Distinct electrophysiological properties of long-range GABAergic and glutamatergic neurons from the lateral amygdala to the auditory cortex of the mouse

Differentiating between auditory signals of various emotional significance plays a crucial role in an individual's ability to thrive and excel in social interactions and in survival. Multiple approaches, including anatomical studies, electrophysiological investigations, imaging techniques, optogenetics and chemogenetics, have confirmed that the auditory cortex (AC) impacts fear-related behaviours driven by auditory stimuli by conveying auditory information to the lateral amygdala (LA) through long-range excitatory glutamatergic and GABAergic connections. In addition, the LA provides glutamatergic projections to the AC which are important to fear memory expression and are modified by associative fear learning. Here we test the hypothesis that the LA also sends long-range direct inhibitory inputs to the cortex. To address this fundamental question, we used anatomical and electrophysiological approaches, allowing us to directly assess the nature of GABAergic inputs from the LA to the AC in the mouse. Our findings elucidate the existence of a long-range inhibitory pathway from the LA to the AC (LAC) via parvalbumin-expressing (LAC-Parv) and somatostatin-expressing (LAC-SOM) neurons. This research identifies distinct electrophysiological properties for genetically defined long-range GABAergic neurons involved in the communication between the LA and the cortex (LAC-Parv inhibitory projections → AC neurons; LAC-Som inhibitory projections → AC neurons) within the lateral amygdala cortical network. KEY POINTS: The mouse auditory cortex receives inputs from the lateral amygdala. Retrograde viral tracing techniques allowed us to identify two previously undescribed lateral amygdala to auditory cortex (LAC) GABAergic projecting neurons. Extensive electrophysiological, morphological and anatomical characterization of LAC neurons is provided here, demonstrating key differences in the three populations. This study paves the way for a better understanding of the growing complexity of the cortico-amygdala-cortico circuit.

Microglial activation in the lateral amygdala promotes anxiety-like behaviors in mice with chronic moderate noise exposure

BACKGROUND

Long-term non-traumatic noise exposure, such as heavy traffic noise, can elicit emotional disorders in humans. However, the underlying neural substrate is still poorly understood.

METHODS

We exposed mice to moderate white noise for 28 days to induce anxiety-like behaviors, measured by open-field, elevated plus maze, and light-dark box tests. In vivo multi-electrode recordings in awake mice were used to examine neuronal activity. Chemogenetics were used to silence specific brain regions. Viral tracing, immunofluorescence, and confocal imaging were applied to define the neural circuit and characterize the morphology of microglia.

RESULTS

Exposure to moderate noise for 28 days at an 85-dB sound pressure level resulted in anxiety-like behaviors in open-field, elevated plus maze, and light-dark box tests. Viral tracing revealed that fibers projecting from the auditory cortex and auditory thalamus terminate in the lateral amygdala (LA). A noise-induced increase in spontaneous firing rates of the LA and blockade of noise-evoked anxiety-like behaviors by chemogenetic inhibition of LA glutamatergic neurons together confirmed that the LA plays a critical role in noise-induced anxiety. Noise-exposed animals were more vulnerable to anxiety induced by acute noise stressors than control mice. In addition to these behavioral abnormalities, ionized calcium-binding adaptor molecule 1 (Iba-1)-positive microglia in the LA underwent corresponding morphological modifications, including reduced process length and branching and increased soma size following noise exposure. Treatment with minocycline to suppress microglia inhibited noise-associated changes in microglial morphology, neuronal electrophysiological activity, and behavioral changes. Furthermore, microglia-mediated synaptic phagocytosis favored inhibitory synapses, which can cause an imbalance between excitation and inhibition, leading to anxiety-like behaviors.

CONCLUSIONS

Our study identifies LA microglial activation as a critical mediator of noise-induced anxiety-like behaviors, leading to neuronal and behavioral changes through selective synapse phagocytosis. Our results highlight the pivotal but previously unrecognized roles of LA microglia in chronic moderate noise-induced behavioral changes.

Potentiation of cholinergic and corticofugal inputs to the lateral amygdala in threat learning

The amygdala, cholinergic basal forebrain, and higher-order auditory cortex (HO-AC) regulate brain-wide plasticity underlying auditory threat learning. Here, we perform multi-regional extracellular recordings and optical measurements of acetylcholine (ACh) release to characterize the development of discriminative plasticity within and between these brain regions as mice acquire and recall auditory threat memories. Spiking responses are potentiated for sounds paired with shock (CS+) in the lateral amygdala (LA) and optogenetically identified corticoamygdalar projection neurons, although not in neighboring HO-AC units. Spike- or optogenetically triggered local field potentials reveal enhanced corticofugal-but not corticopetal-functional coupling between HO-AC and LA during threat memory recall that is correlated with pupil-indexed memory strength. We also note robust sound-evoked ACh release that rapidly potentiates for the CS+ in LA but habituates across sessions in HO-AC. These findings highlight a distributed and cooperative plasticity in LA inputs as mice learn to reappraise neutral stimuli as possible threats.

Synaptic and intrinsic plasticity within overlapping lateral amygdala ensembles following fear conditioning

Introduction

New learning results in modulation of intrinsic plasticity in the underlying brain regions. Such changes in intrinsic plasticity can influence allocation and encoding of future memories such that new memories encoded during the period of enhanced excitability are linked to the original memory. The temporal window during which the two memories interact depends upon the time course of intrinsic plasticity following new learning.

Methods

Using the well-characterized lateral amygdala-dependent auditory fear conditioning as a behavioral paradigm, we investigated the time course of changes in intrinsic excitability within lateral amygdala neurons.

