## 5 Effects of Cannabinoids on Intestinal Barrier Permeability
### Effects of Endocannabinoids on Intestinal Barrier Permeability
#### 5.1.2 Effects of Endocannabinoid-like Compounds on Intestinal Barrier Permeability  
A wide variety of endocannabinoid-like compounds have been isolated and studied over the years. Studies examining their effect on intestinal permeability have mostly used the Caco-2 cell culture model, with one study including human in vivo experiments. Endocannabinoid-like compounds differ in their effect on intestinal permeability, as well as in the receptors via which they effect these changes. When applied to the apical membrane of Caco-2 cells, OEA caused an increase in TEER, and a decrease in TEER when given basolaterally. The effect on the apical membrane was inhibited by the TRPV1 antagonist capsazepine, and the effect of basolateral application was inhibited by the PPAR\(\alpha\) antagonist GW6471. OEA given apically at the same time as IFNg and TNFa prevented the development of cytokine-induced hyperpermeability and also recovered TEER values when given 24 h after the cytokines. This was inhibited by capsazepine. basolateral application of OEA at the same time points exacerbated cytokine-induced permeability. This was inhibited by GW6471, but not capsazepine. PEA increased TEER when introduced both apically and basolaterally, and was inhibited by GW6471 at both locations. PEA applied apically had no effect on cytokine-induced changes in permeability; however, when administered basolaterally, it prevented hyperpermeability when co-applied with cytokines and caused recovery of TEER when given 24 h later. This effect was inhibited by GW6471. Both OEA and PEA reduced membrane expression of aquaporin 3 (AQP3) and aquaporin 4 (AQP4) and led to a concentration-dependent increase in fluorescence, indicating potassium channel activation [90]. In another study, N-arachidonoyl dopamine (NADA), OEA, and OA, administered apically, mitigated hypoxia-induced TEER decrease and sped up recovery. Applied basolaterally, the same was true for NADA, OA, NE, and PEA, but OEA had the opposite effect. Apically, NADA, OA, and OEA exerted their effects through TRPV1 activation. When introduced basolaterally, NADA worked via TRPV1, and OA worked via TRPV1 and CB1. NE and PEA worked via PPAR\(\alpha\). OEA worsened permeability via PPAR\(\alpha\) activation [91]. PEA, when applied basolaterally, reduced both FD4 and FD10 flux across Caco-2 membranes in a state of cytokine-induced hyperpermeability. The effect of PEA in reducing dextran flux was inhibited by PPAR\(\alpha\) antagonism as well as by inhibition of protein kinase A (PKA), mitogen-associated protein kinase (MAPK), and adenylyl cyclase signaling pathways. In cell cultures, PEA also prevented the cytokine-induced drop in TRPV1 mRNA and the cytokine-induced rise in AQP3 gene expression. Alone, PEA increased AQP4 gene expression in Caco-2 cells. In both normal and inflamed human colonic tissue, PEA caused a drop in claudin-3 mRNA levels. In human subjects given lactulose and mannitol + PEA and aspirin, PEA lowered the urinary lactulose-to-mannitol ratio, as compared to the group given lactulose and mannitol + placebo. Lactulose is not absorbed in the healthy gut, but is passively absorbed in inflammatory states. Aspirin increases the urinary lactulose-to-mannitol ratio, which is hypothesized to happen via COX inhibition. This implies that PEA can reduce intestinal permeability in humans in vivo [92]