Behavioral and endocrine change following chronic predatory stress

Chemogenetic activation of CRF neurons as a model of chronic stress produces sex-specific physiological and behavioral effects

Trauma and chronic stress exposure are the strongest predictors of lifetime neuropsychiatric disease presentation. These disorders often have significant sex biases, with females having higher incidences of affective disorders such as major depression, anxiety, and PTSD. Understanding the mechanisms by which stress exposure heightens disease vulnerability is essential for developing novel interventions. Current rodent stress models consist of a battery of sensory, homeostatic, and psychological stressors that are ultimately integrated by corticotropin-releasing factor (CRF) neurons to trigger corticosteroid release. These stress paradigms, however, often differ between research groups in the type, timing, and duration of stressors utilized. These inconsistencies, along with the variability of individual animals' perception and response to each stressor, present challenges for reproducibility and translational relevance. Here, we hypothesized that a more direct approach using chemogenetic activation of CRF neurons would recapitulate the effects of traditional stress paradigms and provide a high-throughput method for examining stress-relevant phenotypes. Using a transgenic approach to express the Gq-coupled Designer Receptor Exclusively Activated by Designer Drugs (DREADD) receptor hM3Dq in CRF-neurons, we found that the DREADD ligand clozapine-N-oxide (CNO) produced an acute and robust activation of the hypothalamic-pituitary-adrenal (HPA) axis, as predicted. Interestingly, chronic treatment with this method of direct CRF activation uncovered a novel sex-specific dissociation of glucocorticoid levels with stress-related outcomes. Despite hM3Dq-expressing females producing greater corticosterone levels in response to CNO than males, hM3Dq-expressing males showed significant typical physiological stress sensitivity with reductions in body and thymus weights. hM3Dq-expressing females while resistant to the physiological effects of chronic CRF activation, showed significant increases in baseline and fear-conditioned freezing behaviors. These data establish a novel mouse model for interrogating stress-relevant phenotypes and highlight sex-specific stress circuitry distinct for physiological and limbic control that may underlie disease risk.

Differential c-Fos Response in Neurocircuits Activated by Repeated Predator Stress in Male and Female C57BL/6J Mice with Stress Sensitive or Resilient Alcohol Intake Phenotypes

Comorbidity of post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) worsens the prognosis for each of these individual disorders. The current study aimed to identify neurocircuits potentially involved in regulation of PTSD-AUD comorbidity by mapping expression of c-Fos in male and female C57BL/6J mice following repeated predator stress (PS), modeled by exposure to dirty rat bedding. In experiment 1, the levels of c-Fos in the paraventricular nucleus of the hypothalamus (PVH) and the nucleus accumbens shell were higher after the second PS vs the first PS, indicating a sensitized response to this stressor. Additional brain regions showed varied sex-dependent and independent regulation by the two consecutive PS exposures. In experiment 2, mice that increased voluntary alcohol consumption following four exposures to PS (Sensitive subgroup) showed higher c-Fos induction in the PVH, piriform cortex and ventromedial hypothalamus than mice that decreased consumption following these exposures (Resilient subgroup). In contrast to these brain regions, c-Fos was higher in the anterior olfactory nucleus of Resilient vs Sensitive mice. Taken together, these data demonstrate that repeated PS exposure and voluntary alcohol consumption increase neuronal activity across neurocircuits in which specific components depend on the vulnerability of individual mice to these stressors. Increased PVH activity observed across both experiments suggests this brain area as a potential mediator of PS-induced increases in alcohol consumption. Future investigations of specific neuronal populations within the PVH activated by PS, and manipulation of these specific neuronal populations, could improve our understanding of the mechanisms leading to PTSD-AUD comorbidity.

Assessing stress physiology within a conservation breeding program for an endangered species

Conservation breeding programs typically involve the management of individuals both in and ex situ, so it is vital to understand how the physiology of managed species changes in these environments to maximize program outcomes. The Vancouver Island marmot (VIM; Marmota vancouverensis) is one species that has been managed in a conservation breeding program to recover the critically low wild population. Previous research has shown there are differences in hair glucocorticoid concentrations for VIMs in different managed groups in the program. Therefore, we used >1000 blood samples collected since the program's inception to assess the neutrophil to lymphocyte (N:L) ratio among captive, pre-release, post-release and wild populations as another metric of stress. In situ VIM populations were found to have a significantly higher N:L ratio than ex situ populations, suggesting that the wild is a more physiologically challenging environment than managed care. Moreover, the effect of age, sex and the month of sampling on the N:L ratio were found to be different for each population. Age had the greatest magnitude of effect in the wild population, and sex was only significant in ex situ populations. This study provided previously unknown insights into the physiology of VIMs and increased post-release monitoring will be useful in the future to fully understand how physiology may be contributing to differences in survival of VIMs in the program.

Positive Wild Animal Welfare

With increasing attention given to wild animal welfare and ethics, it has become common to depict animals in the wild as existing in a state dominated by suffering. This assumption is now taken on board by many and frames much of the current discussion; but needs a more critical assessment, both theoretically and empirically. In this paper, we challenge the primary lines of evidence employed in support of wild animal suffering, to provide an alternative picture in which wild animals may often have lives that are far more positive than is commonly assumed. Nevertheless, while it is useful to have an alternative model to challenge unexamined assumptions, our real emphasis in this paper is the need for the development of effective methods for applying animal welfare science in the wild, including new means of data collection, the ability to determine the extent and scope of welfare challenges and opportunities, and their effects on welfare. Until such methods are developed, discussions of wild animal welfare cannot go beyond trading of intuitions, which as we show here can just as easily go in either direction.

Anxiolytic reversal of classically conditioned / chronic stress-induced gene expression and learning in the Stress Alternatives Model

Social decision-making is critically influenced by neurocircuitries that regulate stress responsiveness. Adaptive choices, therefore, are altered by stress-related neuromodulatory peptide systems, such as corticotropin releasing factor (CRF). Experimental designs that take advantage of ecologically salient fear-inducing stimuli allow for revelation of neural mechanisms that regulate the balance between pro- and anti-stress responsiveness. To accomplish this, we developed a social stress and conditioning protocol, the Stress Alternatives Model (SAM), that utilizes a simple dichotomous choice, and produces distinctive behavioral phenotypes (Escape or Stay). The experiments involve repeated social aggression, a potent unconditioned stimulus (US), from a novel larger conspecific (a 3X larger Rainbow trout). Prior to the social interaction, the smaller test fish is presented with an auditory conditioning stimulus (water off = CS). During the social aggression, an escape route is available, but is only large enough for the smaller test animal. Surprisingly, although the new aggressor provides vigorous attacks each day, only 50% of the test fish choose Escape. Stay fish, treated with the CRF1 antagonist antalarmin, a potent anxiolytic drug, on day 4, promotes Escape behavior for the last 4 days of the SAM protocol. The results suggest that the decision to Escape, required a reduction in stress reactivity. The Stay fish that chose Escape following anxiolytic treatment, learned how to use the escape route prior to stress reduction, as the Escape latency in these fish was significantly faster than first time escapers. In Escape fish, the use of the escape route is learned over several days, reducing the Escape latency over time in the SAM. Fear conditioning (water off + aggression) resulted in elevated hippocampal (DL) Bdnf mRNA levels, with coincident reduction in the AMPA receptor subunit Glua1 expression, a result that is reversed following a one-time treatment (during SAM aggression on day 4) with the anxiolytic CRF1 receptor antagonist antalarmin.

