Novel Perspectives on the Development of the Amygdala in Rodents

Coordination and persistence of aggressive visual communication in Siamese fighting fish

Outside acoustic communication, little is known about how animals coordinate social turn taking and how the brain drives engagement in these social interactions. Using Siamese fighting fish (Betta splendens), we discover dynamic visual features of an opponent and behavioral sequences that drive visually driven turn-taking aggressive behavior. Lesions of the telencephalon show that it is unnecessary for coordinating turn taking but is required for persistent participation in aggressive interactions. Circumscribed lesions of the caudal dorsomedial telencephalon (cDm; the fish pallial amygdala) recapitulated the telencephalic lesions. Furthermore, ventral telencephalic regions and the thalamic preglomerular complex, all of which project to cDm, show increased activity during aggressive interactions. Our work highlights how dynamic visual cues shape the rhythm of social interactions at multiple timescales. The results point to the vertebrate pallial amygdala as a region with an evolutionarily conserved role in regulating the persistence of emotional states, including those that promote engagement in social interactions.

Diagnostic Value of Serum miR-499a-5p in Chinese Children with Autism Spectrum Disorders

The purpose of this study was to investigate the expression of miR-499a-5p in children with autism spectrum disorders (ASD) and its value in early diagnosis of ASD. This is a retrospective case-control study that included 40 children with ASD as a case group and 43 healthy children as a control group. Magnetic resonance imaging (MRI) was performed on all subjects, and the children were scored with childhood autism rating scale (CARS) and autism behavior checklist (ABC). The expression of miR-499a-5p in serum was detected by RT-qPCR, and the diagnostic value of miR-499a-5p in ASD was evaluated by ROC curve. Pearson correlation coefficient was used to evaluate the correlation between miR-499a-5p levels and scores. Compared with healthy children, the expression level of serum miR-499a-5p was significantly reduced in children with ASD. ROC curve showed that miR-499a-5p is of high diagnostic value for ASD. The results of MRI suggested that the volume of the amygdala in ASD children was significantly larger than that in healthy children, while the volume of the caudate nucleus was significantly reduced. Correlation results showed that the scores of CARS and ABC in the ASD group were significantly negatively correlated with the levels of miR-499a-5p. In the ASD group, the volume of the amygdala was negatively correlated with the level of miR-499a-5p, while the volume of the caudate nucleus was positively correlated with the level of miR-499a-5p. The decreased expression of miR-499a-5p in the serum of children with ASD was significantly related to the changes in brain volume of children with ASD, and the miRNA showed good diagnostic accuracy in children with ASD.

Development and function of the medial amygdala

Across studied vertebrates, the medial amygdala (MeA) is a central hub for relaying sensory information with social and/or survival relevance to downstream nuclei such as the bed nucleus of stria terminalis (BNST) and the hypothalamus. MeA-driven behaviors, such as mating, aggression, parenting, and predator avoidance are processed by different molecularly defined inhibitory and excitatory neuronal output populations. Work over the past two decades has deciphered how diverse MeA neurons arise from embryonic development, revealing contributions from multiple telencephalic and diencephalic progenitor domains. Here, we first provide a brief overview of current findings regarding the role of the MeA in social behaviors, followed by a deeper dive into current knowledge of how this complex structure is specified during development. We outline a conceptual model of MeA formation that has emerged based on these findings. We further postulate how embryonic developmental programming of the MeA may inform later emergence of stereotypical circuitry governing hardwired behaviors.

Translational Insights From Cell Type Variation Across Amygdala Subnuclei in Rhesus Monkeys and Humans

OBJECTIVE

Theories of amygdala function are central to our understanding of psychiatric and neurodevelopmental disorders. However, limited knowledge of the molecular and cellular composition of the amygdala impedes translational research aimed at developing new treatments and interventions. The aim of this study was to characterize and compare the composition of amygdala cells to help bridge the gap between preclinical models and human psychiatric and neurodevelopmental disorders.

METHODS

Tissue was dissected from multiple amygdala subnuclei in both humans (N=3, male) and rhesus macaques (N=3, male). Single-nucleus RNA sequencing was performed to characterize the transcriptomes of individual nuclei.

