Nitric oxide promotes medial preoptic dopamine release during male rat copulation

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.

Hypothalamic cellular and molecular plasticity linked to sexual experience in male rats and mice

Male sexual behavior is subject to learning, resulting in increased efficiency of experienced males compared to naive ones. The improvement in behavioral parameters is underpinned by cellular and molecular changes in the neural circuit controlling sexual behavior, particularly in the hypothalamic medial preoptic area. This review provides an update on the mechanisms related to the sexual experience in male rodents, emphasizing the differences between rats and mice.

Anatomy, Pathophysiology, Molecular Mechanisms, and Clinical Management of Erectile Dysfunction in Patients Affected by Coronary Artery Disease: A Review

Erectile dysfunction (ED) has been defined as the inability to attain or maintain penile erection sufficient for successful sexual intercourse. ED carries a notable influence on life quality, with significant implications for family and social relationships. Because atherosclerosis of penile arteries represents one of the most frequent ED causes, patients presenting with it should always be investigated for potential coexistent coronary or peripheral disease. Up to 75% of ED patients have a stenosis of the iliac-pudendal-penile arteries, supplying the male genital organ’s perfusion. Recently, pathophysiology and molecular basis of male erection have been elucidated, giving the ground to pharmacological and mechanical revascularization treatment of this condition. This review will focus on the normal anatomy and physiology of erection, the pathophysiology of ED, the relation between ED and cardiovascular diseases, and, lastly, on the molecular basis of erectile dysfunction.

Behavioral and hormonal effects of prolonged Sildenafil treatment in a mouse model of chronic social stress

Chronic social defeat can inhibit the reproductive system of subordinate males and causes behavioral deficits. Sildenafil treatment increases mice testosterone levels through its effects on Leydig cells of mice and it has been found to work as an antidepressant drug both in humans and in animal models. Since previous findings showed that sildenafil can counteract the inhibitory effects of chronic social defeat on agonistic, reproductive and anxiety-like behaviors of subordinate male mice, we investigated whether these behavioral outcomes can be explained by Sildenafil stimulation of testosterone. CD1 mice underwent an intruder-resident paradigm. After the fifth day of test, subordinate mice were injected with either a 10 mg/kg Sildenafil or a saline solution for 4 weeks. The results of the present study showed that Sildenafil treatment increased counterattacking behaviors and sexual motivation of subordinate males in addition to limiting the increase in body weight often observed in subordinate mice following chronic psychosocial stress. Moreover, sildenafil treated mice showed a pattern of behaviors reflecting lower anxiety. In agreement with previous studies, Sildenafil also increased testosterone levels. These data demonstrate that sildenafil can counteract the effects of chronic stress, possibly through its stimulatory effects on Leydig cells. These data demonstrate that sildenafil might counteract the effects of chronic psychosocial stress through centrally and peripherally mediated mechanisms.

Aromatization Is Not Required for the Facilitation of Appetitive Sexual Behaviors in Ovariectomized Rats Treated With Estradiol and Testosterone

Testosterone can be safely and effectively administered to estrogen-treated post-menopausal women experiencing hypoactive sexual desire. However, in the United States and Canada, although it is often administered off-label, testosterone co-administered with estradiol is not a federally approved treatment for sexual arousal/desire disorder, partly because its mechanism is poorly understood. One possible mechanism involves the aromatization of testosterone to estradiol. In an animal model, the administration of testosterone propionate (TP) given in combination with estradiol benzoate (EB) significantly increases sexually appetitive behaviors (i.e., solicitations and hops/darts) in ovariectomized (OVX) Long-Evans rats, compared to those treated with EB-alone. The goal of current study was to test whether blocking aromatization of testosterone to estradiol would disrupt the facilitation of sexual behaviors in OVX Long-Evans rats, and to determine group differences in Fos immunoreactivity within brain regions involved in sexual motivation and reward. Groups of sexually experienced OVX Long-Evans rats were treated with EB alone, EB+TP, or EB+TP and the aromatase inhibitor Fadrozole (EB+TP+FAD). Females treated with EB+TP+FAD displayed significantly more hops and darts, solicitations and lordosis magnitudes when compared to EB-alone females. Furthermore, TP, administered with or without FAD, induced the activation of Fos-immunoreactivity in brain areas implicated in sexual motivation and reward including the medial preoptic area, ventrolateral division of the ventromedial nucleus of the hypothalamus, the nucleus accumbens core, and the prefrontal cortex. These results suggest that aromatization may not be necessary for TP to enhance female sexual behavior and that EB+TP may act via androgenic pathways to increase the sensitivity of response to male-related cues, to induce female sexual desire.

Clinical neuroanatomy and neurotransmitter-mediated regulation of penile erection

Erectile dysfunction (ED) has an adverse impact on men's quality of life. Penile erection, which is regulated by nerves that are innervated into the erectile tissue, can be affected by functional or anatomical trauma of the perineal region, including specific structures of the penis, causing ED. Penile erection is neurologically controlled by the autonomic nervous system. Therefore, it is of utmost importance to understand the neurogenic structure of the erectile tissue and the types of neurotransmitters involved in the penile erection process. Here, we highlight the basic clinical anatomy and erectile function of the penis. Understanding the clinical connotation of the relationship between penile erectile structure and function may provide fresh insights for identifying the main mechanisms involved in ED and help develop surgical techniques for the treatment of ED.

Sexual asthenia: Tradamixina versus Tadalafil 5 mg daily

BACKGROUND

Reduced libido is widely considered the most prominent symptomatic reflection of low testosterone (T) levels in men. Testosterone deficiency (TD) afflicts approximately 30% of men aged 40-79 years. This study seeks to evaluate the effect of a new natural compound "tradamixina "in order to improve male sexual function in elderly men, particularly libido and possible erectile dysfunction, versus administration of tadalafil 5 mg daily.

METHODS

Seventy patients (67.3 ± 3.7 years) with stable marital relations and affected by reduced libido, with or without erectile dysfunction were recruited. They were randomly separated in 2 groups A-B of 35. Group A was administered twice a day a new compound "Tradamixina" (150 mg of Alga Ecklonia Bicyclis, 396 mg of Tribulus Terrestris and 144 mg of D-Glucosamine and N-Acetyl-D-Glucosamine) for two months, while Group B was administered tadalafil 5 mg daily, for two months. At visit and after 60 days of treatment patients were evaluated by means of detailed medical and sexual history, clinical examination, laboratory investigations (Total and Free T), instrumental examination (NPTR- nocturnal penile tumescence and rigidity test- with Rigiscan). Patients completed a self-administered IIEF questionnaire (The international index of erectile function) and SQoLM questionnaire (Sexual quality of life Questionnarie-Male). The results pre and post treatment were compared by Student t test (p<0.005).

