Vasoactive intestinal polypeptide (VIP) as a putative neurotransmitter in penile erection

Testis–Gut-Reproduction Axis: The Key to Reproductive Health

Reproductive health is an important issue for humanity. In the context of the increasing incidence rate of male infertility, it is essential to find the factors that affect male reproductive health. Gastrointestinal health is closely related to reproductive health. Gastrointestinal hormones (GIH) and gut microbiota (GM), as important material foundations for gastrointestinal function, can promote or inhibit testicular reproductive function, including spermatogenesis, sperm maturation, androgen synthesis, and even broader male diseases such as sexual function, prostate cancer, etc. On the contrary, the functional health of the testes is also of great significance for the stability of gastrointestinal function. This review mainly discusses the important regulatory effects of GIH and GM on male reproductive function.

Erectile Dysfunction in Men on the Rise: Is There a Link with Endocrine Disrupting Chemicals?

Erectile dysfunction (ED) is one of the most prevalent chronic conditions affecting men. ED can arise from disruptions during development, affecting the patterning of erectile tissues in the penis and/or disruptions in adulthood that impact sexual stimuli, neural pathways, molecular changes, and endocrine signalling that are required to drive erection. Sexual stimulation activates the parasympathetic system which causes nerve terminals in the penis to release nitric oxide (NO). As a result, the penile blood vessels dilate, allowing the penis to engorge with blood. This expansion subsequently compresses the veins surrounding the erectile tissue, restricting venous outflow. As a result, the blood pressure localised in the penis increases dramatically to produce a rigid erection, a process known as tumescence. The sympathetic pathway releases noradrenaline (NA) which causes detumescence: the reversion of the penis to the flaccid state. Androgen signalling is critical for erectile function through its role in penis development and in regulating the physiological processes driving erection in the adult. Interestingly, estrogen signalling is also implicated in penis development and potentially in processes which regulate erectile function during adulthood. Given that endocrine signalling has a prominent role in erectile function, it is likely that exposure to endocrine disrupting chemicals (EDCs) is a risk factor for ED, although this is an under-researched field. Thus, our review provides a detailed description of the underlying biology of erectile function with a focus on the role of endocrine signalling, exploring the potential link between EDCs and ED based on animal and human studies.

Substance P (SP) and vasoactive intestinal polypeptide (VIP) in the lower uterine segment in first and repeated cesarean sections

The authors studied the presence of substance P (SP) and vasoactive intestinal polypeptide (VIP) and their related fibers in the lower uterine segment (LUS) in 133 women undergoing cesarean sections (CS) during active labor. These were divided into 2 groups: women undergoing repeat or first CSs. Specimens were collected from the LUS and were evaluated by light microscopy and by immunohistochemistry, for the morphometrical quantification of the SP and VIP fibers in the LUS. The SP amount was higher in the post-CS scar, while the VIP amount decreased: nerve fibers contained an SP amount of up to 13 ± 2.6C.U., while nerve fibers contained a VIP amount of up to 7 ± 1.9 C.U. The SP amount counts 10 ± 1.5% of the total Bodian fibers, and the ratio of the VIP is 10 ± 1.8% of their total amount. In normal conditions only 6.61 C.U. of the Bodian surface is occupied by SP related nerve fibers in contrast to 6.63 C.U. of the total surface by VIP; the amount of SP increased up to 13 ± 2.6C.U., while it decreased in the LUS previous scars. The SP levels are higher in repeat CS, while the VIP levels are reduced in the LUS. The increase of SP is probably linked to the attempt to achieve cervical ripening in a post-CS LUS, with the possible consequences of dystocia during vaginal birth after CS. Nevertheless, the decrease of VIP probably affects the relaxation of the internal uterine orifice, compromising the LUS formation and cervical ripening.

Vasoactive intestinal polypeptide/phentolamine for intracavernosal injection in erectile dysfunction

Erectile dysfunction (ED) is becoming an increasingly common problem and although oral therapies offer first-line treatment for many men, they are contraindicated or ineffective in substantial groups of patients. Intracavernosal injection (ICI) therapy is the most effective nonsurgical treatment for ED and offers an effective alternative to oral therapy. Sufficient arterial blood supply and a functional veno-occlusive mechanism are prerequisites in the attainment and maintenance of a functional erection. Invicorp (Plethora Solutions, London, UK) is a combination of vasoactive intestinal polypeptide (VIP) 25 microg and phentolamine mesylate 1 or 2 mg for ICI in the management of moderate to severe ED. The two active components have complementary modes of action; VIP has a potent effect on the veno-occlusive mechanism, but little effect on arterial inflow, whereas phentolamine increases arterial blood flow with no effect on the veno-occlusive mechanism. Clinical studies showed that Invicorp is effective in >or=80% of men with ED, including those who have failed to respond to other therapies and, unlike existing intracavernosal therapies, is associated with a very low incidence of penile pain and virtually negligible risk of priapism. We estimate that there are >5.9 million men in the USA alone for whom oral ED drugs are not a viable treatment option, and for whom Invicorp might offer a safe and effective alternative.

Pharmacology of erectile dysfunction in man

Erectile dysfunction (ED) is defined as the consistent or recurrent inability of a man to attain and/or maintain a penile erection sufficient for sexual activity (2nd International Consultation on Sexual Dysfunction-Paris, June 28th-July 1st, 2003). Following the discovery and introduction of sildenafil, research on the mechanisms underlying penile erection has had an enormous boost and many preclinical and clinical papers have been published in the last 5 years. This review is structured in order to give the reader an overview of the clinical and preclinical data available on the peripheral regulation of and the mediators involved in human penile erection. The most widely accepted risk factors for ED are discussed. The article is focused on human data, and the safety and effectiveness of the 3 commercially available Phosphodiesterase-5 (PDE5) inhibitors used to treat ED are also discussed.

