Effects of aromatase mutation (ArKO) on the sexual differentiation of kisspeptin neuronal numbers and their activation by same versus opposite sex urinary pheromones

Kisspeptin neurons from mice to men: similarities and differences

The discovery that kisspeptin was critical for normal fertility in humans ushered in a new chapter in our understanding of the control of GnRH secretion. In this paper, we will review recent data on the similarities and differences across several mammalian species in the role of kisspeptin in reproductive neuroendocrinology. In all mammals examined to date, there is strong evidence that kisspeptin plays a key role in the onset of puberty and is necessary for both tonic and surge secretion of GnRH in adults, although kisspeptin-independent systems are also apparent in these studies. Similarly, two groups of kisspeptin neurons, one in the arcuate nucleus (ARC) and the other more rostrally, have been identified in all mammals, although the latter is concentrated in a limited area in rodents and more scattered in other species. Estrogen has divergent actions on kisspeptin expression in these two regions across these species, stimulating it the latter and inhibiting expression in the former. There is also strong evidence that the rostral population participates in the GnRH surge, whereas the ARC population contributes to steroid-negative feedback. There may be species differences in the role of these two populations in puberty, with the ARC cells important in rats, sheep, and monkeys, whereas both have been implicated in mice. ARC kisspeptin neurons also appear to participate in the GnRH surge in sheep and guinea pigs, whereas the data on this possibility in rodents are contradictory. Similarly, both populations are sexually dimorphic in sheep and humans, whereas most data in rodents indicate that this occurs only in the rostral population. The functional consequences of these species differences remain to be fully elucidated but are likely to have significance for understanding normal neuroendocrine control of reproduction as well as for use of kisspeptin agonists/antagonists as a therapeutic tool.

Aromatase knockout mice show normal steroid-induced activation of gonadotrophin-releasing hormone neurones and luteinising hormone surges with a reduced population of kisspeptin neurones in the rostral hypothalamus

We recently reported that female aromatase knockout (ArKO) mice show deficits in sexual behaviour and a decreased population of kisspeptin-immunoreactive neurones in the rostral periventricular area of the third ventricle (RP3V), resurrecting the question of whether oestradiol actively contributes to female-typical sexual differentiation. To further address this question, we assessed the capacity of ArKO mice to generate a steroid-induced luteinising hormone (LH) surge. Adult, gonadectomised wild-type (WT) and ArKO mice were given silastic oestradiol implants s.c. and, 1 week later, received s.c. injections of either oestradiol benzoate (EB) followed by progesterone, EB alone, or no additional steroids to activate gonadotrophin-releasing hormone (GnRH) neurones and generate an LH surge. Treatment with EB and progesterone induced significant Fos/GnRH double-labelling and, consequently, an LH surge in female WT and in ArKO mice of both sexes but not in male WT mice. ArKO mice of both sexes had fewer cells expressing Kiss-1 mRNA in the RP3V compared to female WT mice but had more Kiss-1 mRNA-expressing cells compared to WT males, reflecting an incomplete sexual differentiation of this system. To determine the number of cells expressing kisspeptin, the same experimental design was repeated in Experiment 2 with the addition of groups of WT and ArKO mice that were given EB + progesterone and sacrificed 2 h before the expected LH surge. No differences were observed in the number of kisspeptin-immunoreactive cells 2 h before and at the time of the LH surge. The finding that ArKO mice of both sexes have a competent LH surge system suggests that oestradiol has predominantly defeminising actions on the GnRH/LH surge system in males and that the steroid-induced LH surge can occur in females even with a greatly reduced population of kisspeptin neurones in the RP3V.

Kisspeptin-immunoreactivity changes in a sex- and hypothalamic-region-specific manner across rat postnatal development

