Variations in maternal behaviour are associated with differences in oxytocin receptor levels in the rat

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.

Regulation of anxiety during the postpartum period

Healthy mother-infant interactions are critical for the physical, cognitive, and psychological development of offspring. Such interactions rely on numerous factors, including a positive maternal emotional state. However, many postpartum women experience emotional dysregulation, often involving elevated anxiety. Neuroendocrine factors contributing to the onset of postpartum anxiety symptoms are mostly unknown, but irregularities in hypothalamic-pituitary-adrenal axis function, reduced prolactin and oxytocin signaling, or parturitional withdrawal of ovarian, placental and neural steroids could contribute to anxiety in susceptible women. Although the causes of initial onset are unclear, postpartum anxiety can be mitigated by recent contact with infants. Numerous neurochemical systems, including oxytocin, prolactin, GABA, and norepinephrine mediate this anxiolytic effect of infant contact. Insight into the etiology of postpartum anxiety disorders, and how contact with infants helps counter existing anxiety dysregulation, will surely facilitate the diagnosis and treatment of postpartum women at risk for, or experiencing, an anxiety disorder.

Cocaine disrupts pup-induced maternal behavior in juvenile and adult rats

Impaired onset of maternal behavior in first generation rat dams was previously correlated with rearing by cocaine-treated dams and prenatal cocaine exposure. Pup-induced maternal behavior in non-lactating rats has not been examined with regard to cocaine exposure and rearing conditions. First generation male and female juveniles and young adult males reared by cocaine-treated or control dams and prenatally exposed to either cocaine or control conditions were tested for pup-induced maternal behavior at postnatal days 28 and 60. We now report disruptions in pup-induced maternal behavior in both 28 and 60 day old first generation offspring attributable to rearing condition and prenatal cocaine exposure.

Importance of early lighting conditions in maternal care by dam as well as anxiety and memory later in life of offspring

Rodent studies have revealed that the early rearing environment plays an important role in the development of stress vulnerability, memory and cognition. Although early lighting conditions (ELC) are involved in these neuronal developments through both maternal and offspring behavior, their influence has not been fully elucidated. Thus, by using Sprague-Dawley rats, we examined whether ELC affected maternal care by the dam and the subsequent neurodevelopment of the offspring. Prolonged dark phase conditions (PDC) (light/dark, 6/18 h) and prolonged light phase conditions (light/dark, 18/6 h) were administered from postnatal day 2 to postnatal day 14. Throughout this period, maternal care and the circadian rhythmicity of dams were investigated. In adolescence and adulthood of the offspring, we measured anxiety-like behavior, social interaction, object recognition memory, activity rhythm and corticosterone response to stress with hippocampal expression of N-methyl-D-aspartate and glucocorticoid receptor mRNAs. PDC altered maternal care and circadian rhythmicity in the dam compared with normal lighting conditions and prolonged light phase conditions. PDC markedly increased anxiety-like behavior, decreased social interaction and object recognition memory, and inhibited corticosterone feedback in offspring later in life. Furthermore, hippocampal levels of glucocorticoid receptor mRNA and N-methyl-D-aspartate receptor 2B mRNA in rats subjected to PDC were significantly lower than in animals subjected to normal lighting conditions. In the adult offspring, the circadian rhythm of locomotor activity was not affected. These findings suggested that ELC affect mother-infant interactions and subsequently at least partially alter the neurobehavioral development of offspring.

Individual differences in novelty-seeking and emotional reactivity correlate with variation in maternal behavior

Numerous studies have demonstrated that Sprague-Dawley rats exhibit a wide range of locomotor reactivity when placed in a novel environment. High Responder (HR) rats show exaggerated locomotor response to novelty, enhanced neuroendocrine stress reactivity, decreased anxiety-like behavior, and propensity to self-administer psychostimulants, compared to the less active Low Responder (LR) animals. Few studies have explored the early environmental factors which may underlie the HR-LR differences in emotional reactivity. Considering the enormous impact of maternal care on rodent neurodevelopment, we sought to examine maternal behavior in HR-LR dams to determine whether they exhibit differences which could contribute to their offspring's differential temperaments. Females, like males, can be classified as HR versus LR, showing marked differences in novelty-induced locomotor activity and anxiety-like behavior. HR-LR mothers behaved differently with their pups during the first two postpartum weeks. LR dams spent greater time licking and nursing their pups compared to HR dams, with the most prominent differences occurring during the second postpartum week. By contrast, when non-lactating HR-LR females were presented with orphaned pups, the pattern of maternal response was reversed. HR females were more responsive and showed greater maternal care of the novel pups compared to LR females, which were probably inhibited due to fear of the unfamiliar pups. This underscores the critical interplay between the female's emotional phenotype, her hormonal status and her familiarity with the pup as key factors in determining maternal behavior. Future work should explore neural and hormonal mechanisms which drive these HR-LR differences in maternal behavior and their impact on the development of the offspring.

Prenatal stress generates deficits in rat social behavior: Reversal by oxytocin

Neurodevelopmental changes induced by environmental stress exposure play a significant but poorly defined role in the etiology of schizophrenia. Exposure of pregnant female rats to a series of unpredictable stresses during the final week of pregnancy generates behavioral deficits and molecular changes in the offspring similar to those observed in schizophrenic individuals. We used this rat prenatal stress preparation to investigate social withdrawal behaviors that may have relevance to the negative symptoms of schizophrenia. The cumulative time adult male offspring of stress-exposed pregnant female rats actively interacted with a weight-matched, same-sex peer was decreased approximately 76% relative to non-stress exposed control rats. Prenatal stress exposure also diminished the quality of the social interaction behavior indicative of reduced social drive. Analysis of the oxytocinergic system in the prenatally stressed male rats revealed significantly less oxytocin mRNA in the paraventricular nucleus and increased oxytocin receptor binding in the central amygdala. Moreover, oxytocin, but not vasopressin, administration into the central amygdala reversed the social incompetence of the prenatally stressed rats without increasing behavior in non-stressed control animals. In addition, cross-fostering pups from prenatally stressed mothers to non-stressed mothers failed to improve the social deficit of the prenatally stressed male offspring. Two behavioral assays designed to measure anxiety did not differentiate the prenatally stressed rats from non-stressed controls. These data indicate that prenatal stress may be an etiologically appropriate animal model for some aspects of schizophrenic social withdrawal. Furthermore, unpredictable prenatal stress exposure selectively degrades social interaction behaviors without increasing anxiety measures.

