Hippocampal testosterone relates to reference memory performance and synaptic plasticity in male rats

Changes in neuroactive steroids, neurotrophins and immunological biomarkers after monotherapy 8-week rTMS treatment and their relationship with neurocognitive functions in depression

Repetitive transcranial magnetic stimulation (rTMS) has proven effective in the treatment of major depression. The underlying mechanisms of action are still poorly understood. We aimed to evaluate the changes in the levels of neuroactive steroids, neurotrophins and immunological biomarkers before and after rTMS treatment and assess the relationship of this change between clinical response and cognitive functions after monotherapy rTMS treatment. Twenty-three patients with major depressive disorder (MDD) and 25 matched healthy controls were included in the study. The Hamilton Depression Rating Scale (HDRS), Trail Making Test A and B forms and Digit Span Test were administered. Biomarkers (BDNF, TNF-α, IL-1ß, NAS) were run in the peripheral blood at the end of the first month that rTMS was administered daily and at the end of the 2nd month when that rTMS was administered once a week. Appropriate conditions were provided so that the relevant biomarkers were not affected by the biorhythm. After rTMS monotherapy, an increase in BDNF and allopregnanolone, a decrease in TNF-α, IL-1ß, DHEA, and DHEA-S levels was found to be statistically significant. The scores on cognitive tests increased with the treatment. Positive significant correlations was found between BDNF levels and cognitive tests at the end of the first and second months. Our findings suggest that the effects of rTMS treatment may be related to the neuroendocrine, neurotrophin, and immunological mechanisms. rTMS treatment is found to have positive effects on cognitive functions in the short term.

Sex- and cycle-dependent changes in spine density and size in hippocampal CA2 neurons

Sex hormones affect structural and functional plasticity in the rodent hippocampus. However, hormone levels not only differ between males and females, but also fluctuate across the female estrous cycle. While sex- and cycle-dependent differences in dendritic spine density and morphology have been found in the rodent CA1 region, but not in the CA3 or the dentate gyrus, comparable structural data on CA2, i.e. the hippocampal region involved in social recognition memory, is so far lacking. In this study, we, therefore, used wildtype male and female mice in diestrus or proestrus to analyze spines on dendritic segments from identified CA2 neurons. In basal stratum oriens, we found no differences in spine density, but a significant shift towards larger spine head areas in male mice compared to females. Conversely, in apical stratum radiatum diestrus females had a significantly higher spine density, and females in either cycle stage had a significant shift towards larger spine head areas as compared to males, with diestrus females showing the larger shift. Our results provide further evidence for the sexual dimorphism of hippocampal area CA2, and underscore the importance of considering not only the sex, but also the stage of the estrous cycle when interpreting morphological data.

Contrasting parental roles shape sex differences in poison frog space use but not navigational performance

Sex differences in vertebrate spatial abilities are typically interpreted under the adaptive specialization hypothesis, which posits that male reproductive success is linked to larger home ranges and better navigational skills. The androgen spillover hypothesis counters that enhanced male spatial performance may be a byproduct of higher androgen levels. Animal groups that include species where females are expected to outperform males based on life-history traits are key for disentangling these hypotheses. We investigated the association between sex differences in reproductive strategies, spatial behavior, and androgen levels in three species of poison frogs. We tracked individuals in natural environments to show that contrasting parental sex roles shape sex differences in space use, where the sex performing parental duties shows wider-ranging movements. We then translocated frogs from their home areas to test their navigational performance and found that the caring sex outperformed the non-caring sex only in one out of three species. In addition, males across species displayed more explorative behavior than females and androgen levels correlated with explorative behavior and homing accuracy. Overall, we reveal that poison frog reproductive strategies shape movement patterns but not necessarily navigational performance. Together this work suggests that prevailing adaptive hypotheses provide an incomplete explanation of sex differences in spatial abilities.

Aging, testosterone, and neuroplasticity: friend or foe?

Neuroplasticity or neural plasticity implicates the adaptive potential of the brain in response to extrinsic and intrinsic stimuli. The concept has been utilized in different contexts such as injury and neurological disease. Neuroplasticity mechanisms have been classified into neuroregenerative and function-restoring processes. In the context of injury, neuroplasticity has been defined in three post-injury epochs. Testosterone plays a key yet double-edged role in the regulation of several neuroplasticity alterations. Research has shown that testosterone levels are affected by numerous factors such as age, stress, surgical procedures on gonads, and pharmacological treatments. There is an ongoing debate for testosterone replacement therapy (TRT) in aging men; however, TRT is more useful in young individuals with testosterone deficit and more specific subgroups with cognitive dysfunction. Therefore, it is important to pay early attention to testosterone profile and precisely uncover its harms and benefits. In the present review, we discuss the influence of environmental factors, aging, and gender on testosterone-associated alterations in neuroplasticity, as well as the two-sided actions of testosterone in the nervous system. Finally, we provide practical insights for further study of pharmacological treatments for hormonal disorders focusing on restoring neuroplasticity.

Nk3R blockade has sex-divergent effects on memory in mice

Background

Memory consolidation is a process required for the formation of long-term memories. The G-protein-coupled receptor (GPCR) neurokinin-3-receptor (Nk3R) and its interactions with sex hormones seem important for the modulation of fear memory consolidation: Nk3R antagonism in male mice impairs fear memory, but enhances it in females. However, the involvement of the Nk3R as a modulator of other memories in both sexes remains unexplored.

