Dopaminergic neurotransmission at the paraventricular nucleus of hypothalamus in central regulation of penile erection in the rat

Who Knew? Dopamine Transporter Activity Is Critical in Innate and Adaptive Immune Responses

The dopamine transporter (DAT) regulates the dimension and duration of dopamine transmission. DAT expression, its trafficking, protein-protein interactions, and its activity are conventionally studied in the CNS and within the context of neurological diseases such as Parkinson's Diseases and neuropsychiatric diseases such as drug addiction, attention deficit hyperactivity and autism. However, DAT is also expressed at the plasma membrane of peripheral immune cells such as monocytes, macrophages, T-cells, and B-cells. DAT activity via an autocrine/paracrine signaling loop regulates macrophage responses to immune stimulation. In a recent study, we identified an immunosuppressive function for DAT, where blockade of DAT activity enhanced LPS-mediated production of IL-6, TNF-α, and mitochondrial superoxide levels, demonstrating that DAT activity regulates macrophage immune responses. In the current study, we tested the hypothesis that in the DAT knockout mice, innate and adaptive immunity are perturbed. We found that genetic deletion of DAT (DAT^-/-) results in an exaggerated baseline inflammatory phenotype in peripheral circulating myeloid cells. In peritoneal macrophages obtained from DAT^-/- mice, we identified increased MHC-II expression and exaggerated phagocytic response to LPS-induced immune stimulation, suppressed T-cell populations at baseline and following systemic endotoxemia and exaggerated memory B cell expansion. In DAT^-/- mice, norepinephrine and dopamine levels are increased in spleen and thymus, but not in circulating serum. These findings in conjunction with spleen hypoplasia, increased splenic myeloid cells, and elevated MHC-II expression, in DAT^-/- mice further support a critical role for DAT activity in peripheral immunity. While the current study is only focused on identifying the role of DAT in peripheral immunity, our data point to a much broader implication of DAT activity than previously thought. This study is dedicated to the memory of Dr. Marc Caron who has left an indelible mark in the dopamine transporter field.

Performance and safety of treatment options for erectile dysfunction in patients with spinal cord injury: A review of the literature

BACKGROUND

For a large proportion of patients with spinal cord injury, sexuality and reproduction are important issues. However, sparse data exist regarding available treatment options for this patient population.

OBJECTIVES

We sought to review performance and safety rates of all currently available treatment options for erectile dysfunction in spinal cord injury men.

MATERIALS AND METHODS

A systematic literature review without time restrictions was performed using PubMed/EMBASE database for English-, Italian-, German-, and Spanish-language articles. Articles' selection was performed according to the PRISMA guidelines. Relevant papers on erectile dysfunction in spinal cord injury patients were included in the final analyses.

RESULTS AND DISCUSSION

Overall, 47 studies were eligible for inclusion in this review. Of these, most evidence dealt with phosphodiesterase 5-inhibitors and intracavernous drug injection. Both treatment options are associated with high levels of performance and with patients/partners' satisfaction; side effects are acceptable. Overall, penile prostheses and vacuum erection devices are in general less approved by spinal cord injury patients and are correlated with increased rates of complications in comparison with phosphodiesterase 5-inhibitors and intracavernous drug injection. Sacral neuromodulation, transcutaneous electrical nerve stimulation, and intraurethral suppositories have been poorly studied, but preliminary studies did not show convincing results.

CONCLUSION

The best treatment options for erectile dysfunction in spinal cord injury patients emerged to be phosphodiesterase 5-inhibitors and intracavernous drug injection. The choice of erectile dysfunction treatment should be based on several aspects, including residual erectile function, spinal cord injury location, and patients' comorbidities. Future studies assessing the applicability of less well-studied treatments, as well as evaluating innovative options, are needed in this specific population.

Do rats have orgasms?

BACKGROUND

Although humans experience orgasms with a degree of statistical regularity, they remain among the most enigmatic of sexual responses; difficult to define and even more difficult to study empirically. The question of whether animals experience orgasms is hampered by similar lack of definition and the additional necessity of making inferences from behavioral responses.

METHOD

Here we define three behavioral criteria, based on dimensions of the subjective experience of human orgasms described by Mah and Binik, to infer orgasm-like responses (OLRs) in other species: 1) physiological criteria that include pelvic floor and anal muscle contractions that stimulate seminal emission and/or ejaculation in the male, or that stimulate uterine and cervical contractions in the female; 2) short-term behavioral changes that reflect immediate awareness of a pleasurable hedonic reward state during copulation; and 3) long-term behavioral changes that depend on the reward state induced by the OLR, including sexual satiety, the strengthening of patterns of sexual arousal and desire in subsequent copulations, and the generation of conditioned place and partner preferences for contextual and partner-related cues associated with the reward state. We then examine whether physiological and behavioral data from observations of male and female rats during copulation, and in sexually-conditioned place- and partner-preference paradigms, are consistent with these criteria.

RESULTS

Both male and female rats display behavioral patterns consistent with OLRs.

CONCLUSIONS

The ability to infer OLRs in rats offers new possibilities to study the phenomenon in neurobiological and molecular detail, and to provide both comparative and translational perspectives that would be useful for both basic and clinical research.

