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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Exp Gerontol. Author manuscript; available in PMC 2017 August 1.
Published in final edited form as:
PMCID: PMC5479709

The endocrine-brain-aging triad where many paths meet: female reproductive hormone changes at midlife and their influence on circuits important for learning and memory[star]


Female mammals undergo natural fluctuations in sex steroid hormone levels throughout life. These fluctuations span from early development, to cyclic changes associated with the menstrual or estrous cycle and pregnancy, to marked hormone flux during perimenopause, and a final decline at reproductive senescence. While the transition to reproductive senescence is not yet fully understood, the vast majority of mammals experience this spontaneous, natural phenomenon with age, which has broad implications for long-lived species. Indeed, this post-reproductive life stage, and its transition, involves significant and enduring physiological changes, including considerably altered sex steroid hormone and gonadotropin profiles that impact multiple body systems, including the brain. The endocrine-brain-aging triad is especially noteworthy, as many paths meet and interact. Many of the brain regions affected by aging are also sensitive to changes in ovarian hormone levels, and aging and reproductive senescence are both associated with changes in memory performance. This review explores how menopause is related to cognitive aging, and discusses some of the key neural systems and molecular factors altered with age and reproductive hormone level changes, with an emphasis on brain regions important for learning and memory.

Keywords: Estrogen, Androgen, Progesterone, Cholinergic, GABAergic, ERK, Aging, Learning, Memory, Cognition, Gonadotropins, Brain, Ovarian, Hormone, Steroid

1. Introduction to menopause and the aging brain: The endocrine-brain-aging triad

During aging, humans and other animals commonly experience cognitive changes. Neurobiological alterations concomitant with aging can impact brain regions that are critical for the regulation of attention, learning, and memory processes, such as the frontal cortex, basal forebrain, and hippocampal formation. In females, changes in cognitive function during midlife are often associated with reproductive senescence. Reproductive senescence occurs in women when the finite ovarian follicle pool is depleted. Women are born with all of the immature ovarian follicles they will ever have; it has been estimated that human females have over half a million immature ova at birth (Gougeon, 2010; Wallace and Kelsey, 2010). Approximately 400 of these immature follicles fully mature and are ovulated throughout the reproductive life stage between puberty and menopause, and as such, over 99% of follicles undergo atresia, or programmed cell death. This normal apoptotic process begins at birth and continues until the follicle pool is exhausted around the fifth decade of life (Hsueh et al., 1994).

During the reproductive life stage, the ovary is the main synthesis site of circulating sex steroid hormones, including estrogens, progesterone, and androgens. Estrogens are primarily synthesized by growing ovarian follicles, progesterone predominantly by the corpus luteum after ovulation and in small amounts by growing follicles, and androgens by both ovarian interstitial tissue and the adrenal glands. The production and regulation of these hormones are mediated by the hypothalamic-pituitary-gonadal (HPG) axis. The feedback loop includes the hypothalamus, which produces and releases gonadotropin releasing hormone (GnRH) into the anterior pituitary gland, initiating the synthesis and secretion of the gonadotropins follicle stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH are two key hormone regulators of ovarian follicle development and the ovarian cycle. Once the follicle pool is depleted through natural atresia and ovulation, the ovaries do not generate a sufficient amount of estrogens and progesterone to sustain the normal uterine cycle. Although the natural transition to reproductive senescence is not completely understood, it is clear that it is not an abrupt event; rather, it is thought that when a critical threshold of remaining ovarian follicles is reached, women begin to experience the transitional phase to menopause, which involves intermittent ovulatory cycles, significant fluctuations in ovarian hormone levels, and variable but rising FSH and LH levels. This process culminates with an eventual cessation of menses and infertility, referred to as menopause, and occurs at the average age of 51–52 in women (Hoffman et al., 2012; NAMS, 2014). This gradual menopause transition may last up to ten years prior to the final menstrual period, signaling the end of the reproductive life stage (Harlow et al., 2013). In addition to these physiological changes, women often report undesirable symptoms including hot flashes, genitourinary symptoms, and changes in sleep, mood, and memory during the menopause transition (Sullivan Mitchell and Fugate Woods, 2001; Weber et al., 2013; Weber and Mapstone, 2009).

How, then, do these factors associated with menopause relate to the brain and behavior? Basic science research on the role of hormones and behavior began as early as the mid 19th century with Arnold Berthold’s classic experiments evaluating the role of the testes in sex behaviors in roosters, which showed that castration decreased aggression and male sex behaviors. He found that when he surgically implanted testes back into the rooster’s abdominal cavity, they reestablished a blood supply, and both aggressive and sexual behavior were also reinstated (Berthold, 1849). From these experiments, Berthold hypothesized that a substance was secreted from the testes into the bloodstreamto trigger these behaviors. It was not until some fifty years later that the term “hormone” was coined and defined by William Bayliss and Ernest Starling. By the 1930’s and 40’s, Dr. Frank Beach began his influential work on early life hormone manipulations and rodent sexual behavior. Beach and colleagues systematically manipulated sex hormone exposure by surgically removing the gonads of male and female rats and observing altered sex behaviors. He also found that phenotypic sex behaviors (e.g. mounting male sex behavior, lordosis female sex behavior) could be altered or induced by the manipulation of sex hormones in males and females (Beach, 1942, 1941). In the decades following these landmark studies, researchers went on to recognize the role of sex hormones and their influence on the brain with regard to sexual differentiation and neuroendocrine modulation of reproduction, including the discovery of the sexually dimorphic nucleus of the preoptic area, and how the size of this brain region can be altered depending on early life gonadal hormone exposures (Gorski, 1972; Gorski et al., 1978). Additionally, the role of estrogen as an activator of female-typical sexual behavior early in life was noted in Dr. Christina Williams’ research, showing that administering a supraphysiological dose of estradiol benzoate to rat pups as young as 4–6 days old facilitated the expression of lordosis (a receptive behavior) and of ear wiggling (a proceptive behavior) (Williams, 1987). The McEwen laboratory found that male pups that were administered an aromatase inhibitor, which prevents the metabolic conversion of androgens to estrogens, exhibited lordosis behavior in addition to phenotypic male sex behaviors in adulthood. When these aromatase inhibitor-treated males were gonadectomized as adults and subsequently administered estradiol benzoate and progesterone, they also exhibited lordosis and proceptive behaviors, suggesting the mechanisms driving phenotypic female sex behaviors can develop independently of male sex behaviors (Davis et al., 1979). Through the decades, research has shown that just as the male brain is actively masculinized by sex steroids, the female brain is actively feminized by sex steroids (Fitch and Denenberg, 1998); this has been exemplified for gross brain structure (Bimonte et al., 2000a, 2000b), cortical ultrastructure (Kim and Juraska, 1997), as well as behavior (Stewart and Cygan, 1980; Zimmerberg and Farley, 1993). These findings moved the field forward by challenging the traditional tenet that estrogens have a passive role in sexual differentiation of the brain. Since this time, the field of neuroendocrinology and aging has learned that the role of circulating ovarian hormones is not limited to reproductive functions and behaviors, and the brain areas that mediate them, but also extends to modulating memory and other cognitive processes, as well as their related brain regions. In fact, we have learned that in addition to impacts on brain health, the effects of estrogens, progestogens, and androgens on the body are diverse and manifold, including, but not limited to, influences on cardiovascular and bone health (Engler-Chiurazzi et al., 2016; Turgeon et al., 2006; Wise et al., 2009).

While acknowledging that sex steroid hormones have an impact on a myriad of systems with important functional and health outcomes, this review will focus on the brain and cognition. That sex hormones and gonadotropins could impact non-reproductive domains of the brain and behavior is not unexpected given the discovery of mechanisms which could mediate such effects. Indeed, there are steroid hormone and gonadotropin receptors in many areas of the brain, including the hippocampus and frontal cortex, two brain regions that are critical for effective memory functioning in everyday life. These memory functions include spatial, working, and reference memory. Spatial memory is hippocampal dependent and involves the use of distal cues to navigate through an environment. Working memory depends on both the frontal cortex and hippocampus, and is a type of short-term memory that involves updating information. For example, working memory requires manipulating information, such as mental arithmetic. Reference memory is a form of long-term memory that remains consistent across time; for example, one would utilize reference memory to navigate the route driven from home to work each day (Bimonte-Nelson et al., 2015). A substantial amount of research, from which key findings are highlighted and explored below, has been dedicated to elucidating the cognitive effects of ovarian hormones, particularly estrogens, and how changes in circulating hormones can impact the brain and behavior across the lifespan (See Fig. 1).

