Role of estrogen in balancing contributions from multiple memory systems

Oral administration of ethinyl estradiol and the brain-selective estrogen prodrug DHED in a female common marmoset model of menopause: Effects on cognition, thermoregulation, and sleep

Menopausal symptoms of sleep disturbances, cognitive deficits, and hot flashes are understudied, in part due to the lack of animal models in which they co-occur. Common marmosets (Callithrix jacchus) are valuable nonhuman primates for studying these symptoms, and we examined changes in cognition (reversal learning), sleep (48 h/wk of sleep recorded by telemetry), and thermoregulation (nose temperature in response to mild external warming) in middle-aged, surgically-induced menopausal marmosets studied at baseline, during 3-week phases of ethinyl estradiol (EE2, 4 μg/kg/day, p.o.) treatment and after EE2 withdrawal. We also assessed a brain-selective hormonal therapy devoid of estrogenic effects in peripheral tissues on the same measures (cognition, sleep, thermoregulation) after treatment with the estrogen prodrug 10β,17β-dihydroxyestra-1,4-dien-3-one (DHED, 100 μg/kg/day, p.o) and DHED withdrawal. Reversal learning performance was improved with EE2 or DHED treatment relative to phases without hormone administration, as indicated by a faster reversal of the stimulus/reward contingencies. Both EE2 and DHED increased non-REM sleep and reduced nighttime awakenings relative to baseline, but to the detriment of REM sleep which was highest at baseline. Nasal temperature in response to mild external warming was highest, and overnight core body temperature lowest, in the DHED treatment phase compared to both the EE2 and baseline phases. These results suggest that low dose estradiol, delivered either peripherally or centrally via DHED, benefits selective aspects of cognition and sleep in a marmoset menopause model. DHED appears a promising therapeutic candidate for alleviating the cognitive and sleep disruptions associated with estrogen deficiency in primates.

Leukocyte telomere length and memory circuitry and cognition in early aging: Impact of sex and menopausal status

Telomere length (TL) is an important cellular marker of biological aging impacting the brain and heart. However, how it is related to the brain (e.g., cognitive function and neuroanatomic architecture), and how these relationships may vary by sex and reproductive status, is not well established. Here we assessed the association between leukocyte TL and memory circuitry regional brain volumes and memory performance in early midlife, in relation to sex and reproductive status. Participants (N = 198; 95 females, 103 males; ages 45-55) underwent structural MRI and neuropsychological assessments of verbal, associative, and working memory. Overall, shorter TL was associated with smaller white matter volume in the parahippocampal gyrus and dorsolateral prefrontal cortex. In males, shorter TL was associated with worse working memory performance and corresponding smaller white matter volumes in the parahippocampal gyrus, anterior cingulate cortex, and dorsolateral prefrontal cortex. In females, the impact of cellular aging was revealed over the menopausal transition. In postmenopausal females, shorter TL was associated with poor associative memory performance and smaller grey matter volume in the right hippocampus. In contrast, TL was not related to memory performance or grey and white matter volumes in any memory circuitry region in pre/perimenopausal females. Results demonstrated that shorter TL is associated with worse memory function and smaller volume in memory circuitry regions in early midlife, an association that differs by sex and reproductive status. Taken together, TL may serve as an early indicator of sex-dependent brain abnormalities in early midlife.

Enhancement of response learning in male rats with intrastriatal infusions of a BDNF - TrkB agonist, 7,8-dihydroxyflavone

Enhancement of learning and memory by cognitive and physical exercise may be mediated by brain-derived neurotrophic factor (BDNF) acting at tropomyosin receptor kinase B (TrkB). Upregulation of BDNF and systemic administration of a TrkB agonist, 7,8-dihydroxyflavone (7,8-DHF), enhance learning of several hippocampus-sensitive tasks in rodents. Although BDNF and 7,8-DHF enhance functions of other brain areas too, these effects have mainly targeted non-cognitive functions. One goal of the present study was to determine whether 7,8-DHF would act beyond the hippocampus to enhance cognitive functions sensitive to manipulations of the striatum. Here, we examined the effects of intrastriatal infusions of 7,8-DHF on learning a striatum-sensitive response maze and on phosphorylation of TrkB receptors in 3-month-old male Sprague Dawley rats. Most prior studies of BDNF and 7,8-DHF effects on learning and memory have administered the drugs for days to months before assessing effects on cognition. A second goal of the present study was to determine whether a single drug treatment near the time of training would effectively enhance learning. Moreover, 7,8-DHF is often tested for its ability to reverse impairments in learning and memory rather than to enhance these functions in the absence of impairments. Thus, a third goal of this experiment was to evaluate the efficacy of 7,8-DHF in enhancing learning in unimpaired rats. In untrained rats, intrastriatal infusions of 7,8-DHF resulted in phosphorylation of TrkB receptors, suggesting that 7,8-DHF acted as a TrkB agonist and BDNF mimic. The findings that a single, intra-striatal infusion of 7,8-DHF 20 min before training enhanced response learning in rats suggest that, in addition to its trophic effects, BDNF modulates learning and memory through receptor mediated cell signaling events.

Exploring the physiological role of the G protein-coupled estrogen receptor (GPER) and its associations with human diseases

Estrogen is a group of hormones that collaborate with the nervous system to impact the overall well-being of all genders. It influences many processes, including those occurring in the central nervous system, affecting learning and memory, and playing roles in neurodegenerative diseases and mental disorders. The hormone's action is mediated by specific receptors. Significant roles of classical estrogen receptors, ERα and ERβ, in various diseases were known since many years, but after identifying a structurally and locationally distinct receptor, the G protein-coupled estrogen receptor (GPER), its role in human physiology and pathophysiology was investigated. This review compiles GPER-related information, highlighting its impact on homeostasis and diseases, while putting special attention on functions and dysfunctions of this receptor in neurobiology and biobehavioral processes. Understanding the receptor modulation possibilities is essential for therapy, as disruptions in receptors can lead to diseases or disorders, irrespective of correct estrogen levels. We conclude that studies on the GPER receptor have the potential to develop therapies that regulate estrogen and positively impact human health.

