Maternal behavior: activation of the central oxytocin receptor system in parturient rats?

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.

Microglia depletion facilitates the display of maternal behavior and alters activation of the maternal brain network in nulliparous female rats

The peripartum period is accompanied by peripheral immune alterations to promote a successful pregnancy. We and others have also demonstrated significant neuroimmune changes that emerge during late pregnancy and persist postpartum, most prominently decreased microglia numbers within limbic brain regions. Here we hypothesized that microglial downregulation is important for the onset and display of maternal behavior. To test this, we recapitulated the peripartum neuroimmune profile by depleting microglia in non-mother (i.e., nulliparous) female rats who are typically not maternal but can be induced to behave maternally towards foster pups after repeated exposure, a process called maternal sensitization. BLZ945, a selective colony-stimulating factor 1 receptor (CSF1R) inhibitor, was administered systemically to nulliparous rats, which led to ~75% decrease in microglia number. BLZ- and vehicle-treated females then underwent maternal sensitization and tissue was stained for ∆fosB to examine activation across maternally relevant brain regions. We found BLZ-treated females with microglial depletion met criteria for displaying maternal behavior significantly sooner than vehicle-treated females and displayed increased pup-directed behaviors. Microglia depletion also reduced threat appraisal behavior in an open field test. Notably, nulliparous females with microglial depletion had decreased numbers of ∆fosB+ cells in the medial amygdala and periaqueductal gray, and increased numbers in the prefrontal cortex and somatosensory cortex, compared to vehicle. Our results demonstrate that microglia regulate maternal behavior in adult females, possibly by shifting patterns of activity in the maternal brain network.

Involvement of oxytocin receptor deficiency in psychiatric disorders and behavioral abnormalities

Oxytocin and its target receptor (oxytocin receptor, OXTR) exert important roles in the regulation of complex social behaviors and cognition. The oxytocin/OXTR system in the brain could activate and transduce several intracellular signaling pathways to affect neuronal functions or responses and then mediate physiological activities. The persistence and outcome of the oxytocin activity in the brain are closely linked to the regulation, state, and expression of OXTR. Increasing evidence has shown that genetic variations, epigenetic modification states, and the expression of OXTR have been implicated in psychiatric disorders characterized by social deficits, especially in autism. Among these variations and modifications, OXTR gene methylation and polymorphism have been found in many patients with psychiatric disorders and have been considered to be associated with those psychiatric disorders, behavioral abnormalities, and individual differences in response to social stimuli or others. Given the significance of these new findings, in this review, we focus on the progress of OXTR's functions, intrinsic mechanisms, and its correlations with psychiatric disorders or deficits in behaviors. We hope that this review can provide a deep insight into the study of OXTR-involved psychiatric disorders.

Adult mammalian neurogenesis and motivated behaviors

Adult neurogenesis continues to captivate the curiosity of the scientific community; and researchers seem to have a particular interest in identifying the functional implications of such plasticity. While the majority of research focuses on the association between adult neurogenesis and learning and memory (including spatial learning associated with hippocampal neurogenesis and olfactory discrimination associated with neurogenesis in the olfactory system), the following review will explore the link to motivated behaviors. In particular, goal-directed behaviors such as sociosexual, parental, aggressive, as well as depression- and anxiety-like behaviors and their reciprocal association to adult neurogenesis will be evaluated. The review will detail research in humans and other mammalian species. Furthermore, the potential mechanisms underlying these neurogenic alterations will be highlighted. Lastly, the review will conclude with a discussion on the functional significance of these newly generated cells in mediating goal-directed behaviors.

Puerperal and parental experiences alter rat preferences for pup odors via changes in the oxytocin system

