Bulbocavernosus Research Articles

The relationship between pre- and postprostatectomy measures of pelvic floor muscle function and development of early incontinence after surgery.

The aim of this study is to investigate (i) whether pelvic floor muscle (PFM) shortening can be enhanced by provision of training focused on striated urethral sphincter (SUS) with feedback before prostatectomy, (ii) whether PFM shortening during voluntary efforts and coughing before and after prostatectomy differs between men who do and do not report symptoms of urinary incontinence 1 month after prostatectomy, and (iii) the relationship between severity of incontinence after prostatectomy and features of pelvic floor function (muscle shortening) and urethral length before and after prostatectomy. Sixty men referred for preoperative PFM training before radical prostatectomy participated. The International Continence Society Male Short Form questionnaire was used to quantify continence status. Transperineal ultrasound (US) imaging was used to record pelvic displacements related to activation of striated urethral sphincter, bulbocavernosus (BC) and puborectalis muscles during cough, "natural" voluntary contraction following pamphlet instruction, and trained voluntary contraction after formal physiotherapist instruction including US feedback. Pelvic floor displacements following training differed between continent and incontinent men; continent participants demonstrated increased SUS shortening after training (compared with "natural"), but no difference was observed between trained and "natural" contractions for incontinent participants. Motion at ano-rectal junction during cough was reduced following surgery, but voluntary and involuntary activation of SUS or BC was not consistently affected by surgery. Participants' capacity to improve function of the SUS with training appears related to postprostatectomy continence outcome.

Triple Junction Theory and Maneuver: reinforcement of Ischiocavernosus, Bulbocavernosus and Rhabdosphincter muscles

Background: Until today, pelvic floor training protocols are focused on the mm. levator ani, although most dysfunctions are of superficial or urethral muscles. Aims:To present the theoretical and empirical bases of the Triple Junction Theory and introduce the Triple Junction Maneuver to strenght the ischiocavernosus (ICs), bulbocavernosus (BCs) and rhabdosphincter (RE) muscles. Method: Theoretical essay based on anatomical, kinesiological and biomechanical findings and exercise physiology. Results: The ICs, BCs and RE muscles are joined in the region adjacent to the glans of the clitoris and the external urethral meatus, in a Triple Junction. It was possible to develop a maneuver to strengthen these three muscles, the Triple Junction Maneuver, superficial and non-invasive, for women, men and children. Most urinary and sexual dysfunctions are caused by one of these three muscles, so such a maneuver can increase the efficiency of pelvic floor training in general. Conclusion: The Triple Junction between mm. ICs, BCs and RE allowed the creation of a strenght maneuver that, however, still lacks new evidence.

BDNF is upregulated by androgen in the inguinal fat pad of immature mice and may regulate inguinoscrotal testicular descent.

Androgens control rodent inguinoscrotal testicular descent during a "programming window" (E12-17). It is proposed that androgen masculinises the genitofemoral nerve, but the mechanism remains unknown. We investigate androgen receptor (AR)-containing target organs: inguinal fat pad (IFP) and mammary bud (MB), supplied by the genitofemoral nerve, hypothesizing that neurotrophic factors may retrogradely masculinise the GFN. The IFP, MB and bulbocavernosus (BC) muscle were collected at E12.5/E17.5 from androgen receptor knockout (ARKO) mice and wild-type (WT) littermates. Immunofluorescence and gene expression (RT-qPCR; n = 8/group) for Bdnf, active (TrkB) and inactive (truncated TrkB) receptors, Cntf and Cntf receptor were performed. In the IFP at E12.5, ARKO TrkB mRNA expression was significantly downregulated compared to WT males (p < 0.0026). By E17.5, there was increased Bdnf expression (p < 0.0233). The MB had no differences at E12.5 and had regressed in WT males by E17.5. The BC had no differences at E12.5, but at E17.5 had significant upregulation of Bdnf expression in ARKO, compared to WT males. There were no differences in CNTF or CNTF receptor expression. Androgen alters active TrkB and Bdnf expression in the IFP. IFP Bdnf signalling may regulate "masculinisation" of the GFN sensory nerves to indirectly control inguinoscrotal testicular descent. Androgen mediates neurotrophin release in the inguinal fat pad in mice, which may facilitate normal testicular descent by masculinising the GFN by peripheral uptake of neurotrophin. This is the first study to examine the role of neurotrophins in testicular descent. This suggests novel steps in the mechanical process of normal testicular descent that may be abnormal in some children with undescended testes.

