The role of the rhabdosphincter in female rat voiding

Mid-lumbar (L3) epidural stimulation effects on bladder and external urethral sphincter in non-injured and chronically transected urethane-anesthetized rats

Recent pre-clinical and clinical spinal cord epidural stimulation (scES) experiments specifically targeting the thoracolumbar and lumbosacral circuitries mediating lower urinary tract (LUT) function have shown improvements in storage, detrusor pressure, and emptying. With the existence of a lumbar spinal coordinating center in rats that is involved with external urethral sphincter (EUS) functionality during micturition, the mid-lumbar spinal cord (specifically L3) was targeted in the current study with scES to determine if the EUS and thus the void pattern could be modulated, using both intact and chronic complete spinal cord injured female rats under urethane anesthesia. L3 scES at select frequencies and intensities of stimulation produced a reduction in void volumes and EUS burst duration in intact rats. After chronic transection, three different subgroups of LUT dysfunction were identified and the response to L3 scES promoted different cystometry outcomes, including changes in EUS bursting. The current findings suggest that scES at the L3 level can generate functional neuromodulation of both the urinary bladder and the EUS in intact and SCI rats to enhance voiding in a variety of clinical scenarios.

Assessing Neurogenic Lower Urinary Tract Dysfunction after Spinal Cord Injury: Animal Models in Preclinical Neuro-Urology Research

Neurogenic bladder dysfunction is a condition that affects both bladder storage and voiding function and remains one of the leading causes of morbidity after spinal cord injury (SCI). The vast majority of individuals with severe SCI develop neurogenic lower urinary tract dysfunction (NLUTD), with symptoms ranging from neurogenic detrusor overactivity, detrusor sphincter dyssynergia, or sphincter underactivity depending on the location and extent of the spinal lesion. Animal models are critical to our fundamental understanding of lower urinary tract function and its dysfunction after SCI, in addition to providing a platform for the assessment of potential therapies. Given the need to develop and evaluate novel assessment tools, as well as therapeutic approaches in animal models of SCI prior to human translation, urodynamics assessment techniques have been implemented to measure NLUTD function in a variety of animals, including rats, mice, cats, dogs and pigs. In this narrative review, we summarize the literature on the use of animal models for cystometry testing in the assessment of SCI-related NLUTD. We also discuss the advantages and disadvantages of various animal models, and opportunities for future research.

Urodynamic Parameters in Spinal Cord Injury-Induced Neurogenic Bladder Rats after Stem Cell Transplantation: A Narrative Review

Neurogenic bladder (NGB) secondary to spinal cord injury (SCI) is accompanied with several complications such as urinary tract deterioration, urinary incontinence, and consequently lower quality of life (QoL), significant morbidities, and occasionally death. Current therapeutic methods have some side effects and there is no treatment for the upper urinary tract injuries. Stem cell therapy is a promising method for treating this condition. However, the best timing and the best route of its transplantation have not yet been determined. Animal models of SCI, especially in rats, are the most commonly used method for evaluating the efficacy of cell therapy in NGB improvement, and the most common assessment method is the urodynamic studies (UDS). However, there are variations in the range of UDS parameters among the published studies. The current review aimed to discuss the effect of stem cell transplantation on bladder dysfunction recovery based on urodynamic parameters after SCI in rats. For this purpose, the cell source, doses, the route of administration, and the complete UDS equipment and its parameters were summarized in SCI models in rats. In some urodynamic test results, to some extent, an improvement in the lower urinary system function was observed in each treatment group. However, this improvement was far from full functional recovery. The average cell dose was about 1 million cells in every injected site. In most studies, the stem cells (SCs) were transplanted 9 days after the injury using PE-50 and PE-60. Many researchers have recommended further experimental and clinical studies to confirm this treatment modality.

Validation of an efficient and continuous urodynamic monitoring system for awake, unrestrained, chronic rodent studies

The postvoid residual (PVR) is an important measure of bladder function, but obtaining PVR is burdensome because bladder volume must be measured at the time of voiding. The PVR measurement problem has led to experimental tricks in animal studies (infusing the bladder at supraphysiological rates and limiting animal observation windows) to keep the number of observed voids statistically robust while reducing the time an experimenter must be present. Our solution to the PVR measurement problem is a system called Automatic Monitoring for Efficient, Awake, Sensitive, Urine Residual Estimation (AMEASURE). AMEASURE combines metabolic cages and optimization algorithms to estimate continuously PVR for every voiding event 24 h/day for multiple weeks, without artificial bladder infusion, continuous experimenter supervision, anesthesia, or restraints. Using AMEASURE, we obtained voided volumes, PVRs, and other urodynamic parameters continuously for 21 days in 10 healthy female Sprague-Dawley rats. Importantly, this required only one manual measurement of animals' bladder volume every 12 h. We validated the accuracy of the system experimentally and in simulation. We detected marked differences in voiding frequency and efficiency between light and dark cycles and found that voiding frequency increased over time during the dark cycle (but not the light cycle), due to surgical recovery, cage acclimation, and socialization. This tool enhances the relevance of rodent models to the study of human lower urinary tract by expanding observation periods and obviating the need to infuse the bladder and facilitates the study of conditions for which behavioral, social, or circadian factors play essential roles.

Spinal Transection Alters External Urethral Sphincter Activity during Spontaneous Voiding in Freely Moving Rats

The rat is a commonly used model for the study of lower urinary tract function before and after spinal cord injury. We have previously reported that in unanesthetized freely moving rats, although phasic external urethral sphincter (EUS) activity (bursting) is most common during micturition, productive voiding can occur in the absence of bursting, which differs from results seen in anesthetized or unanesthetized restrained animals. The purpose of the present study was to characterize EUS behavior in unanesthetized, freely moving rats before and after mid-thoracic (T8) or thoraco-lumbar (T13-L1) spinal transection to determine how EUS behavior after spinal cord injury differs from that seen in anesthetized or unanesthetized restrained rats. Several abnormalities became evident that were comparable after transection at either level, including the following: repetitive non-voiding EUS contractions; increased prevalence, intensity, and duration of EUS bursting; decreased rate of urine evacuation during bursting; increased void size and decreased number of daily voids; shorter inter-burst silent period and increased frequency of bursting; and loss of the direct linear relationships that are evident in intact animals between void size and bursting silent period. These data suggest that transection-induced delayed initiation of EUS bursting allows co-contraction of the bladder and the EUS that prevents or limits urine evacuation, resulting in a detrusor-sphincter dyssynergia-like phenomenon. In addition, the higher-than-normal frequency at which EUS bursting occurs after transection is associated with shorter silent periods during which urine typically flows, which interferes with voiding by slowing the rate of urine evacuation. That results were comparable after either transection suggests that the central pattern generator responsible for EUS bursting is located caudal to the L1 spinal segment.

