Complex effects of NMDA receptor antagonist APV in the basolateral amygdala on acquisition of two-way avoidance reaction and long-term fear memory

Stress-Induced Enhanced Long-Term Potentiation and Reduced Threshold for N-Methyl-D-Aspartate Receptor- and β-Adrenergic Receptor-Mediated Synaptic Plasticity in Rodent Ventral Subiculum

Stress is a biologically relevant signal and can modulate hippocampal synaptic plasticity. The subiculum is the major output station of the hippocampus and serves as a critical hub in the stress response network. However, stress-associated synaptic plasticity in the ventral subiculum has not been adequately addressed. Therefore, we investigated the impact of a single exposure to an inherently stressful two-way active avoidance conditioning on the induction of long-term potentiation (LTP) at CA1-subiculum synapses in ventral hippocampal slices from young adult rats 1 day after stressor exposure. We found that acute stress enhanced LTP and lowered the induction threshold for a late-onset LTP at excitatory CA1 to subicular burst-spiking neuron synapses. This late-onset LTP was dependent on the activation of β-adrenergic and glutamatergic N-methyl-D-aspartate receptors and independent of D1/D5 dopamine receptor activation. Thereby, we present a cellular mechanism that might contribute to behavioral stress adaptation after acute stressor exposure.

Revisiting the role of anxiety in the initial acquisition of two-way active avoidance: pharmacological, behavioural and neuroanatomical convergence

Two-way active avoidance (TWAA) acquisition constitutes a particular case of approach -avoidance conflict for laboratory rodents. The present article reviews behavioural, psychopharmacological and neuroanatomical evidence accumulated along more than fifty years that provides strong support to the contention that anxiety is critical in the transition from CS (conditioned stimulus)-induced freezing to escape/avoidance responses during the initial stages of TWAA acquisition. Thus, anxiolytic drugs of different types accelerate avoidance acquisition, anxiogenic drugs impair it, and avoidance during these initial acquisition stages is negatively associated with other typical measures of anxiety. In addition behavioural and developmental treatments that reduce or increase anxiety/stress respectively facilitate or impair TWAA acquisition. Finally, evidence for the regulation of TWAA acquisition by septo-hippocampal and amygdala-related mechanisms is discussed. Collectively, the reviewed evidence gives support to the initial acquisition of TWAA as a paradigm with considerable predictive and (in particular) construct validity as an approach-avoidance conflict-based rodent anxiety model.

Comparing brain activity patterns during spontaneous exploratory and cue-instructed learning using single photon-emission computed tomography (SPECT) imaging of regional cerebral blood flow in freely behaving rats

Learning can be categorized into cue-instructed and spontaneous learning types; however, so far, there is no detailed comparative analysis of specific brain pathways involved in these learning types. The aim of this study was to compare brain activity patterns during these learning tasks using the in vivo imaging technique of single photon-emission computed tomography (SPECT) of regional cerebral blood flow (rCBF). During spontaneous exploratory learning, higher levels of rCBF compared to cue-instructed learning were observed in motor control regions, including specific subregions of the motor cortex and the striatum, as well as in regions of sensory pathways including olfactory, somatosensory, and visual modalities. In addition, elevated activity was found in limbic areas, including specific subregions of the hippocampal formation, the amygdala, and the insula. The main difference between the two learning paradigms analyzed in this study was the higher rCBF observed in prefrontal cortical regions during cue-instructed learning when compared to spontaneous learning. Higher rCBF during cue-instructed learning was also observed in the anterior insular cortex and in limbic areas, including the ectorhinal and entorhinal cortexes, subregions of the hippocampus, subnuclei of the amygdala, and the septum. Many of the rCBF changes showed hemispheric lateralization. Taken together, our study is the first to compare partly lateralized brain activity patterns during two different types of learning.

