Postnatal handling reduces emotionality ratings and accelerates two-way active avoidance in female rats

Potential resilience treatments for orangutans (Pongo spp.): Lessons from a scoping review of interventions in humans and other animals

Wild orangutans (Pongo spp.) rescued from human-wildlife conflict must be adequately rehabilitated before being returned to the wild. It is essential that released orangutans are able to cope with stressful challenges such as food scarcity, navigating unfamiliar environments, and regaining independence from human support. Although practical skills are taught to orangutans in rehabilitation centres, post-release survival rates are low. Psychological resilience, or the ability to 'bounce back' from stress, may be a key missing piece of the puzzle. However, there is very little knowledge about species-appropriate interventions which could help captive orangutans increase resilience to stress. This scoping review summarises and critically analyses existing human and non-human animal resilience literature and provides suggestions for the development of interventions for orangutans in rehabilitation. Three scientific databases were searched in 2021 and 2023, resulting in 63 human studies and 266 non-human animal studies. The first section brings together human resilience interventions, identifying common themes and assessing the applicability of human interventions to orangutans in rehabilitation. The second section groups animal interventions into categories of direct stress, separation stress, environmental conditions, social stress, and exercise. In each category, interventions are critically analysed to evaluate their potential for orangutans in rehabilitation. The results show that mild and manageable forms of intervention have the greatest potential benefit with the least amount of risk. The study concludes by emphasising the need for further investigation and experimentation, to develop appropriate interventions and measure their effect on the post-release survival rate of orangutans.

The impact of environmental and social factors on learning abilities: a meta-analysis

Since the 1950s, researchers have examined how differences in the social and asocial environment affect learning in rats, mice, and, more recently, a variety of other species. Despite this large body of research, little has been done to synthesize these findings and to examine if social and asocial environmental factors have consistent effects on cognitive abilities, and if so, what aspects of these factors have greater or lesser impact. Here, we conducted a systematic review and meta-analysis examining how different external environmental features, including the social environment, impact learning (both speed of acquisition and performance). Using 531 mean-differences from 176 published articles across 27 species (with studies on rats and mice being most prominent) we conducted phylogenetically corrected mixed-effects models that reveal: (i) an average absolute effect size |d| = 0.55 and directional effect size d = 0.34; (ii) interventions manipulating the asocial environment result in larger effects than social interventions alone; and (iii) the length of the intervention is a significant predictor of effect size, with longer interventions resulting in larger effects. Additionally, much of the variation in effect size remained unexplained, possibly suggesting that species differ widely in how they are affected by environmental interventions due to varying ecological and evolutionary histories. Overall our results suggest that social and asocial environmental factors do significantly affect learning, but these effects are highly variable and perhaps not always as predicted. Most notably, the type (social or asocial) and length of interventions are important in determining the strength of the effect.

Revisiting the role of anxiety in the initial acquisition of two-way active avoidance: pharmacological, behavioural and neuroanatomical convergence

Two-way active avoidance (TWAA) acquisition constitutes a particular case of approach -avoidance conflict for laboratory rodents. The present article reviews behavioural, psychopharmacological and neuroanatomical evidence accumulated along more than fifty years that provides strong support to the contention that anxiety is critical in the transition from CS (conditioned stimulus)-induced freezing to escape/avoidance responses during the initial stages of TWAA acquisition. Thus, anxiolytic drugs of different types accelerate avoidance acquisition, anxiogenic drugs impair it, and avoidance during these initial acquisition stages is negatively associated with other typical measures of anxiety. In addition behavioural and developmental treatments that reduce or increase anxiety/stress respectively facilitate or impair TWAA acquisition. Finally, evidence for the regulation of TWAA acquisition by septo-hippocampal and amygdala-related mechanisms is discussed. Collectively, the reviewed evidence gives support to the initial acquisition of TWAA as a paradigm with considerable predictive and (in particular) construct validity as an approach-avoidance conflict-based rodent anxiety model.

Behavioral differences of male Wistar rats from different vendors in vulnerability and resilience to chronic mild stress are reflected in epigenetic regulation and expression of p11

Outbred rat lines such as Wistar rats are commonly used for models of depressive disorders. Such rats arise from random mating schedules. Hence, genetic drift occurs in outbred populations which could lead to genotypic and phenotypic heterogeneity between rats from different vendors. Additionally, vendor specific rearing conditions could contribute to intrastrain variability. In the present study differences in behavioral responses to the chronic mild stress (CMS) model of depression within Wistar rat strains from different vendors are described. DNA methylation studies and mRNA expression analysis of p11 revealed that the behavioral differences between the substrains are reflected at the epigenetic and genetic level. The results suggest that there are breeder-dependent differences in vulnerability to stress in the CMS model of depression, which might bear on the validity of the model and contribute to contradictory findings and difficulties of replication between laboratories. P11 mRNA expression seems to be differently regulated depending on the quality of the stress response evoked by CMS exposure.

The impact of early postnatal environmental enrichment on maternal care and offspring behaviour following weaning

The early postnatal period is a sensitive period in rodents as behavioural systems are developing and maturing during this time. However, relatively little information is available about the impact of environmental enrichment on offspring behaviour if enrichment is implemented only during this period. Here, environmental enrichment was provided from postnatal day 1 until weaning. On post-natal day 9, maternal behaviour and nonmaternal behaviour of the dam was observed. Nursing time in the enriched group was reduced but dams showed more non-maternal appetitive behaviours. Offspring were exposed to either the open field or the elevated plus maze (EPM) after weaning. In the open field, rats from the enriched group approached the more aversive inner zone of the open field later than control rats. Offspring from the enriched group made fewer entries into the inner zone and spent less time in this part of the arena. Enrichment had no impact on behaviour in the EPM. The present study provides evidence that postnatal enrichment can interfere with maternal behaviour in rats and can possibly lead to increased anxiety in the offspring. The findings suggest that enrichment procedures can have potentially unintended effects, interfering with the development of emotional behaviours in rats.

