Apomorphine and haloperidol, but not domperidone, affect penile reflexes in rats

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.

Effects of chronic l-DOPA administration on neurogenic and endothelium-dependent relaxation responses in rabbit corpus cavernosum

BACKGROUND

Dopamine is a crucial central neurotransmitter that plays a fundamental role in the autonomic and somatic components of penile reflexes in animals and humans. Similar to the erectile responses of dopamine, systemic administration of l-DOPA induces yawning and penile erection in some species. The possible effects of l-DOPA on nitric oxide (NO)-dependent and -independent non-adrenergic non-cholinergic (NANC) relaxation responses mediated by electrical field stimulation (EFS) and endothelium-dependent relaxation were investigated in this study.

METHODS

Thirty-two adult albino male rabbits, in two- and four-week-treatment groups, were divided into three subgroups: control group (saline-injected) (n=4), 3mg/kg/day (low dose) l-DOPA-injected groups (n=6) and 12mg/kg/day (high dose) l-DOPA-injected groups (n=6). After the intraperitoneal injection treatments, the corpus cavernosum tissues were placed in organ bath chambers. The EFS-mediated responses, and the concentration-response curve to carbachol, sodium nitroprusside (SNP), sildenafil were assessed.

RESULTS

The two-week treatment with high-dose l-DOPA decreased the NO-dependent NANC relaxation responses, while there was no change in the low-dose two- and four-week treatment groups. The NO-independent NANC relaxation responses in the two-week groups decreased, and the responses in the four-week groups were unchanged when compared to the controls. The relaxation responses to carbachol showed no differences among all groups except for the high-dose four-week l-DOPA group. The relaxation responses of SNP and sildenafil were increased in all of the treatment groups when compared to the controls.

CONCLUSIONS

The observed increases in SNP- and sildenafil-induced responses, along with the decreased EFS-mediated responses, suggest increased sensitivity in the NO-signalling pathway following l-DOPA administration.

Colocalization of Mating-Induced Fos and D2-Like Dopamine Receptors in the Medial Preoptic Area: Influence of Sexual Experience

Dopamine in the medial preoptic area (mPOA) stimulates sexual activity in males. This is evidenced by microdialysis and microinjection experiments revealing that dopamine receptor antagonists in the mPOA inhibit sexual activity, whereas agonists facilitate behavior. Microdialysis experiments similarly show a facilitative role for dopamine, as levels of dopamine in the mPOA increase with mating. While the majority of evidence suggests an important role for dopamine receptors in the mPOA in the regulation of male sexual behaviors, whether sexual activity or sexual experience influence dopamine receptor function in the mPOA has not been previously shown. Here we used immunohistochemical assays to determine whether varying levels of sexual activity or experience influence the number of cells containing Fos or D2 receptor immunoreactivity. Results show that sexual experience facilitated subsequent behavior, namely experience decreased latencies. Moreover, the number of cells with immunoreactivity for Fos or D2 correlated with levels of sexual experience and sexual activity. Sexual activity increased Fos immunoreactivity. Sexually experienced animals also had significantly more D2-positive cells. Sexually inexperienced animals copulating for the first time had a larger percentage of D2-positive cells containing Fos, when compared to sexually experienced animals. Finally, regardless of experience, animals that had sex prior to sacrifice had significantly more D2-positive cells that contained Fos, vs. animals that did not copulate. These findings are noteworthy because sexually experienced animals display increased sexual efficiency. The differences in activation of D2 and changes in receptor density may play a role in this efficiency and other behavioral changes across sexual experience.

Neural regulation of sexual function in men

Male sexual response is controlled by a series of neurally mediated phenomena regulating libido, motivation, arousal and genital responses such as penile erection and ejaculation. These neural events that occur in a hormonally defined milieu involve different neurophysiological, neurochemical, and neuropsychological parameters controlled by central mechanisms, spinal reflexes and peripheral nervous system. Epidemiologic studies have suggested the high prevalence of male sexual dysfunction worldwide with significant impact on the quality of life of patients suffering from this problem. The incidence of sexual dysfunction is particularly high among men with neurologic disorders. Sexual dysfunction in men, such as loss of sexual desire, erectile dysfunction (ED), changes in arousal, and disturbances in orgasm and ejaculation may involve organic causes, psychological problems, or both. Organic male sexual disorders include a wide variety of neurologic, vasculogenic, neurovascular or hormonal factors that interfere with libido, erection, ejaculation and orgasm. Neurogenic sexual dysfunction may result from a specific neurologic problem or it could be the presenting symptom of a developing neurologic disease. Neurologic ED could result from complications of chronic neurologic disorders, trauma, surgical injury or iatrogenic causes. These etiologic factors and the underlying pathophysiologic conditions could overlap, which should be considered when making a diagnosis and selecting a treatment. A detailed history of physical examination, neurologic disorders, as well as any past history of psychological and psychiatric disturbances, and a thorough neurological examination will provide better understanding of the underlying causes of neurogenic sexual dysfunction. In patients with spinal cord injury, the location of the lesion and the time of onset of injury should be determined. Therapeutic strategies against erectile dysfunction are initiated with the least invasive options using the phosphodiesterase inhibitors. When oral medication options are exhausted, intraurethral and intracavernosal therapies and ultimately vacuum constriction devices and penile implants are considered. Recent basic research has suggested the potential role of stem cell-based therapeutic strategies to protect penile neural integrity and reverse cavernosal neurodegeneration in experimental models. Further insight into the central, spinal and peripheral neural mechanisms of male sexual response may help precise diagnosis and better management of neurogenic sexual dysfunction in men.

