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The effect of capsaicin (10–80 mg/kg sc) on reflex activity of the urinary bladder was examined in anesthetized normal as well as anesthetized and awake chronic spinal cord-injured (SCI) cats. In normal cats, capsaicin elicited a transient increase in the frequency of isovolumetric bladder contractions and reduced the volume threshold for inducing micturition, but did not depress the amplitude of bladder contractions or the reflex firing on bladder nerves. In anesthetized SCI cats, capsaicin depressed reflex bladder activity and firing on bladder nerves. In awake SCI cats, capsaicin initially decreased the volume threshold for inducing micturition; however, after a delay of 3–6 h the volume threshold increased and intravesical voiding pressure decreased. This effect persisted for 4–12 days. It is concluded that capsaicin-sensitive C fiber bladder afferents are not involved in initiating reflex micturition in normal cats, but play an essential role in triggering automatic micturition in chronic SCI cats. The results are consistent with the clinical data indicating that C fiber bladder afferents contribute to bladder hyperactivity and incontinence in patients with neurogenic bladder dysfunction.

The effect of capsaicin on micturition and associated reflexes was studied in urethan-anesthetized female rats. Capsaicin or vehicle solution were administered 4 days before the experiment in a dose of 125 mg/kg sc or during the experiment in a dose of 50-100 mg/kg sc. Activity of the urinary bladder was recorded by measuring intravesical pressure via a urethral catheter while slowly filling (0.052 ml/min) the bladder or when the bladder was distended beyond the micturition threshold and maintained at a constant volume. Pretreatment with capsaicin did not significantly change various parameters of urinary bladder function including micturition volume threshold or the amplitude, duration, and interval between reflex bladder contractions. However, capsaicin pretreatment significantly reduced (80% decrease) the arterial pressor responses accompanying reflex bladder contractions and reduced by approximately one-half the percentage of animals in which bladder activity was inhibited by stimulation of the uterine cervix. A large dose of capsaicin (50 mg/kg sc) elicited an acute block of bladder activity that persisted for 8-15 h. This effect is attributable to an action on myelinated afferent or efferent components of the micturition reflex pathway. It is concluded that capsaicin-sensitive afferents are not essential for the performance of micturition in the rat. However, these afferents are involved in cervicovesical reflex mechanisms that modulate bladder activity and in vascular reflexes triggered by isometric bladder contractions.