Abstract

The parasubiculum is a major component of the subicular complex, and plays an integral role in the retrohippocampal circuitry. It receives numerous inputs from the hippocampus, medial septum, thalamus, and other subcortical areas, and sends its major output projection to the layer II of the medial entorhinal cortex. Theta-frequency EEG activity (4-12 Hz) is a sinusoidal-like waveform, and has been correlated with numerous cognitive processes. The present studies were aimed at determining whether the superficial layers of the parasubiculum generate theta-frequency EEG activity locally, and whether membrane potential oscillations in parasubicular neurons may contribute to the generation of theta activity in this area. In the urethane-anaesthetized rat, cholinergically-mediated theta activity was recorded as a bipolar electrode passed through the superficial layers of the parasubiculum, and the theta activity was phase-locked to theta activity recorded in the stratum radiatum/lacunosum-moleculare of the hippocampal CA1 region. Using whole-cell current-clamp recordings in acute brain slices, voltage-dependent theta-frequency membrane potential oscillations were observed in the majority of layer II parasubicular neurons when the cells were held near threshold voltages using steady current injection. The frequency of oscillations increased when the cells were heated from 22{493}C to 32{493}C, and persisted in the presence of blockers of fast ionotropic glutamatergic and GABAergic synaptic transmission. The oscillations are therefore likely generated intrinsically by voltage-dependent ionic conductances. These results suggest that the superficial layers of the parasubiculum locally generate theta-frequency EEG activity, and that voltage-dependent membrane potential oscillations may contribute to the generation of this activity in vivo