Neuromodulation of penile erection: an overview of the role of neurotransmitters and neuropeptides

An Acupuncture Protocol for the Treatment of Erectile Dysfunction: A Delphi Process

INTRODUCTION

Erectile dysfunction (ED) is a widespread disorder, and the worldwide incidence is rapidly increasing. Acupuncture, an intervention out of the spectrum of traditional Chinese medicine (TCM), has a long tradition as treatment for ED. Nonetheless, a best-practice treatment protocol is currently missing. A recent systematic review and meta-analysis confirmed a huge diversity of acupuncture treatments for ED and concluded that there is an urgent need to standardise acupuncture treatment for ED. Consequently, the authors conducted a Delphi process with the aim to achieve an expert consensus as a basis for the development of a best-practice protocol.

METHODS

The Delphi process consisted of four rounds of questionnaires with closed and open-ended questions. Eleven acupuncture experts participated. The therapeutic aim was defined as "to achieve an erection sufficient for sexual satisfaction."

RESULTS

Consensus was achieved on 24 acupoints corresponding to 12 TCM syndromes. The syndromes were KI Yang xu, KI Yin xu, KI Qi xu, Ki and HT not harmonised, LR Qi Stagnation, LR Qi stagnation and Heat, Liver Blood xu, Liver Blood xu and Liver Qi stagnation, Damp-heat sinking to the lower Jiao5, HT and GB Qi xu, SP xu and HT Blood xu, Yin xu. The suggested optimal dose was between 11 and 15 treatments given once or twice a week.

CONCLUSION

An expert consensus-based, semi-standardised best-practice treatment protocol for the treatment of ED was developed. Moreover, the Delphi process also revealed inconsistencies as to which signs and symptoms constitute a TCM syndrome. Further Delphi studies including a broader range of experts from various acupuncture traditions are needed to establish further agreement. Nonetheless, the best-practice protocol introduced in this study provides a first point of departure for the implementation of a more standardised treatment approach. Moreover, since a recent meta-analysis concluded that more high-quality clinical studies on the topic are needed, this study provides a first standardised acupuncture treatment protocol for ED.

Neurophysiology of male sexual arousal-Behavioral perspective

In the presented review, we analyzed the physiology of male sexual arousal and its relation to the motivational aspects of this behavior. We highlighted the distinction between these processes based on observable physiological and behavioral parameters. Thus, we proposed the experimentally applicable differentiation between sexual arousal (SA) and sexual motivation (SM). We propose to define sexual arousal as an overall autonomic nervous system response leading to penile erection, triggered selectively by specific sexual cues. These autonomic processes include both spinal and supraspinal neuronal networks, activated by sensory pathways including information from sexual partner and sexual context, as well as external and internal genital organs. To avoid misinterpretation of experimental data, we also propose to precise the term "sexual motivation" as all actions performed by the individual that increase the probability of sexual interactions or increase the probability of exposition to sexual context cues. Neuronal structures such as the amygdala, bed nucleus of stria terminalis, hypothalamus, nucleus raphe, periaqueductal gray, and nucleus paragigantocellularis play crucial roles in controlling the level of arousal and regulating peripheral responses via specific autonomic effectors. On the highest level of CNS, the activity of cortical structures involved in the regulation of the autonomic nervous system, such as the insula and anterior cingulate cortex, can visualize an elevated level of SA in both animal and human brains. From a preclinical perspective, we underlie the usefulness of the non-contact erection test (NCE) procedure in understanding factors influencing sexual arousal, including studies of sexual preference in animal models. Taken together results obtained by different methods, we wanted to focus attention on neurophysiological aspects that are distinctly related to sexual arousal and can be used as an objective parameter, leading to higher translational transparency between basic, preclinical, and clinical studies.

Erectile Dysfunction: Treatments, Advances and New Therapeutic Strategies

Erectile dysfunction (ED) is the inability to get and maintain an adequate penile erection for satisfactory sexual intercourse. Due to its negative impacts on men's life quality and increase during aging (40% of men between 40 and 70 years), ED has always attracted researchers of different disciplines, from urology, andrology and neuropharmacology to regenerative medicine, and vascular and prosthesis implant surgery. Locally and/or centrally acting drugs are used to treat ED, e.g., phosphodiesterase 5 inhibitors (first in the list) given orally, and phentolamine, prostaglandin E1 and papaverine injected intracavernously. Preclinical data also show that dopamine D4 receptor agonists, oxytocin and α-MSH analogues may have a role in ED treatment. However, since pro-erectile drugs are given on demand and are not always efficacious, new strategies are being tested for long lasting cures of ED. These include regenerative therapies, e.g., stem cells, plasma-enriched platelets and extracorporeal shock wave treatments to cure damaged erectile tissues. Although fascinating, these therapies are laborious, expensive and not easily reproducible. This leaves old vacuum erection devices and penile prostheses as the only way to get an artificial erection and sexual intercourse with intractable ED, with penile prosthesis used only by accurately selected patients.

Dopamine, Erectile Function and Male Sexual Behavior from the Past to the Present: A Review

Early and recent studies show that dopamine through its neuronal systems and receptor subtypes plays different roles in the control of male sexual behavior. These studies show that (i) the mesolimbic/mesocortical dopaminergic system plays a key role in the preparatory phase of sexual behavior, e.g., in sexual arousal, motivation and reward, whereas the nigrostriatal system controls the sensory-motor coordination necessary for copulation, (ii) the incertohypothalamic system is involved in the consummatory aspects of sexual behavior (penile erection and copulation), but evidence for its role in sexual motivation is also available, (iii) the pro-sexual effects of dopamine occur in concert with neural systems interconnecting the hypothalamus and preoptic area with the spinal cord, ventral tegmental area and other limbic brain areas and (iv) D₂ and D₄ receptors play a major role in the pro-sexual effects of dopamine. Despite some controversy, increases or decreases, respectively, of brain dopamine activity induced by drugs or that occur physiologically, usually improves or worsens, respectively, sexual activity. These findings suggest that an altered central dopaminergic tone plays a role in mental pathologies characterized by aberrant sexual behavior, and that pro-erectile D₄ receptor agonists may be considered a new strategy for the treatment of erectile dysfunction in men.

The Biology of BDSM: A Systematic Review

INTRODUCTION

BDSM is an abbreviation used to reference the concepts of bondage and discipline, dominance and submission, sadism and masochism, enacted by power exchanges between consensual partners. In recent years, attention has shifted from the idea of BDSM as a pathological and tabooed niche practice towards viewing BDSM as a healthy form of intimacy.

