Role of the basolateral amygdala in memory consolidation

Synaptic interactions underlying synchronized inhibition in the basal amygdala: evidence for existence of two types of projection cells

The basal amygdala (BA) plays a key role in mediating the facilitating effects of emotions on memory. Recent findings indicate that this function depends on the ability of BA neurons to generate coherent oscillatory activity, facilitating synaptic plasticity in target neurons. However, the mechanisms allowing BA neurons to synchronize their activity remain poorly understood. Here, we aimed to shed light on this question, focusing on a slow periodic inhibitory oscillation previously observed in the BA in vitro. Paired patch recordings showed that these large inhibitory postsynaptic potentials (IPSPs) occur almost synchronously in BA projection neurons, that they were typically not preceded by excitatory postsynaptic potentials (EPSPs), and that they had little or no correlate in neighboring amygdala nuclei or cortical fields. The initial phase of the IPSPs was associated with an increase in membrane potential fluctuations at 50-100 Hz. In keeping with this, the IPSPs seen in projection cells were correlated with repetitive firing at 50-100 Hz in presumed interneurons and they could be abolished by picrotoxin. However, the IPSPs were also sensitive to 6-cyano-7-nitroquinoxaline-2,3-dione, implying that they arose from the interplay between glutamatergic and GABAergic BA neurons. In support of this idea, we identified a small subset of projection cells (15%), positively identified as such by retrograde labeling from BA projection sites, that began firing shortly before the IPSP onset and presumably drove interneuronal firing. These results add to a rapidly growing body of data indicating that the BA contains at least two distinct types of projection cells that vary in their relation with interneurons and extra-amygdala targets.

The central amygdala nucleus via corticotropin-releasing factor is necessary for time-limited consolidation processing but not storage of contextual fear memory

The central nucleus of the amygdala (CeA) is traditionally portrayed in fear conditioning as the key neural output that relays conditioned information established in the basolateral amygdala complex to extra-amygdalar brain structures that generate emotional responses. However, several recent studies have questioned this serial processing view of the amygdalar fear conditioning circuit by showing an influence of the CeA on memory consolidation. We previously reported that inhibition of endogenous CeA secretion of corticotropin-releasing factor (CRF) at the time of contextual training effectively impaired fear memory consolidation. However, the time-dependent range of CeA CRF secretion in facilitating consolidation processing has not been examined. Therefore, to address this issue, we performed CeA site-specific microinjections of CRF antisense oligonucleotides (CRF ASO) at several post-training time intervals. Rats microinjected with CRF ASO at post-training intervals up to 24-h subsequently exhibited significant impairments in contextual freezing retention in contrast to animals treated 96-h after training. To further establish the validity of the results, CeA fiber-sparing lesions were made at two distinct post-training periods (24-h and 96-h), corresponding respectively to the temporal intervals when CeA CRF ASO administration disrupted or had no significant effects on memory consolidation. Similar to the CeA CRF ASO results, CeA lesions made 24-h, but not 96-h, after training induced significant freezing deficits in the retention test. In conclusion, the current results demonstrate: (1) an extended involvement of CeA CRF in contextual memory consolidation and (2) that contextual fear memory storage is not dependent on a functional CeA.

fMRI studies of successful emotional memory encoding: A quantitative meta-analysis

Over the past decade, fMRI techniques have been increasingly used to interrogate the neural correlates of successful emotional memory encoding. These investigations have typically aimed to either characterize the contributions of the amygdala and medial temporal lobe (MTL) memory system, replicating results in animals, or delineate the neural correlates of specific behavioral phenomena. It has remained difficult, however, to synthesize these findings into a systems neuroscience account of how networks across the whole-brain support the enhancing effects of emotion on memory encoding. To this end, the present study employed a meta-analytic approach using activation likelihood estimates to assess the anatomical specificity and reliability of event-related fMRI activations related to successful memory encoding for emotional versus neutral information. The meta-analysis revealed consistent clusters within bilateral amygdala, anterior hippocampus, anterior and posterior parahippocampal gyrus, the ventral visual stream, left lateral prefrontal cortex and right ventral parietal cortex. The results within the amygdala and MTL support a wealth of findings from the animal literature linking these regions to arousal-mediated memory effects. The consistency of findings in cortical targets, including the visual, prefrontal, and parietal cortices, underscores the importance of generating hypotheses regarding their participation in emotional memory formation. In particular, we propose that the amygdala interacts with these structures to promote enhancements in perceptual processing, semantic elaboration, and attention, which serve to benefit subsequent memory for emotional material. These findings may motivate future research on emotional modulation of widespread neural systems and the implications of this modulation for cognition.

Increased voluntary ethanol consumption and c-Fos expression in selected brain areas induced by fear memory retrieval in ethanol withdrawn rats

Withdrawal from chronic ethanol administration facilitated the formation of contextual fear memory. The effect of fear memory retrieval on subsequent ethanol consumption, by employing a two-bottle free-choice procedure with either water or ethanol (2-8% v/v), was investigated in ethanol withdrawn rats. The effect of fear memory extinction with or without d-cycloserine (DCS, 5mg/kgi.p.) on subsequent ethanol consumption was also evaluated. In addition, we examined c-Fos expression in different brain areas following the fear memory recall. The retrieval of such fear memory induced a significant increase in ethanol consumption in ethanol withdrawn but not in control animals. Regardless of DCS treatment, this increase was attenuated by extinction training. In ethanol withdrawn rats, context-dependent memory retrieval was accompanied by an increased c-Fos expression in the basolateral amygdala, ventrolateral periaqueductal gray, dentate gyrus and dorsomedial periaqueductal gray. Among these brain areas suggested to be implicated in the modulation of motivation and of emotional states, the basolateral amygdala has a crucial role in the emergence of negative affective state during ethanol withdrawal. These data suggest that retrieval of fear memory in ethanol withdrawn rats affected ethanol consumption and that amygdala may be involved in this effect.

Neuronal localization of m1 muscarinic receptor immunoreactivity in the rat basolateral amygdala

Muscarinic cholinergic neurotransmission in the basolateral nuclear complex (BLC) of the amygdala is critical for memory consolidation in emotional/motivational learning tasks. Although knowledge of the localization of muscarinic receptor subtypes in the BLC would contribute to an understanding of the actions of acetylcholine in mnemonic function, previous receptor binding and in situ hybridization studies lacked the resolution necessary to identify which neurons in the BLC express different receptor subtypes. In the present study immunohistochemistry was used to study the neuronal localization of the m1 receptor. The intensity of m1 immunoreactivity varied in different nuclei of the amygdala, and was most robust in the BLC, and in the adjacent posterolateral cortical nucleus. The density and morphology of labeled neurons in the BLC suggested that the m1+ neuronal population included pyramidal cells, the principal neurons in this amygdalar region. In addition, there was dense punctate m1 immunoreactivity in the neuropil of the BLC. Dual labeling immunofluorescence studies of the BLC using antibodies to cell type specific markers were performed to more definitively determine the phenotype of m1-positive (m1+) neurons. An antibody to calcium/calmodulin protein kinase II (CaMK) was used to label pyramidal cells, whereas an antibody to glutamic acid decarboxylase was used to label interneurons. Virtually all of the intensely labeled m1+ neurons of the BLC were CaMK+ pyramidal cells. These data suggest that the ability of M1 receptor antagonists to impair memory consolidation in the BLC is mainly due to blockade of cholinergic influences on the activity of pyramidal neurons.

