Unpredictable and uncontrollable stress impairs neuronal plasticity in the rat hippocampus

Increased forebrain EAAT3 expression confers resilience to chronic stress

Depression is a disabling and highly prevalent psychiatric illness. Multiple studies have linked glutamatergic dysfunction with the pathophysiology of depression, but the exact alterations in the glutamatergic system that contribute to depressive-like behaviors are not fully understood. Recent evidence suggests that a decreased level in neuronal glutamate transporter (EAAT3), known to control glutamate levels and limit the activation of glutamate receptors at synaptic sites, may contribute to the manifestation of a depressive phenotype. Here, we tested the possibility that increased EAAT3 expression at excitatory synapses could reduce the susceptibility of mice to develop depressive-like behaviors when challenged to a 5-week unpredictable chronic mild stress (UCMS) protocol. Mice overexpressing EAAT3 in the forebrain (EAAT3glo/CMKII) and control littermates (EAAT3glo) were assessed for depressive-like behaviors and long-term memory performance after being subjected to UCMS conditions. We found that, after UCMS, EAAT3glo/CMKII mice did not exhibit depressive-like behaviors or memory alterations observed in control mice. Moreover, we found that EAAT3glo/CMKII mice did not show alterations in phasic dopamine release in the nucleus accumbens neither in long-term synaptic plasticity in the CA1 region of the hippocampus after UCMS, as observed in control littermates. Altogether these results suggest that forebrain EAAT3 overexpression may be related to a resilient phenotype, both at behavioral and functional level, to the deleterious effect of chronic stress, highlighting the importance of neuronal EAAT3 in the pathophysiology of depressive-like behaviors.

The emerging role of rapid corticosteroid actions on excitatory and inhibitory synaptic signaling in the brain

Over the past two decades, there has been increasing evidence for the importance of rapid-onset actions of corticosteroid hormones in the brain. Here, we highlight the distinct rapid corticosteroid actions that regulate excitatory and inhibitory synaptic transmission in the hypothalamus, the hippocampus, basolateral amygdala, and prefrontal cortex. The receptors that mediate rapid corticosteroid actions are located at or close to the plasma membrane, though many of the receptor characteristics remain unresolved. Rapid-onset corticosteroid effects play a role in fast neuroendocrine feedback as well as in higher brain functions, including increased aggression and anxiety, and impaired memory retrieval. The rapid non-genomic corticosteroid actions precede and complement slow-onset, long-lasting transcriptional actions of the steroids. Both rapid and slow corticosteroid actions appear to be indispensable to adapt to a continuously changing environment, and their imbalance can increase an individual's susceptibility to psychopathology.

Memory Under Stress: From Adaptation to Disorder

Stressful events are ubiquitous in everyday life. Exposure to these stressors initiates the temporally orchestrated release of a multitude of hormones, peptides, and neurotransmitters that target brain areas that have been critically implicated in learning and memory. This review summarizes recent insights on the profound impact of stress on 4 fundamental processes of memory: memory formation, memory contextualization, memory retrieval, and memory flexibility. Stress mediators instigate dynamic alterations in these processes, thereby facilitating efficient responding under stress and the creation of a decontextualized memory representation that can effectively aid coping with novel future threats. While they are generally adaptive, the same stress-related changes may contribute to the rigid behaviors, uncontrollable intrusions, and generalized fear responding seen in anxiety disorders and posttraumatic stress disorder. Drawing on recent discoveries in cognitive neuroscience and psychiatry, this review discusses how stress-induced alterations in memory processes can simultaneously foster adaptation to stressors and fuel psychopathology. The transition from adaptive to maladaptive changes in the impact of stress on memory hinges on the nuanced interplay of stressor characteristics and individual predispositions. Thus, taking individual differences in the cognitive response to stressors into account is essential for any successful treatment of stress-related mental disorders.

Neurocognitive effects of stress: a metaparadigm perspective

Stressful experiences, both physical and psychological, that are overwhelming (i.e., inescapable and unpredictable), can measurably affect subsequent neuronal properties and cognitive functioning of the hippocampus. At the cellular level, stress has been shown to alter hippocampal synaptic plasticity, spike and local field potential activity, dendritic morphology, neurogenesis, and neurodegeneration. At the behavioral level, stress has been found to impair learning and memory for declarative (or explicit) tasks that are based on cognition, such as verbal recall memory in humans and spatial memory in rodents, while facilitating those that are based on emotion, such as differential fear conditioning in humans and contextual fear conditioning in rodents. These vertically related alterations in the hippocampus, procedurally observed after subjects have undergone stress, are generally believed to be mediated by recurrently elevated circulating hypothalamic-pituitary-adrenal (HPA) axis effector hormones, glucocorticoids, directly acting on hippocampal neurons densely populated with corticosteroid receptors. The main purposes of this review are to (i) provide a synopsis of the neurocognitive effects of stress in a historical context that led to the contemporary HPA axis dogma of basic and translational stress research, (ii) critically reappraise the necessity and sufficiency of the glucocorticoid hypothesis of stress, and (iii) suggest an alternative metaparadigm approach to monitor and manipulate the progression of stress effects at the neural coding level. Real-time analyses can reveal neural activity markers of stress in the hippocampus that can be used to extrapolate neurocognitive effects across a range of stress paradigms (i.e., resolve scaling and dichotomous memory effects issues) and understand individual differences, thereby providing a novel neurophysiological scaffold for advancing future stress research.

Non-genomic steroid signaling through the mineralocorticoid receptor: Involvement of a membrane-associated receptor?

Corticosteroid receptors in the mammalian brain mediate genomic as well as non-genomic actions. Although receptors mediating genomic actions were already cloned 35 years ago, it remains unclear whether the same molecules are responsible for the non-genomic actions or that the latter involve a separate class of receptors. Here we focus on one type of corticosteroid receptors, i.e. the mineralocorticoid receptor (MR). We summarize some of the known properties and the current insight in the localization of the MR in peripheral cells and neurons, especially in relation to non-genomic signaling. Previous studies from our own and other labs provided evidence that MRs mediating non-genomic actions are identical to the ones involved in genomic signaling, but may be translocated to the plasma cell membrane instead of the nucleus. With fixed cell imaging and live cell imaging techniques we tried to visualize these presumed membrane-associated MRs, using antibodies or overexpression of MR-GFP in COS7 and hippocampal cultured neurons. Despite the physiological evidence for MR location in or close to the cell membrane, we could not convincingly visualize membrane localization of endogenous MRs or GFP-MR molecules. However, we did find punctae of labeled antibodies intracellularly, which might indicate transactivating spots of MR near the membrane. We also found some evidence for trafficking of MR via beta-arrestins. In beta-arrestin knockout mice, we didn't observe metaplasticity in the basolateral amygdala anymore, indicating that internalization of MRs could play a role during corticosterone activation. Furthermore, we speculate that membrane-associated MRs could act indirectly via activating other membrane located structures like e.g. GPER and/or receptor tyrosine kinases.

