Urine contains a variety of toxic waste metabolites that are prevented from accessing the underlying bladder interstitium and sensory nerve endings by the usually impermeable urothelium. The urothelial barrier is maintained by tight junctions between apical urothelial cells, hydrophobic uroplakin plaques, and a considerable glycosaminoglycan (GAG) mucus layer of glycoproteins and proteoglycans that acts as a protective barrier between urine and urothelial cells (Jafari and Rohn, 2022; Klingler, 2016; Wyndaele et al., 2019). Several clinical studies have revealed that IC/BPS patients have a diminished or damaged urothelium (Elbadawi and Light, 1996; Tomaszewski et al., 2001; Keay et al., 2014; Hurst et al., 2015), providing toxic irritants and urea greater access to the cell membranes of urothelial cells (Figure 1). IC/BPS patients also have reduced expression of tight junction proteins, including E-cadherin and zonula-occludens-1 compared with healthy controls (Liu et al., 2012a; Jhang and Kuo, 2016a). A decrease in tight junction proteins allows urinary solutes to diffuse through the urothelium into the lamina propria to activate afferent nerve endings and precipitate urological symptoms consistent with IC/BPS (Davis et al., 2014). The urothelium can also be damaged further when in contact with high concentrations of cationic urinary components (Parsons et al., 2000; Parsons et al., 2014), allowing increasing amounts of urine to leak through to the deeper layers, exacerbating afferent hypersensitivity.

Whilst an increase in urothelial permeability clearly exaggerates bladder pain symptoms in IC/BPS, it is not yet known if urothelial permeability is a crucial component in the pathogenesis of bladder hypersensitivity in IC/PBS or a consequence of inflammation that acts to entrench a chronic disease state.

Inflammatory mediator sensitisation of afferent nerves is a pivotal component of the healing process, providing awareness of an injury to alter behaviour and promote tissue regeneration. However, if inflammation becomes uncontrolled this can be detrimental to tissue repair (Eming et al., 2007; Leoni et al., 2015; Landén et al., 2016), and can trigger long term changes in sensory afferent networks to induce a persistent hypersensitive state.

Only a minority of IC/BPS patients exhibit significant bladder inflammation and the development of Hunner’s lesions (Leiby et al., 2007; Whitmore et al., 2019). However, some degree of inflammation is common in the bladders of IC/BPS patients without Hunner lesions, with higher levels of pro-inflammatory mediators, including cytokines, chemokines, histamine, and nerve growth factor compared to healthy control bladders. IC/BPS bladders have also been shown to overexpress pro-inflammatory genes, exhibit mild oedema and tissue granulation, and have elevated numbers of immune cells, including mast cell, macrophages, eosinophils as well as T and B cell markers compared to healthy control bladders (Grundy et al., 2018a; Jhang and Kuo, 2016b; Peters et al., 1999; Liu and Kuo, 2012; Furuta et al., 2018; Liu et al., 2012a; Jacobs et al., 2010; el-Mansoury et al., 1994; Kastrup et al., 1983; Abernethy et al., 2017a; Hauser et al., 2008a; Sant et al., 2007; Grover et al., 2011; Hauser et al., 2008b; Abernethy et al., 2017a). Preclinical studies have confirmed that pro-inflammatory mediators can directly sensitise afferent nerve endings within the bladder wall (de Groat and Yoshimura, 2009; Hughes et al., 2013; Davidson et al., 2014; Grundy et al., 2020a; Grundy et al., 2021), providing a crucial link between inflammation and exaggerated sensation. Furthermore, it is well known that an inflammatory environment disrupts mucosal homeostasis and is detrimental to epithelial regeneration and repair during wound healing (Raziyeva et al., 2021). As such, localised inflammation within the bladder mucosa has the potential to increase bladder permeability, combining to establish a positive feedback cycle that further promotes an inflammatory state and chronic sensitisation of peripheral afferent endings within the bladder wall (Figure 1) (Sant et al., 2007; Grover et al., 2011; Grundy et al., 2018a). Whilst numerous inflammatory factors are elevated in the bladders of IC/BPS patients, whether these factors are a consequence of IC/BPS pathophysiology or contribute to the pathogenesis of IC/BPS in an otherwise healthy bladder has yet to be determined.