Reflex erection in the rat: reciprocal interplay between hemodynamic and somatic events

BackgroundPenile erection is a complex reflex under spinal control and modulated by the brain. The hemodynamic events under autonomic control and the perineal muscles somatic activity are interconnected during the reflex erection at the spinal level, however if the afferent feedback on the corpus cavernosum pressure during an erection affects the somatic activity (perineal muscles contractions) and vice versa is not known. This study was aimed to test this hypothesis using a rat model.MethodsIntracavernous pressure (ICP) and bulbocavernosus (BC) muscle EMG were recorded during reflex erections elicited with dorsal penile nerve (DNP) electrical stimulation in anaesthetized acutely spinalized SD rats with surgically (bilateral cavernous nerve section, CnX, n = 8) and pharmacologically (trimetaphan infusion, TMPh, n = 8) abolished pressor response, or with surgically (bilateral section of the motor branch of the pudendal nerve, PnX, n = 7) and pharmacologically (1 mg/kg d-tubocurarine, n = 8) blocked perineal muscles contractions, or with interrupted afferent input from the penis (bilateral crush of the dorsal penile nerve, DPnX, n = 7). Control rats (n = 8) received no intervention.ResultsModerate positive correlations were found between net parameters of pressor and somatic activity during DNP-stimulation induced reflex erection in spinal rats, particularly the speed of pressor response development was positively correlated to EMG parameters. No changes of EMG activity were found in CnX rats, while the decrease of BC EMG in TMPh-treated males can be attributed to direct inhibitory action of TMPh on neuromuscular transmission. Pressor response latency was increased and ICP front slope decreased in dTK and PnX rats, indicating that perineal muscles contraction augment pressor response. DPN crush had little effect on ICP and EMG.ConclusionAfferent input on the level of intracavernous pressure and the perineal muscles activity has minimal impact on, correspondingly, the somatic and the autonomic components of the reflex erection in spinal males, once the reflex has been initiated.

Neurotrophin signaling in a genitofemoral nerve target organ during testicular descent in mice

Background/AimIt has been proposed that androgens control inguinoscrotal testicular descent via release of calcitonin gene-related peptide (CGRP) from a masculinised genitofemoral nerve (GFN). As there are androgen receptors in the inguinoscrotal fat pad (IFP) during the window of androgen sensitivity (E14-17 in mouse embryos), we tested the hypothesis that neurotrophins in the IFP may masculinise the sensory fibers of the GFN supplying the gubernaculum and IFP prior to gubernacular migration. MethodsAndrogen-receptor knockout (ARKO) and wild-type (WT) mouse embryos were collected at E17, with ethical approval (AEC 734). Sagittal sections of IFP, mammary area and bulbocavernosus (BC) muscle were processed for standard histology and fluorescent immunohistochemistry for ciliary neurotrophic factor (CNTF), ciliary neurotrophic factor receptor (CNTFR) and cell nuclei (DAPI). ResultsIn the ARKO mouse CNTFR immunoreactivity (CNTFR-IR) was increased in the IFP but decreased in BC. Perinuclear staining of CNTF-IR was seen in mouse sciatic nerve but only weakly in IFP. In the mammary area, also supplied by GFN, there were no differences in IR staining. ConclusionThis study found CNTFR-IR in the IFP was negatively regulated by androgen, suggesting that CNTF signaling may be suppressed in GFN sensory nerves to enable CGRP expression for regulating gubernacular migration in the male, but not the female. The indirect action of androgen via the GFN required for testicular descent may be one of the sites of anomalies in the putative multifactorial cause of cryptorchidism.

Nonmyocytic androgen receptor regulates the sexually dimorphic development of the embryonic bulbocavernosus muscle.