Methods

We use the novel object recognition paradigm to test the effect of a systemic blockade of Nk3R during memory consolidation. Further, we assess the expression of estrogen receptor α, estrogen receptor β, and androgen receptor and heterodimerization with Nk3R in the medial prefrontal cortex (mPFC) and dorsal hippocampus (DH) of mice.

Results

Nk3R systemic antagonism elicited decreased memory consolidation in males while it enhanced it in females during proestrus. Nk3R analysis in the different subregions of the mPFC and the DH showed a higher expression in males than females. Moreover, females presented upregulation of the androgen receptor in the CA1 and the estrogen receptor beta in the cingulate cortex, CA1, and dentate gyrus. Overall, males presented an upregulation of the estrogen receptor alpha. We also explored the heterodimerization of GCPR membrane sex hormone receptors with the Nk3R. We found a higher percentage of Nk3R-membrane G-protein estrogen receptors heterodimers in the prelimbic cortex of the mPFC in females, suggesting an interaction of estradiol with Nk3R in memory consolidation. However, males presented a higher percentage of Nk3R-membrane G-protein androgen receptors heterodimers compared to females, pointing to an interaction of testosterone with Nk3R in memory consolidation.

Conclusion

These data propose novel ideas on functional interactions between Nk3R, sex hormones, estrogen receptors, and androgen receptors in memory consolidation.

Nk3R antagonism reduces recognition memory consolidation in male mice and increases it in proestrus females.

Androgen receptor expression is higher in the CA1 compared to DG, CA3, and the mPFC.

Estrogen repcetor α expression is higher in males than in females in the DH and mPFC.

Estrogen receptor β expression is greater in females than in males in the DG, CA1, and CG.

Over 60% of Nk3R in the DH and mPFC is heterodimerized with membrane estrogen receptor and androgen receptor.

Nk3R–GPAR is more abundant in males than in proestrus females, whereas Nk3R–GPER is greater in proestrus females compared to males.

Influence of Testosterone depletion on Neurotrophin-4 in Hippocampal synaptic plasticity and its effects on learning and memory

Sex steroids are neuromodulators that play a crucial role in learning, memory, and synaptic plasticity, providing circuit flexibility and dynamic functional connectivity in mammals. Previous studies indicate that testosterone is crucial for neuronal functions and required further investigation on various frontiers. However, it is surprising to note that studies on testosterone-induced NT-4 expression and its influence on synaptic plasticity and learning and memory moderation are scanty. The present study is focused on analyzing the localized influence of neurotrophin-4 (NT4) on hippocampal synaptic plasticity and associated moderation in learning and memory under testosterone deprivation. Adult Wistar albino rats were randomly divided into various groups, control (Cont), orchidectomy (ORX), orchidectomy + testosterone supplementation (ORX+T) and control + testosterone (Cont+T). After two weeks, the serum testosterone level was undetectable in ORX rats. The behavioural assessment showed a decline in the learning ability of ORX rats with increased working and reference memory errors in the behavioural assessment in the 8-arm radial maze. The mRNA and protein expressions of NT-4 and androgen receptors were significantly reduced in the ORX group. In addition, there was a decrease in the number of neuronal dendrites in Golgi-Cox staining. These changes were not seen in ORX+T rats with improved learning behaviour. Indicating that testosterone exerts its protective effect on hippocampal synaptic plasticity through androgen receptor-dependent neurotrophin-4 regulation in learning and memory upgrade.

Different Anesthetic Drugs Mediate Changes in Neuroplasticity During Cognitive Impairment in Sleep-Deprived Rats via Different Factors

Background

Perioperative neuro-cognitive disorders (PND) are preoperative and postoperative complications of multiple nervous systems, typically manifested as decreased memory and learning ability after surgery. It was used to replace the original definition of postoperative cognitive dysfunctions (POCD) from 2018. Our previous studies have shown that sevoflurane inhalation can lead to cognitive dysfunction in Sprague-Dawley rats, but the specific mechanism is still unclear.

Material/Methods

Thirty-six male Sprague-Dawley rats were randomly divided into 6 groups (n=6): the SD group was given 24-h acute sleep deprivation; Sevoflurane was inhaled for 2 h in the Sevo group. Two mL propofol was injected into the tail vein of rats in the Prop group. The rats in the SD+Sevo group and SD+Prop group were deprived of sleep before intervention in the same way as before.

Results

We noted significant behavioral changes in rats treated with SIK3 inhibitors or tau phosphorylation agonists before propofol injection or sevoflurane inhalation, with associated protein levels and dendritic spine density documented. Sevoflurane anesthesia-induced cognitive impairment following acute sleep deprivation was more pronounced than sleep deprivation-induced cognitive impairment alone and resulted in increased brain SIK3 levels, increased phosphorylation of total tau and tau, and decreased acetylation modifications. After using propofol, the cognitive function returned to baseline levels with a series of reversals of cognitive dysfunction.

Conclusions

These results suggest that sevoflurane inhalation via the SIK3 pathway aggravates cognitive impairment after acute sleep deprivation and that propofol anesthesia reverses the effects of sleep deprivation by affecting modifications of tau protein.

