The aim of the present study was to investigate the changes of the bladder epithelial barrier in the pathogenesis of ketamine‑induced cystitis (KIC). A total of 60 female mice were randomly allocated into control and ketamine groups, which received daily intraperitoneal injections of saline and ketamine, respectively. Micturition behavior was recorded in 2‑h intervals at the end of 4, 8 and 12 weeks, and bladders were harvested for subsequent analyses. Routine hematoxylin and eosin staining was performed on the bladders and histopathological changes were analyzed using light microscopy. The distribution of zonula occludens‑1 (ZO‑1) protein was determined by immunohistochemical analysis. The ultrastructure of umbrella cells was observed using a transmission electron microscope (TEM). Ketamine‑addicted mice exhibited a significantly increased frequency of micturitions following 8 and 12 weeks of ketamine treatment (P<0.05 and P<0.01, respectively). Suburothelial congestion and infiltration of mononuclear cells was observed in ketamine‑addicted mice following 8 and 12 weeks of treatment. Immunohistochemical examination demonstrated that there was an increased abnormal distribution of ZO‑1 in the bladders of ketamine‑treated mice compared with control mice. TEM analysis demonstrated that the surface of bladder urothelium became flattened, the tight junctions between umbrella cells became thinner and the endothelial cells exhibited cell body shrinkage, chromatin condensation and layer denudation in mice treated with ketamine. The present study indicated that the structural and functional changes to the bladder epithelial barrier caused by long‑term use of ketamine may be key mechanisms in the development of KIC.

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