siRNA silencing of estrogen receptor-α expression specifically in medial preoptic area neurons abolishes maternal care in female mice

Stress and parental behaviors

In nearly all mammalian species, newborn pups are weak and vulnerable, relying heavily on care and protection from parents for survival. Thus, developmentally hardwired neural circuits are in place to ensure the timely expression of parental behaviors. Furthermore, several neurochemical systems, including estrogen, oxytocin, and dopamine, facilitate the emergence and expression of parental behaviors. However, stress can adversely affect these systems, impairing parental behaviors. In this review, we will summarize our current knowledge regarding the impact of stress on pup-directed behavior circuits that lead to infant neglect, abuse, and, in extreme cases, killing. We will discuss various stressors that influence parental behaviors at different life stages and how stress induces changes in the neurochemical systems that support parental care, ultimately leading to its poor performance.

Hormonal Actions in the Medial Preoptic Area Governing Parental Behavior: Novel Insights From New Tools

The importance of hormones in mediating a behavioral transition in mammals from a virgin or nonparenting state to parental state was established around 50 years ago. Extensive research has since revealed a highly conserved neural circuit that underlies parental behavior both between sexes and between mammalian species. Within this circuit, hormonal action in the medial preoptic area of the hypothalamus (MPOA) has been shown to be key in timing the onset of parental behavior with the birth of offspring. However, the mechanism underlying how hormones act in the MPOA to facilitate this change in behavior has been unclear. Technical advances in neuroscience, including single cell sequencing, novel transgenic approaches, calcium imaging, and optogenetics, have recently been harnessed to reveal new insights into maternal behavior. This review aims to highlight how the use of these tools has shaped our understanding about which aspects of maternal behavior are regulated by specific hormone activity within the MPOA, how hormone-sensitive MPOA neurons integrate within the wider neural circuit that governs maternal behavior, and how maternal hormones drive changes in MPOA neuronal function during different reproductive states. Finally, we review our current understanding of hormonal modulation of MPOA-mediated paternal behavior in males.

Reproductive function and behaviors: an update on the role of neural estrogen receptors alpha and beta

Infertility is becoming a major public health problem, with increasing frequency due to medical, environmental and societal causes. The increasingly late age of childbearing, growing exposure to endocrine disruptors and other reprotoxic products, and increasing number of medical reproductive dysfunctions (endometriosis, polycystic ovary syndrome, etc.) are among the most common causes. Fertility relies on fine-tuned control of both neuroendocrine function and reproductive behaviors, those are critically regulated by sex steroid hormones. Testosterone and estradiol exert organizational and activational effects throughout life to establish and activate the neural circuits underlying reproductive function. This regulation is mediated through estrogen receptors (ERs) and androgen receptor (AR). Estradiol acts mainly via nuclear estrogen receptors ERα and ERβ. The aim of this review is to summarize the genetic studies that have been undertaken to comprehend the specific contribution of ERα and ERβ in the neural circuits underlying the regulation of the hypothalamic-pituitary-gonadal axis and the expression of reproductive behaviors, including sexual and parental behavior. Particular emphasis will be placed on the neural role of these receptors and the underlying sex differences.

Father's Absence in the Mongolian gerbil (Meriones unguiculatus) is associated with alterations in paternal behavior, T, cort, presence of ERα, and AR in mPOA/ BNST

Testosterone (T), estrogen receptor alpha (ERα), and androgen receptor (AR) play a significant role in the regulation of paternal behavior. We determined the effects of deprivation of paternal care on alterations in paternal behavior, T concentrations in plasma, and the presence of ERα and AR in the medial preoptic area (mPOA), bed nucleus of the stria terminalis (BNST), medial amygdala (MeA), and olfactory bulb (OB), as well as the corticosterone (CORT) concentrations in plasma caused by deprivation of paternal care in the Mongolian gerbil (Meriones unguiculatus). Twenty pairs of gerbils were formed; the pups were deprived of paternal care (DPC) in 10 pairs. In another 10 pairs, the pups received paternal care (PC). Ten males raised in DPC condition and 10 males raised in PC conditions were mated with virgin females. When they became fathers, each DPC male and PC male was subjected to tests of paternal behavior on day three postpartum. Blood samples were obtained to quantify T and CORT concentrations, and the brains were removed for ERα and AR immunohistochemistry analyses. DPC males gave less care to their pups than PC males, and they had significantly lower T concentrations and levels of ERα and AR in the mPOA and BNST than PC males. DPC males also had higher CORT concentrations than PC males. These results suggest that in the Mongolian gerbil father's absence causes a decrease in paternal care in the offspring, which is associated with alterations in the neuroendocrine mechanisms that regulate it.

Activation of lateral preoptic neurons is associated with nest-building in male mice

Nest-building behavior is a widely observed innate behavior. A nest provides animals with a secure environment for parenting, sleep, feeding, reproduction, and temperature maintenance. Since animal infants spend their time in a nest, nest-building behavior has been generally studied as parental behaviors, and the medial preoptic area (MPOA) neurons are known to be involved in parental nest-building. However, nest-building of singly housed male mice has been less examined. Here we show that male mice spent longer time in nest-building at the early to middle dark phase and at the end of the dark phase. These two periods are followed by sleep-rich periods. When a nest was removed and fresh nest material was introduced, both male and female mice built nests at Zeitgeber time (ZT) 6, but not at ZT12. Using Fos-immunostaining combined with double in situ hybridization of Vgat and Vglut2, we found that Vgat- and Vglut2-positive cells of the lateral preoptic area (LPOA) were the only hypothalamic neuron population that exhibited a greater number of activated cells in response to fresh nest material at ZT6, compared to being naturally awake at ZT12. Fos-positive LPOA neurons were negative for estrogen receptor 1 (Esr1). Both Vgat-positive and Vglut2-positive neurons in both the LPOA and MPOA were activated at pup retrieval by male mice. Our findings suggest the possibility that GABAergic and glutamatergic neurons in the LPOA are associated with nest-building behavior in male mice.

