Iodo-resiniferatoxin, a new potent vanilloid receptor antagonist

The highly potent vanilloid receptor (VR) agonist resiniferatoxin has been radiolabeled with I-125, and the pharmacology to the cloned rodent VR, VR1, and the endogenous VR in rat spinal cord membranes has been characterized. [I-125]RTX binding to human embryonic kidney 293 cells expressing VR1 was reversible and with high affinity (K-d = 4.3 nM) in an apparent monophasic manner. In rat spinal cord membranes, [I-125] RTX bound with a similar high affinity (K-d = 4.2 nM) to a limited number of binding sites (B-max = 51 /- 8 fmol/mg of protein). The pharmacology of recombinant rodent VR1 and th e endogenous rat VR1 was indistinguishable when measuring displacement of [ I-125] RTX binding (i.e., the following rank order of affinity was observed : RTX > I-RTX > olvanil > capsaicin > capsazepine). Capsaicin and RTX induc ed large nondesensitizing currents in Xenopus laevis oocytes expressing VR1 (EC50 values were 1300 nM and 0.2 nM, respectively), whereas I-RTX induced no current per se at concentrations up to 10 muM. However, I-RTX completel y blocked capsaicin-induced currents (IC50 = 3.9 nM). In vivo, I-RTX effect ively blocked the pain responses elicited by capsaicin (ED50 = 16 ng/mouse, intrathecally). The present study showed that I-RTX is at least 40-fold mo re potent than the previously known VR antagonist, capsazepine. Thus, I-RTX as well as its radiolabeled form, should be highly useful for further expl oring the physiological roles of VRs in the brain and periphery.