Gonadal steroid action and brain sex differentiation in the rat

Reproductive consequences of developmental phytoestrogen exposure

Phytoestrogens, estrogenic compounds derived from plants, are ubiquitous in human and animal diets. These chemicals are generally much less potent than estradiol but act via similar mechanisms. The most common source of phytoestrogen exposure to humans is soybean-derived foods that are rich in the isoflavones genistein and daidzein. These isoflavones are also found at relatively high levels in soy-based infant formulas. Phytoestrogens have been promoted as healthy alternatives to synthetic estrogens and are found in many dietary supplements. The aim of this review is to examine the evidence that phytoestrogen exposure, particularly in the developmentally sensitive periods of life, has consequences for future reproductive health.

Sex differences in mechanisms and outcome of neonatal hypoxia-ischemia in rodent models: implications for sex-specific neuroprotection in clinical neonatal practice

Clinical findings show that male infants with hypoxic-ischemic injury (HI) fare more poorly than matched females on cognitive outcomes. Rodent models of neonatal hypoxia-ischemia support this difference, with data showing that perinatal brain injury leads to long-term behavioral deficits primarily in male rodents and in female rodents treated with early androgens. Results support the idea that sex-specific gonadal hormones may modulate developmental response to injury and dovetail with overwhelming evidence of developmental androgen effects on typical brain morphology and behavior. However, mechanisms underlying sex differences in response to early brain injury may be more complicated. Specifically, activation of cell death pathways in response to HI may also differ by sex. In females, the preferential activation of the caspase-dependent apoptotic pathway may actually afford greater protection, potentially due to the actions of X-linked inhibitor of apoptosis (XIAP) within this pathway. This contrasts the pattern of preferential activation of the caspase-independent pathway in males. While an integrated model of sex-specific hormonal and genetic modulation of response to early injury remains to be fully elucidated, these findings suggest that infants might benefit from sex-specific neuroprotection following HI injury.

Developing brain as an endocrine organ: secretion of dopamine

This study was aimed to test our hypothesis that the developing brain operates as an endocrine organ before the establishment of the blood-brain barrier (BBB), in rats up to the first postnatal week. Dopamine (DA) was selected as a marker of the brain endocrine activity. The hypothesis was supported by the observations in rats of: (i) the physiological concentration of DA in peripheral blood of fetuses and neonates, before the BBB establishment, and its drop by prepubertal period, after the BBB development; (ii) a drop of the DA concentration in the brain for 54% and in blood for 74% on the 3rd postnatal day after the intraventricular administration of 50 μg of α-methyl-p-tyrosine, an inhibitor of DA synthesis, with no changes in the DA metabolism in peripheral DA-producing organs. Thus, the developing brain is a principal source of circulating DA which is capable of providing an endocrine regulation of peripheral organs and the brain.

Influence of sex and estrous cycle on synaptic responses of the medial vestibular nuclei in rats: role of circulating 17β-estradiol

We investigated the possible influence of sex and estrous cycle on the synaptic responses of neurons in the medial vestibular nucleus (MVN) and their long-term modifications. In brain stem slices of male and female rats during proestrus (PE) and diestrus (DE), we evaluated the field potential evoked in the MVN by vestibular afferent stimulation. Here we find that in PE females the field potential had a lower threshold and higher amplitude than in DE females and in males and also that the stimulus-response curve was shifted to the left. Such difference is related to the level and cyclic fluctuation of circulating 17β-estradiol (E(2)). This is supported by the exogenous administration of E(2) in DE females and males, with low levels of circulating E(2) that enhanced the field potential amplitude to values close to those of PE females. Sex and estrous cycle also influence the MVN synaptic plasticity. This has been shown by investigating the effect of testosterone (T) on the induction of long-term effects, since T is the precursor for the neural synthesis of E(2) (estrogenic pathway), which is involved in the induction of fast long-term potentiation (LTP), or of 5α-dihydrotestosterone (DHT, androgenic pathway) which mediates slow LTP and long-term depression (LTD). We found that T mostly induced LTD in PE females and no effect in DE females, while it only provoked fast LTP in males. We suggest that high level of circulating E(2) may interfere with the conversion of T, by inhibiting the neural estrogenic pathway and facilitating the androgenic one. On the whole these results demonstrate an influence of circulating E(2) on vestibular synaptic transmission and plasticity that in some cases may contribute to the sex and menstrual cycle dependence of symptoms in human vestibular pathology.

