Learning/memory processes under stress conditions

Effect of traditional medicine brahmi vati and bacoside A-rich fraction of Bacopa monnieri on acute pentylenetetrzole-induced seizures, amphetamine-induced model of schizophrenia, and scopolamine-induced memory loss in laboratory animals

OBJECTIVE

Brahmi vati (BV) is an Ayurvedic polyherbal formulation used since ancient times and has been prescribed in seizures associated with schizophrenia and related memory loss by Ayurvedic practitioners in India. The aim of the study was to investigate these claims by evaluation of anticonvulsant, antischizophreniac, and memory-enhancing activities. Antioxidant condition of brain was determined by malondialdehyde (MDA) and reduced glutathione (GSH) levels estimations. Acetylcholinesterase (AChE) was quantitatively estimated in the brain tissue.

METHODS

Brahmi vati was prepared in-house by strictly following the traditional Ayurvedic formula. Bacoside A rich fraction (BA) of Bacopa monnieri was prepared by extraction and fractionation. It was than standardized by High Performance Liquid Chromatography (HPLC) and given in the dose of 32.5mg/kg body weight to the different groups of animals for 7days. On the seventh day, activities were performed adopting standard procedures.

KEY FINDINGS

Brahmi vati showed significant anticonvulsant, memory-enhancing and antischizophrenia activities, when compared with the control groups and BA. It cause significantly higher brain glutathione levels. Acetylcholinesterase activity was found to be significantly low in BV-treated group.

CONCLUSION

The finding of the present study suggests that BV may be used to treat seizures associated with schizophrenia and related memory loss.

Variable Effects of Acoustic Trauma on Behavioral and Neural Correlates of Tinnitus In Individual Animals

The etiology of tinnitus is known to be diverse in the human population. An appropriate animal model of tinnitus should incorporate this pathological diversity. Previous studies evaluating the effect of acoustic over exposure (AOE) have found that animals typically display increased spontaneous firing rates and bursting activity of auditory neurons, which often has been linked to behavioral evidence of tinnitus. However, only a subset of studies directly associated these neural correlates to individual animals. Furthermore, the vast majority of tinnitus studies were conducted on anesthetized animals. The goal of this study was to test for a possible relationship between tinnitus, hearing loss, hyperactivity and bursting activity in the auditory system of individual unanesthetized animals following AOE. Sixteen mice were unilaterally exposed to 116 dB SPL narrowband noise (centered at 12.5 kHz) for 1 h under ketamine/xylazine anesthesia. Gap-induced prepulse inhibition of the acoustic startle reflex (GPIAS) was used to assess behavioral evidence of tinnitus whereas hearing performance was evaluated by measurements of auditory brainstem response (ABR) thresholds and prepulse inhibition PPI audiometry. Following behavioral assessments, single neuron firing activity was recorded from the inferior colliculus (IC) of four awake animals and compared to recordings from four unexposed controls. We found that AOE increased spontaneous activity in all mice tested, independently of tinnitus behavior or severity of threshold shifts. Bursting activity did not increase in two animals identified as tinnitus positive (T+), but did so in a tinnitus negative (T−) animal with severe hearing loss (SHL). Hyperactivity does not appear to be a reliable biomarker of tinnitus. Our data suggest that multidisciplinary assessments on individual animals following AOE could offer a powerful experimental tool to investigate mechanisms of tinnitus.

An investigation into the ameliorating effect of black soybean extract on learning and memory impairment with assessment of neuroprotective effects

BACKGROUND

The physiological effects of the non-anthocyanin fraction (NAF) in a black soybean seed coat extract on Aβ-induced oxidative stress were investigated to confirm neuroprotection. In addition, we examined the preventive effect of NAF on cognitive defects induced by the intracerebroventricular (ICV) injection of Aβ.