RESULTS

The results reveal substantial heterogeneity between regions, even when restricted to inhibitory or excitatory neurons. Consistent with previous work, the data highlight the complexities of individual marker genes for uniquely targeting specific cell types. Cross-species analyses suggest that the rhesus monkey model is well-suited to understanding the human amygdala, but also identify limitations. For example, a cell cluster in the ventral lateral nucleus of the amygdala (vLa) is enriched in humans relative to rhesus macaques. Additionally, the data describe specific cell clusters with relative enrichment of disorder-related genes. These analyses point to the human-enriched vLa cell cluster as relevant to autism spectrum disorder, potentially highlighting a vulnerability to neurodevelopmental disorders that has emerged in recent primate evolution. Further, a cluster of cells expressing markers for intercalated cells is enriched for genes reported in human genome-wide association studies of neuroticism, anxiety disorders, and depressive disorders.

CONCLUSIONS

Together, these findings shed light on the composition of the amygdala and identify specific cell types that can be prioritized in basic science research to better understand human psychopathology and guide the development of potential treatments.

Altered socio-affective communication and amygdala development in mice with protocadherin10-deficient interneurons

Autism spectrum disorder (ASD) is a group of neurodevelopmental conditions associated with deficits in social interaction and communication, together with repetitive behaviours. The cell adhesion molecule protocadherin10 (PCDH10) is linked to ASD in humans. Pcdh10 is expressed in the nervous system during embryonic and early postnatal development and is important for neural circuit formation. In mice, strong expression of Pcdh10 in the ganglionic eminences and in the basolateral complex (BLC) of the amygdala was observed at mid and late embryonic stages, respectively. Both inhibitory and excitatory neurons expressed Pcdh10 in the BLC at perinatal stages and vocalization-related genes were enriched in Pcdh10-expressing neurons in adult mice. An epitope-tagged Pcdh10-HAV5 mouse line revealed endogenous interactions of PCDH10 with synaptic proteins in the young postnatal telencephalon. Nuanced socio-affective communication changes in call emission rates, acoustic features and call subtype clustering were primarily observed in heterozygous pups of a conditional knockout (cKO) with selective deletion of Pcdh10 in Gsh2-lineage interneurons. These changes were less prominent in heterozygous ubiquitous Pcdh10 KO pups, suggesting that altered anxiety levels associated with Gsh2-lineage interneuron functioning might drive the behavioural effects. Together, loss of Pcdh10 specifically in interneurons contributes to behavioural alterations in socio-affective communication with relevance to ASD.

Effects of prenatal alcohol exposure on the olfactory system development

Fetal Alcohol Spectrum Disorders (FASD), resulting from maternal alcohol consumption during pregnancy, are a prominent non-genetic cause of physical disabilities and brain damage in children. Alongside common symptoms like distinct facial features and neurocognitive deficits, sensory anomalies, including olfactory dysfunction, are frequently noted in FASD-afflicted children. However, the precise mechanisms underpinning the olfactory abnormalities induced by prenatal alcohol exposure (PAE) remain elusive. Utilizing rodents as a model organism with varying timing, duration, dosage, and administration routes of alcohol exposure, prior studies have documented impairments in olfactory system development caused by PAE. Many reported a reduction in the olfactory bulb (OB) volume accompanied by reduced OB neuron counts, suggesting the OB is a brain region vulnerable to PAE. In contrast, no significant olfactory system defects were observed in some studies, though subtle alterations might exist. These findings suggest that the timing, duration, and extent of fetal alcohol exposure can yield diverse effects on olfactory system development. To enhance comprehension of PAE-induced olfactory dysfunctions, this review summarizes key findings from previous research on the olfactory systems of offspring prenatally exposed to alcohol.

Morphological heterogeneity of neurons in the human central amygdaloid nucleus

The central amygdaloid nucleus (CeA) has an ancient phylogenetic development and functions relevant for animal survival. Local cells receive intrinsic amygdaloidal information that codes emotional stimuli of fear, integrate them, and send cortical and subcortical output projections that prompt rapid visceral and social behavior responses. We aimed to describe the morphology of the neurons that compose the human CeA (N = 8 adult men). Cells within CeA coronal borders were identified using the thionine staining and were further analyzed using the "single-section" Golgi method followed by open-source software procedures for two-dimensional and three-dimensional image reconstructions. Our results evidenced varied neuronal cell body features, number and thickness of primary shafts, dendritic branching patterns, and density and shape of dendritic spines. Based on these criteria, we propose the existence of 12 morphologically different spiny neurons in the human CeA and discuss the variability in the dendritic architecture within cellular types, including likely interneurons. Some dendritic shafts were long and straight, displayed few collaterals, and had planar radiation within the coronal neuropil volume. Most of the sampled neurons showed a few to moderate density of small stubby/wide spines. Long spines (thin and mushroom) were observed occasionally. These novel data address the synaptic processing and plasticity in the human CeA. Our morphological description can be combined with further transcriptomic, immunohistochemical, and electrophysiological/connectional approaches. It serves also to investigate how neurons are altered in neurological and psychiatric disorders with hindered emotional perception, in anxiety, following atrophy in schizophrenia, and along different stages of Alzheimer's disease.