Oxytocin, vasopressin and estrogen receptor gene expression in relation to social recognition in female mice

Inter- and intra-species differences in social behavior and recognition-related hormones and receptors suggest that different distribution and/or expression patterns may relate to social recognition. We used qRT-PCR to investigate naturally occurring differences in expression of estrogen receptor-alpha (ERα), ER-beta (ERβ), progesterone receptor (PR), oxytocin (OT) and receptor, and vasopressin (AVP) and receptors in proestrous female mice. Following four 5 min exposures to the same two conspecifics, one was replaced with a novel mouse in the final trial (T5). Gene expression was examined in mice showing high (85-100%) and low (40-60%) social recognition scores (i.e., preferential novel mouse investigation in T5) in eight socially-relevant brain regions. Results supported OT and AVP involvement in social recognition, and suggest that in the medial preoptic area, increased OT and AVP mRNA, together with ERα and ERβ gene activation, relate to improved social recognition. Initial social investigation correlated with ERs, PR and OTR in the dorsolateral septum, suggesting that these receptors may modulate social interest without affecting social recognition. Finally, increased lateral amygdala gene activation in the LR mice may be associated with general learning impairments, while decreased lateral amygdala activity may indicate more efficient cognitive mechanisms in the HR mice.

Sex-dependent changes induced by prenatal stress in cortical and hippocampal morphology and behaviour in rats: an update

Recent prospective studies have shown that gestational stress in humans is more likely to cause cognitive and emotional problems in the offspring if it occurs during weeks 12-20 of pregnancy. There are also suggestions that such problems may be gender dependent. This review describes recent studies that found sex differences in the behaviour and brain morphology of rats stressed prenatally during the equivalent period of neuronal development in humans. Learning deficits are more prevalent in males and anxious behaviour in females but their appearance depends also on the timing and intensity of the stress and the age when the offspring were tested. Cognitive deficits and anxiety are linked to a sex-dependent reduction in neurogenesis and in measures of dendritic morphology in the prefrontal cortex and hippocampal formation. Maternal adrenalectomy prior to the stress prevents the anxiety in both sexes and learning deficits in males. Corticosterone administration to the dam to mimic levels induced by stress reinstates only the anxiety, indicating that it arises from foetal exposure to corticosterone from the maternal circulation. Learning deficits in males may result from a combination of a reduction in testosterone and in aromatase activity, together with the action of other adrenal hormones.

Male or Female? Brains are Intersex

The underlying assumption in popular and scientific publications on sex differences in the brain is that human brains can take one of two forms “male” or “female,” and that the differences between these two forms underlie differences between men and women in personality, cognition, emotion, and behavior. Documented sex differences in brain structure are typically taken to support this dimorphic view of the brain. However, neuroanatomical data reveal that sex interacts with other factors in utero and throughout life to determine the structure of the brain, and that because these interactions are complex, the result is a multi-morphic, rather than a dimorphic, brain. More specifically, here I argue that human brains are composed of an ever-changing heterogeneous mosaic of “male” and “female” brain characteristics (rather than being all “male” or all “female”) that cannot be aligned on a continuum between a “male brain” and a “female brain.” I further suggest that sex differences in the direction of change in the brain mosaic following specific environmental events lead to sex differences in neuropsychiatric disorders.

Chronic exposure to anabolic androgenic steroids alters activity and synaptic function in neuroendocrine control regions of the female mouse

Disruption of reproductive function is a hallmark of abuse of anabolic androgenic steroids (AAS) in female subjects. To understand the central actions of AAS, patch clamp recordings were made in estrous, diestrous and AAS-treated mice from gonadotropin releasing hormone (GnRH) neurons, neurons in the medial preoptic area (mPOA) and neurons in the anteroventroperiventricular nucleus (AVPV); regions known to provide GABAergic and kisspeptin inputs to the GnRH cells. Action potential (AP) frequency was significantly higher in GnRH neurons of estrous mice than in AAS-treated or diestrous animals. No significant differences in AAS-treated, estrous or diestrous mice were evident in the amplitude or kinetics of spontaneous postsynaptic currents (sPSCs), miniature PSCs or tonic currents mediated by GABA(A) receptors or in GABA(A) receptor subunit expression in GnRH neurons. In contrast, the frequency of GABA(A) receptor-mediated sPSCs in GnRH neurons showed an inverse correlation with AP frequency across the three hormonal states. Surprisingly, AP activity in the medial preoptic area (mPOA), a likely source of GABAergic afferents to GnRH cells, did not vary in concert with the sPSCs in the GnRH neurons. Furthermore, pharmacological blockade of GABA(A) receptors did not alter the pattern in which there was lower AP frequency in GnRH neurons of AAS-treated and diestrous versus estrous mice. These data suggest that AAS do not impose their effects either directly on GnRH neurons or on putative GABAergic afferents in the mPOA. AP activity recorded from neurons in kisspeptin-rich regions of the AVPV and the expression of kisspeptin mRNA and peptide did vary coordinately with AP activity in GnRH neurons. Our data demonstrate that AAS treatment imposes a "diestrous-like" pattern of activity in GnRH neurons and suggest that this effect may arise from suppression of presynaptic kisspeptin-mediated excitatory drive arising from the AVPV. The actions of AAS on neuroendocrine regulatory circuits may contribute the disruption of reproductive function observed in steroid abuse.

