Phosphorylated cyclic AMP response element binding protein expression induced in the periaqueductal gray by predator stress: its relationship to the stress experience, behavior and limbic neural plasticity

Inhibition of Protein Synthesis Attenuates Formation of Traumatic Memory and Normalizes Fear-Induced c-Fos Expression in a Mouse Model of Posttraumatic Stress Disorder

Posttraumatic stress disorder (PTSD) is a debilitating psychosomatic condition characterized by impairment of brain fear circuits and persistence of exceptionally strong associative memories resistant to extinction. In this study, we investigated the neural and behavioral consequences of inhibiting protein synthesis, a process known to suppress the formation of conventional aversive memories, in an established PTSD animal model based on contextual fear conditioning in mice. Control animals were subjected to the conventional fear conditioning task. Utilizing c-Fos neural activity mapping, we found that the retrieval of PTSD and normal aversive memories produced activation of an overlapping set of brain structures. However, several specific areas, such as the infralimbic cortex and the paraventricular thalamic nucleus, showed an increase in the PTSD group compared to the normal aversive memory group. Administration of protein synthesis inhibitor before PTSD induction disrupted the formation of traumatic memories, resulting in behavior that matched the behavior of mice with usual aversive memory. Concomitant with this behavioral shift was a normalization of brain c-Fos activation pattern matching the one observed in usual fear memory. Our findings demonstrate that inhibiting protein synthesis during traumatic experiences significantly impairs the development of PTSD in a mouse model. These data provide insights into the neural underpinnings of protein synthesis-dependent traumatic memory formation and open prospects for the development of new therapeutic strategies for PTSD prevention.

Response and recovery of endocrine, behavioral, and neuronal morphology outcomes after different traumatic stressor exposures in male rats

Preclinical models of organismal response to traumatic stress (threat of death or serious injury) can be monitored using neuroendocrine, behavioral, and structural metrics. While many rodent models of traumatic stress have provided a glimpse into select components of the physiological response to acute and chronic stressors, few studies have directly examined the potential differences between stressors and their potential outcomes. To address this gap, we conducted a multi-level comparison of the immediate and longer-term effects of two types of acute traumatic stressors. Adult male rats were exposed to either underwater trauma (UWT), predator exposure (PE), or control procedural handling conditions. Over the next 7 days, yoked cohorts underwent either serial blood sampling for neuroendocrine evaluation across the circadian cycle, or repeated behavioral testing in the elevated plus maze. In addition, a subset of brains from the latter cohort were assessed for dendritic spine changes in the prefrontal cortex and basolateral amygdala. We observed stressor-dependent patterns of response and recovery across all measures, with divergence between endocrine responses despite similar behavioral outcomes. These results demonstrate that different stressors elicit unique behavioral, neuroendocrine, and neuro-structural response profiles and suggest that specific stress models can be used to model desired responses for specific preclinical applications, such as evaluations of underlying mechanisms or therapeutic candidates.

Understanding stress: Insights from rodent models

Through incorporating both physical and psychological forms of stressors, a variety of rodent models have provided important insights into the understanding of stress physiology. Rodent models also have provided significant information with regards to the mechanistic basis of the pathophysiology of stress-related disorders such as anxiety disorders, depressive illnesses, cognitive impairment and post-traumatic stress disorder. Additionally, rodent models of stress have served as valuable tools in the area of drug screening and drug development for treatment of stress-induced conditions. Although rodent models do not accurately reproduce the biochemical or physiological parameters of stress response and cannot fully mimic the natural progression of human disorders, yet, animal research has provided answers to many important scientific questions. In this review article, important studies utilizing a variety of stress models are described in terms of their design and apparatus, with specific focus on their capabilities to generate reliable behavioral and biochemical read-out. The review focusses on the utility of rodent models by discussing examples in the literature that offer important mechanistic insights into physiologically relevant questions. The review highlights the utility of rodent models of stress as important tools for advancing the mission of scientific research and inquiry.

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Stressful life events may lead to the onset of severe psychopathologies in humans.

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Rodents may model many features of stress exposure in human populations.

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Induction of stress via pharmacological and psychological manipulations alter rodent behavior.

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Mechanistic rodent studies reveal key molecular targets critical for new therapeutic targets.

Olfactory Laterality Is Valence-Dependent in Mice

(1) Background: Although olfaction is the predominant sensory modality in rodents, studies focusing on lateralisation of olfactory processing remain scarce, and they are limited to the exploration of brain asymmetries. This study aimed to test whether outbred and inbred mice (NMRI and C57BL/6J mice strains) show nostril-use preference in processing olfactory stimuli differing in terms of emotional valence under unrestrained conditions. (2) Methods: Five odour stimuli were used in the study: vanilla, female urine, garlic, rat, distilled water. We measured the number of times mice used their left or right nostril for each testing session. (3) Results: We here showed that mice preferentially used their right nostril when sniffing attractive stimuli (female urine, vanilla), and their left nostril when sniffing aversive stimuli (rat odour). Results were consistent for both strains. (4) Conclusions: Surprisingly, the results obtained seem opposite to the valence theory assessing that the left and the right hemispheres are dominant in processing stimuli with a positive and a negative valence, respectively. It remains to be determined whether this valence-dependent pattern is specific or not to olfaction in mice. These new findings will be important to better understand how both hemispheres contribute to odour processing in rodents.

Recent Advancements Surrounding the Role of the Periaqueductal Gray in Predators and Prey

Recent advances in neural circuitry techniques, like optogenetics and chemogenetics, have allowed for a greater understanding of the periaqueductal gray (PAG) and its importance in predator and prey behaviors. These studies in rodents have highlighted the role of the rostrolateral PAG in hunting behaviors, and have demonstrated functional differences across the dorsal-ventral/rostral-caudal axes of the PAG associated with defensive behaviors. Human imaging studies have further demonstrated that the PAG is active during situations involving imminent threat suggesting that the function of the PAG is likely largely conserved across species. This mini-review article highlights some of the recent advancements towards our understanding of the functional neuroanatomy of the PAG and its importance in the predator and prey behaviors that are critical for survival.

Intra-periaqueductal gray matter administration of orexin-A exaggerates pulpitis-induced anxiogenic responses and c-fos expression mainly through the interaction with orexin 1 and cannabinoid 1 receptors in rats

Different types of trigeminal pains are frequently associated with psychophysiological concerns. Orexin-A and orexin 1 receptor (OX1R) are involved in modulation of both trigeminal pain and anxiety responses. Ventrolateral periaqueductal gray matter (vlPAG), a controlling site for nociception and emotion, receives orexinergic inputs. Here, the role of vlPAG OX1Rs and their interaction with cannabinoid 1 (CB1) receptor was evaluated in anxiety-like behavior following capsaicin-induced dental pulp pain. Rats were cannulated in the vlPAG and orexin-A was injected at the doses of 0.17, 0.35 and 0.51 μg/rat prior to the induction of pain. The elevated plus maze (EPM) and open field (OF) tests were used for assessing the anxiety responses. In addition, the induction of c-fos, in the vlPAG, was investigated using immunofluorescence microscopy. Capsaicin-treated rats displayed significantly higher anxiogenic behavior on EPM and OF tests. Pretreatment with orexin-A (0.51 μg/rat) attenuated capsaicin-mediated nociception, while exaggerated anxiogenic responses (p < 0.05). In addition, orexin-A effects were diminished by the administration of OX1R (SB-334867, 12 μg/rat) and cannabinoid 1 (AM251, 4 μg/rat) receptor antagonists. Intradental capsaicin induced a significant increase in c-fos expression in the vlPAG that was exaggerated by orexin-A (0.51 μg/rat). Blockage of OX1R and CB1 receptors attenuated the effect of orexin-A on c-fos expression in capsaicin-treated rats. In conclusion, the data suggest that manipulation of OX1R and CB1 receptors in the vlPAG alters capsaicin-evoked anxiety like behaviors and c-fos induction in rats.

