Sex and estrous cycle differences in immediate early gene activation in the hippocampus and the dorsal striatum after the cue competition task

Pregnancy history and estradiol influence spatial memory, hippocampal plasticity, and inflammation in middle-aged rats

Pregnancy and motherhood can have long-term effects on cognition and brain aging in both humans and rodents. Estrogens are related to cognitive function and neuroplasticity. Estrogens can improve cognition in postmenopausal women, but the evidence is mixed, partly due to differences in age of initiation, type of menopause, dose, formulation and route of administration. Additionally, past pregnancy influences brain aging and cognition as a younger age of first pregnancy in humans is associated with poorer aging outcomes. However, few animal studies have examined specific features of pregnancy history or the possible mechanisms underlying these changes. We examined whether maternal age at first pregnancy and estradiol differentially affected hippocampal neuroplasticity, inflammation, spatial reference cognition, and immediate early gene activation in response to spatial memory retrieval in middle-age. Thirteen-month-old rats (who were nulliparous (never mothered) or previously primiparous (had a litter) at three or seven months) received daily injections of estradiol (or vehicle) for sixteen days and were tested on the Morris Water Maze. An older age of first pregnancy was associated with impaired spatial memory but improved performance on reversal training, and increased number of new neurons in the ventral hippocampus. Estradiol decreased activation of new neurons in the dorsal hippocampus, regardless of parity history. Estradiol also decreased the production of anti-inflammatory cytokines based on age of first pregnancy. This work suggests that estradiol affects neuroplasticity and neuroinflammation in middle age, and that age of first pregnancy can have long lasting effects on hippocampus structure and function.

Are there sex differences in spatial reference memory in the Morris water maze? A large-sample experimental study

Sex differences have been found in allocentric spatial learning and memory tasks, with the literature indicating that males outperform females, although this issue is still controversial. This study aimed to explore the behavior of male and female rats during the habituation and learning of a spatial memory task performed in the Morris Water Maze (MWM). The study included a large sample of 89 males and 85 females. We found that females searched slightly faster than males during habituation with a visible platform. During learning, both male and female rats decreased the latency and distance traveled to find the hidden platform over the days, with males outperforming females in the distance traveled. Females swam faster but did not find the platform earlier, suggesting a less directed navigational strategy. Both sexes increased time spent in the target zone over the days, with no sex differences. Although females swam more in the periphery during the first days of the task, both sexes decreased the time spent in this area. Finally, only males increased swimming in the pool's center over the days, spending more time than females in this area across the entire training. In conclusion, we need to register several variables in the MWM and analyze path strategies to obtain more robust results concerning sex differences. Research on spatial learning should include both sexes to achieve a more equitable, representative, and translational science.

Histone deacetylase inhibitor RG2833 has therapeutic potential for Alzheimer's disease in females

Nearly two-thirds of patients with Alzheimer's are women. Identifying therapeutics specific for women is critical to lowering their elevated risk for developing this major cause of adult dementia. Moreover, targeting epigenetic processes that regulate multiple cellular pathways is advantageous given Alzheimer's multifactorial nature. Histone acetylation is an epigenetic process heavily involved in memory consolidation. Its disruption is linked to Alzheimer's. Through our computational studies, we predicted that the investigational drug RG2833 (N-[6-(2-aminoanilino)-6-oxohexyl]-4-methylbenzamide) has repurposing potential for Alzheimer's. RG2833 is a histone deacetylase HDAC1/3 inhibitor that is orally bioavailable and permeates the blood-brain-barrier. We investigated the RG2833 therapeutic potential in TgF344-AD rats, which are a model of Alzheimer's that exhibits age-dependent progression, thus mimicking this aspect of Alzheimer's patients that is difficult to establish in animal models. We investigated the RG2833 effects on cognitive performance, gene expression, and AD-like pathology in 11-month TgF344-AD female and male rats. A total of 89 rats were used: wild type n = 45 (17 females, 28 males), and TgF344-AD n = 44 (24 females, 20 males)] across multiple cohorts. No obvious toxicity was detected in the TgF344-AD rats up to 6 months of RG2833-treatment starting at 5 months of age administering the drug in rodent chow at ∼30mg/kg of body weight. We started treatment early in the course of pathology when therapeutic intervention is predicted to be more effective than in later stages of the disease. The drug-treatment significantly mitigated hippocampal-dependent spatial memory deficits in 11-month TgF344-AD females but not in males, compared to wild type littermates. This female sex-specific drug effect has not been previously reported. RG2833-treatment failed to ameliorate amyloid beta accumulation and microgliosis in female and male TgF344-AD rats. However, RNAseq analysis of hippocampal tissue from TgF344-AD rats showed that drug-treatment in females upregulated the expression of immediate early genes, such as Arc, Egr1 and c-Fos, and other genes involved in synaptic plasticity and memory consolidation. Remarkably, out of 17,168 genes analyzed for each sex, no significant changes in gene expression were detected in males at P < 0.05, false discovery rate < 0.05, and fold-change ≥ 1.5. Our data suggest that histone modifying therapeutics such as RG2833 improve cognitive behavior by modulating the expression of immediate early, neuroprotective and synaptic plasticity genes. Our preclinical study supports that RG2833 has therapeutic potential specifically for female Alzheimer's patients. RG2833 evaluations using other AD-related models is necessary to confirm our findings.