Temporal precedence of emotion over attention modulations in the lateral amygdala: Intracranial ERP evidence from a patient with temporal lobe epilepsy

Previous fMRI studies have reported mixed evidence for the influence of selective attention on amygdala responses to emotional stimuli, with some studies showing "automatic" emotional effects to threat-related stimuli without attention (or even without awareness), but other studies showing a gating of amygdala activity by selective attention with no response to unattended stimuli. We recorded intracranial local field potentials from the intact left lateral amygdala in a human patient prior to surgery for epilepsy and tested, with a millisecond time resolution, for neural responses to fearful faces appearing at either task-relevant or task-irrelevant locations. Our results revealed an early emotional effect in the amygdala arising prior to, and independently of, attentional modulation. However, at a later latency, we found a significant modulation of the differential emotional response when attention was directed toward or away from fearful faces. These results suggest separate influences of emotion and attention on amygdala activation and may help reconcile previous discrepancies concerning the relative responsiveness of the human amygdala to emotional and attentional factors.

Different effects of scopolamine on the retrieval of spatial memory and fear memory

Retrieval of memory is fundamental for our life as individuals. The participation of cholinergic system in memory consolidation process has been extensively studied, but there are few data concerning the function of this system in memory retrieval process. In the current study, we inject non-selective muscarinic antagonist scopolamine peripherally 20 min before training or testing to see whether cholinergic modulation has effects on the acquisition or retrieval of spatial memory by water maze task and fear memory by inhibitory avoidance task. We find that the cholinergic system is essential for the acquisition of both spatial memory and fear memory. As for the memory retrieval, the cholinergic system has a positive role in the retrieval of spatial memory, because mice injected with scopolamine 20 min before the testing in the water maze show impaired spatial memory retrieval. Whereas injection of scopolamine 20 min before the testing in the inhibitory avoidance task does not cause memory retrieval deficits. That indicates the cholinergic system is not essential for the retrieval of fear memory.

The effects of repeated restraint stress on energy balance and behavior of mice with selective deletion of CRF receptors

Mice subjected to restraint stress (RRS) daily for 3 days lose weight. Once stress ends they are slow to recover the weight loss and exhibit increased anxiety and hypothalamus-pituitary-adrenal (HPA) activity in response to novel stressors. We tested the effect of RRS in mice deficient in corticotropin releasing factor receptor one (CRFR1-KO) or two (CRFR2-KO). Wild type (WT) and CRFR2-KO, but not CRFR1-KO, mice lost weight during RRS. All adrenalectomised mice lost weight and CRFR2-KO controls stopped gaining weight on the days of RRS. WT RRS mice returned to the weight of their controls 8 days after restraint. CRFR2-KO mice showed high levels of anxiety in an elevated plus maze (EPM) 11 days after RRS and in a light/dark choice test 14 days after RRS. CRFR1-KO mice displayed low anxiety in both tests, but RRS decreased EPM exploration. By contrast, exploration increased in RRS ADX mice. Testing in the EPM increased serum corticosterone level in all WT and CRFR2-KO mice. Corticosterone increased in RRS CRFR1-KO mice compared with their controls. These results suggest that CRFR1 are required for stress-induced weight loss, but that hyper-reactivity of the HPA axis in RRS mice exposed to a subsequent novel stress is independent of CRFR1.

Quantification of acetylcholine, an essential neurotransmitter, in brain microdialysis samples by liquid chromatography mass spectrometry

Chemical neurotransmission has been the subject of intensive investigations in recent years. Acetylcholine is an essential neurotransmitter in the central nervous system as it has an effect on alertness, memory and learning. Enzymatic hydrolysis of acetylcholine in the synaptic cleft is fast and quickly metabolizes to choline and acetate by acetylcholinesterase. Hence the concentration in the extracellular fluid of the brain is low (0.1-6 nm). Techniques such as microdialysis are routinely employed to measure acetylcholine levels in living brain systems and the microdialysis sample volumes are usually less than 50 microL. In order to develop medicine for the diseases associated with cognitive dysfunction like mild cognitive impairment, Alzheimer's disease, schizophrenia and Parkinson's disease, or to study the mechanism of the illness, it is important to measure the concentration of acetylcholine in the extracellular fluid of the brain. Recently considerable attention has been focused on the development of chromatographic-mass spectrometric techniques to provide more sensitive and accurate quantification of acetylcholine collected from in-vivo brain microdialysis experiments. This review will provide a brief overview of acetylcholine biosynthesis, microdialysis technique and liquid chromatography mass spectrometry, which is being used to quantitate extracellular levels of acetylcholine.

Hostility- and gender-related differences in oscillatory responses to emotional facial expressions

Hostility is associated with biases in the perception of emotional facial expressions, such that ambiguous or neutral expressions tend to be perceived as threatening or angry. In this study, the effects of hostility and gender on the perception of angry, neutral, and happy faces and on the oscillatory dynamics of cortical responses elicited by these presentations were investigated using time-frequency decomposition by means of wavelet transforms. Feelings of hostility predisposed subjects to perceive happy and neutral faces as less friendly. This effect was more pronounced in women. In hostile subjects, presentation of emotional facial expressions also evoked stronger posterior synchronization in the theta and diminished desynchronization in the alpha band. This may signify a prevalence of emotional responding over cognitive processing. These effects were also more pronounced in females. Hostile females, but not hostile males, additionally showed a widespread synchronization in the alpha band. This synchronization is tentatively explained as a manifestation of inhibitory control which is present in aggressive females, but not in aggressive males.

