Relation of intromissions to the female's postejaculatory refractory period in rats

Being friendly: paced mating for the study of physiological, behavioral, and neuroplastic changes induced by sexual behavior in females

Paced mating in rats is an experimental condition that allows the evaluation of sexual behavior in a way that closely resembles what occurs in seminatural and natural conditions enabling the female to control the rate of the sexual interaction. In conventional non-paced mating tests, females cannot escape from male approaches, which may lead to an unrewarding overstimulation. Paced mating is an alternative laboratory procedure that improves animal welfare and has a higher ethological relevance. The use of this procedure contributed to the identification of physiological and behavioral factors that favor reproduction. Paced mating includes motivational and behavioral components differentiating quantitative and qualitative characteristics that are critical for the induction of the rewarding properties of mating. These positive consequences ensure that the behavior will be repeated, favoring the species' survival. Sexual reward is an immediate consequence of paced mating, mediated mainly by the endogenous opioid system. Paced mating also induces long-lasting neuroplastic changes, including gene expression, synthesis of proteins, and neurogenesis in sex-relevant brain areas. The interest in paced mating is growing since the complexity of its elements and consequences at different levels in a laboratory setting resembles what occurs in natural conditions. In this review, we analyze the classic studies and recent publications demonstrating the advantages of using paced mating to evaluate different aspects of sexual behavior in females.

Neural control of female sexual behaviors

Reproduction is the biological process by which new individuals are produced by their parents. It is the fundamental feature of all known life and is required for the existence of all species. All mammals reproduce sexually, a process that involves the union of two reproductive cells, one from a male and one from a female. Sexual behaviors are a series of actions leading to reproduction. They are composed of appetitive, action, and refractory phases, each supported by dedicated developmentally-wired neural circuits to ensure high reproduction success. In rodents, successful reproduction can only occur during female ovulation. Thus, female sexual behavior is tightly coupled with ovarian activity, namely the estrous cycle. This is achieved through the close interaction between the female sexual behavior circuit and the hypothalamic-pituitary-gonadal (HPG) axis. In this review, we will summarize our current understanding, learned mainly in rodents, regarding the neural circuits underlying each phase of the female sexual behaviors and their interaction with the HPG axis, highlighting the gaps in our knowledge that require future investigation.

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.

Previous sexual experience alters the display of paced mating behavior in female rats

The present study tested whether the display of paced mating behavior in female rats over four weekly tests is affected by sexual experience and whether test parameters, i.e., ending the test based on time or number of stimulations received, influence behavioral changes. In Experiment 1A rats with nonpaced sexual experience returned to the male more quickly overall compared to sexually naïve rats in a 30-min test of paced mating behavior. In Experiment 1B, rats received four weekly 30-min tests with one, different, male rat partner each week. Over the four tests, rats returned to the male significantly more quickly after intromissions, but significantly more slowly after ejaculations. Experiment 2A tested whether sexual experience would influence paced mating behavior in tests with a 15-intromission end criterion and the male replaced after ejaculation. Rats tested weekly under 15-intromission test conditions returned to the male significantly more quickly after intromissions, but no behavioral change was observed after ejaculations. When those same rats were given a 30-min test of paced mating behavior (Experiment 2B), they returned to the male significantly more slowly after ejaculations. Collectively, these data show that sexual experience influences the display of paced mating behavior in female rats and that the test parameters interact with sexual experience to influence the nature of the changes. Sexual experience may facilitate behaviors that promote reproductive success in female rats.

Extended paced mating tests induces conditioned place preference without affecting sexual arousal

