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

Role of sexually dimorphic oxytocin receptor-expressing neurons in the anteroventral periventricular nucleus on maternal behavior

Oxytocin is a neuropeptide produced by magnocellular neurosecretory neurons located primarily in the supraoptic nucleus and paraventricular nucleus of the hypothalamus. The long axons of these neurons project to the neurohypophysis where oxytocin is released into the general circulation in response to the physiological demands. Oxytocin plays critical roles in female reproductive physiology, specifically in uterine contraction during labor and milk ejection while nursing. Oxytocin is also called "the love hormone" due to its modulatory roles in prosocial behaviors, including social recognition, maternal behavior, and pair bonding. Oxytocin influences behaviors by binding to oxytocin receptors (OXTR) located in various parts of the brain. Previously, we discovered a group of estrogen-dependent OXTR neurons that is exclusively present in the anteroventral periventricular nucleus (AVPV) of females but not of males. The female-specific expression of OXTR in the AVPV is a rare case of neurochemically-demonstrated, all-or-none sexual dimorphism in the brain. In this review, the cellular characterization and functional significance of the sexually dimorphic OXTR neurons in the AVPV as well as the clinical implications of the research will be discussed.

Neurobiological mechanisms underlying oxytocin-mediated parental behavior in rodents

Parental behavior is essential for mammalian offspring to survive. Because of this significance, elucidating the neurobiological mechanisms that facilitate parental behavior has received strong interest. Decades of studies utilizing pharmacology and molecular biology have revealed that in addition to its facilitatory effects on parturition and lactation, oxytocin (OT) promotes the expression of parental behavior in rodents. Recent studies have also described the modulation of sensory processing by OT and the interaction of the OT system with other brain regions associated with parental behavior. However, the precise neurobiological mechanisms underlying the facilitation of caregiving behaviors by OT remain unclear. In this Review, I summarize the findings from rats and mice with a view toward integrating past and recent progress. I then review recent advances in the understanding of the molecular, cellular, and circuit mechanisms of OT-mediated parental behavior. Based on these observations, I propose a hypothetical model that would explain the mechanisms underlying OT-mediated parental behavior. Finally, I conclude by discussing some major remaining questions and propose potential future research directions.

Lateral septum as a possible regulatory center of maternal behaviors

The lateral septum (LS) is involved in controlling anxiety, aggression, feeding, and other motivated behaviors. Lesion studies have also implicated the LS in various forms of caring behaviors. Recently, novel experimental tools have provided a more detailed insight into the function of the LS, including the specific role of distinct cell types and their neuronal connections in behavioral regulations, in which the LS participates. This article discusses the regulation of different types of maternal behavioral alterations using the distributions of established maternal hormones such as prolactin, estrogens, and the neuropeptide oxytocin. It also considers the distribution of neurons activated in mothers in response to pups and other maternal activities, as well as gene expressional alterations in the maternal LS. Finally, this paper proposes further research directions to keep up with the rapidly developing knowledge on maternal behavioral control in other maternal brain regions.

Parenting behaviors in mice: Olfactory mechanisms and features in models of autism spectrum disorders

Rodents, along with numerous other mammals, heavily depend on olfactory cues to navigate their social interactions. Processing of olfactory sensory inputs is mediated by conserved brain circuits that ultimately trigger social behaviors, such as social interactions and parental care. Although innate, parenting is influenced by internal states, social experience, genetics, and the environment, and any significant disruption of these factors can impact the social circuits. Here, we review the molecular mechanisms and social circuits from the olfactory epithelium to central processing that initiate parental behaviors and their dysregulations that may contribute to the social impairments in mouse models of autism spectrum disorders (ASD). We discuss recent advances of the crucial role of olfaction in parental care, its consequences for social interactions, and the reciprocal influence on social interaction impairments in mouse models of ASD.

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.

High fat diet-induced maternal obesity in mice impairs peripartum maternal behaviour

Obesity during pregnancy represents a significant health issue and can lead to increased complications during pregnancy and impairments with breastfeeding, along with long-term negative health consequences for both mother and offspring. In rodent models, diet-induced obesity (DIO) during pregnancy leads to poor outcomes for offspring. Using a DIO mouse model, consisting of feeding mice a high fat diet for 8 weeks before mating, we recapitulate the effect of high pup mortality within the first 3 days postpartum. To examine the activity of the dam around the time of birth, late pregnant control and DIO dams were recorded in their home cages and the behaviour of the dam immediately before and after birth was analysed. Prior to giving birth, DIO dams spent less time engaging in nesting behaviour, while after birth, DIO dams spent less time in the nest with their pups compared to control dams, indicating reduced pup-engagement in the early postpartum period. We have previously reported that lactogenic hormone action, mediated by the prolactin receptor, in the medial preoptic area of the hypothalamus (MPOA) is critical for the onset of normal postpartum maternal behaviour. We hypothesized that DIO dams may have lower lactogenic hormone activity during late pregnancy, which would contribute to impaired onset of normal postpartum maternal behaviour. Day 16 lactogenic activity, transport of prolactin into the brain, and plasma prolactin concentrations around birth were all similar in control and DIO dams. Moreover, endogenous pSTAT5, a marker of prolactin receptor activity, in the MPOA was unaffected by DIO. Overall, these data indicate that lactogenic activity in late pregnancy of DIO dams is not different to controls and is unlikely to play a major role in impaired onset of normal postpartum maternal behaviour.

