Neurophysiology and therapeutic receptor targets for stress urinary incontinence

Authors

  • Naoki Yoshimura,

    Corresponding author
    1. Departments of Urology
    2. Pharmacology and Chemical Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA
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  • Minoru Miyazato

    1. Departments of Urology
    2. Division of Urology, Department of Organ-Oriented Medicine, Faculty of Medicine, University of the Ryukyus, Nishihara, Okinawa, Japan
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Naoki Yoshimura M.D., Ph.D., Department of Urology, University of Pittsburgh School of Medicine, Suite 700, Kaufmann Medical Bldg, 3471 Fifth Ave, Pittsburgh, PA 15213, USA. Email: nyos@pitt.edu

Abstract

Stress urinary incontinence is the most common type of urinary incontinence in women. Stress urinary incontinence involves involuntary leakage of urine in response to abdominal pressure caused by activities, such as sneezing and coughing. The condition affects millions of women worldwide, causing physical discomfort as well as social distress and even social isolation. This type of incontinence is often seen in women after middle age and it can be caused by impaired closure mechanisms of the urethra as a result of a weak pelvic floor or poorly supported urethral sphincter (urethral hypermobility) and/or a damaged urethral sphincter system (intrinsic sphincter deficiency). Until recently, stress urinary incontinence has been approached by clinicians as a purely anatomic problem as a result of urethral hypermobility requiring behavioral or surgical therapy. However, intrinsic sphincter deficiency has been reported to be more significantly associated with stress urinary incontinence than urethral hypermobility. Extensive basic and clinical research has enhanced our understanding of the complex neural circuitry regulating normal function of the lower urinary tract, as well as the pathophysiological mechanisms that might underlie the development of stress urinary incontinence and lead to the development of potential novel strategies for pharmacotherapy of stress urinary incontinence. Therapeutic targets include adrenergic and serotonergic receptors in the spinal cord, and adrenergic receptors at the urethral sphincter, which can enhance urethral reflex activity during stress conditions and increase baseline urethral pressure, respectively. This article therefore reviews the recent advances in stress urinary incontinence research and discusses the neurophysiology of urethral continence reflexes, the etiology of stress urinary incontinence and potential targets for pharmacotherapy of stress urinary incontinence.

Abbreviations & Acronyms
5HT =

5-hydroxytryptamine

α1 =

α1-adrenergic receptors

ACh =

acetylcholine

ALPP =

abdominal leak point pressure

AT =

angiotensin receptors

A-URS =

amplitude of urethral pressure response during sneezing

β3 =

β3-adrenergic receptors

CTX =

cardiotoxin

EUS =

external urethral sphincter

ISD =

intrinsic sphincter deficiency

LPP =

leak point pressure

mCPP =

meta-chlorophenylpiperazine

MUCP =

maximal urethral closure pressure

N =

nicotinic

NE =

norepinephrine

NO =

nitric oxide

Pabd =

increase of abdominal pressure

PAG =

periaqueductal gray

PMC =

pontine micturition center

PSC =

pontine storage canter

RLPP =

retrograde leak point pressure

RUPP =

retrograde urethral perfusion pressure

S2–S4 =

sacral segments of the spinal cord

SUI =

stress urinary incontinence

T10–L2 =

thoracolumbar segments of the spinal cord

UBP =

urethral baseline pressure

V-LPP =

Valsalva leak point pressure

VD =

vaginal distention

Introduction

SUI is a major urological problem defined as involuntary loss of urine secondary to an increase in abdominal pressure during events, such as sneezing, coughing or laughing in the absence of bladder contraction. This disorder is especially prominent in women and is a significant urological/gynecological problem. A total of 25% of women older than 20 years-of-age have urinary incontinence, and 50% and 36% of these women have SUI either alone or with urgency incontinence (i.e. mixed incontinence), respectively.1 There is increasing risk of SUI at or after middle age, and the prevalence of SUI peaks during perimenopausal years.1 Other studies have also shown that SUI occurs in 37.7% of non-institutionalized women older than 60 years-of-age,2 and in 33.9% of women older than 40 years-of-age.3 SUI also imposes substantial costs both on the individual and the society in the USA and worldwide4–6 with treatment costs being estimated to be up to $16 billion annually in the USA alone.7 With the continued aging of the populations in all developed countries, the problems associated with SUI will certainly continue to increase. Although the etiology of SUI in women seems to be multifactorial, vaginal childbirth and pelvic trauma have been shown to have major impacts on the development of SUI.8,9

Two main pathophysiological conditions are found in women with SUI: (i) loss of bladder neck/proximal urethra support, also known as urethra hypermobility; and (ii) intrinsic sphincter deficiency, characterized by malfunction of the urethral sphincter mechanism resulting in the low-pressure urethra.10,11 These conditions often coexist, but intrinsic sphincter deficiency has been reported to be more prominent than urethral hypermobility in patients with SUI, and contributes significantly to the emergence of SUI.10,12–16 Our or other recent basic studies using SUI animal models, such as rats with VD, which simulates birth trauma, also supports this theory.17,18

Thus, the present review article focuses on neurophysiological mechanisms in the control of lower urinary tract function and their alterations that contribute to the SUI condition, and discusses potential pharmacological approaches for correcting SUI.

