Differential effects of estrogen on hippocampal- and striatal-dependent learning

The effects of estrogens on spatial learning and memory in female rodents - A systematic review and meta-analysis

Estrogens have inconsistent effects on learning and memory in both the clinical and preclinical literature. Preclinical literature has the advantage of investigating an array of potentially important factors contributing to the varied effects of estrogens on learning and memory, with stringently controlled studies. This study set out to identify specific factors in the animal literature that influence the effects of estrogens on cognition, for possible translation back to clinical practice. The literature was screened and studies meeting strict inclusion criteria were included in the analysis. Eligible studies included female ovariectomized rodents with an adequate vehicle for the estrogen treatment, with an outcome of spatial learning and memory in the Morris water maze. Training days of the Morris water maze were used to assess acquisition of spatial learning, and the probe trial was used to evaluate spatial memory recall. Continuous outcomes were pooled using a random effects inverse variance method and reported as standardized mean differences with 95 % confidence intervals. Subgroup analyses were developed a priori to assess important factors. The overall analysis favoured treatment for the later stages of training and for the probe trial. Factors including the type of estrogen, route, schedule of administration, age of animals, timing relative to ovariectomy, and duration of treatment were all found to be important. The subgroup analyses showed that chronic treatment with 17β-estradiol, either cyclically or continuously, to young animals improved spatial recall. These results, observed in animals, can inform and guide further clinical research on hormone replacement therapy for cognitive benefits.

Effects of estrogen on spatial navigation and memory

RATIONALE

Animal studies suggest that the so-called "female" hormone estrogen enhances spatial navigation and memory. This contradicts the observation that males generally out-perform females in spatial navigation and tasks involving spatial memory. A closer look at the vast number of studies actually reveals that performance differences are not so clear.

OBJECTIVES

To help clarify the unclear performance differences between men and women and the role of estrogen, we attempted to isolate organizational from activational effects of estrogen on spatial navigation and memory.

METHODS

In a double-blind, placebo-controlled study, we tested the effects of orally administered estradiol valerate (E2V) in healthy, young women in their low-hormone menstrual cycle phase, compared to healthy, young men. Participants performed several first-person, environmentally rich, 3-D computer games inspired by spatial navigation and memory paradigms in animal research.

RESULTS

We found navigation behavior suggesting that sex effects dominated any E2 effects with men performing better with allocentric strategies and women with egocentric strategies. Increased E2 levels did not lead to general improvements in spatial ability in either sex but to behavioral changes reflecting navigation flexibility.

CONCLUSION

Estrogen-driven differences in spatial cognition might be better characterized on a spectrum of navigation flexibility rather than by categorical performance measures or skills.

Sex steroid hormones, the estrous cycle, and rapid modulation of glutamatergic synapse properties in the striatal brain regions with a focus on 17β-estradiol and the nucleus accumbens

The striatal brain regions encompassing the nucleus accumbens core (NAcc), shell (NAcs) and caudate-putamen (CPu) regulate cognitive functions including motivated behaviors, habit, learning, and sensorimotor action, among others. Sex steroid hormone sensitivity and sex differences have been documented in all of these functions in both normative and pathological contexts, including anxiety, depression and addiction. The neurotransmitter glutamate has been implicated in regulating these behaviors as well as striatal physiology, and there are likewise documented sex differences in glutamate action upon the striatal output neurons, the medium spiny neurons (MSNs). Here we review the available data regarding the role of steroid sex hormones such as 17β-estradiol (estradiol), progesterone, and testosterone in rapidly modulating MSN glutamatergic synapse properties, presented in the context of the estrous cycle as appropriate. Estradiol action upon glutamatergic synapse properties in female NAcc MSNs is most comprehensively discussed. In the female NAcc, MSNs exhibit development period-specific sex differences and estrous cycle variations in glutamatergic synapse properties as shown by multiple analyses, including that of miniature excitatory postsynaptic currents (mEPSCs). Estrous cycle-differences in NAcc MSN mEPSCs can be mimicked by acute exposure to estradiol or an ERα agonist. The available evidence, or lack thereof, is also discussed concerning estrogen action upon MSN glutamatergic synapse in the other striatal regions as well as the underexplored roles of progesterone and testosterone. We conclude that there is strong evidence regarding estradiol action upon glutamatergic synapse function in female NAcs MSNs and call for more research regarding other hormones and striatal regions.

Estradiol Receptors Inhibit Long-Term Potentiation in the Dorsomedial Striatum

Estradiol, a female sex hormone and the predominant form of estrogen, has diverse effects throughout the brain including in learning and memory. Estradiol modulates several types of learning that depend on the dorsomedial striatum (DMS), a subregion of the basal ganglia involved in goal-directed learning, cued action-selection, and motor skills. A cellular basis of learning is synaptic plasticity, and the presence of extranuclear estradiol receptors ERα, ERβ, and G-protein-coupled estrogen receptor (GPER) throughout the DMS suggests that estradiol may influence rapid cellular actions including those involved in plasticity. To test whether estradiol affects synaptic plasticity in the DMS, corticostriatal long-term potentiation (LTP) was induced using theta-burst stimulation (TBS) in ex vivo brain slices from intact male and female C57BL/6 mice. Extracellular field recordings showed that female mice in the diestrous stage of the estrous cycle exhibited LTP similar to male mice, while female mice in estrus did not exhibit LTP. Furthermore, antagonists of ERα or GPER rescued LTP in estrous females and agonists of ERα or GPER reduced LTP in diestrous females. In males, activating ERα but not GPER reduced LTP. These results uncover an inhibitory action of estradiol receptors on cellular learning in the DMS and suggest a cellular mechanism underlying the impairment in certain types of DMS-based learning observed in the presence of high estradiol. Because of the dorsal striatum's role in substance use disorders, these findings may provide a mechanism underlying an estradiol-mediated progression from goal-directed to habitual drug use.

ERα Stimulation Rapidly Modulates Excitatory Synapse Properties in Female Rat Nucleus Accumbens Core

INTRODUCTION

The nucleus accumbens core (NAcc) is a sexually differentiated brain region that is modulated by steroid hormones such as 17β-estradiol (estradiol), with consequential impacts on relevant motivated behaviors and disorders such as addiction, anxiety, and depression. NAcc estradiol levels naturally fluctuate, including during the estrous cycle in adult female rats, which is analogous to the menstrual cycle in adult humans. Across the estrous cycle, excitatory synapse properties of medium spiny neurons rapidly change, as indicated by analysis of miniature excitatory postsynaptic currents (mEPSCs). mEPSC frequency decreases during estrous cycle phases associated with high estradiol levels. This decrease in mEPSC frequency is mimicked by acute topical exposure to estradiol. The identity of the estrogen receptor (ER) underlying this estradiol action is unknown. Adult rat NAcc expresses three ERs, all extranuclear: membrane ERα, membrane ERβ, and GPER1.