Results

We found transient changes in the intrinsic excitability of amygdala neurons. Neuronal excitability was increased immediately following fear conditioning and persisted for up to 4 days post-learning but was back to naïve levels 10 days following fear conditioning. We also determined the relationship between learning-induced intrinsic and synaptic plasticity. Synaptic plasticity following fear conditioning was evident for up to 24 h but not 4 days later. Importantly, we demonstrated that the enhanced neuronal intrinsic excitability was evident in many of the same neurons that had undergone synaptic plasticity immediately following fear conditioning. Interestingly, such a correlation between synaptic and intrinsic plasticity following fear conditioning was no longer present 24 h post-learning.

Discussion

These data demonstrate that intrinsic and synaptic changes following fear conditioning are transient and co-localized to the same neurons. Since intrinsic plasticity following fear conditioning is an important determinant for the allocation and consolidation of future amygdala-dependent memories, these findings establish a time course during which fear memories may influence each other.

A central alarm system that gates multi-sensory innate threat cues to the amygdala

Perception of threats is essential for survival. Previous findings suggest that parallel pathways independently relay innate threat signals from different sensory modalities to multiple brain areas, such as the midbrain and hypothalamus, for immediate avoidance. Yet little is known about whether and how multi-sensory innate threat cues are integrated and conveyed from each sensory modality to the amygdala, a critical brain area for threat perception and learning. Here, we report that neurons expressing calcitonin gene-related peptide (CGRP) in the parvocellular subparafascicular nucleus in the thalamus and external lateral parabrachial nucleus in the brainstem respond to multi-sensory threat cues from various sensory modalities and relay negative valence to the lateral and central amygdala, respectively. Both CGRP populations and their amygdala projections are required for multi-sensory threat perception and aversive memory formation. The identification of unified innate threat pathways may provide insights into developing therapeutic candidates for innate fear-related disorders.

The entorhinal cortex modulates trace fear memory formation and neuroplasticity in the mouse lateral amygdala via cholecystokinin

Although fear memory formation is essential for survival and fear-related mental disorders, the neural circuitry and mechanism are incompletely understood. Here, we utilized trace fear conditioning to study the formation of trace fear memory in mice. We identified the entorhinal cortex (EC) as a critical component of sensory signaling to the amygdala. We adopted both loss-of-function and gain-of-function experiments to demonstrate that release of the cholecystokinin (CCK) from the EC is required for trace fear memory formation. We discovered that CCK-positive neurons project from the EC to the lateral nuclei of the amygdala (LA), and inhibition of CCK-dependent signaling in the EC prevented long-term potentiation of the auditory response in the LA and formation of trace fear memory. In summary, high-frequency activation of EC neurons triggers the release of CCK in their projection terminals in the LA, potentiating auditory response in LA neurons. The neural plasticity in the LA leads to trace fear memory formation.

Turnover of fear engram cells by repeated experience

A sparse population of neurons active during a learning event has been identified as memory engram cells. However, cells that are recruited to support memory when experience is repeated have been scarcely explored. Evidence from previous studies provides contradictory views. To address these questions, we employed learning-dependent cell labeling in the lateral amygdala (LA) and applied electrophysiological recording, spine imaging, and optogenetic tools to the labeled neurons with or without retraining. We found that engram cells established from original fear learning became dispensable for memory retrieval specifically with relearning, and this correlated with a reduction of synaptic transmission and loss of dendritic spines in these neurons. Despite such decreased connectivity, direct activation of these neurons resulted in fear-memory recall. We further identified that repeated memory was encoded in neurons active during relearning. These results suggest a shift in neuronal ensembles encoding fear memory in the LA by relearning through disconnection of the existing engram neurons established from original experience.

Hyperexcitability and Loss of Feedforward Inhibition Contribute to Aberrant Plasticity in the Fmr1KO Amygdala

Fragile X syndrome (FXS) is a neurodevelopmental disorder (NDD) characterized by intellectual disability, autism spectrum disorders (ASDs), and anxiety disorders. The disruption in the function of the FMR1 gene results in a range of alterations in cellular and synaptic function. Previous studies have identified dynamic alterations in inhibitory neurotransmission in early postnatal development in the amygdala of the mouse model of FXS. However, little is known about how these changes alter microcircuit development and plasticity in the lateral amygdala (LA). Using whole-cell patch clamp electrophysiology, we demonstrate that principal neurons (PNs) in the LA exhibit hyperexcitability with a concomitant increase in the synaptic strength of excitatory synapses in the BLA. Further, reduced feed-forward inhibition appears to enhance synaptic plasticity in the FXS amygdala. These results demonstrate that plasticity is enhanced in the amygdala of the juvenile Fmr1 knock-out (KO) mouse and that E/I imbalance may underpin anxiety disorders commonly seen in FXS and ASDs.

Fear Memory Relapse: The Importance of Input Associativity

An inherent property of extinguished fear memories is that the fear may return. A recent study in mice by Li et al. provides novel insights into the mechanisms underlying the relapse of an extinguished memory through converging sensory and contextual cues from the auditory cortex (ACx) and ventral hippocampus (vHPC) to the lateral amygdala (LA).

