Neurokinin-1 (NK-1) receptor is required in antigen-induced cystitis

Abstract

Interstitial cystitis (IC) is a debilitating disease that has been adversely affecting the quality of women's lives for many years. The trigger in IC is not entirely known, and a role for the sensory nerves in its pathogenesis has been suggested. In addition to inflammation, increased mast cell numbers in the detrusor muscle have been reported in a subset of IC patients. Experimentally, several lines of evidence support a central role for substance P and neurokinin-1 (NK-1) receptors in cystitis. The availability of mice genetically deficient in neurokinin-1 receptor (NK-1R(-/-)) allows us to directly evaluate the importance of substance P in cystitis. An unexpected finding of this investigation is that NK-1R(-/-) mice present increased numbers of mast cells in the bladder when compared with wild-type control mice. Despite the increase in mast cell numbers, no concomitant inflammation was observed. In addition, bladder instillation of wild-type mice with a sensitizing antigen induces activation of mast cells and an acute inflammatory response characterized by plasma extravasation, edema, and migration of neutrophils. Antigen-sensitized NK-1R(-/-) mice also exhibit bladder mast cell degranulation in response to antigen challenge. However, NK-1R(-/-) mice are protected from inflammation, failing to present bladder inflammatory cell infiltrate or edema in response to antigen challenge. This work presents the first evidence of participation of NK-1 receptors in cystitis and a mandatory participation of these receptors on the chain of events linking mast cell degranulation and inflammation.

Figure 1.

Histological evaluation of antigen-induced cystitis. a–c: Cell migration and edema. a: Sensitized WT mouse bladder 24 hours after intravesical instillation of saline, showing normal bladder submucosa architecture. b: Sensitized WT mouse bladder 24 hours after intravesical antigen challenge showing vasodilation, edema, and infiltration of cells in the submucosa. c: Sensitized NK-1R^−/− mouse bladder 24 hours after intravesical antigen challenge showing lack of antigen-associated inflammatory changes. a–c were stained with hematoxylin and eosin. d–f: Mast cell distribution. Bladders were isolated from sensitized NK-1R−/− 24 hours after intravesical instillation of antigen. Mast cells were found in the suburothelial layer (d), around the blood vessels (e), and within the detrusor muscle (f). Results are representative of 16 NK-1R^−/− mice. Original magnification, ×500. g–i: Mast cell activation. Bladders were isolated from sensitized NK-1R^−/− 24 hours after intravesical instillation of antigen. Activated mast cells were found in the suburothelial layer (g) around the blood vessels (h) and within the detrusor muscle (j). Original magnification, ×1000. Identification of mast cells’ distribution and activation in d–i was performed in Giemsa-stained sections, because the presence of granules is the identifying characteristic for mast cells. a–i were analyzed by the investigator in a blinded fashion and quantification of mast cells, edema, and PMNs is presented in Table 1 ▶ . Results are representative of WT (n = 16) and NK-1R^−/− mice (n = 16). Arrowsindicate urothelial cells.