Gastric acid inhibitory and gastric protective effects of cannabis and cannabinoids

Omar Abdel-SalamDepartment of Toxicology and Narcotics, Medical Division, National Research Centre, Tahrir Street, Dokki, Cairo, Egypt

ABSTRACT

Cannabis sativa has long been known for its psychotropic effect. Only recently with the discovery of the cannabinoid receptors, their endogenous legends and the enzymes responsible for their synthesis and degradation, the role of this ‘endocannabinoid system’ in diff erent pathophysiologic processes is beginning to be delineated. There is evidence that CB1receptor stimulation with synthetic cannabinoids or Cannabis sativa extracts rich in D9-tetrahydrocannabinol inhibit gastric acid secretion in humans and in experimental animals. This is specially seen when gastric acid secretion is stimulated by pentagastrin, carbachol or 2 deoxy-D-glucose. Cannabis and/or cannabinoids protect the gastric mucosa against noxious challenge with non-steroidal anti-infl ammatory drugs, ethanol as well as against stress induced mucosal damage. Cannabis/cannabinoids might protect the gastric mucosa by virtue of its antisecretory, antioxidant, anti-infl ammatory, and vasodilator properties.

ARTICLE INFO

Article history:

Received 15 January 2016

Received in revised form 16 February 2016

Accepted 15 March 2016

Available online 20 May 2016

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Gastric acid inhibitory and gastric protective effects of cannabis and cannabinoids

Omar Abdel-Salam*
Department of Toxicology and Narcotics, Medical Division, National Research Centre, Tahrir Street, Dokki, Cairo, Egypt

ABSTRACT

Cannabis sativa has long been known for its psychotropic effect. Only recently with the discovery of the cannabinoid receptors, their endogenous legends and the enzymes responsible for their synthesis and degradation, the role of this ‘endocannabinoid system’ in diff erent pathophysiologic processes is beginning to be delineated. There is evidence that CB1receptor stimulation with synthetic cannabinoids or Cannabis sativa extracts rich in D9-tetrahydrocannabinol inhibit gastric acid secretion in humans and in experimental animals. This is specially seen when gastric acid secretion is stimulated by pentagastrin, carbachol or 2 deoxy-D-glucose. Cannabis and/or cannabinoids protect the gastric mucosa against noxious challenge with non-steroidal anti-infl ammatory drugs, ethanol as well as against stress induced mucosal damage. Cannabis/cannabinoids might protect the gastric mucosa by virtue of its antisecretory, antioxidant, anti-infl ammatory, and vasodilator properties.

ARTICLE INFO

Article history:

Received 15 January 2016

Received in revised form 16 February 2016

Accepted 15 March 2016

Available online 20 May 2016

Keywords:

1. Introduction

Cannabis is the most commonly abused illicit substance worldwide. The two commonly used Cannabis preparations are herbal Cannabis or marijuana (prepared from the dried fl owering tops and leaves) and hashish (consists of dried Cannabis resin and compressed flowers). Both are derived from the female plant of Cannabis sativa Linn (family Cannabidaceae) [1] . Research into Cannabis led to discovery of its active constituents or cannabinoids, a terpenophenol compounds; more than 70 of which have been isolated. The most studied cannabinoids are Δ9-tetrahydrocannabinol (THC), cannabinol, cannabidiol, cannabigerol, cannabichromene, Δ9-tetrahydrocannabivarin, cannabidivarin and others [2,3] .Δ9-THC is the primary constituent that is responsible for the psychotropic properties of recreational Cannabis [4] .

Cannabinoids mediate their biological eff ects through interaction with cannabinoid receptors, which belong to the superfamily of G protein-coupled receptors. There are at least two cannabinoid receptor subtypes: the CB1receptor, essentially located in the central nervous system, but also in peripheral tissues, and the CB2receptor, found only at the periphery especially on immune cells[5]. Most of Cannabis eff ects in the central nervous system are mediated by CB1receptors. These are expressed at brain areas that control movements, memory, cognition and emotion and in the spinal cord [6,7] where they mediate retrograde inhibition of neuronal activity [8] .