The mixed legacy of the rat estrous cycle

BACKGROUND

The rat estrous cycle first characterized by Long and Evans in 1922 profoundly affected the course of endocrine research. Investigators took advantage of sex steroid hormone fluctuations associated with the cycle to assess hormonal influences on anxiety, depression, food intake, stress, brain structure and other traits. Similarities of the rat estrous and human menstrual cycles facilitated understanding of human reproductive physiology. I assessed the impact of awareness of the estrous cycle on the emergence of a sex bias that excluded female rats from biomedical research.

METHODS

Beginning with the 1918 volume of the American Journal of Physiology and ending in 1976 when the journal subdivided into several separate disciplinary journals, all studies conducted on rats were downloaded; the use of females, males, both sexes and sex left unspecified was tabulated for 485 articles. A second analysis tracked the number of rat estrous cycle studies across all disciplines listed in PubMed from 1950 to 2021.

RESULTS

The description and awareness of variability associated with the rat estrous cycle was correlated with a precipitous decline in investigations that incorporated both sexes, a marked increase in male-only studies and a striking sex bias that excluded female rats. The number of rat estrous cycles studies increased markedly from earlier decades to a peak in 2021.

CONCLUSIONS

The initial description the rat estrous cycle was correlated with a substantial decline in investigations that incorporated both sexes; one result was a marked increase in male-only studies and a striking sex bias that excluded female rats from biomedical research. Recognition of the advantages of studies that incorporate the rat estrous cycle has resulted in recent years in an increase of such investigations. Female rats and females of several other species are not more variable than their male counterparts across traits, arguing for female inclusion without requiring cycle monitoring. There, remain, however, many advantages of incorporating the estrous cycle in contemporary research.

High estradiol reduces adult neurogenesis but strengthens functional connectivity within the hippocampus during spatial pattern separation in adult female rats

Adult neurogenesis in the dentate gyrus plays an important role for pattern separation, the process of separating similar inputs and forming distinct neural representations. Estradiol modulates neurogenesis and hippocampus function, but to date no examination of estradiol's effects on pattern separation have been conducted. Here, we examined estrogenic regulation of adult neurogenesis and functional connectivity in the hippocampus after the spatial pattern separation task in female rats. Ovariectomized Sprague-Dawley rats received daily injections of vehicle, 0.32 μg (Low) or 5 μg (High) of estradiol benzoate until the end of experiment. A single bromodeoxyuridine (BrdU) was injected one day after initiation of hormone or vehicle treatment and rats were tested in the delayed nonmatching to position spatial pattern separation task in the 8-arm radial maze for 12 days beginning two weeks after BrdU injection. Rats were perfused 90 min after the final trial and brain sections were immunohistochemically stained for BrdU/neuronal nuclei (NeuN) (new neurons), Ki67 (cell proliferation), and the immediate early gene, zif268 (activation). Results showed that high, but not low, estradiol reduced the density of BrdU/NeuN-ir cells and had significant inter-regional correlations of zif268-ir cell density in the hippocampus following pattern separation. Estradiol treatment did not influence pattern separation performance or strategy use. These results show that higher doses of estradiol can reduce neurogenesis but at the same time increases correlations of activity of neurons within the hippocampus during spatial pattern separation.