The Role of Microglia in the (Mal)adaptive Response to Traumatic Experience in an Animal Model of PTSD

The present study investigates whether predator scent-stress (PSS) shifts the microglia from a quiescent to a chronically activated state and whether morphological alterations in microglial activation differ between individuals displaying resilient vs. vulnerable phenotypes. In addition, we examined the role that GC receptors play during PSS exposure in the impairment of microglial activation and thus in behavioral response. Adult male Sprague Dawley rats were exposed to PSS or sham-PSS for 15 min. Behaviors were assessed with the elevated plus-maze (EPM) and acoustic startle response (ASR) paradigms 7 days later. Localized brain expression of Iba-1 was assessed, visualized, and classified based on their morphology and stereological counted. Hydrocortisone and RU486 were administered systemically 10 min post PSS exposure and behavioral responses were measured on day 7 and hippocampal expression of Ionized calcium-binding adaptor molecule 1 (Iba-1) was subsequently evaluated. Animals whose behavior was extremely disrupted (PTSD-phenotype) selectively displayed excessive expression of Iba-1 with concomitant downregulation in the expression of CX3C chemokine receptor 1 (CX3CR1) in hippocampal structures as compared with rats whose behavior was minimally or partially disrupted. Changes in microglial morphology have also been related only to the PTSD-phenotype group. These data indicate that PSS-induced microglia activation in the hippocampus serves as a critical mechanistic link between the HPA-axis and PSS-induced impairment in behavioral responses.

The Effects of Chronic Variable Stress and Photoperiod Alteration on the Hypothalamic-Pituitary-Adrenal Axis Response and Behavior of Mice

The hypothalamic-pituitary-adrenal (HPA) axis mediates the physiological response to stressors and also synchronizes different physiological systems to environmental cues. Changes in day length (i.e., photoperiod) as well as chronic exposure to stressors are known to impact the HPA axis activity regulating the levels of glucocorticoid hormones. Over-exposure to inappropriate levels of glucocorticoids has been implicated in increased disease risk. In the present study, we examined the impact of chronic stress, using a chronic variable stress (CVS) paradigm, in combination with changes in photoperiod on physiological and behavioral measures, as well as on the reactivity and regulation of the HPA axis, in male and female mice. Six weeks of CVS, regardless of the photoperiod condition, decreased the body weight and attenuated the HPA axis reactivity to an acute stressor in both sexes. The attenuated HPA axis reactivity observed in stressed animals was related to reduced Pro-opiomelanocortin (POMC) mRNA levels in the pituitary of females. The gene expression analyses of key regulators of the HPA axis also indicated a sex-dependent effect with opposite patterns in the pituitary and adrenal glands. CVS effects on behavior were limited and related to an anxiety-like phenotype in both sexes, regardless of photoperiod condition. Our findings highlight sex-specific differences in the HPA axis and also sex-dependent effects of CVS on physiological parameters.

Emotional state alters encoding of long-term spatial episodic memory

The neurobiology of emotion and episodic memory are well-researched subjects, as is their intersection: memory of emotional events (i.e. emotional memory). We and others have previously demonstrated that the emotional valence of stimuli is encoded in the dorsal hippocampus, a structure integral to the acquisition, consolidation and retrieval of long-term episodic memories. Such findings are consistent with the idea that the emotional valence of stimuli contributes to the "what" component of episodic memories ("where" and "when" being the other components). We hypothesized that being in a heightened emotional state by itself does not contribute to the "what" component of episodic memories. We tested an inference of this hypothesis - that negative emotional state does not alter re-encoding of a spatial episodic event. Rats from the experimental group explored a novel place at their baseline emotional state (Event 1) and 20 min later re-explored the same place (Event 2) in a negative emotional state induced by a state-altering event prior to Event 2. We examined neuronal ensembles that induced expression of Arc and Homer1a, two immediate-early genes (IEGs) necessary for synaptic plasticity and consolidation of long-term memories, during both events. We found that in dorsal CA1 and dorsal CA3, Event 1 and Event 2 induced IEG expression in different neuronal ensembles. This finding was reflected in a low Fidelity score, which assesses the percentage of the Event 1 IEG-expressing ensemble re-activated during Event 2. The Fidelity score was significantly higher in a control group which was at a baseline emotional state during Event 2. Groups which were matched for non-specific disruptions from the state-altering event had intermediate Fidelity scores in dorsal CA1. The Fidelity scores of the dorsal CA3 in the latter groups were similar to those of the control group. Combined, the findings reject the tested hypothesis and suggest that a negative emotional state is encoded in the hippocampus as part of the long-term memory of episodic events that lack explicit emotion-inducing stimuli. These findings also suggest that individuals who often experience strong negative emotional states incorporate these states into ongoing non-emotional episodic memories.

Short Juvenile Stress Has No Long-Lasting Effects on Anxiety-Like Behavior, Object Recognition Memory, or Gross Brain Morphology but Affects Dendritic Spines in the Hippocampus in Male Rats

PURPOSE

During the juvenile stage, such areas as the hippocampus and corpus callosum (CC) are still immature and sensitive to stress exposure. The present study investigated whether two different types of stressors in the juvenile stage of life have a long-lasting impact on behavior and biological outcomes in adult rats.

METHODS

Male juvenile rats were exposed to restraint or predator stress on postnatal day 25 (P25) for 3 days. Thirty-two days later (P60-74), behavioral and biological analyses were conducted. The behavioral analysis included measures of anxiety-like behavior and recognition memory. The biological analysis investigated gross cerebral morphology, based on volume analysis of the CC and hippocampus, perirhinal cortex thickness, and dendritic spine density.

RESULTS

Neither restraint stress nor predator stress affected anxiety-like behavior or object recognition memory in adulthood. Body weight and adrenal gland weight were unaffected by both types of stress. Overall, volumetric measures of the CC and hippocampus were not significant, with no changes in perirhinal cortex thickness. Spine density in the medial prefrontal cortex also was unaffected, but a decrease in dendritic spine density was found in the hippocampus in response to restraint stress and an increase to predator stress.

CONCLUSION

Short-term and daily restraint and predator stress during the juvenile stage had no long-lasting effects on anxiety-like behavior, object memory, volume of the CC or hippocampus, or perirhinal cortex thickness, but a decrease in dendritic spine density was found in the hippocampus. These findings suggest that different types of stressors have different impacts on microstructures in the brain without affecting behavior or the gross morphology of stress-sensitive brain areas.

