Enriched environment experience overcomes learning deficits and depressive-like behavior induced by juvenile stress

Pathophysiology in cortico-amygdala circuits and excessive aversion processing: the role of oligodendrocytes and myelination

Stress-related psychiatric illnesses, such as major depressive disorder, anxiety and post-traumatic stress disorder, present with alterations in emotional processing, including excessive processing of negative/aversive stimuli and events. The bidirectional human/primate brain circuit comprising anterior cingulate cortex and amygdala is of fundamental importance in processing emotional stimuli, and in rodents the medial prefrontal cortex-amygdala circuit is to some extent analogous in structure and function. Here, we assess the comparative evidence for: (i) Anterior cingulate/medial prefrontal cortex<->amygdala bidirectional neural circuits as major contributors to aversive stimulus processing; (ii) Structural and functional changes in anterior cingulate cortex<->amygdala circuit associated with excessive aversion processing in stress-related neuropsychiatric disorders, and in medial prefrontal cortex<->amygdala circuit in rodent models of chronic stress-induced increased aversion reactivity; and (iii) Altered status of oligodendrocytes and their oligodendrocyte lineage cells and myelination in anterior cingulate/medial prefrontal cortex<->amygdala circuits in stress-related neuropsychiatric disorders and stress models. The comparative evidence from humans and rodents is that their respective anterior cingulate/medial prefrontal cortex<->amygdala circuits are integral to adaptive aversion processing. However, at the sub-regional level, the anterior cingulate/medial prefrontal cortex structure-function analogy is incomplete, and differences as well as similarities need to be taken into account. Structure-function imaging studies demonstrate that these neural circuits are altered in both human stress-related neuropsychiatric disorders and rodent models of stress-induced increased aversion processing. In both cases, the changes include altered white matter integrity, albeit the current evidence indicates that this is decreased in humans and increased in rodent models. At the cellular-molecular level, in both humans and rodents, the current evidence is that stress disorders do present with changes in oligodendrocyte lineage, oligodendrocytes and/or myelin in these neural circuits, but these changes are often discordant between and even within species. Nonetheless, by integrating the current comparative evidence, this review provides a timely insight into this field and should function to inform future studies-human, monkey and rodent-to ascertain whether or not the oligodendrocyte lineage and myelination are causally involved in the pathophysiology of stress-related neuropsychiatric disorders.

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.

Environmental enrichment initiated in adolescence restores the reduced expression of synaptophysin and GFAP in the hippocampus of chronically stressed rats in a sex-specific manner

This study aims at investigating whether environmental enrichment (EE) initiated in adolescence can alter chronic unpredictable stress (CUS)-associated changes in astroglial and synaptic plasticity markers in male and female rats. To this end, we studied possible alterations in hippocampal glial fibrillary acidic protein (GFAP) and synaptophysin (SYN) in CUS rats previously housed in EE. Wistar rats on postnatal day (PND) 23 were housed for 10 weeks in standard housing (SH) or enriched conditions. On PND 66, animals were exposed to CUS for 4 weeks. SYN and GFAP expressions were evaluated in CA1 and CA3 subfields and dentate gyrus (DG). CUS reduced the expression of SYN in all hippocampal areas, whereas lower GFAP expression was evident only in CA1 and CA3. The reduced expression of SYN in DG and CA3 was evident to male SH/CUS rats, whereas the reduced GFAP expression in CA1 and CA3 was limited to SH/CUS females. EE housing increased the hippocampal expression of both markers and protected against CUS-associated decreases. Our findings indicate that the decreases in the expression of SYN and GFAP following CUS are region and sex-specific and underline the neuroprotective role of EE against these CUS-associated changes.

Τhe neuroprotective role of environmental enrichment against behavioral, morphological, neuroendocrine and molecular changes following chronic unpredictable mild stress: A systematic review

Environmental factors interact with biological and genetic factors influencing the development and well-being of an organism. The interest in better understanding the role of environment on behavior and physiology led to the development of animal models of environmental manipulations. Environmental enrichment (EE), an environmental condition that allows cognitive and sensory stimulation as well as social interaction, improves cognitive function, reduces anxiety and depressive-like behavior and promotes neuroplasticity. In addition, it exerts protection against neurodegenerative disorders, cognitive aging and deficits aggravated by stressful experiences. Given the beneficial effects of EE on the brain and behavior, preclinical studies have focused on its protective role as an alternative, non-invasive manipulation, to help an organism to cope better with stress. A valid, reliable and effective animal model of chronic stress that enhances anxiety and depression-like behavior is the chronic unpredictable mild stress (CUMS). The variety of stressors and the unpredictability in the time and sequence of exposure to prevent habituation, render CUMS an ethologically relevant model. CUMS has been associated with dysregulation of the hypothalamic-pituitary-adrenal axis, elevation in the basal levels of stress hormones, reduction in brain volume, dendritic atrophy and alterations in markers of synaptic plasticity. Although numerous studies have underlined the compensatory role of EE against the negative effects of various chronic stress regimens (e.g. restraint and social isolation), research concerning the interaction between EE and CUMS is sparse. The purpose of the current systematic review is to present up-to-date research findings regarding the protective role of EE against the negative effects of CUMS.

Stress in adolescence as a first hit in stress-related disease development: Timing and context are crucial

The two-hit stress model predicts that exposure to stress at two different time-points in life may increase or decrease the risk of developing stress-related disorders later in life. Most studies based on the two-hit stress model have investigated early postnatal stress as the first hit with adult stress as the second hit. Adolescence, however, represents another highly sensitive developmental window during which exposure to stressful events may affect programming outcomes following exposure to stress in adulthood. Here, we discuss the programming effects of different types of stressors (social and nonsocial) occurring during adolescence (first hit) and how such stressors affect the responsiveness toward an additional stressor occurring during adulthood (second hit) in rodents. We then provide a comprehensive overview of the potential mechanisms underlying interindividual and sex differences in the resilience/susceptibility to developing stress-related disorders later in life when stress is experienced in two different life stages.

Olfactory loss is a predisposing factor for depression, while olfactory enrichment is an effective treatment for depression

The loss of olfactory stimulation correlates well with at least 68 widely differing neurological disorders, including depression, and we raise the possibility that this relationship may be causal. That is, it seems possible that olfactory loss makes the brain vulnerable to expressing the symptoms of these neurological disorders, while daily olfactory enrichment may decrease the risk of expressing these symptoms. This situation resembles the cognitive reserve that is thought to protect people with Alzheimer’s neuropathology from expressing the functional deficit in memory through the cumulative effect of intellectual stimulation. These relationships also resemble the functional response of animal models of human neurological disorders to environmental enrichment, wherein the animals continue to have the induced neuropathology, but do not express the symptoms as they do in a standard environment with restricted sensorimotor stimulation.

Sex-dependent effects of postweaning exposure to an enriched environment on visceral pain and anxiety- and depression-like behaviors induced by neonatal maternal separation

BACKGROUND

Neonatal maternal separation (NMS) can lead to visceral pain and anxiety- and depression-like behaviors. An enriched environment (EE) can alleviate NMS-induced pain and mental disorders, but previous studies have mostly been performed in male animals. Therefore, the aim of this study was to investigate whether the effects of EE were sex dependent at different stages of development.

