Variations of maternal care alter offspring levels of behavioural defensiveness in adulthood: evidence for a threshold model

Natural variations of maternal care in the rat influence the development of neuronal systems that regulate defensive responses to threat. Thus, as adults, rats that received higher levels of maternal licking/grooming (LG) in infancy display dramatic reductions in burying in the shock-probe test, relative to offspring of low LG mothers. We sought to replicate that finding and determine whether maternal care similarly influences offspring responses to social threat, using the resident-intruder test. We also examined whether maternal LG influences offspring behaviour along a continuum by comparing defensive responses of offspring of mid LG mothers to those of offspring of high LG and low LG mothers. A final goal was to assess whether the reductions in adult offspring reactivity to threat that typically follow corticosterone (CORT) administration to dams across lactation are mediated through CORT-induced changes in maternal care. Adult offspring of high LG mothers spent less time burying the shock-probe, relative to offspring of mid and low LG mothers, whereas offspring of CORT-treated mothers did not differ from any group. Similarly, offspring of high LG (but not CORT-treated) mothers displayed fewer defensive responses in the resident-intruder test. Thus, only natural variations of maternal care were associated with individual differences in offspring reactivity to threat. Furthermore, because offspring of mid and low LG mothers displayed equivalent levels of defensive responding in both tests, it appears that a critical threshold of maternal LG is necessary to alter the developmental trajectory of neural systems mediating defensive behaviours.     

Variation in maternal care influences ventromedial hypothalamus activation in the rat

Natural variation in maternal care in the rat is an important source of individual differences in the female neuroendocrine system and sexual behaviours. Thus, females reared by low licking and grooming (LG) mothers are sexually more receptive to males, showing higher lordosis ratings, and are more motivated to mate compared to female offspring of high LG mothers. In the present study, we investigated the effect of natural variations in maternal care on the ventrolateral part of the ventromedial hypothalamus (VMHvl) cell population and on the reproductive success of the female rat. Immunohistochemistry for phosphorylated oestrogen-receptor (pER) α and progesterone receptor (PR) were used to study the VMHvl of female offspring of high and low LG mothers at pro-oestrus and dioestrus. A second experiment investigated sexual behaviour and the effect of mating on c-Fos expression in the VMHvl of pro-oestrus and ovariectomised high and how female offspring. Lastly, we investigated the maternal effect on the establishment of the progestational state. A greater number of VMHvl pERα immunoreactive cells was found in the pro-oestrous female offspring of low LG mothers and PR was most abundant at pro-oestrus compared to dioestrus in both high and low LG females. Interestingly it is the less receptive high females that show the greater c-Fos expression in the VMH after mating in the pro-oestrous group. The difference in c-Fos expression after mating disappeared when the two groups were ovariectomised and received steroid replacement. Finally, low LG female offspring reached pseudopregnancy more often when receiving only seven intromissions at a 5-min interval compared to high LG females. Lower levels of maternal care may favour the reproductive success of low LG offspring by increasing pERα and oestrogen-dependent lordosis behaviour and lowering c-Fos after mating, resulting in inhibition of termination of oestrus.     

Epigenetic mechanisms and the transgenerational effects of maternal care

The transmission of traits across generations has typically been attributed to the inheritance by offspring of genomic information from parental generations. However, recent evidence suggests that epigenetic mechanisms are capable of mediating this type of transmission. In the case of maternal care, there is evidence for the behavioral transmission of postpartum behavior from mothers to female offspring. The neuroendocrine and molecular mediators of this transmission have been explored in rats and implicate estrogen-oxytocin interactions and the differential methylation of hypothalamic estrogen receptors. These maternal effects can influence multiple aspects of neurobiology and behavior of offspring and this particular mode of inheritance is dynamic in response to environmental variation. In this review, evidence for the generational transmission of maternal care and the mechanisms underlying this transmission will be discussed as will the implications of this inheritance system for offspring development and for the transmission of environmental information from parents to offspring.     

