Adolescent and adult male rats habituate to repeated isolation, but only adolescents sensitize to partner unfamiliarity

Lasting effects of adolescent social instability stress on dendritic morphology in the nucleus accumbens in female and male Long Evans rats

Social instability stress (SS) in adolescence in rats leads to long-lasting changes in social behaviour and reward-related behaviour relative to control rats. Given the role of the nucleus accumbens (NAc) in such behaviours, we investigated the morphology of medium spiny neurons (MSNs), which are most neurons in the NAc, in adult female and male rats exposed to SS in adolescence. Irrespective of sex, SS rats had increased number of dendritic spines in both the core and shell regions of the NAc (2.3 % and 18.1 % increase, respectively). In the core, SS rats had a 16 % reduction in the total dendritic lengths of MSNs, whereas in the shell, SS rats had a greater dendritic length closer to the soma, and particularly in SS female rats, whereas the opposite was found farther from the soma (SS 10.6 % > CTL overall). Although the extent to which such structural changes may underlie the enduring effects of SS in adolescence requires investigation, the results add to evidence that changes to the social environment in adolescence can determine adult neuronal structural.

Adolescent restraint stress enhances adult nicotine reinforcement in male and female rats

Adolescent stress is a risk factor for the initiation of nicotine use, but whether adolescent stress can enhance nicotine reinforcement when it is initiated later in adulthood is unknown, and it is unclear whether males and females are equally impacted. Therefore, this study assessed physiological responses (body weight and blood serum corticosterone - CORT) to restraint stress (RS) during adolescence (P28-55) or during adulthood (P70-96) in male and female Sprague-Dawley rats. When all subjects reached adulthood (P69 or 110; 2 weeks after termination of stress exposure), they were tested on sucrose preference and intravenous single-dose nicotine (0.03 mg/kg/infusion) self-administration. It was found that all rats displayed a significant CORT response to RS. Importantly, stress during adolescence, but not during adulthood, enhanced subsequent acquisition of nicotine intake tested in adulthood. Although this effect was observed in both sexes, only males displayed reduced body weight gain and adult sucrose preference. Moreover, regardless of stress exposure, females were more stimulated by nicotine, consumed more nicotine overall, and displayed enhanced nicotine seeking. These results suggest that adolescence is a period of heightened sensitivity to the enhancing effect of repeated stress on the susceptibility to develop nicotine dependence later in life in both sexes.

Evaluation of Parameters Which Influence Voluntary Ingestion of Supplements in Rats

Drug safety and efficacy studies frequently use oral gavage, but repetitive usage may cause problems. Administration through voluntary ingestion represents an opportunity for refinement. We aimed to develop a protocol for voluntary ingestion of gelatin-based supplements in rats, assessing the influence of age, sex, fasting (4 h), and additives (vanilla, VF; sucralose, S), and to test it in lactating dams. Three-week-old and 5-month-old Sprague-Dawley rats were placed individually in an empty cage containing a gelatin cube and trained daily (5 days/week), recording the day the whole cube was consumed (latency). Rats trained prior to gestation were offered a gelatin containing 250 mg/kg cocoa shell extract (CSE) during lactation. Rats that did not eat the cube after 8 training days were considered non-habituated, with a proportion similar in young males (7.1%), young females (11.1%), and adult females (10.3%), but significantly higher in adult males (39.3%). Excluding non-habituated rats, latency was 2-3 days, without differences between young and adult rats (p = 0.657) or between males and females (p = 0.189). VF or VF + S in the gelatin did not modify latency, while fasting significantly reduced it in females (p = 0.007) but not in males (p = 0.501). During lactation, trained females ate the CSE-gelatin within 1-5 min without litter problems. Conclusions: Acceptance of a gelatin-based supplement is negatively influenced by male sex, facilitated by fasting, and not modified by additives. Training is remembered after 2 months and does not interfere with lactation. Gelatin-based voluntary ingestion is suitable to administer drugs that need to pass through the digestive system, ensuring adequate dosage, and is important to detect non-habituated rats prior to the study. The current protocol may be implemented by training the rats in their own cage.

