Quinelorane (LY163502), a D2 dopamine receptor agonist, facilitates seminal emission, but inhibits penile erection in the rat

Effects of cabergoline and dimethylcabergoline on the sexual behavior of male rats

RATIONALE

Cabergoline (CAB) is an ergot derivative typically prescribed for the treatment of hyperprolactinemia. It suppresses the release of prolactin through agonist actions on dopamine (DA) D2 receptors; however, it possesses binding affinity for other DA and 5-HT receptors. Side effects that exacerbate valvular heart disease can occur with high doses.

OBJECTIVE

The present study examined the acute, subchronic, and chronic dose-response effects of CAB and a derivative dimethylcabergoline (DMC) which acts as an antagonist instead of agonist at 5-HT 2B receptors, on appetitive and consummatory sexual behaviors of male rats.

METHODS

CAB (0, 0.03, 0.15, or 0.3 mg/kg/ml) was administered daily to sexually experienced male rats (N = 10/dose) by oral gavage for a total of 68 days. Sexual behavior was tested every 4 days during this period for a total of 16 trials. On the 17th trial, rats were administered their dose of CAB, and 4 h after were overdosed with sodium pentobarbital, perfused intracardially, and their brains processed for Fos immunohistochemistry. DMC (0, 0.03, 0.15, 0.3 mg/kg/ml) was administered daily to sexually experienced male rats (N = 10/dose) by oral gavage for a total of 36 days. Sexual behavior was tested every 4 days for a total of 9 trials.

RESULTS

CAB increased anticipatory level changes, intromissions, and ejaculations significantly across all timepoints, with the medium and high doses being most potent. The medium and high doses also increased Fos protein significantly within the medial preoptic area, whereas in the nucleus accumbens shell, the low and medium doses decreased Fos protein but the high dose increased it significantly from control. Similar to CAB, the medium and high doses of DMC increased the number of ejaculations significantly. Rats in all drug dose groups appeared healthy for the duration of the experiments.

CONCLUSIONS

Both CAB and DMC facilitate ejaculations, and CAB further facilitates measures of anticipatory sexual motivation and intromissions. These data suggest that both could be used as treatments for sexual arousal disorders and ejaculation/orgasm disorders with little or no untoward side effects at low doses.

Association of low serum ferritin levels with augmentation in patients with restless legs syndrome: A systematic review and meta-analysis

BACKGROUND

Augmentation of restless legs syndrome (RLS) is an iatrogenic side effect induced by dopaminergic agents, and it is a major cause of therapeutic failure. Iron deficiency is a risk factor for RLS, but its effects on the development of RLS augmentation are unclear. This meta-analysis aimed to elucidate the association between serum ferritin and RLS augmentation.

METHODS

We searched the PubMed, Cochrane Library, Embase, ClinicalKey, ScienceDirect, and ProQuest databases for studies comparing the serum ferritin levels of patients with augmented RLS and nonaugmented RLS. A meta-analysis based on a random-effects model was conducted. Levodopa equivalent dose (LED), International Restless Legs Study Group Severity Rating Scale (IRLS), and serum hemoglobin levels were also analyzed.

RESULTS

Six observational studies fulfilled the eligibility criteria of this meta-analysis. A total of 220 RLS patients with augmentation and 687 RLS patients without augmentation were included. The results revealed that augmented RLS was significantly associated with low serum ferritin levels (p = 0.002), high LEDs (p = 0.026), and nonsignificantly associated with high IRLS scores (p = 0.227).

CONCLUSIONS

A low serum ferritin level is associated with RLS augmentation. For patients with RLS who are iron deficient, iron supplements can not only relieve their fundamental RLS symptoms but also lower the risk of RLS augmentation. Moreover, non-dopminergic agents should be considered as the first-line treatment for patients with persistent low serum ferritin levels or those with moderate to severe RLS to prevent augmentation.

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

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

Central Mechanisms of Apomorphine and m-Chlorophenylpiperazine on Synergistic Action for Ejaculation in Rats

BACKGROUND

We previously reported that the combination of the dopamine (DA) receptor agonist apomorphine and the 5-hydroxytryptamine (5-HT₂) receptor agonist m-chlorophenylpiperazine (m-CPP) in rats potently and selectively facilitates the ejaculatory response through activation of D₂-like and 5-HT_2C receptors, respectively.

AIM

The aim of this study was to clarify the target level of the proejaculatory effects induced by combination of these agonists.

