Neurobiology of Infant Fear and Anxiety: Impacts of Delayed Amygdala Development and Attachment Figure Quality

Abstract

Anxiety disorders are the most common form of mental illness and are more likely to emerge in childhood compared to most other psychiatric disorders. While research on children is the gold standard for understanding the behavioral expression of anxiety and its neural circuitry, the ethical and technical limitations exploring neural underpinnings limit our understanding of the child’s developing brain. Instead, we must rely on animal models to build strong methodological bridges for bi-directional translation to child development research. Using the caregiver-infant context, we review the rodent literature on early-life fear development to characterize developmental transitions in amygdala function underlying age-specific behavioral transitions. We then describe how this system can be perturbed by early-life adversity, including reduced efficacy of the caregiver as a safe haven. We suggest that greater integration of clinically-informed animal research enhances bi-directional translation to permit new approaches to therapeutics for children with early onset anxiety disorders.

Introduction

Anxiety disorders are the most common form of mental illness, although they are more likely to emerge in childhood compared to most other psychiatric disorders (1–3). Emerging evidence suggests, however, that how anxiety is expressed across the life-span changes (4, 5) and this may not be reflected in the current diagnosis criteria, resulting in missed diagnoses (6–8). Since childhood anxiety disorders can be a relatively reliable predictor of psychiatric disorders emerging in later-life, understanding unique features of early-life anxiety and its developing supporting networks is critical (9, 10). While research on children is the gold standard and a critical benchmark for understanding the neurobehavioral expression of anxiety and its neural circuitry, the ethical and technical limitations exploring neural underpinnings limit our understanding of the child’s developing brain.

To address this issue, we must rely on animal models with strong methodological links to the developmental research, combined with clinically-relevant experimental questions. The phylogenetically-preserved neural threat processing system has enabled animal models of threat expression and learning in rodents and nonhuman primates to provide translational bridges and inform the neurobiology of anxiety disorders in adulthood, adolescence, and more recently, in late and early infant development (for review see (11–16). Throughout this review, we will use “fear/threat” when discussing animal data as a means of addressing the challenges of parsing behavioral responses to threats vs. subjective fear states (17). We focus on the young infant rat as a model for studying early fear/threat processing. This permits characterization of typical development of this system as well as how this system becomes perturbed to produce pathology, namely, anxiety-like behavior emerging in early-life -- an approach that includes incorporating the age-specific ecological niche of the parent (or other attachment figure). In this way, this approach includes the context in which early-life anxiety disorders are frequently initially noticed. The goal of this approach is to define unique features of early-life heightened fear/threat processing and identification of neural mechanisms and then consider how this information informs our understanding of early-life anxiety disorders.

Although transient fears and anxieties are normal in early childhood, these fears subside in typically-developing children. Persistent elevation of these threat responses can be considered pathological and can be diagnosed as anxiety (18, 19). This increased fear is typically first highlighted by parents, as they express concerns about their child’s excessive fear of novel people and environments and increased protest upon separation (i.e. Separation Anxiety). This high fear level impacts the child’s daily functions, including avoidance of social interactions with peers and new experiences, which can independently hinder normative development (20).

While anxiety is broader than the concept of fear, understanding child anxiety requires a description of major changes in a child’s fear system as well as the unique age-specific importance of the relationship to the caregiver. As we explore the early expression of heightened fear and the development of the threat system, it is critical to remember that the infant fear system is not an immature version of the adult system and best viewed within the context of pups’ natural environment with the mother (14, 21). The infant cannot engage adult-like defenses such as attack, fleeing, escape, hiding, and freezing, because the neural circuits supporting these diverse behaviors are yet undeveloped. Instead, the infant goes to the caregiver for protection, exhibiting an infant age-specific form of defense (see Figure 1). This age-specific response to threat and its close connection with the caregiver is seen in myriad species and is a key feature of the attachment system, where the caregiver is considered a “Safe Haven” (22, 23). Indeed, as noted above, it is within the parent-infant dyad that early-life heightened fear is typically first identified in children.

Figure 1. Developmental transitions in evoking and expressing fear toward a threat.

