Faculty Bibliography

Early life trauma is a risk factor for life-long disorders related to emotional processing, but knowledge underlying its enduring effect is incomplete. This study was motivated by the hypothesis that early life trauma increases amygdala-dependent threat responses via reduction in inhibition by parvalbumin (PV) interneurons and perineuronal nets (PNN) supporting PV cells, thus increasing excitability of the basolateral amygdala (BLA). From postnatal day (PN) 8-12, rat pups of both sexes were reared under normal bedding or under insufficient nest-building materials to induce maternal-to-infant maltreatment trauma (Scarcity-Adversity Model, SAM). At weaning age of PN23, the SAM group exhibited increased threat responses to predator odor. The SAM-induced increase in threat response was recapitulated in normally reared PN22-23 rats that were unilaterally depleted of PNN in the BLA by the enzymes, chondroitinase-ABC plus hyaluronidase at PN19-20. Light and electron microscopic analysis of the BLA revealed that anterior-to-mid levels of SAM group's BLAs exhibited decreased PNN intensity and decreased axo-somatic synapses between PV-to-principal pyramidal-like neurons and PV-to-PV. PV and PNN densities (cells/ mm² ) in the BLA of both control (CON) and SAM groups were still low at PN12 and SAM delayed the ontogenetic rise of PV intensity and PNN density. Moreover, PV cell density in the anterior-to-mid BLA correlated negatively with threat response of CON animals, but not for SAM animals. Thus, reduction of PNN-supported, PV-mediated somatic inhibition of pyramidal cells provides a mechanistic support for the enduring effect of early life maltreatment manifested as increasing innate threat response at weaning.

Although infants learn and remember, they rapidly forget, a phenomenon known as infantile amnesia. While myriad mechanisms impact this rapid forgetting, the molecular events supporting memory maintenance have yet to be explored. To explore memory mechanisms across development, we used amygdala-dependent odor-shock conditioning and focused on mechanisms important in adult memory, the AMPA receptor subunits GluA1/2 and upstream protein kinases important for trafficking AMPAR, protein kinase M zeta (PKMζ) and iota/lambda (PKCι/λ). We use odor-shock conditioning in infant rats because it is late-developing (postnatal day, PN10) and can be modulated by corticosterone during a sensitive period in early life. Our results show that memory-related molecules did not change in pups too young to learn threat (PN8) but were activated in pups old enough to learn (PN12), with increased PKMζ-PKCι/λ and GluA2 similar to that observed in adult memory, but with an uncharacteristic decrease in GluA1. This molecular signature and behavioral avoidance of the conditioned odor was recapitulated in PN8 pups injected with CORT before conditioning to precociously induce learning. Blocking learning via CORT inhibition in older pups (PN12) blocked the expression of these molecules. PN16 pups showed a more adult-like molecular cascade of increased PKMζ-PKCι/λ and GluA1-2. Finally, at all ages, zeta inhibitory peptide (ZIP) infusions into the amygdala 24 hr after conditioning blocked memory. Together, these results identify unique features of memory processes across early development: AMPAR subunits GluA1/2 and PKC isoform expression are differentially used, which may contribute to mechanisms of early life forgetting.

Animals, including humans, require a highly coordinated and flexible system of social behavior and threat evaluation. However, trauma can disrupt this system, with the amygdala implicated as a mediator of these impairments in behavior. Recent evidence has further highlighted the context of infant trauma as a critical variable in determining its immediate and enduring consequences, with trauma experienced from an attachment figure, such as occurs in cases of caregiver-child maltreatment, as particularly detrimental. This review focuses on the unique role of caregiver presence during early-life trauma in programming deficits in social behavior and threat processing. Using data primarily from rodent models, we describe the interaction between trauma and attachment during a sensitive period in early life, which highlights the role of the caregiver's presence in engagement of attachment brain circuitry and suppressing threat processing by the amygdala. These data suggest that trauma experienced directly from an abusive caregiver and trauma experienced in the presence of caregiver cues produce similar neurobehavioral deficits, which are unique from those resulting from trauma alone. We go on to integrate this information into social experience throughout the lifespan, including consequences for complex scenarios, such as dominance hierarchy formation and maintenance.