AIM

This systematic review brings together all existing literature on the biology of BDSM and places it in a broader biological context.

METHODS

A systematic search was conducted on PubMed, Web of Science and PsycARTICLES, of which 10 articles are included and discussed in this systematic review.

RESULTS

There is evidence for cortisol changes in submissives as a result of a BDSM interaction, suggesting involvement of the physiological stress system. Endocannabinoid changes implicate the pleasure and reward system. In dominants, this biologically measured pleasure seemed to be dependent on power play rather than pain play. Testosterone and oxytocin are also implicated in BDSM, though their role is less evident. Research into brain region activity patterns related to BDSM interest suggests a role for the parietal operculum and ventral striatum in the context of the pleasure and reward system, the primary and secondary somatosensory cortex in the context of pain perception, empathy-related circuits such as the anterior insula, anterior midcingulate cortex and sensorimotor cortex and the left frontal cortex in the context of social and sexual interactions. Pain thresholds are shown to be higher in submissive individuals and a BDSM interaction may cause pain thresholds to rise in submissives as well.

CONCLUSION

BDSM interactions are complex and influenced by several psychological, social and biological processes. Though research is limited, there is emerging evidence for an interaction between several biological systems involved in these types of interests and activities. This means there is an important role for future research to replicate and supplement current results.

Erectile Function and Sexual Behavior: A Review of the Role of Nitric Oxide in the Central Nervous System

Nitric oxide (NO), the neuromodulator/neurotransmitter formed from l-arginine by neuronal, endothelial and inducible NO synthases, is involved in numerous functions across the body, from the control of arterial blood pressure to penile erection, and at central level from energy homeostasis regulation to memory, learning and sexual behavior. The aim of this work is to review earlier studies showing that NO plays a role in erectile function and sexual behavior in the hypothalamus and its paraventricular nucleus and the medial preoptic area, and integrate these findings with those of recent studies on this matter. This revisitation shows that NO influences erectile function and sexual behavior in males and females by acting not only in the paraventricular nucleus and medial preoptic area but also in extrahypothalamic brain areas, often with different mechanisms. Most importantly, since these areas are strictly interconnected with the paraventricular nucleus and medial preoptic area, send to and receive neural projections from the spinal cord, in which sexual communication between brain and genital apparatus takes place, this review reveals that central NO participates in concert with neurotransmitters/neuropeptides to a neural circuit controlling both the consummatory (penile erection, copulation, lordosis) and appetitive components (sexual motivation, arousal, reward) of sexual behavior.

Oxytocin, Erectile Function and Sexual Behavior: Last Discoveries and Possible Advances

A continuously increasing amount of research shows that oxytocin is involved in numerous central functions. Among the functions in which oxytocin is thought to be involved are those that play a role in social and sexual behaviors, and the involvement of central oxytocin in erectile function and sexual behavior was indeed one of the first to be discovered in laboratory animals in the 1980s. The first part of this review summarizes the results of studies done in laboratory animals that support a facilitatory role of oxytocin in male and female sexual behavior and reveal mechanisms through which this ancient neuropeptide participates in concert with other neurotransmitters and neuropeptides in this complex function, which is fundamental for the species reproduction. The second part summarizes the results of studies done mainly with intranasal oxytocin in men and women with the aim to translate the results found in laboratory animals to humans. Unexpectedly, the results of these studies do not appear to confirm the facilitatory role of oxytocin found in male and female sexual behavior in animals, both in men and women. Possible explanations for the failure of oxytocin to improve sexual behavior in men and women and strategies to attempt to overcome this impasse are considered.

Altered Sexual Behavior in Dopamine Transporter (DAT) Knockout Male Rats: A Behavioral, Neurochemical and Intracerebral Microdialysis Study

Central dopamine plays a key role in sexual behavior. Recently, a Dopamine Transporter knockout (DAT KO) rat has been developed, which displays several behavioral dysfunctions that have been related to increased extracellular dopamine levels and altered dopamine turnover secondary to DAT gene silencing. This prompted us to characterize the sexual behavior of these DAT KO rats and their heterozygote (HET) and wild type (WT) counterparts in classical copulatory tests with a sexually receptive female rat and to verify if and how the acquisition of sexual experience changes along five copulatory tests in these rat lines. Extracellular dopamine and glutamic acid concentrations were also measured in the dialysate obtained by intracerebral microdialysis from the nucleus accumbens (Acb) shell of DAT KO, HET and WT rats, which underwent five copulatory tests, when put in the presence of an inaccessible sexually receptive female rat and when copulation was allowed. Markers of neurotropism (BDNF, trkB), neural activation (Δ-FosB), functional (Arc and PSA-NCAM) and structural synaptic plasticity (synaptophysin, syntaxin-3, PSD-95) were also measured in the ventral tegmental area (VTA), Acb (shell and core) and medial prefrontal cortex (mPFC) by Western Blot assays. The results indicate that the sexual behavior of DAT KO vs. HET and WT rats shows peculiar differences, mainly due to a more rapid acquisition of stable sexual activity levels and to higher levels of sexual motivation and activity. These differences occurred with differential changes in dopamine and glutamic acid concentrations in Acb dialysates during sexual behavior, with lower increases of dopamine and glutamic acid in DAT KO vs. WT and HET rats, and a lower expression of the markers investigated, mainly in the mPFC, in DAT KO vs. WT rats. Together these findings confirm a key role of dopamine in sexual behavior and provide evidence that the permanently high levels of dopamine triggered by DAT gene silencing cause alterations in both the frontocortical glutamatergic neurons projecting to the Acb and VTA and in the mesolimbic dopaminergic neurons, leading to specific brain regional changes in trophic support and neuroplastic processes, which may have a role in the sexual behavior differences found among the three rat genotypes.

Neuroanatomy and function of human sexual behavior: A neglected or unknown issue?

INTRODUCTION

Sexual desire, arousal, and orgasm are mediated by complex, yet still not fully understood, interactions of the somatic and autonomic nervous systems operating at the central and peripheral levels. Disruption of endocrine, neural, or vascular response, caused by aging, medical illness, neurological diseases, surgery, or drugs, can lead to sexual dysfunctions, thus significantly affecting patients' quality of life.

PURPOSE

This narrative review aims at characterizing the involvement of the central nervous system in human sexual behavior.

