Offspring-induced nurturance: animal-human parallels

Oxytocin Receptor-Expressing Neurons in the Medial Preoptic Area Are Essential for Lactation, whereas Those in the Lateral Septum Are Not Critical for Maternal Behavior

INTRODUCTION

In nurturing systems, the oxytocin (Oxt)-oxytocin receptor (Oxtr) system is important for parturition, and essential for lactation and parental behavior. Among the nerve nuclei that express Oxtr, the lateral septal nucleus (LS) and medial preoptic area (MPOA) are representative regions that control maternal behavior.

METHODS

We investigated the role of Oxtr- and Oxtr-expressing neurons, located in the LS and MPOA, in regulating maternal behavior by regulating Oxtr expression in a region-specific manner using recombinant mice and adeno-associated viruses. We quantified the prolactin (Prl) concentrations in the pituitary gland and plasma when Oxtr expression in the MPOA was reduced.

RESULTS

The endogenous Oxtr gene in the neurons of the LS did not seem to play an essential role in maternal behavior. Conversely, decreased Oxtr expression in the MPOA increased the frequency of pups being left outside the nest and reduced their survival rate. Deletion of Oxtr in MPOA neurons prevented elevation of Prl levels in plasma and pituitary at postpartum day 2.

DISCUSSION/CONCLUSION

Oxtr-expressing neurons in the MPOA are involved in the postpartum production of Prl. We confirmed the essential functions of Oxtr-expressing neurons and the Oxtr gene itself in the MPOA for the sustainability of maternal behavior, which involved Oxtr-dependent induction of Prl.

Characterization of Behavioral Phenotypes in Heterozygous DAT Rat Based on Pedigree

Dopamine is an essential neurotransmitter whose key roles include movement control, pleasure and reward, attentional and cognitive skills, and regulation of the sleep/wake cycle. Reuptake is carried out by the dopamine transporter (DAT; DAT1 SLC6A3 gene). In order to study the effects of hyper-dopaminergia syndrome, the gene was silenced in rats. DAT-KO rats show stereotypical behavior, hyperactivity, a deficit in working memory, and an altered circadian cycle. In addition to KO rats, heterozygous (DAT-HET) rats show relative hypofunction of DAT; exact phenotypic effects are still unknown and may depend on whether the sire or the dam was KO. Our goal was to elucidate the potential importance of the parental origin of the healthy or silenced allele and its impact across generations, along with the potential variations in maternal care. We thus generated specular lines to study the effects of (grand) parental roles in inheriting the wild or mutated allele. MAT-HETs are the progeny of a KO sire and a WT dam; by breeding MAT-HET males and KO females, we obtained subjects with a DAT -/- epigenotype, named QULL, to reflect additional epigenetic DAT modulation when embryos develop within a hyper-dopaminergic KO uterus. We aimed to verify if any behavioral anomaly was introduced by a QULL (instead of KO) rat acting as a direct father or indirect maternal grandfather (or both). We thus followed epigenotypes obtained after three generations and observed actual effects on impaired maternal care of the offspring (based on pedigree). In particular, offspring of MAT-HET-dam × QULL-sire breeding showed a compulsive and obsessive phenotype. Although the experimental groups were all heterozygous, the impact of having a sire of epigenotype QULL (who developed in the uterus of a KO grand-dam) has emerged clearly. Along the generations, the effects of the DAT epigenotype on the obsessive/compulsive phenotype do vary as a function of the uterine impact on either allele in one's genealogical line.

Mammalian NREM and REM sleep: Why, when and how

This report proposes that fish use the spinal-rhombencephalic regions of their brain to support their activities while awake. Instead, the brainstem-diencephalic regions support the wakefulness in amphibians and reptiles. Lastly, mammals developed the telencephalic cortex to attain the highest degree of wakefulness, the cortical wakefulness. However, a paralyzed form of spinal-rhombencephalic wakefulness remains in mammals in the form of REMS, whose phasic signs are highly efficient in promoting maternal care to mammalian litter. Therefore, the phasic REMS is highly adaptive. However, their importance is low for singletons, in which it is a neutral trait, devoid of adaptive value for adults, and is mal-adaptive for marine mammals. Therefore, they lost it. The spinal-rhombencephalic and cortical wakeful states disregard the homeostasis: animals only attend their most immediate needs: foraging defense and reproduction. However, these activities generate allostatic loads that must be recovered during NREMS, that is a paralyzed form of the amphibian-reptilian subcortical wakefulness. Regarding the regulation of tonic REMS, it depends on a hypothalamic switch. Instead, the phasic REMS depends on an independent proportional pontine control.

Placentophagia and the Tao of POEF

Placentophagia, ingestion of placenta and amniotic fluid, usually during parturition, is a behavioral feature of nearly all nonaquatic, placental mammals, and is a nexus for several interlocking behavioral phenomena. Placentophagia has not been typical of human cultures, but in recent years, some women in affluent societies have engaged in it, thereby bringing publicity to the behavior. First, we summarized benefits of placentophagia for nonhuman mammals, which include increased attractiveness of neonates, enhanced onset of maternal behavior, suppression of pseudopregnancy, and enhancement of opioid hypoalgesia by Placental Opioid-Enhancing Factor (POEF), a benefit that may extend well outside the context of parturition. The research on POEF in animals was discussed in detail. Then we discussed placentophagia (placentophagy) in humans, and whether there is validity to the claims of various benefits reported primarily in the pro-placentophagy literature, and, although human afterbirth shows POEF activity, the POEF effect has not yet been tested in humans. Finally, we discussed the general possible implications, for the management of pain and addiction, of isolating and characterizing POEF.

Resilience to Stress: Lessons from Rodents about Nature versus Nurture

Individual differences in human temperament influence how we respond to stress and can confer vulnerability (or resilience) to emotional disorders. For example, high levels of behavioral inhibition in children predict increased risk of mood and anxiety disorders in later life. The biological underpinnings of temperament are unknown, although improved understanding can offer insight into the pathogenesis of emotional disorders. Our laboratory has used a rat model of temperamental differences to study neurodevelopmental factors that lead to a highly inhibited, stress vulnerable phenotype. Selective breeding for high versus low behavioral response to novelty created two rat strains that exhibit dramatic behavior differences over multiple domains relevant to emotional disorders. Low novelty responder (bLR) rats exhibit high levels of behavioral inhibition, passive stress coping, anhedonia, decreased sociability and vulnerability to chronic stress compared to high novelty responders (bHRs). On the other hand, bHRs exhibit high levels of behavioral dis-inhibition, active coping, and aggression. This review article summarizes our work with the bHR/bLR model showing the developmental emergence of the bHR/bLR phenotypes, the role the environment plays in shaping it, and the involvement of epigenetic processes such as DNA methylation that mediate differences in emotionality and stress reactivity.

