Influence of reactivity to novelty and anxiety on hypothalamic–pituitary–adrenal and prolactin responses to two different novel environments in adult male rats

Influence of footshock number and intensity on the behavioral and endocrine response to fear conditioning and cognitive fear generalization in male rats

Foot-shock paradigms have provided valuable insights into the neurobiology of stress and fear conditioning. An extensive body of literature indicates that shock exposure can elicit both conditioned and unconditioned effects, although delineating between the two is a challenging task. This distinction holds crucial implications not only for the theoretical interpretation of fear conditioning, but also for properly evaluating putative preclinical models of post-traumatic stress disorder (PTSD) involving shock exposure. The characteristics of shocks (intensity and number) affect the strength of learning, but how these characteristics interact to influence conditioned and unconditioned consequences of shocks are poorly known. In this study, we aimed to investigate in adult male rats the impact of varying shock number and intensity on the endocrine and behavioral response to contextual fear conditioning and fear generalization to a novel environment markedly distinct from the shock context (i.e., fear generalization). Classical biological markers of stress (i.e., ACTH, corticosterone, and prolactin) were sensitive to manipulations of shock parameters, whereas these parameters had a limited effect on contextual fear conditioning (evaluated by freezing and distance traveled). In contrast, behavior in different novel contexts (fear generalization) was specifically sensitive to shock intensity. Notably, altered behavior in novel contexts markedly improved, but not completely normalized after fear extinction, hypoactivity apparently being the result of both conditioned and unconditioned effects of foot-shock exposure. The present results will contribute to a better understanding of shock exposure as a putative animal model of PTSD.

Differential Hypothalamic-pituitary-adrenal Response to Stress among Rat Strains: Methodological Considerations and Relevance for Neuropsychiatric Research

The hormones of the hypothalamic-pituitary-adrenal (HPA) axis, particularly glucocorticoids (GCs), play a critical role in the behavioral and physiological consequences of exposure to stress. For this reason, numerous studies have described differences in HPA function between different rodent strains/lines obtained by genetic selection of certain characteristics not directly related to the HPA axis. These studies have demonstrated a complex and poorly understood relationship between HPA function and certain relevant behavioral characteristics. The present review first remarks important methodological considerations regarding the evaluation and interpretation of resting and stress levels of HPA hormones. Then, it presents works in which differences in HPA function between Lewis and Fischer rats were explored as a model for how to approach other strain comparisons. After that, differences in the HPA axis between classical strain pairs (e.g. High and Low anxiety rats, Roman high- and low-avoidance, Wistar Kyoto versus Spontaneously Hypertensive or other strains, Flinder Sensitive and Flinder Resistant lines) are described. Finally, after discussing the relationship between HPA differences and relevant behavioral traits (anxiety-like and depression-like behavior and coping style), an example for main methodological and interpretative concerns and how to test strain differences is offered.

The Role of Annexin A1 and Formyl Peptide Receptor 2/3 Signaling in Chronic Corticosterone-Induced Depression-Like behaviors and Impairment in Hippocampal-Dependent Memory

BACKGROUND

The activity of the Hypothalamic-Pituitary-Adrenal (HPA) axis is commonly dysregulated in stress-related psychiatric disorders. Annexin A1 (ANXA1), an endogenous ligand of Formyl Peptide Receptor (FPR) 2/3, is a member of the family of phospholipid- and calcium-binding proteins with a well-defined role in the delayed early inhibitory feedback of Glucocorticoids (GC) in the pituitary gland and implicated in the occurrence of behavioural disorders such as anxiety.

OBJECTIVE

The present study aimed to evaluate the potential role of ANXA1 and its main receptor, as a cellular mediator of behavioural disorders, in a model of Corticosterone (CORT)-induced depression and subsequently, the possible correlation between the depressive state and impairment of hippocampal memory.

METHODS

To induce the depression model, Wild-Type (WT), ANXA1 Knockout (KO), and FPR2/3 KO mice were exposed to oral administration of CORT for 28 days dissolved in drinking water. Following this, histological, biochemical and behavioural analyses were performed.

RESULTS

FPR2/3 KO and ANXA1 KO mice showed improvement in anxiety and depression-like behaviour compared with WT mice after CORT administration. In addition, FPR2/3 KO and ANXA1 KO mice showed a reduction in histological alterations and neuronal death in hippocampal sections. Moreover, CORT+ FPR2/3 KO and ANXA1 KO, exhibited a higher expression of Brain-Derived Neurotrophic Factor (BDNF), phospho-ERK, cAMP response element-binding protein (pCREB) and a decrease in Serotonin Transporter Expression (SERT) compared to WT(CORT+) mice.

CONCLUSION

In conclusion, the absence of the ANXA1 protein, even more than the absence of its main receptor (FPR 2/3), was fundamental to the inhibitory action of GC on the HPA axis; it also maintained the hippocampal homeostasis by preventing neuronal damage associated with depression.

Male long-Evans rats: An outbred model of marked hypothalamic-pituitary-adrenal hyperactivity

Rat and mouse strains differ in behavioral and physiological characteristics, and such differences can contribute to explain discrepant results between laboratories and better select the most appropriate strain for a particular purpose. Differences in the activity of the hypothalamic-pituitary-adrenal (HPA) axis are particularly important given the pivotal role of this system in determining consequences of exposure to stressors. In this regard, Long-Evans (LE) rats are widely used in stress research, but there is no specific study aiming at thoroughly characterizing HPA activity in LE versus other extensively used strains. In a first experiment, LE showed higher resting ACTH and corticosterone levels only at certain points of the circadian rhythm, but much greater ACTH responsiveness to stressors (novel environment and forced swim) than Sprague-Dawley (SD) rats. Accordingly, enhanced corticotropin-releasing hormone (CRH) expression in the paraventricular nucleus of the hypothalamus and reduced expression of glucocorticoid receptors were observed in the hippocampal formation. Additionally, they are hyperactive in novel environments, and prone to adopt passive-like behavior when compared to SD rats. Supporting that altered HPA function has a marked physiological impact, we observed in another set of animals much lower thymus weight in LE than SD rats. Finally, to demonstrate that LE rats are likely to have higher HPA responsiveness to stressors than most strains, we studied resting and stress levels of HPA hormones in LE versus Wistar and Fischer rats, the latter considered an example of high HPA responsiveness. Again, LE showed higher resting and stress levels of ACTH than both Wistar and Fischer rats. As ACTH responsiveness to stressors in LE rats is stronger than that previously reported when comparing other rat strains and they are commercially available, they could be an appropriate model for studying the behavioral and physiological implications of a hyper-active HPA axis under normal and pathological conditions.

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Strain differences in hypothalamic-pituitary-adrenal (HPA) function were studied.

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Long-Evans (LE) rats show greater HPA response to stressors than other strains.

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CRH expression in critical brain areas is greater in LE than Sprague-Dawley (SD) rats.

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Glucocorticoid receptor expression was lower in the hippocampal formation of LE rats.

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LE rats are more active in novel environments but showed more passive coping.

Can the Hole-Board Test Predict a Rat's Exploratory Behavior in a Free-Exploration Test?

This study focuses on the rat activity in a hole-board setting that we considered a type of exploratory behavior. The general hypothesis is based on the claim that a motivational mechanism is central to both the response to novelty in a highly familiarized environment and the activity in the hole-board apparatus. Our sample consisted of 80 experimentally naive Lister Hooded rats. All rats were tested in the hole-board apparatus. Twenty individuals with the highest hole-board scores and twenty subjects with the lowest hole-board scores subsequently underwent an established free-exploration test. In our study, the scores obtained in the hole-board test had little predictive value for the rats' activity in the free-exploration test. Based on our previous experience in studying exploratory behavior in the free-exploration test and the data presented in this paper, we suggest that the hole-board test is not an appropriate tool for measuring exploratory behavior in laboratory rodents.

