Excitotoxic amino acid injections into the medial amygdala facilitate maternal behavior in virgin female rats

Roadmap for maternal behavior research in domestic dogs: lessons from decades of laboratory rodent work

Maternal behavior research in laboratory rats has revealed important behavioral and neurobiological mechanisms governing the onset, maintenance and decline of maternal behavior. However, the extent to which these mechanisms are evolutionarily conserved across species is less clear. This manuscript proposes that examining these mechanisms in dogs may be a viable approach to test their generality and help bridge the gap between rodent and human research, as domestic dogs show greater individual differences and exhibit more human-like maternal characteristics than rodents. These aspects represent advantages over rodent models, which in turn allow systems biological approaches not available in rodents. Additionally, domestic dogs share similar social environments with humans, suffer from the same mental disorders as humans, and can be treated with the same medications. This paper begins with a summary of key findings and theoretical developments from decades of rat maternal behavior research, followed by a literature review of the extant maternal behavior research on dogs and related methodology, highlighting the unique behavioral characteristics of dog maternal behavior and similarities and differences from rat maternal behavior. Finally, several knowledge gaps in dog maternal behavior research, as well as the future research in this area is discussed. It concludes that research on dog maternal behavior will not only advance our understanding of the universality of the neurobiological and behavioral mechanisms in maternal behavior, but also improve our understanding of risk factors associated with postpartum mental disorders.

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.

Transcriptionally defined amygdala subpopulations play distinct roles in innate social behaviors

Social behaviors are innate and supported by dedicated neural circuits, but the molecular identities of these circuits and how they are established developmentally and shaped by experience remain unclear. Here we show that medial amygdala (MeA) cells originating from two embryonically parcellated developmental lineages have distinct response patterns and functions in social behavior in male mice. MeA cells expressing the transcription factor Foxp2 (MeAFoxp2) are specialized for processing male conspecific cues and are essential for adult inter-male aggression. By contrast, MeA cells derived from the Dbx1 lineage (MeADbx1) respond broadly to social cues, respond strongly during ejaculation and are not essential for male aggression. Furthermore, MeAFoxp2 and MeADbx1 cells show differential anatomical and functional connectivity. Altogether, our results suggest a developmentally hardwired aggression circuit at the MeA level and a lineage-based circuit organization by which a cell's embryonic transcription factor profile determines its social information representation and behavioral relevance during adulthood.

Hardwired to attack: Transcriptionally defined amygdala subpopulations play distinct roles in innate social behaviors

Social behaviors are innate and supported by dedicated neural circuits, but it remains unclear whether these circuits are developmentally hardwired or established through social experience. Here, we revealed distinct response patterns and functions in social behavior of medial amygdala (MeA) cells originating from two embryonically parcellated developmental lineages. MeA cells in male mice that express the transcription factor Foxp2 (MeAFoxp2) are specialized for processing male conspecific cues even before puberty and are essential for adult inter-male aggression. In contrast, MeA cells derived from the Dbx1-lineage (MeADbx1) respond broadly to social cues and are non-essential for male aggression. Furthermore, MeAFoxp2 and MeADbx1 cells show differential anatomical and functional connectivity. Altogether, our results support a developmentally hardwired aggression circuit at the level of the MeA and we propose a lineage-based circuit organization by which a cell's embryonic transcription factor profile determines its social information representation and behavior relevance during adulthood.

Role of Calcr expressing neurons in the medial amygdala in social contact among females

Social animals become stressed upon social isolation, proactively engaging in affiliative contacts among conspecifics after resocialization. We have previously reported that calcitonin receptor (Calcr) expressing neurons in the central part of the medial preoptic area (cMPOA) mediate contact-seeking behaviors in female mice. Calcr neurons in the posterodorsal part of the medial amygdala (MeApd) are also activated by resocialization, however their role in social affiliation is still unclear. Here we first investigated the functional characteristics of MeApd Calcr + cells; these neurons are GABAergic and show female-biased Calcr expression. Next, using an adeno-associated virus vector expressing a short hairpin RNA targeting Calcr we aimed to identify its molecular role in the MeApd. Inhibiting Calcr expression in the MeApd increased social contacts during resocialization without affecting locomotor activity, suggesting that the endogenous Calcr signaling in the MeApd suppresses social contacts. These results demonstrate the distinct roles of Calcr in the cMPOA and MeApd for regulating social affiliation.

Enhanced pup retrieval behaviour in a mouse model of polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is the most common endocrinopathy to affect women of reproductive-age world-wide. Hyperandrogenism is both a hallmark feature of PCOS, and is hypothesised to be an underlying mechanism driving the development of the condition in utero. With circulating hormones known to profoundly influence maternal responses in females, we aimed to determine whether maternal behaviour is altered in a well-described prenatally androgenised (PNA) mouse model of PCOS. Mouse dams were administered with dihydrotestosterone or vehicle on days 16, 17 and 18 of pregnancy. Maternal responses were assessed in both the dihydrotestosterone-injected dams following parturition and in their adult female PNA offspring. Exposure of dams to excess androgens during late pregnancy had no detrimental effects on pregnancy outcomes, including gestation length, pup survival and gestational weight gain, or on subsequent maternal behaviour following parturition. By contrast, PNA virgin females, modelling PCOS, exhibited enhanced maternal behaviour when tested in an anxiogenic novel cage environment, with females rapidly retrieving pups and nesting with them. In comparison, most control virgin females failed to complete this retrieval task in the anxiogenic environment. Assessment of progesterone receptor and oestrogen receptor α immunoreactivity in the brains of virgin PNA and control females revealed increased numbers of oestrogen receptor α positive cells in the brains of PNA females in regions well known to be important for maternal behaviour. This suggests that increased oestrogenic signalling in the neural circuit that underlies maternal behaviour may be a possible mechanism by which maternal behaviour is enhanced in PNA female mice.

