Paroxetine-induced reduction of sexual incentive motivation in female rats is not modified by 5-HT1B or 5-HT2C antagonists

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.

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.

Sexual incentive motivation, sexual behavior, and general arousal: Do rats and humans tell the same story?

Sexual incentive stimuli activate sexual motivation and heighten the level of general arousal. The sexual motive may induce the individual to approach the incentive, and eventually to initiate sexual acts. Both approach and the ensuing copulatory interaction further enhance general arousal. We present data from rodents and humans in support of these assertions. We then suggest that orgasm is experienced when the combined level of excitation surpasses a threshold. In order to analyze the neurobiological bases of sexual motivation, we employ the concept of a central motive state. We then discuss the mechanisms involved in the long- and short-term control of that state as well as those mediating the momentaneous actions of sexual incentive stimuli. This leads to an analysis of the neurobiology behind the interindividual differences in responsivity of the sexual central motive state. Knowledge is still fragmentary, and many contradictory observations have been made. Nevertheless, we conclude that the basic mechanisms of sexual motivation and the role of general arousal are similar in rodents and humans.

Female rat sexual behavior is unaffected by perinatal fluoxetine exposure

Serotonin plays an important role in adult female sexual behavior, however little is known about the influence of serotonin during early development on sexual functioning in adulthood. During early development, serotonin acts as neurotrophic factor, while it functions as a modulatory neurotransmitter in adulthood. The occurrence of serotonin release, could thus have different effects on behavioral outcomes, depending on the developmental period in which serotonin is released. Because serotonin is involved in the development of the HPG axis which is required for puberty establishment, serotonin could also alter expression patterns of for instance the estrogen receptor ɑ (ERɑ). The aim of our study was to investigate the effects of increased serotonin levels during early development on adult female rat sexual behavior during the full behavioral estrus in a seminatural environment. To do so, rats were perinatally exposed with the selective serotonin reuptake inhibitor (SSRI) fluoxetine (10 mg/kg FLX) and sexual performance was tested during adulthood. All facets of female sexual behavior between the first and last lordosis (behavioral estrus), and within each copulation bout of the behavioral estrus were analyzed. Besides the length and onset of the behavioral estrus and the sexual behaviors patterns, other social and conflict behavior were also investigated. In addition, we studied the effects of perinatal FLX exposure on ERɑ expression patterns in the medial preoptic nucleus, ventromedial nucleus of the hypothalamus, medial amygdala, bed nucleus of the stria terminalis, and the dorsal raphé nucleus. The results showed that perinatal fluoxetine exposure has no effect on adult female sexual behavior. The behavioral estrus of FLX-females had the same length and pattern as CTR-females. In addition, FLX- and CTR-females showed the same amount of paracopulatory behavior and lordosis, both during the full behavioral estrus and the "most active bout". Furthermore, no differences were found in the display of social and conflict behaviors, nor in ERɑ expression patterns in the brain. We conclude that increases in serotonin levels during early development do not have long-term consequences for female sexual behavior in adulthood.

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.

Modeling Human Sexual Motivation in Rodents: Some Caveats

Sexual behavior is activated by motivation. An overwhelming majority of experimental studies of the intricacies of sexual motivation has been performed in rodents, most of them in rats. Sometimes it is desirable to generalize results obtained in this species to other species, particularly the human. It is hoped that studies of the neurobiology of rodent sexual behavior may shed light on the central nervous mechanisms operating in the human, and the search for efficient pharmacological treatments of human sexual dysfunctions relies partly on studies performed in rodents. Then the issue of generalizability of the rodent data to the human becomes crucial. We emphasize the importance of distinguishing between copulatory acts, behavior involving the genitals, and the preceding event, the establishment of physical contact with a potential mate. Comparisons between the structure of copulatory behavior in rats and humans show abysmal differences, but there may be some similarity in the underlying mechanisms. The endocrine control of sex behavior is shortly mentioned, and we also compare the effects of the few drugs known to affect both rodent and human copulatory behavior. The stimuli activating sexual motivation, often called desire in the human literature, are examined, and the sexual approach behaviors in rats and humans are compared. There is a striking similarity between these species in how these behaviors respond to drugs. It is then shown that the intensity of sexual approach is unrelated to the intensity of copulatory behavior. Even though the approach is a requisite for copulation, an activity that requires at least two individuals in close physical contact, these two aspects of sexuality do not covary. This is similar to the role of the testosterone in men and male rats: although the hormone is needed for sex behavior, there is no correlation between serum testosterone concentration and the intensity of copulation. It is also pointed out that human sexual behavior is mostly determined by social conventions, whereas this is not the case in rats and other rodents. It is concluded that some observations in rats can be generalized to the human, but extreme caution must be exercised.