In search of common developmental and evolutionary origin of the claustrum and subplate

The human claustrum, a major hub of widespread neocortical connections, is a thin, bilateral sheet of gray matter located between the insular cortex and the striatum. The subplate is a largely transient cortical structure that contains some of the earliest generated neurons of the cerebral cortex and has important developmental functions to establish intra- and extracortical connections. In human and macaque some subplate cells undergo regulated cell death, but some remain as interstitial white matter cells. In mouse and rat brains a compact layer is formed, Layer 6b, and it remains underneath the cortex, adjacent to the white matter. Whether Layer 6b in rodents is homologous to primate subplate or interstitial white matter cells is still debated. Gene expression patterns, such as those of Nurr1/Nr4a2, have suggested that the rodent subplate and the persistent subplate cells in Layer 6b and the claustrum might have similar origins. Moreover, the birthdates of the claustrum and Layer 6b are similarly precocious in mice. These observations prompted our speculations on the common developmental and evolutionary origin of the claustrum and the subplate. Here we systematically compare the currently available data on cytoarchitecture, evolutionary origin, gene expression, cell types, birthdates, neurogenesis, lineage and migration, circuit connectivity, and cell death of the neurons that contribute to the claustrum and subplate. Based on their similarities and differences we propose a partially common early evolutionary origin of the cells that become claustrum and subplate, a likely scenario that is shared in these cell populations across all amniotes.

Hippocampal Representation of Threat Features and Behavior in a Human Approach-Avoidance Conflict Anxiety Task

Decisions under threat are crucial to survival and require integration of distinct situational features, such as threat probability and magnitude. Recent evidence from human lesion and neuroimaging studies implicated anterior hippocampus (aHC) and amygdala in approach-avoidance decisions under threat, and linked their integrity to cautious behavior. Here we sought to elucidate how threat dimensions and behavior are represented in these structures. Twenty human participants (11 female) completed an approach-avoidance conflict task during high-resolution fMRI. Participants could gather tokens under threat of capture by a virtual predator, which would lead to token loss. Threat probability (predator wake-up rate) and magnitude (amount of token loss) varied on each trial. To disentangle effects of threat features, and ensuing behavior, we performed a multifold parametric analysis. We found that high threat probability and magnitude related to BOLD signal in left aHC/entorhinal cortex. However, BOLD signal in this region was better explained by avoidance behavior than by these threat features. A priori ROI analysis confirmed the relation of aHC BOLD response with avoidance. Exploratory subfield analysis revealed that this relation was specific to anterior CA2/3 but not CA1. Left lateral amygdala responded to low and high, but not intermediate, threat probability. Our results suggest that aHC BOLD signal is better explained by avoidance behavior than by threat features in approach-avoidance conflict. Rather than representing threat features in a monotonic manner, it appears that aHC may compute approach-avoidance decisions based on integration of situational threat features represented in other neural structures.SIGNIFICANCE STATEMENT An effective threat anticipation system is crucial to survival across species. Natural threats, however, are diverse and have distinct features. To be able to adapt to different modes of danger, the brain needs to recognize these features, integrate them, and use them to modify behavior. Our results disclose the human anterior hippocampus as a likely arbiter of approach-avoidance decisions harnessing compound environmental information while partially replicating previous findings and blending into recent efforts to illuminate the neural basis of approach-avoidance conflict in humans.

Corticotropin-Releasing Factor Receptor-1 Neurons in the Lateral Amygdala Display Selective Sensitivity to Acute and Chronic Ethanol Exposure

The lateral amygdala (LA) serves as the point of entry for sensory information within the amygdala complex, a structure that plays a critical role in emotional processes and has been implicated in alcohol use disorders. Within the amygdala, the corticotropin-releasing factor (CRF) system has been shown to mediate some of the effects of both stress and ethanol, but the effects of ethanol on specific CRF1 receptor circuits in the amygdala have not been fully established. We used male CRF1:GFP reporter mice to characterize CRF1-expressing (CRF1+) and nonexpressing (CRF1-) LA neurons and investigate the effects of acute and chronic ethanol exposure on these populations. The CRF1+ population was found to be composed predominantly of glutamatergic projection neurons with a minority subpopulation of interneurons. CRF1+ neurons exhibited a tonic conductance that was insensitive to acute ethanol. CRF1- neurons did not display a basal tonic conductance, but the application of acute ethanol induced a δ GABAA receptor subunit-dependent tonic conductance and enhanced phasic GABA release onto these cells. Chronic ethanol increased CRF1+ neuronal excitability but did not significantly alter phasic or tonic GABA signaling in either CRF1+ or CRF1- cells. Chronic ethanol and withdrawal also did not alter basal extracellular GABA or glutamate transmitter levels in the LA/BLA and did not alter the sensitivity of GABA or glutamate to acute ethanol-induced increases in transmitter release. Together, these results provide the first characterization of the CRF1+ population of LA neurons and suggest mechanisms for differential acute ethanol sensitivity within this region.

Serotonergic control of GABAergic inhibition in the lateral amygdala

Much evidence implicates the serotonergic regulation of the amygdala in anxiety. Thus the present study was undertaken to characterize the influence of serotonin (5-HT) on principal neurons (PNs) of the rat lateral amygdala (LA), using whole cell recordings in vitro. Because inhibition is a major determinant of PN activity, we focused on the control of GABAergic transmission by 5-HT. IPSCs were elicited by local electrical stimulation of LA in the presence of glutamate receptor antagonists. We found that 5-HT reduces GABA_A inhibitory postsynaptic currents (IPSCs) via presynaptic 5-HT_1B receptors. While the presynaptic inhibition of GABA release also attenuated GABA_B currents, this effect was less pronounced than for GABA_A currents because 5-HT also induced a competing postsynaptic enhancement of GABA_B currents. That is, GABA_B currents elicited by pressure application of GABA or baclofen were enhanced by 5-HT. In addition, we obtained evidence suggesting that 5-HT differentially regulates distinct subsets of GABAergic synapses. Indeed, GABA_A IPSCs were comprised of two components: a relatively 5-HT-insensitive IPSC that had a fast time course and a 5-HT-sensitive component that had a slower time course. Because the relative contribution of these two components varied depending on whether neurons were recorded at proximity versus at a distance from the stimulating electrodes, we speculate that distinct subtypes of local-circuit cells contribute the two contingents of GABAergic synapses. Overall, our results indicate that 5-HT is a potent regulator of synaptic inhibition in LA.NEW & NOTEWORTHY We report that 5-HT, acting via presynaptic 5-HT_1B receptors, attenuates GABA_A IPSCs by reducing GABA release in the lateral amygdala (LA). In parallel, 5-HT enhances GABA_B currents postsynaptically, such that GABA_B inhibitory postsynaptic currents (IPSCs) are relatively preserved from the presynaptic inhibition of GABA release. We also found that the time course of 5-HT-sensitive and -insensitive GABA_A IPSCs differ. Together, these results indicate that 5-HT is a potent regulator of synaptic inhibition in LA.