Fig. 1
Literature demonstrating the discoveries of brain changes in regions that regulate cognitive processes, and that are sensitive to both aging and ovarian hormones. ChAT=choline acetyltransferase; AChE=acetylcholinesterase; BF=basal forebrain; Ovx=ovariectomy; ...

2. On the role of midlife changes in ovarian hormones, gonadotropins, and cognitive function

2.1. Estrogen and progesterone as the traditional key ovarian hormone players

As aging ensues, female mammals typically experience a cessation of reproductive capacity in mid- to end- of life. Most animals are not long-lived following reproductive senescence; humans are one of the unique exceptions to this rule. People are living longer than ever before, with the average female lifespan surpassing 81 years in developed countries such as the United States (Murray et al., 2015). However, the age at menopause does not seem to be changing with increased longevity (Sherwin, 2003). This means that women are now living in a postmenopausal state, with significantly reduced circulating ovarian hormone levels, for a substantial part of their lives. This underscores the need to understand the effects of aging and related hormone loss on the body, including on the brain and its function.

There has been ample research, both in basic science and human realms, suggesting that the loss of ovarian hormones has a negative impact on a variety of body systems. These adverse effects are especially robust when an abrupt hormone loss occurs, such as that associated with ovariectomy (Ovx; the surgical removal of the ovaries). Ovarian hormone loss is associated with a decline in cognitive function both in humans (Nappi et al., 1999; Rocca et al., 2007; Sherwin, 2003) as well as in animal models (Daniel, 2013; Frick, 2015; Koebele and Bimonte-Nelson, 2015; Korol and Pisani, 2015; Luine, 2014). Animal models have provided an excellent framework to elucidate the effects of ovarian hormones on the brain and behavior. For example, seminal preclinical work in the field of neuroendocrinology, aging, and cognition has shown that Ovx impairs spatial memory performance, and that subsequent estrogen treatment can improve memory performance following Ovx, at least for a period of time (Bimonte and Denenberg, 1999; Bohacek et al., 2008; Savonenko and Markowska, 2003; Talboom et al., 2008; Wallace et al., 2006). Interestingly, transient 17β-estradiol treatment after Ovx can enhance memory performance, as well as increase hippocampal choline acetyltransferase (ChAT; the synthesizing enzyme for acetylcholine) and estrogen receptor alpha (ERα) levels, even after the estrogen treatment has been terminated (Rodgers et al., 2010). However, timing of 17β-estradiol replacement is critical; spatial memory performance was improved only when hormone treatment was initiated immediately after Ovx, but not after five months of hormone deprivation (Daniel et al., 2006). It is also notable that estrogen replacement following Ovx is more efficacious in young and middle-aged animals than in aged animals (Diz-Chaves et al., 2012; Savonenko and Markowska, 2003), and that chronic estrogen treatment can improve cognitive performance, but only after priming with a cyclic regimen of 17β-estradiol injections (Markowska and Savonenko, 2002). Moreover, animals’ responsiveness to the enhancing effects of 17β-estradiol or estradiol benzoate changes with age (Foster et al., 2003; Talboom et al., 2008),which may be related to estrogen receptor expression in the hippocampus (Foster, 2012). ERα and ERβ are associated with a range of intracellular signaling molecules that are rapidly activated in the presence of estrogens. Remarkably, Ovx animals receiving hippocampal lentivirus injection of ERα, which increases ERα expression, displayed enhanced spatial working memory, even in the absence of high circulating estrogen levels (Foster et al., 2008; Witty et al., 2012). Lentiviral delivery of ERα to the hippocampus also increased phosphorylated extracellular regulated kinases (ERK1/2; discussed in more detail below) in rats, suggesting that signal transduction pathways important for learning and memory are, in part, moderated by estrogen receptor expression and activity (Witty et al., 2012). Taken together, these novel findings indicate that serum estrogen levels alone cannot necessarily dictate or predict cognitive outcomes; they are part of a complex and interactive system involving many cellular and molecular mechanisms that impact memory performance in a collaborative fashion.

Further elaborating on this tenet, estrogens do not operate on the brain and body in isolation. Progestogens are a class of steroid hormones that include endogenous progesterone and synthetic progestins that bind to the progesterone receptor. Progesterone is an important component of the reproductive cycle, and is especially critical for the maintenance of pregnancy. In a non-pregnant female, the main release of progesterone occurs during the endogenous female reproductive cycle from the corpus luteum after ovulation. With follicular depletion and ensuing menopause, corpora lutea formation is attenuated, and therefore there is a lack of elevated progesterone. In a broad context, the scientific study regarding the impact of the shifts in progesterone across the female lifespan is important because of the systematic and rapid alterations in progesterone levels across the regular reproductive cycle in adulthood, markedly elevated levels with pregnancy, as well as the decreased levels that occur into old age. Determining the impact of progestogens on the brain and other systems is crucial, given the wide use of bioidentical and synthetic progestogens in hormone therapies and contraceptives. The effects of progestogens specifically on the brain and its functions is a growing area of research; in fact, the work is yielding strong evidence that progestogens have marked impacts on brain areas integral to many reproductive and non-reproductive behaviors, including translating effects to cognition.

Interestingly, our laboratory has found that the beneficial effects of estrogen treatment on spatial memory can be reversed by concomitant progesterone administration (Bimonte-Nelson et al., 2006), and that administering the synthetic progestin medroxyprogesterone acetate (MPA) to Ovx rats impaired performance on a spatial working memory task (Braden et al., 2011, 2010). However, it seems that progestins are not unequivocally harmful to cognition. Our laboratory and others have recently demonstrated that different classes of synthetic progestins commonly used in HT formulations, including levonorgestrel and norethindrone acetate, can have differential effects on spatial memory performance compared to MPA (Braden et al., 2016; Gambacciani et al., 2003; Simone et al., 2015; Tierney et al., 2009).While MPA administration has been shown to produce detrimental, long-lasting cognitive impairments (Braden et al., 2011, 2010), and norethindrone acetate dose-dependently impaired spatial memory, levonorgestrel has been shown to have a null or even enhancing effect on spatial memory performance (Braden et al., 2016). In the context of translational research, finding a null effect of a progestin is a better outcome than the generally detrimental effects of MPA; that is, it is preferable for women to use a progestin that will have no effect or a beneficial effect on memory, rather than utilize a known cognitively impairing option like MPA. Thus, these novel findings regarding the differential effects of progestogens on cognition warrant further investigation into progestin type, dose, and timing of treatment to produce an optimal brain aging profile while maintaining the important protective effects that progestogens provide for other body systems.

Notably, the age at which ovarian hormone changes occur is also an important factor in cognitive outcomes. Some research shows that women who experience surgical menopause prior to natural, transitional menopause onset have poorer verbal memory scores and may have a greater risk for developing cognitive impairments, as well as dementia, later in life (Nappi et al., 1999; Rocca et al., 2007). Our laboratory recently extended these findings using the 4-vinylcyclohexene diepoxide transitional menopause rodent model, which acts by chemically inducing ovarian follicular depletion to produce an ovarian and hormone profile similar to women undergoing the transition to menopause (Koebele and Bimonte-Nelson, 2016).We found that animals that underwent the transition to menopause in young adulthood exhibited working memory impairments compared to normally aging adult rats, whereas transitionally menopausal middle-aged rats performed similarly to middle-aged control rats. These memory impairments were evident early in the menopause transition, particularly when working memory load was taxed (Koebele et al., 2017). These cumulative findings suggest that it is not only essential to consider hormone type, timing, and dosing regimen, but also an individual’s reproductive history and status, as well as age, as important factors for understanding the potential of hormone therapy to have neuroprotective effects.

2.2. Androstenedione: long ignored but not unimportant

Androstenedione is an androgen synthesized by the adrenal glands and interstitial ovarian tissue, as well as by the thecal cells of maturing follicles. The aromatase enzyme converts androstenedione to estrone and 17β-estradiol; androstenedione can also be converted to testosterone via the enzyme17β-HSD; both of these androgens and their metabolites can impact the brain and cognition (Bimonte-Nelson et al., 2003; Camp et al., 2012; Mennenga et al., 2015b). In the context of menopause, research shows that, while estrogen and progesterone production declines substantially with reproductive senescence, the postmenopausal ovary continues to produce androgens in rodents (Mayer, 2004) and in humans (Fogle et al., 2007). In fact, it has been estimated that in the postmenopausal state, the ovaries continue to produce about 30% of the circulating androstenedione levels and 50% of total testosterone levels (Vermeulen, 1976). Recent research in postmenopausal women shows that exogenous testosterone can enhance memory (Davis et al., 2014; Davison et al., 2011), but endogenous testosterone levels may differentially impact cognition, such that a lower testosterone to estrogen ratio is better for memory performance (Ryan et al., 2012). Thus, the cognitive effects of androgens likely depend on a woman’s background hormone profile in the postmenopausal state, and should be taken into consideration when interpreting the effects of ovarian hormones on cognitive outcomes.

