Anandamide activates vanilloid receptor 1 (VR1) at acidic pH in dorsal root ganglia neurons and cells ectopically expressing VR1

The vanilloid receptor type 1 (VR1) is a heat-activated ionophore preferent ially expressed in nociceptive neurons of trigeminal and dorsal root gangli a (DRG). VR1, which binds and is activated by capsaicin and other vanilloid compounds, was noted to interact with the endocannabinoid anandamide (ANA) and certain inflammatory metabolites of arachidonic acid in a pH-dependent manner. At pH less than or equal to 6.5 ANA induced Ca-45(2+) uptake eithe r in primary cultures of DRG neurons or cells ectopically expressing C-term inally tagged recombinant forms of VR1 with an EC50 = similar to 10 mum at PH 5.5. Capsazepine, a potent antagonist of vanilloids, inhibited ANA-induc ed Ca2+ transport in both cell systems. Vanilloids displaced [H-3]ANA in VR I-expressing cells, suggesting competition for binding to VR1. Ratiometric determination of intracellular free calcium and confocal imaging of the VR1 -green fluorescent fusion protein revealed that, at low PH (less than or eq ual to 6.5), ANA could induce an elevation of intracellular free Ca2+ and c onsequent intracellular membrane changes in DRG neurons or transfected cell s expressing VR1. These actions of ANA were similar to the effects determin ed previously for vanilloids. The ligand-induced changes in Ca2+ at PH less than or equal to 6.5 are consistent with the idea that ANA and other eicos anoids act as endogenous ligands of VR1 in a conditional fashion in vivo. T he PH dependence suggests that tissue acidification in inflammation, ischem ia, or traumatic injury can sensitize VR1 to eicosanoids and transduce pain from the periphery.