Glucocorticoids are necessary for enhancing the acquisition of associative memories after acute stressful experience

Exposure to acute stressful experience can enhance the later ability to acquire new memories about associations between stimuli. This enhanced learning is observed during classical eyeblink conditioning of both hippocampal-dependent and -independent learning. It can be induced within minutes of the stressful event and persists for days. Here we examined the role of the major stress hormones glucocorticoids in the enhancement of learning after stress. In the first two experiments, it was determined that adrenalectomy (ADX), with and without replacement of basal levels of corticosterone, prevented the stress-induced enhancement of trace conditioning, a task that is dependent on the hippocampus for acquisition. In a third experiment, demedullation, which removes the adrenal medulla but leaves the adrenal cortex and corticosterone levels intact, did not affect the enhancement of learning after stress. In a fourth experiment, ADX prevented the stress-induced enhancement of delay conditioning, a hippocampal-independent task. In a final experiment, it was determined that one injection of stress levels of corticosterone enhanced new learning within minutes but not new learning 24 h later. Together these results suggest that endogenous glucocorticoids are necessary and sufficient for transiently enhancing acquisition of new associative memories and necessary but insufficient for persistently enhancing their acquisition after exposure to an acute stressful experience.     

Stress hormones receptors in the amygdala mediate the effects of stress on the consolidation, but not the retrieval, of a non aversive spatial task

This study examined the effects of the arousal level of the rat and exposure to a behavioral stressor on acquisition, consolidation and retrieval of a non-aversive hippocampal-dependent learning paradigm, the object location task. Learning was tested under two arousal conditions: no previous habituation to the experimental context (high novelty stress/arousal level) or extensive prior habituation (reduced novelty stress/arousal level). Results indicated that in the habituated rats, exposure to an out-of-context stressor (i.e, elevated platform stress) impaired consolidation and retrieval, but not acquisition, of the task. Non-habituated animals under both stressed and control conditions did not show retention of the task. In habituated rats, RU-486 (10 ng/side), a glucocorticoid receptor (GR) antagonist, or propranolol (0.75 μg/side), a beta-adrenergic antagonist, injected into the basolateral amygdala (BLA), prevented the impairing effects of the stressor on consolidation, but not on retrieval. The CB1/CB2 receptor agonist WIN55,212-2 (WIN, 5 μg/side) microinjected into the BLA did not prevent the effects of stress on either consolidation or retrieval. Taken together the results suggest that: (i) GR and β-adrenergic receptors in the BLA mediate the impairing effects of stress on the consolidation, but not the retrieval, of a neutral, non-aversive hippocampal-dependent task, (ii) the impairing effects of stress on hippocampal consolidation and retrieval are mediated by different neural mechanisms (i.e., different neurotransmitters or different brain areas), and (iii) the effects of stress on memory depend on the interaction between several main factors such as the stage of memory processing under investigation, the animal's level of arousal and the nature of the task (neutral or aversive).     

Ultrasonic vocalization as an indicator of emotional state during active avoidance learning in rats.

Adult male rats subjected to a two-way avoidance task emitted ultrasonic vocalizations (20-30 kHz) both during the presentation of the conditioned stimulus and the intertrial interval. The rate of ultrasonic calling decreased during the 75-trial session indicating that acquisition of the conditioned avoidance response (CAR) was inversely correlated with the rate of vocalization. The rate of acquisition of the CAR was most rapid in those rats that did not emit any vocalization during learning. These data suggest that ultrasonic calling during stressful situations may be sensitive indicator of underlying emotional states that interfere with the acquisition of a complex task.     

