Physiological Dissociation in Hippocampal Subregions in Response to Amygdala Stimulation

Epinephrine injected into the basolateral amygdala affects anxiety-like behavior and memory performance in stressed rats

The amygdala is known to mediate in moderating the impacts of emotional arousal and stress on memory. According to a growing body of research, the basolateral amygdala (BLA) is an important locus for integrating neuromodulator influences coordinating the retrieval of different types of memory and anxiety. This study aimed to investigate how the epinephrine in the BLA affects hippocampal fear memory, anxiety, and plasticity in control and stressed rats. For four days, male Wistar rats were exposed to electrical foot-shock stress. Animals received bilateral micro-injections of either vehicle or epinephrine (1 µg/side) into the BLA over four days (5 min before foot-shock stress). Behavioral characteristics (fear memory and anxiety-like behavior), histological features and electrophysiological parameters were investigated. Epinephrine injection into BLA resulted in a considerable impairment of fear memory in stressed rats. On the other hand, epinephrine effectively affected fear memory in control rats. Under stress conditions, epinephrine in the BLA is thought to increase anxiety-like behaviors. Treatment with epinephrine significantly increases the slope of fEPSP in the CA1 region of the hippocampus in the control and stress rats. In different groups, foot-shock stress had no effect on the apical and basal dendritic length in the CA1 region of the hippocampus. These results indicate that activating adrenergic receptors diminish fear memory and anxiety-like behaviors in the foot-shock stress, which this impact is independent of CA1 long-term potentiation induction.

Alcohol-induced damage to the fimbria/fornix reduces hippocampal-prefrontal cortex connection during early abstinence

INTRODUCTION

Alcohol dependence is characterized by a gradual reduction in cognitive control and inflexibility to contingency changes. The neuroadaptations underlying this aberrant behavior are poorly understood. Using an animal model of alcohol use disorders (AUD) and complementing diffusion-weighted (dw)-MRI with quantitative immunohistochemistry and electrophysiological recordings, we provide causal evidence that chronic intermittent alcohol exposure affects the microstructural integrity of the fimbria/fornix, decreasing myelin basic protein content, and reducing the effective communication from the hippocampus (HC) to the prefrontal cortex (PFC). Using a simple quantitative neural network model, we show how disturbed HC-PFC communication may impede the extinction of maladaptive memories, decreasing flexibility. Finally, combining dw-MRI and psychometric data in AUD patients, we discovered an association between the magnitude of microstructural alteration in the fimbria/fornix and the reduction in cognitive flexibility. Overall, these findings highlight the vulnerability of the fimbria/fornix microstructure in AUD and its potential contribution to alcohol pathophysiology. Fimbria vulnerability to alcohol underlies hippocampal-prefrontal cortex dysfunction and correlates with cognitive impairment.

Date palm spathe extract reverses chronic stress-induced changes in dendritic arborization in the amygdala and impairment of hippocampal long-term potentiation

Chronic restraint stress induces anxiety-like behaviors and emotional abnormalities via an alteration of synaptic remodeling in the amygdala and the hippocampus. Given that the date palm spathe has been shown to have neuroprotective effects on different experimental models, this study aimed to address whether the date palm spathe extract (hydroalcoholic extract of date palm spathe [HEDPP]) can reduce chronic restraint stress-induced behavioral, electrophysiological, and morphological changes in the rat model. Thirty-two male Wistar rats (weight 200-220 g) were randomly divided into control, stress, HEDPP, and stress + HEDPP for 14 days. Animals were submitted to restraint stress for 2 h per day for 14 consecutive days. The animals of the HEDPP and stress + HEDPP groups were supplemented with HEDPP (125 mg/kg) during these 14 days, 30 min before being placed in the restraint stress tube. We used passive avoidance, open-field test, and field potential recording to assess emotional memory, anxiety-like behavioral and long-term potentiation in the CA1 region of the hippocampus, respectively. Moreover, Golgi-Cox staining was used to investigate the amygdala neuron dendritic arborization. Results showed that stress induction was associated with behavioral changes (anxiety-like behavioral and emotional memory impairment), and the administration of HEDPP effectively normalized these deficits. HEDPP remarkably amplified the slope and amplitude of mean-field excitatory postsynaptic potentials (fEPSPs) in the CA1 area of the hippocampus in stressed rats. Chronic restraint stress significantly decreased the dendritic arborization in the central and basolateral nucleus of the amygdala neuron. HEDPP suppressed this stress effect in the central nucleus of the amygdala. Our findings indicated that HEDPP administration improves stress-induced learning impairment and memory and anxiety-like behaviors by preventing adverse effects on synaptic plasticity in the hippocampus and amygdala.

Sex- and exposure age-dependent effects of adolescent stress on ventral tegmental area dopamine system and its afferent regulators

Adolescent stress is a risk factor for schizophrenia. Emerging evidence suggests that age-dependent sensitive windows for childhood trauma are associated more strongly with adult psychosis, but the neurobiological basis and potential sex differences are unknown.Using in vivo electrophysiology and immunohistology in rats, we systematically compared the effects of two age-defined adolescent stress paradigms, prepubertal (postnatal day [PD] 21-30; PreP-S) and postpubertal (PD41-50; PostP-S) foot-shock and restraint combined stress, on ventral tegmental area (VTA) dopaminergic activity, pyramidal neuron activity in the ventral hippocampus (vHipp) and the basolateral amygdala (BLA), corticoamygdalar functional inhibitory control, and vHipp and BLA parvalbumin interneuron (PVI) impairments. These endpoints were selected based on their well-documented roles in the pathophysiology of psychosis.Overall, we found distinct sex- and exposure age-dependent stress vulnerability. Specifically, while males were selectively vulnerable to PreP-S-induced adult VTA dopamine neuron and vHipp hyperactivities, females were selectively vulnerable to PostP-S. These male selective PreP-S effects were correlated with stress-induced aberrant persistent BLA hyperactivity, dysfunctional prefrontal inhibitory control of BLA neurons, and vHipp/BLA PVI impairments. In contrast, female PostP-S only produced vHipp PVI impairments in adults, with the BLA structure and functions largely unaffected.Our results indicated distinct adolescent-sensitive periods during which stress can sex-dependently confer maximal risks to corticolimbic systems to drive dopamine hyperactivity, which provide critical insights into the neurobiological basis for sex-biased stress-related psychopathologies emphasizing but not limited to schizophrenia. Furthermore, our work also provides a framework for future translational research on age-sensitive targeted interventions.

