Sharp wave-ripple complexes (SPW-Rs) are coordinated network events that emerge in the CA3 subregion of the hippocampus and propagate via the "output loop" into the entorhinal cortex (EC). These patterns of activity appear to contribute to memory consolidation, most likely by inducing lasting memory traces in the neocortex. As a consequence, propagating SPW-Rs should induce specific signals in downstream cortical areas. We therefore investigated the postsynaptic currents (PSCs) in layer V (LV) principal neurons of the medial EC (mEC) during SPW-R oscillations in horizontal mouse brain slices. Whole-cell voltage clamp recordings revealed that PSCs had complex waveforms, often showing superimposed rhythmic activity with leading frequencies similar to the dominant ripple frequency observed in CA1. Charge-transfer of SPW-R-associated PSCs was positively correlated with the amplitude of preceding SPW-Rs in CA1. Recordings of GABAergic currents at 0 mV holding potential and glutamatergic currents at the GABA reversal potential (~ -74 mV) indicated that network-associated synaptic input was mostly excitatory. In order to investigate the input-specificity of the postsynaptic signals in mEC LV neurons, we analyzed their correlation with different waveforms of field-SPW-Rs in CA1. Field potentials were sorted into self-organizing maps, and the distribution of cellular events amongst prototypic waveforms was quantified. We found significant values for sparsity and information in comparison to shuffled data, indicating transfer of specific information between both regions. This specificity was almost entirely carried by synaptic excitation, in contrast to CA1 pyramidal cells where GABAergic PSCs contributed significantly to sparsity and information. Our results suggest that SPW-Rs may support specific memory transfer from hippocampal assemblies to upstream networks in the mEC.

Supported by BMBF (01GQ1003A, BCCN Heidelberg/Mannheim, B3) and by IB BMBF (RUS 11/015).