Plasma corticosterone concentrations sensitively reflect levels of stimulus intensity in the rat

Faecal glucocorticoid metabolite profiles in diademed sifakas increase during seasonal fruit scarcity with interactive effects of age/sex class and habitat degradation

Glucocorticoids are metabolic byproducts of animals' physiological responses to ecological or social challenges and are thought to represent an adaptive response allowing beneficial responses to short-term challenges. Glucocorticoid metabolites (GCs) can be assayed non-invasively through faeces and therefore can be a useful tool to gauge the health of populations experiencing natural and/or anthropogenic stressors. However, the response of GCs to anthropogenic stressors varies, with both higher and lower GC levels reported. Here, we describe variation in GC secretion within eight diademed sifaka (Propithecus diadema) groups across 1 year. These groups span a gradient of anthropogenic habitat disturbance, including groups in continuous forest ('CONT') and disturbed fragments ('FRAG'), and indicators of health suggest that FRAG groups are negatively impacted by habitat disturbance. We monitored phenology, used focal animal follows to quantify diet and collected faeces (n = 547) from which we quantified GC content using enzyme immunoassay. All groups showed elevated lean-season GCs, but with a single, brief peak. GCs were inversely correlated with feeding time. No overall effect of habitat (CONT vs. FRAG) was found, but the lean-season peak was significantly higher in CONT groups. There was a significant season*age-sex interaction; adult females had an attenuated lean-season response compared with groupmates. The observed lean-season 'challenge' is consistent with previous lemur studies, as well as mammals in general. Low and largely invariable GC levels in FRAG, within the context of observed health and nutritional declines, suggest that FRAG groups employ a strategy whereby the adrenal response to stressors is downregulated. More research is needed to contextualize our observations of GC variation and health on an individual level, both in terms of corroborating evidence for ecological and social stressors, and longer-term quantification of reproductive success and fitness.

Baleen whale cortisol levels reveal a physiological response to 20th century whaling

One of the most important challenges researchers and managers confront in conservation ecology is predicting a population's response to sub-lethal stressors. Such predictions have been particularly elusive when assessing responses of large marine mammals to past anthropogenic pressures. Recently developed techniques involving baleen whale earplugs combine age estimates with cortisol measurements to assess spatial and temporal stress/stressor relationships. Here we show a relationship between baseline-corrected cortisol levels and corresponding whaling counts of fin, humpback, and blue whales in the Northern Hemisphere spanning the 20th century. We also model the impact of alternative demographic and environmental factors and determine that increased anomalies of sea surface temperature over a 46-year mean (1970-2016) were positively associated with cortisol levels. While industrial whaling can deplete populations by direct harvest, our data underscore a widespread stress response in baleen whales that is peripheral to whaling activities or associated with other anthropogenic change.

Intergenerational Sex-Specific Transmission of Maternal Social Experience

The social environment is a major determinant of individual stress response and lifetime health. The present study shows that (1) social enrichment has a significant impact on neuroplasticity and behaviour particularly in females; and (2) social enrichment in females can be transmitted to their unexposed female descendants. Two generations (F0 and F1) of male and female rats raised in standard and social housing conditions were examined for neurohormonal and molecular alterations along with changes in four behavioural modalities. In addition to higher cortical neuronal density and cortical thickness, social experience in mothers reduced hypothalamic-pituitary-adrenal (HPA) axis activity in F0 rats and their F1 non-social housing offspring. Only F0 social mothers and their F1 non-social daughters displayed improved novelty-seeking exploratory behaviour and reduced anxiety-related behaviour whereas their motor and cognitive performance remained unchanged. Also, cortical and mRNA measurements in the F1 generation were affected by social experience intergenerationally via the female lineage (mother-to-daughter). These findings indicate that social experience promotes cortical neuroplasticity, neurohormonal and behavioural outcomes, and these changes can be transmitted to the F1 non-social offspring in a sexually dimorphic manner. Thus, a socially stimulating environment may form new biobehavioural phenotypes not only in exposed individuals, but also in their intergenerationally programmed descendants.

Salivary cortisol and behavioral response to social evaluative threat in adolescents with autism spectrum disorder

Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social behavior. One possible explanation for social communication deficits in ASD could be differences in biological systems that support responses to environmental stimuli. If so, it is unclear if differences in the arousal response to social stimuli in ASD are due to reduced interest in social information, or to an increased stress response. The hypothalamic pituitary adrenal axis facilitates arousal and the stress response to sensory input, including social stimuli. Previous research shows blunted cortisol response to social evaluative threat in children with ASD. The majority of prior work has focused on children with ASD, but adolescents with ASD are understudied. The adolescent period is of interest, as this developmental epoch is associated with increased salience of social evaluative threat in typically developing (TD) populations. In this study, we employed the Trier Social Stress Test (TSST), a laboratory paradigm that involves exposure to social evaluative threat, to study the cortisol and behavioral response to social evaluative threat in ASD and TD adolescents. Salivary cortisol data were collected at six time points before and after the TSST. Behavioral data were collected using video recordings of the TSST, which were then operationalized and coded. Paired sample t-tests were used to calculate within-group cortisol response to the TSST. Cortisol significantly increased in response to the TSST in the TD group but not the ASD group. The TD group showed a trend for more self-soothing behaviors during the stressor than the ASD group. The lack of a cortisol response to the TSST in the ASD group suggests that the TSST is not interpreted as stressful or salient for ASD adolescents. Autism Res 2017, 10: 346-358. © 2016 International Society for Autism Research, Wiley Periodicals, Inc.

Inoculation stress hypothesis of environmental enrichment

One hallmark of psychiatric conditions is the vast continuum of individual differences in susceptibility vs. resilience resulting from the interaction of genetic and environmental factors. The environmental enrichment paradigm is an animal model that is useful for studying a range of psychiatric conditions, including protective phenotypes in addiction and depression models. The major question is how environmental enrichment, a non-drug and non-surgical manipulation, can produce such robust individual differences in such a wide range of behaviors. This paper draws from a variety of published sources to outline a coherent hypothesis of inoculation stress as a factor producing the protective enrichment phenotypes. The basic tenet suggests that chronic mild stress from living in a complex environment and interacting non-aggressively with conspecifics can inoculate enriched rats against subsequent stressors and/or drugs of abuse. This paper reviews the enrichment phenotypes, mulls the fundamental nature of environmental enrichment vs. isolation, discusses the most appropriate control for environmental enrichment, and challenges the idea that cortisol/corticosterone equals stress. The intent of the inoculation stress hypothesis of environmental enrichment is to provide a scaffold with which to build testable hypotheses for the elucidation of the molecular mechanisms underlying these protective phenotypes and thus provide new therapeutic targets to treat psychiatric/neurological conditions.

Modest elevation of corticosterone in preweanling rats impairs subsequent trace eyeblink conditioning during the juvenile period

The hippocampus is known to be especially sensitive to the deleterious effects of glucocorticoids. Previously, we administered exogenous corticosterone, the major stress-related glucocorticoid in rats, to young developing rats using subcutaneous pellets which produced high pharmacological levels of circulating corticosterone as well as a sex-specific learning deficit for males on a hippocampus-mediated associative learning task, trace eyeblink conditioning [1]. The present study evaluated the effects of corticosterone administered at a physiologically-relevant level by a more consistent release method, osmotic mini-pumps. Pumps were implanted subcutaneously in 15-day-old rats to deliver either corticosterone or the vehicle control (PEG) at a rate of 1 μl/h over 3 days. On Day 28, learning was assessed using trace eyeblink conditioning. The results of the present experiment revealed that a small elevation in corticosterone (11.77 μg/dl versus 6.02 μg/dl for controls) within the normal physiological range impaired learning as determined by a significantly lower percentage and amplitude of total conditioned responses (CRs) and lower amplitude of adaptive responses relative to the control group. There were no significant differences in response timing, although the corticosterone group tended to produce CRs which began and peaked a little later than controls. These findings indicate that even modest elevations of corticosterone for several days can produce later impairments on this hippocampally mediated learning task.

