Characterization of the muscarinic receptor mediating contraction of the dog prostate

Generation and Regulation of Spontaneous Contractions in the Prostate

Spontaneous myogenic contractions have been shown to be significantly upregulated in prostate tissue collected from men with Benign Prostatic Hyperplasia (BPH), an extremely common disorder of the ageing male. Although originally thought likely to be involved in 'housekeeping' functions, mixing prostatic secretions to prevent stagnation, these spontaneous myogenic contractions provide a novel opportunity to understand and treat BPH. This treatment potential differs from previous models, which focused exclusively on attenuating nerve-mediated neurogenic contractions. Previous studies in the rodent prostate have provided an insight into the mechanisms underlying the regulation of myogenic contractions. 'Prostatic Interstitial Cells' (PICs) within the prostate appear to generate pacemaker potentials, which arise from the summation of number of spontaneous transient depolarisations triggered by the spontaneous release of Ca²⁺ from internal stores and the opening of Ca²⁺-activated Cl^- channels. Pacemaker potentials then conduct into neighbouring smooth muscle cells to generate spontaneous slow waves. These slow waves trigger the firing of 'spike-like' action potentials, Ca²⁺ entry and contraction, which are not attenuated by blockers of neurotransmission. However, these spontaneous prostatic contractions can be modulated by the autonomic nervous system. Here, we discuss the mechanisms underlying rodent and human prostate myogenic contractions and the actions of existing and novel pharmacotherapies for the treatment of BPH. Understanding the generation of human prostatic smooth muscle tone will confirm the mechanism of action of existing drugs, inform the identification and effectiveness of new pharmacotherapies, as well as predict patient outcomes.

Correlation between Cholinergic Innervation, Autophagy, and Etiopathology of Benign Prostatic Hyperplasia

BACKGROUND

Whether cholinergic innervations and/or autophagy have a role in the etiopathology of benign prostatic hyperplasia (BPH) is still unknown. This study aimed to investigate the role of cholinergic innervation and autophagy in the etiopathology of BPH.

METHODS

Male, 13-week-old spontaneous hypertension rats (spontaneous BPH animal model) were divided into three groups: an experimental group (EG, n = 24), a control group (CG, n = 24), and a normal control group (NC, n = 10). The EG animals were intragastrically injected with tolterodine (3.5 mg/kg, twice a day), CG animals were intragastrically injected with physiological saline, and the NC animals did not receive any treatment. Rats were sacrificed every 4 weeks, and the prostatic gross morphological changes, wet weight/body weight (ww/bw), dry weight/wet weight (dw/ww), histological changes, ultrastructural changes, and LC3 immunohistochemistry were continuously observed and compared.

RESULTS

The gross morphological and ww/bw changes in the three groups were similar at every stage. The dw/ww (mg/mg) values of the EG at week 17, 21, 25, and 29 were 0.1478 ± 0.0034, 0.1653 ± 0.0036, 0.1668 ± 0.0045, and 0.1755 ± 0.0034, respectively, and the CG values were 0.1511 ± 0.0029, 0.1734 ± 0.0020, 0.1837 ± 0.0052, and 0.1968 ± 0.0045, respectively. The difference between EG and CG for dw/ww showed statistical significance after 21 weeks of age (week 21: P= 0.016, week 25: P= 0.008, and week 29: P= 0.001). Both EG and CG, prostatic glandular epithelial cell proliferation, and secretory function improved with age, but in EG, these improvements were slower than those in CG, and all the differences were statistically significant after 21 weeks. An increasing number of autophagosomes in the prostatic glandular cell cytoplasm, attenuation of LC3-I immunohistochemical staining, enhancement of LC3-II staining, and the ratio of LC3-II/LC3-I staining were all progressive in both groups, but the rate of change in EG was faster than that in CG, and these differences gained statistical significance after 25 weeks. Comparisons with regard to the above indexes between CG and NC showed no statistical significance at any stage.

CONCLUSIONS

Cholinergic innervations and activation of autophagy appear to have important functions in the etiopathology of BPH. Drug-mediated blockade of cholinergic innervations could delay the physiopathology processes. Moreover, overactivation of autophagy may also play an important role in this delay.

Relaxing effect of acetylcholine on phenylephrine-induced contraction of isolated rabbit prostate strips is mediated by neuronal nitric oxide synthase

Purpose

The location of acetylcholinesterase-containing nerve fibers suggests a role for acetylcholine in both contractility and secretion in the prostate gland. The colocalization of nitrergic nerves with cholinergic nerves, and the cotransmission of nitric oxide with acetylcholine in cholinergic nerves, has been demonstrated in the prostate glands of various species. Thus, we investigated the effects of acetylcholine on phenylephrine-induced contraction and the correlation between cholinergic transmission and nitric oxide synthase by using isolated prostate strips of rabbits.