METHODS

For in vivo behavioral studies, apomorphine and m-CPP were given intracerebroventricularly and intrathecally alone or in combination with either drug administered systemically. Male rats were acclimated to observational cages bedded in paper towels, and the occurrence of ex copula ejaculation was assessed by evaluating the presence and weight of ejaculatory plugs dropped from the tip of the penis to the paper towels or adhered to the tip of the penis at 30 min after drug administration. For in vitro contraction studies, seminal vesicles isolated from rats were suspended in an organ bath to test contractile responses to drug combinations, and the effects of the combined drugs on the contractile response of noradrenaline were also tested.

MAIN OUTCOME MEASURES

The presence and weight of ejaculatory plugs produced by drug-induced ejaculation and the contractile responses of the seminal vesicle were evaluated.

RESULTS

Intrathecal m-CPP (10 μg), but not intracerebroventricular m-CPP, evoked the synergistic effects on ejaculation when used in combination with systemically administered apomorphine (0.1 mg/kg, subcutaneous). Moreover, the synergy between m-CPP and apomorphine was completely abolished by the intrathecal 5-HT_2C receptor antagonist SB242084 (10 μg). Intrathecal or intracerebroventricular apomorphine (1-10 μg) evoked proejaculatory effects in combination with systemically administered m-CPP (0.3 mg/kg, intraperitoneal). The selective peripherally acting D₂-like receptor agonist carmoxirole did not evoke ejaculation when used in combination with m-CPP. Furthermore, isolated rat seminal vesicles were completely insensitive to the combination of apomorphine and m-CPP.

CONCLUSION

These results indicated that the synergistic effects of the drugs on ejaculation were induced at the central level but not at peripheral sites. Our findings also suggested that the 5-HT_2C receptor mediated the stimulation of the spinal ejaculatory pattern generator and was synergistically potentiated by the spinal DA receptor and that activation of the supraspinal DA receptor was also involved in mediating these synergistic effects. Yoshizumi M, Yonezawa A, Kimura Y, et al. Central Mechanisms of Apomorphine and m-Chlorophenylpiperazine on Synergistic Action for Ejaculation in Rats. J Sex Med 2021;18:231-239.

Management Options for Premature Ejaculation and Delayed Ejaculation in Men

INTRODUCTION

Many men experience distressing issues regarding the timing of orgasm and ejaculation, such as premature ejaculation (PE) and delayed ejaculation (DE). Despite being highly prevalent, both PE and DE are poorly understood and present a management challenge for sexual medicine specialists.

AIM

To summarize existing data on the medical management of PE and DE.

METHODS

A comprehensive literature review pertaining to the management of PE and DE was conducted using PubMed and clinicaltrials.gov for data published up until May 2019. Our focus was on double-blind, placebo-controlled trials and meta-analyses of such studies.

MAIN OUTCOME MEASURE

Peer-reviewed studies on treatment options for PE and DE were critically analyzed for results and methodological rigor.

RESULTS

The peer-reviewed data on PE management continue to evolve. Psychotherapy, pharmacotherapy, and procedural interventions have all been associated with some degree of efficacy. A strong evidence base supports the off-label use of selective serotonin reuptake inhibitors and local anesthetics in PE given consistent increases in ejaculation latency time. Education and mental health assessments remain important components of PE management despite a dearth of peer-reviewed data on these interventions. Numerous treatment strategies have been evaluated for DE; limited data support psychotherapy, pharmacotherapy, and/or penile vibratory stimulation as management options.

CONCLUSION

A number of management options for PE or DE exist but none has been formally approved by the US Food and Drug Administration. New and novel treatments would be of great value in managing issues regarding the timing of ejaculation/orgasm. Martin-Tuite P, Shindel AW. Management Options for Premature Ejaculation and Delayed Ejaculation in Men. Sex Med Rev 2020; 8:473-485.

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.

The effects of dopamine receptor 1 and 2 agonists and antagonists on sexual and aggressive behaviors in male green anoles

The propensity to exhibit social behaviors during interactions with same-sex and opposite-sex conspecifics is modulated by various neurotransmitters, including dopamine. Dopamine is a conserved neurotransmitter among vertebrates and dopaminergic receptors are also highly conserved among taxa. Activation of D₁ and D₂ dopamine receptor subtypes has been shown to modulate social behaviors, especially in mammalian and avian studies. However, the specific behavioral functions of these receptors vary across taxa. In reptiles there have been few studies examining the relationship between dopaminergic receptors and social behaviors. We therefore examined the effects of D₁ and D₂ agonists and antagonists on sexual and aggressive behaviors in the male green anole lizard (Anolis carolinensis). Treatment with high doses of both D₁ and D₂ agonists was found to impair both sexual and aggressive behaviors. However, the D1 agonist treatment was also found to impair motor function, suggesting that those effects were likely nonspecific. Lower doses of both agonists and antagonists failed to affect social behaviors. These findings provide some evidence for D2 receptor regulation of social behaviors, but in contrast with previous research, these effects are all inhibitory and no effects were found for manipulations of D1 receptors. A potential reason for the lack of more widespread effects on social behaviors using moderate or low drug doses is that systemic injection of drugs resulted in effects throughout the whole brain, thus affecting counteracting circuits which negated one another, making measurable changes in behavioral output difficult to detect. Future studies should administer drugs directly into brain regions known to regulate sexual and aggressive behaviors.