METHODS

In this brief report, we take a first step toward addressing this challenge by testing whether activation of ERs via acute topical agonist application is sufficient for inducing changes in mEPSC properties recorded via whole-cell patch clamp.

RESULTS

An agonist of ERα induced large decreases in mEPSC frequency, while agonists of ERβ and GPER1 did not robustly modulate mEPSC properties.

CONCLUSIONS

These data provide evidence that activation of ERα is sufficient for inducing changes in mEPSC frequency and is a likely candidate underlying the estradiol-induced changes observed during the estrous cycle. Overall, these findings extend our understanding of the neuroendocrinology of the NAcc and implicate ERα as a primary target for future studies.

The estrous cycle and 17β-estradiol modulate the electrophysiological properties of rat nucleus accumbens core medium spiny neurons

The nucleus accumbens core is a key nexus within the mammalian brain for integrating the premotor and limbic systems and regulating important cognitive functions such as motivated behaviors. Nucleus accumbens core functions show sex differences and are sensitive to the presence of hormones such as 17β-estradiol (estradiol) in normal and pathological contexts. The primary neuron type of the nucleus accumbens core, the medium spiny neuron (MSN), exhibits sex differences in both intrinsic excitability and glutamatergic excitatory synapse electrophysiological properties. Here, we provide a review of recent literature showing how estradiol modulates rat nucleus accumbens core MSN electrophysiology within the context of the estrous cycle. We review the changes in MSN electrophysiological properties across the estrous cycle and how these changes can be mimicked in response to exogenous estradiol exposure. We discuss in detail recent findings regarding how acute estradiol exposure rapidly modulates excitatory synapse properties in nucleus accumbens core but not caudate-putamen MSNs, which mirror the natural changes seen across estrous cycle phases. These recent insights demonstrate the strong impact of sex-specific estradiol action upon nucleus accumbens core neuron electrophysiology.

Estrogen receptors observed at extranuclear neuronal sites and in glia in the nucleus accumbens core and shell of the female rat: Evidence for localization to catecholaminergic and GABAergic neurons

Estrogens affect dopamine-dependent diseases/behavior and have rapid effects on dopamine release and receptor availability in the nucleus accumbens (NAc). Low levels of nuclear estrogen receptor (ER) α and ERβ are seen in the NAc, which cannot account for the rapid effects of estrogens in this region. G-protein coupled ER 1 (GPER1) is observed at low levels in the NAc shell, which also likely does not account for the array of estrogens' effects in this region. Prior studies demonstrated membrane-associated ERs in the dorsal striatum; these experiments extend those findings to the NAc core and shell. Single- and dual-immunolabeling electron microscopy determined whether ERα, ERβ, and GPER1 are at extranuclear sites in the NAc core and shell and whether ERα and GPER1 were localized to catecholaminergic or γ-aminobutyric acid-ergic (GABAergic) neurons. All three ERs are observed, almost exclusively, at extranuclear sites in the NAc, and similarly distributed in the core and shell. ERα, ERβ, and GPER1 are primarily in axons and axon terminals suggesting that estrogens affect transmission in the NAc via presynaptic mechanisms. About 10% of these receptors are found on glia. A small proportion of ERα and GPER1 are localized to catecholaminergic terminals, suggesting that binding at these ERs alters release of catecholamines, including dopamine. A larger proportion of ERα and GPER1 are localized to GABAergic dendrites and terminals, suggesting that estrogens alter GABAergic transmission to indirectly affect dopamine transmission in the NAc. Thus, the localization of ERs could account for the rapid effects of estrogen in the NAc.

Intermittent Hypoxia Activates N-Methyl-D-Aspartate Receptors to Induce Anxiety Behaviors in a Mouse Model of Sleep-Associated Apnea

Sleep apnea disrupts physiologic homeostasis and causes neuronal dysfunction. In addition to signs of mental disorders and cognitive dysfunction, patients with sleep apnea have a higher anxiety rate. Here, we examined the mechanisms underlying this critical health issue. We used a mouse model with sleep-associated chronic intermittent hypoxia (IH) to verify the effects of sleep apnea on neuronal dysfunction. To evaluate how IH alters neuronal function to yield anxiety-like behavior and cognitive dysfunction, we examined synaptic plasticity and neuronal inflammation in related brain areas, including the medial prefrontal cortex (mPFC), striatum, and hippocampus. Mice subjected to chronic IH for 10 days exhibited significant anxiety-like behaviors in the elevated plus maze test. IH mice spent less travel time in open arms and more travel time in enclosed arms compared to control mice. However, cognitive impairment was minimal in IH mice. Increased glutamate N-methyl-D-aspartate (NMDA) receptor subunits 2B (GluN2B) and phosphorylated-ERK1/2 were seen in the mPFC, striatum, and hippocampus of IH mice, but no significant microglial and astrocyte activation was found in these brain areas. Chronic IH in mice induced compensatory increases in GluN2B to disturb neuronal synaptic plasticity, without neuronal inflammation. The altered synaptic plasticity subsequently led to anxiety-like behavior in mice. Treatment with the NMDA receptor antagonist dextromethorphan attenuated chronic IH-induced anxiety-like behavior and GluN2B expression. Our findings provide mechanistic evidence of how IH may provoke anxiety and support for the importance of early intervention to alleviate anxiety-associated complications in patients with chronic sleep apnea.

Place vs. Response Learning: History, Controversy, and Neurobiology

The present article provides a historical review of the place and response learning plus-maze tasks with a focus on the behavioral and neurobiological findings. The article begins by reviewing the conflict between Edward C. Tolman's cognitive view and Clark L. Hull's stimulus-response (S-R) view of learning and how the place and response learning plus-maze tasks were designed to resolve this debate. Cognitive learning theorists predicted that place learning would be acquired faster than response learning, indicating the dominance of cognitive learning, whereas S-R learning theorists predicted that response learning would be acquired faster, indicating the dominance of S-R learning. Here, the evidence is reviewed demonstrating that either place or response learning may be dominant in a given learning situation and that the relative dominance of place and response learning depends on various parametric factors (i.e., amount of training, visual aspects of the learning environment, emotional arousal, et cetera). Next, the neurobiology underlying place and response learning is reviewed, providing strong evidence for the existence of multiple memory systems in the mammalian brain. Research has indicated that place learning is principally mediated by the hippocampus, whereas response learning is mediated by the dorsolateral striatum. Other brain regions implicated in place and response learning are also discussed in this section, including the dorsomedial striatum, amygdala, and medial prefrontal cortex. An exhaustive review of the neurotransmitter systems underlying place and response learning is subsequently provided, indicating important roles for glutamate, dopamine, acetylcholine, cannabinoids, and estrogen. Closing remarks are made emphasizing the historical importance of the place and response learning tasks in resolving problems in learning theory, as well as for examining the behavioral and neurobiological mechanisms of multiple memory systems. How the place and response learning tasks may be employed in the future for examining extinction, neural circuits of memory, and human psychopathology is also briefly considered.