Materials and Methods

Isolated prostate strips were contracted with phenylephrine and then treated with cumulative concentrations of acetylcholine. Changes in acetylcholine-induced relaxation after preincubation with NG-nitroarginine methyl ester, 7-nitroindazole, and aminoguanidine were measured. The effects of selective muscarinic receptor antagonists were also evaluated.

Results

In the longitudinal phenylephrine-contracted strip, the cumulative application of acetylcholine (10^-9 to 10^-4 M) elicited a concentration-dependent relaxation effect. Acetylcholine-induced relaxation was inhibited not only by nitric oxide synthase inhibitors (10 µM L-NAME or 10 µM 7-nitroindazole) but also by 10 µM atropine and some selective muscarinic receptor antagonists (10^-6 M 11-([2-[(diethylamino)methyl]-1-piperdinyl]acetyl)-5,11-dihydro-6H-pyrido[2,3-b][1,4]benzodiazepine-6-one and 10^-6 M 4-diphenylacetoxy-N-methyl-piperidine). In contrast, relaxation was significantly increased by pretreatment of the strips with 10 mM L-arginine.

Conclusions

Acetylcholine relaxed phenylephrine-induced contractions of isolated rabbit prostate strips. This relaxation may be mediated via both cholinergic and constitutive nitric oxide synthase with both the M2 and M3 receptors possibly playing key roles.

Age-related changes in the innervation of the prostate gland: implications for prostate cancer initiation and progression

The adult prostate gland grows and develops under hormonal control while its physiological functions are controlled by the autonomic nervous system. The prostate gland receives sympathetic input via the hypogastric nerve and parasympathetic input via the pelvic nerve. In addition, the hypogastric and pelvic nerves also provide sensory inputs to the gland. This review provides a summary of the innervation of the adult prostate gland and describes the changes which occur with age and disease. Growth and development of the prostate gland is age dependent as is the occurrence of both benign prostate disease and prostate cancer. In parallel, the activity and influence of both the sympathetic and parasympathetic nervous system changes with age. The influence of the sympathetic nervous system on benign prostatic hyperplasia is well documented and this review considers the possibility of a link between changes in autonomic innervation and prostate cancer progression.

Novel drug targets for the pharmacotherapy of benign prostatic hyperplasia (BPH)

Benign prostatic hyperplasia (BPH) is the major cause of lower urinary tract symptoms in men aged 50 or older. Symptoms are not normally life threatening, but often drastically affect the quality of life. The number of men seeking treatment for BPH is expected to grow in the next few years as a result of the ageing male population. Estimates of annual pharmaceutical sales of BPH therapies range from $US 3 to 10 billion, yet this market is dominated by two drug classes. Current drugs are only effective in treating mild to moderate symptoms, yet despite this, no emerging contenders appear to be on the horizon. This is remarkable given the increasing number of patients with severe symptoms who are required to undergo invasive and unpleasant surgery. This review provides a brief background on prostate function and the pathophysiology of BPH, followed by a brief description of BPH epidemiology, the burden it places on society, and the current surgical and pharmaceutical therapies. The recent literature on emerging contenders to current therapies and novel drug targets is then reviewed, focusing on drug targets which are able to relax prostatic smooth muscle in a similar way to the α(1) -adrenoceptor antagonists, as this appears to be the most effective mechanism of action. Other mechanisms which may be of benefit are also discussed. It is concluded that recent basic research has revealed a number of novel drug targets such as muscarinic receptor or P2X-purinoceptor antagonists, which have the potential to produce more effective and safer drug treatments.

The residual nonadrenergic contractile response to nerve stimulation of the mouse prostate is mediated by acetylcholine but not ATP in a comparison with the mouse vas deferens

