Differential sensitivity of recent vs. remote memory traces to extinction in a water-maze task in rats

Extinction has mostly been studied in conditioning paradigms, more sparsely in spatial tasks, and never as a function of the age of a spatial memory. Using rats, we compared the time-course of extinction of a recent (5 days) vs. remote (25 days) spatial memory in a water maze, over three probe trials. When the trials were set 24h apart, performance in the remote memory group was significantly worse on the first probe trial and significantly better on the third probe trial, as compared to the recent memory group, thereby showing differences between cognitive operations underlying recent vs. remote memory extinction. In contrast, when trials were given consecutively, both groups showed a similar profile of extinction. Furthermore, in a room with overly-salient cues providing a strong remote memory trace, no difference between groups was observed when the spaced extinction paradigm was used. These results might be related to a balance between reconsolidation and extinction processes occurring after a first retrieval experience, of which the outcome may depend on the extinction protocol, and on the age and strength of a memory.