Bladder sensory signals converge in the periaqueductal gray (PAG) of the midbrain with inputs from the limbic system (amygdala, hypothalamus, thalamus, cingulate gyrus), insula, and prefrontal cortex (Fowler et al., 2008). The hypothalamic pituitary adrenal (HPA) axis mediates the major adaptive component of the stress response and is a significant modulator of both the limbic system and sensory perception. Furthermore, bladder muscle function is under autonomic regulation, with stress imparting direct effects on bladder function. Modulation of the emotional affective state and homeostasis of the HPA axis can thus have overwhelming effects on bladder sensation and function and has been proposed as a key underlying mechanism in the development, persistence, and exacerbation of IC/BPS symptoms. In healthy patients, stress modulation of bladder sensation and function is commonly observed as urinary urgency during acutely stressful situations. However, in addition to the acute impacts of stress on the bladder, strong correlations exist between chronic stress and anxiety in the symptomology of IC/BPS as well as other visceral pain disorders such as irritable bowel syndrome (Pierce and Christianson, 2015; Moloney et al., 2016). Furthermore, acute and chronic stress can exacerbate urgency and the severity of pain in established IC/BPS patients (Koziol et al., 1993; Lutgendorf et al., 2000; Rothrock et al., 2001; Pierce and Christianson, 2015). With this in mind, chronic stress has been identified as a key risk factor in developing IC/BPS in otherwise healthy patients (Birder, 2019), and a number of studies have reported higher incidences of early life stress in IC/BPS patients than healthy controls (Fuentes and Christianson, 2018a). The precise mechanisms regulating stress induced IC/BPS are unclear, however, evidence is accumulating that the functional impacts of stress on bladder function and the perception of painful stimuli are likely mediated by long-term perturbations of the HPA axis and the sympathetic-adrenal medulla pathway (Figure 1) (de Groat et al., 2015; Fuentes and Christianson, 2018b; Bendrick et al., 2022). The downstream effectors of these pathways, including CRF, cortisol, and noradrenaline are well known regulators of urinary function and thought to be crucial in regulating centrally mediated changes that induce IC/BPS symptoms (Ulrich-Lai and Herman, 2009; Pierce and Christianson, 2015). Furthermore, clinical studies have revealed that chronic psychological stress induces heightened inflammatory responses in peripheral tissues, including elevated levels of circulating proinflammatory cytokines, and mastocytosis in the bladder (Charrua et al., 2015). Crucially, stress alleviation has been shown to be effective in reducing the severity of IC/BPS symptoms in some patients (Bosch and Bosch, 2014; Webster and Brennan, 1998; Carrico et al.).

Non-pharmacological treatments including diet and behavioural adaptations are initially offered to all patients to reduce symptom severity. Urine with an acidic pH is thought to exacerbate bladder irritation and can thus be harmful for IC/BPS patients with a diminished urothelium by increasing inflammation (Ueda et al., 2014; Ueda et al., 2021). Dietary modifications that exclude or limit certain foods such as citrus, coffee and alcohol can decrease urine pH to reduce bladder irritation (Koziol et al., 1993; Lai et al., 2019; Garzon et al., 2020b).

Behavioural adaptations incorporate a variety of modifications, including control of fluid intake, bladder training, and stress management. Bladder training is used to control urgency by incrementally and progressively increasing voiding intervals over 1–3 months (Davis et al., 2014; Hanno et al., 2015; Ogawa et al., 2015; Garzon et al., 2020b; Ueda et al., 2021). Bladder training is commonly employed for other urological disorders including overactive bladder syndrome and may be more useful in IC/BPS patients with mild/moderate symptoms (Chaiken et al., 1993; Foster et al., 2010). Patients are also encouraged to implement stress management practices, including increased exercise, as well as non-physical breathing/relaxation techniques, and psychotherapy if deemed necessary.

Pentosan polysulphate (PPS) is the only FDA approved treatment for IC/BPS (Garzon et al., 2020b; Ueda et al., 2021). PPS is a heparin-like agent that is intended to mimic glycosaminoglycans (GAG) within the bladder to restore urothelial impermeability (Ogawa et al., 2015; Garzon et al., 2020b). Meta-analyses of clinical trials using PPS have shown efficacy compared to placebo in providing moderate relief of bladder pain, urinary urgency, and frequency of micturition without significant side effects in specific subsets of patients (van Ophoven et al., 2019; Taneja, 2021; Grigoryan et al., 2022). However, long term use of PPS presents a risk of macular damage, vision-related injuries, gastrointestinal symptoms, and alopecia.

