Cognitive function in aged ovariectomized female rhesus monkeys

Sex differences in the blood-brain barrier: Implications for mental health

Prevalence of mental disorders, including major depressive disorder (MDD), bipolar disorder (BD) and schizophrenia (SZ) are increasing at alarming rates in our societies. Growing evidence points toward major sex differences in these conditions, and high rates of treatment resistance support the need to consider novel biological mechanisms outside of neuronal function to gain mechanistic insights that could lead to innovative therapies. Blood-brain barrier alterations have been reported in MDD, BD and SZ. Here, we provide an overview of sex-specific immune, endocrine, vascular and transcriptional-mediated changes that could affect neurovascular integrity and possibly contribute to the pathogenesis of mental disorders. We also identify pitfalls in current literature and highlight promising vascular biomarkers. Better understanding of how these adaptations can contribute to mental health status is essential not only in the context of MDD, BD and SZ but also cardiovascular diseases and stroke which are associated with higher prevalence of these conditions.

Parameters of Experimental Activity in the Study of the Cognitive Abilities of Rhesus Monkeys (Macaca mulatta) and Baboons (Papio hamadryas)

We compared experimental activity, behavioral activity in the experiment, of rhesus monkeys (Macaca mulatta) and baboons (Papio hamadryas). Baboons showed higher levels of experimental activity. The contrast in the activity between the species is probably associated with species-specific characteristics of the behavior and different ability to adapt under new conditions. In particular, they may be the result of lower plasticity of rhesus monkeys with respect to experimental conditions.

Hysterectomy Uniquely Impacts Spatial Memory in a Rat Model: A Role for the Nonpregnant Uterus in Cognitive Processes

Approximately one-third of women experience hysterectomy, or the surgical removal of the uterus, by 60 years of age, with most surgeries occurring prior to the onset of natural menopause. The ovaries are retained in about half of these surgeries, whereas for the other half hysterectomy occurs concurrently with oophorectomy. The dogma is that the nonpregnant uterus is dormant. There have been no preclinical assessments of surgical variations in menopause, including hysterectomy, with and without ovarian conservation, on potential endocrine and cognitive changes. We present a novel rat model of hysterectomy alongside sham, ovariectomy (Ovx), and Ovx-hysterectomy groups to assess effects of surgical menopause variations. Rats without ovaries learned the working memory domain of a complex cognitive task faster than did those with ovaries. Moreover, uterus removal alone had a unique detrimental impact on the ability to handle a high-demand working memory load. The addition of Ovx, that is, Ovx-hysterectomy, prevented this hysterectomy-induced memory deficit. Performance did not differ amongst groups in reference memory-only tasks, suggesting that the working memory domain is particularly sensitive to variations in surgical menopause. Following uterus removal, ovarian histology and estrous cycle monitoring demonstrated that ovaries continued to function, and serum assays indicated altered ovarian hormone and gonadotropin profiles by 2 months after surgery. These results underscore the critical need to further study the contribution of the uterus to the female phenotype, including effects of hysterectomy with and without ovarian conservation, on the trajectory of brain and endocrine aging to decipher the impact of common variations in gynecological surgery in women. Moreover, findings demonstrate that the nonpregnant uterus is not dormant, and indicate that there is an ovarian-uterus-brain system that becomes interrupted when the reproductive tract has been disrupted, leading to alterations in brain functioning.

Ovarian hormones, sleep and cognition across the adult female lifespan: An integrated perspective

Loss of ovarian function in women is associated with sleep disturbances and cognitive decline, which suggest a key role for estrogens and/or progestins in modulating these symptoms. The effects of ovarian hormones on sleep and cognitive processes have been studied in separate research fields that seldom intersect. However, sleep has a considerable impact on cognitive function. Given the tight connections between sleep and cognition, ovarian hormones may influence selective aspects of cognition indirectly, via the modulation of sleep. In support of this hypothesis, a growing body of evidence indicates that the development of sleep disorders following menopause contributes to accelerated cognitive decline and dementia in older women. This paper draws from both the animal and human literature to present an integrated view of the effects of ovarian hormones on sleep and cognition across the adult female lifespan.

Sex and the stimulus-movement effect: Differences in acquisition of autoshaped responding in cynomolgus monkeys

The stimulus-movement effect refers to the phenomenon in which stimulus discrimination or acquisition of a response is facilitated by moving stimuli as opposed to stationary stimuli. The effect has been found in monkeys, rats, and humans, but the experiments conducted did not provide adequate female representation to investigate potential sex differences. The current experiment analyzed acquisition of stimulus touching in a progressive series of classical conditioning procedures in cynomolgus monkeys (Macaca fascicularis) as a function of sex and stimulus movement. Classical conditioning tasks arrange two or more stimuli in relation to each other with different temporal and predictive relations. Autoshaping procedures overlay operant contingencies onto a classical-conditioning stimulus arrangement. In the present case, a neutral stimulus (a small gray square displayed on a touchscreen) functioned as the conditional stimulus and a food pellet functioned as the unconditional stimulus. Although touching is not required to produce food, with repeated stimulus pairings subjects eventually touch the stimulus. Across conditions of increasing stimulus correlation and temporal contiguity, male monkeys acquired the response faster with a moving stimulus. In contrast, females acquired the response faster with a stationary stimulus. These results demonstrate that the stimulus-movement effect may be differentially affected by sex and indicate that additional experiments with females are needed to determine how sex interacts with behavioral phenomena discovered and elaborated almost exclusively using males.

