Sexual motivation in the female rat after testosterone treatment

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.

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.

Ejaculation latency determines susceptibility to stress in the male rat

Stress induces diverse effects on sexual behavior, ranging from enhanced execution to the complete abolishment of sexual interaction. However, it is not clear whether some characteristics intrinsic to the individual that experiences stress could also explain this differential effect. This study seeks to relate sexual execution to susceptibility to stress (as post-stress sexual motivation). To this end, we designed a three-session experimental paradigm. In the first session, male rats were allowed to copulate with a female. In the second, the male rats received electric foot shocks as they attempted to approach the female. The third and final session was used to determine the effects of stress on sexual behavior by separating the rats into two groups: a motivation-impaired group (rats that did not cross to achieve copulation), and an unimpaired group (rats that did cross). Mount latency was affected immediately by stress in both groups, though only the non-crossing group presented a reduced number of copulatory events. The rats that did not cross showed slower-paced sexual execution even before stress was applied compared to the rats that crossed. These results show that rats that are more susceptible to stress present higher ejaculation latency even before the application of stress.

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.

Facilitation of sexual behavior in ovariectomized rats by estradiol and testosterone: A preclinical model of androgen effects on female sexual desire

In the United States and Canada, there are no approved treatments for hypoactive sexual desire disorder in postmenopausal women. Testosterone improves female sexual desire in naturally- and surgically-menopausal women maintained on estrogen replacement therapy, and long-term safety data from randomized placebo-controlled clinical trials has yielded promising results. However, the mechanisms associated with its efficacy are not known, and could be addressed using preclinical rodent models; yet there is no systematic evaluation of the effects of estradiol and testosterone on female rat sexual behavior. The aim of these studies was to assess whether testosterone propionate (TP) facilitates sexual behaviors, particularly appetitive sexual behaviors, in Long-Evans and Wistar ovariectomized (OVX) rats primed with estradiol benzoate (EB). In Experiment 1, Long-Evans OVX rats were treated with Oil (O), 10μg EB+O, O+200μg TP, 10μg EB+500μg progesterone (P), or 10μg EB+200μg TP. In Experiment 2a, Wistar OVX rats were treated with varying doses of EB (2.5, 5, or 10μg) 48h prior, and TP (0, 200, or 400μg) 4h prior to testing in a Latin-Square design. A subset of animals was used in Experiment 2b and treated sequentially with EB (0, 2.5, 5, or 10μg) followed by TP (0, 200, or 400μg, in a Latin-Square design) 48h prior to sexual behavior testing. All tests occurred in the bilevel pacing chamber. Frequencies of female appetitive (hops/darts, solicitations, level changes) and consummatory (lordosis quotient and magnitude) sexual behaviors as well as the number of defensive behaviors towards males were scored. Number of mounts, intromissions and ejaculations from males were also scored. In EB-primed OVX Long-Evans rats, 200μg TP administered 4h prior to testing facilitated hops/darts and lordosis ratings beyond EB alone, and to levels equivalent to EB+P. In contrast, that regimen was not successful in EB-primed OVX Wistar rats. When EB and TP were co-administered 48h prior to testing, 10μg EB+200μg TP significantly increased hops/darts and level changes beyond that observed by 10μg EB alone. In summary, the administration of EB and TP to OVX Long-Evans and Wistar rats facilitates appetitive measures of sexual behavior. Strain differences exist that likely reflect underlying differences in sensitivities to EB, and the EB-primed OVX Long-Evans rat may be useful for studying mechanisms of TP-facilitation of desire due to higher baseline sexual inhibition.

