Firing Patterns Of Hippocampal CA1 Interneurons

When processing complex information, neural network in the brain will involve co-operative activities of ensemble neurons. One of the main representations is different rhythm oscillations of field potentials in different behaviours and brain states. It has been indicated in some research that interneurons play a very important role in modulating different oscillations in neural network. Through multi-channel recording techniques, we reviewed the correlation between firing patterns of hippocampal CA1 interneurons and rhythm oscillations of field potentials where they located. We found that hippocampal CA1 interneurons had different firing patterns correlated with different field potentials in different behaviours. Thereinto two types of interneurons were correlated with theta oscillation and two with ripple oscillation. Another type of interneuron had no relationship with theta or ripple oscillation, but their firings displayed typical bursts during slow wave sleep.One type of interneurons correlated with theta oscillation were called burst-theta cells, whose firings were strictly phase-locked to theta oscillation. And their firing rates also fluctuated with theta oscillation. In every theta cycle this type of interneuron fired in burst with 3-10 spikes. Their firings incline to the positive peak of theta oscillation. Another type were called single-theta cells, which were active only during exploration and rapid eye movement sleep and kept silent during slow wave sleep. This type of interneuron showed similar firing rates with theta oscillation (10.947±0.658 Hz ) when they were active. In every theta cycle there was only one spike and their firings were not phase-locked to theta oscillation.We called interneurons correlated with ripple oscillation ripple type I cells and ripple type II cells. They associated their firings with high-frequency ripple oscillation (150-250 Hz), a characteristic field potential in hippocampus during slow wave sleep. The firings of these two types of interneurons highly synchronized with ripple oscillation, that is, they fired at a high frequency corresponding to each cycle of ripple oscillation. Type I cells had one spike in each sub-cycle of ripple oscillation, whereas, type II cells had one or two spikes. The peak firing rates of these two types of neurons were up to 310 ±33.17 Hz (type I ) and 410 ± 47.61 Hz (type II), respectively.Burst cells showed no phase-locked relationship with both theta and ripple oscillations although fired in bursts with 3-5 spikes during slow wave sleep. The firing rates of burst cells in active exploration were the highest, then in rapid eye movement sleep and the firing rates in slow wave sleep were the slowest.The results may suggest that different interneurons participate in modulating ensemble activity of neural network in hippocampus with their specific firing patterns.