Pattern of sensory innervation of the perineal skin in the female rat

Time course for urethral neuromuscular reestablishment and its facilitated recovery by transcutaneous neuromodulation after simulated birth trauma in rats

The aims of the study were to determine the time-course of urinary incontinence recovery after vaginal distension (VD), elucidate the mechanisms of injury from VD leading to external urethral sphincter (EUS) dysfunction, and assess if transcutaneous electrical stimulation (TENS) of the dorsal nerve of the clitoris facilitates recovery of urinary continence after VD. Rats underwent 4-h VD, 4-h sham VD (SH-VD), VD plus 1-h DNC TENS, and VD plus 1-h sham TENS (SH-TENS). TENS or SH-TENS were applied immediately and at days 2 and 4 post-VD. Micturition behavior, urethral histochemistry and histology, EUS and nerve electrophysiology, and cystometrograms were evaluated. VD induced urine leakage and significantly disrupted EUS fibers and nerve-conduction (VD vs SH-VD group;p < 0.01). Urine leakage disappeared 13 days post-VD (p < 0.001). Structural and functional recovery of EUS neuromuscular circuitry started by day 6 post-VD, but did not fully recover by day 11 post-VD (p > 0.05). TENS significantly decreased the frequency of urine leakage post-VD (days 5–7;p < 0.01). We conclude that rat urinary continence after VD requires 2 weeks to recover, although urethra structure is not fully recovered. TENS facilitated urinary continence recovery after VD. Additional studies are necessary to assess if TENS could be used in postpartum women.

Anatomical organization and somatic axonal components of the lumbosacral nerves in female rabbits

AIM

To determine the anatomical organization and somatic axonal components of the lumbosacral nerves in female rabbits.

METHODS

Chinchilla adult anesthetized female rabbits were used. Anatomical, electrophysiological, and histological studies were performed.

RESULTS

L7, S1, and some fibers from S2 and S3 form the lumbosacral trunk, which gives origin to the sciatic nerve and innervation to the gluteal region. From S2 to S3 originates the pudendal nerve, whose branches innervates the striated anal and urethra sphincters, as well as the bulbospongiosus, ischiocavernosus, and constrictor vulvae muscles. The sensory field of the pudendal nerve is ∼1800 mm² and is localized in the clitoral sheath and perineal and perigenital skin. The organization of the pudendal nerve varies between individuals, three patterns were identified, and one of them was present in 50% of the animals. From S3 emerge the pelvic nerve, which anastomoses to form a plexus localized between the vagina and the rectum. The innervation of the pelvic floor originates from S3 to S4 fibers.

CONCLUSIONS

Most of the sacral spinal nerves of rabbit are mixed, carrying sensory, and motor information. Sacral nerves innervate the hind limbs, pelvic viscera, clitoris, perineal muscles, inguinal and anal glands and perineal, perigenital, and rump skin. The detailed description of the sacral nerves organization, topography, and axonal components further the knowledge of the innervation in pelvic and perinal structures of the female rabbit. This information will be useful in future studies about the physiology and physiopathology of urinary, fecal, reproductive, and sexual functions.

Neuromodulation of the neural circuits controlling the lower urinary tract

The inability to control timely bladder emptying is one of the most serious challenges among the many functional deficits that occur after a spinal cord injury. We previously demonstrated that electrodes placed epidurally on the dorsum of the spinal cord can be used in animals and humans to recover postural and locomotor function after complete paralysis and can be used to enable voiding in spinal rats. In the present study, we examined the neuromodulation of lower urinary tract function associated with acute epidural spinal cord stimulation, locomotion, and peripheral nerve stimulation in adult rats. Herein we demonstrate that electrically evoked potentials in the hindlimb muscles and external urethral sphincter are modulated uniquely when the rat is stepping bipedally and not voiding, immediately pre-voiding, or when voiding. We also show that spinal cord stimulation can effectively neuromodulate the lower urinary tract via frequency-dependent stimulation patterns and that neural peripheral nerve stimulation can activate the external urethral sphincter both directly and via relays in the spinal cord. The data demonstrate that the sensorimotor networks controlling bladder and locomotion are highly integrated neurophysiologically and behaviorally and demonstrate how these two functions are modulated by sensory input from the tibial and pudental nerves. A more detailed understanding of the high level of interaction between these networks could lead to the integration of multiple neurophysiological strategies to improve bladder function. These data suggest that the development of strategies to improve bladder function should simultaneously engage these highly integrated networks in an activity-dependent manner.

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.

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.