Sexually dimorphic oxytocin receptor-expressing (OXTR) neurons in the anteroventral periventricular nucleus (AVPV) in the postpartum female mouse are involved in maternal behavior

Maternal care is crucial for the survival and development of offspring. Oxytocin modulates maternal behavior by binding to oxytocin receptors (OXTRs) in various parts of the brain. Previously, we showed that OXTRs are expressed in the anteroventral periventricular nucleus (AVPV) of female, but not male mice. Because the AVPV is involved in the regulation of maternal behavior and oxytocin enhances its induction, this finding leads to the hypothesis that the female specific population of OXTR neurons in the AVPV regulates maternal behavior. To address this hypothesis, OXTR-Venus reporter mice were used to assess if expression levels of OXTR in the AVPV are changed during the postpartum period. The total number of OXTR-Venus neurons was significantly greater in postpartum dams compared to virgin females. To assess efferent projections of the AVPV-OXTR neurons, a Cre-dependent fluorescent protein (tdTomato) expressing a viral vector was injected into one side of the AVPV of female OXTR-Cre mice. Fibers expressing tdTomato were found in hypothalamic areas containing oxytocin neurons (the supraoptic and paraventricular nuclei) and the midbrain areas (the ventral tegmental area and periaqueductal gray) that are involved in the regulation of maternal motivation. To assess if activity of the AVPV-OXTR neurons is involved in the regulation of maternal behaviors, a chemogenetic approach was employed. Specific inhibition of activity of AVPV-OXTR neurons completely abolished pup retrieval and nest building behaviors. Collectively, these findings demonstrate that AVPV-OXTR neurons in postpartum female mice constitute an important node in the neural circuitry that regulates maternal behavior.

Effects of oxytocin ablation on pup rescue, nursing behaviors and response to pup separation in early-to-mid postpartum mice

Oxytocin, a neuropeptide hormone, is indispensable for milk ejection during nursing and is important for uterine contractions during parturition. The exact functions of oxytocin in postpartum maternal behaviors and motivations require further investigation. To this end, we characterized the role of oxytocin in components of maternal motivations during the mid-postpartum period, which has not been previously studied. To maintain suckling stimuli, postpartum oxytocin knockout (Oxt^-/- ) and heterozygous (Oxt^+/- ) littermates were co-housed with a wild-type lactating mother and its litter, and were examined for their ability to retrieve pups under standard or high-risk conditions, nursing behavior, maternal aggression towards an unfamiliar intruder, and motivation to regain contact with separated pups. One-third of Oxt^-/- mothers exhibited prolonged parturition but were otherwise grossly healthy. Despite their inability to eject milk, Oxt^-/- mothers displayed nursing behaviors for similar durations to Oxt^+/- mothers during the second postpartum week. In addition, Oxt^-/- mothers were essentially intact for pup retrieval under standard conditions and were motivated to stay close to pups, although they showed a mild decrease in maternal care under high-risk conditions and increased anxiety-like behaviors in pup-related contexts. The present findings indicate that oxytocin is dispensable for nursing behavior and maternal motivations, yet suggest that oxytocin may be relevant for stress resilience in the postpartum period.

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.

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.

Abnormal maternal behavior in mice lacking phospholipase Cβ1

Motherhood goes through preparation, onset and maintenance phases until the natural weaning. A variety of changes in hormonal/neurohormonal systems and brain circuits are involved in the maternal behavior. Hormones, neuropeptides, and neurotransmitters involved in maternal behavior act via G-protein-coupled receptors, many of which in turn activate plasma membrane enzymes including phospholipase C (PLC) β isoforms. In this study, we examined the effect of PLCβ1 knockout (KO) on maternal behavior. There was little difference between PLCβ1-KO and wild-type (WT) dams in the relative time spent in maternal behavior during the period between 24 h prepartum and 12 h postpartum (-24 h ∼ PPH 12). After PPH 18, however, PLCβ1-KO dams neglected their pups so that they all died in 2-3 days. In the pup retrieval test, latency was not different during the period within PPH 12, but after PPH 18, PLCβ1-KO dams could not finish pup retrieval in a given time. During both periods, FosB expression in the nucleus accumbens (NAcc) of PLCβ1-KO dams was significantly lower than WT, but not different in the medial preoptic area (mPOA). Given that mPOA activity is required for initiation of maternal behavior, and that NAcc is known to be involved in maternal motivation and maintenance of maternal behavior, our results suggest that PLCβ1 signaling is essential for transition from the onset to maintenance phase of maternal behavior.

Effects of gestational low dose perfluorooctanoic acid on maternal and "anxiety-like" behavior in dams

Pregnancy is a unique critical window with nearly ubiquitous exposure to low concentrations of endocrine disrupting chemicals, such as per- and poly-fluoroalkyl substances (PFAS). Human and animal research suggests that PFAS compounds disrupt hypothalamic-pituitary-adrenal axis function, with some evidence of altered "anxiety-like" behavior, but little is known about the potential effects on maternal mental health following exposures during pregnancy. Evaluating the consequences of gestational PFAS exposures on maternal health is essential, because approximately 1 in 10 women experience postpartum depression, often with increased anxiety. To address this gap, dams were exposed to a low dose, 0.1 mg/kg, of perfluorooctanoic acid (PFOA) from gestational day 0 to birth. Maternal behavior was then observed from postnatal days 5-9, and "anxiety-like" behavior was measured using open field spontaneous locomotor behavior and elevated plus maze following weaning. No difference was observed in the litter size or sex of offspring. Gestational PFOA exposure altered maternal behavior. Despite similar nursing durations, PFOA dams spent more time nursing in a flat posture and on their side, and less time in kyphosis. Despite significantly quicker first contact, PFOA dams did not return pups to the nest quicker, indicating reduced retrieval latency. At weaning, dams displayed increased "anxiety-like" behaviors in the elevated plus maze with a significantly higher mean duration in the closed arms and reduced choice frequency with significantly lower number of entries in the closed and open arms. PFOA dams showed reductions in ambulatory movement across the session. Pregnancy exposure to PFOA altered both maternal and "anxiety-like" behavior in dams. Additional assays focused on depression-associated behaviors, such as forced swim, anhedonia, and social preference, will further delineate behavioral mechanisms. Further research on the effects of environmental contaminant exposures during pregnancy should investigate how co-exposures to other risk factors, such as stress, may enhance behavioral toxicity. Understanding how environmental contaminant exposure during pregnancy effects maternal depression-associated, and/or "anxiety-like" behavior is necessary for the public health protection of women.

