Period Of Brain Sexual Differentiation Research Articles

Sex-specific Effects of Endocrine-disrupting Chemicals on Brain Monoamines and Cognitive Behavior.

The period of brain sexual differentiation is characterized by the development of hormone-sensitive neural circuits that govern the subsequent presentation of sexually dimorphic behavior in adulthood. Perturbations of hormones by endocrine-disrupting chemicals (EDCs) during this developmental period interfere with an organism's endocrine function and can disrupt the normative organization of male- or female-typical neural circuitry. This is well characterized for reproductive and social behaviors and their underlying circuitry in the hypothalamus and other limbic regions of the brain; however, cognitive behaviors are also sexually dimorphic, with their underlying neural circuitry potentially vulnerable to EDC exposure during critical periods of brain development. This review provides recent evidence for sex-specific changes to the brain's monoaminergic systems (dopamine, serotonin, norepinephrine) after developmental EDC exposure and relates these outcomes to sex differences in cognition such as affective, attentional, and learning/memory behaviors.

Hormonal and genetic factors interact to control aromatase expression in the developing brain.

Brain expression of the enzyme P450-aromatase has been studied extensively. Subsequent to the aromatisation hypothesis having established brain aromatase as a key factor to convert gonadal testosterone to oestradiol, several studies have investigated the regulation of aromatase during the critical period of brain sexual differentiation. We review previous and recent findings concerning regulation of aromatase. The role of gonadal hormones, sex chromosome genes and neurosteroids is analysed in terms of their contribution to aromatase expression, as well as implications for the organisational effect of steroids during development.

Delayed effects of single neonatal subcutaneous exposure of low-dose 17α-ethynylestradiol on reproductive function in female rats

Delayed effects of exposure to small amounts of estrogenic compounds during the critical period of brain sexual differentiation were investigated by subcutaneous treatment of female Sprague-Dawley rats with 0 (vehicle control), 0.08, 0.4, or 2 µg/kg of 17α-ethynylestradiol (EE) on postnatal day (PND) 1. The treatment did not affect growth and development of the treated animals, and the timings of vaginal opening were similar between the EE-treated and control groups. The animals were periodically examined for the estrous cycle from postnatal week (PNW) 8-9 to PNW 32-33. Patterns of the estrous cycle were similar among the groups until PNW 17. None of the control animals showed persistent estrus until PNW 33. The animals treated with 0.4 µg/kg or more EE showed persistent estrus from PNW 20. The alteration was reflected in the number of days judged as proestrus or estrus, and was found to gradually increase in the EE-treated groups. At necropsy on PNW 32-33, ovulation was not confirmed in most EE-treated animals, even on the day of estrus. In addition, sporadic milk accumulations were observed in the mammary gland of the EE-treated animals. Histological evaluation revealed cystic follicle formation in the EE-treated ovaries and also revealed hyperplasia of mammary glands. Furthermore, ovaries from the animals showing persistent estrus lacked corpus luteum, indicating long-term anovulation. These results clearly show that single exposure to EE during the critical period of brain sexual differentiation can exert effects on reproductive functions at a later period in rats.

Neonatal Exposure to Bisphenol A and Reproductive and Endocrine Alterations Resembling the Polycystic Ovarian Syndrome in Adult Rats

BackgroundBisphenol A (BPA), an endocrine disruptor, is a component of polycarbonate plastics, epoxy resins, and polystyrene. Several studies have reported potent in vivo effects, because BPA behaves as an estrogen agonist and/or antagonist and as an androgen and thyroid hormone antagonist.ObjectivesWe investigated the effects of neonatal exposure to BPA on the reproductive axis in adult female Sprague-Dawley rats.MethodsFemale rats were injected subcutaneously, daily from postnatal day 1 (PND1) to PND10 with BPA in castor oil at 500 μg/50 μL [BPA500; ~ 10−4 M, a dose higher than the lowest observed adverse effect level (LOAEL) of 50 mg/kg], 50 μg/50 μL (BPA50), or 5 μg/50 μL (both BPA50 and BPA5 are doses lower than the LOAEL), or castor oil vehicle alone. In adults we studied a) the release of gonadotropin-releasing hormone (GnRH) from hypothalamic explants, b) serum sex hormone levels, and c) ovarian morphology, ovulation, and fertility.ResultsNeonatal exposure to BPA was associated with increased serum testosterone and estradiol levels, reduced progesterone in adulthood, and altered in vitro GnRH secretion. Animals exposed to BPA500 had altered ovarian morphology, showing a large number of cysts. Animals exposed to BPA50 had reduced fertility without changes in the number of oocytes on the morning of estrus, whereas animals exposed to BPA500 showed infertility.ConclusionsExposure to high doses of BPA during the period of brain sexual differentiation altered the hypothalamic–pituitary–gonadal axis in female Sprague-Dawley rats. These results have the potential to link neonatal exposure to high doses of BPA in rats with the development of polycystic ovarian syndrome. Studies of doses and routes of administration more consistent with human exposures are needed to determine the relevance of these findings to human health.