Long-term moderate treadmill exercise promotes stress-coping strategies in male and female rats

Recent evidence has revealed the impact of exercise in alleviating anxiety and mood disorders; however, the exercise protocol that exerts such benefit is far from known. The current study was aimed to assess the effects of long-term moderate exercise on behavioural coping strategies (active vs. passive) and Hypothalamic-Pituitary-Adrenal response in rats. Sprague-Dawley male and female rats were exposed to 32-weeks of treadmill exercise and then tested for two-way active avoidance learning (shuttle-box). Two groups were used as controls: a non-handled sedentary group, receiving no manipulation, and a control group exposed to a stationary treadmill. Female rats displayed shorter escape responses and higher number of avoidance responses, reaching criterion for performance earlier than male rats. In both sexes, exercise shortened escape latencies, increased the total number of avoidances and diminished the number of trials needed to reach criterion for performance. Those effects were greater during acquisition in female rats, but remained over the shuttle-box sessions in treadmill trained male rats. In females, exercise did not change ACTH and corticosterone levels after shuttle-box acquisition. Collectively, treadmill exercise improved active coping strategies in a sex-dependent manner. In a broader context, moderate exercise could serve as a therapeutic intervention for anxiety and mood disorders.

Neonatal handling decreases unconditioned anxiety, conditioned fear, and improves two-way avoidance acquisition: a study with the inbred Roman high (RHA-I)- and low-avoidance (RLA-I) rats of both sexes

The present study evaluated the long-lasting effects of neonatal handling (NH; administered during the first 21 days of life) on unlearned and learned anxiety-related responses in inbred Roman High- (RHA-I) and Low-avoidance (RLA-I) rats. To this aim, untreated and neonatally-handled RHA-I and RLA-I rats of both sexes were tested in the following tests/tasks: a novel object exploration (NOE) test, the elevated zero maze (ZM) test, a “baseline acoustic startle” (BAS) test, a “context-conditioned fear” (CCF) test and the acquisition of two-way active—shuttle box—avoidance (SHAV). RLA-I rats showed higher unconditioned (novel object exploration test -“NOE”-, elevated zero maze test -“ZM”-, BAS), and conditioned (CCF, SHAV) anxiety. NH increased exploration of the novel object in the NOE test as well as exploration of the open sections of the ZM test in both rat strains and sexes, although the effects were relatively more marked in the (high anxious) RLA-I strain and in females. NH did not affect BAS, but reduced CCF in both strains and sexes, and improved shuttle box avoidance acquisition especially in RLA-I (and particularly in females) and in female RHA-I rats. These are completely novel findings, which indicate that even some genetically-based anxiety/fear-related phenotypes can be significantly modulated by previous environmental experiences such as the NH manipulation.

How postnatal insults may program development: studies in animal models

During the postnatal period, the nervous system is modified and shaped by experience, in order to adjust it to the particular environment in which the animal will live. This plasticity, one of the most remarkable characteristics of the nervous system, promotes adaptive changes, but it also makes brain more vulnerable to insults. This chapter will focus on the effects of interventions during the postnatal development in animal models of neonatal handling (usually up to 15 min of handling) and maternal separation (usually at least for 3 h). Sex-specific changes and effects of prepubertal stress such as social isolation later on in life were also considered. These interventions during development induce long-lasting traces in the pups' nervous system, which will be reflected in changes in neuroendocrine functions, including the hypothalamus-pituitary-adrenal and hypothalamus-pituitary-gonadal axes; anxiety and cognitive performance; and feeding, sexual, and social behavior. These enduring changes may be adaptive or maladaptive, depending on the environment in which the animal will live. The challenge researchers facing now is to determine how to reverse the deleterious effects that may result from early-life stress exposure.

Cognitive and emotional alterations in young Alzheimer's disease (3xTgAD) mice: effects of neonatal handling stimulation and sexual dimorphism

Alzheimer disease is the most common neurodegenerative disorder and cause of senile dementia. It is characterized by an accelerated memory loss, and alterations of mood, reason, judgment and language. The main neuropathological hallmarks of the disorder are β-amyloid (βA) plaques and neurofibrillary Tau tangles. The triple transgenic 3xTgAD mouse model develops βA and Tau pathologies in a progressive manner which mimicks the pattern that takes place in the human brain with AD, and showing cognitive alterations characteristic of the disease. The present study intended to examine whether 3xTgAD mice of both sexes present cognitive, emotional and other behavioral alterations at the early age of 4 months, an age in which only some intraneuronal amyloid accumulation is found. Neonatal handling (H) is an early-life treatment known to produce profound and long-lasting behavioral and neurobiological effects in rodents, as well as improvements in cognitive functions. Therefore, we also aimed at evaluating the effects of H on the behavioral/cognitive profile of 4-month-old male and female 3xTgAD mice. The results indicate that, (1) 3xTgAD mice present spatial learning/memory deficits and emotional alterations already at the early age of 4 months, (2) there exists sexual dimorphism effects on several behavioral variables at this age, (3) neonatal handling exerts a preventive effect on some cognitive (spatial learning) and emotional alterations appearing in 3xTgAD mice already at early ages, and 4) H treatment appears to produce stronger positive effects in females than in males in several spatial learning measures and in the open field test.

Neonatal handling: an overview of the positive and negative effects

As one of the first rodent models designed to investigate the effects of early-life experiences, the neonatal handling paradigm has helped us better understand how subtle changes in the infant environment can powerfully drive neurodevelopment of the immature brain in typical or atypical trajectories. Here, we review data from more than 50 years demonstrating the compelling effects of neonatal handling on behavior, physiology, and neural function across the lifespan. Moreover, we present data that challenge the classical view of neonatal handling as an animal model that results only in positive/beneficial outcomes. Indeed, the overall goal of this review is to offer the suggestion that the effects of early-life experiences-including neonatal handling-are nuanced rather than unidirectional. Both beneficial and negative outcomes may occur, depending on the parameters of testing, sex of the subject, and neurobehavioral system analyzed.