New perspectives on sex differences in learning and memory

Females have historically been disregarded in memory research, including the thousands of studies examining roles for the hippocampus, medial prefrontal cortex, and amygdala in learning and memory. Even when included, females are often judged based on male-centric behavioral and neurobiological standards, generating and perpetuating scientific stereotypes that females exhibit worse memories compared with males in domains such as spatial navigation and fear. Recent research challenges these dogmas by identifying sex-specific strategies in common memory tasks. Here, we discuss rodent data illustrating sex differences in spatial and fear memory, as well as the neural mechanisms underlying memory formation. The influence of sex steroid hormones in both sexes is discussed, as is the importance to basic and translational neuroscience of studying sex differences.

Estradiol Receptors Inhibit Long-Term Potentiation in the Dorsomedial Striatum

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

Learning and metabolic brain differences between juvenile male and female rats in the execution of different training regimes of a spatial memory task

Spatial memory is responsible for encoding spatial information to form a path, storing this mental representation, and evaluating and recovering spatial configurations to find a target location in the environment. It is mainly supported by the hippocampus and its interaction with other structures, such as the prefrontal cortex, and emerges in rodents around postnatal day (PND) 20. Sex differences in spatial tasks have been found in adults, with a supposedly better performance in males. However, few studies have examined sex differences in orientation throughout postnatal development. This study aimed to analyse the performance of juvenile (PND 23) male (n=18) and female (n=21) Wistar rats in a spatial reference memory task in the Morris water maze (MWM) with two different training regimes in the acquisition phase, and their subjacent metabolic brain activity. Based on sex, subjects were assigned to two different groups: one that performed four learning trials per day (n=9 males and n=8 females) and the other that was submitted to two trials per day (n=9 males and n=13 females). After the behavioural protocols, metabolic activity was evaluated using cytochrome c oxidase histochemistry. Results showed no metabolic brain or behavioural differences in the four-trial protocol performance, in which both sexes reached the learning criterion on the fourth day. By contrast, the two-trial protocol revealed an advantage for females, who reached the learning criterion on day four, whereas males needed more training and succeeded on day six. The female group showed lower metabolic activity than the male group in the cingulate and prelimbic cortex. These results suggest a faster consolidation process in the female group than the male group. Further research is needed to understand sex differences in spatial memory at early stages.

Sex differences in contextual pattern separation, neurogenesis, and functional connectivity within the limbic system

Background

Females are more likely to present with anxiety disorders such as post-traumatic stress disorder (PTSD) compared to males, which are associated with disrupted hippocampal integrity. Sex differences in the structure and function of hippocampus exist. Here, we examined sex differences in contextual pattern separation, functional connectivity, and activation of new neurons during fear memory.

Methods

Two-month-old male and female Sprague-Dawley rats were injected with the DNA synthesis markers, iododeoxyuridine (IdU) and chlorodeoxyuridine (CldU) 3 weeks and 4 weeks before perfusion, respectively. One week after CldU injection, the rats underwent a context discrimination task in which rats were placed in context A (shock) and context A’ (no shock) every day for 12 days. On the test day, rats were placed in the shock context (context A) to measure fear memory and expression of zif268, an immediate early gene across 16 different limbic and reward regions. Repeated-measures or factorial analysis of variance was conducted on our variables of interest. Pearson product-moment calculations and principal component analyses on zif268 expression across regions were also performed.

Results

We found that females, but not males, showed contextual discrimination during the last days of training. On the test day, both sexes displayed similar levels of freezing, indicating equivalent fear memory for context A. Despite similar fear memory, males showed more positive correlations of zif268 activation between the limbic regions and the striatum, whereas females showed more negative correlations among these regions. Females showed greater activation of the frontal cortex, dorsal CA1, and 3-week-old adult-born dentate granular cells compared to males.