A Role for Preoptic Glutamate in the Regulation of Male Reproductive Behavior

Although much progress has been made toward understanding the role of the medial preoptic area (MPOA) in the regulation of male reproductive behaviors, the precise mechanisms responsible for its activation during mating are largely unclear. Several studies implicate glutamate in this response. However, not until recently was there direct evidence supporting this hypothesis. Results obtained using in vivo microdialysis showed that levels of glutamate increased in the MPOA during mating, particularly with ejaculation. Levels then decreased rapidly following ejaculation, during a period of sexual quiescence. The magnitude of this decrease correlated with time spent in quiescence. Additionally, central administration of glutamate uptake inhibitors increased levels of glutamate and facilitated behavior. Glutamate activation of N-methyl-D-aspartate (NMDA) receptors in the MPOA is at least partly responsible for behavioral effects evoked by increase glutamate. This is evidenced by histological analysis of the MPOA, which shows that nearly all cells containing mating-induced Fos also contained NMDA receptors. Mating also increased phosphorylation of NMDA receptors, indicating receptor activation. Finally, bilateral microinjections of NMDA receptor antagonists inhibited copulation. This neurochemical, anatomical, and behavioral evidence points to a key role of preoptic glutamate in the regulation of sexual behavior in males. The implications of these findings are discussed.

Hormesis and medicine

Evidence is presented which supports the conclusion that the hormetic dose-response model is the most common and fundamental in the biological and biomedical sciences, being highly generalizable across biological model, endpoint measured and chemical class and physical agent. The paper provides a broad spectrum of applications of the hormesis concept for clinical medicine including anxiety, seizure, memory, stroke, cancer chemotherapy, dermatological processes such as hair growth, osteoporosis, ocular diseases, including retinal detachment, statin effects on cardiovascular function and tumour development, benign prostate enlargement, male sexual behaviours/dysfunctions, and prion diseases.

Peripheral Modulation of Dopaminergic Receptors Affects Erectile Responses in Rats

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

The supraspinal network in the control of erection

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

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.

Ac-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-NH2 induces penile erection via brain and spinal melanocortin receptors

Penile erection induced by alpha-melanocyte-stimulating hormone and melanocortin receptors (MC-R) in areas of the spinal cord and periphery has not been demonstrated. To elucidate sites of the proerectile action of melanocortin peptides, in awake male rats we administered the MC-R agonist Ac-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-NH(2) (MT-II) i.c.v., intrathecal (i.th.) and i.v. and scored penile erection and yawning. Injection of the MC-R antagonist Ac-Nle-c[Asp-His-DNal(2')-Arg-Trp-Lys]-NH(2) (SHU-9119) i.c.v. or i.th. in combination with i.th. MT-II differentiated spinal from supraspinal effects. To exclude a site of action in the penis, we recorded intracavernous pressure responses to intracavernosal injection of MT-II in the anesthetized rat.I.c.v., i.th., and i.v. MT-II induced penile erections in a dose-dependent fashion. Yawning was observed with i.c.v. and i.v. MT-II, while spinal injection did not produce this behavior. Intrathecal delivery of MT-II to the lumbosacral spinal cord was more efficacious in inducing erections than i.c.v. or i.v. administration; SHU-9119 blocked the erectile responses to i.th. MT-II when injected i.th. but not i.c.v. Intracavernosal MT-II neither increased intracavernous pressure nor augmented neurostimulated erectile responses. We confirmed the central proerectile activity of MT-II and demonstrated that in addition to a site of action in the brain, the distal spinal cord contains melanocortin receptors that can initiate penile erection independent of higher centers. These results provide new insight into the central melanocortinergic pathways that mediate penile erection and may allow for more efficacious melanotropin-based therapy for erectile dysfunction.