Brain asymmetry is globally different in males and females: exploring cortical volume, area, thickness, and mean curvature

Brain asymmetry is a cornerstone in the development of higher-level cognition, but it is unclear whether and how it differs in males and females. Asymmetry has been investigated using the laterality index, which compares homologous regions as pairwise weighted differences between the left and the right hemisphere. However, if asymmetry differences between males and females are global instead of pairwise, involving proportions between multiple brain areas, novel methodological tools are needed to evaluate them. Here, we used the Amsterdam Open MRI collection to investigate sexual dimorphism in brain asymmetry by comparing laterality index with the distance index, which is a global measure of differences within and across hemispheres, and with the subtraction index, which compares pairwise raw values in the left and right hemisphere. Machine learning models, robustness tests, and group analyses of cortical volume, area, thickness, and mean curvature revealed that, of the three indices, distance index was the most successful biomarker of sexual dimorphism. These findings suggest that left-right asymmetry in males and females involves global coherence rather than pairwise contrasts. Further studies are needed to investigate the biological basis of local and global asymmetry based on growth patterns under genetic, hormonal, and environmental factors.

Contributions of gonadal hormones in the sex-specific organization of context fear learning

It is widely established that gonadal hormones are fundamental to modulating and organizing the sex-specific nature of reproductive behaviors. Recently we proposed that context fear conditioning (CFC) may emerge in a sex-specific manner organized prior to the pubertal surge of gonadal hormones. Here we sought to determine the necessity of male and female gonadal hormones secreted at critical periods of development upon context fear learning. We tested the organizational hypothesis that neonatal and pubertal gonadal hormones play a permanent role in organizing contextual fear learning. We demonstrate that the postnatal absence of gonadal hormones by neonatal orchiectomy (oRX) in males and ovariectomy (oVX) in females resulted in an attenuation of CFC in adult males and an enhancement of CFC in adult females. In females, the gradual introduction of estrogen before conditioning partially rescued this effect. However, the decrease of CFC in adult males was not rescued by introducing testosterone before conditioning. Next, at a further point in development, preventing the pubertal surge of gonadal hormones by prepubertal oRX in males resulted in a reduction in adult CFC. In contrast, in females, prepubertal oVX did not alter adult CFC. However, the adult introduction of estrogen in prepubertal oVX rats reduced adult CFC. Lastly, the adult-specific deletion of gonadal hormones by adult oRX or oVX alone or replacement of testosterone or estrogen did not alter CFC. Consistent with our hypothesis, we provide initial evidence that gonadal hormones at early periods of development exert a vital role in the organization and development of CFC in male and female rats.

Combined effects of the contraceptive hormones, ethinyl estradiol and levonorgestrel, on the use of place and response memory in gonadally-intact female rats

During maze navigation rats can rely on hippocampus-mediated place memory or striatum-mediated response memory. Ovarian hormones bias whether females use place or response memory to reach a reward. Here, we investigated the impact of the contraceptive hormones, ethinyl estradiol (EE) and levonorgestrel (LNG), on memory bias. A total of 63 gonadally-intact female rats were treated with either 10 μg/kg of EE alone, 20 μg/kg of LNG alone, both 10 μg/kg of EE and 20 μg/kg of LNG together, or a sesame oil injection with 5% ethanol as a vehicle control. Rats in the control condition were tested during the diestrus phase of the estrous cycle in order to control for the low circulating levels of gonadotropin and ovarian hormones that occur with oral contraceptive administration. Rats treated with LNG alone had a bias towards the use of place memory compared to diestrus phase control rats. This bias was not observed if LNG was administered in combination with EE. Rats treated with EE or EE+LNG did not have a statistically significant difference in memory bias compared to rats in the control group. These data show that synthetic hormones contained in oral contraceptives administered to females influence which cognitive strategy is predominantly used during navigation.

Methods and considerations for the use of hormonal contraceptives in rat models of neurobehavior

Hormonal contraceptives (HCs), prescribed to millions of women around the world, alter the ovarian hormonal cycle resulting in neurobehavioral changes in HC users. Human epidemiological and experimental data has characterized some of these effects with oftentimes conflicting or irreproducible results, reflecting a dearth of research considering different compositions, routes of administration, or time-courses of HC use. Non-human animal research can model these effects and help elucidate the underlying mechanisms by which different HCs modulate neurobehavioral outcomes. Still, animal models using HCs are not well-established. This may be because the pharmacological profile of HCs - including the metabolism, receptor binding affinity, and neuromodulatory effects - is dynamic and not always clearly translatable between animals and humans. The current review addresses these issues and provides basic methods and considerations for the use of HCs in animal models of neurobehavior to help advance the field of behavioral neuroendocrinology and inform decisions regarding to women's health.

Cyclic estrogen and progesterone during instrumental acquisition contributes to habit formation in female rats

Habit formation is thought to involve two parallel processes that are mediated by distinct neural substates: one that suppresses goal-directed behavior, and one that facilitates stimulus-response (S-R) learning, which underscores habitual behavior. In previous studies we showed that habitual responding emerges early during instrumental training in gonadally-intact female, compared to male, rats. The present study aimed to determine the role of ovarian hormones during instrumental acquisition in the transition from goal-directed to habitual behavior in female rats. Ovariectomized (OVX) female rats were given subcutaneous silastic capsules that released low levels of 17-β estradiol (E2) to maintain estrogen receptor availability. Rats were assigned to one of three hormone treatment conditions: no additional hormone replacement (Control group), replacement with high E2 (High E2 group), or replacement with high E2 followed by progesterone (High E2 + P4 group). Hormone replacement occurred twice during acquisition to mimic natural hormone fluctuations. At test, the Control and High E2 groups demonstrated responding that was sensitive to devaluation by lithium chloride-induced illness, indicating goal-directed behavior. In contrast, the High E2 + P4 group exhibited a pattern of devaluation-insensitive, habitual responding, that suggested the suppression of goal-directed processes. In a follow-up experiment, similar procedures were conducted, however during acquisition, OVX rats were given cyclic high E2 plus medroxy-progesterone (MPA), a form of progesterone that does not metabolize to neuroactive metabolites. In this group, goal-directed behavior was observed. These data indicate that habit formation is not facilitated in low estrogen states, nor in the presence of cyclic high E2. However, cyclic high E2, together with progesterone during acquisition, appears to facilitate the early emergence of habitual responding. Furthermore, these data suggest that a neuroactive progesterone metabolite, like allopregnanolone, in combination with high cyclic E2, supports this phenomenon.