Cannabinoid receptors can also be activated by a number of endogenous ligands, the endocannabinoids. The main endocannabinoids arachidonoyl ethanolamide (anandamide) and 2-arachidonoyl glycerol (2-AG) are selective agonists at the CB1and CB2receptors, respectively. Both are derivatives of arachidonic acid, that are produced and released ‘on demand’ by cleavage of membrane lipid precursors and are hydrolysed by the fatty acid amide hydrolase anandamide or monoglyceride lipase, respectively. Other endocannabinoids are noladin ether and virodhamine [9,10] . The cannabinoid receptors, endocannabinoids as well as theenzymes responsible for their synthesis or degradation, collectively constitute the ‘endocannabinoid system’ [12] .

Cannabis sativa has a wide-world reputation as a psychotropic drug [1] . Cannabis are usually smoked, but may also be eaten, mixed in cakes or cookies or drunk in a liquid infusion [13] . Only recently, did Cannabis and cannabinoid-based medicines come to attention as a remedy for different medical conditions. The sublingual oromucosal spray Sativex, composed of whole plant extract containing both Δ9-THC and cannabidiol (CBD) [THC: CBD=1:1] have recently been approved for the treatment of pain and spasticity in multiple sclerosis [14] . Dronabinol (Marinol) and nabilone (Cesamet) are two oral formulations of a synthetic THC approved for the treatment of nausea and vomiting that complicate chemotherapy and which are refractory to conventional antiemetic therapy. These agents are also being used to improve appetite to treat weight loss associated with human immunodefi ciency virus infection and cancer [15] . Medicinal Cannabis is also being used for a variety of medical conditions including chronic pain, depression, arthritis, and neuropathy[16-18]. The endocannabinoid system is a target for the treatment of neurodegenerative disease eg., tics in Tourette syndrome, levodopa-induced dyskinesia in Parkinson s disease and some forms of tremor and dystonia [19,20] .

Cannabinoid receptors and their endogenous ligands (anandamide and 2-arachidonylglycerol) have been identifi ed in the gastrointestinal tract and are involved in mediation of several gastrointestinal functions eg., relaxation of the lower oesophageal sphincter, gastric acid secretion, gastric emptying, gastrointestinal motility and fluid secretion [21,22] . Evidence thus suggests that cannabinoid-based medicines might be benefi cial in a number of gastrointestinal disorders.

The aim of this review is to compile and discuss the available data pertaining to the eff ect of Cannabis and/or cannabinoids on gastric acid secretion and gastric mucosal integrity.

2. Cannabis and gastric acid secretion

There are no clinical studies on the eff ect of Cannabis on gastric acid secretion. In their study, however, on 90 human volunteers participating in a vaccine development programme, Nalin et al. 1978 [23] found that smoking Cannabis for more than two days a week was associated with low gastric acid output. On the other hand, several preclinical studies suggested inhibition of gastric acid secretion by Cannabis or individual cannabinoids. Thus, in rats subjected to pylorus-ligation for (2-4) h (Shay rat), the administration of an ethanolic extract of Cannabis sativa raised the gastric pH. Rats treated with 0.1 and 0.3 g/kg of Cannabis extract for 4 h had their gastric pH raised from 2 to 4 and 4.5. In the 4 h pylorus ligated rat, Cannabis 1 g/kg raised pH slightly more than 0.05 g/kg of the histamine H-2 receptor blocker ranitidine. In rats subjected to pylorus-ligation for 2h, randitine was more eff ective than Cannabis (pH values were 2.2, 3.5 and 4 for control, Cannabis and ranitidine, respectively)[24].

