Spatial memory under acute cold and restraint stress

Acute stress differentially alters reward-related decision making and inhibitory control under threat of punishment

Acute stress has various effects on cognition, executive function and certain forms of cost/benefit decision making. Recent studies in rodents indicate that acute stress differentially alters reward-related decisions involving particular types of costs and slows choice latencies. Yet, how stress alters decisions where rewards are linked to punishment is less clear. We examined how 1 h restraint stress, followed by behavioral testing 10 min later altered action-selection on two tasks involving reward-seeking under threat of punishment in well-trained male and female rats. One study used a risky decision-making task involving choice between a small/safe reward and a large/risky one that could coincide with shock, delivered with a probability that increased over blocks of trials. Stress increased risk aversion and punishment sensitivity, reducing preference for the larger/risky reward, while increasing decision latencies and trial omissions in both sexes, when rats were teste. A second study used a "behavioral control" task, requiring inhibition of approach towards a readily available reward associated with punishment. Here, food pellets were delivered over discrete trials, half of which coincided with a 12 s audiovisual cue, signalling that reward retrieval prior to cue termination would deliver shock. Stress exerted sex- and timing-dependent effects on inhibitory control. Males became more impulsive and received more shocks on the stress test, whereas females were unaffected on the stress test, and were actually less impulsive when tested 24 h later. None of the effects of restraint stress were recapitulated by systemic treatment with physiological doses of corticosterone. These findings suggest acute stress induces qualitatively distinct and sometimes sex-dependent effects on punished reward-seeking that are critically dependent on whether animals must either choose between different actions or withhold them to obtain rewards and avoid punishment.

Evaluation of the Effect of Hypericum triquetrifolium Turra on Memory Impairment Induced by Chronic Psychosocial Stress in Rats: Role of BDNF

Background

Chronic psychosocial stress impairs memory function and leads to a depression-like phenotype induced by a persistent status of oxidative stress. Hypericum perforatum L. (St. John's wort) is widely used to relieve symptoms of anxiety and depression; however, its long-term use is associated with adverse effects. Hypericum triquetrifolium Turra is closely related to H. perforatum. Both plants belong to Hypericaceae family and share many biologically active compounds. Previous work by our group showed that methanolic extracts of H. triquetrifolium have potent antioxidant activity as well as high hypericin content, a component that proved to have stress-relieving and antidepressant effects by other studies. Therefore, we hypothesized that H. triquetrifolium would reduce stress-induced cognitive impairment in a rat model of chronic stress.

Objective

To determine whether chronic treatment with H. triquetrifolium protects against stress-associated memory deficits and to investigate a possible mechanism.

Methods

The radial arm water maze (RAWM) was used to test learning and memory in rats exposed to daily stress using the resident-intruder paradigm. Stressed and unstressed rats received chronic H. triquetrifolium or vehicle. We also measured levels of brain-derived neurotrophic factor (BDNF) in the hippocampus, cortex and cerebellum.

Results

Neither chronic stress nor chronic H. triquetrifolium administration affected performance during acquisition. However, memory tests in the RAWM showed that chronic stress impaired different post-encoding memory stages. H. triquetrifolium prevented this impairment. Furthermore, hippocampal BDNF levels were markedly lower in stressed animals than in unstressed animals, and chronic administration of H triquetrifolium chronic administration protected against this reduction. No significant difference was observed in the effects of chronic stress and/or H. triquetrifolium treatment on BDNF levels in the cerebellum and cortex.

Conclusion

H. triquetrifolium extract can oppose stress-associated hippocampus-dependent memory deficits in a mechanism that may involve BDNF in the hippocampus.

