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

Activation of NOS-cGMP pathways promotes stress-induced sensitization of behavioral responses in zebrafish

Nitric oxide (NO) is a molecule involved in plasticity across levels and systems. The role of NOergic pathways in stress-induced sensitization (SIS) of behavioral responses, in which a particular stressor triggers a state of hyper-responsiveness to other stressors after an incubation period, was assessed in adult zebrafish. In this model, adult zebrafish acutely exposed to a fear-inducing conspecific alarm substance (CAS) and left undisturbed for an incubation period show increased anxiety-like behavior 24 h after exposure. CAS increased forebrain glutamate immediately after stress and 30 min after stress, an effect that was accompanied by increased nitrite levels immediately after stress, 30 min after stress, 90 min after stress, and 24 h after stress. CAS also increased nitrite levels in the head kidney, where cortisol is produced in zebrafish. CAS-elicited nitrite responses in the forebrain 90 min (but not 30 min) after stress were prevented by a NOS-2 blocker. Blocking NOS-1 30 min after stress prevents SIS; blocking NOS-2 90 min after stress also prevents stress-induced sensitization, as does blocking calcium-activated potassium channels in this latter time window. Stress-induced sensitization is also prevented by blocking guanylate cyclase activation in both time windows, and cGMP-dependent channel activation in the second time window. These results suggest that different NO-related pathways converge at different time windows of the incubation period to induce stress-induced sensitization.

The anxiogenic effects of adolescent psychological stress in male and female mice

Adolescence is a period of transition during which there is extensive development of the brain and the hypothalamic-pituitary-adrenal axis. However, the term adolescence is broad and covers a number of important developmental periods ranging from pre-pubescence to sexual maturity. Using a predator stress model, we investigated the effects of chronic psychological stress on anxiety-like, depression-like, and social behaviours in male and female mice during early adolescence, when mice are pre-pubertal, and late adolescence, when mice are sexually mature. All stressed mice showed hyperactivity and increased anxiety-like behaviours. The anxiogenic effects were generally more pronounced in mice exposed to late, rather than early adolescent stress, but were clearly evident when stress was experienced at either timepoint. Risk assessment behaviours were also affected by the stress treatments, but the direction of these changes were sometimes sex- and age-specific. Surprisingly, mice stressed during adolescence showed no depressive-like behaviours as adults. This study provides evidence that adolescent psychological stress has pronounced long-term anxiogenic effects but that the precise behavioural phenotype differs based on sex and the sub-stage of adolescence during which the individual is exposed.

Resilience to fear: The role of individual factors in amygdala response to stressors

Resilience to stress is an adaptive process that varies individually. Resilience refers to the adaptation, or the ability to maintain or regain mental health, despite being subject to adverse situation. Resilience is a dynamic concept that reflects a combination of internal individual factors, including age and gender interacting with external factors such as social, cultural and environmental factors. In the last decade, we have witnessed an increase in the prevalence of anxiety disorders, including post-traumatic stress disorder. Given that stress in unavoidable, it is of great interest to understand the neurophysiological mechanisms of resilience, the individual factors that may contribute to susceptibility and promote efficacious approaches to improve resilience. Here, we address this complex question, attempting at defining clear and operational definitions that may allow us to improve our analysis of behavior incorporating individuality. We examine how individual perception of the stressor can alter the outcome of an adverse situation using as an example, the fear-conditioning paradigm and discuss how individual differences in the reward system can contribute to resilience. Given the central role of the endocannabinoid system in regulating fear responses and anxiety, we discuss the evidence that polymorphisms in several molecules of this signaling system contribute to different anxiety phenotypes. The endocannabinoid system is highly interconnected with the serotoninergic and dopaminergic modulatory systems, contributing to individual differences in stress perception and coping mechanisms. We review how the individual variability in these modulatory systems can be used towards a multivariable assessment of stress risk. Incorporating individuality in our research will allow us to define biomarkers of anxiety disorders as well as assess prognosis, towards a personalized clinical approach to mental health.

