Systems mediating acute glucocorticoid effects on memory consolidation and retrieval

Class I histone deacetylases inhibition reverses memory impairment induced by acute stress in mice

While chronic stress induces learning and memory impairments, acute stress may facilitate or prevent memory consolidation depending on whether it occurs during the learning event or before it, respectively. On the other hand, it has been shown that histone acetylation regulates long-term memory formation. This study aimed to evaluate the effect of two inhibitors of class I histone deacetylases (HDACs), 4-phenylbutyrate (PB) and IN14 (100 mg/kg/day, ip for 2 days), on memory performance in mice exposed to a single 15-min forced swimming stress session. Plasma corticosterone levels were determined 30 minutes after acute swim stress in one group of mice. In another experimental series, independent groups of mice were trained in one of three different memory tasks: Object recognition test, Elevated T maze, and Buried food location test. Subsequently, the hippocampi were removed to perform ELISA assays for histone deacetylase 2 (HDAC2) expression. Acute stress induced an increase in plasma corticosterone levels, as well as hippocampal HDAC2 content, along with an impaired performance in memory tests. Moreover, PB and IN14 treatment prevented memory loss in stressed mice. These findings suggest that HDAC2 is involved in acute stress-induced cognitive impairment. None of the drugs improved memory in non-stressed animals, indicating that HDACs inhibitors are not cognitive boosters, but rather potentially useful drugs for mitigating memory deficits.

Does high fidelity neonatal resuscitation simulation increase salivary cortisol levels of health care providers?

BACKGROUND

The use of simulation-based medical education is strongly recommended to insure neonatal resuscitation skills for health caregivers. High fidelity simulation was executed to allow the evaluation of technical and non-technical skills. Salivary cortisol level was considered reliable biomarkers of adrenocortical activity and useful tool to learning assessment and stress response.

METHODS

Our primary aim was to test changes in salivary cortisol levels before and after the simulation for neonatal resuscitation between high and low fidelity setting. Secondary aim was to evaluate salivary cortisol level in the participants, leader and not leader. Fifty-two health care providers were divided in ratio 1:1 into low-fidelity (LF group) and high-fidelity scenario (HF group) of neonatal resuscitation. In each group the participants assumed the role of team leader or not team leader. Salivary samples were collected from all participants 5 minutes before and after each simulation scenario by using oral swab. Analysis of difference was analyzed by Kruskal Wallis Test.

RESULTS

Salivary cortisol levels were significantly higher in HF group (N.=26) than LF group (N.=26) before the performance (5.407 mmol/L vs. 3.090 mmol/L; P=0.018). In the HF group, salivary cortisol levels were significantly lower after simulation than before (P=0.007), moreover not team leader showed higher salivary cortisol levels before of the simulation than after (P=0.003). Team leaders showed higher salivary cortisol levels than not team leader after high-fidelity scenario (P=0.039).

CONCLUSIONS

High-fidelity simulation scenario had a great impact on stress level, furthermore leaders showed higher salivary cortisol levels than not team leaders.

Mitochondrial cannabinoid receptors gate corticosterone impact on novel object recognition

Corticosteroid-mediated stress responses require the activation of complex brain circuits involving mitochondrial activity, but the underlying cellular and molecular mechanisms are scantly known. The endocannabinoid system is implicated in stress coping, and it can directly regulate brain mitochondrial functions via type 1 cannabinoid (CB1) receptors associated with mitochondrial membranes (mtCB1). In this study, we show that the impairing effect of corticosterone in the novel object recognition (NOR) task in mice requires mtCB1 receptors and the regulation of mitochondrial calcium levels in neurons. Different brain circuits are modulated by this mechanism to mediate the impact of corticosterone during specific phases of the task. Thus, whereas corticosterone recruits mtCB1 receptors in noradrenergic neurons to impair NOR consolidation, mtCB1 receptors in local hippocampal GABAergic interneurons are required to inhibit NOR retrieval. These data reveal unforeseen mechanisms mediating the effects of corticosteroids during different phases of NOR, involving mitochondrial calcium alterations in different brain circuits.

A systematic review of the pharmacological modulation of autobiographical memory specificity

Background

Over-general autobiographical memory (AM) retrieval is proposed to have a causal role in the maintenance of psychological disorders like depression and PTSD. As such, the identification of drugs that modulate AM specificity may open up new avenues of research on pharmacological modeling and treatment of psychological disorders.

Aim

The current review summarizes randomized, placebo-controlled studies of acute pharmacological modulation of AM specificity.

Method

A systematic search was conducted of studies that examined the acute effects of pharmacological interventions on AM specificity in human volunteers (healthy and clinical participants) measured using the Autobiographical Memory Test.

Results

Seventeen studies were identified (986 total participants), of which 16 were judged to have low risk of bias. The presence and direction of effects varied across drugs and diagnostic status of participants (clinical vs. healthy volunteers). The most commonly studied drug-hydrocortisone-produced an overall impairment in AM specificity in healthy volunteers [g = -0.28, CI (-0.53, -0.03), p = 0.03], although improvements were reported in two studies of clinical participants. In general, studies of monoamine modulators reported no effect on specificity.

Conclusion

Pharmacological enhancement of AM specificity is inconsistent, although monaminergic modulators show little promise in this regard. Drugs that reduce AM specificity in healthy volunteers may be useful experimental-pharmacological tools that mimic an important transdiagnostic impairment in psychological disorders.

Systematic review registration

PROSPERO, identifier CRD42020199076, https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42020199076.

Randomized controlled experimental study of hydrocortisone and D-cycloserine effects on fear extinction in PTSD

Fear extinction underlies prolonged exposure, one of the most well-studied treatments for posttraumatic stress disorder (PTSD). There has been increased interest in exploring pharmacological agents to enhance fear extinction learning in humans and their potential as adjuncts to PE. The objective of such adjuncts is to augment the clinical impact of PE on the durability and magnitude of symptom reduction. In this study, we examined whether hydrocortisone (HC), a corticosteroid, and D-Cycloserine (DCS), an N-methyl-D-aspartate receptor partial agonist, enhance fear extinction learning and consolidation in individuals with PTSD. In a double-blind placebo-controlled 3-group experimental design, 90 individuals with full or subsyndromal PTSD underwent fear conditioning with stimuli that were paired (CS+) or unpaired (CS-) with shock. Extinction learning occurred 72 h later and extinction retention was tested one week after extinction. HC 25 mg, DCS 50 mg or placebo was administered one hour prior to extinction learning. During extinction learning, the DCS and HC groups showed a reduced differential CS+/CS- skin conductance response (SCR) compared to placebo (b = -0.19, CI = -0.01 to -37, p = 0.042 and b = -0.25, CI = -08 to -0.43, p = 0.005, respectively). A nonsignificant trend for a lower differential CS+/CS- SCR in the DCS group, compared to placebo, (b = -0.25, CI = 0.04 to -0.55, p = 0.089) was observed at retention testing, one week later. A single dose of HC and DCS facilitated fear extinction learning in participants with PTSD symptoms. While clinical implications have yet to be determined, our findings suggest that glucocorticoids and NMDA agonists hold promise for facilitating extinction learning in PTSD.

