Localisation of cannabinoid CB(1) receptor immunoreactivity in the guinea pig and rat myenteric plexus

TRPV1-mediated calcium signal couples with cannabinoid receptors and sodium-calcium exchangers in rat odontoblasts

Odontoblasts are involved in the transduction of stimuli applied to exposed dentin. Although expression of thermo/mechano/osmo-sensitive transient receptor potential (TRP) channels has been demonstrated, the properties of TRP vanilloid 1 (TRPV1)-mediated signaling remain to be clarified. We investigated physiological and pharmacological properties of TRPV1 and its functional coupling with cannabinoid (CB) receptors and Na(+)-Ca(2+) exchangers (NCXs) in odontoblasts. Anandamide (AEA), capsaicin (CAP), resiniferatoxin (RF) or low-pH evoked Ca(2+) influx. This influx was inhibited by capsazepine (CPZ). Delay in time-to-activation of TRPV1 channels was observed between application of AEA or CAP and increase in [Ca(2+)](i). In the absence of extracellular Ca(2+), however, an immediate increase in [Ca(2+)](i) was observed on administration of extracellular Ca(2+), followed by activation of TRPV1 channels. Intracellular application of CAP elicited inward current via opening of TRPV1 channels faster than extracellular application. With extracellular RF application, no time delay was observed in either increase in [Ca(2+)](i) or inward current, indicating that agonist binding sites are located on both extra- and intracellular domains. KB-R7943, an NCX inhibitor, yielded an increase in the decay time constant during TRPV1-mediated Ca(2+) entry. Increase in [Ca(2+)](i) by CB receptor agonist, 2-arachidonylglycerol, was inhibited by CB1 receptor antagonist or CPZ, as well as by adenylyl cyclase inhibitor. These results showed that TRPV1-mediated Ca(2+) entry functionally couples with CB1 receptor activation via cAMP signaling. Increased [Ca(2+)](i) by TRPV1 activation was extruded by NCXs. Taken together, this suggests that cAMP-mediated CB1-TRPV1 crosstalk and TRPV1-NCX coupling play an important role in driving cellular functions following transduction of external stimuli to odontoblasts.

Plasticity of mouse enteric synapses mediated through endocannabinoid and purinergic signaling

BACKGROUND

The enteric nervous system (ENS) possesses extensive synaptic connections which integrate information and provide appropriate outputs to coordinate the activity of the gastrointestinal tract. The regulation of enteric synapses is not well understood. Cannabinoid (CB)(1) receptors inhibit the release of acetylcholine (ACh) in the ENS, but their role in the synapse is not understood. We tested the hypothesis that enteric CB(1) receptors provide inhibitory control of excitatory neurotransmission in the ENS.

METHODS

Intracellular microelectrode recordings were obtained from mouse myenteric plexus neurons. Interganglionic fibers were stimulated with a concentric stimulating electrode to elicit synaptic events on to the recorded neuron. Differences between spontaneous and evoked fast synaptic transmission was examined within preparations from CB(1) deficient mice (CB(1)(-/-)) and wild-type (WT) littermate controls.

KEY RESULTS

Cannabinoid receptors were colocalized on terminals expressing the vesicular ACh transporter and the synaptic protein synaptotagmin. A greater proportion of CB(1)(-/-) neurons received spontaneous fast excitatory postsynaptic potentials than neurons from WT preparations. The CB(1) agonist WIN55,212 depressed WT synapses without any effect on CB(1)(-/-) synapses. Synaptic activity in response to depolarization was markedly enhanced at CB(1)(-/-) synapses and after treatment with a CB(1) antagonist in WT preparations. Activity-dependent liberation of a retrograde purine messenger was demonstrated to facilitate synaptic transmission in CB(1)(-/-) mice.

CONCLUSIONS & INFERENCES

Cannabinoid receptors inhibit transmitter release at enteric synapses and depress synaptic strength basally and in an activity-dependent manner. These actions help explain accelerated intestinal transit observed in the absence of CB(1) receptors.

Alternative targets within the endocannabinoid system for future treatment of gastrointestinal diseases

Many beneficial effects of herbal and synthetic cannabinoids on gut motility and inflammation have been demonstrated, suggesting a vast potential for these compounds in the treatment of gastrointestinal disorders. These effects are based on the so-called 'endocannabinoid system' (ECS), a cooperating network of molecules that regulate the metabolism of the body's own and of exogenously administered cannabinoids. The ECS in the gastrointestinal tract quickly responds to homeostatic disturbances by de novo synthesis of its components to maintain homeostasis, thereby offering many potential targets for pharmacological intervention. Of major therapeutic interest are nonpsychoactive cannabinoids or compounds that do not directly target cannabinoid receptors but still possess cannabinoid-like properties. Drugs that inhibit endocannabinoid degradation and raise the level of endocannabinoids are becoming increasingly promising alternative therapeutic tools to manipulate the ECS.

The Endocannabinoid System as Pharmacological Target Derived from Its CNS Role in Energy Homeostasis and Reward. Applications in Eating Disorders and Addiction

The endocannabinoid system (ECS) has been implicated in many physiological functions, including the regulation of appetite, food intake and energy balance, a crucial involvement in brain reward systems and a role in psychophysiological homeostasis (anxiety and stress responses). We first introduce this important regulatory system and chronicle what is known concerning the signal transduction pathways activated upon the binding of endogenous cannabinoid ligands to the G_i/0-coupled CB1 cannabinoid receptor, as well as its interactions with other hormones and neuromodulators which can modify endocannabinoid signaling in the brain. Anorexia nervosa (AN) and bulimia nervosa (BN) are severe and disabling psychiatric disorders, characterized by profound eating and weight alterations and body image disturbances. Since endocannabinoids modulate eating behavior, it is plausible that endocannabinoid genes may contribute to the biological vulnerability to these diseases. We present and discuss data suggesting an impaired endocannabinoid signaling in these eating disorders, including association of endocannabinoid components gene polymorphisms and altered CB1-receptor expression in AN and BN. Then we discuss recent findings that may provide new avenues for the identification of therapeutic strategies based on the endocannabinod system. In relation with its implications as a reward-related system, the endocannabinoid system is not only a target for cannabis but it also shows interactions with other drugs of abuse. On the other hand, there may be also a possibility to point to the ECS as a potential target for treatment of drug-abuse and addiction. Within this framework we will focus on enzymatic machinery involved in endocannabinoid inactivation (notably fatty acid amide hydrolase or FAAH) as a particularly interesting potential target. Since a deregulated endocannabinoid system may be also related to depression, anxiety and pain symptomatology accompanying drug-withdrawal states, this is an area of relevance to also explore adjuvant treatments for improving these adverse emotional reactions.

The differential effect of CB1 receptors on the discharge of afferent and efferent fibres supplying the rat jejunum

BACKGROUND

The cannabinoid receptor (CB(1) ) is expressed on GI sensory neurons and is suggested to play a role in food intake, inflammation and nociception. Expression of CB(1) in the nodose is influenced by nutritional status. Our aim was to determine the functional response of afferent and efferent fibres supplying the proximal jejunum to the CB(1) agonist docosatetraenylethanolamide (DEA) in fed and fasted animals.

METHODS

Experiments were performed on anesthetized rats, either fed ad libitum or fasted for 24 h. Blood pressure was recorded via the carotid artery and the proximal jejunum intubated to measure intraluminal pressure. A single paravascular nerve bundle was isolated and attached to an electrode for recording either afferent or efferent impulse traffic.

KEY RESULTS

Docosatetraenylethanolamide (1 mg kg(-1) , i.v.) had a depressor effect on blood pressure but surprisingly had little effect on afferent nerve activity in fed animals. In fasted animals the afferent response to DEA was augmented, however, the blood pressure effect was attenuated. In contrast, DEA caused a significant and prolonged increase in efferent firing, which was diminished in fasted animals. Bilateral cervical vagotomy had no effect on the DEA-mediated efferent response, however, hexamethonium (10 mg kg(-1) ) abolished excitation and unmasked an inhibitory effect of DEA.

CONCLUSIONS & INFERENCES

Docosatetraenylethanolamide has only a modest effect on intestinal afferent firing but a profound effect on efferent function, which is modulated by changes in nutritional status. The persistent response after vagotomy and block by hexamethonium suggests DEA is acting centrally, although there may be an inhibitory effect at the level of the postganglionic sympathetic neuron.

Small intestinal cannabinoid receptor changes following a single colonic insult with oil of mustard in mice

Cannabinoids are known to be clinically beneficial for control of appetite disorders and nausea/vomiting, with emerging data that they can impact other GI disorders, such as inflammation. Post-inflammatory irritable bowel syndrome (PI-IBS) is a condition of perturbed intestinal function that occurs subsequent to earlier periods of intestinal inflammation. Cannabinoid 1 receptor (CB1R) and CB2R alterations in GI inflammation have been demonstrated in both animal models and clinically, but their continuing role in the post-inflammatory period has only been implicated to date. Therefore, to provide direct evidence for CBR involvement in altered GI functions in the absence of overt inflammation, we used a model of enhanced upper GI transit that persists for up to 4 weeks after a single insult by intracolonic 0.5% oil of mustard (OM) in mice. In mice administered OM, CB1R immunostaining in the myenteric plexus was reduced at day 7, when colonic inflammation is subsiding, and then increased at 28 days, compared to tissue from age-matched vehicle-treated mice. In the lamina propria CB2R immunostaining density was also increased at day 28. In mice tested 28 day after OM, either a CB1R-selective agonist, ACEA (1 and 3 mg/kg, s.c.) or a CB2R-selective agonist, JWH-133 (3 and 10 mg/kg, s.c.) reduced the enhanced small intestinal transit in a dose-related manner. Doses of ACEA and JWH-133 (1 mg/kg), alone or combined, reduced small intestinal transit of OM-treated mice to a greater extent than control mice. Thus, in this post-colonic inflammation model, both CBR subtypes are up-regulated and there is increased efficacy of both CB1R and CB2R agonists. We conclude that CBR remodeling occurs not only during GI inflammation but continues during the recovery phase. Thus, either CB1R- or CB2-selective agonists could be efficacious for modulating GI motility in individuals experiencing diarrhea-predominant PI-IBS.

