Central monoamine activity in genetically distinct strains of mice following a psychogenic stressor: effects of predator exposure

Genetic disruption of dopamine β-hydroxylase dysregulates innate responses to predator odor in mice

In rodents, exposure to predator odors such as cat urine acts as a severe stressor that engages innate defensive behaviors critical for survival in the wild. The neurotransmitters norepinephrine (NE) and dopamine (DA) modulate anxiety and predator odor responses, and we have shown previously that dopamine β-hydroxylase knockout (Dbh -/-), which reduces NE and increases DA in mouse noradrenergic neurons, disrupts innate behaviors in response to mild stressors such as novelty. We examined the consequences of Dbh knockout on responses to predator odor (bobcat urine) and compared them to Dbh-competent littermate controls. Over the first 10 min of predator odor exposure, controls exhibited robust defensive burying behavior, whereas Dbh -/- mice showed high levels of grooming. Defensive burying was potently suppressed in controls by drugs that reduce NE transmission, while excessive grooming in Dbh -/- mice was blocked by DA receptor antagonism. In response to a cotton square scented with a novel "neutral" odor (lavender), most control mice shredded the material, built a nest, and fell asleep within 90 min. Dbh -/- mice failed to shred the lavender-scented nestlet, but still fell asleep. In contrast, controls sustained high levels of arousal throughout the predator odor test and did not build nests, while Dbh -/- mice were asleep by the 90-min time point, often in shredded bobcat urine-soaked nesting material. Compared with controls exposed to predator odor, Dbh -/- mice demonstrated decreased c-fos induction in the anterior cingulate cortex, lateral septum, periaqueductal gray, and bed nucleus of the stria terminalis, but increased c-fos in the locus coeruleus and medial amygdala. These data indicate that relative ratios of central NE and DA signaling coordinate the type and valence of responses to predator odor.

Genetic disruption of dopamine β-hydroxylase dysregulates innate responses to predator odor in mice

In rodents, exposure to predator odors such as cat urine acts as a severe stressor that engages innate defensive behaviors critical for survival in the wild. The neurotransmitters norepinephrine (NE) and dopamine (DA) modulate anxiety and predator odor responses, and we have shown previously that dopamine β-hydroxylase knockout (Dbh -/-), which reduces NE and increases DA in mouse noradrenergic neurons, disrupts innate behaviors in response to mild stressors such as novelty. We examined the consequences of Dbh knockout (Dbh -/-) on responses to predator odor (bobcat urine) and compared them to Dbh-competent littermate controls. Over the first 10 min of predator odor exposure, controls exhibited robust defensive burying behavior, whereas Dbh -/- mice showed high levels of grooming. Defensive burying was potently suppressed in controls by drugs that reduce NE transmission, while excessive grooming in Dbh -/- mice was blocked by DA receptor antagonism. In response to a cotton square scented with a novel "neutral" odor (lavender), most control mice shredded the material, built a nest, and fell asleep within 90 min. Dbh -/- mice failed to shred the lavender-scented nestlet, but still fell asleep. In contrast, controls sustained high levels of arousal throughout the predator odor test and did not build nests, while Dbh -/- mice were asleep by the 90-min time point, often in shredded bobcat urine-soaked nesting material. Compared with controls exposed to predator odor, Dbh -/- mice demonstrated decreased c-fos induction in the anterior cingulate cortex, lateral septum, periaqueductal gray, and bed nucleus of the stria terminalis, but increased c-fos in the locus coeruleus and medial amygdala. These data indicate that relative ratios of central NE and DA signaling coordinate the type and valence of responses to predator odor.

Subcortico-amygdala pathway processes innate and learned threats

Behavioral flexibility and timely reactions to salient stimuli are essential for survival. The subcortical thalamic-basolateral amygdala (BLA) pathway serves as a shortcut for salient stimuli ensuring rapid processing. Here, we show that BLA neuronal and thalamic axonal activity in mice mirror the defensive behavior evoked by an innate visual threat as well as an auditory learned threat. Importantly, perturbing this pathway compromises defensive responses to both forms of threats, in that animals fail to switch from exploratory to defensive behavior. Despite the shared pathway between the two forms of threat processing, we observed noticeable differences. Blocking β-adrenergic receptors impairs the defensive response to the innate but not the learned threats. This reduced defensive response, surprisingly, is reflected in the suppression of the activity exclusively in the BLA as the thalamic input response remains intact. Our side-by-side examination highlights the similarities and differences between innate and learned threat-processing, thus providing new fundamental insights.

Brain Monoamine Dysfunction in Response to Predator Scent Stress Accompanies Stress-Susceptibility in Female Rats

Post-traumatic stress disorder (PTSD) is prevalent in women; however, preclinical research on PTSD has predominantly been conducted in male animals. Using a predator scent stress (PSS) rodent model of PTSD, we sought to determine if stress-susceptible female rats show altered monoamine concentrations in brain regions associated with PTSD: the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), and dorsal (dHIPP) and ventral (vHIPP) hippocampus. Female Sprague-Dawley rats were exposed to a single, 10-min PSS exposure and tested for persistent anhedonia, fear, and anxiety-like behavior over four weeks. Rats were phenotyped as stress-Susceptible based on sucrose consumption in the sucrose preference task and time spent in the open arms of the elevated plus maze. Brain tissue was collected, and norepinephrine, dopamine, serotonin, and their metabolites were quantified using high-performance liquid chromatography. Stress-susceptibility in female rats was associated with increased dopamine and serotonin turnover in the mPFC. Susceptibility was also associated with elevated dopamine turnover in the NAc and increased norepinephrine in the vHIPP. Our findings suggest that stress-susceptibility after a single stress exposure is associated with long-term effects on monoamine function in female rats. These data suggest interventions that decrease monoamine turnover, such as MAOIs, may be effective in the treatment of PTSD in women.

Prior stress and vasopressin promote corticotropin-releasing factor inhibition of serotonin release in the central nucleus of the amygdala

Corticotropin-releasing factor (CRF) is essential for coordinating endocrine and neural responses to stress, frequently facilitated by vasopressin (AVP). Previous work has linked CRF hypersecretion, binding site changes, and dysfunctional serotonergic transmission with anxiety and affective disorders, including clinical depression. Crucially, CRF can alter serotonergic activity. In the dorsal raphé nucleus and serotonin (5-HT) terminal regions, CRF effects can be stimulatory or inhibitory, depending on the dose, site, and receptor type activated. Prior stress alters CRF neurotransmission and CRF-mediated behaviors. Lateral, medial, and ventral subdivisions of the central nucleus of the amygdala (CeA) produce CRF and coordinate stress responsiveness. The purpose of these experiments was to determine the effect of intracerebroventricular (icv) administration of CRF and AVP on extracellular 5-HT as an index of 5-HT release in the CeA, using in vivo microdialysis in freely moving rats and high performance liquid chromatography (HPLC) analysis. We also examined the effect of prior stress (1 h restraint, 24 h prior) on CRF- and AVP-mediated release of 5-HT within the CeA. Our results show that icv CRF infusion in unstressed animals had no effect on 5-HT release in the CeA. Conversely, in rats with prior stress, CRF caused a profound dose-dependent decrease in 5-HT release within the CeA. This effect was long-lasting (240 min) and was mimicked by CRF plus AVP infusion without stress. Thus, prior stress and AVP functionally alter CRF-mediated neurotransmission and sensitize CRF-induced inhibition of 5-HT release, suggesting that this is a potential mechanism underlying stress-induced affective reactivity in humans.