Parental brain through time: The origin and development of the neural circuit of mammalian parenting

This review consolidates current knowledge on mammalian parental care, focusing on its neural mechanisms, evolutionary origins, and derivatives. Neurobiological studies have identified specific neurons in the medial preoptic area as crucial for parental care. Unexpectedly, these neurons are characterized by the expression of molecules signaling satiety, such as calcitonin receptor and BRS3, and overlap with neurons involved in the reproductive behaviors of males but not females. A synthesis of comparative ecology and paleontology suggests an evolutionary scenario for mammalian parental care, possibly stemming from male-biased guarding of offspring in basal vertebrates. The terrestrial transition of tetrapods led to prolonged egg retention in females and the emergence of amniotes, skewing care toward females. The nocturnal adaptation of Mesozoic mammalian ancestors reinforced maternal care for lactation and thermal regulation via endothermy, potentially introducing metabolic gate control in parenting neurons. The established maternal care may have served as the precursor for paternal and cooperative care in mammals and also fostered the development of group living, which may have further contributed to the emergence of empathy and altruism. These evolution-informed working hypotheses require empirical validation, yet they offer promising avenues to investigate the neural underpinnings of mammalian social behaviors.

Distinct roles of amylin and oxytocin signaling in intrafamilial social behaviors at the medial preoptic area of common marmosets

Calcitonin receptor (Calcr) and its brain ligand amylin in the medial preoptic area (MPOA) are found to be critically involved in infant care and social contact behaviors in mice. In primates, however, the evidence is limited to an excitotoxic lesion study of the Calcr-expressing MPOA subregion (cMPOA) in a family-living primate species, the common marmoset. The present study utilized pharmacological manipulations of the cMPOA and shows that reversible inactivation of the cMPOA abolishes infant-care behaviors in sibling marmosets without affecting other social or non-social behaviors. Amylin-expressing neurons in the marmoset MPOA are distributed in the vicinity of oxytocin neurons in the anterior paraventricular nucleus of the hypothalamus. While amylin infusion facilitates infant carrying selectively, an oxytocin's inverse agonist, atosiban, reduces physical contact with non-infant family members without grossly affecting infant care. These data suggest that the amylin and oxytocin signaling mediate intrafamilial social interactions in a complementary manner in marmosets.

Sex differences in androgen receptor, estrogen receptor alpha, and c-Fos co-expression with corticotropin releasing factor expressing neurons in restrained adult mice

Gonadal hormone actions through androgen receptor (AR) and estrogen receptor alpha (ERα) regulate sex differences in hypothalamic-pituitary-adrenal (HPA) axis responsivity and stress-related behaviors. Here we tested whether corticotropin releasing factor (CRF) expressing neurons, which are widely known to regulate neuroendocrine and behavioral stress responses, co-express AR and ERα as a potential mechanism for gonadal hormone regulation of these responses. Using Crh-IRES-Cre::Ai9 reporter mice we report high co-localization of AR in CRF neurons within the medial preoptic area (MPOA), bed nucleus of the stria terminalis (BST), medial amygdala (MeA), and ventromedial hypothalamus (VMH), moderate levels within the central amygdala (CeA) and low levels in the paraventricular hypothalamus (PVN). Sex differences in CRF/AR co-expression were found in the principal nucleus of the BST (BSTmpl), CeA, MeA, and VMH (males>females). CRF co-localization with ERα was generally lower relative to AR co-localization. However, high co-expression was found within the MPOA, AVPV, and VMH, with moderate co-expression in the arcuate nucleus (ARC), BST, and MeA and low levels in the PVN and CeA. Sex differences in CRF/ERα co-localization were found in the BSTmpl and PVN (males>females). Finally, we assessed neural activation of CRF neurons in restraint-stressed mice and found greater CRF/c-Fos co-expression in females in the BSTmpl and periaqueductal gray, while co-expression was higher in males within the ARC and dorsal CA1. Given the known role of CRF in regulating behavioral stress responses and the HPA axis, AR/ERα co-expression and sex-specific activation of CRF cell groups indicate potential mechanisms for modulating sex differences in these functions.

Imprinted genes and the manipulation of parenting in mammals

Genomic imprinting refers to the parent-of-origin expression of genes, which originates from epigenetic events in the mammalian germ line. The evolution of imprinting may reflect a conflict over resource allocation early in life, with silencing of paternal genes in offspring soliciting increased maternal provision and silencing of maternal genes limiting demands on the mother. Parental caregiving has been identified as an area of potential conflict, with several imprinted genes serendipitously found to directly influence the quality of maternal care. Recent systems biology approaches, based on single-cell RNA sequencing data, support a more deliberate relationship, which is reinforced by the finding that imprinted genes expressed in the offspring influence the quality of maternal caregiving. These bidirectional, reiterative relationships between parents and their offspring are critical both for short-term survival and for lifelong wellbeing, with clear implications for human health.

Hypothalamic control of innate social behaviors

Sexual, parental, and aggressive behaviors are central to the reproductive success of individuals and species survival and thus are supported by hardwired neural circuits. The reproductive behavior control column (RBCC), which comprises the medial preoptic nucleus (MPN), the ventrolateral part of the ventromedial hypothalamus (VMHvl), and the ventral premammillary nucleus (PMv), is essential for all social behaviors. The RBCC integrates diverse hormonal and metabolic cues and adjusts an animal's physical activity, hence the chance of social encounters. The RBCC further engages the mesolimbic dopamine system to maintain social interest and reinforces cues and actions that are time-locked with social behaviors. We propose that the RBCC and brainstem form a dual-control system for generating moment-to-moment social actions. This Review summarizes recent progress regarding the identities of RBCC cells and their pathways that drive different aspects of social behaviors.