Effect of Ovarian Hormone Therapy on Cognition in the Aged Female Rhesus Macaque

Studies of the effect of hormone therapy on cognitive function in menopausal women have been equivocal, in part due to differences in the type and timing of hormone treatment. Here we cognitively tested aged female rhesus macaques on (1) the delayed response task of spatial working memory, (2) a visuospatial attention task that measured spatially and temporally cued reaction times, and (3) a simple reaction time task as a control for motor speed. After task acquisition, animals were ovariectomized (OVX). Their performance was compared with intact controls for 2 months, at which time no group differences were found. The OVX animals were then assigned to treatment with either a subcutaneous sham implant (OVX), 17-β estradiol (E) implant (OVX+E) or E implant plus cyclic oral progesterone (OVX+EP). All groups were then tested repeatedly over 12 months. The OVX+E animals performed significantly better on the delayed response task than all of the other groups for much of the 12 month testing period. The OVX+EP animals also showed improved performance in the delayed response task, but only at 30 s delays and with performance levels below that of OVX+E animals. The OVX+E animals also performed significantly better in the visuospatial attention task, particularly in the most challenging invalid cue condition; this difference also was maintained across the 12 month testing period. Simple reaction time was not affected by hormonal manipulation. These data demonstrate that chronic, continuous administration of E can exert multiple beneficial cognitive effects in aged, OVX rhesus macaque females.

SIGNIFICANCE STATEMENT

Hormone therapy after menopause is controversial. We tested the effects of hormone replacement in aged rhesus macaques, soon after surgically-induced menopause [ovariectomy (OVX)], on tests of memory and attention. Untreated ovarian-intact and OVX animals were compared with OVX animals receiving estradiol (E) alone or E with progesterone (P). E was administered in a continuous fashion via subcutaneous implant, whereas P was administered orally in a cyclic fashion. On both tests, E-treated animals performed better than the other 3 experimental groups across 1 year of treatment. Thus, in this monkey model, chronic E administered soon after the loss of ovarian hormones had long-term benefits for cognitive function.

Evidence of estrogen modulation on memory processes for emotional content in healthy young women

PURPOSE

It is well accepted that emotional content can affect memory, interacting with the encoding and consolidation processes. The aim of the present study was to verify the effects of estrogens in the interplay of cognition and emotion.

METHODS

Images from the International Affective Pictures System, based on valence (pleasant, unpleasant and neutral), maintaining arousal constant, were viewed passively by two groups of young women in different cycle phases: a periovulatory group (PO), characterized by high level of estrogens and low level of progesterone, and an early follicular group (EF), characterized by low levels of both estrogens and progesterone. The electrophysiological responses to images were measured, and P300 peak was considered. One week later, long-term memory was tested by means of free recall.

FINDINGS

Intra-group analysis displayed that PO woman had significantly better memory for positive images, while EF women showed significantly better memory for negative images. The comparison between groups revealed that women in the PO phase had better memory performance for positive pictures than women in the EF phase, while no significant differences were found for negative and neutral pictures. According to the free recall results, the subjects in the PO group showed greater P300 amplitude, and shorter latency, for pleasant images compared with women in the EF group.

CONCLUSION

Our results showed that the physiological hormonal fluctuation of estrogens during the menstrual cycle can influence memory, at the time of encoding, during the processing of emotional information.

Estrogens, inflammation and cognition

The effects of estrogens are pleiotropic, affecting multiple bodily systems. Changes from the body's natural fluctuating levels of estrogens, through surgical removal of the ovaries, natural menopause, or the administration of exogenous estrogens to menopausal women have been independently linked to an altered immune profile, and changes to cognitive processes. Here, we propose that inflammation may mediate the relationship between low levels of estrogens and cognitive decline. In order to determine what is known about this connection, we review the literature on the cognitive effects of decreased estrogens due to oophorectomy or natural menopause, decreased estrogens' role on inflammation--both peripherally and in the brain--and the relationship between inflammation and cognition. While this review demonstrates that much is unknown about the intersection between estrogens, cognition, inflammation, we propose that there is an important interaction between these literatures.

Neurocognitive effects of estrogens across the adult lifespan in nonhuman primates: State of knowledge and new perspectives

This article is part of a Special Issue "Estradiol and cognition". This review discusses the unique contribution of nonhuman primate research to our understanding of the neurocognitive effects of estrogens throughout the adult lifespan in females. Mounting evidence indicates that estrogens affect many aspects of hippocampal, prefrontal and cholinergic function in the primate brain and the underlying mechanisms are beginning to be elucidated. In addition, estrogens may also influence cognitive function indirectly, via the modulation of other systems that impact cognition. We will focus on the effects of estrogens on sleep and emphasize the need for primate models to better understand these complex interactions. Continued research with nonhuman primates is essential for the development of therapies that are optimal for the maintenance of women's cognitive health throughout the lifespan.

Long-term effects of ageing and ovariectomy on aversive and recognition memory and DNA damage in the hippocampus of female rats

OBJECTIVE

This study investigated the influence of ageing - in particular the decrease of gonadal hormone levels during the ageing process - on the memory and the levels of DNA damage in the hippocampus of female rats.

METHODS

Three groups of female Wistar rats were investigated: Group I consisted of non-ovariectomised, adult animals (6 months old); Group II consisted of non-ovariectomised, aged animals (18 months old); and Group III consisted of ovariectomised, aged animals (18 months old). The memory of the animals in these groups was examined via novel object recognition and inhibitory avoidance tests. The hippocampus tissue samples of all animals were obtained via biopsy and used to quantify the DNA damage using a Comet Assay.

