The role of the GABAA receptor Alpha 1 subunit in the ventral hippocampus in stress resilience.

Pre-pubertal stress increases post-traumatic stress disorder (PTSD) susceptibility. We have previously demonstrated that enriched environment (EE) intervention immediately after pre-pubertal stress protects from the effects of trauma in adulthood. Here, we examined whether exposure to EE would also be beneficial if applied after exposure to trauma in adulthood. We have recently shown that exposure to juvenile stress and under-water trauma (UWT) is associated with increased expression of GABAA receptor subunit α1 in the ventral hippocampus. However, differentiating between affected and unaffected individuals, this increased expression was confined to stress-exposed, behaviorally unaffected individuals, suggesting upregulation of α1 expression as a potential mechanism of resilience. We now examined whether EE-induced resilience renders increased expression of α1 in the ventral hippocampus redundant when facing a trauma later in life. Adult rats were exposed to UWT, with pre-exposure to juvenile stress, and tested in the open field and elevated plus maze paradigms four weeks later. EE exposure during juvenility prevented pre-pubertal stress-induced vulnerability, but not if performed following UWT in adulthood. Furthermore, juvenile EE exposure prevented the trauma-associated increase in α1 expression levels. Our findings emphasize the importance of early interventions in order to reduce the likelihood of developing psychopathologies in adulthood.

Pre-trauma Methylphenidate in rats reduces PTSD-like reactions one month later.

In basic research, the etiology of fear-related pathologies, such as post-traumatic stress disorder (PTSD), is conceptualized using fear-conditioning protocols that pair environmental stimuli (that is, a conditioned stimulus-CS) with an aversive, unconditioned stimulus (US) to elicit an assessable conditioned fear response. Although pathophysiological models agree that regulatory dysfunctions in this associative process may instigate fear-related pathology, current opinions differ in regard to the nature of these dysfunctions. Primarily derived from studies in rodents, the prevailing perspective proposes that pathological fear-reactions develop from intensified and overly consolidated CS-US associations. Alternatively, models derived from studies in humans suggest that tempospatial inaccuracies in representations of associative fear might precipitate pathology by engendering failure to differentiate present experiences and past memories of threat. To test this concept in rodents, we administered rats with cognition enhancing doses of Methylphenidate before or after fear conditioning and measured long-term alterations in their conditioned fear behaviors and PTSD-like reactions. The administration of Methylphenidate before fear-memory formation indeed reduced anxious-like responses during fear-memory retrieval one month later. An individual profiling analysis revealed that Methylphenidate onset had opposing effects on the risk for PTSD-like classification. The modulation of initial learning and formation of associative fear normalized the risk for developing PTSD-like reaction. In contrast, when the effects of Methylphenidate were exerted only over later consolidation this risk increased markedly. When examined under current psychiatric and neuropharmacologic literature, these results reveal a possible strategy of using low-dose Methylphenidate for the prevention of PTSD in high risk populations.

Exposure to prolonged controllable or uncontrollable stress affects GABAergic function in sub-regions of the hippocampus and the amygdala.

The degree of behavioral control that an individual has over a stressor can critically determine its behavioral and neurochemical outcomes. Exposure to uncontrollable stress was previously shown to have detrimental effects on behavior, whereas exposure to equivalent controllable stress prevented these negative outcomes and even improved later stress coping. As many lines of evidence show, stress exposure can have maladaptive changes on inhibitory circuitry, and these effects were largely shown in the hippocampus and amygdala. In the current study we set out to examine alterations in GABAergic activity following exposure to the prolonged two way shuttle (TWS) avoidance task, focusing on the GABA-related factors glutamate decarboxylase (GAD)65, cholecystokinin (CCK) and neuropeptide Y (NPY). As recent views of the hippocampus assume regional specificity in hippocampal function, we examined different regions in the hippocampus, as well as the basolateral amygdala (BLA). Our findings reveal similar alterations in GAD65 in BLA for both controllable and uncontrollable stress exposure, but differential alterations in GAD65 and NPY in the dorsal dentate gyrus (DG). Synaptic plasticity and inhibitory activity in the dorsal DG was further assessed by applying different stimulation protocols and measuring evoked field potentials in vivo. Our results support a role for the DG in stress processing, emphasizing its sensitivity to the nature of the stressor.