Altered hippocampal microstructure and function in children who experienced Hurricane Irma

Hurricane Irma was the most powerful Atlantic hurricane in recorded history, displacing 6 million and killing over 120 people in the state of Florida alone. Unpredictable disasters like Irma are associated with poor cognitive and health outcomes that can disproportionately impact children. This study examined the effects of Hurricane Irma on the hippocampus and memory processes previously related to unpredictable stress. We used an innovative application of an advanced diffusion-weighted imaging technique, restriction spectrum imaging (RSI), to characterize hippocampal microstructure (i.e., cell density) in 9- to 10-year-old children who were exposed to Hurricane Irma relative to a non-exposed control group (i.e., assessed the year before Hurricane Irma). We tested the hypotheses that the experience of Hurricane Irma would be associated with decreases in: (a) hippocampal cellularity (e.g., neurogenesis), based on known associations between unpredictable stress and hippocampal alterations; and (b) hippocampal-related memory function as indexed by delayed recall. We show an association between decreased hippocampal cellularity and delayed recall memory in children who experienced Hurricane Irma relative to those who did not. These findings suggest an important role of RSI for assessing subtle microstructural changes related to functionally significant changes in the developing brain in response to environmental events.

Predator-based psychosocial stress model of PTSD differentially influences voluntary ethanol consumption depending on methodology

Post-traumatic stress disorder (PTSD) is a debilitating psychological disorder typified by diagnostic symptom clusters including hyperarousal, avoidance, negative cognitions and mood, and intrusive re-experiencing of the traumatic event. Patients with PTSD have been reported to self-medicate with alcohol to ameliorate hyperarousal symptoms associated with the disorder. Research utilizing rodent models of PTSD to emulate this behavioral phenomenon has thus far yielded inconsistent results. In the present study, we examined the effects of a predator-based psychosocial stress model of PTSD on voluntary ethanol consumption. In the first of two experiments, following exposure to a 31-day stress or control paradigm, rats were singly housed during the dark cycle with free access to 1% sucrose solution or 10% ethanol, which was also sweetened with 1% sucrose. Over the course of a 20-day period of ethanol access, stressed rats consumed significantly less ethanol than non-stressed rats. These counterintuitive results prompted the completion of a second experiment which was identical to the first, except rats were also exposed to the two-bottle paradigm for 20 days before the stress or control paradigm. In the second experiment, after the stress manipulation, stressed rats exhibited significantly greater ethanol preference than non-stressed rats. These findings suggest that prior exposure to ethanol influences the subsequent effect of stress on ethanol intake. They also validate the use of the present model of PTSD to examine potential mechanisms underlying stress-related changes in ethanol-seeking behavior.

How could stress lead to major depressive disorder?

Stress is associated with major depressive disorder (MDD), but the underlying mechanism remains elusive. However, some experiences, referred to as stress, may actually lead to resilience. It is thus critical first to define what type of stress may lead to MDD. Long-term potentiation (LTP) and long-term depression (LTD) are both sensitive to stress, but particularly to inescapable and not escapable stress. Thus, these are the psychological aspects of stress which contribute to the development of MDD, but by which mechanisms remains still elusive. Interestingly, the same stress may facilitate LTD and impair LTP in the CA1 region. In addition, repeated efforts are often required for learning under neutral conditions but single- or few learning trials are sufficient for forming stress-related memories. If LTP is crucial for normal learning, a combination of limited LTP and facilitated LTD appears to have higher efficiency for storing stress-related memories. Chronic psychological stress may cause a hyper-link among stress-related memories across the spatiotemporal due to shared quality of inescapability, leading to automatically negative appraisal through memory generalization mechanisms in MDD patients when encountering new distinct events which are perceived to share such similarity with previous experiences.

Repeated shock stress facilitates basolateral amygdala synaptic plasticity through decreased cAMP-specific phosphodiesterase type IV (PDE4) expression

Previous studies have shown that exposure to stressful events can enhance fear memory and anxiety-like behavior as well as increase synaptic plasticity in the rat basolateral amygdala (BLA). We have evidence that repeated unpredictable shock stress (USS) elicits a long-lasting increase in anxiety-like behavior in rats, but the cellular mechanisms mediating this response remain unclear. Evidence from recent morphological studies suggests that alterations in the dendritic arbor or spine density of BLA principal neurons may underlie stress-induced anxiety behavior. Recently, we have shown that the induction of long-term potentiation (LTP) in BLA principal neurons is dependent on activation of postsynaptic D1 dopamine receptors and the subsequent activation of the cyclic adenosine 5'-monophosphate (cAMP)-protein kinase A (PKA) signaling cascade. Here, we have used in vitro whole-cell patch-clamp recording from BLA principal neurons to investigate the long-term consequences of USS on their morphological properties and synaptic plasticity. We provided evidence that the enhanced anxiety-like behavior in response to USS was not associated with any significant change in the morphological properties of BLA principal neurons, but was associated with a changed frequency dependence of synaptic plasticity, lowered LTP induction threshold, and reduced expression of phosphodiesterase type 4 enzymes (PDE4s). Furthermore, pharmacological inhibition of PDE4 activity with rolipram mimics the effects of chronic stress on LTP induction threshold and baseline startle. Our results provide the first evidence that stress both enhances anxiety-like behavior and facilitates synaptic plasticity in the amygdala through a common mechanism of PDE4-mediated disinhibition of cAMP-PKA signaling.

Probiotic gut effect prevents the chronic psychological stress-induced brain activity abnormality in mice

BACKGROUND

A probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175 combination, Probio'Stick(®) ) displays anxiolytic-like activity and reduces apoptosis in the lymbic system in animal models of depression. Based on the hypothesis that modulation of gut microbiota by this probiotic formulation has beneficial effects on brain activity in stress conditions, we report a set of probiotic-evoked physiological, cellular, and molecular events in the brain of Probio'Stick(®) pretreated mice submitted to chronic psychological stress.

METHODS

Water avoidance stress (WAS) was applied or not (sham). Hypothalamic-pituitary-adrenal (HPA) axis and autonomic nervous system (ANS) responses to the chronic stress were assessed through plasma corticosterone and catecholamine measurements. Specific markers for neuronal activity, neurogenesis, and synaptic plasticity were used to assess brain activity. In addition, gut permeability and tight junction (TJ) proteins levels were also determinated.

KEY RESULTS

We observed that a pretreatment with the probiotic formulation attenuated HPA axis and ANS activities in response to WAS, and reduced cFos expression in different brain areas but Lactobacillus salivarius (a negative control) treatment was ineffective on these parameters. Moreover, probiotic pretreatment prevented the WAS-induced decrease hippocampal neurogenesis and expression changes in hypothalamic genes involved in synaptic plasticity. These central effects were associated with restoration of TJ barrier integrity in stressed mice.