Glutamatergic Neurotransmission Controls the Functional Lateralization of the mPFC in the Modulation of Anxiety Induced by Social Defeat Stress in Male Mice

The rodent medial prefrontal cortex (mPFC) is anatomically divided into cingulate (Cg1), prelimbic (PrL), and infralimbic (IL) subareas. The left and right mPFC (L and RmPFC) process emotional responses induced by stress-related stimuli, and LmPFC and RmPFC inhibition elicit anxiogenesis and anxiolysis, respectively. Here we sought to investigate (i) the mPFC functional laterality on social avoidance/anxiogenic-like behaviors in male mice subjected to chronic social defeat stress (SDS), (ii) the effects of left prelimbic (PrL) inhibition (with local injection of CoCl₂) on the RmPFC glutamatergic neuronal activation pattern (immunofluorescence assay), and (iii) the effects of the dorsal right mPFC (Cg1 + PrL) NMDA receptor blockade (with local injection of AP7) on the anxiety induced by left dorsal mPFC inhibition in mice exposed to the elevated plus maze (EPM). Results showed that chronic SDS induced anxiogenic-like behaviors followed by the rise of ΔFosB labeling and by ΔFosB + CaMKII double-labeling bilaterally in the Cg1 and IL subareas of the mPFC. Chronic SDS also increased ΔFosB and by ΔFosB + CaMKII labeling only on the right PrL. Also, the left PrL inhibition increased cFos + CaMKII labeling in the contralateral PrL and IL. Moreover, anxiogenesis induced by the left PrL inhibition was blocked by NMDA receptor antagonist AP7 injected into the right PrL. These findings suggest the lateralized control of the glutamatergic neurotransmission in the modulation of emotional-like responses in mice subjected to chronic SDS.

Predator odor exposure increases social contact in adolescents and parental behavior in adulthood in Brandt's voles

Research suggests that predation risk during adolescence can program adult stress response and emotional behavior; however, little is known about the short-term and lasting residual effects of this experience on social behavior. We explored this concept in social Brandt's voles (Lasiopodomys brandtii). Adolescent male and female voles were exposed to distilled water, rabbit urine (as a non-predator stimulus), and cat urine for 60 min daily from postnatal day (PND) 28-49. Social play tests were conducted immediately following exposure on PND 28, 35, 42, and 49. In the social play test, repeated cat odor (CO) exposure enhanced the contact behavior of voles with their cagemate. Adolescent exposure to CO did not affect behavioral responses toward unrelated pups in the alloparental behavior test or same-sex individuals in the social interaction test. However, exposure to CO significantly enhanced the licking/grooming behavior of voles towards their own pups in the home cage parental behavior test. Repeated CO exposure significantly inhibited weight gain in male voles during adolescence. This effect was transmitted to the next generation, with lower weight gain in offspring before weaning. Following repeated CO exposure, males tended to have more female offspring whereas females produced more offspring, suggesting an adaptive strategy to increase inclusive fitness under predatory risk. These findings demonstrate that adolescent exposure to predatory risk augments adolescent social contact and adult parental behavior and suggest a role for improved inclusive fitness in mediating long-term outcomes.

Are physiological and behavioural responses to stressors displayed concordantly by wild urban rodents?

Understanding wild animal responses to stressors underpins effective wildlife management. In order for responses to stressors to be correctly interpreted, it is critical that measurements are taken on wild animals using minimally invasive techniques. Studies investigating wild animal responses to stressors often measure either a single physiological or behavioural variable, but whether such responses are comparable and concordant remains uncertain. We investigated this question in a pilot study that measured responses of wild-caught urban brown and black rats (Rattus norvegicus, Rattus rattus) to fur-based olfactory cues from a predator, the domestic cat (Felis catus); a novel herbivore, the koala (Phascolarctos cinereus); and a familiar herbivore and competitor, the common brushtail possum (Trichosurus vulpecula). Physiological responses, measured by assaying faecal glucocorticoid metabolites, were compared to behavioural responses observed via video recordings. We found that physiological and behavioural responses to stressors were expressed concordantly. There was no sizeable physiological response observed, and the behavioural response when considered across the night was negligible. However, the behavioural response to the predator and competitor cues changed across the observation period, with activity increasing with increasing hours of exposure. Our results indicate that responses of wild rodents to cues are nuanced, with stress responses modulated by behaviour changes that vary over time according to the severity of the perceived threat as animals gather further information. If the physiological response alone had been assessed, this moderated response may not have been evident, and in terms of wildlife management, vital information would have been lost.

The Herbicide Atrazine Potentiates Angiotensin II-Induced Aldosterone Synthesis and Release From Adrenal Cells

Atrazine is one of the most commonly used pre-emergence and early post-emergence herbicides in the world. We have shown previously that atrazine does not directly stimulate the pituitary or adrenal to trigger hormone release but acts centrally to activate a stress-like activation of the hypothalamic-pituitary-adrenal axis. In doing so, atrazine treatment has been shown to cause adrenal morphology changes characteristic of repeated stress. In this study, adrenals from atrazine treated and stressed animals were directly compared after 4 days of atrazine treatment or restraint stress. Both atrazine and stressed animals displayed reduced adrenocortical zona glomerulosa thickness and aldosterone synthase (CYP11B2) expression, indicative of repeated adrenal stimulation by adrenocorticotropic hormone. To determine if reduced CYP11B2 expression resulted in attenuated aldosterone synthesis, stressed and atrazine treated animals were challenged with angiotensin II (Ang II). As predicted, stressed animals produced less aldosterone compared to control animals when stimulated. However, atrazine treated animals had higher circulating aldosterone concentrations compared to both stressed and control groups. Ang II-induced aldosterone release was also potentiated in atrazine pretreated human adrenocortical carcinoma cells (H295R). Atrazine pretreated did not alter the expression of the rate limiting steroidogenic StAR protein or angiotensin II receptor 1. Atrazine treated animals also presented with higher basal blood pressure than vehicle treated control animals suggesting sustained elevations in circulating aldosterone levels. Our results demonstrate that treatment with the widely used herbicide, atrazine, directly increases stimulated production of aldosterone in adrenocortical cells independent of expression changes to rate limiting steroidogenic enzymes.

Differential Regulation of the Glucocorticoid Receptor in a Rat Model of Inflammatory Pain

BACKGROUND

Anti-inflammatory corticosteroids are a common treatment for different conditions involving chronic pain and inflammation. Clinically used steroids target the glucocorticoid receptor (GR) for its anti-inflammatory effects. We previously reported that GR in sensory neurons may play central roles in some pain models and that GR immunoreactivity signal in dorsal root ganglia (DRG) decreased after local inflammation of the DRG (a model of low back pain). In the current study, we aimed to determine if similar changes in GR signal also exist in a skin inflammation model, the complete Freund's adjuvant (CFA) model (a model of peripheral inflammatory pain), in which the terminals of the sensory neurons rather than the somata are inflamed.

METHODS

A low dose of CFA was injected into the hind paw to establish the peripheral inflammation model in Sprague-Dawley rats of both sexes, as confirmed by measurements of behavior and paw swelling. Immunohistochemical and western blotting techniques were used to determine the expression pattern of the GR in the inflamed hind paw and the DRGs. Plasma corticosterone levels were measured with radioimmunoassay.