No new pharmacotherapies specifically designed for treating IC/BPS have been successfully developed, however, clinical data is now accumulating that repurposing immunosuppressive agents, such as Cyclosporine A (CyA) (Sairanen et al., 2004; Forrest et al., 2012; Ehrén et al., 2013; Crescenze et al., 2017), and Certolizumab Pegol (Bosch, 2018) may be efficacious in treating IC/BPS symptoms in patients refractory to approved oral and intravesical treatments. In particular CyA has been shown to have greater efficacy in patients with Hunner lesions, and the AUA now recommends oral CyA as fifth-line therapy for patients with Hunner lesions refractory to current treatments (Clemens et al., 2022a). Larger, longer, and multicenter randomized controlled trials are still required to further investigate certolizumab pegol as a treatment for IC/BPS. Whilst it is common for early-stage drug development not to translate into the clinic, there has been a significant and obvious lack of new oral medications for the treatment of IC/BPS. As a consequence, a variety of pre-existing medications have been trialled and are commonly prescribed in the hope of managing symptoms, including tricyclic antidepressants (amitriptyline), histamine receptor inhibitors (cimetidine and hydroxyzine) for which there is some evidence of efficacy (Henry Lai and Moldwin, 2017; Garzon et al., 2020b; Colemeadow et al., 2020).

For those patients who do not respond to non-pharmacological or oral medications, intravesical instillations may be recommended.

Dimethylsulfoxide (DMSO) via temporary urethral catheter is an FDA-approved treatment for IC/BPS (Garzon et al., 2020b; Ueda et al., 2021), however, the optimal dwell time, length of induction therapy or length of maintenance therapy is unknown. DMSO induces smooth muscle relaxation, blocks sensory nerve activity, and is anti-inflammatory, and has been used effectively to relieve pain and urinary frequency in IC/BPS patients (Birder et al., 1997; Garzon et al., 2020b). DMSO is especially beneficial for IC/BPS with Hunner lesions (Tomoe, 2015; Yoshimura et al., 2021), however, a large proportion of patients relapse within 2 months of treatment.

Lidocaine/heparin: Lidocaine is a local anaesthetic that blocks voltage gated sodium channels present on the peripheral ends of bladder-innervating sensory nerves (Grundy et al., 2018c). Alkalinisation of lidocaine with sodium bicarbonate increase absorption via the urothelium and increases absorption into the neuronal cytoplasm to enhance the therapeutic effect. The inclusion of heparin within the infusion formulation, a naturally occurring glycosaminoglycan, is considered to provide additional benefits to the treatment of IC/BPS by restoring urothelial impermeability (Parsons et al., 1994). Clinical trials of intravesical instillation of lidocaine/heparin show efficacy in relieving IC/BPS symptoms (Henry et al., 2001; Parsons, 2005; Welk and Teichman, 2008; Nickel et al., 2009; Parsons et al., 2012; Henry et al., 2015; Parsons et al., 2015; Digesu et al., 2020), however, an optimal formulation of combined lidocaine and heparin has not been agreed upon, and its widespread use is limited by the requirement for urethral catheterisation.

If behavioural, oral pharmacology, and intravesical instillations are unsuccessful at controlling symptoms, patients may be recommended for more invasive procedures including bladder hydrodistension or neuromodulation.

Hydrodistension of the bladder under high pressure (60–80 cm H₂O) for a short duration (less than 10 min) can offer relief from urinary symptoms in 30%–55% of patients. However, symptom improvement decreases over time, requiring repeated procedures after only a few months (Hanno et al., 2011a; Garzon et al., 2020b; Ueda et al., 2021).

Neuromodulation has not been FDA-approved as a treatment for IC/BPS but has recently been clinically approved for select patients who have success in a nerve stimulation trial (Clemens et al., 2022b). Neuromodulation can lead to control over urinary symptoms through the emission of electrical stimulation that targets nerve activity due to bladder filling (Intern ational Neuromodulation Society, 2013; Padilla-Fernandez et al., 2022). There are two main neuromodulation techniques that are currently being explored to treat IC/BPS; sacral nerve stimulation and pudendal nerve stimulation (Clemens et al., 2022b; Padilla-Fernandez et al., 2022).