Object recognition and Morris water maze to detect cognitive impairment from mild hippocampal damage in rats: A reflection based on the literature and experience

Object recognition (OR) and the Morris water maze (MWM) are classical tasks widely used to assess memory parameters and deficits in rodents. Learning processes in both tasks involve integrity of the hippocampus and associated regions, and prefrontal cortex connections. Here, we highlight the idea that these classical tests can be used to indicate memory deficits caused by models of disease that affect hippocampal function in rats, and identify some practical issues of OR and MWM, based on the literature and our experience. Additionally, we have shown that the performance of both tasks does not alter blood levels of corticosterone, considering exposure to a single task. Hence, taking into consideration the difficulties and care required during task execution, the infrastructure needed and the training of the experimenter, we suggest that OR and its variations offer minimal manageable stressful conditions, representing an effective and practical tool for hippocampal-related memory assessment of rats. Thus, OR may provide similar information to that of the MWM, despite controversy regarding hippocampus participation in OR and given due differences in the types of memory evaluated and researchers' objectives. We recommend the observation of some important precautions and details, also based on the literature and our own experience.

Behavioral tagging as a mechanism for aversive-memory formation under acute stress

The behavioral tagging (BT) hypothesis postulates that a weak learning experience, which only induces short-term memory, may benefit from another event that provides plasticity-related proteins (PRPs) to establish a long-lasting memory. According to BT, the weak experience sets a transient learning tag at specific activated sites, and its temporal and spatial convergence with the PRPs allows the long-term memory (LTM) formation. In this work, rats were subjected to a weak inhibitory avoidance (IAw) training and we observed that acute stress (elevated platform, EP) experienced 1 hr before IAw promoted IA-LTM formation. This effect was dependent on glucocorticoid-receptor activity as well as protein synthesis in the dorsal hippocampus. However, the same stress has negative effects on IA-LTM formation when training is strong, probably by competing for necessary PRPs. Furthermore, our experiments showed that EP immediately after training did not impair the setting of the learning tag and even facilitated IA-LTM formation. These findings reveal different impacts of a given acute stressful experience on the formation of an aversive memory that could be explained by BT processes.

Contributions of glucocorticoid receptors in cortical astrocytes to memory recall

Dysfunctions in memory recall lead to pathological fear; a hallmark of trauma-related disorders, like posttraumatic stress disorder (PTSD). Both, heightened recall of an association between a cue and trauma, as well as impoverished recall that a previously trauma-related cue is no longer a threat, result in a debilitating fear toward the cue. Glucocorticoid-mediated action via the glucocorticoid receptor (GR) influences memory recall. This literature has primarily focused on GRs expressed in neurons or ignored cell-type specific contributions. To ask how GR action in nonneuronal cells influences memory recall, we combined auditory fear conditioning in mice and the knockout of GRs in astrocytes in the prefrontal cortex (PFC), a brain region implicated in memory recall. We found that knocking out GRs in astrocytes of the PFC disrupted memory recall. Specifically, we found that knocking out GRs in astrocytes in the PFC (AstroGRKO) after fear conditioning resulted in higher levels of freezing to the CS+ tone when compared with controls (AstroGRintact). While we did not find any differences in extinction of fear toward the CS+ between these groups, AstroGRKO female but not male mice showed impaired recall of extinction training. These results suggest that GRs in cortical astrocytes contribute to memory recall. These data demonstrate the need to examine GR action in cortical astrocytes to elucidate the basic neurobiology underlying memory recall and potential mechanisms that underlie female-specific biases in the incidence of PTSD.

Sleep, Cognition and Cortisol in Addison's Disease: A Mechanistic Relationship

Sleep is a critical biological process, essential for cognitive well-being. Neuroscientific literature suggests there are mechanistic relations between sleep disruption and memory deficits, and that varying concentrations of cortisol may play an important role in mediating those relations. Patients with Addison's disease (AD) experience consistent and predictable periods of sub- and supra-physiological cortisol concentrations due to lifelong glucocorticoid replacement therapy, and they frequently report disrupted sleep and impaired memory. These disruptions and impairments may be related to the failure of replacement regimens to restore a normal circadian rhythm of cortisol secretion. Available data provides support for existing theoretical frameworks which postulate that in AD and other neuroendocrine, neurological, or psychiatric disorders, disrupted sleep is an important biological mechanism that underlies, at least partially, the memory impairments that patients frequently report experiencing. Given the literature linking sleep disruption and cognitive impairment in AD, future initiatives should aim to improve patients' cognitive performance (and, indeed, their overall quality of life) by prioritizing and optimizing sleep. This review summarizes the literature on sleep and cognition in AD, and the role that cortisol concentrations play in the relationship between the two.

Understanding stress: Insights from rodent models

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

•

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

•

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

•

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

•

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

False Opposing Fear Memories Are Produced as a Function of the Hippocampal Sector Where Glucocorticoid Receptors Are Activated

Injection of corticosterone (CORT) in the dorsal hippocampus (DH) can mimic post-traumatic stress disorder (PTSD)-related memory in mice: both maladaptive hypermnesia for a salient but irrelevant simple cue and amnesia for the traumatic context. However, accumulated evidence indicates a functional dissociation within the hippocampus such that contextual learning is primarily associated with the DH whereas emotional processes are more linked to the ventral hippocampus (VH). This suggests that CORT might have different effects on fear memories as a function of the hippocampal sector preferentially targeted and the type of fear learning (contextual vs. cued) considered. We tested this hypothesis in mice using CORT infusion into the DH or VH after fear conditioning, during which a tone was either paired (predicting-tone) or unpaired (predicting-context) with the shock. We first replicate our previous results showing that intra-DH CORT infusion impairs contextual fear conditioning while inducing fear responses to the not predictive tone. Second, we show that, in contrast, intra-VH CORT infusion has opposite effects on fear memories: in the predicting-tone situation, it blocks tone fear conditioning while enhancing the fear responses to the context. In both situations, a false fear memory is formed based on an erroneous selection of the predictor of the threat. Third, these opposite effects of CORT on fear memory are both mediated by glucocorticoid receptor (GR) activation, and reproduced by post-conditioning stress or systemic CORT injection. These findings demonstrate that false opposing fear memories can be produced depending on the hippocampal sector in which the GRs are activated.

