Muscarinic Receptors in the Urinary Bladder - The role of the urothelium regarding cholinergic and nitrergic effects in inflammation

Abstract: Inflammation alters the functional properties of the urinary bladder. Interstitial cystitis (IC) is a chronic inflammatory syndrome in man that is characterized by urgency, frequency and visceral pain. The overall aim of this study was to investigate how the rat urinary bladder is affected by inflammation, and what specific part the urothelium plays in this. Methods: Cystitis was induced in rats by a single injection of cyclophosphamide (CYP; 100 mg/kg). This treatment causes a disease state which is highly comparable to IC. Data comparing the properties of the healthy and inflamed bladder were gathered from (1) contraction experiments in vitro in an organ bath setup, (2) cystometrical studies in vivo in anaesthetized rats and (3) wake, freely moving rats in a metabolism cage. Cell cultures were also cultivated in order to investigate if proliferation of urothelial cells is influenced by receptor activation. Key findings: Induction of cystitis by CYP altered the cholinergic response of the urinary bladder. In vitro studies showed a significantly lower response to carbachol in the inflamed bladder. Both in vitro and in vivo, the altered cholinergic response could be normalized by either removal of the urothelium, blockade of nitric oxide (NO) synthase or blockade of muscarinic M1/M3/M5 receptors. These findings indicate that during CYP-induced cystitis, NO is released from the urothelium upon muscarinic receptor activation. Further characterization in vitro revealed the M5 receptor as the most likely candidate for mediating this release. In vivo experiments in the metabolism cage showed that micturition parameters are affected by CYP-induced cystitis. Increasing doses of a muscarinic antagonist eliminated these differences, and a connection between the effects of antimuscarinic and antinitrergic drugs was indicated. These findings underline the importance of muscarinic receptors and NO in the alterations seen during cystitis. Proliferation experiments indicated that adrenergic, but not muscarinic, nicotinic or EGF receptors, are involved in the regulation of urothelial cell proliferation. Conclusions: In CYP-induced cystitis in the rat, the urothelium exerts an inhibitory influence on the cholinergic response of the urinary bladder. We conclude that this is caused by the release of NO upon activation of urothelial muscarinic M5 receptors.