Effect of excitatory amino acid, dopamine, and oxytocin receptor antagonists on noncontact penile erections and paraventricular nitric oxide production in male rats

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.

ICV galanin-like peptide stimulates non-contact erections but not touch-based erections in adult, sexually experienced male rats

Galanin-like peptide (GALP) is a neuropeptide transcribed only within the arcuate nucleus of the hypothalamus and is thought to be a mediator between energetics and reproductive function. Intracerebroventricular (ICV) injection of GALP is known to have effects on feeding, and to significantly increase gonadotropin releasing hormone- (GnRH-) mediated luteinizing hormone (LH) secretion. Furthermore, ICV GALP is known to stimulate fos production in the medial pre-optic area (mPOA) and to a lesser extent, the paraventricular nucleus (PVN). ICV injection of 5.0nmol GALP profoundly stimulates male rat sexual behavior. It is not known if GALP's effects on sex behavior are due to an increase in appetitive or mechanical (erectile) aspects of male sexual behavior. To determine this, sexually experienced male rats were cannulated in the lateral ventricle and injected with 5.0nmol GALP or vehicle. Immediately after injections, male rats were placed in an arena connected to a second arena via a tube with a fan. The second arena contained a steroid-primed female and her bedding. The male rat had olfactory but not visual or tactile contact with the female. We analyzed the amount of time the male rats spent investigating the air intake and the number of non-contact erections (NCEs) in a 30minute test. ICV GALP significantly (p<0.05) increased both the amount of time of olfactory investigations and NCEs compared to vehicle. In a second set of animals, we tested if ICV GALP could stimulate touch-based erections. GALP had no significant effect on touch-based erections compared to vehicle. These data suggest that GALP's activation of fos within the mPOA is indicative of its action to stimulate the appetitive aspects of male sexual behavior.

Neuropeptides and central control of sexual behaviour from the past to the present: a review

Of the numerous neuropeptides identified in the central nervous system, only a few are involved in the control of sexual behaviour. Among these, the most studied are oxytocin, adrenocorticotropin, α-melanocyte stimulating hormone and opioid peptides. While opioid peptides inhibit sexual performance, the others facilitate sexual behaviour in most of the species studied so far (rats, mice, monkeys and humans). However, evidence for a sexual role of gonadotropin-releasing hormone, corticotropin releasing factor, neuropeptide Y, galanin and galanin-like peptide, cholecystokinin, substance P, vasoactive intestinal peptide, vasopressin, angiotensin II, hypocretins/orexins and VGF-derived peptides are also available. Corticotropin releasing factor, neuropeptide Y, cholecystokinin, vasopressin and angiotensin II inhibit, while substance P, vasoactive intestinal peptide, hypocretins/orexins and some VGF-derived peptide facilitate sexual behaviour. Neuropeptides influence sexual behaviour by acting mainly in the hypothalamic nuclei (i.e., lateral hypothalamus, paraventricular nucleus, ventromedial nucleus, arcuate nucleus), in the medial preoptic area and in the spinal cord. However, it is often unclear whether neuropeptides influence the anticipatory phase (sexual arousal and/or motivation) or the consummatory phase (performance) of sexual behaviour, except in a few cases (e.g., opioid peptides and oxytocin). Unfortunately, scarce information has been added in the last 15 years on the neural mechanisms by which neuropeptides influence sexual behaviour, most studied neuropeptides apart. This may be due to a decreased interest of researchers on neuropeptides and sexual behaviour or on sexual behaviour in general. Such a decrease may be related to the discovery of orally effective, locally acting type V phosphodiesterase inhibitors for the therapy of erectile dysfunction.

Exercise training improves the defective centrally mediated erectile responses in rats with type I diabetes

INTRODUCTION

Erectile dysfunction is a serious and common complication of diabetes mellitus. Apart from the peripheral actions, central mechanisms are also responsible for the penile erection.

AIM

The goal of the present study was to determine the impact of exercise training (ExT) on the centrally mediated erectile dysfunction in streptozotocin (STZ)-induced type I diabetic (T1D) rats.

METHODS

Male Sprague-Dawley rats were injected with STZ to induce diabetes mellitus. Three weeks after STZ or vehicle injections, rats were assigned to either ExT (treadmill running for 3-4 weeks) or sedentary groups to produce four experimental groups: control + sedentary, T1D + sedentary, control + ExT, and T1D + ExT.

MAIN OUTCOME MEASURE

After 3-4 weeks ExT, central N-methyl-D-aspartic acid (NMDA) or sodium nitroprusside (SNP)-induced penile erectile responses were measured. Neuronal nitric oxide synthase (nNOS) expression in the paraventricular nucleus (PVN) of the hypothalamus was measured by using histochemistry, real time polymerase chain reaction (PCR) and Western blot approaches.