A ventral root avulsion injury model for neurogenic underactive bladder studies

Detrusor underactivity (DU) is defined as a contraction of reduced strength and/or duration during bladder emptying and results in incomplete and prolonged bladder emptying. The clinical diagnosis of DU is challenging when present alone or in association with other bladder conditions such as detrusor overactivity, urinary retention, detrusor hyperactivity with impaired contractility, aging, and neurological injuries. Several etiologies may be responsible for DU or the development of an underactive bladder (UAB), but the pathobiology of DU or UAB is not well understood. Therefore, new clinically relevant and interpretable models for studies of UAB are much needed in order to make progress towards new treatments and preventative strategies. Here, we review a neuropathic cause of DU in the form of traumatic injuries to the cauda equina (CE) and conus medullaris (CM) portions of the spinal cord. Lumbosacral ventral root avulsion (VRA) injury models in rats mimic the clinical phenotype of CM/CE injuries. Bilateral VRA injuries result in bladder areflexia, whereas a unilateral lesion results in partial impairment of lower urinary tract and visceromotor reflexes. Surgical re-implantation of avulsed ventral roots into the spinal cord and pharmacological strategies can augment micturition reflexes. The translational research need for the development of a large animal model for UAB studies is also presented, and early studies of lumbosacral VRA injuries in rhesus macaques are discussed.

Phasic activation of the external urethral sphincter increases voiding efficiency in the rat and the cat

OBJECTIVE

Electrical stimulation of the pudendal nerve (PN) is a potential therapy for bladder dysfunction, but voiding efficiency (VE) produced by PN stimulation appears limited to 60-70%. We conducted experiments in rats and cats to investigate the hypothesis that introduction of artificial phasic bursting activity of the external urethral sphincter (EUS) would enhance VE under conditions where such activity was absent.

MATERIALS AND METHODS

Cystometry experiments were conducted in 17 urethane anesthetized female Sprague-Dawley rats and 4 α-chloralose anesthetized male cats. The effects of phasic stimulation of the pudendal motor branch on VE were quantified in intact conditions, following bilateral transection of the motor branch of the PN, and following subsequent bilateral transection of the sensory branch of the PN.

RESULTS

Artificial phasic bursting activity in the EUS generated by electrical stimulation of the motor branch of the PN increased VE in both rats and cats. Subsequent transection of the sensory branch of the PN abolished the increased VE elicited by phasic stimulation in both rats and cats.

CONCLUSIONS

Artificial phasic EUS bursting restored efficient voiding in rats. Introduction of artificial phasic bursting in cats, which normally exhibit EUS relaxation while voiding, was also effective in promoting efficient voiding. In both species phasic EUS activity increased voiding efficiency via activation of pudendal sensory pathways. These results provide further insight into the function of phasic EUS activity in efficient voiding and highlight a novel approach to increase VE generated by pudendal afferent nerve stimulation.

Dynamics of the sensory response to urethral flow over multiple time scales in rat

KEY POINTS

Sensory information from the urethra is essential to maintain continence and to achieve efficient micturition and when compromised by disease or injury can lead to substantial loss of function. Despite the key role urethral sensory information plays in the lower urinary tract, the relationship between physiological urethral stimuli, such as fluid flow, and the neural sensory response is poorly understood. This work systematically quantifies pudendal afferent responses to a range of fluid flows in the urethra in vivo and describes a previously unknown long-term neural accommodation phenomenon in these afferents. We present a compact mechanistic mathematical model that reproduces the pudendal sensory activity in response to urethral flow. These results have implications for understanding urinary tract dysfunction caused by neuropathy or nerve damage, such as urinary retention or incontinence, as well as for the development of strategies to mitigate the symptoms of these conditions. The pudendal nerve carries sensory information from the urethra that controls spinal reflexes necessary to maintain continence and achieve efficient micturition. Despite the key role urethral sensory feedback plays in regulation of the lower urinary tract, there is little information about the characteristics of urethral sensory responses to physiological stimuli, and the quantitative relationship between physiological stimuli and the evoked sensory activation is unknown. Such a relation is critical to understanding the neural control of the lower urinary tract and how dysfunction arises in disease states. We systematically quantified pudendal afferent responses to fluid flow in the urethra in vivo in the rat. We characterized the sensory response across a range of stimuli, and describe a previously unreported long-term neural accommodation phenomenon. We developed and validated a compact mechanistic mathematical model capable of reproducing the pudendal sensory activity in response to arbitrary profiles of urethral flows. These results describe the properties and function of urethral afferents that are necessary to understand how sensory disruption manifests in lower urinary tract pathophysiology.

Contribution of the external urethral sphincter to urinary void size in unanesthetized unrestrained rats

AIMS

In anesthetized rats, voiding is typically associated with phasic activation (bursting) of the external urethral sphincter (EUS). During spontaneous voiding in unanesthetized, unrestrained rats, EUS bursting is the most common form of EUS activity exhibited, but it is not necessary for productive voiding to occur. The aim of the present study was to determine which aspects of EUS activity contributed to void size during bursting and non-bursting voiding in conscious, freely moving rats.

METHODS

Female rats were implanted with electrodes adjacent to the EUS for recording electromyographic activity (EMG). EUS EMG recordings were performed during 24-hr sessions in a metabolic cage while voided urine was continuously collected and weighed.

RESULTS

Void size was positively correlated with the duration of the intra-burst silent and active periods and variables reflecting the overall intensity and duration of bursting, particularly at lower frequencies within the 3-10 Hz range of EUS bursting. In addition, void size was inversely related to the frequency of bursting and to the average EMG amplitude during voiding, both in voids with and without bursting.