The role of neonatal NMDA receptor activation in defeminization and masculinization of sex behavior in the rat

Normal development of the male rat brain involves two distinct processes, masculinization and defeminization, that occur during a critical period of brain sexual differentiation. Masculinization allows for the capacity to express male sex behavior in adulthood, and defeminization eliminates or suppresses the capacity to express female sex behavior in adulthood. Despite being separate processes, both masculinization and defeminization are induced by neonatal estradiol exposure. Though the mechanisms underlying estradiol-mediated masculinization of behavior during development have been identified, the mechanisms underlying defeminization are still unknown. We sought to determine whether neonatal activation of glutamate NMDA receptors is a necessary component of estradiol-induced defeminization of behavior. We report here that antagonizing glutamate receptors during the critical period of sexual differentiation blocks estradiol-induced defeminization but not masculinization of behavior in adulthood. However, enhancing NMDA receptor activation during the same critical period mimics estradiol to permanently induce both defeminization and masculinization of sexual behavior.

Mecp2 Organizes Juvenile Social Behavior in a Sex-Specific Manner

Methyl-CpG-binding protein 2 (MeCP2) binds methylated DNA and recruits corepressor proteins to modify chromatin and alter gene transcription. Mutations of the MECP2 gene can cause Rett syndrome, whereas subtle reductions of MeCP2 expression may be associated with male-dominated social and neurodevelopmental disorders. We report that transiently decreased amygdala Mecp2 expression during a sensitive period of brain sexual differentiation disrupts the organization of sex differences in juvenile social play behavior. Interestingly, neonatal treatment with Mecp2 small interfering RNA within the developing amygdala reduced juvenile social play behavior in males but not females. Reduced Mecp2 expression did not change juvenile sociability or anxiety-like behavior, suggesting that this disruption is associated with subtle behavioral modification. This suggests that Mecp2 may have an overlooked role in the organization of sexually dimorphic behaviors and that male juvenile behavior is particularly sensitive to Mecp2 disruption during this period of development.

Could neonatal testosterone replacement prevent alterations induced by prenatal stress in male rats?

The present study was designed to examine whether testosterone replacement is able to prevent some effects of maternal restraint stress — during the period of brain sexual differentiation — on endocrine system and sexual behavior in male rat descendants. Pregnant rats were exposed to restraint stress for 1 h/day from gestational days 18 to 22. At birth, some male pups from these stressed rats received testosterone propionate. The neonatal testosterone replacement was able to prevent the reduction in anogenital distance at 22 days of age observed in pups from stressed pregnant rats as well as prevents the decrease in testosterone levels during the adulthood of these animals. Testosterone replacement in these males also presented an improvement in sexual performance. In this way, testosterone replacement probably through increasing neonatal level of this hormone was able to prevent the later alterations caused by the prenatal stress during the period of brain sexual differentiation.

D-Amphetamine-related reinforcing effects are reduced in mice exposed prenatally to estrogenic endocrine disruptors

Estrogenic endocrine disruptors are hormonally active compounds that can bind to estradiol receptors. Central dopamine pathways have been reported to be affected by early developmental exposure to estrogenic endocrine disruptors. In the present study, pregnant female CD-1 mice were allowed to drink spontaneously either oil or environmentally relevant low doses of two estrogenic compounds, methoxychlor (20 μg/kg) or bisphenol-A (10 μg/kg) during gestation days 11–18. Their adult offspring were assessed for conditioned place preference produced by d-amphetamine (0, 1 or 2 mg/kg). Interestingly, prenatal treatment effects were sex-dependent and no changes in conditioned place preference emerged for the male offspring. Conversely, a clear-cut profile of d-amphetamine-induced conditioned place preference was only shown by oil-exposed females, whereas exposure to bisphenol-A or methoxychlor resulted in little or no place conditioning. Locomotor effects of acute d-amphetamine were not affected by prenatal exposure to bisphenol-A or methoxychlor. As a whole, prenatal exposure to estrogenic endocrine disruptors affected some steps in the organization of the brain dopaminergic systems in the female offspring, thus leading to long-term alterations in neurobehavioral function. These data confirm that exposure to weak environmental estrogens in the period of brain sexual differentiation can influence adult behavior.