Mechanisms of penile erection and basis for pharmacological treatment of erectile dysfunction

Erection is basically a spinal reflex that can be initiated by recruitment of penile afferents, both autonomic and somatic, and supraspinal influences from visual, olfactory, and imaginary stimuli. Several central transmitters are involved in the erectile control. Dopamine, acetylcholine, nitric oxide (NO), and peptides, such as oxytocin and adrenocorticotropin/α-melanocyte-stimulating hormone, have a facilitatory role, whereas serotonin may be either facilitatory or inhibitory, and enkephalins are inhibitory. The balance between contractant and relaxant factors controls the degree of contraction of the smooth muscle of the corpora cavernosa (CC) and determines the functional state of the penis. Noradrenaline contracts both CC and penile vessels via stimulation of α₁-adrenoceptors. Neurogenic NO is considered the most important factor for relaxation of penile vessels and CC. The role of other mediators, released from nerves or endothelium, has not been definitely established. Erectile dysfunction (ED), defined as the "inability to achieve or maintain an erection adequate for sexual satisfaction," may have multiple causes and can be classified as psychogenic, vasculogenic or organic, neurologic, and endocrinologic. Many patients with ED respond well to the pharmacological treatments that are currently available, but there are still groups of patients in whom the response is unsatisfactory. The drugs used are able to substitute, partially or completely, the malfunctioning endogenous mechanisms that control penile erection. Most drugs have a direct action on penile tissue facilitating penile smooth muscle relaxation, including oral phosphodiesterase inhibitors and intracavernosal injections of prostaglandin E₁. Irrespective of the underlying cause, these drugs are effective in the majority of cases. Drugs with a central site of action have so far not been very successful. There is a need for therapeutic alternatives. This requires identification of new therapeutic targets and design of new approaches. Research in the field is expanding, and several promising new targets for future drugs have been identified.

Testostérone et contrôle central de l’érection

La testostérone orchestre l’organisation périnatale et l’activation adulte des structures nerveuses cérébrales et spinales impliquées dans l’expression du comportement sexuel mâle. Cette revue décrit brièvement les différents effets de la testostérone dans la régulation de la motivation sexuelle et de l’érection, et les modèles génétiques générés, jusqu’à présent, dans le but d’élucider ses mécanismes d’action centraux.

Differential brain processing of audiovisual sexual stimuli in men: Comparative positron emission tomography study of the initiation and maintenance of penile erection during sexual arousal

The human male psychosexual cycle consists of four phases: excitation, plateau, orgasm, and resolution. Identification of the specific neural substrates of each phase may provide information regarding the brain's pathophysiology of sexual dysfunction. We previously analyzed regional cerebral blood flow (rCBF) with H(2)15O-positron emission tomography (PET) during the excitation phase (initiation of penile erection) induced by audiovisual sexual stimuli (AVSS) and identified activation of the cerebellar vermis, the bilateral extrastriate cortex, and right orbitofrontal cortex, suggesting a role of cognition/emotion in the excitement phase. In the present study, we analyzed rCBF of the same six healthy volunteers during the plateau phase (maintenance of penile erection) induced by AVSS and compared the results with those of the excitation phase. Penile rigidity was monitored in real time with RigiScan Plus during PET scanning. Images were analyzed by statistical parametric mapping (SPM) software, and rCBF in the amygdala, hypothalamus, anterior cingulate, and insula was measured. During the plateau phase, primary subcortical activation was noted in the right ventral putamen, indicating motivational factors in the sexual response via the limbic reward circuit. A significant increase in rCBF in the left hypothalamus was also observed during the plateau phase. The right anterior cingulate and left insula were specifically activated during the excitation phase but not during the plateau phase. These results indicate a significant role of the ventral putamen and the hypothalamus in the plateau phase and confirm that paralimbic and limbic components of the human brain differentially coordinate the sexual response in a psychosexual phase-dependent manner.

Peripheral Modulation of Dopaminergic Receptors Affects Erectile Responses in Rats

Clinical observations have suggested that dopaminergic mechanisms are involved in the regulation of male sexual responses, including erection. Apomorphine was initially reported to exert its erectogenic effects by modifying central dopaminergic activity. This study aimed primarily at evaluating and investigating the effect of apomorphine on erectile functions in rats and its potential effects on the cardiovascular system, as well as the possible role of dopaminergic stimulation in the peripheral control of erection. Measurement of intracavernosal pressure changes elicited by electrical stimulation of the cavernosal nerve in anaesthetized rats and mating tests were used. SCH23390, the D1 receptor antagonist, attenuated penile response to electrical stimulation. Intravenous administration of apomorphine in low dose (100 microg/kg), but not in high dose, significantly potentiated erectile responses to electrical stimulation. Intracavernosally injected apomorphine (50 microg/kg) significantly potentiated the filling rate of the corpora cavernosa 5 min. after injection, and did not induce erection in absence of electrical stimulation. In addition, apomorphine amplified the male sexual and copulatory behaviour by reducing ejaculation, mount and intromission latencies, and significantly increasing the number of ejaculations per session. However, apomorphine produced rapid and long-lasting hypotension and potentiated the hypotension and tachycardia associated with nerve-stimulated penile erection. Our results suggest that D1-dopaminergic receptors may be functionally involved in the peripheral mediation of penile erection. Apomorphine may amplify sexual and copulatory behaviour and may also, by a complementary role, amplify neurogenically mediated erections by acting in the periphery.