Chronic Stress Induces a Hyporeactivity of the Autonomic Nervous System in Response to Acute Mental Stressor and Impairs Cognitive Performance in Business Executives

The present study examined the incidence of chronic stress in business executives (109 subjects: 75 male and 34 female) and its relationship with cortisol levels, cognitive performance, and autonomic nervous system (ANS) reactivity after an acute mental stressor. Blood samples were collected from the subjects to measure cortisol concentration. After the sample collection, the subjects completed the Lipp Inventory of Stress Symptoms for Adults and the Stroop Color-Word Test to evaluate stress and cognitive performance levels, respectively. Saliva samples were collected prior to, immediately after, and five minutes after the test. The results revealed that 90.1% of the stressed subjects experienced stress phases that are considered chronic stress. At rest, the subjects with chronic stress showed higher cortisol levels, and no gender differences were observed. No differences were found between the stressed and non-stressed subjects regarding salivary amylase activity prior to test. Chronic stress also impaired performance on the Stroop test, which revealed higher rates of error and longer reaction times in the incongruent stimulus task independently of gender. For the congruent stimulus task of the Stroop test, the stressed males presented a higher rate of errors than the non-stressed males and a longer reaction time than the stressed females. After the acute mental stressor, the non-stressed male group showed an increase in salivary alpha-amylase activity, which returned to the initial values five minutes after the test; this ANS reactivity was not observed in the chronically stressed male subjects. The ANS responses of the non-stressed vs stressed female groups were not different prior to or after the Stroop test. This study is the first to demonstrate a blunted reactivity of the ANS when male subjects with chronic psychological stress were subjected to an acute mental stressor, and this change could contribute to impairments in cognitive performance.     

Distinctive stress effects on learning during puberty

Puberty is a time of significant change in preparation for adulthood. Here, we examined how stressful experience affects cognitive and related hormonal responses in male and female rats prior to, during and after puberty. Groups were exposed to an acute stressor of brief periodic tailshocks and tested 24 h later in an associative memory task of trace eyeblink conditioning. Exposure to the stressor did not alter conditioning in males or females prior to puberty but enhanced conditioning in both males and females during puberty. The enhancement occurred in pubescent females irrespective of the estrous cycle. In adulthood, sex differences in trace conditioning and the response to stress emerged: females outperformed males under unstressed conditions, but after stressor exposure, trace conditioning in females was impaired whereas that in males was enhanced. These differences were not related to changes in gross motor activity or other nonspecific measures of performance. The effects of acute stress on corticosterone, estradiol, progesterone, and testosterone were also measured. Stressor exposure increased the concentration of corticosterone in all age groups, although sex differences were only evident in adults. All reproductive hormones except estradiol increased with age in a predictable and sex dependent fashion and none were affected by stressor exposure. Estradiol decreased in male rats across age, and remained stable for female rats. Together, these data indicate that males and female respond similarly to learning opportunities and stressful experience before and during puberty; it is in adulthood that sex differences and the opposite responses to stress arise.     

Effects of penitrem A on rat's performances in passive avoidance and Morris water maze tests

Intraperitoneal administration of the mycotoxin penitrem A 30 min before a training session in passive avoidance task, impaired performance of rats subjected to a test-session 24 h after. This effect was not antagonised by pretraining administration of physostigmine or bicuculline. Administration of penitrem A 20 min before a training session or 30 min before a test-session did not impair performance. In the Morris water maze, doses of penitrem A that induces slight to moderate tremors, but not a lower dose, disrupted place learning. These results suggest that penitrem A disrupts the processes that take place at the time of acquisition, but not those just after acquisition, and does not alter the restitution of information. This effect would not be related to a decrease of cholinergic neurotransmission nor to a stimulation of GABA A receptors. Nevertheless, it could not be totally excluded that the performance impairments induced by penitrem A would be secondary to a motor disruption. This revised version was published online in June 2006 with corrections to the Cover Date.     

Impaired passive avoidance acquisition in Wistar rats after restraint/cold stress and/or stresscopin administration

Stresscopin (SCP) and related peptides are new members of the corticotropin-releasing factor (CRF) peptide family that are selective ligands for CRF type 2 receptor; these ligands are essential for maintaining homeostasis after stress. SCP (i.p. injections) was tested on the passive avoidance learning task in stressed Wistar rats; it impaired the formation of memory trace. The retention performance deficit induced by SCP was comparable with the deficit induced by the stressor of restraint/cold. More profound impairment of avoidance response occurred following combined application of SCP and stressor. More specific actions of SCP can be expected from its studies with targeted intracerebral applications.     