RESULTS

The immunohistochemical staining revealed that GR is widely expressed in the normal DRG and skin tissues. Paw injection with CFA caused upregulation of the GR in the skin tissue on postinjection day 1, mostly detected in the dermis area. However, paw inflammation significantly reduced the GR signal in the L5 DRG 1 day after the injection. The GR downregulation was still evident 14 days after CFA inflammation. On day 1, western blotting confirmed this downregulation and showed that it could also be observed in the contralateral L5 DRG, as well as in the L2 DRG (a level which does not innervate the paw). Plasma corticosterone levels were elevated in both sexes on day 14 after CFA compared to day 1, suggesting autologous downregulation of the GR by corticosterone may have contributed to the downregulation observed on day 14 but not day 1.

CONCLUSIONS

There are distinctive patterns of GR activation under different pain conditions, depending on the anatomical location. The observed downregulation of the GR in sensory neurons may have a significant impact on the use of steroids as treatment in these conditions and on the regulatory effects of endogenous glucocorticoids.

Inhibition of the left medial prefrontal cortex (mPFC) prolongs the social defeat-induced anxiogenesis in mice: Attenuation by NMDA receptor blockade in the right mPFC

Chemical inhibition and nitrergic stimulation of the left and right medial prefrontal cortex (L and RmPFC), respectively, provoke anxiety in mice. Moreover, LmPFC inhibition immediately followed by a single social defeat stress (SDS) led to anxiogenesis in mice exposed to the elevated plus maze (EPM) 24 h later. Given that glutamate NMDA (N-methyl-D-aspartate) receptors are densely present in the mPFC, we investigated (i) the time course of LmPFC inhibition + SDS-induced anxiogenesis and (ii) the effects of intra-RmPFC injection of AP-7 (a NMDA receptor antagonist) on this long-lasting anxiety. Male Swiss mice received intra-LmPFC injection of CoCl₂ (1 mM) and 10 min later were subjected to a single SDS episode and then (i) exposed to the EPM 2, 5, or 10 days later or (ii) 2 days later, received intra-RmPFC injection of AP-7 (0.05 nmol) and were exposed to the EPM to observe the percentage of open arm entries and time (%OE; %OT) and frequency of closed arm entries (CE). Dorsal but not ventral LmPFC inhibition + SDS reduced open arm exploration 2, 5, and 10 days later relative to that of saline-treated or non-defeated mice. Moreover, this effect is not due to locomotor impairment as assessed using the general activity. Intra-RmPFC AP-7 injection 2 days after LmPFC inhibition + SDS prevented this type of anxiogenesis. These results suggest that the integrity of the LmPFC is important for mice to properly cope with SDS, and that NMDA receptor blockade in the RmPFC facilitates resilience to SDS-induced anxiogenesis in mice.

Interoceptive Insular Cortex Mediates Both Innate Fear and Contextual Threat Conditioning to Predator Odor

The insular cortex (IC), among other brain regions, becomes active when humans experience fear or anxiety. However, few experimental studies in rats have implicated the IC in threat responses. We have recently reported that inactivation of the primary interoceptive cortex (pIC) during pre-training, or the intra-pIC blockade of protein synthesis immediately after training, impaired the consolidation of auditory fear conditioning. The present study was designed to investigate the role of the pIC in innate and learned defensive responses to predator odor. Freezing behavior was elicited by single or repetitive exposures to a collar that had been worn by a domestic cat. Sessions were video-recorded and later scored by video observation. We found that muscimol inactivation of the pIC reduced the expression of freezing reaction in response to a single or repeated exposure to cat odor. We also found that pIC inactivation with muscimol impaired conditioning of fear to the context in which rats were exposed to cat odor. Furthermore, neosaxitoxin inactivation of the pIC resulted in a prolonged and robust reduction in freezing response in subsequent re-exposures to cat odor. In addition, freezing behavior significantly correlated with the neural activity of the IC. The present results suggest that the IC is involved in the expression of both innate and learned fear responses to predator odor.

Effect of chronic corticosterone treatment on expression and distribution of serotonin 5-HT7 receptors in rat adrenal glands

Sensitized stress-induced corticosterone (CORT) secretion in chronically stressed rats involves 5-HT7 receptor activation. The effect of 14-day chronic CORT and vehicle (VEH) administration on 5-HT7 receptor expression in adrenal glands, adrenal 5-HT content, and adrenocorticotropic hormone and CORT secretion was analysed. On day 15, VEH- and CORT-treated animals were perfused or decapitated without stress exposure (0 min) or after 10 and 30 min of restraint for collection of trunk blood and tissues. 5-HT7 receptor-like immunoreactivity (5-HT7R-LI), 5-HT7 receptor protein, and mRNA levels were determined by immunohistochemistry, Western blot, and reverse transcription polymerase chain reaction assays, respectively; 5-HT levels and hormones were quantified using HPLC and ELISA kits, respectively. An undisturbed control group was included for most experimental comparisons. Chronic CORT strongly increased 5-HT7R-LI in the outer adrenal cortex, as well as 5-HT7 receptor protein and mRNA in whole adrenal glands; adrenal 5-HT content also increased in these animals. Decreased adrenocorticotropic hormone and CORT secretion at 30 min of restraint occurred in CORT-treated rats. The results support the notion that chronic stress-induced increase of adrenocortical 5-HT7 receptors and adrenal 5-HT content is a glucocorticoid-dependent phenomenon; the development of magnified stress-induced 5-HT7 receptor-mediated CORT responses in chronically stressed animals nevertheless likely involves additional mechanisms.

Parental pre-conception stress status and risk for anxiety in rat offspring: specific and sex-dependent maternal and paternal effects

Prenatal stressful events have long-lasting consequences on behavioral responses of offspring. While the effects of gestational and maternal stress have been extensively studied on psychological alterations in the progeny, little is known about effects of each parent's pre-conception life events on emotional responses in offspring. Here, the effect of maternal and/or paternal pre-conception stress was investigated on anxiogenic responses of offspring. Male and female adult rats were subjected to predatory stress (contactless exposure to a cat for 1 + 1 h per day) for 50 (male, n: 12) and 15 (female, n: 24) consecutive days; controls were not exposed. After the stress procedure, the control and stressed rats were mated to create four types of breeding pairs: control female/control male, stressed female/control male, control female/stressed male, and stressed female/stressed male. On postnatal days 30-31, the offspring were tested on the elevated plus maze and plasma corticosterone concentration was measured. Half of the pups were exposed to acute predatory stress before the elevated plus maze test. In most subgroups, corticosterone and anxiety-like behaviors in the offspring with both or only one parent exposed to pre-gestational stress increased compared to their control counterparts. However, under acute stress conditions, a different sex-dependent pattern of anxiety responses emerged. The combined effects of maternal and paternal stress were not additive. Hence, individual offspring behaviors can be influenced by the former life stress experiences of either parent. Incorporation of genetic and epigenetic aspects in development of neurobehavioral abnormalities and reprograming of the hypothalamic-pituitary-adrenal axis may contribute to this phenomenon. Lay summary Early life stress (including during pregnancy) is known to have long-lasting effects on offspring, including emotional behaviors. Whether individual anxiety behaviors can be influenced by stress experiences of each parent even before a pregnancy is less well-understood. Our findings from this study on rats exposed to predator stress before mating suggest that maternal or paternal adult life events prior to pregnancy can lead to maladaptive behavior in their offspring later in life.

