastric distention. Stimulation of GABA(B) receptors potently inhibits triggering of TLESR by gastric loads. The functional similarity between GABA(B) and cannabinoid receptors (CBRs) prompted us to study the role of CBRs on mechanisms of gastric distention-induced TLESRs.

METHODS

Gastric nutrient infusion and air insufflation was performed during gastroesophageal manometry in conscious dogs. The effects of the CBR agonist WIN 55,212-2 were assessed alone and in combination with the CBR1 antagonist SR141716A or the CBR2 antagonist SR144528. The effects of WIN 55,212-2 were also studied on firing of gastric vagal mechanosensitive afferents in an isolated preparation of ferret stomach.

RESULTS

WIN 55,212-2 (57 nmol/kg) inhibited the occurrence of TLESR after gastric loads by 80% (P < 0.01). The latency to the first TLESR after the load was prolonged (P < 0.001), and the occurrence of swallowing was reduced (P < 0.05). The CBR1 antagonist SR141716A reversed the effects of WIN 55,212-2, whereas the CBR2 antagonist SR144528 did not. The CBR1 antagonist alone increased occurrence of TLESR (P < 0.05). The responses of gastric vagal mechanoreceptors to distention were unaffected by WIN 55,212-2 at a concentration of 3 micromol/L.

CONCLUSIONS

Exogenous and endogenous activation of the CBR1 receptor inhibits TLESRs. The effects of CBR1 are not mediated peripherally on gastric vagal afferents, and therefore are most likely in the brain stem.

Cannabinoids and multiple sclerosis

There is a growing amount of evidence to suggest that cannabis and individual cannabinoids may be effective in suppressing certain symptoms of multiple sclerosis and spinal cord injury, including spasticity and pain. Anecdotal evidence is to be found in newspaper reports and also in responses to questionnaires. Clinical evidence comes from trials, albeit with rather small numbers of patients. These trials have shown that cannabis, Delta(9)-tetrahydrocannabinol, and nabilone can produce objective and/or subjective relief from spasticity, pain, tremor, and nocturia in patients with multiple sclerosis (8 trials) or spinal cord injury (1 trial). The clinical evidence is supported by results from experiments with animal models of multiple sclerosis. Some of these experiments, performed with mice with chronic relapsing experimental allergic encephalomyelitis (CREAE), have provided strong evidence that cannabinoid-induced reductions in tremor and spasticity are mediated by cannabinoid receptors, both CB(1) and CB(2). Endocannabinoid concentrations are elevated in the brains and spinal cords of CREAE mice with spasticity, and in line with this observation, spasticity exhibited by CREAE mice can be ameliorated by inhibitors of endocannabinoid membrane transport or enzymic hydrolysis. Research is now needed to establish whether increased endocannabinoid production occurs in multiple sclerosis. Future research should also be directed at obtaining more conclusive evidence about the efficacy of cannabis or individual cannabinoids against the signs and symptoms of these disorders, at devising better modes of administration for cannabinoids and at exploring strategies that maximize separation between the sought-after therapeutic effects and the unwanted effects of these drugs.

Cannabinoids inhibit pre- and postjunctionally sympathetic neurotransmission in rat mesenteric arteries

The effects of cannabinoids on sympathetic neurotransmission in the rat isolated perfused mesenteric arterial bed, were investigated. Electrically evoked sympathetic neurogenic vasocontraction was inhibited by the cannabinoid receptor agonists 11-hydroxy-dimethylheptyl-Delta(8)-tetrahydrocannabinol (HU210), (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]trans-4-(3-hydroxypropyl)-cyclohexanol (CP55,940) and methanandamide, and by (+)-11-hydroxy-Delta(8)-tetrahydrocannabinol (HU211), a (+)-stereoisomer of HU210. The inhibition was unaffected by cannabinoid CB(1) and CB(2) receptor antagonists. Electrically evoked release of endogenous noradrenaline from sympathetic nerves was inhibited by HU210, but not by HU211. Inhibition was blocked by a cannabinoid CB(1), but not a CB(2), receptor antagonist. HU210 attenuated contractions to noradrenaline, and all of the cannabinoids blocked contractions to KCl. Capsaicin pre-treatment had no significant effect on HU210- and CP55,940-mediated inhibition of sympathetic neurogenic contraction, but partly blocked inhibition mediated by methanandamide. These data show that cannabinoids can inhibit, by distinct pre- and postjunctional actions, sympathetic neurotransmission in the rat mesenteric arterial bed. The pre-junctional action is mediated by a cannabinoid CB(1)-like receptor, but the postjunctional action does not appear to involve either cannabinoid CB(1) or CB(2) receptors.