Stimulation of Cortisol During Mental Task Performance in a Provocative Virtual Environment

Fully immersive and stereoscopic Virtual Environments (VE) represent a powerful multimedia tool for laboratory-based simulations of distinct scenarios including scenarios for evaluating stressful situations resembling reality. Thus far, cortisol secretion as a neuroendocrine parameter of stress has not been evaluated within a Virtual Reality (VR)-based paradigm. In this study 94 healthy volunteers were subjected to a provocative VR-paradigm and a cognitive stress task. Provocative in this context means the VE was designed to provoke physiological reactions (cortisol secretion) within the respective users by purpose. It was tested (a) if a fully dynamic VE as opposed to a static VE can be regarded as a stressor and (b) if such a fully dynamic VE can modify an additional response to a cognitive stressor presented within the VE additionally. Furthermore, possible gender-related impacts on cortisol responses were assessed. A significant cortisol increase was observed only after the combined application of the fully dynamic VE and the cognitive stressor, but not after application of the dynamic VE or the cognitive stressor alone. Cortisol reactivity was greater for men than for women. We conclude that a fully dynamic VE does not affect cortisol secretion per se, but increases cortisol responses to a dual task paradigm that includes performance of a stressful mental task. This provides the basis for the application of VR-based technologies in neuroscientific research, including the assessment of the human Hypothalamus-Pituitary-Adrenal (HPA) axis regulation.     

Influence of acoustic signal with ultrasonic components on the development of defensive conditioned reflex in Wistar rats

Short-term (90 s) effects of the complex acoustic signal (CAS) with ultrasonic components on the development of defensive conditioned reflex of two-way active avoidance in a shuttle-box were studied in female Wistar rats. The learning ability of rats was measured on a scale designed in our laboratory. It was shown that CAS stimulation triggered an audiogenic seizure of different strength in 59% of animals. The CAS was extremely stressful for Wistar rats: it prevented the active avoidance learning in early terms after its application (the first training session in 4 or 6 days). This effect did not depend on the presence or intensity of audiogenic seizures during CAS. In the second training session in 9 days (the first session was in 4 days), learning was impaired as compared to control without CAS. However, during repeated training procedure 1.5 months after the CAS (the first session in 6 days), rats rapidly reached the criterion of learning (10 consecutive avoidance reactions). On the other hand, if the CAS was presented with different time lags (immediately, in 3 or in 45 days) after the first training session, the ability of animals to learn during the second session was not impaired both in early and late terms after exposure to the stressor. The results suggest that exposure to CAS prevents development of short-term memory but does not affect consolidation process and long-term memory.     

Stress-induced effects on feeding behavior and growth performance of the sea bass (<Emphasis Type="Italic">Dicentrarchus labrax</Emphasis>): a self-feeding approach

Repetitive aquaculture-related protocols may act as cyclic stressors that induce chronic stress in cultured fish. The sea bass is particularly sensitive to stressful conditions and the mere presence of humans will disturb feeding behavior. In this paper, we study whether chronic stress induced by repetition of acute stress protocols affects long-term feeding behavior and growth performance in sea bass and whether exogenous cortisol may induce stress-like changes in these parameters. We demonstrate that both chronic stress and dietary cortisol decrease food intake and have a negative effect on feed conversion efficiency, severely impairing sea bass performance. Both experimental approaches induced changes in the daily feeding activity by lengthening the active feeding periods. Fish subjected to a cyclic stressor modify their daily feeding pattern in an attempt to avoid interference with the time of the stressor. The delay in feeding when fish are acutely and repeatedly stressed could be of substantial adaptive importance.     

Stressors and tasks: how and when should stressors be introduced during training for task performance in stressful situations?

Studies on the training of individuals for task performance in stressful situations have typically evaluated procedures that simultaneously expose trainees to tasks and to stressors. Such procedures might create a mutual interference of the stressor with task acquisition, or conversely, of preoccupation with task acquisition with familiarization with the stressors. Using a sample of 180 males, the present study compared a procedure that temporally separates task acquisition from exposure to stressors ("phased training") with the more typical approach which combines the two ("combined training"). The comparison was carried out under varying degrees of stressor-fidelity representation in the course of training, and under two degrees of contingency between quality of task performance and the possibility of avoiding stress. The main result indicates that phased and combined training are equally effective under conditions of noncontingency. In a contingent condition, on the other hand, phased training proves to be significantly superior.     

