Fast network oscillations in the newborn rat hippocampus in vitro

Spontaneous neural activity is crucial for the formation of the intricate p atterns of cortical connectivity during development. In particular, tempora l correlations in presynaptic and postsynaptic activity have been hypothesi zed to be a critical determinant in the selection of neurons that are to be come wired together. To date, however, temporally correlated activity in th e neonatal brain has been believed to take place with a precision of tens o f milliseconds to seconds. Here we describe a novel type of a fast network oscillation associated with millisecond synchronization of pyramidal cell firing in newborn rat hippoc ampus in vitro. Individual pyramidal neurons fired mainly at lower gamma fr equencies (20-40 Hz) but were synchronized into a high-frequency (100-400 H z) population oscillation that was reflected in field potential spikes and intracellular AMPA-kainate receptor-mediated currents. The high-frequency p opulation oscillation was patterned by a gamma-frequency modulatory oscilla tion. The gamma modulation was imposed by GABAergic currents, which exerted an inhibitory action on pyramidal neurons. Patterned activity based on GAB Aergic inhibition and glutamatergic excitation thus occurs already in newbo rn hippocampus. The network oscillations described here may be a mechanism for selective coincidence detection with a millisecond range temporal preci sion to shape the patterns of connectivity within the emerging hippocampal synaptic circuitry.