Whereas adults can run, hide or fight when faced with a threat, infants lack both the motoric and neural networks to engage these defensive strategies. Instead, children approach the caregiver or other attachment figure for protection. These developmentally-specific behavioral transitions are conserved across species, including the infant rodent pup. Throughout this review, we refer to the period when rodents are completely dependent on the mother as infancy alongside discussions of infancy and very young childhood in humans.

An Evolving Role for Amygdala Engagement in Human Fear Behavior

Within the fear/threat circuit (Figure 2), it is well-documented that the brain’s core structure for processing threat is the amygdala, and this structure is critically important across mammalian species in supporting responses to threat in adults (24–26). Though our discussion centers on the role of the amygdala in threat/fear processing, similar to other brain areas, the amygdala does not have a unitary function. Indeed, it has been shown to be causal in functions associated with e.g. signaling salience, reward, pain, and punishment (27–31). The amygdala is strongly associated with anxiety disorders and this association appears to be maintained throughout development (32). This amygdala-anxiety association has even been found during the first days of life: fMRI measuring newborn amygdala connectivity shows this measure is predictive of fear at six months of age, and individual differences in amygdala connectivity are relevant for the expression of fear over the first two years of life (33). This correlational association in newborns very likely exists even before the amygdala is mature enough to be engaged in the response to threat and is likely highlighting altered amygdala functioning due to genetic or prenatal programming (see review by Jed Ellison in this Special Issue).

Figure 2. Fear learning circuitry during typical development in humans and rodents.

Although understanding anxiety disorders has been challenging, neural circuit organization and function has emerged as a potential therapeutic target. To fully explore this, we must understanding the timing, trajectories, and mechanisms involved in the normative circuit development that serve these complex behaviors. Here, we present circuit data in rodents to characterize the neurobiology of threat/fear processing and how this can be perturbed by adversity. The neural circuitry supporting fear processing in humans has been shown to primarily involve the prefrontal cortex (PFC), amygdala, and hippocampus, with the amygdala developing first to functionally support fear reactions in early infancy(125, 126). In rodents, amygdala-dependent threat/fear conditioning is causally involved in cue learning at PN10 (58, 75, 76) followed by the PFC days before weaning (13) and hippocampus at weaning age (127, 128), although the hippocampus appears to support context memory for a minute or two post acquisition in younger pups (129, 130). In the service of translation, here we explore how this literature can be integrated with the human data to provide a unique view of the neurobiology of anxiety-like behavior across development. Gray shading indicates the presumed functional maturity of different structures. Functional connectivity in humans is depicted by lines connecting structures, and the presumed maturity of those structures is illustrated by whether the line is solid (mature) or broken (immature). The same scheme is used for rodent connectivity, with the additional complexity of indicating whether the connection is an excitatory (triangle) or inhibitory (perpendicular) projection, a distinction afforded by the greater specificity of the evidence from rodents. In rodents younger than PN10, threatening cues (e.g. odors associated with shock) bypass the BLA/HPC/PFC adult fear circuit (still immature). Instead this info processed by the attachment circuitry (locus coeruleus, OB, anterior piriform) to produce preference behavior for odor cues. After PN10, amygdala engagement and developing PFC connectivity become incorporated into the fear/threat circuit to produce avoidance responses when the animal is alone. After weaning, the hippocampus, PFC and amygdala coordinate avoidant threat responses. Abbreviations: HPC, hippocampus; IL, infralimbic region of the PFC; LC, locus coeruleus; OB, olfactory bulb; Pir, piriform cortex; PFC, prefrontal cortex; PL, prelimbic region of the PFC; PN, postnatal day.