Maternal behavior, novelty confrontation, and subcortical c-Fos expression during lactation period are shaped by gestational environment

The environmental context during gestation may modulate the postpartum variations in maternal behaviors observed within different animal species. Most of our experimental knowledge on this phenomenon and its physiological effects have been gained by confronting the pregnant mother with stressful situations, with the consensual results indicating a reduced maternal behavior and a hyper reactivity of stress-related neural paths. Here, in contrast, by exposing nulliparous rats strictly during pregnancy to a standard laboratory environment (STD) or a highly stimulating sensory and social environment (EE), we investigated the hypothesis that subjects frequently exposed to social stimuli and novel situations during pregnancy will show postpartum changes in subcortical brain areas' activity related to the processing of social stimuli and novelty, such that there will be modifications in maternal behavior. We found that EE mothers doubled the levels of licking and grooming, and active hovering over pups during the first postpartum week than STD dams, without a difference in the time of contact with the pups. Associated with these behaviors, EE dams showed increased c-Fos immunoreaction in hypothalamic nuclei and distinct responses in amygdalar nuclei, than STD dams. In the maternal defensive test, EE dams tripled the levels of aggressive behaviors of the STD rats. Additionally, in two different tests, EE mothers showed lower levels of postpartum anxiety-like behaviors when confronted with novel situations. Our results demonstrate that the activity of brain areas related to social behavior is adaptable by environmental circumstances experienced during gestation, presumably to prepare the progeny for these particular conditions.

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.

Influence of gestational exercise practice and litter size reduction on maternal care

Mother-pup interactions are extremely important to offspring survival and growth. The goal of this study was to evaluate the influence of prenatal and neonatal interventions on maternal care, analyzing the effect of maternal exercise, as a healthy intervention, and also the litter size reduction, a model that has been widely used to study early overfeeding in rats. Female Wistar rats were divided into 1) sedentary, and 2) swimming exercise for four weeks, starting one week before mating (5 days/week, 30 min/session). One day after birth, the litter was culled to 8 pups (normal) or 3 pups (small) per dam, yielding control and overfed subgroups for each maternal group, respectively. From postnatal days 2-9 the litter was observed 5 periods a day, to evaluate maternal behavior. Litter reduction caused important alterations in maternal behavior, reducing the total time out of the nest and increasing the frequency of maternal care and lactation in several observation periods, justifying the increased pup's weight gain already demonstrated by this animal model. The practice of maternal exercise did not prevent, but cause the less intensive frequency of non-maternal behavior and lactation in arched-back position, induced by the reduction of litter size. These data demonstrated that small litter size altered maternal behavior, and gestational exercise does not influence significantly these changes.

Controlling litter effects to enhance rigor and reproducibility with rodent models of neurodevelopmental disorders

Research with rodents is crucial for expanding our understanding of genetic and environmental risk factors for neurodevelopmental disorders (NDD). However, there is growing concern about the number of animal studies that are difficult to replicate, potentially undermining the validity of results. These concerns have prompted funding agencies and academic journals to implement more rigorous standards in an effort to increase reproducibility in research. However, these standards fail to address a major source of variability in rodent research brought on by the “litter effect,” the fact that rodents from the same litter are phenotypically more similar to one other than rodents from different litters of the same strain. We show that the litter effect accounts for 30–60% of the variability associated with commonly studied phenotypes, including brain, placenta, and body weight. Moreover, we show how failure to control for litter-to-litter variation can mask a phenotype in Chd8^V986*/+ mice that model haploinsufficiency of CHD8, a high-confidence autism gene. Thus, if not properly controlled, the litter effect has the potential to negatively influence rigor and reproducibility of NDD research. While efforts have been made to educate scientists on the importance of controlling for litter effects in previous publications, our analysis of the recent literature (2015–2020) shows that the vast majority of NDD studies focused on genetic risks, including mutant mouse studies, and environmental risks, such as air pollution and valproic acid exposure, do not correct for litter effects or report information on the number of litters used. We outline best practices to help scientists minimize the impact of litter-to-litter variability and to enhance rigor and reproducibility in future NDD studies using rodent models.

Playback of Alarm and Appetitive Calls Differentially Impacts Vocal, Heart-Rate, and Motor Response in Rats

Our rudimentary knowledge about rat intraspecific vocal system of information exchange is limited by experimental models of communication. Rats emit 50-kHz ultrasonic vocalizations in appetitive states and 22-kHz ones in aversive states. Both affective states influence heart rate. We propose a behavioral model employing exposure to pre-recorded playbacks in home-cage-like conditions. Fifty-kHz playbacks elicited the most vocalizations (>60 calls per minute, mostly of 50-kHz type), increased heart rate, and locomotor activity. In contrast, 22-kHz playback led to abrupt decrease in heart rate and locomotor activity. Observed effects were more pronounced in singly housed rats compared with the paired housed group; they were stronger when evoked by natural playback than by corresponding artificial tones. Finally, we also observed correlations between the number of vocalizations, heart rate levels, and locomotor activity. The correlations were especially strong in response to 50-kHz playback.

Rats bred for high anxiety exhibit distinct fear-related coping behavior, hippocampal physiology, and synaptic plasticity-related gene expression

The hippocampus is essential for learning and memory but also regulates emotional behavior. We previously identified the hippocampus as a major brain region that differs in rats bred for emotionality differences. Rats bred for low novelty response (LRs) exhibit high levels of anxiety- and depression-like behavior compared to high novelty responder (HR) rats. Manipulating the hippocampus of high-anxiety LR rats improves their behavior, although no work to date has examined possible HR/LR differences in hippocampal synaptic physiology. Thus, the current study examined hippocampal slice electrophysiology, dendritic spine density, and transcriptome profiling in HR/LR hippocampus, and compared performance on three hippocampus-dependent tasks: The Morris water maze, contextual fear conditioning, and active avoidance. Our physiology experiments revealed increased long-term potentiation (LTP) at CA3-CA1 synapses in HR versus LR hippocampus, and Golgi analysis found an increased number of dendritic spines in basal layer of CA1 pyramidal cells in HR versus LR rats. Transcriptome data revealed glutamate neurotransmission as the top functional pathway differing in the HR/LR hippocampus. Our behavioral experiments showed that HR/LR rats exhibit similar learning and memory capability in the Morris water maze, although the groups differed in fear-related tasks. LR rats displayed greater freezing behavior in the fear-conditioning task, and HR/LR rats adopted distinct behavioral strategies in the active avoidance task. In the active avoidance task, HRs avoided footshock stress by pressing a lever when presented with a warning cue; LR rats, on the other hand, waited until footshocks began before pressing the lever to stop them. Taken together, these findings concur with prior observations of HR rats generally exhibiting active stress coping behavior while LRs exhibit reactive coping. Overall, our current findings coupled with previous work suggest that HR/LR differences in stress reactivity and stress coping may derive, at least in part, from differences in the developing and adult hippocampus.

