THE RELATIONSHIP BETWEEN ACUTE GLUCOCORTICOID LEVELS AND HIPPOCAMPAL FUNCTION DEPENDS UPON TASK AVERSIVENESS AND MEMORY PROCESSING STAGE

Mitochondrial cannabinoid receptors gate corticosterone impact on novel object recognition

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

Corticosterone injection into the dorsal and ventral hippocampus impairs fear memory reconsolidation in a time-dependent manner in rats

Reconsolidation is an active process induced following the reactivation of previously consolidated memories. Recent studies suggest brain corticosteroid receptors may participate in the modulation of fear memory reconsolidation. Glucocorticoid receptors (GRs), with 10-fold lower affinity than mineralocorticoid receptors (MRs), are mainly occupied during the peak of the circadian rhythm, and after stress, so they probably have a more critical role than MRs in memory phases during stressful situations. This study investigated the role of dorsal and ventral hippocampal (DH and VH) GRs and MRs on fear memory reconsolidation in rats. Male Wistar rats with surgically implanted bilaterally cannulae at the DH and VH were trained and tested in an inhibitory avoidance task. The animals received bilateral microinjections of vehicle (0.3 µl/side), corticosterone (3 ng/0.3 µl/side), the GRs antagonist RU38486 (3 ng/0.3 µl/side), or the MRs antagonist spironolactone (3 ng/0.3 µl/side) immediately after memory reactivation. Moreover, drugs were injected into VH 90 minutes after memory reactivation. Memory tests were performed 2, 9, 11, and 13 days after memory reactivation. Results indicated that injection of corticosterone into the DH but not VH immediately after memory reactivation significantly impaired fear memory reconsolidation. Moreover, corticosterone injection into VH 90 minutes after memory reactivation impaired fear memory reconsolidation. RU38486 reversed these effects but not spironolactone. These findings indicate that corticosterone injection into the DH and VH via GRs activation impairs the reconsolidation of fear memory in a time-dependent manner.

The infralimbic mineralocorticoid blockage prevents the stress-induced impairment of aversive memory extinction in rats

Individuals deal with adversity and return to a normal lifestyle when adversity ends. Nevertheless, in specific cases, traumas may be preceded by memory distortions in stress-related malaises, and memory extinction impairment is strictly associated with the symptoms of post-traumatic stress disorder. Glucocorticoids (GCs), the central stress mediator, target mineralocorticoid (MR) and glucocorticoid (GR) receptors and coordinate stress responses. Despite MRs being present in brain regions essential to cognition, emotions, and initial stress processing, such as the medial prefrontal cortex (mPFC), most studies attempt to elucidate the stress-induced deleterious actions of GCs via GR. Therefore, it is necessary to understand the relationship between stress, infralimbic mPFC (IL), and memory and how MR-mediated intracellular signaling influences this relationship and modulates memory extinction. We observed that acutely restraint-stressed male Wistar rats showed high corticosterone (CORT) levels, and previous intra-IL-spironolactone administration (a selective MR antagonist) decreased it 60 min after the stress started. Intra-IL-CORT118335, a novel mixed MR/GR selective modulator, increased CORT throughout stress exposure. Ten days after stress, all rats increased freezing in the memory retrieval test and acquired the aversive contextual memory. During the extinction test, intra-IL injection of spironolactone, but not CORT118335, prevented the stress-impaired memory extinction, suggesting that the IL-MR activity controls CORT concentration, and it is crucial to the establishment of late extinction impairment. Also, the concomitant GR full activation overrode MR blockage. It increased CORT levels leading to the stress-induced extinction memory impairment, reinforcing that the MR/GR balance is crucial to predicting stress-induced behavioral outcomes.

The Effect of 12 Hz Extremely Low-frequency Electromagnetic Field on Visual Memory of Male Macaque Monkeys

Introduction

Today, humans live in a world surrounded by electromagnetic fields. Numerous studies have been conducted to discover the biological, physiological, and behavioral effects of electromagnetic fields on humans and animals. Given the biological similarities between monkeys and humans, The present research aimed to examine Visual Memory (VM), hormonal, genomic, and anatomic changes, in the male rhesus macaques exposed to an Extremely Low-Frequency Magnetic Field (ELF-MF).

Methods

Four male rhesus macaques (Macaca mulatta) were used. For the behavioral tests, the animals should be fasting for 17 hours. For the tests such as visual memory, the animal's cooperation was necessary. Using the radiation protocol, we exposed two monkeys to a 12-Hz electromagnetic field with a magnitude of 0.7 μT (electromagnetic radiation) four hours a day for a month. Before and after the exposure, a visual memory test was conducted using a coated device (visible reward) on a movable stand. Ten milliliters of blood was obtained from the femoral artery of each monkey, and half of it was used to examine cortisol serum levels using the MyBioSource kit (made in the USA). The other half of the blood was used to extract lymphocytes for assaying expressions of Glucocorticoid Receptor (GR) genes before and after radiation using the PCR method. Anatomic studies of the amygdala were carried out based on pre- and post-radiation Magnetic Resonance Imaging (MRI).

Results

Research results indicated that visual memory in male primates increased significantly after exposure to the 12-Hz frequency. Hormonal analysis at the 12-Hz frequency showed a decrease in cortisol serum levels. However, visual memory and serum cortisol levels did not change considerably in male primates in the control group. There was no considerable amygdala volumetric difference after exposure to the 12-Hz frequency. The expression of the GR genes decreased in the 12-Hz group compared to the control group.

Conclusion

In short, these results indicated that ELF might benefit memory enhancement because exposure to the 12-HZ ELF can enhance visual memory. This outcome may be due to a decrease in plasma cortisol and or expression of GR genes. Moreover, direct amygdala involvement in this regard cannot be recommended.

Highlights

The effects of Extremely Low-Frequency Electromagnetic Fields (ELF-EMF) of 12 Hz on monkeys were studied.The results showed a reduction in the serum cortisol levels and the expression of GR genes.The amygdala anatomical area changes were not significant in the experimental group.In the experimental group, visual memory (delay of 30- and 60-s evaluation) improved after exposure to a frequency of 12 Hz.

