Steroid Hormone Interaction with Dendritic Spines: Implications for Neuropsychiatric Disease

Dendritic spines, key sites for neural plasticity, are influenced by gonadal steroids. In this chapter, we review the effects of gonadal steroids on dendritic spine density in areas important to cognitive function, the hippocampus, and prefrontal cortex. Most of these animal model studies investigated the effects of estrogen in females, but we also include more recent data on androgen effects in both males and females. The underlying genomic and non-genomic mechanisms related to gonadal steroid-induced spinogenesis are also reviewed. Subsequently, we discuss possible reasons for the observed sex differences in many neuropsychiatric diseases, which appear to be caused, in part, by aberrant synaptic connections that may involve dendritic spine pathology. Overall, knowledge concerning the regulation of dendritic spines by gonadal hormones has grown since the initial discoveries in the 1990s, and current research points to a potential role for aberrant spine functioning in many neuropsychiatric disorders.

Sex differences in cognition following variations in endocrine status

Spatial memory, mediated primarily by the hippocampus, is responsible for orientation in space and retrieval of information regarding location of objects and places in an animal's environment. Since the hippocampus is dense with steroid hormone receptors and is capable of robust neuroplasticity, it is not surprising that changes in spatial memory performance occur following a variety of endocrine alterations. Here, we review cognitive changes in both spatial and nonspatial memory tasks following manipulations of the hypothalamic-pituitary-adrenal and gonadal axes and after exposure to endocrine disruptors in rodents. Chronic stress impairs male performance on numerous behavioral cognitive tasks and enhances or does not impact female cognitive function. Sex-dependent changes in cognition following stress are influenced by both organizational and activational effects of estrogen and vary depending on the developmental age of the stress exposure, but responses to gonadal hormones in adulthood are more similar than different in the sexes. Also discussed are possible underlying neural mechanisms for these steroid hormone-dependent, cognitive effects. Bisphenol A (BPA), an endocrine disruptor, given at low levels during adolescent development, impairs spatial memory in adolescent male and female rats and object recognition memory in adulthood. BPA's negative effects on memory may be mediated through alterations in dendritic spine density in areas that mediate these cognitive tasks. In summary, this review discusses the evidence that endocrine status of an animal (presence or absence of stress hormones, gonadal hormones, or endocrine disruptors) impacts cognitive function and, at times, in a sex-specific manner.

Androgens Enhance Recognition Memory and Dendritic Spine Density in the Hippocampus and Prefrontal Cortex of Ovariectomized Female Rats

Estrogen replacement has been repeatedly shown to enhance memory and increase dendritic spine density in the hippocampus and prefrontal cortex of ovariectomized (OVX) female rats. Given the potential deleterious effects of chronic estrogen administration, the present study assessed cognitive function using recognition memory tasks and measured dendritic spine density in the CA1 region of the hippocampus and medial prefrontal cortex after subchronic androgen replacement to adult OVX female rats. All androgens enhanced recognition memory in OVX rats, but object placement (OP) and object recognition (OR) results differed. Only testosterone enhanced OR. Testosterone had no effect on OP while dehydroepiandrosterone (DHEA), dihydrotestosterone (DHT) and androstenedione (AD) enhanced OP. Dendritic spine density was increased by both TP and DHEA in both brain areas (DHT and AD were not tested). Lastly, we used the aromatase inhibitor, letrozole, to discriminate between potential androgenic and estrogenic effects of androgens on behavior. Letrozole alone did not alter recognition memory in OVX rats and did not block the effects of either TP or DHEA on recognition memory suggesting that effects were mediated via androgenic mechanisms. The present results expand previous information on gonadal hormone actions and show that, in addition to estrogens, androgens also improve memory and increase spine density in brains of OVX female rats. While requiring further investigation, these observations provide a basis for therapeutic interventions in the treatment of menopausal, age or disease related memory loss.

A potential role for dendritic spines in bisphenol-A induced memory impairments during adolescence and adulthood

Developmental exposure to Bisphenol A (BPA), an endocrine disrupting chemical, alters many behaviors and neural parameters in rodents and non-human-primates. The effects of BPA are mediated via gonadal hormone, primarily, estrogen receptors, and are not limited to the perinatal period since recent studies show impairments further into development. The studies described in this chapter address the effects of BPA administration during early adolescence on memory and dendritic spine density in intact male and female rats as well as ovariectomized (OVX) rats in late adolescence and show that some of these adolescent induced changes endure into adulthood. In general, BPA impairs spatial memory and induces decreases in dendritic spine density in the hippocampus and the medial prefrontal cortex, two areas important for memory. The effects of adolescent BPA in intact females are compared to OVX females in an attempt to address the importance of estrogens in the mechanism(s) underlying the profound neuronal alterations occurring during adolescent development. In addition, potential mechanisms by which acute and chronic BPA induce structural alterations are discussed. These studies suggest a complex interaction between low doses of BPA, gonadal state and neural development.

Estrogenic regulation of memory: The first 50 years

This review highlights fifty years of progress in research on estradiol's role in regulating behavior(s). It was initially thought that estradiol was only involved in regulating estrus/menstrual cycles and concomitant sexual behavior, but it is now clear that estradiol also influences the higher order neural function of cognition. We provide a brief overview of estradiol's regulation of memory and some mechanisms which underlie its effects. Given systemically or directly into the hippocampus, to ovariectomized female rodents, estradiol or specific agonists, enhance learning and/or memory in a variety of rodent cognitive tasks. Acute (within minutes) or chronic (days) treatments enhance cognitive functions. Under the same treatment conditions, dendritic spine density on pyramidal neurons in the CA1 area of the hippocampus and medial prefrontal cortex increase which suggests that these changes are an important component of estrogen's ability to impact memory processes. Noradrenergic, dopaminergic and serotoninergic activity are also altered in these areas following estrogen treatments. Memory enhancements and increased spine density by estrogens are not limited to females but are also present in castrate males. In the next fifty years, neuroscientists need to determine how currently described neural changes mediate improved memory, how interactions among areas important for memory promote memory and the potential significance of neurally derived estrogens in normal cognitive processing. Answering these questions may provide significant advances for treatment of dementias as well as age and neuro-degenerative disease related memory loss.

