Epigenetic programming by maternal behavior

Here we report that increased pup licking and grooming (LG) and arched-back nursing (ABN) by rat mothers altered the offspring epigenome at a glucocorticoid receptor (GR) gene promoter in the hippocampus. Offspring of mothers that showed high levels of LG and ABN were found to have differences in DNA methylation, as compared to offspring of 'low-LG-ABN' mothers. These differences emerged over the first week of life, were reversed with cross-fostering, persisted into adulthood and were associated with altered histone acetylation and transcription factor (NGFI-A) binding to the GR promoter. Central infusion of a histone deacetylase inhibitor removed the group differences in histone acetylation, DNA methylation, NGFI-A binding, GR expression and hypothalamic-pituitary-adrenal (HPA) responses to stress, suggesting a causal relation among epigenomic state, GR expression and the maternal effect on stress responses in the offspring. Thus we show that an epigenomic state of a gene can be established through behavioral programming, and it is potentially reversible.

Epigenetic regulation of the glucocorticoid receptor in human brain associates with childhood abuse

Maternal care influences hypothalamic-pituitary-adrenal (HPA) function in the rat through epigenetic programming of glucocorticoid receptor expression. In humans, childhood abuse alters HPA stress responses and increases the risk of suicide. We examined epigenetic differences in a neuron-specific glucocorticoid receptor (NR3C1) promoter between postmortem hippocampus obtained from suicide victims with a history of childhood abuse and those from either suicide victims with no childhood abuse or controls. We found decreased levels of glucocorticoid receptor mRNA, as well as mRNA transcripts bearing the glucocorticoid receptor 1F splice variant and increased cytosine methylation of an NR3C1 promoter. Patch-methylated NR3C1 promoter constructs that mimicked the methylation state in samples from abused suicide victims showed decreased NGFI-A transcription factor binding and NGFI-A-inducible gene transcription. These findings translate previous results from rat to humans and suggest a common effect of parental care on the epigenetic regulation of hippocampal glucocorticoid receptor expression.

Maternal care, hippocampal glucocorticoid receptors, and hypothalamic-pituitary-adrenal responses to stress

Variations in maternal care affect the development of individual differences in neuroendocrine responses to stress in rats. As adults, the offspring of mothers that exhibited more licking and grooming of pups during the first 10 days of life showed reduced plasma adrenocorticotropic hormone and corticosterone responses to acute stress, increased hippocampal glucocorticoid receptor messenger RNA expression, enhanced glucocorticoid feedback sensitivity, and decreased levels of hypothalamic corticotropin-releasing hormone messenger RNA. Each measure was significantly correlated with the frequency of maternal licking and grooming (all r's > -0.6). These findings suggest that maternal behavior serves to "program" hypothalamic-pituitary-adrenal responses to stress in the offspring.

Maternal care, gene expression, and the transmission of individual differences in stress reactivity across generations

Naturally occurring variations in maternal care alter the expression of genes that regulate behavioral and endocrine responses to stress, as well as hippocampal synaptic development. These effects form the basis for the development of stable, individual differences in stress reactivity and certain forms of cognition. Maternal care also influences the maternal behavior of female offspring, an effect that appears to be related to oxytocin receptor gene expression, and which forms the basis for the intergenerational transmission of individual differences in stress reactivity. Patterns of maternal care that increase stress reactivity in offspring are enhanced by stressors imposed on the mother. These findings provide evidence for the importance of parental care as a mediator of the effects of environmental adversity on neural development.

Early, postnatal experience alters hypothalamic corticotropin-releasing factor (CRF) mRNA, median eminence CRF content and stress-induced release in adult rats

Rat pups 2-14 days of age were exposed daily to handling (15 min of separation from mother and home cage), maternal separation (MS; 180 min of comparable separation), or were left entirely undisturbed (non-handled; NH). As adults, MS rats showed increased hypothalamic corticotropin-releasing factor (CRF) mRNA levels compared with NH rats, while CRF mRNA levels in H rats were significantly lower than either MS or NH animals. Hypothalamic CRF content under basal conditions followed exactly the same pattern. A 20-min period of restraint stress produced significant CRF depletion in all groups, although the percentage of depletion was significantly lower in H animals compared with either MS or NH animals. Restraint stress produced significantly higher increases in plasma corticosterone in MS and NH animals than in H animals. These data reflect the importance of early environmental factors in regulating the development of the hypothalamic CRF system and the responsiveness of the hypothalamic-pituitary-adrenal axis to stress.

