Dense GABAergic input on somata of parvalbumin-immunoreactive GABAergic neurons in the hippocampus of the mouse.

GABAergic neurons in the hippocampus proper are greatly diverse in their morphological and physiological features. In the present study we examined whether or not they are also diverse regarding the density of GABAergic input on their somata. GABAergic neurons were immunocytochemically identified with antibodies against glutamic acid decarboxylase (GAD), and the densities of GAD-immunoreactive (GAD-IR) boutons that abutted on GAD-IR somata were estimated by conventional light microscopic, combined light and electron microscopic, and confocal laser scanning microscopic analyses. GAD-IR somata were apparently diverse regarding the density of GABAergic input on them, and those surrounded by higher densities of GAD-IR boutons were distributed mainly in the strata pyramidale and oriens of the CA3 and CA1 regions and could be correlated to a parvalbumin (PV)-IR subpopulation of GABAergic neurons. Quantitative analysis clearly revealed the statistically significant difference between PV-positive and PV-negative GAD-IR neurons in the densities of their somatic GAD-IR boutons. Particularly, most of PV-IR neurons in the CA3 stratum pyramidale as well as some in other layers are characterized by an exceedingly high density of perisomatic GAD-IR boutons. Furthermore, the majority of GAD-IR boutons on PV-IR somata in the stratum pyramidale were also PV-IR. Bilateral transection of the fimbria-fornix, which was supposed to remove GABAergic afferents from the septum, had only partial effects on the densities of PV-IR boutons on PV-IR somata, indicating these PV-IR boutons mainly originated from intrinsic PV-IR neurons. These observations indicate the dense mutual connection between PV-IR GABAergic neurons through perisomatic synaptic contacts, particularly in the stratum pyramidale.