Endogenously elevated androgens alter the developmental programming of the hypothalamic-pituitary axis in male mice

Transgenic male mice that express human chorionic gonadotropin (hCG) α and β subunits constitutively hypersecrete hCG and produce elevated levels of androgens. The aim of this study was to characterize the hypothalamic-pituitary function of these transgenic (hCGαβ+) males by focusing on FSH regulation. Serum FSH levels and pituitary mRNA expression of Fshb, Lhb, Cga, Gnrhr and Esr1 were reduced, whereas Fst expression was increased in prepubertal hCGαβ+ males as compared with wild-type. In the hypothalamus, Cyp19a1 expression, GnRH concentration and ex-vivo GnRH pulsatility were elevated in prepubertal hCGαβ+ mice, whereas Kiss1 expression was decreased prepubertally and Gad67 expression was elevated neonatally. The effect of androgens on the developmental programming of the hypothalamic-pituitary axis of hCGαβ+ males was evaluated by perinatal and prepubertal antiandrogen (flutamide) administration. Our studies identified a critical window between gestational day 18 and postnatal day 14, during which chronically elevated androgens and/or their locally produced metabolites activate the hypothalamus and concomitantly shut-down the gonadotropin axis.

17β-estradiol and progesterone regulate multiple progestin signaling molecules in the anteroventral periventricular nucleus, ventromedial nucleus and sexually dimorphic nucleus of the preoptic area in female rats

Recent work identified novel progestin signaling molecules, including progesterone receptor membrane component 1 (Pgrmc1), Pgrmc2, serpine mRNA binding protein 1 (Serbp1), progestin and adiponectin receptors 7 (Paqr7) and Paqr8. These molecules mediate rapid progesterone (P(4)) effects in non-neural tissue and we recently mapped their expression in the brain. Many rapid effects of P(4) require 17β-estradiol (E(2)) and P(4) priming; therefore, we examined the effects of ovarian hormones on the expression of these non-classical progestin signaling molecules. We focused specifically on the anteroventral periventricular nucleus (AVPV), the sexually dimorphic nucleus of the preoptic area (SDN-POA) and the ventrolateral portion of the ventromedial nucleus (VMNvl). These brain nuclei are important for female reproduction. Ovariectomized adult female rats were implanted with capsules containing sesame oil or E(2), and injected 48 h later with sesame oil or P(4). Brains were collected 8 h later and RNA was isolated from the AVPV, SDN-POA and VMNvl. We assessed the effects of ovarian hormones on mRNA levels using quantitative polymerase chain reaction (QPCR). In the AVPV, Serbp1 mRNA levels were increased by P(4) in the presence of E(2), and Paqr8 was downregulated by P(4) alone. In the SDN-POA, combined E(2) and P(4) increased Pgrmc1 and Serbp1 mRNA levels, and E(2) alone increased Paqr8 mRNA levels. Finally, in the VMNvl, P(4) increased mRNA levels encoding Pgrmc1, Pgrmc2 and Serbp1, and the combination of E(2) and P(4) increased Pgrmc1 and Serbp1 mRNA levels. Paqr7 was not regulated by E(2) or P(4) in any brain region examined. In summary, we showed that ovarian hormones regulate novel progestin signaling molecules in brain regions important for the neuroendocrine control of reproduction.

