Functional roles of bladder α1-adrenoceptors in the activation of single-unit primary bladder afferent activity in rats

Novel pharmacotherapeutic avenues for bladder storage dysfunction in men

INTRODUCTION

Bladder storage dysfunction is associated with low quality of life in men and remains a challenging field in pharmacotherapy because of low persistence followed by patient-perceived lack of efficacy and adverse effects. The persistent desire for the development of novel pharmacotherapy is evident, leading to numerous research efforts based on its pathophysiology.

AREAS COVERED

This review describes the pathophysiology, current pharmacotherapeutic strategies, and emerging novel drugs for male bladder storage dysfunction. The section on emerging pharmacotherapy provides an overview of current research, focusing on high-potential target molecules, particularly those being evaluated in ongoing clinical trials.

EXPERT OPINION

As pharmacotherapies targeting alpha-adrenergic, beta-adrenergic, and muscarinic receptors - the current primary targets for treating male bladder storage dysfunction - have demonstrated insufficient efficacy and side effects, researchers are exploring various alternative molecular targets. Numerous targets have been identified as central to regulating bladder afferent nerve activity, and their pharmacological effects and potential have been evaluated in animal-based experiments. However, there is a limited number of clinical trials for these new pharmacotherapies, and they have not demonstrated clear superiority over current treatments. Further research is needed to develop new effective pharmacotherapies for bladder storage dysfunction in men.

The Influence of an Adrenergic Antagonist Guanethidine on the Distribution Pattern and Chemical Coding of Caudal Mesenteric Ganglion Perikarya and Their Axons Supplying the Porcine Bladder

This study was aimed at disclosing the influence of intravesically instilled guanethidine (GUA) on the distribution, relative frequency and chemical coding of both the urinary bladder intramural sympathetic nerve fibers and their parent cell bodies in the caudal mesenteric ganglion (CaMG) in juvenile female pigs. GUA instillation led to a profound decrease in the number of perivascular nerve terminals. Furthermore, the chemical profile of the perivascular innervation within the treated bladder also distinctly changed, as most of axons became somatostatin-immunoreactive (SOM-IR), while in the control animals they were found to be neuropeptide Y (NPY)-positive. Intravesical treatment with GUA led not only to a significant decrease in the number of bladder-projecting tyrosine hydroxylase (TH) CaMG somata (94.3 ± 1.8% vs. 73.3 ± 1.4%; control vs. GUA-treated pigs), but simultaneously resulted in the rearrangement of their co-transmitters repertoire, causing a distinct decrease in the number of TH⁺/NPY⁺ (89.6 ± 0.7% vs. 27.8 ± 0.9%) cell bodies and an increase in the number of SOM-(3.6 ± 0.4% vs. 68.7 ± 1.9%), calbindin-(CB; 2.06 ± 0.2% vs. 9.1 ± 1.2%) or galanin-containing (GAL; 1.6 ± 0.3% vs. 28.2 ± 1.3%) somata. The present study provides evidence that GUA significantly modifies the sympathetic innervation of the porcine urinary bladder wall, and thus may be considered a potential tool for studying the plasticity of this subdivision of the bladder innervation.

Effect of Alpha 1-Adrnoceptor Antagonists on Postsynaptic Sensitivity in Substantia Gelatinosa Neurons From Lumbosacral Spinal Cord in Rats Using Slice Patch-Clamp Technique for mEPSC

Purpose

Alpha1-adrenoceptors participate in improving storage symptoms of male lower urinary tract symptoms (LUTS). However, the mechanism of action of these compounds remains unclear. To clarify the mechanism of the α1-adrenoceptor antagonists, the amplitude of miniature excitatory postsynaptic currents (mEPSCs) was analyzed in the lumbosacral spinal cord in rats.

Methods

Male adult Sprague-Dawley rats were used. Blind whole-cell patch-clamp recordings were performed on substantia gelatinosa (SG) neurons in spinal cord slice preparations. The amplitude of mEPSCs was recorded in individual SG neurons to which α1-adrenoceptors (100μM naftopidil, 100μM tamsulosin, and 30μM silodosin) were applied sequentially with intervening washout periods. Individual amplitudes were analyzed.

Results

Pearson correlation coefficients (r) for the amplitudes of mEPSCs between the baseline and postadministration of α1-adrenoceptor antagonists indicated changes of the amplitude ranked in the order of naftopidil (r =0.393), tamsulosin (r=0.738), and silodosin (r=0.944). Together, the α1-adrenoceptor antagonists yielded significant increases in the amplitude of mEPSCs in SG neurons (n=108, P=0.012). However, the effects of each α1-adrenoceptor antagonist on the amplitude were as follows (relative to the baseline; n=36 each): naftopidil, P=0.129; tamsulosin, P=0.201; and silodosin, P=0.005. The rate of response to naftopidil for the outward current was relatively high among the α1-adrenoceptor blockers. An inward current was observed only with the naftopidil application.

