Functional connectivity of the main intercalated nucleus of the mouse amygdala

The effect of transcranial direct current and magnetic stimulation on fear extinction and return of fear: A meta-analysis and systematic review

BACKGROUND

We conducted a meta-analysis and qualitative review on the randomized controlled trials investigating the effects of transcranial direct current stimulation and transcranial magnetic stimulation on fear extinction and the return of fear in non-primate animals and humans.

METHODS

The meta-analysis was conducted by searching PubMed, Web of science, PsycINFO, and Cochrane Library and extracting fear response in the active and sham groups in the randomized controlled trials. The pooled effect size was quantified by Hedges' g using a three-level meta-analytic model in R.

RESULTS

We identified 18 articles on the tDCS effect and 5 articles on the TMS effect, with 466 animal subjects and 621 human subjects. Our findings show that tDCS of the prefrontal cortex significantly inhibit fear retrieval in animal models (Hedges' g = -0.50). In human studies, TMS targeting the dorsolateral/ventromedial prefrontal cortex has an inhibiting effect on the return of fear (Hedges' g = -0.24).

LIMITATIONS

The limited number of studies and the heterogeneous designs of the selected studies made cross-study and cross-species comparison difficult.

CONCLUSIONS

Our findings shed light on the optimal non-invasive brain stimulation protocols for targeting the neural circuitry of threat extinction in humans.

Anatomical Connectivity of the Intercalated Cells of the Amygdala

The intercalated cells of the amygdala (ITCs) are a fundamental processing structure in the amygdala that remain relatively understudied. They are phylogenetically conserved from insectivores through primates, inhibitory, and project to several of the main processing and output stations of the amygdala and basal forebrain. Through these connections, the ITCs are best known for their role in conditioned fear, where they are required for fear extinction learning and recall. Prior work on ITC connectivity is limited, and thus holistic characterization of their afferent and efferent connectivity in a genetically defined manner is incomplete. The ITCs express the FoxP2 transcription factor, affording genetic access to these neurons for viral input-output mapping. To fully characterize the anatomic connectivity of the ITCs, we used cre-dependent viral strategies in FoxP2-cre mice to reveal the projections of the main (mITC), caudal (cITC), and lateral (lITC) clusters along with their presynaptic sources of innervation. Broadly, the results confirm many known pathways, reveal previously unknown ones, and demonstrate important novel insights about each nucleus's unique connectivity profile and relative distributions. We show that the ITCs receive information from a wide range of cortical, subcortical, basal, amygdalar, hippocampal, and thalamic structures, and project broadly to areas of the basal forebrain, hypothalamus, and entire extent of the amygdala. The results provide a comprehensive map of their connectivity and suggest that the ITCs could potentially influence a broad range of behaviors by integrating information from a wide array of sources throughout the brain.

Altered neurotransmission in stress-induced depressive disorders: The underlying role of the amygdala in depression

Depression is the second leading cause of disability in the world population, for which currently available pharmacological therapies either have poor efficacy or have some adverse effects. Accumulating evidence from clinical and preclinical studies demonstrates that the amygdala is critically implicated in depressive disorders, though the underlying pathogenesis mechanism needs further investigation. In this literature review, we overviewed depression and the key role of Gamma-aminobutyric acid (GABA) and Glutamate neurotransmission in depression. Notably, we discussed a new cholecystokinin-dependent plastic changes mechanism under stress and a possible antidepressant response of cholecystokinin B receptor (CCKBR) antagonist. Moreover, we discussed the fundamental role of the amygdala in depression, to discuss and understand the pathophysiology of depression and the inclusive role of the amygdala in this devastating disorder.

Amygdala Intercalated Cells: Gate Keepers and Conveyors of Internal State to the Circuits of Emotion

Generating adaptive behavioral responses to emotionally salient stimuli requires evaluation of complex associations between multiple sensations, the surrounding context, and current internal state. Neural circuits within the amygdala parse this emotional information, undergo synaptic plasticity to reflect learned associations, and evoke appropriate responses through their projections to the brain regions orchestrating these behaviors. Information flow within the amygdala is regulated by the intercalated cells (ITCs), which are densely packed clusters of GABAergic neurons that encircle the basolateral amygdala (BLA) and provide contextually relevant feedforward inhibition of amygdala nuclei, including the central and BLA. Emerging studies have begun to delineate the unique contribution of each ITC cluster and establish ITCs as key loci of plasticity in emotional learning. In this review, we summarize the known connectivity and function of individual ITC clusters and explore how different neuromodulators conveying internal state act via ITC gates to shape emotionally motivated behavior. We propose that the behavioral state-dependent function of ITCs, their unique genetic profile, and rich expression of neuromodulator receptors make them potential therapeutic targets for disorders, such as anxiety, schizophrenia spectrum, and addiction.

Dopaminergic circuits underlying associative aversive learning

Associative aversive learning enables animals to predict and avoid threats and thus is critical for survival and adaptive behavior. Anxiety disorders are characterized with deficits in normal aversive learning mechanisms and hence understanding the neural circuits underlying aversive learning and memory has high clinical relevance. Recent studies have revealed the dopamine system as one of the key modulators of aversive learning. In this review, we highlight recent advances that provide insights into how distinct dopaminergic circuits contribute to aversive learning and memory.

Functional connectivity of intercalated nucleus with medial amygdala: A circuit relevant for chemosignal processing

Medial amygdala processes social/reproductive chemosensory input, and its projections to preoptic and hypothalamic areas evoke appropriate behavioral and physiological responses. We and others have shown that different chemosensory signals elicit differential responses in medial amygdala subregions and in adjacent main intercalated nucleus (mICN). The largely GABAergic mICN receives no direct chemosensory input but, as we show, mICN has functional circuit connections with medial amygdala that could be responsible both for mICN chemosensitivity and for a feedforward inhibitory effect on posterior medial amygdala; which, in turn would affect chemosignal response. mICN is subject to inhibition by dopamine and is probably regulated by neuropeptides and input from frontal cortex. Thus, mICN is in position to modify chemosensory processing in medial amygdala and behavioral responses to social signals, according to internal brain state. Patch-clamp recordings from neurons in each relevant nucleus in horizontal brain-slices, with electrical stimulation in adjacent nuclei, reveal multiple functional connections between medial amygdala subregions and mICN. We highlight a triangular circuit which may underlie mICN chemosensitivity and its potential for modifying chemosensory information transmitted to basal forebrain. Anterior medial amygdala, which receives most of the chemosensory input, connects to posterior medial amygdala directly and both areas send information on to basal forebrain. Anterior medial amygdala can also modulate posterior medial amygdala indirectly via the mICN side-loop, which also provides a pathway for modulation by cortical input or, when inhibited by dopamine, could allow a more automatic response - as proposed for other amygdala circuits with similar ICN side loops.