CONCLUSIONS & INFERENCES

These data suggest that chronic stress-induced abnormal brain plasticity and reduction in neurogenesis can be prevented by a pretreatment with the Probio'Stick(®) formulation, suggesting that probiotics modulate neuroregulatory factors and various signaling pathways in the central nervous system involved in stress response.

Low-frequency stimulation induces long-term depression and slow onset long-term potentiation at perforant path-dentate gyrus synapses in vivo

The expression of homosynaptic long-term depression (LTD) is thought to mediate a crucial role in sustaining memory function. Our in vivo investigations of LTD expression at lateral (LPP) and medial perforant path (MPP) synapses in the dentate gyrus (DG) corroborate prior demonstrations that PP-DG LTD is difficult to induce in intact animals. In freely moving animals, LTD expression occurred inconsistently among LPP-DG and MPP-DG responses. Interestingly, following acute electrode implantation in anesthetized rats, low-frequency stimulation (LFS; 900 pulses, 1 Hz) promotes slow-onset LTP at both MPP-DG and LPP-DG synapses that utilize distinct induction mechanisms. Systemic administration of the N-methyl-d-aspartate (NMDA) receptor antagonist (+/-)-cyclopiperidine-6-piperiperenzine (CPP; 10 mg/kg) 90 min before LFS selectively blocked MPP-DG but not LPP-DG slow onset LTP, suggesting MPP-DG synapses express a NMDA receptor-dependent slow onset LTP whereas LPP-DG slow onset LTP induction is NMDA receptor independent. In experiments where paired-pulse LFS (900 paired pulses, 200-ms paired-pulse interval) was used to induce LTD, paired-pulse LFS of the LPP resulted in rapid onset LTP of DG responses, whereas paired-pulse LFS of the MPP induced slow onset LTP of DG responses. Although LTD observations were very rare following acute electrode implantation in anesthetized rats, LPP-DG LTD was demonstrated in some anesthetized rats with previously implanted electrodes. Together, our data indicate in vivo PP-DG LTD expression is an inconsistent phenomenon that is primarily observed in recovered animals, suggesting perturbation of the dentate through surgery-related tissue trauma influences both LTD incidence and LTP induction at PP-DG synapses in vivo.

Brain Physiology and Pathophysiology in Mental Stress

Exposure to various forms of stress is a common daily occurrence in the lives of most individuals, with both positive and negative effects on brain function. The impact of stress is strongly influenced by the type and duration of the stressor. In its acute form, stress may be a necessary adaptive mechanism for survival and with only transient changes within the brain. However, severe and/or prolonged stress causes overactivation and dysregulation of the hypothalamic pituitary adrenal (HPA) axis thus inflicting detrimental changes in the brain structure and function. Therefore, chronic stress is often considered a negative modulator of the cognitive functions including the learning and memory processes. Exposure to long-lasting stress diminishes health and increases vulnerability to mental disorders. In addition, stress exacerbates functional changes associated with various brain disorders including Alzheimer’s disease and Parkinson’s disease. The primary purpose of this paper is to provide an overview for neuroscientists who are seeking a concise account of the effects of stress on learning and memory and associated signal transduction mechanisms. This review discusses chronic mental stress and its detrimental effects on various aspects of brain functions including learning and memory, synaptic plasticity, and cognition-related signaling enabled via key signal transduction molecules.

D-serine as a gliotransmitter and its roles in brain development and disease

The development of new techniques to study glial cells has revealed that they are active participants in the development of functional neuronal circuits. Calcium imaging studies demonstrate that glial cells actively sense and respond to neuronal activity. Glial cells can produce and release neurotransmitter-like molecules, referred to as gliotransmitters, that can in turn influence the activity of neurons and other glia. One putative gliotransmitter, D-serine is believed to be an endogenous co-agonist for synaptic N-methyl-D-aspartate receptors (NMDARs), modulating synaptic transmission and plasticity mediated by this receptor. The observation that D-serine levels in the mammalian brain increase during early development, suggests a possible role for this gliotransmitter in normal brain development and circuit refinement. In this review we will examine the data that D-serine and its associated enzyme serine racemase are developmentally regulated. We will consider the evidence that D-serine is actively released by glial cells and examine the studies that have implicated D-serine as a critical player involved in regulating NMDAR-mediated synaptic transmission and neuronal migration during development. Furthermore, we will consider how dysregulation of D-serine may play an important role in the etiology of neurological and psychiatric diseases.

Chewing ameliorates stress-induced suppression of spatial memory by increasing glucocorticoid receptor expression in the hippocampus

Chewing alters hypothalamic-pituitary-adrenal axis function and improves the ability to cope with stress in rodents. Given that stress negatively influences hippocampus-dependent learning and memory, we aimed to elucidate whether masticatory movements, namely chewing, improve the stress-induced impairment of spatial memory in conjunction with increased hippocampal glucocorticoid receptor expression. Male Sprague-Dawley rats were subjected to restraint stress by immobilization for 2h: the stress with chewing (SC) group were allowed to chew on a wooden stick during the latter half of the immobilization period, whereas the stress without chewing (ST) group were not allowed to do so. Performance in the Morris water maze test was significantly impaired in the ST group compared with the SC group. Further, the numbers of glucocorticoid receptor immunopositive neurons in the hippocampal cornu ammonis 1 region were significantly lower in the ST group than in the control and SC groups. The control and SC rats showed no significant differences in both the water maze performance and the numbers of glucocorticoid receptor-immunopositive neurons. The immunohistochemical finding correlated with the performance in the water maze test. These results suggest that chewing is a behavioral mechanism to cope with stress by increasing hippocampal glucocorticoid receptor expression.

Unpredictable chronic stress in juvenile or adult rats has opposite effects, respectively, promoting and impairing resilience

We evaluated the effects of early maternal deprivation (MD; age 7-14 days) alone or in combination with unpredictable chronic stress (UCS; MDUN; 28-84 days) on anxiety and learning in 90 days old adult rats. We hypothesized that exposure to both stressors (MDUN) would be more detrimental than exposure to one or neither. Unexpectedly, adult rats from the MDUN group did not differ from control animals, whereas adult MD animals exhibited impaired avoidance learning. We next investigated the effect of juvenile-onset (30-90 days) versus adult-onset (60-90 days) stress on avoidance learning in adulthood (90 days). We found that adult-onset chronic stress impaired avoidance learning and memory whereas juvenile-onset stress did not. Thus, the results again indicate that juvenile exposure to UCS induces resilience rather than impairment.