A Gad2 specific Slc6a8 deletion recapitulates the contextual and cued freezing deficits seen in Slc6a8-/y mice

The creatine (Cr)-phosphocreatine shuttle is essential for ATP homeostasis. In humans, the absence of brain Cr causes significant intellectual disability, epilepsy, and language delay. Mutations of the creatine transporter (SLC6A8) are the most common cause of Cr deficiency. In rodents, Slc6a8 deletion causes deficits in spatial learning, novel object recognition (NOR), as well as in contextual and cued freezing. The mechanisms that underlie these cognitive deficits are not known. Due to the heterogeneous nature of the brain, it is important to determine which systems are affected by a loss of Cr. In this study, we generated mice lacking Slc6a8 in GABAergic neurons by crossing Slc6a8FL mice with Gad2-Cre mice. These Gad2-specific Slc6a8 knockout (cKO) mice, along with the ubiquitous Slc6a8 KO (Slc6a8-/y), Gad2-Cre+, and wild-type (WT) mice were tested in the Morris water maze, NOR, conditioned freezing, and the radial water maze. Similar to the Slc6a8-/y mice, cKO mice had reduced contextual and cued freezing compared with WT mice. The cKO mice had a mild spatial learning deficit during the reversal phase of the MWM, however they were not as pronounced as in Slc6a8-/y mice. In NOR, the Gad2-Cre mice spent less time with the novel object, similar to the reduced novel time in the cKO mice. There were no changes in radial water maze performance. Slc6a8 deletion in GABAergic neurons is sufficient to recapitulate the conditioned freezing deficits seen in Slc6a8-/y mice.

Connectivity and molecular profiles of Foxp2- and Dbx1-lineage neurons in the accessory olfactory bulb and medial amygdala

In terrestrial vertebrates, the olfactory system is divided into main (MOS) and accessory (AOS) components that process both volatile and nonvolatile cues to generate appropriate behavioral responses. While much is known regarding the molecular diversity of neurons that comprise the MOS, less is known about the AOS. Here, focusing on the vomeronasal organ (VNO), the accessory olfactory bulb (AOB), and the medial amygdala (MeA), we reveal that populations of neurons in the AOS can be molecularly subdivided based on their ongoing or prior expression of the transcription factors Foxp2 or Dbx1, which delineate separate populations of GABAergic output neurons in the MeA. We show that a majority of AOB neurons that project directly to the MeA are of the Foxp2 lineage. Using single-neuron patch-clamp electrophysiology, we further reveal that in addition to sex-specific differences across lineage, the frequency of excitatory input to MeA Dbx1- and Foxp2-lineage neurons differs between sexes. Together, this work uncovers a novel molecular diversity of AOS neurons, and lineage and sex differences in patterns of connectivity.

From active affordance to active inference: vertical integration of cognition in the cerebral cortex through dual subcortical control systems

In previous papers, we proposed that the dorsal attention system's top-down control is regulated by the dorsal division of the limbic system, providing a feedforward or impulsive form of control generating expectancies during active inference. In contrast, we proposed that the ventral attention system is regulated by the ventral limbic division, regulating feedback constraints and error-correction for active inference within the neocortical hierarchy. Here, we propose that these forms of cognitive control reflect vertical integration of subcortical arousal control systems that evolved for specific forms of behavior control. The feedforward impetus to action is regulated by phasic arousal, mediated by lemnothalamic projections from the reticular activating system of the lower brainstem, and then elaborated by the hippocampus and dorsal limbic division. In contrast, feedback constraint-based on environmental requirements-is regulated by the tonic activation furnished by collothalamic projections from the midbrain arousal control centers, and then sustained and elaborated by the amygdala, basal ganglia, and ventral limbic division. In an evolutionary-developmental analysis, understanding these differing forms of active affordance-for arousal and motor control within the subcortical vertebrate neuraxis-may help explain the evolution of active inference regulating the cognition of expectancy and error-correction within the mammalian 6-layered neocortex.