The role of mGlu receptors in susceptibility to stress-induced anhedonia, fear, and anxiety-like behavior

Stress and trauma exposure contribute to the development of psychiatric disorders such as post-traumatic stress disorder (PTSD) and major depressive disorder (MDD) in a subset of people. A large body of preclinical work has found that the metabotropic glutamate (mGlu) family of G protein-coupled receptors regulate several behaviors that are part of the symptom clusters for both PTSD and MDD, including anhedonia, anxiety, and fear. Here, we review this literature, beginning with a summary of the wide variety of preclinical models used to assess these behaviors. We then summarize the involvement of Group I and II mGlu receptors in these behaviors. Bringing together this extensive literature reveals that mGlu5 signaling plays distinct roles in anhedonia, fear, and anxiety-like behavior. mGlu5 promotes susceptibility to stress-induced anhedonia and resilience to stress-induced anxiety-like behavior, while serving a fundamental role in the learning underlying fear conditioning. The medial prefrontal cortex, basolateral amygdala, nucleus accumbens, and ventral hippocampus are key regions where mGlu5, mGlu2, and mGlu3 regulate these behaviors. There is strong support that stress-induced anhedonia arises from decreased glutamate release and post-synaptic mGlu5 signaling. Conversely, decreasing mGlu5 signaling increases resilience to stress-induced anxiety-like behavior. Consistent with opposing roles for mGlu5 and mGlu2/3 in anhedonia, evidence suggests that increased glutamate transmission may be therapeutic for the extinction of fear learning. Thus, a large body of literature supports the targeting of pre- and post-synaptic glutamate signaling to ameliorate post-stress anhedonia, fear, and anxiety-like behavior.

The Anterior Piriform Cortex and Predator Odor Responses: Modulation by Inhibitory Circuits

Rodents acquire more information from the sense of smell than humans because they have a nearly fourfold greater variety of olfactory receptors. They use olfactory information not only for obtaining food, but also for detecting environmental dangers. Predator-derived odor compounds provoke instinctive fear and stress reactions in animals. Inbred lines of experimental animals react in an innate stereotypical manner to predators even without prior exposure. Predator odors have also been used in models of various neuropsychiatric disorders, including post-traumatic stress disorder following a life-threatening event. Although several brain regions have been reported to be involved in predator odor-induced stress responses, in this mini review, we focus on the functional role of inhibitory neural circuits, especially in the anterior piriform cortex (APC). We also discuss the changes in these neural circuits following innate reactions to odor exposure. Furthermore, based on the three types of modulation of the stress response observed by our group using the synthetic fox odorant 2,5-dihydro-2,4,5-trimethylthiazoline, we describe how the APC interacts with other brain regions to regulate the stress response. Finally, we discuss the potential therapeutic application of odors in the treatment of stress-related disorders. A clearer understanding of the odor–stress response is needed to allow targeted modulation of the monoaminergic system and of the intracerebral inhibitory networks. It would be improved the quality of life of those who have stress-related conditions.

Endogenous Estrogen Influences Predator Odor-Induced Impairment of Cognitive and Social Behaviors in Aromatase Gene Deficiency Mice

Epidemiological studies have suggested that traumatic stress increases vulnerability to various mental disorders, such as dementia and psychiatric disorders. While women are more vulnerable than men to depression and anxiety, it is unclear whether endogenous estrogens are responsible for the underlying sex-specific mechanisms. In this study, the aromatase gene heterozygous (Ar+/-) mice were used as an endogenous estrogen deficiency model and age- and sex-matched wild type mice (WT) as controls to study the predator odor 2,3,5-trimethyl-3-thiazoline- (TMT-) induced short- and long-term cognitive and social behavior impairments. In addition, the changes in brain regional neurotransmitters and their associations with TMT-induced changes in behaviors were further investigated in these animals. Our results showed TMT induced immediate fear response in both Ar+/- and WT mice regardless of sexes. TMT induced an acute impairment of novel object recognition memory and long-term social behavior impairment in WT mice, particularly in females, while Ar+/- mice showed impaired novel object recognition in both sexes and TMT-elevated social behaviors, particularly in males. TMT failed to induce changes in the prepulse inhibition (PPI) test in both groups. TMT resulted in a slight increase of DOPAC/DA ratio in the cortex and a significant elevation of this ratio in the striatum of WT mice. In addition, the ratio of HIAA/5-HT was significantly elevated in the cortex of TMT-treated WT mice, which was not found in TMT-treated Ar+/- mice. Taken together, our results indicate that TMT exposure can cause cognitive and social behavior impairments as well as change catecholamine metabolism in WT mice, and endogenous estrogen deficiency might desensitize the behavioral and neurochemical responses to TMT in Ar+/- mice.

Stress Adaptation and the Brainstem with Focus on Corticotropin-Releasing Hormone

Stress adaptation is of utmost importance for the maintenance of homeostasis and, therefore, of life itself. The prevalence of stress-related disorders is increasing, emphasizing the importance of exploratory research on stress adaptation. Two major regulatory pathways exist: the hypothalamic–pituitary–adrenocortical axis and the sympathetic adrenomedullary axis. They act in unison, ensured by the enormous bidirectional connection between their centers, the paraventricular nucleus of the hypothalamus (PVN), and the brainstem monoaminergic cell groups, respectively. PVN and especially their corticotropin-releasing hormone (CRH) producing neurons are considered to be the centrum of stress regulation. However, the brainstem seems to be equally important. Therefore, we aimed to summarize the present knowledge on the role of classical neurotransmitters of the brainstem (GABA, glutamate as well as serotonin, noradrenaline, adrenaline, and dopamine) in stress adaptation. Neuropeptides, including CRH, might be co-localized in the brainstem nuclei. Here we focused on CRH as its role in stress regulation is well-known and widely accepted and other CRH neurons scattered along the brain may also complement the function of the PVN. Although CRH-positive cells are present on some parts of the brainstem, sometimes even in comparable amounts as in the PVN, not much is known about their contribution to stress adaptation. Based on the role of the Barrington’s nucleus in micturition and the inferior olivary complex in the regulation of fine motoric—as the main CRH-containing brainstem areas—we might assume that these areas regulate stress-induced urination and locomotion, respectively. Further studies are necessary for the field.

The role of the locus coeruleus in the generation of pathological anxiety

This review aims to synthesise a large pre-clinical and clinical literature related to a hypothesised role of the locus coeruleus norepinephrine system in responses to acute and chronic threat, as well as the emergence of pathological anxiety. The locus coeruleus has widespread norepinephrine projections throughout the central nervous system, which act to globally modulate arousal states and adaptive behavior, crucially positioned to play a significant role in modulating both ascending visceral and descending cortical neurocognitive information. In response to threat or a stressor, the locus coeruleus–norepinephrine system globally modulates arousal, alerting and orienting functions and can have a powerful effect on the regulation of multiple memory systems. Chronic stress leads to amplification of locus coeruleus reactivity to subsequent stressors, which is coupled with the emergence of pathological anxiety-like behaviors in rodents. While direct in vivo evidence for locus coeruleus dysfunction in humans with pathological anxiety remains limited, recent advances in high-resolution 7-T magnetic resonance imaging and computational modeling approaches are starting to provide new insights into locus coeruleus characteristics.

