PRECISION AND VARIABILITY IN POSTSYNAPTIC TARGET SELECTION OF INHIBITORY CELLS IN THE HIPPOCAMPAL CA3 REGION

Non-pyramidal cells were filled intracellularly with biocytin in the C A3 region of the guinea-pig hippocampus in vitro, within or close to s tratum pyramidale. On the basis of camera lucida reconstructions and e lectron microscopy, six different cell types with distinct laminar dis tribution of axon terminals could be distinguished. The axon of three axo-axonic cells, three typical basket cells, and atypical basket cell s of two types arborized in the perisomatic and proximal dendritic reg ion of CA3 pyramidal cells. Two cells with axons innervating the dista l dendritic segments of pyramidal cells were also found; one terminate d in stratum radiatum and the other in stratum lacunosum - moleculare. Electron microscopy demonstrated that symmetrical synapses were forme d by the labelled boutons on axon initial segments, somata, and proxim al or distal dendrites of mostly pyramidal neurons. Axo-axonic cells s howed absolute target selectivity for axon initial segments, whereas f or the other cells the distribution of contacted elements was determin ed by the laminar distribution of axon terminals. In two cases, where additional cells were labelled with biocytin, multiple (up to nine) li ght microscopically identified contacts (presumed synaptic contacts) w ere established by the interneurons on several pyramidal cells and on an axo-axonic cell. Our results show that a restricted set of inhibito ry cells, with somata within or close to CA3 stratum pyramidale, posse ss variable patterns of axonal arborization. Various types of postsyna ptic elements are contacted, but precision in selecting certain target s and ignoring others is maintained within a particular cell type and layer. In contrast to the diversity of axonal arbors the structure of the dendritic trees shows no consistent differences, suggesting that t he cells may be activated by a similar set of afferents. It seems prob able that the innervation of precise regions of postsynaptic pyramidal cells by different types of interneurons-often in conjunction with pa rticular excitatory afferents (Han et al., Eur. J. Neurosci., 5, 395-4 10, 1993)-underlies functional differences in inhibitory synaptic acti ons.