Evidence for both inverse agonism at the cannabinoid CB1 receptor and the lack of an endogenous cannabinoid tone in the rat and guinea-pig isolated ileum myenteric plexus-longitudinal muscle preparation

BACKGROUND AND PURPOSE

Cannabinoid receptor agonists reduce intestinal propulsion in rodents through the CB(1) receptor. In addition to its antagonistic activity at this receptor, rimonabant (N-(piperidino)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxyamide) alone augments intestinal transit. Using rat and guinea-pig ileum MPLM (myenteric plexus-longitudinal muscle) preparations, we investigated whether the latter effect was through inverse agonism or antagonism of endocannabinoid agonist(s).

EXPERIMENTAL APPROACH

Inverse agonism was investigated by comparing the maximal enhancement of electrically evoked contractions of the MPLM by two CB(1) receptor antagonists, AM 251 (N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide) and O-2050 [(6aR,10aR)-3-(1-methanesulphonylamino-4-hexyn-6-yl)-6a,7,10,10a-tetrahydro-6,6,9-trimethyl-6-H-dibenzo[b,d]pyran], with that produced by rimonabant. To reveal ongoing endocannabinoid activity, effects of inhibiting endocannabinoid hydrolysis by fatty acid amide hydrolase (FAAH) using AA-5HT (arachidonyl-5-hydroxytryptamine), PMSF (phenylmethylsulphonyl fluoride) or URB-597 (3'-carbamoyl-biphenyl-3-yl-cyclohexylcarbamate), or putative uptake using VDM-11 [(5Z,8Z,11Z,14Z)-N-(4-hydroxy-2-methylphenyl)-5,8,11,14-eicosatetraenamide] was evaluated.

KEY RESULTS

The presence of CB(1) receptors was revealed by antagonism of exogenous anandamide, arachidonylethanolamide (AEA) and WIN 55,212-2 [(R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)-pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate] by rimonabant. The rank order of potentiation of contractions was AM 251 > rimonabant > O-2050. Neither the FAAH inhibitors nor VDM-11 affected electrically evoked contractions. Each FAAH inhibitor increased the potency of AEA but not WIN 55,212-2. VDM-11 did not alter the inhibitory effect of AEA.

CONCLUSIONS AND IMPLICATIONS

The different levels of maximal potentiation of contractions by the CB(1) receptor antagonists suggest inverse agonism. The potentiation of the action of AEA by the FAAH inhibitors showed that FAAH was present. The lack of effect of FAAH inhibitors and VDM-11 alone on electrically evoked contractions, and on the potency of exogenous AEA suggests that pharmacologically active endocannabinoids were not released and the endocannabinoid transporter was absent. Thus, the CB(1) receptor antagonists behave as inverse agonists.

The role of peripheral cannabinoid receptors type 1 in rats with visceral hypersensitivity induced by chronic restraint stress

Background/Aims

This study was designed to investigate the possibility that the enhanced nociceptive responsiveness associated with canabonoid type 1 receptors (CB1Rs) and identify its role in mediating visceral hypersensitivity induced by chronic restraint stress.

Methods

Rats were exposed to daily partial restraint stress or sham partial restraint stress with intraperitoneal injection of the vehicle, CB1R agonist or antagonist for 4 consecutive days. We tested the visceromotor reflex to colorectal distention at day 0 and 5. Reverse-transcription polymerase chain reaction and Western blot were used to assess the expression of CB1Rs.

Results

Intraperitoneal CB1 agonist (ACEA) injection significantly diminished (p < 0.05) the enhanced visceromotor reflex to colorectal distention at day 5 in stressed rats. Change in electromyogram response after ACEA over baseline, at pressure of 40 mmHg (+13.3 ± 2.2), 60 mmHg (+15.3 ± 2.8) and 80 mmHg (+17.0 ± 4.0) were much lower than in the control animals, which were +35.9 ± 5.1, +41.1 ± 6.3 and +54.1 ± 9.6, respectively. Whereas, CB1 antagonist (SR141716A) had an opposite effect. Compared with control group, the change in electromyogram response after SR141716A over baseline was significantly enhanced (p < 0.05) for the distending pressure of 40 mmHg (+56.0 ± 10.3), 60 mmHg (+74.6 ± 12.3) and 80 mmHg (+82.9 ± 11.0), respectively. Reverse-transcription polymerase chain reaction and Western blotting demonstrated the stress-induced up-regulation of colon CB1Rs (p < 0.05).

Conclusions

Our results suggest there is a key contribution of peripheral CB1Rs involved in the maintenance of visceral hyperalgesia after repeated restraint stress, providing a novel mechanism for development of peripheral visceral sensitization.

Pharmacological effects of cannabinoids on the Caco-2 cell culture model of intestinal permeability

Activation of cannabinoid receptors decreases emesis, inflammation, gastric acid secretion, and intestinal motility. However, the effects of cannabinoids on intestinal permeability have not yet been established. The aim of the present study is to examine the effects of cannabinoids on intestinal permeability in an in vitro model. Caco-2 cells were grown until fully confluent on inserts in 12-well plates. Transepithelial electrical resistance (TEER) measurements were made as a measure of permeability. EDTA (50 μM) was applied to reversibly increase permeability (reduce TEER). The effects of cannabinoids on permeability in combination with EDTA, or alone, were assessed. Potential target sites of action were investigated using antagonists of the cannabinoid (CB)(1) receptor, CB(2) receptor, transient receptor potential vanilloid subtype 1 (TRPV1), peroxisome proliferator-activated receptor (PPAR)γ, PPARα, and a proposed cannabinoid receptor. When applied to the apical or basolateral membrane of Caco-2 cells, Δ(9)-tetrahydrocannabinol (THC) and cannabidiol (CBD) enhanced the speed of recovery of EDTA-induced increased permeability. This effect was sensitive to cannabinoid CB(1) receptor antagonism only. Apical application of endocannabinoids caused increased permeability, sensitive to cannabinoid CB(1) receptor antagonism. By contrast, when endocannabinoids were applied basolaterally, they enhanced the recovery of EDTA-induced increased permeability, and this involved additional activation of TRPV1. All cannabinoids tested increased the mRNA of the tight junction protein zona occludens-1, but only endocannabinoids also decreased the mRNA of claudin-1. These findings suggest that endocannabinoids may play a role in modulating intestinal permeability and that plant-derived cannabinoids, such as THC and CBD, may have therapeutic potential in conditions associated with abnormally permeable intestinal epithelium.

Naphthalen-1-yl-(4-pentyloxynaphthalen-1-yl)methanone (SAB378), a peripherally restricted cannabinoid CB1/CB2 receptor agonist, inhibits gastrointestinal motility but has no effect on experimental colitis in mice

The endocannabinoid system is involved in the regulation of gastrointestinal (GI) motility and inflammation. Using the peripherally restricted cannabinoid (CB)(1)/CB(2) receptor agonist naphthalen-1-yl-(4-pentyloxynaphthalen-1-yl)methanone (SAB378), we investigated the role of peripheral cannabinoid receptors in the regulation of GI motility and the development of colitis in mice. The actions of SAB378 on whole gut transit, upper GI transit, colonic propulsion, and locomotor activity were investigated in C57BL/6N, CB(1) receptor knockout, and CB(2) receptor knockout mice. The potential for SAB378 to modify inflammation was studied by using dextran sulfate sodium (DSS) and 2,4,6-trinitrobenzene sulfonic acid (TNBS) models of experimental colitis. SAB378 did not modify locomotor activity. SAB378 slowed all parameters of GI motility, and these effects were significantly reduced by the CB(1) receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3 carboxamide (AM251), but not by the CB(2) receptor antagonist 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H- indol-3-yl](4-methoxyphenyl)methanone (AM630). SAB378 did not inhibit GI transit or colonic propulsion in CB(1) receptor knockout mice, whereas its effects were observed in CB(2) receptor knockout mice. SAB378 did not reduce the degree of colitis induced by DSS or TNBS. The actions of SAB378 on GI motility are mediated by peripherally located CB(1) receptors. SAB378 was not effective against two models of experimental colitis, which may indicate that peripheral cannabinoid receptor stimulation alone may not be sufficient to mediate the anti-inflammatory effects of cannabinoids.

Cannabinoid 1 receptors modulate intestinal sensory and motor function in rat

BACKGROUND

Cannabinoid receptors are involved in visceral pain perception and control of intestinal motility in vivo. The underlying mechanisms are not well characterized. We aimed to determine whether the cannabinoid-1 (CB(1)) receptor modulates intestinal afferent nerve discharge and the peristaltic reflex.

METHODS

Rats were anesthetized and intestinal segments were removed. Afferent nerve discharge from a mesenteric nerve was investigated in vitro in the presence of the CB(1) antagonist SR 141716A or the CB(1) agonist WIN 55212-2. The myenteric peristaltic reflex was induced by electrical field stimulation and influence of SR 141716A or WIN 55212-2 was recorded.