CONCLUSIONS

EUS bursting contributes to productive voiding when bursting is present. Lower bursting frequencies elicit more productive voiding than do higher frequencies. In the absence of bursting, the association of increased void size with smaller average EUS EMG amplitude suggests that conscious rats can perform synergic voiding (i.e., bladder contraction with EUS relaxation) that is comparable to that seen in humans and other typically non-bursting species. Neurourol. Urodynam. 35:696-702, 2016. © 2015 Wiley Periodicals, Inc.

Anatomic and functional properties of bulboglandularis striated muscle support its contribution as sphincter in female rabbit micturition

PURPOSE

To determine anatomic and functional properties of the bulboglandularis muscle (Bgm) for clarifying its role in micturition in female rabbits.

MATERIAL AND METHODS

Virgin female rabbits were used to describe the gross anatomy and innervation of the Bgm, to determine the effect of the Bgm contraction on urethral pressure, and to evaluate the Bgm activity during the induced-micturition. Both electromyogram and cystometrogram activity were simultaneously recorded in urethane-anesthetized rabbits. Bladder function was assessed measuring standard urodynamic variables before and after blocking the Bgm activity for approaching its contribution to micturition. The relevance of the Bgm activation for micturition was approached applying lidocaine injections.

RESULTS

The Bgm was composed of circularly oriented striated fibers enveloping distal urethra and pelvic vagina. Both the venous plexus and urethra were comprised by the Bgm contraction induced by electrical stimulation. The Bgm showed bursts of tonic activity at the storage phase of micturition that gradually decreased until turning off as the onset of the voiding phase. The voided volume, the voiding efficiency, the threshold pressure, and the maximal pressure were decreased after lidocaine injection. Contrastingly, the threshold volume, the residual volume, the voiding duration, and the urethral resistance at voiding were increased.

CONCLUSIONS

Present anatomical and physiological findings support that the Bgm acts as a sphincter during micturition of female rabbits. Neurourol. Urodynam. 35:689-695, 2016. © 2015 Wiley Periodicals, Inc.

Evaluation of voiding assays in mice: impact of genetic strains and sex

Void spot assays (VSA) and cystometry are two of the most common tests performed in mice to assess lower urinary tract function. Assay protocols and methodology vary greatly among laboratories, and little is known about reproducibility of results generated by different laboratories. We performed VSA in four mouse strains, comparing males with females and comparing results between two independent laboratories. Unique aspects of the current study include direct comparison of results of VSA performed in a similar manner in two locations and comparison of cystometry performed using two different rates of infusion in these two laboratories. Both assays were performed in male and female 129S1/SvImJ, C57BL/6J, NOD/ShiLtJ, and CAST/EiJ mice, and cystometry was performed under urethane anesthesia (10/group). Assays were performed and results analyzed as previously described. Results obtained in female mice were compared with previously reported values. Results of lower urinary tract function testing in mice vary in a consistent manner with strain and sex. Variables in husbandry, testing techniques, and analysis of results can significantly affect conclusions, particularly those obtained by cystometry. Although VSA results were remarkably similar between the two laboratories, consistent methods for performing lower urinary tract function testing in mice are required to compare results among studies with confidence.

Flow Starting Point and Voiding Mechanisms Measured by Simultaneous Registrations of Intravesical, Intra-abdominal, and Intraurethral Pressures in Awake Rats

Purpose

The aim of this study was to apply a new surgical procedure that allows for the successful monitoring of intraurethral pressure (IUP) changes in the cystometry of awake Sprague-Dawley rats.

Methods

Twenty-six female Sprague-Dawley rats were grouped according to the catheterization method (bladder only; bladder and urethra; or bladder, urethra, and abdomen). Using an arbitrarily determined initial point of the first phase among four rat micturition phases on the simultaneous curves as a reference point, we compared the time differences to the points on an intravesical pressure (IVP) and those on IUP or a detrusor pressure (DP) curve from intra-abdominal pressure (IAP).

Results

In awake rat, the start of urethral flow on IUP curve corresponded to the initial point of the second phase, which is same to the results on the anesthetized rat. However, certain results, such as micturition pressure (MP) and intraluminal pressure high-frequency oscillations (IPHFOs), differed between awake and anesthetized rats. Most MP values were checked after the end of urethral flow on the IUP curve, which is due to the peculiar methodology such as transvesical catheterization. Urethral flow was not completely interrupted during the IPHFOs, which suggests the presence of urethral wall tension against the flow during voiding. After removal of the superimposed effects of IAP from IVP, the DP curve clearly showed a peculiar shape, highlighting the possibility of using IAP in place of IUP to detect the flow starting point on the IVP curve.

Conclusions

Awake rat cystometry results have been interpreted based on those in anesthetized rats. However, our awake cystometry data were substantially different in terms of voiding time compared to those of anesthetized rats. This discovery warrants careful interpretation of the voiding parameters in awake rat cystometry.

Long-term recording of external urethral sphincter EMG activity in unanesthetized, unrestrained rats

The external urethral sphincter muscle (EUS) plays an important role in urinary function and often contributes to urinary dysfunction. EUS study would benefit from methodology for longitudinal recording of electromyographic activity (EMG) in unanesthetized animals, but this muscle is a poor substrate for chronic intramuscular electrodes, and thus the required methodology has not been available. We describe a method for long-term recording of EUS EMG by implantation of fine wires adjacent to the EUS that are secured to the pubic bone. Wires pass subcutaneously to a skull-mounted plug and connect to the recording apparatus by a flexible cable attached to a commutator. A force transducer-mounted cup under a metabolic cage collected urine, allowing recording of EUS EMG and voided urine weight without anesthesia or restraint. Implant durability permitted EUS EMG recording during repeated (up to 3 times weekly) 24-h sessions for more than 8 wk. EMG and voiding properties were stable over weeks 2-8. The degree of EUS phasic activity (bursting) during voiding was highly variable, with an average of 25% of voids not exhibiting bursting. Electrode implantation adjacent to the EUS yielded stable EMG recordings over extended periods and eliminated the confounding effects of anesthesia, physical restraint, and the potential for dislodgment of the chronically implanted intramuscular electrodes. These results show that micturition in unanesthetized, unrestrained rats is usually, but not always, associated with EUS bursting. This methodology is applicable to studying EUS behavior during progression of gradually evolving disease and injury models and in response to therapeutic interventions.

