The temporal dynamics model of emotional memory processing: a synthesis on the neurobiological basis of stress-induced amnesia, flashbulb and traumatic memories, and the Yerkes-Dodson law

Cycloheximide impairs and enhances memory depending on dose and footshock intensity

This experiment examined the effects on memory of interactions of cycloheximide dose and training foot shock intensity. Mice received injections of cycloheximide (120 mg/kg, s.c.) or saline 30 min prior to inhibitory avoidance training with shock intensities of 100, 150, 250 or 300 μA (1 s duration). Memory was tested 48 h later. The saline control mice showed increasing memory latencies as a function of shock intensity. The ability of cycloheximide to impair memory increased as the training shock intensity increased. In a second experiment, mice were trained with a 200 μA (1 s duration) shock and received injections of saline or cycloheximide at one of several doses (30, 60 or 120 mg/kg). Under these training conditions, cycloheximide enhanced memory in an inverted-U dose-response manner. These findings are consistent with prior findings suggesting that protein synthesis inhibitors act on memory by altering modulators of memory formation as a secondary consequence of the inhibition of protein synthesis rather than by interfering with training-initiated synthesis of proteins required for memory formation.

Fear memory for cue and context: opposite and time-dependent effects of a physiological dose of corticosterone in male BALB/c and C57BL/6J mice

Highly emotional, stress reactive BALB/c mice secrete more corticosterone in response to fear conditioning than the low stress reactive C57BL/6J mice. Fear memory to cue and context differs between the strains. We injected corticosterone at physiological concentrations (250 μg/kg i.p.) 30 min before fear conditioning. Fear memory was tested 48 and 72 h later. Although corticosterone had little effect on acquisition, it differentially affected fear memories in strain dependent manner: while BALB/c mice decreased freezing during cue and context episodes, C57BL/6J mice showed an overall increase in freezing. BALB/c mice showed extinction over days while no such extinction was seen in C57BL/6J mice. Evaluation of these data in the perspective of previous studies using the same fear conditioning paradigm with corticosterone injections 5 min before or immediately after acquisition, revealed the impact of corticosterone during conditioning on the strength of fear memories. In C57BL/6J mice the overall increase in fear memories was higher if corticosterone was injected 30 min pre acquisition than if injected 5 min pre. In contrast, BALB/c mice showed reduced fear memories when injected 30 min pre compared to that seen 5 min pre acquisition. Both strains showed decreased fear memories compared to vehicle if corticosterone was administered immediately after acquisition. We conclude that the timing of physiologically relevant, stress levels increase in corticosterone is essential for the processing of aversive events and the formation of fear memories. However, the quality of the effect depends on the genetic background. These findings contribute to the understanding of the etiology of stress-related disorders.

Immediate recall influences the effects of pre-encoding stress on emotional episodic long-term memory consolidation in healthy young men

The stress-associated activation of the hypothalamus-pituitary-adrenal axis influences memory. Several studies have supported the notion that post-learning stress enhances memory consolidation, while pre-retrieval stress impairs retrieval. Findings regarding the effects of pre-encoding stress, in contrast, have been rather inconsistent. In the current two studies, the impact of an immediate retrieval task on these effects was explored. In the first study, 24 healthy young male participants were exposed to a psychosocial laboratory stressor (Trier Social Stress Test) or a control condition before viewing positive, negative, and neutral photographs, which were accompanied by a brief narrative. Immediate as well as delayed (24 h later) free recall was assessed. Stress was expected to enhance emotional long-term memory without affecting immediate recall performance. Stress caused a significant increase in salivary cortisol concentrations but had no significant effects on immediate or delayed retrieval performance, even though a trend toward poorer memory of the stress group was apparent. Based on these findings, the second experiment tested the hypothesis that the beneficial effects of stress on emotional long-term memory performance might be abolished by an immediate recall test. In the second study (n = 32), the same design was used, except for the omission of the immediate retrieval test. This time stressed participants recalled significantly more negative photographs compared to the control group. The present study indicates that an immediate retrieval attempt of material studied after stress exposure can prevent or even reverse the beneficial effects of pre-encoding stress on emotional long-term memory consolidation.

Cognitive behavioral hypnotherapy for dissociative disorders

Dissociative disorders (DD) prevail as sequelae to overwhelming experiences in childhood. These readily formed post-traumatic responses and trance states develop in high hypnotizable subjects whose dysregulations become organized into ego states. A cognitive behavioral hypnotherapeutic treatment model will effectively contain, explore, metabolize, and resolve these life-endangering conditions. This article will detail the cognitive hypnotic world of DD patients, the relational spaces of the ego states, and the triphasic treatment mode to successfully resolve the dissociative pathology. Structured and phase appropriate hypnotic interventions will be described.

When fear forms memories: threat of shock and brain potentials during encoding and recognition

The anticipation of highly aversive events is associated with measurable defensive activation, and both animal and human research suggests that stress-inducing contexts can facilitate memory. Here, we investigated whether encoding stimuli in the context of anticipating an aversive shock affects recognition memory. Event-related potentials (ERPs) were measured during a recognition test for words that were encoded in a font color that signaled threat or safety. At encoding, cues signaling threat of shock, compared to safety, prompted enhanced P2 and P3 components. Correct recognition of words encoded in the context of threat, compared to safety, was associated with an enhanced old-new ERP difference (500-700 msec; centro-parietal), and this difference was most reliable for emotional words. Moreover, larger old-new ERP differences when recognizing emotional words encoded in a threatening context were associated with better recognition, compared to words encoded in safety. Taken together, the data indicate enhanced memory for stimuli encoded in a context in which an aversive event is merely anticipated, which could assist in understanding effects of anxiety and stress on memory processes.

Culture and Posttraumatic Stress Disorder (PTSD): A Proposed Conceptual Framework

It is widely accepted that the understanding of any psychological disorder needs to be contextualised within cultural parameters. The notion that current diagnostic taxonomies are not always universally applicable does not mean that all symptoms are not applicable. We present a framework that is aimed at being a starting point from which to delineate universal and culture specific elements of PTSD. The framework follows the possible influence of cultural factors on (a) the formation of an intrusive memory, (b) an understanding of how such a memory becomes pathological, and (c) how symptoms are expressed from the intrusive memory core of PTSD. While the framework presents certain elements (e.g. intrusive memory and core schemas) as centrally important, the focus is on a heuristic framework that allows for the study of the dynamic interaction between potentially universal and cultural factors and how this interaction may produce the symptom profile generally seen with PTSD. Tentative recommendations are made for a research agenda and are presented after a brief exploration of the strengths and weaknesses of such a framework.

