Granule cells are the main source of excitatory input to a subpopulation of GABAergic hippocampal neurons as revealed by electron microscopic double staining for zinc histochemistry and parvalbumin immunocytochemistry

Immunocytochemistry was combined with a recent modification of Timm's metho d to evaluate semiquantitatively the mossy fiber innervation of dendrites a nd somata of parvalbumin-containing neurons of the hilus of the dentate gyr us and the CA3 area of Ammon's horn. Using this electron microscopic double staining technique, it was found that (1) the overwhelming majority (95%) of terminals forming asymmetric synapses with parvalbumin-positive dendrite s in the dentate hilus, and the strata pyramidale and lucidum of the CA3 ar ea of Ammon's horn, originated from granule cells; (2) two-thirds of the as ymmetric axosomatic terminals of parvalbumin-positive neurons contained zin c; and (3) no zinc-containing axon terminals formed synapses with somata or main dendritic shafts of the granule cells.