BACKGROUND: Mirabegron is the first β3-adrenoceptor agonist that is clinically 
effective for overactive bladder.
OBJECTIVE: The effects of mirabegron on primary bladder mechanosensitive 
single-unit afferent activities (SAAs) and bladder microcontractions were 
evaluated and compared with the effects of oxybutynin.
DESIGN, SETTING, AND PARTICIPANTS: Female Sprague-Dawley rats were anesthetized. 
The SAAs generated from left L6 dorsal roots were identified by electrical 
stimulation of the left pelvic nerve and bladder distension. Nerves with 
conduction velocities (CVs) >2.5 m/s were designated as Aδ-fibers, and nerves 
with CVs<2.5 m/s were designated as C-fibers.
OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS: Two measurements were performed 
in separate animals. First, after measuring the baselines of SAA during constant 
filling cystometry, the procedure was repeated with each intravenous 
administration of mirabegron at three doses-0.1, 0.3, and 1.0mg/kg-cumulatively. 
Second, the bladder was filled with saline until the intravesical pressure 
reached 30 cm H(2)O and was kept under an isovolumetric condition; then the 
recording was performed for 5 min with vehicle and mirabegron or oxybutynin 
administrated intravenously.
RESULTS AND LIMITATIONS: A total of 74 single-unit afferent fibers were isolated 
from 55 rats (Aδ-fibers: n=34; C-fibers: n=40). SAAs of both Aδ-fibers and 
C-fibers in response to bladder filling significantly decreased after mirabegron 
administration in a dose-dependent manner, which was more remarkable for 
Aδ-fibers. During an isovolumetric condition of the bladder, the mean bladder 
pressure and the number of microcontractions decreased after mirabegron 
administration, whereas these parameters did not change with oxybutynin 
administration. SAAs of Aδ-fibers were significantly decreased by mirabegron 
administration at both 0.3 and 1mg/kg, whereas SAAs of C-fibers decreased only 
at 1mg/kg. In contrast, oxybutynin (1mg/kg) did not alter either type of SAA.
CONCLUSIONS: The present study demonstrates that mirabegron can inhibit 
mechanosensitive bladder afferent activity, especially of Aδ-fibers, which may 
be related to suppression of bladder microcontractions.