It’s all about who you know: Memory retention of a rat’s cagemates during infancy negatively predicts adulthood hippocampal FGF2

Abstract

Recent research has demonstrated that individual differences in infant fear memory positively predict adulthood anxiety-like behavior and conditioned fear expression. However, the physiological mechanisms underlying this relationship and the effect of environmental (e.g., social) influences on the stability of this relationship have not been explored. In the present study, we examined whether individual differences in infant fear memory predict levels of endogenous fibroblast growth factor-2 (FGF2; a biomarker of fear/anxiety) in adulthood, and whether the mean memory retention of a rat’s cagemates predicts conditioned fear expression and FGF2 in adulthood. We conditioned infant rats to associate a white noise with shock, and tested their memory of the association 1 week later. They were then weaned and randomly assigned to cage/cagemates. In adulthood, rats received weak context conditioning (i.e., a single shock) and were tested for fear of the context the following day. Rats were then euthanized and their brains extracted to measure levels of hippocampal FGF2 protein. Across 2 experiments, an individual rat’s fear memory during infancy positively predicted their own fear expression in adulthood, but the mean memory retention of their cagemates did not predict fear expression. In contrast, the mean memory retention of a rat’s cagemates during infancy negatively predicted hippocampal FGF2 protein in adulthood, but an individual rat’s memory retention did not predict their own levels of FGF2. These data support the idea that variations in the fearfulness of a rat’s cagemates predict individual differences on physiological measures in adulthood.