Fig. 5: Inactivation of hippocampal-LS projections ameliorates disruptive social effects of S-SDFC.

a AAV8-hSyn-hM4D(Gi)-mCherry encodes expression of mCherry (brown labeling) for mapping projections sites of hippocampal-septal projections. In the septum, dense projections in the caudal LS were observed. b Schematic illustration of the dual viral approach for inactivating synaptic transmission in LS-projecting hippocampal neurons. AAV8.EF1α.mCherry-WGA-Cre mediates expression of WGA-Cre. When this AAV infects a neuron, WGA-Cre is transneuronally transferred to connected neurons. Double-floxed inverted AAV8.hSyn-Hm4D(Gi)-mCherry encodes expression of mCherry for visualizing infected neurons and Hm4D(Gi) for conditionally blocking synaptic transmission. The coding region of the double-floxed inverted Hm4D(Gi) is not translated until Cre-recombinase flips the inverted coding region into the correct orientation. Scale bars: 250 μm. c Schematics of the neuroanatomical sites are on the left, images showing virus infection (mCherry signals, red) are on the right. LS-projecting hippocampal neurons were seen throughout the hippocampus, with particularly dense labeling in the CA3 field. Anatomic reference content was obtained from the Allen Brain Atlas⁷². d Schematic outline of the behavioral paradigm. Mice were infused with both viruses. Five and a half weeks later, mice were implanted with cannula targeting the DG. Three days later, mice were injected i.h. with GBX before a 2-day eight-shock contextual fear conditioning procedure. On alternating days, mice were tested for memory retrieval on VEH or GBX. Following memory retrieval tests, mice were injected i.h. with VEH or CNO before sociability testing (S-SDFC-VEH and S-SDFC-CNO groups, respectively). e During memory retrieval testing, S-SDFC-VEH and S-SDFC-CNO mice froze more in the presence of GBX compared to VEH. n = 8 mice per group; memory retrieval test drug F_(1,14) = 25.02, P = 0.0002; conditioning drug F_(1,14) = 0.008804, NS; interaction F_(1,14) = 10.69, P = 0.0056. f During sociability testing, S-SDFC-VEH mice did not display a significant preference between the mouse and toy, while S-SDFC-CNO mice did display a significant preference. n = 8 S-SDFC-VEH mice t = 1.113, df = 14, P = 0.2844; n = 8 S-SDFC-CNO mice t = 4.849, df = 14, P = 0.0003. Both control VEH and CNO mice displayed a significant preference between the mouse and toy. n = 7VEH mice t = 3.784, df = 12, P = 0.0026; n = 8CNO mice t = 4.128, df = 14, P = 0.0010. g Schematic outline of the behavioral paradigm. Mice were infused with both viruses. Five and a half weeks later, mice were implanted with cannula targeting the DG. Three days later, mice were i.h. with CNO before sociability testing. h Schematic of proposed theoretical model. (Left) In groups which exhibited sociability (three-compartment social behavior chamber, black outlines), a greater proportion of neurons was active in the suprapyramidal blade and distal CA3, as compared to the infrapyramidal blade and proximal CA3. In contrast, mice with disrupted sociability (three-compartment social behavior chamber, red outline) did not exhibit this asymmetric patterned activity, but instead displayed similar number of active cells between the DG blades and across proximodistal CA3. (Right) Models of hippocampal activity across the transverse axis in the regulation of social, spatial, and nonspatial tasks, respectively. Experimentally derived activity data (solid lines) are denoted from hypothetical activity (dashed lines) based upon known anatomical connectivity. Scale bars: 250 μm. **P < 0.01, ***P < 0.001. Alv. alveus, CA1. cornu ammonis 1, CA2. cornu ammonis 2, CA3. cornu ammonis 3, DG dentate gyrus, Dist. distal, fi. fimbria, hf. hippocampal sulcus, HPF. hippocampal formation, Infra. infrapyramidal granule cell blade, LSc. caudal lateral septum, LSr. rostral lateral septum, mo. startum moleculare, MS. medial septum, po. dentate gyrus polymorph layer, Prox. proximal, sg. granule cell layer, slm. stratum lacunosum moleculare, slu. stratum lucidem, so. stratum oriens, sp. stratum pyramidale, sr. stratum radiatum, Supra. suprapyramidal granule cell blade.