The bulbocavernosus (BC) is a sexually dimorphic muscle observed only in males. Androgen receptor knockout mouse studies show the loss of BC formation. This suggests that androgen signaling plays a vital role in its development. Androgen has been known to induce muscle hypertrophy through satellite cell activation and myonuclei accretion during muscle regeneration and growth. Whether the same mechanism is present during embryonic development is not yet elucidated. To identify the mechanism of sexual dimorphism during BC development, the timing of morphological differences was first established. It was revealed that the BC was morphologically different between male and female mice at embryonic day (E) 16.5. Differences in the myogenic process were detected at E15.5. The male BC possesses a higher number of proliferating undifferentiated myoblasts. To identify the role of androgen signaling in this process, muscle-specific androgen receptor (AR) mutation was introduced, which resulted in no observable phenotypes. Hence, the expression of AR in the BC was examined and found that the AR did not colocalize with any muscle markers such as Myogenic differentiation 1, Myogenin, and paired box transcription factor 7. It was revealed that the mesenchyme surrounding the BC expressed AR and the BC started to express AR at E15.5. AR mutation on the nonmyocytic cells using spalt-like transcription factor 1 (Sall1) Cre driver mouse was performed, which resulted in defective BC formation. It was revealed that the number of proliferating undifferentiated myoblasts was reduced in the Sall1 Cre:AR(L-/Y) mutant embryos, and the adult mutants were devoid of BC. The transition of myoblasts from proliferation to differentiation is mediated by cyclin-dependent kinase inhibitors. An increased expression of p21 was observed in the BC myoblast of the Sall1 Cre:AR(L-/Y) mutant and wild-type female. Altogether this study suggests that the nonmyocytic AR may paracrinely regulate the proliferation of myoblast possibly through inhibiting p21 expression in myoblasts of the BC.

Dynamics of male pelvic floor muscle contraction observed with transperineal ultrasound imaging differ between voluntary and evoked coughs

Coughing provokes stress urinary incontinence, and voluntary coughs are employed clinically to assess pelvic floor dysfunction. Understanding urethral dynamics during coughing in men is limited, and it is unclear whether voluntary coughs are an appropriate surrogate for spontaneous coughs. We aimed to investigate the dynamics of urethral motion in continent men during voluntary and evoked coughs. Thirteen men (28-42 years) with no history of urological disorders volunteered to participate. Transperineal ultrasound (US) images were recorded and synchronized with measures of intraabdominal pressure (IAP), airflow, and abdominal/chest wall electromyography during voluntary coughs and coughs evoked by inhalation of nebulized capsaicin. Temporal and spatial aspects of urethral movement induced by contraction of the striated urethral sphincter (SUS), levator ani (LA), and bulbocavernosus (BC) muscles and mechanical aspects of cough generation were investigated. Results showed coughing involved complex urethral dynamics. Urethral motion implied SUS and BC shortening and LA lengthening during preparatory and expulsion phases. Evoked coughs resulted in greater IAP, greater bladder base descent (LA lengthening), and greater midurethral displacement (SUS shortening). The preparatory inspiration cough phase was shorter during evoked coughs, as was the latency between onset of midurethral displacement and expulsion. Maximum midurethral displacement coincided with maximal bladder base descent during voluntary cough, but followed it during evoked cough. The data revealed complex interaction between muscles involved in continence in men. Spatial and temporal differences in urethral dynamics and cough mechanics between cough types suggest that voluntary coughing may not adequately assess capacity of the continence mechanism.

Electrophysiological study of the bulbocavernosus reflex: normative data.

In the clinical setting the bulbocavernosus reflex (BCR) is elicited by squeezing the glans penis and digitally palpating the contraction of the bulbocavernosus (BC) muscle. In neurophysiology the BCR is obtained by stimulating the dorsal nerve of the penis or clitoris and by recording the response from BC muscle and it should be performed in selected patients with suspected urinary, bowel, or sexual neurogenic dysfunction. The BCR is considered one of the sacral neurophysiological tests of the greatest clinical utility. Previous normative data were obtained on small samples. The aim of this study was to determine normative values for the BCR in a large sample of men. We studied a large population (105 men; mean age 53 years, range 19-73 years) without central or peripheral neurological diseases. In each subject the sacral reflex was elicited by electrical stimulation of the base of the dorsum penis and recorded using a surface electrode from the BC muscle. We recorded the latency, calculated at onset, and the maximal amplitude of response, calculated peak to peak. We were able to detect the BCR in all the men. No correlation between BCR latency and age was found (r=0.136; p=0.160). The mean onset latency value was 33.0±4.85 ms (mean±2SD, range 26.8-39.4). The mean amplitude value was 16.53±12.21 μV (mean±2SD, range 4.2-43.6). Our normative data on the BCR were similar to previously published data.