Single-nucleus genomics in outbred rats with divergent cocaine addiction-like behaviors reveals changes in amygdala GABAergic inhibition

The amygdala processes positive and negative valence and contributes to addiction, but the cell-type-specific gene regulatory programs involved are unknown. We generated an atlas of single-nucleus gene expression and chromatin accessibility in the amygdala of outbred rats with high and low cocaine addiction-like behaviors following prolonged abstinence. Differentially expressed genes between the high and low groups were enriched for energy metabolism across cell types. Rats with high addiction index (AI) showed increased relapse-like behaviors and GABAergic transmission in the amygdala. Both phenotypes were reversed by pharmacological inhibition of the glyoxalase 1 enzyme, which metabolizes methylglyoxal-a GABAA receptor agonist produced by glycolysis. Differences in chromatin accessibility between high and low AI rats implicated pioneer transcription factors in the basic helix-loop-helix, FOX, SOX and activator protein 1 families. We observed opposite regulation of chromatin accessibility across many cell types. Most notably, excitatory neurons had greater accessibility in high AI rats and inhibitory neurons had greater accessibility in low AI rats.

Functional connectivity of amygdala subnuclei in PTSD: a narrative review

While the amygdala is often implicated in the neurobiology of posttraumatic stress disorder (PTSD), the pattern of results remains mixed. One reason for this may be the heterogeneity of amygdala subnuclei and their functional connections. This review used PRISMA guidelines to synthesize research exploring the functional connectivity of three primary amygdala subnuclei, basolateral (BLA), centromedial (CMA), and superficial nuclei (SFA), in PTSD (N = 331) relative to trauma-exposed (N = 155) and non-trauma-exposed controls (N = 210). Although studies were limited (N = 11), preliminary evidence suggests that in PTSD compared to trauma-exposed controls, the BLA shows greater connectivity with the dorsal anterior cingulate, an area involved in salience detection. In PTSD compared to non-trauma-exposed controls, the BLA shows greater connectivity with the middle frontal gyrus, an area involved in attention. No other connections were replicated across studies. A secondary aim of this review was to outline the limitations of this field to better shape future research. Importantly, the results from this review indicate the need to consider potential mediators of amygdala subnuclei connectivity, such as trauma type and sex, when conducting such studies. They also highlight the need to be aware of the limited inferences we can make with such small samples that investigate small subcortical structures on low field strength magnetic resonance imaging scanners. Collectively, this review demonstrates the importance of exploring the differential connectivity of amygdala subnuclei to understand the pathophysiology of PTSD and stresses the need for future research to harness the strength of ultra-high field imaging to gain a more sensitive picture of the neural connectivity underlying PTSD.

Molecular Diversity of Neuron Types in the Salamander Amygdala and Implications for Amygdalar Evolution

The amygdala is a complex brain structure in the vertebrate telencephalon, essential for regulating social behaviors, emotions, and (social) cognition. In contrast to the vast majority of neuron types described in the many nuclei of the mammalian amygdala, little is known about the neuronal diversity in non-mammals, making reconstruction of its evolution particularly difficult. Here, we characterize glutamatergic neuron types in the amygdala of the urodele amphibian Pleurodeles waltl. Our single-cell RNA sequencing data indicate the existence of at least ten distinct types and subtypes of glutamatergic neurons in the salamander amygdala. These neuron types are molecularly distinct from neurons in the ventral pallium (VP), suggesting that the pallial amygdala and the VP are two separate areas in the telencephalon. In situ hybridization for marker genes indicates that amygdalar glutamatergic neuron types are located in three major subdivisions: the lateral amygdala, the medial amygdala, and a newly defined area demarcated by high expression of the transcription factor Sim1. The gene expression profiles of these neuron types suggest similarities with specific neurons in the sauropsid and mammalian amygdala. In particular, we identify Sim1+ and Sim1+ Otp+ expressing neuron types, potentially homologous to the mammalian nucleus of the lateral olfactory tract (NLOT) and to hypothalamic-derived neurons of the medial amygdala, respectively. Taken together, our results reveal a surprising diversity of glutamatergic neuron types in the amygdala of salamanders, despite the anatomical simplicity of their brain. These results offer new insights on the cellular and anatomical complexity of the amygdala in tetrapod ancestors.