Neuropeptides of the pituitary adenylate cyclase-activating polypeptide/vasoactive intestinal polypeptide/growth hormone-releasing hormone/secretin family in testis

Mammalian testicular development and the maintenance of spermatogenesis are hormone-dependent processes that are controlled by the pituitary gonadotropins and testosterone. Recent studies have demonstrated the presence of many neuropeptides and their receptors in the testis, suggesting that these peptides operate as local regulators of testicular germ cell development and function. Among these testicular neuropeptides, the peptides that belong to the pituitary adenylate cyclase-activating polypeptide (PACAP) family, particularly growth hormone-releasing hormone and secretin, appear to show some unique common features in terms of intratesticular localization and the time of expression during the spermatogenic cycle. However, their precise physiologic roles and mechanisms of action remain unknown. This review analyzes the available information on the functional interactions among the testicular cells that appear to be mediated by locally produced neuropeptides, with a special emphasis on the peptides of the PACAP family.

Effect of excitatory amino acid receptor agonists on penile erection after administration into paraventricular nucleus of hypothalamus in the rat

OBJECTIVES

To investigate whether the excitatory amino acid receptor agonists can activate the paraventricular nucleus of the hypothalamus (PVN) and induce penile erections in the rat.

METHODS

Male adult Sprague-Dawley rats were used. A 26-gauge needle was inserted into the corpus cavernosum to monitor the intracavernous pressure (ICP) simultaneously with the systemic arterial pressure and heart rate. The study was divided into seven parts: stereotaxic delivery of N-methyl-d-aspartic acid (NMDA) (50 ng/500 nL) into the PVN; administration of the NMDA noncompetitive antagonist MK-801 (100 ng/250 nL) and NMDA (50 ng/250 nL) into the PVN; administration of the NMDA competitive antagonist (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; 100 ng/250 nL) and NMDA (50 ng/250 nL) into the PVN; microinjection of (+/-)-alpha-(amino)-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA; 100 ng/500 nL) into the PVN; microinjection of trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD; 100 ng/500 nL) into the PVN; saline 500 nL into the PVN; and intracavernous administration of NMDA (50 ng/100 microL).

RESULTS

On administration of NMDA into the PVN, a significant increase occurred in the ICP from a resting 8.3 +/- 1.8 mm Hg to a peak at 59.0 +/- 8.4 mm Hg. No change occurred in the resting ICP after administration of either the mixture of MK-801 and NMDA or CPP and NMDA into the PVN. Microinjection of AMPA, ACPD, or saline into the PVN and intracavernous administration of NMDA were all ineffective to induce an increase in ICP.

CONCLUSIONS

The results of this study suggest that ionotropic excitatory amino acid (NMDA) may have an effect on inducing penile erection through activation of the PVN in the rat.

Neuropeptides of the pituitary adenylate cyclase-activating polypeptide/vasoactive intestinal polypeptide/growth hormone-releasing hormone/secretin family in testis

Mammalian testicular development and the maintenance of spermatogenesis are hormone-dependent processes that are controlled by the pituitary gonadotropins and testosterone. Recent studies have demonstrated the presence of many neuropeptides and their receptors in the testis, suggesting that these peptides operate as local regulators of testicular germ cell development and function. Among these testicular neuropeptides, the peptides that belong to the pituitary adenylate cyclase-activating polypeptide (PACAP) family, particularly growth hormone-releasing hormone and secretin, appear to show some unique common features in terms of intratesticular localization and the time of expression during the spermatogenic cycle. However, their precise physiologic roles and mechanisms of action remain unknown. This review analyzes the available information on the functional interactions among the testicular cells that appear to be mediated by locally produced neuropeptides, with a special emphasis on the peptides of the PACAP family.

Recent advances in the treatment of erectile dysfunction in patients with diabetes mellitus

OBJECTIVE

To present current information on the pathogenesis of and available therapeutic options for erectile dysfunction (ED) in patients with diabetes.

METHODS

We provide a detailed review of the following topics: (1) peripheral and central neurotransmitter pathways involved in the penile erectile process (for example, nitric oxide, acetylcholine, vasoactive intestinal polypeptide, and prostaglandin E(1)), (2) pathogenesis of ED in patients with diabetes (vascular insufficiency, endothelial dysfunction, and autonomic neuropathy), (3) currently available treatment options for ED and their advantages and disadvantages, (4) potential new avenues for future research, and (5) the possibility of preventive treatment.

RESULTS

Clearly a need exists for effective treatment options for ED in patients with diabetes. Because the development of ED in patients with diabetes is often caused by several interrelated mechanisms, including vascular disease, endothelial dysfunction, autonomic neuropathy, hormone imbalance, and certain medications, a thorough understanding of the various pathways involved in penile erection and their modulation in diabetes is essential for physicians to design an effective treatment plan. Interventions that modulate the erectile pathway at different points include therapies that enhance the erectile mechanism (amplification of the nitric oxide pathway), inhibit the detumescence mechanism, or affect the final common pathway by augmenting smooth muscle relaxation. Oral therapy, intracavernosal injections, transurethral pellets, combination therapy, and surgical procedures are available treatment strategies.