The drug treatment of delayed ejaculation

Delayed ejaculation (DE) is an uncommon and a challenging disorder to treat. It is often quite concerning to patients and it can affect psychosocial well-being. Here we reviewed how DE is treated pharmacologically .We also highlighted specific settings where drugs could be introduced to medical practice. Electronic databases were searched from 1966 to February 2016, including PubMed MEDLINE, EMBASE, EBCSO Academic Search Complete, Cochrane Systematic Reviews Database, and Google Scholar using key words; delayed ejaculation, retarded ejaculation, inhibited ejaculation, drugs, treatment, or pharmacology. To achieve the maximum sensitivity of the search strategy and to identify all studies, we combined "delayed ejaculation" as Medical Subject Headings (MeSH) terms or keywords with each of "testosterone" or "cabergoline" or "bupropion" or "amantadine" or "cyproheptadine" or "midodrine" or "imipramine" or "ephedrine" or "pseudoephedrine" or "yohimbine" or "buspirone" or "oxytocin" or "bethanechol" as MeSH terms or keywords. There are a number of drugs to treat patients with DE including: testosterone, cabergoline, bupropion, amantadine, cyproheptadine, midodrine, imipramine, ephedrine, pseudoephedrine, yohimbine, buspirone, oxytocin, and bethanechol. Although there are many pharmacological treatment options, the evidence is still limited to small trials, case series or case reports. Review of literature showed that evidence level 1 (Double blind randomized clinical trial) studies were performed with testosterone, oxytocin, buspirone or bethanechol treatment. It is concluded that successful drug treatment of DE is still in its infancy. The clinicians need to be aware of the pathogenesis of DE and the pharmacological basis underlying the use of different drugs to extend better care for these patients. Various drugs are available to address such problem, however their evidence of efficacy is still limited and their choice needs to be individualized to each specific case.

Neurophysiology of erection and ejaculation

INTRODUCTION

Penile erection and ejaculation are closely associated during sexual intercourse. Erection is a central psychoneuroendocrine and peripheral neuro-vasculo-tissular event, resulting in blood filling the sinusoidal spaces of the corpora cavernosa and corpus spongiosum. Ejaculation represents the climax of the sexual cycle and comprises emission (secretion of semen) and expulsion (propulsion of semen) phases.

AIM

This article provides an overview of the proposed neurophysiology of erection and ejaculation.

METHODS

Review of the literature.

MAIN OUTCOME MEASURES

Current data on the neurophysiology of erection and ejaculation.

RESULTS

In terms of peripheral innervation, the pelvic plexus represents a junction for efferent nerves to the structures involved in erection and ejaculation. At the spinal level, the spinal cord contains three sets of neurons (thoracolumbar sympathetic, sacral parasympathetic, and somatic) innervating the sexual organs involved in erection and ejaculation. The presence of cerebral descending pathways to spinal erection and ejaculation centers indicates that the brain has an excitatory or inhibitory effect on these processes. Brain structures that modulate spinal command of erection and ejaculation are part of a larger network that is dedicated to regulating sexual responses. Neurophysiological and pharmacological research has elucidated that dopamine and serotonin have central roles in modulating erection and ejaculation. Interestingly, erection is not a prerequisite for ejaculation, and each of these sexual responses can exist without the other.

CONCLUSION

Despite the association between erection and ejaculation during intercourse, these two processes can be considered distinct events from an anatomical, physiological, and pharmacological perspective.