Environmental Programming of Phenotypic Diversity in Female Reproductive Strategies

Among invertebrates, certain hermaphroditic species reproduce sexually, but with no process of sexual differentiation. In such cases the brain is bisexual: Each member of the species develops male and female sexual organs and retains the capacity to express both male and female patterns of reproductive behavior. Members of such species can reproduce socially or alone. Mammals and many other species reproduce both sexually and socially, which requires an active process of sexual differentiation of reproductive organs and brain. The primary theme of this chapter is simply that this process admits to variation and thus individual differences in gender-specific patterns of reproductive function. The focus on this chapter is the often neglected variation in the development of reproductive function in the female mammal. The basic premise is that evolution has not defined any single, optimal reproductive phenotype, but rather encourages plasticity in specific reproductive traits among same sex members of the species that are derived from variations in the quality of the prevailing environment during development that are mediated by alterations in parent-offspring interactions. Thus, the variations in parental care that define the reproductive phenotype of the offspring are influenced by the quality of the environment (i.e., nutrient availability, predation, infection, population density, and so on).

Glucocorticoid-mediated effects of systemic oxytocin upon memory retrieval

During the last decade, a considerable amount of evidence has accumulated to show that oxytocin (OT) is involved with functions other than its classical roles in reproduction-associated processes, such as social recognition, maternal behavior and neuroendocrine regulation of the stress response. It has been shown, for instance, that post-training systemic administration of oxytocin in mice produces an amnestic effect on the step-through inhibitory avoidance. Since it is still unclear how systemic levels of OT may affect CNS memory processes, our aim here was to investigate the hypothesis that systemic OT effects on memory retrieval might be mediated through an oxytocin-induced decrease in glucocorticoid release. In our first experiment, we have found an amnestic effect of i.p. pre-test 0.4 microg/kg of OT upon memory retrieval in the inhibitory avoidance task (IA); this OT dose was shown to (a) significantly decrease plasma corticosterone levels when compared to the saline group, and (b) not to cause any anxiety effects by itself in a plus-maze task. At last, an ineffective-by-itself dose of dexamethasone was able to reverse the amnestic effect of this OT dose. Our results suggest that the amnestic effect of systemically administered oxytocin upon memory retrieval in the inhibitory avoidance task was probably caused by an oxytocin-induced decrease in glucocorticoid release from the adrenal gland.

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.

Developmental plasticity of HPA and fear responses in rats: a critical review of the maternal mediation hypothesis

Developmental plasticity of HPA and fear responses in rats has been proposed to be mediated by environment-dependent variation in active maternal care. Here, we review this maternal mediation hypothesis based on the postnatal manipulation literature and on our own recent research in rats. We show that developmental plasticity of HPA and fear responses in rats cannot be explained by a linear single-factor model based on environment-dependent variation in active maternal care. However, by adding environmental stress as a second factor to the model, we were able to explain the variation in HPA and fear responses induced by postnatal manipulations. In this two-factor model, active maternal care and environmental stress (as induced, e.g., by long maternal separations or maternal food restriction) exert independent, yet opposing, effects on HPA reactivity and fearfulness in the offspring. This accounts well for the finding that completely safe and stable, as well as, highly stressful maternal environments result in high HPA reactivity and fearfulness compared to moderately challenging maternal environments. Furthermore, it suggests that the downregulation of the HPA system in response to stressful maternal environments could reflect adaptive developmental plasticity based on the increasing costs of high stress reactivity with increasingly stressful conditions. By contrast, high levels of environmental stress induced by environmental adversity might constrain such adaptive plasticity, resulting in non-adaptive or even pathological outcomes. Alternatively, however, developmental plasticity of HPA and fear responses in rats might be a function of maternal HPA activation (e.g., levels of circulating maternal glucocorticoid hormones). Thus, implying a U-shaped relationship between maternal HPA activation and HPA reactivity and fearfulness in the offspring, increasing maternal HPA activation with increasing environmental adversity would explain the effects of postnatal manipulations equally well. This raises the possibility that variation in active maternal care is an epiphenomenon, rather than a causal factor in developmental plasticity of HPA and fear responses in rats. Developmental plasticity of HPA and fear responses in rats and other animals has important implications for the design of animal experiments and for the well-being of experimental animals, both of which depend on the exact underlying mechanism(s). Importantly, however, more naturalistic approaches are needed to elucidate the adaptive significance of environment-dependent variation of HPA reactivity and fearfulness in view of discriminating between effects reflecting adaptive plasticity, phenotypic mismatch and pathological outcomes, respectively.

Neuropeptidergic regulation of affiliative behavior and social bonding in animals

Social relationships are essential for maintaining human mental health, yet little is known about the brain mechanisms involved in the development and maintenance of social bonds. Animal models are powerful tools for investigating the neurobiological mechanisms regulating the cognitive processes leading to the development of social relationships and for potentially extending our understanding of the human condition. In this review, we discuss the roles of the neuropeptides oxytocin and vasopressin in the regulation of social bonding as well as related social behaviors which culminate in the formation of social relationships in animal models. The formation of social bonds is a hierarchical process involving social motivation and approach, the processing of social stimuli and formation of social memories, and the social attachment itself. Oxytocin and vasopressin have been implicated in each of these processes. Specifically, these peptides facilitate social affiliation and parental nurturing behavior, are essential for social recognition in rodents, and are involved in the formation of selective mother-infant bonds in sheep and pair bonds in monogamous voles. The convergence of evidence from these animal studies makes oxytocin and vasopressin attractive candidates for the neural modulation of human social relationships as well as potential therapeutic targets for the treatment of psychiatric disorders associated with disruptions in social behavior, including autism.

Stress during gestation alters postpartum maternal care and the development of the offspring in a rodent model

BACKGROUND

Epidemiological studies suggest that environmental adversity can alter parental care and thus influence child development. We addressed the question of whether stressors can directly affect parental behavior using a rodent model of stable, individual differences in maternal behavior.

METHODS

Lactating rat mothers were characterized as high or low in pup-directed licking/grooming (LG) behavior, rebred, and subjected to 7 days of intermittent stress or control conditions during gestation. Female rats were mated a third time without any subsequent intervention. Maternal behavior, oxytocin receptor (OTR) binding, and offspring behavior were examined.

