Fig. 4: VMT-dHP photoinhibition increases hippocampal activity and rescues high load incidental LTM in females.

a Representative images of the AAV injection in the VMT and its diffusion in the fiber terminals at the level of the dHP (scale bar 100 µm and 50 µm in the squared magnification). b Schematic of the experimental plans: AAV-hSyn-eNpHR3.0-EYFP was injected in the VMT and after about 4–5 weeks mice underwent ex vivo recordings in hippocampal slices (b′) or in vivo hippocampal multiunit activity recordings under anesthesia (b″). b′ On the top, representative traces from CA1 hippocampal neurons showing EPSCs in baseline and under photoinhibition (green bar). On the bottom, scatterplot showing EPSPs increased frequency and amplitude for females. b″ On the top, representative traces of multiunit activity recorded from hippocampal CA1 pyramidal layer before and during photoinhibition (green bar). On the bottom, bar charts represent increased frequency activity in females between baseline (control) and under photoinhibition (light ON). *p < 0.05 between groups (Wilcoxon test). c Schematic of the experimental design showing the timing of VMT-dHP fibers post-training (30 min) continuous inhibition. c′ Bar charts represent the new object % exploration in light OFF and light ON females in the test conducted 24 h after post-training VMT-dHP photoinhibition, which shows a rescued memory consolidation capacity. d–d′ VMT-dHP photoinhibition increased c-Fos⁺ cells in the CA1 pyramidal layer of females; scale bars: 100 µm. e Schematic of the dHP pyramidal layer (py) and stratum oriens (so) in which the c-Fos⁺ cells and the co-localizing signal with GAD67 (GAD⁺ cells) was analyzed. f–f′ VMT-dHP photoinhibition also increased c-Fos⁺ cells in the so of the CA1, but the percentage of GAD⁺/c-Fos⁺ cells was lowered by VMT-dHP photoinhibition; scale bar 100 µm; magenta arrows indicate c-Fos⁺ cells, yellow arrows GAD⁺ ones, while white arrows indicate c-Fos⁺/GAD⁺ cells. g–g′ Quantitative analyses of immunofluorescence intensity in the CA1 excitatory neurons after VMT-dHP photoinhibition showed decreased p-EIF2α/EIF2α ratio in females; scale bar 10 µm. *p < 0.05 between groups. Data in bar charts are presented as mean values ± SEM. For statistics see Supplementary Table 5.