Lower urinary tract dysfunction, including urinary urgency, frequency, and pain, is frequently associated with bladder inflammation such as that seen in interstitial or chemically induced cystitis. In these conditions, changes in the expression and activity of sensory and mechanosensitive ion channels contribute to dysfunction in both the bladder and urethra [1, 2]. Among these, members of the transient receptor potential vanilloid (TRPV) family have emerged as key regulators of urological sensory processing, particularly under inflammatory conditions [3, 4].

TRPV1 and TRPV4 are nonselective cation channels that participate in sensory and mechanotransductive processes in the lower urinary tract. TRPV1 is activated by noxious stimuli such as heat, low pH, and capsaicin, and plays a central role in nociceptive signaling and inflammatory hypersensitivity, particularly in bladder-associated pain [5]. In contrast, TRPV4 is a mechanosensitive ion channel responsive to shear stress, osmotic changes, and tissue stretch. It plays a pivotal role in sensing mechanical forces and initiating afferent signaling pathways, particularly via adenosine triphosphate (ATP) release from the urothelium during bladder filling or urethral deformation [6-9].

Historically considered a passive conduit, the urethra is now recognized to play an active role in micturition control. Urothelial cells in both the bladder and urethra transduce mechanical stimuli such as stretch or shear stress into chemical signals like ATP, which activate afferent pathways. These afferents contribute significantly to storage and voiding reflexes [1, 10]. Inflammatory conditions such as cystitis may alter this coordinated signaling environment and disrupt lower urinary tract function. However, the specific roles of urethral TRPV channels in the context of bladder inflammation remain poorly understood.

Cyclophosphamide (CYP)-induced cystitis is a well-established experimental model that reproduces key features of bladder inflammation, including mucosal edema, leukocyte infiltration, and altered micturition patterns [11]. In this model, TRPV1 contributes to bladder hyperactivity without necessarily increasing the severity of tissue inflammation, while TRPV4 has been shown to play a critical role in cystitis-induced bladder dysfunction. Genetic deletion or pharmacological blockade of TRPV4 prevents the typical decrease in bladder capacity and increase in voiding frequency seen in CYP-treated animals [12-14]. Although the expression of TRPV1 and TRPV4 in the bladder under inflammatory conditions has been extensively studied, their expression and functional roles in the urethra during cystitis remain largely unexplored.

This study aimed to investigate the effects of CYP-induced cystitis on TRPV1 and TRPV4 expression in the rat urethra. Specifically, we assessed (1) whether cystitis induces upregulation of TRPV1 and TRPV4 in urethral tissues, (2) the differential localization of these channels in urethral compartments, and (3) their potential contributions to altered voiding physiology. Understanding the involvement of urethral TRPV channels in the context of bladder inflammation may provide novel insights into the mechanisms of lower urinary tract dysfunction and support future therapeutic strategies targeting both bladder and urethral sensory pathways.

This study provides novel insights into the upregulation and differential localization of TRPV1 and TRPV4 in the rat urethra in response to CYP-induced cystitis, highlighting a potentially underappreciated mechanism of bladder-urethral dysfunction during inflammation. Our findings suggest that, beyond the well-characterized role of the bladder urothelium and afferent pathways, urethral TRPV channels also participate in the modulation of lower urinary tract reflexes during inflammatory states.

The lower urinary tract functions as an integrated organ system, with the urinary bladder and urethra forming a coordinated “bladder-urethral axis” that is essential for preserving continence and enabling effective voiding reflexes. Disruption of this axis—such as by inflammation, neural injury, or mechanical perturbation—can lead to significant lower urinary tract dysfunction [15]. Recent reports also emphasize that the urethra actively contributes to bladder sensation and reflex control within this axis [6]. Consistent with previous studies that identified TRPV1 and TRPV4 expression in the bladder [3, 4, 12, 13], our results demonstrate that both channels are significantly upregulated in urethral tissues following chemically induced cystitis. The present findings extend the concept of inflammation-induced TRPV channel activation from the bladder to the urethra, indicating that the entire outlet tract is functionally involved in the sensory and contractile remodeling process during cystitis. While previous studies have largely focused on bladder TRPV-mediated pathways, our data reveal that similar molecular adaptations occur in the urethra but with distinct compartmental localization, suggesting an additional and previously unrecognized sensory site contributing to lower urinary tract dysfunction.