Neurophysiology of the lower urinary tract

The lower urinary tract is composed of the bladder and the urethra – the two functional units for storage (the bladder body, or reservoir) and elimination (the bladder neck and urethra, or outlet) of urine. The wall of the bladder body is lined with bundles of intertwining smooth muscle fibers, which comprise the detrusor. The smooth muscles lining the bladder neck and the urethra form the internal sphincter, which is surrounded by striated muscle called the rhabdosphincter. Together, the periurethral striated muscle – striated muscle fibers surrounding the urethra – and the rhabdosphincter constitute the EUS.

The bladder and urethra function reciprocally. As the bladder fills during the urine storage phase, the detrusor remains quiescent, with a little change in intravesical pressure, adapting to the increasing volume by increasing the length of its muscle cells. Furthermore, neural pathways that stimulate the bladder for micturition are quiescent during this phase, and inhibitory pathways are active.19–22 The urethral outlet remains closed, with progressively increasing EUS contractions; this progressive increase in EUS activity in response to increasing bladder volume is known as the guarding reflex. When the bladder volume reaches a critical threshold, the EUS relaxes, detrusor muscles engage in a series of contractions, the bladder neck opens and urine elimination occurs.

The lower urinary tract is innervated by parasympathetic, sympathetic, and somatic peripheral nerves that are components of intricate efferent and afferent circuitry derived from the brain and the spinal cord. The neural circuits act as an integrated complex of reflexes that regulates micturition, allowing the lower urinary tract to be in either a storage or elimination mode. In persons with urinary incontinence, some aspect of the system is dysfunctional and urine leakage occurs during the storage phase.23,24

Efferent pathways

The smooth muscles of the bladder – the detrusor – are innervated primarily by parasympathetic nerves, whereas those of the bladder neck and urethra – the internal sphincter – are innervated by sympathetic nerves. The striated muscles of the EUS receive their primary innervation from somatic nerves (Figs 1,2).

Figure 1.

Major preganglionic and postganglionic neural pathways from the spinal cord to the lower urinary tract. The sympathetic hypogastric nerve, emerging from the inferior mesenteric ganglion, stimulates urethral smooth muscle. The parasympathetic pelvic nerve, emerging from the pelvic ganglion, stimulates bladder detrusor muscle and inhibits urethral smooth muscle. The somatic pudendal nerve stimulates striated muscle of the EUS.

Figure 2.

Innervation of the lower urinary tract: The parasympathetic pelvic nerve stimulates the bladder detrusor muscle, mediated by muscarinic receptors (M3) being activated by ACh. The sympathetic hypogastric nerve stimulates urethral smooth muscle and inhibits bladder detrusor, mediated by α-adrenergic and β3-adrenergic receptors, respectively. The somatic pudendal nerve stimulates striated muscle of the external urethral sphincter, mediated by ACh activating N receptors. Plus and minus signs indicate neural stimulation and inhibition, respectively.

Parasympathetic nerves

The efferent parasympathetic pathway provides the major excitatory innervation of the detrusor.22,24 Preganglionic axons emerge, as the pelvic nerve, from the sacral parasympathetic nucleus in the intermediolateral column of sacral spinal segments S2 to S4, and synapse in the pelvic ganglia, as well as in small ganglia on the bladder wall, releasing ACh. Excitation of postsynaptic neurons by ACh is mediated by nicotinic receptors. Postganglionic axons continue for a short distance in the pelvic nerve and terminate in the detrusor layer, where they release ACh to induce contractions of the smooth muscle fibers of the detrusor. This stimulatory effect of ACh at the postganglionic axon terminal is mediated by muscarinic receptors in detrusor cells. Two muscarinic subtypes, M2 and M3, are known to be present in the bladder; although M2 is most abundant in detrusor cells, the M3 subtype is the major receptor mediating stimulation of detrusor contractions.22,25,26

In addition to the parasympathetic stimulation of bladder smooth muscle, some postsynaptic parasympathetic neurons exert a relaxation effect on urethral smooth muscle, most likely through transmission of NO.19,22,26,27 Thus, as the bladder contracts during the elimination phase, the internal urethral sphincter relaxes.