Hippocampal, amygdala, and neocortical synchronization of theta rhythms is related to an immediate recall during rey auditory verbal learning test

It is well known that theta rhythms (3-8 Hz) are the fingerprint of hippocampus, and that neural activity accompanying encoding of words differs according to whether the items are later remembered or forgotten ["subsequent memory effect" (SME)]. Here, we tested the hypothesis that temporal synchronization of theta rhythms among hippocampus, amygdala, and neocortex is related to immediate memorization of repeated words. To address this issue, intracerebral electroencephalographic (EEG) activity was recorded in five subjects with drug-resistant temporal lobe epilepsy (TLE), under presurgical monitoring routine. During the recording of the intracerebral EEG activity, the subjects performed a computerized version of Rey auditory verbal learning test (RAVLT), a popular test for the clinical evaluation of the immediate and delayed memory. They heard the same list of 15 common words for five times. Each time, immediately after listening the list, the subjects were required to repeat as many words as they could recall. Spectral coherence of the intracerebral EEG activity was computed in order to assess the temporal synchronization of the theta (about 3-8 Hz) rhythms among hippocampus, amygdala, and temporal-occipital neocortex. We found that theta coherence values between amygdala and hippocampus, and between hippocampus and occipital-temporal cortex, were higher in amplitude during successful than unsuccessful immediate recall. A control analysis showed that this was true also for a gamma band (40-45 Hz). Furthermore, these theta and gamma effects were not observed in an additional (control) subject with drug-resistant TLE and a wide lesion to hippocampus. In conclusion, a successful immediate recall to the RAVLT was associated to the enhancement of temporal synchronization of the theta (gamma) rhythms within a cerebral network including hippocampus, amygdala, and temporal-occipital neocortex.

Appetitive vs. defensive responses to emotional cues. Autonomic measures and brain oscillation modulation

The present study explored the effect of the subjective evaluation and the individual differences related to BIS and BAS (Behavioural Inhibition and Activation System) on autonomic measures and brain oscillations, in response to appetitive and aversive emotional stimuli. Multiple measures were recorded, such as psychophysiological (skin conductance response, heart rate, and electromyography) and frequency bands (delta, theta, alpha, and gamma), during viewing IAPS figures, that varied in terms of pleasantness (appetitive vs. aversive) and arousing power (high vs. low intensity). Both BIS and BAS measures were significant in modulating behavioural, autonomic and brain oscillations responses. Withdrawal (BIS system) and appetitive (BAS system) behaviour showed opposite patterns of responses by the subjects. Also, frontal cortical site response was more significant than other sites. Nevertheless, no specific lateralization effect was found as a function of BIS/BAS dichotomy. Moreover, autonomic variables and frequency band modulations were found to be effected by valence and arousal rating per se, with an increased response for high arousing and negative or positive stimuli in comparison with low arousing and neutral stimuli. The effects of subjective evaluation and individual differences were discussed at light of coping activity model of emotion comprehension.

A framework combining delta Event-Related Oscillations (EROs) and Synchronisation Effects (ERD/ERS) to study emotional processing

Event-Related Potentials (ERPs) or Event-Related Oscillations (EROs) have been widely used to study emotional processing, mainly on the theta and gamma frequency bands. However, the role of the slow (delta) waves has been largely ignored. The aim of this study is to provide a framework that combines EROs with Event-Related Desynchronization (ERD)/Event-Related Synchronization (ERS), and peak amplitude analysis of delta activity, evoked by the passive viewing of emotionally evocative pictures. Results showed that this kind of approach is sensitive to the effects of gender, valence, and arousal, as well as, the study of interhemispherical disparity, as the two-brain hemispheres interplay roles in the detailed discrimination of gender. Valence effects are recovered in both the central electrodes as well as in the hemisphere interactions. These findings suggest that the temporal patterns of delta activity and the alterations of delta energy may contribute to the study of emotional processing. Finally the results depict the improved sensitivity of the proposed framework in comparison to the traditional ERP techniques, thereby delineating the need for further development of new methodologies to study slow brain frequencies.

Anxiogenic modulation of spontaneous and evoked neuronal activity in the basolateral amygdala

The amygdala has a well-established role in stress, anxiety, and aversive learning, and anxiolytic and anxiogenic agents are thought to exert their behavioral actions via the amygdala. However, despite extensive behavioral data, the effects of noradrenergic anxiogenic drugs on neuronal activity within the amygdala have not been examined. The present experiments examined how administration of the anxiogenic drug yohimbine affects spontaneous and evoked neuronal activity in the basolateral amygdala (BLA) of rats. Yohimbine produced both excitatory and inhibitory effects on neurons of the BLA, with an increase in spontaneous activity being the predominant response in the lateral and basomedial nuclei of the BLA. Furthermore, yohimbine tended to facilitate neuronal responses evoked by electrical stimulation of the entorhinal cortex, with this facilitation seen more often in lateral and basomedial nuclei of the BLA. These data are the first to examine the effects of the anxiogenic agent yohimbine on BLA neuronal activity, and suggest that neurons in specific subnuclei of the amygdala exhibit unique responses to administration of such pharmacological agents.

Stress, memory and the amygdala

Emotionally significant experiences tend to be well remembered, and the amygdala has a pivotal role in this process. But the efficient encoding of emotional memories can become maladaptive - severe stress often turns them into a source of chronic anxiety. Here, we review studies that have identified neural correlates of stress-induced modulation of amygdala structure and function - from cellular mechanisms to their behavioural consequences. The unique features of stress-induced plasticity in the amygdala, in association with changes in other brain regions, could have long-term consequences for cognitive performance and pathological anxiety exhibited in people with affective disorders.

Psychosocial stress after reactivation of drug-related memory impairs later recall in abstinent heroin addicts

INTRODUCTION

Stress and stress hormone are known to play important roles in modulating different stages of memory including reconsolidation. In a previous study, we found that treatment with stress or corticosterone after a single memory reactivation disrupted reconsolidation of a drug-related memory in rats. Here we presumed that stress after memory reactivation can effectively inhibit drug-related memory by disrupting its reconsolidation in abstinent heroin addicts.

MATERIALS AND METHODS

In the present study, 21 abstinent heroin addicts learned a word list (containing ten neutral, ten heroin-related negative, and ten heroin-related positive words) on day 1; retrieval of a word list (learned 24 h earlier) was made on day 2; and immediately after retrieval, they were exposed to either a standardized psychosocial laboratory stressor (Trier Social Stress Test) or a control condition in a crossover manner. On day 3, free recall of the word list and other psychological and physical responses were assessed.

RESULTS

The stressor induced a significant increase in salivary free cortisol and a decrease in mood. Memory recall was significantly impaired after the stress condition. Follow-up analysis revealed that heroin-related negative and positive words (i.e., heroin-related words) were affected, whereas no effect was observed for neutral words. No changes were detected for cued recall, working memory, or attention. Stress after drug-related memory retrieval significantly decreased its subsequent recall, likely through impaired drug-related memory reconsolidation process.

CONCLUSION

Reconsolidation blockade may thus provide a potential therapeutic strategy for the prevention of relapse in drug addiction.