Effects of Two Types of Restraint Stress on the Learned Behaviour in Rats

To study the effects of stress on cognitive functions, Wistar and Lewis rats were exposed to restraint (immobilization stressor) (IMO) or restraint combined with partial immersion into water (IMO+C). Learned discriminatory avoidance response in Y-maze, with foot-shock as an unconditioned stimulus, was used as a memory test. The latency to enter the correct arm and number of wrong entries were daily recorded during the training period (20 days) until the criterion was reached, which was set at 90 % avoidances (choosing the correct arm). After exposure of rats to one of the stressors for 60 min, the rats were returned to the home cage; the latency to enter the safe arm was recorded in 6 daily trials that started 1 h after application of stressor. Both stressors significantly prolonged the avoidance latencies for 2 or 3 days in Wistar and Lewis rats, respectively; then the latencies returned to the values obtained before the stress exposure. In Lewis rats, the latencies more increased after IMO+C than after IMO stressor, and the maximal increase in latencies was higher in Lewis rats than in Wistar rats. The latency did not reach the time limit for foot-shock delivery, and the number of correct choices remained unchanged in both strains. The results indicate that the used restraint stressors did not affect the long-term memory; rather a transient impairment of retrieval can be considered. Further, differences in response of Lewis and Wistar rats may be interpreted by different activity of hypothalamic-pituitary-adrenal axis activity in used strains.

Effects of Two Types of Restraint Stress on the Spontaneous Behaviour in Rats

Our previous findings suggested the existence of stressor-specific behavioural and cognitive responses in rats. In the present study, restraint stressor (immobilization, IMO) and restraint stressor combined with partial immersion of rats into water (IMO+C) were applied for 1 hour to Wistar male rats and their spontaneous behaviour was examined in the open field test. The classic behavioural parameters were recorded: crossing, rearing, and resting. When tested 1 and 4 hours after IMO+C, animals exhibited strong suppression of locomotor and exploratory activity (crossing and rearing); partial inhibition of both behavioural variables was found after IMO. Thus, substantial differences were observed in dependence on the length of period between the end of stressor application and the start of testing. In testing performed one week later, the locomotor and exploratory activity levels of both IMO and IMO+C animals corresponded to the control ones. These data suggest a differential behavioural response to both used stressors that may result from their different proportion of psychical and physical components. In conclusion, our results provide other data for the support of differential effects of two types of restraint stressors on spontaneous behaviour of animals exposed to a novel environment.

Acute psychosocial stress in mid-aged male rats causes hyperthermia, cognitive decline, and increased deep sleep power, but does not alter deep sleep duration

Aging is associated with altered sleep architecture and worsened hippocampus-dependent cognition, highly prevalent clinical conditions that detract from quality of life for the elderly. Interestingly, exposure to psychosocial stress causes similar responses in young subjects, suggesting that age itself may act as a stressor. In prior work, we demonstrated that young animals show loss of deep sleep, deficits in cognition, and elevated body temperature after acute stress exposure, whereas aged animals are hyporesponsive on these measures. However, it is unclear if these age-altered stress responses occur in parallel over the course of aging. To address this, here we repeated the experiment in mid-aged animals. We hypothesized that mid-aged stress responses would be intermediate between those of young and aged subjects. Sixteen mid-aged (12 months) male F344 rats were implanted with EEG/EMG emitters to monitor sleep architecture and body temperature, and were trained on the Morris water maze for 3 days. On the fourth day, half of the subjects were restrained for 3 hours immediately before the water maze probe trial. Sleep architecture and body temperature were measured during the ensuing inactive period, and on the following day, endpoint measures were taken. Restrained mid-aged animals showed resistance to deep sleep loss, but demonstrated stress-induced water maze probe trial performance deficits as well as postrestraint hyperthermia. Taken in the context of prior work, these data suggest that age-related loss of sleep architecture stress sensitivity may precede both cognitive and body temperature-related stress insensitivity.

Learning aptitude, spatial orientation and cognitive flexibility tested in a virtual labyrinth after virtual stress induction

BACKGROUND

Under stressful conditions such as in an emergency situation, efficient information processing is essential for reasonable responses.

PURPOSE OF THE STUDY

Virtual Reality (VR) technology is used to induce stress and to test three main cognitive functions for decision making in stressful situations.

METHODS

A VR task was developed to induce stress following the Trier Social Stress Test (TSST) protocol and two VR cognitive performance tests to measure learning aptitude, spatial orientation and cognitive flexibility. Participants (N = 31) gave a public speech in front of a virtual audience (TSST) and later had to find their way out of different VR labyrinths. The first exercise tested spatial orientation and learning aptitude where participants had to learn aspects of the ground layout and geometric icons had to be identified as correct in order to be able to exit. The second labyrinth tested cognitive flexibility on the background of the Wisconsin Card Sorting Test.