Individual differences in the neuroendocrine response of male rats to emotional stressors are not trait-like and strongly depend on the intensity of the stressors

Biological response to stressors is critical to understand stress-related pathologies and vulnerability to psychiatric diseases. It is assumed that we can identify trait-like characteristics in biological responsiveness by testing subjects in a particular stressful situation, but there is scarce information on this issue. We then studied, in a normal outbred population of adult male rats (n = 32), the response of well-characterized stress markers (ACTH, corticosterone and prolactin) to different types of stressors: two novel environments (open-field, OF1 and OF2), an elevated platform (EP), forced swim (SWIM) and immobilization (IMO). Based on both plasma ACTH and prolactin levels, the OF1 was the lowest intensity situation, followed by the OF2 and the EP, then SWIM and finally IMO. When correlations between the individual responses to the different stressors were studied, the magnitude of the correlations was most dependent on the similarities in intensity rather than on other characteristics of stressors, with good correlations between similar intensity stressors and no correlations at all were found between stressors markedly differing in intensity. In two additional confirmatory experiments (n = 37 and n = 20) with HPA hormones, we observed good correlation between the response to restraint and IMO, which were close in intensity, and no correlation between OF1 and SWIM. The present results suggest that individual neuroendocrine response to a particular stressor does not predict the response to another stressor greatly differing in intensity, thus precluding characterization of low or high responsive individuals to any stressor in a normal population. The present data have important implications for human studies.

Modeling heritability of temperamental differences, stress reactivity, and risk for anxiety and depression: Relevance to research domain criteria (RDoC)

Animal models provide important tools to study biological and environmental factors that shape brain function and behavior. These models can be effectively leveraged by drawing on concepts from the National Institute of Mental Health Research Domain Criteria (RDoC) Initiative, which aims to delineate molecular pathways and neural circuits that underpin behavioral anomalies that transcend psychiatric conditions. To study factors that contribute to individual differences in emotionality and stress reactivity, our laboratory utilized Sprague-Dawley rats that were selectively bred for differences in novelty exploration. Selective breeding for low versus high locomotor response to novelty produced rat lines that differ in behavioral domains relevant to anxiety and depression, particularly the RDoC Negative Valence domains, including acute threat, potential threat, and loss. Bred Low Novelty Responder (LR) rats, relative to their High Responder (HR) counterparts, display high levels of behavioral inhibition, conditioned and unconditioned fear, avoidance, passive stress coping, anhedonia, and psychomotor retardation. The HR/LR traits are heritable, emerge in the first weeks of life, and appear to be driven by alterations in the developing amygdala and hippocampus. Epigenomic and transcriptomic profiling in the developing and adult HR/LR brain suggest that DNA methylation and microRNAs, as well as differences in monoaminergic transmission (dopamine and serotonin in particular), contribute to their distinct behavioral phenotypes. This work exemplifies ways that animal models such as the HR/LR rats can be effectively used to study neural and molecular factors driving emotional behavior, which may pave the way toward improved understanding the neurobiological mechanisms involved in emotional disorders.

Effects of Kappa opioid receptor blockade by LY2444296 HCl, a selective short-acting antagonist, during chronic extended access cocaine self-administration and re-exposure in rat

RATIONALE

Cocaine addiction is a chronic brain disease characterized by compulsive drug intake and dysregulation of brain reward systems. Few preclinical studies have modeled the natural longitudinal course of cocaine addiction. Extended access self-administration protocols are powerful tools for modeling the advanced stages of addiction; however, few studies have duration of drug access longer than 12 h/session, potentially limiting their construct validity. Identification of changes in cocaine intake patterns during the development of addictive-like states may allow better treatments for vulnerable subjects. The kappa opioid receptor (KOPr) system has been implicated in the neurobiological regulation of addictive states as well as mood and stress disorders, with selective KOPr antagonists proposed as possible pharmacotherapeutic agents. Chronic cocaine exposure increases the expression of KOPr and its endogenous agonists, the dynorphins, in several brain areas in rodents.

OBJECTIVES

To examine the behavioral pattern of intake during chronic (14 days) 18 h intravenous cocaine self-administration (0.5 mg/kg/infusion) and the effect of a novel short-acting KOPr antagonist LY2444296 HCl (3 mg/kg) administered during sessions 8 to 14 of chronic 18 h/day cocaine self-administration and prior to a single re-exposure session after 2 cocaine-free withdrawal days.

RESULTS

Both daily and hourly cocaine intake patterns changed over 14 days of 18 h self-administration. LY pretreatment affected the pattern of self-administration across the second week of extended access cocaine self-administration and prevented the increase in cocaine intake during re-exposure.

CONCLUSIONS

Overall, the KOPr antagonist attenuated escalated cocaine consumption in a rat model of extended access cocaine self-administration.

Increased cortisol awakening response after completing the summer treatment program in children with ADHD

OBJECTIVE

Little is known about the cortisol awakening response (CAR) in children with attention deficit hyperactivity disorder (ADHD). Here, we examined the CAR in children with ADHD and their mothers before, immediately after, and 4months after an intensive summer treatment program (STP).

METHODS

Participants were 37 children aged 7-12years who completed the STP in 2009 and 2010, and their mothers. Daily saliva samples for cortisol measurement were collected twice daily at awakening and 30min afterwards at pre-STP, post-STP, and during a follow-up measurement period. ADHD symptom scores were evaluated by parents, and participants completed the Kid-KINDL^R QOL questionnaire.

RESULTS

CAR was low in children with ADHD before the STP, and increased to the control range 4months after STP. Maternal CAR also tended to increase after STP. Changes in the CAR in children tended to correlate with an improved ADHD inattention scores (p=0.091), physical health (p=0.070), and school life subscales scores in the Kid-KINDL^R (p=0.079).

CONCLUSION

We demonstrated that STP improved the behavior and QOL of children with ADHD. Our results indicate that STP could lead to improvements in HPA axis function, as reflected by increased CAR after STP.

Cortisol levels at baseline and under stress in adolescent males with attention-deficit hyperactivity disorder, with or without comorbid conduct disorder

Reported findings on cortisol reactivity to stress in young people with ADHD are very variable. This inconsistency may be explained by high rates of comorbidity with Conduct Disorder (CD). The present study examined cortisol responses to a psychosocial stressor in a large sample of adolescent males with ADHD (n=202), with or without a comorbid diagnosis of Conduct Disorder (CD). Associations between stress reactivity and callous-unemotional traits and internalizing symptoms were also assessed. The ADHD only (n=95) and ADHD+CD (n=107) groups did not differ in baseline cortisol, but the ADHD+CD group showed significantly reduced cortisol stress reactivity relative to the ADHD only group. Regression analyses indicated that ADHD symptom severity predicted reduced baseline cortisol, whereas CD symptom severity predicted increased baseline cortisol (ADHD β=-0.24, CD β=0.16, R=0.26) and reduced cortisol stress reactivity (β=-0.17, R=0.17). Callous-unemotional traits and internalizing symptoms were not significantly related to baseline or stress-induced cortisol. Impaired cortisol reactivity is hypothesised to reflect fearlessness and is associated with deficient emotion regulation and inhibition of aggressive and antisocial behaviour. Consequently, it may partly explain the greater severity of problems seen in those with comorbid ADHD and CD.