Although the effects of endogenous and exogenous estrogens and progestogens have been the focus of research related to cognitive function, the role of androgens (particularly androstenedione) in learning and memory remains somewhat elusive, and not as well defined as estrogens and progestogens. Our laboratory has shown that in a transitional menopause rat model, animals with higher naturally circulating androstenedione levels tended to make more working memory errors on the water radial-arm maze (Acosta et al., 2009). Given that androgens can convert to estrogens, we recognized this novel finding could inform important innovations in the realm of hormone therapy options for women. Thus, intrigued by this correlation, we continued to explore the role of androstenedione on memory in the middle-aged female rat. We found that in middle-aged Ovx rats, a high dose of androstenedione impaired spatial working memory and reference memory (Camp et al., 2012). Androstenedione can be aromatized to estrone, an estrogen that we have shown to impair memory in the Ovx rat model (Engler-Chiurazzi et al., 2012). Consequently, our laboratory systematically evaluated whether the apparent detrimental effects of androstenedione on memory were due to binding to the androgen receptor or androstenedione’s conversion to estrone via the aromatase enzyme. Results indicated that blocking aromatase enzymatic activity via anastrozole reversed androstenedione-induced spatial memory impairments in young Ovx rats, but blocking the androgen receptor did not prevent detrimental effects on memory, suggesting that the conversion of androstenedione to estrone influences cognitive performance (Mennenga et al., 2015b). Collectively, these findings point to a crucial role of androgens, a long ignored yet ostensibly critical factor in understanding the role of the hormone milieu on cognition in the menopausal woman. Future research should continue to focus on understanding how circulating androgen levels impact the brain and body of aging women, and how maintaining a “golden ratio” of androgens to estrogens may be key to preserving cognition in the postmenopausal life stage.

2.3. What about those gonadotropins? Cognitive effects of LH and FSH during menopause and aging

The reproductive system in females is regulated by communication and interactions with numerous hormones from the hypothalamus, pituitary, and ovaries. Thus, ovarian-derived steroids, such as estrogens, progesterone, and androgens, are not the only hormones to become dysregulated with age and reproductive senescence. Research in recent years has indicated that changes in gonadotropins, namely FSH and LH, have a major role in cognitive changes and risk factors for developing age-related neurodegenerative disorders. FSH and LH are glycoprotein hormones released from the anterior pituitary, and they each have critical effects on body growth and maturation, as well as reproductive functions. FSH is released to stimulate the growth of immature ovarian follicles, resulting in a gradual rise in circulating estrogen levels during the first half of the cycle. Once estrogen levels reach a certain threshold, an LH surge occurs, which triggers ovulation and concurrently initiates corpus luteum development from the remaining ovarian follicle, which produces progesterone in preparation for egg fertilization. Once the follicle pool falls below a critical threshold, typically in midlife, the normal feedback from the ovaries to the hypothalamus and pituitary becomes disrupted. Thought to be a compensatory mechanism, increased FSH and LH levels are released in the system’s attempt to stimulate normal follicular growth and ovulation. Seminal work from the laboratories of Dr. Andrea Gore and Dr. Phyllis Wise has provided evidence that perturbations in the cyclic release of GnRH and subsequent release of gonadotropins occur before alterations in regular estrous or menstrual cyclicity becomes apparent, and that changes in N-methyl-D-aspartate (NMDA) receptor function may play a key role in disrupted GnRH release and feedback (Gore et al., 2000a; Gore et al., 2000b; Scarborough and Wise, 1990; Wise, 1982).

Some human research suggests that it is the alterations in gonadotropin levels, over and above declines in circulating ovarian hormone levels, that result in the cognitive changes observed during aging (Webber et al., 2005). In fact, higher circulating FSH and LH levels have been associated with neurodegenerative disease and pathologies in clinical populations (Bowen et al., 2002, 2000; Short et al., 2001). Basic science research using rodent models has further substantiated this tenet. For example, Dr. Gemma Casadesus and colleagues found that transgenic mice that overexpress LH receptors performed poorly on a hippocampal-dependent Y-maze task, while LH receptor knock out mice were not impaired, despite increased circulating LH levels (Casadesus et al., 2007). Furthermore, this group has shown that pharmacologically down-regulating serum LH improves cognitive performance after Ovx in wildtype and a triple transgenic mouse model of Alzheimer’s disease (Blair et al., 2016; Palm et al., 2014); in wild type animals, decreasing LH serum levels benefitted memory performance, even after exogenous 17β-estradiol treatment was no longer effective in enhancing memory following Ovx (Blair et al., 2016). In addition, our laboratory has shown an inverted-U association between LH levels and cognitive performance in middle-aged female rats. Specifically, for animals with their ovaries (sham and follicle-deplete via experimental induction), higher LH levels were associated with poorer memory performance. Conversely, for Ovx animals, higher LH levels tended to be associated with better memory performance (Acosta et al., 2009). It is clear that in addition to circulating steroid ovarian hormone levels, gonadotropins also play a part in mediating cognitive performance, and these effects likely depend on background hormone milieu. Overall, these exciting findings point to novel pathways to explore to fully understand the impact of a dysregulated hypothalamic-pituitary-ovarian feedback loop, especially regarding the transition to reproductive senescence as related to the trajectory of cognitive aging.

3. Aging, ovarian hormones, and altered neural systems

The brain is a highly plastic organ. It adapts and changes throughout the lifespan, constantly revising and redacting information in order to adjust to an organism’s ever changing environment. Neural systems and biochemical mediators are affected by many factors that are modified with age and interactions with the environment. A fundamental factor influencing the brain beginning early in life is sex steroid hormones. It is well established that androgens and estrogens play a key role in organizing the developing brain, and set it up to respond in a particular way following sexual maturity of an organism. Many of these neural systems and molecular pathways that are impacted by age and reproductive hormones are also associated with learning and memory processes. Here, we focus upon the cholinergic and GABAergic systems, which are two of the most well-studied neural systems critical for learning and memory processes that are concomitantly impacted by age and ovarian hormones. The effects of age and altered ovarian hormone levels on dendritric morphology, as well as ERK1/2 signaling, a ERα-linked signaling pathway, are also discussed.

3.1. The cholinergic system

Age and ovarian hormones, both endogenously circulating and exogenously administered, impact a myriad of factors in the brain, including, but not limited to, growth factors (e.g., neurotropins), the inflammatory response, the immune response, mitochondrial function, and the cholinergic system. The latter involves the neurotransmitter acetylcholine, which also has diverse functions on the brain. One of acetylcholine’s significant functions is to act as a key regulator of learning and memory consolidation. The basal forebrain is a primary synthesis site for acetylcholine in the mammalian forebrain. It is known that there are long-range projections from the basal forebrain to the frontal cortex as well as the hippocampus, crucial brain structures for learning and memory consolidation. Beginning in the 1980’s, landmark research has indicated that age impacts morphology and functionality of the brain’s cholinergic system. For example, aged animals showed a decline in ChAT and acetylcholinesterase (AChE; the enzyme that breaks down acetylcholine) activity in the basal forebrain and hippocampus (Springer et al., 1987). Recently, the Bizon laboratory reported a decreased number of ChAT-immunoreactive (ChAT+) basal forebrain neurons in aged males rats compared to young adult male rats (Bañuelos et al., 2013). By examining p75NTR expression, a growth factor receptor that often co-localizes with cholinergic neurons, Veng and colleagues reported a reduction in density and presence of healthy cholinergic neurons in both aged male and female rats compared to younger animals (Veng et al., 2003). Aged males exhibited smaller cholinergic neuron somas compared to younger males, while aged females did not show a reduction in mean soma size (Veng et al., 2003). In addition to age-related alterations in cholinergic neurons, Ovx has been associated with a decline in ChAT activity, while subsequent 17β-estradiol administration restored ChAT activity in the female rat basal forebrain and projection sites into the frontal cortex and CA1 region of the hippocampus (Gibbs, 1994; Luine, 1985; Singh et al., 1994). Further, lesions to the medial septum and vertical/diagonal bands of the basal forebrain resulted in impaired spatial memory performance and prevented the memory enhancing effects of 17β-estradiol (Gibbs, 2002; Hagan et al., 1988). It is important to note that age and ovarian hormone loss do not necessarily affect the number of ChAT-producing neurons in the basal forebrain, but do impact the functional integrity of the cholinergic system (Gibbs, 2003). Recently, it has been shown that GPR30, a membrane bound G-protein coupled estrogen receptor distinct from ERα and ERβ, exhibits co-localization with basal forebrain cholinergic neurons and likely mediates some of estrogens’ effects on both basal forebrain cholinergic integrity and resulting cognitive outcomes (Hammond et al., 2011; Hammond and Gibbs, 2011; Ping et al., 2008). Thus, ovarian-derived hormones likely play a significant role in the neuroendocrine modulation of the cholinergic-hippocampal pathway. Most research on this subject has been evaluated in the Ovx model, where the ovaries, which are the endogenous source of circulating gonadal hormones, are surgically removed, and subsequent exogenous hormone therapy is administered. The effects of estrogens on cholinergic neurons in the basal forebrain are not always consistent, however. For example, many studies have shown that after Ovx, exogenous administration of 17β-estradiol can increase ChAT+ neurons in the basal forebrain (Engler-Chiurazzi et al., 2012; Gibbs, 1997); it is of note that other estrogen types initiate varied effects on this measurement. Indeed, tonic administration of estrone, a weaker metabolite of 17β-estradiol, failed to impact ChAT+ neurons in the rat basal forebrain (Engler-Chiurazzi et al., 2012), and the synthetic estrogen used in oral contraceptives, ethinyl estradiol, decreased the number of ChAT+ neurons in the basal forebrain following chronic administration in an Ovx rat model (Mennenga et al., 2015a). These diverse effects of estrogens on one system highlight the complexity of estrogens’ actions in the brain, and underscore the importance of taking multiple factors into account when assessing estrogens’ effects on the brain and other body systems, such as type of estrogen, dose, route of administration, and timing of administration (for review, see Koebele and Bimonte-Nelson, 2015).

