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

Role of endogenous cannabinoids in synaptic signaling

Research of cannabinoid actions was boosted in the 1990s by remarkable discoveries including identification of endogenous compounds with cannabimimetic activity (endocannabinoids) and the cloning of their molecular targets, the CB1 and CB2 receptors. Although the existence of an endogenous cannabinoid signaling system has been established for a decade, its physiological roles have just begun to unfold. In addition, the behavioral effects of exogenous cannabinoids such as delta-9-tetrahydrocannabinol, the major active compound of hashish and marijuana, await explanation at the cellular and network levels. Recent physiological, pharmacological, and high-resolution anatomical studies provided evidence that the major physiological effect of cannabinoids is the regulation of neurotransmitter release via activation of presynaptic CB1 receptors located on distinct types of axon terminals throughout the brain. Subsequent discoveries shed light on the functional consequences of this localization by demonstrating the involvement of endocannabinoids in retrograde signaling at GABAergic and glutamatergic synapses. In this review, we aim to synthesize recent progress in our understanding of the physiological roles of endocannabinoids in the brain. First, the synthetic pathways of endocannabinoids are discussed, along with the putative mechanisms of their release, uptake, and degradation. The fine-grain anatomical distribution of the neuronal cannabinoid receptor CB1 is described in most brain areas, emphasizing its general presynaptic localization and role in controlling neurotransmitter release. Finally, the possible functions of endocannabinoids as retrograde synaptic signal molecules are discussed in relation to synaptic plasticity and network activity patterns.

Therapeutic potential of cannabinoids in CNS disease

The major psychoactive constituent of Cannabis sativa, delta(9)-tetrahydrocannabinol (delta(9)-THC), and endogenous cannabinoid ligands, such as anandamide, signal through G-protein-coupled cannabinoid receptors localised to regions of the brain associated with important neurological processes. Signalling is mostly inhibitory and suggests a role for cannabinoids as therapeutic agents in CNS disease where inhibition of neurotransmitter release would be beneficial. Anecdotal evidence suggests that patients with disorders such as multiple sclerosis smoke cannabis to relieve disease-related symptoms. Cannabinoids can alleviate tremor and spasticity in animal models of multiple sclerosis, and clinical trials of the use of these compounds for these symptoms are in progress. The cannabinoid nabilone is currently licensed for use as an antiemetic agent in chemotherapy-induced emesis. Evidence suggests that cannabinoids may prove useful in Parkinson's disease by inhibiting the excitotoxic neurotransmitter glutamate and counteracting oxidative damage to dopaminergic neurons. The inhibitory effect of cannabinoids on reactive oxygen species, glutamate and tumour necrosis factor suggests that they may be potent neuroprotective agents. Dexanabinol (HU-211), a synthetic cannabinoid, is currently being assessed in clinical trials for traumatic brain injury and stroke. Animal models of mechanical, thermal and noxious pain suggest that cannabinoids may be effective analgesics. Indeed, in clinical trials of postoperative and cancer pain and pain associated with spinal cord injury, cannabinoids have proven more effective than placebo but may be less effective than existing therapies. Dronabinol, a commercially available form of delta(9)-THC, has been used successfully for increasing appetite in patients with HIV wasting disease, and cannabinoid receptor antagonists may reduce obesity. Acute adverse effects following cannabis usage include sedation and anxiety. These effects are usually transient and may be less severe than those that occur with existing therapeutic agents. The use of nonpsychoactive cannabinoids such as cannabidiol and dexanabinol may allow the dissociation of unwanted psychoactive effects from potential therapeutic benefits. The existence of other cannabinoid receptors may provide novel therapeutic targets that are independent of CB(1) receptors (at which most currently available cannabinoids act) and the development of compounds that are not associated with CB(1) receptor-mediated adverse effects. Further understanding of the most appropriate route of delivery and the pharmacokinetics of agents that act via the endocannabinoid system may also reduce adverse effects and increase the efficacy of cannabinoid treatment. This review highlights recent advances in understanding of the endocannabinoid system and indicates CNS disorders that may benefit from the therapeutic effects of cannabinoid treatment. Where applicable, reference is made to ongoing clinical trials of cannabinoids to alleviate symptoms of these disorders.