Maternal deprivation has sexually dimorphic long-term effects on hypothalamic cell-turnover, body weight and circulating hormone levels

Maternal deprivation (MD) has numerous outcomes, including modulation of neuroendocrine functions. We previously reported that circulating leptin levels are reduced and hypothalamic cell-turnover is affected during MD, with some of these effects being sexually dimorphic. As leptin modulates the development of hypothalamic circuits involved in metabolic control, we asked whether MD has long-term consequences on body weight, leptin levels and the expression of neuropeptides involved in metabolism. Rats were separated from their mother for 24h starting on postnatal day (PND) 9 and sacrificed at PNDs 13, 35 and 75. In both sexes MD reduced body weight, but only until puberty, while leptin levels were unchanged at PND 35 and significantly reduced at PND 75. Adiponectin levels were also reduced at PND 75 in females, while testosterone levels were reduced in males. At PND 13, MD modulated cell-turnover markers in the hypothalamus of males, but not females and increased nestin, a marker of immature neurons, in both sexes, with males having higher levels than females and a significantly greater rise in response to MD. There was no effect of MD on hypothalamic mRNA levels of the leptin receptor or metabolic neuropeptides or the mRNA levels of leptin and adiponectin in adipose tissue. Thus, MD has long-term effects on the levels of circulating hormones that are not correlated with changes in body weight. Furthermore, these endocrine outcomes are different between males and females, which could be due to the fact that MD may have sexually dimorphic effects on hypothalamic development.

Influence of testosterone on synaptic transmission in the rat medial vestibular nuclei: estrogenic and androgenic effects

In brainstem slices of young male rat, we investigated the influence of the neuroactive steroid testosterone (T) on the synaptic responses by analyzing the field potential evoked in the medial vestibular nucleus (MVN) by vestibular afferent stimulation. T induced three distinct and independent long-term synaptic changes: fast long-lasting potentiation (fLP), slow long-lasting potentiation (sLP) and long-lasting depression (LD). The fLP was mediated by 17β-estradiol (E(2)) since it was abolished by blocking the estrogen receptors (ERs) or the enzyme converting T to E(2). Conversely, sLP and LD were mediated by 5α-dihydrotestosterone (DHT) since they were prevented by blocking the androgen receptors (ARs) or the enzyme converting T to DHT. Therefore, the synaptic effects of T were mediated by its androgenic or estrogenic metabolites. The pathways leading to estrogenic and androgenic conversion of T might be co-localized since, the occurrence of fLP under block of androgenic pathway, and that of sLP and LD under estrogenic block, were higher than those observed without blocks. In case of co-localization, the effect on synaptic transmission should depend on the prevailing enzymatic activity. We conclude that circulating and neuronal T can remarkably influence synaptic responses of the vestibular neurons in different and opposite ways, depending on its conversion to estrogenic or androgenic metabolites.

Prenatal stress diminishes gender differences in behavior and in expression of hippocampal synaptic genes and proteins in rats

The study determined whether there were gender differences in the expression of hippocampal genes in adult rats in association with dissimilarity in their behavior, and how these were affected by prenatal stress. Pregnant Wistar rats were subjected to varied stress once daily on days 14-20 of gestation. Adult female offspring of control rats showed significantly less anxiogenic behavior in the elevated plus maze and better discrimination between a novel and familiar object than males in the object recognition test. These gender differences in behavior were markedly attenuated by prenatal stress. Using Affymetrix DNA chip technology on hippocampal extracts prepared from littermates of the offspring used for behavioral tests, we found that 1,680 genes were differentially expressed in control males and females. The gender difference in gene expression was decreased to 11% (191 genes) by prenatal stress. In both sexes, processes like the translational machinery, mitochondrial activity, and cation transport were downregulated compared to controls, but there was a greater suppression of genes involved in vesicle trafficking, regulation of synaptic plasticity, and neurogenesis in females than in males. This was compensated by a higher expression of other components of vesicle trafficking, microtubule-based processes, and neurite development. Prenatal stress decreased the expression of 19 Rab proteins in females and five Rabs in males, but a compensatory increase of Rab partner proteins and effectors only occurred in females. Exposure to stress decreased the expression of synaptic proteins, synaptophysin, and synaptopodin in prenatally stressed males and females and increased those of PSD-95 and NR1 subunit of the N-methyl-D-aspartic acid (NMDA) glutamate receptor only in females. The study provides an unbiased view of key genes and proteins that act as gender dependent molecular sensors. The disruption of their expression by adverse early life stress may explain the alterations that occur in behavior.