Sympathetic nerves

Sympathetic nerves stimulate smooth muscle contraction in the urethra and bladder neck, and cause relaxation of the detrusor. Preganglionic sympathetic neurons are located in the intermediolateral column of thoracolumbar cord segments T10 to L2.22,26 Most of the preganglionic fibers synapse with postganglionic neurons in the inferior mesenteric ganglia. The preganglionic neurotransmitter is ACh, which acts through nicotinic receptors in the postganglionic neurons. Postganglionic axons travel in the hypogastric nerve and release NE at their terminals. The major terminals are in the urethra and bladder neck, as well as in the bladder body. NE stimulates contraction of urethral and bladder neck smooth muscle through α1-adrenoceptors, and causes relaxation of detrusor through β2-adrenoceptors and β3-adrenoceptors, the latter being most predominant.28

Somatic nerves

Somatic nerves provide excitatory innervation to the striated muscles of the EUS and pelvic floor. The efferent motoneurons are located in Onuf's nucleus, along the lateral border of the ventral horn in sacral spinal cord segments S2–S4.22,29 The motoneuron axons are carried in the pudendal nerve and release ACh at their terminals. The ACh acts on nicotinic receptors in the striated muscle, inducing muscle contraction to maintain closure of the EUS.22,29,30

Afferent pathways

The pelvic, hypogastric and pudendal nerves carry sensory information in afferent fibers from the lower urinary tract to the lumbosacral spinal cord.24,29,31–33 The somata of the pelvic and pudendal afferent nerves are located in dorsal root ganglia at sacral segments S2–S4 and the somata of the hypogastric nerve in dorsal root ganglia at thoracolumbar segments T11–L2. After entering the spinal cord, the primary afferent fibers of the pelvic and pudendal nerves travel rostrally in Lissauer's tract. Sensory information is transmitted to second-order neurons in the spinal cord and then to the PAG in the midbrain.34

Overview: Interacting reflexes of urine storage phase

Until the volume of urine in the bladder reaches a critical threshold for voiding, the detrusor is quiet, the bladder having low and relatively constant levels of internal pressure during filling.24 This is to some extent achieved passively: (i) the intrinsic viscoelasticity of detrusor muscles permits the bladder wall to adjust to increasing volume by stretching; and (ii) the stimulatory parasympathetic pathway is quiescent. Also, there are major neurogenic contributions toward maintaining an inactive bladder during the storage phase (Fig. 3).24,35–37

Figure 3.

Major reflex pathways to the lower urinary tract initiating and maintaining urine storage. Plus and minus signs indicate neural stimulation and inhibition, respectively.

The bladder-to-EUS reflex, the guarding reflex, is initiated by distension of the bladder during filling, which activates stretch-sensitive mechanoceptors in the bladder wall, in turn generating afferent signals to the sacral spinal cord where pudendal motoneuron efferents are activated. The pudendal efferent nerves stimulate EUS contractions, thereby maintaining outlet resistance and urinary continence. The guarding reflex increases in intensity as the bladder volume increases (Fig. 3). This reflex is also important to prevent urine leakage during stress conditions that increase abdominal pressure.24,38

The bladder-to-sympathetic pathway reflex is also triggered by bladder distension: stimulated bladder afferents activate an intersegmental pathway from the sacral cord to the thoracolumbar sympathetic nerves. The activated sympathetic nerves stimulate contraction of the internal urethral sphincter and inhibit bladder activity (Fig. 3).35

The PMC and PSC are the final integrative centers, receiving and integrating input from ascending spinal cord nerves and more rostral brain regions, and controlling an on/off switch for the lower urinary tract. Neurons in the PSC project directly to the motoneurons in Onuf's nucleus, and stimulation of PSC neurons causes EUS contractions. Neurons in the PMC project to the sacral parasympathetic nucleus, and stimulation of PMC neurons results in bladder contractions, as well as relaxation of the internal urethral sphincter and EUS (Fig. 3).

Glutamic acid, the major excitatory neurotransmitter in the central nervous system, appears to function as the on switch for the EUS; while the pudendal nerve is receiving excitatory glutamatergic transmission, EUS contractions continue and the lower urinary tract remains in the storage mode. Suppression of glutamatergic transmission serves as the final signal for EUS relaxation and bladder elimination (Fig. 3).

Numerous other supraspinal neurotransmitters have modulatory roles in lower urinary tract function. Two that appear to have a positive neuromodulatory effect on EUS contractions are NE and serotonin.22,26,39–42 Onuf's nucleus is densely innervated by serotonergic and noradrenergic terminals, largely derived from neurons in the raphe nucleus and locus coeruleus in the brainstem, respectively (Fig. 3). Rajaofetra et al. showed that, in baboons with transected spinal cords, NE had disappeared from Onuf's nucleus; however, some serotonin remained, showing a more local source for a portion of this neurotransmitter.42

The reflexes involved in urine storage are predominantly integrated in the spinal cord and seem to function normally in animals with supraspinal transection. However, voluntary control of micturition is impaired in many patients with lesions that interrupt brain stem pathways. Thus, although initiation of the storage phase seems to be established within the spinal cord, maintaining a stable urethral resistance apparently requires supraspinal input (Fig. 3).24,30