Nongenomic transmission across generations of maternal behavior and stress responses in the rat

In the rat, variations in maternal care appear to influence the development of behavioral and endocrine responses to stress in the offspring. The results of cross-fostering studies reported here provide evidence for (i) a causal relationship between maternal behavior and stress reactivity in the offspring and (ii) the transmission of such individual differences in maternal behavior from one generation of females to the next. Moreover, an environmental manipulation imposed during early development that alters maternal behavior can then affect the pattern of transmission in subsequent generations. Taken together, these findings indicate that variations in maternal care can serve as the basis for a nongenomic behavioral transmission of individual differences in stress reactivity across generations.

Cortisol levels during human aging predict hippocampal atrophy and memory deficits

Elevated glucocorticoid levels produce hippocampal dysfunction and correlate with individual deficits in spatial learning in aged rats. Previously we related persistent cortisol increases to memory impairments in elderly humans studied over five years. Here we demonstrate that aged humans with significant prolonged cortisol elevations showed reduced hippocampal volume and deficits in hippocampus-dependent memory tasks compared to normal-cortisol controls. Moreover, the degree of hippocampal atrophy correlated strongly with both the degree of cortisol elevation over time and current basal cortisol levels. Therefore, basal cortisol elevation may cause hippocampal damage and impair hippocampus-dependent learning and memory in humans.

Maternal care during infancy regulates the development of neural systems mediating the expression of fearfulness in the rat

The mothers of infant rats show individual differences in the frequency of licking/grooming and arched-back nursing (LG-ABN) of pups that contribute to the development of individual differences in behavioral responses to stress. As adults, the offspring of mothers that exhibited high levels of LG-ABN showed substantially reduced behavioral fearfulness in response to novelty compared with the offspring of low LG-ABN mothers. In addition, the adult offspring of the high LG-ABN mothers showed significantly (i) increased central benzodiazepine receptor density in the central, lateral, and basolateral nuclei of the amygdala as well as in the locus ceruleus, (ii) increased alpha2 adrenoreceptor density in the locus ceruleus, and (iii) decreased corticotropin-releasing hormone (CRH) receptor density in the locus ceruleus. The expression of fear and anxiety is regulated by a neural circuitry that includes the activation of ascending noradrenergic projections from the locus ceruleus to the forebrain structures. Considering the importance of the amygdala, notably the anxiogenic influence of CRH projections from the amygdala to the locus ceruleus, as well as the anxiolytic actions of benzodiazepines, for the expression of behavioral responses to stress, these findings suggest that maternal care during infancy serves to "program" behavioral responses to stress in the offspring by altering the development of the neural systems that mediate fearfulness.

Maternal care, hippocampal synaptogenesis and cognitive development in rats

We report that variations in maternal care in the rat promote hippocampal synaptogenesis and spatial learning and memory through systems known to mediate experience-dependent neural development. Thus, the offspring of mothers that show high levels of pup licking and grooming and arched-back nursing showed increased expression of NMDA receptor subunit and brain-derived neurotrophic factor (BDNF) mRNA, increased cholinergic innervation of the hippocampus and enhanced spatial learning and memory. A cross-fostering study provided evidence for a direct relationship between maternal behavior and hippocampal development, although not all neonates were equally sensitive to variations in maternal care.

Effect of neonatal handling on age-related impairments associated with the hippocampus

In rats, an environmental manipulation occurring early in life resulted in changes in the adrenocortical axis that persisted throughout the entire life of the animals and attenuated certain deficits associated with aging. Rats handled during infancy had a permanent increase in concentrations of receptors for glucocorticoids in the hippocampus, a critical region in the negative-feedback inhibition of adrenocortical activity. Increased receptor concentrations led to greater hippocampal sensitivity to glucocorticoids and enhanced negative-feedback efficacy in the handled rats. Thus, at all ages tested, rats that were not handled secreted more glucocorticoids in response to stress than did handled rats. At later ages, nonhandled rats also showed elevated basal glucocorticoid levels, with the result that there was a greater cumulative exposure to glucocorticoids in nonhandled rats. Increased exposure to adrenal glucocorticoids can accelerate hippocampal neuron loss and cognitive impairments in aging. Hippocampal cell loss and pronounced spatial memory deficits emerged with age in the nonhandled rats, but were almost absent in the handled rats. Previous work showed that glucocorticoid hypersecretion, hippocampal neuron death, and cognitive impairments form a complex degenerative cascade of aging in the rat. The present study shows that a subtle manipulation early in life can retard the emergence of this cascade.