Brain-derived neurotrophic factor signaling plays a role in resilience to stress promoted by isoquinoline in defeated mice

Certain stressful life events have been associated with the onset of depression. This study aims to investigate if 7-fluoro-1,3-diphenylisoquinoline-1-amine (FDPI) is effective against social avoidance induced by social defeat stress model in mice. Furthermore, it was investigated the effects of FDPI in the mouse prefrontal cortical plasticity-related proteins and some parameters of toxicity. Adult Swiss mice were subjected to social defeat stress for 10 days. Two protocols with FDPI were carried out: 1- FDPI (25 mg/kg, intragastric) was administered to mice 24 h after the last social defeat stress episode; 2- FDPI (1-25 mg/kg, intragastric) was administered to mice once a day for 10 days concomitant with the social defeat stress. The mice performed social avoidance and locomotor tests. The prefrontal cortical protein contents of kinase B (Akt), extracellular signal-regulated kinase (ERK), cAMP-response element binding protein (CREB), pro-brain-derived neurotrophic factor (proBDNF), p75^NTR, neuronal nuclear protein (NeuN) and nuclear factor-κB (NF-κB) were determined in mice. A single administration of FDPI (25 mg/kg) partially protected against social avoidance induced by stress in mice. Repeated administration of FDPI (25 mg/kg) protected against social avoidance induced by stress in mice. Social defeat stress decreased the protein contents of p75^NTR, NeuN and the pERK/ERK ratio but increased those of proBDNF and the pCREB/CREB ratio, without changing that of NF-κB. Repeated administration of FDPI modulated signaling pathways altered by social defeat stress in mice. The present findings demonstrate that FDPI promoted resilience to stress in mice.

Tonic modulation of anxiety-like behavior by corticotropin-releasing factor (CRF) type 1 receptor (CRF1) within the medial prefrontal cortex (mPFC) in male mice: role of protein kinase A (PKA)

The medial prefrontal cortex (mPFC) and the neuropeptide corticotropin-releasing factor (CRF) have recently been receiving more attention from those interested in the neurobiology of anxiety. Here, we investigated the CRF pathway in the modulation of anxiety-like behaviors in male mice exposed to the elevated plus-maze (EPM), through intra-mPFC injections of CRF, CP376395 [N-(1-ethylpropyl)-3,6-dimethyl-2-(2,4,6-trimethylphenoxy)-4-pyridinamine hydrochloride, a CRF type 1 receptor antagonist (CR F1)] or H-89 [N-[2-[[3-(4-bromophenyl)-2-propenyl]amino]ethyl]-5-isoquinolinesulfonamide dihydrochloride, a protein kinase (PKA) inhibitor]. We also investigated the effects of intra-mPFC injections of H-89 on the behavioral effects induced by CRF. Mice received bilateral intra-mPFC injections of CRF (0, 37.5, 75 or 150pmol), CP376395 (0, 0.75, 1.5 or 3nmol) or H-89 (0, 1.25, 2.5 or 5nmol) and were exposed to the EPM, to record conventional and complementary measures of anxiety for 5min. Results showed that while CRF (75 and 150pmol) produced an anxiogenic-like effect, CP376395 (all doses) and H-89 (5nmol) attenuated anxiety-like behavior. When injected before CRF (150pmol), intra-mPFC H-89 (2.5nmol, a dose devoid of intrinsic effects on anxiety) completely blocked the anxiogenic-like effects of CRF. These results suggest that (i) CRF plays a tonic anxiogenic-like role at CRF1 receptors within the mPFC, since their blockade per se attenuated anxiety indices and (ii) the anxiogenic-like effects following CRF1 receptor activation depend on cAMP/PKA cascade activation in this limbic forebrain area.

Glutamate and GABA imbalance promotes neuronal apoptosis in hippocampus after stress

Background

People who experience traumatic events have an increased risk of post-traumatic stress disorder (PTSD). However, PTSD-related pathological changes in the hippocampus and prefrontal cortex remain poorly understood.

Material/Methods

We investigated the effect of a PTSD-like animal model induced by severe stress. The experimental rats received 20 inescapable electric foot shocks in an enclosed box for a total of 6 times in 3 days. The physiological state (body weight and plasma corticosterone concentrations), emotion, cognitive behavior, brain morphology, apoptosis, and balance of gamma-aminobutyric acid (GABA) and glutamate in the hippocampus and prefrontal cortex were observed. Cell damages were examined with histological staining (HE, Nissl, and silver impregnation), while apoptosis was analyzed with flow cytometry using an Annexin V and propidium iodide (PI) binding and terminal deoxynucleotidyl transferase mediated-dUTP nick end labeling (TUNEL) method.

Results

In comparison with the sham litter-mates, the stressed rats showed decreased body weight, inhibition of hypothalamic-pituitary-adrenal (HPA) axis activation, increase in freezing response to trauma reminder, hypoactivity and anxiety-like behaviors in elevated plus maze and open field test, poor learning in Morris water maze, and shortened latency in hot-plate test. There were significant damages in the hippocampus but not in the prefrontal cortex. Imbalance between glutamate and GABA was more evident in the hippocampus than in the prefrontal cortex.

Conclusions

These results suggest that neuronal apoptosis in the hippocampus after severe traumatic stress is related to the imbalance between glutamate and GABA. Such modifications may resemble the profound changes observed in PTSD patients.

Delayed developmental changes in neonatal vocalizations correlates with variations in ventral medial hypothalamus and central amygdala development in the rodent infant: effects of prenatal cocaine

While variations in neonatal distress vocalizations have long been shown to reflect the integrity of nervous system development following a wide range of prenatal and perinatal insults, a paucity of research has explored the neurobiological basis of these variations. To address this, virgin Sprague-Dawley rats were bred and divided into three groups: [1] untreated, [2] chronic-cocaine treated (30 mg/kg/day, gestation days (GDs) 1-20); or [3] chronic saline treated (2 mg/kg/day, GDs 1-20). Pregnant dams were injected with Bromodeoxyuridine (10 mg/kg) on GDs 13-15 to label proliferating cells in limbic regions of interest. Ultrasonic vocalizations (USVs) were recorded on postnatal days (PNDs) 1, 14, and 21, from one male and female pup per litter. Variations in acoustic properties of USVs following cocaine-exposure were age and sex-dependent including measures of total number, total duration and amplitude of USVs, and percent of USVs with at least one harmonic. Following USV testing brains were stained with standard fluorescent immunohistochemistry protocols and examined for variations in neuronal development and if variations were associated with acoustic characteristics. Limbic region developmental differences following cocaine-exposure were sex- and age-dependent with variations in the ventral medial hypothalamus and central amygdala correlating with variations in vocalizations on PND 14 and 21. Results suggest maturation of the ventral medial hypothalamus and central amygdala may provide the basis for variations in the sound and production of USVs. As vocalizations may serve as a neurobehavioral marker for nervous system integrity, understanding the neurobiological basis of neonatal vocalizations may provide the basis for early intervention strategies in high-risk infant populations.

