Post-training memory facilitation blocks extinction but not retroactive interference

Rats were trained in a step-down inhibitory avoidance task using a 0.2-mA footshock and tested 6 h later. Exposure to a session of extinction (animals placed on the box where they had been trained and left to explore it freely for 100.0 s without footshocks) or to a series of 10 tones presented in another box, in the dark, 2 h after training, hindered retention test performance. The immediate post-training ip administration of epinephrine (5.0 micrograms/kg), ACTH-(1-24) (0.2 microgram/kg), or lysine-vasopressin (10.0 micrograms/kg) facilitated retention test performance and cancelled the effect of extinction, but not the retroactive interference caused by exposure to the tones. These results support the concept that post-training facilitation induced by the hormones is due to a strengthening of the memory trace left by the avoidance task, whereas the retroactive interference caused by the tones occurs independently from that process and is more likely due to the incorporation of postevent information.