Stress inhibits psychomotor performance differently in simple and complex open field environments

Stress affects psychomotor profiles and exploratory behavior in response to environmental features. Here we investigated psychomotor and exploratory patterns induced by stress in a simple open-field arena and a complex, multi-featured environment. Groups of rats underwent seven days of restraint stress or no-stress conditions and were individually tested in three versions of the ziggurat task (ZT) that varied according to environmental complexity. The hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis due to stress procedure was evaluated by the pre- and post-stress levels of circulating corticosterone (CORT). Horizontal activity, exploration, and motivation were measured by the number of fields entered, the time spent in the central fields, path length and speed, and stop duration. In addition, vertical exploratory behavior was measured by the times rats climbed onto ziggurats. Stress-induced psychomotor changes were indicated by reduced path length and path speed and increased duration of stops only within the complex arena of the ZT. Rats in stress groups also showed a significant decline in the vertical movements as measured by the number of climbing onto ziggurats. No stress-induced changes were revealed by the simple open-field arena. The exploratory patterns of stressed animals suggest psychomotor inhibition and reduced novelty-seeking behaviors in an environment-dependent manner. Thus, multi-featured arenas that require complex behavioral strategies are ideally suited to reveal the inhibitory effects of stress on psychomotor capabilities in rodents.

Blue whale earplug reveals lifetime contaminant exposure and hormone profiles

Lifetime contaminant and hormonal profiles have been reconstructed for an individual male blue whale (Balaenoptera musculus, Linnaeus 1758) using the earplug as a natural aging matrix that is also capable of archiving and preserving lipophilic compounds. These unprecedented lifetime profiles (i.e., birth to death) were reconstructed with a 6-mo resolution for a wide range of analytes including cortisol (stress hormone), testosterone (developmental hormone), organic contaminants (e.g., pesticides and flame retardants), and mercury. Cortisol lifetime profiles revealed a doubling of cortisol levels over baseline. Testosterone profiles suggest this male blue whale reached sexual maturity at approximately 10 y of age, which corresponds well with and improves on previous estimates. Early periods of the reconstructed contaminant profiles for pesticides (such as dichlorodiphenyltrichloroethanes and chlordanes), polychlorinated biphenyls, and polybrominated diphenyl ethers demonstrate significant maternal transfer occurred at 0-12 mo. The total lifetime organic contaminant burden measured between the earplug (sum of contaminants in laminae layers) and blubber samples from the same organism were similar. Total mercury profiles revealed reduced maternal transfer and two distinct pulse events compared with organic contaminants. The use of a whale earplug to reconstruct lifetime chemical profiles will allow for a more comprehensive examination of stress, development, and contaminant exposure, as well as improve the assessment of contaminant use/emission, environmental noise, ship traffic, and climate change on these important marine sentinels.

Behavioral and physiological responses of female prairie voles (Microtus ochrogaster) to various stressful conditions

Stressful life events elicit hypothalamic-pituitary-adrenal (HPA) axis activation, which may alter psychological states or behavioral routines. Therefore, the current study focused on the HPA axis response to better understand such manifestations in female prairie voles (Microtus ochrogaster). In Experiment 1, females were stressed for 1 h via one of the four stressors: exposure to a novel environment, immobilization ("plastic mesh"), brief social defeat, or prolonged social defeat. Following a 30-min recovery, the females received a 5-min elevated plus maze (EPM) test and, subsequently, blood was collected to measure plasma corticosterone concentrations. Only immobilization stress induced an anxiety-like behavioral response in the EPM test and elevated plasma corticosterone levels compared to the control groups. Corticosterone concentrations were also significantly elevated following exposure to prolonged social defeat compared to the control conditions, but not after novel environment stress or short social defeat. In Experiment 2, females were exposed to immobilization stress over 1, 3, or 7 days in a daily (predictable; pIMO) or irregular (unpredictable; uIMO) schedule. The biobehavioral stress response in females exposed to pIMO for 3 or 7 days did not differ significantly from controls, suggesting these females habituated. By comparison, females exposed to uIMO over 3 or 7 days did not habituate behaviorally or physiologically, even producing augmented corticosterone levels. In both experiments, positive correlations were found between corticosterone levels and anxiety-like behaviors in the EPM test. Together, our data suggest that the stress response by female prairie voles is dependent on stress intensity, source, previous experience, and predictability. Furthermore, the HPA axis response, as evident by corticosterone levels, is associated with the impact that these factors have on behavioral routine.

Cannabinoid receptors activation and glucocorticoid receptors deactivation in the amygdala prevent the stress-induced enhancement of a negative learning experience

The enhancement of emotional memory is clearly important as emotional stimuli are generally more significant than neutral stimuli for surviving and reproduction purposes. Yet, the enhancement of a negative emotional memory following exposure to stress may result in dysfunctional or intrusive memory that underlies several psychiatric disorders. Here we examined the effects of stress exposure on a negative emotional learning experience as measured by a decrease in the magnitude of the expected quantity of reinforcements in an alley maze. In contrast to other fear-related negative experiences, reward reduction is more associated with frustration and is assessed by measuring the latency to run the length of the alley to consume the reduced quantity of reward. We also examined whether the cannabinoid receptors agonist WIN55,212-2 (5 μg/side) and the glucocorticoid receptors (GRs) antagonist RU-486 (10 ng/side) administered into the rat basolateral amygdala (BLA) could prevent the stress-induced enhancement. We found that intra-BLA RU-486 or WIN55,212 before stress exposure prevented the stress-induced enhancement of memory consolidation for reduction in reward magnitude. These findings suggest that cannabinoid receptors and GRs in the BLA are important modulators of stress-induced enhancement of emotional memory.

Effects of recovery from immobilization stress on striatal preprodynorphin- and kappa opioid receptor-mRNA levels of the male rat

Previously, we have reported that brain regions that are thought to be involved in motivated behavior are altered in animals undergoing repeated exposures to immobilization stress. The goal of the present study was to determine the effects of recovery from this type of stress on these same mesolimbic brain regions. For this purpose, adult male Sprague-Dawley rats were initially exposed to immobilization stress either once (2 h) or repeatedly (2 h×10 days). Rats were then either allowed to recover from the stressor for a shorter (2 days) or longer period of time (9 days) in their home cages. At the end of this recovery period, rats were euthanized and trunk blood and brains were processed for serum corticosterone (CORT) and neurochemistry, respectively. Brain mRNA levels were determined via in situ hybridization for the opioid preprodynorphin (DYN) and its cognate receptor (kappa, KOR), in striatal and accumbal subregions. A pattern of selective transcriptional activation emerged in the four resultant treatment conditions where a short recovery from either a single or repeated exposure to immobilization produced increases in KOR-mRNA levels in striatal and nucleus accumbens (Acb) subregions. Relative to controls, these differences were diminished after a longer recovery period. Interestingly, DYN-mRNA levels were unchanged after the shorter recovery period and after single or repeated immobilizations but appeared to be induced after a longer recovery period after repeated immobilizations. A relative amount of weight loss occurred after immobilization following repeated but not single exposure to stress. In addition, only those rats recovering from repeated stress exposures had higher CORT levels compared with non-immobilized controls. These results suggest that recovery from immobilization stress may alter the motivational system after as little as a single immobilization and that a possible dysphoric effect on appetitive behavior may be reflected by an altered striatal dynorphin system.

