Mechanisms of memory retrieval in the hippocampus

The discovery that hippocampal cells have location-specific activation (place cells), which is extremely reliable and observable during almost any behaviour in a moving rat made the possibility to study neuronal processes and behaviour together. Recent ad vances suggest that place is represented not only in hippocampus but by interacting cell types in the entorhinal-hippocampal circuits. However, up to this date studies of hippocampal representation have largely been based on time-averaged approaches and t o exploit our knowledge of hippocampal remapping we need to develop methods for monitoring the activity in large neural populations at the time scale of milliseconds, not minutes. Although there is plenty of data of hippocampal CA1 area, considerably less is known about its role. To understand the contribution of CA1 to memory, we need to know how CA1 cells interact, on a second-by-second basis, with their inputs from the CA3 and the entorhinal cortex. In the CA1, gamma rhythms split into distinct fast an d slow frequency components. Fast CA1 gamma oscillations are synchronized with fast gamma in medial entorhinal cortex (MEC), and slow CA1 gamma oscillations with slow gamma in CA3. CA3 cells are thought to store information about the past experiences of a nimals in the environment, whereas MEC cells may be more responsive to the current location. Our observations raise the possibility that flickering between slow and fast gamma reflects switches between representations of past and present information, expr essed respectively in CA3 and MEC in different cycles of the theta rhythm. The aim of the present project is to provide a mechanism for temporally distinct retrieval from the CA3 network, in a manner that enables CA1 to keep retrieved information separat e from incoming spatial information from the entorhinal cortex.