Androgen action at the target musculature regulates brain‐derived neurotrophic factor protein in the spinal nucleus of the bulbocavernosus

We have previously demonstrated that brain-derived neurotrophic factor (BDNF) interacts with testosterone to regulate dendritic morphology of motoneurons in the highly androgen-sensitive spinal nucleus of the bulbocavernosus (SNB). Additionally, in adult male rats testosterone regulates BDNF in SNB motoneurons and its target muscle, the bulbocavernosus (BC). Because BDNF is retrogradely transported from skeletal muscles to spinal motoneurons, we hypothesized that testosterone could regulate BDNF in SNB motoneurons by acting locally at the BC muscle. To test this hypothesis, we restricted androgen manipulation to the SNB target musculature. After castration, BDNF immunolabeling in SNB motoneurons was maintained at levels similar to those of gonadally intact males by delivering testosterone treatment directly to the BC muscle. When the same implant was placed interscapularly in castrated males it was ineffective in supporting BDNF immunolabeling in SNB motoneurons. Furthermore, BDNF immunolabeling in gonadally intact adult males given the androgen receptor blocker hydroxyflutamide delivered directly to the BC muscle was decreased compared with that of gonadally intact animals that had the same hydroxyflutamide implant placed interscapularly, or when compared with castrated animals that had testosterone implants at the muscle. These results demonstrate that the BC musculature is a critical site of action for the androgenic regulation of BDNF in SNB motoneurons and that it is both necessary and sufficient for this action. Furthermore, the local action of androgens at the BC muscle in regulating BDNF provides a possible mechanism underlying the interactive effects of testosterone and BDNF on motoneuron morphology. © 2013 Wiley Periodicals, Inc. Develop Neurobiol 73: 587-598, 2013.

A New Method to Quantify Male Pelvic Floor Displacement From 2D Transperineal Ultrasound Images

To develop a method to quantify displacement of pelvic structures during contraction of the pelvic floor muscles from transperineal ultrasound images in men and investigate the reliability of the method between days. Ten healthy male volunteers (aged 28-41 years) attended 2 separate data collection sessions. Ultrasound images were recorded during voluntary pelvic floor muscle contractions in cine-loop (video) format with the transducer aligned in the midsagittal plane on the perineum. Five anatomic points were defined to represent contraction from striated urethral sphincter (SUS), levator ani (LA), and bulbocavernosus (BC) muscles. Displacement of each point was calculated between the relaxed and contracted-state images. Intraclass correlation coefficient (ICC) values were calculated from displacement data to assess reliability of the method between days. Displacements of the 5 anatomic points closely matched predictions based on anatomic considerations of the male pelvic musculature. ICC values for displacement data calculated from 1, 2, and 3 repetitions ranged between 0.82 and 0.95 for ICC (2,1), 0.85 and 0.97 for ICC (2,2), and 0.86 and 0.97 for ICC (2,3), respectively. The new method reliably calculates displacements of points previously validated for women (ano-rectal junction and bladder base) in addition to new measures of muscle actions (SUS and BC) specific to men. Future use might include assessment of clinical populations to understand how these displacements relate to symptoms of incontinence.

Maintenance of the spinal nucleus of the bulbocavernosus neuromuscular system is not influenced by physiological levels of glucocorticoids

The spinal nucleus of the bulbocavernosus (SNB) neuromuscular system mediates sexual reflexes, and is highly sexually dimorphic in rats. While maintenance of this system in adulthood is mainly dependent on androgens, there is also evidence to suggest that glucocorticoids may have a catabolic effect. We conducted a series of studies to fully examine the influence of basal glucocorticoids on the size of the SNB motoneurons and the associated bulbocavernosus (BC) and levator ani (LA) muscles. Specifically, we examined whether the muscles and motoneurons of the SNB neuromuscular system are affected by: (1) blockade of endogenous glucocorticoids via delivery of the antagonist RU-486 at doses ranging from low to high, (2) removal of endogenous glucocorticoids via adrenalectomy, or (3) restoration of physiological corticosterone levels via implants following adrenalectomy. In each study, we found that muscle and motoneuron size were unaffected by glucocorticoid manipulation. In contrast to previous results with supraphysiological levels of glucocorticoids, our results indicate that basal, nonstress levels of glucocorticoids do not influence the size of the BC/LA muscles or their associated SNB motoneurons.