Rapid effects on memory consolidation and spine morphology by estradiol in female and male rodents

Contribution to Special Issue on Fast effects of steroids. Rapid, neurosteroid-like effects of estrogens on memory consolidation during recognition memory tasks in both male and female rodents are described. We discuss how these mnemonic changes are related to rapid estrogenic effects on dendritic spine density, the distribution of spine types and the expression of PSD95 and GluA2 within spines in the hippocampus and medial prefrontal cortex, two areas critical for learning and memory. Overall, these data lead to the conclusion that estrogens are capable of exerting rapid and potent influences on memory and spine morphology in both sexes. The demonstration of estrogenic effects in males, which are used in the majority of memory studies, may provide a model for better understanding how hormone dependent changes in signaling pathways mediating memory and spinogenesis are coordinated to promote memory consolidation.

Sex differences in chronic stress effects on cognition in rodents

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

Estradiol: Mediator of memories, spine density and cognitive resilience to stress in female rodents

Estradiol rapidly activates, within minutes, various physiological functions and behaviors including cognition in rodents. This review describes rapid effects of estradiol on hippocampal dependent learning and memory tasks in rodents. Mechanisms underlying the memory enhancements including the activation of signaling molecules and the enhancement of dendritic spinogenesis are briefly reviewed. In addition, the role of estradiol in the cognitive resilience to chronic stress exhibited only in females is discussed including contributions of ovarian as well as intra-hippocampally derived estrogens to this sex difference. Finally, speculations on possible physiologic functions for rapid mnemonic changes mediated by estrogens are made. Overall, the emergence of a novel and powerful mechanism for regulation of cognition by estradiol is described.

The evolving role of dendritic spines and memory: Interaction(s) with estradiol

This article is part of a Special Issue "Estradiol and Cognition". Memory processing is presumed to depend on synaptic plasticity, which appears to have a role in mediating the acquisition, consolidation, and retention of memory. We have studied the relationship between estrogen, recognition memory, and dendritic spine density in the hippocampus and medial prefrontal cortex, areas critical for memory, across the lifespan in female rodents. The present paper reviews the literature on dendritic spine plasticity in mediating both short and long term memory, as well as the decreased memory that occurs with aging and Alzheimer's disease. It also addresses the role of acute and chronic estrogen treatments in these processes.

Bisphenol-A exposure during adolescence leads to enduring alterations in cognition and dendritic spine density in adult male and female rats

We have previously demonstrated that adolescent exposure of rats to bisphenol-A (BPA), an environmental endocrine disrupter, increases anxiety, impairs spatial memory, and decreases dendritic spine density in the CA1 region of the hippocampus (CA1) and medial prefrontal cortex (mPFC) when measured in adolescents in both sexes. The present study examined whether the behavioral and morphological alterations following BPA exposure during adolescent development are maintained into adulthood. Male and female, adolescent rats received BPA, 40μg/kg/bodyweight, or control treatments for one week. In adulthood, subjects were tested for anxiety and locomotor activity, spatial memory, non-spatial visual memory, and sucrose preference. Additionally, stress-induced serum corticosterone levels and dendritic spine density in the mPFC and CA1 were measured. BPA-treated males, but not females, had decreased arm visits on the elevated plus maze, but there was no effect on anxiety. Non-spatial memory, object recognition, was also decreased in BPA treated males, but not in females. BPA exposure did not alter spatial memory, object placement, but decreased exploration during the tasks in both sexes. No significant group differences in sucrose preference or serum corticosterone levels in response to a stress challenge were found. However, BPA exposure, regardless of sex, significantly decreased spine density of both apical and basal dendrites on pyramidal cells in CA1 but had no effect in the mPFC. Current data are discussed in relation to BPA dependent changes, which were present during adolescence and did, or did not, endure into adulthood. Overall, adolescent BPA exposure, below the current reference safe daily limit set by the U.S.E.P.A., leads to alterations in some behaviors and neuronal morphology that endure into adulthood.

Adolescent bisphenol-A exposure decreases dendritic spine density: role of sex and age

Bisphenol-A (BPA), a common environmental endocrine disruptor, modulates estrogenic, androgenic, and antiandrogenic effects throughout the lifespan. We recently showed that low dose BPA exposure during adolescence increases anxiety and impairs spatial memory independent of sex. In this study, six week old Sprague Dawley rats (n=24 males, n=24 females) received daily subcutaneous injections (40 µg/kg bodyweight) of BPA or vehicle for one week. Serum corticosterone levels in response to a 1 h restraint stress and spine density were examined at age 7 (cohort 1) and 11 (cohort 2) weeks. Adolescent BPA exposure did not alter stress dependent corticosterone responses but decreased spine density on apical and basal dendrites of pyramidal cells in the medial prefrontal cortex (mPFC) and hippocampal CA1 region (CA1). Sex differences in spine density were observed on basal dendrites of the mPFC and CA1 with females having greater spine density than males. This sex difference was further augmented by both age and treatment, with results indicating that BPA-dependent decreases in spine density were more pronounced in males than females on mPFC basal dendrites. Importantly, the robust neuronal alterations were observed in animals exposed to BPA levels below the current U.S.E.P.A. recommended safe daily limit. These results are the first demonstrating that BPA given during adolescence leads to enduring effects on neural morphology at adulthood. Given that humans are routinely exposed to low levels of BPA through a variety of sources, the decreased spine density reported in both male and female rats after BPA exposure warrants further investigation.

