The etiology of chronic pelvic pain syndrome including interstitial cystitis/bladder pain syndrome (IC/BPS) seems to be multifactorial.  Recent studies have shown that pelvic venous congestion is an important cause of chronic pelvic pain in humans and animal models [1]. Furthermore, bladder-centric IC/BPS identified by bladder pathologies such as Hunner lesions is often associated with severe inflammatory changes in the bladder, and the prevalence of this type of IC/BPS seems to be higher than previously thought [2].  Thus, animal models with cystitis have been used to study the inflammation-related aspects of IC/BPS pathophysiology.  However, previous experimental animal models of cystitis, for example, those induced by a single intraperitoneal or intravesical application of injection of chemical irritants such as cyclophosphamide or its metabolite, acrolein, exhibit only short-term inflammatory responses and urinary symptoms within several days. Recent studies reported that animals with an intravesical application of hydrogen peroxide (H2O2) exhibited bladder inflammation and hypersensitivity, which lasted longer than 7 days [3].  In this study, we sought to establish a new model of longer-lasting bladder hypersensitivity induced by intravesical application of H2O2 using rats with pelvic congestion.

We used adult female Sprague–Dawley rats. (I) Pelvic vein congestion: Rats were anesthetized with 2% isoflurane, and an incision was made in the lower abdomen. In the pelvic vein congestion groups, the bilateral common iliac veins and uterine veins were ligated. In the sham operation groups, the bilateral common iliac veins were dissected free from the common iliac arteries [1]. (II) Intravesical H2O2 injection: Rats were anesthetized with 2% isoflurane, and a polyethylene tube (PE-50) was introduced into the bladder transurethrally and then the lower abdomen was pressed gently to withdraw urine. Thereafter, 300 µL of 1.5% H2O2 solution or vehicle was injected into the bladder through the catheter. The H2O2 solution or vehicle was drained from the bladder after 30 min by pressing the lower abdomen [3]. 
Rats were divided into four groups; Sham operation-Vehicle injected group (S-V; n = 8), Pelvic vein congestion-Vehicle injected group (C-V; n = 8), Sham operation-H2O2 injected group (S-H; n = 8) and Pelvic vein congestion- H2O2 injected group (C-H; n = 8). Vehicle or H2O2 solution was instilled into the bladder at 2 weeks post-pelvic congestion or sham surgery. Voiding behavior was evaluated using metabolic cages before the operation, 2-weeks after pelvic congestion or sham surgery, 1-week and 2-weeks after vehicle or H2O2 application in each animal. Cystometry was also performed at 2-weeks after vehicle or H2O2 application in each group. Under isoflurane anesthesia, a polyethylene catheter was inserted into the bladder through the bladder dome. Saline was continuously infused into the bladder at a rate of 0.04 mL/min in an awake condition. Intercontraction intervals (ICIs), basal pressure and peak pressure were measured after rhythmic bladder contractions became stable for at least 60 min.

The table shows the number of micturition and average urine volume per voiding at pre-operation, 2-weeks after pelvic congestion or sham surgery, 1-week and 2-weeks after vehicle or H2O2 application into the bladder. Pelvic congestion alone for 2 weeks did not alter these voiding parameters.  However, the number of micturition of the C-H group was significantly increased compared to the S-V group at two weeks after bladder H2O2 or vehicle application (21.8 vs 15.7; p<0.05). The average urine volume per voiding of the C-H group is significantly decreased compared to the S-V or S-H group at two weeks after H2O2 or vehicle application into the bladder, respectively (0.26 vs 0.55 or 0.46 mL, p<0.05). In cystometry (Figure), ICIs were significantly smaller in the C-H group compared to the S-V group (4.4 vs 10.0 min; p<0.05). Basal pressure and peak pressure were not significantly difference among group.

In this study, bladder hypersensitivity evident as frequent urination with smaller voided volume lasted up to 14 days after a single intravesical application of H2O2 into rats with pelvic congestion although pelvic congestion alone for 2 weeks did not alter bladder function.  It has been suggested that increased vascular permeability may be involved in lower urinary tract dysfunction due to pelvic congestion or chemical cystitis [1][3]. Thus, it is likely that two pathological changes dues to chemical irritation and pelvic congestion act synergistically to induce a longer-lasting animal model of bladder hypersensitivity than previous models including the H2O2-induced cystitis model.

This study revealed that a single intravesical application of H2O2 into rats with pelvic congestion induces relatively long-lasting bladder hypersensitivity with frequent urination. This model would be suitable for the study of the inflammation-related pathophysiologies of IC/BPS, especially the bladder-centric phenotype with tissue inflammation.  Further studies are warranted to investigate the molecular or histological changes in relation to bladder dysfunction in this animal model.

Tadalafil improves bladder dysfunction and object recognition in rats with pelvic venous congestion. Nishijima S, Sugaya K, Kadekawa K, Ashitomi K, Ueda T, Yamamoto H.  Int J Urol. 2019 May;26(5):578-585.
Hunner lesion versus non-Hunner lesion interstitial cystitis/bladder pain syndrome. Whitmore KE, Fall M, Sengiku A, Tomoe H, Logadottir Y, Kim YH. Int J Urol. 2019 Jun;26 Suppl 1:26-34.
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