Sub-millimeter variation in human locus coeruleus is associated with dimensional measures of psychopathology: An in vivo ultra-high field 7-Tesla MRI study

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We combined ultra-high field 7-Tesla and 0.4 × 0.4 × 0.5 mm quantitative MR imaging with a computational LC localization and segmentation algorithm.

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LC was delineated in 29 human subjects including subjects with and without an anxiety or stress-related disorder.

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Patients with an anxiety or stress-related disorder had larger LC compared to controls (Cohen's d = 1.08, p = 0.024).

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Larger LC was additionally associated with poorer attentional and inhibitory control and higher anxious arousal (FDR-corrected p's<0.025), trans-diagnostically across the full sample.

The locus coeruleus (LC) has a long-established role in the attentional and arousal response to threat, and in the emergence of pathological anxiety in pre-clinical models. However, human evidence of links between LC function and pathological anxiety has been restricted by limitations in discerning LC with current neuroimaging techniques. We combined ultra-high field 7-Tesla and 0.4 × 0.4 × 0.5 mm quantitative MR imaging with a computational LC localization and segmentation algorithm to delineate the LC in 29 human subjects including subjects with and without an anxiety or stress-related disorder. Our automated, data-driven LC segmentation algorithm provided LC delineations that corresponded well with postmortem anatomic definitions of the LC. There was variation of LC size in healthy subjects (125.7 +/- 59.3 mm³), which recapitulates histological reports. Patients with an anxiety or stress-related disorder had larger LC compared to controls (Cohen's d = 1.08, p = 0.024). Larger LC was additionally associated with poorer attentional and inhibitory control and higher anxious arousal (FDR-corrected p's<0.025), trans-diagnostically across the full sample. This study combined high-resolution and quantitative MR with a mixture of supervised and unsupervised computational techniques to provide robust, sub-millimeter measurements of the LC in vivo, which were additionally related to common psychopathology. This work has wide-reaching applications for a range of neurological and psychiatric disorders characterized by expected LC dysfunction.

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.

Current understanding of fear learning and memory in humans and animal models and the value of a linguistic approach for analyzing fear learning and memory in humans

Fear is an emotion that serves as a driving factor in how organisms move through the world. In this review, we discuss the current understandings of the subjective experience of fear and the related biological processes involved in fear learning and memory. We first provide an overview of fear learning and memory in humans and animal models, encompassing the neurocircuitry and molecular mechanisms, the influence of genetic and environmental factors, and how fear learning paradigms have contributed to treatments for fear-related disorders, such as posttraumatic stress disorder. Current treatments as well as novel strategies, such as targeting the perisynaptic environment and use of virtual reality, are addressed. We review research on the subjective experience of fear and the role of autobiographical memory in fear-related disorders. We also discuss the gaps in our understanding of fear learning and memory, and the degree of consensus in the field. Lastly, the development of linguistic tools for assessments and treatment of fear learning and memory disorders is discussed.

Reversal of aging-related emotional memory deficits by norepinephrine via regulating the stability of surface AMPA receptors

Aging-related emotional memory deficit is a well-known complication in Alzheimer's disease and normal aging. However, little is known about its molecular mechanism. To address this issue, we examined the role of norepinephrine (NE) and its relevant drug desipramine in the regulation of hippocampal long-term potentiation (LTP), surface expression of AMPA receptor, and associative fear memory in rats. We found that there was a defective regulation of NE content and AMPA receptor trafficking during fear conditioning, which were accompanied by impaired emotional memory and LTP in aged rats. Furthermore, we also found that the exogenous upregulation of NE ameliorated the impairment of LTP and emotional memory via enhancing AMPA receptor trafficking in aged rats, and the downregulation of NE impaired LTP in adult rats. Finally, acute treatment with NE or desipramine rescued the impaired emotional memory in aged rats. These results imply a pivotal role for NE in synaptic plasticity and associative fear memory in aging rats and suggest that desipramine is a potential candidate for treating aging-related emotional memory deficit.

Odor cues released by Ehrlich tumor-bearing mice are aversive and induce psychological stress

BACKGROUND/AIMS

This study aimed to verify if odor cues released by Ehrlich tumor-bearing mice are aversive and stressful.

METHODS

Female mice were divided into a control group and an experimental group. One animal of each experimental pair of mice was inoculated with 5 × 10(6) Ehrlich tumor cells intraperitoneally; the other animal was kept undisturbed and was referred to as a CSP (companion of sick partner). One mouse of each control pair was treated intraperitoneally with 0.9% NaCl (1 mg/kg); the other animal (CHP, companion of healthy partner) was kept undisturbed.

RESULTS

It was shown that, in relation to CHP, CSP mice (1) spent less time within the companion zone in a T-maze place preference test, (2) had increased levels of social interaction, (3) had increased levels of plasmatic adrenaline and noradrenaline and (4) displayed no changes in serum corticosterone levels before and after an immobilization stress challenge. It was also shown that (5) cohabitation with 2 tumor-bearing mice was more effective in decreasing neutrophil oxidative burst than cohabitation with 1 sick partner and (6) the presence of a healthy conspecific within the cage of the tumor-injected/CSP pair abrogated the effects of cohabitation on neutrophil activity. These results show that odor cues released by Ehrlich tumor-injected mice are aversive and induce psychological stress.

CONCLUSION

We postulate that the aversive response induced by the chemosignals released by Ehrlich tumor-injected animals activates the sympathetic nervous system and causes the neuroimmunal changes that occur in the mice cohabiting with the sick mice.

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.

Chronic exposure to a predator or its scent does not inhibit male-male competition in male mice lacking brain serotonin

Although it is well-known that defective signaling of the 5-HT system in the brain and stressful stimuli can cause psychological disorders, their combined effects on male–male aggression and sexual attractiveness remain unknown. Our research aimed at examining such effects using tryptophan hydroxylase 2 (Tph2) knockout male mice vs. a rat- or rat scent-based chronic stress model. Tph2^+/+ and Tph2^−/− male mice were placed individually into the rat home cage (rat), a cage containing soiled rat bedding (rat scent) or a cage containing fresh bedding (control) for 5 h every other day for 56 consecutive days. In Tph2^+/+ male mice, rat-exposure decreased male–male aggression and sexual attractiveness of urine odor relative to either rat scent-exposure or control; and rat scent-exposure decreased aggression rather than sexual attractiveness of urine odor compared with control. However, such dose-dependent and long-lasting behavioral inhibitory effects vanished in Tph2^−/− male mice. RT-PCR assay further revealed that putative regulatory genes, such as AR, ERα and GluR4 in the prefrontal cortex, and TrkB-Tc and 5-HTR1A in the hippocampus, were down-regulated at the mRNA level in either rat- or rat scent-exposed Tph2^+/+ male mice, but partially in the Tph2^−/− ones. Hence, we suggest that the dose-dependent and long-lasting inhibitory effects of chronic predator exposure on male–male aggression, sexual attractiveness of urine odor, and mRNA expression of central regulatory genes might be mediated through the 5-HT system in the brain of male mice.

Pattern of Stress-Induced Hyperglycemia according to Type of Diabetes: A Predator Stress Model

BACKGROUND

We aimed to quantify stress-induced hyperglycemia and differentiate the glucose response between normal animals and those with diabetes. We also examined the pattern in glucose fluctuation induced by stress according to type of diabetes.

