ACTH- and noncontingent footshock-induced recovery of an extinguished passive avoidance response.

The effectiveness of both exogenous adrenocorticotropic hormone and noncontingent footshock as agents for the recovery of an extinguished passive avoidance response was assessed. Six groups of male Holtzman rats were administered either adrenocorticotropic hormone (4IU), or a single noncontingent footshock, either 15 minutes, 24 hours, or 7 days prior to retention testing. Two control groups received an injection of the inactive gel vehicle either 15 minutes or 7 days prior to test. Adrenocorticotropic hormone administered either 15 minutes or 24 hours prior to test, as well as noncontingent footshock delivered 24 hours (but not 15 minutes) prior to test, served as effective reinstatement agents. Substantial "spontaneous recovery" of responding precluded evaluation of reinstatement effects at the 7 day retention interval. The persistence of the recovery for at least 24 hours is consistent with the notion that both adrenocorticotropic hormone and noncontingent footshock not only enhance memory retrieval of the original fear conditioning, but also cause the training memory to be further processed.