RESULTS

In rats with T1D, ExT significantly improved the blunted erectile response, and the intracavernous pressure changes to NMDA (50 ng) microinjection within the PVN (T1D + ExT: 3.0 ± 0.6 penile erection/rat; T1D + sedentary: 0.5 ± 0.3 penile erection/rat within 20 minutes, P < 0.05). ExT improved erectile dysfunction induced by central administration of exogenous nitric oxide (NO) donor, SNP in T1D rats. Other behavior responses including yawning and stretching, induced by central NMDA and SNP microinjection were also significantly increased in T1D rats after ExT. Furthermore, we found that ExT restored the nNOS mRNA and protein expression in the PVN in T1D rats.

CONCLUSIONS

These results suggest that ExT may have beneficial effects on the erectile dysfunction in diabetes through improvement of NO bioavailability within the PVN. Thus, ExT may be used as therapeutic modality to up-regulate nNOS within the PVN and improve the central component of the erectile dysfunction in diabetes mellitus.

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.

Genetic and clinical predictors of sexual dysfunction in citalopram-treated depressed patients

Sexual dysfunction is a major contributor to treatment discontinuation and nonadherence among patients treated with selective serotonin reuptake inhibitors (SSRIs). The mechanisms by which depressive symptoms in general, as well as SSRI exposure in particular, may worsen sexual function are not known. We examined genetic polymorphisms, including those of the serotonin and glutamate systems, for association with erectile dysfunction, anorgasmia, and decreased libido during citalopram treatment. Clinical data were drawn from a nested case-control cohort derived from the STAR(*)D study, a multicenter, prospective, effectiveness trial in outpatients with nonpsychotic major depressive disorder (MDD). Self-reports of erectile dysfunction, decreased libido, or difficulty achieving orgasm based on the Patient-Rated Inventory of Side Effects were examined among Caucasian subjects (n=1473) for whom DNA and adverse effect measures were available, and who were treated openly with citalopram for up to 14 weeks. Of 1473 participants, 799 (54%) reported decreased libido; 525 (36%) reported difficulty achieving orgasm. Of 574 men, 211 (37%) reported erectile dysfunction. Using a set-based test for association, single nucleotide polymorphisms in glutamatergic genes were associated with decreased libido (GRIA3; GRIK2), difficulty achieving orgasm (GRIA1), and difficulty achieving erection (GRIN3A) (experiment-wide permuted p<0.05 for each). Evidence of association persisted after adjustment for baseline clinical and sociodemographic differences. Likewise, evidence of association was similar when the cohort was limited to those who did not report a given adverse event at the first post-baseline visit (ie, those whose adverse events were known to be treatment emergent). These hypothesis-generating analyses suggest the potential for glutamatergic treatment targets for sexual dysfunction during major depressive episodes.

Oxytocin: the great facilitator of life

Oxytocin (Oxt) is a nonapeptide hormone best known for its role in lactation and parturition. Since 1906 when its uterine-contracting properties were described until 50 years later when its sequence was elucidated, research has focused on its peripheral roles in reproduction. Only over the past several decades have researchers focused on what functions Oxt might have in the brain, the subject of this review. Immunohistochemical studies revealed that magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei are the neurons of origin for the Oxt released from the posterior pituitary. Smaller cells in various parts of the brain, as well as release from magnocellular dendrites, provide the Oxt responsible for modulating various behaviors at its only identified receptor. Although Oxt is implicated in a variety of "non-social" behaviors, such as learning, anxiety, feeding and pain perception, it is Oxt's roles in various social behaviors that have come to the fore recently. Oxt is important for social memory and attachment, sexual and maternal behavior, and aggression. Recent work implicates Oxt in human bonding and trust as well. Human disorders characterized by aberrant social interactions, such as autism and schizophrenia, may also involve Oxt expression. Many, if not most, of Oxt's functions, from social interactions (affiliation, aggression) and sexual behavior to eventual parturition, lactation and maternal behavior, may be viewed as specifically facilitating species propagation.