Physiological and hormonal responses to novelty exposure in rats are mainly related to ongoing behavioral activity

Stress research has been dominated by a circular type of reasoning that occurrence of a stress response is bad. Consequently, the stimulus is often interpreted as stressful in terms of aversiveness involving uncontrollability and unpredictability, which may have maladaptive and pathological consequences. However, the hypothalamic-pituitary-adrenal (HPA) axis and sympathico-adrenomedullary (SAM) system are not only activated in response of the organism to challenges, but also prepare and support the body for behavior. Therefore, a considerable part of the physiological and hormonal responses to a certain situation can be a direct reflection of the metabolic requirements for the normal ongoing behavioral activity, rather than of the stressful nature. In order to clarify this, behavioral, physiological, hormonal and electroencephalographic (EEG) responses to novel cage exposure were studied in male Sprague-Dawley rats. Forced confrontation with a novel cage has been interpreted as a psychological and aversive stressor. However, this interpretation is simply based on the occurrence of a stress response. This study aimed at detailed analysis of the time course of the novelty-induced responses. Different parameters were measured simultaneously in freely moving rats, which allowed correlational comparisons. Hereto, radio telemetry using a small implantable transmitter combined with permanent catheters and an automated blood sampling system was used. A camera placed above the cage allowed behavioral observations. The results show that novelty exposure induced significant increases in locomotor activity, heart rate, blood pressure and plasma corticosterone together with a complete lack of sleep as compared to the undisturbed control situation. The latency to reach significance and the duration of responses varied across parameters but all had recovered within 30min after termination of novelty. The behavioral activity (locomotor activity and EEG wakefulness duration) response pattern was significantly correlated with that of heart rate, blood pressure and plasma corticosterone. Behavioral observations showed mainly explorative behavior in response to novelty. Therefore, the present results indicate that the novelty-induced physiological and hormonal responses are closely related to the ongoing, mainly explorative behavioral activity induced by novelty. An interpretation in terms of metabolic support of ongoing behavior seems to be more appropriate than the frequently used stress interpretation. The present study also emphasizes the added value of simultaneous assessment of behavioral, physiological and hormonal parameters under controlled, non-confounding conditions.

Repeated ferret odor exposure induces different temporal patterns of same-stressor habituation and novel-stressor sensitization in both hypothalamic-pituitary-adrenal axis activity and forebrain c-fos expression in the rat

Repeated exposure to a moderately intense stressor typically produces attenuation of the hypothalamic-pituitary-adrenal (HPA) axis response (habituation) on re-presentation of the same stressor; however, if a novel stressor is presented to the same animals, the HPA axis response may be augmented (sensitization). The extent to which this adaptation is also evident within neural activity patterns is unknown. This study tested whether repeated ferret odor (FO) exposure, a moderately intense psychological stressor for rats, leads to both same-stressor habituation and novel-stressor sensitization of the HPA axis response and neuronal activity as determined by immediate early gene induction (c-fos mRNA). Rats were presented with FO in their home cages for 30 min a day for up to 2 wk and subsequently challenged with FO or restraint. Rats displayed HPA axis activity habituation and widespread habituation of c-fos mRNA expression (in situ hybridization) throughout the brain in as few as three repeated presentations of FO. However, repeated FO exposure led to a more gradual development of sensitized HPA-axis and c-fos mRNA responses to restraint that were not fully evident until after 14 d of prior FO exposure. The sensitized response was evident in many of the same brain regions that displayed habituation, including primary sensory cortices and the prefrontal cortex. The shared spatial expression but distinct temporal development of habituation and sensitization neural response patterns suggests two independent processes with opposing influences across overlapping brain systems.

Neotic preferences in laboratory rodents: Issues, assessment and substrates

Neotic preference refers to the extent to which animals prefer stimuli of differing novelty value. Degree of novelty is determined by within- and between-trials habituation and amount of temporal (novelty) and spatial change (complexity) in stimulation which in turn will determine the amount of curiosity-based approach (neophilia) or fear-based avoidance (neophobia) of novel stimuli. Tests of genuine neotic preferences enable direct assessments of responsiveness to temporal and spatial changes and include measurements of novel versus familiar locations (such as novelty-related location preferences), responsiveness to stimulus complexity (such as object exploration) and learning for exploratory rewards (such as light-contingent bar-pressing). Effects of brain lesions and peripherally administered drugs have implicated several brain areas and neurotransmitters that subserve memory, fear and reward in neotic preferences namely the hippocampus and ACh (memory), the amygdala, GABA and 5-HT (fear), and the mesolimbic DA reward system. However, more attention should be paid to the complexity of interactions between different brain and neurotransmitter systems and improvements in methodology before conclusions should be drawn about the neurobiological basis of neotic preferences.

A tryptic hydrolysate from bovine milk alphaS1-casein improves sleep in rats subjected to chronic mild stress

The putative effects of a tryptic bovine alphaS1-casein hydrolysate on stress-induced sleep disorders were investigated and their possible link with typical blood stress parameters such as plasma corticosterone concentrations and glycaemia was assessed. Rats were subjected to chronic stress in the form of environmental disturbances, while receiving an oral administration of the alphaS1-casein hydrolysate (CH). Chronic stress significantly reduced sleep duration in control rats during the first 2 days of the stress period, but stress-induced sleep disturbance was prevented in CH-treated rats. Indeed, CH administration allowed the maintenance of slow wave sleep (SWS) duration and even a slight increase in paradoxical sleep (PS) duration in treated rats. Results on plasma corticosterone concentrations and on glycemia values were inconclusive with respect to the implication of the HPA axis in this study. However, the protective effect of the alphaS1-casein hydrolysate on sleep during exposure to our chronic mild stress conditions may be mediated by modulation of the central adrenergic response.

Immediate-early gene induction in hippocampus and cortex as a result of novel experience is not directly related to the stressfulness of that experience

The stressful quality of an experience, as perceived by rats, is believed to be largely represented by the magnitude of a hypothalamic-pituitary-adrenal (HPA) axis response. The hippocampus may be especially important for assessing the stressfulness of psychological stressors such as novel experience. If such is the case then experience-dependent immediate-early gene expression levels within the hippocampus may parallel relative levels of HPA axis activity. We examined this prospect in rats that were placed in four different novel environments (empty housing tub, circular arena, elevated pedestal or restraint tube). Restraint and pedestal produced the largest magnitude of increased ACTH and corticosterone secretion, arena an intermediate level (Experiment 2) and tub the least magnitude of increase. We saw a very similar experience-dependent pattern of relative Fos protein, c-fos mRNA and zif268 mRNA expression in the paraventricular nucleus of the hypothalamus. However, in hippocampus (and select regions of cortex), immediate-early gene expression was associated with the exploratory potential of the novel experience rather than level of HPA axis activity; pedestal and arena elicited the greatest immediate-early gene expression, tub an intermediate level and restraint the least amount of expression. We conclude that the stressfulness of psychological stressors is not represented by the amount of immediate-early gene induction elicited in hippocampus and cortex, nor does there appear to be a general enhancing or depressive influence of acute stress on immediate-early gene induction in those brain regions.