Arousal effect on emotional face comprehension: frequency band changes in different time intervals

The study aimed to explore the modulation of frequency bands (delta, theta, alpha-1 and alpha-2, and gamma) in response to emotional face within different post-stimulus time intervals (50-450 ms). Twenty adults looked at emotional (happy, sad, angry, fearful) or neutral faces. EEG results showed that motivational significance of face can modulate frequency bands, specifically for theta and gamma. Moreover, gamma can be varied related to degree of arousing feature (high or low) of facial expression. As a function of time, ANOVA and regression analysis revealed that emotional discrimination by gamma and theta is observable mainly within 150-250 time interval and, as revealed also by coherence analysis, that it is more distributed on the anterior-right (theta) or right (gamma) side of the scalp for the emotional stimuli, whereas delta is maximally increased within 250-350 interval and it is more posteriorly (parietal site) distributed for all the stimulus type. We proposed that band modulations respond to variations in processing emotional face, and, whereas delta reflects updating of the stimulus, and theta responds to the emotional significance of face, gamma reflects differences in the arousing power of facial expression.

Beta-adrenergic facilitation of synaptic plasticity in the rat basolateral amygdala in vitro is gradually reversed by corticosterone

The rat basolateral amygdala is important for emotional learning; this is modulated by noradrenaline and corticosterone. We report that the beta-adrenergic agonist isoproterenol markedly enhances synaptic plasticity induced in the basolateral amygdala by a weak stimulation paradigm but is ineffective with stronger protocols. Simultaneous application of corticosterone gradually reversed the facilitatory effect of isoproterenol. When corticosterone was briefly applied several hours prior to isoproterenol, facilitatory effects of the beta-agonist were entirely suppressed. This suggests that in the basolateral amygdala, beta-adrenergic influences promote synaptic plasticity; this is gradually normalized by corticosterone, preventing the network from overshooting.

Chronic stress-induced alterations in amygdala responsiveness and behavior--modulation by trait anxiety and corticotropin-releasing factor systems

The basolateral nucleus of the amygdala (BLA) plays a key role in emotional arousal and anxiety, and expresses high levels of corticotropin-releasing factor receptor (CRFR)1. In rat brain slices, we have recently shown that afferent activation of the BLA is increased following application of exogenous corticotropin-releasing factor (CRF). Here we examined the impact of chronic unpredictable stress (CUS) on this effect of CRF and whether blockade of CRFR1 could prevent stress-induced changes in the electrophysiological response, the animal's behavior and in cell proliferation in the hippocampus. The behavior of the rats was monitored via a series of tests that formed part of the CUS. Electrophysiological measures of the BLA response to CRF, cell proliferation in the dentate gyrus and the expression of CRF and CRFR1 mRNA in amygdaloid nuclei were determined ex vivo after completion of the CUS. CRF-induced potentiation of afferent activation of the BLA was reduced in rats exposed to CUS, an effect that was inhibited by chronic antagonism of CRFR1. Furthermore, the reduction in BLA response to CRF was correlated with the anxiety trait of the animals, determined prior to initiation of the CUS. These results implicate CRFR1 in chronic stress-induced alterations in amygdala function and behavior. Furthermore, they show that CRFR1 antagonists can prevent changes induced by chronic stress, in particular in those animals that are highly anxious.

Age-related dendritic hypertrophy and sexual dimorphism in rat basolateral amygdala

Little research has examined the influence of aging or sex on anatomical measures in the basolateral amygdala. We quantified spine density and dendritic material in Golgi-Cox stained tissue of the basolateral nucleus in young adult (3-5 months) and aged (20-24 months) male and female Long-Evans rats. Dendritic branching and spine density were measured in principal neurons. Age, but not sex, influenced the dendritic tree, with aged animals displaying significantly more dendritic material. Previous findings from our laboratory in the same set of subjects indicate an opposite effect of aging on dendritic material in the medial prefrontal cortex and hippocampus. We also report here a sex difference across ages in dendritic spine density, favoring males.

CRF1 receptor activation increases the response of neurons in the basolateral nucleus of the amygdala to afferent stimulation

The basolateral nucleus (BLA) of the amygdala contributes to the consolidation of memories for emotional or stressful events. The nucleus contains a high density of CRF1 receptors that are activated by corticotropin-releasing factor (CRF). Modulation of the excitability of neurons in the BLA by CRF may regulate the immediate response to stressful events and the formation of associated memories. In the present study, CRF was found to increase the amplitude of field potentials recorded in the BLA following excitatory afferent stimulation, in vitro. The increase was mediated by CRF1 receptors, since it could be blocked by the selective, non-peptide antagonists, NBI30775 and NBI35583, but not by the CRF2-selective antagonist, astressin 2B. Furthermore, the CRF2-selective agonist, urocortin II had no effect on field potential amplitude. The increase induced by CRF was long-lasting, could not be reversed by subsequent administration of NBI35583, and required the activation of protein kinase C. This effect of CRF in the BLA may be important for increasing the salience of aversive stimuli under stressful conditions, and for enhancing the consolidation of associated memories. The results provide further justification for studying the efficacy of selective antagonists of the CRF1 receptor to reduce memory formation linked to emotional or traumatic events, and suggest that these compounds might be useful as prophylactic treatments for stress-related illnesses such as post-traumatic stress disorder.

Switching on and off fear by distinct neuronal circuits

Switching between exploratory and defensive behaviour is fundamental to survival of many animals, but how this transition is achieved by specific neuronal circuits is not known. Here, using the converse behavioural states of fear extinction and its context-dependent renewal as a model in mice, we show that bi-directional transitions between states of high and low fear are triggered by a rapid switch in the balance of activity between two distinct populations of basal amygdala neurons. These two populations are integrated into discrete neuronal circuits differentially connected with the hippocampus and the medial prefrontal cortex. Targeted and reversible neuronal inactivation of the basal amygdala prevents behavioural changes without affecting memory or expression of behaviour. Our findings indicate that switching between distinct behavioural states can be triggered by selective activation of specific neuronal circuits integrating sensory and contextual information. These observations provide a new framework for understanding context-dependent changes of fear behaviour.

Rhythms in expression of PER1 protein in the amygdala and bed nucleus of the stria terminalis of the diurnal grass rat (Arvicanthis niloticus)

In the diurnal rodent Arvicanthis niloticus (grass rats) the pattern of expression of the clock genes and their proteins in the suprachiasmatic nucleus (SCN) is very similar to that seen in nocturnal rodents. Rhythms in clock gene expression have been also documented in several forebrain regions outside the SCN in nocturnal Ratus norvegicus (lab rats). To investigate the neural basis for differences in the circadian systems of diurnal and nocturnal mammals, we examined PER1 expression in the oval nucleus of the bed nucleus of the stria terminalis (BNST-OV), and in the basolateral (BLA) and the central (CEA) amygdala of male grass rats kept in a 12:12 light/dark cycle. In the BNST-OV, peak levels of PER1 expression were seen early in the light phase of the cycle, 12h out of phase with what has been reported for nocturnal lab rats. In the BLA the pattern of PER1 expression featured sustained high levels during the day and low levels at night. PER1 expression in the CEA was also at its highest early in the light phase, but the effect of sampling time was not statistically significant (p<0.06). The results are consistent with the hypothesis that differences between nocturnal and diurnal species are due to differences in neural systems downstream from the SCN.

