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During the course of ontogenesis does long-lasting memory emerge progressively or abruptly, and when? To examine this question, rat pups were conditioned at different ages (3-, 10-, 12-, 15- or 18-day-old) and tested at different retention intervals: from 3 days to 1 year. Conditioned aversion memory established before 12-day-old lasts for only 1 week, but when acquired after 15 days, memory survives for more than 1 year. This defines a short temporal window of 3 days for sudden emergence of a remote memory. Our result offers a precise temporal target to explore the mechanisms involved in long maintenance of memory., (Copyright 2009 Elsevier B.V. All rights reserved.)

The ability to form long-term memories exists very early during ontogeny; however, the properties of early memory processes, brain structures involved and underlying cellular mechanisms are poorly defined. Here, we examine the role of extracellular signal-regulated kinase (ERK), a member of the mitogen-activated protein kinase/ERK signaling cascade, which is crucial for adult memory, in the consolidation and reconsolidation of an early memory using a conditioned taste aversion paradigm in 3-day-old rat pups. We show that intraperitoneal injection of SL327, the upstream mitogen-activated protein kinase kinase inhibitor, impairs both consolidation and reconsolidation of early memory, leaving short-term memory after acquisition and after reactivation intact. The amnesic effect of SL327 diminishes with increasing delays after acquisition and reactivation. Biochemical analyses revealed ERK hyperphosphorylation in the amygdala but not the hippocampus following acquisition, suggesting functional activation of the amygdala as early as post-natal day 3, although there was no clear evidence for amygdalar ERK activation after reactivation. These results indicate that, despite an immature brain, the basic properties of memory and at least some of the molecular mechanisms and brain structures implicated in aversion memory share a number of similarities with the adult and emerge very early during ontogeny.

Ontogenetic modification of an early memory is relatively poorly understood. And an important question is whether the memory output is more determined by the age at acquisition or at retention? Here we explore the expression of odor-shock conditioning in the rat pup. Acquisition at post-natal day 6 (P6) leads to an approach response and at post-natal day 12 (P12) to an avoidance response when the retention test is 24h later. In both cases, anisomycin injected immediately post-acquisition induced a retrograde amnesia. Controls show that, in either case, short-term memory measured 4h after acquisition is not impaired and that anisomycin given after a 4h delay has no effect. Thus, at the two ages, memory involves a consolidation process. The main result is the spontaneous reversal of the conditioned response from approach acquired at P6 to avoidance when tested at P13. This phenomenon is robust as it is observed in three conditions. Moreover, amnesia induced at P6 is maintained at P13. Results are discussed in terms of maturation and/or competition of the memory traces.