Mu opioid receptor regulation of glutamate efflux in the central amygdala in response to predator odor

The amygdala plays an important role in the responses to predator threat. Glutamatergic processes in amygdala regulate the behavioral responses to predator stress, and we have found that exposure to ferret odor activates glutamatergic neurons of the basolateral amygdala [BLA] which are known to project to the central amygdala [CeA]. Therefore, we tested if predator stress would increase glutamate release in the rat CeA using in vivo microdialysis, while monitoring behavioral responses during a 1 h exposure to ferret odor. Since injections of mu opioid receptor [MOR] agonists and antagonists into the CeA modulate behavioral responses to predator odor, we locally infused the MOR agonist DAMGO or the MOR antagonist CTAP into the CeA during predator stress to examine effects on glutamate efflux and behavior. We found that ferret odor exposure increased glutamate, but not GABA, efflux in the CeA, and this effect was attenuated by tetrodotoxin. Interestingly, increases in glutamate efflux elicited by ferret odor exposure were blocked by infusion of CTAP, but CTAP did not alter the behavioral responses during predator stress. DAMGO alone enhanced glutamate efflux, but did not modulate glutamate efflux during predator stress. These studies demonstrate that ferret odor exposure, like other stressors, enhances glutamate efflux in the CeA. Further, they suggest that activation of MOR in the CeA may help shape the defensive response to predator odor and other threats.

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Predator odor stress increased glutamate efflux in the central amygdala.

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Predator stress-induced increases in glutamate were blocked by a mu opioid receptor antagonist.

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Blocking glutamate efflux in the amygdala did not alter behavioral responses to predator odor.

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Infusion of a mu opioid receptor agonist also increased glutamate efflux in the central amygdala.

Stress Across Generations: DNA Methylation as a Potential Mechanism Underlying Intergenerational Effects of Stress in Both Post-traumatic Stress Disorder and Pre-clinical Predator Stress Rodent Models

Although most humans will experience some type of traumatic event in their lifetime only a small set of individuals will go on to develop post-traumatic stress disorder (PTSD). Differences in sex, age, trauma type, and comorbidity, along with many other elements, contribute to the heterogenous manifestation of this disorder. Nonetheless, aberrant hypothalamus-pituitary-adrenal (HPA) axis activity, especially in terms of cortisol and glucocorticoid receptor (GR) alterations, has been postulated as a tenable factor in the etiology and pathophysiology of PTSD. Moreover, emerging data suggests that the harmful effects of traumatic stress to the HPA axis in PTSD can also propagate into future generations, making offspring more prone to psychopathologies. Predator stress models provide an ethical and ethologically relevant way to investigate tentative mechanisms that are thought to underlie this phenomenon. In this review article, we discuss findings from human and laboratory predator stress studies that suggest changes to DNA methylation germane to GRs may underlie the generational effects of trauma transmission. Understanding mechanisms that promote stress-induced psychopathology will represent a major advance in the field and may lead to novel treatments for such devastating, and often treatment-resistant trauma and stress-disorders.

Maternal Predator Odor Exposure in Mice Programs Adult Offspring Social Behavior and Increases Stress-Induced Behaviors in Semi-Naturalistic and Commonly-Used Laboratory Tasks

Maternal stress has a profound impact on the long-term behavioral phenotype of offspring, including behavioral responses to stressful and social situations. In this study, we examined the effects of maternal exposure to predator odor, an ethologically relevant psychogenic stressor, on stress-induced behaviors in both semi-naturalistic and laboratory-based situations. Adult C57BL/6 mice offspring of dams exposed to predator odor during the last half of pregnancy showed increased anti-predatory behavior, more cautious foraging behavior and, in the elevated plus maze, avoidance of elevated open areas and elevated open areas following restraint stress challenge. These offspring also exhibited alterations in social behavior including reduced free interaction and increased initial investigation despite normal social recognition. These changes in behavior were associated with increased transcript abundance of corticotropin-releasing factor, mineralocorticoid receptor and oxytocin (Oxt) in the periventricular nucleus of the hypothalamus. Taken together, the findings are consistent with a long-term increase in ethologically-relevant behavioral and neural responses to stress in male and female offspring as a function of maternal predator odor exposure.