Sacral nerve stimulation involves the implantation of a generator under the skin and in the upper buttock area. A small electrode is also placed near the sacral nerve, which will receive electrical impulses from the neurotransmitter, that controls voiding function in the lower spine (Hernández-Hernández et al., 2020; International Neuromodulation Society, 2021a). Pudendal nerve stimulation is seen as an alternative method to sacral nerve stimulation. Similarly, the generator is placed in the upper buttock area, but the electrode is implanted near the pudendal nerve (Peters et al., 2007). The impulses from the generator will stimulate the pudendal nerve and control the pelvic floor muscle during bladder filling (International Neuromodulation Society, 2021b).

Both sacral and pudendal nerve stimulation has been shown to improve urinary symptoms, including bladder capacity, urinary frequency, voided volume, nocturia and pain (Peters et al., 2007; Clemens et al., 2022b; Padilla-Fernandez et al., 2022). However, only a small number of patients have been studied and there is a lack of evidence to suggest that neuromodulation is effective for long periods of time. The AUA guidelines state that sacral/pudendal neuromodulation may be effective in carefully selected patients and emphasise that the procedure can improve frequency/urgency symptoms but is less effective for pelvic pain (Clemens et al., 2022b).

Despite the availability of multiple treatment options for IC/BPS, no currently available treatment has been shown to permanently reverse disease symptoms, and many patients remain refractory to treatment. As a result, patients continue to suffer with symptoms indefinitely, with available treatments generally only providing temporary relief of chronic pain or are sufficient in a sub-population of patients. At the time of writing, there are 30 clinical trials recruiting or active for interstitial cystitis (ClinicalTrials.gov, 2023), however, the only pharmacological tool being tested is the opioid antagonist Naltrexone (NorthShore University, 2022; Stanford, 2023).

Developing novel and efficacious treatments for IC/BPS is extremely challenging. The diversity of symptoms means that patients may need to take multiple medications or engage in additional interventions that target distinct symptomology. Furthermore, the lack of a defined pathophysiology means that the origin of IC/BPS symptoms may be highly distinct between patients. These clinical challenges are also replicated preclinically, with the diversity of disease and symptoms translating into a difficulty in establishing animal models that can faithfully recapitulate the full spectrum of IC/BPS pathophysiology and symptoms.

Despite the wealth of clinical evidence supporting a role for increased urothelial permeability in the pathophysiology of IC/BPS, there are relatively few animal models that exclusively target this pathophysiology.

In vivo bladder instillation of protamine sulphate is the most common method for specifically inducing urothelial permeability (Figure 3) (Lavelle et al., 2002; Shin et al., 2011; Hurst et al., 2015). Protamine sulphate promotes an increase in urothelial permeability by inactivating the sulphated polysaccharides of the GAG layer, increasing transcellular permeability of the urothelium and thus increasing absorption of urine solutes (Lasič et al., 2015). At low doses (1–10 mg/ml) protamine sulphate induces only mild urothelial damage, including urothelial sloughing and an increase in transcellular permeability that returns to normal over a period of 7 days (Lavelle et al., 2002; Meerveld et al., 2015). At higher doses (50 mg/ml), however, bladders have been shown to develop urothelial ulceration and infiltration of neutrophils into the mucosa (Soler et al., 2008). Whilst this goes beyond an isolated urothelial permeability model, it provides insight into IC/BPS pathophysiology by confirming that a significant increase in bladder permeability is able to induce bladder inflammation (Figure 4) (Soler et al., 2008). The impact of protamine sulphate on bladder function has not been reported, and the two studies that have assessed bladder sensory output have described contrasting results. A single low dose of protamine sulphate (1 mg/ml) was found to induce bladder afferent hypersensitivity ex vivo at 1 day post infusion (Grundy et al., 2020b). Afferent hypersensitivity was characterised by an increase in peak firing and decreased activation thresholds to bladder distension that returned to baseline by day 7 post infusion (Grundy et al., 2020b). In contrast, Stemler et al. reported protamine sulphate treated mice (10 mg/ml) had significantly blunted visceromotor responses (VMR) to bladder distension at noxious bladder distension pressures (Stemler et al., 2013), indicative of reduced peripheral sensory drive from the bladder into the spinal cord. A recent study utilising a cocktail mixture of chondroitinase ABC and heparanase III to deglycosylate the proteoglycans of the GAG layer as an alternate method of urothelial barrier disruption (Offiah et al., 2017) induces acute increases in c-fos immunoreactivity in the spinal cord, significant decreases in abdominal mechanical withdrawal threshold to von-Frey hair (VFH) probing, and a significant increase in micturition reflex excitability. However, increased pelvic sensitivity and voiding parameters returned to control levels by day 7 post treatment (Offiah et al., 2017), which corresponds with urothelial barrier recovery in low-dose protamine sulphate treated bladders.