3.2. The GABAergic system

Adding complexity to understanding the system, many cholinergic projections from the basal forebrain synapse onto [Latin small letter gamma]-aminobutyric acid (GABA) ergic cortical neurons; GABA is the primary inhibitory neurotransmitter in the brain and an important neuromodulator for normal cognitive processes, including hippocampal and cortical function. Acetylcholine release onto these GABAergic neurons in the hippocampus may modulate hippocampal theta wave oscillations through both direct and indirect pathways; these hippocampal theta rhythms play a role in regulating memory consolidation and synaptic plasticity (Dannenberg et al., 2015). The basal forebrain also has long-range GABAergic projections to the frontal cortex and hippocampus, both of which are thought to play a regulatory role in normal neural activity. Among its many roles, GABA signaling in the brain is a key regulatory factor in normal memory formation and maintenance (Kalueff and Nutt, 1997; Katz and Liebler, 1978). Inhibitory GABAergic neurons and signaling appear to become dysregulated with aging (Shetty and Turner, 1998; Stanley and Shetty, 2004). Animal models with altered GABA signaling, both systematically and with normal aging, show altered cognition with changes to the system, both in relation to cognitive aging and other psychiatric disorders (Bañuelos et al., 2013; McQuail et al., 2015). For example, the Bizon laboratory found that younger animals had better performance on the probe trial of the spatial reference memory Morris water maze compared to aged rats. The basal forebrain was immunohistochemically processed for ChAT and glutamate decarboxylase 67 (GAD67; the synthesizing enzyme for GABA). For GAD67-immunoreactive (GAD67+) neurons, there was no overall difference between young and aged rats. However, when aged rats were sub-classified into spatially- unimpaired and impaired groups, aged-spatially-impaired rats were found to have significantly more GAD67 + neurons compared to both young and aged-spatially-unimpaired animals. Further, this group showed a negative correlation with spatial memory performance in aged rats, such that a greater number of GAD67 + neurons was associated with poorer memory performance (Bañuelos et al., 2013). This laboratory also recently found that aged male rats have impaired performance on a set-shifting task, and that poorer performance on this task was associated with fewer GABA(B) receptors in the medial prefrontal cortex of the aged animals, but not the young animals. Directly infusing a GABA(B) receptor agonist into this brain region enhanced performance on the set shifting task for the aged animals (Beas et al., 2016), suggesting that cognitive changes with age are in part modulated by GABAergic signaling.

In addition to age-related changes in the GABAergic system and subsequent memory performance, ovarian hormones influence the GABAergic system. While some research suggests that 17β-estradiol can influence GABAergic signaling in the hippocampus (Wójtowicz and Mozrzymas, 2010), the majority of studies thus far have focused on the role of progesterone and GABAergic functioning. For example, our laboratory and others have shown that progesterone decreased GAD65 + 67 protein levels in the hippocampus and increased GAD65 + 67 protein levels in the entorhinal cortex (Braden et al., 2010) as well as decreased GAD activity in several brain regions, including the dorsal hippocampus, as measured by kinetic studies (Wallis and Luttge, 1980). Furthermore, an in situ hybridization study revealed that 12 h after treatment, progesterone, but not MPA, reduced hippocampal mRNA expression of the α4 subunit of the GABA(A) receptor (Pazol et al., 2009), suggesting that different progestogens can have variable impacts on the GABAergic system. We recently showed that in a middle-age Ovx rat model, progesterone administration resulted in transient working memory impairments on a spatial memory task, but concomitant delivery of bicuculline, a GABA(A) receptor antagonist, obviated these memory impairments (Braden et al., 2015). Finally, the recent finding that cholinergic neurons may also co-release GABA adds an additional level of complexity wherein the full impact on cognitive function has yet to be determined (Tritsch et al., 2016). Nonetheless, whether there are sex differences in how GABAergic circuitry and signaling are affected by aging, as well as how endogenous alterations and exogenous administrations of other sex steroid hormone levels impact this system, remains somewhat elusive and warrants further investigation.

3.3. MAPK/ERK1/2 signaling pathway

A wide range of intracellular pathways and kinases are known to be important for normal learning and memory processes (Giese and Mizuno, 2013), many of which are recruited downstream of estrogen receptor activation. One pathway in particular, the extracellular signal-regulated kinases, known as ERK1/2, p44/42, and classical mitogen-activated protein kinases (MAPKs; in humans, ERK1 = MAPK3), has diverse functions in regulating learning and memory (Atkins et al., 1998; Bozon et al., 2003; Fasano and Brambilla, 2011). Estrogen receptors are thought to activate ERK1/2 via production of cyclic adenosine monophosphate (cAMP) and/or interactions with growth factor receptors. Age-related brain changes in ERK1/2 signal transduction have not yet been extensively studied; however, one experiment found that aged male rats had decreased ERK1/2 activity in the cortex compared to younger rats (Zhen et al., 1999). Further investigations into how aging alters ERK1/2 signaling are necessary to elucidate whether aberrant signal transduction has functional consequences on cognitive outcomes across the lifespan. Given that ERK1/2 is ubiquitously expressed throughout the brain and other organs, it is important to evaluate potential age-related changes in multiple cognitive brain regions, as well as consider sex, age, and hormone status as factors influencing ERK1/2 expression and signaling. Indeed, in recent years, 17β-estradiol has been proposed to regulate ERK1/2 activity in both in vitro and in vivo studies. Dr. Karyn Frick’s laboratory has demonstrated that intraperitoneal injections and intracerebroventricular or hippocampal infusions of 17β-estradiol activated ERK2 and enhanced object recognition memory (Fernandez et al., 2008; Frick, 2015; Lewis et al., 2008). Temporal parameters may impact the outcome of estrogen effects on this system; in aged Ovx rats, the amount of time since Ovx (and therefore ovarian hormone deprivation) impacted subsequent effects of 17β-estradiol on ERK1/2 phosphorylation, which were dependent upon brain region (Pinceti et al., 2016). Additionally, findings from Dr. Thomas Foster’s laboratory revealed that ERα lentivirus injection directly into the hippocampus of middle-aged Ovx rats (i.e. animals with low endogenous estrogen levels) increased ERK1/2 phosphorylation and enhanced memory performance (Witty et al., 2012), suggesting that estrogen receptor activity, and possibly brain-derived estrogens, can activate and alter signal transduction pathways critical for learning and memory formation. These novel findings point to ERK1/2 signaling as another important biochemical mediator to investigate in the context of aging-and menopause- related brain changes. The interactions between ERK1/2 and the multitude of other hormone-linked pathways on learning and memory processes are only beginning to be explored. Further investigations in this newer field of how ovarian hormone fluctuations and aging impact these biochemical signaling pathways are currently underway.