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.

Human Primary Olfactory Amygdala Subregions Form Distinct Functional Networks, Suggesting Distinct Olfactory Functions

Three subregions of the amygdala receive monosynaptic projections from the olfactory bulb, making them part of the primary olfactory cortex. These primary olfactory areas are located at the anterior-medial aspect of the amygdala and include the medial amygdala (MeA), cortical amygdala (CoA), and the periamygdaloid complex (PAC). The vast majority of research on the amygdala has focused on the larger basolateral and basomedial subregions, which are known to be involved in implicit learning, threat responses, and emotion. Fewer studies have focused on the MeA, CoA, and PAC, with most conducted in rodents. Therefore, our understanding of the functions of these amygdala subregions is limited, particularly in humans. Here, we first conducted a review of existing literature on the MeA, CoA, and PAC. We then used resting-state fMRI and unbiased k-means clustering techniques to show that the anatomical boundaries of human MeA, CoA, and PAC accurately parcellate based on their whole-brain resting connectivity patterns alone, suggesting that their functional networks are distinct, relative both to each other and to the amygdala subregions that do not receive input from the olfactory bulb. Finally, considering that distinct functional networks are suggestive of distinct functions, we examined the whole-brain resting network of each subregion and speculated on potential roles that each region may play in olfactory processing. Based on these analyses, we speculate that the MeA could potentially be involved in the generation of rapid motor responses to olfactory stimuli (including fight/flight), particularly in approach/avoid contexts. The CoA could potentially be involved in olfactory-related reward processing, including learning and memory of approach/avoid responses. The PAC could potentially be involved in the multisensory integration of olfactory information with other sensory systems. These speculations can be used to form the basis of future studies aimed at clarifying the olfactory functions of these under-studied primary olfactory areas.

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.

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.

Organization of neural circuits underlying social behavior: A consideration of the medial amygdala

The medial amygdala (MeA) is critical for the expression of a broad range of social behaviors, and is also connected to many other brain regions that mediate those same behaviors. Here, we summarize recent advances toward elucidating mechanisms that enable the MeA to regulate a diversity of social behaviors, and also consider what role the MeA plays within the broader network of regions that orchestrate social sensorimotor transformations. We outline the molecular, anatomical, and electrophysiological features of the MeA that segregate distinct social behaviors, propose experimental strategies to disambiguate sensory representations from behavioral function in the context of a social interaction, and consider to what extent MeA function may overlap with other regions mediating similar behaviors.

Neural circuits of social behaviors: Innate yet flexible

Social behaviors, such as mating, fighting, and parenting, are fundamental for survival of any vertebrate species. All members of a species express social behaviors in a stereotypical and species-specific way without training because of developmentally hardwired neural circuits dedicated to these behaviors. Despite being innate, social behaviors are flexible. The readiness to interact with a social target or engage in specific social acts can vary widely based on reproductive state, social experience, and many other internal and external factors. Such high flexibility gives vertebrates the ability to release the relevant behavior at the right moment and toward the right target. This maximizes reproductive success while minimizing the cost and risk associated with behavioral expression. Decades of research have revealed the basic neural circuits underlying each innate social behavior. The neural mechanisms that support behavioral plasticity have also started to emerge. Here we provide an overview of these social behaviors and their underlying neural circuits and then discuss in detail recent findings regarding the neural processes that support the flexibility of innate social behaviors.

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.

Psychological and neurobiological mechanisms underlying the decline of maternal behavior

The maternal behavior decline is important for the normal development of the young and the wellbeing of the mother. This paper reviews limited research on the factors and mechanisms involved in the rat maternal behavior decline and proposes a multi-level model. Framed in the parent-offspring conflict theory (an ultimate cause) and the approach-withdrawal model (a proximate cause), the maternal behavior decline is viewed as an active and effortful process, reflecting the dynamic interplay between the mother and her offspring. It is instigated by the waning of maternal motivation, coupled with the increased maternal aversion by the mother in responding to the changing sensory and motoric patterns of pup stimuli. In the decline phase, the neural circuit that mediates the inhibitory ("withdrawal") responses starts to increase activity and gain control of behavioral outputs, while the excitatory ("approach") maternal neural circuit is being inhibited or reorganized. Various hormones and certain monoamines may play a critical role in tipping the balance between the excitatory and inhibitory neural circuits to synchronize the mother-infant interaction.

Parenting - a paradigm for investigating the neural circuit basis of behavior

Parenting is essential for survival and wellbeing in many species. Since it can be performed with little prior experience and entails considerable sacrifices without immediate benefits for the caregiver, this behavior is likely orchestrated by evolutionarily shaped, hard-wired neural circuits. At the same time, experience, environmental factors and internal state also make parenting highly malleable. These characteristics have made parenting an attractive paradigm for linking complex, naturalistic behavior to its underlying neural mechanisms. Recent work - based on the identification of critical neuronal populations and improved tools for dissecting neural circuits - has uncovered novel functional principles and challenged simplistic models of parenting control. A better understanding of the neural basis of parenting will provide crucial clues to how complex behaviors are organized at the level of cells, circuits and computations. Here I review recent progress, discuss emerging functional principles of parental circuits, and outline future opportunities and challenges.

