Acute effects of glucocorticoids: behavioral and pharmacological perspectives

Effects of Peripubertal Experiences on Competitive Behavior in Male Rats at Different Stages of Adulthood

Past studies in animal models have extensively investigated the impact of early life experiences on behavioral development, yet relatively few have specifically examined the implications of peripubertal experiences on the evolution of competitive behavior across distinct stages of adulthood. In the current research, we probed potential differences in competitive behavior during emerging adulthood (3 months old) and middle adulthood (12 months old) in 81 Sprague-Dawley male rats exposed to three different peripubertal (postnatal Days 37-60) environments: an enriched environment (EE), a chronic unpredictable mild stress (CUMS) condition, and a control condition. Anxiety-like behavior served as a positive control in our study. Results revealed significant variations in competitive behavior among the groups during emerging adulthood. The EE group displayed the least anxiety and outperformed their peers in food-reward-oriented competition, whereas the CUMS group excelled in status-driven, agonistic competition. However, these behavioral differentiations gradually attenuated by middle adulthood, at which point the control group began to show an advantage. Our findings suggest that although peripubertal experiences significantly shape competitive behavior in the emerging adulthood stage, this effect diminishes over time and is nearly non-detectable during mid-adulthood, underscoring the fluidity of behavioral development and demonstrating that the effects of peripubertal experiences can be modulated by subsequent life experiences.

The emerging role of rapid corticosteroid actions on excitatory and inhibitory synaptic signaling in the brain

Over the past two decades, there has been increasing evidence for the importance of rapid-onset actions of corticosteroid hormones in the brain. Here, we highlight the distinct rapid corticosteroid actions that regulate excitatory and inhibitory synaptic transmission in the hypothalamus, the hippocampus, basolateral amygdala, and prefrontal cortex. The receptors that mediate rapid corticosteroid actions are located at or close to the plasma membrane, though many of the receptor characteristics remain unresolved. Rapid-onset corticosteroid effects play a role in fast neuroendocrine feedback as well as in higher brain functions, including increased aggression and anxiety, and impaired memory retrieval. The rapid non-genomic corticosteroid actions precede and complement slow-onset, long-lasting transcriptional actions of the steroids. Both rapid and slow corticosteroid actions appear to be indispensable to adapt to a continuously changing environment, and their imbalance can increase an individual's susceptibility to psychopathology.

Effect of a Territorial Challenge on the Steroid Profile of a Juvenile Songbird

Aggression is a social behavior that is critical for survival and reproduction. In adults, circulating gonadal hormones, such as androgens, act on neural circuits to modulate aggressive interactions, especially in reproductive contexts. In many species, individuals also demonstrate aggression before reaching gonadal maturation. Adult male song sparrows, Melospiza melodia, breed seasonally but maintain territories year-round. Juvenile (hatch-year) males aggressively compete for territory ownership during their first winter when circulating testosterone is low. Here, we characterized the relationship between the steroid milieu and aggressive behavior in free-living juvenile male song sparrows in winter. We investigated the effect of a 10 min simulated territorial intrusion (STI) on behavior and steroid levels in blood, 10 microdissected brain regions, and four peripheral tissues (liver, pectoral muscle, adrenal glands, and testes). Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), we quantified 12 steroids: pregnenolone, progesterone, corticosterone, 11-dehydrocorticosterone, dehydroepiandrosterone, androstenedione, testosterone, 5α-dihydrotestosterone, 17β-estradiol, 17α-estradiol, estrone, and estriol. We found that juvenile males are robustly aggressive, like adult males. An STI increases progesterone and corticosterone levels in blood and brain and increases 11-dehydrocorticosterone levels in blood only. Pregnenolone, androgens, and estrogens are generally non-detectable and are not affected by an STI. In peripheral tissues, steroid concentrations are very high in the adrenals. These data suggest that adrenal steroids, such as progesterone and corticosterone, might promote juvenile aggression and that juvenile and adult songbirds might rely on distinct neuroendocrine mechanisms to support similar aggressive behaviors.

Linking microbiome and stress hormone responses in wild tropical treefrogs across continuous and fragmented forests

The amphibian skin microbiome is an important component of anti-pathogen defense, but the impact of environmental change on the link between microbiome composition and host stress remains unclear. In this study, we used radiotelemetry and host translocation to track microbiome composition and function, pathogen infection, and host stress over time across natural movement paths for the forest-associated treefrog, Boana faber. We found a negative correlation between cortisol levels and putative microbiome function for frogs translocated to forest fragments, indicating strong integration of host stress response and anti-pathogen potential of the microbiome. Additionally, we observed a capacity for resilience (resistance to structural change and functional loss) in the amphibian skin microbiome, with maintenance of putative pathogen-inhibitory function despite major temporal shifts in microbiome composition. Although microbiome community composition did not return to baseline during the study period, the rate of microbiome change indicated that forest fragmentation had more pronounced effects on microbiome composition than translocation alone. Our findings reveal associations between stress hormones and host microbiome defenses, with implications for resilience of amphibians and their associated microbes facing accelerated tropical deforestation.

Expanding the frame around social dynamics and glucocorticoids: From hierarchies within the nest to competitive interactions among species

The effect of the social environment on individual state or condition has largely focused on glucocorticoid levels (GCs). As metabolic hormones whose production can be influenced by nutritional, physical, or psychosocial stressors, GCs are a valuable (though singular) measure that may reflect the degree of "stress" experienced by an individual. Most work to date has focused on how social rank influences GCs in group-living species or how predation risk influences GCs in prey. This work has been revealing, but a more comprehensive assessment of the social environment is needed to fully understand how different features of the social environment influence GCs in both group living and non-group living species and across life history stages. Just as there can be intense within-group competition among adult conspecifics, it bears appreciating there can also be competition among siblings from the same brood, among adult conspecifics that do not live in groups, or among heterospecifics. In these situations, dominance hierarchies typically emerge, albeit, do dominants or subordinate individuals or species have higher GCs? We examine the degree of support for hypotheses derived from group-living species about whether differential GCs between dominants and subordinates reflect the "stress of subordination" or "costs of dominance" in these other social contexts. By doing so, we aim to test the generality of these two hypotheses and propose new research directions to broaden the lens that focuses on social hierarchies and GCs.

Glucocorticoids and Aggression: A Tripartite Interaction

The effects of glucocorticoids on aggression can be conceptualized based on its mechanisms of action. These hormones can affect cell function non-genomically within minutes, primarily by affecting the cell membrane. Overall, such effects are activating and promote both metabolic preparations for the fight and aggressive behavior per se. Chronic increases in glucocorticoids activate genomic mechanisms and are depressing overall, including the inhibition of aggressive behavior. Finally, excessive stressors trigger epigenetic phenomena that have a large impact on brain programming and may also induce the reprogramming of neural functions. These induce qualitative changes in aggression that are deemed abnormal in animals, and psychopathological and criminal in humans. This review aims at deciphering the roles of glucocorticoids in aggression control by taking in view the three mechanisms of action often categorized as acute, chronic, and toxic stress based on the duration and the consequences of the stress response. It is argued that the tripartite way of influencing aggression can be recognized in all three animal, psychopathological, and criminal aggression and constitute a framework of mechanisms by which aggressive behavior adapts to short-term and log-term changes in the environment.