Assessment of spatial memory in mice

Improvements in health care have greatly increased life span in the United States. The focus is now shifting from physical well-being to improvement in mental well-being or maintenance of cognitive function in old age. It is known that elderly people suffer from cognitive impairment, even without neurodegeneration, as a part of 'normal aging'. This 'age-associated memory impairment' (AAMI), can have a devastating impact on the social and economic life of an individual as well as the society. Scientists have been experimenting to find methods to prevent the memory loss associated with aging. The major factor involved in these experiments is the use of animal models to assess hippocampal-based spatial memory. This review describes the different types of memory including hippocampal-based memory that is vulnerable to aging. A detailed overview of various behavioral paradigms used to assess spatial memory including the T-maze, radial maze, Morris water maze, Barnes maze and others is presented. The review also describes the molecular basis of memory in hippocampus called as 'long-term potentiation'. The advantages and limitations of the behavioral models in assessing memory and the link to the long-term potentiation are discussed. This review should assist investigators in choosing suitable methods to assess spatial memory in mice.

Epigenetics and the biological definition of gene x environment interactions

Variations in phenotype reflect the influence of environmental conditions during development on cellular functions, including that of the genome. The recent integration of epigenetics into developmental psychobiology illustrates the processes by which environmental conditions in early life structurally alter DNA, providing a physical basis for the influence of the perinatal environmental signals on phenotype over the life of the individual. This review focuses on the enduring effects of naturally occurring variations in maternal care on gene expression and phenotype to provide an example of environmentally driven plasticity at the level of the DNA, revealing the interdependence of gene and environmental in the regulation of phenotype.

Acute stress impairs hippocampal mossy fiber-CA3 long-term potentiation by enhancing cAMP-specific phosphodiesterase 4 activity

The mossy fiber synapses onto hippocampal CA3 neurons show unique molecular features and a wide dynamic range of plasticity. Although acute stress has been well recognized to alter bidirectional long-term synaptic plasticity in the hippocampal CA1 region and dentate gyrus, it remains unclear whether the same effect may also occur at the mossy fiber-CA3 synapses. Here, we report that hippocampal slices prepared from adult mice that had experienced an acute unpredictable and inescapable restraint tail-shock stress showed a marked impairment of long-term potentiation (LTP) induced by high-frequency stimulation or adenylyl cyclase activator forskolin. This effect was prevented when animals were submitted to bilateral adrenalectomy or given the glucocorticoid receptor antagonist RU38486 before experiencing stress. In contrast, stress has no effect on synaptic potentiation induced by the non-hydrolysable and membrane-permeable cyclic adenosine 5'-monophosphate (cAMP) analog Sp-8-bromo-cAMPS. No obvious differences were observed between control and stressed mice in the basal synaptic transmission, paired-pulse facilitation, or frequency facilitation at the mossy fiber-CA3 synapses. We also found that the inhibitory effect of stress on mossy fiber LTP was obviated by the adenosine A(1) receptor antagonist 8-cyclopentyl-1,3,-dipropylxanthine, the non-specific phosphodiesterase (PDE) inhibitor 3-isobutyl-methylxanthine, and the specific PDE4 inhibitor 4-(3-butoxy-4-methoxyphenyl)methyl-2-imidazolidone. In addition, stress induces a sustained and profound increase in cAMP-specific PDE4 activity. These results suggest that the inhibition of mossy fiber LTP by acute stress treatment seems originating from a corticosterone-induced sustained increase in the PDE4 activity to accelerate the metabolism of cAMP to adenosine, in turn triggering an adenosine A(1) receptor-mediated impairment of transmitter release machinery.

Transitions in sensitive period attachment learning in infancy: the role of corticosterone

Survival of altricial infants, including humans and rats, depends on attachment to the caregiver - a process that requires infants to recognize, learn, and remember their attachment figure. The demands of a dynamic environment combined with a maturing organism require frequent neurobehavioral reorganization. This restructuring of behavior and its supporting neural circuitry can be viewed through the unique lens of attachment learning in rats in which preference learning is enhanced and aversion learning is attenuated. Behavioral restructuring is well adapted to securing the crucial infant-caregiver relationship regardless of the quality of care. With maturation and the end of the infant-caregiver attachment learning period, the complex interplay of neural structures, hormones, and social behavior coordinates the developing rat's eventual transition to life outside of the nest. Nevertheless, early-life environmental and physiological stressors can alter the resilient nature of this system, particularly with respect to the amygdala, and these changes may provide important clues to understanding the lasting effects of early stress.

Gender-related qualitative differences in baseline and post-stress anxiety responses are not reflected in the incidence of criterion-based PTSD-like behaviour patterns

BACKGROUND

Most epidemiological studies report higher prevalence rates of stress-related disorders such as acute stress disorder, post-traumatic stress disorder (PTSD) and major depressive disorder in women than in men. Few animal models of PTSD have taken gender differences into account and have typically used male subjects. In order to explore gender-related PTSD-like stress-responses more thoroughly, we applied an animal model that focuses selectively on individual patterns of behavioural responses.

METHODS

Prevalence rates of individual behavioural response to a single exposure to predator scent stress (PSS) were assessed by both elevated plus-maze and startle response paradigms. Prevalence rates of extreme behavioural disruption (EBR) on both tests were assessed, correlated to corticosterone levels, and compared to global population response data. In addition, we assessed learning and memory in the Morris water-maze (MWM).

RESULTS

There were no significant differences between the behavioural responses related to oestrous cycle phase in terms of global data for the groups or in terms of prevalence rates of EBR. The overall patterns of response of males and females were affected, yet females demonstrated greater levels of baseline anxiety-like behaviour and lower peak levels of post-exposure anxiety-like behaviour than males. However, the prevalence rates of individual subjects who responded with PTSD-like behaviour were equal for female and male subjects. PSS-exposed female subjects demonstrated significantly compromised performance in the MWM compared to males.

CONCLUSIONS

In this animal model, the results clarified that the assumption that females are more vulnerable is true for the magnitude of the response, but not for the prevalence of pathological response patterns in rat populations.

A behavioral analysis of the impact of voluntary physical activity on hippocampus-dependent contextual conditioning

Voluntary physical activity induces molecular changes in the hippocampus consistent with improved hippocampal function, but few studies have explored the effects of wheel running on specific hippocampal-dependent learning and memory processes. The current studies investigated the impact of voluntary wheel running on learning and memory for context and extinction using contextual fear conditioning which is known to be dependent on the hippocampus. When conditioning occurred prior to the start of 6 weeks of wheel running, wheel running had no effect on memory for context or extinction (assessed with freezing). In contrast, when wheel running occurred for 6 weeks prior to conditioning, physical activity improved contextual memory during a retention test 24 h later, but did not affect extinction learning or memory. Wheel running had no effect on freezing immediately after foot shock presentation during conditioning, suggesting that physical activity does not affect the acquisition of the context-shock association or alter the expression of freezing, per se. Instead, it is argued that physical activity improves the consolidation of contextual memories in the hippocampus. Consistent with improved hippocampus-dependent context learning and memory, 6 weeks of wheel running also improved context discrimination and reduced the context pre-exposure time required to form a strong contextual memory. The effect of wheel running on brain-derived neurotrophic factor (BDNF) messenger ribonucleic acid (mRNA) in hippocampal and amygdala subregions was also investigated. Wheel running increased BDNF mRNA in the dentate gyrus, CA1, and the basolateral amygdala. Results are consistent with improved hippocampal function following physical activity.