Preweaning Paternal Deprivation Impacts Parental Responses to Pups and Alters the Serum Oxytocin and Corticosterone Levels and Oxytocin Receptor, Vasopressin 1A Receptor, Oestrogen Receptor, Dopamine Type I Receptor, Dopamine Type II Receptor Levels in Relevant Brain Regions in Adult Mandarin Voles

Although maternal separation and neonatal paternal deprivation (PD) have been found to exert a profound and persistent effects on the physiological and behavioural development of offspring, whether preweaning PD (PPD; from PND 10 to 21) affects maternal and parental responses to pups and the underlying neuroendocrine mechanism are under-investigated. Using monogamous mandarin voles (Microtus mandarinus), the present study found that PPD increased the latency to approach a pup-containing ball, decreased the total durations of sniffing and contacting a pup-containing ball and walking and increased the total duration of inactivity in both sexes. Moreover, PPD decreased serum oxytocin levels and increased corticosterone levels, but only in females. Furthermore, in both males and females, PPD decreased the expression of oxytocin receptor mRNA and protein in the medial preoptic area (MPOA), nucleus accumbens (NAcc) and medial prefrontal cortex (mPFC), but increased it in the medial amygdala (MeA) and decreased the expression of oestrogen receptor mRNA and protein in the MPOA. PPD increased the expression of dopamine type I receptor in the NAcc, but decreased it in the mPFC. PPD decreased dopamine type II receptor (D2R) in the NAcc both in males and females, but increased D2R in the mPFC in females and decreased D2R protein expression in males. Moreover, PPD decreased vasopressin 1A receptor (V1AR) in the MPOA, MeA and mPFC, but only in males. Our results suggest that the reduction of parental responses to pups induced by PPD may be associated with the sex-specific alteration of several neuroendocrine parameters in relevant brain regions.

Pharmacological manipulation of DNA methylation normalizes maternal behavior, DNA methylation, and gene expression in dams with a history of maltreatment

The quality of parental care received during development profoundly influences an individual's phenotype, including that of maternal behavior. We previously found that female rats with a history of maltreatment during infancy mistreat their own offspring. One proposed mechanism through which early-life experiences influence behavior is via epigenetic modifications. Indeed, our lab has identified a number of brain epigenetic alterations in female rats with a history of maltreatment. Here we sought to investigate the role of DNA methylation in aberrant maternal behavior. We administered zebularine, a drug known to alter DNA methylation, to dams exposed during infancy to the scarcity-adversity model of low nesting resources, and then characterized the quality of their care towards their offspring. First, we replicate that dams with a history of maltreatment mistreat their own offspring. Second, we show that maltreated-dams treated with zebularine exhibit lower levels of adverse care toward their offspring. Third, we show that administration of zebularine in control dams (history of nurturing care) enhances levels of adverse care. Lastly, we show altered methylation and gene expression in maltreated dams normalized by zebularine. These findings lend support to the hypothesis that epigenetic alterations resulting from maltreatment causally relate to behavioral outcomes.

The parental brain and behavior: A target for endocrine disruption

During pregnancy, the sequential release of progesterone, 17β-estradiol, prolactin, oxytocin and placental lactogens reorganize the female brain. Brain structures such as the medial preoptic area, the bed nucleus of the stria terminalis and the motivation network including the ventral tegmental area and the nucleus accumbens are reorganized by this specific hormonal schedule such that the future mother will be ready to provide appropriate care for her offspring right at parturition. Any disruption to this hormone pattern, notably by exposures to endocrine disrupting chemicals (EDC), is therefore likely to affect the maternal brain and result in maladaptive maternal behavior. Development effects of EDCs have been the focus of intense study, but relatively little is known about how the maternal brain and behavior are affected by EDCs. We encourage further research to better understand how the physiological hormone sequence prepares the mother's brain and how EDC exposure could disturb this reorganization.

Sexually Dimorphic Control of Parenting Behavior by the Medial Amygdala

Social behaviors, including behaviors directed toward young offspring, exhibit striking sex differences. Understanding how these sexually dimorphic behaviors are regulated at the level of circuits and transcriptomes will provide insights into neural mechanisms of sex-specific behaviors. Here, we uncover a sexually dimorphic role of the medial amygdala (MeA) in governing parental and infanticidal behaviors. Contrary to traditional views, activation of GABAergic neurons in the MeA promotes parental behavior in females, while activation of this population in males differentially promotes parental versus infanticidal behavior in an activity-level-dependent manner. Through single-cell transcriptomic analysis, we found that molecular sex differences in the MeA are specifically represented in GABAergic neurons. Collectively, these results establish crucial roles for the MeA as a key node in the neural circuitry underlying pup-directed behaviors and provide important insight into the connection between sex differences across transcriptomes, cells, and circuits in regulating sexually dimorphic behavior.

Neuronal activation associated with paternal and aversive interactions toward pups in the Mongolian gerbils (Meriones unguiculatus)

Approach/avoid model is used to analyze the neural regulation of maternal behavior in the laboratory rat. This model proposes that the medial preoptic area (mPOA) and bed nucleus of stria terminalis (BNST) are brain regions involved in facilitating mechanisms. By contrast, anterior hypothalamic nucleus (AHN), ventromedial hypothalamic nucleus (VMH), and periaqueductal gray participate in the inhibiting mechanisms of neural regulation of maternal behavior. We hypothesized that there are also facilitating and inhibiting mechanisms in the neural regulation of paternal behavior. Here, we determined which neural areas are activated during paternal and aversive interactions with pups in the Mongolian gerbils (Meriones unguiculatus). By testing paternal behavior, we selected 40 males aggressive toward pups and 20 paternal males. These males were organized into six groups of 10 animals in each group: aggressive males that interacted with pups (AGG-pups) or candy (AGG-candy), paternal males that interacted with pups (PAT-pups) or candy (PAT-candy), and males with testosterone (T)-induced paternal behavior that interacted with pups (IPAT-pups) or candy (IPAT-candy). After interacting with pups or candy, the brains were extracted and analyzed for immunoreactivity (ir) with c-fos. Males that interacted with pups had significantly higher c-fos-ir in the mPOA/BNST than males that interacted with candy. Males that displayed aggression had significantly higher c-fos-ir in the AHN, VMH, and periaqueductal gray than aggressive males that interacted with candy. These results suggest that in the neural regulation of paternal behavior in the Mongolian gerbil underlie positive and negative mechanisms as occurs in maternal behavior.

