A contextual definition of male sexual arousal

Sexual arousal is a construct without a widely shared definition. Historically, sexual arousal has usually referred to a central physiological state, but there has been much less agreement on its relation to motivation, emotion, and - for males - penile erection and ejaculation. Many behavioral and physiological measures have been used as operational definitions of sexual arousal, but the relation of the measure to arousal is often assumed rather than tested. For men, penile erection in the presence of erotic stimuli has been considered the most reliable and valid indicator of sexual arousal. The adoption of analogous criteria is recommended for research on other male mammals in order to establish a minimal basis for inferring that they are sexually aroused. That is, sexual arousal should be inferred only when penile erection is observed in a sexual context. A sexual context is provisionally defined as an environment that tends in most reproductively active males of the species to provoke further sexual stimulation, e.g., copulation or self-stimulation to ejaculation. Erection occurring outside of a sexual context, as during REM sleep or from injection of drugs, is not grounds for inferring arousal. Conversely, males engaging in behavior directed toward estrous females may be sexually motivated, but in the absence of erection, the males should not be assumed to be sexually aroused. Implications of other erection-context interactions are also considered. Adoption of these more conservative criteria for inferring sexual arousal may promote greater precision in identifying the physiological systems mediating this hypothetical construct.     

Sexual arousal, is it for mammals only?

Sexual arousal has many dimensions and has consequently been defined in various ways. In humans, sexual arousal can be assessed based in part on verbal communication. In male non-human mammalian species, it has been argued that arousal can only be definitively inferred if the subject exhibits a penile erection in a sexual context. In non-mammalian species that lack an intromittent organ, as is the case for most avian species, the question of how to assess sexual arousal has not been thoroughly addressed. Based on studies performed in male Japanese quail, we argue that several behavioral or physiological characteristics provide suitable measures of sexual arousal in birds and probably also in other tetrapods. These indices include, the performance of appetitive sexual behavior in anticipation of copulation (although anticipation and arousal are not synonymous), the activation of specific brain area as identified by the detection of the expression of immediate early genes (fos, egr-1) or by 2-deoxygucose quantitative autoradiography, and above all, by the release of dopamine in the medial preoptic area as measured by in vivo dialysis. Based on these criteria, it is possible to assess in birds sexual arousal in its broadest sense but meeting the more restrictive definition of arousal proposed for male mammals (erection in an explicit sexual context) is and will probably remain impossible in birds until refinement of in vivo imaging techniques such fMRI allow us to match in different species, with and without an intromittent organ, the brain areas that are activated in the presence of specific stimuli.     

Restoration of male sexual behavior by adult exogenous estrogens in male aromatase knockout mice

We previously found that male aromatase knockout (ArKO) mice that carry a targeted mutation in exons 1 and 2 of the CYP19 gene and as a result cannot aromatize androgen to estrogen show impaired sexual behavior in adulthood. To determine whether this impairment was due to a lack of activation of sexual behavior by estradiol, we studied here male coital behavior as well as olfactory investigation of sexually relevant odors in male ArKO mice following adult treatment with estradiol benzoate (EB) or dihydrotestosterone propionate (DHTP). Again, we found that gonadally intact ArKO males show pronounced behavioral deficits affecting their male coital behavior as well as their olfactory investigation of volatile body odors but not that of soiled bedding. Deficits in male coital behavior were largely corrected following adult treatment with EB and the androgen DHTP, suggesting that estradiol has prominent activational effects on this behavior. By contrast, adult treatment with EB to either castrated or gonadally intact ArKO males did not stimulate olfactory investigation of volatile body odors, suggesting that this impairment may result from a lack of proper organization of this behavior during ontogeny due to the chronic lack of estrogens. In conclusion, the present studies suggest that the behavioral deficits in sexual behavior in male ArKO mice result predominantly from a lack of activation of the behavior by estrogens. This is in contrast with earlier pharmacological studies performed on rats and ferrets that have suggested strong organizational effects of estradiol on male sexual behavior.     