The very young child’s amygdala is engaged during threat and anxious behavior (26, 33–40). There is clear evidence that the child’s amygdala is not engaged by pain in the early postnatal days following birth (41, 42), whereas it is strongly engaged in adults (32). Behaviorally, fear of heights and fear of strangers functionally emerge around the later part of the first year of life and is thought to signal amygdala maturation. Young children begin showing fear to widening of the eyes (more of the white sclera shown) at around 7 months and this has also been suggested to involve the amygdala (43, 44) since observing eye widening in adults is strongly correlated with amygdala activation (45). A causal link in humans between the amygdala and fear was strengthened by the identification of clinical cases of adult amygdala ablation patients that showed greatly reduced fear (46, 47). Overall, there is strong suggestion that these newly emerging fears in infants are dependent upon a newly emerging functional development of the amygdala. It has been suggested that the coincidental alignment of the emergence of crawling and the fears listed above may keep the newly mobile infant safe by engaging the amygdala and fear to inhibit behavior and promote avoidance (48, 49).

A Threat Can Generate Learned Approach Behaviors Before Amygdala Development

Infant rats younger than 10 days-old, including fetal rats, show learning when an odor is paired with high shock (≥1mA, produces slight tissue damage and illness/malaise) or lithium chloride (LiCl, induces malaise). These pups readily learn to avoid that odor (50, 51), although subsequent research showed that, unlike adults, this learning is engaging the piriform cortex, not the amygdala (52), as is used by adults (53). The literature on the cellular, molecular, volumetric, and connectivity development of the amygdala supports this view (54, 55). Less painful stimuli (0.5mA shock, tailpinch) paired with odor fail to produce aversion/fear learning and instead produce a behavioral preference in these young pups, expressed as approach (52, 56–58). The inability of the paired cue-pain procedure to produce fear learning is not due to pups’ inability to detect the aversive stimulus or feel pain (59). Rather, the noxious stimulus is entering the amygdala and not engaging plasticity mechanisms (41, 60, 61), instead engaging the same learning system pups use to learn about the mother (the attachment figure) as the odor takes on the qualities of the mother’s odor (62, 63). How this infant attachment learning and attachment maintenance circuitry relates to attachment mechanisms used in adults is unclear. While there has been considerable research on attachment neurobiology in adults focusing on mates and reproduction, the infant has received little attention (29, 64, 65)). However, the scant literature suggests a specific locus coeruleus, noradrenergic network for attachment focus on olfactory bulb and anterior piriform cortex plasticity to learn the attachment figure ((66) (see Figure 2). Hereafter, we will use the model system of the infant rat and focus on how once this attachment is acquired, the attachment figure regulates or fails to regulate circuits supporting threat processing. However, we direct the reader to a broader body of work in other rodent species, including monogamous California mice and species of vole, and how such manipulations might impact amygdala development and its coordination of learning about threat within the context of attachment figure presence or absence (67–71).

Due to the amygdala’s failure to be recruited in odor-shock fear/threat learning in PN8 rat pups, odor-shock pairings engage the attachment circuit and pups subsequently express an approach toward the odor previously paired with the shock, rather than show the typical adult behavior of freezing to the odor. Lesioning or suppressing the amygdala during this age range (<PN10) does not alter learning, indicating it is not used in these behaviors. A direct comparison of this odor-shock induced preferred odor with the maternal odor shows striking similarities: both the shock-paired odor and maternal odor are approached in a Y-maze, both support nipple attachment, and both blunt the stress response induced by a threat (58). During very early life, before the amygdala is sufficiently mature to be engaged in fear learning, an odor-pain paradigm (fear conditioning) engages the attachment system rather than the amygdala-dependent fear system. Further, there is a developmental bias to use the attachment system for learning within a wide range of contexts, including slight to moderate adversity (see below). For example, this can be demonstrated within the nest with a mother that roughly handles pups. If the mother is washed and scented with a novel odor during this treatment, this novel odor takes on qualities of the natural maternal odor: pups learn to approach this odor when tested outside the nest, and this learning fails to engage the amygdala (72).

What does this paradoxical, odor-shock, pre-amygdala learning tell us about the development of fear/threat and anxiety in children? These data suggest a few general principles. First, young infants can show fear/threat learning behaviors [using paradigms involving high shock and malaise (LiCl)] that appear similar to adults, but the infant uses a nonamygdala circuit, while adults use the amygdala. Second, infant neurobehavioral functioning is not always an immature version of the adult – a threat can support approach behaviors in infants before the amygdala is functional in producing aversions.