Of rodents and humans: A comparative review of the neurobehavioral effects of early life SSRI exposure in preclinical and clinical research

Selective serotonin reuptake inhibitors (SSRIs) have been a mainstay pharmacological treatment for women experiencing depression during pregnancy and postpartum for the past 25 years. SSRIs act via blockade of the presynaptic serotonin transporter and result in a transient increase in synaptic serotonin. Long-lasting changes in cellular function such as serotonergic transmission, neurogenesis, and epigenetics, are thought to underlie the therapeutic benefits of SSRIs. In recent years, though, growing evidence in clinical and preclinical settings indicate that offspring exposed to SSRIs in utero or as neonates exhibit long-lasting behavioral adaptions. Clinically, children exposed to SSRIs in early life exhibit increased internalizing behavior reduced social behavior, and increased risk for depression in adolescence. Similarly, rodents exposed to SSRIs perinatally exhibit increased traits of anxiety- or depression-like behavior. Furthermore, certain individuals appear to be more susceptible to early life SSRI exposure than others, suggesting that perinatal SSRI exposure may pose greater risks for negative outcome within certain populations. Although SSRIs trigger a number of intracellular processes that likely contribute to their therapeutic effects, early life antidepressant exposure during critical neurodevelopmental periods may elicit lasting negative effects in offspring. In this review, we cover the basic development and structure of the serotonin system, how the system is affected by early life SSRI exposure, and the behavioral outcomes of perinatal SSRI exposure in both clinical and preclinical settings. We review recent evidence indicating that perinatal SSRI exposure perturbs the developing limbic system, including altered serotonergic transmission, neurogenesis, and epigenetic processes in the hippocampus, which may contribute to behavioral domains (e.g., sociability, cognition, anxiety, and behavioral despair) that are affected by perinatal SSRI treatment. Identifying the molecular mechanisms that underlie the deleterious behavioral effects of perinatal SSRI exposure may highlight biological mechanisms in the etiology of mood disorders. Moreover, because recent studies suggest that certain individuals may be more susceptible to the negative consequences of early life SSRI exposure than others, understanding mechanisms that drive such susceptibility could lead to individualized treatment strategies for depressed women who are or plan to become pregnant.

Novel environment influences the effect of paradoxical sleep deprivation upon brain and peripheral cytokine gene expression

Sleep loss increases inflammatory mediators in brain and peripheral tissues, but the mechanisms underlying this association are not fully understood. Male C57BL/6j mice were exposed to paradoxical sleep deprivation (PSD) for 24h using the modified multiple platform (MMP) technique (platforms over water) or two different controls: home cage or a dry platform cage, which constituted a novel environment. PSD mice exhibited increased IL-1β and TNF-α pro-inflammatory gene expression in brain (hypothalamus, hippocampus, pre-frontal cortex), as well as in peripheral tissues (liver, spleen), when compared with home-cage controls. In addition, among PSD mice, TGFβ1, an anti-inflammatory cytokine, was increased in pre-frontal cortex, liver, and spleen in conjunction with elevated serum corticosterone concentration relative to home-cage controls. However, these differences were nearly abolished when PSD mice were compared with control mice subjected to a dry MMP cage, suggesting that simply exposing mice to a novel environment can induce an acute inflammatory response.

Motor activity in young APPswe + PS1/A246E bigenic mice as a predicting variable for memory decline

Reports of individuality in rodent species have been a subject of debate in pharmacology and other fields. In the current study, APPswe  + PS1/A246E bigenic mice with Alzheimer's-like pathogenesis and wild-type controls were subdivided at 3 months of age into low, intermediate, and high responders in open-field activity. The mice were then evaluated longitudinally at 3 and 9 months for object recognition. Irrespective of genotype, mice with a high level of motor activity had better scores in object recognition. However, a significant correlation was established between open-field activity measured at 3 months of age and recognition memory measured at 9 months of age in the bigenic group only. These results indicate that motor activity in young mice with amyloid neuropathology may serve as a predicting variable for cognitive dysfunction in more mature mice.

Formyl peptide receptor as a novel therapeutic target for anxiety-related disorders

Formyl peptide receptors (FPR) belong to a family of sensors of the immune system that detect microbe-associated molecules and inform various cellular and sensorial mechanisms to the presence of pathogens in the host. Here we demonstrate that Fpr2/3-deficient mice show a distinct profile of behaviour characterised by reduced anxiety in the marble burying and light-dark box paradigms, increased exploratory behaviour in an open-field, together with superior performance on a novel object recognition test. Pharmacological blockade with a formyl peptide receptor antagonist, Boc2, in wild type mice reproduced most of the behavioural changes observed in the Fpr2/3^-/- mice, including a significant improvement in novel object discrimination and reduced anxiety in a light/dark shuttle test. These effects were associated with reduced FPR signalling in the gut as shown by the significant reduction in the levels of p-p38. Collectively, these findings suggest that homeostatic FPR signalling exerts a modulatory effect on anxiety-like behaviours. These findings thus suggest that therapies targeting FPRs may be a novel approach to ameliorate behavioural abnormalities present in neuropsychiatric disorders at the cognitive-emotional interface.

Early-life exposure to the SSRI paroxetine exacerbates depression-like behavior in anxiety/depression-prone rats

Selective serotonin reuptake inhibitor (SSRI) antidepressants are the mainstay treatment for the 10-20% of pregnant and postpartum women who suffer major depression, but the effects of SSRIs on their children's developing brain and later emotional health are poorly understood. SSRI use during pregnancy can elicit antidepressant withdrawal in newborns and increase toddlers' anxiety and social avoidance. In rodents, perinatal SSRI exposure increases adult depression- and anxiety-like behavior, although certain individuals are more vulnerable to these effects than others. Our study establishes a rodent model of individual differences in susceptibility to perinatal SSRI exposure, utilizing selectively bred Low Responder (bLR) and High Responder (bHR) rats that were previously bred for high versus low behavioral response to novelty. Pregnant bHR/bLR females were chronically treated with the SSRI paroxetine (10 mg/kg/day p.o.) to examine its effects on offspring's emotional behavior and gene expression in the developing brain. Paroxetine treatment had minimal effect on bHR/bLR dams' pregnancy outcomes or maternal behavior. We found that bLR offspring, naturally prone to an inhibited/anxious temperament, were susceptible to behavioral abnormalities associated with perinatal SSRI exposure (which exacerbated their Forced Swim Test immobility), while high risk-taking bHR offspring were resistant. Microarray studies revealed robust perinatal SSRI-induced gene expression changes in the developing bLR hippocampus and amygdala (postnatal days 7-21), including transcripts involved in neurogenesis, synaptic vesicle components, and energy metabolism. These results highlight the bLR/bHR model as a useful tool to explore the neurobiology of individual differences in susceptibility to perinatal SSRI exposure.

Reduced sleep, stress responsivity, and female sex contribute to persistent inflammation-induced mechanical hypersensitivity in rats

Studies in humans suggest that female sex, reduced sleep opportunities and biological stress responsivity increase risk for developing persistent pain conditions. To investigate the relative contribution of these three factors to persistent pain, we employed the Sciatic Inflammatory Neuritis (SIN) model of repeated left sciatic perineurial exposures to zymosan, an inflammatory stimulus, to determine their impact upon the development of persistent mechanical hypersensitivity. Following an initial moderate insult, a very low zymosan dose was infused daily for eight days to model a sub-threshold inflammatory perturbation to which only susceptible animals would manifest or maintain mechanical hypersensitivity. Using Sprague Dawley rats, maintaining wakefulness throughout the first one-half of the 12-h light phase resulted in a bilateral reduction in paw withdrawal thresholds (PWTs); zymosan infusion reduced ipsilateral PWTs in all animals and contralateral PWTs only in females. This sex difference was validated in Fischer 344, Lewis and Sprague Dawley rats, suggesting that females are the more susceptible phenotype for both local and centrally driven responses to repeated low-level inflammatory perturbations. Hypothalamic-pituitary-adrenal (HPA) axis hyporesponsive Lewis rats exhibited the most robust development of mechanical hypersensitivity and HPA axis hyperresponsive Fischer 344 rats matched the Lewis rats' mechanical hypersensitivity throughout the latter four days of the protocol. If HPA axis phenotype does indeed influence these findings, the more balanced responsivity of Sprague Dawley rats would seem to promote resilience in this paradigm. Taken together, these findings are consistent with what is known regarding persistent pain development in humans.