Hormone-mediated neural remodeling orchestrates parenting onset during pregnancy

During pregnancy, physiological adaptations prepare the female body for the challenges of motherhood. Becoming a parent also requires behavioral adaptations. Such adaptations can occur as early as during pregnancy, but how pregnancy hormones remodel parenting circuits to instruct preparatory behavioral changes remains unknown. We found that action of estradiol and progesterone on galanin (Gal)-expressing neurons in the mouse medial preoptic area (MPOA) is critical for pregnancy-induced parental behavior. Whereas estradiol silences MPOAGal neurons and paradoxically increases their excitability, progesterone permanently rewires this circuit node by promoting dendritic spine formation and recruitment of excitatory synaptic inputs. This MPOAGal-specific neural remodeling sparsens population activity in vivo and results in persistently stronger, more selective responses to pup stimuli. Pregnancy hormones thus remodel parenting circuits in anticipation of future behavioral need.

Modulation of fetoplacental growth, development and reproductive function by endocrine disrupters

Maternal endocrine homeostasis is vital to a successful pregnancy, regulated by several hormones such as human chorionic gonadotropin, estrogen, leptin, glucocorticoid, insulin, prostaglandin, and others. Endocrine stress during pregnancy can modulate nutrient availability from mother to fetus, alter fetoplacental growth and reproductive functions. Endocrine disrupters such as bisphenols (BPs) and phthalates are exposed in our daily life's highest volume. Therefore, they are extensively scrutinized for their effects on metabolism, steroidogenesis, insulin signaling, and inflammation involving obesity, diabetes, and the reproductive system. BPs have their structural similarity to 17-β estradiol and their ability to bind as an agonist or antagonist to estrogen receptors to elicit an adverse response to the function of the endocrine and reproductive system. While adults can negate the adverse effects of these endocrine-disrupting chemicals (EDCs), fetuses do not equip themselves with enzymatic machinery to catabolize their conjugates. Therefore, EDC exposure makes the fetoplacental developmental window vulnerable to programming in utero. On the one hand prenatal BPs and phthalates exposure can impair the structure and function of the ovary and uterus, resulting in placental vascular defects, inappropriate placental expression of angiogenic growth factors due to altered hypothalamic response, expression of nutrient transporters, and epigenetic changes associated with maternal endocrine stress. On the other, their exposure during pregnancy can affect the offspring's metabolic, endocrine and reproductive functions by altering fetoplacental programming. This review highlights the latest development in maternal metabolic and endocrine modulations from exposure to estrogenic mimic chemicals on subcellular and transgenerational changes in placental development and its effects on fetal growth, size, and metabolic & reproductive functions.

The transition to motherhood: linking hormones, brain and behaviour

We are witnessing a stark increase in scientific interest in the neurobiological processes associated with pregnancy and maternity. Convergent evidence suggests that around the time of labour, first-time mothers experience a specific pattern of neuroanatomical changes that are associated with maternal behaviour. Here we provide an overview of the human neurobiological adaptations of motherhood, focusing on the interplay between pregnancy-related steroid and peptide hormones, and neuroplasticity in the brain. We discuss which brain plasticity mechanisms might underlie the structural changes detected by MRI, which hormonal systems are likely to contribute to such neuroanatomical changes and how these brain mechanisms may be linked to maternal behaviour. This Review offers an overarching framework that can serve as a roadmap for future investigations.

Sex-dependent control of pheromones on social organization within groups of wild house mice

Dominance hierarchy is a fundamental social phenomenon in a wide range of mammalian species, critically affecting fitness and health. Here, we investigate the role of pheromone signals in the control of social hierarchies and individual personalities within groups of wild mice. For this purpose, we combine high-throughput behavioral phenotyping with computational tools in freely interacting groups of wild house mice, males and females, in an automated, semi-natural system. We show that wild mice form dominance hierarchies in both sexes but use sex-specific strategies, displaying distinct male-typical and female-typical behavioral personalities that were also associated with social ranking. Genetic disabling of VNO-mediated pheromone detection generated opposite behavioral effects within groups, enhancing social interactions in males and reducing them in females. Behavioral personalities in the mutated mice displayed mixtures of male-typical and female-typical behaviors, thus blurring sex differences. In addition, rank-associated personalities were abolished despite the fact that both sexes of mutant mice formed stable hierarchies. These findings suggest that group organization is governed by pheromone-mediated sex-specific neural circuits and pave the way to investigate the mechanisms underlying sexual dimorphism in dominance hierarchies under naturalistic settings.

Neural activation associated with maternal and aversive interactions with pups in the Mongolian gerbil (Meriones unguiculatus)

According to approach-avoidance model, virgin female laboratory rats display maternal behaviour when the tendency to approach and interact with the pup is stronger than avoiding it. A positive neural mechanism that includes the medial preoptic area (mPOA)/bed nucleus of the stria terminalis (BNST) and a negative mechanism that involves the anterior hypothalamic nucleus (AHN)/ventromedial nucleus (VMN)/ periaqueductal grey (PAG) underlie to these behaviours. Unlike virgin rats, which avoid the pups, virgin females Mongolian gerbils (Meriones unguiculatus) can be immediately either maternal or aggressive with the pups. Furthermore, the Mongolian gerbil is monogamous and biparental species. Despite these difference, we hypothesised that maternal and aggressive interaction with the pups could activate mPOA/BNST and AHN/VMH/PAG, respectively, and that maternal response could be associated with high concentrations of estradiol (E₂). Twenty virgin maternal females and 20 aggressive toward the pups were selected. Ten maternal females interacted with the pups (MAT-pups) and 10 with candy (MAT-candy). Of the 20 aggressive females, 10 interacted with the pups (AGG-pups) and 10 with candy (AGG-candy). Immediately after the test, blood samples were taken to quantify E₂. The brains were dissected for c-Fos immunohistochemistry. MAT-pups females had significantly higher activation in mPOA/BNST than MAT-candy females, while AGG-pups showed significant activation in AHN/VMH/PAG compared with AGG-candy females. The maternal response was associated with high concentrations of E₂. These results suggested a positive and a negative mechanism in the regulation of maternal behaviour in the Mongolian gerbil, and that the immediate maternal response could be due to high E₂ concentrations.