METHODS

To load psychological stress on animal models, we used a predator stress model by exposing rats to a cat for 60 minutes and measured glucose level from the beginning to the end of the test to monitor glucose fluctuation. We induced type 1 diabetes model (T1D) for ten Sprague-Dawley rats using streptozotocin and used five Otsuka Long-Evans Tokushima Fatty rats as obese type 2 diabetes model (OT2D) and 10 Goto-Kakizaki rats as nonobese type 2 diabetes model (NOT2D). We performed the stress loading test in both the normal and diabetic states and compared patterns of glucose fluctuation among the three models. We classified the pattern of glucose fluctuation into A, B, and C types according to speed of change in glucose level.

RESULTS

Increase in glucose, total amount of hyperglycemic exposure, time of stress-induced hyperglycemia, and speed of glucose increase were significantly increased in all models compared to the normal state. While the early increase in glucose after exposure to stress was higher in T1D and NOT2D, it was slower in OT2D. The rate of speed of the decrease in glucose level was highest in NOT2D and lowest in OT2D.

CONCLUSION

The diabetic state was more vulnerable to stress compared to the normal state in all models, and the pattern of glucose fluctuation differed among the three types of diabetes. The study provides basic evidence for stress-induced hyperglycemia patterns and characteristics used for the management of diabetes patients.

Psychological stress on female mice diminishes the developmental potential of oocytes: a study using the predatory stress model

Although the predatory stress experimental protocol is considered more psychological than the restraint protocol, it has rarely been used to study the effect of psychological stress on reproduction. Few studies exist on the direct effect of psychological stress to a female on developmental competence of her oocytes, and the direct effect of predatory maternal stress on oocytes has not been reported. In this study, a predatory stress system was first established for mice with cats as predators. Beginning 24 h after injection of equine chorionic gonadotropin, female mice were subjected to predatory stress for 24 h. Evaluation of mouse responses showed that the predatory stress system that we established increased anxiety-like behaviors and plasma cortisol concentrations significantly and continuously while not affecting food and water intake of the mice. In vitro experiments showed that whereas oocyte maturation and Sr(2+) activation or fertilization were unaffected by maternal predatory stress, rate of blastocyst formation and number of cells per blastocyst decreased significantly in stressed mice compared to non-stressed controls. In vivo embryo development indicated that both the number of blastocysts recovered per donor mouse and the average number of young per recipient after embryo transfer of blastocysts with similar cell counts were significantly lower in stressed than in unstressed donor mice. It is concluded that the predatory stress system we established was both effective and durative to induce mouse stress responses. Furthermore, predatory stress applied during the oocyte pre-maturation stage significantly impaired oocyte developmental potential while exerting no measurable impact on nuclear maturation, suggesting that cytoplasmic maturation of mouse oocytes was more vulnerable to maternal stress than nuclear maturation.

Long-lasting effects of inescapable-predator stress on brain tryptophan metabolism and the behavior of juvenile mice

The kynurenine (KYN) pathway, which is initiated by indoleamine 2,3-dioxygenase, is the main tryptophan (TRP) metabolic pathway. It shares TRP with the serotonin (5-HT) pathway. We investigated the influence of inescapable-predator (rat) stress on behavior and brain TRP metabolism in mice. Male ICR mice (4W) were exposed to 20-min inescapable-predator stress. Behavior on an elevated plus-maze, and TRP, KYN, and 5-HT levels in the prefrontal cortex, hippocampus, amygdala, and dorsal raphe nuclei were measured 1 and 4 weeks after stress exposure. Predator stress increased the number of open-arm entries (NOA) 4 weeks after stress exposure without altering the number of closed-arm entries (NCA). Thus, the open/closed-arm entry ratio (NOA/NCA) increased after stress exposure. Predator stress increased KYN levels in the prefrontal cortex (until 4 weeks after stress exposure) and dorsal raphe nuclei (for 1 week after stress exposure), decreased 5-HT levels in all brain regions (until 4 weeks after stress exposure). Thus, predator stress increased the KYN/5-HT ratio in all regions, in particular in the prefrontal cortex and hippocampus until 4 weeks after stress exposure. Predator stress shifted the balance between the KYN and 5-HT pathways to the KYN pathway, and induced behavioral disinhibition.

Context-dependent modulation of auditory processing by serotonin

Context-dependent plasticity in auditory processing is achieved in part by physiological mechanisms that link behavioral state to neural responses to sound. The neuromodulator serotonin has many characteristics suitable for such a role. Serotonergic neurons are extrinsic to the auditory system but send projections to most auditory regions. These projections release serotonin during particular behavioral contexts. Heightened levels of behavioral arousal and specific extrinsic events, including stressful or social events, increase serotonin availability in the auditory system. Although the release of serotonin is likely to be relatively diffuse, highly specific effects of serotonin on auditory neural circuitry are achieved through the localization of serotonergic projections, and through a large array of receptor types that are expressed by specific subsets of auditory neurons. Through this array, serotonin enacts plasticity in auditory processing in multiple ways. Serotonin changes the responses of auditory neurons to input through the alteration of intrinsic and synaptic properties, and alters both short- and long-term forms of plasticity. The infrastructure of the serotonergic system itself is also plastic, responding to age and cochlear trauma. These diverse findings support a view of serotonin as a widespread mechanism for behaviorally relevant plasticity in the regulation of auditory processing. This view also accommodates models of how the same regulatory mechanism can have pathological consequences for auditory processing.

Activation patterns of cells in selected brain stem nuclei of more and less stress responsive rats in two animal models of PTSD - predator exposure and submersion stress

This study had two purposes. First: compare predator and water submersion stress cFos activation patterns in dorsal raphe (DR), locus coeruleus (LC) and periaqueductal gray (PAG). Second: identify markers of vulnerability to stressors within these areas. Rats were either predator or submersion stressed and tested 1.75 h later for anxiety-like behavior. Immediately thereafter, rats were sacrificed and cFos expression examined. In DR, serotonergic cells expressing or not expressing cFos were also counted. Predator and submersion stress increased anxiety-like behavior (in the elevated plus maze- EPM) equally over controls. Moreover, stressed rats spent equally less time in the center of the hole board than handled controls, another indication of increased anxiety-like behavior. To examine vulnerability, rats which were less anxious (LA) and more (highly) anxious (MA) in the EPM were selected from among handled control and stressed animals. LA rats in the stressed groups were considered stress non-responsive and MA stressed rats were considered stress responsive. LA and MA rats did not differ in cFos expression in any brain area, though stressors did increase cFos cell counts in all areas over controls. Intriguingly, the number of serotonergic DR neurons not activated by stress predicted degree of anxiety response to submersion stress only. LA submersion stressed rats had more serotonergic cells than all other groups, and MA submersion stressed rats had fewer serotonergic cells than all other groups, which did not differ. Moreover, these cell counts correlated with EPM anxiety. We conclude that a surplus of such cells protects against anxiogenic effects of submersion, while a paucity of such cells enhances vulnerability to submersion stress. Other data suggest serotonergic cells may exert their effects via inhibition of dorsolateral PAG cells during submersion stress. Findings are discussed with respect to serotonergic transmission in vulnerability to predator stress and relevance of findings for post traumatic stress disorder (PTSD). This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Serotonin in the inferior colliculus fluctuates with behavioral state and environmental stimuli

Neuromodulation by serotonin (5-HT) could link behavioral state and environmental events with sensory processing. Within the auditory system, the presence of 5-HT alters the activity of neurons in the inferior colliculus (IC), but the conditions that influence 5-HT neurotransmission in this region of the brain are unknown. We used in vivo voltammetry to measure extracellular 5-HT in the IC of behaving mice to address this issue. Extracellular 5-HT increased with the recovery from anesthesia, suggesting that the neuromodulation of auditory processing is correlated with the level of behavioral arousal. Awake mice were further exposed to auditory (broadband noise), visual (light) or olfactory (2,5-dihydro-2,4,5-trimethylthiazoline, TMT) stimuli, presented with food or confined in a small arena. Only the auditory stimulus or restricted movement increased the concentration of extracellular 5-HT in the IC. Changes occurred within minutes of stimulus onset, with the auditory stimulus increasing extracellular 5-HT by an average of 5% and restricted movement increasing it by an average of 14%. These findings suggest that the neuromodulation of auditory processing by 5-HT is a dynamic process that is dependent on internal state and behavioral conditions.