Distinct contributions of ventral CA1/amygdala co-activation to the induction and maintenance of synaptic plasticity

The amygdala is known to modulate hippocampal synaptic plasticity. One role could be an immediate effect of basolateral amygdala (BLA) in priming synaptic plasticity in the hippocampus. Another role could be through associative synaptic co-operation and competition that triggers events involved in the maintenance of synaptic potentiation. We present evidence that the timing and activity level of BLA stimulation are important factors for the induction and maintenance of long-term potentiation (LTP) in ventral hippocampal area CA1. A 100 Hz BLA co-stimulation facilitated the induction of LTP, whereas 200 Hz co-stimulation attenuated induction. A 100 Hz BLA co-stimulation also caused enhanced persistence, sufficient to prevent synaptic competition. This maintenance effect is likely through translational mechanisms, as mRNA expression of primary response genes was unaffected, whereas protein level of plasticity-related products was increased. Further understanding of the neural mechanisms of amygdala modulation on hippocampus could provide insights into the mechanisms of emotional disorders.

Amygdala-hippocampal interactions in synaptic plasticity and memory formation

Memories of emotionally arousing events tend to endure longer than other memories. This review compiles findings from several decades of research investigating the role of the amygdala in modulating memories of emotional experiences. Episodic memory is a kind of declarative memory that depends upon the hippocampus, and studies suggest that the basolateral complex of the amygdala (BLA) modulates episodic memory consolidation through interactions with the hippocampus. Although many studies in rodents and imaging studies in humans indicate that the amygdala modulates memory consolidation and plasticity processes in the hippocampus, the anatomical pathways through which the amygdala affects hippocampal regions that are important for episodic memories were unresolved until recent optogenetic advances made it possible to visualize and manipulate specific BLA efferent pathways during memory consolidation. Findings indicate that the BLA influences hippocampal-dependent memories, as well as synaptic plasticity, histone modifications, gene expression, and translation of synaptic plasticity associated proteins in the hippocampus. More recent findings from optogenetic studies suggest that the BLA modulates spatial memory via projections to the medial entorhinal cortex, and that the frequency of activity in this pathway is a critical element of this modulation.

Stress enhances hippocampal neuronal synchrony and alters ripple-spike interaction

Adverse effects of chronic stress include anxiety, depression, and memory deficits. Some of these stress-induced behavioural deficits are mediated by impaired hippocampal function. Much of our current understanding about how stress affects the hippocampus has been derived from post-mortem analyses of brain slices at fixed time points. Consequently, neural signatures of an ongoing stressful experiences in the intact brain of awake animals and their links to later hippocampal dysfunction remain poorly understood. Further, no information is available on the impact of stress on sharp-wave ripples (SPW-Rs), high frequency oscillation transients crucial for memory consolidation. Here, we used in vivo tetrode recordings to analyze the dynamic impact of 10 days of immobilization stress on neural activity in area CA1 of mice. While there was a net decrease in pyramidal cell activity in stressed animals, a greater fraction of CA1 spikes occurred specifically during sharp-wave ripples, resulting in an increase in neuronal synchrony. After repeated stress some of these alterations were visible during rest even in the absence of stress. These findings offer new insights into stress-induced changes in ripple-spike interactions and mechanisms through which chronic stress may interfere with subsequent information processing.

Region-specific involvement of interneuron subpopulations in trauma-related pathology and resilience

Only a minority of trauma-exposed individuals develops Posttraumatic stress disorder (PTSD) and active processes may support trauma resilience. Individual behavioral profiling allows investigating neurobiological alterations related to resilience or pathology in animal models of PTSD and is utilized here to examine the activation of different interneuron subpopulations of the dentate gyrus-amygdala system associated with trauma resilience or pathology. To model PTSD, rats were exposed to juvenile stress combined with underwater trauma (UWT) in adulthood. Four weeks later, individual anxiety levels were assessed in the elevated plus maze test for classifying rats as highly anxious 'affected' vs. 'non-affected', i.e. behaving as control animals. Analyzing the activation of specific interneuron subpopulations in the dorsal and ventral dentate gyrus (DG), the basolateral (BLA) and central amygdala by immunohistochemical double-labeling for cFos and different interneuron markers, revealed an increased activation of cholecystokinin (CCK)-positive interneurons in the ventral DG, together with increased activation of parvalbumin- and CCK-positive interneurons in the BLA of affected trauma-exposed rats. By contrast, increased activation of neuropeptide Y (NPY)-positive interneurons was observed in the dorsal DG of trauma-exposed, but non-affected rats. To test for a direct contribution of NPY in the dorsal DG to trauma resilience, a local shRNA-mediated knock down was performed after UWT. Such a treatment significantly reduced the prevalence of resilient animals. Our results suggest that distinct interneuron populations are associated with resilience or pathology in PTSD with high regional specificity. NPY within the dorsal DG was found to significantly contribute to trauma resilience.

GABAergic Transmission in the Basolateral Amygdala Differentially Modulates Plasticity in the Dentate Gyrus and the CA1 Areas

The term "metaplasticity" is used to describe changes in synaptic plasticity sensitivity following an electrical, biochemical, or behavioral priming stimulus. For example, priming the basolateral amygdala (BLA) enhances long-term potentiation (LTP) in the dentate gyrus (DG) but decreases LTP in the CA1. However, the mechanisms underlying these metaplastic effects are only partly understood. Here, we examined whether the mechanism underlying these effects of BLA priming involves intra-BLA GABAergic neurotransmission. Low doses of muscimol, a GABA_A receptor (GABA_AR) agonist, were microinfused into the rat BLA before or after BLA priming. Our findings show that BLA GABA_AR activation via muscimol mimicked the previously reported effects of electrical BLA priming on LTP in the perforant path and the ventral hippocampal commissure-CA1 pathways, decreasing CA1 LTP and increasing DG LTP. Furthermore, muscimol application before or after tetanic stimulation of the ventral hippocampal commissure-CA1 pathways attenuated the BLA priming-induced decrease in CA1 LTP. In contrast, muscimol application after tetanic stimulation of the perforant path attenuated the BLA priming-induced increase in DG LTP. The data indicate that GABA_AR activation mediates metaplastic effects of the BLA on plasticity in the CA1 and the DG, but that the same GABA_AR activation induces an intra-BLA form of metaplasticity, which alters the way BLA priming may modulate plasticity in other brain regions. These results emphasize the need for developing a dynamic model of BLA modulation of plasticity, a model that may better capture processes underlying memory alterations associated with emotional arousing or stressful events.

