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1Department of Neurobiology, Duke University Medical Center; 2Center for Neuroengineering, 3Department of Biomedical Engineering, and 4Department of Psychological and Brain Sciences, Duke University, Durham, North Carolina; and 5Centre National de la Recherche Scientifique, Université Claude Bernard, Lyon, France
Submitted 16 May 2006; accepted in final form 19 August 2006
Traditionally, most basal forebrain (BF) functions have been attributed to its cholinergic neurons. However, the majority of cortical-projecting BF neurons are noncholinergic and their in vivo functions remain unclear. We investigated how BF modulates cortical dynamics by simultaneously recording
50 BF single neurons along with local field potentials (LFPs) from the prefrontal cortex (PFCx) in different wakesleep states of adult rats. Using stereotypical spike time correlations, we identified a large (roughly 70%) subset of BF neurons, which we named BF tonic neurons (BFTNs). BFTNs fired tonically at 28 Hz without significantly changing their average firing rate across wakesleep states. As such, these cannot be classified as cholinergic neurons. BFTNs substantially increased the spiking variability during waking and rapid-eye-movement sleep, by exhibiting frequent spike bursts with <50-ms interspike interval. Spike bursts among BFTNs were highly correlated, leading to transient population synchronization events of BFTN ensembles that lasted on average 160 ms. Most importantly, BFTN synchronization occurred preferentially just before the troughs of PFCx LFP oscillations, which reflect increased cortical activity. Furthermore, BFTN synchronization was accompanied by transient increases in prefrontal cortex gamma oscillations. These results suggest that synchronization of BFTN ensembles, which are likely to be formed by cortical-projecting GABAergic neurons from the BF, could be primarily responsible for fast cortical modulations to provide transient amplification of cortical activity.
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