Frequency-Dependent Synaptic Dynamics Differentially Tune CA1 and CA2 Pyramidal Neuron Responses to Cortical Input.

Entorhinal cortex (EC) neurons make monosynaptic connections onto distal apical dendrites of CA1 and CA2 pyramidal neurons (PNs) through the perforant path (PP) projection. Previous studies show that differences in dendritic properties and synaptic input density enable the PP inputs to produce a much stronger excitation of CA2 compared to CA1 PNs. Here, using mice of both sexes, we report that the difference in PP efficacy varies substantially as a function of presynaptic firing rate. Although a single PP stimulus evokes a 5-6 fold greater EPSP in CA2 compared to CA1, a brief high-frequency train of PP stimuli evokes a strongly facilitating postsynaptic response in CA1, with relatively little change in CA2. Furthermore, we demonstrate that blockade of NMDARs significantly reduces strong temporal summation in CA1, but has little impact on that in CA2. As a result of the differences in the frequency- and NMDAR-dependent temporal summation, naturalistic patterns of presynaptic activity evoke CA1 and CA2 responses with distinct dynamics, differentially tuning CA1 and CA2 responses to bursts of presynaptic firing versus single presynaptic spikes, respectively. Significance Statement Recent studies have demonstrated abundant entorhinal cortical innervation and efficient dendritic propagation enable hippocampal CA2 pyramidal neurons to produce robust excitation evoked by single cortical stimuli, compared to CA1. Here we uncovered, unexpectedly, that the difference in efficacy of cortical excitation varies substantially as a function of presynaptic firing rate. A burst of stimuli evokes a strongly facilitating response in CA1, but not in CA2. As a result, postsynaptic response of CA1 and CA2 to presynaptic naturalistic firing displays contrasting temporal dynamics, which depends on the activation of NMDARs. Thus our findings provide a synaptic mechanism by which hippocampal neurons dynamically respond to irregular, high-frequency burst firing in entorhinal cortex in behaving animals.