Animal Models of PTSD: A Critical Review

The goals of animal research in post-traumatic stress disorder (PTSD) include better understanding the neurophysiological etiology of PTSD, identifying potential targets for novel pharmacotherapies, and screening drugs for their potential use as PTSD treatment in humans. Diagnosis of PTSD relies on a patient interview and, as evidenced by changes to the diagnostic criteria in the DSM-5, an adequate description of this disorder in humans is a moving target. Therefore, it may seem insurmountable to model the construct of PTSD in animals such as rodents. Fortunately, the neural circuitry involved in fear and anxiety, thought to be essential to the etiology of PTSD in humans, is highly conserved throughout evolution. Furthermore, many symptoms can be modeled using behavioral tests that have face, construct, and predictive validity. Because PTSD is precipitated by a definite traumatic experience, animal models can simulate the induction of PTSD, and test causal factors with longitudinal designs. Accordingly, several animal models of physical and psychological trauma have been established. This review discusses the widely used animal models of PTSD in rodents, and overviews their strengths and weaknesses in terms of face, construct, and predictive validity.

Amygdala ensembles encode behavioral states

Internal states, including affective or homeostatic states, are important behavioral motivators. The amygdala regulates motivated behaviors, yet how distinct states are represented in amygdala circuits is unknown. By longitudinally imaging neural calcium dynamics in freely moving mice across different environments, we identified opponent changes in activity levels of two major, nonoverlapping populations of basal amygdala principal neurons. This population signature does not report global anxiety but predicts switches between exploratory and nonexploratory, defensive states. Moreover, the amygdala separately processes external stimuli and internal states and broadcasts state information via several output pathways to larger brain networks. Our findings extend the concept of thalamocortical "brain-state" coding to include affective and exploratory states and provide an entry point into the state dependency of brain function and behavior in defined circuits.

Updates in PTSD Animal Models Characterization

Post-traumatic stress disorder (PTSD) is a chronic, debilitating mental disorder afflicting more than 7% of the US population and 12% of military service members. Since the Afghanistan and Iraq wars, thousands of US service members have returned home with PTSD. Despite recent progress, the molecular mechanisms underlying the pathology of PTSD are poorly understood. To promote research on PTSD (especially its molecular mechanisms) and to set a molecular basis for discovering novel medications for this disorder, well-validated animal models are needed. However, to develop PTSD animal models is a challenging process, due to predisposing factors such as physiological, behavioral, emotional, and cognitive changes that emerge after trauma. Currently, there is no well-validated animal model of PTSD, although several stress paradigms mimic the behavioral symptoms and neurological alterations seen in PTSD. In this chapter, we will provide an overview of animal models of PTSD including learned helplessness, footshock, restraint stress, inescapable tail shock, single-prolonged stress, underwater trauma, social isolation, social defeat, early-life stress, and predator-based stress. We emphasize rodent models because they reproduce some of the behavioral and biotical phenotypes seen in PTSD. We will also present data showing that homologous biological measures are increasingly incorporated in studies to assess markers of risk and therapeutic response in these models. Therefore, PTSD animal models may be refined in hopes of capitalizing on the understanding of the molecular mechanisms and delivering tools in order to develop new and more efficacious treatments for PTSD.

Palatable Food Affects HPA Axis Responsivity and Forebrain Neurocircuitry in an Estrous Cycle-specific Manner in Female Rats

Eating palatable foods can provide stress relief, but the mechanisms by which this occurs are unclear. We previously characterized a limited sucrose intake (LSI) paradigm in which twice-daily access to a small amount of 30% sucrose (vs. water as a control) reduces hypothalamic-pituitary-adrenocortical (HPA) axis responses to stress and alters neuronal activation in stress-regulatory brain regions in male rats. However, women may be more prone to 'comfort feeding' behaviors than men, and stress-related eating may vary across the menstrual cycle. This suggests that LSI effects may be sex- and estrous cycle-dependent. The present study therefore investigated the effects of LSI on HPA axis stress responsivity, as well as markers of neuronal activation/plasticity in stress- and reward-related neurocircuitry in female rats across the estrous cycle. We found that LSI reduced post-restraint stress plasma ACTH in female rats specifically during proestrus/estrus (P/E). LSI also increased basal (non-stress) FosB/deltaFosB- and pCREB-immunolabeling in the basolateral amygdala (BLA) and central amygdala specifically during P/E. Finally, Bayesian network modeling of the FosB/deltaFosB and pCREB expression data identified a neurocircuit that includes the BLA, nucleus accumbens, prefrontal cortex, and bed nucleus of the stria terminalis as likely being modified by LSI during P/E. When considered in the context of our prior results, the present findings suggest that palatable food reduces stress responses in female rats similar to males, but in an estrous cycle-dependent manner. Further, the BLA may contribute to the LSI effects in both sexes, whereas the involvement of other brain regions appears to be sex-dependent.

Developmental stress and social phenotypes: integrating neuroendocrine, behavioural and evolutionary perspectives

The social world is filled with different types of interactions, and social experience interacts with stress on several different levels. Activation of the neuroendocrine axis that regulates the response to stress can have consequences for innumerable behavioural responses, including social decision-making and aspects of sociality, such as gregariousness and aggression. This is especially true for stress experienced during early life, when physiological systems are developing and highly sensitive to perturbation. Stress at this time can have persistent effects on social behaviours into adulthood. One important question remaining is to what extent these effects are adaptive. This paper initially reviews the current literature investigating the complex relationships between the hypothalamic-pituitary-adrenal (HPA) axis and other neuroendocrine systems and several aspects of social behaviour in vertebrates. In addition, the review explores the evidence surrounding the potential for 'social programming' via differential development and activation of the HPA axis, providing an insight into the potential for positive effects on fitness following early life stress. Finally, the paper provides a framework from which novel investigations could work to fully understand the adaptive significance of early life effects on social behaviours.This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.

Dietary Manipulations That Induce Ketosis Activate the HPA Axis in Male Rats and Mice: A Potential Role for Fibroblast Growth Factor-21

In response to an acute threat to homeostasis or well-being, the hypothalamic-pituitary-adrenocortical (HPA) axis is engaged. A major outcome of this HPA axis activation is the mobilization of stored energy, to fuel an appropriate behavioral and/or physiological response to the perceived threat. Importantly, the extent of HPA axis activity is thought to be modulated by an individual's nutritional environment. In this study, we report that nutritional manipulations signaling a relative depletion of dietary carbohydrates, thereby inducing nutritional ketosis, acutely and chronically activate the HPA axis. Male rats and mice maintained on a low-carbohydrate high-fat ketogenic diet (KD) exhibited canonical markers of chronic stress, including increased basal and stress-evoked plasma corticosterone, increased adrenal sensitivity to adrenocorticotropin hormone, increased stress-evoked c-Fos immunolabeling in the paraventricular nucleus of the hypothalamus, and thymic atrophy, an indicator of chronic glucocorticoid exposure. Moreover, acutely feeding medium-chain triglycerides (MCTs) to rapidly induce ketosis among chow-fed male rats and mice also acutely increased HPA axis activity. Lastly, and consistent with a growing literature that characterizes the hepatokine fibroblast growth factor-21 (FGF21) as both a marker of the ketotic state and as a key metabolic stress hormone, the HPA response to both KD and MCTs was significantly blunted among mice lacking FGF21. We conclude that dietary manipulations that induce ketosis lead to increased HPA axis tone, and that the hepatokine FGF21 may play an important role to facilitate this effect.