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

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

Lack of opioid or dopaminergic effects on unconditioned sexual incentive motivation in male rats

The effects of dopaminergic and opioidergic drugs on sexual incentive motivation were evaluated in sexually inexperienced male rats subjected to a choice procedure. Various parameters of ambulatory activity were recorded as well. Two drugs stimulating dopaminergic neurotransmission, amphetamine and apomorphine, failed to affect sexual incentive motivation, although ambulatory activity was enhanced by amphetamine. The dopamine antagonist cis(Z)-flupenthixol reduced sexual incentive motivation, but only at a dose that severely disrupted motor function. Morphine had marginal effects on sexual motivation but reduced ambulatory activity. These effects were not reduced by a peripheral opioid antagonist, methylnaloxone. Loperamide, a peripheral opioid agonist, reduced sexual motivation through an opioid-independent action. Naloxone was ineffective. Neither dopamine nor opioids seem to be important for sexual incentive motivation.

Effect of apomorphine on intracavernous pressure and blood pressure in conscious, spinalized rats

Apomorphine, given subcutaneously (s.c.), induces erection and bladder overactivity in rats through stimulation of dopamine (D1- and D2-like) receptors in the central nervous system. In paraplegic patients, apomorphine was reported to cause bladder overactivity. This suggests that apomorphine may have a spinal site of action also for stimulation of erection. The present study was initiated to evaluate the effect of apomorphine on erectile function in spinalized rats. Apomorphine (100 microg/kg, s.c.) was given to awake, unrestrained male Sprague-Dawley rats (300 g) with or without spinal cord injury, made at the Th 8 level 2 weeks before the experiment. Intracavernous pressure changes from baseline were evaluated as time to first response to apomorphine (TFR; sec), number of phasic pressure changes in the first 30 min (PP30), duration (D; sec) of the phasic pressure changes, the amount of increase in tonic peak pressure (TPP; cmH2O), and burst peak pressure (BPP; cmH2O). Blood pressure (cmH2O) was recorded via an intra-arterial catheter. Apomorphine, 100 microg/kg, caused no significant differences in TFR (217.8 vs 271.2), PP30 (6.4 vs 6.5), D (38.9 vs 37.6.), TPP (51.0 vs 54.0) and BPP (128.9 vs 160.4) between normal (n=8) and spinalized rats (n=6). However, blood pressure decreased significantly more in spinalized than in normal animals (17.7 vs 43.3; P<0.05). The results suggest that both in normal rats, and in rats with spinal cord injury, apomorphine given s.c., can produce erection. This finding supports the use of apomorphine for treatment of erectile dysfunction in paraplegia patients. However, due consideration should be given to possible decreases in blood pressure.

Adenosine: biphasic dose responses

This article characterizes the occurrence of biphasic dose responses by adenosine and its stable analogs in numerous systems, including the respiratory tract, kidney, cardiovascular system, and brain. Considerable mechanistic research on various systems explored and clarified the interactions of the A-1 and A-2 receptors and their interactive influence on the occurrence of the biphasic dose responses. In general, the maximum stimulatory response was 2.5-fold or less than controls, while the stimulatory concentration range was quite variable ranging from 10- to 10(5)-fold.

Regulation by the medial amygdala of copulation and medial preoptic dopamine release

The medial preoptic area (MPOA) is a critical integrative site for male copulatory behavior in most vertebrate species. Extracellular dopamine (DA) is increased in the MPOA of male rats immediately before and during copulation. DA agonists microinjected into the MPOA of male rats facilitate and DA antagonists inhibit sexual behavior. A major source of input to the MPOA is the medial amygdala (MeA), which processes and relays olfactory information to the MPOA. We now report that microinjections of a DA agonist into the MPOA of animals with excitotoxic lesions of the amygdala restored copulatory ability that was lost after the lesions. Moreover, radio-frequency lesions of the MeA impaired copulation and blocked the increases in extracellular DA seen in animals with sham lesions during exposure to a receptive female and during copulation. Thus, both copulatory ability and the MPOA DA response, during exposure to a receptive female and during copulation, are facilitated by input from the MeA to the MPOA.

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.

Prenatal morphine exposure differentially alters TH-immunoreactivity in the stress-sensitive brain circuitry of adult male and female rats

Previously, we demonstrated that exposure to morphine during gestation increases hypothalamic norepinephrine (NE) content and turnover rate in adult male rats and decreases these measures in adult females. To investigate the basis of these alterations, the present study examined the effects of prenatal exposure to morphine on tyrosine hydroxylase immunoreactivity (TH-IR) in the brains of adult male and female progeny. In male rats, prenatal morphine exposure significantly increased the density of TH-IR in cells and fibers in the caudal paraventricular nucleus of the hypothalamus (PVN) and locus coeruleus (LC), but had no effects in the lateral hypothalamus (LH). In female rats that were ovariectomized (OVX), prenatal morphine exposure significantly decreased the density of TH-IR in cells and fibers in the LC. Interestingly, an injection of estrogen in OVX control females reduced the mean optical density of TH-IR in the LC, but it was ineffective in drug-exposed females in the same brain region. Estrogen injections also reduced the mean optical density of TH-IR in the LH but not in the PVN of females, regardless of prenatal drug exposure. Thus, the present study suggests that prenatal morphine exposure induces long-term, sex-specific alterations in TH-IR in the PVN and LC of adult progeny.

