Sex Differences in Sexual Motivation in Humans and Other Mammals: The Role of Conscious and Unconscious Processes

In self-report questionnaires, men report higher scores than women on variables such as desire for sex, frequency of sexual thoughts, number of sex partners, etc. Based on this, men are considered to have a higher level of sexual motivation than women. However, retrospective self-reports may be unsuitable for estimations of the inherent level of sexual motivation. We review data on automatic (unconsciously controlled) responses and measures of implicit motivation during exposure to sexual stimuli. These responses and measures are inaccessible to willful manipulations and make it possible to determine whether the sex difference in answers to questionnaires is replicated when volitional response manipulations are unlikely. We complement the human data with observations from some rodent and non-human primate species. The attentional resources allotted to stimuli with sexual relevance as well as genital responses to such stimuli are similar in men and women. Measures of implicit motivation also fail to detect any sex difference. Finally, the frequency of masturbation is superior in female infants before the age at which social expectations begin to determine behavior. Neither in rodents nor in non-human primates is there any clear-cut evidence for sex differences in motivation. It seems that males and females are similar with regard to the intensity of sexual motivation. The responses to questionnaires may be affected by social learning of sexual scripts and/or the inferior quality of sexual experiences in women, among other things.

Androgen receptors and sociosexual behaviors in mammals: The limits of generalization

Circulating testosterone is easily aromatized to estradiol and reduced to dihydrotestosterone in target tissues and elsewhere in the body. Thus, the actions of testosterone can be mediated either by the estrogen receptors, the androgen receptor or by simultaneous action at both receptors. To determine the role of androgens acting at the androgen receptor, we need to eliminate actions at the estrogen receptors. Alternatively, actions at the androgen receptor itself can be eliminated. In the present review, I will analyze the specific role of androgen receptors in male and female sexual behavior as well as in aggression. Some comments about androgen receptors and social recognition are also made. It will be shown that there are important differences between species, even between strains within a species, concerning the actions of the androgen receptor on the behaviors mentioned. This fact makes generalizations from one species to another or from one strain to another very risky. The existence of important species differences is often ignored, leading to many misunderstandings and much confusion.

Sexual Incentive Motivation and Sexual Behavior: The Role of Consent

The generalized social concern with sexual harassment and nonconsensual sex makes it imperative to incorporate notions of consent in any analysis of human sexual interactions. Such interactions follow an ordered sequence of events, starting with the perception of a sexual incentive, followed by an approach to it, genital interaction, and eventually orgasm. Consent from the partner is needed at every stage. At some points in this chain of events, the individuals involved make cognitive evaluations of the context and predictions of the likelihood for obtaining consent for proceeding to the next phase. Processes such as communication of consent or lack thereof, sexual decision making, and interpretation of cues emitted by the partner are decisive. Increased sexual motivation may influence these processes. However, available data make it possible to ascertain that enhanced motivation has no, or at most minor, effects, thereby invalidating the old assumption that heightened sexual motivation leads to impaired control.

The elusive concept of sexual motivation: can it be anchored in the nervous system?

Sexual motivation is an abstract concept referring to the mechanisms determining the responsivity to sexually relevant stimuli. This responsivity determines the likelihood of producing a sexual response and the intensity of that response. Both responsivity to stimuli and the likelihood of making a response as well as the intensity of response are characteristics of an individual. Therefore, we need to assume that the concept of sexual motivation materializes in physiological mechanisms within the individual. The aim of the present communication is to analyze the requisites for the endeavor to materialize sexual motivation. The first requisite is to provide an operational definition, making the concept quantifiable. We show that parameters of copulatory behavior are inappropriate. We argue that the intensity of sexual approach behaviors provides the best estimate of sexual motivation in non-human animals, whereas the magnitude of genital responses is an exquisite indicator of human sexual motivation. Having assured how to quantify sexual motivation, we can then proceed to the search for physiological or neurobiological underpinnings. In fact, sexual motivation only manifests itself in animals exposed to appropriate amounts of gonadal hormones. In female rats, the estrogen receptor α in the ventrolateral part of the ventromedial nucleus of the hypothalamus is necessary for the expression of sexual approach behaviors. In male rats, androgen receptors within the medial preoptic area are crucial. Thus, in rats sexual motivation can be localized to specific brain structures, and even to specific cells within these structures. In humans, it is not even known if sexual motivation is materialized in the brain or in peripheral structures. Substantial efforts have been made to determine the relationship between the activity of neurotransmitters and the intensity of sexual motivation, particularly in rodents. The results of this effort have been meager. Likewise, efforts of finding drugs to stimulate sexual motivation, particularly in women complaining of low sexual desire, have produced dismal results. In sum, it appears that the abstract concept of sexual motivation can be reliably quantified, and the neurobiological bases can be described in non-human animals. In humans, objective quantification is feasible, but the neurobiological substrate remains enigmatic.

The Sexual Incentive Motivation Model and Its Clinical Applications

Sexual motivation (desire) requires the simultaneous presence of an active central motive state and a stimulus with sexual significance. Once activated, sexual motivation leads to visceral responses and approach behaviors directed toward the emitter of the sexual stimulus. In humans, such behaviors follow cognitive evaluation of the context, including predictions of the approached individual's response. After successful approach and establishment of physical contact, manifest sexual activities may be initiated. Sexual interaction is associated with and followed by a state of positive affect in most animals, whereas aversive consequences may be experienced by humans. The affective reactions may become associated with stimuli present during sexual interaction, and these stimuli may thereby alter their incentive properties. Here we show how the incentive motivation model can be used to explain the origins and possible treatments of sexual dysfunctions, notably disorders of desire. We propose that associations formed between negative outcomes of sexual interaction and the salient stimuli, for example, the partner, underlies hypoactive desire disorder. Highly positive outcomes of sexual interaction enhance the incentive value of the stimuli present, and eventually lead to hyperactive sexual desire. Treatments aim to alter the impact of sexual incentives, mainly by modifying cognitive processes.