Objectives: Human infants born prematurely experience repeated noxious medical procedures, but maternal contact can attenuate the behavioral response to these procedures. However, the mechanisms by which the mother reduces pain or the enduring impact of using the mother as an analgesic stimulus are unknown. By use of an animal model of early life pain with and without the mother, we monitored behavior at different points across the lifespan. Methods: Infant rats were given mild tail shocks (0.5 mA, 1 second every 4 minutes for 32 minutes) either with the mother present or absent for five consecutive days. Two age ranges were chosen to represent the sensitive period for pain programming (PN5-9) and the age at which maternal presence has major neurobehavioral effects, including suppression of amygdala activity (PN10-14). In infancy, neural pain responses were assessed via cFos expression in brain areas associated with pain, as well as behavioral measures related to pain response. In adulthood, pain thresholds, social behavior, and unlearned fear behavior were assessed as a function of infant pain experience. Results: Activity and ultrasonic vocalizations after treatment from PN5-9 and PN10-14 were reduced by the mother's presence during exposure to painful shock. After treatment at PN10-14 pups only, Fos expression in the periaqueductal gray and basolateral and medial amygdala was elevated by the shock alone and reduced by the mother. Adults treated at PN5-9 had reduced carrageenan-induced hyperalgesia and reduced social behavior but no changes in fear behavior if they had undergone pain-mother pairings. In contrast, when treated at PN10-14, the adult had no change in hyperalgesia and showed disruption of social behavior. Shock with or without the mother decreased unlearned fear responding only if treated at PN10-14. The social behavior deficits were normalized by 2 weeks of environmentally enriched rearing after weaning. Conclusions: Our results provide evidence that maternal presence during early life pain reduces pain responses in both infancy and adulthood, but it is also associated with long-term changes in emotionality. The results of these studies aid in our understanding of the impact of nursery procedures that are used to attenuate pain on later outcomes focused on affective behaviors and potentially provide a strategy to reduce those effects

Objectives: During infancy, rapid learning associated with attachment and orientation to a caregiver is essential to survival. This developmental period also prevents the acquisition of avoidance learning. In rodent models, this developmental time window occurs prior to post-natal day 10 (PND 10), during which pups display heightened preference learning accompanied by decreased aversion learning. PND 10 rats presented with odor-shock pairings fail to avoid the odor associated with shock, and show a preference for the paired odor. Older pups (PND 12) given odorshock pairings develop an aversion to the odor at subsequent test. One key developmental mechanism that appears to direct the change from a preference for the odor associated with shock to an aversion, involves the activation of the amygdala by corticosterone. Corticosterone is low in pups during the sensitive period and increases at PND 10. We investigated synaptic markers which may be important for establishing the avoidance memory and that are likely activated by corticosterone. Protein kinase M zeta (PKMzeta), which is important for late-phase LTP and long-term memory, is also upregulated during stress (Sebastian et al 2013, PLoS One, vol 8, e79077). Therefore, we investigated the role of PKMzeta in avoidance vs preference learning in rat pups using the paired odor-0.5mA shock fear-conditioning paradigm. Methods: PND 8 and PND 12 pups were given either paired (simultaneous odor and shock) or unpaired (shock 2 min after odor) training and tested 24hr later on a Y maze with one arm containing the conditioned stimulus (CS) odor and the other a familiar odor. Immediately after Y maze test, pups were sacrificed and amygdalae were harvested. The tissues were separated into cytosolic and synaptic cellular fractions. Each fraction was analyzed by Western blots. Results: Pups in the unpaired condition showed no preference for either arm. PND 8 pups in the paired condition preferred the CS odor and PND 12 pups in the paired condition avoided the CS odor (p<0.01). PND 12 in the paired condition had higher cytosolic PKMzeta in the amygdala compared to unpaired pups (p<0.05) with no change in synaptic PKMzeta. PN8 paired did not show any changes in cytosolic or synaptic PKMzeta Conclusions: Increased PKMzeta expression following the sensitive period plays a role in the activation process of the amygdala and the formation of aversive memories

Research on social instability has focused on its detrimental consequences, but most people are resilient and respond by invoking various coping strategies. To investigate cellular processes underlying such strategies, a dominance hierarchy of rats was formed and then destabilized. Regardless of social position, rats from disrupted hierarchies had fewer new neurons in the hippocampus compared with rats from control cages and those from stable hierarchies. Social disruption produced a preference for familiar over novel conspecifics, a change that did not involve global memory impairments or increased anxiety. Using the neuropeptide oxytocin as a tool to increase neurogenesis in the hippocampus of disrupted rats restored preference for novel conspecifics to predisruption levels. Conversely, reducing the number of new neurons by limited inhibition of adult neurogenesis in naive transgenic GFAP-thymidine kinase rats resulted in social behavior similar to disrupted rats. Together, these results provide novel mechanistic evidence that social disruption shapes behavior in a potentially adaptive way, possibly by reducing adult neurogenesis in the hippocampus. SIGNIFICANCE STATEMENT: To investigate cellular processes underlying adaptation to social instability, a dominance hierarchy of rats was formed and then destabilized. Regardless of social position, rats from disrupted hierarchies had fewer new neurons in the hippocampus compared with rats from control cages and those from stable hierarchies. Unexpectedly, these changes were accompanied by changes in social strategies without evidence of impairments in cognition or anxiety regulation. Restoring adult neurogenesis in disrupted rats using oxytocin and conditionally suppressing the production of new neurons in socially naive GFAP-thymidine kinase rats showed that loss of 6-week-old neurons may be responsible for adaptive changes in social behavior.