Conclusions

Alpha1-adrenoceptor antagonists changed the amplitudes of mEPSCs in a subset of SG neurons in slices prepared from the L6–S1 levels of rat spine. Although the α1-adrenoceptor antagonists generated inward or outward currents in the SG neurons, different rates of response were observed with each antagonist. These results are important for understanding the mechanisms of action (at the spinal level) of α1-adrenoceptor antagonists for the storage symptoms of male LUTS.

Effects of silodosin and tadalafil on bladder dysfunction in spontaneously hypertensive rats: Possible role of bladder blood flow

OBJECTIVES

To investigate the effects of an alpha1-adrenoceptor antagonist, silodosin, or a phosphodiesterase type 5 inhibitor, tadalafil, on bladder overactivity in spontaneously hypertensive rats.

METHODS

Twelve-week-old male spontaneously hypertensive rats were perorally administered silodosin (100 µg/kg), tadalafil (2 or 10 mg/kg) or vehicle once daily for 6 weeks. Wistar rats were used as normotensive controls and were treated with the vehicle. At 18-weeks-old, the effects of silodosin or tadalafil on blood pressure, bladder blood flow, urodynamic parameters (i.e. micturition frequency, urine output, inter-contraction interval, maximum voiding pressure, single voided volume and post-voiding residual urine volume), and bladder tissue levels of malondialdehyde, interleukin-6 and tumor necrosis factor-alpha were measured.

RESULTS

A significant increase in blood pressure, micturition frequency and bladder tissue levels of malondialdehyde, interleukin-6 and tumor necrosis factor-alpha was noted in spontaneously hypertensive rats. The single voided volume, bladder capacity and bladder blood flow were significantly lower in the spontaneously hypertensive rats than in the Wistar rats. Treatment with silodosin and the higher dose of tadalafil improved the urodynamic parameters, bladder blood flow and bladder tissue levels of malondialdehyde in the spontaneously hypertensive rats without affecting the blood pressure and bladder tissue levels of interleukin-6 and tumor necrosis factor-alpha.

CONCLUSIONS

Treatment with silodosin or tadalafil might improve hypertension-related bladder overactivity, as shown in spontaneously hypertensive rats through an improvement in the bladder blood flow and bladder tissue levels of oxidative stress.

[Afferent nerve activity in relation to bladder sensation]

Bladder afferent nerves are composed by myelinated Aδ- and unmyelinated C-fibers. During the storage phase of urine, distention of the bladder has long been considered to evoke afferent activity via Aδ-fibers connected in series with the smooth muscle fibers. In contrast, a previous study in cats revealed that more than 90% of C-fibers do not respond to normal bladder distension, being so called 〝silent〟 fibers. However, at least in rats, C-fibers can respond to normal bladder distension like Aδ-fibers, although they may also fulfill a potentially different role in the bladder sensory function in response to abnormal stimuli. The symptoms of overactive bladder (OAB) or interstitial cystitis (IC) are believed to be commonly related to the sensory (afferent) function. In addition, it has been suggested that bladder myogenic microcontractions or micromotions may partly contribute to the development of urgency in OAB related to bladder outlet obstruction (BOO), which is one of cause of benign prostatic hyperplasia (BPH). We have investigated the direct effects of drugs (anticholinergics, β3-adrenoceptor agonists, α1-adrenoceptor antagonists, PDE type5 inhibitors, etc.) on the bladder afferent function in rodents. In our results, almost all drugs may act on the bladder afferent function, and some of drug (e.g. mirabegron) inhibits the afferent activities through the suppression of the bladder myogenic microcontractions in normal or pathophysiological conditions.

Capsaicin-Sensitive Sensory Nerves Indirectly Modulate Motor Function of the Urinary Bladder

Purpose

The urinary bladder (UB) is innervated by both sensory and autonomic nerves. Recent studies have shown that sensory neuropeptides induced contractions in the detrusor muscle. Therefore, in a mouse model, we investigated the presence of interactions between the submucosal sensory nerves and the autonomic nerves that regulate the motor function of the detrusor muscle.

Methods

UB samples from male C57BL/6 mice were isolated, cut into strips, and mounted in an organ bath. Dose-response curves to norepinephrine and phenylephrine were studied in UB strips with and without mucosa, and the effects of preincubation with a receptor antagonist and various drugs on relaxation were also studied using tissue bath myography.

Results

Phenylephrine-induced relaxation of the UB strips showed concentration-related effects. This relaxation appeared in both mucosa-intact and mucosa-denuded UB strips, and was significantly inhibited by lidocaine, silodosin, and guanethidine (an adrenergic neuronal blocker). Meanwhile, phenylephrine-induced relaxation was inhibited by pretreatment with propranolol and calcitonin gene-related peptide (CGRP)–depletory capsaicin in UB strips with and without mucosa.

Conclusions

The present study suggests that phenylephrine activates the α-1A adrenergic receptor (AR) of the sensory nerve, and then activates capsaicin-sensitive sensory nerves to release an unknown substance that facilitates the release of norepinephrine from adrenergic nerves. Subsequently, norepinephrine stimulates β-ARs in the detrusor muscle in mice, leading to neurogenic relaxation of the UB. Further animal and human studies are required to prove this concept and to validate its clinical usefulness.