STATISTICAL TESTS

Correlations were analyzed using Kendall Tau Correlation (One-tailed tests with p set to 0.05 for all analyses). Heart rate (HR) was calculated from the RR time values and averaged across the TSST- speech and the post-stress period. Autonomic nervous system reactivity was defined as the deviation of HR during TSST- speech condition from post-stress baseline measurement. A repeated-measures t-test was used to analyze differences.

RESULTS

The newly developed virtual stress test was successfully adapted from the original TSST. Participants perceived the task as stressful and scored an average of 5.7 points on a 1-8 Likert Scale. As a physiological stress parameter, increased heart rates of the participants showed that they were more stressed during the TSST procedure compared to the post-stress period. Also, the subjective stress perception, has a strong correlation with the results of the cognitive tasks performed after the stress induction.

CONCLUSIONS

The more a participant experienced the TSST as stressful, the lower their learning aptitude and spatial orientation were found to be at the end of the study. On the other hand, if someone perceived the virtual TSST as "unexpected", as an indicator for a mild stress response, their cognitive flexibility was improved. Potential Implications: The findings indicate that both, the VR stress induction scenario using TSST, as well as the VR cognitive tests, are a first successful step towards a better ecological validity in neuropsychological testing.

Aged rats are hypo-responsive to acute restraint: implications for psychosocial stress in aging

Cognitive processes associated with prefrontal cortex and hippocampus decline with age and are vulnerable to disruption by stress. The stress/stress hormone/allostatic load hypotheses of brain aging posit that brain aging, at least in part, is the manifestation of life-long stress exposure. In addition, as humans age, there is a profound increase in the incidence of new onset stressors, many of which are psychosocial (e.g., loss of job, death of spouse, social isolation), and aged humans are well-understood to be more vulnerable to the negative consequences of such new-onset chronic psychosocial stress events. However, the mechanistic underpinnings of this age-related shift in chronic psychosocial stress response, or the initial acute phase of that chronic response, have been less well-studied. Here, we separated young (3 month) and aged (21 month) male F344 rats into control and acute restraint (an animal model of psychosocial stress) groups (n = 9–12/group). We then assessed hippocampus-associated behavioral, electrophysiological, and transcriptional outcomes, as well as blood glucocorticoid and sleep architecture changes. Aged rats showed characteristic water maze, deep sleep, transcriptome, and synaptic sensitivity changes compared to young. Young and aged rats showed similar levels of distress during the 3 h restraint, as well as highly significant increases in blood glucocorticoid levels 21 h after restraint. However, young, but not aged, animals responded to stress exposure with water maze deficits, loss of deep sleep and hyperthermia. These results demonstrate that aged subjects are hypo-responsive to new-onset acute psychosocial stress, which may have negative consequences for long-term stress adaptation and suggest that age itself may act as a stressor occluding the influence of new onset stressors.

Prenatal loud music and noise: differential impact on physiological arousal, hippocampal synaptogenesis and spatial behavior in one day-old chicks

Prenatal auditory stimulation in chicks with species-specific sound and music at 65 dB facilitates spatial orientation and learning and is associated with significant morphological and biochemical changes in the hippocampus and brainstem auditory nuclei. Increased noradrenaline level due to physiological arousal is suggested as a possible mediator for the observed beneficial effects following patterned and rhythmic sound exposure. However, studies regarding the effects of prenatal high decibel sound (110 dB; music and noise) exposure on the plasma noradrenaline level, synaptic protein expression in the hippocampus and spatial behavior of neonatal chicks remained unexplored. Here, we report that high decibel music stimulation moderately increases plasma noradrenaline level and positively modulates spatial orientation, learning and memory of one day-old chicks. In contrast, noise at the same sound pressure level results in excessive increase of plasma noradrenaline level and impairs the spatial behavior. Further, to assess the changes at the molecular level, we have quantified the expression of functional synapse markers: synaptophysin and PSD-95 in the hippocampus. Compared to the controls, both proteins show significantly increased expressions in the music stimulated group but decrease in expressions in the noise group. We propose that the differential increase of plasma noradrenaline level and altered expression of synaptic proteins in the hippocampus are responsible for the observed behavioral consequences following prenatal 110 dB music and noise stimulation.

