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

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.

I. Antidepressants and sexual behavior: Weekly ketamine injections increase sexual behavior initially in female and male rats

The present study characterized the effects of weekly ketamine injections on sexual behavior and anxiety in female and male rats, using a dosing protocol that mimics human therapeutic treatment for depression. In Experiment 1A, ketamine (10 mg/kg, i.p.) or saline was injected once per week for four consecutive weeks. The partner preference paradigm was used to measure sexual motivation 30 min after each weekly injection. Briefly, subjects were first given a 10-min test during which they could choose to spend time in the vicinity of a sexually receptive female stimulus or a sexually experienced male stimulus, however physical contact was restricted (no-contact). Immediately after, subjects were given unrestricted access to the stimulus animals (contact). After a washout period, subjects received four additional weekly injections of ketamine or saline, and then were tested for anxiety-like behavior on the elevated plus maze (EPM) after the last injection (Experiment 1B). For Experiment 2, similar procedures were used to test the effects of weekly ketamine injections on sexual motivation (Experiment 2A) and anxiety (Experiment 2B) in male subjects. In female subjects, ketamine increased sexual motivation as measured by greater time spent with the male stimulus, decreased likelihood of leaving after receiving mounts, and shorter return latencies after receiving intromissions, when compared to saline controls. In male subjects, ketamine shortened latency to first mount and first intromission, as well as increased time spent with the female stimulus. Very little anxiety was observed in either group (ketamine or saline) of female or male subjects when tested on the EPM. In conclusion, even after four weeks of ketamine exposure, sexual dysfunction did not emerge in either females or males. In contrast, ketamine increased sexual motivation in both females and males, with an initial robust response. However, as both groups gained sexual experience, the impact of ketamine diminished.

II. Antidepressants and sexual behavior: Acute fluoxetine, but not ketamine, disrupts paced mating behavior in sexually experienced female rats

Female sexual dysfunction is both a symptom of depression and exacerbated by treatments for depression. Ketamine, a novel treatment for depression, has been shown to enhance, whereas fluoxetine has been shown to impair sexual motivation. Sexual experience leads to more robust partner preference and paced mating behavior in female rats. Whether acute ketamine and fluoxetine similarly affect sexual motivation and mating behavior in sexually experienced female rats is unknown. Sexually experienced female rats received 10 mg/kg i.p. of ketamine or saline vehicle (Experiment 1) or 10 mg/kg i.p. of fluoxetine or water vehicle (Experiment 2) 30 min before a 10-min no-contact partner preference test followed immediately by a 15-intromission paced mating test. Partner preference and paced mating behavior did not differ between ketamine- and saline-treated rats. In contrast, rats treated with fluoxetine spent significantly less time with either stimulus animal and were less active during the partner preference test than water-treated rats. Additionally, contact-return latency to ejaculation was significantly longer in fluoxetine-treated rats and they spent less time with the male during paced mating in comparison to water-treated rats. Thus, even with sexual experience, fluoxetine disrupts sexual function whereas ketamine has no detrimental effects on sexual behavior in female rats. A growing body of evidence suggests that ketamine is an encouraging new approach to treat depression particularly because it is not associated with sexual dysfunction.

"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.

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.