Characterizing the behavioral and neuroendocrine features of susceptibility and resilience to social stress

Evaluating and coping with stressful social events as they unfold is a critical strategy in overcoming them without long-lasting detrimental effects. Individuals display a wide range of responses to stress, which can manifest in a variety of outcomes for the brain as well as subsequent behavior. Chronic Social Defeat Stress (CSDS) in mice has been widely used to model individual variation following a social stressor. Following a course of repeated intermittent psychological and physical stress, mice diverge into separate populations of social reactivity: resilient (socially interactive) and susceptible (socially avoidant) animals. A rich body of work reveals distinct neurobiological and behavioral consequences of this experience that map onto the resilient and susceptible groups. However, the range of factors that emerge over the course of defeat have not been fully described. Therefore, in the current study, we focused on characterizing behavioral, physiological, and neuroendocrine profiles of mice in three separate phases: before, during, and following CSDS. We found that following CSDS, traditional read-outs of anxiety-like and depression-like behaviors do not map on to the resilient and susceptible groups. By contrast, behavioral coping strategies used during the initial social stress encounter better predict which mice will eventually become resilient or susceptible. In particular, mice that will emerge as susceptible display greater escape behavior on Day 1 of social defeat than those that will emerge as resilient, indicating early differences in coping mechanisms used between the two groups. We further show that the social avoidance phenotype in susceptible mice is specific to the aggressor strain and does not generalize to conspecifics or other strains, indicating that there may be features of threat discrimination that are specific to the susceptible mice. Our findings suggest that there are costs and benefits to both the resilient and susceptible outcomes, reflected in their ability to cope and adapt to the social stressor.

Personality and physiological traits predict contest interactions in Kryptolebias marmoratus

Personality and physiological traits often have close relationships with dominance status, but the significance and/or direction of the relationships vary between studies. This study examines whether two personality traits (aggressiveness and boldness) and three physiological traits (testosterone and cortisol levels and oxygen consumption rates) are associated with contest decisions/performance using a mangrove killifish Kryptolebias marmoratus. The results show that individuals that attacked their own mirror images (an aggressiveness index) at higher rates or had higher levels of testosterone were more likely to attack their opponent and win non-escalated contests, while individuals that had higher levels of cortisol were more likely to lose. After the contests, (1) individuals that had attacked their opponents or won had higher post-contest oxygen consumption rates, and (2) individuals that had attacked their opponents also had higher post-contest levels of cortisol. Although no significant correlations were detected among pre-contest physiological traits, post-contest levels of cortisol were positively correlated with oxygen consumption rates. Overall, personality and physiological traits provide useful predictors for the fish's contest decisions/performance. Contest interactions subsequently modified post-contest physiological traits and potentially also promoted associations between them. Nevertheless, the fish's physiological traits remained rather consistent over the entire study period.

Aggressive behavior and brain neuronal activation in sexually naïve male Mongolian gerbils

Aggressive behavior plays an important role in animal's survival and reproductive success. Although there has been growing interests in studying neural mechanisms underlying aggressive behavior using traditional laboratory animal models, little is known about mechanisms controlling naturally occurring aggression in sexually naïve animals. In the present study, we characterized aggressive behavior displayed by sexually naïve male Mongolian gerbils (Meriones unguiculatus) and examined the subsequent neuronal activation in the brain measured by Fos-immunoreactive (Fos-ir) staining. We found that resident males initiated attacks and showed intense levels of aggression (including chase, bite, offensive sideway, lunge and on-top) towards a conspecific male intruder. Furthermore, attacks from the resident males towards the intruder produced a nonrandom distribution of bites, with the most on the rump, flank, back and tail and few on the limbs, ventrum and head. In contrast, control males that were exposed to a woodblock (control for novelty) never attacked the woodblock and showed higher levels of object/environmental investigation. Male gerbils exposed to an intruder had significantly higher levels of Fos-ir density in the medial (MeA) and anterior cortical (ACo) subnuclei of the amygdala, principal nucleus (BSTpr) and interfascicular nucleus (BSTif) of the bed nucleus of the stria terminalis, ventrolateral subdivision of the ventromedial hypothalamus (VMHvl), and paraventricular nucleus of the hypothalamus (PVN), compared to control males. Together, our results indicate that sexually naïve, group housed male gerbils naturally display aggression towards conspecific strangers, and such aggressive behavior is associated with special patterns of neuronal activation in the brain.

Transgenerational hypocortisolism and behavioral disruption are induced by the antidepressant fluoxetine in male zebrafish Danio rerio

The global prevalence of depression is high during childbearing. Due to the associated risks to the mother and baby, the selective serotonin reuptake inhibitor fluoxetine (FLX) is often the first line of treatment. Given that FLX readily crosses the placenta, a fetus may be susceptible to the disruptive effects of FLX during this highly plastic stage of development. Here, we demonstrate that a 6-day FLX exposure to a fetus-relevant concentration at a critical developmental stage suppresses cortisol levels in the adult zebrafish (F₀). This effect persists for three consecutive generations in the unexposed descendants (F₁ to F₃) without diminution and is more pronounced in males. We also show that the in vivo cortisol response of the interrenal (fish "adrenal") to an i.p. injection of adrenocorticotropic hormone was also reduced in the males from the F₀ and F₃ FLX lineages. Transcriptomic profiling of the whole kidney containing the interrenal cells revealed that early FLX exposure significantly modified numerous pathways closely associated with cortisol synthesis in the male adults from the F₀ and F₃ generations. We also show that the low cortisol levels are linked to significantly reduced exploratory behaviors in adult males from the F₀ to F₂ FLX lineages. This may be a cause for concern given the high prescription rates of FLX to pregnant women and the potential long-term negative impacts on humans exposed to these therapeutic drugs.

The role of glucocorticoids in the vertebrate response to weather

Changes in the environment related to inclement weather can threaten survival and reproductive success both through direct adverse exposure and indirectly by decreasing food availability. Glucocorticoids, released during activation of the hypothalamic-pituitary-adrenal axis as part of the stress response, are an important candidate for linking vertebrate coping mechanisms to weather. This review attempts to determine if there is a consensus response of glucocorticoids to exposure to weather-related stimuli, including food availability, precipitation, temperature and barometric pressure. The included studies cover field and laboratory studies for all vertebrate taxa, and are separated into four exposure periods, e.g., hours, days, weeks and months. Each reported result was assigned a score based on the glucocorticoid response, e.g., increased, no change, or decreased. Short-term exposure to weather-related stimuli, of up to 24 h, is generally associated with increased glucocorticoids (79% of studies), suggesting that these stimuli are perceived as stressors by most animals. In contrast, the pattern for exposures longer than 24 h shows more variation, even though a majority of studies still report an increase (64%). Lack of glucocorticoid increases appeared to result from instances where: (1) prolonged exposure was a predictable part of the life history of an animal; (2) environmental context was important for the ultimate effect of a stimulus (e.g., precipitation limited food availability in one environment, but increased food in another); (3) prolonged exposure induced chronic stress; and (4) long-term responses appeared to reflect adaptations to seasonal shifts, instead of to short-term weather. However, there is a strong bias towards studies in domesticated laboratory species and wild animals held in captivity, indicating a need for field studies, especially in reptiles and amphibians. In conclusion, the accumulated literature supports the hypothesis that glucocorticoids can serve as the physiological mechanism promoting fitness during inclement weather.