Swimming exercise before and during pregnancy: Promising preventive approach to impact offspring´s health

Several environmental factors affect child development, such as the intrauterine environment during the embryonic and fetal development and early postnatal environment provided by maternal behavior. Although mechanistic effects of maternal exercise on offspring health improvement are not yet completely understood, the number of reports published demonstrating the positive influence of maternal exercise have increase. Herein, we addressed issues related to early postnatal environment provided by maternal behavior and early developmental physical landmarks, sensorimotor reflexes, and motor movements ontogeny. In brief, adult female rats underwent involuntary swimming exercise, in a moderated intensity, one week before mating and throughout pregnancy, 30 min a day, 5 days a week. Maternal exercised dams have unchanged gestational outcomes compared to sedentary dams. We found no differences concerning the frequency of pup-directed behavior displayed by dams. However, sedentary dams displayed a poorer pattern of maternal care quality during dark cycle than exercised dams. Physical landmarks and sensorimotor reflexes development of female and male littermates did not differ between maternal groups. Developmental motor parameters such as immobility, lateral head movements, head elevation, pivoting, rearing with forelimb support and crawling frequencies did not differ between groups. Pups born to exercised dams presented higher frequency of walking and rearing on the hind legs. These data suggest that female and male littermates of exercised group present a high frequency of exploratory behavior over sedentary littermates. Taken together, the present findings reinforce that maternal exercise throughout pregnancy represent a window of opportunity to improve offspring's postnatal health.

Intergenerational transmission of maternal care deficiency and offspring development delay induced by perinatal protein malnutrition

Objectives: Early life represents a sensitive and critical period for an individual. Nutrition plays a crucial role in the maturation and functional development of the central nervous system. Inadequate nutrition before birth and during the postnatal life can seriously interfere with brain development and lead to behavioral and neurological disorders such as learning disabilities and psychiatric diseases. In addition, the quality of mother-infant interactions represents an important adaptive pathway that prepares offspring for the conditions of life. In this work, we asked if protein malnutrition alters maternal care and offspring development and if these phenotypes can be transmitted to next generation.Methods: Female mice were fed with a normal or hypoproteic diet during pregnancy and lactation. Nurturing behaviors, i.e. arched, blanket and passive nursing, and liking and grooming of the pups, were evaluated from postnatal day 1 (PD1) to postnatal day 7 (PD7). The same protocol was employed to evaluate maternal behavior for filial generation 1 (F1) and filial generation 2 (F2) dams. Offspring development was evaluated for F1, F2, and F3 generations. Developmental landmarks and neurological reflexes were assessed from PD8 until complete development of the landmark or acquisition of the reflex.Results: Our results show that malnourished dams provide a lesser and more fragmented maternal care than their normally fed counterparts. This altered maternal behavior as well as the delay in the physical and neurological development observed in the offspring from malnourished mothers was transmitted up to two generations at least.Conclusion: These results highlight the harmful effects of protein malnutrition even for generations that are not directly exposed to this environmental adversity.

Use of fast-scan cyclic voltammetry to assess phasic dopamine release in rat models of early postpartum maternal behavior and neglect

Maternal behavior (MB) is a complex response to infant cues, orchestrated by postpartum neurophysiology. Although mesolimbic dopamine contributes toward MB, little is known about real-time dopamine fluctuations during the postpartum period. Thus, we used fast-scan cyclic voltammetry to measure individual dopamine transients in the nucleus accumbens of early postpartum rats and compared them with dopamine transients in virgins and in postpartum females exposed to cocaine during pregnancy, which is known to disrupt MB. We hypothesized that dopamine transients are normally enhanced postpartum and support MB. In anesthetized rats, electrically evoked dopamine release was larger and clearance was faster in postpartum females than in virgins and gestational cocaine exposure blocked the change in clearance. In awake rats, control mothers showed more dopamine transients than cocaine-exposed mothers during MB. Salient pup-produced stimuli may contribute toward differences in maternal phasic dopamine by evoking dopamine transients; supporting the feasibility of this hypothesis, urine composition (glucose, ketones, and leukocytes) differed between unexposed and cocaine-exposed infants. These data, resulting from the novel application of fast-scan cyclic voltammetry to models of MB, support the hypothesis that phasic dopamine signaling is enhanced postpartum. Future studies with additional controls can delineate which aspects of gestational cocaine reduce dopamine clearance and transient frequency.

Combined effect of gestational stress and postpartum stress on maternal care in rats

Variations in maternal care in the rat influence the development of individual differences in behavioral and endocrine responses to stress. This study aimed to examine the interaction between intragastric intubation during late gestation and postpartum stress, induced by pup separation, on maternal behavior and on dams' emotional state and HPA axis function. Rats received intragastric intubation of water on days 12-20 of gestation or remained untreated in their home cage (naïve dams). Pup separation was used as a model of postpartum stress. The procedure consisted of a daily separation of the dam from its litter for 3h from PND 3 until PND 15. Pup separation was carried out in both naïve and intubated dams. The behavioral results indicate that the association of these two stressors significantly decreased arched-back nursing (ABN) and licking and grooming (LG), behaviors considered important parameters to discriminate the high quality of maternal care. Moreover, dams that received both stressors displayed less nest building and blanket nursing behaviors; no effect on the frequency of passive and total nursing was recorded. The analysis of single effects on ABN and LG, revealed that dams that underwent gestational stress induced by intragastric intubation displayed less LG, but ABN was overall unchanged. On the contrary, pup separation stress significantly increased ABN and LG upon reunion of naïve dams with their pups. Treatments per se or the association of both induced modest changes in plasma levels of allopregnanolone and corticosterone that likely did not influence maternal care. These data show that the association of a mild stress during gestation with an unfavorable experience after parturition had a significant impact on maternal care. This effect seems independent from HPA axis activation or from changes in emotional state; further studies would be necessary to ascertain the neural changes that could contribute to altered maternal behavior in stressed mothers. Moreover, these results suggest that the use of intragastric intubation during gestation would interfere with measures of drug-induced changes in maternal behavior and likely their consequences on the offspring.

Pup exposure facilitates retrieving behavior via the oxytocin neural system in female mice

Parental behavior in mammals is innate, but it is also facilitated by social experience, specifically social interactions between the parent and infant. Social interactions with infants also induce the alloparental behavior of virgin animals. Oxytocin (OT) plays an important role in mediating alloparental behavior. Although parental behavior is modulated by the medial preoptic area (MPOA) and adjacent regions, it is unclear how OT acts in these regions as a control mechanism of alloparental behavior promoted by adult-pup interaction. The aim of this study was to investigate the role of OT for facilitating effects of adult-pup interactions on alloparental behavior via neural activity of preoptic area (POA), including MPOA and adjacent area. For this purpose, we conducted behavioral tests and examined the neural activity of the OT system in POA. Virgin female mice that were repeatedly exposed to pups showed shorter retrieving latencies and higher number of c-Fos expressing neurons in POA, particular in lateral preoptic area (LPO) compared to control animals that were exposed to pups only one time. In addition, repeated pup exposure increased the proportion of OT neurons and OTR neurons expressing c-Fos in POA. The concentration of OT also significantly increased in the POA. Finally, infusion of an OT antagonist into the POA area blocked the facilitating effects of repeated pup exposure on retrieving behavior. These results demonstrated that the facilitating effects of repeated pup exposure on alloparental behavior occurred via an organizational role of the OT system.