Recognition memory tasks in neuroendocrine research

The recognition memory tasks, novel object and novel object location, have been beneficial to neuroendocrine research concerning the effects of gonadal and adrenal hormones on cognitive function. This review discusses the advantages of these tasks in comparison with other learning and memory tasks. Experiments conducted across a number of laboratories show that gonadal hormones, both estradiol and testosterone, promote memory while the adrenal hormone, corticosterone, impairs memory. The effects of these steroid hormones on spine density in the prefrontal cortex and hippocampus are also briefly presented. Overall, results show that these steroid hormones are potent modulators of memory consolidation in rodent models.

Interactions between estradiol, BDNF and dendritic spines in promoting memory

Several lines of evidence have converged to indicate that memory formation involves plasticity of dendritic spines in the medial prefrontal cortex (PFC) and the hippocampus. Memory varies with estrogen levels throughout the lifespan of the female. Generally, increased levels of estrogen are related to greater dendritic spine density on pyramidal cells in the PFC and the hippocampus and to improved memory function. Brain-derived neurotrophic factor (BDNF) is a growth factor which increases dendritic spines and enhances memory function. Estrogens increase BDNF levels in the PFC and the hippocampus. In the present review we provide evidence that estradiol and BDNF may work in concert to enhance cognition. In adult females, fluctuations in recognition memory following ovariectomy and estradiol replacement, during the estrous cycle, in pregnancy and with aging are accompanied by similar changes in circulating estradiol, BDNF levels and spine density alterations in the PFC and the hippocampus. In addition, both estradiol and BDNF induce spine plasticity via rapid membrane effects and slower transcriptional regulation via the CREB pathway. Moreover, estradiol increases BDNF levels through action on nuclear receptors. While the exact mechanism(s) for the influence of estrogens and BDNF on memory remain unclear, this combination may provide the basis for new and more effective strategies for treating age-related and neurodegenerative memory loss.

Estrogen-induced memory enhancements are blocked by acute bisphenol A in adult female rats: role of dendritic spines

Acute effects of bisphenol (BPA), an environmental chemical, on estradiol (17α or β-E2)-dependent recognition memory and dendritic spines in the medial prefrontal cortex and hippocampus were investigated in adult female rats. Ovariectomized rats received BPA 30 min before or immediately after a sample trial (viewing objects), and retention trials were performed 4 h later. Retention trials tested discrimination between old and new objects (visual memory) or locations (place memory). When given immediately after the sample trial, BPA, 1-400 μg/kg, did not alter recognition memory, but 1 and 40 μg/kg BPA, respectively, blocked 17β-E2-dependent increases in place and visual memory. When ovariectomized rats were tested with 17α-E2, 1 μg/kg BPA blocked place memory, but up to 40 μg did not block visual memory. BPA, given to cycling rats at 40 μg/kg, blocked visual, but not place, memory during proestrus when 2 h intertrial delays were given. Spine density was assessed at times of memory consolidation (30 min) and retention (4 h) after 17β-E2 or BPA + 17β-E2. In prefrontal cortex, BPA did not alter E2-dependent increases. In the hippocampus, BPA blocked E2 increases in basal spines at 4 h and was additive with E2 at 30 min. Thus, these novel data show that doses of BPA, below the current Environmental Protection Agency safe limit of 50 μg/kg, rapidly alter neural functions dependent on E2 in adult female rats.

Bisphenol-A impairs memory and reduces dendritic spine density in adult male rats

Exposure to Bisphenol-A (BPA), an endocrine disruptor used in plastics, occurs in the United States on a daily basis. Recent studies suggest exposure during development causes memory deficits later in life; however, the ramifications of exposure in adulthood are unclear. We examined the effects of acute BPA administration (40 μg/kg) on memory and synaptic plasticity in adult male rats. BPA significantly impaired both visual and spatial memory and decreased dendritic spine density on pyramidal cells in CA1 and the medial prefrontal cortex (mPFC). Additionally, BPA significantly decreased PSD-95, a synaptic marker, in the hippocampus and increased cytosolic pCREB, a transcription factor, in mPFC. Together, these findings show that a single dose of BPA, below the USEPA reference safe daily limit of 50 μg/kg/day, may block the formation of new memories by interfering with neural plasticity processes in the adult brain.

Cocaine alters dendritic spine density in cortical and subcortical brain regions of the postpartum and virgin female rat

Cocaine use during pregnancy induces profound neural and behavioral deficits in both mother and offspring. The present study was designed to compare the effects of cocaine exposure on spine density of postpartum and virgin female rat brains. Timed, pregnant, primiparous rats were injected with either cocaine (30 mg/kg) or saline, once daily, from gestational day 8 to 20. Twenty-four hours after giving birth, dam brains were processed for Golgi-impregnation. Virgin females were also injected with the same dose of cocaine or saline for 12 days and sacrificed 24 h after the last injection for comparison. Pregnant rats had significantly greater spine density in the medial amygdala (MeA) and medial preoptic area (MPOA) and lower spine density in CA1 than virgin females independent of cocaine treatment. Cocaine significantly increased dendritic spine density on the apical branch of pyramidal cells in the prefrontal cortex (PFC, 15%), both apical (13%) and basal (14.8%) branches of CA1 and cells in the MeA (28%) of pregnant rats. In the MPOA, cocaine administration resulted in a decrease in dendritic spine density (14%) in pregnant rats. In virgin females, cocaine had fewer effects but did increase dendritic spine density on both branches of CA1 neurons and in the MeA. The present study is the first to demonstrate that spine density differs between pregnant and virgin females and that pregnancy makes the brain more vulnerable to cocaine, which has important clinical implications.