Anxiolytic and antidepressant-like effects of monoterpene tetrahydrolinalool and in silico approach of new potential targets

INTRODUCTION- The drugs currently available for treatment of anxiety and depression act through modulation of the neurotransmission systems involved in the neurobiology of the disorder, yet they of-ten present side effects, which can impair patient adherence to treatment. METHOD- This, has driven the search for new molecules with anxiolytic and antidepressant potential. Aromatic plants are rich in essential oils, and their chemical constituents, such as monoterpenes, are be-ing studied for these disorders. This study aims to evaluate the anxiolytic and antidepressant-like poten-tial of the monoterpene tetrahydrolinalool in in vivo animal models, and review pharmacological targets with validation through molecular docking. Male Swiss mice (Mus musculus) were treated with THL (37.5-600 mg kg-1 p.o.) and submitted to the elevated plus maze, open field, rota rod, and forced swim tests. In the elevated plus-maze, THL at doses of 37.5 and 75 mg kg-1 induced a significant increase in the percentage of entries (72.7 and 64.3% respectively), and lengths of stay (80.3 and 76.8% respective-ly) in the open arms tests. RESULT- These doses did not compromise locomotor activity or motor coordination in the animals. In the open field, rota rod tests, and the forced swimming model, treatment with THL significantly reduced immobility times at doses of 150, 300, and 600 mg kg-1, and by respective percentages of 69.3, 60.9 and 68.7%. CONCLUSION- In molecular docking assay, which investigated potential targets, THL presented sat-isfactory energy values for: nNOs, SGC, IL-6, 5-HT1A, NMDAr, and D1. These demonstrate the po-tential of THL (a derivative of natural origin) in in vivo and in silico models, making it a drug candidate.

Selective δ-Opioid Receptor Agonist, KNT-127, Facilitates Contextual Fear Extinction via Infralimbic Cortex and Amygdala in Mice

Facilitation of fear extinction is a desirable action for the drugs to treat fear-related diseases, such as posttraumatic stress disorder (PTSD). We previously reported that a selective agonist of the δ-opioid receptor (DOP), KNT-127, facilitates contextual fear extinction in mice. However, its site of action in the brain and the underlying molecular mechanism remains unknown. Here, we investigated brain regions and cellular signaling pathways that may mediate the action of KNT-127 on fear extinction. Twenty-four hours after the fear conditioning, mice were reexposed to the conditioning chamber for 6 min as extinction training (reexposure 1). KNT-127 was microinjected into either the basolateral nucleus of the amygdala (BLA), hippocampus (HPC), prelimbic (PL), or infralimbic (IL) subregions of the medial prefrontal cortex, 30 min before reexposure 1. Next day, mice were reexposed to the chamber for 6 min as memory testing (reexposure 2). KNT-127 that infused into the BLA and IL, but not HPC or PL, significantly reduced the freezing response in reexposure 2 compared with those of control. The effect of KNT-127 administered into the BLA and IL was antagonized by pretreatment with a selective DOP antagonist. Further, the effect of KNT-127 was abolished by local administration of MEK/ERK inhibitor into the BLA, and PI3K/Akt inhibitor into the IL, respectively. These results suggested that the effect of KNT-127 was mediated by MEK/ERK signaling in the BLA, PI3K/Akt signaling in the IL, and DOPs in both brain regions. Here, we propose that DOPs play a role in fear extinction via distinct signaling pathways in the BLA and IL.

Deletion of ErbB4 Disrupts Synaptic Transmission and Long-Term Potentiation of Thalamic Input to Amygdalar Medial Paracapsular Intercalated Cells

Identification of candidate risk genes and alteration in the expression of proteins involved in regulating inhibitory neuron function in various psychiatric disorders, support the notion that GABAergic neuron dysfunction plays an important role in disease etiology. Genetic variations in neuregulin and its receptor kinase ErbB4, expressed exclusively by GABAergic neurons in the CNS, have been linked with schizophrenia. In the amygdala, ErbB4 is highly expressed in GABAergic intercalated cell clusters (ITCs), which play a critical role in amygdala-dependent behaviors. It is however unknown whether ErbB4 deletion from ITCs affects their synaptic properties and function in amygdala circuitry. Here, we examined the impact of ErbB4 deletion on inhibitory and excitatory circuits recruiting medial paracapsular ITCs (mpITCs) using electrophysiological techniques. Ablation of ErbB4 in mpITCs suppressed NMDA receptor-mediated synaptic transmission at thalamo-mpITC synapses and enhanced thalamic driven GABAergic transmission onto mpITCs. Furthermore, long-term potentiation (LTP) at thalamo-mpITC synapses was compromised in ErbB4 mutant mice, indicating that ErbB4 activity is critical for LTP at these synapses. Together, our findings suggest that ErbB4 deletion from mpITCs disrupts excitation-inhibition balance and learning mechanisms in amygdala circuits.

Modulation of glutamatergic synaptic transmission and neuronal excitability in the prelimbic medial prefrontal cortex via delta-opioid receptors in mice

The medial prefrontal cortex (mPFC) plays a vital role in the processing of emotional events. It has been shown that activation of the glutamatergic transmission in prelimbic subregion of the mPFC (PL-PFC) evoked anxiety-like behavior in rodents. We previously reported that local perfusion of a selective agonist to delta-opioid receptor (DOP), KNT-127, attenuated the veratrine-induced elevation of extracellular glutamate in the PL-PFC and anxiety-like behavior in mice. These results suggested the possibility that KNT-127 suppresses glutamate release from the presynaptic site in the PL-PFC. To examine this possibility directly, we performed whole-cell patch-clamp recording from principal neurons in the PL-PFC and examined the spontaneous and electrically-evoked excitatory postsynaptic currents (EPSC)s. We found that bath application of KNT-127 significantly decreased the frequency of spontaneous and miniature EPSCs. Conversely, amplitude, rise time, and decay time of spontaneous and miniature EPSCs were not affected by bath application of KNT-127. Also, KNT-127 increased paired-pulse ratios of electrically-evoked EPSCs in the PL-PFC principal neurons tested. Further, we analyzed the firing properties of pyramidal neurons in the PL-PFC and found that KNT-127 treatment significantly reduced the number of action potentials and firing threshold. These results suggested that KNT-127 suppresses glutamatergic synaptic transmission by inhibiting glutamate release from the presynaptic site and reduces neuronal excitability in the mouse PL-PFC. We propose the possibility that these suppressing effects of KNT-127 on PL-PFC activity are part of the underlying mechanisms of its anxiolytic-like effects.

Intercalated amygdala clusters orchestrate a switch in fear state

Adaptive behaviour necessitates the formation of memories for fearful events, but also that these memories can be extinguished. Effective extinction prevents excessive and persistent reactions to perceived threat, as can occur in anxiety and 'trauma- and stressor-related' disorders¹. However, although there is evidence that fear learning and extinction are mediated by distinct neural circuits, the nature of the interaction between these circuits remains poorly understood^2-6. Here, through a combination of in vivo calcium imaging, functional manipulations, and slice physiology, we show that distinct inhibitory clusters of intercalated neurons (ITCs) in the mouse amygdala exert diametrically opposed roles during the acquisition and retrieval of fear extinction memory. Furthermore, we find that the ITC clusters antagonize one another through mutual synaptic inhibition and differentially access functionally distinct cortical- and midbrain-projecting amygdala output pathways. Our findings show that the balance of activity between ITC clusters represents a unique regulatory motif that orchestrates a distributed neural circuitry, which in turn regulates the switch between high- and low-fear states. These findings suggest that the ITCs have a broader role in a range of amygdala functions and associated brain states that underpins the capacity to adapt to salient environmental demands.

Midbrain dopaminergic inputs gate amygdala intercalated cell clusters by distinct and cooperative mechanisms in male mice

Dopaminergic signaling plays an important role in associative learning, including fear and extinction learning. Dopaminergic midbrain neurons encode prediction error-like signals when threats differ from expectations. Within the amygdala, GABAergic intercalated cell (ITC) clusters receive one of the densest dopaminergic projections, but their physiological consequences are incompletely understood. ITCs are important for fear extinction, a function thought to be supported by activation of ventromedial ITCs that inhibit central amygdala fear output. In mice, we reveal two distinct novel mechanisms by which mesencephalic dopaminergic afferents control ITCs. Firstly, they co-release GABA to mediate rapid, direct inhibition. Secondly, dopamine suppresses inhibitory interactions between distinct ITC clusters via presynaptic D1 receptors. Early extinction training augments both GABA co-release onto dorsomedial ITCs and dopamine-mediated suppression of dorso- to ventromedial inhibition between ITC clusters. These findings provide novel insights into dopaminergic mechanisms shaping the activity balance between distinct ITC clusters that could support their opposing roles in fear behavior.