Brief daily postpartum separations from the litter alter dam response to psychostimulants and to stress

Neonatal handling induces several behavioral and neurochemical alterations in pups, including decreased responses to stress and reduced fear in new environments. However, there are few reports in the literature concerning the behavioral effects of this neonatal intervention on the dams during the postpartum period. Therefore, the aim of the current study was to determine if brief postpartum separation from pups has a persistent impact on the dam's stress response and behavior. Litters were divided into two neonatal groups: 1) non-handled and 2) handled [10 min/day, from postnatal day (PND) 1 to 10]. Weaning occurred at PND 21 when behavioral tasks started to be applied to the dams, including sweet food ingestion (PND 21), forced swimming test (PND 28), and locomotor response to a psychostimulant (PND 28). On postpartum day 40, plasma was collected at baseline for leptin assays and after 1 h of restraint for corticosterone assay. Regarding sweet food consumption, behavior during the forced swimming test or plasma leptin levels did not differ between dams briefly separated and non-separated from their pups during the postpartum period. On the other hand, both increased locomotion in response to diethylpropion and increased corticosterone secretion in response to acute stress were detected in dams briefly separated from their pups during the first 10 postnatal days. Taken together, these findings suggest that brief, repeated separations from the pups during the neonatal period persistently impact the behavior and induce signs of dopaminergic sensitization in the dam.

Brief daily postpartum separations from the litter alter dam response to psychostimulants and to stress

Neonatal handling induces several behavioral and neurochemical alterations in pups, including decreased responses to stress and reduced fear in new environments. However, there are few reports in the literature concerning the behavioral effects of this neonatal intervention on the dams during the postpartum period. Therefore, the aim of the current study was to determine if brief postpartum separation from pups has a persistent impact on the dam's stress response and behavior. Litters were divided into two neonatal groups: 1) non-handled and 2) handled [10 min/day, from postnatal day (PND) 1 to 10]. Weaning occurred at PND 21 when behavioral tasks started to be applied to the dams, including sweet food ingestion (PND 21), forced swimming test (PND 28), and locomotor response to a psychostimulant (PND 28). On postpartum day 40, plasma was collected at baseline for leptin assays and after 1 h of restraint for corticosterone assay. Regarding sweet food consumption, behavior during the forced swimming test or plasma leptin levels did not differ between dams briefly separated and non-separated from their pups during the postpartum period. On the other hand, both increased locomotion in response to diethylpropion and increased corticosterone secretion in response to acute stress were detected in dams briefly separated from their pups during the first 10 postnatal days. Taken together, these findings suggest that brief, repeated separations from the pups during the neonatal period persistently impact the behavior and induce signs of dopaminergic sensitization in the dam.

Early life manipulations alter learning and memory in rats

Much research shows that early life manipulations have enduring behavioral, neural, and hormonal effects. However, findings of learning and memory performance vary widely across studies. We reviewed studies in which pre-weaning rat pups were exposed to stressors and tested on learning and memory tasks in adulthood. Tasks were classified as aversive conditioning, inhibitory learning, or spatial/relational memory. Variables of duration, type, and timing of neonatal manipulation and sex and strain of animals were examined to determine if any predict enhanced or impaired performance. Brief separations enhanced and prolonged separations impaired performance on spatial/relational tasks. Performance was impaired in aversive conditioning and enhanced in inhibitory learning tasks regardless of manipulation duration. Opposing effects on performance for spatial/relational memory also depended upon timing of manipulation. Enhanced performance was likely if the manipulation occurred during postnatal week 3 but performance was impaired if it was confined to the first two postnatal weeks. Thus, the relationship between early life experiences and adulthood learning and memory performance is multifaceted and decidedly task-dependent.

Postnatal maternal separation modifies the response to an obesogenic diet in adulthood in rats

An early-life adverse environment has been implicated in the susceptibility to different diseases in adulthood, such as mental disorders, diabetes and obesity. We analyzed the effects of a high-fat sucrose (HFS) diet for 35 days in adult female rats that had experienced 180 minutes daily of maternal separation (MS) during lactancy. Changes in the obesity phenotype, biochemical profile, levels of glucocorticoid metabolism biomarkers, and the expression of different obesity- and glucocorticoid-metabolism-related genes were analyzed in periovaric adipose tissue. HFS intake increased body weight, adiposity and serum leptin levels, whereas MS decreased fat pad masses but only in rats fed an HFS diet. MS reduced insulin resistance markers but only in chow-fed rats. Corticosterone and estradiol serum levels did not change in this experimental model. A multiple gene expression analysis revealed that the expression of adiponutrin (Adpn) was increased owing to MS, and an interaction between HFS diet intake and MS was observed in the mRNA levels of leptin (Lep) and peroxisome proliferator-activated receptor gamma coactivator 1 alpha (Ppargc1a). These results revealed that early-life stress affects the response to an HFS diet later in life, and that this response can lead to phenotype and transcriptomic changes.

Early-life stress-induced anxiety-related behavior in adult mice partially requires forebrain corticotropin-releasing hormone receptor 1

Early-life stress may lead to persistent changes in central corticotropin-releasing hormone (CRH) and the CRH receptor 1 (CRHR1) system that modulates anxiety-related behavior. However, it remains unknown whether CRH-CRHR1 signaling is involved in early-life stress-induced anxiety-related behavior in adult animals. In the present study, we used conditional forebrain CRHR1 knockout (CRHR1-CKO) mice and examined the potential role of forebrain CRHR1 in the anxiogenic effects of early-life stress. As adults, wild-type mice that received unstable maternal care during the first postnatal week showed reduced body weight gain and increased anxiety levels in the open field test, which were prevented in stressed CRHR1-CKO mice. In the light-dark box test, control CRHR1-CKO mice were less anxious, but early-life stress increased anxiety levels in both wild-type and CRHR1-CKO mice. In the elevated plus maze test, early-life stress had only subtle effects on anxiety-related behavior. Moreover, early-life stress did not alter the basal home cage activity and gene expression levels of key hypothalamic-pituitary-adrenal axis regulators in adult wild-type and CRHR1-CKO mice, but enhanced neuroendocrine reactivity to acute immobilization stress in CRHR1-CKO mice. Our findings highlight the importance of forebrain CRHR1 in modulating some of the anxiogenic effects of early-life stress, and suggest that other neural circuits are also involved in the programming effects of early-life stress on anxiety-related behavior.