Sex and age differences in cognitive bias and neural activation in response to cognitive bias testing

Cognitive symptoms of depression, including negative cognitive bias, are more severe in women than in men. Current treatments to reduce negative cognitive bias are not effective and sex differences in the neural activity underlying cognitive bias may play a role. Here we examined sex and age differences in cognitive bias and functional connectivity in a novel paradigm. Male and female rats underwent an 18-day cognitive bias procedure, in which they learned to discriminate between two contexts (shock paired context A, no-shock paired context B), during either adolescence (postnatal day (PD 40)), young adulthood (PD 100), or middle-age (PD 210). Cognitive bias was measured as freezing behaviour in response to an ambiguous context (context C), with freezing levels akin to the shock paired context coded as negative bias. All animals learned to discriminate between the two contexts, regardless of sex or age. However, adults (young adults, middle-aged) displayed a greater negative cognitive bias compared to adolescents, and middle-aged males had a greater negative cognitive bias than middle-aged females. Females had greater neural activation of the nucleus accumbens, amygdala, and hippocampal regions to the ambiguous context compared to males, and young rats (adolescent, young adults) had greater neural activation in these regions compared to middle-aged rats. Functional connectivity between regions involved in cognitive bias differed by age and sex, and only adult males had negative correlations between the frontal regions and hippocampal regions. These findings highlight the importance of examining age and sex when investigating the underpinnings of negative cognitive bias and lay the groundwork for determining what age- and sex-specific regions to target in future cognitive bias studies.

Steroid hormones and hippocampal neurogenesis in the adult mammalian brain

Hippocampal neurogenesis persists across the lifespan in many species, including rodents and humans, and is associated with cognitive performance and the pathogenesis of neurodegenerative disease and psychiatric disorders. Neurogenesis is modulated by steroid hormones that change across development and differ between the sexes in rodents and humans. Here, we discuss the effects of stress and glucocorticoid exposure from gestation to adulthood as well as the effects of androgens and estrogens in adulthood on neurogenesis in the hippocampus. Throughout the review we highlight sex differences in the effects of steroid hormones on neurogenesis and how they may relate to hippocampal function and disease. These data highlight the importance of examining age and sex when evaluating the effects of steroid hormones on hippocampal neurogenesis.

Age-dependent changes in hormonal stress reactivity following repeated restraint stress throughout adolescence in male rats

Stress-related psychological dysfunctions show a marked increase during adolescence, yet the mechanisms that mediate these vulnerabilities are unknown. Notably, however, adolescence is associated with changes in hormonal stress reactivity mediated by the hypothalamic-pituitary-adrenal (HPA) axis, which might contribute to these dysfunctions. Specifically, pre-adolescent animals display prolonged stress-induced HPA responses compared to adults. Previous experience with stressors further modify these changes in stress reactivity, such that repeated exposure to the same stressor results in an augmented HPA response prior to adolescence, but a habituated response in adulthood. It is unclear when during adolescence the habituated, adult-like response develops to a repeated stressor. Using male rats at various ages that span adolescence (30-70 days of age), we show that by mid-adolescence (i.e. 42 days of age), animals show neither a facilitated nor a habituated HPA hormonal response following four days of repeated restraint stress (4RS) compared to a single restraint session (1RS). We also show that the habituated HPA response to 4RS develops between late-adolescence and young adulthood (i.e. between 56 and 70 days of age, respectively). Further, we find age- and experience-dependent changes in progesterone and testosterone secretion, indicating that the interaction between development and experience affects stress-induced hormonal responses outside of canonical HPA-related hormones. Despite these hormonal differences mediated by age and experience, repeated restraint stress resulted in decreased fecal boli production at all four ages, suggesting dissociation between hormonal and autonomic reactivity during adolescence. These data indicate that HPA plasticity is significantly affected by adolescence and that a habituated hormonal response to homotypic stress does not occur until young adulthood. A greater appreciation of these changes in stress reactivity will contribute to our understanding of the psychological vulnerabilities often associated with stressful adolescence.

Methods and Challenges in Investigating Sex-Specific Consequences of Social Stressors in Adolescence in Rats: Is It the Stress or the Social or the Stage of Development?

Adolescence is a time of social learning and social restructuring that is accompanied by changes in both the hypothalamic-pituitary-gonadal axis and the hypothalamic-pituitary-adrenal (HPA) axis. The activation of these axes by puberty and stressors, respectively, shapes adolescent development. Models of social stress in rats are used to understand the consequences of perturbations of the social environment for ongoing brain development. This paper reviews the challenges in investigating the sex-specific consequences of social stressors, sex differences in the models of social stress used in rats and the sex-specific effects on behaviour and provides an overview of sex differences in HPA responding to stressors, the variability in pubertal development and in strains of rats that require consideration in conducting such research, and directions for future research.

