Maternal regulation of estrogen receptor alpha methylation

Postnatal maternal care moderates the effects of prenatal bisphenol exposure on offspring neurodevelopmental, behavioral, and transcriptomic outcomes

Bisphenols (BP), including BPA and "BPA-free" structural analogs, are commonly used plasticizers that are present in many plastics and are known endocrine disrupting chemicals. Prenatal exposure to BPA has been associated with negative neurodevelopmental and behavioral outcomes in children and in rodent models. Prenatal BPA exposure has also been shown to impair postnatal maternal care provisioning, which can also affect offspring neurodevelopment and behavior. However, there is limited knowledge regarding the biological effects of prenatal exposure to bisphenols other than BPA and the interplay between prenatal bisphenol exposure and postnatal maternal care on adult behavior. The purpose of the current study was to determine the interactive impact of prenatal bisphenol exposure and postnatal maternal care on neurodevelopment and behavior in rats. Our findings suggest that the effects of prenatal bisphenol exposure on eye-opening, adult attentional set shifting and anxiety-like behavior in the open field are dependent on maternal care in the first five days of life. Interestingly, maternal care might also attenuate the effects of prenatal bisphenol exposure on eye opening and adult attentional set shifting. Finally, transcriptomic profiles in male and female medial prefrontal cortex and amygdala suggest that the interactive effects of prenatal bisphenol exposure and postnatal maternal care converge on estrogen receptor signaling and are involved in biological processes related to gene expression and protein translation and synthesis. Overall, these findings indicate that postnatal maternal care plays a critical role in the expression of the effects of prenatal bisphenol exposure on neurodevelopment and adult behavior. Understanding the underlying biological mechanisms involved might allow us to identify potential avenues to mitigate the adverse effects of prenatal bisphenol exposure and improve health and well-being in human populations.

Postnatal maternal care moderates the effects of prenatal bisphenol exposure on offspring neurodevelopmental, behavioral, and transcriptomic outcomes

Bisphenols (BPs), including BPA and "BPA-free" structural analogs, are commonly used plasticizers that are present in many plastics and are known endocrine disrupting chemicals. Prenatal exposure to BPA has been associated with negative neurodevelopmental and behavioral outcomes in children and rodent models. Prenatal BPA exposure has also been shown to impair postnatal maternal care provisioning, which can also affect offspring neurodevelopment and behavior. However, there is limited knowledge regarding the biological effects of prenatal exposure to bisphenols other than BPA and the interplay between prenatal BP exposure and postnatal maternal care on adult behavior. The purpose of the current study was to determine the interactive impact of prenatal BP exposure and postnatal maternal care on neurodevelopment and behavior. Our findings suggest that the effects of prenatal BP exposure on eye-opening, adult attentional set shifting and anxiety-like behavior in the open field are dependent on maternal care in the first five days of life. Interestingly, maternal care might also attenuate the effects of prenatal BP exposure on eye opening and adult attentional set shifting. Finally, transcriptomic profiles in male and female medial prefrontal cortex and amygdala suggest that the interactive effects of prenatal BP exposure and postnatal maternal care converge on estrogen receptor signaling and are involved in biological processes related to gene expression and protein translation and synthesis. Overall, these findings indicate that postnatal maternal care plays a critical role in the expression of the effects of prenatal BP exposure on neurodevelopment and adult behavior. Understanding the underlying biological mechanisms involved might allow us to identify potential avenues to mitigate the adverse effects of prenatal BP exposure and improve health and well-being in human populations.

Effects of limited bedding and nesting on postpartum mood state in rats

This study examined the effect of limited bedding and nesting (LBN) stress on postpartum anhedonia, maternal behaviors, anxiety-like behaviors, and neuroendocrine and neuroimmune function as a potential model of postpartum depression. Dams underwent sucrose preference tests prior to breeding, during gestation and again postpartum, to examine the potential onset of anhedonia. On embryonic day 19, dams were placed into either a LBN or control housing condition. Contrary to our predictions, LBN stress had no effect on postpartum sucrose preference. We also found no effect of LBN condition on fecal estradiol or corticosterone levels, both of which increased at birth and decreased postpartum. Regardless of housing conditions, approximately 40% of new mothers exhibited a decrease in sucrose preference, while others show no change, suggesting an individual susceptibility to postpartum anhedonia. In a separate cohort of LBN and control dams, we measured pup retrieval, hoarding behavior, elevated plus maze (EPM), and marble burying. LBN dams exhibited increased anxiety, associated with decreased time spent in the open arms of the EPM. We also measured a significant increase in IL-6 expression in the dorsal hippocampus and medial prefrontal cortex of postpartum dams compared to nonpregnant dams. These findings suggest that while LBN stress has effects on anxiety and maternal care, it does not induce postpartum anhedonia. Rather, there are inherent differences in susceptibility to anhedonia in individual dams, and future studies should be conducted to better understand individual vulnerability and resilience to postpartum anhedonia.

Social Network Plasticity of Mice Parental Behavior

Neural plasticity occurs during developmental stages and is essential for sexual differentiation of the brain and the ensuing sex-dependent behavioral changes in adults. Maternal behavior is primarily affected by sex-related differences in the brain; however, chronic social isolation even in mature male mice can induce maternal retrieving and crouching behavior when they are first exposed to pups. Social milieus influence the inherent behavior of adults and alter the molecular architecture in the brain, thereby allowing higher levels of associated gene expression and molecular activity. This review explores the possibility that although the development of neural circuits is closely associated with maternal behavior, the brain can still retain its neuroplasticity in adults from a neuromolecular perspective. In addition, neuronal machinery such as neurotransmitters and neuropeptides might influence sociobehavioral changes. This review also discusses that the neural circuits regulating behaviors such as parenting and infanticide (including neglect behavior), might be controlled by neural relay on melanin concentrating hormone (MCH)–oxytocin in the hypothalamus during the positive and negative mode of action in maternal behavior. Furthermore, MCH–oxytocin neural relay might contribute to the anxiolytic effect on maternal behavior, which is involved with reward circuits.

Effect of adverse childhood experiences and DNA methylation on male sexual orientation

The causes for male sexual orientation are complicated, which have not yet been clarified. Recent years have witnessed fruitful progress in the field of biology, while the impact of environment has received little attention. Adverse childhood experiences (ACEs), identified as a special environment in the early stage of development, can affect the individual phenotype by DNA methylation. Given the relationships among male sexual orientation, ACEs, and DNA methylation, as well as based on the existing theory, this article proposes the model "ACEs-DNA methylation-male sexual orientation"from the perspective of environment and epigenetics, aiming to provide a theoretical basis for future research.