Effects of muscarinic receptor antagonism in the basolateral amygdala on two-way active avoidance

The aim of the present study was to investigate whether the blockade of muscarinic receptors (mRs) in the basolateral amygdala (BLA), which receives important cholinergic inputs related to avoidance learning, affects the consolidation of two-way active avoidance (TWAA). In Experiment 1, adult male Wistar rats were bilaterally infused with scopolamine (SCOP, 20 μg/site) or PBS (VEH) in the BLA immediately after a single 30-trial acquisition session. Twenty-four hours later, avoidance retention was tested in an identical session. Results indicated that scopolamine in the BLA did not affect TWAA performance measured by the number of avoidance responses. Experiment 2 was conducted to test whether such a negative outcome might be due to the occurrence of overtraining during acquisition, which may indeed have a protective effect against scopolamine-induced memory deficits. In this experiment, rats were infused with scopolamine in the BLA immediately after a brief 10-trial acquisition session and tested 24 h later in a 30-trial retention session. The SCOP group showed significantly more avoidances and inter-trial crossings in the retention session than the VEH rats. Together, these results reveal that mRs blockade in the BLA does not disrupt TWAA consolidation and may even enhance avoidance performance when infused after a low number of acquisition trials. Performance factors, such as locomotor activity in the shuttle-box, may account, at least in part, for the facilitative effects of muscarinic antagonism in the BLA.

Nicotine ameliorates NMDA receptor antagonist-induced deficits in contextual fear conditioning through high-affinity nicotinic acetylcholine receptors in the hippocampus

NMDA glutamate receptors (NMDARs) and nicotinic acetylcholine receptors (nAChRs) are both involved in learning and synaptic plasticity. Increasing evidence suggests processes mediated by these receptors may interact to modulate learning; however, little is known about the neural substrates involved in these interactive processes. The present studies investigated the effects of nicotine on MK-801 hydrogen maleate (MK-801) and DL-2-Amino-5-phosphonovaleric acid (APV)-induced disruption of contextual fear conditioning in male C57BL/6J mice, using direct drug infusion and selective nAChR antagonists to define the brain regions and the nAChR subtypes involved. Mice treated with MK-801 showed a deficit in contextual fear conditioning that was ameliorated by nicotine. Direct drug infusion demonstrated that the NMDAR antagonists disrupted hippocampal function and that nicotine acted in the dorsal hippocampus to ameliorate the deficit in learning. The high-affinity nAChR antagonist Dihydro-β-erythroidine hydrobromide (DhβE) blocked the effects of nicotine on MK-801-induced deficits while the α7 nAChR antagonist methyllycaconitine citrate salt hydrate (MLA) did not. These results suggest that NMDARs and nAChRs may mediate similar hippocampal processes involved in contextual fear conditioning. Furthermore, these results may have implications for developing effective therapeutics for the cognitive deficits associated with schizophrenia because a large subset of patients with schizophrenia exhibit cognitive deficits that may be related to NMDAR dysfunction and smoke at much higher rates than the healthy population, which may be an attempt to ameliorate cognitive deficits.

Central noradrenergic lesion induced by DSP-4 impairs the acquisition of avoidance reactions and prevents molecular changes in the amygdala

The noradrenergic system plays and an important modulatory role in memory consolidation of emotionally arousing tasks. However, the molecular cascades regulated in the brain by norepinephrine and involved in memory formation are still largely unknown. The purpose of the present study was to evaluate the role of the noradrenergic system on the acquisition of a highly emotionally arousing task-two-way active avoidance training-and its molecular and cellular substrates. The selective norepinephrine neurotoxin N-(2-chloroethyl)-N-ethyl-2 bromobenzylamine (DSP-4, 50mg/kg) was used. DSP-4-treated rats were trained in a shuttle box to avoid a footshock signaled by an auditory stimulus. Immunohistochemical mapping of the neuronal plasticity-related molecules c-Fos protein and the activated form of extracellular signal-regulated kinase (phosphorylated ERK [pERK]) was then employed. We found that DSP-4 treatment depleted the expression of the norepinephrine marker dopamine -hydroxylase (DBH) in the locus coeruleus and its projection area, the basolateral nucleus of the amygdala, confirming locus coeruleus noradrenergic lesion in the experimental animals. Furthermore, DSP-4 treatment impaired the acquisition of the avoidance reaction. We also found that acquisition of the active avoidance reaction induced c-Fos expression and ERK activation in the amygdala and piriform cortex. This upregulation was prevented by DSP-4 treatment. Thus, our data suggest that the noradrenergic system is involved in the acquisition of the active avoidance reaction by regulating ERK pathway activity and c-Fos expression in the amygdala and piriform cortex.