Sex-dependent effect of APOE on Alzheimer's disease and other age-related neurodegenerative disorders

The importance of apolipoprotein E (APOE) in late-onset Alzheimer's disease (LOAD) has been firmly established, but the mechanisms through which it exerts its pathogenic effects remain elusive. In addition, the sex-dependent effects of APOE on LOAD risk and endophenotypes have yet to be explained. In this Review, we revisit the different aspects of APOE involvement in neurodegeneration and neurological diseases, with particular attention to sex differences in the contribution of APOE to LOAD susceptibility. We discuss the role of APOE in a broader range of age-related neurodegenerative diseases, and summarize the biological factors linking APOE to sex hormones, drawing on supportive findings from rodent models to identify major mechanistic themes underlying the exacerbation of LOAD-associated neurodegeneration and pathology in the female brain. Additionally, we list sex-by-genotype interactions identified across neurodegenerative diseases, proposing APOE variants as a shared etiology for sex differences in the manifestation of these diseases. Finally, we present recent advancements in 'omics' technologies, which provide a new platform for more in-depth investigations of how dysregulation of this gene affects the development and progression of neurodegenerative diseases. Collectively, the evidence summarized in this Review highlights the interplay between APOE and sex as a key factor in the etiology of LOAD and other age-related neurodegenerative diseases. We emphasize the importance of careful examination of sex as a contributing factor in studying the underpinning genetics of neurodegenerative diseases in general, but particularly for LOAD.

Differences in Hypothalamic Lipid Profiles of Young and Aged Male Rats With Impaired and Unimpaired Spatial Cognitive Abilities and Memory

Lipids play a major role for several brain functions, including cognition and memory. There is a series of work on individual lipids showing involvement in memory mechanisms, a concise lipidome was not reported so far. Moreover, there is no evidence for age-related memory decline and there is only work on brain of young vs. aging animals. Aging animals, however, are not a homogeneous group with respect to memory impairments, thus animals with impaired and unimpaired memory can be discriminated. Following recent studies of hippocampal lipid profiles and hypothalamus controlled hormone profiles, the aim of this study was to compare hypothalamic, lipidomic changes in male Sprague-Dawley rats between young (YM), old impaired (OMI) and old unimpaired (OMU) males. Grouping criterions for aged rats were evaluated by testing them in a spatial memory task, the hole-board. YMs were also tested. Subsequently brains were removed, dissected and hypothalami were kept at −80°C until sample preparation and analysis on liquid chromatography / mass spectrometry (LC-MS). Significant differences in the amounts of a series of lipids from several classes could be detected between young and aged and between OMI and OMU. A large number of lipids were increased in OMI and a smaller number in OMU as compared to young rats. Differences of lipid ratios (log2 of ratio) between OMI and OMU consisted of glycerophosphocholines (aPC 36:2 and 36:3; PC 34:0, 36:1, 36:3 and 40:2); Glycerophosphoethanolamines (aPE 34:2, 38:5 and 40:5; LPE 18:1, 20:1, 20:4, 22:4 and 22:6; PE36:1 and 38:4); glycerophosphoserines (PS 36:1, 40:4, and 40:6); triacylglycerol TG 52:4; ceramide Cer 17:2 and sphingomyelin SM 20:0. Thus, hypothalamic lipid profiles across different lipid classes discriminate aged male animals into OMU and OMI. The underlying mechanisms may be related to different functional networks of lipids in memory mechanisms and differences in metabolic processes. The study underlines the importance of lipidomics in the pathophysiology of age-related cognitive decline. The necessity of evaluating the cognitive status of aged subjects by behavioral tests results in more specific detection of critical lipids in memory decline, on which now can be focused in subsequent memory studies in animals and humans.

Sex differences in Alzheimer's-related Tau biomarkers and a mediating effect of testosterone

Women show greater pathological Tau biomarkers than men along the Alzheimer's disease (AD) continuum, particularly among apolipoprotein ε-E4 (APOE4) carriers; however, the reason for this sex difference in unknown. Sex differences often indicate an underlying role of sex hormones. We examined whether testosterone levels might influence this sex difference and the modifying role of APOE4 status. Analyses included 172 participants (25 cognitively normal, 97 mild cognitive impairment, 50 AD participants) from the Alzheimer's Disease Neuroimaging Initiative (34% female, 54% APOE4 carriers, aged 55-90). We examined the separate and interactive effects of plasma testosterone levels and APOE4 on cerebrospinal fluid phosphorylated-tau181 (p-Tau) levels in the overall sample and the sex difference in p-Tau levels before and after adjusting for testosterone. A significant APOE4-by-testosterone interaction revealed that lower testosterone levels related to higher p-Tau levels among APOE4 carriers regardless of sex. As expected, women had higher p-Tau levels than men among APOE4 carriers only, yet this difference was eliminated upon adjustment for testosterone. Results suggest that testosterone is protective against p-Tau particularly among APOE4 carriers. The lower testosterone levels that typically characterize women may predispose them to pathological Tau, particularly among female APOE4 carriers.