Predator-based psychosocial stress animal model of PTSD: Preclinical assessment of traumatic stress at cognitive, hormonal, pharmacological, cardiovascular and epigenetic levels of analysis

Research on post-traumatic stress disorder (PTSD) is faced with the challenge of understanding how a traumatic experience produces long-lasting detrimental effects on behavior and brain functioning, and more globally, how stress exacerbates somatic disorders, including cardiovascular disease. Moreover, the design of translational research needs to link animal models of PTSD to clinically relevant risk factors which address why only a subset of traumatized individuals develop persistent psychopathology. In this review, we have summarized our psychosocial stress rodent model of PTSD which is based on well-described PTSD-inducing risk factors, including a life-threatening experience, a sense of horror and uncontrollability, and insufficient social support. Specifically, our animal model of PTSD integrates acute episodes of inescapable exposure of immobilized rats to a predator with chronic daily social instability. This stress regimen produces PTSD-like effects in rats at behavioral, cognitive, physiological, pharmacological and epigenetic levels of analysis. We have discussed a recent extension of our animal model of PTSD in which stress exacerbated coronary pathology following an ischemic event, assessed in vitro. In addition, we have reviewed our research investigating pharmacological and non-pharmacological therapeutic strategies which may have value in clinical approaches toward the treatment of traumatized people. Overall, our translational approach bridges the gap between human and animal PTSD research to create a framework with which to enhance our understanding of the biological basis of trauma-induced pathology and to assess therapeutic approaches in the treatment of psychopathology.

Stress as a one-armed bandit: Differential effects of stress paradigms on the morphology, neurochemistry and behavior in the rodent amygdala

Neuroplasticity may be defined as the ability of the central nervous system (CNS) to respond to changes in the internal and external environment and it is well established that some stimuli have the ability to facilitate or impair neuroplasticity depending on the pre-existing milieu. A classic example of a stimulus that can both facilitate and impair neuroplasticity is stress. Indeed, the ability of CNS to respond to acute stress is often dependent upon the prior stress history of the individual. While responses to acute stress are often viewed as adaptive in nature, stress reactivity in subjects with prior chronic stress experiences are often linked to neuropsychiatric disorders, including major depressive disorder, post-traumatic stress disorder (PTSD) and anxiety. In rodent studies, chronic stress exposure produces structural and functional alterations in the hippocampus and medial prefrontal cortex that are consistent across different types of stress paradigms. Conversely, the amygdala appears to exhibit differential structural and functional responses to stress that are dependent on a variety of factors, including the type of stressor performed and the duration of the stress paradigm. This is most evident in output measures including morphological analysis of amygdala neurons, measurement of glutamatergic tone in amygdalar subdivisions and the analysis of amygdala-centric behaviors. Accordingly, this review will provide an overview of the effects of stress on the structural and functional plasticity of the rodent amygdala, especially in relation to the differential effects of repeated or chronic stress paradigms on dendritic architecture, neurochemistry of the glutamatergic system and behavior.

Time-dependent effects of rapamycin on consolidation of predator stress-induced hyperarousal

Previous studies have indicated that rapamycin, a potent inhibitor of the mammalian target of rapamycin (mTOR) pathway, blocks consolidation of shock-induced associative fear memories. Moreover, rapamycin's block of associative fear memories is time-dependent. It is unknown, however, if rapamycin blocks consolidation of predator stress-induced non-associative fear memories. Furthermore, the temporal pattern of mTOR activation following predator stress is unknown. Thus, the goal of the current studies was to determine if rapamycin blocks consolidation of predator stress-induced fear memories and if so, whether rapamycin's effect is time-dependent. Male rats were injected systemically with rapamycin at various time points following predator stress. Predator stress involves an acute, unprotected exposure of a rat to a cat, which causes long-lasting non-associative fear memories manifested as generalized hyperarousal and increased anxiety-like behaviour. We show that rapamycin injected immediately after predator stress blocked consolidation of stress-induced startle. However, rapamycin injected 9, 24 or 48h post predator stress potentiated stress-induced startle. Consistent with shock-induced associative fear memories, we show that mTOR signalling is essential for consolidation of predator stress-induced hyperarousal. However, unlike shock-induced fear memories, a second, persistent, late phase mTOR-dependent process following predator stress actually dampens startle. Consistent with previous findings, our data support the potential role for rapamycin in treatment of stress related disorders such as posttraumatic stress disorder. However, our data suggest timing of rapamycin administration is critical.

