Studies on human and animals shed light on the unique hippocampus contributions to relational memory. However, the particular role of each hippocampal subregion in memory processing is still not clear. Hippocampal computational models and theories have emphasized a unique function in memory for each hippocampal subregion, with the CA3 area acting as an autoassociative memory network and the CA1 area as a critical output structure. In order to understand the respective roles of the CA3-and CA1-hippocampal areas in the formation of contextual memory, we studied the effects of the reversible inactivation by lidocaine of the CA3 or CA1 areas of the dorsal hippocampus on acquisition, consolidation, and retrieval of a contextual fear conditioning. Whereas infusions of lidocaine never impaired elementary tone conditioning, their effects on contextual conditioning provided interesting clues about the role of these two hippocampal regions. They demonstrated first that the CA3 area is necessary for the rapid elaboration of a unified representation of the context. Secondly, they suggested that the CA1 area is rather involved in the consolidation process of contextual memory. Third, they showed that CA1 or CA3 inactivation during retention test has no effect on contextual fear retrieval when a recognition memory procedure is used. In conclusion, our findings point as evidence that CA1 and CA3 subregions of the dorsal hippocampus play important and different roles in the acquisition and consolidation of contextual fear memory, whereas they are not required for context recognition.Studies in higher primates and humans have led to the idea that the hippocampus is required for different types of memory, such as declarative (Squire 1992) or episodic memory (Tulving 1983). Independently of the type of memory, the hippocampus might be engaged in different memory processes, such as encoding, consolidation, and retrieval. Lesion studies in animals provide further support for the transient requirement of the hippocampus in the process of long-term memory formation Anagnostaras et al. 1999) in agreement with the theory of Squire (1992), describing the hippocampus as a temporary memory buffer that enlists the prefrontal cortex, where the information is ultimately stored.The hippocampal structure is functionally heterogeneous, with different portions of the longitudinal axis having different functional roles, certainly due to differences in connectivity (Moser and Moser 1998). Indeed, the dorsal hippocampus seems to be highly involved in spatial learning (Moser et al. 1993(Moser et al. , 1995, which is consistent with the major visuo-spatial inputs received from the temporal and parietal cortices, whereas the ventral hippocampus presents a strong connectivity with both the hypothalamus and the amygdala, which potentially accounts for some effects of the hippocampal lesion on emotionality (Kjelstrup et al. 2002). Moreover, all hippocampal subregions are highly interconnected, and their arrangement suggests that, individually, they may su...

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Cortical and hippocampal hypersynchrony of neuronal networks seems to be an early event in Alzheimer’s disease pathogenesis. Many mouse models of the disease also present neuronal network hypersynchrony, as evidenced by higher susceptibility to pharmacologically-induced seizures, electroencephalographic seizures accompanied by spontaneous interictal spikes and expression of markers of chronic seizures such as neuropeptide Y ectopic expression in mossy fibers. This network hypersynchrony is thought to contribute to memory deficits, but whether it precedes the onset of memory deficits or not in mouse models remains unknown. The earliest memory impairments in the Tg2576 mouse model of Alzheimer’s disease have been observed at 3 months of age. We thus assessed network hypersynchrony in Tg2576 and non-transgenic male mice at 1.5, 3 and 6 months of age. As soon as 1.5 months of age, Tg2576 mice presented higher seizure susceptibility to systemic injection of a GABAA receptor antagonist. They also displayed spontaneous interictal spikes on EEG recordings. Some Tg2576 mice presented hippocampal ectopic expression of neuropeptide Y which incidence seems to increase with age among the Tg2576 population. Our data reveal that network hypersynchrony appears very early in Tg2576 mice, before any demonstrated memory impairments.