When do we eat? Ingestive behavior, survival, and reproductive success

The neuroendocrinology of ingestive behavior is a topic central to human health, particularly in light of the prevalence of obesity, eating disorders, and diabetes. The study of food intake in laboratory rats and mice has yielded some useful hypotheses, but there are still many gaps in our knowledge. Ingestive behavior is more complex than the consummatory act of eating, and decisions about when and how much to eat usually take place in the context of potential mating partners, competitors, predators, and environmental fluctuations that are not present in the laboratory. We emphasize appetitive behaviors, actions that bring animals in contact with a goal object, precede consummatory behaviors, and provide a window into motivation. Appetitive ingestive behaviors are under the control of neural circuits and neuropeptide systems that control appetitive sex behaviors and differ from those that control consummatory ingestive behaviors. Decreases in the availability of oxidizable metabolic fuels enhance the stimulatory effects of peripheral hormones on appetitive ingestive behavior and the inhibitory effects on appetitive sex behavior, putting a new twist on the notion of leptin, insulin, and ghrelin "resistance." The ratio of hormone concentrations to the availability of oxidizable metabolic fuels may generate a critical signal that schedules conflicting behaviors, e.g., mate searching vs. foraging, food hoarding vs. courtship, and fat accumulation vs. parental care. In species representing every vertebrate taxa and even in some invertebrates, many putative "satiety" or "hunger" hormones function to schedule ingestive behavior in order to optimize reproductive success in environments where energy availability fluctuates.

Quantitative assessment of female sexual motivation in the rat: Hormonal control of motivation

While a good deal of information has been garnered in the last few decades regarding the neural and hormonal control of female sexual behavior, literature elucidating these mechanisms with respect to female sexual motivation has been scarce. We believe that one reason for this is the lack of a standardized paradigm that will quantify female sexual motivation while allowing for sexual interaction to occur. Here we describe a two-chambered apparatus that utilizes operant responding (nose poking) to quantify female sexual motivation. During the test, the female exhibits nose pokes to gain access to a sexually active male, with whom she is allowed to mate. Therefore, this apparatus allows for examination of sexual behavior as well as quantification of sexual motivation by assessing the number of nose pokes the female will exhibit within a fixed interval to gain access to the male. We report that hormone priming significantly increases sexual motivation in the female as indicated by the number of nose pokes she will exhibit to gain access to the male. Additionally, hormone primed females enter the male compartment after a shorter period and spend more time in direct contact with the male compared to when they are not hormone primed. In contrast, when females are not hormone primed they spend more time in view, but out of reach, of the male. This paradigm will help to advance the study of female sexual motivation, providing a method for quantifiable assessment of female sexual motivation while allowing for sexual activity to occur.

The effects of paradoxical sleep deprivation on estrous cycles of the female rats

The present purpose was to examine how sleep deprivation affects the estrous cycle of the female rat. Paradoxical sleep-deprived (PSD) adult female Wistar rats were compared to home-cage control (CTRL) on their estrous cyclicity. Forty-four PSD and forty-four CTRL female rats were distributed into 4 subgroups of 11 animals each according to the phase of estrous cycle and were subjected to sleep deprivation for 96 h by the multiple platform technique. After PSD period, vaginal estrous cycles were taken for an additional 9 days. Animals that were submitted to PSD in diestrus phase (PSD-diestrus) had their estrous cycles disrupted during the recovery period by showing a constant diestrus during the first week. As for hormone alterations, progesterone concentrations were statistically higher in PSD-diestrus compared to respective phase control and to PSD-proestrus and PSD-estrus rats while CTRL-metestrus had higher levels than CTRL-proestrus and estrus groups. Testosterone was significantly decreased in PSD-estrus in relation to PSD-proestrus and PSD-diestrus groups and was lower in CTRL-diestrus rats than in home-cage rats in proestrus. In addition, PSD-diestrus phase exhibited higher concentrations of corticosterone and lower estrogen than the respective control rats. These data indicate that PSD may modulate the ovarian hormone release through alterations in hormonal-neurochemical mechanisms.

Eurycoma longifolia Jack enhances sexual motivation in middle-aged male mice

Eurycoma longifolia Jack was investigated for sexual motivation activity in adult, middle-aged male mice and in retired breeders, using the modified open field and the modified runway choice methods. Each mouse received 500 mg/kg of one of 4 fractions of E. longifolia Jack, viz. chloroform, methanol, butanol, and water, whereas the mice in the control and yohimbine groups received 3 ml/kg of normal saline and 30 mg/kg of yohimbine daily respectively for 10 d. The results show a transient increase in the percentage of male mice responding to the right choice after chronic consumption of the fractions with 50 percent of the adult middle-aged male mice treated with E. longifolia Jack and yohimbine scoring the right choice after 8 and 5 days post-treatment respectively. In conclusion, this study has shown that E. longifolia Jack continues to enhance sexual motivation in adult, middle-aged male mice and in retired breeders.

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.