Localization of Contextual and Context Removed Auditory Fear Memory within the Basolateral Amygdala Complex

Debilitating and persistent fear memories can rapidly form in humans following exposure to traumatic events. Fear memories can also be generated and studied in animals via Pavlovian fear conditioning. The current study was designed to evaluate basolateral amygdala complex (BLC) involvement following the formation of different fear memories (two contextual fear memories and one adjusted auditory fear memory). Fear memories were created in the same context with five 1.0 mA (0.50 s) foot-shocks and, where necessary, five auditory tones (5 kHz, 75 dB, 20 s). The adjusted auditory fear conditioning protocol was employed to remove background contextual fear and produce isolated auditory fear memories. Immunofluorescent labeling was utilized to identify neurons expressing immediate early genes (IEGs). We found the two contextual fear conditioning (CFC) procedures to produce similar levels of fear-related freezing to context. Contextual fear memories produced increases in BLC IEG expression with distinct and separate patterns of expression. These data suggest contextual fear memories created in slightly altered contexts, can produce unique patterns of amygdala activation. The adjusted auditory fear conditioning procedure produced memories to a tone, but not to a context. This group, where no contextual fear was present, had a significant reduction in BLC IEG expression. These data suggest background contextual fear memories, created in standard auditory fear conditioning protocols, contribute significantly to increases in amygdala activation.

Glucocorticoid receptor-mediated amygdalar metaplasticity underlies adaptive modulation of fear memory by stress

Glucocorticoid receptor (GR) is crucial for signaling mediated by stress-induced high levels of glucocorticoids. The lateral nucleus of the amygdala (LA) is a key structure underlying auditory-cued fear conditioning. Here, we demonstrate that genetic disruption of GR in the LA (LAGRKO) resulted in an auditory-cued fear memory deficit for strengthened conditioning. Furthermore, the suppressive effect of a single restraint stress (RS) prior to conditioning on auditory-cued fear memory in floxed GR (control) mice was abolished in LAGRKO mice. Optogenetic induction of long-term depression (LTD) at auditory inputs to the LA reduced auditory-cued fear memory in RS-exposed LAGRKO mice, and in contrast, optogenetic induction of long-term potentiation (LTP) increased auditory-cued fear memory in RS-exposed floxed GR mice. These findings suggest that prior stress suppresses fear conditioning-induced LTP at auditory inputs to the LA in a GR-dependent manner, thereby protecting animals from encoding excessive cued fear memory under stress conditions.

Blocking constitutive activity of GHSR1a in the lateral amygdala facilitates acquisition of conditioned taste aversion

Ghrelin is a circulating peptide hormone promoting feeding and regulating energy metabolism in human and rodents. Ghrelin functions by binding to its receptor, the growth hormone secretagogue receptor 1a (GHSR1a), which are widely distributed throughout the brain including the amygdala, a brain region important for regulating valenced behavior, such as aversion. Interestingly, GHSR1a was once characterized by highly constitutive, ligand-independent activity. However, the physiological importance of such ligand-independent signaling on aversive memory processing has not been tested yet. Here, we applied [D-Arg¹, D-Phe⁵, D-Trp^7,9, Leu¹¹]-Substance P (D-SP), a full inverse agonist for GHSR1a, into the lateral amygdala (LA) and investigated the effect of blocking GHSR1a constitutive activity on conditioned taste aversion (CTA) in rats. We found that intra-LA infusion of a single low dose of D-SP (8ng/0.5μl/side) facilitates CTA acquisition. Moreover, pre-administration of a high dose of D-SP into the LA abolishes the suppressive effect of exogenous ghrelin on CTA acquisition. In contrast, pre-administration of the same dose of D-SP does not affect the suppression of substance P, a potent neurokinin-1 (NK1) receptor ligand, on CTA. Therefore, our data indicated that the spontaneous or basal activity of GHSR1a signaling in the LA might interfere with CTA memory formation. D-SP decreases the constitutive activity of GHSR1a and thus facilitates CTA. Altogether, our present findings along with previous results support the idea that ghrelin/GHSR1a signaling in the LA circuit blocks conditioned taste aversion.

Amount of fear extinction changes its underlying mechanisms

There has been a longstanding debate on whether original fear memory is inhibited or erased after extinction. One possibility that reconciles this uncertainty is that the inhibition and erasure mechanisms are engaged in different phases (early or late) of extinction. In this study, using single-session extinction training and its repetition (multiple-session extinction training), we investigated the inhibition and erasure mechanisms in the prefrontal cortex and amygdala of rats, where neural circuits underlying extinction reside. The inhibition mechanism was prevalent with single-session extinction training but faded when single-session extinction training was repeated. In contrast, the erasure mechanism became prevalent when single-session extinction training was repeated. Moreover, ablating the intercalated neurons of amygdala, which are responsible for maintaining extinction-induced inhibition, was no longer effective in multiple-session extinction training. We propose that the inhibition mechanism operates primarily in the early phase of extinction training, and the erasure mechanism takes over after that.

