Reduced prepubertal expression of progesterone receptor in the hypothalamus of female aromatase knockout mice

Neural and Hormonal Basis of Opposite-Sex Preference by Chemosensory Signals

In mammalian reproduction, sexually active males seek female conspecifics, while estrous females try to approach males. This sex-specific response tendency is called sexual preference. In small rodents, sexual preference cues are mainly chemosensory signals, including pheromones. In this article, we review the physiological mechanisms involved in sexual preference for opposite-sex chemosensory signals in well-studied laboratory rodents, mice, rats, and hamsters of both sexes, especially an overview of peripheral sensory receptors, and hormonal and central regulation. In the hormonal regulation section, we discuss potential rodent brain bisexuality, as it includes neural substrates controlling both masculine and feminine sexual preferences, i.e., masculine preference for female odors and the opposite. In the central regulation section, we show the substantial circuit regulating sexual preference and also the influence of sexual experience that innate attractants activate in the brain reward system to establish the learned attractant. Finally, we review the regulation of sexual preference by neuropeptides, oxytocin, vasopressin, and kisspeptin. Through this review, we clarified the contradictions and deficiencies in our current knowledge on the neuroendocrine regulation of sexual preference and sought to present problems requiring further study.

Progesterone Actions During Central Nervous System Development

Although progesterone is a steroid hormone mainly associated with female reproductive functions, such as uterine receptivity and maintenance of pregnancy, accumulating data have shown its physiological actions to extend to several non-reproductive functions in the central nervous system (CNS) both in males and females. In fact, progesterone is de novo synthesized in specific brain regions by neurons and glial cells and is involved in the regulation of various molecular and cellular processes underlying myelination, neuroprotection, neuromodulation, learning and memory, and mood. Furthermore, progesterone has been reported to be implicated in critical developmental events, such as cell differentiation and neural circuits formation. This view is supported by the increase in progesterone synthesis observed during pregnancy in both the placenta and the fetal brain. In the present review, we will focus on progesterone actions during CNS development.

Expression of progesterone receptor, estrogen receptors α and β, and kisspeptin in the hypothalamus during perinatal development of gonad-lacking steroidogenic factor-1 knockout mice

Gonadal hormones contribute to brain sexual differentiation. We analyzed expression of progesterone receptor (PR), estrogen receptor-α (ERα), ERβ, and kisspeptin, in the preoptic area (POA) and/or the arcuate nucleus (ARC), in gonad-lacking steroidogenic factor-1 knockout (KO) mice during perinatal development. At postnatal-day (P) 0-P7, POA PR levels were higher in wild-type (WT) males compared with WT females, while those in KO males were lower than in WT males and similar to those in WT and KO females. At P14-P21, PR levels in all groups increased similarly. POA ERα levels were similar in all groups at embryonic-day (E) 15.5-P14. Those in WT but not KO males reduced during postnatal development to be significantly lower compared with females at P21. POA ERβ levels were higher in WT males than in WT females, while those in KO males were lower than in WT males and similar to those in WT and KO females at P0-P21. POA kisspeptin expression was female-biased in WT mice, while levels in KO females were lower compared with WT females and similar to those in WT and KO males. ARC kisspeptin levels were equivalent among groups at E15.5-P0. At P7-P21, ARC levels in WT but not KO males became lower compared with WT females. Diethylstilbestrol exposure during P0-P6 and P7-P13 increased POA PR and ERβ, and decreased POA ERα and ARC kisspeptin levels at P7 and/or P14 in both sexes of KO mice. These data further understanding of gonadal hormone action on neuronal marker expression during brain sexual development.

Female sexual behavior in mice is controlled by kisspeptin neurons

Sexual behavior is essential for the survival of many species. In female rodents, mate preference and copulatory behavior depend on pheromones and are synchronized with ovulation to ensure reproductive success. The neural circuits driving this orchestration in the brain have, however, remained elusive. Here, we demonstrate that neurons controlling ovulation in the mammalian brain are at the core of a branching neural circuit governing both mate preference and copulatory behavior. We show that male odors detected in the vomeronasal organ activate kisspeptin neurons in female mice. Classical kisspeptin/Kiss1R signaling subsequently triggers olfactory-driven mate preference. In contrast, copulatory behavior is elicited by kisspeptin neurons in a parallel circuit independent of Kiss1R involving nitric oxide signaling. Consistent with this, we find that kisspeptin neurons impinge onto nitric oxide-synthesizing neurons in the ventromedial hypothalamus. Our data establish kisspeptin neurons as a central regulatory hub orchestrating sexual behavior in the female mouse brain.

