Effects of early weaning on anxiety and autonomic responses to stress in rats

Minimizing Variability in Developmental Fear Studies in Mice: Toward Improved Replicability in the Field

In rodents, the first weeks of postnatal life feature remarkable changes in fear memory acquisition, retention, extinction, and discrimination. Early development is also marked by profound changes in brain circuits underlying fear memory processing, with heightened sensitivity to environmental influences and stress, providing a powerful model to study the intersection between brain structure, function, and the impacts of stress. Nevertheless, difficulties related to breeding and housing young rodents, preweaning manipulations, and potential increased variability within that population pose considerable challenges to developmental fear research. Here we discuss several factors that may promote variability in studies examining fear conditioning in young rodents and provide recommendations to increase replicability. We focus primarily on experimental conditions, design, and analysis of rodent fear data, with an emphasis on mouse studies. The convergence of anatomical, synaptic, physiological, and behavioral changes during early life may increase variability, but careful practice and transparency in reporting may improve rigor and consensus in the field. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC.

Preadolescent exposure to a sexually mature, unrelated male rat reduces postadolescent social recognition memory and CA2 c-Fos labeling

Introduction

Social memory involves social recognition: the ability to discriminate between two or more conspecifics when one has been previously encountered. The CA2 region of the hippocampus has been implicated in social memory, as lesions and dysfunction to this area lead to social memory impairments. A variety of psychogenic manipulations during postnatal sensitive developmental periods are associated with social memory impairments later in life.

Methods

In this study, we exposed preadolescent rats to a sexually, mature unrelated male and examined whether this was associated with changes in postadolescent social memory and c-Fos labeling in the CA2 region. Male and female Long-Evans rats were exposed to a male, adult rat on postnatal days 19-21 (P19-21). Social memory was measured during the postadolescent period and defined as increased interactions towards a novel age-matched rat in contrast to a previously-encountered age-matched rat. After the test, rats were euthanized and brain tissue was then collected to quantify c-Fos labeling within the CA2 region.

Results

Compared to home cage controls and controls not exposed to the adult male, male and female rats exposed to the unrelated adult during preadolescence were unable to discriminate between a novel and previously encountered conspecific during the postadolescent test showing social memory deficits. The groups that showed social recognition deficits also had significantly fewer c-Fos-positive cells within the CA2 region compared to the control groups.

Discussion

These findings indicate that threatening psychogenic encounters during preadolescence can have detrimental long-term effects on social memory potentially via disrupted activity in the CA2 hippocampal region.

Maternal separation in mice leads to anxiety-like/aggressive behavior and increases immunoreactivity for glutamic acid decarboxylase and parvalbumin in the adolescence ventral hippocampus

It has been reported that stressful events in early life influence behavior in adulthood and are associated with different psychiatric disorders, such as major depression, post-traumatic stress disorder, bipolar disorder, and anxiety disorder. Maternal separation (MS) is a representative animal model for reproducing childhood stress. It is used as an animal model for depression, and has well-known effects, such as increasing anxiety behavior and causing abnormalities in the hypothalamic-pituitary-adrenal (HPA) axis. This study investigated the effect of MS on anxiety or aggression-like behavior and the number of GABAergic neurons in the hippocampus. Mice were separated from their dams for four hours per day for 19 d from postnatal day two. Elevated plus maze (EPM) test, resident-intruder (RI) test, and counted glutamic acid decarboxylase 67 (GAD67) or parvalbumin (PV) positive cells in the hippocampus were executed using immunohistochemistry. The maternal segregation group exhibited increased anxiety and aggression in the EPM test and the RI test. GAD67-positive neurons were increased in the hippocampal regions we observed: dentate gyrus (DG), CA3, CA1, subiculum, presubiculum, and parasubiculum. PV-positive neurons were increased in the DG, CA3, presubiculum, and parasubiculum. Consistent with behavioral changes, corticosterone was increased in the MS group, suggesting that the behavioral changes induced by MS were expressed through the effect on the HPA axis. Altogether, MS alters anxiety and aggression levels, possibly through alteration of cytoarchitecture and output of the ventral hippocampus that induces the dysfunction of the HPA axis.

Effects of Early Weaning Associated with Alimentary Stress on Emotional and Feeding Behavior of Female Adult Wistar Rats

Maternal lactation proves crucial for mammals’ nutrition during their early development, influencing the development of adult physiological mechanisms. Its premature termination has been associated with several disorders, but these have been primarily documented in males, when they are most prevalent in women. Therefore, we subjected adult female Wistar rats to Early Weaning through maternal separation at age 15 days to acute alimentary stress in the form of visual and olfactory exposition to a cafeteria diet sans consumption for 22 days. We measured standard diet intake and water intake daily and cafeteria diet intake every 7 days. Additionally, we evaluated anxiety using the elevated plus maze and measured body weight in similar intervals. Results showed less consumption of the cafeteria diet among Early Weaning rats on day 2 and more time spent in the maze’s central area by the Early Weaning rats during the basal evaluation and in the maze’s open arms by control rats on day 7 when compared to the same group’s basal time. No other significant differences were found. These results show the importance of determining the impact that female steroidal gonadal hormones such as estradiol have upon feeding behavior and anxiety and determining to what degree these parameters are influenced by hormonal action.