Increased corticosterone levels in mice subjected to the rat exposure test

In recent years, there has been a notable interest in studying prey-predator relationships to develop rodent-based models for the neurobehavioral aspects of stress and emotion. However, despite the growing use of transgenic mice and results showing important differences in the behavioral responses of rats and mice, little research has been conducted regarding the responses of mice to predators. The rat exposure test (RET), a recently developed and behaviorally validated prey-predator (mouse-rat)-based model, has proven to be a useful tool in evaluating the defensive responses of mice facing rats. To further validate the RET, we investigated the endocrine and behavioral responses of mice exposed to this apparatus. We first constructed a plasma corticosterone secretion curve in mice exposed to a rat or to an empty cage (control). Rat-exposed mice showed a pronounced rise in corticosterone levels that peaked 15 min from the beginning of the predator exposure. The corticosterone levels and behavioral responses of mice exposed to a rat or to a toy in the RET apparatus were then measured. We observed high plasma corticosterone levels along with clear avoidance behaviors represented by decreases in tunnel and surface area exploration and increases in risk assessment behaviors and freezing. This strongly suggests that the test elicits a repertoire of behavioral responses compatible with an aversion state and indicates that it is a promising model for the evaluation of prey-predator interactions. However, more physiological, neurochemical, and pharmacological studies are needed to further validate the test.

A single fear-inducing stimulus induces a transcription-dependent switch in synaptic AMPAR phenotype

Changes in emotional state are known to alter neuronal excitability and can modify learning and memory formation. Such experience–dependent neuronal plasticity can be long-lasting and is thought to involve the regulation of gene transcription. Here we show that a single fear-inducing stimulus increases GluR2 mRNA abundance and promotes synaptic incorporation of GluR2-containing AMPA receptors (AMPARs) in mouse cerebellar stellate cells. The switch in synaptic AMPAR phenotype is mediated by noradrenaline and action potential prolongation. The subsequent rise in intracellular Ca²⁺ and activation of Ca²⁺-sensitive ERK /MAPK signaling trigger new GluR2 gene transcription and a switch in the synaptic AMPAR phenotype from GluR2-lacking, Ca²⁺-permeable, to GluR2-containing Ca²⁺-impermeable receptors on the order of hours. The change in glutamate receptor phenotype alters synaptic efficacy in cerebellar stellate cells. Thus, a single fear-inducing stimulus can induce a long-term change in synaptic receptor phenotype and may alter the activity of an inhibitory neural network.

Interferon-gamma deficiency modifies the motor and co-morbid behavioral pathology and neurochemical changes provoked by the pesticide paraquat

In addition to nigrostriatal pathology and corresponding motor disturbances, Parkinson's disease (PD) is often characterized by co-morbid neuropsychiatric symptoms, most notably anxiety and depression. Separate lines of evidence indicate that inflammatory processes associated with microglial activation and cytokine release may be fundamental to the progression of both PD and its co-morbid psychiatric pathology. Accordingly, we assessed the contribution of the pro-inflammatory cytokine, interferon-gamma (IFN-gamma), to a range of PD-like pathology provoked by the ecologically relevant herbicide and dopamine (DA) toxin, paraquat. To this end, paraquat provoked overt motor impairment (reduced home-cage activity and impaired vertical climbing) and signs of anxiety-like behavior (reduced open field exploration) in wild-type but not IFN-gamma-deficient mice. Correspondingly, paraquat promoted somewhat divergent variations in neurochemical activity among wild-type and IFN-gamma null mice at brain sites important for both motor (striatum) and co-morbid affective pathologies (dorsal hippocampus, medial prefrontal cortex, and locus coeruleus). Specifically, the herbicide provoked a dosing regimen-dependent reduction in striatal DA levels that was prevented by IFN-gamma deficiency. In addition, the herbicide influenced serotonergic and noradrenergic activity within the dorsal hippocampus and medial prefrontal cortex; and elevated noradrenergic activity within the locus coeruleus. Although genetic ablation of IFN-gamma had relatively few effects on monoamine variations within the locus coeruleus and prefrontal cortex, loss of the pro-inflammatory cytokine did normalize the paraquat-induced noradrenergic alterations within the hippocampus. These findings further elucidate the functional implications of paraquat intoxication and suggest an important role for IFN-gamma in the striatal and motor pathology, as well as the co-morbid behavioral and hippocampal changes induced by paraquat.

Morphological correlates of emotional and cognitive behaviour: insights from studies on inbred and outbred rodent strains and their crosses

Every study in rodents is also a behavioural genetic study even if only a single strain is used. Outbred strains are genetically heterogeneous populations with a high intrastrain variation, whereas inbred strains are based on the multiplication of a unique individual. The aim of the present review is to summarize findings on brain regions involved in three major components of rodent behaviour, locomotion, anxiety-related behaviour and cognition, by paying particular attention to the genetic context, genetic models used and interstrain comparisons. Recent trends correlating gene expression in inbred strains with behavioural data in databases, morpho-behavioural-haplotype analyses and problems arising from large-scale multivariate analyses are discussed. Morpho-behavioural correlations in multiple strains are presented, including correlations with projection neurons, interneurons and fibre systems in the striatum, midbrain, amygdala, medial septum and hippocampus, by relating them to relevant transmitter systems. In addition, brain areas differentially activated in different strains are described (hippocampus, prefrontal cortex, nucleus accumbens, locus ceruleus). Direct interstrain comparisons indicate that strain differences in behavioural variables and neuronal markers are much more common than usually thought. The choice of the appropriate genetic model can therefore contribute to an interpretation of positive results in a wider context, and help to avoid misleading interpretations of negative results.

Vulnerability to lasting anxiogenic effects of brief exposure to predator stimuli: sex, serotonin and other factors-relevance to PTSD

Lasting anxiogenic effects of predator stress in rodents may model aspects of post-traumatic stress disorder (PTSD). There is a link between genetic variation in the serotonin (5-HT) transporter (SERT) and anxiety in humans, prompting the generation of SERT knockout mice. This review brings together studies of SERT knockout male mice, normal female mice, and different 5-HT receptors in predator stress effects on anxiety. These studies provide for a link between vulnerability to the anxiogenic effects of predator stress and abnormalities of 5-HT transmission induced by a life long reduction in 5-HT reuptake in male mice, which creates a vulnerability like that seen in normal female mice. Data reviewed suggest abnormalities in 5-HT transmission contribute to vulnerability to lasting anxiogenic effects of species relevant stressors. To the extent to which predator stress effects model aspects of PTSD, and in the light of relevant human literature, these considerations implicate abnormalities of 5-HT transmission in vulnerability to PTSD per se, and as a potential contributor to enhanced female vulnerability to PTSD.

