Retrieval and Savings of Contextual Fear Memories Across an Extended Retention Interval in Juvenile and Adult Male and Female Rats

Abstract

Adult rodents exhibit an exceptional ability to retrieve context fear memories across lengthy retention intervals. In contrast, these same memories established in juvenile rodents are susceptible to significant forgetting. While much of this original work has focused on male rodents, emerging research indicates that the mechanisms of context fear conditioning in females may be sexually dimorphic. If so, these female memories may exhibit a unique mnemonic profile from male rats that may be probed using single-trial conditioning, extended retention intervals, and savings procedures. The current study aimed to examine the longevity of contextual fear memories established in 24-day-old juvenile and 90-day-old Long-Evans male and female rats. Testing at the 1-day retention interval, adults exhibited greater conditioning than juvenile males, while in females, conditioning did not differ between juvenile and adult rats. In adults, males displayed greater conditioning than females, while in juveniles, males and females reached similar conditioning levels. At the 60-day retention interval, adult sex differences were maintained; however, juvenile rats failed to retrieve this remote contextual fear memory. Next, we examined whether a savings trial and test procedure could recover these remotely established juvenile memories in both males and females. Following a 60-day retention test, the now adult rats were presented with an additional context-shock pairing to assess the level of saving in both male and female rats. While this procedure produced greater conditioning in males than females, the relative amounts of the savings of this early life memory were similar in males and females. Lastly, the recovery of this memory exhibited significant generalization relative to previously unshocked juveniles and was similar in males and females. The results of these experiments indicate that adult sex differences in contextual fear memory are maintained across an extended retention interval, while in juveniles, there were no significant sex differences. In addition, male and female rats failed to retrieve an initial juvenile memory at an extended retention interval. However, these memories could be recovered with a single reminder of the original juvenile experience.