The role of capsaicin-sensitive C-fiber afferent pathways in the control of micturition in spinal-intact and spinal cord-injured mice

Sexual Dimorphism of Spinal Neural Circuits Controlling the Mouse External Urethral Sphincter With and Without Spinal Cord Injury

Spinal cord injury (SCI) disrupts coordination between the bladder and the external urinary sphincter (EUS), leading to transient or permanent voiding impairment, which is more severe in males. Male versus female differences in spinal circuits related to the EUS as well as post-SCI rewiring are essential for understanding of sex-/gender-specific impairments and possible recovery mechanisms. To quantitatively assess differences between EUS circuits in males versus females and in spinal intact (SI) versus SCI animals, we retrogradely traced and counted EUS-related neurons. In transgenic ChAT-GFP mice, motoneurons (MNs), interneurons (INs), and propriospinal neurons (PPNs) were retrogradely trans-synaptically traced with PRV614-red fluorescent protein (RFP) injected into EUS. EUS-MNs in dorsolateral nucleus (DLN) were separated from other GFP+ MNs by tracing them with FluoroGold (FG). We found two morphologically distinct cell types in DLN: FG+ spindle-shaped bipolar (SB-MNs) and FG- rounded multipolar (RM-MNs) cholinergic cells. Number of MNs of both types in males was twice as large as in females. SCI caused a partial loss of MNs in all spinal nuclei. After SCI, males showed a fourfold rise in the number of RFP-labeled cells in retro-DLN (RDLN) innervating hind limbs. This suggests (a) an existence of direct synaptic interactions between spinal nuclei and (b) a post-SCI increase of non-specific inputs to EUS-MNs from other motor nuclei. Number of INs and PPNs deferred between males and females: In SI males, the numbers of INs and PPNs were ∼10 times larger than in SI females. SCI caused a twofold decrease of INs and PPNs in males but not in females.

Chronic chemogenetic inhibition of TRPV1 bladder afferent promotes micturition recovery post SCI

Spinal cord injury often results in chronic loss of micturition control, which is featured by bladder hyperreflexia and detrusor sphincter dyssynergia. Previous studies showed that treatment of capsaicin reduces non-voiding bladder contractions in multiple animal injury models and human patients. However, its underlying neural mechanisms remain largely unknown. Here, by injecting a RetroAAV into the bladder wall, we specifically targeted TRPV1+, a capsaicin receptor, bladder afferent neurons. Morphometric analysis revealed borderline increase of the soma size and significant spinal axon sprouting of TRPV1+ bladder afferent neurons post a complete T8 spinal cord crush. We further demonstrated that chronic chemogenetic inhibition of these DRG neurons improved micturition recovery after SCI by increasing voiding efficiency and alleviating bladder hyperreflexia, along with reduced morphological changes caused by injury. Our study provided novel insights into the structural and functional changes of TRPV1+ bladder afferent post SCI and further supports the clinical use of capsaicin as an effective treatment to improve bladder functions in patients with SCI.

Optogenetic stimulation of neurons in the anterior cingulate cortex induces changes in intravesical bladder pressure and the micturition reflex

Lower urinary tract (LUT) function is controlled by the central nervous system, including higher-order cognitive brain regions. The anterior cingulate cortex (ACC) is one of these regions, but the role of its activity in LUT function remains poorly understood. In the present study, we conducted optogenetic experiments to manipulate neural activity in mouse ACC while monitoring bladder pressure to elucidate how the activity of ACC regulates LUT function. Selective optogenetic stimulation of excitatory neurons in ACC induced a sharp increase in bladder pressure, whereas activation of inhibitory neurons in ACC prolonged the interval between bladder contractions. Pharmacological manipulation of ACC also altered bladder contractions, consistent with those observed in optogenetic experiments. Optogenetic mapping of the cortical area responsible for eliciting the increase in bladder pressure revealed that stimulation to ACC showed more potent effects than the neighboring motor cortical areas. These results suggest that ACC plays a crucial role in initiating the bladder pressure change and the micturition reflex. Thus, the balance between excitation and inhibition in ACC may regulate the reflex bidirectionally.

Pathophysiology of Overactive Bladder and Pharmacologic Treatments Including β3-Adrenoceptor Agonists -Basic Research Perspectives

Overactive bladder (OAB) is a symptom-based syndrome defined by urinary urgency, frequency, and nocturia with or without urge incontinence. The causative pathology is diverse; including bladder outlet obstruction (BOO), bladder ischemia, aging, metabolic syndrome, psychological stress, affective disorder, urinary microbiome, localized and systemic inflammatory responses, etc. Several hypotheses have been suggested as mechanisms of OAB generation; among them, neurogenic, myogenic, and urothelial mechanisms are well-known hypotheses. Also, a series of local signals called autonomous myogenic contraction, micromotion, or afferent noises, which can occur during bladder filling, may be induced by the leak of acetylcholine (ACh) or urothelial release of adenosine triphosphate (ATP). They can be transmitted to the central nervous system through afferent fibers to trigger coordinated urgency-related detrusor contractions. Antimuscarinics, commonly known to induce smooth muscle relaxation by competitive blockage of muscarinic receptors in the parasympathetic postganglionic nerve, have a minimal effect on detrusor contraction within therapeutic doses. In fact, they have a predominant role in preventing signals in the afferent nerve transmission process. β3-adrenergic receptor (AR) agonists inhibit afferent signals by predominant inhibition of mechanosensitive Aδ-fibers in the normal bladder. However, in pathologic conditions such as spinal cord injury, it seems to inhibit capsaicin-sensitive C-fibers. Particularly, mirabegron, a β3-agonist, prevents ACh release in the BOO-induced detrusor overactivity model by parasympathetic prejunctional mechanisms. A recent study also revealed that vibegron may have 2 mechanisms of action: inhibition of ACh from cholinergic efferent nerves in the detrusor and afferent inhibition via urothelial β3-AR.

Therapeutic effects of p38 mitogen-activated protein kinase inhibition on hyperexcitability of capsaicin sensitive bladder afferent neurons in mice with spinal cord injury

AIMS

Nerve growth factor (NGF) has been implicated as a key molecule of pathology-induced changes in C-fiber afferent nerve excitability, which contributes to the emergence of neurogenic detrusor overactivity due to spinal cord injury (SCI). It is also known that the second messenger signaling pathways activated by NGF utilize p38 Mitogen-Activated Protein Kinase (MAPK). We examined the roles of p38 MAPK on electrophysiological properties of capsaicin sensitive bladder afferent neurons with SCI mice.

MAIN METHODS

We used female C57BL/6 mice and transected their spinal cord at the Th8/9 level. Two weeks later, continuous administration of p38 MAPK inhibitor (0.51 μg/h, i.t. for two weeks) was started. Bladder afferent neurons were labelled with a fluorescent retrograde tracer, Fast-Blue (FB), injected into the bladder wall three weeks after SCI. Four weeks after SCI, freshly dissociated L6-S1 dorsal root ganglion neurons were prepared and whole cell patch clamp recordings were performed in FB-labelled neurons. After recording action potentials or voltage-gated K+ currents, the sensitivity of each neuron to capsaicin was evaluated.

