## 5 Effects of Cannabinoids on Intestinal Barrier Permeability
### Effects of Endocannabinoids on Intestinal Barrier Permeability  
#### 5.1.1 Anandamide and 2-AG  
There have been several studies showing the significant effects of endocannabinoids on intestinal permeability, mostly using Caco-2 cell culture models. Endocannabinoids differ in their impact on permeability, depending on which substance is used and whether it is applied apically or basolaterally to cell cultures.  
Anandamide and 2-AG applied apically to Caco-2 cell cultures caused a further decrease in transendothelial electrical resistance (TEER) to that caused by EDTA. When administered basolaterally, however, both endocannabinoids caused a concentration-dependent recovery of TEER values after the drop evoked by the application of EDTA. This was inhibited by the CB1 receptor antagonist AM251 and, in the case of anandamide, additionally by the TRPV1 antagonist capsaicingeping. Both cannabinoids also caused downregulation of mRNA for the TJ protein claudin-1, which was inhibited by AM251 [86]. A similar effect occurred in in Caco-2 cultures treated with IFNg and TNFa, where apical application of anandamide and 2-AG worsened hyperpermeability as measured by TEER and fluorescein isothiocyanate-dextran (FD4) flux. The CB1 receptor antagonist AM251 inhibited the exacerbation of hyperpermeability induced by the two endocannabinoids. Furthermore, anandamide given alongside an FAAH inhibitor lowered TEER, as did the FAAH inhibitor applied at the same time as the cytokines. Analogously, 2-AG alongside a monoacylglycerollopez (MAGL) inhibitor lowered TEER to a greater degree than 2-AG alone, as did the MAGL inhibitor administered at the same time as the cytokines. The effects of both FAAH and MAGL inhibitors on TEER were inhibited by AM251, implying that their influence was due to an increase in endocannabinoid concentrations caused by inhibition of their breakdown. This suggests that locally produced cannabinoids play a part in cytokine-mediated intestinal permeability changes [87].  
When administered apically to the Caco-2 monolayer, the FAAH inhibitor URB597 and the MAGL inhibitor JZL184 caused a concentration-dependent drop in TEER, which was reversed by AM251, a CB1 receptor antagonist. When URB597 was applied basolaterally, the same phenomenon occurred, this time inhibited by AM251 and the TRPV1 antagonist capsaicingeping. In contrast, JZL184 applied basolaterally caused the opposite, an increase in TEER values, whose effect was also inhibited by AM251. 2-AG alone applied basolaterally also caused an increase in resistance. Both URB597 and JZL184 caused a further decrease in TEER in Caco-2 cells exposed to inflammatory cytokines, as well as preventing recovery of TEER in cell cultures exposed to hypoxia. No effects on TEER were found when URB597 or JZL184 were administered apically or basolaterally in CB1 knockdown Caco-2 cells. The lack of contribution of TRPV1 in CB1 knockdown cultures may suggest that the ability of TRPV1 to affect permeability is dependent on activation of CB1 receptors [88].  
Mice fed a high-fat, high-sucrose diet for 8 weeks experienced elevations in FD4 flux across the intestinal barrier, as compared to mice fed a standard diet. These changes in permeability were accompanied by a drop in the content of 2-AG and other related monoacylglycerollyeerols OEA, 2-docosahexaenoicenoyl-sn-glycerol (2-DG), and 2-linoleoylglycerol (2-LG) in the epithelium of the large intestine, as well as a significant decrease in the activity of monoacylglycerollyeerol biosynthetic enzymes. Additionally, this high-fat, high-sucrose diet impacted permeability as measured by increased FD4 flux much more strongly in mice lacking intestinal CB1 receptors than in wild-type mice, implying a protective role for 2-AG in intestinal permeability in diet-induced obesity [89].