COMPLETE AXON ARBORIZATION OF A SINGLE CA3 PYRAMIDAL CELL IN THE RAT HIPPOCAMPUS, AND ITS RELATIONSHIP WITH POSTSYNAPTIC PARVALBUMIN-CONTAINING INTERNEURONS

The complete axon arborization of a single CA3 pyramidal cell has been reconstructed from 32 (60 mum thick) sections from the rat hippocampu s following in vivo intracellular injection of neurobiotin. The same s ections were double-immunostained for parvalbumin-a calcium-binding pr otein selectively present in two types of GABAergic interneurons, the basket and chandelier cells-in order to map boutons of the pyramidal c ell in contact with dendrites and somata of these specific subsets of interneurons visualized in a Golgi-like manner. The axon of the pyrami dal cell formed 15 295 boutons, 63.8% of which were in stratum oriens, 15.4% in stratum pyramidale and 20.8% in stratum radiatum. Only 2.1% of the axon terminals contacted parvalbumin-positive neurons. Most of these were single contacts (84.70/o), but double or triple contacts (1 5.3%) were also found. The majority of the boutons terminated on dendr ites (84.1%) of parvalbumin-positive cells, less frequently on cell bo dies (15.9%). In order to estimate the proportion of contacts represen ting synapses, 16 light microscopically identified contacts between bo utons of the filled pyramidal cell axon and the parvalbumin-positive t argets were examined by correlated electron microscopy. Thirteen of th em were found to be asymmetrical synapses, and in the remaining three cases synapses between the labelled profiles could not be confirmed. W e conclude that the physiologically effective excitatory connections b etween single pyramidal cells and postsynaptic inhibitory neurons are mediated by a small number of contacts, mostly by a single synapse. Th is results in a high degree of convergence and divergence in hippocamp al networks.