The supraspinal network in the control of erection

Penile erection is a complex event controlled by vascular, hormonal and neuronal systems. The neuronal system involved in erection is often divided into spinal and supraspinal networks. It is generally accepted that the spinal system directly controls erection and that the supraspinal network modulates this control mechanism through different ascending and descending pathways. In contrast to the spinal control of erection, relatively little is known about the supraspinal network. In the present review, the authors outline the supraspinal network involved in the control of penile erection. Firstly, the brain regions reported to be involved in erection are described and the brain circuit of erection is outlined. Subsequently, the neuromediators involved in erection are summarised. Finally, these data are discussed in the light of therapeutic possibilities in the management of erectile dysfunction by targeting the supraspinal system.

PIP3EA and PD-168077, two selective dopamine D4 receptor agonists, induce penile erection in male rats: site and mechanism of action in the brain

PIP3EA (2-[4-(2-methoxyphenyl)piperazin-1-yl-methyl]imidazo[1,2-a]pyridine) and PD-168077 (N-[4-2-cyanophenylpiperazin-1-ylmethyl]-3-methylbenzamide maleate), two selective dopamine D4 agonists, administered systemically, intracerebroventricularly or into the paraventricular nucleus of the hypothalamus induce penile erection in male Sprague-Dawley rats. A U-inverted dose-response curve was found with either compound when given subcutaneously (1-100 microg/kg) or intracerebroventricularly (0.1-20 microg/rat), but not into the paraventricular nucleus (10-200 ng/rat). The pro-erectile effect of PIP3EA and of PD-168077 occurs concomitantly with an increased nitric oxide (NO) production in the paraventricular nucleus, as measured by the increased concentration of nitrites and nitrates found in the dialysate obtained from the paraventricular nucleus by intracerebral microdialysis. These effects of PIP3EA and PD-168077 were reduced by L-745,870 (3-[4-(4-chlorophenyl)piperazin-1-ylmethyl]-1H-pyrrolo[2,3-b]pyridine trihydrochloride), a selective dopamine D4 receptors antagonist, by omega-conotoxin, a blocker of voltage-dependent Ca2+ channels of the N-type, by S-methyl-thiocitrulline, a neuronal nitric oxide synthase inhibitor, and by d(CH2)5Tyr(Me)2-Orn8-vasotocin, an oxytocin receptor antagonist, given into the lateral ventricles, but not into the paraventricular nucleus. Comparison of the dose-response curves of PIP3EA and PD-168077 revealed that PIP3EA is as potent as PD-168077 when given into the paraventricular nucleus, but more potent when given systemically. However, both compounds are less efficacious (e.g. induce a lower number of penile erection episodes) than apomorphine, a classical mixed dopamine receptor agonist, irrespective of the route of administration. These results confirm previous findings showing that central D4 receptors mediate penile erection and show that dopamine D4 receptor agonists act in the paraventricular nucleus to facilitate penile erection by increasing central oxytocinergic neurotransmission.

Melanotan-II: Investigation of the inducer and facilitator effects on penile erection in anaesthetized rat

The effects of melanotan-II, a non-specific agonist of melanocortin receptors, on erection and its possible sites of action were investigated in anesthetized rats. Delivered i.v. (0.1, 0.3 and 1 mg/kg) or within the paraventricular nucleus of the hypothalamus (0.1 and 1 microg), melanotan-II exerted a dose-dependent inducer activity on erection by eliciting erectile events and shortening latency of the first erectile event to occur. Erectile events were of higher amplitude in rats treated with melanotan-II i.t. (0.2 microg) delivered at the L6-S1 level than in animals treated with the vehicle i.t. delivered. Erectile responses elicited by cavernous nerve stimulation were increased after i.v. melanotan-II (1 mg/kg), thereby exerting facilitator effect on erection. In contrast, melanotan-II injected within the corpus cavernosum (1 microg) did not display any facilitator activity. To investigate the neural pathways involved in the facilitator effect of melanotan-II, we performed acute spinalization (T8 level) and differential selective nerve transections. Neither spinalization nor bilateral transection of pelvic nerves or dorsal penile nerves impaired facilitator activity of i.v. melanotan-II (1 mg/kg). Conversely, the facilitator effect of melanotan-II was abolished after acute removal of the lumbar paravertebral sympathetic chain. These results lead to the conclusion that central and peripheral melanocortin pathways are recruited by melanotan-II, depending on its route of delivery, to exert both inducer and facilitator activities on erection.