Rat strain differences in sleep after acute mild stressors and short-term sleep loss

Genetic and physiological diversity amongst rodent strains provide the potential for developing models that may give insight into factors that regulate sleep in response to environmental challenges. We examined home cage activity, behavioral performance in the open field and sleep after a number of mild stressors (cage change [CC], open field [OF]) and after 1 and 4h of sleep deprivation (1hSD and 4hSD) in rat strains (Fischer 344 [F344], Lewis [LEW], Wistar [WST] and Sprague-Dawley [Sp-D], n=16 per strain) that differ in behavior and sleep. F344 and WST rats had greater home cage locomotion than LEW and Sp-D rats, but F344 rats exhibited the least relative locomotion in OF. In 24h baseline recordings of sleep, strain rankings were LEW=WST=Sp-D>F344 in rapid eye movement sleep (REM), and LEW=Sp-D>F344 and LEW>WST in non-REM (NREM). Compared to baseline, total sleep was reduced in all four strains after CC, OF and 1hSD, but not after 4hSD, in the first hour after treatment. Afterwards, increases in REM and NREM were seen after all treatments with the amount and time course varying across treatments and strains. CC induced the weakest and 4hSD the largest effects on sleep, whereas OF and 1hSD had intermediate effects. Among strains, the more anxious F344 rats exhibited the greatest sleep increases during the light period after OF, 1hSD and 4hSD. The results are discussed with respect to the relationship between behavioral and sleep responses to stressors, and to potential mechanisms underlying the strain differences.

Home cage activity and activity-based measures of anxiety in 129P3/J, 129X1/SvJ and C57BL/6J mice

We investigated the home cage activity and emotional behavior in mouse strains used as background for many studies of altered genes [C57BL/6J (B6, n=20), 129X1/SvJ (X1, n=20) and 129P3/J (P3, n=19)]. In their home cages, X1 and P3 mice exhibited less locomotion than did B6 mice, and the X1 mice showed significantly greater rearing than B6 and P3 mice did. A battery of three tests conducted in an open field (open field, emergence and novel object) revealed strain rankings of B6>X1>P3 or B6>X1=P3 in most activity variables. Significant correlations were found between home cage activity and activity in each of three tests, but not in all observation periods. Strain rankings on the elevated zero maze test were B6=X1>P3 in the number of stretched-attend body postures (SAPS) during the initial 6-min exposure for naive mice. Naive and nonnaive mice showed significantly different behaviors on the elevated zero maze. The results suggest that rankings on anxiety are P3>X1>B6 and that B6 mice have greater exploration in a novel environment compared with X1 and P3 mice. However, anxiety-like behaviors differed among strains in open-field-based tests and in the zero maze, and testing experience impacted performance on the zero maze. The findings illustrate that test variations and experience can influence performance and suggest the need for the consideration of how these factors interact with background strains in assessing gene-altered mice.

Strain differences in the influence of open field exposure on sleep in mice

The open field (OF) is thought to induce anxiety in rodents. It also allows an opportunity for exploration in a novel environment. Less activity in the OF is thought to indicate greater anxiety whereas more activity may reflect greater exploration, and possibly greater exploratory learning. Anxiety and learning have poorly understood relationships to sleep. In order to determine how anxiety and exploration in the OF could influence sleep, we recorded sleep in mouse strains (C57BL/6J (B6), BALB/cJ (C), DBA/2J (D2), and CB6F1/J (CB6)) with different levels of anxiety and exploration after 30 min in an OF. In all strains, OF exposure induced immediate decreases in rapid eye movement sleep (REM) followed by longer latency increases in REM. The time course and amount of REM decreases and increases varied among strains. Compared to less anxious B6, D2 and CB6 mice, C mice had greater and longer lasting immediate decreases in REM. C mice also displayed longer periods of decreases REM and a smaller, longer latency increase in REM. OF exploratory activity was positively correlated to percentage of REM increases from 6 to 10h after OF exposure. The results suggest that the anxiogenic component of the OF produced an immediate decrease in REM that was greater in more "anxious" mice. In contrast, exploration in the OF was associated with increased REM, with the increase greater in less anxious mice. The results are discussed with respect to the potential influences of anxiety and learning on sleep.

Cross-institutional assessment of stress responses in zoo animals using longitudinal monitoring of faecal corticoids and behaviour

Cross-institutional studies that combine non-invasive physiological measures of stress responses and the assessment of individual differences in behaviour and temperament have great potential as tools for assessing the well-being of zoo animals and for identifying key environmental stimuli relevant to well-being. In addition, such studies allow comparison of animals under a wide variety of conditions and enable researchers to obtain sufficiently large samples sizes for statistical data analyses. Faecal corticoid measurements, a method recently developed to monitor adrenal activity in wildlife and domestic species, can be obtained non-invasively as part of the normal husbandry routine. While basic techniques still need improvement, and interpretation of the acquired measures can be challenging, assessment of faecal corticoid concentrations can provide a useful indicator of stress responses under a variety of captive conditions. Here we report on three studies that illustrate this approach and the results that can be obtained. An on-going study reveals significant differences in the pattern of variability of faecal corticoid concentrations between polar bears that are reported by keepers to perform stereotypic behaviour and those that do not. In another study, faecal corticoid measures indicated that stress responses to certain extraneous noises might interfere with the breeding of Hawaiian honeycreepers in captivity. In a study of clouded leopards, higher faecal corticoid concentrations were measured when cats were kept on public display or near potential predators compared to individuals maintained off exhibit or in the absence of visible predators. The findings of an on-going experimental study suggest a causal relationship between the provision of additional hiding spaces and a decline in faecal corticoid concentrations in clouded leopards.

Stereotaxic localization of corticosterone to the amygdala enhances hypothalamo-pituitary-adrenal responses to behavioral stress

The amygdala is involved in behavioral, autonomic, and neuroendocrine responses to stressful stimuli. The goal of the current study was to determine the effect of directly elevating glucocorticoids in the amygdala on hypothalamo-pituitary-adrenocortical (HPA) responses to the elevated plus maze, a behavioral stressor known to activate the amygdala. Micropellets (30 microg) of crystalline corticosterone or cholesterol (control) were implanted bilaterally at the dorsal margin of the CeA in male Wistar rats; vascular catheters were also placed at this time. Five days post-surgery, blood samples were drawn at 07:00 and 19:00 h to assess diurnal rhythm of plasma corticosterone. At 7 days post-implantation, rats were subjected to behavioral stress using an elevated plus maze and blood was collected 15 min prior to stress, and at 15, 45, and 90 min after the initiation of the stressor. Corticotropin releasing factor (CRF) and arginine vasopressin (AVP) mRNA levels were analyzed by in situ hybridization in the medial parvocellular division of the hypothalamic paraventricular nucleus (mpPVN) in corticosterone- and cholesterol-implanted rats either not exposed to the elevated plus maze (control) or 4 h post-behavioral stress. Localization of corticosterone to the amygdala had no effect on diurnal rhythm of corticosterone secretion. Behavioral stress significantly increased peak plasma corticosterone levels in both groups to a similar level. However, in the corticosterone implanted rats, plasma corticosterone concentrations at 45 and 90 min post-stress were significantly greater compared to control rats indicating a prolonged corticosterone response to behavioral stress. In non-stressed rats, corticosterone delivery to the amygdala elevated basal CRF mRNA in the mpPVN to levels similar to those observed post-stress in control animals; no further increase was observed in CRF mRNA following stress. Behavioral stress resulted in a significant elevation in CRF mRNA in cholesterol controls. Basal AVP mRNA levels were unaffected by corticosterone implants. AVP mRNA did not increase in cholesterol implanted rats in response to behavioral stress. However, AVP mRNA levels were higher in corticosterone implanted rats post stress compared to cholesterol treated controls. In conclusion, direct administration of corticosterone to the amygdala increases plasma corticosterone in response to a behavioral stressor without altering the diurnal rhythm in plasma corticosterone. Elevated basal levels of mpPVN CRF mRNA, and the induction of a mpPVN AVP mRNA response to the behavioral stressor implicate enhanced ACTH secretagogue expression in the increased HPA response to corticosterone modulation of amygdala function.