Estrogen receptor (ER) subtype agonists alter monoamine levels in the female rat brain

We assessed the effects of subtype-selective ER agonists on monoamine levels in discrete regions of the female rat brain. Ovariectomized (ovx) rats were treated for 4 days with vehicle, 17β-estradiol (E; 0.05mg/kg), an ERβ agonist (C19; 3mg/kg) or an ERα agonist (PPT; 3mg/kg) and samples from brain regions were assessed for monoamines and metabolites. We also assessed effects of ERβ modulation on baseline and fenfluramine-induced release of monoamines in hippocampus using microdialysis. In the first study, E and the ERα agonist increased norepinephrine in cortex and all three ER ligands increased it in the ventral hippocampus. Changes in levels of the noradrenergic metabolite, MHPG and the dopaminergic metabolite, DOPAC were noted in brain areas of ER ligand-treated animals. E also increased levels of 5HIAA in three brain areas. In the microdialysis study, there were no differences among groups in baseline levels of monoamines. However, E and the ERβ agonist increased levels of the dopaminergic metabolite, HVA following fenfluramine. In summary, activation of the two nuclear ERs with selective agonists affects monoamine and metabolite levels in discrete brain areas, a number of which are known to play key roles in cognitive and affective function.

Estradiol and ERβ agonists enhance recognition memory, and DPN, an ERβ agonist, alters brain monoamines

Effects of estradiol benzoate (EB), ERα-selective agonist, propyl pyrazole triol (PPT) and ERβ-selective agonists, diarylpropionitrile (DPN) and Compound 19 (C-19) on memory were investigated in OVX rats using object recognition (OR) and placement (OP) memory tasks. Treatments were acute (behavior 4h later) or sub chronic (daily injections for 2 days with behavior 48 h later). Objects were explored in sample trials (T1), and discrimination between sample (old) and new object/location in recognition trials (T2) was examined after 2-4h inter-trial delays. Subjects treated sub chronically with EB, DPN, and C-19, but not PPT, discriminated between old and new objects and objects in old and new locations, suggesting that, at these doses and duration of treatments, estrogenic interactions with ERβ contribute to enhancements in recognition memory. Acute injections of DPN, but not PPT, immediately after T1, also enhanced discrimination for both tasks (C19 was not investigated). Effects of EB, DPN and PPT on anxiety and locomotion, measured on elevated plus maze and open field, did not appear to account for the mnemonic enhancements. Monoamines and metabolites were measured following DPN treatment in subjects that did not receive behavioral testing. DPN was associated with alterations in monoamines in several brain areas: indexed by the metabolite, 3-methoxy-4-hydroxyphenylglycol (MHPG), or the MHPG/norepinephrine (NE) ratio, NE activity was increased by 60-130% in the prefrontal cortex (PFC) and ventral hippocampus, and NE activity was decreased by 40-80% in the v. diagonal bands and CA1. Levels of the dopamine (DA) metabolite, homovanillic acid (HVA), increased 100% in the PFC and decreased by 50% in the dentate gyrus following DPN treatment. The metabolite of serotonin, 5-hydroxyindole acetic acid (5-HIAA), was increased in the PFC and CA3, by approximately 20%. No monoaminergic changes were noted in striatum or medial septum. Results suggest that ERβ mediates sub chronic and acute effects of estrogens on recognition memory and that memory enhancements by DPN may occur, in part, through alterations in monoaminergic containing systems primarily in PFC and hippocampus.

Acute estrogen treatment facilitates recognition memory consolidation and alters monoamine levels in memory-related brain areas

Acute effects of estrogens on mnemonic processes were examined at the behavioral and neurochemical levels. 17beta-estradiol and 17alpha-estradiol influences on memory consolidation were assessed using object placement (OP) and object recognition (OR) tasks. Subjects received treatment immediately after a sample trial (exploring two novel objects), and memory of objects (OR memory) or location of objects (OP memory) was tested 4h later. Both isomers of estradiol enhanced memory. For spatial memory, 15 and 20 microg/kg of 17beta-estradiol facilitated OP, while lower and higher doses were ineffective. 17alpha-estradiol had a similar pattern, but a lower dose was effective. When treatment was delayed until 45 min after a sample trial, memory was not enhanced. For non-spatial memory, OR was facilitated at 5 microg/kg of 17beta-estradiol and at 1 and 2 microg/kg of 17alpha-estradiol and, similar to OP, lower and higher doses were ineffective. These data demonstrate that beneficial effects of estrogens are dose, time and task dependent, and the dose-response pattern is an inverted U. Because monoamines are known to have contributions to memory, brains were removed 30 min after treatment for measurements of dopamine (DA), norepinephrine (NE), serotonin (5-HT), and metabolites. Estrogen elevated 5HT, NE metabolite MHPG, turnover ratio of NE to MHPG, and DA metabolite DOPAC levels in the prefrontal cortex, while NE and MHPG were decreased in the hippocampus. Thus, acute estrogens exert rapid effects on memory consolidation and neural function, which suggests that its mnemonic effects may involve activation of membrane associated estrogen receptors and subsequent signaling cascades, and that monoamines may contribute to this process.

Therapeutics for cognitive aging

This review summarizes the scientific talks presented at the conference "Therapeutics for Cognitive Aging," hosted by the New York Academy of Sciences and the Alzheimer's Drug Discovery Foundation on May 15, 2009. Attended by scientists from industry and academia, as well as by a number of lay people-approximately 200 in all-the conference specifically tackled the many aspects of developing therapeutic interventions for cognitive impairment. Discussion also focused on how to define cognitive aging and whether it should be considered a treatable, tractable disease.

Impaired Recognition Memory and Decreased Prefrontal Cortex Spine Density in Aged Female Rats

Aged F344 female rats (21 months) showed decreased performance, as compared to young rats (4 months), on an object recognition memory task. Golgi impregnation measured dendritic spine density of pyramidal neurons in the prefrontal cortex (layer II-III), a brain area important for recognition memory. Densities of spines in aged rats were 16% lower in tertiary, apical dendrites, but not significantly different in secondary basal dendrites. Concurrent measures of memory and spine density in the young and aged subjects show that age-related declines in recognition memory are associated with decreased cortical spine density.