Protective effects of morphine in a rat model of post-traumatic stress disorder: Role of hypothalamic-pituitary-adrenal axis and beta- adrenergic system

Post-traumatic stress disorder (PTSD) arises after tremendous traumatic experiences. Recently, we have reported that morphine has time-dependent protective effects against behavioral and morphological deficits in the single prolonged stress (SPS) as an experimental model of PTSD in adult male rats. To find the mechanisms underlying the protective effects of morphine against SPS-induced PTSD-like symptoms, the present study investigated the interaction between morphine and hypothalamic-pituitary-adrenal (HPA) axis and beta - adrenergic system, which crucially involved in the stress response, on PTSD-like symptoms in male rats. The animals were exposed to the SPS procedure (restraint for 2 h, forced swimming for 20 min, and ether anesthesia) and morphine (10 mg/kg) or saline was injected 24 h following the SPS. The glucocorticoid receptor antagonist RU486 (20 mg/kg), the mineralocorticoid receptor antagonist spironolactone (50 mg/kg), and the corticosterone synthesis inhibitor metyrapone (50 mg/kg) were injected 90 min before morphine administration to block the HPA axis activity. The beta - adrenergic receptor blocker propranolol (10 mg/kg) and the peripheral beta-adrenergic receptor blocker nadolol (5 mg/kg) were administered 30 min before morphine injection to block the beta - adrenergic system. Anxiety-like behaviors were evaluated using the elevated plus maze (EPM) 11 days after the SPS. After that, animals were conditioned in a fear-conditioning task and extinction training was performed on days 1, 2, 3, 4 and 11 after fear conditioning. SPS increased anxiety-like behaviors and impaired fear extinction. Morphine injection 24 h after SPS significantly improved anxiety-like behaviors and enhanced fear extinction. The RU486, spironolactone and metyrapone prevented the protective effects of morphine on both SPS-induced anxiety-like behaviors and impaired fear extinction. The propranolol, and nadolol did not prevent the effect of morphine on anxiety-like behaviors, but the propranolol prevented morphine effects on fear extinction in SPS animals. These findings together suggest that the protective effects of morphine on PTSD-like symptoms in rats require a certain level of the HPA axis and central beta - adrenergic activity and any alteration in the function of these systems can impede the protective effects of morphine.

Mechanisms underlying remediation of depression-associated anxiety by chronic N-acetyl cysteine treatment

RATIONALE

Anxiety is one of the most comorbid conditions with major depressive disorder (MDD). Depression-associated anxiety often stems from the dysfunctional hypothalamic-pituitary-adrenal (HPA) axis and its altered regulation by the amygdala. Furthermore, MDD is associated with altered glutamatergic processing leading to anxiety and impaired regulation of the HPA axis. Recent studies have demonstrated that N-acetyl cysteine (NAC), a pleiotropic drug, exerts antidepressant-like effect by modulation of hippocampal functions, periterminal release of glutamate, and/or redox systems. However, the effects of NAC on depression-associated anxiety, HPA axis hyperactivity, and amygdalar dysfunctions are relatively unknown.

OBJECTIVES

Accordingly, we evaluated the effect of NAC on neonatal clomipramine (CLI)-induced adulthood anxiety and accompanying changes in plasma corticosterone levels, amygdalar volumes, neuronal/glial densities, levels of monoamines, and their metabolites in the amygdalar complex.

RESULTS

We found that chronic treatment with NAC reverses CLI-induced anhedonia and enhanced anxiety. Interestingly, attenuation of CLI-associated anxiety in NAC-treated rats were accompanied by a reversal of adrenal and spleen hypertrophy, and normalization of enhanced plasma corticosterone levels, indicating improved HPA axis functioning. Furthermore, NAC treatment was sufficient to reverse volumetric hypertrophy of basolateral amygdala (BLA), and altered noradrenaline (NA) metabolism in the amygdalar complex. The effects of NAC in the reversal of CLI-induced impairments were similar to that of fluoxetine (FLX).

CONCLUSIONS

We suggest that beneficial effects of NAC on antidepressive- and antianxiety-like behaviors are at least in part mediated via restoration of amygdalar and HPA axis functioning. Our results support the hypothesis that NAC might be evolved as a therapeutic strategy for reversal of amygdalar dysfunction in depression.

Stress before training alters memory retrieval of a non-declarative memory in Lymnaea

Stress alters both memory formation and its retrieval. Here, we show that a combination of stressors before an associative learning event alters memory retrieval of a non-declarative memory in an invertebrate model system. Previously, two combinations of stressors were purported to prevent long-term memory (LTM) formation in 'smart' Lymnaea and this inability to form LTM was considered to be a cost of being smart. Here, we show that is not the case. The specific combinations of stressors used here cause emotional memory formation. Previously, it was shown that propranolol, a synthetic beta-blocker, altered emotional memory in Lymnaea. We show here that when propranolol but not saline is injected into smart snails before they perceive the combination of stressors, these snails form LTM. We then show that the injection of propranolol but not saline before a memory activation session allowed the memory to be recalled. That is, LTM formed but was not retrievable unless propranolol was injected pre-retrieval. Thus, the smart snails formed LTM in the face of the stressors but could not retrieve it.

A review on how stress modulates fear conditioning: Let's not forget the role of sex and sex hormones

Stress and fear are two fields of research that have evolved simultaneously. It was not until the eighties that these domains converged in order to better characterize the impact of stress on fear memory formation. Here, we reviewed the effects of stress occurring before fear acquisition on the main phases of fear conditioning protocols (acquisition training, extinction training, extinction retention test), with a specific focus on sex and sex hormones. We also paid close attention to methodological aspects in order to better understand and characterize discrepant findings across studies. In men, stress appears to potentiate fear acquisition at a physiological level but induces lower activations of fear-related brain regions. In women, results are inconsistent. Although some studies have shown that stress lowers physiological fear responses and heightens brain activations in women during fear acquisition, many studies report no significant effects. Irrespective of sex, pre-acquisition stress seems to induce fear extinction learning resistance. Overall, few studies have taken into account sex hormones, despite their impact on both the fear and stress brain networks. As methodological variability makes it complex to draw strong conclusions, several methodological aspects are discussed with the aim of orienting future research.

Retrieval-Extinction and Relapse Prevention: Rewriting Maladaptive Drug Memories?

Addicted individuals are highly susceptible to relapse when exposed to drug-associated conditioned stimuli (CSs; "drug cues") even after extensive periods of abstinence. Until recently, these maladaptive emotional drug memories were believed to be permanent and resistant to change. The rediscovery of the phenomenon of memory reconsolidation-by which retrieval of the memory can, under certain conditions, destabilize the previously stable memory before it restabilizes in its new, updated form-has led to the hypothesis that it may be possible to disrupt the strong maladaptive drug-memories that trigger a relapse. Furthermore, recent work has suggested that extinction training "within the reconsolidation window" may lead to a long-term reduction in relapse without the requirement for pharmacological amnestic agents. However, this so-called "retrieval-extinction" effect has been inconsistently observed in the literature, leading some to speculate that rather than reflecting memory updating, it may be the product of facilitation of extinction. In this mini review article, we will focus on factors that might be responsible for the retrieval-extinction effects on preventing drug-seeking relapse and how inter-individual differences may influence this therapeutically promising effect. A better understanding of the psychological and neurobiological mechanisms underpinning the "retrieval-extinction" paradigm, and individual differences in boundary conditions, should provide insights with the potential to optimize the translation of "retrieval-extinction" to clinical populations.