Neuroendocrine Mechanisms of Social Bonds and Separation Stress in Rodents, Dogs, and Other Species

Mammalian species form unique bonds between mothers and infants. Maternal care, including suckling, is necessary for infant survival, and the mother and, sometimes, the father require a lot of effort in nurturing infants. An infant's probability of survival depends on the extent of the investment of care by the mother. In parallel, mothers must identify their offspring and invest only in those who possess their genes to achieve evolutionary benefits. Therefore, they need to recognize their offspring and show a strong preference for them. For this reason, bond formation between mothers and infants is important. The mother monitors her offspring's physical condition and stays close to them. The offspring also form strong bonds with their mothers. Therefore, a separation between the mother and infant causes severe stress for both parties. Although it was initially thought that such bonds between mother and infant are limited to the same species, we have also observed a similar phenomenon in the human-dog relationship. In this article, we discuss the neuroendocrine mechanisms that underlie bond formation and separation based on findings of neurobiological research in mice and the relationship between humans and dogs.

The effects of early weaning on Pavlovian fear conditioning in young rats

Early life stress enhances memory for Pavlovian fear conditioning. Stress enhancements on fear conditioning following early weaning, however, have yet to be studied in periweaning rats. Early weaning is a relevant animal model for human early life trauma, and Pavlovian fear conditioning is useful for identifying links between stress-induced developmental changes and behavior. We hypothesized that early weaning-on postnatal day (P)15-would lead to higher levels of conditional freezing relative to rats weaned later in life. Periweaning rats were trained with a discrete conditional stimulus (CS) and a shock unconditional stimulus (US), and tested 1 or 15 days later. Enhanced retention was observed in early weaned rats receiving forward paired CS-US training in Experiment 1, though this did not replicate in the second experiment. Despite overall enhancements in early weaned rats in Experiment 1, infantile amnesia effects were not overcome in young rats tested 15 days after training. Enhanced freezing levels in early weaned rats were not observed in subjects receiving unpaired CS, US training, and sensitivity to the US was not different due to age at weaning. Potential mechanisms underlying weaning-related enhancements and considerations for future studies including the role of social transmission of fear information are discussed.

Blockade of Opiodergic System During Early Weaning Reverts Feeding Behavior Altered Patterns

Adverse experiences that occur during the early stages of life can have permanent repercussions in adulthood. Among these experiences, early weaning is one that can alter the molecular, cellular, and behavior patterns in later life. Centered on this fact, the objective of the current study was to evaluate the effect of early weaning at 15 days of life of Wistar rats on their feeding behavior and if the opioidergic system blockade would cause a reversal of these outcomes. Experimental groups were formed based on the weaning period of each litter. On postnatal day 15, the group D15 was weaned and, on postnatal day 30 (natural weaning), the group D30 was weaned. The rats weaned on postnatal day 15, and administered subcutaneous Naltrexone (3 mg/kg) were from group D15 + NTX. Those weaned at 15 days of age exhibited higher depressive-like behavior, lesser reactivity time to sucrose, and higher intake of palatable food than the control group. The Naltrexone administration was observed to reverse some outcomes, such as increasing the reactivity time to sucrose and decreasing the quantity of palatable food consumed, to levels similar to those of the control group. Together, the findings of the present study are indicative of the vital role played by the opioidergic system in inducing the changes noted in the eating behavior patterns during adulthood, post early weaning.

Attachment Insecurity in Rats Subjected to Maternal Separation and Early Weaning: Sex Differences

Attachment insecurity in the forms of attachment anxiety and avoidance is associated with mental disorders in humans. In this research field, rodents, especially mice and rats, are commonly used to study social behaviors and underlying biological mechanisms due to their pronounced sociability. However, quantitative assessment of attachment security/insecurity in rodents has been a major challenge. The present study identified attachment insecurity behaviors in rats subjected to maternal separation (MS) during postnatal days (PD) 2-16 and early weaning (EW) during PD 17-21. This MSEW procedure has been used to mimic early life neglect in humans. After MSEW, rats continued to survive until early adulthood when they were subjected to open-field, social interaction, and elevated-plus maze tests. Compared to CNT rats in either gender, MSEW rats moved longer distances at higher velocities in the open-field. The MSEW rats also showed lower ratios of travel distance at central zone over that on whole arena of the open-field compared to CNT rats. In social interaction test, male CNT rats preferred to investigate an empty cage than females; whereas female CNT rats spent more time with a partner-containing cage as compared to males. This gender-specific difference was reversed in MSEW rats. On elevated-plus maze female CNT rats exhibited more risk-taking behaviors as compared to male counterparts. Moreover, female MSEW rats experienced a greater difficulty in making a decision on whether approaching to or averting from which arms of elevated-plus maze. Taken together, male MSEW rats behaved like attachment anxiety while females' phenotype is alike to attachment avoidance described in humans. These results shall prompt further application of MSEW rat in abnormal psychology and biological psychiatry research.

How to Handle Your Dragon: Does Handling Duration Affect the Behaviour of Bearded Dragons (Pogona Vitticeps)?

Simple Summary

Keeping reptiles as pets has become relatively common; therefore, it is important for us to understand more about how different aspects of their life as pets might affect their welfare. Very little is known about the effects of handling on reptiles, particularly the type of gentle handling (i.e., without restraint) likely to be experienced by pet animals. Therefore, this study investigated whether the amount of time that bearded dragons, a commonly kept lizard species, experienced gentle handling impacted on their behaviour in tests that give insight into how they may be feeling. We found that longer durations of handling appeared to increase stress-related behaviour slightly. This finding suggests that handling bearded dragons for longer periods of time, even in a gentle way, may be mildly stressful for them. However, we do not know whether animals would become accustomed to handling for longer periods with more experience.