Ovariectomized rats show decreased recognition memory and spine density in the hippocampus and prefrontal cortex

Effects of ovariectomy (OVX) on performance of the memory tasks, Object Recognition (OR) and Object Placement (OP), and on dendritic spine density in pyramidal neurons in layer II/III of the prefrontal cortex and the CA1 and CA3 regions of the hippocampus were determined. OVX was associated with a significant decline in performance of the memory tasks as compared to intact rats beginning at 1 week post OVX for OR and 4 weeks post OVX for OP. Golgi impregnation at 7 weeks post OVX showed significantly lower spine densities (17-53%) in the pyramidal neurons of the medial prefrontal cortex and the CA1, but not the CA3, region of the hippocampus in OVX compared to intact rats. These results suggest that cognitive impairments observed in OVX rats may be associated with morphological changes in brain areas mediating memory.

Dietary phytoestrogens enhance spatial memory and spine density in the hippocampus and prefrontal cortex of ovariectomized rats

Long-term maintenance of ovariectomized rats (9 weeks) on chow containing high phytoestrogen levels (Purina LabDiet 5001) as compared to chow with minimal phytoestrogens (Harlan 2016 Teklad) was associated with better performance of the spatial memory task, object placement, increased dendritic spine density in CA1 and prefrontal cortex pyramidal neurons, and higher uterine weights. Object recognition memory, anxiety on an elevated plus maze and body weight were unaffected by phytoestrogen levels in the diet.

Changes in cerebrospinal fluid neurochemistry during pregnancy

BACKGROUND

Little is known about changes in brain function that may occur during pregnancy. Studies in rodents and sheep suggest that several brain neurotransmitter and neurohormonal systems known to modulate anxiety may be altered during pregnancy.

METHODS

Cerebrospinal fluid (CSF) and plasma samples were obtained from 21 women (during weeks 38-39 of pregnancy) who were undergoing elective cesarean section and from 22 healthy nonpregnant women.

RESULTS

The CSF levels of g-aminobutyric acid (GABA) and 3-methoxy-4-hydroxyphenylglycolwere reduced in pregnant women. There were no changes in CSF glutamate, 5-hydroxyindoleactic acid, and homovanillic acid. There was a large increase in CSF prolactin in pregnant women and also a trend toward an elevation in CSF oxytocin. Levels of prolactin, but not oxytocin, in CSF and plasma were correlated in pregnant women.

CONCLUSIONS

These results suggest that pregnancy alters regulation of brain GABA, norepinephrine, and prolactin, which may play a role in changes in vulnerability to anxiety and depression during pregnancy and postpartum. Prolactin circulating in the bloodstream seems to be the major source of CSF prolactin during pregnancy.

Estrogen alters hippocampal dendritic spine shape and enhances synaptic protein immunoreactivity and spatial memory in female mice

Estrogen (E) treatment induces axospinous synapses in rat hippocampus in vivo and in cultured hippocampal neurons in vitro. To better explore the molecular mechanisms underlying this phenomenon, we have established a mouse model for E action in the hippocampus by using Golgi impregnation to examine hippocampal dendritic spine morphology, radioimmunocytochemistry (RICC) and silver-enhanced immunocytochemistry to examine expression levels of synaptic protein markers, and hippocampal-dependent object-placement memory as a behavioral readout for the actions of E. In ovariectomized mice of several strains and F(1) hybrids, the total dendritic spine density on neurons in the CA1 region was not enhanced by E treatment, a finding that differs from that in the female rat. E treatment of ovariectomized C57BL/6J mice, however, caused an increase in the number of spines with mushroom shapes. By RICC and silver-enhanced immunocytochemistry, we found that the immunoreactivity of postsynaptic markers (PSD95 and spinophilin) and a presynaptic marker (syntaxin) were enhanced by E treatment throughout all fields of the dorsal hippocampus. In the object-placement tests, E treatment enhanced performance of object placement, a spatial episodic memory task. Taken together, the morphology and RICC results suggest a previously uncharacterized role of E in synaptic structural plasticity that may be interpreted as a facilitation of the spine-maturation process and may be associated with enhancement of hippocampal-dependent memory.

Functional aspects of estrogen neuroprotection

Evidence that estrogen protects neurons against toxic/ ischemic insults or degenerative/aging processes is evident in a variety of in vitro and in vivo systems. However, a critical remaining question is: Does the demonstrated morphologic and neurochemical protection by estrogen lead to a preservation of brain function or an enhanced ability to recover? To date, little basic research is available on this issue. Cognition is a critical function that might provide a sensitive way to examine this question. As a first step, we present results showing that two chronic environmental insults, psychoactive drugs and stress, produce gender-specific responses in cognitive abilities. Specifically, females appear less sensitive than males to cognitive impairments following chronic exposure to these factors. Results are presented in male and female rats utilizing cognitive tests that assess visual (object recognition) and spatial memory (object placement and radial arm maze) following chronic amphetamine, methamphetamine, or daily restraint stress. Following regimes of chronic stress or amphetamine, males were impaired on these tasks while females were either unaffected, less affected, or enhanced in performance. These observations suggest that differences in circulating gonadal hormone levels between the sexes may contribute to the differential sensitivity of the sexes and provide endogenous neuroprotection for females. Surprisingly, ovariectomized females were still not impaired following a stress regimen that impaired males (21 d of daily restraint). These data taken together with neurochemical data on estrogen neuroprotective effects indicate that it is possible that neuroprotection by estrogen may result from hormone action both during sexual differentiation (organizational effect) and in adulthood (activational effect). These considerations and possible unwanted/untoward effects of chronic estrogen use are discussed in relation to the use of selective estrogen receptor modulators for chronic treatment of both males and females. In conclusion, although compelling evidence for neuroprotection by estrogen has been presented in anatomic and neurochemical studies, it is clear that the functional/ behavioral aspects need further investigation.