Urocortin-3 neurons in the perifornical area are critical mediators of chronic stress on female infant-directed behavior

Infant avoidance and aggression are promoted by activation of the Urocortin-3 expressing neurons of the perifornical area of hypothalamus (PeFA^Ucn3) in male and female mice. PeFA^Ucn3 neurons have been implicated in stress, and stress is known to reduce maternal behavior. We asked how chronic restraint stress (CRS) affects infant-directed behavior in virgin and lactating females and what role PeFA^Ucn3 neurons play in this process. Here we show that infant-directed behavior increases activity in the PeFA^Ucn3 neurons in virgin and lactating females. Chemogenetic inhibition of PeFA^Ucn3 neurons facilitates pup retrieval in virgin females. CRS reduces pup retrieval in virgin females and increases activity of PeFA^Ucn3 neurons, while CRS does not affect maternal behavior in lactating females. Inhibition of PeFA^Ucn3 neurons blocks stress-induced deficits in pup-directed behavior in virgin females. Together, these data illustrate the critical role for PeFA^Ucn3 neuronal activity in mediating the impact of chronic stress on female infant-directed behavior.

Neurobiology of Maternal Behavior in Nonhuman Mammals: Acceptance, Recognition, Motivation, and Rejection

Among the different species of mammals, the expression of maternal behavior varies considerably, although the end points of nurturance and protection are the same. Females may display passive or active responses of acceptance, recognition, rejection/fear, or motivation to care for the offspring. Each type of response may indicate different levels of neural activation. Different natural stimuli can trigger the expression of maternal and paternal behavior in both pregnant or virgin females and males, such as hormone priming during pregnancy, vagino-cervical stimulation during parturition, mating, exposure to pups, previous experience, or environmental enrichment. Herein, we discuss how the olfactory pathways and the interconnections of the medial preoptic area (mPOA) with structures such as nucleus accumbens, ventral tegmental area, amygdala, and bed nucleus of stria terminalis mediate maternal behavior. We also discuss how the triggering stimuli activate oxytocin, vasopressin, dopamine, galanin, and opioids in neurocircuitries that mediate acceptance, recognition, maternal motivation, and rejection/fear.

Exposure to environmentally relevant doses of plasticizers alters maternal behavior and related neuroendocrine processes in primiparous and multiparous female mice

Phthalates are organic pollutants frequently detected in the environment. The effects of these substances on male reproduction have been extensively studied but their potential impact on female reproductive behaviors in particular at environmental doses still remains to be documented. In the present study, we examined the effects of chronic exposure to di (2-ethylhexyl) phthalate (DEHP) alone at 5 or 50 μg/kg/d, or in an environmental phthalate mixture on maternal behavior of lactating female mice after a first (primiparous) and a second gestation (multiparous). Exposure of DEHP alone or in a phthalate mixture reduced pup-directed behaviors, increased self-care and forced nursing behaviors and altered nest quality for both primiparous and multiparous dams. In pup-retrieval test, primiparous and multiparous dams exposed to DEHP alone or in a phthalate mixture retrieved their pups more rapidly, probably due to a higher emission of ultrasonic vocalizations by the pups. At lactational day 2 following the third and last gestational period, the neural circuitry of maternal behavior was analyzed. A lower number of oxytocin-immunoreactive neurons in the paraventricular and anterior commissural nuclei was found in dams exposed to DEHP alone or in a phthalate mixture, while no changes were observed in the number of arginine-vasopressin immunoreactive cells. In the medial preoptic area, exposure to DEHP alone or in a phthalate mixture reduced ERα-immunoreactive cell number. Dendritic spine density assessed for DEHP at 5 μg/kg/d was also reduced. Thus, exposure to DEHP alone or in a phthalate mixture altered maternal behavior probably through a neuroendocrine mode of action involving oxytocin and estrogen through ERα, key pathways necessary for neuroplasticity and behavioral processing.

A calcitonin receptor-expressing subregion of the medial preoptic area is involved in alloparental tolerance in common marmosets

Like humans, common marmoset monkeys utilize family cooperation for infant care, but the neural mechanisms underlying primate parental behaviors remain largely unknown. We investigated infant care behaviors of captive marmosets in family settings and caregiver-infant dyadic situations. Marmoset caregivers exhibited individual variations in parenting styles, comprised of sensitivity and tolerance toward infants, consistently across infants, social contexts and multiple births. Seeking the neural basis of these parenting styles, we demonstrated that the calcitonin receptor-expressing neurons in the marmoset medial preoptic area (MPOA) were transcriptionally activated during infant care, as in laboratory mice. Further, site-specific neurotoxic lesions of this MPOA subregion, termed the cMPOA, significantly reduced alloparental tolerance and total infant carrying, while sparing general health and other social or nonsocial behaviors. These results suggest that the molecularly-defined neural site cMPOA is responsible for mammalian parenting, thus provide an invaluable model to study the neural basis of parenting styles in primates.

