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

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.

Endocrine-disrupting chemicals alter the neuromolecular phenotype in F2 generation adult male rats

Endocrine-disrupting chemical (EDC) exposures to the fetus have long-lasting effects on health and disease in adulthood. Such EDC exposure to the F1 fetuses also reaches the germ cells that become the F2 generation. Previously, we demonstrated that adult social and communicative behaviors such as ultrasonic vocalizations and mating behaviors were altered by EDCs in F2 rats, especially males. In the current study, we used the brains of these F2 males to ascertain the underlying molecular changes in the hypothalamus related to these behavioral outcomes. Their progenitors were Sprague-Dawley rat dams, treated on pregnancy days 8 to 18 with one of three treatments: a polychlorinated biphenyl (PCB) mixture, Aroclor 1221, selected because it is weakly estrogenic; the anti-androgenic fungicide vinclozolin (VIN); or the vehicle, 6% dimethylsulfoxide in sesame oil (VEH). In adulthood, F1 male and female offspring were bred with untreated partners to generate paternal or maternal lineages of the F2 offspring, the subjects of molecular work. Quantitative real-time PCR was conducted in the medial preoptic area (POA) and the ventromedial nucleus (VMN) of the hypothalamus, selected for their roles in social and sexual behaviors. Of the genes assessed, steroid hormone receptors (estrogen receptor α, androgen receptor, progesterone receptor) but not dopamine receptors 1 and 2 or DNA methyltransferase 3a expression were altered, particularly in the VIN males. Several significant correlations between behavior and gene expression were also detected. These results suggest that preconceptional exposure of male rats to EDCs at the germ cell stage alters the neuromolecular phenotype in adulthood in a lineage-dependent manner.

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.

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

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

Perinatal exposure to low levels of the environmental antiandrogen vinclozolin alters sex-differentiated social play and sexual behaviors in the rat

In this study we examined the effects of exposure to the antiandrogenic fungicide vinclozolin (Vz) on the development of two sex-differentiated behaviors that are organized by the perinatal actions of androgens. Pregnant Long-Evans rats were administered a daily oral dose of 0, 1.5, 3, 6, or 12 mg/kg Vz from the 14th day of gestation through postnatal day (PND)3. The social play behavior of juvenile offspring was examined on PND22 and again on PND34 during play sessions with a same-sex littermate. After they reached adulthood, the male offspring were examined with the ex copula penile reflex procedure to assess erectile function. Vz did not produce any gross maternal or neonatal toxicity, nor did it reduce the anogenital distance in male pups. We observed no effects of Vz on play behavior on PND22. However, the 12-mg/kg Vz dose significantly increased play behavior in the male offspring on PND34 compared with controls. The most dramatic increases were seen with the nape contact and pounce behavior components of play. The Vz effect was more pronounced in male than in female offspring. As adults, male offspring showed a significant reduction of erections at all dose levels during the ex copula penile reflex tests. The 12-mg/kg dose was also associated with an increase in seminal emissions. These effects demonstrate that perinatal Vz disrupts the development of androgen-mediated behavioral functions at exposure levels that do not produce obvious structural changes or weight reductions in androgen-sensitive reproductive organs.

Dopamine and serotonin: influences on male sexual behavior

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

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.

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.

Direct evidence of facilitative actions of dopamine in the medial preoptic area on reflexive and noncontact erections in male rats

PURPOSE

We examined the effects of alterations of the extracellular dopamine level in the medial preoptic area on 2 erectile contexts, namely reflexive and noncontact erections, in male rats.

MATERIALS AND METHODS

The extracellular dopamine level was measured in the medial preoptic area after administering the dopamine reuptake inhibitor bupropion hydrochloride (Sigma Chemical Co., St. Louis, Missouri) into the same area through a micro-dialysis tube. We measured the frequency and latency of reflexive erections, and the frequency of noncontact erection during infusion of bupropion.

RESULTS

Administration of 10 mM. bupropion was associated with significant elevation in the extracellular dopamine level in the medial preoptic area. Bupropion (1 mM.) and Ringer's solution did not induce significant alterations in dopamine in the medial preoptic area. The number of reflexive erections significantly increased and erection latency decreased during infusion of 10 mM. bupropion into the medial preoptic area. The number of noncontact erections was also increased by administering 10 mM. of drug.

CONCLUSIONS

The altered dopamine level in the medial preoptic area affected 2 distinct penile erectile contexts, suggesting that the dopamine levels in the medial preoptic area may be involved in the regulation of erection. These results may have important implications for the central regulation of penile erection.