RESULTS

Stress reduced OTR levels and pup LG of high LG mothers to levels comparable with those of low LG mothers. The adult offspring of the gestational stress/high LG mothers resembled those of low LG mothers on behavioral measures of anxiety and maternal behavior, as well as OTR levels. The results of the third mating revealed an enduring effect of gestational stress on both mother and offspring maternal LG.

CONCLUSIONS

These findings suggest that stress can directly alter maternal care through the neuroendocrine systems that normally regulate this behavior. Thus, the effects of environmental adversity can be transmitted across generations through a nongenomic mechanism involving maternal care.

Functional magnetic resonance imaging shows oxytocin activates brain regions associated with mother-pup bonding during suckling

Oxytocin is released in the maternal brain during breastfeeding and may help strengthen the mother-infant relationship. Here, we used functional magnetic resonance imaging to determine whether oxytocin modulates brain activity in postpartum day 4-8 dams receiving suckling stimulation. During imaging sessions, dams were exposed to pup suckling before and after administration of an oxytocin receptor antagonist. Another group of dams received oxytocin alone. Changes in brain activation in response to suckling closely matched that elicited by oxytocin administration. The overlapping brain areas included the olfactory system, nucleus accumbens, insular cortex, prefrontal cortex, ventral tegmental area, cortical amygdala, and several cortical and hypothalamic nuclei. Blockade of oxytocin receptors largely attenuated activation in these regions. The data suggest that oxytocin may strengthen mother-infant bond formation partly by acting through brain areas involved in regulating olfactory discrimination, emotions, and reward.

Species and individual differences in juvenile female alloparental care are associated with oxytocin receptor density in the striatum and the lateral septum

The neuropeptide oxytocin has been implicated in the regulation of affiliative behavior and maternal responsiveness in several mammalian species. Rodent species vary considerably in the expression of juvenile alloparental behavior. For example, alloparental behavior is spontaneous in juvenile female prairie voles (approximately 20 days of age), takes 1-3 days of pup exposure to develop in juvenile rats, and is nearly absent in juvenile mice and meadow voles. Here, we tested the hypothesis that species differences in pup responsiveness in juvenile rodents are associated with oxytocin receptor (OTR) density in specific brain regions. We found that OTR density in the nucleus accumbens (NA) is highest in juvenile prairie voles, intermediate in juvenile rats, and lowest in juvenile mice and meadow voles. In the caudate putamen (CP), OTR binding was highest in prairie voles, intermediate in rats and meadow voles, and lowest in mice. In contrast, the lateral septum (LS) shows an opposite pattern, with OTR binding being high in mice and meadow voles and low in prairie voles and rats. Thus, alloparental responsiveness in juvenile rodents is positively correlated with OTR density in the NA and CP and negatively correlated with OTR density in the LS. We then investigated whether a similar receptor-behavior relationship exists among juvenile female prairie voles by correlating individual variation in alloparental behavior with variation in OTR density. The time spent adopting crouching postures, the most distinctive component of alloparental behavior in juveniles, was positively correlated with OTR density in the NA (r = 0.47) and CP (r = 0.45) and negatively correlated with OTR density in the lateral septum (r = -0.53). Thus, variation in OTR density in the NA, CP, and LS may underlie both species and individual differences in alloparental care in rodents.

Maternal programming of defensive responses through sustained effects on gene expression

There are profound maternal effects on individual differences in defensive responses and reproductive strategies in species ranging literally from plants to insects to birds. Maternal effects commonly reflect the quality of the environment and are most likely mediated by the quality of the maternal provision (egg, propagule, etc.), which in turn determines growth rates and adult phenotype. In this paper we review data from the rat that suggest comparable forms of maternal effects on defensive responses stress, which are mediated by the effects of variations in maternal behavior on gene expression. Under conditions of environmental adversity maternal effects enhance the capacity for defensive responses in the offspring. In mammals, these effects appear to 'program' emotional, cognitive and endocrine systems towards increased sensitivity to adversity. In environments with an increased level of adversity, such effects can be considered adaptive, enhancing the probability of offspring survival to sexual maturity; the cost is that of an increased risk for multiple forms of pathology in later life.

Biological Aspects of Social Bonding and the Roots of Human Violence

The brain systems that motivate humans to form emotional bonds with others probably first evolved to mobilize the high-quality maternal care necessary for reproductive success in placental mammals. In these species, the helplessness of infants at birth and their dependence upon nutrition secreted from their mothers' bodies (milk) and parental body heat to stay warm required the evolution of a new motivational system in the brain to stimulate avid and sustained mothering behavior. Other types of social bonds that emerged subsequently in placental mammals, in particular monogamous bonds between breeding pairs, appear to have evolved from motivational brain systems that stimulate maternal behavior. This chapter focuses on aspects of the evolution and neurobiology of maternal and pair bonding and associated behavioral changes that may provide insights into the origins of human violence. The roles of the neuropeptides oxytocin and vasopressin as well as the neurotransmitter dopamine will be emphasized. Maternal and pair bonding are accompanied by increased aggressiveness toward perceived threats to the object of attachment as well as diminished fear and anxiety in stressful situations. The sustained closeness with mother required for the survival of infant mammals opened a new evolutionary niche in which aspects of the mother's care became increasingly important in regulating development in offspring. The quantity and quality of maternal care received during infancy determines adult social competence, ability to cope with stress, aggressiveness, and even preference for addictive substances. Indeed, the development of neurochemical systems within the brain that regulate mothering, aggression, and other types of social behavior, such as the oxytocin and vasopressin systems, are strongly affected by parental nurturing received during infancy. Evidence will be reviewed that the neural circuitry and neurochemistry implicated in studies of lower mammals also facilitate primate/human interpersonal bonding. It is hypothesized that neural bonding systems may also be important for the development in individuals of loyalty to the social group and its culture. Neglect and abuse during early life may cause bonding systems to develop abnormally and compromise capacity for rewarding interpersonal relationships and commitment to societal and cultural values later in life. Other means of stimulating reward pathways in the brain, such as drugs, sex, aggression, and intimidating others, could become relatively more attractive and less constrained by concern about violating trusting relationships. The ability to modify behavior based on negative experiences may be impaired. Unmet needs for social bonding and acceptance early in life might increase the emotional allure of groups (gangs, sects) with violent and authoritarian values and leadership. Social neurobiology has the potential to provide new strategies for treating and preventing violence and associated social dysfunction.