Early Infancy Amygdala-Dependent Fear Learning Requires Stress Hormones

By PN10, the amygdala participates in odor-shock conditioning, as expressed by odor avoidance and freezing, as well as plasticity in the basolateral amygdala (58). Thus, in sharp contrast to pups just 1 day younger, we found robust amygdala-dependent fear learning. The literature on amygdala anatomical and physiological development also supports this gradual postnatal amygdala development on the cellular, molecular, volumetric, and connectivity levels of analysis (54, 73, 74). Specifically, BLA long-term potentiation (LTP), a robust measure of plasticity, could not be induced by tetanic stimulation in pups younger than PN10 yet could be induced in pups older than PN10 (60).

Capitalizing on animal research to further understand mechanisms, we first showed that the BLA was truly causal in this newly emerging fear/threat learning. Specifically, amygdala microinfusions of the GABA_A agonist, muscimol (suppresses amygdala neural firing), were able to block pup threat learning (75, 76). Next we asked, what developmental change was responsible for the pups’ new learning ability? Previous literature had identified this age range (<PN10) as the Stress Hyporesponsive Period (SHRP), and the gradual increase of pups’ threat-induced stress hormone response (corticosterone, homologous to cortisol in primates) as critical for both innate and learned fear expression. Specifically, research had shown that infant rats’ responses to a predator odor (innate) required stress hormone elevation (77): fear could be induced in pups younger than PN10 by an exogenous increase in the stress hormone, while pups older than PN10 could have fear blocked by pharmacologically halting stress hormone production or blocking hormone receptors.

We next confirmed a role for corticosterone-BLA engagement in learned fear by showing that pharmacologically increasing corticosterone in younger pups could precociously induce fear learning, and blocking stress hormones in older pups completely blocks fear learning. These hormone manipulations controlled pups’ fear learning when given systemically, as well as via microinfusions localized to the BLA. The local BLA control in switching fear learning on and off indicates it is necessary and sufficient for the emergence of pup learning and that the BLA requires this stress hormone to promote fear learning in infants. This is in sharp contrast to older pups and adults, where stress hormones are not needed for amygdala-dependent threat responding, instead only modulating the fear response (78). Importantly, this research highlights that during development, unique physiological and neural controls are used to alter amygdala function, with these mechanisms showing age-specific abilities of the amygdala to block plasticity (79).

Once the attachment figure is learned, the infant maintains contact with the attachment figure, but the attachment figure also regulates the infant immature physiological system (reviewed in (80)) One aspect of regulation is “social buffering” and the finding that maternal presence during infant experience of pain or stress blocked pups’ stress hormone increase (81). Briefly, social buffering refers to a social partner’s ability to blunt the neurobehavioral response to a threat across development (82). It should be noted that a young infant’s stress response is blocked by the caregiver, whereas in older pups it is attenuated, similarly to adults (83). This infant blockade of stress hormones was first shown in rodents (84, 85) and quickly replicated in nonhuman primates(86) and children (87, 88) (Figure 3). Since fear/ threat responding was shown to be closely tied to the level of stress hormones (77), and the social buffering blocked stress hormone release, we questioned if the mother was capable of blocking amygdala and/or fear conditioning. Specifically, our next step involved using the mother rat, combined with pharmacological manipulation of stress hormones within the hypothalamic-pituitary-adrenal (HPA) axis and amygdala, to reduce pups’ shock-induced corticosterone release during fear conditioning. Results showed the presence of the mother also blocked fear learning in PN10–15 pups, because the mother blocked corticosterone release. The effect was observed even when the mother was anesthetized or if pups could only smell the odor of an awake mother (unstressed, unaware her pups were being shocked). Maternal presence blocked the release of the stress hormone at the level of the hypothalamic paraventricular nucleus (PVN) of the HPA axis, preventing the corticosterone release required by the amygdala for plasticity to induce learning (89). This phase of maternal blockade of pups’ fear learning terminates at PN16 (90), although the mother continues to attenuate fear learning, similar to what is seen in adult social pairs (61). While it was initially surprising to learn that mothers (or other attachment figures) could block fear learning, developmental psychologists have long known that the mother (or other attachment figure) is a “Safe Haven” for the child: contact with the mother produces a decrease in the behavioral expression of fear (22, 91). It should be noted that the system described above requires the mother to be calm. Indeed, if the mother rat responds to the threat, pups will express and learn fear through corticosterone-dependent amygdala engagement (92–94), mimicking social transmission of fear shown in human mother-infant dyads (95) (Figure 4).