A novel approach to PTSD modeling in rats reveals alternating patterns of limbic activity in different types of stress reaction.

Human reactions to trauma exposure are extremely diverse, with some individuals exhibiting only time-limited distress and others qualifying for posttraumatic stress disorder diagnosis (PTSD). Furthermore, whereas most PTSD patients mainly display fear-based symptoms, a minority of patients display a co-morbid anhedonic phenotype. We employed an individual profiling approach to model these intriguing facets of the psychiatric condition in underwater-trauma exposed rats. Based on long-term assessments of anxiety-like and anhedonic behaviors, our analysis uncovered three separate phenotypes of stress response; an anxious, fear-based (38%), a co-morbid, fear-anhedonic (15%), and an exposed-unaffected group (47%). Immunohistochemical assessments for cellular activation (c-Fos) and activation of inhibition (c-Fos+GAD67) revealed a differential involvement of limbic regions and distinct co-activity patterns for each of these phenotypes, validating the behavioral categorization. In accordance with recent neurocognitive hypotheses for posttraumatic depression, we show that enhanced pretrauma anxiety predicts the progression of posttraumatic anhedonia only in the fear-anhedonic phenotype.

5-HT receptor-mediated modulation of granule cell inhibition after juvenile stress recovers after a second exposure to adult stress.

Aversive experiences in early life are thought to dispose to psychopathologies such as mood or anxiety disorders. In a two-hit stress model, we assessed the effects of juvenile and/or adult stress on the 5-HT-mediated modulation of synaptic inhibition of ventral dentate gyrus granule cells. Combined but not single stress exposure led to a significant reduction in activity and increased anxiety-like behavior. Similarly, the 5-HT1A receptor-mediated inhibition of evoked inhibitory postsynaptic currents (IPSCs) of granule cells was only reduced in single stress exposed animals. This was also true for the number of granule cells responding with a 5-HT3 receptor-dependent burst of miniature IPSCs. 5-HT3 receptors are expressed on cholecystokinin (CCK)+ basket cells in the hippocampus. In fact, we observed a reduction of steady-state mRNA levels of CCK+ basket cell markers after single juvenile or adult stress and partial recovery after combined stress, thus matching the electrophysiological findings. Adaptive changes in 5-HT-mediated modulation of synaptic inhibition and CCK+ basket cells in the DG may help to maintain normal levels of anxiety after single juvenile or adult stress exposure, as indicated by the increased anxiety that accompanies the loss of this regulation upon combined stress.

A rat model of pre-puberty (juvenile) stress-induced predisposition to stress-related disorders: sex similarities and sex differences in effects and symptoms.

OBJECTIVES: This study assessed the interactive effect of two risk factors: "Juvenile stress" and sex in the long-term consequences of "Juvenile stress" in male and female rats. METHODS: Rats were exposed to "Juvenile stress" and to additional stress in adulthood. Measurements of anxiety and depressive-like behaviours were assessed in relation to each stress exposure and "Sex-specific" sets of criteria in order to characterize individual profiles of altered behaviours. RESULTS: While both male and female rats were affected by exposure to "Juvenile stress", sex difference were evident in saccharine preference, coping with the stressful challenge of the two-way shuttle avoidance task, and on "Adult stress" induced changes in saccharine preference. "Profiling" altered behaviours revealed sex differences also in the prevalence of rats exhibiting different categories of "Affected" behaviours, indicating that female rats are more susceptible to the long-term effects of "Juvenile stress" and to the immediate effects of "Adulthood stress". Additionally, the prevalence of "Affected" animals among "Juvenile+ Adulthood stress" was similar, yet the profile of altered behaviours was significantly different. CONCLUSIONS: The "Behavioural Profiling" approach presented here is of importance to understanding gender differences in the aetiology of predisposition to stress-related disorders, and of gender symptomatology differences in stress-related disorders.