Glutamatergic neurons from the medial prefrontal cortex to the dorsal raphe nucleus regulate maternal aggression in lactating mice

Glutamatergic signals in the dorsal raphe nucleus (DRN) regulate maternal aggression and care in mice. We examined whether glutamatergic input from the medial prefrontal cortex (mPFC) to the DRN might regulate maternal aggression and care in mice. In the maternal aggression test, each dam was exposed to an identical intruder male twice for 5 min, 60 min apart. During the latter trial (opt trial), the terminals of glutamatergic neurons from the mPFC to the DRN were manipulated using optogenetic techniques. Compared to the former trial (pre-opt trial), the inhibition of glutamatergic input in the opt trial decreased bite frequency and prevented the shortening of biting latency. In contrast, the activation of glutamatergic input at 5 Hz increased the biting frequency. Meanwhile, the activation of glutamatergic input at 1, 10, and 20 Hz prevented the shortening of biting latency without affecting biting frequency. In the maternal care test, activation of glutamatergic input at 5 Hz did not affect maternal care. Our results suggest that glutamatergic neurons from the mPFC to the DRN differently regulate maternal aggression, depending on temporal patterns of their activation, and that the glutamatergic signals that enhance maternal aggression are not involved in the regulation of maternal care.

Parenting and the Serotonin Transporter Gene (5HTTLPR), Is There an Association? A Systematic Review of the Literature

The current systematic review examines whether there is an association between the genetic 5-HTTPLR polymorphism and parenting, and the mechanisms by which this association operates. The literature was searched in various databases such as PubMed, Scopus, and ScienceDirect. In line with our inclusion criteria, nine articles were eligible out of 22. Most of the studies analysed in this review found an association between 5HTTLPR and parenting. Four studies found a direct association between 5-HTTLPR and parenting with conflicting findings: two studies found that mothers carrying the short variant were more sensitive to their infants, while two studies found that parents carrying the S allele were less sensitive. In addition, several studies found strong interaction between genetic and environmental factors, such as childhood stress and disruptive child behaviour, quality of early care experiences, poor parenting environment, and quality of the environment. Only one study found an association between children's 5HTTLPR and parenting. Parenting can be described as a highly complex construct influenced by multiple factors, including the environment, as well as parent and child characteristics. According to the studies, maternal 5-HTTLPR polymorphism is most likely to be associated with sensitive parenting.

Experience-Regulated Neuronal Signaling in Maternal Behavior

Maternal behavior is shaped and challenged by the changing developmental needs of offspring and a broad range of environmental factors, with evidence indicating that the maternal brain exhibits a high degree of plasticity. This plasticity is displayed within cellular and molecular systems, including both intra- and intercellular signaling processes as well as transcriptional profiles. This experience-associated plasticity may have significant overlap with the mechanisms controlling memory processes, in particular those that are activity-dependent. While a significant body of work has identified various molecules and intracellular processes regulating maternal care, the role of activity- and experience-dependent processes remains unclear. We discuss recent progress in studying activity-dependent changes occurring at the synapse, in the nucleus, and during the transport between these two structures in relation to maternal behavior. Several pre- and postsynaptic molecules as well as transcription factors have been found to be critical in these processes. This role reflects the principal importance of the molecular and cellular mechanisms of memory formation to maternal and other behavioral adaptations.

Oxytocin Facilitates Allomaternal Behavior under Stress in Laboratory Mice

Oxytocin (Oxt) controls reproductive physiology and various kinds of social behaviors, but the exact contribution of Oxt to different components of parental care still needs to be determined. Here, we illustrate the neuroanatomical relations of the parental nurturing-induced neuronal activation with magnocellular Oxt neurons and fibers in the medial preoptic area (MPOA), the brain region critical for parental and alloparental behaviors. We used genetically-targeted mouse lines for Oxt, Oxt receptor (Oxtr), vasopressin receptor 1a (Avpr1a), vasopressin receptor 1b (Avpr1b), and thyrotropin-releasing hormone (Trh) to systematically examine the role of Oxt-related signaling in pup-directed behaviors. The Oxtr-Avpr1a-Avpr1b triple knock-out (TKO), and Oxt-Trh-Avpr1a-Avpr1b quadruple KO (QKO) mice were grossly healthy and fertile, except for their complete deficiency in milk ejection and modest deficiency in parturition secondary to maternal loss of the Oxt or Oxtr gene. In our minimal stress conditions, pup-directed behaviors in TKO and QKO mothers and fathers, virgin females and males were essentially indistinguishable from those of their littermates with other genotypes. However, Oxtr KO virgin females did show decreased pup retrieval in the pup-exposure assay performed right after restraint stress. This stress vulnerability in the Oxtr KO was abolished by the additional Avpr1b KO. The general stress sensitivity, as measured by plasma cortisol elevation after restraint stress or by the behavioral performance in the open field (OF) and elevated plus maze (EPM), were not altered in the Oxtr KO but were reduced in the Avpr1b KO females, indicating that the balance of neurohypophysial hormones affects the outcome of pup-directed behaviors.

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.

A novel mouse model of postpartum depression using emotional stress as evaluated by nesting behavior

Postpartum depression is an important mental health issue not only for the mother but also for the child’s development, other family members, and the society. An appropriate animal model is desired to elucidate the pathogenesis of postpartum depression. However, methods for stress loading during pregnancy have not been established. Behavioral experiments to investigate postpartum depression-like behaviors should be conducted without stress because behavioral tests affect rearing behaviors such as lactation. Therefore, we developed a new mouse model of postpartum depression using a psychological stress method. Mating partners were made to witness their partners experiencing social defeat stress and then listen to their cries. Emotional stress loading during pregnancy significantly increased postpartum depression-like behaviors. Postpartum depression also affected nurturing behaviors and caused disturbances in pup care. Furthermore, nesting behavior was impaired in the stressed group, suggesting that the observation of nesting behavior may be useful for assessing social dysfunction in postpartum depression. These results demonstrate the utility of this new mouse model of postpartum depression.