Paclitaxel Chemotherapy Elicits Widespread Brain Anisotropy Changes in a Comprehensive Mouse Model of Breast Cancer Survivorship: Evidence From In Vivo Diffusion Weighted Imaging

Breast cancer is one of the most common diseases in the United States with 1 in 8 women developing the disease in her lifetime. Women who develop breast cancer are often post-menopausal and undergo a complex sequence of treatments including surgery, chemotherapy, and aromatase inhibitor therapy. Both independently and through potential interactions, these factors and treatments are associated with behavioral comorbidities reported in patients (e.g., fatigue), although the underlying neurobiological mechanisms are poorly understood. Currently, brain imaging is the most feasible way to assess neurobiology in patients. Indeed, breast cancer patients display alterations in white matter connections and chemotherapy is associated with decreased white and gray matter in the corpus callosum and cortex as well as decreased hippocampal volume. However, imaging in breast cancer rodent models is lacking, impeding translation of the mechanistic neurobiological findings made possible through modeling. Furthermore, current rodent models of breast cancer often lack the complexity of typical multimodal breast cancer treatments, thereby limiting translational value. The present study aimed to develop a comprehensive model of post-menopausal breast cancer survival using immunocompetent ovariectomized mice, including an orthotopic syngeneic tumor, surgical tumor removal, chemotherapy, and aromatase inhibitor therapy. Using this model, we systematically investigated the cumulative effects of chemotherapy and hormone replacement therapy on neurostructure and behavior using diffusion weighted imaging, open field test, and spontaneous alternation test. Our previous findings, in a simplified chemotherapy-only model, indicate that this regimen of chemotherapy causes circulating and central inflammation concurrent with reduced locomotor activity. The current study, in the more comprehensive model, has recapitulated the peripheral inflammation coincident with reduced locomotor activity as well as demonstrated that chemotherapy also drives widespread changes in brain anisotropy. Validating the clinical relevance of this comprehensive rodent breast cancer model will allow for additional neurobiological investigations of the interactions among various cancer components associated with behavioral comorbidities, as well as the relationship between these mechanisms and neurostructural imaging changes that can be measured in cancer patients.

Time-dependent changes in hippocampal and striatal glycogen long after maze training in male rats

Long-lasting biological changes reflecting past experience have been studied in and typically attributed to neurons in the brain. Astrocytes, which are also present in large number in the brain, have recently been found to contribute critically to learning and memory processing. In the brain, glycogen is primarily found in astrocytes and is metabolized to lactate, which can be released from astrocytes. Here we report that astrocytes themselves have intrinsic neurochemical plasticity that alters the availability and provision of metabolic substrates long after an experience. Rats were trained to find food on one of two versions of a 4-arm maze: a hippocampus-sensitive place task and a striatum-sensitive response task. Remarkably, hippocampal glycogen content increased while striatal levels decreased during the 30 days after rats were trained to find food in the place version, but not the response version, of the maze tasks. A long-term consequence of the durable changes in glycogen stores was seen in task-by-site differences in extracellular lactate responses activated by testing on a working memory task administered 30 days after initial training, the time when differences in glycogen content were most robust. These results suggest that astrocytic plasticity initiated by a single experience may augment future availability of energy reserves, perhaps priming brain areas to process learning of subsequent experiences more effectively.

Neural systems and the emotion-memory link

The present brief review for this Special Issue summarizes some of the original research on the emotional modulation of memory. The review begins by highlighting the pioneering research from James L. McGaugh and colleagues demonstrating modulatory effects of post-training drug administration on memory consolidation, in particular the stress hormone epinephrine. The subsequent discovery of a critical role for the basolateral amygdala in emotional modulation of memory is described. Within the context of a multiple systems approach to memory focusing on selective roles for the hippocampus and dorsolateral striatum in cognitive and habit memory, the original studies indicating that robust emotional arousal can bias animals and humans toward the predominant use of habit memory are reviewed. This effect of emotional arousal on the relative use of multiple memory systems depends on a modulatory role of the basolateral amygdala. Finally, we briefly consider how an emotion-induced enhancement of dorsolateral striatal-dependent memory may be relevant to understanding maladaptive habitual behaviors in certain human psychopathologies.

Extracellular levels of glucose in the hippocampus and striatum during maze training for food or water reward in male rats

Glucose potently enhances cognitive functions whether given systemically or directly to the brain. The present experiments examined changes in brain extracellular glucose levels while rats were trained to solve hippocampus-sensitive place or striatum-sensitive response learning tasks for food or water reward. Because there were no task-related differences in glucose responses, the glucose results were pooled across tasks to form combined trained groups. During the first 1-3 min of training for food reward, glucose levels in extracellular fluid (ECF) declined significantly in the hippocampus and striatum; the declines were not seen in untrained, rewarded rats. When trained for water reward, similar decreases were observed in both brain areas, but these findings were less consistent than those seen with food rewards. After the initial declines in ECF glucose levels, glucose increased in most groups, approaching asymptotic levels ∼15-30 min into training. Compared to untrained food controls, training with food reward resulted in significant glucose increases in the hippocampus but not striatum; striatal glucose levels exhibited large increases to food intake in both trained and untrained groups. In rats trained to find water, glucose levels increased significantly above the values seen in untrained rats in both hippocampus and striatum. The decreases in glucose early in training might reflect an increase in brain glucose consumption, perhaps triggering increased brain uptake of glucose from blood, as evident in the increases in glucose later in training. The increased brain uptake of glucose may provide additional neuronal metabolic substrate for metabolism or provide astrocytic substrate for production of glycogen and lactate.

Sex steroids-induced neurogenesis in adult brain: a better look at mechanisms and mediators

Adult neurogenesis is the production of new nerve cells in the adult brain. Neurogenesis is a clear example of the neuroplasticity phenomenon which can be observed in most of mammalian species, including human beings. This phenomenon occurs, at least, in two regions of the brain: the subgranular zone of the dentate gyrus in hippocampus and the ventricular zone of lateral ventricles. Numerous studies have investigated the relationship between sex steroid hormones and neurogenesis of adult brain; of which, mostly concentrated on the role of estradiol. It has been shown that estrogen plays a significant role in this process through both classic and non-classic mechanisms, including a variety of different growth factors. Therefore, the objective of this review is to investigate the role of female sex steroids with an emphasis on estradiol and also its potential implications for regulating the neurogenesis in the adult brain.

Sex and Menstrual Phase Influences on Sleep and Memory

Purposes of Review

This review highlights the effect of sex differences in sleep mediated memory consolidation and cognitive performance. In addition, the role of menstrual cycle and the fluctuating level of sexual hormones (mainly oestrogen and progesterone) are stressed.