KEY FINDINGS

In capsaicin-sensitive FB-labelled neurons, SCI significantly reduced the spike threshold and increased the number of action potentials during 800 ms membrane depolarization. Densities of slow-decaying A-type K+ (KA) and sustained delayed rectifier-type K+ (KDR) currents were significantly reduced by SCI. The reduction of KA, but not KDR, current density was reversed by the treatment with p38 MAPK inhibitor.

SIGNIFICANCE

P38 MAPK plays an important role in hyperexcitability of capsaicin-sensitive bladder afferent neurons due to the reduction in KA channel activity in SCI mice.

Molecular Mechanisms of Neurogenic Lower Urinary Tract Dysfunction after Spinal Cord Injury

This article provides a synopsis of current progress made in fundamental studies of lower urinary tract dysfunction (LUTD) after spinal cord injury (SCI) above the sacral level. Animal models of SCI allowed us to examine the effects of SCI on the micturition control and the underlying neurophysiological processes of SCI-induced LUTD. Urine storage and elimination are the two primary functions of the LUT, which are governed by complicated regulatory mechanisms in the central and peripheral nervous systems. These neural systems control the action of two functional units in the LUT: the urinary bladder and an outlet consisting of the bladder neck, urethral sphincters, and pelvic-floor striated muscles. During the storage phase, the outlet is closed, and the bladder is inactive to maintain a low intravenous pressure and continence. In contrast, during the voiding phase, the outlet relaxes, and the bladder contracts to facilitate adequate urine flow and bladder emptying. SCI disrupts the normal reflex circuits that regulate co-ordinated bladder and urethral sphincter function, leading to involuntary and inefficient voiding. Following SCI, a spinal micturition reflex pathway develops to induce an overactive bladder condition following the initial areflexic phase. In addition, without proper bladder-urethral-sphincter coordination after SCI, the bladder is not emptied as effectively as in the normal condition. Previous studies using animal models of SCI have shown that hyperexcitability of C-fiber bladder afferent pathways is a fundamental pathophysiological mechanism, inducing neurogenic LUTD, especially detrusor overactivity during the storage phase. SCI also induces neurogenic LUTD during the voiding phase, known as detrusor sphincter dyssynergia, likely due to hyperexcitability of Aδ-fiber bladder afferent pathways rather than C-fiber afferents. The molecular mechanisms underlying SCI-induced LUTD are multifactorial; previous studies have identified significant changes in the expression of various molecules in the peripheral organs and afferent nerves projecting to the spinal cord, including growth factors, ion channels, receptors and neurotransmitters. These findings in animal models of SCI and neurogenic LUTD should increase our understanding of pathophysiological mechanisms of LUTD after SCI for the future development of novel therapies for SCI patients with LUTD.

Therapeutic effects of KCC2 chloride transporter activation on detrusor overactivity in mice with spinal cord injury

This study aimed to clarify whether downregulation of K⁺-Cl^- cotransporter 2 (KCC2) in the sacral parasympathetic nucleus (SPN) of the lumbosacral spinal cord, from which the efferent pathway innervating the bladder originates, causes cellular hyperexcitability and triggers detrusor overactivity (DO) in spinal cord injury (SCI). SCI was produced by Th8-9 spinal cord transection in female C57BL/6 mice. At 4 wk after SCI, CLP290, a KCC2 activator, was administered, and cystometry was performed. Thereafter, neuronal activity with c-fos staining and KCC2 expression in cholinergic preganglionic parasympathetic neurons in the SPN was examined using immunohistochemistry. Firing properties of neurons in the SPN region were evaluated by extracellular recordings in the spinal cord slice preparations. DO evident as nonvoiding contractions was significantly reduced by CLP290 treatment in SCI mice. The number of c-fos-positive cells and coexpression of c-fos in choline acetyltransferase-positive cells were decreased in the SPN region of the SCI CLP290-treated group versus the SCI vehicle-treated group. KCC2 immunoreactivity was present on the cell membrane of SPN neurons and normalized fluorescence intensity of KCC2 in choline acetyltransferase-positive SPN neurons was decreased in the SCI vehicle-treated group versus the spinal intact vehicle-treated group but recovered in the SCI CLP290-treated group. Extracellular recordings showed that CLP290 suppressed the high-frequency firing activity of SPN neurons in SCI mice. These results indicated that SCI-induced DO is associated with downregulation of KCC2 in preganglionic parasympathetic neurons and that activation of KCC2 transporters can reduce DO, increase KCC2 expression in preganglionic parasympathetic neurons, and decrease neuronal firing of SPN neurons in SCI mice.NEW & NOTEWORTHY This study is the first report to suggest that activation of the Cl^- transporter K⁺-Cl^- cotransporter 2 may be a therapeutic modality for the treatment of spinal cord injury-induced detrusor overactivity by targeting bladder efferent pathways.

Acupuncture for Women with Overactive Bladder: Perspective of Traditional Chinese Medicine and Related Mechanism

Purpose

Overactive bladder (OAB) syndrome is one of the most common diseases in urology and affects quality of life. Although the current treatment for OAB is based on oral medications, there are limitations and many patients have difficulty accepting drug-induced adverse effects. This review aimed to analyze the efficacy of acupuncture and its related mechanisms and provide a preliminary therapeutic regimen.

Methods

Two authors independently searched PubMed, Embase, and Cochrane Library up to April 2022. They searched related English literature and extracted the data under a standard form based on the search strategy. Clinical trials which included OAB women with the treatment of acupuncture were included. Common acupuncture alone without other pharmacotherapy, external treatments was in the treatment group. The control interventions may include any active treatments, sham placebo, or no establishment of a control group. Outcomes included 3-day or 24-hour voiding diary, overactive bladder symptom score, etc. The Cochrane risk of bias tool was also used to assess the methodological quality of the randomized controlled trials (RCTs).

Results

We analyzed five RCTs and one comparative study on acupuncture for OAB to review and discuss the acupoint location, treatment course, and retention time based on clinical evidence and treatment ideas in traditional Chinese medicine. Additionally, we used the available evidence to reveal and discuss the acupuncture mechanisms for OAB. Acupuncture may regulate bladder function by inhibiting C-fibers, modulating nerve growth factors and reducing spontaneous contractions of the detrusor muscle.

Conclusion

Combined with the available evidence, the combination of local acupoints and distal acupoints should be necessary to consider, especially the lumbosacral acupoints, the small abdomen acupoints and the lower limb acupoints. Among them, acupuncture at SP4, CV4 and KI3 are strongly recommended. The treatment course of acupuncture should be no less than 4 weeks and maintain the frequency of acupuncture no less than once a week. The duration of each session should be no less than 20 minutes. In addition, investigations remain necessary to verify acupuncture's efficacy and precise mechanism for OAB treatment in further exploration.

