Colocalization of parvalbumin and somatostatin-like immunoreactivity in the mouse hippocampus: Quantitative analysis with optical disector

The colocalization of parvalbumin (PV) and somatostatin (SS)-like immunorea ctivity was studied quantitatively in the mouse hippocampus, with particula r reference to their areal and dorsoventral differences. The optical disect or method was applied by using a confocal laser scanning microscope with im munofluorescent double-labeling. In the present study, we found a particula r subpopulation of hippocampal nonprincipal neurons that contained both PV and SS-like immunoreactivity, i.e., PV-immunoreactive (IR)/SS-like immunore active (LIR) neurons. In the CA1 region, PV-IR/SS-LIR, neurons were restric ted to the stratum oriens (SO). In the CA3 region, they were scattered in t he SO, stratum pyramidale (SP), and stratum radiatum (SR). However, they we re rarely seen in the dentate gyrus (DG). The proportion of PV-IR/SS-LIR ne urons in the PV-IR neurons or SS-LIR neurons was about 10% in the CA1 regio n, 15-30% in the CA3 region, 0-5% in the DG, and 10-20% in total. Laminar a nalysis revealed that the proportions of PV-IR/SS-LIR neurons in the PV-IR neurons were high in the SO (about 25%) of the CA1 region, and in the SO (a bout 50%) and SR (30-45%) of the CA3 region. The proportion of PV-IR/SS-LIR neurons in the SS-LIR neurons was low in the SO of the CA1 region (about 1 0%), but high in the SO (35-65%) and SR (35-45%) of the CA3 region. Morphol ogically, medium-sized horizontal fusiform and multipolar PV-IR/SS-LIR neur ons were frequently observed, and they showed weak immunoreactivity for PV. Large-sized vertical bitufted and triangular PV-ER neurons lacked SS-like immunoreactivity, and most of them showed moderate to intense immunoreactiv ity for PV. Tn addition, we provide direct Evidence that some PV-IR/SS-LIR neurons projected to the medial septum by using retrograde labeling with Fl uoro-Gold injection. These observations indicate that PV-IR/SS-LIR neurons constitute a particular subpopulation of hippocampal nonprincipal neurons. (C) 2000 Wiley-Liss, Inc.