Cannabinoid receptor antagonism and inverse agonism in response to SR141716A on cAMP production in human and rat brain

The effects of cannabinoid drugs on cAMP production were examined in mammalian brain. The cannabinoid receptor agonist (R)-(+)-[2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrrolo[1,2,3,-d,e-1,4-benzoxazin-6-yl]-(1-naphthalenyl) methanone (WIN55,212-2) decreased forskolin-induced cAMP accumulation in a concentration-dependent manner (10(-8)-10(-5) M) in membranes from several rat and human brain regions, this effect being antagonized by 10(-5) M N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A). Furthermore, high micromolar concentrations of SR141716A evoked a dose-dependent increase in basal cAMP in rat cerebellum and cortex, as well as in human frontal cortex. This effect was antagonized by WIN55,212-2 and abolished by N-ethylmaleimide, consistent with the involvement of cannabinoid CB(1) receptors through the activation of G(i/o) proteins. These results suggest a ligand-independent activity for cannabinoid CB(1) receptor signaling cascade in mammalian brain.

Cannabinoid receptors and their ligands

There are at least two types of cannabinoid receptors, CB(1) and CB(2), both coupled to G proteins. CB(1) receptors exist primarily on central and peripheral neurons, one of their functions being to modulate neurotransmitter release. CB(2) receptors are present mainly on immune cells. Their roles are proving more difficult to establish but seem to include the modulation of cytokine release. Endogenous agonists for cannabinoid receptors (endocannabinoids) have also been discovered, the most important being arachidonoyl ethanolamide (anandamide), 2-arachidonoyl glycerol and 2-arachidonyl glyceryl ether. Other endocannabinoids and cannabinoid receptor types may also exist. Although anandamide can act through CB(1) and CB(2) receptors, it is also a vanilloid receptor agonist and some of its metabolites may possess yet other important modes of action. The discovery of the system of cannabinoid receptors and endocannabinoids that constitutes the "endocannabinoid system" has prompted the development of CB(1)- and CB(2)-selective agonists and antagonists/inverse agonists. CB(1)/CB(2) agonists are already used clinically, as anti-emetics or to stimulate appetite. Potential therapeutic uses of cannabinoid receptor agonists include the management of multiple sclerosis/spinal cord injury, pain, inflammatory disorders, glaucoma, bronchial asthma, vasodilation that accompanies advanced cirrhosis, and cancer. Following their release onto cannabinoid receptors, endocannabinoids are removed from the extracellular space by membrane transport and then degraded by intracellular enzymic hydrolysis. Inhibitors of both these processes have been developed. Such inhibitors have therapeutic potential as animal data suggest that released endocannabinoids mediate reductions both in inflammatory pain and in the spasticity and tremor of multiple sclerosis. So too have CB(1) receptor antagonists, for example for the suppression of appetite and the management of cognitive dysfunction or schizophrenia.

Structural domains of the CB1 cannabinoid receptor that contribute to constitutive activity and G-protein sequestration