One way to evaluate sexual arousal is by measuring approach behavior to sexual incentive stimuli. In our case we measure approach behavior to an originally non-preferred compartment which is associated with the physiological state induced by mating. This change of preference indicative of a positive affective (reward) state can be evaluated by conditioned place preference (CPP). We have shown that the CPP induced by paced mating is mediated by opioids. The administration of opioids also induces a reward state. The present study was designed to compare the rewarding properties of paced mating and a morphine injection. One group of females was allowed to pace the sexual interaction before being placed in the non-preferred compartment. In alternate sessions they received a morphine injection before being placed in the preferred compartment. In another group of females, the treatments were reversed. Only the females placed in the originally non-preferred compartment after paced mating changed their original preference, suggesting that paced mating induces a positive affective, reward, state of higher intensity than a morphine injection of 1mg/kg. In a second experiment we determined if females allowed to pace the sexual interaction for 1h would still developed CPP. No change in preference was observed in the females that mated for 1h without pacing the sexual interaction. On the other hand, females that received between 10 and 15 paced intromissions as well as females that paced the sexual interaction for 1h developed a clear CPP. The second experiment demonstrated that pacing is rewarding even in an extended mating session in which the females received around 25 intromissions and several ejaculations. These results further demonstrate the biological relevance associated with the ability of the female to space coital stimulation received during mating. This positive affective state will contribute to increase sexual arousal the next time a rat finds an appropriate mate.

Paced mating behavior persists in rats with vaginocervical Lidocaine

The present study tested whether the topical application of a local anesthetic (Lidocaine) to the vaginocervical region altered the pattern of paced mating behavior displayed by gonadectomized, hormone-primed female rats. Both rats receiving Lidocaine and rats receiving vehicle exhibited the expected lengthening of contact-return latency as the intensity of the mating stimuli increased (mount<intromission<ejaculation). Although rats given Lidocaine versus vehicle received a greater number of intromissions, no other group differences were observed. The present study found no evidence for a change in behavioral responsiveness following the vaginocervical application of Lidocaine.

Hormonal and testing conditions for the induction of conditioned place preference by paced mating

The ability to control or pace the sexual interaction has important physiological and behavioral consequences for the female rat. Paced mating favors reproduction and induces a positive affective state as revealed by conditioned place preference (CPP). In the present experiment we evaluated: 1) If paced mating induces CPP in naturally cycling females; 2) If females developed a positive affective state if they paced the sexual interaction through a 1- or a 3-hole pacing chamber; 3) If females that mate with the same male without pacing the sexual interaction develop CPP. In the first experiment intact females were divided in 4 different groups; 2 paced the sexual interaction until receiving 1 or 3 ejaculations; the other 2 groups mated, without pacing the sexual interaction, until receiving 1 or 3 ejaculations. Only the group that paced the sexual interaction until receiving 3 ejaculations developed a positive affective state. In experiments 2 and 3 hormonally treated ovariectomized females were used. In experiment 2 females were allowed to pace the sexual interaction through a 1- or a 3-hole pacing chamber: A clear positive affective state was induced in both testing conditions. Finally, in experiment 3 females did not develop CPP for non-paced sex despite the fact that they mated with the same male in the conditioning sessions. These results demonstrate that the pattern of vaginocervical stimulation that the females received by engaging in approach and avoidance behaviors to pace the sexual interaction can induce a positive affective state in naturally cycling females. They also confirm the existence of a threshold of vaginocervical stimulation for paced mating to induce CPP in female rats.

Revisiting post-ejaculation refractory time-what we know and what we do not know in males and in females

INTRODUCTION

The post-ejaculation refractory time (PERT), the period after a single ejaculation when further erections and ejaculations are inhibited, has been studied and well-documented in male rats. Since its first attribution in men by Masters and Johnson and its inaccurate delineation in their graphic sexual response model in 1966 it has been infrequently studied whereas scant attention has been paid to any such possible activity in women after female ejaculation.

AIM

To critically review our current knowledge about PERT in rats and humans and describe and correct shortcomings and errors in previous publications and propose corrections.

METHODS

Review of published literature.

MAIN OUTCOME MEASURES

Identifying evidence-based data to support authority-based facts.

RESULTS

The review exposes the extremely limited evidence-based data that our knowledge of PERT is based on. The paucity of data for most aspects of human PERT is remarkable; even the generally accepted statement that the duration of PERT increases with age has no published support data.