Most animal models of IC/BPS attempt to create an inflammatory phenotype. This has been achieved by irritating the bladder urothelium with chemicals, chemotherapeutics, bacterial products and fungal ligands, and via induced urothelial autoimmunity. Importantly, these inflammatory models are not used because they are thought to be part of the underlying pathophysiology of IC/BPS in humans, although this may be true in some specific cases of cystitis, but because they induce bladder inflammation which leads to bladder hypersensitivity and recapitulates the key symptoms of IC/BPS patients, including increased urinary frequency and pelvic pain.

Experimental autoimmune cystitis (EAC) models have been developed based on the hypothesis that IC/BPS is caused by out-of-control bladder autoimmunity (Akiyama et al., 2020). Several methods of stimulating autoimmunity within the bladder wall have been explored.

Feline interstitial cystitis (FIC) is a naturally occurring idiopathic condition of domestic cats that exhibits physiological changes similar to those associated with IC/BPS without Hunner lesions (Birder et al., 2003; Kullmann et al., 2018; Mohamaden et al., 2019; Jones et al., 2021). FIC is broadly described as the presence of chronic, waxing and waning clinical signs of irritative voiding (pollakiuria, stranguria and dysuria) with an absence of neoplasia or bacteriuria, with or without the presence of uroliths or urethral plugs (Kruger et al., 1991; Kruger et al., 2009; Defauw et al., 2010; Lulich et al., 2010; Mohamaden et al., 2019; Jones et al., 2021). FIC exhibits similar histological features to IC/BPS without Hunner lesions including urothelial denudation or ulceration, submucosal oedema, chronic inflammatory cell infiltrates and muscularis fibrosis (Jones et al., 2021).

FIC cats have shown denudation and thinning of the urothelium (Lavelle et al., 2000), and urothelial spongiosis suggestive of a loss of cell-cell adhesion. Tight junctions were also found to have come apart leading to the disruption of the epithelial layer (Lavelle et al., 2000) and expression of E-cadherin tight junction protein was also significantly downregulated in FIC bladders compared with those of controls (Kullmann et al., 2018; Mohamaden et al., 2019). As a result, transepithelial resistance (TER) of the urothelium was significantly reduced, while water and urea permeability were significantly increased (Lavelle et al., 2000). Bladder oedema, haemorrhage, congestion of blood vessels, and infiltration of inflammatory cells in the bladder interstitium is also common in FIC (Kullmann et al., 2018; Mohamaden et al., 2019). FIC cats have been shown to have significantly higher serum concentrations of IL-1β, IL-6 and TNF-α compared to control cats (Mohamaden et al., 2019). Urine levels for IL-1β and IL-6, but not TNF-α were significantly higher than controls (Mohamaden et al., 2019), and a significant increase in mast cell number is observed in both the urothelium and lamina propria of FIC cats (Kullmann et al., 2018; Mohamaden et al., 2019).

Bladder Aδ afferents from FIC cats are hypersensitive to bladder distension (Roppolo et al., 2005), and urothelial cells from the bladders of FIC cats also demonstrate altered ATP release during cell swelling (Birder et al., 2003), which may contribute to additional activation or sensitisation of sensory afferent nerves (Birder and Andersson, 2013).

Chronic stress has been identified as a key factor in the development of IC/BPS, and early life stress has been shown to significantly increase the chances of developing chronic pelvic pain in later life (Fuentes and Christianson, 2018b). A large proportion of IC/BPS patients have experienced early-life adverse or traumatic events (Kullmann et al., 2018). Rodent psychological stress models are designed to recapitulate these human stressors via the induction of one or more stressful events that vary in their induction method and intensity. The most widely employed stress models to generate an IC/BPS phenotype are Water Avoidance Stress (WAS) or Neonatal Maternal-Separation (NMS).