D2-like receptors mediate the expulsion phase of ejaculation elicited by 8-hydroxy-2-(di-N-propylamino)tetralin in rats

The mechanism of action by which 8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) facilitates ejaculation in conscious rats is not clearly established. The serotonin (5-HT) 1A agonist 8-OH-DPAT may actually act on cerebral dopaminergic receptors to exert its proejaculatory effect. The present work was undertaken to clarify this issue by testing various compounds i.c.v. delivered in an experimental model of the expulsion phase of ejaculation in anesthetized Wistar rats. Intracerebroventricular delivery of 8-OH-DPAT dose-dependently (ED(50) = 17 microg) induced rhythmic contractions of bulbospongiosus (BS) muscles, which are of paramount importance for the expulsion of semen, occurring in the form of cluster of bursts evidenced by the recording of BS muscle electrical activity. The 5-HT1A antagonist WAY100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide) (20 microg) i.c.v. coadministered with 8-OH-DPAT (20 microg) was unable to inhibit the effect of 8-OH-DPAT on BS muscle contractile activity. Conversely, raclopride (40 microg) and spiperone (10 microg), both dopamine D2-like receptor antagonists, i.c.v. coinjected with 8-OH-DPAT (20 microg), abolished BS muscle contractions. The involvement of D2-like receptors was further supported by the fact that the D2-like agonist quinelorane (20 microg i.c.v.) also induced BS muscle rhythmic contractions. Our data demonstrate that D2-like receptors mediate the induction by 8-OH-DPAT of rhythmic BS muscle contractions and suggest that i.c.v. delivery of D2-like receptor agonists to anesthetized rats represents a relevant experimental model to study the expulsion phase of ejaculation.

Neural control of erection

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

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.

Localization of peripheral dopamine D1 and D2 receptors in rat and human seminal vesicles

Dopamine, an established neurotransmitter in the central nervous system, is recognized for its role in penile erection and ejaculation in rats. However, its complete mechanism of action in the genitourinary tract is unknown. The objective of this study was to investigate the existence and expression of peripheral dopamine D1 and D2 receptor messenger RNAs (mRNAs) and corresponding proteins in rat and human seminal vesicles. The seminal vesicle tissues of male Sprague-Dawley rats and human radical prostatectomy specimens were used to extract total RNA and proteins, and to prepare slide sections. Rat hypothalamus tissue served as a control for dopamine D1 and D2 receptors. Testing for the presence and expression of peripheral dopamine D1 and D2 receptor mRNAs in rat and human seminal vesicle tissues was performed by reverse transcription-polymerase chain reaction. Western blotting was used to detect corresponding proteins of D1 and D2 receptors. Immunohistochemical staining using rabbit antipeptide polyclonal antibodies was employed to identify and anatomically localize dopamine D1 and D2 receptor proteins in rat and human seminal vesicles. Dopamine D1 and D2 receptor transcripts were detected in both human and rat seminal vesicle tissues. Western blot analysis demonstrated that peripheral dopamine D1 and D2 receptor proteins exist in both human and rat seminal vesicle tissues. Immunohistochemical analysis demonstrated the localization of peripheral dopamine D1 and D2 receptors to the smooth muscle layer of human and rat seminal vesicles. The results of this study demonstrate that peripheral dopamine D1 and D2 receptors are present in the seminal vesicle tissue in both rats and humans. Although these results suggest that seminal emission may be mediated in part by the stimulation of peripheral dopamine receptors located in the seminal vesicles, the functional significance of dopamine in male reproductive tract has yet to be fully defined.

Central neural regulation of penile erection

Penile erection is caused by a change of the activity of efferent autonomic pathways to the erectile tissues and of somatic pathways to the perineal striated muscles. The spinal cord contains the cell bodies of autonomic and somatic motoneurons that innervate the peripheral targets. The sympathetic outflow is mainly antierectile, the sacral parasympathetic outflow is proerectile, and the pudendal outflow, through contraction of the perineal striated muscles, enhances an erection already present. The shift from flaccidity to erection suggests relations among these neuronal populations in response to a variety of informations. Spinal neurons controlling erection are activated by information from peripheral and supraspinal origin. Both peripheral and supraspinal information is capable of eliciting erection, or modulating or inhibiting an erection already present. One can hypothesize a spinal network consisting of primary afferents from the genitals, spinal interneurons and sympathetic, parasympathetic and somatic nuclei. This system is capable of integrating information from the periphery and eliciting reflexive erections. The same spinal network, eventually including different populations of spinal interneurons, would be the recipient of supraspinal information. Premotor neurons that project directly onto spinal sympathetic, parasympathetic or somatic motoneurons, are present in the medulla, pons and diencephalon. Several of these premotor neurons may in turn be activated by sensory information from the genitals. Aminergic and peptidergic descending pathways in the vicinity of spinal neurons, exert complex effects on the spinal network that control penile erection. This is caused by the potential interaction of a great variety of receptors and receptor subtypes present in the spinal cord. Brainstem and hypothalamic nuclei (among the latter, the paraventricular nucleus and the medial preoptic area) may not necessarily reach spinal neurons directly. However they are prone to regulate penile erection in more integrated and coordinated responses of the body, such as those occurring during sexual behavior. Finally, the central and spinal role of regulatory peptides (oxytocin, melanocortins, endorphins) has only recently been elucidated.