Hypomorphism of a Novel Long ERα Isoform Causes Severe Reproductive Dysfunctions in Female Mice

Estrogen receptor alpha (ERα)-mediated estrogen signaling plays a pivotal role in both reproductive and nonreproductive functions. Transcriptional regulation of the ERα gene is highly complex, with multiple transcript variants being differentially produced across the tissues. However, tissue-specific variation and physiological specificity of the ERα variants are not yet fully understood. In an attempt to generate a Cre-dependently restorable ERα-null mouse for functional genetic studies, we unexpectedly produced ERα hypomorphic mice with biased downregulation of a previously unappreciated long ERα isoform that is enriched in the female reproductive organs (uterus and ovaries) and the pituitary but minimally expressed in the brain. Female homozygous mutant mice were capable of pregnancy but displayed irregular estrus cycle and rarely kept newborn pups alive. No significant morphological and pathological changes in reproductive system or disruption of body weight homeostasis were seen in female homozygous mutant mice. Collectively, our results define a tissue-specific enriched long ERα isoform and its preferential role in female reproductive function rather than body weight homeostasis.

Social Network Plasticity of Mice Parental Behavior

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

Microplastics and Their Impact on Reproduction-Can we Learn From the C. elegans Model?

Biologically active environmental pollutants have significant impact on ecosystems, wildlife, and human health. Microplastic (MP) and nanoplastic (NP) particles are pollutants that are present in the terrestrial and aquatic ecosystems at virtually every level of the food chain. Moreover, recently, airborne microplastic particles have been shown to reach and potentially damage respiratory systems. Microplastics and nanoplastics have been shown to cause increased oxidative stress, inflammation, altered metabolism leading to cellular damage, which ultimately affects tissue and organismal homeostasis in numerous animal species and human cells. However, the full impact of these plastic particles on living organisms is not completely understood. The ability of MPs/NPs to carry contaminants, toxic chemicals, pesticides, and bioactive compounds, such as endocrine disrupting chemicals, present an additional risk to animal and human health. This review will discusses the current knowledge on pathways by which microplastic and nanoplastic particles impact reproduction and reproductive behaviors from the level of the whole organism down to plastics-induced cellular defects, while also identifying gaps in current knowledge regarding mechanisms of action. Furthermore, we suggest that the nematode Caenorhabditis elegans provides an advantageous high-throughput model system for determining the effect of plastic particles on animal reproduction, using reproductive behavioral end points and cellular readouts.

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.

Immunofluorescent Verification of Silencing Estrogen Receptor α with siRNA in the Intact Rodent Brain

The ability to silence the expression of gene products in a chemically, spatially, and temporally specific manner in the brains of animals has enabled key breakthroughs in the field of behavioral neuroscience. Using this technique, estrogen receptor alpha (ERα) has been specifically implicated in a multitude of behaviors in mice, including sexual, aggressive, locomotor, and maternal behaviors, in a variety of brain regions, including the medial preoptic area, ventromedial hypothalamus, and amygdala. In this chapter, we describe the techniques involved in the generation of the small hairpin RNAs (shRNAs) specifically designed to silence ERα, the construction of the adeno-associated viral (AAV) vector for delivery of the shRNA, the procedures to confirm the silencing of ERα (in vitro and in vivo) and in vivo delivery of the shRNAs to the brains of animals.

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.

Neural Contributions of the Hypothalamus to Parental Behaviour

Parental behaviour is a comprehensive set of neural responses to social cues. The neural circuits that govern parental behaviour reside in several putative nuclei in the brain. Melanin concentrating hormone (MCH), a neuromodulator that integrates physiological functions, has been confirmed to be involved in parental behaviour, particularly in crouching behaviour during nursing. Abolishing MCH neurons in innate MCH knockout males promotes infanticide in virgin male mice. To understand the mechanism and function of neural networks underlying parental care and aggression against pups, it is essential to understand the basic organisation and function of the involved nuclei. This review presents newly discovered aspects of neural circuits within the hypothalamus that regulate parental behaviours.

The social network: Neural control of sex differences in reproductive behaviors, motivation, and response to social isolation

Social animal species present a vast repertoire of social interactions when encountering conspecifics. Reproduction-related behaviors, such as mating, parental care, and aggression, are some of the most rewarding types of social interactions and are also the most sexually dimorphic ones. This review focuses on rodent species and summarizes recent advances in neuroscience research that link sexually dimorphic reproductive behaviors to sexual dimorphism in their underlying neuronal circuits. Specifically, we present a few possible mechanisms governing sexually-dimorphic behaviors, by hypothalamic and reward-related brain regions. Sex differences in the neural response to social isolation in adulthood are also discussed, as well as future directions for comparative studies with naturally solitary species.

Calcitonin receptor signaling in the medial preoptic area enables risk-taking maternal care

Maternal mammals exhibit heightened motivation to care for offspring, but the underlying neuromolecular mechanisms have yet to be clarified. Here, we report that the calcitonin receptor (Calcr) and its ligand amylin are expressed in distinct neuronal populations in the medial preoptic area (MPOA) and are upregulated in mothers. Calcr+ MPOA neurons activated by parental care project to somatomotor and monoaminergic brainstem nuclei. Retrograde monosynaptic tracing reveals that significant modification of afferents to Calcr+ neurons occurs in mothers. Knockdown of either Calcr or amylin gene expression hampers risk-taking maternal care, and specific silencing of Calcr+ MPOA neurons inhibits nurturing behaviors, while pharmacogenetic activation prevents infanticide in virgin males. These data indicate that Calcr+ MPOA neurons are required for both maternal and allomaternal nurturing behaviors and that upregulation of amylin-Calcr signaling in the MPOA at least partially mediates risk-taking maternal care, possibly via modified connectomics of Calcr+ neurons postpartum.