Predator odor fear conditioning: current perspectives and new directions

Predator odor fear conditioning involves the use of a natural unconditioned stimulus, as opposed to aversive electric foot-shock, to obtain novel information on the neural circuitry associated with emotional learning and memory. Researchers are beginning to identify brain sites associated with conditioned contextual fear such as the ventral anterior olfactory nucleus, dorsal premammillary nucleus, ventrolateral periaqueductal gray, cuneiform nucleus, and locus coeruleus. In addition, a few studies have reported an involvement of the basolateral and medial nucleus of the amygdala and hippocampus in fear conditioning. However, several important issues concerning the effectiveness of different predator odor unconditioned stimuli to produce fear conditioning, the precise role of brain nuclei in fear conditioning, and the general relation between the current predator odor and the traditional electric foot-shock fear conditioning procedures remain to be satisfactorily addressed. This review discusses the major behavioral results in the current predator odor fear conditioning literature and introduces two novel contextual and auditory fear conditioning models using cat odor. The new models provide critical information on the acquisition of conditioned fear behavior during training and the expression of conditioned responses in the retention test. Future studies adopting fear conditioning procedures that incorporate measures of both unconditioned and conditioned responses during training may lead to broad insights into predator odor fear conditioning and identify specific brain nuclei mediating conditioned stimulus-predator odor unconditioned stimulus associations.

Activation of phenotypically distinct neuronal subpopulations in the anterior subdivision of the rat basolateral amygdala following acute and repeated stress

The effects of acute and repeated stress on expression of the early immediate gene c-fos in the basolateral amygdala have previously been reported; however, characterization of which neuronal subpopulations are activated by these stimuli has not been investigated. This question is of considerable relevance, insofar as the basolateral amygdala houses a heterogeneous population of neurons, including those of gamma-aminobutyric acid (GABA)-ergic and glutamatergic phenotypes that may be subcategorized based on their expression of various calcium-binding proteins, including parvalbumin, calbindin, calretinin, and the calcium-sensitive enzyme calcium/calmodulin-dependent kinase II. Characterization of these subpopulations has revealed unique differences in their physiology, synaptology, and morphology, suggesting that each distinct phenotype may have profound effects on the local circuitry of the amygdala. Therefore, we examined the effects of acute and repeated restraint stress on expression of the immediate early gene c-fos in neurons containing parvalbumin, calbindin, calretinin, or calcium/calmodulin-dependent kinase II in the basolateral amygdala. Double-label immunohistochemistry revealed that acute restraint stress activated a proportion of parvalbumin-, calbindin-, or calcium/calmodulin-dependent kinase II-positive neurons. Prior exposure to repeated restraint stress markedly attenuated acute-stress mediated activation of these neuronal populations, although not equally. Expression of c-Fos protein was not detected in calretinin-positive neurons in any experimental group. These results demonstrate that distinct neuronal phenotypes in the basolateral amygdala are activated by acute restraint stress and that prior repeated restraint stress differentially affects this response.

The basolateral nucleus of the amygdala is necessary to induce the opposing effects of stressful experience on learning in males and females

The basolateral nucleus of the amygdala (BLA) has been implicated in the modulation of learning after stress. Acute inescapable stress enhances classical eyeblink conditioning in male rats, whereas the same stressor impairs eyeblink conditioning in female rats. The experiments here directly assessed whether inactivation of the BLA during stress exposure would block both the stress-induced facilitation in males and the retardation of eyeblink conditioning in females. To this end, the BLA was temporarily inactivated by infusion of the GABA agonist muscimol before acute stressor exposure. All rats were trained in a different context 24 h later. Males infused with muscimol before the stressful event did not exhibit facilitated eyeblink conditioning, whereas those infused with the vehicle emitted more conditioned responses than unstressed males. Females infused with muscimol before stress did not express a deficit in conditioning, whereas those infused with vehicle and stressed emitted fewer conditioned responses than unstressed vehicle controls. These data demonstrate that neuronal activity within the BLA during stress exposure is necessary to modulate learning 24 h later in a new context. Thus, the BLA is necessary to induce the long-term effect of stressful experience on conditioning regardless of sex and direction of modulation.

Age-related changes in conditioned flavor preference in rats

Age-related changes have been documented in regions of the brain shown to process reward information. However, few studies have examined the effects of aging on associative memory for reward. The present study tested 7- and 24-month-old rats on a conditioned flavor preference task. Half of the rats in each age group received an unsweetened grape-flavored solution (CS-) on odd-numbered days and a sweetened cherry-flavored solution (CS+) on even-numbered days. The remaining rats in each age group received a sweetened grape-flavored solution (CS+) on odd-numbered days and an unsweetened cherry-flavored solution (CS-) on even-numbered days. During the acquisition phase of testing, the designated solution (CS+ or CS-) was presented to each rat for 15 min daily across six consecutive days. On the preference phase, each rat received unsweetened cherry and unsweetened grape-flavored solutions simultaneously for 15 min daily across four consecutive days. The 7-month-old rats showed a significant preference for the flavor that was previously sweetened during the acquisition phase (CS+) compared to the previously unsweetened solution (CS-) when the two unsweetened solutions were presented simultaneously during the preference phase of testing. In contrast, the 24-month-old rats did not show a preference and consumed roughly equal amounts of the previously sweetened (CS+) and unsweetened (CS-) solutions. Thus, the data suggest that the ability to form flavor-reward associations declines with increasing age, resulting in impaired conditioned flavor preference.

Predator (cat hair)-induced enhancement of hippocampal long-term potentiation in rats: involvement of acetylcholine

Extensive literature has demonstrated that arousal and fear modify memory acquisition and consolidation. Predator hair and odors increase arousal in rats and, therefore, may influence information encoding and synaptic plasticity in the rodent nervous system. In behavioral experiments, we confirm that laboratory-bred Long Evans rats avoid cat hair. Electrophysiological work in vivo showed that long-term potentiation (LTP) in the dentate gyrus induced by perforant path stimulation was enhanced for 5-7 days when LTP induction occurred in the presence of cat hair relative to fake hair. The muscarinic receptor antagonist scopolamine (i.p.) reversed the cat hair-elicited LTP enhancement without affecting weaker LTP elicited in the presence of fake hair. Thus, exposure to a predator stimulus elicits a cholinergically-dependent state of heightened plasticity that may serve to facilitate information storage in hippocampal circuits.

