“Some guys have all the luck”: Mate preference influences paced-mating behavior in female rats

Decrease in ERɑ within the BNST of sexually naïve male rats following an encounter with a novel female

Testosterone and its metabolites facilitate male-typical social behaviors in sexually experienced animals. The metabolite estradiol acts on estrogen receptors (ERs) within the bed nucleus of the stria terminalis (BNST) to facilitate socio-sexual behaviors. While circulating testosterone does not increase in naïve males, aromatase-expressing neurons within the BNST of naïve males are necessary for sex recognition, suggesting that local estradiol production may be responsible. In the present study, we examined ERɑ-immunoreactive (ir) cell number within the brain of sexually naïve male rats 24 h after an encounter with a novel animal. As expected, males investigated females more than males. Additionally, males that encountered females had fewer ERɑ-ir cells within both anterior and posterior BNST compared to those who encountered a novel male or a non-social control. There were no changes within the AVPV, MPN, or MeA. The decrease in ERɑ-ir cell number within the posterior BNST only occurred in males that encountered estrus females whereas the decrease in the anterior BNST occurred only in males that encountered non-estrus females. Additionally, anogenital investigations were correlated with fewer ERɑ-ir cells in the posterior BNST, while cage sniffing correlated with the number ERɑ-ir cells in the anterior BNST. There were no differences in serum testosterone 45 min or 24 h after the encounter, suggesting changes in ERɑ were due to local changes in estradiol levels. Our results expand upon previous research regarding the role of estradiol within the subregions of the BNST in naïve male rat socio-sexual behavior.

Paced Mating Behaviour Is Influenced by Duration of Female Post-Ejaculatory Interval

BACKGROUND

Laboratory paradigms that enable the female rat to control the frequency and temporal distribution of sexual stimulation are well-suited to gaining knowledge about female sexual function; however, the variety of procedures used influence the specific behaviors exhibited by female rats and bring uncertainty into the conclusions that can be drawn.

AIM

In this study, we evaluated the effects of test parameters on the display of paced mating behavior in female rats to develop better preclinical models for exploring female sexual health.

METHODS

Sexually experienced, estradiol- and progesterone-primed female rats were tested under paced mating conditions to determine whether sexual behavior differed as a function of number of male partners (1 or 3; Experiment 1), the time span between receipt of an ejaculation and the next intromission (ie, the post-ejaculatory interval or PEI; Experiment 2), or the duration of ejaculations (Experiment 3).

OUTCOMES

Contact-return latency, exit latency, and withdrawal duration after mounts, intromissions, and ejaculations.

RESULTS

The shorter withdrawal latency after intromission and longer return latency after ejaculation observed in 30-minute paced mating tests is best attributed to the female's PEI. The duration of the PEI is a specific factor that affects the display of paced mating behavior.

CLINICAL TRANSLATION

Understanding of neurobiological mechanisms and sensory factors influencing sexual behavior learned from these models can be applied to studies of human female sexual health.

STRENGTHS & LIMITATIONS

Sexual motivation is indicated by analogous behaviors and supported by evolutionarily conserved systems in women and rats, meaning that animal models can be used to identify neurobiological mechanisms subserving these behaviors. Nonetheless, data related specifically to physiological effects of different mating paradigms are needed in future research.

CONCLUSION

Procedures that mimic naturalistic settings, and thus enable female sexual motivation to drive behavior, are apt for studies aimed at understanding mechanisms supporting female sexual function whereas artificially lengthening the interval between an ejaculation and subsequent intromission may provide a model to study female sexual dysfunction.

Hearing, touching, and multisensory integration during mate choice

Mate choice is a potent generator of diversity and a fundamental pillar for sexual selection and evolution. Mate choice is a multistage affair, where complex sensory information and elaborate actions are used to identify, scrutinize, and evaluate potential mating partners. While widely accepted that communication during mate assessment relies on multimodal cues, most studies investigating the mechanisms controlling this fundamental behavior have restricted their focus to the dominant sensory modality used by the species under examination, such as vision in humans and smell in rodents. However, despite their undeniable importance for the initial recognition, attraction, and approach towards a potential mate, other modalities gain relevance as the interaction progresses, amongst which are touch and audition. In this review, we will: (1) focus on recent findings of how touch and audition can contribute to the evaluation and choice of mating partners, and (2) outline our current knowledge regarding the neuronal circuits processing touch and audition (amongst others) in the context of mate choice and ask (3) how these neural circuits are connected to areas that have been studied in the light of multisensory integration.

