Central effects of RDS-127: sexual behavior after intracerebroventricular administration and in vitro receptor binding studies

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.

Sexual dysfunction with major depressive disorder and antidepressant treatments: impact, assessment, and management

INTRODUCTION

Sexual dysfunction (SD) is a symptom of depression in ≈70% of patients presenting with major depressive disorder (MDD). Antidepressant medications (AD) and adjunctive treatments may further contribute to SD and complicate evaluation and management.

AREAS COVERED

A systematic literature search of PubMed, Ovid MEDLINE and Cochrane databases for MDD, SD, classes of antidepressants, etc. was performed with a focus on 2014 to June 2021. SSRIs are associated with 70% treatment-emergent sexual dysfunction (TESD), SNRIs and tricyclics have rates of TESD of 40 - 45%, and antidepressant medications without SRI effects or with additional unique mechanisms of action have rates similar to placebo (<10%). Appropriate assessment at baseline and throughout treatment, consideration of patient preferences in prescribing, addressing modifiable factors (comorbid medical/psychiatric conditions, substances, relationship difficulties), and utilizing management strategies of switching to an AD with less SD, adding an antidote/adjunctive therapy or lowering the dose are discussed.

EXPERT OPINION

MDD and antidepressant treatment contribute to SD in a high percentage of patients. Treating to remission reduces SD as a symptom of depression. Frequent assessment and targeted management strategies may be effective in preventing or addressing SD. Secondary outcomes like impact on adherence, relationships and self-image should also be considered.

Adrenergic receptors: biphasic dose responses

Agonists and antagonists of the adrenergic receptor system were assessed for their capacity to affect dose-response relationships across biological model, tissue and endpoint. U shaped dose responses were commonly reported, affecting multiple endpoints such as smooth muscle contraction/relaxation, memory, blood pressure, sexual behavior, platelet aggregation, and others. In six of the endpoints studied, mechanistic foundations of the biphasic nature of the dose response were established. The quantitative features of the dose-response relationships indicated that the maximum stimulatory responses, with but one exception, were less than twofold greater than the controls. The range of stimulatory responses was generally within 10- to 1000-fold, except for the platelet aggregation endpoint where the range was 10(3) to 10(5).

Specific involvement of central 5-HT1A receptors in the mediation of male rat ejaculatory behavior

The aminotetralin 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), pharmacologically characterized as a 5-HT1A receptor agonist, produces a pronounced decrease in ejaculation latency in the male rat. Stimulation of 5-HT receptors by a pharmacologically induced increase in the synaptic availability of 5-HT has been shown to produce the opposite effect. The 8-OH-DPAT-induced decrease in ejaculation latency is specific for this compound, and some chemically related ergot derivatives. In this paper we review the evidence in support for stimulation of serotonergic autoreceptors of the 5-HT1A receptor subtype as a mechanism of action for effects by 8-OH-DPAT on male rat ejaculatory behavior. We also present the questions posed by the fact that quinpirole and lisuride both produce 8-OH-DPAT-like effects on male rat ejaculatory behavior. The effects by quinpirole, lisuride of 8-OH-DPAT are not sensitive to pretreatment with the DA D2/3 receptor antagonist raeclopride. Continued studies will show whether the effects of quinpirole and lisuride can be related to stimulation of 5-HT1A receptors, of all these compounds have as yet undefined common properties.

Serotonin reuptake inhibitor-induced sexual dysfunction and its treatment: a large-scale retrospective study of 596 psychiatric outpatients

In the present study, a large-scale retrospective case review was undertaken to assess the incidence and type of sexual dysfunctions associated with serotonin reuptake inhibitor (SRI) therapy, in addition to the effects of three pharmacological antidotes (yohimbine, amantadine, cyproheptadine) on SRI-induced sexual dysfunctions. A retrospective chart review was conducted on 596 patients treated with SRIs in an outpatient psychiatric practice between July 1991 and September 1994. Patients who reported new-onset sexual dysfunction during this time were categorized as having SRI-induced sexual dysfunctions. Sexual difficulties were characterized by type and duration, and the background characteristics and psychiatric diagnoses of all patients were recorded. Psychiatric outcome and sexual functioning at follow-up were independently assessed by a single psychiatrist by means of a 4-point rating scale. Sexual dysfunction symptoms were clearly associated with SRI administration in 97 (16.3%) cases. The most common problems reported were orgasmic delay or anorgasmia and hypoactive sexual desire. Sexual difficulties were more frequent among men (23.4%) and married patients of both sexes (22.3%), whereas psychiatric diagnosis and type of SRI were unrelated to the occurrence of sexual problems. Of the patients with sexual dysfunction, 45 (46.4%) opted for a trial of antidote therapy with yohimbine, amantadine, or cyproheptadine. All three antidotes were found to be safe and relatively effective, although yohimbine was significantly more effective than amantadine or cyproheptadine in reversing SRI-induced sexual dysfunction.