The CB1 cannabinoid receptor is a constitutively active receptor that can sequester G(i/o)-proteins and prevent other G(i/o)-coupled receptors from signaling (Bouaboula et al., 1997; Pan et al., 1998; Vasquez and Lewis, 1999). G-protein sequestration occurs because the population of CB1 cannabinoid receptors exists in both an inactive G-protein-precoupled RG(GDP) state and a constitutively active R*G(GTP) state. We tested the hypothesis that the distal C-terminal tail acts to prevent G-protein activation. We found that truncation of the distal C-terminal tail of the CB1 receptor (CB1-417) enhanced both the constitutive activity and the ability of the receptor to sequester G-proteins. In addition, we tested the hypothesis that the conserved aspartate (D2.50) in the second transmembrane domain of the CB1 cannabinoid receptor is crucial for constitutive activity and G-protein sequestration. We found that the mutation of aspartate to asparagine (CB1-D164N) abolished G-protein sequestration and constitutive receptor activity without disrupting agonist-stimulated activity. We conclude that the CB1-D164N mutation and the C-terminal truncation shift the population of receptors in opposite directions. The CB1-D164N mutation shifts the receptor into an inactive R state upcoupled from G-proteins, whereas the C-terminal truncation (CB1-417) shifts the receptor into the active R*G(GTP) state. Thus the distal C-terminal tail acts to constrain the receptor from activating G-proteins, whereas the aspartate (D2.50) in the second transmembrane domain stabilizes the receptor in both the inactive RG(GDP) state and the active R*G(GTP) state.

Structural domains of the CB1 cannabinoid receptor that contribute to constitutive activity and G-protein sequestration

The CB1 cannabinoid receptor is a constitutively active receptor that can sequester G(i/o)-proteins and prevent other G(i/o)-coupled receptors from signaling (Bouaboula et al., 1997; Pan et al., 1998; Vasquez and Lewis, 1999). G-protein sequestration occurs because the population of CB1 cannabinoid receptors exists in both an inactive G-protein-precoupled RG(GDP) state and a constitutively active R*G(GTP) state. We tested the hypothesis that the distal C-terminal tail acts to prevent G-protein activation. We found that truncation of the distal C-terminal tail of the CB1 receptor (CB1-417) enhanced both the constitutive activity and the ability of the receptor to sequester G-proteins. In addition, we tested the hypothesis that the conserved aspartate (D2.50) in the second transmembrane domain of the CB1 cannabinoid receptor is crucial for constitutive activity and G-protein sequestration. We found that the mutation of aspartate to asparagine (CB1-D164N) abolished G-protein sequestration and constitutive receptor activity without disrupting agonist-stimulated activity. We conclude that the CB1-D164N mutation and the C-terminal truncation shift the population of receptors in opposite directions. The CB1-D164N mutation shifts the receptor into an inactive R state upcoupled from G-proteins, whereas the C-terminal truncation (CB1-417) shifts the receptor into the active R*G(GTP) state. Thus the distal C-terminal tail acts to constrain the receptor from activating G-proteins, whereas the aspartate (D2.50) in the second transmembrane domain stabilizes the receptor in both the inactive RG(GDP) state and the active R*G(GTP) state.

Cannabinoids inhibit emesis through CB1 receptors in the brainstem of the ferret

BACKGROUND & AIMS

Marijuana and other cannabinoids are effective anti-emetics. Despite ongoing controversy over their usage, the receptor distribution and the site of the anti-emetic action of these compounds are not known. Our aim was to investigate whether the cannabinoid 1 receptor (CB1r) and endocannabinoids play a role in the anti-emetic action of cannabinoids.

METHODS

Ferrets were given an emetic stimulus and the number of episodes of retching and vomiting were observed after administration of CB1r agonists and a CB1r antagonist. CB1r and fatty acid amide hydrolase (FAAH), which degrades endocannabinoids, were localized by immunohistochemistry.

RESULTS

CB1r and FAAH were localized in the dorsal vagal complex, consisting of the area postrema, nucleus of the solitary tract, and the dorsal motor nucleus of the vagus in the brainstem. CB1r was found in the myenteric plexus of the stomach and duodenum. Activation of CB1r by the agonists (delta)(9)-tetrahydrocannabinol, WIN 55,212-2, and methanandamide inhibited emesis and their action was reversed by a selective CB1r antagonist, which alone had no effect, but potentiated vomiting in response to an emetic stimulus.

CONCLUSIONS

CB1r mediates the anti-emetic action of cannabinoids in the dorsal vagal complex. Endocannabinoids are a novel neuroregulatory system involved in the control of emesis.