Sex differences in chronic stress effects on memory in rats

Recent studies in rodent models and in humans have shown that the status of both the gonadal and adrenal axes (hypothalamic-pituitary-gonadal, HPG and hypothalamic-pituitary-adrenal, HPA, respectively) can influence learning and memory function. In this article, the effects of activating the HPA axis (stress) on performance of memory tasks in rats are reviewed. More importantly, results are presented which show that chronic stress has a different impact on performance of these tasks depending upon the sex of the rat. These observations are novel and potentially important since few studies, animal or human, have utilized females as subjects in studies of the stress response. Sex differences in the effects of chronic stress on memory were investigated in rats using an object recognition task and two spatial memory tasks, radial arm maze and object location. Given the same chronic stress--21 days of restraint for 6 h each day--males were impaired in all of the memory tests while females showed enhanced performance of the spatial memory tasks and no changes in object recognition performance. Levels of neurotransmitters and metabolites were measured in brain areas important for cognition in the subjects in order to determine neural systems that may respond to stress and mediate the cognitive responses. These results show that responses of monoamine and amino acid containing neural systems may contribute to or underlie sex differences in stress effects on cognition. Stress decreased dopaminergic activity in the frontal cortex and amygdala of males but not females; whereas, in CA3 of the hippocampus, stress increased levels of 5-HT and norepinephrine in females, but not males, and increased GABA in males, but not females. Finally, a possible role for estradiol in mediating sexually differentiated responses to stress was examined. Behavioral and neurochemical evaluations in ovariectomized, stressed females, with or without estrogen replacement, suggest that sex differences in response to stress are influenced by both the organizing and activating effects of estradiol. A few, recent studies in humans, that show sexually dimorphic relationships between chronic stress and cognition, are also highlighted. These results in humans are consistent with the pattern of results in rats. Clearly, further studies are necessary to substantiate sex differences in stress effects on memory function in humans and to understand mechanisms whereby estrogen may influence the stress response in rats. Nonetheless, recent studies show sexually differentiated cognitive responses to chronic stress and underline the importance of considering the sex/gender of subjects when studying the stress response.

Serotonin mediates CA1 spine density but is not crucial for ovarian steroid regulation of synaptic plasticity in the adult rat dorsal hippocampus

The activity of the serotonin (5-hydroxytryptamine, 5-HT) system is sensitive to estradiol and progesterone. During the ovarian cycle, dendritic spines on CA1 pyramidal neurons of the dorsal hippocampus are increased by estradiol and later decreased by progesterone. We sought to determine whether 5-HT is involved in maintaining CA1 spine density and/or in steroid regulation of synaptic plasticity in dorsal hippocampus. Ovariectomized rats were treated (sc) over 10 days with the tryptophan hydroxylase inhibitor parachlorophenylalanine (pCPA) to deplete 5-HT, followed by estradiol benzoate on days 10 and 11. A subset of animals received progesterone on day 12. The day after the last treatment, rats were perfused and brains were processed for Golgi impregnation. Separate groups were processed for radioimmunocytochemistry (RICC) for the spine-associated protein, spinophilin, or high-performance liquid chromatography (HPLC) for monoamine analysis. Golgi and RICC data indicate that CA1 apical spine density was significantly decreased by pCPA (17-20%). Estradiol increased spine density in both saline- and pCPA-treated rats compared to respective controls (30%); however, pCPA animals maintained significantly fewer spines. No differences in spine densities were observed between saline- and pCPA-treated rats given estradiol and progesterone. Depletion of 5-HT by pCPA was confirmed in the CA1 (-90%) and dorsal raphe (-80%) by HPLC analysis. While 5-HT depletion was associated with a 57% decrease in CA1 norepinephrine (NE), there was no difference in dorsal raphe NE. Thus, whereas 5-HT is involved in maintaining spine density in the adult female rat CA1, it is not crucial for steroid-mediated plasticity. 5-HT-regulated spines/synapses may represent distinct populations from those modulated by estradiol and progesterone in dorsal hippocampus.

The gene encoding proline dehydrogenase modulates sensorimotor gating in mice

Hemizygous cryptic deletions of the q11 band of human chromosome 22 have been associated with a number of psychiatric and behavioural phenotypes, including schizophrenia. Here we report the isolation and characterization of PRODH, a human homologue of Drosophila melanogaster sluggish-A (slgA), which encodes proline dehydrogenase responsible for the behavioural phenotype of the slgA mutant. PRODH is localized at chromosome 22q11 in a region deleted in some psychiatric patients. We also isolated the mouse homologue of slgA (Prodh), identified a mutation in this gene in the Pro/Re hyperprolinaemic mouse strain and found that these mice have a deficit in sensorimotor gating accompanied by regional neurochemical alterations in the brain. Sensorimotor gating is a neural filtering process that allows attention to be focused on a given stimulus, and is affected in patients with neuropsychiatric disorders. Furthermore, several lines of evidence suggest that proline may serve as a modulator of synaptic transmission in the mammalian brain. Our observations, in conjunction with the chromosomal location of PRODH, suggest a potential involvement of this gene in the 22q11-associated psychiatric and behavioural phenotypes.

