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

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 differences in inhibitory control in socially-housed baboons (Papio papio)

Inhibitory control is an important component of executive function. An emerging literature in humans suggests that inhibitory control is sexually dimorphic and modulated by sex steroids, but evidence for such a link in nonhuman animals is scarce. In this study, we examined the effects of menstrual cycle and biological sex on response inhibition, as measured by a Stop-Signal task, in the baboon (Papio papio). The monkeys (n=13) were socially-housed, with voluntary access to multiple touchscreen computerized stations. The task required monkeys to inhibit prepotent responses (touching a target, "Go" trials) following the appearance of a visual stop signal on 25% of the trials ("Stop" trials). The cognitive data, consisting of computerized records of the monkeys' performance on the Stop-Signal task over a year of testing, were matched to records of female sexual swellings. Same-day menstrual and cognitive data were available for 5 females, aged 5-18 years. These data were compared to those of 8 males (5-14 years old) performing the Stop-Signal task over the same time period. Contrary to our hypothesis, performance on the task was not significantly affected by the phase (ovulatory vs. luteal) of the cycle in females. However, males were slower than females on Go trials and were less efficient in inhibiting responses on Stop trials. Slower responses in males were indicative of a speed-accuracy trade-off, as overall accuracy was also better in males than in females. Analyses of trial history indicated that males did not speed as much as females following a successful Go trial, but did not differ from females in post-error slowing or post-inhibiting responses. Overall, the data show that biological sex modulates Stop-Signal performance in the baboon, with males exhibiting slower response execution overall, less efficient inhibition, but greater accuracy than females. This pattern of sex differences may reflect motivational sex differences in which males emphasize accuracy rather than speed. Interestingly, these sex differences do not seem to vary as a function of ovarian hormones in females. Males' greater focus on accuracy is possibly due to enhanced sensitivity to reward mediated by testosterone levels.

Sex differences in visuospatial abilities persist during induced hypogonadism

BACKGROUND

Despite well-established sex differences in the performance on tests of several cognitive domains (e.g., visuospatial ability), few studies in humans have evaluated if these sex differences are evident both in the presence of circulating sex hormones and during sex steroid hormonal suppression. Sex differences identified in the relative absence of circulating levels of estradiol and testosterone suggest that differences in brain structure or function exist independent of current hormonal environment and are more likely a reflection of differing developmental exposures and/or genetic substrates.

OBJECTIVE

To evaluate cognitive performance in healthy eugonadal men and women before and again during GnRH agonist-induced hypogonadism.

METHODS

Men (n=16) and women (n=15) without medical or psychiatric illness were matched for IQ. Cognitive tests were performed at baseline (when eugonadal) and after 6-8 weeks of GnRH agonist-induced gonadal suppression. The test batteries included measures of verbal and spatial memory, spatial ability, verbal fluency, motor speed/dexterity, and attention/concentration. Data were analyzed using repeated-measures models.

RESULTS

During both eugonadism and hypogonadism, men performed significantly better than women on several measures of visuospatial performance including mental rotation, line orientation, Money Road Map, Porteus maze, and complex figure drawing. Although some test performances showed an effect of hormone treatment, the majority of these differences reflected an improved performance during hypogonadism compared with baseline (and probably reflected practice effects).

CONCLUSION

The well-documented male advantage in visuospatial performance, which we observed during eugonadal conditions, was maintained in the context of short-term suppression of gonadal function in both men and women. These findings suggest that, in humans, sex differences in visuospatial performance are not merely dependent on differences in the current circulating sex steroid environment. Thus sex differences in visuospatial performance in adulthood could reflect early developmental effects of sex steroid exposure or other environmental exposures differing across the sexes as our data confirm that these differences are independent of circulating estradiol or testosterone levels in men and women.

Understanding the cognitive impact of the contraceptive estrogen Ethinyl Estradiol: tonic and cyclic administration impairs memory, and performance correlates with basal forebrain cholinergic system integrity

