Effects of cellular excitatory-inhibitory composition on neuronal dynamics

Excitation/Inhibition balance is essential for stable neuronal dynamics. It is considered to be strongly related to the relative counts of excitatory and inhibitory neurons. However, it is not clear if the relative counts indeed change the excitation/inhibition balance on a synaptic level and affect the neuronal dynamics. To investigate these effects, we recorded Ca-activity of hippocampal cultures with various numbers of inhibitory neurons. In experiments, all cultures developed network bursting. The cultures with various fractions of inhibitory neurons showed stable average inter-burst intervals. The variance of inter-burst intervals, however, grew with the number of inhibitory neurons. We reproduced the results of experiments in a model network of leaky integrate-and-fire neurons with different numbers of inhibitory neurons, but balanced strength of excitation and inhibition, and adaptation. The model showed that the stable mean and increasing variance of inter-burst intervals can be achieved by the synaptic balance between excitation and inhibition that regulates effects of adaptation. We also show that an equivalent mean-field model of excitatory and inhibitory rate-neurons with adaptation can account for these effects in terms of simple attractor dynamics. Overall, our results suggest that hippocampal cultures with various cellular compositions tend to maintain the balance