Repeated variate stress in male rats induces increased voiding frequency, somatic sensitivity, and urinary bladder nerve growth factor expression

Stress exacerbates symptoms of functional lower urinary tract disorders including interstitial cystitis (IC)/bladder pain syndrome (BPS) and overactive bladder (OAB) in humans, but mechanisms contributing to symptom worsening are unknown. These studies address stress-induced changes in the structure and function of the micturition reflex using an animal model of stress in male rats. Rats were exposed to 7 days of repeated variate stress (RVS). Target organ (urinary bladder, thymus, adrenal gland) tissues were collected and weighed following RVS. Evans blue (EB) concentration and histamine, myeloperoxidase (MPO), nerve growth factor (NGF), brain-derived neurotropic factor (BDNF), and CXCL12 protein content (ELISA) were measured in the urinary bladder, and somatic sensitivity of the hindpaw and pelvic regions was determined following RVS. Bladder function was evaluated using continuous, open outlet intravesical infusion of saline in conscious rats. Increases in body weight gain were significantly (P ≤ 0.01) attenuated by day 5 of RVS, and adrenal weight was significantly (P ≤ 0.05) increased. Histamine, MPO, NGF, and CXCL12 protein expression was significantly (P ≤ 0.01) increased in the urinary bladder after RVS. Somatic sensitivity of the hindpaw and pelvic regions was significantly (P ≤ 0.01) increased at all monofilament forces tested (0.1-4 g) after RVS. Intercontraction interval, infused volume, and void volume were significantly (P ≤ 0.01) decreased after RVS. These studies demonstrate increased voiding frequency, histamine, MPO, NGF, and CXCL12 bladder content and somatic sensitivity after RVS suggesting an inflammatory component to stress-induced changes in bladder function and somatic sensitivity.

External urethral sphincter motor unit recruitment patterns during micturition in the spinally intact and transected adult rat

In the rat, external urethral sphincter (EUS) activation during micturition consists of three sequential phases: 1) an increase in tonic EUS activity during passive filling and active contraction of the bladder (guarding reflex), 2) synchronized phasic activity (EUS bursting) associated with voiding, and 3) sustained tonic EUS activity that persists after bladder contraction. These phases are perturbed following spinal cord injury. The purpose of the present study was to characterize individual EUS motor unit (MU) patterns during micturition in the spinally intact and transected adult rat. EUS MU activity was recorded from either the L5 or L6 ventral root (intact) or EUS muscle (transected) during continuous flow cystometry in urethane-anesthetized adult female Sprague-Dawley rats. With the use of bladder pressure threshold and timing of activation, four distinct patterns of EUS MU activity were identified in the intact rat: low threshold sustained, medium/high threshold sustained, medium/high threshold not sustained, and burst only. In general, these MUs displayed little frequency modulation during active contraction, generated high-frequency bursts of action potentials during EUS bursting, and varied in terms of the duration of sustained tonic activity. In contrast, three general patterns of EUS MU activity were identified in the transected rat: low threshold, medium threshold, and high threshold. These MUs exhibited considerable frequency modulation during active contraction of the bladder, no bursting behavior and little to no sustained firing. The prominent frequency modulation of EUS MUs is likely due to the enhanced guarding reflex seen in EUS whole muscle electromyogram recordings in transected rats (D'Amico SC, Schuster IP, Collins WF 3rd. Exp Neurol 228: 59–68, 2011). In addition, EUS MU recruitment in transected rats more closely followed predictions by the size principle than in intact rats. This may reflect the influence of local synaptic circuits or intrinsic properties of EUS motoneurons that are active in intact rats but attenuated or absent in transected rats.

Evaluation of biomaterials for bladder augmentation using cystometric analyses in various rodent models

Renal function and continence of urine are critically dependent on the proper function of the urinary bladder, which stores urine at low pressure and expels it with a precisely orchestrated contraction. A number of congenital and acquired urological anomalies including posterior urethral valves, benign prostatic hyperplasia, and neurogenic bladder secondary to spina bifida/spinal cord injury can result in pathologic tissue remodeling leading to impaired compliance and reduced capacity¹. Functional or anatomical obstruction of the urinary tract is frequently associated with these conditions, and can lead to urinary incontinence and kidney damage from increased storage and voiding pressures². Surgical implantation of gastrointestinal segments to expand organ capacity and reduce intravesical pressures represents the primary surgical treatment option for these disorders when medical management fails³. However, this approach is hampered by the limitation of available donor tissue, and is associated with significant complications including chronic urinary tract infection, metabolic perturbation, urinary stone formation, and secondary malignancy^4,5.

Current research in bladder tissue engineering is heavily focused on identifying biomaterial configurations which can support regeneration of tissues at defect sites. Conventional 3-D scaffolds derived from natural and synthetic polymers such as small intestinal submucosa and poly-glycolic acid have shown some short-term success in supporting urothelial and smooth muscle regeneration as well as facilitating increased organ storage capacity in both animal models and in the clinic^6,7. However, deficiencies in scaffold mechanical integrity and biocompatibility often result in deleterious fibrosis⁸, graft contracture⁹, and calcification¹⁰, thus increasing the risk of implant failure and need for secondary surgical procedures. In addition, restoration of normal voiding characteristics utilizing standard biomaterial constructs for augmentation cystoplasty has yet to be achieved, and therefore research and development of novel matrices which can fulfill this role is needed.

In order to successfully develop and evaluate optimal biomaterials for clinical bladder augmentation, efficacy research must first be performed in standardized animal models using detailed surgical methods and functional outcome assessments. We have previously reported the use of a bladder augmentation model in mice to determine the potential of silk fibroin-based scaffolds to mediate tissue regeneration and functional voiding characteristics.^11,12 Cystometric analyses of this model have shown that variations in structural and mechanical implant properties can influence the resulting urodynamic features of the tissue engineered bladders^11,12. Positive correlations between the degree of matrix-mediated tissue regeneration determined histologically and functional compliance and capacity evaluated by cystometry were demonstrated in this model^11,12. These results therefore suggest that functional evaluations of biomaterial configurations in rodent bladder augmentation systems may be a useful format for assessing scaffold properties and establishing in vivo feasibility prior to large animal studies and clinical deployment. In the current study, we will present various surgical stages of bladder augmentation in both mice and rats using silk scaffolds and demonstrate techniques for awake and anesthetized cystometry.

