The expression of allocentric object-place recognition memory during development.

The allocentric representation of space is a fundamental pillar of episodic experience. In infant rats, the neural circuitry underlying the formation of allocentric spatial representations is functioning from early on when eyes are opening, i.e., before postnatal day (PD) 15. However, it remains unclear when and how during early development rats use these representations to regulate spatial behavior. Here, we studied indicators of memory-based spatial navigation using a classical object-place recognition (OPR) task set-up in infant (PD15), pre-weanling (PD18), juvenile (PD25), peri-adolescent (PD31), adolescent (PD38, PD48), and young adult rats (PD84). On the task, rats explored an arena with two identical objects, and memory was tested in a recall phase 3 h later in the same arena with, one object displaced from its original location. Only at adolescence (PD38), rats showed the typical adult-like expression of allocentric spatial memory with a preferential exploration of the object at the novel location. However, the first expression of allocentric spatial memory was revealed already in PD18 rats, which contrasting with PD84 rats, showed a preference to explore the object at the familiar location. At PD31, rats showed a null preference between the object-locations. Nevertheless, spatial memory at this age expressed in a preference for the zone including the familiar object-location. In PD15 rats, we found no evidence for a memory-based organization of spatial behavior. In conclusion, although rats might be able to form allocentric neuronal representations of space already earlier, only from PD18 on, such representations are used to organize spatial behavior, with a motivational shift from familiarity to novelty-driven navigation occurring during adolescence.