Neuroendocrinology and Adaptive Physiology of Maternal Care

The role of maternal hormones in regulating autonomic functions during pregnancy

Offspring development relies on numerous physiological changes that occur in a mother's body, with hormones driving many of these adaptations. Amongst these, the physiological functions controlled by the autonomic nervous system are required for the mother to survive and are adjusted to meet the demands of the growing foetus and to ensure a successful birth. The hormones oestrogen, progesterone, and lactogenic hormones rise significantly during pregnancy, suggesting they may also play a role in regulating the maternal adaptations linked to autonomic nervous system functions, including respiratory, cardiovascular, and thermoregulatory functions. Indeed, expression of pregnancy hormone receptors spans multiple brain regions known to regulate these physiological functions. This review examines how respiratory, cardiovascular, and thermoregulatory functions are controlled by these pregnancy hormones by focusing on their action on central nervous system circuits. Inadequate adaptations in these systems during pregnancy can give rise to several pregnancy complications, highlighting the importance in understanding the mechanistic underpinnings of these changes and potentially identifying ways to treat pregnancy-associated afflictions using hormones.

Prolactin Action Is Necessary for Parental Behavior in Male Mice

Parental care is critical for successful reproduction in mammals. Recent work has implicated the hormone prolactin in regulating male parental behavior, similar to its established role in females. Male laboratory mice show a mating-induced suppression of infanticide (normally observed in virgins) and onset of paternal behavior 2 weeks after mating. Using this model, we sought to investigate how prolactin acts in the forebrain to regulate paternal behavior. First, using c-fos immunoreactivity in prolactin receptor (Prlr) Prlr-IRES-Cre-tdtomato reporter mouse sires, we show that the circuitry activated during paternal interactions contains prolactin-responsive neurons in multiple sites, including the medial preoptic nucleus, bed nucleus of the stria terminalis, and medial amygdala. Next, we deleted Prlr from three prominent cell types found in these regions: glutamatergic, GABAergic, and CaMKIIα. Prlr deletion from CaMKIIα, but not glutamatergic or GABAergic cells, had a profound effect on paternal behavior as none of these KO males completed the pup-retrieval task. Prolactin was increased during mating, but not in response to pups, suggesting that the mating-induced secretion of prolactin is important for establishing the switch from infanticidal to paternal behavior. Pharmacological blockade of prolactin secretion at mating, however, had no effect on paternal behavior. In contrast, suppressing prolactin secretion at the time of pup exposure resulted in failure to retrieve pups, with exogenous prolactin administration rescuing this behavior. Together, our data show that paternal behavior in sires is dependent on basal levels of circulating prolactin acting at the time of interaction with pups, mediated through Prlr on CaMKIIα-expressing neurons.SIGNIFICANCE STATEMENT Parental care is critical for offspring survival. Compared with maternal care, however, the neurobiology of paternal care is less well understood. Here we show that the hormone prolactin, which is most well known for its female-specific role in lactation, has a role in the male brain to promote paternal behavior. In the absence of prolactin signaling specifically during interactions with pups, father mice fail to show normal retrieval behavior of pups. These data demonstrate that prolactin has a similar action in both males and females to promote parental care.

Prolactin-mediated restraint of maternal aggression in lactation

Heightened intruder-directed aggressive behavior in female mice is displayed during lactation to enable a mother to protect her offspring. Although recent work has identified that the ventromedial nucleus of the hypothalamus plays an important role in governing aggressive behavior, it is unknown how the changing hormones of pregnancy and lactation might regulate this behavior during specific reproductive states. Here, we show that prolactin-responsive neurons are activated during aggression and project to multiple brain regions with known roles in regulating behavior. Through neuron-specific and region-specific deletion of the prolactin receptor, our data reveal that prolactin is an important modulator of maternal aggression. By acting on glutamatergic neurons in the ventromedial nucleus, prolactin restrains maternal aggression, specifically in lactating female mice.