Plain Language Summary

Extremely low-frequency electromagnetic fields are among the most important factors affecting humans. This study aimed to determine the fields of 12-Hz frequency on the visual memory changes of male monkeys. The importance of research is due to the cognitive similarity of monkeys to humans. The findings of the research can be attributed to humans. Behavioral, hormonal, genetic, and anatomical studies indicated improvement in visual memory (test monkeys versus control monkeys). This study demonstrates the effect of the 12-Hz frequency on the monkey's visual memory. Researchers can study 12-Hz frequency in other cognitive indices.

Peroxiredoxin 6 Knockout Mice Demonstrate Anxiety Behavior and Attenuated Contextual Fear Memory after Receiving Acute Immobilization Stress

Stress can elicit glucocorticoid release to promote coping mechanisms and influence learning and memory performance. Individual memory performance varies in response to stress, and the underlying mechanism is not clear yet. Peroxiredoxin 6 (PRDX6) is a multifunctional enzyme participating in both physiological and pathological conditions. Several studies have demonstrated the correlation between PRDX6 expression level and stress-related disorders. Our recent finding indicates that lack of the Prdx6 gene leads to enhanced fear memory. However, it is unknown whether PRDX6 is involved in changes in anxiety response and memory performance upon stress. The present study reveals that hippocampal PRDX6 level is downregulated 30 min after acute immobilization stress (AIS) and trace fear conditioning (TFC). In human retinal pigment epithelium (ARPE-19) cells, the PRDX6 expression level decreases after being treated with stress hormone corticosterone. Lack of PRDX6 caused elevated basal H₂O₂ levels in the hippocampus, basolateral amygdala, and medial prefrontal cortex, brain regions involved in anxiety response and fear memory formation. Additionally, this H₂O₂ level was still high in the medial prefrontal cortex of the knockout mice under AIS. Anxiety behavior of Prdx6^-/- mice was enhanced after immobilization for 30 min. After exposure to AIS before a contextual test, Prdx6^-/- mice displayed a contextual fear memory deficit. Our results showed that the memory performance of Prdx6^-/- mice was impaired when responding to AIS, accompanied by dysregulated H₂O₂ levels. The present study helps better understand the function of PRDX6 in memory performance after acute stress.

Vitamin A deficiency impairs contextual fear memory in rats: Abnormalities in the glucocorticoid pathway

Vitamin A and its active metabolite, retinoic acid (RA), play a key role in the maintenance of cognitive functions in the adult brain. Depletion of RA using the vitamin A deficiency (VAD) model in Wistar rats leads to spatial memory deficits in relation to elevated intrahippocampal basal corticosterone (CORT) levels and increased hippocampal 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) activity. All of these effects are normalised by vitamin A supplementation. However, it is unknown whether vitamin A status also modulates contextual fear conditioning (CFC) in a glucocorticoid-associated fear memory task dependent on the functional integrity of the hippocampus. In the present study, we investigated the impact of VAD and vitamin A supplementation in adult male rats on fear memory processing, plasma CORT levels, hippocampal retinoid receptors and 11β-HSD1 expression following a novelty-induced stress. We also examined whether vitamin A supplementation or a single injection of UE2316, a selective 11β-HSD1 inhibitor, known to modulate local glucocorticoid levels, had any beneficial effects on contextual fear memory and biochemical parameters in VAD rats. We provide evidence that VAD rats exhibit a decreased fear conditioning response during training with a poor contextual fear memory 24 hours later. These VAD-induced cognitive impairments are associated with elevated plasma CORT levels under basal conditions, as well as following a stressful event, with saturated CORT release, altered hippocampal retinoid receptors and 11β-HSD1 expression. Vitamin A supplementation normalises VAD-induced fear conditioning training deficits and all biochemical effects, although it cannot prevent fear memory deficits. Moreover, a single injection of UE2316 not only impairs contextual fear memory, but also reduces plasma CORT levels, regardless of the vitamin A status and decreases slightly hippocampal 11β-HSD1 activity in VAD rats following stress. The present study highlights the importance of vitamin A status with respect to modulating fear memory conditioning in relation to plasma CORT levels and hippocampal 11β-HSD1.

Antagonizing the GABAA receptor during behavioral training improves spatial memory at different doses in control and chronically stressed rats

Chronic stress leads to a dysregulated inhibitory tone that could impact hippocampal-dependent spatial learning and memory. The present study examined whether spatial memory deficits resulting from chronic stress could be overcome by antagonizing the GABA_A receptor, a prominent inhibitory receptor of GABA in the hippocampus. Young adult male Sprague-Dawley rats were chronically stressed (STR, wire mesh restraint, 6h/d/21d) or placed in a no-stress control group (CON). When chronic restraint ended, rats were tested on a 2-trial object placement (OP) task at a delay (3h) that would result in chance performance without intervention and then on novel object recognition (NOR) and the elevated plus maze (EPM) to assess non-spatial memory and anxiety profile. In CON rats, Bicuculline (BIC, 0, 0.25, 0.5mg/kg), a GABA_A antagonist, injected 30min prior to training led to facilitated OP performance with 0.25 and 0.5mg/kg doses. In contrast, STR rats required BIC at the highest dose (0.5mg/kg) to improve OP performance. While overall object exploration was decreased by chronic stress, motivation or anxiety profile were unlikely to explain these results. These findings reveal two different dose response functions for BIC in control and chronically stressed rats, with the dose response function of BIC being shifted to the right for chronically stressed rats compared to controls in order to improve spatial memory. While the literature demonstrates that chronic stress disrupts hippocampal inhibitory tone, the current study reveals that a single injection to antagonize the GABA_A receptor can restore hippocampal-dependent spatial memory in chronically stressed subjects.