Corticolimbic analysis of microRNAs and protein expressions in scopolamine-induced memory loss under stress

The aim of the present study was to assess thealterations of corticolimbic microRNAs and protein expressions in the effect of scopolamine with or without stress on passive-avoidance memory in male Wistar rats. The expressions of miR-1, miR-10 and miR-26 and also the levels of p-CREB, CREB, C-FOS and BDNF in the prefrontal cortex (PFC), the hippocampus and the amygdala were evaluated using RT-qPCR and Western blotting techniques. The data showed that the administration of a muscarinic receptor antagonist, scopolamine or the exposure to 30 min stress significantly induced memory loss. Interestingly, the injection of an ineffective dose of scopolamine (0.5 mg/kg) alongside with exposure to an ineffective time of stress (10 min) impaired memory formation, suggesting a potentiative effect of stress on scopolamine response. Our results showed that memory formation was associated with the down-regulated expression of miR-1, miR-10 and miR-26 in the PFC and the hippocampus, but not the amygdala. The relative expression increase of miR-1 and miR-10 in the PFC and the hippocampus was shown in memory loss induced by scopolamine administration or 30-min stress. The PFC level of miR-10 and also hippocampal level of miR-1 and miR-10 were significantly up-regulated, while amygdala miR-1 and miR-26 were down-regulated in scopolamine-induced memory loss under stress. Memory formation increased BDNF, C-FOS and p-CREB/CREB in the PFC, the hippocampus and the amygdala. In contrast, the PFC, hippocampal and amygdala protein expressions were significantly decreased in memory loss induced by scopolamine administration (2 mg/kg), stress exposure (for 30 min) or scopolamine (0.5 mg/kg) plus stress (10 min). One of the most significant findings to emerge from this study is that the stress exposure potentiated the amnesic effect of scopolamine may via affecting the expressions of miRs and proteins in the PFC, the hippocampus and the amygdala. It is possible to hypothesis that corticolimbic signaling pathways play a critical role in relationship between stress and Alzheimer's disease.

Glucocorticoids and resilience

All organisms endure frequent challenges to homeostasis, or stressors, that require adaptation. Depending on the individual, the context, and the magnitude of stress, this active adaptation can lead to behavioral susceptibility or resilience. The latter is an under-appreciated consequence of stress, as the damaging effects of chronic stress and chronically elevated glucocorticoids have received much more attention. We suggest here that neural pathways promoting resilience are initiated at the time of stress, and that they involve glucocorticoid signaling. By focusing on the neurobiology of resilience induction and the identification of vulnerable individuals, we may be able to intervene in the future at the time of stress to promote positive adaptation.

The impact of maternal separation and isolation stress during stress hyporesponsive period on fear retention and extinction recall memory from 5-week- to 1-year-old rats

The purpose of the present study was to determine whether age would disrupt fear retention and extinction memory in rats pre-exposed to maternal separation and isolation stress; these rats are called MS rats. MS stress was induced by exposing rat pups into maternal separation followed by isolation stress from peer groups (MS) daily/6 h during stress hyporesponsive period, while controls rats that were undisturbed during this period are called NMS rats. 5, 8, 15 and 52 weeks later, these animals were exposed to classical fear conditioning test by pairing auditory stimulus (conditioned stimulus, CS+) with electric footshock. 24 h later, conditioned freezing response to CS+ was measured during fear retention, extinction and extinction recall trials. The normal ageing per se did not affect the formation of fear memory, retention and fear extinction memory. MS stress, on the other hand, disrupted fear memory at young adulthood age exhibiting increased freezing response to CS+ during retention test and reduced during fear extinction memory test when compared to NMS groups. On the other hand, rats at adolescence age exhibited reduced freezing during fear retention and enhanced freezing response to CS+ during extinction recall test. However, MS-induced changes in freezing response during fear retention and extinction tests were not seen in adulthood and 1-year-old age groups. These data demonstrate the young adulthood age is highly vulnerable to fear memory and extinction processes. The differences in freezing response to CS+ during fear conditioning from adolescence to old age, thus, appear to be related to the maturation of the limbic circuit.

Repeated cortisol administration does not reduce intrusive memories - A double blind placebo controlled experimental study

PTSD is a severe mental disorder, which may develop after exposure to traumatic events and is characterized by intrusive memories. Intrusions are sudden brief sensory memories of the traumatic event, that cause immense distress and impairment in every day functioning. Thus, the reduction of intrusive memories is one of the main aims of PTSD therapy. Recently, the glucocorticoid cortisol has been proposed as a pharmacological option to reduce intrusive memories, because cortisol is known to have memory retrieval inhibiting effects. However, the research on the effects of cortisol administration on intrusive memories is not conclusive. The aim of the present study was to examine if repeated cortisol administration inhibits intrusions and recognition memory in an experimental study using the trauma film paradigm. In a randomized double-blind placebo controlled design participants were exposed to a traumatic film (known to induce intrusions in healthy participants) and received either a low dose of cortisol (20mg) or placebo on the three days following "trauma exposure". Intrusive memories were assessed with an Electronic Diary and an Intrusion Triggering Task. Furthermore, we assessed explicit memory for the traumatic film clip with a recognition test. Contrary to our predictions, the cortisol group did not report fewer intrusions than the placebo group nor did it show diminished performance on the recognition test. Our results show that sole cortisol administration after a traumatic experience cannot reduce intrusive re-experiencing.

Exposure to a contextually neutral stressor potentiates fear conditioning in juvenile rainbow trout, Oncorhynchus mykiss

Organisms faced with stressors deploy a suite of adaptive responses in the form of behavioral, physiological and cognitive modifications to overcome the challenge. Interactive effects of these responses are known to influence learning and memory processes and facilitation is thought to be dependent, in part, upon contextual relevance of the stressor to the learning task. Predation is one such stressor for prey animals, and their ability to manage reliable information about predators is essential for adaptive antipredator strategies. Here, we investigated (i) the influence cortisol has on the ability of juvenile rainbow trout to learn and retain conditioned antipredator responses to predatory cues, and (ii) whether conditioned behavioral and physiological responses to predator cues are fixed or deployed in a threat-sensitive manner. Trout were fed cortisol-coated pellets minutes prior to a conditioning event where they were exposed to novel predator odor paired with chemical alarm cues (unconditioned stimulus). We tested for conditioned responses by exposing trout to predator cues after 2, 4 or 10days and subsequently documented physiological and behavioral responses. Both control and cortisol-fed trout learned the predator odor and responded 2 and 4days post conditioning. However, at 10days only cortisol-fed trout maintained strong behavioral responses to predator cues. Interestingly, we failed to find conditioned physiological responses to predator odor despite the presence of threat-sensitive cortisol responses to the unconditioned stimulus. Our findings suggest cortisol exposure prior to predator-learning may enhance retention of conditioned responses, even without a contextual link between stressor source and learning task.