A Basomedial Amygdala to Intercalated Cells Microcircuit Expressing PACAP and Its Receptor PAC1 Regulates Contextual Fear

Trauma can cause dysfunctional fear regulation leading some people to develop disorders, such as post-traumatic stress disorder (PTSD). The amygdala regulates fear, whereas PACAP (pituitary adenylate activating peptide) and PAC1 receptors are linked to PTSD symptom severity at genetic/epigenetic levels, with a strong link in females with PTSD. We discovered a PACAPergic projection from the basomedial amygdala (BMA) to the medial intercalated cells (mICCs) in adult mice. In vivo optogenetic stimulation of this pathway increased CFOS expression in mICCs, decreased fear recall, and increased fear extinction. Selective deletion of PAC1 receptors from the mICCs in females reduced fear acquisition, but enhanced fear generalization and reduced fear extinction in males. Optogenetic stimulation of the BMA-mICC PACAPergic pathway produced EPSCs in mICC neurons, which were enhanced by the PAC1 receptor antagonist, PACAP 6-38. Our findings show that mICCs modulate contextual fear in a dynamic and sex-dependent manner via a microcircuit containing the BMA and mICCs, and in a manner that was dependent on behavioral state.SIGNIFICANCE STATEMENT Traumatic stress can affect different aspects of fear behaviors, including fear learning, generalization of learned fear to novel contexts, how the fear of the original context is recalled, and how fear is reduced over time. While the amygdala has been studied for its role in regulation of different aspects of fear, the molecular circuitry of this structure is quite complex. In addition, aspects of fear can be modulated differently in males and females. Our findings show that a specific circuitry containing the neuropeptide PACAP and its receptor, PAC1, regulates various aspects of fear, including acquisition, generalization, recall, and extinction in a sexually dimorphic manner, characterizing a novel pathway that modulates traumatic fear.

Dopamine in Fear Extinction

The ability to extinguish fear memories when threats are no longer present is critical for adaptive behavior. Fear extinction represents a new learning process that eventually leads to the formation of extinction memories. Understanding the neural basis of fear extinction has considerable clinical significance as deficits in extinction learning are the hallmark of human anxiety disorders. In recent years, the dopamine (DA) system has emerged as one of the key regulators of fear extinction. In this review article, we highlight recent advances that have demonstrated the crucial role DA plays in mediating different phases of fear extinction. Emerging concepts and outstanding questions for future research are also discussed.

The expression of tyrosine hydroxylase and DARPP-32 in the house crow (Corvus splendens) brain

Birds of the family Corvidae which includes diverse species such as crows, rooks, ravens, magpies, jays, and jackdaws are known for their amazing abilities at problem-solving. Since the catecholaminergic system, especially the neurotransmitter dopamine, plays a role in cognition, we decided to study the distribution of tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of catecholamines in the brain of house crows (Corvus splendens). We also studied the expression of DARPP-32 (dopamine and cAMP-regulated phosphoprotein), which is expressed in dopaminoceptive neurons. Our results demonstrated that as in other avian species, the expression of both TH and DARPP-32 was highest in the house crow striatum. The caudolateral nidopallium (NCL, the avian analogue of the mammalian prefrontal cortex) could be differentiated from the surrounding pallial regions based on a larger number of TH-positive "baskets" of fibers around neurons in this region and greater intensity of DARPP-32 staining in the neuropil in this region. House crows also possessed distinct nuclei in their brains which corresponded to song control regions in other songbirds. Whereas immunoreactivity for TH was higher in the vocal control region Area X compared to the surrounding MSt (medial striatum) in house crows, staining in RA and HVC was not as prominent. Furthermore, the arcopallial song control regions RA (nucleus robustus arcopallialis) and AId (intermediate arcopallium) were strikingly negative for DARPP-32 staining, in contrast to the surrounding arcopallium. Patterns of immunoreactivity for TH and DARPP-32 in "limbic" areas such as the hippocampus, septum, and extended amygdala have also been described.

Mechanisms of fear learning and extinction: synaptic plasticity-fear memory connection

RATIONALE

The ability to memorize threat-associated cues and subsequently react to them, exhibiting escape or avoidance responses, is an essential, often life-saving behavioral mechanism that can be experimentally studied using the fear (threat) conditioning training paradigm. Presently, there is substantial evidence supporting the Synaptic Plasticity-Memory (SPM) hypothesis in relation to the mechanisms underlying the acquisition, retention, and extinction of conditioned fear memory.

OBJECTIVES

The purpose of this review article is to summarize findings supporting the SPM hypothesis in the context of conditioned fear control, applying the set of criteria and tests which were proposed as necessary to causally link lasting changes in synaptic transmission in corresponding neural circuits to fear memory acquisition and extinction with an emphasis on their pharmacological diversity.

RESULTS

The mechanisms of synaptic plasticity in fear circuits exhibit complex pharmacological profiles and satisfy all four SPM criteria-detectability, anterograde alteration, retrograde alteration, and mimicry.

CONCLUSION

The reviewed findings, accumulated over the last two decades, provide support for both necessity and sufficiency of synaptic plasticity in fear circuits for fear memory acquisition and retention, and, in part, for fear extinction, with the latter requiring additional experimental work.

Administration of a delta opioid receptor agonist KNT-127 to the basolateral amygdala has robust anxiolytic-like effects in rats

RATIONALE

We previously reported that systemic administration of a selective delta opioid receptor (DOP) agonist, KNT-127, produced potent anxiolytic-like effects in rats. Interestingly, DOPs are highly distributed in the basolateral region of the amygdala (BLA).

OBJECTIVES

In this study, we investigated the effect of intra-BLA administration of KNT-127 on anxiety-like behaviors in rats.

METHODS AND RESULTS

In the elevated plus maze test, bilateral injection of KNT-127 into the BLA significantly and dose-dependently increased time spent in the open arms. The magnitude of KNT-127 (0.08 μg/0.2 μl)-induced anxiolytic-like effects was similar to muscimol (0.1 μg/0.2 μl), which is a selective agonist for the gamma amino butyric acid type A receptors. Further, anxiolytic-like effects of KNT-127 were abolished by pretreatment with naltrindole, a selective DOP antagonist, suggesting that KNT-127-induced anxiolytic-like effects are mediated by DOPs. These anxiolytic-like effects were confirmed using another innate anxiety model, the open field test. Interestingly, intra-BLA administration of KNT-127 also induced anxiolytic-like effects in the contextual fear conditioning test. Moreover, these effects were also abolished by naltrindole pretreatment. Finally, we demonstrated that intra-BLA administration of KNT-127 facilitates extinction learning of contextual fear in conditioned rats.

CONCLUSIONS

Altogether, our findings clearly demonstrate that intra-BLA administration of KNT-127 in rats has robust anxiolytic-like effects not only in innate anxiety-like behavioral tests but also in the contextual fear conditioning test.