Kisspeptins are potent secretagogues of gonadotrophin-releasing hormone, playing a key role in puberty onset. These peptides are produced by distinct neuronal populations of the hypothalamus located in the rostral periventricular area of the third ventricle (RP3V) and arcuate nucleus (ARC). The present immunohistochemical study aimed to determine the spatiotemporal onset of kisspeptin-immunoreactivity (-IR) in the neonatal hypothalamus of male and female rats and to evaluate changes in kisspeptin-IR around puberty. Kisspeptin-IR cells and fibres could be detected from the day of birth in the ARC of both males and females. At this stage, only females displayed some kisspeptin-IR fibres in the RP3V. From postnatal day 7 to adulthood, males displayed lower levels of kisspeptin-IR than females in both regions. During infancy, kisspeptin-IR fibre density in the female decreased in the ARC, whereas it increased in the RP3V. A sex-independent decline in RP3V kisspeptin-IR fibre density was observed in the juvenile, followed by a peripubertal increase in RP3V and ARC kisspeptin-IR. These peripubertal increases in kisspeptin-IR occurred at different timings dependent on sex and region. In females specifically, the increase in kisspeptin-IR fibre density occurred first in the ARC and later in the RP3V under constant levels of circulating oestradiol. In conclusion, the present study highlights the expression of hypothalamic kisspeptins soon after birth, as well as the neonatal establishment of a strong and persisting sex difference in ARC kisspeptin-IR in rats. Moreover, a female-specific desynchronisation of the ARC and RP3V was observed with respect to the increase in kisspeptin-IR fibre density around puberty, which was not related to peripubertal variations in circulating oestradiol.

Kisspeptins and Reproduction: Physiological Roles and Regulatory Mechanisms

Procreation is essential for survival of species. Not surprisingly, complex neuronal networks have evolved to mediate the diverse internal and external environmental inputs that regulate reproduction in vertebrates. Ultimately, these regulatory factors impinge, directly or indirectly, on a final common pathway, the neurons producing the gonadotropin-releasing hormone (GnRH), which stimulates pituitary gonadotropin secretion and thereby gonadal function. Compelling evidence, accumulated in the last few years, has revealed that kisspeptins, a family of neuropeptides encoded by the Kiss1 gene and produced mainly by neuronal clusters at discrete hypothalamic nuclei, are pivotal upstream regulators of GnRH neurons. As such, kisspeptins have emerged as important gatekeepers of key aspects of reproductive maturation and function, from sexual differentiation of the brain and puberty onset to adult regulation of gonadotropin secretion and the metabolic control of fertility. This review aims to provide a comprehensive account of the state-of-the-art in the field of kisspeptin physiology by covering in-depth the consensus knowledge on the major molecular features, biological effects, and mechanisms of action of kisspeptins in mammals and, to a lesser extent, in nonmammalian vertebrates. This review will also address unsolved and contentious issues to set the scene for future research challenges in the area. By doing so, we aim to endow the reader with a critical and updated view of the physiological roles and potential translational relevance of kisspeptins in the integral control of reproductive function.

Female mice deficient in alpha-fetoprotein show female-typical neural responses to conspecific-derived pheromones

The neural mechanisms controlling sexual behavior are sexually differentiated by the perinatal actions of sex steroid hormones. We recently observed using female mice deficient in alpha-fetoprotein (AFP-KO) and which lack the protective actions of AFP against maternal estradiol, that exposure to prenatal estradiol completely defeminized the potential to show lordosis behavior in adulthood. Furthermore, AFP-KO females failed to show any male-directed mate preferences following treatment with estradiol and progesterone, indicating a reduced sexual motivation to seek out the male. In the present study, we asked whether neural responses to male- and female-derived odors are also affected in AFP-KO female mice. Therefore, we compared patterns of Fos, the protein product of the immediate early gene, c-fos, commonly used as a marker of neuronal activation, between wild-type (WT) and AFP-KO female mice following exposure to male or estrous female urine. We also tested WT males to confirm the previously observed sex differences in neural responses to male urinary odors. Interestingly, AFP-KO females showed normal, female-like Fos responses, i.e. exposure to urinary odors from male but not estrous female mice induced equivalent levels of Fos protein in the accessory olfactory pathways (e.g. the medial part of the preoptic nucleus, the bed nucleus of the stria terminalis, the amygdala, and the lateral part of the ventromedial hypothalamic nucleus) as well as in the main olfactory pathways (e.g. the piriform cortex and the anterior cortical amygdaloid nucleus), as WT females. By contrast, WT males did not show any significant induction of Fos protein in these brain areas upon exposure to either male or estrous female urinary odors. These results thus suggest that prenatal estradiol is not involved in the sexual differentiation of neural Fos responses to male-derived odors.