Maternal programming of sexual attractivity in female Long Evans rats

In mammals, maternal care influences the developing offspring across multiple domains. In Long Evans rats, for example, the quality of maternal care received as a pup influences later cognitive function, neuroendocrine responses to stress and behavioral measures of emotionality. Data from humans, non-human primates, and rodents also suggest that early life events may similarly perturb measures of sexual reproduction, with possible consequences for reproductive fitness. The current study examined whether or not male conspecifics differentially prefer females, as adult mating partners, that were reared under varying maternal conditions (assessed via the quantity of licking and grooming received; LG). Additionally, the impact of maternal care on adult female sexual motivation and behavior were quantified to determine if these behavioral characteristics are associated with any preference observed. In a mate preference task, male rats chose, almost exclusively, to mount, copulate and ejaculate with female rats reared under Low LG conditions. Under non-paced mating conditions, female Low LG rats display significantly more paracopulatory and copulatory behaviors compared to High LG rats. Due to its critical role in female paracopulatory behavior, progesterone receptor immunoreactivity (PR-ir) in the ventromedial nucleus of the hypothalamus (VMH) was also assessed in both groups of female rats. Estradiol induced PR-ir in the VMH was significantly higher in Low LG relative to High LG rats. Together, these data suggests that early life parental care may developmentally program aspects of behavior and physiology that subsequently influence sexual attractivity and behavior in adult females.     

Naturally occurring differences in maternal care are associated with the expression of oxytocin and vasopressin (V1a) receptors: gender differences

Variations in maternal care have been associated with long-term changes in neurochemistry and behaviour in adult rats. Rats receiving high levels of licking and grooming as pups are less fearful and more maternal than rats receiving low levels of maternal licking and grooming. Central pathways for oxytocin and vasopressin have been implicated in the neurobiology of anxiety and social behaviours. We assessed whether variations in maternal care were associated with differences in oxytocin receptors (OTR) or vasopressin (V1a) receptors in the brains of adult offspring. In the central nucleus of the amygdala and bed nucleus of the stria terminalis, OTR binding was increased in adult females, but not adult males, that had received high levels of maternal licking and grooming as pups. Conversely, amygdala V1a receptor binding was increased in males, but not females, that had received high levels of maternal licking and grooming. These findings suggest that variations in maternal care may influence the expression of oxytocin and vasopressin receptors in a gender-specific manner.     

Maternal care influences neuronal survival in the hippocampus of the rat

Maternal care during the first week of postnatal life influences hippocampal development and function (Liu et al., 2000; Nature Neurosci., 3, 799-806). Offspring reared by mothers who exhibit increased levels of pup licking/grooming (LG) show increased hippocampal synaptic density and enhanced spatial learning and memory. Using 5-bromo-2'-deoxyuridine (BrdU), a thymidine analogue incorporated into cells during DNA synthesis, we examined the effects of early maternal care on hippocampal cell proliferation and neuronal survival in the rat. Twenty-four hours following injection on day 7 of life (P7) there were no differences in BrdU labelling in the offspring of high- compared with low-LG mothers, suggesting no maternal effect on the rate of proliferation at this age. However, 14 and 83 days following injection (P21 and P90), the offspring of high-LG mothers had significantly more surviving BrdU-labelled cells and BrdU-NeuN+-colabelled neurons in the dentate gyrus subgranular zone and granule cell layer. At P21, the offspring of high-LG mothers showed increased protein expression of basic fibroblast growth factor and significantly decreased levels of pyknosis. These findings suggest an influence of maternal care on neuronal survival in the hippocampus. Conversely, at the same time point there was a significantly higher level of hippocampal glial fibrillary acidic protein expression in the offspring of low-LG mothers. These findings emphasize the importance of early maternal care for hippocampal development.     

Perinatal Testosterone Exposure and Maternal Care Effects on the Female Rat's Development and Sexual Behaviour

Natural variations in maternal care have profound influences on offspring behaviour, brain activity and hormone release. Measuring the amount of time that a rat dam spends licking/grooming (LG) her pups during their first week of life allows for characterisation of distinctive Low, Mid and High LG phenotypes. We have previously found that female offspring of High LG mothers are less sexually receptive, less motivated to mate and show a later onset of puberty relative to Low LG offspring. Given that High LG females are exposed to greater levels of testosterone in utero, we hypothesise that differences in sexual behaviour between High and Low LG female offspring are driven in part by differences in prenatal hormone exposure. To test this hypothesis, pregnant dams pre-characterised as Low, Mid, or High LG mothers were implanted with testosterone or placebo on gestational day (GD) 16. Offspring body weight and anogenital index were assessed at GD 21 and in adulthood. Age of vaginal opening and oestrous cyclicity were assessed to determine the timing of pubertal onset. Testosterone exposure removed the difference between LG phenotypes in pubertal onset by delaying vaginal opening and the appearance of first pro-oestrus. In adulthood, sexual behaviour in a paced mating chamber after sham surgery or ovariectomy with steroid-replacement was examined. Our findings show that Low, Mid and High LG female offspring are differentially affected by perinatal testosterone exposure, and that this exposure removes the precocial pubertal onset of Low LG offspring and increases the sexual proceptivity and receptivity of High LG offspring. These results suggest that maternal programming of the female reproductive system may be mediated, in part, through differences in perinatal testosterone exposure, instead of solely through maternal behaviour.     