L-tetrahydropalmatine ameliorates development of anxiety and depression-related symptoms induced by single prolonged stress in rats

Abnormal adaptation of the stress-response system following traumatic stress can lead to alterations in the hypothalamic-pituitary-adrenal (HPA) axis that may contribute to the development of post-traumatic stress disorder (PTSD). The present study used several behavioral tests to investigate the anxiolytic-like and antidepressant activity of L-tetrahydropalmatine (L-THP) in an experimental rat model of anxiety and depression induced by single prolonged stress (SPS), an animal model of PTSD. Male rats were treated intraperitoneally (i.p.) with vehicle or varied doses of THP 30 min prior to SPS for 8 consecutive days. Daily THP (50 mg/kg) administration significantly increased the number and duration of open arm visits in the elevated plus maze (EPM) test, reduced the anxiety index, increased the risk assessment, and increased the number of head dips over the borders of the open arms after SPS. THP was also associated with increased time spent at the center of the open field, reduced grooming behaviors in the EPM test, and reduced time spent immobile in the forced swimming test (FST). It also blocked the decrease in neuropeptide Y (NPY) and the increase in corticotrophin-releasing factor (CRF) expression in the hypothalamus. This is the first study to determine that THP exerts pronounced anxiolytic-like and antidepressant effects on the development of the behavioral and biochemical symptoms associated with PTSD, indicating its prophylactic potential. Thus, THP reversed several behavioral impairments triggered by the traumatic stress of SPS and is a potential non-invasive therapeutic intervention for PTSD.

Alpha-Asarone, a Major Component of Acorus gramineus, Attenuates Corticosterone-Induced Anxiety-Like Behaviours via Modulating TrkB Signaling Process

We investigated the anxiolytic-like activity of α-asarone (AAS) from Acorus gramineus in an experimental rat model of anxiety induced by repeated administration of the exogenous stress hormone corticosterone (CORT). The putative anxiolytic effect of AAS was studied in behavioral tests of anxiety, such as the elevated plus maze (EPM) test and the hole-board test (HBT) in rats. For 21 consecutive days, male rats received 50, 100, or 200 mg/kg AAS (i.p.) 30 min prior to a daily injection of CORT. Dysregulation of the HPA axis in response to the repeated CORT injections was confirmed by measuring serum levels of CORT and the expression of corticotrophin-releasing factor (CRF) in the hypothalamus. Daily AAS (200 mg/kg) administration increased open-arm exploration significantly in the EPM test, and it increased the duration of head dipping activity in the HBT. It also blocked the increase in tyrosine hydroxylase (TH) expression in the locus coeruleus (LC) and decreased mRNA expression of brain-derived neurotrophic factor (BDNF) and its receptor, TrkB, in the hippocampus. These results indicated that the administration of AAS prior to high-dose exogenous CORT significantly improved anxiety-like behaviors, which are associated with modification of the central noradrenergic system and with BDNF function in rats. The current finding may improve understanding of the neurobiological mechanisms responsible for changes in emotions induced by repeated administration of high doses of CORT or by elevated levels of hormones associated with chronic stress. Thus, AAS did exhibit an anxiolytic-like effects in animal models of anxiety.

Tests of unconditioned anxiety - pitfalls and disappointments

The plus-maze, the light-dark box and the open-field are the main current tests of unconditioned anxiety for mice and rats. Despite their disappointing achievements, they remain as popular as ever and seem to play an important role in an ever-growing demand for behavioral phenotyping and drug screening. Numerous reviews have repeatedly reported their lack of consistency and reliability but they failed to address the core question of whether these tests do provide unequivocal measures of fear-induced anxiety, that these measurements are not confused with measures of fear-induced avoidance or natural preference responses - i.e. discriminant validity. In the present report, I examined numerous issues that undermine the validity of the current tests, and I highlighted various flaws in the aspects of these tests and the methodologies pursued. This report concludes that the evidence in support of the validity of the plus-maze, the light/dark box and the open-field as anxiety tests is poor and methodologically questionable.

Sex-dependent effects of an early life treatment in rats that increases maternal care: vulnerability or resilience?

Early life stress (ELS) in rodents has profound long-term effects that are partially mediated by changes in maternal care. ELS not only induces “detrimental” effects in adulthood, increasing psychopathology, but also promotes resilience to further stressors. In Long-Evans rats, we evaluated a combination of two procedures as a model of ELS: restriction of bedding during the first post-natal days and exposure to a “substitute” mother. The maternal care of biological and “substitute” mothers was measured. The male and female offspring were evaluated during adulthood in several contexts. Anxiety was measured by the elevated plus-maze (EPM), acoustic startle response (ASR) and forced swim test (FST). In other group of animals, novelty-seeking was measured (activity in an inescapable novel environment, preference for novel environments and exploration of novel objects). Plasmatic ACTH and corticosterone in basal conditions and in response to stress were also measured. Cognitive impulsivity was assessed by a delay-discounting paradigm, and impulsive action, attention and compulsive-like behavior by a five choice serial reaction time task (5CSRTT). ELS decreased pup body weight and increased the care of the biological mother; however, the “substitute” mother did not exhibit overt maltreatment. A mixture of “detrimental” and “beneficial” effects was shown. In the 5CSRTT, attention was impaired in both genders, and in females, ELS increased compulsive-like behavior. Novel object exploration was only increased by ELS in males, but the preference for novel spaces decreased in both genders. Baseline anxiety (EPM and ASR) and recognition memory were not affected. Unexpectedly, ELS decreased the ACTH response to novelty and swim stress and increased active coping in the FST in both genders. Cognitive impulsivity was decreased only in females, but impulsive action was not affected. The enhancement in maternal care may “buffer” the effects of ELS in a context-dependent manner.

Individual differences and the characterization of animal models of psychopathology: a strong challenge and a good opportunity

Despite the development of valuable new techniques (i.e., genetics, neuroimage) for the study of the neurobiological substrate of psychiatric diseases, there are strong limitations in the information that can be gathered from human studies. It is thus critical to develop appropriate animal models of psychiatric diseases to characterize their putative biological bases and the development of new therapeutic strategies. The present review tries to offer a general perspective and several examples of how individual differences in animals can contribute to explain differential susceptibility to develop behavioral alterations, but also emphasizes methodological problems that can lead to inappropriate or over-simplistic interpretations. A critical analysis of the approaches currently used could contribute to obtain more reliable data and allow taking full advantage of new and sophisticated technologies. The discussion is mainly focused on anxiety-like and to a lower extent on depression-like behavior in rodents.

Male rats that differ in novelty exploration demonstrate distinct patterns of sexual behavior

High- versus low-novelty exploration predicts a variety of behavioral differences. For example, rats selectively bred for high-novelty exploration (bred-high responders, bHR) exhibit exaggerated aggression, impulsivity, and proclivity to addictive behaviors compared with low-novelty reactive rats (bred-low responders, bLRs), which are characterized by a high anxiety/depressive-like phenotype. Since bHR/bLR rats exhibit differences in dopaminergic circuitry and differential response to rewarding stimuli (i.e., psychostimulants, food), the present study examined whether they also differ in another key hedonic behavior-sex. Thus, adult bHR/bLR males were given five 30-min opportunities to engage in sexual activity with a receptive female. Sexual behavior and motivation were examined and compared between the groups. The bHR/bLR phenotype affected both sexual motivation and behavior, with bLR males demonstrating reduced motivation for sex compared with bHR males (i.e., fewer animals copulated, longer latency to engage in sex). The bHR males required more intromissions at a faster pace per ejaculation than did bLR males. Thus, neurobiological differences that affect motivation for drugs of abuse, aggression, and impulsivity in rats also affect sexual motivation and performance.

