Role of the amygdalo-hippocampal transition area in the fear expression: evaluation by behavior and immediate early gene expression.

Using pre- and post-training lesions of the amygdalo-hippocampal transition area (AHi), the role of the AHi in the fear conditioning of rats was examined. Pretraining lesions by N-methyl-d-aspartate led to the enhancement of freezing behavior in auditory fear conditioning and contextual conditioning. However, the freezing of post-training-lesioned rats did not differ from that of the sham-lesioned rats. There were several regions of the brain observed in this study in which c-Fos and/or Egr-1 immunoreactive-positive cell expression changed in diverse manners after the test session. In the pretraining lesioned rats that were trained for auditory conditioning, the number of c-Fos and Egr-1 decreased in the infralimbic cortex (IL) and the number of Egr-1 increased in the basomedial amygdaloid nucleus (BM). In the pretraining AHi-lesioned rats that were trained for contextual conditioning, the number of c-Fos increased in the lateral periaqueductal gray (LPAG) and the number of Egr-1 increased in the BM. These results suggest that the AHi plays an important role in the acquisition of memory during conditioning alone, whereas it is improbable that the AHi had an effect on consolidation, retrieval, and expression in the case of either auditory or contextual fear conditioning. The findings also suggest that the freezing behavior was related to the changes in c-Fos and/or Egr-1 in the IL, BM, and LPAG. As in the case of the BM, the number of Egr-1 immunoreactive-positive cells was increased in both experiments, and it was possible that the activation of neurons with high basal levels of expression might be associated with memory retrieval or expression as a freezing behavior observed in the test session.