Adolescent social defeat alters neural, endocrine and behavioral responses to amphetamine in adult male rats

Adolescent stress differentially modifies dopamine and norepinephrine release in the medial prefrontal cortex of adult rats

Adolescent stress (AS) has been associated with higher vulnerability to psychiatric disorders such as schizophrenia, depression, or drug dependence. Moreover, the alteration of brain catecholamine (CAT) transmission in the medial prefrontal cortex (mPFC) has been found to play a major role in the etiology of psychiatric disturbances. We investigated the effect of adolescent stress on CAT transmission in the mPFC of freely moving adult rats because of the importance of this area in the etiology of psychiatric disorders, and because CAT transmission is the target of a relevant group of drugs used in the therapy of depression and psychosis. We assessed basal dopamine (DA) and norepinephrine (NE) extracellular concentrations (output) by brain microdialysis in in the mPFC of adult rats that were exposed to chronic mild stress in adolescence. To ascertain the role of an altered release or reuptake, we stimulated DA and NE output by administering either different doses of amphetamine (0.5 and 1.0 mg / kg s.c.), which by a complex mechanism determines a dose dependent increase in the CAT output, or reboxetine (10 mg/kg i.p.), a selective NE reuptake inhibitor. The results showed the following: (i) basal DA output in AS rats was lower than in controls, while no difference in basal NE output was observed; (ii) amphetamine, dose dependently, stimulated DA and NE output to a greater extent in AS rats than in controls; (iii) reboxetine stimulated NE output to a greater extent in AS rats than in controls, while no difference in stimulated DA output was observed between the two groups. These results show that AS determines enduring effects on DA and NE transmission in the mPFC and might lead to the occurrence of psychiatric disorders or increase the vulnerability to drug addiction.

Quality not quantity: Deficient juvenile play experiences lead to altered medial prefrontal cortex neurons and sociocognitive skill deficits

Reduced play experience over the juvenile period leads to adults with impoverished social skills and to anatomical and physiological aberrations of the neurons found in the medial prefrontal cortex (mPFC). Even rearing rats from high-playing strains with low-playing strains show these developmental consequences. In the present study, we evaluated whether low-playing rats benefit from being reared with higher playing peers. To test this, we reared male Fischer 344 rats (F344), typically thought to be a low-playing strain, with a Long-Evans (LE) peer, a relatively high-playing strain. As juveniles, F344 rats reared with LE rats experienced less play and lower quality play compared to those reared with another F344. As adults, the F344 rats reared with LE partners exhibited poorer social skills and the pyramidal neurons of their mPFC had larger dendritic arbors than F344 rats reared with same-strain peers. These findings show that being reared with a more playful partner does not improve developmental outcomes of F344 rats, rather the discordance in the play styles of F344 and LE rats leads to poorer outcomes.

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.

Play fighting and the development of the social brain: The rat's tale

The benefits gained by young animals engaging in play fighting have been a subject of conjecture for over a hundred years. Progress in understanding the behavioral development of play fighting and the underlying neurobiology of laboratory rats has produced a coherent model that sheds light on this matter. Depriving rats of typical peer-peer play experience during the juvenile period leads to adults with socio-cognitive deficiencies and these are correlated with physiological and anatomical changes to the neurons of the prefrontal cortex, especially the medial prefrontal cortex. Detailed analysis of juvenile peer play has shown that using the abilities needed to ensure that play fighting is reciprocal is critical for attaining these benefits. Therefore, unlike that which was posited by many earlier hypotheses, play fighting does not train specific motor actions, but rather, improves a skill set that can be applied in many different social and non-social contexts. There are still gaps in the rat model that need to be understood, but the model is well-enough developed to provide a framework for broader comparative studies of mammals from diverse lineages that engage in play fighting.

The brain in social context: A systematic review of substance use and social processing from adolescence to young adulthood

Substance use escalates between adolescence and young adulthood, and most experimentation occurs among peers. To understand underlying mechanisms, research has focused on neural response during relevant psychological processes. Functional magnetic resonance imaging (fMRI) research provides a wealth of information about brain activity when processing monetary rewards; however, most studies have used tasks devoid of social stimuli. Given that adolescent neurodevelopment is sculpted by the push-and-pull of peers and emotions, identifying neural substrates is important for intervention. We systematically reviewed 28 fMRI studies examining substance use and neural responses to stimuli including social reward, emotional faces, social influence, and social stressors. We found substance use was positively associated with social-reward activity (e.g., in the ventral striatum), and negatively with social-stress activity (e.g., in the amygdala). For emotion, findings were mixed with more use linked to heightened response (e.g., in amygdala), but also with decreased response (e.g., in insula). For social influence, evidence supported both positive (e.g., cannabis and nucleus accumbens during conformity) and negative (e.g., polydrug and ventromedial PFC during peers' choices) relations between activity and use. Based on the literature, we offer recommendations for future research on the neural processing of social information to better identify risks for substance use.

Effect of early life social adversity on drug abuse vulnerability: Focus on corticotropin-releasing factor and oxytocin

Early life adversity can set the trajectory for later psychiatric disorders, including substance use disorders. There are a host of neurobiological factors that may play a role in the negative trajectory. The current review examines preclinical evidence suggesting that early life adversity specifically involving social factors (maternal separation, adolescent social isolation and adolescent social defeat) may influence drug abuse vulnerability by strengthening corticotropin-releasing factor (CRF) systems and weakening oxytocin (OT) systems. In adulthood, pharmacological and genetic evidence indicates that both CRF and OT systems are directly involved in drug reward processes. With early life adversity, numerous studies show an increase in drug abuse vulnerability measured in adulthood, along a concomitant strengthening of CRF systems and a weakening of OT systems. Mechanistic studies, while relatively few in number, are generally consistent with the theme that strengthened CRF systems and weakened OT systems mediate, at least in part, the link between early life adversity and drug abuse vulnerability. Establishing a direct role of CRF and OT in mediating the relation between early life social stressors and drug abuse vulnerability will inform clinical researchers and practitioners toward the development of intervention strategies to reduce risk among those suffering from early life adversities. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

The relationship between childhood trauma, dopamine release and dexamphetamine-induced positive psychotic symptoms: a [11C]-(+)-PHNO PET study

Childhood trauma is a risk factor for psychosis. Amphetamine increases synaptic striatal dopamine levels and can induce positive psychotic symptoms in healthy individuals and patients with schizophrenia. Socio-developmental hypotheses of psychosis propose that childhood trauma and other environmental risk factors sensitize the dopamine system to increase the risk of psychotic symptoms, but this remains to be tested in humans. We used [¹¹C]-(+)-PHNO positron emission tomography to measure striatal dopamine-2/3 receptor (D_2/3R) availability and ventral striatal dexamphetamine-induced dopamine release in healthy participants (n = 24). The relationships between dexamphetamine-induced dopamine release, dexamphetamine-induced positive psychotic symptoms using the Positive and Negative Syndrome Scale (PANSS), and childhood trauma using the Childhood Trauma Questionnaire (CTQ) were assessed using linear regression and mediation analyses, with childhood trauma as the independent variable, dexamphetamine-induced dopamine release as the mediator variable, and dexamphetamine-induced symptoms as the dependent variable. There was a significant interaction between childhood trauma and ventral striatal dopamine release in predicting dexamphetamine-induced positive psychotic symptoms (standardized β = 1.83, p = 0.003), but a mediation analysis was not significant (standardized β = -0.18, p = 0.158). There were no significant effects of dopamine release and childhood trauma on change in negative (p = 0.280) or general PANSS symptoms (p = 0.061), and there was no relationship between ventral striatal baseline D_2/3R availability and positive symptoms (p = 0.368). This indicates childhood trauma and dopamine release interact to influence the induction of positive psychotic symptoms. This is not consistent with a simple sensitization hypothesis, but suggests that childhood trauma moderates the cognitive response to dopamine release to make psychotic experiences more likely.

