Recovery after chronic stress within spatial reference and working memory domains: correspondence with hippocampal morphology.

Chronic stress results in reversible spatial learning impairments in the Morris water maze that correspond with hippocampal CA3 dendritic retraction in male rats. Whether chronic stress impacts different types of memory domains, and whether these can similarly recover, is unknown. This study assessed the effects of chronic stress with and without a post-stress delay to evaluate learning and memory deficits within two memory domains, reference and working memory, in the radial arm water maze (RAWM). Three groups of 5-month-old male Sprague-Dawley rats were either not stressed [control (CON)], or restrained (6 h/day for 21 days) and then tested on the RAWM either on the next day [stress immediate (STR-IMM)] or following a 21-day delay [stress delay (STR-DEL)]. Although the groups learned the RAWM task similarly, groups differed in their 24-h retention trial assessment. Specifically, the STR-IMM group made more errors within both the spatial reference and working memory domains, and these deficits corresponded with a reduction in apical branch points and length of hippocampal CA3 dendrites. In contrast, the STR-DEL group showed significantly fewer errors in both the reference and working memory domains than the STR-IMM group. Moreover, the STR-DEL group showed better RAWM performance in the reference memory domain than did the CON group, and this corresponded with restored CA3 dendritic complexity, revealing long-term enhancing actions of chronic stress. These results indicate that chronic stress-induced spatial working and reference memory impairments, and CA3 dendritic retraction, are reversible, with chronic stress having lasting effects that can benefit spatial reference memory, but with these lasting beneficial effects being independent of CA3 dendritic complexity.

Chronic variable stress and intravenous methamphetamine self-administration - Role of individual differences in behavioral and physiological reactivity to novelty.

Stress is a contributing factor to the development and maintenance of addiction in humans. However, few studies have shown that stress potentiates the rewarding and/or reinforcing effects of methamphetamine in rodent models of addiction. The present study assessed the effects of exposure to 14 days of chronic variable stress (CVS), or no stress as a control (CON), on the rewarding and reinforcing effects of methamphetamine in adult rats using the conditioned place preference (Experiment 1) and intravenous self-administration (Experiment 2) paradigms. In Experiment 2, we also assessed individual differences in open field locomotor activity, anxiety-like behavior in the elevated plus maze (EPM), and physiological responses to a novel environment as possible predictors of methamphetamine intake patterns. Exposure to CVS for 14 days did not affect overall measures of methamphetamine conditioned reward or reinforcement. However, analyses of individual differences and direct vs. indirect effects revealed that rats exhibiting high physiological reactivity and locomotor activity in the EPM and open field tests self-administered more methamphetamine and reached higher breakpoints for drug reinforcement than rats exhibiting low reactivity. In addition, CVS exposure significantly increased the proportion of rats that exhibited high reactivity, and high reactivity was significantly correlated with increased levels of methamphetamine intake. These findings suggest that individual differences in physiological and locomotor reactivity to novel environments, as well as their interactions with stress history, predict patterns of drug intake in rodent models of methamphetamine addiction. Such predictors may eventually inform future strategies for implementing individualized treatment strategies for amphetamine use disorders.

Chronic stress enhances spatial memory in ovariectomized female rats despite CA3 dendritic retraction: possible involvement of CA1 neurons.

Emerging data report sex differences in how the brain responds to chronic stress. Here, we investigated the effects of chronic restraint stress (6 h/day/21 days) on hippocampal morphology and function in ovariectomized female rats. Chronic restraint stress caused CA3 apical dendritic retraction in short- and long-shafted neurons, while it reduced basal dendritic arbors in long-shafted neurons only. Chronic restraint did not affect CA1 dendritic arborization, although it increased the proportion of CA1 spine heads compared with controls. Both stressed and control animals performed well on the Y-maze, a spatial memory task. However, chronic stress enhanced Y-maze performance compared with controls, which may reflect facilitated spatial memory or reduced habituation. Y-maze performance correlated with CA1 spine head proportion. This relationship suggests that spatial ability in females may be more tightly coupled with CA1 morphology, which may override the influence of CA3 dendritic retraction. Thus, this research provides additional evidence that CA3 morphology does not always parallel spatial memory.

Chronic stress enhances ibotenic acid-induced damage selectively within the hippocampal CA3 region of male, but not female rats.