Maternal depression and family adversity: Linked pathways to offspring depression?

There is conflicting evidence about the contribution of maternal depression and family adversity to depression experienced by offspring. Because maternal depression and family adversity are related, there is a need to determine how they independently contribute to offspring depression. Data are from a long-running prospective birth cohort study (Mater-University of Queensland Study of Pregnancy and its outcomes – MUSP). For this study some 2200 offspring were followed up at 30 years of age. We first examine the association between maternal depression and family adversity over the period from the pregnancy to the child reaching adulthood. Then we consider the extent to which maternal depression and family adversity trajectories over this period predict CIDI/DSM-IV episodes of depression in the offspring of these mothers at 30 years of age. We find a strong bi-directional association between maternal depression and family experiences of adverse life events over the entire period the child is at home. After adjustment, children reared in a family experiencing high levels of adverse life events are more likely to experience a lifetime ever DSM-IV diagnosis of depression, are more likely to have experienced multiple episodes of lifetime ever depression, and are more likely to report their first episode of depression was at a younger age. The findings suggest the association between maternal depression and offspring depression appears to be partly attributable to the higher levels of family adversity characteristic of depressed mothers.     

Adult hippocampal glucocorticoid receptor expression and dentate synaptic plasticity correlate with maternal care received by individuals early in life

Maternal care in mammals is the prevailing environmental influence during perinatal development. The adult rat offspring of mothers exhibiting increased levels of pup licking/grooming (LG; High LG mothers), compared to those reared by Low LG dams, show increased hippocampal glucocorticoid receptor expression, complex dendritic tree structure, and an enhanced capacity for synaptic potentiation. However, these data were derived from studies using the total amount of maternal care directed toward the entire litter, thus ignoring possible within-litter variation. We show that the amount of LG received by individual pups within a litter varies considerably. Therefore, we questioned if the amount of LG received by individual pups correlates with and thus putatively predicts later hippocampal structure and function. To this end, LG-scores were determined during the first postnatal week for all pups in 32 litters and correlated with neuroendocrine and hippocampal parameters in young-adulthood. Pup LG-score positively correlated with the glucocorticoid receptor mRNA expression in the adult hippocampus. Moreover, the ability to induce synaptic potentiation in the dentate gyrus in vitro was enhanced in animals with high LG-scores. Structural plasticity correlated less reliably with LG-scores early in life and differed between sexes. Male offspring with high LG-scores displayed fewer newborn neurons, higher brain derived neurotrophic factor expression and tended to have more complex granule cell dendritic trees. We conclude that even moderate variations in early life environment have a major impact on adult hippocampal function. This principle could provide a mechanistic basis for individual differences in susceptibility to psychopathology.     

Peripubertal environmental enrichment reverses the effects of maternal care on hippocampal development and glutamate receptor subunit expression

Maternal care in the rat influences the development of cognitive function in the offspring through neural systems known to mediate activity-dependent synaptic plasticity. The offspring of mothers that exhibit increased levels of pup licking/grooming (high-LG mothers) show increased hippocampal N-methyl-D-aspartate (NMDA) subunit mRNA expression, enhanced synaptogenesis and improved hippocampal-dependent spatial learning in comparison with animals reared by low-LG mothers. The effects of reduced maternal care on cognitive function are reversed with peripubertal environmental enrichment; however, the neural mechanisms mediating this effect are not known. In these studies we exposed the offspring of high- and low-LG mothers to environmental enrichment from days 22 to 70 of life, and measured the expression of genes encoding for glutamate receptor subunits and synaptophysin expression as a measure of synaptic density. Environmental enrichment reversed the effects of maternal care on synaptic density and this effect was, in turn, associated with a reversal of the effect of maternal care on the NR2A and NR2B subunits of the NMDA receptor, as well as effects on (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits. Finally, direct infusion of an NR2B-specific NMDA receptor antagonist into the hippocampus eliminated the effects of maternal care on spatial learning/memory in the Morris water maze. These findings suggest that: (1) the effects of maternal care are mediated by changes in NR2B gene expression; and (2) that environmental enrichment reverses the effects of reduced maternal care through the same genomic target, the NR2B gene, and possibly effects on other subunits of the NMDA and AMPA receptors.     

