Characterization of bladder and external urethral activity in mice with or without spinal cord injury--a comparison study with rats

Abstract

To clarify the lower urinary tract function in mice, we compared bladder and urethral activity between rats and mice with or without spinal cord injury (SCI). Female Sprague-Dawley rats and C57BL/6N mice were divided into five groups:1) spinal intact (SI) rats,2) SI mice,3) pudendal nerve transection (PNT) SI mice,4) spinal cord injury (SCI) rats, and 5) SCI mice. Continuous cystometry (CMG) and external urethral sphincter (EUS)-electromyogram (EMG) analyses were conducted under an awake, restrained condition. During voiding bladder contractions, SI animals exhibited EUS bursting with alternating active and silent periods, which, in rats but not mice, coincided with small-amplitude intravesical pressure oscillations in CMG recordings. In SI mice with bursting-like EUS activity, the duration of active periods was significantly shorter by 46% (32 ± 5 ms) compared with SI rats (59 ± 9 ms). In PNT-SI mice, there were no significant differences in any of cystometric parameters compared with SI mice. In SCI rats, fluid elimination from the urethra and the EUS bursting occurred during small-amplitude intravesical pressure oscillations. However, SCI mice did not exhibit clear EUS bursting activity or intravesical pressure oscillations but rather exhibited intermittent voiding with slow large-amplitude reductions in intravesical pressure, which occurred during periods of reduced EUS activity. These results indicate that EUS pumping activity is essential for generating efficient voiding in rats with or without spinal cord injury. However, EUS bursting activity is not required for efficient voiding in SI mice and does not reemerge in SCI mice in which inefficient voiding occurs during periods of reduced tonic EUS activity.

Keywords: electromyogram; external urethral sphincter; mouse; pudendal nerve transection; urinary bladder.

Fig. 1.

Representative recordings of simultaneous measurement of intravesical pressure (IVP) and external urethral sphincter (EUS)-electromyogram (EMG) activity in a spinal intact unanesthetized rat. Traces B, C, and D depict sections of trace A on expanded time scales. Traces B, C, and D are the expanded portion of traces A, B, and C, respectively, indicated by rectangular boxes. The EUS-EMG exhibits tonic activity before the onset of voiding and bursting activity during voiding (A, B, and C). The EUS bursting is characterized by clusters of high-frequency spikes (active periods: APs) separated by low tonic activity (silent periods: SPs) (C and D). The bursting produces rhythmic contractions and relaxations of the EUS and is thought to generate a urethral pumping action during voiding, which is seen as pressure oscillations on the continuous cystometry (CMG) tracing (D).

Fig. 2.

Simultaneous measurements of IVP and EUS-EMG in spinal intact (SI) mouse without pressure oscillation (PO) (A1–A4), a SI mouse with PO (B1–B4), and a SI mouse with pudendal nerve transection (PNT) (C1–C4). Traces A2–A4, B2–B4, and C2–C4 are expanded from the traces A1, B1, and C1, respectively, with different time scales. Eight of twelve SI mice exhibited reduced EUS activity without bursting and no obvious pressure oscillation during voiding (A1–A4). The remaining four SI mice exhibited bursting-like EUS activity during voiding bladder contractions, similar to EUS busting in SI rats (B1–B4), consisting of alternating tonic (APs), and reduced EMG activity (SPs), which coincided with pressure oscillations on CMG tracing, but pressure oscillations of CMG tracing was less obvious compared with SI rats. In the PNT-SI-mouse (C1–C4), EUS was silent during voiding.

Fig. 3.

Comparison of duration of APs and SPs of EUS bursting during voiding bladder contractions in SI rats and SI mice. The total duration of APs or SPs were calculated by the sum of each AP or SP duration during a voiding bladder contraction, respectively. The duration of APs was significantly shorter (32 ± 5 ms, 46% shorter) in SI mice compared with SI rats (59 ± 9 ms). #P < 0.05, significant difference between SI rats and SI mice, using Mann-Whitney U-test.

Fig. 4.

Simultaneous measurements of IVP and EUS-EMG in a spinal cord-injured (SCI) rat (A1–A4) and a SCI mouse (B1–B4). Traces A2–A4 and B2–B4 are expanded from the traces A1 and B1, respectively, with different time scales. The SCI rat (A1–A4) exhibited EUS bursting with alternating APs and SPs during voiding bladder contractions, which coincided with rapid pressure oscillations in the CMG tracing. The SCI mouse (B1–B4) had intermittent voiding coinciding with more prolonged reductions in intravesical pressure in the CMG recording, which occurred during periods of low-tonic, synergic EUS-EMG activity (B3 and B4). Clear EUS bursting activity or intravesical pressure oscillations were not seen in the SCI mouse. # symbols in B3 and B4 indicate the reduced EUS activity (R-EA) during voiding. Please note that the time scales of SCI mouse traces in Fig. 4, B1–B4 are different from those used for SI mice in Fig. 2, A–C.

Fig. 5.

Comparison of contraction duration (CD) and total duration of reduced EUS activity (R-EA) during voiding bladder contractions in SI and SCI mice. The CD was significantly longer (23.1 ± 7.5 s; 65% longer) in SCI mice compared with SI mice (14.0 ± 2.4 s). The R-EA, which was calculated by the sum of SP durations in SI mice with EMG bursting or reduced EMG durations in those without EMG bursting and the sum of durations of reduced tonic EUS EMG activity during a voiding contraction in SCI mice, was significantly shorter (1.3 ± 0.7 s: 55% shorter) in SCI mice compared with SI mice (2.9 ± 0.8 s). The ratio of R-EA to CD was significantly smaller (0.05 ± 0.02; 76% smaller) in SCI mice compared with SI mice (0.21 ± 0.04). #P < 0.05, ##P < 0.01, ###P < 0.001, significant difference between SI rats and SI mice, using Mann-Whitney U-test.