Central propranolol and pindolol, but not atenolol nor metoprolol, inhibit sexual behavior in male rats

Are the anti-sexual effects of propranolol and pindolol due to actions within the brain? To answer this, these agents were administered directly into the brain ventricular system (ICV). Additionally, atenolol and metoprolol were evaluated to see whether differential delivery to the brain contributed to the observed lack of effect of systemically administered beta 1-adrenoceptor antagonists. ICV administration of pindolol (45 or 90 micrograms) was followed by a suppression of copulation. At 45 micrograms, inhibition was limited to performance aspects of copulation, whereas at 90 micrograms, decrements in motivational and performance aspects of copulation were evident. ICV administration of propranolol also suppressed copulatory behavior. At 45 micrograms, no significant effects were observed, whereas at 90 micrograms decrements in motivational and performance aspects of copulation were evident. In contrast, ICV administration of the beta 1-adrenoceptor antagonists, atenolol and metoprolol, was not associated with any major modifications in copulatory behavior. We suggest that the inhibitory effects of propranolol and pindolol may involve interactions with 5-HT1A receptors in the CNS. Alternatively, it may be that the adverse effects of pindolol and propranolol are due to the simultaneous blockade of both beta 1- and beta 2-adrenoceptors.

The inhibition of sexual behavior in male rats by propranolol is stereoselective

We have previously reported that administration of racemic mixtures of propranolol was associated with a marked inhibition of mating behavior in male rats. To compare the effects of (+)-propranolol, (-)-propranolol, and (+/-)-propranolol in sexually experienced males, rats ejaculating in four or more mating tests were divided into three groups (N = 16 per group) such that no differences in parameters of copulatory behavior were evidence in preexperimental tests. No major effect of propranolol on parameters of behavior associated with initiation of sexual behavior was evident. In contrast, other measures of behavior were profoundly modified. The ejaculatory threshold, indicated by the number of intromissions preceding ejaculation, was increased after (+)- and (+/-)-propranolol, but not (-)-propranolol. The number of mounts without intromission preceding ejaculation was increased only after (+/-)-propranolol. A decrease in copulatory efficacy was evident after (-)- or (+/-)-propranolol, but not after (+)-propranolol. Increases in ejaculation latency, intercopulatory interval, and postejaculatory interval were observed after (-)- and (+/-)-propranolol, but not after (+)-propranolol. In summary, the present data indicate that the (-) isomer of propranolol is the active form necessary for the inhibitory effects of propranolol on male sexual function. We suggest that this inhibition is due to specific receptor-mediated mechanisms, involving beta-adrenoceptors and 5-HT1A receptor interactions.

Sexual function in altered physiological states: comparison of effects of hypertension, diabetes, hyperprolactinemia, and others to "normal" aging in male rats

In this review, we examine the changes in sexual function that accompany deviations from "normal" physiological states. We propose that the changes one observes in many altered physiological states should not be viewed in isolation. We describe our paradigms for assessing sexual function, and proceed to evaluate how sexual function changes with hormonal deprivation and aging, in rat models for hypertension, in severe hyperprolactinemia, in streptozotocin-induced diabetes, after chronic alcohol intake, after chronic morphine administration, and after exposure to the heavy metal, cadmium. We will provide evidence for the involvement of adrenergic transmitters and two neuropeptides, neuropeptide Y and somatostatin, in the neuroendocrine regulation of sexual behavior. Finally, we compare and contrast the changes observed relative to the changes seen in "normal" aging in rats. The sequence of age-related changes in sexual function is distinct. The first change observed is a decrement in ex copula erectile reflexes. Next are decreases in ejaculatory threshold, followed shortly by increases in initiation and reinitiation of copulation after ejaculation. This is followed by a decrement in the number of males copulating to ejaculation. Finally, there is a failure to initiate the copulatory process. This sequelae is relatively common, being evident after castration, with hyperprolactinemia, and after exposure to cadmium. The data available for sexual function in hypertension is incomplete and modified by the etiology, but a suggestion for this sequelae is seen in SHR. In contrast, sexual dysfunction associated with chronic morphine administration appears to be due to an initial deficit in motivational aspects. Testosterone reverses sexual dysfunction associated with castration, but not with idiopathic sexual inactivity, nor with sexual dysfunction associated with aging, diabetes, or chronic morphine administration. Comparing sexual function in rat models for hypertension, diabetes and chronic ethanol leads to the conclusion that increases in blood pressure, like decreases in testosterone, cannot be the primary causal factor for sexual dysfunction. Age, hormonal history of the subject, and the age at castration influence changes in sexual function. Age-related sexual dysfunction appears to be contributed to by changes in adrenergic-neuropeptidergic, to include sympathetic, systems. Site-specific administration of NPY induces alterations in parameters of copulatory behavior which mimic those seen in aging and the retention of ejaculatory behavior with aging is associated with site-selective attenuation (or reversal) of age-associated changes in NPY content. Yohimbine enhances copulatory activity in castrated and aging rats, and attenuates or reverses the antisexual effects of clonidine, epinephrine and somatostatin.(ABSTRACT TRUNCATED AT 400 WORDS)