Risk-taking behavior in adolescent mice: psychobiological determinants and early epigenetic influence

Epidemiological research has emphasized that adolescence is associated with some temperamental and behavioral traits that are typical of this age and that might substantially contribute to both psychological and psychobiological vulnerability. The contribution of the important developmental rearrangements in neurobiological and neuroendocrinological processes has received surprisingly little investigation. The present review summarizes recent work in animal models, indicating that adolescent rodents exhibit marked peculiarities in their spontaneous behavioral repertoire. When compared to adults, adolescents show an unbalanced and 'extremes-oriented' behavior, consisting of an increased novelty seeking, together with decreased novelty-induced stress and anxiety, an increased risk-taking behavior in the plus-maze, as well as elevated levels of impulsivity and restlessness. Age-related discontinuities in the function of monoaminergic systems, which are a main target of abused drugs, can perhaps account for such a profile. In particular, a peculiar function within reward-related dopaminergic brain pathways actually seems to underlie the search for novel and rewarding sensations, as well as changes in the magnitude of psychostimulant effects. The role played by early epigenetic factors in the shaping of novelty-seeking behavior of adolescent and adult rodents are also reviewed. Two examples are considered, namely, subtle variations in the hormonal milieu as a function of intrauterine position and precocious or delayed maturation of nutritional independence as a function of changes in time of weaning. As for spontaneous drug consumption, a prominent vulnerability to the oral intake of nicotine during early adolescence is reported. In conclusion, adolescence in rodents may represent a suitable animal model with enough face- and construct-validity. Actually, this model is able to show behavioral features that resemble those found in human adolescents, including vulnerability to the consumption of psychoactive drugs.

Home cage activity and behavioral performance in inbred and hybrid mice

Locomotor activity is a key component in many behavioral tests, suggesting that genetic differences in activity levels may be a critical consideration when comparing mouse strains. In order to assess the relationship between activity and performance, we recorded home cage activity, and locomotion and defecation, a non-activity-linked behavior, in tests of anxiety in inbred (C57BL/6J (B6), n = 25; BALB/cJ (C), n = 24; DBA/2J (D2), n = 28) and hybrid (CB6F1/6J (CB6: B6 x C) n = 19) mice. Under our test conditions, the strains showed significant differences in home cage activity levels: C > B6 > D2. The CB6 mice were similar to the B6 mice in horizontal activity and were intermediate between the parental strains in vertical movement. Based on measures of locomotion and defecation in the open field, emergence and novel object tests, and the elevated zero maze, the C mice appeared to be the most anxious and the B6 were the least anxious. The D2 mice were intermediate on some measures but more similar to B6 mice on others, making ranking them more difficult. In addition, the CB6 mice displayed characteristics of both parental strains. They had greater similarity to B6 mice in measures of horizontal movement in the home cage and locomotion in the open field and emergence tests, but exhibited defecation responses similar to those of C mice in the novel object test and elevated zero maze. The results suggest that strain differences in spontaneous locomotion should be considered when interpreting strain differences in behavioral tests, and that home cage activity may be a useful interpretive aid.

The effects of chronic mild stress on male Sprague-Dawley and Long Evans rats: I. Biochemical and physiological analyses

The chronic unpredictable mild stress (CMS) is a paradigm developed in animals to model the relatively minor and unanticipated irritants that lead to a state of anhedonia in some individuals. However, the effectiveness of CMS is sometimes difficult to establish, for which unique strain sensitivities has been attributed as one contributing factor. These considerations led us to design the present study, which was an investigation of the corticosterone response to CMS in two outbred rat strains--Sprague-Dawley and Long Evans. Animals were exposed to one of two conditions--control or CMS--for 3 weeks during which body weight and fecal count were regularly monitored. At the end of this period, blood was sampled at a variety of time intervals following induction of a brief restraint stressor. First, a significant effect of CMS on corticosterone levels was evident at time 0 (prior to the application of the acute restraint stressor) in both strains. Second, the typical quadratic pattern of stressor-elicited fluctuations in this measure was similar in both Sprague-Dawley and Long Evans rats, with consistently elevated levels for the first hour following exposure to the acute stressor; near baseline values were observed at 2 h. However, only in the Long Evans strain were CMS related values much less than that observed in the control group after restraint stress. Third, both strains showed a reduced weight gain in the CMS groups relative to control groups. Fourth, spleen and adrenal weights were similar across all groups. Fifth, fecal counts remained stable across weeks of treatment in all groups with the exception of the Long Evans rats exposed to CMS; in this group, average counts were systematically reduced over the treatment period. We conclude that a history of chronic stress significantly blunts corticosterone levels in Long Evans but not Sprague-Dawley rats following exposure to an acute stressor. Physiological indices however are less influenced by this experience, at least when the exposure is limited to 3 weeks.

Psychological stress and environmental adaptation in enriched vs. impoverished housed rats

In this study, we report differential behavioural and cognitive effects, as assessed in the open-field and the Morris water maze, following psychological stress in enriched vs. impoverished housed rats. Three stress conditions were evaluated: nonstress, mild stress and powerful stress. Mild stress consisted of exposure to an avoidance box but without shock, while in the powerful stress condition animals were exposed to an electric shock. The results revealed distinct effects in the differentially housed animals. Prior exposure to a mild stress enhanced escape performance in the water maze in enriched but not impoverished animals. However, preexposure to powerful stress negatively affected animals from both housing conditions in the water maze task, but with the enriched animals less affected than impoverished animals. In the open-field test, stress preexposure reduced locomotion counts in both the differentially housed animals. In addition, the results showed that the enrichment effect on emotional reactivity in the open-field is long-lasting and persists even after extensive training and housing in standard laboratory conditions. The results are discussed in relation to the nature of the behavioural and learning differences between the differentially housed animals.