Self-supplementation and effects of dietary antioxidants during acute thermal stress

Thermal stress leads to increased production of Reactive Oxygen Species (ROS). If the organism is not able to simultaneously mount an efficient antioxidant defense system, this may lead to increased oxidative damage, potentially deleterious in terms of health and fitness. Exposure to cold or heat is therefore expected to be associated with a high demand for antioxidants. In agreement, several studies have shown that supplementing the diet of thermally-stressed organisms with antioxidants leads to a reduction of oxidative damage. However, whether organisms can actively supplement their diet with antioxidants to alleviate temperature-induced oxidative damage is unknown. Here, we show that captive Gouldian finches (Erythrura gouldiae) supplement their diet more with seeds rich in antioxidants below than within their thermoneutral zone. Moreover, having access to seeds rich in antioxidants at temperature below thermoneutrality decreases their oxidative damage. These results indicate that, when facing a thermal challenge, animals are able to take advantage of the antioxidant properties of their food to improve their oxidative balance. Having access to food resources rich in antioxidants may therefore be of primary importance for organisms in their natural habitat, as it may help them to cope with oxidative constraints due to challenging temperature regimes.

Acute stress induces selective alterations in cost/benefit decision-making

Acute stress can exert beneficial or detrimental effects on different forms of cognition. In the present study, we assessed the effects of acute restraint stress on different forms of cost/benefit decision-making, and some of the hormonal and neurochemical mechanisms that may underlie these effects. Effort-based decision-making was assessed where rats chose between a low effort/reward (1 press=2 pellets) or high effort/reward option (4 pellets), with the effort requirement increasing over 4 blocks of trials (2, 5, 10, and 20 lever presses). Restraint stress for 1 h decreased preference for the more costly reward and induced longer choice latencies. Control experiments revealed that the effects on decision-making were not mediated by general reductions in motivation or preference for larger rewards. In contrast, acute stress did not affect delay-discounting, when rats chose between a small/immediate vs larger/delayed reward. The effects of stress on decision-making were not mimicked by treatment with physiological doses of corticosterone (1-3 mg/kg). Blockade of dopamine receptors with flupenthixol (0.25 mg/kg) before restraint did not attenuate stress-induced effects on effort-related choice, but abolished effects on choice latencies. These data suggest that acute stress interferes somewhat selectively with cost/benefit evaluations concerning effort costs. These effects do not appear to be mediated solely by enhanced glucocorticoid activity, whereas dopaminergic activation may contribute to increased deliberation times induced by stress. These findings may provide insight into impairments in decision-making and anergia associated with stress-related disorders, such as depression.

Short-term spatial memory responses in aged Japanese quail selected for divergent adrenocortical stress responsiveness

Stress-induced glucocorticoids can dampen learning and spatial memory via neuronal damage to the hippocampus. Cognition losses can be transient (associated with acute stress episodes) or permanent as in aged individuals who show chronic glucocorticoid-induced accelerated brain aging and neurodegeneration (dementia). Thus, chronic versus acute stress effects on spatial memory responses of quail selected for reduced (low stress, LS) or exaggerated (high stress, HS) plasma corticosterone (B) response to brief restraint were assessed. Aged food-motivated male LS and HS quail were tested for 10 min in a feed-baited 8-arm radial arm maze (RAM) 1) at 255 d of age (quail who had experienced lifelong management stressors but who were otherwise never intentionally stressed; that is, chronically stressed birds), 2) on the next day post-acute stressor treatment (5 min of restraint), and 3) on the next day without treatment (acute stress recovery). The RAM tests used the win-shift procedure in which visited arms were not rebaited. Radial arm maze performance was measured by determination of the total number of arm choices made, the number of correct entries made into baited arms out of the first 8 choices, the time required to make a choice, and the number of pellets eaten. Line effects (P < 0.001 in all cases) were detected for total number of arm choices made (HS < LS), number of correct entries made into baited arms out of the first 8 choices (HS < LS), time required to make a choice (HS > LS), and number of pellets eaten (HS < LS). However, neither the effects of day of RAM testing nor its interaction with line further influenced these variables. Thus, although selection for divergent plasma B responsiveness to an acute stressor was found to be associated with severe impairment of spatial memory in aged male HS compared with LS quail, the observed spatial memory impairments (HS > LS) could not be further altered by acute stressor treatment. Line differences in cognition may reflect lifelong management-induced stress episodes that periodically produce higher plasma B responses in HS than LS quail, which underlie HS quail memory deficits, or other etiologies, or both.