Pudendal neuromodulation improves voiding efficiency in diabetic rats

AIMS

Diabetic cystopathy is typically manifested as bladder voiding dysfunction, and numerous patients are refractory to standard therapy. In this study, we determined whether electrical stimulation (ES) of the sensory branch of the pudendal nerve could engage an augmenting reflex and thereby improve bladder emptying in a diabetic animal model with cystopathy.

METHODS

The efficiency of bladder emptying with ES of the sensory branch of the pudendal nerve at different stimulation intensities was measured in rats at 8 or 18 weeks after the induction of diabetes with streptozotocin.

RESULTS

The voiding efficiency (VE) was reduced from 74 ± 4% to 30 ± 8% in rats with diabetes for 8 weeks and from 73 ± 6% to 20 ± 6% in rats with diabetes for 18 weeks. ES at lower intensities (0.025-0.05 mA) applied to the pudendal sensory nerve did not affect the VE in rats with diabetes for 18 weeks but increased the VE in rats with diabetes for 8 weeks. Subsequently, when the stimulation intensity was elevated to 0.1-0.3 mA, the VEs in rats with diabetes for both 8 and 18 weeks increased to 40-50%.

CONCLUSIONS

The results of the present study are consistent with the essential role for pudendal sensory feedback in efficient bladder emptying, and electrical activation of the sensory branch of the pudendal nerve was efficient restoring the voiding function in diabetic animals with cystopathy. This could provide an approach to improve bladder emptying in diabetic patients with voiding dysfunction.

Rodent models for urodynamic investigation

Rodents, most commonly rats, mice, and guinea pigs are widely used to investigate urinary storage and voiding functions, both in normal animals and in models of disease. An often used methodology is cystometry. Micturitions in rodents and humans differ significantly and this must be considered when cystometry is used to interpret voiding in rodent models. Cystometry in humans requires active participation of the investigated patient (subject), and this can for obvious reasons not be achieved in the animals. Cystometric parameters in rodents are often poorly defined and do not correspond to those used in humans. This means that it is important that the terminology used for description of what is measured should be defined, and that the specific terminology used in human cystometry should be avoided. Available disease models in rodents have limited translational value, but despite many limitations, rodent cystometry may give important information on bladder physiology and pharmacology. The present review discusses the principles of urodynamics in rodents, techniques, and terminology, as well as some commonly used disease models, and their translational value.

Bladder outlet physiology in the context of lower urinary tract dysfunction

Physiological function of the bladder outlet is complex and symptomatic consequences can result from outlet dysfunction. Within the outlet, smooth, and skeletal muscles constitute the contractile apparatus, but additional cell types include interstitial cells and neuroendocrine cells, and various transmitters are present in the innervation, raising the possibility of unrecognized functional subtleties. Key outlet functions are; maintained closure for urine storage, increased closure (guarding) during exertion, sustained opening for voiding, transient opening for territorial marking in animals and orthograde male ejaculation. These are co-ordinated by several spinal and higher CNS centers, with overlap of the somatic, sympathetic and parasympathetic nervous systems. During voiding, urethral afferents may be important in maintaining detrusor contraction until completion of bladder emptying. Some of the bladder outlet afferents may be shared with the anal sphincter. Dysfunction of the outlet leads to conditions such as retrograde ejaculation, Fowler's syndrome, and detrusor sphincter dyssynergia. Urethral relaxation during urine storage may lead to urinary urgency, which may be misleadingly labeled as overactive bladder. Research priorities are numerous, including; peripheral cellular integrative physiology, interactions with other pelvic organs, interconnectivity of the CNS centers at all levels of the neuraxis, and standardized animal models of outlet functions such as reflex-driven voiding.

Bilateral pudendal afferent stimulation improves bladder emptying in rats with urinary retention

OBJECTIVE

To determine whether bilateral electrical stimulation (BiES) of the transected pudendal sensory nerves could further enhance the voiding efficiency beyond that produced by unilateral electrical stimulation (UniES) of transected pudendal afferents in rats with urinary retention.

MATERIALS AND METHODS

The efficiency of bladder emptying with either UniES or BiES of pudendal nerve afferents was measured after acute bilateral transection of the sensory branch of the pudendal nerve. The effects of UniES and BiES on voiding in a partially denervated bladder and acute spinal transection, respectively, were determined.

RESULTS

The voiding efficiency (VE) was reduced from 69 to 22% after bilateral transection of the sensory branch of the pudendal nerve. UniES or BiES increased the VE to 49-62%. Although in most instances BiES consistently generated more efficient bladder emptying than did UniES, these differences were not significant. Both UniES and BiES increased VE after unilateral pelvic nerve transection, demonstrating efficacy in a partially denervated bladder. The enhancement of VE by either UniES or BiES was preserved after acute T(9)-T(10), demonstrating the spinal origin of this augmenting reflex.

CONCLUSIONS

The results of the present study are consistent with an essential role for pudendal sensory feedback in efficient bladder emptying, and unilateral and bilateral electrical activation of pudendal nerve afferents are equally efficient in improving bladder emptying in an animal model of urinary retention. This could provide an approach to improve bladder emptying in patients with non-obstructive urinary retention.

Effect of ovariectomy on external urethral sphincter activity in anesthetized female rats

PURPOSE

The postmenopausal hypoestrogen condition is associated with various lower urinary tract dysfunctions, including frequency, urgency, stress urinary incontinence and recurrent urinary infection. We determined whether hypoestrogen induced lower urinary tract dysfunction after ovariectomy is also associated with an alteration in external urethral sphincter activity.

MATERIALS AND METHODS

Bilateral ovariectomy was performed in female Sprague-Dawley® rats and sham operated rats served as controls. Transvesical cystometry and external urethral sphincter electromyogram activity were monitored 4, 6 and 12 weeks after sham operation or bilateral ovariectomy and at 6 weeks in bilaterally ovariectomized rats treated with estrogen.

RESULTS

The micturition reflex was elicited in sham operated and bilaterally ovariectomized, urethane anesthetized animals. Post-void residual urine increased and voiding efficiency decreased in rats with 4 to 12 weeks of bilateral ovariectomy. The silent period of external urethral sphincter electromyogram activity was shortened significantly and progressively at increased times after bilateral ovariectomy. These effects were prevented by estradiol treatment.

