Nitric oxide synthase-containing nerves and ganglia in the dog prostate: a comparison with other transmitters

Emerging drugs to target lower urinary tract symptomatology (LUTS)/benign prostatic hyperplasia (BPH): focus on the prostate

OBJECTIVES

The benign prostatic syndrome, comprising lower urinary tract symptomatology secondary to benign prostatic hyperplasia/enlargement, represents a major health care issue in westernized countries. The pharmacological management involves alpha-adrenoceptor antagonists, intervention into the hormonal control of prostate growth using inhibitors of the enzyme 5-alpha-reductase, and stimulation of the nitric oxide/cyclic GMP pathway by tadalafil, an inhibitor of the phosphodiesterase type 5.

METHODS

This review summarizes the achievements which have been made in the development of drug candidates assumed to offer opportunities as beneficial treatment options in the management of the benign prostatic syndrome.

RESULTS

A review of the literature has revealed that the line of development is focusing on drugs interfering with peripheral neuromuscular/neuronal mechanisms (nitric oxide donor drugs, agonists/antagonists of endogenous peptides, botulinum toxin, NX-1207), the steroidal axis (cetrorelix) or the metabolic turn-over (lonidamine), as well as the combination of drugs already established in the treatment of lower urinary tract symptomatology/benign prostatic hyperplasia (phosphodiesterase 5 inhibitor plus alpha-adrenoceptor antagonist).

CONCLUSION

Many research efforts have provided the basis for the development of new therapeutic modalities for the management of lower urinary tract dysfunctions, some of which might be offered to the patients in the near future.

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.

Stereological Quantification of Nerve Fibers Immunoreactive to PGP 9.5, NPY, and VIP in Rat Prostate During Postnatal Development

This work was undertaken to study prostate innervation during the postnatal development of rats. It deals with the quantification of nervous fibers throughout all the regions of the rat prostate during the postnatal development using a general marker for nervous tissue, protein gene product 9.5, and 2 neuropeptides (NPY and VIP). Forty male Wistar rats (prepubertals, pubertals, young, and aged adults) were studied for immunohistochemistry of protein gene product (PGP 9.5), neuropeptide Y (NPY), and vasoactive intestinal polypeptide (VIP). They were also evaluated for length density of nerve fibers (L(V) PGP 9.5, L(V) NPY, L(V) VIP). Nerve fibers immunoreactive to the 3 antigens studied were detected in all the groups and in all the prostate zones. Periductal L(V) NPY evidenced a significant increase in the pubertal group, maintained throughout adult life. Periductal L(V) VIP showed a significant increase in young adults. The length densities of VIP and NPY fibers were significantly higher in periductal and ampular locations in comparison with dorsal and ventral sites. It can be concluded that the relative amount of nerve fibers in rat prostate, detected by PGP 9.5, does not change during postnatal development. There were significant changes in NPY and VIP fibers, showing an increase in periurethral ducts at puberty. The abundance of peptidergic innervation around the excretory ducts is related to their contractility. The development of innervation of periurethral ducts is regulated by androgens.

Phosphodiesterases (PDEs) and PDE inhibitors for treatment of LUTS

Lower urinary tract (LUT) smooth muscle can be relaxed by drugs that increase intracellular concentrations of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Both of these substances are degraded by phosphodiesterases (PDEs), which play a central role in the regulation of smooth muscle tone. The distribution and functional significance of PDE enzymes vary in different tissues of the LUT. Targeting specific PDE isoenzymes should thus allow organ selectivity. PDE 4 and 5 appear to predominate in the prostate, PDE 1 and 4 are thought to influence detrusor smooth muscle function, and PDE 5 may be functionally important in the urethra and vasculature. Studies on the use of PDE inhibitors to treat various LUT symptoms (LUTS), have yielded favorable results. Thus, positive effects of the PDE 5 inhibitors sildenafil and tadalafil on symptoms and quality of life in men with LUTS, erectile dysfunction, and BPH have also been demonstrated. These effects may be due to effects on cGMP signaling and/or modification of afferent input from bladder, urethral, and prostate tissue. This review gives an update on the distribution of PDEs in structures relevant for LUT function, and discusses how inhibition of these enzymes can contribute to beneficial effects on LUTS. Information for the review was obtained from searches of the PubMed database, and from the authors' files.

Nitric oxide/cGMP-mediated effects in the outflow region of the lower urinary tract--is there a basis for pharmacological targeting of cGMP?

Treatment with alpha-adrenoceptor antagonists that reduce the tone of prostatic stromal and urethral smooth muscle has beneficial effects in patients with benign prostatic hyperplasia (BPH) and lower urinary tracts symptoms (LUTS) and has brought attention to regulatory mechanisms of smooth muscle contractility of the outflow region. The prostate, urethra and bladder neck are densely supplied by nitric oxide (NO)-synthase-containing nerves that cause relaxation upon activation. In various experimental models, altered function or activity of the NO/cGMP pathway of the bladder neck and urethra may be related to inappropriate or un-coordinated functions of the bladder outlet and detrusor, but causal connections between alterations in this signaling system, a dysfunctional bladder outlet, and the development of LUTS are not established for humans. The present review focuses on regulatory functions of smooth muscle contractility by the NO/cGMP-pathway in the bladder neck, urethra, and prostate. Disease-related alterations in the NO/cGMP-pathway, and putative options for pharmacological modification of this signaling pathway in the out-flow region are briefly discussed.