Optogenetic Stimulation of the Basolateral Amygdala Increased Theta-Modulated Gamma Oscillations in the Hippocampus

The amygdala can modulate declarative memory. For example, previous research in rats and humans showed that brief electrical stimulation to the basolateral complex of the amygdala (BLA) prioritized specific objects to be consolidated into long term memory in the absence of emotional stimuli and without awareness of stimulation. The capacity of the BLA to influence memory depends on its substantial projections to many other brain regions, including the hippocampus. Nevertheless, how activation of the BLA influences ongoing neuronal activity in other regions is poorly understood. The current study used optogenetic stimulation of putative glutamatergic neurons in the BLA of freely exploring rats to determine whether brief activation of the BLA could increase in the hippocampus gamma oscillations for which the amplitude was modulated by the phase of theta oscillations, an oscillatory state previously reported to correlate with good memory. BLA neurons were stimulated in 1-s bouts with pulse frequencies that included the theta range (8 Hz), the gamma range (50 Hz), or a combination of both ranges (eight 50-Hz bursts). Local field potentials were recorded in the BLA and in the pyramidal layer of CA1 in the intermediate hippocampus. A key question was whether BLA stimulation at either theta or gamma frequencies could combine with ongoing hippocampal oscillations to result in theta-modulated gamma or whether BLA stimulation that included both theta and gamma frequencies would be necessary to increase theta–gamma comodulation in the hippocampus. All stimulation conditions elicited robust responses in BLA and CA1, but theta-modulated gamma oscillations increased in CA1 only when BLA stimulation included both theta and gamma frequencies. Longer bouts (5-s) of BLA stimulation resulted in hippocampal activity that evolved away from the initial oscillatory states and toward those characterized more by prominent low-frequency oscillations. The current results indicated that one mechanism by which the amygdala might influence declarative memory is by eliciting neuronal oscillatory states in the hippocampus that benefit memory.

Cellular and Molecular Differences Between Area CA1 and the Dentate Gyrus of the Hippocampus

A distinct feature of the hippocampus of the brain is its unidirectional tri-synaptic pathway originating from the entorhinal cortex and projecting to the dentate gyrus (DG) then to area CA3 and subsequently, area CA1 of the Ammon's horn. Each of these areas of the hippocampus has its own cellular structure and distinctive function. The principal neurons in these areas are granule cells in the DG and pyramidal cells in the Ammon's horn's CA1 and CA3 areas with a vast network of interneurons. This review discusses the fundamental differences between the CA1 and DG areas regarding cell morphology, synaptic plasticity, signaling molecules, ability for neurogenesis, vulnerability to various insults and pathologies, and response to pharmacological agents.

Extrafield Activity Shifts the Place Field Center of Mass to Encode Aversive Experience

Hippocampal place cells are known to have a key role in encoding spatial information. Aversive stimuli, such as predator odor, evoke place field remapping and a change in preferred firing locations. However, it remains unclear how place cells use positive or negative experiences to remap. We investigated whether CA1 place cells, recorded from behaving rats, remap randomly or whether their reconfiguration depends on the perceived location of the aversive stimulus. Exposure to trimethylthiazoline (TMT; an innately aversive odor), increased the amplitude of hippocampal β oscillations in the two arms of the maze in which TMT exposure occurred. We found that a population of place cells with fields located outside the TMT arms increased their activity (extrafield spiking) in the TMT arms during the aversive episodes. Moreover, in the subsequent post-TMT recording, these cells exhibited a significant shift in their center of mass (COM) towards the TMT arms. The induction of extrafield plasticity was mediated by the basolateral amygdala complex (BLA). Photostimulation of the BLA triggered aversive behavior, synchronized hippocampal local field oscillations, and increased the extrafield spiking of the hippocampal place cells for the first 100 ms after light delivery. Optogenetic BLA activation triggered an increase in extrafield spiking activity that was correlated with the degree of place field plasticity. Furthermore, BLA-mediated increase of the extrafield activity predicts the degree of subsequent field plasticity. Our findings demonstrate that that the remapping of hippocampal place cells during aversive episodes is not random but it depends on the location of the aversive stimulus.

Influence of pharmacological and epigenetic factors to suppress neurotrophic factors and enhance neural plasticity in stress and mood disorders

Stress-induced major depression and mood disorders are characterized by behavioural abnormalities and psychiatric illness, leading to disability and immature mortality worldwide. Neurobiological mechanisms of stress and mood disorders are discussed considering recent findings, and challenges to enhance pharmacological effects of antidepressant, and mood stabilizers. Pharmacological enhancement of ketamine and scopolamine regulates depression at the molecular level, increasing synaptic plasticity in prefrontal regions. Blood-derived neurotrophic factors facilitate mood-deficit symptoms. Epigenetic factors maintain stress-resilience in hippocampal region. Regulation of neurotrophic factors blockades stress, and enhances neuronal survival though it paralyzes limbic regions. Molecular agents and neurotrophic factors also control behavioral and synaptic plasticity in addiction and stress disorders. Future research on neuronal dynamics and cellular actions can be directed to obtain the etiology of synaptic dysregulation in mood disorder and stress. For the first time, the current review contributes to the literature of synaptic plasticity representing the role of epigenetic mechanisms and glucocorticoid receptors to predict depression and anxiety in clinical conditions.