Socioeconomic status and the brain: mechanistic insights from human and animal research

Human brain development occurs within a socioeconomic context and childhood socioeconomic status (SES) influences neural development--particularly of the systems that subserve language and executive function. Research in humans and in animal models has implicated prenatal factors, parent-child interactions and cognitive stimulation in the home environment in the effects of SES on neural development. These findings provide a unique opportunity for understanding how environmental factors can lead to individual differences in brain development, and for improving the programmes and policies that are designed to alleviate SES-related disparities in mental health and academic achievement.

Brain charts for the human lifespan

Over the past few decades, neuroimaging has become a ubiquitous tool in basic research and clinical studies of the human brain. However, no reference standards currently exist to quantify individual differences in neuroimaging metrics over time, in contrast to growth charts for anthropometric traits such as height and weight1. Here we assemble an interactive open resource to benchmark brain morphology derived from any current or future sample of MRI data ( http://www.brainchart.io/ ). With the goal of basing these reference charts on the largest and most inclusive dataset available, acknowledging limitations due to known biases of MRI studies relative to the diversity of the global population, we aggregated 123,984 MRI scans, across more than 100 primary studies, from 101,457 human participants between 115 days post-conception to 100 years of age. MRI metrics were quantified by centile scores, relative to non-linear trajectories2 of brain structural changes, and rates of change, over the lifespan. Brain charts identified previously unreported neurodevelopmental milestones3, showed high stability of individuals across longitudinal assessments, and demonstrated robustness to technical and methodological differences between primary studies. Centile scores showed increased heritability compared with non-centiled MRI phenotypes, and provided a standardized measure of atypical brain structure that revealed patterns of neuroanatomical variation across neurological and psychiatric disorders. In summary, brain charts are an essential step towards robust quantification of individual variation benchmarked to normative trajectories in multiple, commonly used neuroimaging phenotypes.

Variations in maternal care in the rat as a mediating influence for the effects of environment on development

Variations in maternal care have been widely considered as a critical influence in development. In the rat, variations in maternal behavior, particularly in licking/grooming, regulate the development of endocrine, emotional and cognitive responses to stress. These studies form the basis of a potentially useful model for the study of maternal effects in mammals. In this paper we provide a detailed methodological investigation into this model of maternal behavior, providing an analysis of the frequency, temporal dynamics, and transmission of maternal licking/grooming in several large cohorts. Frequency data indicate that licking/grooming is normally distributed across dams. The peak in licking/grooming occurs in the first few days postpartum and gradually declines. Dams designated as High or Low LG mothers differ in this behavior only during the first week postpartum. Observations over Days 2 to 5 postpartum are essential for the reliable assessments of individual differences in maternal behavior. Individual differences in licking/grooming behavior are stable across multiple litters, and are not associated with differences in litter size, weaning weight of pups, or gender ratio of the litter. We also observed no significant differences in the amount of licking/grooming received by individual pups within a litter, though variation does exist. Finally, maternal licking/grooming is transmitted to female offspring, though there is considerable within-litter variation in the expression of this behavior. Overall, these findings indicate considerable, normal variations in licking/grooming in the rat that are a stable, individual characteristic of rat dams.

Early environmental regulation of forebrain glucocorticoid receptor gene expression: implications for adrenocortical responses to stress