Temporal characteristics of stress-induced decrease in benzodiazepine reception in C57BL/6 and BALB/c mice

We studied the duration of the drop of specific (3)H-flunitrazepam binding by synaptosomal membranes from the brain of C57Bl/6 and BALB/c mice after open-field and "contact with predator" tests. It was found that reduced benzodiazepine reception in BALB/c mice after open-field test persisted for 1.5 h, but no changes of this parameter were found in C57Bl/6 mice. After contact with predator, the binding capacity of the benzodiazepine site of GABAA receptor was reduced for 8 h in BALB/c mice and for 24 h in C57Bl/6 mice.

Distributed circuits underlying anxiety

Anxiety is of paramount importance for animals, as it allows assessment of the environment while minimizing exposure to potential threats. Furthermore, anxiety disorders are highly prevalent. Consequently, the neural circuitry underlying anxiety has been a topic of great interest. In this mini review, we will discuss current views on anxiety circuits. We will focus on rodent anxiety paradigms, but we will also consider results from human neuroimaging and clinical studies. We briefly review studies demonstrating the central role that the amygdala and the bed nucleus of the stria terminals (BNST) play in modulating anxiety and present evidence showing how the bed nucleus uses different output pathways to influence specific features of anxiolysis. Lastly, we propose that several brain regions, such as the medial prefrontal cortex (mPFC) and the ventral hippocampus (vHPC), act in a coordinated fashion with the amygdala and BNST, forming a distributed network of interconnected structures that control anxiety both in rodents and humans.

Inhibition of mTOR kinase via rapamycin blocks persistent predator stress-induced hyperarousal

Traumatic, stressful life events are thought to trigger acquired anxiety disorders such as post-traumatic stress disorder (PTSD). Recent data suggests that the mammalian target of rapamycin (mTOR) plays a key role in the formation of traumatic memories. The predator stress paradigm allows us to determine whether mTOR mediates the formation of both context-dependent (associative) and context-independent (non-associative) fear memories. Predator stress involves an acute, unprotected exposure of a rat to a cat which causes long-lasting non-associative fear memories manifested as generalized hyperarousal and increased anxiety-like behavior. Here, we show that rapamycin, an mTOR inhibitor, attenuates predator stress-induced hyperarousal, lasting at least three weeks. In addition, rapamycin blocks a subset of anxiety-like behaviors as measured in the elevated plus maze and hole board. Furthermore, when re-exposed to the predator stress context, rapamycin-treated stressed rats showed increased activity compared to vehicle controls suggesting that rapamycin blocks predator stress-induced associative fear memory. Taken together with past research, our results indicate that mTOR regulation of protein translation is required for the formation of both associative and non-associative fear memories. Overall, these data suggest that mTOR activation may contribute to the development of acquired anxiety disorders such as PTSD.

Translationally relevant modeling of PTSD in rodents

Post-traumatic stress disorder (PTSD) is clinically defined in DSM-4 by exposure to a significantly threatening and/or horrifying event and the presence of a certain number of symptoms from each of three symptom clusters at least one month after the event. Since humans clearly do not respond homogeneously to a potentially traumatic experience, the heterogeneity in animal responses might be regarded as confirming the validity of animal studies, rather than as representing a problem. A model of diagnostic criteria for psychiatric disorders could therefore be applied to animal responses to augment the validity of study data, providing that the criteria for classification are clearly defined, reliably reproducible and yield results that conform to findings in human subjects. The method described herein was developed in an attempt to model diagnostic criteria in terms of individual patterns of response by using behavioral measures and determining cut-off scores to distinguish between extremes of response or non-response, leaving a sizeable proportion of subjects in a middle group, outside each set of cut-off criteria. The cumulative results of our studies indicate that the contribution of animal models can be further enhanced by classifying individual animal study subjects according to their response patterns. The animal model also enables the researcher to go one step further and correlate specific anatomic, bio-molecular and physiological parameters with the degree and pattern of the individual behavioral response and introduces "prevalence rates" as a parameter. The translational value of the classification method and future directions are discussed.