Aggressive behavior is rarely observed in virgin female mice but is specifically triggered in lactation where it facilitates protection of offspring. Recent studies demonstrated that the hypothalamic ventromedial nucleus (VMN) plays an important role in facilitating aggressive behavior in both sexes. Here, we demonstrate a role for the pituitary hormone, prolactin, acting through the prolactin receptor in the VMN to control the intensity of aggressive behavior exclusively during lactation. Prolactin receptor deletion from glutamatergic neurons or specifically from the VMN resulted in hyperaggressive lactating females, with a marked shift from intruder-directed investigative behavior to very high levels of aggressive behavior. Prolactin-sensitive neurons in the VMN project to a wide range of other hypothalamic and extrahypothalamic regions, including the medial preoptic area, paraventricular nucleus, and bed nucleus of the stria terminalis, all regions known to be part of a complex neuronal network controlling maternal behavior. Within this network, prolactin acts in the VMN to specifically restrain male-directed aggressive behavior in lactating females. This action in the VMN may complement the role of prolactin in other brain regions, by shifting the balance of maternal behaviors from defense-related activities to more pup-directed behaviors necessary for nurturing offspring.

Prenatal health behaviours as predictors of human placental lactogen levels

Placental lactogen (hPL) is a key hormone of pregnancy responsible for inducing maternal adaptations critical for a successful pregnancy. Low levels of placental lactogen have been associated with lower birth weight as well as symptoms of maternal depression and anxiety. Lower placental lactogen has been reported in women with higher body mass index (BMI) but it is unclear whether prenatal health behaviours predict hPL levels or if hPL is associated with infant weight outcomes. This study utilised data from the longitudinal Grown in Wales cohort, based in South Wales. Participants were recruited at the pre-surgical appointment for an elective caesarean section. This study incorporates data from recruitment, post-delivery and a 12 month follow-up. Measures of maternal serum hPL were available for 248 participants. Analysis included unadjusted and adjusted linear and binary regression. Unadjusted, prenatal smoking and a Health Conscious dietary pattern were associated with hPL levels, however this was lost on adjustment for BMI at booking, Welsh Index of Multiple Deprivation (WIMD) score and placental weight. When stratified by maternal BMI at booking, a Health Conscious dietary pattern remained associated with increased hPL levels in women with a healthy BMI (p=.024, B=.59. 95% CI=.08,1.11) following adjustment for WIMD score and placental weight. When adjusted for a wide range of confounders, maternal hPL was also associated with increased custom birthweight centiles (CBWC) (p=.014, B=1.64. 95% CI=.33,2.94) and increased odds of large for gestational age deliveries (p=<.001, Exp(B)=1.42. 95% CI=1.17,1.72). This study identified that consuming a Health Conscious dietary pattern in pregnancy was associated with increased hPL, within women of a healthy BMI. Moreover, higher hPL levels were associated with increased CBWC and increased odds of delivering a large for gestational age infant. This improves the current limited evidence surrounding the nature of hPL in these areas.

Parental Behavior in Rodents

Members of the order Rodentia are among the best-studied mammals for understanding the patterns, outcomes, and biological determinants of maternal and paternal caregiving. This research has provided a wealth of information but has historically focused on just a few rodents, mostly members of the two Myomorpha families that easily breed and can be studied within a laboratory setting (including laboratory rats, mice, hamsters, voles, gerbils). It is unclear how well this small collection of animals represents the over 2000 species of extant rodents. This chapter provides an overview of the hormonal and neurobiological systems involved in parental care in rodents, with a purposeful eye on providing information known or could be gleaned about parenting in various less-traditional members of Rodentia. We conclude from this analysis that the few commonly studied rodents are not necessarily even representative of the highly diverse members of Myomorpha, let alone other rodent suborders, and that additional laboratory and field studies of members of this order more broadly would surely provide invaluable information toward revealing a more representative picture of the rich diversity in rodent parenting.

In support of the placental programming hypothesis: Placental endocrine insufficiency programs atypical behaviour in mothers and their offspring

New Findings

What is the topic of this review?

More than half of all pregnancies in the UK are exposed to adversity linked to increased problems in pregnancy for mothers and adverse outcomes for their children, but we do not know the mechanism(s) underpinning these relationships.

What advances does it highlight?