Gestational and lactational exposition to di-n-butyl phthalate increases neurobehavioral perturbations in rats: A three generational comparative study

Di-n-butyl phthalate (DBP) cause significant deficits in cognition and memory, however the neuroanatomical basis for impairments remain poorly understood. This study evaluates neurobehavioral changes in rats for three successive generations between non-siblings by administering DBP at 500mg/kg bw dose through oral gavage from gestation day-6 to 21 and lactation (3-weeks). Weaning period evaluations and developmental deficits assessed showed variations specific to generation and the toxic potential of DBP was confounded by behavioral deficits that include changes in sensorimotor development reflex response, poor performance, low memory retention and greater latency period. The cytoarchitectural alterations witnessed in hippocampus include condensed nuclei, vacuole formation and remarkable degeneration, shrinkage of pyramidal neurons in CA1 and CA3 regions; disorganized hilar cells and hyperplasia in dentate gyrus. Comparatively, the enlisted changes were high in subsequent generations than preceding and correlates assessed between cognitive impairment(s) and endocrine function confirm a link indicating vulnerability of immature animals as target to disrupt neural and endocrine functions.

Amyloid deposition is associated with different patterns of hippocampal connectivity in men versus women

Compared to men, women are disproportionally affected by Alzheimer's disease (AD) and have an accelerated trajectory of cognitive decline and disease progression. Neurobiological factors underlying gender differences in AD remain unclear. This study investigated brain beta-amyloid (Aβ)-related neural system differences in cognitively normal older men and women (N = 61; 41 females, 65-93 years old). We found that men and women showed different associations between Aβ load and hippocampal functional connectivity. During associative memory encoding, in men greater Aβ burden was accompanied by greater hippocampus-prefrontal connectivity (i.e., more synchronized activities), whereas in women hippocampal connectivity did not vary by Aβ burden. For resting-state data, the interaction of gender × Aβ on hippocampal connectivity did not survive multiple comparison in the whole-brain analyses. In the region of interest-based analyses, resting-state hippocampal-prefrontal connectivity was positively correlated with Aβ load in men and was negatively correlated with Aβ load in women. The observed Aβ-related neural differences may explain the accelerated trajectory of cognitive decline and AD progression in women.

Neuroplasticity pathways and protein-interaction networks are modulated by vortioxetine in rodents

Background

The identification of biomarkers that predict susceptibility to major depressive disorder and treatment response to antidepressants is a major challenge. Vortioxetine is a novel multimodal antidepressant that possesses pro-cognitive properties and differentiates from other conventional antidepressants on various cognitive and plasticity measures. The aim of the present study was to identify biological systems rather than single biomarkers that may underlie vortioxetine’s treatment effects.

Results

We show that the biological systems regulated by vortioxetine are overlapping between mouse and rat in response to distinct treatment regimens and in different brain regions. Furthermore, analysis of complexes of physically-interacting proteins reveal that biomarkers involved in transcriptional regulation, neurodevelopment, neuroplasticity, and endocytosis are modulated by vortioxetine. A subsequent qPCR study examining the expression of targets in the protein–protein interactome space in response to chronic vortioxetine treatment over a range of doses provides further biological validation that vortioxetine engages neuroplasticity networks. Thus, the same biology is regulated in different species and sexes, different brain regions, and in response to distinct routes of administration and regimens.

Conclusions

A recurring theme, based on the present study as well as previous findings, is that networks related to synaptic plasticity, synaptic transmission, signal transduction, and neurodevelopment are modulated in response to vortioxetine treatment. Regulation of these signaling pathways by vortioxetine may underlie vortioxetine’s cognitive-enhancing properties.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-017-0376-x) contains supplementary material, which is available to authorized users.

Postmenopausal cognitive changes and androgen levels in the context of apolipoprotein E polymorphism

INTRODUCTION

The focus of this study was to assess cognitive functions in relation to androgens and specifically testosterone and dehydroepiandrosterone in postmenopausal women as well as the correlation between cognitive functions and these two androgens according to polymorphism of the apolipoprotein E gene (APOE).

MATERIAL AND METHODS

A group of 402 women was recruited to the study (minimum 2 years after the last menstruation, follicle-stimulating hormone (FSH) more than 30 U/ml and no dementia signs on Montreal Cognitive Assessment). The computerized battery of the Central Nervous System Vital Signs test was used to diagnose cognitive functions. APOE genotyping was performed by multiplex polymerase chain reaction (PCR). Testosterone (TTE) and dehydroepiandrosterone (DHEA) in the blood serum were assessed for further statistical correlations analysis.

RESULTS

In the group of postmenopausal women, higher testosterone concentration was associated with lower scores for Neurocognition Index (NCI) (p = 0.028), memory (p = 0.008) and psychomotor speed (p < 0.001). Presence of at least one APOE ε4 allele potentiated testosterone's negative influence on cognitive functions (p < 0.05). Woman with a high normal level of DHEA scored significantly better in verbal (p = 0.027) and visual memory (p < 0.001) than other participants. APOE polymorphism did not modify the relationship between DHEA concentration and scores for cognitive functions.

CONCLUSIONS

Hormonal balance variations after menopause may influence brain processes concerned with cognition, especially memory and psychomotor speed. The observed effects may be related to androgens' influence on higher cortical functions in the changed hormonal dynamics of the postmenopausal period.