ESTROGEN REPLACEMENT THERAPY FOR STROKE

Stroke is the third most common cause of death and severe disability among Western populations. Overall, the incidence of stroke is uniformly higher in men than in women. Stroke is rare in women during the reproductive years, and rapidly increases after menopause, strongly suggesting that estrogen (E2) plays an important role in the prevention of stroke. Ongoing studies are currently evaluating both the benefits and risks associated with E2 replacement therapy and hormone replacement therapy in stroke. Equally important is the role of E2 receptor (ER), as studies indicate that ER populations in several tissue sites may significantly change during stress and aging. Such changes may affect the patient's susceptibility to neurological disorders including stroke, and greatly affect the response to selective E2 receptor modulators (SERMs). Replacement therapies may be inefficient with low ER levels. The goal of this review paper is to discuss an animal model that will allow investigations of the potential therapeutic effects of E2 and its derivatives in stroke. We hypothesize that E2 neuroprotection is, in part, receptor mediated. This hypothesis is a proof of principle approach to demonstrate a role for specific ER subtypes in E2 neuroprotection. To accomplish this, we use a retroviral mediated gene transfer strategy that express subtypes of the ER gene in regions of the rat brain most susceptible to neuronal damage, namely the striatum and cortex. The animal model is exposed to experimental stroke conditions involving middle cerebral artery occlusion (MCAo) method, and eventually the extent of neuronal damage will be evaluated. A reduction in neuronal damage is expected when E2 is administered with specific ER subtypes. From this animal model, an optimal E2 dose and treatment regimen can be determined. The animal model can help identify potential E2-like therapeutics in stroke, and screen for beneficial or toxic additives present in commercial E2 preparations that are currently available. Such studies will be informative in designing drug therapies for stroke.

Agonistic activity of ICI 182 780 on activation of GSK 3β/AKT pathway in the rat uterus during the estrous cycle

We examined the ability of ICI 182,780 (ICI) to block uterine cell proliferation via protein kinase b/AKT pathway in the uterus of the rat during the estrous cycle. Intact rats, with regular estrous cycles, received a subcutaneous (s.c.) injection of either vehicle or ICI at 08:00 h on the day of proestrus or at 00:00 h on the day of estrus and sacrificed at 13:00 h of metaestrus. Estradiol (E₂) and progesterone (P₄) plasma levels were measured by radioimmunoassay. Both ICI treatments, induced a significant decrease (p<0.01) in uterine estrogen receptor alpha (ERα) content, had no effect on uterine progesterone receptor (PR) protein expression and caused marked nuclear localization of cyclin D1, in both luminal and glandular uterine epithelium, as compared to vehicle-treated animals. Furthermore, we detected that ICI treatment induced glycogen synthase kinase (Gsk3-β) Ser 9 phosphorylation, which correlates with cyclin D1 nuclear localization. However, some differences were observed between the two different time schedules of administration. We observed that the administration of ICI at 08:00 h on proestrus day produced a 15% inhibition of luminal epithelial cell proliferation, reduced uterine wet weight by 21% and caused reduction of Akt phosphorylation at Ser 473 as compared to vehicle-treated animals, whereas ICI treatment at 00:00 h on estrus day had no effect on these parameters. The overall results indicate that ICI may exert agonistic and antagonistic effects on uterine cell proliferation through differential activation of the Akt pathway depending on the administration period during the estrous cycle, and indicates that the mechanism of cell proliferation during the physiological conditions of the estrous cycle, is under a different and more complex regulation than in the ovariectomized + E₂ animal model.

Effect of DHEA therapy on sexual behavior in female rats

Delta-5 androgen therapies seem to enhance the sexual response in experimental animal models and in clinical trial. This study analyzed the influence of dehydroepiandrosterone (DHEA) administration on receptive and proceptive components of female rat sexual behavior. Ovariectomized (OVX) adult rats were divided in six groups submitted to the following treatments for 4 weeks: DHEA 0.5 and 5 mg/kg, by oral gavage, alone or in combination with estradiol benzoate 3 µg/rat; EB 3 and 10 µg/rat as control groups. All animals received progesterone (500 µg/rat) 4 h before the behavioral tests. All animals were tested for the following: receptivity and proceptivity weekly for 4 weeks; partner preference and paced mating behavior at the end of the treatments. Oral administration of DHEA at 5 mg/kg in EB primed rats was able to significantly increase proceptive behaviors, already after 1 week of treatment. The increase was more marked after 3 and 4 weeks of treatment. Behavioral changes were associated to modifications of circulating and brain level of allopregnanolone and beta-endorphin, although circulating hormonal levels were within a physiological range. Hormonal treatment using physiological doses of delta-5 androgens (DHEA) positively affects sexual motivation in OVX rats.