AC pulsed electromagnetic fields-induced sexual arousal and penile erections in Parkinson's disease

Sexual dysfunction is common in patients with Parkinson's disease (PD) since brain dopaminergic mechanisms are involved in the regulation of sexual behavior. Activation of dopamine D2 receptor sites, with resultant release of oxytocin from the paraventricular nucleus (PVN) of the hypothalamus, induces sexual arousal and erectile responses in experimental animals and humans. In Parkinsonian patients subcutaneous administration of apomorphine, a dopamine D2 receptor agonist, induces sexual arousal and penile erections. It has been suggested that the therapeutic efficacy of transcranial administration of AC pulsed electromagnetic fields (EMFs) in the picotesla flux density in PD involves the activation of dopamine D2 receptor sites which are the principal site of action of dopaminergic pharmacotherapy in PD. Here, 1 report 2 elderly male PD patients who experienced sexual dysfunction which was recalcitrant to treatment with anti Parkinsonian agents including selegiline, levodopa and tolcapone. However, brief transcranial administrations of AC pulsed EMFs in the picotesla flux density induced in these patients sexual arousal and spontaneous nocturnal erections. These findings support the notion that central activation of dopamine D2 receptor sites is associated with the therapeutic efficacy of AC pulsed EMFs in PD. In addition, since the right hemisphere is dominant for sexual activity, partly because of a dopaminergic bias of this hemisphere, these findings suggest that right hemispheric activation in response to administration of AC pulsed EMFs was associated in these patient with improved sexual functions.

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

PURPOSE

To investigate whether the paraventricular nucleus of hypothalamus (PVN) is involved in the central regulation of apomorphine-induced penile erection in the rat, and to decipher dopamine receptor subtypes in the PVN that are involved in apomorphine-induced penile erection.

MATERIALS AND METHODS

Male adult Sprague-Dawley rats (200 to 300 gm.) anesthetized with pentobarbital sodium were used. The intracavernous pressure (ICP), recorded along with systemic and mean arterial pressure (SAP, MAP) as well as heart rate (HR), was measured via a 26-gauge needle inserted into one corpus cavernosum. The PVN was activated by stereotaxically delivered apomorphine hydrochloride (0.1 nmol./100 nl.). Injection of saline into PVN served as a vehicle control. To investigate the participation of dopamine receptor subtypes in the PVN on apomorphine-induced penile erection, D1 or D2 receptor antagonist, SCH-23390 (100 pmol./100 nl.) or sulpiride (100 pmol./100 nl.) respectively, was administered into the PVN prior to subcutaneous application of apomorphine (80 microg./kg.). The effects on ICP of microinjection of D1, D2 or D3 receptor agonist, SKF-38393 (200 pmol./100 nl.), lisuride (200 pmol./100 nl.) or 7-hydroxy-DPAT (200 pmol./100 nl.) respectively, into the PVN were also evaluated.

RESULTS

The mean resting ICP was 5.2+/-0.4 mm. Hg. Upon administration of apomorphine into the PVN, there was a significant increase in ICP that peaked at 50.7+/-5.3 mm. Hg and persisted for 45.2+/-18.0 minutes after an onset latency of 677.7+/-311.6 seconds. Yawning and teeth gnashing were also observed in most of animals during the period of ICP increase. There was no significant change in SAP, MAP or HR. In addition, there was no elevation in ICP after administration of saline to the PVN or direct injection of apomorphine into the cavernous tissue. Microinjection of D1 or D2 receptor antagonist into the PVN blocked the increase in ICP after subcutaneous administration ofapomorphine. Direct application of D2, but not D1 or D3 receptor agonist into the PVN, on the other hand, increased the ICP.

CONCLUSIONS

Our results demonstrate that application of apomorphine to the paraventricular nucleus of hypothalamus elicited penile erection in the rat. Such an increase in ICP to apomorphine was due mainly to activation of the D2 receptor subtype in the PVN. These observations indicate that PVN may be involved in the central regulation of apomorphine-induced penile erection in the rat.

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.

