Previous Midlife Oestradiol Treatment Results in Long‐Term Maintenance of Hippocampal Oestrogen Receptor α Levels in Ovariectomised Rats: Mechanisms and Implications for Memory

Abstract

Ovariectomized rats that received previous administration of oestradiol in midlife display enhanced cognition and increased hippocampal levels of oestrogen receptor alpha (ERα) months after oestradiol treatment ended as compared to ovariectomized controls. Objectives of the current work were to investigate mechanisms by which ERα levels are maintained following midlife oestradiol exposure and the role of ERα in memory in aging females in the absence of circuiting oestrogens. Unliganded ERα has increased interaction with the ubiquitin ligase, C terminus of Hsc-70 interacting protein (CHIP) leading to increased degradation of the receptor. In our first experiment, we tested the hypothesis that midlife oestradiol exposure in ovariectomized rats results in decreased interaction between CHIP and hippocampal ERα, leading to increased levels of ERα. Middle-aged rats were ovariectomized and received oestradiol or vehicle implants. After 40 days, implants were removed. One month later, rats were killed and hippocampi processed for whole protein western blotting and co-immunoprecipitation, in which ERα was immunoprecipitated from lysate. As expected, ERα protein expression was increased in rats previously treated with oestradiol compared to vehicle-treated rats. In rats treated with oestradiol, there was a decrease in CHIP-ERα interaction, suggesting that previous oestradiol treatment reduces interaction, slowing degradation of ERα. In a second experiment, we determined the impact on memory of antagonism of ER in the absence of circulating oestrogens. Rats were ovariectomized and implanted with oestradiol capsules. Capsules were removed after 40 days. Rats received chronic i.c.v. infusion of ER antagonist, ICI 182,780 or aCSF vehicle and were tested on a spatial memory radial maze task. Rats treated with ICI 182,780 had significantly worse performance (more errors). These experiments provide evidence that previous midlife oestradiol treatment maintains hippocampal ERα by decreasing its interaction with CHIP and that activation of these receptors provides cognitive benefits in the absence of circulating oestrogens.

Previous estradiol treatment in ovariectomized mice provides lasting enhancement of memory and brain estrogen receptor activity

“…Results of the current study demonstrate that short-term estradiol exposure can have effects on hippocampus-dependent memory in ovariectomized mice that persist after treatment has ceased and in absence of ovarian-derived or exogenously administered estrogens. The results reported here are consistent with both previous studies from our lab in rats (Black et al, 2016; Rodgers et al, 2010; Witty et al, 2013) and clinical results (Bagger et al, 2005; Whitmer et al, 2011). The current results extend our previous findings from rats into the mouse species to validate the use of the transgenic ERE-LUC reporter mouse in subsequent experiments.…”

“…Short-term continuous, unopposed estradiol exposure also provides long-term enhancement of classic ERE-dependent gene activation in the hippocampus. This is consistent with previous findings from our lab highlighting the importance of ERα expression (Witty et al, 2012) and activity (Black et al, 2016) in the hippocampus in mediating memory enhancement after previous estradiol exposure. The results of the current experiment extend our previous findings by confirming that canonical ERE-dependent gene expression is enhanced in the hippocampus by previous estradiol exposure 28 days after termination of treatment.…”

“…Memory enhancement coincided with lasting increased expression of estrogen receptor alpha (ERα) and the estrogen-sensitive target protein choline acetyltransferase in the hippocampus. In subsequent studies we found that antagonism of the nuclear estrogen receptors in the hippocampus following previous estradiol exposure abrogates memory enhancement (Black et al, 2016) and exogenously increasing expression of ERα in the hippocampus alone of ovariectomized rats is sufficient to induce memory enhancement (Witty et al, 2012). Together these observations indicate that estrogen receptor signaling in the hippocampus is an important mediator of the observed hippocampus-dependent memory enhancement that persists long after previous exposure to continuous, unopposed estradiol.…”

SK3-overexpression drives sex-dimorphic working and social memory impairment in mice

“…In the hippocampus, estrogen might facilitate the induction of LTP. Studies also offer evidence that estrogen treatment in mid-age would improve aging-induced cognitive decline (Black et al, 2016). Based on the general hypothesis that estrogen actions in the hippocampus account for the sex dimorphism observed during memory process (McEween, 2002) and that levels of 17-β estradiol are related to SK3 expression levels, we sought the analysis of the levels of these sex hormones in the DG or hippocampus of WT or T/T mice (both sexes).…”

The Role of Estrogen in Anxiety-Like Behavior and Memory of Middle-Aged Female Rats

“…Therefore, studies performed in middle-aged rats are valuable and important for understanding the causal relationship between estrogen deficiency and psychological features of post-menopausal syndrome. Although there are published studies on the effect of ovariectomy (29,30,(42)(43)(44) and estradiol treatment (21-23, 25, 29, 31) on memory and anxietylike behavior in middle-aged (∼12-month old) and old rodents (>18-month old), studies on such behavioral effects of letrozole treatment are rare (45).…”