In the rat, induction of maternal behavior depends on the parity of the female. For example, nulliparous (NP) females need longer exposure to pups than multiparous (MP) or lactating (L) females to exhibit similar maternal behavior. In this study, we investigated the role of brain oxytocin in the approaching behavior of these female rats. Olfactory preferences for pup odors were examined for 8 consecutive days. Each preference test was followed by direct overnight exposure to pups. On the 8th day, MP and L, but not NP females showed robust pup-odor preferences. After the behavioral test, half of the females were exposed to pups for 2 h, whereas the other half were not. The females were then sacrificed to analyze brain oxytocin (OXT) and vasopressin (AVP) activities by cFos immunohistochemistry and to quantify their receptor mRNA expression using real-time PCR. In the paraventricular nucleus (PVN), the percentage of cFos-positive OXT neurons was significantly larger in MP and L females than in NP females after pup exposure. No significant differences were found in cFos expression in OXT neurons of the supraoptic nucleus (SON) or in AVP neurons of either the PVN or SON. Expression of OXT receptor mRNA in the medial preoptic area and amygdala of the control groups was also higher in MP females than in NP females. Finally, we demonstrated that infusion of OXT into the lateral ventricle of NP females promoted preferences for pup odors. These results indicate that puerperal and parental experiences enhance the responsiveness of OXT neurons in the PVN to pup stimuli and establish olfactory preferences for these odors in a parity-dependent manner.

Preoptic inputs and mechanisms that regulate maternal responsiveness

The preoptic area is a well-established centre for the control of maternal behaviour. An intact medial preoptic area (mPOA) is required for maternal responsiveness because lesion of the area abolishes maternal behaviours. Although hormonal changes in the peripartum period contribute to the initiation of maternal responsiveness, inputs from pups are required for its maintenance. Neurones are activated in different parts of the mPOA in response to pup exposure. In the present review, we summarise the potential inputs to the mPOA of rodent dams from the litter that can activate mPOA neurones. The roles of potential indirect effects through increased prolactin levels, as well as neuronal inputs to the preoptic area, are described. Recent results on the pathway mediating the effects of suckling to the mPOA suggest that neurones containing the neuropeptide tuberoinfundibular peptide of 39 residues in the posterior thalamus are candidates for conveying the suckling information to the mPOA. Although the molecular mechanism through which these inputs alter mPOA neurones to support the maintenance of maternal responding is not yet known, altered gene expression is a likely candidate. Here, we summarise gene expression changes in the mPOA that have been linked to maternal behaviour and explore the idea that chromatin remodelling during mother-infant interactions mediates the long-term alterations in gene expression that sustain maternal responding.

Hypothalamic expression of Peg3 gene is associated with maternal care differences between SM/J and LG/J mouse strains

Maternal care is essential in mammals, and variations in the environment provided by mothers may directly influence the viability of newborns and emotional behavior later in life. A previous study investigated genetic variations associated with maternal care in an intercross of LG/J and SM/J inbred mouse strains and identified two single-locus QTLs (quantitative trait loci). Here, we selected three candidate genes located within these QTLs intervals; Oxt on chromosome 2, and FosB and Peg3 on chromosome 7 and tested their association with maternal care. LG/J females showed impaired postpartum nest building and pup retrieval, a one-day delay in milk ejection, reduced exploratory activity, and higher anxiety-like behavior when compared to SM/J females. The nucleotide sequences of Oxt and FosB were similar between strains, as were their hypothalamic expression levels. Conversely, Peg3 nucleotide sequences showed four nonsynonymous replacement substitutions on LG/J dams, T11062G, G13744A, A13808G, and G13813A, and a 30 base pair (10 aa) in tandem repeat in the coding region with three copies in SM/J and five copies in LG/J. Maternal care impaired LG/J mothers express 37% lower Peg3 mRNA levels in the hypothalamus on the second postpartum day. We also found an association of the Peg3 repeat-variant and poor maternal care in F(2) heterozygote females derived from a LG/J × SM/J intercross. These results may suggest that the maternally imprinted Peg3 gene is responsible for the single-locus QTL on chromosome 7 that has been shown to influence maternal care in these strains. Furthermore, these data provide additional support for an epigenetic regulation of maternal behavior.

The social environment and neurogenesis in the adult Mammalian brain

Adult neurogenesis - the formation of new neurons in adulthood - has been shown to be modulated by a variety of endogenous (e.g., trophic factors, neurotransmitters, and hormones) as well as exogenous (e.g., physical activity and environmental complexity) factors. Research on exogenous regulators of adult neurogenesis has focused primarily on the non-social environment. More recently, however, evidence has emerged suggesting that the social environment can also affect adult neurogenesis. The present review details the effects of adult-adult (e.g., mating and chemosensory interactions) and adult-offspring (e.g., gestation, parenthood, and exposure to offspring) interactions on adult neurogenesis. In addition, the effects of a stressful social environment (e.g., lack of social support and dominant-subordinate interactions) on adult neurogenesis are reviewed. The underlying hormonal mechanisms and potential functional significance of adult-generated neurons in mediating social behaviors are also discussed.

