Eight experiments examined the role of the intertrial interval in the extinction of conditioned fear to a context. Rats were shocked in one context (A) but not in another (B) and freezing responses to Context A were extinguished. The interval between extinction trials was spent in the home cages. Experiments 1a and 1b showed that massed extinction trials produced better response loss but worse learning than spaced trials. Experiment 2 demonstrated that the interval between the final extinction trial and test mediated the level of responding on a test exposure. Experiments 3 and 4 showed that the duration of the extinction trial affected long term response loss, whereby long durations facilitate response loss compared to shorter durations. Subsequent experiments (Experiments 5 to 8) demonstrated that the first in the series of massed extinction trials reduced the associability of subsequent trials. Associability was restored by alternating extinction trials between Context A and Context B. The results are discussed in terms of the role accorded to self-generated priming in the models developed by A. R. Wagner (1978; 1981).

Eight experiments examined the role of the intertrial interval in the extinction of conditioned fear to a context. Rats were shocked in one context (A) but not in another (B) and freezing responses to Context A were extinguished. The interval between extinction trials was spent in the home cages. Experiments 1a and 1b showed that massed extinction trials produced better response loss but worse learning than spaced trials. Experiment 2 demonstrated that the interval between the final extinction trial and test mediated the level of responding on a test exposure. Experiments 3 and 4 showed that the duration of the extinction trial affected long term response loss, whereby long durations facilitate response loss compared to shorter durations. Subsequent experiments (Experiments 5 to 8) demonstrated that the first in the series of massed extinction trials reduced the associability of subsequent trials. Associability was restored by alternating extinction trials between Context A and Context B. The results are discussed in terms of the role accorded to self-generated priming in the models developed by A. R. Wagner (1978; 1981).

Typescripe (photocopy). / Digitized by Kansas Correctional Industries

Enhancing stress resilience could prevent a variety of stress-induced disorders and thus reduce the global burden of psychiatric disease. It was previously reported that a single administration of the N-methyl-D-aspartate receptor (NMDAR) antagonist (R,S)-ketamine prior to stress could prevent stress-induced fear and behavioral despair in male mice, suggesting the possibility of developing prophylactic drugs to prevent psychiatric disorders. However, it was still unknown whether prophylactic agents could be effective in female mice, if other drugs could exert prophylactic actions, and how prophylactics could alter mechanisms in the brain to increase stress resilience. We hypothesized that targeting different receptors that could significantly alter neuroplasticity in the hippocampus (HPC) would be protective against stress. We first sought to determine whether stereospecific metabolites of (R,S)-ketamine, (2S,6R)-hydroxnorketamine ((2S,6S)-HNK) and (2R,6R)-hydroxynorketamine ((2R,6R)-HNK), could attenuate stress-induced behaviors in male and female mice. Next, we aimed to test the prophylactic efficacy of fluoroethylnormemantine (FENM), a novel-composition NMDAR antagonist derived from memantine. Subsequently, we examined the protective effects of 3 different serotonin type IV receptor (5-HT4R) agonists against stress. Finally, we sought to determine whether dual targeting of the NMDAR and 5-HT4R could exert additive effects in enhancing resilience to stress against a wide variety of stress-induced behaviors. Drug efficacy was assayed in male and female mice using a battery of stress models and behavioral tests including: contextual fear conditioning (CFC), cued fear conditioning, contextual fear discrimination (CFD), learned helplessness (LH), chronic immobilization stress (CIS), paired-pulse inhibition (PPI), the forced swim test (FST), sucrose splash test (SST), open field (OF), elevated plus maze (EPM), marble burying (MB), and novelty-suppressed feeding (NSF). Liquid chromatography-mass spectrometry (LC-MS), immunohistochemistry, Western blotting, patch clamp electrophysiology, ovariectomy (OVX), and hormone replacement techniques were used to examine how prophylactic drugs alter brain function and test whether ovarian hormones mediate the protective effects of prophylactic compounds in female mice. We show that (R,S)-ketamine, (2S,6S)-HNK, (2R,6R)-HNK, FENM, and 3 different 5-HT4R agonists are protective against specific stress-induced behaviors when administered in both male and female mice. We demonstrate that multimodally targeting NMDARs and 5-HT4Rs can broadly enhance resilience to protect against a variety of stressinduced maladaptive behaviors in both sexes. Finally, we determine a common mechanism by which various prophylactic compounds, despite targeting different receptors, attenuate bursts of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated activity to exert their protective effects. Together, these data: 1) uncover a variety of novel drug candidates for further preclinical and clinical development, 2) indicate a potential neural substrate underlying resilience to stress, and 3) reveal neurobiological mechanisms contributing to the psychopathology of psychiatric disease.