The neuroendocrinology of sexual attraction

Sexual attraction has two components: Emission of sexually attractive stimuli and responsiveness to these stimuli. In rodents, olfactory stimuli are necessary but not sufficient for attraction. We argue that body odors are far superior to odors from excreta (urine, feces) as sexual attractants. Body odors are produced by sebaceous glands all over the body surface and in specialized glands. In primates, visual stimuli, for example the sexual skin, are more important than olfactory. The role of gonadal hormones for the production of and responsiveness to odorants is well established. Both the androgen and the estrogen receptor α are important in male as well as in female rodents. Also in primates, gonadal hormones are necessary for the responsiveness to sexual attractants. In males, the androgen receptor is sufficient for sustaining responsiveness. In female non-human primates, estrogens are needed, whereas androgens seem to contribute to responsiveness in women.

Female Reproductive Behavior

Reproductive behavior is the behavior related to the production of offspring and includes all aspects from the establishment of mating systems, courtship, sexual behavior, and parturition to the care of young. In this chapter, I outline the hormonal regulation of the estrous cycle, followed by a description of the neural regulation of female sexual behavior. Ovarian hormones play an important role in the induction of ovulation and behavioral estrus, in which they interact closely with several neurotransmitters and neuropeptides to induce sexual behavior. This chapter discusses the latest research on the role of estrogen, progesterone, serotonin, dopamine, noradrenaline, oxytocin, and GABA in female mating behavior. In addition, the most relevant brain areas, such as the preoptic area and the ventromedial nucleus of the hypothalamus, in which these regulations take place, are discussed.

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.

The adrenergic α2-receptor, sexual incentive motivation and copulatory behavior in the male rat

Adrenergic α2 antagonists are known to enhance sexual incentive motivation and modify copulatory behavior while agonists are consistently inhibitory. However, many of the drugs employed in earlier studies were of modest specificity for the α2 receptor, and the importance of the different subtypes of this receptor remains completely unknown. In the present series of experiments we determined the effects on sexual incentive motivation and copulatory behavior of additional, highly specific compounds, as well as of agonists selective for each of the three subtypes of the α2 receptor. Sexual incentive motivation and copulatory behavior were evaluated in male rats in well established procedures. Among the α2 antagonists, RX 821002 reliably enhanced sexual incentive motivation while fluparoxan only had a modest effect. In large doses both drugs reduced copulatory behavior. The agonist S 18616 reduced both incentive motivation and copulation. None of the subtype selective agonists (BRL 44408, ARC 239, JP 1302) had any consistent effect. A peripheral α2 antagonist, L 659,066 was also ineffective. Even though there are some differences between α2 antagonists with regard to their effects on sexual incentive motivation and copulatory behavior it seems safe to conclude that antagonism of the adrenergic α2 receptor enhances motivation without any concomitant stimulation of copulatory behavior. It appears that antagonism of a single receptor subtype is insufficient for having this effect. Perhaps non-selective α2 antagonists could be used for the treatment of male sexual dysfunction.

Animal Models for the Study of Female Sexual Dysfunction

INTRODUCTION

Significant progress has been made in elucidating the physiological and pharmacological mechanisms of female sexual function through preclinical animal research. The continued development of animal models is vital for the understanding and treatment of the many diverse disorders that occur in women.

AIM

To provide an updated review of the experimental models evaluating female sexual function that may be useful for clinical translation.

METHODS

Review of English written, peer-reviewed literature, primarily from 2000 to 2012, that described studies on female sexual behavior related to motivation, arousal, physiological monitoring of genital function and urogenital pain.

MAIN OUTCOMES MEASURES

Analysis of supporting evidence for the suitability of the animal model to provide measurable indices related to desire, arousal, reward, orgasm, and pelvic pain.

RESULTS

The development of female animal models has provided important insights in the peripheral and central processes regulating sexual function. Behavioral models of sexual desire, motivation, and reward are well developed. Central arousal and orgasmic responses are less well understood, compared with the physiological changes associated with genital arousal. Models of nociception are useful for replicating symptoms and identifying the neurobiological pathways involved. While in some cases translation to women correlates with the findings in animals, the requirement of circulating hormones for sexual receptivity in rodents and the multifactorial nature of women's sexual function requires better designed studies and careful analysis. The current models have studied sexual dysfunction or pelvic pain in isolation; combining these aspects would help to elucidate interactions of the pathophysiology of pain and sexual dysfunction.