METHODS

Levels of cellular oxidative stress were measured using 2',7'-dichlorofluorescein diacetate (DCF-DA). Neuronal cell viability was investigated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and lactate dehydrogenase (LDH) assay. To investigate in vivo anti-amnesic effects of NAF by using Y-maze and passive avoidance tests, the learning and memory impairment in mice was induced by Aβ. After in vivo assays, acetylcholinesterase (AChE) activity and level of malondialdehyde (MDA) in the mouse brain were determined to confirm the cognitive effect. Individual phenolics of NAF were qualitatively analyzed by using an ultra-performance liquid chromatography (UPLC) Accurate-Mass Quadrupole Time of-Flight (Q-TOF) UPLC/MS.

RESULTS

A NAF showed cell protective effects against oxidative stress-induced cytotoxicity. Intracellular ROS accumulated through Aβ1-40 treatment was significantly reduced in comparison to cells only treated with Aβ1-40. In MTT and LDH assay, the NAF also presented neuroprotective effects on Aβ1-40-treated cytotoxicity. Finally, the administration of this NAF in mice significantly reversed the Aβ1-40-induced cognitive defects in in vivo behavioral tests. After behavioral tests, the mice brains were collected in order to examine lipid peroxidation and AChE activity. AChE, preparation was inhibited by NAF in a dose-dependent manner. MDA generation in the brain homogenate of mice treated with the NAF was decreased. Q-TOF UPLC/MS analyses revealed three major phenolics from the non-anthocyanin fraction; epicatechin, procyanidin B1, and procyanidin B2.

CONCLUSIONS

The results suggest that the NAF in black soybean seed coat extracts may improve the cytotoxicity of Aβ in PC12 cells, possibly by reducing oxidative stress, and also have an anti-amnesic effect on the in vivo learning and memory deficits caused by Aβ. Q-TOF UPLC/MS analyses showed three major phenolics; (-)-epicatechin, procyanidin B1, and procyanidin B2. Above results suggest that (-)-epicatechins are the major components, and contributors to the anti-amnesic effect of the NAF from black soybean seed coat.

Impact of a multi-nutrient diet on cognition, brain metabolism, hemodynamics, and plasticity in apoE4 carrier and apoE knockout mice

Lipid metabolism and genetic background together strongly influence the development of both cardiovascular and neurodegenerative diseases like Alzheimer's disease (AD). A non-pharmacological way to prevent the genotype-induced occurrence of these pathologies is given by dietary behavior. In the present study, we tested the effects of long-term consumption of a specific multi-nutrient diet in two models for atherosclerosis and vascular risk factors in AD: the apolipoprotein ε4 (apoE4) and the apoE knockout (apoE ko) mice. This specific multi-nutrient diet was developed to support neuronal membrane synthesis and was expected to contribute to the maintenance of vascular health. At 12 months of age, both genotypes showed behavioral changes compared to control mice and we found increased neurogenesis in apoE ko mice. The specific multi-nutrient diet decreased anxiety-related behavior in the open field, influenced sterol composition in serum and brain tissue, and increased the concentration of omega-3 fatty acids in the brain. Furthermore, we found that wild-type and apoE ko mice fed with this multi-nutrient diet showed locally increased cerebral blood volume and decreased hippocampal glutamate levels. Taken together, these data suggest that a specific dietary intervention has beneficial effects on early pathological consequences of hypercholesterolemia and vascular risk factors for AD.