RESULTS

According to our findings, the process of ageing results in a change during the behavioural tests. To prevent genotoxic damage to the hippocampus caused by the ageing process, lowered hormone levels seem to be part of a protective biochemical mechanism in the body of rats. Animals that were previously submitted to an ovariectomy adapted better to these lower levels of hormones.

CONCLUSION

Our results indicate that ovariectomy can provide beneficial long-term effects on the memory. However, this could be specific to the kind of memory examined, as the aversive memory deficits caused by ageing were not affected by ovariectomy.

Cognitive and motor aging in female chimpanzees

We present the first longitudinal data on cognitive and motor aging in the chimpanzee (Pan troglodytes). Thirty-eight adult female chimpanzees (10-54 years old) were studied. The apes were tested longitudinally for 3 years in a modified Primate Cognition Test Battery, which comprised 12 tests of physical and social cognition. The chimpanzees were also administered a fine motor task requiring them to remove a steel nut from rods of various complexity. There was little evidence for an age-related decline in tasks of Physical Cognition: for most tasks, performance was either stable or improved with repeated testing across age groups. An exception was Spatial Memory, for which 4 individuals more than 50 years old experienced a significant performance decline across the 3 years of testing. Poorer performance with age was found in 2 tasks of Social Cognition, an attention-getting task and a gaze-following task. A slight motor impairment was also observed, with old chimpanzees improving less than younger animals with repeated testing on the simplest rod. Hormonal status effects were restricted to spatial memory, with non-cycling females outperforming cycling females independently of age. Unexpectedly, older chimpanzees were better than younger individuals in understanding causality relationships based on sound.

Systemic and intra-rhinal-cortical 17-β estradiol administration modulate object-recognition memory in ovariectomized female rats

Previous studies using the novel-object-preference (NOP) test suggest that estrogen (E) replacement in ovariectomized rodents can lead to enhanced novelty preference. The present study aimed to determine: 1) whether the effect of E on NOP performance is the result of enhanced preference for novelty, per se, or facilitated object-recognition memory, and 2) whether E affects NOP performance through actions it has within the perirhinal cortex/entorhinal cortex region (PRh/EC). Ovariectomized rats received either systemic chronic low 17-β estradiol (E2; ~20 pg/ml serum) replacement alone or in combination with systemic acute high administration of estradiol benzoate (EB; 10 μg), or in combination with intracranial infusions of E2 (244.8 pg/μl) or vehicle into the PRh/EC. For one of the intracranial experiments, E2 was infused either immediately before, immediately after, or 2 h following the familiarization (i.e., learning) phase of the NOP test. In light of recent evidence that raises questions about the internal validity of the NOP test as a method of indexing object-recognition memory, we also tested rats on a delayed nonmatch-to-sample (DNMS) task of object recognition following systemic and intra-PRh/EC infusions of E2. Both systemic acute and intra-PRh/EC infusions of E enhanced novelty preference, but only when administered either before or immediately following familiarization. In contrast, high E (both systemic acute and intra-PRh/EC) impaired performance on the DNMS task. The findings suggest that while E2 in the PRh/EC can enhance novelty preference, this effect is probably not due to an improvement in object-recognition abilities.

Impaired spatial working memory learning and performance in normal aged rhesus monkeys

Aged non-human primates may have deficits in a variety of cognitive functions. However, it is possible that at least some age-related performance deficits relate to a deficit in initial task learning. To assess this, aged rhesus monkeys were trained to perform a Self-Ordered Spatial Search (SOSS) task using the same training and testing parameters used previously with normal young animals. Aged animals failed to reach criterion at the easiest task level. In an attempt to improve learning, a group of aged animals were first trained on SOSS using a standard 5s ITI, followed by trials with low inter-trial interference (e.g., a stimulus used in a trial would not be used again for the next 2 trials) or with trials in which the spatial distance between the stimuli on the screen was maximized. Because performance improved but failed to reach criterion, this was followed by sessions with increasing ITIs (from 5 s to 10 or 15 s). Only increasing the ITI improved the performance of the aged animals enough to allow them to learn the task to criterion. Once the criterion was reached, memory was taxed by increasing the delay between stimulus presentations and increasing the number of spatial positions to be remembered. Performance declined for young animals, but even more so for aged animals. The results of the present study suggests that aged primates have difficulty initially learning a complex working memory task, and that the ITI may be an important parameter to manipulate to improve learning. However, once the task is learned, performance of aged animals is inferior to that of young animals, particularly when memory demands are increased.

