Little is known about the ontogenesis of memory, whether it appears with its full characteristics or whether they emerge progressively with development. In the adult, basic characteristics of memory processing are consolidation of memory after acquisition and reconsolidation after retrieval. Here, using a conditioned aversion paradigm and postlearning or postreactivation injection of a protein synthesis inhibitor, we show that memory acquired by rat pups as early as postnatal day 3 requires time-dependent protein synthesis after both learning and reactivation. These results present the first evidence suggesting that consolidation and reconsolidation are original properties of memory function.
Pregnant rats were subjected to garlic essential oil as the conditioned stimulus and 45 min later to LiCl as the unconditioned stimulus either on embryonic Days 15 and 16 (E15 and E16) or on 18 and 19 (E18 and E19). Control dams received only garlic, LiCl, or water. Progenies were tested on garlic drinking 6 weeks after the exposure to the stimuli via the mothers. In the E18 to 19 group, rats that were exposed to paired garlic-LiCl expressed a significant aversion for garlic. In the E15 to 16 group, no significant differences appeared between subgroups. These results confirm that an associative memory can be established before birth and suggests that this ability potentially emerges in a short time window of 3 days at the end of gestation. Moreover, it appears that a long-term memory can be acquired in utero and retained to be expressed postnatally when animals are autonomous.
The temporal dynamics of consolidation and reconsolidation of taste/odor aversion memory are evaluated during rat pup growth at postnatal days 3, 10, and 18. This is assessed through the temporal gradients of efficacy of a protein synthesis inhibitor (anisomycin) in inducing amnesia after either acquisition (consolidation) or reactivation (reconsolidation). The results show a progressive reduction with age of the delay during which the inhibitor is able to induce amnesia. Control experiments rule out a reduction of anisomycin efficacy due to blood brain barrier growth or decrease in protein synthesis inhibition. Thus, these results present the first evidence that the protein synthesis-dependent phase of memory stabilization requires less time with age. This decrease occurs in parallel for consolidation and reconsolidation. Such changes in the dynamics of memory processing could contribute to the cognitive improvement associated with development.Neurobiological changes occurring during development have been extensively explored, but the functional consequences of these changes for cognition are still not well understood. Research on cognitive development has demonstrated very early learning capacities in vertebrates; different kinds of associative learning may be acquired by neonates and even by the fetus (Smotherman et al. 1982;Kehoe and Blass 1986;Cheslock et al. 2000;Sullivan 2001;Smotherman 2002;Gruest et al. 2004a;Shionoya et al. 2006). Some of these early acquisitions have been shown to be retained in the long term, but the characteristics of long-term memory in the course of development are still poorly known, particularly concerning its stabilization.In the adult, it is well known that memory stabilization is not instantaneous but requires time for the biological processes involved. To be stored in the long term, the memory trace must be consolidated (McGaugh 1966(McGaugh , 2000 and this consolidation requires new protein synthesis (Davis and Squire 1984). Thus, protein synthesis inhibitors given at the time of acquisition, or just afterward, cause amnesia by blocking consolidation. Recently, numerous experiments have enriched this notion by showing that retrieval of a previously consolidated memory induces a new period of lability and that stabilization of the memory in the long term requires a new wave of protein synthesis (for reviews, see Despite the theoretical importance of consolidation and reconsolidation, little is known about the emergence of these characteristics of memory during development. We have recently shown that consolidation and reconsolidation can be detected in the neonate and so may be considered as innate properties of memory processing (Gruest et al. 2004b). However, that does not rule out any possibilities of ontogenetic modifications of these memory characteristics. An indication of this possibility is that retrograde amnesia may be induced in the pup one hour after conditioned aversion training, a delay which has never been observed in the adult in the same kind of learning. Th...
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