Timing in the absence of supraspinal input II: regularly spaced stimulation induces a lasting alteration in spinal function that depends on the NMDA receptor, BDNF release, and protein synthesis

The detection of temporal regularity allows organisms to predict the occurrence of future events. When events occur in an irregular manner, uncertainty is increased, and negative outcomes can ensue (e.g., stress). The present study shows that spinal neurons can discriminate between variable- and fixed-spaced stimulation and that the detection of regularity requires training and engages a form of NMDA receptor-mediated plasticity. The impact of stimulus exposure was assessed using a spinally mediated instrumental response, wherein spinally transected rats are given legshock whenever one hindlimb is extended. Over time, they learn to maintain the leg in a flexed position that minimizes net shock exposure. Prior exposure to 180-900 tailshocks given in a variable (unpredictable) manner inhibited this learning. A learning deficit was not observed when 900 tailshocks were applied using a fixed (predictable) spacing. Fixed-spaced stimulation did not have a divergent effect when fewer (180) shocks were presented, implying that the abstraction of temporal regularity required repeated exposure (training). Moreover, fixed-spaced stimulation both prevented and reversed the learning deficit. The protective effect of fixed-spaced shock lasted 48 h, and was prevented by pretreatment with the NMDA receptor antagonist MK-801. Administration of the protein synthesis inhibitor cycloheximide after training blocked the long-term effect. Inhibiting BDNF function, using TrkB-IgG, also eliminated the beneficial effect of fixed-spaced stimulation. The results suggest that spinal systems can detect regularity and that this type of stimulation promotes adaptive plasticity, which may foster recovery after spinal injury.

A distinct pattern of intracellular glucocorticoid-related responses is associated with extreme behavioral response to stress in an animal model of post-traumatic stress disorder

BACKGROUND

Activation of glucocorticoid receptors (GR) increases expression of the mitogen-activated protein kinase (MAPK) pathway leading to increased expression of Zif/268, an effector immediate early gene involved in cellular growth, intracellular signaling, and synaptic modification. Glucocorticoids induce expression of Zif/268 through two distinct mechanisms: a rapid-onset, MAPK-independent pathway and a slower-onset, MAPK-dependent mechanism.

METHOD

This study investigated both rapid and long-term expression of GR protein in the cytosolic extract, its translocation to the nucleus, and expression of mRNA for the Zif/268 gene in selected brain areas, associated with circulating levels of corticosterone, in an animal model of PTSD. Trauma cue-triggered Zif/268 expression was assessed eight days after stress exposure.

RESULTS

The results demonstrated a pattern of response that was common to all exposed individuals at 30 min after exposure, characterized by a significant elevation in GR translocation to the nucleus and elevated levels of Zif/268 mRNA in the hippocampus. A distinct pattern associated with extreme behavioral response (EBR) was revealed upon further bioassay of behavioral response groups, classified according to their individual patterns of behavioral response at seven days. These EBR individuals displayed significantly higher circulating corticosterone and nuclear GR levels, compared to minimal behavioral responders and controls. No difference in Zif/268 mRNA levels was observed between the exposed and naïve animals.

CONCLUSION

Following the uniform acute response, the patterns of GR protein levels and Zif/268 mRNA levels are associated with degree of behavioral disruption. Since the slower-onset mechanism for glucocorticoid-induced Zif/268 expression depends on activation of the MAPK pathway, the pattern observed only in EBR rats may be related to disruptions of this pathway.

Evaluation of spatial memory of C57BL/6J and CD1 mice in the Barnes maze, the Multiple T-maze and in the Morris water maze

Evaluation of spatial learning and memory is mainly carried out using the Morris water maze as a single paradigm. We intended to test whether mice in the Barnes maze and Multiple T-maze would lead to comparable results and to test two individual mouse strains with different anxiety levels. C57BL/6J and CD1 male mice were used in the experiments. During the acquisition phase, learning was measured using parameters latency, path length, errors in the BM and correct decisions in MTM. Mice were trained for 4 days and probe trials were performed on days 5 and 12. Latencies reduction over the training period indicated that both strains learned all tasks. During retention phase at days 5 and 12 C57BL/6J performed the Barnes maze and Multiple T-maze task better than CD1 mice while CD1 performed better than C57BL/6J in the Morris water maze. In the BM at day 12, C57BL/6J kept the level of visits to target observed at day 5 whereas CD1 performed worse. Strain- and task-dependent differences were observed using the three mazes. Therefore, fair evaluation of spatial memory demands application of (at least) two different test systems, a water- and a land maze. Different anxiety-related behaviour as well as stress-responses in the strains used may help to interpret the findings reported and again may propose the use of at least two mouse strains when robust evaluation of spatial memory is considered.

Protein synthesis inhibition before or after stress exposure results in divergent endocrine and BDNF responses disassociated from behavioral responses

This study aimed to assess the effects of anisomycin, a protein synthesis inhibitor, on behavioral responses, brain-derived neurotrophic factor (BDNF) and TrkB mRNA levels, and circulating corticosterone in rats-when administered before or after initial exposure to a predator scent stress stimulus. Magnitude of changes in prevalence of anxiety-like behaviors on the elevated plus-maze and exaggerated startle reaction as well as corticosterone levels and mRNA BDNF and TrkB were compared in rats exposed to predator stress, microinjected with anisomycin before or after stress exposure. Administration of anisomycin before or after stress exposure reduced anxiety-like behavior in the elevated plus-maze and reduced the mean startle amplitude 7 days postexposure. Although the behavioral responses were similar when anisomycin was microinjected before or after stress exposure, the levels of mRNAs for BDNF and TrkB, which play a role in modulation of synaptic plasticity and the consolidation process, showed varying responses.

Enduring effects of infant memories: infant odor-shock conditioning attenuates amygdala activity and adult fear conditioning

BACKGROUND

Early life adverse experience alters adult emotional and cognitive development. Here we assess early life learning about adverse experience and its consequences on adult fear conditioning and amygdala activity.