Molecular Mechanism Operating in Animal Models of Neurogenic Detrusor Overactivity: A Systematic Review Focusing on Bladder Dysfunction of Neurogenic Origin

Neurogenic detrusor overactivity (NDO) is a severe lower urinary tract disorder, characterized by urinary urgency, retention, and incontinence, as a result of a neurologic lesion that results in damage in neuronal pathways controlling micturition. The purpose of this review is to provide a comprehensive framework of the currently used animal models for the investigation of this disorder, focusing on the molecular mechanisms of NDO. An electronic search was performed with PubMed and Scopus for literature describing animal models of NDO used in the last 10 years. The search retrieved 648 articles, of which reviews and non-original articles were excluded. After careful selection, 51 studies were included for analysis. Spinal cord injury (SCI) was the most frequently used model to study NDO, followed by animal models of neurodegenerative disorders, meningomyelocele, and stroke. Rats were the most commonly used animal, particularly females. Most studies evaluated bladder function through urodynamic methods, with awake cystometry being particularly preferred. Several molecular mechanisms have been identified, including changes in inflammatory processes, regulation of cell survival, and neuronal receptors. In the NDO bladder, inflammatory markers, apoptosis-related factors, and ischemia- and fibrosis-related molecules were found to be upregulated. Purinergic, cholinergic, and adrenergic receptors were downregulated, as most neuronal markers. In neuronal tissue, neurotrophic factors, apoptosis-related factors, and ischemia-associated molecules are increased, as well as markers of microglial and astrocytes at lesion sites. Animal models of NDO have been crucial for understanding the pathophysiology of lower urinary tract (LUT) dysfunction. Despite the heterogeneity of animal models for NDO onset, most studies rely on traumatic SCI models rather than other NDO-driven pathologies, which may result in some issues when translating pre-clinical observations to clinical settings other than SCI.

Spinal Cord Injury Causes Marked Tissue Rearrangement in the Urethra-Experimental Study in the Rat

Traumatic spinal cord injury (SCI) results in the time-dependent development of urinary impairment due to neurogenic detrusor overactivity (NDO) and detrusor-sphincter-dyssynergia (DSD). This is known to be accompanied by massive changes in the bladder wall. It is presently less clear if the urethra wall also undergoes remodelling. To investigate this issue, female rats were submitted to complete spinal transection at the T8/T9 level and left to recover for 1 week and 4 weeks. To confirm the presence of SCI-induced NDO, bladder function was assessed by cystometry under urethane anesthesia before euthanasia. Spinal intact animals were used as controls. Urethras were collected and processed for further analysis. Following thoracic SCI, time-dependent changes in the urethra wall were observed. Histological assessment revealed marked urethral epithelium reorganization in response to SCI, as evidenced by an increase in epithelial thickness. At the muscular layer, SCI resulted in strong atrophy of the smooth muscle present in the urethral sphincter. Innervation was also affected, as evidenced by a pronounced decrease in the expression of markers of general innervation, particularly those present in sensory and sympathetic nerve fibres. The present data show an evident impact of SCI on the urethra, with significant histological rearrangement, accompanied by sensory and sympathetic denervation. It is likely that these changes will affect urethral function and contribute to SCI-induced urinary dysfunction, and they deserve further investigation.

Therapeutic effects of a soluble guanylate cyclase activator, BAY 60-2770, on lower urinary tract dysfunction in mice with spinal cord injury

We aimed to evaluate the effects of a soluble guanylate cyclase (sGC) activator, BAY 60-2770, on neurogenic lower urinary tract dysfunction in mice with spinal cord injury (SCI). Mice were divided into the following three groups: spinal cord intact (group A), SCI + vehicle (group B), and SCI + BAY 60-2770 (group C). SCI mice underwent Th8-Th9 spinal cord transection and treatment with BAY 60-2770 (10 mg/kg/day) once daily for 2-4 wk after SCI. We evaluated urodynamic parameters using awake cystometry and external urethral sphincter electromyograms (EMG); mRNA levels of mechanosensory channels, nitric oxide (NO)-, ischemia-, and inflammation-related markers in L6-S1 dorsal root ganglia, the urethra, and bladder tissues; and protein levels of cGMP in the urethra at 4 wk after SCI. With awake cystometry, nonvoiding contractions, postvoid residual, and bladder capacity were significantly larger in group B than in group C. Voiding efficiency (VE) was significantly higher in group C than in group B. In external urethral sphincter EMGs, the duration of notch-like reductions in intravesical pressure and reduced EMG activity time were significantly longer in group C than in group B. mRNA expression levels of transient receptor potential ankyrin 1, transient receptor potential vanilloid 1, acid-sensing ion channel (ASIC)1, ASIC2, ASIC3, and Piezo2 in the dorsal root ganglia, and hypoxia-inducible factor-1α, VEGF, and transforming growth factor-β1 in the bladder were significantly higher in group B than in groups A and C. mRNA levels of neuronal NO synthase, endothelial NO synthase, and sGCα1 and protein levels of cGMP in the urethra were significantly lower in group B than in groups A and C. sGC modulation might be useful for the treatment of SCI-related neurogenic lower urinary tract dysfunction.NEW & NOTEWORTHY This is the first report to evaluate the effects of a soluble guanylate cyclase activator, BAY 60-2770, on neurogenic lower urinary tract dysfunction in mice with spinal cord injury.

Moxibustion attenuates neurogenic detrusor overactivity in spinal cord injury rats by inhibiting M2/ATP/P2X3 pathway

PURPOSE

Activation of muscarinic receptors located in bladder sensory pathways is generally considered to be the primary contributor for driving the pathogenesis of neurogenic detrusor overactivity following spinal cord injury. The present study is undertaken to examine whether moxibustion improves neurogenic detrusor overactivity via modulating the abnormal muscarinic receptor pathway.

MATERIALS AND METHODS

Female Sprague-Dawley rats were subjected to spinal cord injury with T9-10 spinal cord transection. Fourteen days later, animals were received moxibustion treatment for one week. Urodynamic parameters and pelvic afferents discharge were measured. Adenosine triphosphate (ATP) content in the voided cystometry fluid was determined. Expressions of M2, M3, and P2X3 receptors in the bladder mucosa were evaluated.

RESULTS

Moxibustion treatment prevented the development of detrusor overactivity in spinal cord injury rats, with an increase in the intercontraction interval and micturition pressure threshold and a decrease in afferent activity during filling. The expression of M2 was markedly suppressed by moxibustion, accompanied by a reduction in the levels of ATP and P2X3. M2 receptor antagonist methoctramine hemihydrate had similar effects to moxibustion on bladder function and afferent activity, while the M2-preferential agonist oxotremorine methiodide abolished the beneficial effects of moxibustion.

CONCLUSION

Moxibustion is a potential candidate for treating neurogenic bladder overactivity in a rat model of spinal cord injury, possibly through inhibiting the M2/ATP/P2X3 pathway.

Current knowledge and novel frontiers in lower urinary tract dysfunction after spinal cord injury: Basic research perspectives

This review article aims to summarize the recent advancement in basic research on lower urinary tract dysfunction (LUTD) following spinal cord injury (SCI) above the sacral level. We particularly focused on the neurophysiologic mechanisms controlling the lower urinary tract (LUT) function and the SCI-induced changes in micturition control in animal models of SCI. The LUT has two main functions, the storage and voiding of urine, that are regulated by a complex neural control system. This neural system coordinates the activity of two functional units in the LUT: the urinary bladder and an outlet including bladder neck, urethra, and striated muscles of the pelvic floor. During the storage phase, the outlet is closed and the bladder is quiescent to maintain a low intravesical pressure and continence, and during the voiding phase, the outlet relaxes and the bladder contracts to promote efficient release of urine. SCI impairs voluntary control of voiding as well as the normal reflex pathways that coordinate bladder and sphincter function. Following SCI, the bladder is initially areflexic but then becomes hyperreflexic due to the emergence of a spinal micturition reflex pathway. However, the bladder does not empty efficiently because coordination between the bladder and urethral sphincter is lost. In animal models of SCI, hyperexcitability of silent C-fiber bladder afferents is a major pathophysiological basis of neurogenic LUTD, especially detrusor overactivity. Reflex plasticity is associated with changes in the properties of neuropeptides, neurotrophic factors, or chemical receptors of afferent neurons. Not only C-fiber but also Aδ-fiber could be involved in the emergence of neurogenic LUTD such as detrusor sphincter dyssynergia following SCI. Animal research using disease models helps us to detect the different contributing factors for LUTD due to SCI and to find potential targets for new treatments.