Stress-induced metaplasticity: from synapses to behavior.

Synaptic plasticity, specifically long-term potentiation and long-term depression, is thought to be the underlying cellular mechanism for learning and memory processes in the brain. About two decades ago a new concept was introduced, namely metaplasticity, which comprises changes that modify the properties of synaptic plasticity due to a priming or preconditioning event. While metaplasticity was initially defined and studied predominantly on a synaptic and cellular level, it soon became apparent that the term could also be very useful to describe plasticity changes on a more global level, including environmental stressors as priming events and altered behavior as outcome measures. We consider here whether it is helpful to conceptualize these latter effects as "behavioral metaplasticity", and in which sense this view fits into the original concept of metaplasticity. By integrating the literature on environmental effects on plasticity, especially stress, plus developmental aspects as well as genetic and epigenetic modifications, we shape the framework in which the term "behavioral metaplasticity" should be considered and discuss research directions that can help to unravel the mechanisms involved in both synaptic and behavioral metaplasticity.

Age and sex-dependent differences in activity, plasticity and response to stress in the dentate gyrus.

In the last decade, early-onset of affective illness has been recognized as a major public health problem. However, clinical studies indicate that although children experience the symptoms of anxiety and depression in much the same way as adults, they display and react to those symptoms differently (Bostic et al., 2005). Recently, we have demonstrated that similar differences in symptoms are found also between adult and juvenile rats (Jacobson-Pick and Richter-Levin, 2010). Especially the hippocampus is believed to be vulnerable to stress-related illness, as this brain region has a high density of corticoid receptors. The hippocampus is known to finalize its development, and particularly that of GABA-related functions, into adolescence (Bergmann et al., 1991; Harris et al., 1992; Nurse and Lacaille, 1999; Lopez-Tellez et al., 2004; Jacobson-Pick et al., 2008) and may thus be differentially sensitive to environmental challenges in childhood and in adulthood. In this study we explored the differences in activity and plasticity of the dentate gyrus between pre-pubertal and adult rats in vivo. Furthermore, we have examined the impact of exposure to stress either during pre-puberty or in adulthood on dentate gyrus electrophysiology. In both male and female rats, marked differences were found for intrinsic excitability and local circuit activity between pre-pubertal and adult animals. Exposure to forced swim stress resulted in significant alterations of dentate gyrus activity and plasticity in male rats with differences between adult and pre-pubertal animals. Stress had far less impact on females' dentate electrophysiology. The results are in agreement with the differences in behavioral response to stress between pre-pubertal and adult rats, and with reported differences for the sensitivity of male and female rats in performing hippocampus-dependent tasks under stress, such as the active avoidance task.

Priming stimulation of basal but not lateral amygdala affects long-term potentiation in the rat dentate gyrus in vivo.

The amygdaloid complex, or amygdala, has been implicated in assigning emotional significance to sensory information and producing appropriate behavioral responses to external stimuli. The lateral and basal nuclei (lateral and basal amygdala), which are termed together as basolateral amygdala, play a critical role in emotional and motivational learning and memory. It has been established that the basolateral amygdala activation by behavioral manipulations or direct electrical stimulation can modulate hippocampal long-term potentiation (LTP), a putative cellular mechanism of memory. However, the specific functional role of each subnucleus in the modulation of hippocampal LTP has not been studied yet, even though studies have shown cytoarchitectural differences between the basal and lateral amygdala and differences in the connections of each one of them to other brain areas. In this study we have tested the effects of lateral or basal amygdala pre-stimulation on hippocampal dentate gyrus LTP, induced by theta burst stimulation of the perforant path, in anesthetized rats. We found that while priming stimulation of the lateral amygdala did not affect LTP of the dentate gyrus, priming stimulation of the basal amygdala enhanced the LTP response when the priming stimulation was relatively weak, but impaired it when it was relatively strong. These results show that the basal and lateral nuclei of the amygdala, which have been already shown to differ in their anatomy and connectivity, may also have different functional roles. These findings raise the possibility that the lateral and basal amygdala differentially modulate memory processes in the hippocampus under emotional and motivational situations.