The Role of the Lateral Hypothalamus in Violent Intraspecific Aggression-The Glucocorticoid Deficit Hypothesis

This review argues for a central role of the lateral hypothalamus in those deviant forms of aggression, which result from chronic glucocorticoid deficiency. Currently, this nucleus is considered a key region of the mechanisms that control predatory aggression. However, recent findings demonstrate that it is strongly activated by aggression in subjects with a chronically downregulated hypothalamus-pituitary-adrenocortical (HPA) axis; moreover, this activation is causally involved in the emergence of violent aggression. The review has two parts. In the first part, we review human findings demonstrating that under certain conditions, strong stressors downregulate the HPA-axis on the long run, and that the resulting glucocorticoid deficiency is associated with violent aggression including aggressive delinquency and aggression-related psychopathologies. The second part addresses neural mechanisms in animals. We show that the experimental downregulation of HPA-axis function elicits violent aggression in rodents, and the activation of the brain circuitry that originally subserves predatory aggression accompanies this change. The lateral hypothalamus is not only an integral part of this circuitry, but can elicit deviant and violent forms of aggression. Finally, we formulate a hypothesis on the pathway that connects unfavorable social conditions to violent aggression via the neural circuitry that includes the lateral hypothalamus.

Effects of acute corticosterone treatment on male prairie voles (Microtus ochrogaster): Territorial aggression does not accompany induced social preference

Corticosterone (CORT) is a stress-related steroid hormone found in vertebrates, and is known to interact with behavior. In the socially monogamous prairie vole (Microtus ochrogaster), acute stress and specifically acute CORT administration have been shown to facilitate male social preference for a familiar female, and this effect has been described as facilitation of the monogamous pair bond. It is possible, however, that the effects of stress on social preference may initially represent a short-term coping strategy. Here we test whether the effect of acute CORT administration extends to territoriality, a defining component of the prairie vole monogamous suite of behaviors. Onset of territoriality would provide further support for an induced pair bond, whereas no increase in aggression would suggest an initial coping response. Using acute exogenous CORT injections followed by behavioral trials, we found a facilitation of social preference, but we did not find increased aggression. This result suggests that the social preference that develops in response to CORT is at least in part a coping response rather than facilitation of comprehensive monogamous pair bond behavior. Our results are consistent with previous studies both within prairie voles and across other taxa that suggest that social contact may be involved in the regulation of stress responses. (PsycINFO Database Record

Group housing during adolescence has long-term effects on the adult stress response in female, but not male, zebra finches (Taeniopygia guttata)

Adolescent social interactions can have long-term effects on physiological responses to stressors in later-life. A larger adolescent group size can result in higher stressor-induced secretion of glucocorticoids in adulthood. The effect may be due to a socially-mediated modulation of gonadal hormones, e.g. testosterone. However, group size (number of animals) has been conflated with social density (space per animal). Therefore it is hard to determine the mechanisms through which adolescent group size can affect the stress response. The current study aimed to tease apart the effects of group size and social density during adolescence on the physiological stress response and gonadal hormone levels in adulthood. Adolescent zebra finches were housed in groups varying in size (2 vs. 5 birds per cage) and density (0.03m³ vs. 0.06m³ per bird) during early adolescence (day 40-60). Density was only manipulated in birds raised in groups of five. Glucocorticoid concentration secreted in response to a standard capture and restraint stressor was quantified in adolescence (day 55±1) and adulthood (day 100+). Basal gonadal hormone concentrations (male testosterone, female estradiol) were also quantified in adulthood. Female birds housed in larger groups, independent of social density, secreted a higher glucocorticoid concentration 45min into restraint regardless of age, and had higher peak glucocorticoid concentration in adulthood. Adult gonadal hormone concentrations were not affected by group size or density. Our results suggest that group size, not density, is a social condition that influences the development of the endocrine response to stressors in female zebra finches, and that these effects persist into adulthood. The findings have clear relevance to the social housing conditions necessary for optimal welfare in captive animals, but also elucidate the role of social rearing conditions in the emergence of responses to stressors that may persist across the lifespan and affect fitness of animals in wild populations.

Both serotonergic and noradrenergic systems modulate the development of tolerance to chronic stress in rats with lesions of the serotonergic neurons of the median raphe nucleus

Acute exposure to stress induces significant behavioural changes, while repeated exposure to the same stressor leads to the development of tolerance to stress. The development of tolerance appears to involve the serotonergic projections from the Median Raphe Nucleus (MnRN) to the dorsal Hippocampus (dH), since rats with lesions of this pathway does not develop tolerance to stress. Previous data from our laboratory showed that treatment with imipramine, a serotonin (5-HT) and noradrenaline (NA) reuptake inhibitor, lead to the development of tolerance. However, it remains to be elucidated whether such tolerance involves the participation of the noradrenergic system, apart from the serotonergic projections. Therefore, the aim of this work was to investigate the behavioural and neurochemical effects of chronic treatment with desipramine (NA reuptake inhibitor) or fluoxetine (5-HT reuptake inhibitor) in chronically stressed rats with lesions of the serotonergic neurons of the MnRN. Male Wistar rats with or without lesion in the MnRN were submitted or not to acute (2 h) or chronic restraint (2 h/seven days) stress and tested in the elevated pus maze (EPM). Treatment with fluoxetine, desipramine (10 mg/kg) or saline was performed twice daily (12-12 h interval), for 7 consecutive days. EPM test was conducted 24 h after the treatment. Fluoxetine attenuated the anxiogenic-induced effect of lesion in chronically restrained rats, without changing serotonin and noradrenaline levels in the hippocampus of lesioned rats. A similar profile was also observed after treatment with desipramine. These results suggest that both the serotonergic and the noradrenergic systems are involved in the development of tolerance to chronic stress. Additionally, the integrity of the serotonergic pathway of the MnRN-dH is not essential for the anxiolytic-like effects of these drugs.

Long-term effects of adolescent stress on neophobic behaviors in zebra finches are modulated by social context when in adulthood

Experiencing stress during adolescence can increase neophobic behaviors in adulthood, but most tests have been conducted in the absence of conspecifics. Conspecifics can modulate responses to stressors, for example by acting as 'social buffers' to attenuate the aversive appraisal of stressors. Here, we investigate the long-term effects of adolescent stress on the behavioral responses to novel stimuli (a mild stressor) across social contexts in an affiliative passerine bird, the zebra finch. During early (days 40-60) or late (days 65-85) adolescence the birds (n=66) were dosed with either saline or the hormone corticosterone (CORT). CORT was given in order to mimic a physiological stress response and saline was given as a control. In adulthood, the birds' behavioral responses to a novel environment were recorded in both the presence and absence of conspecifics. An acute CORT response was also quantified in adolescence and adulthood. Our findings show clear evidence of social context mediating any long-term effects of adolescent stress. In the presence of familiar conspecifics no treatment effects were detected. Individually, birds dosed with CORT in early adolescence were slower to enter a novel environment, spent more time perching in the same novel environment, and, if female, engaged in more risk assessment. Birds dosed in late adolescence were unaffected. No treatment effects were detected on CORT, but adolescents had a higher CORT concentration than adults. Our results are the first to suggest that familiar conspecifics in adulthood can buffer the long-term effects of stress that occurred during early adolescence.

Disruption of the Glutamate-Glutamine Cycle Involving Astrocytes in an Animal Model of Depression for Males and Females

Background: Women are twice as likely as men to develop major depression. The brain mechanisms underlying this sex disparity are not clear. Disruption of the glutamate–glutamine cycle has been implicated in psychiatric disturbances. This study identifies sex-based impairments in the glutamate–glutamine cycle involving astrocytes using an animal model of depression.