Oxytocin receptors in the nucleus accumbens facilitate “spontaneous” maternal behavior in adult female prairie voles

Oxytocin and the nucleus accumbens have been extensively implicated in the regulation of maternal behavior, and the processing of pup-related stimuli relevant for this behavior. Oxytocin receptor density in the nucleus accumbens is highly variable in virgin female prairie voles, as is their behavioral response to pups, ranging from neglecting and infanticidal to full maternal behavior. We hypothesized that oxytocin receptor in the nucleus accumbens facilitates the expression of "spontaneous" maternal behavior in prairie voles. Forty sexually-naive adult females were exposed to pups for the first time and tested for maternal behavior. Oxytocin receptor binding in the nucleus accumbens and other brain regions was later determined using autoradiography. Females that showed maternal behavior (lick and groom the pups and hover over them for at least 30 s, n=24) had higher oxytocin receptor density in the nucleus accumbens (shell subregion) (P<0.05) than females that did not show maternal behavior or attacked the pups (n=16). No differences were found in other brain regions (medial preoptic area, septum, prelimbic cortex). In a second experiment, we tested whether infusions of the oxytocin receptor antagonist (d(CH2)5(1),Tyr(Me)2,Orn8)-AVT into the nucleus accumbens would block "spontaneous" maternal behavior. As a control region, oxytocin receptor antagonist was also infused into the caudate putamen. Ten females were infused bilaterally into the nucleus accumbens or caudate putamen with either 2 ng/0.5 microl of oxytocin receptor antagonist or CSF (vehicle). While five of 10 nucleus accumbens CSF-infused animals showed maternal behavior, none of the nucleus accumbens oxytocin receptor antagonist-infused subjects did (0/10; chi2, P<0.01). Nucleus accumbens oxytocin receptor antagonist-infused females recovered the next day and were not different from controls. Animals infused with CSF or oxytocin receptor antagonist into the caudate putamen did not differ (four/10, four/10). This is the first study to show that the nucleus accumbens is involved in the regulation of "spontaneous" maternal behavior and that oxytocin receptor in this brain region facilitates maternal responses.

Environmental programming of stress responses through DNA methylation: life at the interface between a dynamic environment and a fixed genome

Early experience permanently alters behavior and physiology. These effects are, in part, mediated by sustained alterations in gene expression in selected brain regions. The critical question concerns the mechanism of these environmental “programming” effects. We examine this issue with an animal model that studies the consequences of variations in mother-infant interactions on the development of individual differences in behavioral and endocrine responses to stress in adulthood. Increased levels of pup licking/grooming by rat mothers in the first week of life alter DNA structure at a glucocorticoid receptor gene promoter in the hippocampus of the offspring. Differences in the DNA methylation pattern between the offspring of high- and low-lickinglgrooming mothers emerge over the first week of life; they are reversed with cross-fostering; they persist into adulthood; and they are associated with altered histone acetylation and transcription factor (nerve growth factor-induced clone A [NGFIA]) binding to the glucocorticoid receptor promoter. DNA methylation alters glucocorticoid receptor expression through modifications of chromatin structure. Pharmacological reversal of the effects on chromatin structure completely eliminates the effects of maternal care on glucocorticoid receptor expression and hypothalamic-pituitary-adrenal (HPA) responses to stress, thus suggesting a causal relation between the maternally induced, epigenetic modification of the glucocorticoid receptor gene and the effects on stress responses in the offspring. These findings demonstrate that the structural modifications of the DNA can be established through environmental programming and that, in spite of the inherent stability of this epigenomic marker, it is dynamic and potentially reversible.

Effects of oxytocin microinjected into the central amygdaloid nucleus and bed nucleus of stria terminalis on maternal aggressive behavior in rats

The central effect of oxytocin (OT) on the aggressive behavior of lactating rats was studied. Female rats are more aggressive than nonlactating resident females, vigorously attacking conspecific intruder male or females. This behavior is considered important for pup protection against infanticide. The present work aimed to test the effects on maternal aggressive behavior of OT infused into the central amygdaloid nucleus (CeM) or bed nucleus of stria terminalis (BNST). The surgeries for bilateral cannula implantation were performed between the 2nd and 4th postpartum day. Three days after the surgery, saline or OT was infused and 5 min later a male intruder was placed in the home-cage and the behaviors were videotaped for 10 min. The frequency of the aggressive behaviors and the duration of locomotion during the aggressive behavior test were measured. The latency to retrieve the pups was also evaluated. The results showed that OT injected into CeM (10 and 20 ng/nucleus) decreased frequency of biting and frontal attack while in the BNST (10 and 20 ng/nucleus) decreased the frequency of biting. No significant change on retrieval activity was detected. OT in CeM and BNST has an inhibitory effect on the aggressive behavior of lactating female rats.

Maternal programming of steroid receptor expression and phenotype through DNA methylation in the rat

Increased levels of pup licking/grooming and arched-back nursing by rat mothers over the first week of life alter the epigenome at a glucocorticoid receptor gene promoter in the hippocampus of the offspring. Differences in the DNA methylation pattern between the offspring of High and Low licking/grooming--arched-back mothers emerge over the first week of life, are reversed with cross-fostering, persist into adulthood and are associated with altered histone acetylation and transcription factor (NGFI-A) binding to the glucocorticoid receptor promoter. Central infusion of the adult offspring with the histone deacetylase inhibitor trichostatin A removes the previously defined epigenomic group differences in histone acetylation, DNA methylation, NGFI-A binding, glucocorticoid receptor expression, and hypothalamic-pituitary-adrenal responses to stress, thus suggesting a causal relation between the epigenomic state, glucocorticoid receptor expression and the effects of maternal care on stress responses in the offspring. These findings demonstrate that an epigenomic state of a gene can be established through a behavioral mode of programming and that in spite of the inherent stability of this epigenomic mark, it is dynamic and potentially reversible.

Neuropeptides and non-human primate social deficits associated with pathogenic rearing experience

There is a persuasive evidence that autism is highly heritable and likely to be substantially determined by polygenic mechanisms. Nevertheless, some intriguing findings in children raised in conditions of extreme social deprivation suggest that an autistic-like syndrome may occur as a consequence of environmental conditions. A particularly close model of this human syndrome has been studied in rhesus monkeys for almost half a century. Monkeys reared in pathogenic rearing conditions manifest considerable deficits in social interaction and increased self-directed behaviors. We have been interested in the possibility that disruptions in normal social development in non-human primates might be expressed in neuropeptide systems which have emerged in rodent studies as important candidates for a unique social biology. In recent studies, we have described persistently reduced CSF OT levels in male rhesus monkeys with significant social deficits. We also found that OT levels were positively related to the expression of affiliative social behaviors. Alterations were also detected in both CRH and AVP receptor binding patterns in limbic structures likely to influence social and emotional development. Taken together, these data suggest that abnormal rearing influences the development of brain systems critical to normal social and emotional competence in rhesus monkeys and may contribute to the development of autistic-like symptomatology associated with pathogenic rearing histories.