Effects of the cannabinoid antagonist SR 141716 on sexual and motor behaviour in receptive female rats

1. The aim of the present study was to investigate the effect of the cannabinoid antagonist/inverse agonist SR 141716 (SR) on the receptive behaviour and sexual motivation of female rats. 2. Partner preference, receptivity and proceptivity were evaluated in ovariectomized female rats primed with oestrogen and progesterone and administered SR (1 or 2.5 mg/kg, i.p.) 20 min prior to testing. 3. In the partner preference test, a reduced interest in both stimulus animals (a sexually active male and an ovariectomized hormone-primed female) was detected in rats treated with SR at both doses, but no effect on preference score was observed. In the receptivity test, pronounced reductions in lordosis quotient, lordosis rating and in the percentage of receptive females were found in SR-treated rats compared with control rats. Proceptive behaviours were not significantly affected by either dose of SR. 4. In addition, we explored the behavioural effects induced by SR in female rats using the open field test. Only at the higher dose (i.e. 2.5 mg/kg) did SR markedly increased grooming and scratching behaviour. 5. The results demonstrate the ability of SR to reduce female sexual receptivity, but not sexual motivation. The reduction does not seem strictly related to the motor alterations induced by the cannabinoid antagonist.

"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.

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.

The phytoestrogen ferutinin improves sexual behavior in ovariectomized rats

The present study was designed to examine the effect of ferutinin chronic administration on sexual behavior of ovariectomized non-estrogen-primed rats. Starting from 3 weeks after ovariectomy, female rats were orally treated with ferutinin at the doses of 0.2 and 0.5 mg/kg, daily for 4 weeks. Ferutinin's effect was compared with that of estradiol benzoate, subcutaneously injected at the dose of 1.5 microg/rat twice a week. Animals were tested for sexual motivation, receptivity and proceptivity after 1, 2 and 3 weeks of treatment and for paced mating behavior after 4 weeks of treatment. Before each experimental test, they received progesterone injection (500 microg/rat). Both dosages of ferutinin significantly increased the receptive behavior in a time-dependent manner, as well as estradiol benzoate did. Also proceptive behaviors increased in ferutinin-treated animals in comparison with control ones. During the partner preference test ferutinin was able to induce a significant preference for a sexually active male over a sexually receptive female. Moreover, ferutinin restored a normal paced mating behavior, which had been suppressed by ovariectomy. These results show that ferutinin exerts an estrogenic activity in ovariectomized non-estrogen-primed female rats.

The phytoestrogen ferutinin affects female sexual behavior modulating ERalpha expression in the hypothalamus

This study was designed to assess the effect of the phytoestrogenic compound ferutinin, chronically administered in ovariectomized progesterone primed rats, alone or in combination with estradiol benzoate. After 2, 3 and 4 weeks of treatments, female rats were tested for receptive (lordosis) and proceptive behaviors (hops, darts and ear wigglings). Ferutinin given alone markedly increased the intensity of the lordotic response in ovariectomized rats but failed to significantly affect proceptivity. On the other hand estradiol benzoate significantly increased both receptive and proceptive behaviors. When administered in combination with estradiol, ferutinin reduced the increase in receptivity and proceptivity due to estrogen effects, acting as an antiestrogen. At the end of the behavioral experiments, animals were sacrificed and Western blot analysis of estrogen receptor alpha (ERalpha) levels was performed in the dissected hypothalami. Ferutinin increased ERalpha expression when administered alone, as estradiol did, but decreased the response to estradiol when administered in combination. These results suggest that ferutinin displays estrogenic or antiestrogenic activity through ERalpha in the hypothalamus, depending on the absence or the presence of estrogen priming.