Lipopolysaccharide and a social stressor influence behaviour, corticosterone and cytokine levels: divergent actions in cyclooxygenase-2 deficient mice and wild type controls

Administration of the endotoxin, lipopolysaccharide (LPS) diminished motor activity and increased plasma corticosterone as well as circulating levels of interleukin-1beta (IL-1beta), IL-6, tumor necrosis-factor-alpha (TNF-alpha) and IL-10. Among cyclooxygenase-2 (COX-2) knockout mice the behavioural, corticosterone and cytokine variations promoted by LPS were moderately (home cage activity, corticosterone, TNF-alpha) or largely (IL-6) reduced. However, if mice were exposed to a psychosocial stressor (social disruption associated with grouping mice with novel cage-mates after a period of isolation) coupled with LPS treatment, then the effects of the COX-2 deletion were absent, or there was a synergistic or additive elevation apparent (e.g., in the case of TNF-alpha, IL-6 and corticosterone). Evidently, COX-2 deletion may have either pro- or anti-inflammatory actions, depending upon the psychosocial context in which immune activation occurs.

Synergistic and additive actions of a psychosocial stressor and endotoxin challenge: Circulating and brain cytokines, plasma corticosterone and behavioral changes in mice

Activation of the inflammatory immune response may provoke neuroendocrine and central neurochemical effects that are reminiscent of those elicited by traditional stressors, and when administered concurrently may have synergistic effects. The present investigation assessed whether a psychosocial stressor, comprising social disruption, would augment the effects of lipopolysaccharide in mice. It was indeed observed that the social disruption engendered by a period of 2-4 weeks of social isolation (but not 1-7 days of this treatment) followed by regrouping, enhanced the effects of lipopolysaccharide (LPS: 10mug) in the provocation of sickness behavior, as well as plasma corticosterone, IL-6, TNF-alpha and IL-10 levels. Similar effects were not apparent with respect to IL-1beta, IL-4, or IFN-gamma. Synergy between LPS and other stressors (restraint, tail pinch, and loud noise) was not apparent with respect to sickness or plasma corticosterone, provisionally suggesting that social stressors, such as regrouping, may be more powerful or may engage unique neural or neuroendocrine circuits that favour synergistic outcomes. Within the CNS, the LPS and the regrouping stressor synergistically enhanced NE utilization within the prefrontal cortex, and additively influenced hippocampal NE utilization. In contrast to the effects on circulating cytokines, the LPS-induced elevation of IL-1beta, IL-6 and TNF-alpha mRNA expression in the hippocampus, PFC and nucleus tractus solitarius was diminished in animals that had experienced the regrouping stressor. In view of the combined actions of LPS challenge and a social stressor, these data are interpreted as suggesting that models of depression based on immune activation ought to consider the stressor backdrop upon which immune challenges are imposed.

Experiential and genetic contributions to depressive- and anxiety-like disorders: clinical and experimental studies

Stressful events have been implicated in the precipitation of depression and anxiety. These disorders may evolve owing to one or more of an array of neuronal changes that occur in several brain regions. It seems likely that these stressor-provoked neurochemical alterations are moderated by genetic determinants, as well as by a constellation of experiential and environmental factors. Indeed, animal studies have shown that vulnerability to depressive-like behaviors involve mechanisms similar to those associated with human depression (e.g., altered serotonin, corticotropin releasing hormone and their receptors, growth factors), and that the effects of stressors are influenced by previous stressor experiences, particularly those encountered early in life. These stressor effects might reflect sensitization of neuronal functioning, phenotypic changes of processes that lead to neurochemical release or receptor sensitivity, or epigenetic processes that modify expression of specific genes associated with stressor reactivity. It is suggested that depression is a life-long disorder, which even after effective treatment, has a high rate of re-occurrence owing to sensitized processes or epigenetic factors that promote persistent alterations of gene expression.

Emotion enhances learning via norepinephrine regulation of AMPA-receptor trafficking

Emotion enhances our ability to form vivid memories of even trivial events. Norepinephrine (NE), a neuromodulator released during emotional arousal, plays a central role in the emotional regulation of memory. However, the underlying molecular mechanism remains elusive. Toward this aim, we have examined the role of NE in contextual memory formation and in the synaptic delivery of GluR1-containing alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA)-type glutamate receptors during long-term potentiation (LTP), a candidate synaptic mechanism for learning. We found that NE, as well as emotional stress, induces phosphorylation of GluR1 at sites critical for its synaptic delivery. Phosphorylation at these sites is necessary and sufficient to lower the threshold for GluR1 synaptic incorporation during LTP. In behavioral experiments, NE can lower the threshold for memory formation in wild-type mice but not in mice carrying mutations in the GluR1 phosphorylation sites. Our results indicate that NE-driven phosphorylation of GluR1 facilitates the synaptic delivery of GluR1-containing AMPARs, lowering the threshold for LTP, thereby providing a molecular mechanism for how emotion enhances learning and memory.

Behavioral effects of chronic fluoxetine in BALB/cJ mice do not require adult hippocampal neurogenesis or the serotonin 1A receptor

We previously reported that chronic, but not subchronic, treatment with the selective serotonin reuptake inhibitor (SSRI) fluoxetine altered behavior in the forced swimming test (FST) in BALB/cJ mice. We now use this model to investigate mechanisms underlying the delayed onset of the behavioral response to antidepressants, specifically (1) adult hippocampal neurogenesis and (2) expression of the 5-HT1A receptor. Here, we show data validating this model of chronic antidepressant action. We found the FST to be selectively responsive to chronic administration of the SSRI fluoxetine (18 mg/kg/day) and the tricyclic antidepressant desipramine (20 mg/kg/day), but not to the antipsychotic haloperidol (1 mg/kg/day) in BALB/cJ mice. The behavioral effects of fluoxetine emerged by 12 days of treatment, and were affected neither by ablation of progenitor cells of the hippocampus nor by genetic deletion of the 5-HT1A receptor. The effect of fluoxetine in the BALB/cJ mice was also neurogenesis-independent in the novelty-induced hypophagia test. We also found that chronic fluoxetine does not induce an increase in cell proliferation or the number of young neurons as measured by BrdU and doublecortin immunolabeling, respectively, in BALB/cJ mice. These data are in contrast to our previous report using a different strain of mice (129SvEvTac). In conclusion, we find that BALB/cJ mice show a robust response to chronic SSRI treatment in the FST, which is not mediated by an increase in new neurons in the hippocampus, and does not require the 5-HT1A receptor. These findings suggest that SSRIs can produce antidepressant-like effects via distinct mechanisms in different mouse strains.

