Age-dependent sensitivity to glucocorticoids in the developing mouse basolateral nucleus of the amygdala

Associations of harsh, unpredictable environment, amygdala connectivity and overeating for children

OBJECTIVE

In harsh and unpredictable environments, individuals tend to engage in activities that yield immediate rewards as delayed benefits can be unavailable. Substantial evidence suggests that a harsh and unpredictable childhood environment is associated with overeating. However, the neuromechanisms underlying this association remain unclear. This study aimed to investigate amygdala connectivity in relation to environmental harshness and unpredictability (EHU) from an evolutionary perspective and examine their relationship with overeating in children.

METHODS

Eighty-five children aged 8 to 12 years were scanned using a magnetic resonance imaging machine to assess resting-state functional connectivity (RSFC) of the two subregions of the amygdala (i.e., centromedial amygdala [CMA]; basolateral amygdala [BLA]). Self-reports of EHU and parental reports of overeating, including food responsiveness and enjoyment of food, were obtained cross-sectionally. Furthermore, findings indicated that children completed high- and low-calorie food portion choice tasks in the absence of hunger at 12 months of follow-up.

RESULTS

EHU was positively associated with parental reports of overeating, including food responsiveness and enjoyment, as well as children's selection of high-calorie food portion sizes. Moreover, static RSFC analyses revealed that EHU was negatively associated with bilateral BLA-left inferior frontal gyrus (IFG) connectivity, while dynamic RSFC analyses found that EHU was negatively associated with right CMA, left inferior parietal lobule, and right CMA-right precuneus connectivity. Particularly, the left BLA-left IFG connectivity mediated the association between EHU and parental reports of food responsiveness.

CONCLUSION

EHU was negatively associated with amygdala connectivity, which is implicated in the intrinsic processing of emotional regulation. Furthermore, deficits in emotional regulation resulted in increased energy intake. These insights provide a new perspective for understanding the developmental neuromechanisms underlying obesity.

Aversive memory reactivation: A possible role for delta oscillations in the hippocampus-amygdala circuit

Memory labilization, the process by which memories become susceptible to update, is essential for memory reconsolidation and has been a target for novel therapies for traumatic memory-associated disorders. Maternal separation (MS) in male rats produced memories resistant to labilization in adulthood. Based on previous results, we hypothesized that temporal desynchronization between the dorsal hippocampus (DHc) and the basolateral amygdala (BLA), during memory retrieval, could be responsible for this impairment. Our goal was to investigate possible differences in oscillatory activity and synchrony between the DHc and BLA during fear memory reactivation, between MS and non-handled (NH) rats. We used male adult Wistar rats, NH or MS, with electrodes for local field potential (LFP) recordings implanted in the DHc and BLA. Animals were submitted to aversive memory reactivation by exposure to the conditioned context (Reat) or to pseudo-reactivation in a neutral context (pReat), and LFP was recorded. Plasticity markers linked to reconsolidation were evaluated one hour after reactivation. The power of delta oscillations and DHc-BLA synchrony in Reat animals was increased, during freezing. Besides, delta modulation of gamma oscillations amplitude in the BLA was associated with the increase in DHc Zif268 levels, an immediate early gene specifically associated with reconsolidation. Concerning early life stress, we found lower power of delta and strength of delta-gamma oscillations coupling in MS rats, compared to NH, which could explain the low Zif268 levels in a subgroup of MS animals. These results suggest a role for delta oscillations in memory reactivation that should be further investigated.

Early life stress from allergic dermatitis causes depressive-like behaviors in adolescent male mice through neuroinflammatory priming

Allergic dermatitis (AD), associated with pruritus and itchiness, is one of the major stressful conditions early in life. AD also influences the incidence of neuropsychiatric disorders and developmental disorders through neuro-immune interactions. To the best of our knowledge, there is no report that assesses the effects of early childhood dermatitis on psychiatric disorders later in life using an animal model. Here, we developed an oxazolone (Ox)-induced AD model in the early life of male C57BL/6J mice whose ears were challenged by Ox repeatedly from postnatal days (PD) 2 to PD30. On PD30, the Ox-treated ears were remarkably thickened and showed epidermal hyperplasia coupled with increased expression of T helper 2 cytokines, interleukin (IL)-4, and IL-13 in the ear tissue. Additionally, serum immunoglobulin E levels and serum corticosterone levels were higher in the Ox-treated mice than those in the control mice. Although Ox-treated PD40 mice showed neither behavioral abnormalities nor increases in pro-inflammatory cytokine expression in the brain, this study revealed that they experienced downregulation of CD200R1 expression in the amygdala under basal conditions and that additional lipopolysaccharide (LPS) administration induced enhanced neuroinflammatory reaction as the priming effect was accompanied by an increase of Iba-1-positive microglia in the amygdala and hippocampus. Furthermore, the Ox-treated PD40 mice showed depressive-like behaviors 24 h after LPS administration, whereas the control mice did not. Interestingly, the expression of indoleamine 2,3-dioxygenase and kynurenine 3-monooxygenase, key rate-limiting enzymes of the kynurenine metabolism pathway, was upregulated in the hippocampus, prefrontal cortex, and amygdala of the Ox-treated mice 4 h after LPS administration. Based on these results, we suggest that early life stress from AD aggravates susceptibility to systemic inflammation in the adolescent brain, leading to depressive behaviors with abnormal kynurenine metabolism.