Preliminary data indicating a connection between stress-induced prefrontal dopamine release and hippocampal TSPO expression in the psychosis spectrum

Prolonged stress can cause neuronal loss in the hippocampus resulting in disinhibition of glutamatergic neurons proposed to enhance dopaminergic firing in subcortical regions including striatal areas. Supporting this, imaging studies show increased striatal dopamine release in response to psychosocial stress in healthy individuals with low childhood maternal care, individuals at clinical high risk for psychosis (CHR) and patients with schizophrenia. The prefrontal cortex (PFC) is connected to the hippocampus and a key region to control neurochemical responses to stressful stimuli. We recently reported a disrupted PFC dopamine-stress regulation in schizophrenia, which was intact in CHR. Given the available evidence on the link between psychosocial stress, PFC dopamine release and hippocampal immune activation in psychosis, we explored, for the first time in vivo, whether stress-induced PFC dopamine release is associated with hippocampal TSPO expression (a neuroimmune marker) in the psychosis spectrum. We used an overlapping sample of antipsychotic-naïve subjects with CHR (n = 6) and antipsychotic-free schizophrenia patients (n = 9) from our previously published studies, measuring PFC dopamine release induced by a psychosocial stress task with [¹¹C]FLB457 positron emission tomography (PET) and TSPO expression with [¹⁸F]FEPPA PET. We observed that participants on the psychosis spectrum with lower stress-induced dopamine release in PFC had significantly higher TSPO expression in hippocampus (β = -2.39, SE = 0.96, F(1,11) = 6.17, p = 0.030). Additionally, we report a positive association between stress-induced PFC dopamine release, controlled for hippocampal TSPO expression, and Global Assessment of Functioning. This is the first exploration of the relationship between PFC dopamine release and hippocampal TSPO expression in vivo in humans.

Comparative evaluation of adolescent repeated psychological or physical stress effects on adult cognitive performance, oxidative stress, and heart rate in female rats

Multiple adult health problems are associated with adolescent stress. As the brain discriminates physical and psychological stressors by activation of different neural networks, we hypothesized that behavioral and physiological performance would be modulated differently based on the nature of the stressors. Thus, we studied the comparative effects of adolescent repeated physical and psychological stresses on adult cognitive performance, pro-oxidant-antioxidant balance (PAB) and heart rate in female rats. The aim was to differentiate disparate potency of chronic psychological and physical stresses leading to long-term behavioral and physiological alterations. Twenty-one female rats were divided randomly into three groups of seven rats each; control, physical, and psychological stress. Experimental rats were exposed to the stressors for five consecutive days (10 min daily) via a two-communication box. After verifying stress induction by serum corticosterone measurement, the rats were returned to their home cage for 6 weeks, until adulthood, elevated plus maze (EPM), forced swimming test (FST), Y-maze, object recognition task (ORT), and passive avoidance test (PAT) were used as five different behavioral tests to evaluate cognitive performance of each group. Serum PAB and heart rate were measured to assess long-term stress-induced physiological disorders. The results showed exposure to adolescent psychological stress resulted in a larger set of significant changes (in behavioral variation, oxidative stress, and elevated heart rate) 6 weeks post-stress compared to adolescent physical stress. Hence, mental health care in adolescence and therapies targeting PAB and heart rate could be prevention and treatment approaches to confront persistent adolescent stress-induced disorders. Lay summaryThe aim of our study on female laboratory rats was to differentiate disparate potency of chronic psychological and physical stresses in adolescence leading to long-term behavioral and physiological alterations. The results suggest that psychological stresses result in a greater extent of changes compared to physical stress. Adolescent chronic psychological stress may reveal itself in the form of certain behavioral and physiological variations in adulthood. Therefore, mental health care in adolescence could be a valuable prevention approach to confront a variety of adult stress-induced disorders.

Social stress during adolescence activates long-term microglia inflammation insult in reward processing nuclei

The experience of social stress during adolescence is associated with higher vulnerability to drug use. Increases in the acquisition of cocaine self-administration, in the escalation of cocaine-seeking behavior, and in the conditioned rewarding effects of cocaine have been observed in rodents exposed to repeated social defeat (RSD). In addition, prolonged or severe stress induces a proinflammatory state with microglial activation and increased cytokine production. The aim of the present work was to describe the long-term effects induced by RSD during adolescence on the neuroinflammatory response and synaptic structure by evaluating different glial and neuronal markers. In addition to an increase in the conditioned rewarding effects of cocaine, our results showed that RSD in adolescence produced inflammatory reactivity in microglia that is prolonged into adulthood, affecting astrocytes and neurons of two reward-processing areas of the brain (the prelimbic cortex, and the nucleus accumbens core). Considered as a whole these results suggest that social stress experience modulates vulnerability to suffer a loss of glia-supporting functions and neuronal functional synaptic density due to drug consumption in later life.

Cortical stress regulation is disrupted in schizophrenia but not in clinical high risk for psychosis

While alterations in striatal dopamine in psychosis and stress have been well studied, the role of dopamine in prefrontal cortex is poorly understood. To date, no study has investigated the prefrontocortical dopamine response to stress in the psychosis spectrum, even though the dorsolateral and medial prefrontal cortices are key regions in cognitive and emotional regulation, respectively. The present study uses the high-affinity dopamine D2/3 receptor radiotracer 11C-FLB457 and PET together with a validated psychosocial stress challenge to investigate the dorsolateral and medial prefrontocortical dopamine response to stress in schizophrenia and clinical high risk for psychosis. Forty participants completed two 11C-FLB457 PET scans (14 antipsychotic-free schizophrenia, 14 clinical high risk for psychosis and 12 matched healthy volunteers), one while performing a Sensory Motor Control Task (control) and another while performing the Montreal Imaging Stress Task (stress). Binding potential (BPND) was estimated using Simplified Reference Tissue Model with cerebellar cortex as reference region. Dopamine release was defined as per cent change in BPND between control and stress scans (ΔBPND) using a novel correction for injected mass. Salivary cortisol response (ΔAUCI) was assessed throughout the tasks and its relationship with dopamine release examined. 11C-FLB457 binding at control conditions was significantly different between groups in medial [F(2,37) = 7.98, P = 0.0013] and dorsolateral [F(2,37) = 6.97, P = 0.0027] prefrontal cortex with schizophrenia patients having lower BPND than participants at clinical high risk for psychosis and healthy volunteers, but there was no difference in ΔBPND among groups [dorsolateral prefrontal cortex: F(2,37) = 1.07, P = 0.35; medial prefrontal cortex: F(2,37) = 0.54, P = 0.59]. We report a positive relationship between ΔAUCI and 11C-FLB457 ΔBPND in dorsolateral and medial prefrontal cortex in healthy volunteers (r = 0.72, P = 0.026; r = 0.76, P = 0.014, respectively) and in participants at clinical high risk for psychosis (r = 0.76, P = 0.0075; r = 0.72, P = 0.018, respectively), which was absent in schizophrenia (r = 0.46, P = 1.00; r = 0.19, P = 1.00, respectively). Furthermore, exploratory associations between ΔBPND or ΔAUCI and stress or anxiety measures observed in clinical high risk for psychosis were absent in schizophrenia. These findings provide first direct evidence of a disrupted prefrontocortical dopamine-stress regulation in schizophrenia.