Abstract

Reptiles are popular as pets and it is, therefore, important to understand how different aspects of housing and husbandry impact on their behaviour and welfare. One potential cause of stress in captive reptiles is interaction with humans; in particular, the effect of handling. However, little research on handling has been carried out with reptiles, particularly relating to the type of gentle handling likely to be experienced by pet animals. The aim of this study was therefore to determine whether the amount of time that bearded dragons (Pogona vitticeps), a commonly kept pet species, experienced gentle handling induced no or differing levels of anxiety, as reflected in their subsequent behavioural response to novelty. We found that there appeared to be a mildly aversive effect of handling time on subsequent behavioural response to novelty. Longer durations of handling (5 min or 15 min) appeared to increase anxiety-related behaviour, with handled animals showing more frequent tongue flicking behaviour when they experienced a novel environment and reduced time spent in close proximity to a novel object. These results suggest that handling bearded dragons, even in a gentle way, may increase their anxiety. However, it is not yet known whether animals may habituate to handling for longer periods if provided with additional experience.

Perineuronal Nets, Inhibitory Interneurons, and Anxiety-Related Ventral Hippocampal Neuronal Oscillations Are Altered by Early Life Adversity

BACKGROUND

In humans, accumulated adverse experiences during childhood increase the risk of anxiety disorders and attention-deficit/hyperactivity disorder. In rodents, the ventral hippocampus (vHIP) is associated with anxiety regulation, and lesions in this region alter both anxiety-like behavior and activity levels. Neuronal oscillations in the vHIP of the theta frequency range (4-12 Hz) have been implicated in anxious states and derive in part from the activity of inhibitory interneurons in the hippocampus, some of which are enwrapped with perineuronal nets (PNNs), extracellular matrix structures known to regulate plasticity. We sought to investigate the associations among early life stress-induced anxiety and hyperactivity with vHIP neuronal oscillations, inhibitory interneurons, and PNNs in mice.

METHODS

We used repeated maternal separation with early weaning (MSEW) to model accumulated early life adversity in mouse offspring and studied the underlying cellular and electrophysiological changes in the vHIP that are associated with excessive anxiety and hyperactivity.

RESULTS

We found increased anxiety-like behavior and activity levels in MSEW adult males, along with increased theta power and enhanced theta-gamma coupling in the vHIP. MSEW mice showed reduced intensity of parvalbumin as well as increased PNN intensity around parvalbumin-positive interneurons in the vHIP. We further observed that MSEW increased orthodenticle homeobox protein 2, a transcription factor promoting PNN development, in the choroid plexus, where it is produced, as well as in parvalbumin-positive interneurons, where it is sequestered.

CONCLUSIONS

These findings raise the possibility of causal links among parvalbumin-positive interneurons, PNNs, orthodenticle homeobox protein 2, and MSEW-induced anxiety and hyperactivity.

Early weaning increases anxiety via brain-derived neurotrophic factor signaling in the mouse prefrontal cortex

Deprivation of maternal care during early development markedly affects emotional development, but the underlying neuromolecular mechanisms are not fully understood. In a mouse model of disrupted mother-infant relationship, early weaning causes long-term impacts on pups to exhibit increased corticosterone secretion, anxiety, and stress responses in their adulthood. Revealing the molecular mechanisms behind it would beneficial to ameliorating mental problems caused by abuse in childhood. We report that normalizing circulating corticosterone in early-weaned mice, either in adulthood or soon after weaning, ameliorated anxiety levels assessed in the plus maze test. Administering a glucocorticoid receptor antagonist into the prefrontal cortex (PFC) reversed the effects of early weaning, whereas administering corticosterone increased anxiety levels, suggesting that the PFC is corticosterone’s target brain region. In the PFCs of early-weaned mice, we observed prolonged reductions in the expression of brain-derived neurotrophic factor (BDNF) and associated mRNAs. Anxiety in early-weaned mice was ameliorated by pretreatment with BDNF or a BDNF receptor agonist. In summary, early weaning increased anxiety levels by modulating glucocorticoid and BDNF signaling in the PFC.

Pre-reproductive Parental Enriching Experiences Influence Progeny's Developmental Trajectories

While the positive effects of environmental enrichment (EE) applied after weaning, in adulthood, during aging, or even in the presence of brain damage have been widely described, the transgenerational effects of pre-reproductive EE have been less examined. And yet, this issue is remarkable given that parental environmental experience may imprint offspring’s phenotype over generations through many epigenetic processes. Interactions between individual and environment take place lifelong even before conception. In fact, the environment pre-reproductively experienced by the mother and/or the father exerts a substantial impact on neural development and motor and cognitive performances of the offspring, even if not directly exposed to social, cognitive, physical and/or motor enrichment. Furthermore, pre-reproductive parental enrichment exerts a transgenerational impact on coping response to stress as well as on the social behavior of the offspring. Among the effects of pre-reproductive parental EE, a potentiation of the maternal care and a decrease in global methylation levels in the frontal cortex and hippocampus of the progeny have been described. Finally, pre-reproductive EE modifies different pathways of neuromodulation in the brain of the offspring (involving brain-derived neurotrophic factor, oxytocin and glucocorticoid receptors). The present review highlights the importance of pre-reproductive parental enrichment in altering the performances not only of animals directly experiencing it, but also of their progeny, thus opening the way to new hypotheses on the inheritance mechanisms of behavioral traits.