Gonadal steroid induction of kisspeptin peptide expression in the rostral periventricular area of the third ventricle during postnatal development in the male mouse

Kisspeptin and its G-protein coupled receptor Gpr54 are essential for the pubertal activation of gonadotrophin-releasing hormone (GnRH) neurones, with Gpr54 mutation or deletion resulting in failed puberty and infertility in humans and mice. The number of kisspeptin-immunoreactive neurones in the rostral periventricular area of the third ventricle (RP3V) increases during pubertal development in concert with the appearance of kisspeptin appositions with GnRH neurones in the mouse rostral preoptic area. We recently demonstrated that the pubertal increase in RP3V kisspeptin neuronal number in females is dependent upon circulating oestradiol levels. The present experiments investigated the potential role of gonadal steroids in the induction of kisspeptin expression in the RP3V during pubertal development in the male mouse. Using immunocytochemistry (ICC), we show that gonadectomy of male pups at postnatal day (P) 20 resulted in a 60-70% reduction in the number of kisspeptin immunoreactive (IR) neurones within the RP3V of P45 mice (P<0.05) compared to sham-treated littermates. We established a profile of circulating testosterone levels during postnatal development in male mice and found that circulating testosterone was low throughout early postnatal development and increased from P35-40 to reach adult levels. Treatment of P20-gonadectomised male mice with 17β-oestradiol or testosterone from P38-45 restored kisspeptin-IR neurone number in the RP3V to intact control levels (P>0.05). Using double-label ICC, we demonstrate that the majority of RP3V kisspeptin neurones express androgen receptors and oestrogen receptor α, indicating that RP3V kisspeptin neurones in the male mouse are equipped to respond to both androgen and oestrogen signals. These results indicate that, as in females, gonadal steroids are essential for the increase in kisspeptin immunoreactive cell number that occurs in the RP3V during pubertal development in the male mouse.

Postnatal aromatase blockade increases c-fos mRNA responses to acute restraint stress in adult male rats

Recent evidence suggests that the aromatization of testosterone to estrogen is important for the organizing effects of neonatal testosterone on neuroendocrine responses to acute challenges. However, the extent to which neonatal inhibition of aromatase alters the stress-induced activation of neural pathways has not been examined. Here we assessed central patterns of c-fos mRNA induced by 30 min of restraint in 65-d-old adult male rats that were implanted with sc capsules of the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD), introduced within 12 h of birth and removed on d 21 of weaning. Neonatal ATD decreased the expression of arginine vasopressin within extrahypothalamic regions in adults, confirming reduced estrogen exposure during development. As adults, ATD-treated animals showed higher corticosterone responses at 30 min of restraint exposure compared with control animals as well as higher c-fos expression levels in the paraventricular nucleus of the hypothalamus. ATD treatment also increased stress-induced c-fos within several limbic regions of the forebrain, in addition to areas involved in somatosensory processing. Based on these results, we propose that the conversion of testosterone to estrogen during the neonatal period exerts marked, system-wide effects to organize adult neuroendocrine responses to homeostatic threat.

A potential mechanism for the sexual dimorphism in the onset of puberty and incidence of idiopathic central precocious puberty in children: sex-specific kisspeptin as an integrator of puberty signals

The major determinants of the variability in pubertal maturation are reported to be genetic and inherited. Nonetheless, nutritional status contributes significantly to this variability. Malnutrition delays puberty whereas obesity has been associated to a rise in Idiopathic Central Precocious Puberty (ICPP) in girls. However, epidemiology data indicate that contribution of obesity to early puberty varies significantly among ethnic groups, and that obesity-independent inheritable genetic factors are the strongest predictors of early puberty in any ethnic group. In fact, two human mutations with confirmed association to ICPP have been identified in children with no history of obesity. These mutations are in kisspeptin and kisspeptin receptor, a ligand/receptor pair with a major role on the onset of puberty and female cyclicity after puberty. Progressive increases in kisspeptin expression in hypothalamic nuclei known to regulate reproductive function has been associated to the onset of puberty, and hypothalamic expression of kisspeptin is reported to be sexually dimorphic in many species, which include humans. The hypothalamus of females is programmed to express significantly higher levels of kisspeptin than their male counterparts. Interestingly, incidence of ICPP and delayed puberty in children is markedly sexually dimorphic, such that ICPP is at least 10-fold more frequent in females, whereas prevalence of delayed puberty is about 5-fold higher in males. These observations are consistent with a possible involvement of sexually dimorphic kisspeptin signaling in the sexual dimorphism of normal puberty and of pubertal disorders in children of all ethnicities. This review discusses the likelihood of such associations, as well as a potential role of kisspeptin as the converging target of environmental, metabolic, and hormonal signals, which would be integrated in order to optimize reproductive function.