D₁ receptor activation improves vigilance in rats as measured by the 5-choice continuous performance test

RATIONALE

Impaired attention/vigilance is putatively core to schizophrenia. The dopaminergic D(1) receptor system has been reported as one of the most promising targets for improving cognition in patients with schizophrenia, with some evidence suggesting D(1) activation may improve sustained attention.

OBJECTIVES

The purpose of this study was twofold: firstly assessing the applicability of using rats in the 5-Choice Continuous Performance Test (5 C-CPT), recently validated in mice. Secondly, the effect of systemic administration of a D(1) partial agonist, SKF 38393, on task performance during baseline, and a challenge session consisting of a reduced event-rate was investigated.

METHODS

Animals were trained to perform the 5 C-CPT with performance assessed following systemic SKF 38393 (2, 4 and 6 mg/kg) vs. vehicle administration.

RESULTS

Rats could discriminate between target (requiring a response) and non-target (requiring the inhibition of response) trials within the 5 C-CPT. Moreover, SKF 38393 treatment impaired performance during the baseline session reducing target detection, yet improved performance during the reduced event-rate challenge session, increasing target detection and improving signal discrimination indicating an SKF 38393-induced enhancement of vigilance. Thus, these data suggest that activation of the D(1) system affected 5 C-CPT performance in a baseline dependent manner.

CONCLUSION

Rats can be trained to perform the 5 C-CPT, appropriately withholding from responding to non-target trials. Systemic administration of SKF 38393 impaired performance during baseline conditions. Following a task-related challenge, which reduced the event rate, activation of the dopamine (DA) D(1) system improved performance by heightening the animals' vigilance levels, quantified using signal detection theory.

The fade-in--short stimulation--fade out approach to sham tDCS--reliable at 1 mA for naïve and experienced subjects, but not investigators

OBJECTIVE

Slowly ramping down initial current intensity after a minimal interval of stimulation is the de facto standard for sham stimulation in transcranial electrical stimulation research. The aim of this study is to further investigate the effectiveness of this method of blinding.

METHODS

We have investigated the time course of the cutaneous perception during 10 min of anodal, cathodal, and sham transcranial direct current stimulation, probing the perceived strength and site of the perceived sensation. We have also utilized post-stimulation assessment and measurements of sleepiness prior to and after the intervention. Previous exposure to tDCS has also been taken into account: the experiment has been repeated in naïve and experienced subject groups, and a group consisting of investigators who use tDCS as a research tool.

RESULTS

Although we have observed a general reduction in the perceived strength of the stimulation with time, we have not found the complete disappearance of the cutaneous perception during either the verum or the sham conditions. Experienced subjects were more likely to be able to differentiate between trials with stimulation and non-stimulation trials and to correctly identify sham and verum stimulation conditions.

CONCLUSION

When taking only naïve and experienced subjects into account, there was no significant difference between the strength of the perceived stimulation in the verum and sham conditions. The fade-in - short stimulation - fade-out sham stimulation can be indistinguishable from verum stimulation, but not because it mimics the disappearance of the cutaneous sensations associated with the verum stimulation, but because these sensations persist also in the sham stimulation. The significance of this finding with potential confounding factors and limitations are discussed.

Individual development and evolution: experiential canalization of self-regulation

In this article, we contrast evolutionary and psychobiological models of individual development to address the idea that individual development occurring in prototypically risky and unsupportive environments can be understood as adaptation. We question traditional evolutionary explanations of individual development, calling on the principle of probabilistic epigenesis to suggest that individual development resulting from the combined activity of genes and environments is best understood to precede rather than follow from evolutionary change. Specifically, we focus on the ways in which experience shapes the development of stress response physiology, with implications for individual development and intergenerational transmission of reactive, as opposed to reflective, phenotypes. In doing so, we describe results from several analyses conducted with a longitudinal data set of 1,292 children and their primary caregivers followed from birth. Our results indicate that the effects of poverty on stress response physiology and on the development of the self-regulation of behavior represent instances of the experiential canalization of development with implications for understanding the genesis and "adaptiveness" of risk behavior.

Deficient plasticity in the hippocampus and the spiral of addiction: focus on adult neurogenesis

Addiction is a complex neuropsychiatric disorder which causes disruption at multiple levels, including cognitive, emotional, and behavioral domains. Traditional biological theories of addiction have focused on the mesolimbic dopamine pathway and the nucleus accumbens as anatomical substrates mediating addictive-like behaviors. More recently, we have begun to recognize the engagement and dynamic influence of a much broader circuitry which encompasses the frontal cortex, the amygdala, and the hippocampus. In particular, neurogenesis in the adult hippocampus has become a major focus of attention due to its ability to influence memory, motivation, and affect, all of which are disrupted in addiction. First, I summarize toxicological data that reveal strongly suppressive effects of drug exposure on adult hippocampal neurogenesis. Then, I discuss the impact of deficient neurogenesis on learning and memory function, stress responsiveness and affective behavior, as they relate to addiction. Finally, I examine recent behavioral observations that implicate neurogenesis in the adult hippocampus in the emergence and maintenance of addictive behavior. The evidence reviewed here suggests that deficient neurogenesis is associated with several components of the downward spiraling loop that characterizes addiction, including elevated sensitivity to drug-induced reward and reinforcement, enhanced neurohormonal responsiveness, emergence of a negative affective state, memory impairment, and inflexible behavior.