Peculiar response of adolescent mice to acute and chronic stress and to amphetamine: evidence of sex differences

Although final brain size and the number of available neurons and axons appear to be established early in infancy, plasticity of the brain continues during adolescence through an integrated process of overproduction and elimination of synapses and receptors. In addition, hormonal levels change dramatically during this period, as a result of the onset of puberty. This age-specific condition has been suggested to serve as a permissive factor for the emergence of a number of early-onset neuropsychiatric disorders, including schizophrenia, attention-deficit hyperactivity disorder (ADHD), and perhaps substance abuse. However, relatively few investigations have focused on animal models of this developmental phase. The periadolescent rodent (similar30-45-day-old), has been proposed as a useful model. Periadolescent rats and mice are generally associated with a peculiar behavioral profile, consisting of basal hyperactivity, high attraction towards novel stimuli and a marked involvement in affiliative and playful behaviors. Moreover, a unique profile of psychopharmacological responsivity characterizes rodents around this age. Recent experiments by our group investigated age-related discontinuities in the response of the hypothalamic-pituitary-adrenal axis (HPA) to both stress and psychostimulants. The latter are often administered as therapeutic drugs to children with ADHD, which have been also associated with an impaired response to stress and abnormalities in HPA axis function. Indeed, an altered functioning of the HPA axis has been proposed as a possible risk factor and a potential marker for such a behavioral vulnerability. Animals were studied at adulthood (> pnd 70) or during periadolescence. Experiment I characterized basal corticosterone (CORT) levels in naive mice kept undisturbed in standard social conditions from weaning to sacrifice. Periadolescent male mice showed higher basal CORT levels than adult subjects, suggesting that the set up of the HPA axis is physiologically elevated during adolescence. In experiment II, we investigated age-related differences in the response to both acute and chronic stress conditions. Periadolescent and adult mice were housed either in a standard (three animals per cage) or in a crowding condition (nine animals per cage). The latter has been indeed reported to potentiate the subsequent reaction to acute stress in adult rodents. At the end of this period and following 24 h individual housing, mice were injected with either saline (SAL) or a standard amphetamine (AMPH) dose (2 mg/kg), and faced with a mild acute psychological stress, namely removal of sawdust from the home cage. Important sex differences emerged in animals of the two ages. Periadolescent females showed a reduced CORT response to acute stress. Within the adult male group, the chronic crowding condition produced a prominent potentiation of CORT response to the acute stress challenge. Conversely, this profile was not evidenced in periadolescents. These results indicate a strong role for gender and social variables in the response of periadolescent subjects to the various aspects of stress. As for AMPH effects, in the absence of significant changes in adult subjects, the drug produced a marked CORT release in periadolescent mice. A better understanding of neuroendocrine-related AMPH effects as a function of social and environmental risk factors during adolescence, might deepen our knowledge on the neurobiological bases of genetically determined neuropsichiatric disorders and possibly improve the therapeutical efficacy of psychostimulant drugs.

Circadian characteristics of corticosterone secretion in red-backed voles (Clethrionomys gapperi)

To provide necessary background for study of stress response in red-backed voles (Clethrionomys gapperi), the circadian and ultradian rhythm in corticosterone release was characterized. Animals were maintained under a 16h light, 8h dark cycle. A total of 55 males and 46 females provided 101 independent blood samples over a 6-month span. Samples were obtained at 1h to 2h intervals during the light and at 2h intervals during the dark. Using edited data (5 values beyond the upper 95% limit were removed), a significant time effect was found by analysis of variance (ANOVA) for both sexes at P < .001. The composite single cosine best describing the circadian wave-form for each sex consisted of three components (24h, 12h, and 6h), each significant at P < .05 (overall model P < .001). The 24h mean (mesor) was about 60% higher in females than males (646 ng/mL vs. 412 ng/mL, P = .01), with amplitudes of 429 and 298 ng/mL being proportional (66% vs. 72%) to the respective mesor. The predictable range of change within a 24h span (determined by the double amplitude of a 24h + 12h + 6h cosine model) was large: It was more than 1600 ng/mL for females and more than 900 ng/ mL for males. Highest values were found during the dark phase, with the 24h acrophase located at 2h into the dark span for both sexes. With the caveat of fewer samples obtained during dark than during light, the actual peak values for females occurred at 2h and for males at 6h into the 8h dark span. These results provide baseline information about the circadian time structure for serum corticosterone in red-backed voles under normal light-dark, low-stress conditions.

Environmental enrichment in BALB/c mice: effects in classical tests of anxiety and exposure to a predatory odor

Early stimulation by environmental enrichment generally leads to improved learning abilities in rodents. However, the effects of environmental enrichment on emotional reactivity remain more questionable and were mostly studied by using classical tests of anxiety based on confrontation with a novel environment. The main goal of our study was to use different tests of anxiety to compare BALB/c mice reared in either a standard condition (SC) or enriched condition (EC). Exposure to cat feces was used to assess anxiety according to an ethoexperimental approach and a comparison was made with the elevated plus maze and the open field as classical tests of anxiety. In accordance with previous works, our results show that EC mice were more active than SC mice in the elevated plus maze and the open field. Thus, possibly as a direct consequence of frequent changes in their breeding conditions, reactivity to a novel environment was reduced in EC mice. However, the cat odor test revealed no intergroup differences for behavior, although corticosterone levels were reduced in EC mice. These results indicate that classical (i.e. reaction to novel environments) and ethoexperimental-based tests (i.e. exposure to predator cues) measure different aspects of emotional reactivity. Further studies using ECs should be useful for the delineation of the neurobiological substrates of these different reactions.

A unique hormonal and behavioral hyporesponsivity to both forced novelty and d-amphetamine in periadolescent mice

The identification of critical ontogenetic periods of increased vulnerability to the effects of drugs of abuse could have a great psychobiological and clinical-therapeutical importance. Potential age-related differences in the response of the hypothalamic-pituitary-adrenal (HPA) axis to both stress and psychostimulants has been tested here in an animal model of adolescence. Periadolescent (PND 33-43) and Adult (PND>60) mice of both sexes were injected with d-amphetamine (AMPH, 0, 2, or 10 mg/kg i.p.) and immediately faced with a mild psychological stress experience, i.e. placement in a novel environment. A detailed time-course analysis of both hormonal and behavioral profiles was performed, with animals being sacrificed for trunk-blood collection at different time-points during the test (before the injection, NT group; 15, 30, or 120 min after the injection). Basal corticosterone (CORT) levels (NT group) were consistently higher in periadolescents than in adults. As a whole, a marked increment of blood CORT levels was found in mice of both ages exposed to forced novelty. However, important age-related differences were also observed, with Saline-injected periadolescents still exhibiting elevated levels of locomotion at the end of the 120-min test session and failing to show the increasing profile of CORT release over the baseline that was typical of adults. Upon an AMPH 2 administration, periadolescents exhibited a much lower profile of locomotor hyperactivity than adults, and also failed to show an increase across the course of the session in CORT release, that was observed in adults. When treated with the high AMPH 10 dose, a marked locomotor hyperactivity was found in periadolescents, which however showed much lower levels of the stereotyped licking and gnawing behavior, that was typical of adults. The present results suggest a unique profile of integrated behavioral and physiological hyporesponsivity in mice during periadolescence. The latter also represents a very useful model for the study of the issue of psychobiological risk factors involved in vulnerability to drugs of abuse in human adolescents.

The enhancement of hippocampal primed burst potentiation by dehydroepiandrosterone sulfate (DHEAS) is blocked by psychological stress

This series of studies investigated the effects of psychological stress and the neurosteroid dehydroepiandrosterone sulfate (DHEAS) on hippocampal primed burst (PB) and long-term (LTP) potentiation, two electrophysiological models of memory. The DHEAS and stress manipulations were performed on awake rats, and then PB and LTP were recorded while the rats were anesthetized. DHEAS enhanced PB potentiation when administered to rats under non-stress conditions, but had no effect when given to stressed rats. Further study showed that DHEAS enhanced PB potentiation only when it was administered before, but not after, the rats were stressed. The DHEAS and stress manipulations had no effect on LTP. This study provides three major findings regarding stress, neurosteroids and hippocampal plasticity. First, DHEAS enhanced a threshold form of plasticity (PB potentiation), but had no effect on a supra-threshold form of plasticity (LTP). Second, stress blocked the DHEAS-induced enhancement of PB potentiation. Third, stress and DHEAS effects on the hippocampus were so durable they could be performed on awake animals and then be studied while the animals were anesthetized. That DHEAS enhanced a subset of forms of hippocampal plasticity under restricted behavioral conditions may help to resolve conflicting observations of DHEAS effects on cognition and mood in people.