CONCLUSION

Despite the availability of many treatment options for ED, early intervention and prevention (by such measures as improved glycemic control and general reduction of associated risk factors) should be emphasized because many of the diabetes-related complications leading to ED are irreversible.

Characterization of VIP and PACAP receptors in cultured rat penis corpus cavernosum smooth muscle cells and their interaction with guanylate cyclase-B receptors

Penile corpus cavernosum smooth muscle relaxation can be induced by both cyclic AMP and cyclic GMP-elevating agents, but possible interactions between these two signalling pathways are still poorly understood. Using in vitro cultured rat penile corpus cavernosum smooth muscle (CCSM) cells, we have characterized the local expression and functional activities of receptors for the cAMP-elevating peptides, PACAP and VIP, and for the cGMP-elevating peptides, CNP and ANP. Stimulation of the cells with various concentrations of PACAP(-27/-38) or VIP resulted in rapid and dose-dependent increases in cyclic AMP levels. RT-PCR analyses revealed gene expression of PAC(1) and VPAC(2) but not of VPAC(1) receptors in the cells. The natriuretic peptide, CNP, and the nitric oxide donor, sodium nitroprusside, were capable of enhancing cyclic GMP formation, indicating the presence of membrane-associated in addition to soluble guanylate cyclase (sGC) activities in these cells. Findings that cyclic GMP formation was preferentially activated by CNP but not by the related peptide, ANP, were consistent with RT-PCR analyses, demonstrating gene expression of the CNP receptor, GC-B, but not of the ANP receptor, GC-A, in these cells. Prior exposure of the cells to 10(-8) M PACAP resulted in a marked down-regulation of GC-B activity, whereas sGC was not affected. These findings provide functional and molecular evidence for the presence of three receptors, PAC(1), VPAC(2) and GC-B, involved in cyclic nucleotide signalling in penile CCSM cells. The observed cross-talk of the PACAP/VIP receptors with GC-B but not with sGC may have implications for the therapy of erectile dysfunction.

Involvement of nitric oxide and vasoactive intestinal peptide in the nonadrenergic-noncholinergic relaxation of the porcine retractor penis muscle

Neurotransmitters mediating nonadrenergic-noncholinergic (NANC) relaxation were investigated in strips of porcine retractor penis muscle (RPM). Muscle tone was raised by phenylephrine (1 microM) in the presence of atropine (1 microM) and guanethidine (50 microM). Upon electrical field stimulation (1 ms, 80 V, 1-32 Hz for 10 s), the initial fast relaxation was followed by the slow relaxation. Although the fast and the slow relaxation were completely abolished by tetrodotoxin (1 microM), they showed different pharmacological sensitivities to the nitric oxide (NO) synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 0.1 mM). The fast relaxation was markedly inhibited by L-NAME in an L-arginine reversible manner and by oxyhemoglobin (50 microM), while the slow relaxation was hardly blocked by L-NAME. L-NAME and alpha-chymotrypsin (alpha-CT, 3 U/ml) selectively inhibited the fast and the slow relaxation, respectively. Alpha-CT abolished L-NAME-resistant slow relaxation, and L-NAME completely abolished the alpha-CT-resistant fast relaxation. Alpha-CT-resistant relaxation was not significantly different from the digitally calculated L-NAME-sensitive component, and L-NAME-resistant relaxation was similar to the digitally calculated alpha-CT-sensitive component. Vasoactive intestinal peptide (VIP, 0.003-0.1 microM) relaxed porcine RPM in a concentration-dependent manner. The effect of a VIP was partially inhibited by a VIP receptor antagonist, VIP(10-28) (1 and 3 microM). L-NAME-resistant relaxation was also reduced by VIP(10-28) (3 microM) and by another putative antagonist, VIP(6-28) (1 microM), although the effects of the two antagonists were somewhat inconsistent. From the histochemical staining, it was verified that nerve bundles that showed VIP-like immunoreactivities were also positive for the NADPH diaphorase reaction. These results suggest that NO and peptide neurotransmitter(s) including VIP mediate the NANC relaxation in porcine RPM.

Immunocytochemical detection of vasoactive intestinal peptide-like and peptide histidine isoleucine-like peptides in the nervous system and the excretory system of adult Nippostrongylus brasiliensis

Vasoactive intestinal peptide-like and peptide histidine isoleucine-like immunoreactivities were detected in the excretory duct of adult male and female Nippostrongylus brasiliensis, thus indicating the source of these two physiologically active peptides previously isolated from the excretory/secretory products of adult N. brasiliensis. In the nervous system immunoreactivity to both these peptides was confined to females and was found in the neurons of the ovijector associated ganglion. This is consistent with co-synthesis of vasoactive intestinal peptide-like and peptide histidine isoleucine-like peptides which has also been shown to occur in all mammalian vasoactive intestinal peptid-ergic neurons studied to date. However, in addition to this, and in common to some previous studies on helminth vasoactive intestinal peptide and peptide histidine isoleucine immunoreactivities, co-synthesis of the peptides was not indicated in a pair of branched neurons which projected posteriorly and peripherally from the ganglion associated with the ovijector of females and which terminated in two pairs of ganglia also exhibiting vasoactive intestinal peptide-like immunoreactivity only. The position of these ganglia indicated that they innervate muscles close to the body wall and may be responsible for the muscular contractions required for expulsion of eggs from female Nippostrongylus brasiliensis. This is also the first study to successfully detect these peptides in the excretory system of gastrointestinal nematodes.