Individual Differences in Male Rats in a Behavioral Test Battery: A Multivariate Statistical Approach

Animal models for anxiety, depressive-like and cognitive diseases or aging often involve testing of subjects in behavioral test batteries. The large number of test variables with different mean variations and within and between test correlations often constitute a significant problem in determining essential variables to assess behavioral patterns and their variation in individual animals as well as appropriate statistical treatment. Therefore, we applied a multivariate approach (principal component analysis) to analyse the behavioral data of 162 male adult Sprague-Dawley rats that underwent a behavioral test battery including commonly used tests for spatial learning and memory (holeboard) and different behavioral patterns (open field, elevated plus maze, forced swim test) as well as for motor abilities (Rota rod). The high dimensional behavioral results were reduced to fewer components associated with spatial cognition, general activity, anxiety-, and depression-like behavior and motor ability. The loading scores of individual rats on these different components allow an assessment and the distribution of individual features in a population of animals. The reduced number of components can be used also for statistical calculations like appropriate sample sizes for valid discriminations between experimental groups, which otherwise have to be done on each variable. Because the animals were intact, untreated and experimentally naïve the results reflect trait patterns of behavior and thus individuality. The distribution of animals with high or low levels of anxiety, depressive-like behavior, general activity and cognitive features in a local population provides information of the probability of their appeareance in experimental samples and thus may help to avoid biases. However, such an analysis initially requires a large cohort of animals in order to gain a valid assessment.

Multiple Mechanisms Linking Type 2 Diabetes and Alzheimer's Disease: Testosterone as a Modifier

Evidence in support of links between type-2 diabetes mellitus (T2DM) and Alzheimer's disease (AD) has increased considerably in recent years. AD pathological hallmarks include the accumulation of extracellular amyloid-β (Aβ) and intracellular hyperphosphorylated tau in the brain, which are hypothesized to promote inflammation, oxidative stress, and neuronal loss. T2DM exhibits many AD pathological features, including reduced brain insulin uptake, lipid dysregulation, inflammation, oxidative stress, and depression; T2DM has also been shown to increase AD risk, and with increasing age, the prevalence of both conditions increases. In addition, amylin deposition in the pancreas is more common in AD than in normal aging, and although there is no significant increase in cerebral Aβ deposition in T2DM, the extent of Aβ accumulation in AD correlates with T2DM duration. Given these similarities and correlations, there may be common underlying mechanism(s) that predispose to both T2DM and AD. In other studies, an age-related gradual loss of testosterone and an increase in testosterone resistance has been shown in men; low testosterone levels can also occur in women. In this review, we focus on the evidence for low testosterone levels contributing to an increased risk of T2DM and AD, and the potential of testosterone treatment in reducing this risk in both men and women. However, such testosterone treatment may need to be long-term, and would need regular monitoring to maintain testosterone at physiological levels. It is possible that a combination of testosterone therapy together with a healthy lifestyle approach, including improved diet and exercise, may significantly reduce AD risk.

High-intensity treadmill running impairs cognitive behavior and hippocampal synaptic plasticity of rats via activation of inflammatory response

Although appropriate exercise is beneficial for enhancing brain functions, high-intensity exercise (HIE)-induced cognitive dysfunction is causing more and more concerns nowadays. In the present study, we observed the effects of high-intensity treadmill running on the spatial learning of the adult Sprague Dawley male rats in Y-maze (n = 16 per group), and investigated its possible electrophysiological and molecular mechanisms by examining in vivo hippocampal long-term potentiation (LTP), central inflammatory responses, and JNK/p38/ERK signal pathway. The Y-maze active avoidance test showed that high-intensity treadmill running impaired spatial learning ability of rats, with increased error times and prolonged training time in recognizing safety condition. Associated with the cognitive dysfunction, the induction and maintenance of hippocampal LTP were also impaired by the HIE. Furthermore, accompanied by elevated levels of inflammatory factors IL-1β, TNF-α, and iNOS, overactivation of microglia and astrocytes was also found in the CA1 region of hippocampus in the excessive exercise group, indicating an inflammatory response induced by HIE. In addition, Western blot assay showed that the phosphorylation of JNK/p38/ERK proteins was enhanced in the exercise group. These results suggest that exercise stress-induced neuronal inflammatory responses in the hippocampus are associated with HIE-induced cognitive deficits, which may be involved in the upregulation of the JNK/p38/ERK pathway. © 2016 Wiley Periodicals, Inc.