Central Nervous System Agents in the Treatment of Erectile Dysfunction

In the last two decades, a better understanding of the mechanisms governing erectile function and the pathophysiologies underlying erectile dysfunction (ED) have led re-searchers to investigate novel treatment concepts. Selective type-5 phosphodiesterase inhibitors are recommended as first-line therapy because of their high efficacy, but 30% to 40% of patients who have ED do not respond adequately to these agents and require alternative methods. The central nervous system plays a fundamental role in sexual behavior. Animal models have advanced our understanding of the neuroanatomic and neuropharmacologic basis of centrally induced penile erections. Clinical research with apomorphine has demonstrated efficacy in men who have a range of ED. Recent interest has focused on other centrally acting agents for ED treatment, including the melanocortin receptor agonists.

Central control of penile erection: Role of the paraventricular nucleus of the hypothalamus

The paraventricular nucleus of the hypothalamus is an integration centre between the central and peripheral autonomic nervous systems. It is involved in numerous functions from feeding, metabolic balance, blood pressure and heart rate, to erectile function and sexual behaviour. In particular, a group of oxytocinergic neurons originating in this nucleus and projecting to extra-hypothalamic brain areas (e.g., hippocampus, medulla oblongata and spinal cord) control penile erection in male rats. Activation of these neurons by dopamine and its agonists, excitatory amino acids (N-methyl-D-aspartic acid) or oxytocin itself, or by electrical stimulation leads to penile erection, while their inhibition by gamma-amino-butyric acid (GABA) and its agonists or by opioid peptides and opiate-like drugs inhibits this sexual response. The activation of these neurons is secondary to the activation of nitric oxide synthase, which produces nitric oxide. Nitric oxide in turn causes, by a mechanism that is as yet unidentified, the release of oxytocin in extra-hypothalamic brain areas. Other compounds recently identified that facilitate penile erection by activating central oxytocinergic neurons are peptide analogues of hexarelin, a growth hormone releasing peptide, pro-VGF-derived peptides, endogenous peptides that may be released by neuronal nerve endings impinging on oxytocinergic cell bodies, SR 141716A, a cannabinoid CB1 receptor antagonist, and, less convincingly, adrenocorticotropin-melanocyte-stimulating hormone (ACTH-MSH)-related peptides. Paraventricular oxytocinergic neurons and similar mechanisms are also involved in penile erection occurring in physiological contexts, namely noncontact erections that occur in male rats in the presence of an inaccessible receptive female, and during copulation. These findings show that the paraventricular nucleus of the hypothalamus plays an important role in the control of erectile function and sexual activity. As the male rat is a model of sexual behaviour and penile physiology, which has largely increased in the last years our knowledge of peripheral and central mechanisms controlling erectile function (drugs that induce penile erection in male rats usually do so also in man), the above results may have great significance in terms of a human perspective for the treatment of erectile dysfunction.

Nitric oxide-cyclic GMP pathway with some emphasis on cavernosal contractility

Nitric oxide (NO) is formed from the conversion of L-arginine by nitric oxide synthase (NOS), which exists in three isoforms: neuronal (nNOS), endothelial (eNOS), and inducible (iNOS). nNOS is expressed in penile neurons innervating the corpus cavernosum, and eNOS protein expression has been identified primarily in both cavernosal smooth muscle and endothelium. NO is released from nerve endings and endothelial cells and stimulates the activity of soluble guanylate cyclase (sGC), leading to an increase in cyclic guanosine-3',5'-monophosphate (cGMP) and, finally, to calcium depletion from the cytosolic space and cavernous smooth muscle relaxation. The effects of cGMP are mediated by cGMP dependent protein kinases, cGMP-gated ion channels, and cGMP-regulated phosphodiesterases (PDE). Thus, cGMP effect depends on the expression of a cell-specific cGMP-receptor protein in a given cell type. Numerous systemic vasculature diseases that cause erectile dysfunction (ED) are highly associated with endothelial dysfunction, which has been shown to contribute to decreased erectile function in men and a number of animal models of penile erection. Based on the increasing knowledge of intracellular signal propagation in cavernous smooth muscle tone regulation, selective PDE inhibitors have recently been introduced in the treatment of ED. Phosphodiesterase 5 (PDE5) inactivates cGMP, which terminates NO-cGMP-mediated smooth muscle relaxation. Inhibition of PDE5 is expected to enhance penile erection by preventing cGMP degradation. Development of pharmacologic agents with this effect has closely paralleled the emerging science.

Has dopamine a physiological role in the control of sexual behavior? A critical review of the evidence