METHODS

Female and Male C57BL/6 J mice that had been subjected to NMS alone and those subjected to both NMS and exposed to EE were used in this study. The visceral pain threshold test (PTT), open field test (OFT), sucrose preference test (SPT), and forced swimming test (FST) were conducted to evaluate visceral pain, anxiety-like behavior, and depression-like behavior in mice, respectively.

RESULTS

Compared with the male mice in the NMS group without EE exposure, those exposed to EE from postnatal day (P)21 to 41 showed an increase of the visceral pain threshold in the PTT, an increase of the central time and central distance in the OFT, an increase of the sucrose preference rate in the SPT, and a decrease of the time of immobility in the FST. Compared with both female and male mice in the NMS group without EE exposure, those exposed to EE from P21 to P61 had an increase of the visceral pain threshold in the PTT, an increase of the central time and central distance in the OFT, an increase in the sucrose preference rate in the SPT, and a decrease of the time of immobility in the FST.

CONCLUSIONS

EE is more effective in male NMS mice, while longer EE is required in female NMS mice for positive effects.

The influence of the enriched environment in different periods on neonatal maternal separation-induced visceral pain, anxiousness, and depressive behaviors

BACKGROUND

Neonatal maternal separation (NMS) is a major early life stress that can induce visceral pain and mental disorders. We have shown that an enriched environment (EE) can alleviate NMS-induced negative effects, but the time window over which EE works is unclear. Therefore, this study aimed to investigate the time window through which EE alleviates NMS-induced visceral pain, anxiousness, and depressive behaviors.

METHODS

In this study, we used male C57BL/6J mice. The mice were randomly divided into five groups: control group, NMS group, prepubertal EE group (EE1 group), pubertal EE group (EE2 group), and adult EE group (EE3 group). The visceral pain threshold test (PTT), open field test (OFT), elevated plus maze (EPM), forced swimming test (FST), and sucrose preference test (SPT) were performed in all five groups to assay visceral pain, anxiety-, and depression-like behaviors in mice, respectively. Enzyme-linked immunosorbent assay (ELISA) for corticosterone was performed in all five groups to assess the function of the hypothalamic-pituitary-adrenal (HPA) axis.

RESULTS

There was no significant change in weight between groups. It was shown that NMS induced visceral pain, anxiety, and depression, and EE1 and EE2 reversed these negative effects, but EE3 had no significant effect. Likewise, EE1 and EE2 reversed the NMS-induced increase of corticosterone, but EE3 did not.

CONCLUSIONS

Adverse life experiences in early life can lead to visceral pain, anxiety, and depression in adulthood, which can be effectively prevented by EE interventions in prepuberty and puberty.

Investigating the role of environmental enrichment initiated in adolescence against the detrimental effects of chronic unpredictable stress in adulthood: Sex-specific differences in behavioral and neuroendocrinological findings

Environmental Enrichment (EE) improves cognitive function and enhances brain plasticity, while chronic stress increases emotionality, impairs learning and memory, and has adverse effects on brain anatomy and biochemistry. We explored the beneficial role of environmental enrichment initiated in adolescence against the negative outcomes of Chronic Unpredictable Stress (CUS) during adulthood on emotional behavior, cognitive function, as well as somatic and neuroendocrine markers in both sexes. Adolescent Wistar rats housed either in enriched or standard housing conditions for 10 weeks. On postnatal day 66, a subgroup from each housing condition was daily exposed to a 4-week stress protocol. Following stress, adult rats underwent behavioral testing to evaluate anxiety, exploration/locomotor activity, depressive-like behavior and spatial learning/memory. Upon completion of behavioral testing, animals were exposed to a 10-m stressful event to test the neuroendocrine response to acute stress. CUS decreased body weight gain and increased adrenal weight. Some stress-induced behavioral adverse effects were sex-specific since learning impairments were limited to males while depressive-like behavior to females. EE housing protected against CUS-related behavioral deficits and body weight loss. Exposure to CUS affected the neuroendocrine response of males to acute stress as revealed by the increased corticosterone levels. Our findings highlight the significant role of EE in adolescence as a protective factor against the negative effects of stress and underline the importance of inclusion of both sexes in animal studies.

Juvenile Stress Exerts Sex-independent Effects on Anxiety, Antidepressant-like Behaviours and Dopaminergic Innervation of the Prelimbic Cortex in Adulthood and Does Not Alter Hippocampal Neurogenesis

Stress, particularly during childhood, is a major risk factor for the development of depression. Depression is twice as prevalent in women compared to men, which suggests that that biological sex also contributes to depression susceptibility. However, the neurobiology underpinning sex differences in the long-term consequences of childhood stress remains unknown. Thus, the aim of this study was to determine whether stress applied during the prepubertal juvenile period (postnatal day 27-29) in rats induces sex-specific changes in anxiety-like behaviour, anhedonia, and antidepressant-like behaviour in adulthood in males and females. The impact of juvenile stress on two systems in the brain associated with these behaviours and that develop during the juvenile period, the mesocorticolimbic dopaminergic system and hippocampal neurogenesis, were also investigated. Juvenile stress altered escape-oriented behaviours in the forced swim test in both sexes, decreased latency to drink a palatable substance in a novel environment in the novelty-induced hypophagia test in both sexes, and decreased open field supported rearing behavior in females. These behavioural changes were accompanied by stress-induced increases in tyrosine hydroxylase immunoreactivity in the prefrontal cortex of both sexes, but not other regions of the mesocorticolimbic dopaminergic system. Juvenile stress did not impact anhedonia in adulthood as measured by the saccharin preference test and had no effect hippocampal neurogenesis across the longitudinal axis of the hippocampus. These results suggest that juvenile stress has long-lasting impacts on antidepressant-like and reward-seeking behaviour in adulthood and these changes may be due to alterations to catecholaminergic innervation of the medial prefrontal cortex.

Environmental enrichment rescues survival and function of adult-born neurons following early life stress

Adverse experiences early in life are associated with the development of psychiatric illnesses. The hippocampus is likely to play pivotal role in generating these effects: it undergoes significant development during childhood and is extremely reactive to stress. In rodent models, stress in the pre-pubertal period impairs adult hippocampal neurogenesis (AHN) and behaviours which rely on this process. In normal adult animals, environmental enrichment (EE) is a potent promoter of AHN and hippocampal function. Whether exposure to EE during adolescence can restore normal hippocampal function and AHN following pre-pubertal stress (PPS) is unknown. We investigated EE as a treatment for reduced AHN and hippocampal function following PPS in a rodent model. Stress was administered between post-natal days (PND) 25-27, EE from PND 35 to early adulthood, when behavioural testing and assessment of AHN took place. PPS enhanced fear reactions to a conditioned stimulus (CS) following a trace fear protocol and reduced the survival of 4-week-old adult-born neurons throughout the adult hippocampus. Furthermore, we show that fewer adult-born neurons were active during recall of the CS stimulus following PPS. All effects were reversed by EE. Our results demonstrate lasting effects of PPS on the hippocampus and highlight the utility of EE during adolescence for restoring normal hippocampal function. EE during adolescence is a promising method of enhancing impaired hippocampal function resulting from early life stress, and due to multiple benefits (low cost, few side effects, widespread availability) should be more thoroughly explored as a treatment option in human sufferers of childhood adversity.