RESULTS

After 2 months of treatment in group A serum TT levels (230 ± 18 ng/dl vs 671 ± 14 ng/dl ) and FT levels(56 ± 2.4 pg/ml vs 120 ± 3.9 pg/ml) increased, while in group B serum TT levels (245 ± 12 ng/dl vs 247 ± 15 ng/dl ) and FT levels(53 ± 0.3 pg/ml vs 55 ± 0.5 pg/ml) increased not statistically significant. The patient's numbers with negative NPTR improved after treatment in group A and B (15 vs 18 and 13 vs 25 respectively). The IIEF total score in group A increased after treatment with tradamixina (15 ± 1.5 vs 29.77 ± 1.2); the IIEF total score in group B increased slightly (12 ± 1.3 vs 23.40 ± 1.2). The SQoLM total score improved in both groups (A:16 ± 2,3 vs 33 ± 4,1 and B: 16 ± 3,4 vs 31 ± 2,1).

CONCLUSION

The treatment twice a day with "Tradamixina" for 2 months improved libido in elderly men without side effects of Tadalafil.

Sex, drugs and gluttony: how the brain controls motivated behaviors

Bart Hoebel has forged a view of an integrated neural network that mediates both natural rewards and drug use. He pioneered the use of microdialysis, and also effectively used electrical stimulation, lesions, microinjections, and immunohistochemistry. He found that feeding, stimulant drug administration, and electrical stimulation of the lateral hypothalamus (LH) all increased dopamine (DA) release in the nucleus accumbens (NAc). However, whereas DA in the NAc enhanced motivation, DA in the LH inhibited motivated behaviors. The Hull lab has pursued some of those ideas. We have suggested that serotonin (5-HT) in the perifornical LH inhibits sexual behavior by inhibiting orexin/hypocretin neurons (OX/HCRT), which would otherwise excite neurons in the mesocorticolimbic DA tract. We have shown that DA release in the medial preoptic area (MPOA) is very important for male sexual behavior, and that testosterone, glutamate, nitric oxide (NO) and previous sexual experience promote MPOA DA release and mating. Future research should follow Bart Hoebel's emphasis on neural systems and interactions among brain areas and neurotransmitters.

Nitric oxidergic cells related to ejaculation in gerbil forebrain contain androgen receptor and respond to testosterone

Two clusters of forebrain neurons-one in the posterodorsal preoptic nucleus (PdPN) and one in the lateral part of the posterodorsal medial amygdala (MeApd)-are activated at ejaculation in male rats and gerbils as seen with Fos immunocytochemistry. To understand the functions of these cells and how they respond synchronously, it may be useful to identify their neurotransmitters. Nitric oxide (NO) was of interest because its levels in the preoptic area affect ejaculation, and it could synchronize clustered neurons through paracrine/volume transmission. Thus, we determined whether the ejaculation-related cells produce NO by assessing Fos co-localization with NO synthase (NOS) in recently mated male gerbils. We also studied NOS-Fos co-localization in the medial part of the medial preoptic nucleus (MPNm), where half of the neurons that express Fos after mating reflect ejaculation. We also quantified NOS co-localization with androgen receptor (AR) and NOS sensitivity to androgens at these sites. Without quantification, we extended these analyses throughout the hypothalamus and amygdala. Many mating-activated PdPN, lateral MeApd, and MPNm cells contained NOS (32-54%), and many NOS neurons at these sites expressed Fos (34-51%) or AR (25-69%). PdPN and MPNm NOS cells were sensitive to testosterone but not its androgenic metabolite dihydrotestosterone. The overall distribution of NOS and NOS-AR cells was similar to that in rats. These data suggest that NO may help to synchronize the activation of PdPN and lateral MeApd neurons at ejaculation and that NOS in PdPN and MPNm cells is regulated by testosterone acting via estradiol or without undergoing metabolism.

Dopamine release in the medial preoptic area is related to hormonal action and sexual motivation

To help elucidate how general the role of dopamine (DA) release in the medial preoptic area (mPOA) is for the activation of male sexual behavior in vertebrates, we recently developed an in vivo microdialysis procedure in the mPOA of Japanese quail. Using these techniques in the present experiment, the temporal pattern of DA release in relation to the precopulatory exposure to a female and to the expression of both appetitive and consummatory aspects of male sexual behavior was investigated. Extracellular samples from the mPOA of adult sexually experienced male quail were collected every 6 min before, while viewing, while in physical contact with, and after exposure to a female. In the absence of a precopulatory rise in DA, males failed to copulate when the barrier separating them from the female was removed. In contrast, males that showed a substantial increase in mPOA DA during precopulatory interactions behind the barrier, copulated with females after its removal. However, there was no difference in DA during periods when the quail were copulating as compared to when the female was present but the males were not copulating. In addition, we show that precopulatory DA predicts future DA levels and copulatory behavior frequency. Furthermore, the size of the cloacal gland, an accurate indicator of testosterone action, is positively correlated with precopulatory DA. Taken together, these results provide further support for the hypothesis that DA action in the mPOA is specifically linked to sexual motivation as compared to copulatory behavior per se.

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.

Estrous cycle influences the expression of neuronal nitric oxide synthase in the hypothalamus and limbic system of female mice

BACKGROUND

Nitric oxide plays an important role in the regulation of male and female sexual behavior in rodents, and the expression of the nitric oxide synthase (NOS) is influenced by testosterone in the male rat, and by estrogens in the female. We have here quantitatively investigated the distribution of nNOS immunoreactive (ir) neurons in the limbic hypothalamic region of intact female mice sacrificed during different phases of estrous cycle.

