Characteristics of retrograde amnesia for CS preexposure.

Two experiments using rats evaluated the susceptibility of CS preexposure to retrograde amnesia induced by the protein synthesis inhibitor cycloheximide and tested whether amnesia for CS preexposure shares similar characteristics with amnesia for other memories. In Experiment 1, rats received cycloheximide either immediately, 60 minutes, or 120 minutes after preexposure. Following preexposure, rats received fear conditioning. When later tested, the subjects that received the amnestic treatment shortly after preexposure showed no CS preexposure effect (i.e., no reduction of fear). The amnesia for CS preexposure was attenuated with longer post-preexposure delays, showing a temporal gradient. In Experiment 2, following the replication of amnesia for CS preexposure, the amnestic treatment was readministered to the rats prior to testing. It was demonstrated that the amnestic-preexposure memory could be recovered (i.e., readministration of the drug alleviated the amnesia for CS preexposure). These two experiments show that memories for CS preexposure are susceptible to retrograde amnesia and share similar characteristics with memories for original acquisition and extinction. The results are explained using a retrieval hypothesis of retrograde amnesia.