Effects of enriched environment on depression and anxiety-like behavior induced by early life stress: A comparison between different periods

BACKGROUND

Brain development is a prolonged process and it is sensitive to the environment during critical periods. Stress in early life is believed to increase vulnerability to depression, while enriched environment (EE) has beneficial effects on neural plasticity and depression. In this study, we compared the therapeutic effect of EE during different periods on early life stress-induced depression, and investigated the role of brain-derived neurotrophic factor (BDNF) and protein kinase B (AKT) on the effect of EE. Plasma corticosterone level was also detected to evaluate the reactivity of hypothalamic-pituitary-adrenal axis.

METHODS

C57BL/6 mice were subjected to a 4-h maternal separation (MS) procedure during postnatal days 2-21. After this separation, the mice were assigned to standard environment groups (SE), EE in the early period groups (3-8 weeks, EEE) and EE in the adult groups (8-13 weeks, EEA). Depression and anxiety behavior were evaluated at 14-weeks of age. The plasma corticosterone was quantified utilizing enzyme-linked immunosorbent assay. Hippocampus BDNF and AKT/p-AKT were detected using Western blotting.

RESULTS

The results showed that MS increased depression and anxiety level, while EE in both intervention periods alleviated the symptoms of depression and anxiety. The EEE group showed better effects in terms of anhedonia and anxiety than the EEA group. The difference in despair behavior between the EEE and EEA groups was not significant. MS increased plasma corticosterone level, while EE decreased corticosterone level in both intervention periods. EE increased BDNF and p-AKT expression in the hippocampus, with stronger effects in the EEE group.

CONCLUSION

EE during the early development period was more effective in alleviating depression and anxiety induced by early life stress. BDNF and AKT may play a significant role in the effect of EE, and further research is needed to explore the detailed neurobiological mechanisms.

Environmental enrichment modulates HPA axis reprogramming in adult male rats exposed to early adolescent stress

Exposure to early stressful events increases susceptibility to post-traumatic stress disorder (PTSD) in adulthood, in which the hypothalamic-pituitary-adrenal (HPA) axis plays a crucial role. Studies have found that environmental enrichment (EE) mitigates the detrimental outcomes of early adversity. However, the HPA-related mechanism remains unclear. In this study, we used the single prolonged stress (SPS) paradigm to explore the long-term effects of early adolescent stress on behavior, HPA axis activity, as well as expression levels of the glucocorticoid receptor (GR), mineralocorticoid receptor (MR), corticotropin-releasing hormone receptor 1 (CRF1R) and CRF2R in the hypothalamus and hippocampus. Meanwhile, the protective effects of EE intervention were examined. We found that adult male rats exposed to adolescent stress showed reduced locomotor activity, increased anxiety-like behaviors, enhanced contextual fear memory, elevated basal plasma ACTH levels, and enhanced HPA negative feedback inhibition, as indicated by decreased plasma ACTH levels in the dexamethasone suppression test (DST). Furthermore, EE normalized the behavioral abnormalities and enhanced HPA negative feedback in stressed rats, possibly through down-regulating GR expression in the hippocampus and hypothalamus. These findings suggested that EE could ameliorate adolescent stress-induced PTSD-like behaviors and aberrant reprogramming of the HPA axis, reducing the risk of developing PTSD in adulthood.

The downstream effects of forced exercise training and voluntary physical activity in an enriched environment on hippocampal plasticity in preadolescent rats

During critical periods of brain development, exercise-induced physical fitness may greatly impact the brain structure and function. Nevertheless, forced and intensive physical activities may display negative effects, particularly in the pre-pubertal period. Preadolescent rats were exposed to an enriched environment and combined exercise training for three consecutive weeks in the present study. There was a large cage with enriching stimuli and voluntary physical activity opportunities as an enriched environment (EE). The combined exercise training (CET) consisted of aerobic and resistance training programs. The protein levels of corticosterone (CORT), glucocorticoid receptors (GR_s), insulin-like growth factor-1 (IGF-1), brain-derived neurotrophic factor (BDNF), and vascular endothelial growth factor (VEGF) were assessed using Enzyme-linked immunosorbent assay and western blotting. Cresyl violet staining was also used to evaluate the number of cells in the hippocampus. While GR_s levels were significantly increased in both EE and CET groups (P < 0.001), decreased CORT levels were found in enriched rats (P < 0.05). Moreover, elevated BDNF levels were found in the EE (P < 0.01) and CET (P < 0.05) groups. Similarly, VEGF significantly increased in the EE (P < 0.01) and CET (P < 0.05) animals. However, IGF-1 levels were high only in trained rats (P < 0.05). The number of cells also significantly increased in the DG and CA1 region of the hippocampus after each intervention (P < 0.001). These findings clarified that combined exercise training and voluntary physical activity in an enriched environment during the preadolescent period might promote the downstream plasticity effects on the hippocampus.

Understanding stress: Insights from rodent models

Through incorporating both physical and psychological forms of stressors, a variety of rodent models have provided important insights into the understanding of stress physiology. Rodent models also have provided significant information with regards to the mechanistic basis of the pathophysiology of stress-related disorders such as anxiety disorders, depressive illnesses, cognitive impairment and post-traumatic stress disorder. Additionally, rodent models of stress have served as valuable tools in the area of drug screening and drug development for treatment of stress-induced conditions. Although rodent models do not accurately reproduce the biochemical or physiological parameters of stress response and cannot fully mimic the natural progression of human disorders, yet, animal research has provided answers to many important scientific questions. In this review article, important studies utilizing a variety of stress models are described in terms of their design and apparatus, with specific focus on their capabilities to generate reliable behavioral and biochemical read-out. The review focusses on the utility of rodent models by discussing examples in the literature that offer important mechanistic insights into physiologically relevant questions. The review highlights the utility of rodent models of stress as important tools for advancing the mission of scientific research and inquiry.

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Stressful life events may lead to the onset of severe psychopathologies in humans.

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Rodents may model many features of stress exposure in human populations.

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Induction of stress via pharmacological and psychological manipulations alter rodent behavior.

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Mechanistic rodent studies reveal key molecular targets critical for new therapeutic targets.

Juvenile stress facilitates safety learning in male and female high alcohol preferring mice

Adversities during juvenility increase the risk for stress-related disorders, such as post-traumatic stress disorder (PTSD) and alcohol use disorder. However, stress can also induce coping mechanisms beneficial for later stressful experiences. We reported previously that mice selectively bred for high alcohol preference (HAP) exposed to stress during adolescence (but not during adulthood) showed enhanced fear-conditioned responses in adulthood, as measured by fear-potentiated startle (FPS). However, HAP mice also showed enhanced responding to safety cues predicting the absence of foot shocks in adulthood. Here, we pursue these findings in HAP mice by investigating in further detail how juvenile stress impacts the acquisition of safety and fear learning. HAP mice were subjected to three days of juvenile stress (postnatal days 25, 27, 28) and discriminative safety/fear conditioning in adulthood. FPS was used to assess safety versus fear cue discrimination, fear learning, and fear inhibition by the safety cue. Both stressed and unstressed HAP mice were able to discriminate between both cues as well as learn the fear cue-shock association. Interestingly, it was only the previously stressed mice that were able to inhibit their fear response when the fear cue was co-presented with the safety cue, thus demonstrating safety learning. We also report an incidental finding of alopecia in the juvenile stress groups, a phenotype seen in stress-related disorders. These results in HAP mice may be relevant to understanding the influence of juvenile trauma for individual risk and resilience toward developing PTSD and how individuals might benefit from safety cues in behavioral psychotherapy.