Do rats really express neophobia towards novel objects? Experimental evidence from exposure to novelty and to an object recognition task in an open space and an enclosed space

Three set of experiments were performed in an enclosed space (open-field) and in an open space (elevated platform). The surface of the open-field and the elevated platform were divided in nine equal squares. Rats were exposed (without previous habituation) in a unique session (experiment 1) or three consecutive sessions (experiment 2) either to an open-field (enclosed space) or to an elevated platform (open space) with and without an object on the centre of the field. In experiment 3, rats were exposed (without previous habituation) either to an enclosed or an open space on five consecutive sessions, one session a day. They were tested in an object recognition test in sessions 1, 3 and 5. In sessions 2 and 4, no objects were present. In experiment 1, we recorded the latency, frequency and duration of entries into different areas of the field. In experiment 3, we recorded the latency, frequency and duration of contacts with objects in addition to entries into different areas of the field. The first experiment demonstrates that rats exposed for the first time to an enclosed or an open space do not express neophobia toward novel objects in the absence of walls that surround an open-field. They crossed frequently into and spent more time in areas occupied with an object than in unoccupied areas. After two sessions of habituation to an empty open space or an empty enclosed space, the latency of first approach to a novel object is reduced while the frequency and duration of approaches are increased. The third experiment on object recognition confirmed that rats do not avoid novel objects; they made frequent visit and spent more time in the corner of the field occupied with an object than in empty corners. Recording of crossings provided detailed information about the patterns of exploratory behavior of rats but failed to reveal discrimination between novel and familiar objects which was evident in both open and enclosed space with recording of contacts with objects on the fifth exposure.

Effect of neonatal handling on adult rat spatial learning and memory following acute stress

Brief neonatal handling permanently alters hypothalamic-pituitary-adrenal axis function resulting in increased ability to cope with stress. Since stress is known to affect cognitive abilities, in the present study we investigated the effect of brief (15 min) handling on learning and memory in the Morris water maze, following exposure to an acute restraint stress either before training or recall. Exposure of non-handled rats to the acute stress prior to training resulted in quicker learning of the task, than in the absence of the stressor. When acute stress preceded acquisition, male handled rats showed an overall better learning performance, and both sexes of handled animals were less impaired in the subsequent memory trial, compared to the respective non-handled. In addition, the number of neurons immunoreactive for GR was higher in all areas of Ammon's horn of the handled rats during the recall. In contrast, the number of neurons immunoreactive for MR was higher in the CA1 and CA2 areas of the non-handled males. When the acute restraint stress was applied prior to the memory test, neonatal handling was not effective in preventing mnemonic impairment, as all animal groups showed a similar deficit in recall. In this case, no difference between handled and non-handled rats was observed in the number of GR positive neurons in the CA2 and CA3 hippocampal areas during the memory test. These results indicate that early experience interacts with sex and acute stress exposure in adulthood to affect performance in the water maze. Hippocampal corticosterone receptors may play a role in determining the final outcome.

Decreased hippocampal cholinergic neurostimulating peptide precursor protein associated with stress exposure in rat brain by proteomic analysis

The stress response alters behavior, autonomic function, and secretion of multiple hormones, including corticotropin-releasing factor, adrenocorticotropin hormone, and cortisol, through the hypothalamic-pituitary-adrenal axis. Constitutive stress responses lead to a number of psychiatric disorders, including depression, posttraumatic stress disorder, Alzheimer's disease (AD), and other anxiety disorders through increased stress hormones and other unknown factors. Here, we performed a proteomic analysis of rat brain exposed to restraint stress compared with a nonstress group by using 2D-DIGE and MALDI-TOF analysis. Several proteins were identified by peptide mass fingerprint (PMF), including down-regulated hippocampal cholinergic neurostimulating peptide precursor protein (HCNP-pp). The current study demonstrates that HCNP-pp mRNA and protein expression are decreased in rat hippocampus after stress exposure. The level of HCNP-pp in H19-7, a rat hippocampal cell line, significantly decreases with dexamethasone treatment, a synthetic glucocorticoid. Thus, this finding suggests that HCNP-pp expression may decrease in response to stress exposure. Decreased HCNP-pp from stress exposure may result in lower levels of HCNP that might contribute to a loss of acetylcholine production.

