The role of capsaicin-sensitive afferent fibers in the lower urinary tract dysfunction induced by chronic spinal cord injury in rats.

The role of capsaicin-sensitive afferents in neurogenic voiding dysfunction was studied in chronic spinal cord injured rats (SCI). Cystometry and external urethral sphincter (EUS) electromyography were performed on 2 consecutive days after induction of urethane anesthesia in SCI rats 6-8 weeks after spinal cord injury. SCI rats exhibited voiding abnormalities including: non-voiding contractions (NVCs) before micturition, increased volume threshold (VT) for initiating voiding, increased amplitude and duration of voiding contractions, decreased voiding efficiency, increased residual urine, and changes in the pattern of the EUS-EMG. In SCI rats, the EUS electromyogram (EUS-EMG) consisted of more prominent tonic activity, shorter periods of bursting activity, and a reduction in the ratio of silent to active periods during bursting. These changes were more prominent during deeper levels of anesthesia on day 1. Capsaicin (125 mg/kg, s.c., 4 days before urodynamic examination) reduced VT and the number of NVCs, decreased the amplitude and duration of voiding contractions, partially normalized the pattern of EUS-EMG activity, and improved the voiding efficiency on day 1 after induction of anesthesia but not on day 2. Capsaicin treatment increased the percentage of animals (from 55% to 80%) that voided on day 1. The results indicate that capsaicin-sensitive C-fiber bladder afferents are not essential for reflex micturition in SCI rats. However, these afferents do contribute to overactivity of the bladder and detrusor sphincter dyssynergia in deeply anesthetized SCI rats.