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 apomorphine on sexual behavior in male quail

In the rat, dopamine (DA) facilitates male copulatory behavior. Indirect evidence based largely on neuroanatomical data suggest that in quail DA is also implicated in the control of male reproductive behavior but there is no pharmacological evidence to support this conclusion. To test this idea, castrated testosterone (T)-treated male quail were injected with various doses of the dopaminergic agonist apomorphine (APO) in the range 1-10,000 micrograms/kg. The sexual behavior of birds was recorded starting 15 min after APO injection for a duration of 30 min. A dose-dependent inhibition of male reproductive behavior that lasted for the entire duration of the test was observed. In a second experiment, gonadectomized T-treated male Japanese quail were injected daily with APO (0, 10, or 1,000 micrograms/kg) during 8 days. Their sexual interactions with a partner were quantified either 24 h or 15 min after the last injection. No influence of the treatment on copulatory behavior was observed 24 h after the last injection, but a strong inhibition was present when the test was performed 15 min after. To research whether the inhibitory effects of APO were due to a preferential action on D2 presynaptic autoreceptors, male quail were pretreated with two different D2 antagonists (spiperone or pimozide; 0.5 or 2 mg/kg) before being injected with APO (100 micrograms or 1 mg/kg). Spiperone facilitated male sexual behavior but did not suppress the inhibitory effect of APO. No significant effect of pimozide was observed. These results support the notion that DA modulates male sexual activity in the Japanese quail. The specific role of the different dopaminergic receptor subtypes remains, however, to be elucidated.

Suppression of copulatory behavior in male rats following central administration of clonidine

Adrenergic transmitters have been implicated in the regulation of male sexual behavior. In the present study the contribution of alpha 2-adrenoceptors, located within the central nervous system, was evaluated. Sexually experienced male Long-Evans rats were implanted with a permanent cannula in either the third cerebral ventricle or, unilaterally, in the medial preoptic area. Baseline mating tests were administered 4, 7 and 10 days after surgery and only males ejaculating (at least) in the 7 and 10 day tests were used. Clonidine-evoked dose-dependent decrements in the number of males mounting, intromitting and ejaculating, with administration into the medial preoptic area more effective than into the 3rd ventricle. In those animals mating, administration of 20 nmol into the 3rd ventricle was associated with decreases in the latency to ejaculation and intercopulatory interval, whereas administration of 2 nmol into the medial preoptic area was associated with increases in these parameters and decreases in the ejaculatory threshold. Administration of yohimbine into the medial preoptic area attenuated the effects of systemically-administered clonidine and the systemic administration of yohimbine completely prevented the copulatory suppression induced by administration of clonidine into the medial preoptic area. It is suggested that central alpha 2-adrenoceptors are important in the control of male sexual behavior and that alterations in adrenergic mechanisms in the medial preoptic area may underlie sexual dysfunction of various etiologies.