Involvement of cyclin-dependent kinase-like 2 in cognitive function required for contextual and spatial learning in mice

Cyclin-dependent kinase-like 2 (Cdkl2) is a cdc2-related serine/threonine protein kinase that is postnatally expressed in various brain regions, including the cerebral cortex, entorhinal cortex, hippocampus, amygdala, and dorsal thalamus. The extremely high Cdkl2 expression in these regions suggests that it has a role in cognition and emotion. Recent genetic studies indicate that mutations of Cdkl family kinases are associated with neurodevelopmental and neuropsychiatric disorders in humans. To elucidate the physiologic role of Cdkl2, we behaviorally analyzed Cdkl2(LacZ/LacZ) mice lacking Cdkl2. Cdkl2(LacZ/LacZ) mice had reduced latencies to enter the dark compartment after electric footshock in an inhibitory avoidance task and attenuated contextual fear responses when exposed to mild training conditions. Hippocampal spatial learning in the Morris water maze was slightly anomalous with mice exhibiting an abnormal swimming pattern. The aversive response in a two-way avoidance task was slightly, but not significantly, enhanced. On the other hand, Cdkl2(LacZ/LacZ) mice did not exhibit altered sensitivity to aversive stimuli, such as electric footshock and heat, or deficits in the elevated plus maze or rotating rod test. These findings suggest that Cdkl2 is involved in cognitive function and provide in vivo evidence for the function of Cdkl family kinases expressed in terminally differentiated neurons in mice.

Cognitive Deficit in Amyloid-β–Injected Mice Was Improved by Pretreatment With a Low Dose of Telmisartan Partly Because of Peroxisome Proliferator-Activated Receptor-γ Activation

The pathological hallmark of Alzheimer disease is deposition of amyloid-β protein (Aβ) in the brain. Telmisartan is a unique angiotensin II receptor blocker with peroxisome proliferator-activated receptor-γ (PPAR-γ)–stimulating activity. Activation of PPAR-γ is expected to prevent inflammation and Aβ accumulation in the brain. We investigated the possible preventive effect of telmisartan on cognitive decline in an Alzheimer disease mouse model via PPAR-γ activation. Here, male ddY mice underwent ICV injection of Aβ 1-40. Cognitive function was evaluated by the Morris water maze test. A low dose of telmisartan (0.35 mg/kg per day) was administered in drinking water with or without GW9662, a PPAR-γ antagonist. Cerebral blood flow was evaluated by laser speckle flowmetry. Inflammatory cytokine levels were measured by quantitative RT-PCR. Aβ 1-40 ICV injection significantly impaired cognitive function. Pretreatment with telmisartan improved this cognitive decline to a similar level to that in control mice. Cotreatment with GW9662, a PPAR-γ antagonist, attenuated this telmisartan-mediated improvement of cognition. Treatment with telmisartan enhanced cerebral blood flow and attenuated the Aβ-induced increase in expression of cytokines, such as tumor necrosis factor-α and inducible NO synthase in the brain. Interestingly, coadministration of GW9662 cancelled these beneficial effects of telmisartan. Aβ 1-40 concentration in the brain was significantly decreased by treatment with telmisartan, whereas administration of GW9662 attenuated the decrease in telmisartan-mediated Aβ 1-40 concentration. Taken together, our findings suggest that even a low dose of telmisartan had a preventive effect on cognitive decline in an Alzheimer disease mouse model, partly because of PPAR-γ activation.

ERK2 and CREB activation in the amygdala when an event is remembered as "Fearful" and not when it is remembered as "Instructive"

A training protocol was developed based on durable exposure to the two-way shuttle avoidance task, in which the conditioned stimulus (CS), which was fear evoking for both training conditions on the first day of training, becomes instructive at the end of training under controllable conditions but remains fear evoking under the uncontrollable conditions. The protocol was utilized to examine whether, depending on the training regime, the memory formed will result in a different level of involvement of the amygdala. Three groups of rats were tested: controllable, subjected to durable avoidance learning; uncontrollable, subjected to the same training schedule but with no control over the stressor; and naive. Two weeks later, after the introduction of a reminder cue, freezing response, defecation, and blood corticosterone (CORT) of the uncontrollable group were higher than in the controllable and naive groups, indicating that indeed, for this group, the CS remained fear evoking. Significantly higher than chance shuttling responses of the controllable group indicated that, for them, the CS became "instructive." Activation of ERK2 and CREB in the basolateral amygdala (BLA) was highest in the uncontrollable group compared with the controllable and naive groups. Overall, the results indicate that the training procedure succeeded in dissociating between the physical (electric shock) and the psychological (control) attributes of the experience. Also, our findings support the view that an emotionally charged reminder cue activates the amygdala but that, as a previously fear-evoking memory cue becomes instructive, the involvement of the amygdale lessens.