METHODS

A literature search was conducted using PubMed in its entirety up to June 2018, analyzing the studies dealing with the neurobiological and neurophysiological basis of human sexuality.

RESULTS

Sexual behavior is regulated by both subcortical structures, such as the hypothalamus, brainstem, and spinal cord, and several cortical brain areas acting as an orchestra to finely adjust this primitive, complex, and versatile behavior. At the central level, dopaminergic and serotonergic systems appear to play a significant role in various factors of sexual response, although adrenergic, cholinergic, and other neuropeptide transmitter systems may contribute as well.

CONCLUSIONS

Providing healthcare professionals with information concerning sexual behavior may overcome useless and sometimes dangerous barriers and improve patient management, since sexual well-being is considered one of the most important aspects of one's quality of life.

Rats selectively bred for showing divergent behavioral traits in response to stress or novelty or spontaneous yawning with a divergent frequency show similar changes in sexual behavior: the role of dopamine

Sexual behavior plays a fundamental role for reproduction in mammals and other animal species. It is characterized by an anticipatory and a consummatory phase, and several copulatory parameters have been identified in each phase, mainly in rats. Sexual behavior varies significantly across rats even when they are of the same strain and reared under identical conditions. This review shows that rats of the same strain selectively bred for showing a divergent behavioral trait when exposed to stress or novelty (i.e. Roman high and low avoidance rats, bred for their different avoidance response to the shuttle box, and high and low novelty exploration responders rats, bred for their different exploratory response to a novel environment) or a spontaneous behavior with divergent frequency (i.e. low and high yawning frequency rats, bred for their divergent yawning frequency) show similar differences in sexual behavior, mainly in copulatory pattern, but also in sexual motivation. As shown by behavioral pharmacology and intracerebral microdialysis experiments carried out mainly in Roman rats, these sexual differences may be due to a more robust dopaminergic tone present in the mesocorticolimbic dopaminergic system of one of the two sub-lines (e.g. high avoidance, high novelty exploration, and low yawning rat sub-lines). Thus, differences in genotype and/or in prenatal/postnatal environment lead not only to individual differences in temperament and environmental/emotional reactivity but also in sexual behavior. Because of the highly conserved mechanisms controlling reproduction in mammals, this may occur not only in rats but also in humans.

Oxytocin induces penile erection and yawning when injected into the bed nucleus of the stria terminalis: Involvement of glutamic acid, dopamine, and nitric oxide

Oxytocin (5-100ng), but not Arg⁸-vasopressin (100ng), injected unilaterally into the bed nucleus of the stria terminalis (BNST) induces penile erection and yawning in a dose-dependent manner in male rats. The minimal effective dose was 20ng for penile erection and 5ng for yawning. Oxytocin responses were abolished not only by the oxytocin receptor antagonist d(CH₂)₅Tyr(Me)²-Orn⁸-vasotocin (1μg), but also by (+) MK-801 (1μg), an excitatory amino acid receptor antagonist of the N-methyl-d-aspartic acid (NMDA) subtype, SCH 23390 (1μg), a D1 receptor antagonist, but not haloperidol (1μg), a D2 receptor antagonist, and SMTC (40μg), an inhibitor of neuronal nitric oxide synthase, injected into the BNST 15min before oxytocin. Oxytocin-induced penile erection, but not yawning, was also abolished by CNQX (1μg), an excitatory amino acid receptor antagonist of the AMPA subtype. In contrast, oxytocin responses were not reduced by bicuculline (20ng), a GABA_A receptor antagonist, phaclofen (5μg), a GABA_B receptor antagonist, CP 376395, a CRF receptor-1 antagonist (5μg), or astressin 2B, a CRF receptor-2 antagonist (150ng). Considering the ability of NMDA (100ng) to induce penile erection and yawning when injected into the BNST and the available evidence showing possible interaction among oxytocin, glutamic acid, and dopamine in the BNST, oxytocin possibly activates glutamatergic neurotransmission in the BNST. This in turn leads to the activation of neural pathways projecting back to the paraventricular nucleus, medial preoptic area, ventral tegmental area, and/or ventral subiculum/amygdala, thereby inducing penile erection and yawning.

Assessment of hormonal activity in patients with premature ejaculation

Purpose

Premature ejaculation is considered the most common type of male sexual dysfunction. Hormonal controls of ejaculation have not been exactly elucidated. The aim of our study is to investigate the role of hormonal factors in patients with premature ejaculation.

Materials and Methods

Sixty-three participants who consulted our outpatient clinics with complaints of premature ejaculation and 39 healthy men as a control group selected from volunteers were included in the study. A total of 102 sexual active men aged between 21 and 76 years were included. Premature ejaculation diagnostic tool questionnaires were used to assessment of premature ejaculation. Serum levels of follicle stimulating hormone, luteinizing hormone, prolactin, total and free testosterone, thyroid-stimulating hormone, free triiodothyronine and thyroxine were measured.

Results

Thyroid-stimulating hormone, luteinizing hormone, and prolactin levels were significantly lower in men with premature ejaculation according to premature ejaculation diagnostic tool (p=0.017, 0.007 and 0.007, respectively). Luteinizing hormone level (OR, 1.293; p=0.014) was found to be an independent risk factor for premature ejaculation.

Conclusions

Luteinizing hormone, prolactin, and thyroid-stimulating hormone levels are associated with premature ejaculation which was diagnosed by premature ejaculation diagnostic tool questionnaires. The relationship between these findings have to be determined by more extensive studies.

The role of prolactin in andrology: what is new?

Prolactin (PRL) has been long deemed as a hormone involved only in female reproduction. However, PRL is a surprising hormone and, since its identification in the 1970s, its attributed functions have greatly increased. However, its specific role in male health is still widely unknown. Recently, low PRL has been associated with reduced ejaculate and seminal vesicle volume in infertile subjects. In addition, in men consulting for sexual dysfunction, hypoprolactinemia has been associated with erectile dysfunction and premature ejaculation, findings further confirmed in the general European population and infertile men. Several metabolic derangements, recapitulating metabolic syndrome, have also been associated with low PRL both in men with sexual dysfunction and from the general European population. In men with sexual dysfunction, followed-up for more than 4 years, low PRL was identified as an independent predictor of the incidence of major adverse cardiovascular events. Finally, an association with anxiety or depressive symptoms has been found in men with sexual dysfunction and from the general European population. While a direct role for impaired PRL function in the pathogenesis of these reproductive, sexual, metabolic and psychological disorders is conceivable, the possibility that low PRL is a mirror of an increased dopaminergic or a decreased serotonergic tone cannot be ruled-out. Hyperactivity of the dopaminergic system can explain only a few of the aforementioned findings, whereas a hypo-serotonergic tone fits well with the clinical features associated with low PRL, and there is significant evidence supporting the hypothesis that PRL could be a mirror of serotonin in the brain.