Differential effects of tianeptine on the dorsal hippocampal volume of rats submitted to maternal separation followed by chronic unpredictable stress in adulthood

Early maternal separation (MS) may produce lasting effects in the dorsal hippocampus (DH) that can change its response to chronic stress in adulthood. Chronic stress affects DH morphology and function, but tianeptine (an anti-depressant) can reverse the stress-induced morphological impairments. Morphologic alterations of hippocampus can affect contextual memory. Therefore, we evaluated the effect of tianeptine in MS and chronically stressed rats on: 1) volume of the DH and its areas using stereology and 2) hippocampal-dependent memory using a fear conditioning test. Male Wistar rats were subjected to daily MS for 4.5 h between postnatal days (PND) 1-21, or to animal facility rearing (AFR). Between (PND) days 50 and 74, rats were exposed to chronic unpredictable stress and were treated daily with tianeptine (10 mg/kg) or vehicle, providing eight groups: AFR-unstressed/vehicle (n = 5 for stereology, n = 18 for fear conditioning test); AFR unstressed/tianeptine (n = 6 and n = 10); AFR-chronic stress/vehicle (n = 6 and n = 14); AFR-chronic stress/tianeptine (n = 6 and n = 10), MS-unstressed/vehicle (n = 5 and n = 19), MS-unstressed/tianeptine (n = 6 and n = 10), MS-chronic stress/vehicle (n = 6 and n = 18), and MS-chronic stress/tianeptine (n = 6 and n = 10). MS-chronic stress/tianeptine rats showed a diminished CA1 area than the corresponding MS-unstressed/tianeptine rats. The combination of stressors produced a freezing response similar to those of the control group during postconditioning. During retrieval, MS led to a diminished freezing response compared to the AFR-unstressed groups. Tianeptine had no effect on freezing behavior. Our results show that tianeptine can affect the CA1 area volume differently depending on the nature and quantity of stressors but cannot alter freezing to context.

The effect of pre-eclampsia-like syndrome induced by L-NAME on learning and memory and hippocampal glucocorticoid receptor expression: A rat model

OBJECTIVE

We aimed to study the impacts of pre-eclampsia on the cognitive and learning capabilities of adolescent rat offspring and to explore the possible underlying mechanisms at the molecular level.

METHODS

Pregnant rats were subcutaneously injected with saline solution (control) (n = 16) or NG-nitro-L-arginine methyl ester (L-NAME) (n = 16) from the 13th day of gestation until parturition. The brain tissues from fetal rats delivered by cesarean section were examined in both groups with hematoxylin and eosin (H&E) staining. Rats born vaginally in both groups were subjected to the Morris water maze test when 8-week-old and their hippocampi were analyzed for glucocorticoid receptor (GR) expression.

RESULTS

A pre-eclampsia-like model was successfully built in pregnant rats by infusion of the NO synthase inhibitor L-NAME, including phenotypes as maternal hypertension and proteinuria, high stillbirth rate, and fetal growth retardation. Neuroepithelial cell proliferation was found in the hippocampus of fetal rats in the L-NAME group. Grown to 8-week-old, the L-NAME group showed significantly longer escape latency than the control group in the beginning as well as in the end of navigation trials. At the same time, the swimming distance achieved by the L-NAME group was significantly longer than that of the control group. Such differences in cognitive and learning capabilities between the two groups were not gender dependent. Besides, the 8-week-old rats in the L-NAME group had increased GR expression in the hippocampus than the control group.

CONCLUSION

Pre-eclampsia would impair cognitive and learning capabilities in adolescent offspring, and the upregulated expression of hippocampal GR may be involved in the underlying mechanisms.

Stress as a one-armed bandit: Differential effects of stress paradigms on the morphology, neurochemistry and behavior in the rodent amygdala

Neuroplasticity may be defined as the ability of the central nervous system (CNS) to respond to changes in the internal and external environment and it is well established that some stimuli have the ability to facilitate or impair neuroplasticity depending on the pre-existing milieu. A classic example of a stimulus that can both facilitate and impair neuroplasticity is stress. Indeed, the ability of CNS to respond to acute stress is often dependent upon the prior stress history of the individual. While responses to acute stress are often viewed as adaptive in nature, stress reactivity in subjects with prior chronic stress experiences are often linked to neuropsychiatric disorders, including major depressive disorder, post-traumatic stress disorder (PTSD) and anxiety. In rodent studies, chronic stress exposure produces structural and functional alterations in the hippocampus and medial prefrontal cortex that are consistent across different types of stress paradigms. Conversely, the amygdala appears to exhibit differential structural and functional responses to stress that are dependent on a variety of factors, including the type of stressor performed and the duration of the stress paradigm. This is most evident in output measures including morphological analysis of amygdala neurons, measurement of glutamatergic tone in amygdalar subdivisions and the analysis of amygdala-centric behaviors. Accordingly, this review will provide an overview of the effects of stress on the structural and functional plasticity of the rodent amygdala, especially in relation to the differential effects of repeated or chronic stress paradigms on dendritic architecture, neurochemistry of the glutamatergic system and behavior.

Post-training corticosterone inhibits the return of fear evoked by platform stress and a subthreshold conditioning procedure in Sprague-Dawley rats

The return of fear is an important issue in anxiety disorder research. Each time a fear memory is reactivated, it may further strengthen overactivation of the fear circuit, which may contribute to long-term maintenance of the fear memory. Recent evidence indicates that glucocorticoids may help attenuate pathological fear, but its role in the return of fear is unclear. In the present study, systemic corticosterone (CORT; 25mg/kg) administration 1h after fear conditioning did not impair the consolidation process but significantly suppressed the return of fear evoked by a subthreshold conditioning (SC) procedure and elevated platform (EP) stress. Compared with the SC-induced return of fear, acute stress-induced return was state-dependent. In addition, post-training CORT treatment increased the adrenocorticotropic response after EP stress, which indicates that the drug-induced suppression of the return of fear may possibly derive from its regulation effect of the hypothalamic-pituitary-adrenal axis reactivity to stress. These results suggest that post-training CORT administration may help inhibit the return of fear evoked by EP or SC stress. The possible mechanisms involved in the high-dose CORT-induced suppression of the SC- and EP-induced return of fear are discussed.