Neuromodulatory property of standardized extract Ginkgo biloba L. (EGb 761) on memory: behavioral and molecular evidence

Although it has been suggested that the standardized Ginkgo biloba leaf extract (Egb 761) may have a beneficial effect on memory, the cellular and molecular changes that underlie this process are not yet well defined. The present study evaluated the effects of acute (one dose) or subacute treatments (one daily dose/seven days) with EGb 761 (0.5 g kg(-1) and 1.0 g kg(-1)) on rats submitted to a conditioned emotional response (CER) in comparison with positive (4 mg kg(-1) Diazepam) and negative (12%Tween 80) control groups. To this end, eighty (n=10/group) adult, male, Wistar rats (+/-250-300 g) were used in an off-baseline CER procedure. We here observed that the rats submitted to an acute and subacute EGb 761 treatments had acquisition of fear conditioning. Additionally, we investigate if the expression of genes previously associated with classical conditioning (CREB-1 and GAP-43) and new candidate genes (GFAP) are modulated following EGb 761 acute treatment. CREB-1, GAP-43 and GFAP mRNA and protein expressions were evaluated using both quantitative PCR (qPCR) and immunohistochemical analysis, respectively. We here show, for the first time, that EGb 761 modulated GAP-43, CREB-1 and GFAP expression in the prefrontal cortex, amygdala and hippocampus. We observed an underexpression of GAP-43 in all structures evaluated and over-expression of GFAP in the amygdala and hippocampus following acute G. biloba treatment when compared to control group (Tween; p<0.01). GAP-43 expression was decreased in prefrontal cortex and hippocampus in the subacute treatment with EGb 761. Subacute treatment with EGb 761 lead to a decreased CREB-1 in mPFC (p<0.001) and increased in the hippocampus to 1.0 g kg(-1)G. biloba group (p<0.001). The results obtained from immunohistochemical analysis support our aforementioned findings and revealed that the changes in expression occurred within specific regions in the areas evaluated. All together, our findings not only provide new evidence for a role of EGb 761 on memory but also identify molecular changes that underlie the fear memory consolidation.

Induction of c-Fos expression by electrical stimulation of the nucleus basalis magnocellularis

The present study examined the expression of the immediate-early gene c-fos in different brain regions following a single 20-min session of unilateral electrical stimulation of the nucleus basalis magnocellularis (NBM). Current findings confirm that NBM stimulation provides specific activation of several cortical and subcortical regions closely related to the NBM and involved in learning and memory processes, such as the cingulate, parietal, piriform and perirhinal cortices, dorsal subiculum, and the parafascicular, central lateral and central medial nuclei of the thalamus. In contrast, NBM stimulation did not increase c-Fos expression in some expected areas that receive direct NBM projections such as the entorhinal cortex or amygdala nuclei. Results are discussed in terms of the possibility that NBM electrical stimulation facilitates learning by inducing neural changes related to transcription factors such as c-Fos.

Enhancement of active shuttle avoidance response by the NO-cGMP-PKG activator YC-1

Although much has been learned about the role of the amygdala in Pavlovian fear conditioning, relatively little is known about the signaling pathway involved in the acquisition of an active avoidance reaction. The aim of this study is to investigate the potentiating effects of the NO-guanylate cyclase activator YC-1 on learning and memory of shuttle avoidance test in rats. YC-1 enhanced the induction of long-term potentiation (LTP) in amygdala through NO-cGMP-PKG-ERK pathway and the increase of BDNF expression. The Western blot and PCR methods were used to examine the signaling pathways involved in fear memory. It was found that YC-1 increased the avoidance responses during learning period and the memory retention lasted longer than one week. The enhancement of learning behavior by YC-1 was antagonized by intracerebroventricular injection of NOS inhibitor l-NAME, PKG inhibitor Rp-8-Br-PET-cGMPS and MEK inhibitor PD98059, indicating that NO-cGMP-PKG and ERK pathways are involved in the learning potentiating action of YC-1. In addition, YC-1 increased the activation of ERK and Akt 30 min after Day-1 training in amygdala. YC-1 also potentiated the expression of BDNF and CREB in response to fear memory test. Taken together, these findings suggest that NO-cGMP-PKG-ERK signaling pathway is involved in the action of YC-1 in enhancing the fear memory.