Early life adversity (ELA) produces long lasting developmental changes to the postnatal brain, increasing predisposition to a number of physical and psychiatric disorders. The mechanisms through which ELA is able to create lasting detrimental changes to neuronal development remains unclear. This thesis tested the hypothesis that increases in fear generalization, a common symptom in psychiatric disorders, follows ELA exposure in age dependent and sexually dimorphic ways in alignment with the findings of clinical studies. The effects of ELA often impact fear circuitry and we confirmed, using electrophysiology and tissue imaging, that 5-HT circuitry from the median raphe nucleus (MRN), integral to fear response, was impaired following ELA. Using a transgenic mouse model that allows for modulation of serotonergic release, we showed that circumventing serotonergic pathways disrupted by ELA and increasing whole brain 5-HT release was enough to rescue hippocampal dependent fear responses and fear generalization. Involvement of the hippocampus in ELA effects, particularly the ventral dentate gyrus (vDG), in fear overgeneralization was confirmed as hyperactivity in thevDG following exposure to novel contexts was rescued by increased 5-HT release. In addition to ELA-induced hyperactivity of the vDG, known to potentiate stress susceptibility, I demonstrated that ELA resulted in an increase in passive coping strategies, HPA axis dysfunction and elevated stress hormone release. These effects were seen predominantly in adult females and rescued in those with increased 5-HT release. Together these data suggest that increased predisposition to psychiatric disorders following ELA exposure involves the disruption of fear circuitry regulated by 5-HT activity. Identifying the underlying circuits altered by ELA not only provides insight about disrupted postnatal brain development, but also increases our knowledge of the timeline, trajectory and factors affecting healthy postnatal brain development.

Experiencing early life adversity (ELA) increases the risk of anxiety disorders, such as generalized anxiety disorder and post-traumatic stress disorder, with disproportionally higher risk in women compared to men. Neurodevelopmental and behavioral outcomes following ELA are multifaceted and are influenced heavily by the type of adversity experienced and sex of the individual. A major contributor to emotional dysfunction and anxiety disorders resulting from ELA are changes in fear and threat circuitry. Children who experienced ELA have been reported to show an accelerated development of the amygdala, a region involved in processing threat, and greater cerebrospinal levels of corticotrophin releasing hormone (Crh), an orchestrator of neuroendocrine and behavioral responses to stress. Work in rodents have linked Crh signaling within the lateral central amygdala (CeAL) with processing and responding to threat, core features disrupted in anxiety-related disorders. Further, sex biases in risk and symptom presentation have been proposed to be related to sexual dimorphic signaling of Crh across the brain that differentially influence a variety of Crh-dependent behaviors. However, it remains unclear what properties of ELA portend differential neurobiological risk, what is the basis of sex-differences for negative outcomes, and how specific mechanistic changes give rise to certain endophenotypes. In this work, I use genetic, cellular, and behavioral approaches to explore the impact of ELA and sex on perinatal development in mice and the functional consequences of altered Crh neuron activity in the CeAL on threat responding in adulthood. In Chapter 1, I review how factors such as sex and type of ELA influence amygdala development and Crh. In Chapter 2, I assess the impact of two forms of ELA, maternal separation (MS) and limited bedding and nesting (LBN) on perinatal development and anxiety-like behavior. Both forms of ELA shifted the timing of somatic maturation and basal CORT levels and led to increased anxiety-like behaviors, but the degree of the impact depended on the sex and type of adversity experienced. In Chapter 3, I demonstrate that a distinguishing feature between types of ELA was the predictability of maternal care. The type of ELA also contributed to sex-differences in Crh related gene expression in the perinatal amygdala. Increased expression was primarily observed in males following MS and in females following LBN. In Chapter 4, I investigate the functional consequences of ELA in the form of LBN on the activity of CeALCrh+ neurons in vivo and their causal role in threat reactivity indexed by the startle response. LBN rearing led to sustained activity of CeALCrh+ in female mice but diminished in male mice. Persistent activity of this population was necessary for and predicted the magnitude of startle responding. In Chapter 5, I discuss important considerations when integrating new advancements in the study of ELA and the use of sex as a biological variable. Collectively, this work deepens our understanding of the neurobiological mechanisms impacted by sex and ELA and holds promise for future strategies that may consider the sex and specific experiences of the individual to target specific endophenotypes and address the underlying root causes of anxiety disorders.