Mechanisms of Reciprocal Regulation of Gonadotropin-Releasing Hormone (GnRH)-Producing and Immune Systems: The Role of GnRH, Cytokines and Their Receptors in Early Ontogenesis in Normal and Pathological Conditions

Different aspects of the reciprocal regulatory influence on the development of gonadotropin-releasing hormone (GnRH)-producing- and immune systems in the perinatal ontogenesis and their functioning in adults in normal and pathological conditions are discussed. The influence of GnRH on the development of the immune system, on the one hand, and the influence of proinflammatory cytokines on the development of the hypothalamic-pituitary-gonadal system, on the other hand, and their functioning in adult offspring are analyzed. We have focused on the effects of GnRH on the formation and functional activity of the thymus, as the central organ of the immune system, in the perinatal period. The main mechanisms of reciprocal regulation of these systems are discussed. The reproductive health of an individual is programmed by the establishment and development of physiological systems during critical periods. Regulatory epigenetic mechanisms of development are not strictly genetically controlled. These processes are characterized by a high sensitivity to various regulatory factors, which provides possible corrections for disorders.

Gender-Affirming Hormone Therapy Modifies the CpG Methylation Pattern of the ESR1 Gene Promoter After Six Months of Treatment in Transmen

BACKGROUND

Brain sexual differentiation is a process that results from the effects of sex steroids on the developing brain. Evidence shows that epigenetics plays a main role in the formation of enduring brain sex differences and that the estrogen receptor α (ESR1) is one of the implicated genes.

AIM

To analyze whether the methylation of region III (RIII) of the ESR1 promoter is involved in the biological basis of gender dysphoria.

METHODS

We carried out a prospective study of the CpG methylation profile of RIII (-1,188 to -790 bp) of the ESR1 promoter using bisulfite genomic sequencing in a cisgender population (10 men and 10 women) and in a transgender population (10 trans men and 10 trans women), before and after 6 months of gender-affirming hormone treatment. Cisgender and transgender populations were matched by geographical origin, age, and sex. DNAs were treated with bisulfite, amplified, cloned, and sequenced. At least 10 clones per individual from independent polymerase chain reactions were sequenced. The analysis of 671 bisulfite sequences was carried out with the QUMA (QUantification tool for Methylation Analysis) program.

OUTCOMES

The main outcome of this study was RIII analysis using bisulfite genomic sequencing.

RESULTS

We found sex differences in RIII methylation profiles in cisgender and transgender populations. Cismen showed a higher methylation degree than ciswomen at CpG sites 297, 306, 509, and at the total fragment (P ≤ .003, P ≤ .026, P ≤ .001, P ≤ .006). Transmen showed a lower methylation level than trans women at sites 306, 372, and at the total fragment (P ≤ .0001, P ≤ .018, P ≤ .0107). Before the hormone treatment, transmen showed the lowest methylation level with respect to cisgender and transgender populations, whereas transwomen reached an intermediate methylation level between both the cisgender groups. After the hormone treatment, transmen showed a statistically significant methylation increase, whereas transwomen showed a non-significant methylation decrease. After the hormone treatment, the RIII methylation differences between transmen and transwomen disappeared, and both transgender groups reached an intermediate methylation level between both the cisgender groups.

CLINICAL IMPLICATIONS

Clinical implications in the hormonal treatment of trans people.

STRENGTHS & LIMITATIONS

Increasing the number of regions analyzed in the ESR1 promoter and increasing the number of tissues analyzed would provide a better understanding of the variation in the methylation pattern.

CONCLUSIONS

Our data showed sex differences in RIII methylation patterns in cisgender and transgender populations before the hormone treatment. Furthermore, before the hormone treatment, transwomen and transmen showed a characteristic methylation profile, different from both the cisgender groups. But the hormonal treatment modified RIII methylation in trans populations, which are now more similar to their gender. Therefore, our results suggest that the methylation of RIII could be involved in gender dysphoria. Fernández R, Ramírez K, Gómez-Gil E, et al. Gender-Affirming Hormone Therapy Modifies the CpG Methylation Pattern of the ESR1 Gene Promoter After Six Months of Treatment in Transmen. J Sex Med 2020;17:1795-1806.

Depression-related disturbances in rat maternal behaviour are associated with altered monoamine levels within mesocorticolimbic structures

The ability of mothers to sensitively attune their maternal responses to the needs of their developing young is fundamental to a healthy mother-young relationship. The biological mechanisms that govern how mothers adjust caregiving to the dynamic changes in the demands of the young remain an open question. In the present study, we examined whether changes in monoamine levels, within discrete mesocorticolimbic structures involved in cognitive and motivational processes key to parenting, modulate this flexibility in caregiving across the postpartum period. The present study used a Wistar-Kyoto (WKY) animal model of depression and control Sprague-Dawley (SD) rats, which differ dramatically in their cognitive, motivational, and parenting performance. Levels of the monoamine neurotransmitters, dopamine, noradrenaline and serotonin, as well as their major metabolites, were measured within the medial prefrontal cortex, striatum, nucleus accumbens and medial preoptic area of SD and WKY mothers at early (postpartum day [PPD]7-8), late (PPD15-16) and weaning (PPD25) postpartum stages using high-performance liquid chromatography with electrochemical detection. Consistent with our prior work, we find that caregiving of SD mothers declined as the postpartum period progressed. Relative to nulliparous females, early postpartum mothers had lower intracellular concentrations of monoamines, as well as lower noradrenaline turnover, and an elevated serotonin turnover within most structures. Postpartum behavioural trajectories subsequently corresponded to a progressive increase in all three monoamine levels within multiple structures. Compared to SD mothers, WKY mothers were inconsistent and disorganised in caring for their offspring and exhibit profound deficits in maternal behaviour. Additionally, WKY mothers had generally lower levels of all three monoamines, as well as different patterns of change across the postpartum period, compared to SD mothers, suggesting dysfunctional central monoamine pathways in WKY mothers as they transition and experience motherhood. Taken together, the results of the present study suggest a role for monoamines at multiple mesocorticolimbic structures with repect to modulating caregiving behaviours attuned to the changing needs of the young.