Early weaning deprives mouse pups of maternal care and decreases their maternal behavior in adulthood

Weaning is one of the most important events in the early stage of life, and recently we have found that precocious weaning augments anxiety and aggressiveness in mice. Maternal behavior has been reported to be transmitted from one generation to the next; that is, female pups that received intensive maternal care showed higher maternal behavior in their adulthood. In the present study, the following three experiments were conducted to understand maternal behavior transmission in early-weaned mice that were separated from the dam on postnatal day 14. First, the maternal behavior observed from the postpartum day 15 to 21, which was deprived in the early-weaned mice, were analyzed. Mothers spent 3% of their time on licking/grooming and arched-back nursing of their pups on postpartum day 15, and the time spent on these behaviors was gradually decreased until postpartum day 21; however, they spent 50% of their time attending to their pups through postpartum days 15-21. Simultaneously, the behavior of the pups was monitored, and it was found that the early-weaned mice had higher activity and lower resting behavior over the period from postnatal day 15 to 21. Secondly, the early- and normally weaned female mice were subjected to an elevated plus maze test at the age of 8 weeks to assess their anxiety level. The early-weaned mice showed a lower frequency of entering the open arms, and a shorter duration of time spent within them, as compared to the normally weaned mice, suggesting that early-weaned females had a higher anxiety level. In the third experiment, the two groups of female mice were paired with adult male mice for 2 weeks, and the mother's maternal behavior was analyzed. The early-weaned female mice showed lower frequency of licking/grooming and arched-back nursing of their pups as compared to the normally weaned mice, whereas the time of mother-off pups and attending to pups were not different between groups. These results suggest that early-weaning manipulation deprives offspring of a certain level of maternal care, and as a consequence, the offspring show higher anxiety levels and lower maternal behavior in their own adulthood.

Exposure to PCB 77 affects the maternal behavior of rats

Polychlorinated biphenyls are environmental contaminants known to affect neurobehavioral development in many laboratory studies using different animal models. Because of their bioaccumulation and long half-life they are a serious concern for our own species. The dioxin-like PCB congener 3,4,3',4'-tetrachlorobiphenyl (PCB 77) has estrogenic and anti-estrogenic properties, and has been shown to affect brain chemistry and behavior of developing rats when administered during gestation. Since many developmental outcomes in mammals depend upon the type of maternal care provided by the dams, we investigated the effects of two doses of PCB 77 (2 and 4 mg/kg administered during gestational days 6-18) on the maternal behavior of the treated dams. Both doses of PCB 77 reduced the amount of nursing time in which the dams displayed the high-crouch posture over postnatal days 1-6. In addition, the high dose increased the amount of maternal licking and grooming of the litters and the amount of time the dams spent on the nest. The high dose also increased pup mortality, and both doses reduced the weight gain of the litters during the first 6 days of life. These results document effects of PCB 77 on maternal behavior and serve to raise questions about the importance of maternal contributions to the developmental effects of this and similar contaminants.

The programming of individual differences in defensive responses and reproductive strategies in the rat through variations in maternal care

There are profound maternal effects on individual differences in defensive responses and reproductive strategies in species ranging literally from plants to insects to birds. Maternal effects commonly reflect the quality of the environment and are most likely mediated by the quality of the maternal provision (egg, propagule, etc.), which in turn determines growth rates and adult phenotype. In this paper, we review data from the rat that suggest comparable forms of maternal effects on both defensive responses to threat and reproductive behavior and which are mediated by variations in maternal behavior. Ultimately, we will need to contend with the reality that neural development, function and health are defined by social and economic influences.

Prepartum plasma estradiol and postpartum cortisol, but not oxytocin, are associated with interindividual and breed differences in the expression of maternal behaviour in sheep

Consistent, individual differences in the expression of maternal behaviour have been described in several species including the sheep. The neuroendocrine mechanisms underlying the onset of maternal behaviour in the sheep have been described, although the relationship between hormonal events and individual differences in behaviour has yet to be determined. In this study, we examined whether the individual differences in plasma estradiol, progesterone, oxytocin and cortisol concentrations were related to observed individual and breed differences in maternal behaviours in two breeds of sheep (Scottish Blackface and Suffolk) known to differ in maternal behavioural expression. Maternal estradiol concentration increased rapidly before parturition and was higher in Blackface ewes than Suffolk ewes. Plasma progesterone declined before parturition and was higher in Suffolk than Blackface ewes. Prepartum estradiol, but not progesterone, was related to individual differences in maternal grooming of the lamb. Plasma oxytocin did not differ between breeds in late gestation. There was a tendency for oxytocin to be higher in Blackface than Suffolk ewes immediately after birth. However, there were no significant relationships between prepartum or postpartum oxytocin and any maternal behaviours. Plasma cortisol was higher in Blackface than Suffolk ewes in the last days of pregnancy but rose in both breeds over the last 24 h before parturition and did not differ at delivery. Cortisol peripartum was negatively related to individual differences in maternal affiliative behaviours. These data suggest that estradiol, and potentially cortisol, may mediate individual differences in maternal behaviour in sheep.

Maternal Programming of Individual Differences in Defensive Responses in the Rat

This paper describes the results of a series of studies showing that variations in mother-pup interactions program the development of individual differences in behavioral and endocrine stress responses in the rat. These effects are associated with altered expression of genes in brain regions, such as the amygdala, hippocampus, and hypothalamus, that regulate the expression of stress responses. Studies from evolutionary biology suggest that such "maternal effects" are common and often associated with variations in the quality of the maternal environment. Together these findings suggest an epigenetic process whereby the experience of the mother alters the nature of the parent-offspring interactions and thus the phenotype of the offspring.

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.