RESULTS

Changes were observed in the medial preoptic area (MPA) (significantly higher number in estrus) and in the arcuate nucleus (Arc) (significantly higher number in proestrus). In the ventrolateral part of the ventromedial nucleus (VMHvl) and in the bed nucleus of the stria terminalis (BST) no significant changes have been observed. In addition, by comparing males and females, we observed a stable sex dimorphism (males have a higher number of nNOS-ir cells in comparison to almost all the different phases of the estrous cycle) in the VMHvl and in the BST (when considering only the less intensely stained elements). In the MPA and in the Arc sex differences were detected only comparing some phases of the cycle.

CONCLUSION

These data demonstrate that, in mice, the expression of nNOS in some hypothalamic regions involved in the control of reproduction and characterized by a large number of estrogen receptors is under the control of gonadal hormones and may vary according to the rapid variations of hormonal levels that take place during the estrous cycle.

Estrous cycle influences the expression of neuronal nitric oxide synthase in the hypothalamus and limbic system of female mice

BACKGROUND

Nitric oxide plays an important role in the regulation of male and female sexual behavior in rodents, and the expression of the nitric oxide synthase (NOS) is influenced by testosterone in the male rat, and by estrogens in the female. We have here quantitatively investigated the distribution of nNOS immunoreactive (ir) neurons in the limbic hypothalamic region of intact female mice sacrificed during different phases of estrous cycle.

RESULTS

Changes were observed in the medial preoptic area (MPA) (significantly higher number in estrus) and in the arcuate nucleus (Arc) (significantly higher number in proestrus). In the ventrolateral part of the ventromedial nucleus (VMHvl) and in the bed nucleus of the stria terminalis (BST) no significant changes have been observed. In addition, by comparing males and females, we observed a stable sex dimorphism (males have a higher number of nNOS-ir cells in comparison to almost all the different phases of the estrous cycle) in the VMHvl and in the BST (when considering only the less intensely stained elements). In the MPA and in the Arc sex differences were detected only comparing some phases of the cycle.

CONCLUSION

These data demonstrate that, in mice, the expression of nNOS in some hypothalamic regions involved in the control of reproduction and characterized by a large number of estrogen receptors is under the control of gonadal hormones and may vary according to the rapid variations of hormonal levels that take place during the estrous cycle.

The effects of nitric oxide-cGMP pathway stimulation on dopamine in the medial preoptic area and copulation in DHT-treated castrated male rats

Dopamine (DA) in the medial preoptic area (MPOA) provides important facilitative influence on male rat copulation. We have shown that the nitric oxide-cGMP (NO-cGMP) pathway modulates MPOA DA levels and copulation. We have also shown that systemic estradiol (E(2)) maintains neuronal NO synthase (nNOS) immunoreactivity in the MPOA of castrates, as well as relatively normal DA levels. This effect of E(2) on nNOS probably accounts for at least some of the previously demonstrated behavioral facilitation by intra-MPOA E(2) administration in castrates. Therefore, we hypothesized that stimulation of the MPOA NO-cGMP pathway in dihydrotestosterone (DHT)-treated castrates should restore DA levels and copulatory behaviors. Reverse-dialysis of a NO donor, sodium nitroprusside (SNP), increased extracellular DA in the MPOA of DHT-treated castrates and restored the ability to copulate to ejaculation in half of the animals. A cGMP analog, 8-Br-cGMP, also increased extracellular DA, though not as robustly, but did not restore copulatory ability. The effectiveness of the NO donor in restoring copulation and MPOA DA levels is consistent with our hypothesis. However, the lack of behavioral effects of 8-Br-cGMP, despite its increase in MPOA DA, suggests that NO may have additional mediators in the MPOA in the regulation of copulation. Furthermore, the suboptimal copulation seen in the NO donor-treated animals suggests the importance of extra-MPOA systems in the regulation of copulation.

Impaired nitric oxide synthase signaling dissociates social investigation and aggression

A combination of social withdrawal and increased aggression is characteristic of several mental disorders. Most previous studies have investigated the neurochemical bases of social behavior and aggression independently, as opposed to how these behaviors are regulated in concert. Neuronal nitric oxide synthase (nNOS) produces gaseous nitric oxide, which functions as a neurotransmitter and is known to affect several types of behavior including mating and aggression. Compared with wild-type mice, we observed that nNOS knockout mice showed reduced behavioral responses to an intruder behind a wire barrier. Similar results were observed in mice treated with the selective nNOS inhibitor 3-bromo-7-nitroindazole (3BrN). In habituation-dishabituation tests, treatment with 3BrN did not block recognition of male urine but did attenuate investigation time compared with oil-treated animals. Finally, nNOS knockout mice and 3BrN treated mice were significantly more aggressive than wild-type and oil-treated males, respectively. In general, these behavioral effects are less pronounced in pair-housed males compared with singly-housed males. Thus, nNOS inhibition results in a phenotype that displays reduced social investigation and increased aggression. These data suggest that further study of nNOS signaling is warranted in mental disorders characterized by social withdrawal and increased aggression.

Involvement of nitric oxide in cocaine-induced erections and ejaculations after paradoxical sleep deprivation

OBJECTIVES

As nitric oxide (NO) is involved in penile erectile (PE) function and also influences the sleep-wake cycle, we speculated that NO could play a role in PE and ejaculation of paradonical sleep deprivation (PSD) rats.

METHODS

Animals were pretreated with N(G)-nitro-L-arginine methyl ester (L-NAME, ip) and L-arginine (ip and icv) prior to saline or cocaine injection.

RESULTS

Cocaine-induced PE in 90% of PSD rats, 60% of which ejaculated. L-NAME reduced the frequency of erection, but had no effect in the proportion of PSD-cocaine-injected rats displaying this response. L-NAME had no effect in saline groups. L-Arginine in PSD-saline rats reduced the proportion of animals displaying PE at the highest dose and reduced the frequency of PE at all doses in both saline and cocaine groups. The icv administration of L-arginine reduced PE only in PSD-cocaine rats. Results indicate that common to both drugs, whether it was NO synthase (NOS) inhibitor or NO precursor, was their capacity to strongly reduce PE frequency in cocaine-treated rats. Moreover, L-arginine (ip) played a relevant inhibitory role in the erection displayed by PSD rats.

CONCLUSIONS

Our findings suggest that the stimulating effects of PSD associated or not with cocaine on erection can be modified by alterations in the NO system.

