Behavioral stress impairs long-term potentiation in rodent hippocampus

A number of hormones secreted from the pituitary-adrenal system during stress affect learning and memory processes. The phenomenon of hippocampal long-term potentiation (LTP) is viewed by many as a putative mechanism of memory storage and has proved a most valuable model for study of neuronal plasticity at the cellular level. The present study was conducted to investigate the possibility that stressful events which occur prior (in vivo) to the preparation of brain slices may influence the electrophysiology of the in vitro hippocampal explant when tested for LTP. Adult male rats (Long-Evans male X Sprague-Dawley female) were pair-housed 1 week prior to testing. One animal in each pair was either placed in a restraining tube for 30 min and received no tail shocks (Restraint) or placed in a restraining tube and received tail shocks (1 microA, 1 s) every minute for 30 min (Restraint + Shock). The other animal in each pair was taken directly from the home cage and received no restraint or tail shock (Control). In vitro hippocampal slices were then prepared immediately from these animals according to standard methods. Our results demonstrate a marked impairment of LTP in hippocampal explants taken from rats exposed to stress. The significance of this result with respect to cellular mechanisms underlying the relationship between stress, cognition, and learning is discussed.     

The effects of acute 17beta-estradiol treatment on gene expression in the young female mouse hippocampus

Previous studies have demonstrated that treatment with 17beta-estradiol (E(2)) improves both spatial and nonspatial memory in young female mice. Still unclear, however, are the molecular mechanisms underlying the beneficial effects of E(2) on memory. We have previously demonstrated that a single post-training intraperitoneal (i.p.) injection of 0.2 mg/kg E(2) can enhance hippocampal-dependent spatial and object memory consolidation (e.g., Gresack & Frick, 2006b). Therefore, in the present study, we performed a microarray analysis on the dorsal hippocampi of 4-month-old female mice injected i.p. with vehicle or 0.2 mg/kg E(2). Genes were considered differentially expressed following E(2) treatment if they showed a greater than 2-fold change in RNA expression levels compared to controls. Overall, out of a total of approximately 25,000 genes represented on the array, 204 genes showed altered mRNA expression levels upon E(2) treatment, with 111 up-regulated and 93 down-regulated. Of these, 17 of the up-regulated and 6 of the down-regulated genes are known to be involved in learning and memory. mRNA expression changes in 5 of the genes were confirmed by real-time quantitative PCR analysis, and protein changes in these same genes were confirmed by Western blot analysis: Hsp70, a heat shock protein known to be estrogen responsive; Igfbp2, an IGF-I binding protein; Actn4, an actin binding protein involved in protein trafficking; Tubb2a, the major component of microtubules; and Snap25, a synaptosome-specific protein required for neurotransmitter release. The types of genes altered indicate that E(2) may induce changes in the structural mechanics of cells within the dorsal hippocampus that could be conducive to promoting memory consolidation.     

The hippocampus and prefrontal cortex are differentially involved in serial memory retrieval in non-stress and stress conditions

We previously showed that 24 h after learning, mice significantly remembered the first (D1) but not the second (D2) discrimination in a serial spatial task and that an acute stress delivered 5 min before the test phase reversed this memory retrieval pattern.A first experiment evaluated the effects of dorsal hippocampus (HPC) or prefrontal cortex (PFC) lesions, these two brain areas being well-known for their involvement in serial and spatial memory processes. For this purpose, six independent groups of mice were used: non-lesioned (controls), PFC or HPC-lesioned animals, submitted or not to an acute stress (electric footshocks; 0.9 mA). Results show that (i) non-stressed controls as well as PFC-lesioned mice (stressed or not) remembered D1 but not D2; (ii) stressed controls and HPC-lesioned mice (stressed or not) remembered D2 but not D1; (iii) stress significantly increased plasma corticosterone in controls and PFC-lesioned mice, but not in HPC-lesioned mice which already showed a significant plasma corticosterone increase in non-stressed condition.Since data from this first experiment showed that stress inhibited the hippocampal-dependent D1 memory retrieval, a second experiment evaluated the behavioral effect of intrahippocampal corticosterone injection in non-stressed mice. Results show that intrahippocampal corticosterone injection induced a reversal of serial memory retrieval pattern similar to that induced by acute stress.Overall, our study shows that (i) in non-stress condition, the emergence of D1 is HPC-dependent; (ii) in stress condition, the emergence of D2 requires the PFC integrity; moreover, intrahippocampal corticosterone injection mimicked the effects of stress in the CSD task.     

The effects of experimentally induced stress on false recognition.

The fallibility of memory has become an issue of considerable practical and theoretical importance. Here we studied the impact of experimentally induced stress on the ability of human participants to accurately recognise words presented on a list. We found that stress selectively disrupted participants' ability to distinguish words that were presented for study from critical lure words that were semantically related, but not presented for study. This finding indicates that stress, possibly through its impact on the hippocampus and prefrontal cortex, can potentiate false memories.     

