Motor pudendal nerve characterization in the female rat

Histochemical Identification of Motor Fascicles Using Cholinesterase Staining in Rats Using a Mixed Nerve Model

BACKGROUND

Inappropriate matching of motor and sensory fibers after nerve repair or grafting can lead to nerve recovery failure. Identifying the motor and sensory fascicles enables surgeons to match them accurately and correctly align nerve stumps, which is crucial for neural regeneration. Very few methods have been reported to differentiate between the sensory and motor nerve fascicles, and the replicability of these techniques remains unestablished. In this study, we aimed to assess the accuracy of axonal cholinesterase (CE) histochemical staining in distinguishing motor and sensory nerve fibers.

METHODS

The femoral and sciatic nerves were harvested from rats. The specimens were immediately cut, frozen in isopentane, and cooled with liquid nitrogen. Nerve serial cross-sections were processed for hematoxylin and eosin staining, followed by CE histochemistry. The staining protocol solutions included acetylthiocholine iodide, phosphate buffer, cobalt sulfate hydrate, potassium phosphate monobasic, sulfuric acid, sodium bicarbonate, glutaraldehyde, and ammonium sulfide.

RESULTS

Cross-sections of nerves containing efferent and afferent nerve fibers in segregated fascicles showed that CE activity was confined to motor neurons. A histochemical study revealed that motor fibers with high cholinesterase activity can be differentiated from sensory fibers. The motor branches of the femoral and sciatic nerves showed specific axonal staining, whereas the sensory branch did not show any specific staining.

CONCLUSION

CE histochemical staining is a useful technique for distinguishing between motor and sensory nerve fibers. It can be potentially useful in improving the outcomes of nerve grafts or extremity allotransplantation surgery.

Swine Pudendal Nerve as a Model for Neuromodulation Studies to Restore Lower Urinary Tract Dysfunction

Lower urinary tract dysfunction, such as incontinence or urinary retention, is one of the leading consequences of neurological diseases. This significantly impacts the quality of life for those affected, with implications extending not only to humans but also to clinical veterinary care. Having motor and sensory fibers, the pudendal nerve is an optimal candidate for neuromodulation therapies using bidirectional intraneural prostheses, paving the way towards the restoration of a more physiological urination cycle: bladder state can be detected from recorded neural signals, then an electrical current can be injected to the nerve based on the real-time need of the bladder. To develop such prostheses and investigate this novel approach, animal studies are still required since the morphology of the target nerve is fundamental to optimizing the prosthesis design. This study aims to describe the porcine pudendal nerve as a model for neuromodulation studies aiming at restoring lower urinary tract dysfunction. Five male farm pigs were involved in the study. First, a surgical procedure to access the porcine pudendal nerve without muscle resection was developed. Then, an intraneural interface was implanted to confirm the presence of fibers innervating the external urethral sphincter by measuring its electromyographic activity. Finally, the morphophysiology of the porcine pudendal nerve at the level of surgical exposure was described by using histological and immunohistochemical characterization. This analysis confirmed the fasciculate nature of the nerve and the presence of mixed fibers with a spatial and functional organization. These achievements pave the way for further pudendal neuromodulation studies by using a clinically relevant animal model with the potential for translating the findings into clinical applications.

State-dependent bioelectronic interface to control bladder function

Electrical stimulation therapies to promote bladder filling and prevent incontinence deliver continuous inhibitory stimulation, even during bladder emptying. However, continuous inhibitory stimulation that increases bladder capacity (BC) can reduce the efficiency of subsequent voiding (VE). Here we demonstrate that state-dependent stimulation, with different electrical stimulation parameters delivered during filling and emptying can increase both BC and VE relative to continuous stimulation in rats and cats of both sexes. We show that continuous 10 Hz pudendal nerve stimulation increased BC (120–180% of control) but decreased VE (12–71%, relative to control). In addition to increasing BC, state-dependent stimulation in both rats and cats increased VE (280–759% relative to continuous stimulation); motor bursting in cats increased VE beyond the control (no stimulation) condition (males: 323%; females: 161%). These results suggest that a bioelectronic bladder pacemaker can treat complex voiding disorders, including both incontinence and retention, which paradoxically are often present in the same individual.

