Sexual reinforcement in the female rat

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.

Membrane estrogen receptor signaling impacts the reward circuitry of the female brain to influence motivated behaviors

Within the adult female, estrogen signaling is well-described as an integral component of the physiologically significant hypothalamic-pituitary-gonadal axis. In rodents, the timing of ovulation is intrinsically entwined with the display of sexual receptivity. For decades, the importance of estradiol activating intracellular estrogen receptors within the hypothalamus and midbrain/spinal cord lordosis circuits has been appreciated. These signaling pathways primarily account for the ability of the female to reproduce. Yet, often overlooked is that the desire to reproduce is also tightly regulated by estrogen receptor signaling. This lack of emphasis can be attributed to an absence of nuclear estrogen receptors in brain regions associated with reward, such as the nucleus accumbens, which are associated with motivated behaviors. This review outlines how membrane-localized estrogen receptors affect metabotropic glutamate receptor signaling within the rodent nucleus accumbens. In addition, we discuss how, as estrogens drive increased motivation for reproduction, they also produce the untoward side effect of heightening female vulnerability to drug addiction.

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.

Use of an operant paradigm for the study of antidepressant-induced sexual dysfunction

These studies were designed to develop a paradigm for the detection of antidepressant-induced sexual dysfunction in female rats. Ovariectomized, Fischer rats were conditioned to nose poke to open a guillotine door to gain access to a sexually active male. To develop the procedure, we examined the acquisition and stability of the response with a 15-s fixed interval, compared rats treated with 10 μg estradiol benzoate and 500 μg progesterone with those that received only estradiol benzoate, and carried out a preliminary analysis of the effects of 5, 10, and 15 mg/kg fluoxetine. We then more fully evaluated the effects of 5 mg/kg fluoxetine. Fluoxetine reduced sexual motivation, as assessed by the number of nose pokes, the number of nose poke episodes, and the latency to approach the male. In addition, changes in the females' sexual motivation were examined before and after ejaculation during the final conditioning trials. The number of nose pokes was reduced and the latency to initiate a new nose poke episode was increased following ejaculation. The robustness of the antidepressant-induced decline in sexual motivation is in marked contrast to the findings with several other animal models for sexual dysfunction and illustrates the usefulness of the operant procedure.

Who, what, where, when (and maybe even why)? How the experience of sexual reward connects sexual desire, preference, and performance

Although sexual behavior is controlled by hormonal and neurochemical actions in the brain, sexual experience induces a degree of plasticity that allows animals to form instrumental and Pavlovian associations that predict sexual outcomes, thereby directing the strength of sexual responding. This review describes how experience with sexual reward strengthens the development of sexual behavior and induces sexually-conditioned place and partner preferences in rats. In both male and female rats, early sexual experience with partners scented with a neutral or even noxious odor induces a preference for scented partners in subsequent choice tests. Those preferences can also be induced by injections of morphine or oxytocin paired with a male rat's first exposure to scented females, indicating that pharmacological activation of opioid or oxytocin receptors can "stand in" for the sexual reward-related neurochemical processes normally activated by sexual stimulation. Conversely, conditioned place or partner preferences can be blocked by the opioid receptor antagonist naloxone. A somatosensory cue (a rodent jacket) paired with sexual reward comes to elicit sexual arousal in male rats, such that paired rats with the jacket off show dramatic copulatory deficits. We propose that endogenous opioid activation forms the basis of sexual reward, which also sensitizes hypothalamic and mesolimbic dopamine systems in the presence of cues that predict sexual reward. Those systems act to focus attention on, and activate goal-directed behavior toward, reward-related stimuli. Thus, a critical period exists during an individual's early sexual experience that creates a "love map" or Gestalt of features, movements, feelings, and interpersonal interactions associated with sexual reward.

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.

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.

Operant animal welfare: productive approaches and persistent difficulties

Operant procedures occupy a prominent role within animal welfare science because they provide information about the strength of animals’ preferences. It is assumed that strongly motivated choices commonly indicate conditions necessary for uncompromised welfare. A review of the literature shows that members of many species will work for access to resources not commonly provided to them; including a secure resting place (perches for hens or boxes for rodents) and substrates for species-typical activities such as nesting, digging and rooting (in hens, rats, mice and pigs). Despite a recent surge in popularity, operant techniques remain under-utilised and studies employing them struggle to find the best method for prioritising resources. In order to fully exploit the potential of operant procedures a wider appreciation of the relevant theories and techniques might be beneficial; including greater employment of the basic principles of reinforcement theory and further development of more complex economic analogies. If these two strands of research develop together, operant approaches have a key role to play in refining and replacing husbandry practices that undermine animal welfare.