Interactive effects of age and estrogen on cortical neurons: implications for cognitive aging

In the past few decades it has become clear that estrogen signaling plays a much larger role in modulating the cognitive centers of the brain than previously thought possible. We have developed a nonhuman primate (NHP) model to investigate the relationships between estradiol (E) and cognitive aging. Our studies of cyclical E treatment in ovariectomized (OVX) young and aged rhesus monkeys have revealed compelling cognitive and synaptic effects of E in the context of aging. Delayed response (DR), a task that is particularly dependent on integrity of dorsolateral prefrontal cortex (dlPFC) area 46 revealed the following: (1) that young OVX rhesus monkeys perform equally well whether treated with E or vehicle (V), and (2) that aged OVX animals given E perform as well as young adults with or without E, whereas OVX V-treated aged animals display significant DR impairment. We have analyzed the structure of layer III pyramidal cells in area 46 in these same monkeys. We found both age and treatment effects on these neurons that are consistent with behavioral data. Briefly, reconstructions of pyramidal neurons in area 46 from these monkeys showed that cyclical E increased the density of small, thin spines in both young and aged monkeys. However, this effect of E was against a background of age-related loss of small, thin spines, leaving aged V-treated monkeys with a particularly low density of these highly plastic spines, and vulnerable to cognitive decline. Our current interpretation is that E not only plays a critically important role in maintaining spine number, but also enables synaptic plasticity through a cyclical increase in small highly plastic spines that may be stabilized in the context of learning. Interestingly, recent studies demonstrate that chronic E is less effective at inducing spinogenesis than cyclical E. We have begun to link certain molecular attributes of excitatory synapses in area 46 to E effects and cognitive performance in these monkeys. Given the importance of synaptic estrogen receptor α (ER-α) in rat hippocampus, we focused our initial studies on synaptic ER-α in area 46. Three key findings have emerged from these studies: (1) synaptic ER-α is present in axospinous synapses in area 46; (2) it is stable across treatment and age groups (which is not the case in rat hippocampus); and (3) the abundance and distribution of synaptic ER-α is a key correlate of individual variation in cognitive performance in certain age and treatment groups. These findings have important implications for the design of hormone treatment strategies for both surgically and naturally menopausal women. This article is part of a Special Issue entitled: Neuroactive Steroids: Focus on Human Brain.

The impact of age-related ovarian hormone loss on cognitive and neural function

On average, women now live one-third of their lives after menopause. Because menopause has been associated with an elevated risk of dementia, an increasing body of research has studied the effects of reproductive senescence on cognitive function. Compelling evidence from humans, nonhuman primates, and rodents suggests that ovarian sex-steroid hormones can have rapid and profound effects on memory, attention, and executive function, and on regions of the brain that mediate these processes, such as the hippocampus and prefrontal cortex. This chapter will provide an overview of studies in humans, nonhuman primates, and rodents that examine the effects of ovarian hormone loss and hormone replacement on cognitive functions mediated by the hippocampus and prefrontal cortex. For humans and each animal model, we outline the effects of aging on reproductive function, describe how ovarian hormones (primarily estrogens) modulate hippocampal and prefrontal physiology, and discuss the effects of both reproductive aging and hormone treatment on cognitive function. Although this review will show that much has been learned about the effects of reproductive senescence on cognition, many critical questions remain for future investigation.

Chronic estradiol replacement impairs performance on an operant delayed spatial alternation task in young, middle-aged, and old rats

The current study examined effects of chronic estradiol replacement on a prefrontally-mediated working memory task at different ages in a rodent model. Ovariectomized young, middle-aged, and old Long-Evans rats were given 5% or 10% 17beta-estradiol in cholesterol vehicle via Silastic implants and tested on an operant delayed spatial alternation task (DSA). The two estradiol exposed groups did not perform as well as the vehicle control group did. Deficits were present at all but the longest delay, where all groups including the vehicle control group performed poorly. Surprisingly, there was not a significant effect of age or an age by estradiol interaction, despite the fact that old rats had longer latencies to respond after both correct and incorrect lever presses. These data confirm our earlier finding that chronic estradiol treatment has an impairing effect on working memory as measured on DSA task. However, contrary to expectations, young, middle-aged and old rats were similarly impaired by chronic estradiol treatment; there were no indications of differential effects at different periods of the lifespan. Also contrary to expectations, there were no indications of a decline in DSA performance with advancing age. Overall, the results demonstrate that chronic estradiol exposure causes deficits in the DSA performance of ovariectomized female rats, not only in young adulthood, but also at older ages analogous to those at which hormone replacement therapy is commonly prescribed in humans.

Estrogens and age-related memory decline in rodents: what have we learned and where do we go from here?

The question of whether ovarian hormone therapy can prevent or reduce age-related memory decline in menopausal women has been the subject of much recent debate. Although numerous studies have demonstrated a beneficial effect of estrogen and/or progestin therapy for certain types of memory in menopausal women, recent clinical trials suggest that such therapy actually increases the risk of cognitive decline and dementia. Because rodent models have been frequently used to examine the effects of age and/or ovarian hormone deficiency on mnemonic function, rodent models of age-related hormone and memory decline may be useful in helping to resolve this issue. This review will focus on evidence suggesting that estradiol modulates memory, particularly hippocampal-dependent memory, in young and aging female rats and mice. Various factors affecting the mnemonic response to estradiol in aging females will be highlighted to illustrate the complications inherent to studies of estrogen therapy in aging females. Avenues for future development of estradiol-based therapies will also be discussed, and it is argued that an approach to drug development based on identifying the molecular mechanisms underlying estrogenic modulation of memory may lead to promising future treatments for reducing age-related mnemonic decline.