Effects of repeated low dose administration and withdrawal of haloperidol on sexual behaviour of male rats

Neuroleptics are known to cause anhedonia and attenuate sexual behaviour at therapeutic doses in humans. These effects are assumed to result from the dopamine antagonism of the drugs. It has been observed that a mixed dopamine D1/D2 antagonist, haloperidol, may cause a reduction in the number of intromissions required to achieve ejaculation. On the other hand, dopamine antagonists are considered unable to modify sexual behaviour once the copulatory sequence is initiated. In this study, male rats received low doses of haloperidol (30 or 60 microg/kg) before the investigation of sexual behaviour in five consecutive days and the mating test was repeated after withdrawal periods of four and five days. Haloperidol dose-dependently reduced intromission frequency, and this effect was maintained for four days after withdrawal. Ejaculation latency was reduced in all groups, including controls. The results indicate that at low doses haloperidol dose-dependently reduces intromission frequency, and the effect of a repeated dosage may persist several days after cessation of medication.

Partial antagonism of 8-OH-DPAT'S effects on male rat sexual behavior with a D2, but not a 5-HT1A, antagonist

The serotonin agonist 8-hydroxy-di-propylaminotetralin (8-OH-DPAT), injected systemically or directly into the medial preoptic area (MPOA), reduces the ejaculatory threshold in male rats. While 8-OH-DPAT has been characterized as an agonist at the 5-HT1A receptor, it also acts at other receptor sites including the dopamine D2 receptor. The current experiments investigated whether 8-OH-DPAT injected into the MPOA facilitates male sexual behavior through stimulation of the 5-HT1A receptor or the dopamine D2 receptor. Experiment 1 co-administered 8-OH-DPAT (6 microgram) with either the 5-HT1A antagonist 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-ben zamide hydrochloride (MPPI) (10 microgram) or the D2 antagonist raclopride (10 microgram). Raclopride blocked 8-OH-DPAT's facilitative effects on ejaculation frequency and latency, while the 5-HT1A antagonist was ineffective. In Experiment 2, 8-OH-DPAT (500 microM), retrodialyzed into the MPOA through a microdialysis probe, enhanced male copulatory behavior similarly to the microinjection, increasing ejaculation frequency and decreasing ejaculation latency, postejaculatory interval and mount frequency. Retrodialyzing 8-OH-DPAT through a microdialysis probe in the MPOA had been previously shown to increase extracellular levels of dopamine and serotonin. The data from the present studies suggest that the effects of 8-OH-DPAT in the MPOA on male rat copulatory behavior may be mediated, at least in part, either directly through 8-OH-DPAT's activity at D2 receptors or indirectly through 8-OH-DPAT's ability to increase extracellular dopamine.

Spinal control of penile erection

Smooth muscle relaxation of penile arteries, the corpus cavernosum, and the corpus spongiosum, leading to penile erection, results from parasympathetic neural pathway activation and, likely, simultaneous inhibition of sympathetic outflow. Proerectile parasympathetic outflow is reflexively activated by sensory information of peripheral origin, conveyed by the dorsal penile nerve, and reflexive erections are supported by an intraspinal circuitry. Supraspinal influences modulate the reflex. Information integrated at or originating from supraspinal structures may also elicit penile erection. Several neurotransmitters are involved in either the modulation of the spinal reflex or the mediation of supraspinal influences. Spinal cord injury differently alters reflexive penile erection or erection from a central origin, depending on the neurologic level of injury.

Amantadine stimulates sexual behavior in male rats

The effects of amantadine on sexual behavior, penile erection, and seminal emission of male rats was studied. Amantadine significantly decreased latency of mounts in all doses (1.25 to 50 mg/kg), and decreased the number of mounts and intromission latency at the highest doses used. The lowest dose of amantadine significantly increased ejaculation latency and intromission frequency, while higher doses significantly reduced it, which indicates a biphasic response of the drug. Additionally, seminal emission, erections, and genital grooming were significantly induced by amantadine. Amantadine-induced seminal emissions were impaired by spinal cord transection, which suggests the involvement of supraspinal structures in the drug action. Haloperidol and atropine sulphate significantly reduced seminal emissions and penile erections induced by amantadine. These results demonstrate that amantadine stimulates sexual behavior and genital reflexes in male rats and suggest a facilitatory effect of the drug that probably involves different mechanisms of action.

