The neural mechanisms and consequences of paternal caregiving.

In recent decades, human sociocultural changes have increased the numbers of fathers that are involved in direct caregiving in Western societies. This trend has led to a resurgence of interest in understanding the mechanisms and effects of paternal care. Across the animal kingdom, paternal caregiving has been found to be a highly malleable phenomenon, presenting with great variability among and within species. The emergence of paternal behaviour in a male animal has been shown to be accompanied by substantial neural plasticity and to be shaped by previous and current caregiving experiences, maternal and infant stimuli and ecological conditions. Recent research has allowed us to gain a better understanding of the neural basis of mammalian paternal care, the genomic and circuit-level mechanisms underlying paternal behaviour and the ways in which the subcortical structures that support maternal caregiving have evolved into a global network of parental care. In addition, the behavioural, neural and molecular consequences of paternal caregiving for offspring are becoming increasingly apparent. Future cross-species research on the effects of absence of the father and the transmission of paternal influences across generations may allow research on the neuroscience of fatherhood to impact society at large in a number of important ways.

Postnatal rearing environment alters pup cues for caregiver-offspring interactions.

Maternal behavior experienced in early life provides essential scaffolding to infant psychobiology with life-long effects on neurobiological and behavioral outcomes. However, infants are not passive recipients of caregiving. Evidence in rodents suggests that pups actively contribute to dam-pup interactions by soliciting maternal care with auditory, tactile, and hormonal cues. The limited bedding and nesting material (LBN) rearing manipulation induces changes in maternal care that have been attributed to maternal stress caused by the low-resource environment. The goal of the current study was to determine whether LBN also alters pup cues for maternal behavior, with implications for the mechanism of LBN-induced effects. Rat dams and pups were randomly assigned to LBN or Control rearing conditions on postnatal day (P) 0-6 and pups were fostered to the same or different condition on P6-13. LBN increased pup-directed maternal behaviors measured through 24 h monitoring using machine learning based automated analysis. LBN altered several pup cues known to affect maternal behavior including reducing pup core body temperature, reducing body weight, and altering pup vocalizations on P6 and P12. P6-13 LBN-exposed pups had elevated serum testosterone, which positively correlated with maternal licking and grooming. LBN reduced pup movement between nest attendance onset and the start of nursing, which was negatively related to dam nursing latency and contributed to longer nursing latency in LBN dams. P0-6 pup exposure to LBN also led to longer nest attendance bouts and shorter licking and grooming bouts on P7 and P9, suggesting lasting effects of LBN on pups. These data demonstrate that LBN changes pup behavioral and hormonal signals consistent with eliciting more maternal care, contributing to augmented pup-directed behaviors. This bidirectional interplay may be a critical mechanism involved in the lasting effects of early life environments.

Sex-specific effects of chronic paternal stress on offspring development are partially mediated via mothers.

Paternal stress exposure is known to impact the development of stress-related behaviors in offspring. Previous work has highlighted the importance of sperm mediated factors, such as RNAs, in transmitting the effects of parental stress. However, a key unanswered question is whether mothers behavior could drive or modulate the transmission of paternal stress effects on offspring development. Here we investigate how chronic variable stress in Balb/C mice influences the sex-specific development of anxiety- and depression-like neural and behavioral development in offspring. Moreover, we examined how stressed fathers influenced mate maternal investment towards their offspring and how this may modulate the transmission of paternal stress effects on offspring. We show that paternal stress leads to sex-specific effects on offspring behavior. Males that are chronically stressed sire female offspring that show increased anxiety and depression-like behaviors. However, male offspring of stressed fathers show reductions in anxiety- and depression-behaviors and are generally more exploratory. Moreover, we show that females mated with stressed males gain less weight during pregnancy and provide less care towards their offspring which additionally influenced offspring development. These data indicate that paternal stress can influence offspring development both directly and indirectly via changes in mothers, with implications for sex-specific offspring development.

The contributions of parental lactation on offspring development: It's not udder nonsense!