Alcohol, psychomotor-stimulants and behaviour: methodological considerations in preclinical models of early-life stress

BACKGROUND

In order to assess the risk associated with early-life stress, there has been an increase in the amount of preclinical studies investigating early-life stress. There are many challenges associated with investigating early-life stress in animal models and ensuring that such models are appropriate and clinically relevant.

OBJECTIVES

The purpose of this review is to highlight the methodological considerations in the design of preclinical studies investigating the effects of early-life stress on alcohol and psychomotor-stimulant intake and behaviour.

METHODS

The protocols employed for exploring early-life stress were investigated and summarised. Experimental variables include animals, stress models, and endpoints employed.

RESULTS

The findings in this paper suggest that there is little consistency among these studies and so the interpretation of these results may not be as clinically relevant as previously thought.

CONCLUSION

The standardisation of these simple stress procedures means that results will be more comparable between studies and that results generated will give us a more robust understanding of what can and may be happening in the human and veterinary clinic.

Impact of juvenile chronic stress on adult cortico-accumbal function: Implications for cognition and addiction

Repeated exposure to stress during childhood is associated with increased risk for neuropsychiatric illness, substance use disorders and other behavioral problems in adulthood. However, it is not clear how chronic childhood stress can lead to emergence of such a wide range of symptoms and disorders in later life. One possible explanation lies in stress-induced disruption to the development of specific brain regions associated with executive function and reward processing, deficits in which are common to the disorders promoted by childhood stress. Evidence of aberrations in prefrontal cortex and nucleus accumbens function following repeated exposure of juvenile (pre- and adolescent) organisms to a variety of different stressors would account not only for the similarity in symptoms across the wide range of childhood stress-associated mental illnesses, but also their persistence into adulthood in the absence of further stress. Therefore, the goal of this review is to evaluate the current knowledge regarding disruption to executive function and reward processing in adult animals or humans exposed to chronic stress over the juvenile period, and the underlying neurobiology, with particular emphasis on the prefrontal cortex and nucleus accumbens. First, the role of these brain regions in mediating executive function and reward processing is highlighted. Second, the neurobehavioral development of these systems is discussed to illustrate how juvenile stress may exert long-lasting effects on prefrontal cortex-accumbal activity and related behavioral functions. Finally, a critical review of current animal and human findings is presented, which strongly supports the supposition that exposure to chronic stress (particularly social aggression and isolation in animal studies) in the juvenile period produces impairments in executive function in adulthood, especially in working memory and inhibitory control. Chronic juvenile stress also results in aberrations to reward processing and seeking, with increased sensitivity to drugs of abuse particularly noted in animal models, which is in line with greater incidence of substance use disorders seen in clinical studies. These consequences are potentially mediated by monoamine and glutamatergic dysfunction in the prefrontal cortex and nucleus accumbens, providing translatable therapeutic targets. However, the predominant use of male subjects and social-based stressors in preclinical studies points to a clear need for determining how both sex differences and stressor heterogeneity may differentially contribute to stress-induced changes to substrates mediating executive function and reward processing, before the impact of chronic juvenile stress in promoting adult psychopathology can be fully understood.

Repeated social defeat and the rewarding effects of cocaine in adult and adolescent mice: dopamine transcription factors, proBDNF signaling pathways, and the TrkB receptor in the mesolimbic system

RATIONALE

Repeated social defeat (RSD) increases the rewarding effects of cocaine in adolescent and adult rodents.

OBJECTIVE

The aim of the present study was to compare the long-term effects of RSD on the conditioned rewarding effects of cocaine and levels of the transcription factors Pitx3 and Nurr1 in the ventral tegmental area (VTA), the dopamine transporter (DAT), the D2 dopamine receptor (D2DR) and precursor of brain-derived neurotrophic factor (proBDNF) signaling pathways, and the tropomyosin-related kinase B (TrkB) receptor in the nucleus accumbens (NAc) in adult and adolescent mice.

METHODS

Male adolescent and young adult OF1 mice were exposed to four episodes of social defeat and were conditioned 3 weeks later with 1 mg/kg of cocaine. In a second set of mice, the expressions of the abovementioned dopaminergic and proBDNF and TrkB receptor were measured in VTA and NAc, respectively.

RESULTS

Adolescent mice experienced social defeats less intensely than their adult counterparts and produced lower levels of corticosterone. However, both adult and adolescent defeated mice developed conditioned place preference for the compartment associated with this low dose of cocaine. Furthermore, only adolescent defeated mice displayed diminished levels of the transcription factors Pitx3 in the VTA, without changes in the expression of DAT and D2DR in the NAc. In addition, stressed adult mice showed a decreased expression of proBDNF and the TrkB receptor, while stressed adolescent mice exhibited increased expression of latter without changes in the former.

CONCLUSION

Our findings suggest that dopaminergic pathways and proBDNF signaling and TrkB receptors play different roles in social defeat-stressed mice exposed to cocaine.

Modeling hypohedonia following repeated social defeat: Individual vulnerability and dopaminergic involvement

Social defeat in rodents putatively can model hypohedonia. The present studies examined models for assessing hypohedonia-like behavior and tested the hypotheses that 1) individual differences in baseline reward sensitivity predict vulnerability, and 2) defeat elicits changes in pharmacological measures of striatal dopaminergic function. Male Wistar rats (n=142) received repeated defeat (3 "triad" blocks of 3 defeats) or control handling. To determine whether defeat influenced consumption of SuperSac (glucose-saccharin) over an isocaloric, less preferred, glucose solution, a 2-choice paradigm was used. To determine repeated defeat effects on the reinforcing efficacy of SuperSac, a progressive-ratio schedule of reinforcement was used. Amphetamine-induced locomotor activity (0.08mg/kg, s.c.) was determined as a measure sensitive to striatal dopaminergic function. Defeat reduced SuperSac consumption during the first two triads-an effect seen in the third triad only in defeated rats with High vs. Low baseline SuperSac intake. The characteristic escalation in PR breakpoint for SuperSac normally seen in controls was absent in defeated rats, leading to a significant difference by the third triad. Defeat-induced blunting of the escalation in PR performance was greater in rats with High antecedent PR breakpoints and persisted 2.5weeks post-defeat. Repeated defeat also blunted amphetamine-induced locomotion 13days post-defeat. Thus, hypohedonic-like effects of social defeat were detected and accompanied by persistently attenuated striatal dopamine function. Early effects were seen for consumption of differentially-palatable solutions, and persistent effects were seen for the "breakpoint" motivational measure. The results implicate initial reward sensitivity as a risk factor for stress-induced hypohedonia.

