Regional differences in sensory afferent innervation of the mouse bladder wall

Abstract

Background and objective: Sensory afferent innervation of the bladder wall is critical for regulating lower urinary tract function. Calcitonin gene-related peptide (CGRP) is a neuropeptide expressed predominantly in unmyelinated C-fibers and serves as a marker of sensory afferent fibers. While previous studies suggested regional differences in bladder innervation, quantitative evidence across the rostro-caudal axis is lacking. Here, we systematically quantified regional differences in CGRP-positive afferent innervation across the mouse bladder. Methods: Bladder tissue from nine mice was processed and stained for CGRP immunofluorescence. Images were segmented into seven equal rostro-caudal regions from bladder dome to bladder neck, and CGRP-positive area was quantified and normalized to total tissue area. A permutation testing approach was applied to determine whether CGRP increased along the rostro-caudal axis. Key findings and limitations: CGRP-positive fibers were detected throughout the bladder wall and showed a gradual increase in CGRP-positive area from dome to bladder neck. Linear regression analysis across segment means yielded a positive slope (beta = 0.0095). Exact permutation testing across all 5040 possible segment arrangements showed that only 1.7% of permuted datasets produced slopes equal to or greater than the observed value (p = 0.0171). These results confirm that CGRP-positive afferent innervation of the mouse bladder wall is not evenly distributed but follows an exponential-like gradient along the rostro-caudal axis, with higher density toward the bladder neck. Conclusions: These findings establish a quantitative baseline for regional sensory organization and may inform studies of pathological remodeling and new region-targeted therapies.