Functional circuit architecture underlying parental behaviour

Parenting is essential for the survival and wellbeing of mammalian offspring but we lack a circuit-level understanding of how distinct components of this behaviour are orchestrated. Here we investigate how Galanin-expressing neurons in the medial preoptic area (MPOA^Gal) coordinate motor, motivational, hormonal and social aspects of parenting. These neurons integrate inputs from a large number of brain areas, whose activation depends on the animal’s sex and reproductive state. Subsets of MPOA^Gal neurons form discrete pools defined by their projection sites. While the MPOA^Gal population is active during all episodes of parental behaviour, individual pools are tuned to characteristic aspects of parenting. Optogenetic manipulation of MPOA^Gal projections mirrors this specificity, affecting discrete parenting components. This functional organization, reminiscent of the control of motor sequences by pools of spinal cord neurons, provides a new model for how discrete elements of a social behaviour are generated at the circuit level.

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.

Oxytocin and Olfaction

Social signals are identified through processing in sensory systems to trigger appropriate behavioral responses. Social signals are received primarily in most mammals through the olfactory system. Individuals are recognized based on their unique blend of odorants. Such individual recognition is critical to distinguish familiar conspecifics from intruders and to recognize offspring. Social signals can also trigger stereotyped responses like mating behaviors. Specific sensory pathways for individual recognition and eliciting stereotyped responses have been identified both in the early olfactory system and its connected cortices. Oxytocin is emerging as a major state modulator of sensory processing with distinct functions in early and higher olfactory brain regions. The brain state induced through Oxytocin influences social perception. Oxytocin acting on different brain regions can promote either exploration and recognition towards same- or other-sex conspecifics, or association learning. Region-specific deletion of Oxytocin receptors suffices to disrupt these behaviors. Together, these recent insights highlight that Oxytocin's function in social behaviors cannot be understood without considering its actions on sensory processing.

The neurobiology of parenting: A neural circuit perspective

Social interactions are essential for animals to reproduce, defend their territory, and raise their young. The conserved nature of social behaviors across animal species suggests that the neural pathways underlying the motivation for, and the execution of, specific social responses are also maintained. Modern tools of neuroscience have offered new opportunities for dissecting the molecular and neural mechanisms controlling specific social responses. We will review here recent insights into the neural circuits underlying a particularly fascinating and important form of social interaction, that of parental care. We will discuss how these findings open new avenues to deconstruct infant-directed behavioral control in males and females, and to help understand the neural basis of parenting in a variety of animal species, including humans. Please also see the video abstract here.

Neuropeptide Regulation of Social Attachment: The Prairie Vole Model

Social attachments are ubiquitous among humans and integral to human health. Although great efforts have been made to elucidate the neural underpinnings regulating social attachments, we still know relatively little about the neuronal and neurochemical regulation of social attachments. As a laboratory animal research model, the socially monogamous prairie vole (Microtus ochrogaster) displays behaviors paralleling human social attachments and thus has provided unique insights into the neural regulation of social behaviors. Research in prairie voles has particularly highlighted the significance of neuropeptidergic regulation of social behaviors, especially of the roles of oxytocin (OT) and vasopressin (AVP). This article aims to review these findings. We begin by discussing the role of the OT and AVP systems in regulating social behaviors relevant to social attachments, and thereafter restrict our discussion to studies in prairie voles. Specifically, we discuss the role of OT and AVP in adult mate attachments, biparental care, social isolation, and social buffering as informed by studies utilizing the prairie vole model. Not only do these studies offer insight into social attachments in humans, but they also point to dysregulated mechanisms in several mental disorders. We conclude by discussing these implications for human health. © 2017 American Physiological Society. Compr Physiol 7:81-104, 2017.

Current Models and Future Directions for Understanding the Neural Circuitries of Maternal Behaviors in Rodents

Maternal behaviors in rodents include a number of subcomponents, such as nursing, nest building, licking and grooming of pups, pup retrieval, and maternal aggression. Because each behavior involves a unique motor pattern, a unique ensemble neural circuitry must underlie each behavior. To what extent there is overlap in terms of brain regions and specific neurons for each circuit is being actively investigated. This review will first examine overlapping and separate components of pup retrieval and maternal aggression circuitries while examining a central role for medial preoptic area (MPA) in both behaviors. With an emphasis on experimental approaches, the review will then highlight recent findings and propose future directions for understanding maternal behavior regulation. Finally, examples for why studying the neural basis of maternal behaviors can bring insights to other areas of neuroscience, such as feeding, addiction, and anxiety and aggression regulation will be provided.

Environmental influences on the female epigenome and behavior

Environmental factors have long-lasting effects on brain development and behavior. One way experiences are propagated is via epigenetic modifications to the genome. Environmentally-driven epigenetic modifications show incredible brain region- and sex-specificity, and many brain regions affected are ones involved in maternal behavior. In rodent models, females are typically the primary caregiver and thus, any environmental factors that modulate the epigenotype of the mother could have consequences for her current and future offspring. Here we review evidence of the susceptibility of the female epigenome to environmental factors, with a focus on brain regions involved in maternal behavior. Accordingly, implications for interventions that target the mother's epigenome and parenting behavior are discussed.