Methods: Male and female adult Long-Evans rats were exposed to chronic social defeat stress (CSDS) for 21 days, using a modified resident-intruder paradigm. Territorial aggression was used for males and maternal aggression was used for females to induce depressive-like deficits for intruders. The depressive-like phenotype was assessed with intake for saccharin solution, weight gain, estrous cycle, and corticosterone (CORT). Behaviors displayed by the intruders during daily encounters with residents were characterized. Rats with daily handling were used as controls for each sex. Ten days after the last encounter, both the intruders and controls were subjected to a no-net-flux in vivo microdialysis to assess glutamate accumulation and extracellular glutamine in the nucleus accumbens (NAc). The contralateral hemispheres were used for determining changes in astrocytic markers, including glial fibrillary acidic protein (GFAP) and glutamate transporter-1 (GLT-1).

Results: Both male and female intruders reduced saccharin intake over the course of CSDS, compared to their pre-stress period and to their respective controls. Male intruders exhibited submissive/defensive behaviors to territorial aggression by receiving sideways threats and bites. These males showed reductions in striatal GLT-1 and spontaneous glutamine in the NAc, compared to controls. Female intruders exhibited isolated behaviors to maternal aggression, including immobility, rearing, and selfgrooming. Their non-reproductive days were extended. Also, they showed reductions in prefrontal and accumbal GFAP⁺ cells and prefrontal GLT-1, compared to controls. When 10 μM of glutamate was infused, these females showed a significant accumulation of glutamate compared to controls. Infusions of glutamate reduced extracellular glutamine for both male and female intruders compared to their respective controls.

Conclusion: Twenty-one days of territorial or maternal aggression produced a depressive-like phenotype and impaired astrocytes in both male and female intruders. Disruption of the glutamate–glutamine cycle in the PFC-striatal network may be linked to depressive-like deficits more in females than in males.

Membrane-initiated non-genomic signaling by estrogens in the hypothalamus: cross-talk with glucocorticoids with implications for behavior

The estrogen receptor and glucocorticoid receptor are members of the nuclear receptor superfamily that can signal using both non-genomic and genomic transcriptional modes. Though genomic modes of signaling have been well characterized and several behaviors attributed to this signaling mechanism, the physiological significance of non-genomic modes of signaling has not been well understood. This has partly been due to the controversy regarding the identity of the membrane ER (mER) or membrane GR (mGR) that may mediate rapid, non-genomic signaling and the downstream signaling cascades that may result as a consequence of steroid ligands binding the mER or the mGR. Both estrogens and glucocorticoids exert a number of actions on the hypothalamus, including feedback. This review focuses on the various candidates for the mER or mGR in the hypothalamus and the contribution of non-genomic signaling to classical hypothalamically driven behaviors and changes in neuronal morphology. It also attempts to categorize some of the possible functions of non-genomic signaling at both the cellular level and at the organismal level that are relevant for behavior, including some behaviors that are regulated by both estrogens and glucocorticoids in a potentially synergistic manner. Lastly, it attempts to show that steroid signaling via non-genomic modes may provide the organism with rapid behavioral responses to stimuli.

A robust and reliable non-invasive test for stress responsivity in mice

Stress and an altered stress response have been associated with many multifactorial diseases, such as psychiatric disorders or neurodegenerative diseases. As currently mouse mutants for each single gene are generated and phenotyped in a large-scale manner, it seems advisable also to test these mutants for alterations in their stress responses. Here we present the determinants of a robust and reliable non-invasive test for stress-responsivity in mice. Stress is applied through restraining the mice in tubes and recording behavior in the Open Field 20 min after cessation of the stress. Two hours, but not 15 or 50 min of restraint lead to a robust and reproducible increase in distance traveled and number of rearings during the first 5 min in the Open Field in C57BL/6 mice. This behavioral response is blocked by the corticosterone synthesis inhibitor metyrapone, but not by RU486 treatment, indicating that it depends on corticosteroid secretion, but is not mediated via the glucocorticoid receptor type II. We assumed that with a stress duration of 15 min one could detect hyper-responsivity, and with a stress duration of 2 h hypo-responsivity in mutant mouse lines. This was validated with two mutant lines known to show opposing effects on corticosterone secretion after stress exposure, corticotropin-releasing hormone (CRH) over-expressing mice and CRH receptor 1 knockout (KO) mice. Both lines showed the expected phenotype, i.e., increased stress responsivity in the CRH over-expressing mouse line (after 15 min restraint stress) and decreased stress responsivity in the CRHR1-KO mouse line (after 2 h of restraint stress). It is possible to repeat the acute stress test several times without the stressed animal adapting to it, and the behavioral response can be robustly evoked at different ages, in both sexes and in different mouse strains. Thus, locomotor and rearing behavior in the Open Field after an acute stress challenge can be used as reliable, non-invasive indicators of stress responsivity and corticosterone secretion in mice.

The glucocorticoid/aggression relationship in animals and humans: an analysis sensitive to behavioral characteristics, glucocorticoid secretion patterns, and neural mechanisms

Glucocorticoids control a wide array of biological processes from glucose homeostasis to neuronal function. The mechanisms mediating their effects are similarly varied and include rapid and transient nongenomic effects on calcium trafficking, various neurotransmitter receptors, and other membrane/cytoplasmic proteins, as well as slowly developing but durable genomic effects that are mediated by a large number of glucocorticoid-sensitive genes that are affected after variable lag-times. Given this complexity, we suggest that the aggression/glucocorticoid relationship cannot be reduced to the simple "stimulation/inhibition" question. Here, we review the effects of glucocorticoids on aggression by taking into account the complexities of glucocorticoid actions. Acute and chronic effects were differentiated because these are mediated by different mechanisms. The effects of chronic increases and decreases in glucocorticoid production were discussed separately, because the activation of mechanisms that are not normally activated and the loss of normal functions should not be confounded. Findings in healthy/normal subjects and those obtained in subjects that show abnormal forms of behavior or psychopathologies were also differentiated, because the effects of glucocorticoids are indirect, and largely depend on the properties of neurons they act upon, which are altered in subjects with psychopathologies. In addition, the conditions of glucocorticoid measurements were also thoroughly evaluated. Although the role of glucocorticoids in aggression is perceived as controversial by many investigators, a detailed analysis that is sensitive to glucocorticoid and behavioral measure as well as to the mediating mechanism suggests that this role is rather clear-cut; moreover, there is a marked similarity between animal and human findings.

Acute and lasting effects of single mineralocorticoid antagonism on offensive aggressive behaviour in rats

Aggression is a major component of territorial behaviour. However, different mechanisms evolved to fulfil the defence function while reducing the cost derived from agonistic interactions, as a differential response to the same stimulus, depending on the outcome of past conflicts - priming, which makes the aggressive response adaptable. Aggressive behaviour is facilitated by the stress response, so, we tested the effect of a single injection of a mineralocorticoid antagonist (spironolactone) on the escalation of territorial aggression in a resident-intruder paradigm, and its modulation by social stimulus. We used naïve Wild Type Groningen - WTG - rats as residents, and naïve and previously defeated Wistar rats as intruders. The first encounter was 1h after the injection, and then repeated in 3 consecutive days. When WTG rats were confronted with naïve Wistar rats, single injections of spironolactone completely abolished the attack behaviour in the short term while enhancing it in the long term. When we used defeated Wistar rats, the spironolactone effect was not as great. The short-term reduction in aggressive behaviour was attributable to the blockade of mineralocorticoid receptors during the first encounters, while the enhancement in aggressive behaviour in the long term was suggested to be related to the imbalance between mineralocorticoid and glucocorticoid receptors during the stress response associated to the encounters.