The adrenal glucocorticoids and catecholamines comprise a frontline of defense for mammalian species under conditions which threaten homeostasis (conditions commonly referred to as stress). Glucocorticoids represent the end product of the hypothalamic-pituitary-adrenal (HPA) axis and along with the catecholamines serve to mobilize the production and distribution of energy substrates during stress. The increased secretion of pituitary-adrenal hormones in response to stress is stimulated by the release of corticotropin-releasing hormone (CRH) and/or arginine vasopressin (AVP) from neurons in the nucleus paraventricularis. In this way, a neural signal associated with the stressor is transduced into a set of endocrine and sympathetic responses. The development of the HPA response to stressful stimuli is altered by early environmental events. Animals exposed to short periods of infantile stimulation or handling show decreased HPA responsivity to stress, whereas maternal separation, physical trauma and endotoxin administration enhance HPA responsivity to stress. In all cases, these effects persist throughout the life of the animal and are accompanied by increased hypothalamic levels of the mRNAs for CRH and often AVP. The inhibitory regulation of the synthesis for these ACTH releasing factors is achieved, in part, through a negative feedback loop whereby circulating glucocorticoids act at various neural sites to decrease CRH and AVP gene expression. Such inhibitory effects are initiated via an interaction between the adrenal steroid and an intracellular receptor (either the mineralocorticoid or glucocorticoid receptor). We have found that these early environmental manipulations regulate glucocorticoid receptor gene expression in the hippocampus and frontal cortex, regions that have been strongly implicated as sites for negative-feedback regulation of CRH and AVP synthesis. When the differences in glucocorticoid receptor density are transiently reversed, so too are those in HPA responses to stress. Taken together, our findings indicate that the early postnatal environment alters the differentiation of hippocampal neurons. This effect involves an altered rate of glucocorticoid receptor gene expression, resulting in changes in the sensitivity of the system to the inhibitory effects of glucocorticoids on the synthesis of CRH and AVP in hypothalamic neurons. Changes in CRH and AVP levels, in turn, determine the responsivity of the axis to subsequent stressors; increased releasing factor production is associated with increased HPA responses to stress. Thus, the early environment can contribute substantially to the development of stable individual differences in HPA responsivity to stressful stimuli. These data provide examples of early environmental programming of neural systems. One major objective of our research is to understand how such programming occurs within the brain.

Reversal of maternal programming of stress responses in adult offspring through methyl supplementation: altering epigenetic marking later in life

Stress responses in the adult rat are programmed early in life by maternal care and associated with epigenomic marking of the hippocampal exon 1(7) glucocorticoid receptor (GR) promoter. To examine whether such epigenetic programming is reversible in adult life, we centrally infused the adult offspring with the essential amino acid L-methionine, a precursor to S-adenosyl-methionine that serves as the donor of methyl groups for DNA methylation. Here we report that methionine infusion reverses the effect of maternal behavior on DNA methylation, nerve growth factor-inducible protein-A binding to the exon 1(7) promoter, GR expression, and hypothalamic-pituitary-adrenal and behavioral responses to stress, suggesting a causal relationship among epigenomic state, GR expression, and stress responses in the adult offspring. These results demonstrate that, despite the inherent stability of the epigenomic marks established early in life through behavioral programming, they are potentially reversible in the adult brain.

Epigenetics and the biological definition of gene x environment interactions

Variations in phenotype reflect the influence of environmental conditions during development on cellular functions, including that of the genome. The recent integration of epigenetics into developmental psychobiology illustrates the processes by which environmental conditions in early life structurally alter DNA, providing a physical basis for the influence of the perinatal environmental signals on phenotype over the life of the individual. This review focuses on the enduring effects of naturally occurring variations in maternal care on gene expression and phenotype to provide an example of environmentally driven plasticity at the level of the DNA, revealing the interdependence of gene and environmental in the regulation of phenotype.

Maternal care effects on the hippocampal transcriptome and anxiety-mediated behaviors in the offspring that are reversible in adulthood

Early-life experience has long-term consequences on behavior and stress responsivity of the adult. We previously proposed that early-life experience results in stable epigenetic programming of glucocorticoid receptor gene expression in the hippocampus. The aim of this study was to examine the global effect of early-life experience on the hippocampal transcriptome and the development of stress-mediated behaviors in the offspring and whether such effects were reversible in adulthood. Adult offspring were centrally infused with saline vehicle, the histone deacetylase inhibitor trichostatin A (TSA), or the essential amino acid l-methionine. The animals were assessed in an unfamiliar open-field arena, and the hippocampal transcriptome of each animal was evaluated by microarray analysis. Here we report that TSA and methionine treatment reversed the effect of maternal care on open-field behavior. We identified >900 genes stably regulated by maternal care. A fraction of these differences in gene expression is reversible by either the histone deacetylase inhibitor TSA or the methyl donor l-methionine. These results suggest that early-life experience has a stable and broad effect on the hippocampal transcriptome and anxiety-mediated behavior, which is potentially reversible in adulthood.