Influence of ovarian hormones on value-based decision-making systems: Contribution to sexual dimorphisms in mental disorders

Women and men exhibit differences in behavior when making value-based decisions. Various hypotheses have been proposed to explain these findings, stressing differences in functional lateralization of the brain, functional activation, neurotransmitter involvement and more recently, sex hormones. While a significant interaction of neurotransmitter systems and sex hormones has been shown for both sexes, decision-making in women might be particularly affected by variations of ovarian hormones. In this review we have gathered information from animal and human studies on how ovarian hormones affect decision-making processes in females by interacting with neurotransmitter systems at functionally relevant brain locations and thus modify the computation of decision aspects. We also review previous findings on impaired decision-making in animals and clinical populations with substance use disorder and depression, emphasizing how little we know about the role of ovarian hormones in aberrant decision-making.

Effects of ∆9-THC on memory in ovariectomized and intact female rats

The effects of marijuana's major psychoactive cannabinoid, ∆9-tetrahydrocannibinol (∆9-THC), were examined on memory in female rats by training subjects to respond under a repeated acquisition and delayed-performance procedure. During this task, subjects acquired a different 4-response sequence each session, which was then recalled after a delay. Sequence retention was tested following various delays, and quantified by a percent savings measure. Response rate and percent errors were also recorded. Subsequent to training, subjects underwent an ovariectomy (OVX) or sham surgery (intact). The OVX group then underwent implantation of subcutaneous 17β-estradiol capsules while the intact group received chronic administration of 1 mg/kg of the estrogen receptor modifier, tamoxifen. Increasing delays from 1 min to 24 h produced delay-dependent decreases in percent savings in both OVX and intact rats. Acute administration of ∆9-THC (0.32-3.2 mg/kg) dose-dependently decreased retention, increased percent errors, and decreased response rate in both groups when the delay was 1 h. However, intact rats showed a significantly lower percent savings than OVX rats at the 0.56-mg/kg dose. Delays of 3 h enhanced the disruptive effects of ∆9-THC more in intact than OVX rats; furthermore, implantation of 17β-estradiol attenuated ∆9-THC-induced disruptions in OVX rats and significantly increased estradiol levels and uterine weight as compared to intact rats. Although chronic tamoxifen administration did not alter ∆9-THC's effects on memory in intact rats, it did significantly decrease response rate. These results demonstrate the capacity of chronic 17β-estradiol for attenuating ∆9-THC's acute memory-disrupting effects in OVX female rats.

Dose-dependent effects of testosterone on spatial learning strategies and brain-derived neurotrophic factor in male rats

Studies suggest that males outperform females on some spatial tasks. This may be due to the effects of sex steroids on spatial strategy preferences. Past experiments with male rats have demonstrated that low doses of testosterone bias them toward a response strategy, whereas high doses of testosterone bias them toward a place strategy. We investigated the effect of different testosterone doses on the ability of male rats to effectively employ these two spatial learning strategies. Furthermore, we quantified concentrations of brain-derived neurotrophic factor (pro-, mature-, and total BDNF) in the prefrontal cortex, hippocampus, and striatum. All rats were bilaterally castrated and assigned to one of three daily injection doses of testosterone propionate (0.125, 0.250, or 0.500 mg/rat) or a control injection of the drug vehicle. Using a plus-maze protocol, we found that a lower testosterone dose (0.125 mg) significantly improved rats' performance on a response task, whereas a higher testosterone dose (0.500 mg) significantly improved rats' performance on a place task. In addition, we found that a low dose of testosterone (0.125 mg) increased total BDNF in the striatum, while a high dose (0.500 mg) increased total BDNF in the hippocampus. Taken altogether, these results suggest that high and low levels of testosterone enhance performance on place and response spatial tasks, respectively, and this effect is associated with changes in BDNF levels within relevant brain regions.

Estradiol and selective estrogen receptor agonists differentially affect brain monoamines and amino acids levels in transitional and surgical menopausal rat models

Estrogens have many beneficial effects in the brain. Previously, we evaluated the effects of two models of menopause (surgical vs. transitional) on multiple monoaminergic endpoints in different regions of the adult rat brain in comparison with levels in gonadally intact rats. Here we evaluated the effects of estrogen receptor (ER) agonist treatments in these same two models of menopause. Neurochemical endpoints were evaluated in the hippocampus (HPC), frontal cortex (FCX), and striatum (STR) of adult ovariectomized (OVX) rats and in rats that underwent selective and gradual ovarian follicle depletion by daily injection of 4-vinylcyclohexene-diepoxide (VCD), after 1- and 6-weeks treatment with 17β-estradiol (E2), or with selective ERα (PPT), ERβ (DPN), or GPR30 (G-1) agonists. Endpoints included serotonin (5-HT) and 5-Hydroxyindoleacetic acid, dopamine (DA), 3,4-Dihydroxyphenylacetic acid and homovanillic acid, norepinephrine (NE) and epinephrine, as well as the amino acids tryptophan (TRP) and tyrosine (TYR). Significant differences between the models were detected. OVX rats were much more sensitive to ER agonist treatments than VCD-treated rats. Significant differences between brain regions also were detected. Within OVX rats, more agonist effects were detected in the HPC than in any other region. One interesting finding was the substantial decrease in TRP and TYR detected in the HPC and FCX in response to agonist treatments, particularly in OVX rats. This is on top of the substantial decreases in TRP and TYR previously reported one week after OVX or VCD-treatments in comparison with gonadally intact controls. Other interesting findings included increases in the levels of 5-HT, DA, and NE in the HPC of OVX rats treated with DPN, increases in DA detected in the FCX of OVX rats treated with any of the ER agonists, and increases in 5-HT and DA detected in the STR of OVX rats treated with E2. Many effects that were observed after 1-week of treatment were no longer observed after 6-weeks of treatment, demonstrating that effects were temporary despite continued agonist treatment. Collectively, the results demonstrate significant differences in the effects of ER agonists on monoaminergic endpoints in OVX vs. VCD-treated rats that also were brain region-specific and time dependent. The fact that agonist treatments had lesser effects in VCD treated rats than in OVX rats may help to explain reports of lesser effects of estrogen replacement on cognitive performance in women that have undergone transitional vs. surgical menopause.

The role of G protein-coupled estrogen receptor 1 on neurological disorders

G protein-coupled estrogen receptor 1 (GPER) is a membrane-associated estrogen receptor (ER) associated with rapid estrogen-mediated effects. Over recent years GPER emerged has a potential therapeutic target to induce neuroprotection, avoiding the side effects elicited by the activation of classical ERs. The putative neuroprotection triggered by GPER selective activation was demonstrated in mood disorders, Alzheimer's disease or Parkinson's disease of male and female in vivo rodent models. In others, like ischemic stroke, the results are contradictory and currently there is no consensus on the role played by this receptor. However, it seems clear that sex is a biological variable that may impact the results. The major objective of this review is to provide an overview about the physiological effects of GPER in the brain and its putative contribution in neurodegenerative disorders, discussing the data about the signaling pathways involved, as well as, the diverse effects observed.