Distress vocalization delay in the neonate lamb as a neurobehavioral assessment tool

Acoustic features of infant distress vocalizations including latency and rate of emission are used as indices of neurological deficit and integrity in human and rodent neonates. This paper investigates the relationship between temporal characteristics of distress calls, elicited by an isolation stimulus, and indicators of neurobehavioral development over 12 hr postpartum in the neonate lamb. Delayed vocalization initiation was found to be associated with poor locomotor and orientation behavior reflecting the capacity of the lamb to reunite with and follow its dam, and a lowered rate of signal emission following commencement of vocalization. Animals demonstrating delayed vocalization initiation also appeared more likely to be of a birth weight predisposed to fetal distress, and to urinate when exposed to a novel environment. Based on these preliminary studies, we propose that compromised emission of vocal signals is indicative of neurobehavioral deficit in the neonate lamb.

Structural and Functional Plasticity in the Maternal Brain Circuitry

Parenting recruits a distributed network of brain structures (and neuromodulators) that coordinates caregiving responses attuned to the young's affect, needs, and developmental stage. Many of these structures and connections undergo significant structural and functional plasticity, mediated by the interplay between maternal hormones and social experience while the reciprocal relationship between the mother and her infant forms and develops. These alterations account for the remarkable behavioral plasticity of mothers. This review will examine the molecular and neurobiological modulation and plasticity through which parenting develops and adjusts in new mothers, primarily discussing recent findings in nonhuman animals. A better understanding of how parenting impacts the brain at the molecular, cellular, systems/network, and behavioral levels is likely to significantly contribute to novel strategies for treating postpartum neuropsychiatric disorders in new mothers, and critical for both the mother's physiological and mental health and the development and well-being of her young.

Stress and the Emerging Roles of Chromatin Remodeling in Signal Integration and Stable Transmission of Reversible Phenotypes

The influence of early life experience and degree of parental-infant attachment on emotional development in children and adolescents has been comprehensively studied. Structural and mechanistic insight into the biological foundation and maintenance of mammalian defensive systems (metabolic, immune, nervous and behavioral) is slowly advancing through the emerging field of developmental molecular (epi)genetics. Initial evidence revealed that differential nurture early in life generates stable differences in offspring hypothalamic-pituitary-adrenal (HPA) regulation, in part, through chromatin remodeling and changes in DNA methylation of specific genes expressed in the brain, revealing physical, biochemical and molecular paths for the epidemiological concept of gene-environment interactions. Herein, a primary molecular mechanism underpinning the early developmental programming and lifelong maintenance of defensive (emotional) responses in the offspring is the alteration of chromatin domains of specific genomic regions from a condensed state (heterochromatin) to a transcriptionally accessible state (euchromatin). Conversely, DNA methylation promotes the formation of heterochromatin, which is essential for gene silencing, genomic integrity and chromosome segregation. Therefore, inter-individual differences in chromatin modifications and DNA methylation marks hold great potential for assessing the impact of both early life experience and effectiveness of intervention programs—from guided psychosocial strategies focused on changing behavior to pharmacological treatments that target chromatin remodeling and DNA methylation enzymes to dietary approaches that alter cellular pools of metabolic intermediates and methyl donors to affect nutrient bioavailability and metabolism. In this review article, we discuss the potential molecular mechanism(s) of gene regulation associated with chromatin modeling and programming of endocrine (e.g., HPA and metabolic or cardiovascular) and behavioral (e.g., fearfulness, vigilance) responses to stress, including alterations in DNA methylation and the role of DNA repair machinery. From parental history (e.g., drugs, housing, illness, nutrition, socialization) to maternal-offspring exchanges of nutrition, microbiota, antibodies and stimulation, the nature of nurture provides not only mechanistic insight into how experiences propagate from external to internal variables, but also identifies a composite therapeutic target, chromatin modeling, for gestational/prenatal stress, adolescent anxiety/depression and adult-onset neuropsychiatric disease.

Neurobehavioral assessment of maternal odor in developing rat pups: implications for social buffering

Social support can attenuate the behavioral and stress hormone response to threat, a phenomenon called social buffering. The mother's social buffering of the infant is one of the more robust examples; yet we understand little about the neurobiology. Using a rodent model, we explore the neurobiology of social buffering by assessing neural processing of the maternal odor, a major cue controlling social buffering in rat pups. We used pups before (postnatal day (PN) 7) and after (PN14, PN23) the functional emergence of social buffering. Pups were injected with 14C 2-deoxyglucose (2-DG) and presented with the maternal odor, a control preferred odor incapable of social buffering (acetophenone), or no odor. Brains were removed, processed for autoradiography and brain areas identified as important in adult social buffering were assessed, including the amygdala basolateral complex (Basolateral Amygdala [BLA]), medial prefrontal cortex (mPFC), and anterior cingulate cortex (ACC). Results suggest dramatic changes in the processing of maternal odor. PN7 pups show mPFC and ACC activation, although PN14 pups showed no activation of the mPFC, ACC, or BLA. All brain areas assessed were recruited by PN23. Additional analysis suggests substantial changes in functional connectivity across development. Together, these results imply complex nonlinear transitions in the neurobiology of social buffering in early life that may provide insight into the changing role of the mother in supporting social buffering.

Animal Models of Maternal Immune Activation in Depression Research

Abstract: Background

Depression and schizophrenia are debilitating mental illnesses with significant socio-economic impact. The high degree of comorbidity between the two disorders, and shared symptoms and risk factors, suggest partly common pathogenic mechanisms. Supported by human and animal studies, maternal immune activation (MIA) has been intimately associated with the development of schizophrenia. However, the link between MIA and depression has remained less clear, in part due to the lack of appropriate animal models.

Objective

Here we aim to summarize findings obtained from studies using MIA animal models and discuss their relevance for preclinical depression research.

Methods

Results on molecular, cellular and behavioral phenotypes in MIA animal models were collected by literature search (PubMed) and evaluated for their significance for depression.

Results

Several reports on offspring depression-related behavioral alterations indicate an involvement of MIA in the development of depression later in life. Depression-related behavioral phenotypes were frequently paralleled by neurogenic and neurotrophic deficits and modulated by several genetic and environmental factors.

Conclusion

Literature evidence analyzed in this review supports a relevance of MIA as animal model for a specific early life adversity, which may prime an individual for the development of distinct psychopathologies later life. MIA animal models may present a unique tool for the identification of additional exogenous and endogenous factors, which are required for the manifestation of a specific neuropsychiatric disorder, such as depression, later in life. Hereby, novel insights into the molecular mechanisms involved in the pathophysiology of depression may be obtained, supporting the identification of alternative therapeutic strategies.

Effects of Paternal Predation Risk and Rearing Environment on Maternal Investment and Development of Defensive Responses in the Offspring

Detecting past experiences with predators of a potential mate informs a female about prevailing ecological threats, in addition to stress-induced phenotypes that may be disseminated to offspring. We examined whether prior exposure of a male rat to a predator (cat) odor influences the attraction of a female toward a male, subsequent mother-infant interactions and the development of defensive (emotional) responses in the offspring. Females displayed less interest in males that had experienced predator odor. Mothers that reared young in larger, seminaturalistic housing provided more licking and grooming and active arched back-nursing behavior toward their offspring compared with dams housed in standard housing, although some effects interacted with paternal experience. Paternal predation risk and maternal rearing environment revealed sex-dependent differences in offspring wean weight, juvenile social interactions, and anxiety-like behavior in adolescence. Additionally, paternal predator experience and maternal housing independently affected variations in crf gene promoter acetylation and crf gene expression in response to an acute stressor in offspring. Our results show for the first time in mammals that variation among males in their predator encounters may contribute to stable behavioral variation among females in preference for mates and maternal care, even when the females are not directly exposed to predator threat. Furthermore, when offspring were exposed to the same threat experienced by the father, hypothalamic crf gene regulation was influenced by paternal olfactory experience and early housing. These results, together with our previous findings, suggest that paternal stress exposure and maternal rearing conditions can influence maternal behavior and the development of defensive responses in offspring.