Pharmacological manipulation of DNA methylation normalizes maternal behavior, DNA methylation, and gene expression in dams with a history of maltreatment

The quality of parental care received during development profoundly influences an individual's phenotype, including that of maternal behavior. We previously found that female rats with a history of maltreatment during infancy mistreat their own offspring. One proposed mechanism through which early-life experiences influence behavior is via epigenetic modifications. Indeed, our lab has identified a number of brain epigenetic alterations in female rats with a history of maltreatment. Here we sought to investigate the role of DNA methylation in aberrant maternal behavior. We administered zebularine, a drug known to alter DNA methylation, to dams exposed during infancy to the scarcity-adversity model of low nesting resources, and then characterized the quality of their care towards their offspring. First, we replicate that dams with a history of maltreatment mistreat their own offspring. Second, we show that maltreated-dams treated with zebularine exhibit lower levels of adverse care toward their offspring. Third, we show that administration of zebularine in control dams (history of nurturing care) enhances levels of adverse care. Lastly, we show altered methylation and gene expression in maltreated dams normalized by zebularine. These findings lend support to the hypothesis that epigenetic alterations resulting from maltreatment causally relate to behavioral outcomes.

Markers as mediators: A review and synthesis of epigenetics literature

Epigenetics, the study of the processes that control gene expression without a change in DNA sequence, highlights the importance of environmental factors in gene regulation. This paper maps the terrain of epigenetics and identifies four main research subfields: gene expression; molecular epigenetics; clinical epigenetics and epigenetic epidemiology. Within and across these fields, we analyse of what is conceptualised as environment and demonstrate the variable ways authors understand epigenetics environments. Then, following an analysis of the discursive strategies employed by epigenetics researchers, we demonstrate how authors portray the interactions between genes, epigenetics, and environment as relationships linking the outside (where the environment is located) with the inside (where the genes are located). We argue that authors assign specific roles to each actor: the environment as the active player initiating the relationship, the genes as recipients, and epigenetics as mediators between environment and genes. Framed as mediators, epigenetic markers can be understood as enablers of communication between environment and genome, capable of processing and organising signals so as to regulate the interactions between the actors of epigenetic relationships. This finding complicates the observation by social science scholars that the interactions between environment and genes can be understood through the concept of signal.

Sex differences in Hippocampal Memory and Learning following Neonatal Brain Injury: Is There a Role for Estrogen Receptor-α?

Neonatal encephalopathy due to hypoxia-ischemia (HI) leads to severe, life-long morbidities in thousands of neonates born in the USA and worldwide each year. Varying capacities of long-term episodic memory, verbal working memory, and learning can present without cerebral palsy and have been associated with the severity of neonatal encephalopathy sustained at birth. Among children who sustain a moderate degree of HI at birth, girls have larger hippocampal volumes compared to boys. Clinical studies indicate that female neonatal brains are more resistant to the effects of neonatal HI, resulting in better long-term cognitive outcomes compared to males with comparable brain injury. Our most recent mechanistic studies have addressed the origins and cellular basis of sex differences in hippocampal neuroprotection following neonatal HI-related brain injury and implicate estrogen receptor-α (ERα) in the neurotrophin receptor-mediated hippocampal neuroprotection in female mice. This review summarizes the recent findings on ERα-dependent, neurotrophin-mediated hippocampal neuroprotection and weighs the evidence that this mechanism plays an important role in preservation of long-term memory and learning following HI in females.

Developmental estrogen exposures and disruptions to maternal behavior and brain: Effects of ethinyl estradiol, a common positive control

Due of its structural similarity to the endogenous estrogen 17β-estradiol (E2), the synthetic estrogen 17α-ethinyl estradiol (EE2) is widely used to study the effects of estrogenic substances on sensitive organs at multiple stages of development. Here, we investigated the effects of EE2 on maternal behavior and the maternal brain in females exposed during gestation and the perinatal period. We assessed several components of maternal behavior including nesting behavior and pup retrieval; characterized the expression of estrogen receptor (ER)α in the medial preoptic area (MPOA), a brain region critical for the display of maternal behavior; and measured expression of tyrosine hydroxylase, a marker for dopaminergic cells, in the ventral tegmental area (VTA), a brain region important in maternal motivation. We found that developmental exposure to EE2 induces subtle effects on several aspects of maternal behavior including time building the nest and time spent engaged in self-care. Developmental exposure to EE2 also altered ERα expression in the central MPOA during both early and late lactation and led to significantly reduced tyrosine hydroxylase immunoreactivity in the VTA. Our results demonstrate both dose- and postpartum stage-related effects of developmental exposure to EE2 on behavior and brain that manifest later in adulthood, during the maternal period. These findings provide further evidence for effects of exposure to exogenous estrogenic compounds during the critical periods of fetal and perinatal development.

Effects of pair bonding on parental behavior and dopamine activity in the nucleus accumbens in male prairie voles

Male parental care is a vital behavior for the development as well as the physical and mental well-being of the young. However, little is known about the neurochemical regulation of male parental behavior, mainly due to the lack of appropriate animal models. In this study, we used the socially monogamous male prairie vole (Microtus ochrogaster) to investigate the effect of pair-bonding experience on paternal behavior and dopamine (DA) signaling in the nucleus accumbens (NAcc) in the brain. We compared sexually naïve males with males that were pair bonded with a female for two weeks. Our data showed that pair-bonded males displayed enhanced paternal behavior, particularly in pup licking/grooming, associated with increased DA type-1 receptor (D1R) protein expression in the NAcc, compared to sexually naïve males. Site-specific brain microdialysis revealed a significant, but transient, increase in DA release in the NAcc associated with pup exposure in both groups of the males. Further, pharmacological blockade of D1R in the NAcc decreased pup licking/grooming in the pair-bonded males. Together, our data demonstrate that pair-bonding experience with a female facilitated male parental behavior via NAcc D1R mediation in male prairie voles.

Low doses of 17α-ethinyl estradiol alter the maternal brain and induce stereotypies in CD-1 mice exposed during pregnancy and lactation

Maternal care is critical for the survival, development and long-term success of offspring. Despite our current understanding of the role of endogenous estrogen in both maternal behavior and the maternal brain, the potential effects of exogenous estrogens on these endpoints remain poorly understood. Here, pregnant CD-1 mice were exposed to low doses of 17α-ethinyl estradiol (EE2), commonly used as a positive control in studies of other xenoestrogens, from day 9 of pregnancy until weaning. Using traditional maternal behavior assays, we document no significant changes in maternal behavior throughout the lactational period. However, EE2 induced increases in repetitive tail retrieval, which may indicate a stereotypy or obsessive compulsive (OCD)-like behavior. We also observed a significant reduction in tyrosine hydroxylase (TH) immunoreactivity in the ventral tegmental area (VTA), a region important for maternal motivation. These results suggest that pregnant adult females are not immune to the effects of this compound.