Mechanisms inducing SUI in women: Clinical evidence

Urine leakage in SUI occurs when bladder pressure is higher than urethral pressure. It has been postulated that pregnancy- and childbirth-associated injuries to muscles, connective tissues and nerves, as well as aging, menopause (estrogen deficiency) and obesity, are potential risk factors of SUI.43 Although it is not fully elucidated how these risk factors induce SUI, two different pathophysiological conditions, such as insufficient functioning of the urethral sphincter (ISD) and/or urethral hypermobility have been proposed as major causes of SUI. Both pathologies (ISD and urethral hypermobility) reportedly coexist to at least some extent in the majority of women with SUI.44,45 Although it is generally believed that a loss of urethral support is the primary factor in SUI because of the success of widely-used midurethral sling operations for SUI that improve urethral support, recent studies showed that impaired urethral closure evidenced by a reduction in MUCP is the factor most strongly associated with SUI.12–14 In DeLancey's studies, MUCP is the parameter that differs the most between groups, with 43% lower MUCP in women with SUI compared with asymptomatic volunteers.12,13 There is also wide variation in MUCP in healthy continent women. For example, in the third decade, MUCP averages 92 cm H2O, ranging from approximately 70 to 115 cm H2O; but by the seventh decade, MUCP is decreased to an average of 37 cm H2O with only few values exceeding 50 cm H2O.15 In contrast, urethral hypermobility as assessed by Q-tip testing angle does not achieve a statistically significant association with V-LPP in women with SUI who underwent Burch bladder neck suspension or autologous rectus fascial sling procedures.16 Overall, the incompetence of urethral closure function (i.e. ISD), which is more common and prominent than previously thought, significantly contributes to the emergence of SUI.

Neural mechanisms preventing SUI: Clinical studies

The mechanisms that maintain urinary continence during elevation of abdominal pressure include both passive and active closure of the urethra. A passive closure mechanism involves the essentially simultaneous transmission of intra-abdominal pressure to the urinary bladder and proximal urethra, and has been considered to play an important role in urinary continence.46 Although the MUCP value corresponds to urethral closure pressure at rest, the active, neurally-mediated urethral closure during abdominal stress conditions, such as coughing, act as another important mechanism to prevent SUI. First, urethral pressure reportedly increases before cough transmission.47,48 Second, the urethral pressure increase during coughing exceeds the increase in bladder pressure.48 Third, the urethral closure pressure during coughing is significantly reduced by bilateral pudendal nerve blockade.49 Furthermore, this active urethral closure mechanism is damaged in SUI patients, as evidenced by lower cough-induced LPP,50 disappearance of the pressure increment in the urethra preceding cough,47,48 decreased number of fast-twitch type II muscle fibers in the pelvic floor,51 partial denervation in pelvic floor musculature because of pudendal neuropathy,52,53 decreased electromyography activity of the striated urethral sphincter,54 and a loss of voluntary increases in urethral pressure.55 Overall, impaired urethral closure function at rest and during stress conditions could be involved in the pathphysiological basis of SUI and impaired urethral closure function (i.e. ISD).

Different modes of urinary continence reflexes: Animal studies

Bladder afferent-dependent reflex

Recent studies in rats using microtransducer-tipped catheters and nerve transection techniques further clarified the detailed mechanisms underlying the active urethral closure reflex during stress conditions.56,57 Passive increases in intravesical pressure by manual bladder compression or a bladder pressure clamping method, which activate afferent pathways in the pelvic nerve, can elicit reflex contractile responses of smooth and striated muscles in the proximal to mid-urethra through efferent pathways in autonomic and somatic nerves (the hypogastric nerve, the pudendal nerve and the nerves to pelvic floor muscles) in rats.57,58 In the LPP measurement, electrical stimulation of abdominal muscles elevated intravesical pressure in a stimulus intensity-dependent manner, and intravesical pressure elevation was almost lost when the abdomen was opened.59 The LPP was lowered in rats whose pelvic nerves or somatic nerves were cut bilaterally, whereas transection of bilateral hypogastric nerves showed smaller effects.59 Based on these findings, it is concluded that the bladder afferent-induced reflex urethral closure mechanisms prevent urinary leakage during a stress condition that momentarily increases intravesical pressure (Fig. 4a).

Figure 4.

Hypothetical schemas of two urethral continence reflexes preventing stress urinary incontinence. (a) Bladder afferent-dependent reflex. Activation of afferent pathways in the pelvic nerve due to intravesical pressure elevation can elicit reflex contractile responses of smooth and striated muscles in the proximal to mid-urethra through efferent pathways in the hypogastric nerve and the pudendal nerve. (b) Sneeze-induced reflex. Spinal descending signals directly enhance Onuf's nucleus, thereby maintaining and augmenting the active urethral closure mechanisms. Sneezing-induced continence reflex can activate pudendal nerves and nerves to pelvic floor muscles (iliococcygeous/pubococcygeous muscles).