Premature ejaculation

Premature ejaculation (PE) is a common male sexual disorder. Recent normative data suggests that men with an intravaginal ejaculatory latency time (IELT) of less than 1 minute have "definite" PE, while men with IELTs between 1 and 1.5 minutes have "probable" PE. Although there is insufficient empirical evidence to identify the etiology of PE, there is limited correlational evidence to suggest that men with PE have high levels of sexual anxiety and inherited altered sensitivity of central 5-HT (5-hydroxytryptamine, serotonin) receptors. Pharmacological modulation of the ejaculatory threshold using off-label daily or on-demand selective serotonin re-uptake inhibitors is well tolerated and offers patients a high likelihood of achieving improved ejaculatory control within a few days of initiating treatment, consequential improvements in sexual desire and other sexual domains. Investigational drugs such as the ejaculo-selective serotonin transport inhibitor, dapoxetine represent a major development in sexual medicine. These drugs offer patients the convenience of on-demand dosing, significant improvements in IELT, ejaculatory control and sexual satisfaction with minimal adverse effects.

Getting his act together: roles of glutamate, nitric oxide, and dopamine in the medial preoptic area

Gonadal hormones have primarily slow, genomically mediated effects, but copulation requires rapid interactions with a partner. A major way in which hormones facilitate male sexual behavior is by increasing production of neurotransmitter receptors or of enzymes that regulate neurotransmitter synthesis or release. Dopamine is an important facilitative neurotransmitter, and the medial preoptic area (MPOA) is a critical integrative site for male sexual behavior. MPOA dopamine is released before and during mating and facilitates copulation, genital reflexes, and sexual motivation. Gonadal hormones regulate dopamine release in the MPOA of male rats in part by increasing nitric oxide synthase (NOS) in the MPOA; the resultant increase in production of nitric oxide (NO) increases both basal and female-stimulated dopamine release. Glutamate also increases dopamine release via increased production of NO. At least some of the glutamatergic inputs to the MPOA are from the medial amygdala (MeA) and bed nucleus of the stria terminalis (BNST), which mediate the female-stimulated increase in dopamine, which in turn enhances copulatory ability. Extracellular glutamate in the MPOA increases during copulation, especially during ejaculation, and increased glutamate facilitates copulation and genital reflexes. Previous sexual experience also facilitates copulation and confers resistance to impairment by various lesions, drugs, and stress. Experience enhances processing of sexual stimuli, and its effects require activation of glutamate NMDA receptors and NOS in the MPOA. Neuronal NOS is increased in the MPOA of experienced males. Therefore, glutamate, NO, and dopamine interact in the MPOA to facilitate mating and to enhance future sexual responsiveness.

Efficacy of type-5 phosphodiesterase inhibitors in the drug treatment of premature ejaculation: a systematic review

This review examines the role of nitric oxide (NO) as a neurotransmitter involved in the central and peripheral control of ejaculation, the methods of phosphodiesterase type 5 inhibitor (PDE5I) drug treatment studies for premature ejaculation (PE), the adherence of methods to the contemporary consensus of ideal PE drug trial design, the impact of methods on treatment outcomes and the role of PDE5Is in the treatment of PE. NO/cGMP transduction is involved in both the central and peripheral control of emission, but evidence for a direct central or peripheral effect of PDE5Is on ejaculation is speculative. Thirteen of the 14 studies reviewed failed to fulfil the evidence-based medicine criteria for ideal PE drug trial design. Limitations of the studies include inadequately defined study populations, the lack of a double-blind placebo-controlled study design, and the absence of consistent objective physiological measures or sensitive, validated outcome assessment instruments as study endpoints. The broad range of intravaginal ejaculatory latency time (IELT) fold-increases reported with PDE5Is, on-demand selective serotonin re-uptake inhibitor (SSRI) drugs, and combined PDE5I/on-demand SSRIs is testament to the unreliability of data and conclusions from methodologically flawed studies. The one study that fulfilled the evidence-based medicine criteria of an ideal clinical trial design reported that treatment with sildenafil failed to significantly increase baseline IELT, supporting our conclusion that there is no convincing evidence to support any role for PDE5Is in the treatment of men with lifelong PE and normal erectile function. However, there is limited evidence to support a potential role for PDE5Is alone or combined with daily or on-demand SSRIs in the treatment of acquired PE in men with comorbid erectile dysfunction. Further controlled studies adhering to the contemporary consensus of ideal clinical trial design are required to clarify the role of PDE5Is in this subgroup of men with acquired PE.

The etiology and management of premature ejaculation

Premature ejaculation (PE) is a common male sexual disorder. Normative data suggest that men with an intravaginal ejaculatory latency time of less than 1 min have 'definite' premature ejaculation, while men with intravaginal ejaculatory latency times of between 1 and 1.5 min have 'probable' premature ejaculation. Although there is insufficient empirical evidence to identify the etiology of PE, there is correlational evidence to suggest that men with PE have high levels of sexual anxiety and altered sensitivity of central 5-hydroxytryptamine receptors. Pharmacological modulation of the ejaculatory threshold using daily or on-demand selective serotonin reuptake inhibitors offers patients a high likelihood of achieving improved ejaculatory control within a few days of initiating treatment, leads to improvements in sexual desire and other sexual domains, and is well tolerated.

Efficacy of sildenafil citrate (Viagra) in men with premature ejaculation

OBJECTIVES

Premature ejaculation (PE) is the most common ejaculatory dysfunction. We assessed the efficacy of sildenafil to increase the time to ejaculation, improve ejaculatory control, and decrease the postejaculatory erectile refractory time in men with PE.

DESIGN AND METHODS

The main study was an 8-week, double-blind, placebo-controlled, parallel group study in men between 18 and 65 years of age with diagnosed PE. A substudy was also conducted using a subset of patients (two-way crossover, one center) before entry to the main study. The primary study measured intravaginal ejaculatory latency (IELT) and responses to the Index of Premature Ejaculation (IPE) questionnaire. The substudy measured vibrotactile stimulation ejaculatory latency time (VTS-ELT) and postejaculatory erectile refractory time. Differences between treatment groups were determined by ancova at the 5% level of significance.