Stress sensitizes the brain: increased processing of unpleasant pictures after exposure to acute stress

A key component of acute stress is a surge in vigilance that enables a prioritized processing of highly salient information to promote the organism's survival. In this study, we investigated the neural effects of acute stress on emotional picture processing. ERPs were measured during a deep encoding task, in which 40 male participants categorized 50 unpleasant and 50 neutral pictures according to arousal and valence. Before picture encoding, participants were subjected either to the Socially Evaluated Cold Pressor Test (SECPT) or to a warm water control procedure. The exposure to the SECPT resulted in increased subjective and autonomic (heart rate and blood pressure) stress responses relative to the control condition. Viewing of unpleasant relative to neutral pictures evoked enhanced late positive potentials (LPPs) over centro-parietal scalp sites around 400 msec after picture onset. Prior exposure to acute stress selectively increased the LPPs for unpleasant pictures. Moreover, the LPP magnitude for unpleasant pictures following the SECPT was positively associated with incidental free recall performance 24 hr later. The present results suggest that acute stress sensitizes the brain for increased processing of cues in the environment, particularly priming the processing of unpleasant cues. This increased processing is related to later long-term memory performance.

Allostasis and allostatic load in the context of poverty in early childhood

This paper examined the relation of early environmental adversity associated with poverty to child resting or basal level of cortisol in a prospective longitudinal sample of 1135 children seen at 7, 15, 24, 35, and 48 months of age. We found main effects for poor housing quality, African American ethnicity, and low positive caregiving behavior in which each was uniquely associated with an overall higher level of cortisol from age 7 to 48 months. We also found that two aspects of the early environment in the context of poverty, adult exits from the home and perceived economic insufficiency, were related to salivary cortisol in a time-dependent manner. The effect for the first of these, exits from the home, was consistent with the principle of allostatic load in which the effects of adversity on stress physiology accumulate over time. The effect for perceived economic insufficiency was one in which insufficiency was associated with higher levels of cortisol in infancy but with a typical but steeper decline in cortisol with age at subsequent time points.

Dose-dependent effects of endotoxin on neurobehavioral functions in humans

Clinical and experimental evidence document that inflammation and increased peripheral cytokine levels are associated with depression-like symptoms and neuropsychological disturbances in humans. However, it remains unclear whether and to what extent cognitive functions like memory and attention are affected by and related to the dose of the inflammatory stimulus. Thus, in a cross-over, double-blind, experimental approach, healthy male volunteers were administered with either placebo or bacterial lipopolysaccharide (LPS) at doses of 0.4 (n = 18) or 0.8 ng/kg of body weight (n = 16). Pro- and anti-inflammatory cytokines, norephinephrine and cortisol concentrations were analyzed before and 1, 1.75, 3, 4, 6, and 24 h after injection. In addition, changes in mood and anxiety levels were determined together with working memory (n-back task) and long term memory performance (recall of emotional and neutral pictures of the International Affective Picture System). Endotoxin administration caused a profound transient physiological response with dose-related elevations in body temperature and heart rate, increases in plasma interleukin (IL)-6, IL-10, tumor necrosis factor (TNF)-α and IL-1 receptor antagonist (IL-1ra), salivary and plasma cortisol, and plasma norepinephrine. These changes were accompanied by dose-related decreased mood and increased anxiety levels. LPS administration did not affect accuracy in working memory performance but improved reaction time in the high-dose LPS condition compared to the control conditon. In contrast, long-term memory performance was impaired selectively for emotional stimuli after administration of the lower but not of the higher dose of LPS. These data suggest the existence of at least two counter-acting mechanisms, one promoting and one inhibiting cognitive performance during acute systemic inflammation.

The potential contribution of stress systems to the transition to chronic whiplash-associated disorders

STUDY DESIGN

A narrative description highlighting preclinical and clinical evidence that physiologic stress systems contribute to whiplash-associated disorders (WAD) pathogenesis.

OBJECTIVE

To present several lines of evidence supporting the hypothesis that physiologic stress systems contribute to WAD pathogenesis.

SUMMARY OF BACKGROUND DATA

In addition to subjecting soft tissue to biomechanical strain, a motor vehicle collision (MVC) event is also an acute stressor which activates physiologic stress systems. Increasing data from animal and human studies suggest that the activation of these stress systems may contribute to long-lasting changes in pain sensitivity after tissue injury.

METHODS

Nonsystematic review of several lines of evidence that together suggest that physiologic systems involved in the stress response may contribute to the development of WAD.

RESULTS

Stress systems which appear capable of producing hyperalgesia and allodynia include catecholaminergic systems, serotonin systems, and the hypothalamic-pituitary-adrenocortical system. Evidence for the role of these systems comes, in part, from studies examining the association between genetic variants and chronic pain outcomes. For example, in a recent study of acute neck pain after MVC, patients with certain genotypes of an enzyme involved in catecholamine metabolism were more than twice as likely to report moderate or severe neck pain in the emergency department. Such pain vulnerability because of stress system function may interact with the effects of biomechanical injury and psychobehavioral responses to influence the development of WAD.

CONCLUSION

More research examining the influence of stress systems on WAD are needed. If these systems do influence WAD outcomes, then treatments which diminish the adverse effects of stress systems may be a useful component of multimodal therapeutic interventions for individuals at risk of chronic pain development after MVC.

The stressed synapse: the impact of stress and glucocorticoids on glutamate transmission

Mounting evidence suggests that acute and chronic stress, especially the stress-induced release of glucocorticoids, induces changes in glutamate neurotransmission in the prefrontal cortex and the hippocampus, thereby influencing some aspects of cognitive processing. In addition, dysfunction of glutamatergic neurotransmission is increasingly considered to be a core feature of stress-related mental illnesses. Recent studies have shed light on the mechanisms by which stress and glucocorticoids affect glutamate transmission, including effects on glutamate release, glutamate receptors and glutamate clearance and metabolism. This new understanding provides insights into normal brain functioning, as well as the pathophysiology and potential new treatments of stress-related neuropsychiatric disorders.

Corticosteroid induced decoupling of the amygdala in men

The amygdala is a key regulator of vigilance and heightens attention toward threat. Its activity is boosted upon threat exposure and contributes to a neuroendocrine stress response via the hypothalamic-pituitary-adrenal (HPA) axis. Corticosteroids are known to control brain activity as well as HPA activity by providing negative feedback to the brain. However, it is unknown how corticosteroids affect the neural circuitry connected to the amygdala. Implementing a randomized, double-blind, placebo-controlled design, we here investigated the effects of 10-mg hydrocortisone on amygdala-centered functional connectivity patterns in men using resting state functional magnetic resonance imaging. Results showed generally decreased functional connectivity of the amygdala by corticosteroids. Hydrocortisone reduced "positive" functional coupling of the amygdala to brain regions involved in the initiation and maintenance of the stress response; the locus coeruleus, hypothalamus, and hippocampus. Furthermore, hydrocortisone reduced "negative" functional coupling of the amygdala to the middle frontal and temporal gyrus; brain regions known to be involved in executive control. A control analysis did not show significant corticosteroid modulation of visual cortex coupling, indicating that the amygdala decoupling was not reflecting a general reduction of network connectivity. These results suggest that corticosteroids may reduce amygdala's impact on brain processing in the aftermath of stress in men.