Agonist-induced internalization and trafficking of cannabinoid CB1 receptors in hippocampal neurons

Agonist-induced internalization of G-protein-coupled receptors is an important mechanism for regulating receptor abundance and availability at the plasma membrane. In this study we have used immunolabeling techniques and confocal microscopy to investigate agonist-induced internalization and trafficking of CB(1) receptors in rat cultured hippocampal neurons. The levels of cell surface CB(1) receptor immunoreactivity associated with presynaptic GABAergic terminals decreased markedly (by up to 84%) after exposure to the cannabinoid agonist (+)-WIN55212, in a concentration-dependent (0.1-1 microm) and stereoselective manner. Inhibition was maximal at 16 hr and abolished in the presence of SR141716A, a selective CB(1) receptor antagonist. Methanandamide (an analog of an endogenous cannabinoid, anandamide) also reduced cell surface labeling (by 43% at 1 microm). Differential labeling of cell surface and intracellular pools of receptor demonstrated that the reduction in cell surface immunoreactivity reflects agonist-induced internalization and suggests that the internalized CB(1) receptors are translocated toward the soma. The internalization process did not require activated G-protein alpha(i) or alpha(o) subunits. A different pattern of cell surface CB(1) receptor expression was observed using an undifferentiated F-11 cell line, which had pronounced somatic labeling. In these cells substantial CB(1) receptor internalization was also observed after exposure to (+)-WIN55212 (1 microm) for relatively short periods (30 min) of agonist exposure. In summary, this dynamic modulation of CB(1) receptor expression may play an important role in the development of cannabinoid tolerance in the CNS. Agonist-induced internalization at presynaptic terminals has important implications for the modulatory effects of G-protein-coupled receptors on neurotransmitter release.

Cannabinoids and the gastrointestinal tract

The enteric nervous system of several species, including the mouse, rat, guinea pig and humans, contains cannabinoid CB1 receptors that depress gastrointestinal motility, mainly by inhibiting ongoing contractile transmitter release. Signs of this depressant effect are, in the whole organism, delayed gastric emptying and inhibition of the transit of non-absorbable markers through the small intestine and, in isolated strips of ileal tissue, inhibition of evoked acetylcholine release, peristalsis, and cholinergic and non-adrenergic non-cholinergic (NANC) contractions of longitudinal or circular smooth muscle. These are contractions evoked electrically or by agents that are thought to stimulate contractile transmitter release either in tissue taken from morphine pretreated animals (naloxone) or in unpretreated tissue (gamma-aminobutyric acid and 5-hydroxytryptamine). The inhibitory effects of cannabinoid receptor agonists on gastric emptying and intestinal transit are mediated to some extent by CB1 receptors in the brain as well as by enteric CB1 receptors. Gastric acid secretion is also inhibited in response to CB1 receptor activation, although the detailed underlying mechanism has yet to be elucidated. Cannabinoid receptor agonists delay gastric emptying in humans as well as in rodents and probably also inhibit human gastric acid secretion. Cannabinoid pretreatment induces tolerance to the inhibitory effects of cannabinoid receptor agonists on gastrointestinal motility. Findings that the CB1 selective antagonist/inverse agonist SR141716A produces in vivo and in vitro signs of increased motility of rodent small intestine probably reflect the presence in the enteric nervous system of a population of CB1 receptors that are precoupled to their effector mechanisms. SR141716A has been reported not to behave in this manner in the myenteric plexus-longitudinal muscle preparation (MPLM) of human ileum unless this has first been rendered cannabinoid tolerant. Nor has it been found to induce "withdrawal" contractions in cannabinoid tolerant guinea pig ileal MPLM. Further research is required to investigate the role both of endogenous cannabinoid receptor agonists and of non-CB1 cannabinoid receptors in the gastrointestinal tract. The extent to which the effects on gastrointestinal function of cannabinoid receptor agonists or antagonists/inverse agonists can be exploited therapeutically has yet to be investigated as has the extent to which these drugs can provoke unwanted effects in the gastrointestinal tract when used for other therapeutic purposes.

