EFFECTS OF GABA-RECEPTOR STIMULATION AND BLOCKADE ON MICTURITION IN NORMAL RATS AND RATS WITH BLADDER OUTFLOW OBSTRUCTION

The effects on micturition of the GABA receptor agonists muscimol (sel ective for GABA(A)-receptors) and baclofen (selective for GABA(B)-rece ptors), given intrathecally and intra-arterially to unanesthetized rat s with and without bladder hypertrophy secondary to outflow obstructio n, were studied by continuous cystometry. When given intrathecally in increasing doses, both muscimol (1 to 10 mug.) and baclofen (0.1 to 5 mug.) produced a dose-dependent inhibition of micturition with progres sive increases in bladder capacity and residual volume, and a decrease in micturition pressure ending with urinary retention and dribbling i ncontinence. Generally, the effects were similar in normal rats and ra ts with bladder hypertrophy. In normal rats, bicuculline (blocking GAB A(A)-receptors) decreased bladder capacity and micturition volume, and increased micturition pressure. In both types of rat, the effects of muscimol and baclofen were partly counteracted by bicuculline and sacl ofen, respectively. In rats with bladder hypertrophy, the amplitude of the spontaneous bladder contractions during filling was significantly increased after administration of muscimol and baclofen. When given i ntra-arterially in high doses, muscimol (2 to 16 mg./kg.) and baclofen (8 to 16 mg./kg.) produced effects on cystometric parameters and spon taneous bladder contractions similar to those obtained with intratheca l administration. These results suggest that muscimol and baclofen hav e insignificant peripheral effects on the lower urinary tract, but dep ress micturition by an effect on the central nervous system. They also suggest that the inhibitory GABA-ergic system does not play a role in the genesis of bladder hyperactivity in rats with outflow obstruction .