Early deprivation leads to altered behavioural, autonomic and endocrine responses to environmental challenge in adult Fischer rats

Age, β-endorphin, and sex dependent effects of maternal separation on locomotor activity, anxiety-like behavior, and alcohol reward

Introduction

Childhood adversity is pervasive and linked to numerous disadvantages in adulthood, including physical health problems, mental illness, and substance use disorders. Initial sensitivity to the rewarding effects of alcohol predicts the risk of developing an alcohol use disorder, and may be linked to developmental stress. The opioid peptide β-endorphin (β-E) regulates the stress response and is also implicated in the risk for excessive alcohol consumption.

Methods

We explored the influence of β-E in an animal model of early life adversity using controlled maternal separation by evaluating changes in locomotor activity, anxiety-like behavior, and the initial rewarding effects of alcohol in a single exposure conditioned place preference paradigm in control C57BL/6J and β-E deficient β-E +/+ 0.129S2-Pomc tm1Low/J; β-E −/− mice. Maternal separation (MS) occurred for 3 h each day from post-natal days (PND) 5–18 in approximately half the subjects.

Results

Maternal interactions increased following the separation protocol equally in both genotypes. MS and control subjects were tested as adolescents (PND 26–32) or adults (PND 58–72); the effects of MS were generally more pronounced in older subjects. Adults were more active than adolescents in the open field, and MS decreased activity in adolescent mice but increased it in adults. The increase in adult activity as a result of early life stress depended on both β-E and sex. β-E also influenced the effect of maternal separation on anxiety-like behavior in the Elevated Plus Maze. MS promoted rewarding effects of alcohol in male β-E deficient mice of either age, but had no effect in other groups.

Discussion

Taken together, these results suggest that the effects of MS develop over time and are β-E and sex dependent and may aid understanding of how individual differences influence the impact of adverse childhood experiences.

Enduring effects of early-life adversity on reward processes: A systematic review and meta-analysis of animal studies

Two-thirds of individuals experience adversity during childhood such as neglect, abuse or highly-stressful events. Early-life adversity (ELA) increases the life-long risk of developing mood and substance use disorders. Reward-related deficits has emerged as a key endophenotype of such psychiatric disorders. Animal models are invaluable for studying how ELA leads to reward deficits. However, the existing literature is heterogenous with difficult to reconcile findings. To create an overview, we conducted a systematic review containing multiple meta-analyses regarding the effects of ELA on reward processes overall and on specific aspects of reward processing in animal models. A comprehensive search identified 120 studies. Most studies omitted key details resulting in unclear risk of bias. Overall meta-analysis showed that ELA significantly reduced reward behaviors (SMD: -0.42 [-0.60; -0.24]). The magnitude of ELA effects significantly increased with longer exposure. When reward domains were analyzed separately, ELA only significantly dampened reward responsiveness (SMD: -0.525[-0.786; -0.264]) and social reward processing (SMD: -0.374 [-0.663; -0.084]), suggesting that ELA might lead to deficits in specific reward domains.

Chronic mild stress paradigm as a rat model of depression: facts, artifacts, and future perspectives

RATIONALE

The chronic mild stress (CMS) paradigm was first described almost 40 years ago and has become a widely used model in the search for antidepressant drugs for major depression disorder (MDD). It has resulted in the publication of almost 1700 studies in rats alone. Under the original CMS procedure, the expression of an anhedonic response, a key symptom of depression, was seen as an essential feature of both the model and a depressive state. The prolonged exposure of rodents to unpredictable/uncontrollable mild stressors leads to a reduction in the intake of palatable liquids, behavioral despair, locomotor inhibition, anxiety-like changes, and vegetative (somatic) abnormalities. Many of the CMS studies do not report these patterns of behaviors, and they often fail to include consistent molecular, neuroanatomical, and physiological phenotypes of CMS-exposed animals.

OBJECTIVES

To critically review the CMS studies in rats so that conceptual and methodological flaws can be avoided in future studies.

RESULTS

Analysis of the literature supports the validity of the CMS model and its impact on the field. However, further improvements could be achieved by (i) the stratification of animals into 'resilient' and 'susceptible' cohorts within the CMS animals, (ii) the use of more refined protocols in the sucrose test to mitigate physiological and physical artifacts, and (iii) the systematic evaluation of the non-specific effects of CMS and implementation of appropriate adjustments within the behavioral tests.

CONCLUSIONS

We propose methodological revisions and the use of more advanced behavioral tests to refine the rat CMS paradigm, which offers a valuable tool for developing new antidepressant medications.

Corticotropin-releasing factor infusion in the bed nucleus of the stria terminalis of lactating mice alters maternal care and induces behavioural phenotypes in offspring

The peripartum period is accompanied by numerous physiological and behavioural adaptations organised by the maternal brain. These changes are essential for adequate expression of maternal behaviour, thereby ensuring proper development of the offspring. The corticotropin-releasing factor (CRF) plays a key role in a variety of behaviours accompanying stress, anxiety, and depression. There is also evidence that CRF contributes to maladaptations during the peripartum period. We investigated the effects of CRF in the bed nucleus of the stria terminalis (BNST) of lactating mice during maternal care and analysed locomotor activity and anxiety-like behaviour in the offspring. The BNST has been implicated in anxiety behaviour and regulation of the stress response. The effects of intra-BNST CRF administration were compared with those induced by the limited bedding (LB) procedure, a model that produces altered maternal behaviour. BALB/cJ dams were exposed to five infusions of CRF or saline into the BNST in the first weeks after birth while the LB dams were exposed to limited nesting material from postnatal days (P) 2–9. Maternal behaviour was recorded in intercalated days, from P1-9. Offspring anxiety-like behaviour was assessed during adulthood using the open-field, elevated plus-maze, and light/dark tests. Both intra-BNST CRF and LB exposure produced altered maternal care, represented by decreased arched-back nursing and increased frequency of exits from the nest. These changes in maternal care resulted in robust sex-based differences in the offspring’s behavioural responses during adulthood. Females raised by CRF-infused dams exhibited increased anxiety-like behaviour, whereas males presented a significant decrease in anxiety. On the other hand, both males and females raised by dams exposed to LB showed higher locomotor activity. Our study demonstrates that maternal care is impaired by intra-BNST CRF administrations, and these maladaptations are similar to exposure to adverse early environments. These procedures, however, produce distinct phenotypes in mice during young adulthood and suggest sex-based differences in the susceptibility to poor maternal care.

Effects of Early Life Stress on Epigenetic Changes of the Glucocorticoid Receptor 17 Promoter during Adulthood

Growing evidence suggests that early life stress (ELS) has long-lasting effects on glucocorticoid receptor (GR) expression and behavior via epigenetic changes of the GR exon 1₇ promoter. However, it remains unclear whether ELS regulates histone modifications of the GR exon 1₇ promoter across the life span. We investigated the effects of maternal separation (MS) on histone acetylation and methylation of GR exon 1₇ promoter in the hippocampus, according to the age of adults. Depression-like behavior and epigenetic regulation of GR expression were examined at young and middle adulthood in mice subjected to MS from postnatal day 1 to 21. In the forced swimming test, young adult MS mice showed no effect on immobility time, but middle-aged MS mice significantly increased immobility time. Young adult and middle-aged MS mice showed decreased GR expression. Their two ages showed decreased histone acetylation with increased histone deacetylases (HDAC5) levels, decreased permissive methylation, and increased repressive methylation at the GR exon 1₇ promoter. The extent of changes in gene expression and histone modification in middle adulthood was greater than in young adulthood. These results indicate that MS in early life causes long-term negative effects on behavior via histone modification of the GR gene across the life span.