Examining the central effects of chronic stressful social isolation on rats

Stress-related disorders are extremely complex and current treatment strategies have limitations. The present study investigated alternative pathological mechanisms using a combination of multiple environmental approaches with biochemical and molecular tools. The aim of the present study was to evaluate blood-brain-barrier (BBB) integrity in socially manipulated animal housing conditions. Multiple environmentally-related models were employed in the current study. The main model proposed (chronically isolated rats) was biochemically validated using the level of peripheral corticosterone. The current study examined and compared the mRNA levels of certain inflammatory and BBB markers in the hippocampal tissue of chronically isolated rats, including claudin-5 (cldn5) and tight junction protein (tjp). Animals were divided into four groups: i) Standard housed rats (controls); ii) chronically isolated rats; iii) control rats treated with fluoxetine, which is a standard selective serotonin reuptake inhibitor; and iv) isolated rats treated with fluoxetine. To further examine the effect of environmental conditions on BBB markers, the current study assessed BBB markers in enriched environmental (EE) housing and short-term isolation conditions. The results demonstrated a significant increase in cldn5 and tjp levels in the chronically isolated group. Despite some anomalous results, alterations in mRNA levels were further confirmed in EE housing conditions compared with chronically isolated rats. This trend was also observed in rats subjected to short-term isolation compared with paired controls. Additionally, levels of IL-6, an inflammatory marker associated with neuroinflammation, were markedly increased in the isolated group. However, treatment with fluoxetine treatment reversed these effects. The results indicated that BBB integrity may be compromised in stress-related disorders, highlighting a need for further functional studies on the kinetics of BBB in stress-related models.

Enriched environment effect on lipopolysaccharide-induced spatial learning, memory impairment and hippocampal inflammatory cytokine levels in male rats

Neuro-inflammation is responsible for cognitive impairments and neurodegenerative diseases such as Alzheimer's disease. In this study, we aimed to investigate the enriched environment (EE) effect on learning and memory impairment as well as on pro-inflammatory cytokines changes induced by lipopolysaccharide (LPS). LPS injection (1 mg/kg/i.p, days 1, 3, 5, and 7) was used to develop the animal model of neuro-inflammation. Twenty-eight male Wistar rats were used in the experiment and randomly divided into 4 groups: 1) sham (S), 2) sham + enriched environment (SE), 3) LPS (L), and 4) LPS + EE (LE). Two different housing conditions, including standard environment (SE) and enriched environment, were used. The Morris Water Maze (MWM) test was used to examine animals learning and memory. IL-1β, IL-10, and TNF-α levels were measured in the brain using ELISA. We found that LPS significantly impaired learning and memory (p < 0.05) in the MWM task, but EE could significantly improve learning and memory impairment (p < 0.05). IL-1 and IL-10 levels dramatically increased in the LPS group (P < 0.05), whereas EE could decrease and increase IL-1β and IL-10 values in the LPS + EE group (P < 0.05), respectively. TNF-α levels were traced but had not detectable values in the hippocampus. Thus, we can conclude that EE has healing effects on LPS induced neuro-inflammation and can improve learning and memory deficit; however, further studies are needed to support the findings of our study.

Lasting Impact of Chronic Adolescent Stress and Glucocorticoid Receptor Selective Modulation in Male and Female Rats

Adolescent animals are vulnerable to the effects of stress on brain development. We hypothesized that long-term effects of adolescent chronic stress are mediated by glucocorticoid receptor (GR) signaling. We used a specific GR modulator (CORT108297) to pharmacologically disrupt GR signaling in adolescent rats during exposure to chronic variable stress (CVS). Male and female rats received 30 mg/kg of drug during a 2-week CVS protocol starting at PND46. Emotional reactivity (open field) and coping behaviors (forced swim test (FST)) were then tested in adulthood, 5 weeks after the end of the CVS protocol. Blood samples were collected two days before FST and serial samples after the onset of the swim test to determine baseline and stress response levels of HPA hormones respectively. Our results support differential behavioral, physiological and stress circuit reactivity to adolescent chronic stress exposure in males and females, with variable involvement of GR signaling. In response to adolescent stress, males had heightened reactivity to novelty and exhibited marked reduction in neuronal excitation following swim stress in adulthood, whereas females developed a passive coping strategy in the FST and enhanced HPA axis stress reactivity. Only the latter effect was attenuated by treatment with the GR modulator C108297. In summary, our data suggest that adolescent stress differentially affects emotional behavior and circuit development in males and females, and that GR manipulation during stress can reverse at least some of these effects.

Chronic unpredictable intermittent restraint stress disrupts spatial memory in male, but not female rats

Chronic stress leads to sex-dependent outcomes on spatial memory by producing deficits in males, but not in females. Recently it was reported that compared to daily restraint, intermittent restraint (IR) produced more robust stress and anxiety responses in male rats. Whether IR would be sufficiently robust to impair hippocampal-dependent spatial memory in both male and female rats was investigated. IR involved mixing restraint with non-restraint days over weeks before assessing spatial memory and anxiety profile on the radial arm water maze, object placement, novel object recognition, Y-maze, open field and novelty suppressed feeding. Experiments 1 and 2 used Sprague-Dawley male rats only and determined that IR for 6 h/d (IR6), but not 2 h/d, impaired spatial memory and that task order was important. In experiment 3, IR6 was extended for 6wks before spatial memory testing commenced using both sexes. Unexpectedly, an extended IR6 paradigm failed to impair spatial memory in either sex, suggesting that by 6wks IR6 may have become predictable. In experiment 4, an unpredictable IR (UIR) paradigm was implemented, in which restraint duration (30 or 60-min) combined with orbital shaking, time of day, and the days off from UIR were varied. UIR impaired spatial memory in males, but not in females. Together with other reports, these findings support the interpretation that chronic stress negatively impairs hippocampal-dependent function in males, but not in females. We interpret these findings to show that females are more resilient to chronic stress than are males as it pertains to spatial ability.

Developmental effects of environmental enrichment on selective and auditory sustained attention

Environmental enrichment (EE) has been used as a positive manipulation in different disease models. However, there is conflicting evidence reported in the literature about the effects of EE. Additionally, the time period that would be most beneficial in implementing environmental enrichment as an intervention is not clear. Our study aimed to systematically compare the prenatal, juvenile, mid-adolescence, and adulthood developmental trajectory to further the understanding of enriched environment's effects on selective and auditory sustained attention, corresponding to behavioral (conceived) and physiological-reflexive (non-conceived) measures. Rats were exposed for 21 days to enriched environment during various developmental periods and compared to age-matched controls. All groups were tested for long-term effects (at postnatal day 120 and onward) on selective and sustained attention. We found that the exposure to enriched environment during mid-adolescence has yielded the most significant and long-term pattern of effects, including selective and auditory sustained attention performance, increased foraging-like behavior and a significant decrease in corticosterone level. Similarly, the exposure to EE at juvenile period improved selective attention, increased foraging-like behavior, and reduced anxiety levels as reflected in the open field as well as in low corticosterone levels. These results specify a crucial period along the developmental trajectory for applying environmental enrichment. Mid-adolescence is suggested, in future basic and translational studies, as the sensitive time period that induces the most beneficial and long-term effects of EE on attention. The current findings suggest that the exposure to EE during mid-adolescence should be further considered and studied as behavioral alternative intervention, or as adjuvant behavioral therapy, aimed to decrease the probability to develop ADHD in post-adolescence period. This suggestion is highly relevant due to the debate regarding the pros and cons of screens usage (e.g. Facebook, online games, etc.) during early life that decreases environmental enrichment, especially, direct social interaction.