Directional spread of activity in synaptic networks of the human lateral amygdala

Spontaneous epileptiform activity has previously been observed in lateral amygdala (LA) slices derived from patients with intractable-temporal lobe epilepsy. The present study aimed to characterize intranuclear LA synaptic connectivity and to test the hypothesis that differences in the spread of flow of neuronal activity may relate to spontaneous epileptiform activity occurrence. Electrical activity was evoked through electrical microstimulation in acute human brain slices containing the LA, signals were recorded as local field potentials combined with fast optical imaging of voltage-sensitive dye fluorescence. Sites of stimulation and recording were systematically varied. Following recordings, slices were anatomically reconstructed using two-dimensional unitary slices as a reference for coronal and parasagittal planes. Local spatial patterns and spread of activity were assessed by incorporating the coordinates of electrical and optical recording sites into the respective unitary slice. A preferential directional spread of evoked electrical signals was observed from ventral to dorsal, rostral to caudal and medial to lateral regions in the LA. No differences in spread of evoked activity were observed between spontaneously and non-spontaneously active LA slices, i.e. basic properties of evoked synaptic responses were similar in the two functional types of LA slices, including input-output relationship, and paired-pulse depression. These results indicate a directed propagation of synaptic signals within the human LA in spontaneously active epileptic slices. We suggest that the lack of differences in local and in systemic information processing has to be found in confined epileptiform circuits within the amygdala likely involving well-known "epileptic neurons".

Diazepam reduces excitability of amygdala and further influences auditory cortex following sodium salicylate treatment in rats

CONCLUSION

Diazepam can reduce the excitability of lateral amygdala and eventually suppress the excitability of the auditory cortex in rats following salicylate treatment, indicating the regulating effect of lateral amygdala to the auditory cortex in the tinnitus procedure.

OBJECTIVE

To study the spontaneous firing rates (SFR) of the auditory cortex and lateral amygdala regulated by diazepam in the tinnitus rat model induced by sodium salicylate.

MATERIALS AND METHODS

This study first created a tinnitus rat modal induced by sodium salicylate, and recorded SFR of both auditory cortex and lateral amygdala. Then diazepam was intraperitoneally injected and the SFR changes of lateral amygdala recorded. Finally, diazepam was microinjected on lateral amygdala and the SFR changes of the auditory cortex recorded.

RESULTS

Both SFRs of the auditory cortex and lateral amygdala increased after salicylate treatment. SFR of lateral amygdala decreased after intraperitoneal injection of diazepam. Microinjecting diazepam to lateral amygdala decreased SFR of the auditory cortex ipsilaterally and contralaterally.

Endothelin-1-induced mini-stroke in the dorsal hippocampus or lateral amygdala results in deficits in learning and memory

Functional and structural alterations in brain connectivity associated with brain ischemia have been extensively studied. However, the mechanism whereby local ischemia in deep brain region affect brain functions is still unknown. Here, we first established a mini-stroke model by infusion of endothelin-1 (ET-1) into the dorsal hippocampus or the lateral amygdala, and then investigated how these mini-infarcts affected brain functions associated with these regions. We found that rats with ET-1 infusion showed deficit in recall of contextual fear memory, but not in learning process and recall of tone fear memory. In novel object task, ET-1 in the hippocampus also eliminated object identity memory. ET-1 in the lateral amygdale affected acquisition of fear conditioning and disrupted retention of tone-conditioned fear, but did not impair retention of contextual fear. These findings suggest that ET-1-induced mini-infarct in deep brain area leads to functional deficits in learning and memory associated with these regions.

Social buffering suppresses fear-associated activation of the lateral amygdala in male rats: behavioral and neurophysiological evidence

In social mammals, the presence of an affiliative conspecific reduces stress responses, a phenomenon referred to as “social buffering.”In a previous study, we found that the presence of a conspecific animal ameliorated a variety of stress responses to an aversive conditioned stimulus (CS), including freezing and Fos expression in the lateral amygdala (LA) of male rats. Although these findings suggest that the presence of a conspecific animal suppresses neural activity in the LA, direct neurophysiological evidence of suppressed activity in the LA during social buffering is still lacking. In the present study, we analyzed freezing behavior and local field potentials in the LA of fear-conditioned rats in response to the CS, in the presence or absence of a conspecific. After auditory aversive conditioning, the CS was presented to the conditioned rats in the presence or absence of a conspecific animal, on 2 successive days. The presence of a conspecific animal significantly decreased the mean peak amplitudes of auditory evoked field potentials, gamma oscillations (25–75 Hz) and high frequency oscillations (100–300 Hz) in the LA. Furthermore, magnitudes of these neural responses positively correlated with freezing duration of the fear-conditioned rats. The results provide the first electrophysiological evidence that social buffering suppresses CS-induced activation in the LA, which consequently reduces conditioned fear responses.

Group I mGluR-dependent depotentiation in the lateral amygdala does not require the removal of calcium-permeable AMPA receptors

There is conflicting evidence regarding whether calcium-permeable receptors are removed during group I mGluR-mediated synaptic depression. In support of this hypothesis, AMPAR rectification, a correlative index of the synaptic expression of GluA2-lacking calcium–permeable AMPARs (CP-AMPARs), is known to decrease after the induction of several types of group I mGluR-mediated long-term depression (LTD), suggesting that a significant proportion of synaptic CP-AMPARs is removed during synaptic depression. We have previously demonstrated that fear conditioning-induced synaptic potentiation in the lateral amygdala is reversed by group 1 mGluR-mediated depotentiation. Here, we examined whether CP-AMPARs are removed by mGluR1-mediated depotentiation of fear conditioning-induced synaptic potentiation. The synaptic expression of CP-AMPARs was negligible before, increased significantly 12 h after, and returned to baseline 48 h after fear conditioning, as evidenced by the changes in the sensitivity of lateral amygdala synaptic responses to NASPM. Importantly, the sensitivity to NASPM was not altered after induction of depotentiation. Our findings, together with previous results, suggest that the removal of CP-AMPARs is not required for the depotentiation of fear conditioning-induced synaptic potentiation at lateral amygdala synapses.

