Pharmacological blockade of the aromatase enzyme, but not the androgen receptor, reverses androstenedione-induced cognitive impairments in young surgically menopausal rats.

Androstenedione, the main circulating ovarian hormone present after menopause, has been shown to positively correlate with poor spatial memory in an ovary-intact rodent model of follicular depletion, and to impair spatial memory when administered exogenously to surgically menopausal ovariectomized rats. Androstenedione can be converted directly to estrone via the aromatase enzyme, or to testosterone. The current study investigated the hormonal mechanism underlying androstenedione-induced cognitive impairments. Young adult ovariectomized rats were given either androstenedione, androstenedione plus the aromatase inhibitor anastrozole to block conversion to estrone, androstenedione plus the androgen receptor blocker flutamide to block androgen receptor activity, or vehicle treatment, and were then administered a battery of learning and memory maze tasks. Since we have previously shown that estrone administration to ovariectomized rats impaired cognition, we hypothesized that androstenedione's conversion to estrone underlies, in part, its negative cognitive impact. Here, androstenedione administration impaired spatial reference and working memory. Further, androstenedione did not induce memory deficits when co-administered with the aromatase inhibitor, anastrozole, whereas pharmacological blockade of the androgen receptor failed to block the cognitive impairing effects of androstenedione. Anastrozole alone did not impact performance on any cognitive measure. The current data support the tenet that androstenedione impairs memory through its conversion to estrone, rather than via actions on the androgen receptor. Studying the effects of aromatase and estrogen metabolism is critical to elucidating how hormones impact women's health across the lifespan, and results hold important implications for understanding and optimizing the hormone milieu from the many endogenous and exogenous hormone exposures across the lifetime.

A component of Premarin(®) enhances multiple cognitive functions and influences nicotinic receptor expression.

In women, ovarian hormone loss at menopause has been related to cognitive decline, and some studies suggest that estrogen-containing hormone therapy (HT) can mitigate these effects. Recently, the Women's Health Initiative study found that conjugated equine estrogens, the most commonly prescribed HT, do not benefit cognition. Isolated components of conjugated equine estrogens (tradename Premarin(®)) have been evaluated in vitro, with delta(8,9)-dehydroestrone (∆(8)E1) and equilin showing the strongest neuroprotective profiles. It has not been evaluated whether ∆(8)E1 or equilin impact cognition or the cholinergic system, which is affected by other estrogens and known to modulate cognition. Here, in middle-aged, ovariectomized rats, we evaluated the effects of ∆(8)E1 and equilin treatments on a cognitive battery and cholinergic nicotinic receptors (nAChR). Specifically, we used (125)I-labeled epibatidine binding to assay brain nicotinic receptor containing 4α and 2ß subunits (α4ß2-nAChR), since this nicotinic receptor subtype has been shown previously to be sensitive to other estrogens. ∆(8)E1 enhanced spatial working, recent and reference memory. ∆(8)E1 also decreased hippocampal and entorhinal cortex α4ß2-nAChR expression, which was related to spatial reference memory performance. Equilin treatment did not affect spatial memory or rat α4ß2-nAChR expression, and neither estrogen impacted (86)Rb(+) efflux, indicating lack of direct action on human α4ß2 nAChR function. Both estrogens influenced vaginal smear profiles, uterine weights, and serum luteinizing hormone levels, analogous to classic estrogens. The findings indicate that specific isolated Premarin(®) components differ in their ability to affect cognition and nAChR expression. Taken with the works of others showing ∆(8)E1-induced benefits on several dimensions of health-related concerns associated with menopause, this body of research identifies ∆(8)E1 as a new avenue to be investigated as a potential component of HT that may benefit brain health and function during aging.

The cognitive effects of conjugated equine estrogens depend on whether menopause etiology is transitional or surgical.