Protective Effects of Spirulina platensis, Voluntary Exercise and Environmental Interventions Against Adolescent Stress-Induced Anxiety and Depressive-Like Symptoms, Oxidative Stress and Alterations of BDNF and 5HT-3 Receptors of the Prefrontal Cortex in Female Rats

BACKGROUND AND PURPOSE

Ample evidence indicates that chronic adolescence stress is associated with an increased risk of developing neuropsychiatric disorders in adulthood. Given the importance of the effective therapeutic ways to overcome adolescent stress-related deficits, the present study investigated the effects of Spirulina platensis (SP), environmental enrichment (EE), and voluntary exercise (EX) and their combination on anxiety or depression-like behaviors, oxidative stress, and alterations of BDNF and 5HT-3 receptors in the prefrontal cortex (PFC) induced by adolescent stress in adult female rats.

METHODS

During the adolescent period (PNDs30-40), rats were subjected to restraint stress. Then, the animals were subjected to SP treatment (200 mg/kg/day), EX, EE, and the combined treatments (SP+EX, and SP+EE) for 15 days between PNDs41-55. Subsequently, anxiety or depression-like behaviors, BDNF levels, oxidative stress markers and mRNA expression of BDNF and 5HT₃ in the PFC were assessed.

RESULTS

Stressed rats demonstrated enhanced anxiety levels and depression-like behaviors in adulthood. Regarding the oxidative stress markers, stressed rats exhibited significantly higher levels of malondialdehyde, a lipid peroxidation product, higher activities of antioxidant enzymes (glutathione peroxidase and superoxide dismutase) and significantly lower total antioxidant reactivity capacity in the PFC. Additionally, adolescent stress significantly increased 5HT3 receptor mRNA expression and decreased BDNF content and its mRNA expression in the PFC. Treatments with SP, EX, EE, and the combined interventions alleviated these deficits.

CONCLUSION

Our findings indicate that appropriate interventions during the adolescent period can protect against adolescent stress-induced behavioral, and biochemical defects and oxidative stress damage in adulthood.

The role of the GABAA receptor Alpha 1 subunit in the ventral hippocampus in stress resilience

Pre-pubertal stress increases post-traumatic stress disorder (PTSD) susceptibility. We have previously demonstrated that enriched environment (EE) intervention immediately after pre-pubertal stress protects from the effects of trauma in adulthood. Here, we examined whether exposure to EE would also be beneficial if applied after exposure to trauma in adulthood. We have recently shown that exposure to juvenile stress and under-water trauma (UWT) is associated with increased expression of GABA_A receptor subunit α1 in the ventral hippocampus. However, differentiating between affected and unaffected individuals, this increased expression was confined to stress-exposed, behaviorally unaffected individuals, suggesting upregulation of α1 expression as a potential mechanism of resilience. We now examined whether EE-induced resilience renders increased expression of α1 in the ventral hippocampus redundant when facing a trauma later in life. Adult rats were exposed to UWT, with pre-exposure to juvenile stress, and tested in the open field and elevated plus maze paradigms four weeks later. EE exposure during juvenility prevented pre-pubertal stress-induced vulnerability, but not if performed following UWT in adulthood. Furthermore, juvenile EE exposure prevented the trauma-associated increase in α1 expression levels. Our findings emphasize the importance of early interventions in order to reduce the likelihood of developing psychopathologies in adulthood.

Stressor during Early Adolescence in Hyperreactive Female Wistar Kyoto Rats Induces a 'Double Hit' Manifested by Variation in Neurobehaviors and Brain Monoamines

Stress is an additive factor in the development of depressive-like profiles that mainly onsets during adolescence. However, effects of early post-weaning stress on developing brain neurochemical pathways in inducing anxiety- and depressive-like profiles in vulnerable females have not been extensively studied. The Wistar Kyoto (WKY) rat, a putative model of adolescent depression and stress-sensitivity could elucidate the pathophysiology of stress-related depression in vulnerability. Through such an approach, links between inherent risk for predisposition to depression and homotypic stress, as in a 'double hit' would unravel endocrine regulation, interference in developing neural pathways and neurobehaviors. Here, early adolescent WKY female rats were subjected to 1-h physical restraint over 7 days followed by neurobehavioral testing in the elevated plus maze (EPM) and forced swim test (FST). The stressor's effectiveness was assayed by plasma corticosterone (CORT) and altered functioning in depression-implicated brain areas by assaying monoamines/metabolites. Homotypic stress induced an anxiolytic-like response in the EPM with learned helplessness and reduced struggling behavior in FST. Significant elevation in CORT levels (p < 0.05) indicated an upregulated HPA axis. Medial prefrontal cortex, a still maturing brain area, exhibited increased serotonin (5-HT) metabolite (p < 0.01) and turnover rates (p < 0.01) indicative of altered/maladaptive serotonergic functioning. Nucleus accumbens (p < 0.05) and dorsal striatum (p < 0.01) also depicted increased 5-HT metabolite, with the latter also demonstrating reduced Dopamine turnover (p < 0.01) as a result of homotypic stress. Hence, female WKY rats could constitute a diathesis-stress model to study underlying mechanisms of stress-related depression.

Beneficial effects of Spirulina platensis, voluntary exercise and environmental enrichment against adolescent stress induced deficits in cognitive functions, hippocampal BDNF and morphological remolding in adult female rats

Chronic exposure to stress during adolescent period has been demonstrated to impair cognitive functions and the dendritic morphology of pyramidal neurons in the rat hippocampal CA3 area. The present study investigated the combined protective effects of Spirulina platensis (SP), a supplement made from blue-green algae with neuroprotective properties, voluntary exercise (EX) and environmental enrichment (EE) against cognitive deficits, alternations in hippocampal BDNF levels, and abnormal neuronal remodeling in adult female rats (PND 60) induced by exposure to chronic restraint stress during adolescent period (PND 30-40). Rats were exposed to restraint stress (2 h/day for 10 days, PND 30-40). Then, the animals were subjected to treatment with SP (200 mg/kg/day), EX, EE and the combined treatments (SP + EX, and SP + EE) between PND 41 and 55 of age. Following the interventions, spatial learning and memory, passive avoidance performance, hippocampal dendritic morphology and BDNF levels were assessed. Results showed that plasma corticosterone levels increased at PND 40 and remained elevated at PND 55 and 70 in the stressed rats. Stressed rats showed deficits in spatial learning and memory and passive avoidance performance, decreased BDNF levels in the hippocampus, and reduced apical dendritic length and branch points of the CA3 pyramidal neurons. These deficits were alleviated by the SP, EX and EE, and the combined treatments, which accompanied with a decline in serum corticosterone in stressed animals. Some treatments even enhanced cognitive functions, and BDNF levels and neuroanatomical remodeling in the hippocampus of non-stressed animals. Our findings provide important evidences that physical activity, exposure to EE, and the SP treatment during adolescent period can protect against adolescent stress induced behavioral, biochemical and neuroanatomical impairments in adulthood.