Centrally truncated neuropeptide Y analog acts as an agonist for Y1 receptors on SK-N-MC cells

The similarity of neuropeptide Y (NPY) to pancreatic polypeptide (PP), whose X-ray crystallographic structure is known, has allowed computer-assisted molecular modelling of NPY and predictions of its three-dimensional structure. Utilizing these techniques, Krstenansky et al. (Proc. Natl. Acad. Sci. U.S.A., 86 (1989) 4377-4381) reported that a centrally truncated analog of porcine NPY, [D-Cys7-Aoc8-17-Cys20]pNPY, which was designed to maintain the tertiary structure of the native molecule, bound to sites on membranes from mouse brain with even higher affinity than native NPY. As brain membranes may represent a heterogeneous mixture of receptor subtypes, we decided to characterize the activity of this analog on a defined cell line. SK-N-MC cells are a human epithelioma cell line with high-affinity receptors of the Y1 subtype which are coupled to inhibition of adenylate cyclase. (D-Cys7-Aoc8-17-Cys20]pNPY bound to receptors on SK-N-MC cells, but in contrast to membranes from mouse brain, with a lower affinity than pNPY. Furthermore, [D-Cys7-Aoc8-17-Cys20]pNPY was able to inhibit isoproterenol-stimulated cAMP production in these cells. Therefore, it appears that the central amino acids deleted from this analog are not involved in NPY binding, and biological activity can be maintained by conservation of the tertiary structure of NPY around the binding surface.

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

Dopaminergic compounds have been shown to facilitate male sexual responses in various contexts. We investigated the effects of a specific D2 dopamine receptor agonist, quinelorane (LY163502), on sexual responses elicited in the restrained supine male rat (i.e., ex copula reflex tests). Penile erections, evoked by retraction of the penile sheath, were inhibited by systemic administration of 10 micrograms/kg quinelorane; however, the occurrence of seminal emission was dramatically increased. A smaller dose of 0.25 ng/kg was without effect. In a second experiment, intracranial microinjection of quinelorane was followed by ex copula reflex tests. The medial preoptic area (MPOA) has been previously implicated in the dopaminergic regulation of male copulatory behavior. The effects of an intra-MPOA injection of quinelorane on seminal emission and erectile responses were similar to those observed following systemic administration. These results are consistent with the hypothesis that DA receptors in the MPOA are important in the regulation of male sexual behavior and suggest that D2 receptors in the MPOA may decrease ejaculatory threshold while inhibiting erectile mechanisms.

The effects of intracranial administration of the dopamine agonist apomorphine on penile reflexes and seminal emission in the rat

Previous studies employing systemic administration of the dopamine agonist apomorphine have shown that the dose response curves for apomorphine's effects on penile reflexes and seminal emission differ, suggesting that experimentally separable populations of dopamine receptors regulate these two responses. The present experiments examined the locations of central nervous system DA receptors mediating genital responses in the restrained, supine rate by injecting apomorphine into the medial preoptic area and the paraventricular nucleus through chronic, indwelling cannulae. Medial preoptic area injections facilitated penile reflexes, but not seminal emission, while paraventricular injections facilitated seminal emission. These results suggest that systemically administered apomorphine may facilitate penile reflexes by acting on the medial preoptic area and may enhance seminal emission by acting on the paraventricular nucleus.

Effects of stereoselective 5-HT1A agonists on male rat sexual behavior

The effects on male rat sexual behavior of some new stereoselective 5-HT agonists, related to 8-OH-DPAT, are presented. It was found that (+)cis-8-hydroxy-1-methyl-2-(di-n-propylamino) tetralin (8-OH-MeDPAT), as well as (-)trans-2-(2-hydroxyphenyl)-N,N-di-n-propylcyclopropylamine (2-OH-DCPA), and its 3-hydroxy-phenyl analog (3-OH-DCPA), stereoselectively facilitated the male rat sexual behavior, as evidenced by a decrease in the number of intromissions preceding ejaculation, and a shortening of the ejaculation latency. For the former two compounds, studied in further detail, the potency and efficacy appear to be of the same magnitude as previously found for 8-OH-DPAT. The results demonstrate specific 5-HT receptor involvement in the mediation of male rat sexual behavior.