Common and divergent psychobiological mechanisms underlying maternal behaviors in non-human and human mammals

Maternal interactions with young occupy most of the reproductive period for female mammals and are absolutely essential for offspring survival and development. The hormonal, sensory, reward-related, emotional, cognitive and neurobiological regulators of maternal caregiving behaviors have been well studied in numerous subprimate mammalian species, and some of the importance of this body of work is thought to be its relevance for understanding similar controls in humans. We here review many of the important biopsychological influences on maternal behaviors in the two best studied non-human animals, laboratory rats and sheep, and directly examine how the conceptual framework established by some of the major discoveries in these animal "models" do or do not hold for our understanding of human mothering. We also explore some of the limits for extrapolating from non-human animals to humans. We conclude that there are many similarities between non-human and human mothers in the biological and psychological factors influencing their early maternal behavior and that many of the differences are due to species-characteristic features related to the role of hormones, the relative importance of each sensory system, flexibility in what behaviors are exhibited, the presence or absence of language, and the complexity of cortical function influencing caregiving behaviors.

Neural control of maternal and paternal behaviors

Parental care, including feeding and protection of young, is essential for the survival as well as mental and physical well-being of the offspring. A large variety of parental behaviors has been described across species and sexes, raising fascinating questions about how animals identify the young and how brain circuits drive and modulate parental displays in males and females. Recent studies have begun to uncover a striking antagonistic interplay between brain systems underlying parental care and infant-directed aggression in both males and females, as well as a large range of intrinsic and environmentally driven neural modulation and plasticity. Improved understanding of the neural control of parental interactions in animals should provide novel insights into the complex issue of human parental care in both health and disease.

Gene Expression Profiling during Pregnancy in Rat Brain Tissue

The neurophysiological changes that occur during pregnancy in the female mammal have led to the coining of the phrases “expectant brain” and “maternal brain”. Although much is known of the hormonal changes during pregnancy, alterations in neurotransmitter gene expression have not been well-studied. We examined gene expression in the ventromedial nucleus of the hypothalamus (VMH) during pregnancy based on the fact that this nucleus not only modulates the physiological changes that occur during pregnancy but is also involved in the development of maternal behavior. This study was designed to identify genes that are differentially expressed between mid- and late-pregnancy in order to determine which genes may be associated with the onset and display of maternal behavior and the development of the maternal brain. A commercially available PCR array containing 84 neurotransmitter receptor and regulator genes (RT² Profiler PCR array) was used. Brains were harvested from rats on days 12 and 21 of gestation, frozen, and micropunched to obtain the VMH. Total RNA was extracted, cDNA prepared, and SYBR Green qPCR was performed. In the VMH, expression of five genes were reduced on day 21 of gestation compared to day 12 (Chrna6, Drd5, Gabrr2, Prokr2, and Ppyr1) whereas Chat, Chrm5, Drd4, Gabra5, Gabrg2, LOC289606, Nmu5r2, and Npy5r expression was elevated. Five genes were chosen to be validated in an additional experiment based on their known involvement in maternal behavior onset. This experiment confirmed that gene expression for both the CCK-A receptor and the GABA_AR γ2 receptor increases at the end of pregnancy. In general, these results identify genes possibly involved in the establishment of the maternal brain in rats and indicate possible new genes to be investigated.

Storing maternal memories: hypothesizing an interaction of experience and estrogen on sensory cortical plasticity to learn infant cues

Much of the literature on maternal behavior has focused on the role of infant experience and hormones in a canonical subcortical circuit for maternal motivation and maternal memory. Although early studies demonstrated that the cerebral cortex also plays a significant role in maternal behaviors, little has been done to explore what that role may be. Recent work though has provided evidence that the cortex, particularly sensory cortices, contains correlates of sensory memories of infant cues, consistent with classical studies of experience-dependent sensory cortical plasticity in non-maternal paradigms. By reviewing the literature from both the maternal behavior and sensory cortical plasticity fields, focusing on the auditory modality, we hypothesize that maternal hormones (predominantly estrogen) may act to prime auditory cortical neurons for a longer-lasting neural trace of infant vocal cues, thereby facilitating recognition and discrimination. This couldthen more efficiently activate the subcortical circuit to elicit and sustain maternal behavior.

Paternal responsiveness is associated with, but not mediated by reduced neophobia in male California mice (Peromyscus californicus)

Hormones associated with pregnancy and parturition have been implicated in facilitating the onset of maternal behavior via reductions in neophobia, anxiety, and stress responsiveness. To determine whether the onset of paternal behavior has similar associations in biparental male California mice (Peromyscus californicus), we compared paternal responsiveness, neophobia (novel-object test), and anxiety-like behavior (elevated plus maze, EPM) in isolated virgins (housed alone), paired virgins (housed with another male), expectant fathers (housed with pregnant pairmate), and new fathers (housed with pairmate and pups). Corticotropin-releasing hormone (CRH) and Fos immunoreactivity (IR) were quantified in brain tissues following exposure to a predator-odor stressor or under baseline conditions. New fathers showed lower anxiety-like behavior than expectant fathers and isolated virgins in EPM tests. In all housing conditions, stress elevated Fos-IR in the hypothalamic paraventricular nucleus (PVN). Social isolation reduced overall (baseline and stress-induced) Fos- and colocalized Fos/CRH-IR, and increased overall CRH-IR, in the PVN. In the central nucleus of the amygdala, social isolation increased stress-induced CRH-IR and decreased stress-induced activation of CRH neurons. Across all housing conditions, paternally behaving males displayed more anxiety-related behavior than nonpaternal males in the EPM, but showed no differences in CRH- or Fos-IR. Finally, the latency to engage in paternal behavior was positively correlated with the latency to approach a novel object. These results suggest that being a new father does not reduce anxiety, neophobia, or neural stress responsiveness. Low levels of neophobia, however, were associated with, but not necessary for paternal responsiveness.