On the organization of partner preference behavior in female Wistar rats

The possible prenatal organizing effects of testosterone (T) on adult sexual partner preference, i.e., sexual orientation in female rats, were studied through prenatal exposure (days 11-22) of female fetuses to the antiandrogens flutamide (Sch 13521; 4'-nitro-3'-trifluoromethylisobutyranilide; 5 or 10 mg/day; Experiment 1) or anandron [RU 23908; 5,5-dimethyl-3-(4-nitro-3-(trifluoromethyl)phenyl)- 2,4-imidazolidinedione; 35 mg/kg/day; Experiment 2]. The neonatal organizing effects of T were further studied by giving T, dihydrotestosterone (DHT) or oil within 9 h after birth to female pups (Experiment 3). In adulthood sexual orientation was ascertained, after ovariectomy followed by hormone treatment, in an automated open field (AOF), with stimulus animals behind wire mesh, and in a 3-compartment box (3-CB), with stimulus animals tethered. When given the choice between an estrous female and a sexually active male in the AOF, flutamide females, as well as controls, preferred the male partner. After long-term T treatment and 3 weekly pair-tests with an estrous female, flutamide females as well as controls switched their preference to the estrous female partner. In anadron females similar results were obtained. Thus the prenatal antiandrogens had no significant effect on sexual orientation in female rats. This suggests that adult sexual orientation in female rats is not organized prenatally through endogenous T. The change in preference after sexual experience corroborates earlier findings from our laboratory. When given the choice between an estrous female and a sexually active male in the 3-CB (sexual interaction with incentives possible), neonatally DHTP-treated females preferred the male; neonatally TP- or oil-treated females showed no preference.(ABSTRACT TRUNCATED AT 250 WORDS)

Sexual experience and preference for males or females in the female rat

The influence of sexual experience on preference behavior was investigated in adult female rats. In the first experiment, preference behavior for sexually active males versus estrous females was investigated in female rats who were experienced with mounting behavior. Preference of these mount experienced females was compared to preference of females who were naive in this respect. Mount experience with estrous females induced a female-directed preference in ovariectomized female rats, irrespective of whether the females were treated with OIL or with testosterone-propionate (500 micrograms TP, injected once 48 h prior to testing). Sexually naive, OIL-treated females did not show a preference for males or females, but TP induced a preference for a male. Individual differences in mount frequency were not correlated with preference behavior. In the second experiment, the influence of experience with feminine sexual responses on preference behavior was investigated. OIL-treated and TP-treated females oriented equally toward males and females when they had been given the opportunity to copulate with males prior to the preference tests. The results strongly suggest that prior sexual experience is an important determinant of preference behavior. The female's behavior during sexual interactions was, however, not predictive of later preference.

Homosexual and heterosexual partner preference in ovariectomized female rats: effects of testosterone, estradiol and mating experience

Ovariectomized female rats, longterm treated with estradiol benzoate (EB, 20 micrograms, 3 x/week) or testosterone propionate (TP, 0.4 mg, 3 X/week), were tested for homo- or heterosexual partner preference behavior with either tethered animals (sexually active male vs. estrous female) or animals behind a wire mesh. A preference score was calculated by subtracting the time spent in the vicinity of the stimulus female from the time spent in the vicinity of the stimulus male. Thus, a positive score indicates preference for the male (heterosexual preference), a negative score preference for the female (homosexual preference). Two weeks of EB treatment caused a clearcut preference for the male incentive. This heterosexual preference was significantly different from the indistinct preference of the TP-treated females. Sexual interaction with a freely moving active male (with or without a vaginal mask which prevented intromissions) did not alter the preference for the male of the EB-treated females. It did affect, however, the preference behavior of the TP-treated females significantly: it changed in the direction of the sexually active estrous incentive female, i.e., a homosexual preference. This change in preference could not be attributed to the experience of penile intromissions, it occurred despite the presence of a vaginal mask. Apparently, being mounted by a sexually active male had a negative reinforcing value in the TP-treated female rats and provoked a homosexual partner preference.