Adolescent conditioning affects rate of adult fear, safety and reward learning during discriminative conditioning

Fear and reward memories formed in adulthood are influenced by prior experiences. Experiences that occur during sensitive periods, such as adolescence, can have an especially high impact on later learning. Fear and reward memories form when aversive or appetitive events co-occur with initially neutral stimuli, that then gain negative or positive emotional load. Fear and reward seeking behaviours are influenced by safety cues, signalling the non-occurrence of a threat. It is unclear how adolescent fear or reward pre-conditioning influences later dynamics of these conditioned emotions, and conditioned safety. In this study, we presented male rats with adolescent fear or reward pre-conditioning, followed by discriminative conditioning in adulthood. In this discriminative task, rats are simultaneously conditioned to reward, fear and safety cues. We show that adolescent reward pre-conditioning did not affect the rate of adult reward conditioning, but instead accelerated adult safety conditioning. Adolescent fear pre-conditioning accelerated adult fear and reward seeking behaviours but delayed adult safety expression. Together, our results suggest that the dynamics of safety conditioning can be influenced by adolescent priming of different valences. Taking adolescent experiences into consideration can have implications on how we approach therapy options for later learned fear disorders where safety learning is compromised.

Enriched Environment Facilitates Anxiolytic Efficacy Driven by Deep-Brain Stimulation of Medial Prefrontal Cortex

Deep brain stimulation (DBS) is a widely used treatment for neurodegenerative disorders like Parkinson’s disease. Recently, several studies have used preclinical animal models to suggest that DBS has a potential to improve emotional symptoms in mental disorders such as treatment-resistant depression and post-traumatic stress disorder. An important difference between neurodegenerative and emotional disorders is the crucial role of environment in the ontogeny of the latter. Thus, it is important to understand the effects of DBS in the context of environmental variation. In this study, we show that DBS of ventromedial prefrontal cortex reduces anxiety in rats when it is coupled with simultaneous exposure to an enriched environment (EE). In contrast, effects of DBS on anxiety-like behaviors remained equivocal when animals were housed in standard laboratory conditions. These results suggest that the ability of DBS to treat anxiety and related phenotypes can be significantly enhanced by EE opportunities.

Beneficial effects of environmental enrichment on behavior, stress reactivity and synaptophysin/BDNF expression in hippocampus following early life stress

Exposure to environmental enrichment can beneficially influence the behavior and enhance synaptic plasticity. The aim of the present study was to investigate the mediated effects of environmental enrichment on postnatal stress-associated impact with regard to behavior, stress reactivity as well as synaptic plasticity changes in the dorsal hippocampus. Wistar rat pups were submitted to a 3 h maternal separation (MS) protocol during postnatal days 1-21, while another group was left undisturbed. On postnatal day 23, a subgroup from each rearing condition (maternal separation, no-maternal separation) was housed in enriched environmental conditions until postnatal day 65 (6 weeks duration). At approximately three months of age, adult rats underwent behavioral testing to evaluate anxiety (Elevated Plus Maze), locomotion (Open Field Test), spatial learning and memory (Morris Water Maze) as well as non-spatial recognition memory (Novel Object Recognition Test). After completion of behavioral testing, blood samples were taken for evaluation of stress-induced plasma corticosterone using an enzyme-linked immunosorbent assay (ELISA), while immunofluorescence was applied to evaluate hippocampal BDNF and synaptophysin expression in dorsal hippocampus. We found that environmental enrichment protected against the effects of maternal separation as indicated by the lower anxiety levels and the reversal of spatial memory deficits compared to animals housed in standard conditions. These changes were associated with increased BDNF and synaptophysin expression in the hippocampus. Regarding the neuroendocrine response to stress, while exposure to an acute stressor potentiated corticosterone increases in maternally-separated rats, environmental enrichment of these rats prevented this effect. The current study aimed at investigating the compensatory role of enriched environment against the negative outcomes of adverse experiences early in life concurrently on emotional and cognitive behaviors, HPA function and neuroplasticity markers.

Fluoxetine treatment is effective in a rat model of childhood-induced post-traumatic stress disorder

Although selective serotonin reuptake inhibitors (SSRIs) are first-line treatment for post-traumatic stress disorder (PTSD) patients, their therapeutic efficacy is limited. Childhood adversities are considered a risk factor for developing PTSD in adulthood but may trigger PTSD without additional trauma in some individuals. Nevertheless, just as childhood is considered a vulnerable period it may also be an effective period for preventive treatment. Using a rat model of childhood-induced PTSD, pre-pubertal stress (juvenile stress, JVS), we compared the therapeutic effects of fluoxetine and examined the effectiveness of 1 month of fluoxetine treatment following JVS and into adulthood compared to treatment in adulthood. Since not all individuals develop PTSD following a trauma, comparing only group means is not the adequate type of analysis. We employed a behavioral profiling approach, which analyzes individual differences compared to the normal behavior of a control group. Animals exposed to JVS exhibited a higher proportion of affected animals as measured using the elevated plus maze 8 weeks after JVS. Fluoxetine treatment following the JVS significantly decreased the proportion of affected animals as measured in adulthood. Fluoxetine treatment in adulthood was not effective. The results support the notion that childhood is not only a vulnerable period but also an effective period for preventive treatment.

Protective Effects of Enriched Environment Against Transient Cerebral Ischemia-Induced Impairment of Passive Avoidance Memory and Long-Term Potentiation in Rats

Introduction

Enriched Environment (EE), a complex novel environment, has been demonstrated to improve synaptic plasticity in both injured and intact animals. The present study investigated the capacity of an early environmental intervention to normalize the impairment of passive avoidance memory and Long-Term Potentiation (LTP) induced by transient bilateral common carotid artery occlusion (2-vessel occlusion, 2VO) in rats.

Methods

After weaning, young Wistar rats (22 days old) were housed in EE or Standard Environment (SE) for 40 days. Transient (30-min) incomplete forebrain ischemia was induced 4 days before the passive avoidance memory test and LTP induction.

Results

The transient forebrain ischemia led to impairment of passive avoidance memory and LTP induction in the Perforant Path-Dentate Gyrus (PP-DG) synapses. Interestingly, housing and growing in EE prior to 2VO was found to significantly reverse 2VO-induced cognitive and LTP impairments.

Conclusion

Our results suggest that early housing and growing in EE exhibits therapeutic potential to normalize cognitive and LTP abnormalities induced by 2VO ischemic model in rats.