Translational relevance of rodent models of hypothalamic-pituitary-adrenal function and stressors in adolescence

Elevations in glucocorticoids that result from environmental stressors can have programming effects on brain structure and function when the exposure occurs during sensitive periods that involve heightened neural development. In recent years, adolescence has gained increasing attention as another sensitive period of development, a period in which pubertal transitions may increase the vulnerability to stressors. There are similarities in physical and behavioural development between humans and rats, and rats have been used effectively as an animal model of adolescence and the unique plasticity of this period of ontogeny. This review focuses on benefits and challenges of rats as a model for translational research on hypothalamic-pituitary-adrenal (HPA) function and stressors in adolescence, highlighting important parallels and contrasts between adolescent rats and humans, and we review the main stress procedures that are used in investigating HPA stress responses and their consequences in adolescence in rats. We conclude that a greater focus on timing of puberty as a factor in research in adolescent rats may increase the translational relevance of the findings.

Effects of repeated social defeat on adolescent mice on cocaine-induced CPP and self-administration in adulthood: integrity of the blood-brain barrier

Social stress in adulthood enhances cocaine self-administration, an effect that has been related with an increase in extracellular signal-regulated kinase and p38α mitogen-activated protein kinase phosphorylation. A detrimental effect of cocaine on blood-brain barrier (BBB) integrity has also been reported. This study evaluates the effects of repeated social defeat (RSD) during adolescence on the reinforcing and motivational effects of cocaine in adult mice and the changes induced by RSD on BBB permeability. Cocaine self-administration, conditioned place preference and quantitative analysis of claudin-5, laminin, collagen-IV and IgG immunoreactivity took place 3 weeks after RSD. Mice socially defeated during adolescence developed conditioned place preference and exhibited reinstated preference with a non-effective dose of cocaine (1 mg/kg). RSD mice needed significantly more sessions than control animals for the preference induced by 25 mg/kg of cocaine to be extinguished. However, acquisition of cocaine self-administration (0.5 mg/kg per injection) was delayed in the RSD group. Mice exposed to RSD displayed significant changes in BBB structure in adulthood, with a marked reduction in expression of the tight junction protein claudin-5 and an increase in basal laminin degradation (reflected by a decrease in laminin and collagen-IV expression) in the nucleus accumbens and hippocampus. The detrimental effect induced by cocaine (25 mg/kg) on collagen-IV expression in the hippocampus was more pronounced in RSD mice. In summary, our findings suggest that stress and cocaine can increase the long-term vulnerability of the brain to subsequent environmental insults as a consequence of a sustained disruption of the BBB.

Adolescent Social Defeat Induced Alterations in Social Behavior and Cognitive Flexibility in Adult Mice: Effects of Developmental Stage and Social Condition

Negative social experiences during adolescence increase the risk of psychiatric disorders in adulthood. Using "resident-intruder" stress, the present study aimed to investigate the effects of adolescent social defeat on emotional and cognitive symptoms associated with psychiatric disorders during adulthood and the effects of the developmental stage and social condition on this process. In Experiment 1, animals were exposed to social defeat or manipulation for 10 days during early adolescence (EA, postnatal days [PND] 28-37), late adolescence (LA, PND 38-47), and adulthood (ADULT, PND 70-79) and then singly housed until the behavioral tests. Behaviors, including social avoidance of the defeat context and cortically mediated cognitive flexibility in an attentional set-shifting task (AST), were assessed during the week following stress or after 6 weeks during adulthood. We determined that social defeat induced significant and continuous social avoidance across age groups at both time points. The mice that experienced social defeat during adulthood exhibited short-term impairments in reversal learning (RL) on the AST that dissipated after 6 weeks. In contrast, social defeat during EA but not LA induced a delayed deficit in extra-dimensional set-shifting (EDS) in adulthood but not during adolescence. In Experiment 2, we further examined the effects of social condition (isolation or social housing after stress) on the alterations induced by social defeat during EA in adult mice. The adult mice that had experienced stress during EA exhibited social avoidance similar to the avoidance identified in Experiment 1 regardless of the isolation or social housing after the stress. However, social housing after the stress ameliorated the cognitive flexibility deficits induced by early adolescent social defeat in the adult mice, and the social condition had no effect on cognitive function. These findings suggest that the effects of social defeat on emotion and cognitive function are differentially affected by the developmental stage and social condition. EA may comprise a particularly sensitive developmental period in which social defeat may produce a delayed impairment in cognitive flexibility during adulthood, and the social condition following stress appears to play an important intermediary role in the development of these cognitive deficits.

CRF type 1 receptor antagonism in ventral tegmental area of adolescent rats during social defeat: prevention of escalated cocaine self-administration in adulthood and behavioral adaptations during adolescence

BACKGROUND

Activation of corticotropin-releasing factor type 1 receptors (CRF-R1) in the ventral tegmental area (VTA) represents a critical mechanism for social defeat to escalate cocaine self-administration in adult rats.

OBJECTIVE

We determined the acute effect of a CRF-R1 antagonist (CP376395) microinfusion into the VTA prior to each episode of social defeat in adolescent rats and determined whether this drug treatment could prevent later escalation of cocaine taking in early adulthood.

METHODS

Rats were implanted with bilateral cannulae aimed at the VTA 5 days before the first social defeat. Bilateral microinfusion of CP376395 (500 ng/side) or vehicle occurred 20 min before each episode of social defeat on postnatal days (P) 35, 38, 41, and 44. Behavior was quantified on P35 and P44. On P57, rats were implanted with intra-jugular catheters, and subsequent cocaine self-administration was analyzed.

RESULTS

CP376395-treated adolescent rats walked less and were attacked more slowly but were socially investigated more than vehicle-treated adolescents. Vehicle-treated rats showed increased social and decreased non-social exploration from P35 to P44, while CP376395-treated rats did not. Socially defeated, vehicle-treated adolescents took more cocaine during a 24-h unlimited access binge during adulthood. The latency to supine posture on P44 was inversely correlated with later cocaine self-administration during fixed and progressive ratio schedules of reinforcement and during the binge.

CONCLUSIONS

CP376395 treatment in adolescence blocked escalation of cocaine taking in adulthood. Episodes of social defeat stress engender neuroadaptation in CRF-R1s in the VTA that alter coping with social stress and that persist into adulthood.