3.4. Age- and ovarian hormone- influenced structural brain changes

In addition to the age- and menopause- related alterations observed in many neural systems and signaling pathways, the field is beginning to understand how aging and the ovarian hormone milieu impact the brain and other systems at the structural level. While neuron number does not necessarily decline in a healthy aging brain, age-related alterations in dendritic length (Pyapali and Turner, 1996), branching (Markham et al., 2005), and spine density and synapses (Adams et al., 2010; Geinisman et al., 1992) occur in several species, including rodents and non-human primates; these changes are seen in many brain regions, including those that regulate cognitive processes (Dickstein et al., 2013). Ovarian hormones can also affect dendritic morphology, and these alterations are sex-specific. For example, Miranda and colleagues demonstrated that aged Ovx rats showed decreased dendritic spine density in the dentate gyrus of the hippocampus compared to younger Ovx females deprived of hormones short-term; however, short-term estrogen administration, even in old age, increased spine density to the level of an adult female, and long-term estrogen replacement did not affect spines. Males did not show the same pattern of responsiveness to hormone deprivation and subsequent estradiol benzoate administration, suggesting that estrogen effects on dendritic spines are both sex- and time- dependent (Miranda et al., 1999). Furthermore, 17β-estradiol administration immediately after Ovx increased CA1 apical spines and enhanced memory performance, but if 17β-estradiol was given after 10 weeks of hormone deprivation, these morphological and behavioral changes were not as pronounced, again suggesting that temporal dynamics of estrogen administration matter for memory effects (McLaughlin et al., 2008). Middle-aged, ovary intact female rats showed impaired object recognition memory and a significant decrease in apical dendritic spines in pyramidal neurons within the CA1 regions of the hippocampus compared to young rats, but no differences in basal dendritic spines or pyramidal neurons in the CA3 region were apparent (Luine et al., 2011). Estrogen and progesterone likely regulate dendritic spines through NMDA receptors (Woolley and McEwen, 1994), and it is noted that there is natural variation in dendritic spines across the estrous cycle (Woolley and McEwen, 1993). These studies collectively indicate that both aging and ovarian hormone fluctuations have the capacity to trigger structural brain changes at multiple levels, and that there is marked plasticity both during aging and across the reproductive cycle; this is true even though we note that the extent and efficacy of this plasticity likely waxes and wanes across the lifespan. It is possible that organizational effects not only occur during early development, but that reorganizational events also exist across the lifespan as natural, significant fluctuations in ovarian hormones occur, such as with puberty, pregnancy, and menopause.

4. Conclusions and future directions for understanding the complex interactions among female reproductive hormones, age, and neurobiological alterations underlying cognitive processes

Accumulating evidence points to roles for both age and reproductive hormones on the brain and behavior throughout life. Knowledge about the complex interactions within this endocrine-brain-aging triad is growing in breadth and depth as scientific discoveries are made, and continuing this work will yield new insights into how these paths meet and influence each other. Given the continuously increasing average human lifespan, it is more important than ever for the field of neuroendocrinology and aging to better understand how aging and the long-lasting changes in gonadal hormones and gonadotropins that occur in midlife affect the neural circuits and molecular mechanisms related to learning and memory. Thus far, discoveries have included multiple neural systems, domains of function, and biochemical mediators, such as the basal forebrain-hippocampal cholinergic pathway, GABAergic transmission, ERK1/2 signal transduction, and structural brain changes. It is of particular interest to understand how the neurobiological and neurochemical changes associated with menopause and aging alter the underlying circuitry of cognitive pathways, and if these systems compensate by using alternative mechanisms or undergo a rewiring to return to homeostasis as aging occurs. Elucidating the changes in these molecular mechanisms with age and ovarian hormone milieu in a systematic and demonstrable fashion will yield insight into how and when the brain responds to endogenous hormone changes as well as to potential exogenous hormone treatment. This will, in turn, drive progress forward toward development and optimization of opportunities and choices for women undergoing the transition to menopause that not only addresses the undesirable symptoms associated with menopause, but also that potentially prevents, attenuates, or postpones the onset of cognitive or affective changes for at-risk women during aging. In order to move toward this realm of discovery, it should be recognized that female reproductive hormones, including sex steroids and gonadotropins, have a powerful impact on many complex and interactive neural systems that influence cognitive outcomes throughout life. Indeed, it seems that exposure to these hormones, whether transient or long-lasting, can change the course of future responses and brain health.


Dr. Heather Bimonte-Nelson is funded by the following grant awards: NIA (AG028084), state of Arizona, and Arizona Department of Health Services Arizona Alzheimer’s Consortium (ADHS 14-052688). No funding was received to write this article. We are grateful to Dr. Jason Newbern for his important contributions and review of this manuscript.


choline acetyltransferase
basal forebrain
glutamate decarboxylase
[Latin small letter gamma]-aminobutyric acid
extracellular regulated kinases
medroxyprogesterone acetate
estrogen receptor
luteinizing hormone
follicle stimulating hormone
hydroxysteroid dehydrogenase
gonadotropin releasing hormone


[star]This was an invited short review to the special issue in Experimental Gerontology (2016 Neurobiology of Aging) by Dr. Holly Brown-Borg following Stephanie Koebele’s attendance and presentation at theNeurobiology and Neuroendocrinology of Aging symposium in Bregenz, Austria (July 2016).