Involvement of the cGMP pathway in the osthole-facilitated glutamate release in rat hippocampal nerve endings

Osthole, an active constituent isolated from Cnidium monnieri (L.) Cusson, has previously been shown to have the capacity to increase depolarization-evoked glutamate release in rat hippocampal nerve terminals. As cGMP-dependent signaling cascade has been found to modulate glutamate release at the presynaptic level, the aim of this study was to further examine the role of cGMP signaling pathway in the regulation of osthole on glutamate release in hippocampal synaptosomes. Results showed that osthole dose-dependently increased intrasynaptosomal cGMP levels. The elevation of cGMP levels by osthole was prevented by the phosphodiesterase 5 inhibitor sildenafil but was insensitive to the guanylyl cyclase inhibitor ODQ. In addition, osthole-induced facilitation of 4-aminopyridine (4-AP)-evoked glutamate release was completely prevented by the cGMP-dependent protein kinase (PKG) inhibitors, KT5823, and Rp-8-Br-PET-cGMPS. Direct activation of PKG with 8-Br-cGMP or 8-pCPT-cGMP also occluded the osthole-mediated facilitation of 4-AP-evoked glutamate release. Furthermore, sildenafil exhibited a dose-dependent facilitation of 4-AP-evoked release of glutamate and occluded the effect of osthole on the 4-AP-evoked glutamate release. Collectively, our findings suggest that osthole-mediated facilitation of glutamate release involves the activation of cGMP/PKG-dependent pathway.

Oxytocin and social motivation

Humans are fundamentally social creatures who are ‘motivated’ to be with others. In this review we examine the role of oxytocin (OT) as it relates to social motivation. OT is synthesized in the brain and throughout the body, including in the heart, thymus, gastrointestinal tract, as well as reproductive organs. The distribution of the OT receptor (OTR) system in both the brain and periphery is even more far-reaching and its expression is subject to changes over the course of development. OTR expression is also sensitive to changes in the external environment and the internal somatic world. The OT system functions as an important element within a complex, developmentally sensitive biobehavioral system. Other elements include sensory inputs, the salience, reward, and threat detection pathways, the hypothalamic-pituitary-gonadal axis, and the hypothalamic-pituitary-adrenal stress response axis. Despite an ever expanding scientific literature, key unresolved questions remain concerning the interplay of the central and peripheral components of this complex biobehavioral system that dynamically engages the brain and the body as humans interact with social partners over the course of development.

Anatomy, physiology, and pathophysiology of erectile dysfunction

INTRODUCTION

Significant scientific advances during the past 3 decades have deepened our understanding of the physiology and pathophysiology of penile erection. A critical evaluation of the current state of knowledge is essential to provide perspective for future research and development of new therapies.

AIM

To develop an evidence-based, state-of-the-art consensus report on the anatomy, physiology, and pathophysiology of erectile dysfunction (ED).

METHODS

Consensus process over a period of 16 months, representing the opinions of 12 experts from seven countries.

MAIN OUTCOME MEASURE

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

RESULTS

ED occurs from multifaceted, complex mechanisms that can involve disruptions in neural, vascular, and hormonal signaling. Research on central neural regulation of penile erection is progressing rapidly with the identification of key neurotransmitters and the association of neural structures with both spinal and supraspinal pathways that regulate sexual function. In parallel to advances in cardiovascular physiology, the most extensive efforts in the physiology of penile erection have focused on elucidating mechanisms that regulate the functions of the endothelium and vascular smooth muscle of the corpus cavernosum. Major health concerns such as atherosclerosis, hyperlipidemia, hypertension, diabetes, and metabolic syndrome (MetS) have become well integrated into the investigation of ED.

CONCLUSIONS

Despite the efficacy of current therapies, they remain insufficient to address growing patient populations, such as those with diabetes and MetS. In addition, increasing awareness of the adverse side effects of commonly prescribed medications on sexual function provides a rationale for developing new treatment strategies that minimize the likelihood of causing sexual dysfunction. Many basic questions with regard to erectile function remain unanswered and further laboratory and clinical studies are necessary.

Oxytocin induces penile erection when injected into the ventral tegmental area of male rats: role of nitric oxide and cyclic GMP

Oxytocin (80 ng) injected into the caudal mesencephalic ventral tegmental area (VTA) of male rats induces penile erection. Such an effect occurs together with an increase in nitric oxide (NO) production, as measured by the augmented concentration of NO(2)(-) and NO(3)(-) found in the dialysate obtained from this brain area by means of intracerebral microdialysis. Both effects are abolished by d(CH(2))(5)Tyr(Me)(2)-Orn(8)-vasotocin (1 microg), an oxytocin receptor antagonist, by S-methyl-l-thiocitrulline acetate (20 microg), a neuronal NO synthase inhibitor, or by omega-conotoxin GVIA (50 ng), a N-type Ca(2+) channel blocker, all injected into the VTA 15 min before oxytocin. In contrast, 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (40 microg), a guanylate cyclase inhibitor, given into the VTA 15 min before oxytocin, abolishes penile erection, but not the increase in NO production, while haemoglobin (40 microg), a NO scavenger, injected immediately before oxytocin reduces the increase in NO production, but not penile erection. 8-Bromo-cyclic guanosine monophosphate (0.5-10 microg) microinjected into the VTA induces penile erection with an inverted U-shaped dose-response curve; the maximal effective dose being 3 microg. Immunohistochemistry reveals that in the caudal VTA oxytocin-containing axons/fibres (originating from the paraventricular nucleus of the hypothalamus) contact cell bodies of mesolimbic dopaminergic (tyrosine hydroxylase-positive) neurons containing both NO synthase and guanylate cyclase. These results suggest that oxytocin injected into the VTA induces penile erection by activating NO synthase in the cell bodies of mesolimbic dopaminergic neurons. NO in turn activates guanylate cyclase present in these neurons, thereby increasing cyclic GMP concentration.