Effect of a socıal defeat experıence on prefrontal actıvıty ın schızophrenıa

The social defeat (SD) hypothesis of schizophrenia posits that repeated experiences of SD may lead to sensitization of the mesolimbic dopaminergic system and to precipitation of psychosis. Based on previous definitions adapted to a human experimental paradigm, we prepared a computer simulation of SD to mimic this subjective experience. We measured prefrontal cortex (PFC) activity in subjects with schizophrenia and healthy controls during exposure to a single SD experience with functional near infrared spectroscopy. PFC activity declined in both groups. Compared with the control condition, SD exposure was associated with a broader decline in left ventromedial, right medial and right lateral PFC activity in healthy controls (n=25), and a sharper decline in right ventrolateral PFC activity in subjects with schizophrenia (n=25). The activity in the right ventrolateral PFC, was significantly lower in patients compared with controls. This may be due to a deficiency in emotion regulation or self-control, or it may be related to impaired empathy in schizophrenia. Different patterns of brain activity during the SD experience in subjects with schizophrenia versus healthy controls may provide indirect evidence regarding the SD hypothesis of schizophrenia.

Escalation of cocaine self-administration in adulthood after social defeat of adolescent rats: role of social experience and adaptive coping behavior

BACKGROUND

The link between adolescent social stress and substance abuse is modeled in social defeat of adolescent male rats, at an age when social experiences are essential for neurobehavioral maturation.

OBJECTIVE

We investigated the role of social experience and social defeat stress during adolescence on social behavior and cocaine self-administration (CocSelfAd) in early adulthood.

METHODS

We manipulated social experience by housing male rats in pairs (PH) or singly (SH) on postnatal day (P) 21. In addition, rats were subjected to social defeat from P35-44. Social behavior was measured during the first and last social defeat in PH and SH adolescents and PH adults. After assessing the behavioral response to novelty and cocaine (P57-61), intrajugular catheters were implanted and CocSelfAd was analyzed.

RESULTS

Residents were less aggressive toward PH adolescent intruders compared to PH adult intruders. Adults were submissive and defensive when attacked, whereas PH adolescents froze. In the course of repeated defeats, adolescent PH rats increased freezing, while SH rats decreased freezing. Longer attack-induced freezing after repeated defeats predicted escalated CocSelfAd in adulthood. PH controls acquired CocSelfAd more slowly than PH defeated and SH rats. Defeated PH rats increased CocSelfAd during progressive ratio schedules of reinforcement and during a 24-h continuous access binge compared to PH controls and SH defeated rats.

CONCLUSIONS

Social defeat in adolescence of PH rats caused persistent increases in adult CocSelfAd. Adolescent PH rats coped with attacks adaptively by increasing freezing behavior after repeated social defeats, a measure that predicted CocSelfAd in adulthood.

Effects of stressors on the reinforcing efficacy of nicotine in adolescent and adult rats

RATIONALE AND OBJECTIVES

Stress increases drug intake. This depends on the stressor, drug, and aspect of drug seeking assessed. The objectives of these experiments done in adolescent and adult male rats were to (1) examine social defeat effects on acquisition of nicotine self-administration (SA) and the reinforcing efficacy of nicotine and (2) determine the effects of acute exposure to intermittent footshock (FS) or yohimbine on the reinforcing efficacy of nicotine.

METHODS

In experiment 1, rats received four defeat exposures prior to nicotine SA acquisition and progressive ratio (PR) SA sessions (30 μg/kg nicotine/infusion). Exposure to an olfactory cue previously paired with defeat was also tested on responding maintained by nicotine on the PR schedule. In experiments 2 and 3, the effects of FS (5 and 10 min) or yohimbine (0.625 and 1.25 mg/kg, i.p.) on PR responding for nicotine (15, 30, or 60 μg/kg/infusion) were assessed. Adolescents were aged PD34-36 and adults PD81-85 at the beginning of nicotine SA training.

RESULTS

Defeat did not affect nicotine SA acquisition. Prior exposure to defeat or a defeat-paired olfactory cue did not affect PR responding for nicotine. FS modestly decreased PR responding in adolescents at the middle nicotine infusion dose. Yohimbine increased PR responding independent of nicotine infusion dose and age.

CONCLUSIONS

Together with previous work with other drugs, our data indicate that the effects of stress on the reinforcing efficacy of nicotine are stressor- and drug-dependent. This suggests that there is heterogeneity among stressors on how they affect neuronal systems underlying drug intake.

Stress in adolescence and drugs of abuse in rodent models: role of dopamine, CRF, and HPA axis

RATIONALE

Research on adolescence and drug abuse increased substantially in the past decade. However, drug-addiction-related behaviors following stressful experiences during adolescence are less studied. We focus on rodent models of adolescent stress cross-sensitization to drugs of abuse.

OBJECTIVES

Review the ontogeny of behavior, dopamine, corticotropin-releasing factor (CRF), and the hypothalamic-pituitary-adrenal (HPA) axis in adolescent rodents. We evaluate evidence that stressful experiences during adolescence engender hypersensitivity to drugs of abuse and offer potential neural mechanisms.

RESULTS AND CONCLUSIONS

Much evidence suggests that final maturation of behavior, dopamine systems, and HPA axis occurs during adolescence. Stress during adolescence increases amphetamine- and ethanol-stimulated locomotion, preference, and self-administration under many conditions. The influence of adolescent stress on subsequent cocaine- and nicotine-stimulated locomotion and preference is less clear. The type of adolescent stress, temporal interval between stress and testing, species, sex, and the drug tested are key methodological determinants for successful cross-sensitization procedures. The sensitization of the mesolimbic dopamine system is proposed to underlie stress cross-sensitization to drugs of abuse in both adolescents and adults through modulation by CRF. Reduced levels of mesocortical dopamine appear to be a unique consequence of social stress during adolescence. Adolescent stress may reduce the final maturation of cortical dopamine through D2 dopamine receptor regulation of dopamine synthesis or glucocorticoid-facilitated pruning of cortical dopamine fibers. Certain rodent models of adolescent adversity are useful for determining neural mechanisms underlying the cross-sensitization to drugs of abuse.

Decreased prefrontal cortex dopamine activity following adolescent social defeat in male rats: role of dopamine D2 receptors

RATIONALE

Adverse social experience in adolescence causes reduced medial prefrontal cortex (mPFC) dopamine (DA) and associated behavioral deficits in early adulthood.

OBJECTIVE

This study aims to determine whether mPFC DA hypofunction following social stress is specific to adolescent experience and if this results from stress-induced DA D2 receptor activation.

MATERIALS AND METHODS

Male rats exposed to repeated social defeat during adolescence or adulthood had mPFC DA activity sampled 17 days later. Separate experiments used freely moving microdialysis to measure mPFC DA release in response to adolescent defeat exposure. At P40, 49 and 56 mPFC DA turnover was assessed to identify when DA activity decreased in relation to the adolescent defeat experience. Finally, nondefeated adolescent rats received repeated intra-mPFC infusions of the D2 receptor agonist quinpirole, while another adolescent group received intra-mPFC infusions of the D2 antagonist amisulpride before defeat exposure.