Enhanced brain performance in mice following postnatal stress

The double postnatal stress model (brief maternal separation plus sham injection daily applied from birth to weaning) induces metabolic alterations similar to type 2 diabetes in young-adult male mice. We verify whether 1) the stress also induces brain metabolic-functional alterations connected to diabetes and 2) different alterations are modulated selectively by two stress-damaged endogenous systems (opioid- and/or ACTH-corticosteroid-linked). Here, diabetes-like metabolic plus neurophysiologic-neurometabolic parameters are studied in adult mice following postnatal stress and drug treatment. Surprisingly, together with 'classic' diabetes-like alterations, the stress model induces in young-adult mice significantly enhanced brain neurometabolic-neurophysiologic performances, consisting of decreased latency to flash-visual evoked potentials (- ~8%); increased level (+ ~40%) and reduced latency (- ~30%) of NAD(P)H autofluorescence postsynaptic signals following electric stimuli; enhanced passive avoidance learning (+ ~135% latency); and enhanced brain-derived neurotrophic factor level (+ ~70%). Postnatal treatment with the opioid receptor antagonist naloxone prevents some alterations, moreover the treatment with antisense (AS; AS vs proopiomelanocortin mRNA) draws all parameters to control levels, thus showing that some alterations are bound to endogenous opioid-system hyper-functioning, while others depend on ACTH-corticosterone system hyper-functioning. Our stress model induces diabetes-like metabolic alterations coupled to enhanced brain neurometabolic-neurophysiologic performances. Taken all together, these findings are compatible with an 'enduring acute-stress' reaction, which puts mice in favorable survival situations vs controls. However, prolonged hormonal-metabolic imbalances are expected to also produce diabetes-like complications at later ages in stressed mice.

BmNPV resistance of silkworm larvae resulting from the ingestion of TiO₂ nanoparticles

Bombyx mori nucleopolyhedrovirus (BmNPV) causes infection in the silkworm that is often lethal. The infection is hard to prevent, partly because of the nature of the virus particles and partly because of the different strains of B. mori. Titanium dioxide nanoparticles (TiO₂ NPs) have been demonstrated to have antimicrobial properties. The present study investigated whether TiO₂ NPs added to an artificial diet can increase the resistance of B. mori larvae to BmNPV and examined the molecular mechanism behind any resistance shown. The results indicated that ingested TiO₂ NPs decreased reactive oxygen species and NO accumulation in B. mori larvae under BmNPV infection, which in turn led to a decrease in their growth inhibition and mortality. In addition, the TiO₂ NPs significantly promoted the expression of resistance-related genes, including those encoding superoxide dismutase, catalase, glutathione peroxidase, acetylcholine esterase, carboxylesterase, heat shock protein 21, glutathione S transferase o1, P53, and transferring and of genes encoding cytochrome p302 and nitric oxide synthase. These findings are a useful addition to the understanding of the mechanism of BmNPV resistance of B. mori larvae in response to TiO₂ NPs addition. Such information also provides a theoretical basis for the use of TiO₂ NPs in sericulture.

Methodological optimization of tinnitus assessment using prepulse inhibition of the acoustic startle reflex

Recently prepulse inhibition of the acoustic startle reflex (ASR) became a popular technique for tinnitus assessment in laboratory animals. This method confers a significant advantage over the previously used time-consuming behavioral approaches utilizing basic mechanisms of conditioning. Although this technique has been successfully used to assess tinnitus in different laboratory animals, many of the finer details of this methodology have not been described enough to be replicated, but are critical for tinnitus assessment. Here we provide detail description of key procedures and methodological issues that provide guidance for newcomers with the process of learning to correctly apply gap detection techniques for tinnitus assessment in laboratory animals. The major categories of these issues include: refinement of hardware for best performance, optimization of stimulus parameters, behavioral considerations, and identification of optimal strategies for data analysis. This article is part of a Special Issue entitled: Tinnitus Neuroscience.