Blunted cortisol response to acute pre-learning stress prevents misinformation effect in a forced confabulation paradigm

Research examining the effects of stress on false memory formation has been equivocal, partly because of the complex nature of stress-memory interactions. A major factor influencing stress effects on learning is the timing of stress relative to encoding. Previous work has shown that brief stressors administered immediately before learning enhance long-term memory. Thus, we predicted that brief stress immediately before learning would decrease participants' susceptibility to subsequent misinformation and reduce false memory formation. Eighty-four male and female participants submerged their hand in ice cold (stress) or warm (no stress) water for 3min. Immediately afterwards, they viewed an 8-min excerpt from the Disney movie Looking for Miracles. The next day, participants were interviewed and asked several questions about the video, some of which forced them to confabulate responses. Three days and three weeks later, respectively, participants completed a recognition test in the lab and a free recall test via email. Our results revealed a robust misinformation effect, overall, as participants falsely recognized a significant amount of information that they had confabulated during the interview as having occurred in the original video. Stress, overall, did not significantly influence this misinformation effect. However, the misinformation effect was completely absent in stressed participants who exhibited a blunted cortisol response to the stress, for both recognition and recall tests. The complete absence of a misinformation effect in non-responders may lend insight into the interactive roles of autonomic arousal and corticosteroid levels in false memory development.

Genome-Wide Identification of Basic Helix-Loop-Helix and NF-1 Motifs Underlying GR Binding Sites in Male Rat Hippocampus

Glucocorticoids regulate hippocampal function in part by modulating gene expression through the glucocorticoid receptor (GR). GR binding is highly cell type specific, directed to accessible chromatin regions established during tissue differentiation. Distinct classes of GR binding sites are dependent on the activity of additional signal-activated transcription factors that prime chromatin toward context-specific organization. We hypothesized a stress context dependency for GR binding in hippocampus as a consequence of rapidly induced stress mediators priming chromatin accessibility. Using chromatin immunoprecipitation sequencing to interrogate GR binding, we found no effect of restraint stress context on GR binding, although analysis of sequences underlying GR binding sites revealed mechanistic detail for hippocampal GR function. We note enrichment of GR binding sites proximal to genes linked to structural and organizational roles, an absence of major tethering partners for GRs, and little or no evidence for binding at negative glucocorticoid response elements. A basic helix-loop-helix motif closely resembling a NeuroD1 or Olig2 binding site was found underlying a subset of GR binding sites and is proposed as a candidate lineage-determining transcription factor directing hippocampal chromatin access for GRs. Of our GR binding sites, 54% additionally contained half-sites for nuclear factor (NF)-1 that we propose as a collaborative or general transcription factor involved in hippocampal GR function. Our findings imply a dose-dependent and context-independent action of GRs in the hippocampus. Alterations in the expression or activity of NF-1/basic helix-loop-helix factors may play an as yet undetermined role in glucocorticoid-related disease susceptibility and outcome by altering GR access to hippocampal binding sites.

Role of Glia in Stress-Induced Enhancement and Impairment of Memory

Both acute and chronic stress profoundly affect hippocampally-dependent learning and memory: moderate stress generally enhances, while chronic or extreme stress can impair, neural and cognitive processes. Within the brain, stress elevates both norepinephrine and glucocorticoids, and both affect several genomic and signaling cascades responsible for modulating memory strength. Memories formed at times of stress can be extremely strong, yet stress can also impair memory to the point of amnesia. Often overlooked in consideration of the impact of stress on cognitive processes, and specifically memory, is the important contribution of glia as a target for stress-induced changes. Astrocytes, microglia, and oligodendrocytes all have unique contributions to learning and memory. Furthermore, these three types of glia express receptors for both norepinephrine and glucocorticoids and are hence immediate targets of stress hormone actions. It is becoming increasingly clear that inflammatory cytokines and immunomodulatory molecules released by glia during stress may promote many of the behavioral effects of acute and chronic stress. In this review, the role of traditional genomic and rapid hormonal mechanisms working in concert with glia to affect stress-induced learning and memory will be emphasized.

Post-learning stress enhances long-term memory and differentially influences memory in females depending on menstrual stage

Most work has shown that post-learning stress enhances long-term memory; however, there have been recent inconsistencies in this literature. The purpose of the present study was to examine further the effects of post-learning stress on long-term memory and to explore any sex differences that may exist. Male and female participants learned a list of 42 words that varied in emotional valence and arousal level. Following encoding, participants completed a free recall assessment and then submerged their hand into a bath of ice cold (stress) or lukewarm (no stress) water for 3 min. The next day, participants were given free recall and recognition tests. Stressed participants recalled more words than non-stressed participants 24h after learning. Stress also enhanced female participants' recall of arousing words when they were in the follicular, but not luteal, phase. These findings replicate previous work examining post-learning stress effects on memory and implicate the involvement of sex-related hormones in such effects.

Short-term, high-dose administration of corticosterone by injection facilitates trace eyeblink conditioning in young male rats

Glucocorticoids released as part of the physiological response to stress are known to affect cognitive function, presumably via effects on the hippocampus. Trace classical eyeblink conditioning is an associative learning task which depends on the hippocampus and has been used to examine the development of learning processes in young mammals. Previously, we demonstrated deficits in trace eyeblink conditioning associated with postnatal administration of the glucocorticoid corticosterone by creating a sustained elevation with methods such as subcutaneous timed-release pellets and osmotic mini-pumps which were active over several days. In the present study, we examined the effects of an oscillating pattern of corticosterone elevation on subsequent trace eyeblink conditioning. Twice daily corticosterone injections (high, low, or vehicle) were administered over a 3-day period, starting at postnatal day 15. Then, on postnatal day 28, animals underwent trace classical eyeblink conditioning to examine the possible influence of earlier corticosterone elevations on the development of learning and memory. Eyeblink conditioning was affected by corticosterone treatments, but only for males, and only very early in acquisition; Males receiving the high dose of corticosterone exhibited facilitation of learning relative to controls. These data demonstrate that oscillating corticosterone elevations produce opposite effects on this associative learning task than do sustained elevations.