What can we know from pituitary-adrenal hormones about the nature and consequences of exposure to emotional stressors?

Exposure to stress induces profound physiological and behavioral changes in the organisms and some of these changes may be important regarding stress-induced pathologies and animal models of psychiatric diseases. Consequences of stress are dependent on the duration of exposure to stressors (acute, chronic), but also of certain characteristics such as intensity, controllability, and predictability. If some biological variables were able to reflect these characteristics, they could be used to predict negative consequences of stress. Among the myriad of physiological changes caused by stress, only a restricted number of variables appears to reflect the intensity of the situation, mainly plasma levels of ACTH and adrenaline. Peripheral hypothalamic-pituitary-adrenal (HPA) hormones (ACTH and corticosterone) are also able to reflect fear conditioning. In contrast, the activation of the HPA axis is not consistently related to anxiety as evaluated by classical tests such as the elevated plus-maze. Similarly, there is no consistent evidence about the sensitivity of the HPA axis to psychological variables such as controllability and predictability, despite the fact that: (a) lack of control over aversive stimuli can induce behavioral alterations not seen in animals which exert control, and (b) animals showed clear preference for predictable versus unpredictable stressful situations. New studies are needed to re-evaluate the relationship between the HPA axis and psychological stress characteristics using ACTH instead of corticosterone and taking advantages of our current knowledge about the regulation of this important stress system.

Different emotional disturbances in two experimental models of temporal lobe epilepsy in rats

Affective symptoms such as anxiety and depression are frequently observed in patients with epilepsy. The mechanisms of comorbidity of epilepsy and affective disorders, however, remain unclear. Diverse models are traditionally used in epilepsy research, including the status epilepticus (SE) model in rats, which are aimed at generating chronic epileptic animals; however, the implications of different SE models and rat strains in emotional behaviors has not been reported. To address this issue, we examined the emotional sequelae of two SE models of temporal lobe epilepsy (TLE)--the lithium-pilocarpine (LIP) model and the kainic acid (KA) model--in two different rat strains (Wistar and Sprague-Dawley), which differ significantly in the pattern and extent of TLE-associated brain lesions. We found differences between LIP- and KA-treated animals in tests for depression-like and anxiety-like behaviors, as well as differences in plasma corticosterone levels. Whereas only LIP-treated rats displayed increased motivation to consume saccharin, both SE models led to reduced motivation for social contact, with LIP-treated animals being particularly affected. Evaluation of behavior in the open field test indicated very low levels of anxiety in LIP-treated rats and a mild decrease in KA-treated rats compared to controls. After exposure to a battery of behavioral tests, plasma corticosterone levels were increased only in LIP-treated animals. This hyperactivity in the hypothalamus-pituitary-adrenocortical (HPA) axis was highly correlated with performance in the open field test and the social interaction test, suggesting that comorbidity of epilepsy and emotional behaviors might also be related to other factors such as HPA axis function. Our results indicate that altered emotional behaviors are not inherent to the epileptic condition in experimental TLE; instead, they likely reflect alterations in anxiety levels related to model-dependent dysregulation of the HPA axis.

Reproductive experience modifies the effects of estrogen receptor alpha activity on anxiety-like behavior and corticotropin releasing hormone mRNA expression

Previous studies have demonstrated that prior reproductive experience can influence anxiety-like behaviors, although neural mechanisms underlying this shift remain unknown. Studies in virgin females suggest that activation of the two estrogen receptor subtypes, ERα and ERβ, have differing effects on anxiety. Specifically, ERβ activation has been shown to reduce anxiety-like behaviors, while ERα activation has no significant effect. The purpose of the present study was to examine the possible roles of ERα and ERβ subtypes in parity-induced alterations in anxiety-like behavior, as tested on the elevated plus maze (EPM). Groups of ovariectomized, age-matched, nulliparous and primiparous females were tested on the EPM following administration of the ERα agonist 4,4',4''-(4-Propyl-{1H}-pyrazole-1,3,5-tryl)trisphenol (PPT; 1 mg/kg), the ERβ agonist Diarylpropionitrile (DPN; 1 mg/kg) or vehicle (DMSO). All drugs were administered once daily for 4 days prior to testing as this dosing paradigm has previously been used to demonstrate anxiolytic effects of DPN in virgin rats. In addition, as exposure to the EPM is a psychological stressor, physiological markers of the stress response were measured in both plasma (corticosterone) and brain (corticotropin releasing hormone; CRH) post-EPM testing. Unexpectedly, the ERα agonist PPT selectively increased the time spent exploring the open arms of the EPM in non-lactating, primiparous females, with no significant effects of DPN observed in either nulliparous or primiparous subjects. All females administered PPT and tested on the EPM demonstrated significantly reduced corticosterone secretion when compared to vehicle-treated controls. In addition, significant effects of both reproductive experience and PPT administration on CRH mRNA expression were observed in both the paraventricular nucleus and amygdala using qPCR. These findings indicate that reproductive experience modulates the effects of ERα activation on both EPM behavior related to anxiety and CRH gene expression.

Adrenocortical and behavioural response to chronic restraint stress in neurokinin-1 receptor knockout mice

Brain substance P and its receptor (neurokinin-1, NK1) have a widespread brain distribution and are involved in an important number of behavioural and physiological responses to emotional stimuli. However, the role of NK1 receptors in the consequences of exposure to chronic stress has not been explored. The present study focused on the role of these receptors in the hypothalamic-pituitary-adrenal (HPA) response to daily repeated restraint stress (evaluated by plasma corticosterone levels), as well as on the effect of this procedure on anxiety-like behaviour, spatial learning and memory in the Morris water maze (MWM), a hippocampus-dependent task. Adult null mutant NK1-/- mice, with a C57BL/6J background, and the corresponding wild-type mice showed similar resting corticosterone levels and, also, did not differ in corticosterone response to a first restraint. Nevertheless, adaptation to the repeated stressor was faster in NK1-/- mice. Chronic restraint modestly increased anxiety-like behaviour in the light-dark test, irrespective of genotype. Throughout the days of the MWM trials, NK1-/- mice showed a similar learning rate to that of wild-type mice, but had lower levels of thigmotaxis and showed a better retention in the probe trial. Chronic restraint stress did not affect these variables in either genotype. These results indicate that deletion of the NK1 receptor does not alter behavioural susceptibility to chronic repeated stress in mice, but accelerates adaptation of the HPA axis. In addition, deletion may result in lower levels of thigmotaxis and improved short-term spatial memory, perhaps reflecting a better learning strategy in the MWM.

Behavioral and endocrine consequences of simultaneous exposure to two different stressors in rats: interaction or independence?