The perception of team engagement reduces stress induced situation awareness overconfidence and risk-taking

Much research has examined how stress restricts objective Situation Awareness (SA). Little research, however, has focused on SA overconfidence, the notion that an individual may grasp a situation when in fact they do not. Even less SA research has examined the motivational and emotional states of individuals operating in teams in stressful environments. Expanding on recent data suggesting that stress creates SA overconfidence, not simply SA loss, the present experiment manipulated stress levels and the perception of team engagement, which is thought to be a positive motivational state of task-related well-being. Teams of Soldiers were tested in a virtual combat scenario testing shared risk-taking, objective (i.e., collaborative ability to answer SA probes), and subjective SA. Results indicated that the mere perception of above average team engagement reduced stress induced SA overconfidence and risk-taking of teams. These results suggest simple, virtually costless strategies for improving elements of SA that may impact the behavior of teams and potentially improve their decision-making.     

Limbic hypertension induced by stress and septal stimulation

For this study, it was postulated that hypertension of emotional origin is generated within the limbic system. To validate this thesis, septal induced blood pressure elevations were combined with a stressful experience in adult rats (spontaneously hypertensive and Wistar Kyoto rats). Stress consisted of intermittent confinement in a plexiglass tube. The results revealed that hypertension induced by electric stimulation of the septum during the stressful state could subsequently be elicited by the stressful state alone, without electric stimulation of the septum. It was postulated that the cortically modulated stress circuit converged with the electrically activated limbic pressor circuit at the level of the septum. This convergence resulted in the formation of one cortico-limbic circuitry which could be activated by stress alone. This underlying mechanism may be considered as a model of neural sensitization in the production of stress-induced limbic hypertension.     

The impact of semantic distance and induced stress on analogical reasoning: A neurocomputational account

In a study of reasoning with four-term verbal analogy problems, we explored the relationship between the effects of an acute, mild stressor and the complexity of the reasoning process. Participants judged whether analogy problems in the form A:B :: C:D were valid or invalid, on the basis of whether the relation in the A:B term matched that in the C:D term. Half of the problems contained a C:D pair semantically near the A:B pair (e.g., NOSE:SCENT :: TONGUE:TASTE), and the other half contained ones semantically far from A:B (e.g., NOSE:SCENT :: ANTENNA:SIGNAL). After an initial block without stress, participants were randomly assigned to count backward by 13?s from 1,000 while being told to go faster, or to count forward by 1?s from 0. The stress-induced participants reported a significant increase in state anxiety as compared to controls immediately after the mental arithmetic task. Stressed participants performed less accurately (as measured by d') on both near and far analogy problems, mainly due to an increase in false alarms. We were able to model the influence of semantic distance using the ??learning and inference with schemas and analogies?? (LISA) model. Our findings indicated that even mild increases in stress impair analogical reasoning. However, the decrement does not seem to directly involve the integration of relations, but rather is due to a shift in decision strategy: Under stress, people show an increased tendency to endorse analogies as valid when the terms in the individual pairs are semantically related to each other, even if the overall analogical relationship is not valid.     

Consistency in physiological stress responses and electromyographic activity during induced stress exposure in women and men

Physiological responses serve the role as objective indicators of stress as well as a link between psychosocial stress and various health outcomes. The aim of the present exposure session was to compare different physiological stress responses (systolic and diastolic blood pressure, heart rate, urinary epinephrine and norepinephrine, salivary cortisol) as well as trapezius muscle activity, measured by surface electromyography, during mental and physical stress in 11 women and ten men. The results show significantly increased activity in all measures but cortisol and significant associations between sympathetic arousal and EMG activity. The association between sympathetic arousal and muscle activity is of importance for understanding the high prevalence of musculoskeletal disorders in mentally stressful but physically light work tasks. Men had higher blood pressure and a more pronounced increase in epinephrine output than women, whereas women had higher heart rate. It was concluded that sympathetic activity is more sensitive to moderately intense stress exposure than pituitary adrenocortical (cortisol) activity and that men respond to performance stress with more epinephrine output than women. Although the correlations between the different indicators of sympathetic arousal were high, together they could still only explain 30–70% of the inter-individual variance. Thus, several parameters are needed in order to obtain a reliable measure of sympathetic activity.     