CD38 gene knockout juvenile mice: a model of oxytocin signal defects in autism

Oxytocin (OXT) in the hypothalamus is the biological basis of social recognition, trust, and bonding. We showed that CD38, a leukaemia cell marker, plays an important role in the hypothalamus in the process of OXT release in adult mice. Disruption of Cd38 (Cd38(-/-)) produced impairment of maternal behavior and male social recognition in mice, similar to the behavior observed in Oxt and OXT receptor (Oxtr) gene knockout (Oxt(-/-) and Oxtr(-/-), respectively) mice. Locomotor activity induced by separation from the dam was higher and the number of ultrasonic vocalization (USV) calls was lower in Cd38(-/-) than Cd38(+/+) pups. These phenotypes seemed to be caused by the high plasma OXT levels during development from neonates to 3-week-old juvenile mice. ADP-ribosyl cyclase activity was markedly lower in the knockout mice from birth, suggesting that weaning for mice is a critical time window of differentiating plasma OXT. Contribution by breastfeeding was an important exogenous source for regulating plasma OXT before weaning by the presence of OXT in milk and the dam's mammary glands. The dissimilarity of Cd38(-/-) infant behaviour to Oxt(-/-) or Oxtr(-/-) mice can be explained partly by this exogenous source of OXT. These results suggest that secretion of OXT into the brain in a CD38-dependent manner may play an important role in the development of social behavior, and mice with OXT signalling deficiency, including Cd38(-/-), Oxt(-/-) and Oxtr(-/-) mice are good animal models for developmental disorders, such as autism.

Oxytocin signal and social behaviour: comparison among adult and infant oxytocin, oxytocin receptor and CD38 gene knockout mice

Oxytocin in the hypothalamus is the biological basis of social recognition, trust, love and bonding. Previously, we showed that CD38, a proliferation marker in leukaemia cells, plays an important role in the hypothalamus in the process of oxytocin release in adult mice. Disruption of Cd38 (Cd38 (-/-)) elicited impairment of maternal behaviour and male social recognition in adult mice, similar to the behaviour observed in Oxt and oxytocin receptor (Oxtr) gene knockout (Oxt (-/-) and Oxtr (-/-), respectively) mice. Locomotor activity induced by separation from the dam was higher and the number of ultrasonic vocalisation calls was lower in Cd38 (-/-) than Cd38( +/+) pups. However, these behavioural changes were much milder than those observed in Oxt (-/-) and Oxtr (-/-) mice, indicating less impairment of social behaviour in Cd38 (-/-) pups. These phenotypes appeared to be caused by the high plasma oxytocin levels during development from the neonatal period to 3-week-old juvenile mice. ADP-ribosyl cyclase activity was markedly lower in the knockout mice from birth, suggesting that weaning for mice is a critical time window of plasma oxytocin differentiation. Breastfeeding was an important exogenous source of plasma oxytocin regulation before weaning as a result of the presence of oxytocin in milk and the dam's mammary glands. The dissimilarity between Cd38 (-/-) infant behaviour and those of Oxt (-/-) or Oxtr (-/-) mice can be explained partly by this exogenous source of oxytocin. These results suggest that secretion of oxytocin into the brain in a CD38-dependent manner may play an important role in the development of social behaviour.

Locomotor activity, ultrasonic vocalization and oxytocin levels in infant CD38 knockout mice

Oxytocin (OT), a neurohormone involved in reproduction, plays a critical role in social behavior in a wide range of mammalian species from rodents to humans. The role of CD38 in regulating OT secretion for social behavior has been demonstrated in adult mice, but has not been examined in pups or during development. Separation from the dam induces stress in 7-day-old mouse pups. During such isolation, locomotor activity was higher in CD38 knockout (CD38(-/-)) pups than in wild-type (CD38(+/+)) or heterozygous (CD38(+/-)) controls. The number of ultrasonic vocalizations was lower in CD38(-/-) pups than in CD38(+/+) pups. However, the difference between the two genotypes was less severe than that in OT knockout or OT receptor knockout mice. To explain this, we measured plasma OT levels. The level was not lower in CD38(-/-) pups during the period 1-3 weeks after birth, but was significantly reduced after weaning (>3 weeks). ADP-ribosyl cyclase activities in the hypothalamus and pituitary were markedly lower from 1 week after birth in CD38(-/-) mice and were consistently lower thereafter to the adult stage (2 months old). These results showed that the reduced severity of behavioral abnormalities in CD38(-/-) pups was due to partial compensation by the high level of plasma OT.