Social and sexual motivation in the mouse

Does a sexual encounter have reward value for a learned operant response? Ovariectomized female mice with or without estradiol replacement were trained to perform a bar-contact operant response for either male or female targets. Response rates of females with estradiol replacement did not differ from those of females without estradiol replacement or females responding for access to females. Reflexive receptive sexual behavior remained responsive to estradiol replacement. Experiment 2 demonstrated that socially isolated females would respond faster for access to a female target than when group housed. Finally, the oxytocin blocker, atosiban, reduced both operant and reflexive social behavior. These results converge on the conclusion that the operant reward value of social and sexual contact is primarily social.

Sexual behavior of female rats in a multiple-partner preference test

In this study, sexually experienced female rats were tested in a multiple-partner preference test (MPPT) in which they were allowed to pace their sexual contacts with four sexually active males. Four cylinders, with a small hole through which only the female could move freely from one cylinder to another, were assembled forming in the center an empty compartment. An intact female was placed in the central compartment and a sexually active male in each cylinder. Female sexual behavior was analyzed throughout the estrus cycle in four consecutive days. Each daily test lasted 15 min. The percentage of exits after intromission or ejaculation was significantly higher than the percentage of exits after each mount. The female spent significantly longer time with one of the males. We designated this male as the preferred male (PM). Although in each of the 4 days studied, females spent significantly longer time with the PM, however, the male selected was not the same throughout the estrus cycle. The number of entries into the compartment of the PM was significantly higher and increased around proestrus. Compared to previous studies, pacing behavior was notably lower in the conditions of the MPPT. No significant differences were observed during the estrous cycle concerning the other parameters recorded. The present results suggest that the MPPT could be a good model to study partner preference in the female rat.

Preclinical models of sexual desire: conceptual and behavioral analyses

The epidemiology, etiology and proposed treatments for the sexual desire disorders are briefly reviewed before turning to an analysis of preclinical models. We suggest that the concept of sexual desire in the human is equivalent to sexual motivation as employed in the scientific literature. Many animal tests for sexual motivation have been described over the years. Most of them are based on the evaluation of the rate or speed of performing learned operant responses. These are not ideal measures for inferring the intensity of sexual motivation. We present a test for sexual incentive motivation, which has been used in male and female rats. No learning is involved, and the test is rather insensitive to variations in ambulatory activity and it does not employ rate measures. A procedure that recently has attracted much attention, paced-mating behavior in the female, does not seem to be as useful as could be expected. In fact, it does not appear to be superior to tests for sexual receptivity (lordosis). The lack of established, clinically efficient treatments for sexual desire disorders makes it difficult to evaluate if any model has predictive validity. However, the model proposed here may be isomorphic and homologous to the human condition.

Only self-paced mating is rewarding in rats of both sexes

When rats are mated in a traditional mating chamber (with one male and one female) in which the male dictates the pace of the copulatory sequence, males develop a reward state as evaluated by conditioned place preference (CPP). In this mating situation no reward state is induced in females. However, when female rats are able to control (pace) the rate of sexual stimulation, thereby reducing the aversive consequences associated with mating, a clear CPP is observed. In the present study the CPP paradigm was used to determine whether if the reinforced state induced by coital interactions in male rats can be maintained when females pace the sexual interaction. Adult male and female rats were mated in one of two different conditions: (1) where subjects were able to pace their coital interactions or (2) where subjects were not able to pace their sexual contacts. The results showed that when males had control over the sexual interaction they developed a clear place preference while males that mated with females that paced their coital contacts did not develop CPP. Similarly, only females that were able to pace their sexual contacts developed place preference. These results suggest that coital interactions in males, as well as in females, can induce a reward state only when they are able to control the sexual interaction. Under seminatural conditions sexual behavior in rats is highly promiscuous, they mate in groups and repeatedly change partners in the middle of copulation. This behavioral sequence allows both, male and female to control the rate of sexual interaction, assuring the induction of a reward state outlasting the actual performance of coital responses.

Estrogens, brain and behavior: studies in fundamental neurobiology and observations related to women's health