Glucocorticoid receptors in the prefrontal cortex regulate stress-evoked dopamine efflux and aspects of executive function

Enhanced dopamine efflux in the prefrontal cortex is a well-documented response to acute stress. However, the underlying mechanism(s) for this response is unknown. Using in vivo microdialysis, we demonstrate that blocking glucocorticoid receptors locally within the rat prefrontal cortex results in a reduction in stress-evoked dopamine efflux. In contrast, blocking glucocorticoid receptors in the ventral tegmental area did not affect stress-evoked dopamine efflux in the prefrontal cortex. Additionally, local administration of corticosterone into the prefrontal cortex increased prefrontal dopamine efflux. The functional impact of enhanced dopamine efflux evoked by acute stress was demonstrated using a cognitive task dependent on the prefrontal cortex and sensitive to impairment in working memory. Notably, stress-induced impairments in cognition were attenuated by blockade of glucocorticoid receptors in the prefrontal cortex. Taken together, these data demonstrate that glucocorticoids act locally within the prefrontal cortex to modulate mesocortical dopamine efflux leading to the cognitive impairments observed during acute stress.

Salivary cortisol mediates effects of poverty and parenting on executive functions in early childhood

In a predominantly low-income population-based longitudinal sample of 1,292 children followed from birth, higher level of salivary cortisol assessed at ages 7, 15, and 24 months was uniquely associated with lower executive function ability and to a lesser extent IQ at age 3 years. Measures of positive and negative aspects of parenting and household risk were also uniquely related to both executive functions and IQ. The effect of positive parenting on executive functions was partially mediated through cortisol. Typical or resting level of cortisol was increased in African American relative to White participants. In combination with positive and negative parenting and household risk, cortisol mediated effects of income-to-need, maternal education, and African American ethnicity on child cognitive ability.

How much is residents' distress detection performance during a clinical round related to their characteristics?

OBJECTIVE

The aim of this study was to investigate residents' characteristics associated with their performance in detecting patients' distress (detection performance).

METHODS

Residents' detection performance was assessed in a clinical round. A mean detection performance score was calculated for each resident by comparing residents' rating of patients' distress (VAS) with patients' reported distress (HADS). Residents' characteristics include general (socio-demographic, professional and psychological), detection (self-efficacy, attitudes and outcome expectancies) and performance characteristics (communication skills (LaComm), psychological arousal (STAI) and physiological arousal (heart rate and blood pressure) in a highly emotional and complex simulated interview task).

RESULTS

Ninety-four residents and 442 inpatients were included. 30% of the variance in residents' detection performance was related to residents' performance characteristics: anxiety level (p=.040) and mean arterial blood pressure (p=.019) before the task; empathy (p=.027) and mean heart rate (p=.043) during the task; mean arterial blood pressure changes (p=.012) during the assessment procedure.

CONCLUSION

Residents' detection performance is partly related to their performance characteristics. Psychological and physiological arousals are key characteristics--beside empathic skills--that need to be considered in models designed to determine detection performance.

PRACTICE IMPLICATIONS

Future interventions designed to improve residents' detection performance should focus notably on their performance characteristics.

The Promotion of Self-Regulation as a Means of Enhancing School Readiness and Early Achievement in Children at Risk for School Failure

This article reviews the literature on self-regulation and the development of school readiness and academic competence in early childhood. It focuses on relations between the development of cognitive aspects of regulation-referred to as executive functions and defined as abilities used to regulate information and to organize thinking in goal-directed activities-and the development of reactivity and regulation in stimulus-driven emotion, attention, and physiological stress response systems. It examines a bidirectional model of cognition-emotion interaction in the development of self-regulation in which top-down executive control of thought and behavior develops in reciprocal and interactive relation to bottom-up influences of emotion and stress reactivity. The bidirectional model is examined within the context of innovative preschool interventions designed to promote school readiness by promoting the development of self-regulation.

Stress increases behavioral resistance to extinction

Behavioral persistence is required to reach a goal but may impede adaptations to changing environments. Given the well-documented effects of stress on learning and memory processes, we asked here whether stress affects the persistence of behavior. Participants were exposed to stress or a control condition before they learned an instrumental action to gain a food reward. During learning, we presented several extinction blocks in which the food reward was not presented. Stress rendered participants' responding shortly after initial learning insensitive to the extinction procedure. Overall learning curves remained unaffected. Thus, the present findings suggest that stress increases the resistance of behavior to extinction. The cause of the behavioral persistence after stress may be its habitual form.

Noradrenergic-glucocorticoid modulation of emotional memory encoding in the human hippocampus

BACKGROUND

Current rodent models emphasize the joint action of the stress mediators noradrenaline (NE) and cortisol (CORT) in conferring a memory advantage of emotional over neutral stimuli.

METHOD

Using a pharmacological strategy of tackling this stress-related mechanism to enhance human episodic (autobiographical) memory, we measured amygdala-hippocampal responses during encoding of emotional and neutral stimuli with functional magnetic resonance imaging in 51 healthy subjects under four pharmacological conditions in a double-blind parallel group design: (i) placebo; (ii) the NE-reuptake inhibitor reboxetine (4 mg); (iii) hydrocortisone (synthetic CORT) (30 mg); or (iv) both agents in combination.

RESULTS

Differential drug effects were found in the left hippocampus, whereas hydrocortisone alone selectively decreased hippocampal responses to emotional relative to neutral stimuli, reboxetine potentiated hippocampal responses to these stimuli. Importantly, the inhibitory influence of hydrocortisone was reversed by co-administration of reboxetine.

CONCLUSIONS

Our results imply that stress levels of CORT alone attenuate hippocampal responses to emotional stimuli, an effect possibly related to a regulatory negative feedback loop. However, when simultaneously elevated to stress levels, NE and CORT act together to synergistically enhance hippocampal activity during encoding of emotional stimuli, a mechanism that may turn maladaptive under circumstances of traumatic stress.