  • Acosta JI, Mayer L, Talboom JS, Tsang CWS, Smith CJ, Enders CK, Bimonte-Nelson HA. Transitional versus surgical menopause in a rodent model: etiology of ovarian hormone loss impacts memory and the acetylcholine system. Endocrinology. 2009;150:4248–4259. [PubMed]
  • Adams MM, Donohue HS, Linville MC, Iversen EA, Newton IG, Brunso-Bechtold JK. Age-related synapse loss in hippocampal CA3 is not reversed by caloric restriction. Neuroscience. 2010;171:373–382. [PMC free article] [PubMed]
  • Atkins CM, Selcher JC, Petraitis JJ, Trzaskos JM, Sweatt JD. The MAPK cascade is required for mammalian associative learning. Nat Neurosci. 1998;1:602–609. [PubMed]
  • Bañuelos C, LaSarge CL, McQuail JA, Hartman JJ, Gilbert RJ, Ormerod BK, Bizon JL. Age-related changes in rostral basal forebrain cholinergic and GABAergic projection neurons: relationship with spatial impairment. Neurobiol Aging. 2013;34:845–862. [PMC free article] [PubMed]
  • Beach FA. Female Mating Behavior Shown by Male Rats after Administration of Testosterone Proprionate. Endocrinology. 1941;29:409–412.
  • Beach FA. Male and Female Mating Behavior in Pre-pubertally Castrated Female Rats Treated with Androgens. Endocrinology. 1942;31:673–678.
  • Beas BS, McQuail JA, Banuelos C, Setlow B, Bizon JL. Prefrontal cortical GABAergic signaling and impaired behavioral flexibility in aged F344 rats. Neuroscience. 2016:1–13. [PMC free article] [PubMed]
  • Berthold AA. Transplantation of testes, English translation by D. P. Quiring, 1944. Bull Hist Med. 1849;16:399–401.
  • Bimonte HA, Denenberg VH. Estradiol facilitates performance as working memory load increases. Psychoneuroendocrinology. 1999;24:161–173. [PubMed]
  • Bimonte HA, Holly Fitch R, Denenberg VH. Adult ovary transfer counteracts the callosal enlargement resulting from prepubertal ovariectomy. Brain Res. 2000a;872:254–257. [PubMed]
  • Bimonte HA, Mack CM, Stavnezer AJ, Denenberg VH. Ovarian hormones can organize the rat corpus callosum in adulthood. Dev Brain Res. 2000b;121:169–177. [PubMed]
  • Bimonte-Nelson HA, Singleton RS, Nelson ME, Eckman CB, Barber J, Scott TY, Granholm ACE. Testosterone, but not nonaromatizable dihydrotestosterone, improves working memory and alters nerve growth factor levels in aged male rats. Exp Neurol. 2003;181:301–312. [PubMed]
  • Bimonte-Nelson HA, Francis KR, Umphlet CD, Granholm AC. Progesterone reverses the spatial memory enhancements initiated by tonic and cyclic oestrogen therapy in middle-aged ovariectomized female rats. Eur J Neurosci. 2006;24:229–242. [PubMed]
  • Bimonte-Nelson HA, Daniel JM, Koebele SV. In: The Maze Book: Theories, Practice, and Protocols for Testing Rodent Cognition. Bimonte-Nelson HA, editor. Neuromethods-Springer, US; 2015. pp. 37–72.
  • Blair JA, Palm R, Chang J, McGee H, Zhu X, Wang X, Casadesus G. Luteinizing hormone downregulation but not estrogen replacement improves ovariectomy-associated cognition and spine density loss independently of treatment onset timing. Horm Behav. 2016;78:60–66. [PMC free article] [PubMed]
  • Bohacek J, Bearl AM, Daniel JM. Long-term ovarian hormone deprivation alters the ability of subsequent oestradiol replacement to regulate choline acetyltransferase protein levels in the hippocampus and prefrontal cortex of middle-aged rats. J Neuroendocrinol. 2008;20:1023–1027. [PubMed]
  • Bowen RL, Isley JP, Atkinson RL. An association of elevated serum gonadotropin concentrations and Alzheimer disease? J Neuroendocrinol. 2000;12:351–354. [PubMed]
  • Bowen RL, Smith MA, Harris PLR, Kubat Z, Martins RN, Castellani RJ, Perry G, Atwood CS. Elevated luteinizing hormone expression colocalizes with neurons vulnerable to Alzheimer’s disease pathology. J Neurosci Res. 2002;70:514–518. [PubMed]
  • Bozon B, Kelly A, Josselyn SA, Silva AJ, Davis S, Laroche S. MAPK, CREB and zif268 are all required for the consolidation of recognition memory. Philos Trans R Soc Lond Ser B Biol Sci. 2003;358:805–814. [PMC free article] [PubMed]
  • Braden BB, Talboom JS, Crain ID, Simard AR, Lukas RJ, Prokai L, Scheldrup MR, Bowman BL, Bimonte-Nelson HA. Medroxyprogesterone acetate impairs memory and alters the GABAergic system in aged surgically menopausal rats. Neurobiol Learn Mem. 2010;93:444–453. [PMC free article] [PubMed]
  • Braden BB, Garcia AN, Mennenga SE, Prokai L, Villa SR, Acosta JI, Lefort N, Simard AR, Bimonte-Nelson HA. Cognitive-impairing effects of medroxyprogesterone acetate in the rat: independent and interactive effects across time. Psychopharmacology. 2011;218:405–418. [PMC free article] [PubMed]
  • Braden BB, Kingston ML, Whitton E, Lavery C, Tsang CWS, Bimonte-Nelson HA. The GABA(A) antagonist bicuculline attenuates progesterone-induced memory impairments in middle-aged ovariectomized rats. Front Aging Neurosci. 2015;7:1–8. [PMC free article] [PubMed]
  • Braden BB, Andrews MG, Acosta JI, Mennenga SE, Lavery C, Bimonte-Nelson HA. A comparison of progestins within three classes: differential effects on learning and memory in the aging surgically menopausal rat. Behav Brain Res. 2016 [PubMed]
  • Camp BW, Gerson JE, Tsang CWS, Villa SR, Acosta JI, Blair Braden B, Hoffman AN, Conrad CD, Bimonte-Nelson HA. High serum androstenedione levels correlate with impaired memory in the surgically menopausal rat: a replication and new findings. Eur J Neurosci. 2012;36:3086–3095. [PMC free article] [PubMed]
  • Casadesus G, Milliken EL, Webber KM, Bowen RL, Lei Z, Rao CV, Perry G, Keri RA, Smith MA. Increases in luteinizing hormone are associated with declines in cognitive performance. Mol Cell Endocrinol. 2007;269:107–111. [PubMed]
  • Daniel JM. Estrogens, estrogen receptors, and female cognitive aging: the impact of timing. Horm Behav. 2013;63:231–237. [PubMed]
  • Daniel JM, Hulst JL, Berbling JL. Estradiol replacement enhances working memory in middle-aged rats when initiated immediately after ovariectomy but not after a long-term period of ovarian hormone deprivation. Endocrinology. 2006;147:607–614. [PubMed]
  • Dannenberg H, Pabst M, Braganza O, Schoch S, Niediek J, Bayraktar M, Mormann F, Beck H. Synergy of direct and indirect cholinergic septo-hippocampal pathways coordinates firing in hippocampal networks. J Neurosci. 2015;35:8394–8410. [PubMed]
  • Davis PG, Chaptal CV, McEwen BS. Independence of the differentiation of masculine and feminine sexual behavior in rats. Horm Behav. 1979;12:12–19. [PubMed]
  • Davis SR, Jane F, Robinson PJ, Davison SL, Worsley R, Maruff P, Bell RJ. Transdermal testosterone improves verbal learning and memory in postmenopausal women not on oestrogen therapy. Clin Endocrinol. 2014;81:621–628. [PubMed]
  • Davison SL, Bell RJ, Gavrilescu M, Searle K, Maruff P, Gogos A, Rossell SL, Adams J, Egan GF, Davis SR. Testosterone improves verbal learning and memory in postmenopausal women: results from a pilot study. Maturitas. 2011;70:307–311. [PubMed]
  • Dickstein DL, Weaver CM, Luebke JI, Hof PR. Dendritic spine changes associated with normal aging. Neuroscience. 2013;251:21032. [PMC free article] [PubMed]
  • Diz-Chaves Y, Kwiatkowska-Naqvi A, Von Hülst H, Pernía O, Carrero P, Garcia-Segura LM. Behavioral effects of estradiol therapy in ovariectomized rats depend on the age when the treatment is initiated. Exp Gerontol. 2012;47:93–99. [PubMed]
  • Engler-Chiurazzi EB, Talboom JS, Braden BB, Tsang CWS, Mennenga S, Andrews M, Demers LM, Bimonte-Nelson HA. Continuous estrone treatment impairs spatial memory and does not impact number of basal forebrain cholinergic neurons in the surgically menopausal middle-aged rat. Horm Behav. 2012;62:1–9. [PMC free article] [PubMed]
  • Engler-Chiurazzi EB, Brown CM, Povroznik JM, Simpkins JW. Estrogens as neuroprotectants: estrogenic actions in the context of cognitive aging and brain injury. Prog Neurobiol. 2016 [PubMed]
  • Fasano S, Brambilla R. Ras–ERK signaling in behavior: old questions and new perspectives. Front Behav Neurosci. 2011;5(79) [PMC free article] [PubMed]
  • Fernandez SM, Lewis MC, Pechenino AS, Harburger LL, Orr PT, Gresack JE, Schafe GE, Frick KM. Estradiol-induced enhancement of object memory consolidation involves hippocampal extracellular signal-regulated kinase activation and membrane-bound estrogen receptors. J Neurosci. 2008;28:8660–8667. [PMC free article] [PubMed]
  • Fitch RH, Denenberg VH. A role for ovarian hormones in sexual differentiation of the brain. Behav Brain Sci. 1998;21:311–352. [PubMed]