Exposure to stressors during juvenility disrupts development-related alterations in the PSA-NCAM to NCAM expression ratio: potential relevance for mood and anxiety disorders

Childhood trauma is associated with higher rates of both mood and anxiety disorders in adulthood. The exposure of rats to stressors during juvenility has comparable effects, and was suggested as a model of induced predisposition for these disorders. The neural cell adhesion molecule (NCAM) and its polysialylated form PSA-NCAM are critically involved in neural development, activity-dependent synaptic plasticity, and learning processes. We examined the effects of exposure to stressors during juvenility on coping with stressors in adulthood and on NCAM and PSA-NCAM expression within the rat limbic system both soon after the exposure and in adulthood. Exposure to stressors during juvenility reduced novel-setting exploration and impaired two-way shuttle avoidance learning in adulthood. Among naive rats, a development-related decrease of about 50% was evident in the PSA-NCAM to NCAM expression ratio in the basolateral amygdala, in the CA1 and dentate gyrus regions of the hippocampus, and in the entorhinal cortex. In juvenile-stressed rats, we found no such decrease, but rather an increase in the polysialylation of NCAM ( approximately 50%), evident soon after the exposure to juvenile stress and also in adulthood. Our results suggest that exposure to stressors during juvenility alters the maturation of the limbic system, and potentially underlies the predisposition to exhibit stress-related symptoms in adulthood.

An animal model of emotional blunting in schizophrenia

Schizophrenia is often associated with emotional blunting—the diminished ability to respond to emotionally salient stimuli—particularly those stimuli representative of negative emotional states, such as fear. This disturbance may stem from dysfunction of the amygdala, a brain region involved in fear processing. The present article describes a novel animal model of emotional blunting in schizophrenia. This model involves interfering with normal fear processing (classical conditioning) in rats by means of acute ketamine administration. We confirm, in a series of experiments comprised of cFos staining, behavioral analysis and neurochemical determinations, that ketamine interferes with the behavioral expression of fear and with normal fear processing in the amygdala and related brain regions. We further show that the atypical antipsychotic drug clozapine, but not the typical antipsychotic haloperidol nor an experimental glutamate receptor 2/3 agonist, inhibits ketamine's effects and retains normal fear processing in the amygdala at a neurochemical level, despite the observation that fear-related behavior is still inhibited due to ketamine administration. Our results suggest that the relative resistance of emotional blunting to drug treatment may be partially due to an inability of conventional therapies to target the multiple anatomical and functional brain systems involved in emotional processing. A conceptual model reconciling our findings in terms of neurochemistry and behavior is postulated and discussed.

Functional internal complexity of amygdala: focus on gene activity mapping after behavioral training and drugs of abuse

The amygdala is a heterogeneous brain structure implicated in processing of emotions and storing the emotional aspects of memories. Gene activity markers such as c-Fos have been shown to reflect both neuronal activation and neuronal plasticity. Herein, we analyze the expression patterns of gene activity markers in the amygdala in response to either behavioral training or treatment with drugs of abuse and then we confront the results with data on other approaches to internal complexity of the amygdala. c-Fos has been the most often studied in the amygdala, showing specific expression patterns in response to various treatments, most probably reflecting functional specializations among amygdala subdivisions. In the basolateral amygdala, c-Fos expression appears to be consistent with the proposed role of this nucleus in a plasticity of the current stimulus-value associations. Within the medial part of the central amygdala, c-Fos correlates with acquisition of alimentary/gustatory behaviors. On the other hand, in the lateral subdivision of the central amygdala, c-Fos expression relates to attention and vigilance. In the medial amygdala, c-Fos appears to be evoked by emotional novelty of the experimental situation. The data on the other major subdivisions of the amygdala are scarce. In conclusion, the studies on the gene activity markers, confronted with other approaches involving neuroanatomy, physiology, and the lesion method, have revealed novel aspects of the amygdala, especially pointing to functional heterogeneity of this brain region that does not fit very well into contemporarily active debate on serial versus parallel information processing within the amygdala.

