Proportion of parvalbumin‐positive basket cells in the GABAergic innervation of pyramidal and granule cells of the rat hippocampal formation

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Abstract
Recent studies have indicated that hippocampal GABAergic neurons in both the dentate gyrus and Ammon's horn contain immunoreactivity for the calcium‐binding protein parvalbumin (PARV). Although the distribution of PARV‐positive neurons has been previously described, detailed quantitative electron microscopic studies of the PARV‐positive axon terminals in the hippocampal formation are lacking. In the present study, immunocytochemical methods were used to localize PARV‐positive neurons and axon terminals to determine their similarity to GABAergic neurons. The PARV‐positive cells and axon terminals are associated closely with the pyramidal and granule cell layers. In agreement with previous studies, the morphology of PARV‐positive neurons is similar to that of GABAergic cells, including the basket cells of both the dentate gyrus and Ammon's horn. The PARV‐positive axon terminals form exclusively symmetric synapses with somata, dendrites, dendritic spines, and axon initial segments. However, these terminals represent only a portion of the total number of terminals that form symmetric synapses. Quantitative results indicate that only 32–38% of the total number of terminals forming symmetric axosomatic synapses with principal cells of the dentate gyrus and Ammon's horn are PARV positive. Together with previous findings from light microscopic double‐labeling studies, these data indicate that the PARV‐positive terminals arise from a subpopulation of GABAergic hippocampal neurons. Finally, it is important to note that the terminal plexus of PARV‐positive hippocampal axons overlaps at all postsynaptic sites with a plexus of PARV‐negative axons.