Wired for behaviors: from development to function of innate limbic system circuitry

The limbic system of the brain regulates a number of behaviors that are essential for the survival of all vertebrate species including humans. The limbic system predominantly controls appropriate responses to stimuli with social, emotional, or motivational salience, which includes innate behaviors such as mating, aggression, and defense. Activation of circuits regulating these innate behaviors begins in the periphery with sensory stimulation (primarily via the olfactory system in rodents), and is then processed in the brain by a set of delineated structures that primarily includes the amygdala and hypothalamus. While the basic neuroanatomy of these connections is well-established, much remains unknown about how information is processed within innate circuits and how genetic hierarchies regulate development and function of these circuits. Utilizing innovative technologies including channel rhodopsin-based circuit manipulation and genetic manipulation in rodents, recent studies have begun to answer these central questions. In this article we review the current understanding of how limbic circuits regulate sexually dimorphic behaviors and how these circuits are established and shaped during pre- and post-natal development. We also discuss how understanding developmental processes of innate circuit formation may inform behavioral alterations observed in neurodevelopmental disorders, such as autism spectrum disorders, which are characterized by limbic system dysfunction.

Why the maternal brain?

In the rat, the change from a virgin/nulliparous female to the maternal animal takes place at many levels. A subtle developmental wave washes over the female nervous system and transforms her from largely self-centred to offspring-directed, from personal care and protection to care of genetically-related offspring, from indifference to ardour. Such change is preceded by substantial and apparently permanent neural alterations, the depth of which results in the maternal brain, and is the basis of the present review. The neuroplasticity of pregnancy, inherent to the female brain and, we believe, representative of the full expression of the female nervous system's capacity, is a result of significant hormonal and other neurochemical actions. It results in the striking brain changes that are associated with, and necessary for, successful reproduction. We discuss some of these changes and their ramifications. Collectively, they represent the culmination of mammalian evolution and have led to the development of the social brain characteristic of higher orders of mammal, including the human. We also examine different facets of the maternal brain, beginning with a review of the genes involved in maternal behaviour, and in the subsequent 'expression' of the maternal brain. We next discuss olfaction and the manner in which this major sense draws from the rich sensory milieu of the mother to regulate and support maternal behaviour. Last, we discuss the 'whys' of maternal behaviour, a theoretical foray into the reasons for such substantial maternal brain alterations. We focus on the male's potential role as the raison d'etre for the manifest alterations in his mate's brain. In the end, it is clear that the female brain undergoes a significant reorganisation en route to motherhood, the results of which are deep and enduring.

Neuromolecular basis of parental behavior in laboratory mice and rats: with special emphasis on technical issues of using mouse genetics

To support the well-being of the parent-infant relationship, the neuromolecular mechanisms of parental behaviors should be clarified. From neuroanatomical analyses in laboratory rats, the medial preoptic area (MPOA) has been shown to be of critical importance in parental retrieving behavior. More recently, various gene-targeted mouse strains have been found to be defective in different aspects of parental behaviors, contributing to the identification of molecules and signaling pathways required for the behavior. Therefore, the neuromolecular basis of "mother love" is now a fully approachable research field in modern molecular neuroscience. In this review, we will provide a summary of the required brain areas and gene for parental behavior in laboratory mice (Mus musculus) and rats (Rattus norvegicus). Basic protocols and technical considerations on studying the mechanism of parental behavior using genetically-engineered mouse strains will also be presented.

Interest in infants by female rhesus monkeys with neonatal lesions of the amygdala or hippocampus

Previous research in our laboratory has shown that damage to the amygdala in neonatal rhesus monkeys profoundly alters behaviors associated with fear processing, while leaving many aspects of social development intact. Little is known, however, about the impact of neonatal lesions of the amygdala on later developing aspects of social behavior. A well-defined phenomenon in the development of young female rhesus monkeys is an intense interest in infants that is typically characterized by initiating proximity or attempting to hold them. The extent to which young females are interested in infants may have important consequences for the development of species-typical maternal behavior. Here we report the results of a study that was designed to assess interest in infants by female rhesus monkeys that received neonatal lesions to the amygdala, hippocampus or a sham surgical procedure. Subjects were first paired with pregnant "stimulus" females to assess social interactions with them prior to the birth of the infants. There were few behavioral differences between lesion groups when interacting with the pregnant females. However, following the birth of the infants, the amygdala-lesioned females showed significantly less interest in the infants than did control or hippocampus-lesioned females. They directed fewer affiliative vocalizations and facial expressions to the mother-infant pair compared to the hippocampus-lesioned and control females. These findings suggest that neonatal damage to the amygdala, but not the hippocampus, impairs important precursors of non-human primate maternal behavior.

Differential expression of oestrogen receptor alpha following reproductive experience in young and middle-aged female rats

Reproductive experience (i.e. pregnancy and lactation) alters a number of physiological and behavioural endpoints, many of which are related to reproductive function and are regulated by oestrogen. For example, reproductive experience significantly attenuates the oestradiol-induced prolactin surge on the afternoon of pro-oestrous and circulating oestradiol levels are reduced at this time. Although parity-related effects on oestrogen receptor (ER) alpha have been observed within the anterior pituitary, there are currently no data regarding possible parity-induced alterations in ERalpha in the brain. Thus, the present study aimed to examine the effect of parity on the expression of ERalpha in reproductively relevant brain regions. Moreover, because previous findings have demonstrated that the long-term effects of reproductive experience are often oestrous cycle-dependent, ERalpha was examined at two stages of the oestrous cycle (i.e. dioestrous and pro-oestrous). Finally, because the expression of ERalpha is significantly influenced by age, both young and middle-aged females were included in the present study. ERalpha status was determined using immunohistochemistry in select brain regions involved in the regulation of reproductive behaviour in age-matched, cycling primiparous (i.e. one pregnancy and lactation) and nulliparous females as well as in age-matched, noncycling (i.e. persistent oestrous) 12 month-old primiparous and nulliparous females. Significant shifts in ERalpha cell numbers were observed in the medial preoptic area and medial amygdala as a consequence of reproductive experience in an oestrous-dependent manner. These findings indicate that significant changes in ERalpha activity occur in the brain as a function of reproductive experience.