Epigenetic legacy of parental experiences: Dynamic and interactive pathways to inheritance

The quality of the environment experienced by an individual across his or her lifespan can result in a unique developmental trajectory with consequences for adult phenotype and reproductive success. However, it is also evident that these experiences can impact the development of offspring with continued effect on subsequent generations. Epigenetic mechanisms have been proposed as a mediator of both these within- and across-generation effects, and there is increasing evidence to support the role of environmentally induced changes in DNA methylation, posttranslational histone modifications, and noncoding RNAs in predicting these outcomes. Advances in our understanding of these molecular modifications contribute to increasingly nuanced perspectives on plasticity and transmission of phenotypes across generations. A challenge that emerges from this research is in how we integrate these "new" perspectives with traditional views of development, reproduction, and inheritance. This paper will highlight evidence suggestive of an epigenetic impact of the environment on mothers, fathers, and their offspring, and illustrate the importance of considering the dynamic nature of reproduction and development and inclusive views of inheritance within the evolving field of behavioral and environmental epigenetics.

Early-Life Stress Paradigm Transiently Alters Maternal Behavior, Dam-Pup Interactions, and Offspring Vocalizations in Mice

Animal models can help elucidate the mechanisms through which early-life stress (ELS) has pathophysiological effects on the developing brain. One model that has been developed for rodents consists of limiting the amount of bedding and nesting material during the first postnatal weeks of pup life. This ELS environment has been shown to induce "abusive" behaviors by rat dams towards pups, while mouse dams have been hypothesized to display "fragmented care". Here, as part of an ongoing study of gene-environment interactions that impact brain development, we analyzed long observation periods of wild-type C57Bl/6J dams caring for wild-type and Met heterozygous pups. Met encodes for the MET receptor tyrosine kinase, which is involved in cortical and hippocampal synaptogenesis. Dams with limited resources from postnatal day (P)2 to P9 preserved regular long on-nest periods, and instead increased the number of discrete dam-pup interactions during regular off-nest periods. Immediately after dams entered the nest during off-nest periods in this ELS environment, pups responded to these qualitatively different interactions with an increased number of ultrasonic vocalizations (USV) and audible vocalizations (AV), communication signals that have been associated with aversive and painful stimuli. After returning to control conditions, nest entry behaviors normalized, and dams did not show altered anxiety-like or contextual fear learning behaviors after pup weaning. Furthermore, female mice that had undergone ELS as pups did not show atypical nest entry behaviors in control conditions in adulthood, suggesting that these specific maternal behaviors are not learned during the ELS period. The results suggest that atypical responses of both mother and pups during exposure to this ELS environment likely contribute to long-term negative outcomes in mice, and that these responses more closely resemble the effects of limited bedding on rat dams and pups than was previously suggested. Discerning how different early-life stressors mediate changes in maternal-pup interactions can help elucidate the mechanisms of ELS on brain development and behavior.

Associations among within-litter differences in early mothering received and later emotional behaviors, mothering, and cortical tryptophan hydroxylase-2 expression in female laboratory rats

This article is part of a Special Issue "Parental Care". The effects of differential maternal care received on offspring phenotype in rodents has been extensively studied between litters, but the consequences of differential mothering within litters on offspring neurobehavioral development have been rarely examined. We here investigated how variability in maternal care received among female rat siblings (measured four times daily on postnatal days 4, 6, 8, and 10) relates to the siblings' later emotional and maternal behaviors. As previously reported, we found that some female pups received up to three times more maternal licking bouts compared to their sisters; this difference was positively correlated with the pups' body weights. The number of maternal licking bouts that females received was negatively correlated with their later neophobic behaviors in an open field during periadolescence, but positively correlated with their anxiety-related behavior in an elevated plus maze during adulthood. Licking received was also positively correlated with females' later likelihood to retrieve pups in a maternal sensitization paradigm. In addition, females' neophobia during adolescence and anxiety-related behavior during adulthood predicted some aspects of both postpartum and sensitized maternal responsiveness. Medial prefrontal cortex expression of tryptophan hydroxylase-2 (TPH2; enzyme necessary for serotonin synthesis) was negatively associated with early maternal licking received. Interestingly, cortical TPH2 was positively associated with the maternal responsiveness of sensitized virgins but negatively associated with it in postpartum females. These results indicate that within-litter differences in maternal care received is an often neglected, but important, contributor to individual differences in offspring socioemotional behaviors as well as to the cortical serotonin neurochemistry that may influence these behaviors.

Parent-offspring transaction: Mechanisms and the value of within family designs

This article is part of a Special Issue "Parental Care". Parenting is best understood as a transactional process between parents and their offspring. Each responds to cues in the other, adapting their own behavior to that of their partner. One of the goals of parenting research in the past twenty years has been to untangle reciprocal processes between parents and children in order to specify what comes from the child (child effects) and what comes from the parent (parent effects). Child effects have been found to relate to genetic, pre and perinatal, family-wide, and child-specific environmental influences. Parent effects relate to stresses in the current context (e.g. financial strain, marital conflict), personality and ethnicity but also to adverse childhood experiences (e.g. parental mental health and substance abuse, poverty, divorce). Rodent models have allowed for the specification of biological mechanisms in parent and child effects, including neurobiological and genomic mechanisms, and of the causal role of environmental experience on outcomes for offspring through random assignment of offspring-mother groupings. One of the methods that have been developed in the human and animal models to differentiate between parent and child effects has been to study multiple offspring in the family. By holding the parent steady, and studying different offspring, we can examine the similarities and differences in how parents parent multiple offspring. Studies have distinguished between family average parenting, child-specific parenting and family-wide dispersion (the within family standard deviation). These different aspects of parenting have been differentially linked to offspring behavioral phenotypes.

Neuroanatomical and neurochemical basis of parenting: Dynamic coordination of motivational, affective and cognitive processes

This article is part of a Special Issue "Parental Care". Becoming a parent is arguably the most profound transforming experience in life. It is also inherently very emotionally and physically demanding, such that the reciprocal interaction with the young changes the brain and behavior of the parents. In this review, we examine the neurobiological mechanisms of parenting primarily discussing recent research findings in rodents and primates, especially humans. We argue that it is essential to consider parenting within a conceptual framework that recognizes the dynamics of the reciprocal mother-young relationship, including both the complexity and neuroplasticity of its underlying mechanisms. Converging research suggests that the concerted activity of a distributed network of subcortical and cortical brain structures regulates different key aspects of parenting, including the sensory analysis of infant stimuli as well as motivational, affective and cognitive processes. The interplay among these processes depends on the action of various neurotransmitters and hormones that modulate the timely and coordinated execution of caregiving responses of the maternal circuitry exquisitely attuned to the young's affect, needs and developmental stage. We conclude with a summary and a set of questions that may guide future research.