Effects of cannabinoids on lithium-induced conditioned rejection reactions in a rat model of nausea

RATIONALE

Marijuana has been reported to suppress nausea produced by chemotherapy treatment in human cancer patients. Although there is abundant evidence that cannabinoid agonists attenuate vomiting in emetic species, there has been little experimental evidence of their anti-nausea potential. Considerable evidence suggests that conditioned rejection reactions in rats reflect nausea. The present experiments evaluated the potential of low doses of the cannabinoid agonists, delta-9-tetrahydrocannabinol (THC; 0.5 mg/kg, i.p.), and HU-210 (0.001 mg/kg and 0.01 mg/kg, i.p.), and the CB(1) antagonist SR-141716A in modulating the establishment and the expression of lithium-induced conditioned rejection reactions in rats.

OBJECTIVES

To evaluate the effect of cannabinoids on conditioned rejection reactions, a rat model of nausea.

METHODS

In experiments 1 and 2, respectively, rats were injected with cannabinoid agonists, THC (0.5 mg/kg, i.p.) and HU-210 (0.001, 0.005 or 0.01 mg/kg), 30 min prior to exposure to 0.1% saccharin solution by intraoral infusion. Immediately following saccharin exposure, they were injected with 20 ml/kg 0.15 M lithium chloride or saline. On each of two test trials, the rats were injected with the cannabinoid or vehicle 30 min prior to exposure to saccharin. In experiment 3, rats were injected with the CB(1) antagonist, SR-141716A (2.5 mg/kg) or a combination of SR-141716A and HU-210 (0.01 mg/kg) 30 min prior to an infusion of saccharin followed by injection of lithium or saline. They were given a single drug-free test trial. Experiment 4 replicated and extended the findings of experiment 3.

RESULTS

delta-9-THC and HU-210 interfered with the establishment and the expression of lithium-induced conditioned rejection reactions. The suppressive effect of HU-210 on rejection reactions was reversed by pretreatment with SR-141716A. Administration of SR-141716A prior to conditioning potentiated lithium-induced conditioned rejection reactions.

CONCLUSIONS

These results indicate that the establishment and the expression of lithium-induced conditioned rejection reactions are suppressed by pretreatment with cannabinoid agents. These effects appear to be mediated by their action on the CB(1) receptor, because they are reversed by pretreatment with SR-141716A. Finally, our results suggest that endogenous cannabinoids play a role in modulation of nausea, because the antagonist potentiated lithium-induced nausea.

Antiemetic and motor-depressive actions of CP55,940: cannabinoid CB1 receptor characterization, distribution, and G-protein activation

