1. ^* Institute of Experimental Medicine, Hungarian Academy of Sciences, Budapest, PO Box 67, H-1450, Hungary
2. ^† Laboratoire de Neurobiologie Cellulaire, INSERM U261, Institut Pasteur, 25 rue de Dr Roux, 75264 Paris Cedex 15, France
3. ^‡ Department of Anatomy, Fukui Medical School, Matsuoka, Fukui 910-11, Japan
4. ^§ To whom correspondence should be addressed.

Abstract

MORPHOLOGICALLY a synapse consists of a presynaptic release site containing vesicles, a postsynaptic element with membrane specialization, and a synaptic cleft between them¹. The number of release sites shapes the properties of synaptic transmission between neurons^2–4. Although excitatory interactions between cortical neurons have been examined^5–9, the number of release sites remains unknown. We have now recorded excitatory postsynaptic potentials evoked by single pyramidal cells in hippocampal interneurons and visualized both cells using biocytin injections. Light and electron microscopy showed that excitatory postsynaptic 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 axoaxonic cells^10,11. Single synaptic contacts between pyramidal cells and parvalbumin-containing neurons were dominant (>80%), providing evidence for high convergence and divergence in hippocampal networks¹².