Interaction does Count: A Cross-Fostering Study on Transgenerational Effects of Pre-reproductive Maternal Enrichment

Pre-reproductive environmental enrichment of female rats influences sensorimotor development and spatial behavior of the offspring, possibly through the changed maternal nurturing. Nevertheless, maternal care could be not the solely responsible for changing offspring developmental trajectories. To disentangle the specific contribution to the transgenerational inheritance of pre- and post-natal factors, a cross-fostering study was performed. Female rats were reared in an enriched environment from weaning to sexual maturity, while control female rats were reared under standard conditions. Following mating with standard-reared males, all females were housed individually. Immediately after delivery, in- or cross-fostering manipulations were performed so that any foster dams received pups born to another dam of the same (in-fostering) or the opposite (cross-fostering) pre-reproductive rearing condition. In lactating dams maternal care and nesting activities were assessed, while in their male pups spatial abilities were assessed through Morris Water Maze (MWM) test at post-natal day 45. Moreover, the expression of Brain-Derived-Neurotrophic-Factor (BDNF) was evaluated in the hippocampus and frontal cortex of dams and pups at weaning. Pre-reproductive maternal environmental enrichment, followed by adoption procedures, loosened its potential in modifying maternal care and offspring developmental trajectories, as indicated by the lack of differences between in-fostered groups of dams and pups. In addition, enriched dams rearing standard pups showed the least complex maternal repertoire (the highest sniffing duration and the lowest nest quality), and their pups showed a reduced spatial learning in the MWM. Nevertheless, pre-reproductive maternal enrichment kept influencing neurotrophic pattern, with enriched dams expressing increased frontal BDNF levels (regardless of the kind of fostered pups), and their offspring expressing increased hippocampal BDNF levels. The present findings enlighten the crucial importance of the early mother-pups interactions in influencing maternal care and offspring phenotype, with the enriched dam-standard pups couple resulting in the most maladaptive encounter. Our study thus sustains that the bidirectional interactions between mother and pups are able to deeply shape offspring phenotype.

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

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

Effects of diesel engine exhaust origin secondary organic aerosols on novel object recognition ability and maternal behavior in BALB/c mice

Epidemiological studies have reported an increased risk of cardiopulmonary and lung cancer mortality associated with increasing exposure to air pollution. Ambient particulate matter consists of primary particles emitted directly from diesel engine vehicles and secondary organic aerosols (SOAs) are formed by oxidative reaction of the ultrafine particle components of diesel exhaust (DE) in the atmosphere. However, little is known about the relationship between exposure to SOA and central nervous system functions. Recently, we have reported that an acute single intranasal instillation of SOA may induce inflammatory response in lung, but not in brain of adult mice. To clarify the whole body exposure effects of SOA on central nervous system functions, we first created inhalation chambers for diesel exhaust origin secondary organic aerosols (DE-SOAs) produced by oxidation of diesel exhaust particles caused by adding ozone. Male BALB/c mice were exposed to clean air (control), DE and DE-SOA in inhalation chambers for one or three months (5 h/day, 5 days/week) and were examined for memory function using a novel object recognition test and for memory function-related gene expressions in the hippocampus by real-time RT-PCR. Moreover, female mice exposed to DE-SOA for one month were mated and maternal behaviors and the related gene expressions in the hypothalamus examined. Novel object recognition ability and N-methyl-D-aspartate (NMDA) receptor expression in the hippocampus were affected in male mice exposed to DE-SOA. Furthermore, a tendency to decrease maternal performance and significantly decreased expression levels of estrogen receptor (ER)-α, and oxytocin receptor were found in DE-SOA exposed dams compared with the control. This is the first study of this type and our results suggest that the constituents of DE-SOA may be associated with memory function and maternal performance based on the impaired gene expressions in the hippocampus and hypothalamus, respectively.

On the causes of early life experience effects: evaluating the role of mom

Early life experiences are thought to have long-lasting effects on cognitive, emotional, and social function during adulthood. Changes in neuroendocrine function, particularly the hypothalamic-pituitary-adrenal (HPA) axis, contribute to these systems-level behavioral effects. In searching for causal mechanisms underlying these early experience effects, pioneering research has demonstrated an important role for maternal care in offspring development, and this has led to two persistent ideas that permeate current research and thinking: first, environmental impact on the developing infant is mediated through maternal care behavior; second, the more care that a mother provides, the better off her offspring. While a good beginning, the reality is likely more complex. In this review, we critically examine these ideas and propose a computationally-motivated theoretical framework, and within this framework, we consider evidence supporting a hypothesis of maternal modulation. These findings may inform policy decisions in the context of child health and development.

Neonatal handling alters the structure of maternal behavior and affects mother-pup bonding

During early life, a mother and her pups establish a very close relationship, and the olfactory learning of the nest odor is very important for the bond formation. The olfactory bulb (OB) is a structure that plays a fundamental role in the olfactory learning (OL) mechanism that also involves maternal behavior (licking and contact). We hypothesized that handling the pups would alter the structure of the maternal behavior, affect OL, and alter mother-pup relationships. Moreover, changes in the cyclic AMP-response element binding protein phosphorylation (CREB) and neurotrophic factors could be a part of the mechanism of these changes. This study aimed to analyze the effects of neonatal handling, 1 min per day from postpartum day 1 to 10 (PPD 1 to PPD 10), on the maternal behavior and pups' preference for the nest odor in a Y maze (PPD 11). We also tested CREB's phosphorylation and BDNF signaling in the OB of the pups (PPD 7) by Western blot analysis. The results showed that handling alters mother-pups interaction by decreasing mother-pups contact and changing the temporal pattern of all components of the maternal behavior especially the daily licking and nest-building. We found sex-dependent changes in the nest odor preference, CREB and BDNF levels in pups OB. Male pups were more affected by alterations in the licking pattern, and female pups were more affected by changes in the mother-pup contact (the time spent outside the nest and nursing).

Integrating early life experience, gene expression, brain development, and emergent phenotypes: unraveling the thread of nature via nurture

Adaptation to environmental changes is based on the perpetual generation of new phenotypes. Modern biology has focused on the role of epigenetic mechanisms in facilitating the adaptation of organisms to changing environments through alterations in gene expression. Inherited and/or acquired epigenetic factors are relatively stable and have regulatory roles in numerous genomic activities that translate into phenotypic outcomes. Evidence that dietary and pharmacological interventions have the potential to reverse environment-induced modification of epigenetic states (e.g., early life experience, nutrition, medication, infection) has provided an additional stimulus for understanding the biological basis of individual differences in cognitive abilities and disorders of the brain. It has been suggested that accurate quantification of the relative contribution of heritable genetic and epigenetic variation is essential for understanding phenotypic divergence and adaptation in changing environments, a process requiring stable modulation of gene expression. The main challenge for epigenetics in psychology and psychiatry is to determine how experiences and environmental cues, including the nature of our nurture, influence the expression of neuronal genes to produce long-term individual differences in behavior, cognition, personality, and mental health. To this end, focusing on DNA and histone modifications and their initiators, mediators and readers may provide new inroads for understanding the molecular basis of phenotypic plasticity and disorders of the brain. In this chapter, we review recent discoveries highlighting epigenetic aspects of normal brain development and mental illness, as well as discuss some future directions in the field of behavioral epigenetics.