Recording Single Neurons' Action Potentials from Freely Moving Pigeons Across Three Stages of Learning

While the subject of learning has attracted immense interest from both behavioral and neural scientists, only relatively few investigators have observed single-neuron activity while animals are acquiring an operantly conditioned response, or when that response is extinguished. But even in these cases, observation periods usually encompass only a single stage of learning, i.e. acquisition or extinction, but not both (exceptions include protocols employing reversal learning; see Bingman et al.¹ for an example). However, acquisition and extinction entail different learning mechanisms and are therefore expected to be accompanied by different types and/or loci of neural plasticity.Accordingly, we developed a behavioral paradigm which institutes three stages of learning in a single behavioral session and which is well suited for the simultaneous recording of single neurons'' action potentials. Animals are trained on a single-interval forced choice task which requires mapping each of two possible choice responses to the presentation of different novel visual stimuli (acquisition). After having reached a predefined performance criterion, one of the two choice responses is no longer reinforced (extinction). Following a certain decrement in performance level, correct responses are reinforced again (reacquisition). By using a new set of stimuli in every session, animals can undergo the acquisition-extinction-reacquisition process repeatedly. Because all three stages of learning occur in a single behavioral session, the paradigm is ideal for the simultaneous observation of the spiking output of multiple single neurons. We use pigeons as model systems, but the task can easily be adapted to any other species capable of conditioned discrimination learning.     

Study of long-lasting effects of acute prenatal stress induced forced swimming

The aim of the present work was to assess long-lasting effects of acute prenatal stress in white rats. Forced swimming in cold water on the 7th or the 14th gestational day was used as a prenatal stressor. The prenatal stress led to low birthweight of offspring and their delayed growth rate during the second month of life. Prenatally stressed animals showed abnormalities in exploratory behavior and anxiety, increased emotionality and impaired learning capabilities at the age of 1-2 month. Consequently, acute stress on the 7th and at the 14th day of pregnancy induced long-lasting negative behavioral changes in offspring of stressed white rats.     

Pre- and postnatally administered ACTH, Organon 2766 and CRF facilitate or inhibit active avoidance task performance in young adult mice

This study investigated the effect of learning/memory-related neuropeptides on behavioral task performance in later life. A 1 mg/kg dosage of adrenocorticotropic hormone 4-9, Organon 2766, ACTH/MSH 4-10, ACTH 1-24, CRF, or diluent was subcutaneously injected into either pregnant females or into newborn pups during specific neural developmental windows. Each of the progeny was trained in an active-avoidance task and tested for acquisition on postpartum days 35-37. The mice were then tested for memory task performance and reacquisition on days 42-44 postpartum using the identical experimental paradigm as that used in the training sessions. Prenatal treatment with these memory-related neuropeptides resulted in significant facilitation of learning/memory task performance in male and female mice treated with Organon 2766 (p less than 0.001), and a significant inhibition of learning/memory task performance in males and females treated with ACTH 1-24 (p less than 0.01). Additional sex-specific performance facilitations and inhibitions resulted from the pre- or postnatal administration of the various neuropeptides used in this study. These results suggest that neuropeptides, when available in increased amounts during specific neural developmental windows, can significantly improve or suppress related behavioral performance capability in later life.     

Effects of stress and of amphetamine on passive avoidance conditioning in rats

This study examined the effects of immobilization stress combined with water immersion (ICS) and/or amphetamine (AM) on different memory phases in the passive avoidance task in rats. The performance of rats was evaluated in the retention tests 24 and 48 h after a single acquisition trial. ICS exposure lasting 1 h impaired retention of the learned avoidance response if applied 2 to 4 h before or immediately after training. The stressor did not affect retrieval if presented 5 or 2 h before the retention test. AM was used i.p. at the dose of 8 or 1 mg/kg. Neither 8 mg AM administered 4 h before nor 8 or 1 mg doses given after training did not impair the retention performance in unstressed rats. The 1 mg AM prevented the impairment of retention in animals exposed to the stressor 3 or 4 h before training but had no effect when the stronger impairment was induced by ICS 2 h before training. However, when given 1 h before retention testing, 1 mg AM attenuated even the severe impairment induced by the pre-training stressor exposure. Our results suggest that ICS impairs primarily the early phase of memory consolidation and a low dose of AM can prevent this effect.     