Dynamic changes in oxytocin receptor expression and activation at parturition in the rat brain

Oxytocin plays a pivotal role in rat parturition, acting within the brain to facilitate its own release in the supraoptic nucleus (SON) and paraventricular nucleus, and to stimulate maternal behavior. We investigated oxytocin receptor (OTR) expression and activation perinatally. Using a (35)S-labeled riboprobe complementary to OTR mRNA, OTR expression was quantified in proestrus virgin, 21- and 22-day pregnant, parturient (90 min. from pup 1 birth), and postpartum (4-12 h from parturition) rats. Peak OTR mRNA expression was observed at parturition in the SON, brainstem regions, medial preoptic area (mPOA), bed nucleus of the stria terminalis (BnST), and olfactory bulbs, but there was no change in the paraventricular nucleus and lateral septum. OTR mRNA expression was increased on the day of expected parturition in the SON and brainstem, suggesting that oxytocin controls the pathway mediating input from uterine signals. Likewise, OTR mRNA expression was increased in the mPOA and BnST during labor/birth. In the olfactory bulbs and medial amygdala, parturition induced increased OTR mRNA expression compared with pre-parturition, reflecting their immediate response to new stimuli at birth. Postpartum OTR expression in all brain regions returned to levels observed in virgin rats. Parturition significantly increased the number of double-immunolabeled cells for Fos and OTR within the SON, brainstem, BnST, and mPOA regions compared with virgin rats. Thus, there are dynamic region-dependent changes in OTR-expressing cells at parturition. This altered OTR distribution pattern in the brain perinatally reflects the crucial role oxytocin plays in orchestrating both birth and maternal behavior.

Neural circuits underlying crying and cry responding in mammals

Crying is a universal vocalization in human infants, as well as in the infants of other mammals. Little is known about the neural structures underlying cry production, or the circuitry that mediates a caregiver's response to cry sounds. In this review, the specific structures known or suspected to be involved in this circuit are identified, along with neurochemical systems and hormones for which evidence suggests a role in responding to infants and infant cries. In addition, evidence that crying elicits parental responses in different mammals is presented. An argument is made for including 'crying' as a functional category in the vocal repertoire of all mammalian infants (and the adults of some species). The prevailing neural model for crying production considers forebrain structures to be dispensable. However, evidence for the anterior cingulate gyrus in cry production, and this structure along with the amygdala and some other forebrain areas in responding to cries is presented.

Heterotrimeric G proteins of the Gq/11 family are crucial for the induction of maternal behavior in mice

Heterotrimeric G proteins of the G(q/11) family transduce signals from a variety of neurotransmitter receptors and have therefore been implicated in several functions of the central nervous system. To investigate the potential role of G(q/11) signaling in behavior, we generated mice which lack the alpha-subunits of the two main members of the G(q/11) family, Galpha(q) and Galpha(11), selectively in the forebrain. We show here that forebrain Galpha(q/11)-deficient females do not display any maternal behavior such as nest building, pup retrieving, crouching, or nursing. However, olfaction, motor behavior and mammary gland function are normal in forebrain Galpha(q/11)-deficient females. We used c-fos immunohistochemistry to investigate pup-induced neuronal activation in different forebrain regions and found a significant reduction in the medial preoptic area, the bed nucleus of stria terminalis, and the lateral septum both in postpartum females and in virgin females after foster pup exposure. Pituitary function, especially prolactin release, was normal in forebrain Galpha(q/11)-deficient females, and activation of oxytocin receptor-positive neurons in the hypothalamus did not differ between genotypes. Our findings show that G(q/11) signaling is indispensable to the neuronal circuit that connects the perception of pup-related stimuli to the initiation of maternal behavior and that this defect cannot be attributed to either reduced systemic prolactin levels or impaired activation of oxytocin receptor-positive neurons of the hypothalamus.