Intracavernous pharmacotherapy: treatment for the aging erectile response

Fueled by serendipity, observation, and direct scientific investigations over the last 15 years the normal mechanisms of erection, and the pathophysiology of erectile dysfunction have been revealed. The demonstrations that vasoactive injections could produce penile erection without benefit of psychic or tactile stimuli revolutionized the diagnosis and treatment of erectile dysfunction by providing a direct test of end organ integrity or deficiency. This article reviews the clinical pharmacology of vasoactive corporal agents and outlines what has been surmised about the aging erectile response.

Nitric oxide control of lower genitourinary tract functions: a review

It is apparent that evolving concepts of the regulatory basis for functions in the pelvis must take into account the role exerted by nitric oxide. A recently characterized messenger molecule, nitric oxide has been associated with numerous physiologic processes. Intense investigations of this molecule have extended its importance to several genitourinary functions. Penile erection, micturition, peristalsis of the male excurrent duct system, contractile properties of the prostate, and lumbosacral spinal cord neurotransmission are all functions that may transpire under some degree of control by nitric oxide. Impotence, urinary obstruction, or ejaculatory problems, in turn, may represent alterations of nitric oxide production or action. The strategic manipulation of nitric oxide or its mechanism of action, possibly by pharmacologic means, may restore or produce desired functional effects. These possibilities, therefore, suggest that the advancing knowledge of nitric oxide in the genitourinary tract may be of enormous clinical value in the future.

Vasoactive intestinal polypeptide and other preprovasoactive intestinal polypeptide-derived peptides in the female and male genital tract: localization, biosynthesis, and functional and clinical significance

Vasoactive intestinal polypeptide, a neuropeptide with wide distribution in the central and peripheral nervous system, has a broad spectrum of biologic actions. The demonstration of vasoactive intestinal polypeptide containing nerve fibers within the female and male genital tract 17 years ago indicated a putative role for this peptide in the local nervous control of reproductive functions. The genes encoding the preprovasoactive intestinal polypeptide precursor molecule and the vasoactive intestinal polypeptide receptor have been identified. The gene expression has been studied by the use of specific antibodies against the functional domains of the vasoactive intestinal polypeptide precursor and the biologic action elucidated by both in vivo and in vitro methods. Evidence has been provided to support vasoactive intestinal polypeptide as a neurotransmitter in several physiologic events in the genital tracts (i.e., blood flow and nonvascular smooth muscle relaxation). In the ovary vasoactive intestinal polypeptide seems to play an important role as regulator and/or modulator of folliculogenesis and steroidogenesis. In the male genital tract vasoactive intestinal polypeptide seems to participate in the control of erection. Vasoactive intestinal polypeptide has been suggested as a causative factor in some diseases of the genital organs (e.g., it may play a pathophysiologic role in male impotence and the peptide is currently used in the treatment of this condition). Vasoactive intestinal polypeptide may be important for control of the low resistance in the fetomaternal vascular bed and is therefore a putative factor involved in the development of preeclampsia. The therapeutic potential of vasoactive intestinal polypeptide and future agonists and antagonists will be revealed by ongoing and forthcoming studies.

Expression and characterization of preproVIP derived peptides in the human male urogenital tract

Expression of the gene sequence encoding vasoactive intestinal polypeptide (VIP) leads to the synthesis of a 170 amino acid precursor molecule which can be processed to five fragments: preproVIP 22-79, peptide histidine methionine (PHM), or peptide histidine valine (PHV), preproVIP 111-122, VIP and preproVIP 156-170. Using region specific radioimmunoassays and antisera against the functional domains of the VIP precursor in combination with immunocytochemistry and chromatography, the localization, distribution and identity of the preproVIP derived peptides within the human male urogenital tract were investigated. Postmortem as well as fresh tissue specimens were used. All the preproVIP derived peptides were expressed and could be demonstrated in nerve fibres throughout the urogenital tract in close relation to the epithelial lining and in vascular as well as non-vascular smooth muscle. The VIP-related peptide containing fibres were most abundant in the prostate parenchyma and the seminal vesicle. Using double immunostaining, co-localization of the various preproVIP derived peptides could be evidenced. The fact that all preproVIP derived peptides are present in the urogenital tract, should be taken into consideration when the regulatory aspects of neuropeptides in physiological and pathophysiological functions are discussed.

Penile erection in the primate: induction with nitric-oxide donors

The primate model has been used for investigations on the physiology and pharmacology of erection. Recent in vitro investigations indicate that nitric oxide acts as the mediator of penile erection, but in vivo primate studies are needed to corroborate these findings. Penile erections were induced in a primate model using intracavernosal injections of nitric oxide donors s-nitrocysteine (NO-CYS) and sodium nitroprusside (SNP), and acetylcholine (ACh) which stimulates the formation of nitric oxide. Penile length and intracavernosal pressures following agonist injection were compared with baseline (flaccid) and control erections (elicited by injection of a papaverine/phentolamine/PGE1 standard mixture). Dose-response curves for each drug were determined with respect to maximal intracavernosal pressure, duration of effect and penile length, and systemic arterial pressure was monitored. All three agents induced erections, with dose-dependent increases in cavernosal pressure and penile length. The maximal cavernosal pressure attained was similar for all three agents, but the duration of action was significantly shorter with ACh (p < .05). Injection of L-nitro-arginine-methyl-ester (L-NAME), a nitric oxide synthase inhibitor, before injection of the nitric oxide donor shortened the duration of effect but did not alter maximal cavernosal pressure or penile length attained. Although systemic hypotension was induced by each agent, digital compression at the base of the penis at the time of injection prevented such changes. These results suggest that the primate is a useful model to evaluate the action of substances that induce or inhibit penile erection. The findings provide support for the hypothesis that nitric oxide is a mediator of penile erection and that nitric oxide donors may be useful in the treatment of erectile dysfunction.