Figure 3. Cross-species alignment of fear learning and the relevant neural circuits across typical maturation within the developmental ecology framework.

Top: Developmental milestones in fear/threat across species. Bottom: Cross-species evidence for social buffering in infants and adults ((75, 88, 131–138); for review, see (139)). As with any cross-species developmental comparison, it is important to note that no consensus has been reached regarding how rodent neural development aligns with human neural development. Given the importance of the timing in development, this lack of information might result in imprecise inferences and difficulties in translating major preclinical studies to the human. One approach compares developmental milestones such as neurogenesis, synaptogenesis, connectivity, and specific functions of a given circuit across species, rather than comparing development of the whole brain (140). In contrast, our approach to assessing early fear development uses ecologically relevant behavioral milestones, such as the emergence of crawling and developmental transitions in maternal regulation of the infant, to facilitate bi-directional translation between human and animal models (15). The value of this framework is that it permits coordinating/identifying clinically-relevant questions and assessing neural mechanisms in the rodent (12, 141). Extensive cross-species analysis has shown that both fear conditioning ((17, 25, 142) and attachment (including social buffering and safe haven characteristics of the caregiver) (81, 136, 143) are useful models for cross species analysis when used with caution (17, 144). Although understanding threat processing deficits associated with psychiatric disorders such as anxiety has been challenging, neural circuit organization and function has emerged as a potential therapeutic target (145, 146). To fully explore this, we must understand the timing, trajectories, and mechanisms involved in the normative circuit development that serve early fear expression (147).

Figure 4. Transitions in amygdala-dependent fear/threat in typical and atypical development.

Using a rodent model, we characterize how the amygdala becomes causal in fear/threat expression at 10 days of age (age when pups become mobile, corresponds to crawling in the human infant (16, 111). The caregiver or other attachment figure can block the fear response in young pups, which transitions to social buffering or the attenuation of fear, via blockade or reduction of stress hormones, respectively (92, 112). Social buffering continuous into adulthood but is most robust in early life in the caregiver-infant dyad (88, 103, 113–116). With atypical development, this timeline can be shifted earlier (dashed red line) to produce early fear and anxiety-like behavior through multiple causes, including a hyperactive amygdala induced by infant experience in adversity-rearing, poor social buffering by a calm caregiver, or the presence of an anxious parent where anxiety/fear is socially transmitted to the child. This latter example represents a deviation from the phenomenon observed during typical development, when fear to a real threat is expressed by the caregiver and socially transferred to the infant. In these studies, mother rats (pregnant or postpartum) were conditioned to express threat/fear toward a previously neutral peppermint odor. Pups (PN6–12) were exposed to a peppermint odor and a fearful mother or just her odor. This simple pairing, in the absence of shock to pups, produced amygdala-dependent fear behavior in the pups. The mechanism for this social transmission of fear was dependent upon the mother increasing her pups’ corticosterone levels, mediated by her release of an alarm pheromone (92). The ability of alarm pheromone to alter stress hormone levels, anxiety and the brain had been shown in adult rats(148), transmitted through a more recently identified olfactory pathway via the Grueneberg ganglion cells within the nasal cavity (148, 149). This phenomenon of social transmission of fear has been shown in human adults (95) as well as from the caregiver to the child (150), suggesting it is conserved across mammalian species and can be probed mechanistically using animal models. Since children do not possess a Grueneberg ganglion, mechanisms of social transmission diverge across species, with children using a variety of sensory systems for social information. However, since stress hormone increase is also a key feature of social transmission of fear, a potential common cross-species importance for stress hormone can be explored (95). This also provides a template in which to assess how this system goes awry, as in the case of an overly anxious caregiver failing to regulate the infant.