The Developmental Origins of Health and Disease (DOHaD) hypothesis describes how maternal stress exposures experienced during critical periods of perinatal life are linked to altered developmental trajectories in offspring. Perinatal stress also induces changes in lactogenesis, milk volume, maternal care, and the nutritive and non-nutritive components of milk, affecting short and long-term developmental outcomes in offspring. For instance, selective early life stressors shape the contents of milk, including macro/micronutrients, immune components, microbiota, enzymes, hormones, milk-derived extracellular vesicles, and milk microRNAs. In this review, we highlight the contributions of parental lactation to offspring development by examining changes in the composition of breast milk in response to three well-characterized maternal stressors: nutritive stress, immune stress, and psychological stress. We discuss recent findings in human, animal, and in vitro models, their clinical relevance, study limitations, and potential therapeutic significance to improving human health and infant survival. We also discuss the benefits of enrichment methods and support tools that can be used to improve milk quality and volume as well as related developmental outcomes in offspring. Lastly, we use evidence-based primary literature to convey that even though select maternal stressors may modulate lactation biology (by influencing milk composition) depending on the severity and length of exposure, exclusive and/or prolonged milk feeding may attenuate the negative in utero effects of early life stressors and promote healthy developmental trajectories. Overall, scientific evidence supports lactation to be protective against nutritive and immune stressors, but the benefits of lactation in response to psychological stressors need further investigation.

Distinct immune and transcriptomic profiles in dominant versus subordinate males in mouse social hierarchies.

Social status is a critical factor determining health outcomes in human and nonhuman social species. In social hierarchies with reproductive skew, individuals compete to monopolize resources and increase mating opportunities. This can come at a significant energetic cost leading to trade-offs between different physiological systems. In particular, changes in energetic investment in the immune system can have significant short and long-term effects on fitness and health. We have previously found that dominant alpha male mice living in social hierarchies have increased metabolic demands related to territorial defense. In this study, we tested the hypothesis that high-ranking male mice favor adaptive immunity, while subordinate mice show higher investment in innate immunity. We housed 12 groups of 10 outbred CD-1 male mice in a social housing system. All formed linear social hierarchies and subordinate mice had higher concentrations of plasma corticosterone (CORT) than alpha males. This difference was heightened in highly despotic hierarchies. Using flow cytometry, we found that dominant status was associated with a significant shift in immunophenotypes towards favoring adaptive versus innate immunity. Using Tag-Seq to profile hepatic and splenic transcriptomes of alpha and subordinate males, we identified genes that regulate metabolic and immune defense pathways that are associated with status and/or CORT concentration. In the liver, dominant animals showed a relatively higher expression of specific genes involved in major urinary production and catabolic processes, whereas subordinate animals showed relatively higher expression of genes promoting biosynthetic processes, wound healing, and proinflammatory responses. In spleen, subordinate mice showed relatively higher expression of genes facilitating oxidative phosphorylation and DNA repair and CORT was negatively associated with genes involved in lymphocyte proliferation and activation. Together, our findings suggest that dominant and subordinate animals adaptively shift immune profiles and peripheral gene expression to match their contextual needs.

Elevated prenatal maternal sex hormones, but not placental aromatase, are associated with child neurodevelopment.

Fetal exposure to testosterone may contribute to vulnerability for autism spectrum disorder (ASD). It is hypothesized that placental aromatase prevents fetal exposure to maternal testosterone, however, this pathway and the implications for child neurodevelopment have not been fully explored. We examined the relationships between prenatal maternal testosterone and estradiol at 19.2 ± 1.3 weeks, cord blood testosterone and estradiol at birth, placental aromatase mRNA expression, and neurodevelopment using the Social Communication Questionnaire (SCQ), the Behavioral Assessment System for Children, 3rd Edition (BASC-3), and the Empathizing Quotient for Children (EQ-C) at 4.5-6.5 years of age in a sample of 270 Nulliparous-Mothers-to-be (nuMoM2b) study participants. Maternal testosterone levels were positively associated with SCQ scores, but the association was not significant after adjusting for maternal age at delivery, nor was there a significant interaction with sex. Maternal estradiol levels were negatively associated with BASC-3 Clinical Probability scores among males (n = 139). We report a significant interaction effect of cord blood testosterone and fetal sex on both total SCQ scores and t-scores on the Developmental Social Disorders subscale. Placental aromatase was not associated with any neurodevelopmental or hormone measure, but under conditions of low placental aromatase expression, high maternal testosterone was positively associated with SCQ scores in males (n = 46). No other associations between hormone levels and neurodevelopment were significant. Our findings provide a foundation for further investigation of the mechanisms through which maternal sex hormones and placental steroidogenesis may affect fetal hormone production and neurobehavior.