Non-invasive electromyographic estimation of motor unit number in the external anal sphincter of the rat

AIMS

The external anal sphincter (EAS) is essential for maintaining fecal continence. Neurological disorders or traumatic injuries to muscle and nervous systems could lead to EAS denervation. Currently, there are no techniques available to document global innervation changes in the EAS in vivo. The aim of this study was to develop a novel approach to non-invasively estimate the number of functioning motor units (MUs) in the EAS and validate with immunofluorescent techniques in rats.

METHODS

Intra-rectal surface electromyography (EMG) signals of the EAS, induced by a series of intra-vaginally delivered pudendal nerve stimulations with different intensities, were recorded. Variation in EMG responses at different intensities was used to estimate the value of a single motor unit potential (SMUP) in order to perform the proposed EAS motor unit number estimation (MUNE) approach. The EAS MUNE was tested in 12 female Sprague-Dawley rats, and validated by comparing against the EAS myofiber counting results achieved by performing immunostaining of acetylcholine receptors in 7 of the 12 rats.

RESULTS

The mean MU number was 35 ± 9, with an averaged SMUP size of 52.49 ± 20.39 μV. The mean number of successfully identified myofibers was 652.7 ± 130.6 myofiber/EAS. Significance of linear regression between the immunofluorescent results and the MUNE was confirmed (P < 0.01).

CONCLUSIONS

Our study represents the first effort to non-invasively assess the innervation of the EAS in vivo using the rat as a pre-clinical model. This approach can potentially enable future clinical applications for advanced diagnosis and treatment of neurogenic EAS disorders.

Urethral musculature and innervation in the female rat

AIMS

The urethral sphincter and urethral muscle innervation are critically involved in maintaining continence, especially in the female. However, the urethral muscle type and distribution, as well as the urethral nerves are far from being well documented. Our aim was to clearly identify the distribution of urethral striated muscle, smooth muscle, and urethral nerves.

METHODS

In a cohort analysis of 3-month-old female Sprague-Dawley rats, cross and longitudinal sections of female rat urethra were extensively investigated using morphological techniques. Urethras were harvested to the sections, in order to provide both global and detailed visions of the urethra. H&E, Masson's Trichrome, phalloidin and immunoflourence stains were used. The cytoarchitecture, nitrergic, and cholinergic innervations were mainly investigated. Different layers of the segments of urethra were traced to draw curve graphs that represent the thickness of each muscle layer of urethral wall.

RESULTS

The results showed that the primary peak of striated muscle is in the middle urethra. The inner layer close to mucosa was found to contain longitudinal smooth muscle. Near the bladder orifice, the circular smooth muscle dominates, which becomes thinner distally throughout the rest of urethra. In the middle urethra the vast majority of the urethral muscle are circularly oriented striated muscle cells. Typical nerve endings were present in high power images to show the different characteristic features of nerve innervation.

CONCLUSIONS

This study has illustrated the detailed morphological structure and innervations of the normal female rat urethra and can serve as a basis for further study of stress urinary incontinence (SUI).

