Activity-stress induces atrophy of apical dendrites of hippocampal pyramidal neurons in male rats

Chronic stress- and sex-specific neuromorphological and functional changes in limbic structures

Chronic stress produces sex-specific neuromorphological changes in a variety of brain regions, which likely contribute to the gender differences observed in stress-related illnesses and cognitive ability. Here, we review the literature investigating the relationship between chronic stress and sex differences on brain plasticity and function, with an emphasis on morphological changes in dendritic arborization and spines in the hippocampus, prefrontal cortex, and amygdala. These brain structures are highly interconnected and sensitive to stress and gonadal hormones, and influence a variety of cognitive abilities. Although much less work has been published using female subjects than with male subjects, the findings suggest that the relationship between brain morphology and function is very different between the sexes. After reviewing the literature, we present a model showing how chronic stress influences the morphology of these brain regions and changes the dynamic of how these limbic structures interact with each other to produce altered behavioral outcomes in spatial ability, behavioral flexibility/executive function, and emotional arousal.

Forced and voluntary exercise differentially affect brain and behavior

The potential of physical exercise to decrease body weight, alleviate depression, combat aging and enhance cognition has been well-supported by research studies. However, exercise regimens vary widely across experiments, raising the question of whether there is an optimal form, intensity and duration of exertion that would produce maximal benefits. In particular, a comparison of forced and voluntary exercise is needed, since the results of several prior studies suggest that they may differentially affect brain and behavior. In the present study, we employed a novel 8-week exercise paradigm that standardized the distance, pattern, equipment and housing condition of forced and voluntary exercisers. Exercising rats were then compared with sedentary controls on measures previously shown to be influenced by physical activity. Our results indicate that although the distance covered by both exercise groups was the same, voluntary exercisers ran at higher speed and for less total time than forced exercisers. When compared with sedentary controls, forced but not voluntary exercise was found to increase anxiety-like behaviors in the open field. Both forms of exercise increased the number of surviving bromodeoxyuridine (BrdU)+ cells in the dentate gyrus after 8 weeks of exercise, although forced exercisers had significantly more than voluntary exercisers. Phenotypic analysis of BrdU+ cells showed no difference between groups in the percentage of newborn cells that became neurons, however, because forced exercise maximally increased the number of BrdU+ cells, it ultimately produced more neurons than voluntary exercise. Our results indicate that forced and voluntary exercise are inherently different: voluntary wheel running is characterized by rapid pace and short duration, whereas forced exercise involves a slower, more consistent pace for longer periods of time. This basic difference between the two forms of exercise is likely responsible for their differential effects on brain and behavior.

Physiology and neurobiology of stress and adaptation: central role of the brain

The brain is the key organ of the response to stress because it determines what is threatening and, therefore, potentially stressful, as well as the physiological and behavioral responses which can be either adaptive or damaging. Stress involves two-way communication between the brain and the cardiovascular, immune, and other systems via neural and endocrine mechanisms. Beyond the "flight-or-fight" response to acute stress, there are events in daily life that produce a type of chronic stress and lead over time to wear and tear on the body ("allostatic load"). Yet, hormones associated with stress protect the body in the short-run and promote adaptation ("allostasis"). The brain is a target of stress, and the hippocampus was the first brain region, besides the hypothalamus, to be recognized as a target of glucocorticoids. Stress and stress hormones produce both adaptive and maladaptive effects on this brain region throughout the life course. Early life events influence life-long patterns of emotionality and stress responsiveness and alter the rate of brain and body aging. The hippocampus, amygdala, and prefrontal cortex undergo stress-induced structural remodeling, which alters behavioral and physiological responses. As an adjunct to pharmaceutical therapy, social and behavioral interventions such as regular physical activity and social support reduce the chronic stress burden and benefit brain and body health and resilience.

Chronic stress: implications for neuronal morphology, function and neurogenesis

In normal life, organisms are repeatedly exposed to brief periods of stress, most of which can be controlled and adequately dealt with. The presently available data indicate that such brief periods of stress have little influence on the shape of neurons or adult neurogenesis, yet change the physiological function of cells in two time-domains. Shortly after stress excitability in limbic areas is rapidly enhanced, but also in brainstem neurons which produce catecholamines; collectively, during this phase the stress hormones promote focused attention, alertness, vigilance and the initial steps in encoding of information linked to the event. Later on, when the hormone concentrations are back to their pre-stress level, gene-mediated actions by corticosteroids reverse and normalize the enhanced excitability, an adaptive response meant to curtail defense reactions against stressors and to enable further storage of relevant information. When stress is experienced repetitively in an uncontrollable and unpredictable manner, a cascade of processes in brain is started which eventually leads to profound, region-specific alterations in dendrite and spine morphology, to suppression of adult neurogenesis and to inappropriate functional responses to a brief stress exposure including a sensitized activation phase and inadequate normalization of brain activity. Although various compounds can effectively prevent these cellular changes by chronic stress, the exact mechanism by which the effects are accomplished is poorly understood. One of the challenges for future research is to link the cellular changes seen in animal models for chronic stress to behavioral effects and to understand the risks they can impose on humans for the precipitation of stress-related disorders.

