Pyramidal Cell-Interneuron Circuit Architecture and Dynamics in Hippocampal Networks

Abstract

Circuit activity is modulated by feedback inhibition. Excitatory control of this inhibition is poorly understood due to the difficulty of studying synaptic connectivity in vivo. Here, we infer such connectivity through analysis of spike timing. The validity of this inference was confirmed by juxtacellular and optogenetic control of spikes in behaving rodents. We found that neighboring CA1 neurons were more likely to be connected and to have stronger connections, though superficial pyramidal cells projected more to deep interneurons. Connection probability was skewed, with a small minority of highly connected hubs. In general, high frequency pre-synaptic firing led to short-term depression. Synaptic drive was moderately affected by the firing rate of the post-synaptic cell and strongly modulated by the prior spike timing. Finally, we established a short-latency integration window for convergent pre-synaptic inputs. These results support a nonhomogenous circuit organization and provide a framework for constructing transiently active cell assemblies.