Bisphenol S (BPS) Alters Maternal Behavior and Brain in Mice Exposed During Pregnancy/Lactation and Their Daughters

Estrogenic endocrine disrupting chemicals have been shown to disrupt maternal behavior in rodents. We investigated the effects of an emerging xenoestrogen, bisphenol S (BPS), on maternal behavior and brain in CD-1 mice exposed during pregnancy and lactation (F0 generation) and in female offspring exposed during gestation and perinatal development (F1 generation). We observed different effects in F0 and F1 dams for a number of components of maternal behavior, including time on the nest, time spent on nest building, latency to retrieve pups, and latency to retrieve the entire litter. We also characterized expression of estrogen receptor α in the medial preoptic area (MPOA) and quantified tyrosine hydroxylase immunoreactive cells in the ventral tegmental area, 2 brain regions critical for maternal care. BPS-treated females in the F0 generation had a statistically significant increase in estrogen receptor α expression in the caudal subregion of the central MPOA in a dose-dependent manner. In contrast, there were no statistically significant effects of BPS on the MPOA in F1 dams or the ventral tegmental area in either generation. This work demonstrates that BPS affects maternal behavior and brain with outcomes depending on generation, dose, and postpartum period. Many studies examining effects of endocrine disrupting chemicals view the mother as a means by which offspring can be exposed during critical periods of development. Here, we demonstrate that pregnancy and lactation are vulnerable periods for the mother. We also show that developmental BPS exposure alters maternal behavior later in adulthood. Both findings have potential public health implications.

Early life stress elicits visceral hyperalgesia and functional reorganization of pain circuits in adult rats

Early life stress (ELS) is a risk factor for developing functional gastrointestinal disorders, and has been proposed to be related to a central amplification of sensory input and resultant visceral hyperalgesia. We sought to characterize ELS-related changes in functional brain responses during acute noxious visceral stimulation. Neonatal rats (males/females) were exposed to limited bedding (ELS) or standard bedding (controls) on postnatal days 2–9. Age 10–11 weeks, animals were implanted with venous cannulas and transmitters for abdominal electromyography (EMG). Cerebral blood flow (rCBF) was mapped during colorectal distension (CRD) using [¹⁴C]-iodoantipyrine autoradiography, and analyzed in three-dimensionally reconstructed brains by statistical parametric mapping and functional connectivity. EMG responses to CRD were increased after ELS, with no evidence of a sex difference. ELS rats compared to controls showed a greater significant positive correlation of EMG with amygdalar rCBF. Factorial analysis revealed a significant main effect of ‘ELS’ on functional activation of nodes within the pain pathway (somatosensory, insular, cingulate and prefrontal cortices, locus coeruleus/lateral parabrachial n. [LC/LPB], periaqueductal gray, sensory thalamus), as well as in the amygdala, hippocampus and hypothalamus. In addition, ELS resulted in an increase in the number of significant functional connections (i.e. degree centrality) between regions within the pain circuit, including the amygdala, LC/LPB, insula, anterior ventral cingulate, posterior cingulate (retrosplenium), and stria terminalis, with decreases noted in the sensory thalamus and the hippocampus. Sex differences in rCBF were less broadly expressed, with significant differences noted at the level of the cortex, amygdala, dorsal hippocampus, raphe, sensory thalamus, and caudate-putamen. ELS showed a sexually dimorphic effect (‘Sex x ELS’ interaction) at the LC/LPB complex, globus pallidus, hypothalamus, raphe, septum, caudate-putamen and cerebellum. Our results suggest that ELS alters functional activation of the thalamo-cortico-amydala pathway, as well as the emotional-arousal network (amygdala, locus coeruleus), with evidence that ELS may additionally show sexually dimorphic effects on brain function.

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Early life stress (ELS) elicits visceral hyperalgesia in adult offspring.

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ELS alters functional activation of the thalamo-cortico-amydala pathway.

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ELS shows a sexually dimorphic effects on brain function.

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Functional imaging-based endpoints promise improved animal-to-human translation.

Perinatally Influenced Autonomic System Fluctuations Drive Infant Vocal Sequences

The variable vocal behavior of human infants is the scaffolding upon which speech and social interactions develop. It is important to know what factors drive this developmentally critical behavioral output. Using marmoset monkeys as a model system, we first addressed whether the initial conditions for vocal output and its sequential structure are perinatally influenced. Using dizygotic twins and Markov analyses of their vocal sequences, we found that in the first postnatal week, twins had more similar vocal sequences to each other than to their non-twin siblings. Moreover, both twins and their siblings had more vocal sequence similarity with each other than with non-sibling infants. Using electromyography, we then investigated the physiological basis of vocal sequence structure by measuring respiration and arousal levels (via changes in heart rate). We tested the hypothesis that early-life influences on vocal output are via fluctuations of the autonomic nervous system (ANS) mediated by vocal biomechanics. We found that arousal levels fluctuate at ∼0.1 Hz (the Mayer wave) and that this slow oscillation modulates the amplitude of the faster, ∼1.0 Hz respiratory rhythm. The systematic changes in respiratory amplitude result in the different vocalizations that comprise infant vocal sequences. Among twins, the temporal structure of arousal level changes was similar and therefore indicates why their vocal sequences were similar. Our study shows that vocal sequences are tightly linked to respiratory patterns that are modulated by ANS fluctuations and that the temporal structure of ANS fluctuations is perinatally influenced.

At the frontier of epigenetics of brain sex differences

The notion that epigenetics may play an important role in the establishment and maintenance of sex differences in the brain has garnered great enthusiasm but the reality in terms of actual advances has been slow. Two general approaches include the comparison of a particular epigenetic mark in males vs. females and the inhibition of key epigenetic enzymes or co-factors to determine if this eliminates a particular sex difference in brain or behavior. The majority of emphasis has been on candidate genes such as steroid receptors. Only recently have more generalized survey type approaches been achieved and these promise to open new vistas and accelerate discovery of important roles for DNA methylation, histone modification, genomic imprinting and microRNAs (miRs). Technical challenges abound and, while not unique to this field, will require novel thinking and new approaches by behavioral neuroendocrinologists.

