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Merrill, Liana

Neuroscience Graduate Program

2014

Ph. D.

The ability to respond to an acute stressor is a necessary part of survival. However, the transition from acute to chronic stressor exposure can have negative impacts on quality of life. Micturition, a seemingly simple behavior, can be sensitive to stress. Disorders of the lower urinary tract, including bladder pain syndrome (BPS)/interstitial cystitis(IC) and overactive, bladder (OAB), can worsen due to stressor exposure. This dissertation project examines the effects of stress on micturition reflex function, with the long-term goal of discovering novel targets for therapeutic intervention. The overall hypothesis is that stress leads to changes in bladder sensory physiology characterized by a hyperactive bladder state similar to pre-clinical models of BPS/IC. Further, we hypothesize that these stress-induced changes are due in part to increased expression and function of the sensory transducer, transient receptor potential vanilloid (TRPV) family member 4 (TRPV4). We examined the effects of repeated variate stress (RVS) on the function and inflammatory milieu of the urinary bladder, as well as somatic sensitivity. We also assessed the role of the TRPV4 channel in RVS-induced changes in bladder sensory physiology using protein and transcript quantification techniques, and in vivo bladder function studies including pharmacological manipulation of the TRPV4 channel.
Other animal models oflower urinary tract disorders, many involving direct insult to the urinary bladder, have demonstrated changes in both the structure and function of the urinary bladder, as well as changes in referred somatic sensitivity. Since stress is knowh to exacerbate symptoms of these disorders, we aimed to determine if RVS could produce similar changes in bladder function and structure in the absence of direct bladder insult. We hypothesized that 7 days of RVS would produce a hyperactive bladder state and increased somatic sensitivity. We found that RVS produced changes in bladder sensory physiology characterized by decreased bladder capacity and void volume and increased micturition frequency. Furthermore, RVS altered somtic sensation characterized by increased referred sensitivity ofhe pelvic and hindpaw regions. Changes in the inflammatory milieu of the urinary bladder following RVS were evaluated, and we found that 7 days of RVS increased levels of inflammatory mediators [e.g., histamine, myeloperoxidase (MPO), nerve growth factor (NGF), and the chemokine CXCL12] in the urinary bladder.
TRP channels, including TRPV4, have been demonstrated on urothelial cells. The TRPV4 channel may act as a sensor of stretch in the urinary bladder, but its specific role in the micturition reflex pathway remains unclear. We hypothesized that increased expression and function of TRPV4 on bladder urothelium may play a role in stress-induced changes in the micturition reflex. Western blot and qPCR studies demonstrated increased protein and transcript levels, respectively, of TRPV4 in the urinary bladder, specifically the urothelium, of stressed rats. Furthermore, pharmacological blockade of the TRPV4 channel at the level of the urinary bladder ameliorated the stress-induced bladder hyperactivity seen following RVS. TRPV4 blockade increased bladder capacity and void volume and decreased micturition frequency compared to RVS-exposed rats.
These studies characterize an animal model of stress in terms of the relationship between stress and mIcturItion reflex pathways. These studies provide additional support for the TRPV4 channel as a potentIal therapeutic target for lower urinary tract disorders.