METHODS

Neonatal rats were conditioned daily from 8-12 days-old with paired odor (conditioned stimulus, CS) .5mA shock, unpaired, odor-only, or naive (no infant conditioning). In adulthood, each infant training group was divided into three adult training groups: paired, unpaired or odor-only, using either the same infant CS odor, or a novel adult CS odor without or with the infant CS present as context. Adults were cue tested for freezing (odor in novel environment), with amygdala (14)C 2-DG autoradiography and electrophysiology assessment.

RESULTS

Infant paired odor-shock conditioning attenuated adult fear conditioning, but only if the same infant CS odor was used. The (14)C 2-DG activity correlated with infant paired odor-shock conditioning produced attenuated amygdala but heightened olfactory bulb activity. Electrophysiological amygdala assessment further suggests early experience causes changes in amygdala processing as revealed by increased paired-pulse facilitation in adulthood.

CONCLUSIONS

This suggests some enduring effects of early life adversity (shock) are under CS control and dependent upon learning for their impact on later adult fear learning.

Modulation of stress consequences by hippocampal monoaminergic, glutamatergic and nitrergic neurotransmitter systems

Several findings relate the hippocampal formation to the behavioural consequences of stress. It contains a high concentration of corticoid receptors and undergoes plastic modifications, including decreased neurogenesis and cellular remodelling, following stress exposure. Various major neurotransmitter systems in the hippocampus are involved in these effects. Serotonin (5-HT) seems to exert a protective role in the hippocampus and attenuates the behavioural consequences of stress by activating 5-HT1A receptors in this structure. These effects may mediate the therapeutic actions of several antidepressants. The role of noradrenaline is less clear and possibly depends on the specific hippocampal region (dorsal vs. ventral). The deleterious modifications induced in the hippocampus by stress might involve a decrease in neurotrophic factors such as brain derived neurotrophic factor (BDNF) following glutamate N-methyl-D-aspartate (NMDA) receptor activation. In addition to glutamate, nitric oxide (NO) could also be related to these effects. Systemic and intra-hippocampal administration of nitric oxide synthase (NOS) inhibitors attenuates stress-induced behavioural consequences. The challenge for the future will be to integrate results related to these different neurotransmitter systems in a unifying theory about the role of the hippocampus in mood regulation, depressive disorder and antidepressant effects.

Hippocampal expression analyses reveal selective association of immediate-early, neuroenergetic, and myelinogenic pathways with cognitive impairment in aged rats

Although expression of some genes is known to change during neuronal activity or plasticity, the overall relationship of gene expression changes to memory or memory disorders is not well understood. Here, we combined extensive statistical microarray analyses with behavioral testing to comprehensively identify genes and pathways associated with aging and cognitive dysfunction. Aged rats were separated into cognitively unimpaired (AU) or impaired (AI) groups based on their Morris water maze performance relative to young-adult (Y) animals. Hippocampal gene expression was assessed in Y, AU, and AI on the fifth (last) day of maze training (5T) or 21 d posttraining (21PT) and in nontrained animals (eight groups total, one array per animal; n = 78 arrays). ANOVA and linear contrasts identified genes that differed from Y generally with aging (differed in both AU and AI) or selectively, with cognitive status (differed only in AI or AU). Altered pathways/processes were identified by overrepresentation analyses of changed genes. With general aging, there was downregulation of axonal growth, cytoskeletal assembly/transport, signaling, and lipogenic/uptake pathways, concomitant with upregulation in immune/inflammatory, lysosomal, lipid/protein degradation, cholesterol transport, transforming growth factor, and cAMP signaling pathways, primarily independent of training condition. Selectively, in AI, there was downregulation at 5T of immediate-early gene, Wnt (wingless integration site), insulin, and G-protein signaling, lipogenesis, and glucose utilization pathways, whereas Notch2 (oligodendrocyte development) and myelination pathways were upregulated, particularly at 21PT. In AU, receptor/signal transduction genes were upregulated, perhaps as compensatory responses. Immunohistochemistry confirmed and extended selected microarray results. Together, the findings suggest a new model, in which deficient neuroenergetics leads to downregulated neuronal signaling and increased glial activation, resulting in aging-related cognitive dysfunction.

The temporal dynamics model of emotional memory processing: a synthesis on the neurobiological basis of stress-induced amnesia, flashbulb and traumatic memories, and the Yerkes-Dodson law

We have reviewed research on the effects of stress on LTP in the hippocampus, amygdala and prefrontal cortex (PFC) and present new findings which provide insight into how the attention and memory-related functions of these structures are influenced by strong emotionality. We have incorporated the stress-LTP findings into our "temporal dynamics" model, which provides a framework for understanding the neurobiological basis of flashbulb and traumatic memories, as well as stress-induced amnesia. An important feature of the model is the idea that endogenous mechanisms of plasticity in the hippocampus and amygdala are rapidly activated for a relatively short period of time by a strong emotional learning experience. Following this activational period, both structures undergo a state in which the induction of new plasticity is suppressed, which facilitates the memory consolidation process. We further propose that with the onset of strong emotionality, the hippocampus rapidly shifts from a "configural/cognitive map" mode to a "flashbulb memory" mode, which underlies the long-lasting, but fragmented, nature of traumatic memories. Finally, we have speculated on the significance of stress-LTP interactions in the context of the Yerkes-Dodson Law, a well-cited, but misunderstood, century-old principle which states that the relationship between arousal and behavioral performance can be linear or curvilinear, depending on the difficulty of the task.

Opiate withdrawal modifies synaptic plasticity in subicular-nucleus accumbens pathway in vivo

Subiculum receives output of hippocampal CA1 neurons and projects glutamatergic synapses onto nucleus accumbens (NAc), the subicular-NAc pathway linking memory and reward system. It is unknown whether morphine withdrawal influences synaptic plasticity in the subicular-NAc pathway. Here, we recorded the field excitatory postsynaptic potential (EPSP) within the shell of NAc by stimulating ventral subiculum in anesthetized adult rats. We found that high frequency stimulation (HFS, 200 Hz) induced long-term potentiation (LTP) but low frequency stimulation (LFS, 1 Hz) failed to induce long-term depression (LTD) in control animals. However, behavioral stress enabled LFS to induce a reliable LTD (sLTD) that was dependent on the glucocorticoid receptors. Both LTP and sLTD were prevented by the N-methyl-d-aspartate receptor antagonist AP-5. After repeated morphine treatment for 12 days, acute withdrawal (12 h) impaired LTP but had no effect on sLTD; prolonged withdrawal (4 days) restored the LTP but impaired the sLTD. Remarkably, basal synaptic efficacy reflected by baseline EPSP amplitude was potentiated in acute withdrawal but was depressed in prolonged withdrawal. Thus, acute and prolonged opiate withdrawal may cause endogenous LTP and LTD in the subicular-NAc pathway that occludes the subsequent induction of synaptic plasticity, demonstrating adaptive changes of the NAc functions during opiate withdrawal.

