2.2.3. Anti-inflammatory Effects of BTX-A in Bladder Urothelium

Inflammation-induced afferent sensitization to recruit immune cells is an important protective mechanism to resist infection in the urinary tract [44,45] (Figure 1, part C). However, prolonged inflammation may result in the long-lasting sensitization of afferents and lead to chronic pain [45]. Most experts believe that bladder pain can be attributed to chronic inflammation although the pathogenesis is still inconclusive due to contradictory histopathology results. However, an increase in proinflammatory mediators (histamine, NGF, and those released from mast cells) within the bladder and urine of IC/BPS patients has been widely reported, which is consistent with the hypothesis of inflammation causing bladder pain [46–48]. Cui et al. [49] were the first to describe the direct involvement of BTX-A in pain modulation after inhibiting inflammation. Formalin-induced edema and accompanying peripheral glutamate release were reduced by intraplantar BTX-A injection in rats.

To investigate the inflammatory-mediated pathophysiology of bladder pain, a range of irritants or immune stimulants, including CYP, lipopolysaccharide, acetic acid, acrolein, and protamine sulfate, have been administered in animal models [50]. Altered cystometry and an enhanced visceromotor response during bladder distension were shown in experimental animals, which mimicked a reduced bladder capacity and the hyperalgesia and allodynia observed in response to bladder distension in humans [45,51].

Chuang et al. [52] also reported that BTX-A intravesical injection reduced cyclooxygenase 2 (COX2) and PGE2 receptor expression in the bladders of rats with CYP-induced cystitis. In humans, Shie et al. [53] disclosed that repeated BTX-A injections significantly reduced the number of activated mast cells in the bladder. In another study, vascular endothelial growth factor (VEGF) expression and the apoptotic cell count were decreased in patients with IC/BPS after repeated BTX-A injections [54].

#### Increased Urothelial Permeability after Inflammation/Infection

Increased bladder permeability is believed to be a part of the underlying pathology of bladder hypersensitivity and hyperalgesia in IC/BPS patients or occur secondary to localized inflammation [45]. Many studies have pointed out that patients with IC/BPS, but not OAB, have a damaged or ulcerative, thin urothelium [55]. Liu et al. [56] confirmed results that identified mast cell infiltration in both the OAB and IC/BPS bladder wall, but showed reduced expression of the tight junction protein, zona occludens-1; E-cadherin was only detected in IC/BPS tissues. Glycosaminoglycan (GAG) replacement therapy with pentosane polysulfate (PPS) has been shown to improve bladder pain for some IC/BPS patients [57]. This may be due to the repair and recovery of a tight urothelial barrier maintained by GAG, the anti-inflammatory actions of PPS, and the inhibition of mast cell histamine release [58]. Although effects of BTX-A on urothelial barrier proteins have not been reported, it is reasonable to suggest that BTX-A injection may be beneficial for relieving bladder pain by inhibiting the localized inflammation of urothelium, reducing the numbers of activated mast cells [53], and blocking the subsequent release of inflammatory mediators, such as histamine, cytokines, and proteases, therefore desensitizing peripheral afferent nerve endings [59].