The estrous cycle modulates rat caudate-putamen medium spiny neuron physiology

The neuroendocrine environment in which the brain operates is both dynamic and differs by sex. How differences in neuroendocrine state affect neuron properties has been significantly neglected in neuroscience research. Behavioral data across humans and rodents indicate that natural cyclical changes in steroid sex hormone production affect sensorimotor and cognitive behaviors in both normal and pathological contexts. These behaviors are critically mediated by the caudate-putamen. In the caudate-putamen, medium spiny neurons (MSNs) are the predominant and primary output neurons. MSNs express membrane-associated estrogen receptors and demonstrate estrogen sensitivity. However, how the cyclical hormone changes across the estrous cycle may modulate caudate-putamen MSN electrophysiological properties remains unknown. Here, we performed whole-cell patch-clamp recordings on male, diestrus female, proestrus female, and estrus female caudate-putamen MSNs. Action potential, passive membrane, and miniature excitatory post-synaptic current properties were assessed. Numerous MSN electrical properties robustly differed by cycle state, including resting membrane potential, rheobase, action potential threshold, maximum evoked action potential firing rate, and inward rectification. Strikingly, when considered independent of estrous cycle phase, all but one of these properties do not significantly differ from male MSNs. These data indicate that female caudate-putamen MSNs are sensitive to the estrous cycle, and more broadly, the importance of considering neuroendocrine state in studies of neuron physiology.

Organizational effects of testosterone on learning strategy preference and muscarinic receptor binding in prepubertal rats

Prior to puberty, male rats, but not female rats, prefer a striatum-based stimulus-response learning strategy rather than a hippocampus-based place strategy on a water maze task that can be solved using either strategy. Neurochemically, learning strategy preference has been linked to the ratio of cholinergic muscarinic receptor binding in the hippocampus relative to the striatum, with lower ratios displayed by males compared to females and by stimulus-response learners compared to place learners. Sex differences in a variety of different behaviors are established by the organizational influence of testosterone on brain development. Therefore, the current study investigated the potential organizational effects of neonatal testosterone on learning strategy preference and the hippocampus:striatum ratio of muscarinic receptor binding in prepubertal male and female rats. Similar to vehicle-treated control males, prepubertal females treated with testosterone propionate on the first two days of life preferred a stimulus-response strategy on a dual-solution water maze task. Conversely, vehicle-treated prepubertal females were more likely to use a place strategy. Consistent with previous findings, the hippocampus:striatum ratio of muscarinic receptor binding was lower in rats preferring a stimulus-response strategy compared to those using a place strategy and lower in control males compared to control females. However, the hippocampus:striatum ratio was not reversed by neonatal testosterone treatment of females as predicted. The current study is the first to show that sex differences in how a navigational task is learned prior to puberty is impacted by the presence of testosterone during vulnerable periods in brain development.

Licorice root components mimic estrogens in an object location task but not an object recognition task

This study investigated the efficacy of components of licorice root to alter performance on two different recognition tasks, a hippocampus-sensitive metric change in object location (MCOL) task and a striatum-sensitive double object recognition (DOR) task. Isoliquiritigenin (ISL), licorice root extract (LRE), and whole licorice root powder (LRP) were assessed. Young adult female rats were ovariectomized (OVX) and exposed to ISL, LRE or LRP at 0.075%, 0.5% or 5% respectively in the diet. An estradiol group was included as a positive control based on our prior findings. Rats were allowed to explore two objects for three 5-min study trials (separated by 3-min intervals) before a fourth 5-min test trial where the objects were moved closer together (MCOL task) or replaced with two new objects (DOR task). Rats typically habituate to the objects across the three study trials. An increase in object exploration time in the test trial suggests the rat detected the change. Estradiol improved MCOL performance and impaired DOR performance, similar to previously shown effects of estradiol and other estrogens, which tend to improve learning and memory on hippocampus-sensitive tasks and impair striatum-sensitive cognition. LRP had no effect on recognition while exposure to ISL and LRE improved MCOL performance. Exposure to ISL, LRE and LRP failed to attenuate DOR, contrary to effects of estradiol shown here and to previous reports in young-adult OVX rats. These findings suggest components of licorice root may prove to be effective therapies targeting memory enhancement without unintended deleterious cognitive effects.

Using a memory systems lens to view the effects of estrogens on cognition: Implications for human health

Understanding the organizing and activating effects of gonadal steroids on adult physiology can guide insight into sex differences in and hormonal influences on health and disease, ranging from diabetes and other metabolic disorders, emotion and stress regulation, substance abuse, pain perception, immune function and inflammation, to cognitive function and dysfunction accompanying neurological disorders. Because the brain is highly sensitive to many forms of estrogens, it is not surprising that many adult behaviors, including cognitive function, are modulated by estrogens. Estrogens are known for their facilitating effects on learning and memory, but it is becoming increasingly clear that they also can impair learning and memory of some classes of tasks and may do so through direct actions on specific neural systems. This review takes a multiple memory systems approach to understanding how estrogens can at the same time enhance hippocampus-sensitive place learning and impair striatum-sensitive response learning by exploring the role estrogen receptor signaling may play in the opposing cognitive effects of estrogens. Accumulating evidence suggests that neither receptor subtype nor the timing of treatment, i.e. rapid vs slow, explain the bidirectional effects of estrogens on different types of learning. New findings pointing to neural metabolism and the provision of energy substrates by astrocytes as a candidate mechanism for cognitive enhancement and impairment are discussed.

Sex Differences in Medium Spiny Neuron Excitability and Glutamatergic Synaptic Input: Heterogeneity Across Striatal Regions and Evidence for Estradiol-Dependent Sexual Differentiation

Steroid sex hormones and biological sex influence how the brain regulates motivated behavior, reward, and sensorimotor function in both normal and pathological contexts. Investigations into the underlying neural mechanisms have targeted the striatal brain regions, including the caudate-putamen, nucleus accumbens core (AcbC), and shell. These brain regions are of particular interest to neuroendocrinologists given that they express membrane-associated but not nuclear estrogen receptors, and also the well-established role of the sex steroid hormone 17β-estradiol (estradiol) in modulating striatal dopamine systems. Indeed, output neurons of the striatum, the medium spiny neurons (MSNs), exhibit estradiol sensitivity and sex differences in electrophysiological properties. Here, we review sex differences in rat MSN glutamatergic synaptic input and intrinsic excitability across striatal regions, including evidence for estradiol-mediated sexual differentiation in the nucleus AcbC. In prepubertal animals, female MSNs in the caudate-putamen exhibit a greater intrinsic excitability relative to male MSNs, but no sex differences are detected in excitatory synaptic input. Alternatively, female MSNs in the nucleus AcbC exhibit increased excitatory synaptic input relative to male MSNs, but no sex differences in intrinsic excitability were detected. Increased excitatory synaptic input onto female MSNs in the nucleus AcbC is abolished after masculinizing estradiol or testosterone exposure during the neonatal critical period. No sex differences are detected in MSNs in prepubertal nucleus accumbens shell. Thus, despite possessing the same neuron type, striatal regions exhibit heterogeneity in sex differences in MSN electrophysiological properties, which likely contribute to the sex differences observed in striatal function.