Neonatal stress and litter composition alter sucrose intake in both rat dam and offspring

The early postnatal environment can have long lasting effects on the physiology and behavior of both mother and offspring. A great deal of evidence indicates that stress during this time period is a risk factor for the future development of a multitude of disorders including substance abuse. The maternal separation paradigm is used to model such stress in rats. The current study evaluated the effects of maternal separation and litter composition on sucrose consumption, a non-drug measure of reward, in both male Long-Evans rat offspring and mother. On postnatal day 2, rats were cross-fostered, placed in single-sex or mixed-sex litters, and subsequently stressed by daily dam-pup separations during the first two postnatal weeks. The length of the separation (15 min, 1 h, or 3 h) was randomly assigned to each day. A two-bottle choice test was given to maternally separated and nonhandled offspring when they were adults, and to dams 2-4 weeks after weaning. Intake of a 10% sucrose solution or water was compared for 1 h daily across five consecutive days. Rearing condition had a profound effect on total fluid intake and sucrose solution intake by both offspring and dam, with the separated offspring and dams generally consuming a greater amount of total fluid and 10% sucrose solution than their nonhandled counterparts. Litter composition also affected consumption, with the offspring from maternally separated mixed-sex litters consuming more total fluid and 10% sucrose solution than offspring from maternally separated single-sex litters. These results indicate that early postnatal and postpartum stress can lead to changes in sucrose consumption, a non-drug measure of reward, indicating that such stress may alter the underlying brain reward mechanisms.

Novelty exploration elicits a reversal of acute stress-induced modulation of hippocampal synaptic plasticity in the rat

Acute behavioural stress has been recognized as a strong influence on the inducibility of hippocampal long-term synaptic plasticity. We have reported previously that in adult male rats, acute behavioural stress impairs long-term potentiation (LTP) but enhances long-term depression (LTD) in the hippocampal CA1 region. In this study we report that the effects of stress on LTP and LTD were reversed when animals were introduced into a novel 'stimulus-rich' environment immediately after the stress. Novelty exploration-induced reversal of stress effects was prevented when the animals were given the NMDA receptor antagonist D-(-)-2-amino-5-phosphonopentanoic acid, the cholinergic antagonist atropine and the protein phosphatase (PP) 2B inhibitors cyclosporin A and cypermethrin, but not the alpha1-adrenergic antagonist prazosin, the beta-adrenergic antagonist propranolol or the PP1/2A inhibitor okadaic acid, respectively before being subjected to the novel environment. In addition, the ability of novelty exploration to reverse the stress effects was mimicked by a direct application of the cholinergic agonist carbachol. Exposure to the novel environment following stress was accompanied by the activation of both PP2B and striatal-enriched tyrosine phosphatase (STEP). Taken together, these findings suggest that the activation of the cholinergic system and, in turn, the triggering of an NMDA receptor-mediated activation of PP2B to increase STEP activity appear to mediate the novelty exploration-induced reversal of stress-related modulation of hippocampal long-term synaptic plasticity.

Stress evoked by opiate withdrawal facilitates hippocampal LTP in vivo

Stress impairs hippocampal long-term potentiation (LTP), but it is unknown whether the stress evoked by opiate withdrawal has the same effect. Here the authors report that opiate withdrawal for 4 days does not influence basal synaptic transmission, but results in a greatly increased LTP in hippocampal CA1 area in anesthetized rats. Elevated-platform stress enabled a large LTP in rats withdrawn for only 18 h, but the glucocorticoid receptor antagonist RU38486 (twice per day for 3 days) prevented the large LTP on 4 days withdrawal. Moreover, 4 days withdrawal enhanced the NMDAR-mediated EPSCs, in which the NR2A-containing NMDAR-mediated EPSC was increased but the NR2B-containing NMDAR-mediated EPSC was decreased. These results suggest that adaptive changes of the NMDAR and glucocorticoid receptor functions during 4 days of opiate withdrawal may enable stress to facilitate hippocampal LTP, potentially contributing to the opiate withdrawal experience-dependent modifications of hippocampal functions.

Hippocampal neurogenesis: Opposing effects of stress and antidepressant treatment

The hippocampus is one of several limbic brain structures implicated in the pathophysiology and treatment of mood disorders. Preclinical and clinical studies demonstrate that stress and depression lead to reductions of the total volume of this structure and atrophy and loss of neurons in the adult hippocampus. One of the cellular mechanisms that could account for alterations of hippocampal structure as well as function is the regulation of adult neurogenesis. Stress exerts a profound effect on neurogenesis, leading to a rapid and prolonged decrease in the rate of cell proliferation in the adult hippocampus. In contrast, chronic antidepressant treatment up-regulates hippocampal neurogenesis, and could thereby block or reverse the atrophy and damage caused by stress. Recent studies also demonstrate that neurogenesis is required for the actions of antidepressants in behavioral models of depression. This review discusses the literature that has lead to a neurogenic hypothesis of depression and antidepressant action, as well as the molecular and cellular mechanisms that underlie the regulation of adult neurogenesis by stress and antidepressant treatment.

Effects of unconditioned and conditioned aversive stimuli in an intense fear conditioning paradigm on synaptic plasticity in the hippocampal CA1 area in vivo

Repeated vivid recalls or flashbacks of traumatic memories and memory deficits are the cardinal features of post-traumatic stress disorder (PTSD). The underlying mechanisms are not fully understood yet. Here, we examined the effects of very strong fear conditioning (20 pairings of a light with a 1.5-mA, 0.5-s foot shock) and subsequent reexposure to the conditioning context (chamber A), a similar context (chamber B), and/or to the fear conditioned stimulus (CS) (a light) on synaptic plasticity in the hippocampal CA1 area in anesthetized Sprague-Dawley rats. The conditioning procedure resulted in very strong conditioned fear, as reflected by high levels of persistent freezing, to both the contexts and to the CS, 24 h after fear conditioning. The induction of long-term potentiation (LTP) was blocked immediately after fear conditioning. It was still markedly impaired 24 h after fear conditioning; reexposure to the conditioning chamber A (CA) or to a similar chamber B (CB) did not affect the impairment. However, presentation of the CS in the CA exacerbated the impairment of LTP, whereas the CS presentation in a CB ameliorated the impairment so that LTP induction did not differ from that of control groups. The induction of long-term depression (LTD) was facilitated immediately, but not 24 h, after fear conditioning. Only reexposure to the CS in the CA, but not reexposure to either chamber A or B alone, or the CS in chamber B, 24 h after conditioning, reinstated the facilitation of LTD induction. These data demonstrate that unconditioned and conditioned aversive stimuli in an intense fear conditioning paradigm can have profound effects on hippocampal synaptic plasticity, which may aid to understand the mechanisms underlying impairments of hippocampus-dependent memory by stress or in PTSD.