The role of dopaminergic systems in the control of sexual behavior has been a subject of study for at least 40 years. Not surprisingly, reviews of the area have been published at variable intervals. However, the earlier reviews have been summaries of published research rather than a critical analysis of it. They have focused upon the conclusions presented in the original research papers rather than on evaluating the reliability and functional significance of the data reported to support these conclusions. During the last few years, important new knowledge concerning dopaminergic systems and their behavioral functions as well as the possible role of these systems in sexual behavior has been obtained. For the first time, it is now possible to integrate the data obtained in studies of sexual behavior into the wider context of general dopaminergic functions. To make this possible, we first present an analysis of the nature and organization of sexual behavior followed by a summary of current knowledge about the brain structures of crucial importance for this behavior. We then proceed with a description of the dopaminergic systems within or projecting to these structures. Whenever possible, we also try to include data on the electrophysiological actions of dopamine. Thereafter, we proceed with analyses of pharmacological data and release studies, both in males and in females. Consistently throughout this discussion, we make an effort to distinguish pharmacological effects on sexual behavior from a possible physiological role of dopamine. By pharmacological effects, we mean here drug-induced alterations in behavior that are not the result of the normal actions of synaptically released dopamine in the untreated animal. The conclusion of this endeavor is that pharmacological effects of dopaminergic drugs are variable in both males and females, independently of whether the drugs are administered systemically or intracerebrally. We conclude that the pharmacological data basically reinforce the notion that dopamine is important for motor functions and general arousal. These actions could, in fact, explain most of the effects seen on sexual behavior. Studies of dopamine release, in both males and females, have focused on the nucleus accumbens, a structure with at most a marginal importance for sexual behavior. Since accumbens dopamine release is associated with all kinds of events, aversive as well as appetitive, it can have no specific effect on sexual behavior but promotes arousal and activation of non-specific motor patterns. Preoptic and paraventricular nucleus release of dopamine may have some relationship to mechanisms of ejaculation or to the neuroendocrine consequences of sexual activity or they can be related to other autonomic processes associated with copulation. There is no compelling indication in existing experimental data that dopamine is of any particular importance for sexual motivation. There is experimental evidence showing that it is of no importance for sexual reward.

Pharmacotherapy for Erectile Dysfunction

INTRODUCTION

Advances in understanding of the biochemistry and physiology of penile erection have led to breakthroughs in pharmacotherapy of erectile dysfunction.

AIM

To provide recommendations/guidelines concerning state-of-the-art knowledge for the putative molecular and cellular mechanisms of action of centrally and peripherally acting drugs currently utilized in pharmacotherapy of erectile dysfunction.

METHODS

An international consultation in collaboration with the major urology and sexual medicine associations assembled over 200 multidisciplinary experts from 60 countries into 17 committees. Committee members established specific objectives and scopes for various male and female sexual medicine topics. The recommendations concerning state-of-the-art knowledge in the respective sexual medicine topic represent the opinion of experts from five continents developed in a process over a two-year period. Concerning the Pharmacotherapy for Erectile Dysfunction Committee there were 25 experts from 10 countries.

MAIN OUTCOME MEASURE

Expert opinion was based on grading of evidence-based medical literature, widespread internal committee discussion, public presentation and debate.

RESULTS

Selective and potent oral PDE5 inhibitors have significantly more affinity than cGMP and form broader molecular interactions with multiple amino acids, thereby blocking access to cGMP in the catalytic sites of the PDE5 enzyme. PDE5 inhibitors, which vary as to biochemical potency, selectivity and pharmacokinetics, lead to cGMP elevation and relaxation facilitation of penile corpus cavernosum smooth muscle cells following sexual stimulation. Various centrally acting drugs influence sexual behaviour. In particular, the dopaminergic substance apomorphine is a central enhancer that acts in the paraventricular nucleus of the hypothalamus as a dopamine (D2) receptor agonist, induces and increases penile erection responses via disinhibition, following sexual stimulation.

CONCLUSIONS

There is a need for more research in the pharmacotherapeutic development of central and peripheral agents for safe and effective erectile dysfunction treatment.

Melanocortin Receptors and Erectile Function

OBJECTIVE

Review the historical and current evidence that suggests that activation of melanocortin receptors modulates erectile activity.

METHODS

The available literature was reviewed.

RESULTS

Melanocortin peptides derived from the pro-opiomelanocortin (POMC) precursor protein exert a host of diverse physiological effects in the periphery and in the CNS through interactions with one or more of the five cloned melanocortin receptors. Natural and synthetic melanocortin peptide agonists influence erectile and sexual function in a range of preclinical species. Emerging clinical evidence now suggests that the proerectile effects observed in preclinical species are evident in man as well.

CONCLUSIONS

Preclinical and clinical results support the involvement of melanocortins in the modulation of erectile and sexual function. Current evidence indicates that the melanocortin 4 receptor subtype contributes to the proerectile effects observed with pan-receptor agonists. However, the putative receptor subtypes, pathways and mechanisms implicated in mediating the proerectile effects of melanocortins remain to be fully elucidated.

Apomorphine sublingual as primary or secondary treatment for erectile dysfunction in patients with spinal cord injury

OBJECTIVES

To evaluate the effectiveness of apomorphine sublingual (SL) 3 mg, as a primary or secondary treatment for erectile dysfunction (ED) in patients with spinal cord injury (SCI), and to determine possible differences in efficacy considering clinical, urodynamic and neurophysiological findings.

PATIENTS AND METHODS

The study included 22 patients with chronic SCI and neurogenic ED who were examined physically and by a video-urodynamic evaluation. A neurophysiological evaluation included somatosensory evoked potentials of the pudendal nerve, palmar and plantar sympathetic skin responses and bulbocavernous reflex recordings. Thereafter the patients received 8 tablets of apomorphine SL 3 mg and were asked to complete the International Index of Erectile Function questionnaire before and after treatment. Side-effects, subjective efficacy compared with other treatments and satisfaction with the SL administration were recorded.