Seminatural environments for rodent behavioral testing: a representative design improving animal welfare and enhancing replicability

The low replicability of scientific studies has become an important issue. One possible cause is low representativeness of the experimental design employed. Already in the 1950's, Egon Brunswick pointed out that experimental setups ideally should be based on a random sample of stimuli from the subjects' natural environment or at least include basic features of that environment. Only experimental designs satisfying this criterion, representative designs in Brunswikian terminology, can produce results generalizable beyond the procedure used and to situations outside the laboratory. Such external validity is crucial in preclinical drug studies, for example, and should be important for replicability in general. Popular experimental setups in rodent research on non-human animals, like the tail suspension test or the Geller-Seifter procedure, do not correspond to contexts likely to be encountered in the animals' habitat. Consequently, results obtained in this kind of procedures can be generalized neither to other procedures nor to contexts outside the laboratory. Furthermore, many traditional procedures are incompatible with current notions of animal welfare. An approximation to the natural social and physical context can be provided in the laboratory, in the form of a seminatural environment. In addition to satisfy the basic demands for a representative design, such environments offer a far higher level of animal welfare than the typical small cages. This perspective article will briefly discuss the basic principles of the generalizability of experimental results, the virtues of representative designs and the coincidence of enhanced scientific quality and animal welfare provided by this kind of design.

Neuroendocrinology of sexual behavior

One of the consequences of sexual behavior is reproduction. Thus, this behavior is essential for the survival of the species. However, the individual engaged in sexual behavior is rarely aware of its reproductive consequences. In fact, the human is probably the only species in which sexual acts may be performed with the explicit purpose of reproduction. Most human sexual activities as well as sex in other animals is performed with the aim of obtaining a state of positive affect. This makes sexual behavior important for wellbeing as well as for reproduction. It is not surprising, then, that sexual health has become an increasingly important issue, and that knowledge of the basic mechanisms controlling that behavior are urgently needed. The endocrine control of sexual behavior has been extensively studied, and although it is established that gonadal hormones are necessary, some controversy still exists concerning which hormone does what in which species. The brain areas necessary for sexual behavior have been determined in almost all vertebrates except the human. The medial preoptic area is crucial in males of all non-human vertebrates, whereas the ventromedial nucleus of the hypothalamus is important in females. Modulatory functions have been ascribed to several other brain areas.

Sexual incentive motivation, sexual behavior, and general arousal: Do rats and humans tell the same story?

Sexual incentive stimuli activate sexual motivation and heighten the level of general arousal. The sexual motive may induce the individual to approach the incentive, and eventually to initiate sexual acts. Both approach and the ensuing copulatory interaction further enhance general arousal. We present data from rodents and humans in support of these assertions. We then suggest that orgasm is experienced when the combined level of excitation surpasses a threshold. In order to analyze the neurobiological bases of sexual motivation, we employ the concept of a central motive state. We then discuss the mechanisms involved in the long- and short-term control of that state as well as those mediating the momentaneous actions of sexual incentive stimuli. This leads to an analysis of the neurobiology behind the interindividual differences in responsivity of the sexual central motive state. Knowledge is still fragmentary, and many contradictory observations have been made. Nevertheless, we conclude that the basic mechanisms of sexual motivation and the role of general arousal are similar in rodents and humans.

Sexual Incentive Motivation and Copulatory Behavior in Male Rats Treated With the Adrenergic α2-Adrenoceptor Agonists Tasipimidine and Fadolmidine: Implications for Treatment of Premature Ejaculation

BACKGROUND

Premature ejaculation is the most common sexual dysfunction in young men, and it often leads to reduced relationship satisfaction and quality of life.

AIM

To determine the role of central and peripheral α₂-adrenoceptors in the control of ejaculation and sexual incentive motivation in rats.

METHODS

Sexual incentive motivation was studied in a large arena in which a male subject could choose between approaching and remaining close to a sexually receptive female or another male. Sexual behavior was studied in standard observation cages in which a male was allowed to freely interact with a receptive female for 30 minutes. Two highly selective agonists at the α₂-adrenoceptors, tasipimidine and fadolmidine, were administered before the tests. Low peripheral doses of fadolmidine have been reported to have effects mainly outside of the central nervous system, whereas at large doses also the central effects are evident.

OUTCOMES

The time spent close to the receptive female in relation to the time spent with the male and measures of ambulatory activity were obtained from the test for sexual incentive motivation, while the habitual parameters of sexual behavior were recorded with the copulation test.

RESULTS

Tasipimidine prolonged ejaculation latency and the interintromission interval at the dose of 200 µg/kg when data from fast-ejaculating rats were used. No other sexual parameter was modified. A dose of 100 µg/kg was ineffective. There was no consistent effect on sexual incentive motivation, although modest sedation was observed. Fadolmidine, a drug that does not easily penetrate the blood-brain barrier, had no effect on sexual incentive motivation at any of the doses used (3, 30, and 100 µg/kg). The largest dose had clear sedative effects. The lower doses had no systematic effect on sexual behavior, not even when only fast or very fast ejaculating males were analyzed.

CLINICAL TRANSLATION

The findings are relevant to the search for treatments for premature ejaculation that are specific enough to selectively delay ejaculation.

STRENGTHS & LIMITATIONS

The procedures used here are standard in the field and yield the most reliable data. Whether the effects observed in male rats are directly transferrable to men can only be determined through clinical studies.

CONCLUSION

The observation that drugs acting at central but not peripheral α₂-adrenoceptors prolong ejaculation latency without affecting any other parameter of sexual behavior or sexual incentive motivation suggests that this kind of drug may be suitable for treating premature ejaculation. Jyrki L., Elisa V.-A., Xi C., et al. Sexual Incentive Motivation and Copulatory Behavior in Male Rats Treated With the Adrenergic α₂-Adrenoceptor Agonists Tasipimidine and Fadolmidine: Implications for Treatment of Premature Ejaculation. J Sex Med 2021;18:1677-1689.