Childhood Physical Neglect Is Associated With Exaggerated Systemic and Intracellular Inflammatory Responses to Repeated Psychosocial Stress in Adulthood

Experiences of child maltreatment are associated with a host of adverse mental and physical health outcomes in adulthood. Altered reactivity to psychosocial stress exposure may partially explain known associations between early experiences of maltreatment and later life health. The present study focuses on examining whether experiences of child maltreatment are associated with physiological reactions to initial and repeated psychosocial stress in adulthood. To this end, 44 healthy adults (52% male, aged 18-65) completed the Childhood Trauma Questionnaire to provide information about exposure to child maltreatment and completed the Trier Social Stress Test (TSST) on 2 consecutive days. Peripheral blood was collected prior to as well as 30 and 120 min following the TSST on each day. Plasma Interleukin-6 (IL-6) and gene expression of IL-6, IL-1β, nuclear factor-kB (NF-kB), and inhibitor of kB (IkB) were measured from each blood sample. Total CTQ scores were unrelated to plasma IL-6 and gene expression (ps > .10) but a history of childhood physical neglect was associated with increased interleukin-1β (β =.35; p =.02; R² =.19) and nuclear factor-kB (β =.30; p =.046; R² =.13) expression following initial stress. Following repeated exposure to the TSST, childhood physical neglect was associated with increased plasma IL-6 reactivity (β =.34; p =.02; R² =.16) and increased expression of nuclear factor-kB (β =.31; p =.04; R² =.08). Finally, childhood physical neglect was associated with decreased habituation following repeated exposure to the TSST. Other CTQ subscales were not related to plasma IL-6 and gene expression when considered individually. Results from this study are suggestive of a unique effect of childhood physical neglect on the physiological stress response following initial and repeated exposure to a common psychosocial stressor. This provides important directions for future research because the effect of childhood physical neglect on long-term neglect are not well understood and in need of further investigation.

Cocaine- and amphetamine-regulated transcript peptide promotes reward seeking behavior in socially isolated rats

Reward deficit, expressed as anhedonia, is one of the major symptoms associated with neuropsychiatric disorders, but the underlying maladaptations have not been understood. Herein, we test the hypothesis that the neuropeptide cocaine- and amphetamine-regulated transcript (CART) may participate in the process. The study is justified since the peptide is a major player in inducing satiety and also processing of reward. The rats were socially isolated to induce reward deficit and conditioned to self-stimulate via an electrode in lateral hypothalamus (LH)-medial forebrain bundle (MFB) region. Compared to group-housed control rats, the socially isolated animals showed decreased lever press activity and elevated ICSS threshold indicating anhedonia-like condition. However, the effects of social isolation were alleviated by CART administered via intracerebroventricular route. The changes in the expression of CART protein and mRNA were screened using immunofluorescence and qRT-PCR methods, respectively. Socially isolated rats showed reduction in the expression of CART in the LH, nucleus accumbens shell (AcbSh) and posterior ventral tegmental area (pVTA) and CART mRNA in the Acb and LH. Double immunostaining with antibodies against CART and synaptophysin revealed significant loss of colabeled elements in LH, AcbSh and pVTA. We suggest that down-regulation of endogenous CARTergic system in the LH-pVTA-AcbSh reward circuitry may be causal to motivational anhedonia like phenotype seen in neuropsychiatric conditions.

Early-Life Adversity Induces Epigenetically Regulated Changes in Hippocampal Dopaminergic Molecular Pathways

Early-life adversity (ELA) represents a major risk factor for the development of behavioral dysfunctions and mental disorders later in life. On the other hand, dependent on type, time point, and duration, ELA exposure can also induce adaptations, which result in better stress coping and resilience later in life. Guided by the hypothesis that chronic exposure to ELA results in dysfunctional brain and behavior, whereas short exposure to ELA may result in resilience, the behavioral and neurobiological consequences of long-term separation stress (LTSS) and short-term separation stress (STSS) were compared in a mouse model for ELA. In line with our hypothesis, we found that LTSS induced depressive-like behavior, whereas STSS reduced depressive-like behavioral symptoms. We then tested the hypothesis that the opposite behavioral outcomes of the two stress paradigms may be mediated by functional, epigenetically regulated changes of dopaminergic modulation in the hippocampal formation. We found that STSS exposure elevated dopamine receptor D1 (DRD1) gene expression and decreased gene expression of its downstream modulator DARPP-32 (32-kDa dopamine- and cAMP-regulated phosphoprotein), which was paralleled by decreased H3 acetylation at its gene promoter region. In contrast, LTSS elevated DARPP-32 gene expression, which was not paralleled by changes in histone acetylation and DRD1 gene expression. These findings indicate that short- and long-term neonatal exposure to ELA induces changes in dopaminergic molecular pathways, some of which are epigenetically regulated and which either alleviate or aggravate depressive-like symptoms later in life.

Early life stress in rats sex-dependently affects remote endocrine rather than behavioral consequences of adult exposure to contextual fear conditioning

Exposure to electric foot-shocks can induce in rodents contextual fear conditioning, generalization of fear to other contexts and sensitization of the hypothalamic-pituitary-adrenal (HPA) axis to further stressors. All these aspects are relevant for the study of post-traumatic stress disorder. In the present work we evaluated in rats the sex differences and the role of early life stress (ELS) in fear memories, generalization and sensitization. During the first postnatal days subjects were exposed to restriction of nesting material along with exposure to a "substitute" mother. In the adulthood they were exposed to (i) a contextual fear conditioning to evaluate long-term memory and extinction and (ii) to a novel environment to study cognitive fear generalization and HPA axis heterotypic sensitization. ELS did not alter acquisition, expression or extinction of context fear conditioned behavior (freezing) in either sex, but reduced activity in novel environments only in males. Fear conditioning associated hypoactivity in novel environments (cognitive generalization) was greater in males than females but was not specifically affected by ELS. Although overall females showed greater basal and stress-induced levels of ACTH and corticosterone, an interaction between ELS, shock exposure and sex was found regarding HPA hormones. In males, ELS did not affect ACTH response in any situation, whereas in females, ELS reduced both shock-induced sensitization of ACTH and its conditioned response to the shock context. Also, shock-induced sensitization of corticosterone was only observed in males and ELS specifically reduced corticosterone response to stressors in males but not females. In conclusion, ELS seems to have only a minor impact on shock-induced behavioral conditioning, while affecting the unconditioned and conditioned responses of HPA hormones in a sex-dependent manner.

Sex differences in associations between maternal deprivation and alterations in hippocampal calcium-binding proteins and cognitive functions in rats

Background and objective

Adverse early-life experiences have been suggested as one of the key contributors to neurodevelopmental disorders, such that these experiences influence brain development, cognitive ability and mental health. Previous studies indicated that hippocampal levels of the calcium-binding proteins calretinin (CALR) and calbindin-D28k (CALB) changed in response to maternal deprivation (MD), a model for adverse early-life experiences. We investigated the effects of MD on hippocampal CALR and CALB protein levels and cognitive behaviors, and explored whether these effects were sex-related.