Context alters the ability of clitoral stimulation to induce a sexually-conditioned partner preference in the rat

We have shown previously that clitoral stimulation (CLS) of female rats induces significant conditioned place preference (CPP), indicating that it is rewarding. The present study asked whether CLS could induce a conditioned partner preference. In the first experiment, sexually naïve females received 10 alternating trials of CLS and No-CLS in the presence of a male rat behind a wire-mesh screen. For one group, CLS was made in the presence of the male scented with almond extract. On alternating trials, those females received sham CLS in the presence of an unscented male behind the screen. The order was reversed for the other group. After 5 trials in each condition, females were placed into an open field with two sexually vigorous males, one scented and the other unscented. Contrary to expectation, females displayed a preference for the male associated with sham CLS. The second experiment examined whether a partner preference could be conditioned by associating CLS with the almond odor alone. A new group of sexually naive females received the same CLS-odor, No-CLS-No Odor pairings as above, but with the odor presented on cotton gauze in the chamber. During the final open field test, those females selectively solicited the scented male. We conclude that CLS that induces CPP also induces conditioned partner preference. However, we propose that CLS in the presence of an inaccessible male created a sexual inhibitory state for female rats.

Pelvic floor muscles and the external urethral sphincter have different responses to applied bladder pressure during continence

OBJECTIVES

To determine the functional innervation of the pelvic floor muscles (PFM) and whether there is PFM activity during an external pressure increase to the bladder in female rats.

METHODS

Thirty-one female adult virgin Sprague-Dawley rats received an external increase in bladder pressure until urinary leakage was noted while bladder pressure was recorded (leak point pressure [LPP]) under urethane anesthesia. Six of the rats underwent repeat LPP testing after bilateral transection of the levator ani nerve. Another 6 rats underwent repeat LPP testing after bilateral transection of the pudendal nerve. Simultaneous recordings of PFM (pubo- and iliococcygeus muscles), electromyogram (EMG), and external urethral sphincter (EUS) EMG were recorded during cystometry and LPP testing.

RESULTS

Thirteen rats (42%) showed tonic PFM EMG activity during filling cystometry. Eighteen rats (58%) showed no tonic PFM EMG activity at baseline, but PFM EMG could be activated by pinching the perineal skin. This activity could be maintained unless voiding occurred. The external increase in bladder pressure caused significantly increased EUS EMG activity as demonstrated by increased amplitude and frequency. However, there was no such response in PFM EMG. LPP was not significantly different after levator ani nerve transection, but was significantly decreased after pudendal nerve transection.

CONCLUSIONS

PFM activity was not increased during external pressure increases to the bladder in female rats. Experimental designs using rats should consider this result. The PFM, unlike the EUS, does not contribute to the bladder-to-urethra continence reflex. PFM strengthening may nonetheless facilitate urinary continence clinically by stabilizing the bladder neck.

Identification of perineal sensory neurons activated by innocuous heat

C-fiber sensory neurons comprise nociceptors and smaller populations of cells detecting innocuous thermal and light tactile stimuli. Markers identify subpopulations of these cells, aiding our understanding of their physiological roles. The transient receptor potential vanilloid 1 (TRPV1) cation channel is characteristic of polymodal C-fiber nociceptors and is sensitive to noxious heat, irritant vanilloids, and protons. By using immunohistochemistry, in situ hybridization, and retrograde tracing, we anatomically characterize a small subpopulation of C-fiber cells that express high levels of TRPV1 (HE TRPV1 cells). These cells do not express molecular markers normally associated with C-fiber nociceptors. Furthermore, they express a unique complement of neurotrophic factor receptors, namely, the trkC receptor for neurotrophin 3, as well as receptors for neurturin and glial cell line-derived neurotrophic factor. HE TRPV1 cells are distributed in sensory ganglia throughout the neuraxis, with higher numbers noted in the sixth lumbar ganglion. In this ganglion and others of the lumbar and sacral regions, 75% or more of such HE TRPV1 cells express estrogen receptor alpha, suggestive of their regulation by estrogen and a role in afferent sensation related to reproduction. Afferents from these cells provide innervation to the hairy skin of the perineal region and can be activated by thermal stimuli from 38 degrees C, with a maximal response at 42 degrees C, as indicated by induction of extracellular signal-regulated kinase phosphorylation. We hypothesize that apart from participating in normal thermal sensation relevant to thermoregulation and reproductive functions, HE TRPV1 cells may mediate burning pain in chronic pain syndromes with perineal localization.