Concerted Gene Expression of Hippocampal Steroid Receptors during Spatial Learning in Male Wistar Rats: A Correlation Analysis

Adrenal and gonadal steroid receptor activities are significantly involved and interact in the regulation of learning, memory and stress. Thus, a coordinated expression of steroid receptor genes during a learning task can be expected. Although coexpression of steroid receptors in response to behavioral tasks has been reported the correlative connection is unclear. According to the inverted U-shape model of the impact of stress upon learning and memory we hypothesized that glucocorticoid (GR) receptor expression should be correlated to corticosterone levels in a linear or higher order manner. Other cognition modulating steroid receptors like estrogen receptors (ER) should be correlated to GR receptors in a quadratic manner, which describes a parabola and thus a U-shaped connection. Therefore, we performed a correlational meta-analyis of data of a previous study (Meyer and Korz, 2013a) of steroid receptor gene expressions during spatial learning, which provides a sufficient data basis in order to perform such correlational connections. In that study male rats of different ages were trained in a spatial holeboard or remained untrained and the hippocampal gene expression of different steroid receptors as well as serum corticosterone levels were measured. Expressions of mineralocorticoid (MR) and GR receptors were positively and linearly correlated with blood serum corticosterone levels in spatially trained but not in untrained animals. Training induced a cubic (best fit) relationship between mRNA levels of estrogen receptor α (ERα) and androgen receptor (AR) with MR mRNA. GR gene expression was linearly correlated with MR expression under both conditions. ERα m RNA levels were negatively and linearily and MR and GR gene expressions were cubicely correlated with reference memory errors (RME). Due to only three age classes correlations with age could not be performed. The findings support the U-shape theory of steroid receptor interaction, however the cubic fit suggest a more complex situation, which mechanisms may be revealed in further studies.

Gonadectomy reduces the density of androgen receptor-immunoreactive neurons in male rat's hippocampus: testosterone replacement compensates it

BACKGROUND

In the present study, the role of gonadectomy on memory impairment and the density of androgen receptor-immunoreactive neurons in rats' hippocampus as well as the ability of testosterone to compensate of memory and the density of androgen receptors in the hippocampus was evaluated.

METHODS

Adult male rats (except intact-no testosterone group) were bilaterally castrated, and behavioral tests performed 2 weeks later. Animals bilaterally cannulated into lateral ventricles and then received testosterone (10, 40 and 120 µg/0.5 µl DMSO) or vehicle (DMSO; 0.5 µl) for gonadectomized-vehicle group, 30 min before training in water maze test. The androgen receptor-immunoreactive neurons were detected by immunohistochemical technique in the hippocampal areas.

RESULTS

In the gonadectomized male rats, a memory deficit was found in Morris water maze test on test day (5th day) after DMSO administration. Gonadectomy decreased density of androgen receptor-immunoreactive neurons in the rats' hippocampus. The treatment with testosterone daily for 5 days attenuated memory deficits induced by gonadectomy. Testosterone also significantly increased the density of androgen receptor-immunoreactive neurons in the hippocampal areas. The intermediate dose of this hormone (40 µg) appeared to have a significant effect on spatial memory and the density of androgen receptor-immunoreactive neurons in gonadectomized rats' hippocampus.

CONCLUSIONS

The present study suggests that testosterone can compensate memory failure in gonadectomized rats. Also testosterone replacement can compensate the reduction of androgen receptor-immunoreactive neurons density in the rats' hippocampus after gonadectomy.

On the effects of testosterone on brain behavioral functions

Testosterone influences the brain via organizational and activational effects. Numerous relevant studies on rodents and a few on humans focusing on specific behavioral and cognitive parameters have been published. The results are, unfortunately, controversial and puzzling. Dosing, timing, even the application route seem to considerably affect the outcomes. In addition, the methods used for the assessment of psychometric parameters are a bit less than ideal regarding their validity and reproducibility. Metabolism of testosterone contributes to the complexity of its actions. Reduction to dihydrotestosterone by 5-alpha reductase increases the androgen activity; conversion to estradiol by aromatase converts the androgen to estrogen activity. Recently, the non-genomic effects of testosterone on behavior bypassing the nuclear receptors have attracted the interest of researchers. This review tries to summarize the current understanding of the complexity of the effects of testosterone on brain with special focus on their role in the known sex differences.

Testosterone and social evaluative stress: the moderating role of basal cortisol

Research has suggested that stressful situations lead to a decrease in testosterone, whereas concern with one's social status increases testosterone. However, results from studies examining testosterone reactivity in stressful situations that involve evaluation by others (hence status concerns) are inconsistent. Furthermore, there is a lack of research examining individual differences in testosterone responses in such situations. In this study 85 male participants underwent the Trier Social Stress Test (TSST, which includes performing speech and arithmetic tasks in front of two critical evaluators) and practiced solving puzzles. Testosterone and cortisol levels were assessed from saliva. Across participants, testosterone increased from baseline to peak levels following the stressor tasks. Importantly, the increase in testosterone was larger for participants with lower basal cortisol. Hence, lower basal cortisol (which is known to be associated with low social fearfulness) may help one to mobilize a larger testosterone response in situations that involve social-evaluative stress. Given the hypothesized adaptive role of a larger testosterone response in social competition situations, the results suggest that there may be long-term benefits in learning to lower one's social fearfulness in situations involving potential for negative evaluation by others.