Somatomotor and sensory urethral control of micturition in female rats

In rats, axons of external urethral sphincter (EUS) motoneurons travel through the anastomotic branch of the pudendal nerve (ABPD) and anastomotic branch of the lumbosacral trunk (ABLT) and converge in the motor branch of the sacral plexus (MBSP). The aim of the present study was to determine in female rats the contribution of these somatomotor pathways and urethral sensory innervation from the dorsal nerve of the clitoris on urinary continence and voiding. EUS electromyographic (EMG) activity during cystometry, leak point pressure (LPP), and voiding efficiency (VE) were assessed in anesthetized virgin Sprague-Dawley female rats before and after transection of the above nerve branches. Transection of the MBSP eliminated EUS EMG, decreased LPP by 50%, and significantly reduced bladder contraction duration, peak pressure, intercontraction interval, and VE. Transection of the ABPD or ABLT decreased EUS EMG discharge and LPP by 25% but did not affect VE. Transection of the dorsal nerve of the clitoris did not affect LPP but reduced contraction duration, peak pressure, intercontraction interval, and VE. We conclude that somatomotor control of micturition is provided by the MBSP with axons travelling through the ABPD and ABLT. Partial somatomotor urethral denervation induces mild urinary incontinence, whereas partial afferent denervation induces voiding dysfunction. ABPD and ABLT pathways could represent a safeguard ensuring innervation to the EUS in case of upper nerve damage. Detailed knowledge of neuroanatomy and functional innervation of the urethra will enable more accurate animal models of neural development, disease, and dysfunction in the future.

The distribution of muscles fibers and their types in the female rat urethra: cytoarchitecture and three-dimensional reconstruction

An attempt to explore urethral cytoarchitecture including the distribution of smooth muscles and fast and slow striated muscles of adult female Sprague Dawley rat--a popular model in studying lower urinary tract function. Histological and immunohistochemical stainings were carried out to investigate the distribution of urethral muscle fibers and motor end plates. The urethral sphincter was furthermore three-dimensionally reconstructed from serial histological sections. The mucosa at the distal urethra was significantly thicker than that of other segments. A prominent inner longitudinal and outer circular layer of smooth muscles covered the proximal end of urethra. Thick circular smooth muscles of the bladder neck region (urethral portion) decreased significantly distalward and longitudinal smooth muscles became 2- to 3-fold thicker in the rest of the urethra. An additional layer of striated muscles appeared externally after neck region (urethra) and in association with motor end plates ran throughout the remaining urethra as the striated sphincter layer. Most striated muscles were fast fibers while relatively fewer slow fibers often concentrated at the periphery. A pair of extraneous striated muscles, resembling the human urethrovaginal sphincter muscles, connected both sides of mainly the distal vagina to the dorsal striated muscles in the wall of the middle urethra. The tension provided by this pair of muscles, and in conjunction with the striated sphincter of the urethral wall, was likely to function to suspend the middle urethra and facilitates its closure. Comprehensive morphological data of urethral sphincter offers solid basis for researchers conducting studies on dysfunction of bladder outlet.

Comparison of the motor discharge to the voluntary sphincters of continence in the rat

BACKGROUND

  The rat external anal sphincter (EAS) and external urethral sphincter (EUS) are voluntary muscles of continence that can display similar synchronous electromyographic (EMG) activity patterns. However, the two sphincters are quite different in structure and function. The EUS is a fast twitch muscle and contains fibers expressing type 2B myosin. In contrast, the EAS exhibits slower kinetics and lacks type 2B fibers. This striking contrast in kinetics and fiber type profiles may be shaped by differences in the basal motor drive that each sphincter receives.

METHODS

  A double EMG approach was used to obtain spontaneously active single motor unit action potentials from the EUS and EAS simultaneously and compare their basal discharge frequencies in urethane anaesthetized rats.

KEY RESULTS

  The basal firing rates of motor units of the EUS and EAS were not significantly different (3.9 ± 0.9 Hz vs. 3.1 ± 1.6 Hz, respectively, n = 7 animals, P = 0.32, paired Student's t-test). However, auto-correlogram analysis showed that EUS is driven by neurons with faster instantaneous firing frequencies: 30.5 ± 2.4 Hz vs 14.3 ± 0.9 Hz in the EAS (P = 0.03, paired Student's t-test).

CONCLUSIONS & INFERENCES

  The oscillator(s) driving the EUS operate(s) at a frequency twice that of the EAS. This may explain the presence of type 2B fibers in the EUS. In the inter-micturition periods no cross correlation was found in motor discharge to the sphincters suggesting that the two muscles do not share a common central drive to sustain the continent tonus of the two outlet tracts.

