eYFP fibers labeled after injection of AAV-DJ-hSyn-Con/Fon-hChR2(H134R)-eYFP-WPRE into cNOS-fDLX mice, or for comparison cSOM-fDLX mice, were quantified in the medial septum (MS; [a]), diagonal band of broca (DB; [b]), and dorsal tenia tecta (TT; [c]). Left: example confocal images of eYFP+ fibers from the MS (sagittal; [a]), DB (coronal; [b]), and TT (sagittal; [c]). Right: Histograms of fiber length by region and mouse line; cumulative data from n = 3 animals are shown (three cNOS-fDLX mice and three cSOM-fDLX mice; one 50 μm section each). Note that cSOM-fDLX mice were used as a point of comparison because SOM+ GABAergic projection cells are a known population of hippocampal neurons that project to the MS/DB area (referred to in the literature as hippocampal-septal cells [Jinno et al., 2007; Klausberger, 2009]). While LINCs robustly project to the MS and DB (a, b), LINC projections in the MS branch less profusely than SOM+ projections, as seen in the reduced number of short fibers (a), while the number of longer fibers (passing through the MS) are similar (inset in panel [a], right). (d) Fiber density, however, is lower in MS and DB in cNOS-fDLX compared to cSOM-fDLX animals. Substantial eYFP fibers are also seen in the TT in all cNOS-fDLX animals, although similar fibers are essentially lacking in cSOM-fDLX mice (c, d), supporting the hypothesis that LINCs, but not SOM+ hippocampal-septal cells, project to the TT. Open dots in panel (d) represent data from individual animals; bars represent the mean. (e) These basic findings based on eYFP expression were confirmed upon FG injection into the medial septum, diagonal band, or tenia tecta, and they suggest both some overlap between SOM+ hippocampal-septal cells and LINCs, as well as the existence of distinct SOM-immunonegative LINCs. A portion of FG-labeled LINCs were SOM immuno+ after FG injection into the medial septum (8 of 12 FG+ LINCs were SOM+, n = 3 animals, cells counted in every other 50 µm section), and after FG injection into the diagonal band of Broca (4 of 29 FG+ LINCs in CA1, n = 3 animals, cells counted in every fourth 50 µm section). However, paralleling the eYFP fiber quantification, no FG+ LINCs were SOM+ after FG injection into the tenia tecta (n = 3 animals, cells counted in every other 50 µm section). Scale bars in panels (a–c) = 100 µm.