Figure 4.

ERK-driven histone H3 acetylation is necessary for E₂ to enhance novel object recognition memory consolidation. (A) Bilateral infusion of β-cyclodextrin encapsulated E₂ (5 μg/hemisphere) or TSA (16.5 mM/hemisphere) into the dorsal hippocampus significantly increased histone H3 acetylation in the dorsal hippocampus relative to vehicle 30 minutes after infusion (*p < 0.05). (B) Infusion of E₂ (10 μg) into the dorsal third ventricle significantly increased histone H3 acetylation in the dorsal hippocampus relative to vehicle 30 minutes after infusion (*p < 0.05). Infusion of the ERK pathway inhibitor U0126 (0.5 μg/hemisphere) into the dorsal hippocampus blocked this increase, but had no effect on H3 acetylation on its own. (C) Mice were given bilateral infusions of vehicle or one of four doses of the HAT inhibitor garcinol into the dorsal hippocampus immediately after novel object recognition training. Mice infused with 0.1, 1, or 10 μg/hemisphere spent no more time than chance with the novel object. In contrast, mice infused with vehicle or 0.001 μg garcinol exhibited a significant preference for the novel object (*p < 0.05 relative to chance), suggesting that all but the lowest dose of garcinol impaired novel object recognition memory consolidation. (D) When this lowest dose (0.001 μg) of garcinol was infused into the dorsal hippocampus with E₂, it blocked the effects of E₂ on memory, demonstrating that histone acetylation is necessary for E₂ to enhance novel object recognition memory consolidation. (E) Bilateral infusion of E₂ into the dorsal hippocampus significantly reduced levels of HDAC2 protein in the dorsal hippocampus four hours after infusion (*p < 0.05 relative to vehicle). (F) The E₂-induced decrease in HDAC2 protein was blocked by 0.001 μg garcinol, indicating that histone acetylation is necessary for E₂ to reduce HDAC2 levels. Bars in all panels represent the mean ± SEM. Insets in panels A, B, E, and F illustrate representative Western blot images. Acetylated H3 protein was normalized to total H3, and HDAC2 protein was normalized to β-actin. Reprinted with permission from (32, 33).