Dopamine D4 receptor-knock-out mice exhibit reduced exploration of novel stimuli

The involvement of dopamine neurotransmission in behavioral responses to novelty is suggested by reports that reward is related to increased dopamine activity, that dopamine modulates exploratory behavior in animals, and that Parkinson's disease patients report diminished responses to novelty. Some studies have reported that polymorphisms of the human dopamine D4 receptor (D4R) gene are associated with personality inventory measures of the trait called "novelty-seeking". To explore a potential role for the D4R in behavioral responses to novelty, we evaluated D4R-knock-out (D4R-/-) and wild-type (D4R+/+) mice in three approach-avoidance paradigms: the open field, emergence, and novel object tests. These three paradigms differ in the degree to which they elicit approach, or exploratory behavior, and avoidance, or anxiety-related behavior. Thus, we used these three tests to determine whether the D4R primarily influences the exploratory or the anxious component of responses to approach-avoidance conflicts. D4R-/- mice were significantly less behaviorally responsive to novelty than D4R+/+ mice in all three tests. The largest phenotypic differences were observed in the novel object test, which maximizes approach behavior, and the smallest phenotypic differences were found in the open field test, which maximizes avoidance behavior. Hence, D4R-/- mice exhibit reductions in behavioral responses to novelty, reflecting a decrease in novelty-related exploration.

Psychobiological risk factors for vulnerability to psychostimulants in human adolescents and animal models

Adolescence is associated with an increased risk of developing drug abuse/dependence. During this ontogenetic phase, brain and hormonal systems are still undergoing crucial maturational rearrangements, which take place together with significant modifications in psychosocial development. However, the neurohormonal and behavioral facets of adolescence have been poorly investigated in relation to the vulnerability to psychostimulants such as MDMA ("Ecstasy") and amphetamine (AMPH). Novelty-seeking, a temperamental/behavioral trait that is typical of this age period, might substantially contribute to both psychological and psychobiological vulnerability. In humans, an elevated score of novelty-sensation seeking and a derangement of monoaminergic function were both associated with late adolescence MDMA users compared to controls. In animal models of periadolescence, the search for novel stimuli and sensations actually shares a common neurobiological substrate (the reward-related brain mesolimbic pathways) with psychostimulants. The present review summarises recent work in mice, which indicates that periadolescent subjects are characterized by an unbalanced and "extremes-oriented" behavior and by elevated novelty-seeking compared to adults. Repeated and intermittent administration of cocaine or AMPH was associated with the development of a prominent locomotor sensitization in periadolescents, which failed to exhibit the marked sensitization of the stereotyped behavioral syndrome--possibly associated with poor welfare--that was typical of adults. A unique profile of integrated behavioral and physiological hyporesponsivity to both forced novelty and acute AMPH administration during periadolescence was also found. As a whole, these results, together with previous work on this topic, suggest that periadolescents may be more "protected" from AMPH-related aversive properties, and perhaps more vulnerable to the experience of internal states of reward, than older animals. Thus, the present animal model of adolescence seems to represent a reliable and useful method for the investigation of vulnerability to a variety of habit-forming agents or emotional experiences whose positive reinforcing properties may rely on common neurobiological substrates. A deeper understanding of psychostimulant effects during adolescence on the complex interaction between genetic, neurobiologic, psychosocial, and environmental factors will lead to earlier and more effective prevention and treatment.

Stress during pregnancy alters the offspring hypothalamic, pituitary, adrenal, and testicular response to isolation on the day of weaning

Subjecting pregnant female rats to situations that activate the hypothalamic-pituitary-adrenal (HPA) axis can have long-term effects on the development of the offspring. Restraint under bright lights is a common method of stressing pregnant females that results in consistent behavioral changes in the offspring. We investigated the effects of gestationally administered restraint, bright lights, and heat on the HPA axis response of 21-day-old offspring following exposure to isolation in a novel environment or under resting conditions. Corticotropin-releasing factor titers in the hypothalamus were unaffected following isolation. Nonetheless, adrenocorticotropin hormone (ACTH) was found to be lower in the gestationally stressed offspring prior to or following the isolation period. Corticosterone was attenuated in gestationally stressed offspring following the postnatal stressor and there was also a tendency for the gestationally stressed females to have lower concentrations of aldosterone. Plasmatic testosterone levels were higher in the gestationally stressed males following the period of isolation. The present data suggest that the HPA axis of the offspring is differentially affected by the gestational stress procedure, that is, it is attenuated at the level of the pituitary and adrenal, but not at the level of the hypothalamus. These data have implications for behavioral differences observed in gestationally stressed animals.

High versus low reactivity to a novel environment: behavioural, pharmacological and neurochemical assessments

Based on their rearing response to a novel open field, male Wistar rats were divided into two sub-groups with either high or low behavioural activity (high rearings, versus low rearings). These sub-groups were repeatedly exposed to the same open field and tested for behavioural habituation. Since we previously found neurochemical evidence for different cholinergic reactivities in such high rearing and low rearing rats, their behavioural responses to the muscarinic antagonist scopolamine (0.5 mg/kg) were also investigated in the open field. Additionally, they were exposed to the elevated plus-maze to test for possible differences in measures of anxiety. After behavioural testing, tissue concentrations of biogenic amines were determined in the ventral striatum (nucleus accumbens, olfactory tubercle), frontal cortex, striatum, hippocampus and amygdala. The results show that the higher rearing responses of high rearing rats in the novel open field were paralleled by higher locomotor activity. These behavioural differences between groups decreased with repeated open field exposure, an effect which was largely due to between-session habituation in high rearing rats. Thereby, high rearing rats approached the lower levels of low rearing rats, in which locomotor activity and rearings did not habituate between testing. Nevertheless, habituation was also observed in low rearing rats, especially in the measure of thigmotactic scanning, since the levels of scanning declined both between and within test sessions. The anticholinergic challenge with scopolamine induced a general pattern of behavioural activation. Furthermore, scopolamine partly reinstated the behavioural differences between high and low rearing rats that had been observed in the novel open field, since high rearing rats showed more rearing behaviour than low rearing rats under scopolamine. In contrast to the open field, there were no significant differences between high and low rearing rats in the plus-maze. The neurochemical analysis revealed, among others, higher dopamine levels in the ventral striatum of high rearing rats together with lower serotonin levels in the medial frontal cortex. The current findings thus indicate that high and low rearing rats not only differ in their behavioural response to a novel environment, but also in their patterns of behavioural habituation, and with respect to behaviour induced by an anti-cholinergic challenge. These differential behavioural profiles of high and low rearing animals are discussed with respect to the role of dopaminergic mechanisms in the forebrain, and the potential impact of cholinergic mechanisms.

