The capsaicin-sensitive innervation of the rat urinary bladder: further studies on mechanisms regulating micturition threshold.

The functional role of the capsaicin-sensitive innervation of the rat urinary bladder in regulating micturition has been investigated by means of the transvesical cystometrogram which allows study of the effects of drugs on the various phases of the bladder voiding cycle in urethane-anesthetized rats. Capsaicin desensitization (50-125 mg./kg., 4 to 60 days before) significantly increased both the volume and pressure threshold for micturition, indicating functional impairment of the mechanisms which transmit volume information from the bladder to the central nervous system. On the other hand the parameters related to the expulsive phase of the voiding cycle of the rat bladder are unaffected by capsaicin pretreatment. In capsaicin-desensitized rats the bladder weight remained within the limits of controls up to 60 days from the treatment although at this time the volume threshold was still elevated. In capsaicin-desensitized rats an increase of infusion rate above the physiological range produced micturition cycles having a normal volume and pressure threshold. These findings suggest that at high values of volume/pressure threshold micturition could be initiated through capsaicin-resistant sensory mechanism(s); in effect, multiple sensory systems, having different thresholds, relay volume/pressure information from the detrusor to the CNS.