Single-cell activity and synchronous bursting in the rat hippocampus during waking behavior and sleep.

Eighty-one of eighty-five isolated neurons recorded from the rat dorsal hippocampus (Ammon's horn and area dentata) were classifiable into four types according to their firing patterns and behavioral correlates. Sixty-two percent of the total neurons were complex spike cells which fired slowly (less than 2/s) most of the time, but were most active during slow-wave sleep, slightly less active during awake immobility, and least active during awake movement and paradoxical sleep. Theta cells (21% of the total) generally fired much faster than complex spike cells and increased their discharge rate whenever rhythmical slow activity was present in the hippocampal EEG (i.e., during awake movement and paradoxical sleep). The behavioral correlates of seven putative granule cells were similar to those of theta cells. However, the behavior-dependent changes in firing frequency were much more pronounced in the former than in the latter. Three (fast) complex spike cells were observed which fired much faster than any other complex spike cell during all states and showed the behavioral correlates similar to those of theta cells. The complex spike cells, theta cells, and fast complex spike cells were found in both Ammon's horn and dentate hilus. A type of multiunit burst was recorded from the pyramidal cell layer. It occurred most frequently during slow-wave sleep, slightly less frequently during awake immobility, and was virtually absent during awake movement and paradoxical sleep. The multiunit burst was considered to represent a summed activity of synchronously bursting complex spike cells. Possible cellular mechanisms of the state-dependent activities in the different neuronal populations of the hippocampus are discussed.