Beta-adrenergic receptors in the lateral nucleus of the amygdala contribute to the acquisition but not the consolidation of auditory fear conditioning

Beta-adrenergic receptors (βARs) have long been associated with fear disorders and with learning and memory. However, the contribution of these receptors to Pavlovian fear conditioning, a leading behavioral model for studying fear learning and memory, is still poorly understood. The aim of this study was to investigate the involvement of βAR activation in the acquisition, consolidation and expression of fear conditioning. We focused on manipulations of βARs in the lateral nucleus of the amygdala (LA) because of the well-established contribution of this area to fear conditioning. Specifically, we tested the effects of intra-LA microinfusions of the βAR antagonist, propranolol, on learning and memory for auditory Pavlovian fear conditioning in rats. Pre-training propranolol infusions disrupted the initial acquisition, short-term memory (STM), and long-term memory (LTM) for fear conditioning, but infusions immediately after training had no effect. Further, infusion of propranolol prior to testing fear responses did not affect fear memory expression. These findings indicate that amygdala βARs are important for the acquisition but not the consolidation of fear conditioning.

Ultrastructural characterization of noradrenergic axons and Beta-adrenergic receptors in the lateral nucleus of the amygdala

Norepinephrine (NE) is thought to play a key role in fear and anxiety, but its role in amygdala-dependent Pavlovian fear conditioning, a major model for understanding the neural basis of fear, is poorly understood. The lateral nucleus of the amygdala (LA) is a critical brain region for fear learning and regulating the effects of stress on memory. To understand better the cellular mechanisms of NE and its adrenergic receptors in the LA, we used antibodies directed against dopamine beta-hydroxylase (DβH), the synthetic enzyme for NE, or against two different isoforms of the beta-adrenergic receptors (βARs), one that predominately recognizes neurons (βAR 248) and the other astrocytes (βAR 404), to characterize the microenvironments of DβH and βAR. By electron microscopy, most DβH terminals did not make synapses, but when they did, they formed both asymmetric and symmetric synapses. By light microscopy, βARs were present in both neurons and astrocytes. Confocal microscopy revealed that both excitatory and inhibitory neurons express βAR248. By electron microscopy, βAR 248 was present in neuronal cell bodies, dendritic shafts and spines, and some axon terminals and astrocytes. When in dendrites and spines, βAR 248 was frequently concentrated along plasma membranes and at post-synaptic densities of asymmetric (excitatory) synapses. βAR 404 was expressed predominately in astrocytic cell bodies and processes. These astrocytic processes were frequently interposed between unlabeled terminals or ensheathed asymmetric synapses. Our findings provide a morphological basis for understanding ways in which NE may modulate transmission by acting via synaptic or non-synaptic mechanisms in the LA.

GABA(C) Receptors in the Lateral Amygdala: A Possible Novel Target for the Treatment of Fear and Anxiety Disorders?

Activation of GABA(A)Rs in the lateral nucleus of the amygdala (LA), a key site of plasticity underlying fear learning, impairs fear learning. The role of GABA(C)Rs in the LA and other brain areas is poorly understood. GABA(C)Rs could be an important novel target for pharmacological treatments of anxiety-related disorders since, unlike GABA(A)Rs, GABA(C)Rs do not desensitize. To detect functional GABA(C)Rs in the LA we performed whole cell patch clamp recordings in vitro. We found that GABA(A)Rs and GABA(B)Rs blockade lead to a reduction of evoked inhibition and an increase increment of excitation, but activation of GABA(C)Rs caused elevations of evoked excitation, while blocking GABA(C)Rs reduced evoked excitation. Based on this evidence we tested whether GABA(C)Rs in LA contribute to fear learning in vivo. It is established that activation of GABA(A)Rs leads to blockage of fear learning. Application of GABA(C) drugs had a very different effect; fear learning was enhanced by activating and attenuated by blocking GABA(C)Rs in the LA. Our results suggest that GABA(C) and GABA(A)Rs play opposing roles in modulation of associative plasticity in LA neurons of rats. This novel role of GABA(C)Rs furthers our understanding of GABA receptors in fear memory acquisition and storage and suggests a possible novel target for the treatment of fear and anxiety disorders.

Expression of AP-1 family transcription factors in the amygdala during conditioned taste aversion learning: role for Fra-2