Glucocorticoids are required for extinction of predator stress-induced hyperarousal

BACKGROUND

The role of glucocorticoids in extinction of traumatic memories has not been fully characterized despite its potential as a therapeutic target for acquired posttraumatic stress disorder (PTSD). The predator stress paradigm allows us to determine whether glucocorticoids mediate the extinction of both context-dependent and context-independent fear memories.

METHODS

Male C57BL/6J mice were exposed to a predator (cat) then repeatedly exposed to the predator stress context in the absence of the cat. Context-dependent (associative) fear memory was assessed as suppression of activity during re-exposure to the predator stress context without the cat (extinction trials). Context-independent fear (non-associative) was assessed seven days after extinction trials using measures of hyperarousal and anxiety-like behaviours in environments unlike the predator stress context. To assess the role of glucocorticoids, mice were injected with metyrapone (50mg/kg) 90 min prior to extinction trials in predator stressed mice and context-dependent and context-independent fear memories were assessed. Finally, metyrapone-treated predator stressed mice were injected with corticosterone (5 or 10mg/kg) immediately following extinction trials and context-dependent and context-independent fear memories were assessed.

RESULTS

Repeated re-exposure to the predator stress context without the cat present extinguished context-dependent fear memory, and also reduced hyperarousal, a generalized, chronic PTSD-like symptom. We show that extinction of context-independent predator stress-induced hyperarousal is dependent on endogenous glucocorticoids during the extinction trials. Furthermore, the inhibition of extinction by metyrapone on startle amplitude was reduced by exogenous administration of corticosterone following extinction trials. Overall, these data implicate glucocorticoids in the extinction of hyperarousal, a core symptom of PTSD.

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'.

Is the Podarcis muralis lizard left-eye lateralised when exploring a new environment?

The typical lateral eye position in ectotherms likely facilitated the spread of visual lateralisation--i.e., the different use of the eyes--in those species. The diffusion of this form of lateralisation seems due to the possibility of carrying out more than one task simultaneously, some controlled by one eye and the visual structures it feeds and some by the other. Similar to other species, exploratory and monitoring behaviours seem to be under left "eye system" control. Wild individuals of the Common wall lizard Podarcis muralis were tested individually in captivity to ascertain whether they showed lateralisation when exploring a new environment, using preferentially the left eye. In Experiment 1, the lizards explored a maze. A left-turning bias was found, both at individual and population level, indicating a possible right hemisphere visual control. In Experiment 2, lizards explored a T-maze, preferring to enter the left rather than the right arm though without any particular preference in the head turns. In Experiment 3, the lizards had to exit an opaque box within a terrarium. We found a left-eye preference again for head turn while leaving the box. Our findings support the hypothesis of right hemisphere mediation of exploratory and monitoring behaviours in Podarcis muralis. In addition to previous studies on the same species, our results support the hypothesis of a simultaneous control of anti-predatory and exploratory behaviours (left-eye mediated) and predatory behaviour (right-eye mediated).

A single footshock causes long-lasting hypoactivity in unknown environments that is dependent on the development of contextual fear conditioning

Exposure to a single session of footshocks induces long-lasting inhibition of activity in unknown environments that markedly differ from the shock context. Interestingly, these effects are not necessarily associated to an enhanced anxiety and interpretation of this hypoactivity remains unclear. In the present experiment we further studied this phenomenon in male Sprague-Dawley rats. In a first experiment, a session of three shocks resulted in hypoactivity during exposure, 6-12days later, to three different unknown environments. This altered behaviour was not accompanied by a greater hypothalamic-pituitary-adrenal (HPA) activation, although greater HPA activation paralleling higher levels of freezing was observed in the shock context. In a second experiment we used a single shock and two procedures, one with pre-exposure to the context before the shock and another with immediate shock that did not induce contextual fear conditioning. Hypoactivity and a certain level of generalization of fear (freezing) to the unknown environments only appeared in the group that developed fear conditioning, but no evidence for enhanced anxiety in the elevated plus-maze was found in any group. The results suggest that if animals are able to associate an aversive experience with a distinct unknown environment, they would display more cautious behaviour in any unknown environment and such strategy persists despite repeated experience with different environments. This long-lasting cautious behaviour was not associated to greater HPA response to the unknown environment that was however observed in the shock context. The present findings raised some concerns about interpretation of long-lasting behavioural changes caused by brief stressors.

Gender-related qualitative differences in baseline and post-stress anxiety responses are not reflected in the incidence of criterion-based PTSD-like behaviour patterns

BACKGROUND

Most epidemiological studies report higher prevalence rates of stress-related disorders such as acute stress disorder, post-traumatic stress disorder (PTSD) and major depressive disorder in women than in men. Few animal models of PTSD have taken gender differences into account and have typically used male subjects. In order to explore gender-related PTSD-like stress-responses more thoroughly, we applied an animal model that focuses selectively on individual patterns of behavioural responses.

METHODS

Prevalence rates of individual behavioural response to a single exposure to predator scent stress (PSS) were assessed by both elevated plus-maze and startle response paradigms. Prevalence rates of extreme behavioural disruption (EBR) on both tests were assessed, correlated to corticosterone levels, and compared to global population response data. In addition, we assessed learning and memory in the Morris water-maze (MWM).

RESULTS

There were no significant differences between the behavioural responses related to oestrous cycle phase in terms of global data for the groups or in terms of prevalence rates of EBR. The overall patterns of response of males and females were affected, yet females demonstrated greater levels of baseline anxiety-like behaviour and lower peak levels of post-exposure anxiety-like behaviour than males. However, the prevalence rates of individual subjects who responded with PTSD-like behaviour were equal for female and male subjects. PSS-exposed female subjects demonstrated significantly compromised performance in the MWM compared to males.

CONCLUSIONS

In this animal model, the results clarified that the assumption that females are more vulnerable is true for the magnitude of the response, but not for the prevalence of pathological response patterns in rat populations.

Molecular and genetic substrates linking stress and addiction

Drug addiction is one of the top three health concerns in the United States in terms of economic and health care costs. Despite this, there are very few effective treatment options available. Therefore, understanding the causes and molecular mechanisms underlying the transition from casual drug use to compulsive drug addiction could aid in the development of treatment options. Studies in humans and animal models indicate that stress can lead to both vulnerability to develop addiction, and increased drug taking and relapse in addicted individuals. Exposure to stress or drugs of abuse results in long-term adaptations in the brain that are likely to involve persistent alterations in gene expression or activation of transcription factors, such as the cAMP Response Element Binding (CREB) protein. The signaling pathways controlled by CREB have been strongly implicated in drug addiction and stress. Many potential CREB target genes have been identified based on the presence of a CRE element in promoter DNA sequences. These include, but are not limited to CRF, BDNF, and dynorphin. These genes have been associated with initiation or reinstatement of drug reward and are altered in one direction or the other following stress. While many reviews have examined the interactions between stress and addiction, the goal of this review was to focus on specific molecules that play key roles in both stress and addiction and are therefore posed to mediate the interaction between the two. Focus on these molecules could provide us with new targets for pharmacological treatments for addiction.

