Cellular imaging of zif268 expression in the hippocampus and amygdala during contextual and cued fear memory retrieval: Selective activation of hippocampal CA1 neurons during the recall of contextual memories

The neuroanatomical and molecular basis of fear memory retrieval was studie d by analyzing the expression of the plasticity-associated immediate early gene zif268. Cellular quantitative in situ hybridization revealed that zif2 68 is expressed within specific regions of the hippocampus and amygdala dur ing fear memory retrieval. Within the hippocampus, increased expression of zif268 was observed within CA1 neurons, but not dentate gyrus neurons, duri ng the retrieval of contextual, but not cued, fear associations. In contras t, zif268 expression was increased within neurons of the amygdala (lateral, basal, and central nuclei) during the retrieval of both contextual and cue d fear memories. These results demonstrate activation of hippocampal CA1 ne urons in contextual fear memory retrieval that was not merely a correlate o f the behavioral expression of fear itself, because it was limited to the r etrieval of contextual, and not cued, fear memories. Further studies reveal ed that the selective increase in hippocampal CA1 zif268 expression seen af ter contextual fear memory retrieval was limited to the retrieval of recent (24 hr) but not older (28 d) memories. These experiments represent the fir st demonstration that zif268 expression in specific neuronal populations is associated with memory retrieval and suggest that this gene may contribute to plasticity and reconsolidation accompanying the retrieval process.