Targeted disruption of the neuronal nitric oxide synthase gene

By homologous recombination, we have generated mice that lack the neuronal nitric oxide synthase (NOS) gene. Neuronal NOS expression and NADPH-diaphorase (NDP) staining are absent in the mutant mice. Very low level residual catalytic activity suggests that other enzymes in the brain may generate nitric oxide. The neurons normally expressing NOS appear intact, and the mutant NOS mice are viable, fertile, and without evident histopathological abnormalities in the central nervous system. The most evident effect of disrupting the neuronal NOS gene is the development of grossly enlarged stomachs, with hypertrophy of the pyloric sphincter and the circular muscle layer. This phenotype resembles the human disorder infantile pyloric stenosis, in which gastric outlet obstruction is associated with the lack of NDP neurons in the pylorus.

Transmitter role of vasoactive intestinal peptide

Vasoactive intestinal polypeptide (VIP) is a 28 amino acid with a wide-spread neuronal localization. VIP fulfils many of the classical criteria for neurotransmission. In the cerebral cortex bipolar VIP neurones are involved in the coupling between energy metabolism, blood flow and neuronal activity. Furthermore, VIP in the brain plays a role in circadian rhythms and melatonin and pituitary hormone secretion. In the peripheral nervous system VIP is the transmitter of a number of non-cholinergic, non-adrenergic autonomic events. Thus, the peptide is involved in the control of smooth muscle tone and motility, blood flow and secretion in the digestive tract, respiratory tract and urogenital tract. The effects of VIP are mediated by a specific membrane-bound receptor linked to adenylate cyclase via a stimulatory G-protein. It is likely that impairment of VIP nerves is involved in some autonomic dysfunctions, an example being male impotence which is successfully treated with VIP injections.

Evidence against vasoactive intestinal polypeptide as the relaxant neurotransmitter in human cavernosal smooth muscle

1. The putative role of vasoactive intestinal polypeptide (VIP) as the relaxant neurotransmitter in human cavernosal smooth muscle has been studied in isolated tissue preparations. 2. Consistent neurogenic relaxations were evoked by electrical field stimulation (EFS; 2-64 pulses/train, 0.8 ms pulse duration, 10 Hz). VIP (0.1-3 microM) relaxed cavernosal smooth muscle in a dose-dependent fashion. Relaxant responses to both EFS and VIP were reduced in tissue from impotent men. 3. Neurogenic relaxant responses were not diminished in the presence of the VIP-inactivating peptidase, alpha-chymotrypsin (alpha-CT, 2 units ml-1). In contrast VIP-induced relaxations were completely abolished. 4. Inhibition of nitric oxide synthase by NG-nitro-L-arginine (30 microM), and of guanylate cyclase by methylene blue (50 microM) caused highly significant reductions of neurogenic relaxant responses whereas VIP-evoked relaxations were unaffected. 5. It is concluded that VIP-evoked relaxations are not mediated by the NO-guanosine 3':5'-cyclic monophosphate (cyclic GMP) pathway and that VIP release is not essential for neurogenic relaxation of human cavernosal smooth muscle. VIP does not therefore act as the major relaxant neurotransmitter in this tissue.

Effect of intracavernous simultaneous injection of acetylcholine and vasoactive intestinal polypeptide on canine penile erection

We investigated the effects of intracavernous injection of a combination of acetylcholine (ACh) and vasoactive intestinal polypeptide (VIP) on the erectile response in eleven adult male dogs. The minimum dose of ACh which increased the intracavernous pressure in eight dogs varied from 0.2 to 40 micrograms, and the minimum dose of VIP varied from 0.2 to 5 micrograms. When the minimum doses of ACh and VIP were injected simultaneously, a strong increase of intracavernous pressure (the mean increase was 102 cm. H2O from the baseline level) and a sustained erection (mean 5 min.) were observed in all eight dogs. The effect of simultaneous injection of both drugs was not additive but synergistic. Pretreatment with VIP-antibody and atropine intracavernously suppressed the erectile response induced by cavernous nerve stimulation. VIP may increase the affinity of muscarinic receptors for ACh in canine corpus cavernosum because pretreatment with atropine alone before the simultaneous injection of ACh and VIP completely abolished the effect of the combination. We conclude that ACh and VIP may play a cooperative role in canine penile erection.

Nitric oxide: a physiologic mediator of penile erection

Nitric oxide (NO) is a cytotoxic agent of macrophages, a messenger molecule of neurons, and a vasodilator produced by endothelial cells. NO synthase, the synthetic enzyme for NO, was localized to rat penile neurons innervating the corpora cavernosa and to neuronal plexuses in the adventitial layer of penile arteries. Small doses of NO synthase inhibitors abolished electrophysiologically induced penile erections. These results establish NO as a physiologic mediator of erectile function.