Dopamine and opioids inhibit synaptic outputs of the main island of the intercalated neurons of the amygdala

Neural circuits in the amygdala are important for associating the positive experience of drug taking with the coincident environmental cues. During abstinence, cue re-exposure activates the amygdala, increases dopamine release in the amygdala and stimulates relapse to drug use in an opioid dependent manner. Neural circuits in the amygdala and the learning that underlies these behaviours are inhibited by GABAergic synaptic inhibition. A specialised subtype of GABAergic neurons in the amygdala are the clusters of intercalated cells. We focussed on the main-island of intercalated cells because these neurons, located ventromedial to the basolateral amygdala, express very high levels of dopamine D1-receptor and μ-opioid receptor, release enkephalin and are densely innervated by the ventral tegmental area. However, where these neurons project to was not fully described and their regulation by opioids and dopamine was incomplete. To address this issue we electrically stimulated in the main-island of the intercalated cells in rat brain slices and made patch-clamp recordings of GABAergic synaptics from amygdala neurons. We found that main-island neurons had a strong GABAergic inhibitory output to pyramidal neurons of the basolateral nucleus and the medial central nucleus, the major output zones of the amygdala. Opioids inhibited both these synaptic outputs of the intercalated neurons and thus would disinhibit these target zones. Additionally, dopamine acting at D1-receptors inhibited main-island neuron synapses onto other main-island neurons. This data indicates that the inhibitory projections from the main-island neurons could influence multiple aspects of addiction and emotional processing in an opioid and dopamine dependent manner.

Modulation of Neuronal Activity on Intercalated Neurons of Amygdala Might Underlie Anxiolytic Activity of a Standardized Extract of Centella asiatica ECa233

GABAergic intercalated neurons of amygdala (ITCs) have recently been shown to be important in the suppression of fear-like behavior. Effects of ECa233 (a standardized extract of Centella asiatica), previously demonstrated anxiolytic activity, were then investigated on ITCs. Cluster of GABAergic neurons expressing fluorescence of GFP was identified in GAD67-GFP knock-in mice. We found that neurons of medial paracapsular ITC were GABAergic neurons exhibiting certain intrinsic electrophysiological properties similar to those demonstrated by ITC neurons at the same location in C57BL/6J mice. Therefore, we conducted experiments in both C57BL/6J mice and GAD67-GFP knock-in mice. Excitatory postsynaptic currents (EPSCs) were evoked by stimulation of the external capsule during the whole cell patch-clamp recordings from ITC neurons in brain slices. ECa233 was found to increase the EPSC peak amplitude in the ITC neurons by about 120%. The EPSCs in ITC neurons were completely abolished by the application of an AMPA receptor antagonist. Morphological assessment of the ITC neurons with biocytin demonstrated that most axons of the recorded neurons innervated the central nucleus of the amygdala (CeA). Therefore, it is highly likely that anxiolytic activity of ECa233 was mediated by increasing activation, via AMPA receptors, of excitatory synaptic input to the GABAergic ITC leading to depression of CeA neurons.

Endogenous opioids regulate moment-to-moment neuronal communication and excitability

Fear and emotional learning are modulated by endogenous opioids but the cellular basis for this is unknown. The intercalated cells (ITCs) gate amygdala output and thus regulate the fear response. Here we find endogenous opioids are released by synaptic stimulation to act via two distinct mechanisms within the main ITC cluster. Endogenously released opioids inhibit glutamate release through the δ-opioid receptor (DOR), an effect potentiated by a DOR-positive allosteric modulator. Postsynaptically, the opioids activate a potassium conductance through the μ-opioid receptor (MOR), suggesting for the first time that endogenously released opioids directly regulate neuronal excitability. Ultrastructural localization of endogenous ligands support these functional findings. This study demonstrates a new role for endogenously released opioids as neuromodulators engaged by synaptic activity to regulate moment-to-moment neuronal communication and excitability. These distinct actions through MOR and DOR may underlie the opposing effect of these receptor systems on anxiety and fear.

The endogenous opioid system regulates fear and anxiety, but the underlying cellular mechanism is unclear. Winters et al. shows that in the intercalated cells (ITC) of the amygdala, endogenous opioids suppress glutamatergic inputs via the δ-opioid receptor presynaptically, and reduce the excitability of ITCs via the μ-opioid receptor postsynaptically.

Amygdala Circuits for Fear Memory: A Key Role for Dopamine Regulation

In addition to modulating a number of cognitive functions including reward, punishment, motivation, and salience, dopamine (DA) plays a pivotal role in regulating threat-related emotional memory. Changes in neural circuits of the amygdala nuclei are also critically involved in the acquisition and expression of emotional memory. In this review, we summarize the regulation of amygdala circuits by DA. Specifically, we describe DA signaling in the amygdala, and DA regulation of synaptic transmission and synaptic plasticity of the amygdala neurons. Finally, we discuss a potential contribution of DA-related mechanisms to the pathogenesis of posttraumatic stress disorder.

The intercalated nuclear complex of the primate amygdala

The organization of the inhibitory intercalated cell masses (IM) of the primate amygdala is largely unknown despite their key role in emotional processes. We studied the structural, topographic, neurochemical and intrinsic connectional features of IM neurons in the rhesus monkey brain. We found that the intercalated neurons are not confined to discrete cell clusters, but form a neuronal net that is interposed between the basal nuclei and extends to the dorsally located anterior, central, and medial nuclei of the amygdala. Unlike the IM in rodents, which are prominent in the anterior half of the amygdala, the primate inhibitory net stretched throughout the antero-posterior axis of the amygdala, and was most prominent in the central and posterior extent of the amygdala. There were two morphologic types of intercalated neurons: spiny and aspiny. Spiny neurons were the most abundant; their somata were small or medium size, round or elongated, and their dendritic trees were round or bipolar, depending on location. The aspiny neurons were on average slightly larger and had varicose dendrites with no spines. There were three non-overlapping neurochemical populations of IM neurons, in descending order of abundance: (1) Spiny neurons that were positive for the striatal associated dopamine- and cAMP-regulated phosphoprotein (DARPP-32+); (2) Aspiny neurons that expressed the calcium-binding protein calbindin (CB+); and (3) Aspiny neurons that expressed nitric oxide synthase (NOS+). The unique combinations of structural and neurochemical features of the three classes of IM neurons suggest different physiological properties and function. The three types of IM neurons were intermingled and likely interconnected in distinct ways, and were innervated by intrinsic neurons within the amygdala, or by external sources, in pathways that underlie fear conditioning and anxiety.

The basolateral amygdala γ-aminobutyric acidergic system in health and disease

The brain comprises an excitatory/inhibitory neuronal network that maintains a finely tuned balance of activity critical for normal functioning. Excitatory activity in the basolateral amygdala (BLA), a brain region that plays a central role in emotion and motivational processing, is tightly regulated by a relatively small population of γ-aminobutyric acid (GABA) inhibitory neurons. Disruption in GABAergic inhibition in the BLA can occur when there is a loss of local GABAergic interneurons, an alteration in GABAA receptor activation, or a dysregulation of mechanisms that modulate BLA GABAergic inhibition. Disruptions in GABAergic control of the BLA emerge during development, in aging populations, or after trauma, ultimately resulting in hyperexcitability. BLA hyperexcitability manifests behaviorally as an increase in anxiety, emotional dysregulation, or development of seizure activity. This Review discusses the anatomy, development, and physiology of the GABAergic system in the BLA and circuits that modulate GABAergic inhibition, including the dopaminergic, serotonergic, noradrenergic, and cholinergic systems. We highlight how alterations in various neurotransmitter receptors, including the acid-sensing ion channel 1a, cannabinoid receptor 1, and glutamate receptor subtypes, expressed on BLA interneurons, modulate GABAergic transmission and how defects of these systems affect inhibitory tonus within the BLA. Finally, we discuss alterations in the BLA GABAergic system in neurodevelopmental (autism/fragile X syndrome) and neurodegenerative (Alzheimer's disease) diseases and after the development of epilepsy, anxiety, and traumatic brain injury. A more complete understanding of the intrinsic excitatory/inhibitory circuit balance of the amygdala and how imbalances in inhibitory control contribute to excessive BLA excitability will guide the development of novel therapeutic approaches in neuropsychiatric diseases.