Toward a Choice-Based Judgment Bias Task for Horses

Judgment bias tasks for nonhuman animals are promising tools to assess emotional valence as a measure of animal welfare. In view of establishing a valid judgment bias task for horses, the present study aimed to evaluate 2 versions (go/no-go and active choice) of an auditory judgment bias task for horses in terms of acquisition learning and discrimination of ambiguous cues. Five mares and 5 stallions were randomly assigned to the 2 designs and trained for 10 trials per day to acquire different operant responses to a low-frequency tone and a high-frequency tone, respectively. Following acquisition learning, horses were tested on 4 days with 3 ambiguous-tone trials interspersed between the 10 high-tone and low-tone trials. All 5 go/no-go horses but only one active-choice horse successfully learned their task, indicating that it is more difficult to train horses on an active choice task than on a go/no-go task. During testing, however, go/no-go horses did not differentiate between the 3 different ambiguous cues, thereby making the validity of the test results questionable in terms of emotional valence.     

Changes in free and total plasma cortisol levels in juvenile haddock (Melanogrammus aeglefinus) exposed to long-term handling stress

We measured changes in free and total plasma cortisol levels, plasma glucose, gill hsp70 levels, and growth in haddock (Melanogrammus aeglefinus) subjected to a long-term handling stress (15 s out of water, each day, for 4 weeks), and the effect of this long-term stress on the ability of haddock to respond to an acute stressor. The acute stressor was a single handling stress, and fish were sampled at 1, 6, and 12 h post-stress. During the long-term stress study, free and total plasma cortisol levels increased significantly (10-fold) in the stressed group after the second week. However, the percentage of free cortisol was already significantly elevated by the first week (control 17%, stressed 55%), and remained high during the second week (control 35% and stressed 65%). After 3 and 4 weeks of handling, both free and total cortisol declined in stressed fish to levels that were not significantly different from pre-stress values. Control fish grew significantly more than stressed fish (by 32% and 18%, respectively) over the 4 week study, and condition factor only increased in control fish. Although fish from the control group showed elevated total plasma cortisol levels (to 47 ng mL(-1)) 1 h after the acute stress, and the levels in stressed fish were comparable to those for the control fish, no significant increase in plasma cortisol was measured in the group subjected to the long-term stress. Free plasma cortisol levels did not increase significantly in either group following the acute stress. However, free plasma cortisol levels were significantly higher in long-term stress group, as compared with the control group, at 6 h post-stress. Plasma glucose and gill hsp70 levels were not altered by either the long-term stress or acute stressor. Our data indicate that cortisol (free and total), but not glucose or hsp70, appears to be adequate to assess short- and long-term stress in haddock.     