Methods

From postnatal day 2 (PND-2) to PND-14, rat pups in the MD group were separated from their mothers for 3 h/day for comparison with pups raised normally (control). To determine hippocampal CALR and CALB levels, fluorescent immunostaining of hippocampal sections and Western blot analysis of hippocampal tissues were employed at various timepoints (PND-21, -25, -30, -35 and -40). Behavioral and cognitive changes were determined by open field test (PND-21) and Morris water maze (PND-25).

Results

Western blot analysis showed changes in the hippocampal CALR and CALB levels in both male and female MD groups, compared with controls. The open field test showed reduced exploration only in male MD groups but not female MD groups. The Morris water maze tests indicated that MD caused spatial memory impairment both in male and female rats, but there was a sex difference in CALR and CALB levels.

Conclusions

Male rats are relatively more vulnerable to MD stress than female rats, but both male and female rats demonstrate spatial learning impairment after exposure to MD stress. Sex difference in CALR and CALB levels may reveal the different mechanisms behind the behavioral observations.

Neonatal whisker trimming in WAG/Rij rat pups causes developmental delay, encourages maternal care and affects exploratory activity in adulthood

WAG/Rij rats are genetically predisposed to absence epilepsy. Maternal behavior in WAG/Rij female rats is known to differ from that in non-epileptic females. We hypothesize that (1) mother's behavior may be changed as response to changes in pup's conditions; (2) sensory deprivation at the neonatal age affect learning and behavior in adulthood. All whiskers in WAG/Rij rat pups were trimmed daily during PN1-PN8. Maternal behavior was examined during the same period. It was found that in the control group, WAG/Rij females often demonstrated abnormally long (>1 min) repetitive purposeless stereotypical actions that were roughly classified as compulsive-like behavior. Mothers of the trimmed pups showed less compulsive-like behavior and more intensively interacted with pups and built better nests. Rat pups in the trimmed group had lower body weight on PN7-PN19 as compared to the control. In the trimmed group, maturation of motor skills and early behavioral patterns (i.e. walking, grooming, vertical activity, motor functions of forelimbs) showed 1-2 days delay in comparison to the control. At the age of 2-2.5 months, the locomotor activity in the trimmed rats differed from the control, but the level of anxiety was the same (the open field and the elevated plus maze). At the age of 6 months, the trimmed and control rats showed no differences in conditioned avoidance learning test, therefore, neonatal whisker trimming did not influence fear-based learning abilities in adulthood. It is hypothesized that an enhanced maternal care is capable to modulate development of brain functions in sensory deprived progeny.

Modulation of the Hypothalamic-Pituitary-Adrenal Axis by Early Life Stress Exposure

Exposure to stress during critical periods in development can have severe long-term consequences, increasing overall risk on psychopathology. One of the key stress response systems mediating these long-term effects of stress is the hypothalamic-pituitary-adrenal (HPA) axis; a cascade of central and peripheral events resulting in the release of corticosteroids from the adrenal glands. Activation of the HPA-axis affects brain functioning to ensure a proper behavioral response to the stressor, but stress-induced (mal)adaptation of the HPA-axis' functional maturation may provide a mechanistic basis for the altered stress susceptibility later in life. Development of the HPA-axis and the brain regions involved in its regulation starts prenatally and continues after birth, and is protected by several mechanisms preventing corticosteroid over-exposure to the maturing brain. Nevertheless, early life stress (ELS) exposure has been reported to have numerous consequences on HPA-axis function in adulthood, affecting both its basal and stress-induced activity. According to the match/mismatch theory, encountering ELS prepares an organism for similar ("matching") adversities during adulthood, while a mismatching environment results in an increased susceptibility to psychopathology, indicating that ELS can exert either beneficial or disadvantageous effects depending on the environmental context. Here, we review studies investigating the mechanistic underpinnings of the ELS-induced alterations in the structural and functional development of the HPA-axis and its key external regulators (amygdala, hippocampus, and prefrontal cortex). The effects of ELS appear highly dependent on the developmental time window affected, the sex of the offspring, and the developmental stage at which effects are assessed. Albeit by distinct mechanisms, ELS induced by prenatal stressors, maternal separation, or the limited nesting model inducing fragmented maternal care, typically results in HPA-axis hyper-reactivity in adulthood, as also found in major depression. This hyper-activity is related to increased corticotrophin-releasing hormone signaling and impaired glucocorticoid receptor-mediated negative feedback. In contrast, initial evidence for HPA-axis hypo-reactivity is observed for early social deprivation, potentially reflecting the abnormal HPA-axis function as observed in post-traumatic stress disorder, and future studies should investigate its neural/neuroendocrine foundation in further detail. Interestingly, experiencing additional (chronic) stress in adulthood seems to normalize these alterations in HPA-axis function, supporting the match/mismatch theory.

Consequences of early adverse rearing experience(EARE) on development: insights from non-human primate studies

Early rearing experiences are important in one's whole life, whereas early adverse rearing experience(EARE) is usually related to various physical and mental disorders in later life. Although there were many studies on human and animals, regarding the effect of EARE on brain development, neuroendocrine systems, as well as the consequential mental disorders and behavioral abnormalities, the underlying mechanisms remain unclear. Due to the close genetic relationship and similarity in social organizations with humans, non-human primate(NHP) studies were performed for over 60 years. Various EARE models were developed to disrupt the early normal interactions between infants and mothers or peers. Those studies provided important insights of EARE induced effects on the physiological and behavioral systems of NHPs across life span, such as social behaviors(including disturbance behavior, social deficiency, sexual behavior, etc), learning and memory ability, brain structural and functional developments(including influences on neurons and glia cells, neuroendocrine systems, e.g., hypothalamic-pituitary-adrenal(HPA) axis, etc). In this review, the effects of EARE and the underlying epigenetic mechanisms were comprehensively summarized and the possibility of rehabilitation was discussed.

Sensitive periods of substance abuse: Early risk for the transition to dependence

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Early substance use dramatically increases the risk of substance use disorder (SUD).

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Although many try drugs, only a small percentage transition to SUD.

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High reactivity of reward, habit, and stress systems increase risk.

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Identification of early risk enables targeted, preventative interventions for SUD.

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Prevention must start before the sensitive adolescent period to maximize resilience.

Early adolescent substance use dramatically increases the risk of lifelong substance use disorder (SUD). An adolescent sensitive period evolved to allow the development of risk-taking traits that aid in survival; today these may manifest as a vulnerability to drugs of abuse. Early substance use interferes with ongoing neurodevelopment to induce neurobiological changes that further augment SUD risk. Although many individuals use drugs recreationally, only a small percentage transition to SUD. Current theories on the etiology of addiction can lend insights into the risk factors that increase vulnerability from early recreational use to addiction. Building on the work of others, we suggest individual risk for SUD emerges from an immature PFC combined with hyper-reactivity of reward salience, habit, and stress systems. Early identification of risk factors is critical to reducing the occurrence of SUD. We suggest preventative interventions for SUD that can be either tailored to individual risk profiles and/or implemented broadly, prior to the sensitive adolescent period, to maximize resilience to developing substance dependence. Recommendations for future research include a focus on the juvenile and adolescent periods as well as on sex differences to better understand early risk and identify the most efficacious preventions for SUD.

