Calretinin (CR)-containing cells were visualized with immunocytochemistry in the rat dorsal hippocampal formation. CR- immunoreactivity was present exclusively in non-pyramidal cells in all layers of the dentate gyrus and the CA1-CA3 areas. CR-positive neurons and processes were most abundant in the hilus of the dentate gyrus and in stratum lucidum of the CA3 region. Several CR- immunoreactive (CR-IR) cells were located within the hippocampal fissure. A distinct band of CR-IR fibres occupied the superficial part of the granule cell layer and the lowest part of the molecular layer. Closer examination of the CR-positive cells revealed that they formed two distinct cell groups. One group of cells, found exclusively in stratum lucidum of the CA3 area and in the hilus of the dentate gyrus, were covered with numerous spines. Their somata and dendrites were restricted to stratum lucidum and to the hilus. Cells of the other group had smooth, often varicose, radially running dendrites, and were present in all areas and layers of the hippocampal formation.
CR-IR cells with spiny cell bodies and dendrites: 2-3 thick, primary dendrites arose from the irregularly shaped cell body and emitted fine secondary branches only distally (70-100æm) from the soma, where they formed a profuse network. The extensive dendritic tree of the cells spread horizontally within stratum lucidum and span a distance of 400-600 æm both in the septo-temporal and in the transverse directions. The layer-specific location of these cells and their processes suggested that the majority of their input may derive from mossy fibres. This presumption has been confirmed by electron microscopic examination. A large number of asymmetrical synapses were found to cover the soma, the dendritic shafts and the spines (4-6 synapses per spine) of the cells. A large proportion of the synapses were formed by boutons, which showed the distinctive features of mossy fibre terminals. Spine-free CR-IR cells: 3-6 primary dendrites arose from the multipolar, bipolar or pyramidal-shaped somata, which were smaller than the somata of spiny cells. The smooth and frequently varicose dendrites branched proximally and ran primarily radially. Dendrites ascended or descended through several layers and received both asymmetrical and symmetrical synapses. In the CA1 subfield the vertically running dendrites frequently contacted other CR-IR spine-free dendrites or cell bodies. Two or three CR-IR dendrites were often seen to be attached for over a hundred or occasionally 200 micrometers, and several puncta adherentia were observed between them in the electron microscope. The axons of the cells arborized in all layers, had small en passant varicosities, and were found to establish symmetrical synapses with dendritic shafts. In summary, spine-free CR-IR cells showed the characteristic features of GABAergic interneurons both at the light and the electron microscopic level.
Axon terminals of the dense CR-IR fiber network in the superficial part of the granule cell layer formed synapses with dendritic shafts and spines of putative granule cells. The characteristic location and features of the synapses, i.e. a very thick and dense postsynaptic specialization, suggest that these fibres derive from the supramammillary nucleus, rather than from CR-IR neurons in the hippocampus.