Cannabinoid Receptor Activation and the Endocannabinoid System in the Gastrointestinal Tract

This work summarises recent findings relating to the activation of CB1 cannabinoid receptors in the gastrointestinal (GI) tract with a particular emphasis on the endocannabinoid and endovanilloid systems. Of the endogenous cannabinoid ligands discovered thus far (anandamide, 2-arachidonoyl glycerol (2-AG), noladin ether, virodhamine and N-arachidonoyl dopamine), only anandamide and 2-AG have been investigated in the GI tract. Recent experimental evidences regarding the inhibitory influence of CB1 receptor stimulation on fluid secretion and the reduction on GI motility and transit are discussed in the light of their possible interactions with the endocannabinoid system. Recent reports of the actions of anandamide both at cannabinoid and vanilloid receptors in the GI tract are described with particular reference to the distribution of CB1 and VR1 receptors in the intestine, vagal afferent fibres and their central projections. A consistent finding is the presence of CB1 receptors on excitatory cholinergic neurones particularly in the myenteric ganglia of a variety of species. Colocalisation studies of cannabinoid receptor-expressing cells with specific cell markers have been used in an attempt to correlate the immunohistochemical data with functional studies in order to determine the physiological significance of subsets of cholinergic neurones. Evidence regarding cannabinoids as antiemetics and the role of cannabinoid CB1 receptor activation in emesis in vomiting species and nausea in non-vomiting animal models is presented. Observed changes in vanilloid and cannabinoid expression in certain disease states, for example in inflammation, are discussed with a view to their possible therapeutic potential in the treatment of inflammatory bowel conditions.