Social environment as a factor affecting exploration and learning in pre-juvenile rats

Stress associated with social isolation in early life can lead to disturbances in the emotional regulation in adult rats. However, there are no reports on the impact of isolation from the mother while providing contact with peers. Under such conditions, young individuals have the opportunity to interact with others, are able to develop social behaviour, etc. Yet, there is no stimulation and care provided by the mother. We examined the relative impact of maternal contact and sibling contact in the rarely studied pre-juvenile (3rd and 4th week post birth) period on subsequent development. An experiment was designed to compare the impact of different social environments on the animals' behaviour in adulthood. There were three breeding conditions: young with mother, young with peers, and standard breeding conditions. Adult rats were subjected to a T-Maze test to measure the level of exploratory behaviour. Spatial learning was assessed by placing water bottles in the side corridors. The analysis revealed that a distorted environment during the development process has a negative impact on the rats' emotional regulation and a subtle effect on related aspects of adaptive behaviours (i.e. exploration). In the pre-juvenile period, to some degree, contact with peers may be complementary to the mother's influence.

Stress-induced hyperthermia and hypothermia

Stress affects core body temperature (T_c). Many kinds of stress induce transient, monophasic hyperthermia, which diminishes gradually if the stressor is terminated. Stronger stressors produce a longer-lasting effect. Repeated/chronic stress induces anticipatory hyperthermia, reduces diurnal changes in T_c, or slightly increases T_c throughout the day. Animals that are exposed to chronic stress or a cold environment exhibit an enhanced hyperthermic response to a novel stress. These changes persist for several days after cessation of stress exposure. In contrast, long-lasting inescapable stress sometimes induces hypothermia. In healthy humans, psychologic stress induces slight increases in T_c, which are within the normal range of T_c or just above it. Some individuals, however, develop extremely high T_c (up to 41°C) when they are exposed to emotional events or show persistent low-grade high T_c (37-38°C) during or after chronic stress situations. In addition to the nature of the stressor itself, such stress-induced thermal responses are modulated by sex, age, ambient temperature, cage mates, past stressful experiences and cold exposure, and coping. Stress-induced hyperthermia is driven by mechanisms distinct from infectious fever, which requires inflammatory mediators. However, both stress and infection activate the dorsomedial hypothalamus-rostral medullary raphe region-sympathetic nerve axis to increase T_c.

Physical exercise prevents short and long-term deficits on aversive and recognition memory and attenuates brain oxidative damage induced by maternal deprivation

It is known from previous research that physical exercise prevents long-term memory deficits induced by maternal deprivation in rats. But we could not assume similar effects of physical exercise on short-term memory, as short- and long-term memories are known to result from some different memory consolidation processes. Here we demonstrated that, in addition to long-term memory deficit, the short-term memory deficit resultant from maternal deprivation in object recognition and aversive memory tasks is also prevented by physical exercise. Additionally, one of the mechanisms by which the physical exercise influences the memory processes involves its effects attenuating the oxidative damage in the maternal deprived rats' hippocampus and prefrontal cortex.

Early weaning influences short-term synaptic plasticity in the medial prefrontal-anterior basolateral amygdala pathway

Early weaning in rodents reportedly influences behavioral and emotional traits and triggers precocious myelin formation in the anterior basolateral amygdala (aBLA; Ono et al., 2008), where prefrontal efferents terminate. We studied the correlation between behavior and the synaptic properties of the prefrontal-aBLA pathway. Open-field behaviors of adult male rats weaned at either 16 days or 30 days were measured on two consecutive days. On the first day, the rats received a slight footshock that was reportedly insufficient for fear conditioning. Electrophysiological recordings in the prefrontal-aBLA were then performed under urethane anesthesia. Without group differences in the stimulus intensity or the first evoked response, the overall paired-pulse facilitation was significantly lower in the early-weaned group from 25 to 100 ms. At the 25-ms interval, regression values between paired-pulse facilitation and locomotion on the second day were positive/insignificant and negative/significant in early- and control-weaned groups, respectively, and were statistically different between the groups.

The evolution of capture myopathy in hooved mammals: a model for human stress cardiomyopathy?

Background and objectives: Capture myopathy (CM) syndromes in wildlife may be a model for human stress cardiomyopathy, including Takotsubo cardiomyopathy. Emotional stress or grief may trigger heart attack-like symptoms, and occasionally, sudden death in some humans. Similarly, wildlife exposed to predatory stresses, chase, or capture occasionally results in sudden death. To better understand the nature of vulnerability to stress-induced sudden death, we studied cases of CM in hooved mammals—ungulates—and hypothesized that CM would be associated with a syndrome of longevity-related traits.

Methodology: We reconstructed the evolution of CM in ungulates then determined how a set of life history traits explained variation in the likelihood that CM was reported.

Results: CM is broadly reported, but not in all genera, and phylogenetic analyses suggest that it is an evolutionarily labile trait. We found that the following traits were significantly associated with reports of CM: greater brain mass, faster maximum running speed, greater minimum group size and greater maximum longevity.

Conclusions and implications: CM may be an unavoidable consequence of adaptations to reduce predation risk that include increased running speed, sociality and having larger brains. Moreover, longer-lived species seem to be more likely to be susceptible to CM. Exploring variable susceptibility to CM highlights the evolutionary origins of the disorder, potential basic mechanisms that underlie vulnerability to the phenomenon, and the potential for reduction of risk through modification of life history trajectory.