Brain oxytocin receptors mediate ejaculation elicited by 7-hydroxy-2-(di-N-propylamino) tetralin (7-OH-DPAT) in anaesthetized rats

BACKGROUND AND PURPOSE

The involvement of the neuropeptide oxytocin in the control of male sexual responses is documented although its exact mechanisms of action, and especially the site(s) of action, are not fully delineated. In order to clarify this issue, we tested the effects of a peptide oxytocin antagonist delivered through different routes on sexual responses elicited, in anaesthetized male rats, by i.c.v. 7-hydroxy-2-(di-N-propylamino) tetralin (7-OH-DPAT), a dopamine agonist, preferentially active on D3 receptors.

EXPERIMENTAL APPROACH

Seminal vesicle pressure (SVP) and bulbospongiosus muscle (BS) electromyograms were recorded as physiological markers of emission and expulsion phases of ejaculation respectively and intracavernosal pressure (ICP) was monitored as a physiological marker of erection.

KEY RESULTS

When injected i.v., the oxytocin antagonist did not impair 7-OH-DPAT-induced SVP and ICP responses while BS burst frequency was diminished. When delivered i.c.v., the oxytocin antagonist dose-dependently inhibited occurrence of 7-OH-DPAT-induced sexual responses. When delivered intrathecally (i.t.) at the level of the 6th lumbar (L6) segment, but not the 13th thoracic (T13) segment, the oxytocin antagonist reduced the duration of BS responses and the occurrence of ejaculation without impairing ICP responses.

CONCLUSIONS AND IMPLICATIONS

Brain oxytocin receptors mediate male sexual responses elicited by i.c.v. 7-OH-DPAT in anaesthetized rats whereas L6 spinal oxytocin receptors only impair the occurrence of ejaculation. Peripheral oxytocin receptors are marginally involved in 7-OH-DPAT-induced sexual responses. These findings should be considered for the development of potential pharmacological treatment of premature ejaculation in man.

Oxytocin injected into the ventral tegmental area induces penile erection and increases extracellular dopamine in the nucleus accumbens and paraventricular nucleus of the hypothalamus of male rats

The neuropeptide oxytocin (20-100 ng), induces penile erection when injected unilaterally into the caudal but not rostral mesencephalic ventral tegmental area (VTA) of male Sprague-Dawley rats. Such pro-erectile effect started 30 min after treatment and was abolished by the prior injection of d(CH2)5Tyr(Me)(2)-Orn(8)-vasotocin (1 microg), an oxytocin receptor antagonist injected into the same caudal ventral tegmental area or of haloperidol (1 microg), a dopamine receptor antagonist, injected either into the nucleus accumbens shell (NAs) or into the paraventricular nucleus of the hypothalamus (PVN) ipsilateral to the injected ventral tegmental area. Penile erection was seen 15 min after the occurrence of, or concomitantly to, an increase in extracellular dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the dialysate obtained from the nucleus accumbens or the paraventricular nucleus, which was also abolished by d(CH2)5Tyr(Me)(2)-Orn(8)-vasotocin (1 microg), injected into the ventral tegmental area before oxytocin. In the caudal ventral tegmental area oxytocin-containing axons/fibres (originating from the paraventricular nucleus) appeared to closely contact cell bodies of mesolimbic dopaminergic neurons retrogradely labelled with Fluorogold injected into the nucleus accumbens shell, suggesting that oxytocin effects are mediated by the activation of mesolimbic dopaminergic neurons, followed in turn by that of incerto-hypothalamic dopaminergic neurons impinging on oxytocinergic neurons mediating penile erection. As the stimulation of paraventricular dopamine receptors not only induces penile erection, but also increases mesolimbic dopamine neurotransmission by activating oxytocinergic neurons, these results provide further support for the existence of a neural circuit in which dopamine and oxytocin influence both the consummatory and motivational/rewarding aspects of sexual behaviour.

Cannabinoid CB1 receptors in the paraventricular nucleus and central control of penile erection: Immunocytochemistry, autoradiography and behavioral studies

[N-(piperidin-1-yl)-5-(4-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxyamide] (SR 141716A), a selective cannabinoid CB1 receptor antagonist, injected into the paraventricular nucleus of the hypothalamus (PVN) of male rats, induces penile erection. This effect is mediated by the release of glutamic acid, which in turn activates central oxytocinergic neurons mediating penile erection. Double immunofluorescence studies with selective antibodies against CB1 receptors, glutamic acid transporters (vesicular glutamate transporters 1 and 2 (VGlut1 and VGlut2), glutamic acid decarboxylase-67 (GAD67) and oxytocin itself, have shown that CB1 receptors in the PVN are located mainly in GABAergic terminals and fibers surrounding oxytocinergic cell bodies. As GABAergic synapses in the PVN impinge directly on oxytocinergic neurons or on excitatory glutamatergic synapses, which also impinge on oxytocinergic neurons, these results suggest that the blockade of CB1 receptors decreases GABA release in the PVN, increasing in turn glutamatergic neurotransmission to activate oxytocinergic neurons mediating penile erection. Autoradiography studies with [(3)H](-)-CP 55,940 show that chronic treatment with SR 141716A for 15 days twice daily (1 mg/kg i.p.) significantly increases the density of CB1 receptors in the PVN. This increase occurs concomitantly with an almost twofold increase in the pro-erectile effect of SR 141716A injected into the PVN as compared with control rats. The present findings confirm that PVN CB1 receptors, localized mainly in GABAergic synapses that control in an inhibitory fashion excitatory synapses, exert an inhibitory control on penile erection, demonstrating for the first time that chronic blockade of CB1 receptors by SR 141716A increases the density of these receptors in the PVN. This increase is related to an enhanced pro-erectile effect of SR 141716A, which is still present 3 days after the end of the chronic treatment.

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.