Maternal deprivation induces a rapid decline in circulating leptin levels and sexually dimorphic modifications in hypothalamic trophic factors and cell turnover

Pathological outcomes, including metabolic and endocrine disturbances, of maternal deprivation (MD) in Wistar rats depend on gender and the timing of deprivation during development. We analyzed the effect of MD between postnatal days 9 and 10, a critical period in hypothalamic development, on circulating hormones and local production of trophic factors involved in this process, as well as on markers of cell turnover and maturation. Males and females were studied 12 and 24 h after MD and 12 h (MD36) after returning the dam to her pups. Circulating corticosterone levels were increased and glucose and leptin levels decreased throughout the study in both sexes. Hypothalamic mRNA levels of leptin receptor increased significantly at MD24 in both sexes, normalizing in females at MD36, but not in males. In male rats insulin-like growth factor mRNA levels were significantly decreased at MD24 and brain derived neurotrophic factor mRNA levels decreased at MD12 and MD24, with both trophic factors unaffected in females. In males cell proliferation was significantly decreased at MD36, as were the glial structural proteins, glial fibrillary acidic protein and vimentin. In females, nestin levels decreased significantly at MD24. These results indicate that MD differently affects trophic factors and cell-turnover in the hypothalamus of males and females, which may underlie the sex differences seen in the endocrine and metabolic outcome.

Estrogen receptor {alpha} gene promoter 0/B usage in the rat sexually dimorphic nucleus of the preoptic area

The volume of the sexually dimorphic nucleus of the preoptic area (SDN-POA) is two to four times larger in male rats than in females; however, the mechanism for the establishment of sexual dimorphism and the function of this nucleus is almost unknown. Perinatal estrogen can cause sexual dimorphism via the estrogen receptor alpha (ERalpha). Recently, transgenic rats were generated that express enhanced green fluorescent protein (EGFP) under the control of the ERalpha gene promoter 0/B to tag ERalpha-positive neurons in the brain. In the present study, we examined whether this EGFP expression could be a marker for the SDN-POA in adults. EGFP-labeled cells were distributed in the core of the SDN-POA (0/B-SDN) of male and female transgenic rats, in accordance with the Nissl staining and immunoreactivity for the SDN marker, calbindin. They were also immunoreactive for ERalpha. The core was bigger in volume and contained more 0/B-SDN neurons in males than in females. The EGFP-tagged cells were packed more densely in the female core than that in males. Subcutaneous injection of 100 mug 17beta-estradiol to females on the day of birth, or orchidectomy of male neonates, reversed the sexually dimorphic phenotype of the volume of the 0/B-SDN, despite not affecting the cell number. We suggest that this EGFP expression in the SDN-POA could be a useful marker to clarify the sexual differentiation and function of the SDN-POA. Moreover, the ERalpha gene promoter 0/B plays a key role in the organization of the sexual differentiation of the SDN-POA.

Chronic exposure to anabolic androgenic steroids alters neuronal function in the mammalian forebrain via androgen receptor- and estrogen receptor-mediated mechanisms

Anabolic androgenic steroids (AAS) can promote detrimental effects on social behaviors for which GABA type A (GABA(A)) receptor-mediated circuits in the forebrain play a critical role. While all AAS bind to androgen receptors (AR), they may also be aromatized to estrogens and thus potentially impart effects via estrogen receptors (ER). Chronic exposure of wild-type male mice to a combination of chemically distinct AAS increased action potential (AP) frequency, selective GABA(A) receptor subunit mRNAs, and GABAergic synaptic current decay in the medial preoptic area (mPOA). Experiments performed with pharmacological agents and in AR-deficient Tfm mutant mice suggest that the AAS-dependent enhancement of GABAergic transmission in wild-type mice is AR-mediated. In AR-deficient mice, the AAS elicited dramatically different effects, decreasing AP frequency, spontaneous IPSC amplitude and frequency and the expression of selective GABA(A) receptor subunit mRNAs. Surprisingly, in the absence of AR signaling, the data indicate that the AAS do not act as ER agonists, but rather suggest a novel in vivo action in which the AAS inhibit aromatase and impair endogenous ER signaling. These results show that the AAS have the capacity to alter neuronal function in the forebrain via multiple steroid signaling mechanisms and suggest that effects of these steroids in the brain will depend not only on the balance of AR- versus ER-mediated regulation for different target genes, but also on the ability of these drugs to alter steroid metabolism and thus the endogenous steroid milieu.

Estrogen, stress and the brain: progress toward unraveling gender discrepancies in major depressive disorder

Women are twice as likely as men to develop major depressive disorder (MDD) and, while the neurobiological factors underlying this discrepancy are yet to be identified, estrogen almost certainly plays a role. MDD can be precipitated or exacerbated by exposure to stress and there is substantial evidence to suggest that estrogen can interact with stress systems to produce unique stress effects in females. This review integrates current research in animal models regarding estrogen-stress interactions in three areas of the brain known to be relevant to MDD: the hippocampus, the amygdala and the prefrontal cortex. The results from these studies are discussed in the context of MDD, and their implications for future treatment of MDD in women are explored.