Mate preference and copulatory behavior in female rodents are coordinated with the ovulation cycles of the animal. This study shows that hypothalamic kisspeptin neurons control both mate choice and copulation, and therefore, that sexual behavior and ovulation may be synchronized by the same neuropeptide.

The Sexual Differentiation of the Human Brain: Role of Sex Hormones Versus Sex Chromosomes

Men and women differ, not only in their anatomy but also in their behavior. Research using animal models has convincingly shown that sex differences in the brain and behavior are induced by sex hormones during a specific, hormone-sensitive period during early development. Thus, male-typical psychosexual characteristics seem to develop under the influence of testosterone, mostly acting during early development. By contrast, female-typical psychosexual characteristics may actually be organized under the influence of estradiol during a specific prepubertal period. The sexual differentiation of the human brain also seems to proceed predominantly under the influence of sex hormones. Recent studies using magnetic resonance imaging have shown that several sexually differentiated aspects of brain structure and function are female-typical in women with complete androgen insensitivity syndrome (CAIS), who have a 46 XY karyotype but a female phenotype due to complete androgen resistance, suggesting that these sex differences most likely reflect androgen action, although feminizing effects of estrogens or female-typical socialization cannot be ruled out. By contrast, some male-typical neural characteristics were also observed in women with CAIS suggesting direct effects of sex chromosome genes in the sexual differentiation of the human brain. In conclusion, the sexual differentiation of the human brain is most likely a multifactorial process including both sex hormone and sex chromosome effects, acting in parallel or in combination.

Androgenic mechanisms of sexual differentiation of the nervous system and behavior

Testicular androgens are the major endocrine factor promoting masculine phenotypes in vertebrates, but androgen signaling is complex and operates via multiple signaling pathways and sites of action. Recently, selective androgen receptor mutants have been engineered to study androgenic mechanisms of sexual differentiation of the nervous system and behavior. The focus of these studies has been to evaluate androgenic mechanisms within the nervous system by manipulating androgen receptor conditionally in neural tissues. Here we review both the effects of neural loss of AR function as well as the effects of neural overexpression of AR in relation to global AR mutants. Although some studies have conformed to our expectations, others have proved challenging to assumptions underlying the dominant hypotheses. Notably, these studies have called into question both the primacy of direct, neural mechanisms and also the linearity of the relationship between androgenic dose and sexual differentiation of brain and behavior.

Potential contribution of progesterone receptors to the development of sexual behavior in male and female mice

We previously showed that estradiol can have both defeminizing and feminizing effects on the developing mouse brain. Pre- and early postnatal estradiol defeminized the ability to show lordosis in adulthood, whereas prepubertal estradiol feminized this ability. Furthermore, we found that estradiol upregulates progesterone receptors (PR) during development, inducing both a male-and female-typical pattern of PR expression in the mouse hypothalamus. In the present study, we took advantage of a newly developed PR antagonist (ZK 137316) to determine whether PR contributes to either male- or female-typical sexual differentiation. Thus groups of male and female C57Bl/6j mice were treated with ZK 137316 or OIL as control: males were treated neonatally (P0-P10), during the critical period for male sexual differentiation, and females were treated prepubertally (P15-P25), during the critical period for female sexual differentiation. In adulthood, mice were tested for sexual behavior. In males, some minor effects of neonatal ZK treatment on sexual behavior were observed: latencies to the first mount, intromission and ejaculation were decreased in neonatally ZK treated males; however, this effect disappeared by the second mating test. By contrast, female mice treated with ZK during the prepubertal period showed significantly less lordosis than OIL-treated females. Mate preferences were not affected in either males or females treated with ZK during development. Taken together, these results suggest a role for PR and thus perhaps progesterone in the development of lordosis behavior in female mice. By contrast, no obvious role for PR can be discerned in the development of male sexual behavior.