Placing erection in context: the reflexogenic-psychogenic dichotomy reconsidered

Penile erections are usually classified as arising from "reflexogenic" or "psychogenic" causes. In practice this dichotomy has translated, somewhat circularly, to a distinction between spinal vs. supraspinal mediation, pelvic vs. hypogastric neural mediation, and perineal somesthetic stimulation vs. stimulation of receptors innervated by the cranial nerves. Evidence for differential regulation of erection in different contexts is reviewed. Research ascribing a physiological role to the hypogastric nerves in psychogenic erection, exemplified by classic studies of cats and spinally injured men, is suggestive but not compelling. Somewhat stronger is evidence that erection in some contexts (e.g., nocturnal penile tumescence (NPT) in humans or touch-stimulated erection in rats) is more sensitive to androgen levels than in other contexts (e.g., visual erotic stimuli in men or copulation in rats). However, some of these differences may arise from the relative erectogenic strength of the stimuli, rather than from qualitative differences in androgen sensitivity of different contexts. More compelling is the possibility that conflicting interpretations of the role of dopamine in erection may stem in large part from differences among laboratories in the context in which erection is evoked. In light of the evidence reviewed, it seems unlikely that the conventional reflexogenic-psychogenic dichotomy should be retained, at least in its present form. As a first step, it may be worth considering that reflexive erections may not be limited to somesthetic perineal stimulation, but rather may also include stimuli received via the cranial nerves. Two alternatives to the standard reflexogenic-psychogenic dichotomy are proposed. The first is a minor revision in which two senses of psychogenic erection are distinguished: the weak, commonly used, sense would include erection resulting from any extrinsic nonsomesthetic stimulation, whether visual, auditory, or chemosensory. In this sense, reflexive erections and psychogenic erections may not be mutually exclusive. The strong sense of psychogenic erection would be limited to memory and fantasy. The origins of psychogenic erection in both senses need not be available to consciousness, which may account for apparently spontaneous erections. In the second alternative taxonomy, erectogenic stimuli are classified as contact (somesthetic) or noncontact, and their action in evoking erection is placed on a continuum of reflexivity. Erectile contexts could then be considered as orthogonal to the other two dimensions. Even without a change in taxonomy, the conduct and interpretation of research into erectile function may be expected to benefit from closer attention to differences and similarities between contexts and species, and to context-sensitive differences in the regulation of erection.

Dopamine and sexual behavior

Among central neurotransmitters involved in the control of sexual behavior, dopamine is certainly one of the most extensively studied. Our attempt to review old and recent neuropharmacological, biochemical, electrophysiological, and psychobiological studies performed so far only in rats, monkeys, and humans, provides evidence that dopamine through its different neuronal systems and receptor subtypes plays different roles in the control of several aspects of sexual behavior. In fact, while the nigrostriatal system is necessary for the control of the sensory-motor coordination required for copulation, the mesolimbic-mesocortical system plays a key role in the preparatory phase of the behavior, mainly in sexual arousal, motivation and possibly reward. Conversely, the incertohypothalamic system plays a major role in the consummation of the behavior, mainly in seminal emission and erectile performance, but evidence for its involvement in sexual motivation also exists. The dopaminergic receptors playing the major role in the control of male sexual behavior belong to the D2 receptor subtype. However a D1/D2 receptor interaction is well established and an opposite role for D1 and D2 receptors in the preoptic area suggested. Despite some differences, most studies show that treatments that increase or decrease, respectively, brain dopaminergic activity improve or worsen, respectively, several parameters of copulatory activity, supporting a facilitatory role of dopamine in male sexual behavior. In contrast, no conclusion can be deduced from the available studies on the role of central dopaminergic systems in the control of proceptivity and receptivity, the two main components of female sexual behavior.