Need for gender-specific pre-analytical testing: the dark side of the moon in laboratory testing

Many international organisations encourage studies in a sex-gender perspective. However, research with a gender perspective presents a high degree of complexity, and the inclusion of sex-gender variable in experiments presents many methodological questions, the majority of which are still neglected. Overcoming these issues is fundamental to avoid erroneous results. Here, pre-analytical aspects of the research, such as study design, choice of utilised specimens, sample collection and processing, animal models of diseases, and the observer's role, are discussed. Artefacts in this stage of research could affect the predictive value of all analyses. Furthermore, the standardisation of research subjects according to their lifestyles and, if female, to their life phase and menses or oestrous cycle, is urgent to harmonise research worldwide. A sex-gender-specific attention to pre-analytical aspects could produce a decrease in the time for translation from the bench to bedside. Furthermore, sex-gender-specific pre-clinical pharmacological testing will enable adequate assessment of pharmacokinetic and pharmacodynamic actions of drugs and will enable, where appropriate, an adequate gender-specific clinical development plan. Therefore, sex-gender-specific pre-clinical research will increase the gender equity of care and will produce more evidence-based medicine.

Breathing and temperature control disrupted by morphine and stabilized by clonidine in neonatal rats

BACKGROUND

Sedative-analgesics are often given to newborn infants and are known to affect many components of the autonomic nervous system. While morphine is most frequently used, α-2 adrenergic receptor agonists are being increasingly used in this population. Alpha-2 adrenergic receptors agonists also have anti-shivering properties which may make it a desirable drug to give to infants undergoing therapeutic hypothermia. The aim of this study was to systematically compare two different classes of sedative-analgesics, morphine, a μ-opioid receptor agonist, and clonidine an α-2 adrenergic receptor agonist on breathing, metabolism and core body temperature (CBT) in neonatal rodents.

METHODS

Breathing parameters, oxygen consumption (VO2) and carbon dioxide production (VCO2), were measured prior to, 10 and 90 min after intraperitoneal (IP) administration of morphine (2, 10 or 20 mg/kg), clonidine (40, 200 or 400 μg/kg), or saline in Sprague-Dawley rat pups at postnatal day 7 (p7) while continuously monitoring CBT.

RESULTS

Morphine reduced the respiratory rate, VO2 and VCO2 greater than clonidine at all dosages used (p<0.05, morphine vs. clonidine, for all metabolic and respiratory parameters). Furthermore, morphine induced prolonged respiratory pauses, which were not observed in animals treated with clonidine or saline. Morphine caused hypothermia which was dose dependent, while clonidine stabilized CBT in comparison to saline treated animals (p<0.0001).

CONCLUSION

In the newborn rat, morphine causes profound respiratory depression and hypothermia while clonidine causes minimal respiratory depression and stabilizes CBT. All together, we suggest that clonidine promotes autonomic stability and may be a desirable agent to use in infants being treated with therapeutic hypothermia.

Reconceptualizing sex, brain and psychopathology: interaction, interaction, interaction

In recent years there has been a growing recognition of the influence of sex on brain structure and function, and in relation, on the susceptibility, prevalence and response to treatment of psychiatric disorders. Most theories and descriptions of the effects of sex on the brain are dominated by an analogy to the current interpretation of the effects of sex on the reproductive system, according to which sex is a divergence system that exerts a unitary, overriding and serial effect on the form of other systems. We shortly summarize different lines of evidence that contradict aspects of this analogy. The new view that emerges from these data is of sex as a complex system whose different components interact with one another and with other systems to affect body and brain. The paradigm shift that this understanding calls for is from thinking of sex in terms of sexual dimorphism and sex differences, to thinking of sex in terms of its interactions with other factors and processes. Our review of data obtained from animal models of psychopathology clearly reveals the need for such a paradigmatic shift, because in the field of animal behaviour whether a sex difference exists and its direction depend on the interaction of many factors including, species, strain, age, specific test employed and a multitude of environmental factors. We conclude by explaining how the new conceptualization can account for sex differences in psychopathology.

Age at weaning, immunocompetence and ectoparasite performance in a precocial desert rodent

We studied the effects of early weaning on immunocompetence and parasite resistance in a precocial rodent Acomys cahirinus. We hypothesized that if parasite resistance is energetically expensive and nutritional and immunological support from mothers are necessary for the long-term health of offspring, then early weaned animals would be immunologically weaker and less able to defend themselves against parasites than later weaned animals. We weaned pups at 14, 21, or 28 d after birth and assessed their immunocompetence and resistance against fleas Parapulex chephrenis when they attained adulthood. Immunocompetence was assessed using leukocyte concentrations (LC) and a phytohaemagglutinin injection assay (PHA test). To estimate resistance against fleas, we measured performance of fleas via the number of produced eggs and duration of development and resistance to starvation of the flea offspring. We found a significant positive effect of weaning age on the PHA response but not on LC. The effect of age at weaning on flea egg production was manifested in male but not female hosts, with egg production being higher if a host was weaned at 14 than at 28 d. Weaning age of the host did not affect either duration of development or resistance to starvation of fleas produced by mothers fed on these hosts. We concluded that even in relatively precocial mammals, weaning age is an important indicator of future immunological responses and the ability of an animal to resist parasite infestations. Hosts weaned at an earlier age make easier, less-resistant targets for parasite infestations than hosts weaned later in life.

Organizational effects of estrogen on male-type vulnerability to early weaning

We previously reported that early-weaned (postnatal day 14) male ICR mice, compared to normally weaned animals, exhibited a persistent increase in anxiety-related behavior in the elevated plus maze test. In this study, we examined whether steroid hormone manipulations on postnatal day 0 and at the ages of 2 or 3 weeks affected male-type vulnerability to early weaning. Neither castration nor ovariectomy at the age of 3 weeks affected male-type vulnerability. However, in males, castration at the age of 2 weeks attenuated the increased anxiety levels induced by early weaning, and the implantation of testosterone or estradiol, but not of dihydrotestosterone, restored the effects of early weaning. In contrast, in females, neonatal treatment with testosterone propionate together with testosterone at the age of 2 weeks, which reversed sexual behavior to the male type, did not affect anxiety levels in response to early weaning. When pregnant females were repeatedly treated with testosterone propionate on embryonic days 14, 17, and 19, in addition to testosterone treatment at the age of 2 weeks, the anxiety levels in female were increased by early weaning. Furthermore, the prenatal treatment of estradiol benzoate, but not dihydrotestosterone, induced enhanced anxiety levels by early weaning in females. These results suggest that neural systems are masculinized by estrogen from the embryonic phase to the early postnatal period and are responsible for the high levels of anxiety elicited by early weaning.