Excessive Sensory Stimulation during Development Alters Neural Plasticity and Vulnerability to Cocaine in Mice

Early life experiences affect the formation of neuronal networks, which can have a profound impact on brain function and behavior later in life. Previous work has shown that mice exposed to excessive sensory stimulation during development are hyperactive and novelty seeking, and display impaired cognition compared with controls. In this study, we addressed the issue of whether excessive sensory stimulation during development could alter behaviors related to addiction and underlying circuitry in CD-1 mice. We found that the reinforcing properties of cocaine were significantly enhanced in mice exposed to excessive sensory stimulation. Moreover, although these mice displayed hyperactivity that became more pronounced over time, they showed impaired persistence of cocaine-induced locomotor sensitization. These behavioral effects were associated with alterations in glutamatergic transmission in the nucleus accumbens and amygdala. Together, these findings suggest that excessive sensory stimulation in early life significantly alters drug reward and the neural circuits that regulate addiction and attention deficit hyperactivity. These observations highlight the consequences of early life experiences and may have important implications for children growing up in today's complex technological environment.

Exposure to early adversity: Points of cross-species translation that can lead to improved understanding of depression

The relationship between developmental exposure to adversity and affective disorders is reviewed. Adversity discussed herein includes physical and sexual abuse, neglect, or loss of a caregiver in humans. While these stressors can occur at any point during development, the unique temporal relationship to specific depressive symptoms was the focus of discussion. Further influences of stress exposure during sensitive periods can vary by gender and duration of abuse as well. Data from animal studies are presented to provide greater translational and causal understanding of how sensitive periods, different types of psychosocial stressors, and sex interact to produce depressive-like behaviors. Findings from maternal separation, isolation rearing, chronic variable stress, and peer-peer rearing paradigms clarify interpretation about how various depressive behaviors are influenced by age of exposure. Depressive behaviors are broken down into the following categories: mood and affect, anhedonia, energy, working memory, sleep-wake, appetite changes, suicide, and general malaise. Cross-species evidence from humans, nonhuman primates, rats, and mice within each of these categories is discussed. In conclusion, sensitive periods for affective-related behaviors (anxiety, mood, and controllability) occur earlier in life, while other aspects of depression are associated with adversity later during adolescence.

Stereological analyses of reward system nuclei in maternally deprived/separated alcohol drinking rats

The experience of early life stress can trigger complex neurochemical cascades that influence emotional and addictive behaviors later in life in both adolescents and adults. Recent evidence suggests that excessive alcohol drinking and drug-seeking behavior, in general, are co-morbid with depressive-like behavior. Both behaviors are reported in humans exposed to early life adversity, and are prominent features recapitulated in animal models of early life stress (ELS) exposure. Currently, little is known about whether or how ELS modulates reward system nuclei. In this study we use operant conditioning of rats to show that the maternal separation stress (MS) model of ELS consumes up to 3-fold greater quantities of 10% vol/vol EtOH in 1-h, consistently over a 3-week period. This was correlated with a significant 22% reduction in the number of dopaminergic-like neurons in the VTA of naïve MS rats, similar to genetically alcohol-preferring (P) rats which show a 35% reduction in tyrosine hydroxylase (TH)-positive dopaminergic neurons in the VTA. MS rats had a significantly higher 2-fold immobility time in the forced swim test (FST) and reduced sucrose drinking compared to controls, indicative of depressive-like symptomology and anhedonia. Consistent with this finding, stereological analysis revealed that amygdala neurons were 25% greater in number at P70 following MS exposure. Our previous examination of the dentate gyrus of hippocampus, a region involved in encoding emotional memory, revealed fewer dentate gyrus neurons after MS, but we now report this reduction in neurons occurs without effect on the number of astrocytes or length of astrocytic fibers. These data indicate that MS animals exhibit neuroanatomical changes in reward centers similar to those reported for high alcohol drinking rats, but aspects of astrocyte morphometry remained unchanged. These data are of high relevance to understand the breadth of neuronal pathology that ensues in reward loci following ELS.

Stress and the social brain: behavioural effects and neurobiological mechanisms

Stress often affects our social lives. When undergoing high-level or persistent stress, individuals frequently retract from social interactions and become irritable and hostile. Predisposition to antisocial behaviours - including social detachment and violence - is also modulated by early life adversity; however, the effects of early life stress depend on the timing of exposure and genetic factors. Research in animals and humans has revealed some of the structural, functional and molecular changes in the brain that underlie the effects of stress on social behaviour. Findings in this emerging field will have implications both for the clinic and for society.

Environmental insults in early life and submissiveness later in life in mouse models

Dominant and subordinate dispositions are not only determined genetically but also nurtured by environmental stimuli during neuroendocrine development. However, the relationship between early life environment and dominance behavior remains elusive. Using the IntelliCage-based competition task for group-housed mice, we have previously described two cases in which environmental insults during the developmental period altered the outcome of dominance behavior later in life. First, mice that were repeatedly isolated from their mother and their littermates (early deprivation; ED), and second, mice perinatally exposed to an environmental pollutant, dioxin, both exhibited subordinate phenotypes, defined by decreased occupancy of limited resource sites under highly competitive circumstances. Similar alterations found in the cortex and limbic area of these two models are suggestive of the presence of neural systems shared across generalized dominance behavior.

Early maternal separation: a rodent model of depression and a prevailing human condition

The early life of most mammals is spent in close contact with the mother, and for the neonate, early maternal separation is a traumatic event that, depending on various conditions, may shape its behavioral and neurochemical phenotype in adulthood. Studies on rodents demonstrated that a very brief separation followed by increased maternal care may positively affect the development of the offspring but that prolonged separation causes significant amounts of stress. The consequences of this stress (particularly the hyperreactivity of the HPA (hypothalamic-pituitary-adrenal) axis are expressed in adulthood and persist for life. Maternal separation in rodents, particularly rats, was used as a model for various psychotic conditions, especially depression. The most popular separation procedure of a 3-h daily separation from the second to the 12th postpartum day yields a depression model of high construct and predictive validity. The results of studies on maternal separation in rats and monkeys prompt a discussion of the consequences of traditional procedures in the maternity wards of developed countries where attention is focused on the hygiene of the neonates and not on their psychological needs. This alternate focus results in a drastic limitation of mother-infant contact and prolonged periods of separation. It is tempting to speculate that differences in the course and severity of various mental disorders, which are usually less prevalent in underdeveloped countries than in developed countries (as noted by Kraepelin), may be related to different modes of infant care. Only recently has so-called kangaroo mother care (establishing mother-infant skin-to-skin contact immediately after birth) become popular in developed countries. In addition to its instant benefits for the neonates, this procedure may also be beneficial for the mental health of the offspring in adulthood.

Effects of adverse early-life events on aggression and anti-social behaviours in animals and humans

We review the impact of early adversities on the development of violence and antisocial behaviour in humans, and present three aetiological animal models of escalated rodent aggression, each disentangling the consequences of one particular adverse early-life factor. A review of the human data, as well as those obtained with the animal models of repeated maternal separation, post-weaning social isolation and peripubertal stress, clearly shows that adverse developmental conditions strongly affect aggressive behaviour displayed in adulthood, the emotional responses to social challenges and the neuronal mechanisms activated by conflict. Although similarities between models are evident, important differences were also noted, demonstrating that the behavioural, emotional and neuronal consequences of early adversities are to a large extent dependent on aetiological factors. These findings support recent theories on human aggression, which suggest that particular developmental trajectories lead to specific forms of aggressive behaviour and brain dysfunctions. However, dissecting the roles of particular aetiological factors in humans is difficult because these occur in various combinations; in addition, the neuroscientific tools employed in humans still lack the depth of analysis of those used in animal research. We suggest that the analytical approach of the rodent models presented here may be successfully used to complement human findings and to develop integrative models of the complex relationship between early adversity, brain development and aggressive behaviour.