The effects of chronic stress on hippocampal morphology and function: an evaluation of chronic restraint paradigms

Chronic restraint stress for 6 h/21 days causes hippocampal CA3 apical dendritic retraction, which parallels spatial memory impairments in male rats. Recent research suggests that chronic immobilization stress for 2 h/10 days induces CA3 dendritic retraction [Vyas, A., Mitra, R., Shankaranarayana Rao, B.S., Chattarji, S., 2002. Chronic stress induces contrasting patterns of dendritic remodeling in hippocampal and amygdaloid neurons. J. Neurosci. 22, 6810-6818.] and questions whether CA3 dendritic retraction and spatial memory deficits can be produced sooner than found following 6 h/21 days of restraint stress. Therefore, this study investigated the effects of four different durations of chronic restraint stress (varied by hours/day and total number of days) and the subsequent effects on hippocampal CA3 morphology and spatial memory in the same male Sprague-Dawley rats. The results showed that only rats exposed to the 6 h/21 days restraint paradigm exhibited CA3 apical dendritic retraction, consistent spatial memory deficits, and decreased body weight gain compared to experimental counterparts and controls. While chronically stressing a rat with wire mesh restraint has a physical component, it acts primarily as a psychological stressor, and these findings support the interpretation that chronic psychological stress produces hippocampal-dependent cognitive deficits that are consistent with hippocampal structural changes. Differences in stress effects observed across different studies may be due to rat strain, type of stressor, and housing conditions; however, the current findings support the use of chronic restraint stress, with wire mesh, for 6 h/21 days as a reliable and efficient method to produce psychological stress and to cause CA3 dendritic retraction and spatial memory deficits in male Sprague-Dawley rats.

Explorations of Coping Strategies, Learned Persistence, and Resilience in Long-Evans Rats: Innate versus Acquired Characteristics

In the current investigation, predispositions for coping styles (i.e., passive, flexible, and active) were determined in juvenile male rats. In subsequent behavioral tests, flexible copers exhibited more active responses. In another study, animals were exposed to chronic stress and flexible coping rats had lower levels of corticosteroids. Focusing on the acquired nature of coping strategies, rats receiving extensive training in a task requiring them to dig for food rewards (i.e., effort-based rewards) persisted longer in a challenging task than control animals. Thus, the results suggest that both predisposed coping strategies and acquired behavioral experience contribute to resilience in challenging situations.

Chronic stress impairs spatial memory and motivation for reward without disrupting motor ability and motivation to explore

This study uses an operant, behavioral model to assess the daily changes in the decay rate of short-term memory, motivation, and motor ability in rats exposed to chronic restraint. Restraint decreased reward-related motivation by 50% without altering memory decay rate or motor ability. Moreover, chronic restraint impaired hippocampal-dependent spatial memory on the Y maze (4-hr delay) and produced CA3 dendritic retraction without altering hippocampal-independent maze navigation (1-min delay) or locomotion. Thus, mechanisms underlying motivation for food reward differ from those underlying Y maze exploration, and neurobiological substrates of spatial memory, such as the hippocampus, differ from those that underlie short-term memory. Chronic restraint produces functional, neuromorphological, and physiological alterations that parallel symptoms of depression in humans.

What is the functional significance of chronic stress-induced CA3 dendritic retraction within the hippocampus?

Chronic stress produces consistent and reversible changes within the dendritic arbors of CA3 hippocampal neurons, characterized by decreased dendritic length and reduced branch number. This chronic stress-induced dendritic retraction has traditionally corresponded to hippocampus-dependent spatial memory deficits. However, anomalous findings have raised doubts as to whether a CA3 dendritic retraction is sufficient to compromise hippocampal function. The purpose of this review is to outline the mechanism underlying chronic stress-induced CA3 dendritic retraction and to explain why CA3 dendritic retraction has been thought to mediate spatial memory. The anomalous findings provide support for a modified hypothesis, in which chronic stress is proposed to induce CA3 dendritic retraction, which then disrupts hypothalamic-pituitary-adrenal axis activity, leading to dysregulated glucocorticoid release. The combination of hippocampal CA3 dendritic retraction and elevated glucocorticoid release contributes to impaired spatial memory. These findings are presented in the context of clinical conditions associated with elevated glucocorticoids.