Temporal dynamics of stress-induced alternations of intrinsic amygdala connectivity and neuroendocrine levels

Stress-induced changes in functional brain connectivity have been linked to the etiology of stress-related disorders. Resting state functional connectivity (rsFC) is especially informative in characterizing the temporal trajectory of glucocorticoids during stress adaptation. Using the imaging Maastricht Acute Stress Test (iMAST), we induced acute stress in 39 healthy volunteers and monitored the neuroendocrine stress levels during three runs of resting state functional magnetic resonance imaging (rs-fMRI): before (run 1), immediately following (run 2), and 30 min after acute stress (run 3). The iMAST resulted in strong increases in cortisol levels. Whole-brain analysis revealed that acute stress (run 2 - 1) was characterized by changes in connectivity of the amygdala with the ventrolateral prefrontal cortex (vlPFC), ventral posterior cingulate cortex (PCC), cuneus, parahippocampal gyrus, and culmen. Additionally, cortisol responders were characterized by enhanced amygdala - medial prefrontal cortex (mPFC) connectivity. Stress recovery (run 3 - 2) was characterized by altered amygdala connectivity with the dorsolateral prefrontal cortex (dlPFC), ventral and dorsal anterior cingulate cortex (ACC), anterior hippocampal complex, cuneus, and presupplementary motor area (preSMA). Opposite to non-responders, cortisol responders were characterized by enhanced amygdala connectivity with the anterior hippocampal complex and parahippocampal gyrus, and reduced connectivity with left dlPFC, dACC, and culmen during early recovery. Acute stress responding and recovery are thus associated with changes in the functional connectivity of the amygdala network. Our findings show that these changes may be regulated via stress-induced neuroendocrine levels. Defining stress-induced neuronal network changes is pertinent to developing treatments that target abnormal neuronal activity.

The neuroenergetics of stress hormones in the hippocampus and implications for memory

Acute stress causes rapid release of norepinephrine (NE) and glucocorticoids (GCs), both of which bind to hippocampal receptors. This release continues, at varying concentrations, for several hours following the stressful event, and has powerful effects on hippocampally-dependent memory that generally promote acquisition and consolidation while impairing retrieval. Several studies have characterized the brain's energy usage both at baseline and during memory processing, but there are few data on energy requirements of memory processes under stressful conditions. Because memory is enhanced by emotional arousal such as during stress, it is likely that molecular memory processes under these conditions differ from those under non-stressful conditions that do not activate the hypothalamic-pituitary-adrenal (HPA) axis. Mobilization of peripheral and central energy stores during stress may increase hippocampal glucose metabolism that enhances salience and detail to facilitate memory enhancement. Several pathways activated by the HPA axis affect neural energy supply and metabolism, and may also prevent detrimental damage associated with chronic stress. We hypothesize that alterations in hippocampal metabolism during stress are key to understanding the effects of stress hormones on hippocampally-dependent memory formation. Second, we suggest that the effects of stress on hippocampal metabolism are bi-directional: within minutes, NE promotes glucose metabolism, while hours into the stress response GCs act to suppress metabolism. These bi-directional effects of NE and GCs on glucose metabolism may occur at least in part through direct modulation of glucose transporter-4. In contrast, chronic stress and prolonged elevation of hippocampal GCs cause chronically suppressed glucose metabolism, excitotoxicity and subsequent memory deficits.

Acute stress in adulthood impoverishes social choices and triggers aggressiveness in preclinical models

Adult C57BL/6J mice are known to exhibit high level of social flexibility while mice lacking the β2 subunit of nicotinic receptors (β2(-/-) mice) present social rigidity. We asked ourselves what would be the consequences of a restraint acute stress (45 min) on social interactions in adult mice of both genotypes, hence the contribution of neuronal nicotinic receptors in this process. We therefore dissected social interaction complexity of stressed and not stressed dyads of mice in a social interaction task. We also measured plasma corticosterone levels in our experimental conditions. We showed that a single stress exposure occurring in adulthood reduced and disorganized social interaction complexity in both C57BL/6J and β2(-/-) mice. These stress-induced maladaptive social interactions involved alteration of distinct social categories and strategies in both genotypes, suggesting a dissociable impact of stress depending on the functioning of the cholinergic nicotinic system. In both genotypes, social behaviors under stress were coupled to aggressive reactions with no plasma corticosterone changes. Thus, aggressiveness appeared a general response independent of nicotinic function. We demonstrate here that a single stress exposure occurring in adulthood is sufficient to impoverish social interactions: stress impaired social flexibility in C57BL/6J mice whereas it reinforced β2(-/-) mice behavioral rigidity.

Cortisol augmentation of a psychological treatment for warfighters with posttraumatic stress disorder: Randomized trial showing improved treatment retention and outcome

BACKGROUND

Prolonged exposure (PE) therapy for post-traumatic stress disorder (PTSD) in military veterans has established efficacy, but is ineffective for a substantial number of patients. PE is also associated with high dropout rates. We hypothesized that hydrocortisone augmentation would enhance symptom improvement and reduce drop-out rates by diminishing the distressing effects of traumatic memories retrieved during imaginal exposure. We also hypothesized that in responders, hydrocortisone augmentation would be more effective in reversing glucocorticoid indices associated with PTSD than placebo augmentation.

METHOD

Twenty-four veterans were randomized to receive either 30 mg oral hydrocortisone or placebo prior to PE sessions 3-10 in a double-blind protocol. Glucocorticoid receptor sensitivity was assessed in cultured peripheral blood mononuclear cells (PBMC) using the in vitro lysozyme inhibition test and was determined before and after treatment. Intent-to-treat analysis was performed using latent growth curve modeling of treatment effects on change in PTSD severity over time. Veterans who no longer met diagnostic criteria for PTSD at post-treatment were designated as responders.

RESULTS

Veterans randomized to hydrocortisone or placebo augmentation did not differ significantly in clinical severity or glucocorticoid sensitivity at pre-treatment. Hydrocortisone augmentation was associated with greater reduction in total PTSD symptoms compared to placebo, a finding that was explained by significantly greater patient retention in the hydrocortisone augmentation condition. A significant treatment condition by responder status interaction for glucocorticoid sensitivity indicated that responders to hydrocortisone augmentation had the highest pre-treatment glucocorticoid sensitivity (lowest lysozyme IC50-DEX) that diminished over the course of treatment. There was a significant association between decline in glucocorticoid responsiveness and improvement in PTSD symptoms among hydrocortisone recipients.

CONCLUSIONS

The results of this pilot study suggest that hydrocortisone augmentation of PE may result in greater retention in treatment and thereby promote PTSD symptom improvement. Further, the results suggest that particularly elevated glucocorticoid responsiveness at pre-treatment may identify veterans likely to respond to PE combined with an intervention that targets glucocorticoid sensitivity. Confirmation of these findings will suggest that pharmacologic interventions that target PTSD-associated glucocorticoid dysregulation may be particularly helpful in promoting a positive clinical response to PTSD psychotherapy.