Oxidative injury in the brain of mice caused by lanthanid

The organ toxicity of lanthanides (Ln) on organisms had been recognized, but very little is known about the oxidative injury of brain caused by Ln. In order to study the mechanisms underlying the effects of Ln on the brain, ICR mice were injected with a single 20 mg/kg body weight dose of LaCl(3), CeCl(3), and NdCl(3) into the abdominal cavity daily for 14 days. We then examined the coefficient of the brain, the brain pathological changes and oxidative stress-mediated responses, and the accumulation of Ln and levels of neurochemicals in the brain. The results showed that CeCl(3) and NdCl(3) could induce some neurons to turn inflammatory cells and slight edema but did not observe the brain pathological changes from LaCl(3)-treated group. The concentrations of La, Ce, and Nd in the brain were significantly different and ranked in the order of Ce, Nd, and La. The injury of the brain and oxidative stress occurred as Ln appeared to trigger a cascade of reactions such as lipid peroxidation, the decreases of the total antioxidation capacity and activities of antioxidative enzymes, the excessive release of nitric oxide, the increase of glutamic acid, and the downregulated level of acetylcholinesterase activities. Furthermore, both Ce(3+) and Nd(3+) exhibited higher oxidative stress and toxicity on brain than La(3+), and Ce(3+) caused more severe brain injuries and oxidative stress than Nd(3+), implying that the differences in the brain injuries caused by Ln might be related to the number of 4f electrons of Ln.

Effect of a polyphenol-rich wild blueberry extract on cognitive performance of mice, brain antioxidant markers and acetylcholinesterase activity

The aim of this study was to examine the effect of a polyphenol-rich extract (PrB) of Vaccinium angustifolium (wild blueberries) introduced intraperitoneally (i.p.) at 30 (PrB30) and 60 (PrB60) mg/kg body weight for 7 days, on cognitive performance, brain oxidative status and acetylcholinesterase activity in adult, male, 3-4-month-old Balb-c mice. Evaluation of rodent learning and memory was assessed by a step-through test on day 6 after a double training and an initial acquisition trial on day 5. Antioxidant status was determined by ferric reducing antioxidant power (FRAP), ascorbic acid concentration (FRASC), malondialdehyde and reduced glutathione levels in whole brain homogenates. Acetylcholinesterase (AChE) activity was determined by Ellman's colorimetric method. Results showed that the PrB60-treated mice exhibited a significant improvement in learning and memory (step-through latency time of 228+/-38 s compared to 101+/-32 s of the control group). PrB extract administration also resulted in reduced lipid peroxidation products (38 and 79%) and higher brain ascorbic acid levels (21 and 64%) in both PrB30 and PrB60-treated groups, respectively, and higher glutathione levels (28%) in the PrB60-treated group. Furthermore, salt- and detergent soluble AChE activity significantly decreased in both PrB-treated groups. Thus, the significant cognitive enhancement observed in adult mice after short-term i.p. supplementation with the blueberry extract concentrated in polyphenols, is closely related to higher brain antioxidant properties and inhibition of AChE activity. These findings stress the critical impact of wild blueberry bioactive components on brain function.

Central muscarinic blockade interferes with retrieval and reacquisition of active allothetic place avoidance despite spatial pretraining

Animal navigation to hidden goals (place navigation) ranks among the most intensively studied types of behaviour because it requires brain representations of environments in the form of cognitive maps, demonstrated to depend on hippocampal function. Intact function of muscarinic receptors in the brain was originally assumed to be crucial for place navigation, however, recent studies using non-spatial pretraining demonstrated that animals with central blockade of muscarinic acetylcholine receptors can also learn and retrieve spatial memory engrams. In the present study we addressed whether navigation in the active allothetic place avoidance (AAPA) task, which requires animals to separate spatial stimuli into coherent representations and navigate according to the representation relevant for the task, is dependent on intact muscarinic receptors in the brain. We studied the effect of three doses of scopolamine (0.5, 1.0 and 2.0mg/kg) administered 20 min prior to training on the retention of the AAPA and re-acquisition of the AAPA in a new environment. The dose of 2.0mg/kg was found to impair both AAPA retention and re-acquisition of the AAPA in a new environment, whereas the 1.0mg/kg dose only impaired the reinforced retention of AAPA. It is concluded that, unlike navigation in classic paradigms, efficient orientation in the AAPA task is critically dependent on muscarinic receptors in the brain.