Altered activity of the medial prefrontal cortex and amygdala during acquisition and extinction of an active avoidance task

Altered medial prefrontal cortex (mPFC) and amygdala function is associated with anxiety-related disorders. While the mPFC-amygdala pathway has a clear role in fear conditioning, these structures are also involved in active avoidance. Given that avoidance perseveration represents a core symptom of anxiety disorders, the neural substrate of avoidance, especially its extinction, requires better understanding. The present study was designed to investigate the activity, particularly, inhibitory neuronal activity in mPFC and amygdala during acquisition and extinction of lever-press avoidance in rats. Neural activity was examined in the mPFC, intercalated cell clusters (ITCs) lateral (LA), basal (BA) and central (CeA) amygdala, at various time points during acquisition and extinction, using induction of the immediate early gene product, c-Fos. Neural activity was greater in the mPFC, LA, BA, and ITC during the extinction phase as compared to the acquisition phase. In contrast, the CeA was the only region that was more activated during acquisition than during extinction. Our results indicate inhibitory neurons are more activated during late phase of acquisition and extinction in the mPFC and LA, suggesting the dynamic involvement of inhibitory circuits in the development and extinction of avoidance response. Together, these data start to identify the key brain regions important in active avoidance behavior, areas that could be associated with avoidance perseveration in anxiety disorders.

Prefrontal dopamine regulates fear reinstatement through the downregulation of extinction circuits

Prevention of relapses is a major challenge in treating anxiety disorders. Fear reinstatement can cause relapse in spite of successful fear reduction through extinction-based exposure therapy. By utilising a contextual fear-conditioning task in mice, we found that reinstatement was accompanied by decreased c-Fos expression in the infralimbic cortex (IL) with reduction of synaptic input and enhanced c-Fos expression in the medial subdivision of the central nucleus of the amygdala (CeM). Moreover, we found that IL dopamine plays a key role in reinstatement. A reinstatement-inducing reminder shock induced c-Fos expression in the IL-projecting dopaminergic neurons in the ventral tegmental area, and the blocking of IL D1 signalling prevented reduction of synaptic input, CeM c-Fos expression, and fear reinstatement. These findings demonstrate that a dopamine-dependent inactivation of extinction circuits underlies fear reinstatement and may explain the comorbidity of substance use disorders and anxiety disorders.

μ-Opioid Receptor-Mediated Inhibition of Intercalated Neurons and Effect on Synaptic Transmission to the Central Amygdala

The amygdala is a key region for the processing of information underlying fear, anxiety, and fear extinction. Within the local neuronal networks of the amygdala, a population of inhibitory, intercalated neurons (ITCs) modulates the flow of information among various nuclei of amygdala, including the basal nucleus (BA) and the centromedial nucleus (CeM) of the amygdala. These ITCs have been shown to be important during fear extinction and are target of a variety of neurotransmitters and neuropeptides. Here we provide evidence that the activation of μ-opioid receptors (MORs) by the specific agonist DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]-Enkephalin) hyperpolarizes medially located ITCs (mITCs) in acute brain slices of mice. Moreover, we use whole-cell patch-clamp recordings in combination with local electrical stimulation or glutamate uncaging to analyze the effect of MOR activation on local microcircuits. We show that the GABAergic transmission between mITCs and CeM neurons is attenuated by DAMGO, whereas the glutamatergic transmission on CeM neurons and mITCs is unaffected. Furthermore, MOR activation induced by theta burst stimulation in BA suppresses plastic changes of feedforward inhibitory transmission onto CeM neurons as revealed by the MOR antagonist CTAP d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2. In summary, the mITCs constitute a target for the opioid system, and therefore, the activation of MOR in ITCs might play a central role in the modulation of the information processing between the basolateral complex of the amygdala and central nuclei of the amygdala.

Large intercalated neurons of amygdala relay noxious sensory information

Various GABAergic neuron types of the amygdala cooperate to control principal cell firing during fear-related and other behaviors, and understanding their specialized roles is important. Among GABAergic neurons, the so-called intercalated cells (ITCcs) are critically involved in the expression and extinction of fear memory. Tightly clustered small-sized spiny neurons constitute the majority of ITCcs, but they are surrounded by sparse, larger neurons (L-ITCcs) for which very little information is known. We report here a detailed neurochemical, structural and physiological characterization of rat L-ITCcs, as identified with juxtacellular recording/labeling in vivo. We supplement these data with anatomical and neurochemical analyses of nonrecorded L-ITCcs. We demonstrate that L-ITCcs are GABAergic, and strongly express metabotropic glutamate receptor 1α and GABAA receptor α1 subunit, together with moderate levels of parvalbumin. Furthermore, L-ITCcs are innervated by fibers enriched with metabotropic glutamate receptors 7a and/or 8a. In contrast to small-sized spiny ITCcs, L-ITCcs possess thick, aspiny dendrites, have highly branched, long-range axonal projections, and innervate interneurons in the basolateral amygdaloid complex. The axons of L-ITCcs also project to distant brain areas, such as the perirhinal, entorhinal, and endopiriform cortices. In vivo recorded L-ITCcs are strongly activated by noxious stimuli, such as hindpaw pinches or electrical footshocks. Consistent with this, we observed synaptic contacts on L-ITCc dendrites from nociceptive intralaminar thalamic nuclei. We propose that, during salient sensory stimulation, L-ITCcs disinhibit local and distant principal neurons, acting as "hub cells," to orchestrate the activity of a distributed network.

Anxiety disorders and GABA neurotransmission: a disturbance of modulation

Lines of evidence coming from many branches of neuroscience indicate that anxiety disorders arise from a dysfunction in the modulation of brain circuits which regulate emotional responses to potentially threatening stimuli. The concept of anxiety disorders as a disturbance of emotional response regulation is a useful one as it allows anxiety to be explained in terms of a more general model of aberrant salience and also because it identifies avenues for developing psychological, behavioral, and pharmacological strategies for the treatment of anxiety disorder. These circuits involve bottom-up activity from the amygdala, indicating the presence of potentially threatening stimuli, and top-down control mechanisms originating in the prefrontal cortex, signaling the emotional salience of stimuli. Understanding the factors that control cortical mechanisms may open the way to identification of more effective cognitive behavioral strategies for managing anxiety disorders. The brain circuits in the amygdala are thought to comprise inhibitory networks of γ-aminobutyric acid-ergic (GABAergic) interneurons and this neurotransmitter thus plays a key role in the modulation of anxiety responses both in the normal and pathological state. The presence of allosteric sites on the GABAA receptor allows the level of inhibition of neurons in the amygdala to be regulated with exquisite precision, and these sites are the molecular targets of the principal classes of anxiolytic drugs. Changes in the levels of endogenous modulators of these allosteric sites as well as changes in the subunit composition of the GABAA receptor may represent mechanisms whereby the level of neuronal inhibition is downregulated in pathological anxiety states. Neurosteroids are synthesized in the brain and act as allosteric modulators of the GABAA receptor. Since their synthesis is itself regulated by stress and by anxiogenic stimuli, targeting the neurosteroid-GABAA receptor axis represents an attractive target for the modulation of anxiety.