Striatum on the anxiety map: Small detours into adolescence

Adolescence is the most sensitive period for the development of pathological anxiety. Moreover, specific neural changes associated with the striatum might be related to adolescent vulnerability to anxiety. Up to now, the study of anxiety has primarily focused on the amygdala, bed nucleus of the stria terminalis (BNST), hippocampus and ventromedial prefrontal cortex (vmPFC), while the striatum has typically not been considered as part of the anxiety system. This review proposes the addition of the striatum, a complex, multi-component structure, to the anxiety network by underscoring two lines of research. First, the co-occurrence of the adolescent striatal development with the peak vulnerability of adolescents to anxiety disorders might potentially reflect a causal relationship. Second, the recognition of the role of the striatum in fundamental behavioral processes that do affect anxiety supports the putative importance of the striatum in anxiety. These behavioral processes include (1) attention, (2) conditioning/prediction error, and (3) motivation. This review proposes a simplistic schematic representation of the anxiety circuitry that includes the striatum, and aims to promote further work in this direction, as the role of the striatum in shaping an anxiety phenotype during adolescence could have critical implications for understanding and preventing the peak onset of anxiety disorders during this period. This article is part of a Special Issue entitled SI: Adolescent plasticity.

Modulation of spatial and response strategies by phase of the menstrual cycle in women tested in a virtual navigation task

Different memory systems are employed to navigate an environment. It has been consistently shown in rodents that estrogen impacts multiple memory system bias such that low estradiol (E2) is associated with increased use of a striatal-mediated response strategy whereas high E2 increases use of a hippocampal-dependent spatial memory. Low E2 also enhances performance on a response-based task whereas high E2 levels improve learning on a spatial task. The purpose of the present cross-sectional study was to investigate navigational strategies in young, healthy, naturally cycling women. Participants were split into either an early follicular (i.e., when E2 levels are low), ovulatory (i.e., when E2 levels are high) or mid/late luteal (i.e., end of the cycle, when E2 levels decrease and progesterone levels rise) phase group, using self-reported date of the menstrual cycle. Serum hormone level measurements (E2, progesterone, testosterone) were used to confirm cycle phase assignment. Participants were administered a verbal memory task as well as a virtual navigation task that can be solved by using either a response or spatial strategy. Women tested in the ovulatory phase, under high E2 conditions, performed better on a verbal memory task than women tested during the other phases of the cycle. Interestingly, women tested in the mid/late luteal phase, when progesterone is high, predominantly used a spatial strategy, whereas the opposite pattern was observed in the early follicular and ovulatory groups. Our data suggest that the specific memory system engaged differs depending on the phase of the menstrual cycle and may be mediated by both E2 and progesterone, rather than E2 alone.

Can Exercise Ameliorate Aromatase Inhibitor-Induced Cognitive Decline in Breast Cancer Patients?

Aromatase inhibitors (AIs) have been commonly used as an effective adjuvant therapy in treatment of breast cancer, especially for menopausal women with estrogen receptor-positive breast cancer. Due to the nature of aromatase, the key enzyme for endogenous estrogen synthesis, inhibitory of aromatase-induced side effects, such as cognitive impairment has been reported in both human and animal studies. While extensive evidence suggested that physical exercises can improve learning and memory activity and even prevent age-related cognitive decline, basic research revealed some common pathways between exercise and estrogen signaling that affected cognitive function. This review draws on clinical and basic studies to assess the potential impact of exercise in cognitive function from women treated with AIs for breast cancer and explore the potential mechanism and effects of exercise on estrogen-related cognition.

Estrogen Receptor-Selective Agonists Modulate Learning in Female Rats in a Dose- and Task-Specific Manner

Estrogens are well known for their enhancing effects on hippocampus-sensitive cognition. However, estrogens can also impair learning and memory, particularly the acquisition of striatum-sensitive tasks. These cognitive shifts appear to be mediated through local estrogen receptor (ER) activation in each neural structure, but little information is known regarding which specific ER subtypes drive the opposing effects on learning. Elucidating the mnemonic roles of discrete ER subtypes is essential for predicting how treatments with distinct ER pharmacology such as drugs, hormone therapies, and phytoestrogen supplements affect cognitive abilities in and thus the daily lives of the women who take them. The present study examined the effects of the ERα-selective compound propyl pyrazole triol and the ERβ-selective compounds diarylpropionitrile and Br-ERb-041 on place and response learning in young adult female rats. Long-Evans rats were ovariectomized and maintained on phytoestrogen-free chow for 3 weeks before behavioral training, with treatments administered via subcutaneous injection 48 and 24 hours before testing. A dose-response paradigm was used, with each compound tested at 4 different doses in separate groups of rats. Propyl pyrazole triol, diarylpropionitrile, and Br-ERb-041 all enhanced place learning and impaired response learning, albeit with distinct dose-response patterns for each compound and task. These results are consistent with the detection of ERα and ERβ in the hippocampus and striatum and suggest that learning is modulated via activation of either ER subtype.

Genetic sex and the volumes of the caudate-putamen, nucleus accumbens core and shell: original data and a review

Sex differences are widespread across vertebrate nervous systems. Such differences are sometimes reflected in the neural substrate via neuroanatomical differences in brain region volume. One brain region that displays sex differences in its associated functions and pathologies is the striatum, including the caudate-putamen (dorsal striatum), nucleus accumbens core and shell (ventral striatum). The extent to which these differences can be attributed to alterations in volume is unclear. We thus tested whether the volumes of the caudate-putamen, nucleus accumbens core, and nucleus accumbens shell differed by region, sex, and hemisphere in adult Sprague-Dawley rats. As a positive control for detecting sex differences in brain region volume, we measured the sexually dimorphic nucleus of the medial preoptic area (SDN-POA). As expected, SDN-POA volume was larger in males than in females. No sex differences were detected in the volumes of the caudate-putamen, nucleus accumbens core or shell. Nucleus accumbens core volume was larger in the right than left hemisphere across males and females. These findings complement previous reports of lateralized nucleus accumbens volume in humans, and suggest that this may possibly be driven via hemispheric differences in nucleus accumbens core volume. In contrast, striatal sex differences seem to be mediated by factors other than striatal region volume. This conclusion is presented within the context of a detailed review of studies addressing sex differences and similarities in striatal neuroanatomy.

