Autoradiographic visualization and pharmacological characterization of vanilloid (capsaicin) receptors in several...

Sensory neurons sensitive to vanilloids (the paradigm of which is capsaicin, the pungent principle in hot peppers) were visualized by [3H]resiniferatoxin (RTX) autoradiography in several species, including man. Vanilloid binding sites were detected in somatic (trigeminal and dorsal root) and visceral (nodose) sensory ganglia, peripheral (vagal and sciatic) nerves, dorsal horn of the spinal cord, as well as in nuclei in the central nervous system receiving sensory input, such as the nucleus of the solitary tract (containing vagal afferents) and the spinal trigeminal nucleus. Twenty four hrs after ligation of the vagal or the sciatic nerves, a strong accumulation of specific RTX binding sites was observed proximal to the ligature, implying anterograde intraaxonal receptor transport from the nodose and dorsal root ganglia, respectively, to the periphery. RTX induced a dose-dependent loss of vanilloid receptors in the spinal cord and urinary bladder of the rat which was entirely due to a reduction in Bmax. This receptor loss was reversible in the bladder, where the recovery of the binding was accompanied by a restoration of the neurogenic plasma extravasation response, but was irreversible in the spinal cord. These findings suggest that vanilloid receptor loss after RTX treatment can be either reversible (desensitization) or irreversible (most likely reflecting neurotoxicity). Comparably high levels of specific RTX binding were found in human, guinea pig and rat bronchi (species known to respond to vanilloids differently), suggesting that vanilloid receptors can mediate distinct patterns of biological activities among species. Of the species examined, none showed a close resemblance in RTX binding parameters to human vanilloid receptors in spinal cord. The vanilloid receptor antagonist capsazepine was shown to inhibit RTX binding consistent with a competitive mechanism. Both inter- and intraspecies heterogeneity was observed in the affinity by which vanilloid receptors recognize capsazepine. Protons were shown to inhibit RTX binding to rat spinal cord membranes. Thus, protons and/or putative proton-generated substances might represent endogenous modulators of vanilloid receptors. A novel vanilloid ligand, phorbol 12-phenylacetate 13-acetate 20-homovanillate (PPAHV), was developed which bound to cultured dorsal root ganglion neurons and induced calcium uptake by them in a non-cooperative fashion. RTX bound to vanilloid receptors in a positive cooperative manner; however, in the presence of PPAHV, cooperative binding was no longer observed. These results suggest that positive cooperativity is a ligand-induced feature rather than an inherent property of vanilloid receptors. Neuroleptic drugs (trifluoperazine and rimcazole) were found to inhibit RTX binding to porcine dorsal horn membranes consistent with a non-competitive or mixed binding mechanism. this interaction may represent a mechanism for their adjuvant analgesic action. In conclusion, specific binding of [3H]RTX provides a unique tool to visualize vanilloid receptors and to study their pharmacology. A heterogenous vanilloid receptor system is emerging with types/subtypes, and marked species-related differences. The interaction of protons and neuroleptics at vanilloid receptors may provide a rational basis to explain their actions on on primary sensory neurons.