Stress induces insertion of calcium-permeable AMPA receptors in the OFC-BLA synapse and modulates emotional behaviours in mice

Stress increases the risk of neuropsychiatric disorders, such as major depression. Exposure to stress has been reported to induce various neuronal changes, such as alterations in synaptic transmission and structure. However, a causal link between stress-induced neural circuit alterations and changes in emotional behaviours is not well understood. In the present study, we focused on a projection pathway from the orbitofrontal cortex (OFC) to the basolateral nucleus of the amygdala (BLA) as a crucial circuit for negative emotions and examined the effect of stress on OFC-BLA excitatory synaptic transmission using optogenetic and whole-cell patch-clamp methods in mice. As a stress-inducing procedure, we used repeated tail-shock, which increased stress-related behaviours. We found greater α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/N-methyl-D-aspartate current ratios and insertion of calcium-permeable AMPA receptors (AMPARs) in the OFC-BLA synapse after stress. These stress-induced synaptic and behavioural changes were reduced by a blockade of protein kinase A, which plays a principal role in stress-induced targeting of AMPARs into the synaptic membrane. To examine a possible causal relationship between alterations in synaptic transmission in the OFC-BLA pathway and stress-related behaviour, we performed optogenetic activation or chemogenetic inactivation of OFC-BLA transmission in mice. We found that optogenetic activation and chemogenetic inactivation of OFC-BLA transmission increased and decreased stress-related behaviour, respectively. In conclusion, we have demonstrated that stress altered the postsynaptic properties of the OFC-BLA pathway. These synaptic changes might be one of the underlying mechanisms of stress-induced behavioural alterations.

Early Adversity and the Neotenous Human Brain

Human brain development is optimized to learn from environmental cues. The protracted development of the cortex and its connections with subcortical targets has been argued to permit more opportunity for acquiring complex behaviors. This review uses the example of amygdala-medial prefrontal cortex circuitry development to illustrate a principle of human development-namely, that the extension of the brain's developmental timeline allows for the (species-expected) collaboration between child and parent in co-construction of the human brain. The neurobiology underlying affective learning capitalizes on this protracted timeline to develop a rich affective repertoire in adulthood. Humans are afforded this luxuriously slow development in part by the extended period of caregiving provided by parents, and parents aid in scaffolding the process of maturation during childhood. Just as adequate caregiving is a potent effector of brain development, so is adverse caregiving, which is the largest environmental risk factor for adult mental illness. There are large individual differences in neurobiological outcomes following caregiving adversity, indicating that these pathways are probabilistic, rather than deterministic, and prolonged plasticity in human brain development may also allow for subsequent amelioration by positive experiences. The extant research indicates that the development of mental health cannot be considered without consideration of children in the context of their families.

Using a Developmental Ecology Framework to Align Fear Neurobiology Across Species

Children's development is largely dependent on caregiving; when caregiving is disrupted, children are at increased risk for numerous poor outcomes, in particular psychopathology. Therefore, determining how caregivers regulate children's affective neurobiology is essential for understanding psychopathology etiology and prevention. Much of the research on affective functioning uses fear learning to map maturation trajectories, with both rodent and human studies contributing knowledge. Nonetheless, as no standard framework exists through which to interpret developmental effects across species, research often remains siloed, thus contributing to the current therapeutic impasse. Here, we propose a developmental ecology framework that attempts to understand fear in the ecological context of the child: their relationship with their parent. By referring to developmental goals that are shared across species (to attach to, then, ultimately, separate from the parent), this framework provides a common grounding from which fear systems and their dysfunction can be understood, thus advancing research on psychopathologies and their treatment.

Development- and experience-dependent plasticity in the dorsomedial habenula

Of the two major subdivisions of the habenula, the medial and lateral nuclei, the medial habenula is the least understood in terms of synaptic transmission, intrinsic properties and plasticity. The medial habenula (MHb) is composed of glutamatergic neurons which receive the majority of their inputs from the septal region and project predominantly to the interpeduncular nucleus (IPN). To understand the synaptic transmission, we studied both glutamatergic and GABAergic synaptic transmission in the dorsal region of the medial habenula (dMHb). While glutamatergic transmission dominates during early development, an attenuation of glutamatergic transmission and an enhancement of GABAergic transmission occur during development leading into adulthood. Furthermore, as reported previously, GABA_A receptor-mediated transmission is excitatory in the adult dMHb, which is consistent with the reduced expression of the K-Cl co-transporter KCC2. Given the potential role of the dMHb in aversive behaviors, we examined whether fear conditioning or exposure to foot shock affects excitability in dMHb neurons. We observed a suppression of the excitability of dMHb neurons in mice that either underwent fear conditioning or were exposed to foot shock. Furthermore, we observed a suppression of GABAergic but not glutamatergic transmission in the dMHb neurons following fear conditioning. These results suggest that aversive experience produces a suppression of the dMHb neuronal activity. Given that the medial habenula is upstream of the median raphe nucleus which is believed to be involved in the negative regulation of aversive memory, the suppression of dMHb neurons following an aversive experience might play a role in strengthening of aversive memories.