Cellular Composition of the Preoptic Area Regulating Sleep, Parental, and Sexual Behavior

The preoptic area (POA) has long been recognized as a sleep center, first proposed by von Economo. The POA, especially the medial POA (MPOA), is also involved in the regulation of various innate functions such as sexual and parental behaviors. Consistent with its many roles, the MPOA is composed of subregions that are identified by different gene and protein expressions. This review addresses the current understanding of the molecular and cellular architecture of POA neurons in relation to sleep and reproductive behavior. Optogenetic and pharmacogenetic studies have revealed a diverse group of neurons within the POA that exhibit different neural activity patterns depending on vigilance states and whose activity can enhance or suppress wake, non-rapid eye movement (NREM) sleep, or rapid eye movement (REM) sleep. These sleep-regulating neurons are not restricted to the ventrolateral POA (VLPO) region but are widespread in the lateral MPOA and LPOA as well. Neurons expressing galanin also express gonadal steroid receptors and regulate motivational aspects of reproductive behaviors. Moxd1, a novel marker of sexually dimorphic nuclei (SDN), visualizes the SDN of the POA (SDN-POA). The role of the POA in sleep and other innate behaviors has been addressed separately; more integrated observation will be necessary to obtain physiologically relevant insight that penetrates the different dimensions of animal behavior.

Estrogen-sensitive medial preoptic area neurons coordinate torpor in mice

Homeotherms maintain a stable internal body temperature despite changing environments. During energy deficiency, some species can cease to defend their body temperature and enter a hypothermic and hypometabolic state known as torpor. Recent advances have revealed the medial preoptic area (MPA) as a key site for the regulation of torpor in mice. The MPA is estrogen-sensitive and estrogens also have potent effects on both temperature and metabolism. Here, we demonstrate that estrogen-sensitive neurons in the MPA can coordinate hypothermia and hypometabolism in mice. Selectively activating estrogen-sensitive MPA neurons was sufficient to drive a coordinated depression of metabolic rate and body temperature similar to torpor, as measured by body temperature, physical activity, indirect calorimetry, heart rate, and brain activity. Inducing torpor with a prolonged fast revealed larger and more variable calcium transients from estrogen-sensitive MPA neurons during bouts of hypothermia. Finally, whereas selective ablation of estrogen-sensitive MPA neurons demonstrated that these neurons are required for the full expression of fasting-induced torpor in both female and male mice, their effects on thermoregulation and torpor bout initiation exhibit differences across sex. Together, these findings suggest a role for estrogen-sensitive MPA neurons in directing the thermoregulatory and metabolic responses to energy deficiency.

Evolutionary-adaptive and nonadaptive causes of infant attack/desertion in mammals: Toward a systematic classification of child maltreatment

Behaviors comparable to human child maltreatment are observed widely among mammals, in which parental care is mandatory for offspring survival. This article first reviews the recent findings on the neurobiological mechanisms for nurturing (infant caregiving) behaviors in mammals. Then the major causes of attack/desertion toward infants (conspecific young) in nonhuman mammals are classified into five categories. Three of the categories are 'adaptive' in terms of reproductive fitness: (i) attack/desertion toward non-offspring; (ii) attack/desertion toward biological offspring with low reproductive value; and (iii) attack/desertion toward biological offspring under unfavorable environments. The other two are nonadaptive failures of nurturing motivation, induced by: (iv) caregivers' inexperience; or (v) dysfunction in caregivers' brain mechanisms required for nurturing behavior. The proposed framework covering both adaptive and nonadaptive factors comprehensively classifies the varieties of mammalian infant maltreatment cases and will support the future development of tailored preventive measures for each human case. Also included are remarks that are relevant to interpretation of available animal data to humans: (1) any kind of child abuse/neglect is not justified in modern human societies, even if it is widely observed and regarded as adaptive in nonhuman animals from the viewpoint of evolutionary biology; (2) group-level characteristics cannot be generalized to individuals; and (3) risk factors are neither deterministic nor irreversible.

Sexual experience with a known male modulates c-Fos expression in response to mating and male pheromone exposure in female mice

Sexually naïve female mice are not sexually receptive in their first mating opportunity. Four to five sexual encounters are needed to display high sexual receptivity as assessed by the lordosis reflex. The neuronal changes induced by sexual experience are not well understood. In this study, we evaluated if repeated sexual stimulation with the same male was associated with an increase in the neuronal activity evaluated by c-Fos expression in brain structures associated with the control of sexual behavior such as the accessory olfactory bulb (AOB), ventromedial hypothalamus (VMH), and the medial preoptic area (MPOA). Ovariectomized female mice were randomly distributed into three groups: sexually naïve (SN), with no prior sexual stimulation; sexually inexperienced (SI), with one prior mating session; and sexually experienced (SE), with six prior mating sessions. Females were primed with estradiol benzoate and progesterone once a week for 7 weeks. Neuronal activation in response to mating or soiled bedding was evaluated in the 7^th week. Each group was subdivided into three subgroups: clean (exposure to clean bedding), male bedding (exposure to sawdust soiled with secretions from a male), or mating. Each female mated with her assigned male; in the exposure subgroup, soiled bedding was obtained from the male with whom she mated. Neuronal activity data showed that SE females had a higher c-Fos response in the VMH when they mated in comparison to females exposed to clean bedding. SI females that mated had a decrease c-Fos expression in the glomerular cell layer of the AOB, compared to females exposed to male bedding. The mitral cell layer showed a higher c-Fos response in SI females that mated in comparison to those exposed to male bedding. Comparisons between groups presented with the same stimulus indicate that SI females exposed to male bedding showed a decrease in c-Fos response in the mitral cell layer in comparison to SE and SN females. Correlation analysis demonstrated that the lordosis quotient from the last mating test correlated positively with the number of c-Fos-positive cells in the mitral cell layer in SE and SI groups. A similar correlation was found in the MPOA in SI females. Prior mating in female mice is required to increase sexual receptivity. Changes in the neuronal activity in the AOB and VMH may be involved in the neuronal plasticity induced by repeated sexual stimulation.

