Nonredundant roles for hippocampal and entorhinal cortical plasticity in spatial memory storage

Recently, it has been demonstrated that targeted blockade of the extracellular signal-regulated kinase (ERK) cascade in either the entorhinal cortex (EC) or hippocampus (HIP) results in spatial memory deficits. However, it is unclear if ERK-mediated plasticity in these structures has redundant functions or unique roles. In this report, we contrast the role of long-term plasticity in these two structures with side-by-side comparisons of the effects of PD098059 infusion following water maze training. Analysis of performance during the long-term retention test indicates a role for plasticity in the EC in storing broad location information. In contrast, blocking plasticity in the HIP resulted in deficits in indices of precise location information and goal-directed navigational error. To distinguish between a navigational and location deficit, a "two-room" experimental design was employed. Training in the first room allowed animals to consolidate information regarding navigational strategies prior to training and drug infusion in the second room. Hippocampal-PD098059-infused animals demonstrated behavior suggestive of an expanded representation of the platform location and, thus, a loss of precise location information, suggesting that plasticity in these structures is involved in nonredundant, but complementary, processes necessary for spatial memory.