Signaling mechanisms mediating the regulation of synaptic plasticity and memory by estradiol

This article is part of a Special Issue "Estradiol and Cognition". Estradiol participates in the regulation of the function and plasticity of synaptic circuits in key cognitive brain regions, such as the prefrontal cortex and the hippocampus. The mechanisms elicited by estradiol are mediated by the regulation of transcriptional activity by nuclear estrogen receptors and by intracellular signaling cascades activated by estrogen receptors associated with the plasma membrane. In addition, the mechanisms include the interaction of estradiol with the signaling of other factors involved in the regulation of cognition, such as brain derived neurotrophic factor, insulin-like growth factor-1 and Wnt. Modifications in these signaling pathways by aging or by a long-lasting ovarian hormone deprivation after menopause may impair the enhancing effects of estradiol on synaptic plasticity and cognition.

Estrogens and cognition: Friends or foes?: An evaluation of the opposing effects of estrogens on learning and memory

This article is part of a Special Issue "Estradiol and cognition". Estrogens are becoming well known for their robust enhancement on cognition particularly for learning and memory that relies upon functioning of the hippocampus and related neural systems. What is also emerging is that estrogen modulation of cognition is not uniform, at times enhancing yet at other times impairing learning. This review explores the bidirectional effects of estrogens on learning from a multiple memory systems view, focusing on the hippocampus and striatum, whereby modulation by estrogens sorts according to task attributes and neural systems engaged during cognition. We highlight our findings showing that the ability to solve hippocampus-sensitive tasks typically improves under relatively high estrogen status while the ability to solve striatum-sensitive tasks degrades with estrogen exposures. Though constrained by dose and timing of exposure, these opposing enhancements and impairments of cognition can be observed following treatments with different estrogenic compounds including the hormone estradiol, the isoflavone genistein found in soybeans, and agonists that are selective for specific estrogen receptors, suggesting that activation of a single receptor type is sufficient to produce the observed shifts in learning strategies. Using this multi-dimensional framework will allow us to extend our thinking of the relationship between estrogens and cognition to other brain regions and cognitive functions.

Hippocampal learning, memory, and neurogenesis: Effects of sex and estrogens across the lifespan in adults

This article is part of a Special Issue "Estradiol and Cognition". There are sex differences in hippocampus-dependent cognition and neurogenesis suggesting that sex hormones are involved. Estrogens modulate certain forms of spatial and contextual memory and neurogenesis in the adult female rodent, and to a lesser extent male, hippocampus. This review focuses on the effects of sex and estrogens on hippocampal learning, memory, and neurogenesis in the young and aged adult rodent. We discuss how factors such as the type of estrogen, duration and dose of treatment, timing of treatment, and type of memory influence the effects of estrogens on cognition and neurogenesis. We also address how reproductive experience (pregnancy and mothering) and aging interact with estrogens to modulate hippocampal cognition and neurogenesis in females. Given the evidence that adult hippocampal neurogenesis plays a role in long-term spatial memory and pattern separation, we also discuss the functional implications of regulating neurogenesis in the hippocampus.

Estradiol and cognitive function: past, present and future

A historical perspective on estradiol's enhancement of cognitive function is presented, and research, primarily in animals, but also in humans, is reviewed. Data regarding the mechanisms underlying the enhancements are discussed. Newer studies showing rapid effects of estradiol on consolidation of memory through membrane interactions and activation of inter-cellular signaling pathways are reviewed as well as studies focused on traditional genomic mechanisms. Recent demonstrations of intra-neuronal estradiol synthesis and possible actions as a neurosteroid to promote memory are discussed. This information is applied to the critical issue of the current lack of effective hormonal (or other) treatments for cognitive decline associated with menopause and aging. Finally, the critical period hypothesis for estradiol effects is discussed along with novel strategies for hormone/drug development. Overall, the historical record documents that estradiol positively impacts some aspects of cognitive function, but effective therapeutic interventions using this hormone have yet to be realized.

Recognition memory tasks in neuroendocrine research

The recognition memory tasks, novel object and novel object location, have been beneficial to neuroendocrine research concerning the effects of gonadal and adrenal hormones on cognitive function. This review discusses the advantages of these tasks in comparison with other learning and memory tasks. Experiments conducted across a number of laboratories show that gonadal hormones, both estradiol and testosterone, promote memory while the adrenal hormone, corticosterone, impairs memory. The effects of these steroid hormones on spine density in the prefrontal cortex and hippocampus are also briefly presented. Overall, results show that these steroid hormones are potent modulators of memory consolidation in rodent models.

Puberty and adolescence as a time of vulnerability to stressors that alter neurobehavioral processes

Puberty and adolescence are major life transitions during which an individual's physiology and behavior changes from that of a juvenile to that of an adult. Here we review studies documenting the effects of stressors during pubertal and adolescent development on the adult brain and behavior. The experience of complex or compound stressors during puberty/adolescence generally increases stress reactivity, increases anxiety and depression, and decreases cognitive performance in adulthood. These behavioral changes correlate with decreased hippocampal volumes and alterations in neural plasticity. Moreover, stressful experiences during puberty disrupt behavioral responses to gonadal hormones both in sexual performance and on cognition and emotionality. These behavioral changes correlate with altered estrogen receptor densities in some estrogen-concentrating brain areas, suggesting a remodeling of the brain's response to hormones. A hypothesis is presented that activation of the immune system results in chronic neuroinflammation that may mediate the alterations of hormone-modulated behaviors in adulthood.

DREAM/calsenilin/KChIP3 modulates strategy selection and estradiol-dependent learning and memory

Downstream regulatory element antagonist modulator (DREAM)/calsenilin(C)/K⁺ channel interacting protein 3 (KChIP3) is a multifunctional Ca²⁺-binding protein highly expressed in the hippocampus that inhibits hippocampus-sensitive memory and synaptic plasticity in male mice. Initial studies in our lab suggested opposing effects of DR/C/K3 expression in female mice. Fluctuating hormones that occur during the estrous cycle may affect these results. In this study, we hypothesized that DR/C/K3 interacts with 17β-estradiol, the primary estrogen produced by the ovaries, to play a role in hippocampus function. We investigated the role of estradiol and DR/C/K3 in learning strategy in ovariectomized (OVX) female mice. OVX WT and DR/C/K3 knockout (KO) mice were given three injections of vehicle (sesame oil) or 17β-estradiol benzoate (0.25 mg in 100 mL sesame oil) 48, 24, and 2 h before training and testing. DR/C/K3 and estradiol had a time-dependent effect on strategy use in the female mice. Male KO mice exhibited enhanced place strategy relative to WT 24 h after pre-exposure. Fear memory formation was significantly reduced in intact female KO mice relative to intact WT mice, and OVX reduced fear memory formation in the WT, but had no effect in the KO mice. Long-term potentiation in hippocampus slices from female mice was enhanced by circulating ovarian hormones in both WT and DR/C/K3 KO mice. Paired-pulse depression was not affected by ovarian hormones but was reduced in DR/C/K3 KO mice. These results provide the first evidence that DR/C/K3 plays a timing-dependent role in estradiol regulation of learning, memory, and plasticity.