Dibenzopyran (Delta(9)-tetrahydrocannabinol) and aminoalkylindole [R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrolol[1,2,3-de]-1,4-benzoxazin-yl]-(1-naphthalenyl) methanone mesylate; (WIN55,212-2)] cannabinoids suppress vomiting produced by cisplatin via cannabinoid CB(1) receptors. This study investigates the antiemetic potential of the "nonclassical" cannabinoid CP55,940 [1alpha,2beta-(R)-5alpha]-(-)-5-(1,1-dimethyl)-2-[5-hydroxy-2-(3-hydroxypropyl) cyclohexyl-phenol] against cisplatin-induced vomiting and assesses the presence and functionality of cannabinoid CB(1) receptors in the least shrew (Cryptotis parva) brain. CP55,940 (0.025-0.3 mg/kg) reduced both the frequency of cisplatin-induced emesis (ID(50)=0.025 mg/kg) and the percentage of shrews vomiting (ID(50)=0.09 mg/kg). CP55,940 also suppressed shrew motor behaviors (ID(50)=0.06- 0.21 mg/kg) at such doses. The antiemetic and motor-suppressant actions of CP55,940 were countered by SR141716A [N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methylpyrazole-3-carboxamide], indicating both effects are cannabinoid CB(1) receptor-mediated. Autoradiographic studies with [3H]-SR141716A and [35S]-GTPgammaS binding revealed that the distribution of the cannabinoid CB(1) receptor and its activation pattern are similar to rodent brain and significant levels are present in brain loci (e.g., nucleus tractus solitarius (NTS)) that control emesis. The affinity rank order of structurally diverse cannabinoid ligands for cannabinoid CB(1) receptor in shrew brain is similar to rodent brain: HU-210=CP55,940=SR141716A>/=WIN55,212-2>/=delta-9-tetrahydrocannabinol>methanandamide=HU-211=cannabidiol=2-arachidonoylglycerol. This affinity order is also similar and is highly correlated to the cannabinoid EC(50) potency rank order for GTPgammaS stimulation except WIN55,212-2 and delta-9-tetrahydrocannabinol potency order were reversed. The affinity and the potency rank order of tested cannabinoids were significantly correlated with their antiemetic ID(50) potency order against cisplatin-induced vomiting (CP55,940>WIN55,212-2=delta-9-tetrahydrocannabinol) as well as emesis produced by 2-arachidonoylglycerol or SR141716A (CP55,940>WIN55,212-2>delta-9-tetrahydrocannabinol).

Cannabinoids for gastrointestinal diseases: potential therapeutic applications

Delta(9)-Tetrahydrocannabinol (the active ingredient of marijuana), as well as endogenous and synthetic cannabinoids, exert many biological functions by activating two types of cannabinoid receptors, CB(1) and CB(2) receptors. CB(1) receptors have been detected on enteric nerves, and pharmacological effects of their activation include gastroprotection, reduction of gastric and intestinal motility and reduction of intestinal secretion. The digestive tract also contains endogenous cannabinoids (i.e., the endocannabinoids anandamide and 2-aracidonylglycerol) and mechanisms for endocannabinoid inactivation (i.e., endocannabinoids uptake and enzymatic degradation). Cannabinoid receptors, endocannabinoids and the proteins involved in endocannabinoids inactivation are collectively referred as the 'endogenous cannabinoid system'. A pharmacological modulation of the endogenous cannabinoid system could provide new therapeutics for the treatment of a number of gastrointestinal diseases, including nausea and vomiting, gastric ulcers, irritable bowel syndrome, Crohn's disease, secretory diarrhoea, paralytic ileus and gastroesophageal reflux disease. Some cannabinoids are already in use clinically, for example, nabilone and delta(9)-tetrahydrocannabinol are used as antiemetics.

Central mechanisms of lower esophageal sphincter control

Lower esophageal sphincter (LES) tone is decreased during swallowing, during transient LES relaxations (TLESRs), and before emesis, and this decrease is due primarily to increasing inhibitory vagal output to the LES. Reflex-evoked relaxation of the LES is mediated by long-loop vagovagal reflexes that are coordinated by the dorsal vagal complex in the hindbrain medulla. A sequence of events occurs. Central control of TLESRs has not been studied directly; the information on how drugs may work centrally to reduce TLESRs is extrapolated from knowledge of how the brain evokes LES relaxation. Reduction of the frequency of TLESRs by a GABAB agonist, baclofen, is due to inhibition of vagal afferents, information transfer between the nucleus tractus solitarius and dorsal motor nucleus of the vagus, and vagal efferent outflow. Preliminary data show that cannabinoid receptor activation reduces information transfer between the nucleus tractus solitarius and dorsal motor nucleus of the vagus. The potential therapeutic usefulness of these types of agents that reduce TLESRs by acting centrally is promising.