Long maternal separation has protective effects in rats exposed to activity-based anorexia

This study examined the effect of three neonatal treatments of maternal separation during infancy in young adult rats exposed to standard activity-based anorexia (ABA) consisting of food restriction plus free access to an activity wheel. During the first 20 postnatal days of life rat pups were exposed to periods of either brief maternal separation (BMS, 15 min), long maternal separation (LMS, 180 min), or were non-handled (NH). Thereafter, male and female rats were exposed to ABA. Neonatal treatment produced no significant differences in the survival time of male rats, whereas survival was greater in female rats exposed to LMS than in NH rats under ABA procedure. In conclusion, prolonged maternal separation appears to promote resistance in female animals subjected to harsh ABA life-threatening conditions.

Stress and GABA receptors

GABA(A) receptors are sensitive to subtle changes in the environment in both early-life and adulthood. These neurochemical responses to stress in adulthood are sex-dependent. Acute stress induces rapid changes in GABA(A) receptors in experimental animals, with the direction of the changes varying according to the sex of the animals and the stress-paradigm studied. These rapid alterations are of particular interest as they provide an example of fast neurotransmitter system plasticity that may be mediated by stress-induced increases in neurosteroids, perhaps via effects on phosphorylation and/or receptor trafficking. Interestingly, some studies have also provided evidence for long-lasting changes in GABA(A) receptors as a result of exposure to stressors in early-life. The short- and long-term stress sensitivity of the GABAergic system implicates GABA(A) receptors in the non-genetic etiology of psychiatric illnesses such as depression and schizophrenia in which stress may be an important factor.

Long-term effects of neonatal handling on mu-opioid receptor levels in the brain of the offspring

Neonatal handling is an experimental paradigm of an early experience which permanently alters hypothalamic-pituitary-adrenal axis function resulting in increased ability to cope with stress, and decreased emotionality. In the present work we investigated the effect of neonatal handling on adult rat brain mu-opioid receptor levels, since the opioid system is known to play an important role in emotional processing, anxiety and stress responses. Neonatal handling resulted in increased levels of mu-opioid receptors in the basolateral and central amygdaloid nuclei, in the CA3 and CA4 hippocampal areas, in the ventral tegmental area, the nucleus accumbens and the prefrontal cortex. Handled animals of both sexes had lower anxiety as measured in the elevated plus maze. The increased mu receptor levels could participate in the molecular mechanisms underlying the well-documented decreased stress and anxiety responses of handled animals.

Two repeated maternal separation procedures differentially affect brain 5-hydroxytryptamine transporter and receptors in young and adult male and female rats

Early environment is a known determinant for individual differences in vulnerability for adult psychopathology, e.g., ethanol addiction. One underlying mechanism could be dysfunction in serotonergic neurotransmission. This study focused on the methodological considerations regarding an animal model for studying effects of early environment, maternal separation (MS), using two different paradigms. Age- and sex-specific effects on brain stem 5-hydroxytryptamine (5-HT) transporter and receptors were examined. Male and female rat pups were assigned to either litter-wise MS for 15 or 360 min (MS15l or MS360l) or individual MS for 15 or 360 min (MS15i or MS360i) daily during postnatal days 1-21. Normal animal facility reared rats were used as controls. Analyses were performed in young and adult rats. As compared to the other males, MS15l males had lower 5-HT(1A) and 5-HT(2C) receptor mRNA expression at both ages, lower 5-HT(2A) receptor mRNA when young and lower 5-HTT mRNA expression when adult. In contrast, adult MS15l females had higher 5-HT(2C) receptor mRNA expression than other female rats. The strong impact of MS15l on 5-HT-related genes was either transient or persistent depending on sex and fewer effects on gene expression were observed in females than in males. This study shows the importance of tactile contact for the consequences of short but not prolonged MS, as evidenced by major differences between MS15l and MS15i. The results suggest that MS15i is less suitable than MS15l to simulate a protective environment in studies of, for instance, ethanol addiction processes.

Effects of maternal separation on hypothalamic-pituitary-adrenal responses, cognition and vulnerability to stress in adult female rats

We studied the long term effects of neonatal stress in female rats and subsequent responses to stress when adults. Female rats that experienced maternal separation (MS) showed in adulthood depressive-like behavior in the forced swimming test and cognitive impairments in the novel object recognition test, which were reverted by the glucocorticoid receptor antagonist mifepristone or the beta-adrenoceptor antagonist propranolol. Markers of HPA axis (corticosterone levels, CRF mRNA levels in the paraventricular nucleus and glucocorticoid receptor density in the hippocampus) were altered by MS, suggesting that an altered HPA axis function may be associated to behavioral and cognitive deficits in MS female rats. In addition, MS rats were found to be more vulnerable to chronic stress than controls as shown by decreases in open field activity, increases in immobility time in the forced swim test, and changes in markers of HPA axis (decreases in the density of glucocorticoid receptors). These present findings are discussed in terms of gender differences in adulthood.

