The Molecular Substrates of Second-Order Conditioned Fear in the Basolateral Amygdala Complex

Rats quickly learn to fear a stimulus (e.g., a light) that signals brief but painful footshock. The consolidation of this first-order conditioned fear requires transcription and translation of specific genes in the basolateral amygdala complex (BLA). Rats also learn to fear the associates of first-order conditioned stimuli, such as a sound paired with the already-conditioned light. The consolidation of this second-order conditioned fear also requires transcriptional processes in the BLA, but unlike consolidation of first-order fear, it is unaffected by BLA infusions of the protein synthesis inhibitor, cycloheximide. Accordingly, this study sought to identify genes/pathways that regulate second-order conditioned fear in male rats. It focused on the involvement of immediate early genes, as these can be transcribed in the presence of protein synthesis inhibitors. In Experiment 1, we used a principal components analysis of PCR data and found that second-order fear conditioning involves region- and time-specific changes in a network of genes including the immediate early genes <i>bdnf</i> and <i>egr1</i> In Experiments 2-4, we used BLA infusions of antisense oligonucleotides and found that <i>bdnf</i> is required for consolidation of both first- and second-order fears but is not required for reconsolidation of either and <i>egr1</i> is not required for consolidation of first-order fear but is required for acquisition of second-order fear and reconsolidation of both types of fear. Hence, <i>bdnf</i> and <i>egr1</i> regulate different aspects of first- and second-order fear conditioning in the BLA. These findings are discussed with respect to novel forms of information processing in the mammalian brain.