CONCLUSIONS

As evidenced by shortening of the external urethral sphincter electromyogram silent period in ovariectomized rats, the disruption of coordination between the external urethral sphincter and the detrusor muscle could decrease urine outflow and in turn voiding efficiency. Estrogen replacement reverses these changes, suggesting that the central pathways responsible for detrusor-sphincter coordination are modulated by gonadal hormones.

An exploration of the control of micturition using a novel in situ arterially perfused rat preparation

Our goal was to develop and refine a decerebrate arterially perfused rat (DAPR) preparation that allows the complete bladder filling and voiding cycle to be investigated without some of the restrictions inherent with in vivo experimentation [e.g., ease and speed of set up (30 min), control over the extracellular milieu and free of anesthetic agents]. Both spontaneous (naturalistic bladder filling from ureters) and evoked (in response to intravesical infusion) voids were routinely and reproducibly observed which had similar pressure characteristics. The DAPR allows the simultaneous measurement of bladder intra-luminal pressure, external urinary sphincter-electromyogram (EUS-EMG), pelvic afferent nerve activity, pudendal motor activity, and permits excellent visualization of the entire lower urinary tract, during typical rat filling and voiding responses. The voiding responses were modulated or eliminated by interventions at a number of levels including at the afferent terminal fields (intravesical capsaicin sensitization-desensitization), autonomic (ganglion blockade with hexamethonium), and somatic motor (vecuronium block of the EUS) outflow and required intact brainstem/hindbrain-spinal coordination (as demonstrated by sequential hindbrain transections). Both innocuous (e.g., perineal stimulation) and nociceptive (tail/paw pinch) somatic stimuli elicited an increase in EUS-EMG indicating intact sensory feedback loops. Spontaneous non-micturition contractions were observed between fluid infusions at a frequency and amplitude of 1.4 ± 0.9 per minute and 1.4 ± 0.3 mmHg, respectively and their amplitude increased when autonomic control was compromised. In conclusion, the DAPR is a tractable and useful model for the study of neural bladder control showing intact afferent signaling, spinal and hindbrain co-ordination and efferent control over the lower urinary tract end organs and can be extended to study bladder pathologies and trial novel treatments.

Continuous uroflow cystometry in the urethane-anesthetized mouse

PURPOSE

In vivo animal cystometry represents an accepted methodology for the study of lower urinary tract physiology. A particular advantage of the mouse model is the availability of genetically modified strains, offering the possibility of linking individual genes to relevant physiological events. However, small voided volumes complicate the ability to obtain reliable pressure-flow data by gravimetric methods, due to non-continuous drop formation and release during voiding. We investigated the feasibility of a simple non-gravimetric continuous urine collection system during cystometry under urethane anesthesia, and compared urethane-anesthetized with awake cystometry.

METHODS

Cystometry was performed in awake and urethane-anesthetized female mice using a suprapubic tube. A simple, novel non-gravimetric method of urine collection was used in urethane-anesthetized animals to assess voided volume and permit flow rate calculations. Pressure and time-related variables were compared between groups.

RESULTS

Voided urine collection appears to be complete and continuous in this model. Mean voided volume was 0.09 ± 0.020 ml, with an average flow rate of 0.029 ± 0.007 ml/sec. Urethane anesthesia delayed cystometric pressure/volume responses. However, micturition reflexes were intact and otherwise comparable between groups. Female mice void with pulsatile pressurization previously described in rats.

CONCLUSION

Suprapubic voiding cystometry using a simple and reliable urine collection method under urethane anesthesia is feasible in mice, permitting the integration of voided volumes with pressure and time data. The inclusion of volume and flow data enhances the usefulness of the mouse model for in vivo assessment of detrusor and potentially sphincteric performance.

External urethral sphincter motoneuron properties in adult female rats studied in vitro

The external urethral sphincter (EUS) muscle plays a crucial role in lower urinary tract function: its activation helps maintain continence, whereas its relaxation contributes to micturition. To determine how the intrinsic properties of its motoneurons contribute to its physiological function, we have obtained intracellular current-clamp recordings from 49 EUS motoneurons in acutely isolated spinal cord slices from adult female rats. In all, 45% of EUS motoneurons fired spontaneously and steadily (average rate = 12-27 pulses/s). EUS motoneurons were highly excitable, having lower rheobase, higher input resistance, and smaller threshold depolarization than those of rat hindlimb motoneurons recorded in vitro. Correlations between these properties and afterhyperpolarization half-decay time are consistent with EUS motoneurons having characteristics of both fast and slow motor unit types. EUS motoneurons with a slow-like spectrum of properties exhibited spontaneous firing more often than those with fast-like characteristics. During triangular current ramp-induced repetitive firing, recruitment typically occurred at lower current levels than those at derecruitment, although the opposite pattern occurred in 10% of EUS motoneurons. This percentage was likely underestimated due to firing rate adaptation. These findings are consistent with the presence of a basal level of persistent inward current (PIC) in at least some EUS motoneurons. The low EUS motoneuron current and voltage thresholds make them readily recruitable, rendering them well suited to their physiological role in continence. The expression of firing behaviors consistent with PIC activation in this highly reduced preparation raises the possibility that in the intact animal, PICs contribute to urinary function not only through neuromodulator-dependent but also through neuromodulator-independent mechanisms.

Improved bladder emptying in urinary retention by electrical stimulation of pudendal afferents

Urinary retention is the inability to empty the bladder completely, and may result from bladder hypocontractility, increases in outlet resistance or both. Chronic urinary retention can lead to several urological complications and is often refractory to pharmacologic, behavioral and surgical treatments. We sought to determine whether electrical stimulation of sensory fibers in the pudendal nerve could engage an augmenting reflex and thereby improve bladder emptying in an animal model of urinary retention. We measured the efficiency of bladder emptying with and without concomitant electrical stimulation of pudendal nerve afferents in urethane-anesthetized rats. Voiding efficiency (VE = voided volume/initial volume) was reduced from 72 +/- 7% to 29 +/- 7% following unilateral transection of the sensory branch of the pudendal nerve (UST) and from 70 +/- 5% to 18 +/- 4% following bilateral transection (BST). Unilateral electrical stimulation of the proximal transected sensory pudendal nerve during distention-evoked voiding contractions significantly improved VE. Low-intensity stimulation at frequencies of 1-50 Hz increased VE to 40-51% following UST and to 39-49% following BST, while high-intensity stimulation was ineffective at increasing VE. The increase in VE was mediated by increases in the duration of distention-evoked voiding bladder contractions, rather than increases in contraction amplitude. These results are consistent with an essential role for pudendal sensory feedback in efficient bladder emptying, and raise the possibility that electrical activation of pudendal nerve afferents may provide a new approach to restore efficient bladder emptying in persons with urinary retention.