Sneeze-induced direct reflex

Another type of the neurally-mediated urethral continence reflex has been studied during the sneeze reflex in rats. The sneeze reflex, which is a highly coordinated reflex evoked by irritation of the nasal mucosa, serves to remove irritations, and clean the airway. It has been shown that the urethral closure mechanism activated during sneezing was mediated by somatic nerves (pudendal nerves and nerves to pelvic floor muscles) and was crucial for preventing urinary leakage during sneezing in rats.56 Because this urethral closure response during sneezing was not affected by opening the abdominal cavity or by bilateral transection of pelvic nerves and hypogastric nerves,56 the sneeze-induced continence mechanism is likely to be activated directly by sneezing, but not by activation of afferent pathways from the bladder (Fig. 4b). At the bladder neck, on the contrary, the urethral pressure changes on sneezing were very similar to those in the bladder, and were abolished on opening the abdomen, suggesting that “abdominal pressure transmission” does play an important part in bladder neck closure.56

Thus, there seems to be at least two different urethral closure mechanisms to prevent SUI. It might be possible to assume that bladder-to-urethral continence reflexes induced by passive intravesical pressure elevation can be activated during abdominal pressure rises induced by Valsalva-like stress conditions, such as laughing, jogging or lifting heavy objects, and that another continence reflex can additionally be recruited by even stronger, phasic stress conditions, such as sneezing or coughing.46

Animal models of SUI

Animal models of human diseases are essential for understanding the clinical pathophysiology and developing new and effective modalities for the treatment of the diseases. SUI is no exception, and various animal models of SUI have been developed to study the pathophysiological process involved in SUI.18,34,60

Vaginal distention

SUI after vaginal childbirth is common, as approximately 30% of mothers become urinary incontinent after their first vaginal delivery. However, it is usually short-lived in most women;61 despite that, long-term, insidious damage of pelvic floor nerves and musculature induced by pregnancy or parturition remains,62,63 leading to the emergence of SUI during the perimenopausal years in women who have given birth to their children at an earlier age. Thus, rats with VD have been used to simulate the maternal injuries of childbirth to study the pathophysiology of SUI. A Foley balloon catheter is inserted into the rat's vagina and the balloon is inflated to distend the vagina and produce the damage of surrounding organs and tissues, including the urethra.64 VD results in decreased blood flow to the urethra, and hypoxia of the bladder, urethra and vagina, suggesting that hypoxic injury is a possible mechanism of urethral damage leading to SUI.65

Previous studies have also shown that in a VD rat model, the active urethral continence reflex induced by sneezing is impaired, resulting in sneeze-induced SUI.17 It has also been reported that using an ex vivo system, which can assess activity of the excised whole urethra using a laser micrometer in rats, showed that VD alters urethral basal tone, proximal urethral compliance, distal stiffness and adrenergic responses. Lack of basal smooth muscle tone in the proximal and middle urethra might contribute to SUI induced by VD.66,67 Thus, these studies confirm that, similar to humans, urethral closure mechanisms participate in the urinary continence in rats and that impairment of both neural and muscular components in the urethra contribute to the emergence of SUI after VD. Therefore, VD can simulate the postchildbirth urethral dysfunction, which is mainly a result of impaired urethral closure mechanisms rather than urethral hypermobility. Also, as in humans, SUI after VD is recovered after 20 days in female mice.68 They have reported that recovery of continence function after VD is associated with a repair of EUS activity and reinnervation of the urethra.

Pudendal nerve injury (crush or transection)

Partial denervation in urethral and/or pelvic floor musculature as a result of pudendal nerve neuropathy is often found in women with SUI.52,53 Similarly, rats with bilateral pudendal nerve transection showed a reduction in LPP, providing value as a SUI animal model.69,70 It has also been shown that pudendal nerve transection causes electromyographic abnormalities of the EUS, and striated muscle atrophy in the EUS.69 Furthermore, a recent study showed that there is a compensatory mechanism in urethral smooth muscles that counteracts EUS deficiency after pudendal nerve transection, because decreased LPP was recovered 4 weeks after pudendal nerve transection owing to upregulation of α1-adrenoceptor and muscarinic receptor-mediated contractility of the proximal urethra in female rats.71 In addition to pudendal nerve transection, Damaser et al. developed an animal model of SUI using pudendal nerve crush.72 The pudendal nerve is located in the ischiorectal fossa and the pudendal nerve was crushed twice for 30 s bilaterally just proximal to the branch point of the obturator nerve.

