Vesico-inhibitory responses and capsaicin-sensitive afferents in rats.

(1) The effect of perineal pinching and distension of a balloon inserted into the colon on motility of the urinary bladder has been investigated in adult urethane-anesthetized rats pretreated with capsaicin (50 mg/kg s.c.) or its vehicle 4 days before the experiments. (2) At bladder volumes which were sufficient to elicit reflex micturition, perineal pinching or colonic distension transiently inhibited the ongoing bladder voiding contraction. The somato-vesical inhibitory response was markedly reduced or even abolished by division of pudendal nerves. Neither the somato-vesical nor the colovesical inhibitory response were modified by desensitization with systemically administered capsaicin. (3) Intraurethral administration of capsaicin produced a transient inhibition of the reflexly-activated bladder contractions. A second administration of the drug was less effective, indicating desensitization. Intravenously administered capsaicin had a similar inhibitory effect on bladder motility. (4) The vesico-inhibitory response produced by intraurethral administration of capsaicin was not affected by phentolamine, propranolol, guanethidine, picrotoxin or naloxone, while it was greatly reduced or even abolished by bilateral section of the pudendal nerves. (5) These findings provide evidence that capsaicin-sensitive chemoreceptors in the rat urethra are involved in generating a vesico-inhibitory response via pudendal nerves. On the other hand, no evidence was found for the participation of capsaicin-sensitive nerves in the generation of the somato- or colo-vesical inhibitory response.