Characterization and comparison of recombinant human and rat TRPV1 receptors: effects of exo- and endocannabinoids.

BACKGROUND: TRPV1 is a ligand-gated ion channel whose activation by capsaicin increases intracellular Ca(2+) ([Ca(2+)](i)). TRPV1 and cannabinoid CB(1) receptor activation are capable of eliciting analgesia. In this study, using recombinant human (h) and rat (r) TRPV1 receptors expressed in HEK293 cells, we have performed a comparison of both TRPV1 species at 22 and 37 degrees C and compared endo- and exocannabinoid activity at both receptors.

METHODS: [Ca(2+)](i) was measured in Fura-2-loaded HEK293(hTRPV1) and HEK293(rTRPV1) cells. To assess native CB(1) receptor activity, [(35)S]GTPgammaS binding to membranes prepared from rat cerebellum was measured.

RESULTS: Both capsaicin (pEC(50) rat approximately 6.9 and pEC(50) human approximately 6.8 at 37 degrees C) and anandamide (pEC(50) rat approximately 5.3 and pEC(50) human approximately 5.8 at 37 degrees C) produced a concentration-dependent increase in [Ca(2+)](i) in rat and human systems and at 22 and 37 degrees C. In HEK293(rTRPV1) cells, anandamide appeared to be a partial agonist. Capsazepine demonstrated competitive antagonism at both human and rat TRPV1 receptors and at both temperatures studied. Capsazepine effects were not temperature dependent: pK(B) at rTRPV1 was 5.98 at 22 degrees C and 6.02 at 37 degrees C, and pK(B) at hTRPV1 was 6.76 at 22 degrees C and 6.75 at 37 degrees C. However, there was a consistent 6-fold increase in capsazepine potency for hTRPV1 relative to rTRPV1. The exocannabinoid Delta(9)-tetrahydrocannabinol failed to increase [Ca(2+)](i), although its solvent ethanol was an effective TRPV1 activator. In the [(35)S]GTPgammaS binding assay using rat cerebellar membranes, anandamide (pEC(50) approximately 5.8) and Delta(9)-tetrahydrocannabinol (pEC(50) approximately 7.1), but not capsaicin, stimulated binding. Delta(9)-tetrahydrocannabinol was a partial agonist. pEC(50) values for anandamide at rTRPV1 and rCB(1) were similar.

CONCLUSIONS: There were small differences in the pharmacology of rat and human TRPV1 receptors. Whilst capsaicin activated TRPV1 and Delta(9)-tetrahydrocannabinol activated CB(1), anandamide is an endogenous agonist for both receptor systems.