The purpose of this investigation was to assess the ability of the hippocampus to withstand a metabolic challenge following chronic stress. An N-methyl-d-aspartate receptor excitotoxin (ibotenic acid, IBO) was infused into the CA3 region of the hippocampus following a period of restraint for 6 h/day/21 days. Following the end of restraint when CA3 dendritic retraction persists (3 to 4 days), rats were infused with IBO (or vehicle) into the CA3 region of the hippocampus. Stressed male rats showed significantly more CA3 damage after IBO infusion relative to controls and the saline-infused side. Moreover, IBO-exacerbation of damage in males was not observed in the CA3 region 3 to 4 days after acute stress (6 h restraint), nor in the CA1 region after chronic stress. Females were also examined and chronic stress did not exacerbate IBO damage in the CA3 region. Overall, these results demonstrate that chronic stress compromises the ability of the hippocampus to withstand a metabolic challenge days after the chronic stress regimen has subsided in male rats. Whether the conditions surrounding CA3 dendritic retraction in females represents vulnerability is less clear and warrants further investigation.

Acute stress increases neuropeptide Y mRNA within the arcuate nucleus and hilus of the dentate gyrus.

The effects acute restraint stress on neuropeptide Y (NPY) mRNA expression were determined within the dentate gyrus and arcuate nucleus, where the effects of adrenal steroid action were previously reported. Adult male rats were exposed to 1 h of restraint stress and then sacrificed immediately, 6 h, or 24 h later. Controls were undisturbed. Stress increased NPY mRNA levels in both the arcuate nucleus and in the hilar region of the hippocampus with different time courses. NPY mRNA increased in the arcuate at 24 h, but not earlier, as determined by film autoradiography. Single cell grain analysis was performed in the dentate gyrus hilus because the NPY mRNA was heterogeneously distributed and revealed that the number of cells expressing NPY mRNA increased 6 h after stress, returning to control levels within 24 h. These results fit with previously reported effects of adrenal steroids modulating arcuate nucleus NPY expression through the adrenal steroid Type II receptors. In the hilus where adrenal steroid Type I receptors have been reported to suppress NPY mRNA levels, the effect of stress is in the opposite direction to that of adrenal steroid action and a more complex regulation of NPY expression is indicated.

Long-term adrenalectomy can decrease or increase hippocampal dentate gyrus volumes.

Male and female Long-Evans adult rats were adrenalectomized and sacrificed 6 weeks later to determine whether dentate gyrus damage would differ in females and males. A subset of adrenalectomized rats of both sexes had significantly reduced dentate gyrus volumes compared to the same sex SHAM operated rats. The remainder of the male and female adrenalectomized rats which did not have clear dentate gyrus damage had significantly larger dentate gyrus volumes compared to the same sex SHAM rats. The dentate gyrus volumes of all adrenalectomized rats were significantly correlated with two indices of residual hormonal levels (Na+/K+ ratios and body weight gain 6 weeks after surgery), indicating that endogenous corticosterone levels may be a determining factor in the response of the dentate gyrus to adrenalectomy. These dentate gyrus volumetric changes could not be attributed to tissue shrinkage as there were no changes in CA3 volumes in any of the groups. These results suggest that long-term adrenalectomy can result in either increased or decreased dentate gyrus volumes and that the adrenal steroid levels of each individual adrenalectomized rat may be the factor determining the direction of the dentate gyrus volumetric response.

Chronic stress impairs rat spatial memory on the Y maze, and this effect is blocked by tianeptine pretreatment.

Chronic restraint stress causes significant dendritic atrophy of CA3 pyramidal neurons that reverts to baseline within a week. Therefore, the authors assessed the functional consequences of this atrophy quickly (within hours) using the Y maze. Experiments 1-3 demonstrated that rats relied on extrinsic, spatial cues located outside of the Y maze to determine arm location and that rats with hippocampal damage (through kainic acid, colchicine, or trimethyltin) had spatial memory impairments. After the Y maze was validated as a hippocampally relevant spatial task, Experiment 4 showed that chronic restraint stress impaired spatial memory performance on the Y maze when rats were tested the day after the last stress session and that tianeptine prevented the stress-induced spatial memory impairment. These data are consistent with the previously demonstrated ability of tianeptine to prevent chronic stress-induced atrophy of the CA3 dendrites.