Maternal prenatal stress phenotypes associate with fetal neurodevelopment and birth outcomes.

Maternal prenatal stress influences offspring neurodevelopment and birth outcomes including the ratio of males to females born; however, there is limited understanding of what types of stress matter, and for whom. Using a data-driven approach with 27 variables from questionnaires, ambulatory diaries, and physical assessments collected early in the singleton pregnancies of 187 women, 3 latent profiles of maternal prenatal stress emerged that were differentially associated with sex at birth, birth outcomes, and fetal neurodevelopment. Most women (66.8%) were in the healthy group (HG); 17.1% were in the psychologically stressed group (PSYG), evidencing clinically meaningful elevations in perceived stress, depression, and anxiety; and 16% were in the physically stressed group (PHSG) with relatively higher ambulatory blood pressure and increased caloric intake. The population normative male:female secondary sex ratio (105:100) was lower in the PSYG (2:3) and PHSG (4:9), and higher in the HG (23:18), consistent with research showing diminished male births in maternal stress contexts. PHSG versus HG infants were born 1.5 wk earlier (P < 0.05) with 22% compared to 5% born preterm. PHSG versus HG fetuses had decreased fetal heart rate-movement coupling (P < 0.05), which may indicate slower central nervous system development, and PSYG versus PHSG fetuses had more birth complications, consistent with previous findings among offspring of women with psychiatric illness. Social support most strongly differentiated the HG, PSYG, and PHSG groups, and higher social support was associated with increased odds of male versus female births. Stress phenotypes in pregnant women are associated with male vulnerability and poor fetal outcomes.

Added sugar intake during pregnancy: Fetal behavior, birth outcomes, and placental DNA methylation.

Pregnancy is a critical time for the effects of environmental factors on children's development. The effect of added sugar intake on fetal development and pregnancy outcomes remains understudied despite increasing dietary intake in the United States. This study investigated the effect of added sugar on fetal programming by examining the association between maternal added sugar consumption, fetal movement, birth outcomes, and placental DNA methylation. Further, primary human fibroblasts were cultured under normal or high glucose conditions to assess the effect of high glucose exposure on cells' DNA methylation. We found that higher added sugar intake across pregnancy was associated with reduced 3rd-trimester fetal movement (p < .05) and shorter gestation (p < .01). Our sample size was not powered to detect the alteration of individual placental CpG with genome-wide significance. However, a secondary analysis suggested that added sugar consumption was associated with differential methylation of functionally related gene families across pregnancy. Consistent with this, high glucose exposure in primary cultured human fibroblasts altered the methylation of 17% of all CpGs, providing converging evidence for an effect of sugar on DNA methylation. Our results suggest that diets high in added sugar during pregnancy may have implications for offspring health via prenatal programming effects measurable before birth.

Maternal modulation of paternal effects on offspring development.

The paternal transmission of environmentally induced phenotypes across generations has been reported to occur following a number of qualitatively different exposures and appear to be driven, at least in part, by epigenetic factors that are inherited via the sperm. However, previous studies of paternal germline transmission have not addressed the role of mothers in the propagation of paternal effects to offspring. We hypothesized that paternal exposure to nutritional restriction would impact male mate quality and subsequent maternal reproductive investment with consequences for the transmission of paternal germline effects. In the current report, using embryo transfer in mice, we demonstrate that sperm factors in adult food restricted males can influence growth rate, hypothalamic gene expression and behaviour in female offspring. However, under natural mating conditions females mated with food restricted males show increased pre- and postnatal care, and phenotypic outcomes observed during embryo transfer conditions are absent or reversed. We demonstrate that these compensatory changes in maternal investment are associated with a reduced mate preference for food restricted males and elevated gene expression within the maternal hypothalamus. Therefore, paternal experience can influence offspring development via germline inheritance, but mothers can serve as a modulating factor in determining the impact of paternal influences on offspring development.

DNA methylation of BDNF as a biomarker of early-life adversity.