Cholinergic dependence of taste memory formation: Evidence of two distinct processes

Learning the aversive or positive consequences associated with novel taste solutions has a strong significance for an animal's survival. A lack of recognition of a taste's consequences could prevent ingestion of potential edibles or encounter death. We used conditioned taste aversion (CTA) and attenuation of neophobia (AN) to study aversive and safe taste memory formation. To determine if muscarinic receptors in the insular cortex participate differentially in both tasks, we infused the muscarinic antagonists scopolamine at distinct times before or after the presentation of a strong concentration of saccharin, followed by either an i.p. injection of a malaise-inducing agent or no injection. Our results showed that blockade of muscarinic receptors before taste presentation disrupts both learning tasks. However, the same treatment after the taste prevents AN but not CTA. These results clearly demonstrate that cortical cholinergic activity participates in the acquisition of both safe and aversive memory formation, and that cortical muscarinic receptors seem to be necessary for safe but not for aversive taste memory consolidation. These results suggest that the taste memory trace is processed in the insular cortex simultaneously by at least two independent mechanisms, and that their interaction would determine the degree of aversion or preference learned to a novel taste.

Neuronal overexpression of "readthrough" acetylcholinesterase is associated with antisense-suppressible behavioral impairments

Molecular origin(s) of the diverse behavioral responses to anticholinesterases were explored in behaviorally impaired transgenic (Tg) FVB/N mice expressing synaptic human acetylcholinesterase (hAChE-S). Untreated hAChE-S Tg, unlike naïve FVB/N mice, presented variably intense neuronal overexpression of the alternatively spliced, stress-induced mouse "readthrough" mAChE-R mRNA. Both strains displayed similar diurnal patterns of locomotor activity that were impaired 3 days after a day-to-night switch. However, hAChE-S Tg, but not FVB/N mice responded to the circadian switch with irregular, diverse bursts of increased locomotor activity. In social recognition tests, controls displayed short-term recognition, reflected by decreased exploration of a familiar, compared to a novel juvenile conspecific as well as inverse correlation between social recognition and cortical and hippocampal AChE specific activities. In contrast, transgenics presented poor recognition, retrievable by tetrahydroaminoacridine (tacrine, 1.5 mg kg(-1)). Tacrine's effect was short-lived (24 h) suppression of the abnormal social recognition pattern in transgenics. Efficacy of antisense treatment was directly correlated with AChE-R levels and the severity of the impaired phenotype, being most apparent in transgenics presenting highly abnormal pre-treatment behavior. These findings demonstrate that neuronal AChE-R overproduction is involved in various behavioral impairments and anticholinesterase responses, and point to the antisense strategy as a potential approach for re-establishing cholinergic balance.

Effect of intraperitoneal mRNA antisense-oligodeoxynucleotides to cholecystokinin on anxiety-like and learning behaviors in rats: association with pre-experimental stress

RATIONALE

Cholecystokinin and its analogs generate anxiety in humans and measurable anxiety-like behaviors in rats. Cholecystokinin receptor blockers have been reported to have variable effects in the treatment of anxiety disorders. We demonstrated that intracerebroventricular administration of Cholecystokinin-antisense oligodeoxynucleotides (ASODN) for 3 days significantly diminished anxiety-like behavior in rats.

OBJECTIVE

This study was designed to examine the effects of peripheral (intraperitoneal) administration of Cholecystokinin-ASODN on anxiety-like and learning behaviors in rats, in general and in a pre-experiment stress paradigm.

METHODS

In the first study Cholecystokinin-ASODN was injected intraperitoneally to rats five times at 24-h intervals. Control groups received injections of either a scrambled oligodeoxynucleotide (ScrODN) or vehicle. On the sixth day, the rats were assessed in the elevated plus-maze paradigm and in the Morris water maze. In the second study, rats were pre-exposed to a cat for 10 min as a model for psychological stress, and then treated with intraperitoneal Cholecystokinin-ASODN and tested in both paradigms.

