Synaptic Organization of Perisomatic GABAergic Inputs onto the Principal Cells of the Mouse Basolateral Amygdala

Basolateral amygdala parvalbumin and cholecystokinin-expressing GABAergic neurons modulate depressive and anxiety-like behaviors

The basolateral amygdala (BLA) is increasingly recognized as a key regulator of depression and anxiety-like behaviors. However, the specific contribution of individual BLA neurons to these behaviors remains poorly understood. Building on our previous study, which demonstrated increased activity in glutamatergic BLA neurons in response to aversive stimuli and that enhancing inhibition in the BLA can alleviate depressive-like behaviors, we investigated the role of individual BLA GABAergic neurons (BLAGABA) in depressive and anxiety-like phenotypes. To address this question, we employed a comprehensive array of techniques, including c-fos staining, fiber photometry recording, optogenetic and chemogenetic manipulation, and behavior analysis. Our findings indicate that BLAGABA neurons show decreased activity during tail suspension and after chronic social defeat stress (CSDS) during social interaction. High-frequency activation of BLAGABA neurons attenuated depressive and anxiety-like behaviors, while low-frequency activation had no effect. Fiber photometry recordings revealed increased activity in BLA GABAergic neurons expressing somatostatin (SST), parvalbumin (PV), and cholecystokinin (CCK) during footshock aversive stimuli. Moreover, we found increased activity in PV and SST neurons and decreased activity in CCK-GABA neurons in the BLA during tail suspension stress. However, after CSDS, BLAPV neurons displayed decreased activity, while SST and CCK neurons showed no changes during the social interaction test. Behavioral analysis demonstrated that chemogenetic inhibition of PV and CCK-GABA neurons induced depressive and anxiety-like behaviors. whereas SST neuron inhibition had no effect. Conversely, chemogenetic activation of BLAPV neurons alleviated depressive behaviors, and activation of BLACCK-GABA neurons alleviated at least partly both depressive and anxiety-like behaviors. This study provides compelling evidence that BLAPV neurons play a critical role in regulating depressive-like behaviors, and that BLACCK-GABA neurons are involved, at least in part, in modulating both depressive-like and anxiety-like behaviors in mice.

Compartmentalized signaling in the soma: Coordination of electrical and protein kinase A signaling at neuronal ER-plasma membrane junctions

Neuronal information processing depends on converting membrane depolarizations into compartmentalized biochemical signals that can modify neuronal activity and structure. However, our understanding of how neurons translate electrical signals into specific biochemical responses remains limited, especially in the soma where gene expression and ion channel function are crucial for neuronal activity. Here, I emphasize the importance of physically compartmentalizing action potential-triggered biochemical reactions within the soma. Emerging evidence suggests that somatic endoplasmic reticulum-plasma membrane (ER-PM) junctions are specialized organelles that coordinate electrical and biochemical signaling. The juxtaposition of ion channels and signaling proteins at a prominent subset of these sites enables compartmentalized calcium and cAMP-dependent protein kinase (PKA) signaling. I explore the hypothesis that these PKA-containing ER-PM junctions serve as critical sites for translating membrane depolarizations into PKA signals and identify key gaps in knowledge of the assembly, regulation, and neurobiological functions of this somatic signaling system.

Multi-level therapeutic actions of cannabidiol in ketamine-induced schizophrenia psychopathology in male rats

Repeated administration of ketamine (KET) has been used to model schizophrenia-like symptomatology in rodents, but the psychotomimetic neurobiological and neuroanatomical underpinnings remain elusive. In parallel, the unmet need for a better treatment of schizophrenia requires the development of novel therapeutic strategies. Cannabidiol (CBD), a major non-addictive phytocannabinoid has been linked to antipsychotic effects with unclear mechanistic basis. Therefore, this study aims to clarify the neurobiological substrate of repeated KET administration model and to evaluate CBD's antipsychotic potential and neurobiological basis. CBD-treated male rats with and without prior repeated KET administration underwent behavioral analyses, followed by multilevel analysis of different brain areas including dopaminergic and glutamatergic activity, synaptic signaling, as well as electrophysiological recordings for the assessment of corticohippocampal and corticostriatal network activity. Repeated KET model is characterized by schizophrenia-like symptomatology and alterations in glutamatergic and dopaminergic activity mainly in the PFC and the dorsomedial striatum (DMS), through a bi-directional pattern. These observations are accompanied by glutamatergic/GABAergic deviations paralleled to impaired function of parvalbumin- and cholecystokinin-positive interneurons, indicative of excitation/inhibition (E/I) imbalance. Moreover, CBD counteracted the schizophrenia-like behavioral phenotype as well as reverted prefrontal abnormalities and ventral hippocampal E/I deficits, while partially modulated dorsostriatal dysregulations. This study adds novel insights to our understanding of the KET-induced schizophrenia-related brain pathology, as well as the CBD antipsychotic action through a region-specific set of modulations in the corticohippocampal and costicostrtiatal circuitry of KET-induced profile contributing to the development of novel therapeutic strategies focused on the ECS and E/I imbalance restoration.

Neuromodulation of inhibitory synaptic transmission in the basolateral amygdala during fear and anxiety

The basolateral amygdala plays pivotal roles in the regulation of fear and anxiety and these processes are profoundly modulated by different neuromodulatory systems that are recruited during emotional arousal. Recent studies suggest activities of BLA interneurons and inhibitory synaptic transmission in BLA principal cells are regulated by neuromodulators to influence the output and oscillatory network states of the BLA, and ultimately the behavioral expression of fear and anxiety. In this review, we first summarize a cellular mechanism of stress-induced anxiogenesis mediated by the interaction of glucocorticoid and endocannabinoid signaling at inhibitory synapses in the BLA. Then we discuss cell type-specific activity patterns induced by neuromodulators converging on the Gq signaling pathway in BLA perisomatic parvalbumin-expressing (PV) and cholecystokinin-expressing (CCK) basket cells and their effects on BLA network oscillations and fear learning.

Acetylcholine Engages Distinct Amygdala Microcircuits to Gate Internal Theta Rhythm

Acetylcholine (ACh) is released from basal forebrain cholinergic neurons in response to salient stimuli and engages brain states supporting attention and memory. These high ACh states are associated with theta oscillations, which synchronize neuronal ensembles. Theta oscillations in the basolateral amygdala (BLA) in both humans and rodents have been shown to underlie emotional memory, yet their mechanism remains unclear. Here, using brain slice electrophysiology in male and female mice, we show large ACh stimuli evoke prolonged theta oscillations in BLA local field potentials that depend upon M3 muscarinic receptor activation of cholecystokinin (CCK) interneurons (INs) without the need for external glutamate signaling. Somatostatin (SOM) INs inhibit CCK INs and are themselves inhibited by ACh, providing a functional SOM→CCK IN circuit connection gating BLA theta. Parvalbumin (PV) INs, which can drive BLA oscillations in baseline states, are not involved in the generation of ACh-induced theta, highlighting that ACh induces a cellular switch in the control of BLA oscillatory activity and establishes an internally BLA-driven theta oscillation through CCK INs. Theta activity is more readily evoked in BLA over the cortex or hippocampus, suggesting preferential activation of the BLA during high ACh states. These data reveal a SOM→CCK IN circuit in the BLA that gates internal theta oscillations and suggest a mechanism by which salient stimuli acting through ACh switch the BLA into a network state enabling emotional memory.

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.

Structural Organization of Perisomatic Inhibition in the Mouse Medial Prefrontal Cortex

Perisomatic inhibition profoundly controls neural function. However, the structural organization of inhibitory circuits giving rise to the perisomatic inhibition in the higher-order cortices is not completely known. Here, we performed a comprehensive analysis of those GABAergic cells in the medial prefrontal cortex (mPFC) that provide inputs onto the somata and proximal dendrites of pyramidal neurons. Our results show that most GABAergic axonal varicosities contacting the perisomatic region of superficial (layer 2/3) and deep (layer 5) pyramidal cells express parvalbumin (PV) or cannabinoid receptor type 1 (CB1). Further, we found that the ratio of PV/CB1 GABAergic inputs is larger on the somatic membrane surface of pyramidal tract neurons in comparison with those projecting to the contralateral hemisphere. Our morphologic analysis of in vitro labeled PV+ basket cells (PVBC) and CCK/CB1+ basket cells (CCKBC) revealed differences in many features. PVBC dendrites and axons arborized preferentially within the layer where their soma was located. In contrast, the axons of CCKBCs expanded throughout layers, although their dendrites were found preferentially either in superficial or deep layers. Finally, using anterograde trans-synaptic tracing we observed that PVBCs are preferentially innervated by thalamic and basal amygdala afferents in layers 5a and 5b, respectively. Thus, our results suggest that PVBCs can control the local circuit operation in a layer-specific manner via their characteristic arborization, whereas CCKBCs rather provide cross-layer inhibition in the mPFC.SIGNIFICANCE STATEMENT Inhibitory cells in cortical circuits are crucial for the precise control of local network activity. Nevertheless, in higher-order cortical areas that are involved in cognitive functions like decision-making, working memory, and cognitive flexibility, the structural organization of inhibitory cell circuits is not completely understood. In this study we show that perisomatic inhibitory control of excitatory cells in the medial prefrontal cortex is performed by two types of basket cells endowed with different morphologic properties that provide inhibitory inputs with distinct layer specificity on cells projecting to disparate areas. Revealing this difference in innervation strategy of the two basket cell types is a key step toward understanding how they fulfill their distinct roles in cortical network operations.