Effects of adenosine analogues on apomorphine-induced penile erection in rats

1. In the present work, the effect of adenosine agonists and antagonists on apomorphine-induced penile erection (PE) has been studied. 2. Subcutaneous (s.c.) injection of the nonselective D1/D2 dopamine receptor agonist apomorphine (0.05-0.5 mg/kg) induced PE in a biphasic manner. The maximum effect was obtained with 0.1 mg/kg of the drug. The response decreased with increasing doses of apomorphine, from 0.1 to 0.5 mg/kg. 3. Intraperitoneal (i.p.) injections of adenosine agonists 5'-N-ethylcarboxamidoadenosine (NECA) and N6-cyclohexyladenosine (CHA) decreased the response of apomorphine. Apomorphine-induced PE was increased by low doses (25, 50 mg/kg, i.p.) and decreased by high doses (75, 100 mg/kg, i.p.) of the adenosine antagonist theophylline, respectively. Inhibition of PE induced by NECA and CHA was antagonized by 8-PT pretreatment. 4. Intracerebroventricular (i.c.v.) administration of CHA, NECA, and theophylline produced the same effects as i.p. injections of these agents on PE responses. It is concluded that A-1 and A-2 adenosine receptor activation may inhibit PE induced by dopaminergic mechanism(s), which can be prevented by 8-PT pretreatment.

Effects of chronic lithium pretreatment on apomorphine-induced penile erection

1. The effects of chronic lithium pretreatment (600 mg/l in drinking rats, 30 days) on penile erection (PE) induced by apomorphine were investigated in rats. This treatment resulted in a serum Li concentration after 30 days of 0.31 +/- 0.01 mmol/l. 2. Subcutaneous (s.c.) administration of mixed D1/D2 dopamine receptor agonist apomorphine (0.05-0.5 mg/kg) induced PE in a biphasic manner. The maximum effect was obtained with 0.1 mg/kg of the drug while the response decreased with increasing doses of apomorphine from 0.1 to 0.5 mg/kg. 3. Pretreatment of animals with 0.0125-0.1 mg/kg of D1 dopamine receptor antagonist SCH 23390 or D2 dopamine receptor antagonist sulpiride (12.5-100 mg/kg) decreased apomorphine-induced PE. Combination of SCH 23390 (0.025 mg/ kg) with sulpiride (12.5 mg/kg) caused a stronger inhibitory effect on apomorphine response. This indicates that both D1 and D2 dopamine receptors may be involved in PE induced by apomorphine. 4. The response induced by apomorphine (0.05-0.05 mg/kg) was decreased in animals pretreated with chronic lithium. The inhibitory effect of sulpiride on apomorphine response, increased in animals pretreated with lithium, in contrast the inhibitory effect of SCH 23390 did not change in this condition. However, a combination of SCH 23390 with sulpiride increased inhibitory effect on apomorphine response in lithium pretreated rats. 5. It is concluded that chronic lithium inhibits PE induced by dopaminergic mechanism(s).

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.

Effects of GABA-ergic drugs on penile erection induced by apomorphine in rats

The effects of GABA agonists and antagonists on penile erection (PE) induced by apomorphine were investigated in rats. Subcutaneous (SC) administration of apomorphine (0.01-0.1 mg/kg) induces a dose-dependent PE in rats. The maximum effect was obtained with 0.1 mg/kg of the drug. The response was decreased with increasing doses of apomorphine from 0.1 to 0.5 mg/kg. The response induced by apomorphine (0.1-0.5 mg/kg) was decreased in animals pretreated with either the GABA-A agonist muscimol or the GABA-B agonist baclofen. Combination of muscimol with baclofen caused a stronger inhibitory effect on apomorphine-induced PE. Bicuculline or picrotoxin but not phaclofen reduced the inhibitory effect of muscimol on PE induced by apomorphine, whereas phaclofen but not GABA-A antagonists decreased the inhibitory action of baclofen on apomorphine-induced PE. Pretreatment of animals with higher doses of the GABA-A antagonists bicuculline and picrotoxin or the GABA-B antagonist phaclofen elicited inhibition of apomorphine-induced PE. However, the inhibitory effects of GABA-A and GABA-B antagonists are lost on combination. Administration of GABA-A and GABA-B receptor stimulation inhibit PE induced by dopaminergic mechanism(s).

Effects of copulation on apomorphine-induced erection in rats

By testing the effects of antecedent copulation on subsequent apomorphine-induced penile erection we sought to test an implicit assumption in the research on drug-induced "spontaneous" erection--namely, that this research provides information relevant to the regulation of erection in copula. In experiment 1, male rats were observed after being injected SC with 0, 15, 30, 60, or 120 micrograms/kg apomorphine (APO); 60 micrograms/kg yielded the maximum probability of erection and yawning. In experiment 2, males were injected with 60 micrograms/kg APO after no exposure to females, after three intromissions, or after copulation to sexual satiety. There was no significant effect of three intromissions, but sexually sated males displayed no erections, the first evidence that copulation affects drug-induced erections. In experiment 3, males had one ejaculation, three intromissions, or no exposure to females immediately before injection with APO (60 micrograms/kg, SC) or ascorbic acid vehicle. APO induced both erection and yawning, but neither behavior was reliably affected by copulation in APO-treated males. Among vehicle-treated males, those having three intromissions or one ejaculation before the test had shorter erection latencies and more erections than males not exposed to females. Thus, a relatively small amount of copulation resulted in a level of erectile response similar to that of APO-treated males. Optimal doses of APO may be no more effective in promoting erection in male rats than are the natural neurochemical sequelae to copulation.