Sex, hormones and neurogenesis in the hippocampus: hormonal modulation of neurogenesis and potential functional implications

The hippocampus is an area of the brain that undergoes dramatic plasticity in response to experience and hormone exposure. The hippocampus retains the ability to produce new neurones in most mammalian species and is a structure that is targeted in a number of neurodegenerative and neuropsychiatric diseases, many of which are influenced by both sex and sex hormone exposure. Intriguingly, gonadal and adrenal hormones affect the structure and function of the hippocampus differently in males and females. Adult neurogenesis in the hippocampus is regulated by both gonadal and adrenal hormones in a sex- and experience-dependent way. Sex differences in the effects of steroid hormones to modulate hippocampal plasticity should not be completely unexpected because the physiology of males and females is different, with the most notable difference being that females gestate and nurse the offspring. Furthermore, reproductive experience (i.e. pregnancy and mothering) results in permanent changes to the maternal brain, including the hippocampus. This review outlines the ability of gonadal and stress hormones to modulate multiple aspects of neurogenesis (cell proliferation and cell survival) in both male and female rodents. The function of adult neurogenesis in the hippocampus is linked to spatial memory and depression, and the present review provides early evidence of the functional links between the hormonal modulation of neurogenesis that may contribute to the regulation of cognition and stress.

Scatter hoarding and hippocampal cell proliferation in Siberian chipmunks

Food hoarding, especially scatter hoarding and retrieving food caches, requires spatial learning and memory and is an adaptive behavior important for an animal's survival and reproductive success. In the present study, we examined the effects of hoarding behavior on cell proliferation and survival in the hippocampus of male and female Siberian chipmunks (Tamias sibiricus). We found that chipmunks in a semi-natural enclosure displayed hoarding behavior with large individual variations. Males ate more scatter-hoarded seeds than females. In addition, the display of hoarding behavior was associated with increased cell proliferation in the hippocampus and this increase occurred in a brain region-specific manner. These data provide further evidence to support the notion that new cells in the adult hippocampus are affected by learning and memory tasks and may play an important role in adaptive behavior.

Estradiol and striatal dopamine receptor antagonism influence memory system bias in the female rat

Estradiol (E2) has been shown to influence learning and memory systems used by female rats to find a reward. Rats with high levels of E2 tend to use allocentric, or place, memory while rats with low levels of E2 use egocentric, or response, memory. It has been shown that systemic dopamine receptor antagonism interacts with E2 to affect which memory system is used. Here, dopamine antagonists were administered directly into either the dorsal striatum or nucleus accumbens to determine where in the brain this interaction takes place. Seventy-four young adult, female, Sprague-Dawley rats were trained and tested in a modified plus-maze. All rats were ovariectomized, received a subcutaneous low E2 implant, and were implanted with bilateral cannulae into either the dorsal striatum or the nucleus accumbens. Additionally, high E2 rats received daily injections of E2 in a sesame oil solution while low E2 rats received daily injections of vehicle. After reaching criterion levels of performance in a plus-maze task, rats were administered microinjections of either a dopamine D1 receptor (SCH 23390; 0.1 μg/ml and 0.01 μg/ml) or D2 receptor (raclopride; 2 μg/ml and 0.5 μg/ml) antagonist or a vehicle control (saline) in a counterbalanced manner. High E2 rats exhibited a trend towards a place memory bias while low E2 rats showed a response memory bias. Dorsal striatal administration of a D1, but not D2, dopamine receptor antagonist caused a switch in the memory system used by both high and low E rats. There was no significant effect of dopamine receptor antagonism in the nucleus accumbens group. Thus, E2 determined which memory system controlled behavior in a plus-maze task. Moreover, this effect was modulated by dopamine D1R antagonism in the dorsal but not ventral striatum suggesting that memory systems are, in part, mediated by E2 and dopamine in this region.

Voluntary exercise impairs initial delayed spatial alternation performance in estradiol treated ovariectomized middle-aged rats

Estrogens differentially modulate behavior in the adult female rodent. Voluntary exercise can also impact behavior, often reversing age associated decrements in memory processes. Our research group has published a series of papers reporting a deficit in the acquisition of an operant working memory task, delayed spatial alternation (DSA), following 17β-estradiol treatment to middle-aged ovariectomized (OVX) rats. The current study examined if voluntary exercise could attenuate the 17β-estradiol induced deficits on DSA performance. OVX 12-month old Long-Evans rats were implanted with a Silastic capsule containing 17β-estradiol (10% in cholesterol: low physiological range) or with a blank capsule. A subset of the 17β-estradiol and OVX untreated rats were given free access to a running wheel in their home cage. All rats were tested for 40 sessions on the DSA task. Surprisingly, we found running wheel access to impair initial acquisition of the DSA task in 17β-estradiol treated rats, an effect not seen in OVX untreated rats given running wheel access. This deficit was driven by an increase in perseverative responding on a lever no longer associated with reinforcement. We also report for the first time a 17β-estradiol induced impairment on the DSA task following a long intertrial delay (18-sec), an effect revealed following more extended testing than in our previous studies (15 additional sessions). Overall, running wheel access increased initial error rate on the DSA task in 17β-estradiol treated middle-aged OVX rats, and failed to prevent the 17β-estradiol induced deficits in performance of the operant DSA task in later testing sessions.

Pubertal immune challenge blocks the ability of estradiol to enhance performance on cognitive tasks in adult female mice

Puberty is a period characterized by brain reorganization that contributes to the development of neural and behavioral responses to gonadal steroids. Previously, we have shown that a single injection of the bacterial endotoxin, lipopolysaccharide (LPS; 1.5mg/kg IP), during the pubertal period (around 6weeks old) in mice decreases sexual receptivity in response to estradiol and progesterone in adulthood. These findings suggest that pubertal immune challenge has an enduring effect of decreasing the behavioral responsiveness to gonadal steroid hormones. Since estradiol improves cognitive function in certain tasks in mice, we investigated the effect of pubertal immune challenge on the ability of estradiol to enhance cognitive function. We hypothesized that estradiol would be less effective at enhancing performance on particular cognitive tasks in female mice treated with LPS during puberty. Six-week old (pubertal) and 10-week old (adult) female CD1 mice were injected with either saline or LPS. Five weeks later, they were ovariectomized and implanted subcutaneously with either an estradiol- or oil-filled Silastic(©) capsule followed 1week later with testing for cognitive function. The duration of juvenile investigation during social discrimination and recognition tests was used as a measure of social memory, and the duration of object investigation during object recognition and placement tests was used as a measure of object memory. Chronic estradiol treatment enhanced social and object memory in saline-treated females and in females treated with LPS in adulthood. In contrast, in females treated with LPS at 6weeks old, estradiol failed to improve social and object memories. These results support the hypothesis that exposure to an immune challenge during puberty reduces at least some of the cognitive effects of estradiol. Moreover, these results support the idea that pubertal immune challenge compromises a wide variety of behavioral influences of ovarian hormones.