Monoacylglycerol lipase inhibition-induced changes in plasma corticosterone levels, anxiety and locomotor activity in male CD1 mice

The hypothalamus-pituitary-adrenal-axis is strongly controlled by the endocannabinoid system. The specific impact of enhanced 2-arachidonoylglycerol signaling on corticosterone plasma levels, however, was not investigated so far. Here we studied the effects of the recently developed monoacylglycerol lipase inhibitor JZL184 on basal and stress-induced corticosterone levels in male CD1 mice, and found that this compound dramatically increased basal levels without affecting stress responses. Since acute changes in corticosterone levels can affect behavior, JZL184 was administered concurrently with the corticosterone synthesis inhibitor metyrapone, to investigate whether the previously shown behavioral effects of JZL184 are dependent on corticosterone. We found that in the elevated plus-maze, the effects of JZL184 on "classical" anxiety-related measures were abolished by corticosterone synthesis blockade. By contrast, effects on the "ethological" measures of anxiety (i.e. risk assessment) were not affected by metyrapone. In the open-field, the locomotion-enhancing effects of the compound were not changed either. These findings show that monoacylglycerol lipase inhibition dramatically increases basal levels of corticosterone. This endocrine effect partly affects the anxiolytic, but not the locomotion-enhancing effects of monoacylglycerol lipase blockade.

Winner and loser effects are modulated by hormonal states

Introduction

Many animals use information acquired from recent experiences to modify their responses to new situations. Animals’ decisions in contests also depend on their previous experience: after recent victories individuals tend to behave more aggressively and after defeats more submissively. Although these winner and/or loser effects have been reported for animals of different taxa, they have only recently been shown to be flexible traits, which can be influenced by extrinsic factors. In a mangrove killifish (Kryptolebias marmoratus), for instance, individuals which lost an earlier contest were more likely than others to alter contest decisions after a recent win/loss. This result suggests that individuals perceiving themselves to have worse fighting abilities are more inclined to adjust contest strategy based on new information. If this is the case, an individual’s propensity to modify behaviour after a win/loss might also be modulated by intrinsic mechanisms related to its ability to fight. Stress and sex steroid hormones are often associated with an individual’s contest behaviour and performance, so, in this study, we tested the hypothesis that an individual’s propensity to change behaviour after wins or losses also depends on its hormonal state.

Results

Our results show that an individual’s propensity to adjust contest decisions after wins and losses does depend on its hormonal state: individuals with lower levels of cortisol (F), testosterone (T) and 11-ketotestosterone (KT) are more receptive than others to the influence of recent contest experiences, especially losing experiences, and the influences last longer. Furthermore, although winning and losing experiences resulted in significant changes in behaviour, they did not bring about a significant change in the levels of F, T, KT or oestradiol (E2).

Conclusions

This study shows that an individual’s receptivity to the influence of recent wins and losses is modulated by its internal state, as well as by extrinsic factors. Individuals with hormonal profiles corresponding to lower aggressiveness and a reduced likelihood of winning were more likely to alter contest decisions after a recent win/loss. The results also suggest that F, T, KT and E2 are not the primary physiological mechanisms mediating winner-loser effects in this fish.

Aggressive interactions differentially modulate local and systemic levels of corticosterone and DHEA in a wild songbird

During the nonbreeding season, when gonadal androgen synthesis is basal, recent evidence suggests that neurosteroids regulate the aggression of male song sparrows. In particular, dehydroepiandrosterone (DHEA) is rapidly converted in the brain to androgens in response to aggressive interactions. In other species, aggressive encounters increase systemic glucocorticoid levels. However, the relationship between aggression and local steroid levels is not well understood. Here, during the breeding and nonbreeding seasons, we tested the effects of a simulated territorial intrusion (STI) on DHEA and corticosterone levels in the brachial and jugular plasma. Jugular plasma is enriched with neurosteroids and provides an indirect index of brain steroid levels. Further, during the nonbreeding season, we directly measured steroid levels in the brain and peripheral tissues. Both breeding and nonbreeding males displayed robust aggressive responses to STI. During the breeding season, STI increased brachial and jugular corticosterone levels and jugular DHEA levels. During the nonbreeding season, STI did not affect plasma corticosterone levels, but increased jugular DHEA levels. During the nonbreeding season, STI did not affect brain levels of corticosterone or DHEA. However, STI did increase corticosterone and DHEA concentrations in the liver and corticosterone concentrations in the pectoral muscle. These data suggest that 1) aggressive social interactions affect neurosteroid levels in both seasons and 2) local steroid synthesis in peripheral tissues may mobilize energy reserves to fuel aggression in the nonbreeding season. Local steroid synthesis in brain, liver or muscle may serve to avoid the costs of systemic increases in corticosterone and testosterone.

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.

Aggression and anxiety: social context and neurobiological links

Psychopathologies such as anxiety- and depression-related disorders are often characterized by impaired social behaviours including excessive aggression and violence. Excessive aggression and violence likely develop as a consequence of generally disturbed emotional regulation, such as abnormally high or low levels of anxiety. This suggests an overlap between brain circuitries and neurochemical systems regulating aggression and anxiety. In this review, we will discuss different forms of male aggression, rodent models of excessive aggression, and neurobiological mechanisms underlying male aggression in the context of anxiety. We will summarize our attempts to establish an animal model of high and abnormal aggression using rats selected for high (HAB) vs. low (LAB) anxiety-related behaviour. Briefly, male LAB rats and, to a lesser extent, male HAB rats show high and abnormal forms of aggression compared with non-selected (NAB) rats, making them a suitable animal model for studying excessive aggression in the context of extremes in innate anxiety. In addition, we will discuss differences in the activity of the hypothalamic–pituitary–adrenal axis, brain arginine vasopressin, and the serotonin systems, among others, which contribute to the distinct behavioural phenotypes related to aggression and anxiety. Further investigation of the neurobiological systems in animals with distinct anxiety phenotypes might provide valuable information about the link between excessive aggression and disturbed emotional regulation, which is essential for understanding the social and emotional deficits that are characteristic of many human psychiatric disorders.

Single-dose and chronic corticosterone treatment alters c-Fos or FosB immunoreactivity in the rat cerebral cortex