Sexual behavior of mares

Behavior during the estrous phase of the ovulatory cycle of the mare is analogous in most ways to that of estrous females of other species. Proceptive behaviors bring the mare into the proximity of the male and attract his attention. Positioning facilitates mounting, intromission, and ejaculation. Estrous signs appear to be more intense in the few days prior to ovulation than during the transition periods that separate the recurring estrous and diestrous phases. Sexual behavior is absent during diestrus. Detection of estrus in mares is problematic in that it requires the presence (or at least facsimile acoustic or tactile stimuli) or a stallion. Unexplained conditions such as silent or subestrus make reliable determinations even more difficult. The choice of methods for distinguishing estrous from nonestrous mares depends, among other factors, upon characteristics of individual mares, as well as the number of mares to be evaluated. Detection of true estrus--that is, sexual behavior that is associated with follicular growth and ovulation--is confounded by the occurrence of estrus-like behavior independent of gonadal stimulation. Seasonally anovulatory and ovariectomized mares may regularly solicit and accept copulation despite low or even no circulating levels of ovarian steroids. Instead, endocrine support of sexual behavior in these mares appears to be provided by adrenal cortical androgens and/or estrogens. A smaller percentage of pregnant mares may also show some signs of estrus but are not likely to be receptive to mating. The ability of the mare to dissociate sexual behavior from ovulation is unique among infraprimate species studied. The display of estrous behavior and willingness to copulate, even at times when conception is impossible, may have functional significance in promoting band cohesiveness in wild and feral populations. Even without copulation, continued motivation to remain in proximity to the band stallion may contribute to the stability of the social unit outside the ovulatory season.

Androgens are specifically implicated in female rat sexual motivation. The influence of methyltrienelone (R1881) on sexual orientation

The present experiment was designed to investigate whether androgens are specifically involved in the induction of a male-directed orientation in adult female rats. Ovariectomized female rats were either treated with the non-aromatizable androgen methyltrienelone (R1881), with testosterone propionate (TP), with estradiol benzoate (EB) or with an equal volume of the solvent. Sexual orientation of these females towards either sexually active males or estrous females was then investigated and related to levels of receptive and mounting behavior. Compared to the solvent-treated females, females treated with R1881, TP or EB spent more time near sexually active males. Mounting behavior was stimulated in the R1881- and TP-treated females, but EB-treated females mounted as often as females treated with the solvent only. Lordosis behavior was only observed in TP-treated or EB-treated females. Mount frequency of the females of the different treatment groups was positively correlated with time spent near males. These correlations reached statistical significance in the TP-treated and EB-treated females. In the TP-treated females, the lordosis quotient was negatively correlated with time spent near males. The results of the present experiment suggest that androgens need not be converted into estrogens in order to facilitate a male-directed orientation in ovariectomized female rats.

Animal models for human sexuality

The value of animal models in biomedical research is firmly established, and many basic principles of human psychology have been explicated as the result of comparative studies. There is pressing need for non-human models in the behavioural sciences as represented by psychiatry, psychology and ethology; and such models should be constructed, provided their validity can be assured. Valid models cannot be based exclusively on similarity in the formal properties of behaviour. Commonality of descriptive terms as applied to different species does not guarantee identity of the concepts to which the terms apply. Model builders must evaluate interspecific similarities and differences in the causes, mediating mechanisms and functional outcomes of behaviour. The validity of interspecific generalization can never exceed the reliability of intraspecific analysis; and the latter is an indispensable antecedent of the former. Existing and potential models for homosexuality and other psychosexual characteristics of human beings are evaluated within the perspective provided by the foregoing generalizations.

Effect of ACTH 4-10 on copulatory bahavior and on the response for socio-sexual motivation in the female rat

Female copulatory behavior and the urge of a female rat to seek contact with a sexually active male was studied after treatment with the peptide ACTH 4-10. No effects of this peptide were seen on the copulatory response of ovariectomized females treated with estradiol benzoate (EB) alone or EB and progesterone combined. An increasing barrier technique was used to measure how much of an average stimulus (cresssing an electrified grid) a female was willing to endure to gain contact with sexually active male. It has been shown in previous investigations (15) with ovariectomized rats that after a certain time of adaptation and training in this apparatus the response attained remains at a certain basal level from which it is increased by treatment with EB. Animals treated with ACTH 4-10 during the phase of adaptation and training attained a significantly higher basal response level than controls. This effect was only observed when the peptide treatment covered the period of adaptation and was not seen when the treatment was started after this critical period. The EB-induced response was not influenced by ACTH 4-10 given during the period of adaptation or given in connection with the EB treatment. It is concluded that ACTH 4-10 influenced the acquired response level but had no effect on steroid induced responses, which are probably controlled by more innate mechanisms.