Distribution and origin of neuropeptide Y-immunoreactive fibers in the penis of the rat

The present study investigated the distribution of neuropeptide Y-immunoreactive fibers to the penis of the rat. In the corpora cavernosa penis, a dense plexus of fibers was associated with arteries, intrinsic cavernosal muscle, and veins, including the deep dorsal vein. In the corpus spongiosum, immunoreactive fibers were present around vascular smooth muscle and at the periphery of the acini of the paraurethral glands. Immunohistochemistry of penile neurons identified by retrograde tracer injection into the penis indicates that about 5% of the penile neurons in the pelvic plexus contained the neuropeptide while larger percentages of penile neurons in the sympathetic chains were immunoreactive for neuropeptide Y. Chemical and surgical sympathectomy greatly reduced the neuropeptide Y- and catecholamine-containing fibers in the erectile tissue but had no clear effect on the neuropeptide Y fibers around the paraurethral glands; a tissue that is not innervated by adrenergic fibers. It is concluded that (1) the widespread distribution of neuropeptide Y indicates that it may function in the control of penile blood flow, (2) with the possible exception of the paraurethral glands, the sympathetic chain is the most likely source of neuropeptide Y fibers in both erectile bodies of the penis, and (3) this peptide may play a role in the secretory functions of the paraurethral glands.

K(+)-channel openers for relaxation of isolated penile erectile tissue from rabbit

The effects of the K(+)-channel openers (KCOs) cromakalim (BRL 34915) and pinacidil were investigated and compared with those of papaverine on isolated corpus cavernosum from rabbit. Preparations were mounted in organ baths and isometric tension was recorded. Spontaneous contractile activity was effectively abolished by the KCOs tested, cromakalim being the most potent of them. The KCOs concentration-dependently and effectively depressed electrically induced contractions and also contractions induced by exogenously applied noradrenaline and by low (less than or equal to 20 mM) concentrations of K+. Cromakalim was three to four times more potent than pinacidil. Pinacidil and cromakalim were shown to cause increases in the efflux of 86Rb from preloaded cavernous tissue. Papaverine also effectively depressed spontaneous contractile activity, and contractions evoked by electrical stimulation and noradrenaline. It had a potency 19 to 36 times lower than that of cromakalim. However, papaverine did not increase 86Rb efflux from preloaded tissue. The results show that cromakalim and pinacidil effectively relax penile erectile tissue, probably by the opening of K(+)-channels and subsequent hyperpolarization. Further investigations on human material seems motivated in order to elucidate if the principle of K(+)-channel opening offers any therapeutic advantages to other drugs in the diagnosis and treatment of penile erectile dysfunction.

Actions of endothelin on isolated corpus cavernosum from rabbit and man

The effects of endothelin, a vasoconstrictor peptide produced by vascular endothelial cells, were investigated in isolated rabbit and human corpus cavernosum (CC). Preparations from both rabbit and man were potently contracted by endothelin in a concentration-dependent manner. The contractions developed slowly, could not be reversed despite frequent washings, and were only partly inhibited by the Ca2+ channel blocker nimodipine. Even in Ca2(+)-free medium containing the chelator EGTA a small contractile component persisted. In rabbit CC, the contractions in Ca2(+)-free medium were not affected by nimodipine, the Ca2(-)-channel agonist BAY K 86(44), or by depletion of intracellular Ca2(+) stores sensitive to noradrenaline (NA) and caffeine, but were almost abolished by the protein kinase C inhibitor H7. In both rabbit and man, carbachol and vasoactive intestinal polypeptide concentration-dependently relaxed preparations contracted by endothelin. The relaxations induced by carbachol were antagonized by atropine. Endothelin enhanced concentration-dependently the contractions induced by exogenously applied NA in rabbit CC. The enhancement was more pronounced at low concentrations of NA. This study shows that endothelin potently contracts isolated penile erectile tissue. The contraction seems to be mediated mainly by influx of Ca2+ through the cell membrane, which partly occurs through a pathway other than voltage-operated calcium channels. However, involvement of other mechanisms cannot be excluded. The results suggest that endothelin can play a role in penile erectile mechanisms.

A clinical trial of intracavernous vasoactive intestinal peptide to induce penile erection

A double-blind, placebo-controlled clinical trial of the intracavernous injection of vasoactive intestinal peptide was conducted and studied in 24 men with erectile dysfunction of diabetic, neurogenic and psychogenic etiology. The patients were randomized into 6 groups of 4 subjects each and received either placebo, or 200 or 400 pmol. vasoactive intestinal peptide at each of 3 consecutive weekly visits. An increase in penile length was associated significantly with treatment (F equals 5.10, p equals 0.01), dose-related, and independent of the time and sequence of treatment. An increase in penile diameter was associated significantly with treatment (F equals 8.14, p equals 0.001) and time (F equals 8.14, p equals 0.001), dose-related and independent of the sequence of treatment. Penile rigidity was associated significantly with time (F equals 5.44, p equals 0.008), associated nearly significantly with treatment (F equals 3.11, p equals 0.056) and independent of sequence of treatment. Despite some measurable treatment-related increase none of the patients achieved penile rigidity adequate for intromission.

Central effect of yohimbine on sexual behavior in the rat

A large range of doses of yohimbine (Y) was administered intracerebroventricularly (ICV) (5-100 micrograms/rat) or intraperitoneally (IP) (0.35-10 mg/kg) to male rats and the effects on sexual, locomotor and general behavior were evaluated. For both routes there was a clear-cut inverted-U effect (stimulating/depressing), calculable as parabolic regressions on the log of administered doses. The maximal stimulating doses (15 micrograms/rat ICV and 1 mg/kg IP) significantly shortened mount, intromission and ejaculation latencies and the mean interintromission interval. These data indicate the importance of CNS mechanisms in the sexual effect of Y.