Developmental expression of anxiety and depressive behaviours after prenatal predator exposure and early life homecage enhancement

Stressful events during gestation can have sex-specific effects on brain and behaviour, and may contribute to some of the differences observed in adult stress responding and psychopathology. We investigated the impact of a novel repeated prenatal psychological stress (prenatal predator exposure - PPS) during the last week of gestation in rats on offspring behaviours related to social interaction (play behaviour), open field test (OFT), forced swim test (FST) and sucrose preference test (SP) during the juvenile period and in adulthood. We further examined the role of postnatal environmental, using an enhanced housing condition (EHC), to prevent/rescue any changes. Some effects on anxiety, anhedonia, and stress-related coping behaviours (e.g., OFT, SP and OFT) did not emerge until adulthood. PPS increased OFT anxiety behaviours in adult males, and some OFT and SP behaviours in adult females. Contrary to this, EHC had few independent effects; most were apparent only when combined with PPS. In keeping with age-group differences, juvenile behaviours did not necessarily predict the same adult behaviours although juvenile OFT rearing and freezing, and juvenile FST immobility did predict adult FST immobility and sucrose preference, suggesting that some aspects of depressive behaviours may emerge early and predict adult vulnerability or coping behaviours. Together, these results suggest an important, though complex, role for early life psychological stressors and early life behaviours in creating an adult vulnerability to anxiety or depressive disorders and that environmental factors further modulate the effects of the prenatal stressors.

Analysis of Social Process in Two Inbred Strains of Male Mice: A Predominance of Contact-Based Investigation in BALB/c Mice

Developing mouse models for social communication deficits requires a better understanding of the nature of social investigatory processes between mice. Mice use different investigatory strategies based on a possibility of contacts with social sources. A detailed investigation of contact distance revealed strain differences in behavioral strategy between two male inbred C57BL/6 (B6) and BALB/c (BALB) mouse strains. When direct physical contact with stimulus mice was restricted, BALB mice displayed lower social approaches than B6 mice, accompanied by heightened innate anxiety in an unfamiliar environment. However, both BALB and B6 mice expressed distinct object and social recognition in the habituation/dishabituation paradigm. When allowed direct contact with stimulus mice, both B6 and BALB mice showed approach and discrimination of strain differences in the stimulus mice. Furthermore, BALB mice discriminated individuals of the same strain among cagemates and showed a discrete aversion to the anogenital but not facial region of the stranger mice. This anogenital aversion disappeared when the stranger mice received a buspirone injection that reduced anxiety or when familiar cagemates were exposed. These strain differences in investigatory strategies illustrate that B6 mice are able to respond to and process social cues in a vicinity, which does not require physical contact with the source, while BALB mice predominantly process social cues by direct contact with the source. Although BALB mice exhibit marked anxiety and defensive responses to unfamiliarity, there is no evidence of any defect in sociability in BALB mice as a possible autism model.

Mitigation of adverse behavioral impact from predator exposure by the nociceptin/orphanin FQ peptide antagonist J-113397 in rats

The nociceptin/orphanin FQ peptide (NOP) receptor is believed to have an integral modulatory function in the stress response system. We evaluated the highly selective NOP antagonist J-113397 (7.5 and 20.0 mg/kg), using a predator exposure in which rats were exposed to predator cats as a stressor. A single dose of J-113397 or vehicle was administered (intraperitoneally) shortly before exposure to the predators or a sham exposure. Behavioral impact was measured using elevated plus maze (EPM), open field activity (OFA), and an olfactory discrimination (OD). The predator exposure produced a relatively long-lasting deficit (decreased time in open arms, decreased basic activity) on the EPM while having little effect on performance on the OFA or OD. J-113397 mitigated the performance deficits on the EPM in a dose-dependent manner while having little effect on performance on the OFA or OD. The largest dose of J-113397, administered with a sham exposure, was essentially devoid of effects on the EPM, OFA, and OD. These results demonstrate that J-113397 can significantly and selectively mitigate the effects of a stressor typically used in a preclinical model of post-traumatic stress disorder. Furthermore, these results are consistent with and extend previous results showing that the NOP receptor has an important role in the response to stress and that NOP antagonism may, potentially, have therapeutic benefit in stress disorders.

Maternal programming of sex-specific responses to predator odor stress in adult rats

Prenatal stress mediated through the mother can lead to long-term adaptations in stress-related phenotypes in offspring. This study tested the long-lasting effect of prenatal exposure to predator odor, an ethologically relevant and psychogenic stressor, in the second half of pregnancy. As adults, the offspring of predator odor-exposed mothers showed increased anxiety-like behaviors in commonly used laboratory tasks assessing novelty-induced anxiety, increased defensive behavior in males and increased ACTH stress reactivity in females in response to predator odor. Female offspring from predator odor-exposed dams showed increased transcript abundance of glucocorticoid receptor (NR3C1) on the day of birth and FK506 binding protein 5 (FKBP5) in adulthood in the amygdala. The increase in FKBP5 expression was associated with decreased DNA methylation in Fkbp5 intron V. These results indicate a sex-specific response to maternal programming by prenatal predator odor exposure and a potential epigenetic mechanism linking these responses with modifications of the stress axis in females. These results are in accordance with the mismatch hypothesis stating that an animal's response to cues within its life history reflects environmental conditions anticipated during important developmental periods and should be adaptive when these conditions are concurring.

Post-traumatic stress disorder and beyond: an overview of rodent stress models

Post-traumatic stress disorder (PTSD) is a psychiatric disorder of high prevalence and major socioeconomic impact. Patients suffering from PTSD typically present intrusion and avoidance symptoms and alterations in arousal, mood and cognition that last for more than 1 month. Animal models are an indispensable tool to investigate underlying pathophysiological pathways and, in particular, the complex interplay of neuroendocrine, genetic and environmental factors that may be responsible for PTSD induction. Since the 1960s, numerous stress paradigms in rodents have been developed, based largely on Seligman's seminal formulation of 'learned helplessness' in canines. Rodent stress models make use of physiological or psychological stressors such as foot shock, underwater trauma, social defeat, early life stress or predator-based stress. Apart from the brief exposure to an acute stressor, chronic stress models combining a succession of different stressors for a period of several weeks have also been developed. Chronic stress models in rats and mice may elicit characteristic PTSD-like symptoms alongside, more broadly, depressive-like behaviours. In this review, the major existing rodent models of PTSD are reviewed in terms of validity, advantages and limitations; moreover, significant results and implications for future research-such as the role of FKBP5, a mediator of the glucocorticoid stress response and promising target for therapeutic interventions-are discussed.