  • Fogle RH, Stanczyk FZ, Zhang X, Paulson RJ. Ovarian androgen production in postmenopausal women. J Clin Endocrinol Metab. 2007;92:3040–3043. [PubMed]
  • Foster TC. Role of estrogen receptor alpha and beta expression and signaling on cognitive function during aging. Hippocampus. 2012;22:656–669. [PMC free article] [PubMed]
  • Foster TC, Sharrow KM, Kumar A, Masse J. Interaction of age and chronic estradiol replacement on memory and markers of brain aging. Neurobiol Aging. 2003;24:839–852. [PubMed]
  • Foster TC, Rani A, Kumar A, Cui L, Semple-Rowland SL. Viral vector–mediated delivery of estrogen receptor-alpha to the hippocampus improves spatial learning in estrogen receptor-alpha knockout mice. Mol Ther. 2008;16:1587–1593. [PMC free article] [PubMed]
  • Frick KM. Molecularmechanisms underlying the memory-enhancing effects of estradiol. Horm Behav. 2015;74:4–18. [PMC free article] [PubMed]
  • Gambacciani M, Ciaponi M, Cappagli B, Monteleone P, Benussi C, Bevilacqua G, Genazzani AR. Effects of low-dose, continuous combined estradiol and noretisterone acetate on menopausal quality of life in early postmenopausal women. Maturitas. 2003;44:157–163. [PubMed]
  • Geinisman Y, de Toledo-Morrell L, Morrell F, Persina IS, Rossi M. Age-related loss of axospinous synapses formed by two afferent systems in the rat dentate gyrus as revealed by the unbiased stereological dissector technique. Hippocampus. 1992;2:437–444. [PubMed]
  • Gibbs RB. Estrogen and nerve growth factor-related systems in brain. Effects on basal forebrain cholinergic neurons and implications for learning and memory processes and aging. Ann N Y Acad Sci. 1994;743:165–196. [PubMed]
  • Gibbs RB. Effects of estrogen on basal forebrain cholinergic neurons vary as a function of dose and duration of treatment. Brain Res. 1997;757:10–16. [PubMed]
  • Gibbs RB. Basal forebrain cholinergic neurons are necessary for estrogen to enhance acquisition of a delayed Matching-to-position T-maze task. Horm Behav. 2002;42:245–257. [PubMed]
  • Gibbs RB. Effects of ageing and long-term hormone replacement on cholinergic neurones in the medial septum and nucleus basalis magnocellularis of ovariectomized rats. J Neuroendocrinol. 2003;15:477–485. [PubMed]
  • Giese KP, Mizuno K. The roles of protein kinases in learning and memory. Learn Mem. 2013;20(10):540–552. [PubMed]
  • Gore AC, Oung T, Yung S, Flagg RA, Woller MJ. Neuroendocrine mechanisms for reproductive senescence in the female rat: gonadotropin-releasing hormone neurons. Endocrine. 2000a;13:315–323. [PubMed]
  • Gore AC, Yeung G, Morrison JH, Oung T. Neuroendocrine aging in the female rat: the changing relationship of hypothalamic gonadotropin-releasing hormone neurons and N-methyl-D-aspartate-receptors. Endocrinology. 2000b;141:4757–4767. [PubMed]
  • Gorski RA. Steroid hormones and brain function: progress, principles, and problems. UCLA Forum Med Sci. 1972;15:1–26. [PubMed]
  • Gorski RA, Gordon JH, Shryne JE, Southam AM. Evidence for a morphological sex difference within the medial preoptic area of the rat brain. Brain Res. 1978;148:333–346. [PubMed]
  • Gougeon A. Human ovarian follicular development: from activation of resting follicles to preovulatory maturation. Ann Endocrinol (Paris) 2010;71:132–143. [PubMed]
  • Hagan JJ, Salamone JD, Simpson J, Iversen SD, Morris RGM. Place navigation in rats is impaired by lesions of medial septum and diagonal band but not nucleus basalis magnocellularis. Behav Brain Res. 1988;27:9–20. [PubMed]
  • Hammond R, Gibbs RB. GPR30 is positioned to mediate estrogen effects on basal forebrain cholinergic neurons and cognitive performance. Brain Res. 2011;1379:53–60. [PMC free article] [PubMed]
  • Hammond R, Nelson D, Gibbs RB. GPR30 co-localizes with cholinergic neurons in the basal forebrain and enhances potassium-stimulated acetylecholine release in the hippocampus. Psychoneuroendocrinology. 2011;36:182–192. [PMC free article] [PubMed]
  • Harlow SD, Gass M, Hall JE, Lobo R, Maki P, Rebar RW, Sherman S, Sluss PM, de Villiers TJ. FRCOG, FCOG(SA) Executive summary of the Stages of Reproductive Aging Workshop+10: addressing the unfinished agenda of staging reproductive aging. Menopause. 2013;19:387–395. [PMC free article] [PubMed]
  • Hoffman BL, Schorge JO, Schaffer JI, Halvorson LM, Bradshaw KD, Cunningham FG, Calver LE. In: Williams Gynecology. 2. Fried A, Boyle PJ, editors. McGraw-Hill; New York: 2012.
  • Hsueh AJ, Billig H, Tsafriri A. Ovarian follicle atresia: a hormonally controlled apoptotic process. Endocr Rev. 1994;15:707–724. [PubMed]
  • Kalueff A, Nutt DJ. Role of GABA in memory and anxiety. Depress Anxiety. 1997;4:100–110. [PubMed]
  • Katz RJ, Liebler L. GABA involvement in memory consolidation: evidence from posttrial amino-oxyacetic acid. Psychopharmacology. 1978;56:191–193. [PubMed]
  • Kim JHY, Juraska JM. Sex difference in the development of axon number in the splenium of the rat corpus callosum from postnatal day 15 through 60. Dev Brain Res. 1997;102:77–85. [PubMed]
  • Koebele SV, Bimonte-Nelson HA. Trajectories and phenotypes with estrogen exposures across the lifespan: what does Goldilocks have to do with it? Horm Behav. 2015;74:86–104. [PMC free article] [PubMed]
  • Koebele SV, Bimonte-Nelson HA. Modeling menopause: the utility of rodents in translational behavioral endocrinology research. Maturitas. 2016;87:5–17. [PMC free article] [PubMed]
  • Koebele SV, Mennenga SE, Hiroi R, Quihuis AM, Hewitt LT, Poisson ML, George C, Mayer LP, Dyer CA, Aiken LS, Demers LM, Carson C, Bimonte-Nelson HA. Cognitive changes across the menopause transition: a longitudinal evaluation of the impact of age and ovarian status on spatial memory. Horm Behav. 2017;87:96–114. [PubMed]
  • Korol DL, Pisani SL. Estrogens and cognition: friends or foes? An evaluation of the opposing effects of estrogens on learning and memory. Horm Behav. 2015;74:105–115. [PMC free article] [PubMed]
  • Lewis MC, Kerr KM, Orr PT, Frick KM. Estradiol-induced enhancement of object memory consolidation involves NMDA receptors and protein kinase A in the dorsal hippocampus of female C57BL/6 mice. Behav Neurosci. 2008;122:716–721. [PMC free article] [PubMed]
  • Luine VN. Estradiol increases choline acetyltransferase activity in specific basal forebrain nuclei and projection areas of female rats. Exp Neurol. 1985;89:484–490. [PubMed]
  • Luine VN. Estradiol and cognitive function: past, present and future. Horm Behav. 2014;66:602–618. [PMC free article] [PubMed]
  • Luine VN, Wallace ME, Frankfurt M. Age-related deficits in spatial memory and hippocampal spines in virgin, female fischer 344 rats. Curr Gerontol Geriatr Res. 2011;2011 [PMC free article] [PubMed]
  • Markham JA, McKian KP, Stroup TS, Juraska JM. Sexually dimorphic aging of dendritic morphology in CA1 of hippocampus. Hippocampus. 2005;15:97–103. [PubMed]
  • Markowska AL, Savonenko AV. Effectiveness of estrogen replacement in restoration of cognitive function after long-term estrogen withdrawal in aging rats. J Neurosci. 2002;22:10985–10995. [PubMed]
  • Mayer LP. The follicle-deplete mouse ovary produces androgen. Biol Reprod. 2004;71:130–138. [PubMed]
  • McLaughlin KJ, Bimonte-Nelson H, Neisewander JL, Conrad CD. Assessment of estradiol influence on spatial tasks and hippocampal CA1 spines: evidence that the duration of hormone deprivation after ovariectomy compromises 17β-estradiol effectiveness in altering CA1 spines. Horm Behav. 2008;54:386–395. [PMC free article] [PubMed]
  • McQuail JA, Frazier CJ, Bizon JL. Molecular aspects of age-related cognitive decline: the role of GABA signaling. Trends Mol Med. 2015;21:450–460. [PMC free article] [PubMed]
  • Mennenga SE, Gerson JE, Koebele SV, Kingston ML, Tsang CWS, Engler-Chiurazzi EB, Baxter LC, Bimonte-Nelson HA. Understanding the cognitive impact of the contraceptive estrogen Ethinyl Estradiol: tonic and cyclic administration impairs memory, and performance correlates with basal forebrain cholinergic system integrity. Psychoneuroendocrinology. 2015a;54:1–13. [PMC free article] [PubMed]
  • Mennenga SE, Koebele SV, Mousa AA, Alderete TJ, Tsang CWS, Acosta JI, Camp BW, Demers LM, Bimonte-Nelson HA. Pharmacological blockade of the aromatase enzyme, but not the androgen receptor, reverses androstenedione-induced cognitive impairments in young surgically menopausal rats. Steroids. 2015b;99:16–25. [PMC free article] [PubMed]
  • Miranda P, Williams CL, Einstein G. Granule cells in aging rats are sexually dimorphic in their response to estradiol. J Neurosci. 1999;19:3316–3325. [PubMed]
