• cannabinoid receptors;
  • colon cancer;
  • endocannabinoids;
  • inflammatory bowel disease;
  • intestinal motility;
  • irritable bowel syndrome


  1. Top of page
  2. Abstract
  3. CB2 receptors and intestinal motility
  4. CB2 receptors and gut inflammation
  5. CB2 receptors and intestinal cancer
  6. Conclusions
  7. References

Abstract  Mammalian tissues express the cannabinoid 1 (CB1) receptor and the cannabinoid 2 (CB2) receptor, the latter being involved in inflammation and pain. In somatic nerve pathways, the analgesic effects of CB2 agonism are well documented. Two papers published in the Journal have provided evidence that CB2 receptor activation inhibits visceral afferent nerve activity in rodents. These exciting findings are discussed in the context of recent data highlighting the emerging role of CB2 receptor as a critical target able to counteract hypermotility in pathophysiological states, gut inflammation and possibly colon cancer.





CB1 receptor

cannabinoid 1 receptor

CB2 receptor

cannabinoid 2 receptor


colorectal distension


fatty acid amide hydrolase


inflammatory bowel disease


irritable bowel syndrome


mustard oil


2,4,6,-trinitrobenzenesulphonic acid


transient receptor potential vanilloid 1

The plant Cannabis has been known for centuries to improve a variety of gastrointestinal conditions, including emesis, diarrhoea, inflammatory bowel disease (IBD) and intestinal pain.1–5 Two subtypes of G-protein-coupled receptors for Δ9-tetrahydrocannabinol, the main active ingredient of Cannabis, have been cloned to date: the cannabinoid 1 (CB1) and cannabinoid 2 (CB2) receptors. CB1 receptors are expressed by central and peripheral neurones,6,7 including the enteric nervous system, which represents a major site of action of cannabinoids in the digestive tract.1,2 CB2 receptors are mostly expressed by inflammatory/immune cells, although recent data support the expression of CB2 receptors in localized brain areas implicated in neuroinflammatory processes, in the survival of transformed neural cells8,9 and, importantly, in emesis.10,11 In human intestinal tissues, CB2 receptor expression has been observed primarily in macrophages and, to a lesser extent, on plasma cells in the lamina propria.12 However, in the colon from IBD patients, CB2 receptors were upregulated and were also observed in colonic epithelia. The presence of CB2 receptors on enteric nerves or in gastrointestinal afferents has not been demonstrated to date. Moreover, no CB2 receptor has been identified in mouse dorsal root ganglia or spinal cord, in basal conditions.13

Several types of endogenous agonists for cannabinoid receptors have also identified in mammals. These compounds, termed endocannabinoids, are derived from arachidonic acid, with anandamide and 2-arachidonoylglycerol (2-AG) having been most studied.14 Endocannabinoids are biosynthesized ‘on demand’ and released from cells immediately after their production. Endocannabinoid levels in the gut appear to be elevated as an adaptive reaction to re-establish normal homeostasis when this is acutely and pathologically perturbed. For example, noxious stimuli,15–17 food deprivation,18 or clinically diagnosed gut diseases (e.g. IBD, colorectal cancer, diverticulitis and celiac disease)19–22 produced measurable increases in intestinal endocannabinoid (mainly anandamide) levels.

The endogenous cannabinoid system includes not only cannabinoid receptors and endocannabinoids, but also mechanisms for endocannabinoid biosynthesis and inactivation. The latter occurs via cellular reuptake, which might be facilitated by a putative membrane transporter, and enzymatic degradation by hydrolytic enzymes including fatty acid amide hydrolase (FAAH).6,7 There is no direct evidence for the existence of this putative membrane transporter in the gut as yet, although functional studies suggest that this process might be involved in some experimental pathophysiological states (e.g. ileus, diarrhoea and IBD).15,17,21 FAAH mRNA and activity have been detected in different regions of the rodent intestinal tract and FAAH inhibition results in anti-inflammatory effects21,23 and inhibition of motility in the small intestine.24

Although cannabinoids have a favourable drug safety profile, their use in the clinic is severely limited by their psychoactivity. Because the unwanted psychotropic effects of cannabinoids are mediated by brain CB1 receptors, the most obvious alternative possibility is to target CB2 receptors alone.9,10,25,26 There is strong evidence that CB2 agonists have potential for the relief of pain.25–27 However, the experimental data are mainly related to somatic pain and the role of cannabinoids in the control of visceral nociception has been largely neglected. This is a major gap in our understanding of the role of endocannabinoids in the gut, since heightened visceral sensation is a commonly accepted as an explanation for the symptoms of pain and discomfort of IBD and irritable bowel syndrome (IBS).28