Innervation of vas deferens and accessory male genital glands in the water buffalo (Bubalus bubalis). Neurochemical characteristics and relationships to the reproductive activity

Autonomic nerves supplying mammalian male internal genital organs have an important role in the regulation of reproductive function. To find out the relationships between the neurochemical content of these nerves and the reproductive activity, we performed a histochemical and immunohistochemical study in a species, the water buffalo, exhibiting a seasonal sexual behaviour. The distribution of noradrenergic and nitric oxide synthase (NOS)- and peptide-containing nerves was evaluated during the mating and non-mating periods. Fresh segments of vas deferens and accessory genital glands were collected immediately after slaughter and immersed in 4% paraformaldehyde. Frozen sections were obtained and processed according to single and double labelling immunofluorescent procedures or NADPH-diaphorase histochemistry. During the mating period, a dense noradrenergic innervation was observed to supply the vas deferens as well as the accessory genital glands. NOS- and peptide-containing nerves were also observed but with a lower density. During the non-mating period noradrenergic nerves dramatically reduced. In addition, neuropeptide Y (NPY)- and vasoactive intestinal peptide (VIP)-containing nerves were also reduced. These findings suggest the presence of complex interactions between androgen hormones and the autonomic nerve supply in the regulation of male water buffalo reproductive functions.

Morphological and functional characterization of a rat vaginal smooth muscle sphincter

The aim of the present study was to gain information about adrenergic-, cholinergic- and non-adrenergic, non-cholinergic (NANC)- transmitter systems/mediators in the rat vagina, and to characterize its smooth muscles functionally. Tissue sections from vagina of Sprague Dawley rats were immunolabelled with antibodies against protein gene product 9.5 (PGP), synaptophysin (Syn), tyrosine hydroxylase (TH), vesicular acetylcholine transporter (VAChT), neuropeptide Y (NPY), nitric oxide synthase (NOS), vasoactive intestinal polypeptide (VIP), calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating polypeptide (PACAP). Circularly cut vaginal smooth muscle preparations from the distal vagina were studied in organ baths. In the paravaginal tissue, a large number of PGP-, NOS-, TH-, VIP-immunoreactive (IR) and few CGRP-IR nerve trunks were observed, giving off branches to the smooth muscle wall. The smooth muscle wall was supplied by a large number of PGP-, Syn-, VAChT-, NPY-, NOS- and TH- IR nerve terminals, whilst only a moderate to few numbers of CGRP-, VIP- and PACAP-IR terminals were identified. Especially the distal part of the vaginal wall, where the circularly running smooth muscle was thickened into a distinct sphincter structure, was very richly innervated, predominantly by PGP- and NOS-IR terminals. Below and within the basal parts of the epithelium in the distal half of the vagina, a large number of PGP- and few NOS- and PACAP-IR varicose terminals were observed. The vaginal arteries were encircled by plexuses of nerve terminals. A large number of these were PGP-, Syn-, VAChT-, NOS-, TH-, NPY- and VIP-IR, and few were CGRP- and PACAP-IR. In isolated preparations of the distal vagina, electrical field stimulation (EFS) caused frequency-dependent contractions, which were reduced by sildenafil, tetrodotoxin (TTX) and phentolamine. In preparations contracted by norepinephrine (NA), EFS produced frequency-dependent relaxations. Pretreatment with the NOS-inhibitor N(G)-nitro-L-arginine, TTX, or the inhibitor of soluble guanylate cyclase, ODQ, abolished the EFS relaxations. In NE precontracted preparations, cumulative addition of sildenafil caused concentration-dependent relaxation. Carbachol contracted the strips concentration-dependently from baseline. It can be concluded that the distal part of the rat vagina forms a distinct smooth muscle sphincter, which is richly innervated by adrenergic, cholinergic and NANC nerves. The present studies suggest that in the rat the L-arginine/NO-system not only plays an important role in the regulation of vaginal smooth muscle tone, but also affects blood flow, and may have sensory functions.

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.