Morphine inhibits dopaminergic and cholinergic induced ejaculation in rats

1. Subcutaneous injection (s.c.) of apomorphine (0.1-0.5 mg/kg) and intraperitoneal administration (i.p.) of quinpirole (0.01-0.25 mg/kg), physostigmine (0.05-0.2 mg/kg) and pilocarpine (0.75-3 mg/kg, i.p.) but not neostigmine (0.1-1 mg/kg) induced ejaculation in rats. 2. The responses of drugs were reduced by morphine (1-6 mg/kg, s.c.) pretreatment. 3. The inhibitory effect of morphine was reversed by naloxone (1.5 mg/kg, s.c.). 4. Naloxone (0.75-3 mg/kg, s.c.) alone induced slight but significant ejaculation. 5. Ejaculatory responses induced by apomorphine and quinpirole but not those by physostigmine and pilocarpine were reduced by sulpiride (100 mg/kg, i.p.) pretreatment. 6. Domperidone (1-30 mg/kg, i.p.) did not change the response induced by apomorphine. 7. Pretreatment of animals with the cholinergic antagonist atropine (10 mg/kg, i.p.) decreased the frequency of ejaculation induced by apomorphine, quinpirole, physostigmine or pilocarpine. 8. It may be concluded that D-2 activation induces ejaculation through influence on cholinergic mechanisms and morphine inhibits the ejaculation induced by activation of both cholinergic and dopaminergic systems via opiate receptor sites.

Stimulatory effects of quinelorane on yawning and penile erection in the rat

Quinelorane, a highly selective D2 dopamine agonist, was assessed for its ability to induce the penile erection/stretch-yawn syndrome. Quinelorane (0.1-100 micrograms/kg s.c.) or saline vehicle was administered to adult male Sprague-Dawley rats just prior to a 30 min. observation period. Significant dose-related increases in erections were observed in the drug treated animals at 3-100 micrograms/kg. Yawning was also increased at 3-100 micrograms/kg, with highest levels occurring at 10 micrograms/kg. Defecation was stimulated between 10 and 100 micrograms/kg. The stimulatory effects of 30 micrograms/kg of quinelorane on erection, yawning and defecation were blocked by haloperidol (0.1-0.3 mg/kg) but not by domperidone (0.1-1.0 mg/kg). No significant effects of quinelorane on seminal emission were observed. These findings indicate that in addition to its stimulatory effects on sexual activity, quinelorane also acts on D2 receptors in the central nervous system to stimulate erection in the penile erection/stretch-yawn model.

Dopaminergic influences on male sexual behavior of rhesus monkeys: Effects of dopamine agonists

Previous research has demonstrated that the mixed D1/D2 dopamine receptor agonist, apomorphine, and the specific D2 dopamine receptor agonist, quinelorane, facilitated penile erections and masturbatory behavior of male rhesus monkeys when they were tested in the presence of a female monkey that they could see, hear, and smell but not physically contact. The present study was designed to further examine dopaminergic influences on male sexual behavior of rhesus monkeys by evaluating male copulatory behavior following administration of these dopaminergic agents, as well as a D1 agonist, CY 208-243. Apomorphine and quinelorane treatment produced dose-dependent effects on male sexual responding. Compared to vehicle-based performance, postejaculatory intervals were shortened following treatment with either 100-200 micrograms/kg apomorphine or 2.5-10 micrograms/kg quinelorane. Higher doses of apomorphine or quinelorane did not reliably influence the postejaculatory interval. Ejaculation latency, intromission frequency, and number of thrusts/intromission increased following administration of 200-400 micrograms/kg apomorphine and 25 micrograms/kg quinelorane, indicating that dopaminergic stimulation in this dose range raised the monkeys' ejaculatory threshold. No behavioral effects of the D1 agonist, CY 208-243, were observed in this testing situation. These experiments provide further evidence that dopaminergic mechanisms may play a role in the regulation of male sexual behavior of rhesus monkeys and, in particular, demonstrate that the direction of the effect depends on the dopamine receptor subtype and dosage of the dopamine agonist being administered.

Apomorphine induces ejaculation in chronic decerebrate rats

Chronically maintained supracollicular decerebrate (CD) rats, but not rats with midthoracic (T10) spinal transections (SPI) or neurologically intact controls, displayed erection and ejaculation following systemic administration of the dopamine (DA) agonist, apomorphine (APO). The response was blocked by pretreatment with the DA receptor antagonist, cis-flupentixol. CD rats, like intact controls, failed to display erections or penile cupping in response to the same mechanical stimulation that elicited short-latency responses in SPI rats. The present results demonstrate excitatory and inhibitory mechanisms of relevance to the copulatory behavior of the male rat that are endemic to the caudal brainstem neurally isolated from forebrain influence.