Sex steroids and the control of the Kiss1 system: developmental roles and major regulatory actions

Kisspeptins, encoded by the Kiss1 gene, and their canonical receptor, GPR54 (also termed Kiss1R), are unanimously recognised as essential regulators of puberty onset and gonadotrophin secretion. These key reproductive functions stem from the capacity of kisspeptins to stimulate gonadotrophin-releasing hormone (GnRH) secretion in the hypothalamus, where discrete populations of Kiss1 neurones have been identified. In rodents, two major groups of hypothalamic Kiss1 neurones exist: one present in the arcuate nucleus (ARC) and the other located in the anteroventral periventricular area (AVPV/RP3V). In recent years, numerous signals have been identified as putative modulators of the hypothalamic Kiss1 system. Among them, the prominent role of sex steroids as being important regulators of Kiss1 neurones has been documented in different species and developmental stages, such as early brain sex differentiation, puberty, adulthood and senescence. These regulatory actions are (mainly) conducted via oestrogen receptor (ER)α, which is expressed in almost all Kiss1 neurones, and likely involve both classical and nonclassical pathways. The regulatory effects of sex steroids are nucleus-specific. Thus, sex steroids inhibit the expression of Kiss1/kisspeptin at the ARC, as a mechanism to conduct their negative-feedback actions on gonadotrophin secretion. By contrast, oestrogens enhance Kiss1 expression at the AVPV/RP3V in rodents, suggesting the involvement of this population in the positive-feedback actions of oestradiol to generate the preovulatory surge of gonadotrophins. In addition, sex steroids have been shown to act post-transcriptionally, modulating GnRH/gonadotrophin responsiveness to kisspeptin. Finally, sex steroids also regulate the expression of co-transmitters of Kiss1 neurones, such as neurokinin B, whose mRNA content in the ARC fluctuates in parallel to that of Kiss1 in response to changes in the circulating levels of sex steroids, therefore suggesting the contribution of this neuropeptide in the feedback control of gonadotrophin secretion. In sum, compelling experimental evidence obtained in different mammalian (and non-mammalian) species, including primates, demonstrates that sex steroids are essential regulators of hypothalamic Kiss1 neurones, which in turn operate as conduits for their effects on GnRH neurones. The physiological relevance of such regulatory phenomena is thoroughly discussed.

The main and accessory olfactory systems of female mice are activated differentially by dominant versus subordinate male urinary odors

Previous studies have shown that female preferences for male pheromones depend on the female's reproductive condition and the dominance status of the male. However, it is unknown which olfactory system detects the odors that result in a preference for a dominant male. Therefore, in the present study, we asked whether dominant versus subordinate male urinary odors differentially activate the main and accessory olfactory systems in female (C57Bl/6j) mice by monitoring the induction of the immediate early gene, c-fos. A more robust induction of Fos was observed in female mice which had direct nasal contact with dominant male urinary odors in four specific segments of the accessory olfactory system, i.e., the posteroventral part of the medial amygdala, the bed nucleus of the stria terminalis, the medial part of the preoptic nucleus and the ventrolateral part of the ventromedial hypothalamus, compared to females that were exposed to subordinate male urine. This greater activation of the accessory olfactory pathway by dominant male urine suggests that there are differences in the nonvolatile components of dominant versus subordinate male urine that are detected by the vomeronasal organ. By contrast, subordinate male urinary odors induced a greater activation in the piriform cortex which is part of the main olfactory system, suggesting that female mice discriminate between dominant and subordinate male urine using their main olfactory system as well.

Neuroendocrine effects of endocrine disruptors in teleost fish

Because a large proportion of potential endocrine disruptors (EDC) end up in surface waters, aquatic species are particularly vulnerable to their potential adverse effects. Recent studies identified a number of brain targets for EDC commonly present in environmentally relevant concentrations in surface waters. Among those neuronal systems disrupted by EDC are the gonadotropin-releasing hormone (GnRH) neurons, the dopaminergic and serotoninergic circuits, and more recently the Kiss/GPR54 system, which regulates gonadotropin release. However, one of the most striking effects of EDC, notably estrogen mimics, is their impact on the cyp19a1b gene that encodes the brain aromatase isoform in fish. Moreover, this is the only example in which the molecular basis of endocrine disruption is fully understood. The aims of this review were to (1) synthesize the most recent discoveries concerning the EDC effects upon neuroendocrine systems of fish and (2) provide, when possible, the underlying molecular basis of disruption for each system concerned. The potential adverse effects of EDC on neurogenesis, puberty, and brain sexualization are also described. It is important to point out the future environmental, social, and economical issues arising from endocrine disruption studies in the context of risk assessment.