Cannabinoid inhibition of capsaicin-sensitive sensory neurotransmission in the rat mesenteric arterial bed

The present study investigated whether cannabinoids can modulate neurotransmission mediated by capsaicin-sensitive sensory nerves in the rat isolated mesenteric arterial bed. Sensory neurogenic vasorelaxation mediated by electrical field stimulation was concentration-dependently attenuated by HU210 (0.1-3 microM), a cannabinoid receptor agonist (from 62+/-8.3% to 6+/-2.1% at 3 microM HU210). HU210 had no effect on relaxation to exogenous calcitonin gene-related peptide, indicating a prejunctional action. The action of HU210 (1 microM) was not affected by LY320135 (1 microM) or SR144528 (1 microM), cannabinoid CB(1) and CB(2) receptor antagonists, respectively. SR141716A (0.01-1 microM), a cannabinoid CB(1) receptor antagonist, concentration-dependently augmented vasorelaxation to electrical field stimulation, but had no effect on responses to calcitonin gene-related peptide and capsaicin, indicating a possible role of endogenous cannabinoids in sensory neurotransmission in rat mesenteric arteries. These data show that the cannabinoid receptor agonist HU210 inhibits prejunctionally sensory neurotransmission in rat mesenteric arteries and that this action is independent of cannabinoid CB(1)- or CB(2)-like receptors.

Agonist-induced internalization and trafficking of cannabinoid CB1 receptors in hippocampal neurons

Agonist-induced internalization of G-protein-coupled receptors is an important mechanism for regulating receptor abundance and availability at the plasma membrane. In this study we have used immunolabeling techniques and confocal microscopy to investigate agonist-induced internalization and trafficking of CB(1) receptors in rat cultured hippocampal neurons. The levels of cell surface CB(1) receptor immunoreactivity associated with presynaptic GABAergic terminals decreased markedly (by up to 84%) after exposure to the cannabinoid agonist (+)-WIN55212, in a concentration-dependent (0.1-1 microm) and stereoselective manner. Inhibition was maximal at 16 hr and abolished in the presence of SR141716A, a selective CB(1) receptor antagonist. Methanandamide (an analog of an endogenous cannabinoid, anandamide) also reduced cell surface labeling (by 43% at 1 microm). Differential labeling of cell surface and intracellular pools of receptor demonstrated that the reduction in cell surface immunoreactivity reflects agonist-induced internalization and suggests that the internalized CB(1) receptors are translocated toward the soma. The internalization process did not require activated G-protein alpha(i) or alpha(o) subunits. A different pattern of cell surface CB(1) receptor expression was observed using an undifferentiated F-11 cell line, which had pronounced somatic labeling. In these cells substantial CB(1) receptor internalization was also observed after exposure to (+)-WIN55212 (1 microm) for relatively short periods (30 min) of agonist exposure. In summary, this dynamic modulation of CB(1) receptor expression may play an important role in the development of cannabinoid tolerance in the CNS. Agonist-induced internalization at presynaptic terminals has important implications for the modulatory effects of G-protein-coupled receptors on neurotransmitter release.

Cannabinoid receptors and pain

Mammalian tissues contain at least two types of cannabinoid receptor, CB(1) and CB(2), both coupled to G proteins. CB(1) receptors are expressed mainly by neurones of the central and peripheral nervous system whereas CB(2) receptors occur centrally and peripherally in certain non-neuronal tissues, particularly in immune cells. The existence of endogenous ligands for cannabinoid receptors has also been demonstrated. The discovery of this 'endocannabinoid system' has prompted the development of a range of novel cannabinoid receptor agonists and antagonists, including several that show marked selectivity for CB(1) or CB(2) receptors. It has also been paralleled by a renewed interest in cannabinoid-induced antinociception. This review summarizes current knowledge about the ability of cannabinoids to produce antinociception in animal models of acute pain as well as about the ability of these drugs to suppress signs of tonic pain induced in animals by nerve damage or by the injection of an inflammatory agent. Particular attention is paid to the types of pain against which cannabinoids may be effective, the distribution pattern of cannabinoid receptors in central and peripheral pain pathways and the part that these receptors play in cannabinoid-induced antinociception. The possibility that antinociception can be mediated by cannabinoid receptors other than CB(1) and CB(2) receptors, for example CB(2)-like receptors, is also discussed as is the evidence firstly that one endogenous cannabinoid, anandamide, produces antinociception through mechanisms that differ from those of other types of cannabinoid, for example by acting on vanilloid receptors, and secondly that the endocannabinoid system has physiological and/or pathophysiological roles in the modulation of pain.