Parental enrichment and offspring development: modifications to brain, behavior and the epigenome

Environmental enrichment has been shown to have profound effects on the healthy adult brain and as a remedial tool for brains compromised by injury, disease, or negative experience. Based upon these findings and evidence from the prenatal stress literature, we ventured an exploratory study to examine the effects of parental enrichment on offspring development. Using Long Evans rats, paternal enrichment was achieved by housing sires in enriched environments for 28 days prior to mating with a control female. For the maternal enrichment paradigm, female rats were also housed in enriched environments for 28 days (7 days prior to conception and for the duration of pregnancy). Increased size, multiple levels for exploration, an abundance of stimulating toys, and numerous cagemates for social interaction were characteristic of the enriched environments. Offspring were assessed using two early behavioral tests and then sacrificed at postnatal day 21 (P21). Brain tissue from the frontal cortex and hippocampus was harvested for global DNA methylation analysis. Parental enrichment, preconceptionally and prenatally, altered offspring behavior on the negative geotaxis task and openfield exploratory behavior task. Paternal enrichment significantly decreased offspring brain weight at P21. Additionally, both environmental enrichment paradigms significantly decreased global methylation levels in the hippocampus and frontal cortex of male and female offspring. This study demonstrates that positive prenatal experiences; preconceptionally in fathers and prenatally in mothers, have the ability to significantly alter offspring developmental trajectories.     

Parental representations in drug-dependent patients and their parents

The Parental Bonding Instrument (PBI), a measure of perceived parental care and protection, was administered to drug-dependent patients and their parents with the aim to assess the reliability of the instrument in such samples and to compare the parental representations across generations. Ninety drug-dependent patients and 44 mothers and 35 fathers participated. Reliability indices were calculated, and parental representations of parents and their offspring were compared. Linear regression analyses were performed with the patient's PBI score as the dependent variable and the mother's and father's PBI scores as predictor variables. The reliability indices were highly satisfactory and varied between 0.61 and 0.91. The parental bonding of patients, fathers, and mothers was similar. All three groups reported high maternal and paternal control and low maternal care, a pattern characteristic of an "affectionless control" rearing style. Maternal care received by the fathers and paternal protection received by the mothers predicted the care and protection they themselves gave to their drug-dependent offspring.     

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 22 kHz 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.     

Maternal care differs in mice bred for high vs. low trait anxiety: Impact of brain vasopressin and cross-fostering

Brain arginine vasopressin (AVP) not only regulates male social behavior and emotionality, but also promotes maternal behavior, as has been shown in rats. In our CD1 mice breed for high (HAB) or low (LAB) anxiety-related behavior, LAB mice have markedly less AVP mRNA expression in the hypothalamic paraventricular nucleus compared with HAB mice. Together these findings suggest that HAB and LAB mice represent a good model to assess the role of AVP in mouse maternal behavior. Therefore, we studied maternal care of HAB and LAB mouse dams and investigated the impact of maternal care on the offspring's anxiety in a cross-fostering paradigm. In comparison with HAB dams, LABs displayed less maternal care. Daily acute intracerebroventricular infusions of AVP in early lactation increased maternal care of LAB dams and acted anxiogenically. Cross-fostering on postnatal day 5 did not alter separation-induced high and low ultrasonic vocalization calling frequency, a measure of inborn anxiety, in HAB and LAB offspring, respectively. However, adult cross-fostered HAB mice displayed a trend towards decreased anxiety on the elevated plus-maze, which was still significantly higher compared with LAB mice. The low levels of depressive-like behavior, stress-reactivity, and hypothalamic AVP mRNA expression in adult LAB offspring were found to be independent of cross-fostering. In conclusion, the HAB/LAB differences in maternal care and anxiety are robust and strongly depend on differences in the AVP system. The seemingly rigid genetic predisposition to hyperanxiety can only be moderately attenuated by the received nurturing.     