Ethinyl Estradiol (EE), a synthetic, orally bio-available estrogen, is the most commonly prescribed form of estrogen in oral contraceptives, and is found in at least 30 different contraceptive formulations currently prescribed to women as well as hormone therapies prescribed to menopausal women. Thus, EE is prescribed clinically to women at ages ranging from puberty to reproductive senescence. Here, in two separate studies, the cognitive effects of cyclic or tonic EE administration following ovariectomy (Ovx) were evaluated in young female rats. Study I assessed the cognitive effects of low and high doses of EE, delivered tonically via a subcutaneous osmotic pump. Study II evaluated the cognitive effects of low, medium, and high doses of EE administered via a daily subcutaneous injection, modeling the daily rise and fall of serum EE levels with oral regimens. Study II also investigated the impact of low, medium and high doses of EE on the basal forebrain cholinergic system. The low and medium doses utilized here correspond to the range of doses currently used in clinical formulations, and the high dose corresponds to doses prescribed to a generation of women between 1960 and 1970, when oral contraceptives first became available. We evaluate cognition using a battery of maze tasks tapping several domains of spatial learning and memory as well as basal forebrain cholinergic integrity using immunohistochemistry and unbiased stereology to estimate the number of choline acetyltransferase (ChAT)-producing cells in the medial septum and vertical/diagonal bands. At the highest dose, EE treatment impaired multiple domains of spatial memory relative to vehicle treatment, regardless of administration method. When given cyclically at the low and medium doses, EE did not impact working memory, but transiently impaired reference memory during the learning phase of testing. Of the doses and regimens tested here, only EE at the highest dose impaired several domains of memory; tonic delivery of low EE, a dose that corresponds to the most popular doses used in the clinic today, did not impact cognition on any measure. Both medium and high injection doses of EE reduced the number of ChAt-immunoreactive cells in the basal forebrain, and cell population estimates in the vertical/diagonal bands negatively correlated with working memory errors.

Oestradiol modulation of cognition in adult female marmosets (Callithrix jacchus)

The common marmoset (Callithrix jacchus) provides many advantages over traditional rodent and macaque species as a model for human ageing and may be very useful for studying the effects of sex steroids on cognitive and brain ageing. We present the first study examining the effects of oestrogens on cognitive function in female marmosets. Adult monkeys (3-5 years of age) were trained to a specific learning criterion on a battery of cognitive tasks preoperatively (object discrimination, delayed response with increasing delays and detour reaching with opaque box) and were tested on different versions of these tasks (object reversals, delayed response with randomised delays and detour reaching with clear box) after ovariectomy and simultaneous implantation with 17β-oestradiol (E2 ) (n = 6) or blank (n = 6) Silastic capsules. Acquisition of a delayed matching-to-position task with a 1-s delay was also administered after completion of these tests. E2 -treated monkeys were significantly impaired on the second reversal and showed an increase in perseverative responding from reversals 1-3. Their performance also tended to be worse than that of control monkeys on the delayed response task. Performance acquisition on the delayed matching-to-position tended to be better in E2 -treated relative to control monkeys, although the group difference did not reach statistical significance. No effect of treatment was detected for detour reaching or affiliative behaviours. Overall, the findings indicate that E2 compromises performance on prefrontally-mediated tasks. The suggestion that E2 may improve acquisition on tasks dependent on the hippocampus will require further validation. These results are discussed in the context of dopaminergic and serotonergic signalling. We conclude that the marmoset is a useful new primate model for examining the effects of oestrogens on cognitive function.

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 stress, cyclic 17β-estradiol, and daily handling influences on fear conditioning in the female rat

Chronic stress and estrogens alter many forebrain regions in female rats that affect cognition. In order to investigate how chronic stress and estrogens influence fear learning and memory, we ovariectomized (OVX) female Sprague-Dawley rats and repeatedly injected them (s.c.) with 17β-estradiol (E, 10 μg/250 g or sesame oil vehicle, VEH). Concurrently, rats were restrained for 6 h/d/21 d (STR) or left undisturbed (CON). Rats were then fear conditioned with 4 tone-footshock pairings and then after 1 h and 24 h delays, given 15 tone extinction trials. Regardless of E treatment, chronic stress (VEH, E) facilitated freezing to tone during acquisition and extinction following a 1h delay, but not during extinction after a 24 h delay. E did not influence freezing to tone during any phase of fear conditioning for either the control or chronically stressed rats, but did influence contextual conditioning that may have been carried predominately by the STR group. In the second experiment, we investigated "handling" influences on fear conditioning acquisition, given the disparate findings from the current study and previous work (Baran, Armstrong, Niren, & Conrad, 2010; Baran, Armstrong, Niren, Hanna, & Conrad, 2009). Female rats remained gonadally-intact since E did not influence tone fear conditioning. Indeed, brief daily handling (1-3 m/d/21 d) facilitated acquisition of fear conditioning in chronically stressed female rats, and either had no effect or slightly attenuated fear conditioning in controls. Thus, chronic stress impacts amygdala-mediated fear learning in both OVX- and gonadally-intact females as found previously in males, with handling significantly influencing these outcomes.