Repeated restraint stress facilitates fear conditioning independently of causing hippocampal CA3 dendritic atrophy.

This study investigated whether 21 days of restraint stress (6 hr/day) and the subsequent hippocampal dendritic atrophy would affect fear conditioning, a memory task with hippocampal-dependent and hippocampal-independent components. Restraint-stressed rats were injected daily (21 days) with tianeptine (10 mg/kg; to prevent hippocampal atrophy) or vehicle then tested on fear conditioning (Days 23-25, with 2 tone-shock pairings) and open field (Day 25). Restraint stress enhanced freezing to context (hippocampal-dependent behavior) and tone (hippocampal-independent) and decreased open-field exploration, irrespective of whether tianeptine was given. Results confirmed that stress produced CA3 dendritic atrophy and tianeptine prevented it. Moreover, CA3 dendritic atrophy was not permanent but reversed to control levels by 10 days after the cessation of restraint stress. These data argue that different neural substrates underlie spatial recognition memory and fear conditioning.

Calcitonin gene-related peptide level in the rat dentate gyrus increases after damage.

Calcitonin gene related peptide-like immunoreactivity (CGRP-LI) was examined in rat dentate gyrus (DG) following damage to granule cells by adrenalectomy or intrahippocampal colchicine injections. In normal DG, CGRP-LI was present in a diffuse hand within the inner third of the molecular layer (ITML) and in hilar cells. Following the experimental procedures, levels of CGRP-LI increased bilaterally in the ITML and in hilar interneurons resembling mossy cells. Ultrastructural analysis of the ITML revealed that CGRP-LI is associated with large, dense-core vesicles within axon terminals which form asymmetrical synapses with dendritic spines, and within dendritic spines. The increase in CGRP-LI level following granule cell damage suggests a protective role for CGRP in the response to hippocampal injury.

The effects of type I and type II corticosteroid receptor agonists on exploratory behavior and spatial memory in the Y-maze.

We investigated the effects of two adrenal steroid agonists on adrenalectomized (ADX) rats' performance on the Y-maze. The Y-maze was chosen because memory can be assessed quickly and because it is sensitive to various parameters of exploratory behavior and spatial memory performance. Four days after surgery, ADX rats were injected with aldosterone (ALDO, a selective Type I receptor agonist), RU362 (a selective Type II receptor agonist) or sesame vehicle at three different time points (120 min prior to Trial 1, immediately after Trial 1 or 120 min after Trial 1). SHAM-operated rats injected with vehicle were also tested. The results indicate that vehicle-treated ADX rats were impaired on spatial recognition memory compared to SHAM rats. Treatment with ALDO restored spatial recognition memory performance of ADX rats to a level comparable to SHAM-treated rats by acting on acquisition and consolidation, whereas treatment with RU362 did not change the poor spatial recognition memory performance of ADX rats. Discrimination memory was improved only when either agonist was injected prior to the first trial, strongly suggesting a non-selective effect of corticosteroids on discrimination memory such as increasing arousal. A detailed analysis of exploratory behavior showed that both the ALDO- and RU362-treated rats explored the Y-maze more than the ADX and SHAM groups at all periods of the experiment. These results show that the non-specific increase in exploratory behavior induced by replacing corticosteroids targeted at Type I and Type II receptors was used differentially with the ALDO-treated rats learning and consolidating spatial information better than the RU362-treated rats. These data are discussed along with other evidence to suggest that Type II receptors may require the simultaneous occupancy of Type I receptors to affect learning and memory processes.

Prevention of stress-induced morphological and cognitive consequences.