Social status, glucocorticoids, immune function, and health: can animal studies help us understand human socioeconomic-status-related health disparities?

For humans in developed nations, socioeconomic status (SES)--relative income, education and occupational position in a society--is a strong predictor of morbidity and mortality rates, with increasing SES predicting longer life span (e.g. Marmot et al., 1991). Mechanisms underlying this relationship have been examined, but the relative role of each mechanism still remains unknown. By understanding the relative role of specific mechanisms that underlie dramatic health disparities between high and low social status individuals we can begin to identify effective, targeted methods to alleviate health disparities. In the current paper, we take advantage of a growing number of animal studies that have quantified biological health-related correlates (glucocorticoid production and immune function) of social status and compare these studies to the current literature on human SES and health to determine if and how animal studies can further our understanding of SES-associated human health disparities. Specifically, we compared social-status related glucocorticoid production and immune function in humans and animals. From the review, we show that our present understanding of the relationships between social status and glucocorticoid production/immune function is still growing, but that there are already identifiable parallels (and non-parallels) between humans and animals. We propose timely areas of future study focused on (1) specific aspects of social status that may influence stress-related physiology, (2) mechanisms underlying long-term influences of social status on physiology and health, and (3) intervention studies to alleviate potentially negative physiological correlates of social status.

Reducing behavioral inhibition to novelty via systematic neonatal novelty exposure: the influence of maternal hypothalamic-pituitary-adrenal regulation

BACKGROUND

Behavioral inhibition (BI) to novelty is thought to be a stable temperament type that appears early in life and is a major risk factor for anxiety disorders. In the rat, habituation of such inhibition can be facilitated via neonatal novelty exposure (NNE), thus reducing BI to novelty. Here, we tested the hypothesis that this early intervention effect is modulated by the context of maternal self-stress regulation.

METHODS

The NNE was carried out during postnatal days 1-21, in which one half of each litter was exposed to a relatively novel nonhome environment for 3-min daily while the remaining one half stayed in the home cage. After weaning, BI to novelty was assessed in an open field with a measure of disinhibition defined as a greater increase in exploration across two brief trials. Maternal context was characterized by trait measures of hypothalamic-pituitary-adrenal (HPA) axis reactivity, including basal and stress-evoked corticosterone (CORT) responses.

RESULTS

Family-to-family variations in the NNE effect were associated with variations in maternal HPA function-a low-basal CORT and high-evoked CORT response profile constituting the context for a novelty-induced facilitation of disinhibition (i.e., a greater increase in exploratory activity over repeated trials) and an opposite HPA profile constituting the context for a novelty-induced reduction of disinhibition.

CONCLUSIONS

This result is consistent with the hypothesis that maternal self-stress regulation modulates the effect of early life intervention on BI to novelty and suggests that effective interventions should include strategies to help mothers improve their self-stress regulation.

Maternal care associates with play dominance rank among adult female rats

Variations in maternal care influence important life history traits that determine reproductive fitness. The adult female offspring of mothers that show reduced levels of pup licking/grooming (LG; i.e., low-LG mothers) show increased defensive responses to stress, accelerated pubertal development, and greater sexual receptivity than the female offspring of high-LG mothers. Amongst several species an accelerated pattern of reproductive development is associated with increased dominance-related behaviors and higher social rank. We hypothesize that rats from low-LG dams may thus also secure higher social rank as a means to compete for limited resources with conspecifics. In this study, social interactions were observed in triads of adult female rats aged p90 that received low, mid, and high levels of pup LG over the first week of life. Low- and mid-LG females had the highest pinning scores and high-LG rats the lowest, showing that low- and mid-LG adult females engage in greater play dominance-related behavior. Likewise, low- and mid-LG rats spent significantly more time drinking following 24 hr of water deprivation in a water competition test thus allowing them to secure a limited resource more easily than high-LG rats. Interestingly, pinning by play dominant females was increased when subordinates were sexually receptive (proestrus/estrus), suggestive of a process of reproductive suppression. Some evidence suggests that low-LG and mid-LG rats also show greater fecundity than high-LG rats. Variations in maternal care may thus have a long-term influence on the development of play dominance and possibly social rank in the female rat, which might contribute to reproductive success within a competitive environment.

Regulation of corticoid and serotonin receptor brain system following early life exposure of glucocorticoids: long term implications for the neurobiology of mood

Potent glucocorticoids (GC) administered early in life have improved premature infant survival dramatically. However, these agents may increase the risk for physical, neurological and behavior alterations. Anxiety, depression and attention difficulties are commonly described in adolescent and young adult survivors of prematurity. In the present study we administered vehicle, dexamethasone, or hydrocortisone to Sprague-Dawley rat pups on postnatal days 5 and 6, mimicking a short term clinical protocol commonly used in human infants. Two systems that are implicated in the regulation of stress and behavior were assessed: the limbic-hypothalamic-pituitary-adrenal axis [LHPA; glucocorticoid and mineralocorticoid receptors within] and the Serotonin (5-HT) system. We found that as adults, male Sprague-Dawley pups treated with GC showed agent specific altered growth, anxiety-related behavior, changes in corticoid response to novelty and gene expression changes within LHPA and 5-HT-related circuitry. The data suggest that prolonged GC-receptor stimulation during the early neonatal period can contribute to the development of individual differences in stress response and anxiety-related behavior later in life.

Prenatal Cocaine Disrupts Serotonin Signaling-Dependent Behaviors: Implications for Sex Differences, Early Stress and Prenatal SSRI Exposure

Prenatal cocaine (PC) exposure negatively impacts the developing nervous system, including numerous changes in serotonergic signaling. Cocaine, a competitive antagonist of the serotonin transporter, similar to selective serotonin reuptake inhibitors (SSRIs), also blocks dopamine and norepinephrine transporters, leaving the direct mechanism through which cocaine disrupts the developing serotonin system unclear. In order to understand the role of the serotonin transporter in cocaine's effect on the serotonergic system, we compare reports concerning PC and prenatal antidepressant exposure and conclude that PC exposure affects many facets of serotonergic signaling (serotonin levels, receptors, transporters) and that these effects differ significantly from what is observed following prenatal SSRI exposure. Alterations in serotonergic signaling are dependent on timing of exposure, test regimens, and sex. Following PC exposure, behavioral disturbances are observed in attention, emotional behavior and stress response, aggression, social behavior, communication, and like changes in serotonergic signaling, these effects depend on sex, age and developmental exposure. Vulnerability to the effects of PC exposure can be mediated by several factors, including allelic variance in serotonergic signaling genes, being male (although fewer studies have investigated female offspring), and experiencing the adverse early environments that are commonly coincident with maternal drug use. Early environmental stress results in disruptions in serotonergic signaling analogous to those observed with PC exposure and these may interact to produce greater behavioral effects observed in children of drug-abusing mothers. We conclude that based on past evidence, future studies should put a greater emphasis on including females and monitoring environmental factors when studying the impact of PC exposure.

