In humans, a Single Nucleotide Polymorphism (SNP) in Fatty Acid Amide Hydrolase (FAAH), C385A or rs324420 is associated with increased risk of Functional Gastro-Intestinal Disorders in general (P=0.01) and more specifically with diarrhea-type Irritable Bowel Syndrome (IBS)(P=0.008) and mixed-type IBS (P=0.01). As this mutation reduces functional expression of FAAH and FAAH is the major degrading enzyme for anandamide this suggests the involvement of the endocannabinoid system in gastro-intestinal function (Camilleri et al., 2008).
DAGLα is expressed in the enteric nervous system including the gastrointestinal tract. Genetically constipated mice and CB1 deficient mice reversed their symptoms of slow gastrointestinal motility, intestinal contractility and constipation after DAGLα inhibition. These effects were mediated by 2-AG and CB1 receptors (Bashashati et al., 2015).
Polymorphisms (small, single nucleotide mutations) in the CB1 gene/receptor are linked to the susceptibility to develop Crohn’s Disease, suggesting the involvement of the endocannabinoid system in Crohn’s Disease (Storr et al., 2010).
Intracerebrovascular application of anandamide and 2AG appeared gastro-protective in ethanol-induced ulcers suggesting the involvement of endocannabinoids in the central nervous system (Gyires and Zádori, 2016).
In patients with diarrhea-type IBS higher levels of 2AG and lower levels of OEA and PEA were found. In contrast, patients with constipation-type IBS had higher levels of OEA and lower levels of FAAH. Also, PEA levels were inversely correlated with abdominal pain suggesting substantial involvement of the endocannabinoid system in the pathophysiology of IBS (Fichna et al., 2013).
Apart from CB1 and CB2, there is evidence for the involvement of PPARγ and TRPV1 in Crohn’s Disease (de Fontgalland et al., 2014; Schicho and Storr, 2014) Patients with Crohn’s Disease have significantly reduced levels of anandamide, but not 2AG or PEA, supporting a role for the endocannabinoid system in Crohn’s Disease (Di Sabatino et al., 2011).
Cannabinoid-mediated reduction in gastro-intestinal motility appears to be mediated by CB1 but not CB2 (Aviello et al., 2008).
CB1 and TRPV1 signaling are both required for the development of stress-induced visceral hyperalgesia and TRPV4 and TRPA1 may also be involved (Lin et al., 2013).
TRP receptors (TRPV1-4, TRPA1, TRPM8) are classically known for their role in pain sensation but may also be involved in inflammation. TRPs bind to most plant cannabinoids and endocannabinoids with varying affinities (De Petrocellis et al., 2011, 2012), tentatively making TRPs excellent targets and plant cannabinoids excellent substrates for pain and inflammation management.
A study in mice showed that GPR55 is involved in neurogenic gut contractions (Ross et al., 2012).
In mice, selective inhibition of FAAH inhibited colonic motility (Fichna et al., 2014).
Many Crohn’s Disease patients self-administer cannabis suggesting a role for cannabinoids in the treatment of Crohn’s or in the alleviation of its symptoms. Although many patients reported symptomatic improvement of abdominal pain (83.9%), abdominal cramping (76.8%), joint pain (48.2%) and diarrhea (28.6%), cannabis use was also associated with increased hospitalization (Storr et al., 2014). This could be explained as cannabis (or the vehicle it comes in, like tobacco) being harmful in Crohn’s. Alternatively, patients with more severe Crohn’s Disease may be sooner inclined to use cannabis to alleviate the symptoms.
In rats, intra-colonic application of 1 to 10 mg/kg cannabis extract dose-dependently reduced colitis severity but oral application did not (Wallace et al., 2013). This effect was independent of CB1 or CB2 receptors. However, oral extract did prevent NSAID-induced gastric damage at 10 mg/kg in a CB1-dependent way. Cannabis extract also reduced visceral pain at 3 mg/kg in a CB2-dependent way suggesting cannabis extract has distinct beneficial effects in gastro-intestinal disorders via CB1/2-dependent and independent pathways (Wallace et al., 2013).
Interestingly, injection of 100 mg/kg THC produced strong diarrhea in CB1 deficient mice but not in controls suggesting complex involvement of CB1 in the regulation of intestinal transit (Zimmer et al., 1999).
Apart from THC, (relatively) non-psychotropic cannabinoids such as THCV, CBD and CBG were found to have anti-inflammatory effects in experimental intestinal inflammation (Alhouayek and Muccioli, 2012).
In the TNBS mouse model of colitis, 5 mg/kg CBD i.p. twice daily for three days attenuated colitis and promoted endothelial and epithelial wound healing (Krohn et al., 2016).
In the DNBS mouse model of colitis, both oral and i.p. CBD decreased tissue damage and intestinal hypermotility. CBD in extract was more effective than pure CBD, suggesting a significant entourage effect (Pagano et al., 2016).
In the LPS mouse model of colitis, 10 mg/kg i.p. CBD decreased reactive gliosis, mast cell and macrophage recruitment, TNFα expression and intestinal apoptosis. In ulcerative colitis patient rectal biopsies also reduced reactive gliosis, at least partially through PPARγ (De Filippis et al., 2011).
In mice, CBD seems to inhibit intestinal motility in both CB1 dependent and independent ways (Fride et al., 2005).
CBG attenuates colitis in animal models, reduces nitric oxide production in macrophages and reduces ROS formation in intestinal epithelial cells, showing therapeutic potential to treat gastrointestinal inflammation (Borrelli et al., 2013).
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