Prominent differences in sharp waves, ripples and complex spike bursts between the dorsal and the ventral rat hippocampus.

Functions of the hippocampus are segregated along its long axis and emerging evidence shows that the local circuitry is specialized accordingly. Sharp waves (SPWs) and ripples are a basic hippocampal network activity implicated in memory processing. Using recordings from the CA1 field of both dorsal (DH) and ventral (VH) rat hippocampal slices we found that SPWs are larger, shorter and occur much more frequently in the VH than in the DH. Clusters of SPWs (i.e. multiple consecutive events grouped in sequences that depend on NMDA receptors) occur with higher probability in the VH and the frequency of occurrence of consecutive intra-cluster events is higher in the VH (∼10Hz) than in the DH (∼5Hz). The ripple oscillation displays higher amplitude and frequency in the VH than in DH and the associated multiunit firing peaks at a later phase of the ripple waves in the VH than in the DH. Isolated unit complex spike bursts display a significantly lower number of spikes and longer inter-spike intervals in the VH than in the DH suggesting that the synaptically driven neuronal excitability is lower in the VH. We propose that to some extent these differences result from the relatively higher network excitability of the VH compared with DH. Furthermore, they might reflect specializations that provide the local circuitries of the DH and VH with the required optimal ability for synaptic plasticity and might also suggest that the VH could be a favored site of SPW-Rs initiation.