The endocannabinoid system and the molecular basis of paralytic ileus in mice

The endocannabinoid system (i.e., the cannabinoid receptors and their endogenous ligands) plays an important role in the physiological control of intestinal motility. However, its participation in intestinal pathological states is still poorly understood. In the present study, we investigated the possible role of the endocannabinoid system in the pathogenesis of paralytic ileus, a pathological state consisting of decreased intestinal motility following peritonitis, surgery, or other noxious situations. Ileus was induced by i.p. administration of acetic acid, and gastrointestinal propulsion was assessed by the charcoal method. Endocannabinoid levels were measured by isotope-dilution gas chromatography-mass spectrometry, whereas cannabinoid CB1 receptors were identified by immunohistochemistry. Acetic acid administration inhibited gastrointestinal transit (ileus), and this effect was accompanied by increased levels of the endocannabinoid anandamide compared with control mice and by overexpression of CB1 receptors in myenteric nerves. Furthermore, acetic acid-induced ileus was alleviated by the CB1 receptor antagonist SR141716A and worsened by VDM11, a selective inhibitor of anandamide cellular uptake (and hence inactivation). From these findings, it can be concluded that the intestinal hypomotility typical of paralytic ileus is due, at least in part, to the enhancement of anandamide levels and CB1 expression during this condition, and that selective, nonpsychotropic CB1 receptor antagonists could represent new drugs to treat this disorder.

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.

Nausea and Vomiting

Nausea and vomiting are rather stereotyped symptoms. The challenge is that nausea and vomiting have many different causes and, in some patients, management may be rather complex. The clue is to determine the causal factor early. It helps to separate acute vomiting (<48 hours onset) from chronic vomiting. In acute vomiting, the causal factor or factors are most often evident. Symptomatic treatment with parenteral central-type antiemetics is the preferred treatment. Histamine-1 receptor antagonists, phenothiazines, butyrophenones, and corticosteroids are suitable drugs. For specific types of acute vomiting, for example, chemotherapy-related vomiting, the 5-HT3 receptor antagonists are costlier but effective drugs with minimal side effects. Sometimes, oral and parenteral administration of the above-mentioned drugs may be combined. The origin of chronic vomiting is often obscure, requires specialized investigation, and the causative factor may be uncorrectable. Symptomatic treatment requires a value judgement. If delayed gastric emptying is a contributing factor, prokinetic agents (metoclopramide, erythromycin, cisapride in special cases, if authorized) may prove useful. Otherwise, symptomatic treatment with central antiemetics is the only recourse. Some patients with unexplained vomiting present with psychological disturbances that act as magnifying or contributing factors and may be helped by psychotherapeutics.

Endocannabinoids as physiological regulators of colonic propulsion in mice

BACKGROUND & AIMS

Activation of enteric cannabinoid CB1 receptors inhibits motility in the small intestine; however, it is not known whether endogenous cannabinoids (anandamide and 2-arachidonylglycerol) play a physiologic role in regulating intestinal motility. In the present study, we investigated the possible involvement of endocannabinoids in regulating intestinal propulsion in the mouse colon in vivo.

METHODS

Intestinal motility was studied measuring the expulsion of a glass bead inserted into the distal colon; endocannabinoid levels were measured by isotope-dilution gas chromatography-mass spectrometry; anandamide amidohydrolase activity was measured by specific enzyme assays. CB1 receptors were localized by immunohistochemistry.

RESULTS

Anandamide, WIN 55,212-2, cannabinol (nonselective cannabinoid agonists), and ACEA (a selective CB1 agonist) inhibited colonic propulsion; this effect was counteracted by SR141716A, a CB1 receptor antagonist. Administered alone, SR141716A increased motility, whereas the inhibitor of anandamide cellular reuptake, VDM11, decreased motility. High amounts of 2-arachidonylglycerol and particularly anandamide were found in the colon, together with a high activity of anandamide amidohydrolase. CB1 receptor immunoreactivity was colocalized to a subpopulation of choline acetyltransferase-immunoreactive neurons and fiber bundles in the myenteric plexus.

CONCLUSIONS

We conclude that endocannabinoids acting on myenteric CB1 receptors tonically inhibit colonic propulsion in mice.