Spinal and supraspinal components of GABAergic inhibition of the micturition reflex in rats

The effect of gamma-aminobutyric acid (GABA)ergic drugs on micturition reflexes was investigated in urethane-anesthetized rats. Intracisternally administered GABA or homotaurine inhibited, in a bicuculline-sensitive manner, the supraspinal micturition reflex. Bicuculline or picrotoxin increased the amplitude and duration of the micturition contractions of supraspinal origin, suggesting the existence of a tonic GABAergic inhibitory mechanism. Neither diazepam nor bicuculline or picrotoxin had any significant effect on threshold of the spinal vesico-vesical micturition reflex. Aminooxyacetic acid, an inhibitor of GABA catabolism, increased threshold of the spinal but not of the supraspinal vesico-vesical reflex in a bicuculline-sensitive manner. Volume threshold for eliciting the supraspinal reflex was increased by diazepam and decreased by picrotoxin. Neither picrotoxin nor aminooxyacetic acid modified amplitude of bladder contractions elicited by pinching of the perineal skin (spinal somato-vesical reflex) nor of that produced by preganglionic nerve stimulation of the excitatory nerves. It is concluded that endogenous GABAergic mechanisms involving the activation of GABA A receptors modulate both the spinal and the supraspinal vesico-vesical micturition reflexes. These endogenous GABAergic mechanism(s) are operating tonically in inhibiting micturition at supraspinal but not spinal level.