THE SMALL IRREGULAR ACTIVITY STATE IN THE RAT HIPPOCAMPUS
The sleeping rat cycles between two well characterized physiological states, slow-wave sleep (SWS) and rapid eye-movement sleep (REM), often identified by the presence of large irregular activity (LIA) and theta activity, respectively, in the hippocampal EEG. Inspection of the activity of ensembles of hippocampal CA1 complex-spike cells along with the EEG reveals the presence of a third physiological state, distinctly different from both REM and SWS in both hippocampal EEG and population activity. The EEG during this state abruptly flattens for a few seconds, appearing very similar to the small-amplitude irregular activity (SIA) hippocampal EEG state reported in the literature to occur when rats are startled out of sleep. The flattening of the EEG is accompanied by a striking pattern of spike activity in the population of hippocampal pyramidal cells, wherein a small subset of cells becomes very active while the rest become quiet; the same subset of cells is usually active across long sequences of SIA. This dissertation shows (1) that these active cells are place cells whose place fields are in the location in which the rat is sleeping; (2) that the spontaneous SIA observed during sleep corresponds to the SIA state of increased alertness that has been reported in the literature to occur when rats are startled out of sleep; (3) that SIA is accompanied by a desynchronized neocortical EEG and low amplitude EMG; (4) that the cells active in SIA reflect a memory for the location in which the rat fell asleep, rather than an assessment of its location based on current sensory information; and (5) that the generation of SIA is likely to involve an increase of serotonin levels in the medial septal nucleus. It is proposed that SIA serves as a neural substrate for maintaining context memory during sleep, and that it reflects a partial arousal in response to internal or external stimuli that allows the animal to assess whether full awakening is warranted, without disrupting the sleep cycle.
Advisor:Bruce McNaughton; David Touretzky; James Ranck; Carl Olson; Anthony Grace; James McClelland; William E. Skaggs
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication:01/16/2004