Nasal congestion during pregnancy

Twenty-one pregnant women with nasal congestion, verified by rhinomanometry, did not differ significantly from 8 pregnant women without nasal congestion regarding the serum levels of the hormones oestradiol, progesterone and VIP. The congested group had a significantly lower serum level of oxytocin than the reference group. There were no differences in the symptoms urinary incontinence, constipation, and heartburn between the groups. The pathophysiology of nasal congestion during pregnancy is still veiled in obscurity.

Effect of yohimbine hydrochloride on erectile impotence: a double-blind study

A double-blind, partial crossover study on the therapeutic effect of yohimbine hydrochloride on erectile dysfunction was done in 82 sexually impotent patients. All patients underwent a multifactorial evaluation, including determination of penile brachial blood pressure index, cavernosography, sacral evoked response, testosterone and prolactin determination, Derogatis sexual dysfunction inventory and daytime arousal test. After 1 month of treatment with a maximum of 42.0 mg. oral yohimbine hydrochloride daily 14 per cent of the patients experienced restoration of full and sustained erections, 20 per cent reported a partial response to the therapy and 65 per cent reported no improvement. Three patients reported a positive placebo effect. Maximum effect takes 2 to 3 weeks to manifest itself. Yohimbine was active in some patients with arterial insufficiency and a unilateral sacral reflex arc lesion, and in 1 with low serum testosterone levels. The 34 per cent response is encouraging, particularly in a Veterans Administration population presenting with a high incidence of diabetes and vascular pathological conditions not found in regular office patients. Only few and benign side effects were recorded, which makes this medication worth an attempt, often as a first line of treatment even at a dose of 8 tablets.

VIP and autonomic neurotransmission

A variety of peptides have been proposed as transmitter candidates in non-cholinergic, non-adrenergic nerves. The nerves containing vasoactive intestinal polypeptide (VIP), which innervate blood vessels, non-vascular smooth muscle, mucosal epithelium and glands comprise a major and wide-spread population of the peptide-containing systems. There is now experimental data supporting the view that VIP is a transmitter in non-adrenergic, non-cholinergic nerves in the digestive tract, respiratory tract and urogenital tract, controlling smooth muscle tone and motility, blood flow and secretion. It is possible that impairment of VIP-containing nerves is involved in a number of autonomic dysfunctions.

Regulatory peptides in the mammalian urogenital system

By immunocytochemistry a number of the gut/brain peptides have been demonstrated in nerve fibers of the mammalian urogenital tract. These peptides are localized to large vesicles in nerve terminals of afferent fibers or efferent nerves innervating blood vessels, non-vascular smooth muscle, lining epithelium and glands. There is evidence that some neuropeptides (VIP, NPY) participate in the local non-cholinergic, non-adrenergic nervous control of smooth muscle activity and blood flow, while other peptides (substance P, CGRP) seem to be sensory transmitters. It is likely that impaired function of the peptidergic nerves is involved in sexual dysfunction such as male impotence.

Drug-induced priapism. Its aetiology, incidence and treatment

Priapism is characterised by a persistent erection that cannot be relieved by sexual intercourse or masturbation. Although priapism subsides spontaneously in a few days, impotence frequently follows. Both vascular and neural mechanisms are implicated in the pathophysiology of priapism, but it is not clear which initiates the process. Idiopathic cases of priapism are the most frequent (near 50%); other medical conditions that can result in priapism are haematological diseases (mainly sickle cell anaemia and leukaemia), traumatism, and neoplastic processes. Drug-induced priapism comprises about 30% of cases. The drugs most frequently implicated are psychotropic drugs (phenothiazines and trazodone), antihypertensives (mainly prazosin) and heparin. Recently, the intracavernosal injection of vasoactive drugs (papaverine and phentolamine) has been described in patients treated for impotence. With the exception of heparin, an alpha-adrenergic blocking mechanism has been suggested in the priapism-inducing action of these drugs. A significant number of anecdotal case reports link priapism and drugs, and it is possible that certain cases of idiopathic priapism could be reclassified if accurate pharmacological anamnesis were to be performed. Priapism must be considered a urological emergency. Surgical procedures are the most preferred treatment for this condition but, in selected cases, drug treatment seems to be an alternative approach.

Parasympathetic innervation of cutaneous blood vessels by vasoactive intestinal polypeptide-immunoreactive and acetylcholinesterase-positive nerves: histochemical and experimental study on rat lower lip

The distribution and origin of perivascular acetylcholinesterase-active and vasoactive intestinal polypeptide-immunoreactive nerve fibers were studied in the rat lower lip by means of acetylcholinesterase histochemistry and vasoactive intestinal polypeptide immunohistochemistry. The perivascular nerve fibers stained intensely with both histochemical techniques and were widely distributed on small arteries and arterioles of the lower lip, especially in the transitional zone between the hairy skin and the mucous membrane. The distributions of the two types of fibers were very similar and most of them showed overlapping coloration, on consecutive staining for vasoactive intestinal polypeptide and acetylcholinesterase. Both acetylcholinesterase-positive and vasoactive intestinal polypeptide-immunoreactive fibers were completely lost on removal of the otic ganglion, while they were not affected by sympathetic ganglion removal or sensory nerve sectioning. In the otic ganglion, most cells exhibited acetylcholinesterase activity, and about 60% of the cells showed light to heavy vasoactive intestinal polypeptide immunoreactivity. These findings indicate that vessels in the rat lip are innervated by parasympathetic fibers originating from the otic ganglion and support the view that vasoactive intestinal polypeptide is present in cholinergic neurons. This may suggest the possible control by the parasympathetic nervous system of cutaneous blood vessels through vasoactive intestinal polypeptide-containing cholinergic neurons, in general or at least in the facial area.

