Immunocytochemical characterization of hippocamposeptal projecting GABAergic nonprincipal neurons in the mouse brain: a retrograde labeling study

The neurochemical contents of hippocamposeptal projecting nonprincipal neurons were examined in the mouse brain by using retrograde labeling techniques. We used the immunofluorescent multiple labeling method with a confocal laser-scanning microscope. First of all, the hippocamposeptal projecting nonprincipal neurons were glutamic acid decarboxylase 67-immunoreactive (IR), i.e., these hippocamposeptal projecting nonprincipal neurons were immunocytochemically GABAergic in the mouse brain. Next, most (93.0%) of the hippocamposeptal projecting GABAergic neurons were somatostatin-like immunoreactive (SS-LIR). The SS-LIR hippocamposeptal projecting neurons were frequently found in the stratum oriens of the CA1 and CA3 regions, and were also occasionally found in the stratum radiatum, stratum lucidum, and stratum pyramidale of the CA3 region. They were also frequently found in the dentate hilus. On the other hand, at least 40.6% of SS-LIR neurons in the hippocampus projected to the medial septum. Next, 38.0% of hippocamposeptal projecting GABAergic neurons were calbindin D28K (CB)-IR. Although the distribution of the CB-IR hippocamposeptal projecting neurons was generally similar to that of the SS-LIR projecting neurons in Ammon's horn, they were never seen in the dentate hilus. At least 22.1% of CB-IR GABAergic neurons in the hippocampus projected to the medial septum. Furthermore, 5.8% of hippocamposeptal projecting GABAergic neurons were parvalbumin-IR, which were most always found in Ammon's horn. Finally, no hippocamposeptal projecting GABAergic neurons were neuronal nitric oxide synthase-IR nor calretinin-IR. These results indicate that the SS-LIR neurons play a crucial role in the hippocamposeptal projection of the mouse brain, and they are also assumed to be involved in the theta oscillation of the mouse hippocampus.