Pre-test metyrapone impairs memory recall in fear conditioning tasks: lack of interaction with β-adrenergic activity

Cognitive processes, such as learning and memory, are essential for our adaptation to environmental changes and consequently for survival. Numerous studies indicate that hormones secreted during stressful situations, such as glucocorticoids (GCs), adrenaline and noradrenaline, regulate memory functions, modulating aversive memory consolidation and retrieval, in an interactive and complementary way. Thus, the facilitatory effects of GCs on memory consolidation as well as their suppressive effects on retrieval are substantially explained by this interaction. On the other hand, low levels of GCs are also associated with negative effects on memory consolidation and retrieval and the mechanisms involved are not well understood. The present study sought to investigate the consequences of blocking the rise of GCs on fear memory retrieval in multiple tests, assessing the participation of β-adrenergic signaling on this effect. Metyrapone (GCs synthesis inhibitor; 75 mg/kg), administered 90 min before the first test of contextual or tone fear conditioning (TFC), negatively affected animals’ performances, but this effect did not persist on a subsequent test, when the conditioned response was again expressed. This result suggested that the treatment impaired fear memory retrieval during the first evaluation. The administration immediately after the first test did not affect the animals’ performances in contextual fear conditioning (CFC), suggesting that the drug did not interfere with processes triggered by memory reactivation. Moreover, metyrapone effects were independent of β-adrenergic signaling, since concurrent administration with propranolol (2 mg/kg), a β-adrenergic antagonist, did not modify the effects induced by metyrapone alone. These results demonstrate that pre-test metyrapone administration led to negative effects on fear memory retrieval and this action was independent of a β-adrenergic signaling.

Effects of pretraining and water temperature on female rats' performance in the Morris water maze

The water maze is a complex spatial task that requires the coordination of multiple systems to perform efficiently. Various factors have been shown to influence performance in this task, including motivational state and prior experience. Although a consistent sex difference has been observed in acquiring the water maze in rats, the contribution of the various factors in female rat performance has not been fully assessed. Therefore, the current study tested the effects of motivation as manipulated by water temperature of the maze and prior experience in the maze on the performance of female rats. It was hypothesized that females pretrained in the maze would perform better than those without exposure to the water maze, regardless of water temperature, but in naïve rats, colder water would improve performance as shown previously in male rats. For pretraining, female rats were taught to find a visible platform in cold (19 °C, 4 trials on one day) and warm (25 °C, 4 trials on one day) water before acquisition trials, with the order of the water temperature randomly assigned. Control rats were not given any training and were naïve to the water maze procedure. Pretrained and control rats were then tested to locate a hidden platform in either cold or warm water for 5 consecutive days. Overall, pretraining had a significant effect on distance, latency, and directness of path to the platform. Water temperature did not show a significant effect on any measure or a significant interaction with pretraining. Thus, while our hypothesis that pretraining would improve performance was supported, the results did not support the prediction that water temperature would also significantly influence performance. These results show that non-spatial pretraining can critically improve the performance of females in acquiring a place strategy for the hidden platform, irrespective of water temperature.

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).

Glucocorticoids act on glutamatergic pathways to affect memory processes

Glucocorticoids can acutely affect memory processes, with both facilitating and impairing effects having been described. Recent work has revealed that glucocorticoids may affect learning and memory processes by interacting with glutamatergic mechanisms. In this opinion article I describe different glutamatergic pathways that glucocorticoids can affect to modulate memory processes. Furthermore, glucocorticoid-glutamatergic interactions during information processing are proposed as a potential model to explain many of the diverse actions of glucocorticoids on cognition. The model suggests that direct modulation of glutamatergic pathways by glucocorticoids could serve as an important mechanism for these hormones to directly alter cognitive functions.

Diabetes as a chronic metabolic stressor: causes, consequences and clinical complications

Diabetes mellitus is an endocrine disorder resulting from inadequate insulin release and/or reduced insulin sensitivity. The complications of diabetes are well characterized in peripheral tissues, but there is a growing appreciation that the complications of diabetes extend to the central nervous system (CNS). One of the potential neurological complications of diabetes is cognitive deficits. Interestingly, the structural, electrophysiological, neurochemical and anatomical underpinnings responsible for cognitive deficits in diabetes are strikingly similar to those observed in animals subjected to chronic stress, as well as in patients with stress-related psychiatric illnesses such as major depressive disorder. Since diabetes is a chronic metabolic stressor, this has led to the suggestion that common mechanistic mediators are responsible for neuroplasticity deficits in both diabetes and depression. Moreover, these common mechanistic mediators may be responsible for the increase in the risk of depressive illness in diabetes patients. In view of these observations, the aims of this review are (1) to describe the neuroplasticity deficits observed in diabetic rodents and patients; (2) to summarize the similarities in the clinical and preclinical studies of depression and diabetes; and (3) to highlight the diabetes-induced neuroplasticity deficits in those brain regions that have been implicated as important pathological centers in depressive illness, namely, the hippocampus, the amygdala and the prefrontal cortex.

l-histidine induces state-dependent memory deficit in mice mediated by H(1) receptor

This study investigated the role of H(1) receptor in the state-dependent memory deficit induced by l-histidine (LH) in mice using Trial 1/2 protocol in the elevated plus-maze (EPM). The test was performed for two consecutive days: Trial 1 (T1) and Trial 2 (T2). Before both trials, mice received a combined injection i.p. of saline+saline (SAL/SAL), 500 mg/kg L-histidine+saline (LH/SAL), 500 mg/kg L-histidine+16 mg/kg chlorpheniramine (LH/CPA) or saline+16 mg/kg chlorpheniramine (SAL/CPA). The trials were performed in the EPM 10 min after the last injection. Each animal was placed in the center of the maze facing the open arm and had five minutes to explore it. On both days, test sessions were videotaped. The behavioral measures were scored from videotape. Data were analyzed based on Analysis of Variance (ANOVA) and the Fisher's LSD test. The data showed no effects on anxiety since there was no difference between the SAL/SAL and the other groups in Trial 1, respectively, open arm entries (OAE), open arm time (OAT) and their percentages (%OAE and %OAT). During Trial 2, OAE, OAT, %OAE and %OAT were reduced in mice treated with SAL/SAL, LH/CPA and SAL/CPA, while the group LH/SAL did not show any difference in these measures. No significant changes were observed in enclosed arm entries (EAE), an EPM index of general exploratory activity. Thus, it can be suggested that LH induces emotional memory deficit and the treatment with chlorpheniramine was able to revert this effect, suggesting this action of LH was mediated by the H(1) receptor.