Adolescence and Postpartum: Two Life Periods to Deepen Our Understanding of the Complexity of Female Rat Sexual Behavior

The sexual behavior of the female rat has been extensively studied and used as a model for human female sexuality. The development of models that focus on sexual interaction from the female’s perspective has demonstrated the complexity of this behavior, in which motivational and affective aspects have great relevance. This review describes, from a historical perspective, some of the studies that have shed light on this complexity and presents the potential of two periods in the female’s life to expand our understanding of sexual behavior: adolescence and postpartum estrus. Cycling adolescent female rats can express sexual behavior even though some of the underlying circuitry is still developing. Therefore, this is an ideal time for studying the ontogeny of sexual behavior, the interaction of sexual and social motivations, and the neuroendocrine changes that lead to adult behavior. Female rats also show a fertile estrus a few hours after parturition. During this period, rats are simultaneously sexually and maternally motivated, making postpartum estrus an excellent model for exploring the interaction of these two social motivations. What we know so far about female sexual behavior in these unique periods of life reaffirms its richness and shows that much remains to be known.

Sexual behavior in rodents: Where do we go from here?

Hormones and Behavior was first published 50 years ago including some articles related to the hormonal regulation of sexual behavior in different species. Since then, this research field has produced outstanding discoveries that have contributed to our understanding of the control of sexual behavior. The refinement of classical techniques and the development of new experimental tools has opened the door to a new era of research that will allow us to understand different aspects of sexual behavior. It would also expand the possible extrapolation from animal models to understand human sexuality and its dysfunctions. In this review, we summarize some of the most recent findings about sexual behavior in both sexes including the refinement of classical methods of study with new approaches and questions as well as the development of new methods trying to explain mechanisms of action on motivational and consummatory elements of mating behavior. We also reviewed other aspects that modulate sexual behavior such as attractivity, olfactory signals and learning which model mate selection. Additionally, we described studies demonstrating that sexual behavior induces permanent brain modifications in neuronal circuits. Finally, we briefly describe recent contributions on animal models of human sexuality dysfunctions which, although with their own limitations, are under continuous refining.

"What a Girl Wants": What Can We Learn From Animal Models of Female Sexual Motivation?

Sexual motivation is notably different than other motivations such as hunger and thirst, because it lacks homeostatic drive. Sexual motivation poses no threat to physical well-being; individual survival is not at stake. Nevertheless, sexual motivation is a powerful drive and is critical for species survival. Understanding the complexity of sexual motivation has the potential to advance our understanding of other motivations, even pathological motivations, such as those associated with substance abuse. The study of motivation that is unique to females has often been neglected. A number of paradigms have been developed to investigate female sexual motivation beyond measuring only the lordosis reflex. Lordosis is a reflexive posture displayed by female mammals in response to male sexual stimulation to facilitate intromission. The lordosis reflex is essential, but studying the drive to mate is compromised in the absence of robust lordosis. Therefore, appetitive measures of sexual behavior (e.g., preferences, solicitation behaviors) are more specific and more sensitive indicators of sexual motivation than lordosis alone. Paradigms designed to study female sexual motivation often provide a female subject with the choice to interact with a sexually vigorous male or either a non-sexual partner (i.e., female, castrated male) or to remain alone. The study of appetitive measures of sexual motivation has elucidated the role of hormones in female sexual motivation, as well as the underlying neural pathways. The present review describes methods for studying female rats to advance our understanding of sexual motivation and sexual dysfunction.

Sex, Drugs, and the Medial Amygdala: A Model of Enhanced Sexual Motivation in the Female Rat

Methamphetamine (METH) is a psychomotor stimulant that is reported to enhance sexual desire and behavior in both men and women, leading to increases in unplanned pregnancies, sexually-transmitted infections, and even comorbid psychiatric conditions. Here, we discuss our rodent model of increased sexually-motivated behaviors in which the co-administration of METH and the ovarian hormones, estradiol and progesterone, intensify the incentive properties of a sexual stimulus and increases measures of sexually-motivated behavior in the presence of an androgen-specific cue. We then present the neurobiological mechanisms by which this heightened motivational salience is mediated by the actions of METH and ovarian hormones, particularly progestins, in the posterodorsal medial nucleus of the amygdala (MePD), a key integration site for sexually-relevant sensory information with generalized arousal. We finally demonstrate the cellular and molecular mechanisms underlying this facilitation of sexual motivation by METH, including the upregulation, increased phosphorylation, and activation of progestin receptors (PRs) in the MePD by METH in the presence of ovarian hormones. Taken together, this work extends our understanding of the neurobiology of female sexual motivation.

