Impact of Early Life Stress on Safety Learning in Adulthood

Adverse childhood experiences (ACEs) are potentially traumatic experiences that occur in childhood that can affect brain development. Three in four high school students are reported to having experienced one or more ACEs. ACEs have detrimental effects on prefrontal-hypothalamic-amygdala circuit pathways and can increase the likelihood of developing PTSD in adulthood. ACEs can be modeled as reproducible acute early life stress (aELS) exposure to foot shocks in rodents during early development. Foot shocks in rodents induce anxiety, hyperarousal and PTSD symptoms. This study was based on Wang et al. (2024)'s study that proposed a circuit mediating safety learning/expression that involves the paraventricular nucleus of the thalamus (PVT), the prelimbic cortex (PL), and the amygdala (AMY). Activation of this PVT-PL-AMY circuit drives threat responding, so safety learning/expression requires reduced activity within this circuit. This study analyzes the effects of aELS in safety learning in adulthood and cell activity in the proposed circuit.

Research Question

How does aELS affect adult safety learning and cell activity in certain brain regions?

Hypothesis

Female rats exposed to aELS will show more fear and have a lower safety score compared to male rats exposed to aELS. Additionally, female rats with exposure to aELS will have higher cell activity in PVT and AMY compared to male rats with no aELS.

Early Life Stress

On postnatal day 17 (PND17), rats are transported to a conditioning chamber (context A), then receive zero or 15 foot shocks (1s, 1mA) over 93 minutes.

Safety Training

From PND90, rats received two days of habituation in a novel conditioning chamber (context B), then undergo 3 days of 12 minutes of safety training with unpaired footshocks (1mA) and a safety cue (30 second tone (75dB) with a flash of white light for the first second of the tone) signaling no footshock.

Safety Testing

Rats are tested the next day by presenting the safety cue without footshock in the same context that they received training. Rats demonstrate fear to the context alone. A safety index is calculated and analyzed by comparing freezing behavior during the 30 second context-only period before each cue and each 30-second safety cue period.

Intracardial Perfusion and Brain Harvesting

90 minutes after behavioral testing, rats were intracardially perfused with phosphate buffered saline and 4% paraformaldehyde. Brains were fixed, extracted, and stored for immunohistochemical analyses. 

There is an aELS and sex difference between rats. Our current findings show that rats are freezing less throughout the safety training and in the safety test. Female rats exposed to aELS in particular freeze more than male rats exposed and not exposed to aELS based on the three days of safety training and the safety test following training.