The cannabinoid CB1 receptor antagonist SR 141716A induces penile erection by increasing extra-cellular glutamic acid in the paraventricular nucleus of male rats

The cannabinoid CB1 receptor antagonist SR 141716A (0.1, 0.5 and 1 microg) induces penile erection when injected into the paraventricular nucleus of the hypothalamus of male rats. The pro-erectile effect of SR 141716A occurs concomitantly with an increase in the concentration of glutamic acid in the paraventricular dialysate obtained by means of intra-cerebral microdialysis. Glutamic acid increase and penile erection did not occur when SR 141716A was given after tetrodotoxin, a voltage-dependent Na(+) channel blocker. Both penile erection and glutamic acid increases were also reduced by the cannabinoid CB1 receptor agonists WIN 55,212-2 or HU 210 given into the paraventricular nucleus before SR 141716A at doses unable to induce penile erection or to modify glutamic acid. In contrast, dizocilpine ((+)MK-801), an antagonist of excitatory amino acid receptors of the N-methyl-d-aspartic acid (NMDA) subtype, given into the paraventricular nucleus reduced penile erection, but was ineffective on the glutamic acid increase induced by the CB1 receptor antagonist. 6-Cyano-7-nitro-quinoxaline-2,3-dione (CNQX) and (+/-)-2-amino-4-phosphono-butanoic acid (AP(4)), antagonists of the excitatory amino acid receptors of the AMPA subtype and of the metabotropic subtype, respectively, were ineffective on both penile erection and glutamic acid increase. SR 141716A responses were also reduced by muscimol, a GABA(A) receptor agonist, but not by baclofen, a GABA(B) receptor agonist, given into the paraventricular nucleus before SR 141716A. The present results show that SR 141716A induces penile erection by activating glutamic acid neurotransmission, which causes in turn the activation of paraventricular oxytocinergic neurons mediating penile erection.

PD-168077, a selective dopamine D4 receptor agonist, induces penile erection when injected into the paraventricular nucleus of male rats

The effect of PD-168077 (N-methyl-4-(2-cyanophenyl)piperazynil-3-methylbenzamide maleate), a selective D4 dopamine receptor agonist, injected into the paraventricular nucleus of the hypothalamus on penile erection was studied in male rats. PD-168077 (1-200 ng) induced penile erection in a dose-dependent manner. The minimal effective dose was 50 ng, while the maximal response was found with 200 ng of the compound, which increased penile erection episodes from 0.3+/-0.03 to 1.7+/-0.21. The proerectile effect of PD-168077 was reduced almost completely by L-745,870 (3-(4-[chlorophenyl]piperazin-1-yl)-methyl-1H-pyrrolo[2,3-B]pyridine trihydrochloride), a selective D4 dopamine receptor antagonist, (1 microg) given into the paraventricular nucleus before the D4 dopamine agonist, and by other nonselective dopamine receptor antagonists, such as haloperidol (1 microg) and clozapine (1 microg), which block all dopamine receptor subtypes. The pro-erectile effect of PD-168077 was also reduced by the NO synthase inhibitor NG-nitro-L-arginine methylester (25 microg), but not by the oxytocin receptor antagonist d(CH2)5Tyr(Me)2-Orn8-vasotocin (1 microg), when given into the paraventricular nucleus. In spite of its inability to prevent the pro-erectile effect of PD-168077 when given in the paraventricular nucleus, d(CH2)5Tyr(Me)2-Orn8-vasotocin (1 microg) reduced almost completely PD-168077-induced penile erection when given into the lateral ventricles. The present results show that D4 dopamine receptors present in the paraventricular nucleus may influence penile erection by modulating the activity of paraventricular oxytocinergic neurons mediating erectile function.

Regulation of Oxytocin Secretion

A baby sucks at a mother's breast for comfort and, of course, for milk. Milk is made in specialized cells of the mammary gland, and for a baby to feed, the milk must be released into a collecting chamber from where it can be extracted by sucking. Milk "let-down" is a reflex response to the suckling and kneading of the nipple--and sometimes in response to the sight, smell, and sound of the baby--and is ultimately affected by the secretion of oxytocin. Oxytocin has many physiological roles, but its only irreplaceable role is to mediate milk let-down: oxytocin-deficient mice cannot feed their young; the pups suckle but no milk is let down, and they will die unless cross-fostered. Most other physiological roles of oxytocin, including its role in parturition, are redundant in the sense that the roles can be assumed by other mechanisms in the absence of oxytocin throughout development and adult life. Nevertheless, physiological function in these roles can be altered or impaired by acute interventions that alter oxytocin secretion or change the actions of oxytocin. Here we focus on the diverse stimuli that regulate oxytocin secretion and on the apparent diversity of the roles for oxytocin.

The role of oxytocin and the paraventricular nucleus in the sexual behaviour of male mammals

The paraventricular nucleus of the hypothalamus contains the cell bodies of a group of oxytocinergic neurons projecting to extrahypothalamic brain areas and to the spinal cord, which are involved in the control of erectile function and copulation. In male rats, these neurons can be activated by dopamine, excitatory amino acids, nitric oxide (NO), hexarelin analogue peptides and oxytocin itself to induce penile erection and facilitate copulation, while their inhibition by gamma-aminobutyric acid (GABA) and GABA agonists and by opioid peptides and opiate-like drugs inhibits sexual responses. The activation of paraventricular oxytocinergic neurons by dopamine, oxytocin, excitatory amino acids and hexarelin analogue peptides is apparently mediated by the activation of nitric oxide (NO) synthase. NO in turn activates, by a mechanism that is as yet unidentified, the release of oxytocin from oxytocinergic neurons in extrahypothalamic brain areas. Paraventricular oxytocinergic neurons and mechanisms similar to those reported above are also involved in the expression of penile erection in physiological contexts, namely, when penile erection is induced in the male by the presence of an inaccessible receptive female, which is considered a model for psychogenic impotence in man, as well as during copulation. These findings show that paraventricular oxytocinergic neurons projecting to extrahypothalamic brain areas and to the spinal cord and the paraventricular nucleus play an important role in the control of erectile function and male sexual behaviour in mammals.

Neural control of erection

Penile erection is a vascular event controlled by the autonomic nervous system. The spinal cord contains the autonomic preganglionic neurons that innervate the penile erectile tissue and the pudendal motoneurons that innervate the perineal striated muscles. Sympathetic pathways are anti-erectile, sacral parasympathetic pathways are pro-erectile, and contraction of the perineal striated muscles upon activity of the pudendal nerves improves penile rigidity. Spinal neurons controlling erection are activated by information from peripheral and supraspinal origin. Both peripheral and supraspinal information is capable of either eliciting erection or modulating or inhibiting an erection already present. Sensory information from the genitals is a potent activator of pro-erectile spinal neurons and elicits reflexive erections. Some pre-motor neurons of the medulla, pons and diencephalon project directly onto spinal sympathetic, parasympathetic and pudendal motoneurons. They receive in turn sensory information from the genitals. These spinal projecting pathways release a variety of neurotransmitters, including biogenic amines (serotonin, dopamine, noradrenaline, and adrenaline) and peptides that, through interactions with many receptor subtypes, exert complex effects on the spinal network that controls penile erection. Some supraspinal structures (e.g. the paraventricular nucleus and the medial preoptic area of the hypothalamus, the medial amygdala), whose roles in erection have been demonstrated in animal models, may not project directly onto spinal pro-erectile neurons. They are nevertheless prone to regulate penile erection in more integrated and coordinated responses of the body, as those occurring during sexual behavior. The application of basic and clinical research data to treatment options for erectile dysfunction has recently proved successful. Pro-erectile effects of phosphodiesterase type 5 inhibitors, acting in the penis, and of melanocortin agonists, acting in the brain, illustrate these recent developments.