Sleep Deprivation in the Dark Period Does Not Impair Memory in OF1 Mice

There is increasing evidence that sleep facilitates memory acquisition and consolidation. Moreover, the sleep-wake history preceding memory acquisition and retention as well as circadian timing may be important. We showed previously that sleep deprivation (SD) following learning in OF1 mice impaired their performance on an object recognition task. The learning task was scheduled at the end of the 12 h dark period and the test 24 h later. To investigate the influence of the prominent circadian sleep-wake distribution typical for rodents, we now scheduled the learning task at the beginning of the dark period. Wakefulness following immediately after the learning task was attained either by gentle interference (SD; n?=?20) or by spontaneous wheel running (RW; n?=?20). Two control groups were used: one had no RW throughout the experiment (n?=?23), while the other group's wheel was blocked immediately after acquisition (n?=?16), thereby preventing its use until testing. Recognition memory, defined as the difference in exploration of a novel and of familiar objects, was assessed 24 h later during the test phase. Motor activity and RW use were continuously recorded. Remarkably, performance on the object recognition task was not influenced by the protocols; the waking period following acquisition did not impair memory, independent of the method inducing wakefulness (i.e., sleep deprivation or spontaneous running). Thus, all groups explored the novel object significantly longer than the familiar ones during the test phase. Interestingly, neither the amount of rest lost during the SD interventions nor the amount of rest preceding acquisition influenced performance. However, the total amount of rest obtained by the control and SD mice subjected to acquisition at “dark offset” correlated positively (r?=?0.66) with memory at test, while no such relationship occurred in the corresponding groups tested at dark onset. Neither the amount of running nor intermediate rest correlated with performance at test in the RW group. We conclude that interfering with sleep during the dark period does not affect object recognition memory consolidation.     

Effects of lesion of the inferior olivary complex by 3-acetylpyridine on learning and memory in the rat

Summary DA/HAN-strained male rats (pigmented rats) were submitted to two experimental tasks consisting of spatial learning (water-escape) and a passive avoidance conditioning. Both these tasks were performed by different animals. In order to destroy the inferior olivary complex, the animals were injected with 3-acetylpyridine either 9 days prior to the initial learning session or 24 h after completion of the learning task. They were retested (retrieval test) 10 days after the initial learning was achieved. Learning and retention were compared to those noted in control rats. Administration of 3-acetylpyridine before the initial learning did not prevent the spatial learning but the scores were greatly altered and the number of trials needed to reach the fixed learning criterion was much greater than in controls. However, 10 days later the animals had memorized their initial experience. Injection of 3-acetylpyridine after the initial learning session impaired memory: the animals had completely forgotten their initial learning. It can therefore be concluded that lesion of the afferent climbing fibres to the cerebellar cortex alters learning and retention of a spatial task. Such a lesion does not interfere with learning and retention of a passive avoidance conditioning, since in this condition the experimental animals injected with 3-acetylpyridine either before or after the initial learning behave similarly to controls. The effects of the inferior olivary complex lesion are obviously different according to the task to be learnt, suggesting that these two tasks do not require the integrity of the same nervous structures.Abbreviations 3-AP 3-acetylpyridine - C control - ILR initial learning-lesion-retrieval - IOC inferior olivary complex - LIR lesion-initial learning-retrieval     

Learning performances in young horses using two different learning tests

To achieve optimal performance in equine sports as well as in leisure not only the physical abilities of the horse should be considered, but also the horse’s personality. Besides temperamental aspects, like emotionality, or the horse’s reactivity towards humans in handling situations, the learning ability of the horse is another relevant personality trait. To study whether differences in learning performance are consistent over time and whether individual learning performance differs between learning tests or is affected by emotionality, 39 young horses (Dutch Warmblood) were tested repeatedly in two learning tests. An aversive stimulus (AS) was used in one learning test (the avoidance learning test) and a reward was used in the other learning test (the reward learning test). During both learning tests behaviour as well as heart rate were measured. Each test was executed four times, twice when horses were 1 year of age, and twice when they were 2 years of age. Half of the horses received additional physical training from 6 months onwards. In both tests horses could be classified as either performers, i.e. completing the daily session, or as non-performers, i.e. returning to the home environment without having completed the daily session. There were some indications that emotionality might have caused non-performing behaviour, but these indications are not convincing enough to exclude other causes. Furthermore, there seem to be no simple relationships between measures of heart rate, behavioural responses putatively related to emotionality and learning performance. Horses revealed consistent individual learning performances within years in both tests, and in the avoidance learning test also between years. There was no significant correlation between learning performances in the avoidance learning test and the learning performances in the reward learning test. It is concluded that individual learning abilities are consistent over a short time interval for an avoidance learning test and a reward learning test and over a longer time for the avoidance learning test. Furthermore, results indicate that some horses perform better when they have to learn to avoid an aversive stimulus while others perform better when they are rewarded after a correct response. It is suggested that these differences may be relevant to design optimal individual training programmes and methods.