GAMMA (40-100-HZ) OSCILLATION IN THE HIPPOCAMPUS OF THE BEHAVING RAT

The cellular generation and spatial distribution of gamma frequency (4 0-100 Hz) activity was examined in the hippocampus of the awake rat. F ield potentials and unit activity were recorded by multiple site silic on probes (5- and 16-site shanks) and wire electrode arrays. Gamma wav es were highly coherent along the long axis of the dentate hilus, but average coherence decreased rapidly in the CA3 and CA1 directions. Ana lysis of short epochs revealed large fluctuations in coherence values between the dentate and CA1 gamma waves, Current source density analys is revealed large sinks and sources in the dentate gyrus with spatial distribution similar to the dipoles evoked by stimulation of the perfo rant path, The frequency changes of gamma and theta waves positively c orrelated (40-100 Hz and 5-10 Hz, respectively), Putative interneurons in the dentate gyrus discharged at gamma frequency and were phase-loc ked to the ascending part of the gamma waves recorded from the hilus, Following bilateral lesion of the entorhinal cortex the power and freq uency of hilar gamma activity significantly decreased or disappeared. Instead, a large amplitude but slower gamma pattern (25-50 Hz) emerged in the CA3-CA1 network, We suggest that gamma oscillation emerges fro m an interaction between intrinsic oscillatory properties of interneur ons and the network properties of the dentate gyrus. We also hypothesi ze that under physiological conditions the hilar gamma oscillation may be entrained by the entorhinal rhythm and that gamma oscillation in t he CA3-CA1 circuitry is suppressed by either the hilar region or the e ntorhinal cortex.