Estradiol effects on memory depend on hormone levels and the interaction of different estrogen receptors within neural circuits. Estradiol induces gene transcription and rapid membrane signaling mediated by estrogen receptor-alpha (ERα), estrogen receptor-beta (ERβ), and a recently characterized G-protein coupled estrogen receptor, each with distinct distributions and ability to influence estradiol-dependent signaling. Investigations using receptor specific agonists suggest that all three receptors rapidly activate kinase-signaling and have complex dose-dependent influences on memory. Research employing receptor knockout mice demonstrate that ERα maintains transcription and memory as estradiol levels decline. This work indicates a regulatory role of ERβ in transcription and cognition, which depends on estradiol levels and the function of ERα. The regulatory role of ERβ is due in part to ERβ acting as a negative regulator of ERα-mediated transcription. Vector-mediated expression of estrogen receptors in the hippocampus provides an innovative research approach and suggests that memory depends on the relative expression of ERα and ERβ interacting with estradiol levels. Notably, the ability of estradiol to improve cognition declines with advanced age along with decreased expression of estrogen receptors. Thus, it will be important for future research to determine the mechanisms that regulate estrogen receptor expression during aging.

The expression of the ERα and ERβ estrogen receptors in the hippocampus may be important in the etiology of age-related cognitive decline. To examine the role of ERα and ERβ in regulating transcription and learning, ovariectomized wild-type (WT) and ERα and ERβ knockout (KO) mice were used. Hippocampal gene transcription in young ERαKO mice was similar to WT mice 6 h after a single estradiol treatment. In middle-age ERαKO mice, hormone deprivation was associated with a decrease in the expression of select genes associated with the blood–brain barrier; cyclic estradiol treatment increased transcription of these select genes and improved learning in these mice. In contrast to ERαKO mice, ERβKO mice exhibited a basal hippocampal gene profile similar to WT mice treated with estradiol and, in the absence of estradiol treatment, young and middle-age ERβKO mice exhibited preserved learning on the water maze. The preserved memory performance of middle-age ERβKO mice could be reversed by lentiviral delivery of ERβ to the hippocampus. These results suggest that one function of ERβ is to regulate ERα-mediated transcription in the hippocampus. This model is supported by our observations that knockout of ERβ under conditions of low estradiol allowed ERα-mediated transcription. As estradiol levels increased in the absence of ERα, we observed that other mechanisms, likely including ERβ, regulated transcription and maintained hippocampal-dependent memory. Thus, our results indicate that ERα and ERβ interact with hormone levels to regulate transcription involved in maintaining hippocampal function during aging.

Estradiol rapidly modulates hippocampal synaptic plasticity and synaptic transmission; however, the contribution of the various estrogen receptors to rapid changes in synaptic function is unclear. The current study examined the effect of estrogen receptor selective agonists on hippocampal synaptic transmission in slices obtained from 3-5 month old wild type (WT), estrogen receptor alpha (ERαKO), and beta (ERβKO) knockout female ovariectomized mice. Hippocampal slices were prepared 10-16 days following ovariectomy and extracellular excitatory postsynaptic field potentials were recorded from CA3-CA1 synaptic contacts before and following application of 17β-estradiol-3-benzoate (EB, 100 pM), the G-protein estrogen receptor 1 (GPER1) agonist G1 (100 nM), the ERα selective agonist propyl pyrazole triol (PPT, 100 nM), or the ERβ selective agonist diarylpropionitrile (DPN, 1 μM). Across all groups, EB and G1 increased the synaptic response to a similar extent. Furthermore, prior G1 application occluded the EB mediated enhancement of the synaptic response and the GPER1 antagonist, G15 (100 nM), inhibited the enhancement of the synaptic response induced by EB application. We confirmed that the ERα and ERβ selective agonists (PPT, DPN) had effects on synaptic responses specific to animals that expressed the relevant receptor; however, PPT and DPN produced only a small increase in synaptic transmission relative to EB or the GPER1 agonist. We demonstrate that the increase in synaptic transmission is blocked by inhibition of extracellular signal-regulated kinase (ERK) activity. Furthermore, EB was able to increase ERK activity regardless of genotype. These results suggest that ERK activation and enhancement of synaptic transmission by EB involves multiple estrogen receptor subtypes.

We examine similar and differential effects of two senolytic treatments, ABT‐263 and dasatinib + quercetin (D + Q), in preserving cognition, markers of peripheral senescence, and markers of brain aging thought to underlie cognitive decline. Male F344 rats were treated from 12 to 18 months of age with D + Q, ABT‐263, or vehicle, and were compared to young (6 months). Both senolytic treatments rescued memory, preserved the blood–brain barrier (BBB) integrity, and prevented the age‐related decline in hippocampal N‐methyl‐D‐aspartate receptor (NMDAR) function associated with impaired cognition. Senolytic treatments decreased senescence‐associated secretory phenotype (SASP) and inflammatory cytokines/chemokines in the plasma (IL‐1β, IP‐10, and RANTES), with some markers more responsive to D + Q (TNFα) or ABT‐263 (IFNγ, leptin, EGF). ABT‐263 was more effective in decreasing senescence genes in the spleen. Both senolytic treatments decreased the expression of immune response and oxidative stress genes and increased the expression of synaptic genes in the dentate gyrus (DG). However, D + Q influenced twice as many genes as ABT‐263. Relative to D + Q, the ABT‐263 group exhibited increased expression of DG genes linked to cell death and negative regulation of apoptosis and microglial cell activation. Furthermore, D + Q was more effective at decreasing morphological markers of microglial activation. The results indicate that preserved cognition was associated with the removal of peripheral senescent cells, decreasing systemic inflammation that normally drives neuroinflammation, BBB breakdown, and impaired synaptic function. Dissimilarities associated with brain transcription indicate divergence in central mechanisms, possibly due to differential access.

A decline in estradiol (E2)-mediated cognitive benefits denotes a critical window for the therapeutic effects of E2, but the mechanism for closing of the critical window is unknown. We hypothesized that upregulating the expression of estrogen receptor ␣ (ER␣) or estrogen receptor ␤ (ER␤) in the hippocampus of aged animals would restore the therapeutic potential of E2 treatments and rejuvenate E2-induced hippocampal plasticity. Female rats (15 months) were ovariectomized, and, 14 weeks later, adeno-associated viral vectors were used to express ER␣, ER␤, or green fluorescent protein (GFP) in the CA1 region of the dorsal hippocampus. Animals were subsequently treated for 5 weeks with cyclic injections of 17␤-estradiol-3-benzoate (EB, 10 g) or oil vehicle. Spatial memory was examined 48 h after EB/oil treatment. EB treatment in the GFP (GFP ϩ EB) and ER␤ (ER␤ ϩ EB) groups failed to improve episodic spatial memory relative to oil-treated animals, indicating closing of the critical window. Expression of ER␤ failed to improve cognition and was associated with a modest learning impairment. Cognitive benefits were specific to animals expressing ER␣ that received EB treatment (ER␣ ϩ EB), such that memory was improved relative to ER␣ ϩ oil and GFP ϩ EB. Similarly, ER␣ ϩ EB animals exhibited enhanced NMDAR-mediated synaptic transmission compared with the ER␣ ϩ oil and GFP ϩ EB groups. This is the first demonstration that the window for E2-mediated benefits on cognition and hippocampal E2 responsiveness can be reinstated by increased expression of ER␣.