Hydrocortisone responsiveness in Gulf War veterans with PTSD: effects on ACTH, declarative memory hippocampal [(18)F]FDG uptake on PET

Neuroendocrine, cognitive and hippocampal alterations have been described in Gulf War (GW) veterans, but their inter-relationships and significance for posttraumatic stress disorder (PTSD) have not been described. Hydrocortisone (Hcort) was administered to GW veterans with (PTSD+ n=12) and without (PTSD- n=8) chronic PTSD in a randomized, placebo-controlled, double-blind challenge. Changes in plasma ACTH, memory, and hippocampal [(18)F]FDG uptake on positron emission tomography were assessed. The low-dose dexamethasone suppression test was also administered. The PTSD+ group showed greater cortisol and ACTH suppression, reflecting greater peripheral glucocorticoid receptor (GR) responsiveness, and did not show an Hcort-induced decrement in delayed recall or retention. The groups had comparable relative regional hippocampal [(18)F]FDG uptake at baseline, but only the PTSD- group had an Hcort-associated decrease in hippocampal [(18)F]FDG uptake. Asymmetry in hippocampal hemispheric volumes differed between PTSD+ and PTSD- groups. This asymmetry was associated with cortisol, ACTH, retention and functional hippocampal asymmetry before, but not after, Hcort administration. Differences in brain metabolic responses between GW veterans with and without PTSD may reflect differences in peripheral and central GR responsiveness.

A critical review of chronic stress effects on spatial learning and memory

The purpose of this review is to evaluate the effects of chronic stress on hippocampal-dependent function, based primarily upon studies using young, adult male rodents and spatial navigation tasks. Despite this restriction, variability amongst the findings was evident and how or even whether chronic stress influenced spatial ability depended upon the type of task, the dependent variable measured and how the task was implemented, the type and duration of the stressors, housing conditions of the animals that include accessibility to food and cage mates, and duration from the end of the stress to the start of behavioral assessment. Nonetheless, patterns emerged as follows: For spatial memory, chronic stress impairs spatial reference memory and has transient effects on spatial working memory. For spatial learning, however, chronic stress effects appear to be task-specific: chronic stress impairs spatial learning on appetitively motivated tasks, such as the radial arm maze or holeboard, tasks that evoke relatively mild to low arousal components from fear. But under testing conditions that evoke moderate to strong arousal components from fear, such as during radial arm water maze testing, chronic stress appears to have minimal impairing effects or may even facilitate spatial learning. Chronic stress clearly impacts nearly every brain region and thus, how chronic stress alters hippocampal spatial ability likely depends upon the engagement of other brain structures during behavioral training and testing.

Chronic 17beta-estradiol or cholesterol prevents stress-induced hippocampal CA3 dendritic retraction in ovariectomized female rats: possible correspondence between CA1 spine properties and spatial acquisition

Chronic stress may have different effects on hippocampal CA3 and CA1 neuronal morphology and function depending upon hormonal status, but rarely are manipulations of stress and gonadal steroids combined. Experiment 1 investigated the effects of chronic restraint and 17beta-estradiol replacement on CA3 and CA1 dendritic morphology and spatial learning in ovariectomized (OVX) female Sprague-Dawley rats. OVX rats were implanted with 25% 17beta-estradiol, 100% cholesterol, or blank silastic capsules and then chronically restrained (6h/d/21d) or kept in home cages. 17beta-Estradiol or cholesterol prevented stress-induced CA3 dendritic retraction, increased CA1 apical spine density, and altered CA1 spine shape. The combination of chronic stress and 17beta-estradiol facilitated water maze acquisition compared to chronic stress + blank implants and nonstressed controls + 17beta-estradiol. To further investigate the interaction between 17beta-estradiol and stress on hippocampal morphology, experiment 2 was conducted on gonadally intact, cycling female rats that were chronically restrained (6h/d/21d), and then euthanized at proestrus (high ovarian hormones) or estrus (low ovarian hormones). Cycling female rats failed to show chronic stress-induced CA3 dendritic retraction at either estrous phase. Chronic stress enhanced the ratio of CA1 basal spine heads to headless spines as found in experiment 1. In addition, proestrous rats displayed increased CA1 spine density regardless of stress history. These results show that 17beta-estradiol or cholesterol protect against chronic stress-induced CA3 dendritic retraction in females. These stress- and 17beta-estradiol-induced morphological changes may provide insight into how dendritic complexity and spine properties contribute to spatial ability.

Role of the neural cell adhesion molecule (NCAM) in amygdalo-hippocampal interactions and salience determination of contextual fear memory

Evidence suggests that the neural cell adhesion molecule (NCAM) is an important molecular constituent of adaptive and maladaptive circuit (re-)organization in the central nervous system. Here, we further investigate its putative involvement in amygdala and hippocampus functions during context fear memory formation. Using laser capture microdissection and quantitative RT-PCR, we show high NCAM mRNA expression levels in the lateral and basolateral subnuclei of the amygdala, as well as their training intensity- and context-dependent regulation during fear memory consolidation. Moreover, we demonstrate that deficits of NCAM-/- mice in context fear memory can be overcome through contextual pre-exposure, i.e. by reducing the modulatory influence of the amygdala on this hippocampus-dependent memory. On the contrary, NCAM-/- mice failed to increase contextual fear memory after salient overtraining, although they adequately increased their response to auditory-cued fear stimuli. Finally, we demonstrate a reduction of amygdalo-hippocampal theta synchronization in NCAM-/- mice during fear memory retrieval. Together, these results suggest an involvement of NCAM-mediated cell recognition processes in information processing of the amygdalo-hippocampal system and in the amygdala-mediated modulation of context fear memory according to stimulus salience.

