Thalamic and cortical correlates of sleep spindles in rats

INTRODUCTION Sleep spindles are among the most prominent oscillations that appear on the encephalogram (EEG) of all mammalian species and humans during non-REM sleep. International Federation of Electroencephalography and Clinical Neurophysiology (IFSECN, 1974) defines sleep spindle as a “group of rhythmic waves characterized by progressively increasing then decreasing amplitude” (1974) [1]. It was clearly demonstrated that sleep spindles are derived from the thalamus due to inhibitory feedback interaction between neurons in the rely thalamic nuclei and reticular thalamic nuclei (RTN). Further, sequences of sleep spindles are distributed over widespread cortical territories. It is known that thalamic sleep spindles co-occur with their cortical equivalents [1]. However, majority of EEG studies deal with sleep spindles, which appear in the cortex, and little is known about expression of sleep spindles in the thalamus. Our study describes spindle activity on the two levels of thalamo-cortical loop, in the cortex and the thalamus (in the ventroposteromedial nucleus of thalamus (VPM) and in the RTN). In the cortex, sleep spindles are topographically discontinuous: two local, anterior and posterior, spindles types known in Wistar [1] and WAG/Rij rats[2]. It is assumed such a topographical divergence of spindle activity originates form the thalamus. Here we estimate the role of the specific thalamus (VPM) and the RTN in two spindle types by means cross-spectral analysis. This allowed us to characterize subcortical mechanisms that may govern spatial distribution of sleep spindles over the cortex.