Emotional memory function, personality structure and psychopathology: a neural system approach to the identification of vulnerability markers

It is well established that emotional events are ingrained stronger into memory relative to neutral events. Facilitated emotional memory is highly variable between individuals within the normal population and is particularly exacerbated in those diagnosed with mood and anxiety disorders. In order to elucidate how variation of enhanced emotional memory within the normal population may manifest into psychopathological states, we explored the convergence between studies investigating the neural systems engaged in emotional memory facilitation and studies investigating how these systems differ from person to person. Converging evidence highlights the roles of three neural systems (1. Amygdala function and attention, 2. Neuroendocrine function, 3. Interactive effects with mood) that all govern emotional memory facilitation and are highly variable between individuals as a function of personality. We applied this neural system approach to models of vulnerability of three forms of psychopathology that are particularly characterized by atypical emotional memory function (depression, generalized anxiety disorder and post-traumatic stress disorder). This application suggests that the incorporation of known vulnerability markers across psychological, neuroimaging and neuroendocrinological domains is cardinal to how susceptibility is conceptualized and assessed in these disorders.

Activation of basolateral amygdala corticotropin-releasing factor 1 receptors modulates the consolidation of contextual fear

The basolateral amygdala complex (BLA) and central amygdala nucleus (CeA) are involved in fear and anxiety. In addition, the BLA contains a high density of corticotropin-releasing factor 1 (CRF(1)) receptors in comparison to the CeA. However, the role of BLA CRF(1) receptors in contextual fear conditioning is poorly understood. In the present study, we first demonstrated in rats that oral administration of DMP696, the selective CRF(1) receptor antagonist, had no significant effects on the acquisition of contextual fear but produced a subsequent impairment in contextual freezing suggesting a role of CRF(1) receptors in the fear memory consolidation process. In addition, oral administration of DMP696 significantly reduced phosphorylation of cyclic AMP response element-binding protein (pCREB) in the lateral and basolateral amygdala nuclei, but not in the CeA, during the post-fear conditioning period. We then demonstrated that bilateral microinjections of DMP696 into the BLA produced no significant effects on the acquisition of conditioned fear but reduced contextual freezing in a subsequent drug-free conditioned fear test. Importantly, bilateral microinjections of DMP696 into the BLA at 5 min or 3 h, but not 9 h, after exposure to contextual fear conditioning was also effective in reducing contextual freezing in the conditioned fear test. Finally, microinfusions of either DMP696 into the CeA or a specific corticotropin-releasing factor 2 receptor antagonist in the BLA were shown to have no major effects on disrupting either contextual fear conditioning or performance of contextual freezing in the drug-free conditioned fear test. Collectively, results implicate a role of BLA CRF(1) receptors in activating the fear memory consolidation process, which may involve BLA pCREB-induced synaptic plasticity.

Amygdala activation in affective priming: a magnetoencephalogram study

We employed magnetoencephalography (MEG) to examine amygdala activity during a linguistic affective priming task. The experimental design included positive and negative word pairs. Using synthetic aperture magnetometry in the analysis of MEG data, we identified a left amygdala power increase in the theta frequency range during priming involving negative words. We found that the amygdala displayed a time-dependent intensification in responsiveness to negative stimuli, specifically between 150 and 400 ms after target presentation. This study provides evidence for theta power changes in the amygdala and demonstrates that the analysis of brain oscillations provides a powerful tool to explore mechanisms implicated in emotional processing.

Anatomical analysis of afferent projections to the medial prefrontal cortex in the rat

The medial prefrontal cortex (mPFC) has been associated with diverse functions including attentional processes, visceromotor activity, decision making, goal directed behavior, and working memory. Using retrograde tracing techniques, we examined, compared, and contrasted afferent projections to the four divisions of the mPFC in the rat: the medial (frontal) agranular (AGm), anterior cingulate (AC), prelimbic (PL), and infralimbic (IL) cortices. Each division of the mPFC receives a unique set of afferent projections. There is a shift dorsoventrally along the mPFC from predominantly sensorimotor input to the dorsal mPFC (AGm and dorsal AC) to primarily 'limbic' input to the ventral mPFC (PL and IL). The AGm and dorsal AC receive afferent projections from widespread areas of the cortex (and associated thalamic nuclei) representing all sensory modalities. This information is presumably integrated at, and utilized by, the dorsal mPFC in goal directed actions. In contrast with the dorsal mPFC, the ventral mPFC receives significantly less cortical input overall and afferents from limbic as opposed to sensorimotor regions of cortex. The main sources of afferent projections to PL/IL are from the orbitomedial prefrontal, agranular insular, perirhinal and entorhinal cortices, the hippocampus, the claustrum, the medial basal forebrain, the basal nuclei of amygdala, the midline thalamus and monoaminergic nuclei of the brainstem. With a few exceptions, there are few projections from the hypothalamus to the dorsal or ventral mPFC. Accordingly, subcortical limbic information mainly reaches the mPFC via the midline thalamus and basal nuclei of amygdala. As discussed herein, based on patterns of afferent (as well as efferent) projections, PL is positioned to serve a direct role in cognitive functions homologous to dorsolateral PFC of primates, whereas IL appears to represent a visceromotor center homologous to the orbitomedial PFC of primates.

Evolution of the amygdaloid complex in vertebrates, with special reference to the anamnio-amniotic transition

Numerous studies over the last few years have demonstrated that the amygdaloid complex in amniotes shares basic developmental, hodological and neurochemical features. Furthermore, homologous territories of all the main amygdaloid subdivisions have been recognized among amniotes, primarily highlighted by the common expression patterns for numerous developmental genes. Thus, derivatives from the lateral pallium, ventral pallium and subpallium constitute the fundamental parts of the amygdaloid complex. With the development of new technical approaches, study of the precise neuroanatomy of the telencephalon of the anuran amphibians (anamniotes) has been possible. Current embryological, hodological and immunohistochemical evidence strongly suggests that most of the structures present in amniotes are recognizable in these anamniotes. These investigations have yielded enough results to support the notion that the organization of the anuran amygdaloid complex includes subdivisions with their origin in ventral pallial and subpallial territories; a strong relationship with the vomeronasal and olfactory systems; abundant intra-amygdaloid connections; a main output centre involved in the autonomic system; recognizable amygdaloid fibre systems; and distinct chemoarchitecture. Therefore, the new ideas regarding the amygdaloid evolution based on the recent findings in anamniotes, and especially in anurans, strongly support the notion that basic amygdaloid structures were present at least in the brain of ancestral tetrapods organized following a basic plan shared by tetrapods.