Impact of dietary exposure to methoxychlor, genistein, or diisononyl phthalate during the perinatal period on the development of the rat endocrine/reproductive systems in later life.

To evaluate the impact of dietary exposure to endocrine disrupting chemicals (EDCs) during the sensitive period of brain sexual differentiation, maternal Sprague-Dawley rats were fed three representative chemicals, methoxychlor (MXC; 24, 240, and 1200 ppm), genistein (GEN; 20, 200, and 1000 ppm), or diisononyl phthalate (DINP; 400, 4000, and 20,000 ppm), from gestational day 15 to postnatal day 10. Soy-free diet was used as a basal diet to eliminate possible estrogenic effects from the standard diet. Offspring were examined in terms of anogenital distances, prepubertal organ weights, onset of puberty, estrous cyclicity, and organ weights and histopathology of endocrine organs at adult stage (week 11) as well as the volumes of sexually dimorphic nucleus of preoptic area (SDN-POA). All chemicals caused signs of maternal toxicity at high doses. MXC, at 1200 ppm, facilitated and delayed the onset of puberty in females and males, respectively, females also showing endocrine disrupting effects thereafter, such as irregular estrous cyclicity and histopathological alterations in the reproductive tract and anterior pituitary. GEN, at all doses, reduced body weight (BW) at week 11, but did not affect endocrine parameters. Treatment with DINP at 20,000 ppm resulted in degeneration of meiotic spermatocytes and Sertoli cells in the testis and decrease of corpora lutea in the ovary at week 11, although changes remained minimal or slight. The SDN-POA volume remained unchanged with all three chemicals. The results demonstrated that perinatal dietary exposure to EDCs for a limited period causes endocrine disruption in offspring only at high doses.

Cholinergic responses of seminal vesicles isolated from rats exposed perinatally to hydrocortisone

This study was performed in order to investigate the cholinomimetic response of seminal vesicles isolated from rats treated with hydrocortisone acetate during perinatal life. At the adult phase, the body weight and the wet weight of the seminal vesicle of these animals were unchanged. However, these male rats exhibited a significant reduction in plasma testosterone concentration. A significant increase in the sensitivity of the seminal vesicle to acetylcholine was also observed. Despite this, there was a significant reduction in the maximum contractile response of the organ to this transmitter. These results indicate that exposure to hydrocortisone during the critical period of brain sexual differentiation has a long-term effect on testosterone production of male rats. In addition, physiological levels of cortisone in perinatal life are also essential to support the contractile response pattern of the seminal vesicle to acetylcholine in adult life, probably crucial to the reproductive process.

Effects of neonatal clomiphene citrate on fertility and sexual behavior in male rats

In order to investigate the participation of estrogen during the period of brain sexual differentiation, male rats were treated with clomiphene citrate in the neonatal phase. Fertility and sexual behavior were assessed during adult life. Sexual maturation, body weight, and wet weight of the testes were unchanged. Although the adult male rats treated with clomiphene in the neonatal phase presented a significant reduction in the frequency of mounts, 90% of these rats were able to mate with normal females, which became pregnant. However, these females exhibited a significantly increased number of pre- and post-implantation losses. When these adult male rats were castrated and received estrogen, 60% presented female sexual behavior (receptive behavior and acceptance of mount). Thus, treatment of pups with clomiphene immediately after birth has a long-term effect on the reproductive physiology and sexual behavior of male rats.