Effects of sex and rearing environment on imipramine response in mice

RATIONALE

Antidepressant treatments are commonly prescribed, but few studies have been published, even in animal models, on differences in reactivity to antidepressants with respect to rearing environment and sex.

OBJECTIVES

Using a mouse model, we investigated the hypothesis that rearing environment (weaning time) and sex could impact responses to imipramine treatment.

METHODS

ICR mice were assigned to four groups for each sex: early weaned saline or imipramine treated, normally weaned saline or imipramine treated. Early weaning was conducted on postnatal day 14. All groups were injected intraperitoneally with drug or vehicle for 2 weeks from the age of 6 weeks and tested in the elevated plus maze to estimate anxiety levels. Hippocampal neurogenesis was also assessed with immunostaining for calretinin and calbindin because it has suggested that neurogenesis is required for the behavioral effects of antidepressants.

RESULTS

Imipramine treatment decreased anxiety levels in females, but not in males, in the normal weaning condition. By contrast, the same treatment decreased anxiety levels in males, but not females, in the early weaning condition. Hippocampal changes, which correlated these behavioral responses to imipramine, were seen in the extragranule cell layer: the number of calretinin-positive cells was increased by imipramine in females, but not in males, in the normal weaning condition. In the early weaning condition, however, the treatment was associated with similar changes in males but not in females.

CONCLUSIONS

The results suggest that rearing environment and sex differences are implicated in responses to imipramine treatment with respect to anxiety behavior and neurogenesis in the hippocampus.

Behavioral effects of environmental enrichment during gestation in WKY and Wistar rats

Effects of prenatal environmental enrichment (EE) were examined in Wistar Kyoto (WKY) "depressive- and anxious-like" rats and Wistar rats. During gestation, dams lived in standard cages or in EE cages. Their behavior during gestation and lactation was observed. On weaning day, they were tested in the forced swimming test, and corticosterone concentration was measured from their plasma. The offspring, reared in standard environment, were tested as juveniles or young adults in the elevated plus maze, open field and forced swimming tests. Corticosterone concentration in feces was analyzed. EE offspring showed more anxiety-like behaviors and less activity, compared to controls. Effects were more prominent in youth than in adulthood and in Wistar rats more than in WKY. EE lowered corticosterone concentration in young WKY rats' feces. EE induced changes in the dams' behavior during gestation and lactation. These changes in dams' behavior could be mediators of the effects on the offspring.

Developmental consequences and biological significance of mother-infant bonding

Mother-infant bonding is universal to all mammalian species. Here, we review how mutual communication between the mother and infant leads to mother-infant bonding in non-primate species. In rodents, mother-infant bond formation is reinforced by various pup stimuli, such as tactile stimuli and ultrasonic vocalizations. Evidence suggests that the oxytocin neural system plays a pivotal role in each aspect of the mother-infant bonding, although the mechanisms underlying bond formation in the brain of infants has not yet been clarified. Impairment of mother-infant bonding strongly influences offspring sociality. We describe the negative effects of mother-infant bonding deprivation on the neurobehavioral development in rodent offspring, even if weaning occurs in the later lactating period. We also discuss similar effects observed in pigs and dogs, which are usually weaned earlier than under natural conditions. The comparative understanding of the developmental consequences of mother-infant bonding and the underlying mechanisms provide insight into the biological significance of this bonding in mammals, and may help us to understand psychiatric disorders related to child abuse or childhood neglect.

Pilot trial of Melissa officinalis L. leaf extract in the treatment of volunteers suffering from mild-to-moderate anxiety disorders and sleep disturbances

Botanicals are an alternative option to prescription drugs for the alleviation of symptoms due to anxiety disorders and insomnia. Melissa officinalis L. has been shown as an anti-stress and anxiolytic agent. We previously reported moderate stress improvement in mice in which Cyracos(®), a standardized Melissa officinalis L. extract, was administrated. Cyracos(®) contains phytochemicals that inhibit gamma-aminobutyric acid catabolism. This was a prospective, open-label, 15-day study to evaluate the efficacy of Cyracos(®) on stressed volunteers, who have mild-to-moderate anxiety disorders and sleep disturbances. Using clinician rating criteria, primary outcomes showed improvement of symptoms. Cyracos(®) reduced anxiety manifestations by 18% (p < 0.01), ameliorated anxiety-associated symptoms by 15% (p < 0.01) and lowered insomnia by 42% (p < 0.01). As much as 95% of subjects (19/20) responded to treatment, of which 70% (14/20) achieved full remission for anxiety, 85% (17/20) for insomnia, and 70% (14/20) for both. Our study demonstrates, for the first time that chronic administration of Melissa officinalis L. relieves stress-related effects. It is critical that further studies incorporate a placebo and investigate physiological stress markers.