Conclusions

These results highlight the importance of studying sex differences in fear memory and the contribution of adult neurogenesis to the neuronal network and may contribute to differences in susceptibility to fear-related disorders such as post-traumatic stress disorder.

Highlights

Female rats, but not male rats, show faster discrimination during a contextual pattern separation task.

Three-week-old adult-born neurons are more active in response to fear memory in females compared to males.

Females had greater neural activation compared to males in the frontal cortex and dorsal CA1 region of the hippocampus in response to fear memory.

Males and females show distinct patterns in functional connectivity for fear memory across limbic regions.

Males have many positive correlations between activated new neurons of different ages between the dorsal and ventral hippocampus, while females show more correlations between activated new neurons and other limbic regions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13293-022-00450-2.

The removal and addition of cues does not impair spatial retrieval and leads to a different metabolic activity of the limbic network in female rats

The retrieval of spatial memories does not always occur in an environment with the same stimuli configuration where the memory was first formed. However, re-exposure to a partial portion of the previously encountered cues can elicit memory successfully. Navigation with contextual changes has received little attention, especially in females. Thus, we aimed to assess memory retrieval using the Morris Water Maze spatial reference protocol in female adult Wistar rats. Rats were trained with five allocentric cues, and retrieval was explored one week later either with the same cues, or with four removed, or with three added cues. We studied the underlying brain oxidative metabolism of the hippocampus, prefrontal, parietal, retrosplenial, entorhinal, and perirhinal cortices through cytochrome c oxidase (CCO) histochemistry. Neither cue removal nor cue addition impaired retrieval performance. Retrieval with a degraded subset of cues led to increased prefrontal, hippocampal, retrosplenial, parietal, and perirhinal CCO activity. Retrieval with extra cues led to an enhancement of CCO activity in the hippocampus and retrosplenial cortex. Different patterns of network intercorrelations were found. The cue-removal group presented a closed reciprocal network, while the group with extra cues had separate parallel networks. Both groups showed a simpler network than the group with no cue modifications. Future research is needed to delve into behavioral and brain-related functions of spatial memory processes under modified environmental conditions.

Changes in sex differences in neuroanatomical structure and cognitive behavior across the life span

Sex differences occur in the structure and function of the rat cerebral cortex and hippocampus, which can change from the juvenile period through old age. Although the evidence is incomplete, it appears that in at least some portions of the cortex these differences develop due to the rise of ovarian hormones at puberty and are potentially not dependent on the perinatal rise in testosterone, which is essential for sexual differentiation of the hypothalamus and sexual behavior. During aging of female rats, the presence of continued ovarian hormone secretion after cessation of the estrous cycle also influences sex differences in neuroanatomical structure and cognitive behavior, resulting in nullification or reversal of sex differences seen in younger adults. Sex differences can be altered by experience in a stimulating environment during the juvenile/adolescent period, and sex differences in performance even can be affected by the parameters of a task. Thus, broad generalizations about differences such as "spatial ability" are to be avoided. It is clear that to understand how the brain produces behavior, sex and hormones have to be taken into account.

Choosing memory retrieval strategies: A critical role for inhibition in the dentate gyrus

Remembering the location of food is essential for survival. Rodents and humans employ mainly hippocampus-dependent spatial strategies, but when being stressed they shift to striatum-mediated stimulus-based strategies. To investigate underlying brain circuits, we tested mice with a heightened stress susceptibility due to a lack of the GABA-synthetizing enzyme GAD65 (GAD65−/− mice) in a dual solution task. Here, GAD65−/− mice preferred to locate a food reward in an open field via a proximal cue, while their wildtype littermates preferred a spatial strategy. The analysis of cFos co-activation across brain regions and of stress-induced mRNA expression changes of GAD65 pointed towards the hippocampal dorsal dentate gyrus (dDG) as a central structure for mediating stress effects on strategy choices via GAD65. Reducing the GAD65 expression locally in the dDG by a shRNA mediated knock down was sufficient to replicate the phenotype of the global GAD65 knock out and to increase dDG excitability. Using DREADD vectors to specifically interfere with dDG circuit activity during dual solution retrieval but not learning confirmed that the dDG modulates strategy choices and that a balanced excitability of this structure is necessary to establish spatial strategy preference. These data highlight the dDG as a critical hub for choosing between spatial and non-spatial foraging strategies.