In three brain regions central to maternal behaviour, neither male nor female Phodopus dwarf hamsters show changes in oestrogen receptor alpha distribution with mating or parenthood

Oestrogen receptor (ER)alpha immunoreactivity in three brain regions relevant to maternal behaviour (medial preoptic area, bed nucleus of the stria terminalis and medial amygdala) was measured in two species of dwarf hamster that both mate during a postpartum oestrous but differ in expression of paternal behaviour. Male and female Phodopus campbelli and Phodopus sungorus were sampled as sexually naive adults, following mating to satiety, and as new parents. In all brain regions, females expressed higher levels of ER alpha than males. Species did not have an effect on ER alpha distribution except in the medial amygdala, where P. sungorus females had higher expression levels than all other groups. Behavioural status was not associated with altered ER alpha expression. These results were not expected for females and suggest that a primary activational role for oestrogen, acting through ER alpha in these regions, does not generalize to maternal behaviour in Phodopus. In males, these results are consistent with previous manipulations of the ER alpha ligand, oestrogen, and suggest that paternal behaviour in P. campbelli is likely to be regulated by developmental effects of oestrogen on the brain during early life (similar to Microtus ochrogaster), rather than through activation by oestrogen at the time of fatherhood (similar to Peromyscus californicus).

Paternal aggression in a biparental mouse: parallels with maternal aggression

Environmental and social factors have important effects on aggressive behaviors. We examined the effect of reproductive experience on aggression in a biparental species of mouse, Peromyscus californicus. Estrogens are important in mediating aggressive behavior so we also examined estrogen receptor expression and c-fos for insights into possible mechanisms of regulation. Parental males were significantly more aggressive than virgin males, but no significant differences in estrogen receptor alpha or beta expression were detected. Patterns of c-fos following aggression tests suggested possible parallels with maternal aggression. Parental males had more c-fos positive cells in the medial amygdala, and medial preoptic area relative to virgin males. The medial preoptic area is generally considered to be relatively less important for male-male aggression in rodents, but is known to have increased activity in the context of maternal aggression. We also demonstrated through habituation-dishabituation tests that parental males show exaggerated investigation responses to chemical cues from a male intruder, suggesting that heightened sensory responses may contribute to increased parental aggression. These data suggest that, in biparental species, reproductive experience leads to the onset of paternal aggression that may be analogous to maternal aggression.

Arousal regulation, emotional flexibility, medial amygdala function, and the impact of early experience: comments on the paper of Lewis et al

The balance between optimal levels of emotional arousal and cognitive performance reflects the integration of several dopaminergically and adrenergically regulated neural systems. The amygdalar system is a key region for gating stimulation to cortical regions and the medial amygdala appears to play an especially key role in mediating the fear response. More generally, these arousal regulatory neural systems are key to frustration or stress impact prefrontal cortical function. Further, the threshold for when the level of stress is overwhelming and hence impairs cognitive function reflects minimally genetic and experiential influence. An important interface between Drs. Lewis and Davis's work is how early experience, especially through early parenting, may set the threshold of responsiveness for these arousal regulatory neural systems.

Motivational systems and the neural circuitry of maternal behavior in the rat

Jay Rosenblatt's approach-avoidance model of maternal behavior proposes that maternal behavior occurs when the tendency to approach infant stimuli is greater than the tendency to avoid such stimuli. Our research program has uncovered neural circuits which conform to such a model. We present evidence that the medial preoptic area (MPOA: located in the rostral hypothalamus) may regulate maternal responsiveness by depressing antagonistic neural systems which promote withdrawal responses while also activating appetitive neural systems which increase the attractiveness of infant-related stimuli. These MPOA circuits are activated by the hormonal events of late pregnancy. Preoptic efferents may suppress a central aversion system which includes an amygdala to anterior hypothalamic circuit. Preoptic efferents are also shown to interact with components of the mesolimbic dopamine (DA) system to regulate proactive voluntary maternal responses. We make a distinction between specific (MPOA neurons) and nonspecific motivational systems (mesolimbic DA system) in the regulation of maternal responsiveness.

Hypothalamic neural circuits regulating maternal responsiveness toward infants

A theoretical neural model is developed, along with supportive evidence, to explain how the medial preoptic area (MPOA) of the hypothalamus can regulate maternal responsiveness toward infant-related stimuli. It is proposed that efferents from a hormone-primed MPOA (a) depress a central aversion system (composed of neural circuits between the amygdala, medial hypothalamus, and midbrain) so that novel infant stimuli do not activate defensive or avoidance behavior and (b) excite the mesolimbic dopamine system so that active, voluntary maternal responses are promoted. The effects of oxytocin and maternal experience are included in the model, and the specificity of MPOA effects are discussed. The model may be relevant to the mechanisms through which other hypothalamic nuclei regulate other basic motivational states. In addition, aspects of the model may define a core neural circuitry for maternal behavior in mammals.

Brain basis of early parent-infant interactions: psychology, physiology, and in vivo functional neuroimaging studies

Parenting behavior critically shapes human infants' current and future behavior. The parent-infant relationship provides infants with their first social experiences, forming templates of what they can expect from others and how to best meet others' expectations. In this review, we focus on the neurobiology of parenting behavior, including our own functional magnetic resonance imaging (fMRI) brain imaging experiments of parents. We begin with a discussion of background, perspectives and caveats for considering the neurobiology of parent-infant relationships. Then, we discuss aspects of the psychology of parenting that are significantly motivating some of the more basic neuroscience research. Following that, we discuss some of the neurohormones that are important for the regulation of social bonding, and the dysregulation of parenting with cocaine abuse. Then, we review the brain circuitry underlying parenting, proceeding from relevant rodent and nonhuman primate research to human work. Finally, we focus on a study-by-study review of functional neuroimaging studies in humans. Taken together, this research suggests that networks of highly conserved hypothalamic-midbrain-limbic-paralimbic-cortical circuits act in concert to support aspects of parent response to infants, including the emotion, attention, motivation, empathy, decision-making and other thinking that are required to navigate the complexities of parenting. Specifically, infant stimuli activate basal forebrain regions, which regulate brain circuits that handle specific nurturing and caregiving responses and activate the brain's more general circuitry for handling emotions, motivation, attention, and empathy--all of which are crucial for effective parenting. We argue that an integrated understanding of the brain basis of parenting has profound implications for mental health.