The effect of long-term treatment with Cannabis extracts rich in Δ9-THC on gastric acid secretion was studied in the pylorusligated rat model (Shay rat). Rats were treated with 5, 10 and 20 mg/kg of Cannabis extract (expressed as Δ9-THC) subcutaneously for 4 weeks and then subjected to pylorus-ligation (for 4 h) with or without gastric acid stimulation (using pentagastrin, histamine or carbachol). The administration of low doses of Cannabis i.e. 5 or 10 mg/kg Δ9-THC stimulated basal gastric acid output and gastric volume. The high dose of 20 mg/kg, however, had no eff ect on basal gastric acid secretion. The eff ect of Cannabis on stimulated gastric acid secretion was somehow diff erent in that it inhibited gastric acid secretory responses stimulated by pentagastrin or carbachol in a dose-dependent manner. On the other hand, Cannabis prtetreatment had no signifi cant eff ect on acid output stimulated by histamine [25] .

Cannabis’s most active constituent Δ9-THC is CB1receptor agonist [6,7] . When administered intravenously (i.v.), synthetic CB1receptor agonists inhibited gastric acid secretion in the anaesthetized rat preparation. Thus, WIN55, 212-2, which is a non-selective cannabinoid agonist decreased gastric acid secretion stimulated by pentagastrin (10 mg/kg, i.v.) in anaesthetized rats. The inhibitory effect of WIN55, 212-2 on gastric acid secretion is likely to be mediated via CB1receptors, since selective CB1receptor antagonists SR141716A and LY320135t antagonized its action. WIN55, 212-2, however, failed to aff ect basal gastric acid secretion[26].

Similar data were provided by Adami et al. [27] who reported inhibition of pentagastrin stimulated gastric acid secretion in anaesthetized rats with lumen-perfused stomach by the nonselective cannabinoid agonists WIN 55,212-2 and HU-210. Gastric acid secretion stimulated by 2-deoxy-D-glucose (a centrally acting secretagogue which stimulates gastric acid by increasing eff erent vagus activity) is inhibited by the cannabinoid agonists, thereby suggesting a centrally mediated inhibition of gastric acid secretion by these synthetic cannabinoid agonists. But in contrast to their effect on gastric acid stimulation by pentagastrin or 2-deoxy-D-glucose, the two cannabinoid agonists did not aff ect acid secretion stimulated by histamine. The study pointed again to a role for CB1receptors in inhibition of gastric acid secretion by the synthetic cannabinoids since their eff ect was blocked by a CB1but not CB2receptor antagonist. Moreover, vagal involvement is suggested by fi nding that the inhibitory eff ect of HU-210 on pentagastrin-induced acid secretion decreased following bilateral cervical vagotomy and ganglionic blockade with hexamethonium.

Using rat isolated parietal cells, Rivas and Garc?a [28] , however, reported inhibition of gastric acid secretion stimulated by histamine after high concentration of Δ9-THC (20 μM). Basal gastric acid secretion was unaff ected.

Experiments in the isolated mouse stomach indicated the ability of CB1antagonism to increase gastric acid secretion. Stimulation with ouabain (an inhibitor of Na+/K+-ATPase) increased gastric acid secretion (by releasing acetylcholine from cholinergic nerves).The addition of the CB1receptor antagonist SR141716A further increased the ouabain-stimulated acid secretion. In contrast, the cannabinoid agonist WIN55212-2 was without eff ect [29] . These data suggest a role for CB1receptors in inhibiting gastric acid secretion.

The above in vivo and in vitro studies thus suggest that CB1receptor stimulation with synthetic cannabinoids or Cannabis sativa extracts rich in D9-THC inhibits gastric acid secretion. Given the data suggesting that the CB1agonist THC reduces transient lower oesophageal sphincter relaxations and gastro-oesophageal refl ux [30] , cannabinoid based medicines might fi nd utility in the treatment of peptic ulcer disease including refl ux oesophagitis. Interestingly, a study on the symptoms of withdrawal in human marijuana smokers reported ‘Stomach pain’ on the fourth day of abstinence among the abstinence symptoms [31] . One might thus speculate that the stomach pain was due to a rebound increase in gastric acid secretion and/or increased mucosal sensitivity.