Alterations in corticotropin-releasing factor receptor type 1 in the preoptic area and hypothalamus in mice during the postpartum period

Corticotropin-releasing factor (CRF) signaling through CRF receptor 1 (CRFR1) regulates autonomic, endocrine, and behavioral responses to stress, as well as behavioral changes during the maternal period. Previous work in our lab reported higher levels of CRFR1 in female, compared to male, mice within the rostral anteroventral periventricular nucleus (AVPV/PeN), a brain region involved in maternal behaviors. In this study, we used CRFR1-GFP reporter mice to investigate whether the reproductive status (postpartum vs. nulliparous) of acutely stressed females affects levels of CRFR1 in the AVPV/PeN and other regions involved in maternal functions. Compared to nulliparous, postpartum day 14 females showed increased AVPV/PeN CRFR1-GFP immunoreactivity and an elevated number of restraint stress-activated AVPV/PeN CRFR1 cells as assessed by immunohistochemical co-localization of CRFR1-GFP and phosphorylated CREB (pCREB). The medial preoptic area (MPOA) and paraventricular hypothalamus (PVN) of postpartum mice showed modest decreases in CRFR1-GFP immunoreactivity, while increased CRFR1-GFP/pCREB co-expressing cells were found in the PVN following restraint stress relative to nulliparous mice. Tyrosine hydroxylase (TH) and CRFR1-GFP co-localization was also assessed in the AVPV/PeN and other regions and revealed a decrease in co-localized neurons in the AVPV/PeN and ventral tegmental area of postpartum mice. Corticosterone analysis of restrained mice revealed blunted peak, but elevated recovery, levels in postpartum compared to nulliparous mice. Finally, we investigated projection patterns of AVPV/PeN CRFR1 neurons using female CRFR1-Cre mice and revealed dense efferent projections to several preoptic, hypothalamic, and hindbrain regions known to control stress-associated and maternal functions. Together, these findings contribute to our understanding of the neurobiology that might underlie changes in stress-related functions during the postpartum period.

Changes in maternal motivation across reproductive states in mice: A role for prolactin receptor activation on GABA neurons

The survival of newborn offspring in mammals is dependent on sustained maternal care. Mammalian mothers are highly motivated to interact with and care for offspring, however, it is unclear how hormonal signals act on neural circuitry to promote maternal motivation during the transition to motherhood. In this study we aimed to establish methods that enable us to evaluate change in maternal motivation across the reproductive life cycle in female mice. Using two behavioural testing paradigms; a novel T-maze retrieval test and a barrier climbing test, we found that pup retrieval behaviour was low in virgin and pregnant mice compared to lactating females, indicating that maternal motivation arises around the time of parturition. Furthermore, in reproductively experienced females, maternal motivation declined over time after weaning of pups. As we have previously shown that lactogenic action mediated through the prolactin receptor (Prlr) in the medial preoptic area (MPOA) is essential for the expression of maternal behaviour, we aimed to investigate the role of lactogenic hormones in promoting pup-related motivational behaviours. With GABAergic neurons expressing Prlr in multiple brain regions important for maternal behaviour, we conditionally deleted Prlr from GABA neurons. Compared to control females, lactating GABA neuron-specific Prlr knockout mice showed slower and incomplete pup retrieval behaviour in the T-maze test. Testing of anxiety behaviour on an elevated plus maze indicated that these mice did not have increased anxiety levels, suggesting that lactogenic action on GABA neurons is necessary for the full expression of motivational aspects of maternal behaviour during lactation.

Neural and Molecular Mechanisms of Biological Embedding of Social Interactions

Animals operate in complex environments, and salient social information is encoded in the nervous system and then processed to initiate adaptive behavior. This encoding involves biological embedding, the process by which social experience affects the brain to influence future behavior. Biological embedding is an important conceptual framework for understanding social decision-making in the brain, as it encompasses multiple levels of organization that regulate how information is encoded and used to modify behavior. The framework we emphasize here is that social stimuli provoke short-term changes in neural activity that lead to changes in gene expression on longer timescales. This process, simplified-neurons are for today and genes are for tomorrow-enables the assessment of the valence of a social interaction, an appropriate and rapid response, and subsequent modification of neural circuitry to change future behavioral inclinations in anticipation of environmental changes. We review recent research on the neural and molecular basis of biological embedding in the context of social interactions, with a special focus on the honeybee.

Does neonatal manipulation on continuous or alternate days change maternal behavior?

Maternal separation and neonatal manipulation of pups produce changes in maternal behavior after the dam-pup reunion. Here, we examined whether continuous versus alternating days of neonatal manipulation during the first 8 postnatal days produces differential changes in maternal and non-maternal behaviors in rats. We found that both maternal separation protocols increased anogenital licking after dam-pup reunion, reflecting increased maternal care of pups.

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.

Dorsal raphe serotonin neurotransmission is required for the expression of nursing behavior and for pup survival

Proper maternal care is an essential factor of reproductive success in mammals, involving a repertoire of behaviors oriented toward the feeding and care of the offspring. Among the neurotransmitters involved in the initiation of these behaviors, serotonin (5-HT) seems to play an important role. Here we compared pup-oriented maternal behaviors in mice with constitutive 5-HT depletion, the tryptophan hydroxylase 2-knock-out (Tph2-KO) and the Pet1-KO mice. We report that the only common pup-oriented defect in these 2 hyposerotoninergic models is a defective nursing in parturient mice and altered nursing-like (crouching) behavior in virgin mice, while pup retrieval defects are only present in Tph2-KO. Despite a normal mammary gland development and milk production, the defect in appropriate nursing is responsible for severe growth retardation and early lethality of pups born to hyposerotonergic dams. This nursing defect is due to acute rather constitutive 5-HT depletion, as it is reproduced by adult knockdown of Tph2 in the dorsal raphe nucleus in mothers with a prior normal maternal experience. We conclude that 5-HT innervation from the dorsal raphe is required for both the initiation and maintenance of a normal nursing behavior. Our findings may be related to observations of reduced maternal/infant interactions in human depression.