Estrogen therapy and cognition: a review of the cholinergic hypothesis

The pros and cons of estrogen therapy for use in postmenopausal women continue to be a major topic of debate in women's health. Much of this debate focuses on the potential benefits vs. harm of estrogen therapy on the brain and the risks for cognitive impairment associated with aging and Alzheimer's disease. Many animal and human studies suggest that estrogens can have significant beneficial effects on brain aging and cognition and reduce the risk of Alzheimer's-related dementia; however, others disagree. Important discoveries have been made, and hypotheses have emerged that may explain some of the inconsistencies. This review focuses on the cholinergic hypothesis, specifically on evidence that beneficial effects of estrogens on brain aging and cognition are related to interactions with cholinergic projections emanating from the basal forebrain. These cholinergic projections play an important role in learning and attentional processes, and their function is known to decline with advanced age and in association with Alzheimer's disease. Evidence suggests that many of the effects of estrogens on neuronal plasticity and function and cognitive performance are related to or rely upon interactions with these cholinergic projections; however, studies also suggest that the effectiveness of estrogen therapy decreases with age and time after loss of ovarian function. We propose a model in which deficits in basal forebrain cholinergic function contribute to age-related changes in the response to estrogen therapy. Based on this model, we propose that cholinergic-enhancing drugs, used in combination with an appropriate estrogen-containing drug regimen, may be a viable therapeutic strategy for use in older postmenopausal women with early evidence of mild cognitive decline.

Impact of dietary genistein and aging on executive function in rats

Genistein is an estrogenic soy isoflavone widely promoted for healthy aging, but its effects on cognitive function are not well-understood. We examined the cognitive effects of once daily oral genistein treatment at two doses (approximately 162 microg/kg/day low dose and a 323 microg/kg/day high dose) in ovariectomized young (7 month), middle-aged (16 month), and old (22 month) Long-Evans rats. Operant tasks including delayed spatial alternation (DSA), differential reinforcement of low rates of responding (DRL), and reversal learning that tap prefrontal cortical function were used to assess working memory, inhibitory control/timing, and strategy shifting, respectively. At the conclusion of cognitive testing, brains were collected and relative densities of D1 and D2 dopamine receptors and dopamine transporter (DAT) were measured in the prefrontal cortex. On the DSA task, the high dose old group performed worse than both the high dose young and middle-aged groups. On the DRL task, the high dose of genistein resulted in a marginally significant impairment in the ratio of reinforced to non-reinforced lever presses. This effect was present across age groups. Age effects were also found as old rats performed more poorly than the young and middle-aged rats on the DSA overall. In contrast, middle-aged and old rats made fewer lever presses on the DRL than did the young rats, a pattern of behavior associated with better performance on this task. Moreover, while DAT levels overall decreased with age, genistein treatment produced an increase in DAT expression in old rats relative to similarly aged control rats. D1 and D2 densities did not differ between genistein dose groups or by age. These results highlight the fact that aspects of executive function are differentially sensitive to both genistein exposure and aging and suggest that altered prefrontal dopamine function could potentially play a role in mediating these effects.

Executive function and attention are preserved in older surgically menopausal monkeys receiving estrogen or estrogen plus progesterone

Animal models of menopause have been used to further define the cognitive processes that respond to hormone therapy and to investigate parameters that may influence the cognitive effects of estrogen. Many investigations in animals have focused on memory; however, the effects of hormone therapy on executive function and attention processes have not been well studied. Thus, the purpose of this set of investigations was to assess the effects of estrogen therapy alone or with progesterone on executive and attention processes in middle-aged ovariectomized monkeys. Monkeys were preoperatively trained on a modified version of the Wisconsin card sort task and on a visual cued reaction time task. Hormone therapy was initiated at the time of ovariectomy and cognitive function was reassessed at 2, 12, and 24 weeks postoperatively. Relative to monkeys receiving either of the estrogen therapies, monkeys receiving placebo were impaired in their ability to shift a cognitive set in the Wisconsin card sort task and were impaired in shifting visuospatial attention in the visual cued reaction time task. Our findings are consistent with clinical studies that indicate that hormone therapy can improve executive function and attention processes in postmenopausal women.

Effects of chronic estradiol treatment on delayed spatial alternation and differential reinforcement of low rates of responding

Estrogens have been shown to both enhance and impair cognitive function depending on several factors, including regimen of hormone treatment, age of subject, and task attributes. In rodent models, estradiol tends to enhance spatial learning and impair response or cued learning, but effects on executive functions are less well-studied. In this experiment, spatial working memory and response inhibition were tested using delayed spatial alternation (DSA) and differential reinforcement of low rates of responding (DRL) tasks in ovariectomized rats that were given chronic estradiol via Silastic implants resulting in serum estradiol concentrations of 86.2 +/- 8.2 (SEM) pg/ml. Rats were tested for 25 days DSA with variable delays of 0, 3, 6, 9, and 18 seconds between lever presentations, followed by 30 days on a DRL-15s operant schedule. Estradiol-replaced rats showed a significantly lower proportion of correct responses on the DSA task compared to vehicle-implanted ovariectomized animals. On DRL, estradiol-treated rats showed a lower ratio of reinforced to nonreinforced presses. These data suggest that chronic estrogen exposure may impair rats' abilities on measures of executive function including working memory and response inhibition.