Conditioned taste aversion (CTA) learning occurs after the pairing of a novel taste with a toxin (e.g. sucrose with LiCl). The immediate early gene c-Fos is necessary for CTA learning, but c-Fos alone cannot be sufficient for consolidation. The expression of other AP-1 proteins from the Fos- and Jun-families may also be required shortly after conditioning for CTA consolidation. To screen for the expression of AP-1 transcription factors within small subregions, RT-PCR analysis was used after laser capture microdissection of the amygdala. Rats were infused intraorally with 5% sucrose (6 ml/6 min) or injected with LiCl (12 ml/kg, 0.15 M, i.p.) or given sucrose paired with LiCl (sucrose/LiCl), or not treated; 1 h later their brains were dissected. The lateral (LA), basolateral (BLA), and central (CeA) subnuclei of the amgydala of single 5 microm sections from individual rats were dissected using the Arcturus PixCell II system. Semi-quantitative RT-PCR showed the consistent presence of c-Fos, Fra-2, c-Jun, and JunD in the amygdala. In situ hybridization confirmed that c-Fos and Fra-2 mRNA expression was increased in the CeA after LiCl and sucrose/LiCl treatment. Immunohistochemistry for Fra-2 revealed high baseline levels of Fra-2 protein in the BLA and CeA, but also an increase in Fra-2 in the BLA and CeA after LiCl and sucrose/LiCl treatment. The similarity of response in LiCl and sucrose/LiCl treated groups might reflect activation by LiCl in both groups. To control for the effects of LiCl, rats were tested in a learned safety experiment. Fra-2 and c-Fos were examined in response to sucrose/LiCl in rats with prior familiarity with sucrose compared to rats without prior exposure to sucrose. The familiar (pre-exposure) group showed a significantly decreased number of Fra-2-positive cells compared with the novel group in the BLA, but not in the CeA. Because pre-exposure to sucrose attenuates CTA learning, a decreased cellular response in pre-exposed rats suggests a specific correlation with CTA learning. Changes in Fra-2 and c-Fos expression in the BLA and CeA at the time of conditioning, together with constitutive expression of c-Jun and JunD, may contribute to CTA learning.

Amygdala depotentiation and fear extinction

Auditory fear memory is thought to be maintained by fear conditioning-induced potentiation of synaptic efficacy, which involves enhanced expression of surface AMPA receptor (AMPAR) at excitatory synapses in the lateral amygdala (LA). Depotentiation, reversal of conditioning-induced potentiation, has been proposed as a cellular mechanism for fear extinction; however, a direct link between depotentiation and extinction has not yet been tested. To address this issue, we applied both ex vivo and in vivo approaches to rats in which fear memory had been consolidated. A unique form of depotentiation reversed conditioning-induced potentiation at thalamic input synapses onto the LA (T-LA synapses) ex vivo. Extinction returned the enhanced T-LA synaptic efficacy observed in conditioned rats to baseline and occluded the depotentiation. Consistently, extinction reversed conditioning-induced enhancement of surface expression of AMPAR subunits in LA synaptosomal preparations. A GluR2-derived peptide that blocks regulated AMPAR endocytosis inhibited depotentiation, and microinjection of a cell-permeable form of the peptide into the LA attenuated extinction. Our results are consistent with the use of depotentiation to weaken potentiated synaptic inputs onto the LA during extinction and provide strong evidence that AMPAR removal at excitatory synapses in the LA underlies extinction.

Context-dependent neuronal activity in the lateral amygdala represents fear memories after extinction

The context in which fear memories are extinguished has important implications for treating human fear and anxiety disorders. Extinction of Pavlovian fear conditioning is context specific; after extinction, fear responses are reduced only in the extinction context and remain elevated in every other context. Contextual modulation of spike firing in the amygdala is a putative mechanism for the context-specific expression of extinguished fear. To test this possibility, we conditioned rats to fear two auditory conditional stimuli (CSs) and then extinguished each CS in separate and distinct contexts. Thereafter, single-unit activity in the lateral nucleus of the amygdala (LA) and freezing behavior were recorded during tests in which each CS was presented in each extinction context. Hence, each CS was tested in its own extinction context and in the context of the other CS. Conditional freezing was context dependent; fear to an extinguished CS was low in its own extinction context and high in the other test context. Similarly, the majority of LA neurons exhibited context-dependent spike firing; short-latency spike firing was greater to both CSs when they were presented outside of their own extinction context. In contrast, behavioral and neuronal responses to either non-extinguished CSs or habituated auditory stimuli were not contextually modulated. Context-dependent neuronal activity in the LA may be an important mechanism for disambiguating the meaning of fear signals, thereby enabling appropriate behavioral responses to such stimuli.

Contribution of NR2B subunits to synaptic transmission in amygdaloid interneurons

Synaptic responses of interneurons in the rat lateral amygdala (LA) to electrical microstimulation of putative cortical and thalamic afferents were studied in slice preparations in situ. The EPSPs at both thalamic and cortical inputs were composed of two major components that were sensitive to 6,7-dinitroxaline-2,3-dione and DL-2-amino-5-phosphonovaleric acid (APV), indicating mediation through AMPA and NMDA receptors. NMDA receptor activation contributed to basal synaptic transmission, as evidenced through a reduction of EPSP amplitudes and integrals by APV. NMDA receptor-mediated postsynaptic currents showed magnesium-regulated voltage dependence, and current-voltage relationships displayed a region of negative slope conductance negative to resting potential. Deactivation of NMDA receptor-mediated currents followed a two exponential time course, with both components being significantly reduced by ifenprodil (10 microm), an antagonist of the NR2B subunit of NMDA receptors. Significant differences were not observed between NMDA currents or ifenprodil effects at thalamic and cortical inputs. Furthermore, recordings from a sample of projection neurons in the LA provided additional evidence for the existence of ifenprodil-sensitive components of thalamically and cortically evoked NMDA receptor-mediated responses. Immunohistochemical double-labeling and combined in situ hybridization/immunohistochemistry demonstrated that GABA-immunoreactive as well as GABA-negative cells express the NR2B subunit. Overall, these results show that GABAergic interneurons in the LA express functional NMDA receptors, which participate in basal synaptic transmission at both thalamic and cortical inputs. The finding that NR2B subunits are critically involved in NMDA receptor-mediated signaling at the two major input pathways to interneurons and projection cells in the LA is particularly interesting in the light of previous observations that NR2B antagonists interfere with plastic changes in the LA related to associative fear conditioning.