Pre and post-natal antigen exposure can program the stress axis of adult zebra finches: evidence for environment matching

Both maternal exposure to stressors and exposure of offspring to stressors during early life can have lifelong effects on the physiology and behavior of offspring. Stress exposure can permanently shape an individual's phenotype by influencing the development of the hypothalamic-pituitary-adrenal (HPA) axis, which is responsible for the production and regulation of glucocorticoids such as corticosterone (CORT). In this study we used captive zebra finches (Taeniopygia guttata) to examine the effects of matching and mismatching maternal and early post-natal exposure to one of two types of antigens or a control on HPA axis reactivity in adult offspring. Prior to breeding, adult females were injected with lipopolysaccharide (LPS), keyhole limpet hemocyanin (KLH) or a control. Offspring of females in each of the three treatments were themselves exposed to LPS, KLH or a control injection at 5 and 28days post-hatch. When offspring were at least 18months of age, standardized capture and restraint stress tests were conducted to determine the impact of the treatments on adult stress responsiveness. We found significant interaction effects between maternal and offspring treatments on stress-induced CORT levels, and evidence in support of the environment matching hypothesis for KLH-treated birds, not LPS-treated birds. KLH-treated offspring of KLH-treated mothers exhibited reduced stress-induced CORT levels, whereas LPS-treated or control offspring of KLH-treated mothers exhibited elevated stress-induced CORT levels. Although the treatment effects on baseline CORT were non-significant, the overall pattern was similar to the effects observed on stress-induced CORT levels. Our results highlight the complex nature of HPA axis programming, and to our knowledge, provide the first evidence that a match or mismatch between pre and post-natal antigen exposure can have life-long consequences for HPA axis function.

The social brain meets the reactive genome: neuroscience, epigenetics and the new social biology

The rise of molecular epigenetics over the last few years promises to bring the discourse about the sociality and susceptibility to environmental influences of the brain to an entirely new level. Epigenetics deals with molecular mechanisms such as gene expression, which may embed in the organism "memories" of social experiences and environmental exposures. These changes in gene expression may be transmitted across generations without changes in the DNA sequence. Epigenetics is the most advanced example of the new postgenomic and context-dependent view of the gene that is making its way into contemporary biology. In my article I will use the current emergence of epigenetics and its link with neuroscience research as an example of the new, and in a way unprecedented, sociality of contemporary biology. After a review of the most important developments of epigenetic research, and some of its links with neuroscience, in the second part I reflect on the novel challenges that epigenetics presents for the social sciences for a re-conceptualization of the link between the biological and the social in a postgenomic age. Although epigenetics remains a contested, hyped, and often uncritical terrain, I claim that especially when conceptualized in broader non-genecentric frameworks, it has a genuine potential to reformulate the ossified biology/society debate.

Epigenetics, plasticity, and evolution: How do we link epigenetic change to phenotype?

Epigenetic mechanisms are proposed as an important way in which the genome responds to the environment. Epigenetic marks, including DNA methylation and Histone modifications, can be triggered by environmental effects, and lead to permanent changes in gene expression, affecting the phenotype of an organism. Epigenetic mechanisms have been proposed as key in plasticity, allowing environmental exposure to shape future gene expression. While we are beginning to understand how these mechanisms have roles in human biology and disease, we have little understanding of their roles and impacts on ecology and evolution. In this review, we discuss different types of epigenetic marks, their roles in gene expression and plasticity, methods for assaying epigenetic changes, and point out the future advances we require to understand fully the impact of this field.

Linking DNA methylation to the onset of human tubal ectopic pregnancy

Ectopic pregnancy is a common reproductive disorder of unknown etiology and is a leading cause of maternal and fetal mortality. Because of the asymptomatic nature of early tubal ectopic pregnancy and the lack of specific biomarkers for early diagnosis, a better understanding of the complex cellular and molecular interactions that contribute to tubal ectopic pregnancy is required. DNA methylation is the most studied epigenetic process in various tissues and cells, and the goal of this article is to provide a brief review of recent work describing the potential mechanisms of DNA methylation and the biological function of such methylation in normal intrauterine pregnancy. Further, novel findings from our laboratory highlight the possible role of DNA methylation in human Fallopian tube dysfunction and suggest a possible correlation between methylation of estrogen receptor α in women and the occurrence of tubal ectopic pregnancies.

Child abuse and epigenetic mechanisms of disease risk

BACKGROUND

Child abuse is highly prevalent and associated with increased risk for a range of health problems, including cancer, cardiovascular disease, diabetes, psychiatric disorders, and other health problems. Little is currently known about the mechanism by which early adversity confers risk for health problems later in life.

PURPOSE

To determine if there are epigenetic differences associated with child maltreatment that may help explain association between adverse childhood experiences and later health problems.

METHODS

As part of a study examining genetic and environmental factors associated with depression, saliva DNA specimens were collected on 96 maltreated children removed from their parents due to abuse or neglect and 96 demographically matched control children between 2003 and 2010. In 2011, the Illumina 450K BeadChip was used on stored DNA specimens and analyzed to examine whole-genome methylation differences between maltreated and control children.

RESULTS

After controlling for multiple comparisons, maltreated and control children had significantly different methylation values at 2868 CpG sites (p<5.0 × 10(-7), all sites; average methylation difference per site=17%; range=1%-62%). The gene set contained numerous markers of diseases and biological processes related to the health problems associated with early childhood adversity.

CONCLUSIONS

Although replication is required, this study suggests that epigenetic mechanisms may be associated with risk for health problems later in life in maltreated children. This study lays the groundwork for future studies examining health and methylation measures to further characterize the role of epigenetic mechanisms in conferring risk for medical problems in individuals with histories of early adversity.

Derailed estrogen signaling and breast cancer: an authentic couple

Estrogen or 17β-estradiol, a steroid hormone, plays a critical role in the development of mammary gland via acting through specific receptors. In particular, estrogen receptor-α (ERα) acts as a transcription factor and/or a signal transducer while participating in the development of mammary gland and breast cancer. Accumulating evidence suggests that the transcriptional activity of ERα is altered by the action of nuclear receptor coregulators and might be responsible, at least in part, for the development of breast cancer. In addition, this process is driven by various posttranslational modifications of ERα, implicating active participation of the upstream receptor modifying enzymes in breast cancer progression. Emerging studies suggest that the biological outcome of breast cancer cells is also influenced by the cross talk between microRNA and ERα signaling, as well as by breast cancer stem cells. Thus, multiple regulatory controls of ERα render mammary epithelium at risk for transformation upon deregulation of normal homeostasis. Given the importance that ERα signaling has in breast cancer development, here we will highlight how the activity of ERα is controlled by various regulators in a spatial and temporal manner, impacting the progression of the disease. We will also discuss the possible therapeutic value of ERα modulators as alternative drug targets to retard the progression of breast cancer.