Although behavioral and endocrine consequences of acute exposure to stressors have been extensively studied, little is known about how simultaneous exposure to two different stressors interacts to induce short- and long-term effects. In the present experiment we studied this interaction in adult male rats exposed to cat fur odor (impregnated cloth) or immobilization on boards either separately or simultaneously. We reasoned that exposure to the odor of a potential predator while immobilized, may potentiate its negative consequences as compared to exposure to only one of the stressors. Exposure to cat odor elicited the expected reduction of activity and avoidance of the area where the impregnated cloth was located. The endocrine response (plasma levels of ACTH and corticosterone, as a measure of the hypothalamic-pituitary-adrenal axis, HPA) was markedly greater after immobilization than after cat fur odor and no additive effects were found by simultaneous exposure to both stressors. Cat odor, but not immobilization, increased anxiety-like behavior as evaluated in the elevated plus-maze 7 days after the stressors, with no evidence of enhanced HPA activation. In addition, cat odor exposure resulted in long-lasting (8 days later) fear conditioning to the box containing a clean cloth, which was reflected by hypoactivity, avoidance of the cloth area and enhanced HPA activation. All these effects were similarly observed in rats exposed simultaneously to cat odor and immobilization. In rats only exposed to immobilization, only some weak behavioral signs of fear conditioning were found, but HPA activation in response to the context paired to immobilization was enhanced to the same extent as in cat odor-exposed animals, supporting a certain degree of endocrine conditioning. The present results did not reveal important behavioral interactions between the two stressors when animals experienced both simultaneously, whereas some interactions were found regarding HPA activation. Theoretical implications are discussed.

Differential impact of polysialyltransferase ST8SiaII and ST8SiaIV knockout on social interaction and aggression

Previous studies using neuronal cell adhesion molecule (NCAM) -/- knockout (KO) mice provided evidence for a role of NCAMs in social behaviors. However, polysialic acid (PSA), the most important post-translational modification of NCAM, was also absent in these mice, which makes it difficult to distinguish between the specific involvement of either PSA or NCAM in social interactions. To address this issue, we assessed two lines of mice deficient for one of the two sialyltransferase enzymes required for the polysialylation of NCAM, sialyltransferase-X (St8SiaII or STX) and polysialyltransferase (ST8SiaIV or PST), in a series of tests for social behaviors. Results showed that PST KO mice display a decreased motivation in social interaction. This deficit can be partly explained by olfactory deficits and was associated with a clear decrease in PSA-NCAM expression in all brain regions analyzed (amygdala, septum, bed nucleus of the stria terminalis and frontal cortices). STX KO mice displayed both a decreased social motivation and an increased aggressive behavior that cannot be explained by olfactory deficits. This finding might be related to the reduced anxiety-like behavior, increased locomotion and stress-induced corticosterone secretion observed in these mice. Moreover, STX KO mice showed mild increase of PSA-NCAM expression in the lateral septum and the orbitofrontal cortex. Altogether, these findings support a role for PSA-NCAM in the regulation of social behaviors ranging from a lack of social motivation to aggression. They also underscore STX KO mice as an interesting animal model that combines a behavioral profile of violence and hyperactivity with reduced anxiety-like behavior.

Sex differences in behavioral and corticosterone responses to mild stressors in ICR mice are altered by ovariectomy in peripubertal period

Among rodents, females are generally considered to be highly responsive in terms of emotionality under stressful conditions, and have higher corticosterone levels and activity. In this study, we examined sex differences in mice by evaluating anxiety behaviors and corticosterone responses to mild stressors. In our first experiment, we analyzed the behavioral and corticosterone responses to the elevated plus-maze test and open-field test in male and female mice, and compared sex differences. Principal component analysis (PCA) was used to investigate the correlation of these responses between males and females. The corticosterone level was higher in females under both basal and stressed conditions. In the behavioral response, higher locomotor activity was seen in females in the elevated plus-maze test. PCA showed little association among anxiety behavior, locomotor activity, and corticosterone secretion. In our second experiment, we examined the activational effects of sex steroids on the corticosterone response to the elevated plus-maze test by gonadectomizing male and female mice and using testosterone or estrogen capsules as hormonal replacements. Sex differences at the basal corticosterone level were not altered by the hormonal milieu in adults, however the higher corticosterone level of females in response to stress was diminished by ovariectomy, although replacement with neither testosterone nor estrogen had any effect. These results suggest that the sex difference in novelty exposure observed in the form of a greater hypothalamic-pituitary-adrenal (HPA) axis response in female ICR mice is controlled by ovary-derived factors in adults.

Locomotor and stress responses to nicotine differ in adolescent and adult rats

Since adolescence is a critical period for the initiation of tobacco use, we have systematically compared behavioral and endocrine responses to nicotine in Sprague-Dawley rats of both sexes at early adolescence (postnatal day (P) 28), mid- adolescence (P38) and adulthood (P90). Locomotion and center time in a novel open field were evaluated for 30min following intravenous injection of saline or nicotine (60microg/kg), followed by measurement of plasma corticosterone. Complex age and sex differences in behavioral and endocrine response were observed, which were dependent on the functional endpoint examined. Whereas there were age differences in nicotine effects on all functional measures, sex differences were largely restricted to adult stress-related corticosterone and center-time responses. Although significant drug effects were detected at P28 and P90, there was no effect of nicotine at P38 on any measure examined. In saline-treated males, but not females, there were significant positive correlations across age between initial ambulatory counts and both initial vertical counts and total center time. Nicotine treatment increased correlations in both sexes, and yielded a significant negative interaction between initial ambulatory counts and plasma corticosterone. The unique responses of adolescents to nicotine are consistent with an immature function of nicotinic acetylcholine receptors at this age.

Do odors from different cats induce equivalent unconditioned and conditioned responses in rats?

Since cats are predators of rats and mice, interest in the consequences of exposure to cat-associated odors has increased in the last decade, particularly regarding the development of putative animal models of post-traumatic stress disorder (PTSD). Although in the literature there are some comments on the variability of the effects depending on the individual cat used, there are no reports on this subject. In the present study, we demonstrated, using male Sprague-Dawley rats and cloths impregnated with fur/skin odors from three different cats (one ovariectomized female and two intact males), that the unconditioned endocrine (release of corticosterone) and behavioral (inhibition of activity and avoidance of the cloth area) responses to the presence of the cat odors were statistically significant and similar among the three cats. The conditioned behavioral response to a clean cloth, studied 7 days after the initial exposure to odors, was also evident with the three cats, with minor differences among them. In contrast, only the fur odor from one of the male cats clearly increased anxiety-like behavior in the elevated plus-maze 6 days later. These data indicate that: (i) evaluation of unconditioned and conditioned responses to cat odors does not appear to strongly predict long-lasting increases in anxiety-like behavior; and, therefore, both types of responses are partially dissociated; and (ii) differences among cats mainly affected the induction of long-lasting changes in anxiety-like behavior. The ultimate reasons for these differences are not known, but their characterization is critical for a proper understanding of putative PTSD models.

Dopamine D1 and D2 dopamine receptors regulate immobilization stress-induced activation of the hypothalamus-pituitary-adrenal axis

RATIONALE

Whereas the role of most biogenic amines in the control of the hypothalamus-pituitary-adrenal (HPA) response to stress has been extensively studied, the role of dopamine has not.

OBJECTIVES

We studied the effect of different dopamine receptor antagonists on HPA response to a severe stressor (immobilization, IMO) in adult male Sprague-Dawley rats.

RESULTS

Haloperidol administration reduced adrenocorticotropin hormone and corticosterone responses to acute IMO, particularly during the post-IMO period. This effect cannot be explained by a role of dopamine to maintain a sustained activation of the HPA axis as haloperidol did not modify the response to prolonged (up to 6 h) IMO. Administration of more selective D1 and D2 receptor antagonists (SCH23390 and eticlopride, respectively) also resulted in lower and/or shorter lasting HPA response to IMO.

CONCLUSIONS

Dopamine, acting through both D1 and D2 receptors, exerts a stimulatory role on the activation of the HPA axis in response to a severe stressor. The finding that dopamine is involved in the maintenance of post-stress activation of the HPA axis is potentially important because the actual pathological impact of HPA activation is likely to be related to the area under the curve of plasma glucocorticoid levels, which is critically dependent on how long after stress high levels of glucocorticoid are maintained.