Characterization of the Spinal Nucleus of the Bulbocavernosus Neuromuscular System in Male Mice Lacking Androgen Receptor in the Nervous System

Motoneurons in the spinal nucleus of the bulbocavernosus (SNB) and their target bulbocavernosus (BC) and levator ani (LA) muscles play a role in male copulation and fertility. Testosterone (T) induces sexual differentiation of this SNB neuromuscular system during development and maintains its activation in adulthood. In the rat, T-induced effects mostly involve the androgen receptor (AR). However, the role of central AR in T-induced effects remains to be studied with pertinent genetic models. We addressed this question by using specific motoneuron immunolabeling and retrograde tracing in mice selectively disrupted for AR in the nervous system. This work reveals that nervous system AR is not required either for T-induced development of BC-LA muscles and perinatal sparing of SNB motoneurons from atrophy or for adult sensitivity of BC-LA muscles to T. By contrast, loss of AR expression in the nervous system resulted in SNB motoneurons having smaller somata and shorter dendrites than controls. We studied the effects of adult castration and T supplementation on SNB cell morphology in control and mutant males; these experiments showed that central AR is involved in the developmental regulation of soma size and dendritic length and in the adult maintenance of soma size of SNB motoneurons. T seemed to act indirectly through BC-LA muscles to maintain dendritic length in adulthood. Our results also suggest that central AR functions may contribute to normal activity of SNB motoneurons and perineal muscles because mutant mice displayed diminished copulatory behavior and fertility.

Bulbocavernosus Muscle Flap for the Repair of Vesicovaginal Fistula

Introduction: Anatomical studies on the bulbocavernosus muscle (BCM) and its supporting blood vessels were performed to explore the clinical roles of BCM flaps in the repair of complicated vesicovaginal fistulas. Methods: BCM and the supporting blood vessels of 15 female adult cadavers were anatomized. Pedicled BCM flaps were transposed to repair complicated vesicovaginal fistulas in 11 patients. Results: The left side of BCMs was 9.1 ± 1.8 cm in length and 1.5 ± 0.2 cm in width, whereas the right side was 9.0 ± 1.9 cm in length and 1.5 ± 0.4 cm in width. No significant difference between the two sides was identified (p > 0.05). The bulbocavernosus (BC) artery forms an angle with the superficial transverse perineal muscle at 64 ± 14° to the left and 68 ± 15° to the right. No significant difference between the two sides was found (p > 0.05). None of the 11 patients reported any postoperative urine leakage in the vagina or urinary incontinence. Follow-ups ranging from 6 months to 10 years found no urinary incontinence or urethral stricture; all patients have normal sexual function. Conclusion: Anatomical studies of BCM and its supporting blood vessels provide useful data for clinical application, especially the reduction of muscle flap death and the increase in the cure rate of complicated vesicovaginal fistulas.

The role of cell death in sexually dimorphic muscle development: Male‐specific muscles are retained in female bax/bak knockout mice

The bulbocavernosus (BC) and levator ani (LA) muscles are present in males but absent or severely reduced in females, and the fate of these muscles controls the survival of motoneurons in the sexually dimorphic spinal nucleus of the bulbocavernosus. However, the mechanism underlying the sex difference in BC and LA development has been controversial. We examined the role of cell death in sexual differentiation of the bulbocavernosus BC/LA muscles in mice. Muscle development was mapped from embryonic day 16 (E16) to postnatal day 5 (P5). A sex difference (male>female) first arose on E17 (BC) or E18 (LA), and increased in magnitude postnatally. TUNEL labeling revealed dying cells in the BC and LA muscles of both sexes perinatally. However, females had a significantly higher density of TUNEL-positive cells than did males. A role for the proapoptotic factors, Bax and Bak, in BC/LA development was tested by examining mice lacking one or both of these proteins. In females lacking either Bax or Bak, the BC was absent and the LA rudimentary. Deletion of both bax and bak genes, however, rescued the BC, increased LA size approximately 20-fold relative to controls, and virtually eliminated TUNEL-positive cells in both muscles. We conclude that cell death plays an essential role in sexual differentiation of the BC/LA muscles. The presence of either Bax or Bak is sufficient for cell death in the BC/LA, whereas the absence of both prevents sexually dimorphic muscle cell death.