CONCLUSIONS

Despite numerous studies in rats the mechanisms and site(s) of the activity are poorly understood. Dopaminergic and adrenergic pathways are thought to shorten PERT whereas serotonergic pathways lengthen its duration. Raising the brain serotonin levels in men using SSRIs helps reduce early or premature ejaculation. Rats have an absolute PERT (aPERT) during which erection and ejaculation is inhibited and a relative PERT (rPERT) when a stronger or novel stimulus can, whether such phases exist in men is unexamined. Apart from possible depressed activity in the amygdala and penile dorsal nerve and rejection of prolactin as a major factor in PERT little or no significant advance in understanding human male PERT has occurred. No evidence-based data on women's PERT after female ejaculation exists. New investigations in young and older men utilizing brain imaging and electromagnetic tomography are priority studies to accomplish.

Epigenetic programming of phenotypic variations in reproductive strategies in the rat through maternal care

Studies across multiple organisms reveal considerable phenotypic variation in reproductive tactics. In some species, this variation is associated with maternal effects in which variation in maternal investment results in stable individual differences in reproductive function. Recent studies with the rat suggest that maternal effects can alter the function of neuroendocrine systems associated with female sexual behaviour as well as maternal behaviour. These maternal effects appear to be mediated by epigenetic modifications at the promoter for oestrogen receptor alpha (ERalpha) and subsequent effects on gene expression. The tissue-specific nature of such effects may underlie the co-ordinated variation in multiple forms of reproductive function, resulting in distinct reproductive strategies.

Environmental Programming of Phenotypic Diversity in Female Reproductive Strategies

Among invertebrates, certain hermaphroditic species reproduce sexually, but with no process of sexual differentiation. In such cases the brain is bisexual: Each member of the species develops male and female sexual organs and retains the capacity to express both male and female patterns of reproductive behavior. Members of such species can reproduce socially or alone. Mammals and many other species reproduce both sexually and socially, which requires an active process of sexual differentiation of reproductive organs and brain. The primary theme of this chapter is simply that this process admits to variation and thus individual differences in gender-specific patterns of reproductive function. The focus on this chapter is the often neglected variation in the development of reproductive function in the female mammal. The basic premise is that evolution has not defined any single, optimal reproductive phenotype, but rather encourages plasticity in specific reproductive traits among same sex members of the species that are derived from variations in the quality of the prevailing environment during development that are mediated by alterations in parent-offspring interactions. Thus, the variations in parental care that define the reproductive phenotype of the offspring are influenced by the quality of the environment (i.e., nutrient availability, predation, infection, population density, and so on).

The programming of individual differences in defensive responses and reproductive strategies in the rat through variations in maternal care

There are profound maternal effects on individual differences in defensive responses and reproductive strategies in species ranging literally from plants to insects to birds. Maternal effects commonly reflect the quality of the environment and are most likely mediated by the quality of the maternal provision (egg, propagule, etc.), which in turn determines growth rates and adult phenotype. In this paper, we review data from the rat that suggest comparable forms of maternal effects on both defensive responses to threat and reproductive behavior and which are mediated by variations in maternal behavior. Ultimately, we will need to contend with the reality that neural development, function and health are defined by social and economic influences.

Has dopamine a physiological role in the control of sexual behavior? A critical review of the evidence