Recent Findings

The literature indicates that sex hormones mediate and orchestrate the differences observed in performance of females in comparison with males in a variety of tasks and can also be related to how sleep benefits cognition. Although the exact mechanism of such influence is not clear, it most likely involves differential activation of brain areas, sensitivity to neuromodulators (mainly oestrogen), circadian regulation of sleep and temperature, as well as modification of strategies to solve tasks across the menstrual cycle.

Summary

With the evidence presented here, we hope to encourage researchers to develop appropriate paradigms to study the complex relationship between menstrual cycle, sleep (its regulation, architecture and electrophysiological hallmarks) and performance in memory and other cognitive tasks.

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.

Endogenous variation in estradiol in women affects the weighting of metric and categorical information in spatial location memory

Recent work has suggested that sex differences may exist in the strategies or types of cues that are utilized by men and women to remember discrete spatial locations or routes through a visual environment. The current study investigated the effects of circulating estradiol levels in women on the relative weighting of categorical versus fine-grained 'metric' information in a test of short-term memory for spatial locations, either presented within a simple geometric surround (a circular enclosure) or within more visually complex landscape scenes. Patterns of displacement error in the point location estimates made by men and women were analyzed. Results confirmed a sex difference in the weighting of metric versus categorical cues. Relative to men, women's estimates of locations were more strongly biased toward the center of the surrounding category (i.e., toward the category 'prototype'). Furthermore, objective measures of estradiol via saliva collected at the time of memory testing showed that, among naturally-cycling women, estradiol concentrations correlated in a positive, graded, fashion with the degree of emphasis that women placed on categorical information when estimating point locations. No associations were found for progesterone. These findings are consistent with a wider body of research showing that biological sex and reproductive hormone levels, including 17β-estradiol, can subtly influence performance on certain spatial tasks. This is the first study to show that circulating estradiol levels may influence the relative emphasis placed on categorical versus metric cues when remembering simple point locations.

Testicular hormones mediate robust sex differences in impulsive choice in rats

Impairments in choosing optimally between immediate and delayed rewards are associated with numerous psychiatric disorders. Such ‘intertemporal’ choice is influenced by genetic and experiential factors; however, the contributions of biological sex are understudied and data to date are largely inconclusive. Rats were used to determine how sex and gonadal hormones influence choices between small, immediate and large, delayed rewards. Females showed markedly greater preference than males for small, immediate over large, delayed rewards (greater impulsive choice). This difference was neither due to differences in food motivation or reward magnitude perception, nor was it affected by estrous cycle. Ovariectomies did not affect choice in females, whereas orchiectomies increased impulsive choice in males. These data show that male rats exhibit less impulsive choice than females and that this difference is at least partly maintained by testicular hormones. These differences in impulsive choice could be linked to gender differences across multiple neuropsychiatric conditions.

Aging is not equal across memory systems

The present experiments compared the effects of aging on learning several hippocampus- and striatum-sensitive tasks in young (3-4 month) and old (24-28 month) male Fischer-344 rats. Across three sets of tasks, aging was accompanied not only by deficits on hippocampal tasks but also by maintained or even enhanced abilities on striatal tasks. On two novel object recognition tasks, rats showed impaired performance on a hippocampal object location task but enhanced performance on a striatal object replacement task. On a dual solution task, young rats predominately used hippocampal solutions and old rats used striatal solutions. In addition, on two maze tasks optimally solved using either hippocampus-sensitive place or striatum-sensitive response strategies, relative to young rats, old rats had impaired learning on the place version but equivalent learning on the response version. Because glucose treatments can reverse deficits in learning and memory across many tasks and contexts, levels of available glucose in the brain may have particular importance in cognitive aging observed across tasks and memory systems. During place learning, training-related rises in extracellular glucose levels were attenuated in the hippocampus of old rats compared to young rats. In contrast, glucose levels in the striatum increased comparably in young and old rats trained on either the place or response task. These extracellular brain glucose responses to training paralleled the impairment in hippocampus-sensitive learning and the sparing of striatum-sensitive learning seen as rats age, suggesting a link between age-related changes in learning and metabolic substrate availability in these brain regions.

Impact of BDNF and sex on maintaining intact memory function in early midlife

Sex steroid hormones and neurotrophic factors, such as brain-derived neurotrophic factor (BDNF), play a significant neuroprotective role in memory circuitry aging. Here, we present findings characterizing the neuroprotective effects of BDNF on memory performance, as a function of sex and reproductive status in women. Participants (N = 191; mean age = 50.03 ± 2.10) underwent clinical and cognitive testing, fMRI scanning, and hormonal assessments of menopausal staging. Memory performance was assessed with the 6-Trial Selective Reminding Test and the Face-Name Associative Memory Exam. Participants also performed a working memory (WM) N-back task during fMRI scanning. Results revealed significant interactions between menopausal status and BDNF levels. Only in postmenopausal women, lower plasma BDNF levels were associated with significantly worse memory performance and altered function in the WM circuitry. BDNF had no significant impact on memory performance or WM function in pre/perimenopausal women or men. These results suggest that in postmenopausal women, BDNF is associated with memory performance and memory circuitry function, thus providing evidence of potential sex-dependent factors of risk and resilience for early intervention.

Fornix lesions impair place-, but not response-learning in the open-field tower maze

The purpose of the present study was to examine hippocampal function for spatial learning in a land-based circular maze (i.e., the open-field tower maze [OFTM]). The OFTM, a task designed to be non-stressful, has been previously used to demonstrate the influence of gonadal hormones on spatial learning. Thus, determination of brain function for navigating in the OFTM provides an important extension to previous knowledge. Fornix lesions were used in the present experiment to disrupt hippocampal processing. After initial pre-training, rats received either an electrolytic fornix lesion surgery or a sham surgery. The rats from each surgical group were given either place- or response-training in the OFTM. The results showed that (1) lesioned place-learners required more trials than sham place-learners to solve the OFTM and (2) lesioned response-learners solved the OFTM at the same rate as sham response-learners. Our findings support the hypothesis that the hippocampus is necessary for place-, but not response-learning in the OFTM task. The OFTM is an appetitive task that does not depend on a choice between restricted directions that a rat would be required to make in a T-maze or a radial arm-maze, and does not include aversive components inherent to a Morris Water Maze or Barnes Maze. Thus, the OFTM can be used to investigate the manipulations of hippocampus-dependent spatial learning without confounding variables related to an animal's stress level.