No effect of different estrogen receptor ligands on cognition in adult female monkeys

Many studies in women and animal models suggest that estrogens affect cognitive function. Yet, the mechanisms by which estrogens may impact cognition remain unclear. The goal of the present study was to assess the effects of different estrogen receptor (ER) ligands on cognitive function in adult ovariectomized female rhesus monkeys. The monkeys were tested for 6 weeks on a battery of memory and attentional tasks administered on a touchscreen: the object, face, and spatial versions of the Delayed Recognition Span Test (DRST) and a Visual Search task. Following a 2-week baseline period with oil vehicle treatment, monkeys were randomly assigned to one of 3 treatment groups: estradiol benzoate (EB), selective ERbeta agonist (diarylpropionitrile DPN) or selective ER modulator tamoxifen (TAM). In each treatment group, monkeys received oil vehicle for 2 weeks and the drug for 2 weeks, in a cross-over design. After a 4-week washout, a subset of monkeys was re-tested on the battery when treated with a selective ERalpha agonist (propyl-pyrazole-triol, PPT) or oil vehicle. Overall, drug treatments had no or negligible effects on cognitive performance. These results support the contention that exogenous estrogens and selective estrogen receptor modulators (SERMS) do not significantly affect cognition in young adult female macaques. Additional studies are needed to determine whether the cognitive effects of estrogens in monkeys of more advanced age are mediated by ERbeta, ERalpha or complex interactions between the two receptors.

Working memory for emotional facial expressions: role of the estrogen in young women

Physiological hormonal fluctuations during the menstrual cycle, postpartum, and menopause have been implicated in the modulation of mood, cognition, and affective disorders. Taking into account that women's performance in memory tasks can also fluctuate with circulating hormones levels across the menstrual cycle, the cognitive performance in a working memory task for emotional facial expressions, using the six basic emotions as stimuli in the delayed matching-to-sample, was evaluated in young women in different phases of the menstrual cycle. Our findings suggest that high levels of estradiol in the follicular phase could have a negative effect on delayed matching-to-sample working memory task, using stimuli with emotional valence. Moreover, in the follicular phase, compared to the menstrual phase, the percent of errors was significantly higher for the emotional facial expressions of sadness and disgust. The evaluation of the response times (time employed to answer) for each facial expression with emotional valence showed a significant difference between follicular and luteal in reference to the emotional facial expression of sadness. Our results show that high levels of estradiol in the follicular phase could impair the performance of working memory. However, this effect is specific to selective facial expressions suggesting that, across the phases of the menstrual cycle, in which conception risk is high, women could give less importance to the recognition of the emotional facial expressions of sadness and disgust. This study is in agreement with research conducted on non-human primates, showing that fluctuations of ovarian hormones across the menstrual cycle influence a variety of social and cognitive behaviors. Moreover, our data could also represent a useful tool for investigating emotional disturbances linked to menstrual cycle phases and menopause in women.

Differential effects on visual and spatial recognition memory of a novel hormone therapy regimen of estrogen alone or combined with progesterone in older surgically menopausal monkeys

Building upon our initial studies in young adult surgically menopausal monkeys, this study examined the effects of a novel schedule of administration of estradiol therapy alone, or in combination with progesterone, on visual and spatial recognition memory in older monkeys. Monkeys were preoperatively trained on a delayed matching-to-sample task and a delayed response task. At the time of ovariectomy, monkeys began their hormonal treatments and were cognitively assessed at 2, 12 and 24 weeks following treatment initiation. A schedule of hormone administration was used that closely modeled the normal fluctuations of hormones during the course of a normal primate menstrual cycle. Monkeys receiving placebo had lower levels of accuracy than monkeys receiving estrogen therapies on the delayed matching-to-sample task that were not apparent until 12 weeks following initiation of therapy and were no longer detected at the 24-week assessment. There was no effect of hormone therapy on accuracy in the delayed response task at any of the postoperative assessments. In both tasks, monkeys treated with estrogen plus progesterone had longer choice response latencies, especially on trials in which they made errors; however these effects did not influence accuracy measures in these animals. Our findings indicate that visual recognition ability may be more sensitive than spatial recognition memory to this novel hormone therapy regimen, that treatment with estradiol plus progesterone was equivalent to that of estradiol alone, and that neither therapy had significant negative impact on memory profiles.

Brain aging modulates the neuroprotective effects of estrogen on selective aspects of cognition in women: a critical review

Although there is now a substantial literature on the putative neuroprotective effects of estrogen on cognitive functioning in postmenopausal women, it is replete with inconsistencies. The critical period hypothesis, posited several years ago, attempts to account for the discrepancies in this literature by positing that estrogen treatment (ET) will protect aspects of cognition in older women only when treatment is initiated soon after the menopause. Indeed, evidence from basic neuroscience and from the animal and human literature reviewed herein provides compelling support for the critical period hypothesis. Although it is not known with certainty why estrogen does not protect cognition and may even cause harm when administered to women over the age of 65years, it is likely that the events that characterize brain aging, such as a reduction in brain volume and in neuronal size, alterations in neurotransmitter systems, and a decrease in dendritic spine numbers, form an unfavorable background that precludes a neuroprotective effects of exogenous estrogen on the brain. Other factors that have likely contributed to the discrepancies in the estrogen-cognition literature include differences in the estrogen compounds used, their route of administration, cyclic versus continuous regimens, and the concomitant use of progestins. This critical analysis attempts to define conditions under which ET may protect aspects of cognition in aging women while also considering the cost/benefit ratio for the treatment of women aged 50-59years. Suggestions for specific future research questions are also addressed.