Associations between childhood adversity, adult stressful life events, and past-year drug use disorders in the National Epidemiological Study of Alcohol and Related Conditions (NESARC)

Stress sensitization, whereby CA lowers tolerance to later stressors, has been proposed as a potential mechanism explaining the association between exposure to childhood adversities (CA) and drug use disorders in adulthood. However, this mechanism remains untested. This paper begins to address this gap through exploring associations between CA exposure and stressful events in adulthood for predicting drug use disorders. We used data drawn from Wave 2 of the U.S. National Epidemiological Survey of Alcohol and Related Conditions (n = 34,653) to explore whether the association between past-year stressful life events and the 12-month prevalence of disordered cannabis, stimulant, and opiate use varied by the number of types of CA that an individual was exposed to. Past-year stressful life events were associated with an increased risk of cannabis, stimulant, and opiate use disorders among men and women. Exposure to CA was associated with increased risk for disordered cannabis use among men and women and opiate use among men only. Finally, we found significant associations between exposure to CA and past-year stressful life events in predicting disordered drug use, but only for women in relation to disordered stimulant and opiate use. Findings are suggestive of possible stress sensitization effects in predicting disordered stimulant and opiate use among women. Implications of these findings for the prevention and treatment of drug use disorders and for future research are discussed.

Early deprivation induces competitive subordinance in C57BL/6 male mice

Rodent models have been widely used to investigate the impact of early life stress on adult health and behavior. However, the social dimension has rarely been incorporated into the analysis due to methodological limitations. This study characterized the effects of neonatal social isolation (early deprivation, ED) on adult C57BL/6 mouse behavior in a social context using our recently developed behavioral test protocols for group-housed mice. During the first two postnatal weeks, half of the pups per dam were separated from their dam and littermates for 3h per day (ED group). Post weaning, ED and control pups were electronically tagged and co-housed. At 12weeks, the mixed cohorts were transferred to IntelliCages, equipped with computer-controlled operant chambers. Access to the chambers was used as an index to analyze novel object response, behavioral flexibility, and competitive dominance with minimal experimenter intervention. In general, ED had greater effects on males; ED males exhibited reduced body weight, increased novelty response, and were subordinate to control littermates when competing for reward access. Male ED mice also demonstrated mildly impaired reversal learning. Analyzing gene expression changes in brain regions controlling emotion, stress, spatial memory, and executive function revealed reduced BDNF and c-Fos in hippocampal CA1, enhanced c-Fos in the basolateral amygdala, reduced Map2 while enhanced HSD11β2 in prefrontal cortex of ED males. In male mice, it was suggested that neonatal social isolation results in sustained changes in social behavior with altered function of limbic and frontal cortices.

Animal models as tools to study the pathophysiology of depression

The incidence of depressive illness is high worldwide, and the inadequacy of currently available drug treatments contributes to the significant health burden associated with depression. A basic understanding of the underlying disease processes in depression is lacking; therefore, recreating the disease in animal models is not possible. Popular current models of depression creatively merge ethologically valid behavioral assays with the latest technological advances in molecular biology. Within this context, this study aims to evaluate animal models of depression and determine which has the best face, construct, and predictive validity. These models differ in the degree to which they produce features that resemble a depressive-like state, and models that include stress exposure are widely used. Paradigms that employ acute or sub-chronic stress exposure include learned helplessness, the forced swimming test, the tail suspension test, maternal deprivation, chronic mild stress, and sleep deprivation, to name but a few, all of which employ relatively short-term exposure to inescapable or uncontrollable stress and can reliably detect antidepressant drug response.

Sex-dependent effects of an early life treatment in rats that increases maternal care: vulnerability or resilience?

Early life stress (ELS) in rodents has profound long-term effects that are partially mediated by changes in maternal care. ELS not only induces “detrimental” effects in adulthood, increasing psychopathology, but also promotes resilience to further stressors. In Long-Evans rats, we evaluated a combination of two procedures as a model of ELS: restriction of bedding during the first post-natal days and exposure to a “substitute” mother. The maternal care of biological and “substitute” mothers was measured. The male and female offspring were evaluated during adulthood in several contexts. Anxiety was measured by the elevated plus-maze (EPM), acoustic startle response (ASR) and forced swim test (FST). In other group of animals, novelty-seeking was measured (activity in an inescapable novel environment, preference for novel environments and exploration of novel objects). Plasmatic ACTH and corticosterone in basal conditions and in response to stress were also measured. Cognitive impulsivity was assessed by a delay-discounting paradigm, and impulsive action, attention and compulsive-like behavior by a five choice serial reaction time task (5CSRTT). ELS decreased pup body weight and increased the care of the biological mother; however, the “substitute” mother did not exhibit overt maltreatment. A mixture of “detrimental” and “beneficial” effects was shown. In the 5CSRTT, attention was impaired in both genders, and in females, ELS increased compulsive-like behavior. Novel object exploration was only increased by ELS in males, but the preference for novel spaces decreased in both genders. Baseline anxiety (EPM and ASR) and recognition memory were not affected. Unexpectedly, ELS decreased the ACTH response to novelty and swim stress and increased active coping in the FST in both genders. Cognitive impulsivity was decreased only in females, but impulsive action was not affected. The enhancement in maternal care may “buffer” the effects of ELS in a context-dependent manner.

Effects of denial of reward through maternal contact in the neonatal period on adult hypothalamic-pituitary-adrenal axis function in the rat

Emotional behavioral traits associated with stress response are well documented to be affected by early life events. In the present work, we used a novel paradigm of neonatal experience, in which pups were trained in a T-maze and either received (RER rats) or were denied (DER) the reward of maternal contact, during postnatal days 10-13. We then evaluated stress coping and key factors controlling the function of the hypothalamic-pituitary-adrenal axis in adulthood. Adult male DER rats exposed to a single session of forced swim stress (FSS) showed increased immobility, while RER rats exhibited increased escape attempts. The corticosterone response following this stressor was higher although not prolonged in the DER rats. Their CRH mRNA levels in the PVN were increased up to 2h after the forced swim. However, basal levels of these hormones did not differ among groups. In addition, the DER neonatal experience induced an increase in hippocampal GR but a decrease in CRH-R1 immunopositive cells in the CA1 area of the hippocampus and the central amygdala. Overall, these data show a distinct stress response profile in the DER male rats, characterized by passive coping during the forced swim, increased hormonal response following stress, increased inhibitory control through GR and an indirect contribution of CRH-R1, the latter two factors resulting in a modified regulation of the response termination. It thus appears that DER rats have an enhanced potential for appropriate reactivity upon an incoming challenge, while maintaining in parallel an adequate control of the duration of their stress responses.