Sexual incentive motivation in male rats requires both androgens and estrogens

In Experiment 1 castrated male rats were implanted with a Silastic capsule containing either E or cholesterol (CHOL) 35 days after castration. They were then tested for sexual incentive motivation and copulatory behaviors every 5th day for 3 weeks. None of the treatments affected sexual incentive motivation. After the last test, all subjects were implanted with DHT-containing Silastic capsules, and tests continued for another 3 weeks. While E + DHT enhanced sexual incentive motivation and copulatory behavior, DHT alone failed to do so. In Experiment 2 the aromatase inhibitor fadrozole (F) was combined with testosterone (T). T restored all behaviors to the level seen in intact rats, and F significantly reduced these effects. In fact, T + F was not different from DHT. T and DHT restored the weight of the prostate and seminal vesicles to levels close to those of intact rats. In Experiment 3 a lower dose of E was employed. Also this dose of E failed to affect sexual incentive motivation while E + DHT restored it to the level of intact animals. Castration enhanced the serum concentrations of LH and FSH. E alone caused a marked reduction, and E + DHT brought both gonadotropins back to the level of intact animals. It was concluded that the doses of E and DHT employed in these experiments were within or close to the physiological range, and that such doses of E completely fail to enhance sexual incentive motivation in castrated animals. DHT has small or no effects. It appears that sexual incentive motivation and copulation require simultaneous stimulation of androgen and estrogen receptors.     

Neuropeptide Y: A potent inducer of consummatory behavior in rats

Neuropeptide Y (NPY) is a 36 amino acid peptide with potent cardiovascular effects. In the present study, intraventricular injection of NPY was shown to markedly stimulate feeding and drinking during the illuminated period of the light/dark cycle, a time when rats ingest small amounts of food. It also enhanced nocturnal food and water intake following a 24 hour period of food deprivation and during nocturnal feeding. The NPY induction of food intake was suppressed by the opiate antagonist, naloxone, and by the dopamine antagonist, haloperidol. Phentolamine, an alpha adrenergic antagonist, failed to suppress NPY-induced feeding. Based on the maximum quantity of food which was ingested following central administration of NPY, this peptide appears to represent one of the most potent stimulators of feeding yet to be described.     

Oxytocin in the medial preoptic area facilitates male sexual behavior in the rat

Oxytocin (OT) is a versatile neuropeptide that is involved in a variety of mammalian behaviors, and its role in reproductive function and behavior has been well established. The majority of pharmacological studies of the effects of OT on male sexual behavior have focused on the paraventricular nucleus (PVN), ventral tegmental area (VTA), hippocampus, and amygdala. Less attention has been given to the medial preoptic area (MPOA), a major integrative site for male sexual behavior. The present study investigated the effects of intra-MPOA administration of OT and (d(CH2)51, Tyr(Me)2, Thr4, Orn8, Tyr-NH29)-vasotocin, an OT antagonist (OTA), on copulation in the male rat. The relationship between OT receptor (OTR) binding levels in the MPOA and sexual efficiency was also explored. Microinjection of OT into the MPOA facilitated copulation in sexually experienced male rats, whereas similar injections of an OTA inhibited certain aspects of copulation but had no significant effect on locomotor activity in an open field. Contrary to expectation, sexually efficient males had lower levels of OTR binding in the rostral MPOA compared to inefficient animals. The present data suggest that OT activity in the MPOA is not necessary for the expression of male sexual behavior but is sufficient to facilitate copulatory behaviors and improve sexual efficiency in sexually experienced male rats. These data also suggest that OTR activity in the MPOA stimulates anogenital investigation, facilitates the initiation of copulation, and plays a role in the sensitization effect of the first ejaculation on subsequent ejaculations.     

Rapid effects of aromatase inhibition on male reproductive behaviors in Japanese quail