Pharmacology of cognitive enhancers for exposure-based therapy of fear, anxiety and trauma-related disorders

Pathological fear and anxiety are highly debilitating and, despite considerable advances in psychotherapy and pharmacotherapy they remain insufficiently treated in many patients with PTSD, phobias, panic and other anxiety disorders. Increasing preclinical and clinical evidence indicates that pharmacological treatments including cognitive enhancers, when given as adjuncts to psychotherapeutic approaches [cognitive behavioral therapy including extinction-based exposure therapy] enhance treatment efficacy, while using anxiolytics such as benzodiazepines as adjuncts can undermine long-term treatment success. The purpose of this review is to outline the literature showing how pharmacological interventions targeting neurotransmitter systems including serotonin, dopamine, noradrenaline, histamine, glutamate, GABA, cannabinoids, neuropeptides (oxytocin, neuropeptides Y and S, opioids) and other targets (neurotrophins BDNF and FGF2, glucocorticoids, L-type-calcium channels, epigenetic modifications) as well as their downstream signaling pathways, can augment fear extinction and strengthen extinction memory persistently in preclinical models. Particularly promising approaches are discussed in regard to their effects on specific aspects of fear extinction namely, acquisition, consolidation and retrieval, including long-term protection from return of fear (relapse) phenomena like spontaneous recovery, reinstatement and renewal of fear. We also highlight the promising translational value of the preclinial research and the clinical potential of targeting certain neurochemical systems with, for example d-cycloserine, yohimbine, cortisol, and L-DOPA. The current body of research reveals important new insights into the neurobiology and neurochemistry of fear extinction and holds significant promise for pharmacologically-augmented psychotherapy as an improved approach to treat trauma and anxiety-related disorders in a more efficient and persistent way promoting enhanced symptom remission and recovery.

Fear-potentiated startle during extinction is associated with white matter microstructure and functional connectivity

BACKGROUND

Extinction of conditioned fear is an associative learning process that involves communication among the hippocampus, medial prefrontal cortex, and amygdala. Strength of connectivity between the hippocampus and the anterior cingulate cortex (ACC), and between the amygdala and ventromedial prefrontal cortex (vmPFC), may influence fear-potentiated startle (FPS) responses during extinction. Specific white matter tracts, the cingulum and uncinate fasciculus (UF), serve as primary routes of communication for these areas. Our objective was to investigate associations between FPS during extinction and cingulum and UF connectivity.

METHOD

Diffusion tensor imaging (DTI) and probabilistic tractography analyses were used to examine cingulum and UF structural connectivity in 40 female African-Americans with psychological trauma exposure. FPS responses during fear conditioning and extinction were assessed via electromyography (EMG) of the right orbicularis oculi muscle. Secondarily, functional connectivity analyses were performed with the seed regions of interest (ROIs) used for tractography.

RESULTS

A significant negative association between cingulum microstructure and FPS during early extinction (r = -.42, p = .01) and late extinction (r = -.36, p = .03) was observed after accounting for the effects of age, trauma exposure, and psychopathology (post-traumatic stress disorder symptoms); this pattern was similar for early extinction and functional connectivity between these regions (p < .05(corrected)). No significant correlations were observed between FPS and UF microstructure.

CONCLUSIONS

These data indicate that structural integrity of the cingulum is directly associated with extinction learning and appears to influence functional connectivity between these regions. Decrements in cingulum microstructure may interfere with extinction learning, thereby increasing risk for the development of pathological anxiety.

Long-term potentiation at excitatory synaptic inputs to the intercalated cell masses of the amygdala

The intercalated cell masses (ITCs) of the amygdala are clusters of GABAergic interneurons that surround the basolateral complex of the amygdala. ITCs have been increasingly implicated in the acquisition and extinction of conditioned fear responses, but the underlying cellular mechanisms remain unexplored. Here, we report that repetitive stimulation of lateral amygdala (LA) afferents with a modified theta burst stimulation (TBS) protocol and induces long-term potentiation (LTP) of excitatory synapses onto medial paracapsular ITC (Imp) neurons. This TBS-induced LTP is; (1) induced and expressed post-synaptically, (2) involves a rise in post-synaptic Ca2+ and the activation of NR2B-containing N-methyl-D-aspartate receptors (NMDARs), (3) dependent on calcium/calmodulin-dependent protein kinase II and cAMP-dependent protein kinase activation, and (4) associated with increased exocytotic delivery of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) to the post-synaptic membrane. Remarkably, auditory fear conditioning led to a persistent increase in AMPAR/NMDAR ratio of glutamatergic synaptic currents and occluded TBS-induced LTP at LA-Imp synapses. Furthermore, extinction training rescued the effect of fear conditioning on AMPAR/NMDAR ratio and LTP induction. These results show that a prominent form of LTP can be elicited at LA-Imp synapses and suggest that this synaptic plasticity may contribute to the expression of fear conditioning.

Mechanisms to medicines: elucidating neural and molecular substrates of fear extinction to identify novel treatments for anxiety disorders

The burden of anxiety disorders is growing, but the efficacy of available anxiolytic treatments remains inadequate. Cognitive behavioural therapy for anxiety disorders focuses on identifying and modifying maladaptive patterns of thinking and behaving, and has a testable analogue in rodents in the form of fear extinction. A large preclinical literature has amassed in recent years describing the neural and molecular basis of fear extinction in rodents. In this review, we discuss how this work is being harnessed to foster translational research on anxiety disorders and facilitate the search for new anxiolytic treatments. We begin by summarizing the anatomical and functional connectivity of a medial prefrontal cortex (mPFC)-amygdala circuit that subserves fear extinction, including new insights from optogenetics. We then cover some of the approaches that have been taken to model impaired fear extinction and associated impairments with mPFC-amygdala dysfunction. The principal goal of the review is to evaluate evidence that various neurotransmitter and neuromodulator systems mediate fear extinction by modulating the mPFC-amygdala circuitry. To that end, we describe studies that have tested how fear extinction is impaired or facilitated by pharmacological manipulations of dopamine, noradrenaline, 5-HT, GABA, glutamate, neuropeptides, endocannabinoids and various other systems, which either directly target the mPFC-amygdala circuit, or produce behavioural effects that are coincident with functional changes in the circuit. We conclude that there are good grounds to be optimistic that the progress in defining the molecular substrates of mPFC-amygdala circuit function can be effectively leveraged to identify plausible candidates for extinction-promoting therapies for anxiety disorders.

Synaptic plasticity in depression: molecular, cellular and functional correlates

Synaptic plasticity confers environmental adaptability through modification of the connectivity between neurons and neuronal circuits. This is achieved through changes to synapse-associated signaling systems and supported by complementary changes to cellular morphology and metabolism within the tripartite synapse. Mounting evidence suggests region-specific changes to synaptic form and function occur as a result of chronic stress and in depression. Within subregions of the prefrontal cortex (PFC) and hippocampus structural and synapse-related findings seem consistent with a deficit in long-term potentiation (LTP) and facilitation of long-term depression (LTD), particularly at excitatory pyramidal synapses. Other brain regions are less well-studied; however the amygdala may feature a somewhat opposite synaptic pathology including reduced inhibitory tone. Changes to synaptic plasticity in stress and depression may correlate those to several signal transduction pathways (e.g. NOS-NO, cAMP-PKA, Ras-ERK, PI3K-Akt, GSK-3, mTOR and CREB) and upstream receptors (e.g. NMDAR, TrkB and p75NTR). Deficits in synaptic plasticity may further correlate disrupted brain redox and bioenergetics. Finally, at a functional level region-specific changes to synaptic plasticity in depression may relate to maladapted neurocircuitry and parallel reduced cognitive control over negative emotion.