Intriguing roles of hippocampus-synthesized 17β-estradiol in the modulation of hippocampal synaptic plasticity

Accumulated studies have shown that 17β-estradiol (E2) can be de novo synthesized in the hippocampus, and its role in the regulation of hippocampal synaptic plasticity, which is the basis of learning and memory, has long been exploring. Steroidogenic enzymes (e.g., aromatase) that are essential to the hippocampus-synthesized synthesis of E2 have been detected in the hippocampus. Inhibition of E2 synthesis by aromatase inhibitors significantly reduces the density of hippocampal spine synapses, levels of some synaptic proteins such as spinopholin and synaptophysin. Moreover, the electrophysiological properties of hippocampal neurons are also changed in response to this inhibition. The influences of gonadal and hippocampal E2 on synaptic plasticity may exist some differences, since some reports showed that gonadal (or circulating) estrogens have no obvious effects in the modulation of hippocampal synaptic proteins as evidenced in some ovariectomized animals and postmenopausal women who suffered from Alzheimer's disease (AD). These evidences leads to a hypothesis that hippocampal E2 may play a more important role in modulation of synaptic plasticity than gonadal E2. The signaling pathways, whereby hippocampal E2 modulates synaptic plasticity, insist of classical chronic genomic pathway and rapid nongenomic pathway, which mediated by nonnuclear estrogen receptor (GPER) and/or nuclear or nonnuclear estrogen receptors, which require coactivators for their transcription activity. Among which steroid receptor coactivator-1 (SRC-1) is the predominant coactivator p160 family members in the brain. Several clues have shown that SRC-1 is expressed in hippocampus and is highly correlated with some key synaptic proteins developmentally or after orchidectomy but not ovariectomy, indicating SRC-1 may be regulated by hippocampus-synthesized E2 and profoundly involved in the mediation of hippocampal E2 regulation of hippocampal synaptic plasticity. Further studies about the exact roles of hippocampus-synthesized E2 and therefore SRC-1 are urgently needed in order to facilitate our understanding of hippocampal E2, which will be very important to the development of novel strategies of estrogen replacement therapy against neurodegenerative deficits such as Alzheimer's disease (AD).

Estrogen receptor α functions in the regulation of motivation and spatial cognition in young male rats

Estrogenic functions in regulating behavioral states such as motivation, mood, anxiety, and cognition are relatively well documented in female humans and animals. In males, however, although the entire enzymatic machinery for producing estradiol and the corresponding receptors are present, estrogenic functions have been largely neglected. Therefore, and as a follow-up study to previous research, we sub-chronically applied a specific estrogen receptor α (ERα) antagonist in young male rats before and during a spatial learning task (holeboard). The male rats showed a dose-dependent increase in motivational, but not cognitive, behavior. The expression of hippocampal steroid receptor genes, such as glucocorticoid (GR), mineralocorticoid (MR), androgen (AR), and the estrogen receptor ERα but not ERβ was dose-dependently reduced. The expression of the aromatase but not the brain-derived neurotrophic factor (BDNF) encoding gene was also suppressed. Reduced gene expression and increased behavioral performance converged at an antagonist concentration of 7.4 µmol. The hippocampal and blood serum hormone levels (corticosterone, testosterone, and 17β-estradiol) did not differ between the experimental groups and controls. We conclude that steroid receptors (and BDNF) act in a concerted, network-like manner to affect behavior and mutual gene expression. Therefore, the isolated view on single receptor types is probably insufficient to explain steroid effects on behavior. The steroid network may keep motivation in homeostasis by supporting and constraining the behavioral expression of motivation.

Gender, sex steroid hormones, and Alzheimer's disease

Age-related loss of sex steroid hormones is a established risk factor for the development of Alzheimer's disease (AD) in women and men. While the relationships between the sex steroid hormones and AD are not fully understood, findings from both human and experimental paradigms indicate that depletion of estrogens in women and androgens in men increases vulnerability of the aging brain to AD pathogenesis. We review evidence of a wide range of beneficial neural actions of sex steroid hormones that may contribute to their hypothesized protective roles against AD. Both estrogens and androgens exert general neuroprotective actions relevant to a several neurodegenerative conditions, some in a sex-specific manner, including protection from neuron death and promotion of select aspects of neural plasticity. In addition, estrogens and androgens regulate key processes implicated in AD pathogenesis, in particular the accumulation of β-amyloid protein. We discuss evidence of hormone-specific mechanisms related to the regulation of the production and clearance of β-amyloid as critical protective pathways. Continued elucidation of these pathways promises to yield effective hormone-based strategies to delay development of AD.

Stress induced hippocampal mineralocorticoid and estrogen receptor β gene expression and long-term potentiation in male adult rats is sensitive to early-life stress experience

Glucocorticoid hormones and their receptors have been identified to be involved in emotional and cognitive disorders in early stressed subjects during adulthood. However, the impact of other steroid hormones and receptors has been considered less. Especially, functional roles of estrogen and estrogen receptors in male subjects are largely unknown. Therefore, we measured hippocampal concentrations of 17β-estradiol, corticosterone and testosterone, as well as the gene expression of estrogen receptor α and β (ERα, β), androgen receptor (AR), glucocorticoid (GR) and mineralocorticoid (MR) receptors after stress in adulthood in maternally separated (MS+; at postnatal days 14-16 for 6h each day) and control (MS-) male rats. In vivo hippocampal long-term potentiation (LTP) serves as a cellular model of learning and memory formation. Population spike- (PSA) and the fEPSP-LTP within the dentate gyrus (DG) were reinforced by elevated-platform-stress (EP-stress) in MS- but not in MS+ rats. MR- and ERβ-mRNA were upregulated 1h after EP-stress in MS- but not in MS+ rats as compared to non-stressed littermates. Infusion of an MR antagonist before LTP induction blocked early- and late-PSA- and -fEPSP-LTP, whereas blockade of ERβ impaired only the late PSA-LTP. Application of a DNA methyltransferase (DNMT) inhibitor partly restored the LTP-reinforcement in MS+ rats, accompanied by a retrieval of ERβ- but not MR-mRNA upregulation. Basal ERβ gene promoter methylation was similar between groups, whereas MS+ and MS- rats showed different methylation patterns across CpG sites after EP-stress. These findings indicate a key role of ERβ in early-stress mediated emotionality and emotion-induced late-LTP in adult male rats via DNA methylation mechanisms.

