Involvement of alpha 2-adrenoceptors in the sacral micturition reflex in rats.

We have studied the effects of intrathecally-injected drugs that act on alpha-adrenoceptors in the urinary bladder reflex contractile activity evoked by continuous infusion of fluid into the bladder of anesthetized rats. Clonidine (10 and 30 micrograms) facilitated and yohimbine (100 micrograms) abolished the bladder contractile activity, and pretreatment with yohimbine (30 micrograms) inhibited the effect of clonidine (10 micrograms). Phenylephrine (60 micrograms) abolished the bladder contractile activity, but prazosin (40 micrograms) had no significant effect on it. The bladder contractions induced by electrical stimulation of the pontine micturition center were inhibited by yohimbine in a dose-dependent manner. These results suggest that transmission in the descending neurons from the pontine micturition center to the sacral parasympathetic neurons that control bladder motility is mediated by alpha 2-adrenoceptors in rats.