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Stress reduces spatial memory preferences for locating rewards in an open field.

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GAD65 deficient mice show reduced preferences for spatial memory strategy.

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Dorsal dentate gyrus knock down of GAD65 is sufficient to reduce spatial strategies.

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Excitability in the dorsal dentate gyrus modulates retrieval strategy choices.

Sex Differences in Cognition Across Aging

Sex and gender differences are seen in cognitive disturbances in a variety of neurological and psychiatry diseases. Men are more likely to have cognitive symptoms in schizophrenia whereas women are more likely to have more severe cognitive symptoms with major depressive disorder and Alzheimer's disease. Thus, it is important to understand sex and gender differences in underlying cognitive abilities with and without disease. Sex differences are noted in performance across various cognitive domains - with males typically outperforming females in spatial tasks and females typically outperforming males in verbal tasks. Furthermore, there are striking sex differences in brain networks that are activated during cognitive tasks and in learning strategies. Although rarely studied, there are also sex differences in the trajectory of cognitive aging. It is important to pay attention to these sex differences as they inform researchers of potential differences in resilience to age-related cognitive decline and underlying mechanisms for both healthy and pathological cognitive aging, depending on sex. We review literature on the progressive neurodegenerative disorder, Alzheimer's disease, as an example of pathological cognitive aging in which human females show greater lifetime risk, neuropathology, and cognitive impairment, compared to human males. Not surprisingly, the relationships between sex and cognition, cognitive aging, and Alzheimer's disease are nuanced and multifaceted. As such, this chapter will end with a discussion of lifestyle factors, like education and diet, as modifiable factors that can alter cognitive aging by sex. Understanding how cognition changes across age and contributing factors, like sex differences, will be essential to improving care for older adults.

Age-related cognitive decline in rats is sex and context dependent

Previously, we had observed age-related cognitive decline in male rats compared to adolescent and adult rats. This was shown in both a multi-branched maze test (MBM), as well as in the Morris water maze test (MWM). In the present study, we compared the behavior of similar age groups in both male and female rats using the same paradigms. The results confirmed the increase in errors and time spent in MBM in aged male rats compared to other age groups. However, no such differences were observed in female rats. In the acquisition phase of MWM, aged male rats did not differ significantly from the other two groups in terms of time spent in quadrants, whereas aged female rats spent significantly more time in quadrants compared to the other 2 age groups. Aged male rats also travelled significantly more than the other 2 age groups during the acquisition phase, whereas no such differences were observed in female rats. In both short term (30 min post acquisition) and long term (24 h after acquisition) retrieval phases of MWM, significant gender-related differences were also observed in all age groups. These findings suggest gender- and context-dependent alterations in cognitive functions during aging.

Mate Choice, Sex Roles and Sexual Cognition: Neuronal Prerequisites Supporting Cognitive Mate Choice