3. The site of action of Cannabis

The secretion of gastric acid is controlled at different neural, hormonal and paracrine levels. The parietal cells in the gastric glands are the cells secreting and releasing hydrochloric acid. The parietal cell bears receptors for acetylcholine, histamine, and gastrin, the major stimuli for gastric acid secretion. Cholinergic stimulation is carried out by acetylcholine released from postganglionic (i.e., intramural) cholinergic neurons and binds to muscarinic M3receptors. Acetylcholine also stimulates acid secretion indirectly by activating muscarinic M2and M4receptors on somatostatin D cells coupled to inhibition of somatostatin secretion. Histamine which is released from enterochromaffi n-like cells, binds to and activates histamine H2receptors located on parietal cells, is a powerful stimulus for gastric acid secretion as well as gastrin released from G cells of the pyloric antrum. Gastrin reaches parietal cells via the circulation and stimulates the parietal cell directly and also indirectly by releasing histamine from enterochromaffin-like cells cells. Gastrin release from antral gastrin cells is stimulated by gastrin releasing peptide and inhibited by somatostatin[32,33].

The precise site of action for Cannabis and/or cannabinoids in mediating inhibition of gastric acid secretion is yet to be elucidated. The presence of CB1cannabinoid receptor messenger RNA within the rat stomach was demonstrated in full-wall thickness preparations of rat oesophagus and stomach [34] . In the rat, CB1receptors are present in pre-and postganglionic cholinergic neural elements innervating smooth muscle, mucosal, and submucosal blood vessels [27] . Accordingly, it has been suggested that the gastric antisecretory eff ects of cannabinoids are mediated by suppressing the vagal drive through activation of CB1receptor located on the vagal eff erent pathways to the gastric mucosa and not on parietal cells[35].

With the use of different techniques (immunohistochemical staining, Western blot, polymerase chain reaction), the presence of CB1receptor has been shown on the acid-secreting parietal cells within the gastric glands in biopsy samples from the gastric mucosa of patients with dyspeptic symptoms [36] . This suggested a role for CB1receptors in control of gastric acid production. Cannabis therefore might inhibit gastric acid secretion by a direct action on the CB1receptors located on parietal cells.

Cannabinoid CB1receptors are also abundant in the central nervous system [20] . Following absorption THC as well as other cannabinoids are distributed to all tisuues and accumulate in fatty tissues and are slowly released thereafter [1] . Because of their lipophilic properties, cannabinoids can easily cross the blood- brain barrier and act on brain cannabinoid receptors [7,37] . There is also an evidence that the antiemetic action of THC (0.05-1.00 mg/kg i.p.) is due to an eff ect at CB1receptors in specifi c regions of the dorsal vagal complex [38] . It is thus possible that the gastric antisecretory eff ect of Cannabis or cannabinoids is due to a central rather than a peripheral site of action i.e. by decreasing central eff erent vagus activity.

In their study, Adami et al. [39] , however, have shown that the central (intracerebroventricular: i.c.v.) administration of the synthetic cannabinoid agonists, WIN55, 212-2 or HU-210, failed to inhibit basal gastric acid secretion or that stimulated by pentagastrin in anaesthetized rats with lumen-perfused stomach. This suggested that a peripheral rather than a central CB1receptor mechanism is likely to be involved in the inhibitory eff ect of cannabinoids on gastric acid section[39].

4. Cannabis and gastric mucosal damage

Several preclinical studies provided data that supports a protective eff ect for Cannabis or cannabinoids in the stomach. In rats, D9-THC (100 mg/kg) given via subcutaneous or oral routes inhibited the development of gastric ulcers induced by pyloric-ligation (Shay rat) with the protective eff ect of D9-THC being most evident following subcutaneous compared with the oral route of administration. D9-THC decreased gastric juice volume but not free and total acidity[40].