Neurobiological consequences of juvenile stress: A GABAergic perspective on risk and resilience

ALBRECHT, A., MÜLLER, I., ARDI, Z., ÇALIŞKAN, G., GRUBER, D., IVENS, S., SEGAL, M., BEHR, J., HEINEMANN, U., STORK, O., and RICHTER-LEVIN, G. Neurobiological consequences of juvenile stress: A GABAergic perspective on risk and resilience. NEUROSCI BIOBEHAV REV XXX-XXX, 2016.- Childhood adversity is among the most potent risk factors for developing mood and anxiety disorders later in life. Therefore, understanding how stress during childhood shapes and rewires the brain may optimize preventive and therapeutic strategies for these disorders. To this end, animal models of stress exposure in rodents during their post-weaning and pre-pubertal life phase have been developed. Such 'juvenile stress' has a long-lasting impact on mood and anxiety-like behavior and on stress coping in adulthood, accompanied by alterations of the GABAergic system within core regions for the stress processing such as the amygdala, prefrontal cortex and hippocampus. While many regionally diverse molecular and electrophysiological changes are observed, not all of them correlate with juvenile stress-induced behavioral disturbances. It rather seems that certain juvenile stress-induced alterations reflect the system's attempts to maintain homeostasis and thus promote stress resilience. Analysis tools such as individual behavioral profiling may allow the association of behavioral and neurobiological alterations more clearly and the dissection of alterations related to the pathology from those related to resilience.

Stress effects on the hippocampus: a critical review

Uncontrollable stress has been recognized to influence the hippocampus at various levels of analysis. Behaviorally, human and animal studies have found that stress generally impairs various hippocampal-dependent memory tasks. Neurally, animal studies have revealed that stress alters ensuing synaptic plasticity and firing properties of hippocampal neurons. Structurally, human and animal studies have shown that stress changes neuronal morphology, suppresses neuronal proliferation, and reduces hippocampal volume. Since the inception of stress research nearly 80 years ago, much focus has been on the varying levels of hypothalamic-pituitary-adrenal (HPA) axis neuroendocrine hormones, namely glucocorticoids, as mediators of the myriad stress effects on the hippocampus and as contributing factors to stress-associated psychopathologies such as post-traumatic stress disorder (PTSD). However, reports of glucocorticoid-produced alterations in hippocampal functioning vary widely across studies. This review provides a brief history of stress research, examines how the glucocorticoid hypothesis emerged and guides contemporary stress research, and considers alternative approaches to understanding the mechanisms underlying stress effects on hippocampal functioning.

Basolateral amygdala bidirectionally modulates stress-induced hippocampal learning and memory deficits through a p25/Cdk5-dependent pathway

Repeated stress has been suggested to underlie learning and memory deficits via the basolateral amygdala (BLA) and the hippocampus; however, the functional contribution of BLA inputs to the hippocampus and their molecular repercussions are not well understood. Here we show that repeated stress is accompanied by generation of the Cdk5 (cyclin-dependent kinase 5)-activator p25, up-regulation and phosphorylation of glucocorticoid receptors, increased HDAC2 expression, and reduced expression of memory-related genes in the hippocampus. A combination of optogenetic and pharmacosynthetic approaches shows that BLA activation is both necessary and sufficient for stress-associated molecular changes and memory impairments. Furthermore, we show that this effect relies on direct glutamatergic projections from the BLA to the dorsal hippocampus. Finally, we show that p25 generation is necessary for the stress-induced memory dysfunction. Taken together, our data provide a neural circuit model for stress-induced hippocampal memory deficits through BLA activity-dependent p25 generation.

Alterations of hippocampal place cells in foraging rats facing a "predatory" threat

Fear is an adaptive mechanism evolved to influence the primal decisions of foragers in "approach resource-avoid predator" conflicts. To survive and reproduce, animals must attain the basic needs (food, water, shelter, and mate) while avoiding the ultimate cost of predation. Consistent with this view, ecological studies have found that predatory threats cause animals to limit foraging to fewer places in their habitat and/or to restricted times. However, the neurophysiological basis through which animals alter their foraging boundaries when confronted with danger remains largely unknown. Here, we investigated place cells in the hippocampus, implicated in processing spatial information and memory, in male Long-Evans rats foraging for food under risky situations that would be common in nature. Specifically, place cells from dorsal cornu ammonis field 1 (CA1) were recorded while rats searched for food in a semi-naturalistic apparatus (consisting of a nest and a relatively large open area) before, during, and after encountering a "predatory" robot situated remotely from the nest. The looming robot induced remapping of place fields and increased the theta rhythm as the animals advanced toward the vicinity of threat, but not when they were around the safety of the nest. These neurophysiological effects on the hippocampus were prevented by lesioning of the amygdala. Based on these findings, we suggest that the amygdalar signaling of fear influences the stability of hippocampal place cells as a function of threat distance in rats foraging for food.

Adaptive emotional memory: the key hippocampal-amygdalar interaction

For centuries philosophical and clinical studies have emphasized a fundamental dichotomy between emotion and cognition, as, for instance, between behavioral/emotional memory and explicit/representative memory. However, the last few decades cognitive neuroscience have highlighted data indicating that emotion and cognition, as well as their underlying neural networks, are in fact in close interaction. First, it turns out that emotion can serve cognition, as exemplified by its critical contribution to decision-making or to the enhancement of episodic memory. Second, it is also observed that reciprocally cognitive processes as reasoning, conscious appraisal or explicit representation of events can modulate emotional responses, like promoting or reducing fear. Third, neurobiological data indicate that reciprocal amygdalar-hippocampal influences underlie such mutual regulation of emotion and cognition. While supporting this view, the present review discusses experimental data, obtained in rodents, indicating that the hippocampal and amygdalar systems not only regulate each other and their functional outcomes, but also qualify specific emotional memory representations through specific activations and interactions. Specifically, we review consistent behavioral, electrophysiological, pharmacological, biochemical and imaging data unveiling a direct contribution of both the amygdala and hippocampal-septal system to the identification of the predictor of a threat in different situations of fear conditioning. Our suggestion is that these two brain systems and their interplay determine the selection of relevant emotional stimuli, thereby contributing to the adaptive value of emotional memory. Hence, beyond the mutual quantitative regulation of these two brain systems described so far, we develop the idea that different activations of the hippocampus and amygdala, leading to specific configurations of neural activity, qualitatively impact the formation of emotional memory representations, thereby producing either adaptive or maladaptive fear memories.