A component analysis of the effects of DPAT on male rat sexual behavior

Male rat sexual behavior was examined in a variety of tests following a single acute IP injection of the serotonin1A (5-HT1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (DPAT). The objective was to determine the effects of 5-HT1A receptor stimulation on the components of sexual behavior (arousal/motivation, erection and ejaculation) using this prototypical 5-HT1A ligand. In the ex copula genital reflex test, DPAT dramatically inhibited ejaculation and the display of penile erections. When examined in the mating behavior test, DPAT caused a significant reduction in intromission frequency (IF), ejaculation latency (EL), intercopulatory intervals (ICI) and postejaculatory interval (PEI). The decrease in IF and EL indicated a lowering of the behavioral-ejaculatory threshold, while the reduction in ICI and PEI indicated a stimulation of two aspects of sexual arousal. Further tests, however, revealed complexities. Using the mounting test (with genital anesthesia) as an assessment of sexual arousal, no effect of DPAT was found. Collection and examination of the coagulated ejaculates resulting from mating (copulatory plugs) provided evidence of an impairment in ejaculation, as a result of DPAT treatment. The data suggests that 5-HT1A receptor stimulation lowers the behavioral-ejaculatory threshold despite inhibition of ejaculation in and ex copula; as well as stimulating specific aspects of sexual arousal. It remains to be determined whether the effects on arousal were either due to nonselectivity as currently believed or because 5-HT1A receptor stimulation affects alpha 2-adrenoceptor activity by some as yet undefined mechanism; and as a result modulates sexual arousal.

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

Previous studies have shown that systemic administration of the dopamine agonist apomorphine produced a biphasic effect on erection in the freely moving rat, with lower doses facilitating, and high doses inhibiting, erection. However, those studies did not distinguish between erection per se and seminal emission. The present results demonstrate that apomorphine produces a similar biphasic effect on penile reflexes in the restrained, supine rat, while facilitating seminal emission in a monophasic fashion. Haloperidol, a centrally-acting dopamine antagonist, either blocked the effects produced by apomorphine administration, or had actions opposite to those of apomorphine. Domperidone, a dopamine antagonist that does not readily penetrate the blood-brain barrier, did not antagonize apomorphine's effects, and did not affect penile responses when administered alone. These results suggest that dopamine receptors in the central nervous system regulate genital responses, and that effects on penile reflexes and seminal emission can be experimentally dissociated.

Regulation of male rat copulatory behavior by preoptic incertohypothalamic dopamine neurons

The role of dopaminergic terminals in the medial preoptic area (MPO) in the regulation of male rat copulatory behavior was investigated. A 6-hydroxydopamine (6-OHDA) injection into the MPO of animals pretreated with desipramine resulted in a small (23%) depletion of DA, and no impairment of copulatory activity. Further depletion of catecholamines with alpha-methyl p-tyrosine (AMPT) produced several deficits in the copulatory behavior of 6-OHDA-treated males, at a dose of AMPT that did not adversely affect copulation prior to 6-OHDA administration. The dose-related effects of intracranial apomorphine (APO) injections were also altered by 6-OHDA injections into the MPO. The inhibition previously found with 0.2 microgram of APO into the lateral ventricle of normal males was abolished by 6-OHDA treatment. A facilitation of copulatory behavior was observed following the injection of 0.2 microgram of APO into the MPO of 6-OHDA-treated animals, whereas this treatment did not affect the copulatory behavior of intact animals. Finally, inhibitory effects observed following an injection of 0.1 microgram of APO into the MPO of normal males were blocked by 6-OHDA administration. The relative roles of presynaptic autoreceptors and postsynaptic DA receptors in the MPO in mediating the dose-related effects of APO on copulatory behavior are discussed.

Pharmacological analysis of male rat sexual behavior

Pharmacological influences on male rat sexual behavior are reviewed in an attempt to identify neurotransmitters and their respective receptor types that regulate various factors comprising the behavioral pattern. Evidence is presented that: (1) serotonergic influence is generally inhibitory to sexual behavior, although two receptor subtypes may lower ejaculation threshold; (2) dopaminergic agonists facilitate several aspects of copulatory behavior and ex copula genital responses; (3) noradrenergic activity appears to increase sexual arousal; (4) cholinergic agonists facilitate ejaculation, or in some cases, delay or prevent initiation of copulation; (5) GABA agonists inhibit sexual responses both in and ex copula; (6) opiate agonists appear to inhibit copulation and penile reflexes, although antagonists have mixed effects; (7) ACTH and MSH peptides promote copulatory behavior and genital responses; (8) oxytocin facilitates ex copula penile responses, but may contribute to postejaculatory refractoriness; and (9) long-term exposure to prolactin inhibits sexual behavior and penile responses. Although some progress has been made in identifying neurotransmitter-receptor effects on behavioral components, copulatory behavior is complex and no drug has been found to affect only a single component. Furthermore, drug specificity is only relative.