Viral vector induction of CREB expression in the periaqueductal gray induces a predator stress-like pattern of changes in pCREB expression, neuroplasticity, and anxiety in rodents

Predator stress is lastingly anxiogenic. Phosphorylation of CREB to pCREB (phosphorylated cyclic AMP response element binding protein) is increased after predator stress in fear circuitry, including in the right lateral column of the PAG (periaqueductal gray). Predator stress also potentiates right but not left CeA-PAG (central amygdala-PAG) transmission up to 12 days after stress. The present study explored the functional significance of pCREB changes by increasing CREB expression in non-predator stressed rats through viral vectoring, and assessing the behavioral, electrophysiological and pCREB expression changes in comparison with handled and predator stressed controls. Increasing CREB expression in right PAG was anxiogenic in the elevated plus maze, had no effect on risk assessment, and increased acoustic startle response while delaying startle habituation. Potentiation of the right but not left CeA-PAG pathway was also observed. pCREB expression was slightly elevated in the right lateral column of the PAG, while the dorsal and ventral columns were not affected. The findings of this study suggest that by increasing CREB and pCREB in the right lateral PAG, it is possible to produce rats that exhibit behavioral, brain, and molecular changes that closely resemble those seen in predator stressed rats.

Cat odour-induced anxiety--a study of the involvement of the endocannabinoid system

RATIONALE

Recent evidence suggests the involvement of the endocannabinoid (EC) system in the regulation of anxiety.

OBJECTIVES

The aim of present work was to study the role of the EC system in cat odour-induced anxiety in rats. Materials and methods Male Wistar rats were exposed to cat odour in home and motility cages. Exposure of rats to elevated zero-maze was used to determine changes in anxiety. Effect of rimonabant (0.3-3 mg/kg), antagonist of CB1 receptors, was studied on cat odour-induced alterations in exploratory behaviour. Real-time PCR was used to determine gene expression levels of EC-related genes in the brain.

RESULTS

Anxiogenic-like action of cat odour was evident in the elevated zero-maze. Cat odour increased the expression of FAAH, the enzyme responsible for the degradation of anandamide, in the mesolimbic area. By contrast, in the amygdala and periaqueductal grey (PAG) levels of NAPE-PLD, the enzyme related to the synthesis of anandamide, and FAAH were remarkably decreased. Cat odour also decreased the expression of enzymes related to metabolism of 2-archidonoyl-glycerol in the amygdala and PAG. Pre-treatment of rats with rimonabant (0.3-3 mg/kg) reduced the exploratory behaviour of rats, but did not affect cat odour-induced changes.

CONCLUSION

Exposure to cat odour induces anxiogenic-like effect on the behaviour in rats. Cat odour also causes moderate increase in expression of EC-related genes in the mesolimbic area, whereas significant down-regulation is established in the amygdala and PAG. Relation of predator odour-induced anxiety to the inhibition of the EC system in the amygdala and PAG is supported by behavioural studies where blockade of CB1 receptors by rimonabant induces anxiogenic-like action.

Hemispheric lateralization of a molecular signal for pain modulation in the amygdala

The extracellular signal-regulated kinase (ERK) cascade has been shown to be a key modulator of pain processing in the central nucleus of the amygdala (CeA) in mice. ERK is activated in the CeA during persistent inflammatory pain and this activation is both necessary and sufficient to induce peripheral tactile hypersensitivity. Interestingly, biochemical studies show that inflammation-induced ERK activation in the CeA only occurs in the right, but not the left hemisphere. This inflammation-induced ERK activation in the right CeA is independent of the side of peripheral inflammation, suggesting that there is a dominant role of the right hemisphere in the modulation of pain by ERK activation in the CeA. However, the functional significance of this biochemical lateralization has yet to be determined. In the present study, we tested the hypothesis that modulation of pain by ERK signaling in the CeA is functionally lateralized. We acutely blocked ERK activation in the CeA by infusing the MEK inhibitor U0126 into the right or the left hemisphere and then measured the behavioral effects on inflammation-induced mechanical hypersensitivity in mice. Our results show that blockade of ERK activation in the right, but not the left CeA, decreases inflammation-induced peripheral hypersensitivity independent of the side of peripheral injury. These findings demonstrate that modulation of pain by ERK signaling in the CeA is functionally lateralized to the right hemisphere, suggesting a dominant role of the right amygdala in pain processing.

Long-term neuroendocrine and behavioural effects of a single exposure to stress in adult animals

There is now considerable evidence for long-lasting sequels of stress. A single exposure to high intensity predominantly emotional stressors such as immobilisation in wooden-boards (IMO) induces long-term (days to weeks) desensitization of the hypothalamic-pituitary-adrenal (HPA) response to the same (homotypic) stressor, whereas the response to novel (heterotypic) stressors was enhanced. In addition, long-lasting changes in behaviour have been described after a single exposure to brief or more prolonged sessions of shocks, predator, predator odour, underwater stress or a combination of three stressors on 1 day. The most consistent changes are reduced entries into the open arms of the elevated plus-maze and enhanced acoustic startle response, both reflecting enhanced anxiety. However, it is unclear whether there is any relationship between the intensity of the stressors, as evaluated by the main physiological indexes of stress (e.g. HPA axis), the putative traumatic experience they represent and their long-term behavioural consequences. This is particularly critical when trying to model post-traumatic stress disorders (PTSD), which demands a great effort to validate such putative models.

The immediate early gene Arc is associated with behavioral resilience to stress exposure in an animal model of posttraumatic stress disorder

Mechanisms involved in adaptative and maladaptive changes in neural plasticity and synaptic efficacy in various brain areas are pivotal to understanding the physiology of the response to stress and the pathophysiology of posttraumatic stress disorder (PTSD). Activity-regulated cytoskeletal-associated protein (Arc) is an effector immediate early gene (IEG) which has direct effects on intracellular homeostatic functions. Increased expression of Arc has been associated with increased neuronal activity and with consolidation of long-term memory. It may thus play an important role in mediating experience-induced reorganization and/or development of synaptic connections. This study sought to characterize the pattern of expression of mRNA for the Arc gene in selected brain areas of test subjects classified according to their individual pattern of behavioral response to a stressor, correlated with circulating levels of corticosterone (as a physiological marker of stress response). The hippocampal CA1 and CA3 subregions of individuals whose behavior was minimally or partially disrupted in response to predator scent stress demonstrated significantly increased levels of mRNA for Arc, compared to unexposed controls. The group whose behavior was severely disrupted demonstrated no such upregulation. Consistent with the hypothesis that the Arc gene has a promoting effect on neuronal function and/or structural changes, the lack of Arc expression in the behaviorally and physiologically more severely affected individuals raises the possibility that Arc may be associated with resilience and/or recovery after stress exposure.

The role of the read through variant of acetylcholinesterase in anxiogenic effects of predator stress in mice

This study examined the role of the read through variant of acetylcholinesterase (AChE-R) in lasting changes in murine affective behavior produced by a brief predator stress. AChE-R is elevated by stress in limbic cholinergic circuits implicated in anxiogenic effects of predator stress. The expression of AChE-R was blocked with a systemically administered central acting antisense oligonucleotide for AChE-R (EN101). EN101 was injected at multiple points prior to and after a predator stress in male C57 mice. Seven days after the last injection, behavior was tested. Predator stress caused a significant increase in startle amplitude, which EN101 blocked. This effect was specific to EN101, as the negative control inactive form of EN101, INVEN101 was without effect on stress effects on startle. Neither EN101 nor INVEN101 altered the anxiogenic effects of predator stress on behavior in the elevated plus maze, and both drugs partially reduced stress suppression of time active in the hole board. In the light dark box test, INVEN101 exhibited a weak block of stress effects on behavior for reasons which are unclear. Taken together, findings support the view that multiple neural systems are responsible for the different changes in behavior produced by predator stress. Present findings also suggest a role for AChE-R in specific anxiogenic (hyperarousal) effects following predator stress. Since AChE-R manipulations took place starting 23 h prior to predator stress and continued 48 h after predator stress, further research is necessary to determine the role of AChE-R in initiation and/or consolidation of hyperarousal effects of predator stress.

