Figure 2. HIL cells form long-range projections to the medial septum and vertical diagonal band of Broca (MSvDB).

(A) Left, schematic represents the bilateral injection of rAAV-FLEX-GFP in dorsal DGs of SOM-Cre mice. Middle, confocal image stack shows SOM⁺ somata in the hilus and fiber projections in the molecular layer (ml) and hilus (hil). Right, colocalization of immunohistochemically identified SOM (red) and virally expressed GFP. Blue indicates DAPI nuclear staining. Arrows point to somata colocalizing SOM and GFP. (B) Left, schematic illustration of the MSvDB location. Middle, three-dimensional Clarity-processed hippocampal SOM-Cre whole mount after rAAV-GFP injection in the dorsal DG. GFP⁺ projections leave the dorsal DG (dDG) and project toward the fimbria/fornix. Inset, illustrates the depicted area of the dorsal DG in relation to the orientation of the entire brain. Right, confocal image stack of a frontal 50-µm section of the MSvDB. Dashed line indicates the boarder of the MSvDB. Inset, higher magnification of two types of axons in the MSvDB: thick axon with few varicosities and thin axons with several varicosities. (C–E) Identification of DG-septal projecting SOMIs. (C) Schematic illustrates the injection of the retrograde tracer RedRetroBead (red) in the medial septum. Bar graphs summarize colocalization of SOM (antibody labeling) in the DG of rAAV-GFP injected SOM-Cre mice (left) and retrogradely labeled DG cells in SOM-GIN mice (right). (D) Confocal image stack shows colocalization of GFP in SOM-GIN mice and retrogradely labeled cells (red) after RedRetroBead injection in the MSvDB. Insets, arrow points to a RedRetroBead-labelled soma co-expressing SOM-GFP and intracellularly labeled with biocytin (white) during whole-cell recordings. (E) Morphological identification of a retrogradely tagged HIL cell. Dotted line indicate area of high axonal density. Continuous line depicts boarders of the granule cell layer (gcl). Inset, bar graph summarizes the number of morphologically identified HIL cells projecting to the MSvDB. Bars with lines represent means ± SEM, single circles represent values of individual slices. Abbreviations: gcl, granule cell layer; ml, molecular layer; hil, hilus; dDG, dorsal dentate gyrus; MS, medial septum; LS, lateral septum; vDB vertical limb of the diagonal band of Broca.

DOI: http://dx.doi.org/10.7554/eLife.21105.006

Figure 2—figure supplement 1. Morphological reconstructions of DG-SOMI cells retrogradely labeled from the medial septum.

Reconstructions of 2 HIL cells intracellularly labeled with biocytin during whole-cell recordings, 3–8 days after RedRetroBead injection into the medial septum. Somata and dendrites are depicted in black and the axons in red. Note, the majority of retrogradelly labeled DG-SOMIs were morphologically identified as HIL cells with axon collaterals preferentially distributed in the hilus (in total: 13 HIL cells, two additonal SOMIs with axon in the gcl, iml and hilus but not in the oml; five injected mice). Abbreviations: gcl, granule cell layer; hil, hilus; iml, inner molecular layer; oml, outer molecular layer.

DOI: http://dx.doi.org/10.7554/eLife.21105.007