Psychological stress induced bladder overactivity in female mice is associated with enhanced afferent nerve activity

Psychological stress has been linked to the development and exacerbation of overactive bladder symptoms, as well as afferent sensitisation in other organ systems. Therefore, we aimed to investigate the effects of water avoidance stress on bladder afferent nerve activity in response to bladder filling and pharmaceutical stimulation with carbachol and ATP in mice. Adult female C57BL/6J mice were exposed to either water avoidance stress (WAS) for 1 h/day for 10 days or normal housing conditions. Voiding behaviour was measured before starting and 24-h after final stress exposure and then animals were euthanised to measure afferent nerve activity in association with bladder compliance, spontaneous phasic activity, contractile responses, as well as release of urothelial mediators. WAS caused increased urinary frequency without affecting urine production. The afferent nerve activity at low bladder pressures (4-7 mmHg), relevant to normal physiological filling, was significantly increased after stress. Both low and high threshold nerves demonstrated enhanced activity at physiological bladder pressures. Urothelial ATP and acetylcholine release and bladder compliance were unaffected by stress as was the detrusor response to ATP (1 mM) and carbachol (1 µM). WAS caused enhanced activity of individual afferent nerve fibres in response bladder distension. The enhanced activity was seen in both low and high threshold nerves suggesting that stressed animals may experience enhanced bladder filling sensations at lower bladder volumes as well as increased pain sensations, both potentially contributing to the increased urinary frequency seen after stress.

Spinal interneurons of the lower urinary tract circuits

The storage and elimination of urine requires coordinated activity between muscles of the bladder and the urethra. This coordination is orchestrated by a complex system containing spinal, midbrain and forebrain networks. Normally there is a reciprocity between patterns of activity in urinary bladder sacral parasympathetic efferents and somatic motoneurons innervating the striatal external urethral sphincter muscle. At the spinal level this reciprocity is mediated by ensembles of excitatory and inhibitory interneurons located in the lumbar-sacral segments. In this review I will present an overview of currently identified spinal interneurons and circuits relevant to the lower urinary tract and will discuss their established or hypothetical roles in the cycle of micturition. In addition, a recently discovered auxiliary spinal neuronal ensemble named lumbar spinal coordinating center will be described. Sexual dimorphism and developmental features of the lower urinary tract which may play a significant role in designing treatments for patients with urine storage and voiding dysfunctions are also considered. Spinal cord injuries seriously damage or even eliminate the ability to urinate. Treatment of this abnormality requires detailed knowledge of supporting neural mechanisms, therefore various experiments in normal and spinalized animals will be discussed. Finally, a possible intraspinal mechanism will be proposed for organization of external urethral sphincter (EUS) bursting which represents a form of intermittent EUS relaxation in rats and mice.

Deciphering Spinal Endogenous Dopaminergic Mechanisms That Modulate Micturition Reflexes in Rats with Spinal Cord Injury

Spinal neuronal mechanisms regulate recovered involuntary micturition after spinal cord injury (SCI). It was recently discovered that dopamine (DA) is synthesized in the rat injured spinal cord and is involved in lower urinary tract (LUT) activity. To fully understand the role of spinal DAergic machinery in micturition, we examined urodynamic responses in female rats during pharmacological modulation of the DA pathway. Three to four weeks after complete thoracic SCI, the DA precursor L-DOPA administered intravenously during bladder cystometrogram (CMG) and external urethral sphincter (EUS) electromyography (EMG) reduced bladder overactivity and increased the duration of EUS bursting, leading to remarkably improved voiding efficiency. Apomorphine (APO), a non-selective DA receptor (DR) agonist, or quinpirole, a selective DR₂ agonist, induced similar responses, whereas a specific DR₂ antagonist remoxipride alone had only minimal effects. Meanwhile, administration of SCH 23390, a DR₁ antagonist, reduced voiding efficiency by increasing tonic EUS activity and shortening the EUS bursting period. Unexpectedly, SKF 38393, a selective DR₁ agonist, increased EUS tonic activity, implying a complicated role of DR₁ in LUT function. In metabolic cage assays, subcutaneous administration of quinpirole decreased spontaneous voiding frequency and increased voiding volume; L-DOPA and APO were inactive possibly because of slow entry into the CNS. Collectively, tonically active DR₁ in SCI rats inhibit urine storage and enhance voiding by differentially modulating EUS tonic and bursting patterns, respectively, while pharmacologic activation of DR₂, which are normally silent, improves voiding by enhancing EUS bursting. Thus, enhancing DA signaling achieves better detrusor-sphincter coordination to facilitate micturition function in SCI rats.

Efficacy of vibegron, a novel β3-adrenoreceptor agonist, for lower urinary tract dysfunction in mice with spinal cord injury

OBJECTIVES

To investigate the effect of vibegron, a new clinically approved β3-adrenoceptor agonist in lower urinary tract dysfunction in mice with spinal cord injury.

METHODS

Investigators performed cystometry under awake conditions in 4-week spinal cord injury female mice. Two weeks after spinal cord injury, saline or vibegron (30 mg/kg) was orally administered for 2 weeks prior to the urodynamic study. Investigators removed L6-S1 dorsal root ganglia from the saline- or vibegron-treated spinal cord injury mice as well as from saline-treated normal (spinal intact) mice to evaluate the levels of transient receptor potential cation channel subfamily V member 1, transient receptor potential cation channel subfamily A member 1, activating transcription factor 3, and inducible nitric oxide synthase transcripts using real-time polymerase chain reaction.

RESULTS

In vibegron-treated spinal cord injury mice, nonvoiding contractions during bladder filling, which were increased in spinal cord injury compared to spinal intact mice, were significantly decreased. Micturition pressure or voiding efficiency was not significantly increased in comparison to measurements in saline-treated spinal cord injury mice. The expression of transient receptor potential cation channel subfamily V member 1, transient receptor potential cation channel subfamily A member 1, activating transcription factor 3, and inducible nitric oxide synthase messenger RNA was increased in spinal cord injury mice compared to spinal intact mice, but significantly decreased after vibegron treatment.

CONCLUSIONS

Vibegron improves spinal cord injury-induced detrusor overactivity in addition to significantly reducing C-fiber afferent receptors such as transient receptor potential cation channel subfamily V member 1, transient receptor potential cation channel subfamily A member 1, and inflammatory cytokines/markers, such as activating transcription factor 3 and inducible nitric oxide synthase, in spinal cord injury mice. Thus, vibegron might be effective in the treatment of storage lower urinary tract dysfunction induced by C-fiber afferent activation after spinal cord injury.

