Mild daily stressors in adulthood may counteract behavioural effects after constant presence of mother during early life

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.

Early-life stress biases responding to negative feedback and increases amygdala volume and vulnerability to later-life stress

Early-life stress (ELS) or adversity, particularly in the form of childhood neglect and abuse, is associated with poor mental and physical health outcomes in adulthood. However, whether these relationships are mediated by the consequences of ELS itself or by other exposures that frequently co-occur with ELS is unclear. To address this question, we carried out a longitudinal study in rats to isolate the effects of ELS on regional brain volumes and behavioral phenotypes relevant to anxiety and depression. We used the repeated maternal separation (RMS) model of chronic ELS, and conducted behavioral measurements throughout adulthood, including of probabilistic reversal learning (PRL), responding on a progressive ratio task, sucrose preference, novelty preference, novelty reactivity, and putative anxiety-like behavior on the elevated plus maze. Our behavioral assessment was combined with magnetic resonance imaging (MRI) for quantitation of regional brain volumes at three time points: immediately following RMS, young adulthood without further stress, and late adulthood with further stress. We found that RMS caused long-lasting, sexually dimorphic biased responding to negative feedback on the PRL task. RMS also slowed response time on the PRL task, but without this directly impacting task performance. RMS animals were also uniquely sensitive to a second stressor, which disproportionately impaired their performance and slowed their responding on the PRL task. MRI at the time of the adult stress revealed a larger amygdala volume in RMS animals compared with controls. These behavioral and neurobiological effects persisted well into adulthood despite a lack of effects on conventional tests of 'depression-like' and 'anxiety-like' behavior, and a lack of any evidence of anhedonia. Our findings indicate that ELS has long-lasting cognitive and neurobehavioral effects that interact with stress in adulthood and may have relevance for understanding the etiology of anxiety and depression in humans.

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

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

Maternal deprivation induces cytoskeletal alterations and depressive-like behavior in adult male rats by regulating the AKT/GSK3β/CRMP2 signaling pathway

Early-life adverse events exert persistent effects on brain functions and may increase the risk of psychopathology in adulthood. However, the underlying mechanism remains unclear. The purpose of our study was to study the long-lasting effects of maternal deprivation (MD) on depression-related behaviors and microtubule dynamics, and to illuminate the underlying molecular mechanism. Rat pups were separated from the dams for 360 min (MD) or 15 min (brief maternal separation) each day from postnatal day 4 through 10. Rats with MD experience showed significant depressive-like behaviors in adulthood, while brief maternal separation did not alter the behaviors. Behavioral alterations in the MD group were accompanied by alterations in the AKT/GSK3β/CRMP2 signaling pathway and hyperphosphorylation of CRMP2. CRMP2 interacted and colocalized with the cytoskeleton in the hippocampus, and the overlap of CRMP2 and tubulin staining in the hippocampus of MD rats was decreased. In MD rats, the expression of the α-tubulin isoforms Acet-tubulin and Tyr-tubulin changed, and the ratio of Tyr/Acet-tubulin, which is an important marker of microtubule dynamics, was decreased, indicating decreased microtubule dynamics. Furthermore, regulation of the AKT/GSK3β/CRMP2 signaling pathway by an LY294002 microinjection in the hippocampus resulted in cytoskeletal alterations and depressive-like behaviors in rats. These findings suggest that early-life MD induces depressive-like behaviors and cytoskeletal alterations in adult male rats and that the effects may be partly mediated by the AKT/GSK3β/CRMP2 signaling pathway. An understanding of the mechanism underlying the effect of MD on behaviors is crucial for developing pharmacological and psychological interventions for childhood neglect.

The effects of early life stress on reward processing

Early life stress (ELS), in the form of childhood maltreatment, abuse, or neglect, increases the risk for psychiatric sequelae later in life. The neurobiology of response to early stress and of reward processing overlap substantially, leading to the prediction that reward processing may be a primary mediator of the effects of early life stress. We describe a growing body of literature investigating the effects of early life stressors on reward processing in animals and humans. Despite variation in the reviewed studies, an emerging pattern of results indicates that ELS results in deficits of ventral striatum-related functions of reward responsiveness and approach motivation, especially when the stressor is experienced in early in development. For stressors experienced later in the juvenile period and adolescence, the animal literature suggests an opposite effect, in which ELS results in increased hedonic drive. Future research in this area will help elucidate the transdiagnostic impact of early life stress, and therefore potentially identify and intervene with at-risk youth, prior to the emergence of clinical psychopathology.

Sex-dependent impact of different degrees of maternal separation experience on OFT behavioral performances after adult chronic unpredictable mild stress exposure in rats

Early-life social-environmental factors are important for normal development, and different degrees of early-life stress experience have different impacts on adult behaviors and stress responsiveness. The aim of present study was to investigate the long-term effects of different degrees of maternal separation (MS) on male and female rats and subsequent responsiveness to chronic unpredictable mild stress (CUMS) exposure in adults. Sprague-Dawley (SD) newborn pups were exposed to either 15 min/day of MS (MS15), 360 min/day of MS (MS360) or no separation (NS) during postnatal day (PND)4-PND10. At PND56, behavioral tasks, including sucrose preference test (SPT), forced swimming test (FST) and open field test (OFT), were used to explore depressive and anxiety-like behaviors. Then the rats received a series of CUMS for 28 days, behavioral tasks were recorded after CUMS. Prior to CUMS, the behavioral performances in male and female rats were consistent, MS360 led to increased immobile time in FST and decreased activity in OFT, while MS15 rats exhibited behavioral performances similar to NS group. After CUMS, sexual dimorphism was observed in the OFT behavioral responses to adult stress re-exposure, but no differences in FST were observed. CUMS male rats with MS360 experiences showed the worst behavioral performances in OFT compared to those of the other male rats groups, while CUMS female rats without MS experience showed the worst behavioral performances in OFT compared to those of the other female rats groups. Both CUMS male and female rats with MS15 experiences showed better trend in OFT performances than those of CUMS rats with MS360 experience and without MS experiences. These results suggest that brief MS experiences increase the OFT behavioral resilience of rats to adult stress re-exposure, and prolonged MS promotes OFT behavioral resilience of female rats to adult stress re-exposure, while increases vulnerability of male rats to adult stress re-exposure.