Quinelorane (LY163502), a D2 dopamine receptor agonist, acts centrally to facilitate penile erections of male rhesus monkeys

The present study examined the effects of a specific D2 dopamine receptor agonist, quinelorane (LY163502), on male sexual responding of rhesus monkeys. The effects of quinelorane were assessed by observing the behavioral responses of male rhesus monkeys to a sexually receptive female monkey that they could see, hear, and smell, but not physically contact. Quinelorane (IM) treatment produced dose-dependent effects on male sexual responding. Penile erections and masturbation were markedly facilitated following treatment with either 2.5 or 5 micrograms/kg quinelorane. Higher doses of quinelorane (10 and 25 micrograms/kg) generally did not further augment sexual responding, but rather resulted in a return in sexual responding to control vehicle levels. Quinelorane had a biphasic effect on yawning behavior of the monkeys with low doses (2.5 and 5 micrograms/kg) facilitating yawning and high doses (25 micrograms/kg) inhibiting yawning. Quinelorane in the dose-range (1-25 micrograms/kg) being evaluated did not reliably influence stereotypic behavior. In order to determine whether quinelorane acts centrally or peripherally to stimulate male sexual behavior, the ability of the peripherally active dopamine antagonist, domperidone, and the centrally active dopamine antagonist, haloperidol, to block the facilitation of sexual behavior produced by quinelorane treatment was examined. Administration of domperidone (50-200 micrograms/kg) failed to block quinelorane's effects on sexual behavior, whereas treatment with haloperidol (5-20 micrograms/kg) prevented quinelorane from stimulating male sexual responding. These experiments provide further evidence that dopaminergic mechanisms may play a role in the regulation of male sexual behavior of rhesus monkeys and, in particular, demonstrate the sexual stimulant properties of agents that provide central stimulation to D2 dopamine receptor sites.

D2 receptors in the paraventricular nucleus regulate genital responses and copulation in male rats

The D2 dopamine receptor agonist quinelorane (LY-163502), microinjected into the paraventricular nucleus (PVN), affected genital response of restrained supine male rats in a biphasic dose-dependent fashion. A moderate dose (1 microgram) facilitated penile responses (intense erections and penile movements), and decreased the latency to the first response. A high dose of quinelorane (10 micrograms) facilitated seminal emission while inhibiting penile responses. The addition of the D1 antagonist SCH-23390 to the 1 microgram dose of quinelorane potentiated quinelorane's increase in seminal emission. We suggest that D1 receptors in the PVN may be antagonistic to D2 receptor-mediated seminal emission, and possibly also penile responses. In copulation tests 1 microgram quinelorane decreased mount latency, whereas 10 micrograms quinelorane increased mount and intromission latencies and slowed copulatory rate. Both 1 and 10 micrograms quinelorane, and also 1 and 10 micrograms of the mixed D1 and D2 agonist apomorphine, decreased the number of intromissions preceding ejaculation.

Dose dependent D2 effects on genital reflexes after MPOA injections of quinelorane and apomorphine

This study investigated the effects on genital reflexes of unilateral MPOA injections of 0.1, 1, 3, and 10 micrograms of the D2 agonist quinelorane (LY-163502), and of 3 micrograms quinelorane administered together with 3 micrograms of the D1 antagonist SCH-23390. In addition, the effects of an MPOA injection of 10 micrograms apomorphine were tested. All but the lowest dose of quinelorane significantly decreased the latency to the first reflex. The 3 and 10 micrograms doses of quinelorane, and the combination of quinelorane and SCH-23390, decreased the total number of reflexes. In addition, 10 micrograms quinelorane increased the number of seminal emissions. 10 micrograms apomorphine, like 10 micrograms quinelorane, decreased the latency to the first reflex and increased the number of seminal emissions, but did not decrease the numbers of erections or penile movements. The ratio of D1/D2 activity may influence the number of erections displayed during ex copula testing.

Septohippocampal cholinergic pathway and penile erections induced by dopaminergic and cholinergic stimulants

Penile erection was studied in rats injected with pilocarpine, amantadine and apomorphine. Pilocarpine and apomorphine are direct stimulants of muscarinic and dopaminergic receptors, respectively, while amantadine is reported to be an indirect stimulant of dopaminergic nerves. Single intraperitoneal injection of pilocarpine (0.1-3.2 mg/kg), amantadine (1-32 mg/kg) and apomorphine (0.01-1 mg/kg) increased the number of penile erections with bell-shaped dose-response curves. The penile erections, irrespective of the stimulant used, were antagonized by doses (0.032-1 mg/kg) of scopolamine but not by those of methylscopolamine. Sulpiride treatment (1-32 mg/kg) had little effect on pilocarpine-induced penile erection while it attenuated dose-dependently the apomorphine- and amantadine-induced ones. Medial septum and fimbria-fornix lesions abolished penile erections by amantadine and apomorphine but not by pilocarpine. The similar results with amantadine and apomorphine suggest that the drugs activate postsynaptic dopamine receptors, which indirectly activate the septohippocampal cholinergic pathway to induce penile erections. Possible linkage between dopaminergic and cholinergic nerves is discussed.