In bladder-based models, TRPV1 has been recognized as a mediator of nociceptive signaling and inflammatory hypersensitivity, while TRPV4 functions as a mechanotransducer that regulates voiding frequency through stretch- and osmotic-sensitive mechanisms [3-7, 12-14]. Our results show that a similar pattern of channel-specific regulation occurs in the urethra: TRPV1 is mainly localized in the cytoplasm of urothelial and smooth muscle cells, whereas TRPV4 is concentrated along cell membranes and perimuscular connective tissue. This distinct spatial organization implies that inflammation can reprogram the urethral sensory network, increasing both nociceptive and mechanotransductive responsiveness. These adaptations may cooperate with bladder changes to generate the overall symptom complex of urgency, frequency, and pain seen in cystitis.

The current study also supports the growing evidence that the urethra serves as an active sensory organ capable of influencing bladder reflexes. Birder and Andersson [1] first emphasized the role of urothelial signaling as a bidirectional interface between the epithelium and afferent neurons. Cho et al. [10] further demonstrated molecular alterations in urethral caveolin-1, -2, and -3 expression in overactive bladder models, suggesting that membrane scaffolding proteins modulate the functional interaction between urethral and bladder signaling pathways. In this context, our observation of TRPV4 enrichment in the perimuscular region is compatible with the possibility that caveolin-associated membrane remodeling may facilitate TRPV clustering and enhance mechanosensitive signaling. Thus, both caveolin- and TRPV-mediated changes likely contribute to the heightened sensory transmission within the bladder-urethral axis during inflammation.

It is noteworthy that our findings not only confirm previous concepts derived from bladder studies but also broaden their interpretation by identifying the urethra as a parallel site of TRPV activation. This regional extension is supported by the fact that voiding reflexes involve complex afferent-efferent coordination between bladder and urethral components. The amplified urethral TRPV activity observed here may potentiate afferent input to the spinal micturition centers and facilitate detrusor overactivity, thereby coupling urethral inflammation to bladder dysfunction. Such an integrated mechanism may explain why some patients with interstitial cystitis or bladder pain syndrome exhibit prominent urethral symptoms despite minimal bladder pathology.

These findings also have potential therapeutic implications. Pharmacologic blockade or genetic deletion of TRPV4 has been shown to attenuate bladder overactivity in cystitis models [12, 13], and recent reviews have highlighted TRPV4 as a promising therapeutic target for urinary storage dysfunction [2]. Based on our results, selective inhibition of urethral TRPV4 could represent an additional strategy to alleviate urgency and frequency by dampening urethral mechanosensory gain without substantially affecting nociceptive pathways mediated by TRPV1. Likewise, modulation of urethral TRPV1 may help control the pain component of chronic pelvic pain syndromes. Considering that TRPV activation can also be induced by inflammatory mediators such as lipopolysaccharide or cytokine cascades, these pathways may serve as upstream targets for broader anti-inflammatory interventions.

The present study has several limitations. Although we identified protein-level upregulation and specific localization patterns of TRPV1 and TRPV4, functional assays such as urethral manometry, afferent nerve recordings, or selective pharmacologic tests were not performed. Therefore, we cannot directly conclude that TRPV upregulation translates into altered urethral contractility or afferent firing. Furthermore, only female rats were used, and sex-specific factors such as hormonal or anatomical differences may influence TRPV channel expression and function; these potential effects should be considered in future studies including both sexes. It should also be noted that urethral alterations observed in this study are most likely secondary to bladder inflammation, rather than a result of direct urethral injury or primary urethritis, since CYP was administered systemically without urethral exposure. Finally, this study focused on the acute phase (72 hours after CYP injection); temporal evaluation at chronic stages would clarify whether these changes persist or normalize with resolution of inflammation.

In summary, CYP-induced cystitis markedly upregulated TRPV1 and TRPV4 expression in the urethra; however, functional assays such as urethral manometry or selective pharmacologic tests were not performed. Therefore, we cannot directly conclude that TRPV upregulation translates into altered urethral contractility or afferent firing. These results extend bladder-centric TRP research to the urethra and underscore the urethra’s role as an active sensory organ that contributes to lower urinary tract dysfunction during inflammation. Understanding this integrated bladder-urethral signaling mechanism provides a new perspective for future therapies targeting both bladder and urethral TRP channels to alleviate urgency, frequency, and pain.

In conclusion, this study provides new evidence that the urethra, traditionally regarded as a passive structure, actively responds to inflammatory stimuli through upregulation of TRPV1 and TRPV4. The distinct expression patterns of these channels under cystitis conditions support their involvement in urethral sensory modulation and reflex regulation. These findings broaden the current understanding of lower urinary tract pathophysiology by highlighting the urethra’s contribution to inflammation-induced dysfunction, and suggest that urethral TRP channels may represent promising targets for therapeutic intervention in diseases such as IC/BPS and overactive bladder.