Neuronal release of noradrenaline is primarily responsible for the contraction of prostatic smooth muscle in all species, and this forms the basis for the use of α(1)-adrenoceptor antagonists as pharmacotherapies for benign prostatic hyperplasia. Previous studies in mice have demonstrated that a residual nonadrenergic component to nerve stimulation remains after α(1)-adrenoceptor antagonism. In the guinea pig and rat prostate and the vas deferens of guinea pigs, rats, and mice, ATP is the mediator of this residual contraction. This study investigates the mediator of residual contraction in the mouse prostate. Whole prostates from wild-type, α(1A)-adrenoceptor, and P2X1-purinoceptor knockout mice were mounted in organ baths, and the isometric force that tissues developed in response to electrical field stimulation or exogenously applied agonists was recorded. Deletion of the P2X1 purinoceptor did not affect nerve-mediated contraction. Furthermore, the P2-purinoceptor antagonist suramin (30 μM) failed to attenuate nerve-mediated contractions in wild-type, α(1A)-adrenoceptor, or P2X1-purinoceptor knockout mice. Atropine (1 μM) attenuated contraction in prostates taken from wild-type mice. In the presence of prazosin (0.3 μM) or guanethidine (10 μM), or in prostates taken from α(1A)-adrenoceptor knockout mice, residual nerve-mediated contraction was abolished by atropine (1 μM), but not suramin (30 μM). Exogenously administered acetylcholine elicited reproducible concentration-dependent contractions of the mouse prostate that were atropine-sensitive (1 μM), but not prazosin-sensitive (0.3 μM). Acetylcholine, but not ATP, mediates the nonadrenergic component of contraction in the mouse prostate. This cholinergic component of prostatic contraction is mediated by activation of muscarinic receptors.

Current models of human prostate contractility

1. The human prostate is a compact gland contributing to seminal fluid. With increasing age, most humans will develop benign prostatic hyperplasia, a condition of prostatic enlargement and contractility that leads to occlusion of the urethra. Over many years, investigators have used a variety of animal and cell culture models to elucidate some of the contractile and proliferative mechanisms that may be associated with the development of this condition. 2. This review briefly assesses the current state of knowledge of the mechanisms underlying human prostatic contractility and compares it with that of animal and cell culture models. It is not intended as a comprehensive methodological review, nor is it intended to indicate our preferences for either model. Our aim is to correlate findings from animal and cell culture models with the current understanding of human prostate contractility. 3. We hope that the present review will increase awareness of the suitability of the current models in developing our understanding of benign prostatic hyperplasia.

Effect of neurokinins on canine prostate cell physiology

BACKGROUND

Sensory peptide neurotransmitters have been implicated as significant regulators of prostate growth. This study was designed to evaluate the role of neurokinins in proliferation, differentiation, and contraction of canine prostate cells in culture.

METHODS

NK1, NK2, and NK3 receptor subtypes were localized in canine prostate tissue by immunocytochemistry and ligand binding studies. Functional effects of neurokinin agonists were tested on cell differentiation (expression of smooth muscle actin (SMA)), proliferation (MTS assay), and contraction of canine prostate cells in culture.

RESULTS

Immunocytochemical staining of canine prostate sections revealed strong stromal staining for NK1 together with weak stromal staining for NK2 and even weaker staining for NK3. Furthermore, there was overlapping localization of NK1 receptors, substance P (SP), and calcitonin gene-regulated peptide (CGRP) in prostate tissue sections. SP caused concentration-dependent increase in SMA expression that was attenuated in a concentration-dependent manner by YM-44778, a non-selective antagonist for neurokinin receptors, but not by either the NK2 antagonist (SR-48968) nor by the NK3 antagonist (SB-223412). SP and neurokinin A (NKA) also caused a modest contraction of stromal cells in collagen gels. NKA stimulated proliferation of prostate epithelial cells without any apoptotic effect, which was attenuated by SR-48968. Surprisingly, in binding studies NK3 appeared to be the most abundant neurokinin receptor subtype, although functional studies failed to reveal significant coupling of this receptor.

CONCLUSIONS

Our results suggest that, at least in vitro, neurokinins have modest effects on canine prostate epithelial cell proliferation, stromal differentiation, and contraction.

Cholinergic innervation and function in the prostate gland

The mammalian prostate is densely innervated by hypogastric and pelvic nerves that play an important role in regulating the growth and function of the gland. While there has been much interest in the role of the noradrenergic innervation and adrenoceptors in prostate function, the role of cholinergic neurones in prostate physiology and pathophysiology is not well understood. This review focuses on the role of acetylcholine and cholinoceptors in prostate function. Nitric oxide, vasoactive intestinal polypeptide, and/or neuropeptide Y are co-localised with cholinesterase and/or acetylcholine transporter in some of the nerve fibres supplying the prostate. Their roles are also briefly discussed in this review. A dense network of cholinesterase-staining fibres supplies both prostate epithelium and stroma, suggesting a role of acetylcholine and/or co-localised neuropeptides in the modulation of prostatic secretions, as well as smooth muscle tone. A predominantly epithelial location for prostate muscarinic receptors indicated a major secretomotor role for acetylcholine. The muscarinic receptor subtype mediating muscarinic agonist-induced smooth muscle contraction or enhancement of contractions evoked by nerve stimulation differs in different species. In the human, there is evidence for M(1) receptors on the epithelium, M(2) receptors on the stroma, and both M(1) and M(3) receptors in some prostate cancer cell lines. Several recent investigations indicate that muscarinic receptors may also mediate or modulate normal, benign, and malignant prostate growth. The role of muscarinic agonists and their receptors and the influences of age, testicular, and other steroids in regulating the effects are reviewed.