Electrocauterization

Chermansky et al. reported the use of electrocauterization of the urethra as a long-lasting SUI model in rats.73 This electrocauterization model had lowered LPP (approximately 50% of control values in sham-operated rats) that was maintained for up to 16 weeks. Histological findings showed muscle disruption of the EUS and denervation in the mid-urethra 16 weeks after electrocauterization.

Urethrolysis

Rodriguez et al. developed a SUI model in rats by using transabdominal urethrolysis.74 Following a lower abdominal incision, the proximal and distal urethra was detached circumferentially by incising the endopelvic fascia and detaching the urethra from the anterior vaginal wall and pubic bone by sharp dissection. Kinebuchi et al. reported another SUI model in rats with more damage to the urethra.75 They combined urethrolysis with an injection of a myotoxin CTX, which is widely used to induce experimental damage of skeletal and cardiac muscle, into the distal urethra under the pubic bone. CTX action is reversible, and therefore they carried out urethrolysis in conjunction with the injection of CTX to produce damage directly to the EUS.

Periurethral injection of botulinum-A toxin

Takahashi et al. injected botulinum-A toxin periurethrally at the mid-urethra to create a rat SUI model.76 Because periurethral botulinum-A toxin injection does not require a laparotomy, it is an easy and useful model for inducing SUI as a result of impaired EUS function.

Ovariectomy

A recent study reported that urethral baseline pressure and the amplitude of urethral response during sneezing are significantly decreased in 6-week ovariectomized rats, and that estrogen replacement increases urethral baseline pressure, but not urethral response amplitudes during sneezing, leading to a decrease in the incidence of SUI.77 Therefore, ovariectomized rats could be a useful animal model to study the pathophysiology of SUI as a result of estrogen deficiency that occurs in peri- and postmenopausal women.

Pubo-urethral ligament transection

Kefer et al. introduced a rat model of SUI as a result of a loss of periurethral support by transecting the pubo-urethral ligament.78 These rats showed significantly decreased LPP compared with the sham control rats at 4 days and 10 days after the procedure. Pubo-urethral ligament transection might simulate urethral hypermobility in women with SUI.

Retroflexed bladder

Kawamorita et al. reported a rat SUI model with the retroflexed bladder that mimics the loss of the posterior urethrovesical angle often associated with SUI.79 The retroflexed bladder is simply created by stitching the bladder posteriorly to the psoas muscle. In rats with the retroflexed bladder, both urethral pressure responses using microtransducer-tipped catheters and LPP during sneezing are significantly decreased. This model could be used to explain why sling operation without tension can improve SUI.

Functional assessment of animal models

LPP measurements

LPP measurements are the most commonly used methods to evaluate the urethral dysfunction in SUI animal models. LPP can be tested by three different methods: sneezing (sneeze-LPP), the Crede maneuver (Crede-LPP) and the vertical tilt table method (tilt-LPP).58

Sneeze-LPP measurement

The sneeze reflex is induced to examine whether fluid leakage from the urethral orifice occurs during sneezing while intravesical pressure changes are recorded in rats (Fig. 5a).80 No urine leakage is observed during sneezing in normal rats; however, sneeze-induced leakage is seen in SUI rat models induced by VD81–83 or ovariectomy.77 The lowest pressure value that induces fluid leakage from the urethral orifice is defined as the sneeze-LPP.81–83

Figure 5.

Measurements of urethral pressure responses during sneezing using a microtransducer-tipped urethral catheter. (a) A 3.5-Fr-size nylon catheter with a side-mounted microtransducer (white arrow) located 1 mm from the catheter tip, which is inserted into the urethra from the urethral orifice. (b) A-URS, UBP and Pabd during sneezing. (a) Urethral pressure response. (b) Abdominal pressure.

Crede-LPP measurement

Manual abdominal compression (the Crede maneuver), in which the abdomen is slowly depressed manually, usually with cotton swabs or fingers to increase the abdominal pressure.57,71,84 Pressure on the abdomen is immediately removed when fluid leakage occurs at the urethral meatus. Intravesical pressure is recorded while the LPP is tested, and the lowest peak pressure is taken as the Crede-LPP.

Tilt-LPP measurement

LPP is evaluated by the pressure clamp method on a vertical tilt table.9 With the rat hanging vertically, intravesical pressure is clamped by connecting the bladder catheter to a saline reservoir and a pressure transducer. The reservoir is then elevated along a pole in 2-cm increases and the lowest fluid leakage from the urethral orifice is defined as the tilt-LPP (Fig 5b).