A comparison of activation patterns of cells in selected prefrontal cortical and amygdala areas of rats which are more or less anxious in response to predator exposure or submersion stress

This study had two purposes. First: to compare predator and water submersion stress cFos activation in medial prefrontal cortices (mPFC) and the medial amygdala (MeA). Second: to identify markers of vulnerability to stressors within these areas. Rats were either predator or submersion stressed and tested 1.75 h later for anxiety. Immediately thereafter, rats were sacrificed and cFos expression was examined. Predator and submersion stress equally increased anxiety-like behavior in the elevated plus maze (EPM) and hole board. 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 stressed rats were considered stress non-responsive while MA stressed rats were considered stress responsive. Predator stress, but not submersion stress, activated MeA cFos. CFos expression of mPFC cells was elevated in LA rats and reduced in MA rats in predator stressed animals only, correlating negatively with anxiety. These findings are consistent with data implicating greater mPFC excitability in protection against the effects on affect of traumatic stress. The findings also suggest that this conclusion is stressor specific, applying to predator stress but not submersion stress. Both stressors have been suggested to model hyperarousal and comorbid anxiety aspects of PTSD in humans. Hence the use of these paradigms to identify brain bases of vulnerability and resilience to traumatic stress in PTSD has translation potential. On the other hand, our evidence of stressor specificity of vulnerability/resilience markers raises a caution. The data suggest that preclinical markers of vulnerability/resilience in a given stress paradigm are at best suggestive, and translational value must ultimately be confirmed in humans.

CRF receptor blockade prevents initiation and consolidation of stress effects on affect in the predator stress model of PTSD

Post traumatic stress disorder (PTSD) is a chronic anxiety disorder initiated by an intensely threatening, traumatic event. There is a great need for more efficacious pharmacotherapy and preventive treatments for PTSD. In animals, corticotropin-releasing factor (CRF) and the CRF1 receptor play a critical role in behavioural and neuroendocrine responses to stress. We tested the hypothesis that CRF1 activation is required for initiation and consolidation of long-term effects of trauma on anxiety-like behaviour in the predator exposure (predator stress) model of PTSD. Male C57BL6 mice were treated with the selective CRF1 antagonist CRA0450 (2, 20 mg/kg) 30 min before or just after predator stress. Long-term effects of stress on rodent anxiety were measured 7 d later using acoustic startle, elevated plus maze (EPM), light/dark box, and hole-board tests. Predator stress increased startle amplitude and delayed startle habituation, increased time in and decreased exits from the dark chamber in the light/dark box test, and decreased risk assessment in the EPM. CRF1 antagonism had limited effects on these behaviours in non-stressed controls, with the high dose decreasing risk assessment in the EPM. However, in stressed animals CRF1 antagonism blocked initiation and consolidation of stressor effects on startle, and returned risk assessment to baseline levels in predator-stressed mice. These findings implicate CRF1 activation in initiation and post-trauma consolidation of predator stress effects on anxiety-like behaviour, specifically on increased arousal as measured by exaggerated startle behaviours. These data support further research of CRF1 antagonists as potential prophylactic treatments for PTSD.

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.

Exposure to extreme stress impairs contextual odour discrimination in an animal model of PTSD

Post-traumatic stress disorder (PTSD) patients respond to trauma-related danger cues even in objectively safe environments as if they were in the original event, seemingly unable to adequately modulate their responses based on the contextual cues present. In order to model this inability to utilize contextualized memory, in an animal model of PTSD, a novel experimental paradigm of contextual cue processing was developed--the differential contextual odour conditioning (DCOC) paradigm--and tested in trauma-exposed animals and controls. In the DCOC paradigm, animals encountered cinnamon odour in both an aversive environment and a rewarding (safe) environment. Response (freezing) to cinnamon odour was tested in a third, neutral environment to examine the ability of animals to modulate their responses based on the contextual cues. The effect of exposure to traumatic stressors, e.g. predator scent stress (PSS) and underwater trauma (UWT), on contextual cue discrimination was assessed. Rats trained in the DCOC paradigm acquired the ability to modulate their behavioural responses to odour cue based on contextual cues signalling safe vs. dangerous environment. The PSS and UWT stressors abolished the ability to modulate their responses based on contextual cues, both when exposure preceded DCOC training, and when it followed successfully completed training. The DCOC paradigm offers a promising model for studying the neurobiological basis of contextual modulation of response to potential threat in animals, a process that is disrupted by exposure to severe stress/trauma, and thus might be particularly salient for the study of PTSD.

