Updating memories: Changing the involvement of the prelimbic cortex in the expression of an infant fear memory

Recent work has found that infant rats (postnatal day (P) 18) do not require the prelimbic cortex (PL) to express learned fear, whereas older animals (adults and juveniles) do. In other words, there is a switch from a PL-independent fear expression system during infancy to a PL-dependent system later in life. The present study investigated whether the PL would be involved in fear expression in rats trained at P17 but tested at P23 (that is, as juveniles). The first two experiments showed that PL involvement in fear expression was determined by the age of the animal at the time of training rather than the animal's age at the time of test. More specifically, experiment 1 showed that expression of learned fear (measured by freezing, and elicited by a white noise previously paired with a shock) was PL-independent for memories that were acquired when the rat was P17 but then tested at P23. In experiment 2, rats trained at P23, when the PL is functionally mature, still required the PL to express fear when tested at P37. In the last experiment, using two different reactivation procedures, we showed that it is possible to update an infant memory and switch it from being PL-independent to being PL-dependent. Combined, these results have important implications for our understanding of the neural circuitry underlying fear expression across development and show that, at least in some cases, expression of fear responses learned early in life remain PL-independent even as the animal matures. © 2012.