Atrophy and dysfunction of the human hippocampus is a feature of aging in some individuals, and this dysfunction predicts later dementia. There is reason to believe that adrenal glucocorticoids may contribute to these changes, since the elevations of glucocorticoids in Cushing's syndrome and during normal aging are associated with atrophy of the entire hippocampal formation in humans and are linked to deficits in short-term verbal memory. We have developed a model of stress-induced atrophy of the hippocampus of rats at the cellular level, and we have been investigating underlying mechanisms in search of agents that will block the atrophy. Repeated restraint stress in rats for 3 weeks causes changes in the hippocampal formation that include suppression of 5-HT1A receptor binding and atrophy of dendrites of CA3 pyramidal neurons, as well as impairment of initial learning of a radial arm maze task. Because serotonin is released by stressors and may play a role in the actions of stress on nerve cells, we investigated the actions of agents that facilitate or inhibit serotonin reuptake. Tianeptine is known to enhance serotonin uptake, and we compared it with fluoxetine, an inhibitor of 5-HT reuptake, as well as with desipramine. Tianeptine treatment (10 mg/kg/day) prevented the stress-induced atrophy of dendrites of CA3 pycamidal neurons, whereas neither fluoxetine (10 mg/kg/day) nor desipramine (10 mg/kg/day) had any effect. Tianeptine treatment also prevented the stress-induced impairment of radial maze learning. Because corticosterone- and stress-induced atrophy of CA3 dendrites is also blocked by phenytoin, an inhibitor of excitatory amino acid release and actions, these results suggest that serotonin released by stress or corticosterone may interact pre- or post-synaptically with glutamate released by stress or corticosterone, and that the final common path may involve interactive effects between serotonin and glutamate receptors on the dendrites of CA3 neurons innervated by mossy fibers from the dentate gyrus. We discuss the implications of these findings for treating cognitive impairments and the risk for dementia in the elderly.

Chronic stress may facilitate the recruitment of habit- and addiction-related neurocircuitries through neuronal restructuring of the striatum.

Chronic stress is an established risk factor in the development of addiction. Addiction is characterized by a progressive transition from casual drug use to habitual and compulsive drug use. The ability of chronic stress to facilitate the transition to addiction may be mediated by increased engagement of the neurocircuitries underlying habitual behavior and addiction. In the present study, striatal morphology was evaluated after 2 weeks of chronic variable stress in male Sprague-Dawley rats. Dendritic complexity of medium spiny neurons was visualized and quantified with Golgi staining in the dorsolateral and dorsomedial striatum, as well as in the nucleus accumbens core and shell. In separate cohorts, the effects of chronic stress on habitual behavior and the acute locomotor response to methamphetamine were also assessed. Chronic stress resulted in increased dendritic complexity in the dorsolateral striatum and nucleus accumbens core, regions implicated in habitual behavior and addiction, while decreased complexity was found in the nucleus accumbens shell, a region critical for the initial rewarding effects of drugs of abuse. Chronic stress did not affect dendritic complexity in the dorsomedial striatum. A parallel shift toward habitual learning strategies following chronic stress was also identified. There was an initial reduction in acute locomotor response to methamphetamine, but no lasting effect as a result of chronic stress exposure. These findings suggest that chronic stress may facilitate the recruitment of habit- and addiction-related neurocircuitries through neuronal restructuring in the striatum.

Cholesterol and perhaps estradiol protect against corticosterone-induced hippocampal CA3 dendritic retraction in gonadectomized female and male rats.

Chronic stress or glucocorticoid exposure simplifies hippocampal Cornu Ammonis region 3 (CA3) apical dendritic arbors in male rats. In contrast to males, chronic stress either reduces CA3 basal branching or exerts no observable morphological effects in gonadally intact female rats. Under conditions that females display stress-induced CA3 dendritic retraction, such as that following ovariectomy, chronic exposure to 17ß-estradiol or cholesterol can negate these changes. Whether glucocorticoids produce CA3 dendritic retraction in ovariectomized females and whether neuroprotection from 17ß-estradiol or cholesterol is sex-specific remains unknown. The current study examined the effects of chronic glucocorticoid exposure, in conjunction with 17ß-estradiol or cholesterol administration, on hippocampal CA3 dendritic complexity. Adult male and female Sprague-Dawley rats were gonadectomized and implanted with 25% 17ß-estradiol in cholesterol, 100% cholesterol, or blank Silastic capsules. Rats were then assigned to either a 21-day corticosterone (CORT) drink (400µg/ml CORT, 2.4% ethanol in tap water) or tap water (Tap, 2.4% ethanol in tap water) treatment. Brains were processed for Golgi staining, and hippocampal CA3 dendritic architecture was quantified. Results showed 21-day CORT administration reduced hippocampal CA3 apical dendritic branch points, CA3 apical dendritic length, body weight gain, and adrenal weights compared to male and female control counterparts. Furthermore, male and female rats implanted with Silastic capsules containing cholesterol or 25% 17ß-estradiol in cholesterol were protected from CORT-induced CA3 apical dendritic branch reduction. No effects were observed in the CA3 basal dendritic arbors. The present results demonstrate that CORT produces hippocampal CA3 dendritic retraction in gonadectomized male and female rats and that cholesterol and 25% 17ß-estradiol in cholesterol prevent this dendritic simplification.