Neurotoxic lesion of the rostral perirhinal cortex blocks stress-induced exploratory behavioral changes in male rats

Exposure to stress leads to adaptive responses including both behavioral and physiological changes. This process is induced by the activation of multiple brain regions. The present study examined the role of the rostral perirhinal cortex (rPRh) in behavioral changes following electrical foot shock-induced stress. The rPRh of rats was lesioned bilaterally by local microinjections of 10 microg N-methyl-D-aspartic acid (NMDA) before foot shocks (0.7 mA, 1 s). The effects of these lesions on foot shock-induced changes in exploratory behaviors were tested in the open field (4 h, 48 h, 72 h, and 14 days after foot shocks) and the light-dark box (7 days after foot shocks). Foot-shocked and sham-lesioned rats showed several well known behavioral changes in the open field (e.g., immobility, reduction of exploratory activity) most marked at 48 h after foot shocks, and the light-dark box (e.g., reduction of time spent and activity in the lit compartment). All these stress-induced behavioral changes were blocked by neurotoxic lesions of the rPRh. Furthermore, rPRh lesions did not affect behavior in the open field and the light-dark box in unstressed rats. Taken together, these data indicate that the rPRh is involved in neurophysiological mechanisms that mediate changes induced by foot-shock stress in exploratory behaviors which indicate unconditioned fear or anxiety.

Acute and chronic effects of ferret odor exposure in Sprague-Dawley rats

This manuscript describes several behavioral and functional studies evaluating the capacity of ferret odors to elicit a number of acute and long-term responses in male Sprague-Dawley rats. Acute presentation elicits multiple responses, suggesting that ferret odor, likely from skin gland secretions, provides an anxiogenic-like stimulus in this strain of rats. Compared to cat odor, however, ferret odor did not produce rapid fear conditioning, a result perhaps attributable to methodological factors. Inactivation of the olfactory system and medial nucleus of the amygdala, combined with induction of the immediate-early gene c-fos, suggest the necessity of the accessory olfactory system in mediating the effects of ferret odor. Repeated exposures to ferret odor produce variable habituation of neuroendocrine and behavioral responses, perhaps indicative of the lack of control over the exact individual origin or concentration of ferret odor. Ferret odor induces rapid and long-term body weight regulation, thymic involution, adrenal hyperplasia and facilitation of the neuroendocrine response to additional challenges. It is argued that the use of such odors is exquisitely suited to investigate the brain regions coordinating anxiety-like responses and the long-term changes elicited by such stimuli.

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 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.

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.

Prolonged stress will induce Alzheimer’s disease in elderly people by increased release of homocysteic acid

Recently, many papers have reported the physiological functions of amyloid beta and amyloid precursor protein (APP). In particular, one of its functions is of importance for synaptic plasticity. Extracellular amyloid beta may suppress synaptic plasticity or inhibit long-term potentiation (LTP) from outside the cell. LTP is now considered the molecular basis of memory. Amyloid beta may induce the inhibition or loss of memory. We propose that suppression of LTP by amyloid beta induces a kind of physiological forgetfulness. On the other hand, homocysteic acid (HA) which is released from astrocytes under stress conditions accumulates the amyloid beta into neuronal cell, which consequently induces the inhibition of amyloid beta physiological function and induces strong LTP. We propose that HA induces strong unforgetful memory under stress condition such as PTSD and emotional depression. The situation is different in the elderly people. Prolonged stress in the elderly people may induce neurodegenerative diseases such as Alzheimer's disease. We observed that in the presence of excess methionine, HA induced alpha-synuclein protein in cultured cells, suggesting a hypermethylation model in vivo. Usually hypermethylation is observed in the ageing process. We have shown that HA promotes the accumulation of amyloid beta in cells, and that the production of alpha-synuclein, which induces the aggregation of amyloid beta, impairing the cell function. LTP is inhibited by deficient cellular function, which means that memories cannot be formed. In fact, there is confusion of memories in the early stages of Alzheimer's disease. Finally, the aggregated alpha-synuclein induces tau pathology, which induces cell death, leading to Alzheimer's pathology. In conclusion, we propose that HA induces Alzheimer's pathology in the elderly people because of prolonged stress.

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.

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.