RESULTS

The change in IELT (1.6 +/- 6.08 vs. 0.6 +/- 2.07 minutes) and VTS-ELT (2.9 +/- 0.4 vs. 2.4 +/- 0.4 minutes) were higher after taking sildenafil, compared with placebo, but did not reach statistical significance. However, patients who took sildenafil (vs. placebo) reported significantly (P < 0.05) increased ejaculatory control (1.8 +/- 0.3 vs. 1.5 +/- 0.3), increased ejaculatory confidence (2.2 +/- 0.2 vs. 1.9 +/- 0.2), and improved overall sexual satisfaction scores (3.1 +/- 0.2 vs. 2.8 +/- 02) on the IPE, and had a decreased postejaculatory erectile refractory time (3.2 +/- 0.7 vs. 6.4 +/- 0.7 minutes). The most common adverse events for sildenafil (vs. placebo) were headache (15% vs. 1%), flushing (15% vs. 0%), dyspepsia (5% vs. 1%), abnormal vision (5% vs. 0%), and rhinitis (5% vs. 0%).

CONCLUSIONS

Although IELT and VTS-ELT were not significantly improved, sildenafil increased confidence, the perception of ejaculatory control, and overall sexual satisfaction, and decreased the refractory time to achieve a second erection after ejaculation in men with PE.

The nitric oxide-guanosine 3',5'-cyclic monophosphate pathway regulates dopamine efflux in the medial preoptic area and copulation in male rats

Dopamine in the medial preoptic area (MPOA) plays a significant role in regulation of male copulation. One mediator of the MPOA dopamine level is nitric oxide. In the current study, we investigated the role of the nitric oxide-guanosine 3',5'-cyclic monophosphate (cGMP) pathway in the regulation of MPOA dopamine and copulation in male rats. The reverse-dialysis of a membrane-permeable analog, 8-Br-cGMP, increased, while a soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ), significantly reduced basal dopamine and its metabolite levels. ODQ successfully blocked a nitric oxide donor-induced increase in dopamine levels, while a neuronal nitric oxide synthase (nNOS) inhibitor was ineffective in blocking an 8-Br-cGMP-induced increase in dopamine, indicating that cGMP is "downstream" of nitric oxide. Furthermore, 8-Br-cGMP facilitated, while ODQ inhibited copulation. Given the steroid-sensitive nature of nNOS functions and the multiple roles nitric oxide plays in the MPOA, we propose that nitric oxide provides important integration of various neurochemical and neuroendocrine signals. The involvement of the central nitric oxide-cGMP pathway in the regulation of copulation also raises an interesting therapeutic possibility, as the manipulation of the same pathway in peripheral tissue is already utilized in treatment of male sexual dysfunction.

Effects of gonadal hormones on central nitric oxide producing systems

Nitric oxide-containing neurons are widely distributed within the CNS, including regions involved in the control of reproduction and sexual behavior. The expression of neuronal nitric oxide synthase is influenced by testosterone in male rat, and by estrogens in female. Moreover, nitric oxide synthase may co-localize with gonadal hormones' receptors. Gonadal hormones may influence nitric oxide synthase expression in adulthood as well as during the development. In fact, in mice knockout for estrogen receptor alpha, the nitric oxide synthase-expressing population is deeply reduced in specific regions. In physiological conditions, the female in mammalian species is exposed to short-term changes of gonadal hormones levels (estrous cycle). Our recent studies, performed in the rat vomeronasal system and in mouse hypothalamic and limbic systems reveal that, in rodents, the expression of nitric oxide synthase-producing elements within regions relevant for the control of sexual behavior is under the control of gonadal hormones. The expression of nitric oxide synthase may vary according to the rapid variations of hormonal levels that take place during the estrous cycle. This seems in accordance with the hypothesis that gonadal hormone activation of nitric oxide-cyclic guanosine-monophosphate pathway is important for lordosis behavior, as well as that this system is activated during mating behavior. Finally, comparative data available for other vertebrates suggest that class-specific and species-specific differences occur in the nitric oxide synthase system of hypothalamus and limbic structures. Therefore, particular caution is needed to generalize data obtained from studies in rodents.

Impaired male sexual behavior in activin receptor type II knockout mice

Integration of multiple hormonal and neuronal signaling pathways in the medial preoptic area (mPOA) is required for elicitation of male sexual behavior in most vertebrates. Perturbation of nitric oxide synthase (NOS) activity in the mPOA causes significant defects in male sexual behavior. Although activins and their signaling components are highly expressed throughout the brain, including the mPOA, their functional significance in the central nervous system (CNS) is unknown. Here, we demonstrate a neurophysiologic role for activin signaling in male reproductive behavior. Adult activin receptor type II null (Acvr2-/-) male mice display multiple reproductive behavioral deficits, including delayed initiation of copulation, reduced mount, and intromission frequencies, and increased mount, intromission, and ejaculation latencies. These behavioral defects in the adult mice are independent of gonadotropin-releasing hormone (GnRH) homeostasis or mating-induced changes in luteinizing hormone (LH) and testosterone levels. The impairment in behavior can be correlated to the nitric oxide content in the CNS because Acvr2-/- males have decreased NOS activity in the mPOA but not the rest of the hypothalamus or cortex. Olfactory acuity tests confirmed that Acvr2-/- mice have no defects in general odor or pheromone recognition. In addition, motor functions are not impaired and the mutants demonstrate normal neuromuscular coordination and balance. Furthermore, the penile histology in mutant mice appears normal, with no significant differences in the expression of penile differentiation marker genes compared with controls, suggesting the observed behavioral phenotypes are not due to structural defects in the penis. Our studies identify a previously unrecognized role of activin signaling in male sexual behavior and suggest that activins and/or related family members are upstream regulators of NOS activity within the mPOA of the forebrain.

Neuronal nitric oxide synthase and gonadal steroid interaction in the MPOA of male rats: Co-localization and testosterone-induced restoration of copulation and nNOS-immunoreactivity

Neuronal nitric oxide synthase (nNOS) in the medial preoptic area (MPOA) has been implicated in various physiological functions, including male rat copulation. Based on their apparent sensitivity to gonadal steroid manipulation, we hypothesized that nNOS cells contain steroid receptors, and the testosterone-induced restoration of nNOS-immunoreactivity in castrates should accompany the restoration of copulation. In Experiment 1, we investigated co-localization of nNOS with the androgen receptor (AR) and the estrogen receptor alpha (ERalpha) using immunocytochemistry. We found regionally specific co-localizations of nNOS-AR and nNOS-ERalpha. In Experiment 2, we investigated the relationship between MPOA nNOS-immunoreactivity (ir) and copulatory measures in the testosterone-induced restoration paradigm in castrates. The restoration of various copulatory measures was accompanied by an increase in optical density of nNOS-ir, but not in the number of nNOS-ir cells. These data provide additional evidence supporting the role of MPOA nitric oxide in male rat copulation.