Effects of the menstrual cycle on looking preferences for faces in female rhesus monkeys

Fluctuations of ovarian hormones across the menstrual cycle influence a variety of social and cognitive behaviors in primates. For example, female rhesus monkeys exhibit heightened interest for males and increased agonistic interactions with other females during periods of high estrogen levels. In the present study, we hypothesized that females' preference for males during periods of high estrogen levels is also expressed at the level of face perception. We tested four intact females on two face-tasks involving neutral portraits of male and female rhesus monkeys, chimpanzees and humans. In the visual preference task (VP), monkeys had to touch a button to view a face image. The image remained on the screen as long as the button was touched, and the duration of pressing was taken as an index of the monkey's looking time for the face stimulus. In the Face-Delayed Recognition Span Test (Face-DRST), monkeys were rewarded for touching the new face in an increasing number of serially presented faces. Monkeys were tested 5 days a week across one menstrual cycle. Blood was collected every other day for analysis of estradiol and progesterone. Two of the four females were cycling at the time of testing. We did not find an influence of the cycle on Face-DRST, likely due to a floor effect. In the VP however, the two cycling individuals looked longer at conspecific male faces than female faces during the peri-ovulatory period of the cycle. Such effects were absent for human and chimpanzee faces and for the two noncycling subjects. These data suggest that ovarian hormones may influence females' preferences for specific faces, with heightened preference for male faces during the peri-ovulatory period of the cycle. Heightened interest for stimuli of significant reproductive relevance during periods of high conception risk may help guide social and sexual behavior in the rhesus monkey.

Attentional validity effect across the human menstrual cycle varies with basal temperature changes

This study examined the correlation between covert attention and basal temperature change during menstrual cycle phase in 22 adult females. Previous work showing beneficial effects of estrogen on working memory led us to hypothesize that attentional function would be facilitated at the apparent time of ovulation. Menstrual phase was determined through questionnaires and objective measurements of basal body temperature (BBT) spikes over a 1 month period. The cued target detection (CTD) task was used to assess visuospatial attentional performance at three times during the menstrual cycle. The mean reaction times (RTs) to visual targets were measured as a function of menstrual cycle phase, cue type and target location. As predicted, the onset of ovulation showed decreased reaction times and a significant increase in the cue validity effect on the days immediately preceding and following ovulation. The magnitude of the attention validity effect was negatively correlated with the basal temperature rise. Women lacking basal temperature shifts failed to show these changes. Results support the conclusion that the natural fluctuations of body temperature, and possibly reproductive hormones, during the menstrual cycle may enhance the attentional component of cognitive performance.

Executive function is less sensitive to estradiol than spatial memory: performance on an analog of the card sorting test in ovariectomized aged rhesus monkeys

Functions supported by the frontal lobes are particularly sensitive to the detrimental effects of aging. Recent studies on postmenopausal women find that estrogen replacement therapy benefits performance on tasks dependent on the frontal lobes. To determine whether estrogen has a similar influence in a rhesus monkey model of menopause, we tested five aged, long-term ovariectomized rhesus monkeys in a modified version of the Wisconsin Card Sort test which had been adapted to the nonhuman primate. In this test, monkeys had to select 3-D objects based either on color (blue, red, yellow) or shape (block, tube, cup) and had to be able to switch their response as a function of reinforcement contingencies. The monkeys were treated with placebo and ethinyl estradiol (EE2, 450 ng/kg/day) in alternation with each successive test. Contrary to our hypothesis, estradiol treatment did not affect performance. Because previous studies in the same monkeys [Neurobiol. Aging 23 (2002) 589] had shown that EE2 improves performance on a spatial memory task dependent on the hippocampus, but not on another task dependent upon the frontal lobes (the delayed response), we conclude that executive processes may be less sensitive to the effects of estradiol than hippocampal-dependent tasks.

Sex, Age, and Training Modulate Spatial Memory in the Rhesus Monkey (Macaca mulatta)

The authors tested 90 rhesus monkeys (Macaca mulatta) on a task of spatial memory, the spatial Delayed Recognition Span Test. The results showed that performance declined significantly with age, males had greater scores than females, and the rate of apparent decline with age was greater in males than in females. Both working and reference memory declined with age, but only working memory showed sex differences. The authors compared these data with that of 22 monkeys who were trained on a simpler version of the task before formal testing. Training had no effect on males but dramatically improved working memory in young females. The results confirm a male advantage in spatial working memory at a young age and confirm a greater decline with age in males than in females. It is important to note that prior training completely reverses the deficits of young females.

Ovarian hormones and cognition in the aged female rat: II. progesterone supplementation reverses the cognitive enhancing effects of ovariectomy

The authors hypothesized that the progesterone component of some hormone replacement therapies in women is detrimental to cognition. A previous study showed that ovariectomy (ovx) in aged rats enhanced spatial working memory and decreased elevated progesterone levels. The current study evaluated whether progesterone administration counteracts these cognitive enhancing effects of ovx. Aged sham and aged ovx rats given progesterone exhibited compromised learning of the working and reference memory components of the task, and made more working memory errors on the latter testing days compared with aged ovx rats not given progesterone. Results suggest that whereas ovx of the aged female rat enhances learning and the ability to handle numerous items of spatial working memory information, progesterone is detrimental to these aspects of performance. These findings may speak to studies in menopausal women which suggest that combination hormone therapies have a negative impact on cognition.