Researching bladder afferents-determining the effects of β(3) -adrenergic receptor agonists and botulinum toxin type-A

A substantial portion of the current research on lower urinary tract dysfunction is focused on afferent mechanisms. The main goals are to define and modulate the signaling pathways by which afferent information is generated, enhanced and conveyed to the central nervous system. Alterations in bladder afferent mechanisms are a potential source of voiding dysfunction and an emerging source for drug targets. Established drug therapies such as muscarinic receptor antagonists, and two emerging therapies, β(3) -adrenergic receptor agonists and botulinum toxin type-A, may act partly through afferent mechanisms. This review focuses on these two new principles and new and established methods for determining their sites of action. It also provides brief information on the innervation of the bladder, afferent receptors and transmitters and how these may communicate with the urothelium, interstitial cells and detrusor smooth muscle to regulate micturition. Peripheral and central mechanisms of afferent sensitization and myogenic mechanisms that lead to detrusor overactivity, overactive bladder symptoms and urgency sensations are also covered. This work is the result from 'Think Tank' presentations, and the lengthy discussions that followed, at the 2010 International Consultation on Incontinence Research Society meeting in Bristol, UK.

Role of vasopressin V1A receptor in the urethral closure reflex in rats

An enhanced urethral closure reflex via the spinal cord is related to urethral resistance elevation during increased abdominal pressure. However, with the exception of monoamines, neurotransmitters modulating this reflex are not understood. We investigated whether the vasopressin V1A receptor (V1AR) is involved in the urethral closure reflex in urethane-anesthetized female rats. V1AR mRNA was highly expressed among the vasopressin receptor family in the total RNA purified from lamina IX in the spinal cord L6–S1 segment. In situ hybridization analysis of the spinal L6–S1 segment confirmed that these positive signals from the V1ARs were only detected in lamina IX. Intrathecally injected Arg8-vasopressin (AVP), an endogenous ligand, significantly increased urethral resistance during an intravesical pressure rise, and its effect was blocked by the V1AR antagonist. AVP did not increase urethral resistance in rats in which the pelvic nerves were transected bilaterally. Urethral closure reflex responses to the intravesical pressure rise increased by up to threefold compared with the baseline response after AVP administration in contrast to no increase by vehicle. In addition, intravenously and intrathecally injected V1AR antagonists decreased urethral resistance. These results suggest that V1AR stimulation in the spinal cord enhances the urethral closure reflex response, thereby increasing urethral resistance during an abdominal pressure rise and that V1AR plays a physiological role in preventing urine leakage.

Animal models of stress urinary incontinence

Stress urinary incontinence (SUI) is a common health problem significantly affecting the quality of life of women worldwide. Animal models that simulate SUI enable the assessment of the mechanism of risk factors for SUI in a controlled fashion, including childbirth injuries, and enable preclinical testing of new treatments and therapies for SUI. Animal models that simulate childbirth are presently being utilized to determine the mechanisms of the maternal injuries of childbirth that lead to SUI with the goal of developing prophylactic treatments. Methods of assessing SUI in animals that mimic diagnostic methods used clinically have been developed to evaluate the animal models. Use of these animal models to test innovative treatment strategies has the potential to improve clinical management of SUI. This chapter provides a review of the available animal models of SUI, as well as a review of the methods of assessing SUI in animal models, and potential treatments that have been tested on these models.

Central representation of the inferior rectal nerve of the rat

PURPOSE

Obstetric injury to the pudendal nerve contributes significantly to fecal incontinence. The inferior rectal nerve, a terminal branch of the motor division of the pudendal nerve, innervates the external anal sphincter. Animal models have been developed to establish the scientific basis of sacral neuromodulation. The aims of this study were to determine the spinal location of inferior rectal nerve motoneurons projecting to the external anal sphincter and whether the inferior rectal nerve carries sensory fibers.

METHODS

Ten female virgin Wistar rats were used; 7 underwent bilateral inferior rectal nerve section and application of the neuronal tracer fluorogold. Five days later dorsal root ganglia L5 to S2 and the lumbosacral spinal cord were harvested and stained for activating transcription factor 3, a molecular marker of nerve injury. Three animals were used to confirm the specificity of activating transcription factor 3 nuclear labeling as a marker of axotomy.