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.

Gender differences in executive functions following traumatic brain injury

The present study used the National Institute on Disability Rehabilitation and Research (NIDRR) funded Traumatic Brain Injury Model Systems (TBIMS) database to examine the effect of gender on presentation of executive dysfunction following traumatic brain injury (TBI) and variables that might impact the course and degree of recovery. The Wisconsin Card Sort Test (WCST) was chosen as a measure of executive function which has good credentials without reports of gender effects. Female subjects performed significantly better on the WCST than male subjects as shown by analyses of variance on scores of 1,331 patients for Categories Achieved (means for females = 4.09, males = 3.67, p = .003) and Perseverative Responses (means for females = 32.17, males = 36.42, p = .003). Outperformance by females was also noted in additional ANOVAs examining the interaction of education and gender, and ethnicity and gender in relation to Categories Achieved (p < .01), and for ethnicity and gender in relation to Perseverative Responses (p < .01). A multiple logistic regression revealed that gender, minority status, education level, history of illicit drug use, cause of injury, and length of coma each contributed uniquely to predicting Categories Achieved on the WCST. Simple logistic regression analyses showed that, of these variables, gender and cause of injury (violent vs. non-violent) were the strongest predictors. In contrast, when examining Perseverative Responses, regression analyses found gender, minority status and length of coma predicted impairment. Simple logistic regression analyses showed that, of these three variables, gender and minority status were most robust in predicting impaired Perseverative Responses scores. Implications of these findings are discussed and recommendations for further research are offered.

Effects of continuous and intermittent estrogen treatments on memory in aging female mice

The manner in which hormone therapy is given to postmenopausal women may significantly influence its ability to reduce age-associated memory loss. To test the hypothesis that a regimen that approximates the timing of estrogen surges in the natural cycle is more beneficial for memory than a regimen that provides continuous levels of estrogen, we examined the effects of continuous and intermittent estrogen regimens on spatial and object memory in aging female mice. Mice (18 months) were treated with 0.2 mg/kg 17beta-estradiol (E(2)) or vehicle (VEH) for 3 months following ovariectomy. A fast-acting water-soluble cyclodextrin-encapsulated E(2) was used to ensure metabolism within 24 h. Vehicle-treated mice received daily injections of 2-hydroxypropyl-beta-cyclodextrin vehicle. The continuous estradiol group (Contin E(2)) was injected daily with estradiol. The intermittent group (Twice/wk E(2)) received estradiol every 4 days and vehicle on all other days. Mice (21 months) were tested in water-escape motivated 8-arm radial arm maze (WRAM) and object recognition tasks. During WRAM acquisition, the Twice/wk E(2) group committed significantly more reference memory errors than VEH and Contin E(2) groups, and tended to make more working memory errors than the VEH group. The Contin E(2) group did not differ from VEH on either WRAM measure. Additionally, the Twice/wk E(2) group tended to exhibit impaired object recognition. Thus, neither treatment improved spatial or object memory. Indeed, intermittent estradiol was detrimental to both types of memory. These results suggest that the timing of administration may play an important role in the mnemonic response of aging females to estrogen.

Effects of ovarian hormones on cognitive function in nonhuman primates

Several studies have suggested that estrogen benefits verbal memory and lowers the risk of Alzheimer's disease in women, and improves cognitive function in animal models. However, the negative outcome of the Women's Health Initiative Memory Study has challenged the rationale for using estrogen as a protective agent against age-related cognitive decline. In view of the limitations of the Women's Health Initiative Memory Study, it is clear that our understanding of estrogen effects would greatly benefit from further interactions between clinical and basic science. Animal models of menopause can provide crucial information regarding the consequences of estrogen loss and replacement on several systems, including cognition. In this paper, I review the evidence that nonhuman primates, who share numerous cognitive and physiological characteristics with humans, can substantially contribute to our understanding of estrogen influences on the brain and cognition. Studies in young adult females suggest that some aspects of cognition fluctuate with the menstrual cycle, but that ovariectomy and estrogen replacement have only modest effects on cognitive function. In contrast, data in aged, naturally or surgically menopausal monkeys indicate that estrogen modulates a broad range of cognitive domains. Neurobiological data are consistent with the cognitive findings and demonstrate an array of morphological and physiological changes in brain areas important for cognition following ovariectomy and/or estrogen replacement. It is concluded that nonhuman primates, by providing a bridge between rodent and human data, constitute invaluable models to further our understanding of hormonal actions on the brain and cognition and to develop effective hormonal interventions against brain and cognitive aging.