Non-genomic effects of steroid hormones on cell physiology have been reported in the brain. However, relatively little is known about the behavioral significance of these actions. Male sexual behavior is activated by testosterone partly through its conversion to estradiol via the enzyme aromatase in the preoptic area (POA). Brain aromatase activity (AA) changes rapidly which might in turn be important for the rapid regulation of behavior. Here, acute effects of Vorozole, an aromatase inhibitor, injected IP at different doses and times before testing (between 15 and 60 min), were assessed on male sexual behavior in quail. To limit the risk of committing both types of statistical errors (I and II), data of all experiments were entered into a meta-analysis. Vorozole significantly inhibited mount attempts (P < 0.05, size effect [g] = 0.527) and increased the latency to first copulation (P < 0.05, g = 0.251). The treatment had no effect on the other measures of copulatory behavior. Vorozole also inhibited appetitive sexual behavior measured by the social proximity response (P < 0.05, g = 0.534) or rhythmic cloacal sphincter movements (P < 0.001, g = 0.408). Behavioral inhibitions always reached a maximum at 30 min. Another aromatase inhibitor, androstatrienedione, induced a similar rapid inhibition of sphincter movements. Radioenzyme assays demonstrated that within 30 min Vorozole had reached the POA and completely blocked AA measured in homogenates. When added to the extracellular milieu, Vorozole also blocked within 5 min the AA in POA explants maintained in vitro. Together, these data demonstrate that aromatase inhibition rapidly decreases both consummatory and appetitive aspects of male sexual behavior.     

Relationship between male-male and male-female sexual behavior in 5-6-month-old male lambs

To characterize male–male sexual behavior during lamb development, to relate it with lamb body and testicular growth, and with sexual behavior toward estrual ewes, 40 Milchschaf male lambs, weaned at 45 days of age, were kept with ewes that were nursing younger lambs. Experimental lambs were weighed and scrotal circumference was measured every 2 weeks. Male–male sexual behavior was observed during 1–2 h every 2 weeks after birth until 7 months of age. Observations were recorded more intensively (3–4 h on five different days) for 2 weeks (5–6 months of age) as male–male sexual behavior increased during that period. Both mounting and mounted lambs were identified. An individual mounting index (MI) was calculated. To study male–female sexual behavior, lambs were individually located with two estrual ewes, and during 5 min the number of ano-genital sniffing, lateral approaches, mounts, and mounts with ejaculation were recorded. From those data, a libido index was also calculated. Male–male mounts (n = 308) were observed. Courtship behavior was displayed in 25% of interactions; mounts were accepted in 72.1% of attempted mounts. Mounts without previous courtship were accepted more frequently than mounts with previous courtship (P = 0.002). Lamb weight and scrotal circumference were not different according to MI groups. Lambs that mounted more times estrual ewes (first tertile) had greater (P = 0.04) MI (0.61 ± 0.10) than lambs with medium (0.27 ± 0.09) and less (0.30 ± 0.10) MI. The regression between MI and heterosexual libido index was r = 0.33 (P < 0.05). In summary, intensive male–male sexual activity during a short period of male lamb development was observed. There was a positive relationship between sexual behavior of male lambs towards other male lambs and towards estrual ewes.     

Inhibitory and disinhibitory effects of psychomotor stimulants and depressants on the sexual behavior of male and female rats

Drugs of abuse comprise several pharmacological classes, including psychomotor stimulants, such as amphetamine and cocaine, and CNS depressants, such as morphine and alcohol. Few studies have examined the effects of those drugs systematically on human sexual behavior, although substantial clinical and epidemiological literatures suggest that drugs in both classes either inhibit sexual responding or can be “prosexual” in certain situations, thereby increasing the potential of risky sexual activity and the spread of sexually transmitted diseases. This paper reviews original data in rats showing that both classes of drug inhibit or disinhibit sexual behavior depending on the animal's baseline level of sexual responding, hormonal status, whether the drug is given acutely or chronically, and whether the animal has learned to inhibit sexual responding toward nonreceptive partners or in the presence of conditioned olfactory cues that predict sexual nonreward.     

Role of neurotransmitters in coordination of male sexual behavior

The review considers the published data and the results obtained by the author on the role of monoamines (serotonin, dopamine, and noradrenaline) in the control of male sexual behavior. In the above respect, the central mechanisms of action of the neurotransmitters are discussed.     

Effects of Microdesmis keayana roots on sexual behavior of male rats

In the present study, the aphrodisiac properties of Microdesmis keayana J. Léonard root extract and major isolated alkaloids were evaluated by observing the sexual behavior of male rats.Aqueous extract (150 mg/kg body weight) and pure alkaloids (3 mg/kg body weight) were administered orally by gavage to male rats. Latent times of observation, intromission and ejaculation, mounting behavior, number of intromissions and mating performances were evaluated and compared to those obtained with untreated rats in the presence of receptive and non-receptive females. The results have shown that aqueous extract and alkaloids of M. keayana stimulate sexual parameters in rats’ sexual behavior.A short-term toxicity study undertaken to establish the therapeutic index of aqueous extract, showed that a high dose of the extract (2 g/kg body weight) caused no mortality or changes in rats’ behavior.     