Both oxytocin and vasopressin may influence alloparental behavior in male prairie voles

Neuropeptides, especially oxytocin (OT) and arginine vasopressin (AVP), have been implicated in several features of monogamy including alloparenting. The purpose of the present study was to examine the role of OT and AVP in alloparental behavior in reproductively naïve male prairie voles. Males received intracerebroventricular (ICV) injections of artificial cerebrospinal fluid (aCSF), OT, an OT receptor antagonist (OTA), AVP, an AVP receptor antagonist (AVPA), or combinations of OTA and AVPA and were subsequently tested for parental behavior. Approximately 45 min after treatment, animals were tested for behavioral responses to stimulus pups. In a 10-min test, spontaneous alloparental behavior was high in control animals. OT and AVP did not significantly increase the number of males that showed parental behavior, although more subtle behavioral changes were observed. Combined treatment with AVPA and OTA (10 ng each) significantly reduced male parental behavior and increased attacks; following a lower dose (1 ng OTA/1 ng AVPA), males were less likely to display kyphosis and tended to be slower to approach pups than controls. Since treatment with only one antagonist did not interfere with the expression of alloparenting, these results suggest that access to either OT or AVP receptors may be sufficient for the expression of alloparenting.

Variations in nucleus accumbens dopamine associated with individual differences in maternal behavior in the rat

Lactating rats exhibit stable individual differences in pup licking/grooming. We used in vivo voltammetry to monitor changes in extracellular dopamine (DA) in the nucleus accumbens (n. Acc) shell of lactating rats interacting with pups and found that (1) the DA signal increased significantly with pup licking/grooming; (2) the onset of such increases preceded pup licking/grooming; and (3) the magnitude and duration of the increase in the DA signal were significantly correlated with the duration of the licking/grooming bout. In females characterized on the basis of behavioral observations as high-licking/grooming mothers, the magnitude of the increase in the DA signal associated with licking/grooming was significantly greater than in low-licking/grooming dams. Dopamine transporter binding in the n. Acc was increased in low-compared with high-licking/grooming mothers. Injection of the selective DA uptake inhibitor GBR 12909 [1-(2-(Bis-(4-fluorophenyl)methoxy)ethyl)-4-(3 phenypropyl)piperazine dihydrochloride] (5 mg/kg, s.c.) increased the DA signal in the n. Acc and pup licking/grooming in low-licking/grooming mothers to levels comparable with those observed in high-licking/grooming dams. Receptor autoradiographic studies showed elevated levels of D1 and D3 receptors in the n. Acc shell region in high-licking/grooming dams. These results suggest that high- and low-licking/grooming dams differ in mesolimbic dopaminergic activity associated with mother-pup interactions. Such differences may serve as neural substrates for individual differences in the motivational component of maternal behavior.

Oxytocin antagonism alters rat dams' oral grooming and upright posturing over pups

Studies involving intracerebral administration of antiserum or antagonists have demonstrated that central oxytocin (OT) plays a prominent role in initiating but not maintaining postpartum maternal behavior in rats. There has been little investigation, however, of OT's influence on the levels of maternal behavior exhibited during the maintenance phase. We measured rat dam behavior during the 105-min observation periods preceding and beginning 2 h after intracerebroventricular infusion of the selective OT antagonist (OTA) (1 microg), or normal saline (NS) vehicle (5 microl) on postpartum days 2-3 and 6-7. Compared to NS, OTA significantly decreased pup licking as a proportion of dams' total oral grooming, increased self-grooming, decreased the frequency of elevated upright posture over pups and increased the frequency of lying prone on pups. Quiescent, kyphotic nursing was also significantly lower in OTA-treated dams. Other components of maternal behavior were not significantly affected by OTA or NS treatment. These findings suggest that central OT may shift the focus of the dams' oral grooming from self to pups and may also facilitate elevation of dams' upright posture over pups. Acute stress responses, maternal behavior and central OT receptor binding in adult rats have been linked to the amount of maternal licking and arched back, upright nursing received during infancy. OT activity in dams' brains may influence these developmental outcomes in their offspring by selectively regulating their pup licking and crouching posture.

Natural variations in maternal care are associated with estrogen receptor alpha expression and estrogen sensitivity in the medial preoptic area

Lactating rats exhibit stable individual differences in pup licking/grooming (LG) over the first week postpartum. Such naturally occurring variations in maternal behavior are associated with differences in estrogen-inducible oxytocin receptors in the medial preoptic area (MPOA) of the hypothalamus. We compared levels of ER alpha and ER beta mRNA in the MPOA of lactating High or Low LG mothers as well as in their nonlactating, female offspring, which inherit the maternal phenotype of their mothers. Among lactating females, High LG females exhibited significantly elevated levels of ER alpha mRNA compared with Low LG females. Likewise, the adult, virgin female offspring of High LG mothers showed higher levels of ER alpha mRNA in the MPOA compared with those of Low LG mothers. There were no group differences in levels of ER beta mRNA. Differences in ER alpha protein expression in the MPOA were confirmed using Western blot analysis. To further characterize the effects of estrogen in the MPOA, cFos immunoreactivity was compared in ovariectomized, adult offspring of High and Low LG dams treated with estradiol or oil. Increased cFos activity in the anterior ventral nucleus of the MPOA was observed in estradiol-treated High LG, but not Low LG females. These findings suggest that natural variations in maternal care are associated with differences in ER alpha expression in the MPOA and that such differences are transmitted from the mother to her female offspring.

Complete maternal deprivation affects social, but not spatial, learning in adult rats

The effects of maternal deprivation on learning of social and spatial tasks were investigated in female adult rats. Pups were reared artificially and received "lickinglike" tactile stimulation (AR animals) or were reared with their mothers (MR animals). In adulthood, subjects were tested on paradigms of spatial learning and on paradigms involving learning of social cues. Results showed that maternal deprivation did not affect performance on spatial learning, but it did impair performance on the three social learning tasks. The AR animals made no distinction between a new and a previously presented juvenile conspecific. AR animals also responded less rapidly than MR animals at test for maternal behavior 2 weeks after a postpartum experience with pups. Finally, AR animals did not develop a preference for a food previously eaten by a familiar conspecific whereas MR animals did. This study indicates that animals reared without mother and siblings show no deficits in spatial tasks while showing consistent deficits in learning involving social interactions.

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

Parturition plays a critical role in the full expression of maternal behavior in postpartum females, yet the precise mechanism remains unclear. Here we examined the role of parturition in the activation of Fos and FosB in the central oxytocin receptor (OTR) system in rats. Although expression of FosB, not Fos, was seen in the piriform cortex (Pir) and caudate putamen of virgin and pregnant females, activation of Fos and FosB with extensive co-localization was found in the medial preoptic area, the bed nucleus of the stria terminalis and Pir of parturient brain. This parturition induced activation of Fos and FosB was identified in the central OTR-expressing cells as well as in oxytocinergic neurons. Our data provide direct evidence, for the first time, that parturition activates Fos and FosB in the central OTR system. We propose that Fos and FosB may have comparable functions on initiating maternal behavior at parturition.