Rat ultrasonic vocalizations and novelty-induced social and non-social investigation behavior in a seminatural environment

Although rats are known to emit ultrasonic vocalizations (USVs), it remains unclear whether these calls serve an auditory communication purpose. For USVs to be part of communication, the vocal signals will need to be a transfer of information between two or more conspecifics, and with the possibility to induce changes in the behavior of the recipient. Therefore, the aim of our study was to investigate the role of USVs in adult rats' social and non-social investigation strategies when introduced into a large novel environment with unfamiliar conspecifics. We quantified a wide range of social and non-social behaviors in the seminatural environment, which could be affected by subtle signals, including USVs. We found that during the first hour in the seminatural environment the ability to vocalize did not affect how quickly adult rats met each other, their overall social investigation behavior, their passive social behavior nor their aggressive behavior. Furthermore, the non-social exploratory behaviors and behaviors reflecting anxiety/stress-like states were also unaffected. These results demonstrated that a disability to vocalize did not result in significant disadvantages (or changes) compared to intact conspecifics regarding social and non-social behaviors. This suggests that other (multi)sensory cues are more relevant in social interactions than USVs.

Sexual incentive motivation and male and female copulatory behavior in female rats given androgen from postnatal day 20

Masculinization and feminization of rat sexual behavior has been supposed to occur during a short postnatal period. However, much data have made it evident that these processes may continue until adolescence. In the present study, we evaluated whether androgen treatment of females from postnatal day 20 and onwards could alter sexual motivation and behavior in a male direction. Juveniles were ovariectomized on day 20 and concurrently implanted with Silastic capsules containing either testosterone or dihydrotestosterone. Controls were implanted with an empty capsule. Tests for sexual incentive motivation and male sexual behavior were performed every fifth day when the females were between 50 and 75 days of age. At day 80, a test for female sexual behavior was performed. Females treated with testosterone approached a female sexual incentive far more than a male incentive, showing that sexual motivation had been changed in a male-like direction. Dihydrotestosterone had a similar, albeit smaller, effect. Females implanted with an empty capsule approached both incentives equally. Testosterone produced a high level of mounting behavior, whereas intromission-like behavioral patterns were rare and ejaculation-like behavior was absent. In the test for female sexual behavior, the testosterone-treated animals displayed a relatively high lordosis quotient, far above that displayed in females implanted with dihydrotestosterone or an empty capsule. It is concluded that treatment with an aromatizable androgen during the peripubertal-adolescent period masculinizes sexual motivation and partly sexual behavior. A non-aromatizable androgen weakly masculinize sexual motivation without enhancing male sexual behavior. It appears that simultaneous actions on androgen and estrogen receptors are needed for significant masculinization during the period studied here. Since the testosterone-treated females displayed lordosis, sexual behavior was not defeminized. In sum, these results suggest that sexual differentiation continues well into the peripubertal and adolescent periods.

Pubertal stress decreases sexual motivation and supresses the relation between cerebral theta rhythms and testosterone levels in adult male rats

This study evaluated the effect of stress during puberty on sexual motivation and the correlation between serum testosterone levels (T) and the absolute power of the theta electroencephalographic rhythms, recorded in the medial prefrontal cortex (mPFC) and basolateral amygdala (BLA) of adult male rats. Thirty males of the stressed group (SG, housed 1 per cage from days 25-50) and 30 controls (CG, housed 5 per cage), were tested in copulatory interactions at 90 days of age. The above mentioned physiological parameters were obtained during the awake-quiet state in a sub-group without sexual motivation (WSM, n = 15, stimulated with a nonreceptive female) and a sub-group with sexual motivation (SM, n = 15, stimulated with a receptive-female). Pearson correlations (r) between these parameters were calculated for each sub-group and brain structure and then compared between sub-groups. SG presented higher mount and intromission latencies than CG. While CG-WSM showed a positive r between T levels and theta band (0.23-0.59), those CG-SM presented a negative r (-0.23 to -0.67). An r that tended towards zero (-0.31 to 0.29) was obtained in both stressed sub-groups. This study shows that pubertal stress suppresses the relation between serum T levels and theta rhythms in the mPFC and BLA in adult male rats. This is one of the first studies evaluating the association between these two physiological parameters specifically in the context of sexual motivation; thus increasing our understanding of the effect of pubertal stress on prefrontal-amygdaline functioning during the sexually-motivated state in male rats.

Male and female immediate fear reaction to white noise in a semi-natural environment: A detailed behavioural analysis of the role of sex and oestrogen receptors

In classical rodent anxiety models, females usually display lower anxiety than males, whereas anxiety disorders are more prevalent in women. Perhaps this contradiction is caused by the use of behavioural models with low external validity. Therefore, we analysed immediate reactions to a sudden 90-dB white noise in a semi-natural environment. We observed mixed-sex groups of rats for the 60 seconds preceding noise onset and the first 60 seconds of exposure. White noise elicited fear-specific behaviours hiding alone and huddling. It also increased exploratory and ambulatory behaviours, although only in the burrow zone farthest from the open area. Thus, in a semi-natural environment, white noise enhanced motor activity as a product of fear-induced general arousal. Then, we compared male and female sexual, social, exploratory and anxiety-related behaviour, and found little sex difference. This absence of behavioural effect, also observed in other studies, might be a result of our study design, a familiar environment with an ecologically relevant social context. Fear and anxiety responses are modulated by oestrogens through the activation of oestrogen receptors α and β. Thus, in a third part of out study, we analysed how treatment with either oil, oestradiol benzoate (EB), an agonist to the oestrogen receptor α (propylpyrazoletriol [PPT]) or β (diarylpropionitrile [DPN]) influenced female behaviour. The effect of treatment was limited, both EB and PPT stimulated motor activity in the open area before white noise, probably because of sexual activity. PPT increased the probability of fleeing from the noise, and decreased the latency to do so, which is consistent with a pattern of anxiogenic properties found in previous studies. Contrary to reports in classical procedures, we failed to detect any effect of DPN on immediate fear reactions in a semi-natural environment.