Across taxa, mate choice is a highly selective process involving both intra- and intersexual selection processes aiming to pass on one’s genes, making mate choice a pivotal tool of sexual selection. Individuals adapt mate choice behavior dynamically in response to environmental and social changes. These changes are perceived sensorily and integrated on a neuronal level, which ultimately leads to an adequate behavioral response. Along with perception and prior to an appropriate behavioral response, the choosing sex has (1) to recognize and discriminate between the prospective mates and (2) to be able to assess and compare their performance in order to make an informed decision. To do so, cognitive processes allow for the simultaneous processing of multiple information from the (in-) animate environment as well as from a variety of both sexual and social (but non-sexual) conspecific cues. Although many behavioral aspects of cognition on one side and of mate choice displays on the other are well understood, the interplay of neuronal mechanisms governing both determinants, i.e., governing cognitive mate choice have been described only vaguely. This review aimed to throw a spotlight on neuronal prerequisites, networks and processes supporting the interaction between mate choice, sex roles and sexual cognition, hence, supporting cognitive mate choice. How does neuronal activity differ between males and females regarding social cognition? Does sex or the respective sex role within the prevailing mating system mirror at a neuronal level? How does cognitive competence affect mate choice? Conversely, how does mate choice affect the cognitive abilities of both sexes? Benefitting from studies using different neuroanatomical techniques such as neuronal activity markers, differential coexpression or candidate gene analyses, modulatory effects of neurotransmitters and hormones, or imaging techniques such as fMRI, there is ample evidence pointing to a reflection of sex and the respective sex role at the neuronal level, at least in individual brain regions. Moreover, this review aims to summarize evidence for cognitive abilities influencing mate choice and vice versa. At the same time, new questions arise centering the complex relationship between neurobiology, cognition and mate choice, which we will perhaps be able to answer with new experimental techniques.

Sex-Independent Cognition Improvement in Response to Kaempferol in the Model of Sporadic Alzheimer's Disease

Alzheimer's disease (AD) is associated with neural oxidative stress and inflammation, and it is assumed to affect more women than men with unknown mechanisms. Kaempferol (KMP) as a potent natural antioxidant has been known to exhibit various biological and pharmacological functions, including antioxidant and anti-inflammatory. We aimed here to evaluate the role of gender difference in response to KMP on the rat model of sporadic AD. Forty-six female and male Wistar rats were divided into six groups of sham, streptozotocin (STZ) + saline (SAL), STZ + KMP. Female rats were ovariectomized, and then all animals received an intracerebroventricular bilateral injection of STZ (3 mg/kg) to induce the AD model. KMP (10 mg/kg) was intraperitoneally administered for 21 consecutive days. Afterward, spatial learning and memory were assessed via the Morris water maze task (MWM). Finally, the hippocampus level of superoxide dismutase (SOD), glutathione, and malondialdehyde were measured using calorimetric kits. Data showed a significant cognition deficit in STZ + SAL compared with the sham. To sum up, we reported that chronic KMP treatment increase significantly improved acquisition and retrieval of spatial memory as evident by longer TTS (total time spent) and short-latency to the platform in MWM. In addition, KMP increased the levels of SOD and glutathione in the hippocampus of rats. Also, KMP decreased hippocampal levels of malondialdehyde in both genders. In conclusion, KMP successfully restores spatial memory impairment independent of gender difference. This memory restoration may at least in part be mediated through boosting the hippocampal level of SOD and glutathione.

The unique plasticity of hippocampal adult-born neurons: Contributing to a heterogeneous dentate

The dentate gyrus (DG) of the hippocampus is evolutionarily conserved as one of the few sites of adult neurogenesis in mammals. Although there is clear evidence that neurogenesis is necessary for healthy hippocampal function, whether adult-born neurons are simply integrated into existing hippocampal networks to serve a similar purpose to that of developmentally born neurons or whether they represent a discrete cell population with unique functions remains less clear. In this review, we consider evidence for discrete cellular, synaptic, and structural features of adult-born DG neurons, suggesting that neurogenesis contributes to the formation of a heterogeneous DG. We therefore propose that hippocampal neurogenesis creates a specialized neuronal subpopulation that may play a key role in hippocampal functions like episodic memory. We note critical gaps in this extensive body of work, including a general failure to include female animals in relevant research and a need for more precise consideration of intrahippocampal neuroanatomy.

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.

Sex Differences in Maturation and Attrition of Adult Neurogenesis in the Hippocampus

Sex differences exist in the regulation of adult neurogenesis in the hippocampus in response to hormones and cognitive training. Here, we investigated the trajectory and maturation rate of adult-born neurons in the dentate gyrus (DG) of male and female rats. Sprague Dawley rats were perfused 2 h, 24 h, one week (1w), 2w, or 3w after bromodeoxyuridine (BrdU) injection, a DNA synthesis marker that labels dividing progenitor cells and their progeny. Adult-born neurons (BrdU/NeuN-ir) matured faster in males compared with females. Males had a greater density of neural stem cells (Sox2-ir) in the dorsal, but not in the ventral, DG and had higher levels of cell proliferation (Ki67-ir) than non-proestrous females. However, males showed a greater reduction in neurogenesis between 1week and 2weeks after mitosis, whereas females showed similar levels of neurogenesis throughout the weeks. The faster maturation and greater attrition of new neurons in males compared with females suggests greater potential for neurogenesis to respond to external stimuli in males and emphasizes the importance of studying sex on adult hippocampal neurogenesis.