Forebrain glucocorticoid receptor overexpression increases environmental reactivity and produces a stress-induced spatial discrimination deficit

Reactivity to environmental stressors influences vulnerability to neurological and psychiatric illnesses, but little is known about molecular mechanisms that control this reactivity. Since mice with forebrain-specific glucocorticoid receptor overexpression (GRov mice) display anxiety-like behaviors in novel environments and have difficulty adjusting to change in memory tasks, we hypothesized that these may be facets of a broader phenotype of altered reactivity to environmental demands. Male GRov and wild-type mice were tested in a multiple-trial object interaction test comprising environmental and object habituation and spatial and object novelty trials. Half the mice received restraint stress before testing. GRov mice exhibited more locomotor activity and, without stress, more object interaction than wild-type mice. Following acute stress, GRov mice no longer showed increased object exploration. While stress dampened responses to object novelty in both groups, GRov mice were particularly impaired in discrimination of spatial novelty post-stress. These data demonstrate that GRov leads to increased environmental reactivity, responsiveness to salience, and vulnerability to stress-induced cognitive deficits. They implicate forebrain glucocorticoid receptor (GR) in fine-tuning interactions with the environment and the interplay of emotional salience, coping abilities, and cognitive function.

Corticosterone synthesis inhibitor-induced decrease in the age-dependant expression of nitric oxide synthase 3 in the preterm ductus arteriosus of rats

Previous studies have shown that the dilating effect of nitric oxide (NO) on the fetal ductus arteriosus (DA) is age dependent and more marked in the premature stages in rats, but the factors that mediate this effect are poorly understood. The purpose of this study is to determine the changes in the expression of NO synthase (NOS) mRNA in the fetal DA and to examine the effect of an 11-beta-hydroxylase inhibitor of corticosterone synthesis, namely, metyrapone, on NOS expression. NOS 3 mRNA expression was observed in 17.5-day-old rat fetuses; thereafter, its level significantly increased and reached its peak on day 19.5 and then decreased until the end of the gestation period (day 21.5). To inhibit corticosterone synthesis, a constant infusion of metyrapone was administered to rats; this significantly decreased the fetal plasma corticosterone concentration as well as NOS 3 mRNA expression in the DA in a time-dependent manner. These results indicate that NO is generated by NOS 3 in the DA and that the age-dependant expression of NOS 3 in the premature DA is attributable to corticosterone-associated activity.

Sex differences in salivary cortisol in response to acute stressors among healthy participants, in recreational or pathological gamblers, and in those with posttraumatic stress disorder

Sex differences in incidence and severity of some stress-related, neuropsychiatric disorders are often reported to favor men, suggesting that women may be more vulnerable to aberrant hypothalamic-pituitary-adrenal (HPA) axis responses to stress. In this review, we discuss several investigations that we, and others, have conducted assessing salivary cortisol as a measure of HPA function. We have examined basal cortisol among healthy men and women and also following acute exposure to stressors. Among healthy participants, men had higher basal cortisol levels than did women. In response to acute stressors, such as carbon dioxide or noise, respectively, cortisol levels were comparable between men and women or higher among women. We have also examined cortisol levels among those with problem eating, gambling, or posttraumatic stress disorder (PTSD). Women with restrained eating habits have higher basal cortisol levels than do women without restrained eating habits. Pathological gamblers have more aberrant stress response to gambling stimuli than do recreational gamblers, and these effects are more prominent among men than women. Men who have motor vehicle accident related PTSD, demonstrate more aberrant cortisol function, than do their female counterparts. Although these sex differences in cortisol seem to vary with type of stress exposure and/or pathophysiological status of the individual, other hormones may influence cortisol response. To address this, cortisol levels among boys and girls with different stress-related experiences, will be the subject of future investigation.

Mineralocorticoid receptor overexpression in basolateral amygdala reduces corticosterone secretion and anxiety

BACKGROUND

The amygdala plays a critical role in the development of anxiety and the regulation of stress hormone secretion. Reciprocally, stress and stress hormones can induce amygdala hypertrophy, a phenomenon related to enhanced anxiety. As such, modulating amygdaloid function can potentially reduce maladaptive features of the stress response. The amygdala contains two kind of receptor for corticosteroids, the adrenal steroid hormone released during stress: glucocorticoid receptors (GRs) and mineralocorticoid receptors (MRs). Although high-affinity MRs are heavily occupied during basal conditions, low-affinity GRs are heavily occupied only by stress levels of glucocorticoids. Prolonged and heavy occupancy of GRs tends to mediate the deleterious effects of glucocorticoids on neurons, whereas MR occupancy tends to mediate beneficial effects.

METHODS

In this report, we overexpress MR in neurons of adult rat basolateral amygdala, with a herpes simplex viral vector coding for two copies of MR.

RESULTS

Such overexpression reduced anxiety, as measured on an elevated plus-maze, and reduced the magnitude of glucocorticoid secretion after an acute stressor.

CONCLUSIONS

Thus, increasing MR signaling in basolateral amygdala could be valuable in management of stress disorders.