Endocannabinoids in Bladder Sensory Mechanisms in Health and Diseases

The recent surge in research on cannabinoids may have been fueled by changes in legislation in several jurisdictions, and by approval for the use of cannabinoids for treatment of some chronic diseases. Endocannabinoids act largely, but not exclusively on cannabinoid receptors 1 and 2 (CBR1 and CBR2) which are expressed in the bladder mainly by the urothelium and the axons and endings of motor and sensory neurons. A growing body of evidence suggests that endocannabinoid system constitutively downregulates sensory bladder function during urine storage and micturition, under normal physiological conditions. Similarly, exogenous cannabinoid agonists have potent modulatory effects, as do inhibitors of endocannabinoid inactivation. Results suggest a high potential of cannabinoids to therapeutically ameliorate lower urinary tract symptoms in overactive bladder and painful bladder syndromes. At least part of this may be mediated via effects on sensory nerves, although actions on efferent nerves complicate interpretation. The sensory innervation of bladder is complex with at least eight classes identified. There is a large gap in our knowledge of the effects of endocannabinoids and synthetic agonists on different classes of bladder sensory neurons. Future studies are needed to reveal the action of selective cannabinoid receptor 2 agonists and/or peripherally restricted synthetic cannabinoid receptor 1 agonists on bladder sensory neurons in animal models of bladder diseases. There is significant potential for these novel therapeutics which are devoid of central nervous system psychotropic actions, and which may avoid many of the side effects of current treatments for overactive bladder and painful bladder syndromes.

Time-dependent progression of neurogenic lower urinary tract dysfunction after spinal cord injury in the mouse model

This study evaluated the time-course changes in bladder and external urinary sphincter (EUS) activity and the expression of mechanosensitive channels in lumbosacral dorsal root ganglia (DRG) after spinal cord injury (SCI). Female C57BL/6N mice in the SCI group underwent transection of the Th8/9 spinal cord. Spinal intact mice and SCI mice at 2, 4, and 6 wk post-SCI were evaluated by single-filling cystometry and EUS-electromyography (EMG). In another set of mice, the bladder and L6-S1 DRG were harvested for protein and mRNA analyses. In SCI mice, nonvoiding contractions were confirmed at 2 wk post-SCI and did not increase over time to 6 wk. In 2-wk SCI mice, EUS-EMG measurements revealed detrusor sphincter dyssynergia, but periodic EMG reductions during bladder contraction were hardly observed. At 4 wk, SCI mice showed increases of EMG activity reduction time with increased voiding efficiency. At 6 wk, SCI mice exhibited a further increase in EMG reduction time. RT-PCR of L6-S1 DRG showed increased mRNA levels of transient receptor potential vanilloid 1 and acid-sensing ion channels (ASIC1-ASIC3) in SCI mice with a decrease of ASIC2 and ASIC3 at 6 wk compared with 4 wk, whereas Piezo2 showed a slow increase at 6 wk. Protein assay showed SCI-induced overexpression of bladder brain-derived neurotrophic factor with a time-dependent decrease post-SCI. These results indicate that detrusor overactivity is established in the early phase, whereas detrusor sphincter dyssynergia is completed later at 4 wk with an improvement at 6 wk post-SCI, and that mechanosensitive channels may be involved in the time-dependent changes.NEW & NOTEWORTHY This is the first paper to evaluate the time-course changes of bladder dysfunction associated with mechanosensitive channels in a mouse model.

Voiding behavior in awake unrestrained untethered spontaneously hypertensive and Wistar control rats

The spontaneously hypertensive rat (SHR), a genetic model of high blood pressure, has also been studied as a potential model of overactive bladder. In vivo studies have confirmed the presence of surrogate markers of overactive bladder, including detrusor overactivity, increased urinary frequency, decreased bladder capacity and voided volume (VV), and afferent hypersensitivity to bladder irritation. However, these observations were during awake cystometry using implanted bladder catheters tethered to an infusion pump and artificially filled. We conducted experiments in awake unrestrained untethered age-matched female SHRs and Wistar rats to quantify naïve consumption and voiding behavior and the effect of capsaicin desensitization on consumption and voiding behavior. Food and water consumption, body weight, voiding frequency, and VV were recorded. Rats were placed in metabolism cages for 24 h, up to twice a week, from 17 to 37 wk of age. Compared with Wistar rats, SHRs exhibited decrease in VV and did not exhibit diurnal variation in VV between light and dark periods, suggesting that SHRs may have bladder hypersensitivity. Furthermore, SHRs may also have smaller bladder capacities, as they consumed less water, voided less volume (regardless of light cycle), and had equal urinary frequencies compared with age-matched Wistar rats. We detected no change in SHR voiding behavior following capsaicin desensitization, which was in contrast to a prior awake in vivo cystometry study describing increased VV and micturition interval in SHRs and suggests that C-fiber activity may not contribute to bladder hypersensitivity in SHRs.NEW & NOTEWORTHY We characterized the long-term (20 wk) voiding, defecation, and consumption behavior of age-matched spontaneously hypertensive and Wistar rats without the influence of anesthesia or catheters. Spontaneously hypertensive rats exhibited bladder hypersensitiviy that persisted for the 20-wk duration and was unaffected by capsacin desensitization.

Bladder and bowel responses to lumbosacral epidural stimulation in uninjured and transected anesthetized rats

Spinal cord epidural stimulation (scES) mapping at L5-S1 was performed to identify parameters for bladder and bowel inhibition and/or contraction. Using spinally intact and chronic transected rats of both sexes in acute urethane-anesthetized terminal preparations, scES was systematically applied using a modified Specify 5-6-5 (Medtronic) electrode during bladder filling/emptying cycles while recording bladder and colorectal pressures and external urethral and anal sphincter electromyography activity. The results indicate frequency-dependent effects on void volume, micturition, bowel peristalsis, and sphincter activity just above visualized movement threshold intensities that differed depending upon neurological intactness, with some sex-dependent differences. Thereafter, a custom-designed miniature 15-electrode array designed for greater selectivity was tested and exhibited the same frequency-dependent urinary effects over a much smaller surface area without any concurrent movements. Thus, select activation of autonomic nervous system circuitries with scES is a promising neuromodulation approach for expedient translation to individuals with SCI and potentially other neurologic disorders.

Age-related differences in responses to hydrogen sulfide in the bladder of spontaneously hypertensive rats

OBJECTIVES

To investigate whether a response to hydrogen sulfide donors (GYY4137 and sodium hydrosulfide) and the endogenous hydrogen sulfide system (hydrogen sulfide level and expression of cysteine aminotransferase, cystathionine β-synthase, cystathionine γ-lyase, and 3-mercaptopyruvate sulfurtransferase) in the spontaneously hypertensive rat bladder differ with age, we compared the responses of hydrogen sulfide donors to micturition and bladder relaxation, and the endogenous hydrogen sulfide system in the bladder of 18-week versus 12-week-old spontaneously hypertensive rats.

METHODS

GYY4137 was intravesically administered and cystometry was performed in anesthetized rats. The responses of sodium hydrosulfide were evaluated in carbachol-mediated precontracted bladder strips. Bladder hydrogen sulfide levels and expression levels of each enzyme were investigated using the methylene blue method and Western blotting, respectively.

