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Key Points: Functional disorders (i.e. interstitial cystitis/painful bladder syndrome and irritable bowel syndrome) are associated with hyperexcitability of afferent nerves innervating the urinary tract and the bowel, respectively. Various non-5-HT 3 receptor mRNA transcripts are expressed in mouse urothelium and exert functional responses to 5-HT. Whilst 5-HT 3 receptors were not detected in mouse urothelium, 5-HT 3 receptors expressed on bladder sensory neurons plays a role in bladder afferent excitability both under normal conditions and in a mouse model of chronic visceral hypersensitivity. These data suggest that the role 5-HT 3 receptors play in bladder afferent signalling warrants further study as a potential therapeutic target for functional bladder disorders., Abstract: Serotonin (5-HT) is an excitatory mediator that in the gastrointestinal (GI) tract plays a physiological role in gut-brain signalling and is dysregulated in functional GI disorders such as irritable bowel syndrome (IBS). Patients suffering from IBS frequently suffer from urological symptoms characteristic of interstitial cystitis/painful bladder syndrome, which manifests due to cross-sensitization of shared innervation pathways between the bladder and colon. However, a direct modulatory role of 5-HT in bladder afferent signalling and its role in colon-bladder neuronal crosstalk remain elusive. The aim of this study was to investigate the action of 5-HT on bladder afferent signalling in normal mice and mice with chronic visceral hypersensitivity (CVH) following trinitrobenzenesulfonic acid-induced colitis. Bladder afferent activity was recorded directly using ex vivo afferent nerve recordings. Expression of 14 5-HT receptor subtypes, the serotonin transporter (SERT) and 5-HT-producing enzymes was determined in the urothelium using RT-PCR. Retrograde labelling of bladder-projecting dorsal root ganglion neurons was used to investigate expression of 5-HT 3 receptors using single cell RT-PCR, while sensory neuronal and urothelial responses to 5-HT were determined by live cell calcium imaging. 5-HT elicited bladder afferent firing predominantly via 5-HT 3 receptors expressed on afferent terminals. CVH animals showed a downregulation of SERT mRNA expression in urothelium, suggesting increased 5-HT bioavailability. Granisetron, a 5-HT 3 antagonist, reversed bladder afferent hypersensitivity in CVH mice. These data suggest 5-HT exerts a direct effect on bladder afferents to enhance signalling. 5-HT 3 antagonists could therefore be a potential therapeutic target to treat functional bladder and bowel disorders., (© 2019 The Authors. The Journal of Physiology © 2019 The Physiological Society.)

Purpose of Review: Lower urinary tract disorders such as overactive bladder syndrome (OABS) and interstitial cystitis/painful bladder syndrome (IC/PBS) are debilitating conditions with serious adverse effects on quality of life. Common to both OABS and IC/PBS are the sensory symptoms of urgency and frequency, implicating the afferent system in the aetiology of these disorders. Thus, understanding the role that afferent pathways play in the function of the lower urinary tract is the focus of much current research. This review aims to provide an insight into the recent advances in this field., Recent Findings: Sensory transduction in the bladder is not only mediated by direct activation of the afferents via a host of receptors and ion channels located on the afferent terminal but also may be attributed to the interplay between the urothelium and the release of urothelially derived mediators. Recent studies provide compelling evidence to support this concept and highlight the complex nature of the bladder afferent system., Summary: Recent studies provide further evidence that afferent control of the bladder may be dependent on integration of excitatory and inhibitory mediators from the urothelium such as ATP and nitric oxide. A number of studies have examined the role cholinergic and adrenergic mechanisms play in bladder afferent function, and several new potential mechanisms involving the cannabinoid receptors and transient receptor potential channels have emerged as areas which warrant further investigation. A better understanding of afferent mechanisms in the bladder will hopefully lead to more effective treatments of lower urinary tract disorders.

Gastrointestinal vagal afferents transmit satiety signals to the brain via both chemical and mechanical mechanisms. There is indirect evidence that these signals may be attenuated in obesity. We hypothesized that responses to satiety mediators and distension of the gut would be attenuated after induction of diet induced obesity. Obesity was induced by feeding a high fat diet (60% kcal from fat). Low fat fed mice (10% kcal from fat) served as a control. High fat fed mice were obese, with increased visceral fat, but were not hyperglycaemic. Recordings from jejunal afferents demonstrated attenuated responses to the satiety mediators cholecystokinin (CCK, 100 nm) and 5-hydroxytryptamine (5-HT, 10 μm), as was the response to low intensity jejunal distension, while responses to higher distension pressures were preserved. We performed whole cell patch clamp recordings on nodose ganglion neurons, both unlabelled, and those labelled by fast blue injection into the wall of the jejunum. The cell membrane of both labelled and unlabelled nodose ganglion neurons was less excitable in HFF mice, with an elevated rheobase and decreased number of action potentials at twice rheobase. Input resistance of HFF neurons was also significantly decreased. Calcium imaging experiments revealed reduced proportion of nodose ganglion neurons responding to CCK and 5-HT in obese mice. These results demonstrate a marked reduction in afferent sensitivity to satiety related stimuli after a chronic high fat diet. A major mechanism underlying this change is reduced excitability of the neuronal cell membrane. This may explain the development of hyperphagia when a high fat diet is consumed. Improving sensitivity of gastrointestinal afferent nerves may prove useful to limit food intake in obesity.