Stress and adolescent hippocampal neurogenesis: diet and exercise as cognitive modulators

Adolescence is a critical period for brain maturation. Deciphering how disturbances to the central nervous system at this time affect structure, function and behavioural outputs is important to better understand any long-lasting effects. Hippocampal neurogenesis occurs during development and continues throughout life. In adulthood, integration of these new cells into the hippocampus is important for emotional behaviour, cognitive function and neural plasticity. During the adolescent period, maturation of the hippocampus and heightened levels of hippocampal neurogenesis are observed, making alterations to neurogenesis at this time particularly consequential. As stress negatively affects hippocampal neurogenesis, and adolescence is a particularly stressful time of life, it is important to investigate the impact of stressor exposure at this time on hippocampal neurogenesis and cognitive function. Adolescence may represent not only a time for which stress can have long-lasting effects, but is also a critical period during which interventions, such as exercise and diet, could ameliorate stress-induced changes to hippocampal function. In addition, intervention at this time may also promote life-long behavioural changes that would aid in fostering increased hippocampal neurogenesis and cognitive function. This review addresses both the acute and long-term stress-induced alterations to hippocampal neurogenesis and cognition during the adolescent period, as well as changes to the stress response and pubertal hormones at this time which may result in differential effects than are observed in adulthood. We hypothesise that adolescence may represent an optimal time for healthy lifestyle changes to have a positive and long-lasting impact on hippocampal neurogenesis, and to protect against stress-induced deficits. We conclude that future research into the mechanisms underlying the susceptibility of the adolescent hippocampus to stress, exercise and diet and the consequent effect on cognition may provide insight into why adolescence may be a vital period for correct conditioning of future hippocampal function.

Neurobiological consequences of juvenile stress: A GABAergic perspective on risk and resilience

ALBRECHT, A., MÜLLER, I., ARDI, Z., ÇALIŞKAN, G., GRUBER, D., IVENS, S., SEGAL, M., BEHR, J., HEINEMANN, U., STORK, O., and RICHTER-LEVIN, G. Neurobiological consequences of juvenile stress: A GABAergic perspective on risk and resilience. NEUROSCI BIOBEHAV REV XXX-XXX, 2016.- Childhood adversity is among the most potent risk factors for developing mood and anxiety disorders later in life. Therefore, understanding how stress during childhood shapes and rewires the brain may optimize preventive and therapeutic strategies for these disorders. To this end, animal models of stress exposure in rodents during their post-weaning and pre-pubertal life phase have been developed. Such 'juvenile stress' has a long-lasting impact on mood and anxiety-like behavior and on stress coping in adulthood, accompanied by alterations of the GABAergic system within core regions for the stress processing such as the amygdala, prefrontal cortex and hippocampus. While many regionally diverse molecular and electrophysiological changes are observed, not all of them correlate with juvenile stress-induced behavioral disturbances. It rather seems that certain juvenile stress-induced alterations reflect the system's attempts to maintain homeostasis and thus promote stress resilience. Analysis tools such as individual behavioral profiling may allow the association of behavioral and neurobiological alterations more clearly and the dissection of alterations related to the pathology from those related to resilience.

Short environmental enrichment in adulthood reverses anxiety and basolateral amygdala hypertrophy induced by maternal separation

Maternal separation during early childhood results in greater sensitivity to stressors later in adult life. This is reflected as greater propensity to develop stress-related disorders in humans and animal models, including anxiety and depression. Environmental enrichment (EE) reverses some of the damaging effects of maternal separation in rodent models when provided during peripubescent life, temporally proximal to the separation. It is presently unknown if EE provided outside this critical window can still rescue separation-induced anxiety and neural plasticity. In this report we use a rat model to demonstrate that a single short episode of EE in adulthood reduced anxiety-like behaviour in maternally separated rats. We further show that maternal separation resulted in hypertrophy of dendrites and increase in spine density of basolateral amygdala neurons in adulthood, long after initial stress treatment. This is congruent with prior observations showing centrality of basolateral amygdala hypertrophy in anxiety induced by stress during adulthood. In line with the ability of the adult enrichment to rescue stress-induced anxiety, we show that enrichment renormalized stress-induced structural expansion of the amygdala neurons. These observations argue that behavioural plasticity induced by early adversity can be rescued by environmental interventions much later in life, likely mediated by ameliorating effects of enrichment on basolateral amygdala plasticity.

Individual differences in behavioural plasticities

Interest in individual differences in animal behavioural plasticities has surged in recent years, but research in this area has been hampered by semantic confusion as different investigators use the same terms (e.g. plasticity, flexibility, responsiveness) to refer to different phenomena. The first goal of this review is to suggest a framework for categorizing the many different types of behavioural plasticities, describe examples of each, and indicate why using reversibility as a criterion for categorizing behavioural plasticities is problematic. This framework is then used to address a number of timely questions about individual differences in behavioural plasticities. One set of questions concerns the experimental designs that can be used to study individual differences in various types of behavioural plasticities. Although within-individual designs are the default option for empirical studies of many types of behavioural plasticities, in some situations (e.g. when experience at an early age affects the behaviour expressed at subsequent ages), 'replicate individual' designs can provide useful insights into individual differences in behavioural plasticities. To date, researchers using within-individual and replicate individual designs have documented individual differences in all of the major categories of behavioural plasticities described herein. Another important question is whether and how different types of behavioural plasticities are related to one another. Currently there is empirical evidence that many behavioural plasticities [e.g. contextual plasticity, learning rates, IIV (intra-individual variability), endogenous plasticities, ontogenetic plasticities) can themselves vary as a function of experiences earlier in life, that is, many types of behavioural plasticity are themselves developmentally plastic. These findings support the assumption that differences among individuals in prior experiences may contribute to individual differences in behavioural plasticities observed at a given age. Several authors have predicted correlations across individuals between different types of behavioural plasticities, i.e. that some individuals will be generally more plastic than others. However, empirical support for most of these predictions, including indirect evidence from studies of relationships between personality traits and plasticities, is currently sparse and equivocal. The final section of this review suggests how an appreciation of the similarities and differences between different types of behavioural plasticities may help theoreticians formulate testable models to explain the evolution of individual differences in behavioural plasticities and the evolutionary and ecological consequences of individual differences in behavioural plasticities.

Neurogenesis within the adult hippocampus under physiological conditions and in depression

Adult neurogenesis can only be observed in some specific brain regions. One of these areas is the dentate gyrus of the hippocampal formation. The progenitor cells located in the subgranular layer of the dentate gyrus proliferate, differentiate, and give rise to young neurons that can become integrated into existing neuronal circuits. Under physiological conditions, hippocampal neurogenesis is linked to hippocampal-dependent learning, whereas deficits in adult hippocampal neurogenesis have been shown to correlate with disturbances in spatial learning and memory. This review summarizes the phenomenon of adult hippocampal neurogenesis and the use of suitable markers for the investigation of adult hippocampal neurogenesis. In addition, we focused on the disturbances in neurogenesis that can be seen in depression. Interestingly, several antidepressants have been found to be capable of increasing the rate of hippocampal neurogenesis. Based on that, it can be speculated that factors, which directly or indirectly increase the rate of hippocampal neurogenesis, may be helpful in the treatment of depression.