The role of dopaminergic systems in the control of sexual behavior has been a subject of study for at least 40 years. Not surprisingly, reviews of the area have been published at variable intervals. However, the earlier reviews have been summaries of published research rather than a critical analysis of it. They have focused upon the conclusions presented in the original research papers rather than on evaluating the reliability and functional significance of the data reported to support these conclusions. During the last few years, important new knowledge concerning dopaminergic systems and their behavioral functions as well as the possible role of these systems in sexual behavior has been obtained. For the first time, it is now possible to integrate the data obtained in studies of sexual behavior into the wider context of general dopaminergic functions. To make this possible, we first present an analysis of the nature and organization of sexual behavior followed by a summary of current knowledge about the brain structures of crucial importance for this behavior. We then proceed with a description of the dopaminergic systems within or projecting to these structures. Whenever possible, we also try to include data on the electrophysiological actions of dopamine. Thereafter, we proceed with analyses of pharmacological data and release studies, both in males and in females. Consistently throughout this discussion, we make an effort to distinguish pharmacological effects on sexual behavior from a possible physiological role of dopamine. By pharmacological effects, we mean here drug-induced alterations in behavior that are not the result of the normal actions of synaptically released dopamine in the untreated animal. The conclusion of this endeavor is that pharmacological effects of dopaminergic drugs are variable in both males and females, independently of whether the drugs are administered systemically or intracerebrally. We conclude that the pharmacological data basically reinforce the notion that dopamine is important for motor functions and general arousal. These actions could, in fact, explain most of the effects seen on sexual behavior. Studies of dopamine release, in both males and females, have focused on the nucleus accumbens, a structure with at most a marginal importance for sexual behavior. Since accumbens dopamine release is associated with all kinds of events, aversive as well as appetitive, it can have no specific effect on sexual behavior but promotes arousal and activation of non-specific motor patterns. Preoptic and paraventricular nucleus release of dopamine may have some relationship to mechanisms of ejaculation or to the neuroendocrine consequences of sexual activity or they can be related to other autonomic processes associated with copulation. There is no compelling indication in existing experimental data that dopamine is of any particular importance for sexual motivation. There is experimental evidence showing that it is of no importance for sexual reward.

The display of sexual behaviors by female rats administered ICI 182,780

ICI 182,780 (ICI) is a pure antiestrogen that when administered systemically does not cross the blood-brain barrier, thus its actions are limited to the periphery. Four experiments were conducted to test the effects of ICI on the display of sexual behaviors in ovariectomized rats. Experiment 1 examined the effects of three doses of ICI (250, 500, and 750 micro g/rat) on sexual receptivity and paced mating behavior in rats primed with estradiol benzoate (EB) in combination with progesterone (P). Experiments 2 and 3 compared the display of sexual behaviors in rats primed with EB+P or EB alone and administered either 250 micro g ICI (Experiment 2) or 500 micro g ICI (Experiment 3). Experiment 4 tested the effects of ICI (250 and 500 micro g) on the expression of estrogen-induced progestin receptors in the uterus. ICI did not affect the display of sexual receptivity in any experiment. In rats primed with EB+P, paced mating behavior was altered by the 500 and 750 micro g, but not the 250 micro g, doses of ICI. The lowest (250 micro g) dose of ICI did alter paced mating behavior in rats primed with EB alone. The effects of ICI on paced mating behavior were manifested by a substantial lengthening of contact-return latencies following intromissions and ejaculations. The percentage of exits were not affected by ICI. Estrogen stimulation of uterine weight and induction of uterine progestin receptors was suppressed by ICI (250 and 500 micro g). ICI effects on paced mating behavior in hormone-primed female rats are likely to reflect antiestrogenic actions in the periphery, including interference with the estrogen induction of progestin receptors.

What do female rats like about sex? Paced mating

The motivational aspects of female sexual behavior have been evaluated by a variety of methodologies including: the increasing barrier method, the runway procedure, partner preference test, operant behavior and conditioned place preference. When female rats are tested for sexual receptivity under traditional laboratory conditions, usually a small open area, both appetitive and aversive components of the sexual interaction are easily observed. For example, after prolonged testing, subsequent lordosis and the intensity of this response are reduced increasing the rejection behavior by the female. However, when female rats are allowed to pace (control) the rate of sexual stimulation they received, as usually occurs under seminatural and natural conditions, the aversive properties of mating are reduced. The conditioned place preference can be use to measure the positive affect elicited by mating. We have combined pacing and conditioned place preference in an attempt to reduced the possible aversive consequences associated with mating and increase the likelihood of detecting the appetitive effects of coital interaction in female rats. Only female rats that regulated (paced) their coital interactions with a stud male through a two-compartment chamber in which only the female could freely move from one compartment to the other developed a clear place preference. As well, females that received ten or 15 paced intromissions (without ejaculation) also developed place preference. The place preference induce by paced mating is blocked by the systemic administration of naloxone suggesting that opioids are involved in the reward processes associated with paced mating. Paced sexual interactions can induce a positive affect of sufficient intensity and duration to induce conditioning.