Transcriptional and Translational Differences of Microglia from Male and Female Brains

Sex differences in brain structure and function are of substantial scientific interest because of sex-related susceptibility to psychiatric and neurological disorders. Neuroinflammation is a common denominator of many of these diseases, and thus microglia, as the brain's immunocompetent cells, have come into focus in sex-specific studies. Here, we show differences in the structure, function, and transcriptomic and proteomic profiles in microglia freshly isolated from male and female mouse brains. We show that male microglia are more frequent in specific brain areas, have a higher antigen-presenting capacity, and appear to have a higher potential to respond to stimuli such as ATP, reflected in higher baseline outward and inward currents and higher protein expression of purinergic receptors. Altogether, we provide a comprehensive resource to generate and validate hypotheses regarding brain sex differences.

Neurobiological mechanisms underlying sex-related differences in stress-related disorders: Effects of neuroactive steroids on the hippocampus

Men and women differ in their vulnerability to a variety of stress-related illnesses, but the underlying neurobiological mechanisms are not well understood. This is likely due to a comparative dearth of neurobiological studies that assess male and female rodents at the same time, while human neuroimaging studies often don't model sex as a variable of interest. These sex differences are often attributed to the actions of sex hormones, i.e. estrogens, progestogens and androgens. In this review, we summarize the results on sex hormone actions in the hippocampus and seek to bridge the gap between animal models and findings in humans. However, while effects of sex hormones on the hippocampus are largely consistent in animals and humans, methodological differences challenge the comparability of animal and human studies on stress effects. We summarise our current understanding of the neurobiological mechanisms that underlie sex-related differences in behavior and discuss implications for stress-related illnesses.

Inflammation within the neurovascular unit: Focus on microglia for stroke injury and recovery

Neuroinflammation underlies the etiology of multiple neurodegenerative diseases and stroke. Our understanding of neuroinflammation has evolved in the last few years and major players have been identified. Microglia, the brain resident macrophages, are considered sentinels at the forefront of the neuroinflammatory response to different brain insults. Interestingly, microglia perform other physiological functions in addition to their role in neuroinflammation. Therefore, an updated approach in which modulation, rather than complete elimination of microglia is necessary. In this review, the emerging roles of microglia and their interaction with different components of the neurovascular unit are discussed. In addition, recent data on sex differences in microglial physiology and in the context of stroke will be presented. Finally, the multiplicity of roles assumed by microglia in the pathophysiology of ischemic stroke, and in the presence of co-morbidities such as hypertension and diabetes are summarized.

Optimizing brain performance: Identifying mechanisms of adaptive neurobiological plasticity

Although neuroscience research has debunked the late 19^th century claims suggesting that large portions of the brain are typically unused, recent evidence indicates that an enhanced understanding of neural plasticity may lead to greater insights related to the functional capacity of brains. Continuous and real-time neural modifications in concert with dynamic environmental contexts provide opportunities for targeted interventions for maintaining healthy brain functions throughout the lifespan. Neural design, however, is far from simplistic, requiring close consideration of context-specific and other relevant variables from both species and individual perspectives to determine the functional gains from increased and decreased markers of neuroplasticity. Caution must be taken in the interpretation of any measurable change in neurobiological responses or behavioral outcomes, as definitions of optimal functions are extremely complex. Even so, current behavioral neuroscience approaches offer unique opportunities to evaluate adaptive functions of various neural responses in an attempt to enhance the functional capacity of neural systems.

