Hippocampal slow oscillation: a novel EEG state and its coordination with ongoing neocortical activity

State-dependent EEG in the hippocampus (HPC) has traditionally been divided into two activity patterns: theta, a large-amplitude, regular oscillation with a bandwidth of 3-12 Hz, and large-amplitude irregular activity (LIA), a less regular signal with broadband characteristics. Both of these activity patterns have been linked to the memory functions subserved by the HPC. Here we describe, using extracellular field recording techniques in naturally sleeping and urethane-anesthetized rats, a novel state present during deactivated stages of sleep and anesthesia that is characterized by a prominent large-amplitude and slow frequency (< or =1 Hz) rhythm. We have called this activity the hippocampal slow oscillation (SO) because of its similarity and correspondence with the previously described neocortical SO. Almost all hippocampal units recorded exhibited differential spiking behavior during the SO as compared with other states. Although the hippocampal SO occurred in situations similar to the neocortical SO, it demonstrated some independence in its initiation, coordination, and coherence. The SO was abolished by sensory stimulation or cholinergic agonism and was enhanced by increasing anesthetic depth or muscarinic receptor antagonism. Laminar profile analyses of the SO showed a phase shift and prominent current sink-source alternations in stratum lacunosum-moleculare of CA1. This, along with correlated slow oscillatory field and multiunit activity in superficial entorhinal cortex suggests that the hippocampal SO may be coordinated with slow neocortical activity through input arriving via the temporo-ammonic pathway. This novel state may present a favorable milieu for synchronization-dependent synaptic plasticity within and between hippocampal and neocortical ensembles.