Anxiety responses, plasma corticosterone and central monoamine variations elicited by stressors in reactive and nonreactive mice and their reciprocal F1 hybrids

Stressor-provoked anxiety, plasma corticosterone, and variations of brain monoamine turnover are influenced by genetic factors, but may also be moderated by early life experiences. To evaluate the contribution of maternal influences, behavioral and neurochemical stress responses were assessed in strains of mice that were either stressor-reactive or -resilient (BALB/cByJ and C57BL/6ByJ, respectively) as well as in their reciprocal F(1) hybrids. BALB/cByJ mice demonstrated poorer maternal behaviors than did C57BL/6ByJ dams, irrespective of the pups being raised (inbred or F(1) hybrids). The BALB/cByJ mice appeared more anxious than C57BL/6ByJ mice, exhibiting greater reluctance to step-down from a platform and a greater startle response. Although the F(1) behavior generally resembled that of the C57BL/6ByJ parent strain, in the step-down test the influence of maternal factors were initially evident among the F(1) mice (particularly males) with a BALB/cByJ dam. However, over trials the C57BL/6ByJ-like behavior came to predominate. BALB/cByJ mice also exhibited greater plasma corticosterone elevations, 5-HT utilization in the central amygdala (CeA), and greater NE turnover in the paraventricular nucleus of the hypothalamus (PVN). Interestingly, among the F(1)'s corticosterone and 5-HIAA in the CeA resembled that of the BALB/cByJ parent strain, whereas MHPG accumulation in the PVN was more like that of C57BL/6ByJ mice. It seems that, to some extent, maternal factors influenced anxiety responses in the hybrids, but did not influence the corticosterone or the monoamine variations. The inheritance profiles suggest that anxiety was unrelated to either the corticosterone or monoamine changes.

Role of gastrin-releasing peptide and neuromedin B in anxiety and fear-related behavior

Bombesin (BB)-like peptides have been implicated in the mediation and/or modulation of the stress response. However, the impact of manipulating this peptidergic system has only been assessed in a limited number of anxiety and fear paradigms. Given that different behavioral paradigms reflect different aspects of anxiety, the objective of the present investigation was to assess the effects of two mammalian BB-related peptides, namely gastrin-releasing peptide (GRP) and neuromedin B (NMB), in paradigms thought to reflect fear and anxiety-related behaviors. To this end, the effects of central (3rd ventricular; i.c.v.) administration of GRP (0.30 nmol), GRP receptor (BB(2)) antagonist, [Leu(13)-(CH(2)NH)Leu(14)]-BN (1.26 nmol), NMB-30 (0.29 nmol), NMB (BB(1)) receptor antagonist, BIM 23127 (1.70 nmol) and a mixed BB(1)/BB(2) receptor antagonist, PD 176252 (0.621 nmol) were assessed in the elevated plus maze (EPM) and in a fear potentiated startle paradigm (a model thought to reflect conditioned fear). The BB(1) receptor antagonist and the mixed BB(1)/BB(2) receptor antagonist elicited anxiolytic effects in the EPM, whereas, the BB(2) receptor antagonist was without effect. In the fear potentiated startle paradigm, pretreatment with either the BB(1) receptor antagonist or the BB(2) receptor agonist attenuated the fear potentiated startle response, without affecting basal startle amplitude. These data suggest that NMB and GRP do affect the stress response. However, whereas NMB manipulations affected both anxiety and fear responses, GRP alterations selectively affected fear-related responses.

The distinctive effects of acute and chronic psychological stress on airway inflammation in a murine model of allergic asthma

BACKGROUND

Psychological stress has long been recognized to be associated with asthma symptoms. There appear to be individual differences in the susceptibility to even the same kind of stress, and furthermore, stress responses are different between the types of the stress, acute and chronic, even in the same person. However, the mechanisms linking stress to asthma are not well defined. Psychological stress upregulates the expression of endogenous opioids. The opioids stimulate the hypothalamus-pituitary-adrenal axis and sympathetic and adrenomedullary system, through the activation of mu-opioid receptor (MOR) to release stress hormones, such as cortisol and catecholamines, respectively. These hormones can modulate immune responses via the induction of Th1 immunity.

METHODS

Female BALB/c and C57BL/6, wild and MOR-deficient, mice sensitized with ovalbumin (OVA) were exposed to OVA with or without either acute or chronic restraint stress. Airway inflammation was evaluated by the measurement of the number of inflammatory cells and cytokine contents in bronchoalveolar lavage fluids.

RESULTS

In BALB/c mice, but not in C57BL/6 mice, the number of total cells, eosinophils and lymphocytes in the acute stress group were significantly decreased compared with those in the non-acute stress group. In contrast, chronic stress significantly increased the cell numbers and the contents of IL-4 and IL-5 in both mouse strains. Furthermore, these exacerbations were abolished in MOR-deficient mice.

CONCLUSIONS

These results suggest that acute stress modifies the allergic airway responses distinctively depending on the genetic background, and MOR is involved in the chronic psychological stress-induced exacerbation of allergic airway inflammation.

Exposure to predator odor stress increases efflux of frontal cortex acetylcholine and monoamines in mice: comparisons with immobilization stress and reversal by chlordiazepoxide

Psychogenic stress may be associated with the development of mood disorders and schizophrenia. The frontal cortex (FC) regulates stress responses, and its dysfunction contributes to certain neuropsychiatric disorders. We tested the effects of exposure to predator odor stress (POS), a psychogenic stressor, on the concurrent efflux of four major neurotransmitters within the FC in mice in comparison to immobilization stress (IMS), a physical/systemic stressor. POS and IMS significantly increased efflux of acetylcholine (ACh), serotonin (5-HT) and dopamine (DA), but not norephinephrine, within the FC. POS produced a somewhat longer-lasting efflux of 5-HT, as compared to IMS. The effects of POS and IMS on ACh, 5-HT and DA were blocked by chlordiazepoxide. Overall, we demonstrate a novel method to measure the effects of distinctly different stress modalities on FC neurotransmission and suggest that FC responsivity to stressors may be an important marker for evaluating anxiolytic drugs.

Lasting anxiogenic effects of feline predator stress in mice: Sex differences in vulnerability to stress and predicting severity of anxiogenic response from the stress experience

Previous work in male Swiss Webster (CFW) mice demonstrated a long lasting effect of predator stress on risk assessment in the elevated plus maze (EPM). Most severe effects (increases in risk assessment) were seen following a brief unprotected exposure to a cat. Lesser effects were produced by a brief exposure of mice to the cat exposure room without a cat in the room (room stress). This graded response is analogous to the covariation of symptom severity and severity of the precipitating stressor in posttraumatic stress disorder (PTSD). The present study extended these findings to another strain of mice, C57/BL6, and a broader range of tests of anxiety-like behavior, including EPM, acoustic startle response and light/dark box test. Sex was introduced as a variable to investigate if females might be more susceptible to the effects of stressors than males, as has been suggested in human PTSD. Graded and lasting (7 days) effects of a 10 min exposure to a cat (predator stress) or to the cat exposure room only (room stress) were observed on lighted chamber avoidance in the light/dark box. Room stress was without effect on startle responses, but predator stress enhanced peak startle amplitudes measured in the light or in the dark. There was no evidence of light-enhancement of startle in C57 mice. Female mice were more susceptible to the effects of predator and room stress, depending on the measure. Females only responded to cat exposure with a lasting increase in average startle amplitude. This was due to an increased and more prolonged multipeak response to startle after the first and maximal peak startle response. In addition, in females, room and predator stress were equally anxiogenic in measures of open arm avoidance in the EPM. In contrast, room stress was without effect on open arm avoidance in males, but cat exposure was as anxiogenic in males as it was in females. These findings suggest EPM anxiety in females is affected more by the milder stress of room exposure. Severity of effects of predator stress on anxiety-like behaviors in EPM and startle were well predicted (60% of the variance) by measures of cat behavior and probability of mouse defensive response to particular cat behaviors during the cat exposure. Finally, factor analysis indicated that different tests of anxiety-like behavior may be measuring different and independent aspects of mouse affect. Moreover, stressors had no lasting effects on sugar solution consumption. Implications of these findings for modeling PTSD and using transgenic strains of mice to study lasting effects of stress on affect are discussed.

