GAMMA-FREQUENCY-OSCILLATION IN THE HIPPOCAMPUS OF THE RAT - INTRACELLULAR ANALYSIS IN-VIVO

Gamma frequency field oscillations reflect synchronized synaptic poten tials in neuronal populations within the approximate to 10-40 ms range . The generation of gamma activity in the hippocampus was investigated by intracellular recording from principal cells and basket cells in u rethane anaesthetized rats. The recorded neurones were verified by int racellular injection of biocytin. Gamma frequency field oscillations w ere nested within the slower theta waves. The phase and amplitude of i ntracellular gamma were voltage dependent with an almost complete phas e reversal at CI-equilibrium potential in pyramidal cells. Basket cell s fired at gamma frequency and were phase-locked to the same phase of the gamma oscillation as pyramidal cells. Current-induced depolarizati on coupled with synaptically induced inhibition resulted in gamma freq uency discharge (30-80 Hz) of pyramidal cells without accommodation. T hese observations suggest that at least part of the gamma frequency fi eld oscillation reflects rhythmic hyperpolarization of principal cells , brought about by the rhythmically discharging basket neurones. Reson ant properties of pyramidal cells might facilitate network synchrony i n the gamma frequency range.