The endogenous androgen-regulated sialorphin modulates male rat sexual behavior

In sexually mature male rats, sialorphin is synthesized under androgenic control and its surge endocrine secretion is evoked in response to environmental acute stress. These findings led us to suggest that this signaling mediator might play a role in physiological and behavioral integration, especially reproduction. The present study investigates the effects induced by sialorphin on the male sexual behavior pattern. Intact male rats were treated in acute mode, with sialorphin at the 0.3, 1, and 3 microg/kg doses, before being paired with receptive female for 45 min. The data obtained show that sialorphin increased, in a dose-related manner, the occurrence of intromissions across the successive ejaculatory sequences. The rats treated with the highest 3 microg/kg dose significantly ejaculated less often compared to controls; however, 80% of them achieved up to three ejaculations. Further analyses of mount bouts for rats achieving three ejaculations reveal that there were significant stimulatory effects of sialorphin, at all doses, on the frequency of intromissions before ejaculation and on the propensity of males to engage in investigatory behavior directed to the female during the post-ejaculatory interval. Thus, sialorphin has the ability to modulate, at doses related to physiological circulating levels, the male rat mating pattern, that is, exerting a dual facilitative or inhibitory dose-dependent effect on the sexual performance, while stimulating the apparent sexual arousal or motivation. These findings led us to speculate that the endogenous androgen-regulated sialorphin helps modulate the adaptative balance between excitatory and inhibitory mechanisms serving appropriate male rat sexual response, depending on the context.     

Patterns of sexual behavior in male macaques

One large social group of each of three species of macaques (Macaca mulatta, M. fascicularis, M. radiata), housed in half-acre field cages at the California Primate Research Center, were observed for a total of 150 h. Data on sexual behavior and dominance interactions were recorded by pairs of experienced observers using a focal animal technique. Single or multiple mount-to-ejaculation sequences, number of thrusts per mount, number of mounts per sequence, duration of mounts per sequence, duration of sequences, social rank and frequency of sexual activity were recorded for each adult male. M. mulatta used a multiple mount-to-ejaculation (MME) pattern in 91% of their copulations. M. radiata used a single mount-to-ejaculation (SME) pattern in 91% of their copulations. M. fascicularis used both patterns—53% MME and 47% SME. A positive correlation was found between rank and sexual activity in fascicularis and mulatta males. A negative correlation between rank and sexual activity was found in radiata males and also a positive correlation between rank and age indicating that the youngest and most subordinate radiata males were the most sexually active males. In reviewing the literature, a relationship between degree of intermale competition, intermale tolerance and type of mounting pattern was revealed. Macaque species that primarily use an SME pattern also show sa high degree of intermale tolerance and little interrnale competition. Macaque species that primarily use an MME pattern typically show a high degree of intermale competition and a low degree of intermale tolerance. Possible events leading to such relationships are discussed.     

外源性皮质酮对雄性根田鼠交配行为的作用

本文研究了皮质酮对雄性根田鼠交配行为的作用。实验个体分别注射0.10 ug/ g体重、0. 60 ug / g体重和1.00 ug/ g体重剂量的皮质酮,1 h后测定每只雄性根田鼠的交配行为。结果显示,3 个处理组动物具有射精能力的个体比率及其爬跨、抽动和射精潜伏期与对照组动物相比均无显著差异,爬跨和抽动频次也无显著变化。不同处理组个体的血浆睾酮含量无显著差异。因此,皮质酮没有影响雄性根田鼠的交配行为和性激素的分泌。该结果提示,哺乳动物在急性应激条件下所分泌的皮质酮可能不参与对性行为的调控。     

Influence of Griffonia simplicifolia on male sexual behavior in rats: Behavioral and neurochemical study