Maternal care affects male and female offspring working memory and stress reactivity

Variations in maternal care affect the development of individual differences in learning and memory and neuroendocrine responses to stress in adult male offspring, but it is not known how variations in maternal care affect adult female offspring. The present study investigated the performance of adult Sprague-Dawley male and female offspring exposed to either low or high levels of maternal licking/grooming on a spatial memory task (Experiment 1) and the effects of acute stress on corticosterone levels and spatial memory performance (Experiment 2). In Experiment 1 rats were trained for 24 days on the spatial working/reference memory version of the radial arm maze (RAM). In Experiment 2, rats were trained on the same RAM task, exposed to an acute stress, and the effect of stress on corticosterone levels and subsequent spatial memory was examined. In Experiment 1, adult female offspring of low licking/grooming dams had enhanced working memory compared to all other groups. In Experiment 2, all groups of male and female offspring had enhanced working memory 24 h after exposure to acute 2 h restraint stress while reference memory was enhanced after stress in male and female offspring of low licking/grooming dams. Furthermore, female offspring of low licking/grooming dams showed the largest corticosterone response to the acute restraint stress compared to all other groups. Male offspring of low licking/grooming dams showed a flattened corticosterone response to stress. Thus variations in maternal care differentially affect working memory and stress reactivity in male and female offspring.

Timing of stress and testing influence the long-lasting behavioral performance in rats

Three exposures (Days 1, 2 and 3) of rats to immobilization or immobilization combined with cold induced an alteration of exploratory behavior in an open space arena. When tested 1h after both stressors exposure, rats displayed a decrease in locomotor and rearing score. The deficit disappeared when rats were tested five days later and the performance remained unchanged in trials performed on days 9, 10, 15, 22 and 29 of the study. When testing was postponed five days after the third stressor exposure, a gradual reduction of the performance developed and the deficit persisted until the last trial on Day 29. Amphetamine, in a dose of 0.3 mg/kg revealed a sensitized response to the drug in the stressed animals. The results showed short- and long-lasting behavioral consequences of the used stressors, the long-term manifestation of the sequelae being dependent on the sequence and timing of stressor exposure and open space testing.

Exercise effects stress-induced analgesia and spatial learning in rats

Previous studies indicated that intensity level may be a determining factor in the beneficial or detrimental effects of exercise on spatial memory, as chronic low-intensity level exercise appears to enhance learning and memory which stressful situations may impair. This study examines the effects of different intensity levels of acute exercise (treadmill running) on spatial memory in rats. Using the Morris water maze, spatial learning was measured in animals exposed to treadmill running at low- (20-22 m/min for 25 min daily) and high-intensity (25 m/min for 25 min daily) levels of exercise. A stress control using an electric foot shock was used to examine if the high-intensity exercise was sufficient to serve as a stressor. Stress level was estimated by examining tail flick latencies as a measure of stress-induced analgesia. The results indicate that high-intensity exercise at a level that may not induce an analgesic state is sufficient to impair early acquisition of spatial learning. However, with additional trials, all animals are capable of learning the task. Acute exposure to the electric foot shock impaired learning in the Morris water maze. Surprisingly, across all studies, there was a significantly higher analgesic state post-swim as compared to pre-swim. The results indicate that irrespective of stress level prior to water maze testing, swimming in the Morris water maze repeatedly for short durations of time is enough to induce an analgesic state.

Impairments in spatial learning and memory: ethanol, allopregnanolone, and the hippocampus

Acute ethanol administration impairs performance in many cognitive tasks that are dependent on hippocampal function. For example, acute ethanol administration produces dose-dependent impairments in spatial learning. Ethanol also decreases the spatial specificity of hippocampal place cells. Such findings raise the possibility that ethanol affects learning and memory by altering, either directly or indirectly, neuronal activity in the hippocampus and related structures. Acute ethanol administration induces a dose- and time-dependent increase in brain concentration of the neuroactive steroid allopregnanolone. Allopregnanolone is a potent GABAA receptor agonist and produces effects similar to the effects produced by ethanol. Blockade of de novo biosynthesis of allopregnanolone alters many of ethanol's effects including ethanol-induced suppression of spontaneous activity in medial septum/diagonal band of Broca neurons and hippocampal pyramidal neurons. These findings suggest that ethanol-induced increases in allopregnanolone levels might play a central role in the effects of acute ethanol on cognitive processing and hippocampal function. The impact of ethanol on spatial cognitive processing and hippocampal function will be reviewed. In addition, the possibility that ethanol-induced changes in neuroactive steroid levels contribute to the impact of ethanol on spatial learning and hippocampal function will be explored.