Variations in maternal care in the rat as a mediating influence for the effects of environment on development

Variations in maternal care have been widely considered as a critical influence in development. In the rat, variations in maternal behavior, particularly in licking/grooming, regulate the development of endocrine, emotional and cognitive responses to stress. These studies form the basis of a potentially useful model for the study of maternal effects in mammals. In this paper we provide a detailed methodological investigation into this model of maternal behavior, providing an analysis of the frequency, temporal dynamics, and transmission of maternal licking/grooming in several large cohorts. Frequency data indicate that licking/grooming is normally distributed across dams. The peak in licking/grooming occurs in the first few days postpartum and gradually declines. Dams designated as High or Low LG mothers differ in this behavior only during the first week postpartum. Observations over Days 2 to 5 postpartum are essential for the reliable assessments of individual differences in maternal behavior. Individual differences in licking/grooming behavior are stable across multiple litters, and are not associated with differences in litter size, weaning weight of pups, or gender ratio of the litter. We also observed no significant differences in the amount of licking/grooming received by individual pups within a litter, though variation does exist. Finally, maternal licking/grooming is transmitted to female offspring, though there is considerable within-litter variation in the expression of this behavior. Overall, these findings indicate considerable, normal variations in licking/grooming in the rat that are a stable, individual characteristic of rat dams.

Central expression of c-Fos in neonatal male and female prairie voles in response to treatment with oxytocin

Early postnatal exposure to both exogenous and endogenous oxytocin (OT) can have long-term effects on behavior and physiology, although the mechanisms of these effects are not known. c-Fos expression was used to investigate the immediate neural effects of neonatal manipulations of OT in male and female prairie voles. On the day of birth prairie vole pups received an intraperitoneal injection of OT, a selective OT antagonist (OTA), or saline (vehicle control), while an additional group was handled but not injected. One hour after treatment brains were collected and fixed via spinning immersion and immunocytochemistry was then used to label for c-Fos immunoreactivity (IR). There were significant differences between males and females. Handled only females displayed significantly higher levels of c-Fos IR in the mediodorsal thalamic nucleus (MD) than males while handled males had higher c-Fos IR in the paraventricular nucleus of the hypothalamus than females. Exogenous OT stimulated neuronal activity in the supraoptic nucleus (SON) in males, while treatment with OTA increased Fos IR in the SON and was associated with reduced Fos IR in the MD in females. The results indicate that neuronal activity and responses to OT are sexually dimorphic in newborn prairie voles. In females changes in Fos expression were stimulated by treatment with OTA, suggesting that endogenous OT affects cellular activity while males responded to exogenous OT.

Rearing effects on cerebrospinal fluid oxytocin concentration and social buffering in rhesus monkeys

Mother-reared (MR) and nursery-reared (NR) male rhesus monkeys exhibit profound and persistent differences in social and emotional behavior. Compared to MR animals, NR monkeys show reduced reciprocal social behaviors and increased agonistic behavior and high levels of stereotypy. Cerebrospinal fluid oxytocin (CSF OT) in NR monkeys was significantly reduced compared to MR monkeys measured at 18, 24, and 36 months of age. Correlations between OT and individual social behavioral profiles measured across rearing conditions also revealed a significant association between OT and the expression of affiliative social behaviors including allogrooming and reciprocal intermale mounting at each age examined. In contrast, CSF vasopressin levels did not differ according to rearing history, but did correlate with fearful behaviors independent of rearing history. Differential rearing was not associated with differences in basal or stress-related plasma cortisol, although these levels did progressively decline as monkeys matured. MR but not NR monkeys were able to use a social companion to buffer their response to a stressor, but OT levels did not appear to be sensitive to the presence of a social companion in either group. These results are consistent with earlier reports from studies of rodents suggesting an important role for central OT pathways in the development of social affiliation.

Artificial rearing causes changes in maternal behavior and c-fos expression in juvenile female rats

This study investigated the effects of early-rearing experiences on responsiveness to pups and on the pattern of c-fos activation in the brain of juvenile female rats. From Days 4 to 20, littermate females were reared with their mothers (MR) or artificially (AR). AR rats received minimal licking-like tactile stimulation (AR-min) or maximal stimulation (AR-max). On Day 20, rats were exposed to pups for 4 or 8 days, exposed to a playmate for 4 or 8 days, or left in isolation for 4 or 8 days. Compared with MR rats, pup-exposed AR rats engaged in less pup licking, and all AR rats showed significant reductions in c-fos immunoreactivity in the medial preoptic area and the parietal and piriform cortices. The AR-min group showed the greatest difference in Fos-lir compared with the MR groups. Possible mechanisms that mediate the effects of rearing on the development of neural circuits underlying maternal behavior are discussed.

Mothering begets mothering: the transmission of behavior and its neurobiology across generations

Early experiences exert their effects on adult parental behavior in part by altering the development of neurobiological mechanisms that initiate or support the initiation and sustenance of adult parental behavior. The effects of parental behavior on sensory, perceptual and emotional mechanisms in offspring constitute an experientially based mechanism by which neurobiological factors regulating behavior can be transferred from generation to generation somewhat independently of genetic endowment.

Naturally occurring differences in maternal care are associated with the expression of oxytocin and vasopressin (V1a) receptors: gender differences

Variations in maternal care have been associated with long-term changes in neurochemistry and behaviour in adult rats. Rats receiving high levels of licking and grooming as pups are less fearful and more maternal than rats receiving low levels of maternal licking and grooming. Central pathways for oxytocin and vasopressin have been implicated in the neurobiology of anxiety and social behaviours. We assessed whether variations in maternal care were associated with differences in oxytocin receptors (OTR) or vasopressin (V1a) receptors in the brains of adult offspring. In the central nucleus of the amygdala and bed nucleus of the stria terminalis, OTR binding was increased in adult females, but not adult males, that had received high levels of maternal licking and grooming as pups. Conversely, amygdala V1a receptor binding was increased in males, but not females, that had received high levels of maternal licking and grooming. These findings suggest that variations in maternal care may influence the expression of oxytocin and vasopressin receptors in a gender-specific manner.