Both infantile stimulation and exposure to sweet food lead to an increased sweet food ingestion in adult life

We have reported that neonatal handling leads to increased sweet food preference in adult life. Our aim was to verify if these differences in feeding behavior appear before puberty, and whether other types of intervention in periadolescence (such as exposure to toys) could interfere with sweet food consumption later in life. Nests of Wistar rats were (1) non-handled or (2) handled (10 min/day) on days 1-10 after birth. Males from these groups were subdivided in two subgroups: one was habituated to sweet food (Froot Loops-Kellogs) in a new environment for 4 days and tested for sweet food preference at age 27 days, before submitting to a new habituation and test for sweet food ingestion again in adult life. The other subgroup was habituated and tested only in adulthood. In another set of experiments, neonatally non-handled rats were exposed or not to a new environment with toys in periadolescence, and tested for sweet food ingestion as adults. Neonatal handling increases sweet food consumption only if the habituation and tests are performed after puberty. Interestingly, infant exposure to sweet food had a similar effect as neonatal handling, since controls that were exposed to sweet food at age 22 to 27 days increased their ingestion as adults. Exposure to toys in periadolescence had the same effect. We suggest that an intervention during the first postnatal days or exposure to an enriched environment later in the pre-pubertal period leads to behavioral alterations that persist through adulthood, such as increased sweet food ingestion.

Undertaking positive control studies as part of developmental neurotoxicity testing: a report from the ILSI Research Foundation/Risk Science Institute expert working group on neurodevelopmental endpoints

Developmental neurotoxicity testing involves functional and neurohistological assessments in offspring during and following maternal and/or neonatal exposure. Data from positive control studies are an integral component in developmental neurotoxicity risk assessments. Positive control data are crucial for evaluating a laboratory's capability to detect chemical-induced changes in measured endpoints. Positive control data are also valuable in a weight-of-evidence approach to help determine the biological significance of results and provide confidence in negative results from developmental neurotoxicity (DNT) studies. This review is a practical guide for the selection and use of positive control agents in developmental neurotoxicology. The advantages and disadvantages of various positive control agents are discussed for the endpoints in developmental neurotoxicity studies. Design issues specific to positive control studies in developmental neurotoxicity are considered and recommendations on how to interpret and report positive control data are made. Positive control studies should be conducted as an integral component of the incorporation and use of developmental neurotoxicity testing methods in laboratories that generate data used in risk decisions.

Neonatal handling alters learning in adult male and female rats in a task-specific manner

We demonstrated that early life manipulations (neonatal isolation, neonatal handling, maternal separation) impaired fear conditioning in adult rats [Kosten, T.A., Miserendino, M.J.D., Bombace, J.C., Lee, H.J., Kim, J.J., 2005. Sex-selective effects of neonatal isolation on fear conditioning and foot shock sensitivity. Behav. Brain Res. 157, 235-244.; Kosten, T.A., Lee, H.J. and Kim, J.J., 2006. Early life stress impairs fear conditioning in adult male and female rats. Brain Res. 1087, 142-150.]. Although we found few effects on somatic responses to footshock, deficits in conditioned fear may reflect altered emotional reactivity to aversive stimuli not learning deficits. Here we test neonatal handling effects on learning and memory tasks that vary by aversive stimuli. Neonatal handling was chosen because it alters emotional reactivity in adult rats. Litters of Sprague-Dawley rats were assigned to neonatal handling (15-min separation from dam and nest on postnatal days 1-21) or control (nonseparated) conditions. Adult male and female rats with or without neonatal handling experience were compared on: (1) inhibitory avoidance that involves footshock; (2) a circular maze task that involves escape from bright light; and (3) object recognition that presumably does not involve aversive stimuli. Neonatal handling impaired inhibitory avoidance but enhanced object recognition. There were no differences in circular maze performance. In addition, sex differences emerged in both the inhibitory avoidance and object recognition tasks; female rats perform better in inhibitory avoidance and worse in object recognition compared to male rats. These data suggest that neonatal handling alters learning and memory in a task-specific manner that may reflect alterations in emotional reactivity or differential effects of the manipulation on unknown neurohormonal mechanisms.

The communicative functions of touch in humans, nonhuman primates, and rats: a review and synthesis of the empirical research

Although touch is one of the most neglected modalities of communication, several lines of research bear on the important communicative functions served by the modality. The authors highlighted the importance of touch by reviewing and synthesizing the literatures pertaining to the communicative functions served by touch among humans, nonhuman primates, and rats. In humans, the authors focused on the role that touch plays in emotional communication, attachment, bonding, compliance, power, intimacy, hedonics, and liking. In nonhuman primates, the authors examined the relations among touch and status, stress, reconciliation, sexual relations, and attachment. In rats, the authors focused on the role that touch plays in emotion, learning and memory, novelty seeking, stress, and attachment. The authors also highlighted the potential phylogenetic and ontogenetic continuities and discussed suggestions for future research.

Can knowledge of developmental processes illuminate the evolution of parental care?

There are two levels of investigation for elucidating the evolution of parental behavior. The macro level focuses on how parental behavior can evolve as an aspect of reproduction. The micro level focuses on how species variations in parental behavior evolve. Recently, modern evolutionary biology has turned to developmental biology as a source for information about how trait variability (the substrate upon which natural selection and other evolutionary mechanisms can operate) can emerge during development (called "evo-devo"). Application of this evo-devo approach to the phenomenon of parental behavior requires identification of those mechanisms that produce variations in developmental pathways leading to parental behavior. It is these variations that provide the phenotypes for the potential evolution of different parental behavior systems. Variations in rodent maternal behavior affect the development of the HPA and HPG axes in their offspring. These mechanisms are examined to reveal how such developmental variations could underlie the evolution of biparental behavior. Knowledge of the developmental mechanisms responsible for species variations in mammalian parental behavior systems may provide insight into those mechanisms that may have been involved in the evolution of parental behavior itself.

