REACTIVATION OF NEURONAL ENSEMBLES IN HIPPOCAMPAL DENTATE GYRUS DURING SLEEP AFTER SPATIAL EXPERIENCE

Patterns of neuronal activity recorded in CA1 of the hippocampus and i n neocortex during waking-behavior, are reactivated during subsequent slow-wave sleep (SWS), It has been suggested that this reactivation ma y originate in the hippocampal CA3 region, where modifiable excitatory recurrent connections are abundant and where sharpwaves, in which the reactivation is most robust, appear to arise. The present experiment investigated whether ensemble firing patterns of granule cells in the fascia dentata (FD), an area 'upstream' from CA3, are also reactivated during sleep. Populations of FD granule cells were recorded from duri ng spatial behavior and during prior and subsequent SWS. Firing rate c orrelations between cell-pairs with overlapping place fields were sign ificantly enhanced during post behavioral sleep compared to pre behavi oral sleep. Correlations between cells with non-overlapping place fiel ds or which were silent during maze behavior, were not changed. Thus, reactivation of experience specific correlation states also occurs in granule cells during sleep. Because these cells do not have excitatory interconnections, but form a major input to CA3 pyramidal cells, curr ent models predicted that sleep reactivation would appear first in CA3 . There are, however, both extensive polysynaptic excitatory interacti ons among granule cells and feedback from CA3 pyramidal cells. Granule cells also receive indirect input from neocortical regions known to u ndergo trace reactivation. Although a simple model for a CA3 origin of the reactivation phenomenon cannot be confirmed, the present results extend our understanding of the generality of this phenomenon.