Vasoactive intestinal polypeptide (VIP) selectively stimulates prolactin release in healthy women

The neuropeptide, vasoactive intestinal polypeptide (VIP), is released from the hypothalamus to the portal circulation, and experiments on animals provide evidence that it might modulate hormone secretion from the pituitary. Here we report the effects of VIP on the release of different pituitary hormones, including prolactin (PRL), luteinizing hormone (LH), follicle-stimulating hormone (FSH) and thyrotropin-releasing hormone (TSH), in normal women. Seven healthy women (aged 27-32; body weight 53-60 kg), with normal menses and receiving no medication, were tested on days 20-23 of their cycle. Porcine VIP was injected i.v. as a bolus dose of 1 mcg/kg body weight. Blood samples were collected 10 minutes prior to VIP administration and 5, 15, 30, 45, 60 and 90 minutes after VIP injection. Blood pressure and heart rate were continuously monitored. Hormone levels were determined by RIA. Stress, which can stimulate PRL release, was assayed by measuring the effect of placebo on hormone release (5 controls). VIP injection induced a significant (p less than 0.01) increase in plasma PRL levels. Basal PRL was 20.25 +/- 9.14 ng/ml; 5 minutes after VIP injection PRL levels rose to 45.0 +/- 14.9 ng/ml (p less than 0.01). At 15 minutes a plateau was reached (46.0 +/- 14.5 ng/ml), then the levels slowly decreased. VIP administration did not modify the plasma concentration of LH, FSH or TSH at any time during the observation period. The present study indicates that VIP might play a physiological role as a RPL-releasing factor in human beings.

Is yohimbine effective in the treatment of organic impotence? Results of a controlled trial

Yohimbine is an alpha-adrenoceptor blocker that has been used in the treatment of erectile dysfunction. Adequate trials of this substance in a clearly defined organically impotent population are not available. We conducted a randomized, controlled study with partial cross-over of yohimbine versus placebo in 100 organically impotent men. The first phase of the study showed a positive response in 42.6 per cent of the patients receiving yohimbine versus 27.6 per cent in the placebo group. Although favorable to the test medication these values did not reach statistical significance (p equals 0.42). A similar pattern was noted in the second phase of the study. The over-all response rate of 43.5 per cent was consistent with a previous noncontrolled trial but it was much lower than previous studies. The response rate of organically impotent patients to yohimbine is at best marginal. Owing to its ease of administration, safety and modest effect it still is used in those patients who do not accept more invasive methods. Adrenoceptors are involved in the erectile process, although other neurotransmitter systems also are putative modulators of penile erection, including cholinergic, dopaminergic and vasoactive intestinal polypeptide pathways. It is beyond reasonable expectation that a single agent be of value for all cases of organic impotence. However, yohimbine has shown modest effectiveness at the doses used in this trial (18 mg. per day). Higher doses or a different route of administration may produce different effects.

Studies on intracavernosal VIP levels during pharmacologically induced penile erections

Intracavernosal and peripheral venous vasoactive intestinal polypeptide (VIP) levels were measured in men with predominantly organic or predominantly psychogenic impotence. The measurements were taken at intervals up to 30 min following intracavernosal injections of saline, papaverine hydrochloride and papaverine hydrochloride and phentolamine. Levels were also measured after tactile and visual sexual stimulation and following an intravenous injection of papaverine and phentolamine. A penile erection occurred in all men receiving intracavernosal vasoactive compounds. The mean VIP concentration did not alter significantly in either cavernosal or peripheral venous blood during the erection. Mean VIP concentrations were significantly greater in the neurogenic (all diabetic) group than in the other groups studied. Mean cavernosal and peripheral VIP concentrations did not alter following tactile or visual sexual stimulation and no significant alteration in mean peripheral venous VIP concentration occurred following injection of papaverine and phentolamine. The putative role of VIP in the induction of penile erection has not been elucidated in these studies.

Vasoactive intestinal polypeptide levels and distribution in the penis of old rats

Vasoactive intestinal polypeptide (VIP) levels and distribution were studied in the penis of young-adult (3-month-old) and old (30-month-old) Wistar rats by radioimmunoassay and immunofluorescence. No significant changes in tissue VIP concentrations or distribution occurred in the penis of old rats compared to young-adult rats. The present data indicate that the age-related impairment of male sexual function is not dependent on modifications of the VIP-ergic innervation of penile tissue.

Receptors mediating contraction of isolated human vas deferens

A large body of evidences has suggested the role of adrenergic, opioidergic and other peptidergic receptors in the mediation of animal vas deferens motility. Different animal species showed different neurochemical patterns, so it is to be expected that human vas deferens has its own specific response to several substances, in relation to its peculiar function. In this study we report on the effects of monoaminergic (norepinephrine, dopamine, serotonin, isoproterenol, cholinomimetic drugs) and opioidergic (morphine, buprenorphin, beta-endorphin, met-enkephalin and dynorphin) agonists on isolated human vas deferens motility. Norepinephrine and dopamine provoked complex patterns of motility while opioids did not affect the field electroinduced contractions. The implications of this finding are discussed in relation to human vas deferens function.