Testosterone restoration of copulatory behavior correlates with medial preoptic dopamine release in castrated male rats

The medial preoptic area (MPOA) is an important integrative site for male sexual behavior. We have reported an increase in dopamine (DA) release in the MPOA of male rats shortly before and during copulation. Postcastration loss of copulatory ability mirrored the loss of the precopulatory DA response to an estrous female. The present study investigated the time courses of restoration, rather than loss, of the MPOA DA response to a receptive female and of copulation in long-term castrates. Male rats were castrated and tested for loss of copulatory ability 21 days later. They then received 2, 5, or 10 daily subcutaneous injections of testosterone propionate (TP, 500 microg) or oil. Microdialysate samples were collected from the MPOA during baseline, exposure to a female behind a barrier, and copulation. Extracellular DA was measured using HPLC-EC. None of the six 2-day-TP-treated animals copulated, nor did they show elevated DA release in the MPOA in the presence of a receptive female. Five of the nine 5-day-TP-treated animals ejaculated; three intromitted without ejaculating; and one failed to copulate, with all but the noncopulating animal showing elevated DA release. All of the six 10-day-TP-treated animals copulated and also demonstrated an increase in MPOA DA. None of the oil controls copulated or showed an increase in DA release. Therefore, a consistent relationship between MPOA DA release during exposure to a receptive female and the subsequent ability of the male to copulate was observed.

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.

Possible involvement of nitric oxide in pilocarpine induced seminal emission in rats

Intraperitoneal injection of pilocarpine (0.75-3.0 mg/kg) caused a dose-related seminal emission in adult male rats. The seminal emission response to 3 mg/kg of pilocarpine was greatly reduced in atropinized (5 and 10 mg/kg, SC) animals, suggesting a cholinomimetic effect. Nw-nitro-L-arginine methyl ester (5, 10, and 20 mg/kg, SC), a nitric oxide synthesis inhibitor, also inhibited the pilocarpine-induced seminal emission, which was reversed by L-arginine (600 mg/kg, SC) or by coinjection of sodium nitroprusside (0.5 mg/kg, SC). Urine analysis for levels of nitric oxide metabolites, nitrate/nitrite (NO3-/NO2-), showed marked alterations in accordance with the drug treatments. The results suggest that nitric oxide mediates the inhibitory neurotransmission responsible for seminal emission in pilocarpine stimulated rats.

Hormone-neurotransmitter interactions in the control of sexual behavior

The stimuli from a receptive female and/or copulation itself leads to the release of dopamine (DA) in at least three integrative hubs. The nigrostriatal system promotes somatomotor activity; the mesolimbic system subserves numerous types of motivation; and the medial preoptic area (MPOA) focuses the motivation onto specifically sexual targets, increases copulatory rate and efficiency, and coordinates genital reflexes. The previous (but not necessarily concurrent) presence of testosterone is permissive for DA release in the MPOA, both during basal conditions and in response to a female. One means by which testosterone may increase DA release is by upregulating nitric oxide synthase, which produces nitric oxide, which in turn increases DA release. Hormonal priming in females may also increase DA release in the MPOA, and copulatory activity may further increase DA levels in females. One of the intracellular effects of stimulation of DA D1 receptors in the MPOA of male rats may be increased expression of the immediate-early gene c-fos, which may mediate longer term responses to copulation. Furthermore, increased sexual experience led to increased immunoreactivity to Fos, the protein product of c-fos, following copulation to one ejaculation. Another intracellular mediator of DA's effects, particularly in castrates, may be the phosphorylation of steroid receptors. Finally, while DA is facilitative to copulation, 5-HT is generally inhibitory. 5-HT is released in the LHA, but not in the MPOA, at the time of ejaculation. Increasing 5-HT in the LHA by microinjection of a selective serotonin reuptake inhibitor (SSRI) increased the latency to begin copulating and also the latency to the first ejaculation, measured from the time the male first intromitted. These data may at least partially explain the decrease in libido and the anorgasmia of people taking SSRI antidepressants. One means by which LHA 5-HT decreases sexual motivation (i.e. increases the latency to begin copulating) may be by decreasing DA release in the NAcc, a major terminal of the mesolimbic system. Thus, reciprocal changes in DA and 5-HT release in different areas of the brain may promote copulation and sexual satiety, respectively.

A nitric oxide synthesis inhibitor administered into the medial preoptic area increases seminal emissions in an ex copula reflex test

Nitric oxide synthesis inhibitors, when administered systemically or into the ventricles of the brain, affect several indices of male sexual behavior. Some of the systemic effects are assumed to be due to local vasoconstriction at the penis. Others are suggested to be mediated within the brain. In these experiments, the nitric oxide synthesis inhibitor L-NMMA, and its less active enantiomer, D-NMMA, were microinjected into the medial preoptic area of male rats. In an ex copula test of genital reflexes, L-NMMA increased the number of seminal emissions, while D-NMMA had no effect. These results are consistent with the hypothesis that nitric oxide is a tonic inhibitor of sympathetic nervous system tone, possibly in part through an influence on dopamine synthesis or release.