The acute effects of intranasal oxytocin administration on endocrine and sexual function in males

The role of the neuropeptide oxytocin (OT) ranges from the modulation of neuroendocrine physiological effects to the establishment of complex social and bonding behaviours. Experimental studies in animals, as well as case reports in humans, suggest that OT affects different aspects of sexual behaviour and has predominantly facilitating properties for sexual appetence and performance. Using a previously established experimental paradigm of sexual arousal and masturbation-induced orgasm, this study investigated the acute effects of intranasal OT application (24I.U.) on endocrine parameters and measures of sexual appetence and function in healthy men (n=10). In a double-blind, placebo-controlled, balanced cross-over design, sexual arousal, and orgasm were induced by an erotic film and masturbation. In addition to the continuous recording of endocrine (OT, cortisol, prolactin, epinephrine, norepinephrine) and cardiovascular data (heart rate), parameters of appetitive, consummatory, and refractory sexual behaviour were assessed using the acute sexual experience scale (ASES). OT plasma levels were significantly elevated after intranasal OT throughout the whole experiment (>60 min). In addition, OT treatment induced significantly higher increases in epinephrine plasma levels during sexual activity without affecting cortisol levels, prolactin levels or heart rate. OT treatment did not alter appetitive, consummatory, and refractory sexual behaviour according to the ASES. However, when subjects were asked about their subjective perception of whether OT or placebo had been applied, eight out of 10 subjects in the OT group answered correctly, thus pointing to an altered perception of arousal. In conclusion, intranasally administered OT leads to a marked increase in OT plasma levels together with increased secretion of catecholamines when subjects are engaged in sexual activity in a laboratory setting. As the effects of OT on sexual behaviour were equivocal, future studies should examine possible facilitating effects further by including males, females, and couples in a field setting, taking into account that OT exerts the most prominent behavioural effects in pair bond formations.

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.

Morphine reduces penile erection induced by the cannabinoid receptor antagonist SR 141617A in male rats: Role of paraventricular glutamic acid and nitric oxide

The effect of the opiate morphine, on penile erection induced by the cannabinoid CB1 receptor antagonist SR 141716A injected into the paraventricular nucleus of the hypothalamus and on the increase in the concentration of glutamic acid and of NO(2)(-) and NO(3)(-), which occurs concomitantly in the paraventricular dialysate obtained by intracerebral microdialysis, was studied in male rats. Morphine (0.5, 1 and 5 microg), given into the paraventricular nucleus, reduced dose-dependently penile erection induced by SR 141716A (2 microg) injected into the paraventricular nucleus. The reduction of penile erection was parallel to a decrease of the concomitant glutamic acid and NO(2)(-) and NO(3)(-) increase that occurs in the paraventricular dialysate in these experimental conditions. Morphine effects on SR 141716A-induced penile erection, glutamic acid and NO(2)(-) increase were prevented by the prior administration of naloxone, an opioid receptor antagonist (5 microg) given into the paraventricular nucleus. The present results show that the activation of opioid receptors in the paraventricular nucleus impairs penile erection induced by SR 141716A, by reducing the increase in glutamic acid and in NO activity that occurs in this hypothalamic nucleus in these experimental conditions.

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.

Age-related changes of the GABA-B receptor in the lumbar spinal cord of male rats and penile erection

Dorsal horn neurons of lumbosacral spinal cord innervate penile vasculature and regulate penile erection. GABAergic system is involved in the regulation of male sexual behavior. Because aging is frequently accompanied by a progressive decline in erectile function, the aim of this work was to examine age-related changes of the GABA-B receptor in the lumbar spinal cord. Sprague-Dawley rats of 10 and 21 days old, 3, 9 and 20 months old were used. GABA-B receptors were evaluated by quantitative autoradiography using [3H]-Baclofen as ligand with or without GABA (10 microM) to determine the non-specific binding. Ten days after birth a homogeneous neuroanatomical distribution pattern was found in the gray matter, however at 20-day-old adult distribution emerged becoming heterogeneous with the highest binding values at layers II-III and X. In dorsal layers a significant decrease was observed in 9-month-old rats while layer X showed an earlier decrease (21-day-old). GABA-B receptor affinity showed significant age-dependent and regional increase. The GABA-B receptor decrease in aged rats seems not to be related to this receptor inhibitory function in penile erection. Moreover the changes found in GABA-B receptor binding anatomical distribution may indicate its role in the morphological development of the lumbar spinal cord rather than in the decline of the erectile function.

Central Nervous System Agents in the Treatment of Erectile Dysfunction

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

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

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

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.

Effect of excitatory amino acid receptor agonists on penile erection after administration into the CA3 hippocampal region in the rat

OBJECTIVES

To examine whether hippocampal injection of specific agonists for glutamate ionotropic (NMDA, AMPA) and metabotropic (ACPD) receptors modulates erectile response in adult male rats. The paraventricular nuclei of the hypothalamus and hippocampus are believed to play a role in the central regulation of erectile function.

