[3H]resiniferatoxin binding by the vanilloid receptor: species-related differences, effects of temperature and sulfhydryl reagents.

Specific binding of [3H]resiniferatoxin (RTX) is thought to represent the vanilloid (capsaicin) receptor. In the present study, we have used this binding assay to elucidate the contribution of differential receptor expression to the capsaicin-resistance of hamsters and rabbits; binding parameters were compared to those of species (rats, mice) regarded as capsaicin-sensitive. Whereas the 5-fold lower affinity for [3H]RTX binding in the hamster (100 pM) as compared to the rat (20 pM) is unlikely to account for the 100-fold difference in the in vivo responses of RTX-induced inflammation and hypothermia, the lack of detectable specific [3H]RTX binding sites in the rabbit might represent the predominant mechanism of capsaicin-resistance in this species. Regulation of the vanilloid receptor was further characterized in the rat. In accord with the temperature dependence of both in vivo and in vitro capsaicin actions, we found a marked temperature dependence for association rates. Dissociation turned out to have complex kinetics dependent on time and receptor occupancy. Low pH (5.5-7.0) did not affect receptor binding. Preincubation with heavy metal cations and other sulfhydryl-reactive agents inhibited specific [3H]RTX binding indicating that the vanilloid receptor is a thiol-protein, and that free sulfhydryl groups play an essential role in agonist binding activity. Preliminary characterization suggested noncompetitive inhibition.