The spinal nucleus of the bulbocavernosus: Firsts in androgen-dependent neural sex differences

Cell number in the spinal nucleus of the bulbocavernosus (SNB) of rats was the first neural sex difference shown to differentiate under the control of androgens, acting via classical intracellular androgen receptors. SNB motoneurons reside in the lumbar spinal cord and innervate striated muscles involved in copulation, including the bulbocavernosus (BC) and levator ani (LA). SNB cells are much larger and more numerous in males than in females, and the BC/LA target muscles are reduced or absent in females. The relative simplicity of this neuromuscular system has allowed for considerable progress in pinpointing sites of hormone action, and identifying the cellular bases for androgenic effects. It is now clear that androgens act at virtually every level of the SNB system, in development and throughout adult life. In this review we focus on effects of androgens on developmental cell death of SNB motoneurons and BC/LA muscles; the establishment and maintenance of SNB motoneuron soma size and dendritic length; BC/LA muscle morphology and physiology; and behaviors controlled by the SNB system. We also describe new data on neurotherapeutic effects of androgens on SNB motoneurons after injury in adulthood.

Sexual dimorphism and androgen regulation of satellite cell population in differentiating rat levator ani muscle

The bulbocavernosus (BC) and levator ani (LA) muscles of rats show remarkable androgen-dependent sexual dimorphism. These muscles are additionally of interest because they are thought to indirectly mediate sexual differentiation of innervating spinal motoneurons. This sexual differentiation of the BC/LA is thought to be due to an increase in muscle units in the male rat during the first week after birth. We examined the cellular basis of this differentiation by studying satellite cells in the LA of postnatal day 2.5 rats, when sexual dimorphism is already prominent. Two experiments were performed in which LA satellite cells were measured: (1) wild-type (WT) males were compared with females and to Tfm androgen receptor mutant males, which are androgen insensitive despite producing masculine amounts of testosterone, and (2) females treated prenatally and/or postnatally with testosterone proprionate were compared with females receiving vehicle injections. Our results indicate that WT males have a larger LA and a greater number of satellite cells in the LA muscle than females or Tfm males. However, satellite cell density was similar for all three groups. Prenatal testosterone treatment masculinized LA size and resulted in a corresponding increase in satellite cell populations, while postnatal TP treatment resulted in a tendency for increased satellite cell density without a significant increase in LA size. Taken together, these studies indicate that satellite cells in the neonatal LA muscle are sexually dimorphic, and that this dimorphism likely results from perinatal actions of androgens on androgen receptors.

Sacrolumbar intersegmental reflex circuit in men and its relation to the ejaculatory process

Objective We aimed to investigate electrophysiologically the intersegmental reflex circuit from sacral to lumbar cord segments in normopotent adult men, in patients with spinal cord injury and in patients with premature ejaculation. Methods Reflex EMG activity of the cremasteric (CM) and bulbocavernosus (BC) muscles was recorded simultaneously by needle electrodes during electrical stimulation of the upper lumbar and sacral dermatomes, respectively. Thirty-three healthy male volunteers, 16 patients with spinal cord injury (SCI) at the thoracic or cervical levels, and 26 men with premature ejaculation (PME) were included in the study. Results In controls, upper lumbar dermatomal stimulation (ULS) at the inner side of thigh only elicited a reflex response from the CM muscle and did not produce a regular response from the lower sacral myotomes such as in the BC muscle. However lower sacral dermatomal stimulation (LSS) at the dorsal nerve of penis consistently evoked reflex responses from both CM and BC muscles. These basic electrophysiological features were not different in patients with SCI. LSS did not elicit a reflex response from the CM muscle in about 39% of patients with PME, while the BC reflex was obtained from all patients with PME. Conclusions The neurophysiological pattern in BC and CM muscles during sacral or lumbar dermatomal stimulation reflects the sacrolumbar intersegmental reflex linkage that may be related to the ejaculatory process in men. The intersegmental sacrolumbar reflex circuit may be functionally disturbed in some patients with PME. Significance Interaction between the reflex activity of sacral to lumbar dermatomes could prove useful in defining electrophysiological mechanisms related to ejaculation in men.