Differential modulation of synaptic plasticity and local circuit activity in the dentate gyrus and CA1 regions of the rat hippocampus by corticosterone

Depending on its severity, timing and context, stress has been shown to have a differential regional effect on hippocampal synaptic plasticity. While the focus of attention in most recent studies is on excitatory synapses and generation, modifications of inhibitory synapses and local interneurons cannot be ignored. We have now examined the effects of corticosterone (CORT) on extrinsic afferent and local circuit plasticity of the perforant path on the dentate gyrus (DG) and the ventral hippocampal commissure on CA1. Local circuit activity was measured by responses to paired-pulse stimulation. Control rats expressed afferent long-term potentiation (LTP) and local circuit plasticity in both the DG and CA1. Administration of a high dosage of CORT-reduced paired-pulse inhibition and increased facilitation in DG but not in CA1, whereas administration of a moderate CORT dosage had no effect. Moderate CORT doses caused enhancement of LTP in the DG but not in CA1, while high CORT doses converted LTP to long-term depression in the CA1 but had no effect in the DG. CORT blocked theta burst stimulation-induced local circuit plasticity otherwise found in control DG. These findings suggest that elevation of the level of CORT results in a regionally differentiated physiological response. In addition, the results indicate that CORT affects aspects of local circuit activity and plasticity in the DG but less so in the CA1. It is possible that these differentiated alterations underlie some of the behavioral consequences and memory processes under stressful conditions.

The effects of a reminder of underwater trauma on behaviour and memory-related mechanisms in the rat dentate gyrus

Intrusive re-experiencing is a core symptom in post-traumatic stress disorder (PTSD), often triggered by contextual cues associated with the trauma. It is not yet clear if intrusive re-experiencing is only the result, or whether it may contribute to the establishment of PTSD following acute stress. This study aimed at examining the impact of an underwater trauma (UWT) reminder on anxiety-like behaviour and on neuronal activity and plasticity in the hippocampus and the amygdala. Sprague-Dawley rats were exposed to UWT and 24 h later were re-exposed to the context. The effects on behaviour, activation of the amygdala (BLA) and dentate gyrus (DG), and on long-term potentiation (LTP) and local circuit activity (frequency-dependent inhibition (FDI) and paired-pulse inhibition (PPI)) in the DG were assessed. The exposure to UWT by itself resulted in increased anxiety behaviour in the open field, together with increased PPI. Upon exposure to the UWT reminder, an additional increase in anxiety was also observed in the EPM and in FDI. Moreover, reminder exposure resulted in impaired DG LTP and a significant BLA extracellular-signal-regulated kinases (ERK) 2 activation. In conclusion, these observed effects of exposure to a trauma reminder, following the exposure to the initial trauma, might be associated with the progression of trauma-related pathologies and the development of related disorders.

Metaplastic regulation of the median raphe nucleus via serotonin 5-HT1A receptor on hippocampal synaptic plasticity is associated with gender-specific emotional expression in rats

Gender differences in psychiatric disorders are considered to be associated with the serotonergic (5-HTergic) system; however the underlying mechanisms have not been clearly elucidated. In this study, possible involvement of the median raphe nucleus (MRN)-hippocampus 5-HTergic system in gender-specific emotional regulation was investigated, focusing on synaptic plasticity in rats. A behavioral study using a contextual fear conditioning (CFC) paradigm showed that the females exhibited low anxiety-like behavior. Extracellular 5-HT levels in the hippocampus were increased by CFC only in the males. Long-term potentiation (LTP) in the hippocampal CA1 field was suppressed after CFC in the males, which was mimicked by the synaptic response to MRN electrical stimulation. In the MRN, 5-HT immunoreactive cells significantly increased in the females compared with those in the males. Pretreatment with the 5-HT1A receptor agonists tandospirone (10 mg/kg, i.p.) and 8-OH DPAT (3 mg/kg, i.p.) significantly suppressed LTP induction in the males. Synaptic responses to CFC and 5-HT1A receptor interventions were not observed in the females. These results suggest that the metaplastic 5-HTergic mechanism via 5-HT1A receptors in the MRN-hippocampus pathway is a key component for gender-specific emotional regulation and may be a cause of psychiatric disorders associated with vulnerability or resistance to emotional stress.

How the amygdala affects emotional memory by altering brain network properties

The amygdala has long been known to play a key role in supporting memory for emotionally arousing experiences. For example, classical fear conditioning depends on neural plasticity within this anterior medial temporal lobe region. Beneficial effects of emotional arousal on memory, however, are not restricted to simple associative learning. Our recollection of emotional experiences often includes rich representations of, e.g., spatiotemporal context, visceral states, and stimulus-response associations. Critically, such memory features are known to bear heavily on regions elsewhere in the brain. These observations led to the modulation account of amygdala function, which postulates that amygdala activation enhances memory consolidation by facilitating neural plasticity and information storage processes in its target regions. Rodent work in past decades has identified the most important brain regions and neurochemical processes involved in these modulatory actions, and neuropsychological and neuroimaging work in humans has produced a large body of convergent data. Importantly, recent methodological developments make it increasingly realistic to monitor neural interactions underlying such modulatory effects as they unfold. For instance, functional connectivity network modeling in humans has demonstrated how information exchanges between the amygdala and specific target regions occur within the context of large-scale neural network interactions. Furthermore, electrophysiological and optogenetic techniques in rodents are beginning to make it possible to quantify and even manipulate such interactions with millisecond precision. In this paper we will discuss that these developments will likely lead to an updated view of the amygdala as a critical nexus within large-scale networks supporting different aspects of memory processing for emotionally arousing experiences.