Adolescent escitalopram administration modifies neurochemical alterations in the hippocampus of maternally separated rats

Early life stress is a potential precursor of eventual neuropsychiatric diseases and may result in altered neurodevelopment and function of the hippocampus, which thus provides a site at which potential interventions to modify the effects of early life stress may act. In this study, Sprague-Dawley rat pups comprising male and female animals underwent maternal separation (MS) for 180 min from postnatal days (PND) 2 to 14, or were left with their dams. They subsequently received daily administration of saline (0.9%), escitalopram (10 mg/kg), or no treatment during adolescence (PND 43-60). All adult animals underwent brain magnetic resonance imaging (MRI) and bilateral hippocampal proton magnetic resonance spectroscopy ((1)H-MRS). Neither MS nor escitalopram treatment had a significant effect on hippocampal volume. Adult rats that experienced MS displayed significantly increased choline-containing compounds (Cho) and decreased N-acetylaspartate (NAA), glutamate (Glu) and Myo-inositol (MI) relative to the stable neurometabolite creatine (Cr) in hippocampus. Administration of escitalopram during adolescence could modify the alterations of NAA/Cr, Glu/Cr and MI/Cr. The effects of MS on hippocampal neurochemistry were most significant in the right hippocampus. These results indicate that MS in rats has long-term consequences on hippocampal neurochemistry reflective of neural density/functional integrity, especially on the right hippocampus, and adolescent administration with escitalopram can at least partially ameliorate these neurochemical alterations. Furthermore, these metabolite changes seem to be more sensitive indicators of the results from early life stress than volume changes.

Defining age limits of the sensitive period for attachment learning in rat pups

Enhanced odor preference learning and attenuated fear learning characterizes rat pups' attachment learning Sensitive Period for learning the maternal odor. This period terminates at 10 days old (PN10) with increasing endogenous levels of the stress hormone, corticosterone. Increasing Sensitive Period pups' corticosterone prematurely terminates the Sensitive Period, while decreasing corticosterone in older pups delays Sensitive Period termination. Here we extend these findings and define the age range corticosterone alters learning and question whether corticosterone permanently terminates the Sensitive Period. Pups were odor-0.5 mA shock conditioned with either corticosterone increased (PN5-6; 4 mg/kg vs. saline) or decreased (PN15-16; naturally by maternal presence or corticosterone synthesis blocker, Metyrapone). Finally, PN7-8 pups were conditioned with corticosterone and reconditioned without corticosterone to assess whether the Sensitive Period was permanently terminated. Results indicate developmental limits for corticosterone regulation of pup learning are PN6 through PN15. Furthermore, inducing precocious corticosterone induced fear learning was not permanent, since reconditioning without corticosterone enabled odor preference learning. Results suggest pups are protected from learning aversions to maternal odor until approaching weaning.

Transitions in sensitive period attachment learning in infancy: the role of corticosterone

Survival of altricial infants, including humans and rats, depends on attachment to the caregiver - a process that requires infants to recognize, learn, and remember their attachment figure. The demands of a dynamic environment combined with a maturing organism require frequent neurobehavioral reorganization. This restructuring of behavior and its supporting neural circuitry can be viewed through the unique lens of attachment learning in rats in which preference learning is enhanced and aversion learning is attenuated. Behavioral restructuring is well adapted to securing the crucial infant-caregiver relationship regardless of the quality of care. With maturation and the end of the infant-caregiver attachment learning period, the complex interplay of neural structures, hormones, and social behavior coordinates the developing rat's eventual transition to life outside of the nest. Nevertheless, early-life environmental and physiological stressors can alter the resilient nature of this system, particularly with respect to the amygdala, and these changes may provide important clues to understanding the lasting effects of early stress.

Long-term effects of infant learning on adult conditioned odor aversion are determined by the last preweaning experience

We recently showed that odorizing mother's nipples from birth to weaning attenuated adult conditioned odor aversion (COA). The present study evaluated whether shorter durations of preweaning olfactory experiences could induce similar long-term effects. We first showed that late preweaning odorization (PN13-PN25) impaired adult COA similarly to odorization from birth to weaning (PN0-PN25) whereas early odorization (PN0-PN12) had no effect on adult COA. As early odorization was followed by an odorless suckling period, we evaluated whether this odorless suckling could have interfered with early associative learning. We therefore weaned the animals either immediately after early odorization or 7 days later. Early odorization (PN0-PN18) followed by late odorless suckling (PN19-PN25) had no effect on adult COA. However, pups with early odorization (PN0-PN18) but without late odorless suckling (weaned at PN18) showed attenuated COA. These results support the hypothesis that interference between early and late preweaning experiences with the mother determines the long-term impact on adult COA.

The meaning of weaning: influence of the weaning period on behavioral development in mice

Maternal care during the first week postpartum has long-term consequences for offspring development in rodents. However, mother-infant interactions continue well beyond this period, with several physiological and behavioral changes occurring between days 18 and 28 PN. In the present study, we investigate the long-term effects on offspring behavior of being weaned at day 21 PN versus day 28 PN. We found that male and female offspring engage in higher initial levels of social interaction if weaned at day 28 PN, as well as sexually dimorphic changes in exploratory behavior. Females who were themselves weaned earlier also appeared to wean their own pups earlier. Sex-specific effects of weaning age were found on levels of oxytocin and vasopressin V1a receptor density in the hypothalamus, central nucleus of the amygdala and nucleus accumbens. These results indicate that altering weaning age in mice may be a useful model for investigating the development of sexual dimorphism in neurobiology and behavior.