The aim of this study was to examine the effects of single-dose and chronic corticosterone treatment on the inducible transcription factor c-Fos and FosB, and thereby to estimate the effects of high-doses of corticosterone on calcium-dependent neuronal responses in the rat cerebral cortex. At the same time we investigated the distribution of interneurons containing calretinin (CR), vasoactive intestinal polypeptide (VIP) and neuropeptide Y (NPY) in chronically treated animals in order to collect data on the involvement of inhibitory neurons in this process. Adult male rats were injected subcutaneously with 10mg corticosterone, whereas controls received the vehicle (sesame oil). The animals were fixed by transcardial perfusion 12 and 24h following single corticosterone injection, and the brains were processed for c-Fos and FosB immunohistochemistry. To investigate the effects of repeated corticosterone administration, rats were daily treated with the same amount of corticosterone (10mg/animal, subcutaneously) for 21 days. Controls were injected with vehicle. At the end of the experiment, the rats were perfused and immunohistochemistry was used to detect the presence of the FosB protein, CR, VIP and NPY. Quantitative evaluation of immunolabelled cells was performed in the neocortex and the hippocampus. The number of immunoreactive nuclei per unit area was used as a quantitative measure of the effects of corticosterone. It was found that a single-dose administration of corticosterone resulted in a significant, time-dependent increase of c-Fos protein immunoreactivity in the granule cell layer of the dentate gyrus, as well as in regions CA1 and CA3 of the hippocampus 12 and 24h post-injection with respect to control animals. Significant enhancement of c-Fos immunoreactivity was also observed in the neocortex at 12 and 24h post-injection. Single-dose treatment did not significantly alter FosB immunolabelling. Repeated administration of corticosterone produced a complex pattern of changes in FosB immunolabelling: significant increase in FosB immunoreactivity was detected in the granule cell layer of the dentate gyrus, with no significant changes in the CA1 and CA3 layers of the hippocampus and in the neocortex. However, a significant decrease of FosB induction in the neocortex was observed in chronically treated rats in comparison to single-dose injected animals (12h before immunohistochemistry). Analysis of immunohistochemical detection of interneuronal markers revealed a significant reduction of the CR immunolabelling in the CA3 area of the hippocampus. No changes in VIP or NPY immunoreactivity were found in the Ammon's horn 3 weeks following daily corticosterone treatment. NPY immunoreactivity was significantly attenuated in the neocortex. The present data suggest that single-dose corticosterone treatment increases immunoreactivity of c-Fos protein in a time-dependent manner, 12 and 24h post-injection in the rat hippocampus and the neocortex, whereas chronic corticosterone treatment influences FosB immunoreactivity, primarily in the dentate gyrus. Chronic corticosterone administration seems to affect CR levels in the CA3 area of the hippocampus.

Early life stress, the development of aggression and neuroendocrine and neurobiological correlates: what can we learn from animal models?

Early life stress (child and adolescent abuse, neglect and trauma) induces robust alterations in emotional and social functioning resulting in enhanced risk for the development of psychopathologies such as mood and aggressive disorders. Here, an overview is given on recent findings in primate and rodent models of early life stress, demonstrating that chronic deprivation of early maternal care as well as chronic deprivation of early physical interactions with peers are profound risk factors for the development of inappropriate aggressive behaviors. Alterations in the hypothalamic-pituitary-adrenocortical (HPA), vasopressin and serotonin systems and their relevance for the regulation of aggression are discussed. Data suggest that social deprivation-induced inappropriate forms of aggression are associated with high or low HPA axis (re)activity and a generally lower functioning of the serotonin system in adulthood. Moreover, genetic and epigenetic modifications in HPA and serotonin systems influence the outcome of early life stress and may even moderate adverse effects of early social deprivation on aggression. A more comprehensive study of aggression, neuroendocrine, neurobiological and (epi)genetic correlates of early life stress using animal models is necessary to provide a better understanding of the invasive aggressive deficits observed in humans exposed to child maltreatment.

Social and non-social anxiety in adolescent and adult rats after repeated restraint

Adolescence is associated with potentially stressful challenges, and adolescents may differ from adults in their stress responsivity. To investigate possible age-related differences in stress responsiveness, the consequences of repeated restraint stress (90 min/day for 5 days) on anxiety, as indexed using the elevated plus-maze (EPM) and modified social interaction (SI) tests, were assessed in adolescent and adult Sprague-Dawley male and female rats. Control groups at each age included non-stressed and socially deprived animals, with plasma corticosterone (CORT) levels also measured in another group of rats on days 1 and 5 of stress (sampled 0, 30, 60, 90, and 120 min following restraint onset). While repeatedly restrained animals exhibited similar anxiety levels compared to non-stressed controls in the EPM, restraint stress increased anxiety at both ages in the SI test (as indexed by reduced social investigation and social preference). Daily weight gain measurements, however, revealed more marked stress-related suppression of body weight in adolescents versus adults. Analysis of stress-induced increases in CORT likewise showed that adolescents demonstrated less habituation than adults, embedded within typical sex differences in CORT magnitude (females greater than males) and age differences in CORT recovery (adolescents slower than adults). Despite no observable age-related differences in the behavioral response to restraint, adolescents were more sensitive to the repeated stressor in terms of physiological indices of attenuated weight gain and habituation of stress-induced CORT.

Effects of pretest manipulation on elevated plus-maze behavior in adolescent and adult male and female Sprague-Dawley rats

The elevated plus-maze (EPM) is vulnerable to variations in pretest circumstances when testing adult rodents. Because of an increasing interest in adolescence, the present experiments examined the impact of pretest manipulations on anxiety levels in the EPM among adolescent and adult Sprague-Dawley rats of both sexes. In Exp. 1, animals removed from their home cage and immediately placed on the EPM were compared to rats tested following 30 min of social isolation, or following 30-min exposure to a novel context. These pretest manipulations only modestly decreased anxiety levels at both ages. In Exp. 2, more varied pretest conditions were examined: testing directly from the home cage; testing following 30 min of social isolation in a novel environment; or a large saline injection and rehousing 18 h prior to a 30-min period of social isolation in a novelty situation before testing. In adults, anxiety levels decreased linearly as pretest perturbation increased, whereas adolescents showed comparable levels of anxiety with both the moderate and large perturbations. As a result, observed age differences in anxiety differed as a function of pretest circumstances. Therefore, caution is urged when using the EPM for across-age comparisons of anxiolytic and anxiogenic effects of pharmacological or other manipulations.

Novel mechanisms for neuroendocrine regulation of aggression

In 1849, Berthold demonstrated that testicular secretions are necessary for aggressive behavior in roosters. Since then, research on the neuroendocrinology of aggression has been dominated by the paradigm that the brain receives gonadal hormones, primarily testosterone, which modulate relevant neural circuits. While this paradigm has been extremely useful, recent studies reveal important alternatives. For example, most vertebrate species are seasonal breeders, and many species show aggression outside of the breeding season, when gonads are regressed and circulating testosterone levels are typically low. Studies in birds and mammals suggest that an adrenal androgen precursor-dehydroepiandrosterone (DHEA)-may be important for the expression of aggression when gonadal testosterone synthesis is low. Circulating DHEA can be metabolized into active sex steroids within the brain. Another possibility is that the brain can autonomously synthesize sex steroids de novo from cholesterol, thereby uncoupling brain steroid levels from circulating steroid levels. These alternative neuroendocrine mechanisms to provide sex steroids to specific neural circuits may have evolved to avoid the "costs" of high circulating testosterone during particular seasons. Physiological indicators of season (e.g., melatonin) may allow animals to switch from one neuroendocrine mechanism to another across the year. Such mechanisms may be important for the control of aggression in many vertebrate species, including humans.

Reciprocity between endocrine state and contest behavior in the killifish, Kryptolebias marmoratus

Given the dramatic behavioral effects of winning and losing contests, and pronounced changes in stress and sex steroid hormones post-fight, it is reasonable to suppose that these hormones also dictate future behavior. We sampled water-borne cortisol, testosterone (T), and 11-ketotestosterone (KT) before and after contests in the mangrove killifish, Kryptolebias marmoratus, to determine how endogenous steroid hormone levels might predict and respond to contest dynamics or success. Pre-fight cortisol related negatively, and pre-fight T related positively to contest initiation and winning, particularly in the smaller opponent. In the pairs where a larger fish won the contest, winners with higher pre-fight T and lower pre-fight cortisol delivered more attacks to the losers. Contest duration and escalation influenced post-fight hormone concentrations primarily in losers. Escalation significantly increased post-fight cortisol, T, and KT for losers but not for winners. However, winners that attacked losers at higher rates had higher levels of post-fight cortisol. Losers also demonstrate the most consistent post-fight hormone responses, particularly to contest escalation and duration. Despite the bidirectional relationship between hormones and contest behavior, we found no overall mean differences in pre- or post-fight cortisol, T, or KT between eventual winners and losers. Thus, it is evident that the categorical states of winner and loser cannot alone reveal the complex, reciprocal associations between endocrine systems and social behavior.

