CELLULAR-SYNAPTIC GENERATION OF SLEEP SPINDLES, SPIKE-AND-WAVE DISCHARGES, AND EVOKED THALAMOCORTICAL RESPONSES IN THE NEOCORTEX OF THE RAT

Thalamocortical neuronal oscillations underlie various field potential s that are expressed in the neocortex, including sleep spindles and hi gh voltage spike-and-wave patterns (HVSs). The mechanism of extracellu lar current generation in the neocortex was studied in the anesthetize d and awake rat. Field potentials and unit activity were recorded simu ltaneously along trajectories perpendicular to the cortical layers at spatial intervals of 100 mu m by multiple-site recording silicon probe s, Current source density (CSD) analysis revealed that the spatial pos itions of sinks in layers IV, V-VI, and II-III and of the accompanying sources were similar during sleep spindles, HVSs, and thalamic-evoked responses, although their relative strengths and timings differed. Th e magnitude and relative timing of the multiple pairs of sinks anti so urces determined the amplitude variability of HVSs and sleep spindles. The presence of temporally shifted dipoles was also supported by the time distribution of unit discharges in different layers, Putative int erneurons discharged with repetitive bursts of 300-500 Hz. The spike c omponent of HVSs was associated with fast field oscillations (400-600 Hz ''ripples''), Discharges of pyramidal cells were phase-locked to th e ripples. These findings indicate that the major extracellular curren ts underlying sleep spindles, HVSs, and evoked responses result from a ctivation of intracortical circuitries. We hypothesize that the fast f ield ripples reflect summed IPSPs in pyramidal cells resulting from th e high frequency barrage of interneurons.