Capsazepine: a competitive antagonist of the sensory neurone excitant capsaicin

Abstract

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 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 ⁸⁶Rb⁺ 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 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 K_d = 690 nm; 95% confidence limits, 63 nm-1.45 μm). 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 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.