Systematic Review and Methodological Considerations for the Use of Single Prolonged Stress and Fear Extinction Retention in Rodents

Posttraumatic stress disorder (PTSD) is a mental health condition triggered by experiencing or witnessing a terrifying event that can lead to lifelong burden that increases mortality and adverse health outcomes. Yet, no new treatments have reached the market in two decades. Thus, screening potential interventions for PTSD is of high priority. Animal models often serve as a critical translational tool to bring new therapeutics from bench to bedside. However, the lack of concordance of some human clinical trial outcomes with preclinical animal efficacy findings has led to a questioning of the methods of how animal studies are conducted and translational validity established. Thus, we conducted a systematic review to determine methodological variability in studies that applied a prominent animal model of trauma-like stress, single prolonged stress (SPS). The SPS model has been utilized to evaluate a myriad of PTSD-relevant outcomes including extinction retention. Rodents exposed to SPS express an extinction retention deficit, a phenotype identified in humans with PTSD, in which fear memory is aberrantly retained after fear memory extinction. The current systematic review examines methodological variation across all phases of the SPS paradigm, as well as strategies for behavioral coding, data processing, statistical approach, and the depiction of data. Solutions for key challenges and sources of variation within these domains are discussed. In response to methodological variation in SPS studies, an expert panel was convened to generate methodological considerations to guide researchers in the application of SPS and the evaluation of extinction retention as a test for a PTSD-like phenotype. Many of these guidelines are applicable to all rodent paradigms developed to model trauma effects or learned fear processes relevant to PTSD, and not limited to SPS. Efforts toward optimizing preclinical model application are essential for enhancing the reproducibility and translational validity of preclinical findings, and should be conducted for all preclinical psychiatric research models.

Glucocorticoids attenuate interleukin-6-induced c-Fos and Egr1 expression and impair neuritogenesis in PC12 cells

Interleukin-6 (IL-6) is a cytokine primarily known for immune regulation. There is also growing evidence that IL-6 triggers neurogenesis and impacts neural development, both life-long occurring processes that can be impaired by early-life and adult stress. Stress induces the release of glucocorticoids by activation of the hypothalamic-pituitary-adrenal (HPA) axis. On the cellular level, glucocorticoids act via the ubiquitously expressed glucocorticoid receptor. Thus, we aimed to elucidate whether glucocorticoids affect IL-6-induced neural development. Here, we show that IL-6 signalling induces neurite outgrowth in adrenal pheochromocytoma PC12 cells in a mitogen-activated protein kinase (MAPK) pathway-dependent manner, since neurite outgrowth was diminished upon Mek-inhibitor treatment. Using quantitative biochemical approaches, such as qRT-PCR analysis of Hyper-IL-6 treated PC12 cells, we show that neurite outgrowth induced by IL-6 signalling is accompanied by early and transient MAPK-dependent mRNA expression of immediate early genes coding for proteins such as early growth response protein 1 (Egr1) and c-Fos. This correlates with reduced proliferation and prolonged G0/G1 cell cycle arrest as determined by monitoring the cellular DNA content using flow cytometry. These results indicate for IL-6 signalling-induced neural differentiation. Interestingly, the glucocorticoid Dexamethasone impairs early IL-6 signalling-induced mRNA expression of c-Fos and Egr1 and restrains neurite outgrowth. Impaired Egr1 and c-Fos expression in neural development is implicated in the aetiology of neuropathologies. Thus, it appears likely that stress-induced release of glucocorticoids, as well as therapeutically administered glucocorticoids, contribute to the development of neuropathologies by reducing the expression of Egr1 and c-Fos, and by restraining IL-6-dependent neural differentiation.

Long-Term Impacts of Post-weaning Social Isolation on Nucleus Accumbens Function

Adolescence is a period of incredible change, especially within the brain's reward circuitry. Stress, including social isolation, during this time has profound effects on behaviors associated with reward and other neuropsychiatric disorders. Because the Nucleus Accumbens (NAc), is crucial to the integration of rewarding stimuli, the NAc is especially sensitive to disruptions by adolescent social isolation stress. This review highlights the long-term behavioral consequences of adolescent social isolation rearing on the NAc. It will discuss the cellular and molecular changes within the NAc that might underlie the long-term effects on behavior. When available sex-specific effects are discussed. Finally by mining publicly available data we identify, for the first time, key transcriptional profiles induced by adolescence social isolation in genes associated with dopamine receptor 1 and 2 medium spiny neurons and genes associated with cocaine self-administration. Together, this review provides a comprehensive discussion of the wide-ranging long-term impacts of adolescent social isolation on the dopaminergic system from molecules through behavior.