Developmental underpinnings of differences in rodent novelty-seeking and emotional reactivity

Innate differences in human temperament strongly influence how individuals cope with stress and also predispose towards specific types of psychopathology. The present study examines the developing brain in an animal model of temperamental differences to examine how altered neurodevelopment may engender differences in emotional reactivity that are stable throughout the animal's life. We utilize selectively-bred High Responder (bHR) and Low Responder (bLR) rats that exhibit dramatic emotional behavior differences, with bHRs exhibiting exaggerated novelty-exploration, aggression, impulsivity and drug self-administration, and bLRs showing marked behavioral inhibition and exaggerated anxiety-like and depressive-like behavior. Using Affymetrix microarrays, we assessed bLR and bHR gene expression in the developing brain on postnatal days (P)7, 14 and 21, focusing on the hippocampus and nucleus accumbens, two regions related to emotionality and known to differ in adult bLR and bHR rats. We found dramatic gene expression differences between bLR and bHR in the P7 and P14 hippocampus, with minimal differences in the nucleus accumbens. Some of the most profound differences involved genes critical for neurodevelopment and synaptogenesis. Stereological studies evaluated hippocampal structure in developing bHR and bLR pups, revealing enhanced hippocampal volume and cell proliferation in bLR animals. Finally, behavioral studies showed that the characteristic bHR and bLR behavioral phenotypes emerge very early in life, with exploratory differences apparent at P16 and anxiety differences present by P25. Together these data point to specific brain regions and critical periods when the bHR and bLR phenotypes begin to diverge, which may eventually allow us to test possible therapeutic interventions to normalize extreme phenotypes (e.g. the anxiety-prone nature of bLRs or drug addiction proclivity of bHRs).

Rodent model of infant attachment learning and stress

Here we review the neurobiology of infant odor learning in rats, and discuss the unique role of the stress hormone corticosterone (CORT) in the learning necessary for the developing rat. During the first 9 postnatal (PN) days, infants readily learn odor preferences, while aversion and fear learning are attenuated. Such restricted learning may ensure that pups only approach their mother. This sensitive period of preference learning overlaps with the stress hyporesponsive period (SHRP, PN4-14) when pups have a reduced CORT response to most stressors. Neural underpinnings responsible for sensitive-period learning include increased activity within the olfactory bulb and piriform "olfactory" cortex due to heightened release of norepinephrine from the locus coeruleus. After PN10 and with the decline of the SHRP, stress-induced CORT release permits amygdala activation and facilitates learned odor aversions and fear. Remarkably, odor preference and attenuated fear learning can be reestablished in PN10-15 pups if the mother is present, an effect due to her ability to suppress pups' CORT and amygdala activity. Together, these data indicate that functional changes in infant learning are modified by a unique interaction between the developing CORT system, the amygdala, and maternal presence, providing a learning system that becomes more flexible as pups mature.

Oxytocin-dopamine interactions mediate variations in maternal behavior in the rat

Variations in maternal behavior among lactating rats associate with differences in estrogen-oxytocin interactions in the medial preoptic area (mPOA) and in dopamine levels in the nucleus accumbens (nAcc). Thus, stable, individual differences in pup licking/grooming (LG) are abolished by oxytocin receptor blockade or treatments that eliminate differences in the nAcc dopamine signal. We provide novel evidence for a direct effect of oxytocin at the level of the ventral tegmental area (VTA) in the regulation of nAcc dopamine levels. Mothers that exhibit consistently increased pup LG (i.e. high LG mothers) by comparison with low LG mothers show increased oxytocin expression in the mPOA and the paraventricular nucleus of the hypothalamus and increased projections of oxytocin-positive cells from both mPOA and paraventricular nucleus of the hypothalamus to the VTA. Direct infusion of oxytocin into the VTA increased the dopamine signal in the nAcc. Finally, high compared with low LG mothers show greater increases in dopamine signal in the nAcc during bouts of pup LG, and this difference is abolished with infusions of an oxytocin receptor antagonist directly into the VTA. These studies reveal a direct effect of oxytocin on dopamine release within the mesocorticolimbic dopamine system and are consistent with previous reports of oxytocin-dopamine interactions in the establishment and maintenance of social bonds.

Progesterone and maternal aggressive behavior in rats

Females usually display low levels of aggressiveness; however, during lactation, the aggressive behavior against intruders to the nest area is an important component of the maternal behavioral repertoire. The present study aimed to analyze the influence of progesterone (P4) on the maternal aggressive behavior in rats. Lactating rat were ovariectomized on the first day after delivery and, on the 6th postpartum day, aggressive behaviors against a male intruder were recorded. Also in the 6th PPD, the effects of a P4 receptor antagonist (RU 486) as well as of finasteride - which inhibits the conversion of P4 to its metabolite allopregnanolone - on the aggressive behavior of non-ovariectomized lactating rats were analyzed. Finally, plasma concentration of prolactin was measured on the 8th PPD. This study shows, for the first time, that ovariectomy just after parturition reduces some aspects of the maternal behavior (frequency of licking) and the aggressive behavior and increased plasma prolactin. On the other hand, the administration of RU486 induced a marked increase in the aggressiveness of lactating females. No changes were detected after finasteride injection. Gonadal hormones after parturition seem necessary for the development of maternal aggressive behavior. Furthermore, our results suggest that the increase in P4 levels throughout the postpartum period could be one of the causes for the natural reduction of the aggressive behavior in lactating rats.

Neural and environmental factors impacting maternal behavior differences in high- versus low-novelty-seeking rats

Selective breeding of rats exhibiting differences in novelty-induced locomotion revealed that this trait predicts several differences in emotional behavior. Bred High Responders (bHRs) show exaggerated novelty-induced locomotion, aggression, and psychostimulant self-administration, compared to bred Low Responders (bLRs), which are inhibited and prone to anxiety- and depression-like behavior. Our breeding studies highlight the heritability of the bHR/bLR phenotypes, although environmental factors like maternal care also shape some aspects of these traits. We previously reported that HR vs. LR mothers act differently, but it was unclear whether their behaviors were genetically driven or influenced by their pups. The present study (a) used cross-fostering to evaluate whether the bHR/bLR maternal styles are inherent to mothers and/or are modulated by pups; and (b) assessed oxytocin and oxytocin receptor mRNA expression to examine possible underpinnings of bHR/bLR maternal differences. While bHR dams exhibited less maternal behavior than bLRs during the dark/active phase, they were very attentive to pups during the light phase, spending greater time passive nursing and in contact with pups compared to bLRs. Cross-fostering only subtly changed bHR and bLR dams' behavior, suggesting that their distinct maternal styles are largely inherent to the mothers. We also found elevated oxytocin mRNA levels in the supraoptic nucleus of the hypothalamus in bHR versus bLR dams, which may play some role in driving their behavior differences. Overall these studies shed light on the interplay between the genetics of mothers and infants in driving differences in maternal style.