Considerable clinical evidence links diseases of the colon, such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD), with IC/BPS (Grundy and Brierley, 2018). 20%–30% of both men and women with IC/BPS report IBS as among their most common comorbidity (Alagiri et al.; Clemens et al., 2006) and IC/BPS patients are 100 times more likely to have concurrent IBD than healthy controls (Alagiri et al.). More recently, it has been shown that having IBS increases the risk of developing IC/BPS (Chang et al., 2021), however, the underlying mechanisms responsible for the development of comorbid visceral pain syndromes have yet to be fully elucidated. Cross-sensitisation of the peripheral and central sensory pathways that co-innervate pelvic organs such as the bladder and colon has been proposed as a major contributing factor (Grundy and Brierley, 2018).

Animal models have been generated to recapitulate cross-organ sensitisation between the colon and bladder by inducing long term hypersensitivity of the sensory pathways that innervate the colon. By far the most well characterised of these methods is following intracolonic co-administration of ethanol and trinitrobenzene sulfonic acid (TNBS) (Figure 3). Ethanol is required to disrupt the intestinal barrier and enable the interaction of TNBS within the colon wall to elicit an immune response (Antoniou et al., 2016). The severity of colitis in these models is directly related to the doses of ethanol and TNBS, and shows significant variability between species (rat vs. mouse) as well as genotype (Antoniou et al., 2016). In general, the first week following a single TNBS/ethanol enema is characterised by severe colonic inflammation (Hughes et al., 2009; Vannucchi and Evangelista, 2018). By 7 days post-TNBS administration, colonic inflammation has begun to spontaneously resolve, with a corresponding increase in the integrity of the colonic wall (Hughes et al., 2009). By 28 days post-TNBS, there are no observable histological changes in the colon compared with healthy control mice, yet hypersensitivity of peripheral sensory pathways persists (Hughes et al., 2009). Whilst TNBS colitis has not been observed to induce marked inflammation in the bladder (Grundy et al., 2018d; Grundy and Brierley, 2018), studies in rats have demonstrated that urothelial permeability increases during the active phase of TNBS-induced colonic inflammation (0–7 days) in the absence of overt histological damage to the bladder (Figure 4) (Meerveld et al., 2015; Towner et al., 2015).

Mice and rats in the acute phase of TNBS colitis develop bladder hypersensitivity characterised by changes in micturition parameters and exaggerated bladder afferent sensitivity in the absence of overt changes in bladder histology (Lei and Malykhina, 2012; Xia et al.). Despite the resolution of colonic inflammation, reduced bladder capacity, voided volumes, and intermicturition intervals and changes in bladder voiding patterns indicative of bladder overactivity persist in both rats and mice up to 90 days post-TNBS (Lamb et al., 2006; Liang et al., 2007; Ustinova et al., 2007; Fitzgerald et al., 2013; Grundy et al., 2018d). TNBS colitis has also been shown to enhance the VMR to urinary bladder distension (Lamb et al., 2006) and induce hypersensitivity of bladder-innervating sensory nerves to bladder distension both during the active and post-inflammatory phase of TNBS colitis (Ustinova et al., 2006; Ustinova et al., 2007; Grundy et al., 2018d). Together, these studies show that experimental colitis can induce chronic changes in the sensory networks that regulate bladder sensory signalling and provide important information on the mechanisms that might underlie the development of IC/BPS in patients that have comorbid visceral pain disorders such as IBS.

Chronic pelvic or bladder pain are the defining and most debilitating symptoms of IC/BPS. IC/BPS patients display hypersensitivity to bladder distension that translates into increased pain during bladder filling and the development of lower urinary tract symptoms including increased urinary urgency and frequency (Hanno et al., 2011a; Hanno et al., 2011b; Lai et al., 2015). Patients will also demonstrate mechanical hypersensitivity to noxious stimulation in the suprapubic area during sensory testing that reflects the development of referred hyperalgesia from the bladder (Lai et al., 2014; Warren, 2014). As such, it is essential that any animal model intended to be used in the preclinical evaluation of therapeutics for IC/BPS patients develops bladder and/or pelvic pain. Furthermore, as patients with IC/BPS have painful symptoms that persist indefinitely, and often develop additional comorbidities over time (Driscoll and Teichman, 2001), animal models of IC/BPS should ideally see the development of a pelvic/bladder pain phenotype that endures or even increases in intensity over time.