Family discord and stress predictors of depression and other disorders in adolescent children of depressed and nondepressed women

OBJECTIVE: To test the hypothesis that family stress variables are associated with the effects of maternal depression on offspring diagnoses and examined whether such factors may be differentially associated with disorders in offspring of depressed and never-depressed women. METHOD: Eight hundred sixteen mothers and their 15-year-old children in an Australian community completed cross-sectional assessments of mother and youth diagnoses, interviewer-rated and self-reported quality of marital relationship/status, quality of parent-child relationship, and interviews for youth chronic and episodic stress. Women with depression histories were oversampled and included 458 never-depressed and 358 women with current or past major depressive episodes or dysthymic disorder. RESULTS: Significant interaction effects were found between maternal depression and family discord/stress variables such that high levels of environmental risk factors were significantly associated with youth depression in children of depressed women compared with low levels of adverse conditions and were generally less associated with depression in children of nondepressed women. Nondepressive disorders were associated with adverse family and stress factors for both groups of children. CONCLUSIONS: The results are consistent with a multiple risk factor model of depression transmission in high-risk families and suggest a pattern of reactivity to adverse conditions among children of depressed women. The results suggest that psychosocial factors may contribute to diagnoses in offspring of depressed women in community samples.     

Deficient maternal care resulting from immunological stress during pregnancy is associated with a sex-dependent enhancement of conditioned fear in the offspring

Activation of maternal stress response systems during pregnancy has been associated with altered postpartum maternal care and subsequent abnormalities in the offspring's brain and behavioral development. It remains unknown, however, whether similar effects may be induced by exposure to immunological stress during pregnancy. The present study was designed to address this issue in a mouse model of prenatal immune activation by the viral mimic polyriboinosinic-polyribocytidilic acid (PolyI:C). Pregnant mice were exposed to PolyI:C-induced immune challenge or sham treatment, and offspring born to PolyI:C- and sham-treated dams were simultaneously cross-fostered to surrogate rearing mothers, which had either experienced inflammatory or vehicle treatment during pregnancy. We evaluated the effects of the maternal immunological manipulation on postpartum maternal behavior, and we assessed the prenatal and postnatal maternal influences on anxiety- and fear-related behavior in the offspring at the peri-adolescent and adult stage of development. We found that PolyI:C treatment during pregnancy led to changes in postpartum maternal behavior in the form of reduced pup licking/grooming and increased nest building activity. Furthermore, the adoption of neonates by surrogate rearing mothers, which had experienced PolyI:C-induced immunological stress during pregnancy, led to enhanced conditioned fear in the peri-adolescent and adult offspring, an effect that was exclusively seen in female but not male subjects. Unconditioned (innate) anxiety-related behavior as assessed in the elevated plus maze and open field explorations tests were not affected by the prenatal and postnatal manipulations. Our results thus highlight that being raised by gestationally immune-challenged surrogate mothers increases the vulnerability for specific forms of fear-related behavioral pathology in later life, and that this association may be mediated by deficits in postpartum maternal care. This may have important implications for the identification and characterization of early-life risk factors involved in the developmental etiology of fear-related neuropsychiatric disorders.     

Maternal programming of defensive responses through sustained effects on gene expression

There are profound maternal effects on individual differences in defensive responses in species ranging from plants to insects to birds. In this paper, we review data from the rat that suggest comparable forms of maternal effects on defensive responses to stress, which are mediated by the effects of variations in maternal behaviour on gene expression. Under conditions of environmental adversity, maternal effects enhance the capacity for defensive responses in the offspring. These effects appear to "program" emotional, cognitive and endocrine systems toward increased sensitivity to adversity. In environments with an increased level of adversity, such effects can be considered adaptive, enhancing the capacity for responses that have immediate adaptive value; the cost is an increased risk for multiple forms of pathology in later life.     

Inter‐individual maternal care received and genotype interactions affect dopaminergic phenotypes in female rat offspring

Rat mothers exhibit natural variations in care and can shape offspring adult behaviour and their maternal care by affecting the dopaminergic system. We explored whether genotype and gene × environment interactions are involved in these processes in nulliparous female offspring. We assessed maternal licking/grooming toward individual female pups during the first week postpartum and dopamine‐related behaviour of the offspring in adulthood. Behaviours explored included strategy shifting, impulsive action and sucrose preference. Single nucleotide polymorphisms in the dopamine receptor 2, dopamine transporter and catechol‐O‐methyltransferase genes were examined in relation to offspring behaviour and baseline dopamine turnover in select brain regions. Dopamine receptor 2 (RS107017253) variation moderated, or interacted with, the relationship between early‐life licking received and behaviour. Specifically, offspring with the A/A genotype showed a significant correlation between early‐life licking received and behaviour. Offspring with the A/G and G/G genotypes did not show this relationship. Dopamine transporter gene variation affected offspring behaviour regardless of early‐life licking received. Our findings suggest that genotype can directly affect dopamine‐related behaviours and alter the sensitivity of offspring to the maternal environment. This could be informative on how maternal care is transmitted between generations of female offspring.