RESULTS

Of the 22 men, 11 had upper motor neurone lesions (six complete, five incomplete), eight lower motor neurone lesions (seven complete, one incomplete) and three had mixed lesions. In all, 12 patients took sildenafil citrate and five alprostadil intracavernosally beforehand, and five had used nothing to treat their ED. Seven patients had some response and reported that the drug helped them to obtain an erection, but only two reported erections sufficient for intercourse and would agree to continue apomorphine SL as their standard treatment; all the others reported being disappointed. Nine patients reported side-effects. There were no significant correlations for electrophysiological or urodynamic findings and treatment success. Of the 22 patients 20 preferred SL rather than the normal administration.

CONCLUSIONS

Apomorphine SL, a D1/D2 dopamine agonist, facilitates erectile function in a heterogeneous group of patients with no significant relationship with any of the assessed urodynamic or electrophysiological variables. The overall low rates of response either for primary or secondary treatment suggests that apomorphine will have limited applicability in patients with SCI.

Central Mechanisms Regulating Penile Erection in Conscious Rats: The Dopaminergic Systems Related to the Proerectile Effect of Apomorphine

Apomorphine has been used as a pharmacological probe of dopaminergic receptors in a variety of central nervous system disorders. The utility of apomorphine as an agent for the treatment of erectile dysfunction has also been demonstrated clinically. Apomorphine is a nonselective dopaminergic receptor agonist with potent binding affinity (Ki) of 101, 32, 26, 2.6, and 10 nM for D1, D2, D3, D4, and D5, respectively. When administered either subcutaneously (s.c.) or intracerebroventricularly (i.c.v.), apomorphine fully evoked penile erections in conscious rats with maximum effect at 0.1 micromol/kg s.c. and 3 nmol/rat i.c.v., respectively. Apomorphine was less efficacious when injected intrathecally (i.t.) to L4-L6 spinal levels (50% at 30-100 nmol/rat i.t.). Penile erection facilitated by apomorphine was blocked by haloperidol and clozapine (i.p. and i.c.v.) but not by domperidone (a peripherally acting dopaminergic receptor antagonist). In this model using conscious rats, penile erection was significantly induced by quinpirole (D2-D3-D4 receptor agonist), but not by R(+)-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol (SKF38393) and R(+)-6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzapine (SKF81297) (D1 receptor agonists), or a D2 receptor agonist R-5,6-dihydro-N,N-dimethyl-4H-imidazo[4,5,1-ij]quinolin-5-amine (PNU-95666E). The role of D4 receptors in penile erection was demonstrated using selective D4 receptor agonists [(4-phenylpiperazinyl)-methyl]benzamide (PD168077) and 5-fluoro-2-[[4-(2-pyridinyl)-1-piperazinyl]methyl]-1H-indole (CP226269), whether administered systemically (s.c.) or locally in the brain (i.c.v.). The ability of apomorphine to activate D3 receptors in relation to its proerectile activity remains to be elucidated by use of selective subtype agonists. These results suggest that the proerectile action of apomorphine in rats is mediated at supraspinal levels and that this effect is not mimicked by a D2 receptor agonist but associated with activation of D4 receptors.

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.

Apomorphine-induced brain modulation during sexual stimulation: a new look at central phenomena related to erectile dysfunction

It is well recognized that sexual stimulation leading to penile erection is controlled by different areas in the brain. Animal erection studies have shown that apomorphine (a D2>D1 dopamine receptors nonselective agonist) seems to act on neurons located within the paraventricular nucleus and the medial preoptic area of the hypothalamus. Yet, only recently, was a centrally acting agent, apomorphine sublingual, approved for the treatment of erectile dysfunction. The present functional magnetic resonance imaging placebo-controlled study presents the first in vivo demonstration of the apomorphine-induced modulation of cortical and subcortical brain structures in patients with psychogenic erectile dysfunction. Noteworthy, patients in comparison with potent controls, showed an increased activity in frontal limbic areas that was downregulated by apomorphine. This suggests that psychogenic impotence may be associated with previously unrecognized underlying functional abnormalities of the brain.

Central mechanisms of erectile dysfunction: what a clinician may want to know

The interplay between peripheral and central mechanisms of erectile function are not fully elucidated although basic science is moving ahead in this area. It is important from a clinical point of view to understand these mechanisms so that we may begin to make further therapeutic advances in the treatment of erectile dysfunction (ED). It is now widely understood that central disinhibition plays a crucial role in the induction of erectile responses and this has led to the development of the central initiator, apomorphine SL (Ixense ) [apo SL]. Apo SL acts in the paraventricular nucleus of the hypothalamus as a dopamine receptor agonist. It works as a proerectile conditioner at this level to increase the responses of the erectile pathway following appropriate sexual stimulation. This unique central mode of action of apo SL has thus proved efficacious in approximately 70% of ED patients although persistence may be required to produce a robust effect for the maximum number of patients.