A review on animal models for screening potential anti-stress agents

Stress is a state of threatened homeostasis that produces different physiological as well as pathological changes depending on severity, type and duration of stress. The animal models are pivotal for understanding the pathophysiology of stress-induced behavioral alterations and development of effective therapy for its optimal management. A battery of models has been developed to simulate the clinical pain conditions with diverse etiology. An ideal animal model should be able to reproduce each of the aspects of stress response and should be able to mimic the natural progression of the disease. The present review describes the different types of acute and chronic stress models including immersion in cold water with no escape, cold environment isolation, immobilization/restraint-induced stress, cold-water restraint stress, electric foot shock-induced stress, forced swimming-induced stress, food-deprived activity stress, neonatal isolation-induced stress, predatory stress, day-night light change-induced stress, noise-induced stress, model of post-traumatic stress disorder and chronic unpredictable stress models.

Towards a glutamate hypothesis of depression: an emerging frontier of neuropsychopharmacology for mood disorders

Half a century after the first formulation of the monoamine hypothesis, compelling evidence implies that long-term changes in an array of brain areas and circuits mediating complex cognitive-emotional behaviors represent the biological underpinnings of mood/anxiety disorders. A large number of clinical studies suggest that pathophysiology is associated with dysfunction of the predominant glutamatergic system, malfunction in the mechanisms regulating clearance and metabolism of glutamate, and cytoarchitectural/morphological maladaptive changes in a number of brain areas mediating cognitive-emotional behaviors. Concurrently, a wealth of data from animal models have shown that different types of environmental stress enhance glutamate release/transmission in limbic/cortical areas and exert powerful structural effects, inducing dendritic remodeling, reduction of synapses and possibly volumetric reductions resembling those observed in depressed patients. Because a vast majority of neurons and synapses in these areas and circuits use glutamate as neurotransmitter, it would be limiting to maintain that glutamate is in some way 'involved' in mood/anxiety disorders; rather it should be recognized that the glutamatergic system is a primary mediator of psychiatric pathology and, potentially, also a final common pathway for the therapeutic action of antidepressant agents. A paradigm shift from a monoamine hypothesis of depression to a neuroplasticity hypothesis focused on glutamate may represent a substantial advancement in the working hypothesis that drives research for new drugs and therapies. Importantly, despite the availability of multiple classes of drugs with monoamine-based mechanisms of action, there remains a large percentage of patients who fail to achieve a sustained remission of depressive symptoms. The unmet need for improved pharmacotherapies for treatment-resistant depression means there is a large space for the development of new compounds with novel mechanisms of action such as glutamate transmission and related pathways. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Stress effects on memory: an update and integration

It is well known that stressful experiences may affect learning and memory processes. Less clear is the exact nature of these stress effects on memory: both enhancing and impairing effects have been reported. These opposite effects may be explained if the different time courses of stress hormone, in particular catecholamine and glucocorticoid, actions are taken into account. Integrating two popular models, we argue here that rapid catecholamine and non-genomic glucocorticoid actions interact in the basolateral amygdala to shift the organism into a 'memory formation mode' that facilitates the consolidation of stressful experiences into long-term memory. The undisturbed consolidation of these experiences is then promoted by genomic glucocorticoid actions that induce a 'memory storage mode', which suppresses competing cognitive processes and thus reduces interference by unrelated material. Highlighting some current trends in the field, we further argue that stress affects learning and memory processes beyond the basolateral amygdala and hippocampus and that stress may pre-program subsequent memory performance when it is experienced during critical periods of brain development.

Epigenetic modification of hippocampal Bdnf DNA in adult rats in an animal model of post-traumatic stress disorder

Epigenetic alterations of the brain-derived neurotrophic factor (Bdnf) gene have been linked with memory, stress, and neuropsychiatric disorders. Here we examined whether there was a link between an established rat model of post-traumatic stress disorder (PTSD) and Bdnf DNA methylation. Adult male Sprague-Dawley rats were given psychosocial stress composed of two acute cat exposures in conjunction with 31 days of daily social instability. These manipulations have been shown previously to produce physiological and behavioral sequelae in rats that are comparable to symptoms observed in traumatized people with PTSD. We then assessed Bdnf DNA methylation patterns (at exon IV) and gene expression. We have found here that the psychosocial stress regimen significantly increased Bdnf DNA methylation in the dorsal hippocampus, with the most robust hypermethylation detected in the dorsal CA1 subregion. Conversely, the psychosocial stress regimen significantly decreased methylation in the ventral hippocampus (CA3). No changes in Bdnf DNA methylation were detected in the medial prefrontal cortex or basolateral amygdala. In addition, there were decreased levels of Bdnf mRNA in both the dorsal and ventral CA1. These results provide evidence that traumatic stress occurring in adulthood can induce CNS gene methylation, and specifically, support the hypothesis that epigenetic marking of the Bdnf gene may underlie hippocampal dysfunction in response to traumatic stress. Furthermore, this work provides support for the speculative notion that altered hippocampal Bdnf DNA methylation is a cellular mechanism underlying the persistent cognitive deficits which are prominent features of the pathophysiology of PTSD.

Influence of Pre-Training Predator Stress on the Expression of c-fos mRNA in the Hippocampus, Amygdala, and Striatum Following Long-Term Spatial Memory Retrieval

We have studied the influence of pre-training psychological stress on the expression of c-fos mRNA following long-term spatial memory retrieval. Rats were trained to learn the location of a hidden escape platform in the radial-arm water maze, and then their memory for the platform location was assessed 24 h later. Rat brains were extracted 30 min after the 24-h memory test trial for analysis of c-fos mRNA. Four groups were tested: (1) Rats given standard training (Standard); (2) Rats given cat exposure (Predator Stress) 30 min prior to training (Pre-Training Stress); (3) Rats given water exposure only (Water Yoked); and (4) Rats given no water exposure (Home Cage). The Standard trained group exhibited excellent 24 h memory which was accompanied by increased c-fos mRNA in the dorsal hippocampus and basolateral amygdala (BLA). The Water Yoked group exhibited no increase in c-fos mRNA in any brain region. Rats in the Pre-Training Stress group were classified into two subgroups: good and bad memory performers. Neither of the two Pre-Training Stress subgroups exhibited a significant change in c-fos mRNA expression in the dorsal hippocampus or BLA. Instead, stressed rats with good memory exhibited significantly greater c-fos mRNA expression in the dorsolateral striatum (DLS) compared to stressed rats with bad memory. This finding suggests that stressed rats with good memory used their DLS to generate a non-spatial (cue-based) strategy to learn and subsequently retrieve the memory of the platform location. Collectively, these findings provide evidence at a molecular level for the involvement of the hippocampus and BLA in the retrieval of spatial memory and contribute novel observations on the influence of pre-training stress in activating the DLS in response to long-term memory retrieval.