Fear learning and aversive stimuli differentially change excitatory synaptic transmission in perisomatic inhibitory cells of the basal amygdala

Inhibitory circuits in the basal amygdala (BA) have been shown to play a crucial role in associative fear learning. How the excitatory synaptic inputs received by BA GABAergic interneurons are influenced by memory formation, a network parameter that may contribute to learning processes, is still largely unknown. Here, we investigated the features of excitatory synaptic transmission received by the three types of perisomatic inhibitory interneurons upon cue-dependent fear conditioning and aversive stimulus and tone presentations without association. Acute slices were prepared from transgenic mice: one group received tone presentation only (conditioned stimulus, CS group), the second group was challenged by mild electrical shocks unpaired with the CS (unsigned unconditioned stimulus, unsigned US group) and the third group was presented with the CS paired with the US (signed US group). We found that excitatory synaptic inputs (miniature excitatory postsynaptic currents, mEPSCs) recorded in distinct interneuron types in the BA showed plastic changes with different patterns. Parvalbumin (PV) basket cells in the unsigned US and signed US group received mEPSCs with reduced amplitude and rate in comparison to the only CS group. Coupling the US and CS in the signed US group caused a slight increase in the amplitude of the events in comparison to the unsigned US group, where the association of CS and US does not take place. Excitatory synaptic inputs onto cholecystokinin (CCK) basket cells showed a markedly different change from PV basket cells in these behavioral paradigms: only the decay time was significantly faster in the unsigned US group compared to the only CS group, whereas the amplitude of mEPSCs increased in the signed US group compared to the only CS group. Excitatory synaptic inputs received by PV axo-axonic cells showed the least difference in the three behavioral paradigm: the only significant change was that the rate of mEPSCs increased in the signed US group when compared to the only CS group. These results collectively show that associative learning and aversive stimuli unpaired with CS cause different changes in excitatory synaptic transmission in BA perisomatic interneuron types, supporting the hypothesis that they play distinct roles in the BA network operations upon pain information processing and fear memory formation.

An adaptive behavioral control motif mediated by cortical axo-axonic inhibition

Genetically defined subgroups of inhibitory interneurons are thought to play distinct roles in learning, but heterogeneity within these subgroups has limited our understanding of the scope and nature of their specific contributions. Here we reveal that the chandelier cell (ChC), an interneuron type that specializes in inhibiting the axon-initial segment (AIS) of pyramidal neurons, establishes cortical microcircuits for organizing neural coding through selective axo-axonic synaptic plasticity. We found that organized motor control is mediated by enhanced population coding of direction-tuned premotor neurons, with tuning refined through suppression of irrelevant neuronal activity. ChCs contribute to learning-dependent refinements by providing selective inhibitory control over individual pyramidal neurons rather than global suppression. Quantitative analysis of structural plasticity across axo-axonic synapses revealed that ChCs redistributed inhibitory weights to individual pyramidal neurons during learning. These results demonstrate an adaptive logic of the inhibitory circuit motif responsible for organizing distributed neural representations. Thus, ChCs permit efficient cortical computation in a targeted cell-specific manner.

Post-weaning social isolation in male mice leads to abnormal aggression and disrupted network organization in the prefrontal cortex: Contribution of parvalbumin interneurons with or without perineuronal nets

Adverse social experiences during childhood increase the risk of developing aggression-related psychopathologies. The prefrontal cortex (PFC) is a key regulator of social behavior, where experience-dependent network development is tied to the maturation of parvalbumin-positive (PV+) interneurons. Maltreatment in childhood could impact PFC development and lead to disturbances in social behavior during later life. However, our knowledge regarding the impact of early-life social stress on PFC operation and PV+ cell function is still scarce. Here, we used post-weaning social isolation (PWSI) to model early-life social neglect in mice and to study the associated neuronal changes in the PFC, additionally distinguishing between the two main subpopulations of PV+ interneurons, i.e. those without or those enwrapped by perineuronal nets (PNN). For the first time to such detailed extent in mice, we show that PWSI induced disturbances in social behavior, including abnormal aggression, excessive vigilance and fragmented behavioral organization. PWSI mice showed altered resting-state and fighting-induced co-activation patterns between orbitofrontal and medial PFC (mPFC) subregions, with a particularly highly elevated activity in the mPFC. Surprisingly, aggressive interaction was associated with a higher recruitment of mPFC PV+ neurons that were surrounded by PNN in PWSI mice that seemed to mediate the emergence of social deficits. PWSI did not affect the number of PV+ neurons and PNN density, but enhanced PV and PNN intensity as well as cortical and subcortical glutamatergic drive onto mPFC PV+ neurons. Our results suggest that the increased excitatory input of PV+ cells could emerge as a compensatory mechanism for the PV+ neuron-mediated impaired inhibition of mPFC layer 5 pyramidal neurons, since we found lower numbers of GABAergic PV+ puncta on the perisomatic region of these cells. In conclusion, PWSI leads to altered PV-PNN activity and impaired excitatory/inhibitory balance in the mPFC, which possibly contributes to social behavioral disruptions seen in PWSI mice. Our data advances our understanding on how early-life social stress can impact the maturing PFC and lead to the development of social abnormalities in adulthood.

Ligand-gated mechanisms leading to ictogenesis in focal epileptic disorders

We review here the neuronal mechanisms that cause seizures in focal epileptic disorders and, specifically, those involving limbic structures that are known to be implicated in human mesial temporal lobe epilepsy. In both epileptic patients and animal models, the initiation of focal seizures - which are most often characterized by a low-voltage fast onset EEG pattern - is presumably dependent on the synchronous firing of GABA-releasing interneurons that, by activating post-synaptic GABA_A receptors, cause large increases in extracellular [K⁺] through the activation of the co-transporter KCC2. A similar mechanism may contribute to seizure maintenance; accordingly, inhibiting KCC2 activity transforms seizure activity into a continuous pattern of short-lasting epileptiform discharges. It has also been found that interactions between different areas of the limbic system modulate seizure occurrence by controlling extracellular [K⁺] homeostasis. In line with this view, low-frequency electrical or optogenetic activation of limbic networks restrain seizure generation, an effect that may also involve the activation of GABA_B receptors and activity-dependent changes in epileptiform synchronization. Overall, these findings highlight the paradoxical role of GABA_A signaling in both focal seizure generation and maintenance, emphasize the efficacy of low-frequency activation in abating seizures, and provide experimental evidence explaining the poor efficacy of antiepileptic drugs designed to augment GABAergic function in controlling seizures in focal epileptic disorders.

An adaptive behavioral control motif mediated by cortical axo-axonic inhibition

Neural circuits are reorganized with specificity during learning. Genetically-defined subgroups of inhibitory interneurons are thought to play distinct roles in learning, but heterogeneity within these subgroups has limited our understanding of the scope and nature of their specific contributions to learning. Here we reveal that the chandelier cell (ChC), an interneuron type that specializes in inhibiting the axon-initial segment (AIS) of pyramidal neurons, establishes cortical microcircuits for organizing neural coding through selective axo-axonic synaptic plasticity. We find that organized motor control is mediated by enhanced population coding of direction-tuned premotor neurons, whose tuning is refined through suppression of irrelevant neuronal activity. ChCs are required for learning-dependent refinements via providing selective inhibitory control over pyramidal neurons rather than global suppression. Quantitative analysis on structural plasticity of axo-axonic synapses revealed that ChCs redistributed inhibitory weights to individual pyramidal neurons during learning. These results demonstrate an adaptive logic of the inhibitory circuit motif responsible for organizing distributed neural representations. Thus, ChCs permit efficient cortical computation in a target cell specific manner, which highlights the significance of interneuron diversity.

