Learning twice is different from learning once and from learning more

The rat hippocampus plays a crucial role in the consolidation of a variety of memories, including that for a one trial inhibitory avoidance learning task in which stepping down from a platform is associated with a footshock. Here we show that this is the case regardless of the intensity of the footshock used and hence, of the strength of the learned response. However, additional learning produced by a second training session in this task does not involve the hippocampus but, instead, the striatum. Memory consolidation of the second trial requires glutamate α-amino-3-hydroxy-5-methyl-4-isoxazole propionate, N-methyl-d-aspartate and metabotropic receptors, activation of signaling pathways, gene expression and protein synthesis in the striatum, as are required in the hippocampus during memory consolidation of the first trial.