Twice daily long maternal separations in Wistar rats decreases anxiety-like behaviour in females but does not affect males

Prolonged daily separations of rat pups from their mother have been reported to increase anxiety-like behaviour in adult offspring. However, there are an increasing number of studies not showing this. It has been proposed that the effect of long maternal separation (LMS) is partly due to the disruption of maternal care caused by the separations. The aim of the present study was to investigate whether increasing the number of daily separations would produce more robust effects in the adult offspring on anxiety-like behaviour in the defensive withdrawal test, and on spontaneous motor activity. Since previous studies of LMS have revealed sex differences in behaviour, we included both males and females. In our separation paradigm we subjected rat pups to either two daily 3h maternal separations during the first 2 weeks postpartum (LMS), two daily 15 min maternal separations (brief maternal separations, BMS) during the same time period to control for the effects of handling, or to normal husbandry conditions. As adults we found no effects of this LMS paradigm in male rats, although BMS males showed a tendency toward decreased anxiety-like behaviour. In contrast, LMS females showed a decrease in anxiety-like behaviour. We also found significant sex differences that were most prominent in the LMS group, indicating that females are more sensitive to our maternal separation paradigm. The present study suggests that increasing the number of maternal separations does not increase anxiety-like behaviour in neither male nor female Wistar rats.

Effects of neonatal handling on the basal forebrain cholinergic system of adult male and female rats

Neonatal handling is an early experience which results in improved function of the hypothalamic-pituitary-adrenal axis, increased adaptability and coping as a response to stress, as well as better cognitive abilities. In the present study, we investigated the effect of neonatal handling on the basal forebrain cholinergic system, since this system is known to play an important role in cognitive processes. We report that neonatal handling results in increased number of choline-acetyl transferase immunopositive cells in the septum/diagonal band, in both sexes, while no such effect was observed in the other cholinergic nuclei, such as the magnocellular preoptic nucleus and the nucleus basalis of Meynert. In addition, neonatal handling resulted in increased M1 and M2 muscarinic receptor binding sites in the cingulate and piriform cortex of both male and female rats. A handling-induced increase in M1 muscarinic receptor binding sites was also observed in the CA3 and CA4 (fields 3 and 4 of Ammon's horn) areas of the hippocampus. Furthermore, a handling-induced increase in acetylcholinesterase staining was found only in the hippocampus of females. Our results thus show that neonatal handling acts in a sexually dimorphic manner on one of the cholinergic parameters, and has a beneficial effect on BFCS function, which could be related to the more efficient and adaptive stress response and the superior cognitive abilities of handled animals.

Early handling reduces vulnerability of rats to activity-based anorexia

Resistance to restricted feeding with and without wheel access was tested in rats handled (H) for 20 days since birth. Weight loss produced by 1.5-hr restricted food access was less in H than in non-handled (NH) males when tested aged 41 days. At this age combining food restriction with access to a running wheel (a procedure commonly known as activity-based anorexia, ABA) produced very rapid weight loss and no effect of handling was detected. When 75-day females were tested in the same way, under the ABA procedure H rats took longer than NH controls to reach the removal criterion. Simply restricting food access in these females produced variable weight loss, without detection of any handling effect. No differences in food intake or running were detected between H and NH rats in either males or females. In conclusion, handling seems to have a direct effect on rats' later response to either food deprivation alone or to an ABA procedure.

Alterations in stress-associated behaviors and neurochemical markers in adult rats after neonatal short-lasting local inflammatory insult

Recently, there has been a growing interest in long-term consequences of neonatal pain because modern neonatal intensive care units routinely employ procedures that cause considerable pain and may be followed by local inflammation and hyperalgesia lasting for several hours or even days. To address this question, we developed a rat model of short lasting (<2 days) early local inflammatory insult produced by a single injection of 0.25% carrageenan (CAR) into the plantar surface of a hindpaw. Previously, we demonstrated that rats receiving this treatment within the first week after birth grow into adults with a global reduction in responsiveness to acute pain. Here, we report that these animals also manifest a low anxiety trait associated with reduced emotional responsiveness to stress. This conclusion is based in the following observations: (a) rats in our model display reduced anxiety on an elevated plus-maze; (b) in the forced swim test, these rats exhibit behavioral characteristics associated with stronger ability for stress coping; and (c) these animals have reduced basal and stress-induced plasma levels of such stress-related neuroendocrine markers as corticotropin-releasing factor, vasopressin, and adrenocorticotrophic hormone. In addition, we used DNA microarray and real-time reverse-transcriptase polymerase chain reaction to profile long-term changes in gene expression in the midbrain periaqueductal gray (PAG; a region involved in both stress and pain modulation) in our animal model. Among the affected genes, serotonergic receptors were particularly well represented. Specifically, we detected increase in the expression of 5-HT1A, 5-HT1D, 5-HT2A, 5-HT2C and 5-HT4 receptors. Several of these receptors are known to be involved in the anxiolytic and analgesic activity of the PAG. Finally, to determine whether neonatal inflammatory insult induces elevation in maternal care, which may play a role in generating long-term behavioral alterations seen in our model, we examined maternal behavior for 3 days following CAR injection. Indeed, we observed a substantial increase in maternal attention to the pups at the time of inflammation, but this increase was not without its cost: a period of significant maternal neglect afterward.

Early learning failure impairs adult learning in rats

Early life experiences may affect adult learning ability. In two experiments we tested the effect of early learning failure on adult performance in Wistar rats. In the first experiment 17-day-old rats (PN17), but not 25-day-old rats (PN25), trained in a hidden platform water maze task showed deficits in tone-shock avoidance learning when they were 3-months-old. The second experiment, which included random-platform and non-platform control groups, confirmed the effect of early (PN18) spatial learning failure on adult avoidance learning. However, post-weaning training (PN25) without platform also tended to induce adult learning deficits as long as the adult task difficulty was increased. The older non-platform group did not differ from the impaired group which received early training in a fixed hidden platform task. The results are discussed in terms of the relevance of early learning outcome and developmental stage on adult general learning deficits which may be related to the learned helplessness phenomenon and developmental neural plasticity.