Inhibiting silence information regulator 2 and glutaminase in the amygdala can improve social behavior in autistic rats

OBJECTIVE

To investigate the underlying molecular mechanisms by which silence information regulator (SIRT) 2 and glutaminase (GLS) in the amygdala regulate social behaviors in autistic rats.

METHODS

Rat models of autism were established by maternal sodium valproic acid (VPA) exposure in wild-type rats and SIRT2-knockout ( SIRT2 -/-) rats. Glutamate (Glu) content, brain weight, and expression levels of SIRT2, GLS proteins and apoptosis-associated proteins in rat amygdala at different developmental stages were examined, and the social behaviors of VPA rats were assessed by a three-chamber test. Then, lentiviral overexpression or interference vectors of GLS were injected into the amygdala of VPA rats. Brain weight, Glu content and expression level of GLS protein were measured, and the social behaviors assessed.

RESULTS

Brain weight, amygdala Glu content and the levels of SIRT2, GLS protein and pro-apoptotic protein caspase-3 in the amygdala were increased in VPA rats, while the level of anti-apoptotic protein Bcl-2 was decreased (all P<0.01). Compared with the wild-type rats, SIRT2 -/- rats displayed decreased expression of SIRT2 and GLS proteins in the amygdala, reduced Glu content, and improved social dysfunction (all P<0.01). Overexpression of GLS increased brain weight and Glu content, and aggravated social dysfunction in VPA rats (all P<0.01). Knockdown of GLS decreased brain weight and Glu content, and improved social dysfunction in VPA rats (all P<0.01).

CONCLUSIONS

The glutamate circulatory system in the amygdala of VPA induced autistic rats is abnormal. This is associated with the upregulation of SIRT2 expression and its induced increase of GLS production; knocking out SIRT2 gene or inhibiting the expression of GLS is helpful in maintaining the balanced glutamate cycle and in improving the social behavior disorder of rats.

Prolonged contextual fear memory in AMPA receptor palmitoylation-deficient mice

Long-lasting fear-related disorders depend on the excessive retention of traumatic fear memory. We previously showed that the palmitoylation-dependent removal of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors prevents hyperexcitation-based epileptic seizures and that AMPA receptor palmitoylation maintains neural network stability. In this study, AMPA receptor subunit GluA1 C-terminal palmitoylation-deficient (GluA1C811S) mice were subjected to comprehensive behavioral battery tests to further examine whether the mutation causes other neuropsychiatric disease-like symptoms. The behavioral analyses revealed that palmitoylation-deficiency in GluA1 is responsible for characteristic prolonged contextual fear memory formation, whereas GluA1C811S mice showed no impairment of anxiety-like behaviors at the basal state. In addition, fear generalization gradually increased in these mutant mice without affecting their cued fear. Furthermore, fear extinction training by repeated exposure of mice to conditioned stimuli had little effect on GluA1C811S mice, which is in line with augmentation of synaptic transmission in pyramidal neurons in the basolateral amygdala. In contrast, locomotion, sociability, depression-related behaviors, and spatial learning and memory were unaffected by the GluA1 non-palmitoylation mutation. These results indicate that impairment of AMPA receptor palmitoylation specifically causes posttraumatic stress disorder (PTSD)-like symptoms.

Dynorphin/kappa opioid receptor system regulation on amygdaloid circuitry: Implications for neuropsychiatric disorders

Amygdaloid circuits are involved in a variety of emotional and motivation-related behaviors and are impacted by stress. The amygdala expresses several neuromodulatory systems, including opioid peptides and their receptors. The Dynorphin (Dyn)/kappa opioid receptor (KOR) system has been implicated in the processing of emotional and stress-related information and is expressed in brain areas involved in stress and motivation. Dysregulation of the Dyn/KOR system has also been implicated in various neuropsychiatric disorders. However, there is limited information about the role of the Dyn/KOR system in regulating amygdala circuitry. Here, we review the literature on the (1) basic anatomy of the amygdala, (2) functional regulation of synaptic transmission by the Dyn/KOR system, (3) anatomical architecture and function of the Dyn/KOR system in the amygdala, (4) regulation of amygdala-dependent behaviors by the Dyn/KOR system, and (5) future directions for the field. Future work investigating how the Dyn/KOR system shapes a wide range of amygdala-related behaviors will be required to increase our understanding of underlying circuitry modulation by the Dyn/KOR system. We anticipate that continued focus on the amygdala Dyn/KOR system will also elucidate novel ways to target the Dyn/KOR system to treat neuropsychiatric disorders.