Inhibitory circuits in fear memory and fear-related disorders

Fear learning and memory rely on dynamic interactions between the excitatory and inhibitory neuronal populations that make up the prefrontal cortical, amygdala, and hippocampal circuits. Whereas inhibition of excitatory principal cells (PCs) by GABAergic neurons restrains their excitation, inhibition of GABAergic neurons promotes the excitation of PCs through a process called disinhibition. Specifically, GABAergic interneurons that express parvalbumin (PV+) and somatostatin (SOM+) provide inhibition to different subcellular domains of PCs, whereas those that express the vasoactive intestinal polypeptide (VIP+) facilitate disinhibition of PCs by inhibiting PV+ and SOM+ interneurons. Importantly, although the main connectivity motifs and the underlying network functions of PV+, SOM+, and VIP+ interneurons are replicated across cortical and limbic areas, these inhibitory populations play region-specific roles in fear learning and memory. Here, we provide an overview of the fear processing in the amygdala, hippocampus, and prefrontal cortex based on the evidence obtained in human and animal studies. Moreover, focusing on recent findings obtained using genetically defined imaging and intervention strategies, we discuss the population-specific functions of PV+, SOM+, and VIP+ interneurons in fear circuits. Last, we review current insights that integrate the region-specific inhibitory and disinhibitory network patterns into fear memory acquisition and fear-related disorders.

Cortical control of chandelier cells in neural codes

Various cortical functions arise from the dynamic interplay of excitation and inhibition. GABAergic interneurons that mediate synaptic inhibition display significant diversity in cell morphology, electrophysiology, plasticity rule, and connectivity. These heterogeneous features are thought to underlie their functional diversity. Emerging attention on specific properties of the various interneuron types has emphasized the crucial role of cell-type specific inhibition in cortical neural processing. However, knowledge is still limited on how each interneuron type forms distinct neural circuits and regulates network activity in health and disease. To dissect interneuron heterogeneity at single cell-type precision, we focus on the chandelier cell (ChC), one of the most distinctive GABAergic interneuron types that exclusively innervate the axon initial segments (AIS) of excitatory pyramidal neurons. Here we review the current understanding of the structural and functional properties of ChCs and their implications in behavioral functions, network activity, and psychiatric disorders. These findings provide insights into the distinctive roles of various single-type interneurons in cortical neural coding and the pathophysiology of cortical dysfunction.

Quantitative analysis of the GABAergic innervation of the soma and axon initial segment of pyramidal cells in the human and mouse neocortex

Perisomatic GABAergic innervation in the cerebral cortex is carried out mostly by basket and chandelier cells, which differentially participate in the control of pyramidal cell action potential output and synchronization. These cells establish multiple synapses with the cell body (and proximal dendrites) and the axon initial segment (AIS) of pyramidal neurons, respectively. Using multiple immunofluorescence, confocal microscopy and 3D quantification techniques, we have estimated the number and density of GABAergic boutons on the cell body and AIS of pyramidal neurons located through cortical layers of the human and mouse neocortex. The results revealed, in both species, that there is clear variability across layers regarding the density and number of perisomatic GABAergic boutons. We found a positive linear correlation between the surface area of the soma, or the AIS, and the number of GABAergic terminals in apposition to these 2 neuronal domains. Furthermore, the density of perisomatic GABAergic boutons was higher in the human cortex than in the mouse. These results suggest a selectivity for the GABAergic innervation of the cell body and AIS that might be related to the different functional attributes of the microcircuits in which neurons from different layers are involved in both human and mouse.

Long term effects of 24-h-restraint stress on the connectivity and structure of interneurons in the basolateral amygdala

The effects of intense stressors can last a long time and may lead to the development of psychiatric disorders, including posttraumatic stress disorder. The basolateral amygdala (BLA) plays a critical role in these diseases and is extremely sensitive to stress. Here, we explored in male and female mice the long-term (35 days) impact of a 24-h restraint stress on BLA circuitry. We used Thy1-YFP mice to discriminate 2 subpopulations of excitatory neurons, which participate in "Fear-On" (Thy1-) and "Fear-Off" (Thy1+) circuits. The stress decreased the density of parvalbumin (PV) + inhibitory neurons in both sexes but did not alter their dendritic complexity. We also analyzed the perisomatic input of basket interneurons on Thy1+ and Thy1- neurons, finding sex dependent effects. In males, we did not find alterations in the density of PV+ puncta or in that of cannabinoid receptor 1 (CB1R) + puncta from cholecystokinin+ basket cells. By contrast, in females we found increased the density of PV+ puncta on Thy1+ neurons and reduced on the Thy1- neurons. This adverse experience also reduced in the long term the density of CB1R+ puncta both on Thy1+ and Thy1- cells in females. The expression of the activity marker FosB was not altered in PV+ interneurons and in Thy1+ neurons of stressed animals. The density of perineuronal nets, a specialized region of the extracellular matrix, which covers particularly PV+ interneurons and regulates their connectivity, was increased by stress in male mice. Our findings indicate that a single stressful event can produce long-term alterations in the inhibitory circuits of the BLA, especially on PV+ neurons and their plasticity, and that there is a differential impact depending on the sex and the fear-related circuits involved.

Gq neuromodulation of BLA parvalbumin interneurons induces burst firing and mediates fear-associated network and behavioral state transition in mice

Patterned coordination of network activity in the basolateral amygdala (BLA) is important for fear expression. Neuromodulatory systems play an essential role in regulating changes between behavioral states, however the mechanisms underlying this neuromodulatory control of transitions between brain and behavioral states remain largely unknown. We show that chemogenetic Gq activation and α1 adrenoreceptor activation in mouse BLA parvalbumin (PV) interneurons induces a previously undescribed, stereotyped phasic bursting in PV neurons and time-locked synchronized bursts of inhibitory postsynaptic currents and phasic firing in BLA principal neurons. This Gq-coupled receptor activation in PV neurons suppresses gamma oscillations in vivo and in an ex vivo slice model, and facilitates fear memory recall, which is consistent with BLA gamma suppression during conditioned fear expression. Thus, here we identify a neuromodulatory mechanism in PV inhibitory interneurons of the BLA which regulates BLA network oscillations and fear memory recall.

The authors study mechanisms underlying neuromodulatory control of transitions between brain and behavioral states. They identify a mechanism whereby modulation of Gq activity in basolateral amygdala parvalbumin interneurons mediates the transition to a fear-associated network and behavioral state.

Medial prefrontal area reductions, altered expressions of cholecystokinin, parvalbumin, and activating transcription factor 4 in the corticolimbic system, and altered emotional behavior in a progressive rat model of type 2 diabetes

Metabolic disorders are associated with a higher risk of psychiatric disorders. We previously reported that 20-week-old Otsuka Long-Evans Tokushima fatty (OLETF) rats, a model of progressive type 2 diabetes, showed increased anxiety-like behavior and regional area reductions and increased cholecystokinin-positive neurons in the corticolimbic system. However, in which stages of diabetes these alterations in OLETF rats occur remains unclear. We aimed to investigate anxiety-like behavior and its possible mechanisms at different stages of type 2 diabetes in OLETF rats. Eight- and 30-week-old OLETF rats were used as diabetic animal models at the prediabetic and progressive stages of type 2 diabetes respectively, and age-matched Long-Evans Tokushima Otsuka rats served as non-diabetic controls. In the open-field test, OLETF rats showed less locomotion in the center zone and longer latency to leave the center zone at 8 and 30 weeks old, respectively. The areas of the medial prefrontal cortex were smaller in the OLETF rats, regardless of age. The densities of cholecystokinin-positive neurons in OLETF rats were higher in the lateral and basolateral amygdala only at 8 weeks old and in the anterior cingulate and infralimbic cortices and hippocampal cornu ammonis area 3 at both ages. The densities of parvalbumin-positive neurons of OLETF rats were lower in the cornu ammonis area 2 at 8 weeks old and in the prelimbic and infralimbic cortices at both ages. No apoptotic cell death was detected in OLETF rats, but the percentage of neurons co-expressing activating transcription factor 4 and cholecystokinin and parvalbumin was higher in OLETF rats at both ages in the anterior cingulate cortex and basolateral amygdala, respectively. These results suggest that altered emotional behavior and related neurological changes in the corticolimbic system are already present in the prediabetic stage of OLETF rats.