Food deprivation modulates chronic stress effects on object recognition in male rats: role of monoamines and amino acids

An object recognition task was used to determine if chronic restraint stress (6 h/day for 21 days) impairs non-spatial memory, since chronic restraint is known to impair spatial memory. In addition, food deprivation was tested as a possible modulating factor of any stress effect in this non-reward-dependent task. Following 3 weeks of daily restraint, subjects were tested for open field activity and object recognition (over different delay intervals) during one week in two separate experiments. Experiment 1 involved testing under low demand conditions (small arena) while experiment 2 involved testing under higher-demand conditions (large arena). Basal monoamine and amino acid levels (home cage) were assessed in experiment one and monoamine arousal levels (following a sample trial) were assessed in experiment two. We observed that chronic stress impaired object recognition when the delay was extended beyond 1 h, and that food deprivation could attenuate the degree of impairment. In addition, chronic stress was associated with increased norepinephrine levels in both the amygdala and hippocampus, and dopamine (HVA/DA, DOPAC/DA) in prefrontal cortex. These changes were not observed in stress subjects that were subsequently food deprived. Food deprived subjects had higher basal serotonin activity in prefrontal cortex and hippocampus as well as higher serum CORT levels. Results suggest that food deprivation may act as a novel stress, thereby increasing subjects' arousal and attention toward the objects, which aids stressed subjects, especially in low-demand conditions. Both restraint and food deprivation affected select limbic areas associated with memory functioning.

Individual differences in behavioral reactivity: correlation with stress-induced norepinephrine efflux in the hippocampus of Sprague-Dawley rats

The present studies investigate the hypothesis that the locus coeruleus-norepinephrine (LC-NE) system plays a role in the neural substrates underlying individual differences in behavioral reactivity to stress. Individuals were selected from a random sample of Sprague-Dawley rats and categorized as a high responder (HR), middle responder (MR), or low responder (LR) based on the initial locomotor response to a novel open field. Rats with behavioral scores at least 1 SD away from the mean for the subject sample were categorized as HR or LR rats. Middle responder rats exhibited locomotor scores representative of the mean locomotor activity of the population sample. Locomotor activity scores measured 6 days after the initial determination were similar to scores obtained in the original screening, suggesting that the locomotor response to novelty is a stable individual trait. Additionally, locomotor activity during the dark phase of the diurnal cycle was not different among the groups, suggesting that differences in locomotor activity in response to a novel open field are an index of behavioral reactivity to the stressful situation rather than an indicator of global differences in motoric activity. In vivo microdialysis was used to measure extracellular levels of hippocampal NE in the hippocampus. During baseline conditions, the efflux of hippocampal NE was similar among HR, MR, and LR rats. In response to tail-pinch stress, hippocampal NE release was elevated in all groups. This response was significantly greater in HR compared to LR rats. Across all groups, locomotor response in the novel open field was significantly correlated with the magnitude of NE release in response to subsequent application of tail-pinch stress. In contrast, administration of 1.5 mg/kg, i.p., amphetamine resulted in a similar elevation of extracellular NE level among HR, LR, and MR rats. These data suggest that activation of the LC-NE system may be involved in determining the behavioral response of individuals to environmental stress.

Cholinergic activation in frontal cortex and nucleus accumbens related to basic behavioral manipulations: handling, and the role of post-handling experience

The present experiment is part of a series of studies designed to investigate cerebral cholinergic activity during basic behavioral testing procedures. Using in vivo microdialysis, we monitored extracellular acetylcholine levels in rats which were picked up manually (termed handling) and exposed to an open field, or animals which were picked up and returned to their home cage. These procedures were repeated on two consecutive days. In the lateral precentral area of the frontal cortex, both procedures increased cholinergic activity. However, on the 1 st day of testing, the degree of cholinergic activation was of even greater magnitude in animals which were returned to the home cage after handling than in animals which were exposed to a novel open field. This neurochemical pattern was dissociated from behavioral indices of activation, since rearing and locomotor activity were more pronounced in the open field than in the home cage. In the nucleus accumbens core and shell, where extracellular acetylcholine is provided by cholinergic interneurons, we also found cholinergic activation on both days of testing. However, unlike the frontal cortex, there were no substantial neurochemical differences between animals which were exposed to the open field after handling vs. those which were returned to their home cage. Together, our data suggest that a simple interaction like handling provides a significant stimulus for the animal to which cholinergic activity responds in several forebrain areas. Here, frontal cortical acetylcholine appears to be especially sensitive, with a pattern of activation which is dependent on post-handling experience. These results are discussed with respect to their possible functional implications, and the role of handling as an experimental factor. Since handling is part of many neurobehavioral procedures, handling-induced changes can interact with the imposed independent variables under investigation, such as post-trial pharmacological manipulations, requiring consideration in the interpretation of any experiment employing handling of the subjects.

Testosterone modifies response to chronic heat exposure in rats

Eight weeks of heat exposure (34 +/- 0.5 degrees C) in sham-orchiectomized rats leads to an increase of body temperature, slowing of body growth rate, and decrease of serum corticosterone level, as compared with animals maintained at 21 +/- 2 degrees C. Orchiectomy decreases body temperature, slows growth rate, and increases plasma corticosterone concentration both in control and heat exposed animals. Testosterone administration reverts these parameters to initial values. We conclude that testosterone plays a role in the regulation of heat balance in male rats.

Stress during pregnancy alters rat offspring morphology and ultrasonic vocalizations

Stress during pregnancy, or prenatal stress, is known to alter offspring behavior, morphology and physiology. We found that a heat, light and restraint stressor applied during the third trimester of pregnancy: 1) decreased the weight gain of adult female rats during pregnancy; 2) reduced the weight of pups, as well as the anogenital distance of male offspring, at birth; and 3) increased the number of ultrasonic vocalizations emitted by pups during isolation in a novel environment on Postnatal Day 14. These results closely approximate those we previously observed after peripheral administration of corticotropin-releasing factor to pregnant females during the third trimester. Together, the studies strongly suggest a role for corticotropin-releasing factor and/or other hormones of the hypothalamic-pituitary-adrenal system in mediating some of the effects of gestational stress.

Circadian variation in basal plasma corticosterone and adrenocorticotropin in the rat: sexual dimorphism and changes across the estrous cycle

Sexual dimorphism in the rat hypothalamic-pituitary-adrenal axis was investigated by determination of plasma corticosterone and immunoreactive (I-) ACTH in males and in females at each stage of the estrous cycle. A serial blood-sampling technique enabled assessment of covariation of the two hormones across the full circadian range of their concentrations within individual animals. Distinct diurnal rhythms in plasma corticosterone were evident in all rats, and the degree and timing of this rhythmicity, determined by cosinor analyses, did not vary with gender or cycle stage. There were, however, marked differences in absolute levels of corticosterone across the estrous cycle, with the average daily concentration (mesor) increasing progressively from a minimum at estrus (129 +/- 11 ng/ml) to a maximum 3 days later at proestrus (246 +/- 14 ng/ml). The mesor corticosterone value in male rats (102 +/- 21 ng/ml) was not different from that in estrous females, but was lower than that in females at all other stages of the cycle. In contrast, no gender- or cycle-related differences were detected in absolute levels of I-ACTH, although distinct diurnal rhythms, synchronous with those for corticosterone, were evident in all groups. Accordingly, a strong and positive within-rat relationship between plasma corticosterone and I-ACTH was observed in all groups, but there was a clear shift in the nature of this relationship across the estrous cycle, such that the slope (i.e. concentration of plasma corticosterone per unit concentration of I-ACTH) was minimal in males and estrous females and maximal in proestrous females. In conclusion, this study shows that the extent of sexual dimorphism in resting plasma corticosterone levels is dependent on estrous cycle stage, being absent at estrus and maximal at proestrus. Moreover, this variation in plasma corticosterone was not accompanied by corresponding changes in plasma I-ACTH, suggestive of cycle-related changes in responsiveness of the adrenal cortex to trophic stimulation.