Effects of morphine or naloxone on cocaine-induced genital reflexes in paradoxical sleep-deprived rats

The involvement of opioidergic neurotransmission in the modulation of genital reflexes induced by paradoxical sleep deprivation (PSD) and cocaine in rats was the aim of the present study. Morphine (0, 1, 5 and 10 mg/kg) and naloxone (0, 0.3, 3 and 30 mg/kg) were administered prior to saline or cocaine to rats that had been deprived of sleep and the incidence of penile erections (PE) and ejaculations (EJ) was measured. PSD alone induced PE in 50% and EJ in 20% of the rats, but these behaviors were not influenced by morphine or naloxone. Cocaine potentiated the incidence of genital reflexes in PSD rats to 90% (PE) and 70% (EJ). Morphine and not naloxone significantly reduced the percentage of rats displaying this response at the highest doses. Morphine also significantly reduced PE and EJ frequencies at 10 mg/kg. Furthermore, this inhibitory effect of morphine on genital reflexes was prevented by the prior injection of naloxone. Although a number of factors are involved in such a complex phenomenon as PE and EJ, our data show that activation of the opioidergic systems by the agonist morphine reduces genital reflexes-induced by cocaine in PSD males while the antagonist, naloxone, did not have any significant effect. The findings suggest that the stimulating effects of cocaine in potentiating genital reflexes in PSD rats can be unidirectionally modified by opioidergic systems.

De novo design, synthesis, and pharmacology of alpha-melanocyte stimulating hormone analogues derived from somatostatin by a hybrid approach

A number of alpha-melanotropin (alpha-MSH) analogues have been designed de novo, synthesized, and bioassayed at different melanocortin receptors from frog skin (fMC1R) and mouse/rat (mMC1R, rMC3R, mMC4R, and mMC5R). These ligands were designed from somatostatin by a hybrid approach, which utilizes a modified cyclic structure (H-d-Phe-c[Cys---Cys]-Thr-NH(2)) related to somatostatin analogues (e.g. sandostatin) acting at somatostatin receptors, CTAP which binds specifically to micro opioid receptors, and the core pharmacophore of alpha-MSH (His-Phe-Arg-Trp). Ligands designed were H-d-Phe-c[XXX-YYY-ZZZ-Arg-Trp-AAA]-Thr-NH(2) [XXX and AAA = Cys, d-Cys, Hcy, Pen, d-Pen; YYY = His, His(1'-Me), His(3'-Me); ZZZ = Phe and side chain halogen substituted Phe, d-Phe, d-Nal(1'), and d-Nal(2')]. The compounds showed a wide range of bioactivities at the frog skin MC1R; e.g. H-d-Phe-c[Hcy-His-d-Phe-Arg-Trp-Cys]-Thr-NH(2) (6, EC(50) = 0.30 nM) and H-d-Phe-c[Cys-His-d-Phe-Arg-Trp-d-Cys]-Thr-NH(2) (8, EC(50) = 0.10 nM). In addition, when a lactam bridge was used as in H-d-Phe-c[Asp-His-d-Phe-Arg-Trp-Lys]-Thr-NH(2) (7, EC(50) = 0.10 nM), the analogue obtained is as potent as alpha-MSH in the frog skin MC1R assay. Interestingly, switching the bridge of 6 to give H-d-Phe-c[Cys-His-d-Phe-Arg-Trp-Hcy]-Thr-NH(2) (5, EC(50) = 1000 nM) led to a 3000-fold decrease in agonist activity. An increase in steric size in the side chain of d-Phe(7) reduced the bioactivity significantly. For example, H-d-Phe-c[Cys-His-d-Nal(1')-Arg-Trp-d-Cys]-Thr-NH(2) (24) is 2000-fold less active than 9. On the other hand, H-d-Phe-c[Cys-His-d-Phe(p-I)-Arg-Trp-d-Cys]-Thr-NH(2) (23) lost all agonist activity and became a weak antagonist (IC(50) = 1 x 10(-5) M). Furthermore, the modified CTAP analogues with a d-Trp at position 7 all showed weak antagonist activities (EC(50) = 10(-6) to 10(-7) M). Compounds bioassayed at mouse/rat MCRs displayed intriguing results. Most of them are potent at all four receptors tested (mMC1R, rMC3R, mMC4R, and mMC5R) with poor selectivities. However, two of the ligands, H-d-Phe-c[Cys-His-d-Phe-Arg-Trp-Pen]-Thr-NH(2) (9, EC(50) = 6.9 x 10(-9) M, 6.4 x 10(-8) M, 2.0 x 10(-8) M, and 1.4 x 10(-10) M at mMC1R, rMC3R, mMC4R, and mMC5R, respectively) and H-d-Phe-c[Cys-His(3'-Me)-d-Phe-Arg-Trp-Cys]-Thr-NH(2) (16, EC(50) = 3.5 x 10(-8) M, 3.1 x 10(-8) M, 8.8 x 10(-9) M, and 5.5 x 10(-10) M at mMC1R, rMC3R, mMC4R, and mMC5R, respectively) showed significant selectivities for the mMC5R. Worthy of mention is that neither of these two ligands is potent in the frog skin MC1R assay (EC(50) = 10(-7) M for 9 and EC(50) = 10(-5) M for 16). These results clearly demonstrated that binding behaviors in rodent MCRs are quite different from those in the classical frog skin (R pipiens) assay.

