A bladder-cooling reflex in the anaesthetised guinea-pig: A model of the positive clinical ice-water test

Abstract

Introduction: In patients with detrusor hyperreflexia, intravesical instillation of ice-cold water results in the development of involuntary bladder contractions at volumes less than normal cystometric capacity. This is referred to as a positive ice-water test (+IWT) and can be reversed by vanilloid receptor agonists and potentiated by menthol. The present study was designed to investigate the existence of an analogous cooling reflex in the guinea-pig bladder that could be used as a small animal model in order to test the effects of drugs on the reflex. Methods: Bladder pressure and external urethral sphincter electromyogram (EUS EMG) were recorded in α-chloralose/urethane anaesthetised guinea-pigs during rapid infusion of cold or warm saline into the bladder with or without prior intravesical exposure to menthol or resiniferatoxin (RTX). Results: The mean control micturition threshold volume (TV) of 2.58 ml at 38 °C was reduced to 1.52 ml in response to saline infusion at 3 °C (P = 0.001). The cold-induced reduction in TV was reproducible during several subsequent repeat infusions at 38 °C and 3 °C and was accompanied by decreases in bladder voiding pressure. The duration of the micturition reflex was markedly increased following cold compared with warm saline infusion (mean 24.5 s at 3 °C, 10.2 s at 38 °C, P = 0.001) and was associated with oscillations in bladder pressure and concomitant bursting activity in the EUS EMG. During step-wise decreases in infusate temperature from 38 °C to 23 °C, 15 °C, 7 °C and 3 °C, the threshold infusate temperature to elicit a significant reduction in TV was 15 °C. The reduction in TV at 3 °C was potentiated by intravesical infusion of 0.6 mM menthol whilst intravesical infusion of 500 nM RTX reversed the reduction in TV at 3 °C. Discussion: These data suggest that a bladder-cooling reflex is present in the anaesthetised guinea-pig and represents a useful small animal model of the clinical +IWT.

Introduction

A positive ice-water test (+IWT), which can be evoked by instillation of ice-cold water or saline into the bladder, is characterised by involuntary sustained bladder contractions with fluid expulsion at a threshold volume (TV) considerably lower than the normal cystometric capacity for that individual. This bladder-cooling reflex has been demonstrated in patients with detrusor hyperreflexia associated with upper but not lower spinal motor neurone lesions and in a large proportion of patients suffering from overactive bladder associated with neurological disorders such as Parkinson's disease, cerebrovascular lesions and multiple sclerosis (Bors and Blinn, 1957, Geirsson et al., 1993a, Geirsson et al., 1994, Geirsson et al., 1999, Ishigooka et al., 1997, Ronzoni et al., 1997). Although a +IWT has been reported not to be present in neurologically normal subjects (Geirsson et al., 1993a, Ishigooka et al., 1997), such a reflex was recorded in the majority of a population of elderly patients who exhibited uninhibited overactive bladder but lacked a neurological diagnosis; the existence of a limited but undetected neuropathy in such patients is however a possibility (Geirsson, Fall, & Lindstrom, 1993b). Furthermore, a +IWT has also been demonstrated in patients with high-grade bladder outlet obstruction due to benign prostatic hyperplasia (Hirayama, Fujimoto, Matsumoto, Ozono, & Hirao, 2003) and in neurologically normal infants up to 4 years of age, this likely becoming suppressed in later years by development of a descending inhibitory pathway during maturation of the central nervous system (Geirsson et al., 1994). Improvements in symptoms of overactive bladder in patients with neurological deficits and reversion of their previously +IWT to negative have been demonstrated following intravesical instillation of the vanilloid receptor (TRPV1) agonists capsaicin or resiniferatoxin (RTX) which desensitise bladder sensory nerve fibres (Geirsson et al., 1995, Silva et al., 2000). This suggests the potential involvement of a common pathway in the +IWT and overactive bladder.

A similar reflex has also been reported in the α-chloralose anaesthetised cat with an intact neuraxis (Fall, Lindstrom, & Mazieres, 1990), described as an excitatory bladder-to-bladder cooling reflex that is mediated by a spinal pathway triggered by activation of unmyelinated C-fibre afferents in the bladder wall (Jiang et al., 2002, Mazieres et al., 1998). In contrast, normal micturition in response to bladder distension is thought to involve activation of myelinated (A-δ) mechanoreceptor afferents (Habler, Janig, & Koltzenburg, 1993) and a spinal-ponto-spinal reflex pathway (Holstege, Griffiths, de Wall, & Dalm, 1986). Unlike the bladder-cooling reflexes demonstrated in human and the α-chloralose anaesthetised cat, infusion of cold saline into the bladder of urethane anaesthetised rats has been reported to elicit a bladder-to-external urethral sphincter reflex that results in marked increases in micturition pressure threshold and bladder contraction amplitude with no evidence of a decrease in TV (Cheng, Chai, & de Groat, 1997).

The human +IWT may represent a biomarker of the hypersensitivity of bladder sensory C-fibres that has been demonstrated in patients with detrusor hyperreflexia (Ukimura et al., 2004). The present study was designed to investigate the existence and characteristics of a bladder-cooling reflex in the guinea-pig and to determine its responsiveness to RTX and also to menthol, which sensitises the reflex in humans (Geirsson, 1993), in order to establish its usefulness as a small animal model of the clinical +IWT.

The data presented here has previously been published in abstract form (Gardiner & Westbrook, 2003).