RESULTS

Fluorogold-labeled motoneurons were found exclusively at L6 in the dorsomedial sections of Onuf's nuclei (left and right), which contained 30 +/- 9 motoneurons with a median diameter of 28.3 microm (24.4-31.0) (25th-75th centiles). Double-labeled sensory neurons (fluorogold-labeled cytoplasm and activating transcription factor 3-labeled nuclei) were found in dorsal root ganglia L6 to S2 and were smaller in diameter (20.5 microm (17.8-26.7); median (25th-75th centiles)) than motoneurons (P < .0,001).

CONCLUSIONS

The external anal sphincter receives both motor and sensory innervation from the inferior rectal nerve. Activating transcription factor 3 nuclear staining may prove useful for quantifying the degree of nerve injury in animal models of fecal incontinence.

Endogenous ephrinB2 mediates colon-urethra cross-organ sensitization via Src kinase-dependent tyrosine phosphorylation of NR2B

Recently, the role of EphB receptor (EphBR) tyrosine kinase and their ephrinB ligands in spinal pain-related neural plasticity has been identified. To test whether Src-family non-receptor tyrosine kinase-dependent glutamatergic N-methyl-d-aspartate receptor (NMDAR) NR2B subunit phosphorylation underlies lumbosacral spinal EphBR activation to mediate cross-organ sensitization between the colon and the urethra, external urethra sphincter electromyogram activity evoked by pelvic nerve stimulation and protein expression in the lumbosacral (L6-S2) dorsal horn were studied before and after intracolonic mustard oil (MO) instillation. We found MO instillation produced colon-urethra reflex sensitization along with an upregulation of endogenous ephrinB2 expression as well as phosphorylation of EphB 1/2, Src-family kinase, and NR2B tyrosine residues. Intrathecal immunoglobulin fusion protein of EphB1 and EphB2 as well as PP2 reversed the reflex sensitization and NR2B phosphorylation caused by MO. All these results suggest that EphBR-ephrinB interactions, which provoke Src-family kinase-dependent NMDAR NR2B phosphorylation at the lumbosacral spinal cord level, are involved in cross-organ sensitization, contributing to the development of viscero-visceral referred pain between the bowel and the urethra.

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.

Time course of neuroanatomical and functional recovery after bilateral pudendal nerve injury in female rats

The pudendal nerve innervates the external urethral sphincter (EUS) and is among the tissues injured during childbirth, which may lead to symptoms of stress urinary incontinence (SUI). To understand the mechanisms of injury and repair, urethral leak-point pressure (LPP) was measured 4 days, 2 wk, or 6 wk after bilateral pudendal nerve crush. Morphometric changes in the distal nerve and EUS were examined by light and electron microscopy. To determine whether recovery resulted from pudendal neuroregeneration, LPP was measured before and after pudendal nerve transection 2 wk after nerve crush. LPP was significantly decreased 4 days after pudendal nerve crush compared with sham-injured animals as well as 2 or 6 wk after nerve crush. LPP was not significantly different 2 or 6 wk after nerve crush compared with sham-injured animals, suggesting that urethral function had returned to normal. Four days after pudendal nerve crush, the EUS branch of the pudendal nerve distal to the injury site showed evidence of nerve degeneration and the EUS appeared disrupted. Two weeks after nerve crush, the distal nerve and EUS both showed evidence of both nerve degeneration and recovery. Two weeks after nerve crush, LPP was significantly decreased after nerve transection. Six weeks after nerve injury, evidence of neuroregeneration was observed in the pudendal nerve and the EUS. This study has demonstrated that functional recovery and neuroregeneration are significant 2 wk after nerve crush, although by anatomical assessment, recovery appears incomplete, suggesting that 2 wk represents an early time point of initial neuroregeneration.

