Rectal distention inhibits bladder activity via glycinergic and gabaergic mechanisms in rats

Purpose: We examined the influence of rectal distention on the spinobulbospinal micturition reflex and the mechanism underlying the inhibition of bladder contraction. Materials and Methods: A total of 22 female Sprague-Dawley rats were used in this study. Using urethane anesthesia isovolumetric cystometry was performed before and after distention of the rectum by inflation of a rectal balloon (0 to 3 cm(3)), followed by the intrathecal injection of strychnine (a glycine receptor antagonist, 0.001 to 10 mug) and/or bicuculline (a gamma-aminobutyric acid(A) receptor antagonist, 0.001 to 1 mug) at the lumbosacral level of the spinal cord. Results: Rectal distention (1.5 to 3.0 cm(3)) prolonged the interval, decreased the amplitude and shortened the duration of bladder contraction and finally almost abolished bladder activity. After intrathecal injection of strychnine or bicuculline in animals with inhibition of the bladder by rectal distention the interval and duration of bladder contraction returned to baseline but amplitude only recovered to 47% to 54% of the control level. However, simultaneous intrathecal injection of strychnine and bicuculline (0.001 mug each) restored amplitude to the control level. There were no differences between strychnine and bicuculline with respect to their effects on the interval, amplitude and duration of bladder contraction. Conclusions: An inhibitory rectovesical reflex exists in the lumbosacral cord of rats. The afferent limb of the spinobulbospinal micturition reflex pathway may be additionally and redundantly inhibited by glycinergic and GABAergic mechanisms, while the efferent limb of this pathway may be synergistically inhibited by these mechanisms.