Early-life adversity increases the risk for psychopathology in later life. The underlying mechanism(s) is unknown, but epigenetic variation represents a plausible candidate. Early-life exposures can disrupt epigenetic programming in the brain, with lasting consequences for gene expression and behavior. This evidence is primarily derived from animal studies, with limited study in humans due to inaccessibility of the target brain tissue. In humans, although there is evidence for DNA methylation changes in the peripheral blood of psychiatric patients, a fundamental question remains as to whether epigenetic markers in the blood can predict epigenetic changes occurring in the brain. We used in utero bisphenol A (BPA) exposure as a model environmental exposure shown to disrupt neurodevelopment and exert long-term effects on behavior in animals and humans. We show that prenatal BPA induces lasting DNA methylation changes in the transcriptionally relevant region of the Bdnf gene in the hippocampus and blood of BALB/c mice and that these changes are consistent with BDNF changes in the cord blood of humans exposed to high maternal BPA levels in utero. Our data suggest that BDNF DNA methylation in the blood may be used as a predictor of brain BDNF DNA methylation and gene expression as well as behavioral vulnerability induced by early-life environmental exposure. Because BDNF expression and DNA methylation are altered in several psychiatric disorders that are associated with early-life adversity, including depression, schizophrenia, bipolar disorder, and autism, BDNF DNA methylation in the blood may represent a novel biomarker for the early detection of psychopathology.

Influence of maternal care on the developing brain: Mechanisms, temporal dynamics and sensitive periods.

Variation in maternal care can lead to divergent developmental trajectories in offspring with implications for neuroendocrine function and behavioral phenotypes. Study of the long-term outcomes associated with mother-infant interactions suggests complex mechanisms linking the experience of variation in maternal care and these neurobiological consequences. Through integration of genetic, molecular, cellular, neuroanatomical, and neuroendocrine approaches, significant advances in our understanding of these complex pathways have been achieved. In this review, we will consider the impact of maternal care on male and female offspring development with a particular focus on the issues of timing and mechanism. Identifying the period of sensitivity to maternal care and the temporal dynamics of the molecular and neuroendocrine changes that are a consequence of maternal care represents a critical step in the study of mechanism.

Beyond DNA: integrating inclusive inheritance into an extended theory of evolution.

Many biologists are calling for an 'extended evolutionary synthesis' that would 'modernize the modern synthesis' of evolution. Biological information is typically considered as being transmitted across generations by the DNA sequence alone, but accumulating evidence indicates that both genetic and non-genetic inheritance, and the interactions between them, have important effects on evolutionary outcomes. We review the evidence for such effects of epigenetic, ecological and cultural inheritance and parental effects, and outline methods that quantify the relative contributions of genetic and non-genetic heritability to the transmission of phenotypic variation across generations. These issues have implications for diverse areas, from the question of missing heritability in human complex-trait genetics to the basis of major evolutionary transitions.

Sex-specific fitness effects of unpredictable early life conditions are associated with DNA methylation in the avian glucocorticoid receptor.

Organisms can adapt to variable environments by using environmental cues to modulate developmental gene expression. In principle, maternal influences can adaptively adjust offspring phenotype when early life and adult environments match, but they may be maladaptive when future environments are not predictable. One of the best-studied 'maternal effects' is through modification of the offspring's hypothalamic-pituitary-adrenal (HPA) axis, the neuroendocrine system that controls responses to stress. In addition to the direct transfer of glucocorticoids from mother to offspring, offspring HPA function and other phenotypes can also be affected by epigenetic modifications like DNA methylation of the glucocorticoid receptor promoter. Here we examine how among-year variation in rainfall is related to DNA methylation during development and fitness in adulthood in the superb starling (Lamprotornis superbus), which lives in a climatically unpredictable environment where early life and adult environments are unlikely to match. We found that DNA methylation in the putative promoter of the glucocorticoid receptor gene is reduced in chicks - particularly in males - born following drier prebreeding periods. Additionally, DNA methylation is lower in males that become breeders than those that never breed. However, there is no relationship in females between DNA methylation and the likelihood of dispersing from the natal group to breed elsewhere. These results suggest that early life conditions may positively affect fitness in a sex-specific manner through chemical modification of an HPA-associated gene. This study is the first to show that epigenetic modifications during early life may influence the fitness of free-living organisms adapted to unpredictable environments.