Two recent intriguing articles, published in the Journal, provide compelling evidence that CB2 receptor activation inhibits visceral afferent nerve activity in rodents. In the first of the two papers,29 the authors evaluated abdominal sensitivity to colorectal distension (CRD) in rats in basal conditions and after 2,4,6,-trinitrobenzenesulphonic acid (TNBS)-induced colitis, an experimental model of IBD which is known to trigger a hypersensitivity to CRD. In basal conditions, both the cannabinoid receptor agonist WIN55-212-2 (in a CB1 antagonist-sensitive manner) and the CB2 receptor agonist JWH-015 reduced the response to CRD. Most importantly, in the inflamed gut, both compounds were active at a lower dose abolishing the hypersensitivity induced by colitis. Administered alone, the CB1 receptor antagonist rimonabant and the CB2 receptor antagonist SR144528 had no effect on basal sensitivity. In contrast, rimonabant, but not SR144528, enhanced colitis-induced hyperalgesia suggesting that the endogenous cannabinoid system is only involved in the inflammatory hyperalgesia and only through CB1 receptors.

In the second of the two articles,30 published in the current issue of the Journal, the authors evoked a visceral afferent response by the administration of the algesic agent bradykinin. It was found that the CB2 receptor agonist AM1241 inhibited the bradykinin response of murine mesenteric afferent nerves. The effect of AM1241 on the bradykinin response was demonstrably due to activation of the CB2 receptor as the response to AM1241 was blocked by the selective CB2 receptor antagonist AM630 and, most importantly, no effect of AM1241 was observed in CB2-deficient mice. In line with these observations, a relevant study31 has recently shown that probiotics, which play a role in the clinical management of IBS,32,33 may induce the expression of CB2 receptors on intestinal epithelial cells that locally contribute to the restoration of the normal perception of visceral pain in rodents.

The results of these elegant studies, lend additional support to previous intriguing findings which highlight the emerging role of the CB2 receptor in the digestive tract. In addition to the effect on visceral sensation, other experimental evidence suggests that activation of intestinal CB2 receptors may, (i) counteract the exaggerated intestinal motility under pathophysiological states; (ii) exert anti-inflammatory effects; and (iii) inhibit the proliferation of tumour cells.

CB2 receptors and intestinal motility

  1. Top of page
  2. Abstract
  3. CB2 receptors and intestinal motility
  4. CB2 receptors and gut inflammation
  5. CB2 receptors and intestinal cancer
  6. Conclusions
  7. References

Cannabinoids have been shown to reduce gastric and intestinal motility in randomized clinical trials.34,35 Experimental evidence suggests that cannabinoid agonists act on prejunctional CB1 receptors to reduce excitatory enteric transmission (mainly cholinergic transmission) in different regions of the gastrointestinal tract,2 including the human intestine.22,36 Consistently, cannabinoids reduce gastrointestinal transit in rodents through activation of CB1, but not via CB2, receptors, in vivo.37,38 Interestingly, a recent report39 demonstrated that the CB1-mediated reduction of gastrointestinal transit was absent in rats treated with an endotoxic inflammatory agent, and was replaced by CB2 mediated inhibition of stimulated transit. Indeed the CB2 agonist JWH-133, but not the CB1 agonist arachidonoyl-2-chloroethylamide (ACEA), reduced the increase in gastrointestinal transit induced by lipopolysaccharide; this inhibition by the CB2 agonist was dose-dependent and prevented by a selective CB2 antagonist. By evaluating the inhibition of lipopolysaccharide-enhanced gastrointestinal transit by different antagonists, the effects of the CB2 agonist were found to act via cyclooxygenase, and to act independently of inducible nitric oxide synthase and platelet-activating factor. Thus, activation of CB2 receptors in response to lipopolysaccharide represents a mechanism for the re-establishment of normal gastrointestinal transit after an inflammatory stimulus.

CB2 receptors and gut inflammation

  1. Top of page
  2. Abstract
  3. CB2 receptors and intestinal motility
  4. CB2 receptors and gut inflammation
  5. CB2 receptors and intestinal cancer
  6. Conclusions
  7. References

Many patients with IBD anecdotally report that they experience relief from smoking marijuana. Moreover, experimental inflammation enhances cannabinoid signalling, as revealed by the increased expression of cannabinoid receptors and/or increased intestinal levels of endocannabinoids.40 The endocannabinoid anandamide is increased in intestinal tissues from patients with ulcerative colitis,21 diverticular disease,22 and from celiac patients.20 By using a pharmacological approach and genetically modified mice, it was first found that cannabinoids exerted anti-inflammatory actions in the gut through activation of CB1 receptors.23 However, more recent data have highlighted the importance of the CB2 receptor in mediating protective effects in the inflamed gut. Kimball et al.41 evaluated the effect of cannabinoid drugs on the experimental model of colitis induced by oil of mustard (OM, a potent neuronal activator that is know to elicit visceral hyperalgesia when given intracolonically). It was found that both ACEA (a CB1 receptor agonist) and JWH-133 (a CB2 receptor agonist), reduced colon weight gain, colon shrinkage, colon inflammatory damage score and diarrhoea. CB1 receptor expression was increased in the endothelial layer and myenteric ganglia in OM colitis. CB2 receptor immunostaining was more marked in infiltrated immune cells in OM colitis. Nearly, all of the infiltrating cells in the submucosa and a large proportion of those that infiltrated the epithelium were CB2 receptor positive. Consistent with these data, Wright et al.12 showed that CB2 receptor immunoreactivity was seen in the epithelium of colonic tissue characteristic of IBD and appeared more evident in the epithelium at the crypt fissure where ulceration had occurred.