Effects of testosterone metabolites on copulation, medial preoptic dopamine, and NOS-immunoreactivity in castrated male rats

The medial preoptic area (MPOA) is an important integrative site for male sexual behavior. Dopamine (DA) is released in the MPOA of male rats shortly before and during copulation. In a previous study, we identified 17beta-estradiol (E(2)) as the metabolite of testosterone (T) that maintains MPOA basal extracellular DA levels. However, the presence of dihydrotestosterone (DHT), an androgenic metabolite of T, is required for the female-induced increase in MPOA DA observed during copulation. Recently, we reported that assays of MPOA tissue DA content showed that castrates actually had more stored DA than did gonadally intact males. Therefore, the reduction in extracellular levels in castrates was not due to decreased availability of DA; most likely it was due to decreased release. Furthermore, T upregulates neuronal nitric oxide synthase (nNOS) in the MPOA. NO has been implicated in the regulation of DA release in the MPOA. It is not known, however, which metabolite(s) of T regulate(s) tissue stores of DA and/or nNOS in the MPOA of male rats. The present experiments were designed to test the following: (1) whether E(2), DHT, or the combination of the two influences MPOA DA tissue levels, an indication of stored DA, in male rat castrates; and (2) whether E(2), DHT, or the combination of the two influences NOS-ir in the MPOA of castrated male rats. The results indicate that E(2) up-regulates nNOS-ir in the MPOA and maintains tissue content of DA at levels similar to those in T-treated rats. DHT did not influence nNOS-ir, while attenuating the effect of castration on tissue DA content.

Central nitric oxide synthase inhibition disrupts maternal behavior in the rat

Blocking nitric oxide (NO) production, by 3rd ventricle administration of a nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME; 250 microg/5 microl, postpartum [pp]) decreased milk ejections in Day 10 pp rats. On Day 4 pp, L-NAME treatment eliminated pup retrieval and at both stages of lactation suppressed maternal aggression. Fewer rats treated with L-NAME on Day 10 pp retrieved 4-day-old pups than controls, although all nursed older litters. Following exposure to a mobile intruder, Fos expression was lower in the medial preoptic area and the bed nucleus of the stria terminalis in L-NAME-treated rats than in controls but was lower in the medial amygdala only following exposure to an anaesthetized intruder. Thus, the elevated levels of NO observed in lactation may contribute to the mechanism(s) that mediate maternal behavior and aggression.

Dopamine D1 and D2 Receptor Antagonism in the Preoptic Area Produces Different Effects on Maternal Behavior in Lactating Rats

The preoptic area (POA) is critical for maternal behavior in rats but little is known about what neurotransmitters released here influence maternal responding. POA infusion of 10 microg (but not 2 microg) of the dopamine D1 receptor antagonist SCH-23390 greatly impaired retrieval and licking of pups but not other maternal or nonmaternal behaviors in lactating rats. In contrast, POA infusion of 10 microg (but not 2 microg) of the D2 receptor antagonist raclopride facilitated nursing but did not affect oral maternal behaviors. SCH-23390 in the medial hypothalamus tended to impair licking but not retrieval. Raclopride in the medial hypothalamus had no effects. Therefore, D1 and D2 receptor activity, particularly in the POA, is important for regulating different maternal behaviors.

Dopamine and serotonin: influences on male sexual behavior

Steroid hormones regulate sexual behavior primarily by slow, genomically mediated effects. These effects are realized, in part, by enhancing the processing of relevant sensory stimuli, altering the synthesis, release, and/or receptors for neurotransmitters in integrative areas, and increasing the responsiveness of appropriate motor outputs. Dopamine has facilitative effects on sexual motivation, copulatory proficiency, and genital reflexes. Dopamine in the nigrostriatal tract influences motor activity; in the mesolimbic tract it activates numerous motivated behaviors, including copulation; in the medial preoptic area (MPOA) it controls genital reflexes, copulatory patterns, and specifically sexual motivation. Testosterone increases nitric oxide synthase in the MPOA; nitric oxide increases basal and female-stimulated dopamine release, which in turn facilitates copulation and genital reflexes. Serotonin (5-HT) is primarily inhibitory, although stimulation of 5-HT(2C) receptors increases erections and inhibits ejaculation, whereas stimulation of 5-HT(1A) receptors has the opposite effects: facilitation of ejaculation and, in some circumstances, inhibition of erection. 5-HT is released in the anterior lateral hypothalamus at the time of ejaculation. Microinjections of selective serotonin reuptake inhibitors there delay the onset of copulation and delay ejaculation after copulation begins. One means for this inhibition is a decrease in dopamine release in the mesolimbic tract.

Nitric oxide mediates glutamate-evoked dopamine release in the medial preoptic area

Dopamine (DA) release in the medial preoptic area (MPOA) of the hypothalamus is an important facilitator of male sexual behavior. The presence of a receptive female increases extracellular DA in the MPOA, which increases further during copulation. However, the neurochemical events that mediate the increase of DA in the MPOA are not fully understood. Here we report that glutamate, reverse-dialyzed into the MPOA, increased extracellular DA, which returned to baseline after the glutamate was removed. This increase was prevented by co-administration of the nitric oxide synthase inhibitor NG-nitro-l-arginine methyl ester (L-NAME), but not by the inactive isomer, Nw-nitro-d-arginine methyl ester (D-NAME). In contrast, extracellular concentrations of the major metabolites of DA were decreased by glutamate, suggesting that the DA transporter was inhibited. These decreases were also inhibited by L-NAME, but not D-NAME. These results indicate that glutamate enhances extracellular DA in the MPOA, at least in part, via nitric oxide activity. Therefore, glutamatergic stimulation of nitric oxide synthase may generate the female-induced increase in extracellular DA in the MPOA, which is important for the expression of male sexual behavior.

