Differential regulation of glutamic acid decarboxylase gene expression after extinction of a recent memory vs. intermediate memory

Extinction reduces fear to stimuli that were once associated with an aversive event by no longer coupling the stimulus with the aversive event. Extinction learning is supported by a network comprising the amygdala, hippocampus, and prefrontal cortex. Previous studies implicate a critical role of GABA in extinction learning, specifically the GAD65 isoform of the GABA synthesizing enzyme glutamic acid decarboxylase (GAD). However, a detailed analysis of changes in gene expression of GAD in the subregions comprising the extinction network has not been undertaken. Here, we report changes in gene expression of the GAD65 and GAD67 isoforms of GAD, as measured by relative quantitative real-time RT-PCR, in subregions of the amygdala, hippocampus, and prefrontal cortex 24-26 h after extinction of a recent (1-d) or intermediate (14-d) fear memory. Our results show that extinction of a recent memory induces a down-regulation of Gad65 gene expression in the hippocampus (CA1, dentate gyrus) and an up-regulation of Gad67 gene expression in the infralimbic cortex. Extinguishing an intermediate memory increased Gad65 gene expression in the central amygdala. These results indicate a differential regulation of Gad gene expression after extinction of a recent memory vs. intermediate memory.