RESULTS

GYY4137 treatment significantly prolonged intercontraction intervals only in 12-week-old rats. Sodium hydrosulfide-induced bladder relaxation was significantly attenuated in the strips of 18-week-old rats compared with that in 12-week-old rats. In the bladder dome, significant increases in hydrogen sulfide levels and in the expression of cystathionine β-synthase, 3-mercaptopyruvate sulfurtransferase, and cysteine aminotransferase were observed in 18-week-old rats compared with 12-week-old rats. However, cystathionine γ-lyase bands were not detected in bladder tissues of either group.

CONCLUSIONS

Bladder relaxation induced by hydrogen sulfide may be attenuated in spontaneously hypertensive rats in an age-dependent manner.

Effects of a new β3-adrenoceptor agonist, vibegron, on neurogenic bladder dysfunction and remodeling in mice with spinal cord injury

AIMS

To examine vibegron effects on lower urinary tract dysfunction (LUTD) in mice with spinal cord injury (SCI).

METHODS

Female mice underwent Th8-9 spinal cord transection and were orally administered vehicle or vibegron after SCI. We evaluated urodynamic parameters at 4 weeks after SCI with or without vibegron. Fibrosis- and ischemia-related messenger RNA (mRNA) and protein levels of collagen and elastin were measured in bladders of vehicle- and vibegron-treated SCI mice, and spinal intact mice.

RESULTS

Non-voiding contractions (NVCs) were significantly fewer (15.3 ± 8.9 vs 29.7 ± 11.4 contractions; P < .05) and the time to the first NVC was significantly longer (1488.0 ± 409.5 vs 782.7 ± 399.7 seconds; P < .01) in vibegron-treated SCI mice vs vehicle-treated SCI mice. mRNAs levels of collagen types 1 and 3, transforming growth factor-β1 (TGF-β1), and hypoxia-inducible factor-1α (HIF-1α) were significantly upregulated in vehicle-treated SCI mice compared with spinal intact and vibegron-treated SCI mice (Col 1: 3.5 vs 1.0 and 2.0-fold; P < .01 and P < .05, Col 3: 2.1 vs 1.0 and 1.2-fold; P < .01 and P < .05, TGF-β1: 1.2 vs 1.0 and 0.9-fold; P < .05 and P < .05, HIF-1α: 1.4 vs 1.0 and 1.0-fold; P < .05 and P < .01). Total collagen and elastin protein levels in vehicle- and vibegron-treated SCI mice did not differ.

CONCLUSIONS

Vibegron reduced NVCs, delayed the first NVC, and improved collagen types 1 and 3, TGF-β1, and HIF-1α mRNA expression in SCI mice. Vibegron might be effective for SCI-induced LUTD.

Response of hypogastric afferent fibers to bladder distention or irritation in cats

The goal of this study in anesthetized cats was to identify silent hypogastric nerve (HGN) afferent fibers that do not respond to bladder distention but become responsive after chemical irritation of the bladder. The HGN was split into multiple filaments small enough for recording action potentials from single or multiple afferent fibers. The bladder was distended by infusion of either saline or 0.5% acetic acid (AA) through a urethral catheter while recording intravesical pressure. A total of 90 HGN filaments from 17 cats responded to bladder distention with saline or AA. Three types of HGN afferents were identified. The first type was non-nociceptive mechano-sensitive that responded to bladder distention at normal physiological pressures (10-40 cmH₂O). The second type was nociceptive mechano-sensitive that only responded to high-pressure (50-80 cmH₂O) bladder distention with saline but responded to low-pressure bladder distention after sensitization with AA. The third type was chemo-sensitive nociceptive that was silent even during high-pressure bladder distention but after sensitization with AA did respond to low-pressure bladder distention. These results indicate that HGN afferents as well as pelvic nerve afferents may play a role in bladder nociception. The HGN afferent fibers that are silent during bladder distention at normal physiological pressures but become responsive after chemical irritation are important for understanding the possible pathophysiological mechanism underlying bladder allodynia in painful bladder syndrome.

[Overview of pharmacological mechanisms controlling micturition in the central nervous system]

The functions of the lower urinary tract, to storage and periodically release urine, are dependent on the activity of smooth and striated muscles in the bladder, urethra and external urethral sphincter. This activity is in turn controlled by neural circuits not only in the periphery, but also in the central nervous system (CNS). During urine storage, the outlet is closed and the bladder smooth muscle is quiescent by the neural control mechanism mainly organized in the spinal cord. When bladder volume reaches the micturition threshold, activation of a micturition center in the dorsolateral pons (the pontine micturition center) induces micturition through activation of sacral parasympathetic (pelvic) nerves. The brain rostral to the pons (diencephalon and cerebral cortex) is also involved in excitatory and inhibitory regulation of the micturition reflex. Various transmitters including dopamine, serotonin, norepenephrine, GABA, excitatory and inhibitory amino acids, opioids and acetylcholine are implicated in the modulation of the micturition reflex in the CNS. Therefore, injury or neurodegenerative diseases of the CNS as well as drugs can produce bladder and urethral dysfunctions such as urinary frequency, urgency and incontinence or inefficient bladder emptying.

Propriospinal Neurons of L3-L4 Segments Involved in Control of the Rat External Urethral Sphincter

Coordination of activity of external urethral sphincter (EUS) striated muscle and bladder (BL) smooth muscle is essential for efficient voiding. In this study we examined the morphological and electrophysiological properties of neurons in the L3/L4 spinal cord (SC) that are likely to have an important role in EUS-BL coordination in rats. EUS-related SC neurons were identified by retrograde transsynaptic tracing following injection of pseudorabies virus (PRV) co-expressing fluorescent markers into the EUS of P18-P20 male rats. Tracing revealed not only EUS motoneurons in L6/S1 but also interneurons in lamina X of the L6/S1 and L3/L4 SC. Physiological properties of fluorescently labeled neurons were assessed during whole-cell recordings in SC slices followed by reconstruction of biocytin-filled neurons. Reconstructions of neuronal processes from transverse or longitudinal slices showed that some L3/L4 neurons have axons projecting toward and into the ventro-medial funiculus (VMf) where axons extended caudally. Other neurons had axons projecting within laminae X and VII. Dendrites of L3/L4 neurons were distributed within laminae X and VII. The majority of L3/L4 neurons exhibited tonic firing in response to depolarizing currents. In transverse slices focal electrical stimulation (FES) in the VMf or in laminae X and VII elicited antidromic axonal spikes and/or excitatory synaptic responses in L3/L4 neurons; while in longitudinal slices FES elicited excitatory synaptic inputs from sites up to 400 μm along the central canal. Inhibitory inputs were rarely observed. These data suggest that L3/L4 EUS-related circuitry consists of at least two neuronal populations: segmental interneurons and propriospinal neurons projecting to L6/S1.

Role of p38 MAP kinase signaling pathways in storage and voiding dysfunction in mice with spinal cord injury

AIM

To investigate the role of p38 MAP kinase in lower urinary tract dysfunction in mice with spinal cord injury (SCI).