Optimizing laboratory animal stress paradigms: The H-H* experimental design

Major advances in behavioral neuroscience have been facilitated by the development of consistent and highly reproducible experimental paradigms that have been widely adopted. In contrast, many different experimental approaches have been employed to expose laboratory mice and rats to acute versus chronic intermittent stress. An argument is advanced in this review that more consistent approaches to the design of chronic intermittent stress experiments would provide greater reproducibility of results across laboratories and greater reliability relating to various neural, endocrine, immune, genetic, and behavioral adaptations. As an example, the H-H* experimental design incorporates control, homotypic (H), and heterotypic (H*) groups and allows for comparisons across groups, where each animal is exposed to the same stressor, but that stressor has vastly different biological and behavioral effects depending upon each animal's prior stress history. Implementation of the H-H* experimental paradigm makes possible a delineation of transcriptional changes and neural, endocrine, and immune pathways that are activated in precisely defined stressor contexts.

Learning about stress: neural, endocrine and behavioral adaptations

In this review, nonassociative learning is advanced as an organizing principle to draw together findings from both sympathetic-adrenal medullary and hypothalamic-pituitary-adrenocortical (HPA) axis responses to chronic intermittent exposure to a variety of stressors. Studies of habituation, facilitation and sensitization of stress effector systems are reviewed and linked to an animal's prior experience with a given stressor, the intensity of the stressor and the appraisal by the animal of its ability to mobilize physiological systems to adapt to the stressor. Brain pathways that regulate physiological and behavioral responses to stress are discussed, especially in light of their regulation of nonassociative processes in chronic intermittent stress. These findings may have special relevance to various psychiatric diseases, including depression and post-traumatic stress disorder (PTSD).

Olfaction variation in mouse husbandry and its implications for refinement and standardization: UK survey of animal scents

Olfaction plays a crucial role in mouse communication, providing information about genetic identity, physiological status of conspecifics and alerting mice to potential predators. Scents of animal origin can trigger physiological and behavioural responses that could affect experimental responses and impact positively or negatively on mouse welfare. Additionally, differing olfactory profiles could help explain variation in results between laboratories. A survey was sent to animal research units in the UK to investigate potential transfer of scents of animal origin during routine husbandry procedures, and responses were obtained from animal care workers and researchers using mice in 51 institutions. The results reveal great diversity between animal units regarding the relevant husbandry routines covered. Most [71%] reported housing non-breeding male and female mice in the same room, with 76% reporting that hands were not washed and gloves not changed between handling male and female mice. The most commonly reported species housed in the same facility as mice was the rat (91%), and 41% of respondents were aware that scents from rats could affect mice. Changing of gloves between handling mice and other species was reported by 79% of respondents. Depending on the aspect considered, between 18 and 33% of respondents believed human and non-human animal odours would strongly affect mouse physiology, behaviour or standardization, while approximately 32–54% believed these effects would be weak. This indicates uncertainty regarding the significance of these factors. Understanding and controlling these practices could reduce unwanted variability in experimental results and maximize welfare.

The role of the hypothalamus-pituitary-adrenal/interrenal axis in mediating predator-avoidance trade-offs

Maintaining energy balance and reproducing are important for fitness, yet animals have evolved mechanisms by which the hypothalamus-pituitary-adrenal/interrenal (HPA/HPI) axis can shut these activities off. While HPA/HPI axis inhibition of feeding and reproduction may have evolved as a predator defense, to date there has been no review across taxa of the causal evidence for such a relationship. Here we review the literature on this topic by addressing evidence for three predictions: that exposure to predators decreases reproduction and feeding, that exposure to predators activates the HPA/HPI axis, and that predator-induced activation of the HPA/HPI axis inhibits foraging and reproduction. Weight of evidence indicates that exposure to predator cues inhibits several aspects of foraging and reproduction. While the evidence from fish and mammals supports the hypothesis that predator cues activate the HPA/HPI axis, the existing data in other vertebrate taxa are equivocal. A causal role for the HPA axis in predator-induced suppression of feeding and reproduction has not been demonstrated to date, although many studies report correlative relationships between HPA activity and reproduction and/or feeding. Manipulation of HPA/HPI axis signaling will be required in future studies to demonstrate direct mediation of predator-induced inhibition of feeding and reproduction. Understanding the circuitry linking sensory pathways to their control of the HPA/HPI axis also is needed. Finally, the role that fear and anxiety pathways play in the response of the HPA axis to predator cues is needed to better understand the role that predators have played in shaping anxiety related behaviors in all species, including humans.

Adulthood stress responses in rats are variably altered as a factor of adolescent stress exposure

Stress exposure during development may influence adulthood stress response severity. The present study investigates persisting effects of two adolescent stressors upon adulthood response to predator exposure (PE). Rats were exposed to underwater trauma (UWT) or PE during adolescence, then to PE after reaching adulthood. Rats were then exposed to predator odor (PO) to test responses to predator cues alone. Behavioral and neuroendocrine assessments were conducted to determine acute effects of each stress experience. Adolescent stress altered behavioral response to adulthood PE. Acoustic startle response was blunted. Bidirectional changes in plus maze exploration were revealed as a factor of adolescent stress type. Neuroendocrine response magnitude did not predict severity of adolescent or adult stress response, suggesting that different adolescent stress events may differentially alter developmental outcomes regardless of acute behavioral or neuroendocrine response. We report that exposure to two different stressors in adolescence may differentially affect stress response outcomes in adulthood. Acute response to an adolescent stressor may not be consistent across all stressors or all dependent measures, and may not predict alterations in developmental outcomes pertaining to adulthood stress exposure. Further studies are needed to characterize factors underlying long-term effects of a developmental stressor.

Long-term continuous allopregnanolone elevation causes memory decline and hippocampus shrinkage, in female wild-type B6 mice

Chronic stress in various forms increases the risk for cognitive dysfunction, dementia and Alzheimer's disease. While the pathogenesis behind these findings is unknown, growing evidence suggests that chronic increase in neurosteroid levels, such as allopregnanolone, is part of the mechanism. We treated wild-type C57BL/6J mice with allopregnanolone for 5months, using osmotic pumps. This treatment led to moderately increased levels of allopregnanolone, equivalent to that of mild chronic stress. After an interval of no treatment for 1month, female mice showed impaired learning and memory function in the Morris water maze (MWM) in combination with diminished hippocampus weight and increased cerebellum weight, both correlating to MWM performance. Male mice showed a minor reduction in memory function and no differences in brain structure. We conclude that chronic allopregnanolone elevation can lead to cognitive dysfunction and negative brain alterations. We suggest that allopregnanolone could play a key role in the pathogenesis of stress-induced cognitive disturbances and perhaps dementia.