Transgenerational epigenetic effects on animal behaviour

Over the last decade a shift in paradigm has occurred with respect to the interaction between environment and genes. It is now clear that animal genomes are regulated to a large extent as a result of input from environmental events and experiences, which cause short- and long-term modifications in epigenetic markings of DNA and histones. In this review, the evidence that such epigenetic modifications can affect the behaviour of animals is explored, and whether such acquired behaviour alterations can transfer across generation borders. First, the mechanisms by which experiences cause epigenetic modifications are examined. This includes, for example, methylation of cytosine in CpG positions and acetylation of histones, and studies showing that this can be modified by early experiences. Secondly, the evidence that specific modifications in the epigenome can be the cause of behaviour variation is reviewed. Thirdly, the extent to which this phenotypically active epigenetic variants can be inherited either through the germline or through reoccurring environmental conditions is examined. A particularly interesting observation is that epigenetic modifications are often linked to stress, and may possibly be mediated by steroid effects. Finally, the idea that transgenerationally stable epigenetic variants may serve as substrates for natural selection is explored, and it is speculated that they may even predispose for directed, non-random mutations.

Self-awareness, self-regulation, and self-transcendence (S-ART): a framework for understanding the neurobiological mechanisms of mindfulness

Mindfulness-as a state, trait, process, type of meditation, and intervention has proven to be beneficial across a diverse group of psychological disorders as well as for general stress reduction. Yet, there remains a lack of clarity in the operationalization of this construct, and underlying mechanisms. Here, we provide an integrative theoretical framework and systems-based neurobiological model that explains the mechanisms by which mindfulness reduces biases related to self-processing and creates a sustainable healthy mind. Mindfulness is described through systematic mental training that develops meta-awareness (self-awareness), an ability to effectively modulate one's behavior (self-regulation), and a positive relationship between self and other that transcends self-focused needs and increases prosocial characteristics (self-transcendence). This framework of self-awareness, -regulation, and -transcendence (S-ART) illustrates a method for becoming aware of the conditions that cause (and remove) distortions or biases. The development of S-ART through meditation is proposed to modulate self-specifying and narrative self-networks through an integrative fronto-parietal control network. Relevant perceptual, cognitive, emotional, and behavioral neuropsychological processes are highlighted as supporting mechanisms for S-ART, including intention and motivation, attention regulation, emotion regulation, extinction and reconsolidation, prosociality, non-attachment, and decentering. The S-ART framework and neurobiological model is based on our growing understanding of the mechanisms for neurocognition, empirical literature, and through dismantling the specific meditation practices thought to cultivate mindfulness. The proposed framework will inform future research in the contemplative sciences and target specific areas for development in the treatment of psychological disorders.

Self-awareness, self-regulation, and self-transcendence (S-ART): a framework for understanding the neurobiological mechanisms of mindfulness

Mindfulness—as a state, trait, process, type of meditation, and intervention has proven to be beneficial across a diverse group of psychological disorders as well as for general stress reduction. Yet, there remains a lack of clarity in the operationalization of this construct, and underlying mechanisms. Here, we provide an integrative theoretical framework and systems-based neurobiological model that explains the mechanisms by which mindfulness reduces biases related to self-processing and creates a sustainable healthy mind. Mindfulness is described through systematic mental training that develops meta-awareness (self-awareness), an ability to effectively modulate one's behavior (self-regulation), and a positive relationship between self and other that transcends self-focused needs and increases prosocial characteristics (self-transcendence). This framework of self-awareness, -regulation, and -transcendence (S-ART) illustrates a method for becoming aware of the conditions that cause (and remove) distortions or biases. The development of S-ART through meditation is proposed to modulate self-specifying and narrative self-networks through an integrative fronto-parietal control network. Relevant perceptual, cognitive, emotional, and behavioral neuropsychological processes are highlighted as supporting mechanisms for S-ART, including intention and motivation, attention regulation, emotion regulation, extinction and reconsolidation, prosociality, non-attachment, and decentering. The S-ART framework and neurobiological model is based on our growing understanding of the mechanisms for neurocognition, empirical literature, and through dismantling the specific meditation practices thought to cultivate mindfulness. The proposed framework will inform future research in the contemplative sciences and target specific areas for development in the treatment of psychological disorders.

Behavior genetics and postgenomics

The science of genetics is undergoing a paradigm shift. Recent discoveries, including the activity of retrotransposons, the extent of copy number variations, somatic and chromosomal mosaicism, and the nature of the epigenome as a regulator of DNA expressivity, are challenging a series of dogmas concerning the nature of the genome and the relationship between genotype and phenotype. According to three widely held dogmas, DNA is the unchanging template of heredity, is identical in all the cells and tissues of the body, and is the sole agent of inheritance. Rather than being an unchanging template, DNA appears subject to a good deal of environmentally induced change. Instead of identical DNA in all the cells of the body, somatic mosaicism appears to be the normal human condition. And DNA can no longer be considered the sole agent of inheritance. We now know that the epigenome, which regulates gene expressivity, can be inherited via the germline. These developments are particularly significant for behavior genetics for at least three reasons: First, epigenetic regulation, DNA variability, and somatic mosaicism appear to be particularly prevalent in the human brain and probably are involved in much of human behavior; second, they have important implications for the validity of heritability and gene association studies, the methodologies that largely define the discipline of behavior genetics; and third, they appear to play a critical role in development during the perinatal period and, in particular, in enabling phenotypic plasticity in offspring. I examine one of the central claims to emerge from the use of heritability studies in the behavioral sciences, the principle of minimal shared maternal effects, in light of the growing awareness that the maternal perinatal environment is a critical venue for the exercise of adaptive phenotypic plasticity. This consideration has important implications for both developmental and evolutionary biology.

Experience-facilitated improvements in pup retrieval; evidence for an epigenetic effect

The quality and quantity of maternal care received during infancy are highly predictive of successful infant development. It has been well established, primarily in rats, that the combination of hormonal and infant stimuli at birth modifies neural circuits that regulate maternal responsiveness. During subsequent interactions, infant stimuli are more likely to elicit rapid maternal responsiveness. Some species, such as humans, can display maternal care in the absence of the endocrine events of pregnancy and birth. Similarly, virgin C57BL/6J female mice, display maternal care toward infants, and experience with infants elicits long-lasting increases in maternal care. We hypothesized that these experience-induced changes in behavior may be mediated by chromatin modifications, which in turn change expression of genes that promote maternal care. One site of action is the medial preoptic area (MPOA). To test our hypothesis we treated virgin female mice with sodium butyrate, a histone deacetylase inhibitor. This treatment potentiated maternal responsiveness as well as the expression of several genes: estrogen receptor β (Esr2), oxytocin (Oxt), and cyclicAMP response element binding protein (CREB) binding protein (Crebbp; a histone acetyltransferase) in the MPOA. These data suggest that experience induces high levels of maternal care via epigenetic modifications.