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.

Dopaminergic drugs in the medial preoptic area and nucleus accumbens: effects on motor activity, sexual motivation, and sexual performance

In two experiments, dopamine agonists and/or antagonists were injected into the medial preoptic area (MPOA) or the nucleus accumbens (NAcc) of male rats. The animals were then tested in an X-mase with four goal boxes, which contained a receptive female, a male, or were empty. In Experiment 1, the D1 antagonist SCH-23390 and the D2 antagonist raclopride in the MPOA decreased the percentage of trials on which the female's chamber was chosen, a measure of sexual motivation. Raclopride also decreased the number of animals that copulated after choosing the female's chamber. The 10-micrograms dose of the D3/D2 agonist quinelorane increased the latency to reach the female's chamber, slowed the onset of copulation, and decreased the number of intromissions preceding an ejaculation. In Experiment 2, 1- and 5-micrograms doses of quinelorane and of the mixed D1/D2 agonist apomorphine were injected bilaterally into the NAcc. Both doses of quinelorane increased the number of times that the subject did not select a chamber within 60 s. No drug in the NAcc affected specifically sexual motivation or performance. The results are consistent with differential influence of the MPOA and the NAcc on motor activity, sexual motivation, and sexual performance.

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.

Effects of haloperidol, quinelorane, and lithium on regional neurotensin/neuromedin N concentrations: further evidence for neurotensin/neuromedin N-dopamine interactions

In order to further characterize the pharmacologic mechanisms that mediate the antipsychotic drug-induced increase in neurotensin (NT) in nucleus accumbens and striatum, the effects of three weeks treatment with psychotherapeutic levels of lithium alone or in conjunction with haloperidol were compared to the ability of haloperidol alone to alter NT and neuromedin N (NMN) regional brain concentrations in rats. A separate experiment examined the ability of a selective dopamine D2 receptor agonist, quinelorane, to alter NT/NMN regional concentrations after three weeks of treatment as compared to haloperidol, a D2 receptor antagonist. Haloperidol (1 mg/kg) increased both NT and NMN concentrations in several brain regions and these parallel peptide increases were highly correlated. Lithium chloride (0.4 mM) had no effect, either alone or with haloperidol, on NT/NMN concentrations. Quinelorane (1 mg/kg), however, effectively increased both NT and NMN concentrations in the caudate nucleus and nucleus accumbens, as did haloperidol (2 mg/kg). These data indicate that the induction of NT and NMN, whose adjacent sequences are contained in a pro-hormone product of a single gene, occurs in tandem and remains proportional, as well as demonstrating that putative D2 receptor agonists can produce effects on NT/NMN systems that are similar to D2 receptor antagonists.

A D1 agonist in the MPOA facilitates copulation in male rats

The classic dopamine agonist apomorphine, microinjected into the medial preoptic area (MPOA), enhances the copulatory behavior of male rats, while pharmacological blockade of endogenous dopamine inhibits sexual behavior. We now report that MPOA injections of 10 micrograms of the selective D1 agonist dihydroxyphenyl-tetrahydrothienopyridine (THP) significantly increased the number of ejaculations, while decreasing the latency to ejaculate in a 30-min test. These effects were not observed following coadministration of the selective D1 antagonist SCH-23390 with 10 micrograms THP. This enhancement may be related to a D1-stimulated facilitation of penile erections.

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.

Opposite influence of medial preoptic D1 and D2 receptors on genital reflexes: Implications for copulation

Dopamine D1 and D2 receptors may synergize with or oppose each other's effects. We suggest that stimulation of D1 and D2 receptors in the medial preoptic area (MPOA) of male rats have opposing effects on genital reflexes. In Experiment 1 a D1 agonist injected into the MPOA increased the number of ex copula erections but decreased the number of seminal emissions. In Experiment 2 a D1 antagonist had the opposite effects (decreased erections and increased seminal emissions), as had a D2 agonist previously. We also suggest that D1 and D2 mechanisms in the MPOA have different thresholds of activation. In Experiment 3 a low dose of the mixed D1/D2 agonist apomorphine increased erections and anteroflexions, an effect blocked by the D1 antagonist. In Experiments 3 and 4 a high dose of apomorphine increased seminal emissions, an effect blocked by the D2 antagonist. Thus, low levels of dopaminergic stimulation may facilitate erections and anteroflexions (controlled by the parasympathetic system and striated muscles) via D1 receptors; higher or more prolonged stimulation may shift to seminal emission (controlled by the sympathetic system) via D2 receptors. This may explain the progression from erectile to ejaculatory mechanisms during copulation.

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.