Dopamine release in response to a psychological stress in humans and its relationship to early life maternal care: a positron emission tomography study using [11C]raclopride

Mesolimbic dopamine is thought to play a role in the processing of rewards. However, animal studies also demonstrate dopamine release in response to aversive stressful stimuli. Also, in animal studies, disruptions of the mother-infant relationship have been shown to have long-lasting effects on the mesolimbic dopamine system and the hypothalamic-pituitary adrenal axis. We therefore investigated dopamine release in response to stress in human subjects, considering the relationship to early life parental care. We screened 120 healthy young college students for parental care in early life using a combination of telephone interviews and questionnaires. Five students from the top end and five students from the bottom end of the parental care distribution were then invited for a positron emission tomography study using [11C]raclopride and a psychosocial stress task. The psychosocial stressor caused a significant release of dopamine in the ventral striatum as indicated by a reduction in [11C]raclopride binding potential in the stress versus resting condition in subjects reporting low parental care. Moreover, the magnitude of the salivary cortisol response to stress was significantly correlated with the reduction in [11C]raclopride binding in the ventral striatum (r = 0.78), consistent with a facilitating effect of cortisol on dopamine neuron firing. These data suggest that aversive stressful events can be associated with mesolimbic dopamine release in humans, and that the method presented here may be useful to study the effects of early life events on neurobiological stress systems.

Environmental programming of stress responses through DNA methylation: life at the interface between a dynamic environment and a fixed genome

Early experience permanently alters behavior and physiology. These effects are, in part, mediated by sustained alterations in gene expression in selected brain regions. The critical question concerns the mechanism of these environmental “programming” effects. We examine this issue with an animal model that studies the consequences of variations in mother-infant interactions on the development of individual differences in behavioral and endocrine responses to stress in adulthood. Increased levels of pup licking/grooming by rat mothers in the first week of life alter DNA structure at a glucocorticoid receptor gene promoter in the hippocampus of the offspring. Differences in the DNA methylation pattern between the offspring of high- and low-lickinglgrooming mothers emerge over the first week of life; they are reversed with cross-fostering; they persist into adulthood; and they are associated with altered histone acetylation and transcription factor (nerve growth factor-induced clone A [NGFIA]) binding to the glucocorticoid receptor promoter. DNA methylation alters glucocorticoid receptor expression through modifications of chromatin structure. Pharmacological reversal of the effects on chromatin structure completely eliminates the effects of maternal care on glucocorticoid receptor expression and hypothalamic-pituitary-adrenal (HPA) responses to stress, thus suggesting a causal relation between the maternally induced, epigenetic modification of the glucocorticoid receptor gene and the effects on stress responses in the offspring. These findings demonstrate that the structural modifications of the DNA can be established through environmental programming and that, in spite of the inherent stability of this epigenomic marker, it is dynamic and potentially reversible.

Naturally occurring variations in maternal behavior in the rat are associated with differences in estrogen-inducible central oxytocin receptors

Naturally occurring variations in maternal licking/grooming influence neural development and are transmitted from mother to female offspring. We found that the induction of maternal behavior in virgin females through constant exposure to pups (pup sensitization) was significantly shorter in the offspring of High compared with Low licking/grooming mothers, suggesting differences in maternal responsivity. In randomly selected females screened for individual differences in maternal responsivity and subsequently mated, there was a significant and negative correlation (r = -0.73) between the latency to exhibit maternal behavior in the pup sensitization paradigm and the frequency of pup licking/grooming during lactation. Females that were more maternally responsive to pups and that showed increased levels of pup licking/grooming also showed significantly higher oxytocin receptor levels in the medial preoptic area, the lateral septum, the central nucleus (n.) of the amygdala, the paraventricular n. of the hypothalamus, and the bed n. of the stria terminalis. Intracerebroventricular administration of an oxytocin receptor antagonist to mothers on postpartum day 3 completely eliminated the differences in pup licking/grooming, suggesting that differences in oxytocin receptor levels are functionally related to maternal behavior. Finally, estrogen treatment of virgin females significantly increased oxytocin receptor binding in the medial preoptic area and lateral septum of female offspring of High, but not Low, licking/grooming mothers. These findings suggest that maternal licking/grooming influences the development of estrogen sensitivity in brain regions that regulate maternal behavior, providing a potential mechanism for the intergenerational transmission of individual differences in maternal behavior.