Factors influencing behavior in the forced swim test

The forced swim test (FST) is a behavioral test in rodents which was developed in 1978 by Porsolt and colleagues as a model for predicting the clinical efficacy of antidepressant drugs. A modified version of the FST added the classification of active behaviors into swimming and climbing, in order to facilitate the differentiation between serotonergic and noradrenergic classes of antidepressant drugs. The FST is now widely used in basic research and the pharmaceutical screening of potential antidepressant treatments. It is also one of the most commonly used tests to assess depressive-like behavior in animal models. Despite the simplicity and sensitivity of the FST procedure, important differences even in baseline immobility rates have been reported between different groups, which complicate the comparison of results across studies. In spite of several methodological papers and reviews published on the FST, the need still exists for clarification of factors which can influence the procedure. While most recent reviews have focused on antidepressant effects observed with the FST, this one considers the methodological aspects of the procedure, aiming to summarize issues beyond antidepressant action in the FST. The previously published literature is analyzed for factors which are known to influence animal behavior in the FST. These include biological factors, such as strain, age, body weight, gender and individual differences between animals; influence of preconditioning before the FST: handling, social isolation or enriched environment, food manipulations, various kinds of stress, endocrine manipulations and surgery; schedule and routes of treatment, dosage and type of the drugs as well as experimental design and laboratory environmental effects. Consideration of these factors in planning experiments may result in more consistent FST results.

Treatment with tianeptine induces antidepressive-like effects and alters the neurotrophin levels, mitochondrial respiratory chain and cycle Krebs enzymes in the brain of maternally deprived adult rats

Maternally deprived rats were treated with tianeptine (15 mg/kg) once a day for 14 days during their adult phase. Their behavior was then assessed using the forced swimming and open field tests. The BDNF, NGF and energy metabolism were assessed in the rat brain. Deprived rats increased the immobility time, but tianeptine reversed this effect and increased the swimming time; the BDNF levels were decreased in the amygdala of the deprived rats treated with saline and the BDNF levels were decreased in the nucleus accumbens within all groups; the NGF was found to have decreased in the hippocampus, amygdala and nucleus accumbens of the deprived rats; citrate synthase was increased in the hippocampus of non-deprived rats treated with tianeptine and the creatine kinase was decreased in the hippocampus and amygdala of the deprived rats; the mitochondrial complex I and II-III were inhibited, and tianeptine increased the mitochondrial complex II and IV in the hippocampus of the non-deprived rats; the succinate dehydrogenase was increased in the hippocampus of non-deprived rats treated with tianeptine. So, tianeptine showed antidepressant effects conducted on maternally deprived rats, and this can be attributed to its action on the neurochemical pathways related to depression.

Baseline prepulse inhibition expression predicts the propensity of developing sensitization to the motor stimulant effects of amphetamine in C57BL/6 mice

RATIONALE

The startle reflex to a sudden intense acoustic pulse stimulus is attenuated if the pulse is shortly preceded by a weak prepulse stimulus. This represents a form of sensory gating, known as prepulse inhibition (PPI), observable across species. PPI is modulated by dopamine and readily disrupted by acute amphetamine. Prior repeated exposures to amphetamine also disrupt PPI even when the drug is not present during test, suggesting that a sensitized mesolimbic dopamine system-inducible even by a single exposure to amphetamine-might be responsible. However, this causative link has been challenged by inconsistent efficacy between different amphetamine pre-treatment regimes, which all robustly sensitize the behavioral response to amphetamine.

METHODS

Here, the presence of such a link in reverse was tested by comparing the propensity to develop amphetamine sensitization between high- and low-PPI expressing individuals identified within a homogeneous cohort of C57BL/6 mice. Comparison of dopamine content including its metabolites was performed separately in drug naïve mice by post-mortem HPLC.

RESULTS

Behavioral sensitization was substantially stronger in the low-PPI group compared with the high-PPI group, while the magnitude of their response to the first amphetamine challenge was similar. Dopamine content within the nucleus accumbens and medial prefrontal cortex was significantly higher in low-PPI relative to high-PPI mice.

CONCLUSION

Individuals with weak sensory gating characterized by low basal PPI expression may be more susceptible to the development of dopamine sensitization and therefore at greater risk of developing schizophrenia. Conversely, high baseline expression might predict a resistance to dopaminergic sensitization.

Maternal deprivation induces deficits in temporal memory and cognitive flexibility and exaggerates synaptic plasticity in the rat medial prefrontal cortex

Early life adverse events can lead to structural and functional impairments in the prefrontal cortex (PFC). Here, we investigated whether maternal deprivation (MD) alters PFC-dependent executive functions, neurons and astrocytes number and synaptic plasticity in adult male Long-Evans rats. The deprivation protocol consisted of a daily separation of newborn Long-Evans pups from their mothers and littermates 3h/day postnatal day 1-14. Cognitive performances were assessed in adulthood using the temporal order memory task (TMT) and the attentional set-shifting task (ASST) that principally implicates the PFC and the Morris water maze task (WMT) that does not essentially rely on the PFC. The neurons and astrocytes of the prelimbic (PrL) area of the medial PFC (mPFC) were immunolabelled respectively with anti-NeuN and anti-GFAP antibodies and quantified by stereology. The field potentials evoked by electrical stimulation of ventral hippocampus (ventral HPC) were recorded in vivo in the PrL area. In adulthood, MD produced cognitive deficits in two PFC-dependent tasks, the TMT and ASST, but not in the WMT. In parallel, MD induced in the prelimbic area of the medial PFC an upregulation of long-term potentiation (LTP), without any change in the number of neurons and astrocytes. We provide evidence that MD leads in adults to an alteration of the cognitive abilities dependent on the PFC, and to an exaggerated synaptic plasticity in this region. We suggest that this latter phenomenon may contribute to the impairments in the cognitive tasks.

Risky shifts: How the timing and course of mothers' depressive symptoms across the perinatal period shape their own and infant's stress response profiles

We investigated the effects of timing and the course of maternal perinatal depressive symptoms on mother–infant hypothalamic–pituitary–adrenal (HPA) response profiles during an attachment stressor, as well as on within-dyad synchrony of stress profiles: coordination of HPA and sympathetic nervous system and infant–mother HPA attunement. Mothers (n = 86) completed the Center for Epidemiological Studies Depression Scale during pregnancy (Time 1 [T1]) and at 5 months (T2) and 18 months (T3) postnatal. At T3 mother–infant dyads completed the Strange Situation, and four saliva samples collected from both mothers and infants were assayed for cortisol and α-amylase. Hierarchical linear modeling was used to predict mother–infant cortisol response trajectories and within-dyad synchronies by main and interactive effects of T1–T3 Center for Epidemiological Studies Depression Scale scores. Main effects of earlier (T1, T2) depressive symptoms predicted mothers' cortisol trajectories and coordination, and interactions of T1 with postnatal (T2 and T3) symptoms predicted infants' cortisol trajectories, coordination, and attunement. Decomposition of interactions revealed more marked effects on infant cortisol trajectories when the mother shifted from higher to lower depressive symptoms (or vice versa) across the perinatal period. Shifts from lower to higher symptoms also predicted inverse coordination of cortisol with salivary α-amylase and greater attunement of infant with mother cortisol. Implications for the development and transmission of stress dysregulation are discussed.