The effects of attentional training on physiological stress recovery after induced social threat

Social stress has been related to both physiological and psychological responses. The recovery of stress is influenced by the way environmental information is processed, i.e., what information is attended to and how it is interpreted. The present study investigated the effect of attention modification training on physiological stress recovery following induced stress. A group of applicants preparing for a job interview were randomly allocated to an attention training condition versus a control condition. Afterwards, they were asked to give an artificial job interview to a real life jury while being videotaped. Participants in both conditions showed decreased heart rate variability during the job interview, which indicates an overall physiological stress response. However, during a 30-minute follow-up period after the training, indices of heart rate variability showed improved stress recovery only in the attention training condition. Attentional training is proposed as a promising strategy to prevent enduring stress reactions.     

The effects of attentional training on physiological stress recovery after induced social threat

Abstract

Social stress has been related to both physiological and psychological responses. The recovery of stress is influenced by the way environmental information is processed, i.e., what information is attended to and how it is interpreted. The present study investigated the effect of attention modification training on physiological stress recovery following induced stress. A group of applicants preparing for a job interview were randomly allocated to an attention training condition versus a control condition. Afterwards, they were asked to give an artificial job interview to a real life jury while being videotaped. Participants in both conditions showed decreased heart rate variability during the job interview, which indicates an overall physiological stress response. However, during a 30-minute follow-up period after the training, indices of heart rate variability showed improved stress recovery only in the attention training condition. Attentional training is proposed as a promising strategy to prevent enduring stress reactions.     

The hippocampus and memory of verbal and pictorial material

Recognition of words and kaleidoscope pictures showed a double dissociation of left and right hippocampal activity using magnetic source imaging (MSI). MSI has advantages over alternative imaging techniques that measure hemodynamic changes for identifying regional changes in brain activity in real time and on an individual subject basis without the need for image subtraction. In this study, lists of words or kaleidoscope pictures were presented for memorization followed by tests of list items and foils during which brain activity was recorded. There was greater activation in the left than the right hippocampus with abstract nouns (e.g., relief) and greater activation in the right than the left hippocampus with kaleidoscope pictures. This dissociation was evident on a case by case basis. This study demonstrates the specialization of the two medial temporal lobe (MTL) regions, including the hippocampi, for mnemonic processing of verbal and pictorial items that are difficult to encode verbally.     

The hippocampus as an odour encoder

Summaries of recently published papers of interest to cognitive scientists. Readers who would like to contribute to this section, by identifying appropriate papers and writing short summaries, should contact the Editor (tics@current-trends.com).     

Differential effects of predator stress and the antidepressant tianeptine on physiological plasticity in the hippocampus and basolateral amygdala

Stress can profoundly affect memory and alter the functioning of the hippocampus and amygdala. Studies have also shown that the antidepressant tianeptine can block the effects of stress on hippocampal and amygdala morphology and synaptic plasticity. We examined the effects of acute predator stress and tianeptine on long-term potentiation (LTP; induced by 100 pulses in 1 s) and primed burst potentiation (PB; a low threshold form of LTP induced by only five physiologically patterned pulses) in CA1 and in the basolateral nucleus (BLA) of the amygdala in anesthetized rats. Predator stress blocked the induction of PB potentiation in CA1 and enhanced LTP in BLA. Tianeptine blocked the stress-induced suppression of PB potentiation in CA1 without affecting the stress-induced enhancement of LTP in BLA. In addition, tianeptine administered under non-stress conditions enhanced PB potentiation in the hippocampus and LTP in the amygdala. These findings support the hypothesis that acute stress impairs hippocampal functioning and enhances amygdaloid functioning. The work also provides insight into the actions of tianeptine with the finding that it enhanced electrophysiological measures of plasticity in the hippocampus and amygdala under stress, as well as non-stress, conditions.     

Acute stress responses after indirect exposure to the MH17 airplane crash

People can experience disasters vicariously (indirectly) via conversation, social media, radio, and television, even when not directly involved in a disaster. This study examined whether vicarious exposure to the MH17‐airplane crash in Ukraine, with 196 Dutch victims, elicited affective and somatic responses in Dutch adults about 2,600 km away, who happened to participate in an ongoing diary study. Participants (n = 141) filled out a diary three times a day for 30 days on their smartphones. Within‐person changes in positive affect (PA) and negative affect (NA) and somatic symptoms after the crash were studied. Additionally, we tested whether between‐person differences in response could be explained by age, baseline personality (NEO‐FFI‐3), and media exposure. The MH17 crash elicited a small within‐person decrease in PA and an increase in NA and somatic symptoms. This response waned after 3 days and returned to baseline at day four. The decrease in PA was larger in more extraverted participants but smaller in those higher on neuroticism or conscientiousness. The NA response was smaller in elderly. Personality did not seem to moderate the NA and somatic response, and neither did media exposure. Dutch participants showed small acute somatic and affective responses up till 3 days to a disaster that they had not directly witnessed. Vicariously experienced disasters can thus elicit affective‐visceral responses indicative of acute stress reactions. Personality and age explained some of the individual differences in this reaction.