Antagonism of cannabinoid CB1 receptors in the paraventricular nucleus of male rats induces penile erection

The effect of cannabinoid CB1 receptor agonists and antagonists on penile erection was studied in male rats when injected into the paraventricular nucleus of the hypothalamus. The CB1 receptor antagonist SR 141716A [N-(piperidin-1-yl)-5-(4-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxyamide] (0.5-5 microg) induced penile erection in a dose-dependent manner. The minimal effective dose was 1 microg, while the maximal response was found with 5 microg of the compound. In contrast, the CB1 receptor agonists WIN 55,212-2 [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenyl-carbonyl)-6H-pyrrolo[3,2,1-I,j]quinolin-6-one] (0.5-5 microg) and CP 55,940 [1alpha,2beta-(R)-5alpha]-5-(1,1-dimethylheptyl)-2-[5-hydroxy-2-(3-hydroxy-propyl)cyclohexyl]phenol (0.5-5 microg) were ineffective at all the doses tested. Nevertheless, both compounds reduced the enhancing effect of SR 141716A on penile erection when given into the paraventricular nucleus at the above doses before SR 141716A. The pro-erectile effect of SR 141716A was also reduced by the non-competitive NMDA receptor antagonist dizolcipine (MK-801) (0.2 microg) and by the NO synthase inhibitor NG-nitro-l-arginine methylester (L-NAME) (20 microg) but not by the dopamine receptor antagonist cis-flupenthixol (10 microg) or the oxytocin receptor antagonist d(CH2)5Tyr(Me)2-Orn8-vasotocin (0.1 microg), when given into the paraventricular nucleus. In spite of its inability to prevent the pro-erectile effect of SR 141716A when given in the paraventricular nucleus, d(CH2)5Tyr(Me)2-Orn8-vasotocin) (1 microg) reduced almost completely SR 141716A-induced penile erection when given into the lateral ventricles. The present results show that cannabinoid CB1 receptors present in the paraventricular nucleus may influence erectile function and sexual activity by modulating paraventricular oxytocinergic neurons mediating erectile function.

Activation of GABAA and opioid receptors reduce penile erection induced by hexarelin peptides

The effect of muscimol, a GABA(A) receptor agonist, and of morphine, an opioid receptor agonist, on penile erection induced by the hexarelin analogue peptide EP 80661 (GAB-D-Trp(2-Me)-D-Trp(2-Me)-LysNH(2)) and on the increase in the concentration of NO(2)(-) and NO(3)(-) that occurs concomitantly in the dialysate obtained from the paraventricular nucleus (PVN) of the hypothalamus by intracerebral microdialysis, was studied in male rats. Muscimol (50, 100 and 200 ng) and morphine (0.1, 0.5, 1 and 5 microg) given into the PVN dose-dependently reduced penile erection induced by EP 80661 (1 microg) injected into the PVN. The reduction of penile erection was parallel to a decrease of the concomitant NO(2)(-) and NO(3)(-) increase that occurs in the paraventricular dialysate in these experimental conditions. Muscimol and morphine effects on EP 80661-induced penile erection and NO(2)(-) increase were prevented by the prior administration into the PVN of bicuculline (250 ng) and naloxone (5 microg), respectively. The present results show that the activation of GABA(A) receptors and of opioid receptors in the PVN reduces penile erection induced by hexarelin analogue peptides by reducing the increase in NO activity that occurs in this hypothalamic nucleus in these experimental conditions.

Potentiation of penile erection and yawning responses to apomorphine by cannabinoid receptor antagonist in rats

The effect of systemic administration of the cannabinoid antagonist SR 141716A (N-(piperidin-1-yl)-5-(4-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxyamide) on penile erection and yawning induced by apomorphine was investigated in rats. SR 141716A (2 mg/kg, i.p.) administered 40 min before apomorphine (40 and 80 microg/kg, s.c.) increased the number of penile erection and yawning responses. The administration of cannabinoid agonist Delta9-tetrahydrocannabinol (1.25 mg/kg, i.p.) 15 min before apomorphine (40 and 80 microg/kg, s.c.) did not affect penile erection, however it decreased yawning. The present results provide additional evidence that cannabinoid agonists interfere with dopaminergic systems and that SR 141716A together with a dopaminergic agonist could be useful to potentiate dopaminergic activity.

Systemic apomorphine alters HPA axis responses to interleukin-1 beta administration but not sound stress

Apomorphine is a dopamine receptor agonist that was recently licensed for the treatment of erectile dysfunction. However, although sexual activity can be stressful, there has been little investigation into whether treatments for erectile dysfunction affect stress responses. We have examined whether a single dose of apomorphine, sufficient to produce penile erections (50 microg/kg, i.a.), can alter basal or stress-induced plasma ACTH levels, or activity of central pathways thought to control the hypothalamic-pituitary-adrenal axis in rats. An immune challenge (interleukin-1 beta, 1 microg/kg, i.a.) was used as a physical stressor while sound stress (100 dB white noise, 30 min) was used as a psychological stressor. Intravascular administration of apomorphine had no effect on basal ACTH levels but did substantially increase the number of Fos-positive amygdala and nucleus tractus solitarius catecholamine cells. Administration of apomorphine prior to immune challenge augmented the normal ACTH response to this stressor at 90 min and there was a corresponding increase in the number of Fos-positive paraventricular nucleus corticotropin-releasing factor cells, paraventricular nucleus oxytocin cells and nucleus tractus solitarius catecholamine cells. However, apomorphine treatment did not alter ACTH or Fos responses to sound stress. These data suggest that erection-inducing levels of apomorphine interfere with hypothalamic-pituitary-adrenal axis inhibitory feedback mechanisms in response to a physical stressor, but have no effect on the response to a psychological stressor. Consequently, it is likely that apomorphine acts on a hypothalamic-pituitary-adrenal axis control pathway that is unique to physical stressors. A candidate for this site of action is the nucleus tractus solitarius catecholamine cell population and, in particular, A2 noradrenergic neurons.

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.

PT-141: a melanocortin agonist for the treatment of sexual dysfunction

PT-141, a synthetic peptide analogue of alpha-MSH, is an agonist at melanocortin receptors including the MC3R and MC4R, which are expressed primarily in the central nervous system. Administration of PT-141 to rats and nonhuman primates results in penile erections. Systemic administration of PT-141 to rats activates neurons in the hypothalamus as shown by an increase in c-Fos immunoreactivity. Neurons in the same region of the central nervous system take up pseudorabies virus injected into the corpus cavernosum of the rat penis. Administration of PT-141 to normal men and to patients with erectile dysfunction resulted in a rapid dose-dependent increase in erectile activity. The results suggest that PT-141 holds promise as a new treatment for sexual dysfunction.