Effect of chronic stress on spatial memory in rats is attenuated by lithium treatment

Stress is known to alter cognitive functions, such as memory, and it has been linked to the pathophysiology of mood and anxiety disorders. Chronic lithium treatment is used in some psychiatric disorders and has been suggested to act upon mechanisms which can enhance neuronal viability. The purpose of this work is to investigate a possible effect of lithium treatment in a chronic stress model. Adult male Wistar rats were divided in two groups, control and chronically stressed, treated either with normal chow or with chow containing LiCl for 40 days. Stress treatment was a chronic variable stress model, consisting of different stressors which were applied in a random fashion, once a day, every day. Memory was assessed by using the water maze task. The results demonstrated a marked decrease in reference memory in the water maze task in chronically stressed rats. This effect was attenuated by lithium treatment in all the parameters considered. No effect was observed in the working memory. These results indicate that lithium treatment may counteract some effects of chronic stress situations, particularly concerning spatial memory.

Combined restraint and cold stress in rats: effects on memory processing in passive avoidance task and on plasma levels of ACTH and corticosterone

The effect of restraint stress combined with water immersion (IMO+C), applied at various intervals before and after the acquisition of a passive avoidance task, was studied in rats. The procedure started with two pre-training trials. On the single training trial the rats received a footshock (0.3 mA, 3s) after they entered the preferred dark compartment. The exposure to IMO+C lasting 1 h terminated 4 or 1 h before application of the footshock or started immediately or 3 h after this aversive stimulus. Retention tests were performed 1 and 2 days after the acquisition trial. In an attempt to relate the behavioural responses to the stressor with plasma levels of two stress hormones we measured ACTH and corticosterone under similar conditions as were used in the behavioural experiments. IMO+C exposure terminating 1 h before the training resulted in very short avoidance latencies during retention testing. A similar impairment of retention test performance was found in animals exposed to the stressor immediately after training. When IMO+C exposure terminated 4 h before training the stressed rats exhibited comparably long avoidance latencies as shown by the controls. IMO+C presented 3 h after acquisition trial also did not influence retention of avoidance learning. The hormones were estimated 1 and 4 h after IMO+C, both in the absence and presence of footshock. Both ACTH and corticosterone were significantly increased 1 h after IMO+C termination, and their plasma levels returned to control values within 4 h. Footshock alone increased plasma corticosterone, however, the hormone levels were significantly lower than those estimated after IMO+C terminating 1 h before blood collection. Footshock substantially increased ACTH levels in rats exposed to IMO+C 1 h before footshock, but not in stressed rats with already high levels of corticosterone. In conclusion, IMO+C represents a strong stress stimulus exerting amnesic effect when applied shortly before or after the acquisition trial. Further, the findings indicate the restraint and cold stressor to interfere with consolidation of passive avoidance response. We suggest that the moderate circulating levels of corticosterone found after footshock may be positively related to the memory consolidation, while the exceedingly high levels have an opposite effect.

Behavioural changes after an acute stress: stressor and test types influences

Behavioural consequences of different acute stressors (30 min of restraint, 20 min of forced swim stress, 15 min of inescapable footshocks) applied at the beginning of the active period were assessed in using two behavioural tests: a 20 min light extinction test 24 h after the stressor exposure in order to explore the psychomotor ability and a 10 min open field session within the dark period 48 h after the stressor exposure to estimate the emotional status and the locomotor activity of the rat. Different behavioural responses were observed depending on the nature of the applied stressor. In the light extinction test, the footshock-stressed rats developed a very low activity independent on light conditions whereas the rats submitted to forced swim and restraint exhibited an activity level depending on the strain. Moreover, restrained rats had a higher transient activity than forced swim rats under light condition. In the open field test, none of the stressed rats did develop differences in behaviour. The efficacy of a 24 h recovery period on the behavioural response to an acute stressor exposure depends on the intensity of the applied stressor and the behavioural demands.