Androgen Receptor Expression in the Levator Ani Muscle of Male Mice

The spinal nucleus of the bulbocavernosus (SNB) is a sexually dimorphic group of motoneurones that innervates the bulbocavernosus (BC) and levator ani (LA), skeletal muscles that attach to the base of the penis. In many species, including mice, rats and hamsters, the LA and BC have been found to be highly responsive to androgen and, in rats, these muscles mediate several effects of androgen on the SNB system. However, characterising the SNB system in mice is important because of the availability of genetic models in this species. In the present study, we examined AR expression in skeletal muscles of C57/BlJ6 adult male mice using immunoblotting and immunocytochemistry, comparing the BC/LA to the androgen-unresponsive extensor digitorum longus (EDL). We found similar differences in AR expression for these muscles in the mouse as previously reported for rats. In mice, the BC/LA contains more AR protein than does the EDL. At the cellular level, the LA contains a higher percentage of AR positive myonuclei and fibroblasts than does the EDL. Finally, AR expression is enriched at the neuromuscular junction of mouse LA fibres. The increased expression of AR in the LA compared to the EDL in both muscle fibres and fibroblasts indicates that each cell type may critically mediate androgen action on the SNB system in mice.

Effects of Bax Gene Deletion on Muscle and Motoneuron Degeneration in a Sexually Dimorphic Neuromuscular System

Motoneurons in the spinal nucleus of the bulbocavernosus (SNB) and their target muscles in the perineum, bulbocavernosus (BC), and levator ani (LA) normally degenerate in female rodents. Death of the motoneurons and muscles can be prevented by androgen treatments around the time of birth. To identify the intracellular mechanisms underlying hormone-dependent survival of this neuromuscular system, we examined mice with a targeted disruption of the pro-death gene Bax. SNB motoneuron number was increased in female Bax-/- mice, whether measured using immunolabeling for a motoneuron-specific marker or retrograde labeling with the fluorescent tracer Fluoro-Gold. Based on retrograde tracing, the sex difference in SNB cell number is eliminated in Bax-/- mice. Thus, Bax is required for sexually dimorphic motoneuron death in the SNB, and motoneurons rescued by Bax deletion project their axons to the periphery. Mean soma size in the SNB of Bax-/- females is reduced, however, and there is a subpopulation of very small cells in the SNB of female knock-outs. In addition, the BC muscle was not identified in any female, regardless of Bax gene status. All females possessed a small LA muscle, and Bax deletion resulted in a tripling of LA fiber number in females. This increase was small, however, relative to the >50-fold sex difference in LA muscle fiber number. Thus, the sex difference in the perineal muscles is mostly unaffected by the absence of Bax protein, and SNB motoneuron number is dissociated from target muscle size in Bax-/- animals.

Hormone sensitivity of muscle activation in the sexually dimorphic SNB/BC neuromuscular system of the rat

Rat penile reflexes are mediated in part by motoneurons in the sexually dimorphic spinal nucleus of the bulbocavernosus (SNB) and a muscle it innervates, the bulbocavernosus (BC). Recruitment in the M-wave component of electromyographic recording in the SNB/BC neuromuscular circuit is sensitive to testosterone and estradiol. To localize the site of the hormonal effect, we recorded muscle activity in a reduced preparation that isolated the peripheral structures involved in generating an M-wave. Castration reduced recruitment amplitude and increased response latency, and treatment with estradiol or the non-aromatizable androgen dihydrotestosterone prevented these changes. Dihydrotestosterone, but not estradiol, maintained BC muscle mass. These results indicate that functional changes in the SNB/BC circuit can result in part from hormonal sensitivity in the neuromuscular periphery and are independent of muscle mass.