Resiniferatoxin-type phorboid vanilloids display capsaicin-like selectivity at native vanilloid receptors on rat DRG neurons and at the cloned vanilloid receptor VR1

1 Although the cloned rat vanilloid receptor VRI appears to account for bot h receptor binding and calcium uptake, the identification of vanilloids sel ective for one or the other response is of importance because these ligands may induce distinct patterns of biological activities. 2 Phorbol 12,13-didecanoate 20-homovanillate (PDDNV) evoked Ca-45(2+)-uptak e by rat dorsal root ganglion neurons (expressing native vanilloid receptor s) in culture with an EC50 of 70 nM but inhibited [H-3]-resiniferatoxin (RT X) binding to rat dorsal root ganglion membranes with a much lower potency (K-i > 10,000 nM). This difference in potencies represents a more than 100 fold selectivity for capsaicin-type pharmacology. 3 Ca-45(2+) influx by PDDHV was fully inhibited by the competitive vanilloi d receptor antagonist capsazepine, consistent with the calcium uptake occur ring via vanilloid receptors. 4 PDDHV induced calcium mobilization in CHO cells transfected with the clon ed rat vanilloid receptor VRI with an EC50, of 125 nhl and inhibited [H-3]- RTX binding to these cells with an estimated K-i of 10,000 nhl. By contrast , PDDHV failed to evoke a measurable calcium response in non-transfected CH O cells, confirming its action through VRI. 5 We conclude that PDDHV is two orders of magnitude more potent for inducin g calcium uptake than for inhibiting RTX binding at vanilloid receptors, ma king this novel vanilloid a ligand selective for capsaicin-type pharmacolog y. These results emphasize the importance of monitoring multiple endpoints for evaluation of vanilloid receptor structure-activity relations. Furtherm ore, PDDHV now provides a tool to explore the biological correlates of caps aicin-type vanilloid pharmacology.