Significant progress has been made in developing animal models of IC/BPS that exhibit pelvic/bladder pain, including the development and validation of techniques to accurately assess evoked bladder pain and hypersensitivity. These include the VMR to measure abdominal contractions as a surrogate for bladder pain during bladder distension, von-Frey hair probing of the suprapubic region to assess referred hyperalgesia, and sensory nerve recordings during bladder distension to directly record sensory nerve output in response to bladder distension or stretch. Combinations of these techniques have been used extensively to characterise the development of an IC/BPS phenotype in the models presented in this review. The use of different techniques to assess bladder hypersensitivity and pain both across and within animal models reflects the diversity of experimental techniques available to individual research groups. However, as most models have now been validated by multiple research groups, a reliable assessment of each model has started to emerge (Table 2). Bladder permeability, cyclophosphamide, zymosan, lipopolysaccharide, autoimmune cystitis, feline interstitial cystitis, psychological stress, and cross-organ sensitisation models all report the development of evoked bladder hypersensitivity and/or pain reflective of an IC/BPS phenotype. The variability in the degree of bladder pain/hypersensitivity across these models is dependent on both the intensity of the sensitising stimulus as well as the method of induction. In bladder centric models, increased bladder pain/hypersensitivity is typically correlated with a greater severity of inflammation/permeability, with models that combine to simultaneously impact permeability and inflammation shown to impart greater effects on peripheral afferent sensitisation and pain signaling.

Psychological stress and cross-organ sensitisation models can induce both bladder pain and hypersensitivity in the absence of significant bladder inflammation. However, the intensity of the stimulus, including the duration or timing of psychological stress, or magnitude of the colonic insult, remains a crucial factor in determining whether animals develop measurable bladder pain/hypersensitivity. Furthermore, for psychological stress models, utilising specific rodent species and strains that are predisposed to anxiety are crucial for the successful development of bladder pain/hypersensitivity.

The evidence generated from these models supports comprehensive clinical data showing that the aetiology of IC/BPS can be highly diverse, with multiple mechanisms capable of contributing to the development of bladder centric symptoms. The ability to accurately recapitulate the major clinical features of IC/BPS in animal models also supports their continued use in unravelling the mechanisms responsible for the development and maintenance of IC/BPS symptoms. However, as described in detail for each model, the relative transience of bladder pain/hypersensitivity in many of these models remains a limiting factor in their utilisation for exploring novel therapeutics for IC/BPS. Furthermore, whilst these studies comprehensively characterise evoked pain in animal models of IC/BPS, experimental assessment of non-evoked pain, such as grimace scales, burrowing, and automated behavioural analysis that have been employed in a variety of other pain models is not commonly reported (Deuis et al., 2017). As IC/BPS patients report both evoked pain relating to bladder filling as well as non-evoked pain, incorporating experimental techniques that assess non-evoked pain into future research designs may be useful in fully characterising the clinical translatability of IC/BPS animal models. Non-evoked pain assessments are primarily observation based and therefore have the advantage that they can be easily incorporated into an experimental design without the need for additional animal cohorts or specialised equipment.

Although many animal models accurately recapitulate the evoked bladder pain and hypersensitivity phenotype of IC/BPS, animal models that develop a chronic pain phenotype are scarce. In the most explored bladder-centric permeability and inflammatory models, pain and/or hypersensitivity effects are commonly reported only in the day/s, or first week after ceasing treatment. Whether this is due to not exploring more chronic time points or reflects a resolution of hypersensitivity and pain is not always clear, but it is of paramount importance, as IC/BPS is by definition a chronic disorder. As a result, the last decade has seen a discernible drive to refine preclinical models of IC/BPS to induce a more chronic pain state. This has been achieved by modulating the strength, timing, and duration of the model induction, such as with multiple lower doses of cyclophosphamide administered over a longer period, several LPS administrations over a longer time course, neonatal insult, or chronic exposure to stressful stimuli during a specific developmental phase. These refinements will likely prove crucial in developing the next-generation of treatments for IC/BPS, as they provide greater opportunities to interrogate the mechanisms underlying long term changes in bladder sensation.