Cognition, mood disorders, and sex hormones

Macaques (Macaca spp.) are useful models to evaluate effects of ovarian sex steroids and selective estrogen receptor modulators (SERMs) on mood and cognitive function due to similarities to women in their reproductive and central nervous systems. The results of nonhuman primate studies support the hypothesis that estrogen mediates specific aspects of attention and memory, yet much work is needed to understand which cognitive processes are affected, whether natural versus surgical menopause effects are different, and the interaction of age and ovarian senescence on cognitive function. This knowledge is necessary to determine whether to support the cognitive function of women in the menopausal phase of life and, if so, to determine efficacious therapeutic interventions. Mood disorders are prevalent in women and are associated with reproductive function in women and macaques. Exogenous steroid therapies, including oral contraceptives and postmenopausal hormone replacement therapies, have behavioral effects in women and appear to affect the behavior and underlying neural substrates of monkeys. Additional research is necessary to confirm and extend these observations. Ovarian steroids have multiple effects on serotonin synthesis, reuptake, and degradation, on neural activity that drives serotonin release, and on receptor activation in primates. This system modulates cognitive function and mood and is the target of a broad class of antidepressant therapies. Understanding the effects of ovarian steroids on the neural serotonergic system is necessary to understand depression in women. These studies are best carried out in primate models, which are more similar to humans in neural serotonergic function than other animal models.

Ovarian hormones and cognition in the aged female rat: I. Long-term, but not short-term, ovariectomy enhances spatial performance

Although research suggests that ovariectomy (ovx) is detrimental to spatial cognition in young rats, little work has evaluated the cognitive effects of ovx in aged rats. The authors investigated the effects of ovx in aged rats using the water radial-arm maze. In Study 1, young rats and aged rats receiving ovx 1.5 months before testing outperformed aged rats receiving sham surgery or ovx 21 days before testing. In Study 2, young rats and aged rats receiving ovx 2.0 or 6.0 months before testing outperformed aged sham rats. Aged rats exhibited estradiol and elevated progesterone levels comparable to those of young rats. The findings suggest that 1.5-6.0 months, but not 21 days, of ovx improves spatial memory in aged rats. The hypothesis that long-term ovarian hormone loss is detrimental to spatial memory in aged rats was not supported. The authors hypothesize that removal of elevated progesterone levels is related to the ovx-induced cognitive enhancement.

Estradiol selectively affects processing of conspecifics' faces in female rhesus monkeys

Estrogen deficiency following ovariectomy or menopause increases the risk of developing diseases such as osteoporosis and may also lead to memory impairment. Although estrogen replacement therapy (ERT) alleviates many symptoms associated with estrogen loss, it is not clear whether it also benefits cognitive function. The effect of estrogens upon cognition can best be studied in an animal model of human menopause, in which estrogen levels can be experimentally manipulated. Six young ovariectomized female rhesus monkeys (6-9 years old) were tested on a battery of touchscreen-based cognitive tasks, including the Matching-to-Sample (MTS) task with mixed delays and the spatial, object, and face conditions of the Delayed Recognition Span Test (DRST). Monkeys were tested 5 days a week, one task per week, for a total of 8 months, while undergoing treatments with placebo and ethinyl estradiol (EE2) in alternating 28-days blocks. Blood samples were collected to verify EE2 levels. We also observed the monkeys by video monitor during test sessions and recorded locomotor activity and response topology. Performance on the face-DRST, a task that involved selecting the new face in an increasing array of rhesus monkey faces, was impaired by EE2 treatment, as compared to placebo. Other tasks were unaffected by EE2. There was no clear evidence of EE2 effects upon motor activity or anxiety. In order to test the reliability of our findings, we conducted an additional experiment in which the monkeys were again given the face-DRST with different categories of face stimuli for 4 months, while receiving placebo and EE2 in alternating 7-days blocks. They performed each task 4-5 days/week for 4 weeks with (1) the same rhesus monkey faces as in the first experiment, (2) human faces, (3) chimpanzee faces, and (4) novel rhesus monkey faces. Face-DRST performance did not vary as a function of treatment when human or chimpanzee faces were used as stimuli. In contrast, periods of EE2 treatment were associated with a lower performance for both sets of rhesus monkey faces. These findings suggest that EE2 treatment has a detrimental effect on processing faces of conspecifics by female rhesus monkeys. We speculate that estrogens may produce this effect by enhancing emotional reactivity to socially relevant stimuli.

Estrogen and cognitive functioning in women

Research in basic neuroscience has provided biological plausibility for the hypothesis that estrogen replacement therapy (ERT) would protect against cognitive aging in healthy women. The weight of the evidence from randomized controlled trials of estrogen and cognition in women shows that this hormone preferentially protects verbal memory in postmenopausal women, whereas findings from observational studies are less consistent and show a more diffuse effect of estrogen on a range of cognitive functions. There is fairly consistent evidence from epidemiological studies that ERT significantly reduces the risk of Alzheimer's disease (AD) in women. On the other hand, findings from controlled treatment trials of women diagnosed with probable AD failed to show that physiological doses of ERT ameliorate existing deficits in cognitive functioning and/or prevent further deterioration in memory that inevitably occurs in these women over time. Finally, an accumulating body of evidence is beginning to suggest that the immediate postmenopausal period may constitute a critical window for treatment with ERT that maximizes its potential to protect against cognitive decline with aging and/or to reduce the risk of AD.