Alpha 2-adrenoceptor antagonists and male sexual behavior: I. Mating behavior

Three alpha 2-adrenoceptor antagonists yohimbine, idazoxan, and imiloxan were compared by examining the effects of a single injection on male rat copulatory behavior. Dose ranges were: yohimbine: 0.25-8.0 mg/kg; idazoxan: 0.25-8.0 mg/kg; imiloxan: 12.5-50.0 mg/kg. Yohimbine and idazoxan administration produced significant increases in the number of animals copulating to ejaculation and all three drugs increased the rate of copulation as evidenced by reductions in ejaculation latency and intercopulatory interval. Only yohimbine significantly reduced mount latency and postejaculatory interval, but yohimbine and imiloxan significantly reduced intromission latency and idazoxan showed a similar trend. The highest yohimbine dose suppressed sexual activity. A time-course experiment with yohimbine (2.0 mg/kg) and idazoxan (4.0 mg/kg) showed stimulation at 75 min and a trend at 5. To further explore the arousal-stimulating capacity of the two more effective drugs, a mounting test with genital anesthetization was used. Yohimbine but not idazoxan showed marked increases in mounting at 1.0-4.0 mg/kg. Both drugs had a suppressive effect at the highest doses. These data support the involvement of alpha 2-adrenoceptors in the regulation of male sexual behavior, specifically by facilitating sexual arousal, with no effects on ejaculatory threshold, as measured by intromission frequency. Yohimbine is the most globally effective agent and it is likely that factors other than yohimbine's alpha 2-antagonism may play a role in its unique, consistent and broad behavioral effects.

Effects of a selective alpha 1-adrenoceptor agonist, methoxamine, on sexual behavior and penile reflexes

Methoxamine, an adrenergic agonist with selectivity for the alpha 1-adrenoceptor, when administered intraperitoneally 10 minutes prior to mating tests (1 to 5 mg/kg), effected reductions in the ejaculatory threshold, evidenced by a decrease in the number of intromissions preceding ejaculation. In mounting tests after penile anesthetization, a test which specifically assesses sexual motivation, 3 mg/kg methoxamine was without a stimulatory effect. Further, in penile reflex tests (ex copula) 1 mg/kg methoxamine was without effect, whereas 5 mg/kg decreased the number of erections, cups and flips per test, and increased the incidence of seminal emission. These data indicate a facilitation of the ejaculatory mechanism, both in and ex copula, coupled with an inhibition of erectile responses for moderate doses of methoxamine. Treatment of male rats with the alpha 2-adrenoceptor agonist clonidine (0.25 mg/kg, IP, five minutes pretest) drastically reduced the number of animals exhibiting intromissive and ejaculatory behavior in mating tests. This suppressive effect of clonidine was not prevented by prior treatment with methoxamine (3 mg/kg, 10 minutes pretest and five minutes preclonidine). Further, ST-91, a polar analog of clonidine which does not readily enter the central nervous system, was without effect on male sexual behavior. Since (1) the effects of methoxamine administration are not of similar quality or magnitude to those reported earlier after yohimbine, an alpha 2-adrenoceptor antagonist, (2) since concurrent stimulation of alpha 1- (by methoxamine) and alpha 2- (by clonidine) adrenoceptors is followed by a suppression of sexual behavior similar to that seen after clonidine alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Failure of pimozide and metergoline to antagonize the RDS-127-induced facilitation of ejaculatory behavior

Metergoline, a serotonin-receptor antagonist, when administered in either an ascorbic acid or ethanol containing vehicle was without effect on male rat copulatory behavior (3 mg/kg, 90 minutes pretest). When initially dissolved in several drops of acetic acid, however, the same dose of metergoline dramatically suppressed male rat sexual behavior. Thus, one-half of the treated rats failed to intromit and ejaculate, and those displaying the behaviors exhibited elongated intercopulatory and postejaculatory intervals. The administration of the putative dopamine-receptor agonist RDS-127 (2-N,N-di-n-propylamino-4,7-dimethoxyindane; 3 mg/kg, six minutes pretest) induced seminal emission ex copula and drastically reduced intromission frequency and ejaculation latency in copula, as well as effecting lesser reductions in the intercopulatory and postejaculatory intervals in two sequential copulatory series. RDS-127-induced seminal emission was effectively antagonized by pretreatment with the dopamine-receptor antagonist pimozide (250 micrograms/kg, two hours pretest), but not by pretreatment with metergoline. In contrast to seminal emission ex copula, pimozide pretreatment failed to antagonize the RDS-127 facilitation of ejaculatory behavior in copula. Metergoline pretreatment also failed to antagonize the RDS-127-induced facilitation of ejaculatory behavior in copula. However, RDS-127 prevented the suppressive effects of metergoline treatment, suggesting that RDS-127 has some agonistic action at serotonergic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)