Further development of IC/BPS animal models has revealed neonatal maternal stress, neonatal zymosan, and colon-bladder cross-sensitisation models exhibit bladder hypersensitivity and pain long after the resolution of the initial sensitising stimulus and in the absence of overt bladder damage. This is crucial, as it reveals that chronic changes in bladder sensory pathways can occur to embed a hypersensitive state long after an initiating stimulus. Many IC/BPS patients present without obvious bladder pathophysiology at the time of diagnosis, but have a history of chronic psychological stress, urinary tract infections, or comorbidities including IBS. As such, the development of animal models that see a chronic pelvic/bladder pain phenotype in the absence or following resolution of inflammation represents a crucial step towards accurate pre-clinical modelling of these IC/BPS patient subsets.

A logical next step in the development of chronic IC/BPS models is to expand the dual insult models established using intrabladder zymosan to include heterogenous elements of distinct animal models. This may include combining an early in life event, such as neonatal inflammation to mirror childhood UTI, followed by chronic stress in adulthood or visa-versa.

The ability to accurately recapitulate the major symptoms of IC/BPS in animal models is a crucial step towards the development of novel therapeutics for IC/BPS. However, the variety of animal models available that develop an IC/BPS like phenotype, and the stark differences in induction methods, raises important questions as to which model is most suited for the pre-clinical evaluation of therapeutics for future clinical translation. Consideration must be given to the pathophysiological origin, the type/severity of model, and the time after model induction that assessments are made.

A balance must be sought between how closely an animal model of IC/BPS shares pathophysiological traits with IC/BPS patients, and model presentation. For example, chemical models such as acetic/hydrochloric acid and hydrogen peroxide induce a well characterised cystitis phenotype. However, chemical cystitis methods appear limited in their translational potential by an artificial method of induction that lacks an obvious link to the pathophysiology of IC/BPS. In contrast, models of urothelial permeability induce an acute IC/BPS phenotype that closely mirrors the presentation of many IC/BPS patients with a diminished urothelial barrier. However, as is well documented, in the absence of inflammation, the urothelial barrier is rapidly restored, and bladder hypersensitivity normalises, undermining its relevance to studying the chronic nature of IC/BPS. Current studies indicate zymosan and LPS models may represent a good choice for investigating an inflammatory IC/BPS phenotype. Both LPS and zymosan induce an IC/BPS like phenotype characterised by bladder inflammation and bladder pain. However, in contrast to chemical cystitis, inflammation is induced via the recruitment of natural inflammatory pathways associated with receptor activation rather than direct tissue damage. By more closely mirroring the natural inflammatory pathways reported to be activated in the bladders of IC/BPS patients, these models appear to have greater translational potential for investigating therapies that target bladder inflammation and/or inflammation induced hypersensitivity. Experimental autoimmune cystitis (EAC) mice also develop IC/BPS like symptoms, but whilst the inflammation is initiated by a cellular mediated mechanism, the severity is beyond the pathophysiology characterised for most IC/BPS patients. However, as some evidence supports an autoimmunological inflammatory process as an underlying contributor to pathophysiology in IC/BPS with Hunner lesions (Akiyama et al., 2020), EAC models may represent the best choice for investigating novel therapies for this severe IC/BPS phentypes.

The translational potential of an animal model also requires consideration of the mechanism of action of any proposed intervention in relation to the pathology that is thought to underlie the development of IC/BPS symptoms in a specific patient cohort. For instance, bladder hypersensitivity and pain in psychological stress models is caused by chronic dysregulation of the HPA axis. As such, preclinical evaluation of therapies that have peripheral sites of action, such as direct bladder infusions or anti-inflammatory agents are unlikely to be as efficacious as those that are tailored to central stress mechanisms such as psychotherapy, exercise, or bladder training. Similarly, targeting the HPA axis to treat IC/BPS symptoms that develop due to localised bladder inflammation and the sensitisation of bladder-innervating sensory nerves is unlikely to be as effective as directly targeting the bladder or peripheral sensory pathways. Exploring therapies in animal models of cross-organ sensitisation or early in life intervention will likely need a different approach entirely, as the pathophysiology of these disorders is embedded within the chronic remodelling of peripheral and central sensory circuits that establishes a chronically sensitised state. Considering these factors in the design of clinical trials for IC/BPS to aid patient stratification would also have major implications for interpreting success/failure of a particular intervention.