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

The basolateral nuclear complex (BNC) of the amygdala plays an important role in the generation of emotional/motivational behavior and the consolidation of emotional memories. Activation of M1 cholinergic receptors (M1Rs) in the BNC is critical for memory consolidation. Previous receptor binding studies in the monkey amygdala demonstrated that the BNC has a high density of M1Rs, but did not have sufficient resolution to identify which neurons in the BNC expressed them. This was accomplished in the present immunohistochemical investigation using an antibody for the m1 receptor (m1R). Analysis of m1Rs in the monkey BNC using immunoperoxidase techniques revealed that their expression was very dense in the BNC, and suggested that virtually all of the pyramidal projection neurons (PNs) in all of the BNC nuclei were m1R-immunoreactive (m1R+). This was confirmed with dual-labeling immunofluorescence using staining for calcium/calmodulin-dependent protein kinase II (CaMK) as a marker for BNC PNs. However, additional dual-labeling studies indicated that one-third of inhibitory interneurons (INs) expressing glutamic acid decarboxylase (GAD) were also m1R+. Moreover, the finding that 60% of parvalbumin (PV) immunoreactive neurons were m1R+ indicated that this IN subpopulation was the main GAD+ subpopulation exhibiting m1R expression. The cholinergic innervation of the amygdala is greatly reduced in Alzheimer's disease and there is currently considerable interest in developing selective M1R positive allosteric modulators (PAMs) to treat the symptoms. The results of the present study indicate that M1Rs in both PNs and INs in the primate BNC would be targeted by M1R PAMs.

Interneuron Types and Their Circuits in the Basolateral Amygdala

The basolateral amygdala (BLA) is a cortical structure based on its cell types, connectivity features, and developmental characteristics. This part of the amygdala is considered to be the main entry site of processed and multisensory information delivered via cortical and thalamic afferents. Although GABAergic inhibitory cells in the BLA comprise only 20% of the entire neuronal population, they provide essential control over proper network operation. Previous studies have uncovered that GABAergic cells in the basolateral amygdala are as diverse as those present in other cortical regions, including the hippocampus and neocortex. To understand the role of inhibitory cells in various amygdala functions, we need to reveal the connectivity and input-output features of the different types of GABAergic cells. Here, I review the recent achievements in uncovering the diversity of GABAergic cells in the basolateral amygdala with a specific focus on the microcircuit organization of these inhibitory cells.

Inhibitory Circuits in the Basolateral Amygdala in Aversive Learning and Memory

Neural circuits in the basolateral amygdala (BLA) play a pivotal role in the learning and memory formation, and processing of emotionally salient experiences, particularly aversive ones. A diverse population of GABAergic neurons present in the BLA orchestrate local circuits to mediate emotional memory functions. Targeted manipulation of GABAergic neuronal subtypes has shed light on cell-type specific functional roles in the fear learning and memory, revealing organizing principles for the operation of inhibitory circuit motifs in the BLA.

Total Number and Ratio of GABAergic Neuron Types in the Mouse Lateral and Basal Amygdala

GABAergic neurons are key circuit elements in cortical networks. Despite growing evidence showing that inhibitory cells play a critical role in the lateral (LA) and basal (BA) amygdala functions, neither the number of GABAergic neurons nor the ratio of their distinct types has been determined in these amygdalar nuclei. Using unbiased stereology, we found that the ratio of GABAergic neurons in the BA (22%) is significantly higher than in the LA (16%) in both male and female mice. No difference was observed between the right and left hemispheres in either sex. In addition, we assessed the ratio of the major inhibitory cell types in both amygdalar nuclei. Using transgenic mice and a viral strategy for visualizing inhibitory cells combined with immunocytochemistry, we estimated that the following cell types together compose the vast majority of GABAergic cells in the LA and BA: axo-axonic cells (5.5%-6%), basket cells expressing parvalbumin (17%-20%) or cholecystokinin (7%-9%), dendrite-targeting inhibitory cells expressing somatostatin (10%-16%), NPY-containing neurogliaform cells (14%-15%), VIP and/or calretinin-expressing interneuron-selective interneurons (29%-38%), and GABAergic projection neurons expressing somatostatin and neuronal nitric oxide synthase (5.5%-8%). Our results show that these amygdalar nuclei contain all major GABAergic neuron types as found in other cortical regions. Furthermore, our data offer an essential reference for future studies aiming to reveal changes in GABAergic cell number and in inhibitory cell types typically observed under different pathologic conditions, and to model functioning amygdalar networks in health and disease.SIGNIFICANCE STATEMENT GABAergic cells in cortical structures, as in the lateral and basal nucleus of the amygdala, have a determinant role in controlling circuit operation. In this study, we provide the first estimate for the total number of inhibitory cells in these two amygdalar nuclei. In addition, our study is the first to define the ratio of the major GABAergic cell types present in these cortical networks. Taking into account that hyperexcitability in the amygdala, arising from the imbalance between excitation and inhibition typifies many altered brain functions, including anxiety, post-traumatic stress disorder, schizophrenia, and autism, uncovering the number and ratio of distinct amygdalar inhibitory cell types offers a solid base for comparing the changes in inhibition in pathologic brain states.

Maturation of amygdala inputs regulate shifts in social and fear behaviors: A substrate for developmental effects of stress

Stress can negatively impact brain function and behaviors across the lifespan. However, stressors during adolescence have particularly harmful effects on brain maturation, and on fear and social behaviors that extend beyond adolescence. Throughout development, social behaviors are refined and the ability to suppress fear increases, both of which are dependent on amygdala activity. We review rodent literature focusing on developmental changes in social and fear behaviors, cortico-amygdala circuits underlying these changes, and how this circuitry is altered by stress. We first describe changes in fear and social behaviors from adolescence to adulthood and parallel developmental changes in cortico-amygdala circuitry. We propose a framework in which maturation of cortical inputs to the amygdala promote changes in social drive and fear regulation, and the particularly damaging effects of stress during adolescence may occur through lasting changes in this circuit. This framework may explain why anxiety and social pathologies commonly co-occur, adolescents are especially vulnerable to stressors impacting social and fear behaviors, and predisposed towards psychiatric disorders related to abnormal cortico-amygdala circuits.

Immunohistochemical Identification of Interneuronal Subpopulations in the Basolateral Amygdala of the Rhesus Monkey (Macaca mulatta)

Inhibitory circuits in the basolateral nuclear complex of the amygdala (BNC) critical for controlling the acquisition, expression, and extinction of emotional responses are mediated by GABAergic interneurons (INs). Studies in rodents have demonstrated that separate IN subpopulations, identified by their expression of calcium-binding proteins and neuropeptides, play discrete roles in the intrinsic circuitry of the BNC. Far less is known about IN subpopulations in primates. In order to fill in this gap in our understanding of primate INs, the present investigation used dual-labeling immunohistochemistry for IN markers to identify subpopulations expressing cholecystokinin (CCK), calbindin (CB), calretinin (CR), and somatostatin (SOM) in somata and axon terminals in the monkey BNC. In general, colocalization patterns seen in somata and axon terminals were similar. It was found that there was virtually no colocalization of CB and CR, the two calcium-binding proteins investigated. Three subtypes of CCK-immunoreactive (CCK+) INs were identified on the basis of their expression of CR or CB: (1) CCK+/CR+; (2) CCK+/CB+); and (3) CCK+/CR-/CB-. Almost no colocalization of CCK with SOM was observed, but there was extensive colocalization of SOM and CB. CCK+, CR+, and CCK+/CR+ double-labeled axon terminals were seen surrounding pyramidal cell somata in basket-like plexuses, as well as in the neuropil. CB+, SOM+, and CB+/SOM+ terminals did not form baskets, suggesting that these IN subpopulations are mainly dendrite-targeting neurons. In general, the IN subpopulations in the monkey are not dissimilar to those seen in rodents but, unlike rodents, CB+ INs in the monkey are not basket cells.

Expression of the type 1 cannabinoid receptor (CB1R) in CCK-immunoreactive axon terminals in the basolateral amygdala of the rhesus monkey (Macaca mulatta)

Studies in rodents have shown that interactions between cholecystokinin (CCK) and the endogenous cannabinoid system in the basolateral nuclear complex of the amygdala (BNC) modulate anxiety-like behavior and fear learning/expression. One of the main cell types implicated is a CCK-immunoreactive (CCK+) basket cell that innervates the somata of pyramidal projection neurons (PNs) and expresses the type 1 cannabinoid receptor (CB1R) in its axon terminals. Although numerous studies have elucidated the anatomy and physiology of these CCK+/CB1R + interneurons in rodents, it has not been determined if they exist in primates. The present investigation used immunohistochemical techniques in the monkey to answer this question. It was found that the monkey BNC, as in rodents, has a very high density of CB1R + axons, including CB1R + axon terminals that form basket-like plexuses contacting somata of PNs. These axons, as well as axons in the neuropil, exhibit extensive colocalization of CCK and CB1R. These findings suggest that the same synaptic mechanisms involved in CCK-CB1R interactions in rodents may also apply to primates, and that therapies that target the cannabinoid system in the BNC may be useful for treating fear and anxiety in human patients.

