Memory consolidation and reconsolidation in the rat pup require protein synthesis

Abstract

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.

Figure 1.

Effect of anisomycin given during training. Percentage of time attachment to the surrogate nipple on the test for the six groups of rat pups. The lower the index, the more the aversion. Pups were trained on P3 and tested on P6. Pups of group CS-US (n = 11) received aromatized milk paired with LiCl, and those of CS-AN-US (n = 10) received anisomycin between CS and US. Two control conditions were used: aromatized milk alone (CS; n = 10) and LiCl preceding milk from 10 hr (US-CS; n = 12). The effect of AN was tested in these two conditions (CS-AN; n = 12) and (US-CS-AN; n = 9). Error bars indicate SE. ^***p < 0.001. Note that anisomycin abolishes the conditioned aversion.

Figure 2.

Effect of anisomicin given after training. Pups of group CS-US (n = 10) received aromatized milk paired with LiCl without anisomycin treatment. In the other paired groups, pups received anisomycin at different delays, either immediately (AN0; n = 11) or 15 min (AN15; n = 9), 1 hr (AN1h; n = 9), 6 hr (AN6h; n = 9), or 24 hr (AN24h; n = 10) after training. Two groups were submitted to an unpaired condition with a long delay between aromatized milk presentation and LiCl either without anisomycin (CS-US; n = 14) or followed by anisomycin (CS-US+AN; n = 13). Error bars indicate SE. ^**p < 0.01. Note that anisomycin abolishes the conditioned aversion at 0 and 15 min delay; at 6 and 24 hr it has no more effect.

Figure 3.

Effect of anisomycin given after reactivation. After training on P3, pups were cued on P5 by a brief presentation of aromatized milk followed by anisomycin treatment at different delays; test was on P7. One group was paired and reactivated without anisomycin (CS-US/R; n = 11), and the other ones received anisomycin either immediately (AN0; n = 12) or 15 min (AN15; n = 12), 30 min (AN30; n = 12), 1 hr (AN1h; n = 12), 6 hr (AN6h; n = 12), or 24 hr (AN24h; n = 12) after cueing. A control group was unpaired but reactivated (CS-US/R; n = 12), and another one was paired and received anisomycin on P5, but was not reactivated (CS-US/nonR+AN; n = 12). Error bars indicate SE. ^***p < 0.0001. Note that anisomycin is able to impair conditioned aversion 48 hr after training but only if preceded by reactivation, and its effect is time-dependent.