Timing and completion of puberty in female mice depend on estrogen receptor alpha-signaling in kisspeptin neurons

Puberty onset is initiated by activation of neurons that secrete gonadotropin-releasing hormone (GnRH). The timing and progression of puberty may depend upon temporal coordination of two opposing central mechanisms--a restraint of GnRH secretion before puberty onset, followed by enhanced stimulation of GnRH release to complete reproductive maturation during puberty. Neuronal estrogen receptor α (ERα) has been implicated in both controls; however, the underlying neural circuits are not well understood. Here we test whether these mechanisms are mediated by neurons that express kisspeptin, a neuropeptide that modulates GnRH neurosecretion. Strikingly, conditional ablation of ERα in kisspeptin neurons results in a dramatic advancement of puberty onset in female mice. Furthermore, subsequent pubertal maturation is arrested in these animals, as they fail to acquire normal ovulatory cyclicity. We show that the temporal coordination of juvenile restraint and subsequent pubertal activation is likely mediated by ERα in two separate kisspeptin neuronal populations in the hypothalamus.

Mapping of kisspeptin fibres in the brain of the pro-oestrous rat

Kisspeptins are a family of small peptides that play a key role in the neuroendocrine regulation of the reproductive function through neural pathways that have not yet been completely identified. The present study aimed to investigate the distribution of kisspeptin neurone fibres in the female rat brain by comparing precisely the immunoreactive pattern obtained with two antibodies: one specifically directed against kisspeptin-52 (Kp-52), the longest isoform, and the other directed against kisspeptin-10 (Kp-10), whose sequence is common to all putative mature isoforms. With both antibodies, immunoreactive cell bodies were exclusively observed in the arcuate nucleus, and immunoreactive fibres were confined to the septo-preoptico-hypothalamic continuum of the brain. Fibres were observed in the preoptic area, the diagonal band of Broca, the septohypothalamic area, the anteroventral periventricular, suprachiasmatic, supraoptic, paraventricular and periventricular nuclei, the dorsal border of the ventromedian nucleus, the dorsomedial and arcuate nuclei, and the median eminence. In the latter structure, varicose fibres were mainly distributed in the internal layer and were detected to a lesser extent throughout the external layer, including around the deeper part of the infundibular recess. Most regions of immunoreactive cells and fibres matched perfectly for the two antibodies. However, fibres in the dorsolateral septum, anterior fornix, accumbens nucleus and the lateral bed nucleus of the stria terminalis were only recognised by antibody anti-Kp-10, suggesting that anti-Kp-10 may recognise a wider range of kisspeptin isoforms than anti-Kp-52 or cross-react with molecules other than kisspeptin in rat tissue. Overall, these results illustrate the variety of projection sites of kisspeptin neurones in the rat and suggest that these peptides play a role in different functions.

Coming of age in the kisspeptin era: sex differences, development, and puberty

The status of the neuroendocrine reproductive axis differs dramatically during various stages of development, and also differs in several critical ways between the sexes, including its earlier pubertal activation in females than males and the presence of neural circuitry that generates preovulatory hormone surges in females but not males. The reproductive axis is controlled by various hormonal and neural pathways that converge upon forebrain gonadotropin-releasing hormone (GnRH) neurons, and many of the critical age and sex differences in the reproductive axis likely reflect differences in the "upstream" circuits and factors that regulate the GnRH system. Recently, the neural kisspeptin system has been implicated as an important regulator of GnRH neurons. Here I discuss the evidence supporting a critical role of kisspeptin signaling at different stages of life, including early postnatal and pubertal development, as well as in adulthood, focusing primarily on information gleaned from mammalian studies. I also evaluate key aspects of sexual differentiation and development of the brain as it relates to the Kiss1 system, with special emphasis on rodents. In addition to discussing recent advances in the field of kisspeptin biology, this paper will highlight a number of unanswered questions and future challenges for kisspeptin investigators, and will stress the importance of studying the kisspeptin system in both males and females, as well as in multiple species.