A translational research framework for enhanced validity of mouse models of psychopathological states in depression

Depression presents as a disorder of feelings and thoughts that debilitate daily functioning and can be life threatening. Increased understanding of these specific emotional-cognitive pathological states and their underlying pathophysiologies and neuropathologies is fundamental to an increased understanding of the disorder and, therefore, to development of much-needed improved therapies. Despite this, there is a current lack of emphasis on development and application of translational (i.e. valid) neuropsychological measures in depression research. The appropriate strategy is neuropsychological research translated, bi-directionally, between epidemiological and clinical human research and in vivo - ex vivo preclinical research conducted, primarily, with mice. This paper presents a translational framework to stimulate and inform such research, in four inter-dependent sections. (1) A depression systems-model describes the pathway between human environment-gene (E-G) epidemiology, pathophysiology, psycho- and neuropathology, symptoms, and diagnosis. This model indicates that G→emotional-cognitive endophenotypes and E-G/endophenotype→emotional-cognitive state markers are central to experimental and translational depression research. (2) Human neuropsychological tests with (potential) translational value for the quantitative study of these endophenotypes and state markers are presented. (3) The analogous rodent behavioural tests are presented and their translational validity in terms of providing analogue emotional-cognitive endophenotypes and state markers are discussed. (4) The need for aetiological validity of mouse models in terms of G→endophenotypes and E-G→state markers is presented. We conclude that the informed application of the proposed neuropsychological translational framework will yield mouse models of high face, construct and aetiological validity with respect to emotional-cognitive dysfunction in depression. These models, together with the available technological tools, can then be studied to increase understanding of depression pathophysiology and neuropathology, leading to identification and validation of novel therapeutic targets and the development of effective, personalized antidepressant treatments.

Early life stress paradigms in rodents: potential animal models of depression?

RATIONALE

While human depressive illness is indeed uniquely human, many of its symptoms may be modeled in rodents. Based on human etiology, the assumption has been made that depression-like behavior in rats and mice can be modulated by some of the powerful early life programming effects that are known to occur after manipulations in the first weeks of life.

OBJECTIVE

Here we review the evidence that is available in literature for early life manipulation as risk factors for the development of depression-like symptoms such as anhedonia, passive coping strategies, and neuroendocrine changes. Early life paradigms that were evaluated include early handling, separation, and deprivation protocols, as well as enriched and impoverished environments. We have also included a small number of stress-related pharmacological models.

RESULTS

We find that for most early life paradigms per se, the actual validity for depression is limited. A number of models have not been tested with respect to classical depression-like behaviors, while in many cases, the outcome of such experiments is variable and depends on strain and additional factors.

CONCLUSION

Because programming effects confer vulnerability rather than disease, a number of paradigms hold promise for usefulness in depression research, in combination with the proper genetic background and adult life challenges.

Early life stress stimulates hippocampal reelin gene expression in a sex-specific manner: evidence for corticosterone-mediated action

Early life stress predisposes to the development of psychiatric disorders. In this context the hippocampal formation is of particular interest, because it is affected by stress on the structural and cognitive level. Since little is known how early life stress is translated on the molecular level, we mimicked early life stress in mouse models and analyzed the expression of the glycoprotein Reelin, a master molecule for development and differentiation of the hippocampus. From postnatal day 1 (P1) to P14, mouse pups were subjected to one of the following treatments: nonhandling (NH), handling (H), maternal separation (MS), and early deprivation (ED) followed by immediate (P15) or delayed (P70) real time RT-PCR analysis of reelin mRNA expression. We show that at P15, reelin mRNA levels were significantly increased in male H and ED groups when compared with the NH group. In contrast, no stress-induced alterations of reelin mRNA expression were found in female animals. This sex difference in stress-mediated stimulation of reelin expression was maintained into adulthood, since at P70 intergroup differences were still found in male, but not in female mice. On the cellular level, however, we did not find any significant differences in cell densities of Reelin-immunolabeled neurons between treatment groups or sexes, but an overall reduction of Reelin-expressing neurons in the adult hippocampus when compared to P15. To address the question whether corticosterone mediates the stress-induced up-regulation of reelin gene expression, we used age-matched hippocampal slice cultures derived from male and female mouse pups. Quantitative determination of mRNA levels revealed that corticosterone treatment significantly up-regulated reelin mRNA expression in male, but not in female hippocampi. Taken together, these results show a sex-specific regulation of reelin gene expression by early life experience, most likely mediated by corticosterone.

Neonatal maternal separation exacerbates the reward-enhancing effect of acute amphetamine administration and the anhedonic effect of repeated social defeat in adult rats

Early life adversity or parental neglect is linked to the development of a number of psychiatric illnesses, including major depression and substance use disorder. These two disorders are often comorbid and characterized by anhedonia, defined as the reduced ability to experience pleasure or reward. The aim of the present study was to determine the effects of neonatal maternal separation in Long Evans rats, a model of early life stress, on anhedonia under baseline conditions and in response to drug and stress exposure during adulthood. Three hours of daily maternal separation from postnatal day 1 to 14 led to marked decreases in arched-back nursing, licking, and grooming of pups by their dams. In adulthood, brain reward function was assessed using intracranial self-stimulation of the lateral hypothalamus. Lowered current thresholds derived from this procedure are interpreted as reward-enhancing effects, whereas elevations in thresholds are an operational measure of anhedonia. Maternally separated rats did not exhibit anhedonia under baseline conditions compared with non-handled controls but exhibited a greater reward-enhancing effect of acute amphetamine administration. Acute social defeat produced anhedonia in non-handled controls, but not in maternally separated rats. Conversely, control rats habituated to 7 days of repeated social defeat, whereas maternally separated rats developed an increased anhedonic response to the repeated stressor. One week after termination of stress exposure, maternally separated rats still exhibited an increased reward-enhancing effect of acute amphetamine administration compared with non-handled controls, regardless of prior social defeat experience. These data indicate that early life stress increases the reward-enhancing properties of amphetamine, protects against the anhedonic effects of acute stress exposure, and exacerbates the anhedonic response to repeated stress. Thus, early life stress may increase an individual's vulnerability to depressive or addictive disorders when confronted with stress or drug challenge in adulthood.

Effects of the probiotic Bifidobacterium infantis in the maternal separation model of depression

The concept that intestinal microbial composition not only affects the health of the gut, but also influences centrally-mediated systems involved in mood, is supported by a growing body of literature. Despite the emergent interest in brain-gut communication and its possible role in the pathogenesis of psychiatric disorders such as depression, particularly subtypes with accompanying gastrointestinal (GI) symptoms, there are few studies dedicated to the search for therapeutic solutions that address both central and peripheral facets of these illnesses. This study aims to assess the potential benefits of the probiotic Bifidobacterium infantis in the rat maternal separation (MS) model, a paradigm that has proven to be of value in the study of stress-related GI and mood disorders. MS adult rat offsprings were chronically treated with bifidobacteria or citalopram and subjected to the forced swim test (FST) to assess motivational state. Cytokine concentrations in stimulated whole blood samples, monoamine levels in the brain, and central and peripheral hypothalamic-pituitary-adrenal (HPA) axis measures were also analysed. MS reduced swim behavior and increased immobility in the FST, decreased noradrenaline (NA) content in the brain, and enhanced peripheral interleukin (IL)-6 release and amygdala corticotrophin-releasing factor mRNA levels. Probiotic treatment resulted in normalization of the immune response, reversal of behavioral deficits, and restoration of basal NA concentrations in the brainstem. These findings point to a more influential role for bifidobacteria in neural function, and suggest that probiotics may have broader therapeutic applications than previously considered.