KEY RESULTS

Afferent nerve discharge to the algesic mediator bradykinin was reduced to 11 +/- 5.1 imp s(-1) following pretreatment with SR 141716A and unchanged after WIN 55212-2 compared to 63 +/- 15.4 imp s(-1) in controls. At maximum distension pressure (80 cmH(2)O) during ramp distension, 92 +/- 12.4 imp s(-1) were reached following SR 141716A compared to 260 +/- 13.2 in vehicle controls and 227 +/- 15.4 in WIN 55212-2 pretreated animals. In contrast, afferent discharge to 5-HT (500 micromol L(-1)) was increased to 75 +/- 24.6 imp s(-1) following WIN 55212-2 compared to 18 +/- 5.9 imp s(-1) in controls, whereas SR 141716A had no effect. Ascending neuronal contractions were dose-dependently attenuated in the presence of SR 141716A and latency of these contractions was reduced. WIN 55212-2 had opposite effects that were abolished by SR 141716A.

CONCLUSIONS & INFERENCES

Activation of the CB(1) receptor differentially alters afferent intestinal nerve sensitivity to bradykinin, 5-HT, and noxious mechanical distension, while it strengthens ascending neuronal contractions. Further studies are needed to determine the physiological relevance of these observations.

Characteristics of tolerance in the guinea pig ileum produced by chronic in vivo exposure to opioid versus cannabinoid agonists

Few studies have compared the nature of tolerance that develops following chronic opioid treatment with that which develops after chronic cannabinoid exposure in the same tissue and species. The degree and character of tolerance induced by 7 twice daily injections of morphine or 5 daily injections of the cannabinoid receptor agonist, WIN-55,212-2, was examined by comparing the ability of DAMGO, 2-chloroadenosine (CADO) and WIN-55,212-2 to inhibit neurogenic contractions of the longitudinal muscle/myenteric plexus preparation (LM/MP) and the ability of nicotine to elicit contractions in the LM/MP. Chronic morphine treatment resulted in subsensitivity to all inhibitory agonists (rightward shift in IC(50) values of 4-5-fold) and an increased responsiveness to the excitatory effect of nicotine while chronic WIN-55,212-2 exposure resulted in subsensitivity only to WIN-55,212-2 and a reduction in maximum response to both WIN-55,212-2 and DAMGO but no change in responsiveness to CADO. Chronic WIN-55,212-2 treatment significantly reduced CB(1) but not MOR receptor protein abundance while chronic morphine treatment did not change either. Assessment of the distribution of MOR and CB(1) receptors in myenteric neurons revealed distinct individual receptor expression as well as co-localization which was unaffected by either cannabinoid or opioid treatment. Thus, in contrast to the heterologous tolerance that develops after opioid treatment, tolerance in the LM/MP following chronic in vivo WIN-55,212-2 exposure appears to be homologous in character and is accompanied by a selective decrease in CB(1) receptor protein abundance. The data suggest that the cellular basis of tolerance differs between the two systems.

Pharmacological characterization of cannabinoid receptor activity in the rat-isolated ileum myenteric plexus-longitudinal muscle preparation

BACKGROUND AND PURPOSE

Cannabinoid effects on intestinal transit are commonly evaluated in rats. We characterized the cannabinoid receptors mediating the inhibitory effect of 5-(1,1-dimethylheptyl)-2-[5-hydroxy-2-(3-hydroxypropyl)-cyclohexyl]-phenol (CP 55,940), (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN 55,212-2), arachidonylethanolamide (AEA) and Delta(9)-tetrahydrocannabinol (Delta(9)-THC) on contractions of the rat ileum myenteric plexus-longitudinal muscle (MPLM) preparation.

EXPERIMENTAL APPROACH

The interaction of each agonist was examined with the CB(1) and CB(2) receptor antagonist rimonabant and SR 144,528 respectively, on contractions elicited by electrical field stimulation (EFS) or exogenous ACh. The interaction of AEA with capsazepine, a TRPV(1) receptor antagonist, was also investigated.

KEY RESULTS

EFS with single and trains of pulses evoked neurogenic ACh-mediated twitch and rebound contractions respectively. The rank order of potency for inhibition was CP 55,940 = WIN 55,212-2 > AEA > Delta(9)-THC and AEA > WIN 55,212-2 =Delta(9)-THC = CP 55,940 respectively. The stereoisomer WIN 55,212-3 was without effect. Rimonabant antagonized the inhibition of the twitches with pK(B) values of around 8.60, but only antagonized rebound contractions induced by WIN 55,212-2, AEA and Delta(9)-THC, with pA(2) values of around 6.80. Rimonabant increased the twitches but inhibited the rebound contractions. Contractions to exogenous ACh were not altered. These observations extended to the guinea pig ileum MPLM.

CONCLUSIONS AND IMPLICATIONS

The rat MPLM contains CB(1) receptors and at least two non-CB(1)-non-CB(2)-non-TRPV(1) receptors attenuating EFS-evoked ACh-mediated contractions in an EFS frequency-dependent pre-synaptic and stereo-specific manner. Augmentation of the twitches by rimonabant may be through antagonism of an endocannabinoid tone or inverse agonism, whereas inhibition of the rebound contractions involved partial agonism.

Cannabinoids and the gut: new developments and emerging concepts

Cannabis has been used to treat gastrointestinal (GI) conditions that range from enteric infections and inflammatory conditions to disorders of motility, emesis and abdominal pain. The mechanistic basis of these treatments emerged after the discovery of Delta(9)-tetrahydrocannabinol as the major constituent of Cannabis. Further progress was made when the receptors for Delta(9)-tetrahydrocannabinol were identified as part of an endocannabinoid system, that consists of specific cannabinoid receptors, endogenous ligands and their biosynthetic and degradative enzymes. Anatomical, physiological and pharmacological studies have shown that the endocannabinoid system is widely distributed throughout the gut, with regional variation and organ-specific actions. It is involved in the regulation of food intake, nausea and emesis, gastric secretion and gastroprotection, GI motility, ion transport, visceral sensation, intestinal inflammation and cell proliferation in the gut. Cellular targets have been defined that include the enteric nervous system, epithelial and immune cells. Molecular targets of the endocannabinoid system include, in addition to the cannabinoid receptors, transient receptor potential vanilloid 1 receptors, peroxisome proliferator-activated receptor alpha receptors and the orphan G-protein coupled receptors, GPR55 and GPR119. Pharmacological agents that act on these targets have been shown in preclinical models to have therapeutic potential. Here, we discuss cannabinoid receptors and their localization in the gut, the proteins involved in endocannabinoid synthesis and degradation and the presence of endocannabinoids in the gut in health and disease. We focus on the pharmacological actions of cannabinoids in relation to GI disorders, highlighting recent data on genetic mutations in the endocannabinoid system in GI disease.

Molecules Acting on CB1 Receptor and their Effects on Morphine Withdrawal In Vitro

Several pharmacological studies indicate that CB1 cannabinoid receptors (CB1Rs) are present in guinea pig ileum (GPI) and their activation reduce the acetylcholine (Ach) release. Dependence can be induced and measured in vitro by using GPI and the contraction due to opioid withdrawal is caused by acetylcholine release.

Design of molecules acting on the CB1Rs are widely studied and the large availaibility of CB1Rs agonists and antagonists provides powerful tools to determine the role of these receptors in mediating some of physiological and pharmacological effects in the myenteric neurones.

Given the relationship between CB1Rs/Opioid Withdrawal/Ach system, in the present paper we have designed six new CB1Rs agonists named A-F and evaluated their role in mediating morphine withdrawal in GPI. Also, a comparative study was performed by using the CB1Rs synthetic cannabinoid WIN 55,212-2 and CP 55,940. The results of our experiments indicate that both WIN 55,212-2 and CP 55,940 (1x10^-8-5x10^-8-1x10^-7 M) were able to reduce morphine withdrawal in a concentration-dependent manner. Very similar results were obtained with the new CB1Rs agonists (A-F) used at same concentrations. The results of our experiments indicate that CB1Rs are involved in the control of morphine withdrawal in vitro thus confirming an important functional interaction between the cannabinoid and opioid system.

Salvinorin A inhibits colonic transit and neurogenic ion transport in mice by activating kappa-opioid and cannabinoid receptors

The major active ingredient of the plant Salvia divinorum, salvinorin A (SA) has been used to treat gastrointestinal (GI) symptoms. As the action of SA on the regulation of colonic function is unknown, our aim was to examine the effects of SA on mouse colonic motility and secretion in vitro and in vivo. The effects of SA on GI motility were studied using isolated preparations of colon, which were compared with preparations from stomach and ileum. Colonic epithelial ion transport was evaluated using Ussing chambers. Additionally, we studied GI motility in vivo by measuring colonic propulsion, gastric emptying, and upper GI transit. Salvinorin A inhibited contractions of the mouse colon, stomach, and ileum in vitro, prolonged colonic propulsion and slowed upper GI transit in vivo. Salvinorin A had no effect on gastric emptying in vivo. Salvinorin A reduced veratridine-, but not forskolin-induced epithelial ion transport. The effects of SA on colonic motility in vitro were mediated by kappa-opioid receptors (KORs) and cannabinoid (CB) receptors, as they were inhibited by the antagonists nor-binaltorphimine (KOR), AM 251 (CB(1) receptor) and AM 630 (CB(2) receptor). However, in the colon in vivo, the effects were largely mediated by KORs. The effects of SA on veratridine-mediated epithelial ion transport were inhibited by nor-binaltorphimine and AM 630. Salvinorin A slows colonic motility in vitro and in vivo and influences neurogenic ion transport. Due to its specific regional action, SA or its derivatives may be useful drugs in the treatment of lower GI disorders associated with increased GI transit and diarrhoea.