Symptom exacerbation in psychotically depressed adolescents due to high desipramine plasma concentrations.

The pharmacological treatment of two female adolescent patients meeting DSM-III criteria for psychotic depression is described. A combined antipsychotic and tricyclic antidepressant regimen led to clinical remission. However, a recrudescence in both psychotic and depressive symptoms developed as plasma desipramine levels rose 4 times higher than anticipated from the oral doses prescribed. Clinical improvement occurred in both cases when plasma desipramine levels were reestablished below 200 ng/ml. Thus, we recommend prospective monitoring of desipramine plasma levels, especially when an antipsychotic agent that inhibits the metabolism of the tricyclic antidepressant is also used. We further suggest that deterioration with the reemergence of the presenting clinical syndrome, without signs of delirium, represents a distinct manifestation of antidepressant toxicity. Finally, these cases support the existence of a therapeutic upper limit for desipramine plasma concentrations, above which clinical deterioration occurs.

Selective loss of hippocampal granule cells following adrenalectomy: implications for spatial memory.

We examined the effects of long-term adrenalectomy (ADX) on hippocampal anatomy and behavioral learning in two spatial memory tasks. We assessed damage throughout the hippocampus by stereological analysis of the dentate gyrus and Ammon's horn. Rats were ADX or sham operated, and then tested in the Morris water maze 12 weeks after surgery, followed by testing on an eight-arm, alternating-baited radial maze at 22 weeks postsurgery. Animals were killed 7 1/2 months after surgery. ADX rats had selective volume reduction in the dentate gyrus with no changes in pyramidal regions CA1, CA2, CA3, or CA4. Dentate gyrus damage in some cases occurred throughout the entire rostrocaudal extent of the hippocampus. Analysis of corticosterone serum levels, serum Na+/K+ ratios, and body weight gain suggested that individual differences in dentate gyrus damage appear to be due to incomplete adrenalectomies or remaining ectopic tissue. ADX rats were able to learn in both the Morris water maze and eight-arm radial maze, even when the dentate gyrus was severely damaged (80% volume reduction). However, in the Morris water maze, the ADX rats' learning rate was significantly slower compared to controls. There was no difference between ADX and controls during reversal in either task. These data indicate that damage to the dentate gyrus following long-term ADX is severe enough to cause learning impairment in selected learning tasks. Such damage is restricted to the dentate gyrus and can occur throughout the rostrocaudal regions of the hippocampus.

Dentate gyrus destruction and spatial learning impairment after corticosteroid removal in young and middle-aged rats.

We investigated the functional and behavioral implications of chronic corticosteroid removal in young and middle-aged rats. Prepubertal and 13-month-old rats were adrenalectomized (ADX) or sham operated (SHAM). The young ADX rats were divided further into three groups: ADX with no hormone replacement, ADX given corticosterone chronically, (chCORT), and ADX given corticosterone acutely at the time of Morris water maze testing (acCORT). All rats were run on the Morris water maze 12 weeks after surgery. They were then sacrificed and the brains were removed for histological analysis. The results showed that prolonged corticosteroid absence caused major damage to the dentate gyrus and learning impairment on the Morris water maze. The chCORT rats had little dentate gyrus cell loss and were as efficient as the controls in Morris water maze performance, whereas the acCORT rats had dentate gyrus cell loss and were impaired in the spatial acquisition task. Furthermore, exogenously administered corticosterone had an interactive effect on ADX rats. Water maze performance was improved in dentate gyrus damaged rats (acCORT) compared to ADX rats not given corticosterone, whereas ADX rats with very little dentate gyrus damage (chCORT) did not exhibit better water maze performance relative to controls. Middle-aged ADX rats lost cells only in the dorsal blade of the dentate gyrus but they did not show a learning impairment in the Morris water maze relative to the middle-aged controls. These results indicate that corticosteroids are trophic for the dentate gyrus, that mature granule cells are less affected by adrenalectomy, that corticosteroid absence is responsible for some water maze impairment in ADX rats, but that in addition to corticosteroid absence, a substantial amount of dentate gyrus damage is necessary to impair spatial learning.

