Chronic spinal cord injury (SCI) rostral to the lumbosacral level induces detrusor overactivity (DO) during the storage phase, which is mediated by spinal reflexes triggered by hyperexcitable C-fiber afferent pathways. In addition, during the voiding phase, inefficient voiding is commonly observed due to detrusor-sphincter dyssynergia (DSD) after SCI.  Vibegron is a new β3-adrenoceptor agonist that was approved for the treatment of overactive bladder in Japan in 2018; however, it remains to be elucidated whether vibegron has any therapeutic effects on LUTD induced by SCI. Therefore, we investigated the effects of vibegron to clarify the role of β3 in storage and voiding dysfunction using SCI mice.

Female C57BL/6N (8-9 weeks old) mice were used, and SCI was induced by complete transection of the Th8/9 spinal cord under isoflurane anesthesia. SCI mice were then divided into 2 groups; (1) SCI mice treated with vibegron (30mg/kg/day) (n=13), (2) control SCI mice with saline (n=7).  Two weeks after SCI, vibegron or saline (treatment or control group, respectively) was administered daily by oral gavage for 14 days. After spinal cord transection, their bladders were manually squeezed to eliminate urine once daily for 4 weeks until cystometric evaluation, which was performed using cystometry (CMG) under an awake condition. In CMG recordings, the number of non-voiding contractions (NVCs), micturition pressure (MP), post-void residual volume (PVR) and voided volume (VV) were evaluated in each SCI mouse (Figure 1). 
In real-time PCR analyses, another set of mice was used, and L6-S1 dorsal root ganglia (DRG) that contain bladder afferent neurons and the bladder were removed from saline-treated control SCI mice (n=10) and vibegron-treated SCI mice (n=12) as well as saline treated normal (spinal intact) mice (n=12).  The bladder was divided into mucosal and detrusor layers, and the mRNA levels of TRPV1, TRPA1, iNOS and ATF3 transcripts were evaluated by real-time PCR.

Compared to saline-treated SCI mice, NVCs during bladder filling were significantly reduced in vibegron-treated (Figure 1). The mRNA levels of TRPV1, TRPA1, ATF3 and iNOS mRNA in L6-S1 DRG were increased in SCI mice vs. spinal intact mice, and significantly decreased after vibegron treatment in SCI mice (Figure 2). Also, the TRPV1 and TRPA1 mRNA levels in the detrusor muscle layer were increased in SCI mice vs. spinal intact mice, and significantly decreased after vibegron treatment in SCI mice.

The treatment with vibegron improved DO evident as a decrease in NVCs in association with the reduction in the expression of inflammation markers such as ATF3 and iNOS as well as TRPV1 and TRPA1 in L6-S1 DRG, which contain bladder afferent neurons.  Because previous studies have shown that TRPV1 and TRPA1 channels are predominantly expressed in C-fiber afferent pathways and that NVCs are significant reduced by C-fiber-targeting therapies such as anti-nerve growth factor treatment [1], it is likely that β3 adrenoceptor activation is effective to reduce activation of C-fiber bladder afferent pathways along with suppression of inflammatory responses, thereby reducing DO.  Also, vibegron treatment reduced the expression of TRPV1 and TRPA1 in detrusor muscles in SCI mice. Overall, the results of this study demonstrated that vibegron significantly improves storage LUTD as well as hyperexcitability of bladder afferent pathways after SCI.

Vibegron improved SCI-induced detrusor overactivity along with significant reductions in C-fiber afferent receptors such as TRPV1, TRPA1 and inflammatory cytokines/markers such as ATF3 and iNOS in SCI mice. Thus, vibegron could be an effective therapeutic option for storage LUTD after SCI.

Neurourol Urodyn. 2018 Aug;37(6):1889-1896.