Reversibility of the inhibitory effect of intravesical capsaicin on the micturition reflex in rats

The reversibility of the inhibitory effect of capsaicin on the micturition reflex was investigated in adult rats. The experimental group (n = 38) were given 0.5 mL of 1 mmol/L intravesical capsaicin. After 30 minutes the bladder was evacuated, then the rats were allowed to recover for various times from 0 to 24 hours before cystometrography. Control rats (n = 6) were injected with saline. Immediately after capsaicin treatment, the bladders showed detrusor hyperactivity and high intravesical pressure during cystometrography. All capsaicin-treated bladders showed final urinary retention and hematuria developed in 32 of 38. At 6 hours after intravesical instillation of capsaicin, detrusor hyperactivity was reduced and the micturition reflex gradually reappeared. By 12 hours, micturition reflexes were noted in seven of eight bladders with a volume threshold equal to that of the control group. At 24 hours, the volume threshold for the micturition reflex was significantly greater in the capsaicin-treated group than in the control group. The amplitude of detrusor contractions at 6, 12, and 24 hours showed no significant difference from that in the controls. In vitro whole bladder contractility in response to electrical field stimulation, bethanechol, and KCl also showed no significant difference between the control and the experimental groups. The bladder weight increased as the recovery period increased, indicating the presence of neurogenic inflammation. From this study we conclude that capsaicin-induced micturition reflex inhibition in rats is reversible at 12 hours and the volume threshold for eliciting the micturition reflex continues to increase up to 24 hours after capsaicin treatment. These results may provide insight into the clinical application of capsaicin in the treatment of various voiding disorders in humans.