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.

Post-weaning to pre-pubertal ('juvenile') stress: a model of induced predisposition to stress-related disorders.

Human studies suggest that childhood trauma predisposes individuals to develop stress-related disorders such as depression and post-traumatic stress disorder (PTSD). Recent years have witnessed growing interest in effectively modeling in animals the long-term effects of childhood emotional trauma on stress responses in adulthood. Most studies concerned with the impact of early-life stress on subsequent stress responses in adulthood in rodents have focused on the post-natal pre-weaning period. However, psychiatric studies often refer to human childhood rather than infancy when investigating the patients' traumatic history of stress-related psychopathologies. In accordance with that, we have examined the consequences of stress exposure at a later early-life period, the post-weaning, pre-puberty (juvenile) period, which holds greater resemblance to human childhood. This review summarizes a series of studies examining the impact of exposure of rats to stressors during 'juvenility' ('juvenile stress') on the ability of these animals to cope with stress later in life. Exposure to relatively brief but significant stress experience during juvenility was found to impair the ability of animals to cope with stressful challenges in adulthood. These behavioral manifestations were associated with lasting alterations in limbic system brain regions of neuromodulatory pathways, such as alterations in the expression of cell adhesion molecules, GABAergic system functioning and alterations in levels of circulating corticosterone. Importantly, these studies have also demonstrated considerable individual and sex differences, which call for the development of adequate analysis approaches. The juvenile stress model combined with characterization of individual profiles is presented as a useful model to study in rodents different facets of stress-related disorders and neural mechanisms of vulnerability and resilience to stress.

Stress revisited: a critical evaluation of the stress concept.

With the steadily increasing number of publications in the field of stress research it has become evident that the conventional usage of the stress concept bears considerable problems. The use of the term 'stress' to conditions ranging from even the mildest challenging stimulation to severely aversive conditions, is in our view inappropriate. Review of the literature reveals that the physiological 'stress' response to appetitive, rewarding stimuli that are often not considered to be stressors can be as large as the response to negative stimuli. Analysis of the physiological response during exercise supports the view that the magnitude of the neuroendocrine response reflects the metabolic and physiological demands required for behavioural activity. We propose that the term 'stress' should be restricted to conditions where an environmental demand exceeds the natural regulatory capacity of an organism, in particular situations that include unpredictability and uncontrollability. Physiologically, stress seems to be characterized by either the absence of an anticipatory response (unpredictable) or a reduced recovery (uncontrollable) of the neuroendocrine reaction. The consequences of this restricted definition for stress research and the interpretation of results in terms of the adaptive and/or maladaptive nature of the response are discussed.

"Juvenile stress" alters maturation-related changes in expression of the neural cell adhesion molecule L1 in the limbic system: relevance for stress-related psychopathologies.

L1 is critically involved in neural development and maturation, activity-dependent synaptic plasticity, and learning processes. Among adult rats, chronic stress protocols that affect L1 functioning also induce impaired cognitive and neural functioning and heightened anxiety reminiscent of stress-induced mood and anxiety disorders. Epidemiological studies indicate that childhood trauma is related predominantly to higher rates of both mood and anxiety disorders in adulthood and is associated with altered limbic system functioning. Exposing rats to stress during the juvenile period ("juvenile stress") has comparable effects and was suggested as a model of induced predisposition for these disorders. This study examined the effects of juvenile stress on rats aversive learning and on L1 expression soon after exposure and in adulthood, both following additional exposure to acute stress and in its absence. Adult juvenile-stressed rats exhibited enhanced cued fear conditioning, reduced novel-setting exploration, and impaired avoidance learning. Furthermore, juvenile stress increased L1 expression in the BLA, CA1, DG, and EC both soon after the stressful experience and during adulthood. It appears that juvenile stress affects the normative maturational decrease in L1 expression. The results support previous indications that juvenile stress alters the maturation of the limbic system and further support a role for L1 regulation in the mechanisms that underlie the predisposition to exhibit mood and/or anxiety disorders in adulthood. Furthermore, the findings support the "network hypothesis," which postulates that information-processing problems within relevant neural networks might underlie stress-induced mood and anxiety disorders.