Critical period exists in the effects of isolation rearing on sensorimotor gating function but not locomotor activity in rat

Isolation-reared (IR) rats exhibit various cognitive and behavioral abnormalities in adulthood of which locomotor hyperactivity and impaired prepulse inhibition (PPI) of an acoustic startle reflex are the two cardinal characteristics. Using an amended social deprivation-resocialization paradigm, the present study examined the role of the developmental specificity of the effects of IR and its interaction with the non-competitive NMDA receptor antagonist, MK-801. Locomotor activity and PPI were measured in three groups of adult rats: social control, IR throughout life, and IR for the first two weeks after weaning followed by social housing. MK-801 was injected intraperitoneally (i.p.) 30 min prior to testing at doses of 0, 0.02, 0.1, and 0.2 mg/kg. The results revealed that locomotor activity increased only in rats in the IR throughout life group but not in the other two groups. The impairment of PPI was seen in rats in the IR for the first two weeks and the IR throughout life groups. Furthermore, the effect of MK-801 on PPI was bidirectional in rats of the IR for the first two weeks and IR throughout life groups but not in the social control group. These results suggest that the IR-induced behavioral effects are developed distinctively in terms of the critical period hypothesis, which strengthens the developmental hypothesis of schizophrenic-like dysfunctions.

Electromagnetic field effect or simply stress? Effects of UMTS exposure on hippocampal longterm plasticity in the context of procedure related hormone release

Harmful effects of electromagnetic fields (EMF) on cognitive and behavioural features of humans and rodents have been controversially discussed and raised persistent concern about adverse effects of EMF on general brain functions. In the present study we applied radio-frequency (RF) signals of the Universal Mobile Telecommunications System (UMTS) to full brain exposed male Wistar rats in order to elaborate putative influences on stress hormone release (corticosteron; CORT and adrenocorticotropic hormone; ACTH) and on hippocampal derived synaptic long-term plasticity (LTP) and depression (LTD) as electrophysiological hallmarks for memory storage and memory consolidation. Exposure was computer controlled providing blind conditions. Nominal brain-averaged specific absorption rates (SAR) as a measure of applied mass-related dissipated RF power were 0, 2, and 10 W/kg over a period of 120 min. Comparison of cage exposed animals revealed, regardless of EMF exposure, significantly increased CORT and ACTH levels which corresponded with generally decreased field potential slopes and amplitudes in hippocampal LTP and LTD. Animals following SAR exposure of 2 W/kg (averaged over the whole brain of 2.3 g tissue mass) did not differ from the sham-exposed group in LTP and LTD experiments. In contrast, a significant reduction in LTP and LTD was observed at the high power rate of SAR (10 W/kg). The results demonstrate that a rate of 2 W/kg displays no adverse impact on LTP and LTD, while 10 W/kg leads to significant effects on the electrophysiological parameters, which can be clearly distinguished from the stress derived background. Our findings suggest that UMTS exposure with SAR in the range of 2 W/kg is not harmful to critical markers for memory storage and memory consolidation, however, an influence of UMTS at high energy absorption rates (10 W/kg) cannot be excluded.

Time-dependent corticosteroid modulation of prefrontal working memory processing

Corticosteroids are potent modulators of human higher cognitive function. They are released in response to stress, and are thought to be involved in the modulation of cognitive function by inducing distinct rapid nongenomic, and slow genomic changes, affecting neural plasticity throughout the brain. However, their exact effects on the neural correlates of higher-order cognitive function as performed by the prefrontal cortex at the human brain system level remain to be elucidated. Here, we targeted these time-dependent effects of corticosteroids on prefrontal cortex processing in humans using a working memory (WM) paradigm during functional MRI scanning. Implementing a randomized, double-blind, placebo-controlled design, 72 young, healthy men received 10 mg hydrocortisone either 30 min (rapid corticosteroid effects) or 240 min (slow corticosteroid effects), or placebo before a numerical n-back task with differential load (0- to 3-back). Corticosteroids' slow effects appeared to improve working memory performance and increased neuronal activity during WM performance in the dorsolateral prefrontal cortex depending on WM load, whereas no effects of corticosteroids' rapid actions were observed. Thereby, the slow actions of corticosteroids seem to facilitate adequate higher-order cognitive functioning, which may support recovery in the aftermath of stress exposure.

Glucocorticoids act on glutamatergic pathways to affect memory processes

Glucocorticoids can acutely affect memory processes, with both facilitating and impairing effects having been described. Recent work has revealed that glucocorticoids may affect learning and memory processes by interacting with glutamatergic mechanisms. In this opinion article I describe different glutamatergic pathways that glucocorticoids can affect to modulate memory processes. Furthermore, glucocorticoid-glutamatergic interactions during information processing are proposed as a potential model to explain many of the diverse actions of glucocorticoids on cognition. The model suggests that direct modulation of glutamatergic pathways by glucocorticoids could serve as an important mechanism for these hormones to directly alter cognitive functions.

The impact of cortisol reactivity to acute stress on memory: sex differences in middle-aged people

Stress has been identified as a main factor involved in the cognitive changes that occur during the aging process. This study investigated sex differences in the relationship between the magnitude of the acute stress-induced salivary cortisol response and memory performance among middle-aged people. To this end, 16 men and 16 women (aged 54-72 years) were exposed to the Trier Social Stress Test and a control condition in a crossover design. Afterwards their memory performance was measured using a standardized memory test (Rey's Auditory Verbal Learning Test). Only among women, there was an acute impact of stress on memory performance and a significant relationship between a higher cortisol response to the stressor and poorer memory performance in both the stress and control conditions. Additionally, a poorer memory performance was related to earlier timing of sexual maturation (age at menarche), which was also marginally related to higher cortisol reactivity to stress. These results confirm that sex is a critical factor in the relationship between cortisol and poor memory performance. Furthermore, the findings emphasize a strong link between the individual cortisol response to stress and memory functioning among postmenopausal women.