The question of whether to take hormone therapy (HT) will impact every woman as she enters reproductive senescence. In women, studies suggest that ovarian hormone loss associated with menopause has deleterious cognitive effects. Results from clinical studies evaluating whether estrogen-containing HT mitigates these effects, and benefits cognition, are discrepant. Type of menopause, surgical vs. transitional, impacts cognitive outcome in women. However, whether type of menopause impacts cognitive effects of HT has not been methodically tested in women or an animal model. We used the 4-vinylcyclohexene diepoxide rodent model of ovarian follicle depletion, which mimics transitional menopause, and the traditional rat model of menopause, ovariectomy, to cognitively test the most commonly prescribed estrogen therapy in the United States, conjugated equine estrogens (Premarin). Here we show conjugated equine estrogens benefited cognition in surgically menopausal rats, but, in contrast, impaired cognition in transitionally menopausal rats. Androstenedione, released from the residual transitional menopausal ovary, was positively associated with impaired performance, replicating our previous findings in 4-vinylcyclohexene diepoxide animals. The current findings are especially salient given that no clinical study testing cognition has methodically separated these two populations of menopausal women for analysis. That we now show surgical vs. transitional modes of menopause result in disparate cognitive effects of HT has implications for future research and treatments optimizing HT for menopausal women.

Transitional versus surgical menopause in a rodent model: etiology of ovarian hormone loss impacts memory and the acetylcholine system.

Clinical research suggests that type of ovarian hormone loss at menopause influences cognition. Until recently ovariectomy (OVX) has been the primary rodent model to examine effects of ovarian hormone loss on cognition. This model limits evaluations to abrupt and complete ovarian hormone loss, modeling less than 13% of women who receive surgical menopause. The majority of women do not have their ovaries surgically removed and undergo transitional hormone loss via ovarian follicular depletion. 4-Vinylcyclohexene-diepoxide (VCD) produces gradual ovarian follicular depletion in the rodent, with hormone profiles more similar to naturally menopausal women vs. OVX. We directly compared VCD and OVX models to examine whether type of hormone loss (transitional vs. surgical) impacted cognition as assessed on a maze battery as well as the cholinergic system tested via scopolamine mnemonic challenge and brain acetylcholinesterase activity. Middle-aged rats received either sham surgery, OVX surgery, VCD, or VCD then OVX to assess effects of removal of residual ovarian output after transitional menopause and follicular depletion. VCD-induced transitional menopause impaired learning of a spatial recent memory task; surgical removal of residual ovarian hormones by OVX abolished this negative effect of transitional menopause. Furthermore, transitional menopause before OVX was better for memory than an abrupt loss of hormones via OVX only. Surgical ovarian hormone loss, regardless of menopause history, increased hippocampal acetylcholinesterase activity. Circulating gonadotropin and androstenedione levels were related to cognitive competence. Collectively, findings suggest that in the rat, initiation of transitional menopause before surgical ovary removal can benefit mnemonic function and could obviate some negative cognitive consequences of surgical menopause alone.

Premarin improves memory, prevents scopolamine-induced amnesia and increases number of basal forebrain choline acetyltransferase positive cells in middle-aged surgically menopausal rats.

Conjugated equine estrogen (CEE) is the most commonly prescribed estrogen therapy, and is the estrogen used in the Women's Health Initiative study. While in-vitro studies suggest that CEE is neuroprotective, no study has evaluated CEE's effects on a cognitive battery and brain immunohistochemistry in an animal model. The current experiment tested whether CEE impacted: I) spatial learning, reference memory, working memory and long-term retention, as well as ability to handle mnemonic delay and interference challenges; and, II) the cholinergic system, via pharmacological challenge during memory testing and ChAT-immunoreactive cell counts in the basal forebrain. Middle-aged ovariectomized (Ovx) rats received chronic cyclic injections of either Oil (vehicle), CEE-Low (10 microg), CEE-Medium (20 microg) or CEE-High (30 microg) treatment. Relative to the Oil group, all three CEE groups showed less overnight forgetting on the spatial reference memory task, and the CEE-High group had enhanced platform localization during the probe trial. All CEE groups exhibited enhanced learning on the spatial working memory task, and CEE dose-dependently protected against scopolamine-induced amnesia with every rat receiving the highest CEE dose maintaining zero errors after scopolamine challenge. CEE also increased number of ChAT-immunoreactive neurons in the vertical diagonal band of the basal forebrain. Neither the ability to remember after a delay nor interference, nor long-term retention, was influenced by the CEE regimen used in this study. These findings are similar to those reported previously for 17 beta-estradiol, and suggest that CEE can provide cognitive benefits on spatial learning, reference and working memory, possibly through cholinergic mechanisms.