Estrous Cycle Monitoring in Mice with Rapid Data Visualization and Analysis

The estrous cycle provides a readout of reproductive health in female laboratory rodents, and estrous cycle stage can be an important physiological variable. Accurate assessment of estrous cycle stage is also important in producing timed pregnancies for developmental studies. Here, we provide a protocol for evaluation of estrous cycle stage through a minimally invasive procedure of acquiring cells lining the vaginal cavity and immediate microscopic visual assessment of these cells without drying or staining. When performed over several consecutive days, the pattern of progression through the four main stages of the estrous cycle, and disruptions to this pattern, can be determined. We also present software that enables more efficient cycle stage data analysis and pattern visualization. These protocols and tools will thus facilitate the incorporation of female animals in laboratory experiments and enhance the assessment of relationships between the reproductive cycle and overall physiology and behavior.

Sex differences in hippocampal cognition and neurogenesis

Sex differences are reported in hippocampal plasticity, cognition, and in a number of disorders that target the integrity of the hippocampus. For example, meta-analyses reveal that males outperform females on hippocampus-dependent tasks in rodents and in humans, furthermore women are more likely to experience greater cognitive decline in Alzheimer's disease and depression, both diseases characterized by hippocampal dysfunction. The hippocampus is a highly plastic structure, important for processing higher order information and is sensitive to the environmental factors such as stress. The structure retains the ability to produce new neurons and this process plays an important role in pattern separation, proactive interference, and cognitive flexibility. Intriguingly, there are prominent sex differences in the level of neurogenesis and the activation of new neurons in response to hippocampus-dependent cognitive tasks in rodents. However, sex differences in spatial performance can be nuanced as animal studies have demonstrated that there are task, and strategy choice dependent sex differences in performance, as well as sex differences in the subregions of the hippocampus influenced by learning. This review discusses sex differences in pattern separation, pattern completion, spatial learning, and links between adult neurogenesis and these cognitive functions of the hippocampus. We emphasize the importance of including both sexes when studying genomic, cellular, and structural mechanisms of the hippocampal function.

Retrieval of Inhibitory Avoidance Memory Induces Differential Transcription of arc in Striatum, Hippocampus, and Amygdala

Similar to the hippocampus and amygdala, the dorsal striatum is involved in memory retrieval of inhibitory avoidance, a task commonly used to study memory processes. It has been reported that memory retrieval of fear conditioning regulates gene expression of arc and zif268 in the amygdala and the hippocampus, and it is surprising that only limited effort has been made to study the molecular events caused by retrieval in the striatum. To further explore the involvement of immediate early genes in retrieval, we used real-time PCR to analyze arc and zif268 transcription in dorsal striatum, dorsal hippocampus, and amygdala at different time intervals after retrieval of step-through inhibitory avoidance memory. We found that arc expression in the striatum increased 30 min after retrieval while no changes were observed in zif268 in this region. Expression of arc and zif268 also increased in the dorsal hippocampus but the changes were attributed to context re-exposure. Control procedures indicated that in the amygdala, arc and zif268 expression was not dependent on retrieval. Our data indicate that memory retrieval of inhibitory avoidance induces arc gene expression in the dorsal striatum, caused, very likely, by the instrumental component of the task. Striatal arc expression after retrieval may induce structural and functional changes in the neurons involved in this process.

Sex differences in the brain: Implications for behavioral and biomedical research

Biological differences between males and females are found at multiple levels. However, females have too often been under-represented in behavioral neuroscience research, which has stymied the study of potential sex differences in neurobiology and behavior. This review focuses on the study of sex differences in the neurobiology of social behavior, memory, emotions, and recovery from brain injury, with particular emphasis on the role of estrogens in regulating forebrain function. This work, presented by the authors at the 2016 meeting of the International Behavioral Neuroscience Society, emphasizes varying approaches from several mammalian species in which sex differences have not only been documented, but also become the focus of efforts to understand the mechanistic basis underlying them. This information may provide readers with useful experimental tools to successfully address recently introduced regulations by granting agencies that either require (e.g. the National Institutes of Health in the United States and the Canadian Institutes of Health Research in Canada) or recommend (e.g. Horizon 2020 in Europe) the inclusion of both sexes in biomedical research.