Stress controversies: post-traumatic stress disorder, hippocampal volume, gastroduodenal ulceration*

Stress in mammals triggers a neuroendocrine response mediated by the hypothalamic-pituitary-adrenal axis and the autonomic nervous system. Increased activity of these two systems induces behavioural, cardiovascular, endocrine and metabolic cascades that enable the individual to fight or flee and cope with the stress. Our understanding of stress and stress-response mechanisms is generally robust. Here, however, we review three themes that remain controversial and perhaps deserve further scrutiny and investigation before they achieve canonical status. The themes are, first, hypocortisolaemia in post-traumatic stress disorder (PTSD). A reduction rather than a stress-induced increase in adrenal glucocorticoid levels, as seen in major depressive disorder (MDD), is puzzling and furthermore is not a consistent feature of PTSD. Overall, studies on PTSD show that glucocorticoid levels may be normal or higher or lower than normal. The second theme concerns the reduction in volume of the hippocampus in MDD attributed to the neurotoxicity of hypercortisolaemia. Again, as for hypocortisolaemia in PTSD, reduced hippocampal volume in MDD has been found in some but not all studies. Third, the discovery of a causal association between Helicobacter pylori and peptic ulcers apparently brought to an end the long-held view that peptic ulceration was caused predominantly by stress. However, recent studies suggest that stress can cause peptic ulceration in the absence of H. pylori. Predictably, the aetiological pendulum of gastric and duodenal ulceration has swung from 'all stress' to 'all bacteria' followed by a sober realisation that both factors may play a role. This raises the question as to whether stress and H. pylori interact, and if so how? All three controversies are of clinical significance, pose fundamental questions about stress mechanisms and offer important areas for future research.

Stress-induced modulation of instrumental behavior: from goal-directed to habitual control of action

Actions that are directed at achieving pleasant or avoiding unpleasant states are referred to as instrumental. The acquisition of instrumental actions can be controlled by two anatomically and functionally distinct processes: a goal-directed process that is based on the prefrontal cortex and dorsomedial striatum and encodes the causal relationship between an action and the motivational value of the outcome and a dorsolateral striatum-based habit process that learns associations between actions and antecedent stimuli. Here, we review recent research showing that stress modulates the control of instrumental action in a manner that favors habitual over goal-directed action. At the neuroendocrine level, this stress-induced shift towards habit action requires the concerted action of glucocorticoids and noradrenergic arousal and is most likely accompanied by opposite functional changes in the corticostriatal circuits underlying goal-directed and habitual actions. Although generally adaptive, these changes in the control of instrumental action under stress may promote dysfunctional behaviors and the development of psychiatric disorders such as addiction.

Understanding the effects of stimulant medications on cognition in individuals with attention-deficit hyperactivity disorder: a decade of progress

The use of stimulant drugs for the treatment of children with attention-deficit hyperactivity disorder (ADHD) is one of the most widespread pharmacological interventions in child psychiatry and behavioral pediatrics. This treatment is well grounded on controlled studies showing efficacy of low oral doses of methylphenidate and amphetamine in reducing the behavioral symptoms of the disorder as reported by parents and teachers, both for the cognitive (inattention and impulsivity) and non-cognitive (hyperactivity) domains. Our main aim is to review the objectively measured cognitive effects that accompany the subjectively assessed clinical responses to stimulant medications. Recently, methods from the cognitive neurosciences have been used to provide information about brain processes that underlie the cognitive deficits of ADHD and the cognitive effects of stimulant medications. We will review some key findings from the recent literature, and then offer interpretations of the progress that has been made over the past decade in understanding the cognitive effects of stimulant medication on individuals with ADHD.

Tail-pinch stress and REM sleep deprivation differentially affect sensorimotor gating function in modafinil-treated rats

Prepulse inhibition (PPI) is a phenomenon in which a mild stimulus attenuates a cross-modality startle response to later intense stimulation. PPI is thought to index the central inhibitory mechanism through which behavioural responses are filtered. The present study compared the effects of two stress paradigms on the acoustic startle response (ASR) and on PPI in a rat model. The tail-pinch (TP) method produces an acute and immediate stressful condition, whereas rapid eye movement (REM) sleep deprivation (REMSD) leads to a more persistent and long-term stress. Our results demonstrated that in rats, TP stress reduced the size of the ASR, and REMSD impaired PPI. The wake-promoting agent modafinil (MOD) had no effect on PPI if given alone. However, MOD reduced the ASR and PPI under TP stress, whereas only PPI was reduced by MOD after 96 h of REMSD. These results suggest that distinct stress paradigms differentially mediated sensorimotor gating abilities in terms of either responsiveness to the stimulus or information-filtering capabilities.

Comparative neuroscience of stimulant-induced memory dysfunction: role for neurogenesis in the adult hippocampus

The discovery that the addictive drugs impair neurogenesis in the adult hippocampus has prompted the elaboration of new biological hypotheses to explain addiction and drug-induced cognitive dysfunction. Considerable evidence now implicates the process of adult neurogenesis in at least some critical components of hippocampal-dependent memory function. In experimental models, psychomotor stimulant drugs produce alterations in the rate of birth, survival, maturation and functional integration of adult-born hippocampal neurons. Thus some of the deleterious consequences of drug abuse on memory could result from the neurotoxic actions of drugs on adult hippocampal neurogenesis. In this review, we will first summarize preclinical and clinical literature on the disruptive effects of drugs such as cocaine and ecstasy in the areas of learning, memory and attention. We will also summarize data that document the widespread effects of stimulant drugs on progenitor activity and precursor incorporation in the adult dentate gyrus. Finally, we will examine evidence that supports the involvement of hippocampal neurogenesis in specific aspects of learning and memory function and we will consider critically the hypothesis that some of the negative consequences of drug abuse on cognition might be ascribed to deficits in adult hippocampal neurogenesis. Evidence suggests that stimulant abuse impacts negatively on at least four areas of memory/cognitive function that may be influenced by adult hippocampal neurogenesis: contextual memory, spatial memory, working memory and cognitive flexibility.