Maternal deprivation induces depressive-like behaviours only in female rats

Maternal deprivation (MD) has been developed to study the effects of early adverse experiences on behaviour and neurobiology. It has been proposed to represent a potential animal model of major depression. The purpose of our study was to examine the responses induced by MD in male and female adult Long-Evans rats in tasks designed to explore depressive-like behaviours (forced swimming test (FST), repeated open space swim test (OSST), sucrose solution consumption) and in the novel object recognition and object location tasks. A consistent sexual dimorphism was observed in the responses of male and female rats that underwent MD. In male rats, MD led to increased transitions between behaviours in the FST and increased consumption and preference for sucrose (1%) in comparison with non-deprived rats. In female rats, MD induced a decreased swimming activity on the second day of the OSST and reduced the cognitive performance in an object location task. In both sexes, MD did not alter the swimming activity in the FST and the performance in a novel object recognition task. These divergent responses in male and female rats can be related to the gender differences which exist in depression. However, due to the low amplitude of responses obtained in our study, the MD model in Long-Evans rats does not seem to mimic symptoms of major depression. In contrast, our present results suggest the use of the MD model, especially in females, as a model of the dysthymia, a mild chronic-depressive condition, which has been related to poorer maternal relationship.

Models of depression

The incidence of depressive illness is high in the United States and worldwide, and the inadequacy of currently available drug treatments contributes to the significant health burden associated with depression. A basic understanding of the underlying disease processes in depression is lacking, and therefore, recreating the disease in animal models is not possible. Currently used models of depression attempt to produce quantifiable correlates of human symptoms in experimental animals. The models differ in the degree to which they produce features that resemble a depressive-like state, and models that include stress exposure are widely used. Paradigms that employ acute or subchronic stress exposure include learned helplessness, forced swim test, and tail suspension test, which employ relatively short-term exposure to inescapable or uncontrollable stress and can reliably detect antidepressant drug response. Longer-term models include chronic mild stress models, early-life stress models, and social conflict models, which may more accurately simulate processes that lead to depression. These models each have varying degrees of face, construct, and predictive validity for depression and contribute differently to our understanding of antidepressant processes.

Adverse rearing experiences enhance responding to both aversive and rewarding stimuli in juvenile rhesus monkeys

BACKGROUND

While adverse rearing is thought to alter threat responding, the effects on appetitive behavior remains minimally explored. This study examines the effects that early life emotional adversity has on response to both threatening and appetitive stimuli in juvenile rhesus monkeys.

METHODS

Twenty-four, 2-year-old monkeys with differential rearing histories were tested for fear-potentiated startle responding and consumption of an artificially sweetened solution.

RESULTS

Relative to monkeys reared under typical conditions, monkeys removed from their mothers at birth and reared with peers demonstrated both increases in reward responding, as evidenced by greater consumption of a palatable solution in a free choice test, and increased threat responding, as evidenced by enhanced fear-potentiated startle responding.

CONCLUSIONS

Findings suggest that early rearing impacts juvenile manifestations of both appetitive and aversive emotional systems. Results are discussed in the context of development, anxiety, depression, and substance abuse.

Early life stress, the development of aggression and neuroendocrine and neurobiological correlates: what can we learn from animal models?

Early life stress (child and adolescent abuse, neglect and trauma) induces robust alterations in emotional and social functioning resulting in enhanced risk for the development of psychopathologies such as mood and aggressive disorders. Here, an overview is given on recent findings in primate and rodent models of early life stress, demonstrating that chronic deprivation of early maternal care as well as chronic deprivation of early physical interactions with peers are profound risk factors for the development of inappropriate aggressive behaviors. Alterations in the hypothalamic-pituitary-adrenocortical (HPA), vasopressin and serotonin systems and their relevance for the regulation of aggression are discussed. Data suggest that social deprivation-induced inappropriate forms of aggression are associated with high or low HPA axis (re)activity and a generally lower functioning of the serotonin system in adulthood. Moreover, genetic and epigenetic modifications in HPA and serotonin systems influence the outcome of early life stress and may even moderate adverse effects of early social deprivation on aggression. A more comprehensive study of aggression, neuroendocrine, neurobiological and (epi)genetic correlates of early life stress using animal models is necessary to provide a better understanding of the invasive aggressive deficits observed in humans exposed to child maltreatment.

Resilience and vulnerability are dose-dependently related to neonatal stressors in mice

Early life experiences have been shown to adjust cognitive abilities, stress reactivity, fear responses and immune activity in adult mammals of many species. However, whereas severe stressors have been generally associated with the emergence of hypothalamic pituitary adreno-cortical (HPA)-mediated pathology, mild neonatal stressful experiences have been traditionally associated with 'positive' effects or resilience. External stressors stimulate the HPA axis to induce a corticosterone secretion in mouse dams, which, in turn is directly transmitted to the progeny through lactation. Such corticosteroid transfer may offer a unitary mechanism whereby early low corticosterone exposure may favor resilience in the offspring and high corticosterone increase vulnerability to pathology. In this study we further investigated this hypothesis by evaluating the long-term effects of a neonatal exposure to low (33 mg/l) and high (100 mg/l) doses of corticosterone during the first 10 days of life in outbred CD-1 mice through supplementation in the maternal drinking water. Offspring attentional set-shifting abilities, central neurotrophic regulation and levels of natural auto-antibodies (na-Abs) directed to serotonin (SERT) and dopamine (DAT) transporters were assessed in adulthood. While low levels of neonatal corticosterone improved adult cognitive abilities and increased na-Abs levels directed to SERT, high doses of neonatal corticosterone reduced hippocampal BDNF levels and na-Abs directed to DAT. These findings confirm and extend our previous findings, supporting the view that both adaptive plasticity and pathological outcomes in adulthood may depend on circulating neonatal corticosterone levels and that these effects follow a U-shaped profile.

Disruptions of the mother-infant relationship and stress-related behaviours: altered corticosterone secretion does not explain everything

The hypothalamic-pituitary-adrenal (HPA) axis is the main neuroendocrine system of response to stress, and an imbalance of this system's activity is believed to be at the core of numerous psychiatric pathologies. During the neonatal period, the glucocorticoid response to stress is maintained at low levels by specific maternal behaviours, which is essential for proper brain development. Effective evaluation of the impact of increased secretion of corticosterone during an essentially anabolic developmental period on adulthood behaviour involved separation of the neonate from its mother for periods ranging from 3 to 24h. It has been shown that disinhibition of the stress response is achieved by such procedures. The pioneering studies by Seymour Levine set the stage for a prolific and promising field of study that may help neuroscientists unveil the neurobiological underpinnings of stress-related disorders. Based on a series of studies, we propose that maternal separation and maternal deprivation change stress-related behaviours, but that corticosterone seem to be only partially involved in these changes in adulthood. It appears that extra-hypothalamic corticotrophin-releasing factor and neurotransmitter systems may be the primary mediators of these behavioural outcomes.

Adolescent female rats are more resistant than males to the effects of early stress on prefrontal cortex and impulsive behavior

We tested the hypothesis that adolescent Sprague-Dawley females may be more resistant than males to display impulsive behavior and lower prefrontal cortex thickness after mother-infant separation (MS). Starting at postnatal day 2 (P2), the MS group was separated 6 hr/day and the early handled (EH) group 15 min/day for 10 days, and another group was standard facility reared (SFR). Subjects were examined for novel open-field activity (P28), light-dark apparatus (P29), familiar open-field (P30) and frontal cortical thickness. This protocol resulted in impulsive behavior in MS rats relative to EH and SFR, but this effect was less pronounced in females than males. MS affected the two sexes differently in terms of decreased prefrontal cortex dorsoventral thickness, with this effect being significant in males but not females. Neuroanatomical and behavioral documentation that adolescent females are more resistant than males to ADHD-like effects of maternal separation have not been previously reported.