Direct visual input to the limbic system: crossed retinal projections to the nucleus anterodorsalis thalami in the tree shrew.

Evidence for a direct projection from the retina to the nucleus anterodorsalis thalami in the insectivore, Tupaia glis, is presented. 100 muCi tritiated amino acid or amino acid/monosaccharide cocktail were administered as two separate intravitreal injections of 50 muCi each. Fibers were traced using thawmount autoradiography in which tissue is frozen in liquid propane, cryostat-cut, and mounted on photographic emulsion precoated slides (Conrad and Stumpf, 1974). From the dorsolateral geniculate body, contralateral retinal fibers continue as a dorsomedial extension of the optic tract. When traced rostrally from this point, the fibers form a thin fascicle coursing medially under the third ventricle. In the anterior thalamus the bundle arborizes within the n. anterodorsalis, infiltrating its caudal pole completely but only encapsulating the nucleus anteriorly. This new retino-anterodorsal thalamic projection, together with the known anterior thalamic-retrosplenial projection, represents a third visual pathway in Tupaia glis distinct from the retino-geniculo-striatal and retino-tecto-pulvinar-peristriatal systems. It is significant that each of these thalamic relay nuclei projects to an architectonically different cortical region, with the nucleus anterodorsalis sending afferents to the phylogenetically most primitive of the visual cortices, the retrosplenial proisocortex or prostriata (Vitzthum and Sanides, 1967). It is proposed that the retino-anterior thalamic-retrosplenial circuit forms an anatomical substrate by which light cues may affect emotional behavior and corresponding neurovisceral responses.

Support for a bimodal role for type II adrenal steroid receptors in spatial memory.

We investigated the effects of acute adrenal steroid treatment on spatial memory using the Y-maze and employing adrenal steroid receptor antagonists and agonists. For receptor activation, adrenalectomized rats were injected 2 h prior to their first Y-maze trial with sesame oil (adrenalectomy or SHAM), stress levels of corticosterone, a Type I receptor agonist (aldosterone), or a Type II receptor agonist (RU362). For receptor inactivation, unoperated rats were injected with a Type I receptor antagonist (RU318), a Type II receptor antagonist (RU555), sesame oil, or not injected at all. The findings indicated that spatial memory was impaired when the Type II receptors were blocked (RU555) or highly occupied (corticosterone or RU362) and normal for the other treatment conditions. These data suggest that the Type II receptors may be responsible for the inverted U-shaped relationship between spatial memory and corticosterone levels reported by others.

Metyrapone reveals that previous chronic stress differentially impairs hippocampal-dependent memory.

Chronic stress facilitates fear conditioning in rats with hippocampal neuronal atrophy and in rats in which the atrophy is prevented with tianeptine, a serotonin re-uptake enhancer. The purpose of this study was to determine whether the lack of dissociation between fear conditioning performance and hippocampal integrity was masked by the presence of endogenous corticosteroids during training. As in previous studies, rats were stressed by daily restraint (6 h/day for 21 days), trained in the conditioning chamber (day 23), and then assessed for conditioned fear (day 25) at a time when hippocampal dendritic atrophy persists. On the training day, half of the control and stressed rats were. injected with metyrapone to reduce corticosterone release. Two hours later, two paired or unpaired presentations of tone and footshock were delivered. Although metyrapone reduced conditioned fear in all rats, only stressed rats showed dissociated fear conditioning (i.e. tone conditioning was reduced while contextual conditioning was eliminated). Chronically stressed rats, regardless of metyrapone treatment displayed more rearing in the open field when tested immediately after the completion of fear conditioning. These data support the hypothesis that increased emotionality and enhanced fear conditioning exhibited by chronically stressed rats maybe due to endogenous corticosterone secretion at the time of fear conditioned training. Moreover,these data suggest that chronic stress impairs hippocampal-dependent processes more robustly than hippocampal-independent processes after metyrapone to reduce corticosterone secretion during aversive training.