METHODS

Cystometry and external urethral sphincter-electromyography were performed under an awake condition in 4-week SCI female mice. Two weeks after SCI, a catheter connected to an osmotic pump filled with a p38 mitogen-activated protein kinase (MAPK) inhibitor or artificial cerebrospinal fluid (CSF) was implanted into the intrathecal space of L6-S1 spinal cord for continuous intrathecal instillation at infusion rate of 0.51 μL/h for 2 weeks before the urodynamic study. L6 dorsal root ganglia were then removed from CSF and p38 MAPK inhibitor-treated SCI mice as well as from CSF-treated normal (spinal intact) mice to evaluate the levels of transient receptor potential cation channel subfamily V member 1 (TRPV1), tumor necrosis factor-α (TNF-α), and inducible nitric oxide synthase (iNOS) transcripts by real-time polymerase chain reaction.

RESULTS

In p38 MAPK inhibitor-treated SCI mice, nonvoiding contractions during bladder filling, bladder capacity, and post-void residual volume were significantly reduced while micturition pressure and voiding efficiency were significantly increased in comparison to these measurements in CSF-treated SCI mice. The expression of TRPV1, TNF-α, and iNOS messenger RNA was increased in SCI mice compared with expression in spinal intact mice and significantly decreased after p38 MAPK inhibitor treatment.

CONCLUSIONS

The p38 MAPK signaling pathway in bladder sensory neurons or in the spinal cord plays an important role in storage and voiding problems such as detrusor overactivity and inefficient voiding after SCI.

Therapeutic effects of inhibition of brain-derived neurotrophic factor on voiding dysfunction in mice with spinal cord injury

We investigated the involvement of brain-derived neurotrophic factor (BDNF) in bladder and urethral dysfunction using spinal cord-injured mice. We evaluated bladder and urethral function of female mice with 4-wk spinal cord injury (SCI) by filling cystometry and electromyography (EMG) of the external urethral sphincter (EUS) under a conscious condition. Anti-BDNF antibodies (10 μg·kg^-1·h^-1) were administered in some mice for 1 wk before the evaluation. Bladder and spinal (L6-S1) BDNF protein levels were examined by ELISA. Transcript levels of transient receptor potential channels or acid-sensing ion channels (Asic) in L6-S1 dorsal root ganglia were evaluated by RT-PCR. Voided volume and voiding efficiency were significantly increased without any changes in nonvoiding contractions, and the duration of reduced EMG activity during the voiding phase was significantly prolonged in anti-BDNF antibody-treated SCI mice. Compared with spinal cord-intact mice, SCI mice showed increased concentrations of bladder and spinal BDNF. Anti-BDNF antibody treatment decreased bladder and spinal BDNF protein concentrations of SCI mice. Asic2 and Asic3 transcripts were significantly increased after SCI but decreased after anti-BDNF antibody administration. These results indicate that upregulated expression of bladder and spinal BDNF is involved in the emergence of inefficient voiding in SCI mice. Thus, BDNF-targeting treatment could be an effective modality for the treatment of voiding problems, including inefficient voiding and detrusor sphincter dyssynergia after SCI.

Influence of sildenafil on the purinergic components of nerve-mediated and urothelial ATP release from the bladder of normal and spinal cord injured mice

Background and purpose

PDE inhibitors such as sildenafil alleviate lower urinary tract symptoms; however, a complete understanding of their action on the bladder remains unclear. We are investigating the effects of sildenafil, on post and preganglionic nerve‐mediated contractions of the mouse bladder, and neuronal and urothelial ATP release.

Experimental approach

Bladders were used from young (12 weeks), aged (24 months), and spinal cord transected (SCT), mice, for in vitro contractility experiments. An arterially perfused in situ whole mouse model was used to record bladder pressure. Nerve‐mediated contractions were generated by electrical field stimulation (EFS) of postganglionic nerve terminals or the pelvic nerve. ATP release during EFS in intact detrusor strips, and during stretch of isolated mucosa strips, was measured using a luciferin‐luciferase assay.

Key results

Sildenafil (20 μM) inhibited nerve‐mediated contractions in young mice, with an increase in f_1/2 values in force–frequency relationships, demonstrating a greater effect at low frequencies. Sildenafil reduced the atropine‐resistant, purinergic component of nerve‐mediated contractions, and suppressed neuronal ATP release upon EFS in vitro. Sildenafil reduced the preganglionic pelvic nerve stimulated bladder pressure recordings in situ; comparable to in vitro experiments. Sildenafil reduced stretch‐induced urothelial ATP release. Sildenafil also relaxed nerve‐mediated contractions in aged and SCT mice.

Conclusion and implications

Sildenafil has a greater effect on the low‐frequency, purinergic‐mediated contractions and suppresses neuronal ATP release. In addition, sildenafil reduces stretch‐induced urothelial ATP release. These results demonstrate a novel action of sildenafil to selectively inhibit ATP release from nerve terminals innervating detrusor smooth muscle and the urothelium.

Modulatory effects of intravesical P2X2/3 purinergic receptor inhibition on lower urinary tract electromyographic properties and voiding function of female rats with moderate or severe spinal cord injury

OBJECTIVES

To evaluate the role that intravesical P2X2/3 purinergic receptors (P2X2/3Rs) play in early and advanced neurogenic lower urinary tract (LUT) dysfunction after contusion spinal cord injury (SCI) in female rats.

MATERIALS AND METHODS

Female Sprague-Dawley rats received a thoracic Th8/Th9 spinal cord contusion with either force of 100 kDy (cN); moderate) or 150 kDy (cN; severe); Sham rats had no injury. Evaluations on urethane-anesthetised rats were conducted at either 2 or 4 weeks after SCI. LUT electrical signals and changes in bladder pressure were simultaneously recorded using cystometry and a set of custom-made flexible microelectrodes, before and after intravesical application of the P2X2/3R antagonist AF-353 (10 μM), to determine the contribution of P2X2/3R-mediated LUT modulation.

RESULTS

Severe SCI significantly increased bladder contraction frequency, and reduced both bladder pressure amplitude and intraluminal-pressure high-frequency oscillations (IPHFO). Intravesical P2X2/3R inhibition did not modify bladder pressure or IPHFO in the Sham and moderate-SCI rats, although did increase the intercontractile interval (ICI). At 2 weeks after SCI, the Sham and moderate-SCI rats had significant LUT electromyographic activity during voiding, with a noticeable reduction in LUT electrical signals seen at 4 weeks after SCI. Intravesical inhibition of P2X2/3R increased the ICI in the Sham and moderate-SCI rats at both time-points, but had no effect on rats with severe SCI. The external urethral sphincter (EUS) showed strong and P2X2/3R-independent electrical signals in the Sham and moderate-SCI rats in the early SCI stage. At 4 weeks after SCI, the responsiveness of the EUS was significantly attenuated, independently of SCI intensity.

CONCLUSIONS

This study shows that electrophysiological properties of the LUT are progressively impaired depending on SCI intensity and that intravesical P2X2/3R inhibition can attenuate electrical activity in the neurogenic LUT at early, but not at semi-chronic SCI. This translational study should be useful for planning clinical evaluations.

