Effects of TRPV1 receptor antagonists on stimulated iCGRP release from isolated skin of rats and TRPV1 mutant mice.

Capsaicin antagonists including ruthenium red, capsazepine and iodo-resiniferatoxin (I-RTX) have recently been shown to inhibit the activation by noxious heat of the capsaicin receptor (TRPV1) expressed in non-neuronal host cells, and natively, in cultured dorsal root ganglion cells. Noxious heat has been shown to release immunoreactive calcitonin gene-related peptide (iCGRP) from the isolated rat skin. In this model, ruthenium red, I-RTX as well as capsazepine 10 microM caused no alteration in iCGRP release at 32 degrees C by themselves whereas capsazepine 100 microM doubled it reversibly. In wild-type mice 100 microM capsazepine also stimulated iCGRP release while it was without effect in TRPV1 knockout littermates. In the rat skin, both ruthenium red and capsazepine (10/100 microM) reduced and abolished, respectively, capsaicin-induced iCGRP release while I-RTX (1/10 microM) was ineffective. Only ruthenium red 100 microM showed an unspecific effect inhibiting iCGRP release induced by KCl. Ruthenium red and capsazepine (10/100 microM) caused no significant alteration of iCGRP release induced by heat stimulation at 47 degrees C. Employing 45 degrees C stimulation intensity, capsazepine and I-RTX (in the higher concentrations) showed a significant facilitatory effect on the heat response suggesting a partial agonistic action of the compounds. It is concluded that noxious heat-induced iCGRP release in the isolated rat skin occurs through a mechanism that is not inhibited by TRPV1 antagonism reflecting a different pharmacological profile of noxious heat transduction in terminals of sensory neurons compared to that in cultured cell bodies and TRPV1-transfected host cells.