Monosynaptic excitatory transmission from the hippocampal CA1 region to the subiculum

The subiculum is a major output region of the hippocampus, receiving inputs from the CA1 region. We obtained paired patch-clamp recordings from synaptically coupled pairs of CA1 pyramidal cells (CA1PCs) and subicular principal cells (SubPCs), using rat hippocampal organotypic slice cultures. A single action potential in a presynaptic CA1PC evoked a unitary excitatory postsynaptic current in a SubPC (EPSCCA1→Sub). The failure rate of the monosynaptic transmission was remarkably low (0.08). Paired-pulse depression in SubPCs was apparent when an interval of presynaptic action potentials was shorter than 50 ms. When trains of action potentials were induced in a CA1PC, EPSCCA1→Sub was significantly depressed with increasing spike frequency (20-100 Hz). Thus the unitary monosynaptic transmission from a CA1PC to a SubPC is reliable, and depressed in response to frequent inputs, suggesting that the subiculum may function as a low pass filter to provide the downstream brain regions with appropriate information.