Sex chromosome complement determines sex differences in aromatase expression and regulation in the stria terminalis and anterior amygdala of the developing mouse brain

Aromatase, which converts testosterone in estradiol, is involved in the generation of brain sex dimorphisms. Here we used the "four core genotypes" mouse model, in which the effect of gonadal sex and sex chromosome complement is dissociated, to determine if sex chromosomes influence the expression of brain aromatase. The brain of 16 days old XY mouse embryos showed higher aromatase expression in the stria terminalis and the anterior amygdaloid area than the brain of XX embryos, independent of gonadal sex. Furthermore, estradiol or dihydrotestosterone increased aromatase expression in cultures of anterior amygdala neurons derived from XX embryos, but not in those derived from XY embryos. This effect was also independent of gonadal sex. The expression of other steroidogenic molecules, estrogen receptor-α and androgen receptor was not influenced by sex chromosomes. In conclusion, sex chromosomes determine sex dimorphisms in aromatase expression and regulation in the developing mouse brain.

Roles of sex and gonadal steroids in mammalian pheromonal communication

A brain circuit (the accessory olfactory system) that originates in the vomeronasal organ (VNO) and includes the accessory olfactory bulb (AOB) plus additional forebrain regions mediates many of the effects of pheromones, typically comprised of a variety of non-volatile and volatile compounds, on aspects of social behavior. A second, parallel circuit (the main olfactory system) that originates in the main olfactory epithelium (MOE) and includes the main olfactory bulb (MOB) has also been shown to detect volatile pheromones from conspecifics. Studies are reviewed that point to specific roles of several different steroids and their water-soluble metabolites as putative pheromones. Other studies are reviewed that establish an adult, 'activational' role of circulating sex hormones along with sex differences in the detection and/or processing of non-steroidal pheromones by these two olfactory circuits. Persisting questions about the role of sex steroids in pheromonal processing are posed for future investigation.

The two kisspeptin neuronal populations are differentially organized and activated by estradiol in mice

In rodents, kisspeptin-expressing neurons are localized in 2 hypothalamic brain nuclei (anteroventral periventricular nucleus/periventricular nucleus continuum [AVPv/PeN] and arcuate nucleus [ARC]) and modulated by sex steroids. By using wild-type (WT) and aromatase knockout (ArKO) mice (which cannot convert testosterone into estradiol) and immunohistochemistry, we observed that WT females showed a continuous increase in kisspeptin peptide expression in the ARC across postnatal ages (postnatal day 5 [P5] to P25), whereas WT males did not show any expression before P25. Kisspeptin peptide expression was also present in ArKO females but did not increase over this early postnatal period, suggesting that kisspeptin peptide expression in the ARC is organized by estradiol-dependent and -independent mechanisms. We also compared kisspeptin peptide expression between groups of adult male and female mice that were left gonadally intact or gonadectomized and treated or not with estradiol (E(2)) or DHT. In the ARC, kisspeptin peptide expression decreased after gonadectomy but was completely rescued by either E(2) or DHT treatment in each sex/genotype. However, kisspeptin peptide expression was lower in ArKO compared with WT subjects. In the AVPv/PeN, ArKO females showed a male-typical kisspeptin peptide expression, and adult E(2) treatment partially restored kisspeptin peptide expression. Finally, we showed that, after E2 treatment of WT and ArKO mice between either P5 and P15 or P15 and P25, AVPv/PeN kisspeptin peptide expression could be still masculinized at P5, but was feminized from P15 onward. In conclusion, the 2 kisspeptin neuronal populations (AVPv/PeN vs ARC) seem to be differentially organized and activated by E(2).

Contribution of pheromones processed by the main olfactory system to mate recognition in female mammals

Until recently it was widely believed that the ability of female mammals (with the likely exception of women) to identify and seek out a male breeding partner relied on the detection of non-volatile male pheromones by the female's vomeronasal organ (VNO) and their subsequent processing by a neural circuit that includes the accessory olfactory bulb (AOB), vomeronasal amygdala, and hypothalamus. Emperical data are reviewed in this paper that demonstrate the detection of volatile pheromones by the main olfactory epithelium (MOE) of female mice which, in turn, leads to the activation of a population of glomeruli and abutting mitral cells in the main olfactory bulb (MOB). Anatomical results along with functional neuroanatomical data demonstrate that some of these MOB mitral cells project to the vomeronasal amygdala. These particular MOB mitral cells were selectively activated (i.e., expressed Fos protein) by exposure to male as opposed to female urinary volatiles. A similar selectivity to opposite sex urinary volatiles was also seen in mitral cells of the AOB of female mice. Behavioral data from female mouse, ferret, and human are reviewed that implicate the main olfactory system, in some cases interacting with the accessory olfactory system, in mate recognition.