In their experiments, De Souza et al.[41] demonstrated that treatment with a Cannabis sativa extract was able to protect the rat stomach against restraint induced ulcers. Rats were treated with diff erent doses of the extract (5.0, 10.0, 20.0, 40.0 and 60.0 mg/kg, i.p.) both 24 h and immediately before immobilization. Alternatively, the extract (40 and 60 mg/kg) was given for 20 d prior to immobilization. The percentage of rats with lesions decreased with acute treatment reaching 41.7% for the dose of 60 mg/kg vs. control value of 65.6%-82.7%. This contrasted with chronic administration where the percentage of rats displaying lesions was 94.7% vs. control value of 100%, indicating that no protection occurred. These results also demonstrated that chronic Cannabis injection for 20 d resulted in the development of tolerance to the mechanisms of the anti-stress ulcer effect of Cannabis. Interestingly, in unrestrained animals, treatment with Cannabis extract at 40 or 60 mg/kg wasassociated with the development of gastric ulceration. Thus, only in the presence of stress, did Cannabis prevented gastric lesions, but the eff ect is evident in the acute and not in the long-term treatment. Other researchers have shown that 2 h pretreatment with Δ9-THC (10 mg/kg, i.p.) prevented the gastric mucosal haemorrhaic streaks evoked by administration of the nonsteroidal anti-infl ammatory drug (NSAID) diclofenac in mice; the eff ect being attenuated by the CB1receptor antagonist rimonabant [42] . Subsequent experiments in mice showed that Δ9-THC given via oral or intraperitoneal routes prior to diclofenac, decreased the development of gastric hemorrhagic streaks. Δ9-THC given i.p. was more potent in reducing diclofenacinduced gastric ulcerations compared to the oral route. Thus while i.p. Δ9-THC decreased diclofenac-induced gastric hemorrhages at a dose of 0.1 mg/kg and higher, the eff ect of orally given Δ9-THC was evident at a dose of 2.5 mg/kg and above. However, at a dose of 10 or 50 mg/kgΔΔ9-THC given via i.p. or oral route inhibited the development of lesions to almost the same extent. Moreover, there was no difference between 10 or 50 mg/kg Δ9-THC given via i.p. or oral route in the degree of their ulcer preventive eff ect [43] . Using a simple ethanolic Cannabis extract, Wallace et al. [44] found that oral (but not systemic) administration resulted in a decrease in the severity of gastric damage caused by the NSAID naproxen. The extract was administered either orally at doses of 1, 3 and 10 mg/kg or i.p. at a dose of 30 mg/kg thirty minutes prior to oral administration of naproxen and rats euthanized 3 h later. The authors found that oral pretreatment with Cannabis inhibited the development of gastric lesions. Complete protection occurred with the 10 mg/kg of Cannabis extract, while at 3 mg/kg there was 80% inhibition of the lesions. In contrast, Cannabis at 10 mg/kg given via i.p. route was without eff ect. The gastroprotective eff ect of the extract (10 mg/kg, orally) was blocked by a CB1antagonist (but not a CB2antagonist) and thus CB1-mediated [44] . The discrepancy between the oral and i.p. routes is not expected since orally administered THC has a reduced systemic bioavailability owing to gastric degradation with the presence of acids and extensive fi rst-pass metabolism in the liver[1,45].

Studies have also assessed the eff ect of long-term treatment with Cannabis extract on the chemically-induced gastric damage. Rats received daily subcutaneous injections of D9-THC rich Cannabis extract for 4 weeks prior to pylorus-ligation and oral administration of either acidified acetylsalicylic acid or ethanol (96%). In these experiments, Cannabis given at 5, 10 and 20 mg/kg of Cannabis extract (expressed as Δ9-THC) inhibited the development of gastric mucosal damage in a dose-dependent manner [25] . These data does not support that tolerance to the gastroprotective action of Cannabis develops after repeated administration.

Gastric mucosal protective effects have also been reported for synthetic cannabinoids as well as endocannabinoids. In their study, Germano et al.[46] provided data that the non-selective cannabinoid receptor agonist WIN 55,212-2 decreased stress-induced gastric ulcers in rats. The cannabinoid CB1receptor antagonist SR 141716A itself had no eff ect on stress induced lesions. SR 141716A (but not by the cannabinoid CB2 receptor antagonist SR 144528), however, reversed the protective eff ect of WIN 55, 212-2, thus suggesting the involvement of CB1 receptors.