Effects of stress, corticosterone, and epinephrine administration on learning in place and response tasks

These experiments examined the effects of prior stress, corticosterone, or epinephrine on learning in mazes that can be solved efficiently using either place or response strategies. In a repeated stress condition, rats received restraint stress for 6h/day for 21 days, ending 24h before food-motivated maze training. In two single stress conditions, rats received a 1-h episode of restraint stress ending 30 min or 24h prior to training. Single stress ending 30 min prior to training resulted in a significant interaction of stress and learning on the two tasks, with significant enhancement of learning in the response task and non-significant impairment in the place task. Neither acute nor chronic stress significantly altered learning in either task when the stress ended 24h before training. Thus, the anterograde effects of stress on maze learning ended within a single day. Two stress-related hormones, corticosterone and epinephrine, were tested for effects on learning parallel to those of acute stress. When administered 30 min prior to training, a corticosterone dose (40 mg/kg) that enhanced memory on a spontaneous alternation task did not significantly enhance or impair learning in either task. Two doses of epinephrine that modulate memory in other settings were used to test the effects of epinephrine on learning. Pre-training injections of 0.03 mg/kg epinephrine impaired place learning, while 0.1mg/kg epinephrine impaired response learning. The epinephrine results mimicked those seen with acute stress on the place task, but were opposite those seen after acute stress on the response task. Thus, corticosterone does not appear to be a major factor mediating the effects of acute stress on place and response learning and epinephrine is, at most, a partial contributor to these effects.

Stress impairs acquisition of delay eyeblink conditioning in men and women

In rodents stress impairs delay as well as trace eyelid conditioning in females, but enhances it in males. The present study tested the effects of acute psychosocial stress exposure on classical delay eyeblink conditioning in healthy men and women. In a between subject design, participants were exposed to psychosocial stress using the Trier Social Stress Test (TSST) or a control condition which was followed by a delay eyeblink classical conditioning procedure. Stress exposure led to a significant increase in salivary cortisol and impaired acquisition of conditioned eyeblink responses (CRs). This was evident by a later first CR and an overall lower CR rate of the stress group. The stress-induced acquisition impairment was observed in both women and men. Subjects failing to show a stress-induced cortisol increase (cortisol non-responder) were not impaired in acquisition. Our findings indicate that acute stress, possibly via activation of the hypothalamus-pituitary-adrenal (HPA) axis, reduces the ability to acquire a simple conditioned motor response in humans.

Assessment of estradiol influence on spatial tasks and hippocampal CA1 spines: evidence that the duration of hormone deprivation after ovariectomy compromises 17beta-estradiol effectiveness in altering CA1 spines

Two pulses of 17beta-estradiol (10 microg) are commonly used to increase hippocampal CA1 apical dendritic spine density and alter spatial performance in ovariectomized (OVX) female rats, but rarely are the measures combined. The goal of this study was to use this two-pulse injection protocol repeatedly with intervening wash-out periods in the same rats to: 1) measure spatial ability using different tasks that require hippocampal function and 2) determine whether ovarian hormone depletion for an extended 10-week period reduces 17beta-estradiol's effectiveness in elevating CA1 apical dendritic spine density. Results showed that two injections of 10 microg 17beta-estradiol (72 and 48 h prior to testing and timed to maximize CA1 apical spine density at behavioral assessment) corresponded to improved spatial memory performance on object placement. In contrast, two injections of 5 microg 17beta-estradiol facilitated spatial learning on the water maze compared to rats given two injections of 10 microg 17beta-estradiol or the sesame oil vehicle. Neither 17beta-estradiol dose altered Y-maze performance. As expected, the intermittent two-pulse injection protocol increased CA1 apical spine density, but 10 weeks of OVX without estradiol treatment decreased the effectiveness of 10 microg 17beta-estradiol to increase CA1 apical spine density. Moreover, two pulses of 5 microg 17beta-estradiol injected intermittently failed to alter CA1 apical spine density and decreased basal spine density. These results demonstrate that extended time without ovarian hormones reduces 17beta-estradiol's effectiveness to increase CA1 apical spine density. Collectively, these findings highlight the complex interactions among estradiol, CA1 spine density/morphology, and task requirements, all of which contribute to behavioral outcomes.

Acute episodes of predator exposure in conjunction with chronic social instability as an animal model of post-traumatic stress disorder

People who are exposed to horrific, life-threatening experiences are at risk for developing post-traumatic stress disorder (PTSD). Some of the symptoms of PTSD include persistent anxiety, exaggerated startle, cognitive impairments and increased sensitivity to yohimbine, an alpha(2)-adrenergic receptor antagonist. We have taken into account the conditions known to induce PTSD, as well as factors responsible for long-term maintenance of the disorder, to develop an animal model of PTSD. Adult male Sprague-Dawley rats were administered a total of 31 days of psychosocial stress, composed of acute and chronic components. The acute component was a 1-h stress session (immobilization during cat exposure), which occurred on Days 1 and 11. The chronic component was that on all 31 days the rats were given unstable housing conditions. We found that psychosocially stressed rats had reduced growth rate, reduced thymus weight, increased adrenal gland weight, increased anxiety, an exaggerated startle response, cognitive impairments, greater cardiovascular and corticosterone reactivity to an acute stressor and heightened responsivity to yohimbine. This work demonstrates the effectiveness of acute inescapable episodes of predator exposure administered in conjunction with daily social instability as an animal model of PTSD.

Acute predator stress impairs the consolidation and retrieval of hippocampus-dependent memory in male and female rats

We have studied the effects of an acute predator stress experience on spatial learning and memory in adult male and female Sprague-Dawley rats. All rats were trained to learn the location of a hidden escape platform in the radial-arm water maze (RAWM), a hippocampus-dependent spatial memory task. In the control (non-stress) condition, female rats were superior to the males in the accuracy and consistency of their spatial memory performance tested over multiple days of training. In the stress condition, rats were exposed to the cat for 30 min immediately before or after learning, or before the 24-h memory test. Predator stress dramatically increased corticosterone levels in males and females, with females exhibiting greater baseline and stress-evoked responses than males. Despite these sex differences in the overall magnitudes of corticosterone levels, there were significant sex-independent correlations involving basal and stress-evoked corticosterone levels, and memory performance. Most importantly, predator stress impaired short-term memory, as well as processes involved in memory consolidation and retrieval, in male and female rats. Overall, we have found that an intense, ethologically relevant stressor produced a largely equivalent impairment of memory in male and female rats, and sex-independent corticosterone-memory correlations. These findings may provide insight into commonalities in how traumatic stress affects the brain and memory in men and women.