Zaprinast, a phosphodiesterase type-5 inhibitor, alters paced mating behavior in female rats

Nitric oxide (NO) is the primary mediator of blood flow in female genital tissues and drugs that enhance the activity of nitric oxide, such as phosphodiesterase type-5 (PDE-5) inhibitors, increase vaginal blood flow in anesthetized rats. The goal of the present study was to test the effects of one PDE-5 inhibitor, zaprinast, on the display of sexual behaviors in gonadectomized, estrogen- and progesterone-treated female rats. Experiment 1 demonstrates that zaprinast alters paced mating behavior by lengthening the contact-return latency to ejaculation; there is a significant relationship between dose of zaprinast (range 1.5-6 mg/kg) and contact-return latency to ejaculation. Experiment 2 illustrates that zaprinast has no effect on preference for an intact male as measured in a No Contact partner preference test. Rats receiving zaprinast tend to exhibit reduced locomotor activity in both experiments. Collectively, these findings demonstrate that modulation of the NO-cGMP pathway using a PDE-5 inhibitor alters the display of paced mating behaviors in rats.

The hop phytoestrogen, 8-prenylnaringenin, reverses the ovariectomy-induced rise in skin temperature in an animal model of menopausal hot flushes

The mechanisms underlying menopausal hot flushes are poorly understood, although it is generally assumed they result from disturbances of thermoregulatory centres in the hypothalamus. 8-Prenylnaringenin (8-PN) has been identified as a potent phytoestrogen in hops (Humulus lupulus) and there are claims that hop-containing preparations can reduce hot flushes. We have investigated the site of action of 8-PN in a rat model of menopausal hot flushes, in which the tail skin temperature (TST) is increased after oestrogen withdrawal induced by ovariectomy. Daily s.c. administration of either 17beta-oestradiol (E2; 4 microg/kg) or 8-PN (400 microg/kg) significantly reduced the elevated TST after 2 days of treatment. Subcutaneous co-administration of either E2 or 8-PN with the oestrogen receptor (ER) antagonist, ICI 182,780 (200 microg/kg), which is thought not to cross the blood-brain barrier, completely blocked the effect of E2 and 8-PN on TST. The ERalpha- and ERbeta-specific agonists, 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (100 microg/kg) and 2,3-bis(4-hydroxyphenyl)-propionitrile (60 microg/kg) respectively, both significantly reversed the raised TST in ovariectomised rats. These observations suggest that the regulation of the vasomotor response by oestrogens and phytoestrogens is mediated, at least in part, by peripheral mechanisms involving both ERalpha and ERbeta.

Role of ferutinin in the impairment of female sexual function induced by Ferula hermonis

In the present study, we evaluated the effects of single components of Ferula hermonis extract on female rat sexual behaviour. Ovariectomized rats hormonally primed with estradiol benzoate (1.5 or 10 microg/rat s.c.) and progesterone (500 microg/rat s.c.) were acutely treated by oral gavage with ferutinin, teferin and teferdin (2.5 mg/kg). Thereafter they were tested for: a) partner preference, b) receptivity, c) proceptivity, d) paced mating behaviour. In the partner preference test, the choice of the female for a sexually active male was not influenced by the different treatments. Similarly, during the paced mating test, the contact-return latencies as well as the percentage of exits from the male compartment were not different in control and treated rats. Therefore none of the three compounds showed the capacity to alter sexual motivation. On the other hand, ferutinin, but not teferin and teferdin, significantly inhibited female receptivity. These results suggest a primary role of ferutinin in the impairment of sexual behaviour elicited by F. hermonis extract in hormone primed-female rats.

Ferula hermonis impairs sexual behavior in hormone-primed female rats

The influence of Ferula hermonis root extract on sexual behavior was studied in female rats. Sexual receptivity, proceptivity and paced mating behavior were evaluated in ovariectomized females primed with estradiol benzoate (EB) and progesterone (P) and then treated with F. hermonis extract acutely (30 and 60 mg/kg) or subchronically (1 and 10 mg/kg daily for 10 consecutive days). A significant reduction in lordosis responses was observed in rats after acute (60 mg/kg) or subchronic (1 and 10 mg/kg) administration of the plant extract. Similarly, a decrease in proceptive behaviors was exhibited by F. hermonis treated rats in comparison with EB+P controls. No difference was found in the patterns of paced mating behavior between control and treated animals. The present results demonstrate that the acute or repeated ingestion of F. hermonis specifically impairs the receptive and proceptive components of female sexual behavior. The effect could be the consequence of an antiestrogenic action of the extract in hormone-primed female rats.

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.