Long-term down-regulation of BDNF mRNA in rat hippocampal CA1 subregion correlates with PTSD-like behavioural stress response

Brain-derived neurotrophic factor (BDNF) and its intracellular kinase-activating receptor TrkB, have been implicated in the neurobiological mechanisms underlying the clinical manifestations of PTSD, especially those related to synaptic efficacy and neural plasticity. BDNF interacts with components of the stress response such as corticosterone, and plays an important role in growth, maintenance and functioning of several neuronal systems. This study employed an animal model of PTSD to investigate the relationship between prevalence rates of distinct patterns of behavioural responses to predator stress, circulating levels of corticosterone and local levels of mRNA for BDNF, TrkB and two other neurotrophic factors in selected brain areas. Animals whose behaviour was extremely disrupted by exposure selectively displayed significant down-regulation of mRNA for BDNF and up-regulation of TrkB mRNA in the CA1 subregion of the hippocampus, compared to animals whose behaviour was minimally or partially affected and to unexposed controls. The response was consistent throughout the entire study only in CA1. The consistent long-term the BDNF down-regulation and TrkB up-regulation associated with extreme behavioural compromise may be associated with chronic stress-induced psychopathological processes, especially in the hippocampus. The corresponding changes in neural plasticity and synaptic functioning may mediate clinical manifestations of PTSD.

Screen for genes in periaqueductal grey of male Wistar rats related to reduced exploratory activity in the elevated plus-maze

Aim of a present study was to find genes in the periaqueductal grey (PAG) related to the exploratory behavior in rats. Male Wistar rats were divided according to their exploratory behavior in the elevated plus-maze model of anxiety into two groups: high (non-anxious) and low (anxious) exploratory activity. Differential expression of genes was analyzed using the cDNA representational difference analysis (RDA). Q-RT-PCR was used to confirm most prominent changes and functional annotation of the identified genes was performed to establish pathways related to exploratory behavior of rats. We found different genetic activation related to the exploratory activity of rats. Rats with low exploratory activity showed increase in the intracellular signal transduction and in GABA, vasopressin and adrenergic receptor activities. Functional annotation confirmed significant induction of cAMP system and GTPases in rats with anxious-type behavior. On the other hand, rats with high exploratory activity in the elevated plus-maze (non-anxious type of behavior) had increased activity of genes forming "behavioral fear response" system. These changes were specific to PAG, because they were not found in the cerebellum. In addition, plasma corticosterone levels were significantly higher in rats with non-anxious behavior compared to anxious behavior. Our results show that non-anxious behavior is related to activation of "fear response system" and more intense activation of HPA axis. Possibly it means that this system helps animals to cope with the threatening circumstances. More detailed analysis of this potential "fear response system" is necessary in the further studies for understanding its role in the regulation of emotional behavior.

Involvement of group II metabotropic glutamate receptors in stress-induced behavioural sensitization

RATIONALE

A short session of repeated foot shocks in rats causes long-lasting sensitization of behavioural, hormonal and autonomic responses to novel stressful challenges. The behavioural sensitization can be reduced by anxiolytics and mimics aspects of stress-induced changes in patients with post-traumatic stress disorder.

OBJECTIVES

The aim of this study was to test the efficacy of a group II metabotropic glutamate receptor (mGluR) agonist and assess altered brain mGluR receptor expression in shock-sensitized rats.

MATERIALS AND METHODS

Male Wistar rats were exposed to a 15-min session with ten 6-s foot shocks (preshocked). One and 2 weeks later, rats were intraperitoneally injected with the group II metabotropic glutamate receptor agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) or vehicle, and 30 min later exposed to 5 min of 85 dB noise. For in situ hybridization with probes for mGluR1, mGluR2, mGluR3 and mGluR5, preshocked and control rats were killed under basal conditions 2 weeks after foot shocks and their brains cryosectioned.

RESULTS

APDC had no clear effect in controls, but dose-dependently reduced high immobility and increased low locomotion and rearing seen in preshocked rats to the levels of controls. mGluR3 expression was increased in the basolateral nucleus of the amygdala, and mGluR2 expression was increased in the agranular insular cortex of preshocked rats compared to controls.

CONCLUSIONS

Shock-induced behavioural sensitization in rats is reduced by acute treatment with a group II metabotropic glutamate receptor agonist. This effect may depend on the increased expression of amygdala mGluR3, which could be hypothesized as an endogenous mechanism to counteract stress-induced neuronal sensitization.

Social defeat, a paradigm of depression in rats that elicits 22-kHz vocalizations, preferentially activates the cholinergic signaling pathway in the periaqueductal gray

Gene expression profiles in the periaqueductal gray (PAG) of adult Long-Evans rats as a function of a stressful social defeat in inter-male fighting encounters were examined. This social subordination model mimics prototypical behavioral changes that parallel aspects of clinical depression, has been postulated to simulate early changes in the onset of depression in the losers, and has been successfully utilized for the evaluation of antidepressant activity. The 22-kHz ultrasonic vocalizations (USVs) have been shown to reflect negative emotional states akin to anxiety and depression. Social defeat is the most robust and reliable method of eliciting these calls. The PAG has been shown to be a key brain region for the generation of 22-kHz ultrasonic vocalizations, and 22-kHz USVs have been shown to be controlled by the mesolimbic cholinergic system. In this present study, we examined gene expression changes in the PAG of social subordinate rats compared to dominant rats that do not Exhibit 22-kHz USVs. We found that social defeat significantly altered the genes associated with cholinergic synaptic transmission in the PAG. The most robust of these were the increased expression of the beta2 subunit of the nicotinic acetylcholine receptor (CHRNB2) and the T subunit of acetylcholinesterase (ACHE) in the subordinate animals. These changes were corroborated by quantitative real-time polymerase chain reaction (qRT-PCR) and found to be exclusive to the PAG compared to seven other brain regions examined. These data suggest that cholinergic transmission in the PAG is involved in the generation of 22-kHz USVs and provide potential therapeutic targets for the treatment of affective disorders.

Activation of the extracellular signal-regulated kinase in the amygdala modulates pain perception

The amygdala has been proposed to serve as a neural center for the modulation of pain perception. Numerous anatomical and behavioral studies demonstrate that exogenous manipulations of the amygdala (i.e., lesions, drug infusions) modulate behavioral responses to acute noxious stimuli; however, little is known about the endogenous molecular changes in the amygdala that contribute to alterations in nociceptive processing during persistent noxious stimuli that resemble pathological pain conditions. In the present study, we demonstrate that endogenous molecular changes in the amygdala play a crucial role in modulating long-lasting peripheral hypersensitivity associated with persistent inflammation and we further identify the extracellular signal-regulated kinase (ERK) as a molecular substrate underlying this behavioral sensitization. Using the formalin test as a mouse model of persistent inflammatory pain, we show that activation of ERK in the amygdala is both necessary for and sufficient to induce long-lasting peripheral hypersensitivity to tactile stimulation. Thus, blockade of inflammation-induced ERK activation in the amygdala significantly reduced long-lasting peripheral hypersensitivity associated with persistent inflammation, and pharmacological activation of ERK in the amygdala induced peripheral hypersensitivity in the absence of inflammation. Importantly, blockade of ERK activation in the amygdala did not affect responses to acute noxious stimuli in the absence of inflammation, indicating that modulation of nociceptive responses by amygdala ERK activation is specific to the persistent inflammatory state. Altogether, our results demonstrate a functional role of the ERK signaling cascade in the amygdala in inflammation-induced peripheral hypersensitivity.

