Pyramidal cell dendrites are the primary targets of calbindin D28k-immunoreactive interneurons in the hippocampus

The axonal arborization and postsynaptic targets of calbindin D28k (CB)-immunoreactive nonprincipal neurons have been studied in the rat dorsal hippocampus. Two types of neurons were distinguished on the basis of soma location, the characteristics of the dendritic free, and the axon arborisation pattern. Type I cells were located in stratum radiatum of the CA1 and CA3 regions and occasionally in strata pyramidale and oriens. These cells had multipolar or bitufted dendritic trees primarily located in stratum radiatum. Their axons could be followed for a considerable distance, arborised within stratum radiatum, and were covered with regularly spaced small boutons. As demonstrated with postembedding immunogold staining, their axon terminals were gamma-aminobutyric acid (GABA) immunoreactive, and formed symmetrical synapses predominantly on proximal and distal dendrites of pyramidal cells (28% and 58%, respectively), and occasionally on spines (9%) or on GABA-positive dendrites (5%). Type II cells were found exclusively in stratum oriens of the CA1 and CA3 regions and possessed large, fusiform cell bodies and long, horizontally oriented dendrites. Their axon initial segments turned towards the alveus and disappeared in a myelin sheet, which was often possible to follow into the white matter. We conclude that type I CB-immunoreactive cells are likely to represent a major source of inhibitory synapses in the dendritic region of pyramidal cells, which are responsible for the control of dendritic electrogenesis. The distribution of local collaterals of type II cells-if they have any-remains unknown, but their main axon is likely to project to the medial septum.