Actions of cannabinoid receptor ligands on rat cultured sensory neurones: implications for antinociception

Cannabinoids modulate nociceptive processing in models of acute, inflammatory and neuropathic pain. We have investigated the location and function of cannabinoid receptors on cultured neonatal dorsal root ganglion (DRG) neurones and F-11 cells, a dorsal root ganglionxneuroblastoma hybridoma which displays several of the features of authentic DRG neurones. CB(1) receptor immunolabelling was observed on the cell bodies and as fine puncta on processes of both cultured DRG neurones and F-11 cells. Additionally, fluorescence-activated cell sorting (FACS) analysis provided evidence that both CB(1) and CB(2) receptors are expressed on populations of cells within the cultured DRG and F-11 cells. The cannabinoid receptor agonist (+)-WIN55212 (10 and 100 nM) inhibited the mean voltage-activated Ca(2+) current in DRG neurones by 21% and 30%, respectively. The isomer, (-)-WIN55212 (10 and 100 nM) produced significantly less inhibition of 6% and 10% respectively. The CB(1) selective receptor antagonist SR141716A (100 nM) enhanced the peak high voltage-activated Ca(2+) current by 24% and simultaneous application of SR141716A (100 nM) and (+)-WIN55212 (100 nM) resulted in a significant attenuation of the inhibition obtained with (+)-WIN55212 alone. These data give functional evidence for the hypothesis that the analgesic actions of cannabinoids may be mediated by presynaptic inhibition of transmitter release in sensory neurones.

Inhibitory effects of SR141716A on G-protein activation in rat brain

N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A), a cannabinoid CB(1) receptor antagonist, has inverse agonist effects in cannabinoid CB(1) receptor-expressing cell lines, brain and peripheral organs. These studies characterized SR141716A-inhibited G-protein activity by measuring [35S]GTPgammaS binding. Maximal inhibition of basal [35S]GTPgammaS binding in cerebellar membranes was 50%. The EC(50) value for inhibition of [35S]GTPgammaS binding was 4.4 microM, whereas the K(e) for inhibition of R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone mesylate (WIN 55,212-2)-stimulated [35S]GTPgammaS binding was 0.6 nM. [35S]GTPgammaS autoradiography was used to examine the regional specificity of SR141716A inhibition. SR141716A inhibited basal [35S]GTPgammaS binding in all regions examined, with inhibition ranging from approximately 20% in caudate-putamen to 40% in hippocampus. These studies demonstrate that SR141716A is a competitive antagonist at nanomolar concentrations, whereas it inhibits basal receptor-mediated G-protein activity at micromolar concentrations. These data suggest that the apparent inverse agonist effect is either not cannabinoid CB(1) receptor-specific or that SR141716A is binding to different sites on the cannabinoid CB(1) receptor to produce inverse agonist versus competitive antagonist effects.

Pharmacological analysis of cannabinoid receptor activity in the rat vas deferens