A study by Dembi?ski et al. [47] found that anandamide (a natural endogenous ligand for CB1receptor) given i.p. prior to water immersion and restrain stress decreased the development of gastric mucosal lesions. The synthetic CB1receptor antagonist AM 251 antagonized this effect of anandamide, suggesting that CB1receptors are involved. In the study of Rutkowska and Fereniec-Go?ebiewska [48] ACEA (arachidonyl-2-chloroethylamide), the selective cannabinoid CB1, was given (i.p.) 1 h prior to oral administration of acetylsalicylic acid and rats euthanized 3 h later. In this study, ACEA inhibited the development of gastric mucosal lesions due to the NSAID with almost total protection being observed after 5 mg/kg of ACEA. Meanwhile, the reference drug randitidine at 60 mg/kg reduced gastric lesions to 5.6% of control value.

Shujaa et al.[49] provided data that activation of central cannabinoid receptors resulted in gastric mucosal protection. The authors found that anandamide (an endocannabinoid), its biologically stable analog methanandamide and the synthetic agonist WIN55, 212-2 reduced the ethanol-induced gastric mucosal lesions. The protective eff ect was evident after either peripheral (intravenous) or central (i.c.v.) administration. Centrally administered CB1receptor antagonist reversed the effect of centrally administered anandamide and methanandamide while naloxone (a non-selective opioid receptor antagonist) reversed the effect of intravenously administered anandamide, methanandamide and WIN 55,212-2. Thus, central cannabinoid CB1and opioid receptors were involved in the gastric protection by cannabinoids.

Moreover, increasing the levels of endogenous cannabinoids resulted in gastric protection. Fatty-acid amide hydrolase is an enzyme which catalyzes the intracellular hydrolysis of the endocannabinoid anandamide and other bioactive lipid amides [50] . Using URB937, an inhibitor of FAAH, Sasso et al. [51] observed a reduction in both the number and severity of gastric lesions produced by indomethacin in mice. 2-arachidonylglycerol is degraded mainly by monoacylglycerol lipase, but also by fatty acid amide hydrolase [6,7, 52] .

Kinsey et al. [42] administered diclofenac (100 mg/kg, p.o.) to mice so as to induce gastric mucosal lesions. The authors found that pretreatment with JZL184 (an inhibitor of 2-arachidonoyl-glycerol inactivation by monoacylglycerol lipase) attenuated diclofenacinduced gastric hemorrhagic streaks. Meanwhile, 2-AG administered i.p. 2 h prior to diclofenac failed to prevent the NSAID-induced gastric lesions. JZL184 signifi cantly increased 2-AG in the stomach. Proinflammatory cytokines (IL-1β, IL-6, tumour necrosis factoralpha) increased in the stomach of diclofenac-treated mice and these were mitigated by JZL184. Rimonabant, a CB1receptor antagonist (but not the CB2 receptor antagonist SR144528) antagonized theeff ect of Δ9-THC, thereby, suggesting a CB1mechanism. Further experiments in mice given diclofenac showed that repeated daily injection of JZL184 for 6 d protected against gastric mucosal damage caused by the NSAID. In contrast to the eff ect of the high dose of the agent (≥16 mg/kg), there was no tolerance associated with the low dose (≤8 mg/kg) [53] . The above data collectively indicated that stimulation of the endocannabinoid system mediates gastric mucosal protection.

5. Mechanism (s) of gastric protection by Cannabis

The integrity of the gastric mucosa is maintained due to a balance between ‘mucosal aggressive factors’ and the so called ‘gastric mucosal protective mechanisms’[54]. The gastric mucosa is constantly exposed to high concentrations of luminal acid. Other aggressive factors in the lumen are pepsins, bile refluxed from incompetent pyloric sphincter, bacteria, ethanol and drugs especially the non-steroidal anti-inflammatory drugs (NSAIDs) capable of inhibiting the synthesis of cytoprotective prostaglandins. The mucosa’s ability to withstand acid and other injurious agents is due to several mechanisms collectively is known as the gastric mucosal barrier. The mucus-bicarbonate layer together with surfaceactive phospholipids barrier constitute the fi rst line of defense or the pre-epithelial barrier. The surface epithelial cells capable of rapid turnover and migration (restitution) and releasing mucins, bicarobonate, phospholipids, prostaglandins, trefoil peptides form the second line of defense. Other important defense mechanisms of gastric mucosa are cytoprotective prostaglandins, mucosal sulfhydryl content, adequate mucosal blood flow, and sensory afferent innervations. The development of gastric mucosa damage implies a breach in the balance between aggressive and defensive factors [55-58].