Involvement of lactate transport in two object recognition tasks that require either the hippocampus or striatum

Growing evidence indicates that hippocampal lactate, released from astrocytes, is an important regulator of learning and memory processing. This study evaluated the selective involvement of hippocampal and striatal lactate in two object recognition tasks. The tasks tested recognition memory after a change in location of two target objects (double object location; dOL) or after replacement of familiar targets with two new objects set in the original locations (double object replacement; dOR). Rats received three study sessions across which exploration times decreased. The recognition index was the change in exploration time of both objects on a test trial from the exploration times on the final study trial. We first verified a double dissociation between hippocampus and striatum across these tasks. The sodium channel blocker, lidocaine, was infused into one of the two brain regions after the study sessions and before the test trial. To test the role of neuronal lactate in recognition memory, an inhibitor of the neuronal lactate transporter, α-cyano-4-hydroxycinnamate (4-CIN), was similarly infused. For both drugs, infusions into the hippocampus but not the striatum impaired recognition in the dOL, whereas infusions into the striatum but not hippocampus impaired recognition in the dOR. The findings obtained with 4-CIN demonstrate for the first time the importance of neuronal lactate uptake in the hippocampus and the striatum for object recognition memory processing. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Impact of adrenal hormones, reproductive aging, and major depression on memory circuitry decline in early midlife

Dehydroepiandrosterone-sulfate (DHEAS) is an adrenal androgen that is, in part, aromatized to estradiol. It continues to be produced after menopause and provides estrogenicity after depletion of ovarian hormones. Estradiol depletion contributes to memory circuitry changes over menopause, including changes in hippocampal (HIPP) and dorsolateral- and ventrolateral-prefrontal cortex (DLPFC; VLPFC) function. Further, major depressive disorder (MDD) patients have, in general, lower levels of estradiol and lower DHEAS than healthy controls, thus potentially a higher risk of adverse menopausal outcomes. We investigated whether higher DHEAS levels after menopause is associated with better memory circuitry function, especially in women with MDD. 212 adults (ages 45-55, 50% women) underwent clinical and fMRI testing. Participants performed a working memory (WM) N-back task and an episodic memory verbal encoding (VE) task during fMRI scanning. DHEAS levels were significantly associated with memory circuitry function, specifically in MDD postmenopausal women. On the WM task, lower DHEAS levels were associated with increased HIPP activity. On the VE task, lower DHEAS levels were associated with decreased activity in the HIPP and VLPFC. In contrast, there was no association between DHEAS levels and memory circuitry function in MDD pre/perimenopausal women, men, and non-MDD participants regardless of sex and reproductive status. In fact, MDD postmenopausal women with higher levels of DHEAS were similar to MDD pre/perimenopausal women and men. Thus, memory circuitry deficits associated with MDD and a lower ability of the adrenal gland to produce DHEAS after menopause may contribute to a lower ability to maintain intact memory function with age.

Dorsal and ventral hippocampal adult-born neurons contribute to context fear memory

The hippocampus contains one of the few neurogenic niches within the adult brain-the subgranular zone of the dentate gyrus. The functional significance of adult-born neurons in this region has been characterized using context fear conditioning, a Pavlovian paradigm in which animals learn to associate a location with danger. Ablation or silencing of adult-born neurons impairs both acquisition and recall of contextual fear conditioning, suggesting that these neurons contribute importantly to hippocampal memory. Lesion studies indicate that CFC depends on neural activity in both the dorsal and ventral hippocampus, subregions with unique extrahippocampal connectivity and behavioral functions. Because most studies of adult neurogenesis have relied on methods that permanently ablate neurogenesis throughout the entire hippocampus, little is known about how the function of adult-born neurons varies along the dorsal-ventral axis. Using a Nestin-CreERT2 mouse line to target the optogenetic silencer Archaerhodopsin to adult-born neurons, we compared the contribution of dorsal and ventral adult-born neurons to acquisition, recall, and generalization of CFC. Acquisition of CFC was impaired when either dorsal or ventral adult-born neurons were silenced during training. Silencing dorsal or ventral adult-born neurons during test sessions decreased context-evoked freezing but did not impair freezing in a hippocampus-independent tone-shock freezing paradigm. Silencing adult-born neurons modestly reduced generalization of fear. Our data indicate that adult-born neurons in the dorsal and ventral hippocampus contribute to both memory acquisition and recall. The comparatively large behavioral effects of silencing a small number of adult-born neurons suggest that these neurons make a unique and powerful contribution to hippocampal function.

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.

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.

Training-induced elevations in extracellular lactate in hippocampus and striatum: Dissociations by cognitive strategy and type of reward

Recent evidence suggests that astrocytes convert glucose to lactate, which is released from the astrocytes and supports learning and memory. This report takes a multiple memory perspective to test the role of astrocytes in cognition using real-time lactate measurements during learning and memory. Extracellular lactate levels in the hippocampus or striatum were determined with lactate biosensors while rats were learning place (hippocampus-sensitive) or response (striatum-sensitive) versions of T-mazes. In the first experiment, rats were trained on the place and response tasks to locate a food reward. Extracellular lactate levels in the hippocampus increased beyond those of feeding controls during place training but not during response training. However, striatal lactate levels did not increase beyond those of controls when rats were trained on either the place or the response version of the maze. Because food ingestion itself increased blood glucose and brain lactate levels, the contribution of feeding may have confounded the brain lactate measures. Therefore, we conducted a second similar experiment using water as the reward. A very different pattern of lactate responses to training emerged when water was used as the task reward. First, provision of water itself did not result in large increases in either brain or blood lactate levels. Moreover, extracellular lactate levels increased in the striatum during response but not place learning, whereas extracellular lactate levels in the hippocampus did not differ across tasks. The findings from the two experiments suggest that the relative engagement of the hippocampus and striatum dissociates not only by task but also by reward type. The divergent lactate responses of the hippocampus and striatum in place and response tasks under different reward conditions may reflect ethological constraints tied to foraging for food and water.

Physical Activity Is Associated with Reduced Implicit Learning but Enhanced Relational Memory and Executive Functioning in Young Adults

Accumulating evidence suggests that physical activity improves explicit memory and executive cognitive functioning at the extreme ends of the lifespan (i.e., in older adults and children). However, it is unknown whether these associations hold for younger adults who are considered to be in their cognitive prime, or for implicit cognitive functions that do not depend on motor sequencing. Here we report the results of a study in which we examine the relationship between objectively measured physical activity and (1) explicit relational memory, (2) executive control, and (3) implicit probabilistic sequence learning in a sample of healthy, college-aged adults. The main finding was that physical activity was positively associated with explicit relational memory and executive control (replicating previous research), but negatively associated with implicit learning, particularly in females. These results raise the intriguing possibility that physical activity upregulates some cognitive processes, but downregulates others. Possible implications of this pattern of results for physical health and health habits are discussed.