In a different study, the CB1 antagonist rimonabant was found to inhibit the lipopolysaccharide-induced increase in plasma levels of TNF-α in rats and wild-type mice, but not in CB1-null mice. This paradoxical effect of CB1 blockade might be due to the unmasking of CB2-mediated anti-inflammatory effects exerted by enhanced endocannabinoid levels when CB1 receptors are blocked, although this possibility has not yet been investigated.42

Data obtained from colonic epithelial cell lines strengthen the concept that the CB2 receptor is protective against gut inflammation. The non-selective cannabinoid receptor agonists CP55,940, Δ9-tetrahydrocannabinol and WIN55,212-2, the selective CB2 receptor agonist JWH-015 (but not the selective CB1 receptor agonist ACEA) inhibited TNF-α induced release of interleukin-8 in a CB2-antagonist-sensitive manner.43 Furthermore, Western immunoblotting revealed an immunoreactive protein in this cell line at a region with a size consistent with that of CB2 receptors.43

Overall, it seems that cannabinoids might regulate the tissue response to gut inflammation via CB2 receptor activation, at two levels: firstly, involving the smooth-muscle response to pro-inflammatory mediators that affect gastrointestinal transit time (see above); and secondly, involving the direct suppression of pro-inflammatory mediators production.

CB2 receptors and intestinal cancer

  1. Top of page
  2. Abstract
  3. CB2 receptors and intestinal motility
  4. CB2 receptors and gut inflammation
  5. CB2 receptors and intestinal cancer
  6. Conclusions
  7. References

Cannabinoids have been licensed for clinical use as palliative treatment of chemotherapy, but increasing evidence shows direct antiproliferative actions of cannabinoid agonists on several tumour cells in vitro and in animal models.44 The proposed mechanisms are complex and may involve induction of apoptosis in tumour cells, antiproliferative action, and an antimetastatic effect through inhibition of angiogenesis and tumour cell migration.6 Cannabinoids may exert antitumour actions via CB1/CB2- and transient receptor potential vanilloid 1 (TRPV1) receptor-dependent or independent (e.g. cyclooxygenase) mechanisms.6 Notably, CB2 receptor activation leads to inhibition of cell growth or apoptosis in prostate and pancreatic tumour cells.45,46 Ligresti et al.19 showed that the levels of anandamide and 2-AG were increased relative to controls in adenomatous polyps and carcinomas, but there appeared to be no differences in the expression of CB1 and CB2 receptors or FAAH levels among the tissues. To determine if cannabinoids affected colorectal cancer cell growth and to understand the possible molecular target, the authors used CaCo-2 (which express CB1 receptors but not CB2 receptors) and DLD-1 cells (which express both CB1 and CB2 receptors, with CB1 receptor less expressed than in CaCo-2 cells). Anandamide, 2-AG and HU-210 (non-selective cannabinoid receptor agonists), and an inhibitor of anandamide inactivation, potently inhibited CaCo-2 cell proliferation, while DLD-1 cells were less responsive to cannabimimetics than CaCo-2 cells. In DLD-1 cells (but not in CaCo-2 cells), the effect of HU-210 was counteracted by both rimonabant (CB1 receptor antagonist) and SR144528 (CB2 receptor antagonist), suggesting that activation of both CB1 and CB2 receptors led to inhibition of cell growth. A more recent study suggests that anandamide inhibits cell death in colorectal carcinoma cells by targeting cells that are high expressors of cyclooxygenase-2.47


  1. Top of page
  2. Abstract
  3. CB2 receptors and intestinal motility
  4. CB2 receptors and gut inflammation
  5. CB2 receptors and intestinal cancer
  6. Conclusions
  7. References

In conclusion, convincing evidence suggests that the CB2 receptor may represent an intrinsic mechanism in the gut able to counteract hypermotility, intestinal inflammation and possibly cell proliferation. In the current issue of the Journal, CB2 receptor activation was shown to inhibit mesenteric afferent firing, so adding visceral pain to this growing list of regulatory actions of this receptor. Because the CB2 receptor is upregulated in inflammatory bowel conditions, CB2 receptor agonists, which are devoid of the characteristic psychotropic CB1-mediated effects, might represent attractive beneficial therapeutic compounds for the management of gut diseases, including IBD and IBS.


  1. Top of page
  2. Abstract
  3. CB2 receptors and intestinal motility
  4. CB2 receptors and gut inflammation
  5. CB2 receptors and intestinal cancer
  6. Conclusions
  7. References
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