Comparison of effects of anandamide at recombinant and endogenous rat vanilloid receptors

Background. Anandamide, an endogenous lipid, activates bothcannabinoid (CB₁) and vanilloid (VR1) receptors, both of whichare co-expressed in rat dorsal root ganglion (DRG) cells. Activationof either receptor results in analgesia but the relative contributionof CB₁ and VR1 in anandamide-induced analgesia remains controversial.Here we compare the in vitro pharmacology of recombinant andendogenous VR1 receptors using calcium imaging, in clonal andDRG cells, respectively. We also consider the contribution ofCB₁ and VR1 receptors to anandamide-induced analgesia. Methods. Using a Flurometric Imaging Plate Reader (FLIPR^TM),calcium imaging has been used to study the effects of severalvanilloid and cannabinoid ligands in rat VR1-transfected HEK293(rVR1-HEK) cells and in DRG cells. The effect of pre-exposureof several vanilloid and cannabinoids has also been comparedin DRG cells. Results. The VR1 agonists capsaicin, olvanil, (N-(4-hydroxyphenyl-arachinoylamide)(AM404) and anandamide caused a concentration-dependent increasein intracellular calcium concentration (Ca²⁺]_i), with similartemporal profiles in both rVR1-HEK and DRG cells, and potency(pEC₅₀) values of 8.25 (SEM 0.11), 8.37 (0.04), 6.96 (0.06),5.85 (0.01) and 7.45 (0.10), 7.55 (0.07), 6.10 (0.13), approximately5.5, respectively. These responses were inhibited by the VR1antagonist capsazepine (1 µM). In contrast, applicationof synthetic cannabinoid antagonists failed to inhibit the anandamide-inducedincrease in Ca²⁺]_i. Reapplication of VR1 agonists significantlyinhibited a subsequent challenge to either capsaicin or anandamidein either cell type, whilst pre-exposure to cannabinoid agonistswere without effect. Conclusion. Here we provide evidence that the pharmacology ofrecombinant rVR1 receptors is similar to those endogenouslyexpressed in DRG cells. Moreover, we have shown that VR1, butnot CB₁, receptors are involved in anandamide-induced responsesin dorsal root primary neurones in vitro. Therefore, the analgesicproperties of anandamide are likely to be mediated, at leastin part, by VR1 activation in DRG cells in vivo. Br J Anaesth 2002; 89: 882–7