Cholecystokinin-Mediated Neuromodulation of Anxiety and Schizophrenia: A "Dimmer-Switch" Hypothesis

Cholecystokinin (CCK), the most abundant brain neuropeptide, is involved in relevant behavioral functions like memory, cognition, and reward through its interactions with the opioid and dopaminergic systems in the limbic system. CCK excites neurons by binding two receptors, CCK1 and CCK2, expressed at low and high levels in the brain, respectively. Historically, CCK2 receptors have been related to the induction of panic attacks in humans. Disturbances in brain CCK expression also underlie the physiopathology of schizophrenia, which is attributed to the modulation by CCK1 receptors of the dopamine flux in the basal striatum. Despite this evidence, neither CCK2 receptor antagonists ameliorate human anxiety nor CCK agonists have consistently shown neuroleptic effects in clinical trials. A neglected aspect of the function of brain CCK is its neuromodulatory role in mental disorders. Interestingly, CCK is expressed in pivotal inhibitory interneurons that sculpt cortical dynamics and the flux of nerve impulses across corticolimbic areas and the excitatory projections to mesolimbic pathways. At the basal striatum, CCK modulates the excitability of glutamate, the release of inhibitory GABA, and the discharge of dopamine. Here we focus on how CCK may reduce rather than trigger anxiety by regulating its cognitive component. Adequate levels of CCK release in the basal striatum may control the interplay between cognition and reward circuitry, which is critical in schizophrenia. Hence, it is proposed that disturbances in the excitatory/ inhibitory interplay modulated by CCK may contribute to the imbalanced interaction between corticolimbic and mesolimbic neural activity found in anxiety and schizophrenia.

Neurons Expressing Parvalbumin and Calretinin in the Human Amygdaloid Complex: A Quantitative and Qualitative Analysis in Every Nucleus and Nuclear Subdivision

The primate amygdaloid complex (AC) contains projection neurons as well as subsets of interneurons (IN), many of which express calcium-binding proteins, that through their local circuits control the activity of the projection neurons. The inhibitory parvalbumin (PV) and calretinin (CR)-positive (+) AC IN have a crucial role in the appearance of synchronized oscillations in local ensembles of projection neurons that mediate the consolidation and recall of fear memories. The GABAergic transmission of these subsets of IN is modulated by dopamine. To expand the knowledge regarding the cellular composition and distribution of IN in the human AC, we focused on two non-overlapping populations: the PV+ and CR+. We have analyzed the distribution of these IN throughout the AC from subjects without any neurological or psychiatric disorders and estimated their absolute number and density using stereological methods. We have also provided percentages of the IN with respect to the total AC neurons. The CR + IN were distributed throughout the AC, whereas the PV+ were only present in the basolateral nuclear group. The quantity of CR + IN was four times higher than that of PV+ and the percentages varied from less than 1% for PV + IN to 6-20% for CR+. The differences in quantity and distribution of CR+ and PV + IN could be related to their differential inhibitory properties and to the intrinsic and extrinsic connections of every amygdaloid region.

Region-specific brain area reductions and increased cholecystokinin positive neurons in diabetic OLETF rats: implication for anxiety-like behavior

Metabolic disorders can induce psychiatric comorbidities. Both brain and neuronal composition imbalances reportedly induce an anxiety-like phenotype. We hypothesized that alterations of localized brain areas and cholecystokinin (CCK) and parvalbumin (PV) expression could induce anxiety-like behavior in type 2 diabetic Otsuka Long-Evans Tokushima fatty (OLETF) rats. Twenty-week-old OLETF and non-diabetic Long-Evans Tokushima Otsuka (LETO) rats were used. The areas of corticolimbic regions were smaller in OLETF rats. The densities of CCK positive neurons in the lateral and basolateral amygdala, hippocampal cornu ammonis area 2, and prelimbic cortex were higher in OLETF rats. The densities of PV positive neurons were comparable between OLETF and LETO rats. Locomotion in the center zone in the open field test was lower in OLETF rats. These results suggest that imbalances of specific brain region areas and neuronal compositions in emotion-related areas increase the prevalence of anxiety-like behaviors in OLETF rats.

Light microscopy based approach for mapping connectivity with molecular specificity

Mapping neuroanatomy is a foundational goal towards understanding brain function. Electron microscopy (EM) has been the gold standard for connectivity analysis because nanoscale resolution is necessary to unambiguously resolve synapses. However, molecular information that specifies cell types is often lost in EM reconstructions. To address this, we devise a light microscopy approach for connectivity analysis of defined cell types called spectral connectomics. We combine multicolor labeling (Brainbow) of neurons with multi-round immunostaining Expansion Microscopy (miriEx) to simultaneously interrogate morphology, molecular markers, and connectivity in the same brain section. We apply this strategy to directly link inhibitory neuron cell types with their morphologies. Furthermore, we show that correlative Brainbow and endogenous synaptic machinery immunostaining can define putative synaptic connections between neurons, as well as map putative inhibitory and excitatory inputs. We envision that spectral connectomics can be applied routinely in neurobiology labs to gain insights into normal and pathophysiological neuroanatomy.

What Happened in the Hippocampal Axon in a Rat Model of Posttraumatic Stress Disorder

Studies from postmortem and animal models have revealed altered synapse morphology and function in the brain of posttraumatic stress disorder (PTSD). And the effects of PTSD on dendrites and spines have been reported, however, the effection on axon include microtubule (MT) and synaptic vesicles of presynaptic elements remains unknown. Hippocampus is involved in abnormal memory in PTSD. In the present study, we used the single prolonged stress (SPS) model to mimic PTSD. Quantitative real-time polymerase chain reaction (RT-qPCR) and high-throughput sequencing (GSE153081) were utilized to analyze differentially expressed genes (DEGs) in the hippocampus of control and SPS rats. Immunofluorescence and western blotting were performed to examine change in axon-related proteins. Synaptic function was evaluated by measuring miniature excitatory postsynaptic currents (mEPSCs). RNA-sequencing analysis revealed 230 significantly DEGs between the control and SPS groups. Gene Ontology analysis revealed upregulation in axonemal assembly, MT formation, or movement, but downregulation in axon initial segment and synaptic vesicles fusion in the hippocampus of SPS rats. Increased expression in tau, β-tubulin MAP1B, KIF9, CCDC40, DNAH12 and decreased expression in p-tau, stathmin suggested SPS induced axon extension. Increased protein expression in VAMP, STX1A, Munc18-1 and decreased expression in synaptotagmin-1 suggested SPS induced more SNARE complex formation but decreased ability in synaptic vesicle fusion to presynaptic active zone membrane in the hippocampus of SPS rats. Further, low mEPSC frequency in SPS rats indicated dysfunction in presynaptic membrane. These results suggest that axon extension and synaptic vesicles fusion abnormality are involved in dysfunction of PTSD.

Neuronal Circuits in Barrel Cortex for Whisker Sensory Perception

The array of whiskers on the snout provides rodents with tactile sensory information relating to the size, shape and texture of objects in their immediate environment. Rodents can use their whiskers to detect stimuli, distinguish textures, locate objects and navigate. Important aspects of whisker sensation are thought to result from neuronal computations in the whisker somatosensory cortex (wS1). Each whisker is individually represented in the somatotopic map of wS1 by an anatomical unit named a 'barrel' (hence also called barrel cortex). This allows precise investigation of sensory processing in the context of a well-defined map. Here, we first review the signaling pathways from the whiskers to wS1, and then discuss current understanding of the various types of excitatory and inhibitory neurons present within wS1. Different classes of cells can be defined according to anatomical, electrophysiological and molecular features. The synaptic connectivity of neurons within local wS1 microcircuits, as well as their long-range interactions and the impact of neuromodulators, are beginning to be understood. Recent technological progress has allowed cell-type-specific connectivity to be related to cell-type-specific activity during whisker-related behaviors. An important goal for future research is to obtain a causal and mechanistic understanding of how selected aspects of tactile sensory information are processed by specific types of neurons in the synaptically connected neuronal networks of wS1 and signaled to downstream brain areas, thus contributing to sensory-guided decision-making.