New drug targets in the signaling pathways activated by antidepressants

Studies of twins, genetic linkage and association support the existence of a genetic component to the susceptibility to depression. Biochemical parameters that change in depressive patients are also observed in their relatives. This indicates that genetic factors control these physiological changes and the susceptibility to depression. The study of the mechanisms by which different genetic factors affect the organism's response to drugs can identify better diagnostic tools as well as drug targets. Pharmacogenomics can, therefore, help to find the most efficacious treatment for patients with specific genetic profiles. However, the genetic factors that determine the susceptibility to depression and the different responses to antidepressant treatment are largely unknown. Pharmacogenetic studies have used three different categories of genes to search for associations with the outcome of drug treatments. These gene categories are drug-metabolizing enzymes, known drug targets and known susceptibility genes. The present review will explore the different signal transduction pathways involved in antidepressant action and their possible use in psychopharmacogenomics and drug discovery.

Activity and plasticity in the CA1, the dentate gyrus, and the amygdala following controllable vs. uncontrollable water stress

The level of controllability has been shown to modulate the effects of stress on physiology and behavior. In the present study, we investigated the effects of controllable vs. uncontrollable stressors on plasticity in hippocampal CA1, the dentate gyrus (DG), and basal amygdala nucleus (B) in the rat, using the electrophysiological procedure of long-term potentiation (LTP). A naive group was left undisturbed until the electrophysiological recording commenced. Rats of the two controllable stress groups were trained in the Morris water maze to locate an invisible underwater platform (the first group), or visible platform (the second group), thus escaping from the water, before the recording. The uncontrollable stress group underwent the same procedure (exposure time to water was adjusted to the averaged exposure time of the first controllable group) without the escape platform. We first assessed the effects of stress and controllability on LTP in CA1. Both controllable stressors and the uncontrollable stress impaired CA1 LTP, with a more robust effect induced by the uncontrollable stress. We further assessed the effects of the same procedures on LTP in DG and B. The uncontrollable stress enhanced LTP in DG and increased baseline responses (suggesting uncontrollable stress-induced plasticity) in the amygdala. All the stressors decreased amygdalar LTP. An assessment of plasma levels of corticosterone (CORT), following the behavioral procedures, revealed an enhancement in CORT release following the uncontrollable, but not controllable stress, indicating the uncontrollable condition as the most stressful. These findings provide insight into the differential effects of stress and stress controllability on different hippocampal subregions and the amygdala.

Distinct modulatory effects of sleep on the maintenance of hippocampal and medial prefrontal cortex LTP

Both human and animal studies support the idea that memory consolidation of waking experiences occurs during sleep. In experimental models, rapid-eye-movement (REM) sleep has been shown to be necessary for cortical synaptic plasticity and for the acquisition of spatial and nonspatial memory. Because the hippocampus and medial prefrontal cortex (mPFC) play distinct and important roles in memory processing, we sought to determine the role of sleep in the maintenance of long-term potentiation (LTP) in the dentate gyrus (DG) and mPFC of freely behaving rats. Animals were implanted with stimulating and recording electrodes, either in the medial perforant path and DG or CA1 and mPFC, for the recording of field potentials. Following baseline recordings, LTP was induced and the animals were assigned to three different groups: REM sleep-deprived (REMD), total sleep-deprived (TSD) and control which were allowed to sleep (SLEEP). The deprivation protocol lasted for 4 h and the recordings were made during the first hour and at 5, 24 and 48 h following LTP induction. Our results show that REMD impaired the maintenance of late-phase (48-h) LTP in the DG, whereas it enhanced it in the mPFC. Sleep, therefore, could have distinct effects on the consolidation of different forms of memory.

Effects of chronic stress on hippocampal long-term potentiation

Chronic stress causes atrophy of the apical dendrites of CA3 pyramidal neurons and deficits in spatial memory. We investigated the effects of chronic stress on hippocampal physiology and long-term potentiation (LTP) in the CA3 and dentate gyrus (DG). Rats were subjected to chronic (21 days, 6 h/day) restraint stress and tested for LTP 48 h following the last stress episode. Control animals were briefly handled each day, similar to the experimental group but without restraint. To eliminate acute stress effects, a second control group of rats was subjected to a single acute (6 h) restraint stress and tested for LTP 48 h later. Field potential recordings were made, under chloropent anesthesia, from the stratum lucidum of CA3, with stimulation of either the mossy fiber or commissural/associational pathways, or in the DG granule-cell layer, with stimulation of the medial perforant pathway. Chronic stress produced a suppression of LTP at 48 h compared to controls in a site-specific manner, namely, significantly lower LTP in the medial perforant input to the DG and also in the commissural/associational input to the CA3, but not in the mossy fiber input to CA3. The animals subjected to acute stress and tested 48 h later did not show a suppression in LTP. High-frequency stimulation (HFS) of the commissural/associational and mossy fiber inputs to CA3 produced epileptic afterdischarges in 56% of acutely stressed animals and in 29% of chronically stressed animals, whereas HFS caused afterdischarges in only 9% of nonstressed controls. No afterdischarges were seen in the medial perforant path input to DG. In order to explore the basis for these changes, we performed paired-pulse inhibition/facilitation (PPI/F) and current-source-density (CSD) analysis in stressed and control animals. For PPI/F, acute stress caused an overall significant enhancement of excitation in the commissural/associational input to CA3 and medial perforant path input to DG. In contrast, chronic stress did not produce significant changes in PPI/F. The CSD analysis revealed significant chronic stress-induced shifts in the current sources and sinks in the apical dendrites and pyramidal cell layers of the CA3 field but not in the DG. These results are consistent with the morphological findings for stress effects upon dendrites of CA3 neurons. Furthermore, they suggest that chronic stress produces changes in the input-output relationship in the hippocampal trisynaptic circuit which could affect information flow through this structure.

Footshock stress alters early postnatal development of electrophysiological responses and caspase-3 activity in rat hippocampus

Postnatal changes in population spike (PS) amplitudes and caspase-3 activity were compared in the hippocampi of control rats and experimental animals subjected to a brief footshock on postnatal day (PD) 13. Footshock induced an increase in maximal PS amplitudes during the early period (from PD 14 to PD 16), however, the difference between stressed and control animals gradually decreased with age up to PD 21. No difference between hippocampal caspase-3 activity in control and footshock groups was revealed within the PD 14-17. However, caspase-3 activity in the latter group was significantly lower over the next period of postnatal development (PD 18-21). PS amplitudes in the slices of the footshock group significantly increased over PD 22-27. We suggest that footshock activates the development of hippocampal circuitry during early phases, this phenomenon mediating enhanced responsiveness as a result of an increased production of synaptic connections and related decrease in neuronal loss.