Morphine state-dependent learning: interactions with alpha2-adrenoceptors and acute stress

The interactions of -adrenoceptors and acute restraint stress with morphine state-dependent memory of passive avoidance were examined in mice. Memory acquired following pre-training morphine administration (5 mg/kg, i.p.) was dose- and time-dependently retrieved by pre-test morphine; this effect was reversible by yohimbine (1 mg/kg). Pre-test clonidine (0.005-0.1 mg/kg) was also effective in restoring morphine-induced memory. Pre-training clonidine (2 mg/kg) induced an amnestic effect that was restorable by pre-test clonidine or morphine; this effect was also blocked by yohimbine. Acute pre-training stress for 2 h induced an amnestic effect that was reversible by pre-test morphine (1 and 5 mg/kg) or clonidine (0.01 and 0.1 mg/kg). Finally, acute pre-test stress could restore the impairment of memory induced by pre-training morphine. The data are suggestive of a functional interaction between -opioid, -adrenergic receptors and stress in modulating state-dependent learning and memory.

Impaired passive avoidance acquisition in Sprague-Dawley and Lewis rats after restraint and cold stress

The study examined the effects of restraint combined with cold water stress (IMO+C) on learning and memory of Sprague-Dawley (S-D) and Lewis (LE) rats in the passive avoidance task. The procedure started with 6 days of adaptation to the apparatus during which the recorded latencies to enter the dark compartment were used to assess the process of habituation. On the training day rats were exposed to IMO+C for 60 min and the stressor exposure terminated 1 h before the acquisition trial. Retention trials started 24 h later. To evaluate the possible long-term consequences of the acute and repeated stress presentation on the performance of the two strains with diverse activity of hypothalamic-pituitary-adrenal axis, this procedure was performed three times including stress application (Parts 1-3). Finally, an identical procedure was performed without stress (Part 4). An immediate behavioural effect of the stressor exposure was observed in an increase of latencies to enter the dark compartment before the shock delivery in rats of both strains; this enhancement approached significance after the second and third exposure to the stressor (Parts 2 and 3). Control animals of both strains acquired passive avoidance response after training in Parts 2-4. IMO+C produced significant impairment of this response irrespective of the strain. The three-time repeated exposure did not influence the ability to learn the task in the final procedure without stress. Differences in behaviour of S-D and LE rats were observed already during the first adaptation period. LE rats exhibited longer latencies upon the first exposure to the novel environment compared to S-D rats. Also only LE rats displayed habituation. In Part 4 marked strain differences in the latencies both before and after training were recorded. The results show that the repeated exposure to the IMO+C stressor proved to be a strong amnestic stimulus but without persisting consequences for the ability of rats to acquire the learning task.

Chronic psychosocial stress impairs learning and memory and increases sensitivity to yohimbine in adult rats

BACKGROUND

It is well known that intense and prolonged stress can produce cognitive impairments and hippocampal damage and increase noradrenergic activity in humans. This study investigated the hypothesis that chronic psychosocial stress would affect behavior, drug sensitivity, and hippocampal-dependent learning and memory in rats. The work provides a novel connection between animal and human studies by evaluating the effects of stress on a rat's response to yohimbine, an alpha(2) adrenergic receptor antagonist.

METHODS

Rats were exposed to a cat for 5 weeks and randomly housed with a different group of cohorts each day (psychosocial stress). The effects of the stress manipulations were then assessed on open field behavior, spatial learning and memory in the radial arm water maze and the behavioral response to a low dose of yohimbine (1.5 mg/kg).

RESULTS

Stressed rats displayed impaired habituation to a novel environment, heightened anxiety, and increased sensitivity to yohimbine. In addition, the stressed rats exhibited impaired learning and memory.

CONCLUSIONS

There are commonalities between the current findings on stressed rats and from studies on traumatized people. Thus, psychosocial stress manipulations in rats may yield insight into the basis of cognitive and neuroendocrine disturbances that commonly occur in people with anxiety disorders.

Stress, hippocampal plasticity, and spatial learning

During the last two decades numerous studies have been conducted in an attempt to correlate the mechanisms of long-term potentiation (LTP) of hippocampal synaptic transmission with those required for spatial memory formation in the hippocampus. Because stressful events block the induction of hippocampal LTP, it has been suggested that deficits in spatial learning following stress may be related to suppression of LTP-like phenomena in the hippocampus. Here I review these studies and discuss them in light of the emerging view that stress may induce changes in thresholds for synaptic plasticity necessary for both LTP induction and spatial memory formation. This phenomenon, known as metaplasticity, may involve a glucocorticoid modulation of calcium homeostasis.