Role of pudendal afferents in voiding efficiency in the rat

The reciprocal activities of the bladder and external urethral sphincter (EUS) are coordinated by descending projections from the pontine micturition center but are subjected to modulation by peripheral afferent inputs. Transection of the somatic pudendal nerve innervating the striated EUS decreases voiding efficiency and increases residual urine in the rat. The reduction in voiding efficiency was attributed to the lack of phasic bursting activity of the EUS following denervation. However, transection of the pudendal nerve also eliminates somatic sensory feedback that may play a role in voiding. We hypothesized that feedback from pudendal afferents is required for efficient voiding and that the loss of pudendal sensory activity contributes to the observed reduction in voiding efficiency following pudendal nerve transection. Quantitative cystometry in urethane anesthetized female rats following selective transection of pudendal nerve branches, following chemical modulation of urethral afferent activity, and following neuromuscular blockade revealed that pudendal nerve afferents contributed to efficient voiding. Sensory feedback augmented bladder contraction amplitude and duration, thereby increasing the driving force for urine expulsion. Second, sensory feedback was necessary to pattern appropriately the EUS activity into alternating bursts and quiescence during the bladder contraction. These findings demonstrate that the loss of pudendal sensory activity contributes to the reduction in voiding efficiency observed following pudendal nerve transection, and illustrate the importance of urethral sensory feedback in regulating bladder function.

Is abdominal wall contraction important for normal voiding in the female rat?

Background

Normal voiding behavior in urethane-anesthetized rats includes contraction of the abdominal wall striated muscle, similar to the visceromotor response (VMR) to noxious bladder distension. Normal rat voiding requires pulsatile release of urine from a pressurized bladder. The abdominal wall contraction accompanying urine flow may provide a necessary pressure increment for normal efficient pulsatile voiding. This study aimed to evaluate the occurrence and necessity of the voiding-associated abdominal wall activity in urethane-anesthetized female rats

Methods

A free-voiding model was designed to allow assessment of abdominal wall activity during voiding resulting from physiologic bladder filling, in the absence of bladder or urethral instrumentation. Physiologic diuresis was promoted by rapid intravascular hydration. Intercontraction interval (ICI), voided volumes and EMG activity of the rectus abdominis were quantified. The contribution of abdominal wall contraction to voiding was eliminated in a second group of rats by injecting botulinum-A (BTX, 5 U) into each rectus abdominis to induce local paralysis. Uroflow parameters were compared between intact free-voiding and BTX-prepared animals.

Results

Abdominal wall response is present in free voiding. BTX preparation eliminated the voiding-associated EMG activity. Average per-void volume decreased from 1.8 ml to 1.1 ml (p < 0.05), and reduced average flow from 0.17 ml/sec to 0.11 ml/sec (p < 0.05). Intercontraction interval (ICI) was not changed by BTX pretreatment.

Conclusion

The voiding-associated abdominal wall response is a necessary component of normal voiding in urethane anesthetized female rats. As the proximal urethra may be the origin of the afferent signaling which results in the abdominal wall response, the importance of the bladder pressure increment due to this response may be in maintaining a normal duration intermittent pulsatile high frequency oscillatory (IPHFO)/flow phase and thus efficient voiding. We propose the term Voiding-associated Abdominal Response (VAR) for the physiologic voiding-associated EMG/abdominal wall response, to distinguish it from the visceromotor response (VMR) to noxious bladder distension.

Spinal mechanisms contributing to urethral striated sphincter control during continence and micturition: "how good things might go bad"

The external urethral sphincter motoneurons in the sacral ventral horn control the striated external urethral sphincter muscles that circle the urethra. Activity in these motoneurons and muscle normally contribute to continence but during micturition, when urine must pass through the urethra, the motoneurons and striated muscle must be silenced. Following injury to descending pathways in the spinal cord, the ability to inhibit sphincter activity is disrupted or lost, resulting in bladder-sphincter dyssynergia and functional obstruction of the urethra during voiding. This chapter will first review the various reflex pathways and neuronal properties that contribute to continence, and which must be modulated during micturition in the spinal intact animal. A discussion about how the dyssynergia seen with spinal cord injury may be produced will then be presented.

Animal models in urological disease and sexual dysfunction

There are several conditions associated with dysfunction of the lower urinary tract or which result in a reduction in the ability to engage in satisfactory sexual function and result in significant bother to sufferers, partners and/or carers. This review describes some of the animal models that may be used to discover safe and effective medicines with which to treat them. While alpha adrenoceptor antagonists and 5-alpha-reductase inhibitors deliver improvement in symptom relief in benign prostatic hyperplasia sufferers, the availability of efficacious and well-tolerated medicines to treat incontinence is less well served. Stress urinary incontinence (SUI) has no approved medical therapy in the United States and overactive bladder (OAB) therapy is limited to treatment with muscarinic antagonists (anti-muscarinics). SUI and OAB are characterised by high prevalence, a growing ageing population and a strong desire from sufferers and physicians for more effective treatment options. High patient numbers with low presentation rates characterizes sexual dysfunction in men and women. The introduction of Viagra in 1998 for treating male erectile dysfunction and the success of the phosphodiesterase type 5 inhibitor class (PDE5 inhibitor) have indicated the willingness of sufferers to seek treatment when an effective alternative to injections and devices is available. The main value of preclinical models in discovering new medicines is to predict clinical outcomes. This translation can be established relatively easily in areas of medicine where there are a large number of drugs with different underlying pharmacological mechanisms in clinical usage. However, apart from, for example, the use of PDE5 inhibitors to treat male erectile dysfunction and the use of anti-muscarinics to treat OAB, this clinical information is limited. Therefore, current confidence in existing preclinical models is based on our understanding of the biochemical, physiological, pathophysiological and psychological mechanisms underlying the conditions in humans and how they are reflected in preclinical models. Confidence in both the models used and the pharmacological data generated is reinforced if different models of related aspects of the same disorder generate confirmatory data. However, these models will only be fully validated in retrospect once the pharmacological agents they have helped identify are tested in humans.