Penile neuronal nitric oxide synthase and its regulatory proteins are present in hypothalamic and spinal cord regions involved in the control of penile erection

Control of penile erection requires the coordination of the hypothalamus and the L6-S1 region of the spinal cord. Erection requires the activation of neuronal nitric oxide synthase (nNOS), which is tightly regulated. Because variants of nNOS (penile nNOS: PnNOS) and the N-methyl-D-aspartate receptor (truncated NMDAR subunit 1: NMDAR1-T) as well as protein inhibitor of NOS (PIN) have all been located in the pelvic ganglia and penile nerves, this work aims to determine whether these proteins are also present in the hypothalamus. It was found that PnNOS, the brain-type nNOS, and PIN, were expressed in the hypothalamus. In contrast, NMDAR1-T was expressed only in the penis, whereas the brain-type NMDAR1 was present in the brain and sacral spinal cord and not in the penis. PnNOS was found in the media preoptic area, posterior magnocellular, and the parvocellular regions of the paraventricular nucleus, supraoptic nucleus, septohypothalamic nucleus, medial septum, cortex, and in some of the nNOS staining neurons throughout the brain. It was absent in the organum vasculosum of the lamina terminalis. PIN staining was present in neurons of the medial preoptic area, paraventricular nucleus, medial septum, and cortex, but not in the supraoptic nucleus, septohypothalamic nucleus, or organum vasculosum of the lamina terminalis. Colocalization between PnNOS and PIN was found in the medial preoptic area, medial septum, and cortex, and less in the paraventricular nucleus. PnNOS and oxytocin were colocalized in the paraventricular nucleus and supraoptic nucleus. In hypothalamic extracts, recombinant PIN-GST protein bound to PnNOS in the extracts and partially inhibited NOS activity. These results indicate that both nNOS variants, and their respective regulatory proteins are present and colocalize in the hypothalamic and spinal cord regions involved in penile erection.

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.

Selegiline enhances erectile activity induced by dopamine injection in the paraventricular nucleus of the hypothalamus in anesthetized rats

Apomorphine delivered in the paraventricular nucleus of the hypothalamus (PVN) induces penile erection in rats, suggesting a role of dopaminergic projection to the PVN in the control of penile erection. We assessed whether the selective inhibitor of monoamine oxidase B, selegiline, could enhance the erectile activity induced by dopamine delivery in the PVN. Intracavernous and blood pressure (ICP and BP) were monitored in anesthetized rats to quantify ICP rises (number and percentage of ICP maximum/mean BP (ICPmax/BP x 100)) elicited by 10 micro g dopamine injection in the PVN after saline or 3 mg/kg i.v. selegiline (8 rats per group). The number of ICP rises (mean+/-s.d.: 4.5+/-2.9 vs 1.4+/-1.9; P=0.017) and their ICPmax/BP x 100 (49+/-8% vs 34+/-9%; P=0.015) were significantly greater upon dopamine injection in the PVN than upon vehicle. Compared to saline i.v., 3 mg/kg selegiline pretreatment significantly increased the number of ICP rises induced by dopamine injection in the PVN (9.4+/-2.6 vs 4.5+/-2.9; P<0.001), without affecting their amplitude. This suggests that drugs potentiating dopaminergic responses in the central nervous system might enhance proerectile commands of supraspinal origin.

Current oral treatments for erectile dysfunction

Erectile dysfunction (ED) is defined as the inability to achieve and maintain a penile erection adequate for satisfactory sexual intercourse. It is a significant male health problem of global dimensions affecting approximately 150 million men worldwide. A broad range of options are currently available for the management of ED. They include oral agents (phosphodiesterase 5 inhibitors, dopamine agonists and alpha-receptor blocking drugs), intracavernosal injection (papaverine, phentolamine, prostaglandin E1, vasoactive intestinal peptide), transurethral vasoactive agents (prostaglandin E1), vacuum erection devices, vascular surgery and penile prostheses. Here we review the physiology of penile erection and the currently available oral preparations. In addition, novel therapeutic strategies to improve erectile function are discussed.

Involvement of L-arginine/nitric oxide pathway at the paraventricular nucleus of hypothalamus in central neural regulation of penile erection in the rat

The objective of this study is to investigate whether the L-arginine/nitric oxide pathway is involved in the neurotransmission of paraventricular nucleus of hypothalamus (PVN) activation-induced penile erection in the rat. Male adult Sprague-Dawley rats anesthetized with pentobarbital were used. The femoral artery was cannulated to measure systemic and mean arterial pressure (SAP and MAP), and heart rate (HR). A 26-gauge needle was inserted into corpus cavernosum to measure the intracavernous pressure (ICP) simultaneously with SAP, MAP and HR on a polygraph. Four groups of study were arranged: (1) stereotaxically delivery of L-arginine (500 nmol/500 nl) into PVN; (2) administration of a mixture (1 microl) containing N(G)-Nitro-L-arginine methyl ester (L-NAME) 500 nmol and L-arginine 500 nmol into PVN; (3) microinjection of saline 500 nl into PVN as a vehicle control; and (4) intracavernous injection of L-arginine (100 nmol/50 microl). The ICP, SAP, MAP and HR were monitored for at least 2 h after each administration of the experimental agents. Upon administration of L-arginine into PVN, there was a significant increase of ICP from resting 9.6+/-2.5 mmHg to peaked at 64.4+/-9.8 mmHg after a latency of 3016.0+/-1749.7 s and with a duration of 27.6+/-15.8 min. There was no change of resting ICP after administration of the mixture of L-NAME and L-arginine into PVN. Application of saline to PVN and intracavernous injection of L-arginine failed to increase ICP. Based on elicitation of penile erection upon administration of L-arginine into PVN, and elimination of this L-arginine induced penile erection by co-administration of L-NAME with L-arginine, the results of this study suggest that L-arginine/nitric oxide pathway may be involved in the neurotransmission of PVN activation-induced penile erection in the rat.

