Capsaicin inhibits detrusor contractility but not the micturition reflex in capsaicin-desensitized rats.

Intravesical capsaicin at high concentration (1 mmol/L) inhibits the micturition reflex and detrusor contractility. To investigate the mechanism, we compared rats that had been desensitized by capsaicin at birth with control rats. In vivo cystometrography was performed with normal saline (n = 5) or 1 mmol/L capsaicin (n = 5), then an isolated whole bladder contractility study with electrical field stimulation, bethanechol, and KCl was carried out. In the bladders with normal saline cystometrography and after isolated bladder study, 0.5 mL of 1 mmol/L capsaicin was instilled into the bladders and another set of stimulations by electrodes, bethanechol, and KCl was given. The bladders of capsaicin-desensitized rats showed the presence of detrusor reflexes but diminished amplitudes of detrusor contractions. There was no urinary retention or increased intravesical pressure in these desensitized bladders after capsaicin cystometrography. The detrusor contractility in the isolated whole bladder study showed no significant difference between the control and experimental bladders. After 1 mmol/L capsaicin cystometrography the bladders had little contractility compared with bladders after saline cystometrography. The bladders of the desensitized rats increased in weight compared to the controls. Intravesical administration of 1 mmol/L capsaicin induced an initial excitatory effect on the micturition reflex and final urinary retention in control rats. After capsaicin treatment at birth, these effects no longer existed but the depressant effect of capsaicin on detrusor contractility persisted. We conclude that a high concentration of intravesical capsaicin acutely inhibits the micturition reflex and detrusor contractility. Neonatal desensitization by capsaicin can depress the detrusor contractility in rats but the micturition reflex is not affected.