Modeling Human Sexual Motivation in Rodents: Some Caveats

Sexual behavior is activated by motivation. An overwhelming majority of experimental studies of the intricacies of sexual motivation has been performed in rodents, most of them in rats. Sometimes it is desirable to generalize results obtained in this species to other species, particularly the human. It is hoped that studies of the neurobiology of rodent sexual behavior may shed light on the central nervous mechanisms operating in the human, and the search for efficient pharmacological treatments of human sexual dysfunctions relies partly on studies performed in rodents. Then the issue of generalizability of the rodent data to the human becomes crucial. We emphasize the importance of distinguishing between copulatory acts, behavior involving the genitals, and the preceding event, the establishment of physical contact with a potential mate. Comparisons between the structure of copulatory behavior in rats and humans show abysmal differences, but there may be some similarity in the underlying mechanisms. The endocrine control of sex behavior is shortly mentioned, and we also compare the effects of the few drugs known to affect both rodent and human copulatory behavior. The stimuli activating sexual motivation, often called desire in the human literature, are examined, and the sexual approach behaviors in rats and humans are compared. There is a striking similarity between these species in how these behaviors respond to drugs. It is then shown that the intensity of sexual approach is unrelated to the intensity of copulatory behavior. Even though the approach is a requisite for copulation, an activity that requires at least two individuals in close physical contact, these two aspects of sexuality do not covary. This is similar to the role of the testosterone in men and male rats: although the hormone is needed for sex behavior, there is no correlation between serum testosterone concentration and the intensity of copulation. It is also pointed out that human sexual behavior is mostly determined by social conventions, whereas this is not the case in rats and other rodents. It is concluded that some observations in rats can be generalized to the human, but extreme caution must be exercised.

Behavioral response is absent under the mating competition in rats (Rattus norvegicus)

Sexually receptive female rats normally copulate with several males during estrus, and multiple paternity is common. Sperm competition is therefore likely to occur. One response to competitive mating is to enhance sperm output per ejaculation and another is to augment the number of ejaculations. The latter alternative requires more intense copulatory behavior. In studies in a seminatural environment we observed that male rats did not modify their behavior according to the intensity of competition, whereas observations from standard observation cages suggested that they do so. In order to further evaluate the potential response to competitive mating, we observed male rats copulating in a pair situation, i.e. one male and one female, and in a situation where three males simultaneously copulated with one female. In addition to sexual behavior, social interactions were quantified. It was found that the males in the multiple male condition prolonged mount and intromission latencies, and displayed a reduced number of mounts. There was no change in the number of preejaculatory intromissions or the ejaculation latency. The multiple mating did not affect non-sexual interactions with the female, whereas the female displayed more nose-offs and rejections when copulating with three males. It is concluded that mating competition does alter the initiation of copulation in the male rat, whereas copulatory behavior, i.e. intromission and ejaculation, remains unchanged.

Behavioral responses to emotional challenges in female rats living in a seminatural environment: The role of estrogen receptors

Estrogen receptors (ERs) are involved in sexual as well as non-sexual behaviors. In the present study we assessed the effects of stimuli inducing positive or negative affect on sociosexual, exploratory and fear-related behaviors of female rats housed in groups (4 females, 3 males) in a seminatural environment. Ovariectomized females were treated with oil, 17β‑estradiol benzoate (EB, 18 μg/kg), the ERα agonist propylpyrazoletriol (PPT), or the ERβ agonist diarylpropionitrile (DPN) (both 2 × 10 mg/rat). On the test day, the females were exposed to a sequence of events consisting of lavender odor, Mozart's Sonata for Two Pianos K448, chocolate pellets, white noise and fox odor (2,3,5‑Trimethyl‑3‑thiazoline, TMT). All these events are known to induce positive or negative affect. Behavior was carefully observed from the video record. White noise suppressed sexual behaviors and reduced the time spent in the open area of the environment. TMT had no consistent effect whereas exposure to music caused avoidance of the open area. Exposure to chocolate increased exploratory and social behavior. Lavender odor enhanced exploratory behavior. PPT and EB stimulated sexual behaviors, whereas DPN was ineffective. Co-occurrence analyses of the sequence of behavioral patterns revealed that PPT and EB consistently belonged to clusters different from oil and DPN, whereas DPN was separate from oil only under fear-inducing experimental conditions. These data, from a procedure with external validity, confirm that the ERα is crucial for sexual behaviors, that these behaviors are reduced under stressful conditions, and that the ERβ may have some role in fear-related behaviors.

The Role of Estrogen Receptor β (ERβ) in the Establishment of Hierarchical Social Relationships in Male Mice

Acquisition of social dominance is important for social species including mice, for preferential access to foods and mates. Male mice establish social rank through agonistic behaviors, which are regulated by gonadal steroid hormone, testosterone, as its original form and aromatized form. It is well known that estrogen receptors (ERs), particularly ER α (ERα), mediate effects of aromatized testosterone, i.e., 17β-estradiol, but precise role played by ER β (ERβ) is still unclear. In the present study, we investigated effects of ERβ gene disruption on social rank establishment in male mice. Adult male ERβ knockout (βERKO) mice and their wild type (WT) littermates were paired based on genotype- and weight-matched manner and tested against each other repeatedly during 7 days experimental period. They underwent 4 trials of social interaction test in neutral cage (homogeneous set test) every other day. Along repeated trials, WT but not βERKO pairs showed a gradual increase of agonistic behaviors including aggression and tail rattling, and a gradual decrease of latency to social rank determination in tube test conducted after each trial of the social interaction test. Analysis of behavioral transition further suggested that WT winners in the tube test showed one-sided aggression during social interaction test suggesting WT pairs went through a process of social rank establishment. On the other hand, a dominant-subordinate relationship in βERKO pairs was not as apparent as that in WT pairs. Moreover, βERKO mice showed lower levels of aggressive behavior than WT mice in social interaction tests. These findings collectively suggest that ERβ may play a significant role in the establishment and maintenance of hierarchical social relationships among male mice.

