In spatial memory functionally coupled neurons are activated in a highly reproducible way within the hippocampal formation. Major input from the entorhinal cortex to the hippocampus is processed in the dentate gyrus (DG) by a high number of sparsely firing granule cells enabling multiple different activation patterns. Recent evidence indicates that newborn granule cells support separation of input patterns, while mature neurons play a role for pattern completion. However it still needs to be elucidated how the DG affects memory-related assembly formation in the hippocampus during sharp wave ripple oscillations (SPW-R).

Acute slices of rodent hippocampus provide a well studied model for spontaneous SPW-R. These network oscillations support memory consolidation and involve repetitive activation of functionally coupled neurons. We investigated whether such functional assemblies can be modulated by repetitive firing of defined sets of granule cells. Tetrode recordings and especially subsequent sorting of SPW-R by an unbiased sorting algorithms enable the investigation of underlying assembly activity in the cornu ammonis (CA). Weak stimulation of distinct sets of granule cells elicited transient field potentials resembling spontaneous SPW-R. Sorting of evoked events to a sub-set of spontaneous events indicates the activation of defined assemblies. Stimulation of different sites within DG evoked clearly different downstream response patterns in CA3 and CA1. Repetitive extracellular stimulation within DG appeared to modulate the variability of evoked events in an input-specific manner. Our data suggest a modulating impact of the DG for pattern formation within the hippocampus proper. Supported by the DFG (SFB 636, B6).