Testosterone influences spatial strategy preferences among adult male rats

Males outperform females on some spatial tasks, and this may be partially due to the effects of sex steroids on spatial strategy preferences. Previous work with rodents indicates that low estradiol levels bias females toward a striatum-dependent response strategy, whereas high estradiol levels bias them toward a hippocampus-dependent place strategy. We tested whether testosterone influenced the strategy preferences in male rats. All subjects were castrated and assigned to one of three daily injection doses of testosterone (0.125, 0.250, or 0.500 mg/rat) or a control group that received daily injections of the drug vehicle. Three different maze protocols were used to determine rats' strategy preferences. A low dose of testosterone (0.125 mg) biased males toward a motor-response strategy on a T-maze task. In a water maze task in which the platform itself could be used intermittently as a visual cue, a low testosterone dose (0.125 mg) caused a significant increase in the use of a cued-response strategy relative to control males. Results from this second experiment also indicated that males receiving a high dose of testosterone (0.500 mg) were biased toward a place strategy. A third experiment indicated that testosterone dose did not have a strong influence on the ability of rats to use a nearby visual cue (floating ball) in the water maze. For this experiment, all groups seemed to use a combination of place and cued-response strategies. Overall, the results indicate that the effects of testosterone on spatial strategy preference are dose dependent and task dependent.

Is basic research providing answers if adjuvant anti-estrogen treatment of breast cancer can induce cognitive impairment?

Adjuvant treatment of cancer by chemotherapy is associated with cognitive impairment in some cancer survivors. Breast cancer patients are frequently also receiving endocrine therapy with selective estrogen receptor modulators (SERMs) and/or aromatase inhibitors (AIs) to suppress the growth of estradiol sensitive breast tumors. Estrogens are well-known, however, to target brain areas involved in the regulation of cognitive behavior. In this review clinical and basic preclinical research is reviewed on the actions of estradiol, SERMs and AIs on brain and cognitive functioning to see if endocrine therapy potentially induces cognitive impairment and in that respect may contribute to the detrimental effects of chemotherapy on cognitive performance in breast cancer patients. Although many clinical studies may be underpowered to detect changes in cognitive function, current basic and clinical reports suggest that there is little evidence that AIs may have a lasting detrimental effect on cognitive performance in breast cancer patients. The clinical data on SERMs are not conclusive, but some studies do suggest that tamoxifen administration may form a risk for cognitive functioning particularly in older women. An explanation may come from basic preclinical research which indicates that tamoxifen often acts agonistic in the absence of estradiol but antagonistic in the presence of endogenous estradiol. It could be hypothesized that the negative effects of tamoxifen in older women is related to the so-called window of opportunity for estrogen. Administration of SERMs beyond this so-called window of opportunity may not be effective or might even have detrimental effects similar to estradiol.

Working memory in bisphenol-A treated middle-aged ovariectomized rats

Over 90% of the U.S. population has detectable bisphenol-A (BPA) in their urine according to recent biomonitoring data. BPA is best known for its estrogenic properties, and most rodent research on the nervous system effects of BPA has focused on determining if chronic exposures during pre- and perinatal development have organizational effects on brain development and behavior. Estrogens also have important impacts on brain and behavior during adulthood, particularly in females during aging, but the impact of BPA on the adult brain is less studied. We have published a series of studies documenting that chronic exposure to various estrogens including 17β-estradiol, ERβ selective SERMs and soy phytoestrogens impairs performance of middle-aged female rats on an operant working memory task. The purpose of this study was to determine if chronic oral exposure to BPA would alter working memory on this same task. Ovariectomized (OVX) middle-aged Long Evans rats were tested on an operant delayed spatial alternation (DSA) task. Rats were treated for 8-10 weeks with either a 0 (vehicle control), 5 or 50 μg/kg bw/day oral bolus of BPA. A subset of the vehicle control rats was implanted with a Silastic implant containing 17β-estradiol (low physiological range) to serve as a positive control. All rats were tested for 25 sessions on the DSA task. BPA treatment did not influence performance accuracy on the DSA task, whereas 17β-estradiol significantly impaired performance, as previously reported. The results of this study suggest that chronic oral exposure to BPA does not alter working memory processes of middle-aged OVX rats assessed by this operant DSA task.

Making memories matter

This article reviews some of the neuroendocrine bases by which emotional events regulate brain mechanisms of learning and memory. In laboratory rodents, there is extensive evidence that epinephrine influences memory processing through an inverted-U relationship, at which moderate levels enhance and high levels impair memory. These effects are, in large part, mediated by increases in blood glucose levels subsequent to epinephrine release, which then provide support for the brain processes engaged by learning and memory. These brain processes include augmentation of neurotransmitter release and of energy metabolism, the latter apparently including a key role for astrocytic glycogen. In addition to up- and down-regulation of learning and memory in general, physiological concomitants of emotion and arousal can also switch the neural system that controls learning at a particular time, at once improving some attributes of learning and impairing others in a manner that results in a change in the strategy used to solve a problem.

Acute genistein treatment mimics the effects of estradiol by enhancing place learning and impairing response learning in young adult female rats

Endogenous estrogens have bidirectional effects on learning and memory, enhancing or impairing cognition depending on many variables, including the task and the memory systems that are engaged. Moderate increases in estradiol enhance hippocampus-sensitive place learning, yet impair response learning that taps dorsal striatal function. This memory modulation likely occurs via activation of estrogen receptors, resulting in altered neural function. Supplements containing estrogenic compounds from plants are widely consumed despite limited information about their effects on brain function, including learning and memory. Phytoestrogens can enter the brain and signal through estrogen receptors to affect cognition. Enhancements in spatial memory and impairments in executive function have been found following treatment with soy phytoestrogens, but no tests of actions on striatum-sensitive tasks have been made to date. The present study compared the effects of acute exposure to the isoflavone genistein with the effects of estradiol on performance in place and response learning tasks. Long-Evans rats were ovariectomized, treated with 17β-estradiol benzoate, genistein-containing sucrose pellets, or vehicle (oil or plain sucrose pellets) for 2 days prior to behavioral training. Compared to vehicle controls, estradiol treatment enhanced place learning at a low (4.5 μg/kg) but not high dose (45 μg/kg), indicating an inverted pattern of spatial memory facilitation. Treatment with 4.4 mg of genistein over 2 days also significantly enhanced place learning over vehicle controls. For the response task, treatment with estradiol impaired learning at both low and high doses; likewise, genistein treatment impaired response learning compared to rats receiving vehicle. Overall, genistein was found to mimic estradiol-induced shifts in place and response learning, facilitating hippocampus-sensitive learning and slowing striatum-sensitive learning. These results suggest signaling through estrogen receptor β and membrane-associated estrogen receptors in learning enhancements and impairments given the preferential binding of genistein to the ERβ subtype and affinity for GPER.