The neuroendocrinology of sexual attraction

Sexual attraction has two components: Emission of sexually attractive stimuli and responsiveness to these stimuli. In rodents, olfactory stimuli are necessary but not sufficient for attraction. We argue that body odors are far superior to odors from excreta (urine, feces) as sexual attractants. Body odors are produced by sebaceous glands all over the body surface and in specialized glands. In primates, visual stimuli, for example the sexual skin, are more important than olfactory. The role of gonadal hormones for the production of and responsiveness to odorants is well established. Both the androgen and the estrogen receptor α are important in male as well as in female rodents. Also in primates, gonadal hormones are necessary for the responsiveness to sexual attractants. In males, the androgen receptor is sufficient for sustaining responsiveness. In female non-human primates, estrogens are needed, whereas androgens seem to contribute to responsiveness in women.

A cooperative function for multisensory stimuli in the induction of approach behavior of a potential mate

Intrasexual competition is an important element of natural selection in which the most attractive conspecific has a considerable reproductive advantage over the others. The conspecifics that are approached first often become the preferred mate partners, and could thus from a biological perspective have a reproductive advantage. This underlines the importance of the initial approach and raises the question of what induces this approach, or what makes a conspecific attractive. Identification of the sensory modalities crucial for the activation of approach is necessary for elucidating the central nervous processes involved in the activation of sexual motivation and eventually copulatory behavior. The initial approach to a potential mate depends on distant stimuli in the modalities of audition, olfaction, vision, and other undefined characteristics. This study investigated the role of the different modalities and the combination of these modalities in the sexual incentive value of a female rat. This study provides evidence that the presence of a single-sensory stimulus with one modality (olfaction, vision, or 'others', but not audition) is sufficient to attenuate the preference for a social contact with a male rat. However, a multisensory stimulus of multiple modalities is necessary to induce preference for the stimulus over social contact to a level of an intact receptive female. The initial approach behavior, therefore, seems to be induced by the combination of at least two modalities among which olfaction is crucial. This suggests that there is a cooperative function for the different modalities in the induction of approach behavior of a potential mate.

Effect of Sexual Arousal on Cortical Coupling During Performance of the Tower of Hanoi Task in Young Men

Sexual arousal affects cognitive processing, which depends on the coordinated functioning among cortical areas. The aim of this research was to determine whether previous observation of videos with sexual content affects the degree of cortical electroencephalographic (EEG) coupling during performance of an executive task. Cortical EEG correlations were calculated in three groups of heterosexual men under three conditions: at rest; during observation of a video with neutral, aggressive, or erotic content; and while performing the Tower of Hanoi task (TOH). Based on self-reports, it was shown that the erotic video induced general and sexual arousal, while the aggressive video affected valence and general arousal. Task performance was similar in all three groups. During performance of TOH, only the erotic group showed a decreased correlation between prefrontal areas with an increased correlation between parietal and prefrontotemporal areas, specifically in the slow bands. It is likely that these changes in the degree of cortical coupling could be associated with the cognitive strategies or functional adaptations that participants require to adequately solve the task during a state of sexual arousal. These data could contribute to improving our understanding of the central nervous mechanisms that underlie the effect of sexual arousal on the cognitive processes involved in tasks like TOH.

Functions of vocalization in sociosexual behaviors in rats (Rattus norvegicus) in a seminatural environment

Both male and female rats produce vocalizations in the presence of a potential sexual partner. In this study, we evaluated the role of vocalizations in sociosexual behaviors in an ecologically valid procedure. Three males and 4 females were housed in a seminatural environment. In each group, 1 or 2 males and females were devocalized, and the other subjects were sham operated. Sociosexual interactions between males and females were recorded for a period of 1 hr when all 4 females were receptive so that the males had the choice to interact either with vocalizing or with silent females. Devocalized and sham-operated males displayed very similar behavioral patterns. There was no difference in any of the male sexual behavior patterns nor in male-initiated nonsexual social interaction. Female vocalizations do not contribute to the regulation of sociosexual interaction. Devocalized males received as much attention from females as sham-operated males, with the exception of paracopulatory behaviors with short duration, which were more frequently directed toward the sham-operated males than to the devocalized males. This was the case for both silent and vocalizing females. It appears, then, that devocalized males are inferior to sham males with regard to the capacity to induce female paracopulatory behaviors. However, this has no consequence for sexual interaction because devocalized and sham males copulated equally based on the number of mounts, intromissions, and ejaculations. In sum, these data show that vocalizations play a very limited role in rat sociosexual behavior in a seminatural environment. Furthermore, this indicates that vocalizations have no evident function during copulatory interactions. (PsycINFO Database Record

GABA Receptors in the Medial Preoptic Area Modulate the Onset of Oestradiol-Induced Maternal Behaviour in Hysterectomised-Ovariectomised, Pregnant Rats

We studied the participation of GABA neurotransmission in the medial preoptic area (mPOA) with respect to the onset of the pup retrieval response and nest building. Pregnant female rats were implanted with bilateral cannulae in the mPOA on day 12 of pregnancy and, on day 16, the females were hysterectomised and ovariectomised and given 200 μg/kg of oestradiol benzoate. Two days later, the females received one of the following intracerebral drug treatments: GABA_B agonist baclofen (200 ng); GABA_B antagonist phaclofen (1 μg); GABA_A antagonist bicuculline (60 ng); or physiological saline. Five minutes after intracerebral infusion, three foster pups were introduced into the females' home cage. The subjects were observed for pup grouping (retrieval) during 15 min, after which the pups were left with the female. During the next 12 h, an observation was made every 1 h to determine whether the pups had been grouped (retrieved) or not. The GABA_B agonist baclofen reduced the proportion of females retrieving pups from 4 to 8 h following pup introduction. By contrast, both the GABA_A antagonist bicuculline and the GABA_B antagonist phaclofen enhanced the proportion of females retrieving pups during the first 3 h of observation. The latency to pup retrieval in subjects treated with the GABA_B agonist baclofen was significantly longer than that in subjects given any of the antagonists. All females built a nest but baclofen reduced nest quality. These data show that activation of GABA_B receptors in the mPOA has an inhibitory effect on basic maternal behaviours, whereas blockade of either the GABA_A or GABA_B receptor facilitates pup retrieval. It is possible that reduced GABAergic tone in the mPOA is a key element in the initiation of maternal behaviours in postparturient rats.