Striated muscle and nerve fascicle distribution in the female rat urethral sphincter

The anatomical basis for urinary continence depends on a thorough understanding of the tissues in the urethra. The objective of this study was to evaluate the morphology and neuroanatomy of urethral striated muscle, called the rhabdosphincter or external urethral sphincter, in normal female rats. Urethras from 12 female rats were dissected from the bladder, fixed, embedded in paraffin or epon, and sectioned every 1 mm. Striated muscle content was taken as the ratio of the striated muscle area to net urethral area. Nerve fascicles containing myelinated axons near the rhabdosphincter were counted and mapped. Both striated muscle content and number of nerve fascicles peak in the proximal third of the urethra, with a secondary peak at the distal end of the urethra. This secondary peak may correspond to an analog of the combined compressor urethrae/urethrovaginal sphincter located in the distal urethra in human. The rhabdosphincter has a variable distribution along the length of the urethra. In the middle and distal thirds of the urethra, the dorsal striated muscle fibers between the urethra and vagina become more sparse. The majority of nerve fascicles are contained in the lateral quadrants of the urethra, similar to the lateral distribution of somatic nerves in humans. In conclusion, this study demonstrates the normal distribution of the striated musculature and neuroanatomy in the urethra, with similarities to the human. It thus supports and extends the usefulness of the rat as an experimental model for studying urinary incontinence.

Effect of estrogen on urethral function and nerve regeneration following pudendal nerve crush in the female rat

PURPOSE

We tested the hypothesis that estrogen promotes improvement in urethral function and nerve regeneration following bilateral pudendal nerve crush in ovariectomized female rats.

MATERIALS AND METHODS

A total of 52 female rats underwent ovariectomy 6 days before bilateral pudendal nerve crush. Estrogen and sham capsules were subcutaneously implanted at the time of nerve crush in 16 and 14 of these rats, respectively, while 22 served as unoperated controls. Seven days following nerve crush urethral LPP testing was performed using urethane anesthesia. Spinal cord sections containing motoneurons of Onufrowicz's nucleus were subjected to in situ hybridization to detect the expression of beta(II) tubulin mRNA, a marker of the neuroregenerative response.

RESULTS

Mean LPP +/- SEM was significantly decreased after pudendal nerve crush in sham treated animals compared to unoperated controls (32.1 +/- 6.8 vs 54.4 +/- 11.6 cm H2O). Rats with an estrogen implant had an LPP of 42.5 +/- 16.8 cm H2O, which was significantly greater than rats given sham implants and significantly less than unoperated controls. Rats that received an estrogen implant had increased beta(II) tubulin mRNA expression compared to those that received a sham implant.

CONCLUSIONS

The results of this research suggest that estrogen given at the time of pudendal nerve crush promotes and facilitates the recovery of urethral function and an increase in the nerve regenerative response. Future studies will include the investigation of molecular pathways activated by estrogen in response to peripheral nerve injury.

RESTORATION OF EXTERNAL URETHRAL SPHINCTER FUNCTION AFTER PUDENDAL NERVE END-TO-END ANASTOMOSIS IN THE MALE RABBIT

PURPOSE

In this study we rehabilitated external urethral sphincter function by pudendal nerve end-to-end anastomosis after experimental pudendal nerve axotomy in male rabbits.

MATERIALS AND METHODS

A total of 17 animals were included in this study, including group 1-a control group of 5 (29.4%), group 2-6 (35.3%) and group 3-6 (35.3%). Animals from group 2 underwent bilateral axotomy and group 3 underwent pudendal nerve end-to-end anastomosis. In all groups we performed urodynamic investigations prior to axotomy, after axotomy or anastomosis, and 14, 42 and 90 days after axotomy or nerve anastomosis.

RESULTS

In untreated group 1 control sphincter pressure was 28.5 cm H2O. In group 2 average urethral sphincter pressure was 5.6 cm H2O 14 days after axotomy with only a slight increase to 11.05 cm H2O by day 90. In group 3 external urethral pressure increased to 8.26 cm H2O after 14 days and to 21.32 cm H2O by postoperative day 90.

