Capsazepine: a competitive antagonist of the sensory neurone 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 μm) 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 μm γ‐aminobutyric acid (GABA) and 5 μm 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 45Ca2+ uptake in cultures of rat DRG neurones with an IC50 of 420 ± 46 nm (mean ± s.e.mean, n = 6). The 45Ca2+ uptake evoked by resiniferatoxin (RTX), a potent capsaicin‐like agonist was also inhibited. (Log concentration)‐effect curves for RTX (0.3 nm‐1 μm) 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 Kd estimate for capsazepine of 220 nm (95% confidence limits, 57–400 nm). 4 Capsazepine also inhibited the capsaicin‐ and RTX‐evoked efflux of 86Rb+ from cultured DRG neurones. The inhibition appeared to be competitive and Schild plots yielded apparent Kd estimates of 148 nm (95% confidence limits, 30–332 nm) 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 [14C]‐guanidinium efflux from segments of adult rat vagus nerves (apparent Kd = 690 nm; 95% confidence limits, 63 nm‐1.45 μm). No significant difference was noted in the apparent Kd 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 μm, had no significant effects on the efflux of 86Rb+ 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 [14C]‐guanidinium from vagus nerve preparations. 7 Ruthenium Red was also tested for antagonism against capsaicin evoked [14C]‐guanidinium release from vague nerves and capsaicin induced 45Ca2+ uptake in cultures of DRG neurones. In contrast to capsazepine the inhibition by Ruthenium Red (10–500 nm in DRG and 0.5–10 μm in vagus nerve experiments) was not consistent with a competitive antagonism, but rather suggested a more complex, non‐competitive inhibition.