Distinct gene expression profiles in hippocampus and amygdala after fear conditioning.

It is well known that the hippocampus and amygdala are involved in the formations of fear conditioning memories, and both contextual and cued fear memory requires activation of the NMDA receptors. However, the global molecular responses in the hippocampus and amygdala have not been investigated. By applying high-density microarrays containing 11,000 genes and expressed sequence tags, we examined fear conditioning-induced gene expression profiles in these two brain regions at 0.5, 6, and 24 h. We found that 222 genes in the amygdala and 145 genes in the hippocampus showed dynamic changes in their expression levels. Surprisingly, the overall patterns of gene expression as well as the individual genes for the amygdala and hippocampus were drastically different and only small number of genes exhibited the similar regulation in both brain regions. Based on expression kinetics, the genes from the amygdala can be further grouped into eight unique clusters, whereas the genes from the hippocampus were placed into six clusters. Therefore, our study suggests that different genomic responses are initiated in the hippocampus and amygdala which are known to play distinct roles in fear memory formation.