Variations in the hypothalamic-pituitary-adrenal response to stress during the estrous cycle in the rat

To investigate the role of gonadal steroids in the hypothalamic-pituitary-adrenal (HPA) response to stress, we studied adrenocorticotrophin (ACTH) and corticosterone (B) responses to 20-min restraint stress in cycling female rats, and in ovariectomized (OVX) rats replaced with physiological levels of estradiol (E2) and progesterone (P). In cycling rats, we found significantly higher peak ACTH (P less than 0.01) and B (P less than 0.05) responses to stress during proestrus compared to the estrous and diestrous phases. No differences were found in either basal ACTH and B levels across the cycle phases. In a separate study, OVX rats were maintained on low, physiological levels of E2 and P with silastic implants for 3 days, and injected either with oil (O'), 10 micrograms of E2 (E') 24 h before stress testing, or with E2 and 500 micrograms P 24 and 4 h, respectively, prior to stress (EP'). These treatments mimicked endogenous profiles of E2 and P occurring during diestrous, proestrous, and late proestrous-early estrous phases, respectively. In response to stress, ACTH levels were higher (P less than 0.01) in the E' group compared to the EP' and O' groups. Although the peak B response was similar in all groups, the E' and EP' groups secreted more B after the termination of stress than did the O' group. Within the 20 min stress period, ACTH levels in the E' group were significantly (P less than 0.05) higher at 5, 10, and 15 min after the onset of stress, compared to the EP' and O' groups. Plasma B levels were significantly higher in the E' group at 5 and 10 min (P less than 0.05 and P less than 0.01, respectively) compared to the EP' and O' group. beta-endorphin-like immunoreactive responses to restraint stress were also significantly higher in the E' group compared to the EP' (P less than 0.05) and O' (P less than 0.01) groups. In contrast to the effect seen at 24 h, ACTH responses to stress 48 h after E2 injection in the E' group were comparable to O' animals. There was no effect of E2 on ACTH clearance, whereas B clearance was enhanced in E' treated animals vs. O'-treated animals. These results indicate that the HPA axis in the female rat is most sensitive to stress during proestrous. Such enhanced HPA responses to stress are limited to the early portion of proestrous, as progesterone appears to inhibit the facilitatory effects of estrogen on ACTH release during stress. Taken together, these results suggest an ovarian influence on both activational and inhibitory components of HPA activity.

Do early-life events permanently alter behavioral and hormonal responses to stressors?

Early-life stimulation (e.g., brief handling) attenuates the behavioral and neuroendocrine responses to stressors encountered in adulthood, particularly with respect to activation of hypothalamic-pituitary-adrenal (HPA) activity. In contrast, if neonates were subjected to a more severe stressor, such as protracted separation from the dam or exposure to an endotoxin, then the adult response to a stressor was exaggerated. These early-life experiences program HPA functioning, including negative feedback derived from stimulation of hippocampal glucocorticoid receptors, and corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) coexpression in PVN neurons, to modify the response to subsequent stressor experiences. The persistent variations of HPA activity observed in handled/stimulated animals may stem from alterations in dam-pup interactions (e.g. increased arched-back feeding, licking, grooming). In addition genetic makeup is critical in determining stress reactivity. For instance, BALB/cByJ mice are more reactive to stressors than C57BL/6ByJ mice, exhibiting greater HPA hormonal alterations and behavioral disturbances. BALB/cByJ also fail to acquire a spatial learning response in a Morris water-maze paradigm, which has been shown to be correlated with hippocampal cell loss associated with aging. Early-life handling of BALB/cByJ mice prevented these performance deficits and attenuated the hypersecretion of ACTH and corticosterone elicited by stressors. The stressor reactivity may have been related to maternal and genetic factors. When BALB/cByJ mice were raised by a C57BL/6ByJ dam, the excessive stress-elicited HPA activity was reduced, as were the behavioral impairments. However, cross-fostering the more resilient C57BL/6ByJ mice to a BALB/cByJ dam failed to elicit the behavioral disturbances. It is suggested that genetic factors may influence dam-pup interactive styles and may thus proactively influence the response to subsequent stressors among vulnerable animals. In contrast, in relatively hardy animals the early-life manipulations may have less obvious effects.

Epigenetic mechanisms of perinatal programming of hypothalamic-pituitary-adrenal function and health

Environmental effects on the materno-foetal interaction determine birth outcomes that predict health over the lifespan. Thus, maternal undernutrition or stress associate with low birth weight, leading to an increased risk of metabolic and cardiovascular illness in the offspring. We argue that these effects are, in part, mediated by direct and indirect effects on the hypothalamic-pituitary-adrenal (HPA) axis such that (i) the effect of maternal adversity on foetal growth is mediated by adrenal glucocorticoids and (ii) environmental adversity alters maternal physiology and behaviour, which then programs HPA activity in the offspring.