[H-3] RESINIFERATOXIN BINDING BY THE HUMAN VANILLOID (CAPSAICIN) RECEPTOR

We report here that we were able to detect the human vanilloid receptor in all three major central endings of primary afferent neurons - in the dorsal horn of the spinal cord, in the cuneate and gracile nuclei and in the spinal nucleus of the trigeminal nerve - and to characterize the binding properties of the receptor in the dorsal horn. Specific [H-3]resiniferatoxin (RTX) binding is thought to represent the vanilloid (capsaicin) receptor. [H-3]RTX binding to membranes obtained from total human spinal cord and dorsal horn followed sigmoidal saturation kinetics indicating apparent positive cooperativity. The cooperativity index determined by fitting the data to the Hill equation was 1.37 +/- 0.02 in the total spinal cord and 1.77 +/- 0.16 in the dorsal horn. The apparent dissociation constants in whole spinal cord and dorsal horn membranes were 915 +/- 12 and 532 +/- 27 pM; the receptor densities were 140 +/- 6 and 227 +/- 15 fmol/mg protein, respectively. Membrane preparations from the spinal nucleus of the trigeminal nerve and the cuneate and gracile nuclei also bound [H-3]RTX in a similar fashion. In parallel experiments, rat spinal cord membranes bound [H-3]RTX with 20- to 40-fold higher affinity, somewhat greater positive cooperativity, but at a 3-fold lower receptor density. As predicted by the modified Hill equation, non-radioactive RTX at low receptor occupancy produced biphasic competition curves. Capsaicin and the competitive antagonist capsazepine also fully displaced specifically: bound [H-3]RTX from human dorsal horn membranes with Ki values of 1.07 +/- 0.04 mu M and 353 +/- 13 nM, respectively; the corresponding Hill coefficients were 1.69 +/- 0.06 and 1.6 +/- 0.02. [H-3]RTX binding was not inhibited by resiniferonol 9,13,14-orthophenylacetate, the biologically inactive parent diterpene of RTX. Capsaicin and capsazepine inhibited specific binding of [H-3]RTX to rat spinal cord membranes with K-i values of 5.3 +/- 0.37 mu M and 3.55 +/- 0.35 mu M and Hill coefficients of 2.03 +/- 0.08 and 1.9 +/- 0.1, respectively. Our results show that there are species differences in the receptor characteristics between man and rat.