What does this research tell us about early-life anxiety and heightened fear? It suggests that once the amygdala is causally related to fear behavior and assessed within the context of the child alone or with the mother, there are likely multiple pathways by which a child can show heightened fear and anxiety. As summarized in Figure 4, these pathways can include enhanced amygdala response to threat (adversity rearing, described in detail below, (96, 97)), the calm attachment figure’s inability to reduce fear (compromised social buffering (87, 98)), or the anxious attachment figure signaling danger (see above, social transmission of fear, (92)).

Modeling Early Increased Fear and Anxiety Using Adversity-Rearing

Disruption in fear processing and anxiety can be related to genetics (see review by Erika Nurmi in this Special Issue), but environmental input during development is known to be critical, with early adversity being associated with elevated levels of anxiety (19, 99, 100). Here, we operationalize adversity as experience outside the range of species-expected inputs, or experiences beyond the range of normative variation (101). We specifically focus on early caregiving adversity, or a disruption in the relationship with attachment figures, not necessarily involving physical separation from caregivers (101–103). Adverse experiences impact the brain as a whole, although adversity specifically targeting the attachment system, either through neglect, harsh rearing or maltreatment, appears to more robustly produce vulnerabilities to psychiatric disorders, including early onset-anxiety and pathologies expressed in later-life. Interestingly, a wide range of types of early-life adversity ranging from deprivation to maltreatment actually supports and preserves attachment, although the attachment quality can be compromised in children across cultures (104, 105). This has also been demonstrated in our rodent models, which has enabled us to probe neural circuitry (106, 107). Importantly, this retained attachment, despite adversity, is seen across species, including in chicks during imprinting to the caregiver (108, 109), infant dogs that were shocked while interacting with a human caregiver (110), and infant monkeys that were raised with a wire surrogate that inflicted pain, or an abusive mother (111–115).

This wide phylogenetic representation of attachment, which was first identified by Bowlby as he developed Attachment Theory, further supports the use of animal models to understand human attachment and its impact on early-life anxiety. While there are limitations to animal models (such as the inability to fully model social and cultural phenomena), their use in conjunction with human research can isolate candidate mechanisms for cause-effect relationships between adverse experience and neurobehavioral development. Recently, cross-species research has leveraged the comparative strengths of human and animal work to directly illustrate neurobehavioral parallels supporting attachment and loci of dysfunction following early caregiving adversity. For example, this work has shown that in both rodents and humans, caregiving sensitivity is correlated with attachment security, as measured by neurobehavioral responses to reunion with a caregiver (102, 116). In addition, the importance of social odors as attachment cues has also been demonstrated in humans across the lifespan (117–119) (120). Moreover, recent rodent work has shown that during adversity, nurturing inputs such as milk and grooming fail to regulate the infant brain state—thereby testing and generating mechanistic support for a hypothesis initiated by human developmental research (121).

Another strength of clinically-generated animal research questions is that they permit isolated assessment of specific features of adversity which are particularly harmful. For instance, the social context of adversity, as compared to adversity experience alone, seems to robustly target the amygdala and the brain’s processing of the mother (62, 107, 116). Specifically, rat pups reared with a mother maltreating her offspring remain attached to the mother but show slightly slower approach responses and decreased amygdala responses to maternal cues at baseline (no threat)(62). Presenting a stressor or threat uncovers more robust neurobehavioral deficits in animal models (106) and humans, as expressed during the stressful Strange Situation Procedure (91, 104). In addition to behavioral deficits, reduced neural reactivity to the attachment figure following adversity has also recently been shown in children. As described above, the child’s brain shows oscillatory responses to maternal cues and the robustness of these responses is correlated with the quality of attachment (121). This has also been seen in fMRI as a decreased response to maternal cues in the amygdala (101, 122) (see review by Dylan Gee in this Special Issue).