Fearfulness in a large N/Nih genetically heterogeneous rat stock: differential profiles of timidity and defensive flight in males and females

Anxiety-related behaviors were evaluated across various tests in a large sample (n=787, both sexes) of genetically heterogeneous (N/Nih-HS) rats, derived from an eight-way cross of inbred strains. These tests either evoke unlearned (black-white box, BWB-; novel-cage activity, NACT-; elevated "zero" maze, ZM-; baseline acoustic startle response, BAS-) or learned (fear-potentiated startle, FPS-; two-way active-shuttle box-avoidance acquisition, SHAV-) anxious/fearful responses. The results showed that, with the exception of fear-potentiated startle, almost all (unlearned and learned) behaviors assessed fit with a pattern of sex effects characterized by male rats as being more fearful than females. We applied factor analyses (oblique rotation) to each sex, with the final two-factor solution showing: (1) a first factor (labelled as "Timidity") comprising BWB, NACT and ZM variables in both sexes, plus SHAV responding in the case of males, and (2) a second factor (called "Defensive Flight") which grouped BAS, FPS, and SHAV responding in both sexes. An additional regression analysis showed significant influences of (unlearned) risk assessment (i.e. stretch-attendance) behavior on SHAV in males, while FPS was the main variable positively influencing SHAV (in the intermediate and advanced phases of acquisition) in females. This indicates, for the first time, that fear-potentiated startle may have a facilitating role in the rat's active responses (at least in females) to the cue in the intermediate to advanced phases (i.e. when the initial "passive avoidance/active avoidance" begins to fade) of shuttle box avoidance acquisition. The results of this first extensive behavioral evaluation of N/Nih-HS rats are discussed in terms of their potential usefulness for present and future neurobehavioral and genetic studies of fearfulness/anxiety.

Learning and memory deficits in mice lacking protease activated receptor-1

The roles of serine proteases and protease activated receptors have been extensively studied in coagulation, wound healing, inflammation, and neurodegeneration. More recently, serine proteases have been suggested to influence synaptic plasticity. In this context, we examined the role of protease activated receptor 1 (PAR1), which is activated following proteolytic cleavage by thrombin and plasmin, in emotionally motivated learning. We were particularly interested in PAR1 because its activation enhances the function of NMDA receptors, which are required for some forms of synaptic plasticity. We examined several baseline behavioral measures, including locomotor activity, expression of anxiety-like behavior, motor task acquisition, nociceptive responses, and startle responses in C57Bl/6 mice in which the PAR1 receptor has been genetically deleted. In addition, we evaluated learning and memory in these mice using two memory tasks, passive avoidance and cued fear-conditioning. Whereas locomotion, pain response, startle, and measures of baseline anxiety were largely unaffected by PAR1 removal, PAR1-/- animals showed significant deficits in a passive avoidance task and in cued fear conditioning. These data suggest that PAR1 may play an important role in emotionally motivated learning.

Effects of parafascicular excitotoxic lesions on two-way active avoidance and odor-discrimination

To investigate whether the parafascicular (PF) nucleus of the thalamus is involved in different learning and memory tasks, two experiments were carried out in adult male Wistar rats that were submitted to pre-training bilateral N-methyl-d-aspartate PF infusions (0.15M, pH 7.4; 1.2 microl/side, 0.2 microl/min). In Experiment 1, we evaluated the effects of PF lesions in two identical 30-trial training sessions, separated by a 24-h interval, of a two-way active avoidance conditioning. PF-lesioned rats exhibited impaired performance in both sessions, measured by number of avoidance responses. In Experiment 2, the effects of PF lesions were assessed in a training session (5 trials) and a 24-h retention test (2 retention trials and 2 relearning trials) of an odor-discrimination task. PF lesions did not significantly disrupt the acquisition or the first retention trial, which was not rewarded. However, lesioned animals' performance was clearly affected in subsequent trials, following the introduction of the single non-rewarded trial. Current data are discussed considering evidence that lesions of the PF nucleus affect learning and memory functions mediated by anatomically related areas of the frontal cortex and striatum.

Deregulation of NMDA-receptor function and down-stream signaling in APP[V717I] transgenic mice