Involvement of noradrenergic and corticoid receptors in the consolidation of the lasting anxiogenic effects of predator stress

The roles of beta-NER (beta-noradrenergic receptor), GR (glucocorticoid) and mineral corticoid receptors (MR) in the consolidation of anxiogenic effects of predator stress were studied. One minute after predator stress, different groups of rats were injected (ip) with vehicle, propranolol (beta-NER blocker, 5 and 10 mg/kg), mifepristone (RU486, GR blocker, 20 mg/kg), spironolactone (MR blocker, 50 mg/kg), propranolol (5 mg/kg) plus RU486 (20 mg/kg) or the anxiolytic, chloradiazepoxide (CPZ, 10 mg/kg). One week later, rodent anxiety was assessed in elevated plus maze, hole board, light/dark box, social interaction and acoustic startle. Considering all tests except startle, propranolol dose dependently blocked consolidation of lasting anxiogenic effects of predator stress in all tests. GR receptor block alone was ineffective. However, GR block in combination with an ineffective dose of propranolol did blocked consolidation of predator stress effects in all tests, suggesting a synergism between beta-NER and GR. Surprisingly, MR block prevented consolidation of anxiogenic effects in all tests except the light/dark box. CPZ post stress was ineffective against the anxiogenic impact of predator stress. Study of startle was complicated by the fact that anxiogenic effects of stress on startle amplitude manifested as both an increase and a decrease in startle amplitude. Suppression of startle occurred in stressed plus vehicle injected groups handled three times prior to predator stress. In contrast, stressed plus vehicle rats handled five times prior to predator stress showed increases in startle, as did all predator stressed only groups. Mechanisms of consolidation of the different startle responses appear to differ. CPZ post stress blocked startle suppression but not enhancement of startle. Propranolol post stress had no effect on either suppression or enhancement of startle. GR block alone post stress prevented suppression of startle, but not enhancement. In contrast blocking GR and beta-NER together prevented startle enhancement. MR block also prevented startle enhancement. Effects of MR block on startle suppression were not tested. Delay of habituation to startle was found in all stressed rats. Consolidation of delay of habituation was blocked or attenuated by post stress MR block, GR plus beta-NER block and CPZ but not by post stress GR or beta-NER block alone. Taken together, present findings suggest consolidation of lasting anxiogenic effects of predator stress may share some of the same neurochemical mechanisms implicated in some forms of fear memory consolidation. Implications of these findings for the study of stress-induced changes in affect including posttraumatic stress disorder (PTSD) are discussed.

PTSD and stress sensitisation: a tale of brain and body Part 2: animal models

Animal models that are characterised by long-lasting conditioned fear responses as well as generalised behavioural sensitisation to novel stimuli following short-lasting but intense stress have a phenomenology that resembles that of PTSD in humans. These models include brief sessions of shocks, social confrontations, and a short sequence of different stressors. Subgroups of animals with different behavioural traits or coping styles during stress exposure show a different degree or pattern of long-term sensitisation. Weeks to months after the trauma, treated animals on average also show a sensitisation to novel stressful stimuli of neuroendocrine, cardiovascular and gastrointestinal motility responses as well as altered pain sensitivity and immune function. Functional neuroanatomical and pharmacological studies in these animal models have provided evidence for involvement of amygdala and medial prefrontal cortex, and of brain stem areas regulating neuroendocrine and autonomic function and pain processing. They have also generated a number of neurotransmitter and neuropeptide targets that could provide novel avenues for treatment in PTSD.

The NMDA receptor antagonist CPP blocks the effects of predator stress on pCREB in brain regions involved in fearful and anxious behavior

A 5-min unprotected exposure to a cat produces long-lasting anxiogenic effects on behavior which are NMDA receptor-dependent. Since phosphorylation of CREB is regulated by NMDA receptors and pCREB-like-immunoreactivity (lir) is increased after predator stress, we examined the effects of CPP (3-(2-carboxypiperazin4-yl)propyl-l-phosphonic acid), a competitive NMDA receptor antagonist, on predator stress-induced changes in pCREB-lir in brain areas implicated in fearful and anxious behavior. Areas examined included the amygdala, periqueductal gray (PAG), bed nucleus of the stria terminalis (BNST), anterior cingulate cortex (ACC), and dorsal medial hypothalamus (DMH). CPP blocked the predator stress-induced increase in pCREB-lir in the right lateral PAG and in several amygdala nuclei. CPP also reversed the predator stress-induced suppression of pCREB-lir in the BNST. Importantly, at least in the amygdala and PAG, the pattern of pCREB-lir was hemisphere- and AP plane-dependent. Our results suggest that several amygdala nuclei, the PAG, and the BNST, where predator stress changes pCREB-lir in a NMDA receptor-dependent manner, are candidate areas of neuroplastic change contributing to lasting changes in anxiety-like behaviors.

The effect of early poststressor intervention with sertraline on behavioral responses in an animal model of post-traumatic stress disorder

Whereas several well-controlled studies have established the selective serotonin reuptake inhibitors (SSRIs) as the recommended first-line pharmacotherapeutic agents for acute and chronic post-traumatic stress disorder (PTSD), drug interventions in the acute postexposure phase have not been studied to the same extent and tend to be largely speculative. This study employed an animal model which assesses prevalence of individual stress-response behavior patterns in order to assess the short-term effects of a brief treatment regimen with an SSRI (sertraline) administered immediately after stress-exposure, with those of an identical delayed regimen and of saline. Prevalence rates of rats displaying extreme anxiety-like behavioral responses to predator stress, compared to partial and minimal responses, were assessed in the elevated plus maze and startle response paradigms, with and without intraperitoneal administration of sertraline for 7 days immediately postexposure, or 7 days after exposure. Immediate postexposure administration of sertraline reduced anxiety-like and avoidant behavior, decreased hyperarousal responses and diminished the overall incidence of extreme (PTSD-like) behavioral responses, compared to the delayed treatment regimen and to saline controls. Brief immediate poststress exposure treatment with sertraline reduced prevalence rates of extreme behavioral disruption in the short-term. SSRI drugs are thus worthy of further investigation as agents of secondary prevention in the acute aftermath of stress-exposure.