Coding for the initiation of pseudopregnancy by temporally patterned activation of amygdalar NMDA receptors

Female rats modulate the number and interval between the intromissions the female receives during mating. This patterned vaginocervical stimulation (VCS) is critical for triggering long-term changes in prolactin (PRL) secretion necessary for pregnancy or pseudopregnancy (P/PSP). Previous work has shown that NMDA receptor activation in the posterodorsal medial amygdala (MEApd) is required at the time of mating for VCS to induce the twice-daily PRL surges characteristic of P/PSP. The current studies examined whether patterned activation of glutamate receptors within the MEApd induces P/PSP. In anesthetized, cycling females, three 0.27 microg NMDA infusions given at 30 min intervals into the MEApd initiated P/PSP, whereas a single NMDA infusion of the same total dose (0.8 microg) had no effect. In conscious, freely behaving females, three infusions of an excitatory amino acid (EAA) mixture applied at the same interval were more effective in initiating P/PSP and nocturnal PRL surges than were single infusions at the same or higher concentrations. Infusion intervals of 5 and 60 min as well as continuous 1 h infusion did not induce P/PSP. Finally, a synergistic effect was observed between EAA and mating stimulation, because a subthreshold EAA infusion combined with subthreshold numbers of intromissions induced P/PSP. These results demonstrate that repeated, properly spaced, temporally discreet periods of glutamate receptor activation within the MEApd, which mimic mating stimulation, encode for P/PSP. Such findings suggest that single intromissions normally release individually subthreshold quanta of glutamate within the MEApd that summate to induce P/PSP.

Neural steroid hormone receptor gene expression in pregnant rats

Estrogen and progesterone play important roles during pregnancy in stimulating the onset of maternal behavior at parturition. The status of receptor expression of these hormones during pregnancy in neural regions that regulate maternal behavior is unclear. The objective of the present study is to characterize changes in neural gene expression of the estrogen receptors alpha and beta (ERalpha and ERbeta) and the progesterone receptor (PR) during the latter part of pregnancy. Brains from primigravid Sprague-Dawley rats were collected on days 15 and 21 of pregnancy. Micropunches of the olfactory bulb (OB), medial preoptic area (MPOA), bed nucleus of the stria terminalis (BnST), hypothalamus (HYP), medial amygdala (MeA), and the temporal cortex (TCx) were analyzed by real-time RT-PCR (Taqmantrade mark) for levels of gene expression. No changes in either ERalpha or ERbeta mRNA levels were detected in any brain region between days 15 and 21 of pregnancy: however, the MPOA had higher levels of both ERalpha and ERbeta than other brain regions. Progesterone receptor mRNA levels, in contrast, declined significantly in the MPOA, HYP, and TCx, between days 15 and 21 of pregnancy (P < 0.05). In addition, the levels of PR mRNA were significantly higher in the HYP and TCx compared to both the OB and MeA. These data indicate that there is a downregulation of PR prepartum and suggest that this decrease may play a role in the disinhibition of maternal behavior at parturition.

Glutamatergic stimulation of the medial amygdala induces steroid dependent c-fos expression within forebrain nuclei responsive to mating stimulation

Neurons within the posterodorsal medial amygdala of female rats are known to process vaginocervical stimulation received during mating through N-methyl-D-aspartate channel activation, conveying information to downstream hypothalamic cell groups that modulate neuroendocrine function. Stimulation of these neurons with an excitatory amino acid cocktail of glutamate, aspartate and glycine initiates 10-12 days of prolactin surge secretion that normally are observed only after the receipt of vaginocervical stimulation. Posterodorsal medial amygdala neurons responsive to vaginocervical stimulation also contain estrogen and progesterone receptors. The present experiment examined which downstream sites involved in prolactin secretion show c-fos expression following glutamate receptor activation within the posterodorsal medial amygdala and whether ovarian steroids influence cellular activation in these areas. Ovariectomized female rats implanted with unilateral cannulas directed at the posterodorsal medial amygdala received injections of estradiol benzoate and progesterone or oil before infusion treatment with either excitatory amino acid or control PBS. An additional group of estradiol benzoate+progesterone-treated females was infused with 1.0 microM glycine alone in PBS. Infusions were administered three times at 30 min intervals. FOS induction 90 min after infusion was determined immunohistochemically on the sides ipsilateral and contralateral to the infusion. Of the examined regions, excitatory amino acid treatment and hormone treatment induced three patterns of c-fos expression: 1) responses to both excitatory amino acid and hormone treatment [posterodorsal medial amygdala, medial preoptic area, ventrolateral ventromedial hypothalamic nucleus, bed nucleus of the stria terminalis]; 2) responses to estradiol benzoate+progesterone treatment only [anteroventral periventricular nucleus and dorsomedial nucleus]; and 3) responses to excitatory amino acid only [arcuate nucleus, suprachiasmatic nucleus, and paraventricular nucleus]. These data identify possible circuits by which vaginocervical stimulation, via activation of posterodorsal medial amygdala glutamate-type receptors, initiates and coordinates a series of events within a larger neuroendocrine circuit important for pregnancy.