Early Growth Response Gene Upregulation in Epstein-Barr Virus (EBV)-Associated Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS)

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a chronic multisystem disease exhibiting a variety of symptoms and affecting multiple systems. Psychological stress and virus infection are important. Virus infection may trigger the onset, and psychological stress may reactivate latent viruses, for example, Epstein-Barr virus (EBV). It has recently been reported that EBV induced gene 2 (EBI2) was upregulated in blood in a subset of ME/CFS patients. The purpose of this study was to determine whether the pattern of expression of early growth response (EGR) genes, important in EBV infection and which have also been found to be upregulated in blood of ME/CFS patients, paralleled that of EBI2. EGR gene upregulation was found to be closely associated with that of EBI2 in ME/CFS, providing further evidence in support of ongoing EBV reactivation in a subset of ME/CFS patients. EGR1, EGR2, and EGR3 are part of the cellular immediate early gene response and are important in EBV transcription, reactivation, and B lymphocyte transformation. EGR1 is a regulator of immune function, and is important in vascular homeostasis, psychological stress, connective tissue disease, mitochondrial function, all of which are relevant to ME/CFS. EGR2 and EGR3 are negative regulators of T lymphocytes and are important in systemic autoimmunity.

Preclinical methodological approaches investigating of the effects of alcohol on perinatal and adolescent neurodevelopment

Despite much evidence of its economic and social costs, alcohol use continues to increase. Much remains to be known as to the effects of alcohol on neurodevelopment across the lifespan and in both sexes. We provide a comprehensive overview of the methodological approaches to ethanol administration when using animal models (primarily rodent models) and their translational relevance, as well as some of the advantages and disadvantages of each approach. Special consideration is given to early developmental periods (prenatal through adolescence), as well as to the types of research questions that are best addressed by specific methodologies. The zebrafish is used increasingly in alcohol research, and how to use this model effectively as a preclinical model is reviewed as well.

The effects of social instability stress and subsequent ethanol consumption in adolescence on brain and behavioral development in male rats

Excessive drinking in adolescence continues to be a problem, and almost a quarter of young Canadians have reported consuming five or more alcoholic drinks in one occasion in recent surveys. The consequences of such drinking may be more pronounced when commenced in adolescence, given the ongoing brain development during this period of life. Here, we investigated the consequences of 3 weeks' intermittent access to ethanol in mid-adolescence to early adulthood in rats, and the extent to which a stress history moderated the negative consequences of ethanol access. In experiment 1, male rats that underwent adolescent social instability stress (SS; daily 1 h isolation + return to unfamiliar cage partner every day from postnatal day [PND] 30-45) did not differ from control (CTL) rats in intake of 10% ethanol sweetened with 0.1% saccharin (access period; PND 47-66). Ethanol drinking reduced proteins relevant for synaptic plasticity (αCaMKII, βCaMKII, and PSD-95) in the dorsal hippocampus, and in CTL rats only in the prefrontal cortex (αCaMKII and PSD 95), attenuating the difference between CTL and SS rats in the water-drinking group. In experiment 2, ethanol also attenuated the difference between SS and CTL rats in a social interaction test by reducing social interaction in SS rats; CTL rats, however, had a higher intake of ethanol than did SS rats during the access period. Ethanol drinking reduced baseline and fear recall recovery concentrations of corticosterone relative to those exposed only to water, although there was no effect of either ethanol or stress history on fear conditioning. Ethanol drinking did not influence intake after 9 days of withdrawal; however, ethanol-naïve SS rats drank more than did CTL rats when given a 24-h access in adulthood. These results reveal a complex relationship between stress history and ethanol intake in adolescence on outcomes in adulthood.

Effects of oxytocin receptor antagonism on social function and corticosterone release after adolescent social instability in male rats

Oxytocin influences social behaviour and hypothalamic-pituitary-adrenal (HPA) function. We previously found that social instability stress (SS) from postnatal day 30 to 45 increased oxytocin receptor (OTR) densities in the lateral septum and nucleus accumbens of adolescent male rats. Here, we investigated social behaviour and HPA function in adolescent male SS rats compared with age- and sex-matched controls after intraperitoneal treatment with an OTR antagonist L-368,899 (OTR-A). Regardless of OTR antagonism, adolescent SS rats spent more time in social approach (investigation through wire mesh) but less time in social interaction (physical interaction) with unfamiliar same-sex and same-age peers than did controls. However, OTR-A-treatment caused SS rats to be more socially avoidant than OTR-A-treated controls and saline-treated rats of the same condition. Additionally, the predicted rise in plasma corticosterone in response to OTR-A treatment was blunted in SS rats. Fos immunoreactivity (IR) was used as a marker of neural activation in social brain regions and oxytocin-IR was examined in the paraventricular nucleus of the hypothalamus (PVN) in response to interacting with unfamiliar peers in SS and control rats after OTR-A treatment. OTR-A treatment had little effect on Fos-IR and oxytocin-IR in the analyzed brain regions, but SS rats had lower Fos-IR and oxytocin-IR in the PVN and greater Fos-IR in subregions of the prefrontal cortex, and hippocampus, and lateral septum than did controls. Finally, binding density of OTR was measured in the PVN and hippocampus, and greater OTR binding density was found in the PVN of SS rats. Together, these data demonstrate a greater influence of OTR antagonism on social behaviour and a reduced influence of OTR antagonism on HPA responses after adolescent SS in male rats. The results also suggest that differences in neural functioning in the prefrontal cortex, hippocampus and lateral septum of adolescent SS rats may be involved in their altered social behaviour relative to that of controls.

