ABSTRACT
Specific [3H]resiniferatoxin (RTX) binding is thought to represent the postulated vanilloid (capsaicin) receptor. In the present report, this binding has been reevaluated using a modified [3H] RTX binding assay in which the high nonspecific binding, which limited the previous characterization, was reduced by adding alpha 1-acid glycoprotein, a plasma protein that binds RTX, to the usual binding assay after RTX binding by the vanilloid receptor had been terminated. Specific [3H]RTX binding by both dorsal root ganglion (DRG) and spinal cord membranes of the rat followed sigmoidal saturation kinetics indicating apparent positive cooperativity. The cooperativity index determined by fitting the data to the Hill equation was 1.7 in DRG and 1.9 in spinal cord. Apparent dissociation constants were estimated as 24 pM for DRG and 11 pM for spinal cord preparations. As predicted by the modified Hill equation, at low receptor occupancy nonradioactive agonists (RTX, tinyatoxin, capsaicin) produced biphasic competition curves. The initial (enhancement) phase of these curves correlated with the biological potency of the agonist. Dithiothreitol reduced both positive cooperativity and apparent binding affinity; the oxidizing agent 5,5'-dithiobis-(2-nitrobenzoic acid) reduced the cooperativity index without a major effect on binding affinity. These findings suggest that the vanilloid receptor is a receptor cluster in which the subunits cooperate; cooperation is, at least in part, subject to redox modulation.