Amygdala modulation of memory-related processes in the hippocampus: potential relevance to PTSD.

A key assumption in the study of stress-induced cognitive and neurobiological modifications is that alterations in hippocampal functioning after stress are due to an excessive activity exerted by the amygdala on the hippocampus. Research so far focused on stress-induced impairment of hippocampal plasticity and memory but an exposure to stress may simultaneously also result in strong emotional memories. In fact, under normal conditions emotionally charged events are better remembered compared with neutral ones. Results indicate that under these conditions there is an increase in activity within the amygdala that may lead to memory of a different quality. Studying the way emotionality activates the amygdala and the functional impact of this activation we found that the amygdala modulates memory-related processes in other brain areas, such as the hippocampus. However, this modulation is complex, involving both enhancing and suppressing effects, depending on the way the amygdala is activated and the hippocampal subregion examined. The current review summarizes our findings and attempts to put them in context with the impact of an exposure to a traumatic experience, in which there is a mixture of a strong memory of some aspects of the experience but impaired memory of other aspects of that experience. Toward that end, we have recently developed an animal model for the induction of predisposition to stress-related disorders, focusing on the consequences of exposure to stressors during juvenility on the ability to cope with stress in adulthood. Exposing juvenile-stressed rats to an additional stressful challenge in adulthood revealed their impairment to cope with stress and resulted in significant elevation of the amygdala. Interestingly, and similar to our electrophysiological findings, differential effects were observed between the impact of the emotional challenge on CA1 and dentate gyrus subregions of the hippocampus. Taken together, the results indicate that long-term alterations within the amygdala contribute to stress-related mnemonic symptoms and suggest that elucidating further these intra-amygdala alterations and their effects on modulating other brain regions is likely to be beneficial for the development of novel approaches to treat stress-related disorders.

Simultaneous induction of long-term potentiation in the hippocampus and the amygdala by entorhinal cortex activation: mechanistic and temporal profiles.

The medial temporal lobe, including the entorhinal cortex, the amygdala and the hippocampus, has an important role in learning and memory, and its circuits exhibit synaptic plasticity (long-term potentiation [LTP]). The entorhinal cortex is positioned to exert a potent influence on the amygdala and the hippocampus given its extensive monosynaptic projections to both areas. We therefore studied the effects of activation of the entorhinal cortex with simultaneous recording of LTP in the hippocampus and amygdala in the anesthetized rat. theta Burst stimulation of the lateral entorhinal cortex induced LTP simultaneously in the basal amygdaloid nucleus and in the dentate gyrus. However, the mechanisms involved in the induction of LTP in the two areas differed. The N-methyl-D-aspartate receptor antagonist 3-[(+/-)-2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid delivered 1 h before LTP induction (10 mg/kg, i.p.), blocked LTP in the dentate gyrus but not in the amygdala. In addition we found that the basal amygdala as well as the dentate gyrus sustained late-phase LTP (10 h) which may participate in memory encoding and/or modulation processes. Overall, the results suggest a coordinating role for the entorhinal cortex by simultaneously modulating activity and plasticity in these structures, albeit through different mechanisms. Interactive encoding of this sort is believed to endow memories with a different, more integrative, quality than when either pathway is activated alone.

LTP but not seizure is associated with up-regulation of AKAP-150.