Evidence is accumulating for a role for amyloid peptides in impaired synaptic plasticity and cognition, while the underlying mechanisms remain unclear. We here analyzed the effects of amyloid peptides on NMDA-receptor function in vitro and in vivo. A synthetic amyloid peptide preparation containing monomeric and oligomeric A beta (1-42) peptides was used and demonstrated to bind to synapses expressing NMDA-receptors in cultured hippocampal and cortical neurons. Pre-incubation of primary neuronal cultures with A beta peptides significantly inhibited NMDA-receptor function, albeit not by a direct pharmacological inhibition of NMDA-receptors, since acute application of A beta peptides did not change NMDA-receptor currents in autaptic hippocampal cultures nor in xenopus oocytes expressing recombinant NMDA-receptors. Pre-incubation of primary neuronal cultures with A beta peptides however decreased NR2B-immunoreactive synaptic spines and surface expression of NR2B containing NMDA-receptors. Furthermore, we extended these findings for the first time in vivo, demonstrating decreased concentrations of NMDA-receptor subunit NR2B and PSD-95 as well as activated alpha-CaMKII in postsynaptic density preparations of APP[V717I] transgenic mice. This was associated with impaired NMDA-dependent LTP and decreased NMDA- and AMPA-receptor currents in hippocampal CA1 region in APP[V717I] transgenic mice. In addition, induction of c-Fos following cued and contextual fear conditioning was significantly impaired in the basolateral amygdala and hippocampus of APP[V717I] transgenic mice. Our data demonstrate defects in NMDA-receptor function and learning dependent signaling cascades in vivo in APP[V717I] transgenic mice and point to decreased surface expression of NMDA-receptors as a mechanism involved in early synaptic defects in APP[V717I] transgenic mice in vivo.

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.

Extracellular signal-regulated kinases (ERKs) modulate cocaine-induced gene expression in the mouse amygdala

It is known that acute cocaine administration activates the extracellular signal-regulated kinase (ERK) pathway in the striatum, and results in transcription and translation of immediate early genes (IEGs). In the present study we investigated a possible involvement of ERK in the regulation of IEG expression in the amygdala, another brain structure known to be related to an addicted state. The patterns of cocaine-induced c-Fos, JunB and Zif268 protein expression were investigated, using an immunohistochemical approach, within distinct nuclei of the amygdala, either in the presence or absence of a selective inhibitor of the ERK pathway, SL327. Although these IEGs were similarly activated in the various nuclei of the amygdala after acute administration of cocaine, they showed different patterns after chronic injections. They also showed selective sensitivities to ERK inhibition. In particular, whereas c-Fos and JunB expressions were augmented following chronic cocaine treatment, as compared with acute treatment, Zif268 expression was decreased by this chronic treatment. Additionally, chronic blocking of ERK activation affected cocaine-induced c-Fos and JunB but not Zif268 expression. Thus, the differential involvement of ERK in chronic vs. acute regulation of IEGs may account for its specific role in addiction-related behavioral alterations, such as sensitization and tolerance.

Central administration of a caspase inhibitor impairs shuttle-box performance in rats

Recent studies suggest that caspase-3-mediated mechanisms are essential for neuronal plasticity. N-benzyloxycarbonyl-Asp(OMe)-Glu(OMe)-Val- Asp(OMe)-fluoromethyl ketone (z-DEVD-fmk), a caspase inhibitor with predominant specificity toward caspase-3, has been shown to block long-term potentiation in hippocampal slices. Intrahippocampal infusion of a caspase-3 inhibitor to rats has been shown to significantly impair spatial memory in the water maze. The present work was designed to study whether i.c.v. administration of a caspase-3 inhibitor z-DEVD-fmk impairs learning in other tasks related to specific forms of memory in rats. The rats received bilateral injections of z-DEVD-fmk or N-benzyloxycarbonyl-Phe-Ala-fluoromethyl ketone (z-FA-fmk) ("control" peptide) at a dose of 3 nmol. Administration of z-DEVD-fmk significantly decreased the number of avoidance reactions in some blocks of trials in the active avoidance (shuttle box) learning, while z-FA-fmk had no effect as compared with intact rats. However, only a slight effect of the caspase inhibitor across the session was found. z-DEVD-fmk impaired development of some essential components of the two-way active avoidance performance, such as escape reaction, conditioned fear reaction, and inter-trial crossings. Measurement of caspase-3 activity in rat brain regions involved in active avoidance learning revealed most expressed z-DEVD-fmk-related inhibition of the enzyme activity (about 30%) in the fronto-parietal cortex. A similar effect was close to significant in the hippocampus, but not in the other cerebral structures studied. In primary cultures of cerebellar neurons z-DEVD-fmk (2-50 microM) inhibited caspase-3 activity by 60-87%. We suggest that moderate inhibition of caspase-3 resulting from the central administration of z-DEVD-fmk to rats may impair active avoidance learning. Taking into account previous data on the involvement of neuronal caspase-3 in neuroplasticity phenomena we assume that the enzyme may be important for selected forms of learning.