RESULTS

Long-term decreases or increases in mPFC DA turnover were observed following adolescent or adult defeat, respectively. Adolescent defeat exposure elicits sustained increases in mPFC DA release, and DA turnover remains elevated beyond the stress experience before declining to levels below normal at P56. Activation of mPFC D2 receptors in nondefeated adolescents decreases DA activity in a similar manner to that caused by adolescent defeat, while defeat-induced reductions in mPFC DA activity are prevented by D2 receptor blockade.

CONCLUSIONS

Both the developing and mature PFC DA systems are vulnerable to social stress, but only adolescent defeat causes DA hypofunction. This appears to result in part from stress-induced activation of mPFC D2 autoreceptors.

Impact of stress on prefrontal glutamatergic, monoaminergic and cannabinoid systems

Stress has been shown to have marked and divergent effects on learning and memory which involves specific brain regions, such as spatial and declarative memory involving the hippocampus, memory of emotional arousing experiences and fear involving the amygdala, and executive functions and fear extinction involving the prefrontal cortex or the PFC. Response to stress involves a coordinated activation of a constellation of physiological systems including the activation of the hypothalamic-pituitary-adrenal (HPA) axis and other modulatory neurotransmitters and signaling systems. This paper presents a concise review of the effects of stress and glucocorticoids on the glutamatergic and monoaminergic (including noradrenergic, dopaminergic, and serotonergic systems) neurotransmitter systems as well as endocannabinoid signaling. Because of the breadth of the scope of this topic, the review is limited to the effects of stress on these brain systems on the prefrontal cortex, and where relevant, the hippocampus and the amygdala.

Impacts of stress and sex hormones on dopamine neurotransmission in the adolescent brain

RATIONALE

Adolescence is a developmental period of complex neurobiological change and heightened vulnerability to psychiatric illness. As a result, understanding factors such as sex and stress hormones which drive brain changes in adolescence, and how these factors may influence key neurotransmitter systems implicated in psychiatric illness, is paramount.

OBJECTIVES

In this review, we outline the impact of sex and stress hormones at adolescence on dopamine neurotransmission, a signaling pathway which is critical to healthy brain function and has been implicated in psychiatric illness. We review normative developmental changes in dopamine, sex hormone, and stress hormone signaling during adolescence and throughout postnatal life, then highlight the interaction of sex and stress hormones and review their impacts on dopamine neurotransmission in the adolescent brain.

RESULTS AND CONCLUSIONS

Adolescence is a time of increased responsiveness to sex and stress hormones, during which the maturing dopaminergic neural circuitry is profoundly influenced by these factors. Testosterone, estrogen, and glucocorticoids interact with each other and have distinct, brain region-specific impacts on dopamine neurotransmission in the adolescent brain, shaping brain maturation and cognitive function in adolescence and adulthood. Some effects of stress/sex hormones on cortical and subcortical dopamine parameters bear similarities with dopaminergic abnormalities seen in schizophrenia, suggesting a possible role for sex/stress hormones at adolescence in influencing risk for psychiatric illness via modulation of dopamine neurotransmission. Stress and sex hormones may prove useful targets in future strategies for modifying risk for psychiatric illness.

Adolescent social defeat decreases spatial working memory performance in adulthood

Background

Adolescent social stress is associated with increased incidence of mental illnesses in adulthood that are characterized by deficits in cognitive focus and flexibility. Such enhanced vulnerability may be due to psychosocial stress-induced disruption of the developing mesocortical dopamine system, which plays a fundamental role in facilitating complex cognitive processes such as spatial working memory. Adolescent rats exposed to repeated social defeat as a model of social stress develop dopaminergic hypofunction in the medial prefrontal cortex as adults. To evaluate a direct link between adolescent social stress and later deficits in cognitive function, the present study tested the effects of adolescent social defeat on two separate tests of spatial working memory performance.

Methods

Adult rats exposed to adolescent social defeat and their controls were trained on either the delayed win-shift task or the delayed alternating T-Maze task and then challenged with various delay periods. To evaluate potential differences in motivation for the food reward used in memory tasks, consumption and conditioned place preference for sweetened condensed milk were tested in a separate cohort of previously defeated rats and controls.

Results

Compared to controls, adult rats defeated in adolescence showed a delay-dependent deficit in spatial working memory performance, committing more errors at a 90 s and 5 min delay period on the T-maze and win-shift tasks, respectively. Observed memory deficits were likely independent of differences in reward motivation, as conditioned place preference for the palatable food used on both tasks was similar between the adolescent social defeat group and control.

Conclusions

The results demonstrate that severe social stressors during adolescence can produce long term deficits in aspects of cognitive function. Given the dependence of spatial working memory on prefrontal dopamine, pharmacologically reversing dopaminergic deficiencies caused by adolescent social stress has the potential to treat such cognitive deficits.

Behavioural profiles are shaped by social experience: when, how and why

The comprehensive understanding of individual variation in behavioural profiles is a current and timely topic not only in behavioural ecology, but also in biopsychological and biomedical research. This study focuses on the shaping of behavioural profiles by the social environment in mammals. We review evidence that the shaping of behavioural profiles occurs from the prenatal phase through adolescence and beyond. We focus specifically on adolescence, a sensitive phase during which environmental stimuli have distinctive effects on the modulation of behavioural profiles. We discuss causation, in particular, how behavioural profiles are shaped by social stimuli through behavioural and neuroendocrine processes. We postulate a central role for maternal hormones during the prenatal phase, for maternal behaviour during lactation and for the interaction of testosterone and stress hormones during adolescence. We refer to evolutionary history and attempt to place developmental shaping into broader evolutionary historical trends. Finally, we address survival value. We argue that the shaping of behavioural profiles by environmental stimuli from the prenatal phase through adolescence represents an effective mechanism for repeated and rapid adaptation during the lifetime. Notably, the adolescent phase may provide a last chance for correction if the future environment deviates from that predicted in earlier phases.

Effects of adolescent social defeat on adult amphetamine-induced locomotion and corticoaccumbal dopamine release in male rats

Maturation of mesocorticolimbic dopamine systems occurs during adolescence, and exposure to social stress during this period results in behavioral dysfunction including substance abuse disorders. Adult male rats exposed to repeated social defeat in adolescence exhibit reduced basal dopamine tissue content in the medial prefrontal cortex, altered dopamine tissue content in corticoaccumbal dopamine regions following acute amphetamine, and increased amphetamine conditioned place preference following repeated amphetamine treatment. Such changes may reflect altered amphetamine-induced extracellular dopamine release in the corticoaccumbal regions. Therefore, we used in vivo microdialysis to measure extracellular dopamine simultaneously within the medial prefrontal cortex and nucleus accumbens core of previously defeated rats and controls, in response to either acute or repeated (7 daily injections) of amphetamine (1.0 mg/kg). Locomotion responses to acute/repeated amphetamine were also assessed the day prior to taking dopamine measurements. Adolescent defeat potentiated adult locomotion responses to acute amphetamine, which was negatively correlated with attenuated amphetamine-induced dopamine release in the medial prefrontal cortex, but there was no difference in amphetamine-induced accumbal dopamine release. However, both locomotion and corticoaccumbal dopamine responses to repeated amphetamine were equivalent between previously defeated rats and controls. These data suggest adolescent defeat enhances behavioral responses to initial amphetamine exposure as a function of diminished prefrontal cortex dopamine activity, which may be sufficient to promote subsequently enhanced seeking of drug-associated cues. Interestingly, repeated amphetamine treatment appears to normalize amphetamine-elicited locomotion and cortical dopamine responses observed in adult rats exposed to adolescent social defeat, providing implications for treating stress-induced dopamine dysfunction.