A Nitric Oxide Synthesis Inhibitor in the Medial Preoptic Area Inhibits Copulation and Stimulus Sensitization in Male Rats

Dopamine in the medial preoptic area (MPOA) facilitates copulation in male rats, and nitric oxide (NO) regulates basal and female-stimulated MPOA dopamine release. Microinjection of L-nitro-arginine methyl ester (L-NAME, an NO synthesis inhibitor) into the MPOA blocked copulation in naive rats and impaired copulation in sexually experienced males. In other naive rats, L-NAME or saline was microinjected into the MPOA before each of 7 daily exposures to a receptive female placed over their cage. In a drug-free test on Day 8, copulation by L-NAME-treated rats was similar to that of unexposed controls and was impaired relative to saline-treated males. Therefore, NO in the MPOA is important for copulation and stimulus sensitization in male rats.

Erectile physiological and pathophysiological pathways involved in erectile dysfunction

PURPOSE

The importance of signaling pathways in penile smooth muscles involved in normal erection and erectile dysfunction (ED) is discussed based on a review of the literature.

MATERIALS AND METHODS

Erection is basically a spinal reflex that can be initiated by recruitment of penile afferents but also by visual, olfactory and imaginary stimuli. The generated nervous signals will influence the balance between the contractant and relaxant factors, which control the degree of contraction of penile smooth muscles and, thus, determine the functional state of the penis. The different steps involved in neurotransmission, impulse propagation and intracellular transduction of neural signals may be changed in different types of erectile dysfunction.

RESULTS

Recent findings have suggested an important role for RhoA/Rho kinase in the regulation of cavernosal smooth muscle tone and that changes in this pathway may contribute to ED in various patient subgroups, eg diabetes and vascular disease. Neurogenic nitric oxide is still considered the most important factor for immediate relaxation of penile vessels and corpus cavernosum. However, endothelially generated nitric oxide seems essential for maintaining erection. Endothelial dysfunction can contribute to ED in several patient subgroups. In addition, in conditions associated with reduced function of nerves and endothelium, such as aging, hypertension, smoking, hypercholesterolemia and diabetes, circulatory and structural changes in the penile tissues can result in arterial insufficiency and defect muscle relaxation.

CONCLUSIONS

Different types of ED often have overlapping pathophysiologies but may also have common pathways contributing to ED. Such pathways may be potential treatment targets.

Preventing L-NAME inhibitory effects on rat sexual behavior with hydralazine, isradipine or captopril co-treatment

The effects of the chronic oral treatment with N(G)-nitro-L-arginine methyl ester (L-NAME), separately or in combination with isradipine, captopril or hydralazine, on standard and temporal patterning sexual behavior of male rats were evaluated. L-Arginine and filtered water were used as control. L-NAME treatment decreased the copulatory rate and hit rate factors of sexual behavior. However, the initiation factor and temporal patterning were less modified by the drug. After 14 days of L-NAME treatment suspension the male rat sexual response was recovered. The three antihypertensive agents were able to reverse partially or totally the inhibitory effects of L-NAME, suggesting that the chronic oral treatment with L-NAME induces penile erection dysfunction by peripheral mechanisms. The present results suggest that chronic oral treatment with nitric oxide (NO) synthase inhibitor can be a relevant and powerful peripheral erectile dysfunction model to evaluate the effects of drugs on erectile function of male rats.

An NMDA antagonist impairs copulation and the experience-induced enhancement of male sexual behavior in the rat

Sexual experience facilitates subsequent male sexual behavior; activation of the N-methyl-D-aspartate (NMDA) glutamate receptor may play a role in this experience-induced enhancement. In this article, the authors report that systemic injections of MK-801, an NMDA receptor antagonist, impaired male sexual behavior in sexually naive and sexually experienced male rats. Furthermore, saline-treated rats that received 7 daily exposures to an inaccessible estrous female instead of sexual experience displayed enhancement of copulation on the following day. Injections of MK-801 before each of these exposures inhibited the experience-induced enhancement on the drug-free test on Day 8. These data suggest that stimulation of NMDA receptors enhances sexual performance immediately and mediates the experience-induced enhancement of subsequent copulatory behavior.

Anatomical relationships between aromatase-immunoreactive neurons and nitric oxide synthase as evidenced by NOS immunohistochemistry or NADPH diaphorase histochemistry in the quail forebrain

In Japanese quail (Coturnix japonica), previous studies indicated that the distribution of reduced nicotinamide dinucleotide phosphate (NADPH) diaphorase overlaps with steroid-sensitive areas that contain dense populations of aromatase-immunoreactive (ARO-ir) cells. We investigated here the anatomical relationships between aromatase (ARO) and nitric oxide synthase (NOS)-containing cells that were visualized both by NOS-immunohistochemistry and NADPH-histochemistry. The distribution of ARO-ir and of NADPH-positive cells in the forebrain observed here matched exactly the distribution previously reported. The distribution of NOS-immunoreactive material in the vicinity of ARO-ir cell groups appeared similar to the distribution of NADPH-positive structures previously identified by histochemistry. The number of NOS-immunoreactive cells was similar to the number of NADPH-positive cells and they were found in the same brain regions. In contrast, few NOS-immunoreactive fibers were observed whereas numerous NADPH-positive fibers and punctuate structures were present in many areas. Major groups of NOS-immunoreactive/NADPH-positive neurons were adjacent to the main ARO-ir cell groups, such as the medial preoptic nucleus, the bed nucleus of the stria terminalis and the nucleus ventromedialis hypothalamic. However, examination of adjacent sections indicated that there is very little overlap between the NOS-immunoreactive and ARO-ir cell populations. This notion got further support by double-labeled sections where no double-labeled cells could be identified. In sections stained simultaneously by histochemistry for NADPH and immunohistochemistry for ARO, many NADPH-positive fibers and punctate structures were closely associated with ARO-ir perikarya. Taken together, the present data indicate that NOS is not or very rarely colocalized with ARO but that NOS inputs are closely associated with ARO-ir cells. Based on previous work in a variety of model systems, it can be hypothesized that these inputs modulate the expression or activity of ARO in the quail brain.