Tyrosine kinase A, autonomic and transmitter receptors, but not innervation, are upregulated in the aging rat prostate

In the mature rat ventral prostate, epithelial proliferation is accompanied by significant upregulation of tyrosine kinase A, alpha1B and muscarinic acetylcholine M2 receptors as well as the synaptic vesicle-associated membrane proteins synaptobrevin and SV2 as compared with immature prostate tissue. The adrenergic receptors beta1, alpha2A and alpha1 were also up-regulated and translocated in mature rat prostate tissue. Expression of the Schwann cell/axonal marker S100 remained unchanged. These results are suggestive of a marked increase in metabolic activity, calcium influx and autonomic receptor expression in the aging prostate. These changes were not accompanied by an increase in the number of axons.

The autonomic and sensory innervation of the smooth muscle of the prostate gland: a review of pharmacological and histological studies

1. We review literature demonstrating (a) the presence and (b) the actions of substances that mediate or modify neuroeffector transmission to the smooth muscle of the prostrate stroma of a number of species including man. 2. In all species studied prostatic stroma, but not secretory acini, receives rich noradrenergic innervation. Stimulation of these nerves causes contractions of prostate smooth muscle that are inhibited by guanethidine and by alpha1-adrenoceptor antagonists that probably act at the alpha1L-adrenoceptor. Such actions underlie the clinical use of alpha1-adrenoceptor antagonists in benign prostatic hyperplasia (BPH). 3. Acetylcholinesterase-positive nerves innervate prostatic stroma as well as epithelium. Atropine reduces nerve-mediated contractions of stromal muscle in the rat, guinea-pig and rabbit. M1, M2 and M3 muscarinic receptors have been implicated in eliciting or facilitating contraction in the prostate from guinea-pig, dog and rat, respectively. 4. Adenine nucleotides and nucleosides, nitric oxide (NO), opioids, neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP) may act as co-transmitters or modulators in autonomic effector nerves supplying prostate stroma. Adenosine inhibits neurotransmission to the rat prostate, and NO is inhibitory in prostate from human, rat, rabbit, pig and dog. The activity of peptides present in the relatively sparse sensory innervation of the prostate exhibits species variation, but, when effective, calcitonin gene-related peptide is inhibitory while tachykinins are stimulant. The roles of NPY and VIP in modulating stromal contractility remain unclear. 5. Taken together the current literature indicates that, in addition to noradrenaline, other neurotransmitters and neuromodulators may regulate the tone of prostatic smooth muscle. Whether drugs that mimic or modify their actions might be useful in providing symptomatic relief of the urinary symptoms associated with BPH remains to be established.

Cholinergic facilitation of neurotransmission to the smooth muscle of the guinea-pig prostate gland

1. Functional experiments have been conducted to assess the effects of acetylcholine and carbachol, and the receptors on which they act to facilitate neurotransmission to the stromal smooth muscle of the prostate gland of the guinea-pig. 2. Acetylcholine and carbachol (0.1 microM - 0.1 mM) enhanced contractions evoked by trains of electrical field stimulation (20 pulses of 0.5 ms at 10 Hz every 50 s with a dial setting of 60 V) of nerve terminals within the guinea-pig isolated prostate. In these concentrations they had negligible effects on prostatic smooth muscle tone. 3. The facilitatory effects of acetylcholine, but not those of carbachol, were further enhanced in the presence of physostigmine (10 microM). 3. The facilitatory effects of carbachol were unaffected by the neuropeptide Y Y(1) receptor antagonist BIBP 3226 ((R)-N(2)-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]-arginina mide) (0.3 microM, n=3) or suramin (100 microM, n=5). Prazosin (0.1 microM, n=5) and guanethidine (10 microM, n=5) alone and in combination (n=4), reduced responses to field stimulation and produced rightward shifts of the log concentration-response curves to carbachol. 4. The rank orders of potency of subtype-preferring muscarinic receptor antagonists in inhibiting the facilitatory actions of acetylcholine and carbachol were: pirenzepine > HHSiD (hexahydrosiladifenidol) > pF-HHSiD (para-fluoro-hexahydrosiladifenidol)>/= 5 himbacine, and pirenzepine > HHSiD > himbacine>/= 5 pF-HHSiD, respectively. These profiles suggest that muscarinic receptors of the M(1)-subtype mediate the facilitatory effects of acetylcholine and carbachol on neurotransmission to the smooth muscle of the guinea-pig prostate.