High-dose corticosterone after fear conditioning selectively suppresses fear renewal by reducing anxiety-like response

Exposure therapy is widely used to treat anxiety disorders, including posttraumatic stress disorder (PTSD). However, preventing the return of fear is still a major challenge after this behavioral treatment. An increasing number of studies suggest that high-dose glucocorticoid treatment immediately after trauma can alleviate the symptoms of PTSD in humans. Unknown is whether high-dose glucocorticoid treatment following fear conditioning suppresses the return of fear. In the present study, a typical fear renewal paradigm (AAB) was used, in which the fear response to an auditory cue can be restored in a novel context (context B) when both training and extinction occur in the same context (context A). We trained rats for auditory fear conditioning and administered corticosterone (CORT; 5 and 25mg/kg, i.p.) or vehicle with different delays (1 and 24h). Forty-eight hours after drug injection, extinction was conducted with no drug in the training context, followed by a test of tone-induced freezing behavior in the same (AAA) or a shifted (AAB) context. Both immediate and delayed administration of high-dose CORT after fear conditioning reduced fear renewal. To examine the anxiolytic effect of CORT, independent rats were trained for cued or contextual fear conditioning, followed by an injection of CORT (5 and 25mg/kg, i.p.) or vehicle at a 1 or 24h delay. One week later, anxiety-like behavior was assessed in the elevated plus maze (EPM) before and after fear expression. We found that high-dose CORT decreased anxiety-like behavior without changing tone- or context-induced freezing. These findings indicate that a single high-dose CORT administration given after fear conditioning may selectively suppress fear renewal by reducing anxiety-like behavior and not by altering the consolidation, retrieval, or extinction of fear memory.

Basolateral amygdala GABA-A receptors mediate stress-induced memory retrieval impairment in rats

The present study was designed to investigate the involvement of GABA-A receptors of the basolateral amygdala (BLA) in the impairing effect of acute stress on memory retrieval. The BLAs of adult male Wistar rats were bilaterally cannulated and memory retrieval was measured in a step-through type passive avoidance apparatus. Acute stress was evoked by placing the animals on an elevated platform for 10, 20 and 30 min. The results indicated that exposure to 20 and 30 min stress, but not 10 min, before memory retrieval testing (pre-test exposure to stress) decreased the step-through latency, indicating stress-induced memory retrieval impairment. Intra-BLA microinjection of a GABA-A receptor agonist, muscimol (0.005-0.02 μg/rat), 5 min before exposure to an ineffective stress (10 min exposure to stress) induced memory retrieval impairment. It is important to note that pre-test intra-BLA microinjection of the same doses of muscimol had no effect on memory retrieval in the rats unexposed to 10 min stress. The blockade of GABA-A receptors of the BLA by injecting an antagonist, bicuculline (0.4-0.5 μg/rat), 5 min before 20 min exposure to stress, prevented stress-induced memory retrieval. Pre-test intra-BLA microinjection of the same doses of bicuculline (0.4-0.5 μg/rat) in rats unexposed to 20 min stress had no effect on memory retrieval. In addition, pre-treatment with bicuculline (0.1-0.4 μg/rat, intra-BLA) reversed muscimol (0.02 μg/rat, intra-BLA)-induced potentiation on the effect of stress in passive avoidance learning. It can be concluded that pre-test exposure to stress can induce memory retrieval impairment and the BLA GABA-A receptors may be involved in stress-induced memory retrieval impairment.

Hippocampal corticosterone impairs memory consolidation during sleep but improves consolidation in the wake state

We studied the interaction between glucocorticoid (GC) level and sleep/wake state during memory consolidation. Recent research has accumulated evidence that sleep supports memory consolidation in a unique physiological process, qualitatively distinct from consolidation occurring during wakefulness. This appears particularly true for memories that rely on the hippocampus, a region with abundant expression of GC receptors. Against this backdrop we hypothesized that GC effects on consolidation depend on the brain state, i.e., sleep and wakefulness. Following exploration of two objects in an open field, during 80 min retention periods rats received an intrahippocampal infusion of corticosterone (10 ng) or vehicle while asleep or awake. Then the memory was tested in the hippocampus-dependent object-place recognition paradigm. GCs impaired memory consolidation when administered during sleep but improved consolidation during the wake retention interval. Intrahippocampal infusion of GC or sleep/wake manipulations did not alter novel-object recognition performance that does not require the hippocampus. This work corroborates the notion of distinct consolidation processes occurring in sleep and wakefulnesss, and identifies GCs as a key player controlling distinct hippocampal memory consolidation processes in sleep and wake conditions.

Stress and glucocorticoid receptor-dependent mechanisms in long-term memory: from adaptive responses to psychopathologies

A proper response against stressors is critical for survival. In mammals, the stress response is primarily mediated by secretion of glucocorticoids via the hypothalamic-pituitary-adrenocortical (HPA) axis and release of catecholamines through adrenergic neurotransmission. Activation of these pathways results in a quick physical response to the stress and, in adaptive conditions, mediates long-term changes in the brain that lead to the formation of long-term memories of the experience. These long-term memories are an essential adaptive mechanism that allows an animal to effectively face similar demands again. Indeed, a moderate stress level has a strong positive effect on memory and cognition, as a single arousing or moderately stressful event can be remembered for up to a lifetime. Conversely, exposure to extreme, traumatic, or chronic stress can have the opposite effect and cause memory loss, cognitive impairments, and stress-related psychopathologies such as anxiety disorders, depression and post-traumatic stress disorder (PTSD). While more effort has been devoted to the understanding of the negative effects of chronic stress, much less has been done thus far on the identification of the mechanisms engaged in the brain when stress promotes long-term memory formation. Understanding these mechanisms will provide critical information for use in ameliorating memory processes in both normal and pathological conditions. Here, we will review the role of glucocorticoids and glucocorticoid receptors (GRs) in memory formation and modulation. Furthermore, we will discuss recent findings on the molecular cascade of events underlying the effect of GR activation in adaptive levels of stress that leads to strong, long-lasting memories. Our recent data indicate that the positive effects of GR activation on memory consolidation critically engage the brain-derived neurotrophic factor (BDNF) pathway. We propose and will discuss the hypothesis that stress promotes the formation of strong long-term memories because the activation of hippocampal GRs after learning is coupled to the recruitment of the growth and pro-survival BDNF/cAMP response element-binding protein (CREB) pathway, which is well-know to be a general mechanism required for long-term memory formation. We will then speculate about how these results may explain the negative effects of traumatic or chronic stress on memory and cognitive functions.

Stress enhances the consolidation of extinction memory in a predictive learning task

Extinction is not always permanent, as indicated by several types of recovery effects, such as the renewal effect, which may occur after a context change and points towards the importance of contextual cues. Strengthening the retrieval of extinction memory is a crucial aim of extinction-based psychotherapeutic treatments of anxiety disorders to prevent relapse. Stress is known to modulate learning and memory, with mostly enhancing effects on memory consolidation. However, whether such a consolidation-enhancing effect of acute stress can also be found for extinction memory has not yet been examined in humans. In this study, we investigated the effect of stress after extinction learning on the retrieval of extinction memory in a predictive learning renewal paradigm. Participants took the part of being the doctor of a fictitious patient and learned to predict whether certain food stimuli were associated with "stomach trouble" in two different restaurants (contexts). On the first day, critical stimuli were associated with stomach trouble in context A (acquisition phase). On the second day, these associations were extinguished in context B. Directly after extinction, participants were either exposed to a stressor (socially evaluated cold pressor test; n = 22) or a control condition (n = 24). On the third day, we tested retrieval of critical associations in contexts A and B. Participants exposed to stress after extinction exhibited a reduced recovery of responding at test in context B, suggesting that stress may context-dependently enhance the consolidation of extinction memory. Furthermore, the increase in cortisol in response to the stressor was negatively correlated with the recovery of responding in context A. Our findings suggest that in parallel to the known effects of stress on the consolidation of episodic memory, stress also enhances the consolidation of extinction memory, which might be relevant for potential applications in extinction-based psychotherapy.

