Distribution and sub-types of afferent fibre in the mouse urinary bladder

Increased afferent fibre activity contributes to pathological conditions such as the overactive bladder syndrome. Nerve fibres running near the urothelium are considered to be afferent as no efferent system has yet been described. The aim of this study was to identify sub-types of afferent nerve fibres in the mouse bladder wall based on morphological criteria and analyse regional differences.

27 bladders of six month old C57BL/6 mice were removed and tissues were processed for immunohistochemistry. Cryostat sections were cut and stained for Protein Gene Product 9.5 (PGP), calcitonin gene related polypeptide (CGRP), neurofilament (NF), vesicular acetylcholine transporter (VAChT) and neuronal nitric oxide synthase (nNOS).

In the sub-urothelium, different types of afferent nerve fibre were found, i.e. immunoreactive (IR) to; CGRP, NF, VAChT, and/or nNOS. At the bladder base, the sub-urothelium was more densely innervated by CGRP-IR and VAChT-IR nerve fibres, then at the lateral wall. NF- and nNOS nerves were sparsely distributed in the sub-urothelium throughout the bladder. At the lateral wall the inner muscle is densely innervated by CGRP-IR nerve fibres. NF, VAChT and nNOS nerves were evenly distributed in the different muscle layers throughout the bladder. Nerve fibre terminals expressing CGRP and NF were found within the extra-mural ganglia at the bladder base.

Different types of afferent nerve fibres were identified in the sub-urothelium of the mouse bladder. At the bladder base the sub-urothelium is more densely innervated than the lateral wall by CGRP-IR and VAChT-IR afferent nerve fibres. CGRP and NF afferent nerve fibres in the muscle layer probably relay afferent input to external ganglia located near the bladder base. The identification of different afferent nerves in the sub-urothelium suggests a functional heterogeneity of the afferent nerve fibres in the urinary bladder.