We have used differential display to profile and compare the mRNAs expressed in the hippocampus of freely moving animals after the induction of long-term potentiation (LTP) at the perforant path-dentate gyrus synapse with control rats receiving low-frequency stimulation. We have combined this with in situ hybridization and have identified A-kinase anchoring protein of 150 kDa (AKAP-150) as a gene selectively up-regulated during the maintenance phase of LTP. AKAP-150 mRNA has a biphasic modulation in the dentate gyrus following the induction of LTP. The expression of AKAP-150 was 29% lower than stimulated controls 1 h after the induction of LTP. Its expression was enhanced 3 (50%), 6 (239%) and 12 h (210%) after induction, returning to control levels by 24 h postinduction. The NMDA receptor antagonist CPP blocked the tetanus-induced modulation of AKAP-150 expression. Interestingly, strong generalized stimulation produced by electroconvulsive shock did not increase the expression of AKAP-150. This implies that the AKAP-150 harbours a novel property of selective responsiveness to the stimulation patterns that trigger NMDA-dependent LTP in vivo. Its selective up-regulation during LTP and its identified functions as a scaffold for protein kinase A, protein kinase C, calmodulin, calcineurin and ionotropic glutamate receptors suggest that AKAP-150 encodes is an important effector protein in the expression of late LTP.

Local circuit plasticity in the rat dentate gyrus: characterization and aging-related impairment.

We have used frequency-dependent inhibition, a form of short-term plasticity mediated by the activation of inhibitory interneurons, to characterize in vivo alterations in local circuit activity and plasticity in the dentate gyrus of the anesthetized rat. The application of the GABA-A receptor blocker, bicuculline, induced a transient reduction in frequency-dependent inhibition, indicating that this form of local circuit activity is GABA-mediated. Delivering theta burst stimulation to the perforant pathway of the hippocampus induced long-term potentiation of the population excitatory post-synaptic potential, reflecting the potentiation of the perforant path-dentate gyrus granule cell synapses. Concomitantly, theta burst stimulation caused a lasting reduction in frequency-dependent inhibition. In aged rats, long-term potentiation could be induced to the same level as in young rats, but while in young rats frequency-dependent inhibition was concomitantly reduced, frequency-dependent inhibition in the old rats did not show this form of plasticity. Our results indicate that theta burst stimulation induces a form of local circuit plasticity independently of its known capacity to induce synaptic plasticity, and that this form of local circuit plasticity is compromised in aging. Based on these results we propose a potential role for plasticity at the level of the local circuit in learning and memory.

Acute and repeated swim stress effects on peripheral benzodiazepine receptors in the rat hippocampus, adrenal, and kidney.

Peripheral benzodiazepine receptor (PBR) density has been found to be sensitive to stress. We set out to compare the influences of acute and repeated swim stress on behavior and PBR density. Following acute and repeated swim stress, rats were tested in an elevated plus-maze and an open-field test for anxiety levels, and tissues were collected from the adrenal gland, kidney, and hippocampus for measurements of PBR density. The acute rather than the repeated stress led to robust alterations in PBR density. The largest reduction in hippocampal and adrenal gland PBR density was found one hour after acute stress. In the hippocampus, acute stress caused a biphasic change in PBR density: a robust reduction in PBR density one hour after the acute stress and a distinct elevation in PBR density at 24 hours, while 72 hours after stress the elevation in PBR density appeared to be reduced.

A gradient of plasticity in the amygdala revealed by cortical and subcortical stimulation, in vivo.

Projections to the amygdala from various cortical and subcortical areas terminate in different nuclei. In the present study we examined long-term potentiation of synaptic transmission in the lateral or the basal amygdaloid nuclei by theta burst stimulation of thalamic vs. cortical sensory projections in the anesthetized rat. Although both the medial geniculate nucleus and the dorsal perirhinal cortex have direct projections to lateral nucleus, only the thalamic stimulation induced long-term potentiation of field potentials recorded in the lateral nucleus. In contrast, cortical (ventral perirhinal cortex) but not thalamic stimulation induced long-term potentiation in the basal nucleus. Since the thalamic pathway is believed to process simple/unimodal stimulus features, and the perirhinal cortex complex/polymodal sensory representations, the dissociation of long-term potentiation in lateral and basal nuclei suggests that the basal nucleus may serve as an amygdaloid sensory interface for complex stimulus information similar to the role of the lateral nucleus in relation to relatively simple representations. Thus plasticity of simple and complex representations may involve different amygdala inputs and circuits.