Patterns of expression of calcium binding proteins and neuronal nitric oxide synthase in different populations of hippocampal GABAergic neurons in mice

We examined the expression of calcium binding proteins parvalbumin (PV), calretinin (CR), and calbindin D28K (CB), and neuronal nitric oxide synthase (nNOS) in γ-aminobutyric acid (GABA)ergic neurons of the mouse hippocampus, with particular reference to areal and dorsoventral differences. First, we estimated the colocalization of the calcium binding proteins and nNOS. GABAergic neurons containing both PV and nNOS, i.e., PV-immunoreactive (-IR)/nNOS-IR neurons, were rare in Ammon's horn but frequent in the dentate gyrus (DG). CR-IR/nNOS-IR neurons and CB-IR/nNOS-IR neurons were frequent in Ammon's horn but rare in the DG. In the entire hippocampus, the percentage of CR-IR neurons containing nNOS was significantly higher at the ventral level (44.3%) than at the dorsal level (17.0%). The percentage of CB-IR neurons containing nNOS was also significantly higher at the ventral level (42.3%) than at the dorsal level (29.3%). Next, we estimated the numerical densities (NDs) of calcium binding protein-containing GABAergic neurons. The ND of PV-IR neurons was comparable at the dorsal (1.16 × 10³/mm³) and ventral levels (1.23 × 10³/mm³), respectively. The ND of CR-IR neurons was less at the dorsal level (0.52 × 10³/mm³) than at the ventral level (0.64 × 10³/mm³). The ND of CB-IR neurons was also less at the dorsal level (0.91 × 10³/mm³) than at the ventral level (1.57 × 10³/mm³). Overall, approximately half of the GABAergic neurons contained one of the three calcium binding proteins (45% at the dorsal level and 47% at the ventral level). These data establish a baseline for examining potential roles of GABAergic neurons in hippocampal network activity in mice.