Abstract
During Slow Wave Sleep (SWS), cortical activity is dominated by endogenous processes modulated by slow oscillations (0.1–1 Hz): cell ensembles fluctuate between states of sustained activity (UP states) and silent epochs (DOWN states). We investigate here the temporal structure of ensemble activity during UP states by means of multiple single unit recordings in the prefrontal cortex of naturally sleeping rats. As previously shown, the firing rate of each PFC cell peaks at a distinct time lag after the DOWN/UP transition in a consistent order. We show here that, conversely, the latency of the first spike after the UP state onset depends primarily on the session-averaged firing rates of cells (which can be considered as an indirect measure of their intrinsic excitability). This latency can be explained by a simple homogeneous process (Poisson model) of cell firing, with sleep averaged firing rates employed as parameters. Thus, at DOWN/UP transitions, neurons are affected both by a slow process, possibly originating in the cortical network, modulating the time course of firing for each cell, and by a fast, relatively stereotyped reinstatement of activity, related mostly to global activity levels.
Highlights
During Slow Wave Sleep (SWS), cortical activity spontaneously fluctuates between periods of sustained activity, the ‘UP’ state, and periods of generalized silence, the ‘DOWN’ state
887 putative pyramidal cells and 100 interneurons were recorded from the prelimbic (PL) and occasionally from the infralimbic (IL) subdivision of the medial prefrontal cortex (PFC)
In successive pairs of sleep sessions on a given day the same neurons were likely recorded, since the recordings were separated by an active waking period of less than an hour and cluster discrimination was carried out on data combined from the three sessions.) For this analysis, only pyramidal cells with an average firing rate exceeding 1 Hz during SWS were retained (N = 559), because less active cells were inactive during many UP state episodes
Summary
During Slow Wave Sleep (SWS), cortical activity spontaneously fluctuates between periods of sustained activity, the ‘UP’ state, and periods of generalized silence, the ‘DOWN’ state. These are the so-called slow oscillations (0.1–1 Hz) of the neocortex (Steriade et al, 1993). Slow oscillations are generated within the neocortex (Contreras et al, 1996; Timofeev et al, 2000; Cash et al, 2009) and support global and long-scale synchronization of cortical activity (Amzica and Steriade, 1995; Massimini et al, 2004; Dang-Vu et al, 2008; Nir et al, 2008). High synaptic bombardment would keep the network in an active state thereafter (Compte et al, 2003; Destexhe et al, 2003; Steriade, 2006) by means of recurrent excitatory feedback (Contreras et al, 1996; Holcman and Tsodyks, 2006)
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