Alteration of the Centromedial Amygdala Glutamatergic Synapses by the BDNF Val66Met Polymorphism

Fear expression is mediated by an activation of the centromedial amygdala (CEm), the major output nucleus of the amygdaloid complex. Consistently, fear extinction is associated with an increased synaptic inhibition as well as a suppression of the excitability of the CEm neurons. However, little is known about the role of CEm glutamatergic synapses in fear regulation and anxiety-like behaviors. The BDNF Val66Met, a single-nucleotide polymorphism in the human BDNF gene, impairs fear extinction and leads to anxiety-like symptoms. To determine whether the BDNF Val66Met polymorphism affects the CEm excitatory synapses, we examined basal glutamatergic synaptic transmission and plasticity in the CEm neurons of BDNF Val66Met knock-in (BDNF(Met/Met)) mice. The BDNF Val66Met single-nucleotide polymorphism exerted an opposite effect on non-NMDA and NMDA receptor transmission with a potentiation of the former and a suppression of the latter. In addition, the decay time of NMDA currents was decreased in BDNF(Met/Met) mice, suggesting a modification of NMDA receptor subunit composition. Unlike the wild-type mice that exhibited a potentiation of non-NMDA receptor transmission following fear conditioning and a depotentiation upon fear extinction, BDNF(Met/Met) mice failed to show this experience-dependent synaptic plasticity in the CEm neurons. Our results suggest that the elevated non-NMDA receptor transmission, the suppression of NMDA receptor transmission, and an impairment of synaptic plasticity in the CEm neurons might contribute to the fear extinction deficit and increased anxiety-like symptoms in BDNF Val66Met carriers.

Electrophysiological profiles of induced neurons converted directly from adult human fibroblasts indicate incomplete neuronal conversion

The direct conversion of human fibroblasts to neuronal cells, termed human induced neuronal (hiN) cells, has great potential for future clinical advances. However, previous studies have not provided an in-depth analysis of electrophysiological properties of adult fibroblast-derived hiN cultures. We have examined the electrophysiological profile of hiN cells by measuring passive and active membrane properties, as well as spontaneous and evoked neurotransmission. We found that hiN cells exhibited passive membrane properties equivalent to perinatal rodent neurons. In addition, 30% of hiN cells were incapable of action potential (AP) generation and did not exhibit rectifying membrane currents, and none of the cells displayed firing patterns of typical glutamatergic pyramidal neurons. Finally, hiN cells exhibited neither spontaneous nor evoked neurotransmission. Our results suggest that current methods used to produce hiN cells provide preparations in which cells do not achieve the cellular properties of fully mature neurons, rendering these cells inadequate to investigate pathophysiological mechanisms.

Connectivity and circuitry in a dish versus in a brain

In order to understand and find therapeutic strategies for neurological disorders, disease models that recapitulate the connectivity and circuitry of patients’ brain are needed. Owing to many limitations of animal disease models, in vitro neuronal models using patient-derived stem cells are currently being developed. However, prior to employing neurons as a model in a dish, they need to be evaluated for their electrophysiological properties, including both passive and active membrane properties, dynamics of neurotransmitter release, and capacity to undergo synaptic plasticity. In this review, we survey recent attempts to study these issues in human induced pluripotent stem cell-derived neurons. Although progress has been made, there are still many hurdles to overcome before human induced pluripotent stem cell-derived neurons can fully recapitulate all of the above physiological properties of adult mature neurons. Moreover, proper integration of neurons into pre-existing circuitry still needs to be achieved. Nevertheless, in vitro neuronal stem cell-derived models hold great promise for clinical application in neurological diseases in the future.

Environmental insults in early life and submissiveness later in life in mouse models

Dominant and subordinate dispositions are not only determined genetically but also nurtured by environmental stimuli during neuroendocrine development. However, the relationship between early life environment and dominance behavior remains elusive. Using the IntelliCage-based competition task for group-housed mice, we have previously described two cases in which environmental insults during the developmental period altered the outcome of dominance behavior later in life. First, mice that were repeatedly isolated from their mother and their littermates (early deprivation; ED), and second, mice perinatally exposed to an environmental pollutant, dioxin, both exhibited subordinate phenotypes, defined by decreased occupancy of limited resource sites under highly competitive circumstances. Similar alterations found in the cortex and limbic area of these two models are suggestive of the presence of neural systems shared across generalized dominance behavior.