Stress and the Development of Self-Regulation in Context

This article considers the effects of psychosocial stress on child development and describes mechanisms through which early stress in the context of poverty affects the functioning of neural networks that underlie executive functions and self-regulation. It examines the effects of early experience on glucocorticoid and catecholamine levels that influence neural activity in areas of the brain associated with executive functions, primarily as studied in animal models. Finally, it considers the strengths and limitations of this research, its relevance to understanding stress reactivity from the perspective of biological sensitivity to context, and the implications for the study of risk and resilience processes and early intervention to prevent developmental delays.

A combination of high stress-induced tense and energetic arousal compensates for impairing effects of stress on memory retrieval in men

Stress can both impair and enhance memory retrieval. Glucocorticoids mediate impairing effects of stress on memory retrieval. Little is known, however, about factors that facilitate post-stress memory performance. Here, we asked whether stress-induced arousal mediates facilitative stress effects on memory retrieval. Two arousal dimensions were separated: tense arousal, which is characterized by feelings ranging from tension and anxiety to calmness and quietness, and energetic arousal, which is associated with feelings ranging from energy and vigor to states of fatigue and tiredness. Fifty-one men (mean age +/- SEM: 24.57 +/- 0.61 years) learned emotional and neutral words. Memory for these words was tested 165 min later, after participants were exposed to a psychosocial stress or a non-arousing control condition. Changes in heart rate, self-reported (energetic and tense) arousal, and saliva cortisol in response to the stress/control condition were measured. Overall, stress impaired memory retrieval. However, stressed participants with large increases in both tense and energetic arousal performed comparably to controls. Neither salivary cortisol level nor autonomic arousal predicted memory performance after controlling for changes in energetic and tense arousal. The present data indicate that stress-induced concurrent changes in tense and energetic arousal can compensate for impairing effects of stress on memory retrieval. This finding could help to explain some of the discrepancies in the literature on stress and memory.

Cooperation between hippocampal somatostatin receptor subtypes 4 and 2: functional relevance in interactive memory systems

The hippocampal somatostatin (sst) receptor subtype 4 (sst(4)) modulates memory formation by diminishing hippocampus-based spatial function while enhancing striatum-dependent behaviors. sst(4)-mediated regulations on neuronal activity in the hippocampus appear to depend on both competitive and cooperative interactions with sst receptor subtype 2 (sst(2)). Here, we investigated whether interactions with sst(2) receptors are required for sst(4)-mediated effects on hippocampus-dependent spatial memory and striatum-dependent cued memory in a water maze paradigm. Competition was assessed in mice by intrahippocampal injections of the sst(4) agonist L-803,087 alone or combined with sst(2) agonists (L-779,976 or octreotide). Effects of L-803,087 were also tested in sst(2) knockout mice to assess for receptor cooperation. Finally, sst(2a) and sst(4) localizations within hippocampal subregions were analyzed by immunohistochemistry and expression levels of sst(2a) and sst(2b) were quantified by real-time qPCR. Hippocampal injections of L-803,087 impaired spatial memory but enhanced cued memory. The latter effect was lost not only in sst(2) knockout mice but also in the presence of sst(2) agonists, whereas the former effect remained unaffected by sst(2) agonists or gene deletion. Octreotide and L-779,976 did not yield memory effects but reduced swim velocity throughout the acquisition trials suggesting that stimulation of sst(2) affected motivation and/or anxiety. sst(2a) and sst(4) were respectively detected in the dentate gyrus (DG) and the CA1 subfield suggesting that their functional interactions are not mediated by direct receptor coupling. Hippocampus sst(2a) expression was 36-fold higher than sst(2b). Possible neural mechanisms and functional significances for interaction between memory systems in relationship with stress-induced changes in hippocampal functions are discussed.

Concurrent glucocorticoid and noradrenergic activity shifts instrumental behavior from goal-directed to habitual control

Stress modulates instrumental action in favor of habitual stimulus-response processes that are insensitive to changes in outcome value and at the expense of goal-directed action-outcome processes. The neuroendocrine mechanism underlying this phenomenon is unknown. Here, we tested the hypothesis that concurrent glucocorticoid and noradrenergic activity bias instrumental behavior toward habitual performance. To this end, healthy men and women received hydrocortisone, the alpha2-adrenoceptor antagonist yohimbine or both orally before they were trained in two instrumental actions leading to two distinct food outcomes. After training, one of the outcomes was devalued by inviting participants to eat that food to satiety. A subsequent extinction test revealed whether instrumental performance was goal-directed or habitual. Participants that received hydrocortisone or yohimbine alone decreased responding to the devalued action in the extinction test, i.e., they behaved goal-directed. The combined administration of hydrocortisone and yohimbine, however, rendered participants' behavior insensitive to changes in the value of the goal (i.e., habitual). These findings demonstrate that the concerted action of glucocorticoids and noradrenergic activity shifts instrumental behavior from goal-directed to habitual control.

A critical review of chronic stress effects on spatial learning and memory

The purpose of this review is to evaluate the effects of chronic stress on hippocampal-dependent function, based primarily upon studies using young, adult male rodents and spatial navigation tasks. Despite this restriction, variability amongst the findings was evident and how or even whether chronic stress influenced spatial ability depended upon the type of task, the dependent variable measured and how the task was implemented, the type and duration of the stressors, housing conditions of the animals that include accessibility to food and cage mates, and duration from the end of the stress to the start of behavioral assessment. Nonetheless, patterns emerged as follows: For spatial memory, chronic stress impairs spatial reference memory and has transient effects on spatial working memory. For spatial learning, however, chronic stress effects appear to be task-specific: chronic stress impairs spatial learning on appetitively motivated tasks, such as the radial arm maze or holeboard, tasks that evoke relatively mild to low arousal components from fear. But under testing conditions that evoke moderate to strong arousal components from fear, such as during radial arm water maze testing, chronic stress appears to have minimal impairing effects or may even facilitate spatial learning. Chronic stress clearly impacts nearly every brain region and thus, how chronic stress alters hippocampal spatial ability likely depends upon the engagement of other brain structures during behavioral training and testing.