Organic cation transporter 3 is densely expressed in the intercalated cell groups of the amygdala: anatomical evidence for a stress hormone-sensitive dopamine clearance system

The intercalated cell groups of the amygdala (ITCs) are clusters of GABAergic neurons which exert powerful modulatory control of amygdala output, and are thought to play key roles in the extinction of conditioned fear responses. Dopamine, acting through D1 receptors, inhibits ITC neuronal activity, an action that has the potential to disinhibit amygdala activity, leading to changes in behavioral responses. Dopaminergic neurotransmission in the ITC occurs through a combination of synaptic and volume transmission. Thus, mechanisms, including transport mechanisms, that regulate extracellular dopamine concentrations in the ITC, are likely to be important determinants of amygdala function. We have recently demonstrated the expression of organic cation transporter 3 (OCT3), a high-capacity transporter for dopamine and other monoamines, throughout the rat brain. In this study, we used immunohistochemical and immunofluorescence techniques to examine the distribution of OCT3 in the ITC, to identify the phenotype of OCT3-expressing cells, and to describe the spatial relationships of OCT3 to dopaminergic terminals and dopamine D1 receptors in these areas. We observed high densities of OCT3-immunoreactive perikarya and punctae throughout the D1 receptor-rich main, anterior and paracapsular ITCs, in contrast with the basolateral amygdala, where OCT3 immunoreactive perikarya and puncta were observed at much lower density. OCT3-immunoreactive perikarya in the ITC were identified as neurons. Tyrosine hydroxylase-immunoreactive fibers in the ITC were immunonegative for OCT3, though OCT3-immunoreactive punctae were observed in close proximity to TH+ terminals. Punctate OCT3-immunoreactivity in the ITCs was observed in very close proximity (<1 μm) to D1 receptor immunoreactivity. These anatomical data are consistent with the hypothesis that OCT3 plays a central role in regulating dopaminergic neurotransmission in the ITC, and that it represents a post- or peri-synaptic dopamine clearance mechanism. Inhibition of OCT3-mediated transport by corticosterone may represent a mechanism by which acute stress alters dopaminergic neurotransmission in the amygdala, leading to alterations in fear and anxiety-like behavior.

Anatomy and computational modeling of networks underlying cognitive-emotional interaction

The classical dichotomy between cognition and emotion equated the first with rationality or logic and the second with irrational behaviors. The idea that cognition and emotion are separable, antagonistic forces competing for dominance of mind has been hard to displace despite abundant evidence to the contrary. For instance, it is now known that a pathological absence of emotion leads to profound impairment of decision making. Behavioral observations of this kind are corroborated at the mechanistic level: neuroanatomical studies reveal that brain areas typically described as underlying either cognitive or emotional processes are linked in ways that imply complex interactions that do not resemble a simple mutual antagonism. Instead, physiological studies and network simulations suggest that top-down signals from prefrontal cortex realize "cognitive control" in part by either suppressing or promoting emotional responses controlled by the amygdala, in a way that facilitates adaptation to changing task demands. Behavioral, anatomical, and physiological data suggest that emotion and cognition are equal partners in enabling a continuum or matrix of flexible behaviors that are subserved by multiple brain regions acting in concert. Here we focus on neuroanatomical data that highlight circuitry that structures cognitive-emotional interactions by directly or indirectly linking prefrontal areas with the amygdala. We also present an initial computational circuit model, based on anatomical, physiological, and behavioral data to explicitly frame the learning and performance mechanisms by which cognition and emotion interact to achieve flexible behavior.

Exposure to a fearful context during periods of memory plasticity impairs extinction via hyperactivation of frontal-amygdalar circuits

An issue of increasing theoretical and translational importance is to understand the conditions under which learned fear can be suppressed, or even eliminated. Basic research has pointed to extinction, in which an organism is exposed to a fearful stimulus (such as a context) in the absence of an expected aversive outcome (such as a shock). This extinction process results in the suppression of fear responses, but is generally thought to leave the original fearful memory intact. Here, we investigate the effects of extinction during periods of memory lability on behavioral responses and on expression of the immediate-early gene c-Fos within fear conditioning and extinction circuits. Our results show that long-term extinction is impaired when it occurs during time periods during which the memory should be most vulnerable to disruption (soon after conditioning or retrieval). These behavioral effects are correlated with hyperactivation of medial prefrontal cortex and amygdala subregions associated with fear expression rather than fear extinction. These findings demonstrate that behavioral experiences during periods of heightened fear prevent extinction and prolong the conditioned fear response.

Acetylcholine as a neuromodulator: cholinergic signaling shapes nervous system function and behavior

Acetylcholine in the brain alters neuronal excitability, influences synaptic transmission, induces synaptic plasticity, and coordinates firing of groups of neurons. As a result, it changes the state of neuronal networks throughout the brain and modifies their response to internal and external inputs: the classical role of a neuromodulator. Here, we identify actions of cholinergic signaling on cellular and synaptic properties of neurons in several brain areas and discuss consequences of this signaling on behaviors related to drug abuse, attention, food intake, and affect. The diverse effects of acetylcholine depend on site of release, receptor subtypes, and target neuronal population; however, a common theme is that acetylcholine potentiates behaviors that are adaptive to environmental stimuli and decreases responses to ongoing stimuli that do not require immediate action. The ability of acetylcholine to coordinate the response of neuronal networks in many brain areas makes cholinergic modulation an essential mechanism underlying complex behaviors.

A model of amygdala-hippocampal-prefrontal interaction in fear conditioning and extinction in animals

Empirical research has shown that the amygdala, hippocampus, and ventromedial prefrontal cortex (vmPFC) are involved in fear conditioning. However, the functional contribution of each brain area and the nature of their interactions are not clearly understood. Here, we extend existing neural network models of the functional roles of the hippocampus in classical conditioning to include interactions with the amygdala and prefrontal cortex. We apply the model to fear conditioning, in which animals learn physiological (e.g. heart rate) and behavioral (e.g. freezing) responses to stimuli that have been paired with a highly aversive event (e.g. electrical shock). The key feature of our model is that learning of these conditioned responses in the central nucleus of the amygdala is modulated by two separate processes, one from basolateral amygdala and signaling a positive prediction error, and one from the vmPFC, via the intercalated cells of the amygdala, and signaling a negative prediction error. In addition, we propose that hippocampal input to both vmPFC and basolateral amygdala is essential for contextual modulation of fear acquisition and extinction. The model is sufficient to account for a body of data from various animal fear conditioning paradigms, including acquisition, extinction, reacquisition, and context specificity effects. Consistent with studies on lesioned animals, our model shows that damage to the vmPFC impairs extinction, while damage to the hippocampus impairs extinction in a different context (e.g., a different conditioning chamber from that used in initial training in animal experiments). We also discuss model limitations and predictions, including the effects of number of training trials on fear conditioning.

Intercalated and paracapsular cell islands of the adult rat amygdala: a combined rapid-Golgi, ultrastructural, and immunohistochemical account

The anterior and rostral paracapsular intercalated islands (AIC and PIC, respectively) were studied in the context of the amygdaloid modulation of fear/anxiety using horizontal sections. The structural analysis carried out using silver-impregnated specimens revealed that the AIC is composed of tightly packed, medium-sized spiny neurons with distinct dendritic and axonal patterns that send projecting axons to the central nucleus of the amygdala. The AIC occupies a strategic position between the basolateral amygdaloid complex and the caudal limb of the anterior commissure from which it receives fibers en passage and axon terminals. Electron microscopic observation of terminal (i.e., synaptic) degeneration 72 h after the surgical interruption of the anterior commissure, confirms the synaptic interaction between the latter and the AIC neurons. These observations suggest that these islands may gate the activity of neurons from the contralateral basal forebrain and synchronize the anxiogenic output of both amygdalae. Immunohistochemical analysis indicated that, within the AIC and rostral PIC, the distance between tyrosine hydroxylase-immunoreactive terminals and the punctate dopamine D(1) receptor immunoreactivity, was in the micrometer range. These results indicate a short distance and a rapid extrasynaptic form of dopamine volume transmission mediated via D(1) receptors in the AIC and PIC which may enhance fear and anxiety by suppressing feed-forward inhibition in the basolateral and central amygdaloid nuclei. The strong suggestion for a commissural axon projection to the AIC documented here, coupled with the previous evidences indicting an isocortical and amygdalar contributions to the anterior commissure, opens the possibility that the AIC may be involved in decoding nerve impulses arising from both the ipsi- and contra-lateral forebrain to, in turn, modulate the homolateral amygdala.