Social experience during adolescence in female rats increases 50 kHz ultrasonic vocalizations in adulthood, without affecting anxiety-like behavior

Adolescents are highly motivated to engage in social interactions, and researchers have hypothesized that positive social relationships during adolescence can have long term, beneficial effects on stress reactivity and mental well-being. Studies of laboratory rodents provide the opportunity to investigate the relationship between early social experiences and later behavioral and physiological responses to stressors. In this study, female Lister-hooded rats (N = 12 per group) were either (a) provided with short, daily encounters (10 min/day) with a novel partner during mid-adolescence (postnatal day 34-45; "social experience," SE, subjects) or (b) underwent the same protocol with a familiar cagemate during mid-adolescence ("control experience," CE, subjects), or (c) were left undisturbed in the home cage (non-handled "control," C, subjects). When tested in adulthood, the groups did not differ in behavioral responses to novel environments (elevated plus maze, open field, and light-dark box) or in behavioral and physiological (urinary corticosterone) responses to novel social partners. However, SE females emitted significantly more 50 kHz ultrasonic vocalizations than control subjects both before and after social separation from a familiar social partner, which is consistent with previous findings in male rats. Thus, enhanced adolescent social experience appears to have long-term effects on vocal communication and could potentially modulate adult social relationships.

A novel GPR55-mediated satiety signal in the oval Bed Nucleus of the Stria Terminalis

Nestled within feeding circuits, the oval (ov) region of the Bed Nucleus of the Stria Terminalis (BNST) may be critical for monitoring energy balance through changes in synaptic strength. Here we report that bidirectional plasticity at ovBNST GABA synapses was tightly linked to the caloric state of male rats, seesawing between long-term potentiation (iLTP, fed) and depression (iLTD, food restricted). L-α-lysophosphatidylinositol (LPI) acting on GPR55 receptors and 2-arachidonoylglycerol (2-AG) through CB1R were respectively responsible for fed (iLTP) and food restricted (iLTD) states. Thus, we have characterized a potential gating mechanism within the ovBNST that may signal metabolic state within the rat brain feeding circuitry.

Age-dependent regulation by androgens of gene expression in the anterior hypothalamus and stress-induced release of adrenal hormones in adolescent and adult male rats

Adolescents show greater and/or more prolonged activation of the hypothalamic-pituitary-adrenal axis in response to stressors than adults, although the basis for such an age difference is not understood. We investigated developmental shifts in the regulation of HPA function by testosterone using androgen replacement in orchiectomised (OCX) pre-pubertal and post-pubertal adolescent rats and in adults, as well as using inhibitors of testosterone synthesis in non-operated rats. The expected dampening effect of testosterone in adult OCX rats did not meet statistical significance in all of the three experiments. Nevertheless, in each, adolescents had higher post-stress concentrations of corticosterone compared to adults despite similar concentrations of testosterone. The effect of testosterone was in the opposite direction in post-pubertal adolescents compared to that in adults, with testosterone replacement leading to increased rather than lower corticosterone concentration. Testosterone replacement decreased arginine vasopressin and corticotrophin-releasing hormone immune-reactive cell counts in the paraventricular nucleus at all ages. In a fourth experiment, we provide evidence that the basis of the age difference in corticosterone release is because of a greater conversion of testosterone to oestradiol in adolescents and a greater conversion of testosterone to dihydrotestosterone in adults: aromatase inhibition had little effect in adults and attenuated the age difference by decreasing stress-induced corticosterone release in adolescents. By contrast, 5α-reductase inhibition or an androgen receptor antagonist had little effect in adolescents and attenuated the age difference by increasing stress-induced corticosterone release in adults. In the adrenal gland, adolescents had reduced 5α-reductase and androgen receptor gene expression. There also were age differences in the regulation of hypothalamic mRNA expression of androgen receptors, oestrogen receptors and aromatase by testosterone. In sum, the results suggest that developmental shifts in the synthesis of testosterone and the regulation of gene expression are factors with respect to age differences in corticosterone release in response to stressors.