Increased amygdala activation to neutral faces is associated with better face memory performance

Recent evidence suggests that the role of the amygdala may extend beyond threat detection to include processing socially relevant stimuli in general. Thus, we investigated perception and memory for neutral faces; a stimulus-type that lacks emotional valence yet contains relevant social information. Participants viewed neutral faces or houses when undergoing functional MRI. Neutral face memory testing was conducted outside the scanner. In the functional MRI of faces vs. houses contrast, significant bilateral activation in the amygdala and lateral fusiform gyrus was observed. Increased bilateral amygdala activation was associated with better delayed-memory performance. These findings indicate that the role of the amygdala may include processing neutral yet socially relevant stimuli. Further, amygdala activation, independent of emotional valence, appears to be associated with memory enhancement.

Expression of FOS, EGR-1, and ARC in the amygdala and hippocampus of female rats during formation of the intromission mnemonic of pseudopregnancy

Pseudopregnancy (PSP) in the female rat is a neuroendocrine condition that is induced by repeated and intermittent vaginocervical stimulation received during mating and involves the expression of bicircadian prolactin surges and cessation of normal estrous cyclicity for 10-12 days postmating. The temporal patterning and number of intromissions received by the female are critical for PSP initiation, and thus, short-term encoding of VCS occurs during transduction of intromissions into PSP. In this experiment, we characterized and compared the mating-induced neural activation patterns within amygdalar and hippocampal regions using expression of the immediate early genes FOS, EGR-1, and ARC. Cycling female rats mated on proestrus received 15 or 5 intromissions under paced or nonpaced mating conditions. High numbers of intromissions during nonpaced mating or low numbers received during paced mating are sufficient to induce PSP, while five nonpaced intromissions and mounts without intromission are insufficient. Here we demonstrate that the CA1 region of the hippocampus was selectively sensitive to PSP-sufficient but not PSP-insufficient mating stimulation by showing significant effects of paced mating for all three IEGs. Paced mating also stimulated the expression of ARC within the basolateral, cortical, and central nuclei of the amygdala. The posterodorsal medial amygdala also showed selective EGR-1 responses to PSP-sufficient mating stimulation. There was no effect of hemisphere on IEG expression. The postmating expression profiles of these IEGs provide evidence that limbic areas involved in encoding and consolidation of memory are involved in initiating the neuroendocrine memory of PSP.

NMDA Partial agonist reverses blocking of extinction of aversive memory by GABA(A) agonist in the amygdala

The ability to extinguish aversive memories is of significant clinical interest. The amygdala plays an important role in emotional conditioning and its experimental extinction. It has been suggested that gamma-aminobutyric acid (GABA) agonists retard extinction and that consolidation of extinction involves N-methyl-D-aspartate receptor (NMDAR)-mediated plasticity. The aim was to further explore the interaction between GABA and NMDA in the amygdala in consolidation of experimental extinction in the rat. To that end conditioned taste aversion (CTA) was used. In CTA, the amygdala has been reported to subserve both acquisition and extinction. The GABA(A) receptor agonist, muscimol, administered into the amygdala immediately after the first extinction session, caused lasting disruption of extinction of CTA for at least 2 weeks. However, the administration of GABA(A) receptor antagonists had no effect on extinction kinetics. Microinfusing the partial NMDA agonist D-cycloserine together with or after muscimol infusion reversed the blocking effects of muscimol. These findings could bear relevance to the potential involvement of extinction abnormalities in behavioral disorders, and their amelioration.

Muscarinic control of graded persistent activity in lateral amygdala neurons

The cholinergic system is crucially involved in several cognitive processes including attention, learning and memory. Muscarinic actions have profound effects on the intrinsic firing pattern of neurons. In principal neurons of the entorhinal cortex (EC), muscarinic receptors activate an intrinsic cation current that causes multiple self-sustained spiking activity, which represents a potential mechanism for transiently sustaining information about novel items. The amygdala appears to be important for experience-dependent learning by emotional arousal, and cholinergic muscarinic influences are essential for the amygdala-mediated modulation of memory. Here we show that principal neurons from the lateral nucleus of the amygdala (LA) can generate intrinsic graded persistent activity that is similar to EC layer V cells. This firing behavior is linked to muscarinic activation of a calcium-sensitive non-specific cation current and can be mimicked by stimulation of cholinergic afferents that originate from the nucleus basalis of Meynert (n. M). Moreover, we demonstrate that the projections from the n. M. are essential and sufficient for the control and modulation of graded firing activity in LA neurons. We found that activation of these cholinergic afferents (i) is required to maintain and to increase firing rates in a graded manner, and (ii) is sufficient for the graded increases of stable discharge rates even without an associated up-regulation of Ca2+. The induction of persistent activity was blocked by flufenamic acid or 2-APB and remained intact after Ca2+-store depletion with thapsigargin. The internal ability of LA neurons to generate graded persistent activity could be essential for amygdala-mediated memory operations.

Temporary basolateral amygdala lesions disrupt acquisition of socially transmitted food preferences in rats

Lesions of the basolateral amygdala (BLA) have long been associated with abnormalities of taste-related behaviors and with failure in a variety of taste- and odor-related learning paradigms, including taste-potentiated odor aversion, conditioned taste preference, and conditioned taste aversion. Still, the general role of the amygdala in chemosensory learning remains somewhat controversial. In particular, it has been suggested that the amygdala may not be involved in a form of chemosensory learning that has recently received a substantial amount of study-socially transmitted food preference (STFP). Here, we provide evidence for this involvement by pharmacologically inactivating the basolateral amygdala bilaterally during STFP training. The same inactivation sites that impaired taste aversion learning eliminated the normally conditioned preference for a food smelled on a conspecific's breath. Impairments of learned preference persisted even in testing sessions in which BLA was not inactivated, and learning was normal when the BLA was inactivated only during testing sessions; thus, the impairment was a true acquisition deficit. In conjunction with previous results from other paradigms, therefore, our data suggest that the amygdala is vital for learning procedures involving pairings of potent and arbitrary chemosensory stimuli.

The role of the medial prefrontal cortex-amygdala circuit in stress effects on the extinction of fear

Stress exposure, depending on its intensity and duration, affects cognition and learning in an adaptive or maladaptive manner. Studies addressing the effects of stress on cognitive processes have mainly focused on conditioned fear, since it is suggested that fear-motivated learning lies at the root of affective and anxiety disorders. Inhibition of fear-motivated response can be accomplished by experimental extinction of the fearful response to the fear-inducing stimulus. Converging evidence indicates that extinction of fear memory requires plasticity in both the medial prefrontal cortex and the amygdala. These brain areas are also deeply involved in mediating the effects of exposure to stress on memory. Moreover, extensive evidence indicates that gamma-aminobutyric acid (GABA) transmission plays a primary role in the modulation of behavioral sequelae resulting from a stressful experience, and may also partially mediate inhibitory learning during extinction. In this review, we present evidence that exposure to a stressful experience may impair fear extinction and the possible involvement of the GABA system. Impairment of fear extinction learning is particularly important as it may predispose some individuals to the development of posttraumatic stress disorder. We further discuss a possible dysfunction in the medial prefrontal cortex-amygdala circuit following a stressful experience that may explain the impaired extinction caused by exposure to a stressor.