Mild acute stress reactivates memory of a discriminative avoidance task in mice

Previous studies have demonstrated that stress or glucocorticoids impair the retrieval of spatial memory in rodents and declarative memory in humans. However, the effects on memory retrieval of stress introduced a long time after learning have not been well studied. We investigated whether a mild, extrinsic stressor (1-s 0.1 or 0.3 mA foot shock) would reactivate low baseline retrieval of an aversive memory [the plus-maze discriminative avoidance task (PM-DAT)] and if it would be modulated by glucocorticoids. In Experiment 1, male Swiss mice received pre-test foot shock (n = 10 mice/group) 7 days after training and just before testing. A time-retrieval curve for low baseline retrieval for the subsequent experiments was also determined (Experiment 2, n = 10 mice/group). We investigated if pre-test foot shock could modify corticosterone release (Experiment 3, n = 8-9 mice/group) and reinstate retrieval in the PM-DAT (Experiment 4, n = 15 mice/group). The effects of metyrapone (100 mg/kg) on retrieval reinstatement (Experiment 5, n = 15 mice/group) and serum corticosterone enhancement (Experiments 6, n = 7-9 mice/group) induced by foot shock were analyzed. Finally, the effects of foot shock itself on PM-DAT exploration were verified (Experiment 7, n = 10 mice/group). We demonstrated here that foot shock reinstated the retrieval of a low baseline, discriminative avoidance task 30 (but not 7) days after training. This facilitative effect was not dependent on corticosterone secretion because metyrapone abolished the enhancement of corticosterone concentration but did not reverse the stress-induced reinstatement of retrieval.

Stress modulation of reconsolidation

Memories are consolidated and are inscribed as stable traces in the brain; however, once they are retrieved, they are rendered labile and can be modified in a process termed reconsolidation. Studies illustrate the power of behavioral stress and stress hormones to modulate memory processes while focusing on consolidation. However, sparse evidence indicates a critical role of stress in modulating reconsolidation. In this review, we discuss the effects of stress and stress-related neurotransmitter systems on reconsolidation of emotional and non-emotional types of memories. We show that although some general features underlie consolidation and reconsolidation, there is a possible dissimilarity between the two processes that may be dependent on factors such as the cognitive task employed, specific type of stressor, and the arousal state of the animal. The ability to disrupt or facilitate the reconsolidation of emotional and drug-related memories by stress exposure has important implications for the treatment of anxiety disorders linked to traumatic memories, such as post-traumatic stress disorder and of drug-of-abuse memories.

Stimulation of perforant path fibers induces LTP concurrently in amygdala and hippocampus in awake freely behaving rats

Long-term potentiation (LTP) which has long been considered a cellular model for learning and memory is defined as a lasting enhancement in synaptic transmission efficacy. This cellular mechanism has been demonstrated reliably in the hippocampus and the amygdala-two limbic structures implicated in learning and memory. Earlier studies reported on the ability of cortical stimulation of the entorhinal cortex to induce LTP simultaneously in the two sites. However, to retain a stable baseline of comparison with the majority of the LTP literature, it is important to investigate the ability of fiber stimulation such as perforant path activation to induce LTP concurrently in both structures. Therefore, in this paper we report on concurrent LTP in the basolateral amygdala (BLA) and the dentate gyrus (DG) subfield of the hippocampus induced by theta burst stimulation of perforant path fibers in freely behaving Sprague-Dawley rats. Our results indicate that while perforant path-evoked potentials in both sites exhibit similar triphasic waveforms, the latency and amplitude of BLA responses were significantly shorter and smaller than those of DG. In addition, we observed no significant differences in either the peak level or the duration of LTP between DG and BLA.

Order of exposure to pleasant and unpleasant odors affects autonomic nervous system response

When mammals are exposed to an odor, that odor is expected to elicit a physiological response in the autonomic nervous system. An unpleasant aversive odor causes non-invasive stress, while a pleasant odor promotes healing and relaxation in mammals. We hypothesized that pleasant odors might reduce a stress response previously induced by an aversive predator odor. Rats were thus exposed to pleasant and unpleasant odors in different orders to determine whether the order of odor exposure had an effect on the physiological response in the autonomic nervous system. The first trial examined autonomic nerve activity via sympathetic and parasympathetic nerve response while the second trial examined body temperature response. Initial exposure to a pleasant odor elicited a positive response and secondary exposure to an unpleasant odor elicited a negative response, as expected. However, we found that while initial exposure to an unpleasant odor elicited a negative stress response, subsequent secondary exposure to a pleasant odor not only did not alleviate that negative response, but actually amplified it. These findings were consistent for both the autonomic nerve activity response trial and the body temperature response trial. The trial results suggest that exposure to specific odors does not necessarily result in the expected physiological response and that the specific order of exposure plays an important role. Our study should provide new insights into our understanding of the physiological response in the autonomic nervous system related to odor memory and discrimination and point to areas that require further research.

Stress-induced memory retrieval impairments: different time-course involvement of corticosterone and glucocorticoid receptors in dorsal and ventral hippocampus

The present study was aimed at determining the relative contribution of the dorsal (DH) and ventral (VH) hippocampus in stress-induced memory retrieval impairments. Thus, we studied the temporal involvement of corticosterone and its receptors, i.e. mineralocorticoid (MR) and glucocorticoid (GR) in the DH and VH, in relation with the time-course evolution of stress-induced memory retrieval impairments. In a first experiment, double microdialysis allowed showing on the same animal that an acute stress (electric footshocks) induced an earlier corticosterone rise in the DH (15-60 min post-stress) and then in the VH (90-105 min post-stress). The return to baseline was faster in the DH (105 min) than in the VH (120 min). Memory deficits assessed by delayed alternation occurred at 15-, 60-, and 105-min delays after stress and were closely related to the kinetic of corticosterone rises within the DH and VH. In a second experiment, the GR antagonist RU-38486 and the MR antagonist RU-28318 were administered in the DH or VH 15 min before stress. RU-38486 restored memory at 60 but not at 105 min post-stress delays in the DH, whereas the opposite pattern was observed in the VH. By contrast, RU-28318 had no effect on memory impairments at both the 60- and 105-min post-stress delays, showing that MR receptors are not involved at these delays. However, RU-28318 administered in the DH restored memory when administered at a shorter post-stress delay (15 min). Overall, our data are first to evidence that stress induces a functional switch from the DH to VH via different corticosterone time-course evolutions in these areas and the sequential GR receptors involvement in the DH and then in the VH, as regards the persistence of stress-induced memory retrieval deficits over time.