Studies of Sociosexual Interactions in Rats in an Externally Valid Procedure: Are They Relevant for Understanding Human Sexual Behavior?

When a prolonged observation of groups of rats in a seminatural environment is used as testing procedure, different behavioral patterns are shown compared with what observed in a pair housed in a small cage. Males and females copulate simultaneously, they show a promiscuously and random copulatory pattern. Females remain completely receptive from the first lordosis displayed in the period of behavioral estrus until the last. There is no reduction in paracopulatory behaviors and no increase in rejections towards the end of estrus. Female paracopulatory behavior and receptivity change in a most abrupt way at both initiation and termination of behavioral estrus. It appears that, in the seminatural environment, males copulate in bouts, and males do not pursue the females unless they are fully receptive. Non-sexual, social behavior including affiliative and nonaffiliative interaction among rats is rather unrelated to sexual activities in both sex.

The adrenergic α2-receptor, sexual incentive motivation and copulatory behavior in the male rat

Adrenergic α2 antagonists are known to enhance sexual incentive motivation and modify copulatory behavior while agonists are consistently inhibitory. However, many of the drugs employed in earlier studies were of modest specificity for the α2 receptor, and the importance of the different subtypes of this receptor remains completely unknown. In the present series of experiments we determined the effects on sexual incentive motivation and copulatory behavior of additional, highly specific compounds, as well as of agonists selective for each of the three subtypes of the α2 receptor. Sexual incentive motivation and copulatory behavior were evaluated in male rats in well established procedures. Among the α2 antagonists, RX 821002 reliably enhanced sexual incentive motivation while fluparoxan only had a modest effect. In large doses both drugs reduced copulatory behavior. The agonist S 18616 reduced both incentive motivation and copulation. None of the subtype selective agonists (BRL 44408, ARC 239, JP 1302) had any consistent effect. A peripheral α2 antagonist, L 659,066 was also ineffective. Even though there are some differences between α2 antagonists with regard to their effects on sexual incentive motivation and copulatory behavior it seems safe to conclude that antagonism of the adrenergic α2 receptor enhances motivation without any concomitant stimulation of copulatory behavior. It appears that antagonism of a single receptor subtype is insufficient for having this effect. Perhaps non-selective α2 antagonists could be used for the treatment of male sexual dysfunction.

Silencing Estrogen Receptor-α with siRNA in the Intact Rodent Brain

The ability to silence the expression of gene products in a chemically, spatially, and temporally specific manner in the brains of animals has enabled key breakthroughs in the field of behavioral neuroscience. Using this technique, estrogen receptor alpha (ERα) has been specifically implicated in a multitude of behaviors in mice, including sexual, aggressive, locomotor, and maternal behaviors. ERα has been identified in a variety of brain regions, including the medial preoptic area, ventromedial hypothalamus, and amygdala. In this chapter we describe the techniques involved in the generation of the small hairpin RNAs (shRNAs) specifically designed to silence ERα, the construction of the adeno-associated viral (AAV) vector for delivery of the shRNA, the procedures to confirm the silencing of ERα (in vitro and in vivo) and in vivo delivery of the shRNAs to the brains of animals.

Silent or Vocalizing Rats Copulate in a Similar Manner

Both male and female rats produce 50 kHz ultrasonic vocalizations (USVs) in the presence of a sexual partner and during copulation. Previous studies showed that USVs have no incentive value for rats. In this study, we evaluated the role of USVs in behavior during copulation. Three groups of rats were used: sham males paired with sham females, devocalized females paired with sham males, and sham females paired with devocalized males. During the copulation test, the USVs emitted by the sham rat were recorded and the sexual behavior of both the male and the female were observed. The results revealed that devocalized and sham females showed similar patterns of sexual behavior and no difference was found in the copulatory behavior of devocalized and sham males. Also the behavior of the partner of a sham rat was comparable to the partner of a devocalized rat. In addition, almost no changes in USVs emission were found in the 5 seconds before and/or after a copulatory behavior. It can be concluded that USVs play no important role in rat copulatory behavior at least in sexually naïve rats.

Role of Oestrogen α Receptors in Sociosexual Behaviour in Female Rats Housed in a Seminatural Environment

The present study investigated the role of oestrogen receptor (ER)α in the ventromedial nucleus of the hypothalamus (VMN), the preoptic area (POA), the medial amygdala (MePD) and the bed nucleus of stria terminalis (BNST) in sociosexual behaviour in female rats. This was conducted in two sets of experiments, with the VMN and POA investigated in the first set, and the MePD and BNST in the second set. The VMN and POA received intense projections from the MePD and BNST. We used a short hairpin RNA encoded within an adeno-associated viral vector directed against the gene for ERα to reduce the number of ERα in the VMN or POA (first set of experiments) or in the BNST or MePD (second set of experiments) in female rats. The rats were housed in groups of four ovariectomised females and three males in a seminatural environment for 8 days. Compared with traditional test set-ups, the seminatural environment provides an arena in which the rats can express their full behavioural repertoire, which allowed us to investigate multiple aspects of social and sexual behaviour in groups of rats. Behavioural observation was performed after oestrogen and progesterone injections. A reduction of ERα expression in the VMN or POA diminished the display of paracopulatory behaviours and lordosis responses compared to controls, whereas the lordosis quotient remained unaffected. This suggests that ERα in the VMN and POA play an important role in intrinsic sexual motivation. The reduction in ERα did not affect the social behaviour of the females, although the males sniffed and pursued the females with reduced ERα less than the controls. This suggests that the ERα in the VMN and POA is involved in the regulation of sexual attractiveness of females. The ERα in the MePD and BNST, on the other hand, plays no role in sociosexual behaviour.