Elevated pCREB in the PAG after exposure to the elevated plus maze in rats previously exposed to a cat

The elevated plus maze (EPM) is an ethologically based test of anxiety-like behavior. In addition, exposure to the maze itself is stressful and anxiogenic. One of the goals of this study was to examine if the stress of EPM exposure increased pCREB-like-immunoreactivity (lir). The second goal of this study was to determine if prior stress impacted expression of pCREB-lir in animals exposed to the EPM. Toward this end, pCREB-lir was examined after exposure to the EPM in young adult male rats that had been exposed to a cat 7 days earlier. Brain areas investigated included the amygdala, periaqueductal gray (PAG), and bed nucleus of the stria terminalis (BNST), all areas considered to be part of the "fear circuit". Results show that there were no pCREB-lir differences between control rats and rats exposed to the EPM only. However, exposure to the EPM in predator stressed rats showed elevated pCREB-lir in the right lateral column of the PAG and bilaterally in the dorsal column of the PAG. In contrast, EPM exposure did not elevate pCREB-lir in the amygdala or BNST in predator stressed rats. Findings suggest mechanisms associated with neuroplasticity may be engaged by relatively mild stresses in animals with a history of severe stress exposure. This may be clinically relevant, as a key feature of posttraumatic stress disorder (PTSD) is the exaggerated reaction to a mild stressor in which the response is more appropriate to the original traumatic situation than the current conditions. If what happens in animals also occurs in humans, the findings of this study suggest that neural mechanisms of prior traumatic stress may interact with subsequent stress to reinforce psychopathology.

Anisomycin, a protein synthesis inhibitor, disrupts traumatic memory consolidation and attenuates posttraumatic stress response in rats

BACKGROUND

Paradoxical changes in memory represent a troublesome characteristic of posttraumatic stress disorder (PTSD). Exceptionally vivid intrusive memories of some aspects of the trauma are mingled with patchy amnesia regarding other important aspects. Molecular studies of the memory process suggest that the conversion from labile short-term memory into long-term fixed traces involves protein synthesis. This study assessed the effects of administration of anisomycin, a protein synthesis inhibitor, after initial exposure, after exposure to a cue associated with triggering experience, and after reexposure to the triggering trauma in an animal model of PTSD.

METHOD

Magnitude of changes in prevalence of anxiety-like behaviors on the elevated plus-maze and nonhabituated exaggerated startle reaction were compared in rats that were exposed to predator stress, with and without microinjection of anisomycin.

RESULTS

Microinjection of anisomycin before and after stress exposure reduced anxiety-like and avoidant behavior, reduced the mean startle amplitude, and reversed the stress-induced habituation deficit 7 days later. The persistent anxiety-like behaviors that were seen after stress exposure do not appear to be sensitive to anisomycin after reexposure to a cue associated with the event or after reexposure to the index experience.

CONCLUSIONS

Disruption of the process of traumatic memory consolidation may be useful for mitigating PTSD symptoms.

Social stress-related behavior affects hippocampal cell proliferation in mice

Although social stress inhibits neurogenesis in the adult hippocampus, the extent to which individual differences in stress-related behavior affect hippocampal cell proliferation is not well understood. Based on results from resident-intruder stress tests administered to adult male mice, here we report that individual differences in hippocampal cell proliferation are related to the frequency of defensive behavior, and not the amount of aggression received or the frequency of fleeing. In contrast, access to voluntary wheel-running exercise did not affect hippocampal cell proliferation in either stressed or non-stressed mice. Social stress-induced inhibition of cell proliferation was restricted to the hippocampus, as neither stress nor access to wheel-running exercise altered cell proliferation in the amygdala. These findings indicate that individual differences in stress-related behavior influence cell proliferation in the mouse hippocampus, and may have important implications for understanding structural and functional hippocampal impairments in human psychiatric patients.

Expression of egr-1 (zif268) mRNA in select fear-related brain regions following exposure to a predator

Research has demonstrated that immediate-early genes/inducible transcriptional factors (e.g., c-fos, egr-1) are increased in amygdala nuclei (lateral, basal and central nuclei) known to be involved in fear conditioning, footshock stress and novelty. Although these data suggest that expression of inducible transcriptional factors are involved in fear, other non-shock ethologically based paradigms (predator or predator odor exposure) do not appear to increase c-fos in the lateral and basal nuclei. While the lack of c-fos expression may indicate that predator stress does not engage the lateral and basal amygdala nuclei, it may be that c-fos in the amygdala is not responsive to predator exposure. Therefore, egr-1, which increases in the lateral nucleus following fear conditioning, footshock and novelty, was assessed to determine if its expression is induced in rats exposed to a cat. Five minutes of cat exposure did not increase expression of egr-1 mRNA in the lateral nucleus of the amygdala. egr-1 was increased in the paraventricular nucleus of the hypothalamus, indicating cat-induced stress, and visual cortex compared to rats that were either confined for 5 min or handled. In the lateral periaqueductal gray, handled rats displayed a left hemisphere dominance, which disappeared in both the cat-exposed and confined group, suggesting that immobility, induced by either cat-induced stress or unstressed confinement, increased right hemisphere egr-1 expression. The results are discussed in a context of differences and similarities in neural circuitry for conditioned and unconditioned fear.

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.

Prior electrical stimulation of dorsal periaqueductal grey matter or deep layers of the superior colliculus sensitizes rats to anxiety-like behaviors in the elevated T-maze test

Electrical stimulation of the dorsal periaqueductal grey matter (DPAG) and deep layers of the superior colliculus (DLSC) of the rat elicits anxiety-like reactions such as freezing and flight. The temporal course of the effects of the aversive electrical stimulation of the DPAG (5, 15 and 30 min afterward) and DLSC (5, 10 and 15 min afterward) on the defensive response of rats exposed to elevated T-maze were determined. The elevated T-maze generates two defensive behaviors, inhibitory avoidance and one-way escape, which have been related, respectively, to generalized anxiety and panic disorders. Prior electrical stimulation of the DPAG (15 min) and DLSC (5 min) enhanced inhibitory avoidance when compared to no-operated and sham animals, although not affecting escape. Therefore, stimulation of the DPAG and DLSC causes a heightened responsivity to anxiogenic stimulus, but not to panicogenic stimulus, inherent to elevated T-maze. These findings support the participation of the DPAG and DLSC in the elaboration of adaptive responses to stressful situations. Besides, the data supports the view that prior electrical stimulation of DPAG and DLSC is selective in sensitizing rats to anxiety-like behaviors, but not to panic-like behaviors in the elevated T-maze test.

Relationship of the predatory attack experience to neural plasticity, pCREB expression and neuroendocrine response

Aggression takes at least two, an attacker and a target. This paper will address the lasting consequences of being a target of aggression. We review the lasting impact of predatory attack on brain and behavior in rodents. A single brief unprotected exposure of a rat to a cat lastingly alters affective responses of rats in a variety of contexts. Alterations of these behaviors resembles both generalized anxiety comorbid with post traumatic stress disorder (PTSD), and the hyper arousal expressed in enhanced startle in PTSD. Examination of neural transmission and neural plasticity in limbic circuits implicates changes in transmission in two connecting pathways in many but not all of the behavioral changes. Quantification of the predator encounter reveals that both the behavior of the predator and the reaction of the rat to attack are highly predictive of the effects of predatory attack on molecular biological (pCREB expression) and electrophysiological measures of limbic neuroplastic change. Moreover, a case will be made that the pattern of change of corticosteroid level over three hours after the predator encounter, in interaction with the predatory experience, plays an important part in initiation of lasting changes in brain and behavior.