Maternal behavior and developmental psychopathology

This paper reviews recent developments in the phenomenology, neurobiology, and genetics of maternal behavior in animal model systems from an evolutionary perspective on psychopathology. Following a review of the phenomenology and neurobiology of maternal behavior, recent studies addressing the role of genetic factors in the maternal behavior of rodents were identified in a search of literature in peer-reviewed journals. Gene knockout studies were evaluated with regard to mouse strain background, method of behavioral phenotyping, and quantification of the behavioral deficits. Gene knockout data were then analyzed using a cluster analysis technique. At least nine genes have been identified that are necessary for the expression of one or more aspects of maternal behavior. These genes encode for three transcription factors: three enzymes, including dopamine beta hydroxylase and neuronal nitric oxide synthase; two receptors, including the prolactin and the estrogen alpha receptor; and one neuropeptide, oxytocin. Cluster analysis suggested possible relationships between specific genes. Gene knockout technology has provided new insights into the molecular basis of maternal behavior that are congruent with the existing neurobiological literature. Future studies of genetic and environmental influences on maternal behavior have the potential to inform models of disease pathogenesis.

Brain preparations for maternity--adaptive changes in behavioral and neuroendocrine systems during pregnancy and lactation. An overview

Pregnancy, parturition and lactation comprise a continuum of adaptive changes necessary for the development and maintenance of the offspring. The endocrine changes that are driven by the conceptus and are essential for the maintenance of pregnancy and are involved in the preparations for motherhood are outlined. These changes include large increases in the secretion of sex steroid hormones, and the secretion of peptide hormones that are unique to pregnancy. The ability of these pregnancy hormones to alter several aspects of brain function in pregnancy is considered, and the adaptive importance of some of these changes is discussed, for example in metabolic and body fluid adjustments, and the induction of maternal behavior. The importance of sex steroids in determining the timing of the various adaptive changes in preparing for parturition and maternal behavior is emphasized, and the concept that the actions of prolactin and oxytocin, quintessential mammalian motherhood neuropeptides, can serve to coordinate a spectrum of adaptive changes is discussed. The part played by oxytocin neurons and their regulatory mechanisms is reviewed to illustrate how neural systems involved in maternity are prepared in pregnancy via changes in phenotype, synaptic organization and in the relative importance of their different inputs, to function optimally when needed. For oxytocin neurons secreting from the posterior pituitary, important in parturition and essential in lactation, these changes include mechanisms to restrain their premature activation, and adaptations to support synchronized burst firing for pulsatile oxytocin secretion in response to stimulation via afferents from the birth canal, olfactory system or suckled nipples. Within the brain, expression of oxytocin receptors permits centrally released oxytocin to facilitate the expression of maternal behavior. Changes in other neuroendocrine systems are similarly extensive, leading to lactation, suppression of ovulation, reduced stress responses and increased appetite; these changes in lactation are driven by the suckling stimulus. The possible link between these adaptations and changes in cognition and mood in pregnancy and post partum are considered, as well as the dysfunctions that lead to common problems of depression and puerperal psychoses.

Distribution of G-protein-coupled receptor kinase (GRK) isoforms 2, 3, 5 and 6 mRNA in the rat brain

There is limited knowledge about the distribution of the different G-protein-coupled receptor kinases (GRKs) in the rat brain, especially for the recently cloned isoforms GRK5 and GRK6. In this work an overview will be given of the mRNA expression patterns of four G-protein-coupled receptor kinases, GRK2 (betaARK1), GRK3 (betaARK2), GRK5 and GRK6 in the rat brain. As now shown by us and recently by others GRK2 and GRK3 are widely distributed in rat brain with nearly the same expression pattern. But GRK3, in general, appeared to be weaker expressed than GRK2 in most brain areas. Exceptions were the islands of Calleja, the compact part of the substantia nigra and the locus coeruleus. GRK3 mRNA was very low expressed or absent in the striatum and in some hypothalamic and thalamic nuclei. The expression pattern of GRK6 was also similar to GRK2. In the caudate putamen GRK6 yielded the strongest hybridization signal of all GRK types. GRK5 took a special position. The message for this form was not expressed ubiquitously in the brain but was mainly localized in limbic brain regions with a very prominent expression in the lateral septal area. GRK5 may therefore be involved in reward and addiction. Accordingly, a higher expression level of GRK5 mRNA was found in the lateral septum of cocaine-sensitized rats as compared to controls.

Naturally occurring variations in maternal behavior in the rat are associated with differences in estrogen-inducible central oxytocin receptors

Naturally occurring variations in maternal licking/grooming influence neural development and are transmitted from mother to female offspring. We found that the induction of maternal behavior in virgin females through constant exposure to pups (pup sensitization) was significantly shorter in the offspring of High compared with Low licking/grooming mothers, suggesting differences in maternal responsivity. In randomly selected females screened for individual differences in maternal responsivity and subsequently mated, there was a significant and negative correlation (r = -0.73) between the latency to exhibit maternal behavior in the pup sensitization paradigm and the frequency of pup licking/grooming during lactation. Females that were more maternally responsive to pups and that showed increased levels of pup licking/grooming also showed significantly higher oxytocin receptor levels in the medial preoptic area, the lateral septum, the central nucleus (n.) of the amygdala, the paraventricular n. of the hypothalamus, and the bed n. of the stria terminalis. Intracerebroventricular administration of an oxytocin receptor antagonist to mothers on postpartum day 3 completely eliminated the differences in pup licking/grooming, suggesting that differences in oxytocin receptor levels are functionally related to maternal behavior. Finally, estrogen treatment of virgin females significantly increased oxytocin receptor binding in the medial preoptic area and lateral septum of female offspring of High, but not Low, licking/grooming mothers. These findings suggest that maternal licking/grooming influences the development of estrogen sensitivity in brain regions that regulate maternal behavior, providing a potential mechanism for the intergenerational transmission of individual differences in maternal behavior.

Maternal care, gene expression, and the transmission of individual differences in stress reactivity across generations

Naturally occurring variations in maternal care alter the expression of genes that regulate behavioral and endocrine responses to stress, as well as hippocampal synaptic development. These effects form the basis for the development of stable, individual differences in stress reactivity and certain forms of cognition. Maternal care also influences the maternal behavior of female offspring, an effect that appears to be related to oxytocin receptor gene expression, and which forms the basis for the intergenerational transmission of individual differences in stress reactivity. Patterns of maternal care that increase stress reactivity in offspring are enhanced by stressors imposed on the mother. These findings provide evidence for the importance of parental care as a mediator of the effects of environmental adversity on neural development.