Estrogens, androgens and generalized behavioral arousal in gonadectomized female and male C57BL/6 mice

General arousal has been operationally defined as an enhanced motor activity and enhanced intensity of response to sensory stimuli. Even though the effects of gonadal hormones on mating behavior have been much studied, their potential effect on generalized arousal, as defined above, has never been evaluated. In the present study we employed a thoroughly validated assay of general arousal to determine the effects of estradiol (E) and testosterone (T) in gonadectomized female and male mice, respectively. The steroids were administered in three different ways: A fast-acting, water soluble preparation given intraperitoneally, an oil solution given subcutaneously, and an oil solution in a subcutaneous Silastic capsule. Motor activity and responses to sensory stimuli were recorded for 24h, 91h, and seven days following hormone administration, respectively. All measures of arousal varied according to the day/night cycle. The water soluble steroid preparation had no reliable effect. When the same doses of estradiol and testosterone were administered subcutaneously in an oil vehicle no effect of either treatment on arousal was observed. The subcutaneously implanted capsule containing estradiol or testosterone had a delayed effect on motor activity in females (four to seven days) but no effect in males. The long time required by the gonadal hormones for affecting arousal would be consistent with, but does not prove, a genomic action. The limited effects of E and T in our arousal assay suggest to us that the strongest actions of these hormones on arousal occur in the context of sequences of responses to sexually relevant stimuli.

Sociosexual behaviors during the transition from non-receptivity to receptivity in rats housed in a seminatural environment

Female behavioral estrus is defined as the period between the first lordosis displayed during the estrus cycle to the lordosis that is not followed by another within 60 min. In a seminatural environment, an estrous female consistently displays lordosis in response to every male mount from the start of behavioral estrus until the end of it. This means that the female suddenly changes from a state of complete non-receptivity to full receptivity and then abruptly changes back to non-receptivity. It is unlikely that these abrupt changes are caused by sudden changes in serum concentration of ovarian steroids. Here, we present the results of a detailed study of sociosexual behaviors during the transition from non-receptivity to receptivity and vice versa. The frequency of male mounting was close to zero before and after estrus. It remained at a constant, high level throughout estrus. Female paracopulatory behavior and male pursuit of the female increased drastically from a very low level before estrus to a high level during estrus. They returned to low levels immediately after estrus. None of the many other behavior patterns registered changed during the transitions. It appears that the sudden increase in male pursuit and female paracopulatory behavior can explain the beginning of behavioral estrus, and their equally sudden disappearance causes it to end. The neurochemical mechanisms behind these almost instantaneous behavioral changes are unknown.

The role of odors and ultrasonic vocalizations in female rat (Rattus norvegicus) partner choice

Intrasexual competition for access to a female mate is believed to be unusual in wild male rats, which suggests that female choosiness could be more important. It has been shown that females spend more time with one male than with others when tested in a multiple partner paradigm. The male of first entry is visited most. The role of ultrasonic vocalizations (USVs) and male odors in the female rat's initial choice to approach one male instead of another are studied in these experiments. In Experiment 1, female rats were allowed to choose between 3 different intact males, whereas in Experiment 2, females could choose between a devocalized male and 2 intact males. Both experiments started with a 15-min period with inaccessible males followed by a 15-min period with accessible males in which the female could copulate with the males of her choice. The results showed that female rats spent more time with the male of first entry over the males visited subsequently. No differences were found in USV subtype patterns emitted by the different males or the time spent sniffing the different males in the period preceding the choice. In addition, the results of Experiment 2 showed that females visited the silent males as much as the vocalizing males. Thus, the present experiments did not offer any evidence suggesting that USVs or individual differences in male odors play any role in female mate choice. Other factors that were not investigated in this study might be involved in female rat mate selection, but it should also be considered that mate selection could be random.

On the role of individual differences in female odor and ultrasonic vocalizations for male's choice of partner

Intrasexual competition for access to a female mate is believed to be unusual in wild male rats, which suggests that female choosiness could be important. Even if competition is unusual, males still have to inevitably approach one partner first for copulation. In females, it has been shown that females spend longer time with one male compared to the others when tested in a multiple partner paradigm. The male mate preference was investigated in this study. In addition, the role of ultrasonic vocalizations (USVs) and female odors in the male's initial choice to approach one female instead of another was studied in this experiment. Male rats could choose between three different sexually receptive females. The experiment started with a 15-minute period with inaccessible females followed by a 15-minute period with accessible females in which the males could copulate with the females of his choice. The results showed that male rats spent more time with the female of 1st entry over the second or third females visited. No differences were found in USV subtype patterns emitted by the different females or the number of sniff episodes towards the different female chambers. Thus, the present experiments did not offer any evidence suggesting that USVs or individual differences in female odors play any role in male mate choice. Other factors that were not investigated in this study might be involved in male mate selection, but it should also be considered that mate selection could be random.

Sociosexual behaviours in cycling, intact female rats (Rattus norvegicus) housed in a seminatural environment

Groups of 3 males and 4 females were housed in a seminatural environment for a period of 8 days. Female sociosexual behaviours were recorded and analysed from the beginning of behavioural oestrus (defined as the first lordosis response observed in the environment) until the end of oestrus (defined to occur at the moment of a lordosis that was not followed by another one within 60 min). The duration of behavioural oestrus varied between 4.05 and 10.87 h, with a mean of 7.41 ± 0.49 h. In order to analyse changes in behaviour during oestrus, the entire period was divided in 5% segments for each female. The frequency and/or duration of some behaviours changed during the period of oestrus, while others remained stable. Most remarkable among the latter are the frequency of lordosis and the lordosis quotient. From the start of oestrus until the end, the females responded with lordosis to every male mount. Rejections were most frequent at the beginning of the period of oestrus. Then there was a decline, and thereafter rejections remained at a low level, even when the end of oestrus was approached. Paracopulatory behaviours reached a maximum towards the end of the first quarter of the oestrus period. There was no evident decline at the end of oestrus. Females having a high frequency of lordosis showed more paracopulatory behaviours and were pursued more by the males than females with low lordosis frequency. When the behaviour of females with a large number of offspring was compared to that of females with a low number of offspring, no difference in sexual behaviours was found. Comparisons between the females’ behaviour in response to preferred (either defined as the male producing the largest number of lordosis responses or as the male that was most sniffed by the female) and non-preferred males revealed a few minor differences. Location of behaviour was also analysed. Comparisons were made between the environment’s open area and burrow and between different sectors in these parts. Some important differences were found. This is the first detailed description of female rat sociosexual behaviour during the entire period of behavioural oestrus in a seminatural environment.