Clitoral stimulation modulates appetitive sexual behavior and facilitates reproduction in rats

In rats, sexual reward, appetitive sexual behaviors and reproduction are modulated by the amount and rate of vaginocervical stimulation. Here the effect of clitoral stimulation (CLS) on proceptivity was assessed. In Exp 1, ovariectomized, hormone-primed Wistar females formed three groups: G1 (1 CLS every second), G2 (1 CLS every 5s) and G3 (no CLS). Precopulatory CLS consisted of 5cycles of 1min of stimulation with the tip of a cotton swab connected to a vibrator device, followed by 1-2min of rest. CLS increased proceptive behavior in G1 compared to G2, but not compared to G3. In Exp 2, gonadally-intact rats in late proestrous received CLS prior to copulation. No differences in sexual behavior were detected between the groups, but CLS enhanced reproduction in females that received >9 intromissions. 28, 66 and 10% of females became pregnant in G1, G2, and G3, respectively. These data indicate that precopulatory CLS affects proceptive behaviors depending on the pattern and rhythm of stimulation in hormone-primed females. In virgin rats that have received sufficient vagino cervical stimulation CLS also increases fertility.

Clitoral stimulation induces conditioned place preference and Fos activation in the rat

The present study examined the ability of clitoral stimulation (CLS) to induce conditioned place preference (CPP) and Fos protein in the brain. Ovariectomized, hormone-primed Long-Evans rats were randomly assigned to receive either distributed CLS (1 stimulation every 5 s for 1 min prior to being placed in one distinctive side of a nonbiased CPP box for 2 min, after which the cycle of stimulation and CPP exposure were repeated for 4 more cycles, totaling 60 stimulations) or continuous CLS (1 stimulation per second for 1 min with 2 min in one side of the CPP box, repeated for 4 more cycles, totaling 300 stimulations). Two days later, females were placed into the other side of the CPP box without prior stimulation. CPP was tested after 5 sequential exposures each of CLS and no stimulation. Females given distributed stimulation developed a significant CPP whereas females given continuous stimulation did not. CLS induced Fos in hypothalamic and limbic structures, including the nucleus accumbens, piriform cortex, arcuate nucleus, and dorsomedial portion of the ventromedial hypothalamus, compared to no stimulation. However, distributed CLS induced more Fos in the medial preoptic area than continuous CLS or no stimulation. In contrast, continuous CLS induced more Fos in the posteroventral medial amygdala compared to no stimulation. These data indicate that CLS induces a reward state in the rat and a pattern of Fos activation in regions of the brain that process genitosensory input, incentive salience, and reward.

Clitoral sexual arousal: neuronal tracing study from the clitoris through the spinal tracts

PURPOSE

Although genital tactile stimulation is regarded as a precursor to sexual arousal and a recognized initiator of central nervous system arousal, specific afferent neural pathways transmit sensory stimuli of arousal, beginning at the epithelial level on the clitoris and following the course of arousal stimuli through the central nervous system. Limited knowledge exists of the pathway from the cutaneous receptors of nerves originating in the epithelial tissue of the clitoris and continuing to spinal cord afferents. Such information may contribute to an understanding of sexual arousal, particularly in female vertebrates. We further defined the neural pathways and mechanisms responsible for arousal originating in the epithelium of the clitoris as well as related neural pathways to the spinal cord in a murine model.

MATERIALS AND METHODS

We performed a comprehensive review of the published relevant clinical and histological material from human and nonhuman vertebrate studies. In 29 adult female C57B1/6 mice the distribution of pelvic nerves and vessels was mapped. Gross dissection of 4 female mice was facilitated by resin injection of the vascular system in 2. Neuronal tracing was performed in 25 mice that received clitoral injection of wheat germ agglutinin-horseradish peroxidase into the clitoris and were sacrificed after 72 to 96 hours. The spinal cord and periclitoral tissue were removed and fixed. Immunohistochemistry was performed.

RESULTS

Gross anatomy of the mouse clitoris showed that pudendal and hypogastric nerves have a major role in the innervation of the external genitalia. Neuronal tracing revealed that the greatest nerve density was noted in the L5/6 spinal cord. The distribution extended from S1 to L2 with no labeling seen in the L3 spinal cord. Wheat germ agglutinin-horseradish peroxidase labeling was seen caudal in levels S1 through L4 and rostral in L2.

CONCLUSIONS

Understanding the neuroanatomy of the clitoris using a murine model may provide a valuable tool for the study of sexual arousal disorders and the further understanding of sexual function related to neural pathologies and trauma.

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.

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.