Whole-Brain Map of Long-Range Monosynaptic Inputs to Different Cell Types in the Amygdala of the Mouse

The amygdala, which is involved in various behaviors and emotions, is reported to connect with the whole brain. However, the long-range inputs of distinct cell types have not yet been defined. Here, we used a retrograde trans-synaptic rabies virus to generate a whole-brain map of inputs to the main cell types in the mouse amygdala. We identified 37 individual regions that projected to neurons expressing vesicular glutamate transporter 2, 78 regions to parvalbumin-expressing neurons, 104 regions to neurons expressing protein kinase C-δ, and 89 regions to somatostatin-expressing neurons. The amygdala received massive projections from the isocortex and striatum. Several nuclei, such as the caudate-putamen and the CA1 field of the hippocampus, exhibited input preferences to different cell types in the amygdala. Notably, we identified several novel input areas, including the substantia innominata and zona incerta. These findings provide anatomical evidence to help understand the precise connections and diverse functions of the amygdala.

The ins and outs of inhibitory synaptic plasticity: Neuron types, molecular mechanisms and functional roles

GABAergic interneurons are highly diverse, and their synaptic outputs express various forms of plasticity. Compelling evidence indicates that activity-dependent changes of inhibitory synaptic transmission play a significant role in regulating neural circuits critically involved in learning and memory and circuit refinement. Here, we provide an updated overview of inhibitory synaptic plasticity with a focus on the hippocampus and neocortex. To illustrate the diversity of inhibitory interneurons, we discuss the case of two highly divergent interneuron types, parvalbumin-expressing basket cells and neurogliaform cells, which support unique roles on circuit dynamics. We also present recent progress on the molecular mechanisms underlying long-term, activity-dependent plasticity of fast inhibitory transmission. Lastly, we discuss the role of inhibitory synaptic plasticity in neuronal circuits' function. The emerging picture is that inhibitory synaptic transmission in the CNS is extremely diverse, undergoes various mechanistically distinct forms of plasticity and contributes to a much more refined computational role than initially thought. Both the remarkable diversity of inhibitory interneurons and the various forms of plasticity expressed by GABAergic synapses provide an amazingly rich inhibitory repertoire that is central to a variety of complex neural circuit functions, including memory.

Semilunar Granule Cells Are the Primary Source of the Perisomatic Excitatory Innervation onto Parvalbumin-Expressing Interneurons in the Dentate Gyrus

We analyzed the origin and relevance of the perisomatic excitatory inputs on the parvalbumin interneurons of the granule cell layer in mouse. Confocal analysis of the glutamatergic innervation showed that it represents ∼50% of the perisomatic synapses that parvalbumin cells receive. This excitatory input may originate from granule cell collaterals, the mossy cells, or even supramammillary nucleus. First, we assessed the input from the mossy cells on parvalbumin interneurons. Axon terminals of mossy cells were visualized by their calretinin content. Using multicolor confocal microscopy, we observed that less than 10% of perisomatic excitatory innervation of parvalbumin cells could originate from mossy cells. Correlative light and electron microscopy revealed that innervation from mossy cells, although present, was indeed infrequent, except for those parvalbumin cells whose somata were located in the inner molecular layer. Second, we investigated the potential input from supramammillary nucleus on parvalbumin cell somata using anterograde tracing or immunocytochemistry against vesicular glutamate transporter 2 (VGLUT2) and found only occasional contacts. Third, we intracellularly filled dentate granule cells in acute slice preparations using whole-cell recording and examined whether their axon collaterals target parvalbumin interneurons. We found that typical granule cells do not innervate the perisomatic region of these GABAergic cells. In sharp contrast, semilunar granule cells (SGCs), a scarce granule cell subtype often contacted the parvalbumin cell soma and proximal dendrites. Our data, therefore, show that perisomatic excitatory drive of parvalbumin interneurons in the granular layer of the dentate gyrus is abundant and originates primarily from SGCs.

Nonpyramidal neurons in the primate basolateral amygdala: A Golgi study in the baboon (Papio cynocephalus) and long-tailed macaque (Macaca fascicularis)

Nonpyramidal GABAergic interneurons in the basolateral nuclear complex (BNC) of the amygdala are critical for the regulation of emotion. Remarkably, there have been no Golgi studies of these neurons in nonhuman primates. Therefore, in the present study we investigated the morphology of nonpyramidal neurons (NPNs) in the BNC of the baboon and monkey using the Golgi technique. NPNs were scattered throughout all nuclei of the BNC and had aspiny or spine-sparse dendrites. NPNs were morphologically heterogeneous and could be divided into small, medium, large, and giant types based on the size of their somata. NPNs could be further divided on the basis of their somatodendritic morphology into four types: multipolar, bitufted, bipolar, and irregular. NPN axons, when stained, formed dense local arborizations that overlapped their dendritic fields to varying extents. These axons always exhibited varying numbers of varicosities representing axon terminals. Three specialized NPN subtypes were recognized because of their unique anatomical features: axo-axonic cells, neurogliaform cells, and giant cells. The axons of axo-axonic cells formed "axonal cartridges," with clustered varicosities that contacted the axon initial segments of pyramidal neurons (PNs). Neurogliaform cells had small somata and numerous short dendrites that formed a dense dendritic arborization; they also exhibited a very dense axonal arborization that overlapped the dendritic field. Giant cells had very large irregular somata and long, thick dendrites; their distal dendrites often branched extensively and had long appendages. In general, the NPNs of the baboon and monkey BNC, including the specialized subtypes, were similar to those of rodents.

Low-threshold spiking interneurons perform feedback inhibition in the lateral amygdala

Amygdala plays crucial roles in emotional learning. The lateral amygdala (LA) is the input station of the amygdala, where learning related plasticity occurs. The LA is cortical like in nature in terms of its cellular make up, composed of a majority of principal cells and a minority of interneurons with distinct subtypes defined by morphology, intrinsic electrophysiological properties and neurochemical expression profile. The specific functions served by LA interneuron subtypes remain elusive. This study aimed to elucidate the interneuron subtype mediating feedback inhibition. Electrophysiological evidence involving antidromic activation of recurrent LA circuitry via basolateral amygdala stimulation and paired recordings implicate low-threshold spiking interneurons in feedback inhibition. Recordings in somatostatin-cre animals crossed with tdtomato mice have revealed remarkable similarities between a subset of SOM+ interneurons and LTS interneurons. This study concludes that LTS interneurons, most of which are putatively SOM+, mediate feedback inhibition in the LA. Parallels with cortical areas and potential implications for information processing and plasticity are discussed.

Excitation of Diverse Classes of Cholecystokinin Interneurons in the Basal Amygdala Facilitates Fear Extinction

There is growing evidence that interneurons (INs) orchestrate neural activity and plasticity in corticoamygdala circuits to regulate fear behaviors. However, defining the precise role of cholecystokinin-expressing INs (CCK INs) remains elusive due to the technical challenge of parsing this population from CCK-expressing principal neurons (CCK PNs). Here, we used an intersectional genetic strategy in CCK-Cre;Dlx5/6-Flpe double-transgenic mice to study the anatomical, molecular and electrophysiological properties of CCK INs in the basal amygdala (BA) and optogenetically manipulate these cells during fear extinction. Electrophysiological recordings confirmed that this strategy targeted GABAergic cells and that a significant proportion expressed functional cannabinoid CB1 receptors; a defining characteristic of CCK-expressing basket cells. However, immunostaining showed that subsets of the genetically-targeted cells expressed either neuropeptide Y (NPY; 29%) or parvalbumin (PV; 17%), but not somatostatin (SOM) or Ca²⁺/calmodulin-dependent protein kinase II (CaMKII)-α. Further morphological and electrophysiological analyses showed that four IN types could be identified among the EYFP-expressing cells: CCK/cannabinoid receptor type 1 (CB1R)-expressing basket cells, neurogliaform cells, PV+ basket cells, and PV+ axo-axonic cells. At the behavioral level, in vivo optogenetic photostimulation of the targeted population during extinction acquisition led to reduced freezing on a light-free extinction retrieval test, indicating extinction memory facilitation; whereas photosilencing was without effect. Conversely, non-selective (i.e., inclusive of INs and PNs) photostimulation or photosilencing of CCK-targeted cells, using CCK-Cre single-transgenic mice, impaired extinction. These data reveal an unexpectedly high degree of phenotypic complexity in a unique population of extinction-modulating BA INs.

