Spontaneous contractions evoke afferent nerve firing in mouse bladders with detrusor overactivity

Abstract

Purpose: Afferent nerve firing has been linked to spontaneous bladder contractions in a number of lower urinary tract pathologies and it may lead to urgency and incontinence. Using optical mapping, single unit recording and tension measurements we investigated the correlation between afferent nerve firing and spontaneous bladder contractions in spinal cord transected mice.

Materials and methods: Bladder-nerve preparations (bladder sheets and the associated L6-S2 pelvic nerves) were dissected from normal and spinal cord transected mice showing overactivity on cystometry and opened along the ventral aspect from base to dome. Bladder sheets were mounted horizontally in a temperature regulated chamber to simultaneously record Ca(2+) transients across the mucosal surface, single unit afferent nerve firing and whole bladder tension.

Results: Single unit afferent fibers were identified by probing their receptive fields. Fibers showed a graded response to von Frey stimulation and a frequency of afferent firing that increased as a function of the degree of stretch. Optical maps of Ca(2+) transients in control bladders demonstrated multiple initiation sites that resulted in high frequency, low amplitude spontaneous contractions. Alternatively in maps of the bladders of spinal cord transected mice Ca(2+) transients arose from 1 or 2 focal sites, resulting in low frequency, high amplitude contractions and concomitant afferent firing.

Conclusions: Large amplitude, spontaneous bladder contractions evoke afferent nerve activity, which may contribute to incontinence.

Figure 1

Experimental setup to record intracellular Ca²⁺ signals, tension and single unit afferent nerve activity. A, bladder-nerve sheet preparation. B, Ca²⁺ signals recorded in tissue using photodiode array. C, isochronal map derived from data in B. Initiation site corresponds to imaged tissue region with earliest Ca²⁺ transients (white areas). D, optical system arrangement of light source, filters, photodiode array and charge coupled device cameras. E, single unit afferent recordings. s, seconds. F, expanded single unit recording from E. ms, millisecond.

Figure 2

Histograms show afferent fiber firing frequency in response to von Frey stimulation at 0.4, 1.0 and 2.0 gm (g) with 1 millisecond bin width. Afferent recordings correspond to 1.0 gm von Frey stimulation. s, seconds.

Figure 3

Changes in bladder tension and firing of single afferent fiber in response to 0.5, 1.0 and 1.5 mm stretch. s, seconds

Figure 4

Cystometrograms. A, controls. B, SCT decerebrated mice. Arrows indicate voiding

Figure 5

Findings in normal adult and SCT mouse bladders. A and D, tension recordings. g, gm. B and E, Ca²⁺ transients. C and F, isochronal maps.

Figure 6

Simultaneous recordings of spontaneous contractile activity and corresponding afferent firing in normal adult and SCT mouse bladders. A, control. g, gm. B, control expanded time base. C, SCT. D, SCT expanded time base. Arrows indicate action potential at different amplitude recorded in 3 separate fibers that responded to spontaneous contraction, as described. E, effect of 10 µM nifedipine on spontaneous contractions and afferent nerve firing.