CAPSAZEPINE - A COMPETITIVE ANTAGONIST OF THE SENSORY NEURON EXCITANT CAPSAICIN

1 Capsazepine is a synthetic analogue of the sensory neurone excitotoxin, capsaicin. The present study shows the capsazepine acts as a competitive antagonist of capsaicin. 2 Capsazepine (10 muM) reversibly reduced or abolished the current response to capsaicin (500 nM) of voltage-clamped dorsal root ganglion (DRG) neurones from rats. In contrast, the responses to 50 muM gamma-aminobutyric acid (GABA) and 5 muM adenosine 5'-triphosphate (ATP) were unaffected. 3 The effects of capsazepine were examined quantitatively with radioactive ion flux experiments. Capsazepine inhibited the capsaicin (500 nM)-induced Ca-45(2+) uptake in cultures of rat DRG neurones with an IC50 of 420 +/- 46 nM (mean +/- s.e.mean, n = 6). The Ca-45(2+) uptake evoked by resiniferatoxin (RTX), a potent capsaicin-like agonist was also inhibited. (Log concentration)-effect curves for RTX (0.3 nm-1 muM) were shifted in a competitive manner by capsazepine. The Schild plot of the data had a slope of 1.08 +/- 0.15 (s.e.) and gave an apparent K(d) estimate for capsazepine of 220 nm (95% confidence limits, 57 - 400 nM). 4 Capsazepine also inhibited the capsaicin- and RTX-evoked efflux of Rb-86+ from cultured DRG neurones. The inhibition appeared to be competitive and Schild plots yielded apparent K(d) estimates of 148 nm (95% confidence limits, 30-332 muM) with capsaicin as the agonist and 107 nm (95% confidence limits, 49-162 nM) with RTX as agonist. 5 A similar competitive inhibition by capsazepine was seen for capsaicin-induced [C-14]-guanidinium efflux from segments of adult rat vagus nerves (apparent K(d) = 690 nM; 95% confidence limits, 63 nM - 1.45 muM). No significant difference was noted in the apparent K(d) estimates for capsazepine in assays on cultured DRG neurones and vagus nerve as shown by the overlap in the 95% confidence limits. 6 Capsazepine, at concentrations up to 10 muM, had no significant effects on the efflux of Rb-86+ from cultured DRG neurones evoked either by depolarization with high (50 mM) K+ solutions or by acidification of the external medium to pH 5.0 - 5.6. Similarly capsazepine had no significant effect on the depolarization (50 mM KCl)-induced efflux of [C-14]-guanidinium from vagus nerve preparations. 7 Ruthenium Red was also tested for antagonism against capsaicin evoked [C-14]-guanidinium release from vague nerves and capsaicin induced Ca-45(2+) uptake in cultures of DRG neurones. In contrast to capsazepine the inhibition by Ruthenium Red (10 - 500 nM in DRG and 0.5-10 muM in vagus nerve experiments) was not consistent with a competitive antagonism, but rather suggested a more complex, non-competitive inhibition.