Microtransducer-tipped catheter measurements of urethral pressure responses induced by sneezing

The sneeze reflex is induced and the A-URS and the UBP is measured using a microtransducer-tipped catheter inserted into the middle urethra in female rats.56,57,81–83 Intensity of the induced sneeze is evaluated as pressure increases in the abdomen during sneezing through an intra-abdominal balloon catheter inserted through the rectal wall. It has been reported that urethral pressure elevation during sneezing (i.e. A-URS) is mediated by striated muscle contraction of the external urethral sphincter and the pelvic floor as a result of activation of the pudendal nerves and the somatic nerves innervating pelvic floor muscles, respectively.56,57 In contrast, UBP seems to predominantly reflect smooth muscle activity of the urethra because the increase in UBP was blocked by α-adrenoceptor antagonists such as prazosin or hypogastric nerve transection in our pervious study.81 Thus, microtransducer-tipped catheter measurements of urethral activity can allow the investigation of the striated and smooth muscle activity of the urethra, separately (Fig. 6).

Figure 6.

(a) Sneeze LPP and (b) tilt LPP. (a) A polyethylene catheter connected to a pressure transducer is inserted into the bladder through the dome for recording intravesical pressure to detect LPP during sneezing. (b) The LPP is measured with the rat in the vertical position and connected to a cystostomy catheter (adapted from Conway et al.58 with permission).

RUPP or RLPP testing

RUPP or RLPP testing measures the urethral resistance by using pressure-monitored retrograde sphincterometry.74 A PE-50 tubing is placed just inside the urethral meatus. The urethra is sealed by applying a vascular clamp around the distal urethra. Then, saline is retrogradely infused through the urethral catheter. The RLPP or RUPP is the pressure required to achieve and maintain an open urethral sphincter,74 which corresponds to the plateau pressure on the urethral pressure tracing in humans.85

Potential targets of pharmacotherapy

SUI has been considered to be induced primarily by a loss of urethral support as a result of local anatomic damage, and patients were, and still are, treated mainly with physical approaches, which include conservative techniques, such as pelvic floor muscle training, biofeedback and intravaginal urethral compression devices. Periurethral bulking agents and retropubic suspension procedures are among the more invasive physical approaches for correcting SUI conditions. However, recent studies showed that impaired urethral closure mechanisms evidenced by a reduction in MUCP is the factor most strongly associated with SUI, indicating that targeting the urethral closure mechanisms is a desired therapeutic modality for SUI rather than urethral support corrections.12–14 In this regard, there has been a dearth of novel pharmacological strategies for increasing urethral resistance. Two potential targets for SUI therapy that have recently drawn considerable attention are serotonin and NE, which are released at Onuf's nucleus from spinal descending nerves originating from the raphe nucleus and locus coeruleus, respectively.22,26,39–42

A search for effective and well-tolerated drugs for treatment of SUI has shown clinical efficiency of NE and/or serotonin (5HT) reuptake inhibitors, such as duloxetine, in patients with SUI.86–89 In urodynamic studies using urethral pressure measurements, duloxetine reportedly enhances the EUS contraction in responses to transcranial magnetic stimulation or sacral root magnetic stimulation, suggesting that duloxetine can strengthen the neurally-evoked urethral closure mechanisms.90,91 The Onuf's nucleus in the sacral spinal cord, which sends somatic inputs to EUS and pelvic floor muscles, is densely innervated by serotonergic and noradrenergic terminals, derived in large part from neurons in the raphe nucleus and locus coeruleus, respectively (Fig. 7). NE and 5HT can have a stimulatory effect on EUS activity92–94 through interaction with glutamic acid, which is a primary excitatory neurotransmitter for rhabdosphincter motor neurons41,88,95 (Fig. 7). It has also been reported using whole-cell patch-clamp recordings that NE directly increases the excitability of EUS motoneurons through multiple mechanisms in isolated spinal cord slices prepared from neonatal female rats.96

Figure 7.

Hypothetical schema shows the roles of noradrenergic and serotonergic pathways in sneeze-induced urethral continence reflexes. (a) At the spinal level. Descending signals of bulbo-spinal noradrenergic and serotonergic pathways enhance activity of spinal excitatory interneurons (e.g. glutamatergic neurons) that synapse with motoneurons in Onuf's nucleus (dashed lines) or directly activate motoneurons in Onuf's nucleus through α1-adrenergic and 5HT2C receptors, thereby maintaining the sneeze-induced active urethral closure mechanisms, while α2-adrenergic or 5HT1A receptor stimulation can directly or indirectly inhibit activity of Onuf's nucleus. Sympathetic preganglionic neurons also receive facilitatory inputs from both spinal noradrenergic and serotonergic pathways to enhance the sympathetic outflow to urethral smooth muscle and increase UBP. (b) At the peripheral level. Activation of peripheral α1-adrenoceptors in urethral smooth muscles increases UBP. Plus and minus signs indicate neural stimulation and inhibition, respectively (adapted from Miyazato et al.83 with permission).