Amygdala activation and GABAergic gene expression in hippocampal sub-regions at the interplay of stress and spatial learning

Molecular processes in GABAergic local circuit neurons critically contribute to information processing in the hippocampus and to stress-induced activation of the amygdala. In the current study, we determined expression changes in GABA-related factors induced in subregions of the dorsal hippocampus as well as in the BLA of rats 5 h after spatial learning in a Morris water maze (MWM), using laser microdissection and quantitative real-time PCR. Spatial learning resulted in highly selective pattern of changes in hippocampal subregions: gene expression levels of neuropeptide Y (NPY) were reduced in the hilus of the dentate gyrus (DG), whereas somatostatin (SST) was increased in the stratum oriens (SO) of CA3. The GABA-synthesizing enzymes GAD65 and GAD67 as well as the neuropeptide cholecystokinin (CCK) were reduced in SO of CA1. In the BLA, expression of GAD65 and GAD67 were reduced compared to a handled Control group. These expression patterns were further compared to alterations in a group of rats that have been exposed to the water maze but were not provided with an invisible escape platform. In this Water Exposure group, no expression changes were observed in any of the hippocampal subregions, but a differential regulation of all selected target genes was evident in the BLA. These findings suggest that expression changes of GABAergic factors in the hippocampus are associated with spatial learning, while additional stress effects modulate expression alterations in the BLA. Indeed, while in both experimental groups plasma corticosterone (CORT) levels were enhanced, only Water Exposure stress activated the basolateral amygdala (BLA), as indicated by increased levels of phosphorylated ERK 1/2. Altered GABAergic function in the BLA may thus contribute to memory consolidation in the hippocampus, in relation to levels of stress and emotionality associated with the experience.

Different patterns of amygdala priming differentially affect dentate gyrus plasticity and corticosterone, but not CA1 plasticity

Stress-induced activation of the amygdala is involved in the modulation of memory processes in the hippocampus. However, stress effects on amygdala and memory remain complex. The activation of the basolateral amygdala (BLA) was found to modulate plasticity in other brain areas, including the hippocampus. We previously demonstrated a differential effect of BLA priming on long-term potentiation (LTP) in the CA1 and the dentate gyrus (DG). While BLA priming suppressed LTP in CA1, it was found to enhance it in the DG. However, since the amygdala itself is amenable to experience-induced plasticity it is thus conceivable that when activity within the amygdala is modified this will have impact on the way the amygdala modulates activity and plasticity in other brain areas. In the current study, we examined the effects of different patterns of BLA activation on the modulation of LTP in the DG and CA1, as well as on serum corticosterone (CORT). In CA1, BLA-priming impaired LTP induction as was reported before. In contrast, in the DG, varying BLA stimulation intensity and frequency resulted in differential effects on LTP, ranging from no effect to strong impairment or enhancement. Varying BLA stimulation patterns resulted in also differential alterations in Serum CORT, leading to higher CORT levels being positively correlated with LTP magnitude in DG but not in CA1. The results support the notion of a differential role for the DG in aspects of memory, and add to this view the possibility that DG-associated aspects of memory will be enhanced under more emotional or stressful conditions. It is interesting to think of BLA patterns of activation and the differential levels of circulating CORT as two arms of the emotional and stress response that attempt to synchronize brain activity to best meet the challenge. It is foreseeable to think of abnormal such synchronization under extreme conditions, which would lead to the development of maladaptive behavior.

Amygdalar stimulation produces alterations on firing properties of hippocampal place cells

Stress is a biologically ubiquitous factor that, when perceived uncontrollable by humans and animals, can have lingering adverse effects on brain and cognitive functions. We have previously reported that rats that experienced inescapable-unpredictable stress subsequently exhibited decreased stability of firing rates of place cells in the CA1 hippocampus, accompanied by impairments in CA1 long-term synaptic potentiation and spatial memory consolidation. Because the elevated level of glucocorticoid hormones and the heightened amygdalar activity have been implicated in the emergence of stress effects on the hippocampus, we investigated whether administration of corticosterone and electrical stimulation of the amygdala can produce stress-like alterations on hippocampal place cells. To do so, male Long-Evans rats chronically implanted with tetrodes in the hippocampus and stimulating electrodes in the amygdala were placed on a novel arena to forage for randomly dispersed food pellets while CA1 place cells were monitored across two recording sessions. Between sessions, animals received either corticosterone injection or amygdalar stimulation. We found that amygdalar stimulation reliably evoked distress behaviors and subsequently reduced the pixel-by-pixel correlation of place maps across sessions, while corticosterone administration did not. Also, the firing rates of place cells between preamygdalar and postamygdalar stimulation recording sessions were pronouncedly different, whereas those between precorticosterone and postcorticosterone injection recording sessions were not. These results suggest that the heightened amygdalar activity, but not the elevated level of corticosterone per se, reduces the stability of spatial representation in the hippocampus by altering the firing rates of place cells in a manner similar to behavioral stress.

Medial prefrontal cortex: multiple roles in fear and extinction

Anxiety disorders are among the most common mental health problems; deficits in extinction have been implicated as a possible risk factor for the development of these disorders. Fear extinction refers to the ability to adapt as situations change by learning to suppress a previously acquired fear. Attention is directed toward the medial prefrontal cortex (mPFC) and the interaction it has with the amygdala as this circuit has crucial roles in both the acquisition and the extinction of fear associations. Here, we review converging evidence from different laboratories pointing to multiple roles that the mPFC has in fear regulation. Research on rodents indicates opposing roles that the different subregions of the mPFC have in exciting and inhibiting fear. In addition, this review aims to survey the findings addressing the mechanisms by which the mPFC regulates fear. Data from our laboratory and others show that changes in plasticity in the mPFC could be one of the mechanisms mediating extinction of fear. Recent findings on rodents and nonhuman primates report that modifying plasticity in the mPFC alters fear and affects extinction, suggesting that targeting plasticity in the mPFC could constitute a therapeutic tool for the treatment of anxiety disorders.

Stress-induced enhancement of mouse amygdalar synaptic plasticity depends on glucocorticoid and ß-adrenergic activity

Background

Glucocorticoid hormones, in interaction with noradrenaline, enable the consolidation of emotionally arousing and stressful experiences in rodents and humans. Such interaction is thought to occur at least partly in the basolateral nucleus of the amygdala (BLA) which is crucially involved in emotional memory formation. Extensive evidence points to long-term synaptic potentiation (LTP) as a mechanism contributing to memory formation. Here we determined in adolescent C57/Bl6 mice the effects of stress on LTP in the LA-BLA pathway and the specific roles of corticosteroid and β-adrenergic receptor activation in this process.