Neurobiological Mechanisms of Aggression and Stress Coping: A Comparative Study in Mouse and Rat Selection Lines

Aggression causes major health and social problems and constitutes a central problem in several psychiatric disorders. There is a close relationship between the display of aggression and stress coping strategies. In order to gain more insight into biochemical pathways associated with aggression and stress coping, we assessed behavioral and neurobiological responses in two genetically selected rodent models, namely wild house mice selectively bred for a short (SAL) and long (LAL) attack latency and Wistar rats bred for high (HAB) or low (LAB) anxiety-related behavior. Compared to their line counterparts, the SAL mice and the LAB rats display a high level of intermale aggression associated with a proactive coping style. Both the SAL mice and the LAB rats show a reduced hypothalamic-pituitary-adrenal (HPA) axis response to non-social stressors. However, when exposed to social stressors (resident-intruder, sensory contact), SAL mice show an attenuated HPA response, whereas LAB rats show an elevated HPA response. In both rodent lines, the display of aggression is associated with high neuronal activation in the central amygdala, but reduced neuronal activation in the lateral septum. Furthermore, in the lateral septum, SAL mice have a reduced vasopressinergic fiber network, and LAB rats show a decreased vasopressin release during the display of aggression. Moreover, the two lines show several indications of an increased serotonergic neurotransmission. The relevance of these findings in relation to high aggression and stress coping is discussed. In conclusion, exploring neurobiological systems in animals sharing relevant behavioral characteristics might be a useful approach to identify general mechanisms of action, which in turn can improve our understanding of specific behavioral symptoms in human psychiatric disorders.

The effect glucocorticoids on aggressiveness in established colonies of rats

It was repeatedly shown that glucocorticoids increase aggressiveness when subjects are socially challenged. However, the interaction between challenge exposure and glucocorticoid effects was not investigated yet. We studied this interaction by assessing the effects of glucocorticoids in established colonies of rats, i.e. in rats that were not exposed to an acute social challenge. Aggressiveness was high immediately after colony formation but decreased sharply within 4 days and remained stable thereafter. Mild dominance relations were observed in 11 colonies (65%). Approximately three weeks after colony formation, rats remained undisturbed or were injected with vehicle or corticosterone. Routine colony life was followed for 1h after treatments. Injections per se induced a mild and transient behavioral activation: resting was reduced, whereas exploration, social and agonistic interactions were increased. The change lasted about 15min. Corticosterone--although plasma corticosterone levels were increased--had no specific effect, as the behavior of vehicle- and corticosterone-treated rats was similar. Social rank had a minor impact on the results. In contrast, the pro-aggressive effects of corticosterone were robust under conditions of social challenge and were maintained after repeated exposure to aggressive encounters. It occurs that an acute increase in glucocorticoids promotes social challenge-induced aggressiveness, but does not increase aggressiveness under routine conditions. We hypothesize that the pro-aggressive effects of glucocorticoids develop in conjunction with challenge-induced neuronal (e.g. monoaminergic) activation.

Effects of early life stress on adult male aggression and hypothalamic vasopressin and serotonin

Early life stress in humans enhances the risk for psychopathologies, including excessive aggression and violence. In rodents, maternal separation is a potent early life stressor inducing long-lasting changes in emotional and neuroendocrine responsiveness to stress, associated with depression- and anxiety-like symptoms. However, effects of maternal separation on adult male aggression and underlying neurobiological mechanisms remain unknown. Therefore, we investigated the effects of maternal separation on adult intermale aggression in Wistar rats and on hypothalamic arginine vasopressin (AVP) mRNA expression, and AVP and serotonin (5-HT) immunoreactivity, as both AVP and 5-HT have been implicated in stress-coping and aggression. We showed that maternal separation induced depression-like behaviour (increased immobility) and higher adrenocorticotropin hormone responses to an acute stressor (forced swimming). Intermale aggression (lateral threat, offensive upright and keep down) was significantly higher in maternally separated rats compared with control rats. AVP mRNA expression and AVP immunoreactivity were higher in the hypothalamic paraventricular and supraoptic nuclei upon resident-intruder test exposure, whereas 5-HT immunoreactivity was decreased in the anterior hypothalamus of maternally separated rats. Moreover, 5-HT immunoreactivity in the anterior hypothalamus and supraoptic nucleus correlated negatively with aggression. These findings show that exposure to early life stress increases adult male aggression in an animal model of maternal separation. Furthermore, the maternal separation-induced changes in hypothalamic AVP and 5-HT systems may underlie these behavioural alterations.

Evolutionary background for stress-coping styles: relationships between physiological, behavioral, and cognitive traits in non-mammalian vertebrates

Reactions to stress vary between individuals, and physiological and behavioral responses tend to be associated in distinct suites of correlated traits, often termed stress-coping styles. In mammals, individuals exhibiting divergent stress-coping styles also appear to exhibit intrinsic differences in cognitive processing. A connection between physiology, behavior, and cognition was also recently demonstrated in strains of rainbow trout (Oncorhynchus mykiss) selected for consistently high or low cortisol responses to stress. The low-responsive (LR) strain display longer retention of a conditioned response, and tend to show proactive behaviors such as enhanced aggression, social dominance, and rapid resumption of feed intake after stress. Differences in brain monoamine neurochemistry have also been reported in these lines. In comparative studies, experiments with the lizard Anolis carolinensis reveal connections between monoaminergic activity in limbic structures, proactive behavior in novel environments, and the establishment of social status via agonistic behavior. Together these observations suggest that within-species diversity of physiological, behavioral and cognitive correlates of stress responsiveness is maintained by natural selection throughout the vertebrate sub-phylum.

The effects of acute corticosterone on lithium chloride-induced conditioned place aversion and locomotor activity in rats

Acute administration of corticosterone (CORT) facilitates learning in a number of associative paradigms including lithium chloride (LiCl)-induced conditioned taste aversion learning. The present study examined the effects of acute CORT on LiCl-induced conditioned place aversions in male rats. Automated open-fields were partitioned into two chambers distinct in tactile and visual cues. Animals received either LiCl (64 mg/kg, 0.15 M) or saline (NaCl, 0.15 M) followed 10 min later by either CORT (5 mg/kg) or beta-cyclodextrin vehicle (45%) prior to placement in one of the chambers. Control rats received NaCl-Vehicle paired with both chambers. Three experimental groups received either NaCl-CORT, LiCl-Vehicle or LiCl-CORT paired with the preferred chamber and NaCl-Vehicle (control) paired with the non-preferred chamber. During extinction trials, animals were allowed to choose between the two chambers. Locomotor activity and its distribution within the chambers were assessed during both conditioning and extinction trials. CORT administration produced significant increases in a variety of measures of locomotor activity during conditioning trials. During extinction trials both LiCl groups displayed a conditioned place aversion while the NaCl-CORT group did not. In addition, significant increases in vertical activity were recorded in both LiCl groups in the LiCl-paired chamber. Moreover, CORT administration had no effect on LiCl-induced conditioned place aversion as time spent in the LiCl-paired chamber did not significantly differ between LiCl-Vehicle and LiCl-CORT groups. Significant increases in a number of measures of horizontal activity were also observed in both CORT groups. The present study shows that acute CORT administration does not significantly influence LiCl-induced conditioned place aversions and suggests that the facilitatory effects of acute CORT administration on learning are highly context-dependent.