Cat odor causes long-lasting contextual fear conditioning and increased pituitary-adrenal activation, without modifying anxiety

A single exposure to a cat or cat odors has been reported by some groups to induce contextual and auditory fear conditioning and long-lasting changes in anxiety-like behaviour, but there is no evidence for parallel changes in biological stress markers. In the present study we demonstrated in male rats that exposure to a novel environment containing a cloth impregnated with cat fur odor resulted in avoidance of the odor, lower levels of activity and higher pituitary-adrenal (PA) response as compared to those exposed to the novel environment containing a clean cloth, suggesting increased levels of stress in the former animals. When re-exposed 9 days later to the same environment with a clean cloth, previously cat fur exposed rats again showed avoidance of the cloth area and lower levels of activity, suggesting development of contextual fear conditioning, which again was associated with a higher PA activation. In contrast, unaltered both anxiety-like behaviour and PA responsiveness to an elevated plus-maze were found 7 days after cat odor exposure. It is concluded that: (i) PA activation is able to reflect both the stressful properties of cat fur odor and odor-induced contextual fear conditioning; (ii) development of cat odor-induced contextual fear conditioning is independent of the induction of long-lasting changes in anxiety-like behaviour; and (iii) greater PA activation during exposure to the odor context is not explained by non-specific sensitization of the PA axis caused by previous exposure to cat fur odor.

Stable behavioral inhibition and glucocorticoid production as predictors of longevity

Several personality/temperament traits have been linked to health outcomes in humans and animals but underlying physiological mechanisms for these differential outcomes are minimally understood. In this paper, we compared the strength of a behavioral trait (behavioral inhibition) and an associated physiological trait (glucocorticoid production) in predicting life span. In addition, we examined the relative stability of both the behavioral and physiological traits within individuals over a significant portion of adulthood, and tested the hypothesis that a stable behavioral trait is linked with a stable physiological bias. In a sample of 60 Sprague-Dawley male rats, we found that stable inhibition/neophobia was a stronger predictor of life span than stably elevated glucocorticoid production. In addition, these predictors appeared to have an additive influence on life span in that males with both risk factors (stable inhibition and consistently high glucocorticoid production) had the shortest life spans of all, suggesting both traits are important predictors of life span. Across a 4-month period in young adulthood, inhibition and glucocorticoid reactivity were relatively stable traits, however these two traits were not highly correlated with one another. Interestingly, baseline glucocorticoid production was a better predictor of life span than reactivity levels. The results indicate that glucocorticoid production in young adulthood is an important predictor of life span, although not as strong a predictor as inhibition, and that other physiological processes may further explain the shortened life span in behaviorally-inhibited individuals.

Exposure to severe stressors causes long-lasting dysregulation of resting and stress-induced activation of the hypothalamic-pituitary-adrenal axis

Exposure to some predominantly emotional (electric shock) and systemic (interleukin-1beta) stressors has been found to induce long-term sensitization of the hypothalamic-pituitary-adrenal (HPA) responsiveness to further superimposed stressors. Since exposure to immobilization on wooden boards (IMO) is a severe stressor and may have interest regarding putative animal models of post-traumatic stress disorders (PTSD), we have characterized long-lasting effects of a single exposure to IMO and other stressors on the HPA response to the same (homotypic) and to novel (heterotypic) stressors and the putative mechanisms involved. A single exposure to IMO caused a long-lasting reduction of peripheral and central responses of the HPA axis, likely to be mediated by some brain areas, such as the lateral septum and the medial amygdala. This desensitization is not explained by changes in negative glucocorticoid feedback, and, surprisingly, it is positively related to the intensity of the stressors. In contrast, the HPA response to heterotypic stressors (novel environments) was enhanced, with maximal sensitization on the day after IMO. Sensitization progressively vanished over the course of 1-2 weeks and was not modulated by IMO-induced corticosterone release. Moreover, it could not be explained by changes in the sensitivity of the HPA axis to fast or intermediate/delayed negative feedback, as evaluated 1 week after exposure to IMO, using shock as the heterotypic stressor. Long-lasting stress-induced behavioral changes reminiscent of enhanced anxiety and HPA sensitization are likely to be parallel but partially independent phenomena, the former being apparently not related to the intensity of stressors.

Noradrenergic neurons expressing Fos during waking and paradoxical sleep deprivation in the rat

Noradrenaline is known to induce waking (W) and to inhibit paradoxical sleep (PS or REM). Both roles have been exclusively attributed to the noradrenergic neurons of the locus coeruleus (LC, A6), shown to be active during W and inactive during PS. However, the A1, A2, A5 and A7 noradrenergic neurons could also be responsible. Therefore, to determine the contribution of each of the noradrenergic groups in W and in PS inhibition, rats were maintained in continuous W for 3h in a novel environment or specifically deprived of PS for 3 days, with some of them allowed to recover from this deprivation. A double immunohistochemical labeling with Fos and tyrosine hydroxylase was then performed. Thirty percent of the LC noradrenergic cells were found to be Fos-positive after exposure to the novel environment and less than 2% after PS deprivation. In contrast, a significant number of double-labeled neurons (up to 40% of the noradrenergic neurons) were observed in the A1/C1, A2 and A5 groups, after both novel environment and PS deprivation. After PS recovery and in control condition, less than 1% of the noradrenergic neurons were Fos-immunoreactive, regardless of the noradrenergic group. These results indicate that the brainstem noradrenergic cell groups are activated during W and silent during PS. They further suggest that the inhibitory effect of noradrenaline on PS may be due to the A1/C1, A2 and to a lesser degree to A5 neurons but not from those of the LC as previously hypothesized.

Breed differences in behavioural development in kittens

Differences in behaviour of pure breed cats have been suggested but not wholly investigated. Oriental/Siamese/Abyssinian (OSA) kittens (n=43) were weekly compared with Norwegian Forest (NFO) kittens (n=39) from the 4th to the 10th week of age in a repeated Open Field Test (OFT) paradigm. Heart rate (HR) and rectal temperature (RT) before and after the test, and behavioural responses during the OFT were recorded. Behaviours registered were analysed by focal animal sampling. Significant breed differences were found; cats of the northern zones (NFO) seem to develop earlier thermoregulatory abilities. Precocious opening of eyes, higher locomotion scores and longer time spent standing, observed in OSA kittens may indicate an earlier neurological development. Inter breed differences recorded for exploration and locomotion seem to indicate coping style divergences: in the OFT challenging situation OSA kittens presented higher emotional tachycardia and performed more passively, with a faster decline in exploration and locomotion scores. NFO kittens exerted a more active behaviour as they spent more time exploring the arena and in escape attempts. Notwithstanding OSA and NFO cat selection was mainly aimed to improve divergent morphological traits, some different behavioural and physiological traits seem to have been maintained or co-selected within each breed.