The incentive value of males' 50-kHz ultrasonic vocalizations for female rats (Rattus norvegicus)

Rats emit ultrasonic vocalizations (USVs) of 22 kHz and 50 kHz before, during, and after copulation. The 50-kHz vocalizations can be subdivided into flat and frequency-modulated (FM) trill calls. In this study, the role of 50-kHz USVs in sexual incentive motivation in female rats was examined. USVs were recorded from sexually active males during the precopulatory phase. In the first two experiments, "full 10-min song," flat-call, and FM-trill-call stimuli were used. In Experiment 1, a combination of complex and trill calls was used as the FM-trill call, whereas a mixture of multistep and upward ramp calls was used in Experiment 2. The auditory stimuli were played back to sexually receptive female rats in a sexual incentive-motivation test. For comparison, the odor of an intact male rat was also used as incentive stimulus. The flat-call stimulus did not induce approach behavior in any experiment, whereas the FM trill showed a short-lived effect in Experiment 2. The females approached the "full song" during the first minute of stimulus exposure in Experiment 1, but not in Experiment 2. When the entire 10-min test period was analyzed, the females in Experiment 1 did not approach the full song more than background noise, though they did so in Experiment 2. The olfactory stimulus, to the contrary, had a clear incentive value in both experiments. In a third experiment, a devocalized male, an intact vocalizing male and a female rat were used as incentive stimuli. The females did not approach a vocalizing male more than they approached a silent male, showing that USVs do not contribute to the male rat's incentive value. Overall, the results of the present series of experiments show that the male rat's USVs do not have any consistent incentive value for the sexually receptive female rat. This sharply contrasts the strong and reliable effect of male odor.

Female ultrasonic vocalizations have no incentive value for male rats

Ultrasonic vocalizations (USVs) are emitted in response to a sexual partner before, during, and after copulation. These vocalizations are the so-called 50-kHz USVs and can be subdivided into flat and frequency-modulated (FM) 50-kHz trill calls. In the present series of experiments, the potential unconditioned and conditioned incentive properties of female 50-kHz USVs for male rats were examined. USVs were recorded from sexually receptive females during the precopulatory phase. A complete 10-min song, or single flat or FM trill calls were selected as auditory stimuli for Experiments 1 and 2. As FM trill calls, a multistep call was used in Experiments 1 and 2a, and an upward ramp call was used in Experiment 2b. The auditory stimuli were played back with a loudspeaker to naïve and sexually experienced male rats in a sexual incentive motivation test. The odor of a sexually receptive female rat was also used as an incentive stimulus for comparison. In a third experiment, a devocalized female, a sham female and a male rat were used as incentive stimuli. It was found that the auditory stimuli did not induce approach behavior in naïve and sexually experienced male rats, but the olfactory stimulus did. In addition, the males spent equal amounts of time in the vicinity of devocalized and vocalizing females. These data show that 50-kHz USVs neither are unconditioned nor conditioned incentives for male rats.

Behavioral characteristics of pair bonding in the black tufted-ear marmoset (Callithrix penicillata)

The present study describes how the development of a pair bond modifies social, sexual and aggressive behavior. Five heterosexual pairs of marmosets, previously unknown to each other, were formed at the beginning of the study. At the onset of pairing, social, sexual, exploratory and aggressive behaviors were recorded for 40 min. The animals were then observed for 20 min, both in the morning and afternoon for 21 days. The frequency and/or duration of behaviors recorded on Day 1 were compared to those recorded at later observations. The behavior displayed shortly after pairing should be completely unaffected by the pair bond, while such a bond should be present at later observations. Thus, it was possible to determine how the behavior between the pair was modified by the development of a pair bond. Social behaviors increased from Day 1 to Days 2-6 and all subsequent days observed. Conversely, other behaviors, such as open mouth displays (usually considered to be an invitation to sexual activity), had a high frequency during the early part of cohabitation but declined towards the end. Consequently, pair bonding manifests itself in an increased intensity of social behaviors. It is suggested that the intrinsically rewarding properties of grooming and perhaps other social behaviors turn the pair mate into a positive incentive, activating approach and further interactions when possible. Thus, the pair bond may be a motivational state activated by the conditioned incentive properties of the partner. This notion can explain all forms of pair bonds, including those occurring between individuals of the same sex and in promiscuous species.

Estrogen-induced sexual incentive motivation, proceptivity and receptivity depend on a functional estrogen receptor alpha in the ventromedial nucleus of the hypothalamus but not in the amygdala

The display of copulatory behaviors usually requires the presence of a mate and is, therefore, preceded by a search for and approach to a potential partner. The intensity of approach behaviors is determined by a process labeled sexual incentive motivation. Although it is known that female sexual motivation depends on estrogens, their site of action within the brain is unknown. In the present experiment, we obtained data relevant to this issue. An shRNA encoded within an adeno-associated viral (AAV) vector directed against the estrogen receptor alpha (ERalpha) gene (or containing a nonsense base sequence as a control treatment) was injected bilaterally into the ventromedial nucleus of the hypothalamus (VMN) or the posterodorsal amygdala (MePDA) of female rats. After an 80% reduction of the number of ERalpha in the VMN, sexual incentive motivation was absent after treatment with estradiol and progesterone. Proceptivity and receptivity were also much reduced, while the number of rejections was enhanced. Suppression of the ERalpha in the MePDA lacked these effects. Likewise, the inactive control AAV vector failed to modify any behavior. Thus, the ERalpha in the VMN, but not in the MePDA, is important for proceptivity and receptivity as well as for sexual incentive motivation. These results show that ERalpha in the VMN is crucial for the entire sequence of behavioral events from the processes leading to the establishment of sexual contact until the accomplishment of copulatory behaviors.