Modulation of motor and sensory pathways of the peristaltic reflex by cannabinoids

Cannabinoids have long been known to be potent inhibitors of intestinal and colonic propulsion. This effect has generally been attributed to their ability to prejunctionally inhibit release of acetylcholine from excitatory motor neurons that mediate, in part, the ascending contraction phase of the peristaltic reflex. In the present study we examined the effect of cannabinoids on the other transmitters known to participate in the peristaltic reflex using a three-compartment preparation of rat colon that allows separation of ascending contraction, descending relaxation, and the sensory components of the reflex. On addition to the orad motor compartment, anandamide decreased and AM-251, a CB-1 antagonist, increased ascending contraction and the concomitant substance P (SP) release. Similarly, on addition to the caudad motor compartment, anandamide decreased and AM-251 increased descending relaxation and the concomitant vasoactive intestinal peptide (VIP) release. On addition to the central sensory compartment, anandamide decreased and AM-251 increased both ascending contraction and SP release orad, and descending relaxation and VIP release caudad. This suggested a role for CB-1 receptors in modulation of sensory transmission that was confirmed by the demonstration that central addition of anandamide decreased and AM-251 increased release of the sensory transmitter, calcitonin gene-related peptide (CGRP). We conclude that the potent antipropulsive effect of cannabinoids is the result of inhibition of both excitatory cholinergic/tachykininergic and inhibitory VIPergic motor neurons that mediate ascending contraction and descending relaxation, respectively, as well as inhibition of the intrinsic sensory CGRP-containing neurons that initiate the peristaltic reflex underlying propulsive motility.

Cannabinoid signalling in the enteric nervous system

Cannabinoid signalling is an important mechanism of synaptic modulation in the nervous system. Endogenous cannabinoids (anandamide and 2-arachidonyl-glycerol) are synthesized and released via calcium-activated biosynthetic pathways. Exogenous cannabinoids and endocannabinoids act on CB1 and CB2 receptors. CB1 receptors are neuronal receptors which couple via G-proteins to inhibition of adenylate cyclase or to activation or inhibition of ion channels. CB2 receptors are expressed by immune cells and cannabinoids can suppress immune function. In the central nervous system, the endocannabinoids may function as retrograde signals released by the postsynaptic neuron to inhibit neurotransmitter release from presynaptic nerve terminals. Enteric neurons also express CB receptors. Exogenously applied CB receptor agonists inhibit enteric neuronal activity but it is not clear if endocannabinoids released by enteric neurons can produce similar responses in the enteric nervous system (ENS). In this issue of Neurogastroenterology and Motility, Boesmans et al. show that CB1 receptor activation on myenteric neurons maintained in primary culture can suppress neuronal activity, inhibit synaptic transmission and mitochondrial transport along axons. They also provide initial evidence that myenteric neurons (or other cell types present in the cultures) release endocannabinoids and which activate CB1 receptors constitutively. These data provide new information about targets for cannabinoid signalling in the ENS and highlight the potential importance of CB receptors as drug targets. It is necessary that future work extends these interesting findings to intact tissues and ideally to the in vivo setting.

Selective lack of tolerance to delayed gastric emptying after daily administration of WIN 55,212-2 in the rat

The use of cannabinoids to treat gastrointestinal (GI) motor disorders has considerable potential. However, it is not clear if tolerance to their actions develops peripherally, as it does centrally. The aim of this study was to examine the chronic effects of the cannabinoid agonist WIN 55,212-2 (WIN) on GI motility, as well as those in the central nervous and cardiovascular systems. WIN was administered for 14 days, at either non-psychoactive or psychoactive doses. Cardiovascular parameters were measured in anaesthetized rats, whereas central effects and alterations in GI motor function were assessed in conscious animals using the cannabinoid tetrad and non-invasive radiographic methods, respectively. Tests were performed after first (acute effects) and last (chronic effects) administration of WIN, and 1 week after discontinuing treatment (residual effects). Food intake and body weight were also recorded throughout treatment. Blood pressure and heart rate remained unchanged after acute or chronic administration of WIN. Central activity and GI motility were acutely depressed at psychoactive doses, whereas non-psychoactive doses only slightly reduced intestinal transit. Most effects were reduced after the last administration. However, delayed gastric emptying was not and could, at least partially, account for a concomitant reduction in food intake and body weight gain. The remaining effects of WIN administration in GI motility were blocked by the CB1 antagonist AM 251, which slightly accelerated motility when administered alone. No residual effects were found 1 week after discontinuing cannabinoid treatment. The different systems show differential sensitivity to cannabinoids and tolerance developed at different rates, with delayed gastric emptying being particularly resistant to attenuation upon chronic treatment.

Mode of action of cannabinoids on nociceptive nerve endings

In recent years, cannabinoids have emerged as attractive alternatives or supplements to therapy for chronic pain states. However, in humans the activation of cannabinoid receptors in neurons of the central nervous system is associated with psychotropic side effects, temporary memory impairment and dependence, which arise via the effects of cannabinoids on forebrain circuits. For clinical exploitation of the analgesic properties of cannabinoids, a major challenge is to devise strategies that reduce or abolish their adverse effects on cognitive, affective and motor functions without attenuating their analgesic effects. The cannabinoid receptor family currently includes two cloned metabotropic receptors: CB1, CB2 and possibly GPR55 which are distributed widely across many key loci in pain-modulating pathways, including the peripheral terminals of primary afferents. Modulation of transducer ion channels expressed at nociceptive terminals occurs upon activation of metabotropic cannabinoid receptors, but direct cannabinoid action on ion channels involved in sensory transduction or regulation of neuron excitability likely contributes to the peripheral cannabinoid effects.

The role of fatty acid hydrolase gene variants in inflammatory bowel disease

BACKGROUND

Recent studies suggest a role for the endocannabinoid system, including fatty acid amide hydrolase (FAAH), in intestinal inflammation.

AIM

To analyse FAAH expression and the FAAH 385 C/A (p.Pro129Thr; rs324420) single nucleotide polymorphism (SNP) in-patients with Crohn's disease (CD) and ulcerative colitis (UC).

PATIENTS AND METHODS

Genomic DNA from 1008 individuals (CD: n = 435; UC: n = 167; controls: n = 406) was analysed for the FAAH 385 C/A SNP. We determined FAAH mRNA expression by quantitative PCR in CD and UC lesions as well as in intestinal epithelial cells (IECs).

RESULTS

There were no significant differences regarding the frequency of this SNP in the three study groups (CD, UC, controls). However, CD patients homozygous for the FAAH p.Pro129Thr polymorphism were more likely to develop a severe disease phenotype associated with fistulas (P = 0.03, OR 3.12, 95% CI 1.08-8.98) and extra-intestinal manifestations (P = 0.005, OR 4.29, CI 1.49-12.35). In UC, homozygous carriers had an earlier disease onset than wild-type carriers (P = 0.01). FAAH mRNA expression correlated with IL-8 mRNA expression in CD lesions (r = 0.53). However, pro-inflammatory stimuli did not significantly increase FAAH mRNA expression in IECs.

CONCLUSION

The FAAH p.Pro129Thr polymorphism may modulate the CD phenotype.

Influence of intracerebroventricular or intraperitoneal administration of cannabinoid receptor agonist (WIN 55,212-2) and inverse agonist (AM 251) on the regulation of food intake and hypothalamic serotonin levels

The effect of intracerebroventricular or intraperitoneal administration of cannabinoid receptor agonist WIN 55,212-2 or inverse agonist AM 251 on food intake and extracellular levels of serotonin and acetic acid 5-hydroxy-indol from presatiated rats was studied. Compared to the vehicle-injected control, the intracerebroventricular administration of WIN 55,212-2 was associated with a significant increase in food intake, whereas the administration of AM 251 caused a significant reduction in this respect. These results were accompanied by considerable reductions or increases in serotonin and acetic acid 5-hydroxy-indol levels compared to the vehicle-injected control and the baseline values for the different experimental groups studied. Intraperitoneal administration of WIN 55,212-2 at doses of 1 and 2 mg/kg promoted hyperphagia up to 6 h after injection, whereas administration of a higher dose (5 mg/kg) significantly inhibited food intake and motor behaviour in partially satiated rats. Administration of any of the AM 251 doses studied (0.5, 1, 2, 5 mg/kg) led to a significant decrease in the amount of food ingested from 2 h after the injection, compared to the vehicle-injected control group, with the most striking effect being observed when the 5 mg/kg dose was injected.

CB1 receptors mediate the analgesic effects of cannabinoids on colorectal distension-induced visceral pain in rodents

Activation of cannabinoid receptors (CB(1), CB(2) and GPR(55)) produces analgesic effects in several experimental pain models, including visceral pain arising from the gastrointestinal tract. We assessed the role of CB(1), CB(2), and GPR(55) receptors and the endogenous cannabinoid system on basal pain responses and acute mechanical hyperalgesia during colorectal distension (CRD) in rodents. The effects of cannabinoid receptor agonists and antagonists on pain-related responses to CRD were assessed in rats and in wild-type and CB(1) receptor knock-out mice. The dual CB(1/2) agonist, WIN55,212-2, and the peripherally acting CB(1)-selective agonist, SAB-378, inhibited pain-related responses to repetitive noxious CRD (80 mmHg) in a dose-related manner in rats. The analgesic effects of WIN55,212-2 and SAB-378 were blocked by the selective CB(1) antagonist SR141716, but were not affected by the selective CB(2) antagonist SR144528. SR141716, per se, increased the responses to repetitive noxious CRD, indicative of hyperalgesia, and induced pain-related responses during non-noxious CRD (20 mmHg), indicative of allodynia. The cannabinoid receptor agonists anandamide, virodhamine and O-1602 had no effect. At analgesic doses, WIN55,212-2 did not affect colonic compliance. In accordance to the rat data, WIN55,212-2 produced analgesia, whereas SR141716 induced hyperalgesia, during noxious CRD (55 mmHg) in wild-type but not in CB(1)-knock-out mice. These data indicate that peripheral CB(1) receptors mediate the analgesic effects of cannabinoids on visceral pain from the gastrointestinal tract. The allodynic and hyperalgesic responses induced by SR141716 suggest the existence of an endogenous cannabinoid tone and the activation of CB(1) receptors during noxious CRD.

