Comparison of [ 3H]resiniferatoxin binding by the vanilloid (capsaicin) receptor in dorsal root ganglia, spinal cord, dorsal vagal complex, sciatic and vagal nerve and urinary bladder of the rat

Abstract

In the present report we compared the properties of [ 3H]resiniferatoxin (RTX) binding by the vanilloid receptors present at different parts of the primary afferent neurons of the rat. We found no major differences in either the affinity or the cooperativity of [ 3H]RTX binding by vanilloid receptors on the cell body, central terminals, peripheral terminals or axons. Specific binding of [ 3H]RTX to dorsal root ganglia, whole spinal cord, dorsal vagal complex, urinary bladder, and sciatic and vagal nerves all followed sigmoidal saturation kinetics indicating positive cooperativity among the binding sites. The cooperativity indexes determined by fitting the data to the Hill equation were 1.82 ± 0.11, 2.21 ± 0.04, 2.55 ± 0.01, 1.91 ± 0.11, 2.03 ± 0.09 and 2.27 ± 0.04, respectively. The dissociation constants in dorsal root ganglia, spinal cord, dorsal vagal complex, urinary bladder, and sciatic and vagal nerve membranes were 46.5 ± 2.7, 29.3 ± 5.1, 28.2 ± 1.2, 60.8 ± 4.4, 59.9 ± 1.9 and 45.2 ± 0.7 pM; the receptor densities were 219 ± 14, 48 ± 5, 67 ± 1, 32 ± 7, 61 ± 9, and 100 ± 20 fmol/mg protein, respectively. We could not show any major differences in the affinities of capsaicin and capsazepine in inhibition of [ 3H]RTX binding by the different membrane preparations either. In all cases the initial enhancement of [ 3H]RTX binding by nonradioactive RTX, capsaicin, and capsazepine confirmed the existence of positive cooperativity among the binding sites. We were unable to detect specific [ 3H]RTX binding sites in membrane preparations of the preoptic area, locus ceruleus, substantia nigra, striatum and paraventricular nuclei of the rat brain under our present conditions. Our results suggest the uniformity of the vanilloid receptors present at different parts of the primary afferent neuron.