Endogenous cortisol level interacts with noradrenergic activation in the human amygdala

Animal studies show that high cortisol levels exert their effect on stressful task performance via modulation of the amygdala. Availability of noradrenaline in this brain region appears to be a critical prerequisite for this effect. This relationship between noradrenaline and cortisol is explained by an animal model where the amygdala constitutes a crucial region for this interaction. In humans this model has not been extensively tested so far. In a previously reported study human subjects (aged 20.93+/-2.38) were scanned using fMRI when watching sets of emotional and neutral pictures after taking the beta-adrenergic antagonist propranolol or placebo. Stimulus sets consisted of 92 pictures, divided in four emotional categories that ranged from neutral scenes of domestic objects (CAT1) to extremely negative scenes of mutilation or accidents (CAT4). Confrontation with arousing emotional pictures, accompanied by increased noradrenaline levels, evoked increased amygdala activation under placebo but not under betablocker condition. This new and additional analysis of this data set was carried out to determine the effect of differential endogenous cortisol levels on amygdala activation. Cortisol levels during scanning were determined using salivary samples and subjects were post hoc divided in a High (n=14) and Low cortisol group (n=14). When subjects were watching emotional stimuli, presumably associated with enhanced noradrenaline (NA) levels, amygdala activation was contrasted between the two cortisol groups. We hypothesized that emotional stimuli would elicit more amygdala activation in the High than in the Low cortisol group. Here we demonstrate indeed a significant interaction effect of the endogenous cortisol level with increasing activation in the amygdala under placebo but not under betablocker condition, thereby extending the rodent based model of a synergistic effect of the two stress hormones to the human.

Restricted access to food, but not sucrose, saccharine, or salt, synchronizes the expression of Period2 protein in the limbic forebrain

Restricted feeding schedules (RF) in which daily access to food is limited to a few hours each day can entrain the rhythms of expression of circadian clock genes in the brain and periphery in rodents. The critical factors mediating the effect of RF on rhythms of clock gene expression are unknown. Previously, we demonstrated that daytime RF shifts the phase of expression of the clock protein, Period2 (PER2) in the oval nucleus of the bed nucleus of the stria terminalis in rats kept on a 12-h light/dark cycle, and restored the rhythm of PER2 expression in rats housed in constant light. We now report that RF also modifies the rhythms of PER2 expression in the central and basolateral nuclei of the amygdala and in the dentate gyrus, such that all three areas become synchronized, peaking 12 h after the time of food presentation. Daily limited access to sucrose or saccharine in freely fed rats or scheduled access to saline in sodium-deprived rats had no effect on these PER2 rhythms. Thus, it would appear that the rhythms of PER2 in limbic forebrain structures are sensitive to signals that arise from the alleviation of a negative metabolic state associated with scheduled feeding and that access to rewarding substances in the absence of food deprivation or metabolic challenges, per se, is not sufficient to alter the rhythms of PER2 expression in these regions.

Glucocorticoid receptors and β-adrenoceptors in basolateral amygdala modulate synaptic plasticity in hippocampal dentate gyrus, but not in area CA1

The basolateral amygdala (BLA) is a key structure in a memory-modulatory system that regulates stress and stress hormones (glucocorticoid and noradrenaline) effects on hippocampal functioning. We have shown previously that priming the amygdala differentially affects plasticity in the hippocampal dentate gyrus (DG) and CA1, and mimicked acute stress effect on plasticity in these two subregions. In the present study, we investigated the mechanisms that mobilize the BLA to differentially alter plasticity in DG and CA1. Glucocorticoid receptors antagonist RU 38486 or beta-adrenoceptor antagonist propranolol were microinfused in the BLA, 10 min prior to BLA activation-induced modulation of long-term potentiation (LTP) in DG and CA1. The results showed that neither glucocorticoid nor noradrenergic transmissions in the BLA are necessary for LTP induction and for the impairing effect of amygdala activation on CA1 LTP. In contrast, blockade of glucocorticoid or noradrenergic transmission in BLA, increased baseline synaptic transmission in the DG, but suppressed the enhancing effect of BLA activation on DG LTP. These findings provide further evidence for a differential amygdala control of hippocampal subregions as well as for differential memory processes involving CA1 and DG. They also provide insight into how stress hormones exert their actions on the circuits involved in these processes.

NMDA-dependent facilitation of corticostriatal plasticity by the amygdala

Emotions generally improve memory, and the basolateral amygdala (BLA) is believed to mediate this effect. After emotional arousal, BLA neurons increase their firing rate, facilitating memory consolidation in BLA targets. The enhancing effects of BLA activity extend to various types of memories, including motor learning, which is thought to involve activity-dependent plasticity at corticostriatal synapses. However, the underlying mechanisms are unknown. Here we show that the NMDA-to-AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) ratio is nearly twice as high at BLA as compared with cortical synapses onto principal striatal neurons and that activation of BLA inputs greatly facilitates long-term potentiation induction at corticostriatal synapses. This facilitation was NMDA-dependent, but it occurred even when BLA and cortical stimuli were 0.5 s apart during long-term potentiation induction. Overall, these results suggest that BLA activity opens long time windows during which the induction of corticostriatal plasticity is facilitated.

Motivation, emotion, and their inhibitory control mirrored in brain oscillations

Recent studies suggest brain oscillations as a mechanism for cerebral integration. Such integration can exist across a number of functional domains, with different frequency rhythms associated with each domain. Here, evidence is summarized which shows that delta oscillations depend on activity of motivational systems and participate in salience detection. Theta oscillations are involved in memory and emotional regulation. Alpha oscillations participate in inhibitory processes which contribute to a variety of cognitive operations such as attention and memory. The importance of inhibitory functions associated with alpha oscillations increases during the course of evolution. In ontogenesis, these functions develop later and may be more sensitive to a variety of detrimental environmental influences. In a number of developmental stages and pathological conditions, a deficient alpha and/or increased slow-wave activity are associated with cognitive deficits and a lack of inhibitory control. It is shown that slow-wave and alpha oscillations are reciprocally related to each other. This reciprocal relationship may reflect an inhibitory control over motivational and emotional drives which is implemented by the prefrontal cortex.