Patterns of expression of neuropeptides in GABAergic nonprincipal neurons in the mouse hippocampus: Quantitative analysis with optical disector

Neuropeptides are widely distributed in the central nervous system and are considered to play important roles in the regulation of neuronal activity. This study shows the patterns of expression of four neuropeptides [neuropeptide Y (NPY), somatostatin (SOM), cholecystokinin (CCK), and vasoactive intestinal polypeptide (VIP)] in γ-aminobutyric acid (GABA)-ergic neurons of the mouse hippocampus, with particular reference to the areal and dorsoventral difference. First, we estimated the numerical densities (NDs) of GABAergic neurons containing these neuropeptides using the optical disector. The NDs of NPY- and SOM-positive GABAergic neurons were generally higher than those of CCK- and VIP-positive GABAergic neurons. In the whole area of the hippocampus, the ND of NPY-positive GABAergic neurons showed no significant dorsoventral difference (1.90 × 10³/mm³ in the dorsal level, 2.09 × 10³/mm³ in the ventral level), whereas the ND of SOM-positive GABAergic neurons was higher in the ventral level (1.44 × 10³/mm³) than in the dorsal level (0.80 × 10³/mm³). The ND of CCK-positive GABAergic neurons was also higher in the ventral level (0.57 × 10³/mm³) than in the dorsal level (0.33 × 10³/mm³). Similarly, the ND of VIP-positive GABAergic neurons was higher in the ventral level (0.61 × 10³/mm³) than in the dorsal level (0.43 × 10³/mm³). Next, we calculated the proportions of GABAergic neurons containing these neuropeptides among the total GABAergic neurons. In the whole area of the hippocampus, NPY-, SOM-, CCK-, and VIP-positive neurons accounted for about 31%, 17%, 7%, and 8% of GABAergic neurons, respectively. The present data establish a baseline for examining potential roles of GABAergic neurons in the hippocampal network activity in mice.