Cholecystokinin immunoreactive neurons in the basolateral amygdala of the rhesus monkey (Macaca mulatta)

Several distinct subpopulations of interneurons (INs) in the amygdalar basolateral nuclear complex (BNC) of the rat can be recognized on the basis of their expression of calcium-binding proteins and neuropeptides, including parvalbumin (PV), somatostatin (SOM), calretinin (CR), and cholecystokinin (CCK). In the rat BNC CCK is expressed in two separate IN subpopulations, termed large (CCKL ) and small (CCKS ). These subpopulations exhibit distinct connections indicative of discrete functional roles in the circuitry of the BNC. Although there have been several studies of PV+, SOM+, and CR+ INs in the primate BNC, there is almost no information regarding CCK+ INs in these species. Therefore, in the present study the distribution and morphology of CCK+ INs and their axon terminals in the BNC of the monkey was investigated. CCK immunoreactivity in the BNC was observed in somata and proximal dendrites of nonpyramidal neurons, as well as in axon terminals. A moderate density of CCK+ INs was found in all nuclei of the BNC. CCK+ INs in the BNC were morphologically heterogeneous, with both small and large varieties observed. All CCK+ somata gave rise to 2-4 dendrites that branched sparingly and were aspiny. CCK+ axon terminals in the BNC were found both in the neuropil and forming pericellular baskets contacting somata of pyramidal cells. In addition, many CCK+ neurons were contacted by multiple CCK+ terminals, indicative of the existence of a CCK interneuronal network. These data indicate that the morphology of CCK+ INs in the monkey is very similar to that of the rat.

Ultrastructural localization of cannabinoid CB1 and mGluR5 receptors in the prefrontal cortex and amygdala

Stimulation of the postsynaptic metabotropic glutamate receptor mGluR5 triggers retrograde signaling of endocannabinoids that activate presynaptic cannabinoid CB1 receptors on juxtaposing axon terminals. To better understand the synaptic structure that supports mGluR5 mediation of CB1 activation in the prefrontal cortex (PFC) and basolateral amygdala (BLA), we examined electron microscopic dual immunolabeling of these receptors in the prelimbic PFC (prPFC) and BLA of adult male rats. CB1 immunoreactivity was detected in axon terminals that were typically large, complex, and contained dense-core and clear synaptic vesicles. Of terminals forming discernible synaptic specializations, 95% were symmetric inhibitory-type in the prPFC and 90% were inhibitory in the BLA. CB1-immunoreactive terminals frequently contacted dendrites containing mGluR5 adjacent to unlabeled terminals forming excitatory-type synapses. Because most CB1-containing terminals form inhibitory-type synapses, the unlabeled axon terminals forming asymmetric synapses are the likely source of the mGluR5 ligand glutamate. In the prPFC, serial section analysis revealed that GABAergic CB1-containing axon terminals targeted dendrites adjacent to glutamatergic axon terminals, often near dendritic bifurcations. These observations provide ultrastructural evidence that cortical CB1 receptors are strategically positioned for integration of synaptic signaling in response to stimulation of postsynaptic mGluR5 receptors and facilitation of heterosynaptic communication between multiple neurons.

Persistent pain intensifies recall of consolidated fear memories

Ensembles of principal neurons in the basolateral amygdala (BLA) generate the initial engrams for fear memories, while projections from the BLA to the medial prefrontal cortex (mPFC) are essential for the encoding, transfer and storage of remote fear memories. We tested the effects of chronic pain on remote fear memories in mice.

Male mice underwent classic fear conditioning by pairing a single tone (conditional stimulus, CS) with a single electric foot shock (unconditional stimulus, US). Sciatic nerve constriction was used to induce neuropathic pain at various time points before or after the fear conditioning. The mice with sciatic nerve cuffs implanted 48 h after the fear conditioning showed an increased freezing response to CS when compared to mice without cuffs or when compared to mice in which the nerve cuffing was performed 48 h before the fear conditioning. The enhancing effect of pain on consolidated fear memory was further tested and mice in which the nerve cuffing was performed 14 days after the fear conditioning also showed an increased fear response when tested 56 days later.

We used immunostaining to detect morphological changes in the BLA that could suggest a mechanism for the observed increase in fear response. We found an increased number of calbindin/parvalbumin positive neurons in the BLA and increased perisomatic density of GAD65 on projection neurons that connect BLA to mPFC in mice with nerve cuffs. Despite the strong increase of c-Fos expression in BLA and mPFC that was induced by fear recall, neither the BLA to mPFC nor the mPFC to BLA projection neurons were activated in mice with nerve cuffs. Furthermore, non-injured mice had an increased fear response when BLA to mPFC projections were inhibited by a chemogenetic method.

In conclusion, this study provides evidence that persistent pain has a significant impact on consolidated fear memories. Very likely the underlying mechanism for this phenomenon is increased inhibitory input onto the BLA to mPFC projection neurons, possibly from neurons with induced parvalbumin expression. Conceivably, the increased fear response to consolidated fear memory is a harbinger for the later development of anxiety and depression symptoms associated with chronic pain.

Inhibition of Pyramidal Neurons in the Basal Amygdala Promotes Fear Learning

The basolateral amygdala complex, which contains the lateral (LA) and basal (BA) subnuclei, is a critical substrate of associative learning related to reward and aversive stimuli. Auditory fear conditioning studies in rodents have shown that the excitation of LA pyramidal neurons, driven by the inhibition of local GABAergic interneurons, is critical to fear memory formation. Studies examining the role of the BA in auditory fear conditioning, however, have yielded divergent outcomes. Here, we used a neuron-specific chemogenetic approach to manipulate the excitability of mouse BA neurons during auditory fear conditioning. We found that chemogenetic inhibition of BA GABA neurons, but not BA pyramidal neurons, impaired fear learning. Further, either chemogenetic stimulation of BA GABA neurons or chemogenetic inhibition of BA pyramidal neurons was sufficient to generate the formation of an association between a behavior and a neutral auditory cue. This chemogenetic memory required presentation of a discrete cue, and was not attributable to an effect of BA pyramidal neuron inhibition on general freezing behavior, locomotor activity, or anxiety. Collectively, these data suggest that BA GABA neuron activation and the subsequent inhibition of BA pyramidal neurons play important role in fear learning. Moreover, the roles of inhibitory signaling differ between the LA and BA, with excitation of pyramidal neurons promoting memory formation in the former, and inhibition of pyramidal neurons playing this role in the latter.

Vasoactive Intestinal Polypeptide-Immunoreactive Interneurons within Circuits of the Mouse Basolateral Amygdala

In cortical structures, principal cell activity is tightly regulated by different GABAergic interneurons (INs). Among these INs are vasoactive intestinal polypeptide-expressing (VIP+) INs, which innervate preferentially other INs, providing a structural basis for temporal disinhibition of principal cells. However, relatively little is known about VIP+ INs in the amygdaloid basolateral complex (BLA). In this study, we report that VIP+ INs have a variable density in the distinct subdivisions of the mouse BLA. Based on different anatomical, neurochemical, and electrophysiological criteria, VIP+ INs could be identified as IN-selective INs (IS-INs) and basket cells expressing CB1 cannabinoid receptors. Whole-cell recordings of VIP+ IS-INs revealed three different spiking patterns, none of which was associated with the expression of calretinin. Genetic targeting combined with optogenetics and in vitro recordings enabled us to identify several types of BLA INs innervated by VIP+ INs, including other IS-INs, basket and neurogliaform cells. Moreover, light stimulation of VIP+ basket cell axon terminals, characterized by CB1 sensitivity, evoked IPSPs in ∼20% of principal neurons. Finally, we show that VIP+ INs receive a dense innervation from both GABAergic inputs (although only 10% from other VIP+ INs) and distinct glutamatergic inputs, identified by their expression of different vesicular glutamate transporters.

In conclusion, our study provides a wide-range analysis of single-cell properties of VIP+ INs in the mouse BLA and of their intrinsic and extrinsic connectivity. Our results reinforce the evidence that VIP+ INs are structurally and functionally heterogeneous and that this heterogeneity could mediate different roles in amygdala-dependent functions.

SIGNIFICANCE STATEMENT We provide the first comprehensive analysis of the distribution of vasoactive intestinal polypeptide-expressing (VIP+) interneurons (INs) across the entire mouse amygdaloid basolateral complex (BLA), as well as of their morphological and physiological properties. VIP+ INs in the neocortex preferentially target other INs to form a disinhibitory network that facilitates principal cell firing. Our study is the first to demonstrate the presence of such a disinhibitory circuitry in the BLA. We observed structural and functional heterogeneity of these INs and characterized their input/output connectivity. We also identified several types of BLA INs that, when inhibited, may provide a temporal window for principal cell firing and facilitate associative plasticity, e.g., in fear learning.