Akt-dependent phosphorylation of endothelial nitric-oxide synthase mediates penile erection

In the penis, nitric oxide (NO) can be formed by both neuronal NO synthase and endothelial NOS (eNOS). eNOS is activated by viscous drag/shear stress in blood vessels to produce NO continuously, a process mediated by the phosphatidylinositol 3-kinase (PI3kinase)/Akt pathway. Here we show that PI3-kinase/Akt physiologically mediates erection. Both electrical stimulation of the cavernous nerve and direct intracavernosal injection of the vasorelaxant drug papaverine cause rapid increases in phosphorylated (activated) Akt and eNOS. Phosphorylation is diminished by wortmannin and LY294002, inhibitors of PI3-kinase, the upstream activator of Akt. The two drugs also reduce erection. Penile erection elicited by papaverine is reduced profoundly in mice with targeted deletion of eNOS. Our findings support a model in which rapid, brief activation of neuronal NOS initiates the erectile process, whereas PI3-kinase/Akt-dependent phosphorylation and activation of eNOS leads to sustained NO production and maximal erection.

Endogenous nitric oxide-mediated relaxation and nitrinergic innervation in the rabbit prostate: the changes with aging

BACKGROUND

Nitric oxide (NO) plays the key role in the non-adrenergic non-cholinergic (NANC) nerve-mediated relaxation of prostate. We tried to determine whether nitrinergic innervation of the prostate is reduced with aging, and whether a reduction of this innervation alters the relaxant properties of prostatic tissue.

METHODS

The prostate isolated from young rabbits at the age of 3 months and aged rabbits at the age of 24 months was used. Pharmacologic experiments using electrical field stimulation (EFS) were performed on strips of prostate. Nitrinergic nerves were identified histochemically by the presence of NADPH diaphorase reactivity.

RESULTS

The prostate weighed 0.34 +/- 0.06 g in the young and 0.78 +/- 0.13 g in the aged (P < 0.01). Electrical stimulation caused frequency-dependent relaxation that was inhibited by N-nitro-L-arginine and increased by L-arginine. Maximum relaxant rates of noradrenaline-induced tone were 41.3 +/- 1.8% and 22.7 +/- 0.6% in the young and the aged (P < 0.01), respectively. Exogenous NO caused concentration-dependent relaxation of the prostate. Both relaxation induced by electrical stimulation and exogenous NO were inhibited by LY83583 (a guanylate cyclase inhibitor). In all specimens, NADPH diaphorase activity was observed in the smooth muscle layer of the prostatic stroma and capsule. However, NADPH diaphorase positive nerves were significantly less in the aged than in the young (P < 0.01).

CONCLUSIONS

As a result of these findings, it is suggested that both NO-mediated relaxation and nitrinergic innervation are reduced with aging in this animal model.

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.

Isolation and characterization of a novel, human neuronal nitric oxide synthase cDNA

A splice variant of nNOS has recently been identified in both rat and mouse which contains an in-frame insertion of 34 conserved amino acids between the N-terminal oxygenase and the C-terminal reductase domains. In the present study we report the isolation and characterization of a similar, but not identical (76% amino acid identity), human variant (nNOSmu) which arises from the splicing in of an additional exon between exons 16 and 17 of the human nNOS gene. Furthermore, we describe two additional splice variants which, if translated, would give rise to truncated forms of nNOS lacking the C-terminal reductase domain. These additional variants and nNOSmu; have a distinct and more restricted expression pattern compared to nNOS. Further studies are required to elucidate the possible physiological roles of these novel human nNOS splice variants in NO signaling.

Heme oxygenase and NO-synthase in the human prostate--relation to adrenergic, cholinergic and peptide-containing nerves

In the human prostate, the distribution of heme oxygenase (HO-1 and HO-2)-, nitric oxide synthase (NOS)-, and tyrosine hydroxylase (TH)-immunoreactive (IR), acetylcholine-esterase (AChE)-positive, and some peptidergic nerve structures was investigated. Cell bodies and nerve fibers within coarse nerve trunks expressed HO-1-, HO-2-, NOS-, TH-, and vasoactive intestinal polypeptide (VIP)-immunoreactivities, and were AChE-positive, but, as revealed by confocal microscopy. HO- and NOS-immunoreactivities were found in separate nerves. Along strains of smooth muscle, intraglandular septa, and around acini, HO-1-, NOS-, and VIP-IR nerves, and AChE-positive fibers were observed. Double immunostaining showed that NOS- and VIP-immunoreactivities were generally co-localized in varicose nerve terminals. Some TH-IR terminals had profiles that were similar, but not identical, to those of NOS-, HO-1-, or VIP-IR terminals. NPY-IR nerves were similarly distributed as VIP- and NOS-IR fibers, and were found in rich amounts. Calcitonin gene-related peptide (CGRP)-IR nerves were few compared to other nerve populations studies. NOS- and CGRP-IR terminals had similar profiles, but the immunoreactivities were not co-localized. Nitric oxide and electrical stimulation of nerves relaxed noradrenaline-contracted preparations of prostatic stroma. Inhibition of synthesis of nitric oxide abolished the electrically induced relaxations. VIP had small relaxant effects, whereas carbon monoxide was without effect on noradrenaline-contracted strips. The innervation pattern and the functional effects suggest that the L-arginine/nitric oxide pathway may have a role in the control of human prostatic smooth muscle activity and/or in secretory neurotransmission. A physiological role of carbon monoxide in the prostate remains to be established.