Effects of castration and exogenous testosterone supplementation in an animal model of penile erection

The dependence of erectile behavior on androgen functioning is well established. Castration produces loss of both libido and potency in man and animals. The present study, using an animal model for potency, demonstrates the dependence of centrally induced erectile behavior on an intact androgen milieu. Castrated rats failed to produce an erection in response to apomorphine, an agent shown to produce erection in nearly all normal rats. Administration of exogenous testosterone propionate in dosages exceeding 60 micrograms./kg. produced a significant increase in erectile behavior. Yawning, an essentially parallel phenomenon to the stimulation of the erectile response, was also decreased following castration and responded similarly to increasing amounts of exogenous testosterone, demonstrating the influence of androgen functioning on the central nervous system. It was concluded that testosterone is a necessary prerequisite for the maintenance of a centrally induced erectile and yawning response. In an animal model of penile erection, testosterone increases the number of erections in a dose-dependent manner in castrated rats. The dependence of the erectile response on testosterone is, at least in part, centrally mediated.

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.

Effects of streptozotocin-induced diabetes on dopaminergic functioning in the rat: analysis of yawning behavior

Apomorphine, a dopamine receptor agonist, causes yawning in rats. It has been suggested that the analysis of yawning behavior provides an index of dopamine autoreceptor function. Dopamine turnover in the substantia nigra of diabetic rats has been shown to be decreased following administration of amphetamine or apomorphine (17,21). Yawning behavior after 4 weeks of streptozotocin (STZ)-induced diabetes in Wistar rats was significantly lowered when compared with their age-matched normal controls. Yawning behavior was not further diminished after an 8-week duration of diabetes mellitus; however, a significant recovery in yawning was seen by 20 weeks of diabetes. Yawning in rats after 20 weeks of STZ-induced diabetes mellitus is not significantly different from that seen in normal control rats. The results suggest that in STZ-induced diabetes of only 4 weeks duration a measurable change in the substrate for yawning has occurred.

Neuroleptics induce penile erection in the rabbit

1. Intramuscular (i.m.) administration of the neuroleptics chlorpromazine, haloperidol and spiperone at doses ranging from 0.1 to 0.4 mg/kg in male rabbits induced a dose-dependent penile erection. 2. The i.m. administration of the alpha 1-adrenoceptor antagonists prazosin and bunazosin (0.1-0.4 mg/kg), induced a dose-dependent penile erection. However, that of the peripheral dopamine receptor antagonist domperidone (0.4-4.0 mg/kg) and the dopamine receptor agonist apomorphine (0.1-1.0 mg/kg) did not. Penile erection was not induced by i.m. injection of chlorpromazine in combination with intrapenile administration of the alpha 1-adrenoceptor agonist methoxamine. 3. Penile erection was induced by the administration of chlorpromazine (0.25-1.00 mg/body) into the lateral cerebral ventricle. At a low dose, however, the administration of chlorpromazine into the lateral ventricle induced a less notable penile erection than that induced intramuscularly. 4. Penile erection was induced by i.m. injection of the ganglionic blocker hexamethonium (5-20 mg/kg). When chlorpromazine was given after pretreatment with hexamethonium, penile erection was more notable than that induced by either drug given alone. 5. These results suggest that neuroleptics could act locally in the penile structure to cause penile erection by alpha 1-adrenoceptor-blocking actions.