Principal Findings

Exposure to 20 min of restraint stress (compared to control treatment) prior to slice preparation enhanced subsequent LTP induction in vitro, without affecting baseline fEPSP responses. The role of glucocorticoid receptors, mineralocorticoid receptors and β2-adrenoceptors in the effects of stress was studied by treating mice with the antagonists mifepristone, spironolactone or propranolol respectively (or the corresponding vehicles) prior to stress or control treatment. In undisturbed controls, mifepristone and propranolol administration in vivo did not influence LTP induced in vitro. By contrast, spironolactone caused a gradually attenuating form of LTP, both in unstressed and stressed mice. Mifepristone treatment prior to stress strongly reduced the ability to induce LTP in vitro. Propranolol normalized the stress-induced enhancement of LTP to control levels during the first 10 min after high frequency stimulation, after which synaptic responses further declined.

Conclusions

Acute stress changes BLA electrical properties such that subsequent LTP induction is facilitated. Both β-adrenergic and glucocorticoid receptors are involved in the development of these changes. Mineralocorticoid receptors are important for the maintenance of LTP in the BLA, irrespective of stress-induced changes in the circuit. The prolonged changes in BLA network function after stress may contribute to effective memory formation of emotional and stressful events.

Stimulus intensity-dependent modulations of hippocampal long-term potentiation by basolateral amygdala priming

There is growing realization that the relationship between memory and stress/emotionality is complicated, and may include both memory enhancing and memory impairing aspects. It has been suggested that the underlying mechanisms involve amygdala modulation of hippocampal synaptic plasticity, such as long-term potentiation (LTP). We recently reported that while in CA1 basolateral amygdala (BLA) priming impaired theta stimulation induced LTP, it enhanced LTP in the dentate gyrus (DG). However, emotional and stressfull experiences were found to activate synaptic plasticity within the BLA, raising the possibility that BLA modulation of other brain regions may be altered as well, as it may depend on the way the BLA is activated or is responding. In previous studies BLA priming stimulation was relatively weak (1 V, 50 μs pulse duration). In the present study we assessed the effects of two stronger levels of BLA priming stimulation (1 V or 2 V, 100 μs pulse duration) on LTP induction in hippocampal DG and CA1, in anesthetized rats. Results show that 1V-BLA priming stimulation enhanced but 2V-BLA priming stimulation impaired DG LTP; however, both levels of BLA priming stimulation impaired CA1 LTP, suggesting that modulation of hippocampal synaptic plasticity by amygdala is dependent on the degree of amygdala activation. These findings suggest that plasticity-induced within the amygdala, by stressful experiences induces a form of metaplasticity that would alter the way the amygdala may modulate memory-related processes in other brain areas, such as the hippocampus.

Anatomically dependent anticonvulsant properties of temporally-coded electrical stimulation

In the PTZ animal model of epilepsy, electrical stimulation applied to the amygdaloid complex may result in either pro-convulsive or anticonvulsant effect, depending on the temporal pattern used (i.e. periodic-PS and non-periodic-NPS electrical stimulation). Our hypothesis is that the anatomical target is a determinant factor for the differential effect of temporally-coded patterns on seizure outcome. The threshold dose of PTZ to elicit forelimb clonus and generalized tonic-clonic seizure behavior was measured. The effect of amygdaloid complex PS on forelimb clonus threshold showed a pro-convulsive effect while NPS was anticonvulsant. NPS also significantly increased generalized tonic-clonic threshold; while PS, although at lower threshold levels, did not present statistical significance. Thalamus stimulation did not affect forelimb clonus threshold and showed similar anticonvulsant profiles for both PS and NPS on generalized tonic-clonic threshold. In summary, the anatomical target is a determinant factor on whether temporally-coded ES differentially modulates seizure outcome.

Early postnatal stress alters extracellular signal-regulated kinase signaling in the corticolimbic system modulating emotional circuitry in adult rats

The present study elucidated whether early life stress alters the extracellular signal-regulated kinase (ERK) pathway that underlies fear retrieval and fear extinction based on a contextual fear conditioning paradigm, using a juvenile stress model. Levels of phospho-ERK (pERK), the active form of ERK, increased after fear retrieval in the hippocampal CA1 region but not in the medial prefrontal cortex (mPFC). ERK activation in the CA1 following fear retrieval was not observed in adult rats who received aversive footshock (FS) stimuli during the second postnatal period (2wFS), which exhibited low levels of freezing. In fear extinction, pERK levels in the CA1 were increased by repeated extinction trials, but they were not altered after extinction retrieval. In contrast, pERK levels in the mPFC did not change during extinction training, but were enhanced after extinction retrieval. These findings were compatible in part with electrophysiological data showing that synaptic transmission in the CA1 field and mPFC was enhanced during extinction training and extinction retrieval, respectively. ERK activation in the CA1 and mPFC associated with extinction processes did not occur in rats that received FS stimuli during the third postnatal period (3wFS), which exhibited sustained freezing behavior. The repressed ERK signaling and extinction deficit observed in the 3wFS group were ameliorated by treatment with the partial N-methyl-D-aspartate receptor agonist D-cycloserine. These findings suggest that early postnatal stress induced the downregulation of ERK signaling in distinct brain regions through region-specific regulation, which may lead to increased behavioral abnormalities or emotional vulnerabilities in adulthood.

Memory modulation

Our memories are not all created equally strong: Some experiences are well remembered while others are remembered poorly, if at all. Research on memory modulation investigates the neurobiological processes and systems that contribute to such differences in the strength of our memories. Extensive evidence from both animal and human research indicates that emotionally significant experiences activate hormonal and brain systems that regulate the consolidation of newly acquired memories. These effects are integrated through noradrenergic activation of the basolateral amygdala that regulates memory consolidation via interactions with many other brain regions involved in consolidating memories of recent experiences. Modulatory systems not only influence neurobiological processes underlying the consolidation of new information, but also affect other mnemonic processes, including memory extinction, memory recall, and working memory. In contrast to their enhancing effects on consolidation, adrenal stress hormones impair memory retrieval and working memory. Such effects, as with memory consolidation, require noradrenergic activation of the basolateral amygdala and interactions with other brain regions.