CONCLUSIONS

External urethral sphincter deficiency after bilateral pudendal nerve axotomy demonstrates the primacy of the pudendal nerve in the innervation of the external urethral sphincter. We were able to rehabilitate external urethral sphincter function by performing pudendal nerve end-to-end anastomosis.

Early structural effects of oestrogen on pudendal nerve regeneration in the rat

OBJECTIVE

To determine the early effects of oestrogen on the ultrastructure of the pudendal nerve and distal nerve fascicles near the external urethra sphincter (EUS) after a pudendal nerve crush injury. The pudendal nerve is one of the pelvic floor tissues injured during vaginal delivery, possibly contributing to the development of stress urinary incontinence (SUI) in women, the symptoms of which often do not appear until menopause, implicating hormonal factors.

MATERIALS AND METHODS

Twenty-seven virgin female Sprague-Dawley rats were anaesthetized and underwent ovariectomy. Three days later, they had one of four procedures: bilateral pudendal nerve crush plus implant of a subcutaneous oestrogen-containing capsule (NC+E); nerve crush plus implant of a sham saline-containing capsule (NC+S); no nerve crush with an oestrogen capsule; or no nerve crush with a sham capsule. After 2 weeks the pudendal nerves and urethral tissues were prepared for light and electron microscopy. The number of axons, myelin figures and endoneurial nuclei in the pudendal nerve segment distal to the lesion were counted. Nerve fascicles near the EUS were also counted and categorized as normal or showing signs of degeneration and/or regeneration. The location of each nerve fascicle was specified as either ventral or dorsal.

RESULTS

As there were no significant differences between the two control groups they were combined to form a single control group. In the distal pudendal nerve there were significantly fewer myelinated axons and large myelinated axons in the NC+E and NC+S groups than in the control group. There were three times as many large unmyelinated axons in the NC+E group than in either the NC+S or control groups (P < 0.05). There were only half as many nerve fascicles near the ventral side of the EUS in the NC+S group than in both the control and NC+E groups (P < 0.05).

CONCLUSION

Oestrogen appears to affect large unmyelinated axons in both the injured pudendal nerve and at the denervated EUS target. After pudendal nerve crush, nerve fascicles with evidence of degeneration or regeneration near the EUS appear to be spared with oestrogen treatment, particularly in the ventral region. These observations may reflect the early stages of a neuroregenerative effect of oestrogen. Additional studies are needed to confirm these results at later periods and with functional methods.

Functional and neuroanatomical effects of vaginal distention and pudendal nerve crush in the female rat

PURPOSE

We tested the hypothesis that neuroanatomical degeneration near the external urethral sphincter (EUS) would parallel urinary dysfunction after vaginal distention or bilateral pudendal nerve crush in female rats.

MATERIALS AND METHODS

A total of 28 female rats underwent bilateral pudendal nerve crush or vaginal distention, or were unoperated controls. Two days later a catheter was implanted into the bladder dome and 2 days after that (4 days after injury) urethral leak point pressure testing was performed with the rat under urethane anesthesia. The pudendal nerve and urethra were then dissected and prepared for light and electron microscopy.

RESULTS

Leak point pressure was significantly decreased 4 days after pudendal nerve crush and vaginal distention (29.3 +/- 3.4 and 31.0 +/- 2.5 cm H(2)O, respectively) compared with controls (44.3 +/- 3.4 cm H(2)O). The percentage of nerve fascicles with degeneration near the EUS was significantly greater in the nerve crush (13.1% +/- 1.7%) and vaginal distention (7.2% +/- 2.2%) groups than in the control group (0% +/- 0%). There were fewer nerve fascicles near the EUS in the ventral half of the urethral cross section than in the dorsal half in all 3 groups and the percent of fascicles with degeneration was greater in the ventral half than in the dorsal half in the 2 injury groups.

CONCLUSIONS

These results suggest that the pudendal nerve is particularly vulnerable to injury during vaginal distention in this animal model. The 2 injury models may be useful for investigating the pathophysiology of stress urinary incontinence.