Dopamine functions in appetitive and defensive behaviours

The data reviewed here are compatible with the hypothesis that telencephalic dopamine activity is elicited by motivationally significant stimuli which in turn creates a neural state in which animals are more prepared to respond to significant stimuli in the environment. This analysis may be viewed as extensions of both the sensorimotor hypothesis, which depicts dopamine as potentiating the ability of stimuli to elicit responses (Clody and Carlton, 1980; Marshall et al., 1974; White, 1986) and of the incentive motivational hypothesis, which emphasizes the importance of dopamine in responding to stimuli that serve as signals of biologically significant events (Blackburn et al., 1989a; Crow, 1973; Mogenson and Phillips, 1976). In addition, we have sought to emphasize that not all responses are equally dependent upon the integrity of forebrain dopamine activity. Some responses, such as ingestion of standard foods by hungry animals, copulation, and escape, are relatively impervious to dopamine disruption. Further, once other behaviours, such as avoidance or appetitive operant responses, have been acquired, they can be maintained at an initially high rate despite perturbation of dopamine systems, although performance deteriorates with repeated testing. This analysis has emerged from the joint consideration of how both appetitive and defensive behaviours are influenced by dopamine antagonists, along with an examination of dopamine release during sequences of behaviour. The data reviewed suggest that dopamine is involved in fundamental psychological processes through which environmental stimuli come to exert control over certain aspects of behaviour. In the future, as knowledge in this field advances, there will have to be an integration of the literature on dopamine and motivation with the literature on dopamine and motor systems. We expect that dopamine release will be seen as a mechanism by which important environmental cues, of innate or learned significance, lead to a general enhancement of motor skeletal responses directed towards distal cues. We conclude with a caveat: Caution must be exercised when attempting to infer a general role of any neurotransmitter in motivated behaviour based on the study of a limited number of motivational systems. Although neurotransmitter pathways may figure prominently in the control of certain behaviours, it is incorrect to think of neurotransmitters as having a single role in behaviour. However, when comparative analyses reveal a common thread among different motivational systems, as is becoming apparent for the general role of mesotelencephalic dopamine pathways in behaviour, then the goal of generating coherent and comprehensive theory concerning a neurotransmitter's function in behaviour will begin to be realised.(ABSTRACT TRUNCATED AT 400 WORDS)

Opposite influences of dopaminergic receptor subtypes on penile erection

1. Mixed D-1/D-2 dopamine agonist apomorphine induced a penile erection (PE) in rats in a biphasic manner. 2. The response was decreased with increasing doses of the drug. 3. The maximum effect was obtained by 0.1 mg/kg of apomorphine. 4. In animals pretreated with D-1 antagonist SCH 23390, high doses of apomorphine showed higher PE response, while D-2 antagonist sulpiride pretreatment decreased the response of the low doses of the drug. 5. The inhibitory effect of sulpiride was dose-dependent. 6. The D-2 agonists bromocriptine or quinpirole induced a dose-dependent PE. 7. The effects of both drugs were decreased by sulpiride or SKF 38393 pretreatment. 8. Cholinergic drugs physostigmine and neostigmine did not induce PE, but antimuscarinic agent atropine decreased the effects of apomorphine, bromocriptine or quinpirole. 9. It is concluded that D-2 dopamine receptor stimulation may induce PE, while D-1 activation elicit an opposite effect. 10. However, cholinergic stimulation is not able to induce PE, cholinergic inhibition may decrease the PE induced by dopaminergic agents.

Age-related changes in apomorphine-induced erections

Advancing age produces a noticeable and well-documented decline in erectile function in humans. The effects of aging on the ability of apomorphine to stimulate erection and yawning behavior in rats was studied in our bioassay for potency. At the age of seven months, rats failed to respond to the same dose of apomorphine which, just one month earlier, produced erections. Erectile function was then tested in thirty-two seven-month-old rats naive to apomorphine injections, and these rats also failed to respond. Experimentally naive rats of six months of age were then tested and apomorphine produced reliable erections. It is felt that an alteration in dopamine autoreceptor function may be occurring in the central nervous system of rats at approximately seven months of age rendering them incapable of responding to apomorphine with penile erections.

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.

The characterization of a bio-assay of erectile function in a rat model

The investigation of biological phenomena in impotence using an animal system requires a determination of the erectile capabilities of the animal. Rats respond reliably to apomorphine by the exhibition of a phenomenon of erections and yawns. This property has been used to form the basis of a bio-assay of erectile integrity in the rat. We compared rats treated with placebo alone, sham operated rats, rats rendered surgically impotent and castrated rats with and without testosterone. Rats did not respond to placebo. The sham operated rats remained normal in all measured respects (2.66 erections/rat/30 minutes). Surgically impotent rats yawned normally but had no erections. Castrated rats did not have erections and had diminished yawning (3.21 yawns/rat/30 minutes vs. 7.7 for controls p less than .001) but responded normally after testosterone administration. The bio-assay is useful as a standard test of erectile function in the rat.

Metoclopramide decreases apomorphine-induced yawning and penile erection

Acute administration of metoclopramide, a dopamine (D2) antagonist, reduced both apomorphine-induced yawning and penile erections. Metoclopramide, prominent in clinical use as an effective antiemetic, has been shown to be associated with decreased erectile function in humans. Experimentally naive rats were given a standardized dose of apomorphine and one of a range of doses of metoclopramide. The study shows that metoclopramide decreases the erectile response to apomorphine and suggests that the erectile difficulties experienced in humans after metoclopramide treatment may be a result of interference with a central dopaminergic mechanism(s).