Synthesis and characterization of a peripherally restricted CB1 cannabinoid antagonist, URB447, that reduces feeding and body-weight gain in mice

Cannabinoid CB(1) receptor antagonists reduce body weight in rodents and humans, but their clinical utility as anti-obesity agents is limited by centrally mediated side effects. Here, we describe the first mixed CB(1) antagonist/CB(2) agonist, URB447 ([4-amino-1-(4-chlorobenzyl)-2-methyl-5-phenyl-1H-pyrrol-3-yl](phenyl)methanone), which lowers food intake and body-weight gain in mice without entering the brain or antagonizing central CB(1)-dependent responses. URB447 may provide a useful pharmacological tool for investigating the cannabinoid system, and might serve as a starting point for developing clinically viable CB(1) antagonists devoid of central side effects.

Cannabinoid-1 (CB1) receptors regulate colonic propulsion by acting at motor neurons within the ascending motor pathways in mouse colon

Cannabinoid-1 (CB(1)) receptors on myenteric neurons are involved in the regulation of intestinal motility. Our aim was to investigate CB(1) receptor involvement in ascending neurotransmission in mouse colon and to characterize the involved structures by functional and morphological means. Presence of the CB(1) receptor was investigated by RT-PCR, and immunohistochemistry was used for colabeling studies. Myenteric reflex responses were initiated by electrical stimulation (ES) at different distances, and junction potentials (JP) were recorded from circular smooth muscle cells by intracellular recording in an unpartitioned and a partitioned recording chamber. In vivo colonic propulsion was tested in wild-type and CB(1)(-/-) mice. Immunostaining with the cytoskeletal marker peripherin showed CB(1) immunoreactivity both on Dogiel type I and type II neurons. Further neurochemical characterization revealed CB(1) on choline acetyltransferase-, calretinin-, and 5-HT-immunopositive myenteric neurons, but nitrergic neurons appeared immunonegative for CB(1) immunostaining. Solitary spindle-shaped CB(1)-immunoreactive cells in between smooth muscle cells lacked specific markers for interstitial cells of Cajal or glial cells. ES elicited neuronally mediated excitatory JP (EJP) and inhibitory JP. Gradual increases in distance resulted in a wave-like EJP with EJP amplitudes being maximal at the location of stimulating electrode 6 and a maximal EJP projection distance of approximately 18 mm. The CB(1) receptor agonist WIN 55,212-2 reduced the amplitude of EJP and was responsible for shortening the oral spreading of the excitatory impulse. In a partitioned chamber, WIN 55,212-2 reduced EJP at the separated oral sites, proving that CB(1) activation inhibits interneuron-mediated neurotransmission. These effects were absent in the presence of the CB(1) antagonist SR141716A, which, when given alone, had no effect. WIN 55,212-2 inhibited colonic propulsion in wild-type mice but not in SR141716A-pretreated wild-type or CB(1)(-/-) mice. Activation of the CB(1) receptor modulates excitatory cholinergic neurotransmission in mouse colon by reducing amplitude and spatial spreading of the ascending electrophysiological impulses. This effect on electrophysiological spreading involves CB(1)-mediated effects on motor neurons and ascending interneurons and is likely to underlie the here reported in vivo reduction in colonic propulsion.

Differential stimulatory effects of cannabinoids on VIP release and NO synthase activity in synaptosomal fractions from rat ileum

Cannabinoid-1 (CB1) and CB2 receptors are present on neurons of the enteric nervous system. Our aim was to study whether cannabinoid receptor activation is involved in the regulation of VIP release and NO synthesis in isolated fractions of nerve terminals from rat ileum. VIP was measured by RIA and NO synthesis was analyzed using a L-[3H]arginine assay. Anandamide stimulated VIP release (basal: 245.9+/-12.4pg/mg, 10(-6)M: 307.6+/-11.7pg/mg, [n=6, P<0.05], 10(-7)M: 367.0+/-26.1pg/mg, [n=6, P<0.01]). The cannabinoid receptor agonist WIN 55,212-2 had similar effects (basal: 250.5+/-37.4pg/mg, 10(-6)M: 320.9+/-34.7pg/mg; [n=4, P<0.05]). The stimulatory effect of anandamide was blocked by the selective CB2 receptor antagonist, SR144528 (10(-7)M) (anandamide 10(-6)M: 307.6+/-11.7pg/mg; +SR144528: 249.0+/-26.3pg/mg, [n=6, P<0.05]), whereas the selective CB1 receptor antagonist SR141716 A had no effect. NO synthesis was stimulated by anandamide ([fmol/mg/min] basal: 0.08+/-0.01, 10(-6)M: 0.16+/-0.03; 10(-7)M: 0.13+/-0.02, n=4, P<0.05) and WIN 55,212-2 ([fmol/mg/min] basal: 0.05+/-0.01, 10(-6)M: 0.1+/-0.02, n=4, P<0.05). The anandamide reuptake inhibitor, AM 404 increased basal NOS activity ([fmol/mg/min] control: 0.1+/-0.04, 10(-6)M: 0.28+/-0.08, n=7, P<0.05). The stimulatory effect of anandamide on NO synthase was not antagonized by antagonists at the CB1, CB2 or TRPV1 receptor, respectively. In conclusion, in enteric nerves anandamide stimulates VIP release by activation of a CB2 receptor specific pathway, while the stimulation of NO production suggests the existence of an additional type of cannabinoid receptor in the enteric nervous system.

The role of the endocannabinoid system in the pathophysiology and treatment of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a spectrum of disorders characterized by abdominal discomfort and pain, associated with altered bowel habits. Though gut motility, secretion and sensation may be altered in patients with IBS, the pathophysiology of this condition remains to be fully understood. The endocannabinoid system is involved in the regulation of numerous gastrointestinal functions including motility, sensation and secretion under both physiological and pathophysiological conditions. Activation of cannabinoid (CB)(1) and CB(2) receptors under various circumstances reduces motility, limits secretion and decreases hypersensitivity in the gut. Drugs that alter the levels of endocannabinoids in the gut also reduce motility and attenuate inflammation. In this review, we discuss the role of the endocannabinoid system in gastrointestinal physiology. We go on to consider the involvement of the endocannabinoid system in the context of symptoms associated with IBS and a possible role of this system in the pathophysiology and treatment of IBS.

Role of cannabinoid receptors in the control of gastrointestinal motility and perception

The identification of endocannabinoids and cannabinoid CB1 receptors in key areas of the intestinal wall, such as cholinergic neurons, supports a role for cannabinoids in the control of gastrointestinal motility. Activation of CB1 receptors inhibits the peristaltic reflex and slows down gastrointestinal and colonic transit. Endocannabinoids play an important inhibitory role in the control of the occurrence of transient lower esophageal sphincter relaxations. Cannabinoid receptor agonists inhibit gastric emptying and intestinal motility in humans. There is strong anatomical support for a role of CB1 receptors in the control of gastrointestinal perception, since these receptors have been identified in key sites of the neuronal circuitry involved in the transmission of visceral pain. Experimental data indicate a visceral antinociceptive action of cannabinoid receptor agonists, which remains to be confirmed in humans.

Cannabinoid CB1 receptors are expressed by parietal cells of the human gastric mucosa

Experimental data suggest that the endogenous cannabinoid system is involved in gastric function in different animal species. In most of them, CB(1) receptors have been localized on vagal terminals innervating the external wall of the stomach. We aimed at studying the putative presence and distribution of these receptors in the human gastric mucosa. To this end, we first performed Western blotting, RT-PCR, in situ hybridization, and immunohistochemical analysis of CB(1) protein distribution in biopsy samples of healthy individuals. To determine the precise cell populations expressing CB(1) receptors, we performed double immunofluorescence plus confocal microscopy analysis of the same samples. Our results show that CB(1) receptors are present in the gastric epithelium of the mucosa. Specifically, they are expressed by a subpopulation of mucosal cells, the acid-secreting parietal cells, as shown by double immunohistochemical staining and by their differential abundance in subregions of the gastric mucosa. These results reinforce the notion of a prominent role for the endocannabinoid system in the gastric function in humans and postulate the use of cannabinoid CB(1) receptors in parietal cells as new therapeutic targets for the regulation of gastric acid production.

CB1 receptor antagonist AVE1625 affects primarily metabolic parameters independently of reduced food intake in Wistar rats

The objective of the present study was to investigate in fed Wistar rats whether the cannabinoid-1 (CB1) receptor antagonist AVE1625 causes primary effects on metabolic blood and tissue parameters as well as metabolic rate, which are independent of reduced caloric intake. After single administration to rats postprandially, AVE1625 caused a slight dose-dependent increase in basal lipolysis. Six hours after single administration, liver glycogen content was dose-dependently reduced to approximately 60% of that of untreated controls. These findings demonstrate a primary acute effect of AVE1625 on induction of 1) lipolysis from fat tissue (increased FFA) and 2) glycogenolysis from the liver (reduced hepatic glycogen). Measured by indirect calorimetry, AVE1625 caused an immediate increase in total energy expenditure, a long-lasting increase of fat oxidation, and a transient increase of glucose oxidation, which were consistent with the acute findings on metabolic blood and tissue parameters. We conclude that, in addition to the well-investigated effects of CB1 receptor antagonists to reduce caloric intake and subsequently body weight, this pharmacological approach is additionally linked to inherently increased lipid oxidation. This oxidation is driven by persistently increased lipolysis from fat tissues, independently of reduced caloric intake, and might significantly contribute to the weight-reducing effect.