Studies in mice prove that placental endocrine insufficiency driven by genetic manipulation of imprinted genes in the offspring can concurrently drive fetal growth restriction, alterations in maternal caregiving and aberrant behaviour in wild‐type offspring exposed to an adverse environment. This suggests that placental endocrine insufficiency might contribute to the co‐morbidity of low birth weight, maternal depression and neurodevelopmental disorders observed in human populations.

Abstract

Prenatal adversity, which is estimated to impact more than half of all pregnancies in the UK, compromises fetal growth and increases the chances of stillbirth, prematurity and infant mortality. Beyond these immediate and highly visible problems, infants that survive carry the invisible burden of increased risk of some of the most common and pervasive diseases that impact human populations. In utero exposure to depression and anxiety is one adversity that has been linked to these poorer outcomes, suggesting that maternal mood disorders drive the outcomes. However, recent studies in animal models suggest that both the maternal mood disorders and the detrimental outcomes for children could be the result of the same underlying placental pathology. In these studies, genetically wild‐type rodent mothers exposed to placental endocrine insufficiency engaged in less pup‐focused behaviours and less self‐care. Genetically wild‐type rodent offspring raised in this abnormal environment exhibited increased anxiety‐like behaviours, with male offspring additionally exhibiting deficits in cognition and atypical social behaviour, with some evidence of depressive‐like symptoms. This work establishes experimentally that placental endocrine insufficiency alone is sufficient to drive atypical behaviour in both mothers and their offspring. Although there are some data to suggest that this phenomenon is relevant to human pregnancy, considerably more work is required.

The Prolactin Family of Hormones as Regulators of Maternal Mood and Behavior

Transition into motherhood involves profound physiological and behavioral adaptations that ensure the healthy development of offspring while maintaining maternal health. Dynamic fluctuations in key hormones during pregnancy and lactation induce these maternal adaptations by acting on neural circuits in the brain. Amongst these hormonal changes, lactogenic hormones (e.g., prolactin and its pregnancy-specific homolog, placental lactogen) are important regulators of these processes, and their receptors are located in key brain regions controlling emotional behaviors and maternal responses. With pregnancy and lactation also being associated with a marked elevation in the risk of developing mood disorders, it is important to understand how hormones are normally regulating mood and behavior during this time. It seems likely that pathological changes in mood could result from aberrant expression of these hormone-induced behavioral responses. Maternal mental health problems during pregnancy and the postpartum period represent a major barrier in developing healthy mother-infant interactions which are crucial for the child's development. In this review, we will examine the role lactogenic hormones play in driving a range of specific maternal behaviors, including motivation, protectiveness, and mother-pup interactions. Understanding how these hormones collectively act in a mother's brain to promote nurturing behaviors toward offspring will ultimately assist in treatment development and contribute to safeguarding a successful pregnancy.

Love and peace across generations: Biobehavioral systems and global partnerships

Children's environments - especially relationships with caregivers - sculpt not only developing brains but also multiple bio-behavioral systems that influence long-term cognitive and socioemotional outcomes, including the ability to empathize with others and interact in prosocial and peaceful ways. This speaks to the importance of investing resources in effective and timely programs that work to enhance early childhood development (ECD) and, by extension, reach communities at-scale. Given the limited resources currently devoted to ECD services, and the devastating impact of COVID-19 on children and communities, there is a clear need to spur government leaders and policymakers to further invest in ECD and related issues including gender and racial equity. This essay offers concrete examples of scholarly paradigms and leadership efforts that focus on child development to build a peaceful, equitable, just, and sustainable world. As scholars and practitioners, we need to continue to design, implement, assess, and revise high-quality child development programs that generate much-needed evidence for policy and programmatic changes. We must also invest in global partnerships to foster the next generation of scholars, practitioners, and advocates dedicated to advance our understanding of the bio-behavioral systems that underlie love, sociality, and peace across generations. Especially where supported by structural interventions, ECD programs can help create more peaceful, just, and socially equitable societies.