RESULTS

The results show that for intact rats, intraperitoneal Cholecystokinin-ASODN significantly increased anxiety-like behavior and impaired retention performance in the Morris water maze, compared to both control groups. In stressed rats, Cholecystokinin-ASODN reduced anxiety-like behaviors in the plus-maze and improved performance in the water maze compared with controls.

CONCLUSIONS

These results indicate that the anxiolytic effect of intraperitoneal Cholecystokinin-ASODN may be dependent on the baseline endogenous level of stress (i.e., on the Cholecystokinin levels). Basal endogenous levels of Cholecystokinin, as well as exogenous dosage of Cholecystokinin agonists and/or anxiolytic agents, appear to play an important role in the expression and/or control of anxiety-related behaviors in rats.

Repeated exposure to rats has persistent genotype-dependent effects on learning and locomotor activity of apolipoprotein E knockout and C57Bl/6 mice

Recently we have shown that an experimentally controlled encounter of mice with rats ("rat stress") some time before actual behavioural testing either abolished or induced behavioural deficits in the Morris water maze, depending on the genotype of the mice: apolipoprotein E knockout mice (apoE0/0) and wild type mice. Here we report that previous rat stress: (i) facilitated learning of a circular hole board task in apoE0/0 mice and impaired learning in wild type mice, thereby abolishing genotype-dependent differences; (ii) although both genotypes preferred the dark compartment when tested in a light/dark-preference task 3 months after rat stress, locomotor activity was reduced in apoE0/0 and increased in wild type mice, thus genotype differences were amplified; (iii) both genotypes responded with a differential regulation of bodyweight during exposure to rats, which persisted for 3 months: apoE0/0 mice decreased while wild type mice increased their body weight; (iv) the high emotional reactivity (defecation boli) measured during behavioural tasks was not affected in apoE0/0 mice, whereas a decrease was observed in wild type mice. Thus, pre-experimental confrontation of mice with rats shifts behaviour and physiological responses and eliminates some of the genotype-dependent differences.

Repeated acquisition and performance chamber for mice: a paradigm for assessment of spatial learning and memory

Molecular genetic manipulation of the mouse offers the possibility of elucidating the function of individual gene products in neural systems underlying learning and memory. Many extant learning paradigms for mice rely on negative reinforcement, involve simple problems that are relatively rapidly acquired and thus preclude time-course assessment, and may impose the need to undertake additional experiments to determine the extent to which noncognitive behaviors influence the measures of learning. To overcome such limitations, a multiple schedule of repeated acquisition and performance was behaviorally engineered to assess learning vs rote performance within-behavioral test session and within-subject utilizing an apparatus modified from the rat (the repeated acquisition and performance chamber; RAPC). The multiple schedule required mice to learn a new sequence of door openings leading to saccharin availability in the learning component during each session, while the sequence of door openings for the performance component remained constant across sessions. The learning and performance components alternated over the course of each test session, with different auditory stimuli signaling which component was currently in effect. To validate this paradigm, learning vs performance was evaluated in two inbred strains of mice: C57BL/6J and 129/SvJ. The hippocampal dependence of this measure was examined in lesioned C57BL/6J mice. Both strains exhibited longer latencies and higher errors in the learning compared to the performance component and evidenced declines in both measures across the trials of each session, consistent with an acquisition phenomenon. These same measures showed little or no evidence of change in the performance component. Whereas three trials per session were utilized with C57BL/65 mice in each component, behavior of 129/SvJ mice could only be sustained for two trials per component per session, demonstrating differences in testing capabilities between these two strains under these experimental conditions and thus precluding the ability to make systematic strain comparisons of learning capabilities. Hippocampal lesions in C57BL/6J mice resulted in substantially longer latencies and increased errors in the learning but not the performance component, demonstrating the importance of this region to spatial learning as measured in the RAPC. In aggregate, this positive reinforcement-based operant paradigm to evaluate murine spatial learning detects strain differences and hippocampal dependence and permits explicit differentiation of the impact of noncognitive contributions to learning measures on a within-subject, within-session basis.