Repeated stress alters caffeine action on motor coordination in C57Bl6/J male mice

This study was aimed to evaluate the effects of stress on caffeine action on motor coordination in mice. For 6 consecutive days, the mice were subjected to three different stressors. Saline or caffeine (30, 60 or 120 mg kg(-1)) was i.p. administered after the last stressful experience, then the animals were behaviorally tested in the holeboard. Their stumbling frequency was compared to that of unstressed mice injected with either saline or caffeine. (1) There was a strong trend for stress to impair motor coordination. (2) In unstressed mice, caffeine induced a linear dose-dependent increase of stumbling frequency. (3) Stress decreased the stumbling frequency induced by the highest dose of caffeine. The results are discussed in terms of interaction of stress and caffeine on dopaminergic and GABAergic systems.

Further evidences that risk assessment and object exploration behaviours are useful to evaluate emotional reactivity in rodents

In this study, we tested the ability of risk assessment and exploration behaviours to emphasise PH effects. Indeed, postnatal handling (PH) decreases emotional reactivity in rats but inconsistent behavioural results can be observed and may be due to false negative (i.e. existing effects are not detected). Risk assessment behaviours were measured in the elevated plus maze, in the free exploration paradigm and in the open field. In addition, we measured object exploration behaviours towards familiar/new objects in the open field. PH increased general activity in the elevated plus maze and in the free exploration paradigm and risk assessment behaviours allowed demonstrating that these effects were specific to emotional reactivity. In the open field, PH increased object exploration as early as first exposition while general activity was unaffected. PH also decreased behavioural inhibition in response to the introduction of a novel object. On the whole, our results show that risk assessment and object exploration behaviours are valuable tools to measure more precisely emotional reactivity in rodents. This reinforces the idea that these behaviours should be used more frequently in order to avoid false negative when emotional reactivity changes are expected in unconditioned conflict tests.

Neonatal handling alters feeding behavior of adult rats

Stress during the neonatal period leads to a large number of behavioral and biochemical alterations in adult life. The aim of this study is to verify the effects of handling and tactile stimulation during the first 10 days of life on feeding behavior in adult rats. Litters were divided into (1). intact; (2). handled (10 min/day); and (3). handled and tactile stimulated (10 min/day). Procedures were performed on Days 1-10 after birth. When adults, rats were tested for ingestion of sweet and savory snacks. We also measured body weight, ingestion of standard lab chow, and consumption of water and 1% glucose and 1.5% NaCl solutions. Stressed rats (handling and handling+tactile stimulation groups) consumed more sweet (two-way ANOVA, P=.008) or savory snacks (P=.001) than intact ones. This effect was observed in males and females. There were no differences in body weight, ingestion of standard lab chow, water, or in the ingestion of sweetened or salty solutions between groups. The same animals were tested later in life (15 months of age), and the effect was still evident. We suggest that handling during the neonatal period leads to alterations in the CNS of rats, causing an increased ingestion of palatable food in adult life, and this alteration probably persists throughout the whole life.

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

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

Early maternal availability and prefrontal correlates of reward-related memory

Early emotional experiences affect developing brain systems that subsequently mediate adult learning and memory in rodents. Here we test for similar effects in squirrel monkeys (Saimiri sciureus) four years after disruptions in early maternal availability. These conditions were previously shown to generate differences in emotional behavior, hypothalamic-pituitary-adrenal stress physiology, and right ventral medial prefrontal volumes determined in adulthood by magnetic resonance imaging. This report identifies in the same monkeys variability in reward-related memory on tests with a spatial reversal. Adult monkeys that more often selected locations repeatedly rewarded before each reversal had larger right ventral medial prefrontal volumes, but not hippocampal nor dorsolateral prefrontal volumes on the left or right brain side. Differences in performance were also discerned after each spatial reversal. These findings indicate that maternal availability alters developing ventral medial prefrontal brain regions involved in reward-related memory.

Effects of pre- and postnatal stimulation on developmental, emotional, and cognitive aspects in rodents: a review

Interactions between the organism and its environment, during pregnancy as well as during the postnatal period, can lead to important neurobehavioral changes. We briefly review the literature, and successively present the main results from our laboratory concerning the behavioral effects of prenatal stress, differential rearing conditions, and postnatal handling. We show that submitting primiparous DA/HAN rats to an acute emotional stress (exposure to a cat) at gestational day10, 14, or 19 leads to greatly increased mortality of pups and to decreased body weight of surviving animals. The effects of such a stressor on emotional reactivity are less obvious. Cognitive processes are impaired depending on the learning task. Enriched environments restore abnormal behaviors (emotional reactivity, motor skills, motor and spatial learning) due to brain trauma or genetic deficiencies. In any case, environmental enrichment does prevent or slow down aging effects. The effects of postnatal handling noted when using classical tests of emotional reactivity also are clear when defensive reaction paradigms are used. Furthermore, pregnant females that are early handled are less anxious than nonhandled females. We hypothesize that, when subjected to a stressor, the offspring of early-handled females would be protected from the deleterious effects of this stress compared to pups of nonhandled females.

The positive effects of postnatal handling on defensive burying are more obvious in a situation that enlarges the potential coping responses

A recent study reported few effects of postnatal handling in the defensive burying test. Since the importance of testing conditions has been emphasised in this paradigm, the lack of handling effects in this test could be attributed to the testing situation. Our experiment was carried out to test whether postnatal handling in DA/HAN strain rats have positive effects in two situations of the defensive burying test: a no-hide situation, in which avoidance of the probe was not possible, and a hide situation, in which animals were allowed to avoid the probe by sheltering in a hiding compartment. Our results showed no difference between control and handled rats defensive reactions in the no-hide situation. However, the general coping style of handled rats was different, mostly by a higher level of exploration of the apparatus and of the probe. In the hide situation, the time spent burying the probe was significantly lower in handled rats and they also spent less time in the hiding compartment. Those results, along with approach/avoidance behaviours directed towards the probe demonstrate that postnatal handling do have some positive effects in the defensive burying test. In addition, our results also point out that testing situations in which complex coping strategies are available are appropriate to test the effects of early stimulation such as handling.

