Firing rates of hippocampal neurons are preserved during subsequent sleep episodes and modified by novel awake experience

What determines the firing rate of cortical neurons in the absence of exter nal sensory input or motor behavior, such as during sleep? Hero we report t hat, in a familiar environment, the discharge frequency of simultaneously r ecorded individual CA1 pyramidal neurons and the coactivation of cell pairs remain highly correlated across sleep-wake-steep sequences. However, both measures were affected when new sets of neurons were activated in a novel e nvironment. Nevertheless, the grand mean firing rate of the whole pyramidal cell population remained constant across behavioral states and testing con ditions. The findings suggest that long-term firing patterns of single cell s can be modified by experience. We hypothesize that increased firing rates of recently used neurons are associated with a concomitant decrease in the discharge activity of the remaining population, leaving the mean excitabil ity of the hippocampal network unaltered.