Peripheral oxytocin injection modulates vomeronasal sensory activity and reduces pup-directed aggression in male mice

Behaviors are shaped by hormones, which may act either by changing brain circuits or by modifying sensory detection of relevant cues. Pup-directed behaviors have been previously shown to change via action of hormones at the brain level. Here, we investigated hormonal control of pup-induced activity in the vomeronasal organ, an olfactory sensory structure involved in the detection of non-volatile chemosignals. Vomeronasal activity decreases as males switch from a pup-aggressive state to a non-aggressive parenting state, after they socially contact a female. RNA sequencing, qPCR, and in situ hybridization were used to identify expression, in the vomeronasal sensory epithelium, of candidate GPCR hormone receptors chosen by in silico analyses and educated guesses. After identifying that oxytocin and vasopressin receptors are expressed in the vomeronasal organ, we injected the corresponding hormones in mice and showed that oxytocin administration reduced both pup-induced vomeronasal activity and aggressive behavior. Conversely, injection of an oxytocin receptor antagonist in female-primed male animals, which normally exhibit reduced vomeronasal activity, significantly increased the number of active vomeronasal neurons. These data link oxytocin to the modulation of olfactory sensory activity, providing a possible mechanism for changes in male behavior after social experience with females.

MKK7 deficiency in mature neurons impairs parental behavior in mice

c‐Jun N‐terminal kinases (JNKs) are constitutively activated in mammalian brains and are indispensable for their development and neural functions. MKK7 is an upstream activator of all JNKs. However, whether the common JNK signaling pathway regulates the brain's control of social behavior remains unclear. Here, we show that female mice in which Mkk7 is deleted specifically in mature neurons (Mkk7^flox/floxSyn‐Cre mice) give birth to a normal number of pups but fail to raise them due to a defect in pup retrieval. To explore the mechanism underlying this abnormality, we performed comprehensive behavioral tests. Mkk7^flox/floxSyn‐Cre mice showed normal locomotor functions and cognitive ability but exhibited depression‐like behavior. cDNA microarray analysis of mutant brain revealed an altered gene expression pattern. Quantitative RT‐PCR analysis demonstrated that mRNA expression levels of genes related to neural signaling pathways and a calcium channel were significantly different from controls. In addition, loss of neural MKK7 had unexpected regulatory effects on gene expression patterns in oligodendrocytes. These findings indicate that MKK7 has an important role in regulating the gene expression patterns responsible for promoting normal social behavior and staving off depression.

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.

Investigation of the factors that induce maternal aggression towards juveniles among several mouse strains

Maternal care and aggression are representative of maternal behavior among lactating female mice. Even neonates and juveniles, who are not biological offspring, can induce maternal care and aggression in dams. Here, we investigated the factors that induce maternal aggression through exposure to juvenile mice. We first addressed the role of intruder age on the induction of maternal aggression in dams. BALB/c dams displayed attacking behavior towards 14-day-old C57BL/6J male intruders. Consumption of food pellets during the weaning period was unlikely to affect the induction of attacking behavior, as the intruders reared by breastfeeding, without food pellets, induced intensive attacking behavior in dams. Next, we compared the intruder-mediated induction of attacking behavior through different mouse strains. Specifically, BALB/c intruders induced a lower level of attacking behavior in BALB/c or ICR dams, compared to the other strains tested. However, BALB/c intruders induced intense attacking behavior in C57BL/6N dams, indicating that the occurrence of attacking behavior is dependent on the strains of dams as well as intruders. A cross-fostering experiment highlighted that the rearing by an original mother was required for C57BL/6J juveniles to induce attacking behavior. In contrast, BALB/c intruders may emit an inhibitory factor that limits attacking behavior. We finally explored which parts of the body emit these aggression-inducible signals. Removal of body hair around the proximal tail of the intruders significantly decreased the attacking behavior of dams, demonstrating that chemical cues, namely pheromones, attached to the body hair around the proximal tail may be essential for inducing attacking behavior in dams.

Oxytocin in the dorsal raphe nucleus antagonizes the inhibition of maternal care induced by food deprivation

Lactation is indispensable for the pup's survival, but is considered a survival burden in dams under negative energy conditions. In the present study, we tested our hypothesis that oxytocin may facilitate energy investment to pups through behavioral control as well as milk ejection. Maternal care was observed in dams at 3 h but not 8 h after food deprivation. We investigated whether oxytocin in the dorsal raphe nucleus (DRN), which is involved in energy state-dependent regulation of maternal care, regulates maternal care. For this purpose, 2-pmol L368899, an oxytocin receptor antagonist, was injected into the DRN; after treatment, maternal care was inhibited in the dams with 3-h fasting, but not in the fed dams. In contrast, recovery of maternal care was observed in the dams with 8-h fasting who underwent 100-pmol oxytocin injection at the DRN. These results indicate that oxytocin in the DRN is required for displaying maternal behavior under fasting conditions, but not under fed conditions. Next, we investigated the site of oxytocin release. Presentation of pups decreased the oxytocin immunoreactivity at the paraventricular nucleus (PVN) of the hypothalamus in the 3-h-fasted dams, but not in the fed or 8-h-fasted dams. No change of the serum oxytocin level was observed. Few oxytocin-positive neurons projecting from the PVN to the DRN were detected through labeling with the retrograde tracer fluorogold. Oxytocin secreted at the PVN, which reaches the DRN, but not released as a hormone or neurotransmitter may mediate maternal care under food-restricted conditions.

Prefrontal cortex is associated with the rapid onset of parental behavior in inexperienced adult mice (C57BL/6)

There is significant variability in the immediate behavioral response displayed by inexperienced adult mice when exposed to pups for the first time. The aim of this study was to determine which brain regions were engaged (higher c-Fos-immunoreactivity, c-Fos-ir) when virgin females, that were exposed to pups for 15 or 60 min, displayed full parental behavior (FPB), partial parental behavior (PPB), or non-parental behavior (NPB), or virgin males displayed PPB or infanticidal behavior (IB). The number of c-Fos-ir neurons in the prelimbic cortex (PL) was higher in parental females than in the NPB group (after a 15-min exposure), and the group not exposed to pups (NE). C-Fos expression in the nucleus accumbens (NA) was increased in most groups of females exposed to pups compared to NE. Higher c-Fos-ir was also found in the shell subregion of the NA in infanticidal males, compared to males NE. The cortical (CoA) and medial (MA) amygdala also showed higher c-Fos-ir in parental females compared to NE animals. However, PPB and IB male groups also exhibited higher c-Fos-ir in the CoA and MA compared to the NE group. The expression of c-Fos in the different subregions of medial preoptic area and the ventromedial nucleus of the hypothalamus was not specifically associated with either parental or infanticidal behavior. No brain activation in males was specifically associated with infanticidal behavior. Our results suggest that 15 min of exposure to pups is enough to detect brain regions associated with parental behavior (PL) or pups processing (NA, MA, CoA) in mice. The PL might participate in the immediate onset of parental behavior in virgin females, coordinating and planning its rapid execution.

