GABA receptor activation in the lumbosacral spinal cord decreases detrusor overactivity in spinal cord injured rats

Abstract

Purpose: We investigated the effects of intrathecal application of gamma-aminobutyric acid A and B receptor agonists on detrusor overactivity in spinal cord injured rats.

Materials and methods: Adult female Sprague-Dawley rats were used. At 4 weeks after Th9-10 spinal cord transection awake cystometry and recordings of external urethral sphincter electromyogram were performed to examine the effect of intrathecal application of the gamma-aminobutyric acid A and B agonists muscimol and baclofen or the gamma-aminobutyric acid A and B antagonists bicuculline and saclofen (Tocris Cookson, Ellisville, Missouri), respectively, at the level of the L6-S1 spinal cord. The expression of glutamate decarboxylase 67 mRNA in the L6-S1 spinal cord and dorsal root ganglia was also assessed.

Results: Muscimol and baclofen produced a dose dependent inhibition of the number (51% to 73% decrease) and amplitude (35% to 93% decrease) of nonvoiding bladder contractions and a decrease in micturition pressure. The effects of muscimol and baclofen were antagonized by bicuculline and saclofen, respectively. Bursting activity of external urethral sphincter electromyogram was inhibited, corresponding to the inhibition of bladder activity by muscimol and baclofen. Glutamate decarboxylase 67 mRNA levels in the spinal cord and dorsal root ganglia were decreased after spinal cord transection (55% and 84%, respectively).

Conclusions: These results indicate that gamma-aminobutyric acid A and B receptor activation in the spinal cord inhibits detrusor overactivity. The decrease in glutamate decarboxylase 67 mRNA suggests hypofunction of GABAergic inhibitory mechanisms in the spinal cord. Therefore, stimulation of spinal GABAergic mechanisms could be effective for the treatment of detrusor overactivity after spinal cord injury.

Fig. 1

Continuous cystometry in a SCI rat before and after intrathecal (it) application of muscimol (A) or baclofen (B). A. Before voiding (dagger), many NVCs were observed. However, NVCs and voiding bladder contraction (dagger) were inhibited after intrathecal 0.l μg of muscimol. B. Before voiding (dagger), many NVCs were also observed. However, NVCs almost disappeared along with a small inhibition of voiding bladder contraction (dagger), after intrathecal 0.l μg of baclofen.

Fig. 2

Effect of intrathecal application of muscimol (black bars; 0.001–0.1 μg) or baclofen (white bars; 0.001–1 μg) on the number (A) and amplitude (B) of NVCs, as well as the MVP (C) in SCI rats. Measurements after drugs were compared with the values before drug application (control). A: The number of NVCs was significantly decreased by intrathecal application of 0.1 μg of muscimol or baclofen (0.1–1 μg). B: The amplitude of NVCs was significantly and dose-dependently decreased after intrathecal application of muscimol (0.01–0.1 μg) or baclofen (0.01–1 μg). C: The MVP was significantly and dose-dependently decreased after intrathecal application of musimol (0.01–0.1 μg) or 1 μg of baclofen. Values are the mean ± standard error (SE). Significant difference from control (Cont): * p < 0.05, and ** p < 0.01. Increasing doses of drugs were administered at 15–30 min intervals.

Fig. 3

Continuous cystometry (CMG) and EMG in a SCI rat before and after intrathecal application of muscimol (A) or baclofen (B). A. Bursting activity of EUS-EMG and the bladder contractions during voiding (dagger) were inhibited by 0.1 μg of muscimol. B. Bursting activity of EUS-EMG during voiding (dagger) was also inhibited in association with a small inhibition of bladder activity by 0.1 μg of baclofen.

Fig. 4

Effect of intrathecal application of muscimol (white circles; 0.001–0.1 μg) or baclofen (black circles; 0.001–1 μg) on the number (A), maximal (B), and average voltage (C) of EUS-EMG spikes during voiding bladder contractions, compared with the values before injection (control) in SCI rats. A: The number of spikes (expressed as % of control) during voiding bladder contractions was significantly and dose-dependently decreased by intrathecal application of muscimol (0.01–0.1 μg) or baclofen (0.01–1 μg). B: The maximal voltage of spikes during voiding bladder contractions was significantly decreased after intrathecal application of muscimol (0.1 μg) or baclofen (1 μg). C: The average voltage of spikes during voiding bladder contractions was significantly decreased after intrathecal application of muscimol (0.1 μg) or baclofen (1 μg). Values are the mean ± standard error (SE). Significant difference from control (Cont): * p < 0.05, and ** p < 0.01.

Fig. 5

Continuous cystometry in a SCI rat before and after intrathecal (it) application of muscimol (A) or baclofen (B) following bicuculline or saclofen. A. Prior to voiding (dagger), many NVCs are shown. After intrathecal 0.l μg of muscimol following 0.01 μg of bicuculline, NVCs were not changed. B. Prior to voiding (dagger), many NVCs were also observed. After intrathecal 0.l μg of baclofen following 1 μg of saclofen, NVCs were not altered.

Fig. 6

GAD67 mRNA/β-actin mRNA ratio in the L6-S1 DRG (A) and spinal cord (LSC; B). GAD67 mRNA/β-actin mRNA ratio in the L6-S1 DRG (A) and LSC (B) was significantly decreased after SCI. Values are the mean ± SE. Significant differences between intact and SCI rats are indicated by: ** p < 0.01.