The effect of testosterone on the formation of brain structures

It has been confirmed in several studies that testosterone can significantly affect brain development. Following metabolism of this hormone by 5α-reductase to dihydrotestosterone, testosterone may act via androgen receptors, or after conversion by aromatase to estradiol, it may act via estrogen receptors. The parts of the brain which are changed under the influence of sex hormones are known as sexually dimorphic nuclei, especially in the preoptic area of the hypothalamus. Nevertheless, evidence suggests that testosterone also influences the structure of the hippocampus, specifically CA1 and CA3 areas of the hippocampus, as well as the amygdala. These brain areas are designed to convert information from short-term into long-term memory. In this review, we summarize the effects of testosterone on the organization of brain structures with respect to spatial cognitive abilities in small rodents.

Reversal of prenatal morphine exposure-induced memory deficit in male but not female rats

Impaired memory performance in offspring is one of the long-lasting neurobehavioral consequences of prenatal opiate exposure. Here, we studied the effects of prenatal morphine exposure on inhibitory avoidance memory performance in male and female offspring and also investigated whether these deficits are reversible during the postnatal development. Pregnant Wistar rats received morphine sulfate through drinking water, from the first day of gestation up to the day 13, M₁₋₁₃, or to the time of delivery, M₁₋₂₁. Four- and ten-week-old (adolescent and adult, respectively) male and female offspring were subjected to behavioral assays and then analysis of proteins involved in apoptosis or in synaptic plasticity. Results revealed that adolescent and adult female rats failed in passive avoidance retention task in both M₁₋₁₃ and M₁₋₂₁ groups. Adolescent and adult male offspring were similar to control animals in M₁₋₁₃ group. However M₁₋₂₁ impaired retention task in prepubertal male offspring, and this memory loss was repaired in postpubertal stage. Consistently, Bax/Bcl-2 ratio and cleaved caspase-3 were significantly increased in both M₁₋₁₃ and M₁₋₂₁ adolescent and adult female rats, but only in M₁₋₂₁ adolescent male rats. Furthermore, prenatal morphine exposure reduced the expression of brain-derived neurotrophic factor precursor protein in adolescent and adult female offspring and also decreased p-ca(2+)/calmodulin-dependent kinase II/ca(2+)/calmodulin-dependent kinase II ratio in adolescent male and female rats. Altogether, the results show that prenatal morphine exposure, depending on the time or duration of exposure, has distinct effects on male and female rats, and postnatal development may reverse these deficits more likely in males.

Hippocampal temporal-parietal junction interaction in the production of psychotic symptoms: a framework for understanding the schizophrenic syndrome

A framework is described for understanding the schizophrenic syndrome at the brain systems level. It is hypothesized that over-activation of dynamic gesture and social perceptual processes in the temporal-parietal occipital junction (TPJ), posterior superior temporal sulcus (PSTS) and surrounding regions produce the syndrome (including positive and negative symptoms, their prevalence, prodromal signs, and cognitive deficits). Hippocampal system hyper-activity and atrophy have been consistently found in schizophrenia. Hippocampal activity is highly correlated with activity in the TPJ and may be a source of over-excitation of the TPJ and surrounding regions. Strong evidence for this comes from in-vivo recordings in humans during psychotic episodes. Many positive symptoms of schizophrenia can be reframed as the erroneous sense of a presence or other who is observing, acting, speaking, or controlling; these qualia are similar to those evoked during abnormal activation of the TPJ. The TPJ and PSTS play a key role in the perception (and production) of dynamic social, emotional, and attentional gestures for the self and others (e.g., body/face/eye gestures, audiovisual speech and prosody, and social attentional gestures such as eye gaze). The single cell representation of dynamic gestures is multimodal (auditory, visual, tactile), matching the predominant hallucinatory categories in schizophrenia. Inherent in the single cell perceptual signal of dynamic gesture representations is a computation of intention, agency, and anticipation or expectancy (for the self and others). Stimulation of the TPJ resulting in activation of the self representation has been shown to result a feeling of a presence or multiple presences (due to heautoscopy) and also bizarre tactile experiences. Neurons in the TPJ are also tuned, or biased to detect threat related emotions. Abnormal over-activation in this system could produce the conscious hallucination of a voice (audiovisual speech), a person or a touch. Over-activation could interfere with attentional/emotional gesture perception and production (negative symptoms). It could produce the unconscious feeling of being watched, followed, or of a social situation unfolding along with accompanying abnormal perception of intent and agency (delusions). Abnormal activity in the TPJ would also be predicted to create several cognitive disturbances that are characteristic of schizophrenia, including abnormalities in attention, predictive social processing, working memory, and a bias to erroneously perceive threat.