HIPPOCAMPAL PYRAMIDAL CELLS EXCITE INHIBITORY NEURONS THROUGH A SINGLE RELEASE SITE

MORPHOLOGICALLY a synapse consists of a presynaptic release site conta ining vesicles, a postsynaptic element with membrane specialization, a nd a synaptic cleft between them1. The number of release sites shapes the properties of synaptic transmission between neurons2-4. Although e xcitatory interactions between cortical neurons have been examined5-9, the number of release sites remains unknown. We have now recorded exc itatory postsynaptic potentials evoked by single pyramidal cells in hi ppocampal interneurons and visualized both cells using biocytin inject ions. Light and electron microscopy showed that excitatory postsynapti c potentials were mediated by a single synapse. We also reconstructed the entire axon arborization of single pyramidal cells, filled in vivo , in sections counterstained for parvalbumin, which selectively marks basket and axo-axonic cells10,11. Single synaptic contacts between pyr amidal cells and parvalbumin-containing neurons were dominant (>80%), providing evidence for high convergence and divergence in hippocampal networks12.