Impaired experience-dependent maternal care in presynaptic active zone protein CAST-deficient dams

Although sociological studies affirm the importance of parental care in the survival of offspring, maltreatment—including child neglect—remains prevalent in many countries. While child neglect is well known to affect child development, the causes of maternal neglect are poorly understood. Here, we found that female mice with a deletion mutation of CAST (a presynaptic release-machinery protein) showed significantly reduced weaning rate when primiparous and a recovered rate when multiparous. Indeed, when nurturing, primiparous and nulliparous CAST knock out (KO) mice exhibited less crouching time than control mice and moved greater distances. Contrary to expectations, plasma oxytocin (OXT) was not significantly reduced in CAST KO mice even though terminals of magnocellular neurons in the posterior pituitary expressed CAST. We further found that compared with control mice, CAST KO mice drank significantly less water when nurturing and had a greater preference for sucrose during pregnancy. We suggest that deficiency in presynaptic release-machinery protein impairs the facilitation of some maternal behaviours, which can be compensated for by experience and learning.

The Role of Olfactory Genes in the Expression of Rodent Paternal Care Behavior

Olfaction is the dominant sensory modality in rodents, and is crucial for regulating social behaviors, including parental care. Paternal care is rare in rodents, but can have significant consequences for offspring fitness, suggesting a need to understand the factors that regulate its expression. Pup-related odor cues are critical for the onset and maintenance of paternal care. Here, I consider the role of olfaction in the expression of paternal care in rodents. The medial preoptic area shares neural projections with the olfactory and accessory olfactory bulbs, which are responsible for the interpretation of olfactory cues detected by the main olfactory and vomeronasal systems. The olfactory, trace amine, membrane-spanning 4-pass A, vomeronasal 1, vomeronasal 2 and formyl peptide receptors are all involved in olfactory detection. I highlight the roles that 10 olfactory genes play in the expression of direct paternal care behaviors, acknowledging that this list is not exhaustive. Many of these genes modulate parental aggression towards intruders, and facilitate the recognition and discrimination of pups in general. Much of our understanding comes from studies on non-naturally paternal laboratory rodents. Future studies should explore what role these genes play in the regulation and expression of paternal care in naturally biparental species.

Using maternal rescue of pups in a cup to investigate mother-infant interactions in mice/rodents

Efficient parental care is indispensable for survival of the mammalian offspring, and therefore both parents and offspring cooperate to achieve the best performance. For example, when parents transport altricial offspring, the offspring immediately respond by reducing its cry and movement in both human infants and rodent pups. This coordinated set of central, motor and cardiac responses is designated as the Transport Response (TR) and is shown to facilitate maternal carrying in rodents. The present study aims to investigate the core behavioural characteristics of mother-infant interaction, and to investigate the mechanisms underlying the mother-pup cooperation using pharmacological and genetic manipulations (i.e. Oprm1-/). Along with the clear developmental changes of the pups' immobility and posture during maternal carrying as previously reported, there were also adaptations in maternal strategies, particularly in positioning of foothold and oral grasp over the pup's body, with the pups' age and pup's behaviour. Tree-based models elucidated that both of these maternal variables as well as percentage of pups' struggle predict the time required for pup retrieval from a cup. When the sensory-motor control in pups was disturbed by pharmacological or genetic manipulations, these core behaviours were inefficiently performed and impede maternal retrieval. Mother-infant mutual fit is a complex construct where several intermingled mechanisms are involved. Thus mothers and infants, when interacting, should be considered together as one whole system in which any change in one side or the other, affects the output of the whole dyad. The outcome of the interaction relays on a specific dynamic pattern of infant and maternal behaviours, which mutually change and adapt to fit each other's needs. Key features to reach a successful outcome of the interaction were the maternal retrieving strategy and infants' Transport Response behaviour.

The Role of the N-Methyl-D-Aspartate Receptors in Social Behavior in Rodents

The appropriate display of social behaviors is essential for the well-being, reproductive success and survival of an individual. Deficits in social behavior are associated with impaired N-methyl-D-aspartate (NMDA) receptor-mediated neurotransmission. In this review, we describe recent studies using genetically modified mice and pharmacological approaches which link the impaired functioning of the NMDA receptors, especially of the receptor subunits GluN1, GluN2A and GluN2B, to abnormal social behavior. This abnormal social behavior is expressed as impaired social interaction and communication, deficits in social memory, deficits in sexual and maternal behavior, as well as abnormal or heightened aggression. We also describe the positive effects of pharmacological stimulation of the NMDA receptors on these social deficits. Indeed, pharmacological stimulation of the glycine-binding site either by direct stimulation or by elevating the synaptic glycine levels represents a promising strategy for the normalization of genetically-induced, pharmacologically-induced or innate deficits in social behavior. We emphasize on the importance of future studies investigating the role of subunit-selective NMDA receptor ligands on different types of social behavior to provide a better understanding of the underlying mechanisms, which might support the development of selective tools for the optimized treatment of disorders associated with social deficits.