Stress impairs performance in spatial water maze learning tasks

The water maze task has been developed to test spatial learning abilities in rats or mice, and is widely used. Though it has been reported before that numerous cognitive abilities are of importance for learning this task, poor performance is usually interpreted as an impairment of spatial memory formation. Previous investigations that tried to correlate long-term potentiation (LTP) of synaptic transmission with spatial learning abilities in rats reported that injection of drugs or specific gene deletions which blocked the expression of LTP correlated with learning impairments of spatial tasks in a water maze. Recent studies, however, have shown that pretraining enables these animals to learn such spatial tasks even though LTP was still found to be blocked. I investigated to what degree altered fear condition and stress perception could account for the impaired spatial learning when no pretraining is given. In a fear habituation task, unhandled rats preferred a dark over a well lit chamber more than handled animals did, but unhandled rats favoured the lit chamber more in an active avoidance task. They also performed poorly in a spatial water maze task compared with handled rats. Rats pretrained in a radial arm maze performed better in a water maze than non-pretrained rats. No difference between groups was found in a non-spatial water maze task. On the other hand, when pretrained in a water maze, rats performed only marginally better in a radial arm maze compared to non-pretrained animals. Since animals have to be handled to learn a radial arm maze, the difference in this task was not due to stress but most probably due to getting accustomed to the room dimensions prior to learning the spatial task. The results suggest that impaired learning of spatial tasks in the water maze can be due to increased stress and decreased fear conditioning without actually affecting spatial learning abilities. These results question the interpretations of the results of some previously published results of spatial water maze tasks.

Adrenocortical suppression blocks the enhancement of memory storage produced by exposure to psychological stress in rats

Several reports have indicated that various stress stimuli modulate learning and memory processes. In the present study, the effects of adrenocortical suppression with the 11beta-hydroxylase inhibitor metyrapone on the psychological stress-induced changes in memory storage in inhibitory avoidance training and in serotonin turnover in various brain regions were investigated in rats. Retention of one-trial inhibitory avoidance and the plasma corticosterone level were significantly enhanced by post-training exposure to psychological stress for 1 h. Pretreatment with metyrapone (12.5 or 25 mg/kg, s.c.) 90 min beforehand dose-dependently blocked the enhancement of memory storage and of the plasma corticosterone level produced by psychological stress. These results suggest that the adrenocortical system may contribute to the memory-enhancing effect of psychological stress. In a neurochemical study, a significant increase in serotonin turnover in the hippocampus and limbic forebrain, including the nucleus accumbens, were observed in rats that were exposed to psychological stress. In contrast to the behavioral experiments, these changes in serotonin turnover produced by exposure to psychological stress were not antagonized by pretreatment with metyrapone; instead, a further increase in serotonin turnover was observed only in the hippocampus. These results suggest that the serotonergic system in the hippocampus might be selectively regulated by adrenal steroids in response to stress, and imply the existence of negative feedback mechanisms via a hippocampal serotonergic system in the memory enhancement associated with corticosterone and psychological stress.

Vasopressin released within the septal brain area during swim stress modulates the behavioural stress response in rats

The aim of the present study was to investigate the physiological significance of the neuropeptide arginine vasopressin (AVP) released within the septum, in the behavioural response of rats to stress. In the first experiment, rats were chronically implanted with a microdialysis probe aimed at the mediolateral or ventral septum to monitor the local release of AVP in response to 10 min of forced swimming in 20 degrees C warm water. Exposure to this stressor caused a significant increase in AVP release in both the mediolateral (174 +/- 21%, P < 0.01) and ventral septum (220 +/- 33%, P < 0.01). In contrast, microdialysates collected outside the mediolateral septum or in the lateral ventricle remained at prestress levels throughout the dialysis period. Furthermore, unstressed control animals failed to show significant alterations in vasopressin release in the mediolateral septum. In a second experiment, the introduction of the V1 receptor antagonist d(CH2)5Tyr(Me)AVP into the mediolateral septum via inverse microdialysis concomitant with stressor exposure caused the rats to spend an increased time floating and a reduced time swimming compared to vehicle-treated rats. This effect was acute and also detected 24 h after antagonist administration. Taken together, these findings demonstrate a significant activation of the septal vasopressinergic system in response to swim stress. Furthermore, our data support the view that AVP released within this brain area is involved in the generation of active behavioural strategies aimed at coping with new and challenging situations.