It is thus obvious that the management of peptic ulcer disease involves removal or neutralizing aggressive factors especially gastric acid e.g., via antacids or acid inhibitors acting on histamine H2receptors or the proton pump. Strengthening natural defenses is another approach e.g., with the use of drugs such as sucralfate or cytoprotective prostaglandins[59]. Protecting the gastric mucosa independently of gastric acid inhibition is termed cytoprotection. This term was originally introduced by Robert et al. [60] referring to the unique ability of prostaglandins to protect the gastric mucosa from noxious agents such as 0.6 N HCl, 0.2 M NaOH, 25% NaCl or 96% ethanol, independently of gastric acid inhibition. Gastric cytoprotection was also proved for small doses of antisecretory agents, retinoids and growth factors [61] . Clearly, since Cannabis and cannabinoid agonists have been shown to inhibit gastric acid secretion, the protective eff ect of Cannabis cannot be ascribed to a cytoprotective property.

Another mechanism by which the stomach resists the chemicalinduced injury is adaptive cytoprotection. Here, exposure of the gastric mucosa to luminal diluted ulcerogens or mild irritants will result in less damage following later exposure to strong necrotizing agents [62] . Several mechanisms have been postulated to account for adaptive cytoprotection including endogenous prostaglandin synthesis, stimulation of mucus or HCO3secretion, mucosal vasodilation[63], and release of calcitonin gene-related peptide from the sensory nerves [64] . Cannabis or cannabinoid agonists, administered via systemic routes, however, were able to exert protective eff ect [25,41,43,44,46] making adaptive cytoprotection an unlikely mechanism. It remains to be established whether Cannabis administered into the gastric lumen acts as a mild irritant and thereby protecting the stomach via adaptive cytoprotection.

The eff ects of Cannabis are, however, the sum of its constituents. There are more than 70 different cannabinoids and these may have effects that are synergistic with or antagonistic to Δ9-THC effects[65,66]. Other important constituents are terpenoids and the flavonoids flavocannabiside [66] . One terpenoid that is betacaryophyllene has been shown to inhibit the development of gastric lesions evoked by ethanol or 0.6 N HCl when given orally to rats [67] .

6. Cannabis strengthen gastric mucosal defenses

Several mechanisms are likely to account for the ability of Cannabis or individual cannabinoid agonists to protect the stomach against noxious injury. Cannabis and/or individual cannabinoids inhibit gastric acid secretion [24-27] , thereby, lessening the ability of this most powerful aggressive factor to threaten the gastric mucosa. Studies also indicated that Cannabis administration increases mucus secretion in the gastric mucosa [25] . Mucus is secreted by the mucous neck and surface epithelial cells and plays an important role in protecting the surface epithelial cells from luminal acid and other injurious agents. Mucus retards diff usion of luminal acid into the mucosa and together with bicarbonate secreted by the epithelium forms a pH gradient with near-neutral pH at the surface of the mucosa[68,69].

Luminal pepsins constitute an important aggressive factor capable of digesting mucus and thereby increasing the susceptibility of gastric mucosa to other injurious factors [70] . Studies in pylorusligated rats treated with Cannabis extract for 4 weeks indicated that Cannabis did not aff ect basal pepsin secretion. Cannabis, however, decreased pepsin secretion when the stomach is stimulated with pentagastrin and carbachol. Cannabis also decreased pepsin secretion following ethanol administration in rats [25] .