Evidence that sex chromosome genes affect sexual differentiation of female sexual behavior

Female receptivity including the immobile hormone-dependent lordosis posture is essential for successful reproduction in rodents. It is well documented that lordosis is organized during the perinatal period when the actions of androgens decrease the males' ability to display this behavior in adulthood. Conversely the absence of androgens, and the presence of low levels of prepubertal estrogens, preserve circuitry that regulates this behavior in females. The current study set out to determine whether sex chromosomal genes are involved in the differentiation of this behavior. An agonadal mouse model was used to test this hypothesis. The SF-1 gene (Nr5a1) is required for development of gonads and adrenal glands, and knockout mice are consequently not exposed to endogenous gonadal steroids. Thus contributions of sex chromosome genes can be disassociated from the actions of estrogens. Use of this model reveals a direct genetic contribution from sex chromosomes in the display of lordosis and other female-typical sexual behavior patterns. It is likely that the concentrations of gonadal steroids present during normal male development modify the actions of sex chromosome genes on the potential to display female sexual behavior.

Aromatase promoter I.f is regulated by progesterone receptor in mouse hypothalamic neuronal cell lines

Aromatase catalyzes the conversion of C(19) steroids to estrogens. Aromatase and progesterone, both of which function at different steps of steroidogenesis, are crucial for the sexually dimorphic development of the fetal brain and the regulation of gonadotropin secretion and sexual interest in adults. The aromatase gene (Cyp19a1) is selectively expressed in distinct neurons of the mouse hypothalamus through a distal brain-specific promoter, I.f, located ∼40 kb upstream of the coding region. However, the regulation of aromatase expression in the brain is not well understood. In this study, we investigated a short feedback effect of progesterone analogues on aromatase mRNA expression and enzyme activity in estrogen receptor α (Esr1)-positive or -negative mouse embryonic hypothalamic neuronal cell lines that express aromatase via promoter I.f. In a hypothalamic neuronal cell line that highly expresses aromatase, progesterone receptor (Pgr), and Esr1, a progesterone agonist, R5020, inhibited aromatase mRNA level and enzyme activity. The inhibitory effect of R5020 was reversed by its antagonist, RU486. Deletion mutants of promoter I.f suggested that inhibition of aromatase expression by progesterone is conferred by the nt -1000/-500 region, and R5020 enhanced binding of Pgr to the nt -800/-600 region of promoter I.f. Small interfering RNA knockdown of Pgr eliminated progesterone-dependent inhibition of aromatase mRNA and enzyme activity. Taken together, progesterone enhances recruitment of Pgr to specific regions of the promoter I.f of Cyp19a1 and regulates aromatase expression in hypothalamic neurons.

The development of female sexual behavior requires prepubertal estradiol

The classic view of brain and behavioral sexual differentiation holds that the neural mechanisms controlling sexual behavior in female rodents develop in the absence of ovarian sex hormone actions. However, in a previous study, female aromatase knock-out (ArKO) mice, which cannot convert testosterone to estradiol, showed deficient male-oriented partner preference and lordosis behaviors in response to adult ovarian hormones, raising the possibility that estradiol may contribute to the development of these female sexual behaviors. In the present experiments, administering estradiol prepubertally [between postnatal day 15 (P15) and P25] significantly enhanced the ability of ArKO female mice to display lordosis behavior in response to ovarian hormones administered later in adulthood, whereas treatment with estradiol over an earlier postnatal period (P5-P15) had no such effect. Treatment of ArKO females with estradiol between P15 and P25 also rescued their later preference to approach distal cues from an intact male over an estrous female. ArKO females also displayed significantly less female-directed (male-typical) mounting behavior than wild-type control females when treated with testosterone in adulthood. Prepubertal estradiol treatment failed to reverse this deficit in ArKO females, whereas earlier postnatal estradiol augmented later mounting in both genotypes. Our results provide new evidence for an organizing role of prepubertal estradiol in the development of neural mechanisms that control female-typical sexual behavior.