  • Murray CJL, Barber RM, Foreman KJ, Ozgoren AA, Abd-Allah F, Abera SFT, et al. Global, regional, and national disability-adjusted life years (DALYs) for 306 diseases and injuries and healthy life expectancy (HALE) for 188 countries, 1990–2013: quantifying the epidemiological transition. Lancet. 2015;386:2145–2191. [PMC free article] [PubMed]
  • Nappi RE, Sinforiani E, Mauri M, Bono G, Polatti F, Nappi G. Memory functioning at menopause: impact of age in ovariectomized women. Gynecol Obstet Investig. 1999;47:29–36. [PubMed]
  • North American Menopause Society (NAMS) A Clinician’s Guide. 5 2014. Menopause practice.
  • Palm R, Chang J, Blair JA, Garcia-Mesa Y, Lee H, Castellani RJ, Smith MA, Zhu X, Casadesus G. Down-regulation of serum gonadotropins but not estrogen replacement improves cognition in aged-ovariectomized 3xTg AD female mice. J Neurochem. 2014;130:115–125. [PMC free article] [PubMed]
  • Pazol K, Northcutt KV, Patisaul HB, Wallen K, Wilson ME. Progesterone and medroxyprogesterone acetate differentially regulate α4 subunit expression of GABAA receptors in the CA1 hippocampus of female rats. Physiol Behav. 2009;97(1):58–61. [PMC free article] [PubMed]
  • Pinceti E, Shults CL, Rao YS, Pak TR. Differential effects of E2 on MAPK activity in the brain and heart of aged female rats. PLoS ONE. 2016;11:1–21. [PMC free article] [PubMed]
  • Ping SE, Trieu J, Wlodek ME, Barrett GL. Effects of estrogen on basal forebrain cholinergic neurons and spatial learning. J Neurosci Res. 2008;86:1588–1598. [PubMed]
  • Pyapali GK, Turner DA. Increased dendritic extent in hippocampal CA1 neurons from aged F344 rats. Neurobiol Aging. 1996;17:601–611. [PubMed]
  • Rocca WA, Bower JH, Maraganore DM, Ahlskog JE, Grossardt BR, de Andrade M, Melton LJI. Increased risk of cognitive impairment or dementia in women who underwent oophorectomy before menopause. Neurology. 2007;69:1074–1083. [PubMed]
  • Rodgers SP, Bohacek J, Daniel JM. Transient estradiol exposure during middle age in ovariectomized rats exerts lasting effects on cognitive function and the hippocampus. Endocrinology. 2010;151:1194–1203. [PubMed]
  • Ryan J, Stanczyk FZ, Dennerstein L, Mack WJ, Clark MS, Szoeke C, Kildea D, Henderson VW. Hormone levels and cognitive function in postmenopausal midlife women. Neurobiol Aging. 2012;33(617):e11–e22. [PubMed]
  • Savonenko AV, Markowska AL. The cognitive effects of ovariectomy and estrogen replacement are modulated by aging. Neuroscience. 2003;119:821–830. [PubMed]
  • Scarborough K, Wise PM. Age-related changes in pulsatile luteinizing hormone release precede the transition to estrous acyclicity and depend upon estrous cycle history. Endocrinology. 1990;126:884–890. [PubMed]
  • Sherwin BB. Estrogen and cognitive functioning in women. Endocr Rev. 2003;24:133–151. [PubMed]
  • Shetty AK, Turner DA. Hippocampal interneurons expressing glutamic acid decarboxylase and calcium-binding proteins decrease with aging in Fischer 344 rats. J Comp Neurol. 1998;394:252–269. [PubMed]
  • Short RA, Bowen RL, O’Brien PC, Graff-Radford NR. Elevated gonadotropin levels in patients with Alzheimer disease. Mayo Clin Proc. 2001;76:906–909. [PubMed]
  • Simone J, Bogue EA, Bhatti DL, Day LE, Farr NA, Grossman AM, Holmes PV. Ethinyl estradiol and levonorgestrel alter cognition and anxiety in rats concurrent with a decrease in tyrosine hydroxylase expression in the locus coeruleus and brain-derived neurotrophic factor expression in the hippocampus. Psychoneuroendocrinology. 2015;62:265–278. [PubMed]
  • Singh M, Meyer EM, Millard WJ, Simpkins JW. Ovarian steroid deprivation results in a reversible learning impairment and compromised cholinergic function in female Sprague-Dawley rats. Brain Res. 1994;644:305–312. [PubMed]
  • Springer JE, Tayrien MW, Loy R. Regional analysis of age-related changes in the cholinergic system of the hippocampal formation and basal forebrain of the rat. Brain Res. 1987;407:180–184. [PubMed]
  • Stanley DP, Shetty AK. Aging in the rat hippocampus is associated with widespread reductions in the number of glutamate decarboxylase-67 positive interneurons but not interneuron degeneration. J Neurochem. 2004;89:204–216. [PubMed]
  • Stewart J, Cygan D. Ovarian hormones act early in development to feminize adult open-field behavior in the rat. Horm Behav. 1980;14:20–32. [PubMed]
  • Sullivan Mitchell E, Fugate Woods N. Midlife women’s attributions about perceived memory changes: observations from the Seattle Midlife Women’s Health Study. J Womens Health Gend Based Med. 2001;10:351–362. [PubMed]
  • Talboom JS, Williams BJ, Baxley ER, West SG, Bimonte-Nelson HA. Higher levels of estradiol replacement correlate with better spatial memory in surgically menopausal young and middle-aged rats. Neurobiol Learn Mem. 2008;90:155–163. [PMC free article] [PubMed]
  • Tierney MC, Oh P, Moineddin R, Greenblatt EM, Snow WG, Fisher RH, Iazzetta J, Hyslop PSG, MacLusky NJ. A randomized double-blind trial of the effects of hormone therapy on delayed verbal recall in older women. Psychoneuroendocrinology. 2009;34:1065–1074. [PubMed]
  • Tritsch NX, Granger AJ, Sabatini BL. Mechanisms and functions of GABA co-release. Nat Rev Neurosci. 2016;17(3):139–145. [PubMed]
  • Turgeon JL, Carr MC, Maki PM, Mendelsohn ME, Wise PM. Complex actions of sex steroids in adipose tissue, the cardiovascular system, and brain: insights from basic science and clinical studies. Endocr Rev. 2006;27:575–605. [PubMed]
  • Veng LM, Granholm AC, Rose GM. Age-related sex differences in spatial learning and basal forebrain cholinergic neurons in F344 rats. Physiol Behav. 2003;80:27–36. [PubMed]
  • Vermeulen A. The hormonal activity of the postmenopausal ovary. J Clin Endocrinol Metab. 1976;42:247–253. [PubMed]
  • Wallace WHB, Kelsey TW. Human ovarian reserve from conception to the menopause. PLoS ONE. 2010;5(e8772):1–9. [PMC free article] [PubMed]
  • Wallace M, Luine V, Arellanos A, Frankfurt M. Ovariectomized rats show decreased recognition memory and spine density in the hippocampus and prefrontal cortex. Brain Res. 2006;1126:176–182. [PubMed]
  • Wallis CJ, Luttge WG. Influence of estrogen and progesterone on glutamic acid decarboxylase activity in discrete regions of rat brain. J Neurochem. 1980;34:609–613. [PubMed]
  • Webber KM, Casadesus G, Perry G, Atwood CS, Bowen R, Smith MA. Gender differences in Alzheimer disease: the role of luteinizing hormone in disease pathogenesis. Alzheimer Dis Assoc Disord. 2005;19:95–99. [PubMed]
  • Weber MT, Mapstone M. Memory complaints and memory performance in the menopausal transition. Menopause. 2009;16:694–700. [PubMed]
  • Weber MT, Maki PM, McDermott MP. Cognition and mood in perimenopause: a systematic review and meta-analysis. J Steroid Biochem Mol Biol. 2013:20–25. [PMC free article] [PubMed]
  • Williams CL. Estradiol benzoate facilitates lordosis and ear wiggling of 4- to 6-day-old rats. Behav Neurosci. 1987;101:718–723. [PubMed]
  • Wise PM. Alterations in proestrous LH, FSH, and prolactin surges in middle-aged rats. Proc Soc Exp Biol Med. 1982;169:348–354. [PubMed]
  • Wise PM, Suzuki S, Brown CM. Estradiol: a hormone with diverse and contradictory neuroprotective actions. Dialogues Clin Neurosci. 2009;11:297–303. [PMC free article] [PubMed]
  • Witty CF, Foster TC, Semple-Rowland SL, Daniel JM. Increasing hippocampal estrogen receptor alpha levels via viral vectors increases MAP kinase activation and enhances memory in aging rats in the absence of ovarian estrogens. PLoS ONE. 2012;7:1–10. [PMC free article] [PubMed]
  • Wójtowicz T, Mozrzymas JW. Estradiol and GABAergic transmission in the hippocampus. Vitam Horm. 2010;82:279–300. [PubMed]
  • Woolley CS, McEwen BS. Roles of estradiol and progesterone in regulation of hippocampal dendritic spine density during the estrous cycle in the rat. J Comp Neurol. 1993;336(2):293–306. [PubMed]
  • Woolley CS, McEwen BS. Estradiol regulates hippocampal dendritic spine density via an N-methyl-D-aspartate receptor-dependent mechanism. J Neurosci. 1994;14:7680–7687. [PubMed]
  • Zhen X, Uryu K, Cai G, Johnson GP, Friedman E. Age-associated impairment in brain MAPK signal pathways and the effect of caloric restriction in Fischer 344 rats. J Gerontol A Biol Sci Med Sci. 1999;54:B539–B548. [PubMed]
  • Zimmerberg B, Farley MJ. Sex differences in anxiety behavior in rats: role of gonadal hormones. Physiol Behav. 1993;54:1119–1124. [PubMed]