Maternal epigenetic programming of anxiety-like behavior of offspring under developmental predation threat
Date
2024
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Early life stress (ELS), in the form of trauma, abuse, or neglect, has the capacity to increase the propensity for numerous psychopathologies and diseases later in life. However, not all of those who experience ELS go on to develop psychopathology or disease, making elucidating moderator variables a critical research focus. Among moderators, the caregiving relationship may promote either pathology or shift the developmental trajectory toward resilience, dependent on caregiving quality and availability. Indeed, while a deprived caregiver relationship can increase the risk for developing cardiovascular disease, anxiety, or depression, improving the caregiving relationship is known to ameliorate many of the neurobiological and behavioral consequences of ELS exposure. ☐ While underlying mechanisms surrounding these divergent pathologies remain unclear, epigenomics has emerged as a promising candidate. Translating to “above the genome”, epigenomics is a field that seeks to understand how genes are activated or deactivated in different environmental contexts. Among epigenetic markers, previous work from our lab and others have demonstrated aberrant gene expression and DNA methylation associated with deficient caregiving behavior. This suggests that the interaction of caregiving and epigenomics is a sound place to explore how our early life experiences may precipitate psychopathological risk or resilience. ☐ In the present dissertation, we hypothesize that deficiencies in the caregiving relationship, including quality or availability, leads to aberrant anxiety-like and defensive behaviors later in life. Moreover, we hypothesize that the observed aberrant behavior will be paralleled with epigenomic changes in several key brain areas associated with executive function, fear responsivity, and defensive behavior. ☐ To test this, we exposed developing rodents to a chronic predator odor (2,5-dihydro-2,4,5-trimethylthiazoline, TMT, a molecular derivative of fox feces). Previous work using this model determined that exposing dams with their pups to TMT during the first three weeks of life for 20-minutes per day transiently increased maternal caregiving behaviors. Moreover, the progeny displayed dampened fear responses (freezing) upon TMT re-exposure later in life. However, in this work, it is unclear if maternal caregiving (quality or availability) were driving these changes per se, or if it was mere chronic exposure. Additionally, it is unclear if epigenetic programming (gene expression and DNA methylation) was associated with any observed changes in anxiety-like and defensive behaviors. This dissertation sought to address these questions. ☐ Chapters 1-4 of this dissertation serve as a broad introduction to the literature that intersects ELS, psychopathology, and epigenomics. Moreover, these chapters introduce our chronic predator odor (TMT) exposure model and the caregiving relationship as an important moderator for psychopathological or resilient outcomes. Chapters 5 sought to characterize how chamber size and type/dose of odorants alter defensive (freezing) responses in the progeny. Animals were exposed to water, butyric acid (BTA), TMT, or bobcat urine at increasing doses in both a medium and small chamber. Results indicated that TMT, but not bobcat urine, produced a robust freezing response at both a medium and high dose in a medium and small chamber compared to all other odorants. Bobcat urine did produce a marginally increased freezing response compared to water in a small chamber, but this was not different when compared to our noxious control, BTA. Finally, animals froze more when in a small chamber compared to a medium chamber, suggesting that environmental context drives differences in freezing behavior to predator odors. These data expanded the literature of predator odor associated freezing, and provided parameters for further experiments in the dissertation. ☐ Chapter 6 sought to determine how our ELS model of chronic exposure to TMT during the first three postnatal (PN) weeks (PN1-21) alter maternal caregiving behaviors, and defensive and anxiety-like behaviors of the progeny. Rodent dams and pups were exposed either to TMT or BTA (noxious control) with or without their dam present for 20-minutes per day. Results indicated that developmental odor exposure was largely inconsequential in regard to maternal caregiving behavior. In the progeny, developmental odor exposure was similarly inconsequential in regard to freezing to TMT, 40kHz vocalizations, and anxiety-like behaviors later in life. However, maternal separations unexpectedly altered freezing to TMT in a sex-specific manner, increased anxiety-like behavior in the open-field test, and altered infant vocalization patterns. Together, characterizing these infant outcomes in our model helped elucidate any molecular mechanisms in key brain regions that may be associated with ELS exposure and these phenotypic outcomes in later chapters. ☐ Chapter 7 and 8 sought to characterize how gene expression or methylation (respectively) is altered in our aforementioned anxiety-like phenotypes in the medial prefrontal cortex (mPFC), amygdala, and olfactory bulb of several candidate genes. Moreover, to examine if any aberrant gene expression was an immediate consequence of the paradigm or more stable, both PN22 and PN30 tissue were extracted from a separate subset of animals. Results indicated that the expression or methylation of brain-derived neurotrophic factor (Bdnf), transient receptor potential cation channel 1 (Trpa1), or zinc-finger protein 189 (Zfp-189) was altered as a function of environmental conditions created, but expression and methylation were largely in unexpected directions. These data provide potential biomarkers in the literature that can be perturbed or manipulated to examine treatment efficacy, and highlight the importance of experimental parameters on epigenomic and behavioral outcomes. ☐ Finally, chapter 9 highlights the main findings, limitations, and future directions as a function of this dissertation research. Taken together, the data in this dissertation add to a growing body of literature that seeks to understand how environmental stress drives disease risk in the brain. Moreover, these data help inform epigenetic mechanisms that may be measured or manipulated in the treatment of disease to shift developmental trajectories toward resilience.
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Keywords
Amygdala, Anxiety, Developmental odor, Epigenetics, Fear, Stress