Reproductive hormone-dependent and -independent contributions to developmental changes in kisspeptin in GnRH-deficient hypogonadal mice

Kisspeptin is a potent activator of GnRH-induced gonadotropin secretion and is a proposed central regulator of pubertal onset. In mice, there is a neuroanatomical separation of two discrete kisspeptin neuronal populations, which are sexually dimorphic and are believed to make distinct contributions to reproductive physiology. Within these kisspeptin neuron populations, Kiss1 expression is directly regulated by sex hormones, thereby confounding the roles of sex differences and early activational events that drive the establishment of kisspeptin neurons. In order to better understand sex steroid hormone-dependent and -independent effects on the maturation of kisspeptin neurons, hypogonadal (hpg) mice deficient in GnRH and its downstream effectors were used to determine changes in the developmental kisspeptin expression. In hpg mice, sex differences in Kiss1 mRNA levels and kisspeptin immunoreactivity, typically present at 30 days of age, were absent in the anteroventral periventricular nucleus (AVPV). Although immunoreactive kisspeptin increased from 10 to 30 days of age to levels intermediate between wild type (WT) females and males, corresponding increases in Kiss1 mRNA were not detected. In contrast, the hpg arcuate nucleus (ARC) demonstrated a 10-fold increase in Kiss1 mRNA between 10 and 30 days in both females and males, suggesting that the ARC is a significant center for sex steroid-independent pubertal kisspeptin expression. Interestingly, the normal positive feedback response of AVPV kisspeptin neurons to estrogen observed in WT mice was lost in hpg females, suggesting that exposure to reproductive hormones during development may contribute to the establishment of the ovulatory gonadotropin surge mechanism. Overall, these studies suggest that the onset of pubertal kisspeptin expression is not dependent on reproductive hormones, but that gonadal sex steroids critically shape the hypothalamic kisspeptin neuronal subpopulations to make distinct contributions to the activation and control of the reproductive hormone cascade at the time of puberty.

Gonadal and nongonadal regulation of sex differences in hypothalamic Kiss1 neurones

The brains of males and females differ anatomically and physiologically, including sex differences in neurone size or number, synapse morphology and specific patterns of gene expression. Brain sex differences may underlie critical sex differences in physiology or behaviour, including several aspects of reproduction, such as the timing of sexual maturation (earlier in females than males) and the ability to generate a preovulatory gonadotrophin surge (in females only). The reproductive axis is controlled by afferent pathways that converge upon forebrain gonadotrophin-releasing hormone (GnRH) neurones, but GnRH neurones are not sexually dimorphic. Although most reproductive sex differences probably reflect sex differences in the upstream circuits and factors that regulate GnRH secretion, the key sexually-dimorphic factors that influence reproductive status have remained poorly defined. The recently-identified neuropeptide kisspeptin, encoded by the Kiss1 gene, is an important regulator of GnRH secretion, and Kiss1 neurones in rodents are sexually dimorphic in specific hypothalamic populations, including the anteroventral periventricular nucleus-periventricular nucleus continuum (AVPV/PeN) and the arcuate nucleus (ARC). In the adult AVPV/PeN, Kiss1 neurones are more abundant in females than males, representing a sex difference that is regulated by oestradiol signalling during critical periods of postnatal and pubertal development. By contrast, Kiss1 neurones in the ARC are not sexually differentiated in adult rodents but, in mice, the regulation of ARC Kiss1 cells by gonadal hormone-independent factors is sexually dimorphic during prepubertal development. These various sex differences in hypothalamic Kiss1 neurones may relate to known sex differences in reproductive physiology, such as puberty onset and positive feedback.

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

Previous research using alpha-fetoprotein knockout and aromatase knockout (ArKO) female mice suggested that the developing hypothalamic mechanisms that later control feminine sexual behavior are protected prenatally from estradiol, whereas shortly after birth, they may be stimulated by this same sex hormone. In the present study, we found that the amount of progesterone receptor immunoreactivity (PR-ir) in the anteroventral periventricular nucleus and medial part of the medial preoptic nucleus was significantly lower in ArKO female mice than in wild-type (WT) females at several prepubertal ages including postnatal d 15 (P15), P15, P20, and P25 but not neonatally at P0, P5, or P10. Likewise, PR-ir in the lateral subdivision of the ventromedial hypothalamic nucleus was significantly lower at P25 in ArKO vs. WT female mice but not at earlier postnatal ages. PR-ir was consistently higher in male than in female WT mice in the anteroventral periventricular nucleus and medial preoptic nucleus over P0-P10 and in the ventromedial hypothalamic nucleus over P0-P20. In these brain regions across these latter ages, PR-ir in male ArKO mice was significantly lower than in WT males and resembled the values seen in WT females, confirming previous reports that estradiol formed in the developing male hypothalamus from testicular testosterone is responsible for male-typical levels of neural PR expression. Thus, estradiol induces both female- and male-typical expression of PR postnatally in the mouse hypothalamus. Future experiments will determine whether this estradiol-induced PR expression contributes to either female- or male-typical brain and behavioral differentiation.