The seeds of Griffonia simplicifolia Baill. are rich in 5-HTP (5-hydroxytryptophan), a direct precursor of the neurotransmitter serotonin. In the present study we investigated the influence of the plant extract on male sexual behavior. The seed extract was orally administered to Sprague-Dawley male rats at three dose levels (25, 50 and 100 mg/kg) both acutely and subchronically (daily for 9 days). Mating test with receptive female rats was performed 60 min after the acute treatment or the last dose when repetitively administered. Mount, intromission and ejaculation latencies and post-ejaculatory interval were recorded. Food intake and body weight were measured over the 9-day period of treatment. Microdialysis technique was used to detect the extracellular levels of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in rat brain following the acute administration of the extract dosed at 100 mg/kg. The acute treatment significantly increased mount latency (at any dosage), intromission and ejaculation latencies (at 100 mg/kg) and post-ejaculatory interval (at 50 and 100 mg/kg). On the contrary the subchronic treatment failed to exert a significant influence on copulatory behavior. The daily administration of the extract dosed at 50 and 100 mg/kg for 9 days significantly reduced food intake and body weight. Finally in the microdialysis experiments we found a dramatic increase in 5-HT and its metabolite 5-HIAA.     

Ginkgo biloba extract enhances male copulatory behavior and reduces serum prolactin levels in rats

The aim of this study was to investigate the effects of Ginkgo biloba extract (EGb 761) on male copulatory behavior in rats. EGb 761 (1 mg/ml) induced significant production of testosterone (T) in rat Leydig cells in vitro. Its effects on sexual behavior were then tested in Long-Evans male rats after 7, 14, 21, or 28 days of oral gavage of vehicle (distilled water) or EGb 761 at doses of 10, 50, or 100 mg/kg. Administration of 50 mg/kg of EGb 761 for 28 days and of 100 mg/kg for 14 or 21 days significantly increased intromission frequency compared to controls on the same day. An increase in ejaculation frequency was seen after treatment with 50 mg/kg of EGb 761 for 14, 21, or 28 days when compared to either the control group on the same day or the same group on day 0. A reduction in ejaculation latency was only seen after administration of 50 mg/kg of EGb 761 for 14 days compared to the vehicle-treated group. After treatment for 28 days, no significant difference was seen in mount latency, intromission latency, serum T levels, reproductive organ weight, sperm number, or levels of the metabolite of dopamine, 3,4-dihydroxyphenylacetic acid in the brain with any dose of EGb 761, but significantly reduced serum prolactin levels and increased dopamine levels in the medial preoptic area and arcuate nucleus were seen at the dose of 50 mg/kg. These findings show that EGb 761 (especially at the dose of 50 mg/kg) enhances the copulatory behavior of male rats and suggest that the dopaminergic system, which regulates prolactin secretion, may be involved in the facilitatory effect of EGb 761.     

Chronic social stress in puberty alters appetitive male sexual behavior and neural metabolic activity

Repeated social subjugation in early puberty lowers testosterone levels. We used hamsters to investigate the effects of social subjugation on male sexual behavior and metabolic activity within neural systems controlling social and motivational behaviors. Subjugated animals were exposed daily to aggressive adult males in early puberty for postnatal days 28 to 42, while control animals were placed in empty clean cages. On postnatal day 45, they were tested for male sexual behavior in the presence of receptive female. Alternatively, they were tested for mate choice after placement at the base of a Y-maze containing a sexually receptive female in one tip of the maze and an ovariectomized one on the other. Social subjugation did not affect the capacity to mate with receptive females. Although control animals were fast to approach females and preferred ovariectomized individuals, subjugated animals stayed away from them and showed no preference. Cytochrome oxidase activity was reduced within the preoptic area and ventral tegmental area in subjugated hamsters. In addition, the correlation of metabolic activity of these areas with the bed nucleus of the stria terminalis and anterior parietal cortex changed significantly from positive in controls to negative in subjugated animals. These data show that at mid-puberty, while male hamsters are capable of mating, their appetitive sexual behavior is not fully mature and this aspect of male sexual behavior is responsive to social subjugation. Furthermore, metabolic activity and coordination of activity in brain areas related to sexual behavior and motivation were altered by social subjugation.     