Medial geniculate, amygdalar and cingulate cortical training-induced neuronal activity during discriminative avoidance learning in rabbits with auditory cortical lesions

This study addressed the neural mediation of discriminative avoidance learning, wherein rabbits step in a wheel apparatus in response to an acoustic conditional stimulus, the CS+, to avoid a foot shock, and they learn to ignore a different stimulus, the CS-, not followed by foot shock. Previously, muscimol-induced inactivation of the amygdala in the first session of training prevented learning during the inactivation and permanently blocked the development of discriminative training-induced neuronal activity (TIA) in the medial division of the medial geniculate nucleus (MGm). These results suggested that amygdalar neurons induce discriminative TIA in the MGm via basolateral (BL) amygdalar axonal projections to the auditory cortex. To test this hypothesis, the activity of neurons in the MGm was recorded during learning in rabbits with lesions of the auditory cortex. Recordings were also made in the lateral and BL amygdalar nuclei and in the cingulate cortex. In support of the hypothesis, discriminative learning in rabbits with lesions was impaired significantly during early training sessions 1-4; in these same sessions, discriminative TIA was abolished in the MGm, the BL nucleus, and the anterior cingulate cortex. The lesions also blocked posterior cingulate cortical discriminative TIA in training sessions 1-2 but spared TIA in sessions 3-7. Lateral amygdalar neurons showed gradual development of discrimination that was not significantly affected by the lesions. The results demonstrate a critical role of auditory cortex in early discriminative learning and in the production of early discriminative TIA in multiple areas.

Putative cortical and thalamic inputs elicit convergent excitation in a population of GABAergic interneurons of the lateral amygdala

Synaptic circuitry in the rat lateral amygdala (AL) was studied in brain slices using electrophysiological recordings. Electrical stimulation of external and internal capsules evoked an EPSC followed by a sequence of GABA(A) and GABA(B) receptor-mediated IPSC in principal neurons. Paired stimulation of either afferents resulted in a significant reduction ( approximately 45%) of the second GABA(A) receptor-mediated IPSC. A priming stimulation, consisting of a priming pulse to one pathway followed by a pulse to the other pathway, resulted in a strong depression of the second IPSC basically identical to that during paired stimulation. Paired- and primed-pulse depressions were largely relieved by 10 micrometer CGP 55845A, indicating regulation through presynaptic GABA(B) receptors. Furthermore, putative interneurons responded with EPSCs of constant latencies to minimal stimulation of both cortical and thalamic fibers, indicating convergent monosynaptic input. At higher stimulation strength, an approximately 15% reduction of EPSCs occurred in interneurons after paired and primed stimulation, which was not sensitive to CGP 55845A. These findings indicate that a rather homogeneous population of interneurons exists in the AL with respect to their afferent connectivity, in that they receive convergent input through putative thalamic and cortical fibers, both directly and indirectly (through principal neurons), and mediate inhibitory control of postsynaptic principal neurons. This symmetrically built GABAergic circuitry can be of functional significance, given the distinctive role of the two afferent input systems for the mediation of different components of fear responses and the importance of GABAergic mechanisms for limitation of excessive neuronal activity.

Neuronal correlates of fear in the lateral amygdala: multiple extracellular recordings in conscious cats

Much data implicates the amygdala in the expression and learning of fear. Yet, few studies have examined the neuronal correlates of fear in the amygdala. This study aimed to determine whether fear is correlated to particular activity patterns in the lateral amygdaloid (LA) nucleus. Cats, chronically implanted with multiple microelectrodes in the LA and a catheter in the femoral artery, learned that a series of tones interrupted by a period of silence (5 sec) preceded the administration of a footshock. During the silent period, their blood pressure increased, indicating that they anticipated the noxious stimulus. In parallel, the firing rate of LA neurons doubled, and the discharges of simultaneously recorded cells became more synchronized. Moreover, cross-correlation of focal LA waves revealed a significant increase in synchrony restricted to the theta band. In keeping with this, perievent histograms of neuronal discharges revealed rhythmic changes in the firing probability of LA neurons in relation to focal theta waves. Finally, the responsiveness of LA cells to the stimuli predicting the footshock (the tones) increased during the trials, whereas responses to unrelated stimuli (perirhinal shocks) remained stable. Thus, during the anticipation of noxious stimuli, a state here defined anthropomorphically as fear, the firing rate of LA neurons increases, and their discharges become more synchronized through a modulation at the theta frequency. The presence of theta oscillations in the LA might facilitate cooperative interactions between the amygdala and cortical areas involved in memory.

Reciprocal changes in the firing probability of lateral and central medial amygdala neurons

The amygdala is essential for classical fear conditioning. According to the current model of auditory fear conditioning, the lateral nucleus is the input station of the amygdala for conditioned auditory stimuli, whereas the central nucleus is the output station for conditioned fear responses. Yet, the lateral nucleus does not project to the central medial nucleus, where most brainstem projections of the amygdala originate. The available evidence suggests that the basal nuclei could transmit information from the lateral to the central medial nucleus. However, interposed between the basolateral complex and the central nucleus are clusters of GABAergic cells, the intercalated neurons, which receive inputs from the lateral and basal nuclei and contribute a massive projection to the central medial nucleus. Because it is impossible to predict the consequences of these connections, we correlated the spontaneous and auditory-evoked activity of multiple simultaneously recorded neurons of the lateral, basal, and central nuclei. The spontaneous activity of lateral and basolateral neurons was positively correlated to that of central lateral cells but negatively correlated to that of central medial neurons. In response to auditory stimuli, the firing probability of lateral and central medial neurons oscillated in phase opposition, initially being excited and inhibited, respectively. In light of previous anatomical findings, we propose that the lateral nucleus exerts two indirect actions on central medial neurons: an excitation via the basal nuclei and an inhibition via intercalated neurons.