SPATIOTEMPORAL PATTERNS OF GAMMA-FREQUENCY OSCILLATIONS TETANICALLY INDUCED IN THE RAT HIPPOCAMPAL SLICE

1. We used transverse and longitudinal rat hippocampal slices to study the synchronization of gamma frequency (> 20 Hz) oscillations, across distances of up to 4.5 mm. gamma oscillations were evoked in the CA1 region by tetanic stimulation at one or two sites simultaneously, and were associated with population spikes. Tetanic stimuli that were stro ng enough to induce oscillations were associated with depolarization o f both pyramidal cells and interneurones, largely produced by activati on of metabotropic glutamate receptors. 2. Computer simulations of gam ma oscillations were also performed in a model with pyramidal cells an d interneurones, arranged in a chain of five cell groups. This model h ad suggested previously that interneurone networks alone could generat e synchronous gamma oscillations locally but that pyramidal cell firin g, by inducing spike doublets in interneurones, was necessary for the occurrence of highly correlated oscillations with small phase lag (< 2 .5 ms), in a distributed network possessing long axon conduction delay s. 3. In both experiment and model, pyramidal cell spikes occurred in phase with local population spikes, as did the first spike of the inte rneurone doublet. 4. The conductance of the interneurone alpha-amino-3 -hyrdroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor-mediated conductance was manipulated in the model, while the relation between oscillations at opposite ends of the chain was examined. When the cond uctance was large enough for doublet firing to be synaptically induced in interneurones, oscillation phase lags were < 2.25 ms across the ch ain. As predicted, experimental blockade of AMPA receptors resulted in increased phase lags between two sites oscillating simultaneously, co mpared with control conditions. 5. Both in model and in experiment, wh en stimuli to the two ends of the network were slightly different, cro ss-network synchronization occurred with a shorter phase lag at high f requencies than at lower frequencies. 6. These data suggest that, whil e interneurone networks alone can generate locally synchronized gamma oscillations, firing of pyramidal cells, and the synaptically induced doublet firing in interneurones, contribute to the stability and tight synchrony of the oscillations in distributed networks.