The role of the estrogen receptor-α gene, Esr1, in maternal-like behavior in juvenile female and male rats

The estrogen receptor-alpha (ER-α) is an important ligand activated transcription factor that works to control gene transcription in many species. Previous studies have shown estrogen to be an important hormone in the regulation of maternal behavior. Like adult female rats, both male and female juvenile rats exhibit increased level of maternal-like behavior when exposed to pups. The aim of this study was to determine whether ER-α is critical for the expression of maternal-like behavior in juvenile male and female rats. ER-α knock-out and wildtype (WT) juvenile male and female rats were generated and tested for maternal behaviors. Latencies to display maternal-like behaviors that included retrieval, grouping and crouching responses, revealed no genotype differences between KO and WT subjects. Male juvenile rats exhibited slightly shorter latencies than WT juvenile female rats indicating a sex difference in the latency to display these responses. Additionally, ER-α KO females exhibited a delay in onset of vaginal opening compared to WT females, indicating a role for ER-α in sexual maturation. The behavioral findings indicate that ER-α is not obligatory for the expression of full maternal-like behavior in male and female juvenile rats. Understanding this neurobiological system will help to elucidate the developmental involvement of the endocrine and brain networks in the regulation of maternal behaviors in mammals.

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.

A Hypothalamic Midbrain Pathway Essential for Driving Maternal Behaviors

Maternal behaviors are essential for the survival of the young. Previous studies implicated the medial preoptic area (MPOA) as an important region for maternal behaviors, but details of the maternal circuit remain incompletely understood. Here we identify estrogen receptor alpha (Esr1)-expressing cells in the MPOA as key mediators of pup approach and retrieval. Reversible inactivation of MPOA^Esr1+ cells impairs those behaviors, whereas optogenetic activation induces immediate pup retrieval. In vivo recordings demonstrate preferential activation of MPOA^Esr1+ cells during maternal behaviors and changes in MPOA cell responses across reproductive states. Furthermore, channelrhodopsin-assisted circuit mapping reveals a strong inhibitory projection from MPOA^Esr1+ cells to ventral tegmental area (VTA) non-dopaminergic cells. Pathway-specific manipulations reveal that this projection is essential for driving pup approach and retrieval and that VTA dopaminergic cells are reliably activated during those behaviors. Altogether, this study provides new insight into the neural circuit that generates maternal behaviors.

Experience-dependent mechanisms in the regulation of parental care

Maternal behavior is a defining characteristic of mammals, which is regulated by a core, conserved neural circuit. However, mothering behavior is not always a default response to infant conspecifics. For example, initial fearful, fragmented or aggressive responses toward infants in laboratory rats and mice can give way to highly motivated and organized caregiving behaviors following appropriate hormone exposure or repeated experience with infants. Therefore hormonal and/or experiential factors must be involved in determining the extent to which infants access central approach and avoidance neural systems. In this review we describe evidence supporting the idea that infant conspecifics are capable of activating distinct neural pathways to elicit avoidant, aggressive and parental responses from adult rodents. Additionally, we discuss the hypothesis that alterations in transcriptional regulation within the medial preoptic area of the hypothalamus may be a key mechanism of neural plasticity involved in programming the differential sensitivity of these neural pathways.

Tinbergen's challenge for the neuroscience of behavior

Nobel laureate Nikolaas Tinbergen provided clear criteria for declaring a neuroscience problem solved, criteria which despite the passage of more than 50 years and vastly expanded neuroscience tool kits remain applicable today. Tinbergen said for neuroscientists to claim that a behavior is understood, they must correspondingly understand its (i) development and its (ii) mechanisms and its (iii) function and its (iv) evolution. Now, all four of these domains represent hotbeds of current experimental work, each using arrays of new techniques which overlap only partly. Thus, as new methodologies come online, from single-nerve-cell RNA sequencing, for example, to smart FISH, large-scale calcium imaging from cortex and deep brain structures, computational ethology, and so on, one person, however smart, cannot master everything. Our response to the likely “fracturing” of neuroscience recognizes the value of ever larger consortia. This response suggests new kinds of problems for (i) funding and (ii) the fair distribution of credit, especially for younger scientists.

Estrogen receptor-alpha knockouts and maternal memory in nulliparous rats

The current study investigated the role of estrogen receptor alpha (Esr1) in maternal memory in rats, comparing the induction and retention responses of Esr1 knockout (KO) and wild type (WT) nulliparous rats towards foster pups. Thirty days after completion of induction testing, subjects were tested for the retention of maternal care in their home cage and then for maternal behaviors in a novel cage. Both WT and Esr1 KO females displayed similar latencies to respond to foster young during the initial induction testing. Likewise, reinduction latencies to display full maternal responsiveness were similar in the Esr1 KO and WT groups during maternal memory testing in the home cage. However, in the novel cage testing WT subjects displayed modest modifications in maternal care. WT females had shorter latencies to first retrieve and mouth a test pup. These findings suggest that while Esr1 does not appear to affect the establishment of maternal care or the display of maternal memory, it may modulate aspects of pup-directed behaviors associated with the reinduction of maternal care in female rats.

Offspring genetic effects on maternal care

Parental care is found widely across animal taxa and is manifest in a range of behaviours from basic provisioning in cockroaches to highly complex behaviours seen in mammals. The evolution of parental care is viewed as the outcome of an evolutionary cost/benefit trade-off between investing in current and future offspring, leading to the selection of traits in offspring that influence parental behaviour. Thus, level and quality of parental care are affected by both parental and offspring genetic differences that directly and indirectly influence parental care behaviour. While significant research effort has gone into understanding how parental genomes affect parental, and mostly maternal, behaviour, few studies have investigated how offspring genomes affect parental care. In this review, we bring together recent findings across different fields focussing on the mechanism and genetics of offspring effects on maternal care in mammals.