Epigenetics and the power of art

This review presents an epigenetic view on complex factors leading to development and perception of “genius.” There is increasing evidence which indicates that artistic creativity is influenced by epigenetic processes that act both as targets and mediators of neurotransmitters as well as steroid hormones. Thus, perception and production of art appear to be closely associated with epigenetic contributions to physical and mental health.

Why the maternal brain?

In the rat, the change from a virgin/nulliparous female to the maternal animal takes place at many levels. A subtle developmental wave washes over the female nervous system and transforms her from largely self-centred to offspring-directed, from personal care and protection to care of genetically-related offspring, from indifference to ardour. Such change is preceded by substantial and apparently permanent neural alterations, the depth of which results in the maternal brain, and is the basis of the present review. The neuroplasticity of pregnancy, inherent to the female brain and, we believe, representative of the full expression of the female nervous system's capacity, is a result of significant hormonal and other neurochemical actions. It results in the striking brain changes that are associated with, and necessary for, successful reproduction. We discuss some of these changes and their ramifications. Collectively, they represent the culmination of mammalian evolution and have led to the development of the social brain characteristic of higher orders of mammal, including the human. We also examine different facets of the maternal brain, beginning with a review of the genes involved in maternal behaviour, and in the subsequent 'expression' of the maternal brain. We next discuss olfaction and the manner in which this major sense draws from the rich sensory milieu of the mother to regulate and support maternal behaviour. Last, we discuss the 'whys' of maternal behaviour, a theoretical foray into the reasons for such substantial maternal brain alterations. We focus on the male's potential role as the raison d'etre for the manifest alterations in his mate's brain. In the end, it is clear that the female brain undergoes a significant reorganisation en route to motherhood, the results of which are deep and enduring.

Oxytocin and social motivation

Humans are fundamentally social creatures who are ‘motivated’ to be with others. In this review we examine the role of oxytocin (OT) as it relates to social motivation. OT is synthesized in the brain and throughout the body, including in the heart, thymus, gastrointestinal tract, as well as reproductive organs. The distribution of the OT receptor (OTR) system in both the brain and periphery is even more far-reaching and its expression is subject to changes over the course of development. OTR expression is also sensitive to changes in the external environment and the internal somatic world. The OT system functions as an important element within a complex, developmentally sensitive biobehavioral system. Other elements include sensory inputs, the salience, reward, and threat detection pathways, the hypothalamic-pituitary-gonadal axis, and the hypothalamic-pituitary-adrenal stress response axis. Despite an ever expanding scientific literature, key unresolved questions remain concerning the interplay of the central and peripheral components of this complex biobehavioral system that dynamically engages the brain and the body as humans interact with social partners over the course of development.

Early life experience shapes the functional organization of stress-responsive visceral circuits

Emotions are closely tied to changes in autonomic (i.e., visceral motor) function, and interoceptive sensory feedback from body to brain exerts powerful modulatory control over motivation, affect, and stress responsiveness. This manuscript reviews evidence that early life experience can shape the structure and function of central visceral circuits that underlie behavioral and physiological responses to emotive and stressful events. The review begins with a general discussion of descending autonomic and ascending visceral sensory pathways within the brain, and then summarizes what is known about the postnatal development of these central visceral circuits in rats. Evidence is then presented to support the view that early life experience, particularly maternal care, can modify the developmental assembly and structure of these circuits in a way that impacts later stress responsiveness and emotional behavior. The review concludes by presenting a working hypothesis that endogenous cholecystokinin signaling and subsequent recruitment of gastric vagal sensory inputs to the caudal brainstem may be an important mechanism by which maternal care influences visceral circuit development in rat pups. Early life experience may contribute to meaningful individual differences in emotionality and stress responsiveness by shaping the postnatal developmental trajectory of central visceral circuits.

Epigenetic perspective on the developmental effects of bisphenol A

Bisphenol A (BPA) is an estrogenic environmental toxin widely used in the production of plastics and ubiquitous human exposure to this chemical has been proposed to be a potential risk to public health. Animal studies suggest that in utero and early postnatal exposure to this compound may produce a broad range of adverse effects, including impaired brain development, sexual differentiation, behavior, and immune function, which could extend to future generations. Molecular mechanisms that underlie the long-lasting effects of BPA continue to be elucidated, and likely involve disruption of epigenetic programming of gene expression during development. Several studies have provided evidence that maternal exposure to BPA results in postnatal changes in DNA methylation status and altered expression of specific genes in offspring. However, further studies are needed to extend these initial findings to other genes in different tissues, and to examine the correlations between BPA-induced epigenetic alterations, changes in gene expression, and various phenotypic outcomes. It will be also important to explore whether the epigenetic effects of BPA are related to its estrogenic activity, and to determine which downstream effector proteins could mediate changes in DNA methylation. In this review, we will highlight research indicating a consequence of prenatal BPA exposure for brain, behavior, and immune outcomes and discuss evidence for the role of epigenetic pathways in shaping these developmental effects. Based on this evidence, we will suggest future directions in the study of BPA-induced epigenetic effects and discuss the transgenerational implications of exposure to endocrine disrupting chemicals.

Epigenetics meets endocrinology

Although genetics determines endocrine phenotypes, it cannot fully explain the great variability and reversibility of the system in response to environmental changes. Evidence now suggests that epigenetics, i.e. heritable but reversible changes in gene function without changes in nucleotide sequence, links genetics and environment in shaping endocrine function. Epigenetic mechanisms, including DNA methylation, histone modification, and microRNA, partition the genome into active and inactive domains based on endogenous and exogenous environmental changes and developmental stages, creating phenotype plasticity that can explain interindividual and population endocrine variability. We will review the current understanding of epigenetics in endocrinology, specifically, the regulation by epigenetics of the three levels of hormone action (synthesis and release, circulating and target tissue levels, and target-organ responsiveness) and the epigenetic action of endocrine disruptors. We will also discuss the impacts of hormones on epigenetics. We propose a three-dimensional model (genetics, environment, and developmental stage) to explain the phenomena related to progressive changes in endocrine functions with age, the early origin of endocrine disorders, phenotype discordance between monozygotic twins, rapid shifts in disease patterns among populations experiencing major lifestyle changes such as immigration, and the many endocrine disruptions in contemporary life. We emphasize that the key for understanding epigenetics in endocrinology is the identification, through advanced high-throughput screening technologies, of plasticity genes or loci that respond directly to a specific environmental stimulus. Investigations to determine whether epigenetic changes induced by today's lifestyles or environmental 'exposures' can be inherited and are reversible should open doors for applying epigenetics to the prevention and treatment of endocrine disorders.