Sexually dimorphic oxytocin receptor-expressing neurons in the preoptic area of the mouse brain

Oxytocin is involved in the regulation of social behaviors including parental behaviors in a variety of species. Oxytocin triggers social behaviors by binding to oxytocin receptors (OXTRs) in various parts of the brain. OXTRs are present in the preoptic area (POA) where hormone-sensitive sexually dimorphic nuclei exist. The present study was conducted to examine whether sex differences exist in the distribution of neurons expressing OXTRs in the POA. Using OXTR-Venus (an enhanced variant of yellow fluorescent protein) mice, the distribution of OXTR-Venus cells in the POA was compared between sexes. The total number of OXTR-Venus cells in the medial POA (MPOA) was significantly greater in females than in males. No detectable OXTR-Venus cells were observed in the anteroventral periventricular nucleus (AVPV) within the MPOA in most of the brain sections from males. We further examined the total number of OXTR-Venus cells in the AVPV and the rest of the MPOA between the sexes. The total number of OXTR-Venus cells in the AVPV in females (615 ± 43) was significantly greater than that in males (14 ± 2), whereas the total number of OXTR-Venus cells in the rest of the MPOA did not differ significantly between the sexes. Thus, the sexually dimorphic expression of OXTR-Venus specifically occurred in the AVPV, but not in the rest of the MPOA. We also examined whether the expression of OXTR in the AVPV is driven by the female gonadal hormone, estrogen. Immunocytochemistry and single-cell RT-PCR revealed the presence of the estrogen receptor α in OXTR-Venus cells in the female AVPV. Moreover, ovariectomy resulted in the absence of OXTR-Venus expression in the AVPV, whereas estrogen replacement therapy restored OXTR-Venus expression. These results demonstrate that the expression of OXTR in the AVPV is primarily female specific and estrogen dependent. The presence of the sexually dimorphic expression of OXTR in the AVPV suggests the involvement of OXTR neurons in the AVPV in the regulation of female-specific behavior and/or physiology.

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.

Glutamatergic Signals in the Dorsal Raphe Nucleus Regulate Maternal Aggression and Care in an Opposing Manner in Mice

Lactating female mice nurture their pups and attack intruders in their territory. When an intruder invades a dam's territory, she needs to switch her behavior from care to aggression to protect her pups and territory. Although the neuronal mechanisms underlying each distinct behavior have been studied, it is unclear how these behaviors are displayed alternatively. The dorsal raphe nucleus (DRN) regulates both nurturing and aggressive behaviors. In the present study, we examined whether the DRN is involved in regulating alternative display of maternal care and aggression. We first examined neuronal activity in the medial prefrontal cortex (mPFC) and lateral habenula (LHb), which send glutamatergic input to the DRN, in dams by injecting Fluorogold, a retrograde tracer, into the DRN. The number of c-Fos- and Fluorogold-positive neurons in the mPFC and LHb increased in the dams that displayed biting behavior in response to an intruder, but remained unchanged in the dams that displayed nurturing behavior. Injections of N-methyl-d-aspartic acid (NMDA) receptor antagonists or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor antagonists into the DRN inhibited biting behavior but not nurturing behavior. In contrast, injections of NMDA or AMPA into the DRN inhibited nurturing behavior. These results suggest that glutamatergic signals in the DRN, which may originate from the mPFC and/or LHb, regulate the preferential display of biting behavior over nurturing behavior in dams.

Mothering under the influence: how perinatal drugs of abuse alter the mother-infant interaction

Although drug-abusing women try to moderate their drug and alcohol use during pregnancy, they often relapse at a time when childcare needs are high and maternal bonding is critical to an infant's development. In the clinical setting, the search for the neural basis of drug-induced caregiving deficits is complex due to several intervening variables. Rather, the preclinical studies that control for drug dose and regimen, as well as for gestational and postpartum environment, allow a precise determination of the effects of drugs on maternal behaviour. Given the relevance of the issue, this review will gather reports on the phenotypic correlates of maternal behaviour in preclinical studies, and focus on the detrimental consequences on the mother-infant interaction exerted by the perinatal use of alcohol, nicotine, cannabis, cocaine and stimulants and opiates. The drug-induced disruptions of this maternal repertoire are associated with adverse maternal and infant outcomes. A comprehensive overview will help promote the refinement of the treatment approaches toward maternal drug use disorders and maternal misbehaviour, in favour of augmented parenting resiliency.

Energy homeostasis and running wheel activity during pregnancy in the mouse

Pregnancy and lactation are metabolically challenging states, where the mother must supply all the energy requirements for the developing fetus and growing pups respectively. The aim of the current study was to characterize many aspects of energy homeostasis before and during pregnancy in the mouse, and to examine the role of voluntary activity on changes in energy expenditure during pregnancy. In a secondary aim, we evaluate measures of energy homeostasis during pregnancy in mice that successfully reared their litter or in mice that went on to abandon their litter, to determine if an impairment in pregnancy-induced adaptation of energy homeostasis might underlie the abandonment of pups soon after birth. During pregnancy, food intake was increased, characterized by increased meal size and duration but not number of meals per day. The duration of time spent inactive, predicted to indicate sleep behaviour, was increased both early and late in pregnancy compared to pre-pregnancy levels. Increased x + y beam breaks, as a measure of activity increased during pregnancy and this reflected an increase in ambulatory behaviour in mid pregnancy and an increase in non-ambulatory movement in late pregnancy. Energy expenditure, as measured by indirect calorimetry, increased across pregnancy, likely due to the growth and development of fetal tissue. There was also a dramatic reduction in voluntary wheel running as soon as the mice became pregnant. Compared with successful pregnancies and lactations, pregnancies where pups were abandoned soon after birth were associated with reduced body weight gain and an increase in running wheel activity at the end of pregnancy, but no difference in food intake or energy expenditure. Overall, during pregnancy there are multiple adaptations to change energy homeostasis, resulting in partitioning of provisions of energy to the developing fetus and storing energy for future metabolic demands.

CD38 is Required for Dendritic Organization in Visual Cortex and Hippocampus

Morphological screening of mouse brains with known behavioral deficits can give great insight into the relationship between brain regions and their behavior. Oxytocin- and CD38-deficient mice have previously been shown to have behavioral phenotypes, such as restrictions in social memory, social interactions, and maternal behavior. CD38 is reported as an autism spectrum disorder (ASD) candidate gene and its behavioral phenotypes may be linked to ASD. To address whether these behavioral phenotypes relate to brain pathology and neuronal morphology, here we investigate the morphological changes in the CD38-deficient mice brains, with focus on the pathology and neuronal morphology of the cortex and hippocampus, using Nissl staining, immunohistochemistry, and Golgi staining. No difference was found in terms of cortical layer thickness. However, we found abnormalities in the number of neurons and neuronal morphology in the visual cortex and dentate gyrus (DG). In particular, there were arborisation differences between CD38^-/- and CD38^+/+ mice in the apical dendrites of the visual cortex and hippocampal CA1 pyramidal neurons. The data suggest that CD38 is implicated in appropriate development of brain regions important for social behavior.