Microinjection of the dopamine antagonist cis-flupenthixol into the MPOA impairs copulation, penile reflexes and sexual motivation in male rats

Microinjection of the dopamine antagonist cis-flupenthixol into the medial preoptic area was previously shown to impair male rat copulatory behavior. The present experiments provide further evidence of cis-flupenthixol's inhibitory effects on male sexual behavior. Following microinjections of moderate to high doses of cis-flupenthixol, males exhibited slower copulatory rates and fewer ejaculations in copula, fewer ex copula erections and penile movements, and reduced sexual motivation in an X-maze. Locomotion in the X-maze was not significantly affected. Microinjections of the inactive isomer trans-flupenthixol produced no change in any behavioral measure, indicating that cis-flupenthixol's effects were receptor mediated. We suggest that dopamine receptors in the MPOA influence copulation primarily by regulating reflexive and motivational factors, but not locomotion.

Projections of the paraventricular nucleus of the hypothalamus to the sexually dimorphic lumbosacral region of the spinal cord

Lumbar regions L5-L6 of the spinal cord of the male rat contain the sexually dimorphic motor nuclei, the spinal nucleus of the bulbocavernosus (SNB) and the dorsolateral nucleus (DLN), which innervate perineal muscles, the bulbocavernosus and the ischiocavernosus, respectively. This neuromuscular system controls penile reflexes which are essential to male reproductive success. Oxytocin has been shown to induce penile reflexes and the site of action for these effects is the PVN. Since PVN is known to project to cervical and thoracic levels of spinal cord, the present study examined projections of the PVN to the L5-L6 region of the spinal cord. WGA-HRP was injected into the region of L5-L6, aimed at the SNB and DLN and their dendritic extents, in intact male, castrated male and female rats. WGA-HRP-labelled cells bodies were found in the parvocellular subnuclei of PVN, as well as regions of the lateral hypothalamus and the dorsal area of the hypothalamus. These results demonstrate that the PVN projects to lumbar levels of the spinal cord that are sexually dimorphic and androgen-dependent. This suggests that PVN may modulate the activity of these motoneurons.

Sexual motivation: a neural and behavioural analysis of the mechanisms underlying appetitive and copulatory responses of male rats

Experiments investigating the neural mechanisms underlying the expression of masculine sexual behaviour are discussed in the context of the hypothesis set out by Frank Beach that suggested the existence of separate sexual arousal and performance mechanisms. The results indicate that the medial preoptic area is crucially involved in consummatory aspects of sexual behaviour: lesions and chemical manipulations of the area profoundly affect mounts, intromissions and ejaculation, but tend not to alter appetitive sexual responses. By contrast, ventral striatal dopamine-dependent mechanisms primarily affect appetitive sexual responses, measured in a variety of paradigms, but tend not to alter copulatory behaviour itself. Finally, associative mechanisms, for example those by which arbitrary environmental stimuli come to control appetitive sexual responses through their predictive association with sexual reinforcement, are shown to depend at least in part on interactions between the basolateral amygdala and dopamine-dependent events in the ventral striatum. Thus, diverse neural and behavioural procedures have revealed that separable neural mechanisms appear to be involved more or less selectively with different components of the male rat's sexual response system. It may still be useful to conceptualize separate sexual arousal and intromission/ejaculatory mechanisms when studying the neuroendocrine basis of sexual behaviour. However, a major challenge is to understand the way in which elements of the telencephalic limbic system, the striatum and preoptic area, some of which are targets for the action of sex steroids, interact to produce an integrated pattern of sexual behaviour.

Apomorphine facilitates male sexual behavior of rhesus monkeys

In the present study, a novel sexual behavior paradigm was used to study the effects of the dopamine agonist, apomorphine, on male sexual behavior of rhesus monkeys. In Experiment 1, the effects of apomorphine treatment were assessed by observing the behavioral responses of male rhesus to a sexually receptive female monkey that they could see, hear, and smell, but could not physically contact. Apomorphine treatment produced a spectrum of behavioral effects that differed depending on the dose of drug administered. Low doses of apomorphine (25-100 micrograms/kg) stimulated yawning, moderate doses (50-200 micrograms/kg) facilitated male sexual responses associated with the genitals including penile erection and masturbation, and high doses (greater than 200 micrograms/kg) elicited stereotypic behavior. Experiment 2 examined whether the behavioral responses of male monkeys to apomorphine treatment were influenced by the stimulus female. Apomorphine treatment facilitated penile erections in tests in which a stimulus female was present, but did not facilitate erections in tests in which she was absent. In sum, these experiments provide preliminary evidence that dopaminergic mechanisms may play a role in the regulation of male sexual behavior of rhesus monkeys.