Autism spectrum disorder (ASD) is often associated with cognitive deficits and excessive anxiety. Neuroimaging studies have shown atypical structure and neural connectivity in the hippocampus, medial prefrontal cortex (mPFC), and striatum, regions associated with cognitive function and anxiety regulation. Adult hippocampal neurogenesis is involved in many behaviors that are disrupted in ASD, including cognition, anxiety, and social behaviors. Additionally, glial cells, such as astrocytes and microglia, are important for modulating neural connectivity during development, and glial dysfunction has been hypothesized to be a key contributor to the development of ASD. Cells with astroglial characteristics are known to serve as progenitor cells in the developing and adult brain. Here, we examined adult neurogenesis in the hippocampus, as well as astroglia and microglia in the hippocampus, mPFC, and striatum of two models that display autism-like phenotypes, Cntnap2-/- and Shank3+/DeltaC transgenic mice. We found a substantial decrease in the number of immature neurons and radial glial progenitor cells in the ventral hippocampus of both transgenic models compared with wild-type controls. No consistent differences were detected in the number or size of astrocytes or microglia in any other brain region examined. Future work is needed to explore the functional contribution of adult neurogenesis to autism-related behaviors as well as to temporally characterize glial plasticity as it is associated with ASD.

BACKGROUND: Trauma has neurobehavioral effects when experienced at any stage of development, but trauma experienced in early life has unique neurobehavioral outcomes related to later life psychiatric sequelae. Recent evidence has further highlighted the context of infant trauma as a critical variable in determining its immediate and enduring consequences. Trauma experienced from an attachment figure, such as occurs in cases of caregiver child maltreatment, is particularly detrimental. METHODS: Using data primarily from rodent models, we review the literature on the interaction between trauma and attachment in early life, which highlights the role of the caregiver's presence in engagement of attachment brain circuitry and suppressing threat processing by the amygdala. We then consider how trauma with and without the caregiver produces long-term changes in emotionality and behavior, and suggest that these experiences initiate distinct pathways to pathology. RESULTS: Together these data suggest that infant trauma processing and its enduring effects are impacted by both the immaturity of brain areas for processing trauma and the unique functioning of the early-life brain, which is biased toward processing information within the attachment circuitry. CONCLUSION: An understanding of developmental differences in trauma processing as well as the critical role of the caregiver in further altering early life brain processing of trauma is important for developing age-relevant treatment and interventions.

The medial prefrontal cortex is important for cognitive flexibility, a capability that is affected by environmental conditions and specific experiences. Aversive experience, such as chronic restraint stress, is known to impair performance on a task of cognitive flexibility, specifically attentional set-shifting, in rats. Concomitant with this performance decrement, chronic stress reduces the number of dendritic spines on pyramidal neurons in the medial prefrontal cortex. No previous studies have examined whether a rewarding experience, namely mating, affects cognitive flexibility and dendritic spines in the medial prefrontal cortex of male rats. To test this possibility, we exposed adult male rats to sexual receptive females once daily for one week, assessed attentional set-shifting performance, and then analyzed their brains for changes in dendritic spines. We found that sexual experience improved performance on extradimensional set-shifting, which is known to require the medial prefrontal cortex. Additionally, we observed increased dendritic spine density on apical and basal dendrites of pyramidal neurons in the medial prefrontal cortex, but not the orbitofrontal cortex, after sexual experience. We also found that sexual experience enhanced dendritic spine density on granule neurons of the dentate gyrus. The ventral hippocampus sends a direct projection to the medial prefrontal cortex, raising the possibility that experience-dependent changes in the hippocampus are necessary for alterations in medial prefrontal cortex structure and function. As a first attempt at investigating this, we inactivated the ventral hippocampus with the GABA agonist muscimol, after each daily bout of sexual experience to observe whether the beneficial effects on cognitive flexibility were abolished. Contrary to our hypothesis, blocking hippocampal activity after sexual experience had no impact on enhanced cognitive flexibility. Taken together, these findings indicate that sexual experience enhances medial prefrontal cortex dendritic spine density and cognitive flexibility but that these effects may not require continual input from the hippocampus.

A rapidly growing body of literature indicates that adult neurogenesis in the hippocampus is sensitive to a variety of environmental factors. The effects of emotionally salient experiences, such as stress and physical exercise, have been characterized extensively with regard to both adult neurogenesis and behaviors associated with the hippocampus. Experience-dependent changes in the production and function of new neurons may serve as a means to fine-tune the hippocampus to the predicted environment. Here, we discuss this possibility along with the argument that more naturalistic experimental conditions may be a necessary step toward understanding the adaptive significance of neurons born in the adult brain.