Sexual experience confers resilience to restraint stress in female rats

During paced mating, sexually experienced female rats spend more time with the male, return to the male more quickly after intromission, and exhibit shorter interintromission intervals as compared to sexually naïve rats. Factors that trigger the shift in paced mating behavior are unknown. The present study used the elevated plus maze to test whether anxiety-like behavior differs as a function of sexual experience. Ovariectomized, Long-Evans female rats were primed with estradiol benzoate plus progesterone (EB + P) and then either received four, twice weekly, paced mating treatments to gain sexual experience (Experienced) or remained sexually naïve (Naïve) but were exposed to an empty mating apparatus. In Experiment 1, anxiety-like behavior was compared between Experienced or Naïve female rats that were primed with either EB + P or oil. Significantly more time was spent in open arms under EB + P vs. oil, independent of sexual history. To test whether exposure to an acute stressor before elevated plus maze testing affected anxiety-like behavior, EB + P treated, Experienced or Naïve rats received paced mating (Experiment 2) or restraint (Experiment 3) immediately prior to the elevated plus maze task. Restraint, but not mating, led to less anxiety-like behaviors for Experienced rats compared to Naïve rats. Collectively, our data shows that one component of the shift in paced mating behavior that occurs with sexual experience appears to be altered stress responsiveness. We propose that mating is a beneficial stressor that, when repeated, increases the ability to cope with anxiety-producing events such as aversive components of mating or non-voluntary stressors.

Sexually experienced, but not naïve, female rats show a conditioned object preference (COP) for mating after a single training trial

Female rats with mating experience spend more time with the male rat, exhibit shorter contact-return latency to intromission, and display more proceptive behaviors in the male rat's compartment than during the first mating experience. The present study tested 1) whether mating induced conditioned object preference (COP) is possible with a single conditioning trial and 2) whether a preference is induced for an object associated with the first mating encounter or the fifth mating encounter in female rats. Ovariectomized, Long-Evans female rats were primed with estradiol benzoate + progesterone and either exposed to an empty paced mating chamber for 15 min (Naïve) or received a 15 intromission test of paced mating behavior (Experienced) on four separate occasions before undergoing the COP procedure. Experienced, but not Naïve, female rats developed a COP for a single mating bout, indicating that mating is highly rewarding for sexually experienced female rats. The findings raise questions about the effect of sexual experience on reward regions in the brain, the responsiveness of genital tissue, and learning mechanisms.

Mate choice could be random in female rats (Rattus norvegicus)

Female mate choice is often investigated in terms of reproductive success in order to understand how male characteristics contribute to sexual attractiveness. Previous studies have found that females rats prefer mating with their first encounter rather than males visited subsequently, suggesting that the rewarding value of this first encounter is enough to reinforce mating with the first partner. Using a multiple chambers paradigm, we allowed female rats to copulate freely with three males placed each in a different chamber. Then, we switched the males' position, and let the female interact with them freely again within the same session. We tested whether female mate choice was relying rather on a preferred male rat or on a preferred mating location. The results showed that females spent most time with the male in the chamber of 1st entry in the beginning, but as soon as male rats switched chambers, the female rat continued to copulate with the new male in the same chamber of 1st entry, instead of mating with her previously preferred male rat. This suggests that the male preference is an artefact of location preference. Therefore, female mate choice seems to be rather random than the consequence of an individual choice based on male characteristics. This finding, although contradictory with the intuitive feeling that mate choice is a crucial feature in sexual and reproductive behavior, is supported by several recent observations. In the coming years, behavioral neuroscience should bring light to the brain processes at work in random mate choice.

The sexual preference of female rats is influenced by males' adolescent social stress history and social status

Ongoing development of brain systems for social behaviour renders these systems susceptible to the influence of stressors in adolescence. We previously found that adult male rats that underwent social instability stress (SS) in mid-adolescence had decreased sexual performance compared with control males (CTL). Here, we test the hypotheses that SS in adolescence decreases the "attractiveness" of male rats as sexual partners compared with CTL rats and that dominance status is a protective factor against the effects of SS. The main prediction was that females would spend more time with CTL males than SS males, and that this bias would be greater for submissive than for dominant rats. Among dominant pairs (n=16), females preferred SS males, spending more time with and visiting more often SS than CTL males (each pair tested 5×), and SS males had shorter latencies to ejaculation, shorter inter-ejaculation intervals, and made more ejaculations compared with CTL males. Among submissive pairs (n=16), females spent more time with, visited more often, and displayed more paracopulatory behaviour with CTL than with SS males, and differences in sexual performance between SS and CTL males were modest and in the opposite direction from that in dominant pairs. The heightened motivation of SS males relative to CTL males for natural rewards may have attenuated differences in sexual performance in a paced mating context. In sum, the experience of stress in adolescence leads to long-lasting changes in males that are perceptible to females, are moderated by social status, and influence sexual behaviour.