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.

Pup exposure differentially enhances foraging ability in primiparous and nulliparous rats

The role of maternal experience (i.e., pregnancy and pup exposure) on rats' performance in a foraging task was assessed. Primiparous (P) and nulliparous (N) animals were either exposed to pups for 21 days (+) or received no pup exposure (-). Following habituation trials, all animals were tested in spatial and cued versions of the dry land maze (DLM) for three days (three trials per day). In the spatial DLM, the presence of pups decreased latencies in both N and P groups in Trial 5 and P+ rats exhibited shorter latencies to baited food wells than P- animals on Trial 6. In the subsequent probe trial, P+ animals spent significantly more time in proximity to the previously baited well than P- rats. Pups enhanced performance of both P+ and N+ groups in trial 6 of the cued test. Thus, in the spatial task, the individual components of the maternal experience (e.g., pregnancy, parturition, lactation, and pup exposure) converge to produce behavioral modifications in the DLM spatial and probe tasks that enable the female to care for her offspring, in this case, by enhancing foraging behavior. Further, in one trial of each version of the task, pup exposure enhanced performance in N animals suggesting that, in isolation, pup exposure may be a more important influence on ancillary maternal behavior than the pregnancy itself.

The effects of Mexidol on the acquisition of food-related conditioned reflexes and synaptic ultrastructure in field CA1 of the rat hippocampus after single acoustic stimuli with ultrasonic components

The effects of a complex acoustic signal with ultrasonic components on the ultrastructure of synapses field CA1 of the rat hippocampus were studied in conditions of two-week courses of the wide-spectrum antioxidant Mexidol (compared with an untreated group); the effects of complex acoustic signals on the dynamics of acquisition of a food-related conditioned reflex using a standard stimulus (a tone) and on the acquisition of a trace conditioned reflex to estimating time intervals were also studied, in the same groups of rats. Controls consisted of unstressed rats treated and not treated with Mexidol. Ultrastructural analysis of the redistribution of vesicles in the synaptic terminals of hippocampal field CA1 showed that synaptic transmission was impaired when assessed one day after exposure to the complex acoustic signal. Mexidol prevented impairment of synaptic transmission. The complex acoustic signal had negative effects on conditioned reflex activity in rats and Mexidol had normalizing actions on the acquisition of conditioned reflexes in stressed rats. These results lead to the conclusion that the antioxidant Mexidol can be applied to the prophylaxis of the impairments in CNS cognitive functions frequently seen in stress.

BDNF protects against stress-induced impairments in spatial learning and memory and LTP

The present study investigated whether infusion of brain-derived neurotrophic factor (BDNF) could ameliorate stress-induced impairments in spatial learning and memory as well as hippocampal long-term potentiation (LTP) of rats. Chronic immobilization stress (2 h/day x 7 days) significantly impaired spatial performance in the Morris water maze, elevated plasma corticosterone, and attenuated LTP in hippocampal slices from these animals as compared with normal control subjects. BDNF was infused into the left hippocampus (0.5 mul/h) for 14 days, beginning 7 days before the stress exposure. The BDNF group was protected from the deleterious effects of stress and performed at a level indistinguishable from normal control animals despite the presence of elevated corticosterone. BDNF alone and sham infusions had no effect on performance or LTP. These results demonstrate that spatial learning and memory, and LTP, a candidate neural substrate of learning and memory, are compromised during chronic stress, and may be protected by BDNF administration.

Maternal Experience Produces Long-Lasting Behavioral Modifications in the Rat

From 5 to 22 months of age, cognitive and emotional responses of nulliparous, primiparous, and multiparous rats were assessed using a dry land maze (DLM) and an elevated plus-maze (EPM) at 4-month intervals. Parous rats exhibited improved spatial memory in the probe and competitive versions of the DLM, and more exploration in the EPM and a novel stimulus test relative to nulliparous females. The nulliparous females, however, outperformed parous rats during the DLM visual cue test at 17 months of age. At 23 months, no differences in stressed corticosterone levels or Golgi-stained hippocampal neurons were observed. Thus, cognitive and emotional modifications were observed in parous rats; the neurobiological mechanisms for these enduring effects, however, remain to be identified.