Fast glucocorticoid actions on brain: back to the future

Rapid, non-transcriptionally mediated, effects of glucocorticoids affect many behaviors as well as inhibition of function in the hypothalamo-pituitary-adrenal axis. In this short review, it is argued that the fast glucocorticoid actions which are mediated by membrane receptors are an ancient type of sterol/steroid-mediated effect, and that these may be the primordial glucocorticoid receptors. Although the fast feedback actions of the glucocorticoids enjoyed study in the middle of the last century, new results and the availability of new techniques suggest that it is again time for a concerted effort to be made to understand the mechanism(s) of these rapid effects.

Does Individual Variation in Stress Responses and Agonistic Behavior Reflect Divergent Stress Coping Strategies in Juvenile Rainbow Trout?

Individual rainbow trout were transferred to visual isolation in experimental aquaria. As a measure of the speed of acclimation, individual food intake was quantified during the first 6 d following transfer. Following acclimation, aggression was quantified by subjecting the fish to three resident-intruder tests, with 30 d of recovery between the tests. Moreover, between the resident-intruder tests (i.e., two times) the fish were exposed to an unfamiliar environment and their cortisol response was measured. The results of this study show that individuals of juvenile rainbow trout differ distinctly in their response to changes in their environment, and that this diversity in behavior is reflected by consistent behavioral traits displayed by individual fish. These traits have proven to be consistent not only over time but also across situations, revealing two distinct behavioral profiles, in the same manner as shown in studies on proactive and reactive mammals. Our results also show that the reactivity of the hypothalamic-pituitary-interrenal (HPI) axis, when exposed to a stressor, is a consistent physiological trait in juvenile rainbow trout. We found that difference in HPI axis reactivity is linked to the different behavioral profiles. However, HPI axis reactivity could not be linked directly to the singular behavioral traits measured. In other words, we did not find that the consistent behavioral traits shown by the fish were associated with a difference in HPI axis reactivity in the same manner as the reactivity of the hypothalamic-pituitary-adrenocortical axis does in mammals. Taken together, our results show that stress coping strategies akin to what has been described as reactive and proactive stress coping in mammals appear to exist in juvenile rainbow trout.

Serotonin, but not melatonin, plays a role in shaping dominant-subordinate relationships and aggression in rainbow trout

The aim of this study was to clarify to what extent the effects of elevated dietary L-tryptophan (Trp) on aggressive behavior and stress responsiveness in rainbow trout are mediated by circulating melatonin and central serotonin (5-HT), respectively. Isolated rainbow trout were paired for 1h a day for 7 days in order to create fish with experience of being dominant and subordinate. Following this week, the fish were tested for aggressive behavior using a resident-intruder test after which they were subjected to one of four treatments: (1) tryptophan, (2) the selective serotonin reuptake inhibitor (SSRI) citalopram, (3) melatonin, and (4) no treatment (controls). After 7 days of treatment, the fish were subjected to a second resident-intruder test. Trp-supplemented feed resulted in a suppression of aggressive behavior in fish with experience of being dominant. Moreover, fish fed Trp-supplemented feed, regardless of social experience, also displayed lower plasma cortisol levels than controls. These effects of elevated dietary Trp were closely mimicked by citalopram treatment, whereas exogenous melatonin had no effect on either aggressive behavior or plasma cortisol. Thus, the effect of elevated dietary Trp on aggressive behavior and stress responses does not appear to be mediated by melatonin even though elevated dietary intake of Trp resulted in an increase in plasma melatonin concentrations.

Rapid non-genomic effect of glucocorticoid metabolites and neurosteroids on the gamma-aminobutyric acid-A receptor

Glucocorticoids and neurosteroids, such as allopregnanolone and tetrahydrodeoxycorticosterone, are released during stress. A non-genomic effect of glucocorticoids has been established but is not yet fully understood. We have studied the effect of glucocorticoid metabolites on the gamma-aminobutyric acid (GABA) system. In these experiments we studied the effects of the glucocorticoid metabolites allotetrahydrocortisol, tetrahydrocortisol, allotetrahydrocortisone and tetrahydrocortisone in rat cortical microsacs. Our results showed that both these cortisol and cortisone metabolites reduce GABA-mediated chloride ion uptake. This reduction was not observed in the presence of allopregnanolone but allotetrahydrocortisol interacts with allopregnanolone, enhancing the allopregnanolone-stimulated potentiation of GABA-mediated chloride ion uptake. This enhanced effect was completely blocked by the addition of 30 microm of the 3beta-isomer of allopregnanolone, isoallopregnanolone. Our findings show that steroids released during stress interact with each other and GABA in the GABA system.

The glucocorticoid receptor antagonist mifepristone reduces ethanol intake in rats under limited access conditions

There is a substantial amount of evidence indicating control over ethanol intake by steroid hormones, particularly adrenal glucocorticoids. Thus far, however, studies employing pharmacological methods have failed to find effects of glucocorticoid receptor blockade on voluntary ethanol consumption. Since length of ethanol access period can influence ethanol consumption levels as well as potential pharmacological effects in such studies, the present study was conducted to determine the effects of acute administration of the glucocorticoid receptor (GR) antagonist mifepristone on voluntary ethanol intake under limited access conditions. Rats were fluid restricted and given concurrent access to 10% ethanol and water in a two-bottle choice paradigm for 1 h/day, 5 days a week. Both fluids were available ad libitum during the remaining 2 days per week. Administration of mifepristone (1, 5 and 20 mg/kg i.p.) immediately prior to the limited access two-bottle access period dose-dependently suppressed ethanol intake (maximum 40% at 20 mg/kg). The mineralcorticoid receptor (MR) antagonist spironolactone (10, 25 and 50 mg/kg i.p.) was without effect on ethanol intake, and neither compound had an effect on water intake. These data confirm an active role of GRs in modulating voluntary ethanol consumption, particularly under conditions of limited access.

Genomic and non-genomic effects of glucocorticoids on aggressive behavior in male rats

An increasing body of evidence suggests that glucocorticoids--besides their well-known genomic effects--can affect neuronal function via mechanisms that do not involve the genome. Data obtained mainly in amphibians and birds suggest that such mechanisms play a role in the control of behavior. Acute glucocorticoid treatments increase aggressive behavior in rats, but the mechanism of action has not been investigated to date. To clarify the issue, we have assessed the aggressiveness of male rats after treating them with the corticosterone synthesis inhibitor metyrapone, corticosterone, and the protein synthesis inhibitor cycloheximide. Metyrapone applied intraperitoneally (i.p.) decreased the aggressiveness of residents faced with smaller opponents. Corticosterone administered i.p. 20 or 2 min before a 5-min encounter abolished these changes irrespective of the delay of behavioral testing. Thus, the effects of glucocorticoids on aggressive behavior occurred in less than 7 min (the delay and duration of testing taken together), and lasted more than 25 min. Corticosterone applied centrally (infused into the right lateral ventricle) also stimulated aggressive behavior rapidly, which shows that the effect was centrally mediated. The protein synthesis inhibitor cycloheximide did not affect the aggression-promoting effects of corticosterone when the hormone was injected 2 min before the aggressive encounter. Surprisingly, however, the effects were completely abolished when the hormone was injected 20 min before the encounter. These data suggest that glucocorticoids rapidly increase aggressive behavior via non-genomic mechanisms. In later phases of the aggressive encounter, aggressive behavior appears to be stimulated by genomic mechanisms.