We also know that many other early adversity paradigms alter the amygdala and fear, although typically this is assessed in pups older than in the infant rat pup literature reviewed here. For example, early separation from the caregiver (maternal separation) produces early expression of adult-like, persistent fear memory expression patterns before weaning age (PN17), as well as impaired fear extinction in adolescence (13, 40) (see review by Francis Lee in this Special Issue). These studies show that early-life adversity appears to enhance persistent fear/threat in the developing amygdala (see Figure 4). This reinforces the overall message that fear expression shows marked developmental transitions that alter our assessment of behavioral measures of anxiety. This is due, at least in part, to differences between threat expression prior to amygdala causal inclusion into the threat circuit in very young pups and the more mature threat responses as circuit complexity gradually includes the PFC and hippocampus (Figure 2). When early-life adversity disrupts this delicate developmental orchestration of the developing threat network, the expression of fear to threat is further altered due to disrupted amygdala network connectivity to other brain areas. These circuits then interface with other delicate systems impacted by adversity, including development of intracellular molecules, neurotransmitters, hormones, synapses, cellular morphology, and function – some of which show accelerated development, while others are delayed (34, 35, 40, 96, 101, 103, 122, 123). For example, adversity appears to target PNN development to leave programming mechanisms open to change longer and enhances amygdala excitation, but it also directly impacts excitatory synapse as measured by electron microscopy at PN23 (124). This desynchronization of developmental processes may be one mechanism in which adversity generates amygdala hyper-responsivity during early life.

As mentioned above, early adversity also impairs the ability of the caregiver to regulate the amygdala (see Figure 4). Coupled with a hyper-responsive amygdala at baseline, synergy between these two outcomes may further impair threat processing to produce pathology (e.g. anxiety-like behavior). We now understand that this impaired social buffering involves dysregulation of caregiver regulation of amygdala dopamine. Specifically, the mesolimbic dopamine - BLA interface appears to integrate the effects of development, experience, and social buffering (29). In typically developing pups at PN12–14, the caregiver’s presence during threat buffers activation of the amygdala and ventral tegmental area (source of dopamine to BLA). This buffering gradually decreases as pups mature. However, following early adversity, the caregiver’s presence fails to block the VTA and the BLA in the young PN12–14 pup (97). Furthermore, if dopamine is persistently increased in the BLA during threat, pups’ attachment toward the caregiver becomes impaired. Pups with impaired social buffering of amygdala DA go on to demonstrate anxiety-like phenotypes across development.

In Conclusion

A model of early onset high fear in younger rodent pups can have clinical importance, because it models the functional transition of complete dependence on the caregiver and the gradual inclusion of some independence (i.e. crawling away from the caregiver) – as well as pre- and post-amygdala inclusion in attachment and threat responding. Importantly, this very early-life rodent model captures the age when anxiety concerns are first expressed and can potentially highlight the roots of childhood anxiety within the caregiver-infant dyad.

As mentioned above, aligning these early-life neurobehavioral transitions in fear/threat across species can be challenging. To maximize translation, appropriate alignments are necessary with respect to age but also with respect to the developmentally appropriate expression of fear. For instance, in humans, anxiety disorders are typically highlighted during the school-age and adolescent years, whereas anxiety is less impactful than neurodevelopmental and behavioral problems in the infancy and preschool period. This distinction is also important regarding adversity as an antecedent for anxiety, as important similarities and differences exist for clinical anxiety-related problems arising in settings that do or do not involve adversity. With these points in mind, it is not our intent to review clinical issues in early life or claim that the rodent literature described here encapsulates child anxiety in all of its inherent complexity. Rather, it is our intent to leverage the strengths of animal research, such as the ability to test the specific features of experience that impact fear/threat circuitry, in the service of developing optimal age-appropriate treatments and interventions.

Altogether, this approach highlights multiple pathways to heightened anxiety in early life that may have clinical value. These include the infant having a hyper-functioning amygdala or a dysfunctional safety signal from the mother (or other attachment figure, i.e. Safe Haven) that fails to decrease the amygdala’s response to novelty or threat. Capitalizing on the animal literature with this translational approach may permit greater functional and temporal precision in developing therapeutics for children suffering from early anxiety disorders.