Nonpyramidal cells were filled intracellularly with biocytin in the CA3 region of the guinea-pig hippocampus in vitro, within or close to stratum pyramidale. On the basis of camera lucida reconstructions and electron microscopy 6 different cell types, with distinct laminar distribution of axon terminals, could be distinguished. The axon of axo-axonic cells (3), typical basket cells (3) and atypical basket cells (2 different types) arborized in the perisomatic and proximal dendritic region of CA3 pyramidal cells. Two cells with axons innervating the distal dendritic segments of pyramidal cells were also found, one terminated in stratum radiatum, and the other in stratum lacunosum-moleculare. Electron microscopy demonstrated that symmetrical synapses were formed by the labelled boutons on axon initial segments, somata, and proximal or distal dendrites of mostly pyramidal neurons. Axo-axonic cells showed an absolute target selectivity for axon initial segments, whereas for the other cells the distribution of contacted elements changed according to the laminar distribution of axon terminals. In two cases, where additional cells were labelled with biocytin, multiple (up to 9) contacts were established by the interneurons on several pyramidal cells and on an axo-axonic cell.
Our results demonstrate that even a restricted set of nonpyramidal cells (i.e. those within or close to stratum pyramidale of CA3) may display a wide range of axon arborization patterns. Various types of postsynaptic elements are contacted, but precision in selecting certain targets and ignoring others is maintained within a particular cell type. In contrast to the diversity of axonal arbours the structure of the dendritic trees show no consistent differences, suggesting that the cells may be activated by a similar set of afferents. We speculate that a restricted innervation of different segments of pyramidal cells by various types of nonpyramidal neurons - often in conjunction with particular excitatory afferents - reflect functional specialization of inhibition in hippocampal neuronal circuits.