Rat pup social motivation: a critical component of early psychological development

Examining the role of the offspring in early social dynamics is especially difficult. Human developmental psychology has found infant behavior to be a vital part of the early environmental setting. In the rodent model, the different ways that a rodent neonate or pup can influence social dynamics are not well known. Typically, litters of neonates or pups offer complex social interactions dominated by behavior seemingly initiated and maintained by the primary caregiver (e.g., the dam). Despite this strong role for the caregiver, the young most likely influence the litter dynamics in many powerful ways including communication signals, discrimination abilities and early approach behavior. Nelson and Panksepp (1996) developed a preference task to examine early rodent pup social motivation. We have used the same task to examine how variations in maternal care or different environmental perturbations could alter the rat pup preferences for social-related stimuli. Rat pups receiving low levels of maternal licking and grooming were impaired in maternal odor cue learning and emitted lower levels of 22kHz ultrasounds compared to pups from the high licking and grooming cohort. Prenatal stress or early exposure to a toxicant (polychlorinated biphenyl) altered early social preferences in the rat pup in different ways indicating that diverse strategies are expressed and specific to the type of perturbation exposure. A greater focus on the offspring motivation following early 'stressors' will allow for more complete understanding of the dynamics in behavior during early social development.

Facilitating resilience using a society-to-cells framework: a theory of nursing essentials applied to research and practice

The resilience potential of each human being is an essential focus of nursing care and research. An honored nursing tradition is viewing each patient in the context of family and culture and recognizing how these factors affect a patient's health and ability to be resilient. We present a society-to-cells nursing theory that formalizes and extends this holistic view of patients by delineating factors that contribute to resilience potential based on the society, community, and family environment in which people live, as well as individual variables that influence psychological, physiological, and cellular coping abilities. We also summarize how this perspective is essential to optimize patient care and to inform future nursing research.

Developmental and hormone-induced epigenetic changes to estrogen and progesterone receptor genes in brain are dynamic across the life span

Sexual differentiation of the rodent brain occurs during a perinatal critical period when androgen production from the male testis is locally converted to estradiol in neurons, resulting in masculinization of adult sexual behavior. Adult brain responses to hormones are programmed developmentally by estradiol exposure, but the mechanism(s) by which these changes are permanently organized remains poorly understood. Activation of steroid receptors plays a major role in organization of the brain, and we hypothesized that estradiol-induced alteration of steroid-receptor gene methylation is a critical component to this process. Estrogen receptor (ER)-α and ER-β and progesterone receptor are expressed at high levels within the preoptic area (POA) and the mediobasal hypothalamus, two brain regions critical for the expression of male and female sexual behavior. The percent methylation on the ER-α promoter increased markedly across development. During the critical period of sexual differentiation, females had significantly increased methylation than males or females masculinized with estradiol at two CpG sites. By adulthood, the neonatal sex difference and hormonal modulation of methylation were replaced with a new pattern at a different CpG site on the ER-α promoter. In contrast, the percent methylation on the progesterone receptor and ER-β promoter did not change developmentally but was modulated by hormones and exhibited only late emerging transient sex differences. These data indicate that sex differences in the methylation pattern of genes important for sexual behavior are epigenetically modified during development, but the specific changes observed do not endure and are not necessarily temporally associated with neonatal hormone exposure.

Epigenetic mechanisms that underpin metabolic and cardiovascular diseases

Cellular commitment to a specific lineage is controlled by differential silencing of genes, which in turn depends on epigenetic processes such as DNA methylation and histone modification. During early embryogenesis, the mammalian genome is 'wiped clean' of most epigenetic modifications, which are progressively re-established during embryonic development. Thus, the epigenome of each mature cellular lineage carries the record of its developmental history. The subsequent trajectory and pattern of development are also responsive to environmental influences, and such plasticity is likely to have an epigenetic basis. Epigenetic marks may be transmitted across generations, either directly by persisting through meiosis or indirectly through replication in the next generation of the conditions in which the epigenetic change occurred. Developmental plasticity evolved to match an organism to its environment, and a mismatch between the phenotypic outcome of adaptive plasticity and the current environment increases the risk of metabolic and cardiovascular disease. These considerations point to epigenetic processes as a key mechanism that underpins the developmental origins of chronic noncommunicable disease. Here, we review the evidence that environmental influences during mammalian development lead to stable changes in the epigenome that alter the individual's susceptibility to chronic metabolic and cardiovascular disease, and discuss the clinical implications.

Differential expression of oestrogen receptor alpha following reproductive experience in young and middle-aged female rats

Reproductive experience (i.e. pregnancy and lactation) alters a number of physiological and behavioural endpoints, many of which are related to reproductive function and are regulated by oestrogen. For example, reproductive experience significantly attenuates the oestradiol-induced prolactin surge on the afternoon of pro-oestrous and circulating oestradiol levels are reduced at this time. Although parity-related effects on oestrogen receptor (ER) alpha have been observed within the anterior pituitary, there are currently no data regarding possible parity-induced alterations in ERalpha in the brain. Thus, the present study aimed to examine the effect of parity on the expression of ERalpha in reproductively relevant brain regions. Moreover, because previous findings have demonstrated that the long-term effects of reproductive experience are often oestrous cycle-dependent, ERalpha was examined at two stages of the oestrous cycle (i.e. dioestrous and pro-oestrous). Finally, because the expression of ERalpha is significantly influenced by age, both young and middle-aged females were included in the present study. ERalpha status was determined using immunohistochemistry in select brain regions involved in the regulation of reproductive behaviour in age-matched, cycling primiparous (i.e. one pregnancy and lactation) and nulliparous females as well as in age-matched, noncycling (i.e. persistent oestrous) 12 month-old primiparous and nulliparous females. Significant shifts in ERalpha cell numbers were observed in the medial preoptic area and medial amygdala as a consequence of reproductive experience in an oestrous-dependent manner. These findings indicate that significant changes in ERalpha activity occur in the brain as a function of reproductive experience.