Encoding of Fear Memory in High and Low Fear Mice

Abstract

Abstract : Purpose: Determine in a mouse model of high and low fear whether a protein expressed in the lateral amygdala is more abundant in the high fear mice following Pavlovian fear conditioning. Design: Behavioral experiments: compare freezing in naive, tone alone, and Pavlovian fear conditioned mice during training, context test and cue test. Pharmacological inhibition of pMAPK. Measure of key protein: quantify pMAPK-expressing neurons in the lateral amygdala in naive, tone alone, and fear conditioned mice and create density plots of those pMAPK expressing neurons for comparison across groups. Methods: Pavlovian fear conditioning, immunohistochemistry, Western blot, pharmacologically inhibit pMAPK via intraperitoneal injection prior to fear conditioning. Sample: adult male High and Low Fear Phenotype mice. Analysis: Freeze Frame automated scoring system verified by investigator scoring of freezing. Neurolucida software for quantification of pMAPK expressing neurons in the lateral amygdala; Origin software for the creation of density plots. GraphPad Prism for statistical analysis (ANOVA and t-tests). Findings: High fear phenotype mice exhibit greater freezing (fear) compared to low fear mice during training, context test, and cue test. High fear phenotype mice have more pMAPK expressing neurons in a discrete subregion of the lateral amygdala called the dorso-lateral amygdala (LAd) following Pavlovian fear conditioning. No differences exist between controls. Fear conditioned mice (high and low fear) have more pMAPK expressing neurons in the Lad compared to controls. Inhibition of pMAPK prior to fear conditioning reduces high fear freezing to the level of (and below) low fear freezing. Fear conditioned high fear mice have a unique pattern of pMAPK expressing neurons compared to low fear mice. Implications for Military Nursing: Improved understanding of the cellular and molecular mechanisms that underlie high fear, (a component of PTSD) will aid in the treatment.