Lower urinary tract (LUT) functions are storage and urine expulsion. In rats, the external urethral sphincter (EUS) has basal tonic contraction during urine storage and phasic bursting discharge (EUS-PBD) during voiding [1]. EUS-PBD concurs with bladder contraction and facilitates voiding efficiency [2]. The coordinated activity of the bladder and urethra is under neural control. Since EUS-PBD is recruited during voiding we hypothesize that mechanorreceptors localized at the base of the bladder recruits EUS-PBD. The aim of the present study is to investigate in spinally intact rats whether sensory mechanoreceptors recruiting EUS-PBD are regionally distributed in the bladder, as well as to determine the afferent pathways of this viscero-motor reflex.

The experimental protocol was approved by the University Committee on Laboratory Animals, according to the guidelines of the Mexican Council on Laboratory Animals Care (NOM-062-ZOO-1999) and NIH Guide for the Care and Use of Laboratory Animals. We employed 18 (urethane 1.2g/kg) anesthetized adult Wistar female rats (250–300 g) to assess the EUS electromyography (EMG) pattern. In the Experiment 1 (n=6), a suprapubic catheter was implanted. Simultaneous EUS-EMG-cystometrograms (CMG) were recorded, before and during 5-20 min after an injection of 0.05 mL topical anesthetic (Lidocaine-epinephrine 2%) in four points of the bladder dome, body or bladder neck (randomly order). Bladder contraction amplitude and EUS-PBD were evaluated. In Experiment 2 (n=6), the bladder was emptied and the ureters were bilaterally cut to avoid bladder filling. EUS EMG was recorded before and during sequential mechanical distension of the urinary bladder wall. Mechanical distension was externally applied in the bladder dome, with forceps, then this region was transversally cut off. The bladder wall of the cut site was brushed with a cotton bud (~10 g) and slightly distended with forceps, and then was sectioned. This procedure was repeated at one mm long steps, from the dome to the bladder neck. Intersection interval was 5 min interval. In Experiment 3 (n=6), the rostral half of the bladder was sectioned. EUS EMG was recorded before, during and after mechanical stimulation of the bladder neck with a cotton bud; in intact condition and after unilateral and bilateral transection of the hypogastric (HG) and/or viscerocutaneous branch of the pelvic nerve (VCBPN). Mechanical stimulation of the clitoris was performed to corroborate integrity of the motor pathway of the reflex. Statistical analysis was performed using Sigma Plot Software (version 12, Systat Software, Inc.). Paired Student´s t-test was used to analyze the data. P values of <0.05 indicated a statistically significant difference for statistical comparisons.

Experiment 1. EUS tonic activity was observed during bladder filling and EUS-PBD during voiding. Amplitude of bladder contraction significantly decreased (p<0.01) after 3- 5min of application of lidocaine in bladder dome or bladder body, EUS-PBD was not abolished. Normal CMG returned after 25-30 min of lidocaine. Lidocaine injection in the bladder neck not only eliminated EUS-PBD, but also bladder contraction. In 100% of the rats of the Experiment 2, mechanical distension of the bladder dome and bladder body evoked EUS tonic activity. However, distension of the bladder neck evoked EUS-PBD in 83.33% (5/6) of the animals. Experiment 3. Unilateral and bilateral transection of the HG nerve did not significantly (p>0.05) modify the amplitude or frequency of the EUS EMG tonic response, neither abolished EUS-PBD. In contrast, unilateral transection of the VCBPN significantly decreased EUS EMG amplitude (tonic and phasic). Bilateral VCBPN transection abolished both, tonic and EUS-PBD EMG response. However, clitoris stimulation was still discharging EUS EMG activity.

In rats, EUS-PBD discharge during voiding has been considered as an indicator or bladder-sphincter synergy. Our findings show that mechanical distension of the rostral region of the bladder recruits only EUS tonic activity, furthermore, distension on the bladder neck discharges EUS-PBD, which suggest there are two types of EUS motor units. Tonic and bursting motor units have been localized at L6-S1 (tonic) and in L3-L5 (bursting) spinal cord segments [3]. Our data suggest that during micturition, L3-L5 motor units are recruited by mecanoreceptors localized in the bladder neck.

Bladder mechanoreceptors recruiting tonic and bursting EUS motor units are regionally segregated. This segregation may convey to a functional segmented bladder, with the distal region being important to induce bladder-urethral synergy during voiding. Further studies are necessary to determine whether alteration of this segregation system may result in urinary disorders.

D'Amico, S. C., Collins, W. F., 3rd: External urethral sphincter motor unit recruitment patterns during micturition in the spinally intact and transected adult rat. J Neurophysiol, 108: 2554, 2012
Cruz, Y., Pastelin, C. F., Balog, B. M. et al.: Somatomotor and sensory urethral control of micturition in female rats. American journal of physiology. Renal physiology: ajprenal 00255 2014, 2014
Chang, H. Y., Cheng, C. L., Chen, J. J. et al.: Serotonergic drugs and spinal cord transections indicate that different spinal circuits are involved in external urethral sphincter activity in rats. Am J Physiol Renal Physiol, 292: F1044, 2007