Role of NMDA receptors in the lateralized potentiation of amygdala afferent and efferent neural transmission produced by predator stress

The present study investigated the role of NMDA receptors in behavioral and neuroplastic changes in amygdala efferent (central amygdala to periaqueductal gray-ACE-PAG) and amygdala afferent (ventral angular bundle to basolateral amygdala-VAB-BLA) pathways in response to predator stress. Effects on brain and behavioral response to predator stress of competitive block of NMDA receptors with a dose of 10 mg/kg of CPP (3-(2-carboxypiperazin4-yl)propyl-l-phosphonic acid) were studied. Behavioral response to stress was tested with hole board, elevated plus maze, light/dark box, social interaction and acoustic startle tests. CPP was administered i.p. 30 min prior to predator stress and blocked the effects of predator on some but not all behaviors measured 8-9 days later. Effects of predator stress and CPP on potentials evoked in the PAG by single pulse stimulation of the ACE and in the BLA by single pulse stimulation of VAB were assessed 10-11 days after predator stress. Predator stress potentiated ACE-PAG evoked potentials in the right but not the left hemisphere, replicating previous work. Predator stress potentiated VAB-BLA transmission in both hemispheres 10-11 days after predator stress. Right hemisphere VAB-BLA potentiation replicated and extended past studies showing right hemisphere potentiation at 1 and 9 days after stress. Left VAB-BLA potentiation effects differed from the long term depression seen in VAB-BLA at 1 and 9 days after stress in previous studies. CPP blocked predator stress-induced potentiation of ACE-PAG and VAB-BLA evoked potentials in the right hemisphere. CPP did not block left VAB-BLA potentiation, rather CPP amplified it. Left hemisphere effects of CPP were interpreted as reflecting block of NMDA dependent long term depression, which unmasked a non-NMDA dependent potentiation. Taken together, the findings add to a body of evidence suggesting that a syndrome of behavioral changes follows predator stress. Components of this syndrome likely depend on changes in separable neural substrates. Potentiation of ACE-PAG and VAB-BLA evoked potentials in the right hemisphere likely mediates a subset of changes in behavior. Moreover, a medial ACE-PAG pathway is implicated in mediating stress-induced changes in startle amplitude. In contrast, a lateral ACE-PAG pathway is implicated in mediating changes in startle habituation. Finally, consistent with cat and human studies, the right hemisphere appears particularly important in long term response to stress.

Role of NMDA receptors in the syndrome of behavioral changes produced by predator stress

Effects on behavioral response to predator stress of competitive block of NMDA receptors with doses of .1, 1.0 and 10 mg/kg of CPP (3-(2-carboxypiperazin4-yl)propyl-l-phosphonic acid) were studied. An affect test battery assessed behavioral response to stress and employed hole board, elevated plus maze, light/dark box, social interaction, social avoidance and response to acoustic startle tests. Doses of 1-10 mg/kg of CPP administered ip 30 min prior to predator stress blocked the effects of predator stress on some but not all behaviors measured 8-9 days later. Predator stress normally reduces open arm exploration and risk assessment in the plus maze, decreases entries into the lighted arm of the light dark box and delays habituation of the acoustic startle response. CPP blocked all of these effects of predator stress. A dose of 10 mg/kg of CPP was required for all behaviors except habituation to startle. Block of effects on habituation to startle occurred at 1 and 10 mg/kg. Behaviors in which effects of predator stress were not blocked by CPP included reduction in unprotected head dips in the elevated plus maze and reduced social interaction. In addition, predator stress was without effect on social avoidance measured with the Haller test. These findings extend previous work showing NMDA receptor dependence of effects of predator stress on behavior in the elevated plus maze and on amplitude of acoustic startle response. Novel findings include NMDA receptor dependence of predator stress effects on light dark box behavior and startle habituation. Taken together, the findings add to a body of evidence showing that a syndrome of behavioral changes follows predator stress. Components of this syndrome of behavioral changes likely depend on changes in separable neural substrates initiated in part by NMDA receptors as well as by other neurochemical means.

Neural circuit changes mediating lasting brain and behavioral response to predator stress

This paper reviews recent work which points to critical neural circuitry involved in lasting changes in anxiety like behavior following unprotected exposure of rats to cats (predator stress). Predator stress may increase anxiety like behavior in a variety of behavioral tests including: elevated plus maze, light dark box, acoustic startle, and social interaction. Studies of neural transmission in two limbic pathways, combined with path and covariance analysis relating physiology to behavior, suggest long term potentiation like changes in one or both of these pathways in the right hemisphere accounts for stress induced changes in all behaviors changed by predator stress except light dark box and social interaction. Findings will be discussed within the context of what is known about neural substrates activated by predator odor.

Ethological and temporal analyses of anxiety-like behavior: the elevated plus-maze model 20 years on

As well as being considered a reliable measurement instrument of animal anxiety-like behavior, the elevated plus-maze (EPM) is also used as a post-hoc test to evaluate emotionality in genetically modified rodents. The present review considers factors which may further improve the validity (predictive/face/construct) of the EPM model: (1) the importance of measuring defensive patterns of response such as risk assessment in addition to traditional measures such as open arm time; (2) other methodological refinements such as min-by-min scoring and use of a test/retest protocol; and (3) the identification and control of major sources of variability in this test. To estimate whether current use of the EPM by researchers takes the above factor into account, a survey of the recent literature was conducted. Results showed that the majority of studies have not yet assimilated these important considerations into their use of the EPM. For example, although risk assessment measures may be more sensitive to anxiety modulating drugs than traditional measures, only a quarter of studies have adopted them. It is hoped that this review can provide insights into the optimal use of the EPM, a simple task that can be very complex in terms of behavioral analysis.

Effects of systemic injections of vilazodone, a selective serotonin reuptake inhibitor and serotonin 1A receptor agonist, on anxiety induced by predator stress in rats

We examined the effect of Vilazodone, a selective serotonin reuptake inhibitor (SSRI) and serotonin 1A (5-HT(1A)) receptor agonist [Bartoszyk, G.D., Hegenbart, R., Ziegler, H., 1997. EMD 68843, a serotonin reuptake inhibitor with selective presynaptic 5-HT1A receptor agonistic properties. Eur. J. Pharmacol. 322, 147-153.], on change in affect following predator stress. Vilazodone and vehicle injection (intraperitoneal) occurred either 10 min after predator stress (prophylactic testing), or 90 min prior to behavioral testing for the effects of predator stress (therapeutic testing). Predator stress involved unprotected exposure of rats to a domestic cat. Behavioral effects of stress were evaluated with hole board, plus-maze, and acoustic startle tests 1 week after stress. Predator stress increased anxiety-like behavior in the plus-maze and elevated response to acoustic startle. In prophylactic testing, Vilazodone affected stress potentiation of startle at doses above 5 mg/kg. Vilazodone increased stress elevation of startle at 10 mg/kg. Higher doses of Vilazodone (20 and 40 mg/kg) blocked stress potentiation of startle. In contrast, Vilazodone had no effect on stress potentiation of anxiety in the plus-maze. In therapeutic testing, Vilazodone increased stress elevation of startle at all doses. In contrast, therapeutic Vilazodone had no effect on stress potentiation of anxiety in the plus-maze. Taken together, the data suggest a prophylactic potential for Vilazodone in the treatment of changes in hypervigilance following severe stress.

Behavioral Changes Induced in Rats by Exposure to Trimethylthiazoline, a Component of Fox Odor

Trimethylthiazoline (TMT), a component of fox feces, has been used in various studies as a natural predator stimulus to induce autonomic and behavioral signs of fear (e.g., higher levels of stress hormones, freezing, and risk assessment). The present study investigated whether 2 further behavioral signs of fear are induced in rats by TMT exposure: potentiation of the acoustic startle response and inhibition of appetitive behavior. In addition, the authors tested the rats for dose dependency of TMT-induced freezing behavior. The study confirmed that behavioral changes observed during TMT exposure are caused by TMT-induced fear and are dose dependent.