Mechanisms and consequences of the effects of estrogen on the brain have been studied both at the fundamental level and with therapeutic applications in mind. Estrogenic hormones binding in particular neurons in a limbic-hypothalamic system and their effects on the electrophysiology and molecular biology of medial hypothalamic neurons were central in establishing the first circuit for a mammalian behavior, the female-typical mating behavior, lordosis. Notably, the ability of estradiol to facilitate transcription from six genes whose products are important for lordosis behavior proved that hormones can turn on genes in specific neurons at specific times, with sensible behavioral consequences. The use of a gene knockout for estrogen receptor alpha (ERalpha) revealed that homozygous mutant females simply would not do lordosis behavior and instead were extremely aggressive, thus identifying a specific gene as essential for a mammalian social behavior. In dramatic contrast, ERbeta knockout females can exhibit normal lordosis behavior. With the understanding, in considerable mechanistic detail, of how the behavior is produced, now we are also studying brain mechanisms for the biologically adaptive influences which constrain reproductive behavior. With respect to cold temperatures and other environmental or metabolic circumstances which are not consistent with successful reproduction, we are interested in thyroid hormone effects in the brain. Competitive relations between two types of transcription factors - thyroid hormone receptors and estrogen receptors have the potential of subserving the blocking effects of inappropriate environmental circumstances on female reproductive behaviors. TRs can compete with ERalpha both for DNA binding to consensus and physiological EREs and for nuclear coactivators. In the presence of both TRs and ERs, in transfection studies, thyroid hormone coadministration can reduce estrogen-stimulated transcription. These competitive relations apparently have behavioral consequences, as thyroid hormones will reduce lordosis, and a TRbeta gene knockout will increase it. In sum, we not only know several genes that participate in the selective control of this sex behavior, but also, for two genes, we know the causal routes. Estrogenic hormones are also the foci of widespread attention for their potential therapeutic effects improving, for example, certain aspects of mood and cognition. The former has an efficient animal analog, demonstrated by the positive effects of estrogen in the Porsolt forced swim test. The latter almost certainly depends upon trophic actions of estrogen on several fundamental features of nerve cell survival and growth. The hypothesis is raised that the synaptic effects of estrogens are secondary to the trophic actions of this type of hormone in the nucleus and nerve cell body.

Dopamine D2 receptors in the nucleus accumbens are important for social attachment in female prairie voles (Microtus ochrogaster)

The prairie vole (Microtus ochrogaster), a monogamous rodent that forms long-lasting pair bonds, has proven useful for the neurobiological study of social attachment. In the laboratory, pair bonds can be assessed by testing for a partner preference, a choice test in which pair-bonded voles regularly prefer their partner to a conspecific stranger. Studies reported here investigate the role of dopamine D2-like receptors (i.e., D2, D3, and D4 receptors) in the nucleus accumbens (NAcc) for the formation of a partner preference in female voles. Mating facilitated partner preference formation and associated with an approximately 50% increase in extracellular dopamine in the NAcc. Microinjection of the D2 antagonist eticlopride into the NAcc (but not the prelimbic cortex) blocked the formation of a partner preference in mating voles, whereas the D2 agonist quinpirole facilitated formation of a partner preference in the absence of mating. Taken together, these results suggest that D2-like receptors in the NAcc are important for the mediation of social attachments in female voles.

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.

Sexual motivation--an inquiry into events determining the occurrence of sexual behavior

For the individual engaged in it, sexual behavior has no finality or purpose other than its own execution. Data are presented showing that the execution of sexual reflexes can promote learning, i.e. it functions as reinforcement. Furthermore, positive affect is generated. Based on these principles, a model of sexual motivation has been elaborated. The conceptual framework is the incentive motivation theory previously proposed by Bindra D, A motivational view of learning, performance, and behavior modification, Psychol Rev 1974: 81:199-213; A Theory of Intelligent Behavior, New York: Wiley, 1976; How adaptive behavior is produced: a perceptual-motivational alternative to response reinforcement, Behav Brain Sci 1978; 1:41-52. Although the model is intended for application to most mammals, the rat is used as example. Essentially, sexual approach behaviors are activated by appropriate incentives (conditioned in the male, unconditioned in the female). Approach is, in the inexperienced male, followed by the execution of copulatory reflexes as a consequence of accidentally obtained tactile stimulation of the perineal region. In the female, copulatory acts are activated by tactile stimulation of the flanks and hinds provided by the mounting male. The role of conditioning for the execution of copulatory reflexes and for the acquisition of incentive value of neutral stimuli is analyzed. It is also shown that the incentive properties of sexual acts are not substantially different from those of other incentives. Sexual exhaustion is suggested to be either a case of negative alliesthesia or of stimulus habituation and the Coolidge effect is, in consequence, an example of dishabituation. Studies in women and men support this proposal. It is emphasized that sexual behavior is best understood as being entirely mechanistic albeit not deterministic.

Hormones, genes and the structure of sexual arousal

Despite the inherent difficulty of connecting individual genes with integrated mammalian behaviors, it has been determined that a series of genes are turned on by estrogenic hormones acting in forebrain. Their products are, in turn, facilitatory for female reproductive behaviors such as lordosis. The causal routes by which two genes contribute to the control of lordosis behavior, the classical estrogen receptor gene (ER-alpha) and a thyroid hormone (TH) receptor gene (TR-beta), have been delineated. Beyond the mechanisms underlying the expression of concrete, specific natural behaviors, lies the question of sexual motivation. Required as an intervening variable to explain fluctuations in natural behaviors in the face of constant stimuli, motivational states have both general and specific features. Most theoretical and experimental approaches toward the general aspects of motivation have depended heavily on concepts of 'arousal.' Sexual arousal is likely to depend both on very general, broadly distributed neuronal influences and on specific affiliative and sexual tendencies. Is 'general arousal' a monolithic, undifferentiated process? In no way can a review at this time settle such issues, but the reasons behind six new experimental approaches to these questions are described.