Preventing adolescent stress-induced cognitive and microbiome changes by diet

Psychological stress during adolescence may cause enduring cognitive deficits and anxiety in both humans and animals, accompanied by rearrangement of numerous brain structures and functions. A healthy diet is essential for proper brain development and maintenance of optimal cognitive functions during adulthood. Furthermore, nutritional components profoundly affect the intestinal community of microbes that may affect gut-brain communication. We adopted a relatively mild stress protocol, social instability stress, which when repeatedly administered to juvenile rats modifies cognitive behaviors and plasticity markers in the brain. We then tested the preventive effect of a prolonged diet enriched with the ω-3 polyunsaturated fatty acids eicosapentaenoic acid, docosahexaenoic acid, and docosapentaenoic acid and vitamin A. Our findings highlight the beneficial effects of this enriched diet on cognitive memory impairment induced by social instability stress, as stressed rats fed the enriched diet exhibited performance undistinguishable from that of nonstressed rats on both emotional and reference memory tests. Furthermore, in stressed rats, the decline in brain-derived neurotrophic factor expression in the hippocampus and shifts in the microbiota composition were normalized by the enriched diet. The detrimental behavioral and neurochemical effects of adolescent stress, as well as the protective effect of the enriched diet, were maintained throughout adulthood, long after the exposure to the stressful environment was terminated. Taken together, our results strongly suggest a beneficial role of nutritional components in ameliorating stress-related behaviors and associated neurochemical and microbiota changes, opening possible new venues in the field of nutritional neuropsychopharmacology.

Adolescent social stress and social context influence the intake of ethanol and sucrose in male rats soon and long after the stress exposures

Social instability stress in adolescent rats (SS; postnatal day 30-45, daily 1 hr isolation +new cage partner) alters behavioural responses to psychostimulants, but differences in voluntary consumption of natural and drug rewards are unknown. SS also is associated with an atypical behavioural repertoire, for example reduced social interactions. Here, we investigated whether SS rats differ from control (CTL) rats in ethanol (EtOH) or sucrose intake in experiments involving different social contexts: alone, in the presence of an unfamiliar peer, in the presence of its cage partner, or in competition against its cage partner. SS rats drank more EtOH than CTL rats irrespective of social context, although the effects were driven primarily by those tested soon after the test procedure rather than weeks later in adulthood. SS and CTL rats did not differ in sucrose intake, except in adulthood under conditions of competition for limited access (SS>CTL). Adolescent rats drank more sucrose than adults, in keeping with evidence that adolescents are more sensitive to natural rewards than adult animals. Overall, adolescent SS modified the reward value of EtOH and sucrose, perhaps through stress/glucocorticoids modifying the development of the mesocorticolimbic system.

Sex-specific effects of CB1 receptor antagonism and stress in adolescence on anxiety, corticosterone concentrations, and contextual fear in adulthood in rats

There is a paucity of research regarding the role of endogenous cannabinoid signalling in adolescence on brain and behaviour development. We previously demonstrated effects of repeated CB1 receptor antagonism in adolescence on socioemotional behaviours and neural protein expression 24-48 h after the last drug administration in female rats, with no effect in males. Here we investigate whether greater effects would be manifested after a lengthier delay. In Experiment 1, male and female rats were administered either 1 mg / kg of the CB1 receptor-selective antagonist AM251, vehicle (VEH), or did not receive injections (NoINJ) daily on postnatal days (PND) 30-44 either alone (no adolescent confinement stress; noACS), or in tandem with 1 h ACS. On PND 70, adolescent AM251 exposure reduced anxiety in an elevated plus maze in males, irrespective of ACS, with no effects in females. On PND 73, there were no group differences in either sex in plasma corticosterone concentrations before or after 30 min of restraint stress, although injection stress resulted in higher baseline concentrations in males. Brains were collected on PND 74, with negligible effects of either AM251 or ACS on protein markers of synaptic plasticity and of the endocannabinoid system in the hippocampus and medial prefrontal cortex. In Experiment 2, rats from both sexes were treated with vehicle or AM251 on PND 30-44 and were tested for contextual fear conditioning and extinction in adulthood. AM251 females had greater fear recall than VEH females 24 h after conditioning, with no group differences in within- or between-session fear extinction. There were no group differences in long-term extinction memory, although AM251 females froze more during a reconditioning trial compared with VEH females. There were no group differences on any of the fear conditioning measures in males. Together, these findings indicate a modest, sex-specific role of CB1 receptor signalling in adolescence on anxiety-like behaviour in males and conditioned fear behaviour in females.