Sublingual apomorphine for the treatment of erectile dysfunction

Erectile dysfunction (ED) is a prevalent problem in the world and wreaks havoc on the psychosocial well-being of sufferers as well as partners. First-line treatment for ED is oral therapy, however, the currently available oral erectogenic agent is not uniformly successful and requires approximately 1 h to exert its effect. Apomorphine is a novel, oral erectogenic agent taken sublingually. It exerts its effect on hypothalamic centres involved in the triggering of the erection cascade. It dissolves rapidly and results in an erection in responders in approximately 20 min. By avoiding first pass metabolism through its sublingual administration, apomorphine avoids any significant interaction with food or other drugs. In trials in over 4000 subjects representative of the general ED population, beneficial effects were seen in a significant percentage of men. Its exact position in the market remains to be seen, however, its ability to permit spontaneous sexual relations will prove to be of significant benefit to many men.

Central nervous system agents in the treatment of erectile dysfunction: how do they work?

Drugs acting within the central nervous system (CNS) that reduce the sympathetic antierectile flow and enhance the parasympathetic proerectile flow to the penis may restore penile erection in cases of erectile dysfunction of both psychogenic and organic origin. The best characterized of such drugs is the dopaminergic agonist apomorphine, which acts on the hypothalamus and, perhaps, the autonomic nuclei in the spinal cord. Other drugs that target the CNS and have been registered and tested are the a(2)-adrenoceptor antagonists yohimbine and delequamine, the alpha-melanocyte-stimulating hormone agonist melanotan II, and the serotonin reuptake inhibitor trazodone. Androgens also may influence sexual behavior by acting within the CNS, notably by modifying the neurotransmitter system targeted by these drugs. Our knowledge of the mode of action of CNS drugs comes mainly from experiments on rodents. Consequently, explanations regarding the way they work in humans are only speculative.

Erectile dysfunction occurs following substantia nigra lesions in the rat

Erectile function was assessed 6 weeks following uni- and bilateral injections of 6-hydroxydopamine in the substantia nigra nucleus of the brain. Behavioral apomorphine-induced penile erections were reduced (5/8) and increased (3/8) in uni- and bilateral lesioned animals. Intracavernous pressures, following electrical stimulation of the cavernous nerve, decreased in lesioned animals. Lesions of the substantia nigra were confirmed by histology. Concentration of dopamine and its metabolites were decreased in the striatum of substantia nigra lesioned rats. Lesions of the substantia nigra are therefore associated with erectile dysfunction in rats and may serve as a model to study erectile dysfunction in Parkinson's disease.

Oxytocinergic neurotransmission at the hippocampus in the central neural regulation of penile erection in the rat

OBJECTIVES

To investigate whether there is an oxytocinergic neurotransmission at the hippocampus involved in the central regulation of penile erection in the rat.

METHODS

Male adult Sprague-Dawley (n = 27) rats anesthetized with pentobarbital were used. A 26-gauge needle was inserted into the corpus cavernosum to measure the intracavernous pressure (ICP) and the systemic and mean arterial pressure and heart rate simultaneously. The following studies were performed: stereotaxic delivery of oxytocin acetate (3 pmol/100 nL) into the hippocampus; microinjection of a mixture of [d(CH(2))(5)-Tyr(Me)(2)-Orn(8)]-vasotocin (3 pmol/100 nL) and oxytocin (3 pmol/100 nL) into the hippocampus; injection of saline into the hippocampus; and intracavernous injection of oxytocin (3 pmol/100 microL). The ICP and hemodynamic parameters were monitored after each administration of the experimental agents.

RESULTS

After administration of oxytocin into the hippocampus, a significant increase in the ICP occurred from resting (8.8 +/- 1.7 mm Hg) to a peak at 49.6 +/- 12.5 mm Hg and persisted for 18.6 +/- 9.4 minutes after an onset latency of 500.0 +/- 389.7 seconds. However, no change in the ICP occurred after administration of [d(CH(2))(5)-Tyr(Me)(2)-Orn(8)]-vasotocin plus oxytocin into the hippocampus. In addition, no elevation of ICP occurred after administration of saline to the hippocampus or after intracavernous injection of oxytocin.

CONCLUSIONS

The results demonstrate that administration of oxytocin into the hippocampus induces penile erection in the rat. However, concomitant administration of oxytocin and its antagonist was ineffective in eliciting penile erection. These observations suggest that oxytocinergic neurotransmission at the hippocampus may be involved in the central neural regulation of the penile erection in the rat.