Multiple Paternity in the Norway Rat, Rattus norvegicus, from Urban Slums in Salvador, Brazil

The Norway rat, Rattus norvegicus, is one of the most important pest species globally and the main reservoir of leptospires causing human leptospirosis in the urban slums of tropical regions. Rodent control is a frequent strategy in those settings to prevent the disease but rapid growth from residual populations and immigration limit the long-term effectiveness of interventions. To characterize the breeding ecology of R. norvegicus and provide needed information for the level of genetic mixing, which can help identify inter-connected eradication units, we estimated the occurrence of multiple paternity, distances between mothers and sires, and inbreeding in rats from urban slum habitat in Salvador, Brazil. We genotyped 9 pregnant females, their 66 offspring, and 371 males at 16 microsatellite loci. Multiple paternity was observed in 22% (2/9) of the study litters. Of the 12 sires that contributed to the 9 litters, we identified 5 (42%) of those sires among our genotyped males. Related males were captured in close proximity to pregnant females (the mean inter-parent trapping distance per litter was 70 m, ±58 m SD). Levels of relatedness between mother-sire pairs were higher than expected and significantly higher than relatedness between all females and non-sire males. Our findings indicate multiple paternity is common, inbreeding is apparent, and that mother-sire dyads occur in close proximity within the study area. This information is relevant to improve the spatial definition of the eradication units that may enhance the effectiveness of rodent management programs aimed at preventing human leptospirosis. High levels of inbreeding may also be a sign that eradication efforts are successful.

Sexual attractiveness in male rats is associated with greater concentration of major urinary proteins

Female rats show a distinct attraction for males. This attraction remains consistent without the necessity for the physical presence of the male. However, the identity of the olfactory cues contributing to attraction in rats remains unknown. Rat urine contains copious amounts of major urinary proteins (MUPs). Here, we investigated the hypothesis that MUPs mediate sexual attractiveness in rats. We first demonstrated that a member of a male dyad receiving greater copulatory opportunities in competitive mate choice tests excrete greater amounts of MUPs. Furthermore, the amount of male MUPs positively correlated with both copulatory opportunities received and female exploration of the urine. Using females and a two-choice olfactory attraction test, we demonstrated that urinary fractions containing MUPs were sufficient to induce attraction and that male MUPs activated neurons in the posterodorsal medial amygdala in female rats. Taken together, these results suggest that olfactory cues associated with MUPs act as an attractant to female rats in estrus.

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.

Concordant female mate preferences in the cichlid fish Tropheus moorii

Discriminating female mate preferences enhance the variance in reproductive success among males of a population and create a potential for sexual selection, which can account for trait evolution and diversification. Fish color patterns are among the prime targets of mate choice-driven sexual selection. Populations of the cichlid Tropheus from Lake Tanganyika display remarkable geographic color pattern variation, but the role of female choice in their rapid and rich phenotypic diversification is unclear. Males and females establish a pair bond prior to spawning monogamously, but as brood care is strictly maternal, female investment in reproduction is high and the operational sex ratio is male-biased. Therefore, variance in male reproductive success can accrue if individual males succeed repeatedly in securing a mate. To test this prediction in the red colored Tropheus moorii "Chimba", four pairs of males were presented to a series of females and female mate preferences were inferred from pairwise interactions. There was a significant difference in mating success between the males of each pair (P < 0.001 over all trials), as-with one exception-females shared preferences for the same males. Male courtship activity was strongly correlated with female choice. Our experiment suggests that female choice contributes to the variance in male reproductive success in the tested population.

Methamphetamine enhances sexual behavior in female rats

The present study evaluated the effects of methamphetamine (MA) on sexual behavior in female rats. In Experiment 1, ovariectomized, hormone-primed rats were injected with MA (1.0mg/kg, i.p.) or saline prior to a test for mate choice wherein females could mate with two males simultaneously. Female rats treated with saline returned to their preferred mate faster after receiving intromissions and visited their preferred mate at a higher rate than their non-preferred mate. In contrast, MA-treated female rats spent a similar amount of time with their preferred and non-preferred mate and failed to return to their preferred mate faster than to their non-preferred mate following intromissions. Two weeks later, the females received the same drug treatment but were tested for partner preference wherein females could spend time near a male or female stimulus rat. All subjects spent more time near the male stimulus than the female stimulus. However, the MA-treated rats visited the male stimulus more frequently and spent less time near the female stimulus than the saline-treated rats. Similar to Experiment 1, female rats in Experiment 2 were tested for mate choice and then two weeks later tested for partner preference; however, females received three daily injections of MA (1.0mg/kg, i.p.) or saline. Females treated chronically with MA returned to both males faster following intromissions than females treated with saline, independent of preference (i.e., preferred mate and non-preferred mate). Furthermore, MA-treated rats were more likely to leave either male (i.e., preferred or non-preferred mate) than saline-treated rats after receiving sexual stimulation. Although MA-treated subjects spent more time near the male stimulus than the female stimulus, they spent less time near either when compared to saline-treated subjects. The present results demonstrate that MA affects sexual behavior in female rats partly by increasing locomotion and partly by directly affecting sexual behavior.