The roles of noradrenergic and serotonergic systems in the urethral continence have recently been investigated in rats using microtransducer-tipped catheters to elucidate whether activation of 5HT and/or NE pathways can enhance the urethral continence reflex during sneezing in a rat model of SUI. The results indicated that (i) duloxetine and nisoxetine, a NE reuptake inhibitor, can enhance the sneeze-induced active urethral closure mechanism at the spinal level and augment urethral baseline pressure in the periphery through α1-adrenoceptors, thereby improving sneeze-induced SUI; and (ii) synergic activation of spinal noradrenergic and serotonergic systems is important to maintain the urethral continence reflex during sneezing.81,82 In contrast, activation of spinal α2-adrenoceptors suppresses EUS contractions during abdominal compression or sneezing,97,98 and α2-adrenoceptor blockade by idazoxan potentiated the effect of duloxetine on sneeze induced urethral continence reflex in rats.98 The latter finding supports the concept that combination therapy with α2-adrenoceptor antagonists might be an effective, novel strategy to reinforce the clinical efficacy of serotonin and norepinephrine reuptake inhibitors, such as duloxetin for SUI.

A recent study has also clarified 5HT receptors subtypes that contribute to the modulation of the sneeze-induced continence reflex.83 The results showed that intrathecally applied 8-OH-DPAT (a 5HT1A agonist) decreases urethal contractile responses during sneezing by 48.9% and that mCPP (a 5HT2B/2C agonist) increases them by 33.6%, whereas fluoxetine (a serotonin reuptake inhibitor) did not change the responses. The effects of 8-OH-DPAT and mCPP are antagonized by intrathecal applications of WAY-100635, a selective 5HT1A antagonist, and RS-102221, a selective 5HT2C antagonist, respectively. These results indicate that activation of 5HT2C receptors enhances the active urethral closure reflex during sneezing at the spinal level, whereas 5HT1A receptors inhibit it, and that no apparent changes in the sneeze-induced continence reflex after fluoxetine treatment could be a result of coactivation of excitatory 5HT2C receptors and inhibitory 5HT receptors.

Overall, noradrenergic and serotonergic pathways modulate the active urethral continence reflex during sneezing (Fig. 7).81–83,98 Activation of α1-adrenoceptors or 5HT2C receptors enhances the reflex at the spinal cord level, whereas pre- or postsynaptic α2-adrenoceptors and/or 5HT1A receptors inhibit the reflex. Thus, sneeze-induced urethral sphincter reflexes seem to be regulated by a complex balance of facilitatory α1-adrenergic and 5HT2C, and inhibitory α2-adrenergic and 5HT1A receptors. The site of the action of reuptake inhibitors in the spinal cord is probably at Onuf's nucleus, where dense NE- and 5HT-containing terminals and urethral rhabdosphincter motor neurons are located.42,99 Descending signals in the bulbo-spinal noradrenergic and serotonergic pathways might enhance excitatory interneurons, such as glutamatergic neurons,97,99 or directly act on motoneurons located at Onuf's nucleus,96 thereby augmenting the active urethral closure mechanisms. Thus, activation of the noradrenergic system through α1-adrenoceptors could be a promising approach, which can prevent SUI by promoting the descending signals in bulbospinal noradrenergic pathways to strengthen the sneeze-induced continence reflex at the spinal cord level while increasing urethral baseline activity at the peripheral level. In addition, as increased 5HT concentrations at nerve terminals can stimulate both excitatory and inhibitory 5HT receptors, resulting in the negligible effects on urethral continence reflexes, direct activation of excitatory 5HT receptor subtypes, such as 5HT2C, could be more effective than 5HT reuptake inhibitors in enhancing the urethral continence reflex for the treatment of SUI (Fig. 7).

Other pharmacological targets for enhancing the urethral continence mechanism include angiotensin II and vasopression receptors. A previous study has reported that inhibition of angiotensin II receptors (AT-1 and AT-2) reduced RLPP and ALPP values in female rats, and that angiotensin II administration restored RLPP and ALPP to baseline presurgical values in rat models of SUI induced by pudendal nerve injury or circumferential urethrolysis.100 Therefore, activation of AT-1 and/or AT-2 receptors by angiotensin II could be effective to improve the urethral tone to reduce SUI. More recently, Ueno et al.101 reported that vasopressin V1A receptor mRNA is highly expressed among the vasopressin receptor family in Onuf's nucleus in the rat spinal cord L6–S1 segment, and that intrathecally injected Arg8-vasopressin induces a significant increase in urethral resistance during intravesical pressure elevation, which is blocked by a V1A receptor antagonist. These results show that vasopressin V1A receptor activation could be effective for treating SUI.

Taken together, therapeutic targets for SUI can be found in the spinal and peripheral nervous system, and the pharmacological approaches enhancing the active, neurally mediated continence reflexes would be effective for the treatment of SUI.

Acknowledgment

The authors' research has been supported by National Institutes of Health grants (DK067226, AR049398, HD061811 and DK055387).

Conflict of interest

None declared.

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