Individual differences and social influences on the neurobehavioral pharmacology of abused drugs

The interaction of drugs with biologic targets is a critical area of research, particularly for the development of medications to treat substance use disorders. In addition to understanding these drug-target interactions, however, there is a need to understand more fully the psychosocial influences that moderate these interactions. The first section of this review introduces some examples from human behavioral pharmacology that illustrate the clinical importance of this research. The second section covers preclinical evidence to characterize some of the key individual differences that alter drug sensitivity and abuse vulnerability, related primarily to differences in response to novelty and impulsivity. Evidence is presented to indicate that critical neuropharmacological mechanisms associated with these individual differences involve integrated neurocircuits underlying stress, reward, and behavioral inhibitory processes. The third section covers social influences on drug abuse vulnerability, including effects experienced during infancy, adolescence, and young adulthood, such as maternal separation, housing conditions, and social interactions (defeat, play, and social rank). Some of the same neurocircuits involved in individual differences also are altered by social influences, although the precise neurochemical and cellular mechanisms involved remain to be elucidated fully. Finally, some speculation is offered about the implications of this research for the prevention and treatment of substance abuse.

Chronic corticosterone exposure during adolescence reduces impulsive action but increases impulsive choice and sensitivity to yohimbine in male Sprague-Dawley rats

Chronic stress during adolescence is associated with an increased risk for alcoholism and addictive disorders. Addiction is also associated with increased impulsivity, and stress during adolescence could alter cortical circuits responsible for response inhibition. Therefore, the present study determined the effect of chronic exposure to the stress hormone corticosterone (CORT) during adolescence on tests of impulsivity in adulthood and examined possible biochemical mechanisms. Male Sprague-Dawley rats were exposed to CORT by their drinking water during adolescence (post-natal day 30-50). The rats were then tested in adulthood to assess behavior on the 5-choice serial reaction time task (5CSRTT), stop-signal reaction time task (SSRTT), and the delay-discounting task, which differentially assess attention, impulsive action, and impulsive choice. Yohimbine-induced impulsivity on the 5CSRTT and biochemical analysis of the lateral orbital frontal cortex (lOFC) was also assessed owing to the ability of yohimbine to activate the hypothalamic-pituitary-adrenal axis and influence impulsivity. Adolescent CORT-treated rats were found to behave largely like controls on the 5CSRTT, but did show reduced premature responses when the intertrial interval was increased. Nevertheless, the CORT-treated rats tended to have more yohimbine-induced impulsive responses at low doses on this task, which was not found to be due to increased pCREB in the lOFC, but could be related to a higher expression/activity of the AMPA receptor subunit GluR1. Adolescent CORT-treated rats performed more accurately on the SSRTT, but showed greater impulsivity on the delay-discounting task, as indicated by steeper discounting functions. Therefore, adolescent CORT exposure reduced impulsive action but increased impulsive choice, indicating that chronic stress hormone exposure in adolescence can have long-term consequences on behavior.

Adolescent social defeat increases adult amphetamine conditioned place preference and alters D2 dopamine receptor expression

Components of the brain's dopaminergic system, such as dopamine receptors, undergo final maturation in adolescence. Exposure to social stress during human adolescence contributes to substance abuse behaviors. We utilized a rat model of adolescent social stress to investigate the neural mechanisms underlying this correlation. Rats exposed to repeated social defeat in adolescence (P35-P39) exhibited increased conditioned place preference (CPP) for amphetamine (1 mg/kg) in adulthood (P70). In contrast, rats experiencing foot-shock during the same developmental period exhibited amphetamine CPP levels similar to non-stressed controls. Our previous experiments suggested adolescent defeat alters dopamine activity in the mesocorticolimbic system. Furthermore, dopamine receptors have been implicated in the expression of amphetamine CPP. Therefore, we hypothesized that alteration to dopamine receptor expression in the mesocorticolimbic system may be associated with to heightened amphetamine CPP of adult rats exposed to adolescence defeat. We measured D1 and D2 dopamine receptor protein content in the medial prefrontal cortex, nucleus accumbens (NAc), and dorsal striatum following either adolescent social defeat or foot-shock stress and then adult amphetamine CPP. In controls, amphetamine CPP training reduced D2 receptor protein content in the NAc core. However, this down-regulation of NAc core D2 receptors was blocked by exposure to social defeat but not foot-shock stress in adolescence. These results suggest social defeat stress in adolescence alters the manner in which later amphetamine exposure down-regulates D2 receptors. Furthermore, persistent alterations to adult D2 receptor expression and amphetamine responses may depend on the type of stress experienced in adolescence.

Differential sensitivity to amphetamine's effect on open field behavior of psychosocially stressed male rats

RATIONALE

Studies of socially housed rodents have provided significant information regarding the mechanisms of stress and of stress-related disorders.

OBJECTIVE

Since psychosocial stress is known to alter the functional activity of dopaminergic system, we employed amphetamine (AMP) to evaluate the involvement dopamine in mediating the behavioral consequences of psychosocial stress.

METHODS

Male rats housed two per cage were designated as dominant (DOM) or subdominant (Sdom) based on initial evaluations of agonistic behaviors and body weight changes. Diad-housed rats and a group of single-housed (SiH) rats were tested in an open field after injections of saline or amphetamine (0.9 or 2.7 mg/kg IP) prior to and again while diad-housing.

RESULTS

Compared to future DOM rats, saline-injected future Sdom rats entered the open field center less frequently, spent less time in rearing behavior and groomed less. At the pre-diad test AMP treatment elevated locomotor activity of all rats, while stimulation of center entries was more marked in future DOM rats. At the diad test, AMP's locomotor stimulant effect was evident in all experimental groups with DOM rats showing higher effects compared to Sdom and SiH rats. Amphetamine's stimulation of center entries in DOM rats was similar to the pre-diad test, but it was diminished in Sdom rats, while stimulation of rearing behavior was most evident in diad-housed rats.

CONCLUSION

The dopaminergic system modulates the psychosocial stress-induced differences in explorative and emotional behaviors. Furthermore, behavioral traits like frequency of grooming behavior and of center entries were predictive of future hierarchical status.

