Spinal stimulation of the upper lumbar spinal cord modulates urethral sphincter activity in rats after spinal cord injury

Abstract

After spinal cord injury (SCI), the neurogenic bladder is observed to develop asynchronous bladder and external urethral sphincter (EUS) contractions in a condition known as detrusor-sphincter dyssnergia (DSD). Activation of the EUS spinal controlling center located at the upper lumbar spinal cord may contribute to reduce EUS dyssynergic contractions and decrease urethral resistance during voiding. However, this mechanism has not been well studied. This study aimed at evaluating the effects of epidural stimulation (EpS) over the spinal EUS controlling center (L3) in combination with a serotonergic receptor agonist on EUS relaxation in naive rats and chronic (6-8 wk) T8 SCI rats. Cystometrogram and EUS electromyography (EMG) were obtained before and after the intravenous administration of 5HT-1A receptor agonist and antagonist. The latency, duration, frequency, amplitude, and area under curve of EpS-evoked EUS EMG responses were analyzed. EpS on L3 evoked an inhibition of EUS tonic contraction and an excitation of EUS intermittent bursting/relaxation correlating with urine expulsion in intact rats. Combined with a 5HT-1A receptor agonist, EpS on L3 evoked a similar effect in chronic T8 SCI rats to reduce urethral contraction (resistance). This study examined the effect of facilitating the EUS spinal controlling center to switch between urine storage and voiding phases by using EpS and a serotonergic receptor agonist. This novel approach of applying EpS on the EUS controlling center modulates EUS contraction and relaxation as well as reduces urethral resistance during voiding in chronic SCI rats with DSD.

Keywords: electromyography; epidural stimulation; serotonergic receptors; urethral resistance.

Fig. 1.

Continuous bladder infusion in an intact rat. A: regular voiding contractions responding to the saline infusion of the bladder are shown. B: faster time tracing shows external urethral sphincter (EUS) bursting during voiding. To summarize, the rat exhibits EUS tonic activity during storage phase and switches to bursting when voiding. The infusion rate was 0.12 ml/min.

Fig. 2.

Representative examples acquired at different ranges of frequencies (A) at a 0.2-ms pulse width and pulse widths (B) at 40 Hz in a naive rat. The intensity was 5 V, 1.5-fold of threshold without lower extremities' movement. Note the bursting reflex was used as an indicator of voiding reflex because bursting only appeared during voiding in rodents. Therefore, the suggestive optimal stimulation was a 40-Hz, 0.2-ms pulse width at 1- to 2-fold over threshold intensity.

Fig. 3.

Representative examples of EUS bursting reflex elicited by epidural stimulation (EpS) on L6 (A) or L3 (B) spinal segment in 3 intact rats. Note EpS suppressed EUS tonic activity, and elicited EUS bursting reflex. In animals 1 and 3, the duration of suppressed tonic activity was longer and continued after EpS has been stopped. It may indicate that EpS was given close to EUS bursting pattern generator compared with the EpS location at animal 2. Dotted boxes indicate the duration of inhibited EUS tonic activity evoked by EpS. The solid bar above the tracings indicated the duration of EpS (4–6 V, 0.2-ms pulse width at 40 Hz).

Fig. 4.

Verification of the pathway of evoked EUS responses in intact rats (A) when EpS was applied over L3. The animal received bilateral pelvic nerve transection (B), and it did not affect evoked EUS responses. Then, the same animal underwent bilateral pudendal nerve transection (C). The evoked EUS responses were eliminated. In other words, the evoked EUS responses are suggested to be conducted by pudendal nerve. In another rat, an acute T8 transection (D) alone showed that the evoked EUS responses were eliminated when the pathway to the pontine micturition center was blocked acutely.

Fig. 5.

Representative examples of continuous bladder infusion in a rat with chronic spinal cord injury (SCI). The rat produced only EUS tonic activity during storage and voiding phases, representing detrusor sphincter dyssnergia (DSD) after SCI (A). No EUS relaxation was found during the high amplitude part of the tonic contractions during voiding. After administration of 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT; 0.5 mg/kg iv), the EUS bursting was unmasked during voiding (B). Increased EUS relaxation (silent periods between each individual burst) was also demonstrated after drug administration. The bladder infusion rate was 0.12 ml/min. When EpS was applied over L6, only continuous EUS tonic firing activity was seen (C). However, EpS over L3 evoked the inhibition of EUS tonic activity (storage) but no bursting (D). The latency was shortened and the duration of suppressed EUS tonic activity (storage) was prolonged by administration of 8-OH-DPAT (0.5 mg/kg iv; E).

Fig. 6.

Statistical analysis of EpS evoked EUS responses in control (n = 9) and SCI (n = 4) groups without drug administration. AUC, area under the curve. *P < 0.05, significant changes.