Developmental differences in stress responding after repeated underwater trauma exposures in rats

Adolescence is a distinct developmental period characterized by behavioral and physiological maturation. Rapid ongoing changes during neurodevelopment in particular present potential opportunities for stress to have lasting effects on longitudinal outcomes of behavioral and neuroendocrine function. While adult stress effects on outcomes during adulthood have been characterized, little is known about the lasting effects of adolescent repeated stressor exposure on outcomes during adolescence. We have previously reported different stress responses in adolescent rats relative to adult rats, including a blunted fear response outcome in adulthood in rats stressed during adolescence. The present study characterized the ontogeny of behavioral and neuroendocrine responses to eight underwater trauma (UWT) exposures in rats over a two week poststress time period during adolescence (P34) or adulthood (P83) relative to age-matched control groups that underwent eight swimming episodes without UWT. Repeated UWT exposures starting in adolescence, but not adulthood, resulted in adverse behavioral responses on the elevated plus maze 1 day post-stress. Corticosterone responses did not differ between UWT-exposed and controls for either age group at 1 day or at 7 days poststress, although there was an effect of age on corticosterone levels. We conclude that repeated UWT stress events have a lasting, negative behavioral effect on adolescent rats that is not observed in adult rats after the two-week exposure window. These results suggest that neurophysiological mechanisms underlying recovery from a repeated stressor are immature in adolescence relative to adulthood in rats.

The metamorphosis of adolescent hormonal stress reactivity: A focus on animal models

As adolescents transition from childhood to adulthood, many physiological and neurobehavioral changes occur. Shifts in neuroendocrine function are one such change, including the hormonal systems that respond to stressors. This review will focus on these hormonal changes, with a particular emphasis on the pubertal and adolescent maturation of the hypothalamic-pituitary-adrenal (HPA) axis. Furthermore, this review will concentrate on studies using animal models, as these model systems have contributed a great deal to our mechanistic understanding of how factors such as sex and experience with stressors shape hormonal reactivity during development. Continued study of the maturation of stress reactivity will undoubtedly shed much needed light on the stress-related vulnerabilities often associated with adolescence as well as providing us with possible strategies to mitigate these vulnerabilities. This area of research may lead to discoveries that enhance the well-being of adolescents, ultimately providing them with greater opportunities to mature into healthy adults.

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.

Effects of CB1 receptor antagonism and stress exposures in adolescence on socioemotional behaviours, neuroendocrine stress responses, and expression of relevant proteins in the hippocampus and prefrontal cortex in rats

Little is known about the consequences of altered endocannabinoid signalling in adolescence. We hypothesized that CB1 receptor antagonism (AM251, 1 mg/kg) and stress exposures (1 h confinement stress) in adolescence (daily, postnatal days 30-44) would interact to increase neuroendocrine stress responses and anxiety when investigated a minimum of 24 h after drug and stress treatments; these treatment effects were independent of each other. Changes in homecage behaviour and in weight gain confirmed that both males and females were sensitive to the treatments. Nevertheless, in males, repeated AM251 administration was without effect on any of the measures investigated in days post-treatment. Males had reduced corticosterone release to the repeated stress and had increased GAD67 expression in the ventral hippocampus under baseline conditions. In females, AM251 also reduced weight gain and increased stereotypic behaviours in the homecage; these same females showed increased sociality, reduced CB1 receptor expression in the dorsal hippocampus, and increased GAD67 expression in the prefrontal cortex. Further, females exposed to repeated stress had enhanced recovery to baseline corticosterone concentrations after stress. The inclusion of a non-injected comparison group also revealed stress of injection effects in both sexes that otherwise would have been masked. Together, the findings demonstrate effects of CB1 receptor antagonism and stress that were more evident in females than males, suggesting that females may be more vulnerable to the consequences of disrupted endocannabinoid signalling during adolescence.

The Role of Early Growth Response 1 (EGR1) in Brain Plasticity and Neuropsychiatric Disorders