Sex differences in chronic stress effects on cognition in rodents

Chronic stress causes deleterious changes in physiological function in systems ranging from neural cells in culture to laboratory rodents, sub-human primates and humans. It is notable, however, that the vast majority of research in this area has been conducted in males. In this review, we provide information about chronic stress effects on cognition in female rodents and contrast it with responses in male rodents. In general, females show cognitive resilience to chronic stressors which impair male cognitive function using spatial tasks including the radial arm maze, radial arm water maze, Morris water maze, Y-maze and object placement. Moreover, stress often enhances female performance in some of these cognitive tasks. Memory in females is not affected by stress in non-spatial memory tasks like recognition memory and temporal order recognition memory while males show impaired memory following stress. We discuss possible bases for these sex-dependent differences including the use of different strategies by the sexes to solve cognitive tasks. Whether the sex differences result from changes in non-mnemonic factors is also considered. Sex-dependent differences in alcohol and drug influences on stress responses are also described. Finally, the role of neurally derived estradiol in driving sex differences and providing resilience to stress in females is shown. The importance of determining the nature and extent of sex differences in stress responses is that such differences may provide vital information for understanding why some stress related diseases have different incidence rates between the sexes and for developing novel therapeutic treatments.

Reproductive Steroid Regulation of Mood and Behavior

In this article, we examine evidence supporting the role of reproductive steroids in the regulation of mood and behavior in women and the nature of that role. In the first half of the article, we review evidence for the following: (i) the reproductive system is designed to regulate behavior; (ii) from the subcellular to cellular to circuit to behavior, reproductive steroids are powerful neuroregulators; (iii) affective disorders are disorders of behavioral state; and (iv) reproductive steroids affect virtually every system implicated in the pathophysiology of depression. In the second half of the article, we discuss the diagnosis of the three reproductive endocrine-related mood disorders (premenstrual dysphoric disorder, postpartum depression, and perimenopausal depression) and present evidence supporting the relevance of reproductive steroids to these conditions. Existing evidence suggests that changes in reproductive steroid levels during specific reproductive states (i.e., the premenstrual phase of the menstrual cycle, pregnancy, parturition, and the menopause transition) trigger affective dysregulation in susceptible women, thus suggesting the etiopathogenic relevance of these hormonal changes in reproductive mood disorders. Understanding the source of individual susceptibility is critical to both preventing the onset of illness and developing novel, individualized treatments for reproductive-related affective dysregulation. © 2016 American Physiological Society. Compr Physiol 6:1135-1160, 2016e.

The impact of biological sex and sex hormones on cognition in a rat model of early, pre-motor Parkinson's disease

Parkinson's disease (PD) is well known for motor deficits such as bradykinesia. However, patients often experience additional deficits in working memory, behavioral selection, decision-making and other executive functions. Like other features of PD, the incidence and severity of these cognitive symptoms differ in males and females. However, preclinical models have not been used to systematically investigate the roles that sex or sex hormones may play in these complex signs. To address this, we used a Barnes maze spatial memory paradigm to compare the effects of a bilateral nigrostriatal dopamine lesion model of early PD on cognitive behaviors in adult male and female rats and in adult male rats that were gonadectomized or gonadectomized and supplemented with testosterone or estradiol. We found that dopamine lesions produced deficits in working memory and other executive operations, albeit only in male rats where circulating androgen levels were physiological. In males where androgen levels were depleted, lesions produced no additional Barnes maze deficits and attenuated those previously linked to androgen deprivation. We also found that while most measures of Barnes maze performance were unaffected by dopamine lesions in the females, lesions did induce dramatic shifts from their preferred use of thigmotactic navigation to the use of spatially guided place strategies similar to those normally preferred by males. These and other sex- and sex hormone-specific differences in the effects of nigrostriatal dopamine lesions on executive function highlight the potential of gonadal steroids as protective and/or therapeutic for the cognitive symptoms of PD. However, their complexity also indicates the need for a more thorough understanding of androgen and estrogen effects in guiding the development of hormone therapies that might effectively address these non-motor signs.

High Oestradiol Replacement Reverses Response Memory Bias in Ovariectomised Female Rats Regardless of Dopamine Levels in the Dorsal Striatum

Oestrogens influence memory system bias in female rats such that high levels of oestrogen are associated with place (or spatial) memory use, and low oestrogen levels with response (or habitual) memory use. Moreover, striatal-dependent response memory is sensitive to dopamine transmission in the dorsal striatum, and oestrogens have been shown to affect dopamine release in that brain area. In the present study, the effects of oestrogens and dopamine transmission on multiple memory system bias were explored in ovariectomised rats receiving low or high 17β-oestradiol replacement under saline, autoreceptor-activating doses of the dopamine D2 receptor agonist, apomorphine (50 and 80 μg/kg), or amphetamine (0.5 mg/kg) administration. Furthermore, dorsal striatal dopamine release was measured after administration of the same drug conditions using in vivo microdialysis. As expected, high oestradiol rats predominantly used place memory, whereas the opposite pattern was observed in low oestradiol rats. However, the high apomorphine dose statistically significantly altered memory bias in high oestradiol rats from predominant place to predominant response memory, with a similar trend in the low apomorphine dose and the amphetamine group. There was no effect of drugs on memory bias in low oestradiol rats. Rats with high oestradiol replacement receiving amphetamine exhibited greater dorsal striatal dopamine release than low oestradiol replacement rats, and this difference was amplified in the right hemisphere. Furthermore, a logistic regression analysis revealed that oestradiol, but not dorsal striatal dopamine levels, significantly predicted response memory bias. These findings provide further evidence that oestradiol modulates memory system bias, and also that memory bias is changed by systemic apomorphine administration. However, although oestradiol affects dopamine transmission in the dorsal striatum in a lateralised manner, this does not predict memory system bias.

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.

Place and Response Learning in the Open-field Tower Maze

This protocol describes how the Open-field Tower Maze (OFTM) paradigm is used to study spatial learning in rodents. This maze is especially useful for examining how rats learn to use a place- or response-learning to successfully navigate in an open-field arena. Additionally, this protocol describes how the OFTM differs from other behavioral maze paradigms that are commonly used to study spatial learning in rodents. The OFTM described in this article was adapted from the one previously described by Cole, Clipperton, and Walt (2007). Specifically, the OFTM was created to test spatial learning in rodents without the experimenter having to consider how "stress" might play a role as a confounding variable. Experiments have shown that stress-alone can significantly affect cognitive function(1). The representative results section contains data from an experiment that used the OFTM to examine the effects of estradiol treatment on place- and response-learning in adult female Sprague Dawley rats(2). Future studies will be designed to examine the role of the hippocampus and striatum in place- and response-learning in the OFTM.