Neuroendocrinology and Adaptive Physiology of Maternal Care

Parental care is critical for offspring survival in many species. In mammals, parental care is primarily provided through maternal care, due to obligate pregnancy and lactation constraints, although some species also show paternal and alloparental care. These behaviors are driven by specialized neural circuits that receive sensory, cortical, and hormonal input to generate a coordinated and timely change in behavior, and sustain that behavior through activation of reward pathways. Importantly, the hormonal changes associated with pregnancy and lactation also act to coordinate a broad range of physiological changes to support the mother and enable her to adapt to the demands of these states. This chapter will review the neural pathways that regulate maternal behavior, the hormonal changes that occur during pregnancy and lactation, and how these two facets merge together to promote both young-directed maternal responses (including nursing and grooming) and young-related responses (including maternal aggression and other physiological adaptions to support the development of and caring for young). We conclude by examining how experimental animal work has translated into knowledge of human parenting, particularly in regards to maternal mental health issues.

The psychoneuroimmunology of pregnancy

Pregnancy is associated with a number of significant changes in maternal physiology. Perhaps one of the more notable changes is the significant alteration in immune function that occurs during pregnancy. This change in immune function is necessary to support a successful pregnancy, but also creates a unique period of life during which a female is susceptible to disease and, as we'll speculate here, may also contribute to mental health disorders associated with pregnancy and the postpartum period. Here, we review the known changes in peripheral immune function that occur during pregnancy and the postpartum period, while highlighting the impact of hormones during these times on immune function, brain or neural function, as well as behavior. We also discuss the known and possible impact of pregnancy-induced immune changes on neural function during this time and briefly discuss how these changes might be a risk factor for perinatal anxiety or mood disorders.

The Role of Placental Hormones in Mediating Maternal Adaptations to Support Pregnancy and Lactation

During pregnancy, the mother must adapt her body systems to support nutrient and oxygen supply for growth of the baby in utero and during the subsequent lactation. These include changes in the cardiovascular, pulmonary, immune and metabolic systems of the mother. Failure to appropriately adjust maternal physiology to the pregnant state may result in pregnancy complications, including gestational diabetes and abnormal birth weight, which can further lead to a range of medically significant complications for the mother and baby. The placenta, which forms the functional interface separating the maternal and fetal circulations, is important for mediating adaptations in maternal physiology. It secretes a plethora of hormones into the maternal circulation which modulate her physiology and transfers the oxygen and nutrients available to the fetus for growth. Among these placental hormones, the prolactin-growth hormone family, steroids and neuropeptides play critical roles in driving maternal physiological adaptations during pregnancy. This review examines the changes that occur in maternal physiology in response to pregnancy and the significance of placental hormone production in mediating such changes.

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

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

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.

In Utero and Postnatal Propylthiouracil-Induced Mild Hypothyroidism Impairs Maternal Behavior in Mice

Thyroid hormones (THs) play crucial roles in general and brain development. Even if the hypothyroidism is mild, it may alter brain function, resulting in irreversible behavioral alterations. Although various behavioral analyses have been conducted, the effects of propylthiouracil (PTU) treatment during in utero and postnatal periods on maternal behavior have not yet been studied. The present study examined in mice whether THs insufficiency during development induce behavioral changes. Pregnant C57BL/6j mice were divided into three groups, and each group was administered different dosages of PTU (0, 5, or 50 ppm) in drinking water during in utero and postnatal periods (from gestational day 14 to postnatal day 21). First, locomotor activity and cognitive function were assessed in the offspring at 10 weeks. Next, female offspring were mated with normal mice and they and their offspring were used to assess several aspects of maternal behavior (identifying first pup, returning all pups to nest, time spent nursing, and licking pups). As expected, locomotor and cognitive functions in these mice were disrupted in a PTU dose-dependent manner. On postpartum day 2, dams who had been exposed 50 ppm PTU during in utero and postnatal periods displayed a significantly longer time identifying the first pup and returning all three pups back to the nest, less time nursing, and decreased licking behavior. The decrease in maternal behavior was significantly correlated with a decrease in cognition. These results indicate that insufficiency of THs during in utero and postnatal periods impairs maternal behavior, which may be partly induced by impaired cognitive function.

Neural Regulation of Paternal Behavior in Mammals: Sensory, Neuroendocrine, and Experiential Influences on the Paternal Brain

Across the animal kingdom, parents in many species devote extraordinary effort toward caring for offspring, often risking their lives and exhausting limited resources. Understanding how the brain orchestrates parental care, biasing effort over the many competing demands, is an important topic in social neuroscience. In mammals, maternal care is necessary for offspring survival and is largely mediated by changes in hormones and neuropeptides that fluctuate massively during pregnancy, parturition, and lactation (e.g., progesterone, estradiol, oxytocin, and prolactin). In the relatively small number of mammalian species in which parental care by fathers enhances offspring survival and development, males also undergo endocrine changes concurrent with birth of their offspring, but on a smaller scale than females. Thus, fathers additionally rely on sensory signals from their mates, environment, and/or offspring to orchestrate paternal behavior. Males can engage in a variety of infant-directed behaviors that range from infanticide to avoidance to care; in many species, males can display all three behaviors in their lifetime. The neural plasticity that underlies such stark changes in behavior is not well understood. In this chapter we summarize current data on the neural circuitry that has been proposed to underlie paternal care in mammals, as well as sensory, neuroendocrine, and experiential influences on paternal behavior and on the underlying circuitry. We highlight some of the gaps in our current knowledge of this system and propose future directions that will enable the development of a more comprehensive understanding of the proximate control of parenting by fathers.

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

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

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

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