Differential effect of corticosterone deficiency on the expression of LH, prolactin and insulin receptors on rat Leydig cells

The adverse effects of glucocorticoid deficiency on the expression of genes encoding Leydig cell surface receptors and the response to LH/prolactin/insulin to produce testosterone production are yet to be recognized. Following metyrapone-induced corticosterone deficiency, serum corticosterone, testosterone and insulin levels decrease, whereas serum prolactin exhibits a significant increase and serum LH remains unaltered. LH binding and LH receptor mRNA expression were not altered, but a significant decrease in PRL and insulin binding and in the mRNA expressions of their receptors were observed in corticosterone-deficient rats in vivo. Corticosterone deficiency significantly decreases the Leydig cellular basal as well as hormone-stimulated testosterone production in vitro. Simultaneous administration of corticosterone prevented its deficiency-induced changes in Leydig cells both in vivo and in vitro. Our results show that metyrapone-induced corticosterone deficiency impairs Leydig cell insulin and prolactin receptors, and their mRNA expression and the response of Leydig cells to LH/PRL/insulin on testosterone production.

Stress and memory: behavioral effects and neurobiological mechanisms

Stress is a potent modulator of learning and memory processes. Although there have been a few attempts in the literature to explain the diversity of effects (including facilitating, impairing, and lack of effects) described for the impact of stress on memory function according to single classification criterion, they have proved insufficient to explain the whole complexity of effects. Here, we review the literature in the field of stress and memory interactions according to five selected classifying factors (source of stress, stressor duration, stressor intensity, stressor timing with regard to memory phase, and learning type) in an attempt to develop an integrative model to understand how stress affects memory function. Summarizing on those conditions in which there was enough information, we conclude that high stress levels, whether intrinsic (triggered by the cognitive challenge) or extrinsic (induced by conditions completely unrelated to the cognitive task), tend to facilitate Pavlovian conditioning (in a linear-asymptotic manner), while being deleterious for spatial/explicit information processing (which with regard to intrinsic stress levels follows an inverted U-shape effect). Moreover, after reviewing the literature, we conclude that all selected factors are essential to develop an integrative model that defines the outcome of stress effects in memory processes. In parallel, we provide a brief review of the main neurobiological mechanisms proposed to account for the different effects of stress in memory function. Glucocorticoids were found as a common mediating mechanism for both the facilitating and impairing actions of stress in different memory processes and phases. Among the brain regions implicated, the hippocampus, amygdala, and prefrontal cortex were highlighted as critical for the mediation of stress effects.

Chronic stress-induced hippocampal vulnerability: the glucocorticoid vulnerability hypothesis

The hippocampus, a limbic structure important in learning and memory, is particularly sensitive to chronic stress and to glucocorticoids. While glucocorticoids are essential for an effective stress response, their oversecretion was originally hypothesized to contribute to age-related hippocampal degeneration. However, conflicting findings were reported on whether prolonged exposure to elevated glucocorticoids endangered the hippocampus and whether the primate hippocampus even responded to glucocorticoids as the rodent hippocampus did. This review discusses the seemingly inconsistent findings about the effects of elevated and prolonged glucocorticoids on hippocampal health and proposes that a chronic stress history, which includes repeated elevation of glucocorticoids, may make the hippocampus vulnerable to potential injury. Studies are described to show that chronic stress or prolonged exposure to glucocorticoids can compromise the hippocampus by producing dendritic retraction, a reversible form of plasticity that includes dendritic restructuring without irreversible cell death. Conditions that produce dendritic retraction are hypothesized to make the hippocampus vulnerable to neurotoxic or metabolic challenges. Of particular interest is the finding that the hippocampus can recover from dendritic retraction without any noticeable cell loss. When conditions surrounding dendritic retraction are present, the potential for harm is increased because dendritic retraction may persist for weeks, months or even years, thereby broadening the window of time during which the hippocampus is vulnerable to harm, called the 'glucocorticoid vulnerability hypothesis'. The relevance of these findings is discussed with regard to conditions exhibiting parallels in hippocampal plasticity, including Cushing's disease, major depressive disorder (MDD), and post-traumatic stress disorder (PTSD).

Attenuating corticosterone levels on the day of memory assessment prevents chronic stress-induced impairments in spatial memory

This study investigated whether chronic stress-induced spatial memory deficits were caused by changes in the hypothalamic-pituitary-adrenal axis, such as corticosterone (CORT) elevations on the day of memory assessment, rather than the consequence of structural changes in the hippocampus. Male Sprague-Dawley rats were restrained for 6 h/day/21 days, and spatial memory was assessed on the Y-maze on day 22. Ninety minutes before training, rats received a subcutaneous injection of vehicle or metyrapone, a CORT synthesis inhibitor, and then spatial memory was determined 4-h later. The highest dose of metyrapone (75 mg/kg, s.c.) was most effective at preventing stress-induced spatial memory deficits. Chronic stress increased total CORT levels following Y-maze exposure, while acute metyrapone treatment dose-dependently attenuated total and free (unbound) CORT levels in both stress and control conditions. Blood samples taken from a separate subset of chronically stressed rats showed that baseline CORT levels were similar across the restraint period. Finally, chronic stress down-regulated glucocorticoid, but not mineralocorticoid, receptor mRNA expression within the hippocampus (dentate gyrus, CA1, CA2, CA3). These findings suggest that chronic stress-induced spatial memory deficits may be mediated by hypothalamic-pituitary-adrenal axis dysregulation. Specifically, CORT elevations and reductions in hippocampal glucocorticoid receptor expression, at the time of behavioural assessment may be involved, as opposed to a direct effect that is solely dependent upon hippocampal structural changes. These results have significance for treating cognitive decline in conditions associated with elevated glucocorticoids that include subpopulations in ageing, depression, Cushing's disease and Alzheimer's disease.