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.

Strain and sex differences in puberty onset and the effects of THC administration on weight gain and brain volumes

The use of recreational marijuana is widespread and frequently begins and persists through adolescence. Some research has shown negative consequences of adolescent marijuana use, but this is not seen across studies, and certain factors, like genetic background and sex, may influence the results. It is critical to identify which characteristics predispose an individual to be susceptible to the negative consequences of chronic exposure to marijuana in adolescence on brain health and behavior. To this end, using males and females of two strains of rats, Long-Evans hooded (LER) and Wistar (WR) rats, we explored whether these anatomically and behaviorally dimorphic strains demonstrated differences in puberty onset and strain-specific effects of adolescent exposure to Δ9-tetrahydrocannabinol (THC), the main psychoactive component of marijuana. Daily 5 mg/kg treatment began on the day of puberty onset and continued for 14 days. Of particular interest were metrics of growth and volumetric estimates of brain areas involved in cognition that contain high densities of cannabinoid receptors, including the hippocampus and its subregions, the amygdala, and the frontal cortex. Brain volumetrics were analyzed immediately following the treatment period. LER and WR females started puberty at different ages, but no strain differences were observed in brain volumes. THC decreased weight gain throughout the treatment period for all groups. Only the hippocampus and some of its subregions were affected by THC, and increased volumes with THC administration was observed exclusively in females, regardless of strain. Long-term treatment of THC did not affect all individuals equally, and females displayed evidence of increased sensitivity to the effects of THC, and by extension, marijuana. Identifying differences in adolescent physiology of WR and LER rats could help determine the cause for strain and sex differences in brain and behavior of adults and help to refine the use of animal models in marijuana research.

The size of non-hippocampal brain regions varies by season and sex in Richardson's ground squirrel

Sex- and season-specific modulation of hippocampal size and function is observed across multiple species, including rodents. Other non-hippocampal-dependent behaviors exhibit season and sex differences, and whether the associated brain regions exhibit similar variation with sex and season remains to be fully characterized. As such, we examined the brains of wild-caught Richardson's ground squirrels (RGS; Urocitellus richardsonii) for seasonal (breeding, non-breeding) and sex differences in the volumes of specific brain areas, including: total brain volume, corpus callosum (CC), anterior commissure (AC), medial prefrontal cortex (mPFC), total neocortex (NC), entorhinal cortex (EC), and superior colliculus (SC). Analyses of variance and covariance revealed significant interactions between season and sex for almost all areas studied, primarily resulting from females captured during the breeding season exhibiting larger volumes than females captured during the non-breeding season. This was observed for volumes of the AC, mPFC, NC, EC, and SC. Where simple main effects of season were observed for males (the NC and the SC), the volume advantage favoured males captured during the NBr season. Only two simple main effects of sex were observed: males captured in the non-breeding season had significantly larger total brain volume than females captured in the non-breeding season, and females captured during the breeding season had larger volumes of the mPFC and EC than males captured in the breeding season. These results indicate that females have more pronounced seasonal differences in brain and brain region sizes. The extent to which seasonal differences in brain region volumes vary with behaviour is unclear, but our data do suggest that seasonal plasticity is not limited to the hippocampus and that RGS is a useful mammalian species for understanding seasonal plasticity in an ecologically relevant context.

Separate effects of sex hormones and sex chromosomes on brain structure and function revealed by high-resolution magnetic resonance imaging and spatial navigation assessment of the Four Core Genotype mouse model

Males and females exhibit several differences in brain structure and function. To examine the basis for these sex differences, we investigated the influences of sex hormones and sex chromosomes on brain structure and function in mice. We used the Four Core Genotype (4CG) mice, which can generate both male and female mice with XX or XY sex chromosome complement, allowing the decoupling of sex chromosomes from hormonal milieu. To examine whole brain structure, high-resolution ex vivo MRI was performed, and to assess differences in cognitive function, mice were trained on a radial arm maze. Voxel-wise and volumetric analyses of MRI data uncovered a striking independence of hormonal versus chromosomal influences in 30 sexually dimorphic brain regions. For example, the bed nucleus of the stria terminalis and the parieto-temporal lobe of the cerebral cortex displayed steroid-dependence while the cerebellar cortex, corpus callosum, and olfactory bulbs were influenced by sex chromosomes. Spatial learning and memory demonstrated strict hormone-dependency with no apparent influence of sex chromosomes. Understanding the influences of chromosomes and hormones on brain structure and function is important for understanding sex differences in brain structure and function, an endeavor that has eventual implications for understanding sex biases observed in the prevalence of psychiatric disorders.

Medial prefrontal cortical estradiol rapidly alters memory system bias in female rats: ultrastructural analysis reveals membrane-associated estrogen receptors as potential mediators

High plasma levels of estradiol (E2) are associated with use of a place memory system over a response memory system. We examined whether infusing estradiol into the medial prefrontal cortex (mPFC) or anterior cingulate cortex (AC) could affect memory system bias in female rats. We also examined the ultrastructural distribution of estrogen receptor (ER)-α, ERβ, and G protein-coupled estrogen receptor 1 (GPER1) in the mPFC of female rats as a mechanism for the behavioral effects of E2 in the mPFC. Each rat was infused bilaterally with either E2 (0.13 μg) or vehicle into the mPFC or AC. The majority of E2 mPFC rats used place memory. In contrast, the majority of mPFC vehicle rats and AC E2 or vehicle rats used response memory. These data show that mPFC E2 rapidly biases females to use place memory. Electron microscopic analysis demonstrated that ERα, ERβ, and GPER1 are localized in the mPFC, almost exclusively at extranuclear sites. This is the first time that GPER1 has been localized to the mPFC of rats and the first time that ERα and ERβ have been described at extranuclear sites in the rat mPFC. The majority of receptors were observed on axons and axon terminals, suggesting that estrogens alter presynaptic transmission in the mPFC. This provides a mechanism via which ERs could rapidly alter transmission in the mPFC to alter PFC-dependent behaviors, such as memory system bias. The discrete nature of immunolabeling for these membrane-associated ERs may explain the discrepancy in previous light microscopy studies.