Catechol-O-methyltransferase-deficient mice exhibit sexually dimorphic changes in catecholamine levels and behavior

Catechol-O-methyltransferase (COMT) is one of the major mammalian enzymes involved in the metabolic degradation of catecholamines and is considered a candidate for several psychiatric disorders and symptoms, including the psychopathology associated with the 22q11 microdeletion syndrome. By means of homologous recombination in embryonic stem cells, a strain of mice in which the gene encoding the COMT enzyme has been disrupted was produced. The basal concentrations of brain catecholamines were measured in the striatum, frontal cortex, and hypothalamus of adult male and female mutants. Locomotor activity, anxiety-like behaviors, sensorimotor gating, and aggressive behavior also were analyzed. Mutant mice demonstrated sexually dimorphic and region-specific changes of dopamine levels, notably in the frontal cortex. In addition, homozygous COMT-deficient female (but not male) mice displayed impairment in emotional reactivity in the dark/light exploratory model of anxiety. Furthermore, heterozygous COMT-deficient male mice exhibited increased aggressive behavior. Our results provide conclusive evidence for an important sex- and region-specific contribution of COMT in the maintenance of steady-state levels of catecholamines in the brain and suggest a role for COMT in some aspects of emotional and social behavior in mice.

Restraint stress reversibly enhances spatial memory performance

The effects of restraint stress on performance of a spatial memory task, the eight arm radial maze, was examined in rats. When stress was given for 6 h/day for 7 days and performance evaluated days 10-13 post stress, no effect on performance was noted; however, daily restraint stress for 13 days caused a small, but significant, enhancement of performance days 10-13 post stress. Stressed rats performed better than controls: their number of correct choices in the first 8 visits was higher than the controls, and stressed rats took fewer total choices to finish the maze than controls. Stress-dependent, enhanced performance does not appear permanent since further maze testing on days 14 and 15 post stress showed no differences between the groups. Performance of the stressed rats significantly correlated with their stress-induced, serum corticosterone levels measured after 6 h of restraint on the last day of restraint, day 13 (r = -0.63, P < 0.05); rats with higher levels of CORT took fewer choices to finish the task. Examination of hippocampal CA3c pyramidal neurons with Golgi techniques showed no effect of stress on the basal or apical dendritic arbors. Since our previous study showed that 21 days of restraint stress is associated with impaired spatial memory performance (10), these results suggest that the duration of stress may differentially affect learning/memory with shorter periods of stress serving an adaptive function while longer durations causing maladaptive changes.

Effects of estradiol on radial arm maze performance of young and aged rats

Gonadectomized male and female Sprague-Dawley rats, given estradiol (E2) via sc Silastic capsules that generated proestrus levels of hormones, were tested for spatial memory performance on an 8-arm radial maze. Performance of males, with or without E2, exceeded that of females, with or without E2, for choice accuracy parameters over 20 trials. In addition, males reached criterion earlier than females (6 vs 11 trials). There were no significant effects of E2 on performance of either sex. When a 1-h delay was instituted between the 4th and 5th choices, the performance of males remained better than that of the females, and E2 administration was associated with a small, but significant, improvement in performance of the males but not the females. E2 administration to 25-month-old males also did not affect performance in regular trials, but performance was enhanced in trials with delays of 1-3 h after the 4th choice. These results show that estradiol can influence spatial memory performance and suggest that E2 may be beneficial for age and/or disease-related memory impairments.

Repeated stress causes reversible impairments of spatial memory performance

Restraint stress, 6 h/day for 21 days, caused an impairment, during acquisition, of the performance of a spatial memory task, the eight-arm radial maze. The impairment was reversible, temporally limited and blocked by phenytoin, a blocker of excitatory amino acid action, or tianeptine, an antidepressant, which lowers extracellular serotonin. These effects on behavior parallel the reversible stress-induced atrophy of dendrites of hippocampal CA3 neurons that are also blocked by the drugs.

5,7-DHT facilitated lordosis: effects of 5-HT agonists

The role of 5-HT (serotonin) in regulating lordosis was investigated by combining peripheral administration of the 5-HT agonists 8-OH-DPAT (8-hydroxy-2-[di-N-propylamino]tetralin) or TFMPP (1-[m-trifluoromethylphenyl]piperazine), with intrahypothalamic application of the 5-HT neurotoxin 5,7-DHT (5,7-dihydroxytryptamine). The 5-HT1A agonist, 8-OH-DPAT, significantly inhibited lordosis in 5,7-DHT-treated and non-treated rats. TFMPP, an agonist at 5-HT1B and 5-HT1C receptors, significantly facilitated lordosis in 5,7-DHT-treated and non-treated rats. Our results show that both inhibitory and facilitatory influences of hypothalamic 5-HT on lordosis, are modulated via postsynaptic receptors.

GABAergic-serotonergic interactions in regulating lordosis

The GABAB agonist, baclofen, causes a dose-dependent decrease in lordosis, and this effect is attenuated following hypothalamic serotonin, (5-HT) lesions. Baclofen administration is associated with decreased 5-HT activity in the medial preoptic (mPOA) and ventromedial nuclei and enhanced 5-HT activity in the midbrain central gray, and with enhanced norepinephrine activity in the mPOA. Interactions between gamma-aminobutyric acid (GABA) and 5-HT may, therefore, be important for the activation of lordosis.

Spatial memory deficits in aged rats: contributions of monoaminergic systems

Age-dependent changes in monoaminergic systems and their relationship to senescent memory decline were investigated in 4- and 25-26-month-old, female, Fischer 344 rats. Spatial memory performance was tested on an 8-arm radial maze, and levels of norepinephrine (NE), dopamine (DA) and metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid, serotonin (5-HT) and metabolite 5-hydroxyindoleacetic acid were measured in brain areas which contribute to memory function--basal forebrain cholinergic nuclei, subfields of the hippocampus, frontal and entorhinal cortex--and in monoaminergic cell body areas. The performance of aged subjects was significantly impaired as compared to young subjects, and alterations of 20-60% in monoamine and metabolite levels were measured in specific brain areas of aged rats. Decreased NE levels were found in basal forebrain nuclei and cortical areas but not in hippocampal subfields of aged rats. Changes in the 5-HT system were present in hippocampal, cortical and basal forebrain sites. Changes in the DA system were the most pervasive with aged rats showing decreased DA and/or metabolites in several basal forebrain nuclei, cortical areas, and the hippocampus. Aged rats showed 50% decreases of monoamines in locus coeruleus and substantia nigra and 30% decreases in the dorsal raphe nucleus. Some but not all of the changes correlated with memory performance. The present results in rats support evidence that age-dependent changes in monoaminergic function in discrete brain sites contribute to senescent memory decline and suggest that monoaminergic-cholinergic interactions within basal forebrain nuclei may be important in this decline.