Early-life handling stimulation and environmental enrichment: are some of their effects mediated by similar neural mechanisms?

Neonatal (early) handling (EH) and environmental enrichment (EE) of laboratory rodents have been the two most commonly used methods of providing supplementary environmental stimulation in order to study behavioral and neurobiological plasticity. A large body of research has been generated since the 1950s, unequivocally showing that both treatments induce profound and long-lasting behavioral and neural consequences while also inducing plastic brain effects and being "protective" against some age-related deficits. The present work is aimed at reviewing the main neurobehavioral effects of both manipulations, with the final purpose of comparing them and trying to find out to what extent the effects of both treatments may share (or not) possible neural mechanisms.

Long-lasting changes in stress-induced corticosterone response and anxiety-like behaviors as a consequence of neonatal maternal separation in Long-Evans rats

Early neonatal environmental factors appear to have powerful and long-lasting influences on an organism's physiology and behavior. Long-Evans male rats separated from their dam for 3 h daily over the first 2 weeks of life (maternally separated, MS rats) when tested as adults exhibit exaggerated behavioral and neuroendocrine responses to stress compared to 15-min separated (handled, H) animals. The purpose of this study was to compare male and female adult rats that were MS, H or were undisturbed (nonhandled, NH) as neonates in anxiety-like behaviors, in the elevated plus-maze, and in response to startle-inducing auditory stimuli. We confirmed that MS males oversecrete corticosterone (CORT; 2.5-5 times) in response to mild handling stress. MS males and females were less likely to explore open arms of the plus-maze. MS males exhibited 35% higher startle amplitudes compared to controls. Furthermore, MS males were more likely to emit ultrasonic vocalizations in response to startle than were H controls. However, MS and control females did not differ in auditory startle response or in startle-induced ultrasonic vocalizations. Therefore, experiencing maternal separation results in a long-lasting increase in anxiety-like behaviors that occurs in a sex-dependent manner.

Maternal corticosterone influences behavior, stress response and corticosteroid receptors in the female rat

In infancy, glucocorticoids have been shown to affect hypothalamus-pituitary-adrenal (HPA) axis activity and behavior. Both the activity of the HPA axis and many aspects of behavior exhibit important gender-dependent differences physiologically. In our previous studies, male offspring of hypercorticosteronemic mothers show long-lasting changes of learning as well as adrenocortical activity. In the light of these findings, this study aims to determine the long-term effects of glucocorticoids in the early stages of life in female rats. Corticosterone (200 microg/ml) was added to the drinking water of the dams. Female offspring exhibited lower adrenocortical secretory response to stress, improvement in learning (water maze at 21, 30 and 90 days; active avoidance at 15 months) and reduced fearfulness in anxiogenic situations (dark-light test at 1 and 15 months; conditioned suppression of drinking at 3 months; plus maze at 15 months) after weaning, from 21 days up to 15 months of age, but not before. No difference in hippocampal adrenocorticoid receptors was observed. These results, together with previous data on male offspring, show that the outcomes of maternal hypercorticosteronemia on hormonal stress response and behavior are similar in males and females, but the effects on some aspects of the HPA axis activity are gender-dependent. Possible explanations for these differences are discussed.

Chronic stress during prepubertal development: immediate and long-lasting effects on arterial blood pressure and anxiety-related behavior

The present work was undertaken to study the immediate and long-lasting effects of environmental stress during prepubertal life in rats with inherited stress-induced arterial hypertension (ISIAH) and normotensive Wistar rats on blood pressure (BP) levels and anxiety-related behavior. Two models of chronic stress (21-32 postnatal days) were used: repeated handling (HS) and unpredictable stress (US) of daily exposures to a variety of mild physical or psychoemotional stressors. Rats were tested just after the end of the chronic stress period and then at the age of 4 months. Chronic prepubertal stress did not affect the basal or stress-induced BP levels in young or adult Wistar rats. In ISIAH rats, chronic stress during the early phase of hypertension development did not accelerate its formation and did not augment its manifestation in adults. Moreover, the basal BP was decreased in young and adult ISIAH rats exposed to HS or US as compared to the age-matched controls. No long-lasting effect on BP elevation under acute stress in adults was found. Plasma corticosterone levels at resting and acute stress conditions were not changed in adult rats that had experienced prepubertal stress. Hypertensive rats proved to be less anxious in the elevated plus-maze test. The immediate effects of chronic stress were similar in the two rat strains: HS had an anxiolytic action while US stimulated anxiety. Long-lasting consequences depended on the rat strain: the anxiolytic effect of HS was retained in Wistar rats and US caused a greater anxiety in adult ISIAH rats. The data do not evidence that symptoms of anxiety are related to the development and maintenance of stress-sensitive arterial hypertension in ISIAH rats.

The influence of handling and isolation postweaning on open field, exploratory and maternal behavior of female rats

Fifty-three black-hooded female rats, raised from the 28th to the 128th day of life under four types of environmental conditions differing in social and handling factors, had their behavioral performance evaluated in eight tests involving exploration of a complex environment (EX) in two open field (OF) tests and in three pup-retrieving tests. These tests were held before, during and after the period of lactation of their first brood. Females differed from (previously studied) males in that environmental raising conditions had a much smaller influence, especially during the lactation period. Nevertheless, females raised in isolation tended to remain for longer times inside a protected den in the exploration tests and to have greater locomotion in the open field. Previous handling allowed a greater exploration during the first contact with the novel complex environment as well as greater activity in the open field. The presence of small pups drastically reduced the exploratory motivation, but the presence of pups more than 20 days old tended to increase it. Maternal behavior evaluated in the pup retrieval test and in the test of time for the female to leave the nest side of the double box system was markedly refractory to previous environmental influences.