Endogenous activators of the pain receptor TRPV1 From cell to man

N-acylethanolamines (NAEs) belong to a growing family of endogenous signalling molecules acting on a variety of receptors and ion channels. In 1992, it was reported that the N-acylethanolamine anandamide is present in the brain and activates the central cannabinoid (CB) receptor. Anandamide and other NAEs are considered to be generated on demand following receptor activation or cell stress. The tissue level of anandamide increases during ischemic tissue injury and inflammation. Hydrolytic cleavage of the amide bond by fatty acid amide hydrolase is responsible for the elimination of anandamide and other N-acylethanolamines.

Studies on whole animals or isolated tissues have shown that anandamide induces a variety of effects in the cardiovascular system, including vasodilatation, bradycardia and hypotension. Macrophages and monocytes, endothelial cells and sensory nerves are possible cellular sources of anandamide in the vascular system. The mechanisms behind the cardiovascular effects of anandamide are not fully understood, but seem to depend on the bioassay and the mode of administration of anandamide. In isolated arterial segments, anandamide produces vasorelaxation via activation of transient receptor potential vanilloid 1 (TRPV1) on perivascular sensory nerves and the subsequent release of the potent vasodilator calcitonin gene-related peptide. TRPV1 is a nonselective cation channel, belonging to the transient receptor potential ion channel superfamily, and the receptor for the pungent ingredient in hot chilli peppers. In this work, we show that anandamide and its metabolically stable analogue methanandamide are able to cause vasodilatation in intact vascular beds by a similar TRPV1-dependent mechanism. The vasodilator response to methanandamide in rat isolated perfused mesenteric arterial bed is inhibited by the TRPV1 antagonist ruthenium red. More importantly, anandamide together with well known plant-derived or synthetic TRPV1 agonists are able to cause vasodilatation in vivo in human skin, an effect inhibited by the selective TRPV1 antagonist capsazepine.

The endogenous C18 NAEs N-linolenoylethanolamine (18:3 NAE), N-linoleoylethanolamine (18:2 NAE), N-oleoylethanolamine (18:1 NAE) and N-stearoylethanolamine (18:0 NAE) are structurally related to anandamide, but these lipids are poor ligands at cannabinoid CB1 receptors. Here we show that these lipids are present in rat sensory ganglia and vascular tissue in levels that may substantially exceed those of anandamide. 18:3 NAE, 18:2 NAE and 18:1 NAE, but not 18:0 NAE and oleic acid, activate rat TRPV1 on perivascular sensory nerves and human TRPV1 expressed in eukaryotic cells.

In conclusion, this study clearly show that, in the rat isolated mesenteric arterial bed, the vasodilator response to methanandamide is mediated predominantly via activation of TRPV1 on sensory nerves. We also demonstrate for the first time that anandamide has biological activity in man, causing vasodilatation in human skin microcirculation via activation of TRPV1. We also identify novel endogenous TRPV1 ligands, belonging to the C18 NAE family of lipids. These unsaturated C18 NAEs, which are poor ligands at the cannabinoid CB1 receptor, may act in concert with anandamide as endogenous TRPV1 modulators in conditions associated with activation of capsaicin-sensitive sensory nerves, including pain, inflammation, ischemia and bladder instability.