A biophysically detailed computational model of urinary bladder small DRG neuron soma

Bladder small DRG neurons, which are putative nociceptors pivotal to urinary bladder function, express more than a dozen different ionic membrane mechanisms: ion channels, pumps and exchangers. Small-conductance Ca2+-activated K+ (SKCa) channels which were earlier thought to be gated solely by intracellular Ca2+ concentration ([Ca]i) have recently been shown to exhibit inward rectification with respect to membrane potential. The effect of SKCa inward rectification on the excitability of these neurons is unknown. Furthermore, studies on the role of KCa channels in repetitive firing and their contributions to different types of afterhyperpolarization (AHP) in these neurons are lacking. In order to study these phenomena, we first constructed and validated a biophysically detailed single compartment model of bladder small DRG neuron soma constrained by physiological data. The model includes twenty-two major known membrane mechanisms along with intracellular Ca2+ dynamics comprising Ca2+ diffusion, cytoplasmic buffering, and endoplasmic reticulum (ER) and mitochondrial mechanisms. Using modelling studies, we show that inward rectification of SKCa is an important parameter regulating neuronal repetitive firing and that its absence reduces action potential (AP) firing frequency. We also show that SKCa is more potent in reducing AP spiking than the large-conductance KCa channel (BKCa) in these neurons. Moreover, BKCa was found to contribute to the fast AHP (fAHP) and SKCa to the medium-duration (mAHP) and slow AHP (sAHP). We also report that the slow inactivating A-type K+ channel (slow KA) current in these neurons is composed of 2 components: an initial fast inactivating (time constant ∼ 25-100 ms) and a slow inactivating (time constant ∼ 200-800 ms) current. We discuss the implications of our findings, and how our detailed model can help further our understanding of the role of C-fibre afferents in the physiology of urinary bladder as well as in certain disorders.

Effects of nerve growth factor neutralization on TRP channel expression in laser-captured bladder afferent neurons in mice with spinal cord injury

Nerve growth factor (NGF) is reportedly involved in the changes in C-fiber bladder afferent pathways that induce detrusor overactivity (DO) following spinal cord injury (SCI). This study examined the roles of NGF in TRP channel expression in bladder afferent neurons in mice with SCI using laser-capture microdissection (LCM) methods. Spinal intact (SI) and SCI mice were divided into 3 groups: (1) SI with vehicle treatment; (2) SCI with vehicle treatment; and (3) SCI with anti-NGF antibody. Two weeks after SCI, an osmotic pump was placed subcutaneously into the back of the mice and vehicle or anti-NGF antibody was administered at a rate of 10 μg/kg per hour for two weeks. Four weeks after SCI, the L6 dorsal root ganglia (DRG) were removed. Expression of the TRPV1, TRPC1, TRPC3, and TRPC6 genes was analyzed using real-time polymerase chain reaction (PCR) following LCM of the bladder afferent neurons, which were labeled by Fast Blue injected into the bladder wall 1 week prior to tissue removal. The mRNA expression of TRPV1 was found to be higher in vehicle-treated SCI mice than in SI mice. The expression level of TRPC3 and TRPC6 in vehicle-treated SCI mice was lower than in SI mice. However, in SCI mice treated with anti-NGF antibody, the mRNA expression of TRPV1 was lower, and the mRNA levels of TRPC3 and TRPC6 were higher than in vehicle-SCI mice. These results suggest that the NGF-dependent changes in specific TRP channel genes, such as TRPV1, TRPC3, and TRPC6, could be involved in SCI-induced afferent hyperexcitability and DO.

Nerve growth factor-dependent hyperexcitability of capsaicin-sensitive bladder afferent neurones in mice with spinal cord injury

NEW FINDINGS

What is the central question of this study? Nerve growth factor (NGF) is reportedly a mediator inducing urinary bladder dysfunction. Is NGF directly involved in hyperexcitability of capsaicin-sensitive C-fibre bladder afferent pathways after spinal cord injury (SCI)? What is the main finding and its importance? Neutralization of NGF by anti-NGF antibody treatment reversed the SCI-induced increase in the number of action potentials and the reduction in spike thresholds and A-type K⁺ current density in mouse capsaicin-sensitive bladder afferent neurones. Thus, NGF plays an important and direct role in hyperexcitability of capsaicin-sensitive C-fibre bladder afferent neurones attributable to the reduction in A-type K⁺ channel activity in SCI.

ABSTRACT

Nerve growth factor (NGF) has been implicated as an important mediator in the induction of C-fibre bladder afferent hyperexcitability, which contributes to the emergence of neurogenic lower urinary tract dysfunction after spinal cord injury (SCI). In this study, we determined whether NGF immunoneutralization using an anti-NGF antibody (NGF-Ab) normalizes the SCI-induced changes in electrophysiological properties of capsaicin-sensitive C-fibre bladder afferent neurones in female C57BL/6 mice. The spinal cord was transected at the Th8/Th9 level. Two weeks later, continuous administration of NGF-Ab (10 μg kg^-1  h^-1 , s.c. for 2 weeks) was started. Bladder afferent neurones were labelled with Fast-Blue (FB), a fluorescent retrograde tracer, injected into the bladder wall 3 weeks after SCI. Four weeks after SCI, freshly dissociated L6-S1 dorsal root ganglion neurones were prepared. Whole-cell patch-clamp recordings were then performed in FB-labelled neurones. After recording action potentials or voltage-gated K⁺ currents, the sensitivity of each neurone to capsaicin was evaluated. In capsaicin-sensitive FB-labelled neurones, SCI significantly reduced the spike threshold and increased the number of action potentials during membrane depolarization for 800 ms. These SCI-induced changes were reversed by NGF-Ab. Densities of slow-decaying A-type K⁺ (K_A ) and sustained delayed rectifier-type K⁺ currents were significantly reduced by SCI. The NGF-Ab treatment reversed the SCI-induced reduction in the K_A current density. These results indicate that NGF plays an important role in hyperexcitability of mouse capsaicin-sensitive C-fibre bladder afferent neurones attributable to a reduction in K_A channel activity. Thus, NGF-targeting therapies could be effective for treatment of afferent hyperexcitability and neurogenic lower urinary tract dysfunction after SCI.

The effect of neutralization of nerve growth factor (NGF) on bladder and urethral dysfunction in mice with spinal cord injury

AIMS

To investigate the role of nerve growth factor (NGF) in lower urinary tract dysfunction in mice with spinal cord injury (SCI).

METHODS

Using 4-week SCI mice, single-filling cystometry and external urethral sphincter (EUS)-electromyography were performed under an awake condition. In some SCI mice, anti-NGF antibodies (10 µg/kg/h) were administered for 1 or 2 weeks before the urodynamic study. NGF levels in the bladder and L6/S1 spinal cord were assayed by ELISA. The transcript levels of P2X receptors and TRP channels in L6/S1 dorsal root ganglia (DRG) were measured by RT-PCR.

RESULTS

In SCI mice, the area under the curve of non-voiding contractions (NVCs) during the storage phase was significantly decreased in both 1- and 2-week anti-NGF antibody-treated SCI groups. However, EUS-electromyogram parameters during voiding were not altered by the treatment. Bladder mucosal and spinal NGF levels were decreased after 2 weeks of anti-NGF antibody treatment. TRPA1 and TRPV1 transcripts in L6/S1 DRG were significantly decreased after 1- or 2-week anti-NGF treatment.

CONCLUSIONS

In SCI mice, NGF is involved in the emergence of NVCs in association with increased expression of TRP receptors that are predominantly found in C-fiber afferent pathways. Thus, NGF targeting treatments could be effective for treating storage problems such as detrusor overactivity after SCI.