Cannabinoid type 1 receptor modulates intestinal propulsion by an attenuation of intestinal motor responses within the myenteric part of the peristaltic reflex

Cannabinoid-1 (CB1) receptor activation affects gastrointestinal propulsion in vivo. It was our aim to further characterize the involved myenteric mechanisms in vivo and in vitro. In CB1(-/-) mice and wild-type littermates we performed in vivo transit experiments by charcoal feeding and in vitro electrophysiological recordings in mouse small intestinal smooth muscle. Ascending neuronal contraction (ANC) following electrical field stimulation was studied in rat ileum in a partitioned organ bath separating the aboral stimulation site from the oral recording site. The knockout animals displayed an accelerated upper gastrointestinal transit compared to control animals. The CB1 receptor antagonist AM251 stimulated the force of the ANC in a concentration dependent manner when added in the oral chamber. Anandamide significantly inhibited the ANC when added in the oral chamber. Neither AM251 nor anandamide had an influence on the contraction latency. No effects were observed when drugs were added in the aboral chamber, proving a CB1 mediated action on the neuromuscular junction. Resting membrane potentials and neuronal induced inhibitory junction potentials in CB1(-/-) mice were unchanged as compared to wild type. However, the electrophysiological slow waves were more sensitive to blockade of Ca(2+) channels in CB1(-/-) mice. Our data strongly suggest a physiological involvement of the CB-1 receptor in the regulation of small intestinal motility. Therefore, CB1 receptors are a promising target for the treatment of motility disorders.

TRP channels as novel players in the pathogenesis and therapy of itch

Itch (pruritus) is a sensory phenomenon characterized by a (usually) negative affective component and the initiation of a special behavioral act, i.e. scratching. Older studies predominantly have interpreted itch as a type of pain. Recent neurophysiological findings, however, have provided compelling evidence that itch (although it indeed has intimate connections to pain) rather needs to be understood as a separate sensory modality. Therefore, a novel pruriceptive system has been proposed, within which itch-inducing peripheral mediators (pruritogens), itch-selective receptors (pruriceptors), sensory afferents and spinal cord neurons, and defined, itch-processing central nervous system regions display complex, layered responses to itch. In this review, we begin with a current overview on the neurophysiology of pruritus, and distinguish it from that of pain. We then focus on the functional characteristics of the large family of transient receptor potential (TRP) channels in skin-coupled sensory mechanisms, including itch and pain. In particular, we argue that - due to their expression patterns, activation mechanisms, regulatory roles, and pharmacological sensitivities - certain thermosensitive TRP channels are key players in pruritus pathogenesis. We close by proposing a novel, TRP-centered concept of pruritus pathogenesis and sketch important future experimental directions towards the therapeutic targeting of TRP channels in the clinical management of itch.

Effects of a cannabinoid receptor agonist on colonic motor and sensory functions in humans: a randomized, placebo-controlled study

Cannabinoid receptors (CBR) are located on cholinergic neurons in the brain stem, stomach, and colon. CBR stimulation inhibits motility in rodents. Effects in humans are unclear. Dronabinol (DRO), a nonselective CBR agonist, inhibits colonic motility and sensation. The aim of this study was to compare effects of DRO and placebo (PLA) on colonic motility and sensation in healthy volunteers. Fifty-two volunteers were randomly assigned (double-blind) to a single dose of 7.5 mg DRO or PLA postoperative with concealed allocation. A balloon-manometric assembly placed into the descending colon allowed assessment of colonic compliance, motility, tone, and sensation before and 1 h after oral ingestion of medication, and during fasting, and for 1 h after 1,000-kcal meal. There was an overall significant increase in colonic compliance (P = 0.045), a borderline effect of relaxation in fasting colonic tone (P = 0.096), inhibition of postprandial colonic tone (P = 0.048), and inhibition of fasting and postprandial phasic pressure (P = 0.008 and 0.030, respectively). While DRO did not significantly alter thresholds for first gas or pain sensation, there was an increase in sensory rating for pain during random phasic distensions at all pressures tested and in both genders (P = 0.024). In conclusion, in humans the nonselective CBR agonist, DRO, relaxes the colon and reduces postprandial colonic motility and tone. Increase in sensation ratings to distension in the presence of relaxation of the colon suggests central modulation of perception. The potential for CBR to modulate colonic motor function in diarrheal disease such as irritable bowel syndrome deserves further study.

Involvement of CB1 and CB2 receptors in the modulation of cholinergic neurotransmission in mouse gastric preparations

While most of the studies concerning the role of cannabinoids on gastric motility have focused the attention on the gastric emptying in in vivo animal models, there is little information about the cannabinoid peripheral influence in the stomach. In addition, the functional features of CB2 receptors in the gastrointestinal tract have been poorly characterized. The purpose of the present study was to investigate the effects of cannabinoid drugs on the excitatory cholinergic and inhibitory non-adrenergic non-cholinergic (NANC) neurotransmission in mouse isolated gastric preparations. Intraluminal pressure from isolated whole stomach was recorded and mechanical responses induced by electrical field stimulation (EFS) were analyzed in different experimental conditions. EFS (0.5ms duration, supramaximal voltage, in trains of 5s, 2-16Hz) caused a cholinergic contraction, which was abolished by atropine or tetrodotoxin (TTX). The cannabinoid receptor agonist, WIN 55,212-2, the endogenous ligand, anandamide, the selective CB1 receptor agonist ACEA, and the selective CB2 receptor agonists, JWH015 and JWH133, produced a concentration-dependent reduction of the EFS-evoked cholinergic contractions. SR141716A, CB1 receptor antagonist, significantly attenuated the inhibitory effects induced by WIN 55,212-2, anandamide or ACEA, without affecting those caused by JWH133. AM630, CB2 receptor antagonist, reduced the inhibitory effects induced by WIN 55,212-2, anandamide, JWH015 or JWH133, without affecting those caused by ACEA. The joint application of SR141716A and AM630 was able of fully preventing the WIN 55,212-2 and anandamide actions. The cannabinoid antagonists failed per se to affect the neurally evoked responses. Cannabinoids did not modify the contractions produced by exogenous carbachol. In the presence of atropine and guanethidine (NANC conditions) EFS-induced TTX-sensitive relaxation consisting in an early and rapid component followed by a second slow phase, which were unaffected by cannabinoid drugs. In conclusion, the present results suggest that cannabinoids play a prejunctional modulatory role on the cholinergic excitatory transmission without affecting the NANC inhibitory transmission. In addition, this study provides experimental evidence that also the activation of CB2 receptors is able to reduce cholinergic neurotransmission in the mouse stomach.

Cannabinoids mediate analgesia largely via peripheral type 1 cannabinoid receptors in nociceptors

Although endocannabinoids constitute one of the first lines of defense against pain, the anatomical locus and the precise receptor mechanisms underlying cannabinergic modulation of pain are uncertain. Clinical exploitation of the system is severely hindered by the cognitive deficits, memory impairment, motor disturbances and psychotropic effects resulting from the central actions of cannabinoids. We deleted the type 1 cannabinoid receptor (CB1) specifically in nociceptive neurons localized in the peripheral nervous system of mice, preserving its expression in the CNS, and analyzed these genetically modified mice in preclinical models of inflammatory and neuropathic pain. The nociceptor-specific loss of CB1 substantially reduced the analgesia produced by local and systemic, but not intrathecal, delivery of cannabinoids. We conclude that the contribution of CB1-type receptors expressed on the peripheral terminals of nociceptors to cannabinoid-induced analgesia is paramount, which should enable the development of peripherally acting CB1 analgesic agonists without any central side effects.

Cannabinoid-induced hyperphagia: correlation with inhibition of proopiomelanocortin neurons?

We tested the hypothesis that cannabinoids modulate feeding in male guinea pigs, and correlated cannabinoid-induced changes in feeding behavior with alterations in glutamatergic synaptic currents impinging upon proopiomelanocortin (POMC) neurons of the hypothalamic arcuate nucleus. Feeding experiments were performed as follows: after a three-day acclimation period, animals were weighed and injected with either the CB1 receptor agonist WIN 55,212-2 (1 mg/kg, s.c.), antagonist AM251 (3 mg/kg, s.c.) or their cremophore/ethanol/saline vehicle (1:1:18; 1 ml/kg, s.c.) each day for seven days. WIN 55,212-2 increased, whereas AM251 decreased, the rate of cumulative food intake. The agonist effect was manifest primarily by increases in meal frequency and the amount of food eaten per meal. By contrast, the antagonist effect was associated with decreases in meal frequency, duration and weight loss. For the electrophysiological experiments, we performed whole-cell patch-clamp recordings from POMC neurons in hypothalamic slices. WIN 55,212-2 decreased the amplitude of evoked, glutamatergic excitatory postsynaptic currents (eEPSCs) and increased the S2:S1 ratio. Conversely, AM251 increased eEPSC amplitude per se, and blocked the inhibitory effects of the agonist. WIN 55,212-2 also decreased miniature EPSC (mEPSC) frequency; whereas AM251 increased mEPSC frequency per se, and again blocked the inhibitory effect of the agonist. A subpopulation of cells exhibited an agonist-induced outward current, which was blocked by AM251, associated with increased conductance and reversed polarity near the Nernst equilibrium potential for K(+). These data demonstrate that cannabinoids regulate appetite in the guinea pig in part through both presynaptic and postsynaptic actions on anorexigenic POMC neurons.