Behavioral Assays in the Study of Olfaction: A Practical Guide

Olfaction is a fundamental sense in most animal species. In mammals, the olfactory system comprises several subpopulations of sensory neurons located throughout the nasal cavity, which detect a variety of chemostimuli, including odorants, intraspecies and interspecies chemical communication cues. Some of these compounds are important for regulating innate and learned behaviors, and endocrine changes in response to other animals in the environment. With a particular focus on laboratory rodent species, this chapter provides a comprehensive description of the most important behavioral assays used for studying the olfactory system, and is meant to be a practical guide for those who study olfaction-mediated behaviors or who have an interest in deciphering the molecular, cellular, or neural mechanisms through which the sense of smell controls the generation of adaptive behavioral outputs.

Maternal prolactin during late pregnancy is important in generating nurturing behavior in the offspring

Although maternal nurturing behavior is extremely important for the preservation of a species, our knowledge of the biological underpinnings of these behaviors is insufficient. Here we show that the degree of a mother's nurturing behavior is regulated by factors present during her own fetal development. We found that Cin85-deficient (Cin85^-/-) mother mice had reduced pituitary hormone prolactin (PRL) secretion as a result of excessive dopamine signaling in the brain. Their offspring matured normally and produced their own pups; however, nurturing behaviors such as pup retrieval and nursing were strongly inhibited. Surprisingly, when WT embryos were transplanted into the fallopian tubes of Cin85^-/- mice, they also exhibited inhibited nurturing behavior as adults. Conversely, when Cin85^-/- embryos were transplanted into the fallopian tubes of WT mice, the resultant pups exhibited normal nurturing behaviors as adults. When PRL was administered to Cin85^-/- mice during late pregnancy, a higher proportion of the resultant pups exhibited nurturing behaviors as adults. This correlates with our findings that neural circuitry associated with nurturing behaviors was less active in pups born to Cin85^-/- mothers, but PRL administration to mothers restored neural activity to normal levels. These results suggest that the prenatal period is extremely important in determining the expression of nurturing behaviors in the subsequent generation, and that maternal PRL is one of the critical factors for expression. In conclusion, perinatally secreted maternal PRL affects the expression of nurturing behaviors not only in a mother, but also in her pups when they have reached adulthood.

Presence of pups suppresses hunger-induced feeding in virgin adult mice of both sexes

Despite recent progress on neural pathways underlying individual behaviors, how an animal balances and prioritizes behavioral outputs remains poorly understood. While studying the relationship between hunger-induced feeding and pup-induced maternal behaviors in virgin female mice, we made the unexpected discovery that presence of pups strongly delayed and decreased food consumption. Strikingly, presence of pups also suppressed feeding induced by optogenetic activation of Agrp neurons. Such a suppressive effect inversely correlated with the extents of maternal behaviors, but did not rely on the display of these behaviors, and was also present in virgin males. Furthermore, chemogenetic activation of Vglut2+ neurons in the medial preoptic area (mPOA), a region critical for maternal behaviors and motivation, was sufficient to suppress hunger-induced feeding. However, muscimol inhibition of the mPOA, while disrupting maternal behaviors, did not prevent pup suppression of feeding, indicating that neural pathways in other brain regions may also mediate such an effect. Together, these results provide novel insights into neural coordination of pup care and feeding in mice and organizations of animal behaviors in general.

The genetic basis of parental care evolution in monogamous mice

Parental care is essential for the survival of mammals, yet the mechanisms underlying its evolution remain largely unknown. Here we show that two sister species of mice, Peromyscus polionotus and P. maniculatus, have large and heritable differences in parental behaviour. Using quantitative genetics, we identify 12 genomic regions that affect parental care, eight of which have sex-specific effects, suggesting that parental care can evolve independently in males and females. Furthermore, some regions affect parental care broadly, whereas others affect specific behaviours, such as nest building. Of the genes linked to differences in nest-building behaviour, vasopressin is differentially expressed in the hypothalamus of the two species, with increased levels associated with less nest building. Using pharmacology in Peromyscus and chemogenetics in Mus, we show that vasopressin inhibits nest building but not other parental behaviours. Together, our results indicate that variation in an ancient neuropeptide contributes to interspecific differences in parental care.

Oxytocin and Parental Behaviors

The oxytocin/vasopressin ancestor molecule has been regulating reproductive and social behaviors for more than 500 million years. In all mammals, oxytocin is the hormone indispensable for milk-ejection during nursing (maternal milk provision to offspring), a process that is crucial for successful mammalian parental care. In laboratory mice, a remarkable transcriptional activation occurs during parental behavior within the anterior commissural nucleus (AC), the largest magnocellular oxytocin cell population within the medial preoptic area (although the transcriptional activation was limited to non-oxytocinergic neurons in the AC). Furthermore, there are numerous recent reports on oxytocin's involvement in positive social behaviors in animals and humans. Given all those, the essential involvement of oxytocin in maternal/parental behaviors may seem obvious, but basic researchers are still struggling to pin down the exact role oxytocin plays in the regulation of parental behaviors. A major aim of this review is to more clearly define this role. The best conclusion at this moment is that OT can facilitate the onset of parental behavior, or parental behavior under stressful conditions.In this chapter, we will first review the basics of rodent parental behavior. Next, the neuroanatomy of oxytocin systems with respect to parental behavior in laboratory mice will be introduced. Then, the research history on the functional relationship between oxytocin and parental behavior, along with advancements in various techniques, will be reviewed. Finally, some technical considerations in conducting behavioral experiments on parental behavior in rodents will be addressed, with the aim of shedding light on certain pitfalls that should be avoided, so that the progress of research in this field will be facilitated. In this age of populism, researchers should strive to do even more scholarly works with further attention to methodological details.

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.