It is now clearly established that complex interactions between genes and environment are involved in multiple aspects of neuropsychiatric disorders, from determining an individual's vulnerability to onset, to influencing its response to therapeutic intervention. In this perspective, it appears crucial to better understand how the organism reacts to environmental stimuli and provide a coordinated and adapted response. In the central nervous system, neuronal plasticity and neurotransmission are among the major processes integrating such complex interactions between genes and environmental stimuli. In particular, immediate early genes (IEGs) are critical components of these interactions as they provide the molecular framework for a rapid and dynamic response to neuronal activity while opening the possibility for a lasting and sustained adaptation through regulation of the expression of a wide range of genes. As a result, IEGs have been tightly associated with neuronal activity as well as a variety of higher order processes within the central nervous system such as learning, memory and sensitivity to reward. The immediate early gene and transcription factor early growth response 1 (EGR1) has thus been revealed as a major mediator and regulator of synaptic plasticity and neuronal activity in both physiological and pathological conditions. In this review article, we will focus on the role of EGR1 in the central nervous system. First, we will summarize the different factors influencing its activity. Then, we will analyze the amount of data, including genome-wide, that has emerged in the recent years describing the wide variety of genes, pathways and biological functions regulated directly or indirectly by EGR1. We will thus be able to gain better insights into the mechanisms underlying EGR1's functions in physiological neuronal activity. Finally, we will discuss and illustrate the role of EGR1 in pathological states with a particular interest in cognitive functions and neuropsychiatric disorders.

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.

Glucocorticoid receptor translocation and expression of relevant genes in the hippocampus of adolescent and adult male rats

We investigated whether pre-pubertal (postnatal day [P] 35) and post-pubertal adolescent (P45) and adult (P75) male rats differed in stressor-induced hormonal responses and in glucocorticoid receptor (GR) translocation because it has been proposed that negative feedback is maturing in adolescence and may be a basis for the prolonged activation of the HPA axis in adolescents compared with adults. The three age groups did not differ at baseline in plasma corticosterone or progesterone concentrations, and P35 had lower concentrations of testosterone than did both P45 and P75 rats, which did not differ. After 30min of restraint stress, plasma concentrations of corticosterone and progesterone increased to a greater extent in the adolescents than in the adults. Whereas restraint stress increased concentrations of testosterone in adult males, concentrations decreased in adolescents. In all three age groups, restraint stress reduced GR expression in the cytosol and increased expression in the nucleus within the hippocampus, and the increase in nuclear GR was greater in pre-pubertal adolescents compared with adults. In a separate set of rats we investigated age differences in hippocampal mRNA expression of corticosteroid receptors (MR and GR) and of chaperones (FKBP5, FKBP4, BAG-1), which are known to modulate their activity, at baseline and after restraint stress. Restraint stress decreased the expression of GR and increased the expression of FKBP5 mRNA, and age was not a significant factor. Higher expression of FKBP4 mRNA was found at P35 than at P75. Most research of HPA function in adolescent rats has involved pre-pubertal rats; the present findings indicate that despite their increase in gonadal function, responses to stressors in P45 rats are more like those of pre-pubertal than adult rats. The greater stressor-induced GR translocation in pre-pubertal adolescents parallels their greater release of corticosterone in response to stressors, which may contribute to the enhanced sensitivity of adolescent rats to the effects of chronic stress exposures compared with adults.

Adolescence and the ontogeny of the hormonal stress response in male and female rats and mice

Adolescent development is marked by many changes in neuroendocrine function, resulting in both immediate and long-term influences on an individual's physiology and behavior. Stress-induced hormonal responses are one such change, with adolescent animals often showing different patterns of hormonal reactivity following a stressor compared with adults. This review will describe the unique ways in which adolescent animals respond to a variety of stressors and how these adolescent-related changes in hormonal responsiveness can be further modified by the sex and previous experience of the individual. Potential central and peripheral mechanisms that contribute to these developmental shifts in stress reactivity are also discussed. Finally, the short- and long-term programming effects of chronic stress exposure during adolescence on later adult hormonal responsiveness are also examined. Though far from a clear understanding of the neurobehavioral consequences of these adolescent-related shifts in stress reactivity, continued study of developmental changes in stress-induced hormonal responses may shed light on the increased vulnerability to physical and psychological dysfunctions that often accompany a stressful adolescence.

Opposite Sex Contact and Isolation: A Novel Depression/Anxiety Model

To mimic human mood disorders, traditional chronic stresses and social defeat stress have been developed and widely applied. However, these active stresses do not mimic the emotional flaws induced by stresses, and their input levels vary greatly. Also, emotional stresses resulting from social unobtainability remain largely elusive due to the lack of useful animal models. In this study, we developed a mouse model named "opposite sex contact and isolation" (OSCI) and found that OSCI induced significant social avoidance, anhedonia, and anxiety. These behavioral defects developed differently after 7 days of OSCI. The social avoidance behavior was self-curable while anxiety gradually worsened but was alleviated by re-pairing with the same female partner. Corresponding to the behavior changes, the plasma corticosterone and phosphorylated cAMP response element binding protein levels were decreased in the nucleus accumbens of the mice that experienced isolation. Together, this study has developed a novel strategy for depression/anxiety modeling and shows that OSCI may be a useful tool for studying the lovelorn/lovesick type of depression.