A new female rat animal model for hypoactive sexual desire disorder; behavioral and pharmacological evidence

INTRODUCTION

Female sexual dysfunction (FSD) affects 33-48% of women. Female rats with low sexual activity might model FSD.

AIM

In this study, we have investigated whether in a population of normal female rats, subpopulations of rats exist with different levels of sexual behavior.

METHODS

Sexually experienced, intact, estradiol-primed female rats were placed in an empty compartment adjacent to a compartment with a male. The females were allowed, during 30 minutes, to switch between the compartments via a hole through which only the females could pass (paced mating). Next, we investigated the acute effects on female sexual behavior of apomorphine, a D(1) - and D(2) -type dopamine receptor agonist, (+/-)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (± 8-OH-DPAT), a 5-HT1A receptor agonist, and paroxetine, a selective serotonin reuptake inhibitor.

MAIN OUTCOME MEASURES

Time spent in compartments, proceptive behaviors, contact-return latencies, and percentages of exits were quantified.

RESULTS

Based on their behavior in the paced mating sex test, estradiol-primed, intact female rats can be divided into three groups: those that mostly avoid the male, a large middle group, and those that mostly approach the male. The avoiders also showed significantly less proceptive behavior than the male approachers. The sexual behavior of the females was relatively stable over time, suggesting the existence of different endophenotypes in female rats. Apomorphine and ± 8-OH-DPAT had an inhibiting effect on sexual behavior, but only females dosed with apomorphine showed a different response in avoiders and approachers, more inhibiting effect in avoiders than approachers. Paroxetine had no effect on proceptive behavior.

DISCUSSION

The stable, male-avoiding behavior of some females might correspond to the characteristics of women with FSD. Therefore, these avoiders are a promising new model for FSD, specifically for sexual desire and/or arousal disorders. Furthermore, the apomorphine data suggest that differences in the dopamine system may (partly) underlie the differences in sexual behaviors between avoiders and approachers.

"Sex, drugs and the brain": the interaction between drugs of abuse and sexual behavior in the female rat

Preclinical and clinical research investigating female sexual motivation has lagged behind research on male sexual function. The present review summarizes recent advances in our understanding of the specific roles of various brain areas, as well as our understanding of the role of dopaminergic neurotransmission in sexual motivation of the female rat. A number of behavioral paradigms that can be used to thoroughly evaluate sexual behavior in the female rat are first discussed. Although traditional assessment of the reflexive, lordosis posture has been useful in understanding the neuroanatomical and neurochemical systems that contribute to copulatory behavior, the additional behavioral paradigms described in this review have helped us expand our understanding of appetitive and consumatory behavioral patterns that better assess sexual motivation - the equivalent of "desire" in humans. A summary of numerous lesion studies indicates that different areas of the brain, including forebrain and midbrain structures, work together to produce the complex repertoire of female sexual behavior. In addition, by investigating the effects of commonly addictive drugs, we are beginning to elucidate the role of dopaminergic neurotransmission in female sexual motivation. Consequently, research in this area may contribute to meaningful advances in the treatment of human female sexual dysfunction.

Maternal influences on the sexual behavior and reproductive success of the female rat

In many species, including humans, there is evidence for parental effects on within-sex variations in reproductive behavior. In the present studies we found that variations in postnatal maternal care were associated with individual differences in female sexual behavior in the rat. Females born to and reared by dams that showed enhanced pup licking/grooming (i.e., High LG mothers) over the first week postpartum showed significantly reduced sexual receptivity and alterations in the pacing of male mounting (i.e., longer inter-intromission intervals) observed in a paced mating test. There were minimal effects on the sexual behavior of the male offspring. The female offspring of High LG mothers showed a reduced lordosis rating, a decreased mount:intromission ratio, received fewer ejaculations and were less likely to achieve pregnancy following mating in the paced mating context. The data suggest maternal influences on the sexual development of the female rat that are functionally relevant for reproductive success. Together with previous studies these findings imply that maternal care can 'program' reproductive strategies in the female rat.