Fox urine as an aversive stimulus: modification of a passive avoidance task

Predator urine, specifically fox urine, is a noxious but harmless olfactory stimulus. The results of previous studies have shown that fox urine is aversive to rats, and that rats react to fox urine in a similar manner as to other psychostressors. In the present study, the authors further investigated the use of fox urine as an aversive or stressful stimulus, specifically examining behavior change in open-field place-preference task. Three methods of presenting the fox urine were examined. Results indicated that fox urine decreased behavior, especially locomotion, during both fox-urine presentation and during a post-fox-urine recovery session. Data suggested that (a) there were fewer ambulatory episodes and less distance was traveled during the presentation of fox urine, regardless of presentation method, and (b) there were fewer vertical movements during fox-urine presentation when a fox-urine-laced cotton ball was set in the wood shavings than when it was placed on a bare floor or in a cup. The data suggested that fox urine may be an effective but nonharmful stimulus alternative for use in avoidance tasks.

Interactions between corticotropin-releasing hormone and serotonin: implications for the aetiology and treatment of anxiety disorders

The amount of evidence for a role of aberrant serotoninergic neurotransmission in the aetiology of anxiety disorders, such as generalised anxiety and panic disorder, has been increasing steadily during the past several years. Although the picture is far from complete yet--partly due to the large number of serotonin (5-HT) receptors and the often-disparate effects of receptor agonists and antagonists in animal models of anxiety--SSRIs and the 5-HT1A agonist buspirone have now earned their place in the treatment of anxiety disorders. However, these drugs show--as they do in depressed patients--a delayed onset of improvement. Therefore, new therapeutical strategies are being explored. Corticotropin-releasing hormone (CRH), which plays a key role in the autonomic, neuroendocrine and behavioural responses to stress, is a strong anxiogenic neuropeptide and a promising candidate for therapeutical intervention in anxiety disorders. The neuroanatomical localisation of CRH, its congeners (the urocortins) and their receptors within the serotoninergic raphé nuclei suggests that interactions between the CRH system and 5-HT may play a role in fear and anxiety. In this chapter, I will discuss studies from my own and other laboratories showing that CRH and the urocortins influence several aspects of serotoninergic neurotransmission, including the firing rate of 5-HT neurones and the release and synthesis of this monoamine. Moreover, the interactions between CRH and 5-HT during psychologically stressful challenges will be discussed. Finally, I will review data showing that long-term alterations in the CRH system lead to aberrant functioning of serotoninergic neurotransmission under basal and/or stressful conditions. From this growing set of data the picture is emerging that the CRH system exerts a vast modulatory influence on 5-HT neurotransmission. An aberrant cross-talk between CRH and 5-HT may be of crucial importance in the neurobiology of anxiety disorders and represents, therefore, a promising goal for therapeutical intervention in these psychiatric diseases.

The smell of danger: a behavioral and neural analysis of predator odor-induced fear

The odors of predators used in animal models provide, in addition to electric footshock, an important means to investigate the neurobiology of fear. Studies indicate that cat odor and trimethylthiazoline (TMT), a synthetic compound isolated from fox feces, are often presented to rodents to induce fear-related responses including freezing, avoidance, stress hormone and, in some tests, risk assessment behavior. Furthermore, we report that different amounts of cat odor impregnated on small-, medium-, or large-sized cloths impact the display of fear-related behavior when presented to rats. That is, rats exposed to a large cat odor containing cloth exhibit an increase in fear behavior, particularly freezing, which remains at high levels in habituation tests administered over a period of 7 days. The large cloth also induces a long-lasting increase in avoidance behavior during repeated habituation and extinction tests. A review of the brain regions involved in predator odor-induced fear behavior indicates a modulatory role of the medial amygdala, bed nucleus of the stria terminalis, and dorsal premammillary nucleus. In addition, the basolateral amygdala is involved in fear behavior induced by cat odor but not TMT, and the central amygdala does not appear to play a major behavioral role in predator odor-induced fear. Future research involving the use of predator odor is likely to rapidly expand knowledge on the neurobiology of fear, which has implications for understanding fear-related psychopathology.

Effects of exposure to a predator on behaviour and serotonergic neurotransmission in different brain regions of C57bl/6N mice

Clinical studies and animal models have provided evidence that stress and serotonin may play a role in the aetiology of psychiatric diseases such as depression and anxiety. In addition, reciprocal interactions between stress and serotonergic neurotransmission have been demonstrated. However, the relationships between stress, serotonin and behaviour are far from completely understood. In this integrative study, we aimed to elucidate the effect of the psychological stress model predator exposure on behaviour and serotonergic neurotransmission in mice. We used a high time-resolution microdialysis method to measure extracellular levels of serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in the hippocampus, prefrontal cortex, lateral septum and caudate putamen of C57BL/6N [corrected] mice, before (08:30-10:30 h), during (10:30-11:00 h) and after exposure (11:00-14:00 h) to a rat. Detailed behavioural observations were also made. Rat exposure resulted in behavioural activation, with predominant risk-assessment activities, and in increases in hippocampal, cortical, septal but not striatal 5-HT and 5-HIAA. When rat exposure was repeated on the consecutive day, small behavioural differences and reductions in 5-HIAA levels, but no differences in the 5-HT response, as compared with the first exposure were observed. As increases in 5-HT often coincide with behavioural activation, it was particularly interesting to find that 5-HT also increased in periods when mice only made minor movements such as sniffing, and that an effect of predator stress was absent in the caudate putamen. Our results indicate that the presence of the rat leads to differential activation of serotonergic neurotransmission in higher brain structures, probably involved in the coping response to this potentially life-threatening situation.

Stress, depression, and anhedonia: caveats concerning animal models

Numerous animal models of depression have been advanced, each having multiple attributes and some limitations. This review provides caveats concerning etiologically valid animal models of depression, focusing on characteristics of the depressive subtype being examined (e.g. typical vs atypical major depression, dysthymia, melancholia), and factors that contribute to the interindividual behavioral variability frequently evident in stressor-related behavioral paradigms. These include the stressor type (processive vs systemic stressors), and characteristics of the stressor (controllability, predictability, ambiguity, chronicity, intermittence), as well as organismic variables (genetic, age, sex), experiential variables (stressor history, early life events) and psychosocial and personality factors that moderate stressor reactivity. Finally, a model of depression is reviewed that evaluates the effects of stressors on hedonic processes, reflected by responding for rewarding brain stimulation. Anhedonia is a fundamental feature of depression, and assessment of stressor-related reductions in the rewarding value of brain stimulation, especially when coupled with other potential symptoms of depression, provides considerable face, construct and predictive validity. Stressful events markedly impact rewarding brain stimulation, and this effect varies across strains of mice differentially reactive to stressors, is modifiable by antidepressant treatments, and allows for analyses of the contribution of different brain regions to anhedonic processes. The paradigm is sensitive to several factors known to acts as moderators of stress responses, but analyses remain to be conducted with regard to several such variables.