Effects of historically familiar and novel predator odors on the physiology of an introduced prey

Predator odors can elicit fear responses in prey and predator odor recognition is generally associated with physiological responses. Prey species are often more likely to respond to the odor of familiar rather than alien predators. However, predator naïvety in an introduced prey species has rarely been investigated. We examined the physiological response, as shown by changes in ventilatory variables, of an introduced terrestrial herbivore, the European rabbit Oryctolagus cuniculus, in Australia, to the odor of potential predators and to control odors (distilled water and horse), to explore if responses were limited to historical (cat and fox) predators, or extended to historically novel predators (snake and quoll). All odors except distilled water elicited a response, with rabbits showing long-term higher respiratory frequencies and lower tidal volumes after introduction of the odors, indicating an increase in alertness. However, the intensity of the rabbits' reaction could not be directly linked to any pattern of response with respect to the history of predator-prey relationships. Rabbits exhibited significantly stronger reactions in response to both cat and quoll odors than they did to distilled water, but responses to horse, fox, and snake odor were similar to that of water. Our results show that the introduced rabbit can respond to both historical and novel predators in Australia, and suggest that shared evolutionary history is not necessarily a prerequisite to predator odor recognition.

What's wrong with my mouse cage? Methodological considerations for modeling lifestyle factors and gene-environment interactions in mice

The mechanistic understanding of lifestyle contributions to disease has been largely driven by work in laboratory rodent models using environmental interventions. These interventions show an array of methodologies and sometimes unclear collective conclusions, hampering clinical interpretations. Here we discuss environmental enrichment, exercise and stress interventions to illustrate how different protocols can affect the interpretations of environmental factors in disease. We use Huntington's disease (HD) as an example because its mouse models exhibit excellent validity and HD was the first genetic animal model in which environmental stimulation was found to be beneficial. We make a number of observations and recommendations. Firstly, environmental enrichment and voluntary exercise generally show benefits across laboratories and mouse models. However, the extent to which these environmental interventions have beneficial effects depends on parameters such as the structural complexity of the cage in the case of enrichment, the timing of the intervention and the nature of the control conditions. In particular, clinical interpretations should consider deprived control living conditions and the ethological relevance of the enrichment. Secondly, stress can have negative effects on the phenotype in mouse models of HD and other brain disorders. When modeling stress, the effects of more than one type of experimental stressor should be investigated due to the heterogeneity and complexity of stress responses. With stress in particular, but ideally in all studies, both sexes should be used and the randomized group sizes need to be sufficiently powered to detect any sex effects. Opportunities for clinical translation will be guided by the 'environmental construct validity' of the preclinical data, including the culmination of complementary protocols across multiple animal models. Environmental interventions in mouse models of HD provide illustrative examples of how valid preclinical studies can lead to conclusions relevant to clinical populations.

High fat feeding is associated with stimulation of the hypothalamic-pituitary-adrenal axis and reduced anxiety in the rat

Previous studies have shown that diet-induced obesity is associated with insulin resistance and impaired feedback control of the hypothalamic-pituitary-adrenal (HPA) axis. The objective of this study was to test the hypothesis that hyper-secretion of glucocorticoid, evoked by feeding rats a high fat (HF) diet for 12 weeks, also influences behavioural and neural responses to the elevated plus-maze (EPM) test of anxiety. HF-fed animals exhibited anxiolytic-like behaviour in the EPM but were also hyperactive in this test. Covariant analysis established that the anxiolytic-like behaviour was not secondary to the increase in activity. The HF diet significantly increased basal levels of plasma corticosterone. The groups exposed to the EPM also displayed increased plasma corticosterone levels compared to the relevant control group, although the increment was smaller in the HF-fed animals. Glucocorticoid receptor (GR) immunoreactivity in the cytoplasmic fraction of parietal cortex and hypothalamus and the particulate fraction of the parietal cortex were increased by HF feeding. The behavioural changes evoked by HF feeding did not correlate significantly with changes in GR immunoreactivity in each treatment group or 5-HT turnover in the brain areas studied. It is concluded that anxiolytic properties evoked in the EPM by high fat feeding are unlikely to be related to the changes in HPA function seen in animals fed this diet.

How integrated are behavioral and endocrine stress response traits? A repeated measures approach to testing the stress-coping style model

It is widely expected that physiological and behavioral stress responses will be integrated within divergent stress-coping styles (SCS) and that these may represent opposite ends of a continuously varying reactive–proactive axis. If such a model is valid, then stress response traits should be repeatable and physiological and behavioral responses should also change in an integrated manner along a major axis of among-individual variation. While there is some evidence of association between endocrine and behavioral stress response traits, few studies incorporate repeated observations of both. To test this model, we use a multivariate, repeated measures approach in a captive-bred population of Xiphophorus birchmanni. We quantify among-individual variation in behavioral stress response to an open field trial (OFT) with simulated predator attack (SPA) and measure waterborne steroid hormone levels (cortisol, 11-ketotestosterone) before and after exposure. Under the mild stress stimulus (OFT), (multivariate) behavioral variation among individuals was consistent with a strong axis of personality (shy–bold) or coping style (reactive–proactive) variation. However, behavioral responses to a moderate stressor (SPA) were less repeatable, and robust statistical support for repeatable endocrine state over the full sampling period was limited to 11-ketotestosterone. Although post hoc analysis suggested cortisol expression was repeatable over short time periods, qualitative relationships between behavior and glucocorticoid levels were counter to our a priori expectations. Thus, while our results clearly show among-individual differences in behavioral and endocrine traits associated with stress response, the correlation structure between these is not consistent with a simple proactive–reactive axis of integrated stress-coping style. Additionally, the low repeatability of cortisol suggests caution is warranted if single observations (or indeed repeat measures over short sampling periods) of glucocorticoid traits are used in ecological or evolutionary studies focussed at the individual level.

In vivo electrophysiological recordings in amygdala subnuclei reveal selective and distinct responses to a behaviorally identified predator odor

Chemosensory cues signaling predators reliably stimulate innate defensive responses in rodents. Despite the well-documented role of the amygdala in predator odor-induced fear, evidence for the relative contribution of the specific nuclei that comprise this structurally heterogeneous structure is conflicting. In an effort to clarify this we examined neural activity, via electrophysiological recordings, in amygdala subnuclei to controlled and repeated presentations of a predator odor: cat urine. Defensive behaviors, characterized by avoidance, decreased exploration, and increased risk assessment, were observed in adult male hooded Wistar rats (n = 11) exposed to a cloth impregnated with cat urine. Electrophysiological recordings of the amygdala (777 multiunit clusters) were subsequently obtained in freely breathing anesthetized rats exposed to cat urine, distilled water, and eugenol via an air-dilution olfactometer. Recorded units selectively responded to cat urine, and frequencies of responses were distributed differently across amygdala nuclei; medial amygdala (MeA) demonstrated the greatest frequency of responses to cat urine (51.7%), followed by the basolateral and basomedial nuclei (18.8%) and finally the central amygdala (3.0%). Temporally, information transduction occurred primarily from the cortical amygdala and MeA (ventral divisions) to other amygdala nuclei. Interestingly, MeA subnuclei exhibited distinct firing patterns to predator urine, potentially revealing aspects of the underlying neurocircuitry of predator odor processing and defensiveness. These findings highlight the critical involvement of the MeA in processing olfactory cues signaling predator threat and converge with previous studies to indicate that amygdala regulation of predator odor-induced fear is restricted to a particular set of subnuclei that primarily include the MeA, particularly the ventral divisions.