Relationship of gonadal hormone administration, sex, reproductive status and age to monoamine oxidase activity within the hypothalamus

Abstract Activity of Type A monoamine oxidase (MAO) was measured in microdissected samples from preoptic-hypothalamic and hindbrain areas of young male and female rats, and aged female rats. Administration of estradiol and progesterone, in doses sufficient to facilitate lordosis behavior and induce a luteinizing hormone surge in ovariectomized, but not castrated rats, was associated with sexually dimorphic changes of MAO activity within the hypothalamus. Forty-two h following estradiol benzoate administration, increased MAO activity was measured in the ventromedial nucleus (VML) and midbrain central gray of females, while decreased MAO activity was measured in the VML and arcuate-median eminence (ArME) of males. Progesterone administration to estradiol benzoate-primed rats was associated with decreased MAO activity in the VML and medial preoptic nucleus (mPOA) of females and decreased activity in the dorsal raphe nucleus of males. Activity of MAO on diestrus, proestrus and estrus was assessed in ten preoptic-hypothalamic and hindbrain sites. Differences between days of the cycle were limited to the mPOA, ArME and VML. While activities were generally lowest at estrus, these areas exhibited different patterns of activity across the cycle. Activity was highest at proestrus in the mPOA and highest at diestrus in the VML and ArME. Activity of MAO in some areas of 25-month old, diestrus rats was altered as compared to young, cycling rats; however, ageing was not associated with widespread changes in MAO activity. In the suprachiasmatic nucleus, aged rats showed approximately 30% less activity than young rats. In the mPOA, VML and ArME, activity in aged females was different from some, but not all, days of the estrous cycle. These results show that MAO activity changes within specific hypothalamic sites when the neuroendocrine axis is altered. Since the changes are present in areas where activity of rnonoaminergic systems is critical for initiating gonadotrophin surges and inducing lordosis behavior, these results provide initial evidence that catabolism of monoamines by MAO may contribute to rnonoaminergic regulation of reproductive function.

Spatial memory deficits in aged rats: contributions of the cholinergic system assessed by ChAT

Activity of choline acetylase (ChAT) was measured in basal forebrain cholinergic nuclei and in projection sites of these cells in the hippocampus and cortex of young rats and of aged rats who showed impaired performance on the radial arm maze. Decreased ChAT activity was found in the vertical diagonal band nucleus, the dentate gyrus and striatum of aged rats.

Antivascular antibodies in the sera of patients with senile dementia of the Alzheimer's type

We have investigated the specificities of antibrain antibodies in the sera of patients with senile dementia of the Alzheimer's type (SDAT). Using indirect immunofluorescence, we observed a vascular pattern of staining in 6 of 16 sera from patients with typical SDAT. None of 14 sera from age matched controls demonstrated this pattern of staining. The vascular pattern of staining seen with SDAT sera was identical to the immunofluorescent staining of brain by a monoclonal antibody to vascular basement membrane heparan sulfate proteoglycan. Immunoabsorption of SDAT sera with purified vascular proteoglycan abolished the staining of brain vessels. Using an enzyme linked immunoassay, 3 of the 6 vascular-reactive SDAT sera, and none of the 24 sera from aged controls, were shown to contain antibodies to purified vascular heparan sulfate proteoglycan. Proteoglycans play an important role in the barrier function of the blood-brain barrier. Autoimmune injury to the blood-brain barrier by antivascular antibody may play a role in the pathogenesis of dementia by permitting the passage of injurious substances into the brain.

Analysis of temporal and dose-dependent effects of estrogen on monoamines in brain nuclei

Levels of norepinephrine (NE), dopamine (DA) and serotonin (5-HT) were measured in hypothalamic and limbic nuclei of ovariectomized rats after various doses of estradiol and at various intervals after estradiol administration. Of 13 areas examined, time- and dose-dependent effects of estrogen on monoamine content were restricted to only a few, discrete areas which concentrate estradiol. Subcutaneous administration of 1-50 micrograms of estradiol benzoate (EB) and measurement of monoamines 24 h later was associated with dose-dependent increases of NE in the medial preoptic nucleus, diagonal band nucleus and periventricular area of the anterior hypothalamus, and increased levels of DA in the periventricular area of the preoptic area. No changes were found in 5-HT levels, but dose-dependent increases in the level of the 5-HT metabolite, 5-hydroxyindole acetic acid (5-HIAA), were measured in the lateral portion of the ventromedial nucleus. Effects of 5 micrograms of EB were evaluated at 1.5, 6, 12 and 45 h after administration. No changes were noted at 1.5 h, but 5-HIAA in the ventromedial nucleus was elevated at 6 and 12 h. NE levels were elevated at 12 and 45 h in the diagonal band and preoptic nuclei and at 45 h in the lateral septum and periventricular area of the hypothalamus. DA levels decreased in the arcuate-median eminence area 45 h after estrogen. Intravenous administration of 10 micrograms of estrogen and measurement of monoamines 1 h later was not associated with altered levels of any monoamine suggesting that the estrogen-dependent changes are consistent with the genomic model for steroid hormone action.(ABSTRACT TRUNCATED AT 250 WORDS)