Analysis of recordings of single-unit firing and population activity in the dorsal subiculum of unrestrained, freely moving rats.

We examined neuronal activity in the dorsal subiculum of unrestrained, male adult Wistar rats, which were implanted with a moveable eight-electrode microdrive. The subiculum is the primary hippocampal formation output area and is comparatively uninvestigated neurophysiologically. We compared subicular unit activity and the subicular EEG while rats occupied a small, restricted environment and also correlated neuronal activity with the ongoing behavior of the animal. Units were separated using their electrophysiological characteristics into bursting units, regular spiking units, theta-modulated units, and fast spiking units. The bursting and regular spiking unit classes are similar to hippocampal CA1 units, whereas the fast spiking units appear to be interneurons. Bursting units were variable in their behavior: some units bursted regularly, and others bursted only occasionally. Theta-modulated units have not been described before; these were similar to regular spiking units in all respects except that they increased their firing significantly when theta oscillations were present in the simultaneous EEG record. Subicular EEG was similar to hippocampal EEG, with theta oscillations dominating "alert, moving" behaviors, while large amplitude irregular activity (LIA), which included sharp waves, predominated when theta oscillations were not present, mainly during "alert, still" and "quiet" behaviors. A relatively small proportion of subicular recordings (approximately 32%) were phase-locked to theta; this is a smaller proportion than in areas from which the subiculum takes major inputs. The relative lack of entrainment of subicular neurons by this important intrinsic rhythm is suggestive of a limit to which theta might be capable of affecting both subicular and hippocampal information processing more generally.