METHODS

Four groups of 5 male Sprague-Dawley rats each were anesthetized and placed in a stereotaxic apparatus after carotid artery catheterization for measurement of arterial pressure and insertion of a 25-gauge needle in the corpus cavernosum for measurement of intracavernous pressure (ICP). The rats underwent hippocampal injections of saline (1 muL), NMDA (10 nmol/L), AMPA (10 nmol/L), or ACPD (10 nmol/L) followed by perfusion with saline and 10% formalin solutions. Brain slices 24 mum thick were mounted and stained with cresyl violet to identify the drug injection sites.

RESULTS

Hippocampal injection of glutamate receptor subtype agonists produced multiple episodes of ICP elevation. NMDA injection was associated with a greater maximal ICP increase, duration of action, and mean onset of action latency than AMPA injection. ACPD injection had no statistically significant impact on ICP or arterial pressure changes. Histologic evaluation confirmed the injected sites were the CA3 fields of the hippocampus.

CONCLUSIONS

Our results suggest hippocampal glutamate receptors may be involved in initiation of penile erection. Ionotropic receptors appear to play a significant role in these regulatory mechanisms. NMDA receptors appear to be more potent than AMPA receptors, which appear to mediate fast excitatory synaptic transmission.

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.

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

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

Extra-cellular dopamine increases in the paraventricular nucleus of male rats during sexual activity

Dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) concentrations were measured in the dialysate obtained with vertical microdialysis probes implanted into the paraventricular nucleus of the hypothalamus of sexually potent male rats. Animals showed noncontact erections when put in the presence of, and copulated with a receptive (ovarietomized oestrogen and progesterone primed) female rat. Dopamine and DOPAC concentrations in the paraventricular dialysate increased 140% and 19%, respectively, above baseline values during exposure to the receptive female and 280% and 31%, respectively, during copulation. No changes in dopamine and DOPAC concentrations were detected in the paraventricular dialysate when sexually potent male rats were exposed to nonreceptive (ovariectomized not oestrogen plus progesterone primed) female rats. These results confirm the involvement of the paraventricular nucleus in control of erectile function and copulatory behaviour and show for the first time that dopamine neurotransmission is increased in this hypothalamic nucleus when erection occurs in physiological contexts.

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.

Reduction of drug-induced yawning and penile erection and of noncontact erections in male rats by the activation of GABAA receptors in the paraventricular nucleus: involvement of nitric oxide

The effect of muscimol, a GABAA receptor agonist, injected into the paraventricular nucleus (PVN) of the hypothalamus on drug-induced (apomorphine, oxytocin and NMDA) yawning and penile erection, and on the increase in the concentration of NO2- and NO3- occurring in the paraventricular dialysate in these experimental conditions, was studied in male rats. Muscimol (50, 100 and 200 ng) reduced, in a dose-dependent manner, penile erection and yawning induced by apomorphine (50 ng), oxytocin (30 ng) and NMDA (50 ng) delivered into the PVN. The reduction of penile erection and yawning was parallel to a reduction of the concomitant NO2- and NO3- increase that occurs in the paraventricular dialysate in this experimental condition. In contrast, baclofen (200 ng), a GABAB receptor agonist, was ineffective. The muscimol effects on drug-induced penile erection, yawning and NO2- increase were prevented by the prior administration of bicuculline (250 ng into the paraventricular nucleus). Muscimol (200 ng) but not baclofen (200 ng), injected into the PVN, reduced both noncontact erections in male rats placed in the presence of an inaccessible receptive female, and also the NO2- increase that occurs in the paraventricular dialysate in this experimental condition. As found with drug-induced penile erection, the muscimol reduction of noncontact erections and of NO2- increase was prevented by bicuculline. The present results show that the activation of GABAA receptors in the PVN reduces yawning and penile erection induced by drugs or physiological stimuli by reducing the increase in NO activity that occurs in this hypothalamic nucleus in these experimental conditions.

The activation of gamma aminobutyric acid(A) receptors in the paraventricular nucleus of the hypothalamus reduces non-contact penile erections in male rats

Male rats show 4-6 penile erection episodes when put in the presence of an inaccessible receptive female. These non-contact penile erections were reduced dose-dependently by muscimol, a gamma aminobutyric acid (GABA)(A) receptor agonist, when given into the paraventricular nucleus of the hypothalamus (0.1, 0.5, 1 and 2 microg). In contrast, baclofen, a GABA(B) receptor agonist (2 microg) was ineffective. Muscimol reduction of non-contact penile erections was not seen when male rats were pretreated with bicuculline methiodide (2 microg) given 5 min before muscimol into the paraventricular nucleus. Since muscimol injected into the paraventricular nucleus also prevents penile erection induced by drugs (e.g. apomorphine, oxytocin or N-methyl-D-aspartic acid), the present results show that an increased GABAergic activity in the paraventricular nucleus can impair the expression of penile erection induced not only by drugs but also by sexual physiological stimuli.