Effects of estradiol and aging on fine manual performance in female rhesus monkeys

Aging is characterized by a progressive deterioration of motor function related to dysfunctions of the nigrostriatal system. Because estrogen has been reported to protect dopaminergic neurons and to improve the motor deficits associated with Parkinson's disease, we hypothesized that it would partially reverse the age-related decline of motor function in normal aging. We tested the effects of estrogen treatment and withdrawal on fine motor performance in five aged (21-24 years old) and five young (6-9 years old) ovariectomized female rhesus monkeys. The tests required the monkeys to use each hand to retrieve a Life Saver candy from metal rods bent in shapes of different complexity. Monkeys were tested twice a week for 8 consecutive weeks, during treatment with placebo or ethinyl estradiol (EE(2)) in alternating 14-day blocks. Each behavioral test was videotaped and subsequently scored for the duration and the success of the first trial on each shape. Both groups of monkeys improved rapidly with practice in speed and success of retrieval. The older monkeys were slower but as successful as the young monkeys in retrieving the candy. The left hand was faster than the right hand for both the aged and young females. We failed to detect any effect of EE(2) treatment on speed or success of retrieval in either group. These results confirm the slowing of fine motor performance with aging in female rhesus monkeys. They also indicate that estradiol, at least as administered in this study, does not benefit fine manual performance.

Nonhuman primate models of menopause workshop

The Nonhuman Primate Models of Menopause Workshop was held on the National Institutes of Health campus in January 2001. The purpose of this workshop, sponsored by the National Institute on Aging, was to review what is known about the female reproductive aging process in various species of monkeys (particularly rhesus, baboons, cynomolgus, and chimpanzees), including hormone profiles during the menopausal transition, occurrence of hot flashes, extent of age-related and menopause-associated changes in hormone levels on metabolism, bone loss, and impaired cardiovascular and cognitive function. Many aspects of the female reproductive aging process appear to be concordant between humans and these monkey species, but several important features may be species-specific. Those features that appear to parallel human menopause and aging include general similarity of hormone profiles across the menopausal transition, progression to cycle termination through irregular cycles, declining fertility with age, age-related gains in weight and percentage body fat content (with tendencies toward insulin resistance and glucose intolerance), increased low-density lipoprotein cholesterol and decreased high-density lipoprotein cholesterol, declines in serum dehydroepiandrosterone, similarities in temperature-regulation systems, protective responses to estrogen replacement following ovariectomy in terms of bone metabolism, lipid profiles, and cognitive changes. Important differences include relatively short postmenopausal life span, timing in menopause-related changes in hormone secretion, and seasonal menstrual cycles. In addition, the question of whether ovariectomy in young adults is an appropriate model for the consequences of natural or surgical menopause in middle-aged and older adults is unresolved, and the numbers of older female animals available for research on menopause are very limited. The use of animal models is seen by workshop participants to be crucial for a mechanistic understanding of the human menopausal process and its connections to postmenopausal health problems; however, extensive in-depth and broad-based research is required to determine if nonhuman primates are appropriate models of human menopause.

Estradiol, but not raloxifene, improves aspects of spatial working memory in aged ovariectomized rhesus monkeys

Estrogen replacement therapy (ERT) alleviates many postmenopausal symptoms but whether it also benefits cognitive function remains controversial. Further, since estrogen increases the risk of breast and uterine cancers, a new class of compounds, called selective estrogen receptor modulators (SERMs) is being considered as possible alternative to ERT. The SERM raloxifene is particularly interesting because, like estrogen, it improves lipid metabolism and reduces bone loss, without adverse effects on the breast or uterus. Little is known, however, about its effect upon cognitive function. We used a rhesus monkey model of human menopause to examine the effects of ERT and raloxifene on cognitive function. We tested 5 aged females (21-24 years old) ovariectomized long-term (10-16 years) on a battery of age-sensitive tasks, including the Delayed Response (DR), the Delayed Non-Matching-to-Sample-10 min (DNMS-10 min) and the spatial-Delayed Recognition Span Test (DRST). Monkeys were tested 5 days a week on each task for 9 consecutive months, while undergoing treatments with placebo, ethinyl estradiol (EE(2)), and raloxifene in alternating 28-days blocks. EE(2) transiently enhanced the working memory component of the spatial-DRST, but did not affect performance on the other tasks of the battery. Raloxifene had no effect on cognitive performance. These findings indicate that estradiol is able to enhance some aspects of spatial working memory in aged monkeys despite many years of estrogenic deprivation. Further, they suggest that raloxifene does not affect cognitive function after long-term ovarian hormone deprivation.

Fluctuations in spatial recognition memory across the menstrual cycle in female rhesus monkeys

Findings are inconsistent regarding whether women's cognitive performance fluctuates across phases of the menstrual cycle, but differences in methodology and the use of reported cycle phase rather than precise hormonal measures may underlie these disparities. Studies in monkeys may help resolve these discrepant findings, since hormonal status can be reliably determined. We tested four young (5-7 years old) female rhesus monkeys daily during one entire menstrual cycle on three cognitive tasks displayed on a computerized touch-screen system: a Matching to Sample task with a 30 s delay (MTS-30s), a Matching to Sample task without delay (MTS-no delay) and the spatial condition of the Delayed Recognition Span Test (spatial-DRST). Blood samples were collected at specific time intervals throughout the cycle and assayed for estradiol and progesterone in order to identify hormonal status. There was a nonsignificant trend for the MTS-30s scores to be better during the follicular and luteal phases, when estradiol levels were low, than during the peri-ovulatory phase, when estradiol levels were at their highest. MTS-no delay performance did not vary as a function of hormonal status. Spatial-DRST scores were significantly better during the follicular and luteal phases than during the peri-ovulatory phase of the cycle. These data in the female rhesus monkey support the hypothesis that spatial memory performance is sensitive to estradiol variations across the menstrual cycle, with better performance associated with low estradiol levels.