Estrogen receptor alpha influences socially motivated behaviors

Male mice lacking estrogen receptor alpha (ERalphaKO) show reduced social behaviors. We hypothesized that this might be due to either socially elicited or generalized anxiety. Male ERalphaKOs and wild type (WT) mice were given a series of behavioral tests: elevated plus maze, T-maze, and social recognition. Each test included a social dimension by exposing males to ovariectomized (OVX) females. In addition plasma concentrations of corticosterone were measured, and open field activity was assessed. In the elevated plus maze, WT males exposed to an OVX female 1 min prior to the test were more anxious than WT controls. ERalphaKO males showed anxiety in this test whether or not they were preexposed to a female. In the T-maze, WT males increased exploration of a novel arm when it contained an OVX female. The presence or absence of a female in a novel arm did not affect behavior of ERalphaKO males. In social recognition tests, ERalphaKO males spent less time than WT littermates investigating an OVX female that was repeatedly introduced into their home cage. On the final trial, when a novel female was introduced, WT males increased their chemo-investigation but ERalphaKOs did not. Plasma corticosterone levels were lower in ERalphaKO than in WT males when plasma was taken directly after a brief (control) cage disturbance. In the open field WT and ERalphaKO males behaved essentially the same. Taken together, the results of these experiments suggest the ERalphaKO males avoid contact with other conspecifics, perhaps due to an inability to be aroused by social cues.

Roles of estrogen receptor alpha and androgen receptor in the regulation of neuronal nitric oxide synthase

In brain and peripheral tissues, steroid hormones regulate nitric oxide synthase (nNOS). We asked whether estrogen receptor-alpha (ERalpha) and/or androgen receptor (AR) regulated nNOS immunoreactivity in mouse brain. First, we quantified cells singly labeled for nNOS immunoreactivity or labeled dually with ERalpha-immunoreactive (-ir) or AR-ir cells in the nucleus accumbens (Acb), preoptic area (POA), bed nucleus of the stria terminalis (BNST), posterior dorsal and posterior ventral regions of the medial amygdala (MePD and MePV, respectively), and paraventricular nucleus (PVN). The POA and MePD contained the greatest number of double-labeled cells. More nNOS-ir cells were colabeled with ERalpha immunoreactivity compared with AR immunoreactivity. Next, by using a double mutant mouse in which males lacked functional ERalpha, AR, or both, we investigated the roles of these steroid receptors in nNOS-ir cell numbers and immunoreactive area staining under testosterone (T) and estradiol (E2) conditions. Our data show that functional ERalpha is correlated with more nNOS-ir cells under T conditions and more immunoreactive area staining in the POA under both T and E2 conditions. However, ERalpha decreases nNOS-ir cell number in the BNST under E2 treatment. In summary, the data suggest that AR has organizational actions on nNOS-ir cell numbers in the MePV, that interactions between ERalpha and AR genes occur in PVN, and that sex differences in nNOS-ir area staining are limited to the POA. Thus, we show that ERalpha and AR interact to regulate nNOS in male and female brain in a site-specific manner.

Modulatory activity of sildenafil on copulatory behaviour of both intact and castrated male rats

The first experiment of the present study investigates the effects induced by sildenafil (1 or 10 mg/kg po) on the copulatory behaviour of intact male rats, categorized, on the basis of seven consecutive mating pretests, as sluggish or normal ejaculators (SE or NE, respectively). The data obtained show that sildenafil modifies both sexual arousal and the ejaculatory mechanisms of copulation, diminishing ejaculation latency in both categories and increasing copulatory efficacy in SE rats; in addition, it reduced the inter-intromission interval in both SE and NE animals and the post-ejaculatory interval only in SE animals. The second experiment, conducted on rats 3 weeks after their castration, shows that sildenafil alone (1 or 10 mg/kg) did not modify copulatory failure. However, 3 months after castration, and 24 h after the last injection of testosterone (25 microg/kg sc) given twice weekly for 4 weeks, sildenafil (1 or 10 mg/kg) ameliorated rat copulatory performance.

A sexual arousability model involving steroid effects at the plasma membrane

This review will discuss the status of research related to sexual arousability. It will also present a model for sexual arousability based on current knowledge of steroids effects at the membranes of cells. Steroids have multiple rapid actions that are suggested to result from actions at membrane-associated receptors. When stimulated by steroids these receptors alter G-protein coupling in a manner unique to this complex. Initial stimulation of the receptors by steroids alters the coupling pattern of G-proteins and of other binding sites associated with the complex. This change in G-protein coupling is a stable alteration and thus may serve as a long-term change in the system, which is a requirement of sexual arousability. Stimulation of this receptor system by a surge of oxytocin at ejaculation or orgasm then decouples the G-protein and reduces arousability. Sex hormone binding globulin may be an important ligand at this complex. This model suggests completely new relationships among steroids and their receptors that may complement or diverge from actions at known intracellular receptors.

Central modulation of the NO/cGMP pathway affects the MPOA-induced intracavernous pressure response

Alterations in the nitric oxide (NO)/cGMP levels in hypothalamic nuclei, including the medial preoptic area (MPOA), regulate critical aspects of sexual behavior and penile reflexes. However, the effects of altered central nervous system (CNS) NO/cGMP levels at the end organ level, that is, on the magnitude/quality of the erection so achieved [intracavernous pressure (ICP) response], has yet to be evaluated. The goal of this report was to evaluate the effects of intrathecal administration of modulators of NO and cGMP levels on ICP responses to stimulation of the MPOA and cavernous nerve in rats in vivo. In all cases, intrathecal administration of compounds that increase and decrease cGMP and NO levels, respectively, was associated with corresponding increases and decreases in the MPOA-stimulated ICP response. Specifically, sodium nitroprusside (SNP), 8-bromo-cGMP, and sildenafil increased the MPOA-stimulated ICP response, whereas N(omega)-nitro-L-arginine methyl ester reduced it. None of the intrathecal treatments had detectable effects on blood pressure or the cavernous nerve-stimulated ICP response, although intravenous sildenafil increased the latter. These data clearly indicate that intrathecal drug administration affects central and not peripheral neural mechanisms and, moreover, documents that CNS NO/cGMP levels can affect erectile capacity per se (i.e., ICP) in the rat model.

Pharmacology of erectile function and dysfunction

Central regulation of the erectile process involves several transmitters, including dopamine, serotonin, noradrenaline, and nitric oxide, and peptides, such as oxytocin and ACTH/alpha-MSH. These systems may be targets for future drugs designed to treat erectile dysfunction. Peripherally, the different steps involved in neurotransmission, impulse propagation, and intracellular transduction of neural signals in penile smooth muscles need further investigation. Continued studies of the interactions between different transmitters/modulators may reveal new combination therapies. Increased knowledge of the changes in penile tissues associated with erectile dysfunction may explain the pathogenetic mechanisms and help to prevent the disorder.