Factors that influence the relative use of multiple memory systems

Neurobehavioral evidence supports the existence of at least two anatomically distinct "memory systems" in the mammalian brain that mediate dissociable types of learning and memory; a "cognitive" memory system dependent upon the hippocampus and a "stimulus-response/habit" memory system dependent upon the dorsolateral striatum. Several findings indicate that despite their anatomical and functional distinctiveness, hippocampal- and dorsolateral striatal-dependent memory systems may potentially interact and that, depending on the learning situation, this interaction may be cooperative or competitive. One approach to examining the neural mechanisms underlying these interactions is to consider how various factors influence the relative use of multiple memory systems. The present review examines several such factors, including information compatibility, temporal sequence of training, the visual sensory environment, reinforcement parameters, emotional arousal, and memory modulatory systems. Altering these parameters can lead to selective enhancements of either hippocampal-dependent or dorsolateral striatal-dependent memory, and bias animals toward the use of either cognitive or habit memory in dual-solution tasks that may be solved adequately with either memory system. In many learning situations, the influence of such experimental factors on the relative use of memory systems likely reflects a competitive interaction between the systems. Research examining how various factors influence the relative use of multiple memory systems may be a useful method for investigating how these systems interact with one another.

Modulatory effect of 17-β estradiol on performance of ovariectomized rats on the Shock-Probe test

17-β estradiol (E2) has been shown to modulate fear conditioning by influencing freezing behavior following re-exposure to either the conditioning context or a cue associated with shock. Fear-related behaviors other than freezing may be influenced differently by E2 replacement. Accordingly, the present study examined whether E2 modulates fear conditioning using the Shock-Probe test, which allows for the observation of multiple fear responses. Ovariectomized (OVX) rats were divided into three groups: no E2 replacement (Shock-Oil; 0.1ml/kg sesame oil), high E2 replacement (Shock-E2; 3 daily doses of 10μg/kg, s.c.), and a no shock, no E2 replacement (Naïve-oil) group. During the acquisition phase, the two shock groups (Shock-Oil and Shock-E2) demonstrated comparable fear expression on all measures. During the retention tests, rats without E2 replacement demonstrated intact contextual-fear conditioning but impaired cued-fear conditioning, while rats with E2 replacement demonstrated the opposite pattern. In the context paired with shock, the Shock-Oil group spent more time burying the probe than both the Naïve-oil group and Shock-E2 group. The Shock-E2 group avoided the probe area relative to the other two groups when tested in the context unpaired with shock. The present findings demonstrate the importance of using multiple behaviors to measure fear conditioning and suggest that high E2 replacement impairs contextual and facilitates cued fear conditioning.

Estradiol enhances object recognition memory in Swiss female mice by activating hippocampal estrogen receptor α

In rodents, 17β-estradiol (E2) enhances hippocampal function and improves performance in several memory tasks. Regarding the object recognition paradigm, E2 commonly act as a cognitive enhancer. However, the types of estrogen receptor (ER) involved, as well as the underlying molecular mechanisms are still under investigation. In the present study, we asked whether E2 enhances object recognition memory by activating ERα and/or ERβ in the hippocampus of Swiss female mice. First, we showed that immediately post-training intraperitoneal (i.p.) injection of E2 (0.2 mg/kg) allowed object recognition memory to persist 48 h in ovariectomized (OVX) Swiss female mice. This result indicates that Swiss female mice are sensitive to the promnesic effects of E2 and is in accordance with other studies, which used C57/BL6 female mice. To verify if the activation of hippocampal ERα or ERβ would be sufficient to improve object memory, we used PPT and DPN, which are selective ERα and ERβ agonists, respectively. We found that PPT, but not DPN, improved object memory in Swiss female mice. However, DPN was able to improve memory in C57/BL6 female mice, which is in accordance with other studies. Next, we tested if the E2 effect on improving object memory depends on ER activation in the hippocampus. Thus, we tested if the infusion of intra-hippocampal TPBM and PHTPP, selective antagonists of ERα and ERβ, respectively, would block the memory enhancement effect of E2. Our results showed that TPBM, but not PHTPP, blunted the promnesic effect of E2, strongly suggesting that in Swiss female mice, the ERα and not the ERβ is the receptor involved in the promnesic effect of E2. It was already demonstrated that E2, as well as PPT and DPN, increase the phospho-ERK2 level in the dorsal hippocampus of C57/BL6 mice. Here we observed that PPT increased phospho-ERK1, while DPN decreased phospho-ERK2 in the dorsal hippocampus of Swiss female mice subjected to the object recognition sample phase. Taken together, our results suggest that the type of receptor as well as the molecular mechanism used by E2 to improve object memory may differ in Swiss female mice.

Antagonism of brain insulin-like growth factor-1 receptors blocks estradiol effects on memory and levels of hippocampal synaptic proteins in ovariectomized rats

RATIONALE

Treatment with estradiol, the primary estrogen produced by the ovaries, enhances hippocampus-dependent spatial memory and increases levels of hippocampal synaptic proteins in ovariectomized rats. Increasing evidence indicates that the ability of estradiol to impact the brain and behavior is dependent upon its interaction with insulin-like growth factor-1 (IGF-1).

OBJECTIVE

The goal of the current experiment was to test the hypothesis that the ability of estradiol to impact hippocampus-dependent memory and levels of hippocampal synaptic proteins is dependent on its interaction with IGF-1.

METHODS

Adult rats were ovariectomized and implanted with estradiol or control capsules and trained on a radial-maze spatial memory task. After training, rats were implanted with intracerebroventricular cannulae attached to osmotic minipumps (flow rate 0.15 μl/h). Half of each hormone treatment group received continuous delivery of JB1 (300 μg/ml), an IGF-1 receptor antagonist, and half received delivery of aCSF vehicle. Rats were tested on trials in the radial-arm maze during which delays were imposed between the fourth and fifth arm choices. Hippocampal levels of synaptic proteins were measured by western blotting.

RESULTS

Estradiol treatment resulted in significantly enhanced memory. JB1 blocked that enhancement. Estradiol treatment resulted in significantly increased hippocampal levels of postsynaptic density protein 95 (PSD-95), spinophilin, and synaptophysin. JB1 blocked the estradiol-induced increase of PSD-95 and spinophilin and attenuated the increase of synaptophysin.

CONCLUSIONS

Results support a role for IGF-1 receptor activity in estradiol-induced enhancement of spatial memory that may be dependent on changes in synapse structure in the hippocampus brought upon by estradiol/IGF-1 interactions.