CONCLUSIONS

Basic research in animals has been vital for understanding the anatomy, neurobiology, and physiological mechanisms underlying sexual function and urogenital pain. These models are important for understanding the etiology of female sexual function and for future development of pharmacological treatments for sexual dysfunctions with or without pain. Marson L, Giamberardino MA, Costantini R, Czakanski P, and Wesselmann U. Animal models for the study of female sexual dysfunction. Sex Med Rev 2013;1:108-122.

Neuroanatomical dichotomy of sexual behaviors in rodents: a special emphasis on brain serotonin

Much of the social behavior in which rodents engage is related to reproduction, such as maintaining a breeding territory, seeking mates, mating, and caring for their young. Rodents belong to the internally fertilizing species that require sexual behavior for reproduction. The dyadic, heterosexual patterns of most mammalian species are sexually dimorphic, but they also share mutual components in both sexes: sexual attraction is reciprocal, sexual initiative is assumed, appetitive behavior is engaged in, and mating involves consummatory and postconsummatory phases in females as well as in males. Serotonin, a phylogenetically ancient molecule, is the most widely distributed neurotransmitter in the brain and its signaling pathways are essential for numerous functions including sexual behavior. Since the late 1960s, brain serotonergic neurotransmission has been considered to exert an inhibitory influence on the neural mechanisms mediating sexual behavior. This contention was based mainly on the observations that a decrease in central serotonergic activity facilitated the elicitation of sexual behavior, whereas an increase in central serotonergic activity attenuated it. However, the discovery of over 14 types of serotonin receptors has added numerous layers of complexity to the study of serotonin and sexual behavior. Evidence shows that, upon activation, certain receptor subtypes facilitate, whereas some others suppress, sexual behavior, as well as sexual arousal and motivation. Furthermore, the role of these receptors has been shown to be different in the male and female sexes. The use of serotonergic pharmacological interventions, mouse strains with genetic polymorphisms causing alterations in the levels of brain serotonin, and animal models with genetic manipulations of various serotonin effectors has helped delineate the fundamental role of this neurotransmitter in the regulation of sexual behavior. This review aims to examine the basics of the components of female and male sexual behavior and the participation of the serotonin system in the modulation of these behaviors, with emphasis on rodents.

Effect of newly synthesized 1,2,4-triazino[5,6-b]indole-3-thione derivatives on olfactory bulbectomy induced depression in rats

OBJECTIVE

To study the derivatives of 1,2,4-triazino[5,6-b]indole-3-thione for antidepressant activity in olfactory bulbectomized (OBX) rats. Out of various derivatives tested for acute tail suspension test, the two derivatives showing prominent action were selected for bilateral olfactory bulbectomy model of chronic depression in rats.

METHODS

The sub acute effects of 14-day oral pretreatment of two derivatives labeled as 3a (70 mg/kg) and 3r (70 mg/kg), imipramine (20 mg/kg), fluoxetine (30 mg/kg) and moclobemide (15 mg/kg) were evaluated on bilateral bulbectomy induced rise in body weight, hyperphagia, hyperactivity, and on sexual dysfunction. The serum sodium concentration, body temperature, and heart rate were also recorded.

RESULTS

The derivatives 3a and 3r showed reversal of drop in body weight, reversed OBX induced hyperactivity, normalized body temperature, heart rate, and serum sodium concentration. In elevated maze test, moclobemide, 3a, 3r treatment significantly reduced time spent in open arm as compared to OBX rats. 3a and 3r also improved sexual behavior parameters.

CONCLUSIONS

The present study shows promising antidepressant action and provides a proof of concept for the chronic treatment of 3a, 3r to treat depression.

Pharmacology of serotonin and female sexual behavior

In this review, first a historical perspective of serotonin's (5-HT) involvement in female sexual behavior is presented. Then an overview of studies implicating 5-HT is presented. The effect of drugs that increase or decrease CNS levels of 5-HT is reviewed. Evidence is presented that drugs which increase 5-HT have negative effects on female sexual behavior while a decrease in 5-HT is associated with facilitation of sexual behavior. Studies with compounds that act on 5-HT₁, 5-HT₂ or 5-HT₃ receptors are discussed. Most evidence indicates that 5-HT₁A receptor agonists inhibit sexual behavior while 5-HT₂ or 5-HT₃ receptors may exert a positive influence. There is substantial evidence to support a role for 5-HT in the modulation of female consummatory sexual behavior, but studies on the role of 5-HT in other elements of female sexual behavior (e.g. desire, motivation, sexual appetite) are few. Future studies should be directed at determining if these additional components of female sexual behavior are also modulated by 5-HT.