Overactivity of the intestinal endocannabinoid system in celiac disease and in methotrexate-treated rats

The endocannabinoid system is upregulated in both human inflammatory bowel diseases and experimental models of colitis. In this study, we investigated whether this upregulation is a marker also of celiac disease-induced atrophy. The levels of the cannabinoid CB(1) receptor, of the endocannabinoids, anandamide, and 2-arachidonoyl-glycerol (2-AG), and of the anti-inflammatory mediator palmitoylethanolamide (PEA) were analyzed in bioptic samples from the duodenal mucosa of celiac patients at first diagnosis assessed by the determination of antiendomysial antibodies and histological examination. Samples were analyzed during the active phase of atrophy and after remission and compared to control samples from non-celiac patients. The levels of anandamide and PEA were significantly elevated (approx. 2- and 1.8-fold, respectively) in active celiac patients and so were those of CB(1) receptors. Anandamide levels returned to normal after remission with a gluten-free diet. We also analyzed endocannabinoid and PEA levels in the jejunum of rats 2, 3, and 7 days after treatment with methotrexate, which causes inflammatory features (assessed by histopathological analyses and myeloperoxidase activity) similar to those of celiac patients. In both muscle/serosa and mucosa layers, the levels of anandamide, 2-AG, and PEA peaked 3 days after treatment and returned to basal levels at remission, 7 days after treatment. Thus, intestinal endocannabinoid levels peak with atrophy and regress with remission in both celiac patients and methotrexate-treated rats. The latter might be used as a model to study the role of the endocannabinoid system in celiac disease.

Bidirectional alterations of hippocampal cannabinoid 1 receptors and their endogenous ligands in a rat model of alcohol withdrawal and dependence

BACKGROUND

The hippocampus is strongly implicated in memory processes and contains high concentrations of both cannabinoid receptors and their endogenous ligands. Chronic alcohol consumption impairs a variety of cognitive and performance tasks, including memory and learning. As the activation of cannabinoid receptors by their endogenous ligands modulates hippocampal neurotransmission, we hypothesized that the impaired memory and learning in alcoholism may be due to alterations in the hippocampal endocannabinoid system.

METHODS

We used the rat chronic intermittent ethanol (CIE) model for alcohol withdrawal and dependence which involves intermittent episodes of ethanol intoxication (60 doses) and withdrawal (approximating binge drinking episodes in humans). We measured the levels of cannabinoid 1 receptor (CB1R) protein (Western blot using a C-terminal-directed antibody), CB1R mRNA (real-time RT-PCR), CB1R localization (immunocytochemistry), tissue levels of the endocannabinoids N-arachidonoylethanolamine/anandamide (AEA) and 2-arachidonoylglycerol (2-AG), and function (patch-clamp recordings of depolarization-induced suppression of inhibition (DSI), as well as effects of CB1R agonist WIN 55,212-2 on inhibitory currents) in the hippocampus of CIE rats and their saline-treated controls.

RESULTS

Results were obtained in saline and CIE-treated rats after 2 and 40 days of withdrawal (DW) from their respective treatments. In 2 DW CIE rats, CB1R mRNA and protein levels were decreased by 27% (p<0.05) compared with saline controls. Surprisingly, in 40 DW CIE rats, CB1R mRNA increased by 100% and protein increased by 21%, confirmed by immunohistochemistry. Hippocampal [2-AG] increased in both 2 and 40 DW CIE rats; [AEA] increased only at 40 DW. Hippocampal DSI of CIE rats was significantly reduced at 2 DW but not at 40 DW. The CB1R agonist WIN 55,212-2 (0.5 microM) produced a significantly greater decrease in the frequency of spontaneous inhibitory currents from saline-treated rats compared with CIE rats at 2 DW, but not at 40 DW.

CONCLUSIONS

These data demonstrate that CIE treatment and withdrawal transiently down-regulates hippocampal CB1 Rs followed by a long-term up-regulation, including increased levels of endogenous cannabinoids. These findings are consistent with our hypothesis and suggest that long-term up-regulation of hippocampal CB1Rs may contribute to the long-term cognitive impairments in alcoholism. The data further suggest that the effectiveness of CB1R blockade in decreasing alcohol consumption may be greater after protracted abstinence from alcohol.

AM 251 produces sustained reductions in food intake and body weight that are resistant to tolerance and conditioned taste aversion

The cannabinoid 1 (CB(1)) receptor has been implicated in the regulation of food intake. Here, we examine the effect of the CB(1) receptor antagonist AM 251 on food intake and body weight over a prolonged period. Further, we examine whether AM 251 produces conditioned taste aversion (CTA) and if sustained antagonism at central receptors contributes to its anorectic effect. The effect of AM 251 of food intake and body weight was examined in daily (1 mg kg(-1)) and 5-day (5 mg kg(-1)) dosing schedules. Matching reductions in food intake and body weight were observed in both paradigms. A single administration of AM 251 (5 mg kg(-1)) significantly reduced food intake for 4 days. Tolerance to the anorectic effects of AM 251 did not develop in either dosing strategy. Active avoidance of AM 251 (3; 5 mg kg(-1), i.p.) was examined using a CTA assay. Rats showed no evidence of CTA associated with AM 251. We investigated the sustained effect of AM 251 (5 mg kg(-1), i.p.) on CB(1) receptors in the hypothalamus using Delta(9)-tetrahydrocannabinol (8 mg kg(-1), i.p.) induced hypothermia. AM 251 initially blocked hypothermia, but this effect was not seen 2 or 4 days later. The results demonstrate that smaller, or infrequent, administrations of AM 251 can produce sustained reductions in food intake and body weight in rat. Reductions in food intake were sustained longer than AM 251 antagonized the effects of a CB(1) receptor agonist in the hypothalamus, and occurred independently of CTA.

Cannabinoid 1 (CB1) receptors coupled to cholinergic motorneurones inhibit neurogenic circular muscle contractility in the human colon

The effects of cannabinoid subtype 1 (CB(1)) receptor activation were determined on smooth muscle, inhibitory and excitatory motorneuronal function in strips of human colonic longitudinal muscle (LM) and circular muscle (CM) in vitro. Electrical field stimulation (EFS; 0.5-20 Hz, 50 V) evoked a relaxation in LM and CM precontracted with a neurokinin-2 (NK-2) selective receptor agonist (beta-ala(8)-neurokinin A; 10(-6) M) in the presence of atropine (10(-6) M); this was unaltered following pretreatment with the CB(1)-receptor selective agonist arachidonyl-2-chloroethylamide (ACEA; 10(-6) M). In the presence of nitric oxide synthase blockade with N-nitro-L-arginine (10(-4) M), EFS evoked a frequency-dependent 'on-contraction' during stimulation and an 'off-contraction' following stimulus cessation. On-contractions were significantly inhibited in CM strips by pretreatment with ACEA (10(-6) M). These inhibitory effects were reversed in the presence of the CB(1) receptor-selective antagonist N-(piperidine-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (10(-7) M). ACEA did not alter LM or CM contractile responses to acetylcholine or NK-2 receptor-evoked contraction. Immunohistochemical studies revealed a colocalisation of CB(1) receptors to cholinergic neurones in the human colon based on colabelling with choline acetyltransferase, in addition to CB(1) receptor labelling in unidentified structures in the CM. In conclusion, activation of CB(1) receptors coupled to cholinergic motorneurones selectively and reversibly inhibits excitatory nerve transmission in colonic human colonic CM. These results provide evidence of a direct role for cannabinoids in the modulation of motor activity in the human colon by coupling to cholinergic motorneurones.

Involvement of cannabinoid receptors in inflammatory hypersensitivity to colonic distension in rats

Activation of cannabinoid CB1 and CB2 receptors is known to attenuate nociception and hyperalgesia in somatic inflammatory conditions. The aim of this study was to determine whether cannabinoids modulate colonic sensitivity in basal and inflammatory conditions. The effects of CB1 and CB2 receptor agonists and antagonists on the abdominal contractile response to colorectal distension (CRD) in basal conditions and after 2,4,6-trinitrobenzenesulphonic acid-induced colitis were investigated. As previously described, colitis triggered a hypersensitivity to CRD. In basal conditions, both CB1 (WIN 55212-2) and CB2 (JWH 015) agonists reduced the abdominal response to CRD at a dose of 1 mg kg(-1), i.p. Both compounds were active at a lower dose (0.1 mg kg(-1)) abolishing the hypersensitivity induced by colitis. Administered alone, CB1 (Rimonabant) and CB2 (SR 144528) receptor antagonists (10 mg kg(-1)) had no effect on basal sensitivity. In contrast, the CB1, but not the CB2, receptor antagonist enhanced colitis-induced hyperalgesia. It is concluded that colonic inflammation enhances the antinociceptive action of CB1 and CB2 receptor agonists, and activates an endogenous, CB1 receptor mediated, antinociceptive pathway.