Neonatal inhalatory anesthetic exposure: reproductive changes in male rats

We investigated the effects of an inhalatory anesthetic (ethyl ether) during the neonatal period of brain sexual differentiation on the later fertility and sexual behavior of male rats. Animals were exposed to ethyl ether immediately after birth. At adulthood, body weight, testes wet weight, and plasma testosterone levels were not affected; however, neonatal exposure to ether showed alterations on male fertility: a decrease in the number of spermatids and spermatozoa, an increase in the transit time of cauda epididymal spermatozoa and a decrease in daily sperm production. An alteration of sexual behavior was also observed: decreased male sexual behavior and appearance of homosexual behavior when the male rats were castrated and pretreated with exogenous estrogen. Probably, the ether delayed or reduced the testosterone peak of the sexual differentiation period, altering the processes of masculinization and defeminization of the hypothalamus. Our results indicate that perinatal exposure to ethyl ether during the critical period of male brain sexual differentiation, acting as endocrine disruptors, has a long-term effect on the fertility and sexual behavior of male rats, suggesting endocrine disruption through incomplete masculinization and defeminization of the central nervous system.

Transient neonatal elevation in hypothalamic estrogen receptor mRNA in prenatally-stressed male rats

Prenatal stress in rats has been found to alter the sexual dimorphism of brain structures and the sexual behavior of male offspring, pointing to an impaired masculinization of the brain during the perinatal period of brain sexual differentiation. Masculinization of the brain depends on the presence during this critical period of three main elements: adequate levels of testosterone, aromatase activity (locally converting testosterone to estradiol), and brain estrogen receptor (ER) density. In the present study, we measured by reverse transcription-polymerase chain reaction (RT-PCR) the levels of ER messenger RNA (mRNA) expression in the hypothalamus of either prenatally-stressed or control male rats at postnatal (P) days 3, 12 and 90. During the early postnatal period (P3), hypothalamic ER mRNA expression was higher in prenatally stressed male rats (6.12 ± 0.37) than in controls (4.51 ± 0.55) ( P = 0.015). This difference was not, however, found at a later developmental stage (P12, 5.39 ± 0.65 versus 5.39 ± 0.47) or in adult animals (P90, 6.79 ± 1.55 versus 7.07 ± 1.11). This transient elevation of hypothalamic ER mRNA expression resembles the developmental profile of ER mRNA in females. These observations support the idea that androgens play a pivotal role in the demasculinization process, and suggest that testosterone production or aromatization is reduced in prenatally-stressed males during the perinatal period of sexual differentiation, leading to a transient upregulation of unstimulated estrogen receptors.

Transient neonatal elevation in hypothalamic estrogen receptor mRNA in prenatally-stressed male rats

Prenatal stress in rats has been found to alter the sexual dimorphism of brain structures and the sexual behavior of male offspring, pointing to an impaired masculinization of the brain during the perinatal period of brain sexual differentiation. Masculinization of the brain depends on the presence during this critical period of three main elements: adequate levels of testosterone, aromatase activity (locally converting testosterone to estradiol), and brain estrogen receptor (ER) density. In the present study, we measured by reverse transcription-polymerase chain reaction (RT-PCR) the levels of ER messenger RNA (mRNA) expression in the hypothalamus of either prenatally-stressed or control male rats at postnatal (P) days 3, 12 and 90. During the early postnatal period (P3), hypothalamic ER mRNA expression was higher in prenatally stressed male rats (6.12 ± 0.37) than in controls (4.51 ± 0.55) ( P = 0.015). This difference was not, however, found at a later developmental stage (P12, 5.39 ± 0.65 versus 5.39 ± 0.47) or in adult animals (P90, 6.79 ± 1.55 versus 7.07 ± 1.11). This transient elevation of hypothalamic ER mRNA expression resembles the developmental profile of ER mRNA in females. These observations support the idea that androgens play a pivotal role in the demasculinization process, and suggest that testosterone production or aromatization is reduced in prenatally-stressed males during the perinatal period of sexual differentiation, leading to a transient upregulation of unstimulated estrogen receptors.

Embryonic Rodent Brain Contains Estrogen Receptors

Estradiol-binding proteins with the properties of putative estrogen receptors are present in cytosol extracts of embryonic mouse hypothalamus and other brain regions. These embryonic estrogen receptors are adultlike in their high affinity and limited capacity for estradiol, sensitivity to diethylstilbestrol, ability to adhere to DNA, and behavior during DNA-cellulose affinity chromatography. As early as 4 days before birth, mouse hypothalamus has approximately 40 percent of the adult concentration of hypothalamic estrogen receptors with these properties. These observations raise the possibility that embryonic rodent brain has the biochemical potential to respond to sex hormones and that the critical period of brain sexual differentiation could be initiated prenatally.