1. The interaction between the cannabinoid agonists, WIN 55,212-2 or CP 55,940 with the CB(1) receptor-selective antagonists, SR141716A or LY320135 was investigated using the rat electrically-stimulated vas deferens bioassay. 2. Tissues were stimulated by single-field pulses (150 V, 0.5 ms) delivered every 30 mins. In the presence of nifedipine (3 microM), agonists elicited a concentration-dependent inhibition of the contractile response, with pEC(50) values of 7.93 and 6.84 for WIN 55,212-2 and CP 55,940, respectively. 3. SR141716A and LY320135 caused parallel dextral displacements of the agonist concentration-response curves. However, the shift of the agonist curves by either antagonist was accompanied by a concentration-dependent enhancement of basal (agonist-independent) tissue contraction. 4. Addition of the amidase inhibitor, phenylmethylsulphonylfluoride (200 microM), resulted in a significant reduction of the basal twitch response, an effect consistent with the presence of tonic receptor activation mediated by the endogenous cannabinoid, anandamide. 5. In light of these findings, we propose a theoretical model of competitive agonist-antagonist interaction in the presence of endogenous agonist tone that was used to derive an optimized analytical approach for the determination of antagonist potency estimates under conditions of tonic receptor activation. 6. This approach yielded pK(B) estimates for SR141716A and LY320135 that were in good agreement with their activity at cannabinoid CB(1) receptors. 7. It is concluded that the rat vas deferens contains prejunctional cannabinoid CB(1) receptors that are under tonic activation from endogenous substances; under these conditions our analytical approach is preferable to the standard methods for the determination of antagonist potency.

Changes in the cannabinoid receptor binding, G protein coupling, and cyclic AMP cascade in the CNS of rats tolerant to and dependent on the synthetic cannabinoid compound CP55,940

Chronic exposure to CP55,940 produced a significant down-regulation of cannabinoid receptors in the striatum, cortex, hippocampus, and cerebellum of rat brain. At 24 h after SR141716-precipitated withdrawal, we observed a tendency to return to basal levels in the striatum and cortex, whereas the specific binding remained lower in the hippocampus and cerebellum. When we surveyed cannabinoid receptor-activated G proteins, in chronic CP55,940-treated rats the guanosine 5'-O:-(3-[(35)S]thiotriphosphate) ([(35)S]GTPgammaS) binding assay revealed a decrease of activated G proteins in the striatum, cortex, and hippocampus, whereas no significant changes were seen in the cerebellum. At 24 h after the SR141716-precipitated withdrawal, [(35)S]GTPgammaS binding increased compared with that of rats chronically exposed to CP55,940, attaining the control level except for cerebellum, where we observed a trend to overcome the control amounts. Concerning the cyclic AMP (cAMP) cascade, which represents the major intracellular signaling pathway activated by cannabinoid receptors, in the cerebral areas from rats chronically exposed to CP55,940 we found alteration in neither cAMP levels nor protein kinase A activity. In the brain regions taken from CP55, 940-withdrawn rats, we only observed a significant up-regulation in the cerebellum. Our findings suggest that receptor desensitization and down-regulation are strictly involved in the development of cannabinoid tolerance, whereas alterations in the cAMP cascade in the cerebellum could be relevant in the mediation of the motor component of cannabinoid abstinence.

Cannabinoid receptor ligands: clinical and neuropharmacological considerations, relevant to future drug discovery and development

This review highlights some important advances that have taken place in cannabinoid research over the last four years. It focuses on novel ligands that are of interest either as experimental tools or as lead compounds for therapeutic agents and possible clinical applications for some of these ligands. The molecular targets for these compounds are various components of the system of endogenous cannabinoids (endocannabinoids) and receptors that together constitute the 'endocannabinoid system'. These are CB(1) cannabinoid receptors that are present mainly on central and peripheral neurones, CB(2) cannabinoid receptors that are expressed predominantly by immune cells, the biochemical mechanisms responsible for the tissue uptake or metabolism of endocannabinoids and vanilloid receptors. Other cannabinoid receptor types may also exist. Recently developed ligands include potent and selective agonists for CB(1) and CB(2) receptors, a potent CB(2)-selective antagonist/inverse agonist and inhibitors of endocannabinoid uptake or metabolism. Future research should be directed at characterising the endocannabinoid system more completely and at obtaining more conclusive clinical data about the possible beneficial effects of cannabinoids as well as their adverse effects. There is also a need for improved cannabinoid formulations/modes of administration in the clinic and advances in this area should be facilitated by the recent development of a potent water-soluble CB(1)/CB(2) receptor agonist. A growing number of strategies for separating sought-after therapeutic effects of cannabinoid receptor agonists from the unwanted consequences of CB(1) receptor activation are now emerging and these are discussed at the end of this review.