The endocannabinoid system as an emerging target of pharmacotherapy

The recent identification of cannabinoid receptors and their endogenous lipid ligands has triggered an exponential growth of studies exploring the endocannabinoid system and its regulatory functions in health and disease. Such studies have been greatly facilitated by the introduction of selective cannabinoid receptor antagonists and inhibitors of endocannabinoid metabolism and transport, as well as mice deficient in cannabinoid receptors or the endocannabinoid-degrading enzyme fatty acid amidohydrolase. In the past decade, the endocannabinoid system has been implicated in a growing number of physiological functions, both in the central and peripheral nervous systems and in peripheral organs. More importantly, modulating the activity of the endocannabinoid system turned out to hold therapeutic promise in a wide range of disparate diseases and pathological conditions, ranging from mood and anxiety disorders, movement disorders such as Parkinson's and Huntington's disease, neuropathic pain, multiple sclerosis and spinal cord injury, to cancer, atherosclerosis, myocardial infarction, stroke, hypertension, glaucoma, obesity/metabolic syndrome, and osteoporosis, to name just a few. An impediment to the development of cannabinoid medications has been the socially unacceptable psychoactive properties of plant-derived or synthetic agonists, mediated by CB(1) receptors. However, this problem does not arise when the therapeutic aim is achieved by treatment with a CB(1) receptor antagonist, such as in obesity, and may also be absent when the action of endocannabinoids is enhanced indirectly through blocking their metabolism or transport. The use of selective CB(2) receptor agonists, which lack psychoactive properties, could represent another promising avenue for certain conditions. The abuse potential of plant-derived cannabinoids may also be limited through the use of preparations with controlled composition and the careful selection of dose and route of administration. The growing number of preclinical studies and clinical trials with compounds that modulate the endocannabinoid system will probably result in novel therapeutic approaches in a number of diseases for which current treatments do not fully address the patients' need. Here, we provide a comprehensive overview on the current state of knowledge of the endocannabinoid system as a target of pharmacotherapy.

Effect of a cannabinoid agonist on gastrointestinal transit and postprandial satiation in healthy human subjects: a randomized, placebo-controlled study

Cannabinoid receptor (CBR) stimulation inhibits motility and increases food intake in rodents. Effects of CBR stimulation in human gastrointestinal (GI) tract are unclear. We compared effects of dronabinol (DRO) and placebo (PLA) on GI transit, gastric volume and satiation in humans. In a double-blind, randomized study, 30 healthy volunteers were randomly assigned to DRO 5 mg b.i.d. or PLA for three doses. We measured GI functions noninvasively: day 0, Ensure satiation test to measure maximum tolerated volume (MTV) and 30-min post-Ensure symptoms; day 1, scintigraphic transit ((111)In-egg meal) and fasting and postprandial gastric volume ((99Tcm)-SPECT); day 2, 24-h colonic transit and repeat satiation test. ancova was used to compare treatment groups with gender, age, and, for the satiation test, the baseline MTV, as covariates. A log-rank test was used to assess treatment effects on gastric emptying. Planned sample size had 80% power to detect 25-30% differences in primary end points. There was an overall retardation of gastric emptying with DRO (P = 0.018); this was more pronounced in females (P = 0.011), than in males (P = 0.184). No significant treatment differences were detected for gastric volumes, MTV, post-Ensure(R) symptoms, small bowel and colonic transit. Fasting gastric volume was greater in males receiving DRO compared with PLA (238 +/- 17 vs 185 +/- 16, P = 0.04). DRO retards gastric emptying in humans; effects are gender-related. Dronabinol also increases fasting gastric volumes in males.

Agonists of cannabinoid receptor 1 and 2 inhibit experimental colitis induced by oil of mustard and by dextran sulfate sodium

Oil of mustard (OM) is a potent neuronal activator that is known to elicit visceral hyperalgesia when given intracolonically, but the full extent to which OM is also proinflammatory in the gastrointestinal tract is not known. We have previously shown that male CD-1 mice given a single administration of 0.5% OM develop a severe colitis that is maximum at day 3 and that gradually lessens until essentially absent by day 14. OM-induced neuronal stimulation is reported to be reduced by cannabinoid agonists, and cannabinoid receptor 1 (CB1R)-/- mice have exacerbated experimental colitis. Therefore, we examined the role of cannabinoids in this OM-induced 3-day model of colitis in CD-1 mice and in a 7-day dextran sulfate sodium (DSS) colitis model in BALB/c mice. In OM colitis, the CB1R-selective agonist ACEA and the CB2R-selective agonist JWH-133 reduced (P < 0.05) colon weight gain (means +/- SE; 82 +/- 13% and 47 +/- 15% inhibition, respectively), colon shrinkage (98 +/- 24% and 42 +/- 12%, respectively), colon inflammatory damage score (49 +/- 11% and 40 +/- 12%, respectively), and diarrhea (58 +/- 12% and 43 +/- 11%, respectively). Histological damage was similarly reduced by these treatments. Likewise, CBR agonists attenuated DSS colitis, albeit at higher doses; ACEA at 10 mg/kg, twice daily, inhibited (P < 0.05) macroscopic and microscopic scores (46 +/- 9% and 63 +/- 7%, respectively); whereas 20 mg/kg, twice daily, of JWH-133 was required to diminish (P < 0.05) macroscopic and microscopic scores (29 +/- 7% and 43 +/- 5%, respectively). CB1R and CB2R immunostaining of colon sections revealed that CB1R in enteric neurons was more intense in colitic vs. control mice; however, CB1R was also increased in the endothelial layer in OM colitis only. CB2R immunostaining was more marked in infiltrated immune cells in OM colitis. These findings validate the OM colitis model with respect to the DSS model and provide strong support to the emerging idea that cannabinoid receptor activation mediates protective mechanisms in experimental colitis. The demonstration of CB1R agonist effects in colitis support the neurogenic nature of the OM-induced colitis model and reinforce the importance of neuronal activation in intestinal inflammation.

Site-specific increases in peripheral cannabinoid receptors and their endogenous ligands in a model of neuropathic pain

Selective activation of the peripheral cannabinoid receptor 1 (CB1R) has been shown to suppress neuropathic pain symptoms in rodents. However, relatively little is known about changes in CB1R and its endogenous ligands during development or maintenance of neuropathic pain. Using immunohistochemistry, Western blot, real-time reverse transcription polymerase chain reaction, as well as liquid chromatography/mass spectrometry, we studied the changes in CB1Rs and endocannabinoids N-arachidonoylethanolamine/anandamide (AEA) and 2-arachidonoylglycerol (2-AG) in rat lumbar (L4 and L5) dorsal root ganglia (DRG) after neuropathic pain induction (L5 spinal nerve ligation: SNL). Immunohistochemistry revealed that in control rats, CB1R is expressed in the majority (76-83%) of nociceptive neurons as indicated by co-labeling with isolectin B4 (IB4) or antibodies recognizing transient receptor potential vanilloid (TRPV1), calcitonin gene related peptide (CGRP), and the NR2C/2D subunits of the N-methyl-D-aspartate receptor. After L5 SNL, CB1R mRNA and protein increases in the ipsilateral uninjured L4 DRG whereas the percentages of CB1R immunoreactive (CB1R-ir) neurons remain unchanged in L4 and L5 DRG. However, for these CB1R-ir neurons, we observe significant increases in percentage of TRPV1-ir cells in ipsilateral L4 DRG, and decreases in percentage of IB4- and CGRP-co-labeled cells in ipsilateral L5 DRG. Levels of both AEA and 2-AG increase significantly only in the ipsilateral L5 DRG. These results are consistent with the preserved analgesic effects of cannabinoids in neuropathic pain and provide a rational framework for the development of peripherally acting endocannabinoid-based therapeutic interventions for neuropathic pain.

The neurobiology of itch

The neurobiology of itch, which is formally known as pruritus, and its interaction with pain have been illustrated by the complexity of specific mediators, itch-related neuronal pathways and the central processing of itch. Scratch-induced pain can abolish itch, and analgesic opioids can generate itch, which indicates an antagonistic interaction. However, recent data suggest that there is a broad overlap between pain- and itch-related peripheral mediators and/or receptors, and there are astonishingly similar mechanisms of neuronal sensitization in the PNS and the CNS. The antagonistic interaction between pain and itch is already exploited in pruritus therapy, and current research concentrates on the identification of common targets for future analgesic and antipruritic therapy.

A brief history of cannabinoid and endocannabinoid pharmacology as inspired by the work of British scientists

British scientists have played a leading role in the long history of cannabinoid and endocannabinoid research. Such research has progressed from the first crucial evaluation of the medicinal properties of Cannabis sativa in the Western world to pioneering studies of the chemical constituents of this plant, the development of in vitro biological assays to study cannabinoids, the identification of the mechanism of action of cannabinoids, the discovery of endocannabinoids and the assessment of their therapeutic implications. Stemming from the many innovative ideas and achievements of these researchers, I provide a personal view of where these studies have led us thus far and where they are likely to take us in the future.

Effects of cannabinoid receptor agonists on rat gastric acid secretion: discrepancy between in vitro and in vivo data

The effects of the cannabinoid (CB)-receptor agonists WIN55,212-2 and HU-210 and the selective CB(1)-receptor antagonist SR141716A were tested on in vitro and in vivo acid secretion assays from the rat. In the isolated gastric fundus from immature rats, WIN55,212-2 (0.001-30 microM), HU-210 (0.001-10 microM), or SR141716A (0.1-10 microM) did not change the basal acid output or acid responses to histamine, pentagastrin, or electrical field stimulation. HU-210 (0.3 micromol/kg, intravenously) inhibited the acid response to pentagastrin in anesthetized adult, young, or immature rats with lumen-perfused stomachs; moreover, HU-210 reduced vagally induced acid secretion in adult animals, its antisecretory effect being reversed by SR141716A (0.65 micromol/kg, intravenously). In vitro and in vivo data indicate that CB(1) receptors are not located on parietal cells but, rather, on vagal pathways (possibly at preganglionic sites) supplying the gastric mucosa. The lack of effect of CB-receptor ligands in vitro cannot be ascribed to the use of immature rats, since HU-210 inhibited stimulated acid secretion in vivo, irrespective of the animal age.