Abdominal muscle activity during voiding in female rats with normal or irritated bladder

The aims of the present study were to determine in female rats whether abdominal muscle discharges during normal voiding and to describe the effect of bladder irritation on this visceromotor activity. The sensory pathway of this reflex was also determined. Electromyograms (EMGs) indicated that in awake rats, the abdominal muscle was consistently activated during spontaneous voiding and during voiding induced by saline infusion. Similarly, in anesthetized animals, the muscle discharged during urine expulsion. The abdominal EMG activity was not abolished by hypogastric (Hgnx) or sensory pudendal neurectomy (SPdnx). SPdnx dramatically decreased the intercontraction interval and voided volume. Acetic acid infusion reduced the intercontraction interval and increased bladder contraction duration. It also reduced the pressure threshold for evoking the abdominal EMG response and increased the EMG duration and amplitude. Although SPdnx and Hgnx modified some urodynamic parameters, they did not reverse the acetic acid effect on EMG activity. Thus the afferents activating the visceromotor reflex during normal voiding and the increased reflex in response to acetic acid are probably both carried by the pelvic nerve. Abdominal muscle activity induced by bladder distension has been considered to be a pain marker. However, we conclude that in female rats, the abdominal muscle is reflexively activated during physiological urine expulsion. On the other hand, bladder irritation is marked by an exaggeration of this abdominal visceromotor reflex.

Sexually dimorphic micturition in rats: relationship of perineal muscle activity to voiding pattern

In the present study we examined the possibility that striated muscle activity may underlie sexually dimorphic micturition in rats. Micturition dynamics, the gross anatomy of the external urethral sphincter, and the participation of the striated perineal muscles in micturition were compared in urethane-anesthetized adult male and female rats. Bladder contraction characteristics, particularly the magnitude of bladder high-frequency pressure waves during voiding, differed between sexes. Dissections indicated that the sphincter was more extensive and thicker in males than in females. Electromyography showed that in both sexes the sphincter discharged in bursts that correlated with the rising phase of high-frequency bladder pressure oscillations. Regional differences in discharge pattern were seen in the sphincters of males, with the proximal part of the sphincter showing components activated during bladder filling. Bulbospongiosus, ischiocavernosus, and cremaster muscles also were activated during bladder contraction in males. In both sexes transection of the motor branch of the lumbosacral plexus eliminated the bladder high-frequency oscillations and reduced voided volume. Neurectomy did not affect bladder pressure but reduced voiding efficiency by 45% in males. In females the bladder pressure was dramatically decreased, but voiding efficiency only decreased by 24%. Our findings suggest that, in rats, striated perineal muscles contribute to the sexually dimorphic micturition. Activity of the dimorphic perineal muscles may regulate genital and urinary urethra expulsive functions, helping to expel seminal plug and fluids through the long urethra in the male.

Comparison of the urethral properties of the female guinea pig and rat

AIMS

The main focus of this study was to profile and compare urethral function in the female guinea pig and rat, and to characterize urodynamically distinctive patterns in the micturition cycle of the two species. This exercise aimed to investigate potential species-related differences and determine a suitable animal model for the human urethra.

MATERIALS & METHODS

Female Dunkin-Hartley guinea pigs (400-500 g) and Sprague-Dawley rats (200-300 g) were used throughout the study. For in vitro experiments urethral rings were suspended vertically for isometric tension recording in 5 ml organ baths and drugs (phenylephrine 10(-4) M and carbachol 10(-4) M) were applied directly to the bathing solution. In vivo urethral pull-through and urodynamic studies were performed under urethane anesthesia, and phenylephrine (200 microg/kg) was administered during urethral pull-through experiments via the intravenous (i.v.) route. Urethral, bladder and arterial pressures, and external urethral sphincter electromyographic (EMG) spike activity were recorded simultaneously throughout.

RESULTS

Organ bath studies coupled with in vivo pull-through experiments produced urethral profiles with distinct regional variations for both species. Urodynamic studies with the urethral probe fixed in the high pressure zones of the proximal urethra produced similar recordings in both species during bladder filling and marked differences during micturition. The guinea pig showed complete sphincteric inhibition immediately prior and during micturition as also seen in the human, whereas the rat exhibited high frequency bursts in EMG activity at this stage in the cycle.

CONCLUSIONS

The EMG activity seen in the rat during micturition is presumably necessary for efficient voiding, thus the guinea pig is a more suitable comparative model for the human in relation to urethral properties during micturition.

Transmitters contributing to the voiding contraction in female rats

OBJECTIVES

To assess in detail the contribution of acetylcholine and ATP to the different phases of the voiding contraction, urine flow and rhabdosphincter electromyographic (RB-EMG) activity in rats, using alpha,beta-methylene-ATP (desensitizing purinoceptors) and atropine (blocking muscarinic receptors). These agents and possibly other transmitters contribute to bladder emptying in rats, but how they contribute to the different phases of the micturition cycle, including the intraluminal pressure high-frequency oscillations (IPHFOs) is unclear.

MATERIALS AND METHODS

Adult anaesthetized female Sprague-Dawley rats were used; intravesical pressure, RB-EMG and urine flow from the distal urethra were recorded. After baseline recordings, alpha,beta-methylene-ATP (0.5 mg/kg), atropine (1 mg/kg), or both, were injected intravenously.

RESULTS

Alpha,beta-Methylene-ATP significantly decreased the maximum bladder pressure during the first micturition phase, whereas atropine had little effect; the maximum bladder pressure during the second phase was also reduced. IPHFOs were apparent after both treatments. Atropine significantly reduced the maximum bladder pressure during the third phase. The maximum urinary flow rate was reduced by both alpha,beta-methylene-ATP and atropine; after exposure to both agents together, urinary flow was markedly reduced or stopped, and overflow incontinence developed.

CONCLUSIONS

ATP contributes mainly to the initial and acetylcholine to the later phases of the voiding cycle in the rat. Neither agent abolished the IPHFOs; even after blocking the receptors for one transmitter and in the presence of IPHFOs, the bladder can still empty. However, if both receptors are blocked, overflow incontinence develops, suggesting that even if further transmitters are taking part in the voiding contraction, their physiological significance is questionable.