Memory performance in tree shrews: effects of stressful experiences

Male tree shrews were exposed to alternating non-stressful and stressful conditions, and their memory performance was tested during three different stress periods and after rest periods of various lengths using a modified holeboard. This paradigm circumvents confounding factors, e.g. food or water deprivation, transport to a special testing arena, and excludes the development of automatic cognitive processes by complex memory tasks. In experimental tree shrews, psychosocial conflict caused elevated cortisol levels during the stress phases. While this resulted in an impairment of the stress-sensitive declarative memory during the second stress phase, no memory deficit was found during the first and third stress phase. Despite normalized cortisol levels, significant memory deficits in experimental animals were observed even 10 weeks after the last stressful experience. The alternating order of stressful events revealed that the negative correlation between the level of adrenal steroid hormones and memory performance does not account for the long-lasting effects of psychosocial stress in tree shrews.

Differential effects of chronic stress on memory processes in the tree shrew

Unpredictable and uncontrollable stressful events have been shown to affect cognitive processes. Interestingly, only hippocampus-mediated memory processes are thought to be sensitive to the effects of chronic stress. In contrast, the hippocampus-independent memory processes have been shown to be resistant to chronic stressful experiences. A central feature of the stress response is the activation of the hypothalamus-pituitary-adrenocortical (HPA)-axis, resulting in increased plasma levels of glucocorticoids, and several studies suggested that the performance of hippocampus-mediated memory processes might be directly modulated by these adrenal steroids. We investigated the impact of chronic psychosocial stress on hippocampus-mediated and hippocampus-independent memory processes in male tree shrews. By using a modified holeboard we followed memory performance during 23 weeks of alternating stress-free and stressful conditions. This schedule was designed to mimic a more realistic situation with stress-free conditions being sequentially interrupted by challenging events. The results indicate that chronic stress differentially affects hippocampus-mediated and hippocampus-independent memory processes in tree shrews. While hippocampus-independent memory processes remained unimpaired throughout the study, hippocampus-mediated memory was persistently impaired, not only during stress periods but also during recovery periods. This persistent impairment seems not to be exclusively triggered by glucocorticoids because urinary free cortisol concentration returned to normal during recovery periods. The present study is the first to evaluate the consequences of sequential stress exposure on memory performance in animals. Apparently, the mechanisms modulating cognitive processes are far from being understood and need a very systematic analysis in animal models with a high face and predictive validity to human stress-related memory disorders.

Assessing cognitive functions in tree shrews: visuo-spatial and spatial learning in the home cage

We developed a holeboard paradigm to investigate the cognitive abilities of tree shrews (Tupaia belangeri). The design allows animals to be tested in their homecages, which reduces possible confounding and stress factors, such as transfer to a special testing arena, food deprivation or other restraints. Nine male tree shrews performed four visuo-spatial and one spatial trial per day in two blocks of 5 consecutive days. Tree shrews needed only 1 day, that is five trials, to learn the complex holeboard paradigm. From the second day onwards the learning scores remained almost stable. To demonstrate the applicability of this paradigm in studies on the influence of stress on cognitive functions, three animals were subjected to and tested under psychosocial stress conditions on 2 consecutive days. Since the experimental animals also performed the task under stressful conditions the home cage holeboard paradigm is a valid tool to study the development of stress induced cognitive impairments.