Throughout our studies we used

Throughout our studies we used CBD under the assumption that it binds to the GPR55 receptor and has antagonist properties at the GPR55 receptor, as previously suggested (Whyte et al., 2009). To clarify whether the GPR55 receptor is involved in the O-1602-mediated effects on GI motility, we performed whole gut transit time and colonic expulsion experiments in GPR55−/− mice. Whereas the WIN55,212-2 effect was unchanged in GPR55−/− mice, O-1602 was not effective in GPR55−/− mice proving that O-1602 slows GI motility via GPR55 receptors. This important novel observation suggests that the GPR55 receptor is a promising future target for the treatment of GI motility disorders, especially when the goal is slowing of GI motility and transit. These experiments in GPR55−/− mice are of special importance since due to the lack of highly specific or highly selective GPR55 receptor agonists and antagonists, pharmacological studies on GPR55 functioning has to be carefully discussed. In a recent study where we investigated GPR55 involvement in postoperative ileus, we were able to prove that GPR55 740 Y-P is higher in the inflammatory state, but we were not able to characterize GPR55 effects on neurotransmission since in this post-inflammatory state the GRR55 agonist and antagonist had same directed effects, which limited any further interpretation (Lin et al., 2011). CBD was identified to reduce inflammatory hypermotility in mice (Capasso et al., 2010) which was again seen in the study where postoperative ileus was studied. CBD effects were at that time not mediated by CB1 or CB2 involving pathways and it is furthermore unlikely that GPR55 was the mediating receptor since the CBD effect was shown to be located on smooth muscle cells and our present study shows that GPR55 is not located on smooth muscle cells (Capasso et al., 2010). One final result warrants further attention. O-1602 slowed GI motility by acting both at peripheral and central sites, similarly to CB1 receptor agonists (Izzo et al., 2000). Interestingly, we observed that the effects of WIN55,212-2, when given directly into the CNS, can be reversed by CBD, suggesting that GPR55 receptors are involved in the central slowing effects of cannabinoids. Since WIN55,212-2 does not bind to GPR55 receptors (Johns et al., 2007), this effect may be due to downstream activation of GPR55 receptors, following central activation of CB1 binding sites, but it needs to be addressed further in a separate study. However, the pertinent finding in this context here deserves further attention since it suggests future cannabinoid receptor targeting approaches devoid of the undesired CNS side effects, commonly associated with the activation of CB1 receptor.
Competing interests
Funding Supported by the University of Calgary Research Grant Committee (to MS), the Deutsche Forschungsgemeinschaft (DFG; STO 645-6-1 to MS) and the Canadian Institutes of Health Research (KAS). KAS is an Alberta Heritage Foundation for Medical Research Medical Scientist and the Crohn's and Colitis Foundation of Canada Chair in Inflammatory Bowel Disease Research. JF is supported by the Iuventus Plus Program of the Polish Ministry of Science and Higher Education (#0119/IP1/2011/71). RS is supported by the Austrian Science Fund (FWF; P-22771).
Acknowledgements We thank Ms. Winnie Ho from the University of Calgary, Canada for performing the genotyping of the CB1/2 receptor gene deficient mouse colony.