Hormonal and non-hormonal bases of maternal behavior: The role of experience and epigenetic mechanisms.

This article is part of a Special Issue "Parental Care". Though hormonal changes occurring throughout pregnancy and at the time of parturition have been demonstrated to prime the maternal brain and trigger the onset of mother-infant interactions, extended experience with neonates can induce similar behavioral interactions. Sensitization, a phenomenon in which rodents engage in parental responses to young following constant cohabitation with donor pups, was elegantly demonstrated by Rosenblatt (1967) to occur in females and males, independent of hormonal status. Study of the non-hormonal basis of maternal behavior has contributed significantly to our understanding of hormonal influences on the maternal brain and the cellular and molecular mechanisms that mediate maternal behavior. Here, we highlight our current understanding regarding both hormone-induced and experience-induced maternal responsivity and the mechanisms that may serve as a common pathway through which increases in maternal behavior are achieved. In particular, we describe the epigenetic changes that contribute to chromatin remodeling and how these molecular mechanisms may influence the neural substrates of the maternal brain. We also consider how individual differences in these systems emerge during development in response to maternal care. This research has broad implications for our understanding of the parental brain and the role of experience in the induction of neurobiological and behavior changes.

Epigenetic legacy of parental experiences: Dynamic and interactive pathways to inheritance.

The quality of the environment experienced by an individual across his or her lifespan can result in a unique developmental trajectory with consequences for adult phenotype and reproductive success. However, it is also evident that these experiences can impact the development of offspring with continued effect on subsequent generations. Epigenetic mechanisms have been proposed as a mediator of both these within- and across-generation effects, and there is increasing evidence to support the role of environmentally induced changes in DNA methylation, posttranslational histone modifications, and noncoding RNAs in predicting these outcomes. Advances in our understanding of these molecular modifications contribute to increasingly nuanced perspectives on plasticity and transmission of phenotypes across generations. A challenge that emerges from this research is in how we integrate these "new" perspectives with traditional views of development, reproduction, and inheritance. This paper will highlight evidence suggestive of an epigenetic impact of the environment on mothers, fathers, and their offspring, and illustrate the importance of considering the dynamic nature of reproduction and development and inclusive views of inheritance within the evolving field of behavioral and environmental epigenetics.

Sex-specific epigenetic disruption and behavioral changes following low-dose in utero bisphenol A exposure.

Bisphenol A (BPA) is an estrogenic endocrine disruptor widely used in the production of plastics. Increasing evidence indicates that in utero BPA exposure affects sexual differentiation and behavior; however, the mechanisms underlying these effects are unknown. We hypothesized that BPA may disrupt epigenetic programming of gene expression in the brain. Here, we provide evidence that maternal exposure during pregnancy to environmentally relevant doses of BPA (2, 20, and 200 µg/kg/d) in mice induces sex-specific, dose-dependent (linear and curvilinear), and brain region-specific changes in expression of genes encoding estrogen receptors (ERs; ERα and ERß) and estrogen-related receptor-γ in juvenile offspring. Concomitantly, BPA altered mRNA levels of epigenetic regulators DNA methyltransferase (DNMT) 1 and DNMT3A in the juvenile cortex and hypothalamus, paralleling changes in estrogen-related receptors. Importantly, changes in ERα and DNMT expression in the cortex (males) and hypothalamus (females) were associated with DNA methylation changes in the ERα gene. BPA exposure induced persistent, largely sex-specific effects on social and anxiety-like behavior, leading to disruption of sexually dimorphic behaviors. Although postnatal maternal care was altered in mothers treated with BPA during pregnancy, the effects of in utero BPA were not found to be mediated by maternal care. However, our data suggest that increased maternal care may partially attenuate the effects of in utero BPA on DNA methylation. Overall, we demonstrate that low-dose prenatal BPA exposure induces lasting epigenetic disruption in the brain that possibly underlie enduring effects of BPA on brain function and behavior, especially regarding sexually dimorphic phenotypes.

Plasticity of the Maternal Brain Across the Lifespan.