Differential impact of stress on hypothalamic-pituitary-adrenal axis: gene expression changes in Lewis and Fisher rats

The aim of the present work was to study the influence of variable stress on the expression of 11β-hydroxysteroid dehydrogenase type 1 (11HSD1) and the neuropeptides corticotropin-releasing hormone (CRH), urocortins 2 and 3(UCN2, UCN3), arginine vasopressin (AVP), oxytocin (OXT) and adenylate cyclase-activating polypeptide (PACAP) in two inbred rat strains: stress hypo-responsive Lewis (LEW) and hyper-responsive Fisher 344 (F344) rats. We found site-specific and strain-dependent differences in the basal and stress-stimulated expression of 11HSD1, CRH, UCN2, UCN3 and PACAP. In LEW rats, stress upregulated 11HSD1 in the prefrontal cortex and lateral amygdala, whereas in F344 rats 11HSD1 was upregulated in the central amygdala and hippocampal CA2 and ventral but not dorsal CA1 region; no effect was observed in the paraventricular nucleus, pituitary gland and adrenal cortex of both strains. The expression of glucocorticoid receptors did not parallel the upregulation of 11HSD1. Stress also stimulated the expression of paraventricular OXT, CRH, UCN3 and PACAP in both strains but amygdalar CRH only in LEW and UCN2/UCN3 in F344 rats, respectively. The upregulation of PACAP and CRH was paralleled only by increased expression of PACAP receptor PAC1 but not CRH receptor type 1. These observations provide evidence that inbred F344 and LEW rats exhibit not only the well-known phenotypic differences in the activity of the HPA axis but also strain- and stress-dependent differences in the expression of genes encoding 11HSD1 and neuropeptides associated with the HPA axis activity. Moreover, the differences in 11HSD1 expression suggest different local concentration of corticosterone and access to GR in canonical and noncanonical structures of the HPA axis.

Methylphenidate and environmental enrichment ameliorate the deleterious effects of prenatal stress on attention functioning

Either pre- or post-natal environmental factors seem to play a key role in brain and behavioral development and to exert long-term effects. Increasing evidence suggests that exposure to prenatal stress (PS) leads to motor and learning deficits and elevated anxiety, while enriched environment (EE) shows protective effects. The dopaminergic system is also sensitive to environmental life circumstances and affects attention functioning, which serves as the preliminary gate to cognitive processes. However, the effects of methylphenidate (MPH) on the dopaminergic system and attentional functioning, in the context of these life experiences, remain unclear. Therefore, we aimed to examine the effects of EE or PS on distinct types of attention, along with possible effects of MPH exposure. We found that PS impaired selective attention as well as partial sustained attention, while EE had beneficial effects. Both EE and MPH ameliorated the deleterious effects of PS on attention functioning. Considering the possible psychostimulant effect of MPH, we examined both anxiety-like behavior as well as motor learning. We found that PS had a clear anxiogenic effect, whereas EE had an anxiolytic effect. Nevertheless, the treatment with both MPH and/or EE recovered the deleterious effects of PS. In the motor-learning task, the PS group showed superior performance while MPH led to impaired motor learning. Performance decrements were prevented in both the PS + MPH and EE + MPH groups. This study provides evidence that peripubertal exposure to EE (by providing enhanced sensory, motor, and social opportunities) or MPH treatments might be an optional therapeutic intervention in preventing the PS long-term adverse consequences.

The importance of glia in dealing with stress

Glia are starting to be accepted as the equal of neurons. Their impact on intelligence, environmental enrichment, and cerebral dominance forms the basis for understanding the role of glia in stress. Along with neurons, astrocytes, microglia, NG2 cells, and oligodendrocytes all contribute. Glia can even be protective against drug abuse. Glial effects on depression, mood disorders and schizophrenia are reviewed.

Beneficial effects of environmental enrichment and food entrainment in the R6/2 mouse model of Huntington's disease

BACKGROUND

In addition to their cognitive and motor deficits, R6/2 mice show a progressive disintegration in circadian rhythms that mirrors the problems associated with sleep-wake disturbances experienced by patients with Huntington's disease (HD). It has been shown previously that motor and cognitive performance, as well as survival, can be improved in transgenic mouse models of HD through the provision of environmental enrichment.

METHODS

We compared the effect of two different overnight entrainment paradigms presented either separately or in combination. The first was environmental enrichment, the second was temporal food-entrainment. Environmental enrichment was provided in the dark period (the natural active period for mice) in the form of access to a Perspex playground containing running wheels, tunnels, climbing frame, ropes and chew blocks. Food entrainment was imposed by allowing access to food only during the dark period. We assessed a number of different aspects of function in the mice, measuring general health (by SHIRPA testing, body temperature and body weight measurements), cognitive performance in the touchscreen and locomotor behavior in the open field.

RESULTS

There were no significant differences in cognitive performance between groups on different schedules. Environmental enrichment delayed the onset of general health deterioration, while food entrainment slowed the loss of body weight, aided the maintenance of body temperature and improved locomotor behavior. Effects were limited however, and in combination had deleterious effects on survival.

CONCLUSIONS

Our results support previous studies showing that environmental enrichment can be beneficial and might be used to enhance the quality of life of HD patients. However, improvements are selective and 'enrichment' per se is likely to only be useful as an adjunct to a more direct therapy.

The influence of aging on poststroke depression using a rat model via middle cerebral artery occlusion

Poststroke depression (PSD) is the most frequent psychological sequela following stroke. While previous studies describe the impact of age on brain infarct volume, brain edema, and blood-brain barrier (BBB) breakdown following ischemia, the role of age on PSD has yet to be described. Here, we examine the influence of age on PSD progression in a rat model of PSD by middle cerebral artery occlusion (MCAO). One hundred forty-three rats were divided into three groups. 48 rats 20 weeks of age underwent a sham procedure, 51 rats 20 weeks of age had MCAO, and 44 rats 22-26 months of age had MCAO. Groups were further divided into two subgroups. The first subgroup was used to measure infarct lesion volume, brain edema, and BBB breakdown at 24 h. In the second subgroup at 3 weeks after MCAO, rats were subjected to a sucrose preference test, two-way shuttle avoidance task, forced swimming test, and a brain-derived neurotrophic factor (BDNF) protein level measurement. Total and striatal infarct volume, brain edema, and BBB breakdown in the striatum were increased in older rats, as compared with younger rats. While both old and young rats exhibited depressive-like behaviors on each of the behavioral tests and lower BDNF levels post-MCAO, as compared with control rats, there were no differences between old and young rats. Although older rats suffered from larger infarct volumes, increased brain edema and more BBB disruption following MCAO, the lack of behavioral differences between young and old rats suggests that there was no effect of rat age on the incidence of PSD.

Differential neuronal plasticity in mouse hippocampus associated with various periods of enriched environment during postnatal development

Enriched environment (EE) is characterized by improved conditions for enhanced exploration, cognitive activity, social interaction and physical exercise. It has been shown that EE positively regulates the remodeling of neural circuits, memory consolidation, long-term changes in synaptic strength and neurogenesis. However, the fine mechanisms by which environment shapes the brain at different postnatal developmental stages and the duration required to induce such changes are still a matter of debate. In EE, large groups of mice were housed in bigger cages and were given toys, nesting materials and other equipment that promote physical activity to provide a stimulating environment. Weaned mice were housed in EE for 4, 6 or 8 weeks and compared with matched control mice that were raised in a standard environment. To investigate the differential effects of EE on immature and mature brains, we also housed young adult mice (8 weeks old) for 4 weeks in EE. We studied the influence of onset and duration of EE housing on the structure and function of hippocampal neurons. We found that: (1) EE enhances neurogenesis in juvenile, but not young adult mice; (2) EE increases the number of synaptic contacts at every stage; (3) long-term potentiation (LTP) and spontaneous and miniature activity at the glutamatergic synapses are affected differently by EE depending on its onset and duration. Our study provides an integrative view of the role of EE during postnatal development in various mechanisms of plasticity in the hippocampus including neurogenesis, synaptic morphology and electrophysiological parameters of synaptic connectivity. This work provides an explanation for discrepancies found in the literature about the effects of EE on LTP and emphasizes the importance of environment on hippocampal plasticity.