Adolescent social defeat alters markers of adult dopaminergic function

Stressful experiences during adolescence can alter the trajectory of neural development and contribute to psychiatric disorders in adulthood. We previously demonstrated that adolescent male rats exposed to repeated social defeat stress show changes in mesocorticolimbic dopamine content both at baseline and in response to amphetamine when tested in adulthood. In the present study we examined whether markers of adult dopamine function are also compromised by adolescent experience of social defeat. Given that the dopamine transporter as well as dopamine D1 receptors act as regulators of psychostimulant action, are stress sensitive and undergo changes during adolescence, quantitative autoradiography was used to measure [(3)H]-GBR12935 binding to the dopamine transporter and [(3)H]-SCH23390 binding to dopamine D1 receptors, respectively. Our results indicate that social defeat during adolescence led to higher dopamine transporter binding in the infralimbic region of the medial prefrontal cortex and higher dopamine D1 receptor binding in the caudate putamen, while other brain regions analyzed were comparable to controls. Thus it appears that social defeat during adolescence causes specific changes to the adult dopamine system, which may contribute to behavioral alterations and increased drug seeking.

Social instability stress in adolescent male rats alters hippocampal neurogenesis and produces deficits in spatial location memory in adulthood

The ongoing development of the hippocampus in adolescence may be vulnerable to stressors. The effects of social instability stress (SS) in adolescence (daily 1 h isolation and change of cage partner postnatal days 30-45) on cell proliferation in the dentate gyrus (DG) in adolescence (on days 33 and 46, experiment 1) and in adulthood (experiment 2) was examined in Long Evans male rats and compared to nonstressed controls (CTL). Additionally, in experiment 2, a separate group of SS and CTL rats was tested on either a spatial (hippocampal-dependent) or nonspatial (nonhippocampal dependent) version of an object memory test and also were used to investigate hippocampal expression of markers of synaptic plasticity. No memory impairment was evident until the SS rats were adults, and the impairment was only on the spatial test. SS rats initially (postnatal day 33) had increased cell proliferation based on counts of Ki67 immunoreactive (ir) cells and greater survival of immature neurons based on counts of doublecortin ir cells on day 46 and in adulthood, irrespective of behavioral testing. Counts of microglia in the DG did not differ by stress group, but behavioral testing was associated with reduced microglia counts compared to nontested rats. As adults, SS and CTL rats did not differ in hippocampal expression of synaptophysin, but compared to CTL rats, SS rats had higher expression of basal calcium/calmodulin-dependent kinase II (CamKII), and lower expression of the phosphorylated CamKII subunit threonine 286, signaling molecules related to synaptic plasticity. The results are contrasted with those from previous reports of chronic stress in adult rats, and we conclude that adolescent stress alters the ongoing development of the hippocampus leading to impaired spatial memory in adulthood, highlighting the heightened vulnerability to stressors in adolescence.

Early life stress decreases hippocampal BDNF content and exacerbates recognition memory deficits induced by repeated D-amphetamine exposure

Adverse experiences early in life may have profound influences on brain development, for example, determining alterations in response to psychostimulant drugs, an increased risk of developing a substance abuse disorder, and individual differences in the vulnerability to neuropsychiatric disorders later in life. Here, we investigated the effects of exposure to an early adverse life event, maternal deprivation, combined with repeated d-amphetamine (AMPH) administration in adulthood, on recognition memory and brain-derived neurotrophic factor (BDNF) levels in rats' brain and serum. Rats were exposed to one of the following maternal rearing conditions from postnatal days 1 to 14: non-deprived (ND) or deprived (D). In adulthood, both groups received injections of saline (SAL) or AMPH (2.0mg/kg, i.p.) for 7 days. In Experiment I (performed 24h after the last AMPH injection), AMPH induced long-term memory (LTM) impairments in ND and D groups. The D+AMPH group also presented short-term memory (STM) impairments, indicating that the effects of AMPH on memory were more pronounced when the animals where maternally deprived. The group exposed to D+SAL (SAL) showed only LTM impairments. In Experiment II (performed 8 days after the last injection), AMPH detrimental effects on memory persisted in ND and D groups. BDNF levels were decreased in the hippocampus of D+SAL rats. In conclusion, AMPH produces severe and persistent recognition memory impairments that were more pronounced when the animals were maternally deprived, suggesting that an early adverse life event may increase the vulnerability of cognitive function to exposure to a psychostimulant later in life.

Stress during Adolescence Increases Novelty Seeking and Risk-Taking Behavior in Male and Female Rats

Adolescence is a period of major physical, hormonal, and psychological change. It is also characterized by a significant increase in the incidence of psychopathologies and this increase is gender-specific. Likewise, stress during adolescence is associated with the development of psychiatric disorders later in life. Previously, using a rat model of psychogenic stress (exposure to predator odor followed by placement on an elevated platform) during the pre-pubertal period (postnatal days 28–30), we reported sex-specific effects on auditory and contextual fear conditioning. Here, we study the short-term impact of psychogenic stress before and during puberty (postnatal days 28–42) on behavior (novelty seeking, risk taking, anxiety, and depression) and hypothalamus–pituitary–adrenocortical (HPA) axis activation during late adolescence (postnatal days 45–51). Peri-pubertal stress decreased anxiety-like behavior and increased risk taking and novelty seeking behaviors during late adolescence (measured with the elevated plus maze, open field and exposure to novel object tests and intake of chocopop pellets before or immediate after stress). Finally neither depressive-like behavior (measured at the forced-swim test) nor HPA response to stress (blood corticosterone and glucose) were affected by peri-pubertal stress. Nevertheless, when controlling for the basal anxiety of the mothers, animals exposed to peri-pubertal stress showed a significant decrease in corticosterone levels immediate after an acute stressor. The results from this study suggest that exposure to mild stressors during the peri-pubertal period induces a broad spectrum of behavioral changes in late adolescence, which may exacerbate the independence-building behaviors naturally happening during this transitional period (increase in curiosity, sensation-seeking, and risk-taking behaviors).

Adaptive modulation of behavioural profiles by social stress during early phases of life and adolescence

The development of individual behavioural profiles can be powerfully influenced by stressful social experiences. Using a comparative approach, we focus on the role of social stressors for the modulation of behavioural profile during early phases of life and adolescence. For gregarious species, the stability of the social environment in which the pregnant and lactating female lives is of major importance for foetal brain development and the behavioural profile of the offspring in later life. Social instability during these critical periods of development generally brings about a behavioural and neuroendocrine masculinisation in daughters and a less pronounced expression of male-typical traits in sons. Moreover, when mothers live in a socially threatening world during this time, anxiety-like behaviour of their offspring often is elevated in adulthood. These effects of the social environment are likely to be mediated by maternal hormones and/or maternal behaviour. In addition, they can be modulated significantly by offspring genotype. We favour the hypothesis that the behavioural effects of social stress during this phase of life are not necessarily "pathological" (nonadaptive) consequences or constraints of adverse social conditions. Rather, mothers could be adjusting the offspring to their environment in an adaptive way. Adolescence is another period in which behavioural development is particularly susceptible to social influences. There is some evidence that stressful social events experienced at this time alter and canalize behaviour in an adaptive fashion, so that earlier influences on behavioural profile development can be complemented and readjusted, if necessary, to meet current environmental conditions. In terms of underlying neuroendocrine mechanism, a central role for the interaction of testosterone and stress hormones is suggested. In summary, the modulation of behavioural profiles by social stress from the prenatal phase through adolescence appears to represent an effective mechanism for repeated and rapid adaptation.