Appetitive and consummatory sexual behaviors of female rats in bilevel chambers. I. A correlational and factor analysis and the effects of ovarian hormones

This study investigated measures of sexual behavior displayed by female rats in bilevel chambers, the statistical relationships among the measures, and their dependency on hormone priming. Normative data from a standard 35-min test of sexual behavior were gathered from 82 fully primed sexually experienced Long-Evans females and subjected to multiple correlational and factor analyses. Several consummatory measures of copulation were related significantly, whereas appetitive level changing was statistically independent of consummatory measures. Factor analyses were conducted using orthogonal rotations of correlational matrices derived either from (a) measures of female behavior alone or (b) measures of female and male behavior together. The first analysis revealed five factors that accounted for 84% of the intersubject variance: Receptivity, Pacing, Appetitive Level Changing, Lordosis Reflex, and Solicitation. The second factor analysis with male data included revealed seven factors that accounted for 95% of the intersubject variance: Pacing, Copulatory Rate, Mount Count, Receptivity, Appetitive Level Changing, Solicitation, and Lordosis Reflex. Subsequently, subsets of these females were maintained on different steroid priming regimens (oil, low estrogen, high estrogen, high estrogen and progesterone) prior to a standard test of sexual behavior. Although the expression of all sexual behaviors required estrogen priming, appetitive level changing, solicitation, and pacing required progesterone for their full expression. Finally, appetitive level changing developed following hormone treatment alone, regardless of whether the females received access to sexually active males, inactive castrated males, or other females. Use of bilevel chambers allows complex patterns of sexual behavior to be observed in female rats and may thus facilitate the identification of neurochemical or endocrine mechanisms associated with different aspects of female sexual motivation and performance.

Influence of male-related stimuli on female postejaculatory refractory period in rats

Female rats "pace" their sexual contacts with the male when tested in situations where they can escape from the male during copulation. The type and quality of vaginocervical stimulation that the females receive during copulation influences their pacing behavior. This study investigated the effect of several male-related stimuli on the female's postejaculatory refractory period (PER). Females were tested in a two-compartment test chamber from which they could escape the male through one of four openings along the bottom of the barrier separating the two compartments. Experiment 1 examined the influence of the seminal plug, the penile cup, and prostate secretions on the female's PER. Results showed that neither the seminal plug, the penile cup, nor prostate secretions contributed to the female's PER. Experiment 2 investigated the relation of pre-ejaculatory intromission frequency, ejaculation duration, and the number of pelvic thrusts during ejaculation to the female's PER. Results indicated that pre-ejaculatory intromission frequency and ejaculation duration but not the number of pelvic thrusts during ejaculation were significantly correlated with the female's PER. In addition, pre-ejaculatory intromission frequency was significantly correlated with ejaculation duration. Partial correlation analysis suggested that pre-ejaculatory intromission frequency affected ejaculation duration which, in turn, influenced the female's PER. This finding was further supported by the evidence that ejaculation duration and the female's PER were significantly shorter in tests in which the male ejaculated on the first or second intromission.

Function of intromissions on intromission-return latency of female rats during paced sexual behavior

The objectives of this study were to examine how multiple intromissions affect the temporal pattern of the female rat's copulatory behavior; in particular, her latency to return to the male following intromission (intromission-return latency, IRL) and if different hormone replacement regimens affect the temporal aspects of female copulatory behavior. Repeated intromissions alone, without ejaculation, often resulted in prolonged IRLs equal to the postej aculatory refractory period (PER). The first prolonged IRL occurred most frequently between the 24th and 44th intromission. The similar pattern of IRLs around the PER and the prolonged IRLs may indicate that the mechanisms mediating the occurrence of the prolonged IRL are similar to those for the PER. One possible function of the prolonged IRLs may be to facilitate the male's ejaculation after the female has received enough vaginocervical stimulation for the induction of the progestational state of pregnancy. Finally, females receiving a single dose of 50 microg estradiol benzoate (EB) followed by an injection of 0.5 mg progesterone (P) 48 h later showed a significantly longer PER than those receiving 3 daily injections of 0.5 microg EB followed by an injection of 0.5 mg P 24 h after the last EB injection.