Marked dissociation between hypothalamic-pituitary-adrenal activation and long-term behavioral effects in rats exposed to immobilization or cat odor

Exposure of rodents to cats or certain cat odors results in long-term behavioral effects reminiscent of enhanced anxiety that have been considered to model post-traumatic stress disorder. However, other severe stressors such as tail-shock or immobilization in wooden boards (IMO) appear to induce shorter lasting changes in anxiety. In addition, there are controversial results regarding the effects of urine/feces odors. In the present work, we studied in two experiments the relationship between the degree of stress experienced by the animals during exposure to IMO, urine odors or fur odors (as assessed by hypothalamic-pituitary-adrenal activation and plasma glucose) and the short- and long-term behavioral consequences. In the first experiment, rats were individually exposed for 15 min to a novel environment (white large cages) containing either clean cat litter (controls) or litter soiled by cats (urine odors). Half of the rats in each condition were left to freely explore the environment whereas the others were subjected to immobilization (IMO) within the cages. Although ACTH, corticosterone and glucose responses to IMO were much stronger than those to the white cages with clean litter or urine odors (which did not differ from each other), no effect of treatments on anxiety-like behavior in the elevated plus-maze (EPM) were found one week later. However, previous IMO exposure did cause sensitization of the ACTH response to the EPM. In the second experiment, the response to white large cages containing either no odor (controls), litter soiled by cats (urine odor) or a cloth impregnated with cat odor (fur odor) was compared. Urine and fur odors elicited similar ACTH and corticosterone responses that were higher than those of controls, but plasma glucose levels were slightly higher in rats exposed to fur odor. When compared to controls, activity was only diminished in the novel cages containing fur odor. Similarly, fur odor-exposed rats, but not those exposed to urine odor, showed signs of enhanced anxiety in the EPM seven days later, although the ACTH response to the EPM was similar in the three groups. The present data demonstrate: (a) a marked dissociation between the degree of ACTH, corticosterone and glucose responses to stressors and their long-term anxiety-like effects; (b) that the type of cat odor is critical in determining the short-term and long-term physiological and behavioral consequences of exposure; and (c) that plasma ACTH released during brief exposure to the EPM does not appear to reflect anxiety-like behavior.

Behavioural and hormonal effects of social isolation and neophobia in a gregarious bird species, the European starling (Sturnus vulgaris)

Separating gregarious individuals from their group members often results in behavioural and physiological changes, like increased levels of corticosterone. Testosterone and corticosterone, in particular, have been implicated in the response of mammals to novelty. Data in birds are, however, rare. The presence or absence of group members may also influence an individual's response to novel stimuli. We assessed the behaviour and hormonal response of European starlings (Sturnus vulgaris) to a novel object in two different situations and seasons: each starling was tested when separated and when in contact with its group members in May/June (breeding season) and again in September/October (non-breeding season). Starlings are gregarious throughout the year, but as foraging flocks are small during the breeding season and large during the non-breeding season, we assumed that non-breeding starlings would be more affected by social isolation. Overall, starlings had higher levels of corticosterone, lost more body mass, and were more active when they were separated from their group. Isolated individuals, however, did not show a greater neophobic response than individuals in the presence of their group members in either season. Circulating levels of testosterone and corticosterone were higher after a test with novel object than after a test with only the familiar feeding dish in both sexes and seasons. However, control tests for handling effects confirmed only the increase in testosterone. Our study shows that social isolation is stressful for unrelated and unpaired members of a wild flocking bird species and demonstrates that novelty can lead to a rise in testosterone in birds.

The exploratory behaviour of rats in the hole-board apparatus: is head-dipping a valid measure of neophilia?

The exploratory behaviour of laboratory rodents is of interest within a number of areas of behavioural pharmacology. However, how best to measure exploratory behaviour in rodents remains a contentious issue. Many unconditioned tests, such as the open field, potentially confound general locomotor activity with exploration. The hole-board apparatus appears to avoid this confound, as head-dipping into holes in the floor is assumed to be a valid measure of the subject's attraction towards novelty (neophilia). This study aimed to investigate whether head-dipping should be considered a valid measure of neophilia by comparing performance of adult male and female Lister hooded rats on the hole-board task (a) over repeated sessions and (b) when novel objects were absent or present underneath the holes. The results show that head-dipping initially decreased across repeated exposures, while time spent in the aversive central area increased. No change in head-dipping was seen in response to objects being placed underneath the holes. Rather than being a measure of neophilia, these results support the hypothesis that head-dipping represents an escape response, which declines as the subject becomes less fearful. These results are compared with previous studies of repeated exposure to other novel environments.

Stress and memory: behavioral effects and neurobiological mechanisms

Stress is a potent modulator of learning and memory processes. Although there have been a few attempts in the literature to explain the diversity of effects (including facilitating, impairing, and lack of effects) described for the impact of stress on memory function according to single classification criterion, they have proved insufficient to explain the whole complexity of effects. Here, we review the literature in the field of stress and memory interactions according to five selected classifying factors (source of stress, stressor duration, stressor intensity, stressor timing with regard to memory phase, and learning type) in an attempt to develop an integrative model to understand how stress affects memory function. Summarizing on those conditions in which there was enough information, we conclude that high stress levels, whether intrinsic (triggered by the cognitive challenge) or extrinsic (induced by conditions completely unrelated to the cognitive task), tend to facilitate Pavlovian conditioning (in a linear-asymptotic manner), while being deleterious for spatial/explicit information processing (which with regard to intrinsic stress levels follows an inverted U-shape effect). Moreover, after reviewing the literature, we conclude that all selected factors are essential to develop an integrative model that defines the outcome of stress effects in memory processes. In parallel, we provide a brief review of the main neurobiological mechanisms proposed to account for the different effects of stress in memory function. Glucocorticoids were found as a common mediating mechanism for both the facilitating and impairing actions of stress in different memory processes and phases. Among the brain regions implicated, the hippocampus, amygdala, and prefrontal cortex were highlighted as critical for the mediation of stress effects.

Stress and memory: behavioral effects and neurobiological mechanisms

Stress is a potent modulator of learning and memory processes. Although there have been a few attempts in the literature to explain the diversity of effects (including facilitating, impairing, and lack of effects) described for the impact of stress on memory function according to single classification criterion, they have proved insufficient to explain the whole complexity of effects. Here, we review the literature in the field of stress and memory interactions according to five selected classifying factors (source of stress, stressor duration, stressor intensity, stressor timing with regard to memory phase, and learning type) in an attempt to develop an integrative model to understand how stress affects memory function. Summarizing on those conditions in which there was enough information, we conclude that high stress levels, whether intrinsic (triggered by the cognitive challenge) or extrinsic (induced by conditions completely unrelated to the cognitive task), tend to facilitate Pavlovian conditioning (in a linear-asymptotic manner), while being deleterious for spatial/explicit information processing (which with regard to intrinsic stress levels follows an inverted U-shape effect). Moreover, after reviewing the literature, we conclude that all selected factors are essential to develop an integrative model that defines the outcome of stress effects in memory processes. In parallel, we provide a brief review of the main neurobiological mechanisms proposed to account for the different effects of stress in memory function. Glucocorticoids were found as a common mediating mechanism for both the facilitating and impairing actions of stress in different memory processes and phases. Among the brain regions implicated, the hippocampus, amygdala, and prefrontal cortex were highlighted as critical for the mediation of stress effects.

Litter size affects emotionality in adult male rats

The role of natural variations in pre-weaning litter size in rodent adult emotionality and the importance of maternal care as a possible mediating factor have been frequently neglected. To address these issues, maternal behaviour of Sprague-Dawley dams differing in natural number of pups was studied for the first seven postnatal days. Later, adult behaviour of representative male offspring was studied in the elevated plus-maze, the circular corridor, the dark-light box and the forced swimming test. Three groups of offspring were selected in function of the number of littermates: L<10 group (less than 10 pups per dam), L10-15 (between 10 and 15 pups per dam) and L>15 group (more than 15 pups per dam). L<10 litters showed a reduced habituation of activity across time in a circular corridor and as compared to L>15 litters, L<10 litters showed a lower activity during the first 5 min of exposure to the circular corridor. L<10 litters had also higher signs of anxiety in the elevated plus-maze, in comparison to the other two groups. In addition, L<10 litters showed in the forced swimming test reduced struggling and more mild swimming behavior than the other two groups. These abnormalities in L<10 litters are not explained by maternal behavior since they received individually more maternal care than L>15, as assessed by total licking-grooming observed during the whole observation period divided by number of pups. Although previous data from several laboratories have demonstrated that low maternal care is associated with heightened emotionality at adulthood, the present results suggest an important contribution of spontaneous litter size to adult emotional behavior that cannot be explained by concomitant changes in maternal care.