Adult-like action potential properties and abundant GABAergic synaptic responses in amygdala neurons from newborn marmosets

The amygdala plays an important role in the processing of emotional events. This information processing is altered by development, but little is known about the development of electrophysiological properties of neurons in the amygdala. We studied the postnatal development of electrophysiological properties of neurons in the basolateral amygdala (BLA) of the common marmoset (Callithrix jacchus). Whole-cell patch-clamp recordings were obtained from BLA pyramidal neurons in brain slices prepared from developing and adult marmosets, and electrophysiological properties known to change during development in rats were analysed. Two passive electrical properties of the neuronal membrane - the input resistance (R(in)) and the membrane time constant () - significantly decreased with postnatal development. In contrast, the action potential only showed a slight decrease in duration during the first month of life, whereas the amplitude did not change after birth. Passive electrical properties and action potentials in neurons of 4-week-old marmosets were similar to those in neurons of 4-year-old marmosets. The development of the action potential duration was not correlated with the development of R(in) or , whereas the development of R(in) and was correlated with each other. Abundant spontaneous and noradrenaline-induced GABAergic currents were present immediately after birth and did not change during postnatal development. These results suggest that newborn infant marmoset BLA pyramidal neurons possess relatively mature action potentials and receive vigorous GABAergic synaptic inputs, and that they acquire adult-like electrophysiological properties by the fourth week of life.

Functional expression of the GABA(A) receptor α2 and α3 subunits at synapses between intercalated medial paracapsular neurons of mouse amygdala

In the amygdala, GABAergic neurons in the intercalated medial paracapsular cluster (Imp) have been suggested to play a key role in fear learning and extinction. These neurons project to the central (CE) amygdaloid nucleus and to other areas within and outside the amygdala. In addition, they give rise to local collaterals that innervate other neurons in the Imp. Several drugs, including benzodiazepines (BZ), are allosteric modulators of GABA_A receptors. BZ has both anxiolytic and sedative actions, which are mediated through GABA_A receptors containing α2/α3 and α1 subunits, respectively. To establish whether α1 or α2/α3 subunits are expressed at Imp cell synapses, we used paired recordings of anatomically identified Imp neurons and high resolution immunocytochemistry in the mouse. We observed that a selective α3 subunit agonist, TP003 (100 nM), significantly increased the decay time constant of the unitary IPSCs. A similar effect was also induced by zolpidem (10 μM) or by diazepam (1 μM). In contrast, lower doses of zolpidem (0.1–1 μM) did not significantly alter the kinetics of the unitary IPSCs. Accordingly, immunocytochemical experiments established that the α2 and α3, but not the α1 subunits of the GABA_A receptors, were present at Imp cell synapses of the mouse amygdala. These results define, for the first time, some of the functional GABA_A receptor subunits expressed at synapses of Imp cells. The data also provide an additional rationale to prompt the search of GABA_A receptor α3 selective ligands as improved anxiolytic drugs.

Neural substrates for the distinct effects of presynaptic group III metabotropic glutamate receptors on extinction of contextual fear conditioning in mice

The group III metabotropic glutamate (mGlu) receptors mGlu7 and mGlu8 are receiving increased attention as potential novel therapeutic targets for anxiety disorders. The effects mediated by these receptors appear to result from a complex interplay of facilitatory and inhibitory actions at different brain sites in the anxiety/fear circuits. To better understand the effect of mGlu7 and mGlu8 receptors on extinction of contextual fear and their critical sites of action in the fear networks, we focused on the amygdala. Direct injection into the basolateral complex of the amygdala of the mGlu7 receptor agonist AMN082 facilitated extinction, whereas the mGlu8 receptor agonist (S)-3,4-DCPG sustained freezing during the extinction acquisition trial. We also determined at the ultrastructural level the synaptic distribution of these receptors in the basal nucleus (BA) and intercalated cell clusters (ITCs) of the amygdala. Both areas are thought to exert key roles in fear extinction. We demonstrate that mGlu7 and mGlu8 receptors are located in different presynaptic terminals forming both asymmetric and symmetric synapses, and that they preferentially target neurons expressing mGlu1α receptors mostly located around ITCs. In addition we show that mGlu7 and mGlu8 receptors were segregated to different inputs to a significant extent. In particular, mGlu7a receptors were primarily onto glutamatergic afferents arising from the BA or midline thalamic nuclei, but not the medial prefrontal cortex (mPFC), as revealed by combined anterograde tracing and pre-embedding electron microscopy. On the other hand, mGlu8a showed a more restricted distribution in the BA and appeared absent from thalamic, mPFC and intrinsic inputs. This segregation of mGlu7 and mGlu8 receptors in different neuronal pathways of the fear circuit might explain the distinct effects on fear extinction training observed with mGlu7 and mGlu8 receptor agonists. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Amygdala microcircuits mediating fear expression and extinction

This review summarizes the latest developments in our understanding of amygdala networks that support classical fear conditioning, the experimental paradigm most commonly used to study learned fear in the laboratory. These recent advances have considerable translational significance as congruent findings from studies of fear learning in animals and humans indicate that anxiety disorders result from abnormalities in the mechanisms that normally regulate conditioned fear. Because of the introduction of new techniques and the continued use of traditional approaches, it is becoming clear that conditioned fear involves much more complex networks than initially believed, including coordinated interactions between multiple excitatory and inhibitory circuits within the amygdala.

Different fear states engage distinct networks within the intercalated cell clusters of the amygdala

Although extinction-based therapies are among the most effective treatments for anxiety disorders, the neural bases of fear extinction remain still essentially unclear. Recent evidence suggests that the intercalated cell masses of the amygdala (ITCs) are critical structures for fear extinction. However, the neuronal organization of ITCs and how distinct clusters contribute to different fear states are still entirely unknown. Here, by combining whole-cell patch-clamp recordings and biocytin labeling with full anatomical reconstruction of the filled neurons and ultrastructural analysis of their synaptic contacts, we have elucidated the cellular organization and efferent connections of one of the main ITC clusters in mice. Our data showed an unexpected heterogeneity in the axonal pattern of medial paracapsular ITC (Imp) neurons and the presence of three distinct neuronal subtypes. Functionally, we observed that the Imp was preferentially activated during fear expression, whereas extinction training and extinction retrieval activated the main ITC nucleus (IN), as measured by quantifying Zif268 expression. This can be explained by the IPSPs evoked in the IN after Imp stimulation, most likely through the GABAergic monosynaptic innervation of IN neurons by one subtype of Imp cells, namely the medial capsular-projecting (MCp)-Imp neurons. MCp-Imp neurons also target large ITC cells that surround ITC clusters and express the metabotropic glutamate receptor 1α. These findings reveal a distinctive participation of ITC clusters to different fear states and the underlying anatomical circuitries, hence shedding new light on ITC networks and providing a novel framework to elucidate their role in fear expression and extinction.