Immunocytochemical analysis of synaptic proteins provides new insights into diabetes-mediated plasticity in the rat hippocampus

The hippocampus, an important integration center for learning and memory in the mammalian brain, undergoes neurological changes in response to a variety of stimuli that are suggestive of ongoing synaptic reorganization. Accordingly, the aim of this study was to identify markers of synaptic plasticity using rapid and reliable techniques such as radioimmunocytochemistry and confocal microscopy, thereby providing a "birds-eye view" of the whole hippocampus under hypercorticosteronemic conditions. The regulation of microtubule-associated protein 2, synaptophysin and postsynaptic density-95 was examined in two different animal models of hypercorticosteronemia: corticosterone administration and streptozotocin-induced diabetes using both a short-term (1 week) and long-term (5 weeks) treatment. Glucocorticoids and/or hyperglycemia increased synaptophysin expression in CA1, CA3 and the dentate gyrus, regions that exhibit synaptic plasticity in response to glucocorticoid exposure. In these models, postsynaptic density-95 expression increased in the CA3 region, particularly in the diabetic rats, while microtubule-associated protein 2 exhibited more selective changes. Fluoro-Jade histochemistry did not detect neuronal damage, suggesting that glucocorticoids and/or hyperglycemia induce plastic and not irreversible neuronal changes at these time points. Collectively, these results demonstrate that changes in the expression and distribution of synaptic proteins provide another measure of synaptic plasticity in the rat hippocampus in response to glucocorticoid exposure, changes that may accompany or contribute to neuroanatomical, neurochemical, and behavioral changes observed in experimental models of type 1 diabetes.

Neuronal cytoskeletal alterations in an experimental model of depression

It has been proposed that depression is associated with hippocampal morphological changes. The apical dendrite atrophy of hippocampal CA3 pyramidal neurons has been described in experimental models of depression. The aim of the present study was to determine which cytoskeletal components are involved in the morphological changes previously described in the hippocampus of depressed animals. The expression of different neuronal cytoskeletal markers was analyzed by immunohistochemistry in rats exposed to a learned helplessness paradigm, an experimental model of depression. Rats were trained with 60 inescapable foot shocks (0.6 mA/15 s) and escape latencies and failures were tested 4 days after training. Animals in which learned helplessness behavior persisted for 21 days were included in the depressed group. No foot shocks were delivered to control rats. Microtubule-associated protein 2 (MAP-2) and light (NFL; 68 kDa), medium (NFM; 160 kDa) and heavy (NFH; 200 kDa) neurofilament subunit immunostainings were analyzed employing morphometric parameters. In the depressed group, NFL immunostaining decreased 55% (P<0.05) and 60% (P<0.001) in CA3 and dentate gyrus, respectively. In the same areas, MAP-2, NFM and NFH immunostainings did not differ between depressed and control animals. Since NFL is present in the core of mature neurofilament, it is proposed that hippocampal depression-associated plastic alterations may be due to changes in the dynamics of the neurofilament assembly.

Retrieving of pups by neonatally stressed mothers

In the rat, perinatal food and maternal deprivation provoke long-lasting effects upon the retrieving responses of dams to displaced pups. In the current study, the retrieving latency and the disruption in the body area of pups chosen by the mother to transport them to a new location was investigated on days 4, 8 and 12 postpartum in lactating Wistar rats. Rats, neonatally underfed by daily (12 h) mother-litter separation in an incubator from days 1 to 23 postpartum, exhibited prolonged retrieving latencies and disruption in the body area of young ones chosen by the dam to transport them to the nest. Furthermore, neonatally underfed dams frequently transported pups in a rude manner eliciting sonic distress cries from them compared to control mothers. These findings are possibly relevant to understand the impact of epigenetic influences on offspring brain and physiological maturation partly mediated through maternal care.

Activity-stress increases density of GFAP-immunoreactive astrocytes in the rat hippocampus

Although past research has indicated that stress and the accompanying increase in glucocorticoids compromises hippocampal neurons, little is known about the effect of stress on hippocampal glial cells. In the current study, male rats were exposed to activity-stress (A-S) for six days; this comprised housing with an activity wheel and restricted access (1h/day) to food. Physiological data (e.g., relative adrenal and thymus weights, gastric ulceration) suggested that the A-S rats experienced more stress than pair-fed (no wheel) and control (fed ad libitum, no wheel) rats. Whereas stress did not influence the quantitative morphology of glial fibrillary acidic protein (GFAP)-immunoreactive cells, a semi-quantitative analysis revealed that the A-S rats had significantly more (30%) GFAP-immunoreactive cells in the hippocampal CA3 region than the control rats. Based on the present findings, it appears that the hippocampal astrocytic response to chronic stress may be similar to the response found in endangered, or challenged hippocampal environments, such as in ischemia.