Stress-induced dynamic routing of hippocampal connectivity: a hypothesis

Recent observations have caused a drastic shift in the conception of the hippocampus as a homogeneous structure that subserves cognitive functions, either spatial maps or short term episodic memory, to a structure that is associated with both cognitive and emotional functions. In fact, the assignment of cognitive functions to the hippocampus is restricted to its dorsal sector. In contrast, the ventral hippocampus (VH) appears to be associated with control of behavioral inhibition, stress and emotional memory, but not with strictly cognitive functions. Curiously, the VH but not the dorsal hippocampus (DH) is associated with the development of affective disorders. In line with these collective observations, we and others have found that the ability to evoke a sustained long term potentiation (LTP), a cellular correlate of learning and memory, is much lower in the VH compared to the DH. Strikingly, acute stress as well as direct exposure to corticosterone affect DH and VH in an opposite manner; causing facilitation of LTP in the VH and its suppression in the DH. This double dissociative action results from activation of different steroid receptor species in the DH and VH. Since the DH and VH differ in efferent connectivity, and since the strength of LTP can be considered as an indicator of strength of synaptic connectivity, these results suggest that stress regulates the routes by which the hippocampus is functionally linked to the rest of the brain such that under stress, the ventral route to the amygdala is enabled while the dorsal route to the neocortex is suppressed. This selective routing may underlie the complex outcome of stress on hippocampal and amygdala physiology and behavior.

Emotional memory formation under lower versus higher stress conditions

An exposure to stress can enhance memory for emotionally arousing experiences. The phenomenon is suggested to be amygdala-dependent and in accordance with that view the amygdala was found to modulate mnemonic processes in other brain regions. Previously, we illustrated increased amygdala activation and reduced activation of CA1 following spatial learning under higher versus lower stress conditions. When spatial learning was followed by reversal training interference, impaired retention was detected only under higher stress condition. Here we further evaluate the potential implications of the difference in the level of amygdala activation on the quality of the memory formed under these stress conditions. We attempted to affect spatial memory consolidation under lower or higher stress conditions by either introducing a foot shock interference following massed training in the water maze; by manipulating the threshold for acquisition employing either brief (3 trials) or full (12 trials) training sessions; or by employing a spaced training (over 3 days) rather than massed training protocol. The current findings reveal that under heightened emotionality, the process of consolidation seems to become less effective and more vulnerable to interference; however, when memory consolidation is not interrupted, retention is improved. These differential effects might underlie the complex interactions of stress, and, particularly, of traumatic stress with memory formation processes.

From high anxiety trait to depression: a neurocognitive hypothesis

Although exposure to substantial stress has a major impact on the development of depression, there is considerable variability in the susceptibility of individuals to the adverse effects of stress. The personality trait of high anxiety has been identified as a vulnerability factor to develop depression. We propose here a new unifying model based on a series of neurocognitive mechanisms (and fed with crucial information provided by research on the fields of emotion, stress and cognition) whereby individuals presenting a high anxiety trait are particularly vulnerable to develop depression when facing stress and adversity. Our model highlights the importance of developing prevention programs addressed to restrain, in high anxious individuals, the triggering of a dysfunctional neurocognitive cascade while coping with stress.

Distinct patterns of electrical stimulation of the basolateral amygdala influence pentylenetetrazole seizure outcome

Our working hypothesis is that constant interpulse interval (IPI) electrical stimulation would resonate with endogenous epileptogenic reverberating circuits, inducing seizures, whereas a random interinterval electrical stimulation protocol would promote desynchronization of such neural networks, producing an anticonvulsant effect. Male Wistar rats were stereotaxically implanted with a bipolar electrical stimulation electrode in the amygdala. Pentylenetetrazole (10mg/ml/min) was continuously infused through an intravenous catheter to induce seizures while four different patterns of temporally coded electrical stimulation were applied: periodic stimulation (PS), pseudo-randomized IPI stimulation (LH), restrictively randomized IPI stimulation (IH), and bursts of 20-ms IPIs (burst). PS decreased the pentylenetetrazole threshold to forelimb clonus, whereas IH increased the threshold to forelimb clonus and to generalized tonic-clonic seizures. We hypothesize that PS facilitates forelimb clonus by reverberating with epileptogenic circuits in the limbic system, whereas IH delays forelimb clonus and generalized tonic-clonic seizures by desynchronizing the epileptic neural networks in the forebrain-midbrain-hindbrain circuits.

Parallel memory processing by the CA1 region of the dorsal hippocampus and the basolateral amygdala

There is abundant literature on the role of the basolateral amygdala (BLA) and the CA1 region of the hippocampus in memory formation of inhibitory avoidance (IA) and other behaviorally arousing tasks. Here, we investigate molecular correlates of IA consolidation in the two structures and their relation to NMDA receptors (NMDArs) and beta-adrenergic receptors (beta-ADrs). The separate posttraining administration of antagonists of NMDAr and beta-ADr to BLA and CA1 is amnesic. IA training is followed by an increase of the phosphorylation of calcium and calmodulin-dependent protein kinase II (CaMKII) and ERK2 in CA1 but only an increase of the phosphorylation of ERK2 in BLA. The changes are blocked by NMDAr antagonists but not beta-ADr antagonists in CA1, and they are blocked by beta-ADr but not NMDAr antagonists in BLA. In addition, the changes are accompanied by increased phosphorylation of tyrosine hydroxylase in BLA but not in CA1, suggesting that beta-AD modulation results from local catecholamine synthesis in the former but not in the latter structure. NMDAr blockers in CA1 do not alter the learning-induced neurochemical changes in BLA, and beta-ADr blockade in BLA does not hinder those in CA1. When put together with other data from the literature, the present findings suggest that CA1 and BLA play a role in consolidation, but they operate to an extent in parallel, suggesting that each is probably involved with different aspects of the task studied.