Maternal deprivation by early weaning increases corticosterone and decreases hippocampal BDNF and neurogenesis in mice

We previously demonstrated that early weaning increases anxiety and neuroendocrine stress responses in rats and mice. In addition, early-weaned mice show precocious myelin formation, especially in the amygdala, suggesting that these mice are vulnerable to psychological stress. In the present experiments, we examined corticosterone response after early weaning and how early weaning affects hippocampal neurotrophic factor and neurogenesis, which have been linked to depressive behavior in human and animals models. When the mice were weaned at PD14, both male and female mice showed higher corticosterone levels up to 48h after weaning. In contrast, after standard weaning, corticosterone levels returned to the baseline within 2h. Early-weaned males, but not females, had less brain-derived neurotrophic factor (BDNF) protein in the hippocampus at 3 weeks of age than standard-weaned mice. Neural stem cells were labeled with bromodeoxyuridine (BrdU) injections at 2, 3, or 5 weeks of age, and assayed at 3, 5, and 8 weeks of age, respectively. Early-weaned males had fewer BrdU immunoreactive cells in the dentate gyrus at 3, 5, and 8 weeks. In early-weaned females, fewer BrdU-positive cells were observed only at 5 weeks. Double-staining with BrdU and the neuron markers NeuN and Tuj1 demonstrated that neurogenesis was lower in early-weaned mice at 5 weeks of age. These results suggest that lack of mother-infant interaction during the late lactation period leads to an increase in corticosterone synthesis for 2 days and a decrease in BDNF synthesis in males; moreover, this lack of interaction transiently inhibits hippocampal cell proliferation and survival in both males and females, although the effects were more pronounced in males.

Behavioural and neurochemical consequences of early weaning in rodents

Among all mammalian species, pups are highly dependent on their mother not only for nutrition, but also for physical interaction. Therefore, disruption of the mother-pup interaction changes the physiology and behaviour of pups. We review how maternal separation in the early developmental period brings about changes in the behaviour and neuronal systems of the offspring of rats and mice. Early weaning in mice results in adulthood a persistent increase in anxiety-like and aggressive behaviour. The early-weaned mice also show higher hypothalamic-pituitary-adrenal activity in response to novelty stress. Neurochemically, the early-weaned male mice, but not female mice, show precocious myelination in the amygdala, decreased brain-derived neurotrophic factor protein levels in the hippocampus and prefrontal cortex, and reduced bromodeoxyuridine immunoreactivity in the dentate gyrus. Because higher corticosterone levels are persistently observed up to 48 h when the mice are weaned on postnatal day 14, the exposure of the developing brain to higher corticosterone levels may be one of the effects of early weaning. These results suggest that deprivation of the mother-infant interaction during the late lactating period results in behavioural and neurochemical changes in adulthood and that these stress responses are sexually dimorphic (i.e. the male is more vulnerable to early weaning stress).

Effects of early weaning on anxiety and prefrontal cortical and hippocampal myelination in male and female Wistar rats

We investigated developmental changes in myelin formation in the prefrontal cortex and the hippocampus, and behavioral effects of early weaning in Wistar rats. Early-weaned rats showed decreased numbers of open-arm entries in an elevated plus-maze in both sexes at 4 weeks old; this effect persisted in males, but ceased in females after this age. Expression of myelin basic protein (MBP) showed both age-dependent increases and sex differences; 4-week-old males exhibited higher MBP levels in the hippocampus, whereas 7-week-old males showed lower MBP levels in the prefrontal cortex compared to females of the same age. There was a tendency for group differences from weaning for the 21.5-kDa isoform in the prefrontal cortex. Although these results suggest that male rats are more vulnerable than females to early-weaning effects on anxiety-related behaviors, further detailed analysis is needed to clarify the functional relationship between myelination and anxiety-related behaviors.

Early weaning decreases play-fighting behavior during the postweaning developmental period of Wistar rats

We examined the influence of early weaning on the development of play-fighting behaviors and anxiety status in Wistar rats. Pups were divided into two groups, those weaned at postnatal day (PD) 16 (early-weaned group) and those weaned at PD30 (normally weaned group), and were housed in pairs of the same sex. Playful interactions were measured for each pair once a week from 4 to 7 weeks of age. Thereafter, during early adulthood, all the rats were subjected to the elevated plus-maze test. The frequencies of pinning and playful attack were less in the early-weaned group than in the normally weaned group. In the elevated plus-maze test, rat pups in the early-weaned group had higher anxiety levels. The results showed that deprivation of mother-pup interactions during the preweaning period decreases affiliative interactions between cage mates, including play-fighting behaviors during the postweaning developmental period, and increases anxiety levels during early adulthood.

Fostering and environmental enrichment ameliorate anxious behavior induced by early weaning in Balb/c mice

Postnatal stimuli affect many aspects of physiological and behavioral development. In mice, earlier weaning augments anxiety, putatively as a result of removing mother-pup interactions during the weaning period. Here, we examined the ameliorating effects of social and environmental enrichment on anxiety related to early weaning. Mice weaned at postpartum day 14 were fostered by virgin females, who displayed some nursing behavior during the 1-week fostering period. In elevated plus-maze tests, 10-week-old pups reared with a foster mother spent more time in the open arms than early-weaned mice, and entered into the open arms at a rate between that of normally- and early-weaned mice. Subsequently, the mice from each rearing group were transferred into either standard housing or housing enriched with toys that were changed periodically. Elevated plus-maze tests were conducted again when the mice were 18 and 26 weeks old. The enriched environment increased the duration of time spent in the open arms, but the magnitude of the effect varied with the rearing condition. Furthermore, mice that lived in the enriched environment showed lower activity than those kept in standard housing. These results suggest that fostering after early weaning attenuates increases in anxiety levels, and maternal care during this period may be important in the development of an offspring's emotionality. Environmental stimuli in adulthood may act to blunt the effects deprivation in early life.