Effects of Morphine and Maternal Care on Behaviors and Protein Expression of Male Offspring

Genes and environment interact during development to alter gene expression and behavior. Parental morphine exposure before conception has devastating effects on the offspring. In the present study, we evaluated the role of maternal care in the intergenerational effect of maternal morphine exposure. Female rats received morphine or saline for ten days and were drugfree for another ten days. Thereafter, they were allowed to mate with drug-naïve male rats. When pups were born, they were cross-fostered to assess the contribution of maternal care versus morphine effects on the offspring. Adult male offspring were examined for anxiety-like behavior, spatial memory, and obsessive-compulsive-like behavior. To determine the mechanisms underlying the observed behavioral changes, protein levels of acetylated histone H3, BDNF, Trk-B, NMDA subunits, p-CREB, and 5-HT₃R were measured in the brain. Our results indicate that maternal caregiving is impaired in morphine-abstinent mothers. Interestingly, maternal care behaviors were also affected in drug-naïve mothers that raised offspring of morphine-exposed mothers. In addition, the offspring of morphine abstinent and non-drug dependent mothers, when raised by morphine abstinent mothers, exhibited more anxiety, obsessive-compulsive behaviors and impaired spatial memory. These altered behaviors were associated with alterations in the levels of the above-mentioned proteins. These data illustrate the intergenerational effects of maternal morphine exposure on offspring behaviors. Moreover, exposure to morphine before gestation not only affects maternal care and offspring behavior, but also has negative consequences on behaviors and protein expression in adoptive mothers of affected offspring.

Hypoxic Adaptation of Mitochondrial Metabolism in Rat Cerebellum Decreases in Pregnancy

Function of brain amino acids as neurotransmitters or their precursors implies changes in the amino acid levels and/or metabolism in response to physiological and environmental challenges. Modelling such challenges by pregnancy and/or hypoxia, we characterize the amino acid pool in the rat cerebellum, quantifying the levels and correlations of 15 amino acids and activity of 2-oxoglutarate dehydrogenase complex (OGDHC). The parameters are systemic indicators of metabolism because OGDHC limits the flux through mitochondrial TCA cycle, where amino acids are degraded and their precursors synthesized. Compared to non-pregnant state, pregnancy increases the cerebellar content of glutamate and tryptophan, decreasing interdependence between the quantified components of amino acid metabolism. In response to hypoxia, the dependence of cerebellar amino acid pool on OGDHC and the average levels of arginine, glutamate, lysine, methionine, serine, phenylalanine, and tryptophan increase in non-pregnant rats only. This is accompanied by a higher hypoxic resistance of the non-pregnant vs. pregnant rats, pointing to adaptive significance of the hypoxia-induced changes in the cerebellar amino acid metabolism. These adaptive mechanisms are not effective in the pregnancy-changed metabolic network. Thus, the cerebellar amino acid levels and OGDHC activity provide sensitive markers of the physiology-dependent organization of metabolic network and its stress adaptations.

Neurophysiological and cognitive changes in pregnancy

The hormonal fluctuations in pregnancy drive a wide range of adaptive changes in the maternal brain. These range from specific neurophysiological changes in the patterns of activity of individual neuronal populations, through to complete modification of circuit characteristics leading to fundamental changes in behavior. From a neurologic perspective, the key hormone changes are those of the sex steroids, estradiol and progesterone, secreted first from the ovary and then from the placenta, the adrenal glucocorticoid cortisol, as well as the anterior pituitary peptide hormone prolactin and its pregnancy-specific homolog placental lactogen. All of these hormones are markedly elevated during pregnancy and cross the blood-brain barrier to exert actions on neuronal populations through receptors expressed in specific regions. Many of the hormone-induced changes are in autonomic or homeostatic systems. For example, patterns of oxytocin and prolactin secretion are dramatically altered to support novel physiological functions. Appetite is increased and feedback responses to metabolic hormones such as leptin and insulin are suppressed to promote a positive energy balance. Fundamental physiological systems such as glucose homeostasis and thermoregulation are modified to optimize conditions for fetal development. In addition to these largely autonomic changes, there are also changes in mood, behavior, and higher processes such as cognition. This chapter summarizes the hormonal changes associated with pregnancy and reviews how these changes impact on brain function, drawing on examples from animal research, as well as available information about human pregnancy.