Maternal behavior is dynamic and highly sensitive to experiential and contextual factors. In this review, this plasticity will be explored, with a focus on how experiences of females occurring from the time of fetal development through to adulthood impact maternal behavior and the maternal brain. Variation in postpartum maternal behavior is dependent on estrogen sensitivity within the medial preoptic area of the hypothalamus and activation within mesolimbic dopamine neurons. This review will discuss how experiences across the lifespan alter the function of these systems and the multigenerational consequences of these neuroendocrine and behavioral changes. These studies, based primarily on the examination of maternal behavior in laboratory rodents and nonhuman primates, provide mechanistic insights relevant to our understanding of human maternal behavior and to the mechanisms of lifelong plasticity.

Variations in maternal behavior in rats selected for infant ultrasonic vocalization in isolation.

Individual differences in maternal behavior in rodents are associated with altered physiology and behavior in offspring across their lifespan and across generations. Offspring of rat dams that engage in high frequencies of high-arched-back nursing and pup-licking (High-LG) show attenuated stress responses compared to those engaging in lower frequencies (Low-LG). Selective breeding also produces widespread alterations in physiology and behavior that are stable over generations. To examine processes underlying generational and developmental influences on anxiety in an animal model, we developed two lines of rats that emit either extremely high (High-USV) or low (Low-USV) rates of 45kHz ultrasonic vocalizations in isolation at postnatal day 10. Compared to the Low-USV line, High-USV rats display increased indices of anxiety- and depression-like behavior in adulthood. The current study assessed maternal behaviors as well as oxytocin and vasopressin receptor density in High-USV and Low-USV dams to determine if selective breeding had produced differences that paralleled those found in Low- and High-LG dams. We found that Low-USV dams engage in more high-arched nursing and pup-licking than High-USV dams. Differences in oxytocin and vasopressin receptor levels were not widespread throughout the brain, with line differences in the piriform cortex and nucleus accumbens. This research illustrates the potential interplay between genetically determined (USV line) and environmental (postnatal mother-infant interactions) factors in accounting for the phenotypes associated with maternal separation induced postnatal vocalizations.

Paternal social enrichment effects on maternal behavior and offspring growth.

Paternal environmental experiences are significant predictors of developmental outcomes in offspring and can occur even in the absence of paternal care. Although there has been a recent focus on the role of environmentally induced changes in the male germline in producing these effects, the potential mediating role of mothers has not been investigated. A role for mothers in the transmission of paternal effects has been well acknowledged in behavioral ecology, which predicts that females will dynamically adjust their reproductive investment in response to the qualities of their mate. In the present study, we show that a lifetime of socially enriched compared with impoverished housing conditions shifts anxiety-like behavior and gene expression of male mice. Females that mate with enriched-reared males exhibit increased levels of pup nursing and licking toward their offspring, which are associated with changes in gene expression within the maternal hypothalamus. Significantly, these changes in maternal behavior are correlated with the general levels of anxiety exhibited by their male mates. Further, we show that paternal environmental enrichment results in increased growth of their offspring. These results suggest that maternal-paternal interactions at mating may guide offspring development, with significant implications for the transgenerational transmission of paternal environmental experiences.

Food for thought: hormonal, experiential, and neural influences on feeding and obesity.

Obesity is a growing public health problem. Although convenient, the notion that obesity is simply a problem of will power is increasingly antiquated. It is becoming clear that complex interactions of environment, neurohormonal systems, and transgenerational effects directly contribute to obesity. This review highlights data presented at the Society for Neuroscience Annual Meeting in San Diego, California in 2013; and although not meant as an exhaustive review of the area, this reivew will explore seemingly disparate areas of research that, when taken as a whole, illuminate the complex topography of the causes and consequences of obesity. We discuss how disruption of the biological clock, a consequence of modern society, can lead to changes in the brain and periphery that lead to obesity. We explore how obesity can actually cause pathological changes within the hypothalamus of the brain (a key regulator of food intake and metabolic homeostasis). How reward circuitry, particularly the ventral tegmental area, responds to insulin and how these effects modulate feeding and the salience of feeding cues are mechanistically described. We also investigate how nutrition may cross generational boundaries to affect the development and function of offspring, underscoring the long reach of metabolic effects. Finally, the role of the endocannabinoid system is emphasized as a critical node in the transduction of many of these effects. Together, this review should provide perspective into the neural causes and consequences of obesity, and hopefully lead to new areas of interdisciplinary research to tackle this important public health epidemic.