Reactive oxygen intermediates have been implicated in the development of gastric mucosal injury due to ischaemia/reperfusion, ethanol, NSAIDs, and bacteria [71] . Cannabis has been shown to decrease lipid peroxidation and to increase reduced glutathione content and catalase activity in gastric mucosa[25]. Cannabis also inhibited mucosal nitric oxide [25] . Although a vasodilator effect of physiological concentrations of nitric oxide help the mucosa to withstand noxious challenge, high concentrations are likely to have a damaging eff ect[72-74]. Cannabis thus might protect the gastric mucosa by virtue of an antioxidant action.

Mucosal infl ammation plays an important role in the development of gastric ulcers and although initial inflammatory response to the gastric mucosa helps to minimize or limit tissue damage, an exaggerated or uncontrolled response is detrimental to the mucosal integrity[69,75]. Cannabis has been shown to inhibit the proinflammatory cytokine tumour necrosis factor-alpha in mucosal homogenates [25] , an action which might help to minimize the extent of mucosal damage.

Cannabis thus exerts antioxidant and anti-infl ammatory eff ects in the gastric mucosa. It is to be noted, however, that these actions of cannabis were evident only when the gastric mucosa was challenged with increased acid secretion or after exposing the mucosa to noxious agents such as acidifi ed aspirin and ethanol and were not apparent under basal conditions [25] .

One important factor in determining the ability of the gastric mucosa to resist gastric acid and other noxious agents is gastric mucosal blood fl ow[57]. This has been inferred from studies showing that interference with the blood supply to the mucosa i.e. ischaemia resulted in the development of gastric mucosal damage or aggravated the extent of mucosal damage evoked by NSAIDs [76,77] or ethanol[78]. On the other hand, agents which increase gastric mucosal blood fl ow such as isoproterenol [79] , vasodilator prostaglandins [80] or capsaicin-type agents [81-83] helped to protect against noxious challenge. In this ontext, data have been provided that the endocannabinoid anandamide increases gastric mucosal blood flow [47] . There is also an evidence for a vaso-relaxant action for methanandamide in rat gastric arteries. This eff ect was independent of cannabinoid receptors [84] . It is thus possible that a vasodilatory action is involved in the gastric protective eff ects of cannabis and or cannabinoids.

7. Conclusions

Cannabis and/or individual cannabinoids inhibit gastric acid secretion. The inhibitory eff ect of cannabis/cannabinoid agonists on gastric acid secretion is likely to be mediated via CB1receptors. The inhibitory effect might be mediated through activation of CB1receptor located on the vagal eff erent pathways. There is also an evidence for a possible direct effect for cannabis on the CB1receptors located on parietal cells. Cannabis could also inhibit secretion by decreasing central eff erent vagus activity. There appears to be no densistization to the action of cannabis following long-term administration of the herb. Cannabis inhibits the development of gastric ulcers induced by pyloric-ligation (Shay rat), restraint induced ulcers, and NSAIDs. Exogenous administration of endocannabinoids or increasing the levels of endogenous cannabinoids resulted in a gastric protection. The gastroprotective eff ect of cannabis could be blocked by a CB1antagonist. Activation of central cannabinoid receptors results in gastric mucosal protection. Cannabis thus exerts antioxidant and anti-infl ammatory eff ects in the gastric mucosa. It is possible that a vasodilatory action is involved in the gastric protective eff ects of cannabis and or cannabinoids. Casnnabinoidsbased medicines might find utility in treatment of peptic ulcer disease including gastroesophageal refl ux.

Conflict of interest statement

The author declares that he has no confl ict of interest.

Acknowledgements

This study was supported by the National Research Centre (No. 10001004).

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E-mail: omasalam@hotmail.com.

Foundation project: It was supported by the National Research Centre (No. 10001004).

Cannabis sativa

Gastric mucosa

Gastric acid

doi:Document heading 10.1016/j.apjtm.2016.04.021

*Corresponding author:Omar Abdel-Salam, Department of Toxicology and Narcotics, Medical Division, National Research Centre, Tahrir Street, Dokki, Cairo, Egypt.