Amygdala Inhibitory Circuits Regulate Associative Fear Conditioning

Associative memory formation is essential for an animal's survival by ensuring adaptive behavioral responses in an ever-changing environment. This is particularly important under conditions of immediate threats such as in fear learning. One of the key brain regions involved in associative fear learning is the amygdala. The basolateral amygdala is the main entry site for sensory information to the amygdala complex, and local plasticity in excitatory basolateral amygdala principal neurons is considered to be crucial for learning of conditioned fear responses. However, activity and plasticity of excitatory circuits are tightly controlled by local inhibitory interneurons in a spatially and temporally defined manner. In this review, we provide an updated view on how distinct interneuron subtypes in the basolateral amygdala contribute to the acquisition and extinction of conditioned fear memories.

Inhibition in the amygdala anxiety circuitry

Inhibitory neurotransmission plays a key role in anxiety disorders, as evidenced by the anxiolytic effect of the benzodiazepine class of γ-aminobutyric acid (GABA) receptor agonists and the recent discovery of anxiety-associated variants in the molecular components of inhibitory synapses. Accordingly, substantial interest has focused on understanding how inhibitory neurons and synapses contribute to the circuitry underlying adaptive and pathological anxiety behaviors. A key element of the anxiety circuitry is the amygdala, which integrates information from cortical and thalamic sensory inputs to generate fear and anxiety-related behavioral outputs. Information processing within the amygdala is heavily dependent on inhibitory control, although the specific mechanisms by which amygdala GABAergic neurons and synapses regulate anxiety-related behaviors are only beginning to be uncovered. Here, we summarize the current state of knowledge and highlight open questions regarding the role of inhibition in the amygdala anxiety circuitry. We discuss the inhibitory neuron subtypes that contribute to the processing of anxiety information in the basolateral and central amygdala, as well as the molecular determinants, such as GABA receptors and synapse organizer proteins, that shape inhibitory synaptic transmission within the anxiety circuitry. Finally, we conclude with an overview of current and future approaches for converting this knowledge into successful treatment strategies for anxiety disorders.

Perineuronal nets labeled by monoclonal antibody VC1.1 ensheath interneurons expressing parvalbumin and calbindin in the rat amygdala

Perineuronal nets (PNNs) are specialized condensations of extracellular matrix that ensheath particular neuronal subpopulations in the brain and spinal cord. PNNs regulate synaptic plasticity, including the encoding of fear memories by the amygdala. The present immunohistochemical investigation studied PNN structure and distribution, as well as the neurochemistry of their ensheathed neurons, in the rat amygdala using monoclonal antibody VC1.1, which recognizes a glucuronic acid 3-sulfate glycan associated with PNNs in the cerebral cortex. VC1.1+ PNNs surrounded the cell bodies and dendrites of a subset of nonpyramidal neurons in cortex-like portions of the amygdala (basolateral amygdalar complex, cortical nuclei, nucleus of the lateral olfactory tract, and amygdalohippocampal region). There was also significant neuropilar VC1.1 immunoreactivity, whose density varied in different amygdalar nuclei. Cell counts in the basolateral nucleus revealed that virtually all neurons ensheathed by VC1.1+ PNNs were parvalbumin-positive (PV+) interneurons, and these VC1.1+/PV+ cells constituted 60% of all PV+ interneurons, including all of the larger PV+ neurons. Approximately 70% of VC1.1+ neurons were calbindin-positive (CB+), and these VC1.1+/CB+ cells constituted about 40% of all CB+ neurons. Colocalization of VC1.1 with Vicia villosa agglutinin (VVA) binding, which stains terminal N-acetylgalactosamines, revealed that VC1.1+ PNNs were largely a subset of VVA+ PNNs. This investigation provides baseline data regarding PNNs in the rat which should be useful for future studies of their function in this species.

Cellular and oscillatory substrates of fear extinction learning

The mammalian brain contains dedicated circuits for both the learned expression and suppression of fear. These circuits require precise coordination to facilitate the appropriate expression of fear behavior, but the mechanisms underlying this coordination remain unclear. Using a novel combination of chemogenetics, activity-based neuronal-ensemble labeling, and in vivo electrophysiology, we found that fear extinction learning confers parvalbumin-expressing (PV) interneurons in the basolateral amygdala (BLA) with a dedicated role in the selective suppression of a previously encoded fear memory and BLA fear-encoding neurons. In addition, following extinction learning, PV interneurons enable a competing interaction between a 6–12 Hz oscillation and a fear-associated 3–6 Hz oscillation within the BLA. Loss of this competition increases a 3–6 Hz oscillatory signature, with BLA→mPFC directionality signaling the recurrence of fear expression. The discovery of cellular and oscillatory substrates of fear extinction learning that critically depend on BLA PV-interneurons could inform therapies aimed at preventing the pathological recurrence of fear following extinction learning.

Evidence for M2 muscarinic receptor modulation of axon terminals and dendrites in the rodent basolateral amygdala: An ultrastructural and electrophysiological analysis

The basolateral amygdala receives a very dense cholinergic innervation from the basal forebrain that is important for memory consolidation. Although behavioral studies have shown that both M₁ and M₂ muscarinic receptors are critical for these mnemonic functions, there have been very few neuroanatomical and electrophysiological investigations of the localization and function of different types of muscarinic receptors in the amygdala. In the present study we investigated the subcellular localization of M₂ muscarinic receptors (M₂Rs) in the anterior basolateral nucleus (BLa) of the mouse, including the localization of M₂Rs in parvalbumin (PV) immunoreactive interneurons, using double-labeling immunoelectron microscopy. Little if any M₂R-immunoreactivity (M₂R-ir) was observed in neuronal somata, but the neuropil was densely labeled. Ultrastructural analysis using a pre-embedding immunogold-silver technique (IGS) demonstrated M₂R-ir in dendritic shafts, spines, and axon terminals forming asymmetrical (excitatory) or symmetrical (mostly inhibitory) synapses. In addition, about one-quarter of PV+ axon terminals and half of PV+ dendrites, localized using immunoperoxidase, were M₂R+ when observed in single thin sections. In all M₂R+ neuropilar structures, including those that were PV+, about one-quarter to two-thirds of M₂R+ immunoparticles were plasma-membrane-associated, depending on the structure. The expression of M₂Rs in PV+ and PV-negative terminals forming symmetrical synapses indicates M₂R modulation of inhibitory transmission. Electrophysiological studies in mouse and rat brain slices, including paired recordings from interneurons and pyramidal projection neurons, demonstrated M₂R-mediated suppression of GABA release. These findings suggest cell-type-specific functions of M₂Rs and shed light on organizing principles of cholinergic modulation in the BLa.

Differential excitatory control of 2 parallel basket cell networks in amygdala microcircuits

Information processing in neural networks depends on the connectivity among excitatory and inhibitory neurons. The presence of parallel, distinctly controlled local circuits within a cortical network may ensure an effective and dynamic regulation of microcircuit function. By applying a combination of optogenetics, electrophysiological recordings, and high resolution microscopic techniques, we uncovered the organizing principles of synaptic communication between principal neurons and basket cells in the basal nucleus of the amygdala. In this cortical structure, known to be critical for emotional memory formation, we revealed the presence of 2 parallel basket cell networks expressing either parvalbumin or cholecystokinin. While the 2 basket cell types are mutually interconnected within their own category via synapses and gap junctions, they avoid innervating each other, but form synaptic contacts with axo-axonic cells. Importantly, both basket cell types have the similar potency to control principal neuron spiking, but they receive excitatory input from principal neurons with entirely diverse features. This distinct feedback synaptic excitation enables a markedly different recruitment of the 2 basket cell types upon the activation of local principal neurons. Our data suggest fundamentally different functions for the 2 parallel basket cell networks in circuit operations in the amygdala.

The perisomatic region of neurons refers collectively to the membrane surface of the cell body or soma, proximal dendrites, and axon initial segment. This is a unique functional domain in which the activity of a neuron can be controlled in the most effective manner. In the cerebral cortex, the perisomatic region of excitatory principal cells is solely innervated by inhibitory interneurons, which can be divided into 3 functional groups: axo-axonic cells and 2 types of basket cells. The reason why 3 distinct types of inhibitory cells are specialized to control principal cell firing is still unknown. To reveal the possible differences in the role of the 3 interneuron types played in cortical operation, we have investigated the organizing principles of synaptic communication between principal cells and inhibitory cell types in the basal nucleus of the amygdala. In this cortical structure, known to be critical for affective behavior, we revealed that the 2 basket cell types avoid innervating each other but contact axo-axonic cells. Both basket cell types have a similar potency to control principal cell firing, but they receive excitatory input from principal cells with entirely distinct features. Our data suggest fundamentally different functions for the 2 parallel basket cell networks in amygdala operation.