Hormonal responses to different sexually related conditions in male rats

Plasma levels of corticosterone (C) and testosterone (T) increase after sexual activity in males of several species. However, the physiological significance of these increases has not been elucidated. In the present study, hormonal response to different conditions linked to sexual activity was assessed. In the first experiment, plasma levels of C and T were assessed both in sexually experienced and naive male rats after the following conditions: (A) control group, without sexual stimulation; (B) males exposed to ovariectomized females; (C) males exposed to intact, non-receptive females; (D) males exposed to receptive females with the vagina obstructed, to avoid intromission; (E) males exposed to receptive females: but separated by a grid that prevents physical contact; (F) males exposed to receptive females during 30 min. In a second experiment, experienced male rats were allowed to repeatedly copulate until reaching the criteria for sexual exhaustion, and 24 h later, they were allowed to copulate. Once sexually related conditions ended, males were killed and their blood was obtained. C and T plasma levels were assessed by HPLC with ultraviolet (UV) detection. Results indicate that T did not increase significantly in naive male in any sexual condition, while in the experienced males, significant increases were observed with the mere presence of a receptive female and also after ejaculation. These increases were significantly larger in experienced males. On the other hand, C also increased in all sexual conditions, both in experienced and naive rats; however, the increase observed was larger in experienced males. Regarding sexual satiety, both C and T increased after copulating ad libitum to satiety. T increased almost three-fold compared to control, while C increased two-fold. No significant changes were observed in either one of the steroids 24 h after sexual exhaustion, even though males remained with a receptive female during an hour. These results show that sexual experience has an important influence on the hormonal response to sexual activity. C rises could be directly related to sexual arousal involved in the different sexual conditions, while T rises seem to have a direct relationship with both the motivation and execution aspects of masculine sexual behavior.     

Effect of forebrain implants of testosterone or estradiol on scent-marking and sexual behavior in male and female rabbits

Chinning consists of rubbing the chin against an object, thereby depositing secretions from the submandibular glands. As mating, chinning is stimulated in male and female rabbits by testosterone and estradiol, respectively. To investigate the brain sites where steroids act to stimulate chinning and mating we implanted into the ventromedial hypothalamus (VMH) or the medial preoptic area (MPOA) of gonadectomized male and female rabbits testosterone propionate (TP; males) or estradiol benzoate (EB; females) and quantified chinning and sexual behavior. EB implants into the VMH or MPOA reliably stimulated chinning in females. Most of those implanted into the VMH and around half of the ones receiving EB into MPOA or diagonal band of Broca (DBB) showed lordosis. Chinning, but not sexual behavior, was stimulated in males by TP implants into the MPOA or DBB. Neither chinning nor mounting were reliably displayed by males following TP implants into the VMH. Results indicate that, in females, the VMH is an estrogen-sensitive brain area that stimulates both chinning and lordosis while the MPOA seems to contain subpopulations of neurons involved in either behavior. In males, androgen-sensitive neurons of the MPOA, but not the VMH, are involved in chinning stimulation but it is unclear if these areas also participate in the regulation of copulatory behavior.     

Short duration testosterone infusions maintain male sex behavior in Syrian hamsters

In most mammalian species, reduced androgen availability is associated with marked reductions in male sexuality; conversely, androgen replacement in castrated males restores sex behavior within a few weeks. Testosterone (T) pulse duration, amplitude, frequency, and inter-pulse interval may be as important as total amount of hormone in determining target tissue responsiveness. We remain ignorant of the number and duration of daily T pulses necessary and sufficient to sustain male mating behavior. An in-dwelling infusion system was employed to vary T-pulse frequencies and durations. Daily 4 h infusions of aqueous T (100 microg/0.064 ml) and twice daily 4 h pulses of T (each 50 microg/0.064 ml) were sufficient to maintain ejaculatory behavior of sexually experienced castrated hamsters for 11 weeks post-castration; castrated hamsters infused with vehicle ceased to display the ejaculatory pattern 3 weeks after gonadectomy. Circulating T concentrations of hormone-infused hamsters declined markedly 7 h after the termination of each infusion. These results establish that male sex behavior can be sustained with infusions of relatively low T concentrations for 4 h/day and suggests that the basal concentrations of T sustained by the gonad during inter-pulse intervals may not be necessary for maintenance of sex behavior. 4 h T infusions were sufficient to maintain penile and seminal vesicles weights, but not ventral prostate weights or flank gland dimensions; the threshold for maintaining male sex behavior is lower than that for some androgen-dependent peripheral structures. Development of effective androgen replacement regimens that sustain sex behavior in castrated animals may be useful in the design of androgen replacement therapy for hypogonadal men.